Wavelength variable light source and wavelength control method

Abstract

To provide a wavelength variable light source capable of changing wavelength of light to be emitted at a wavelength band of shorter wavelength side, and a wavelength control method.SOLUTION: A wavelength variable light source 1 comprises: an oscillator 2 configured to oscillate ultrashort pulse light L; an acousto-optic modulator 4 and a soliton shift fiber 6 configured to change the wavelength and output first ultrashort pulse light L1 with wavelength in first wavelength band; a branch optical path 21 configured to propagate second ultrashort pulse light L2; and a wavelength conversion part 8 including a wavelength conversion crystal 81, and configured to combine the first ultrashort pulse light L1 and the second ultrashort pulse light L2 on the same optical axis to be made incident into the wavelength conversion crystal 81, and cause the wavelength conversion crystal 81 to emit sum frequency light L3 by sum frequency generation.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a wavelength-tunable light source and a wavelength control method. [Background technology]

[0002] Known wavelength-tunable light sources include an oscillator that oscillates pulsed light and a modulator that modulates the wavelength of the pulsed light oscillated by the oscillator using soliton self-frequency shift, and can tune the wavelength of the output pulsed light by adjusting the intensity of the pulsed light before modulation (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Special Publication No. 2004-527001 Summary of the Invention [Problem to be solved by the invention]

[0004] In recent years, as the above-described wavelength-tunable light source is increasingly applied to various fields, there are cases where it is required to tune the wavelength of emitted light in a wavelength band on the shorter wavelength side.

[0005] The present disclosure has been made in consideration of the above-described circumstances, and aims to provide a wavelength-tunable light source and a wavelength control method that can tune the wavelength of emitted light in a wavelength band on the shorter wavelength side. [Means for solving the problem]

[0006] The tunable light source of the present disclosure is [1] "a tunable light source comprising: an oscillator that oscillates light; a first optical output unit that variably changes the wavelength of the light oscillated by the oscillator and outputs first light having a wavelength in a first wavelength band; a second optical path that propagates second light having a wavelength different from the first light; and a sum frequency light generation unit that includes a nonlinear optical crystal and combines the first light output by the first optical output unit and the second light propagated in the second optical path on the same optical axis and inputs them into the nonlinear optical crystal, and emits sum frequency light having a wavelength in a second wavelength band shorter than the first wavelength band from the nonlinear optical crystal by sum frequency generation."

[0007] In this wavelength-tunable light source, sum frequency generation using the first light and the second light can be performed to emit sum frequency light having a wavelength in a second wavelength band that is shorter than the wavelength in the first wavelength band. In other words, the wavelength-tunable light source of the present disclosure makes it possible to tune the wavelength of the emitted light within a wavelength band on the shorter wavelength side.

[0008] The wavelength-tunable light source of the present disclosure may be [2] "the wavelength-tunable light source according to [1], wherein the second optical path is provided with an optical path adjustment unit that adjusts the optical path length of the second optical path." In this case, the optical path adjustment unit can adjust the timing at which the first light and the second light reach the nonlinear optical crystal so that they arrive closer together.

[0009] The wavelength-tunable light source of the present disclosure may be [3] "the wavelength-tunable light source according to [1] or [2]," wherein the second light path is provided with a dispersion imparting unit that widens the time width of the second light." In this case, by widening the time width of the second light by the dispersion imparting unit, sum frequency light can be generated in the sum frequency light generating unit regardless of the timing at which the first light reaches the nonlinear optical crystal.

[0010] The wavelength-tunable light source of the present disclosure may be [4] "the wavelength-tunable light source according to any one of [1] to [3], wherein the second optical path is connected to an optical path between the oscillator and the first optical output unit via a branching unit, and the light oscillated by the oscillator is propagated as the second light." In this case, the light oscillated by the oscillator can be branched and used as the second light.

[0011] The wavelength-tunable light source of the present disclosure may be [5] "the wavelength-tunable light source according to any one of [1] to [4], in which the sum frequency light generating unit is a wavelength conversion unit and the nonlinear optical crystal is a wavelength conversion crystal." In this case, the wavelength conversion unit can be used as the sum frequency light generating unit to generate sum frequency light.

[0012] The tunable light source of the present disclosure may be [6] "the tunable light source according to any one of [1] to [5], wherein the first wavelength band is 1650 nm to 2200 nm, and the second wavelength band is 800 nm to 900 nm." In this way, the present disclosure makes it possible to realize a tunable light source that can emit light in the wavelength band of 800 nm to 900 nm.

