Pulse light generation device and pulse light generation method

Abstract

A pulse light generation device 1 comprises: an oscillator 2 for oscillating an ultrashort pulse light L; a stretcher fiber 7 for widening the time width of the ultrashort pulse light L; a fiber amplifier 8 for amplifying the ultrashort pulse light L the time width of which has been widened by the stretcher fiber 7; and a compressor 9 for compressing the time width of the ultrashort pulse light L which has been amplified by the fiber amplifier 8. The stretcher fiber 7 is configured by combining a first fiber 71 for widening the time width of the ultrashort pulse light L with a first characteristic, and a second fiber 72 for widening the time width of the ultrashort pulse light L with a second characteristic which is different from the first characteristic.

Description

Pulsed light generating device and pulsed light generating method

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

[0002] A pulsed light generating device is known that includes an oscillator for generating pulsed light and a modulator for modulating the wavelength of the pulsed light generated by the oscillator using soliton self-frequency shift. In such a pulsed light generating device, the intensity of the pulsed light before modulation by the modulator is increased, thereby splitting the pulsed light into multiple pulsed lights with different wavelengths (outputting multicolored solitons) through the modulation (see, for example, Patent Document 1).

[0003] Japanese Patent Application Laid-Open No. 2004-527001

[0004] Here, when an ultrashort pulsed light is used as the pulsed light and the output of the ultrashort pulsed light is strongly amplified, a technique called chirped pulse amplification may be used in which the time width of the ultrashort pulsed light is widened before amplification and then the time width is returned to its original state in order to avoid adverse effects due to nonlinear effects.

[0005] An object of the present disclosure is to provide an optical pulse generating apparatus and an optical pulse generating method that can appropriately perform chirped pulse amplification.

[0006] (1) A pulsed light generating device according to one aspect of the present disclosure includes an oscillator that oscillates pulsed light, a stretcher that expands the time width of the pulsed light, an amplifier that amplifies the pulsed light whose time width has been expanded by the stretcher, and a compressor that compresses the time width of the pulsed light amplified by the amplifier, wherein the stretcher is configured by combining a first fiber that expands the time width of the pulsed light with a first characteristic and a second fiber that expands the time width of the pulsed light with a second characteristic different from the first characteristic.

[0007] The present inventors have noted that, from the standpoint of stability and throughput, it is preferable to use a fiber for stretching and a diffraction grating pair for compression when performing chirped pulse amplification. However, when stretching and compression are performed in this manner, it is difficult to restore the ultrashort pulse light to its pre-stretched state after compression. In this regard, in a pulsed light generating device according to one aspect of the present invention, the stretcher is configured to include a first fiber and a second fiber with different characteristics. By combining fibers with different characteristics as the stretcher, for example, if only the first fiber is used as a stretcher, it may not be possible to restore the pulse light to its pre-stretched state after compression by the compressor. However, by further combining a second fiber with characteristics different from the first fiber, it is possible to bring the compressed pulse light closer to its pre-stretched state, depending on the setting of the second fiber's characteristics. In other words, because the conditions can be adjusted using two fibers with different characteristics, it is possible to restore the state of the pulse light (to its pre-stretched state), which is difficult to achieve using only one fiber. As described above, the pulsed light generating device disclosed herein can perform chirped pulse amplification appropriately.

[0008] (2) In the pulsed light generating device described in (1) above, the second characteristic of the second fiber may be set so that a value of an index related to group delay dispersion (GDD) of each wavelength remaining in the pulsed light approaches a predetermined value when the time width is expanded by the first fiber and then compressed by the compressor in a configuration in which the second fiber is not present. In this way, the second characteristic is set so that a value of an index related to group velocity dispersion of each wavelength remaining after stretching by the first fiber and compression by the compressor approaches a predetermined value, whereby the state of the pulsed light can be appropriately restored (returned to the state before stretching) by appropriately setting the predetermined value.

[0009] (3) In the pulsed light generating device described in (2) above, the second characteristic of the second fiber may be set so that, in a configuration in which the second fiber is not present, when the time width is expanded by the first fiber and then compressed by the compressor, a value of an index related to group delay dispersion (GDD) of each wavelength remaining in the pulsed light approaches 0. In this way, by setting the second characteristic so that a value of an index related to group velocity dispersion of each wavelength remaining after stretching by the first fiber and compression by the compressor approaches 0, it is possible to more appropriately restore the state of the ultrashort pulsed light (return it to the state before stretching).

