Optical device and laser

Abstract

To provide an optical device and a laser that enable adjustment of an oscillation wavelength.SOLUTION: An optical device 100 functions as an artificial saturable absorber, and includes a polarization rotation part 20 that generates non-linear polarization rotation by means of a transmission member PMC of polarization holding type. The polarization rotation part 20 includes a plurality of polarization holding elements 11, 12 that are formed of the transmission member PMC of polarization holding type between polarization rotation start parts R1, R3 and polarization rotation termination parts R2, R3. The polarization rotation part 20 causes independent propagation of a first polarization with a fast axis and a second polarization with a slow axis that are branched at the polarization rotation start part R1 to the polarization rotation termination part R2 such that a phase shift relative to each other is compensated for as a result, suppressing a residual filter effect in the wavelength region of interest.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an optical device that functions as an artificial saturable absorber and a laser incorporating the same.

Background Art

[0002] Ultra-fast mode-locked lasers have great potential in various fields such as medicine, material processing, optical measurement, spectroscopy, and optical communication. In particular, wavelength-tunable mode-locked fiber lasers (MLFL) are essential for applications such as stimulated Raman scattering (SRS) microscopes in the biomedical imaging field. Artificial saturable absorbers (SA) such as non-linear optical loop mirrors (NOLM), non-linear amplifying mirrors (NALM), and non-linear polarization rotation (NPR) are widely used in passive mode-locked lasers. These artificial saturable absorbers, i.e., artificial SAs, have advantages such as a high damage threshold, a wide operating wavelength band, and a sufficient modulation depth compared to material-based SAs. However, the introduction of a polarization controller (PC) used to adjust the polarization state in a non-polarization-maintaining (non-PM) laser cavity is a factor that impairs the reproducibility of the laser. Furthermore, a non-polarized or non-PM configuration is susceptible to environmental influences and has the problem of being difficult to operate stably.

[0003] The implementation of NPR in all polarization-maintaining (all-PM) fibers has the potential to solve the problem of environmental stability that was common in conventional non-linear polarization rotation (NPR) mode-locked lasers. In 2017, Szczepanek et al. greatly contributed to the development of the first all-PM type NPR laser in an ytterbium-doped fiber (YDF)-based all-normal dispersion (ANDi) mode-locked laser (Non-Patent Document 1).

[0004] In this system, three PM fiber segments are cross-spliced at a 90-degree angle to compensate for the walk-off of the polarization components propagating along the slow and fast axes of the PM fiber. This all-PM system is excellent in long-term stability and reproducibility, but strict requirements are imposed on the length of each fiber segment. To solve this problem, Szczepanek et al. proposed a reflective all-PM type NPR laser in 2018 (Non-Patent Document 2). In this configuration, a Faraday rotation mirror (FRM) is used to fold back the NPR section at its midpoint, realizing automatic correction of walk-off. This simplifies the laser design and relaxes the requirements for errors in the fiber segment length.

[0005] However, the nonlinear polarization rotation (NPR) lasers disclosed in Non-Patent Document 1 and the like are not configured to be able to adjust the oscillation wavelength.

[0006] Regarding the adjustment or tuning of the oscillation wavelength, those achieved by mechanical methods or thermal methods are known (for example, Non-Patent Document 3). In the case of an apparatus using such a real filter, the maximum achievable value of the wavelength sweep rate is 500 Hz, and it is difficult to achieve a dramatic speed increase. Moreover, the system becomes complex and unstable. A high speed of the wavelength sweep rate is strongly desired in fields such as SRS microscopes and two-photon microscopes.

Prior Art Documents

Non-Patent Documents

[0007]

Non-Patent Document 1

Non - Patent Document 2

Non - Patent Document 3

Summary of the Invention

[0008] The present invention has been made in view of the above - mentioned background art, and an object thereof is to provide an optical device and a laser that enable the oscillation wavelength to be adjusted or tuned at a dramatically high speed. The present invention aims to achieve high - speed tuning of the oscillation wavelength by providing a SA having a controllable filtering effect.

[0009] To achieve the above object, an optical device according to the present invention is an optical device that functions as an artificial saturable absorber, and has a polarization rotation unit that generates nonlinear polarization rotation by a polarization - maintaining transmission member. The polarization rotation unit has a plurality of polarization - maintaining elements formed by the polarization - maintaining transmission member between a polarization rotation start portion and a polarization rotation end portion. The polarization rotation unit independently propagates the first polarization of the fast axis and the second polarization of the slow axis branched at the polarization rotation start portion to the polarization rotation end portion so as to compensate for the mutual phase shift as a result, and suppresses the residual filter effect in the target wavelength range.

[0010] To achieve the above object, a laser according to the present invention includes an optical resonator including the above - described optical device, and an optical amplification device including an optical amplification element disposed on the optical path of the optical resonator.

Brief Description of the Drawings

[0011]

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Mode for Carrying Out the Invention

[0012] 〔First Embodiment〕 Hereinafter, with reference to FIG. 1 and the like, the structure and the like of the optical device according to the first embodiment of the present invention will be described.

[0013] The optical device 100 of the first embodiment shown in Fig. 1(a) is a non-linear band-pass filter, and functions as an artificial saturable absorber for light LI that passes through a linear unidirectional passing optical path OP1, that is, a one-way optical path from left to right on the paper surface. The optical device 100 includes a polarization rotation unit 20 that generates non-linear polarization rotation by a polarization-maintaining fiber, a first polarizer 2a on the incident side, and a second polarizer 2b on the emission side, and suppresses the residual filter effect in the target wavelength range by its optical structure. The target wavelength range in which the optical device 100 is used is, for example, the L band with a wavelength of 1565 nm - 1625 nm, and corresponds to the anomalous dispersion region of the polarization-maintaining fiber PF that constitutes the optical device 100.

[0014] The polarization rotation unit 20 is formed only by a polarization-maintaining fiber, and has a non-linear polarization rotation region 20A, a polarization rotation start portion R1, and a polarization rotation end portion R2. The non-linear polarization rotation region 20A has a plurality of segments, that is, polarization-maintaining elements 12, each formed by the polarization-maintaining fiber PF, between the polarization rotation start portion R1 and the polarization rotation end portion R2.

[0015] An incident-side additional region 20B exists between the first polarizer 2a and the polarization rotation start portion R1, and an emission-side additional region 20C exists between the polarization rotation end portion R2 and the second polarizer 2b. The incident additional region 20B has a segment formed by the polarization-maintaining fiber PF, that is, a polarization-maintaining element 11. The emission-side additional region 20C has a plurality of segments, that is, polarization-maintaining elements 13, each formed by the polarization-maintaining fiber PF.

[0016] The polarization rotation start portion R1 is formed by joining, for example, by fusion splicing, the polarization-maintaining element 11 of the incident-side additional region 20B and the polarization-maintaining element 12 at the tip of the non-linear polarization rotation region 20A. The polarization rotation end portion R2 is formed by joining, for example, by fusion splicing, the polarization-maintaining element 12 at the rear end of the non-linear polarization rotation region 20A and the polarization-maintaining element 13 of the emission-side additional region 20C.

[0017] The polarization rotation start section R1 is the second-axis rotation section 22. In the polarization rotation start section R1 which is the second-axis rotation section 22, the polarization maintaining element 11 and the polarization maintaining element 12 are connected at a first angle θ1 with reference to the slow axis or the fast axis. This first angle θ1 is set so that the angle formed by the slow axes of these polarization maintaining elements 11 and 12 is not 0 degrees, ±90 degrees, and avoids ±45 degrees. As a result, the first polarization of the fast axis and the second polarization of the slow axis branched at the polarization rotation start section R1 become unequal in intensity.

[0018] The polarization rotation end section R2 is the second-axis rotation section 22. In the polarization rotation end section R2 which is the second-axis rotation section 22, the polarization maintaining element 12 and the polarization maintaining element 13 are connected at a second angle θ2 with reference to the slow axis or the fast axis. This second angle θ2 is set so that the angle formed by the slow axes of these polarization maintaining elements 12 and 13 is not 0 degrees. As a result, the first polarization and the second polarization that are branched at the polarization rotation start section R1 and pass through the non-linear polarization rotation region 20A are branched and combined and propagate to the injection-side addition region 20C.

[0019] In the non-linear polarization rotation region 20A, a first-axis rotation section 21 for switching between the slow axis and the fast axis is provided between adjacent polarization maintaining elements 12. The first-axis rotation section 21 is formed by joining a pair of adjacent polarization maintaining elements 12 by fusion splicing or the like. A pair of adjacent polarization maintaining elements 12 have the same segment length b which is the axial length of each polarization maintaining element 12 for the purpose of canceling out the walk-off of the light LI propagating through them, and are connected so that the fast axis and the slow axis are switched in the first-axis rotation section 21. Specifically, they are connected so that the angle formed by the slow axes of the pair of polarization maintaining elements 12 is 90 degrees, and the optical path lengths are made to match. By connecting a pair of polarization maintaining elements 12 in this way so that the axial directions are switched, the residual filter effects generated in the individual polarization maintaining elements 12 can be superimposed to suppress the wavelength dependence of the transmittance. In the illustrated example, the optical path lengths for each polarization are made to match by two pairs of polarization maintaining elements 12, and the wavelength dependence of the residual filter effect is suppressed.

[0020] In the incident-side additional region 20B, a first-axis rotating portion 21 for switching between the slow axis and the fast axis is provided between adjacent polarization-maintaining elements 11. Further, the segment lengths a, which are the axial lengths of the respective polarization-maintaining elements 11, are equal. Thereby, the walk-off of the light LI propagating through the incident-side additional region 20B can be compensated, and the residual filter effect can be suppressed.

[0021] In the emission-side additional region 20C, a first-axis rotating portion 21 for switching between the slow axis and the fast axis is provided between adjacent polarization-maintaining elements 13. Further, the segment lengths c, which are the axial lengths of the respective polarization-maintaining elements 13, are equal. Thereby, the walk-off of the light LI propagating through the emission-side additional region 20C can be compensated, and the residual filter effect can be suppressed.

[0022] The total fiber length L of the optical device 100 is 2a + 4b + 2c, and in a specific fabrication example, it is in the range of 2 m to 6 m.

