Filter device, tunable laser, and wavelength adjustment method

Abstract

To provide a filter device which can be applied to a wavelength-variable laser that has characteristics such as high-speed wavelength sweeping, environmental stability, and low cost, and has simple design.SOLUTION: A filter device 30 comprises: a polarization holding fiber 31 for filter; a stretcher 32 which adjusts stress to be applied to a stress application part 31c in the polarization holding fiber 31 for filter; and a polarizer 35 which is arranged on the emission side of the stretcher 32.SELECTED DRAWING: Figure 1

Description

[Technical Field] 【0001】 The present invention relates to a wavelength-tunable filter device, a tunable laser using the same, and a wavelength tuning method. [Background technology] 【0002】 Wavelength-swept mode-locked lasers are essential light sources in fields such as optical measurement and imaging, particularly stimulated Raman scattering microscopy. These lasers require characteristics such as high-speed wavelength sweeping, environmental stability, low cost, and a simple design. 【0003】 Wavelength-swept mode-locked lasers are known to have a fiber-based spectral filter with adjustable bandwidth in a full normal dispersion cavity (see Non-Patent Document 1). Additionally, wavelength-swept mode-locked lasers are known to have a galvanometer-driven in-cavity filter (see Non-Patent Document 2). 【0004】 The apparatus described in Non-Patent Documents 1 and 2 does not fully meet the requirements for high-speed wavelength sweeping, environmental stability, low cost, and simple design due to complex cavities, free-space components, and unstable unpolarized fiber configurations. [Prior art documents] [Non-patent literature] 【0005】 [Non-Patent Document 1] Ankita Khanolkar et al., "All normal dispersion fiber laser with a bandwidth tunable fiber based spectral filter," Opt. Lett. 45, 4555 4558 (2020) [Non-Patent Document 2] Yasuyuki Ozeki et al., "Fast wavelength tunable picosecond pulses from a passively mode locked Er fiber laser using a galvanometer driven intracavity filter," Opt. Express 23, 15186 15194 (2015) [Overview of the Initiative] 【0006】 The present invention has been made in view of the above-mentioned background art, and aims to provide a filter device with a simple design that is applicable to tunable lasers having characteristics such as high-speed wavelength sweeping, environmental stability, and low cost. 【0007】 The present invention aims to provide a tunable laser capable of high-speed wavelength sweeping and a wavelength adjustment method using the filter device described above. 【0008】 To achieve the above objective, the filter device according to the present invention comprises a polarization-maintaining fiber, a stretcher for adjusting the stress applied to the stress-applying portion of the polarization-maintaining fiber, and a polarizer disposed on the output side of the stretcher. 【0009】 In the above-described filter device, the tensile strain in the stress-applying section of the polarization-maintaining fiber is adjusted by adjusting the stress applied to the stress-applying section using a stretcher. This allows for adjustment of the change in birefringence in the stress-applying section, enabling tuning to improve the transmittance of light of a specific wavelength. 【0010】 In a specific aspect of the present invention, the filter device further comprises a polarization branching unit located on the incident side of the stretcher for branching polarization, and a polarization restoration unit located on the outgoing side of the stretcher for restoring the polarization that has passed through the stretcher. In this case, the polarization branching unit and the polarization restoration unit function as polarization controllers and can control the polarization direction of the polarization incident on the filter device and the polarization emitted from the filter device. 【0011】 In another aspect of the present invention, the polarization branching portion is a splice in which the velocity axis of the polarization-maintaining fiber is tilted at 45°, and the polarization return portion is a splice in which the velocity axis of the polarization-maintaining fiber is tilted at -45°. 【0012】 In yet another aspect of the present invention, the polarization-maintaining fiber is either a PANDA fiber or a bowtie fiber. 【0013】 To achieve the above objective, the tunable laser according to the present invention comprises a polarization-maintaining ring-shaped resonator, an optical gain unit disposed within the resonator, an optical supply unit that supplies polarization to propagate in the circumferential direction to the resonator, and the aforementioned filter device. 【0014】 In the above-described tunable laser, wavelength adjustment can be performed on the transmittance using a tensile strain-controlled filter device. This wavelength adjustment utilizes changes in birefringence and is approximately two orders of magnitude more efficient than simply adjusting the length. This enables the realization of a tunable laser with a wide tuning range and a linear tuning relationship. The output is extremely stable due to the simple polarization-maintaining fiber structure. 【0015】 In a specific aspect of the present invention, the above-mentioned tunable laser further comprises a transmission adjustment unit located in the resonator, and is a mode-locked laser that performs passive mode-locked operation. In this case, the oscillation state can be stabilized, and polarization can be generated and extracted with high efficiency. This makes it possible to realize a femtosecond pulse mode-locked laser with a wide tuning range and a linear tuning relationship. When the stretcher operates at the resonant frequency, it is possible to achieve wavelength-swept femtosecond pulse laser results on the order of kHz. 【0016】 To achieve the above object, a wavelength adjustment method according to the present invention provides a stretcher for adjusting the stress applied to a stress application portion of a polarization maintaining fiber, and a polarizer disposed on the output side of the stretcher. In the filter device, a predetermined stress is applied to the stress application portion to change the resonance frequency interval. 