Pulse light generation device and pulse light generation method

Abstract

A pulse light generation device (1A) includes: an oscillator (oscillator) (2) that causes oscillation of pulsed light as seed light (L0); a fiber amplifier (broadbanding unit) (3) that broadens the spectrum of the seed light (L0) caused to oscillate by the oscillator (2); a soliton shift fiber (wavelength modulation unit) (7) that uses a soliton self-frequency shift to modulate the wavelength of the seed light (L0) whose spectrum has been broadened by the fiber amplifier (3), thereby generating dispersed waves (LD) and soliton waves (LS) originating from the seed light (L0); and a parametric amplification unit (12) that performs parametric amplification of relatively long-wavelength pulsed light by using relatively short-wavelength pulsed light from among the soliton waves (LS), dispersed waves (LD), seed light (L0), and their harmonics.

Description

Pulse Light Generation Device and Pulse Light Generation Method

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

[0002] A pulse light generation device including an oscillation unit that oscillates pulse light and a modulation unit that modulates the wavelength of the pulse light oscillated by the oscillation unit using soliton self-frequency shift is known (see, for example, Patent Document 1). In such a pulse light generation device, by amplifying the pulse light before it is input to the modulation unit, in the modulation unit, the input pulse light can be split into a plurality of pulse lights (multi-colored soliton waves) having different wavelengths from each other.

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

[0004] The pulse light generation device as described above can be applied, for example, as a light source for 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 an ultrashort pulse laser light in the near-infrared region and detects a signal (fluorescence) from the sample to perform image analysis. When considering application as a light source for such a device, in the method used in a conventional device, after amplifying the seed light with a fiber amplifier, the amplified light is converted to an optimal wavelength. However, in the fiber amplifier at that time, problems such as a decrease in the signal-to-noise ratio due to amplified spontaneous emission (ASE) and distortion of the spectrum of the output light occur.

[0005] For example, in the wavelength range of 1800 nm to 2100 nm, a thulium-doped fiber amplifier is used as an amplifier for pulse light. Considering the influence of amplified spontaneous emission light, it is conceivable that the wavelength region that can be amplified is limited to a very small region. To avoid this, if a filter for cutting amplified spontaneous emission light is provided and the fiber amplifier is configured in multiple stages, there is a problem that the device configuration becomes complicated.

[0006] The present disclosure has been made to solve the above problems, and an object thereof is to provide a pulse light generation device and a pulse light generation method capable of generating amplified light of pulse light in a wide wavelength region.

[0007] The gist of this disclosure is as follows:

[0008] [1] A pulse light generation device comprising: an oscillation unit that oscillates pulse light as seed light; a broadbanding unit that broadens the spectrum of the seed light oscillated by the oscillation unit; a wavelength modulation unit that modulates the wavelength of the seed light whose spectrum has been broadened by the broadbanding unit using soliton self-frequency shift to generate soliton waves and dispersion waves originating from the seed light; and a parametric amplification unit that parametrically amplifies relatively long wavelength pulse light using relatively short wavelength pulse light from among the soliton waves, dispersion waves, seed light, and their harmonics.

[0009] This pulsed light generator broadens the spectrum of the seed light and modulates the wavelength of the broadened seed light using soliton self-frequency shift, thereby converting the soliton wave originating from the seed light into a single soliton. Dispersive waves are also generated during the generation of soliton waves originating from the seed light. Furthermore, this pulsed light generator parametrically amplifies relatively long-wavelength pulsed light using relatively short-wavelength pulsed light from among soliton waves, dispersive waves, seed light, and their harmonics. By using such parametric amplification, unlike conventional amplification using fiber amplifiers, the light to be amplified can be selected from soliton waves, dispersive waves, and seed light, thus enabling the generation of amplified pulsed light over a wide wavelength range.

[0010] [2] The pulsed light generation apparatus according to [1], further comprising an intensity modulation unit that modulates the intensity of the seed light whose spectrum has been broadened by the broadband unit for each pulse. In this case, by modulating the intensity of the seed light for each pulse, it is possible to make the wavelength of the soliton wave or the wavelength of the dispersion wave variable for each pulse.

[0011] [3] The pulse light generation apparatus according to [1] or [2], further comprising a time width compression unit for compressing the time width of the seed light whose spectrum has been broadened in the broadband unit. In this case, the broadening of the time width of the seed light due to the broadbanding of the spectrum in the broadband unit can be corrected.

[0012] [4] A pulse light generation apparatus according to any one of [1] to [3], further comprising a filter for attenuating components of the light components generated in the wavelength modulation section that are not used for the parametric amplification. In this case, components that are not needed for parametric amplification can be attenuated in advance from the generated pulse light.

[0013] [5] A pulse light generation apparatus according to any one of [1] to [4], further comprising a branching section that separates the optical path into the relatively long-wavelength pulse light and the relatively short-wavelength pulse light. In this case, the processing required for the relatively long-wavelength pulse light and the relatively short-wavelength pulse light can be individually performed in each optical path. In addition, by separating the optical paths, it becomes easier to handle the output light finally obtained after parametric amplification.

[0014] [6] The pulse light generating apparatus according to [5], wherein an amplification unit for amplifying the pulse light is arranged in the optical path of the relatively short wavelength pulse light that has been branched at the branching unit. By amplifying the relatively short wavelength pulse light, it is possible to increase the amplification efficiency during parametric amplification.