[0023] The polarization-maintaining fiber PF includes a stress-applying type having birefringence using the photoelastic effect and a structural type in which the effective refractive index is changed in the longitudinal and transverse directions of the core. The stress-applying type is one in which a stress-applying material (SAP: Stress Applying Parts) having a very large thermal shrinkage rate compared to the cladding material is inserted so as to sandwich the core in one direction of the cladding cross section. Generally, the stress-applying type is most commonly used for the polarization-maintaining fiber PF. For example, there are a PANDA (Polarization-maintaining AND Absorption-reducing) type in which the stress-applying material is circular, a Bow-tie type in which the stress-applying material is in the shape of a butterfly necktie, an elliptical jacket fiber in which the stress-applying material is elliptical, and a polarization-maintaining photonic crystal fiber. In the present embodiment, the case where the polarization-maintaining fiber PF is of the PANDA type will be described. By using the PANDA-type polarization-maintaining fiber PF, that is, the PANDA fiber, polarization can be maintained simply and reliably.

[0024] Fig. 2(a) is a cross-sectional view illustrating the structure of a PANDA fiber 10 which is an example of a polarization-maintaining fiber PF. The PANDA fiber 10 has a core 10a, a cladding 10b, and a coating portion (not shown). In the PANDA fiber 10, the core 10a is disposed at the center, the cladding 10b is disposed around the core 10a, and in the cladding 10b, two circular stress-applying portions 10c are arranged so as to sandwich the core 10a. The refractive index of the core 10a is higher than that of the cladding 10b. Each stress-applying portion 10c is formed of, for example, a quartz glass rod doped with B2O3 in order to increase the linear expansion coefficient. The stress-applying portion 3 has a larger linear expansion coefficient than the cladding portion of pure quartz glass, and when tensile strain occurs in the stress-applying portion 10c, stress is applied to the core 10a along the AXs axis in the figure. In Fig. 2(a), the axis AXs indicates the slow axis of the PANDA fiber 10, and the axis AXf indicates the fast axis of the PANDA fiber 10.

[0025] The polarization-maintaining fiber PF is a polarization-transmitting type transmission member PMC. That is, the polarization-maintaining elements 11, 12, 13 are polarization-transmitting type transmission members PMC.

[0026] Fig. 2(b) is a conceptual perspective view for explaining the polarization rotation start portion R1. In the polarization rotation start portion R1, the polarization-maintaining element 12 is spliced in a state of being rotated by a first angle θ1 in the clockwise direction with respect to the polarization-maintaining element 11 with the slow axis AXs as a reference in the traveling direction of the light LI. Here, the first angle θ1 is, for example, -30 degrees, -40 degrees, etc.

[0027] FIG. 2(c) is a conceptual perspective view for explaining the first-axis rotating portion 21 in the non-linear polarization rotation region 20A. In the first-axis rotating portion 21, the second polarization maintaining element 12b on the right side of the drawing is spliced in a state of being rotated by 90 degrees with respect to the first polarization maintaining element 12a on the left side of the drawing, with the slow axis AXs as a reference and the clockwise direction being positive in the traveling direction of the light LI. Although not shown, the third polarization maintaining element on the right side of the drawing is spliced in a state of being rotated by 90 degrees with respect to the second polarization maintaining element 12b, and this is repeated.

[0028] Although not shown, in the first-axis rotating portion 21 of the incident-side addition region 20B shown in FIG. 1(a) as well, similar to the first-axis rotating portion 21 of the non-linear polarization rotation region 20A, splicing is performed such that a relative rotation of 90 degrees is made between a pair of polarization maintaining elements 11 with the slow axis AXs as a reference. In the first-axis rotating portion 21 of the emission-side addition region 20C as well, similar to the first-axis rotating portion 21 of the non-linear polarization rotation region 20A, splicing is performed such that a relative rotation of 90 degrees is made between a pair of polarization maintaining elements 13 with the slow axis AXs as a reference.

[0029] FIG. 2(d) is a conceptual perspective view for explaining the polarization rotation end portion R2. In the polarization rotation end portion R2, the polarization maintaining element 13 is spliced in a state of being rotated by the second angle θ2 with respect to the polarization maintaining element 12, with the slow axis AXs as a reference and the clockwise direction being positive in the traveling direction of the light LI. Here, the second angle θ2 is, for example, -30 degrees, -40 degrees, etc.

[0030] In the example shown in Fig. 1(a), the non-linear polarization rotation region 20A is composed of four polarization maintaining elements 12. However, the non-linear polarization rotation region 20A may be composed of any even number of polarization maintaining elements 12. Also, the segment length b of a pair of adjacent polarization maintaining elements 12 can be appropriately set according to the usage conditions related to the target wavelength range of the optical device 100 and the like. The segment length of a specific pair of adjacent polarization maintaining elements 12 and the segment length of the pair of adjacent polarization maintaining elements 12 next to them do not have to be the same and may be different. Also, the type of polarization maintaining fiber PF of a specific pair of adjacent polarization maintaining elements 12 may be different from the type of polarization maintaining fiber PF of the pair of adjacent polarization maintaining elements 12 next to them.

[0031] Regarding the incident-side additional region 20B and the emission-side additional region 20C, they can also be omitted depending on the system configuration. The incident-side additional region 20B can be composed of not only two polarization maintaining elements 11 but also a large number of polarization maintaining elements 11, and it is only necessary to match the optical path lengths of the orthogonal polarizations or polarization components in this section. The emission-side additional region 20C can also be composed of not only two polarization maintaining elements 13 but also a large number of polarization maintaining elements 13, and it is only necessary to match the optical path lengths of the orthogonal polarizations in this section. The type of polarization maintaining fiber PF constituting the emission-side additional region 20C may be the same as or different from the type of polarization maintaining fiber PF constituting the incident-side additional region 20B.

[0032] FIG. 3 is a diagram for specifically explaining the functions of the polarization rotation start section R1 and the first-axis rotation section 21. When light LI, which is a component of the slow axis, is emitted from the polarization holding element 11 in the front stage of the polarization rotation start section R1 and enters the polarization holding element 12a in the rear stage of the polarization rotation start section R1, this light LI is branched into a first polarization P1 of the slow axis and a second polarization P2 of the fast axis and propagates through the polarization holding element 12a. At this time, a phase shift occurs between the first polarization P1 and the second polarization P2. The first polarization P1 of the slow axis component that has propagated through the polarization holding element 12a enters the polarization holding element 12b as a fast axis component, and the second polarization P2 of the fast axis component that has propagated through the polarization holding element 12a enters the polarization holding element 12b as a slow axis component. When the intensities of the first polarization P1 and the second polarization P2 incident on the polarization holding element 12a are equal, the first polarization P1 of the fast axis component that has propagated through the polarization holding element 12b and the second polarization P2 of the slow axis component that has propagated through the polarization holding element 12b are, as a result, free of phase shift, and the walk-off is compensated. Note that the intensities of the first polarization P1 and the second polarization P2 are different, and the larger the difference between the intensity of the first polarization P1 and the intensity of the second polarization P2, the larger the phase difference between the first polarization P1 and the second polarization P2, and the polarization rotation occurs. Such polarization rotation that increases according to the light intensity is detected by the second polarizer 2b, and by adjusting so that the polarization component when the light intensity is high remains, it is possible to increase the transmittance of short pulses and realize passive mode synchronization.

[0033] Figs. 4(a) to 4(f) are conceptual charts for explaining the function of the optical device 100 shown in Fig. 1(a) etc. as a non-linear bandpass filter. Fig. 4(a) shows the linear filter characteristics of the polarization-maintaining fiber PF (corresponding to the polarization-maintaining element 12a) corresponding to a single segment, and large-amplitude transmittances are formed at wavelength intervals Δλ corresponding to the length ΔL of the polarization-maintaining fiber PF. Here, Δλ corresponding to the FSR is in a proportional relationship with 1 / ΔL. Fig. 4(b) shows the linear filter characteristics of a compensation unit composed of a pair of polarization-maintaining fibers PF spliced with their slow axes orthogonal. In this case, the transmittance is substantially flat and the residual filter effect is zero. The compensation unit composed of a pair of polarization-maintaining fibers PF spliced with their slow axes orthogonal corresponds to the incident-side additional region 20B, the non-linear polarization rotation region 20A, and the emission-side additional region 20C shown in Fig. 1(a). Fig. 4(c) shows the linear filter characteristics when the length of the subsequent polarization-maintaining fiber PF is increased to ΔL + ε. In this case, the amplitude of the transmittance is significantly reduced but remains, and Δλ becomes considerably wider. In this case, the residual filter effect is in a state where it is significantly reduced.

[0034] FIG. 4(d) shows the non-linear filter characteristics of a compensation unit composed of a pair of polarization-maintaining fibers PF spliced with the slow axes orthogonal, and depending on the difference between the powers P1 and P2 of the light LI, a shift occurs in the peak wavelengths λ1 and λ2 of the periodic amplitude. The amplitude of the non-linear filter characteristics, i.e., the modulation depth, is considerably smaller than the amplitude of the linear filter characteristics shown in FIG. 4(a). In FIG. 4(e), the solid line shows the total filter characteristics of the compensation unit assumed to be used in the range of the power P1 - P2, and it is a waveform obtained by superimposing the characteristics shown in FIG. 4(b) and the characteristics shown in FIG. 4(d). In this case, a flat transmittance is achieved in the target wavelength range OW including the peak wavelengths λ1 and λ2. Note that FIG. 4(f) shows the filter characteristics of a comparative example. The filter characteristics of the comparative example are a waveform obtained by superimposing the characteristics shown in FIG. 4(a) and the characteristics shown in FIG. 4(d). That is, as in FIG. 4(e), in the compensation unit with reduced residual filter effect, a flat and relatively high transmittance characteristic is achieved in the target wavelength range OW. Thereby, the degree of freedom regarding the wavelength of the light LI passing through the compensation unit is increased. On the other hand, in the filter characteristics of the comparative example shown in FIG. 4(f), a large amplitude occurs in the target wavelength range OW, and the degree of freedom regarding the wavelength of the light LI passing through is low. Above, the case of superimposing the characteristics shown in FIG. 4(b) and the characteristics shown in FIG. 4(d) has been described, but even when the characteristics shown in FIG. 4(c) and the characteristics shown in FIG. 4(d) are superimposed, the degree of freedom of the wavelength of the light LI passing through the compensation unit can be increased.