[0010] (4) In the pulsed light generating device according to any one of (1) to (3), the compressor may include a diffraction grating pair. By using a diffraction grating pair as the compressor, which performs compression by utilizing the difference in optical path length due to diffraction, it is possible to suppress the occurrence of nonlinear effects that become a problem when, for example, a fiber is used as the compressor. When a diffraction grating pair is used as the compressor, it may be difficult to return the ultrashort pulsed light to its pre-stretched state after compression. However, as described above, in the configuration according to the present disclosure, the conditions can be adjusted using two fibers with different characteristics, so that the state of the pulsed light (returning it to its pre-stretched state) can be appropriately returned.

[0011] (5) In the pulsed light generating device according to any one of (1) to (4), the first fiber may be a normal dispersion fiber. By using a normal dispersion fiber, it is possible to appropriately widen the time width of the pulsed light while avoiding the occurrence of unexpected nonlinear effects.

[0012] (6) In the pulsed light generating device according to any one of (1) to (5), the wavelength band of the pulsed light whose time width is expanded by the stretcher and whose time width is compressed by the compressor may be 1800 nm to 2200 nm. With this configuration, chirped pulse amplification can be performed appropriately within the gain range of, for example, a thulium amplifier.

[0013] (7) A pulsed light generation method according to one aspect of the present disclosure includes an oscillation step of oscillating pulsed light, a stretching step of widening the time width of the pulsed light oscillated in the oscillation step with a first characteristic using a first fiber and further widening the time width with a second characteristic using a second fiber, an amplification step of amplifying the pulsed light whose time width has been widened in the stretching step, and a compression step of compressing the time width of the pulsed light amplified in the amplification step.

[0014] According to the present disclosure, it is possible to provide an optical pulse generating apparatus and an optical pulse generating method that can appropriately perform chirped pulse amplification.

[0015] FIG. 1 is a block diagram showing an optical pulse generating apparatus according to an embodiment. FIG. 2(a) is a graph showing the time waveform of an ultrashort optical pulse output from the oscillator of FIG. 1. FIG. 2(b) is a graph showing the spectrum of an ultrashort optical pulse output from the oscillator of FIG. 1. FIG. 2(c) is a graph showing the time waveform of an ultrashort optical pulse output from the fiber amplifier of FIG. 1. FIG. 2(d) is a graph showing the spectrum of an ultrashort optical pulse output from the fiber amplifier of FIG. 1. FIG. 3 is a graph showing a specific example of the spectrum of an ultrashort optical pulse output from the fiber amplifier of FIG. 1. FIG. 4(a) is a graph showing the time waveform of an ultrashort optical pulse output from the acousto-optic modulator of FIG. 1. FIG. 4(b) is a graph showing the spectrum of an ultrashort optical pulse output from the acousto-optic modulator of FIG. 1. FIG. 5(a) is a graph showing the time waveform of an ultrashort optical pulse output from the soliton-shifted fiber of FIG. 1. FIG. 5(b) is a graph showing the spectrum of an ultrashort optical pulse output from the soliton-shifted fiber of FIG. 1. FIG. 5(c) is a graph showing the time waveform of the ultrashort pulse light output from the filter of FIG. 1. FIG. 5(d) is a graph showing the spectrum of the ultrashort pulse light output from the filter of FIG. 1. FIG. 6 is a diagram explaining chirped pulse amplification. FIG. 7(a) is a diagram explaining a method of generating a difference in optical path length by a difference in refractive index when passing through a substance, and FIG. 7(b) is a diagram explaining a method of generating a difference in optical path length by diffraction. FIG. 8 is a schematic diagram of a configuration related to chirped pulse amplification. FIG. 9 is a schematic diagram of a configuration related to chirped pulse amplification that widens the time width using a single fiber. FIG. 10(a) is a graph showing the characteristics of the stretcher and compressor in the configuration of FIG. 9, and FIG. 10(b) is a graph showing the residual GDD in the configuration of FIG. 9. Fig. 11(a) is a graph showing the characteristics of the first fiber in the configuration of Fig. 6, Fig. 11(b) is a graph showing the characteristics of the compressor in the configuration of Fig. 6, Fig. 11(c) is a graph showing the residual GDD due to the first fiber and the compressor, and Fig. 11(d) is a graph showing the characteristics of the second fiber to be combined with the first fiber. Fig. 12 is a flowchart showing a pulsed light generation method according to an embodiment.FIG. 13( a) is a graph showing the characteristics of a first fiber according to a modified example, FIG. 13( b) is a graph showing the characteristics of a compressor according to a modified example, FIG. 13( c) is a graph showing the residual GDD due to the first fiber and compressor, FIG. 13( d) is a graph showing the characteristics of a second fiber combined with the first fiber, and FIG. 13( e) is a graph showing the characteristics of a third fiber combined with the first fiber and the second fiber.

