Wavelength variable light source and wavelength variable method

Abstract

A wavelength variable light source includes: a light output section that outputs light, and the wavelength of the output light is variable; a first light amplification section that amplifies the light output by the light output section; a filter section that filters the light amplified by the first light amplification section; and a second light amplification section that amplifies the light filtered by the filter section. The first light amplification section has a higher gain on a first wavelength side than on a second wavelength side different from the first wavelength. The second light amplification section has a higher gain on the second wavelength side than on the first wavelength side. The filter section attenuates spontaneous emission amplified light.

Description

Technical Field

[0001] This disclosure relates to wavelength-variable light sources and wavelength-variable methods. Background Technology

[0002] As a wavelength-variable light source, there is a known light source that includes: an oscillation section for oscillating pulsed light; and a modulation section that modulates the wavelength of the pulsed light oscillating by the oscillation section using soliton self-frequency shift, and makes the wavelength of the output pulsed light variable by adjusting the intensity of the pulsed light before modulation (see, for example, Patent Document 1).

[0003] Existing technical documents

[0004] Patent documents

[0005] Patent Document 1: Japanese Patent Publication No. 2004-527001

[0006] Patent Document 2: International Publication No. 2004 / 070898 Summary of the Invention

[0007] The technical problem that the invention aims to solve

[0008] In the wavelength-variable light source described in Patent Document 1, when the output light is amplified by an optical amplifier, spontaneous emission (ASE) is generated, which not only hinders amplification but may also cause parasitic oscillations. In this case, it is possible to use an optical amplifier that matches the desired conditions for not generating spontaneous emission, but there are also cases where optical amplification is difficult to perform in wavelength bands where such an optical amplifier is difficult to use. Furthermore, even assuming that optical amplification is performed in a wavelength band where such an optical amplifier can be used, since the wavelength of light is variable, for example, if an optical amplifier that matches the conditions for light with a shorter wavelength is used, it is difficult to amplify light with a longer wavelength, and vice versa. This is especially significant when the difference between short and long wavelengths becomes large, that is, when optical amplification is performed in a wide wavelength band (for example, when optical amplification is performed in a wavelength band spanning 400 nm, such as optical amplification in a wavelength band of 1800 nm to 2200 nm). This is because as the wavelength band widens, the gain difference of the optical amplifier with respect to wavelength becomes larger. In this case, if a low-gain wavelength is to be amplified, the high-gain wavelength tends to be preferentially amplified. Conversely, in a specific wavelength band, such as one including 1550 nm, optical amplification can be achieved by configuring a filter for gain equalization between two optical amplifiers using Er fiber (see, for example, Patent Document 2). However, even in this case, since the achievable wavelength band is only about 150 nm, reliable optical amplification in a wider wavelength band is required.

[0009] This disclosure was made in view of the above-mentioned actual situation, and its purpose is to reliably amplify the light in a wide wavelength band in a wavelength-variable light source and wavelength-variable method for outputting light with variable wavelength.

[0010] Technical means to solve the problem

[0011] The wavelength-variable light source disclosed herein is, [1] "a wavelength-variable light source, comprising: a light output section that outputs light, and the wavelength of the output light is variable; a first light amplification section that amplifies the light output by the light output section; a filtering section that filters the light amplified by the first light amplification section; and a second light amplification section that amplifies the light filtered by the filtering section, wherein the first light amplification section has a higher gain on a first wavelength side than on a second wavelength side different from the first wavelength side, the second light amplification section has a higher gain on the second wavelength side than on the first wavelength side, and the filtering section reduces the spontaneously emitted amplified light."

[0012] In this wavelength-variable light source, when the wavelength of the output light is varied between a first wavelength side (hereinafter also referred to as "first wavelength side") which is closer to the first wavelength than the second wavelength and a second wavelength side (hereinafter also referred to as "second wavelength side") which is closer to the second wavelength than the first wavelength, the light can be amplified as follows: The light on the first wavelength side is amplified by the first optical amplification unit without the generation of an ASE (associated ester). Here, the second optical amplification unit has a high gain on the second wavelength side, but since the light on the first wavelength side has already been amplified by the first optical amplification unit and easily absorbs energy, the generation of an ASE can be suppressed while the light on the first wavelength side is amplified in the second optical amplification unit. On the other hand, for example, the light on the second wavelength side is not significantly amplified compared to the light on the first wavelength side, and therefore an ASE is generated, but this ASE can be reduced by the filtering unit. Then, the light on the second wavelength side is amplified by the second optical amplification unit without the generation of an ASE. Therefore, according to this disclosure, in a wavelength-variable light source with a variable output light wavelength, the light can be reliably amplified while suppressing an ASE over a wide wavelength band.

