Wavelength variable light source and wavelength variable method

Abstract

To allow a wavelength variable light source and a wavelength variable method that can tune the wavelength of the output light to reliably amplify the light over a wide wavelength range.SOLUTION: A wavelength variable light source 1 includes an optical output unit that outputs ultrashort pulsed light L and can variably change the wavelength of the output ultrashort pulsed light L, 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.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a wavelength-tunable light source and a wavelength-tuning 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 [Patent Document 2] International Publication No. 2004 / 070898 Summary of the Invention [Problem to be solved by the invention]

[0004] In the wavelength-tunable light source described in Patent Document 1, when the output light is amplified by an optical amplifier, amplified spontaneous emission (ASE) is generated, which not only hinders amplification but also may cause parasitic oscillation. In this case, it is possible to use an optical amplifier tailored to desired conditions so that amplified spontaneous emission is not generated, but optical amplification may be performed in a wavelength band where it is difficult to use such an optical amplifier. Even if optical amplification is performed in a wavelength band where such an optical amplifier is possible, because the wavelength of the light is tunable, using an optical amplifier tailored to the conditions for light with shorter wavelengths makes it difficult to amplify light with longer wavelengths, and vice versa. This problem is particularly noticeable when the difference between short and long wavelengths is large, i.e., 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 1800 nm to 2200 nm wavelength band). This is because, as the wavelength band becomes wider, the difference in gain between the wavelengths of the optical amplifier becomes greater, and in such a case, when attempting to amplify a wavelength with low gain, light with a wavelength with high gain tends to be preferentially amplified. In contrast, in a specific wavelength band, such as a wavelength band including 1550 nm, optical amplification can be achieved by placing a filter for gain equalization between two optical amplifiers using Er fibers (see, for example, Patent Document 2). However, even in this case, the wavelength band that can be supported is only about 150 nm, so there is a demand for reliable optical amplification over a wider wavelength band.

[0005] The present disclosure has been made in consideration of the above-described circumstances, and aims to reliably amplify light over a wide wavelength band in a wavelength-tunable light source and wavelength tuning method that can tune the wavelength of output light. [Means for solving the problem]

[0006] The wavelength-tunable light source of the present disclosure is [1] "a wavelength-tunable light source comprising: an optical output unit that outputs light and is capable of varying the wavelength of the output light; 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; and a second optical amplification unit that amplifies the light filtered by the filter unit, wherein the first optical amplification unit has a higher gain on a first wavelength side than on a second wavelength side different from the first wavelength, and 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-tunable light source, when the wavelength of output light is tunable, for example, between a first wavelength side (hereinafter simply referred to as the "first wavelength side") that is closer to the first wavelength than the second wavelength and a second wavelength side (hereinafter simply referred to as the "second wavelength side") that is closer to the second wavelength than the first wavelength, the light can be amplified as follows. That is, the light on the first wavelength side is amplified by the first optical amplifier without generating ASE, and then amplified by the second optical amplifier. Here, although the second optical amplifier has a high gain on the second wavelength side, the light on the first wavelength side is already amplified by the first optical amplifier and therefore easily absorbs energy. Therefore, the second optical amplifier 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, and therefore ASE is generated, but the ASE can be attenuated by the filter. Thereafter, the light on the second wavelength side is amplified by the second optical amplifier without generating ASE. Therefore, according to the present disclosure, in a wavelength-tunable light source that can tune the wavelength of output light, it is possible to reliably amplify the light while suppressing ASE over a wide wavelength band.

[0008] The wavelength-tunable light source of the present disclosure may be [2] "the wavelength-tunable light source according to [1], wherein the optical output unit is capable of outputting light in a wavelength band of 1800 to 2200 nm, and the first optical amplification unit and the second optical amplification unit include Tm fiber." In this case, it is possible to realize an amplification system for light in the wavelength band of 1800 to 2200 nm by using Tm fiber.

[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 first wavelength is shorter than the second wavelength." In this case, the first optical amplification unit enables amplification optimized for short wavelengths shorter than the second wavelength, and the second optical amplification unit enables amplification optimized for long wavelengths longer than the first wavelength.

