Wavelength variable light source and wavelength variable method

Abstract

In a wavelength-variable light source and a wavelength-variable method in which the wavelength of the output light is variable, the light is surely amplified over a wide wavelength band. 【Solution means】The wavelength-variable light source 1 includes an optical output unit that outputs ultrashort pulsed light L and whose output wavelength of the ultrashort pulsed light L is variable, a second fiber amplifier 7 that amplifies the ultrashort pulsed light L output from the optical output unit, a filter 8 that filters the ultrashort pulsed light L amplified by the second fiber amplifier 7, and a third fiber amplifier 9 that amplifies the ultrashort pulsed light L filtered by the filter 8. The second fiber amplifier 7 has a higher gain on the short-wavelength side than on the long-wavelength side. The third fiber amplifier 9 has a higher gain on the long-wavelength side than on the short-wavelength side. The filter 8 attenuates the amplified spontaneous emission light.

Description

Technical Field

[0001] The present disclosure relates to a wavelength-variable light source and a wavelength-variable method.

Background Art

[0002] As a wavelength-variable light source, there is known a light source including an oscillation unit that oscillates pulsed light, and a modulation unit that modulates the wavelength of the pulsed light oscillated by the oscillation unit using soliton self-frequency shift, and the wavelength of the output pulsed light is made variable 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

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the wavelength-variable light source described in Patent Document 1, when the output light is amplified by an optical amplifier, amplified spontaneous emission (ASE) occurs, which not only hinders amplification but also may cause parasitic oscillation. In this case, it is conceivable to use an optical amplifier that meets the desired conditions so that amplified spontaneous emission does not occur. However, there are cases where optical amplification is performed in a wavelength band where it is difficult to use such an optical amplifier. Further, even when optical amplification is performed in a wavelength band where it is possible to use such an optical amplifier, since the wavelength of the light is variable, for example, if an optical amplifier adjusted to the conditions of light with a shorter wavelength is used, it becomes difficult to amplify light with a longer wavelength, and vice versa. This becomes particularly prominent when the difference between the short wavelength and the long wavelength is large, that is, when optical amplification is performed over a wide wavelength band (for example, when optical amplification is performed over a wavelength band spanning 400 nm such as optical amplification in the wavelength band of 1800 nm to 2200 nm). This is because as the wavelength band becomes wider, the difference in gain due to the wavelength of the optical amplifier becomes larger. In such a case, when attempting to amplify a wavelength with a low gain, light with a high gain wavelength tends to be preferentially amplified. On the other hand, in a specific wavelength band, for example, in a wavelength band including 1550 nm, it is possible to perform optical amplification by arranging a filter between optical amplifiers using two Er fibers for gain equalization (see, for example, Patent Document 2). However, even in this case, since the applicable wavelength band is about 150 nm, there is a demand for reliably performing optical amplification over a wider wavelength band.

[0005] The present disclosure has been made in view of the above circumstances, and an object thereof is to reliably amplify light over a wide wavelength band in a wavelength-variable light source and a wavelength-variable method in which the wavelength of the output light is variable.

Means for Solving the Problems

[0006] The wavelength-variable light source of the present disclosure is a "wavelength-variable light source that outputs light, has a light output unit whose output light wavelength is variable, a first optical amplification unit that amplifies the light output by the light output unit, a filter unit that filters the light amplified by the first optical amplification unit, and a second optical amplification unit that amplifies the light filtered by the filter unit, where the first optical amplification unit has a higher gain on the first wavelength side than on the second wavelength side different from the first wavelength, the second optical amplification unit has a higher gain on the second wavelength side than on the first wavelength side, and the filter unit attenuates amplified spontaneous emission light."

