Pulsed light generation apparatus and pulsed light generation method

Abstract

A desired number of pulsed light is formed by modulation utilizing soliton self-frequency shift. [Solution] The pulsed light generation device 100 includes an oscillator 1 that oscillates pulsed light L, a compressor 4 positioned downstream of the oscillator 1 in the optical path of the pulsed light L and controlling the time width of the pulsed light L, and a soliton shift fiber 5 positioned downstream of the compressor 4 in the optical path of the pulsed light L and modulating the wavelength of the pulsed light L using soliton self-frequency shift to form one or more pulsed light L with different wavelengths. The compressor 4 controls the time width of the pulsed light L so that the number of pulsed light L formed by the soliton shift fiber 5 is a predetermined number.

Description

Technical Field

[0001] The present disclosure relates to a pulsed light generation device and a pulsed light generation method.

Background Art

[0002] There is known a pulsed light generation device including an oscillation unit that oscillates pulsed light and a modulation unit that modulates the wavelength of the pulsed light oscillated by the oscillation unit using soliton self-frequency shift. In such a pulsed light generation device, by increasing the intensity of the pulsed light before modulation by the modulation unit, the pulsed light is split by the modulation, and a plurality of pulsed lights with different wavelengths are formed (a multi-colored soliton is output) (see, for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In a pulsed light generation device, there may be a case where it is desired to form a desired number of pulsed lights by modulation of a modulation unit. However, in the above-described technology, the stability of the pulsed lights to be formed is low, and it is not easy to control the number of pulsed lights, so there is a risk that a desired number of pulsed lights cannot be formed. Therefore, an object of the present disclosure is to provide a pulsed light generation device and a pulsed light generation method capable of forming a desired number of pulsed lights by modulation using soliton self-frequency shift.

Means for Solving the Problems

[0005] The pulsed light generation apparatus of the present disclosure is [1] "a pulsed light generation apparatus comprising: an oscillator that oscillates pulsed light; a control unit disposed downstream of the oscillator in the optical path of the pulsed light and controlling the time width of the pulsed light; and a modulation unit disposed downstream of the control unit in the optical path of the pulsed light and modulating the wavelength of the pulsed light using soliton self-frequency shift to form one or a plurality of pulsed lights with different wavelengths, wherein the control unit controls the time width of the pulsed light so that the number of pulsed lights formed by the modulation unit is a predetermined number."

[0006] As a result of diligent research, the Disclosers have found a correlation between the residual dispersion of pulsed light before modulation using soliton self-frequency shifting and the number of pulsed light beams formed by the modulation. Therefore, in this disclosure, the control unit controls the time width of the pulsed light beams so that the number of pulsed light beams formed by the modulation unit becomes a predetermined number. This makes it possible to control the number of pulsed light beams while improving stability by compensating for the dispersion of the pulsed light beams. In other words, it becomes possible to form a desired number of pulsed light beams by modulation using soliton self-frequency shifting.

[0007] The pulsed light generation apparatus of the present disclosure may also be [2] "the pulsed light generation apparatus according to [1], wherein the control unit has a pair of diffraction gratings, and the distance between the pair of diffraction gratings is set to a predetermined distance such that the number of pulsed light beams formed by the modulation unit is a predetermined number." In this case, the number of pulsed light beams formed by modulation using soliton self-frequency shift can be controlled by the distance between the pair of diffraction gratings of the control unit.

[0008] The pulsed light generation apparatus of the present disclosure may also be [3] "the pulsed light generation apparatus according to [2], wherein the pair of diffraction gratings are fixed at the distance between them as the set distance." In this case, the number of pulses of light formed by modulation using soliton self-frequency shift can be set to a desired fixed value.

[0009] The pulsed light generation apparatus of the present disclosure may be the pulsed light generation apparatus according to [2], wherein the pair of diffraction gratings are configured to be adjustable in distance between them. In this case, it is possible to adjust the number of pulses of light formed by modulation using soliton self-frequency shift.

[0010] The pulsed light generation apparatus of the present disclosure may also be the pulsed light generation apparatus according to any one of [1] to [4], wherein the control unit controls the time width of the pulsed light so that the value relating to the wavelength of one or more pulsed light formed by the modulation unit becomes a predetermined value. As a result of diligent research, the inventors have found that there is a correlation between the residual dispersion of the pulsed light before modulation using soliton self-frequency shift and the wavelength of the pulsed light formed by the modulation. Therefore, in the present disclosure, the control unit controls the time width of the pulsed light so that the value relating to the wavelength of the pulsed light formed by the modulation unit becomes a predetermined value. This makes it possible for the control unit to control the wavelength of one or more pulsed light after modulation using soliton self-frequency shift.

[0011] The pulsed light generation apparatus of the present disclosure may also be [6] "a pulsed light generation apparatus according to any one of [1] to [5], comprising: a branching unit arranged downstream of the modulation unit in the optical path of the pulsed light and branching a plurality of the pulsed light; an amplification unit arranged in at least one of the optical paths of the plurality of pulsed light branched by the branching unit and amplifying the pulsed light; and a sum frequency generation unit that combines a part or all of the plurality of pulsed light branched by the branching unit, at least one of which is amplified by the amplification unit, and emits sum frequency light by sum frequency generation." In this case, it is possible to obtain sum frequency light from a plurality of pulsed light formed in the modulation unit.