[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 filter unit attenuates the amplified spontaneous emission light in a predetermined wavelength band including a 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 amplifier unit.

[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], wherein the optical output unit has an oscillator that oscillates pulsed light, an amplifier that broadens the spectrum of the pulsed light oscillated by the oscillator, a modulator that modulates the wavelength of the pulsed light whose spectrum has been broadened by the amplifier using soliton self-frequency shift, and a light intensity controller that is arranged between the amplifier and the modulator on the optical path of the pulsed light and controls the intensity of the pulsed light for each pulse." In this case, the light intensity controller can variably change the wavelength of the generated pulsed light for each pulse. Furthermore, by broadening the spectrum of the pulsed light before modulation using soliton self-frequency shift, it is possible to suppress multi-soliton generation (a phenomenon in which pulsed light is split into multiple pulsed lights due to modulation).

[0012] The wavelength tuning method of the present disclosure is [6] "a wavelength tuning method comprising 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 tuning the wavelength of the light to be output, wherein the first optical amplification step amplifies the light using a first optical amplification unit having a higher gain on a first wavelength side than on a second wavelength side different from the first wavelength, and the second optical amplification step amplifies the light using a second optical amplification unit having a higher gain on the second wavelength side than on the first wavelength side, and the filtering step attenuates amplified spontaneous emission light in a predetermined wavelength band including a wavelength range between the first wavelength and the second wavelength." This disclosure also achieves the same effects as the wavelength tunable light source described above. That is, in a wavelength tuning method in which the wavelength of output light is tunable, it is possible to reliably amplify the light while suppressing ASE over a wide wavelength band.

[0013] The wavelength tuning method of the present disclosure may be [7] "the wavelength tuning 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. [Effects of the Invention]

[0014] According to the present disclosure, it is possible to reliably amplify light over a wide wavelength band in a wavelength-tunable light source and wavelength tuning method that can tune the wavelength of output light. [Brief explanation of the drawings]

[0015] [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 ultrashort pulsed light output from the acousto-optic modulator of Fig. 1. Fig. 4(b) is a graph showing the spectrum of the 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 an ultrashort optical pulse output from the soliton-shifted fiber of Fig. 1. Fig. 5(b) is a graph showing the spectrum of an ultrashort optical pulse output from the soliton-shifted fiber of Fig. 1. Fig. 5(c) is a graph showing the time waveform of an ultrashort optical pulse output from the filter of Fig. 1. Fig. 5(d) is a graph showing the spectrum of an ultrashort optical pulse output from the filter of Fig. 1. [Figure 6] 4 is a flowchart illustrating a wavelength tuning method according to the embodiment. [Figure 7] Fig. 7(a) is a graph for explaining an example of amplifying a short-wavelength ultrashort pulse light. Fig. 7(b) is a graph for explaining a continuation of Fig. 7(a). Fig. 7(c) is a graph for explaining a continuation of Fig. 7(b). Fig. 7(d) is a graph for explaining a continuation of Fig. 7(c). [Figure 8] Fig. 8(a) is a graph for explaining an example of amplifying long-wavelength ultrashort pulsed light. Fig. 8(b) is a graph for explaining a continuation of Fig. 8(a). Fig. 8(c) is a graph for explaining a continuation of Fig. 8(b). Fig. 8(d) is a graph for explaining a continuation of Fig. 8(c). [Figure 9]FIG. 9 is a graph showing the relationship between the absorption cross section and stimulated emission cross section of a Tm fiber and the transmittance of a filter versus wavelength. DETAILED DESCRIPTION OF THE INVENTION

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

[0017] 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 pulse by pulse at high speed and amplify pulsed light of all wavelengths in the tunable wavelength band. For example, the tunable light source 1 is a femtosecond laser device used in, for example, a two-photon microscope. The tunable 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 1550 nm, for example. As shown in FIG. 2(b), the oscillator 2 here oscillates ultrashort pulsed light L having a spectrum with a first spectral width H1 and a first intensity K1. The oscillator 2 is not particularly limited, and various oscillators can be used.

[0019] The first fiber amplifier 3 constitutes an amplification section that broadens the spectrum of the ultrashort optical pulse L oscillated by the oscillator 2. The first 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 first 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.

