Fiber laser device

Abstract

The optical fiber laser device of the present application has a first optical fiber (30) composed of a polarization maintaining optical fiber, a second optical fiber (40), and a third optical fiber (50). The first optical fiber (30) has at least one first section (31) and at least two second sections (32) disposed alternately with the first section (31). Adjacent first sections (31) and second sections (32) are connected to each other in such a manner that the fast axis (31X1) of the first section (31) and the slow axis (32X2) of the second section (32) coincide at the connection site. The total length of the first section (31) is equal to the total length of the second section (32). The mode field diameter of the first optical fiber (30) is smaller than each of the mode field diameters of the second optical fiber (40) and the third optical fiber (50).

Description

Technical Field

[0001] One aspect of this disclosure relates to a fiber laser device. Background Technology

[0002] As a fiber laser device, it is known to use the nonlinear effect of optical fiber to generate mode locking, thereby generating ultrashort pulse lasers (see, for example, Patent Document 1).

[0003] Existing technical documents

[0004] Patent documents

[0005] Patent Document 1: European Patent Application Publication No. 3300191 Summary of the Invention

[0006] The problem the invention aims to solve

[0007] In fiber laser devices as described above, a certain length of fiber is required to generate sufficient nonlinear effects. Therefore, the resonator becomes longer, making it difficult to increase the frequency of the (highly repetitive) output light. Furthermore, to enable self-initiation of mode locking, a sufficiently high internal power density of the resonator is needed. If the fiber is short, the internal power density required to obtain sufficient nonlinear effects becomes higher compared to the case of a long fiber, thus requiring higher excitation power. Additionally, fiber laser devices require light with a good output waveform.

[0008] One aspect of this disclosure is to provide a fiber laser device that can output light with good waveform and achieve high reproducibility or low excitation power.

[0009] Technical means to solve the problem

[0010] A fiber laser device according to one aspect of this disclosure includes: a first fiber composed of a polarization-maintaining fiber; a second fiber composed of a polarization-maintaining fiber and connected to one end of the first fiber; and a third fiber composed of a polarization-maintaining fiber and connected to the other end of the first fiber. The first fiber has at least one first portion and at least two second portions arranged alternately with the first portion. Adjacent first portions and second portions are interconnected such that the fast axis of the first portion coincides with the slow axis of the second portion at the connection point. The length of the first portion is equal to the total length of the second portion. The mode field diameter of the first fiber is smaller than the mode field diameters of the second fiber and the third fiber.

[0011] In this fiber laser device, the first fiber has at least one first section and at least two second sections arranged alternately with the first section. Adjacent first sections and second sections are interconnected such that the fast axis of the first section aligns with the slow axis of the second section at the connection point. Mode locking can be achieved by passing light through such a first fiber. Furthermore, the total length of the first section is equal to the total length of the second section. This compensates for the difference in propagation speed between the component propagating along the fast axis and the component propagating along the slow axis. Additionally, the first fiber has at least one first section and at least two second sections. Therefore, compared to the case where the first fiber contains only two fiber elements, waveform disturbances in the output light caused by the interaction between the component propagating along the fast axis and the component propagating along the slow axis can be suppressed. As a result, light with a good waveform can be output. Furthermore, the mode field diameter of the first fiber is smaller than that of the second fiber and the third fiber, thereby improving the nonlinearity of the first fiber. Therefore, the first fiber can be shortened, thereby achieving high repetition rate and low excitation power. Thus, according to this fiber laser device, light with a good waveform can be output, and high repetition rate and low excitation power can be achieved.

[0012] Alternatively, at least one of the second and third optical fibers can be connected to the first optical fiber by fusion splicing. In this case, the number of components can be reduced, and manufacturing can be simplified.

[0013] Alternatively, the second optical fiber may have a first part and a second part. The first part of the second optical fiber is connected to one end of the first optical fiber such that the fast axis of the first part of the second optical fiber aligns with the slow axis of the first optical fiber at the connection point. The second part of the second optical fiber is connected to the first part of the second optical fiber such that the angle between the fast axis of the second part of the second optical fiber and the fast axis of the first part of the second optical fiber at the connection point is an angle other than 0 degrees or 90 degrees. Connecting polarization-maintaining fibers with different mode field diameters to each other with an angle between their fast axes other than 0 degrees or 90 degrees is difficult and risks reducing yield. In contrast, in this structure, the first and second optical fibers with different mode field diameters are connected with their fast and slow axes aligned (the angle between their fast axes is 90 degrees). This simplifies the connection between the first and second optical fibers and improves the yield.

[0014] Alternatively, the third optical fiber may have a first part and a second part. The first part of the third optical fiber is connected to the other end of the first optical fiber such that the fast axis of the first part of the third optical fiber is aligned with the fast axis of the first optical fiber at the connection point. The second part of the third optical fiber is connected to the first part of the third optical fiber such that the angle between the fast axis of the second part of the third optical fiber and the fast axis of the first part of the third optical fiber at the connection point is an angle other than 0 degrees or 90 degrees. In this case, the connection between the first and third optical fibers can be simplified, thereby further improving the yield.

[0015] Alternatively, the length of the first section of the second optical fiber can be equal to the length of the first section of the third optical fiber. In this case, the difference in propagation speed between the component propagating along the fast axis and the component propagating along the slow axis can be compensated for the first section of both the second and third optical fibers.

[0016] Alternatively, the second optical fiber may have a first portion, which is connected to one end of the first optical fiber such that the fast axis of the first portion of the second optical fiber aligns with the slow axis of the first optical fiber at the connection point. The third optical fiber may also have a first portion, which is connected to the other end of the first optical fiber such that the fast axis of the first portion of the third optical fiber aligns with the fast axis of the first optical fiber at the connection point. The length of the first portion of the second optical fiber is equal to the length of the first portion of the third optical fiber. In this case, the connection between the first and second optical fibers, and between the first and third optical fibers, can be simplified. Furthermore, the difference in propagation speed between the components propagating along the fast axis and those propagating along the slow axis can be compensated for in the first portions of the second and third optical fibers.

[0017] The fiber laser device of this disclosure further includes a first bridging fiber, which is composed of a polarization-maintaining fiber and connected between the first fiber and the second fiber. The mode field diameter of the first bridging fiber is larger than that of the first fiber and smaller than that of the second fiber. If polarization-maintaining fibers with different mode field diameters are connected to each other, losses are likely to occur at the connection point. In contrast, in this structure, a first bridging fiber with a mode field diameter larger than that of the first fiber and smaller than that of the second fiber is connected between the first fiber and the second fiber. This reduces losses at the connection point.

[0018] The fiber laser device of one aspect of this disclosure further includes a second bridging fiber, which is composed of a polarization-maintaining fiber and connected between the first fiber and the third fiber. The mode field diameter of the second bridging fiber is larger than that of the first fiber and smaller than that of the third fiber. In this case, the loss at the connection point can be further reduced.

[0019] Alternatively, the first optical fiber may have an even number of first and second portions, and the difference between the angle between the fast axis of the first bridging fiber and the fast axis of the first optical fiber at the connection point, and the difference between the angle between the fast axis of the second bridging fiber and the fast axis of the first optical fiber at the connection point, is 90 degrees. In this case, the difference in propagation speed between the component propagating along the fast axis and the component propagating along the slow axis can be compensated for for the first and second bridging fibers.