[0013] The wavelength-tunable light source of the present disclosure may be [7] "the wavelength-tunable light source according to any one of [1] to [6], wherein the oscillator oscillates pulsed light as the light, and the first light output unit has a light intensity control unit that controls the intensity of the pulsed light oscillated by the oscillator for each pulse, and a modulation unit that modulates the wavelength of the pulsed light controlled by the light intensity control unit by utilizing soliton self-frequency shift." In this case, the wavelength of the generated pulsed light can be varied for each pulse by the light intensity control unit.

[0014] The wavelength control method of the present disclosure is [8] a wavelength control method comprising: an oscillation step of oscillating light; a wavelength conversion step of tunable the wavelength of the light oscillated in the oscillation step and outputting first light having a wavelength in a first wavelength band; a propagation step of propagating second light having a wavelength different from the first light; and a sum frequency light generation step of multiplexing the first light output in the wavelength conversion step and the second light propagated in the propagation step on the same optical axis, causing the multiplexed light to enter a nonlinear optical crystal, and emitting sum frequency light having a wavelength in a second wavelength band shorter than the first wavelength band from the nonlinear optical crystal by sum frequency generation. The wavelength control method of the present disclosure also makes it possible to tunable the wavelength of the emitted light in a wavelength band on the shorter wavelength side. [Effects of the Invention]

[0015] According to the present disclosure, it is possible to provide a wavelength tunable light source and a wavelength control method that can tune the wavelength of emitted light in a wavelength band on the shorter wavelength side. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a block diagram illustrating a tunable light source according to an embodiment. [Figure 2] Fig. 2(a) is a graph showing the time waveform of the ultrashort pulsed light output from the oscillator of Fig. 1. Fig. 2(b) is a graph showing the spectrum of the ultrashort pulsed light output from the oscillator of Fig. 1. Fig. 2(c) is a graph showing the time waveform of the ultrashort pulsed light output from the fiber amplifier of Fig. 1. Fig. 2(d) is a graph showing the spectrum of the ultrashort pulsed light output from the fiber amplifier of Fig. 1. [Figure 3] FIG. 3 is a graph showing a specific example of the spectrum of the ultrashort pulse light output from the fiber amplifier of FIG. [Figure 4] Fig. 4(a) is a graph showing the time waveform of the first ultrashort pulsed light output from the acousto-optic modulator of Fig. 1. Fig. 4(b) is a graph showing the spectrum of the first ultrashort pulsed light output from the acousto-optic modulator of Fig. 1. [Figure 5]Fig. 5(a) is a graph showing the time waveform of the first ultrashort optical pulse output from the soliton-shifted fiber of Fig. 1. Fig. 5(b) is a graph showing the spectrum of the first ultrashort optical pulse output from the soliton-shifted fiber of Fig. 1. Fig. 5(c) is a graph showing the time waveform of the first ultrashort optical pulse output from the filter of Fig. 1. Fig. 5(d) is a graph showing the spectrum of the first ultrashort optical pulse output from the filter of Fig. 1. [Figure 6] FIG. 6 is a diagram illustrating the configuration of the wavelength conversion unit of FIG. [Figure 7] FIG. 7 is a flowchart showing a wavelength control method according to the embodiment. [Figure 8] Fig. 8(a) is a diagram showing the timing at which the second ultrashort pulsed light, the duration of which has been broadened by the dispersion imparting unit, reaches the wavelength conversion crystal. Fig. 8(b) is a diagram showing the timing at which the first ultrashort pulsed light having a first predetermined wavelength reaches the wavelength conversion crystal. Fig. 8(c) is a diagram showing the timing at which the first ultrashort pulsed light having a second predetermined wavelength reaches the wavelength conversion crystal. Fig. 8(d) is a diagram showing the timing at which the first ultrashort pulsed light having a third predetermined wavelength reaches the wavelength conversion crystal. DETAILED DESCRIPTION OF THE INVENTION

[0017] 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.

[0018] As shown in FIG. 1, the tunable light source 1 is a laser device that can tune the wavelength of the light it outputs. The tunable light source 1 is an ultrashort pulse laser device that can tune the wavelength of pulsed light for each pulse at high speed and shift the tunable wavelength band toward shorter wavelengths by utilizing sum frequency generation (SFG). For example, the tunable light source 1 is a femtosecond laser device that is used in, for example, two-photon microscopes. The tunable light source 1 includes an oscillator 2, a fiber amplifier 3, an acousto-optic modulator 4, a compressor 5, a soliton shift fiber 6, a filter 7, and a wavelength conversion unit 8.