[0016] Hereinafter, the embodiments will be described in detail with reference to the drawings. In each drawing, the same or corresponding parts are designated by the same reference numerals, and duplicated explanations will be omitted.

[0017] 1 , a pulsed light generating apparatus 1 of this embodiment generates long-wavelength ultrashort pulsed light (pulsed light) L by utilizing soliton self-frequency shift (Raman soliton shift). For example, the pulsed light generating apparatus 1 is a femtosecond laser device used in, for example, a two-photon microscope. The pulsed light generating apparatus 1 includes an oscillator 2, a fiber amplifier 3, an acousto-optic modulator 4, a compressor 5, a soliton-shifted fiber 6, a stretcher fiber 7, a fiber amplifier 8, a compressor 9, and a wavelength conversion unit 10.

[0018] The oscillator 2 constitutes an oscillation unit that oscillates ultrashort pulsed light L. As shown in FIG. 2( a), the oscillator 2 generates an ultrashort pulse train with a predetermined period F1. The wavelength band of the ultrashort pulsed light L oscillated from the oscillator 2 may be a band including 1550 nm, for example. As shown in FIG. 2( b), the oscillator 2 here oscillates ultrashort pulsed light L having a spectrum with a first spectral width H1 and a first intensity K1. The oscillator 2 is not particularly limited, and various oscillators can be used.

[0019] The fiber amplifier 3 constitutes an amplifying section that broadens the spectrum of the ultrashort pulsed light L oscillated by the oscillator 2. The fiber amplifier 3 broadens the spectrum of the ultrashort pulsed light L by similariton amplification and also increases the output power of the ultrashort pulsed light L. The fiber amplifier 3 is disposed between the oscillator 2 and the soliton-shifted fiber 6 in the optical path of the ultrashort pulsed light L.

[0020] The fiber amplifier 3 includes a fiber amplifier. The fiber amplifier of the fiber amplifier 3 is a normal dispersion fiber, which is a double-clad fiber co-doped with erbium and ytterbium. That is, the fiber amplifier 3 performs amplification while causing a nonlinear effect by the normal dispersion double-clad fiber so as not to stretch, and obtains ultrashort pulsed light L as broadband amplified light. The normal dispersion fiber is a fiber in which the dispersion parameter D (ps / nm / km) is negative. There are no particular restrictions on the dopant used in the fiber amplifier 3, and various dopant types may be used.

[0021] 2(c) and 2(d), the fiber amplifier 3 broadens the spectral width of the ultrashort pulsed light L to a second spectral width H2 that is wider than the first spectral width H1. The fiber amplifier 3 increases the intensity of the ultrashort pulsed light L to a second intensity K2 that is higher than the first intensity K1. Specifically, as shown in Fig. 3, the fiber amplifier 3 sets the spectral width of the ultrashort pulsed light L to 100 nm or more. In Fig. 3, the horizontal axis represents the wavelength of the ultrashort pulsed light L, and the vertical axis represents the intensity of the ultrashort pulsed light L relative to a predetermined intensity.

[0022] The acousto-optic modulator 4 constitutes a light intensity control unit that controls the intensity of the ultrashort pulsed light L for each pulse. The acousto-optic modulator 4 is a device that modulates the ultrashort pulsed light L by utilizing the force of acoustics (sound waves) 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-shifted fiber 6 in the optical path of the ultrashort pulsed light L. Note that the acousto-optic modulator 4 may be disposed at any position between the oscillator 2 and the soliton-shifted fiber 6. As shown in FIGS. 4( a) and 4(b), the acousto-optic modulator 4 controls the intensity of the ultrashort pulsed light L so that it changes for each pulse. For example, when intensity modulations M1 and M2 are applied as shown in FIG. 4(a), ultrashort pulsed light LM1 and LM2 corresponding to the intensities given by M1 and M2 are generated as shown in FIG. 4(b). The range and precision of intensity modulation of the ultrashort pulsed light L (LM1, LM2) depend on the performance of the acousto-optic modulator 4. The intensity of each pulse light in the pulse train of the ultrashort pulsed light L can be modulated arbitrarily by the acousto-optic modulator 4.

[0023] The compressor 5 constitutes a pulse compression section that compresses the time width of the pulses of the ultrashort pulsed light L. In this embodiment, the compressor 5 is disposed between the acousto-optic modulator 4 and the soliton-shifted fiber 6 in the optical path of the ultrashort pulsed light L. The compressor 5 may be disposed at any position between the fiber amplifier 3 and the soliton-shifted fiber 6. Even if the ultrashort pulsed light L is stretched (for example, by several picoseconds) by the fiber amplifier 3, the compressor 5 compresses the time width of the ultrashort pulsed light L and outputs ultrashort pulsed light L with a time width spread of a certain amount or less (less than 1 picosecond). The compressor 5 is not particularly limited, and various compressors can be used.