[0020] Alternatively, the first optical fiber may have an odd number of first and second portions, and the difference between the angle between the fast axis of the first bridging fiber and the fast axis of the first optical fiber at the connection point, and the difference between the angle between the fast axis of the second bridging fiber and the fast axis of the first optical fiber at the connection point, is 0 degrees. In this case, the difference in propagation speed between the component propagating along the fast axis and the component propagating along the slow axis can be compensated for for the first and second bridging fibers.

[0021] Alternatively, the angle between the fast axis of the first bridging fiber and the fast axis of the second fiber at the connection point, and the angle between the fast axis of the second bridging fiber and the fast axis of the third fiber at the connection point, can be 0 degrees. In this case, it is also possible to compensate for the difference in propagation speed between the component propagating along the fast axis and the component propagating along the slow axis for the second and third fibers.

[0022] Alternatively, the length of the first bridging fiber can be equal to the length of the second bridging fiber. In this case, the difference in propagation speed between the component propagating along the fast axis and the component propagating along the slow axis can be compensated for for both the first and second bridging fibers.

[0023] Alternatively, the fiber laser device of one aspect of this disclosure may further include: a light source that outputs excitation light; and an optical fiber that absorbs the excitation light and emits laser light, the laser light being guided by a first optical fiber, a second optical fiber, and a third optical fiber. In this case, it is possible to output light with a good waveform and to achieve high reproducibility and low excitation power.

[0024] The effects of the invention

[0025] According to one aspect of this disclosure, a fiber laser device can be provided that can output light with a good waveform and can achieve high reproducibility or low excitation power. Attached Figure Description

[0026] Figure 1 This is a structural diagram of the fiber laser device according to the implementation method.

[0027] Figure 2 This is a cross-sectional view of the first optical fiber.

[0028] Figure 3 This is a schematic diagram of the mold locking section.

[0029] Figure 4 It is a diagram showing the pulse waveform and instantaneous wavelength.

[0030] Figure 5 It is a diagram showing the spectral waveform and phase.

[0031] Figure 6 (a) and (b) are graphs showing the spectrum.

[0032] Figure 7 This is a structural diagram of the fiber laser device in the first modified example.

[0033] Figure 8 This is a schematic diagram of the locking part of the first modified example.

[0034] Figure 9 This diagram illustrates the connection method for bridging optical fibers.

[0035] Figure 10 It shows through Figure 7 The image shows the spectral waveform obtained from the structure.

[0036] Figure 11 (a) and (b) are shown by Figure 7 The spectrum obtained by the structure.

[0037] Figure 12 This is a structural diagram of a comparative example fiber laser device.

[0038] Figure 13 It shows through Figure 12 The image shows the spectral waveform obtained from the structure.

[0039] Figure 14 (a) and (b) are shown by Figure 12 The spectrum obtained by the structure.

[0040] Figure 15 This is a structural diagram of the fiber laser device in the second variation.

[0041] Figure 16 It shows through Figure 15 The image shows the spectral waveform obtained from the structure.

[0042] Figure 17 It shows through Figure 15 The pulse waveform and instantaneous wavelength were obtained by the structure.

[0043] Figure 18 It is shown in Figure 15 The diagram shows the angle region where the mold-locking mechanism is generated in the structure.

[0044] Figure 19 This is a structural diagram of the fiber laser device in the third variation.

[0045] Figure 20 It shows through Figure 19 The image shows the spectral waveform obtained from the structure.

[0046] Figure 21 (a) and (b) are shown by Figure 19 The spectrum obtained by the structure. Detailed Implementation

[0047] Hereinafter, one embodiment of the present disclosure will be described with reference to the accompanying drawings. Figure 1 The following explanation will be provided in detail. In the following explanation, the same symbol will be used for the same or equivalent elements, and repeated explanations will be omitted.

[0048] [Fiber Laser Device]

[0049] like Figure 1 The fiber laser device 1 shown includes: a light source 11, a WDM (Wavelength Division Multiplexing) coupler 12, a doped fiber 13, an isolator 14, a mode-locking unit 15, a polarization controller 16, a polarizer 17, an output coupler 18, and an ASE (Amplified Spontaneous Emission) filter 19. Additionally, the fiber laser device 1 includes multiple optical fibers 21-28 for interconnecting these elements. The mode-locking unit 15 includes: a first optical fiber 30, a second optical fiber 40, and a third optical fiber 50.

[0050] Each of the doped fiber 13, fibers 21-28, the first fiber 30, the second fiber 40, and the third fiber 50 is composed of a polarization-maintaining (PM) fiber. A polarization-maintaining fiber is an optical fiber that improves the polarization plane maintenance characteristic of transmitted light by creating a difference in refractive index between mutually orthogonal fast and slow axes. In this example, each of the doped fiber 13, fibers 21-28, the first fiber 30, the second fiber 40, and the third fiber 50 is composed of a stress-applied polarization-maintaining fiber utilizing photoelastic effects, or it could be composed of a polarization-maintaining fiber with a non-axisymmetric core shape.

[0051] Figure 2This is a cross-sectional view of the first optical fiber 30. The first optical fiber 30 has a fast axis X1 and a slow axis X2 that are orthogonal to each other. The first optical fiber 30 includes a core 30a, a cladding 30b, and a pair of stress-applying materials 30c. The core 30a is located at the center of the first optical fiber 31. The refractive index of the core 30a is higher than that of the cladding 30b. The cladding 30b surrounds the core 30a. The pair of stress-applying materials 30c are arranged within the cladding 30b on both sides of the core 30a along the slow axis X2.

[0052] In the first optical fiber 30, tensile stress is applied to the core 30a by utilizing the fact that the thermal shrinkage rate of the stress-applying material 30c is greater than that of the cladding 30b, thereby giving the core 30a birefringence. Due to the refractive index difference, when light is transmitted within the first optical fiber 30, the component transmitted along the fast axis X1 is transmitted faster than the component transmitted along the slow axis X2. The doped optical fibers 13, 21-28, the second optical fiber 40, and the third optical fiber 50 also have the same cross-sectional structure as the first optical fiber 30.

[0053] Refer again Figure 1 The light source 11 outputs excitation light L1. The light source 11 is, for example, a laser diode that outputs laser light with a wavelength of 979 nm. The WDM coupler 12 reflects the excitation light L1 input from the light source 11 via optical fiber 21 and outputs it to optical fiber 22, and allows the signal light L2 input via optical fiber 28 to pass through and be output to optical fiber 22.

[0054] The doped fiber 13 absorbs the excitation light L1 input via fiber 22 and emits laser light (signal light L2). In the doped fiber 13, the emitted signal light L2 is input to the isolator 14 via fiber 23. The doped fiber 13 is, for example, an erbium-doped fiber (EDF) with erbium (Er) added to its core, emitting laser light with a wavelength in the 1.5 μm band. The doped fiber 13 can also be a ytterbium-doped fiber with ytterbium (Yb) added to its core. In this case, the doped fiber 13 emits laser light with a wavelength in the 1.0 μm band.