[0015] [7] The pulse light generating apparatus according to [5] or [6], wherein an amplification unit for amplifying the pulse light is arranged in the optical path of the relatively long wavelength pulse light that has been branched at the branching unit. By amplifying the relatively long wavelength pulse light, the intensity of the output light finally obtained by parametrically amplifying the pulse light can be further increased.

[0016] [8] A pulse light generation method comprising: an oscillation step of oscillating pulse light as seed light; a broadbanding step of broadening the spectrum of the seed light oscillated in the oscillation step; a wavelength modulation step of modulating the wavelength of the seed light whose spectrum has been broadened in the broadbanding step using soliton self-frequency shift to generate soliton waves and dispersion waves originating from the seed light; and a parametric amplification step of parametrically amplifying relatively long wavelength pulse light using relatively short wavelength pulse light among the soliton waves, dispersion waves, seed light, and their harmonics.

[0017] In this pulsed light generation method, the spectrum of the seed light is broadened, and the wavelength of the broadened seed light is modulated using soliton self-frequency shift, thereby converting the soliton wave originating from the seed light into a single soliton. Dispersive waves are also generated during the generation of soliton waves originating from the seed light. Furthermore, in this pulsed light generation method, relatively short-wavelength pulsed light is parametrically amplified using relatively long-wavelength pulsed light from among the soliton wave, dispersion wave, seed light, and their harmonics. By using such parametric amplification, unlike conventional amplification using fiber amplifiers, the light to be amplified can be selected from soliton waves, dispersion waves, and seed light, thus enabling the generation of amplified pulsed light over a wide wavelength range.

[0018] According to this disclosure, amplified light from pulsed light can be generated over a wide wavelength range.

[0019] This is a schematic diagram illustrating the outline of a pulsed light generation device relating to one aspect of the present disclosure. This is a block diagram of a pulsed light generation device according to the first embodiment. (a) is a schematic diagram showing the time waveform of an ultrashort pulse train (seed light) output from an oscillator, (b) is a schematic diagram showing the spectrum of the ultrashort pulse light constituting the ultrashort pulse train in (a), (c) is a schematic diagram showing the time waveform of an ultrashort pulse train (seed light) output from a fiber amplifier, and (d) is a schematic diagram showing the spectrum of the ultrashort pulse light constituting the ultrashort pulse train in (c). This is a graph showing an example of the spectrum of a broadbanded seed light. (a) is a schematic diagram showing the time waveform of an ultrashort pulse train (seed light) output from an acousto-optic modulator, and (b) is a schematic diagram showing the spectrum of the ultrashort pulse light constituting the ultrashort pulse train in (a). (a) is a schematic diagram showing the spectra of soliton waves and dispersed waves output from a soliton-shift fiber, (b) is a graph showing an example thereof, and (c) is a schematic diagram showing the wavelength control of the dispersed wave for each pulse. This is a schematic diagram showing the spectrum of dispersed waves output from a fiber amplifier. This is a flowchart of a pulsed light generation method according to one embodiment of the present disclosure. This is a graph showing the amplification efficiency in a pulsed light generation device according to the first embodiment. This is a block diagram of a pulsed light generation device according to the second embodiment. This is a graph showing the amplification efficiency in a pulsed light generation device according to the second embodiment. This is a block diagram of a pulsed light generation device according to the third embodiment. This is a graph showing the amplification efficiency in a pulsed light generation device according to the third embodiment.

[0020] Hereinafter, with reference to the drawings, preferred embodiments of a pulsed light generation apparatus and a pulsed light generation method relating to one aspect of this disclosure will be described in detail. [Outline of the pulsed light generation apparatus]

[0021] First, an overview of a pulsed light generation device relating to one aspect of this disclosure will be provided. The pulsed light generation device is a device that generates ultrashort pulsed light as output light by utilizing soliton self-frequency shift (Raman soliton shift). The pulsed light generation device can be applied as a light source for optical measuring devices and optical inspection devices, such as microscopes, and especially two-photon microscopes. A two-photon microscope is a type of laser scanning fluorescence microscope that excites fluorescent dyes in a sample by incidenting ultrashort pulsed laser light in the near-infrared region onto the sample, and detects the signal (fluorescence) from the sample to perform image analysis.

[0022] In a pulsed light generator, a seed light, which is an ultrashort pulsed light, is wavelength-modulated by soliton self-frequency shifting to generate soliton waves and dispersed waves from the seed light, as shown in Figure 1. In generating output light, the pulsed light generator parametrically amplifies relatively long-wavelength pulsed light using relatively short-wavelength pulsed light from among the soliton waves, dispersed waves, seed light, and their harmonics.

[0023] In parametric amplification, energy is essentially transferred from short-wavelength light to long-wavelength light. In the case of its use as a light source for a two-photon microscope as described above, if the wavelength of the seed light is 1550 nm, the wavelength range of the resulting soliton wave will be, for example, around 1800 nm to 2200 nm, and the wavelength range of the resulting dispersion wave will be, for example, around 900 nm to 1300 nm. Furthermore, considering the second harmonics of these waves, the wavelength of the second harmonic of the seed light is 775 nm, the wavelength range of the second harmonic of the soliton wave is 900 nm to 1100 nm, and the wavelength range of the second harmonic of the dispersion wave is around 450 nm to 650 nm.