【Brief Description of the Drawings】 【0017】 [Figure 1] (A) and (B) are conceptual perspective views for explaining the filter device of the first embodiment. [Figure 2] It is a conceptual perspective view for explaining the filter device of the first embodiment. [Figure 3] It is a conceptual diagram mainly explaining the structure of the stretcher in the filter device. [Figure 4] It is a diagram for explaining the relationship between the output voltage of the waveform generator and the amount of microstrain by the stretcher in the filter device. [Figure 5] It is a diagram for explaining the characteristics of the transmittance of the filter device. [Figure 6] (A) is a diagram for explaining the transmittance peak shift of the filter device, and (B) is a diagram for explaining the output wavelength of the filter device with respect to stress. [Figure 7] It is a conceptual diagram for explaining the wavelength-variable laser of the second embodiment. [Figure 8] It is a chart showing the optical spectrum of the output light of the wavelength-variable laser shown in FIG. 7. [Figure 9] It is a chart showing the relationship between the center wavelength of the output pulse of the output light and the output voltage of the waveform generator. [Figure 10] It is a conceptual diagram for explaining the wavelength-variable laser of the third embodiment. [Figure 11] It is a conceptual diagram for explaining the filter device of the fourth embodiment. [Figure 12] It is a chart showing the optical spectrum of the output light of a modification of the wavelength-variable laser shown in FIG. 7. 【Embodiments for Carrying Out the Invention】 【0018】 [First Embodiment] Hereinafter, a first embodiment of the filter device according to the present invention will be described with reference to Figures 1(A), 1(B), and 2, etc. Figures 1(A) and 1(B) illustrate an example configuration in which a predetermined stress is applied to the filter device 30, and the transmittance is maximized at a predetermined wavelength. Figure 2 illustrates an example configuration in which a predetermined stress is applied to the filter device 30, and the transmittance is minimized at a predetermined wavelength. The wavelengths of light incident on the filter device 30 shown in Figures 1(A) and 1(B) are assumed to be different, while the wavelengths of light incident on the filter device 30 shown in Figures 1(A) and 2 are assumed to be the same. In Figures 1(A), etc., X, Y, and Z represent the overall coordinate system, while x and y represent the local coordinate system of the area of ​​interest. x and y give the coordinates of a point on the optical axis AX of the filter device 30, and correspond to two directions orthogonal to the optical axis AX, while z corresponds to a direction parallel to the optical axis AX. 【0019】 Figures 1(A), 1(A), and 2 show the case where polarized light P0 is incident on the filter device 30 from the right side. The upper region of Figure 1(A), etc., explains the polarization state or polarization direction at each position passing through the filter device 30 as viewed from the left side of the paper along the optical axis AX. 【0020】 The filter device 30 shown in Figures 1(A), 1(B), etc., utilizes birefringence to transmit only the wavelength with maximum transmittance, thereby generating a narrow passband. It is similar to the basic elements of a Lyot filter and functions as a tunable wavelength optical device. The filter device 30 comprises a polarization-maintaining fiber 31 for filtering, a stretcher 32, a polarizer 35, a polarization splitter 34, and a polarization return unit 36. The stretcher 32 is attached to a part of the polarization-maintaining fiber 31 for filtering. That is, a part of the polarization-maintaining fiber 31 for filtering is wound around and fixed to the stretcher 32, and the applied stress is adjusted. The polarization splitter 34 is located on the incident side of the polarization-maintaining fiber 31 for filtering, i.e., on the incident side of the stretcher 32. The polarization return unit 36 ​​is located on the exit side of the polarization-maintaining fiber 31 for filtering, i.e., on the exit side of the stretcher 32. The polarizer 35 is positioned between the output end 31b and the polarization recovery section 36 of the polarization-maintaining fiber 31 for the filter. The filter device 30 is connected at the input end 31a and the output end 31b to, for example, the fiber FO that constitutes the resonator. A polarization-maintaining fiber is used for the fiber FO. 【0021】 The polarization-maintaining fiber 31 for the filter has different polarization velocities in the fast axis direction and the slow axis direction, and its birefringence can be varied by applying stress. The birefringence variation induced by strain causes changes in the peak position and peak spacing of the transmittance. For example, either a PANDA fiber or a bowtie-type fiber can be used as the polarization-maintaining fiber 31 for the filter. The birefringence strain coefficient of the PANDA fiber is 1.60 × 10⁻⁶. -8 The value is / με, and the strain coefficient for birefringence of a bowtie-type fiber is 1.06 × 10⁻¹⁰. -8 The value is / με. These fibers have a pair of stress-applying materials around the core in cross-sectional view to introduce a strong birefringence. The birefringence strain coefficients of PANDA fibers and bowtie fibers are of the same order and the same effect can be obtained. 