[0055] Isolator 14 directs light along the self-doped fiber 13 toward the mode-locking section 15 in the forward direction, while preventing light from being transmitted in the opposite direction. The mode-locking section 15 has a second fiber 40, a first fiber 30, and a third fiber 50 sequentially arranged upstream of the light transmission direction, through which the signal light L2 is guided. Details of the mode-locking section 15 will be described later.

[0056] The polarization controller 16 has a mechanism for adjusting the polarization state of the signal light L2 output from the mode-locked unit 15. The polarization controller 16 is configured, for example, to include a rotatably held λ / 4 wavelength plate 16a and a λ / 2 wavelength plate 16b. The signal light L2 output from the polarization controller 16 is input to the polarizer 17 via the optical fiber 24. The polarizer 17 allows the component of the signal light L2 propagating along the slow axis to pass through, while reflecting the component propagating along the fast axis.

[0057] The output coupler 18 splits the signal light L2 input from the polarizer 17 via the optical fiber 25 at a specific ratio, outputting a portion of the signal light L2 to the optical fiber 26 and the remainder to the optical fiber 27. For example, the output coupler 18 outputs 25% of the signal light L2 to the optical fiber 26 and the remaining 75% to the optical fiber 27. The signal light L2 output to the optical fiber 26 is, for example, output to the outside as output light. An isolator can also be provided in the output coupler 18 to prevent return light from the outside entering via the optical fiber 26 from returning to the resonator. In this case, it is possible to suppress the instability of the oscillations in the resonator caused by the return light.

[0058] ASE filter 19 allows only specific wavelength range components of the signal light L2 transmitted in optical fiber 27 to pass through and be output to optical fiber 28. In this example, ASE filter 19 only allows light with wavelengths above 1545 nm to pass through. This suppresses oscillations in the wavelength region around 1530 nm.

[0059] As described above, the fiber laser device 1 includes a ring-shaped resonator, i.e., a fully polarization-maintaining fiber resonator (oscillator), composed of polarization-maintaining fibers. In the fiber laser device 1, mode-locking is generated in the mode-locking section 15, and an ultrashort pulse laser with a pulse width of, for example, 50 femtoseconds to 10 picoseconds is output.

[0060] [Mold Locking Department]

[0061] like Figure 1 and Figure 3 The mode-locking section 15 shown includes: a first optical fiber 30, a second optical fiber 40 connected to one end of the first optical fiber 30, and a third optical fiber 50 connected to the other end of the first optical fiber 30. As described later... Figure 1 The mold locking part 15 shown and Figure 3 The locking part 15 shown has a slightly different structure.

[0062] The first optical fiber 30 has a first portion 31 and two second portions 32. Each of the first portion 31 and the two second portions 32 is composed of polarization-maintaining optical fiber. The first portion 31 and the two second portions 32 are arranged alternately. The two second portions 32 are respectively connected to both ends of the first portion 31.

[0063] Each second part 32 is connected to the first part 31 such that the fast axis 32X1 of the second part 32 aligns with the slow axis 31X2 of the first part 31 at the connection point C1. In other words, the angle between the fast axis 32X1 of the second part 32 and the fast axis 31X1 of the first part 31 at the connection point C1 between each second part 32 and the first part 31 is 90 degrees. Each second part 32 is directly connected to the first part 31, for example, by welding. Furthermore, Figure 3 The image depicts optical fibers with gaps between them, but in reality, the fibers are connected without gaps. The phrase "fast axis aligned with slow axis (or fast axis)" means that, when viewed from the direction of light propagation (the direction of fiber extension), the fast axis is aligned with the slow axis (along the slow axis).

[0064] The total length L31 of part 1 31 is equal to the total length L32 of part 2 32. The total lengths L31 and L32 are the lengths of the first optical fiber 30 along its extension direction (the direction of light propagation). The total length L32 of part 2 32 is the length obtained by adding the lengths L32a of each part 2 32. The statement "the total length L31 of part 1 31 is equal to the total length L32 of part 2 32" includes the possibility of a slight, permissible error between the total lengths L31 and L32. The magnitude of this permissible error depends, for example, on whether mode locking occurs. The permissible error is, for example, less than a beat length (around 2 mm). Beat length is an indicator of the magnitude of birefringence and is the distance between the light propagating in the fast axis and the light propagating in the slow axis with a phase difference of 2π. Alternatively, the permissible error may be less than 5 mm. In this regard, the lengths L41 of the first portion 41 of the second optical fiber 40, L51 of the first portion 51 of the third optical fiber 50, L60 of the first bridging optical fiber 60, and L70 of the second bridging optical fiber 70 are also the same.

[0065] The second optical fiber 40 has a first portion 41 and a second portion 42. The first portion 41 is connected to the second portion 32 of the first optical fiber 30 such that the fast axis 41X1 of the first portion 41 coincides with the slow axis 32X2 of the second portion 32 at the connection point C2. In other words, the angle between the fast axis 41X1 of the first portion 41 and the fast axis 32X1 of the second portion 32 at the connection point C2 between the first portion 41 and the second portion 32 is 90 degrees. The first portion 41 is directly connected to the second portion 32, for example, by fusion splicing.

[0066] Part 2 42 is connected to one end of Part 1 41 such that the angle between the fast axis 42X1 of Part 2 42 and the fast axis 41X1 of Part 1 41 at the connection point C3 is an angle θ1 other than 0 degrees or 90 degrees. That is, angle θ1 is neither 0 degrees nor 90 degrees. Angle θ1 is an angle other than 45 degrees. Angle θ1 can be experimentally set, for example, in a way that generates mode-locking, as described later. Angle θ1 is the angle when viewed from the direction of light transmission (the extension direction of the first fiber 30 and the second fiber 40). The same applies to angle θ2, which will be described later. Part 2 42 is directly connected to Part 1 41, for example, by fusion splicing. The other end of Part 2 42 is connected to the aforementioned isolator 14.

[0067] exist Figure 3 In the mode-locking section 15 shown, the third optical fiber 50 has a first portion 51 and a second portion 52. The first portion 51 is connected to the second portion 32 of the first optical fiber 30 such that the fast axis 51X1 of the first portion 51 coincides with the fast axis 32X1 of the second portion 32 at the connection point C4. In other words, the angle between the fast axis 51X1 of the first portion 51 and the fast axis 32X1 of the second portion 32 at the connection point C4 is 0 degrees. The angle between the fast axis 51X1 of the first portion 51 and the fast axis 32X1 of the second portion 32 at the connection point C4 differs by 90 degrees from the angle between the fast axis 41X1 of the first portion 41 and the fast axis 32X1 of the second portion 32 at the connection point C2. The first portion 51 is directly connected to the second portion 32, for example, by fusion splicing.

[0068] Part 2 52 is connected to one end of Part 1 51 such that the angle between the fast axis 52X1 of Part 2 52 and the fast axis 51X1 of Part 1 51 at the connection point C5 is an angle θ2 other than 0 degrees or 90 degrees. That is, angle θ2 is neither 0 degrees nor 90 degrees. Angle θ2 is an angle other than 45 degrees, for example, an angle obtained by adding 90 degrees to angle θ1. One end of Part 2 52 is directly connected to Part 1 51, for example, by welding. The other end of Part 2 52 is connected to the aforementioned polarizer 17.

