Pulsed light generating device and method

Abstract

A pulse light generating device (1) is provided with: an oscillator (2) that oscillates an ultrashort pulse light (L); a stretcher fiber (7) that stretches a time width of the ultrashort pulse light (L); a fiber amplifier (8) that amplifies the ultrashort pulse light (L) whose time width is stretched by the stretcher fiber (7); and a compressor (9) that compresses a time width of the ultrashort pulse light (L) amplified by the fiber amplifier (8), the stretcher fiber (7) being composed of a first fiber (71) that stretches the time width of the ultrashort pulse light (L) with a first characteristic and a second fiber (72) that stretches the time width of the ultrashort pulse light (L) with a second characteristic different from the first characteristic.

Description

Technical Field

[0001] This disclosure relates to a pulse light generating apparatus and a pulse light generating method. Background Technology

[0002] A pulsed light generating device is known, comprising: an oscillating section for oscillating pulsed light; and a modulation section for modulating the wavelength of the pulsed light oscillating in the oscillating section using soliton self-frequency shift. In such a pulsed light generating device, by increasing the intensity of the pulsed light before modulation by the modulation section, it is possible to split the pulsed light into multiple pulsed lights with different wavelengths by the modulation (outputting polychromatic solitons) (see, for example, Patent Document 1).

[0003] Existing technical documents

[0004] Patent documents

[0005] Patent document 1: Japanese Patent Application Publication No. 2004-527001. Summary of the Invention

[0006] The problem the invention aims to solve

[0007] Here, when an ultrashort pulse light is used as a pulse light and its output is strongly amplified, in order to avoid adverse effects caused by nonlinear effects, a technique called chirped pulse amplification is sometimes used, which involves first widening the time width of the ultrashort pulse light before amplification and then restoring the time width to its original state.

[0008] The purpose of this disclosure is to provide a pulse light generating device and a pulse light generating method capable of appropriately amplifying chirped pulses.

[0009] Technical means to solve the problem

[0010] (1) A pulse light generating apparatus according to one aspect of the present disclosure comprises: an oscillating section that oscillates pulse light; a stretcher that stretches the time width of the pulse light; an amplifying section that amplifies the pulse light whose time width has been stretched by the stretcher; and a compressor that compresses the time width of the pulse light amplified by the amplifying section, wherein the stretcher is composed of a first optical fiber that stretches the time width of the pulse light with a first characteristic and a second optical fiber that stretches the time width of the pulse light with a second characteristic different from the first characteristic.

[0011] The inventors have focused on using optical fibers for broadening and diffraction grating pairs for compression during chirped pulse amplification, considering stability and throughput. However, when broadening and compression are performed in this manner, it is difficult to restore the ultrashort pulse light to its pre-broadened state after compression. Regarding this, in one aspect of the pulse light generating apparatus of the present invention, the broadener is configured to include a first optical fiber and a second optical fiber with different characteristics. By combining optical fibers with different characteristics as the broadener, even if the pulse light cannot be restored to its pre-broadened state after compression by the compressor when only the first optical fiber is used as the broadener, by further combining a second optical fiber with characteristics different from the first optical fiber, the compressed pulse light can be brought closer to its pre-broadened state according to the setting of the second characteristic. That is, since the conditions can be adjusted using two optical fibers with different characteristics, it is possible to achieve the state recovery (restoration to the pre-broadened state) of the pulse light, which is difficult with only one optical fiber. As described above, the pulse light generating apparatus according to the present disclosure can perform chirped pulse amplification appropriately.

[0012] (2) In the pulsed light generating apparatus described in (1) above, the second characteristic of the second optical fiber can also be set such that, in a structure where the time width is broadened by the first optical fiber and then compressed by the compressor in the absence of the second optical fiber, the value of the index related to group delay dispersion (GDD) of each wavelength remaining in the pulsed light is close to a predetermined value. In this way, by setting the second characteristic such that, after the broadening using the first optical fiber and the compression using the compressor, the value of the index related to group velocity dispersion remaining in each wavelength is close to a predetermined value, the state recovery of the pulsed light (recovery to the state before broadening) can be appropriately achieved by appropriately setting the predetermined value.

[0013] (3) In the pulsed light generating apparatus described in (2) above, the second characteristic of the second optical fiber can also be set such that the value of the index related to group delay dispersion (GDD) of each wavelength remaining in the pulsed light is close to 0 after the time width is broadened by the first optical fiber and then compressed by the compressor in a structure without the second optical fiber. In this way, by setting the second characteristic such that the value of the index related to group velocity dispersion remaining in each wavelength after the broadening by the first optical fiber and the compression by the compressor is close to 0, the state recovery of the ultrashort pulsed light (recovery to the state before broadening) can be achieved more appropriately.