[0024] The combination of relatively long-wavelength pulsed light and relatively short-wavelength pulsed light can be arbitrarily selected, and amplified pulsed light can be generated over a wide wavelength range. In the following embodiments, details of three embodiments will be described: a form in which a soliton wave is parametrically amplified using a dispersed wave (first embodiment), a form in which a soliton wave is parametrically amplified using the second harmonic of a dispersed wave (second embodiment), and a form in which a soliton wave is parametrically amplified using the second harmonic of a seed light (third embodiment). [First Embodiment]

[0025] Figure 1 is a block diagram showing a pulsed light generation device according to the first embodiment. As shown in Figure 1, the pulsed light generation device 1A according to the first embodiment includes an oscillator (oscillating unit) 2, a fiber amplifier (broadbanding unit) 3, a compressor (time width compression unit) 4, and a branching unit 5. The pulsed light generation device 1A includes a first optical path R1 and a second optical path R2 downstream of the branching unit 5. The first optical path R1 is equipped with an acousto-optic modulator (intensity modulation unit) 6 and a soliton shift fiber (wavelength modulation unit) 7. The second optical path R2 is equipped with a soliton shift fiber (wavelength modulation unit) 7, a filter 8, a stretcher 9, a fiber amplifier 10, and a compressor 11. A parametric amplification unit 12 is equipped downstream of the first optical path R1 and the second optical path R2. Each of these components is optically connected to each other by an optical fiber F. An example of an optical fiber F is a polarization-maintaining fiber.

[0026] Oscillator 2 is an oscillator that oscillates using pulsed light as seed light L0. Various oscillators capable of generating ultrashort pulse light LP having a time width of picoseconds to femtoseconds can be used as oscillator 2. As shown in Figure 3(a), oscillator 2 generates an ultrashort pulse train LT as seed light L0, in which ultrashort pulse light LP is arranged on the time axis at a predetermined time interval F1. Each of the ultrashort pulse light LP included in seed light L0 has a first spectral width H1 and a first intensity K1, as shown in Figure 3(b).

[0027] The fiber amplifier 3 is a broadbanding unit that broadens the spectrum of the seed light L0 oscillated by the oscillator 2. The fiber amplifier 3 is positioned at any position between the oscillator 2 and the soliton-shift fiber 7. In the example shown in Figure 2, the fiber amplifier 3 is positioned between the oscillator 2 and the compressor 4. The fiber amplifier 3 broadens the spectrum of the ultrashort pulse light LP, which is the seed light L0, by, for example, similariton amplification, and also amplifies the ultrashort pulse light LP.

[0028] The fiber amplifier 3 is configured to include, for example, a fiber amplifier. For the fiber amplifier, for example, an erbium-doped single-clad normal dispersion fiber can be used. A normal dispersion fiber is a fiber in which the dispersion parameter D (ps / nm / km) is negative. There are no particular restrictions on the additives used in the fiber, and various additives can be used. The fiber amplifier 3 performs amplification while generating a nonlinear effect using the normal dispersion fiber, and generates ultrashort pulse light LP as broadband amplifier light.

[0029] As shown in Figures 3(c) and 3(d), the fiber amplifier 3 broadens the spectral width of the ultrashort pulse light LP to a second spectral width H2, which is wider than the first spectral width H1. The fiber amplifier 3 also amplifies the intensity of the ultrashort pulse light LP to a second intensity K2, which is higher than the first intensity K1. Figure 4 is a graph showing an example of the spectrum of the broadened seed light. In Figure 4, the horizontal axis shows wavelength (nm), and the vertical axis shows intensity (arbitrary unit). In the example in Figure 4, the spectral width of the seed light L0 in the 1550 nm wavelength band is broadened to 100 nm or more.

[0030] The compressor 4 is a time-width compression unit that compresses the time width of the seed light L0 whose spectrum has been broadened by the fiber amplifier 3. The compressor 4 is positioned at any position between the fiber amplifier 3 and the soliton-shift fiber 7. In the example in Figure 2, the compressor 4 is positioned between the fiber amplifier 3 and the branching unit 5. The compressor 4 corrects the broadening of the time width of the seed light L0 that occurs when the spectrum is broadened by the fiber amplifier 3. For example, even if the time width of the ultrashort pulse light LP is broadened to several picoseconds by the fiber amplifier 3, the compressor 4 can compress the time width of the ultrashort pulse light LP to less than 1 picosecond. Various types of compressors, such as chirp mirror type and prism type, can be used as the compressor 4, but in this embodiment, a diffraction grating type compressor including a pair of diffraction gratings is used.

[0031] The branching section 5 is the part that separates the optical path into relatively short-wavelength pulsed light and relatively long-wavelength pulsed light. In this embodiment, the branching section 5 is composed of an optical fiber coupler capable of dividing the intensity of light. The branching section 5 divides the intensity of the seed light L0 output from the compressor 4 and inputs the intensity-divided seed light L0 to the first optical path R1 and the second optical path R2, respectively. There are no particular restrictions on the intensity division ratio of the seed light L0 to the first optical path R1 and the second optical path R2, but it is set in the range of 1:9 to 9:1, for example.

[0032] The first optical path R1 is an optical path that primarily guides relatively long-wavelength pulsed light (here, soliton waves LS, described later). As mentioned above, the first optical path R1 contains an acousto-optic modulator 6 and a soliton shift fiber 7.

[0033] The acousto-optic modulator 6 is an intensity modulation unit that modulates the intensity of the seed light L0, whose spectrum has been broadened by the fiber amplifier 3, pulse by pulse. The acousto-optic modulator 6 is a device called an AOM (Acousto Optic Modulator) that modulates the intensity of the seed light L0 pulse by pulse using the power of sound (sound waves). The acousto-optic modulator 6 can be placed at any position between the oscillator 2 and the soliton shift fiber 7. The acousto-optic modulator 6 may be placed in the optical path of the first optical path R1 and the second optical path R2 that performs wavelength modulation by the soliton shift fiber 7. Also, multiple acousto-optic modulators 6 may be placed. In the example in Figure 2, the acousto-optic modulator 6 is placed in the first optical path R1, before the soliton shift fiber 7.