[0069] The length L41 of the first part 41 of the second optical fiber 40 is equal to the length L51 of the first part 51 of the third optical fiber 50. The lengths L41 and L51 are the lengths along the extension direction (light transmission direction) of the second optical fiber 40 and the third optical fiber 50.

[0070] exist Figure 1 In the mode-locking section 15 shown, the third optical fiber 50 only has a first portion 51. One end of the first portion 51 is connected to the second portion 32, and the other end of the first portion 51 is connected to the aforementioned polarization controller 16. Figure 3In the mold-locking section 15 shown, the polarization state of light is adjusted by the angle θ2 between the fast axis 51X1 of the first part 51 and the fast axis 52X1 of the second part 52 at the connecting part C5. In contrast, in Figure 1 In the mode-locking unit 15 shown, a polarization controller 16 equipped with a rotatable wavelength plate is used to... Figure 3 The mode-locking section 15 shown similarly adjusts the polarization state of the light. In this way, the second part 52 of the third optical fiber 50 can be replaced with an adjustment mechanism that uses a wavelength plate.

[0071] The mode field diameter (MFD) of the first fiber 30 is smaller than that of the second fiber 40 and the third fiber 50. The mode field diameter is an indicator of the degree to which light propagating within an optical fiber leaks from the core towards the cladding. For example, the mode field diameter can be measured by incident light onto one end of the fiber and obtaining an image of the emitted light from the other end. The MFD of the first fiber 30 is, for example, 2 μm to 4 μm. The MFD of the second fiber 40 and the third fiber 50 is, for example, 4 μm to 10 μm. The MFD is consistent throughout the first fiber 30. The same applies to the second fiber 40 and the third fiber 50. In polarization-maintaining fibers, the smaller the MFD, the greater the nonlinear effect. That is, the first fiber 30 contains a highly nonlinear fiber with a higher nonlinear effect than the polarization-maintaining fibers constituting the second fiber 40 and the third fiber 50. Furthermore, the core diameter of the first optical fiber 30 can be smaller than, or greater than, the core diameters of the second optical fiber 40 and the third optical fiber 50. The core diameter of the first optical fiber 30, for example... Figure 2 The diagram shows the diameter D of the core 30a of the polarization-maintaining fiber constituting the first optical fiber 30. The core diameters of the second optical fiber 40 and the third optical fiber 50 are the diameters D of the core 30a of the polarization-maintaining fibers constituting them.

[0072] [Functions and Effects]

[0073] As described above, in the fiber laser device 1, the first fiber 30 has a first portion 31 and two second portions 32 arranged alternately with the first portion 31. Adjacent first portions 31 and second portions 32 are interconnected such that the fast axis 31X1 of the first portion 31 and the slow axis 32X2 of the second portion 32 coincide at the connection point C1. Mode locking can be achieved by passing light through such a first fiber 30 and by providing necessary optical elements (e.g., polarizer 17) in the rear section.

[0074] That is, the angle between the fast axis 42X1 of the second portion 42 of the second optical fiber 40 at connection point C3 and the fast axis 41X1 of the first portion 41 of the second optical fiber 40 is an angle θ1 other than 0 degrees or 90 degrees. Therefore, when the light transmitted in the second portion 42 is incident on the first portion 41, it is split into a component transmitted along the fast axis 41X1 of the first portion 41 and a component transmitted along the slow axis 41X2 of the first portion 41. Since angle θ1 is set to an angle other than 45 degrees, the intensity of the component transmitted along the fast axis 41X1 is different from the intensity of the component transmitted along the slow axis 41X2. The higher the intensity of the light transmitted within the polarization-maintaining fiber, the greater the nonlinear effect received. Therefore, nonlinear effects of different magnitudes occur between the component transmitted along the fast axis 41X1 and the component transmitted along the slow axis 41X2. The light output from section 41 is guided through the first optical fiber 30 and the first section 51 of the third optical fiber 50, and reaches the second section 52 of the third optical fiber 50. During this guidance, similar to the propagation within section 41, different magnitudes of nonlinear effects are received in the components propagating along the fast axis and along the slow axis. The angle between the fast axis 51X1 of section 51 and the fast axis 52X1 of section 52 at connection point C5 is an angle θ2 other than 0 degrees or 90 degrees. Alternatively, in Figure 1 In the mode-locking unit 15 shown, the polarization state of the light is similarly adjusted by the polarization controller 16. As a result, when light enters from the first part 51 to the second part 52, the components transmitted along each axis combine, generating a phase difference through a nonlinear effect. Since the phase difference varies depending on the intensity, mode-locking can be achieved by increasing the transmittance of high-intensity light while simultaneously decreasing the transmittance of low-intensity light.

[0075] Here, when light is propagated within a polarization-maintaining fiber, a difference in propagation speed arises between the component propagating along the fast axis and the component propagating along the slow axis due to the difference in refractive index. In this respect, in the fiber laser device 1, the total length L31 of the first part 31 is equal to the total length L32 of the second part 32. That is, the lengths of the first part 31 and the second part 32 are set such that the distance propagated along the fast axis is equal to the length propagated along the slow axis. This compensates for the difference in propagation speed between the component propagating along the fast axis and the component propagating along the slow axis.

[0076] Furthermore, in the fiber laser device 1, the first fiber 30 has a first portion 31 and two second portions 32. Therefore, compared to the case where the first fiber 30 contains only two fiber elements, waveform disturbances in the output light caused by the interaction between the component propagating along the fast axis and the component propagating along the slow axis can be suppressed. As a result, light with a good waveform can be output.

[0077] That is, even if the first fiber 30 contains only two fiber elements connected at a 90-degree angle between their fast axes, mode locking can still be achieved. However, there is a situation where the waveform of the output light is disordered because the rear portion of the component that propagates first in the fiber element interacts with the front portion of the component that propagates later in the fiber element (mutual phase modulation). In contrast, in the fiber laser device 1, the first fiber 30 has a first section 31 and two second sections 32. As a result, the distance that causes the difference in propagation speed can be shortened, and waveform disorder caused by the difference in propagation speed can be suppressed. That is, the time difference generated by the second section 32 on the upstream side is compensated by the front portion of the first section 31, and the time difference generated by the rear portion of the first section 31 is compensated by the second section 32 on the downstream side. As a result, waveform disorder of the output light can be suppressed, and light with a good waveform can be output (cross splicing method).

[0078] Furthermore, in fiber laser device 1, the MFD of the first fiber 30 is smaller than that of the second fiber 40 and the third fiber 50, while the nonlinear effect of the first fiber 30 is improved. Therefore, the length of the first fiber 30 can be shortened, achieving high reproducibility. Additionally, the excitation power required to initiate spontaneous mode locking can be reduced. Thus, according to fiber laser device 1, light with a good waveform can be output, achieving both high reproducibility and low excitation power. Furthermore, in fiber laser devices using semiconductor saturable absorber mirrors (SESAMs), the SESAMs are susceptible to optical damage, resulting in significant lifetime deviations and potential problems. However, in fiber laser device 1, since semiconductor saturable absorber mirrors are not used, this situation is avoided.