[0012] The pulsed light generation apparatus of the present disclosure may also be [7] "a pulsed light generation apparatus according to any one of [1] to [6], comprising an optical intensity control unit disposed between the oscillator and the modulation unit in the optical path of the pulsed light, which controls the intensity of the pulsed light for each pulse." In this case, the wavelength of one or more pulsed light after modulation using soliton self-frequency shift can be varied for each pulse by the optical intensity control unit.

[0013] The pulsed light generation method of the present disclosure is [8] "a pulsed light generation method comprising: an oscillation step of oscillating pulsed light; a control step of controlling the time width of the pulsed light oscillated in the oscillation step; and a modulation step of modulating the wavelength of the pulsed light after control by the control step using a soliton self-frequency shift to form one or a plurality of pulsed lights with different wavelengths, wherein the control step controls the time width of the pulsed light so that the number of pulsed lights formed in the modulation step is a predetermined number."

[0014] In the pulsed light generation method of this disclosure, the control step controls the time width of the pulsed light so that the number of pulsed light pulses formed in the modulation step is a predetermined number. Therefore, similar to the pulsed light generation apparatus described above, it is possible to control the number of pulsed light pulses while improving stability by performing dispersion compensation on the pulsed light pulses. In other words, it is possible to form a desired number of pulsed light pulses by modulation utilizing soliton self-frequency shift. [Effects of the Invention]

[0015] According to this disclosure, it is possible to provide a pulsed light generation apparatus and a pulsed light generation method that can form a desired number of pulsed light by modulation utilizing soliton self-frequency shift. [Brief explanation of the drawing]

[0016] [Figure 1] Figure 1 is a block diagram showing a pulsed light generation device according to the first embodiment. [Figure 2]FIG. 2 is a graph showing the spectral waveform of the pulsed light output from the soliton shift fiber of FIG. 1. [Figure 3] FIG. 3 is a graph showing a configuration example of the compressor of FIG. 1. [Figure 4] FIG. 4 is a graph showing the correlation between the group delay dispersion of each wavelength remaining in the pulsed light, the number of pulsed lights, and the central wavelength of the first soliton. [Figure 5] FIG. 5 is a flowchart showing the pulsed light generation method according to the first embodiment. [Figure 6] FIG. 6 is a block diagram showing the pulsed light generation device according to the second embodiment. [Figure 7] FIG. 7 is a diagram showing a configuration example of the wavelength conversion unit of FIG. 6. [Figure 8] FIG. 8 is a block diagram showing the pulsed light generation device according to the third embodiment. [Figure 9] FIG. 9 is a block diagram showing the pulsed light generation device according to the fourth embodiment. [Embodiments for Carrying Out the Invention]

[0017] Hereinafter, embodiments will be described in detail with reference to the drawings. In each figure, the same or corresponding parts are denoted by the same reference numerals, and duplicate descriptions are omitted.

[0018] [First Embodiment] As shown in FIG. 1, the pulsed light generation device 100 according to the first embodiment is a device that generates output light LK, which is ultrashort pulsed light, by using soliton self-frequency shift (Raman soliton shift). The pulsed light generation device 100 can be applied as a light source of an optical measurement device or an optical inspection device, for example, a microscope, particularly a two-photon microscope. A two-photon microscope is a type of laser scanning fluorescence microscope that excites a fluorescent dye in a sample by irradiating the sample with ultrashort pulsed laser light in the near-infrared region, and detects a signal (fluorescence) from the sample to perform image analysis. The pulsed light generation device 100 constitutes a wavelength-variable light source whose wavelength is variable for each pulse.

[0019] The pulsed light generation device 100 comprises an oscillator (oscillator) 1, a fiber amplifier 2, an acousto-optic modulator (light intensity control unit) 3, a compressor (control unit) 4, a soliton shift fiber (modulation unit) 5, and a filter (filter unit) 6.

[0020] Oscillator 1 generates pulsed light L. Various oscillators capable of generating ultrashort pulses L with a time width of picoseconds to femtoseconds can be used as oscillator 1. Oscillator 1 generates a train of ultrashort pulses L that are arranged on the time axis at predetermined time intervals. The pulsed light L generated by oscillator 1 has a repetition rate in the MHz to GHz range.

[0021] The fiber amplifier 2 amplifies the pulsed light L oscillated by the oscillator 1. The fiber amplifier 2 is located downstream of the oscillator 1 in the optical path of the pulsed light L. In this embodiment, the fiber amplifier 2 is located between the oscillator 1 and the acousto-optic modulator 3 in the optical path of the pulsed light L. The fiber amplifier 2 is configured to include, for example, a fiber amplifier. The fiber amplifier of the fiber amplifier 2 is a normally dispersed fiber, for example, a thulium-doped fiber. The laser medium added to the fiber of the fiber amplifier 2 is not particularly limited and may be a rare earth element such as ytterbium, erbium, or neodymium, or it may be Bi, etc. The fiber amplifier 2 has a gain band corresponding to the wavelength range of the pulsed light L.