【0022】 Figure 3 is a conceptual diagram illustrating the structure of the stretcher 32, which is the main component of the filter device 30. The stretcher 32 adjusts the stress applied to the stress application section 31c of the polarization-maintaining fiber 31 for filtering. The stretcher 32 is, for example, a PZT stretcher (piezoelectric ceramic stretcher) and periodically applies stress to the polarization-maintaining fiber 31 for filtering by piezoelectric drive. As shown in Figure 3, the stretcher 32 has a circular shape and includes a plurality of PZT elements 32a and a fiber storage section 32b. The fiber storage section 32b is an annular member whose size can be changed in the radial direction, and the polarization-maintaining fiber 31 for filtering can be wound and stored in the outer groove 32c. In the center of the fiber storage section 32b, a plurality of PZT elements 32a, three PZT elements 32a in the illustrated example, are arranged so that the radial variation due to the expansion and contraction of the PZT elements 32a is applied evenly to the fiber storage section 32b. The stretcher 32 periodically varies the radial magnitude by applying a voltage to the PZT element 32a using the waveform generator 38. The stress application portion 31c of the polarization-maintaining fiber 31 for the filter corresponds to the portion of the polarization-maintaining fiber 31 that is wound around the fiber housing portion 32b a predetermined number of times. Both ends of the fiber in the stress application portion 31c are fixed to the stretcher 32, for example, by adhesive. The stretcher 32 can adjust the stress applied to the stress application portion 31c by changing its radial shape, and as a result can control the strain (tensile strain) in the stress application portion 31c. 【0023】 Figure 4 illustrates the relationship between the output voltage of the waveform generator 38 and the amount of micro-strain applied by the stretcher 32 in the filter device 30. As shown in Figure 4, the output voltage of the waveform generator 38 and the amount of micro-strain applied by the stretcher 32 have a nearly linear correlation. This makes it easy to control the strain in the stress application section 31c due to stress fluctuations in the filter device 30. 【0024】 The stress-applying portion 31c of the polarization-maintaining fiber 31 for the filter is changed by the stretcher 32 in a range of 0 to 88 μm in terms of the amount of diameter variation relative to the original size. The voltage of the waveform generator 38 is in the range of 0 to 10 V, and the frequency is a maximum of 300 Hz. 【0025】 Returning to Figure 1(A), the polarization splitter 34 rotates the polarization direction of the polarization P0 incident on the polarization-maintaining filter fiber 31 relative to the fiber's cross-section, splitting the polarization P0 into a first polarization component P1 and a second polarization component P2. The polarization splitter 34 is not an independent component; its function is integrated into the end face of the polarization-maintaining filter fiber 31. Specifically, the polarization splitter 34 is a splice in which the velocity axis of the polarization-maintaining filter fiber 31, i.e., the x-axis, is tilted at 45° with respect to the velocity axis of the fiber FO, i.e., the x-axis, and is a fusion splice connecting the polarization-maintaining filter fiber 31 and fiber FO. 【0026】 The polarization recovery section 36 rotates the polarization direction of the polarized light P0 that passes through the stretcher 32 and exits to the polarization-maintaining fiber 31 for filtering with respect to the cross-section of the fiber, thereby restoring the polarization P0. The polarization recovery section 36 is not an independent component, but rather its function is integrated into the end face of the polarization-maintaining fiber 31 for filtering. The polarization recovery section 36 is a splice in which the fiber FO is tilted at -45° with respect to the velocity axis, i.e., the x-axis, of the polarization-maintaining fiber 31 for filtering, and the polarization recovery section 36 is fused together with the fiber FO. 【0027】 In Figure 1(A), etc., the fiber is tilted or rotated with respect to the clockwise direction in the polarization branching section 34 and the polarization return section 36, but the fiber may also be tilted or rotated with respect to the counterclockwise direction. In other words, it is sufficient that the polarization direction of the polarized light P0 incident on the polarization branching section 34 matches the polarization direction of the polarized light P0(Pm) emitted from the polarization return section 36. 【0028】 The polarizer 35 is an absorption-type polarizer that restricts the light passing through it to linear polarization in a specific polarization direction. The polarizer 35 is formed from, for example, a dielectric multilayer film, but a coupler can be given a similar function. For example, a polarizing film obtained by stretching a polymer material containing an iodine compound or dye in a specific direction may be bonded to a parallel plate substrate. The polarization axis of the polarizer 35 is set between the -x direction and the +y direction, and is at 45° with respect to both the -x direction and the +y direction. The polarization axis of the polarizer 35 is tilted at 45° with respect to the polarization direction of the polarization P0 incident on the polarization return unit 36 ​​or fiber FO. 【0029】 The operation of the filter device 30 will be explained with reference to Figure 1(A), etc. Figure 1(A) shows an example configuration in which a relatively small stress is applied to the filter device 30, and the transmittance is maximized at a predetermined wavelength λ0. Figure 1(B) shows an example configuration in which a relatively large stress is applied to the filter device 30, and the transmittance is maximized at a predetermined wavelength λ1 different from that in Figure 1(A). Figure 2 shows an example configuration in which a relatively large stress is applied to the filter device 30, and the transmittance is minimized at a predetermined wavelength λ0. Note that wavelength λ0 is assumed to be smaller than wavelength λ1. In the figures, the sine wave E1 drawn before and after the polarization-maintaining fiber 31 for the filter shows the oscillation of the electric field of the first polarization component P1, and the sine wave E2 shows the oscillation of the electric field of the second polarization component P2. 