[0079] The second fiber 40 and the third fiber 50 are connected to the first fiber 30 by fusion splicing. This reduces the number of components and simplifies manufacturing compared to using optical elements (lenses, etc.) in space. Typically, when polarization-maintaining fibers with different MFDs are connected by fusion splicing, losses easily occur at the connection point due to differences in MFD and their susceptibility to deformation during heating. Such losses lead to instabilities such as noise and distortion in the light surrounding the resonator. In the fiber laser device 1, considering these points, the second fiber 40 and the third fiber 50 are connected to the first fiber 30 by fusion splicing, thereby reducing the number of components and simplifying manufacturing.

[0080] Furthermore, in the fiber laser device 1, the first portion 41 of the second fiber 40 is connected to the second portion 32 of the first fiber 30 such that the fast axis 41X1 of the first portion 41 coincides with the slow axis 32X2 of the second portion 32 at the connection point C2. The second portion 42 of the second fiber 40 is connected to the first portion 41 such that the angle θ1 between the fast axis 42X1 of the second portion 42 and the fast axis 41X1 of the first portion 41 at the connection point C3 is an angle other than 0 degrees or 90 degrees. Connecting polarization-maintaining fibers with different MFDs to each other with an angle between their fast axes other than 0 degrees or 90 degrees is difficult and carries the risk of reduced yield. This is particularly evident when polarization-maintaining fibers with different MFDs are connected by fusion splicing. This is because when the angle between the fast axes is other than 0 degrees or 90 degrees, fusion splicing must be performed without achieving stress symmetry, and the deformation during heating differs due to differences in dopants and structures. For example, when the angle between the fast axes is any angle other than 0 degrees or 90 degrees, the success rate is low compared to when the angle between the fast axes is 0 degrees or 90 degrees, and the yield is less than 50%. In contrast, in the fiber laser device 1, the first fiber 30 and the second fiber 40, which have different MFDs, are connected with their fast axes aligned with their slow axes (the angle between their fast axes is 90 degrees). This simplifies the connection between the first fiber 30 and the second fiber 40 and improves the yield. This improved yield is particularly noticeable when the first fiber 30 and the second fiber 40 are connected by fusion splicing, as in this embodiment. Furthermore, when polarization-maintaining fibers with the same MFD are connected to each other, such as at connection points C3 and C5, the success rate is high and the yield is approximately 100%, even when the angle between the fast axes is any angle other than 0 degrees or 90 degrees.

[0081] The first portion 51 of the third optical fiber 50 is connected to the second portion 32 of the first optical fiber 30 such that the fast axis 51X1 of the first portion 51 coincides with the fast axis 32X1 of the second portion 32 at the connection point C4. The second portion 52 of the third optical fiber 50 is connected to the first portion 51 such that the angle θ2 between the fast axis 52X1 of the second portion 52 and the fast axis 51X1 of the first portion 51 at the connection point C5 is an angle other than 0 degrees or 90 degrees. This simplifies the connection between the first optical fiber 30 and the third optical fiber 50 and further improves the yield rate. This improved yield rate is particularly noticeable when the first optical fiber 30 and the third optical fiber 50 are connected by fusion splicing as in this embodiment.

[0082] The length L41 of the first portion 41 of the second optical fiber 40 is equal to the length L51 of the first portion 51 of the third optical fiber 50. Thus, the difference in transmission speed between the component transmitted along the fast axis and the component transmitted along the slow axis can be compensated for for the first portion 41 and the first portion 51.

[0083] [Example]

[0084] pass Figure 1 The fiber laser device 1 shown is mode-locked. It is configured to output laser light with a wavelength in the 1.5 μm band. The MFD of the first fiber 30 is set to approximately 4.9 μm. The length of the first fiber 30 is approximately 2 m. The MFDs of the second fiber 40 and the third fiber 50 are set to approximately 10.1 μm. The overall dispersion value of the resonator is -0.014 ps. 2 The excitation power (output power of light source 11) at the spontaneous start of mode-locking is about 105mW.

[0085] The pulse width of the output light was measured using the Frequency-Resolved Optical Gating (FROG) method. Figure 4 It is a diagram showing the pulse waveform and instantaneous wavelength.

[0086] Figure 5 This is a graph showing the spectral waveform and phase. For example... Figure 4 The output shown is an ultrashort pulse wave with a good waveform. The pulse width is 127 fs. Figure 5 The spectral bandwidth shown is 32 nm, thus achieving sufficient spectral bandwidth. Since the phase decreases in the wavelength region where spectral peaks are formed, the pulse width is well compressed.

[0087] The spectrum of the output light was measured using a high-frequency spectrometer. Figure 6 (a) and Figure 6 (b) is a graph showing the spectrum. Figure 6 In (a), the horizontal axis is spaced 100kHz apart. Figure 6 In (b), the horizontal axis is spaced 100 MHz apart. In both graphs, the vertical axis is spaced 10 dB apart. Figure 6 (a) and Figure 6 (b) is a graph showing the spectrum. The repetition frequency of the output light is 40.6 MHz. Figure 6 The S / N ratio of the output light spectrum shown in (a) is above 70 dB. For example... Figure 6 As shown in (b), even when the bandwidth is widened to 1 GHz, the peak height remains consistent, thus achieving sufficient frequency stability.

[0088] [First Variation]

[0089] exist Figure 7 and Figure 8 In the fiber laser device 1A of the first modified example shown, the mode-locking unit 15 further includes a first bridging fiber 60 and a second bridging fiber 70.

[0090] The first bridging fiber 60 is composed of polarization-maintaining fiber and is connected between the second portion 32 of the first fiber 30 and the first portion 41 of the second fiber 40. One end of the first bridging fiber 60 is connected to the second portion 32 such that the fast axis 60X1 of the first bridging fiber 60 aligns with the slow axis 32X2 of the second portion 32 at connection point C6. The other end of the first bridging fiber 60 is connected to the first portion 41 such that the fast axis 60X1 of the first bridging fiber 60 aligns with the fast axis 41X1 of the first portion 41 at connection point C7. The first bridging fiber 60 is directly connected to the second portion 32 and the first portion 41, for example, by fusion splicing.

[0091] The second bridging fiber 70 is composed of polarization-maintaining fiber and is connected between the second portion 32 of the first fiber 30 and the first portion 51 of the third fiber 50. One end of the second bridging fiber 70 is connected to the second portion 32 such that the fast axis 70X1 of the second bridging fiber 70 aligns with the fast axis 32X1 of the second portion 32 at connection point C8. The other end of the second bridging fiber 70 is connected to the first portion 51 such that the fast axis 70X1 of the second bridging fiber 70 aligns with the fast axis 51X1 of the first portion 51 at connection point C9. The second bridging fiber 70 is directly connected to the second portion 32 and the first portion 51, for example, by fusion splicing.

[0092] The length L60 of the first bridging fiber 60 is equal to the length L70 of the second bridging fiber 70. Lengths L60 and L70 are the lengths of the first bridging fiber 60 and the second bridging fiber 70 along the extension direction (the direction of light transmission).

[0093] The MFD of the first bridging fiber 60 is greater than that of the first fiber 30 and less than that of the second fiber 40. The MFD of the second bridging fiber 70 is greater than that of the first fiber 30 and less than that of the third fiber 50. The MFD of the first bridging fiber 60 is, for example, equal to that of the second bridging fiber 70. The MFDs of the first bridging fiber 60 and the second bridging fiber 70 are, for example, 4μm to 5μm.