[0014] (4) In any of the pulsed light generating apparatuses described in (1) to (3) above, the compressor may also have a diffraction grating pair. By using a diffraction grating pair that utilizes the difference in optical path length caused by diffraction as the compressor, the generation of nonlinear effects that are problematic, for example, when using an optical fiber as the compressor, can be suppressed. When using a diffraction grating pair as the compressor, it is conceivable that it would be difficult to restore the ultrashort pulsed light to its pre-broadened state after compression, but as described above, in the structure according to this disclosure, since the conditions can be adjusted by two optical fibers with different characteristics, the state restoration of the pulsed light (restoring it to its pre-broadened state) can also be appropriately performed.

[0015] (5) In any one of the pulsed light generating apparatuses described in (1) to (4) above, the first optical fiber may also be a normal dispersive optical fiber. By using a normal dispersive optical fiber, the time width of the pulsed light can be appropriately broadened while avoiding unpredictable nonlinear effects.

[0016] (6) In any one of (1) to (5) above, the wavelength band of the pulse light whose time width is broadened by the stretcher and the wavelength band of the pulse light whose time width is compressed by the compressor can be 1800 nm to 2200 nm. According to such a structure, for example, chirped pulse amplification can be performed appropriately within the gain range of the thulium amplifier.

[0017] (7) A pulse light generation method according to one aspect of the present disclosure includes: an oscillation step, oscillating pulse light; a broadening step, broadening the time width of the pulse light oscillated in the oscillation step by using a first optical fiber with a first characteristic, and further broadening it by using a second optical fiber with a second characteristic; an amplification step, amplifying the pulse light whose time width has been broadened in the broadening step; and a compression step, compressing the time width of the pulse light amplified in the amplification step.

[0018] Invention Effects

[0019] According to this disclosure, a pulse light generating apparatus and a pulse light generating method capable of appropriately amplifying chirped pulses can be provided. Attached Figure Description

[0020] Figure 1 This is a block diagram illustrating a pulsed light generating apparatus according to an embodiment.

[0021] Figure 2 (a) shows from Figure 1 A graph showing the time waveform of the ultrashort pulse light output by the oscillator. Figure 2 (b) shows from Figure 1 A graph showing the spectrum of ultrashort pulse light output by the oscillator. Figure 2 (c) is showing from Figure 1A graph showing the time waveform of the ultrashort pulse light output by the fiber amplifier. Figure 2 (d) is showing from Figure 1 A graph showing the spectrum of ultrashort pulse light output from an optical fiber amplifier.

[0022] Figure 3 It shows from Figure 1 A chart showing specific examples of the spectrum of ultrashort pulse light output from an optical fiber amplifier.

[0023] Figure 4 (a) shows from Figure 1 A graph showing the time waveform of the ultrashort pulse light output by the acousto-optic modulator. Figure 4 (b) shows from Figure 1 A graph showing the spectrum of the ultrashort pulse light output by the acousto-optic modulator.

[0024] Figure 5 (a) shows from Figure 1 A graph showing the time waveform of the ultrashort pulse light output from the soliton frequency-shifting fiber. Figure 5 (b) shows from Figure 1 A graph showing the spectrum of ultrashort pulse light output from a soliton frequency-shifting fiber. Figure 5 (c) is showing from Figure 1 A graph showing the time waveform of the ultrashort pulse light output from the filter. Figure 5 (d) is showing from Figure 1 A graph showing the spectrum of ultrashort pulse light output from the filter.

[0025] Figure 6 This is a diagram illustrating the amplification of chirped pulses.

[0026] Figure 7 (a) is a diagram illustrating the method of creating a difference in optical path length by means of the difference in refractive index when a substance passes through it. Figure 7 (b) is a diagram illustrating the method of generating optical path length differences through diffraction.

[0027] Figure 8 This is a schematic diagram of the structure involved in chirped pulse amplification.

[0028] Figure 9 This is a schematic diagram of the structure involved in chirped pulse amplification with extended time width in an optical fiber.

[0029] Figure 10 (a) is shown Figure 9 The diagram shows the stretcher and compressor characteristics in the structure. Figure 10 (b) is shown Figure 9 A diagram of residual GDD in the structure.

[0030] Figure 11(a) is shown Figure 6 A diagram showing the characteristics of the first optical fiber in the structure. Figure 11 (b) is shown Figure 6 A diagram showing the characteristics of the compressor in the structure. Figure 11 (c) is a graph showing the residual GDD caused by the first optical fiber and the compressor. Figure 11 (d) is a graph showing the characteristics of the second optical fiber combined with the first optical fiber.

[0031] Figure 12 This is a flowchart illustrating an embodiment of a pulsed light generation method.