[0019] 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.

[0020] The fiber amplifier 3 constitutes an amplification section that broadens the spectrum of the ultrashort optical pulse L oscillated by the oscillator 2. The fiber amplifier 3 broadens the spectrum of the ultrashort optical pulse L by similariton amplification, and also increases the output power of the ultrashort optical pulse L. The fiber amplifier 3 is disposed between the oscillator 2 and the soliton-shifted fiber 6 in the optical path of the ultrashort optical pulse L.

[0021] The fiber amplifier 3 includes a normal dispersion fiber and a pumping light source. The normal dispersion fiber is a double-clad fiber co-doped with erbium and ytterbium. That is, the fiber amplifier 3 uses a normal dispersion fiber to prevent unexpected nonlinear effects, performs amplification while causing only the desired nonlinear effects, and obtains ultrashort pulsed light L as broadband amplified light. The normal dispersion fiber is a fiber with a negative dispersion parameter D (ps / nm / km). There are no particular restrictions on the dopant used in the fiber amplifier 3, and various dopant types may be used.

[0022] As shown in FIGS. 2(c) and 2(d), the fiber amplifier 3 broadens the spectral width of the ultrashort optical pulse 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 optical pulse 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 optical pulse L to 100 nm or more. In FIG. 3, the horizontal axis represents the wavelength of the ultrashort optical pulse L, and the vertical axis represents the intensity of the ultrashort optical pulse L relative to a predetermined intensity. The ultrashort optical pulse L output from the fiber amplifier 3 is branched by the branching unit 11 into a first ultrashort optical pulse L1 and a second ultrashort optical pulse L2. The branching unit 11 is not particularly limited, and a two-branch optical coupler, for example, may be used.

[0023] The acousto-optic modulator 4 constitutes a light intensity control unit that controls the intensity of the first ultrashort pulsed light L1 for each pulse. The acousto-optic modulator 4 is a device that modulates the first ultrashort pulsed light L1 by utilizing the force of sound (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 first ultrashort pulsed light L1 (hereinafter also referred to as the "first optical path"). Note that the acousto-optic modulator 4 may be disposed at any position between the oscillator 2 and the soliton-shifted fiber 6.

[0024] As shown in Figures 4(a) and 4(b), the acousto-optic modulator 4 controls the intensity of the first ultrashort pulsed light L1 so that it changes for each pulse. For example, when intensity modulations M1 and M2 are given as shown in Figure 4(a), first ultrashort pulsed light LM1 and LM2 are generated according to the intensities given by M1 and M2, as shown in Figure 4(b). The range and accuracy of the intensity modulation of the first ultrashort pulsed light L1 (LM1, LM2) depend on the performance of the acousto-optic modulator 4. The intensity of each pulse light in the pulse train of the first ultrashort pulsed light L1 can be modulated arbitrarily by the acousto-optic modulator 4.

[0025] The compressor 5 constitutes a pulse compression section that compresses the pulse time width of the first ultrashort optical pulse L1. 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 first ultrashort optical pulse L1. The compressor 5 may be disposed at any position between the fiber amplifier 3 and the soliton-shifted fiber 6. Even if the first ultrashort optical pulse L1 is stretched (for example, by several picoseconds) by the fiber amplifier 3, the compressor 5 compresses the time width of the first ultrashort optical pulse L1 and outputs the first ultrashort optical pulse L1 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.

[0026] The soliton-shifted fiber 6 constitutes a modulation section that utilizes soliton self-frequency shift (Raman soliton shift) to modulate the wavelength of the first ultrashort optical pulse L1, the output of which has been increased while broadening the spectrum in the fiber amplifier 3. The soliton-shifted fiber 6 is arranged downstream of the fiber amplifier 3 in the optical path of the first ultrashort optical pulse L1. As shown in Figures 5(a) and 5(b), the soliton-shifted fiber 6 tunes the wavelength of the first ultrashort optical pulse L1 to generate a soliton S1.