[0013] The wavelength-variable light source disclosed herein can also be, [2] "the wavelength-variable light source according to [1], wherein the light output section is capable of outputting light in the wavelength band of 1800-2200 nm, and the first light amplification section and the second light amplification section include Tm optical fiber." In this case, a light amplification system in the wavelength band of 1800-2200 nm can be realized using Tm optical fiber.

[0014] The wavelength-variable light source disclosed herein may also be, [3] "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 section, amplification optimized for the short wavelength side shorter than the second wavelength can be performed, and in the second optical amplification section, amplification optimized for the long wavelength side longer than the first wavelength can be performed.

[0015] The wavelength-variable light source disclosed herein may also be, [4] "a wavelength-variable light source according to any one of [1] to [3], wherein the filtering unit reduces the spontaneous emission amplified light of a predetermined wavelength band including the wavelength region between the first wavelength and the second wavelength." In this case, the filtering unit can effectively reduce the spontaneous emission amplified light generated by the amplification of the first optical amplification unit.

[0016] The wavelength-variable light source disclosed herein may also be, [5] "a wavelength-variable light source according to any one of [1] to [4], wherein the light output unit comprises: an oscillation unit for oscillating pulsed light; an amplification unit for broadbanding the spectrum of the pulsed light oscillated by the oscillation unit; a modulation unit for modulating the wavelength of the pulsed light after the spectrum is broadbanded by the amplification unit using soliton self-frequency shift; and a light intensity control unit disposed between the amplification unit and the modulation unit in the optical path of the pulsed light, controlling the intensity of the pulsed light per pulse." In this case, the wavelength of the generated pulsed light can be changed per pulse by means of the light intensity control unit. Furthermore, by broadbanding the spectrum of the pulsed light before modulation using soliton self-frequency shift, multi-solitonization (the phenomenon that the pulsed light splits and forms multiple pulsed lights due to modulation) can be suppressed.

[0017] The wavelength-variable method disclosed herein is as follows: [6] "A wavelength-variable method includes: a light output step; a first light amplification step for amplifying the light output in the light output step; a filtering step for filtering the light amplified in the first light amplification step; and a second light amplification step for amplifying the light filtered in the filtering step, wherein the light output step includes a step of making the wavelength of the output light variable, wherein in the first light amplification step, the light is amplified by a first light amplification unit having a higher gain on a first wavelength side than on a second wavelength side different from the first wavelength, wherein in the second light amplification step, the light is amplified by a second light 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 comprising a wavelength band including the wavelength region between the first wavelength and the second wavelength is attenuated." In this disclosure, the same effect as the aforementioned wavelength-variable light source is achieved. That is, in the wavelength-variable method for outputting light with variable wavelength, it is possible to reliably amplify the light while suppressing ASE (emission escalation) over a wide wavelength band.

[0018] The wavelength-variable method disclosed herein can also be, [7] "according to the wavelength-variable method described in [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, the wavelength dependence of the gain in both the first and second optical amplification sections can be adjusted.

[0019] The effects of the invention

[0020] According to this disclosure, in a wavelength-variable light source and a wavelength-variable method for outputting light with variable wavelength, the light can be reliably amplified over a wide wavelength band. Attached Figure Description

[0021] Figure 1 This is a block diagram illustrating a wavelength-variable light source according to an embodiment.

[0022] Figure 2 (a) shows from Figure 1 The graph shows the time waveform of the ultrashort pulse light output by the oscillator. Figure 2 (b) shows from Figure 1 The spectrum of the ultrashort pulse light output by the oscillator. Figure 2 (c) is showing from Figure 1 The graph shows the time waveform of the ultrashort pulse light output by the fiber amplifier. Figure 2 (d) is showing from Figure 1 The spectrum of ultrashort pulse light output from the fiber amplifier.

[0023] Figure 3 It shows from Figure 1 A specific example of the spectrum of ultrashort pulse light output by an optical fiber amplifier is shown in the figure.

