Transformation method, optical device, and microscope

Abstract

[Problem] To improve practicality. [Solution] This conversion method includes the feature of converting, by means of a conversion unit, the repetition frequency of pulsed light emitted from a light source, and the feature wherein the repetition frequency converted by the conversion unit is variable.

Description

[Technical Field]

[0001] The present invention relates to a conversion method, an optical device, and an optical microscope. [Background technology]

[0002] Pulsed light may be used as the laser light irradiated onto a sample from an optical microscope (see Patent Document 1). Patent Document 1 discloses that by irradiating a sample with pulsed light having a high repetition frequency and lowering the peak intensity of the pulsed light, photobleaching is suppressed while maintaining the signal intensity obtained when the sample is irradiated with pulsed light having a low repetition frequency. However, from the viewpoint of practicality, it may not necessarily be the case that a higher repetition frequency of pulsed light is better. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] U.S. Patent No. 7,961,764 Summary of the Invention

[0004] According to a first aspect of the present invention, there is provided a conversion method including converting the repetition frequency of pulsed light emitted from a light source using a conversion unit, and the repetition frequency converted by the conversion unit being variable. According to an aspect of the present invention, a repetition frequency of pulsed light emitted from a light source is converted by a conversion unit, and the repetition frequency converted by the conversion unit is variable, and the conversion of the repetition frequency by the conversion unit includes a first step of changing the polarization direction of the pulsed light to a first direction via a half-wave plate, a second step of splitting the pulsed light polarized in the first direction into a first polarized light and a second polarized light orthogonal to the first polarized light, a third step of providing a predetermined optical path length difference between the first polarized light and the second polarized light, and a fourth step of generating pulsed light with a changed repetition frequency by combining the first polarized light and the second polarized light after the predetermined optical path length difference has been provided, and the conversion unit performs a series of processes including the first step, the second step, the third step, and the fourth step n times (n is an integer of 1 or more) to change the repetition frequency from 2 to 3. n The method for converting n times into n times is provided, and the second stage of odd-numbered times and the fourth stage of even-numbered times are performed by a first polarizing beam splitter, and the fourth stage of odd-numbered times and the second stage of even-numbered times are performed by a second polarizing beam splitter different from the first polarizing beam splitter. According to an aspect of the present invention, a repetition frequency of pulsed light emitted from a light source is converted by a conversion unit, and the repetition frequency converted by the conversion unit is variable, and the conversion of the repetition frequency by the conversion unit includes a first step of changing the polarization direction of the pulsed light to a first direction via a half-wave plate, a second step of splitting the pulsed light polarized in the first direction into a first polarized light and a second polarized light orthogonal to the first polarized light, a third step of providing a predetermined optical path length difference between the first polarized light and the second polarized light, and a fourth step of generating pulsed light with a changed repetition frequency by combining the first polarized light and the second polarized light after the predetermined optical path length difference has been provided, and the conversion unit performs a series of processes including the first step, the second step, the third step, and the fourth step n times (n is an integer of 1 or more) to change the repetition frequency from 2 to 3. n The second and fourth stages are performed using a single polarizing beam splitter, the first polarization is P polarization and the second polarization is S polarization, a first quarter-wave plate is installed in the optical path of the P polarization, and a second quarter-wave plate is installed in the optical path of the S polarization, and in the third stage, the P polarization passes through the first quarter-wave plate twice and the S polarization passes through the second quarter-wave plate twice, and every n times in the third stage, the position at which the P polarization passes through the first quarter-wave plate is different and the position at which the S polarization passes through the second quarter-wave plate is different is provided.

[0005] According to a second aspect of the present invention, there is provided an optical device including a converter that converts the repetition frequency of pulsed light, and a switch that can change the repetition frequency converted by the converter. According to an aspect of the present invention, the repetition frequency of the pulsed light is set to 2 n and a switching unit capable of changing the repetition frequency converted by the conversion unit, wherein the conversion unit includes n optical units, each of which includes a first polarizing beam splitter that splits pulsed light polarized in a first direction into a first polarized light and a second polarized light orthogonal to the first polarized light, an optical path length difference providing unit that provides a predetermined optical path length difference between the first polarized light and the second polarized light, and a second polarized light providing unit that combines the first polarized light and the second polarized light after the predetermined optical path length difference has been provided to form pulsed light whose repetition frequency has been changed. and a polarizing beam splitter, wherein the switching unit changes the polarization direction of the pulsed light incident on the first polarizing beam splitter to either a first direction or a second direction different from the first direction, and among the n optical units, odd-numbered first polarizing beam splitters and even-numbered second polarizing beam splitters are used in combination in one polarizing beam splitter, and odd-numbered second polarizing beam splitters and even-numbered first polarizing beam splitters are used in combination in one polarizing beam splitter. According to an aspect of the present invention, the repetition frequency of the pulsed light is set to 2n and a switching unit capable of changing the repetition frequency converted by the conversion unit, wherein the conversion unit comprises n optical units, each of which comprises a first polarizing beam splitter that splits pulsed light polarized in a first direction into a first polarized light and a second polarized light orthogonal to the first polarized light, an optical path length difference providing unit that provides a predetermined optical path length difference between the first polarized light and the second polarized light, and a second polarized beam splitter that combines the first polarized light and the second polarized light after the predetermined optical path length difference has been provided to form pulsed light whose repetition frequency has been changed, and the switching unit is configured to change the polarization direction of the pulsed light incident on the first polarizing beam splitter to and a second direction different from the first direction, the first polarizing beam splitter and the second polarizing beam splitter are used together in one polarizing beam splitter, the first polarization is P polarization and the second polarization is S polarization, a first quarter-wave plate is installed in the optical path of the P polarization and a second quarter-wave plate is installed in the optical path of the S polarization, and in the process of providing a predetermined optical path length difference, the P polarization passes through the first quarter-wave plate twice and the S polarization passes through the second quarter-wave plate twice, and each of the n optical units has a different position at which the P polarization passes through the first quarter-wave plate and a different position at which the S polarization passes through the second quarter-wave plate.

[0006] According to a third aspect of the present invention, there is provided a microscope that irradiates an object to be observed with the pulsed light, the microscope comprising: an optical device that converts the repetition frequency of the pulsed light and is capable of changing the repetition frequency converted by the conversion unit; and an irradiation unit that irradiates the object to be observed with the pulsed light output from the optical device. According to an aspect of the present invention, there is provided a microscope that irradiates an object to be observed with pulsed light, the microscope comprising: an optical device according to the above aspect that converts the repetition frequency of the pulsed light; and an irradiation unit that irradiates the object to be observed with the pulsed light output from the optical device. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 is a diagram illustrating an example of the configuration of a microscope according to a first embodiment. [Figure 2] 1 is a diagram illustrating an example of the configuration of an optical device according to a first embodiment. [Figure 3] FIG. 10 is a diagram illustrating a case where the repetition frequency is changed to 1 according to the first embodiment. [Figure 4] FIG. 10 is a diagram illustrating a case where the repetition frequency is doubled according to the first embodiment. [Figure 5] FIG. 10 is a diagram illustrating a case where the repetition frequency is changed to four times the normal frequency according to the first embodiment. [Figure 6] 6 is a diagram showing the relationship between the polarization of the pulsed light shown in FIG. 5 and the angle between the polarization and the optical axis of the half-wave plate. [Figure 7] 6 is a schematic diagram of the time interval and polarization of incident light and exiting light in the pulse converting unit shown in FIG. 5. [Figure 8] FIG. 10 is a diagram illustrating a case where the repetition frequency is increased by eight times according to the first embodiment. [Figure 9] 4 is a diagram showing the relationship between the polarization of pulsed light and the angle between the polarization of the pulsed light and the optical axis of the half-wave plate according to the first embodiment. FIG. [Figure 10] 3A and 3B are schematic diagrams illustrating the time interval and polarization of incident light and exiting light in the pulse converting unit according to the first embodiment. [Figure 11] FIG. 2 is a diagram illustrating the flow of a conversion method according to the first embodiment. [Figure 12]FIG. 10 is a diagram showing a modified example of the optical device according to the first embodiment. [Figure 13] 13 is a schematic diagram showing the time interval and polarization of incident light and exiting light in the optical device of the modified example shown in FIG. 12. FIG. [Figure 14] FIG. 10 is a diagram illustrating an example of the configuration of an optical device according to a second embodiment. [Figure 15] FIG. 10 is a diagram illustrating an example of the configuration of an optical device according to a third embodiment. [Figure 16] FIG. 10 is a diagram illustrating an example of the configuration of an optical device according to a fourth embodiment. [Figure 17] FIG. 10 is a diagram illustrating an example of the configuration of an optical device according to a fourth embodiment. [Figure 18] FIG. 10 is a diagram illustrating an example of the configuration of an optical device according to a fifth embodiment. [Figure 19] FIG. 10 is a diagram illustrating an optical path of a pulse converting unit according to a fifth embodiment. [Figure 20] FIG. 13 is a diagram illustrating an optical path of a pulse converting unit according to a fifth embodiment. [Figure 21] FIG. 13 is a diagram illustrating an optical path of a pulse converting unit according to a fifth embodiment. [Figure 22] FIG. 10 is a diagram showing the state of an optical path in a pulse converting unit according to a fifth embodiment. [Figure 23] FIG. 11 is a diagram showing a modified example of the optical unit in the optical device according to the fifth embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0008] The present invention will be described below through embodiments of the invention, but the following embodiments do not limit the invention according to the claims. Furthermore, not all of the combinations of features described in the embodiments are necessarily essential to the solution of the invention. In the drawings, the same or similar parts may be designated by the same reference numerals, and redundant explanations may be omitted. In addition, the shapes and sizes of elements in the drawings may be exaggerated for clarity.

[0009] [First embodiment] FIG. 1 is a diagram illustrating an example of the configuration of a microscope 1 according to a first embodiment. FIG. 1 illustrates an example in which the microscope 1 is a multiphoton fluorescence microscope. However, the microscope 1 is not limited to a multiphoton fluorescence microscope and may be a microscope other than a multiphoton fluorescence microscope. In the following, as an example of an embodiment, the microscope 1 will be described using two-photon fluorescence as a nonlinear phenomenon. However, the microscope 1 can also be applied to other microscopic observation techniques using pulsed light as a light source, regardless of whether they are linear or nonlinear. For example, the microscope 1 may be a three-photon fluorescence microscope that utilizes fluorescence generated by three-photon absorption. Furthermore, the microscope 1 may be a CARS (Coherent Anti-Stokes Raman Scattering) microscope or an SRS (Stimulated Raman Scattering) microscope that utilizes nonlinear Raman scattering. The microscope 1 may be a pump-probe microscope that utilizes stimulated emission or transient absorption. The microscope 1 may also be a photoacoustic microscope that utilizes the photoacoustic effect.

[0010] The microscope 1 according to the first embodiment includes, for example, a light source 2, an irradiation unit 3, an optical device 4, a control unit 11, and a setting device 12. However, the control unit 11 and the setting device 12 are not essential components for the microscope 1. In other words, the microscope 1 does not necessarily need to include the control unit 11 and the setting device 12.

[0011] The light source 2 outputs linearly polarized pulsed light (also referred to as pulsed laser light) L1 in a wavelength band (near infrared) of 700 nm to 1300 nm, with a repetition frequency fr and a repetition period Tr. For example, the light source 2 is a femtosecond pulsed laser (pulse width: 50 to 100 fs) or a picosecond pulsed laser. The pulsed light L1 has a repetition frequency fr of about 80 MHz. The light source 2 includes a solid-state light source (which may include an optical fiber) such as an LD (laser diode).

[0012] If the light source 2 is equipped with a dispersion compensation function, the dispersion compensation function of the light source 2 may be used to perform dispersion compensation of the optical system. If the light source 2 is not equipped with a dispersion compensation function, it is preferable that a separate dispersion compensation optical system be provided between the light source 2 and the optical device 4. The dispersion compensation optical system is, for example, a prism pair or a diffraction grating pair. Note that optical elements other than the dispersion compensation optical system may also be disposed.

[0013] The repetition frequency of pulsed light L1 output from the light source 2 is changed by the optical device 4. The pulsed light L1 whose repetition frequency has been changed by the optical device 4 is referred to as pulsed light L2. The polarization of the pulsed light L2 is linearly polarized. The pulsed light L2 is incident on the irradiation unit 3. Note that even when the repetition frequencies of the pulsed light L1 and the pulsed light L2 are the same, it is stated that the repetition frequency is converted to 1 and output from the optical device 4 as pulsed light L2, and this is included in the concept of converting the repetition frequency.

[0014] The irradiation unit 3 irradiates the pulsed light L2 output from the optical device 4 onto the observation object O. The irradiation unit 3 includes, for example, a scanner 5, a scan lens 6a, a tube lens 6b, an objective lens 7, a dichroic mirror 8, a filter 9, and a photodetector 10.

[0015] The scanner 5 is a mechanism that oscillates the pulsed light L2 in a plane perpendicular to the optical axis of the irradiation unit 3, i.e., in the X and Y directions in the figure. The scanner 5, for example, has a pair of mirrors (galvanometer mirrors) for the X and Y axis directions. The pulsed light L2 is oscillated in each of the X and Y axis directions with respect to the optical axis of the irradiation unit 3 by being reflected by the pair of galvanometer mirrors, and the observation object O is scanned in the X and Y directions. This pair of mirrors is installed near a conjugate position with respect to the pupil of the objective lens 7. The scanner 5 may also be a combination of a galvanometer mirror and a resonant mirror.

[0016] A scan lens 6a and a tube lens 6b are provided downstream of the scanner 5. The scan lens 6a focuses the pulsed light L2 emitted from the scanner 5 onto a primary image plane. The tube lens 6b is disposed between the scan lens 6a and the objective lens 7. The tube lens 6b converts the pulsed light L2 into a parallel beam. The pulsed light L2 from the tube lens 6b is incident on a dichroic mirror 8.

[0017] The dichroic mirror 8 transmits the pulsed light L2. The pulsed light L2 that has transmitted through the dichroic mirror 8 is incident on the objective lens 7. The dichroic mirror 8 also reflects light of wavelengths other than the pulsed light L2.

[0018] The objective lens 7 focuses the pulsed light L2, which is excitation light, to illuminate the observation object O. That is, the pulsed light L2, which is excitation light, is focused on the observation object O through the objective lens 7, causing two-photon excitation. The fluorescence generated from the observation object O passes through the objective lens 7 and is reflected by the dichroic mirror 8, and its optical path is separated from that of the pulsed light L2.

[0019] The filter 9 is provided in the optical path between the dichroic mirror 8 and the photodetector 10. The filter 9 transmits only the wavelength component of the fluorescence out of the light reflected by the dichroic mirror 8.

[0020] The photodetector 10 detects the light that has passed through the filter 9. That is, the photodetector 10 detects the fluorescence that has been reflected by the dichroic mirror 8 and passed through the filter 9. For example, the photodetector 10 is a photomultiplier tube. A detection signal according to the amount of fluorescence detected by the photodetector 10 is output to an information processing device (not shown), such as a computer. The information processing device generates a two-dimensional image or a three-dimensional image of the observation object O based on the detection signal.

[0021] The irradiation unit 3 may have a configuration for confocal detection of fluorescence. In this case, the dichroic mirror 8 is disposed upstream of the scanner 5, rather than near the objective lens 7. The light reflected by the dichroic mirror 8 passes through a filter 9 and is focused onto a pinhole by a newly provided lens (not shown). The fluorescence that passes through the pinhole is detected by a photodetector 10 disposed downstream.

[0022] The optical device 4 can convert the repetition frequency fr of the pulsed light L1 emitted from the light source 2 into a repetition frequency different from the repetition frequency fr. For example, the optical device 4 can convert the repetition frequency fr into a repetition frequency different from the repetition frequency fr by up to 2 n For example, the optical device 4 performs a series of processes n (n is an integer between 1 and n) times, in which the pulsed light is split into a first polarized light and a second polarized light orthogonal to the first polarized light, a predetermined optical path length difference is provided between the first polarized light and the second polarized light, and after providing the optical path length difference, the first polarized light and the second polarized light are combined, thereby increasing the repetition frequency by 2. n The first polarized light is, for example, P polarized light. The second polarized light is, for example, S polarized light. The optical device 4 may be configured to be insertable into and detachable from the microscope 1.