[0034] As shown in Figures 5(a) and 5(b), the acousto-optic modulator 6 modulates the intensity of the ultrashort pulse light LP included in the ultrashort pulse train LT pulse by pulse. In the example shown in Figures 5(a) and 5(b), the ultrashort pulse light LP included in the ultrashort pulse train LT is alternately given two different intensities M1 and M2. As a result, in the ultrashort pulse train LT, ultrashort pulse light LP1 with a relatively low intensity M1 and ultrashort pulse light LP2 with a relatively high intensity M2 are arranged alternately in time.

[0035] The soliton shift fiber 7 is a wavelength modulation unit that modulates the wavelength of the seed light L0, whose spectrum has been broadened by the fiber amplifier 3, using soliton self-frequency shifting. The soliton shift fiber 7 can be positioned anywhere on the downstream side of the fiber amplifier 3. In this embodiment, the soliton shift fiber 7 is positioned downstream of the acousto-optic modulator 6 in the first optical path R1. For example, a single-mode anomalous dispersion fiber exhibiting anomalous dispersion in the wavelength band of the seed light L0 can be used as the soliton shift fiber 7.

[0036] When an ultrashort pulse light LP, which is the seed light L0, is input to the soliton shift fiber 7, a soliton wave LS is generated by stimulated Raman scattering, as shown in Figure 6(a). In addition, a dispersion wave LD is generated simultaneously with the soliton wave LS. The dispersion wave LD is a type of soliton wave LS generated by Cherenkov radiation in a highly nonlinear fiber. The soliton wave LS is generated at a wavelength longer than the wavelength of the seed light L0 due to soliton self-frequency shifting. The dispersion wave LD is generated at a wavelength shorter than the wavelength of the seed light L0. The wavelength of the dispersion wave LD corresponds one-to-one with the wavelength of the soliton wave LS.

[0037] Figure 6(b) is a graph showing an example of the spectra of soliton waves and dispersion waves output from a soliton-shift fiber. In Figure 6(b), the horizontal axis shows wavelength (nm) and the vertical axis shows intensity (arbitrary units). In the example in Figure 6(b), a soliton wave LS with a peak around 2050 nm is generated by the soliton self-frequency shift of the seed light L0 after broadbanding to the 1550 nm wavelength band. In addition, in the example in Figure 6(b), secondary diffraction light of the dispersion wave with a peak around 1900 nm can be observed. This indicates that a dispersion wave LD with a peak around 950 nm is generated.

[0038] The wavelength of the soliton wave LS can be controlled by the intensity of the seed light L0 input to the soliton shift fiber 7. Increasing the intensity of the seed light L0 input to the soliton shift fiber 7 shifts the wavelength of the soliton wave LS to the longer wavelength side, and decreasing the intensity of the seed light L0 input to the soliton shift fiber 7 shifts the wavelength of the soliton wave LS to the shorter wavelength side. However, if the intensity of the seed light L0 input to the soliton shift fiber 7 becomes excessive, multi-solitonization may occur, and multiple soliton waves LS may be generated for a single ultrashort pulse light LP.

[0039] In contrast, in this embodiment, the spectral width of the seed light L0 input to the soliton shift fiber 7 is pre-broadened by the fiber amplifier 3. As a result, even when the intensity of the seed light L0 input to the soliton shift fiber 7 is increased, it is possible to lengthen the wavelength of the soliton wave LS, that is, to expand the wavelength tuning range of the soliton wave LS, while suppressing multi-solitonization.

[0040] The wavelength shift of the dispersive wave LD occurs in the opposite direction to the wavelength shift of the soliton wave LS. When the wavelength of the soliton wave LS shifts to the longer wavelength side, the wavelength of the dispersive wave LD shifts to the shorter wavelength side. When the wavelength of the soliton wave LS shifts to the shorter wavelength side, the wavelength of the dispersive wave LD shifts to the longer wavelength side. As described above, the wavelength of the dispersive wave LD corresponds one-to-one with the wavelength of the soliton wave LS. Therefore, in this embodiment, the wavelength of the dispersive wave LD can be controlled by the intensity of the seed light L0 modulated by the soliton self-frequency shift.

[0041] As shown in FIGS. 5(a) and 5(b), when different intensities M1 and M2 are alternately applied to the ultrashort pulse light LP included in the ultrashort pulse train LT, ultrashort pulse light LP1 having a relatively low intensity M1 and ultrashort pulse light LP2 having a relatively high intensity M2 are alternately input to the soliton shift fiber 7. In this case, in the soliton shift fiber 7, as shown in FIG. 6(c), a soliton wave LS1 on the short wavelength side corresponding to the ultrashort pulse light LP1 and a soliton wave LS2 on the long wavelength side corresponding to the ultrashort pulse light LP2 are alternately generated in time. Also, a dispersion wave LD1 on the long wavelength side generated simultaneously with the soliton wave LS on the short wavelength side and a dispersion wave LD2 on the short wavelength side generated simultaneously with the soliton wave LS2 on the long wavelength side are alternately generated in time. The generated soliton waves LS (LS1, LS2) and dispersion waves LD (LD1, LD2) are guided to the parametric amplification unit 12.