[0032] Figure 13 (a) is a graph showing the characteristics of the first optical fiber in the modified example. Figure 13 (b) is a graph showing the characteristics of the compressor in the modified example. Figure 13 (c) is a graph showing the residual GDD caused by the first optical fiber and the compressor. Figure 13 (d) is a graph showing the characteristics of the second optical fiber combined with the first optical fiber. Figure 13 (e) is a diagram showing the characteristics of the third optical fiber combined with the first and second optical fibers. Detailed Implementation

[0033] Hereinafter, the embodiments will be described in detail with reference to the accompanying drawings. In each drawing, the same or equivalent parts are labeled with the same symbols, and repeated descriptions are omitted.

[0034] like Figure 1 As shown, the pulse light generating device 1 of this embodiment utilizes soliton self-frequency shifting (Raman soliton shifting) to generate long-wavelength ultrashort pulse light (pulse light) L. For example, the pulse light generating device 1 is a femtosecond laser device, such as used in a two-photon microscope. The pulse light generating device 1 includes an oscillator 2, an fiber amplifier 3, an acousto-optic modulator 4, a compressor 5, a soliton frequency shifting fiber 6, a stretcher fiber 7, an fiber amplifier 8, a compressor 9, and a wavelength conversion unit 10.

[0035] Oscillator 2 constitutes the oscillation section of the oscillating ultrashort pulse light L. Oscillator 2, as... Figure 2 As shown in (a), an ultrashort pulse sequence with a specified period F1 is generated. The wavelength of the ultrashort pulse light L oscillated from oscillator 2 can, for example, be a wavelength range including 1550 nm. Here, oscillator 2 is as follows: Figure 2 As shown in (b), the oscillation is an ultrashort pulse light L with a spectrum having a first spectral width H1 and a first intensity K1. There is no particular limitation on the oscillator 2; various oscillators can be used.

[0036] The fiber amplifier 3 constitutes an amplification section that broadbands the spectrum of the ultrashort pulse light L oscillated by the oscillator 2. Through self-similar soliton amplification, the fiber amplifier 3 achieves both broadbanding and high power of the ultrashort pulse light L. The fiber amplifier 3 is positioned between the oscillator 2 and the soliton frequency-shifting fiber 6 in the optical path of the ultrashort pulse light L.

[0037] The fiber amplifier 3 includes a fiber amplification unit. This fiber amplification unit of the fiber amplifier 3 is a normally dispersive fiber, specifically a double-clad fiber co-doped with erbium and ytterbium. That is, the fiber amplifier 3 amplifies light using the normally dispersive double-clad fiber without broadening, inducing nonlinear effects while simultaneously producing ultrashort pulse light L as broadband amplified light. A normally dispersive fiber is an fiber with a negative dispersion parameter D (ps / nm / km). The additives used in the fiber amplifier 3 are not particularly limited, and various additives can be used.

[0038] like Figure 2 (c) and Figure 2 As shown in (d), the fiber amplifier 3 broadbands the spectral width of the ultrashort pulse light L to a second spectral width H2, which is wider than the first spectral width H1. The fiber amplifier 3 also increases the power of the ultrashort pulse light L to a second intensity K2, which is higher than the first intensity K1. Specifically, as... Figure 3 As shown, fiber amplifier 3 sets the spectral width of the ultrashort pulse light L to be greater than 100 nm. Figure 3 In the diagram, the horizontal axis represents the wavelength of the ultrashort pulse light L, and the vertical axis represents the relative value of the intensity of the ultrashort pulse light L relative to a specified intensity.

[0039] The acousto-optic modulator 4 constitutes a light intensity control unit that controls the intensity of the ultrashort pulse light L on a pulse-by-pulse basis. The acousto-optic modulator 4 is a device that uses acoustic (sound wave) power to modulate the ultrashort pulse light L, and is called an AOM (Acousto Optic Modulator). In this embodiment, the acousto-optic modulator 4 is disposed between the fiber amplifier 3 and the soliton frequency-shifting fiber 6 in the optical path of the ultrashort pulse light L. It should be noted that the acousto-optic modulator 4 can be disposed at any position, as long as it is between the oscillator 2 and the soliton frequency-shifting fiber 6. The acousto-optic modulator 4 is as follows... Figure 4 (a) and Figure 4 As shown in (b), the intensity of the ultrashort pulse light L is controlled to change with each pulse. For example, as... Figure 4 As shown in (a), when intensity modulation is applied to M1 and M2, as Figure 4As shown in (b), ultrashort pulses LM1 and LM2 are generated, corresponding to the intensities given by M1 and M2. The intensity modulation range and accuracy of the ultrashort pulses L (LM1, LM2) depend on the performance of the acousto-optic modulator 4. The intensity of each pulse in the pulse sequence of the ultrashort pulses L can be arbitrarily modulated by the acousto-optic modulator 4.