[0022] The acousto-optic modulator 3 allows for pulse-by-pulse control of the intensity of the pulsed light L amplified by the fiber amplifier 2. The acousto-optic modulator 3 is a device called an AOM (Acousto Optic Modulator) that modulates the intensity of the pulsed light L pulse by pulse using the power of sound (sound waves). The acousto-optic modulator 3 can modulate the intensity in accordance with the repetition of the pulsed light L. The acousto-optic modulator 3 is positioned between the fiber amplifier 2 and the soliton-shift fiber 5 in the optical path of the pulsed light L. In this embodiment, the acousto-optic modulator 3 is positioned upstream (before) the compressor 4 in the optical path of the pulsed light L, between the fiber amplifier 2 and the soliton-shift fiber 5. The acousto-optic modulator 3 also reduces the repetition rate by thinning out the pulsed light L included in the ultrashort pulse train.

[0023] The compressor 4 is a dispersion control unit that controls (e.g., compresses) the time width of the pulsed light L whose intensity is controlled by the acousto-optic modulator 3. In other words, the compressor 4 applies dispersion in the opposite direction to the dispersion effect that the pulsed light L has received up to the previous stage, thereby achieving dispersion compensation that narrows the time width of the broadened pulsed light L. The compressor 4 is located upstream of the soliton shift fiber 5 in the optical path of the pulsed light L. In this embodiment, the compressor 4 is located between the acousto-optic modulator 3 and the soliton shift fiber 5 in the optical path of the pulsed light L.

[0024] Various types of compressors can be used as compressor 4, such as a compressor containing an anomalous dispersion fiber, a compressor containing a pair of prisms, or a compressor containing a pair of grisms. In this embodiment, a diffraction grating type compressor containing a pair of diffraction gratings is used as compressor 4 (details will be described later).

[0025] The soliton shift fiber 5 modulates the wavelength of pulsed light L, whose time width is controlled by the compressor 4, using soliton self-frequency shifting to form one or more pulsed light L with different wavelengths (hereinafter also simply referred to as "one or more pulsed light L"). The soliton shift fiber 5 is located downstream (after) the compressor 4 in the optical path of the pulsed light L. In this embodiment, the soliton shift fiber 5 is located between the compressor 4 and the filter 6 in the optical path of the pulsed light L. For example, a single-mode anomalous dispersion fiber exhibiting anomalous dispersion in the wavelength band of the pulsed light L can be used as the soliton shift fiber 5.

[0026] The soliton shift fiber 5 outputs one or more solitons with different wavelengths when pulsed light L is input. In the example shown in Figure 2, for example, the soliton shift fiber 5 generates three solitons LS with different wavelengths. The multiple solitons LS generated by the modulation of the soliton shift fiber 5 are sometimes referred to as multisolitons or multisoliton trains. Soliton LS are pulsed light L whose wavelength is modulated by soliton self-frequency shifting. The wavelength of soliton LS can be controlled by the intensity of the pulsed light L input to the soliton shift fiber 5. Increasing the intensity of the pulsed light L input to the soliton shift fiber 5 shifts the wavelength of soliton LS to the longer wavelength side, while decreasing the intensity of the pulsed light L input to the soliton shift fiber 5 shifts the wavelength of soliton LS to the shorter wavelength side. In this embodiment, the wavelength of one or more pulsed light L generated by the soliton shift fiber 5 is controlled by controlling the intensity of multiple pulsed light L using the acousto-optic modulator 3.

[0027] Returning to Figure 1, filter 6 removes the non-soliton component LX (see Figure 2) from the optical component generated by the soliton shift fiber 5, excluding the soliton LS. The non-soliton component LX is the component that did not become soliton LS due to modulation by the soliton shift fiber 5. Filter 6 is located downstream of the soliton shift fiber 5 in the optical path of the pulsed light L. There are no particular restrictions on filter 6 as long as it can remove or attenuate the non-soliton component LX, but it is preferable to use one with an optical density (OD value) of 3 or more. For example, a wavelength-separated filter may be used as filter 6. Filter 6 may be implemented as a standalone unit or as part of a wavelength division multiplexing coupler. The non-soliton component LX removed by filter 6 may be reused. Filter 6 outputs one or more pulsed light L, after the non-soliton component LX has been removed, as output light LK to the next stage.

[0028] In this embodiment, as shown in Figure 3, the compressor 4 has a pair of diffraction gratings 41 and 42. In this embodiment, in the compressor 4, pulsed light L input from the acousto-optic modulator 3 is diffused by lens 43, transmitted through dichroic mirror 44, diffracted by the pair of diffraction gratings 41 and 42, and reflected by mirror 45. The pulsed light L reflected by mirror 45 is diffracted by the pair of diffraction gratings 41 and 42, reflected by dichroic mirror 44, and then focused by lens 46 and output to soliton shift fiber 5.