[0027] The soliton shift fiber 6 can be, for example, a single-mode anomalous dispersion fiber that exhibits anomalous dispersion in the wavelength band of the first ultrashort optical pulse L1 generated by the fiber amplifier 3. In addition, by controlling the acousto-optic modulator 4, it is possible to generate solitons with wavelengths different from the soliton S1. For example, when modulations of intensities M1 and M2 are applied as shown in FIG. 5(c), the wavelength of the soliton S shifts to a wavelength corresponding to the intensities given by M1 and M2 (solitons S1 and S2) as shown in FIG. 5(d).

[0028] The wavelength shift range and precision of the soliton S depend on the performance of the acousto-optic modulator 4. The wavelength of each soliton S in the soliton train generated from the pulse train of the first ultrashort optical pulse L1 can be arbitrarily tuned by applying intensity modulation to the pulse train with the acousto-optic modulator 4. The soliton-shifted fiber 6 is capable of outputting the first ultrashort optical pulse L1 (soliton S) in a first wavelength band (e.g., 1650 nm to 2200 nm). The acousto-optic modulator 4, compressor 5, and soliton-shifted fiber 6 constitute a first optical output unit that tunes the wavelength and outputs the first ultrashort optical pulse L1 having a wavelength in the first wavelength band.

[0029] The filter 7 filters the first ultrashort pulsed light L1, the wavelength of which has been tuned by the soliton-shifted fiber 6. The filter 7 is arranged downstream of the soliton-shifted fiber 6 in the optical path of the first ultrashort pulsed light L1. In the example shown, the filter 7 cuts out the non-soliton component S0 of the first ultrashort pulsed light L1, as shown in FIGS. 5(b) and 5(d). The filter 7 is not particularly limited, and various filters can be used. The wavelength converter 8 converts the wavelength of the emitted ultrashort pulsed light L to a target wavelength (for example, a wavelength corresponding to a two-photon microscope). The wavelength converter 8 is arranged downstream of the filter 7 in the optical path of the first ultrashort pulsed light L1.

[0030] As shown in FIG. 1 , the wavelength-tunable light source 1 of this embodiment includes a branched optical path (second optical path) 21 connected to an optical path between the oscillator 2 and the soliton-shifted fiber 6 via a branching unit 11. The branched optical path 21 propagates second ultrashort pulsed light (second light) L2, which is light oscillated by the oscillator 2 and branched by the branching unit 11. The second ultrashort pulsed light L2 has a wavelength different from that of the first ultrashort pulsed light (first light) L1, the wavelength of which has been tunable by the soliton-shifted fiber 6. As an example, if the first ultrashort pulsed light L1, the wavelength of which has been tunable by the soliton-shifted fiber 6, has a wavelength of 1650 nm to 2200 nm (first wavelength band), the second ultrashort pulsed light L2 has a wavelength of 1550 nm. The branched optical path 21 is connected to the wavelength converting unit 8, and outputs the propagated second ultrashort pulsed light L2 to the wavelength converting unit 8.

[0031] The branched optical path 21 is provided with an optical path adjuster 9 that adjusts the optical path length of the branched optical path 21, and a dispersion imparting unit 10 that amplifies the second ultrashort pulsed light L2 and widens the time width of the second ultrashort pulsed light L2. The optical path adjuster 9 includes, for example, a plurality of mirrors, at least some of which are configured to be movable. The optical path adjuster 9 adjusts, for example, the arrival timing of the second ultrashort pulsed light L2 at the wavelength conversion crystal 81 so that it approaches the arrival timing of the first ultrashort pulsed light L1 at the wavelength conversion crystal 81. The optical path adjuster 9 is not particularly limited, and various known techniques can be employed.

[0032] The dispersion imparting unit 10 includes, for example, a normal dispersion fiber. The dispersion imparting unit 10 is arranged downstream of the optical path adjusting unit 9 in the branched optical path 21. The dispersion imparting unit 10 expands the time width of the pulse of the second ultrashort pulsed light L2 so that the pulse of the first ultrashort pulsed light L1 and the pulse of the second ultrashort pulsed light L2 overlap in the wavelength conversion crystal 81. The dispersion imparting unit 10 is not particularly limited, and various known techniques can be adopted.

[0033] 6, the wavelength conversion unit 8 includes a wavelength conversion crystal 81, which is a nonlinear optical crystal. In the wavelength conversion unit 8, the first ultrashort pulsed light L1 and the second ultrashort pulsed light L2 are multiplexed on the same optical axis and made incident on the wavelength conversion crystal 81, and sum frequency light L3 having a wavelength in a second wavelength band shorter than the first wavelength band is emitted from the wavelength conversion crystal 81 by sum frequency generation.