【0030】 In the cases of Figures 1(A) and 1(B), the polarization P0 is incident on the right side of the filter device 30. The polarization P0 passing through the fiber FO connected to the inlet of the filter device 30 has a polarization plane parallel to the speed axis direction, i.e., the x direction. At the inlet of the filter device 30, i.e., the incident end 31a of the polarization-maintaining fiber 31 for filtering, the x-axis of the incident end 31a of the polarization-maintaining fiber 31 for filtering is rotated by 45° with respect to the x-axis of the fiber FO by the polarization splitter 34, so the polarization direction of the polarization PO has a polarization plane between the +x direction and the +y direction. The polarization P0 incident on the polarization-maintaining fiber 31 for filtering is split into a first polarization component P1 and a second polarization component P2 by the polarization splitter 34. The first polarization component P1 is parallel to the speed axis direction, i.e., the +x direction, and the second polarization component P2 is parallel to the slow axis direction, i.e., the +y direction, and the first polarization component P1 and the second polarization component P2 are orthogonal. In the configuration examples shown in Figures 1(A) and 1(B), the transmittance of the filter device 30 is maximized by stress adjustment in the stretcher 32 for light of predetermined wavelengths λ0 and λ1, and the polarization P0 passes through the polarizer 35 and is guided. Specifically, the first polarization component P1 and the second polarization component P2 are shifted in phase by mλ0 + λ0 / 2 or mλ1 + λ1 / 2 by passing through the stress application section 31c, where m is a natural number. In other words, the first polarization component P1 on the output side of the polarization-maintaining fiber 31 for filtering is parallel to the same +x direction as the incident side. On the other hand, the second polarization component P2 on the output side of the polarization-maintaining fiber 31 for filtering is rotated 180° clockwise with respect to the incident side, with respect to the +y direction, and is parallel to the -y direction. As a result, the linearly polarized light P0 that has passed through the stress application section 31c is inverted by 90° with respect to the y axis and has a polarization plane between the +x direction and the -y direction. The linearly polarized light P0 after passing through the stress application section 31c is incident on the polarizer 35. The polarization direction of the incident linearly polarized light P0 coincides with the polarization axis of the polarizer 35, and it passes through the polarizer 35 with low loss. At the outlet of the filter device 30, i.e., the fiber FO, the x-axis of the fiber FO is rotated by -45° with respect to the x-axis of the output end 31b of the polarization-maintaining filter fiber 31 by the polarization return section 36. Therefore, the polarization direction of the polarized light Pm that has passed through the polarization-maintaining filter fiber 31 and the polarizer 35 has a polarization plane parallel to the +y direction. 【0031】 In the case of Figure 2, for light with wavelength λ0 corresponding to Figure 1(A), the transmittance of the filter device 30 is minimized by the stress adjustment in the stretcher 32, and the polarization P0 does not pass through the polarizer 35 but is blocked. Specifically, the first polarization component P1 and the second polarization component P2 are shifted in phase by mλ0+λ0 as they pass through the stress application section 31c. In other words, the first polarization component P1 on the output side of the polarization-maintaining fiber 31 for filtering is parallel to the same +x direction as the incident side. Also, the second polarization component P2 on the output side of the polarization-maintaining fiber 31 for filtering is parallel to the same +y direction as the incident side. As a result, the linearly polarized light P0 that has passed through the stress application section 31c has a polarization plane between the +x and +y directions, just like the linearly polarized light P0 incident on the stretcher 32. The polarization direction of the linearly polarized light P0 after passing through the stress application section 31c is different from the polarization axis of the polarizer 35, and therefore does not pass through the polarizer 35. 【0032】 Figure 5 illustrates the transmittance characteristics of the filter device 30. The filter device 30 maximizes transmittance at a predetermined wavelength. 【0033】 The Riot filter assumed by the filter device 30 consists of a polarization controller, a birefringent material, and a polarizer. In the filter device 30 of this embodiment, the polarization branching section 34 and the polarization return section 36 function as a fixed polarization controller. The polarization-maintaining fiber 31 for the filter functions as a birefringent material. The filter device 30 is a wavelength-dependent periodic filter, and the function of the Riot filter is defined by the following equation. FSR = Δλ ≈ λ 2 / Δn×L Here, FSR is the resonant frequency interval, λ is the wavelength, Δn is the birefringence value of the polarization-maintaining fiber 31 for the filter (specifically, the difference between the refractive index of the slow axis and the refractive index of the fast axis of the fiber), and L is the length of the birefringent material, i.e., the polarization-maintaining fiber 31 for the filter. Due to the periodic nature of the filter device 30, even a slight fluctuation in FSR causes a large transmittance peak shift. Therefore, by fine-tuning the value of Δn in the above equation, output wavelength tunability can be achieved, and wavelength sweep can be realized. 【0034】 Figure 6(A) illustrates the transmittance peak shift of the filter device 30, and Figure 6(B) illustrates the output wavelength of the filter device 30 in relation to stress. The transmittance peak of the filter device 30 shifts due to the stress applied by the stretcher 32. In other words, the wavelength at which a phase shift occurs changes. The intensity of the filtering effect of the filter device 30 is determined by the fusion angle (45° reference) between the fiber FO and the polarization-maintaining fiber 31 for filtering at the polarization branching section 34 and the polarization return section 36. In other words, the extreme values ​​of transmittance in the filter device 30, i.e., the transmittance at the minimum phase shift (e.g., zero) and the maximum phase shift, do not change with the intensity of the stress. However, the location (wavelength) at which a phase shift of a certain magnitude occurs does change. The filtering effect can be adjusted by changing the fusion angle. 