[0024] The soliton-shifted fiber 6 constitutes a modulation section that utilizes soliton self-frequency shift to modulate the wavelength of the ultrashort pulsed light L, the output of which has been increased while broadening its spectrum in the fiber amplifier 3. The soliton-shifted fiber 6 is disposed downstream of the fiber amplifier 3 in the optical path of the ultrashort pulsed light L. As shown in FIGS. 5( a) and 5(b), the soliton-shifted fiber 6 lengthens the wavelength of the ultrashort pulsed light L to generate soliton S1. The wavelength band of the lengthened ultrashort pulsed light L may be, for example, a band including 1800 nm to 2200 nm. The soliton-shifted fiber 6 may be, for example, a single-mode anomalous dispersion fiber that exhibits anomalous dispersion in the wavelength band of the ultrashort pulsed light L generated in the fiber amplifier 3. In addition, by controlling the acousto-optic modulator 4, solitons with wavelengths different from the soliton S1 can be generated. For example, when the wavelength of the soliton S is modulated by intensities M1 and M2 as shown in FIG. 5C, the soliton S shifts to a wavelength corresponding to the intensities M1 and M2 (solitons S1 and S2) as shown in FIG. 5D. The range and precision of the wavelength shift of the soliton S depend on the performance of the acousto-optic modulator 4. The shift wavelength of each soliton S in the soliton train generated from the pulse train of the ultrashort optical pulse L can be arbitrarily changed by applying intensity modulation to the pulse train using the acousto-optic modulator 4. In the illustrated example, the ultrashort optical pulse L modulated by the soliton self-frequency shift contains a non-soliton component S0 (a component that does not become soliton S1 or S2). A filter (not shown) that cuts the non-soliton component S0 of the ultrashort optical pulse L may be provided downstream of the soliton shift fiber 6. Such a filter may have an OD value of 3 or greater.

[0025] The stretcher fiber 7, the fiber amplifier 8, and the compressor 9 constitute a configuration related to chirped pulse amplification. The configuration related to chirped pulse amplification will be described with reference to FIGS.

[0026] First, chirped pulse amplification will be described. FIG. 6 is a diagram illustrating chirped pulse amplification. When strongly amplifying the output of ultrashort optical pulses, a process called chirped pulse amplification may be performed to prevent adverse effects caused by excessively high peak power and nonlinear effects. In chirped pulse amplification, the time width of ultrashort optical pulses is first expanded (stretched), the stretched ultrashort optical pulses are then amplified, and finally, a compression process is performed to restore the time width of the amplified ultrashort optical pulses to their original state (before stretching). As shown in FIG. 6 , in chirped pulse amplification, the time width of pulsed light I, which is the ultrashort optical pulse L input to the stretcher fiber 7, is first expanded by the stretcher fiber 7. Then, pulsed light II, which is the ultrashort optical pulse L whose time width has been expanded by the stretcher fiber 7, is amplified by the fiber amplifier 8. Finally, pulsed light III, which is the ultrashort optical pulse L amplified by the fiber amplifier 8, is compressed by the compressor 9 to restore its time width to its state before stretching.

[0027] Next, two examples of methods for expanding (or compressing) the time width in chirped pulse amplification will be described. In both examples, the time width is expanded (or compressed) by creating a difference in optical path length. FIG. 7( a) is a diagram illustrating a method for creating a difference in optical path length by using a difference in refractive index when passing through a material, and FIG. 7( b) is a diagram illustrating a method for creating a difference in optical path length by using diffraction. As shown in FIG. 7( a), when ultrashort pulse light, in which light of multiple wavelengths is superimposed and phase-matched with each other, passes through some material, differences in the refractive index of each wavelength cause differences in the optical path length of each wavelength, which changes the group delay time of each wavelength. By utilizing this, the time width can be expanded or, conversely, compressed. Here, the material is, for example, fiber. Furthermore, as shown in FIG. 7( b), differences in the optical path length of each wavelength are created by a diffraction grating pair 91, 92 (described in detail later), which changes the group delay time of each wavelength. This allows the time width to be expanded or, conversely, compressed.