[0034] In the example shown, in the wavelength conversion unit 8, the first ultrashort pulsed light L1, which has been wavelength-tuned by the soliton shift fiber 6 and passed through the filter 7, is reflected by a mirror 82, passes through a dichroic mirror 83, and is focused by a lens 84 before entering the wavelength conversion crystal 81. Furthermore, the second ultrashort pulsed light L2, which has propagated through the branched optical path 21, is reflected by a mirror 85 and the dichroic mirror 83, is coaxially combined with the first ultrashort pulsed light L1, is focused by a lens 84, and is then incident on the wavelength conversion crystal 81. As a result, sum frequency light L3 is emitted from the wavelength conversion crystal 81 via a lens 86 using sum frequency generation spectroscopy. The sum frequency light L3 has a wavelength in a second wavelength band (e.g., 800 nm to 900 nm). The wavelength conversion unit 8 constitutes a sum frequency light generating unit.

[0035] Next, a wavelength control method implemented using the wavelength-tunable light source 1 will be described with reference to the flowchart of FIG.

[0036] First, an oscillator 2 oscillates an ultrashort optical pulse L to generate an ultrashort pulse train with a predetermined period (oscillation step: step S1). A fiber amplifier 3 increases the output of the ultrashort optical pulse L and broadens the spectrum of the ultrashort optical pulse L (step S2). The ultrashort optical pulse L is branched into a first ultrashort optical pulse L1 and a second ultrashort optical pulse L2 via a branching unit 11 (step S3).

[0037] Next, the acousto-optic modulator 4 controls the intensity of the first ultrashort optical pulse L1 for each pulse, for example, in accordance with the specifications or conditions required for the wavelength-tunable light source 1 (step S4). The compressor 5 compresses the time width of the first ultrashort optical pulse L1 (step S5). The soliton shift fiber 6 modulates the spectrally broadened first ultrashort optical pulse L1 using the soliton self-frequency shift, thereby tunable the wavelength of the first ultrashort optical pulse L1 to a wavelength in the first wavelength band (step S6). The filter 7 filters the wavelength-tunable first ultrashort optical pulse L1 to cut its non-soliton component S0 (step S7). The above steps S4 to S7 constitute a wavelength conversion step.

[0038] Meanwhile, the second ultrashort pulsed light L2 is propagated through a branched optical path 21 whose optical path length has been adjusted by the optical path adjusting unit 9 (propagation step: step S8). At this time, the second ultrashort pulsed light L2 is amplified and the time width of the second ultrashort pulsed light L2 is widened by the dispersion imparting unit 10. Then, in the wavelength converting unit 8, the first ultrashort pulsed light L1 after step S7 and the second ultrashort pulsed light L2 after step S8 are combined on the same optical axis and input into the wavelength conversion crystal 81, and sum frequency light L3 having a wavelength in the second wavelength band is emitted from the wavelength conversion crystal 81 by sum frequency generation (sum frequency light generating step: step S9).

[0039] As described above, the wavelength-tunable light source 1 and wavelength control method can emit sum-frequency light L3 having a wavelength in a second wavelength band that is shorter than the first wavelength band by sum-frequency generation using the first ultrashort pulsed light L1 and the second ultrashort pulsed light L2. That is, according to this embodiment, it is possible to tune the wavelength of the emitted ultrashort pulsed light L within a wavelength band on the shorter wavelength side. By switching the wavelength for each pulse, it is possible to output ultrashort pulsed light L in a short-wavelength region that cannot be covered by ordinary light sources.

[0040] In the wavelength-tunable light source 1, the branched optical path 21 is provided with an optical path adjustment unit 9 that adjusts the optical path length of the branched optical path 21. In this case, the optical path adjustment unit 9 can adjust the arrival timing of the first ultrashort pulsed light L1 and the second ultrashort pulsed light L2 at the wavelength conversion crystal 81, for example, so that they arrive closer together.

[0041] In the wavelength-tunable light source 1, the branch optical path 21 is provided with a dispersion-imparting unit 10 that expands the time width of the second ultrashort pulsed light L2. By expanding the time width of the second ultrashort pulsed light L2 using the dispersion-imparting unit 10, sum-frequency light L3 can be generated in the wavelength conversion unit 8 regardless of the timing of arrival of the first ultrashort pulsed light L1 at the wavelength conversion crystal 81. For example, when the wavelength of a soliton is tunable, there is a misalignment in the irradiation timing when obtaining the sum frequency with the second ultrashort pulsed light L2 of 1550 nm. However, by significantly expanding the time width of the second ultrashort pulsed light L2 using the dispersion-imparting unit 10, it is no longer necessary to adjust the timing.