[0094] In the first variation, a polarization controller 90 is provided between the isolator 14 and the mode-locking unit 15. The polarization controller 90 has a mechanism for adjusting the polarization state of the signal light L2 input to the mode-locking unit 15. The polarization controller 90 is configured, for example, to include a rotatably held λ / 4 wavelength plate 90a and a λ / 2 wavelength plate 90b. Figure 7In the example shown, the polarization controller 16 and the polarizer 17 are integrally formed as a single element. The polarization controller 16 consists only of the λ / 2 wavelength plate 16b.

[0095] According to the first modification, similarly to the embodiment described above, light with a good waveform can be output, and high reproducibility and low excitation power can be achieved. Furthermore, in the first modification, a first bridging fiber 60 having an MFD larger than that of the first fiber 30 and smaller than that of the second fiber 40 is connected between the first fiber 30 and the second fiber 40. This reduces the loss at the connection point between the first fiber 30 and the second fiber 40.

[0096] A second bridging fiber 70, having an MFD greater than that of the first fiber 30 and less than that of the third fiber 50, is connected between the first fiber 30 and the third fiber 50. This further reduces losses at the connection point.

[0097] The first bridging fiber 60 is connected to the second part 32 of the first fiber 30 such that the fast axis 60X1 of the first bridging fiber 60 coincides with the slow axis 32X2 of the second part 32 at connection point C6. The second bridging fiber 70 is connected to the second part 32 such that the fast axis 70X1 of the second bridging fiber 70 coincides with the fast axis 32X1 of the second part 32 at connection point C8. The length L60 of the first bridging fiber 60 is equal to the length L70 of the second bridging fiber 70. Therefore, the difference in propagation speed between the component propagating along the fast axis and the component propagating along the slow axis can be compensated for with respect to the first bridging fiber 60 and the second bridging fiber 70.

[0098] The first variation can also be as follows: Figure 9 As shown, it is configured as follows. In this example, at connection point C6, the fast axis 60X1 of the first bridging fiber 60 coincides with the fast axis 32X1 of the second part 32; at connection point C7, the fast axis 60X1 of the first bridging fiber 60 coincides with the slow axis 41X2 of the first part 41. At connection point C8, the fast axis 70X1 of the second bridging fiber 70 coincides with the slow axis 32X2 of the second part 32; at connection point C9, the fast axis 70X1 of the second bridging fiber 70 coincides with the slow axis 51X2 of the first part 51. In this case, the difference in transmission speed between the component transmitted along the fast axis and the component transmitted along the slow axis can also be compensated. That is, in such a case... Figure 9In the case where the first fiber 30 has a total of an odd number of first portions 31 and second portions 32 (for example, having one first portion 31 and two second portions 32), if the difference between the angle between the fast axis 60X1 of the first bridging fiber 60 at connection C6 and the fast axis 32X1 of the second portion 32, and the angle between the fast axis 70X1 of the second bridging fiber 70 at connection C8 and the fast axis 32X1 of the second portion 32, is 90 degrees, then the difference in transmission speed between the component transmitted along the fast axis and the component transmitted along the slow axis can be compensated for for the first bridging fiber 60 and the second bridging fiber 70. Furthermore, in this case, if the difference between the angle between the fast axis 60X1 of the first bridging fiber 60 at connection C7 and the fast axis 41X1 of the first part 41, and the angle between the fast axis 70X1 of the second bridging fiber 70 at connection C9 and the fast axis 51X1 of the first part 51, is 0 degrees, then the difference in transmission speed between the component transmitted along the fast axis and the component transmitted along the slow axis can be compensated for for the first part 41 and the first part 51. On the other hand, in relation to... Figure 9 In cases where the first fiber 30 has a total of an even number of first portions 31 and second portions 32 (for example, having two first portions 31 and two second portions 32), if the difference between the angle between the fast axis 60X1 of the first bridging fiber 60 at connection C6 and the fast axis 32X1 of the second portion 32, and the angle between the fast axis 70X1 of the second bridging fiber 70 at connection C8 and the fast axis 32X1 of the second portion 32, is 0 degrees, then the difference in transmission speed between the component transmitted along the fast axis and the component transmitted along the slow axis can be compensated for for the first bridging fiber 60 and the second bridging fiber 70. In the same case, if the difference between the angle between the fast axis 60X1 of the first bridging fiber 60 at connection C7 and the fast axis 41X1 of the first part 41, and the angle between the fast axis 70X1 of the second bridging fiber 70 at connection C9 and the fast axis 51X1 of the first part 51, is 0 degrees, then the difference in transmission speed between the component transmitted along the fast axis and the component transmitted along the slow axis can be compensated for for the first part 41 and the first part 51.

[0099] An embodiment of the first modified example will be described. (By...) Figure 7 The fiber laser device 1A shown exhibits mode-locked oscillation. It is configured to output laser light with a wavelength in the 1.5 μm band. The MFD (Mean Dispersion) of the first fiber 30 is set to approximately 4 μm. The length of the first fiber 30 is set to approximately 1.5 m. The MFDs of the second fiber 40 and the third fiber 50 are set to approximately 10.1 μm. The overall dispersion value of the resonator is -0.081 ps. 2When the first fiber 30 and the second fiber 40 are directly connected by fusion splicing, the loss at the connection point is 0.8 dB. When a first bridging fiber 60 is connected between the first fiber 30 and the second fiber 40, the loss is 0.4 dB. Therefore, it can be seen that by using the first bridging fiber 60, the loss at the connection point can be reduced.

[0100] Figure 10 It is a diagram showing the shape of the spectrum. Figure 11 (a) and Figure 11 (b) is a graph showing the spectrum. Figure 11 In (a), the horizontal axis is spaced 200 kHz apart. Figure 11 In (b), the horizontal axis is spaced 100 MHz apart. In both graphs, the vertical axis is spaced 10 dB apart. Figure 10 The spectral bandwidth shown is 6.9 nm, thus achieving sufficient spectral bandwidth. Figure 11 In (a), the repetition frequency of the output light is 36.1 MHz. The S / N ratio of the output light spectrum is above 70 dB. Figure 11 As shown in (b), even when the bandwidth is widened to 1 GHz, the peak height remains consistent, thus achieving sufficient frequency stability.

[0101] [Comparative Example]

[0102] exist Figure 12 In the comparative example fiber laser device 100 shown, the mode-locking section 115 includes three fiber elements 115a, each composed of a polarization-maintaining fiber having the same MFD. A polarization controller 116 is connected to one end of the mode-locking section 115, and a polarization controller 117 is connected to the other end. This comparative example fiber laser device 100, like the fiber laser device 1 of the embodiment, is configured to output laser light with a wavelength in the 1.5 μm band at a repetition frequency of approximately 40 MHz. The MFD of the fiber element 115a is set to approximately 10.1 μm. The total length of the mode-locking section 115 is set to approximately 2 m. The overall dispersion value of the resonator is -0.110 ps. 2 .