[0023] Fig. 2 is a diagram showing an example of the configuration of an optical device 4 according to the first embodiment. The optical device 4 includes, for example, n pulse converting units 15. The optical device 4 shown in Fig. 2 includes three pulse converting units 15-1 to 15-3. Note that the reference numerals following the hyphen distinguish multiple components of the same type from one another. When multiple configurations of the same type are not to be distinguished from one another, the reference numerals following the hyphen may be omitted.

[0024] The pulse conversion unit 15 includes an optical unit 20 and a half-wave plate 30. The optical unit 20 is an example of a conversion section. The half-wave plate 30 is an example of a switching section. The optical unit 20 includes a first polarizing beam splitter 21, an optical path length difference providing section 22, and a second polarizing beam splitter 23.

[0025] The first polarizing beam splitter 21 splits pulsed light polarized in a first direction into a first polarized light (P polarized light) and a second polarized light (S polarized light). That is, when the incident pulsed light is polarized in the first direction, the first polarized beam splitter 21 transmits the first polarized light component of the pulsed light and reflects the second polarized light component of the pulsed light in a predetermined direction. The following describes an example in which the first polarized light is P polarized light and the second polarized light is S polarized light.

[0026] The first direction is, for example, 45°. This 45° is a direction rotated 45° counterclockwise from a direction (for example, the horizontal direction) orthogonal to the direction in which the pulsed light travels. On the other hand, the first direction may be −45°, which includes P-polarized and S-polarized components in a 1:1 ratio. That is, based on the respective rotational symmetries, the first direction may be 135° (−225°), 225° (−135°), or 315° (−45°). That is, as an example, when 45° linearly polarized light is incident on the first polarizing beam splitter 21, the P-polarized component is transmitted and the S-polarized component is reflected. Note that, since the magnitudes of the P-polarized component and the S-polarized component in the 45° linearly polarized light are equal, the splitting ratio at the first polarizing beam splitter 21 is 1:1.

[0027] The optical path length difference providing unit 22 provides an optical path length difference between the P polarized light and the S polarized light split by the first polarizing beam splitter 21. For example, the optical path length difference providing unit 22 includes a first optical path 24 and a second optical path 25.

[0028] The first optical path 24 is a path along which the P-polarized light that has passed through the first polarizing beam splitter 21 propagates until it reaches the second polarizing beam splitter 23. For example, of the pulsed light polarized in the first direction, the P-polarized light split by the first polarizing beam splitter 21 propagates through the first optical path 24 and enters the second polarizing beam splitter 23. On the other hand, if the pulsed light polarized in the second direction is P-polarized, it propagates through the first optical path 24 and enters the second polarizing beam splitter 23. On the other hand, if the pulsed light polarized in the second direction is S-polarized, it propagates through the second optical path 25 and enters the second polarizing beam splitter 23.

[0029] The second optical path 25 is a path along which the S-polarized light reflected by the first polarizing beam splitter 21 propagates before reaching the second polarizing beam splitter 23. For example, mirrors M1 and M2 are provided on the second optical path 25. The S-polarized light split by the first polarizing beam splitter 21 is reflected by the mirrors M1 and M2 and enters the second polarizing beam splitter 23. The optical path length of the second optical path 25 is longer than the optical path length of the first optical path 24. Therefore, the optical path length difference providing unit 22 can provide an optical path length difference between the P-polarized light and the S-polarized light.

[0030] The second polarizing beam splitter 23 forms pulsed light with a changed repetition frequency by multiplexing the P polarized light and the S polarized light after the optical path length difference has been imparted by the optical path length difference imparting unit 22. Because an optical path length difference has been imparted between the P polarized light and the S polarized light, a predetermined time difference is imparted between the P polarized light, which is an example of the first polarized light, and the S polarized light, which is an example of the second polarized light, until they propagate to the second polarizing beam splitter 23. Therefore, when the P polarized light and the S polarized light are coaxially multiplexed by the second polarizing beam splitter 23, the repetition frequency of the combined pulsed light is higher than the repetition frequency of the pulsed light incident on the first polarizing beam splitter 21.

[0031] The optical path length difference imparting unit 22-1 imparts a first optical path length difference between the P polarized light and the S polarized light split by the first polarizing beam splitter 21-1. This first optical path length difference imparts a time difference of (Tr / 8) between the P polarized light and the S polarized light.

[0032] The optical path length difference imparting unit 22-2 imparts a second optical path length difference between the P-polarized light and the S-polarized light split by the first polarizing beam splitter 21-2. The second optical path length difference is greater than the first optical path length difference. This second optical path length difference imparts a time difference of (Tr / 4) between the P-polarized light and the S-polarized light.

[0033] The optical path length difference imparting unit 22-3 imparts a third optical path length difference between the P polarized light and the S polarized light split by the first polarizing beam splitter 21-3. The third optical path length difference is greater than the second optical path length difference. This third optical path length difference imparts a time difference of (Tr / 2) between the P polarized light and the S polarized light.

[0034] By setting the angle of the half-wave plate 30, it is possible to switch whether or not the optical unit 20 changes the repetition frequency of the pulsed light L1. For example, the repetition frequency fr of the pulsed light L1 can be changed to a value obtained by multiplying the repetition frequency fr by a power of 2. In other words, by setting the angle of the half-wave plate 30, the repetition frequency fr can be changed from 1 to 2 times the repetition frequency fr. n 2, n=3, and therefore the half-wave plates 30-1 to 30-3 can change the repetition frequency fr of the pulsed light L1 emitted from the light source 2 to one of four repetition frequencies: fr×1, fr×2, fr×4, and fr×8.

[0035] The half-wave plate 30 changes the polarization direction of the pulsed light incident on the first polarizing beam splitter 21 in each of the optical units 20 to either a first direction or a second direction different from the first direction. The half-wave plate 30 is provided on the optical path along which the pulsed light enters the first polarizing beam splitter 21.

[0036] More specifically, the half-wave plate 30 is rotatable. Rotation of the half-wave plate 30 changes the angle (hereinafter simply referred to as "angle") θ between the polarization direction of the pulsed light incident on the half-wave plate 30 and the optical axis (slow axis or fast axis) of the half-wave plate 30. For convenience of explanation, in the following description, the slow axis of the half-wave plate 30 will be referred to as the optical axis of the half-wave plate 30, and the polarization direction of the incident pulsed light will be assumed to be the lateral direction (horizontal direction, corresponding to P-polarized light) perpendicular to the direction in which the pulsed light travels. For example, rotation of the half-wave plate 30 can switch between a first state in which the angle θ of the half-wave plate 30 is within a first angle range and a second state in which the angle θ of the half-wave plate 30 is within a second angle range. When the angle θ is within the first angle range, pulsed light polarized in a first direction is emitted from the half-wave plate 30.

[0037] When the angle θ is in a second angle range different from the first angle range, pulsed light in a second direction different from the first direction is emitted from the half-wave plate 30. The first angle range is a range of 22.5°±1°. As a result, the polarization angle of the pulsed light transmitted through the half-wave plate 30 is 45°. Note that, due to the rotational symmetry of the polarization direction, the first angle range may also be 67.5°±1°. In this case, the polarization angle of the pulsed light transmitted through the half-wave plate 30 is 135°. In this case, the polarization direction is equivalent to -45°. Furthermore, when the polarization direction of the pulsed light is the longitudinal direction (vertical direction, corresponding to S-polarized light), the first angle range of 22.5°±1° can be rephrased as 112.5°±1° or -67.5°±1° in terms of the angle between the slow axis of the half-wave plate 30 and the polarization direction of the pulsed light. In other words, the first angle range can be said to be (22.5°±K×45°)±1°, where K is an integer. The same applies when the slow axis of the half-wave plate 30 is replaced with the fast axis. The second angle range is 0°±1°. Furthermore, due to the rotational symmetry of the polarization direction of the pulsed light, this can be expressed as ±180°±1°. Furthermore, when the polarization direction of the pulsed light is the longitudinal direction (vertical direction, corresponding to S-polarized light), the second angle range 0±1° can be expressed as 90°±1° or −270°±1° in terms of the angle between the slow axis of the half-wave plate 30 and the polarization direction of the pulsed light. In other words, the second angle range can be said to be (0°±K×90°)±1°, where K is an integer. The same applies when the slow axis of the half-wave plate 30 is replaced with the fast axis. The tolerance of the first angle range is ±1° so that the variation in intensity between pulses of the pulsed light L2 after repetition frequency conversion is kept within a range of about ±15% of the average intensity of the pulsed light L2. Also, the tolerance of the second angle range is ±1° so that the output of each of the multiple polarizing beam splitters does not decrease by more than 5% from the input.

[0038] The half-wave plate 30-1 is provided upstream of the first polarizing beam splitter 21-1. The half-wave plate 30-1 changes the polarization direction of the pulsed light incident on the first polarizing beam splitter 21-1 to either a first direction or a second direction different from the first direction. The half-wave plate 30-2 is provided downstream of the second polarizing beam splitter 23-1 and upstream of the first polarizing beam splitter 21-2. The half-wave plate 30-2 changes the polarization direction of the pulsed light emitted from the second polarizing beam splitter 23-1 and incident on the first polarizing beam splitter 21-2 to either a first direction or a second direction different from the first direction. The half-wave plate 30-3 is provided downstream of the second polarizing beam splitter 23-2 and upstream of the first polarizing beam splitter 21-3. The half-wave plate 30-3 changes the polarization direction of the pulsed light emitted from the second polarizing beam splitter 23-2 and incident on the first polarizing beam splitter 21-3 to either a first direction or a second direction different from the first direction.

[0039] A method for varying the repetition frequency according to the first embodiment will be described. Fig. 3 is a diagram illustrating a case where the repetition frequency is changed to fr×1. Fig. 4 is a diagram illustrating a case where the repetition frequency is changed to fr×2. Fig. 5 is a diagram illustrating a case where the repetition frequency is changed to fr×4. Fig. 8 is a diagram illustrating a case where the repetition frequency is changed to fr×8.

[0040] As shown in Fig. 3, when the repetition frequency fr of the pulsed light L1 incident on the optical device 4 is changed to fr × 1, the angles θ of the half-wave plates 30-1, 30-2, and 30-3 are set within a second angle range. Therefore, as shown in Fig. 3(a), the pulsed light L1 (P-polarized) emitted from the light source 2 is transmitted through the first polarizing beam splitters 21-1 to 21-3 without being split. That is, the pulsed light L1 is not branched or time-delayed by the optical units 20-1 to 20-3, and the repetition frequency fr of the pulsed light L1 is maintained. In this case, as shown in Fig. 3(b), the pulsed light L1 having a pulse interval Tr is output from the optical device 4 as pulsed light L2.

[0041] Next, a method of doubling the repetition frequency will be described. As shown in FIG. 4, the repetition frequency fr of the pulsed light L1 incident on the optical device 4 is doubled by fr×2, that is, by doubling the repetition frequency fr. 1 When the angle θ of the half-wave plate 30-1 and the half-wave plate 30-2 is changed to 1 / 4 times (n=1), the angle θ of each of the half-wave plates 30-1 and 30-2 is set to a second angle range. On the other hand, the angle θ of the half-wave plate 30-3 is set to, for example, 22.5°. In this case, the pulsed light L1 (P polarized light) emitted from the light source 2 is transmitted without being split by either the first polarizing beam splitter 21-1 or the first polarizing beam splitter 21-2. That is, no light branching or time delay is applied by the optical units 20-1 and 20-2, and the repetition frequency of the pulsed light emitted from the optical units 20-1 and 20-2 maintains the repetition frequency fr of the pulsed light L1 emitted from the light source 2. Furthermore, the polarization state of the pulsed light emitted from the optical units 20-1 and 20-2 remains P polarized, just like the pulsed light L1.

[0042] The pulsed light L1 output from the half-wave plate 30-3 shown in Fig. 4 is linearly polarized at an angle of 45°. Therefore, as shown in Fig. 4(a), of the pulsed light (linearly polarized at an angle of 45°) output from the half-wave plate 30-3, P-polarized light P1 is transmitted through the first polarizing beam splitter 21-3, and S-polarized light S1 is reflected by the first polarizing beam splitter 21-3. The P-polarized light P1 transmitted through the first polarizing beam splitter 21-3 propagates through the first optical path 24-3 and enters the second polarizing beam splitter 23-3.

[0043] Meanwhile, the S-polarized light S1 reflected by the first polarizing beam splitter 21-3 propagates through the second optical path 25-3 and enters the second polarizing beam splitter 23-3. Therefore, a time difference corresponding to the third optical path difference, i.e., a time difference of Tr / 2, is imparted between the P-polarized light P1 and the S-polarized light S1. The second polarizing beam splitter 23-3 combines the incident P-polarized light P1 and S-polarized light S2 to generate pulsed light with a repetition frequency of (fr×2). This pulsed light with a repetition frequency of (fr×2) is output from the optical device 4 as pulsed light L2. That is, as shown in FIG. 4(b), pulsed light with a pulse interval of (Tr / 2) is emitted from the optical device 4 as pulsed light L2.

[0044] Next, a method for converting the repetition frequency by four times will be described with reference to FIGS. 6 is a diagram illustrating the relationship between the polarization of pulsed light and the angle between the optical axis of the half-wave plate. FIG. 6 illustrates a case where the angle θ of the half-wave plate 30-1 is set to a second angle range (e.g., 0°) and the angles θ of the half-wave plates 30-2 and 30-3 are set to a first angle range (e.g., 22.5°). FIG. 6(a) illustrates the relationship between the polarization of the pulsed light incident on the optical unit 20-1 and the polarization of the pulsed light emitted from the optical unit 20-1 and the angle θ of the half-wave plate 30-1. FIG. 6(b) illustrates the relationship between the polarization of the pulsed light incident on the optical unit 20-2 and the polarization of the pulsed light emitted from the optical unit 20-2 and the angle θ of the half-wave plate 30-2. FIG. 6(c) illustrates the relationship between the polarization of the pulsed light incident on the optical unit 20-3 and the polarization of the pulsed light emitted from the optical unit 20-3 and the angle θ of the half-wave plate 30-3. FIG. 7 is a schematic diagram showing the time interval and polarization of the incident and emitted light in each pulse converting unit.

[0045] The repetition frequency of the pulsed light L1 incident on the optical device 4 is fr×4, that is, 2 2When the angle θ is changed to 1 / 4 times (n=2), the angle θ of the half-wave plate 30-1 is set to a second angle range. On the other hand, the angles θ of the half-wave plates 30-2 and 30-3 are set to a first angle range, for example, 22.5°. In this case, as shown in FIG. 6( a), the pulsed light L1 (P-polarized) emitted from the light source 2 is transmitted through the first polarizing beam splitter 21-1 without being split. That is, the optical unit 20-1 does not branch the light or add a time delay, and the repetition frequency of the pulsed light emitted from the optical unit 20-1 maintains the repetition frequency fr of the pulsed light L1 emitted from the light source 2. Furthermore, the polarization state of the pulsed light emitted from the optical unit 20-1 remains P-polarized, just like the pulsed light L1.

[0046] As shown in FIG. 6(b), the pulsed light immediately after passing through the half-wave plate 30-2 is linearly polarized at 45°. Therefore, of the pulsed light output from the half-wave plate 30-2 shown in FIG. 5(a), i.e., the 45° linearly polarized light, P-polarized light P11 is transmitted through the first polarizing beam splitter 21-2, and S-polarized light S11 is reflected by the first polarizing beam splitter 21-2. The P-polarized light P11 transmitted through the first polarizing beam splitter 21-2 propagates through the first optical path 24-2 and enters the second polarizing beam splitter 23-2. Meanwhile, the S-polarized light S11 reflected by the first polarizing beam splitter 21-2 propagates through the second optical path 25-2 and enters the second polarizing beam splitter 23-2. Therefore, a time difference corresponding to the second optical path difference, i.e., a time difference of (Tr / 4), is imparted between the P-polarized light P11 and the S-polarized light S11 (see FIG. 7). The second polarizing beam splitter 23-2 multiplexes the incident P-polarized light P11 and S-polarized light S11 to generate pulsed light 100. The pulsed light 100 generated by the second polarizing beam splitter 23-2 enters the optical unit 20-3.

[0047] The angle θ of the half-wave plate 30-3 is set to a first angle range, for example, 22.5° for P-polarized light. Therefore, as shown in Fig. 6(c), the polarization of the pulsed light 100 after passing through the half-wave plate 30-3 becomes linearly polarized light at +45° and linearly polarized light at -45°, respectively.