【0035】 In Figure 6(A), the solid line M1 shows the relationship between wavelength and transmittance when a first stress S0 is applied to the filter device 30 and the stress is at its minimum, specifically, at its initial value or zero; the dashed line M2 shows the relationship between wavelength and transmittance when a second stress is applied to the filter device 30 and the stress is S0 + δS; and the dashed line M3 shows the relationship between wavelength and transmittance when a third stress is applied to the filter device 30 and the stress is S0 + 2δS. In the example in Figure 6(A), the transmittance peaks are shifted in the blue, green, and red regions, respectively, but below we will mainly explain the blue region, and the same principle will be applied to the other regions. 【0036】 In the blue region, when the stress is S0, the wavelength at which the transmittance is maximum corresponds to the wavelength λ0 exemplified in Figure 1(A). When the stress is S0 + δS, the wavelength at which the transmittance is maximum corresponds to the wavelength λ1 shown in Figure 1(B). When the stress is S0 + 2δS, the wavelength at which the transmittance is maximum corresponds to the wavelength λ2. As the stress is increased by δS from S0, the characteristic curve of the filter device 30 changes sequentially from the curve corresponding to the first stress S0 to the curve corresponding to the second stress and then to the curve corresponding to the third stress. In this case, for light at wavelength λ0, the transmittance changes, for example, from 100% to 50% and from 50% to 0%. On the other hand, for light at wavelength λ1, the transmittance changes, for example, from 0% to 50% and from 50% to 100%. As shown in Figure 6(B), as the stress increases, the peak of the transmittance shifts and the output wavelength becomes longer. 【0037】 As described above, when stress is applied to the filter device 30, the peak of the transmittance of the filter device 30 shifts. Specifically, when stress is applied to the stress application section 31c of the polarization-maintaining fiber 31 for the filter, the stretcher 32 causes the peak of the transmittance to shift continuously in response to the minimum strain (zero strain) to the maximum strain. In other words, for light of a specific wavelength, the transmittance changes continuously depending on the stress intensity. To put it another way, for a constant stress intensity, the transmittance differs depending on the wavelength. 【0038】 Wavelength adjustment using the filter device 30 of this embodiment can be performed by applying a predetermined stress to the stress application section 31c using a stretcher 32 to change the birefringence value Δn of the stress application section 31c, thereby changing the FSR (resonant frequency interval). 【0039】 In the filter device 30, the strain in the stress application section 31c is controlled by adjusting the stress. Since the value of Δn is affected by the deformation of the fiber, the birefringence value Δn is finely adjusted using the tensile strain as described above. By finely adjusting Δn, the output wavelength of the filter device 30 can be adjusted. 【0040】 In the filter device 30, strain is defined as ΔL / L. L is the length of the stress application portion 31c of the polarization-maintaining fiber 31 for the filter, and ΔL is the amount of change in the length of the stress application portion 31c when stress is applied. For example, when the fiber extends from 1 m to 1.1 m, the strain is 0.1. A strain of ΔL / L of 1 microstrain in the fiber length is 10 -6 . The strain coefficient of the birefringence of the polarization-maintaining fiber is on the order of 10 -8 / με. Here, με means microstrain. On the other hand, in the case of a standard polarization-maintaining fiber, the value of the birefringence, that is, the original value of the birefringence is on the order of 10 -4 . Therefore, a strain of ΔL / L of 1 microstrain in the fiber length (10 -6 ) can give a change in birefringence on the order of 10 -8 / 10 -4 = 10 -4 . Specifically, when the general birefringence is 1.6×10 -4 , the amount of change is 1.6×10 -8 . That is, when the amount of change in birefringence is Δ(Δn), the strain change Δ(Δn) / Δn induced by 1 microstrain is on the order of 10 -4 . This is in the ratio of the order of the strain change amount Δ(Δn) / Δn to the order of 1 microstrain ΔL / L, 10 -4 / 10 -6This results in a magnification control relationship of approximately 100 times. Therefore, compared to the conventional method of adjusting the wavelength by changing the temperature and length of the birefringent material of a Riot filter, the wavelength sweep speed can be increased by several hundred times, and the effect on the resonator can be changed by two orders of magnitude, assuming the above magnification control relationship. Furthermore, by using a resonator composed of all polarization-maintaining fibers, as described later, and a stretcher 32 controlled by a waveform generator 38, a stable and practical femtosecond pulse laser with a wavelength sweep speed on the order of kHz and various tuning modes can be realized. In other words, by incorporating the filter device 30 of this embodiment into a resonator composed of all polarization-maintaining fibers, a highly stable, low-cost, and practical wavelength-swept mode-locked laser can be realized. In addition, high-speed wavelength sweeping is possible with a periodic fiber Riot filter and a magnification control relationship of more than 100 times configured for strain control. 【0041】 In the filter device described above, the tensile strain in the stress-applying section 31c of the polarization-maintaining fiber 31 for filtering is adjusted by adjusting the stress applied to the stress-applying section 31c using the stretcher 32. This allows adjustment of the change in birefringence of the stress-applying section 31c, enabling tuning to improve the transmittance of light of a specific wavelength. 【0042】 [Second Embodiment] The following describes the tunable laser of the second embodiment. The tunable laser of the second embodiment incorporates the filter device of the first embodiment. 