[0103] Figure 13 It is a diagram showing the shape of the spectrum. Figure 14 (a) and Figure 14 (b) is a graph showing the spectrum. Figure 14 In (a), the horizontal axis is spaced 200 kHz apart. Figure 14 In (b), the horizontal axis is spaced 100 MHz apart. In both graphs, the vertical axis is spaced 10 dB apart. Figure 13 The spectral bandwidth shown is 5.8 nm. Figure 14In (a), the repetition frequency of the output light is 40.9 MHz. The S / N ratio of the output light spectrum is above 70 dB. Figure 14 As shown in (b), even when the bandwidth is widened to 1 GHz, the peak height remains consistent, thus achieving sufficient frequency stability.

[0104] As mentioned above Figure 1 In the fiber laser device 1 shown, the excitation power at the spontaneous initiation of mode-locking is approximately 105 mW. On the other hand, in Figure 12 In the comparative example fiber laser device 100 shown, the excitation power at the spontaneous initiation of mode-locking is approximately 225 mW. This satisfies both conditions, and when the fiber length is set to 2 m and the repetition frequency to approximately 40 MHz, in... Figure 1 In the fiber laser device 1 shown, it is possible to Figure 12 The fiber laser device 100 of the comparative example shown uses approximately half the excitation power to cause ultrashort pulse laser oscillation.

[0105] [Second Variation]

[0106] exist Figure 15 In the fiber laser device 1B of the second modified example shown, similar to the first modified example, a polarization controller 90 is provided between the isolator 14 and the mode-locking section 15. The polarization controller 16 and the polarizer 17 are integrally formed as a single element. The second fiber 40 has only a first portion 41. One end of the first portion 41 is connected to the second portion 32 of the first fiber 30, and the other end of the first portion 41 is connected to the polarization controller 90. According to this second modified example, similar to the above embodiment, it is possible to output light with a good waveform, and high reproducibility and low excitation power can be achieved.

[0107] [Mold-locking conditions]

[0108] use Figure 15 The structure shown confirms the conditions for mode-locking. The polarization state of the light input to the upstream second fiber 40 (hereinafter also referred to as the "inlet-side fiber") is adjusted using polarization controller 90, and the polarization state of the light at the output end of the downstream third fiber 50 (hereinafter also referred to as the "outlet-side fiber") is adjusted using polarization controller 16. The angular region for mode-locking is confirmed. Furthermore, the excitation power was increased for confirmation to facilitate understanding of the angular region, but the angle may deviate during actual operation.

[0109] Figure 16 It is a diagram showing the shape of the spectrum. Figure 17 This is a diagram showing the pulse waveform and instantaneous wavelength. In Figure 16 and Figure 17The image shows the results when the rotation angle of the inlet-side fiber (the angle between the fast and slow axes) is set to 12 degrees (192 degrees) and the rotation angle of the outlet-side fiber is set to 100 degrees. Figure 16 The spectral bandwidth shown is 50 nm. For example... Figure 17 The pulse width shown is 635 fs. Figure 17 In the middle, the scale interval of the horizontal axis is 200 fs.

[0110] Figure 18 This shows the angular region where the mold-locking mechanism was generated. Figure 18 In the diagram, the vertical axis is spaced 10 degrees apart, and the horizontal axis is spaced 2 degrees apart. The shaded areas represent combinations of angles that resulted in mold locking. Figure 18 It is known that a mode-locking region exists near the area where the angle obtained by adding 90 degrees to the rotation angle of the inlet-side fiber is equal to the rotation angle of the outlet-side fiber. Based on this experimental result, the rotation angles of the inlet-side and outlet-side fibers can be set. Furthermore, using this experimental result, the angles θ1 and θ2 at the aforementioned connection points C3 and C4 can be set in a mode-locking manner.

[0111] [3rd Variation]

[0112] Figure 19 The fiber laser device 1C shown in the third modification example is configured to output laser light with a wavelength in the 1.0 μm band. The MFD of the first fiber 30 is set to approximately 3.5 μm. The length of the first fiber 30 is set to approximately 6 m. The MFDs of the second fiber 40 and the third fiber 50 are set to approximately 6.9 μm. The overall dispersion value of the resonator is 0.304 ps. 2 In the fiber laser device 1C, a bandpass filter BF is used instead of the ASE filter 19. When outputting laser light in the 1.0 μm band, since the dispersion value is normal and the pulse continues to propagate, it is necessary to limit the pulse propagation using the bandpass filter BF. According to this third modification, similar to the embodiment described above, it is possible to output light with a good waveform, and to achieve high reproducibility and low excitation power.

[0113] Figure 20 It is a diagram showing the shape of the spectrum. Figure 21 (a) and Figure 21 (b) is a graph showing the spectrum. For example... Figure 20 The spectral bandwidth shown is 10.9 nm, thus achieving sufficient spectral bandwidth. Figure 21 In (a), the repetition frequency of the output light is 19.9 MHz. The S / N ratio of the output light spectrum is above 70 dB. Figure 21 As shown in (b), even when the bandwidth is widened to 1 GHz, the peak height remains consistent, thus achieving sufficient frequency stability.

[0114] This disclosure is not limited to the above-described embodiments and variations. In the above embodiments, the second optical fiber 40 may have only a second portion 42, and the second portion 42 may be connected to the second portion 32 of the first optical fiber 30 such that the angle between the fast axis 42X1 of the second portion 42 and the fast axis 32X1 of the second portion 32 is an angle θ1 other than 0 degrees or 90 degrees. Similarly, the third optical fiber 50 may have only a second portion 52, and the second portion 52 may be connected to the second portion 32 such that the angle between the fast axis 52X1 of the second portion 52 and the fast axis 32X1 of the second portion 32 is an angle θ2 other than 0 degrees or 90 degrees.

[0115] In the above embodiment, the angle (first angle) between the fast axis 41X1 of the first portion 41 and the fast axis 32X1 of the second portion 32 at the connection point C2 is 90 degrees, and the angle (second angle) between the fast axis 51X1 of the first portion 51 and the fast axis 32X1 of the second portion 32 at the connection point C4 is 0 degrees. However, it is also possible to reverse this, with the first angle being 0 degrees and the second angle being 90 degrees. That is, as long as the first angle and the second angle differ by 90 degrees, it is acceptable. In this case, similarly to the above embodiment, light with a good waveform can be output, and high reproducibility and low excitation power can be achieved. In other words, in the above embodiment, the second optical fiber 40 can also be regarded as the third optical fiber, and the third optical fiber 50 can be regarded as the second optical fiber. Similarly, in the first variant, the second fiber 40 can be regarded as the third fiber, the third fiber 50 as the second fiber, the first bridging fiber 60 as the second bridging fiber, and the second bridging fiber 70 as the first bridging fiber.

[0116] The first optical fiber 30 may also have two or more first sections 31. In this case, multiple first sections 31 and multiple second sections 32 are arranged alternately. Also in this case, adjacent first sections 31 and second sections 32 are interconnected such that the fast axis 32X1 of the second section 32 coincides with the slow axis 31X2 of the first section 31 at the connection point. When the first optical fiber 30 has two or more first sections 31, the total length L31 of the first sections 31 is the length obtained by adding the lengths of each first section 31. The first optical fiber 30 may also have three or more second sections 32.