[0048] Of the +45° linearly polarized light that precedes in time, P-polarized light P12 passes through first polarizing beam splitter 21-3 and propagates through first optical path 24-3. Of the +45° linearly polarized light, S-polarized light S12 is reflected by first polarizing beam splitter 21-3 and propagates through second optical path 25-3. Also, of the −45° linearly polarized light, P-polarized light P12 passes through first polarizing beam splitter 21-3 and propagates through first optical path 24-3, and S-polarized light S12 is reflected by first polarizing beam splitter 21-3 and propagates through second optical path 25-3. Therefore, a time difference of (Tr / 2) is imparted between P-polarized light P12 and S-polarized light S12 (see FIG. 7). The second polarizing beam splitter 23-3 multiplexes the two incident P-polarized light beams P12 and two incident S-polarized light beams S12 to generate pulsed light 110 having a repetition frequency of (fr×4). That is, as shown in FIG. 5(b), pulsed light 110 having a pulse interval of (Tr / 4) is emitted from the optical device 4 as pulsed light L2.

[0049] Next, a method for converting the repetition frequency by eight times will be described with reference to FIGS.

[0050] FIG. 9 is a diagram illustrating the relationship between the polarization of pulsed light and the angle between the polarization of the pulsed light and the optical axis of the half-wave plate. FIG. 9 illustrates a case where the angle θ is set within a first angle range (e.g., 22.5°) for all half-wave plates 30-1 to 30-3. FIG. 9(a) illustrates the relationship between the polarization of pulsed light incident on optical unit 20-1 and the polarization of pulsed light emitted from optical unit 20-1 and the angle θ of half-wave plate 30-1. FIG. 9(b) illustrates the relationship between the polarization of pulsed light incident on optical unit 20-2 and the polarization of pulsed light emitted from optical unit 20-2 and the angle θ of half-wave plate 30-2. FIG. 9(c) illustrates the relationship between the polarization of pulsed light incident on optical unit 20-3 and the polarization of pulsed light emitted from optical unit 20-2 and the angle θ of half-wave plate 30-3. FIG. 10 is a schematic diagram illustrating the time interval and polarization of incident and emitted light in each pulse conversion unit.

[0051] The repetition frequency fr of the pulsed light L1 incident on the optical device 4 is fr×8, that is, 2 3When changing to a factor of 3 (n=3), all of the angles θ of the half wave plates 30-1, 30-2, and 30-3 are set to the first angle range as shown in FIG.

[0052] As shown in Fig. 9(a), the polarization of the pulsed light immediately after passing through the half-wave plate 30-1 becomes linearly polarized at 45°. Therefore, of the pulsed light output from the half-wave plate 30-1 shown in Fig. 8(a), i.e., the 45° linearly polarized light, P-polarized light P21 is transmitted through the first polarizing beam splitter 21-1, and S-polarized light S21 is reflected by the first polarizing beam splitter 21-1. The P-polarized light P21 that has transmitted through the first polarizing beam splitter 21-1 propagates through the first optical path 24-1 and enters the second polarizing beam splitter 23-1.

[0053] Meanwhile, the S-polarized light S21 reflected by the first polarizing beam splitter 21-1 propagates through the second optical path 25-1 and enters the second polarizing beam splitter 23-1. Therefore, a time difference according to the first optical path difference, i.e., a time difference of Tr / 8, is imparted between the P-polarized light P21 and the S-polarized light S21 (see FIG. 10). The second polarizing beam splitter 23-1 generates pulsed light 200 by multiplexing the incident P-polarized light P21 and S-polarized light S21. The pulsed light 200 enters the optical unit 20-2. Note that the polarization of the pulsed light 200 is linear polarization with the P-polarized light P21 and the S-polarized light S21 being orthogonal to each other, as shown in FIG. 9(a).

[0054] As shown in Fig. 9(b), because the angle θ of the half-wave plate 30-2 is in the first angle range, the polarization of the pulsed light 200 after passing through the half-wave plate 30-2 shown in Fig. 8(a) becomes linearly polarized light at +45° and linearly polarized light at −45°, respectively. Of the +45° polarized light that precedes in time, P-polarized light P22 passes through the first polarizing beam splitter 21-2 and propagates through the first optical path 24-2, and S-polarized light S22 is reflected by the first polarizing beam splitter 21-2 and propagates through the second optical path 25-2.

[0055] Furthermore, of the −45° linearly polarized light, P-polarized light P22 passes through the first polarizing beam splitter 21-2 and propagates through the first optical path 24-2. Of the −45° linearly polarized light, S-polarized light S22 is reflected by the first polarizing beam splitter 21-2 and propagates through the second optical path 25-2. Therefore, a time difference of (Tr / 4) is imparted between the split P-polarized light P22 and the split S-polarized light S22 (see FIG. 10). The second polarizing beam splitter 23-2 generates pulsed light 210 by multiplexing the two incident P-polarized light P22 and the two incident S-polarized light S22. The pulsed light 210 generated by the second polarizing beam splitter 23-2 enters the optical unit 20-3.

[0056] The pulsed light 210 emitted from the optical unit 20-2, i.e., the four pulses, is incident on the half-wave plate 30-3 of the optical unit 20-3. Here, because the angle θ of the half-wave plate 30-3 is within the first angle range, the pulsed light 210 after passing through the half-wave plate 30-3 becomes 45°-polarized light in which the P-polarized light and the S-polarized light are orthogonal to each other, as shown in Fig. 9(c) . In the pulsed light 210 after passing through the half-wave plate 30-3, the two pulses that are temporally leading are linearly polarized at +45°, and the two pulses that are temporally lagging are linearly polarized at -45°.

[0057] In each of the two +45° polarized pulses that precede each other in time, P polarized light P23 passes through the first polarizing beam splitter 21-3 shown in FIG. 8(a) and propagates along the first optical path 24-3, while S polarized light S23 is reflected by the first polarizing beam splitter 21-3 and propagates along the second optical path 25-3. In each of the two −45° polarized pulses that lag each other in time, P polarized light P23 passes through the first polarizing beam splitter 21-3 and propagates along the first optical path 24-3, while S polarized light S23 is reflected by the first polarizing beam splitter 21-3 and propagates along the second optical path 25-3. Therefore, a time difference of (Tr / 2) is imparted between the split P polarized light P23 and S polarized light S23 (see FIG. 10).

[0058] The second polarizing beam splitter 23-3 multiplexes the four P-polarized light beams P23 and four S-polarized light beams S23 that have entered it, thereby generating pulsed light 220 with a repetition frequency of (fr×8). That is, the pulsed light 220 with a pulse interval of (Tr / 8) is emitted from the optical device 4 as pulsed light L2.

[0059] The control unit 11 may control the half-wave plate 30-1, the half-wave plate 30-2, and the half-wave plate 30-3, which are examples of a switching unit. Each of the half-wave plate 30-1, the half-wave plate 30-2, and the half-wave plate 30-3 can perform a switching process to switch the polarization direction of the incident pulsed light between a first direction and a second direction. The control unit 11 may control this switching process.

[0060] As a specific example, the control unit 11 independently controls the rotation of each of the half-wave plates 30-1 to 30-3. For example, the control unit 11 may have a plurality of electric motors for independently rotating each of the half-wave plates 30-1, 30-2, and 30-3. The control unit 11 controls the rotation of the half-wave plate 30-1 to control the angle θ1 of the half-wave plate 30-1 within a first angle range or within a second angle range, thereby controlling the switching of the polarization direction of the pulsed light by the half-wave plate 30-1. The control unit 11 controls the rotation of the half-wave plate 30-2 to control the angle θ2 of the half-wave plate 30-2 within a first angle range or within a second angle range, thereby controlling the switching of the polarization direction of the pulsed light by the half-wave plate 30-2. The control unit 11 controls the rotation of the half-wave plate 30-3 to control the angle θ3 of the half-wave plate 30-3 to be within a first angle range or within a second angle range, thereby controlling the switching of the polarization direction of the pulsed light by the half-wave plate 30-3.

[0061] The control unit 11 controls the rotation of each of the half-wave plates 30-1 to 30-3 based on a command signal from the setting device 12. However, without being limited to this, the control unit 11 may also control the rotation of each of the half-wave plates 30-1 to 30-3 based on a command signal from an external device other than the setting device 12.

[0062] The setting device 12 sets the value of information m (an integer from 0 to n) corresponding to the magnification of the repetition frequency in response to a user operation. For example, the setting device 12 includes an operation unit 121 that accepts a user operation. The operation unit 121 is a pointing device such as a touch panel, a touch pad, or a mouse, a button, a switch, a motion-sensitive controller, a keyboard, a mouse, a gesture input device, or a voice input device (for example, a microphone). When the value of m is set by the operation unit 121, the setting device 12 transmits information (command signal) of the set value of m to the control unit 11. As a result, the control unit 11 controls the rotation of the half-wave plate 30 based on the command signal including the information of the value of m transmitted from the setting device 12 to magnify the repetition frequency fr of the pulsed light by 1 or 2. n Change it to.

[0063] The following describes the repetition frequency of 1 or 2 according to the first embodiment. n An example of a setting method for m will be described below. First, the user operates the operation unit 121 to set the repetition frequency of the pulsed light L2. For example, the user operates the operation unit 121 to set the value of m. When the value of m is set, the setting device 12 transmits a command signal including information about the value of m to the control unit 11. The control unit 11 causes the half-wave plate 30 to change the repetition frequency fr based on the information about the value of m included in the command signal.

[0064] For example, when the value of m included in the command signal is "0," control unit 11 controls the rotation of half-wave plate 30-1, half-wave plate 30-2, and half-wave plate 30-3 so that angles θ1 to θ3 are all within the second angle range. For example, when the value of m included in the command signal is "1," control unit 11 controls the rotation of half-wave plate 30-1, half-wave plate 30-2, and half-wave plate 30-3 so that angles θ1 and θ2 are within the second angle range and angle θ3 is within the first angle range.

[0065] For example, when the value of m included in the command signal is "2," control unit 11 controls the rotation of half-wave plate 30-1, half-wave plate 30-2, and half-wave plate 30-3 so that angle θ1 falls within the second angle range and angles θ2 and θ3 fall within the first angle range. For example, when the value of m included in the command signal is "3," control unit 11 controls the rotation of half-wave plate 30-1, half-wave plate 30-2, and half-wave plate 30-3 so that angles θ1 to θ3 are all within the first angle range.

[0066] However, the repetition frequency is 1 or 2 n The setting method is not limited to the above-described method, and may be set manually by the user. For example, the user may manually adjust the rotation of the half-wave plate 30-1, the half-wave plate 30-2, and the half-wave plate 30-3.

[0067] The repetition frequency (1 or fr×2) according to the first embodiment is as follows: n The flow of the conversion method of (1) will be described with reference to Fig. 11. Fig. 11 is a diagram illustrating the flow of the conversion method according to the first embodiment.

[0068] The value of m is set by the setting device 12 (step S101). Once the value of m is set, the user or the control unit 11 sets the angle θ of the half-wave plate 30 according to the value of m (step S102). If the value of m is other than “0” (step S103: YES), after step S102, when linearly polarized pulsed light L1 having a repetition frequency fr and a repetition period Tr is input to the optical device 4 from the light source 2, the optical device 4 executes a first step of changing the polarization direction of the pulsed light L1 to a first direction via the half-wave plate 30 (step S104). Next, the optical device 4 executes a second step of splitting the pulsed light polarized in the first direction into P-polarized light and S-polarized light using the first polarizing beam splitter 21 (step S105), and executes a third step of providing an optical path length difference between the split P-polarized light and S-polarized light (step S106).

[0069] The optical device 4 executes a fourth stage (step S107) in which the P-polarized light and the S-polarized light, which have been given an optical path length difference in the third stage, are multiplexed using the second polarizing beam splitter 23 to generate pulsed light with a changed repetition frequency. The optical device 4 repeats a series of processes including the first, second, third, and fourth stages n times to generate pulsed light L1 with a repetition frequency fr at a repetition frequency (fr×2 n ) into pulsed light L2 having a repetition frequency fr. In the first stage, in the second and subsequent first stage processes, the optical device 4 changes the polarization direction of the pulsed light generated in the fourth stage, rather than the polarization direction of the pulsed light L1, to the first direction via the half-wave plate 30. Furthermore, if the value of m is "0" (step S103: NO), after step S102, when linearly polarized pulsed light L1 having a repetition frequency fr and a repetition period Tr is input to the optical device 4 from the light source 2, the optical device 4 does not execute the first stage of changing the polarization direction of the pulsed light L1 to the first direction via the half-wave plate 30. Therefore, the pulsed light L1 having the repetition frequency fr is converted into pulsed light L2 having a repetition frequency (1x) without performing a series of processes including the first, second, third, and fourth stages.

[0070] The optical device 4 according to the first embodiment includes an optical unit 20 that converts the repetition frequency fr of pulsed light L emitted from a light source 2, and a repetition frequency (1x or fr×2) converted by the optical unit 20. n The repetition frequency of the pulsed light can be changed as needed depending on the situation, which contributes to improving practicality.

[0071] For example, nonlinear optical microscopes that use ultrashort pulsed light as excitation light have become essential tools in bioimaging. In particular, two-photon fluorescence microscopes that use two-photon excitation are widely used as a method with excellent deep observation capabilities. However, photobleaching caused by irradiation with laser light is a practical issue. One of the causes of this photobleaching is the irradiation of pulsed light with extremely high peak intensity.

[0072] One method to reduce photobleaching is to increase the repetition rate of ultrashort pulsed light and reduce the peak intensity of the excitation pulse. However, in two-photon fluorescence microscopy, for example, when the excitation pulse is set to a high repetition rate (n times higher), the excitation light intensity (average intensity (W) of the excitation light) required to obtain the same amount of fluorescence as pulsed light L1 is √n times higher. The peak intensity (W) of pulsed light L2 is 1 / √n times higher. In other words, the higher the repetition rate, the greater the required light source output. When observing deep inside a biological sample, scattering reduces the excitation light intensity at the focal plane of the objective lens, so the intensity of the excitation light incident on the objective lens must be increased compared to when observing an interface. Therefore, to obtain a sufficient amount of fluorescence signal from deep inside, it may be necessary to maximize the light source output. However, if the repetition rate is set high, the excitation light intensity may be insufficient.

[0073] In the first embodiment, the repetition frequency of the pulsed light L1 converted by the optical device 4 is variable, so that, for example, when the excitation light intensity is insufficient for deep observation, the repetition frequency can be lowered so that a significant signal can be obtained from deep areas. This enables the optical device 4 to perform deep observation while maintaining the advantage of photobleaching for observation.

[0074] Furthermore, in the first embodiment, the optical path length through the optical elements (first polarizing beam splitter 21 and second polarizing beam splitter 23) is the same for all pulses. Therefore, the group delay dispersion (GDD) defined by the thickness of the optical elements is the same regardless of the pulse. The widths of all pulses can be optimized by a dispersion compensation optical system that is attached to the light source 2 or is not shown.

[0075] In the first embodiment, the pulse intervals of the pulsed light L2 emitted from the optical device 4 are uniform. In addition, in the optical device 4, the optical path length differences between the P-polarized light and the S-polarized light are set to Tr / 2, Tr / 4, Tr / 8, ..., Tr / 2 nBy doing so, it is easy to design a system in which the pulse intervals are uniform.

[0076] An adjustment mechanism for beam alignment may be provided in the optical path with the longer optical path length in each pulse conversion unit. The adjustment mechanism can be the angle of two mirrors (M1, M2) installed in the longer optical path. By changing the angle of the mirror, the focusing position of the beam with the objective lens can be made to overlap with the focusing position of the beam passing through the shorter optical path. A specific example of beam alignment in the adjustment mechanism will be described below.

[0077] The object to be observed is a fluorescent bead with a size below the diffraction limit (for example, 100 nm or less). As an example, we will describe the case where the repetition frequency is fr×8 as shown in Figure 8. For convenience, transmission in the pulse conversion unit 15 is represented as "0" and reflection as "1". The transmitted and reflected pulses in each pulse conversion unit 15 are represented as [p3, p2, p1]. pi is the value in the ith pulse conversion unit 15 and has a value of 0 or 1. The focusing positions of the 8 pulses are adjusted in the following steps.