[0024] Figure 4 (a) shows from Figure 1 The diagram shows the time waveform of the ultrashort pulse light output by the acousto-optic modulator. Figure 4 (b) shows from Figure 1 The spectrum of the ultrashort pulse light output by the acousto-optic modulator.

[0025] Figure 5 (a) shows from Figure 1 The figure shows the time waveform of the ultrashort pulse light output from the soliton frequency-shifting fiber. Figure 5 (b) shows from Figure 1 The spectrum of ultrashort pulse light output from a soliton frequency-shifting fiber. Figure 5 (c) is showing from Figure 1 The diagram shows the time waveform of the ultrashort pulse light output by the filter. Figure 5 (d) is showing from Figure 1 The spectrum of the ultrashort pulse light output by the filter is shown in the figure.

[0026] Figure 6 This is a flowchart illustrating the wavelength-variable method involved in the implementation.

[0027] Figure 7 (a) is a diagram illustrating an example of amplification of short-wavelength ultrashort pulses of light. Figure 7 (b) is used to explain Figure 7 The subsequent figure of (a). Figure 7 (c) is used to explain Figure 7 The subsequent figure of (b). Figure 7 (d) is used to explain Figure 7The subsequent figure of (c).

[0028] Figure 8 (a) is a diagram illustrating an example of amplification of long-wavelength ultrashort pulse light. Figure 8 (b) is used to explain Figure 8 The subsequent figure of (a). Figure 8 (c) is used to explain Figure 8 The subsequent figure of (b). Figure 8 (d) is used to explain Figure 8 The subsequent figure of (c).

[0029] Figure 9 This is a graph showing the relationship between the absorption cross-sectional area, stimulated emission cross-sectional area, and transmittance of the filter and wavelength of the Tm optical fiber. Detailed Implementation

[0030] The embodiments will now be described in detail with reference to the accompanying drawings. In the drawings, the same or equivalent parts are labeled with the same symbols, and repeated descriptions are omitted.

[0031] like Figure 1 As shown, 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 capable of rapidly changing the wavelength of the pulsed light with each pulse and amplifying pulsed light of all wavelengths within a variable wavelength band. For example, the wavelength-variable light source 1 is a femtosecond laser device, such as used in two-photon microscopy. 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 frequency-shifting fiber 6, a second fiber amplifier 7, a filter 8, a third fiber amplifier 9, and a wavelength conversion module 10.

[0032] Oscillator 2 constitutes the oscillation section that causes the ultrashort pulse light L to oscillate. Oscillator 2 is as follows: Figure 2 As shown in (a), an ultrashort pulse train with a specified period F1 is generated. The wavelength band of the ultrashort pulse light L oscillating from oscillator 2 can also be, for example, a band including 1550 nm. Here, oscillator 2 is as follows: Figure 2 As shown in (b), an ultrashort pulse light L with a spectrum having a first spectral width H1 and a first intensity K1 is oscillated. The oscillator 2 is not particularly limited, and various oscillators can be used.

[0033] The first fiber amplifier 3 constitutes an amplification section that broadbands the spectrum of the ultrashort pulse light L oscillated by the oscillator 2. The first fiber amplifier 3 broadbands the spectrum of the ultrashort pulse light L and increases its output through similarity amplification. The first fiber amplifier 3 is disposed between the oscillator 2 and the soliton frequency-shifting fiber 6 in the optical path of the ultrashort pulse light L.

[0034] The first fiber amplifier 3 comprises a normally dispersive fiber and an excitation source. The normally dispersive fiber is a double-clad fiber co-doped with erbium and ytterbium. That is, the first fiber amplifier 3 utilizes the normally dispersive double-clad fiber to generate nonlinear effects and amplify the light in a non-stretched manner, obtaining an ultrashort pulse light L as broadband amplified light. The normally dispersive fiber is an fiber with a negative dispersion parameter D (ps / nm / km). The additives used in the first fiber amplifier 3 are not particularly limited, and various additives can be used.

[0035] like Figure 2 (c) and Figure 2 As shown in (d), the first fiber amplifier 3 broadbands the spectral width of the ultrashort pulse light L to a second spectral width H2, which is wider than the first spectral width H1. The first fiber amplifier 3 also increases the output intensity of the ultrashort pulse light L to a second intensity K2, which is higher than the first intensity K1. Specifically, as... Figure 3 As shown, the first fiber amplifier 3 makes the spectral width of the ultrashort pulse light L greater than 100 nm. Figure 3 In the diagram, the horizontal axis represents the wavelength of the ultrashort pulse light L, and the vertical axis represents the relative value of the intensity of the ultrashort pulse light L relative to a specified intensity.