[0042] Fig. 8(a) is a diagram showing the arrival timing of the second ultrashort pulsed light L2, whose time width has been expanded by the dispersion imparting unit 10, at the wavelength conversion crystal 81. Fig. 8(b) is a diagram showing the arrival timing of the first ultrashort pulsed light L1, having a first predetermined wavelength, at the wavelength conversion crystal 81. Fig. 8(c) is a diagram showing the arrival timing of the first ultrashort pulsed light L1, having a second predetermined wavelength, at the wavelength conversion crystal 81. Fig. 8(d) is a diagram showing the arrival timing of the first ultrashort pulsed light L1, having a third predetermined wavelength, at the wavelength conversion crystal 81. In each diagram, the horizontal axis represents time and the vertical axis represents intensity. The horizontal axes in each diagram correspond to each other.

[0043] The first to third predetermined wavelengths of the first ultrashort pulsed light L1 are mutually different, and are, for example, any wavelength between 1650 nm and 2200 nm (first wavelength band). The wavelength of the second ultrashort pulsed light L2 is, for example, 1550 nm. As shown in FIGS. 8(a) to 8(d), in this embodiment, the first ultrashort pulsed light L1 of any wavelength can reach the wavelength conversion crystal 81 at the same timing as the second ultrashort pulsed light L2 (at the timing when the pulses overlap) without adjusting the timing, thereby realizing sum frequency generation.

[0044] In the wavelength-tunable light source 1, the branched optical path 21 is connected via a branching section 11, and propagates the ultrashort pulsed light L oscillated by the oscillator 2 as second ultrashort pulsed light L2. In this case, the ultrashort pulsed light L oscillated by the oscillator 2 can be branched and used as the second ultrashort pulsed light L2.

[0045] In the wavelength-tunable light source 1, the wavelength conversion unit 8 constitutes a sum frequency light generation unit, and the nonlinear optical crystal is a wavelength conversion crystal 81. In this case, the wavelength conversion unit 8 can be used as a sum frequency light generation unit to generate sum frequency light L3.

[0046] In the wavelength-tunable light source 1, the first wavelength band is 1650 nm to 2200 nm, and the second wavelength band is 800 nm to 900 nm. In this manner, in this embodiment, it is possible to realize a wavelength-tunable light source 1 that can emit light in the wavelength band of 800 nm to 900 nm.

[0047] The wavelength-tunable light source 1 has an acousto-optic modulator 4 and a soliton shift fiber 6 as a first optical output unit. In this case, the acousto-optic modulator 4 makes it possible to tune the wavelength of the generated ultrashort optical pulse L for each pulse. In this embodiment, the fiber amplifier 3 broadens the spectrum of the ultrashort optical pulse L before modulation using the soliton self-frequency shift, thereby suppressing multi-soliton formation (a phenomenon in which the ultrashort optical pulse L is split into multiple ultrashort optical pulses due to modulation).

[0048] In this embodiment, as described above, the tunable light source 1 is used in a two-photon microscope. In this case, fluorescent observation is possible using a fluorescent protein, such as eCFP, that is excited at a shorter wavelength than eGFP in a two-photon fluorescence microscope. Furthermore, by using these, the wavelength of a fluorescent protein that can be two-photon excited in the 800 to 900 nm band can be tuned for each pulse, enabling high-speed multi-wavelength imaging.

[0049] As described above, one aspect of the present disclosure is not limited to the above embodiment.

[0050] Although the above embodiment includes the optical path adjuster 9 and the dispersion imparting unit 10, the present invention is not limited to this, and for example, if the dispersion imparting unit 10 is provided, the optical path adjuster 9 may be unnecessary. Although the above embodiment includes the branch optical path 21 as the second optical path, the present invention is not limited to this, and instead of the branch optical path 21, a separate optical path may be provided that propagates second light having a wavelength different from that of the first ultrashort pulsed light L1 (light that can generate a sum frequency by being multiplexed with the first ultrashort pulsed light L1).