[0078] (STEP 1) Pulse conversion unit 15-1, pulse conversion unit 15-2, and pulse conversion unit 15-3 are configured to generate pulsed light with high repetition rate. To achieve this, the angles of half-wave plates 30-1, 30-2, and 30-3 are set to 22.5°. In pulse conversion unit 15-1, pulse conversion unit 15-2, and pulse conversion unit 15-3, shutters (not shown) are installed in the first optical path 24 that passes through first polarizing beam splitter 21 and the second optical path 25 that reflects light. The shutters installed in second optical path 25-2 of pulse conversion unit 15-2 and second optical path 25-3 of pulse conversion unit 15-3 are closed. With this setting, two pulsed lights L2 with a time difference of Tr / 8 are emitted from optical device 4. Hereinafter, in pulse conversion unit 15-1, the pulse that passes through first polarizing beam splitter 21-1 will be expressed as [0,0,0], and the pulse that is reflected by first polarizing beam splitter 21-1 will be expressed as [0,0,1]. These two pulses pass through first polarizing beam splitter 21-2 in pulse conversion unit 15-2, and pass through first polarizing beam splitter 21-3 in pulse conversion unit 15-3.

[0079] Assume that the microscope optical system is adjusted for pulse [0,0,0]. That is, pulse [0,0,0] is focused at the ideal focusing position of the objective lens. The shutter located in the second optical path 25-1 of the pulse conversion unit 15-1 is closed to acquire a fluorescent bead image using only pulse [0,0,0]. Then, the shutter located in the first optical path 24-1 of the pulse conversion unit 15-1 is closed to acquire a fluorescent bead image using only pulse [0,0,1]. If there is a deviation in the fluorescent bead image, two mirrors (M1-1 and M2-1) installed in the pulse conversion unit 15-1 are used to align the focusing position of pulse [0,0,1] with pulse [0,0,0].

[0080] (STEP 2) The beam focusing positions of the pulses that pass through and reflect from the first polarizing beam splitter 21-2 in the pulse conversion unit 15-2 are adjusted. Of the four pulses generated by the pulse conversion units 15-1 and 15-2, two pulses that pass through different optical paths in the pulse conversion unit 15-2 are used. Here, pulses [0,1,0] and [0,0,0] are used as examples. A designated shutter is closed to extract the two pulses, pulses [0,1,0] and [0,0,0], and fluorescent bead images are acquired independently for each. If there is a misalignment in the fluorescent bead images, two mirrors (M1-2 and M2-2) installed in the pulse conversion unit 15-2 are used to align the focusing position of pulse [0,1,0] with pulse [0,0,0].

[0081] (STEP 3) The beam focusing positions of the pulses that pass through and reflect from the first polarizing beam splitter in pulse conversion unit 15-3 are adjusted. Of the eight pulses generated by pulse conversion unit 15-1, pulse conversion unit 15-2, and pulse conversion unit 15-3, two pulses that pass through different optical paths in pulse conversion unit 15-3 are used. Here, pulse [1,0,0] and pulse [0,0,0] are used as examples. A designated shutter is closed to extract the two pulses, pulse [1,0,0] and pulse [0,0,0], and fluorescent bead images are acquired independently for each. If there is a misalignment in the fluorescent bead images, two mirrors (M1-3, M2-3) installed in pulse conversion unit 15-3 are used to align the focusing position of pulse [1,0,0] with pulse [0,0,0].

[0082] By performing the above three steps, STEP 1 to STEP 3, the focusing positions of the eight pulses are overlapped on the focal plane of the objective lens. The overlapping accuracy is preferably equal to or less than the full width at half maximum of the spot diameter (point spread function) generated by the pulse [0,0,0] on the focal plane of the objective lens. More preferably, it is equal to or less than the half width at half maximum.

[0083] If the focusing position on the objective lens focal plane shifts due to aging or other reasons, the acquired image will be a superposition of fluorescence excited by beams focused at different positions, resulting in a deterioration of spatial resolution. In such cases, beam alignment is performed as described above.

[0084] As a countermeasure against the above-mentioned change over time, a mechanism for checking whether or not the beam spot is misaligned may be provided. Specifically, sub-diffraction-limited fluorescent bead images may be acquired using pulses that have passed through different optical paths (pulses with different time delays), and the presence or absence of spot misalignment may be detected by comparing these images. Image correlation, center of gravity analysis, etc. may be used for detection.

[0085] Fig. 12 is a diagram showing a modified example of the optical device 4 according to the first embodiment. The modified optical device 4A shown in Fig. 12 differs from the optical device 4 shown in Fig. 2 in the arrangement order of pulse converting units 15-1 to 15-3, but the other configurations are the same. Fig. 13 is a schematic diagram showing the time intervals and polarizations of incident light and outgoing light in each pulse converting unit in the optical device 4A.

[0086] When the direction of linear polarization of the excitation pulses is orthogonal to that of each successive pulse, the direction of polarization induced in the object of observation differs for each pulse, which is useful for suppressing photobleaching. Furthermore, the polarization dependence of the object of observation O can be averaged, which is expected to enable signal detection independent of the orientation of the object of observation O. The optical device 4A shown in Figure 12 orthogonalizes the direction of linear polarization of the excitation pulses for each successive pulse at high repetition rates.

[0087] The optical device 4A is arranged in the order of pulse conversion unit 15-2, pulse conversion unit 15-3, and pulse conversion unit 15-1 from the direction in which pulsed light L1 is incident. As a result, as shown in Fig. 13, the optical device 4A can orthogonalize the polarization of pulses for each pulse when the repetition frequency is converted to fr × 8 (n = 3).

[0088] In the configuration of the optical device 4A shown in FIG. 12, when n=1, only the angle θ1 of the half-wave plate 30-1 is set to a first angle range, and the angles θ2 and θ3 of the half-wave plate 30-2 and half-wave plate 30-3 are set to a second angle range. When n=2, the angle θ1 of the half-wave plate 30-1 and the angle θ3 of the half-wave plate 30-3 are set to a first angle range, and the angle θ2 of the half-wave plate 30-2 is set to a second angle range. In these cases, the pulse intervals are no longer equal. If n is to be further increased, another pulse conversion unit may be added after the pulse conversion unit 15-1 shown in FIG. 12. When n=4, a Tr / 2 4 In this way, the pulse converting unit that imparts the shortest time difference between the P-polarized light and the S-polarized light may be arranged at the end of the pulse converting units arranged in series. The optical device 4 may not have the function of changing the repetition frequency fr of the pulsed light L1 and may be fixed to a specific frequency. In that case, for example, when used exclusively at fr×2, the optical device 4 includes only an optical unit 20-3 and a pulse converting unit 15-3 having a half-wave plate 30-3, and the angle θ of the half-wave plate 30-3 is fixed to a first angle range (for example, 22.5°). Furthermore, when used exclusively for fr×4x magnification, the optical device 4 includes only an optical unit 20-2, a pulse conversion unit 15-2 having a 1 / 2 wavelength plate 30-2, and a pulse conversion unit 15-3 having an optical unit 20-3 and a 1 / 2 wavelength plate 30-3, and the angles θ of the 1 / 2 wavelength plate 30-2 and the 1 / 2 wavelength plate 30-3 are each fixed to a first angle range (e.g., 22.5°). Furthermore, when used exclusively for fr×8 times magnification, the optical device 4 includes a pulse conversion unit 15-1 having an optical unit 20-1 and a half-wave plate 30-1, a pulse conversion unit 15-2 having an optical unit 20-2 and a half-wave plate 30-2, and a pulse conversion unit 15-3 having an optical unit 20-3 and a half-wave plate 30-3, and the angles θ of the half-wave plate 30-1, the half-wave plate 30-2 and the half-wave plate 30-3 are each fixedly set to a first angle range (e.g., 22.5°).

[0089] [Second embodiment] The microscope according to the second embodiment is obtained by replacing the optical device 4 of the microscope 1 according to the first embodiment shown in FIG. 1 with an optical device 4B shown in FIG.

[0090] The optical device 4B can convert the repetition frequency fr of the pulsed light L1 emitted from the light source 2 into a repetition frequency different from the repetition frequency fr. For example, the optical device 4B can convert the repetition frequency fr into a repetition frequency different from the repetition frequency fr by up to 2 n For example, the optical device 4B divides the pulsed light into P-polarized light and S-polarized light that is orthogonal to the P-polarized light, provides a predetermined optical path length difference between the P-polarized light and the S-polarized light, and then combines the P-polarized light and the S-polarized light after providing the optical path length difference. This process is repeated n times (n is an integer between 1 and n), thereby increasing the repetition frequency by 2. n The optical device 4B may be configured to be insertable into and detachable from the microscope 1.

[0091] Fig. 14 is a diagram showing an example of the configuration of an optical device 4B according to the second embodiment. The optical device 4B includes, for example, n pulse converting units 15B and a plurality of mirrors MB. The optical device 4B shown in Fig. 14 includes three pulse converting units 15B-1 to 15B-3 and two mirrors MB1 and MB2.

[0092] The pulse converting unit 15B includes an optical unit 20B and a half-wave plate 30B. The optical unit 20B is an example of a converting section. The half-wave plate 30B is an example of a switching section. The optical unit 20B includes a polarizing beam splitter 21B and an optical path length difference providing section 22B.

[0093] The polarizing beam splitter 21B splits pulsed light polarized in a first direction into P-polarized light and S-polarized light. That is, when the incident pulsed light is polarized in the first direction, the polarizing beam splitter 21B transmits the P-polarized component of the pulsed light and reflects the S-polarized component of the pulsed light in a predetermined direction. When the incident pulsed light is polarized in a second direction other than the first direction, the polarizing beam splitter 21B does not split the pulsed light. Therefore, pulsed light polarized in the second direction (e.g., P-polarized) passes through the polarizing beam splitter 21B as is. Furthermore, the polarizing beam splitter 21B combines the P-polarized light and the S-polarized light after the optical path length difference is imparted by the optical path length difference imparting unit 22B to form pulsed light with a changed repetition frequency. In other words, the polarizing beam splitter 21B has the function of the first polarizing beam splitter 21 and the function of the second polarizing beam splitter 23 described in the first embodiment.

[0094] The optical path length difference providing unit 22B provides an optical path length difference between the P polarized light and the S polarized light split by the polarizing beam splitter 21 B. For example, the optical path length difference providing unit 22B includes a first optical path 24B and a second optical path 25B.

[0095] The first optical path 24B is provided with a quarter-wave plate 40B, a mirror MB11, and a mirror MB12. The quarter-wave plate 40B converts the P-polarized light split by the polarizing beam splitter 21B into circularly polarized light. The circularly polarized light is reflected by mirrors MB11 and MB12 and enters the quarter-wave plate 40B again. The circularly polarized light that enters the quarter-wave plate 40B is converted into S-polarized light by the quarter-wave plate 40B. The S-polarized light converted by the quarter-wave plate 40B is reflected by the polarizing beam splitter 21B and exits the optical unit 20B.

[0096] The second optical path 25B is provided with a quarter-wave plate 41B, a mirror MB13, and a mirror MB14. The quarter-wave plate 41B converts the S-polarized light split by the polarizing beam splitter 21B into circularly polarized light. The circularly polarized light is reflected by mirrors MB13 and MB14 and enters the quarter-wave plate 41B again. The circularly polarized light entering the quarter-wave plate 41B is converted into P-polarized light by the quarter-wave plate 41B. The P-polarized light converted by the quarter-wave plate 41B is transmitted through the polarizing beam splitter 21B and exits the optical unit 20B. The optical path length of the second optical path 25B is shorter than the optical path length of the first optical path 24B. Therefore, the optical path length difference imparting unit 22 can impart an optical path length difference between the P-polarized light and the S-polarized light.

[0097] Optical path length difference imparting unit 22B-1 imparts a first optical path length difference between the P polarized light and the S polarized light split by polarizing beam splitter 21B-1. This first optical path length difference imparts a time difference of (Tr / 8) between the P polarized light and the S polarized light.

[0098] Optical path length difference imparting unit 22B-2 imparts a second optical path length difference between the P polarized light and the S polarized light split by polarizing beam splitter 21B-2. The second optical path length difference is greater than the first optical path length difference. This second optical path length difference imparts a time difference of (Tr / 4) between the P polarized light and the S polarized light.

[0099] Optical path length difference imparting unit 22B-3 imparts a third optical path length difference between the P polarized light and the S polarized light split by polarizing beam splitter 21B. The third optical path length difference is greater than the second optical path length difference. This third optical path length difference imparts a time difference of (Tr / 2) between the P polarized light and the S polarized light.

[0100] The half-wave plate 30B changes the polarization direction of the pulsed light incident on the polarizing beam splitter 21B in each of the optical units 20B to either a first direction or a second direction different from the first direction. The half-wave plate 30B is rotatable. By rotating the half-wave plate 30B, the angle θ between the polarization direction of the pulsed light incident on the half-wave plate 30B and the optical axis (slow axis or fast axis) of the half-wave plate 30B changes. B For example, the angle θ B is within a first angle range, and the angle θ B The angle θ can be switched between a first state and a second state in which the angle θ is within a second angle range. B is in the first angle range, the half-wave plate 30B emits pulsed light in the first direction. B is in a second angle range different from the first angle range, pulsed light in a second direction different from the first direction is emitted from the half wave plate 30B.

[0101] The half-wave plate 30B-1 is provided upstream of the polarizing beam splitter 21B-1. The half-wave plate 30B-1 changes the direction of the pulsed light incident on the polarizing beam splitter 21B-1 to either a first direction or a second direction different from the first direction. The half-wave plate 30B-2 is provided upstream of the polarizing beam splitter 21B-2. The half-wave plate 30B-2 changes the direction of the pulsed light emitted from the polarizing beam splitter 21B-1 and incident on the polarizing beam splitter 21B-2 to either the first direction or a second direction different from the first direction. The half-wave plate 30B-3 is provided upstream of the polarizing beam splitter 21B-3. The half-wave plate 30B-3 changes the direction of the pulsed light emitted from the polarizing beam splitter 21B-2 and incident on the polarizing beam splitter 21B-3 into either a first direction or a second direction different from the first direction.

[0102] By setting the angle of the half-wave plate 30B, it is possible to switch whether or not the optical unit 20B changes the repetition frequency of the pulsed light L1. For example, the repetition frequency fr of the pulsed light L1 can be changed to a value obtained by multiplying the repetition frequency fr by a power of 2. In other words, by setting the angle of the half-wave plate 30B, the repetition frequency fr can be changed from 1 to 2 times the repetition frequency fr. n 14, n=3, so that the repetition frequency fr of the pulsed light L1 emitted from the light source 2 can be varied to one of four repetition frequencies, fr×1, fr×2, fr×4, and fr×8, depending on the setting angle of the half-wave plate 30B.

[0103] Mirror MB1 reflects the pulsed light emitted from pulse converting unit 15B-1 to pulse converting unit 15B-2, and mirror MB2 reflects the pulsed light emitted from pulse converting unit 15B-2 to pulse converting unit 15B-3.

[0104] The control unit 11 controls the rotation of the half-wave plates 30B-1 to 30B-3. The control unit 11 may also control the rotation of each of the quarter-wave plates 40B-1 to 40B-2 and the rotation of each of the quarter-wave plates 41B-1 to 41B-3.

[0105] In the optical device 4B according to the second embodiment, similarly to the first embodiment, the repetition frequency fr of the pulsed light L1 emitted from the light source 2 can be changed to one of four repetition frequencies, fr×1, fr×2, fr×4, and fr×8, by controlling the rotation of the half-wave plates 30B-1 to 30B-3. In other words, in the optical device 4B, similarly to the first embodiment, the angles θ formed by the half-wave plates 30B-1 to 30B-3 can be changed to one of four repetition frequencies, fr×1, fr×2, fr×4, and fr×8. Bis switched between the first angle range and the second angle range, the repetition frequency fr of the pulsed light L1 emitted from the light source 2 can be varied to one of four repetition frequencies: fr×1, fr×2, fr×4, and fr×8. Note that the method for varying the repetition frequency using the half wave plate 30B is the same as in the first embodiment, and therefore description thereof will be omitted.

[0106] The repetition frequency (1 or fr×2) according to the second embodiment is as follows: n ) is the same as that of the first embodiment shown in FIG. 11, and therefore the description thereof will be omitted.