[0040] Compressor 5 constitutes a pulse compression section that compresses the pulse time width of the ultrashort pulse light L. In this embodiment, compressor 5 is disposed between the acousto-optic modulator 4 and the soliton frequency-shifting fiber 6 in the optical path of the ultrashort pulse light L. It should be noted that compressor 5 can be disposed at any position as long as it is between fiber amplifier 3 and soliton frequency-shifting fiber 6. Even when the ultrashort pulse light L is broadened (e.g., broadened by several picoseconds) using, for example, fiber amplifier 3, compressor 5 can still compress the time width of the ultrashort pulse light L, outputting an ultrashort pulse light L with a broadened time width of less than 1 picosecond. There is no particular limitation on compressor 5, and various compressors can be used.

[0041] The soliton frequency-shifting fiber 6 constitutes a modulation unit that modulates the wavelength of an ultrashort pulse light generated by the fiber amplifier 3, which simultaneously broadbands the spectrum and increases the output power, using soliton self-frequency shifting. The soliton frequency-shifting fiber 6 is positioned downstream of the fiber amplifier 3 in the optical path of the ultrashort pulse light L. The soliton frequency-shifting fiber 6, as... Figure 5 (a) and Figure 5 As shown in (b), the wavelength of the ultrashort pulse light L is lengthened to generate a soliton S1. The wavelength band of the ultrashort pulse light L that is lengthened here can, for example, be a band containing 1800 nm to 2200 nm. The soliton frequency-shifting fiber 6 can be, for example, a single-mode anomalous dispersion fiber that exhibits anomalous dispersion in the wavelength band of the ultrashort pulse light L generated by the fiber amplifier 3. Furthermore, by controlling the acousto-optic modulator 4, a soliton with a wavelength different from that of soliton S1 can also be generated. The wavelength of soliton S1, for example, when... Figure 5 When the intensity of M1 and M2 is modulated as shown in (c), as Figure 5 As shown in (d), soliton S shifts to wavelengths corresponding to the intensities provided by M1 and M2 (solitons S1 and S2). The frequency-shifting wavelength range and accuracy of soliton S depend on the performance of acousto-optic modulator 4. The frequency-shifting wavelength of each soliton S in the soliton sequence generated by the pulse sequence of ultrashort pulse light can be arbitrarily changed by intensity modulation of the pulse sequence using acousto-optic modulator 4. Furthermore, in the illustrated example, the ultrashort pulse light L modulated by soliton self-frequency shifting contains non-soliton components SO (components that fail to become soliton S1 or S2). It should be noted that a filter (not shown) to remove the non-soliton components SO of the ultrashort pulse light L can also be provided at the rear end of the soliton frequency-shifting fiber 6. Such a filter can also have an OD value of 3 or higher.

[0042] The fiber stretcher 7, fiber amplifier 8, and compressor 9 are the structures involved in chirped pulse amplification. For more information on the structures involved in chirped pulse amplification, please refer to [reference needed]. Figures 6 to 11 Please provide an explanation.

[0043] First, the chirped pulse amplification will be explained. Figure 6 This diagram illustrates chirped pulse amplification. When strongly amplifying the output of ultrashort pulse light, a process called chirped pulse amplification is sometimes performed to avoid excessively high peak power and adverse effects caused by nonlinearity. In chirped pulse amplification, the time width of the ultrashort pulse light is first broadened (broadened), then the broadened ultrashort pulse light is amplified, and finally, a compression process is performed to restore the time width of the amplified ultrashort pulse light to its original state (before broadening). For example... Figure 6 As shown, in the chirped pulse amplification, firstly, the pulse light I, which is an ultrashort pulse light L, input to the stretcher fiber 7, has its time width stretched using the stretcher fiber 7. Then, the pulse light II, which is an ultrashort pulse light L with its time width stretched by the stretcher fiber 7, is amplified by the fiber amplifier 8. Finally, the pulse light III, which is an ultrashort pulse light L amplified by the fiber amplifier 8, is compressed using the compressor 9 to restore its time width to the state before stretching.

[0044] Next, two examples will be given regarding methods for widening (or compressing) the time width in chirped pulse amplification. Both examples widen (or compress) the time width by creating a difference in optical path length. Figure 7 (a) is a diagram illustrating the method of creating a difference in optical path length by means of the difference in refractive index when a substance passes through it. Figure 7 (b) is a diagram illustrating the method of generating optical path length differences through diffraction. For example... Figure 7 As shown in (a), an ultrashort pulse of light, formed by multiple wavelengths of light passing through each other with their phases aligned and overlapping, experiences variations in the optical path length of each wavelength due to differences in refractive index caused by the passage through a certain material. This results in variations in the group delay time of each wavelength. This can be used to widen or conversely compress the time width. The material in this case is, for example, optical fiber. Additionally, as... Figure 7 As shown in (b), the diffraction grating creates differences in the optical path lengths of 91 and 92 (details to be described later) at each wavelength, thereby changing the group delay time at each wavelength. This allows for either widening or conversely compressing the time width.