[0035] As is clear from the above, the optical device 100, which is a non-linear bandpass filter, suppresses and reduces the wavelength-dependent residual filter effect, and shows a substantially uniform transmittance characteristic in the target wavelength range OW such as the 1564 - 1625 nm band, for example. It is desirable that the residual filter effect of the optical device 100 has a ripple of 30% or less with respect to the average in the spectral characteristic of the transmittance, more desirably 20% or less, and even more desirably 5% or less.

[0036] In the above description, it was assumed that splicing was performed in the first-axis rotation unit 21 to rotate the slow axis AXs by 90 degrees. However, the rotation of the slow axis AXs does not necessarily have to be exactly 90 degrees. For example, it can be in the range of 75 degrees to 105 degrees, but a range of 80 degrees to 100 degrees is desirable, and a range of 85 degrees to 95 degrees is more desirable.

[0037] In the above description, the non-linear polarization rotation region 20A etc. composed of the polarization-maintaining fiber PF was explained. However, instead of the polarization-maintaining fiber PF, other types of polarization-maintaining transmission members PMC can be used. Even when other polarization-maintaining transmission members PMC are used, a band-pass filter effect similar to that shown in FIGS. 4(a) - 4(e) can be produced. As the polarization-maintaining transmission member PMC, for example, a crystal of a birefringent material can be used.

[0038] FIG. 1(b) is a diagram for explaining the optical device 100 of the modified example. In this case, the non-linear polarization rotation region 20A is composed of three polarization-maintaining elements 12. The segment length of the central polarization-maintaining element 12 is 2b, and the segment lengths of the polarization-maintaining elements 12 at both ends are b. Also in this case, in the entire non-linear polarization rotation region 20A, the optical path lengths of the orthogonal polarization or polarization components are the same, eliminating problems such as walk-off. Note that instead of one central polarization-maintaining element 12, a set of an odd number of polarization-maintaining elements 12 with a segment length of 2b can be incorporated.

[0039] The first-axis rotation unit 21 can be formed not only by fusion splicing but also by mechanical splicing. The second-axis rotation unit 22 can also be formed not only by fusion splicing but also by mechanical splicing. For the second-axis rotation unit 22, a mechanical mechanism that can independently adjust the angles θ1 and θ2 can also be added.

[0040] As described above, the optical device 100 of the first embodiment functions as an artificial saturable absorber (artificial SA), and has a polarization rotation unit 20 that generates non-linear polarization rotation by a polarization-maintaining fiber PF which is a polarization-maintaining transmission member PMC. The polarization rotation unit 20 has a plurality of polarization-maintaining elements 12 formed by the polarization-maintaining fiber PF in a passing optical path (one-way optical path) OP1 from a polarization rotation start portion R1 to a polarization rotation end portion R2. The polarization rotation unit 20 independently propagates the first polarization of the fast axis AXf and the second polarization of the slow axis AXs branched at the polarization rotation start portion R1 in the passing optical path OP1 so as to compensate for the mutual phase shift as a result, and suppresses the residual filter effect in the target wavelength range as the entire optical device 100.

[0041] 〔Second Embodiment〕 Referring to FIG. 5(a), the optical device 2100 of the second embodiment will be described. The optical device 2100 is a partially modified version of the optical device 100 of the first embodiment, and the same reference numerals are given to the same parts and the description thereof is omitted. In the case of the optical device 2100 of the second embodiment, a mirror 3 is used instead of the second polarizer 2b in FIG. 1(a). The optical device 2100 functions as an artificial saturable absorber for light LI passing through a linear reflective passing optical path OP2, that is, a reciprocating optical path. The mirror 3 is a reflector RL that enables the reciprocation of the light LI.

[0042] The optical device 2100 is a non-linear band-pass filter, and includes a polarization rotation unit 20 that generates non-linear polarization rotation by a polarization-maintaining fiber, a first polarizer 2a, and a mirror 3, and suppresses the residual filter effect in the target wavelength range by its optical structure. The target wavelength range in which the optical device 100 is used is, for example, the L band with a wavelength of 1565 nm - 1625 nm, which corresponds to the anomalous dispersion region of the polarization-maintaining fiber PF constituting the optical device 100.

[0043] The polarization rotation section 20 is formed of only polarization-maintaining fibers, and has a non-linear polarization rotation region 120A and a start / end section R3. The non-linear polarization rotation region 120A has a plurality of polarization-maintaining elements 12 and a plurality of polarization-maintaining elements 17 between the start / end section R3 and the mirror 3, and these polarization-maintaining elements 12 and polarization-maintaining elements 17 are formed of polarization-maintaining fibers PF. The start / end section R3 also serves as the polarization rotation start section R1 and the polarization rotation end section R2 in the optical device 100 of the first embodiment shown in Fig. 1(a) etc.

[0044] An incident-side addition region 20B exists between the first polarizer 2a and the start / end section R3. The incident-side addition region 20B has a plurality of segments formed of polarization-maintaining fibers PF, namely, polarization-maintaining elements 11. In the incident-side addition region 20B, a first axis rotation section 21 for switching between the slow axis and the fast axis is provided between adjacent polarization-maintaining elements 11. Further, segment lengths a, which are the axial lengths of the respective polarization-maintaining elements 11, are equal.

[0045] The start / end section R3 is a second axis rotation section 22, and is formed by joining the polarization-maintaining element 11 of the incident-side addition region 20B and the polarization-maintaining element 12 of the non-linear polarization rotation region 120A, for example, by fusion splicing.

[0046] In the non-linear polarization rotation region 120A, a first axis rotation section 21 for switching between the slow axis and the fast axis is provided between adjacent polarization-maintaining elements 12. Also, a first axis rotation section 21 for switching between the slow axis and the fast axis is provided between adjacent polarization-maintaining elements 17. Further, a first axis rotation section 21 for switching between the slow axis and the fast axis is also provided between adjacent polarization-maintaining elements 12 and polarization-maintaining elements 17. These first axis rotation sections 21 are formed by joining adjacent polarization-maintaining elements 11, 17 by fusion splicing or the like.

[0047] Mirror 3 is a polarization-maintaining mirror that rotates the polarization by 90 degrees while maintaining the polarization. Although the polarization-maintaining element 17 is part of the non-linear polarization rotation region 120A from a functional perspective, it is structurally part of mirror 3.

[0048] In addition, when mirror 3 is, for example, a Faraday rotator type, since the polarization components are interchanged in the reciprocating optical path, it may be considered that it is not necessary to strictly adjust the segment lengths a, b, and c of the polarization-maintaining elements 11, 12, and 17 in the non-linear polarization rotation region 120A. However, considering that the Faraday rotator type mirror 3 is incomplete, it is necessary to compensate for walk-off using the polarization-maintaining elements 11, 12, 17, and 12 with controlled segment lengths.

[0049] FIG. 5(b) is a diagram for explaining the optical device 2100 of the modified example. In this case, the non-linear polarization rotation region 120A is composed of three polarization-maintaining elements 12. The segment length of the central polarization-maintaining element 12 is 2b, and the segment lengths of the polarization-maintaining elements 12 at both ends are b. Also in this case, in the entire non-linear polarization rotation region 120A, the optical path lengths of the orthogonal polarizations or polarization components are the same, and problems such as walk-off are solved. Instead of one central polarization-maintaining element 12, a set of an odd number of polarization-maintaining elements 12 with a segment length of 2b can be incorporated.

[0050] As described above, the optical device 2100 of the second embodiment functions as an artificial SA and has a polarization rotation unit 20 that generates non-linear polarization rotation by a polarization-maintaining fiber PF, which is a polarization-maintaining transmission member PMC. The polarization rotation unit 20 has a plurality of polarization-maintaining elements 12 and 17 formed by the polarization-maintaining fiber PF in the passing optical path (reciprocating optical path) OP2 from the start / end portion R3, which also serves as the polarization rotation start portion and the polarization rotation end portion, to the start / end portion R3. The polarization rotation unit 20 independently propagates the first polarization of the fast axis AXf and the second polarization of the slow axis AXs branched at the polarization rotation start portion R1 in the passing optical path OP2 so as to compensate for the mutual phase shift as a result, suppressing the residual filter effect in the target wavelength range for the entire optical device 100.

[0051] 〔Third Embodiment〕 The laser of the third embodiment will be described below. The laser of the third embodiment has a configuration in which the optical device of the first embodiment is incorporated.

[0052] FIG. 6 is a conceptual diagram for explaining the laser 200 of the third embodiment. The laser 200 includes a polarization-maintaining optical fiber loop 40, a WDM (Wavelength Division Multiplexing) 50 disposed on the optical path of the optical fiber loop 40, a fiber amplifier 60 disposed on the optical path of the optical fiber loop 40, an excitation light source 71 for supplying excitation light PL to the fiber amplifier 60, an optical device 100 disposed on the optical path of the optical fiber loop 40, and a driving device 72 for driving the excitation light source 71. The laser 200 has a clockwise optical system.

[0053] The optical fiber loop 40 has loop elements 40a, 40b, 40c made of polarization-maintaining optical fibers and operates in slow-axis propagation. The optical fiber loop 40 includes the fiber amplifier 60 and the optical device 100 in addition to the polarization-maintaining loop elements 40a, 40b, 40c. Both ends of the optical fiber loop 40 are connected to a pair of ports 51, 52 of the WDM 50. The WDM 50 functions as an optical resonator RE together with the optical fiber loop 40.

[0054] The WDM 50 is a polarization-maintaining tap isolator WDM hybrid device in which the functions of an isolator and wavelength-selective optical multiplexing / demultiplexing are integrally incorporated. The WDM 50 also functions as the first polarizer 2a and the second polarizer 2b of the optical device 100. In the following description and drawings, the WDM 50 with an arrow means that it incorporates an isolator. The WDM 50 can introduce the excitation light LE from the excitation light source 71 into the optical fiber loop 40 from the input port 53 while partially emitting the output light or the resonance light LL from the output port 54. The tap ratio or branching ratio of the resonance light LL is, for example, 20%, but is not limited thereto.