[0107] The optical device 4B according to the second embodiment includes an optical unit 20B that converts the repetition frequency fr of pulsed light L emitted from the light source 2, and a repetition frequency (1 or fr×2) converted by the optical unit 20B. n ) and a half wave plate 30B that can change the polarization direction of the polarizing beam splitter 30. This configuration not only produces the same effects as the first embodiment, but also contributes to improving robustness since the number of polarizing beam splitters per optical unit can be reduced by one compared to the first embodiment.

[0108] [Third embodiment] In the microscope according to the third embodiment, the optical device 4 of the microscope 1 according to the first embodiment shown in FIG. 1 is replaced with an optical device 4C shown in FIG.

[0109] The optical device 4C can convert the repetition frequency fr of the pulsed light L1 emitted from the light source 2 into a repetition frequency different from the repetition frequency fr. For example, the optical device 4C can convert the repetition frequency fr into a repetition frequency different from the repetition frequency fr by up to 2 n For example, the optical device 4C divides the pulsed light into P-polarized light and S-polarized light that is orthogonal to the P-polarized light, provides a predetermined optical path length difference between the P-polarized light and the S-polarized light, and then combines the P-polarized light and the S-polarized light after providing the optical path length difference. This process is repeated n times (n is an integer between 1 and n), thereby increasing the repetition frequency by 2. nThe optical device 4C may be configured to be insertable into and detachable from the microscope 1.

[0110] Fig. 15 is a diagram showing an example of the configuration of an optical device 4C according to the first embodiment. The optical device 4C includes, for example, n pulse converting units 15C and a plurality of mirrors MC. The optical device 4C shown in Fig. 15 includes three pulse converting units 15C-1 to 15C-3 and two mirrors MC1 and MC2.

[0111] The pulse converting unit 15C includes an optical unit 20C and a half-wave plate 30C. The optical unit 20C is an example of a converting section. The half-wave plate 30C is an example of a switching section. The optical unit 20C includes a polarizing beam splitter 21C and an optical path length difference providing section 22C.

[0112] The polarizing beam splitter 21C splits pulsed light polarized in a first direction into P-polarized light and S-polarized light. That is, when the incident pulsed light is polarized in the first direction, the polarizing beam splitter 21C transmits the P-polarized component of the pulsed light and reflects the S-polarized component of the pulsed light in a predetermined direction. When the incident pulsed light is polarized in a second direction other than the first direction, the polarizing beam splitter 21C does not split the pulsed light. Therefore, pulsed light polarized in the second direction passes through the polarizing beam splitter 21C as is. Furthermore, the polarizing beam splitter 21C combines the P-polarized light and the S-polarized light after the optical path length difference is imparted by the optical path length difference imparting unit 22C to form pulsed light with a changed repetition frequency. The polarizing beam splitter 21C has the function of the first polarizing beam splitter 21 and the function of the second polarizing beam splitter 23 in the first embodiment.

[0113] The optical path length difference providing unit 22C provides an optical path length difference between the P polarized light and the S polarized light split by the polarizing beam splitter 21 C. For example, the optical path length difference providing unit 22C includes an optical path 24C.

[0114] Mirrors MC11 and MC12 are provided on optical path 24C. The P-polarized light that has passed through polarizing beam splitter 21C propagates along optical path 24C, is reflected by mirrors MC11 and MC12, and then re-enters and passes through polarizing beam splitter 21C. This propagation creates an optical path difference between the S-polarized light and the P-polarized light via optical path 24C, and this optical path difference creates a predetermined time difference between the S-polarized light and the P-polarized light.

[0115] Optical path length difference imparting unit 22C-1 imparts a first optical path length difference between the P polarized light and the S polarized light split by polarizing beam splitter 21C-1. The first optical path length difference imparts a time difference of (Tr / 8) between the P polarized light and the S polarized light.

[0116] Optical path length difference imparting unit 22C-2 imparts a second optical path length difference between the P-polarized light and the S-polarized light split by polarizing beam splitter 21C-2. The second optical path length difference is greater than the first optical path length difference. This second optical path length difference imparts a time difference of (Tr / 4) between the P-polarized light and the S-polarized light.

[0117] Optical path length difference imparting unit 22C-3 imparts a third optical path length difference between the P polarized light and the S polarized light split by polarizing beam splitter 21C-3. The third optical path length difference is greater than the second optical path length difference. This third optical path length difference imparts a time difference of (Tr / 2) between the P polarized light and the S polarized light.

[0118] In each of the optical units 20C, the half-wave plate 30C changes the polarization direction of the pulsed light incident on the polarizing beam splitter 21C to either a first direction or a second direction different from the first direction. The half-wave plate 30C is rotatable. By rotating the half-wave plate 30C, the angle θ of the half-wave plate 30C is changed. C will be changed.

[0119] For example, the angle θ C is within a first angle range, and the angle θ C The angle θ can be switched between a first state and a second state in which the angle θ is within the second angle range.C is in the first angle range, the half-wave plate 30C emits pulsed light in the first direction. C is in a second angle range different from the first angle range, pulsed light in a second direction different from the first direction is emitted from the half wave plate 30.

[0120] The half-wave plate 30C-1 is provided before the polarizing beam splitter 21C-1. The half-wave plate 30C-1 changes the polarization direction of the pulsed light incident on the polarizing beam splitter 21C-1 to either a first direction or a second direction different from the first direction.

[0121] The half-wave plate 30C-2 is provided before the polarizing beam splitter 21C-2. The half-wave plate 30C-2 changes the polarization direction of the pulsed light emitted from the polarizing beam splitter 21C-1 and incident on the polarizing beam splitter 21C-2 to either a first direction or a second direction different from the first direction.

[0122] The half-wave plate 30C-3 is provided before the polarizing beam splitter 21C-3. The half-wave plate 30C-3 changes the polarization direction of the pulsed light emitted from the polarizing beam splitter 21C-2 and incident on the polarizing beam splitter 21C-3 to either a first direction or a second direction different from the first direction.

[0123] By setting the angle of the half-wave plate 30C, it is possible to switch whether or not the optical unit 20C changes the repetition frequency of the pulsed light L1. For example, the repetition frequency fr of the pulsed light L1 can be changed to a value obtained by multiplying the repetition frequency fr by a power of 2. In other words, by setting the angle of the half-wave plate 30C, the repetition frequency fr can be changed from 1 to 2 times the repetition frequency fr. n 15, n=3, and therefore the repetition frequency fr of the pulsed light L1 emitted from the light source 2 can be varied to one of four repetition frequencies: fr×1, fr×2, fr×4, and fr×8, depending on the setting angle of the half-wave plate 30C.

[0124] Mirror MC1 reflects the pulsed light emitted from pulse converting unit 15C-1 to pulse converting unit 15C-2, and mirror MC2 reflects the pulsed light emitted from pulse converting unit 15C-2 to pulse converting unit 15C-3.

[0125] In the optical device 4C according to the third embodiment, similarly to the first embodiment, the repetition frequency fr of the pulsed light L1 emitted from the light source 2 can be varied among four repetition frequencies, fr×1, fr×2, fr×4, and fr×8, by controlling the rotation of the half-wave plates 30C-1 to 30C-3. In other words, in the optical device 4C, similarly to the first embodiment, the angles θ of the half-wave plates 30C-1 to 30C-3 can be varied among four repetition frequencies, fr×1, fr×2, fr×4, and fr×8. C By switching between the first angle range and the second angle range, the repetition frequency fr of the pulsed light L1 emitted from the light source 2 can be changed to one of four repetition frequencies: fr×1, fr×2, fr×4, and fr×8.

[0126] The method of varying the repetition frequency by the half wavelength plate 30C is the same as in the first embodiment, and therefore the explanation will be omitted. n ) is the same as that of the first embodiment shown in FIG. 11, and therefore the description thereof will be omitted.

[0127] The optical device 4C according to the third embodiment includes an optical unit 20C that converts the repetition frequency fr of pulsed light L emitted from the light source 2, and a repetition frequency (1 or fr×2) converted by the optical unit 20C. n ) and a half wave plate 30C that can change the wavelength of the polarizing beam splitter 30. This configuration not only produces the same effects as the first embodiment, but also reduces the number of polarizing beam splitters per optical unit by one compared to the first embodiment and reduces the number of quarter wave plates 40B compared to the second embodiment, thereby contributing to improved robustness.

[0128] [Fourth embodiment] In the microscope according to the fourth embodiment, the optical device 4 of the microscope 1 according to the first embodiment shown in FIG. 1 is replaced with an optical device 4D shown in FIG.

[0129] The optical device 4D can convert the repetition frequency fr of the pulsed light L1 emitted from the light source 2 into a repetition frequency different from the repetition frequency fr. For example, the optical device 4D can convert the repetition frequency fr into a repetition frequency different from the repetition frequency fr by up to 2 n For example, the optical device 4D divides the pulsed light into P-polarized light and S-polarized light orthogonal to the P-polarized light, provides a predetermined optical path length difference between the P-polarized light and the S-polarized light, and combines the P-polarized light and the S-polarized light after providing the optical path length difference. This process is repeated n times (n is an integer between 1 and n), thereby increasing the repetition frequency by 2. n The optical device 4D may be configured to be insertable into and detachable from the microscope 1.

[0130] The principle of pulse height repetition in the optical device 4D is the same as in Embodiment 1. However, the optical device 4D according to the fourth embodiment differs in that the n first polarizing beam splitters 21-1 to 21-n of the first embodiment are shared by a single first polarizing beam splitter 21D, and the n second polarizing beam splitters 23-1 to 23-n of the first embodiment are shared by a single second polarizing beam splitter 23D.

[0131] 16 and 17 are diagrams showing an example of the configuration of an optical device 4D according to the fourth embodiment. Fig. 16 is a schematic diagram of the optical device 4D viewed from above (Y direction). Fig. 17 is a schematic diagram of the optical device 4D viewed from the front (Z direction). The optical device 4D shown in Figs. 16 and 17 includes three pulse converting units 15D-1 to 15D-3 and two roof mirrors RM1 and RM2.

[0132] The optical paths of the pulse converting units 15D-1 to 15D-3 are at different heights. Pulse converting unit 15D-1 is located at the lowest optical path. The pulsed light emitted from pulse converting unit 15D-1 is reflected by roof mirror RM1, and the optical path becomes higher. Thereafter, the pulsed light enters pulse converting unit 15D-2. The pulsed light emitted from pulse converting unit 15D-2 is reflected by roof mirror RM2, and the optical path becomes higher. Thereafter, the pulsed light enters pulse converting unit 15D-3.

[0133] The pulse conversion unit 15D-1 includes an optical unit 20D-1 and a half-wave plate 30D-1. The optical unit 20D-1 includes a first polarizing beam splitter 21D, an optical path length difference providing unit 22D-1, and a second polarizing beam splitter 23D. The optical unit 20D-1 is an example of a conversion unit. The half-wave plate 30D-1 is an example of a switching unit.

[0134] The first polarizing beam splitter 21D splits the pulsed light L1 polarized in a first direction into P-polarized light and S-polarized light. When the incident pulsed light L1 is polarized in the first direction, the first polarizing beam splitter 21D transmits the P-polarized component of the pulsed light L1 and reflects the S-polarized component of the pulsed light L1 in a predetermined direction. When the incident pulsed light L1 is polarized in a second direction other than the first direction, the first polarizing beam splitter 21D does not split the pulsed light L1. Therefore, the pulsed light L1 polarized in the second direction (P-polarized) is transmitted through the polarizing beam splitter 21D-1 as is without being split.

[0135] The optical path length difference providing unit 22D-1 provides a first optical path length difference between the P polarized light and the S polarized light split by the first polarizing beam splitter 21D. For example, the optical path length difference providing unit 22D-1 includes a first optical path 24D-1 and a second optical path 25D-1.

[0136] The first optical path 24D-1 is a path along which the P-polarized light that has passed through the first polarizing beam splitter 21D propagates until it reaches the second polarizing beam splitter 23D. In other words, the P-polarized light that has passed through the first polarizing beam splitter 21D propagates through the first optical path 24D-1 and enters the second polarizing beam splitter 23D.

[0137] The second optical path 25D-1 is a path along which the S-polarized light reflected by the first polarizing beam splitter 21D propagates until it reaches the second polarizing beam splitter 23D. Two mirrors DM1 are provided on the second optical path 25D. The mirrors DM1 are, for example, semicircular. The S-polarized light split by the first polarizing beam splitter 21D is reflected by the two mirrors DM1 and enters the second polarizing beam splitter 23D. The optical path length of the second optical path 25D-1 is longer than the optical path length of the first optical path 24D-1. Therefore, the optical path length difference imparting unit 22D-1 can impart a first optical path length difference between the P-polarized light and the S-polarized light.

[0138] The second polarizing beam splitter 23D forms pulsed light with a changed repetition frequency by multiplexing the P polarized light and the S polarized light after the first optical path difference has been imparted by the optical path difference imparting unit 22-1. When the first optical path difference has been imparted between the P polarized light and the S polarized light, a time difference of (Tr / 8) is imparted between the P polarized light and the S polarized light until they propagate to the second polarizing beam splitter 23D. Therefore, when the P polarized light and the S polarized light are coaxially multiplexed by the second polarizing beam splitter 23D, the repetition frequency of the combined pulsed light is higher than the repetition frequency of the pulsed light L1 incident on the first polarizing beam splitter 21D.

[0139] The roof mirror RM1 has, for example, two reflecting surfaces, and bends the pulsed light emitted from the second polarizing beam splitter 23D by 180 degrees by reflecting it twice. The pulsed light bent by the roof mirror RM1 enters the pulse converting unit 15D-2. In this way, the pulsed light incident on the roof mirror RM1 is reflected twice and then enters the pulse converting unit 15D-2 from the roof mirror RM1.

[0140] The pulse conversion unit 15D-2 includes an optical unit 20D-2 and a half-wave plate 30D-2. The optical unit 20D-2 includes a first polarizing beam splitter 21D, an optical path length difference providing unit 22D-2, and a second polarizing beam splitter 23D. The optical unit 20D-2 is an example of a conversion unit. The half-wave plate 30D-2 is an example of a switching unit.

[0141] The pulsed light from the roof mirror RM1 enters the second polarizing beam splitter 23D via the half-wave plate 30D-2. When the pulsed light entering from the half-wave plate 30D-2 is polarized in a first direction, the second polarizing beam splitter 23D splits the light into P-polarized and S-polarized light. That is, when the pulsed light entering from the half-wave plate 30D-2 is polarized in the first direction, the second polarizing beam splitter 23D transmits the P-polarized component of the pulsed light and reflects the S-polarized component in a predetermined direction. When the pulsed light entering from the half-wave plate 30D-2 is polarized in a second direction other than the first direction, the second polarizing beam splitter 23D does not split the pulsed light. Therefore, the pulsed light polarized in the second direction (P-polarized) passes through the second polarizing beam splitter 23D without being split.

[0142] The optical path length difference providing unit 22D-2 provides a second optical path length difference between the P polarized light and the S polarized light split by the first polarizing beam splitter 21D. For example, the optical path length difference providing unit 22D-2 includes a first optical path 24D-2 and a second optical path 25D-2.

[0143] The first optical path 24D-2 is a path along which the P-polarized light that has passed through the second polarizing beam splitter 23D propagates until it reaches the first polarizing beam splitter 21D. In other words, the P-polarized light that has passed through the second polarizing beam splitter 23D propagates along the first optical path 24D-2 and enters the first polarizing beam splitter 21D.

[0144] The second optical path 25D-2 is a path along which the S-polarized light reflected by the second polarizing beam splitter 23D propagates until it reaches the first polarizing beam splitter 21D. The second optical path 25D-2 includes two mirrors DM2 and one mirror MD1. The mirror DM2 has, for example, a semicircular shape. The S-polarized light split by the second polarizing beam splitter 23D is reflected by the two mirrors DM2 and the one mirror MD1 and enters the first polarizing beam splitter 21D. The optical path length of the second optical path 25D-2 is longer than the optical path length of the first optical path 24D-2. Therefore, the optical path length difference imparting unit 22D-2 can impart a second optical path length difference between the P-polarized light and the S-polarized light.

[0145] The first polarizing beam splitter 21D forms pulsed light with a changed repetition frequency by multiplexing the P polarized light and the S polarized light after the second optical path difference has been imparted by the optical path difference imparting unit 22D-2. Because the second optical path difference has been imparted between the P polarized light and the S polarized light, a time difference of (Tr / 4) is imparted between the P polarized light and the S polarized light until they propagate to the first polarizing beam splitter 21D.