[0042] In addition, as shown in FIG. 6(c), the optical components output from the soliton shift fiber 7 may include, in addition to the soliton wave LS and the dispersion wave LD, a non-soliton component LZ composed of components other than these. The non-soliton component LZ includes components among the seed light L0 that are not affected by the modulation due to the soliton self-frequency shift and do not become the soliton wave LS or the dispersion wave LD, as well as other noise components.

[0043] As shown in FIG. 2, in the second optical path R2, a soliton shift fiber 7, a filter 8, a stretcher 9, a fiber amplifier 10, and a compressor 11 are arranged.

[0044] The soliton shift fiber 7 has the same configuration as the soliton shift fiber 7 arranged in the first optical path R1. In the second optical path R2, since the acousto-optic modulator 6 is not arranged in front of the soliton shift fiber 7, a soliton wave LS and a dispersion wave LD having a certain wavelength are output from the soliton shift fiber 7, respectively.

[0045] Filter 8 is the part that attenuates the optical components generated by the soliton shift fiber 7 that are not used for parametric amplification. Here, filter 8 attenuates the optical components generated by the soliton shift fiber 7, excluding the dispersed wave LD. Filter 8 is positioned at any position downstream of the soliton shift fiber 7. In this embodiment, filter 8 is positioned between the soliton shift fiber 7 and the stretcher 9 in the second optical path R2. There are no particular restrictions on filter 8 as long as it can attenuate the components not used for parametric amplification, but it is preferable to use one with an optical density (OD value) of 3 or more.

[0046] The fiber amplifier 10 is a dispersion wave amplifier that amplifies the dispersion wave LD generated by the soliton shift fiber 7. The fiber amplifier 10 is positioned at any location downstream of the soliton shift fiber 7 in the second optical path R2. In this embodiment, the fiber amplifier 10 is positioned downstream of the filter 8, between the stretcher 9 and the compressor 11. The fiber amplifier 10 is configured to include, for example, a fiber amplifier. For the fiber amplifier, for example, a ytterbium-doped single-clad or double-clad fiber can be used. As shown in Figure 7, for example, the fiber amplifier 10 amplifies the intensity of the dispersion wave LD guiding the second optical path R2 to a level higher than the intensity of the soliton wave LS guiding the first optical path R1.

[0047] The stretcher 9 and compressor 11 are time-width extension and time-width compression units that convert the amplification of the dispersed wave LD in the fiber amplifier 10 to chirp pulse amplification. The stretcher 9 extends the time width of the dispersed wave LD before it is amplified in the fiber amplifier 10, reducing the peak power of the dispersed wave LD. This prevents damage to the amplification medium when amplifying the dispersed wave LD. It also suppresses the occurrence of undesirable nonlinear optical effects during amplification. The compressor 11 compresses the time width of the dispersed wave LD after it has been amplified in the fiber amplifier 10, converting the dispersed wave LD back into ultrashort pulse light.

[0048] In addition, when the stretcher 9 in the front stage of the fiber amplifier 10 or the compressor 11 in the rear stage of the fiber amplifier 10 is a diffraction grating type element including a pair of diffraction gratings, by performing wavelength separation by the diffraction grating, even in a state where the arrangement of the filter 8 is omitted, only the dispersion wave LD can be output to the parametric amplification unit 12.

[0049] In the present embodiment, as shown in FIG. 2, in the first optical path R1, a filter 8', a stretcher 9', a fiber amplifier 10', and a compressor 11' are also arranged. These components are similar to those in the second optical path R2. The filter 8' attenuates components other than the soliton wave LS among the optical components generated by the soliton shift fiber 7. The fiber amplifier 10' is a soliton wave amplification unit that amplifies the soliton wave LS generated by the soliton shift fiber 7. The stretcher 9' and the compressor 11' are a time width extension unit and a time width compression unit that perform chirped pulse amplification of the amplification of the soliton wave LS in the fiber amplifier 10'.

[0050] The parametric amplification unit 12 is a part that parametrically amplifies relatively long wavelength pulsed light using relatively short wavelength pulsed light among the soliton wave LS, the dispersion wave LD, the seed light L0, and their harmonics. In the present embodiment, the parametric amplification unit 12 has a non-linear optical crystal. As the non-linear optical crystal, for example, a BiBO crystal, an LBO crystal, a BBO crystal, etc. can be used. In the parametric amplification unit 12, the soliton wave LS guided through the first optical path R1 and the dispersion wave LD guided through the second optical path R2 are incident on the non-linear optical crystal so as to satisfy the phase matching condition, and the energy of the dispersion wave LD transitions to the soliton wave LS. The soliton wave Ls after amplification by the parametric amplification unit 12 is output outside the pulsed light generation device 1A as the output light LK.

[0051] Figure 8 is a flowchart showing a pulsed light generation method according to one embodiment of the present disclosure. As shown in Figure 8, the pulsed light generation method according to the present embodiment comprises an oscillation step S01, a broadband step S02, a time width compression step S03, a branching step S04, an intensity modulation step S05, a wavelength modulation step S06, a wavelength modulation step S07, a dispersive wave amplification step S08, a parametric amplification step S09, and an output step S10. The pulsed light generation method according to the present embodiment is carried out using the pulsed light generation device 1A described above.

[0052] Oscillation step S01 is a step in which an ultrashort pulse light LP is used as a seed light L0 for oscillation. In oscillation step S01, an ultrashort pulse train LT, in which ultrashort pulse light LP having a first spectral width H1 and a first intensity K1 is arranged on the time axis at a predetermined time interval F1, is output from the oscillator 2 as the seed light.