[0028] In such chirped pulse amplification, when returning the expanded time width of an ultrashort pulse light to its original state, it is preferable to compress the time width in the same way as when the time width was expanded. From this perspective, it is possible to perform stretching and compression using the same method. That is, for example, when using a method in which the optical path length of each wavelength is varied by passing the light through a fiber due to differences in refractive index (see FIG. 7( a)), fibers are used as both the stretcher and compressor, and when using a method in which the optical path difference of each wavelength is varied using a diffraction grating pair (see FIG. 7( b)), a diffraction grating pair is used as both the stretcher and compressor. In this way, by performing stretching and compression using the same type of optical element, the characteristics of stretching and compression (described in detail below) can be matched, and the ultrashort pulse light can be compressed by the same amount as the expanded time width.

[0029] On the other hand, when optimizing the stretcher and compressor from the perspective of stability and throughput, it may be advantageous to use separate stretchers and compressors. For example, when stretching and compressing a fiber, higher throughput (typically nearly zero loss) can be expected for stretching compared to a diffraction grating pair. However, when compressing, nonlinear effects may occur, resulting in light that does not have the expected characteristics. Specifically, complex dispersion may be introduced into the ultrashort pulse light, making it impossible to fully compress the ultrashort pulse light. As a result, the energy that should be concentrated in the main pulse is dispersed to other parts, creating an undesirable state for nonlinear effects. On the other hand, when stretching and compressing a fiber using a diffraction grating pair, for example, nonlinear effects are less likely to occur for compression because the ultrashort pulse light propagates in a spatial system, increasing the beam diameter and reducing energy concentration. Regarding stretching, because diffraction gratings typically have a diffraction efficiency of approximately 90%, a diffraction grating pair with multiple diffraction gratings loses energy with each diffraction, resulting in a final throughput of approximately 60% of the original energy.

[0030] Considering these factors, a configuration using a fiber as a stretcher and a diffraction grating pair as a compressor can be considered to suppress the occurrence of nonlinear effects while maintaining throughput. In a configuration in which stretching and compression are performed using different methods (using different types of optical elements), it is more difficult to match the stretching and compression characteristics (described in detail below) compared to a configuration in which stretching and compression are performed using the same method (using the same types of optical elements), making it more difficult to restore the ultrashort pulse light to its pre-stretched state after compression. This problem becomes even more serious in broadband light sources and broadly wavelength-tunable light sources. To address this issue, the chirped pulse amplification configuration of the pulsed light generating apparatus 1 according to this embodiment includes two fibers (a first fiber 71 and a second fiber 72) as the stretcher fiber 7, as shown in FIG. 8 .

[0031] 8 is a schematic diagram of a configuration related to chirped pulse amplification in a pulsed light generating apparatus 1 according to this embodiment. As shown in FIG. 8 , the pulsed light generating apparatus 1 includes a stretcher fiber 7 (stretcher), a fiber amplifier 8 (amplifying unit), and a compressor 9 as components related to chirped pulse amplification.

[0032] The stretcher fiber 7 is a stretcher that expands the time width of the ultrashort pulsed light L. The wavelength band of the ultrashort pulsed light L whose time width is expanded by the stretcher fiber 7 is, for example, 1800 nm to 2200 nm. The stretcher fiber 7 is configured by combining a first fiber 71 that expands the time width of the ultrashort pulsed light L with a first characteristic and a second fiber 72 that expands the time width of the ultrashort pulsed light L with a second characteristic different from that of the first fiber 71. The first fiber 71 and the second fiber 72 are configured to expand the time width of the ultrashort pulsed light L by generating a difference in the optical path length of each wavelength due to a difference in the refractive index of each wavelength when the ultrashort pulsed light L passes through them. The first fiber 71 expands the time width of the ultrashort pulsed light L including, for example, a wavelength band of 1800 nm to 2200 nm, output from the soliton shift fiber 6, with the first characteristic, and outputs the expanded time width to the second fiber 72. The first fiber 71 may be, for example, a normal dispersion fiber. The second fiber 72 is connected to the first fiber 71, and expands the time width of the ultrashort pulsed light L input from the first fiber 71 using a second characteristic, and outputs the expanded light to the fiber amplifier 8. The second fiber 72 may be, for example, a normal dispersion or anomalous dispersion fiber. The first characteristic and the second characteristic will be described in detail later.

[0033] The fiber amplifier 8 amplifies (increases the output power of) the pulsed light whose time width has been expanded by the stretcher fiber 7. The fiber amplifier 8 includes a fiber amplifier. The fiber amplifier of the fiber amplifier 8 is an anomalous dispersion fiber, for example, a thulium-doped fiber. The laser medium doped in the fiber of the fiber amplifier 8 is not particularly limited, and may be a rare earth element such as ytterbium, erbium, or neodymium, or may be Bi, etc.