[0146] The roof mirror RM2 has, for example, two reflecting surfaces, and bends the pulsed light emitted from the first polarizing beam splitter 21D by 180 degrees by reflecting it twice. The pulsed light bent by the roof mirror RM2 enters the pulse converting unit 15D-3. In this way, the pulsed light incident on the roof mirror RM2 is reflected twice and then enters the pulse converting unit 15D-3 from the roof mirror RM2.

[0147] The pulse conversion unit 15D-3 includes an optical unit 20D-3 and a half-wave plate 30D-3. The optical unit 20D-3 includes a first polarizing beam splitter 21D, an optical path length difference providing unit 22D-3, and a second polarizing beam splitter 23D. The optical unit 20D-2 is an example of a conversion unit. The half-wave plate 30D-2 is an example of a switching unit.

[0148] The pulsed light from the roof mirror RM2 enters the first polarizing beam splitter 21D via the half-wave plate 30D-3. When the pulsed light entering from the half-wave plate 30D-3 is polarized in a first direction, the first polarizing beam splitter 21D splits the light into P-polarized and S-polarized light. That is, when the pulsed light entering from the half-wave plate 30D-3 is polarized in the first direction, the first polarizing beam splitter 21D transmits the P-polarized light of the pulsed light and reflects the S-polarized light in a predetermined direction. When the pulsed light entering from the half-wave plate 30D-3 is polarized in a second direction other than the first direction, the first polarizing beam splitter 21D does not split the pulsed light. Therefore, the pulsed light polarized in the second direction (P-polarized) passes through the first polarizing beam splitter 21D as is without being split.

[0149] The optical path difference providing unit 22D-3 provides a third optical path difference between the P polarized light and the S polarized light split by the first polarizing beam splitter 21D-3. For example, the optical path difference providing unit 22D-3 includes a first optical path 24D-3 and a second optical path 25D-3.

[0150] The first optical path 24D-3 is a path along which the P-polarized light that has passed through the first polarizing beam splitter 21D propagates until it reaches the second polarizing beam splitter 23D. In other words, the P-polarized light that has passed through the first polarizing beam splitter 21D propagates through the first optical path 24D-3 and enters the second polarizing beam splitter 23D.

[0151] The second optical path 25D-3 is a path along which the S-polarized light reflected by the first polarizing beam splitter 21D-3 propagates until it reaches the second polarizing beam splitter 23D. Three mirrors MD2 are provided on the second optical path 25D. The S-polarized light split by the first polarizing beam splitter 21D is reflected by the three mirrors MD2 and enters the second polarizing beam splitter 23D. The optical path length of the second optical path 25D-3 is longer than the optical path length of the first optical path 24D-3. Therefore, the optical path length difference imparting unit 22D-3 can impart a third optical path length difference between the P-polarized light and the S-polarized light.

[0152] The second polarizing beam splitter 23D combines the P polarized light and the S polarized light after the third optical path difference has been imparted by the optical path difference imparting unit 22-3, thereby forming pulsed light with a changed repetition frequency. When the third optical path difference has been imparted between the P polarized light and the S polarized light, a time difference of (Tr / 2) is imparted between the P polarized light and the S polarized light.

[0153] The first polarizing beam splitter 21D has a rectangular parallelepiped shape. Pulse conversion units 15D are stacked in the longitudinal direction (Y direction) of the first polarizing beam splitter 21D. In order to minimize GDD caused by the propagation of pulsed light in the first polarizing beam splitter 21D, it is desirable to set the size of the first polarizing beam splitter 21D in the incident direction of the pulsed light and the reflection direction in which the incident pulsed light is reflected as small as possible in accordance with the beam diameter.

[0154] The second polarizing beam splitter 23D has a rectangular parallelepiped shape. Pulse conversion units 15D are stacked in the longitudinal direction (Y direction) of the second polarizing beam splitter 23D. In order to minimize the GDD caused by the propagation of pulsed light in the second polarizing beam splitter 23D, it is desirable to set the size of the second polarizing beam splitter 23D in the direction of incidence of the pulsed light and the direction of reflection of the incident pulsed light as small as possible in accordance with the beam diameter.

[0155] A method for converting the repetition frequency according to the fourth embodiment will be described below. When the repetition frequency fr of the pulsed light L1 incident on the optical device 4D is changed to fr×1, all of the angles θ of the half-wave plates 30D-1, 30D-2, and 30D-3 are adjusted to 1 / 2. D is set to the second angle range. Therefore, the pulsed light L1 emitted from the light source 2 is not split by the first polarizing beam splitter 21D. That is, the pulsed light L1 is not optically branched or time-delayed by the optical units 20D-1 to 20D-3, and the repetition frequency fr of the pulsed light L1 is maintained. Therefore, the pulsed light L1 is output from the optical device 4D as pulsed light L2.

[0156] Next, a method of doubling the repetition frequency will be described. The repetition frequency fr of the pulsed light L1 incident on the optical device 4D is doubled by fr×2, that is, 2 1 When changing the angle θ of the half wave plate 30D-1 and the half wave plate 30D-2 to 1 / 4 times (n=1), D is set to the second angle range. On the other hand, the angle θ D is set to the first angle range. In this case, the pulsed light L1 irradiated from the light source 2 passes through neither the pulse conversion unit 15D-1 nor the pulse conversion unit 15D-2 without being split. That is, the optical units 20D-1 and 20D-2 do not branch the light nor add a time delay, and the repetition frequency of the pulsed light emitted from the optical units 20D-1 and 20D-2 is maintained to be the repetition frequency fr of the pulsed light L1 irradiated from the light source 2. In addition, the polarization state of the pulsed light emitted from the optical units 20D-1 and 20D-2 remains P-polarized, like the pulsed light L1.

[0157] However, the pulsed light L1 output from the half-wave plate 30D-3 is linearly polarized at 45°. Therefore, of the pulsed light output from the half-wave plate 30D-3, i.e., the 45° linearly polarized light, the P-polarized light passes through the first polarizing beam splitter 21D, and the S-polarized light is reflected by the first polarizing beam splitter 21D. The P-polarized light that passed through the first polarizing beam splitter 21D propagates through the first optical path 24D-3 and enters the second polarizing beam splitter 23D. Meanwhile, the S-polarized light that was reflected by the first polarizing beam splitter 21D propagates through the second optical path 25D-3 and enters the second polarizing beam splitter 23D. Here, the longitudinal directions (Y direction) of the first polarizing beam splitter 21D and the second polarizing beam splitter 23D correspond to the position of the pulse converting unit 15D, so the S-polarized light that passes through the second optical path 25D-3 can avoid the mirrors DM1 and DM2.

[0158] A time difference corresponding to the third optical path difference, i.e., a time difference of Tr / 2, is imparted between the P-polarized light that has propagated through the first optical path 24D-3 and the S-polarized light that has propagated through the second optical path 25D-3. The second polarizing beam splitter 23D generates pulsed light having a repetition frequency of (fr×2) by multiplexing the incident P-polarized light and S-polarized light. This pulsed light having a repetition frequency of (fr×2) is output from the optical device 4D as pulsed light L2. That is, pulsed light having a pulse interval of (Tr / 2) is emitted from the optical device 4D as pulsed light L2.

[0159] Next, a method for converting the repetition frequency by four will be described. The repetition frequency fr of the pulsed light L1 incident on the optical device 4D is converted to fr×4, that is, 2 2 When changing to a factor of 2 (n=2), the angle θ of the half-wave plate 30D-1 is D is set to the second angle range. On the other hand, the angles θ D is set to the first angle range. In this case, the pulsed light L1 irradiated from the light source 2 is transmitted through the pulse conversion unit 15D-1 without being split. That is, the optical unit 20D-1 does not branch the light or add a time delay, and the repetition frequency of the pulsed light emitted from the optical unit 20D-1 is maintained to be the repetition frequency fr of the pulsed light L1 irradiated from the light source 2. In addition, the polarization state of the pulsed light emitted from the optical unit 20D-1 remains P-polarized, just like the pulsed light L1.

[0160] However, the polarization of the pulsed light immediately after passing through the half-wave plate 30D-2 becomes linearly polarized at 45°. Therefore, of the pulsed light output from the half-wave plate 30D-2, i.e., the 45° linearly polarized light, the P-polarized light passes through the second polarizing beam splitter 23D, and the S-polarized light is reflected by the second polarizing beam splitter 23D. The P-polarized light that passed through the second polarizing beam splitter 23D propagates through the first optical path 24D-2 and enters the first polarizing beam splitter 21D. Meanwhile, the S-polarized light reflected by the second polarizing beam splitter 23D propagates through the second optical path 25D-2 and enters the first polarizing beam splitter 21D. Here, the longitudinal directions (Y direction) of the first polarizing beam splitter 21D and the second polarizing beam splitter 23D correspond to the position of the pulse converting unit 15D, so the S-polarized light that passes through the second optical path 25D-2 can avoid the mirror DM1.

[0161] A time difference corresponding to the second optical path difference, i.e., a time difference of Tr / 4, is imparted between the P-polarized light propagating through the first optical path 24D-2 and the S-polarized light propagating through the second optical path 25D-2. The first polarizing beam splitter 21D generates pulsed light by combining the incident P-polarized light and S-polarized light. The pulsed light generated by the first polarizing beam splitter 21D is incident on the half-wave plate 30D-3. When the pulsed light passes through the half-wave plate 30D-3, the polarization of the pulsed light after passing through the plate becomes linearly polarized at +45° and linearly polarized at -45°, respectively.

[0162] Of the +45° linearly polarized light that precedes in time, the P-polarized component passes through first polarizing beam splitter 21D and propagates through first optical path 24D-3. Of the +45° linearly polarized light, the S-polarized component is reflected by first polarizing beam splitter 21D and propagates through second optical path 25D-3. Also, of the −45° linearly polarized light, the P-polarized component passes through first polarizing beam splitter 21D and propagates through first optical path 24D-3. Of the −45° linearly polarized light, the S-polarized component is reflected by first polarizing beam splitter 21D and propagates through second optical path 25D-3. Therefore, a time difference of (Tr / 2) is imparted between the P-polarized light that propagated through first optical path 24D-3 and the S-polarized light that propagated through second optical path 25D-3. The second polarizing beam splitter 23D combines the two incident P-polarized beams and two incident S-polarized beams to generate pulsed light having a repetition frequency of (fr × 4). That is, pulsed light having a pulse interval of (Tr / 4) is emitted from the optical device 4D as pulsed light L2.

[0163] The repetition frequency fr of the pulsed light L1 incident on the optical device 4D is fr×8, that is, 2 3 When the angle θ of the half-wave plate 30D-1, the angle θ of the half-wave plate 30D-2, and the angle θ of the half-wave plate 30D-3 are changed to 3 times (n=3), D is set to the first angle range.

[0164] The pulsed light L immediately after passing through the half-wave plate 30D-1 becomes linearly polarized at 45°. Therefore, of the pulsed light output from the half-wave plate 30D-1, i.e., the 45° linearly polarized light, the P-polarized component passes through the first polarizing beam splitter 21D, and the S-polarized component is reflected by the first polarizing beam splitter 21D. The P-polarized light that passed through the first polarizing beam splitter 21D propagates through the first optical path 24D-1 and enters the second polarizing beam splitter 23D. Meanwhile, the S-polarized light reflected by the first polarizing beam splitter 21D propagates through the second optical path 25D-1 and enters the second polarizing beam splitter 23D. Therefore, a time difference according to the first optical path difference, i.e., a time difference of Tr / 8, is imparted between the P-polarized light that propagated through the first optical path 24D-1 and the S-polarized light that propagated through the second optical path 25D-1.

[0165] The second polarizing beam splitter 23D generates pulsed light by combining the P-polarized light and the S-polarized light to which the first optical path difference has been applied. This pulsed light is incident on the half-wave plate 30D-2 via the roof mirror RM1. The angle θ of the half-wave plate 30D-2 D are in the first angle range, the pulsed light after passing through the half-wave plate 30D-2 becomes linearly polarized light at +45° and linearly polarized light at −45°, respectively.

[0166] Of the +45° linearly polarized light that precedes in time, the P-polarized component passes through the second polarizing beam splitter 23D and propagates along the first optical path 24D-2. Of the +45° linearly polarized light, the S-polarized component is reflected by the second polarizing beam splitter 23D and propagates along the second optical path 25D-2. Furthermore, of the −45° linearly polarized light, the P-polarized component passes through the second polarizing beam splitter 23D and propagates along the first optical path 24D-2. Of the −45° linearly polarized light, the S-polarized component is reflected by the second polarizing beam splitter 23D and propagates along the second optical path 25D-2. Therefore, a time difference of (Tr / 4) is imparted between the P-polarized and S-polarized light split by the second polarizing beam splitter 23D. The first polarizing beam splitter 21D generates pulsed light by combining two incident P-polarized and two incident S-polarized light. The pulsed light generated by the first polarizing beam splitter 21D is incident on the half-wave plate 30D-3 via the roof mirror RM2.

[0167] When the pulsed light generated by the first polarizing beam splitter 21D passes through the half-wave plate 30D-3, the polarization of the passing pulsed light becomes 45° polarization, in which the P-polarized light and the S-polarized light are orthogonal to each other. That is, in the pulsed light after passing through the half-wave plate 30D-3, the two pulses that are earlier in time are linearly polarized at +45°, and the two pulses that are later in time are linearly polarized at -45°.

[0168] Of each of the two +45° linearly polarized light beams that precede each other in time, the P-polarized component passes through first polarizing beam splitter 21D and propagates along first optical path 24D-3, and the S-polarized component is reflected by first polarizing beam splitter 21D and propagates along second optical path 25D-3. Similarly, of each of the −45° linearly polarized light beams that lag in time, the P-polarized component passes through first polarizing beam splitter 21D and propagates along first optical path 24D-3, and the S-polarized component is reflected by first polarizing beam splitter 21D and propagates along second optical path 25D-3. Therefore, a time difference of (Tr / 2) is imparted between the P-polarized light propagating along first optical path 24D-3 and the S-polarized light propagating along second optical path 25D-3.

[0169] The second polarizing beam splitter 23D combines the four incident P-polarized light beams and four incident S-polarized light beams to generate pulsed light having a repetition frequency of (fr × 8). That is, pulsed light having a pulse interval of (Tr / 8) is emitted from the optical device 4D as pulsed light L2.

[0170] The repetition frequency (1 or fr×2) according to the fourth embodiment is as follows: n ) is the same as that of the first embodiment shown in FIG. 11, and therefore the description thereof will be omitted.

[0171] The optical device 4D according to the fourth embodiment includes an optical unit 20D that converts the repetition frequency fr of pulsed light L emitted from the light source 2, and a repetition frequency (1 or fr×2) converted by the optical unit 20D. n ) and a half wave plate 30D that can change the polarization direction of the light beam splitter 15. This configuration not only produces the same effects as the first embodiment, but also reduces the number of polarizing beam splitters per optical unit by one compared to the first embodiment, and reduces the number of optical elements used in the pulse converting unit 15D compared to the second embodiment, contributing to improved robustness and reduced costs.

[0172] [Fifth embodiment] In the microscope according to the fifth embodiment, the optical device 4 of the microscope 1 according to the first embodiment shown in Fig. 1 is replaced with an optical device 4E shown in Fig. 18. The principle of the repetition frequency conversion method of the optical device 4E according to the fifth embodiment is the same as the repetition frequency conversion method according to the second embodiment. The optical device 4E has an equivalent configuration to the optical device 4A according to the second embodiment, but differs from the second embodiment in that the polarizing beam splitters and quarter-wave plates included in the multiple pulse conversion units are common.

[0173] The optical device 4E can convert the repetition frequency fr of the pulsed light L1 emitted from the light source 2 into a repetition frequency different from the repetition frequency fr. For example, the optical device 4E can convert the repetition frequency fr into a repetition frequency different from the repetition frequency fr by up to 2 n For example, the optical device 4E divides the pulsed light into P-polarized light and S-polarized light that is orthogonal to the P-polarized light, provides a predetermined optical path length difference between the P-polarized light and the S-polarized light, and combines the P-polarized light and the S-polarized light after providing the optical path length difference. This process is repeated n times (n is an integer between 1 and n), thereby increasing the repetition frequency by 2. n The optical device 4E may be configured to be insertable into and detachable from the microscope 1.