[0007] In this wavelength-variable light source, when varying the wavelength of the output light between, for example, a first wavelength side closer to the first wavelength than the second wavelength (hereinafter also simply referred to as the "first wavelength side") and a second wavelength side closer to the second wavelength than the first wavelength (hereinafter also simply referred to as the "second wavelength side"), the light can be amplified as follows. That is, the light on the first wavelength side is amplified by the first optical amplification unit without generating amplified spontaneous emission (ASE), and then amplified by the second optical amplification unit. Here, although the second optical amplification unit has a high gain on the second wavelength side, since the light on the first wavelength side has already been amplified by the first optical amplification unit and thus is more likely to absorb energy, the second optical amplification unit can also amplify the light on the first wavelength side while suppressing the generation of ASE. On the other hand, for example, the light on the second wavelength side is not amplified as much as the light on the first wavelength side, so ASE is generated, but the ASE can be attenuated by the filter unit. Thereafter, the light on the second wavelength side is amplified by the second optical amplification unit without generating ASE. Therefore, according to the present disclosure, in a wavelength-variable light source with a variable output light wavelength, it is possible to reliably amplify the light while suppressing ASE over a wide wavelength band.

[0008] The wavelength-variable light source of the present disclosure may also be a "wavelength-variable light source according to [1], where the light output unit can output light in a wavelength band of 1800 to 2200 nm, and the first optical amplification unit and the second optical amplification unit include a Tm fiber." In this case, it is possible to realize an amplification system for light in the wavelength band of 1800 to 2200 nm using a Tm fiber.

[0009] The wavelength-variable light source of the present disclosure may be the "wavelength-variable light source according to [1] or [2], wherein the first wavelength is shorter than the second wavelength". In this case, in the first optical amplification unit, amplification optimized for the shorter-wavelength side than the second wavelength becomes possible, and in the second optical amplification unit, amplification optimized for the longer-wavelength side than the first wavelength becomes possible.

[0010] The wavelength-variable light source of the present disclosure may be the "wavelength-variable light source according to any one of [1] to [3], wherein the filter unit attenuates the amplified spontaneous emission light in a predetermined wavelength band including the wavelength range between the first wavelength and the second wavelength". In this case, the filter unit can effectively attenuate the amplified spontaneous emission light generated by the amplification of the first optical amplification unit.

[0011] The wavelength-variable light source of the present disclosure may be the "wavelength-variable light source according to any one of [1] to [4], wherein the light output unit includes an oscillation unit that oscillates pulsed light, an amplification unit that broadens the spectrum of the pulsed light oscillated by the oscillation unit, a modulation unit that modulates the wavelength of the pulsed light whose spectrum has been broadened by the amplification unit using soliton self-frequency shift, and an optical intensity control unit that is disposed between the amplification unit and the modulation unit in the optical path of the pulsed light and controls the intensity of the pulsed light for each pulse". In this case, the optical intensity control unit can make the wavelength of the generated pulsed light variable for each pulse. Further, by broadening the spectrum of the pulsed light before performing modulation using soliton self-frequency shift, multi-soliton formation (a phenomenon in which pulsed light is split by modulation to form a plurality of pulsed lights) can be suppressed.

[0012] The wavelength variable method of the present disclosure is "[6] an optical output step of outputting light, a first optical amplification step of amplifying the light output in the optical output step, a filtering step of filtering the light amplified in the first optical amplification step, and a second optical amplification step of amplifying the light filtered in the filtering step, wherein the optical output step includes a step of varying the wavelength of the output light, in the first optical amplification step, the light is amplified by a first optical amplification unit having a higher gain on the first wavelength side than on the second wavelength side different from the first wavelength, in the second optical amplification step, the light is amplified by a second optical amplification unit having a higher gain on the second wavelength side of the light than on the first wavelength side, and in the filtering step, spontaneous emission amplified light in a predetermined wavelength band including a wavelength range between the first wavelength and the second wavelength is attenuated, the wavelength variable method". In the present disclosure as well, the same operational effects as those of the above-described wavelength variable light source are achieved. That is, in a wavelength variable method in which the wavelength of the output light is variable, it is possible to reliably amplify the light while suppressing ASE over a wide wavelength band.

[0013] The wavelength variable method of the present disclosure may be "[7] the wavelength variable method according to [6], wherein in the filtering step, energy is transferred from the light of the first wavelength to the light of the second wavelength". In this case, by transferring energy from the light of the first wavelength to the light of the second wavelength, it is possible to adjust the wavelength dependence of the gain in both the first optical amplification unit and the second optical amplification unit.