[0053] The broadbanding step S02 is a step in which the spectrum of the seed light L0 oscillated in the oscillation step S01 is broadened. In the broadbanding step S02, the spectrum of the ultrashort pulse light LP, which is the seed light L0, is broadened to a second spectral width H2 that is wider than the first spectral width H1 by simillariton amplification in the fiber amplifier 3, and the intensity of the ultrashort pulse light LP is amplified to a second intensity K2 that is higher than the first intensity K1.

[0054] The time width compression step S03 is a step in which the time width of the seed light L0, whose spectrum has been broadened in the broadbanding step S02, is compressed. In the time width compression step S03, the compressor 4 corrects the broadening of the time width of the seed light L0 that occurs due to the broadbanding of the spectrum in the fiber amplifier 3.

[0055] The branching step S04 is a step in which the optical path is divided into a relatively long-wavelength pulsed light and a relatively short-wavelength pulsed light. In the branching step S04, the seed light L0 is intensity-divided by the branching unit 5, and the intensity-divided seed light L0 is input to the first optical path R1 and the second optical path R2, respectively.

[0056] The intensity modulation step S05 is a step in which the intensity of the seed light L0, whose spectrum has been broadened in the broadband section, is modulated pulse by pulse. In the intensity modulation step S05, the acousto-optic modulator 6 modulates the intensity of the ultrashort pulse light LP included in the ultrashort pulse train LT of the seed light L0 guiding the first optical path R1, pulse by pulse. Here, for example, different intensities M1 and M2 are assigned to the ultrashort pulse light LP included in the ultrashort pulse train LT such that ultrashort pulse light LP1 having a relatively low intensity M1 and ultrashort pulse light LP2 having a relatively high intensity M2 are arranged alternately in time.

[0057] The wavelength modulation step S06 is a step in which the wavelength of the seed light L0, whose spectrum has been broadened in the broadbanding step S02, is modulated using soliton self-frequency shift to generate soliton waves LS and dispersion waves LD originating from the seed light L0. In the wavelength modulation step S06, the seed light L0, whose spectrum has been broadened, is input to the soliton shift fiber 7 of the first optical path R1, and soliton waves LS and dispersion waves LD are generated by stimulated Raman scattering.

[0058] Wavelength modulation step S07 is the same as wavelength modulation step S06. In wavelength modulation step S06, a broadband seed light L0 is input to the soliton shift fiber 7 in the second optical path R2 to generate soliton waves LS and dispersion waves LD by stimulated Raman scattering. After wavelength modulation step S07, a filter 8 placed in the second optical path R2 may be used to attenuate the optical components generated in the soliton shift fiber 7, excluding the dispersion waves LD.

[0059] The dispersed wave amplification step S08 is a step to amplify the dispersed wave LD generated in the wavelength modulation step S07. In the dispersed wave amplification step S08, the dispersed wave LD is amplified to a level higher than the intensity of the soliton wave LS, for example, by chirp pulse amplification using the stretcher 9, fiber amplifier 10, and compressor 11, while suppressing the occurrence of undesirable nonlinear optical effects. In this embodiment, the step of amplifying the soliton wave LS generated in the wavelength modulation step S07 is performed together with the dispersed wave amplification step S08.

[0060] The parametric amplification step S09 is the part in which a relatively long-wavelength pulsed light is parametrically amplified using a relatively short-wavelength pulsed light from among the soliton wave LS, dispersion wave LD, seed light L0, and their harmonics. In the parametric amplification step S09, the soliton wave LS guided through the first optical path R1 and the short-wavelength dispersion wave LD guided through the second optical path R2 are incident on the nonlinear optical crystal in an amount that satisfies the phase matching condition, thereby transferring the energy of the dispersion wave LD to the soliton wave LS.

[0061] The output step S10 is a step in which the soliton wave Ls amplified in the parametric amplification step S09 is output as output light LK. The soliton wave Ls amplified by the parametric amplification unit 12 is output to the outside of the pulsed light generator 1A as output light LK.

[0062] As explained above, the pulsed light generator 1A broadens the spectrum of the seed light L0 and modulates the wavelength of the broadened seed light L0 using soliton self-frequency shift, thereby converting the soliton wave LS originating from the seed light L0 into a single soliton. When generating the soliton wave LS originating from the seed light L0, a dispersed wave LD is also generated. Furthermore, the pulsed light generator 1A parametrically amplifies the relatively long wavelength soliton wave LS using a relatively short wavelength dispersed wave LD. By using parametric amplification, unlike conventional amplification using fiber amplifiers, the influence of naturally radiated amplified light can be avoided.

[0063] Figure 9 is a graph showing the amplification efficiency in the pulsed light generator 1A. In Figure 9, the horizontal axis shows wavelength (nm), and the vertical axis shows amplification efficiency (arbitrary unit). Here, a BIBO crystal (θ=35.2°, Φ=0°) is used as the nonlinear optical crystal in the parametric amplification unit 12, and pulsed light with a wavelength of 1030 nm is used as the seed light L0 from the oscillator 2. As shown in Figure 9, the pulsed light generator 1A can generate amplified pulsed light in a wide range of wavelengths from approximately 1800 nm to 2400 nm, with peaks around 1900 nm and 2300 nm, respectively.

[0064] The pulsed light generator 1A further includes an acousto-optic modulator 6 that modulates the intensity of the seed light L0, whose spectrum has been broadened by the fiber amplifier 3, for each pulse. By modulating the intensity of the seed light L0 for each pulse, it becomes possible to vary the wavelength of the soliton wave LS or the wavelength of the dispersed wave LD (in this case, the wavelength of the soliton wave LS) for each pulse.