[0045] In such chirped pulse amplification, for ultrashort pulse light, when restoring the broadened time width to its original state, compression is preferably performed in the same manner as when broadening the time width. From this perspective, it is possible to consider performing broadening and compression using the same method. That is, for example, using a method that generates differences in optical path length at each wavelength based on the difference in refractive index allowing the fiber to pass through (see [reference]). Figure 7 In case (a), both the stretcher and the compressor are made of optical fiber, and a method is used to generate differences in the optical path length at each wavelength based on the diffraction grating (see [reference]). Figure 7 In case (b), a diffraction grating pair is used as both the stretcher and the compressor. In this way, by using the same type of optical elements to stretch and compress separately, the characteristics of the stretching and compression (details to follow) are consistent, thereby enabling the ultrashort pulse light to be compressed to the same extent as the stretched time width.

[0046] On the other hand, from the perspective of stability or throughput, it may be more advantageous to configure the stretcher and compressor with different structures when optimizing them. For example, when using optical fiber for stretching and compression, stretching can be expected to have higher throughput (typically with almost zero loss) compared to a diffraction grating pair, but regarding compression, nonlinear effects can be anticipated, leading to light that does not possess the desired characteristics. Specifically, complex dispersion added to ultrashort pulses can sometimes prevent complete compression. As a result, the energy that should be concentrated in the main pulse is dispersed to other parts, thus becoming an undesirable state in terms of causing nonlinear effects. On the other hand, when using a diffraction grating pair for stretching and compression, regarding compression, since the beam diameter increases as the ultrashort pulse propagates in a space system, the energy density is reduced, and therefore, nonlinear effects can be expected to be less likely to occur. Regarding broadening, since diffraction gratings typically have a diffraction efficiency of around 90%, in the case of a diffraction grating pair with multiple diffraction gratings, energy will be lost with each diffraction, and the final throughput will become about 60% of the original energy.

[0047] Based on these considerations, a structure can be considered that uses optical fiber as a stretcher and diffraction grating pairs as compressors, thereby suppressing nonlinear effects while ensuring throughput. Structures that perform stretching and compression using different methods (with different types of optical elements) are less likely to achieve consistent stretching and compression characteristics (details will be described later) compared to structures that perform stretching and compression using the same method (with the same type of optical elements), making it difficult to restore the ultrashort pulse shape to its pre-stretched form after compression. This becomes an even more serious problem in broadband or broadband wavelength-variable light sources. In the structure involved in the chirped pulse amplification of the pulse light generation device 1 of this embodiment, to solve this problem, the stretcher optical fiber 7 is provided with... Figure 8 The two optical fibers shown are (first optical fiber 71 and second optical fiber 72).

[0048] Figure 8 This is a schematic diagram of the structure related to the chirped pulse amplification of the pulse light generating device 1 in this embodiment. Figure 8 As shown, the pulse light generating device 1, as a structure involved in chirped pulse amplification, includes a stretcher fiber 7 (stretcher), an fiber amplifier 8 (amplifier), and a compressor 9.

[0049] The stretcher fiber 7 is a stretcher that stretches the time width of the ultrashort pulse light L. The ultrashort pulse light L whose time width is stretched by the stretcher fiber 7 has a wavelength range of, for example, 1800 nm to 2200 nm. The stretcher fiber 7 is composed of a first fiber 71 that stretches the time width of the ultrashort pulse light L with a first characteristic and a second fiber 72 that stretches the time width of the ultrashort pulse light L with a second characteristic different from that of the first fiber 71. The first fiber 71 and the second fiber 72 are structured to stretch the time width of the ultrashort pulse light L by utilizing the difference in refractive index of each wavelength when the ultrashort pulse light L passes through, thereby creating a difference in the optical path length of each wavelength. The first fiber 71 stretches the time width of the ultrashort pulse light L, for example, the wavelength range of 1800 nm to 2200 nm, output from the soliton frequency-shifting fiber 6 with the first characteristic and outputs it to the second fiber 72. The first fiber 71 can also be, for example, a normal dispersion fiber. The second optical fiber 72 is connected to the first optical fiber 71. For the ultrashort pulse light L input from the first optical fiber 71, the time width is broadened by the second characteristic and output to the optical fiber amplifier 8. The second optical fiber 72 can also be, for example, a normal dispersion or anomalous dispersion fiber. Details regarding the first and second characteristics will be described later.

[0050] Fiber amplifier 8 amplifies (high-powers) the pulsed light whose time width has been broadened by stretcher fiber 7. Fiber amplifier 8 includes a fiber amplification unit. This fiber amplification unit of fiber amplifier 8 is an anomalous dispersion fiber, such as thulium-doped fiber. The laser medium added to the fiber of fiber amplifier 8 is not particularly limited; it can be rare earth elements such as ytterbium, erbium, and neodymium, or it can be Bi, etc.