[0034] The compressor 9 compresses the time width of the ultrashort pulsed light L amplified by the fiber amplifier 8. The compressor 9 has a diffraction grating pair 91, 92. The diffraction grating pair 91, 92 is configured to compress the time width by varying the group delay time of each wavelength by causing a difference in the optical path length of each wavelength. The wavelength band of the ultrashort pulsed light L1, whose time width is compressed by the diffraction grating pair 91, 92, is, for example, 1800 nm to 2200 nm.

[0035] Next, a method for determining the first characteristic of the first fiber 71, the second characteristic of the second fiber 72, and the characteristic (compression characteristic) of the diffraction grating pair 91, 92 will be described.

[0036] The first and second characteristics described above determine how the ultrashort pulsed light L expands. The compression characteristic determines how the ultrashort pulsed light L is compressed. The first, second, and compression characteristics are represented, for example, by information related to group velocity dispersion (GVD). The group velocity is the speed of a wave packet associated with the ultrashort pulsed light L. The group velocity dispersion is the chromatic dispersion of the group velocity. The information related to group velocity dispersion may be the group velocity dispersion itself, group delay dispersion (GDD) obtained by multiplying the group velocity dispersion by the length of the medium (fiber length or the distance between diffraction grating pairs) in consideration of the length, or a third order group delay dispersion (TOD / GVD) ratio, which is the ratio of the group velocity dispersion to the third order group delay dispersion (TOD), which is obtained by differentiating the group velocity dispersion with respect to frequency.

[0037] The above-described characteristics are determined, for example, by the following procedure. First, in a configuration as shown in FIG. 9 , in which the second fiber 72 is not present, the residual group delay dispersion is derived when the time width of the ultrashort pulsed light L is expanded by the first fiber 71, the ultrashort pulsed light L is amplified by the fiber amplifier 8, and the time width of the ultrashort pulsed light L is compressed by the diffraction grating pair 91, 92. In this case, the first characteristic of the first fiber 71 is the group delay dispersion G1 of the first fiber 71 (see FIG. 10( a)), which is determined by arbitrarily determining the fiber length. Furthermore, the characteristic of the diffraction grating pair 91, 92 is the group delay dispersion G2 of the diffraction grating pair 91, 92 (see FIG. 10( a)), which is determined by arbitrarily determining the distance l between the diffraction grating pair 91, 92 (see FIG. 7( b)). Specifically, the group delay dispersion G2 of the diffraction grating pair 91, 92 may be derived by the following equation (1). In the following equation (1), λ is the wavelength, c is the speed of light, d is the grating constant, l is the distance between the diffraction grating pair 91 and 92, and θ is the angle of incidence of light shown in Figure 7(b). Group delay dispersion G2(λ) = -λ３ l / πc ２ d ２ [1-(λ / d-sinθ) ２ ]・・・(1)

[0038] The group delay dispersion G1 of the first fiber 71 and the group delay dispersion G2 of the second fiber 72 are then added together to derive the residual group delay dispersion G3 (see FIG. 10( b)). In FIGS. 10( a) and 10(b), the horizontal axis represents wavelength, and the vertical axis represents the value of group delay dispersion. As shown in FIG. 10( b), the residual group delay dispersion G3 of the portion of the ultrashort pulse light L1 at wavelength X1 is zero, realizing the restoration of the state of the ultrashort pulse light L1 (returning to the state before stretching) and achieving appropriate compression. On the other hand, as shown in FIG. 10( b), the residual group delay dispersion G3 of the portion of the ultrashort pulse light L1 at wavelength X2 is a value that deviates from zero, preventing the restoration of the state of the ultrashort pulse light L1 (returning to the state before stretching) and resulting in appropriate compression.

[0039] Then, with the group delay dispersion G1 (see FIG. 11(a)), which is the first characteristic, the group delay dispersion G2 (see FIG. 11(b)), which is the compression characteristic, and the residual group delay dispersion G3 (see FIG. 11(c)) being determined, the group delay dispersion G4 (see FIG. 11(d)), which is the second characteristic of the second fiber 72, is set so that the value of the residual group delay dispersion G3 approaches a desired value (for example, 0). Specifically, the group delay dispersion G4 may be set to a 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.

[0040] As described above, the second characteristic of the second fiber 72 is set so that when the time width is expanded by the first fiber 71 and then compressed by the diffraction grating pair 91, 92 in a configuration in which the second fiber 72 is not present, 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 approaches a predetermined value, preferably approaches 0.

[0041] 1, the wavelength conversion unit 10 is configured to perform wavelength conversion for each pulse of the ultrashort pulsed light L output from the compressor 9. The wavelength conversion unit 10 may perform wavelength conversion for each pulse to a wavelength band of 900 nm to 1100 nm, for example.