[0029] In this case, since the pulsed light L consists of light of multiple wavelengths superimposed with their phases aligned, when the pulsed light L is diffracted by the pair of diffraction gratings 41 and 42, differences occur in the optical path lengths of each wavelength of the pulsed light. As a result, the group delay time of each wavelength of the pulsed light L changes, and the time width is controlled.

[0030] Here, a correlation is found between the control of the residual dispersion of the pulsed light L by the compressor 4 (control of the residual dispersion of the pulsed light L before modulation using soliton self-frequency shift by the soliton shift fiber 5) and the number of pulsed light L formed by the soliton shift fiber 5. For example, a correlation is found between the group delay dispersion (GDD) of each wavelength remaining in the pulsed light L (hereinafter referred to as "GDD") and the number of pulsed light L formed by the soliton shift fiber 5, as shown in Figure 4 (see "×" in the figure).

[0031] Therefore, in the pulsed light generation device 100 of this embodiment, the compressor 4 controls the time width of the pulsed light so that the number of pulsed light L formed by the soliton shift fiber 5 is a predetermined number. Specifically, since the GDD can be determined based on the distance between a pair of diffraction gratings 41 and 42, the distance between the pair of diffraction gratings 41 and 42 of the compressor 4 is set to a predetermined distance so that the number of pulsed light L formed by the soliton shift fiber 5 is a predetermined number. In other words, for example, the GDD in which the number of pulsed light L formed by the soliton shift fiber 5 is a predetermined number can be determined based on the correlation in Figure 4, and the set distance that realizes this GDD is the distance between the pair of diffraction gratings 41 and 42. The predetermined number is not particularly limited and may be any number, and in the example in Figure 4, it is any number from 1 to 3. The correlation shown in Figure 4 can be obtained in advance, for example, by actual measurement and simulation. The pair of diffraction gratings 41 and 42 are fixed with the distance between them set to a predetermined distance.

[0032] Furthermore, a correlation is found between the control of residual dispersion of the pulsed light L by the compressor 4 and the wavelength of the pulsed light L formed by the soliton shift fiber 5. For example, a correlation is found between the GDD and the center wavelength (wavelength-related value) of the first soliton formed by the modulation, as shown in Figure 4 (see "〇" in the figure). The first soliton is the pulsed light L with the longest wavelength among the one or more pulsed light L formed by the soliton shift fiber 5.

[0033] Therefore, in the pulsed light generation device 100 of this embodiment, the compressor 4 controls the time width of the pulsed light L formed by the soliton shift fiber 5 so that the wavelength value of the pulsed light L becomes a predetermined value. Specifically, since the GDD can be determined based on the distance between a pair of diffraction gratings 41 and 42, the distance between the pair of diffraction gratings 41 and 42 of the compressor 4 is set to a predetermined distance at which the center wavelength of the first soliton formed by the soliton shift fiber 5 becomes a predetermined value. In other words, for example, the GDD at which the center wavelength of the first soliton formed by the soliton shift fiber 5 becomes a predetermined value can be determined based on the correlation shown in Figure 4, and the set distance at which this GDD is realized is the distance between the pair of diffraction gratings 41 and 42. The predetermined value may be any value within the range of the GDD where the number of pulsed light L is predetermined. In the example in Figure 4, if the predetermined number is 1, it is any value between 1800 nm and 1910 nm; if the predetermined number is 2, it is any value between 1840 nm and 2100 nm; and if the predetermined number is 3, it is any value between 1950 nm and 1980 nm.

[0034] Next, the pulsed light generation method performed using the pulsed light generation device 100 will be explained with reference to the flowchart in Figure 5.

[0035] As shown in Figure 5, the pulse light generation method according to this embodiment comprises an oscillation step S01, an amplification step S02, a light intensity control step S03, a time width control step (control step) S04, and a modulation step S05.

[0036] In the oscillation step S01, the oscillator 1 generates pulsed light L. In the amplification step S02, the fiber amplifier 2 amplifies the pulsed light L generated in the oscillation step S01. In the light intensity control step S03, the acousto-optic modulator 3 controls the intensity of the pulsed light L amplified in the amplification step S02 for each pulse. In the time width control step S04, the compressor 4 controls the time width of the pulsed light L whose intensity was controlled in the light intensity control step S03. In particular, in the time width control step S04, the time width of the pulsed light L is controlled so that the number of pulsed light L formed in the subsequent modulation step S05 is a predetermined number and the center wavelength of the first soliton of the pulsed light L is a predetermined value.

[0037] In the modulation step S05, the wavelength of the pulsed light L whose time width was controlled in the time width control step S04 is modulated by the soliton shift fiber 5 using soliton self-frequency shifting to form one or more pulsed light Ls with different wavelengths. After the modulation, the one or more pulsed light Ls are output as output light LK after the non-soliton component LX is removed by the filter 6.

[0038] In the pulsed light generator 100, the compressor 4 controls the time width of the pulsed light L so that the number of pulsed light L formed by the soliton shift fiber 5 is a predetermined number. This makes it possible to control the number of pulsed light L while improving stability by performing dispersion compensation on the pulsed light L modulated by the soliton shift fiber 5. In other words, it becomes possible to form a desired number of pulsed light by modulation using soliton self-frequency shift.