[0055] The fiber amplification unit 60 is a polarization-maintaining optical fiber doped to have an amplification function. The fiber amplification unit 60 is connected between the loop elements 40b and 40c and constitutes a part of the optical fiber loop 40. The fiber amplification unit 60 is a doped fiber added with a rare earth element such as erbium (Er), and amplifies the light circulating in the optical fiber loop 40. The fiber amplification unit 60 can be replaced with an optical amplification element using other amplification media such as a semiconductor optical amplifier or a fiber Raman amplifier.

[0056] The excitation light source 71 is composed of, for example, a semiconductor laser and outputs excitation light LE having a wavelength of, for example, 980 nm. The excitation light LE introduced from the excitation light source 71 into the optical fiber loop 40 via the WDM 50 excites the dopant added to the doped fiber of the fiber amplification unit 60, enabling stimulated emission at the wavelength of the resonance light LL for output (for example, 1565 nm - 1625 nm).

[0057] The optical device 100 functions as an artificial saturable absorber. Among the optical device 100, the non-linear polarization rotation region 20A is connected between the loop elements 40a and 40b of the optical fiber loop 40 and constitutes a part of the optical fiber loop 40. Among the optical device 100, the first polarizer 2a and the second polarizer 2b shown in Fig. 1(a) etc. are incorporated as functions of the WDM 50. Among the optical device 100, the incident-side addition region 20B shown in Fig. 1(a) etc. is unnecessary because it propagates only the polarization of the slow axis in the loop element 40a. Among the optical device 100, the emission-side addition region 20C shown in Fig. 1(a) etc. corresponds to the loop elements 40b, 40c and the fiber amplifier 60. That is, a first-axis rotation part 41 corresponding to the first-axis rotation part 21 shown in Fig. 2(c) is provided in the loop element 40b, and a first-axis rotation part 41 corresponding to the first-axis rotation part 21 shown in Fig. 2(c) is provided in the loop element 40c, and the optical path lengths for each of the two polarizations propagating through these are made to coincide, and the wavelength dependence of the residual filter effect is suppressed. Also, a first-axis rotation part 61 corresponding to the first-axis rotation part 21 shown in Fig. 2(c) is provided in the fiber amplifier 60, and the optical path lengths for each of the two polarizations propagating through this are made to coincide, and the wavelength dependence of the residual filter effect is suppressed.

[0058] In Fig. 6, an orthogonal splicing is performed in which the loop elements 40b, 40c and the fiber amplifier 60 are divided into two and the slow axis is rotated by 90 degrees, but it is also possible to perform the above orthogonal splicing by dividing the loop elements 40b, 40c and the fiber amplifier 60 into three or more while adjusting the segment length.

[0059] The drive device 72 adjusts the amplification function of the fiber amplifier 60 by electrically increasing or decreasing the intensity of the excitation light LE output from the excitation light source 71. Specifically, the drive device 72 adjusts the drive output which is the electric power supplied to the excitation light source 71 and the drive waveform.

[0060] Laser 200 automatically starts the pulse oscillation operation by appropriately setting the drive output of the drive device 72 and supplying excitation light LE with a relatively high intensity from the excitation light source 71 to the optical fiber loop 40. When Laser 200 appropriately adjusts the supply power of the drive device 72 and sets the intensity of the excitation light LE output from the excitation light source 71 (i.e., the pump output) in a relatively low range, the central wavelength of the resonance light LL emitted from the output port 54 of the WDM 50 can be variably adjusted and continuously tuned according to the supply power. During this wavelength tuning, Laser 200 maintains the mode-locked state. Laser 200 operates in the soliton region.

[0061] The wavelength tuning process in the above Laser 200 will be described. The wavelength tuning process involves a transition from an existing mode-locked state (previous wavelength) to a new mode-locked state (new wavelength). This transition is facilitated by the mode-locking mechanism, and in the method of the present application, NPR (nonlinear polarization rotation) is adopted. In NPR, the nonlinear rotation of the polarization state is determined by the phase difference between two orthogonal polarization components. Considering that the parameters of the all-polarization-maintaining (PM) NPR structure of the laser are fixed, in order to maintain mode-locking during the tuning process, the new mode-locked state needs to acquire a phase difference similar to the previous state. This consistency of the phase difference, that is, maintenance or slight variation, is important for stable wavelength-variable mode-locking in an all-PM NPR laser. For the purpose of performing a simplified analysis, it is assumed that the phase difference between the two polarization components is maintained during the tuning process. The nonlinear phase shifts accumulated by the two polarization components through XPM (cross-phase modulation) and SPM (self-phase modulation) are expressed as follows. TIFF2025104343000002.tif10163Here, the nonlinear parameter is γ = 2*πn2 / (λA eff )) and n2 is the nonlinear refractive index, and A effrepresents the effective mode area. L represents the length of the fiber. In the optical device 100 or the non-linear polarization rotation region 20A, etc., the two polarization components travel equal distances along the slow axis and the fast axis of the fiber. Therefore, L is constant in these equations. E s and E f represent the electric field amplitudes of the optical components along the slow axis and the fast axis, respectively. E s and E f The ratio of is determined by the splice angle θ1 (for example, 30°). As a result, the phase difference is expressed as follows. In the spectral adjustment range of TIFF2025104343000003.tif1416320nm, the change in n2 is slight, and during the adjustment process, the factor (2πLn2 / 3) on the right side can be treated as constant. Also, A eff is proportional to λ 2 . Assuming that the phase difference (Δφ) is constant through the wavelength tuning process, the factor (|E s | 2 - |E f | 2 / λ 3 ) must also be constant. The decrease in pump output causes a decrease in the pulse peak power, and accordingly (|E s | 2 - |E f | 2 ) decreases. As a result, λ also decreases, ensuring that this factor is consistently maintained throughout the adjustment process. This mechanism is considered to explain the phenomenon observed in the experiment described later, that is, when the pump output decreases, the central wavelength shifts to the blue side.

[0062] [Fourth Embodiment] Hereinafter, the laser of the fourth embodiment will be described. The laser of the fourth embodiment has a configuration incorporating the optical device of the second embodiment.

[0063] FIG. 7 is a conceptual diagram for explaining the laser 2200 of the fourth embodiment. The laser 2200 includes a polarization-maintaining optical fiber loop 40, a WDM 50 disposed on the optical path of the optical fiber loop 40, a fiber amplifier 60 disposed on the optical path of the optical fiber loop 40, an excitation light source 71 that supplies excitation light PL to the fiber amplifier 60, a circulator 80 disposed on the optical path of the optical fiber loop 40, an optical device 2100 connected to the circulator 80, and a driving device 72 that drives the excitation light source 71.

[0064] In this case, the operations of the optical fiber loop 40, the WDM 50, the fiber amplifier 60, the excitation light source 71, and the driving device 72 are the same as those of the laser 200 of the third embodiment.

[0065] The circulator 80 is a polarization-maintaining circulator. The circulator 80 guides the light incident on the first port from the loop element 40a of the optical fiber loop 40 to the second port and outputs it to the optical device 2100 connected thereto. The circulator 80 guides the light that has returned to the second port after reciprocating through the optical device 2100 to the third port and outputs it to the loop element 40b connected thereto. The circulator 80 also functions as the first polarizer 2a.

[0066] The optical device 2100 functions as an artificial saturable absorber. Among the optical device 2100, the non-linear polarization rotation region 20A is connected to the second port of the circulator 80 and constitutes a part of the optical fiber loop 40. As a result, regarding the first polarizer 2a shown in FIG. 5(a) etc. among the optical device 2100, it is incorporated as a function of the circulator 80. The optical device 2100 includes elements equivalent to the non-linear polarization rotation region 20A and the incident-side addition region 20B shown in FIG. 5(a) etc. In particular, in the incident-side addition region 20B, when the light returning from the non-linear polarization rotation region 20A passes through the pair of polarization-maintaining elements 11, the optical path lengths are made to match by the first-axis rotation unit 21, and the wavelength dependence of the residual filter effect is suppressed.

[0067] 〔Fifth Embodiment〕 The laser of the fifth embodiment will be described below. The laser of the fifth embodiment has a configuration incorporating the optical device of the first embodiment.

[0068] FIG. 8 is a conceptual diagram for explaining the laser 3200 of the fifth embodiment. The laser 3200 includes a polarization-maintaining linear cavity 340, a WDM 150 disposed on the optical path of the linear cavity 340, a fiber amplifier 60 disposed on the optical path of the linear cavity 340, an excitation light source 71 that supplies excitation light PL to the fiber amplifier 60, an optical device 2100 disposed on the optical path of the linear cavity 340, and a drive device (not shown) that drives the excitation light source 71.

[0069] The linear cavity 340 has a fiber element 341 made of a polarization-maintaining optical fiber and operates in slow-axis propagation. The linear cavity 340 includes, in addition to the polarization-maintaining fiber element 341, the fiber amplifier 60 and the optical device 2100. In the linear cavity 340, a WDM 150 is inserted between the polarization-maintaining mirrors 3 and 4 at both ends. The WDM 150 functions as an optical resonator RE together with the linear cavity 340. The WDM 150 functions as the first polarizer 2a, similar to the WDM 50 shown in FIG. 6 and the like, but does not function as an isolator. The polarization-maintaining mirror 4 is, for example, a 90% reflection type and outputs resonance light LL.

[0070] In this case, the fiber amplifier 60 is disposed on the round-trip optical path of the optical device 2100. Therefore, a first-axis rotation part 61 corresponding to the first-axis rotation part 21 shown in FIG. 2(c) is provided in the fiber amplifier 60 to equalize the optical path lengths for each of the two polarizations propagating through it and suppress the wavelength dependence of the residual filter effect. When the mirror 3 is, for example, of the Faraday rotator type, the first-axis rotation part 21 between the pair of polarization-maintaining elements 17 is unnecessary.

[0071] FIG. 9 is a conceptual diagram for explaining a modified example of the laser 3200 shown in FIG. 8. In this case, the linear cavity 340 has fiber elements 341a, 431b, etc. made of polarization-maintaining optical fibers, and the fiber amplifier section 60 is inserted between the fiber elements 341a, 431b. The fiber elements 341a, 431b and the fiber amplifier section 60 are outside the optical device 2100, and there is no need to consider the compensation of walk-off, etc.