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

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

[0022] The acousto-optic modulator 4 constitutes a light intensity control section that controls the intensity of the ultrashort pulsed light L for each pulse. The acousto-optic modulator 4 is a device that modulates the ultrashort pulsed light L by utilizing the force of 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 shifted fiber 6 in the optical path of the ultrashort pulsed light L. Note that the acousto-optic modulator 4 may be disposed at any position between the oscillator 2 and the soliton shifted fiber 6.

[0023] As shown in Figures 4(a) and 4(b), the acousto-optic modulator 4 controls the intensity of the ultrashort pulsed light L so that it changes for each pulse. For example, when intensity modulations M1 and M2 are given as shown in Figure 4(a), 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 ultrashort pulsed light L (LM1, LM2) depend on the performance of the acousto-optic modulator 4. The intensity of each pulse light in the pulse train of the ultrashort pulsed light L can be modulated arbitrarily by the acousto-optic modulator 4.

[0024] The compressor 5 constitutes a pulse compression section that compresses the time width of the pulses of the ultrashort optical pulse L. In this embodiment, the compressor 5 is disposed between the acousto-optic modulator 4 and the soliton-shifted fiber 6 in the optical path of the ultrashort optical pulse L. The compressor 5 may be disposed at any position between the first fiber amplifier 3 and the soliton-shifted fiber 6. Even if the ultrashort optical pulse L is stretched (for example, by several picoseconds) by the first fiber amplifier 3, the compressor 5 compresses the time width of the ultrashort optical pulse L and outputs ultrashort optical pulse L with a time width spread of a certain amount or less (less than 1 picosecond). The compressor 5 is not particularly limited, and various compressors can be used.

[0025] The soliton-shifted fiber 6 constitutes a modulation section that utilizes soliton self-frequency shift (Raman soliton shift) to modulate the wavelength of the ultrashort pulsed light L, the output of which has been increased while broadening the spectrum in the first fiber amplifier 3. The soliton-shifted fiber 6 is disposed downstream of the first fiber amplifier 3 in the optical path of the ultrashort pulsed light L. As shown in Figures 5(a) and 5(b), the soliton-shifted fiber 6 tunes the wavelength of the ultrashort pulsed light L to generate solitons S1.

[0026] 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 ultrashort pulsed light L generated by the first 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).

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

[0028] In the illustrated example, the ultrashort optical pulse L modulated by the soliton self-frequency shift contains non-soliton components S0 (components that do not become solitons S1 or S2). A filter (not shown) that cuts out the non-soliton components S0 of the ultrashort optical pulse L may be provided downstream of the soliton-shifted 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 section that amplifies the ultrashort pulse light L whose wavelength has been tuned by the soliton shift fiber 6. The second fiber amplifier 7 includes a rare-earth doped fiber and a pumping light source. The rare-earth doped fiber is, for example, a Tm fiber. The pumping light source is, for example, a laser diode that outputs pumping light with a wavelength of 1550 nm. The second fiber amplifier 7 is an amplifier optimized for short wavelengths. The second fiber amplifier 7 amplifies the ultrashort pulse light L as signal light (seed light) by absorbing the gain.

[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 achieved, for example, by adjusting the length of the rare earth doped fiber and the concentration of the doped rare earth. As an example, the short wavelength (first wavelength) is a wavelength not less than 1800 nm and less than 2000 nm, and the long wavelength (second wavelength) is a wavelength not less than 2000 nm and not more than 2200 nm.