[0036] The acousto-optic modulator 4 constitutes an optical intensity control unit that controls the intensity of the ultrashort pulse light L with each pulse. The acousto-optic modulator 4 is a device that modulates the ultrashort pulse light L using 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 first fiber amplifier 3 and the soliton frequency-shifting fiber 6 in the optical path of the ultrashort pulse light L. It should be noted that the acousto-optic modulator 4 can be disposed at any position, as long as it is between the oscillator 2 and the soliton frequency-shifting fiber 6.

[0037] Acousto-optic modulator 4 Figure 4 (a) and Figure 4 As shown in (b), the intensity of the ultrashort pulse light L is controlled to vary with each pulse. For example, as... Figure 4 As shown in (a), when modulated with the same intensity as M1 and M2, such as Figure 4 As shown in (b), ultrashort pulses LM1 and LM2 are generated, corresponding to the intensities imparted by M1 and M2. The intensity modulation range and accuracy of the ultrashort pulses L (LM1, LM2) depend on the performance of the acousto-optic modulator 4. The intensity of each pulse in the pulse train of the ultrashort pulses L can be arbitrarily modulated by the acousto-optic modulator 4.

[0038] Compressor 5 constitutes a pulse compression section that compresses the pulse time width of the ultrashort pulse light L. In this embodiment, compressor 5 is disposed between the acousto-optic modulator 4 and the soliton frequency-shifting fiber 6 in the optical path of the ultrashort pulse light L. It should be noted that compressor 5 can be disposed in any position as long as it is between the first fiber amplifier 3 and the soliton frequency-shifting fiber 6. For example, even if the ultrashort pulse light L is broadened (e.g., broadened by several picoseconds) due to the first fiber amplifier 3, compressor 5 still compresses the time width of the ultrashort pulse light L, outputting an ultrashort pulse light L with a time width of less than a certain value (less than 1 picosecond). As for compressor 5, there is no particular limitation, and various compressors can be used.

[0039] The soliton frequency-shifting fiber 6 constitutes a modulation section, which uses soliton self-frequency shifting (Raman soliton frequency shifting) to modulate the wavelength of the ultrashort pulse light L, which is simultaneously broadbanded and has a high output by the first fiber amplifier 3. The soliton frequency-shifting fiber 6 is positioned downstream of the first fiber amplifier 3 in the optical path of the ultrashort pulse light L. The soliton frequency-shifting fiber 6 is as follows... Figure 5 (a) and Figure 5 As shown in (b), the wavelength of the ultrashort pulse light L is made variable, and a soliton S1 is generated.

[0040] The soliton frequency-shifting fiber 6 can, for example, be used in a single-mode anomalous dispersion fiber that exhibits anomalous dispersion in the wavelength band of the ultrashort pulse light L generated by the first fiber amplifier 3. Furthermore, by controlling the acousto-optic modulator 4, a soliton with a wavelength different from that of the soliton S1 can also be generated. The wavelength of the soliton S, for example, is in... Figure 5 As shown in (c), when modulated with the same intensity as M1 and M2, such as Figure 5 As shown in (d), soliton S is shifted to a wavelength corresponding to the intensity given by M1 and M2 (soliton S1 and S2).

[0041] The wavelength range and accuracy of soliton S shifting depend on the performance of the acousto-optic modulator 4. The wavelength of each soliton S in the soliton train generated by the pulse train of ultrashort pulse light L can be arbitrarily changed by intensity modulation of the pulse train by the acousto-optic modulator 4. The soliton frequency-shifting fiber 6 can output ultrashort pulse light L (soliton S) with a wavelength band of, for example, 1800 nm to 2200 nm. The oscillator 2, the first fiber amplifier 3, the acousto-optic modulator 4, the compressor 5, and the soliton frequency-shifting fiber 6 constitute the output light and the output light has a variable wavelength optical output section 50.