[0117] Symbol Explanation

[0118] 1, 1A, 1B, 1C… Fiber laser device; 11… Light source; 30… First fiber; 31… Part 1; 32… Part 2; 40… Second fiber; 41… Part 1; 42… Part 2; 50… Third fiber; 51… Part 1; 52… Part 2; 60… First bridging fiber; 70… Second bridging fiber; L1… Excitation beam; 31X1, 32X1, 41X1, 42X1, 51X1, 52X1, 60X1, 70X1, X1… Fast axis; 32X2, X2… Slow axis; C1, C2, C4, C6, C8… Connection points; θ1, θ2… Angles.

Claims

1. A fiber laser device, wherein, It includes: a first optical fiber, which is composed of polarization-maintaining optical fiber; The second optical fiber, which is composed of polarization-maintaining fiber, is connected to one end of the first optical fiber; and The third optical fiber, which is composed of polarization-maintaining fiber, is connected to the other end of the first optical fiber. The first optical fiber has at least one first portion and at least two second portions configured in interaction with the first portion. The adjacent first and second parts are connected to each other such that the fast axis of the first part and the slow axis of the second part are aligned at the connection point. The total length of the first part is equal to the total length of the second part. The mode field diameter of the first optical fiber is smaller than the mode field diameters of the second optical fiber and the third optical fiber.

2. The fiber laser device as described in claim 1, wherein, At least one of the second optical fiber and the third optical fiber is connected to the first optical fiber by fusion splicing.

3. The fiber laser device as described in claim 1, wherein, The second optical fiber has a first part and a second part. The first portion of the second optical fiber is connected to one end of the first optical fiber such that the fast axis of the first portion of the second optical fiber is aligned with the slow axis of the first optical fiber at the connection point. The second portion of the second optical fiber is connected to the first portion of the second optical fiber such that the angle between the fast axis of the second portion of the second optical fiber and the fast axis of the first portion of the second optical fiber at the connection point is an angle other than 0 degrees or 90 degrees.

4. The fiber laser device as described in claim 2, wherein, The second optical fiber has a first part and a second part. The first portion of the second optical fiber is connected to one end of the first optical fiber such that the fast axis of the first portion of the second optical fiber is aligned with the slow axis of the first optical fiber at the connection point. The second portion of the second optical fiber is connected to the first portion of the second optical fiber such that the angle between the fast axis of the second portion of the second optical fiber and the fast axis of the first portion of the second optical fiber at the connection point is an angle other than 0 degrees or 90 degrees.

5. The fiber laser device as described in claim 3, wherein, The third optical fiber has a first part and a second part. The first portion of the third optical fiber is connected to the other end of the first optical fiber such that the fast axis of the first portion of the third optical fiber is aligned with the fast axis of the first optical fiber at the connection point. The second portion of the third optical fiber is connected to the first portion of the third optical fiber such that the angle between the fast axis of the second portion of the third optical fiber and the fast axis of the first portion of the third optical fiber at the connection point is an angle other than 0 degrees or 90 degrees.

6. The fiber laser device as described in claim 4, wherein, The third optical fiber has a first part and a second part. The first portion of the third optical fiber is connected to the other end of the first optical fiber such that the fast axis of the first portion of the third optical fiber is aligned with the fast axis of the first optical fiber at the connection point. The second portion of the third optical fiber is connected to the first portion of the third optical fiber such that the angle between the fast axis of the second portion of the third optical fiber and the fast axis of the first portion of the third optical fiber at the connection point is an angle other than 0 degrees or 90 degrees.

7. The fiber laser device as described in claim 5, wherein, The length of the first portion of the second optical fiber is equal to the length of the first portion of the third optical fiber.

8. The fiber laser device as described in claim 6, wherein, The length of the first portion of the second optical fiber is equal to the length of the first portion of the third optical fiber.

9. The fiber laser device as described in claim 1, wherein, The second optical fiber has a first portion. The first portion of the second optical fiber is connected to one end of the first optical fiber such that the fast axis of the first portion of the second optical fiber is aligned with the slow axis of the first optical fiber at the connection point. The third optical fiber has a first portion. The first portion of the third optical fiber is connected to the other end of the first optical fiber such that the fast axis of the first portion of the third optical fiber is aligned with the fast axis of the first optical fiber at the connection point. The length of the first portion of the second optical fiber is equal to the length of the first portion of the third optical fiber.

10. The fiber laser device as described in claim 2, wherein, The second optical fiber has a first portion. The first portion of the second optical fiber is connected to one end of the first optical fiber such that the fast axis of the first portion of the second optical fiber is aligned with the slow axis of the first optical fiber at the connection point. The third optical fiber has a first portion. The first portion of the third optical fiber is connected to the other end of the first optical fiber such that the fast axis of the first portion of the third optical fiber is aligned with the fast axis of the first optical fiber at the connection point. The length of the first portion of the second optical fiber is equal to the length of the first portion of the third optical fiber.

11. The fiber laser device as claimed in claim 1, wherein, It also includes: a first bridging fiber, which is composed of polarization-maintaining fiber and is connected between the first fiber and the second fiber. The mode field diameter of the first bridging fiber is greater than that of the first fiber and smaller than that of the second fiber.

12. The fiber laser device as claimed in claim 11, wherein, It also includes: a second bridging fiber, which is composed of polarization-maintaining fiber and is connected between the first fiber and the third fiber. The mode field diameter of the second bridging fiber is larger than that of the first fiber and smaller than that of the third fiber.

13. The fiber laser device as described in claim 12, wherein, The first optical fiber has a total of an even number of the first portion and the second portion. The difference between the angle between the fast axis of the first bridging fiber and the fast axis of the first fiber at the connection point, and the angle between the fast axis of the second bridging fiber and the fast axis of the first fiber at the connection point, is 90 degrees.

14. The fiber laser device as claimed in claim 12, wherein, The first optical fiber has a total of an odd number of the first portion and the second portion. The difference between the angle between the fast axis of the first bridging fiber and the fast axis of the first fiber at the connection point, and the angle between the fast axis of the second bridging fiber and the fast axis of the first fiber at the connection point, is 0 degrees.

15. The fiber laser device as described in claim 13, wherein, The difference between the angle between the fast axis of the first bridging fiber and the fast axis of the second fiber at the connection point, and the angle between the fast axis of the second bridging fiber and the fast axis of the third fiber at the connection point, is 0 degrees.

16. The fiber laser device as claimed in claim 14, wherein, The difference between the angle between the fast axis of the first bridging fiber and the fast axis of the second fiber at the connection point, and the angle between the fast axis of the second bridging fiber and the fast axis of the third fiber at the connection point, is 0 degrees.

17. The fiber laser device according to any one of claims 12 to 16, wherein, The length of the first bridging fiber is equal to the length of the second bridging fiber.

18. The fiber laser device according to any one of claims 1 to 16, wherein, It also features: a light source that outputs excitation light; and An optical fiber that absorbs the excitation light and emits laser light. The laser is guided by the first optical fiber, the second optical fiber, and the third optical fiber.

19. The fiber laser device as claimed in claim 17, wherein, It also features: a light source that outputs excitation light; and An optical fiber that absorbs the excitation light and emits laser light. The laser is guided by the first optical fiber, the second optical fiber, and the third optical fiber.

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