[0031] The filter 8 constitutes a filter section that filters the ultrashort pulse light L amplified by the second fiber amplifier 7. The filter 8 attenuates ASE (Amplified Spontaneous Emission). In this embodiment, the filter 8 attenuates not only ASE generated by the amplification by the second fiber amplifier 7, but also light resulting from amplified noise components generated by the amplification by the second fiber amplifier 7 and light with wavelengths in the high gain region. In this embodiment, light with wavelengths in the high gain region refers to light in a specific wavelength band at short wavelengths that has a particularly high amplification rate 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 specific wavelength band that includes a wavelength range between short and long wavelengths. Light resulting from amplified ASE and noise components not only hinders amplification, but may also cause parasitic oscillations and damage the amplifier system. ASE tends to occur easily, for example, when the wavelength bands of the seed light and the gain do not match, or when the power of the seed light is weak. Furthermore, in filter 8, pulses in the short wavelength region are absorbed, resulting in the gain of pulses in the following long wavelength region, and a process of energy transfer from pulses in the gain region of the gain fiber to pulses in the low gain region also exists. Note that filter 8 may be, for example, one that utilizes an AOM (acousto-optic modulator) or an EOM (electro-optic modulator), or a gain fiber such as a Tm fiber, a Ho fiber, or a Tm / Ho fiber that does not allow pump light to be incident. Furthermore, filter 8 may be a gain fiber in the region of the gain fiber of second fiber amplifier 7 where the pump light is attenuated by 20 dB or more.

[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 includes a rare-earth-doped fiber and a pumping light source. The rare-earth-doped fiber is, for example, a Tm fiber. The pumping light source is, for example, a laser diode that outputs pumping light with a wavelength of 790 nm or a CW laser that outputs pumping 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 pulsed light L as signal light by absorbing the gain. 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 achieved, for example, by adjusting the length of the rare-earth-doped fiber and the concentration of the rare earth to be doped.

[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 disposed downstream 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 tuning method carried out using the wavelength tunable light source 1 will be described with reference to the flowchart of FIG.

[0035] 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 first fiber amplifier 3 increases the output power of the ultrashort optical pulse L and broadens the spectrum of the ultrashort optical pulse L (amplification step: step S2). An acousto-optic modulator 4 controls the intensity of the ultrashort optical pulse L for each pulse, for example, in accordance with the specifications or conditions required of the wavelength-tunable light source 1 (step S3). A compressor 5 compresses the time width of the ultrashort optical pulse L (step S4). A soliton shift fiber 6 modulates and variates the wavelength of the spectrally broadened ultrashort optical pulse L using soliton self-frequency shifting (step S5).

[0036] Next, the ultrashort pulsed light L, the wavelength of which has been tuned in step S5, is amplified by the second fiber amplifier 7 (step S6). In step S6, the ultrashort pulsed light L is amplified with a gain G1 that is higher on the short wavelength side than on the long wavelength side. The ultrashort pulsed light L amplified in step S6 is filtered by the filter 8 (step S7). This attenuates ASE in a predetermined wavelength band generated by amplification by the second fiber amplifier 7, light in which noise components have been amplified, and light with wavelengths in the high gain region.

[0037] Next, the ultrashort pulsed light L filtered in step S7 is amplified by a third fiber amplifier 9 (step S8). In step S8, the ultrashort pulsed light L is amplified at a gain G2 that is higher on the long wavelength side than on the short wavelength side. Then, the wavelength of the amplified ultrashort pulsed light L is converted to a target wavelength by a wavelength conversion module 10 (step S9). Steps S1 to S5 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] The wavelength-tunable light source 1 and wavelength tuning method described above have the following advantages when, for example, the wavelength of the ultrashort pulse light L output from the optical output unit 50 is tuned to a short wavelength λ1 as shown in FIG. 7( a). That is, as shown in FIG. 7( b), the ultrashort pulse light L of the short wavelength λ1 is amplified (pre-amplifier) ​​by the second fiber amplifier 7 having a gain G1 without generating ASE. The pre-amplified ultrashort pulse light L of the short wavelength λ1 passes through the filter 8 without being attenuated, as shown in FIG. 7( c). The ultrashort pulse light L that has passed through the filter 8 is then 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, the ultrashort pulse light L of the short wavelength λ1 is more likely to absorb energy since it has already been amplified by the second fiber amplifier 7, and therefore is amplified in the third fiber amplifier 9 while suppressing ASE generation (without worrying about ASE).