Advantages of the Invention

[0014] According to the present disclosure, in a wavelength variable light source and a wavelength variable method in which the wavelength of the output light is variable, it is possible to reliably amplify the light over a wide wavelength band.

Brief Description of the Drawings

[0015]

Figure 1

Figure 2

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Figure 4

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Figure 9

DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, embodiments will be described in detail with reference to the drawings. In each figure, the same or corresponding parts are denoted by the same reference numerals, and redundant descriptions are omitted.

[0017] As shown in FIG. 1, the wavelength-variable light source 1 is a laser device whose output light wavelength is variable. The wavelength-variable light source 1 is an ultrashort pulse laser device that can rapidly vary the wavelength of pulsed light for each pulse and amplify the pulsed light of all wavelengths in the variable wavelength band. For example, the wavelength-variable light source 1 is a femtosecond laser device and is used, for example, in a two-photon microscope. The wavelength-variable light source 1 includes an oscillator 2, a first fiber amplifier 3, an acousto-optic modulator 4, a compressor 5, a soliton shift fiber 6, a second fiber amplifier 7, a filter 8, a third fiber amplifier 9, and a wavelength conversion module 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, for example, 1550 nm. Here, the oscillator 2 oscillates ultrashort pulsed light L having a spectrum with a first spectral width H1 and a first intensity K1 as shown in FIG. 2(b). The oscillator 2 is not particularly limited, and various oscillators can be used.

[0019] The first fiber amplifier 3 constitutes an amplification unit that broadens the spectrum of the ultrashort pulsed light L oscillated by the oscillator 2. The first fiber amplifier 3 broadens the spectrum of the ultrashort pulsed light L by soliton amplification and increases the output power of the ultrashort pulsed light L. The first fiber amplifier 3 is arranged between the oscillator 2 and the soliton shift fiber 6 in the optical path of the ultrashort pulsed light L.

[0020] The first fiber amplifier 3 is configured to include a normal dispersion fiber and an excitation light source. The normal dispersion fiber is a double-clad fiber co-doped with erbium and ytterbium. That is, the first fiber amplifier 3 performs amplification while causing a non-linear effect with a normal dispersion double-clad fiber so as not to stretch, and obtains the ultrashort pulse light L as broadband amplified light. The normal dispersion fiber is a fiber in a state where the dispersion parameter D (ps / nm / km) is negative. The additive used in the first fiber amplifier 3 is not particularly limited, and various additives may be adopted.

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

[0022] The acousto-optic modulator 4 constitutes an optical intensity control unit that controls the intensity of the ultrashort pulse light L for each pulse. The acousto-optic modulator 4 is a device that modulates the ultrashort pulse light L using the force of sound (sound wave), and is called an AOM (Acousto Optic Modulator). In the present embodiment, the acousto-optic modulator 4 is disposed between the first fiber amplifier 3 and the soliton shift fiber 6 in the optical path of the ultrashort pulse light L. Note that the acousto-optic modulator 4 may be disposed at any position as long as it is between the oscillator 2 and the soliton shift fiber 6.

[0023] As shown in FIGS. 4(a) and 4(b), the acousto-optic modulator 4 controls the intensity of the ultrashort pulse light L to vary from pulse to pulse. For example, when intensity modulations M1 and M2 are applied as shown in FIG. 4(a), ultrashort pulse lights LM1 and LM2 corresponding to the intensities applied by M1 and M2 are generated as shown in FIG. 4(b). The intensity modulation range and accuracy of the ultrashort pulse 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 pulse light L can be arbitrarily modulated by the acousto-optic modulator 4 respectively.

[0024] The compressor 5 constitutes a pulse compression unit that compresses the pulse time width of the ultrashort pulse light L. In the present embodiment, the compressor 5 is disposed between the acousto-optic modulator 4 and the soliton shift fiber 6 in the optical path of the ultrashort pulse light L. Note that the compressor 5 may be disposed at any position as long as it is between the first fiber amplifier 3 and the soliton shift fiber 6. Even when the ultrashort pulse light L is stretched (for example, stretched by several picoseconds) by the first fiber amplifier 3, the compressor 5 compresses the time width of the ultrashort pulse light L and outputs an ultrashort pulse light L having a time width (less than 1 picosecond) with a spread below a certain level. The compressor 5 is not particularly limited, and various compressors can be used.