[0051] Compressor 9 compresses the time width of the ultrashort pulse light L, which has been amplified by fiber amplifier 8. Compressor 9 has diffraction grating pairs 91 and 92. The diffraction grating pairs 91 and 92 are structures that compress the time width by creating differences in the optical path length of each wavelength, thereby changing the group delay time of each wavelength. The wavelength range of the ultrashort pulse light L1 compressed by the diffraction grating pairs 91 and 92 is, for example, 1800 nm to 2200 nm.

[0052] Next, the method for determining the first characteristic of the first optical fiber 71, the second characteristic of the second optical fiber 72, and the characteristics (compression characteristics) of the diffraction grating pairs 91 and 92 will be explained.

[0053] The first and second characteristics mentioned above determine how the ultrashort pulse light L broadens. The compression characteristic determines how the ultrashort pulse light L is compressed. The first, second, and compression characteristics can be represented, for example, using information related to group velocity dispersion (GVD). Group velocity refers to the wave packet velocity associated with the ultrashort pulse light L. Group velocity dispersion refers to the wavelength dispersion of the group velocity. Information related to group velocity dispersion can be the group velocity dispersion itself, or it can be the group delay dispersion (GDD) obtained by multiplying the group velocity dispersion by the length of the medium (fiber length or diffraction grating pair distance), or it can be the TOD / GVD ratio, which is the ratio of the third-order group delay dispersion (TOD) obtained by differentiating the group velocity dispersion with respect to frequency to the group velocity dispersion.

[0054] The above characteristics can be determined, for example, through the following steps. First, in such a case... Figure 9 In the structure shown where the second fiber 72 is absent, the residual group delay dispersion is derived when the time width of the ultrashort pulse L is broadened by the first fiber 71, the ultrashort pulse L is amplified by the fiber amplifier 8, and the time width of the ultrashort pulse L is compressed by the diffraction gratings 91 and 92. At this time, the first characteristic of the first fiber 71 is the group delay dispersion G1 of the first fiber 71, which is determined by arbitrarily deciding the fiber length (refer to...). Figure 10 (a)). Furthermore, the characteristics of diffraction grating pairs 91 and 92 are determined by arbitrarily deciding the distance l between diffraction grating pairs 91 and 92 (see [reference]). Figure 7The group delay dispersion G2 of the diffraction grating for 91 and 92, determined by (b) (see reference). Figure 10 (a)). The group delay dispersion G2 of the diffraction grating pairs 91 and 92 can be derived specifically by the following formula (1). In the following formula (1), λ is the wavelength, c is the speed of light, d is the grating constant, l is the distance between the diffraction grating pairs 91 and 92, and θ is Figure 7 The incident angle of the light shown in (b).

[0055] Group delay dispersion G2(λ)=-λ³l / πc²d² [1-(λ / d-sinθ)²] ···(1)

[0056] Then, by adding the group delay dispersion G1 of the first fiber 71 to the group delay dispersion G2 of the second fiber 72, the aforementioned residual group delay dispersion G3 is derived (refer to...). Figure 10 (b)). In Figure 10 In (a) and (b), the horizontal axis represents wavelength, and the vertical axis represents the group delay dispersion value. For example... Figure 10 As shown in (b), in the ultrashort pulse light L1, the partial residual group delay dispersion G3 at wavelength X1 is 0, achieving state recovery (restored to the state before broadening) and appropriate compression of the ultrashort pulse light L1. On the other hand, as Figure 10 As shown in (b), in the ultrashort pulse light L1, the value of the partial residual group delay dispersion G3 at wavelength X2 becomes a value deviating from 0, and the state recovery (recovery to the state before broadening) of the ultrashort pulse light L1 fails to occur, and it fails to be properly compressed.

[0057] Furthermore, in the group delay dispersion G1 (refer to the first characteristic) Figure 11 (a) ), group delay dispersion G2 as a compression property (refer to Figure 11 (b) and residual group delayed dispersion G3 (refer to) Figure 11 With (c)) determined, in order to make the value of the residual group delay dispersion G3 close to the desired value (e.g., 0), the group delay dispersion G4 of the second fiber 72, which is a second characteristic of the second fiber 72, is set (refer to...). Figure 11 (d) Specifically, the group delay dispersion G4 can be set as the value obtained by multiplying the residual group delay dispersion G3 by -1. The group delay dispersion G4 is determined, for example, by the fiber length of the second fiber 72.

[0058] As described above, the second characteristic of the second optical fiber 72 is set such that, in a structure where the second optical fiber 72 is absent, after the time width is broadened by the first optical fiber 71 and then the time width is compressed by the diffraction grating for 91 and 92, the value of the residual group delay dispersion G3 (an index related to group velocity dispersion) of each wavelength remaining in the ultrashort pulse light L is close to a predetermined value, preferably set to close to 0.

[0059] Back Figure 1 The wavelength conversion unit 10 is a structure that performs wavelength conversion on each pulse of the ultrashort pulse light L output from the compressor 9. For example, the wavelength conversion unit 10 can also perform wavelength conversion on each pulse to the 900nm to 1100nm band.