[0039] On the other hand, for example, as shown in FIG. 8( a), when the wavelength of the ultrashort optical pulse L output from the optical output unit 50 is tuned to the long wavelength λ2, the following effect is achieved. That is, as shown in FIG. 8( b), the ultrashort optical pulse L with the long wavelength λ2 is not amplified as much as the ultrashort optical pulse L with the first wavelength (i.e., the short wavelength λ1), and therefore ASE 11 is generated. Thereafter, as shown in FIG. 8( c), the ASE 11 is attenuated by the filter 8. The ultrashort optical pulse L with the long wavelength λ2 is then amplified by the third fiber amplifier 9 having a gain G2 without generating ASE. Therefore, according to this embodiment, a wavelength-tunable light source 1 and wavelength tuning method capable of tunable wavelengths of output ultrashort optical pulses L can reliably amplify the ultrashort optical pulse L while suppressing ASE 11 over a wide wavelength band. This embodiment is particularly effective when optical amplification is performed over a wide wavelength band and when optical amplification is performed in a wavelength band where it is difficult to use an optical amplifier tailored to the conditions.

[0040] In the wavelength-tunable light source 1, the optical output unit 50 can output light in the wavelength band of 1800 to 2200 nm, and the second fiber amplifier 7 and the third fiber amplifier 9 include Tm fibers. In this case, it is possible to realize an amplification system for ultrashort pulsed light L in the wavelength band of 1800 to 2200 nm by using Tm fibers.

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

[0042] The wavelength-tunable light source 1 has an optical output unit 50 that includes an oscillator 2 that generates ultrashort optical pulses L, a first fiber amplifier 3 that broadens the spectrum of the ultrashort optical pulses L, a soliton shift fiber 6 that modulates the wavelength of the spectrally broadened ultrashort optical pulses L 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 makes it possible to tune the wavelength of the generated ultrashort optical pulses L for each pulse. Furthermore, broadening the spectrum of the ultrashort optical pulses L before modulation using soliton self-frequency shift can suppress multi-soliton generation (a phenomenon in which the ultrashort optical pulses L are split into multiple ultrashort optical pulses due to modulation).

[0043] The wavelength-tunable light source 1 uses a fiber laser, which makes it possible to achieve maintenance-free operation compared to when a titanium sapphire laser is used. The wavelength-tunable light source 1 can freely generate ultrashort pulsed light L of a desired wavelength by adjusting the intensity (output) of the ultrashort pulsed light L input to the soliton-shifted fiber 6. Furthermore, because no configuration requiring physically moving parts is used when converting the wavelength, the wavelength can be tuned at a higher speed compared to when a titanium sapphire laser is used, for example.

[0044] FIG. 9 is a graph showing the relationship between wavelength and the absorption cross section, stimulated emission cross section, and filter transmittance of a Tm fiber. In FIG. 9, the horizontal axis represents the wavelength of the ultrashort pulsed light L, and the vertical axis represents the absorption cross section, stimulated emission cross section, and transmittance of a filter 8 of the Tm fiber. When amplifying light with a wavelength of 1800 to 2200 nm, the Tm fiber has the absorption cross section 22 and stimulated emission cross section 21 shown in FIG. 9. In a fiber amplifier system including a Tm fiber, if the fiber length is short, the gain on the long wavelength side is insufficient, and ASE is likely to occur when amplifying light on the long wavelength side. On the other hand, if the fiber length is long, absorption increases, resulting in excessive absorption of light on the short wavelength side. Therefore, ASE is likely to occur when amplifying light on the short wavelength side. In this regard, in this embodiment, as described above, a two-stage fiber amplifier including a Tm fiber is provided, with a second fiber amplifier 7 and a third fiber amplifier 9. Then, a filter 8 with a transmittance of 23 (dotted line in the figure) is inserted between the second fiber amplifier 7 and the third fiber amplifier 9 to remove ASE, thereby enabling amplification of a broadband wavelength-tunable laser light.

[0045] In this embodiment, broadband ultrashort pulsed light L can be amplified without using a separate mechanism to amplify the broadband ultrashort pulsed light L. Broadband ultrashort pulsed light L can be amplified without using nonlinear optical effects such as Raman amplification. This embodiment can configure a broadband and high-output (e.g., 30 dBm or more) amplifier system. This embodiment can also prevent parasitic oscillation due to increased ASE. In this embodiment, as described above, the wavelength-tunable light source 1 is used in a two-photon microscope. In this case, high-output amplification is possible in a broad wavelength range, and therefore, in combination with a downstream wavelength conversion system, high-output and multi-wavelength imaging is possible in the two-photon microscope.