[0025] The soliton shift fiber 6 constitutes a modulation unit that modulates the wavelength of the ultrashort pulse light L whose output is increased in power while broadening the spectrum by the first fiber amplifier 3, using the soliton self-frequency shift (Raman soliton shift). The soliton shift fiber 6 is disposed on the downstream side of the first fiber amplifier 3 in the optical path of the ultrashort pulse light L. As shown in FIGS. 5(a) and 5(b), the soliton shift fiber 6 varies the wavelength of the ultrashort pulse light L and generates a soliton S1.

[0026] The soliton shift fiber 6 can use a single-mode dispersion-shifted fiber that exhibits anomalous dispersion in the wavelength band of the ultrashort pulsed light L generated by, for example, the first fiber amplifier 3. In addition, by controlling the acousto-optic modulator 4, solitons with wavelengths different from that of the soliton S1 can also be generated. The wavelength of the soliton S shifts to a wavelength corresponding to the intensity applied by M1 and M2 as shown in Fig. 5(d) when intensity modulation is applied by M1 and M2 as shown in, for example, Fig. 5(c) (solitons S1 and S2).

[0027] The shift wavelength range and accuracy 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 ultrashort pulsed light L can be arbitrarily varied by applying intensity modulation to the pulse train with the acousto-optic modulator 4. The soliton shift fiber 6 can output, for example, ultrashort pulsed light L (soliton S) in a wavelength band of 1800 nm to 2200 nm. The oscillator 2, the first fiber amplifier 3, the acousto-optic modulator 4, the compressor 5, and the soliton shift fiber 6 constitute an optical output unit 50 that outputs light and whose output light wavelength is variable.

[0028] In the illustrated example, the ultrashort pulsed light L modulated by soliton self-frequency shift includes a non-soliton component S0 (a component that did not become soliton S1 or S2). A filter (not shown) for cutting the non-soliton component S0 of the ultrashort pulsed light L may be provided at the subsequent stage of the soliton shift fiber 6. Such a filter may have an OD value of 3 or more.

[0029] The second fiber amplifier 7 constitutes a first optical amplification unit that amplifies the ultrashort pulsed light L with a variable wavelength in the soliton shift fiber 6. The second fiber amplifier 7 is configured to include a rare-earth doped fiber and an excitation light source. The rare-earth doped fiber is, for example, a Tm fiber. The excitation light source is, for example, a laser diode that outputs excitation light with a wavelength of 1550 nm. The second fiber amplifier 7 is an amplifier optimized for the short wavelength side. The second fiber amplifier 7 amplifies by absorbing the gain of the ultrashort pulsed light L as the signal light (seed light).

[0030] The second fiber amplifier 7 has a higher gain G1 (see Fig. 7(b)) on the first wavelength side, which is the short wavelength side of the ultrashort pulsed light L, than on the second wavelength side, which is the long wavelength side. Such characteristics of the second fiber amplifier 7 can be realized, for example, by adjusting the length of the rare-earth doped fiber and the concentration of the added rare-earth. As an example, the short wavelength (first wavelength) is a wavelength of 1800 nm or more and less than 2000 nm, and the long wavelength (second wavelength) is a wavelength of 2000 nm or more and 2200 or less.