[0042] It should be noted that in the illustrated example, the ultrashort pulse light L modulated by soliton self-frequency shifting includes a non-soliton component S0 (a component that does not become soliton S1 or S2). Additionally, a filter (not illustrated) can be installed after the soliton frequency-shifting fiber 6 to cut off the non-soliton component S0 of the ultrashort pulse light L. The OD value of such a filter can also be 3 or higher.

[0043] The second fiber amplifier 7 constitutes a first optical amplification section, which amplifies the ultrashort pulse light L, whose wavelength is variable due to the soliton frequency-shifting fiber 6. The second fiber amplifier 7 is configured to include a rare-earth-doped fiber and an excitation source. The rare-earth-doped fiber is, for example, a Tm fiber. The excitation 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 the ultrashort pulse light L, which serves as the signal light (seed light), by absorbing gain.

[0044] The second fiber amplifier 7 has a higher gain G1 on the first wavelength side, which is the short wavelength side of the ultrashort pulse light L, than on the second wavelength side, which is the long wavelength side (see reference). Figure 7 (b)). The characteristics of such a second fiber amplifier 7 can be achieved, 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 less than 2200 nm.

[0045] Filter 8 constitutes a filtering section that filters the ultrashort pulse light L amplified by the second fiber amplifier 7. Filter 8 reduces ASE (Amplified Spontaneous Emission). Furthermore, in this embodiment, filter 8 reduces the ASE generated by the amplification of the second fiber amplifier 7, and also reduces the light after the noise component generated by the amplification of the second fiber amplifier 7 is amplified, as well as the light of wavelengths in the high-gain region. It should be noted that in this embodiment, the light of wavelengths in the high-gain region is light in a defined wavelength band within the short wavelength range, that is, light in the wavelength band with a particularly high amplification ratio compared to light in other wavelength bands. If this light is not reduced, only this light will be preferentially amplified, thus hindering optical amplification in a wide wavelength range. Here, the ASE reduced by filter 8 is the ASE in a defined wavelength band encompassing the wavelength range between short and long wavelengths. In addition to hindering amplification, the ASE and the light after the noise component is amplified may also generate parasitic oscillations and damage the amplifier system. ASE is prone to occur, for example, when the wavelength band of the seed light and the gain are mismatched, or when the power of the seed light is weak. Furthermore, in filter 8, the following process also occurs: pulses in the short-wavelength region are absorbed to become the gain of the pulses in the followed long-wavelength region, and energy transfer occurs from the pulses in the gain region of the gain fiber to the pulses in the region with less gain. It should be noted that filter 8 can also be, for example, a filter using an AOM (acousto-optic modulator) and an EOM (electro-optic modulator), a Tm fiber, a Ho fiber, or a Tm / Ho fiber where no excitation light is incident. Furthermore, filter 8 can also be a gain fiber in the second fiber amplifier 7 where the excitation light attenuation is greater than 20 dB.

[0046] The third fiber amplifier 9 constitutes a second optical amplification section, which amplifies the ultrashort pulse light L filtered by the filter 8. The third fiber amplifier 9 is configured to include a rare-earth-doped fiber and an excitation source. The rare-earth-doped fiber is, for example, a Tm fiber. The excitation 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 the ultrashort pulse light L as the signal light by absorbing gain. The third fiber amplifier 9 has a higher gain G2 (refer to) on the second wavelength side, which is the long wavelength side of the ultrashort pulse light L, than on the first wavelength side, which is the short wavelength side. Figure 7 (d)). The characteristics of such a third fiber amplifier 9 can be achieved, for example, by adjusting the length of the rare earth-doped fiber and the concentration of the added rare earth.

[0047] Wavelength conversion module 10 converts the wavelength of the ultrashort pulse light L to a target wavelength (e.g., the wavelength corresponding to a two-photon microscope). Wavelength conversion module 10 is configured downstream of the third fiber amplifier 9 in the optical path of the ultrashort pulse light L. As an example, wavelength conversion module 10 reduces the intensity of the ultrashort pulse light L and lowers the wavelength band.

[0048] Next, refer to Figure 6 The flowchart illustrates the wavelength-variable method implemented using wavelength-variable light source 1.