[0072] 〔Sixth Embodiment〕 Hereinafter, the laser of the sixth embodiment will be described. The laser of the sixth embodiment is obtained by making a change to the lasers of the third to fifth embodiments.

[0073] FIG. 10 is a conceptual diagram for explaining the laser 200 of the sixth embodiment obtained by making a change to the third embodiment. This laser 200 has a material-type saturable absorber member 81 containing carbon nanotubes (CNT: Carbon Nanotube) and a polarization-maintaining isolator 82 added on the optical path of the optical fiber loop 40. The saturable absorber member 81 and the isolator 82 are inserted between the loop element 40aa and the loop element 40ab. The CNT can be arranged, for example, in a thin film shape so as to cross the core, but it is not limited thereto. Also, graphene, graphene oxide, etc. can be used as the saturable absorber member 81. Furthermore, although it is necessary to change to a reflective optical path as the saturable absorber member 81, a semiconductor saturable absorber mirror (SESAM) can also be used.

[0074] The saturable absorber member 81 is responsible for pulse generation, and the non-linear polarization rotation region 20A controls and adjusts the pulse center wavelength. Also in this case, by appropriately adjusting the supply power of the drive device 72, the center wavelength of the resonant light LL emitted from the WDM 50 can be made variable to enable continuous tuning. By adding the saturable absorber member 81, the length of the optical fiber loop 40 can be shortened, and thereby the repetition frequency of the pulsed laser can be increased.

[0075] FIG. 11 is a conceptual diagram for explaining a laser 2200 according to a sixth embodiment with modifications to the fourth embodiment. This laser 2200 has a material-type saturable absorption member 81 containing CNTs added on the optical path of the optical fiber loop 40. The saturable absorption member 81 is inserted between the loop element 40aa and the loop element 40ab. As the saturable absorption member 81, graphene, graphene oxide, etc. can be used instead of or together with CNTs. Although it is necessary to change to a reflective optical path, a SESAM can also be used.

[0076] Also in this case, by appropriately adjusting the supply power of the driving device 72, the central wavelength of the resonant light LL emitted from the WDM 50 can be made variable to enable continuous tuning.

[0077] FIG. 12(a) is a conceptual diagram for explaining a laser 3200 with modifications to the fifth embodiment (the type shown in FIG. 8). This laser 3200 has a material-type saturable absorption member 81 containing CNTs added on the optical path of the linear cavity 340. The saturable absorption member 81 is inserted between the fiber element 341a and the fiber element 431b. Also, as the saturable absorption member 81, graphene, graphene oxide, etc. can be used. Although it is necessary to change to a reflective optical path, a SESAM can also be used.

[0078] Also in this case, by appropriately adjusting the supply power of the driving device 72, the central wavelength of the resonant light LL emitted from the WDM 150 can be made variable to enable continuous tuning.

[0079] FIG. 12(b) is a conceptual diagram for explaining a laser 3200 obtained by modifying the fifth embodiment (the type shown in FIG. 9). This laser 3200 has a material-type saturable absorber member 81 containing CNTs added on the optical path of the linear cavity 340. The saturable absorber member 81 is disposed adjacent to the fiber amplifier section 60 and is inserted between the fiber element 341ba and the fiber element 431bb. As the saturable absorber member 81, graphene, graphene oxide, etc. can be used instead of or together with CNTs. Although it is necessary to change to a reflective optical path, a SESAM can also be used.

[0080] Also in this case, by appropriately adjusting the supply power of the drive device 72, the central wavelength of the resonant light LL emitted from the WDM 150 can be made variable to enable continuous tuning.

[0081] 〔Seventh Embodiment〕 Hereinafter, the laser of the seventh embodiment will be described. The laser of the seventh embodiment is obtained by modifying the laser of the third embodiment.

[0082] FIG. 13 is a conceptual diagram for explaining the laser 200 of the seventh embodiment. In the laser 200, the polarization rotation start section R1 and the polarization rotation end section R2 are changed to elements of a spatial optical system type.

[0083] Specifically, the polarization rotation start section R1 includes collimators 91a and 91b, a half-wave plate 91c, and a holder (not shown) for positioning and holding these elements. Note that the half-wave plate 91c can be rotatable with respect to the collimators 91a and 91b, but may be fixed after adjustment. By appropriately rotating the optical axis of the half-wave plate 91c around the optical axis OX, the first angle θ1 can be freely adjusted.

[0084] Note that the first angle θ1 can also be adjusted by omitting the half-wave plate 91c and relatively rotating the collimators 91a and 91b.

[0085] FIG. 14 is a conceptual diagram for explaining the structure of the collimators 91a and 91b. The collimator 91a includes a lens 91e that collimates the light LI emitted from the end face 40e of the optical fiber 40f along the optical axis OX, and a holder (not shown) that positions and holds the lens 91e with respect to the optical fiber 40f. The collimator 91b has the same structure as the collimator 91a, but the traveling direction of the light LI is reversed.

[0086] Returning to FIG. 13, the polarization rotation end portion R2 specifically includes the collimators 91a and 91b, a half-wave plate 91c, a PBS 91s for separating orthogonal polarization components by transmission and reflection, and a holder (not shown) that positions and holds these elements. Note that the half-wave plate 91c can be rotatable with respect to the collimators 91a and 91b, or may be fixed after adjustment. By appropriately rotating the optical axis of the half-wave plate 91c around the optical axis OX, the first angle θ2 can be freely adjusted. The PBS 91s functions as the second polarizer 2b. That is, in the laser 200 of FIG. 14, the first axis rotation portions 41 and 61 provided in the laser 200 of FIG. 6 can be omitted.

[0087] 〔Eighth Embodiment〕 Hereinafter, the laser of the eighth embodiment will be described. The laser of the eighth embodiment is a modification of the laser of the third embodiment.

[0088] FIG. 15 is a conceptual diagram for explaining the laser 200 of the eighth embodiment. In the laser 200, the wavelength selective optical combiner / splitter 850 is an element of the spatial optical system type. The wavelength selective optical combiner / splitter 850 performs the same function as the WDM 50 shown in FIG. 6, and also has the functions of the polarization rotation start portion R1 and the polarization rotation end portion R2.

[0089] The wavelength-selective optical multiplexer / demultiplexer 850 includes collimators 191a and 91b, half-wave plates 91c and 91d, and a PBS 91s as optical elements inserted between the loop elements 40a and 40b on the optical path of the optical fiber loop 40. The wavelength-selective optical multiplexer / demultiplexer 850 includes a collimator 93a and an output fiber 93b on the optical path branched from the PBS 91s. The collimators 191a and 91b are similar to those shown in FIG. 14, but the collimator 191a has an additional function of selectively reflecting the excitation light PL. The half-wave plates 91c and 91d are angle-adjustable, but can also be fixed after the angle adjustment.

[0090] Fig. 16(a) shows an example of the structure of a collimator 191a. The collimator 191a has a lens 91e that collimates the light LI emitted from an end face 40e of an optical fiber 40f. A mirror 95a made of a dielectric multilayer film is formed on the end face 40e, and selectively reflects the excitation light PL and transmits the resonance light LL. As shown in Fig. 16(b), a surface mirror 95a may be provided on the emission side of the lens 91e.

[0091] Returning to FIG. 15, the collimator 191a, the half-wave plate 91c, and the PBS 91s on the input side function as the polarization rotation terminal R2. The collimator 91b, the half-wave plate 91d, and the PBS 91s on the output side function as the polarization rotation start portion R1. That is, the second angle θ2 can be adjusted by adjusting the angle of the half-wave plate 91c, and the first angle θ1 can be adjusted by adjusting the angle of the half-wave plate 91d. In this case, the entire optical fiber loop 40 functions as the nonlinear polarization rotation region 20A, and the first axis rotation portion 41 provided in the loop elements 40a and 40b and the first axis rotation portion 61 provided in the fiber amplifier 60 compensate for the walk-off of the light LI propagating through the nonlinear polarization rotation region 20A and suppress the residual filter effect.

[0092] FIG. 17 is a conceptual diagram for explaining a modified example of the laser 200 shown in FIG. 15. This laser 200 has a material-type saturable absorber member 81 containing CNT added on the optical path of the optical fiber loop 40. As the saturable absorber member 81, graphene, graphene oxide, etc. can be used, and although it is necessary to change to a reflective optical path, SESAM can also be used.

[0093] 〔Embodiment 9〕 Hereinafter, the optical device and laser of Embodiment 9 will be described. It is a modification of the optical device of Embodiment 2.

[0094] FIG. 18(a) is a conceptual diagram for explaining the optical device 4100 of Embodiment 9. In this case, in the non-linear polarization rotation region 120A, a ring fiber 112 is connected via a PBS (polarizing beam splitter) 97 after the polarization maintaining element 12. The PBS 97 branches the optical path with p-polarized light and s-polarized light, and makes the clockwise light LI and the counterclockwise light LI incident from both ends 5a, 5b of the ring fiber 112 and returns them to the polarization maintaining element 12. A non-reciprocal phase shifter, that is, an NRPS (non-reciprocal phase shift) 82 is inserted on the optical path of the ring fiber 112. The NRPS 82 gives a phase shift of π to the passing light LI according to the traveling direction of the light LI. That is, a phase bias is generated between the clockwise light LI and the counterclockwise light LI (see "https: / / www.symphotony.com / products / cartridgepc / ").

[0095] In the structure shown in Fig. 18(a), that is, in the optical device 4100, by incorporating the NRPS82, the structure shown in Fig. 18(a) can be made to function as a saturable absorber (SA). The structure excluding the NRPS82 has the effect as an inverse SA. Second, by incorporating the NRPS82 as in the structure of Fig. 18(a), CW light can be strongly blocked, which is advantageous for pulse generation. Furthermore, in the structure of Fig. 18(a), only one 90° splice can be made, simplifying the manufacturing process. It is not easy to completely control the length and splice angle of the polarization-maintaining optical fiber, but by adopting the structure of Fig. 18(a), the manufacturing work can be simplified and the efficiency of pulse generation can be improved.