[0031] The filter 8 constitutes a filter section that filters the ultrashort pulsed light L amplified by the second fiber amplifier 7. The filter 8 attenuates ASE (Amplified Spontaneous Emission). Also, in the present embodiment, the filter 8 attenuates the ASE generated by the amplification by the second fiber amplifier 7, and also attenuates the light in which the noise component generated by the amplification by the second fiber amplifier 7 is amplified, and the light having a wavelength in a high gain region. Note that, in the present embodiment, the light having a wavelength in a high gain region is light in a predetermined wavelength band at a short wavelength, and is light in a wavelength band having a particularly high amplification ratio compared to light in other wavelength bands. If this light is not attenuated, only this light will be preferentially amplified, which will hinder optical amplification over a wide wavelength range. Here, the ASE attenuated by the filter 8 is ASE in a predetermined wavelength band including a wavelength range between a short wavelength and a long wavelength. The light in which the ASE and the noise component are amplified not only hinders amplification but also may cause parasitic oscillation and damage to the amplifier system. ASE tends to occur easily, for example, when the wavelength band of the seed light and the gain do not match, and when the power of the seed light is weak. Also, in the filter 8, there is a process in which the pulses in the short wavelength region are absorbed and become the gain of the pulses in the long wavelength region that follow, and energy transfer occurs from the pulses in the region with gain of the gain fiber to the pulses in the region with less gain. Note that the filter 8 may be, for example, one using an AOM (Acousto-Optic Modulator) and an EOM (Electro-Optic Modulator), a Tm fiber, a Ho fiber, or a gain fiber such as a Tm / Ho fiber that does not receive the excitation light. Also, the filter 8 may be a gain fiber in a region where the excitation light is attenuated by 20 dB or more in the gain fiber of the second fiber amplifier 7.

[0032] The third fiber amplifier 9 constitutes a second optical amplification section that amplifies the ultrashort pulsed light L filtered by the filter 8. The third fiber amplifier 9 is configured to include a rare-earth doped fiber and an excitation light source. The rare-earth doped fiber is, for example, a Tm fiber. The excitation light source is, for example, a laser diode that outputs excitation light with a wavelength of 790 nm or a CW laser that outputs excitation light with a wavelength of 1550 nm. The third fiber amplifier 9 is an amplifier optimized for the long-wavelength side. The third fiber amplifier 9 amplifies by absorbing the gain of the ultrashort pulsed light L as the signal light. The third fiber amplifier 9 has a higher gain G2 (see FIG. 7(d)) on the second wavelength side, which is the long-wavelength side of the ultrashort pulsed light L, than on the first wavelength side, which is the short-wavelength side. Such characteristics of the third fiber amplifier 9 can be realized, for example, by adjusting the length of the rare-earth doped fiber and the concentration of the added rare earth.

[0033] The wavelength conversion module 10 converts the wavelength of the ultrashort pulsed light L to a target wavelength (for example, a wavelength corresponding to a two-photon microscope). The wavelength conversion module 10 is arranged on the downstream side of the third fiber amplifier 9 in the optical path of the ultrashort pulsed light L. As an example, the wavelength conversion module 10 reduces the intensity of the ultrashort pulsed light L and also reduces the wavelength band.

[0034] Next, a wavelength variable method implemented using the wavelength variable light source 1 will be described with reference to the flowchart of FIG. 6.

[0035] First, ultrashort pulsed light L is oscillated in the oscillator 2 to generate an ultrashort pulse train with a predetermined period (oscillation step: step S1). The first fiber amplifier 3 increases the output power of the ultrashort pulsed light L and broadens the spectrum of the ultrashort pulsed light L (amplification step: step S2). The acousto-optic modulator 4 controls the intensity of the ultrashort pulsed light L for each pulse according to, for example, the specifications or conditions required for the wavelength variable light source 1 (step S3). The compressor 5 compresses the time width of the ultrashort pulsed light L (step S4). The soliton shift fiber 6 modulates and varies the wavelength of the ultrashort pulsed light L with a broadened spectrum using the soliton self-frequency shift (step S5).

[0036] Subsequently, the ultrashort pulsed light L with variable wavelength in step S5 above is amplified by the second fiber amplifier 7 (step S6). In step S6 above, the ultrashort pulsed light L is amplified with a gain G1 higher than that on the long-wavelength side on the short-wavelength side. The ultrashort pulsed light L amplified in step S6 is filtered by the filter 8 (step S7). Thereby, the ASE in a predetermined wavelength band generated by the amplification by the second fiber amplifier 7, the light in which the noise component is amplified, and the light with a wavelength in the high-gain region are attenuated.