[0046] In the wavelength tuning method of this embodiment, in the filtering step (step S7 above), energy is transferred from the ultrashort pulsed light L with a short wavelength λ1 to the ultrashort pulsed light L with a long wavelength λ2. In this case, by transferring energy from the ultrashort pulsed light L with a short wavelength λ1 to the ultrashort pulsed 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-described embodiments, a Tm fiber is given as an example of the fiber included in the second fiber amplifier 7 and the third fiber amplifier 9. However, there are no particular limitations on the laser medium doped in the fiber, and a laser medium co-doped with Ho or Tm and Ho may also be used. That is, the rare-earth-doped fiber included in the second fiber amplifier 7 and the third fiber amplifier 9 may be a Ho fiber or a Tm / Ho fiber. Furthermore, the base material of the fiber may be, in addition to silica glass, a fluoride-based glass such as ZBLAN fiber, or a chalcogenide glass such as a selenide, sulfide, or telluride. In the above-described embodiments, for example, a filter and / or an optical amplifier may be added in a subsequent stage in order to achieve gain flattening and / or high output power.

[0049] In the above embodiment, the first wavelength is 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 short wavelengths and the third fiber amplifier 9 is optimized for long wavelengths, but this is not limiting, and the second fiber amplifier 7 may be optimized for long wavelengths and the third fiber amplifier 9 may be optimized for short wavelengths.

[0050] 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.

[0051] In the above embodiment, a fiber amplifier of a double-clad normal dispersion fiber is used as the first 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.

[0052] 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]

[0053] 1...Tunable wavelength 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, light intensity control section), 5...Compressor (optical output section), 6...Soliton shift fiber (optical output section, modulation section), 7...Second fiber amplifier (first optical amplification section), 8...Filter (filter section), 9...Third fiber amplifier (second optical amplification section), 50...Optical output section, G1...Gain, G2...Gain, L...Ultrashort pulse light (light, pulsed light).

Claims

1. an optical output unit that outputs light and has a variable wavelength of the output light; a first optical amplifier that amplifies the light output from the optical output unit; a filter unit that filters the light amplified by the first optical amplifier unit; a second optical amplifier that amplifies the light filtered by the filter, the first optical amplifying unit has a higher gain on a first wavelength side than on a second wavelength side different from the first wavelength, the second optical amplifying unit has a higher gain on the second wavelength side than on the first wavelength side, The filter unit attenuates the amplified spontaneous emission light.

2. the optical output unit is capable of outputting light in a wavelength band of 1800 to 2200 nm, The tunable light source according to claim 1 , wherein the first optical amplification section and the second optical amplification section include Tm fibers.

3. The tunable light source according to claim 1 , wherein the first wavelength is shorter than the second wavelength.

4. 3. The wavelength-tunable light source according to claim 1, wherein the filter section attenuates amplified spontaneous emission light in a predetermined wavelength band including a wavelength range between the first wavelength and the second wavelength.

5. The optical output unit an oscillator that oscillates pulsed light; an amplifier that broadens the spectrum of the pulsed light oscillated by the oscillator; a modulation unit that modulates the wavelength of the pulsed light, the spectrum of which has been broadened by the amplification unit, by utilizing a soliton self-frequency shift; 3. The wavelength-tunable light source according to claim 1, further comprising: a light intensity control unit disposed between the amplifier and the modulator in an optical path of the pulsed light, the light intensity control unit controlling the intensity of the pulsed light for each pulse.

6. a light output step of outputting light; a first optical amplification step of amplifying the light outputted 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, the light output step includes a step of varying the wavelength of the light to be output, In the first optical amplification step, the light is amplified by a first optical amplification unit having a higher gain on a first wavelength side of the light than on a 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; In the filtering step, the amplified spontaneous emission light in a predetermined wavelength band including a wavelength range between the first wavelength and the second wavelength is attenuated.

7. 7. The wavelength tuning method according to claim 6, wherein the filtering step transfers energy from the light of the first wavelength to the light of the second wavelength.

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