[0096] In the optical device 4100 of the ninth embodiment, the PBS97 and the ring fiber 112 function as a reflector RL, and the optical LI passes through a reflective passing optical path, that is, a reciprocating optical path.

[0097] The first polarizer 2a makes the external light LI in a specific polarization direction enter the incident-side addition region 20B, and makes the light LI in the specific polarization direction from the incident-side addition region 20B exit to the outside.

[0098] Fig. 18(b) is a conceptual diagram for explaining another optical device 4100 of the ninth embodiment. The optical device 4100 shown in Fig. 18(b) is a partially modified version of the optical device 4100 shown in Fig. 18(a). Here, a part of the ring fiber 112 is replaced by a fiber amplification unit 60. The fiber amplification unit 60 is excited by the excitation light LE incident via the WDM250 from an excitation light source (not shown). The WDM250 passes not only the slow-axis light LI but also the fast-axis light LI and is not a polarizer. In this optical system, since two lights LI rotating in opposite directions pass through the slow axis, the WDM250 can be replaced by the WDM150 that functions as a polarizer.

[0099] FIG. 19(a) is a conceptual diagram for explaining a modified example of the optical device 4100 shown in FIG. 18(b). In this case, the WDM 250 inserted between the polarization maintaining element 11 and the fiber element 341 passes not only the slow-axis light LI but also the fast-axis light LI, and is not a polarizer. Also, a part of the polarization maintaining element 12 between the WDM 250 and the PBS 97 is replaced by the fiber amplifier section 60.

[0100] FIG. 19(b) is a conceptual diagram for explaining a modified example of the optical device 4100 shown in FIG. 19(a). In this case, the WDM 150 functions as a polarizer.

[0101] FIG. 19(c) is a conceptual diagram for explaining another modified example of the optical device 4100 shown in FIG. 19(a). In this case, the fiber amplifier section 60 is disposed between the polarization maintaining element 11 and the polarization maintaining element 12, and a first-axis rotation section 61 is provided at the center of the fiber amplifier section 60.

[0102] FIG. 20(a) is a conceptual diagram for explaining a modified example of the optical device 4100 shown in FIG. 18(a). In this case, the second-axis rotation section 22 includes the collimators 91a and 91b, a half-wave plate 91c, and a polarizer 191s. Here, the polarizer 191s corresponds to the first polarizer 2a shown in FIG. 18(a). The branching / combining section 197 includes the collimators 91a and 91b and a PBS 91s.

[0103] FIG. 20(b) is a conceptual diagram for explaining a modified example of the optical device 4100 shown in FIG. 20(a). In this case, a part of the ring fiber 112 is replaced by the fiber amplifier section 60. The collimator 291b selectively reflects the excitation light PL, similar to the collimator 191a shown in FIG. 16(a).

[0104] FIG. 20(c) is a conceptual diagram for explaining another modification of the optical device 4100 shown in FIG. 20(a). In this case, a part of the polarization maintaining element 12 instead of the ring fiber 112 is replaced with the fiber amplifier section 60. In the second axis rotation section 22, the collimator 291b selectively reflects the excitation light PL, similarly to the collimator 191a shown in FIG. 16(a).

[0105] FIG. 21 shows a laser 4200 incorporating the optical device 4100 shown in FIG. 18(a) in the laser 2200 shown in FIG. 7.

[0106] FIG. 22(a) shows a laser 4200 incorporating the optical device 4100 shown in FIG. 18(a) in the laser 3200 shown in FIG. 9. Note that the mirror 4 is a polarization maintaining mirror, for example, having a transmittance of 10% and enabling extraction of the resonant light LL.

[0107] FIG. 22(b) shows a laser 4200 incorporating the optical device 4100 shown in FIG. 18(a) in a laser having the same structure as the laser 3200 shown in FIG. 8. Note that the mirror 4 is a polarization maintaining mirror, for example, having a transmittance of 10% and enabling extraction of the resonant light LL.

[0108] FIG. 22(c) is a conceptual diagram for explaining a modification of the laser 4200 shown in FIG. 22(b). In this case, a part of the ring fiber 112 is replaced with the fiber amplifier section 60. The fiber amplifier section 60 is excited by the excitation light LE incident from the excitation light source 71 via the WDM 50. A polarization maintaining coupler WDM 150 incorporating a polarizer is inserted between the polarization maintaining element 11 and the fiber element 341, and the resonant light LL can be extracted. The mirror 304 is a polarization maintaining mirror, but is set to have a transmittance of approximately 0%.

[0109] FIG. 23(a) shows a modified example of the optical device 4100 shown in FIG. 18(a). In this case, the NRPS is omitted. FIG. 23(b) shows a modified example of the optical device 4100 shown in FIG. 18(b). In these cases, the NRPS is omitted.

[0110] In the case of the optical device 4100 shown in FIG. 23(a), that is, the structure, even if the NRPS is omitted, it functions as an SA. This is because the PBS 97 is used instead of the polarizer 2a. In the structure of FIG. 23(a), the input is, for example, a p-polarized component and the output is, for example, an S-polarized component. Assuming a graph showing the relationship between the non-linear phase and the transmittance, when the NRPS 82 is removed from the structure of FIG. 18(a), if the non-linear phase increases with the increase in the light intensity, the transmittance decreases. However, when the polarization components of the input and output are different as in the structure of FIG. 23(a), the transmittance is reversed, and when the non-linear phase increases with the increase in the light intensity, the transmittance also increases. That is, the structure of FIG. 23(a) functions as an SA.

[0111] FIG. 24(a) shows a modified example of the optical device 4100 shown in FIG. 23(b). A part of the polarization maintaining element 12 between the WDM 250 and the PBS 97 is replaced by the fiber amplifier section 60. The WDM 250 passes not only the light LI of the slow axis but also the light LI of the fast axis, not being a polarizer. FIG. 24(b) shows another modified example of the optical device 4100 shown in FIG. 23(b). In this case, the WDM 250 passes both the light LI of the slow axis and the light LI of the fast axis. FIG. 24(c) shows another modified example of the optical device 4100 shown in FIG. 23(b). In this case, the WDM 250 passes both the light LI of the slow axis and the light LI of the fast axis.

[0112] Note that the optical device 4100 shown in FIG. 24(a) etc. becomes a laser by connecting to a ring fiber with a coupler for extracting the resonant light inserted.

[0113] Figure 25(a) shows a modified example of the laser 4200 shown in Figure 21. In this case, the NRPS is omitted, and PBS97 is used instead of the circulator. Also, an isolator 47 is inserted on the optical fiber loop 40. Figure 25(b) shows a modified example of the laser 4200 shown in Figure 5(a). In this case, the NRPS is omitted, and a part of the ring fiber 112 is replaced with the fiber amplifier section 60. On the optical fiber loop 40, a WDM150, which is a polarization-maintaining coupler, is inserted, and the resonant light LL can be extracted.

[0114] Figure 26(a) shows a modified example of the optical device 4100 shown in Figure 20(a). In this case, the NRPS is omitted, and PBS91s is incorporated in the second-axis rotation section 22 instead of the polarizer 191s, and only the slow-axis polarization component branched by PBS91s can be selectively extracted to the side of the collimator 391a arranged to face another surface of PBS91s.

[0115] Figures 26(b) and 26(c) show modified examples of the optical device 4100 shown in Figures 20(b) and 20(c). In this case, the NRPS is omitted, and PBS91s is incorporated in the second-axis rotation section 22 instead of the polarizer 191s, and only the slow-axis polarization component branched by PBS91s can be selectively extracted to the side of the collimator 391a arranged to face another surface of PBS91s.

[0116] In the lasers 200, 2200, and 4200 of Figures 13, 21, 25(a), and 25(b), a saturable absorption member 81 can be added in the same manner as in Figure 17.

[0117] Hereinafter, examples will be described.

[0118] 〔Example 1〕 Next, as Example 1 of the laser of the present invention, the first L-band operation all-PM variable mode-locked fiber laser (MLFL) capable of achieving a sweep rate of up to 19 kHz will be described. This high-speed sweep is realized by modulating the pump current of an external laser diode (LD). This laser employs non-linear polarization rotation (NPR) in an all-PM configuration for mode locking and has a simple cavity design and excellent reproducibility. It is considered that an all-PM L-band variable laser with such a high wavelength sweep capability has been realized for the first time.

[0119] (1-2) Experimental configuration and results This laser has the same structure as the laser of the fourth embodiment shown in FIG. 7. The laser is composed of a pump LD, a slow-axis operation PM type tap isolator WDM hybrid device (PM-TIWDM) with an output ratio of 10%, a PM-EDF (Nufern ESF-7 / 125) with a length of 1.5 m, a slow-axis operation type PM Faraday rotation mirror (FRM), a slow-axis operation PM type circulator (CIR), and a segmented PMF (Fujikura SM15-PS-U25A) with a length of 21 m. The 21 m PMF is divided into a plurality of sections and connected at an angle of 90° each (first-axis rotation part 21), and a part is connected at an angle of 30° (second-axis rotation part 22). The mode locking mechanism is based on NPR in an all-PM configuration. The cavity length of about 50 m shows a net group velocity dispersion of -1.03 ps at 1550 nm, and this laser operates in the soliton region. 2 and this laser is operating in the soliton region.

[0120] The laser automatically started multi-pulse operation in the L-band at a pump power of 250 mW. When the pump power was manually decreased to approximately 150 mW, a single-pulse soliton of approximately 0.1 mW was generated at a central wavelength of 1590 nm. The spectral width was approximately 2.3 nm, and the pulse width was estimated to be approximately 1.3 ps. Furthermore, by steadily decreasing the pump power to approximately 50 mW, it was shown that the central wavelength could be continuously tuned down to 1570 nm (see Fig. 27). Figs. 28(a) and 28(e) show the high-frequency (RF) spectrum and the pulse train at 1590 nm. A signal-to-noise ratio of 70 dB indicates high pulse stability. During this manual tuning process, the laser maintained the mode-locked state, and it was confirmed that wavelength tuning was repeatable by increasing or decreasing the pump power. However, when the pump power was decreased to 40 mW or less, the laser shifted to continuous-wave (CW) oscillation.