[0049] First, the ultrashort pulse light L is oscillated in oscillator 2 to generate an ultrashort pulse train with a specified period (oscillation step: step S1). The ultrashort pulse light L is then amplified by the first fiber amplifier 3 to increase its output and broaden its spectrum (amplification step: step S2). The intensity of the ultrashort pulse light L is controlled for each pulse by the acousto-optic modulator 4, for example, according to the specifications or conditions required by the wavelength-variable light source 1 (step S3). The time width of the ultrashort pulse light L is compressed by the compressor 5 (step S4). Finally, the wavelength of the broadbanded ultrashort pulse light L is modulated by soliton self-frequency shifting using the soliton frequency-shifting fiber 6, making it variable (step S5).

[0050] Next, the ultrashort pulse light L, whose wavelength was made variable in step S5, is amplified by the second fiber amplifier 7 (step S6). In step S6, the ultrashort pulse light L is amplified with a gain G1 that is higher on the short wavelength side than on the long wavelength side. The ultrashort pulse light L amplified in step S6 is then filtered by the filter 8 (step S7). Thus, the light with ASE in the specified wavelength band, the amplified noise component, and the wavelength of the high-gain region generated by the amplification by the second fiber amplifier 7 is reduced.

[0051] Next, the ultrashort pulse light L filtered in step S7 is amplified by the third fiber amplifier 9 (step S8). In step S8, the ultrashort pulse light L is amplified with a gain G2 that is higher on the long wavelength side than on the short wavelength side. Then, the wavelength of the amplified ultrashort pulse light L is converted to the target wavelength by the wavelength conversion module 10 (step S9). Steps S1 to S5 correspond to the optical output steps of the output light. Step S6 corresponds to the first optical amplification step, step S7 corresponds to the filtering step, and step S8 corresponds to the second optical amplification step.

[0052] In the wavelength-variable light source 1 and wavelength-variable method described above, for example... Figure 7 As shown in (a), when the wavelength of the ultrashort pulse light L output from the light output unit 50 is changed to a short wavelength λ1, the following effect is achieved. That is, as... Figure 7As shown in (b), the short-wavelength λ1 ultrashort pulse light L is amplified (pre-amplified) by a second fiber amplifier 7 with gain G1 without the generation of ASE. The pre-amplified short-wavelength λ1 ultrashort pulse light L is as follows: Figure 7 As shown in (c), the light passes directly through filter 8 without being specially attenuated. Then, the ultrashort pulse L passing through filter 8... Figure 7 As shown in (d), the light is further amplified by the third fiber amplifier 9 with a gain G2. At this time, the gain G2 is a higher gain on the long wavelength side, but since the short wavelength λ1 ultrashort pulse light L has already been amplified by the second fiber amplifier 7 and is easy to absorb energy, it is also amplified in the third fiber amplifier 9 while suppressing the generation of ASE (without considering ASE).

[0053] On the other hand, for example, Figure 8 As shown in (a), when the wavelength of the ultrashort pulse light L output from the light output unit 50 is changed to a longer wavelength λ2, the following effect is achieved. That is, as... Figure 8 As shown in (b), the ultrashort pulse L with a long wavelength λ2 is not significantly amplified compared to the ultrashort pulse L on the first wavelength side, which is the short wavelength λ1 side, thus producing ASE11. Then, as Figure 8 As shown in (c), ASE11 is reduced in filter 8. Then, the long-wavelength λ2 ultrashort pulse light L is amplified by the third fiber amplifier 9 with gain G2 without the generation of ASE. Therefore, according to this embodiment, in the wavelength-variable light source 1 and wavelength-variable method of the output ultrashort pulse light L, it is possible to reliably amplify the ultrashort pulse light L while suppressing ASE11 in a wide wavelength band. In particular, this embodiment is effective in cases of optical amplification across a wide wavelength band and in cases of optical amplification in a wavelength band where it is difficult to use an optical amplifier that matches the conditions.

[0054] In the wavelength-variable light source 1, the light output unit 50 is capable of outputting light in the wavelength band of 1800–2200 nm, and the second fiber amplifier 7 and the third fiber amplifier 9 include Tm fiber. In this case, an amplification system for ultrashort pulse light L in the wavelength band of 1800–2200 nm can be realized using Tm fiber.

[0055] 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 reduces the ASE (associated light source) within a defined wavelength band that includes the wavelength region between short and long wavelengths. In this case, the filter 8 can effectively reduce the ASE generated by the amplification of the second fiber amplifier 7.