[0039] In the pulsed light generator 100, the compressor 4 has a pair of diffraction gratings 41 and 42, and the distance between the pair of diffraction gratings 41 and 42 is set to a predetermined distance such that the number of pulsed light L formed by the soliton shift fiber 5 is a predetermined number. In this case, the number of pulsed light L formed by the soliton shift fiber 5 can be controlled by the distance between the pair of diffraction gratings 41 and 42 of the compressor 4.

[0040] In the pulsed light generator 100, the pair of diffraction gratings 41 and 42 are fixed at a set distance. In this case, the number of pulsed light L formed by the soliton shift fiber 5 can be set to a desired fixed value.

[0041] The pulsed light generator 100, with its compressor 4, controls the time width of the pulsed light L so that the wavelength value of one or more pulsed light L formed by the soliton shift fiber 5 becomes a predetermined value. In this case, the compressor 4 can control the wavelength of the pulsed light L after modulation by the soliton shift fiber 5.

[0042] The pulsed light generator 100 includes an acousto-optic modulator 3 positioned between the oscillator 1 and the soliton-shift fiber 5 in the optical path of the pulsed light L. In this case, the wavelength of the pulsed light L after modulation by the soliton-shift fiber 5 can be varied for each pulse by the acousto-optic modulator 3.

[0043] In the pulsed light generation method, the time width of the pulsed light L is controlled in the time width control step S04 so that the number of pulsed light L formed in the modulation step S05 is a predetermined number. This makes it possible to control the number of pulsed light L while improving stability by performing dispersion compensation for the pulsed light L. In other words, it is possible to form a desired number of pulsed light L by modulation using soliton self-frequency shift. .

[0044] Generally, in fields such as the nervous system, the importance of multicolor two-photon imaging is recognized, but it is necessary to prepare multiple light sources for multiple targets (fluorescent proteins), which raises concerns about the difficulty of balancing cost and high-speed observation. In this regard, the pulsed light generation device 100 and pulsed light generation method of this embodiment can suppress these concerns and are an effective device and method.

[0045] In this embodiment, the intensity of the pulsed light L is controlled for each pulse by the acousto-optic modulator 3, but the embodiment is not limited to this, and the intensity of the pulsed light L controlled by the acousto-optic modulator 3 may be fixed to a constant value. In this case, the wavelengths of one or more pulsed light L after modulation by the soliton shift fiber 5 can be fixed to a constant wavelength.

[0046] [Second Embodiment] Next, a second embodiment will be described. In describing this embodiment, the differences from the first embodiment will be explained, and redundant explanations will be omitted.

[0047] As shown in Figure 6, the pulse light generation device 200 according to the second embodiment differs from the first embodiment in that it includes a stretcher 7, a fiber amplifier 8, a compressor 9, and a wavelength conversion unit 10.

[0048] The stretcher 7 collectively stretches the time width of one or more pulsed light L after the non-soliton component LX has been removed by the filter 6, thereby reducing the peak power of the pulsed light L. This suppresses the occurrence of undesirable nonlinear optical effects during amplification by the subsequent fiber amplifier 8. The stretcher 7 is positioned downstream of the soliton-shift fiber 5 in the optical path of the pulsed light L. In this embodiment, the stretcher 7 is positioned between the filter 6 and the fiber amplifier 8 in the optical path of the pulsed light L. The stretcher 7 may stretch the time width of the pulsed light L by transmitting one or more pulsed light L through at least one of the fiber and / or the medium to impart dispersion. Alternatively, the stretcher 7 may stretch the time width of the pulsed light L by imparting dispersion to one or more pulsed light L in a spatial system using a diffraction grating pair or a prism pair, etc. The dispersion imparted by the stretcher 7 may be either normal dispersion or anomalous dispersion.

[0049] The fiber amplifier 8 amplifies one or more pulsed light L whose time width has been stretched by the stretcher 7. The fiber amplifier 8 is located downstream of the soliton-shifted fiber 5 in the optical path of the pulsed light L. In this embodiment, the fiber amplifier 8 is located between the stretcher 7 and the compressor 9 in the optical path of the pulsed light L. The fiber amplifier 8 includes a fiber amplifier. The fiber amplifier of the fiber amplifier 8 is a normally dispersed fiber, for example, a thulium-doped fiber. The laser medium added to the fiber of the fiber amplifier 8 is not particularly limited and may be a rare earth element such as ytterbium, erbium, neodymium, or Bi.

[0050] The compressor 9 controls the time width of one or more pulsed light L amplified by the fiber amplifier 8 in a unified manner. The compressor 9 applies dispersion in the opposite direction to the dispersion effect on the pulsed light L up to the preceding stage, thereby achieving dispersion compensation that narrows the broadened time width of the pulsed light L. The compressor 9 is located downstream of the fiber amplifier 8 in the optical path of the pulsed light L. In this embodiment, the compressor 9 is located between the fiber amplifier 8 and the wavelength conversion unit 10 in the optical path of the pulsed light L. Various compressors can be used as the compressor 9, such as a compressor including an anomalous dispersion fiber, a compressor including a prism pair, or a compressor including a grism pair.