[0060] Next, the pulse light generation method implemented using pulse light generation device 1 will be described, while referring to... Figure 12 process Figure 1 The explanation will be provided later.

[0061] First, the ultrashort pulse light L is oscillated in oscillator 2 to generate an ultrashort pulse sequence with a specified period (oscillation step: step S1). The ultrashort pulse light L is then powered up using fiber amplifier 3, while simultaneously broadbanding its spectrum (step S2). Finally, the intensity of the ultrashort pulse light L is controlled for each pulse using acousto-optic modulator 4, for example, according to the specifications or conditions required for the pulse light generating device 1 (step S3).

[0062] Subsequently, the time width of the ultrashort pulse light L is compressed by compressor 5 (step S4). The wavelength of the ultrashort pulse light L, which has been broadbanded, is modulated by soliton self-frequency shifting fiber 6 to achieve a longer wavelength (step S5).

[0063] Subsequently, the time width of the ultrashort pulse light L oscillated in the oscillation step is broadened using the first optical fiber 71 with the first characteristic and the second optical fiber 72 with the second characteristic (broadening step: step S6).

[0064] Subsequently, the ultrashort pulse light L, whose time width was broadened in the broadening step, is amplified by fiber amplifier 8 (amplification step: step S7).

[0065] Subsequently, the time width of the ultrashort pulse light L, which was amplified in the amplification step, is compressed by the diffraction gratings 91 and 92 of the compressor 9 (compression step: step S8).

[0066] Finally, the wavelength conversion unit 10 performs wavelength conversion on the ultrashort pulse light L for each pulse (step S9).

[0067] Next, the effects of the pulse light generating device 1 in this embodiment will be explained.

[0068] The pulse light generating apparatus 1 of this embodiment includes: an oscillator 2 that oscillates an ultrashort pulse light L; a stretcher fiber 7 that stretches the time width of the ultrashort pulse light L; an fiber amplifier 8 that amplifies the ultrashort pulse light L whose time width has been stretched by the stretcher fiber 7; and a compressor 9 that compresses the time width of the ultrashort pulse light L amplified by the fiber amplifier 8. The stretcher fiber 7 is composed of a first fiber 71 that stretches the time width of the ultrashort pulse light L with a first characteristic and a second fiber 72 that stretches the time width of the ultrashort pulse light L with a second characteristic different from the first characteristic.

[0069] The inventors have focused on using optical fibers for broadening and diffraction grating pairs for compression during chirped pulse amplification, considering stability and throughput. However, when broadening and compression are performed in this manner, it is difficult to restore the ultrashort pulse light to its pre-broadening state after compression. In this regard, in the pulse light generating apparatus 1 of this embodiment, the broadener fiber 7 is configured to include a first fiber 71 and a second fiber 72 with different characteristics. By combining optical fibers with different characteristics as the broadener fiber 7, even if the ultrashort pulse light L cannot be restored to its pre-broadening state after compression by the compressor 9 when only the first fiber 71 is used as the broadener, by further combining the second fiber 72 with characteristics different from the first fiber 71, the compressed ultrashort pulse light L can be brought closer to its pre-broadening state according to the setting of the second characteristic. That is, since the conditions can be adjusted using two optical fibers 71 and 72 with different characteristics, it is possible to restore the state of the ultrashort pulse light L (to its pre-broadening state), which is difficult with only one fiber. As described above, the pulse light generating apparatus 1 according to this embodiment can appropriately perform chirped pulse amplification.

[0070] The second characteristic of the second fiber 72 can also be set such that, in a structure where the time width is broadened by the first fiber 71 and then compressed by the compressor 9 in the absence of the second fiber 72, the values ​​of the indices related to group delay dispersion (GDD) in each wavelength remaining in the ultrashort pulse light L are close to predetermined values. In this way, by setting the second characteristic such that the values ​​of the indices related to group velocity dispersion in each wavelength remaining after broadening using the first fiber 71 and compression using the compressor 9 are close to predetermined values, the state recovery of the ultrashort pulse light L (returning to the state before broadening) can be appropriately achieved by appropriately setting the predetermined values.

[0071] The second characteristic of the second fiber 72 can also be set such that, in a structure where the time width is broadened by the first fiber 71 and then compressed by the compressor 9 in the absence of the second fiber 72, the value of the index related to group delay dispersion (GDD) of each wavelength remaining in the ultrashort pulse light L is close to 0. In this way, by setting the second characteristic such that the value of the index related to group velocity dispersion remaining in each wavelength after broadening using the first fiber 71 and compression using the compressor 9 is close to 0, the state recovery of the ultrashort pulse light L (returning to the state before broadening) can be achieved more appropriately.