[0174] Fig. 18 is a diagram showing an example of the configuration of an optical device 4E according to Embodiment 5. The optical device 4E shown in Fig. 18 includes three pulse converting units 15E-1 to 15E-3.

[0175] The pulse conversion unit 15E-1 includes an optical unit 20E-1 and a half-wave plate 30E-1. The optical unit 20E-1 includes a polarizing beam splitter 21E and an optical path length difference providing unit 22E-1. The optical unit 20E-1 is an example of a conversion unit. The half-wave plate 30E-1 is an example of a switching unit.

[0176] The pulse conversion unit 15E-2 includes an optical unit 20E-2 and a half-wave plate 30E-2. The optical unit 20E-2 includes a polarizing beam splitter 21E, mirrors Mb1 and Mb1, and an optical path length difference providing unit 22E-2. The optical unit 20E-2 is an example of a conversion unit. The half-wave plate 30E-2 is an example of a switching unit.

[0177] The pulse conversion unit 15E-3 includes an optical unit 20E-3 and a half-wave plate 30E-2. The optical unit 20E-3 includes a polarizing beam splitter 21E, mirrors Mg1 and Mg2, and an optical path length difference imparting unit 22E-3. The optical unit 20E-3 is an example of a conversion unit.

[0178] Polarizing beam splitter 21E splits pulsed light polarized in a first direction into P-polarized light and S-polarized light. When the incident pulsed light is polarized in the first direction, polarizing beam splitter 21E transmits the P-polarized light of the pulsed light and reflects the S-polarized light in a predetermined direction. When the incident pulsed light is polarized in a second direction other than the first direction, polarizing beam splitter 21E does not split the pulsed light. Therefore, pulsed light polarized in the second direction (P-polarized light) is not split and passes through polarizing beam splitter 21E as is.

[0179] Furthermore, the polarizing beam splitter 21E combines the P polarized light and the S polarized light after the optical path length difference has been imparted, thereby forming pulsed light with a changed repetition frequency. In the pulse converting units 15E-1 to 15E-3, the polarizing beam splitter that splits the pulsed light into P polarized light and S polarized light and combines the P polarized light and the S polarized light after the optical path difference has been imparted is commonly used as the polarizing beam splitter 21E. For example, the polarizing beam splitter 21E has a rectangular parallelepiped shape. The pulse converting units 15E are stacked in the longitudinal direction of the polarizing beam splitter 21E.

[0180] The optical path length difference providing unit 22E-1 provides a first optical path length difference between the P polarized light and the S polarized light split by the polarizing beam splitter 21E. This first optical path length difference provides a time difference of (Tr / 8) between the P polarized light and the S polarized light. For example, the optical path length difference providing unit 22E-1 includes a first optical path 24E and a second optical path 25E-1.

[0181] The first optical path 24E is a path along which the P-polarized pulsed light that has passed through the polarizing beam splitter 21E propagates. The first optical path 24E-1 is provided with a quarter-wave plate 40E-1 and a mirror ME1. The P-polarized pulsed light that has passed through the polarizing beam splitter 21E is converted into circularly polarized light by the quarter-wave plate 40E-1. The circularly polarized light converted by the quarter-wave plate 40E-1 is reflected by the mirror ME1 and enters the quarter-wave plate 40-1 again. The circularly polarized light that has reflected by the mirror ME1 and entered the quarter-wave plate 40-1 is converted into S-polarized pulsed light and enters the polarizing beam splitter 21E.

[0182] The second optical path 25E-1 is a path along which the S-polarized light reflected by the polarizing beam splitter 21E propagates until it reaches the polarizing beam splitter 21E. The second optical path 25E-1 is provided with a quarter-wave plate 40E-2 and a mirror DM1. The S-polarized light split by the polarizing beam splitter 21E is converted into circularly polarized light by the quarter-wave plate 40E-2. The circularly polarized light converted by the quarter-wave plate 40E-2 is reflected by the mirror DM1 and enters the quarter-wave plate 40E-2 again. The circularly polarized light reflected by the mirror DM1 and enters the quarter-wave plate 40-2 is converted into P-polarized pulsed light and enters the polarizing beam splitter 21E. The optical path length of the second optical path 25E-1 is longer than the optical path length of the first optical path 24E-1. Therefore, the optical path difference providing unit 22E-1 can provide a first optical path difference between the P polarized light and the S polarized light.

[0183] The mirror Mb1 reflects the pulsed light emitted from the polarizing beam splitter 21E toward the mirror Mb2. The mirror Mb2 is disposed at a higher position than the mirror Mb1. Therefore, the mirror Mb1 reflects the incident pulsed light toward the mirror Mb2, which is disposed diagonally above the mirror Mb1. The mirror Mb2 reflects the pulsed light from the mirror Mb1 that has passed through the half-wave plate 30E-2 toward the polarizing beam splitter 21E.

[0184] The optical path length difference providing unit 22E-2 provides a second optical path length difference between the P-polarized light and the S-polarized light of the pulsed light split by the polarizing beam splitter 21E. This second optical path length difference provides a time difference of (Tr / 4) between the P-polarized light and the S-polarized light. For example, the optical path length difference providing unit 22E-1 includes a first optical path 24E and a second optical path 25E-2.

[0185] The second optical path 25E-2 is a path along which the S-polarized pulsed light reflected by the polarizing beam splitter 21E propagates until it reaches the polarizing beam splitter 21E again. The second optical path 25E-2 includes a quarter-wave plate 40E-2, a mirror DM2, and a mirror ME2. The S-polarized light propagating through the second optical path 25E-2 is converted into circularly polarized light by the quarter-wave plate 40E-2. The circularly polarized light that propagates through the second optical path 25E-2 is reflected by the mirror DM2 and returns to the mirror DM2. The circularly polarized light that propagates toward the mirror ME2 is reflected by the mirror ME2 and returns to the mirror DM2, where it is reflected by the mirror DM2 and enters the quarter-wave plate 40E-2. The circularly polarized light that reflects off the mirror DM2 and enters the quarter-wave plate 40E-2 is converted into P-polarized pulsed light and enters the polarizing beam splitter 21E. The optical path length of the second optical path 25E-2 is longer than the optical path length of the second optical path 25E-1. Therefore, the optical path difference providing unit 22E-2 can provide a second optical path difference between the P polarized light and the S polarized light that is greater than the first optical path difference.

[0186] The mirror Mg1 is disposed at a higher position than the mirror Mb1. The mirror Mg1 reflects the pulsed light emitted from the polarizing beam splitter 21E toward the mirror Mg2. The mirror M2g is disposed at a higher position than the mirror Mb2. The mirror Mg2 is disposed at a higher position than the mirror M1g. The mirror Mg1 reflects the incident pulsed light toward, for example, the mirror M2g disposed diagonally above it. The mirror Mg2 reflects the pulsed light from the mirror Mg1 that has passed through the half-wave plate 30E-2 toward the polarizing beam splitter 21E.

[0187] The optical path length difference providing unit 22E-3 provides a third optical path length difference between the P-polarized light and the S-polarized light of the pulsed light from the mirror Mg2, which has been split by the polarizing beam splitter 21E. This third optical path length difference provides a time difference of (Tr / 2) between the P-polarized light and the S-polarized light. For example, the optical path length difference providing unit 22E-1 includes a first optical path 24E and a second optical path 25E-3.

[0188] The second optical path 25E-3 is a path along which the S-polarized pulsed light reflected by the polarizing beam splitter 21E propagates until it reaches the polarizing beam splitter 21E again. The second optical path 25E-3 includes a quarter-wave plate 40E-2 and mirrors ME3 and ME4. The S-polarized light propagating through the second optical path 25E-3 is converted into circularly polarized light by the quarter-wave plate 40E-2. The circularly polarized light that propagates through the second optical path 25E-3 is reflected by the mirror ME3 toward the mirror ME4. The circularly polarized light that propagates toward the mirror ME4 is reflected by the mirror ME4 back to the mirror ME3, and is then reflected by the mirror ME3 and enters the quarter-wave plate 40E-2. The circularly polarized light that reflects off the mirror ME3 and enters the quarter-wave plate 40E-2 is converted into P-polarized pulsed light and enters the polarizing beam splitter 21E. The optical path length of the second optical path 25E-3 is longer than the optical path length of the second optical path 25E-2. Therefore, the optical path difference providing unit 22E-2 can provide a third optical path difference, which is longer than the second optical path difference, between the P polarized light and the S polarized light.

[0189] The half-wave plate 30E-2 changes the polarization direction of the pulsed light incident on the polarizing beam splitter 21E to either a first direction or a second direction different from the first direction. The half-wave plate 30E-1 is rotatable. By rotating the half-wave plate 30E-1, the angle θ of the half-wave plate 30E-1 is changed. E For example, the angle θ of the half-wave plate 30E-1 is changed by rotating the half-wave plate 30E-1. E is within a first angle range, and the angle θ E The angle θ of the half wave plate 30E-1 can be switched between a first state and a second state in which the angle θ is within a second angle range. E is in the first angle range, the half-wave plate 30E-1 emits pulsed light in the first direction. E is in a second angle range different from the first angle range, pulsed light in a second direction different from the first direction is emitted from the half wave plate 30E-1.

[0190] The half-wave plate 30E-2 changes the direction of the pulsed light reflected by the mirror Mb1 to either a first direction or a second direction different from the first direction. The half-wave plate 30E-2 changes the direction of the pulsed light reflected by the mirror Mg1 to either the first direction or a second direction different from the first direction. The half-wave plate 30E-2 is rotatable. By rotating the half-wave plate 30E-2, the angle θ of the half-wave plate 30E-2 can be changed. E For example, the angle θ of the half-wave plate 30E-2 is changed by rotating the half-wave plate 30E-2. E is within a first angle range, and the angle θ E The angle θ of the half-wave plate 30E-2 can be switched between a first state and a second state in which the angle θ is within a second angle range. E is in the first angle range, the half-wave plate 30E-2 emits pulsed light in the first direction. E is in a second angle range different from the first angle range, pulsed light in a second direction different from the first direction is emitted from the half wave plate 30E-2.

[0191] A method for converting the repetition frequency according to the fifth embodiment will be described below. When the repetition frequency fr of the pulsed light L1 incident on the optical device 4E is changed to fr×1, all angles θ of the half-wave plates 30E-1 and 30E-2 are changed. E is set to the second angle range. Therefore, the pulsed light L1 emitted from the light source 2 passes through the polarizing beam splitter 21E without being split. That is, the pulsed light L1 is not optically branched or time-delayed by the optical units 20E-1 to 20E-3, and the repetition frequency fr of the pulsed light L1 is maintained. In other words, the pulsed light L1 is output from the optical device 4E as pulsed light L2.

[0192] The optical device 4E according to the fifth embodiment can change the repetition frequency fr of the pulsed light L1 emitted from the light source 2 to one of three repetition frequencies: fr×1, fr×4, or fr×8 by controlling the rotation of the half-wave plates 30E-1 and 30E-2. Since the half-wave plate 30E-2 is common, the repetition frequency cannot be changed to fr×2. For example, when the angle θ of the half-wave plate 30E-1 is E is set to the second angle range, and the angle θ of the half-wave plate 30E-2 is set to E to the second angle range (for example, 0° for S-polarized light), it is possible to output pulsed light with a repetition frequency of (fr×1) from the optical device 4E.

[0193] For example, the angle θ of the half-wave plate 30E-1 E is set to the second angle range, and the angle θ of the half-wave plate 30E-2 is set to E In the following, a method for converting the repetition frequency by 8 times will be described with reference to Figs. 19 to 22.

[0194] 19 to 22 are diagrams illustrating a method of converting the repetition frequency by 8 times. FIG. 19 is a diagram illustrating the optical path of pulse converting unit 15E-1. FIG. 20 is a diagram illustrating the optical path of pulse converting unit 15E-2. FIG. 21 is a diagram illustrating the optical path of pulse converting unit 15E-3. FIG. 22 is a diagram illustrating the state of the optical path in each pulse converting unit according to the fifth embodiment. (a), (d), and (g) of FIG. 22 are diagrams viewed from the direction of V1 indicated by the arrow in FIG. 18. (b), (e), and (h) of FIG. 22 are diagrams viewed from the direction of V2 indicated by the arrow in FIG. 22. (c) and (f) of FIG. 22 are diagrams viewed from the direction of V3 indicated by the arrow in FIG. 18.

[0195] The repetition frequency fr of the pulsed light L1 incident on the optical device 4E is fr×8, that is, 2 3 When changing to a factor of 3 (n=3), the angles θ of the half-wave plates 30E-1 and 30E-2 are E is set to the first angle range.

[0196] The pulsed light immediately after passing through the half-wave plate 30E-1 becomes linearly polarized at 45°. Therefore, of the pulsed light output from the half-wave plate 30E-1, i.e., the 45° linearly polarized light, the P-polarized component passes through the polarizing beam splitter 21E, and the S-polarized component is reflected by the polarizing beam splitter 21E. The P-polarized light that passes through the polarizing beam splitter 21E propagates along the first optical path 24E. Specifically, the P-polarized light is converted into circularly polarized light by the quarter-wave plate 40E-1 (FIG. 22(a)). The circularly polarized light converted by the quarter-wave plate 40E-1 is then specularly reflected by the mirror ME1 and re-enters the quarter-wave plate 40E-1, where it is converted into S-polarized light.

[0197] The S-polarized light reflected by the polarizing beam splitter 21E propagates through the second optical path 25-1. Specifically, the S-polarized light is converted into circularly polarized light by the quarter-wave plate 40E-2. The circularly polarized light is then specularly reflected by the mirror DM1 and re-enters the quarter-wave plate 40-2, where it is converted into P-polarized light. As a result, as shown in FIG. 19, the polarizing beam splitter 21E generates pulsed light 200E by combining the S-polarized light from the quarter-wave plate 40E-1 and the P-polarized light from the quarter-wave plate 40E-2. That is, by coaxially combining the pulses from both optical paths by the polarizing beam splitter 21E, two pulses (pulsed light) with a time difference of Tr / 8 are obtained, like the light emitted from the pulse conversion unit 15-1 illustrated in FIG. 10. This pulsed light is directed toward the mirror Mb1 (FIG. 22(b)). That is, the S-polarized light from the quarter-wave plate 40E-1 is reflected by the polarizing beam splitter 21E and heads toward the mirror Mb1, while the P-polarized light from the quarter-wave plate 40E-2 is transmitted through the polarizing beam splitter 21E and heads toward the mirror Mb1.

[0198] The pulsed light 200E is reflected by the mirror Mb1 and enters the half-wave plate 30E-2 (FIG. 22(c)). Here, the height of the optical path traveled by the pulsed light 200E increases due to reflection from the mirror Mb1. When the pulsed light 200E enters the half-wave plate 30E-2, the polarization directions of the S-polarized light and the P-polarized light are each rotated by 45°. Having passed through the half-wave plate 30E-2, the pulsed light 200E is reflected by the mirror Mb2 and enters the polarizing beam splitter 21E.

[0199] Each of the two pulses of pulsed light 200E has a P-polarized component and an S-polarized component. As shown in FIG. 20, the S-polarized component of each of the two pulses incident on the polarizing beam splitter 21E is reflected by the polarizing beam splitter 21E and is converted to circularly polarized light by the quarter-wave plate 40E-2. This circularly polarized light is reflected by mirror DM2 and travels toward mirror ME2. The light specularly reflected by mirror ME2 is reflected again by mirror DM2 and passes through the quarter-wave plate 40E-2 again, whereby it is converted to P-polarized light. This P-polarized light is incident on the polarizing beam splitter 21E.

[0200] In each of the two pulses incident on polarizing beam splitter 21E, the P-polarized component passes through polarizing beam splitter 21E and is changed to circularly polarized light by quarter-wave plate 40E-1 (FIG. 22(d)). This circularly polarized light is reflected by mirror ME1 and again incident on quarter-wave plate 40E-1, where it is changed to S-polarized light. This S-polarized light is incident on polarizing beam splitter 21E.