[0051] The wavelength conversion unit 10 performs wavelength conversion for each pulse of one or more pulsed light L whose time width is controlled by the compressor 9. The wavelength conversion unit 10 may, for example, convert the wavelength of pulsed light L between 1800 nm and less than 2000 nm to between 900 nm and less than 1000 nm, which is usable for two-photon microscopy. The wavelength conversion unit 10 may perform the above wavelength conversion by second harmonic generation, a phenomenon included in nonlinear effects. Second harmonic generation is a phenomenon in which the original wavelength (fundamental wave) is converted into light with half the wavelength (second harmonic). The wavelength conversion unit 10 is located downstream of the compressor 9 in the optical path of the pulsed light L. The wavelength conversion unit 10 outputs the pulsed light L after wavelength conversion as output light LK.

[0052] The wavelength conversion unit 10 according to this embodiment performs high-efficiency wavelength conversion using a relatively thick crystal, and without using a drive system, it disperses light for each wavelength band and changes the optical path for each wavelength band, thereby performing broadband wavelength conversion of pulsed light L. A detailed configuration example of the wavelength conversion unit 10 will be described below with reference to Figure 7.

[0053] As shown in Figure 7, the wavelength conversion unit 10 includes a diffraction grating 101, a lens 102, a wavelength conversion element 103, a lens 104, and a diffraction grating 105. The diffraction grating 101 and lens 102 change the optical path for each wavelength band so that light of the wavelength band to be converted at each incident position is incident on the wavelength conversion element 103. The diffraction grating 101 disperses the pulsed light L output from the compressor 9 for each wavelength band so that light of the wavelength band to be converted at each incident position is incident on the wavelength conversion element 103. Lens 102 is a lens that focuses the light from the diffraction grating 101 to the incident position of the wavelength conversion element 103. The distance from the diffraction grating 101 to lens 102, and the distance from lens 102 to wavelength conversion element 103 are both set to match the focal length f of lens 102, for example, the focal length f of lens 102.

[0054] The wavelength conversion element 103 is a crystal in which the converted wavelength band differs depending on the incident position of the light. The wavelength conversion element 103 has a fan-shaped structure so that pseudo-phase matching of different wavelength bands is achieved at each incident position. More specifically, the wavelength conversion element 103 may be a PPLN (Periodically Poled Lithium Niobate) with a fan-shaped structure. That is, the wavelength conversion element 103 may be a so-called fan-out PPLN. Due to the fan-shaped structure of the wavelength conversion element 103, it is possible to achieve pseudo-phase matching of any wavelength band at each incident position of the crystal. In other words, since the period of the region where the second harmonics cancel each other out differs depending on the wavelength band, by using a crystal in which the period is reversed according to the fan-shaped structure, it is possible to make the wavelength bands in which pseudo-phase matching is achieved different at each incident position of the crystal. In such a wavelength conversion element 103, since the pseudo-phase matching of the wavelength band to be achieved at each incident position can be determined in advance, the diffraction grating 101 and lens 102 described above can be set so that light of the wavelength band to be converted is incident at each incident position, thereby enabling appropriate wavelength conversion for light of various wavelength bands.

[0055] Lens 104 is a lens that focuses the light whose wavelength has been converted by the wavelength conversion element 103 onto the diffraction grating 101. The distance from the wavelength conversion element 103 to lens 104, and the distance from lens 104 to diffraction grating 105 are both set to match the focal lengths f of lenses 102 and 104, for example, the focal lengths f of lenses 102 and 104. The diffraction grating 105 outputs the light that has been dispersed by the diffraction grating 101 for each wavelength band and has reached the lens 102, wavelength conversion element 103, and lens 104 as pulsed light L after wavelength conversion. The number of grooves in the diffraction grating 105 may be twice the number of grooves in the diffraction grating 101 in order to restore the dispersion of the light after wavelength conversion.

[0056] As described above, in the pulsed light generation device 200 and pulsed light generation method according to this embodiment, it is possible to form a desired number of pulsed light L by modulation using soliton self-frequency shift. Furthermore, in the pulsed light generation device 200, the wavelength conversion unit 10 makes it possible to perform wavelength conversion for one or more pulsed light L pulse by pulse.

[0057] [Third Embodiment] Next, a third embodiment will be described. In describing this embodiment, the differences from the first embodiment described above will be explained, and redundant explanations will be omitted.

[0058] As shown in Figure 8, the pulse light generation device 300 according to the third embodiment differs from the first embodiment in that it includes a branching unit 11, a stretcher 12, a fiber amplifier (amplification unit) 13, a multiplexing unit 14, a compressor 15, and a wavelength conversion unit 16. In addition, in the pulse light generation device 300, the acousto-optic modulator 3 fixes the intensity of the pulse light L to a constant value, and the compressor 4 controls the time width of the pulse light L so that the number of pulse light L formed by the soliton shift fiber 5 is N (N = an integer of 2 or more), thereby forming N pulse light L (multisoliton) by the soliton shift fiber 5.