[0072] The compressor 9 may also have a pair of diffraction gratings 91 and 92. By using the diffraction gratings 91 and 92, which utilize the difference in optical path length caused by diffraction for compression, as the compressor 9, the generation of nonlinear effects that are problematic, for example, when using optical fibers as compressors, can be suppressed. When using the diffraction gratings 91 and 92 as compressor 9, it is conceivable that it would be difficult to restore the ultrashort pulse light L to its pre-broadening state after compression. However, as described above, in the structure of this embodiment, since the conditions can be adjusted by using two optical fibers 71 and 72 with different characteristics, the state restoration of the ultrashort pulse light L (restoring it to its pre-broadening state) can also be appropriately performed.

[0073] The first fiber 71 can also be a normal dispersion fiber. By using a normal dispersion fiber, the time width of the pulsed light can be appropriately broadened while avoiding unpredictable nonlinear effects.

[0074] The stretcher fiber 7 stretches the wavelength of the ultrashort pulse L with a time width of 1800 nm to 2200 nm, and the compressor 9 compresses the wavelength of the ultrashort pulse L with a time width of 1800 nm to 2200 nm. Based on this structure, for example, chirped pulse amplification can be appropriately performed within the gain range of the thulium amplifier.

[0075] In this embodiment, as described above, the pulsed light generating device 1 is used in a two-photon microscope. In this case, the pulse width of the light in the 1800nm ​​to 2200nm region is appropriately restored to its original state, thus improving the conversion efficiency for subsequent wavelength conversion to 900nm to 1100nm. This allows for the induction of two-photon fluorescence using light with higher output. Consequently, higher brightness fluorescence can be obtained. Furthermore, the light pulses in the 1800nm ​​to 2200nm range can restore the broadened pulses to their original state without changing the inter-diffraction grating distance of the diffraction grating pair, thus enabling application to light sources with wavelength variable per pulse.

[0076] The present disclosure is not limited to the above-described embodiments.

[0077] For example, the example of stretcher fiber 7 consisting of two fibers 71 and 72 has been described, but it is not limited to this. For example, more than three fibers can be combined to form a stretcher fiber.

[0078] In this case, for example, in the group delay dispersion G21 (refer to) which is a first characteristic of the first fiber. Figure 13 (a) ), group delay dispersion G22 as a compression property of diffraction grating pairs (refer to) Figure 13 (b) and residual group delayed dispersion G23 (refer to) Figure 13 With (c)) determined, in order to make the value of the residual group delay dispersion G23 close to the desired value (e.g., 0), the group delay dispersion G24 (refer to) is set as a second characteristic of the two fibers constituting the second fiber. Figure 13 (d) and group delay dispersion G25 (refer to) Figure 13 (e)).

[0079] Symbol Explanation

[0080] 1…Pulse light generating device, 2…Oscillator (oscillation section), 7…Stretcher fiber (stretcher), 8…Fiber amplifier (amplification section), 9…Compressor, 71…First fiber, 72…Second fiber, 91, 92…Diffraction grating pair.

Claims

1. A pulsed light generating device, comprising: The oscillating section emits pulsed light. A stretcher that stretches the time width of the pulsed light; The amplification section amplifies the pulsed light whose time width has been broadened by the stretcher; and A compressor that compresses the time width of the pulsed light, which has been amplified by the amplification unit. The stretcher is composed of a first optical fiber that stretches the time width of the pulsed light with a first characteristic and a second optical fiber that stretches the time width of the pulsed light with a second characteristic different from the first characteristic.

2. The pulsed light generating device according to claim 1, wherein, The second characteristic of the second optical fiber is set such that, in a structure where the time width is broadened by the first optical fiber and then compressed by the compressor in the absence of the second optical fiber, the values ​​of the indices related to the group delay dispersion of each wavelength remaining in the pulse light are close to a specified value.

3. The pulse light generating device according to claim 2, wherein, The second characteristic of the second optical fiber is set such that, in a structure where the time width is broadened by the first optical fiber and then compressed by the compressor in the absence of the second optical fiber, the value of the index related to the group delay dispersion of each wavelength remaining in the pulse light is close to 0.

4. The pulsed light generating apparatus according to any one of claims 1 to 3, wherein, The compressor has a pair of diffraction gratings.

5. The pulsed light generating apparatus according to any one of claims 1 to 3, wherein, The first optical fiber is a normal dispersion optical fiber.

6. The pulsed light generating apparatus according to any one of claims 1 to 3, wherein, The wavelength band of the pulsed light whose time width is broadened by the stretcher and the wavelength band of the pulsed light whose time width is compressed by the compressor are 1800nm ​​to 2200nm.

7. A method for generating pulsed light, comprising: The oscillation step produces pulsed light. In the broadening step, the time width of the pulse light oscillating in the oscillation step is broadened using a first optical fiber with a first characteristic, and then further broadened using a second optical fiber with a second characteristic. The amplification step amplifies the pulse light whose time width was broadened in the stretching step; and The compression step compresses the time width of the pulsed light that was amplified in the amplification step.

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