[0037] Subsequently, the ultrashort pulsed light L filtered in step S7 above is amplified by the third fiber amplifier 9 (step S8). In step S8 above, the ultrashort pulsed light L is amplified with a gain G2 higher than that on the short-wavelength side on the long-wavelength side. Then, the wavelength of the amplified ultrashort pulsed light L is converted to the target wavelength by the wavelength conversion module 10 (step S9). Steps S1 to S5 above correspond to an optical output step for outputting light. Step S6 corresponds to a first optical amplification step, step S7 corresponds to a filtering step, and step S8 corresponds to a second optical amplification step.

[0038] In the wavelength-variable light source 1 and the wavelength-variable method described above, for example, as shown in Fig. 7(a), when the wavelength of the ultrashort pulse light L output from the light output section 50 is variable to a short wavelength λ1, the following operational effects are achieved. That is, as shown in Fig. 7(b), the ultrashort pulse light L with the short wavelength λ1 is amplified by the second fiber amplifier 7 having a gain G1 without generating ASE (pre-amplification). The pre-amplified ultrashort pulse light L with the short wavelength λ1 passes through the filter 8 without being particularly attenuated as shown in Fig. 7(c). Then, the ultrashort pulse light L that has passed through the filter 8 is further amplified by the third fiber amplifier 9 having a gain G2 as shown in Fig. 7(d). At this time, although the gain G2 is higher on the long-wavelength side, since the ultrashort pulse light L with the short wavelength λ1 has already been amplified by the second fiber amplifier 7 and thus is more likely to absorb energy, amplification in the third fiber amplifier 9 is also performed while suppressing the generation of ASE (without worrying about ASE).

[0039] On the other hand, for example, as shown in Fig. 8(a), when the wavelength of the ultrashort pulse light L output from the light output section 50 is variable to a long wavelength λ2, the following operational effects are achieved. That is, as shown in Fig. 8(b), since the ultrashort pulse light L with the long wavelength λ2 is not amplified as much as the ultrashort pulse light L on the first wavelength side which is the short wavelength λ1 side, ASE11 is generated. Then, as shown in Fig. 8(c), the ASE11 is attenuated in the filter 8. And the ultrashort pulse light L with the long wavelength λ2 is amplified by the third fiber amplifier 9 having a gain G2 without generating ASE. Therefore, according to the present embodiment, in the wavelength-variable light source 1 and the wavelength-variable method in which the wavelength of the output ultrashort pulse light L is variable, it is possible to reliably amplify the ultrashort pulse light L while suppressing ASE11 over a wide wavelength band. In particular, the present embodiment is effective when performing optical amplification over a wide wavelength band and when performing optical amplification in a wavelength band where it is difficult to use an optical amplifier that meets the conditions.

[0040] The wavelength-variable light source 1 has a light output unit 50 that can output light in a wavelength band of 1800 to 2200 nm. The second fiber amplifier 7 and the third fiber amplifier 9 include Tm fibers. In this case, an amplification system for ultrashort pulse light L in a wavelength band of 1800 to 2200 nm can be realized using the Tm fiber.

[0041] In the wavelength-variable light source 1, the second fiber amplifier 7 can perform amplification optimized for the short-wavelength side, and the third fiber amplifier 9 can perform amplification optimized for the long-wavelength side. In the wavelength-variable light source 1, the filter 8 attenuates ASE in a predetermined wavelength band including the wavelength range between the short wavelength and the long wavelength. In this case, the filter 8 can effectively attenuate the ASE generated by the amplification of the second fiber amplifier 7.

[0042] The wavelength-variable light source 1 has an oscillator 2 that oscillates ultrashort pulse light L, a first fiber amplifier 3 that broadens the spectrum of the ultrashort pulse light L, a soliton shift fiber 6 that modulates the wavelength of the ultrashort pulse light L with broadened spectrum using soliton self-frequency shift, and an acousto-optic modulator 4 disposed between the first fiber amplifier 3 and the soliton shift fiber 6. In this case, the acousto-optic modulator 4 can vary the wavelength of the generated ultrashort pulse light L for each pulse. Also, by broadening the spectrum of the ultrashort pulse light L before performing modulation using soliton self-frequency shift, multi-soliton formation (a phenomenon in which the ultrashort pulse light L splits by modulation to form a plurality of ultrashort pulse lights) can be suppressed.