[0059] The branching section 11 branches the N pulsed light L after the non-soliton component LX has been removed by the filter 6. Specifically, the branching section 11 branches the N pulsed light L, each having a different wavelength, into N pulsed light L1 to LN, based on the wavelength of each pulsed light L. The branching section 11 is located downstream of the filter 6 in the optical path of the pulsed light L. As the branching section 11, for example, a wavelength division filter such as a dichroic filter, a wavelength division multiplexing coupler, and a spatial division multiplexing optical system or optical element can be used.

[0060] The stretcher 12 stretches the time width of one pulsed light L1 (any one pulsed light L) that is branched at the branching section 11, and reduces the peak power of the pulsed light L1. This suppresses the occurrence of undesirable nonlinear optical effects in amplification by the subsequent fiber amplifier 13. The stretcher 12 is positioned in the optical path of the pulsed light L1. The stretcher 12 may stretch the time width of the pulsed light L1 by transmitting the pulsed light L1 through at least one of the fiber and the medium to impart dispersion. Alternatively, the stretcher 7 may stretch the time width of the pulsed light L1 by imparting dispersion to the pulsed light L1 in the spatial system using a diffraction grating pair or a prism pair, etc. The dispersion may be either normal dispersion or abnormal dispersion.

[0061] The fiber amplifier 13 amplifies a pulse of light L1 whose time width has been stretched by the stretcher 12. The fiber amplifier 13 is located downstream of the stretcher 12 in the optical path of the pulse of light L1. The fiber amplifier 13 includes a fiber amplifier. The fiber amplifier of the fiber amplifier 13 is a normally dispersed fiber, for example, a thulium-doped fiber. The laser medium added to the fiber of the fiber amplifier 13 is not particularly limited and may be a rare earth element such as ytterbium, erbium, neodymium, or Bi. The fiber amplifier 13 may have a gain band corresponding to the wavelength range of the pulse of light L1.

[0062] The multiplexing unit 14 combines the N pulsed light beams L that were branched at the branching unit 11 onto the same optical axis. The multiplexing unit 14 is composed of, for example, a mirror and a dichroic mirror. The multiplexing unit 14 is located upstream of the compressor 15 in the optical path of the pulsed light beams L.

[0063] The compressor 15 controls the time width of the N pulsed light L combined in the multiplexer 14 in a unified manner. The compressor 15 applies dispersion in the opposite direction to the dispersion effect that the N pulsed light L experienced up to the previous stage, thereby achieving dispersion compensation by narrowing the time width of the spread N pulsed light L. The compressor 15 is located downstream of the multiplexer 14 in the optical path of the pulsed light L. Various types of compressors can be used as the compressor 15, such as a compressor containing an anomalous dispersion fiber, a compressor containing a pair of prisms, or a compressor containing a pair of grisms.

[0064] The wavelength conversion unit 16 emits sum-frequency light as output light LK by sum-frequency generation using N pulsed light L combined by the multiplexing unit 14. In the wavelength conversion unit 16, the N pulsed light L combined by the multiplexing unit 14 is incident on a wavelength conversion crystal, and sum-frequency light LW is emitted from the wavelength conversion crystal by sum-frequency generation. The wavelength conversion unit 16 is located downstream of the compressor 15 in the optical path of the pulsed light L. The sum-frequency light LW has a wavelength in the wavelength range of, for example, 800 nm to 900 nm. The multiplexing unit 14 and the wavelength conversion unit 16 constitute a sum-frequency generation unit.

[0065] As described above, in the pulsed light generation apparatus 300 and pulsed light generation method according to this embodiment, it is possible to form a desired number of pulsed light L by modulation using soliton self-frequency shift.

[0066] The pulsed light generator 300 includes a branching unit 11 that branches a plurality of pulsed light L based on the wavelength of the pulsed light L, a fiber amplifier 13 arranged in the optical path of one pulsed light L1 branched by the branching unit 11, and a multiplexing unit 14 and a wavelength conversion unit 16 that combine the plurality of pulsed light L (including the one pulsed light L1 amplified by the fiber amplifier 13) branched by the branching unit and emit sum-frequency light (output light LK) by sum-frequency generation. In this case, it is possible to obtain sum-frequency light from a plurality of pulsed light L formed by the soliton shift fiber 5.

[0067] In this embodiment, the optical path lengths of the N pulsed light L are configured to be the same, for example, the arrival timing of the N pulsed light L1 at the wavelength conversion crystal of the wavelength conversion unit 16 is synchronized. In this embodiment, the multiplexing unit 14 may combine all of the multiple pulsed light L branched at the branching unit 11, or it may combine any part of the multiple pulsed light L branched at the branching unit 11. This makes it possible to obtain any number of sum frequencies. In this embodiment, the stretcher 12 and fiber amplifier 13 are arranged in the optical path of one pulsed light L1 branched at the branching unit 11, but they may be arranged in at least one of the optical paths of the N pulsed light L.

[0068] [Fourth Embodiment] Next, a fourth embodiment will be described. In describing this embodiment, the differences from the third embodiment described above will be explained, and redundant explanations will be omitted.