[0201] The polarizing beam splitter 21E generates pulsed light 210E by combining the S-polarized light from the quarter-wave plate 40E-1 and the P-polarized light from the quarter-wave plate 40E-2. That is, two pulses with a time difference of Tr / 4 are generated from one pulse. Therefore, by coaxially combining the pulses from both optical paths by the polarizing beam splitter 21E, four pulses are obtained, as shown in the example of the light emitted from the pulse conversion unit 15-2 in FIG. 10. This pulsed light 210E is directed toward the mirror Mg1 (FIG. 22(e)). That is, the S-polarized light from the quarter-wave plate 40E-1 is reflected by the polarizing beam splitter 21E and directed toward the mirror Mg1. The P-polarized light from the quarter-wave plate 40E-2 is transmitted through the polarizing beam splitter 21E and directed toward the mirror Mg1.

[0202] The pulsed light 210E is reflected by the mirror Mg1 and enters the half-wave plate 30E-2. Here, due to reflection from the mirror Mg1, the height of the optical path through which the pulsed light 210E passes increases (FIG. 22(f)). When the pulsed light 210E enters the half-wave plate 30E-2, the polarization directions of the S-polarized light and the P-polarized light are each rotated by 45°. Having passed through the half-wave plate 30E-2, the pulsed light 210E is reflected by the mirror Mg2 and enters the polarizing beam splitter 21E.

[0203] Each of the four pulses of pulsed light 210E has a P-polarized component and an S-polarized component. As shown in FIG. 21, the S-polarized component of each of the four pulses incident on the polarizing beam splitter 21E is reflected by the polarizing beam splitter 21E, and its polarization state is changed to circular polarization by the quarter-wave plate 40E-2 (FIG. 22(g)). This circular polarization is reflected by mirror ME3 and proceeds toward mirror ME4. The light specularly reflected by mirror ME4 is reflected again by mirror ME3 and becomes P-polarized by passing through the quarter-wave plate 40E-2. This P-polarized light is incident on the polarizing beam splitter 21E.

[0204] In each of the four pulses incident on polarizing beam splitter 21E, the P-polarized component passes through polarizing beam splitter 21E and is changed to circularly polarized light by quarter-wave plate 40E-1. This circularly polarized light is reflected by mirror ME1 and again incident on quarter-wave plate 40E-1, where it is changed to S-polarized light. This S-polarized light is incident on polarizing beam splitter 21E.

[0205] The polarizing beam splitter 21E generates pulsed light 220E by combining the S-polarized light from the quarter-wave plate 40E-1 and the P-polarized light from the quarter-wave plate 40E-2. That is, two pulses with a time difference of Tr / 2 are generated from one pulse. Therefore, by coaxially combining the pulses from both optical paths by the polarizing beam splitter 21E, eight pulses are obtained, as shown in the output light of the pulse conversion unit 15-3 in FIG. 10. The pulsed light 220E is output from the optical device 4E as pulsed light L2 (FIG. 22(h)). In the fifth embodiment, the position of the pulsed light passing through the quarter-wave plate 40E-2 is controlled using mirrors Mb1, Mb2, Mg1, and Mg2. This reduces the number of quarter-wave plates used to convert the repeating frequency.

[0206] The control unit 11 controls the rotation of the half-wave plates 30E-1 and 30E-2. The control unit 11 may also control the rotation of the quarter-wave plates 40E-1 and 40E-2.

[0207] The repetition frequency (1 or fr×2) according to the fifth embodiment is as follows: n ) is the same as that of the first embodiment shown in FIG. 11, and therefore the description thereof will be omitted.

[0208] FIG. 23 is a diagram showing a modified example of an optical unit 20E-3 in the optical device 4E. The optical path length difference providing unit 22F-3 of the modified optical unit 20F-3 shown in FIG. 23 has a first optical path 24F and a second optical path 25F-3. The first optical path 24F is similar to the first optical path 24E. The second optical path 25F-3 is provided with a plurality of mirrors MF4. The second optical path 25F-3 uses a plurality of mirrors MF4 to multiple-reflect the pulsed light. This configuration allows the optical device 4F to reduce the size required to provide the third optical path length difference. It is desirable to use mirrors MF with as high a reflectivity as possible. This makes it possible to suppress a reduction in light intensity due to the influence of the mirror reflectivity.

[0209] The optical device 4E according to the fifth embodiment includes an optical unit 20E that converts the repetition frequency fr of pulsed light L emitted from the light source 2, and a repetition frequency (1 or fr×2) converted by the optical unit 20E. n This configuration not only produces the same effects as the first embodiment, but also contributes to improved robustness and cost reduction by sharing multiple half-wave plates, multiple polarizing beam splitters, and multiple quarter-wave plates.

[0210] In any of the optical devices according to the first to fifth embodiments, the pulsed light L2 output to the irradiation unit may be circularly polarized. In this case, a quarter-wave plate may be provided to polarize the pulsed light L2 output from the final pulse converting unit among the multiple pulse converting units connected in multiple stages into circularly polarized light. That is, a quarter-wave plate may be connected after the final pulse converting unit. By circularly polarizing the pulsed light L2, it is possible to obtain an isotropic PSF in an objective lens with a high NA.

[0211] Furthermore, in any of the optical devices of the first to fifth embodiments, the pulsed light L2 may be linearly polarized in the same direction. In this case, a polarizer may be installed after the final pulse conversion unit. For example, by setting the transmission direction of the polarizer at 45° to the two orthogonal linearly polarized lights, the polarization direction of the light transmitted through the polarizer can be linearly polarized in the same direction. However, the average output of the light is halved by the polarizer. To change the direction of linear polarization, a half-wave plate may be installed after the polarizer. If the transmission direction of the polarizer is parallel to one of the two orthogonal linearly polarized lights, only S-polarized or P-polarized pulsed light can be extracted.

[0212] Here, the pulse width of the ultrashort pulse may be broadened due to dispersion in optical elements. This may reduce the peak intensity of the pulsed light and reduce the efficiency of signal generation caused by nonlinear effects. Dispersion compensation is generally performed to efficiently generate nonlinear signals. Examples of dispersion compensation methods include a diffraction grating pair or a prism pair. It is desirable to install a dispersion compensation optical system including such an element between the light source 2 and any of the optical devices of the first to fifth embodiments. However, this is not limiting, and the dispersion compensation optical system may be installed between any of the optical devices of the first to fifth embodiments and the irradiation unit 3. When a light source 2 including dispersion compensation optics is used, an additional dispersion compensation optical system is not required.

[0213] The scanner of a microscope equipped with any of the optical devices of the first to fifth embodiments may be either a pair of two galvanometer mirrors or a pair of a galvanometer mirror and a resonant mirror. The former is called galvanometer scanning, and the latter is called resonant scanning. Under the same laser light intensity conditions, resonant scanning has a faster scanning speed and a shorter laser light irradiation time, making it less susceptible to photobleaching than galvanometer scanning. In the time required to acquire one image with galvanometer scanning, resonant scanning can acquire several tens of times more images. For example, 30 images can be captured. By accumulating these 30 images, an image with the same image quality (signal-to-noise ratio) as that of galvanometer scanning can be obtained. Even under these conditions of obtaining similar image quality, resonant scanning is less susceptible to photobleaching than galvanometer scanning because the laser light irradiation time per unit area per image is shorter and the laser light irradiation is intermittent. When the present invention is applied to a resonant scanning method, photobleaching can be reduced more than when the present invention is applied to a galvano scanning method.

[0214] In a microscope equipped with the optical device of any one of the first to fifth embodiments, when the repetition frequency is increased by n times, it is desirable to set the average intensity of the pulsed light L1 (L2) to √n (W). In that case, the peak intensity (W) of the pulsed light L2 becomes 1 / √n times. This allows two-photon fluorescence images of equivalent image quality to be obtained regardless of the repetition frequency. A mechanism for automatically controlling and setting the light intensity in response to changes in the repetition frequency may be provided.

[0215] Although the embodiments have been described above, the technical scope of the present disclosure is not limited to the aspects described in the above embodiments. One or more of the requirements described in the above embodiments may be omitted. Furthermore, the requirements described in the above embodiments may be combined as appropriate. Furthermore, to the extent permitted by law, the disclosures of all documents cited in this specification are incorporated by reference and are included as part of the description of this text. [Explanation of symbols]

[0216] 1, 1A to 1E... microscope, 2... light source, 4, 4A to 4E... optical device, 11... control unit, 12... setting device, 15, 15A to 15F... pulse conversion unit, 20, 20A to 20F... optical unit, 30, 30A to 30E... half wave plate, 40B, 41B, 40E... quarter wave plate

Claims

1. converting the repetition frequency of the pulsed light emitted from the light source by a conversion unit; The repetition frequency converted by the conversion unit is variable; Including, The conversion of the repetition frequency by the conversion unit is a first step of changing the polarization direction of the pulsed light to a first direction via a half-wave plate; a second step of splitting the pulsed light polarized in the first direction into a first polarization and a second polarization orthogonal to the first polarization; a third step of providing a predetermined optical path difference between the first polarized light and the second polarized light; a fourth step of generating the pulsed light having the changed repetition frequency by combining the first polarized light and the second polarized light after the predetermined optical path length difference has been imparted; Including, The conversion unit executes a series of processes including the first stage, the second stage, the third stage, and the fourth stage n times (n is an integer of 1 or more), thereby converting the repetition frequency into 2 ｎ Convert to double, Among the n times, the second step at odd-numbered times and the fourth step at even-numbered times are performed using a first polarizing beam splitter, and the fourth step at odd-numbered times and the second step at even-numbered times are performed using a second polarizing beam splitter different from the first polarizing beam splitter. How to convert.

2. converting the repetition frequency of the pulsed light emitted from the light source by a conversion unit; The repetition frequency converted by the conversion unit is variable; Including, The conversion of the repetition frequency by the conversion unit is a first step of changing the polarization direction of the pulsed light to a first direction via a half-wave plate; a second step of splitting the pulsed light polarized in the first direction into a first polarization and a second polarization orthogonal to the first polarization; a third step of providing a predetermined optical path difference between the first polarized light and the second polarized light; a fourth step of generating the pulsed light having the changed repetition frequency by combining the first polarized light and the second polarized light after the predetermined optical path length difference has been imparted; Including, The conversion unit executes a series of processes including the first stage, the second stage, the third stage, and the fourth stage n times (n is an integer of 1 or more), thereby converting the repetition frequency into 2 ｎ Convert to double, the second step and the fourth step are performed using one polarizing beam splitter; the first polarized light is P polarized light and the second polarized light is S polarized light, a first quarter-wave plate is disposed in the optical path of the P-polarized light, and a second quarter-wave plate is disposed in the optical path of the S-polarized light; the third step includes passing the P-polarized light through the first quarter-wave plate twice, and passing the S-polarized light through the second quarter-wave plate twice; a position where the P-polarized light passes through the first quarter-wave plate is different, and a position where the S-polarized light passes through the second quarter-wave plate is different, in the third stage, every n times; How to convert.

3. when an angle formed between a polarization direction of the pulsed light incident on the half-wave plate and a slow axis or a fast axis of the half-wave plate is within a first angle range, the half-wave plate emits the pulsed light in the first direction, and the first step is executed; when the angle formed is within a second angle range different from the first angle range, the half-wave plate emits the pulsed light in a second direction different from the first direction, and the first step is not executed; The change to either 1 or the value of n is performed by switching between a first state in which the formed angle is within the first angle range and a second state in which the formed angle is within the second angle range. The conversion method according to claim 1 or 2.

4. The first angle range is a range of (22.5° + K × 45°) ± 1° (K is an integer). The conversion method according to claim 3 .

5. The second angle range is a range of (0° + K × 90°) ± 1° (K is an integer). The conversion method according to claim 3 .

6. the half-wave plate is rotatable; The angle is changed by rotating the half-wave plate. The conversion method according to claim 3 .

7. When the repetition period of the pulsed light output from the light source is Tr, when the predetermined optical path difference is given, the difference between the first polarized light and the second polarized light is Tr / 2. ｎ A time difference of The conversion method according to claim 1 or 2.

8. The repetition frequency of the pulsed light is 2 ｎ a conversion unit for converting the data by a factor of (n is an integer of 1 or more); a switching unit capable of changing the repetition frequency converted by the conversion unit; Equipped with the conversion unit includes n optical units, The optical unit comprises: a first polarizing beam splitter that splits the pulsed light polarized in a first direction into a first polarized light and a second polarized light orthogonal to the first polarized light; an optical path length difference imparting unit that imparts a predetermined optical path length difference between the first polarized light and the second polarized light; a second polarizing beam splitter that combines the first polarized light and the second polarized light after the predetermined optical path length difference has been imparted, thereby forming the pulsed light whose repetition frequency has been changed; Equipped with the switching unit changes the polarization direction of the pulsed light incident on the first polarizing beam splitter to either the first direction or a second direction different from the first direction, Among the n optical units, odd-numbered first polarizing beam splitters and even-numbered second polarizing beam splitters are used in combination as one polarizing beam splitter, and odd-numbered second polarizing beam splitters and even-numbered first polarizing beam splitters are used in combination as one polarizing beam splitter. optical equipment.

9. The repetition frequency of the pulsed light is 2 ｎ a conversion unit for converting the data by a factor of (n is an integer of 1 or more); a switching unit capable of changing the repetition frequency converted by the conversion unit; Equipped with the conversion unit includes n optical units, The optical unit comprises: a first polarizing beam splitter that splits the pulsed light polarized in a first direction into a first polarized light and a second polarized light orthogonal to the first polarized light; an optical path length difference imparting unit that imparts a predetermined optical path length difference between the first polarized light and the second polarized light; a second polarizing beam splitter that combines the first polarized light and the second polarized light after the predetermined optical path length difference has been imparted, thereby forming the pulsed light whose repetition frequency has been changed; Equipped with the switching unit changes the polarization direction of the pulsed light incident on the first polarizing beam splitter to either the first direction or a second direction different from the first direction, the first polarizing beam splitter and the second polarizing beam splitter are used together as one polarizing beam splitter; the first polarized light is P polarized light, the second polarized light is S polarized light, a first quarter-wave plate is installed in an optical path of the P polarized light, a second quarter-wave plate is installed in an optical path of the S polarized light, and in the process of imparting the predetermined optical path length difference, the P polarized light passes through the first quarter-wave plate twice, and the S polarized light passes through the second quarter-wave plate twice; Each of the n optical units has a different position at which the P-polarized light passes through the first quarter-wave plate and a different position at which the S-polarized light passes through the second quarter-wave plate. optical equipment.

10. the switching unit has a half-wave plate provided on an optical path along which the pulsed light enters the first polarizing beam splitter.

10. The optical device according to claim 8 or claim 9.

11. when an angle formed between a polarization direction of the pulsed light incident on the half-wave plate and a slow axis or a fast axis of the half-wave plate is within a first angle range, the pulsed light in the first direction is emitted from the half-wave plate, and when the formed angle is within a second angle range different from the first angle range, the pulsed light in the second direction is emitted from the half-wave plate, The angle is switched between a first state in which the angle is within the first angle range and a second state in which the angle is within the second angle range, whereby the angle is changed to either 1 or the value of n.

11. The optical device according to claim 10.

12. the half-wave plate is rotatable; The angle is changed by rotating the half-wave plate.

12. The optical device according to claim 11.

13. The first angle range is a range of (22.5° + K × 45°) ± 1° (K is an integer).

12. The optical device according to claim 11.

14. The second angle range is a range of (0° + K × 90°) ± 1° (K is an integer).

12. The optical device according to claim 11.

15. When the repetition period of the pulsed light output from the light source is Tr, when the predetermined optical path difference is imparted by the optical path difference imparting unit, a difference of Tr / 2 is obtained between the first polarized light and the second polarized light. ｎ A time difference of 10. The optical device according to claim 8 or claim 9.

16. A microscope that irradiates an observation object with pulsed light, the optical device according to claim 8 or 9, which converts the repetition frequency of the pulsed light; an irradiation unit that irradiates an observation object with the pulsed light output from the optical device; A microscope equipped with:

17. A control unit that controls the switching unit is provided.

17. The microscope of claim 16.

18. the switching unit has a half-wave plate provided on an optical path along which the pulsed light enters the first polarizing beam splitter, The control unit controls the rotation of the half-wave plate.

18. The microscope of claim 17.

19. a setting device for setting information corresponding to the magnification of the repetition frequency by a user operation; When information corresponding to the magnification is set, the setting device transmits the information corresponding to the set magnification to the control unit; the control unit causes the switching unit to change the repetition frequency based on information corresponding to the magnification transmitted from the setting device.

18. The microscope of claim 17.

20. The optical device is configured to be insertable into and detachable from the microscope.

17. The microscope of claim 16.

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