【0043】 Figure 7 is a conceptual diagram illustrating the configuration of the tunable laser 100. As shown in Figure 7, the tunable laser 100 is a passive mode-locked laser and comprises a resonator 10, an optical amplifier 20, a filter device 30, a transmission adjustment unit 40, and an isolator-equipped coupler 50. The tunable laser 100 is constructed by joining the optical amplifier 20, the filter device 30, the transmission adjustment unit 40, and the isolator-equipped coupler 50 to a polarization-maintaining ring-shaped resonator 10 by fusion bonding or the like. 【0044】 In the tunable laser 100, the resonator 10 is formed in a ring shape using optical fibers 11. The optical fibers 11 are polarization-maintaining optical fibers (PMF). 【0045】 The filter device 30 is inserted into the optical path of the optical fiber 11 of the resonator 10 by splice connection using a polarization splitter 34 and a polarization return unit 36. The polarization splitter 34 and the polarization return unit 36 ​​enable rotation of the polarization direction of the excitation light PL output from the excitation light source 21a. The filter device 30 has a structure similar to the structure illustrated in Figure 1(A), etc. 【0046】 The optical amplification unit 20 is located inside the resonator 10, attached to the resonator 10, and includes an optical supply unit 21 and a gain fiber 22. 【0047】 The light supply unit 21 includes an excitation light source 21a and a multiplexer 22b. The light supply unit 21 supplies excitation light PL with polarization P0 to the resonator 10 from one direction. Polarization P0, i.e., excitation light PL, propagates in the first circular direction (specifically, clockwise in the loop of the resonator 10). The excitation light source 21a is composed of, for example, a semiconductor laser and outputs excitation light PL with a wavelength of, for example, 980 nm. The multiplexer 22b is part of the isolator coupler 50 and does not hinder the propagation and circulation of light with a wavelength of, for example, 1550 nm in the resonator 10. The excitation light PL introduced into the resonator 10 from the light supply unit 21 excites the dopant added to the doped fiber, which is the gain fiber 22, enabling stimulated emission at the wavelength of the resonant light for output. 【0048】 The gain fiber 22 is an optical gain section located within the resonator 10. The gain fiber 22 is connected inline to the optical fiber 11 that constitutes the resonator 10. The gain fiber 22 is a polarization-maintaining optical fiber doped to provide amplification. Specifically, the gain fiber 22 is a doped fiber with rare earth elements such as erbium (Er) added, and it amplifies the circulating light B1 that circulates clockwise around the resonator 10. 【0049】 The filter device 30 has a configuration similar to that shown in Figure 1(A), and adjusts the light to a specific wavelength through a stress-dependent filtering effect. In the example in Figure 7, the polarizer 35 of the filter device 30 is part of the isolator coupler 50. In the filter device 30, the operation of the stretcher 32 is controlled by periodic voltage fluctuations from the waveform generator 38. The stretcher 32 is used to sweep the center wavelength of the output pulse. 【0050】 The transmission adjustment unit 40 is located in the resonator 10 and enables the generation of ultrashort pulses through its saturable absorption characteristics, which change nonlinearly in transmittance, thereby forming a mode-locked output. The transmission adjustment unit 40 consists of a saturable absorber 40a placed in the optical path. Carbon nanotubes or the like can be used as the saturable absorber 40a. 【0051】 The isolator-equipped coupler 50 is a fast-axis component block polarization-maintaining wavelength division multiplexing coupler (PM-IWDM), and functions as an isolator, wavelength division multiplexing coupler, polarizer, and 50% output coupler. The wavelength division multiplexing coupler of the isolator-equipped coupler 50 corresponds to the multiplexing section 22b of the light supply section 21. The polarizer corresponds to the polarizer 35 of the filter device 30 shown in Figure 1(A). The isolator transmits polarized P0 or polarized Pm in only one direction within the resonator 10. The output coupler extracts a portion of polarized P0 or polarized Pm from the resonator 10. The output coupler has an output port 51a and is coupled to the resonator 10. Output light BO is output from the output port 51a of the output coupler as pulsed light of polarized P0. 【0052】 In the filter device 30, the length of the polarization-maintaining fiber 31 for filtering between the fusion splice points of the polarization branching section 34 and the polarization return section 36, that is, the length of the fiber between the two 45° splices, is set to, for example, 0.21m to determine the length of the filter device 30 corresponding to the Riot filter. The polarization-maintaining optical fiber 11 constituting the resonator 10 is, for example, 2.5m, and the gain fiber 22 is, for example, 1m. The cavity length of the resonator 10 is, for example, 5.5m. In order to prevent the FSR of the filter device 30 from being affected, the optical fiber 11 between the filter device 30 and the isolator coupler 50 is cut in the middle and fusion spliced ​​at, for example, a 90° angle in the orthogonal polarization section 37. The orthogonal polarization section 37 is a splice in which the speed axis of the connecting fiber 11a is tilted by 90°. Specifically, when the length of the optical fiber 11 (hereinafter referred to as the connecting fiber 11a) between the filter device 30 and the isolator-equipped coupler 50 is set to 2L, the connecting fiber 11a is divided into two halves L, and the ends of the first fiber 11b on the filter device 30 side and the ends of the second fiber 11c on the isolator-equipped coupler 50 side are connected by a splice with the speed axis of the connecting fiber 11a tilted by 90°. This corrects the walk-off in the optical fiber 11 of the resonator 10. 