[0042] Next, a pulsed light generating method carried out using the pulsed light generating device 1 will be described with reference to the flowchart of FIG.

[0043] First, an oscillator 2 oscillates an ultrashort pulse light L to generate an ultrashort pulse train with a predetermined period (oscillation step: step S1). A fiber amplifier 3 increases the output power of the ultrashort pulse light L and broadens the spectrum of the ultrashort pulse light L (step S2). An acousto-optic modulator 4 controls the intensity of the ultrashort pulse light L for each pulse, depending on, for example, the specifications or conditions required for the pulsed light generating device 1 (step S3).

[0044] Next, the compressor 5 compresses the time width of the ultrashort pulse light L (step S4). The soliton shift fiber 6 modulates the wavelength of the spectrally broadened ultrashort pulse light L using the soliton self-frequency shift to lengthen the wavelength (step S5).

[0045] Subsequently, the time width of the ultrashort pulsed light L oscillated in the oscillation step is expanded by the first fiber 71 with a first characteristic, and expanded by the second fiber 72 with a second characteristic (stretch step: step S6).

[0046] Subsequently, the ultrashort pulsed light L whose time width has been expanded in the stretching step is amplified by the fiber amplifier 8 (amplification step: step S7).

[0047] Subsequently, the diffraction grating pair 91, 92 of the compressor 9 compresses the time width of the ultrashort pulsed light L amplified in the amplification step (compression step: step S8).

[0048] Finally, the wavelength conversion unit 10 performs wavelength conversion on the ultrashort pulsed light L for each pulse (step S9).

[0049] Next, the effects of the pulsed light generating device 1 according to this embodiment will be described.

[0050] The pulsed light generating device 1 according to this embodiment includes an oscillator 2 that oscillates ultrashort pulsed light L, a stretcher fiber 7 that expands the temporal width of the ultrashort pulsed light L, a fiber amplifier 8 that amplifies the ultrashort pulsed light L whose temporal width has been expanded by the stretcher fiber 7, and a compressor 9 that compresses the temporal width of the ultrashort pulsed light L amplified by the fiber amplifier 8, and the stretcher fiber 7 is configured by combining a first fiber 71 that expands the temporal width of the ultrashort pulsed light L with a first characteristic and a second fiber 72 that expands the temporal width of the ultrashort pulsed light L with a second characteristic different from the first characteristic.

[0051] The inventors have noted that, from the standpoint of stability and throughput, it is preferable to perform chirped pulse amplification by stretching using a fiber and compressing using a diffraction grating pair. However, when stretching and compression are performed in this manner, it is difficult to restore the ultrashort pulse light L to its pre-stretched state after compression. In this regard, in the pulsed light generating device 1 according to this embodiment, the stretcher fiber 7 is configured to include a first fiber 71 and a second fiber 72 having different characteristics. By combining fibers with different characteristics as the stretcher fiber 7, for example, if only the first fiber 71 is used as a stretcher, it may not be possible to restore the ultrashort pulse light L to its pre-stretched state after compression by the compressor 9. However, by further combining the second fiber 72 having characteristics different from those of the first fiber 71, it is possible to bring the ultrashort pulse light L after compression closer to its pre-stretched state, depending on the setting of the second characteristic. In other words, because the conditions can be adjusted using two fibers 71 and 72 with different characteristics, it is possible to restore the ultrashort pulse light L (to its pre-stretched state), which would be difficult using only one fiber. As described above, the pulsed light generating apparatus 1 according to this embodiment can appropriately perform chirped pulse amplification.

[0052] The second characteristic of the second fiber 72 may be set so that, in a configuration in which the second fiber 72 is not present, when the time width is expanded by the first fiber 71 and then compressed by the compressor 9, the value of an index related to group delay dispersion (GDD) of each wavelength remaining in the ultrashort pulsed light L approaches a predetermined value. In this way, by setting the second characteristic so that the value of the index related to the group velocity dispersion of each wavelength remaining after stretching by the first fiber 71 and compression by the compressor 9 approaches a predetermined value, it is possible to appropriately restore the state of the ultrashort pulsed light L (return to the state before stretching) by appropriately setting the predetermined value.

[0053] The second characteristic of the second fiber 72 may be set so that, in a configuration in which the second fiber 72 is not present, when the time width is expanded by the first fiber 71 and then compressed by the compressor 9, the value of an index related to group delay dispersion (GDD) of each wavelength remaining in the ultrashort pulsed light L approaches 0. In this way, by setting the second characteristic so that the value of an index related to group velocity dispersion of each wavelength remaining after stretching by the first fiber 71 and compression by the compressor 9 approaches 0, it is possible to more appropriately restore the state of the ultrashort pulsed light L (to the state before stretching).