[0056] In the wavelength-variable light source 1, the light output unit 50 includes: an oscillator 2 for oscillating an ultrashort pulse light L; a first fiber amplifier 3 for broadbanding the spectrum of the ultrashort pulse light L; a soliton frequency-shifting fiber 6 for modulating the wavelength of the broadbanded ultrashort pulse light L using soliton self-frequency shifting; and an acousto-optic modulator 4 disposed between the first fiber amplifier 3 and the soliton frequency-shifting fiber 6. In this case, the wavelength of the generated ultrashort pulse light L can be changed for each pulse by means of the acousto-optic modulator 4. Furthermore, by broadbanding the spectrum of the ultrashort pulse light L before modulation using soliton self-frequency shifting, multi-solitonization (the phenomenon where the ultrashort pulse light L splits due to modulation and forms multiple ultrashort pulse lights) can be suppressed.

[0057] It should be noted that, in the wavelength-variable light source 1, due to the use of a fiber laser, maintenance-free operation is achieved compared to the case where a Ti:sapphire laser is used. In the wavelength-variable light source 1, by adjusting the intensity (output) of the ultrashort pulse light L input to the soliton frequency-shifting fiber 6, the desired wavelength of the ultrashort pulse light L can be freely generated. Furthermore, since no structure requiring a physical moving part to switch wavelengths is used, the wavelength can be changed at a higher speed, for example, compared to the case where a Ti:sapphire laser is used.

[0058] Figure 9 This is a graph showing the relationship between the absorption cross-sectional area, stimulated emission cross-sectional area, and transmittance of a filter on wavelength for Tm optical fiber. Figure 9 In the diagram, the horizontal axis represents the wavelength of the ultrashort pulse light L, and the vertical axis represents the absorption cross-sectional area, stimulated emission cross-sectional area, and transmittance of the Tm fiber 8. When amplifying light with wavelengths of 1800–2200 nm, the Tm fiber exhibits… Figure 9 The absorption cross-sectional area 22 and the stimulated emission cross-sectional area 21 are shown. In a fiber amplifier system containing Tm fiber, if the fiber length is short, the gain on the long wavelength side is insufficient, and thus ASE is easily generated when amplifying light on the long wavelength side. On the other hand, if the fiber length is long, the absorption also increases, and thus light on the short wavelength side is over-absorbed. Therefore, ASE is easily generated when amplifying light on the short wavelength side. In this regard, in this embodiment, as described above, the fiber amplifier containing Tm fiber is configured as a two-stage system, with a second fiber amplifier 7 and a third fiber amplifier 9. Then, between the second fiber amplifier 7 and the third fiber amplifier 9, a filter 8 with a transmittance of 23 (dashed line in the figure) is inserted to remove ASE, thereby enabling the amplification of broadband wavelength-variable laser light.

[0059] In this embodiment, the broadband ultrashort pulse light L can be amplified without additional mechanisms. The broadband ultrashort pulse light L can be amplified without utilizing nonlinear optical effects such as Raman amplification. This embodiment can construct a broadband and high-output (e.g., 30 dBm or more) amplifier system. According to this embodiment, parasitic oscillations caused by ASE amplification can also be prevented. In this embodiment, as described above, the wavelength-variable light source 1 is used in a two-photon microscope. In this case, since high-output amplification can be performed in a broadband wavelength region, high-output and multi-wavelength imaging can be performed in the two-photon microscope in conjunction with the subsequent wavelength conversion system.

[0060] In this embodiment, the wavelength-variable method transfers energy from a short-wavelength λ1 ultrashort pulse light L to a long-wavelength λ2 ultrashort pulse light L during the filtering step (step S7 above). In this case, by transferring energy from the short-wavelength λ1 ultrashort pulse light L to the long-wavelength λ2 ultrashort pulse light L, the wavelength dependence of the gain in both the second fiber amplifier 7 and the third fiber amplifier 9 can be adjusted.

[0061] The present disclosure is not limited to the embodiments described above.

[0062] In the above embodiments, Tm fiber is used as an example in the second fiber amplifier 7 and the third fiber amplifier 9, but the laser medium doped in the fiber is not particularly limited and can also be Ho, or a laser medium co-doped with Tm and Ho. That is, the rare earth-doped fiber included in the second fiber amplifier 7 and the third fiber amplifier 9 can also be Ho fiber or Tm / Ho fiber. In addition, the matrix material of the fiber can be fluoride-based glass, selenide, sulfide or telluride glass, which are represented by ZBLAN fiber, in addition to quartz glass. In the above embodiments, in order to achieve gain flattening and / or high output, filters and / or optical amplifiers can be added in the subsequent stage, for example.