[0043] Note that since the wavelength-variable light source 1 uses a fiber laser, maintenance-free operation can be realized compared to the case of using a titanium sapphire laser. In the wavelength-variable light source 1, by adjusting the intensity (output) of the ultrashort pulse light L input to the soliton shift fiber 6, ultrashort pulse light L of a desired wavelength can be freely generated. Also, since a configuration that requires a physical movable part when converting the wavelength is not used, the wavelength can be varied at high speed compared to, for example, the case of using a titanium sapphire laser.

[0044] FIG. 9 is a graph showing the relationships between the absorption cross-section, stimulated emission cross-section of the Tm fiber, and the transmittance of the filter and the wavelength. In FIG. 9, the horizontal axis represents the wavelength of the ultrashort pulse light L, and the vertical axis represents the absorption cross-section, stimulated emission cross-section of the Tm fiber, and the transmittance of the filter 8. When the Tm fiber amplifies light with a wavelength of 1800 to 2200 nm, it has the absorption cross-section 22 and stimulated emission cross-section 21 shown in FIG. 9. In a fiber amplifier system including the Tm fiber, when the fiber length is short, the gain on the long-wavelength side is insufficient, so ASE is likely to occur during the amplification of light on the long-wavelength side. On the other hand, when the fiber length is long, the absorption also increases, so the light on the short-wavelength side is absorbed too much. Therefore, ASE is likely to occur during the amplification of light on the short-wavelength side. In this regard, in the present embodiment, as described above, the fiber amplifier including the Tm fiber is made into two stages, and the second fiber amplifier 7 and the third fiber amplifier 9 are provided. Then, by inserting the filter 8 with a transmittance 23 (dotted line in the figure) between the second fiber amplifier 7 and the third fiber amplifier 9, ASE is removed, enabling the amplification of broadband wavelength-variable laser light.

[0045] In the present embodiment, the broadband ultrashort pulse light L can be amplified without separately amplifying the broadband ultrashort pulse light L by another mechanism. The broadband ultrashort pulse light L can be amplified without using a non-linear optical effect such as Raman amplification. The present embodiment can constitute a broadband and high-output (for example, 30 dBm or more) amplifier system. According to the present embodiment, it is also possible to prevent parasitic oscillation due to an increase in ASE. In the present embodiment, as described above, the wavelength-variable light source 1 is used in a two-photon microscope. In this case, since high-output amplification is possible in a broadband wavelength region, combined with the subsequent wavelength conversion system, high-output and multi-wavelength imaging is possible in the two-photon microscope.

[0046] In the wavelength variable method of this embodiment, in the filtering step (the above step S7), energy is transferred from the ultrashort pulse light L with a short wavelength λ1 to the ultrashort pulse light L with a long wavelength λ2. In this case, by transferring energy from the ultrashort pulse light L with a short wavelength λ1 to the ultrashort pulse light L with a long wavelength λ2, the wavelength dependence of the gain in both the second fiber amplifier 7 and the third fiber amplifier 9 can be adjusted.

[0047] As described above, one aspect of the present disclosure is not limited to the above embodiment.

[0048] In the above embodiment, as an example, the fiber included in the second fiber amplifier 7 and the third fiber amplifier 9 is a Tm fiber, but there is no particular limitation on the laser medium added to the fiber, and a laser medium co-doped with Ho, Tm, and Ho may also be used. That is, the rare earth-doped fiber, which is the fiber included in the second fiber amplifier 7 and the third fiber amplifier 9, may be a Ho fiber or a Tm / Ho fiber. Further, the base material of the fiber may be, in addition to silica glass, fluoride glass typified by ZBLAN fiber, selenide, sulfide, or chalcogenide glass such as telluride. In the above embodiment, for gain flattening and / or high output, for example, a filter and / or an optical amplifier may be added to the subsequent stage.

[0049] In the above embodiment, the first wavelength is set to a short wavelength shorter than the second wavelength (the second wavelength is a long wavelength longer than the first wavelength), but the first wavelength and the second wavelength are not particularly limited. The second wavelength may be any wavelength different from the first wavelength. In the above embodiment, the second fiber amplifier 7 is optimized for the short wavelength side and the third fiber amplifier 9 is optimized for the long wavelength side, but the present invention is not limited to this, and the second fiber amplifier 7 may be optimized for the long wavelength side and the third fiber amplifier 9 may be optimized for the short wavelength side.