[0051] In the above embodiment, the wavelength of the ultrashort pulsed light L is tunable by utilizing modulation due to soliton self-frequency shift, but this is not limiting, and various known methods may be used to tunable the wavelength of the ultrashort pulsed light L. In the above embodiment, ultrashort pulsed light L is output, but this is not limiting, and other light may be output.

[0052] In the above embodiment, a fiber amplifier of a double-clad normal dispersion fiber is used as the fiber amplifier 3, but instead, a fiber amplifier of a single-clad normal dispersion fiber (e.g., erbium-doped) may be used. Even in this case, it is possible to at least broaden the spectrum of the ultrashort pulsed light L.

[0053] In the above embodiment, a fiber amplifier may be further provided in the first optical path after the filter 7 and before the wavelength converting unit 8. Alternatively or in addition to this, a fiber amplifier may be further provided in the branch optical path 21, which is the second optical path, after the dispersion imparting unit 10 and before the wavelength converting unit 8. The fiber amplifier broadens the spectrum of the first ultrashort pulsed light L1 (second ultrashort pulsed light L2) and increases its output power, for example, by similariton amplification. The fiber amplifier may be, for example, a normal dispersion fiber, which may be a double-clad fiber co-doped with erbium and ytterbium.

[0054] The components in the above-described embodiments and modifications are not limited to the materials and shapes described above, and various materials and shapes can be applied. Furthermore, the components in the above-described embodiments and modifications can be arbitrarily applied to the components in other embodiments or modifications. [Explanation of symbols]

[0055] 1...Tunable wavelength light source, 2...Oscillator (oscillating section), 4...Acousto-optic modulator (first light output section, light intensity control section), 6...Soliton shift fiber (first light output section, modulating section), 8...Wavelength conversion section (sum frequency light generation section), 9...Optical path adjusting section, 10...Dispersion imparting section, 11...Branching section, 21...Branching optical path (second optical path), 81...Wavelength conversion crystal (nonlinear optical crystal), L...Ultrashort pulsed light (light, pulsed light), L1...First ultrashort pulsed light (light, pulsed light), L2...Second ultrashort pulsed light (light, pulsed light).

Claims

1. an oscillator that emits light; a first light output unit that variably changes the wavelength of the light oscillated by the oscillation unit and outputs a first light having a wavelength in a first wavelength band; a second optical path for propagating a second light having a wavelength different from that of the first light; a sum frequency light generating unit that includes a nonlinear optical crystal, combines the first light output from the first optical output unit and the second light propagated through the second optical path on the same optical axis, and inputs the combined light into the nonlinear optical crystal, and emits sum frequency light having a wavelength in a second wavelength band that is shorter than the first wavelength band from the nonlinear optical crystal by sum frequency generation.

2. The wavelength-tunable light source according to claim 1 , wherein the second optical path is provided with an optical path adjusting unit that adjusts the optical path length of the second optical path.

3. The tunable light source according to claim 1 , wherein the second light path is provided with a dispersion imparting section that widens a time width of the second light.

4. 3. The wavelength-tunable light source according to claim 1, wherein the second optical path is connected to an optical path between the oscillation unit and the first optical output unit via a branching unit, and propagates light oscillated by the oscillation unit as the second light.

5. the sum frequency light generating unit is a wavelength converting unit, 3. The wavelength-tunable light source according to claim 1, wherein the nonlinear optical crystal is a wavelength conversion crystal.

6. the first wavelength band is 1650 nm to 2200 nm; 3. The wavelength-tunable light source according to claim 1, wherein the second wavelength band is from 800 nm to 900 nm.

7. the oscillator oscillates pulsed light as the light, The first optical output unit a light intensity control unit that controls the intensity of the pulsed light oscillated by the oscillation unit for each pulse; 3. The wavelength-tunable light source according to claim 1, further comprising: a modulation section that modulates the wavelength of the pulsed light controlled by the light intensity control section by utilizing a soliton self-frequency shift.

8. an oscillation step of oscillating light; a wavelength conversion step of varying the wavelength of the light oscillated in the oscillation step and outputting first light having a wavelength in a first wavelength band; a propagation step of propagating a second light having a wavelength different from that of the first light; a sum frequency light generation step of combining the first light output in the wavelength conversion step and the second light propagated in the propagation step on the same optical axis and inputting the combined light into a nonlinear optical crystal, and emitting sum frequency light having a wavelength in a second wavelength band shorter than the first wavelength band from the nonlinear optical crystal by sum frequency generation.

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