[0063] In the above embodiments, the first wavelength is set to a short wavelength shorter than the second wavelength (and the second wavelength is set to a long wavelength longer than the first wavelength), but the first wavelength and the second wavelength are not particularly limited. The second wavelength can be any wavelength different from the first wavelength. In the above embodiments, 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 it is not limited to this; it is also possible that the second fiber amplifier 7 is optimized for the long wavelength side, and the third fiber amplifier 9 is optimized for the short wavelength side.

[0064] In the above embodiments, the wavelength of the ultrashort pulse light L is variable by soliton self-frequency shifting and modulation. However, this is not a limitation; various known methods can also be used to make the wavelength of the ultrashort pulse light L variable. In the above embodiments, an ultrashort pulse light L is output, but this is not a limitation; other types of light can also be output.

[0065] In the above embodiment, a fiber amplifier with normal dispersion fiber, i.e., double-clad fiber, was used as the first fiber amplifier 3. However, it is also possible to use a fiber amplifier with normal dispersion fiber, i.e., single-clad fiber (e.g., erbium-doped fiber), instead. Even in this case, it is possible to at least achieve broadbanding of the spectrum of the ultrashort pulse light L.

[0066] The structures in the above embodiments and modifications are not limited to the materials and shapes described above, and can be applied to various materials and shapes. Furthermore, the structures in the above embodiments and modifications can be arbitrarily applied to structures in other embodiments or modifications.

[0067] Explanation of reference numerals in the attached figures

[0068] 1…wavelength variable light source, 2…oscillator (optical output section, oscillation section), 3…first fiber amplifier (optical output section, amplification section), 4…acousto-optic modulator (optical output section, optical intensity control section), 5…compressor (optical output section), 6…soliton frequency shifting fiber (optical output section, modulation section), 7…second fiber amplifier (first optical amplification section), 8…filter (filtering section), 9…third fiber amplifier (second optical amplification section), 50…optical output section, G1…gain, G2…gain, L…ultrashort pulse light (light, pulse light).

Claims

1. A wavelength-variable light source, in, include: A light output unit that outputs light, and the wavelength of the output light is variable; The first optical amplification unit amplifies the light output by the optical output unit; A filtering unit that filters the light amplified by the first optical amplification unit; and The second optical amplification unit amplifies the light filtered by the filtering unit. The first optical amplification section has a higher gain on the first wavelength side than on the second wavelength side, which is different from the first wavelength. The second optical amplification section has a higher gain on the second wavelength side than on the first wavelength side. The filtering section reduces the light emitted by spontaneous emission.

2. The wavelength-variable light source according to claim 1, wherein, The light output unit is capable of outputting light in the wavelength range of 1800–2200 nm. The first optical amplification section and the second optical amplification section both contain Tm optical fibers.

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

4. The wavelength-variable light source according to claim 1 or 2, wherein, The filtering unit reduces the spontaneous emission amplified light of a defined wavelength band that includes the wavelength region between the first wavelength and the second wavelength.

5. The wavelength-variable light source according to claim 1 or 2, wherein, The light output unit has: The oscillating section causes the pulsed light to oscillate; An amplification section that broadens the spectrum of the pulsed light oscillating from the oscillation section; The modulation unit modulates the wavelength of the pulsed light after the spectrum has been broadbanded by the amplification unit using soliton self-frequency shift; as well as A light intensity control unit 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.

6. A wavelength-variable method, in, include: The steps for outputting light; A first optical amplification step that amplifies the light output in the optical output step; A filtering step that filters the light amplified in the first optical amplification step; and A second optical amplification step amplifies the light filtered in the filtering step. The light output step includes the step of making the wavelength of the output light variable. In the first optical amplification step, the light is amplified by a first optical amplification unit that has a higher gain on the first wavelength side of the light than on the second wavelength side, which is different from the first wavelength. In the second optical amplification step, the light is amplified by a second optical amplification unit that has a higher gain on the second wavelength side than on the first wavelength side. In the filtering step, spontaneously emitted amplified light that includes a wavelength band encompassing the wavelength region between the first wavelength and the second wavelength is reduced.

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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