[0050] In the above embodiment, the wavelength of the ultrashort pulsed light L is varied by using modulation by soliton self-frequency shift. However, the present invention is not limited to this, and the wavelength of the ultrashort pulsed light L may be varied by using various known methods. In the above embodiment, the ultrashort pulsed light L is output. However, the present invention is not limited to this, and other light may be output.

[0051] In the above embodiment, as the first fiber amplifier 3, a fiber amplifier of a double-clad fiber which is a normal dispersion fiber is used. However, instead of this, a fiber amplifier of a single-clad fiber (for example, erbium-doped) which is a normal dispersion fiber may be used. Even in this case, at least broadening of the spectrum of the ultrashort pulsed light L is possible.

[0052] Each configuration in the above embodiment and the above modification is not limited to the materials and shapes described above, and various materials and shapes can be applied. Further, each configuration in the above-described embodiment and modification can be arbitrarily applied to each configuration in other embodiments or modifications.

Description of Reference Numerals

[0053] 1... wavelength variable light source, 2... oscillator (light output unit, oscillation unit), 3... first fiber amplifier (light output unit, amplification unit), 4... acousto-optic modulator (light output unit, light intensity control unit), 5... compressor (light output unit), 6... soliton shift fiber (light output unit, modulation unit), 7... second fiber amplifier (first optical amplification unit), 8... filter (filter unit), 9... third fiber amplifier (second optical amplification unit), 50... light output unit, G1... gain, G2... gain, L... ultrashort pulsed light (light, pulsed light).

Claims

1. An optical output unit that outputs light and whose output light wavelength is variable, A first optical amplification unit that amplifies the light output by the optical output unit, A filter unit that filters the light amplified by the first optical amplification unit, A second optical amplification unit that amplifies the light filtered by the filter unit, comprising: The first optical amplification unit has a higher gain on the first wavelength side than on the second wavelength side different from the first wavelength, The second optical amplification unit has a higher gain on the second wavelength side than on the first wavelength side, The filter unit is a wavelength-variable light source that attenuates amplified spontaneous emission light.

2. The optical output unit can output light in a wavelength band of 1800 to 2200 nm, The first optical amplification unit and the second optical amplification unit include a Tm fiber, and the wavelength-variable light source according to Claim 1.

3. The first wavelength is shorter than the second wavelength, and the wavelength-variable light source according to Claim 1 or 2.

4. The filter unit attenuates amplified spontaneous emission light in a predetermined wavelength band including the wavelength range between the first wavelength and the second wavelength, and the wavelength-variable light source according to Claim 1 or 2.

5. The optical output unit is An oscillation unit that oscillates pulsed light, An amplification unit that broadens the spectrum of the pulsed light oscillated by the oscillation unit, A modulation unit that modulates the wavelength of the pulsed light whose spectrum has been broadened by the amplification unit using soliton self-frequency shift, An optical intensity control unit that is disposed between the amplification unit and the modulation unit in the optical path of the pulsed light and controls the intensity of the pulsed light for each pulse, and the wavelength-variable light source according to Claim 1 or 2.

6. An optical output step of outputting light, A first optical amplification step of amplifying the light output in the optical output step, A filtering step of filtering the light amplified in the first optical amplification step, A second optical amplification step of amplifying the light filtered in the filtering step, comprising: The optical output step includes a step of varying the wavelength of the output light, In the first optical amplification step, the light is amplified by a first optical amplification unit having a higher gain on the first wavelength side of the light than on the second wavelength side different from the first wavelength, In the second optical amplification step, the light is amplified by a second optical amplification unit having a higher gain on the second wavelength side than on the first wavelength side. A wavelength variable method for reducing spontaneous emission amplified light in a predetermined wavelength band including a wavelength range between the first wavelength and the second wavelength in the filtering step.

7. The wavelength variable method according to claim 6, wherein in the filtering step, energy is transferred from the light of the first wavelength to the light of the second wavelength.

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