[0121] In the automatic tuning process, the LD driver (Thorlabs CLD1015) was set to the "external modulation" mode, and the pump current or pump power was adjusted by a modulation voltage applied externally. This modulation signal was supplied by an arbitrary waveform generator (AWG, Agilent 3320A), which was capable of generating a sine waveform. By appropriately adjusting the settings of the LD driver and the AWG, the pump power could be swept in the range from 50 mW to 150 mW. This range was consistent with the values used in the above manual adjustment experiment. The optical spectrum analyzer (OSA, YOKOGAWA AQ6370D) had the "maximum value hold" function enabled in the "repeated sweep" mode. This setting was aimed at recording the peak value corresponding to the central wavelength of the optical spectrum during the automatic tuning experiment.

[0122] The modulation frequency of the external signal, that is, the center wavelength sweep rate of the laser, was set to 100 Hz, 200 Hz, 500 Hz, 1 kHz, 10 kHz, and 19 kHz during the experiment. Thereby, the adjustment performance of the laser at different sweep rates was investigated. Figures 28(b), 28(c), 28(d), 28(f), 28(g), and 28(h) show the spectral results during automatic tuning. At all sweep rates, the OSA (optical spectrum analyzer) spectrum stabilized within 1 minute, and the laser maintained a mode-locked state throughout the tuning process. The tuning range reached approximately 20 nm at a sweep rate of 100 Hz, the same as achieved by manual adjustment. As the sweep rate increased, the tuning range decreased, dropping to approximately 14 nm at 200 Hz and 8 nm at 500 Hz. At high frequencies of 1 kHz, 10 kHz, and 19 kHz, it decreased to 6 nm, 5 nm, and 3 nm, respectively.

[0123] The reduction in the adjustment range at high sweep rates observed during the experiment has also been reported in Ref. [3]. This may be due to the mode-locking formation dynamics where it takes time for the pulse to stabilize after the pump conditions change. Our quest for high sweep rates is currently limited by the maximum allowable value of 20 kHz for the LD driver with respect to the external modulation signal. Although the tuning range narrows as the sweep rate increases, our laser has been significantly improved compared to those reported in Ref. [3], achieving a sweep rate approximately 40 times higher. Combining this high sweep capability with a cost-effective and efficient external LD driver control shows new possibilities for rapid and wide tuning of MLFL, opening up new avenues for applications such as optical spectroscopy.

[0124] (1 - 3) Conclusion In conclusion, we report the first L-band all-PM NPR MLFL that realizes wavelength sweeping up to 19 kHz by externally modulating the pump current. This method involves modulating the LD driver, is cost-effective, reliable, and user-friendly. Our design of an all-PM fiber laser with a simple configuration provides a practical solution for rapid wavelength adjustment and shows great potential in various application fields. (1-4) References [3] X. Sun, et al., Opt. Express. 31(8), 12837-12846 (2023).

[0125] 〔Example 2〕 Hereinafter, as Example 2 of the laser of the present invention, an all-PM NPR fiber ring laser operating in the all-abnormal dispersion region will be described. This laser generates soliton pulses at a wavelength of 1575 nm and achieves an average power of 2.3 mW. This design shows a performance improvement of about 6 times compared to a previous study [5] which was 0.388 mW. The repetition frequency is 13.6 MHz, which is comparable to the value reported in the same study of 15.4 MHz. Also, simulations regarding the transmission characteristics of the artificial SA showed that the saturation power, modulation depth, and unsaturated loss can be changed by adjusting the splice angle.

[0126] (2-1) Experimental setup This laser has the same structure as the laser of the third embodiment shown in FIG. 6. This laser or system is basically constructed only with PM-type fibers and optical elements.

[0127] A coupler of a slow-axis operation PM type tap isolator WDM hybrid device (PM-TIWDM) is incorporated, and this coupler has functions of a polarizer, WDM, an isolator, and a 20% output tap. PM-EDF (Nufern ESF-7 / 125) is used as an amplification medium, with a length of 1.4 m and is pumped from the rear through the PM-TIWDM by a 980 nm laser diode (LD). As the remaining passive fiber, Panda PM fiber (Fujikura SM15-PS-U25A) is adopted. This laser operates in the clockwise direction. The NPR section (nonlinear polarization rotation region 20A) is composed of a 11 m PM fiber and is divided into 12 segments. The first and last segments are 0.5 m in length, and the middle 10 segments are each 1 m in length. In the NPR section, each segment is spliced at a 90° angle to the adjacent segment (first axis rotation part 21). The loss at each splicing point is approximately 0.05 dB. Also, the angles of the splicing parts (second axis rotation part 22) between the remaining part of the cavity and the first and last segments are variable. All splicing procedures were carried out using a Fujikura side-view type splicer (FSM-100P).

[0128] PM-TIWDM also functions as a polarizer. After the first splicing angle (first angle θ1), the linearly polarized light is split into two orthogonally polarized components along the slow axis and fast axis of the PM fiber. These components have unequal energies and obtain different nonlinear phase shifts due to self-phase modulation (SPM) and cross-phase modulation (XPM) respectively. When the polarizer is positioned on the slow axis of the PM fiber, the direction of the polarizer can be adjusted by adjusting the second splicing angle (second angle θ2). Generally, changing these two splicing angles performs a function similar to the adjustment of the polarization controller in a non-PM NPR laser. The total cavity length is about 14.7 m, and the group velocity dispersion (GVD) at a wavelength of 1550 nm is -0.3 ps 2 and becomes like this. This laser operates in the all-anomalous dispersion region.

[0129] (2-2) Experimental Results and Discussion The first splicing angle (θ1) was set to 20°, and the second splicing angle (θ2) was set to -30°. When the pump power was set to approximately 400 mW, the laser started self-oscillation in the multi-pulse operation mode. At this stage, an unstable pulse train consisting of 3-5 pulses was observed on the oscilloscope. Subsequently, when the pump power was decreased to approximately 200 mW, the number of pulses decreased, and finally, a stable single pulse was generated.

[0130] Fig. 29(a) shows the output spectrum. The output is centered at a wavelength of 1575 nm, and the 3 dB spectral bandwidth is 5 nm. As shown in Fig. 29(b), the fundamental high-frequency (RF) spectrum was measured with a resolution bandwidth (RBW) of 10 Hz and a span of 10 kHz. The signal-to-noise ratio (SNR) is 68 dB, indicating high pulse-to-pulse stability. The center frequency appears at 13.6 MHz, which is consistent with the total length of the cavity of approximately 14.7 m and corresponds to the pulse interval of approximately 74 ns shown in Fig. 29(c). The fact that the central observed wavelength of 1575 nm is located in the near L-band (1565 nm - 1625 nm) is considered to be due to the shift of the gain profile in the long gain fiber [6], and the EDF provides sufficient gain necessary for mode locking within the near L-band spectrum.

[0131] Figure 29(d) shows the experimental results at different angle sets (θ1, θ2). The central wavelength remains at about 1575 nm, but the continuous wave (CW) component changes. This may be due to the fact that the transmission characteristics of the artificial SA (saturable absorber) change depending on the angle set. As shown in Figure 30(a), in transient laser operation, a CW component of about 1572 nm and a broad spectral base were observed. As shown in Figure 30(b), when the angle is further adjusted, mode locking or continuous wave (CW) oscillation can be obtained. The CW laser oscillation at 1575 nm shown in Figure 30(b) confirmed the oscillation ability in the near L-band. Furthermore, the presence of another CW component at 1557 nm suggests the possibility of a spectral filter existing in the cavity.

[0132] To numerically evaluate the transmission characteristics of the NPR section, a sech pulse with a full width at half maximum (FWHM) of 800 fs was used as the input. This pulse propagates in the PM fiber and was analyzed using the split-step Fourier method based on the coupled-mode nonlinear Schrödinger equation [7]. The peak power was set in the range from 1 W to 1 kW, and the total transmission rate of the pulse energy at different peak power levels was obtained. 2 The simulation results with θ1 fixed (20°) and θ2 varied are shown in Figure 31(a). As the peak power of the pulse increased from zero, the transmission rate increased, confirming the characteristics of the SA. However, when the peak power reached the threshold, the transmission rate decreased without increasing, showing the characteristics of the reverse SA. When the peak power exceeded the transition power value, the peak part suffered greater loss than the peripheral part, and the pulse might split. The results in Figure 31(a) show that θ2 mainly affects the modulation depth and unsaturated loss of the population SA and has little effect on the saturation power. Figure 31(b) shows the case where θ2 was fixed during the simulation, and all three parameters changed with the change of θ1. The high transmission rate at low pulse peak power can explain the CW components with different output powers shown in Figure 29(d).

[0133] ​

[0134] (2-3) Conclusion For the first time, an all-PM NPR fiber ring laser operating in the all-anomalous dispersion regime was experimentally demonstrated. This laser generates pulses at a wavelength of 1575 nm and achieves an average output power of 2.3 mW. This represents an approximately six-fold performance improvement compared to previous studies in the all-anomalous dispersion regime. Also, simulations of the NPR section showed that by adjusting the splice angle, characteristics such as the saturation power, modulation depth, and unsaturated loss of the artificial SA can be controlled.

[0135] (2-4) References [5] Z. Peng, Z. Cheng, X. Bu, C. Hong, H. Li, Y. Shi, and P. Wang, "Study of an er-doped all-pm-fiber laser mode-locked by nonlinear polarization evolution," IEEE Photonics Technol. Lett. 30(24), 2111-2114 (2018). [6] J. Kang, C. Kong, P. Feng, X. Wei, Z.-C. Luo, E. Y. Lam, and K. K. Wong, "Broadband High-Energy All-Fiber Laser at 1.6μm," IEEE Photon. Technol. Lett. 30(4), 311-314 (2017). [7] G. P. Agrawal, Nonlinear Fiber Optics, 5th ed. (Elsevier / Academic, 2013).

[0136] [Example 3] Hereinafter, as Example 3 of the laser of the present invention, an all-PM NPR mode-locked fiber laser that generates wavelength-variable pulses in the L band will be described. By increasing the pump power from 82.5 mW to 135 mW, the wavelength can be adjusted from 1576.2 nm to 1592.2 nm.