[0054] The compressor 9 may have a diffraction grating pair 91, 92. By using the diffraction grating pair 91, 92, which performs compression by utilizing the difference in optical path length due to diffraction, as the compressor 9, it is possible to suppress the occurrence of nonlinear effects that become a problem when, for example, a fiber is used as the compressor. If the diffraction grating pair 91, 92 is used as the compressor 9, it may be difficult to return the ultrashort pulsed light L to the state before it was stretched after compression. However, as described above, in the configuration according to this embodiment, the conditions can be adjusted using the two fibers 71, 72, which have different characteristics, and therefore it is possible to appropriately return the state of the ultrashort pulsed light L (to the state before it was stretched).

[0055] The first fiber 71 may be a normal dispersion fiber, which can appropriately widen the time width of the pulsed light while avoiding the occurrence of unexpected nonlinear effects.

[0056] The wavelength band of the ultrashort pulsed light L whose time width is expanded by the stretcher fiber 7 and whose time width is compressed by the compressor 9 may be 1800 nm to 2200 nm. With this configuration, chirped pulse amplification can be performed appropriately within the gain range of the thulium amplifier, for example.

[0057] 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 light in the 1800-2200 nm region is appropriately restored, improving the conversion efficiency of the subsequent wavelength conversion to 900-1100 nm. This makes it possible to induce two-photon fluorescence using higher-power light. This in turn makes it possible to obtain fluorescence with higher brightness. Furthermore, because the broadened pulses of light in the 1800-2200 nm range can be restored without changing the inter-grating distance of the diffraction grating pair, application to a light source capable of tunable wavelengths for each pulse becomes possible.

[0058] As described above, one aspect of the present disclosure is not limited to the above embodiment.

[0059] For example, although an example in which the stretcher fiber 7 is configured by two fibers 71 and 72 has been described, the present invention is not limited to this, and the stretcher fiber may be configured by combining, for example, three or more fibers.

[0060] In this case, for example, with the group delay dispersion G21 (see FIG. 13( a)) which is the first characteristic of the first fiber, the group delay dispersion G22 (see FIG. 13( b)) which is the compression characteristic of the diffraction grating pair, and the residual group delay dispersion G23 (see FIG. 13( c)) being determined, the group delay dispersion G24 (see FIG. 13( d)) and the group delay dispersion G25 (see FIG. 13( e)) which are second characteristics of the two fibers constituting the second fiber are set so that the value of the residual group delay dispersion G23 approaches a desired value (for example, 0).

[0061] 1... pulsed light generating device, 2... oscillator (oscillating section), 7... stretcher fiber (stretcher), 8... fiber amplifier (amplifying section), 9... compressor, 71... first fiber, 72... second fiber, 91, 92... diffraction grating pair.

Claims

1. A pulsed light generating device comprising: an oscillator that oscillates pulsed light; a stretcher that expands the time width of the pulsed light; an amplifier that amplifies the pulsed light whose time width has been expanded by the stretcher; and a compressor that compresses the time width of the pulsed light amplified by the amplifier, wherein the stretcher is configured by combining a first fiber that expands the time width of the pulsed light with a first characteristic and a second fiber that expands the time width of the pulsed light with a second characteristic different from the first characteristic.

2. A pulsed light generating device as described in claim 1, wherein the second characteristic of the second fiber is set so that when the time width is expanded by the first fiber and then compressed by the compressor in a configuration in which the second fiber is not present, the value of an index related to group delay dispersion (GDD) of each wavelength remaining in the pulsed light approaches a predetermined value.

3. A pulsed light generating device as described in claim 2, wherein the second characteristic of the second fiber is set so that when the time width is expanded by the first fiber and then compressed by the compressor in a configuration in which the second fiber is not present, the value of an index related to group delay dispersion (GDD) of each wavelength remaining in the pulsed light approaches zero.

4. A pulsed light generating device according to any one of claims 1 to 3, wherein the compressor has a diffraction grating pair.

5. A pulsed light generating device according to any one of claims 1 to 3, wherein the first fiber is a normal dispersion fiber.

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

7. A method for generating pulsed light, comprising: an oscillation step of oscillating pulsed light; a stretching step of widening the time width of the pulsed light oscillated in the oscillation step with a first characteristic using a first fiber, and further widening it with a second characteristic using a second fiber; an amplification step of amplifying the pulsed light whose time width has been widened in the stretching step; and a compression step of compressing the time width of the pulsed light amplified in the amplification step.

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