[0065] The pulsed light generator 1A is further equipped with a compressor 4 that compresses the time width of the seed light L0 whose spectrum has been broadened by the fiber amplifier 3. This makes it possible to compensate for the broadening of the time width of the seed light L0 that occurs when the spectrum is broadened by the fiber amplifier 3.

[0066] The pulsed light generator 1A is further equipped with a filter 8 that attenuates components of the light generated by the soliton shift fiber 7 that are not used for parametric amplification. This allows components unnecessary for parametric amplification to be attenuated in advance from the generated pulsed light.

[0067] The pulsed light generator 1A further includes a branching section 5 that separates the optical paths into relatively long-wavelength pulsed light (here, soliton wave LS) and relatively short-wavelength pulsed light (here, dispersed wave LD). This allows the necessary processing to be applied individually to the relatively long-wavelength pulsed light and the relatively short-wavelength pulsed light in the first optical path R1 and the second optical path R2. Furthermore, separating the optical paths makes it easier to handle the output light LK finally obtained after parametric amplification.

[0068] In the pulsed light generator 1A, a fiber amplifier 10 for amplifying the dispersed wave LD is located in the second optical path R2 branched at the branching section 5. By amplifying the dispersed wave LD, which has a relatively short wavelength, with the fiber amplifier 10, it is possible to increase the amplification efficiency during parametric amplification.

[0069] In the pulsed light generator 1A, a fiber amplifier 10' for amplifying the soliton wave LS is arranged in the first optical path R1 branched at the branching section 5. By amplifying the relatively long wavelength soliton wave LS with the fiber amplifier 10', the intensity of the output light LK finally obtained by parametric amplification of the soliton wave LS can be further increased. [Second Embodiment]

[0070] Figure 10 is a block diagram showing a pulsed light generation device according to the second embodiment. As shown in Figure 10, the pulsed light generation device 1B according to the second embodiment differs from the pulsed light generation device 1A according to the first embodiment in that it includes a wavelength conversion unit 13 downstream of the compressor 11 in the second optical path R2.

[0071] The wavelength conversion unit 13 is the part that converts the wavelength of the dispersed wave LD amplified by the fiber amplifier 10. In this embodiment, the wavelength conversion unit 13 is configured to include, for example, a nonlinear optical crystal. Here, the dispersed wave LD output from the compressor 11 is focused by a lens or the like and incident on the nonlinear optical crystal to generate the harmonic LDk of the dispersed wave LD.

[0072] In the parametric amplification unit 12, which is located after the wavelength conversion unit 13, the soliton wave LS guided through the first optical path R1 and the harmonic LDk of the dispersed wave LD generated in the second optical path R2 are incident on the nonlinear optical crystal in an amount that satisfies the phase matching condition, thereby allowing the energy of the harmonic LDk to be converted into the soliton wave LS. The soliton wave Ls amplified by the parametric amplification unit 12 is output to the outside of the pulsed light generator 1B as output light LK.

[0073] In this pulsed light generator 1B, the same effects as in the first embodiment are achieved, and by using parametric amplification, the influence of naturally radiated amplified light can be avoided, unlike when amplification is performed using a conventional fiber amplifier. Therefore, amplified pulsed light can be generated over a wide wavelength range.

[0074] Figure 11 is a graph showing the amplification efficiency in the pulsed light generator 1B. In Figure 11, the horizontal axis shows wavelength (nm), and the vertical axis shows amplification efficiency (arbitrary unit). Here, an LBO crystal (θ=90°, Φ=10°) is used as the nonlinear optical crystal in the parametric amplification unit 12, and pulsed light with a wavelength of 515 nm is used as the seed light L0 from the oscillator 2. As shown in Figure 11, the pulsed light generator 1B can generate amplified pulsed light in a wide range of wavelengths from approximately 1700 nm to 2250 nm. In particular, it can be confirmed that the amplification efficiency has a flat peak in the wavelength range of 1800 nm to 2100 nm. [Third Embodiment]

[0075] Figure 12 is a block diagram showing a pulsed light generation device according to the third embodiment. As shown in Figure 12, the pulsed light generation device 1C according to the third embodiment differs from the first and second embodiments described above in the function and arrangement of some of its components.

[0076] Specifically, in the pulsed light generation device 1C, the acousto-optic modulator 6 and the soliton shift fiber 7 are located upstream of the branching section 5. In the example shown in Figure 12, the acousto-optic modulator 6 is located between the fiber amplifier 3 and the compressor 4, and the soliton shift fiber 7 is located between the compressor 4 and the branching section 5.

[0077] The branching section 5 is composed of an optical fiber coupler capable of separating light of different wavelengths. In the example shown in Figure 12, the branching section 5 is located downstream of the soliton shift fiber 7, inputting the soliton wave LS generated by the soliton shift fiber 7 into the first optical path R1, and inputting the remaining component of the seed light L0 that was not wavelength-modulated by the soliton shift fiber 7 into the second optical path R2.

[0078] No special components are arranged in the first optical path R1, and it only guides the soliton wave LS, which is then input to the parametric amplifier 12. The second optical path R2 is arranged with a stretcher 9, a fiber amplifier 10, a compressor 11, and a wavelength conversion unit 13. Here, the fiber amplifier 10 is a seed light amplifier that amplifies the seed light L0. As the fiber amplifier 10, for example, an erbium-doped single-clad fiber or an erbium and ytterbium-doped double-clad fiber can be used. The fiber amplifier 10 amplifies the intensity of the seed light L0 guided in the second optical path R2 to a level higher than the intensity of the soliton wave LS guided in the first optical path R1.