【0053】 Figure 8 is a chart showing the optical spectrum of the output light BO of the tunable laser 100 shown in Figure 7. Figure 9 is a chart showing the relationship between the center wavelength of the output pulse of the output light BO and the output voltage of the waveform generator 38. 【0054】 The offset and peak voltage of the waveform generator 38 are set to 1.6V and 5.9V, respectively, corresponding to one wavelength sweep cycle. By using the sawtooth wave of the voltage to control the operation of the stretcher 32 of the filter device 30 and adjusting the stress on the stress application unit 31c, an adjustable optical spectrum shown in Figure 8 can be obtained. The sweep frequency of the tunable laser 100 in the illustrated example is 10Hz, which is limited, for example, by the speed of the optical spectrum analyzer. As shown in Figure 9, the center wavelength of the output pulse shifts monotonically and linearly from 1543nm to 1568nm when the voltage of the waveform generator 38 is varied in the range of 1.5 to 6V. The voltage of the waveform generator 38 and the center wavelength of the laser are linear, i.e., correlated, making wavelength adjustment easy to control. 【0055】 As shown in Figure 8, in each cycle, as the voltage increases, the output wavelength is swept from left to right on the paper (from short wavelengths to long wavelengths). Then, when one cycle is complete, the output wavelength jumps back to the left. 【0056】 In the wavelength-tunable laser 100 described above, wavelength adjustment of the transmittance is possible by incorporating a strain-controlled filter device 30, which is equivalent to a Riot filter. Wavelength adjustment using the filter device 30 is approximately 100 times more efficient than conventional wavelength adjustments that change the temperature or length of birefringent materials. By utilizing this control system, a fully polarization-maintaining femtosecond pulse mode-locked laser with a wide tuning range of 25 nm from 1543 nm to 1568 nm and a linear tuning relationship can be realized. The output is very stable due to the simple polarization-maintaining fiber structure. Furthermore, when the stretcher 32 operates at the resonant frequency, wavelength-swept femtosecond pulse laser results on the kHz order can be achieved. 【0057】 As an application of the present invention, we envision a wavelength-swept mode-locked laser used in stimulated Raman scattering microscopes (hereinafter referred to as SRS microscopes). Conventional microscopes stain macromolecules such as proteins using fluorescent dyes. Therefore, it is not possible to detect small molecules, making it difficult to deeply understand life activities at the molecular level. Furthermore, fluorescent dyes can sometimes affect cellular activity. SRS microscopes can detect molecular vibrations. Compared to other microscopy approaches, SRS microscopes have several advantages, including label-free detection, small molecule detection, and real-time imaging. To capture various types of substances and achieve real-time imaging, several requirements must be met for wavelength-tunable pulsed laser light sources for SRS microscopes, such as (i) femtosecond pulse width for efficient signal generation, (ii) near-infrared wavelength range of 0.8 to 1.5 μm, (iii) long-term stable and reproducible operation, (iv) miniaturization of a practical system, and (v) the possibility of fast wavelength tuning. Conventional wavelength-swept lasers have problems with slow sweep speed and instability, making them unsuitable for SRS microscopes. By applying the tunable laser 100 of this embodiment, high stability and high wavelength sweep speed can be ensured. Furthermore, the design of a fully polarization-maintaining fiber simplifies the configuration and keeps the cost low. In the laser of this embodiment, various wavelength sweep ranges and modes can be realized by changing the gain medium, fiber length, waveform generator mode, etc. Therefore, practical application and industrialization in SRS microscopes become possible. 【0058】 [Third Embodiment] The following describes the tunable laser of the third embodiment. The tunable laser of the third embodiment incorporates the filter device of the first embodiment. Furthermore, the tunable laser of the third embodiment is a modified version of the tunable laser of the second embodiment, and matters not specifically described are the same as those of the second embodiment. 【0059】 Figure 10 is a conceptual diagram illustrating the configuration of the tunable laser 100 of the third embodiment. As shown in Figure 10, the tunable laser 100 is a passive mode-locked laser and has a configuration in which one filter device 30 is shared by two resonators 10 and 110. That is, the filter device 30 is a joint stretcher and can simultaneously control the respective stress application units 31c in the two resonators 10 and 110. The tunable laser 100 has a first resonator 10 and a second resonator 110. The first resonator 10 is the same as the resonator 10 shown in Figure 7. The optical amplification unit 20, transmission adjustment unit 40, and isolator coupler 50 connected to the second resonator 110 have the same configuration and function as the optical amplification unit 20, transmission adjustment unit 40, and isolator coupler 50 connected to the resonator 10 shown in Figure 7. 【0060】 In the third embodiment, the gain fiber 22 of the optical amplification unit 20 in the first resonator 10 and the gain fiber 122 of the optical amplification unit 20 in the second resonator 110 are doped fibers with different rare earth elements. Specifically, the gain fiber 22 of the optical amplification unit 20 in the first resonator 10 is a doped fiber with erbium (Er) added, and the gain fiber 122 of the optical amplification unit 20 in the second resonator 110 is a doped fiber with ytterbium (Yb) added. 【0061】 The first resonator 10 operates in the 1500 nm wavelength range using a doped fiber, for example, erbium (Er) doped in the gain fiber 22. The second resonator 110 operates in the 1000 nm wavelength range using a doped fiber, for example, ytterbium (Yb) doped in the gain fiber 122. 【0062】 The tunable laser 100 of this embodiment can be used as a dual-output laser or a dual-comb laser by having two resonators 10 and 110. 