[0137] (3-1) Configuration and Results of the Experiment This laser has the same structure as the laser of the fourth embodiment shown in FIG. 7. In this laser or system, all optical elements and fibers are of the PM type. A slow-axis operation PM type tap · isolator WDM hybrid device (PM-TIWDM) functions as a polarizer, WDM, isolator, and 20% output coupler. A PM-EDF (Nufern ESF-7 / 125) with a length of 1.5 m functions as an amplification medium. An artificial SA, that is, an NPR section (nonlinear polarization rotation region 20A), is composed of a PMF (Fujikura SM15-PS-U25A) with a length of 21 m and a slow-operation PM type Faraday rotation mirror (PM-FRM). The PMF with a length of 21 m is divided into eight sections by a splice section (first-axis rotation section 21) with an angle of 90° between the splice section (second-axis rotation section 22) with an angle of 30° and the FRM. A slow-axis operation PM circulator (PM-CIR) functions as a polarizer. The total cavity length is about 50 m, and the group velocity dispersion (GVD) at a wavelength of 1550 nm is -1.03 ps 2 and the laser is operating in the soliton region.

[0138] The laser that self-oscillates at a pump power of 260 mW shows unstable multi-pulses. However, by gradually reducing the pump power to 135 mW, a stable single soliton pulse is obtained, with an output power of 120 μW and a central wavelength of 1592.2 nm. As shown in Fig. 32, the overall spectrum is shown on a logarithmic scale (dBm), and the inset shows the spectrum at a pump power of 135 mW on a linear scale (μW). The estimated output power of the L-band soliton pulse is 108 μW, taking into account that the ASE (spontaneous emission light) noise accounts for about 10% of the total power. When the pump power is increased to 150 mW, a continuous-wave (CW) laser is generated at 1530 nm, and the L-band soliton pulse does not change. Fig. 33 shows the high-frequency (RF) spectrum with a resolution bandwidth (RBW) of 100 Hz, indicating high pulse-to-pulse stability with a signal-to-noise ratio (SNR) of 63 dB. The central frequency of 3.9 MHz corresponds to a cavity length of approximately 50 m.

[0139] The wavelength tuning performance is shown in Fig. 34, and the central wavelength can be tuned to 1576.2 nm by reducing the pump power to 82.5 mW. Below 82 mW, the laser transitions from mode-locked to a CW laser. This 16-nm tuning range can be repeatedly achieved by increasing or decreasing the pump power and has been tested many times. However, the tuning mechanism of the pump power is non-linear, and it has been found that under the same 5-mW pump power change, the L-band soliton shows a larger tuning range than the mid-L-band soliton. This non-linearity is considered to be due to the change in the amplification profile of the PM-EDF at different pump powers.

[0140] (3-2) Conclusion As a summary, we have achieved the first all-PM NPR fiber laser that can generate wavelength-tunable soliton pulses in the L-band. By increasing the pump power from 82.5 mW to 135 mW, a wavelength tuning range of 16 nm (from 1576.2 nm to 1592.2 nm) was realized. This laser exhibits excellent reproducibility and reliability compared to non-PM mechanically tunable L-band fiber lasers.

[0141] 〔Others〕 Although the present invention has been described in accordance with the above embodiments, the present invention is not limited to the above embodiments.

[0142] As shown in FIG. 35, assuming that the polarization-maintaining fibers PF constituting the nonlinear polarization rotation region 20A and the output-side addition region 20C are individually different, the following conditions TIFF2025104343000004.tif18170 need to be satisfied. Here, Δn(L1) means the difference between the refractive index of the slow axis and the refractive index of the fast axis of the polarization-maintaining element 12 having a length L1, Δn(L2) means the difference between the refractive index of the slow axis and the refractive index of the fast axis of the polarization-maintaining element 12 having a length L2, Δn(L3) means the difference between the refractive index of the slow axis and the refractive index of the fast axis of the polarization-maintaining element 13 having a length L3, and Δn(L4) means the difference between the refractive index of the slow axis and the refractive index of the fast axis of the polarization-maintaining element 13 having a length L4. In the above description, for simplicity, the nonlinear polarization rotation region 20A is composed of two polarization-maintaining elements 12, but for example, it may be composed of four polarization-maintaining elements 12 or more polarization-maintaining elements 12. Also in this case, the lengths of the respective polarization-maintaining elements 12 can be determined in consideration of the refractive index difference between the slow axis and the fast axis for each polarization-maintaining element 12. In the illustrated example, it is assumed that light propagates only in the slow axis in the polarization-maintaining element 11.

[0143] For example, instead of or in addition to the saturable absorption member 81, a NORM (nonlinear optical loop mirror) or a NALM (nonlinear amplifying loop mirror) can be incorporated.

[0144] The wavelengths of the excitation laser light and the amplified laser light used in the excitation light source 71, the optical fiber loop 40, etc. are not limited to those exemplified above, and various wavelengths can be used. That is, the wavelength of the resonance light LL is not limited to the 1.5 μm band, and can be set to the 1.0 μm band, 2.0 μm, visible wavelength region, etc. A general silica optical fiber exhibits normal dispersion in the 1.0 μm band, but by using other optical fibers with different materials and structures, it can be made to operate in the anomalous dispersion region even in the 1.0 μm band. As the fiber amplifier section 60, not only Er-doped fibers but also YDF, EYDF, TDF, etc. can be used. The optical fiber is not limited to those that exhibit anomalous dispersion with respect to the wavelength band used, and may exhibit normal dispersion.

[0145] The mirror 3 is not limited to a Faraday rotator type mirror, and a polarization-maintaining type mirror can be used.

[0146] Regarding the nonlinear polarization rotation region 20A, the filter characteristics can be modified by heating the optical fiber or stretching / bending the optical fiber.

[0147] The number of polarization-maintaining elements 11, 12, 13, 17 that make up the nonlinear polarization rotation region 20A can be changed in the range of about 2 to 100. Also, the length of the polarization-maintaining elements 11, 12, 13, 17, that is, the segment length, can be adjusted in the range of, for example, about 5 cm to 25 m.

[0148] By using a connection of multiple types of doped fibers as the fiber amplifier section 60, the gain profile can be adjusted, and as a result, the setting of the wavelength band and tuning range can be changed.

Explanation of Reference Numerals

[0149] 3,4,304… mirror, 10… PANDA fiber, 11,12,13,17… polarization maintaining element, 20… polarization rotation section, 120A,20A… non-linear polarization rotation region, 20C… emission side addition region, 21… first axis rotation section, 22… second axis rotation section, 40… optical fiber loop, 47… isolator, 50… WDM, 60… fiber amplifier section, 71… excitation light source, 72… drive device, 80… circulator, 81… saturable absorption member, 82… isolator, 91a,91b,93a,191a,291b,391a… collimator, 97… PBS, 112… ring fiber, 191s… polarizer, 197… branching / combining section, 250… coupler, 340… linear cavity, 341,341a,431b… fiber element, 100,2100,4100… optical device, 200,2200,3200,4200… laser, AXf… fast axis, AXs… slow axis, LE… excitation light, LI… light, LL… resonant light, OW… target wavelength range, OX… optical axis, PMC… transmission member, PF… polarization maintaining fiber, PL… excitation light, R1… polarization rotation start section, R2… polarization rotation end section, R3… start / end section, RE… optical resonator, a… segment length, b… segment length, c… segment length, θ1… splice angle (first angle), θ2… splice angle (second angle)

Claims

1. An optical device that functions as an artificial saturable absorber, having a polarization rotation section that generates non-linear polarization rotation by a polarization-maintaining transmission member, wherein the polarization rotation section has a plurality of polarization-maintaining elements formed by the polarization-maintaining transmission member in a passing optical path from a polarization rotation start section to a polarization rotation end section, the polarization rotation section independently propagates the first polarization of the slow axis and the second polarization of the fast axis branched at the polarization rotation start section in the passing optical path so as to compensate for a phase shift therebetween as a result, suppressing a residual filter effect in a target wavelength range, Optical device.

2. The optical device according to claim 1, wherein the polarization-maintaining transmission member is a polarization-maintaining fiber.

3. The optical device according to claim 2, wherein the target wavelength range corresponds to an anomalous dispersion region of the polarization-maintaining fiber.

4. The optical device according to claim 2, wherein the polarization rotation start section and the polarization rotation end section are provided at both ends of a linear one-way optical path that functions as the passing optical path.

5. The optical device according to claim 4, further comprising a pair of polarizers arranged so as to sandwich the one-way optical path.

6. The optical device according to claim 5, wherein the residual filter effect is suppressed by canceling a phase shift between the first polarization and the second polarization at least between the polarization rotation end section and the subsequent polarizer.

7. The optical device according to claim 2, wherein the polarization rotation start section and the polarization rotation end section are one start / end section provided at the other end of a reciprocating optical path that functions as the passing optical path and has a reflector provided at one end.

8. The optical device according to claim 7, further comprising a polarizer arranged on the opposite side of the reciprocating optical path with respect to the start / end section.

9. The optical device according to claim 8, wherein the residual filter effect is suppressed by canceling a phase shift between the first polarization and the second polarization between the start / end section and the polarizer.

10. The optical device according to claim 2, wherein among the plurality of polarization-maintaining elements, there is a first axis rotation section that switches between the slow axis and the fast axis between adjacent polarization-maintaining elements.

11. The optical device according to claim 2, wherein the polarization rotation start section branches the first polarization and the second polarization so that the first polarization and the second polarization become unequal in intensity.

12. The polarization rotation start part and the polarization rotation end part are second-axis rotation parts that perform coupling between fibers so that the relative angle of the fast axis is within a range of 10° to 80°. The optical device according to claim 9.

13. The polarization rotation part does not include a wavelength filter. The optical device according to claim 1.

14. An optical resonator, The optical device according to any one of claims 1 to 13 disposed on the optical path of the optical resonator, An optical amplification device including an optical amplification element disposed on the optical path of the optical resonator, A laser.

15. The optical amplification device adjusts the oscillation wavelength by changing the drive signal of the optical amplification element. The laser according to claim 14.

16. The optical amplification element is a rare earth element doped fiber. The laser according to claim 15.

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