[0079] The stretcher 9 and compressor 11 are time-width extension and time-width compression units that convert the amplification of the seed light L0 in the fiber amplifier 10 into chirp pulse amplification. The stretcher 9 extends the time width of the seed light L0 before it is amplified in the fiber amplifier 10, reducing the peak power of the seed light L0. This prevents damage to the amplification medium when amplifying the seed light L0. It also suppresses the occurrence of undesirable nonlinear optical effects during amplification. The compressor 11 compresses the time width of the seed light L0 after it has been amplified in the fiber amplifier 10, converting the seed light L0 back into ultrashort pulse light.

[0080] The wavelength conversion unit 13 is the part that converts the wavelength of the seed light L0 amplified by the fiber amplifier 10. In this embodiment, the wavelength conversion unit 13 is configured to include, for example, a nonlinear optical crystal. Here, the seed light L0 output from the compressor 11 is focused by a lens or the like and incident on the nonlinear optical crystal to generate the harmonic L0k of the seed light L0.

[0081] In the parametric amplification unit 12, which is located after the wavelength conversion unit 13, the soliton wave LS guided through the first optical path R1 and the harmonic L0k of the seed light L0 generated in the second optical path R2 are incident on the nonlinear optical crystal in an amount that satisfies the phase matching condition, thereby allowing the energy of the harmonic L0k to be converted into the soliton wave LS. The soliton wave Ls amplified by the parametric amplification unit 12 is output to the outside of the pulsed light generator 1C as output light LK.

[0082] In this pulsed light generator 1C, the same effects as in the first embodiment are achieved, and by using parametric amplification, the influence of naturally radiated amplified light can be avoided, unlike when amplification is performed using a conventional fiber amplifier. Therefore, amplified pulsed light can be generated over a wide wavelength range.

[0083] Figure 13 is a graph showing the amplification efficiency in the pulsed light generator 1B. In Figure 13, the horizontal axis shows wavelength (nm), and the vertical axis shows amplification efficiency (arbitrary unit). Here, a BIBO crystal (θ=11.3°, Φ=0°) is used as the nonlinear optical crystal in the parametric amplification unit 12, and pulsed light with a wavelength of 1030 nm is used as the seed light L0 from the oscillator 2. As shown in Figure 13, the pulsed light generator 1C can generate amplified pulsed light over a wide range including at least the wavelength range of 1500 nm to 2300 nm. Furthermore, it can be confirmed that a relatively high and flat amplification efficiency can be obtained in the wavelength range of 1500 nm to 2300 nm.

[0084] 1A-1C...Pulsed light generator, 2...Oscillator (oscillating unit), 3...Fiber amplifier (broadbanding unit), 4...Compressor (time width compression unit), 5...Branching unit, 6...Acousto-optic modulator (intensity modulation unit), 7...Soliton shift fiber (wavelength modulation unit), 8...Filter, 10, 10'...Fiber amplifier (amplification unit), 12...Parametric amplification unit, L0...Seed light, LP...Ultrashort pulse light (pulsed light), LS...Soliton wave, LD...Dispersed wave, R1...First optical path (optical path of relatively long wavelength pulsed light), R2...Second optical path (optical path of relatively short wavelength pulsed light).

Claims

1. A pulse light generation device comprising: an oscillation unit that oscillates pulse light as seed light; a broadbanding unit that broadens the spectrum of the seed light oscillated by the oscillation unit; a wavelength modulation unit that modulates the wavelength of the seed light whose spectrum has been broadened by the broadbanding unit using soliton self-frequency shift to generate soliton waves and dispersion waves originating from the seed light; and a parametric amplification unit that parametrically amplifies relatively long-wavelength pulse light using relatively short-wavelength pulse light from among the soliton waves, dispersion waves, seed light, and their harmonics.

2. The pulsed light generation apparatus according to claim 1, further comprising an intensity modulation unit that modulates the intensity of the seed light whose spectrum has been broadened in the broadband unit for each pulse.

3. The pulse light generation apparatus according to claim 1 or 2, further comprising a time width compression unit for compressing the time width of the seed light whose spectrum has been broadened in the broadband unit.

4. The pulsed light generation apparatus according to any one of claims 1 to 3, further comprising a filter for attenuating components of the light components generated in the wavelength modulation section that are not used for the parametric amplification.

5. The pulsed light generating apparatus according to any one of claims 1 to 4, further comprising a branching section that separates the optical path into the relatively long-wavelength pulsed light and the relatively short-wavelength pulsed light.

6. The pulse light generation apparatus according to claim 5, wherein an amplification unit for amplifying the pulse light is arranged in the optical path of the relatively short wavelength pulse light that has been branched at the branching unit.

7. The pulse light generation apparatus according to claim 5 or 6, wherein an amplification unit for amplifying the pulse light is arranged in the optical path of the relatively long-wavelength pulse light that has been branched at the branching unit.

8. A pulse light generation method comprising: an oscillation step of oscillating pulse light as seed light; a broadbanding step of broadening the spectrum of the seed light oscillated in the oscillation step; a wavelength modulation step of modulating the wavelength of the seed light whose spectrum has been broadened in the broadbanding step using soliton self-frequency shift to generate soliton waves and dispersion waves originating from the seed light; and a parametric amplification step of parametrically amplifying relatively long wavelength pulse light using relatively short wavelength pulse light among the soliton waves, dispersion waves, seed light, and their harmonics.

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