【0063】 [Fourth Embodiment] The filter device of the fourth embodiment will now be described. The filter device of the fourth embodiment is a modification of the filter device of the first embodiment, and matters not specifically described are the same as those of the first embodiment. 【0064】 The filter device 30 of this embodiment has a plurality of stress-applying sections 31c. For example, as shown in Figure 11, in the stretcher 32, a plurality of stress-applying sections 31c are formed so that the number of windings of the polarization-maintaining fiber 31 for the filter is different. 【0065】 Although not shown in the diagram, multiple stretchers 32 may be provided, and the stress in each stress application section 31c may be adjusted in each stretcher 32. For example, the number of windings of the polarization-maintaining fiber 31 for the filter may be set to be the same in each stress application section 31c, and the stress may be adjusted to be different in each stretcher 32, or the number of windings of the polarization-maintaining fiber 31 for the filter may be set to be different in each stress application section 31c, and the stress may be adjusted in each stretcher 32. 【0066】 〔others〕 Although the present invention has been described in reference to the embodiments described above, the present invention is not limited to the embodiments described above. 【0067】 In the second embodiment, by fixing the stretcher 32 to a specific stress, the tunable laser 100 can be made to oscillate at two wavelengths in dual-comb mode. Specifically, dual-wavelength mode-locking shown in Figure 12 can be obtained in the boundary state shown in Figure 8. In the case of dual-wavelength mode-locking, the heights of the two peaks are adjusted, for example, by gain adjustment or wavelength characteristic filtering. By switching between the single-sweep mode shown in Figure 8 and the dual-comb mode shown in Figure 12, the tunable laser 100 can be used in two modes, and high-speed switching is possible. 【0068】 In the above embodiment, the transmission adjustment unit 40 can be made polarization-dependent. For example, the transmission adjustment unit 40 can be made to have transmission characteristics that match the polarization P0. 【0069】 Furthermore, although the above embodiment describes a configuration in which the resonator 10 includes an optical fiber 11, the tunable laser 100 can also be applied to other waveguide-type optical devices that do not use optical fibers. Examples of waveguide-type optical devices include PLCs (Photonics Lightwave Circuits), silicon photonics waveguides, and semiconductor waveguides (InP, GaAs, InGaAsP, etc.). [Explanation of Symbols] 【0070】 10,110…Resonator, 11…Optical fiber, 11a…Connecting fiber, 20…Optical amplification unit, 21…Optical supply unit, 21a…Excitation light source, 22,122…Gain fiber, 22b…Multiplexing unit, 30…Filter device, 31…Polarization-maintaining fiber for filter, 31a…Inlet end, 31b…Outlet end, 31c…Stress application unit, 32…Stretcher, 32a…PZT element, 32b…Fiber housing unit, 32c…Groove, 34…Polarization splitter, 35…Polarizer, 36…Polarization return unit, 37…Orthogonal polarization unit, 38…Waveform generator, 40…Transmission adjustment unit, 40a…Saturable absorber, 50…Coupler with isolator, 51a…Output port, 100…Tunable laser, AX…Optical axis, B1…Circular light, BO…Output light, FO…Fiber, P0,Pm…Polarization, P1,P2…Polarization components, PL…Excitation light

Claims

[Claim 1] Polarization-maintaining fiber, A stretcher that adjusts the stress applied to the stress-applying portion of the polarization-maintaining fiber, A polarizer positioned on the exit side of the stretcher, The stretcher is positioned on the incident side and includes a polarization branching section that branches the polarization, A polarization recovery unit is positioned on the exit side of the stretcher via the polarizer and recovers the polarization that has passed through the stretcher and the polarizer. Equipped with, The polarization branching portion is a splice in which the velocity axis of the polarization-maintaining fiber is tilted 45° with respect to the velocity axis of the fiber connected to the incident end of the polarization-maintaining fiber. The polarization recovery section is a splice in which the velocity axis of the fiber connected to the exit end of the polarization-maintaining fiber is tilted at -45° with respect to the velocity axis of the polarization-maintaining fiber. Filter device. [Claim 2] The filter device according to claim 1, wherein the polarization-maintaining fiber is either a PANDA fiber or a bowtie-type fiber. [Claim 3] A polarization-maintaining ring-shaped resonator, An optical gain section is arranged in the aforementioned resonator, A light supply unit that supplies polarized light to the resonator so as to propagate in the circumferential direction, A filter device according to any one of claims 1 and 2, A tunable laser equipped with a wavelength-tunable laser. [Claim 4] The resonator further comprises a transmission adjustment unit, The tunable laser according to claim 3, which is a mode-locked laser that performs passive mode-locked operation. [Claim 5] A wavelength adjustment method for changing the resonance frequency interval in a filter device comprising: a stretcher for adjusting the stress applied to a stress-applying portion of a polarization-maintaining fiber; a polarizer disposed on the exit side of the stretcher; a polarization splitter disposed on the incident side of the stretcher for splitting polarization; and a polarization recovery portion disposed on the exit side of the stretcher via the polarizer for restoring the polarization that has passed through the stretcher and the polarizer, wherein the polarization splitter is a splice in which the velocity axis of the polarization-maintaining fiber is tilted 45° with respect to the velocity axis of a fiber connected to the incident end of the polarization-maintaining fiber; and the polarization recovery portion is a splice in which the velocity axis of a fiber connected to the exit end of the polarization-maintaining fiber is tilted -45° with respect to the velocity axis of the polarization-maintaining fiber.

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