[0069] As shown in Figure 9, the pulsed light generator 400 according to the fourth embodiment differs from the third embodiment in that it further includes another stretcher 22. The other stretcher 22 further extends the time width of the pulsed light L1 whose time width has been extended by the stretcher 12. The other stretcher 22 is positioned between the stretcher 12 and the fiber amplifier 13 in the optical path of the pulsed light L1. This ensures that the pulse of the pulsed light L1 and at least one of the other pulsed light L2 to LN are reliably superimposed in time by the wavelength conversion unit 16. In addition to or instead of the other stretcher 22, the pulsed light generator 400 may also include a stretcher positioned in at least one of the optical paths of the pulsed light L2 to LN to extend at least one of the pulsed light L2 to LN.

[0070] As described above, in the pulsed light generation apparatus 400 and pulsed light generation method according to this embodiment, it is possible to form a desired number of pulsed light L by modulation using soliton self-frequency shift.

[0071] [Differentiation] The embodiments described above are not limited to the above-described embodiments.

[0072] In the above embodiment, an acousto-optic modulator 3 was used as the light intensity control unit, but the embodiment is not limited to this, and for example, an electro-optic modulator (EOM) may be used as the light intensity control unit. In the above embodiment, multiple acousto-optic modulators 3 may be arranged.

[0073] In the above embodiment, the pair of diffraction gratings 41 and 42 of the compressor 4 are fixed at a set distance between them, but are not limited to this. The pair of diffraction gratings 41 and 42 may be configured so that the distance between them can be adjusted. The configuration for adjusting the distance is not particularly limited, and various known configurations can be adopted. By adjusting the distance between the diffraction gratings 41 and 42, the GDD can be adjusted, and the number of pulsed light L formed by the soliton shift fiber 5 can be adjusted.

[0074] In the above embodiment, a fiber amplifier made of a normally dispersed, double-clad fiber was used as the fiber amplifier 2. However, a fiber amplifier made of a normally dispersed, single-clad fiber (e.g., erbium-doped) may be used instead. In this case as well, it will still be possible to broaden the spectrum of the pulsed light L.

[0075] The components in the above embodiments and modifications are not limited to the materials and shapes described above, and various materials and shapes can be applied. Furthermore, the components in the above embodiments and modifications can be arbitrarily applied to the components in other embodiments or modifications. [Explanation of symbols]

[0076] 1...Oscillator (oscillating unit), 3...Acousto-optic modulator (light intensity control unit), 4...Compressor (control unit), 5...Soliton shift fiber (modulation unit), 11...Branching unit, 13...Fiber amplifier (amplification unit), 14...Multiplier (sum frequency generation unit), 16...Wavelength conversion unit (sum frequency generation unit), 41,42...Diffraction grating, 100,200,300,400...Pulsed light generator, L,L1~LN...Pulsed light, LK...Output light.

Claims

1. An oscillator that emits pulsed light, A control unit is positioned downstream of the oscillator in the optical path of the pulsed light and controls the time width of the pulsed light, The system comprises a modulation unit located downstream of the control unit in the optical path of the pulsed light, which modulates the wavelength of the pulsed light using soliton self-frequency shift to form one or a plurality of pulsed lights with different wavelengths, The control unit controls the time width of the pulsed light so that the number of pulsed light pulses formed by the modulation unit becomes a predetermined number, and is a pulsed light generation device.

2. The control unit has a pair of diffraction gratings, The pulse light generation apparatus according to claim 1, wherein the distance between the pair of diffraction gratings is set to a distance such that the number of pulse light beams formed by the modulation unit is a predetermined number.

3. The pulsed light generating apparatus according to claim 2, wherein the pair of diffraction gratings are fixed at the distance between them as the set distance.

4. The pulsed light generating apparatus according to claim 2, wherein the pair of diffraction gratings are configured to allow adjustment of the distance between them.

5. The pulse light generation apparatus according to claim 1 or 2, wherein the control unit controls the time width of the pulse light so that the value relating to the wavelength of the pulse light formed by the modulation unit becomes a predetermined value.

6. A branching section is located downstream of the modulation section in the optical path of the pulsed light, and branches off a plurality of the pulsed light, An amplification unit is provided in at least one of the optical paths of the multiple pulsed light beams branched at the aforementioned branching section, and amplifies the pulsed light beams. A pulse light generating apparatus according to claim 1 or 2, comprising: a sum frequency generating unit that combines a portion or all of the multiple pulse light beams branched at the branching unit, at least one of which is amplified by the amplification unit, and emits sum frequency light by sum frequency generation.

7. The pulsed light generation apparatus according to claim 1 or 2, further comprising an optical intensity control unit disposed between the oscillator and the modulation unit in the optical path of the pulsed light, which controls the intensity of the pulsed light for each pulse.

8. An oscillation step that generates pulsed light, A control step that controls the time width of the pulse light oscillated in the oscillation step, The system includes a modulation step which modulates the wavelength of the pulsed light after control by the control step using soliton self-frequency shift to form one or a plurality of pulsed lights with different wavelengths, A pulse light generation method comprising the control step, which controls the time width of the pulse light so that the number of pulse light pulses formed in the modulation step becomes a predetermined number.

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