A phase-shift grating writing method of a DFB single-polarization single-frequency fiber laser

Abstract

A phase shift grating writing method of a DFB single polarization single frequency fiber laser, comprising the following steps: step (a): hydrogen treatment of a gain fiber; step (b): the gain fiber is fused on both ends with passive optical fibers to form a writing optical fiber; step (c): the relative positions of an amplitude mask and a phase mask are adjusted to align the middle part, and the writing optical fiber is adjusted to be located near the focal length of a cylindrical lens; step (d): the repetition frequency and the output power of an ultraviolet laser are set; step (e): the writing optical fiber is straightened and clamped on a fiber clamping platform, the writing optical fiber is connected with an ASE light source and a spectrometer, and the grating transmission spectrum is monitored; step (f): the ultraviolet laser is turned on, the spectrometer is observed, and the writing is completed when the appropriate grating parameters are reached. The present application provides a simple, direct, stable, reliable and high-repetition DFB single polarization single frequency fiber laser phase shift grating writing method, which provides a high-quality light source for the fields of nonlinear frequency conversion, gravitational wave detection and coherent polarization beam combination.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of single-frequency fiber lasers, and particularly relates to a phase-shift grating writing method of a DFB single-polarization single-frequency fiber laser. BACKGROUND

[0002] The distributed feedback (DFB) fiber laser is one of effective ways to realize single-frequency fiber laser output by means of a phase-shift fiber grating. The phase-shift fiber grating has a phase jump in the refractive index modulation distribution of the grating region, and is usually written on a gain fiber and used as a resonant cavity of the DFB fiber laser. The existence of the phase shift causes the damping effect on the high-order longitudinal modes in the cavity, thereby ensuring the stability of the laser working in a single-frequency mode. With the development of laser technology, the fields of nonlinear frequency conversion, gravitational wave detection and coherent polarization beam combination have put forward the demand for single-polarization single-frequency fiber lasers. Single-polarization laser output requires good polarization extinction ratio, and the polarization characteristics are stable and not easy to fluctuate.

[0003] However, the current development of high photosensitive polarization-maintaining gain fibers is seriously insufficient, so the phase-shift fiber grating is usually prepared on a non-polarization-maintaining gain fiber. At the same time, the current grating writing process mainly adopts lateral ultraviolet exposure technology. During the writing process of the non-polarization-maintaining fiber, the birefringence phenomenon continuously intensifies with the increase of the refractive index modulation depth. Therefore, the DFB fiber laser naturally starts and works in two orthogonal polarization modes. The two orthogonal polarization modes are in a competitive state in the cavity, which affects the stability of the output laser to some extent.

[0004] With the development of fiber and optical devices, there have been related reports on the use of DFB fiber lasers to realize single-polarization single-frequency laser output. For example, the phase-shift fiber grating of a twisted DFB fiber laser obtains single-polarization single-frequency laser with a power of 92 mW [Optics and Laser Technology, 2022, 145:107519]. B. Yin et al. use the self-injection ring structure of the DFB fiber laser to realize single-polarization single-frequency laser output with a power of 44 mW [IEEE Photonics Journal, 2015, 7(3):1-9]. In the above achievements, the method of applying transverse stress or torsion to the phase-shift fiber grating is difficult to determine the theoretical relationship between the output laser polarization state and the grating parameters; as another effective means of outputting single-polarization single-frequency laser, the phase sensitivity of the injected feedback light is extremely easy to cause the jump of the laser mode. At the same time, the above measures are based on external regulation technology, and do not start from the essential properties of the phase-shift grating, so the core mechanism and key technology have not been effectively broken through. In summary, there is still a lack of a simple, direct and stable and reliable method to realize a DFB single-polarization single-frequency fiber laser. SUMMARY

[0005] The application provides a phase shift grating writing method of a DFB single polarization single frequency fiber laser to solve the defects in the prior art.

[0006] The application is achieved by the following technical solutions:

[0007] A phase shift grating writing method of a DFB single polarization single frequency fiber laser, comprising the following steps:

[0008] Step (a): hydrogenation treatment of the gain fiber;

[0009] Step (b): after the hydrogenation of the gain fiber, the coating layer is removed, and the passive fiber is fused at both ends to form a writing fiber;

[0010] Step (c): the centers of the ultraviolet laser, the circular diaphragm, and the mirror are located on the same horizontal line, the centers of the rectangular diaphragm, the beam expander, the cylindrical lens, the phase mask, and the writing fiber are located on another horizontal line rotated by 90°, so as to ensure that the ultraviolet spot can be vertically incident on the beam expander, the cylindrical lens, the phase mask, and the writing fiber in sequence, the relative positions of the amplitude mask and the phase mask are adjusted to be aligned in the middle, and the writing fiber is located near the focal length of the cylindrical lens;

[0011] Step (d): the repetition frequency and the output power of the ultraviolet laser are set;

[0012] Step (e): the writing fiber is straightened and clamped on the fiber clamping platform, then the gain fiber part of the writing fiber is moved to the interference area of the phase mask, the distance between the writing fiber and the phase mask is controlled within half of the fiber cladding diameter by observing the image of the microscope, finally the writing fiber is connected with the ASE light source and the spectrometer to monitor the grating transmission spectrum;

[0013] Step (f): the ultraviolet laser is turned on, the fiber clamping platform is finely adjusted so that the writing fiber is located at the focal length of the cylindrical lens, the spectrometer is observed, when the appropriate grating parameters are reached, the ultraviolet laser is turned off and the writing fiber is removed to complete the writing, and finally annealing treatment is performed.

[0014] The phase shift grating writing method of the DFB single polarization single frequency fiber laser as described above, the ultraviolet laser is a KrF or ArF excimer ultraviolet laser; the working wavelength range of the mirror, the beam expander, and the phase mask covers the working wavelength range of the ultraviolet laser.

[0015] The phase shift grating writing method of the DFB single polarization single frequency fiber laser as described above, the circular diaphragm, the rectangular diaphragm, and the amplitude mask are made of aluminum alloy and are subjected to black anodic oxidation treatment; the circular diaphragm and the rectangular diaphragm are used to filter the edge diffraction part of the ultraviolet spot and improve the uniformity of the spot energy.

[0016] The phase shift grating writing method of the DFB single polarization single frequency fiber laser as described above, the mirror is mounted on a two-dimensional mirror frame, the reflection angle can be adjusted; the beam expander, the cylindrical lens, the phase mask plate and the fiber clamping platform are installed on a three-dimensional adjusting frame, the relative positions can be adjusted to ensure that they are at the same height and remain horizontal, and the light spot can be vertically incident; the expansion factor of the beam expander is 5-10 times.

[0017] The phase shift grating writing method of the DFB single polarization single frequency fiber laser as described above, the transparent part of the amplitude mask is surrounded by a positive and negative apodization function to form a refractive index distribution modulation; the phase mask plate is a phase shift phase mask plate, the phase shift amount is π, and it is located in the middle of the mask area, and the period and the refractive index of the gain fiber together determine the grating Bragg center wavelength.

[0018] The phase shift grating writing method of the DFB single polarization single frequency fiber laser as described above, the apodization function is used to segment the apodization of the phase shift fiber grating, but it does not compensate for the gain threshold difference between the orthogonal polarization states of the DFB fiber laser resonant cavity, so further regulation of the grating coupling strength is needed to compensate for the gain threshold difference between the orthogonal polarization states of the DFB fiber laser resonant cavity.

[0019] The phase shift grating writing method of the DFB single polarization single frequency fiber laser as described above, the specific operation for compensating for the gain threshold difference between the orthogonal polarization states of the DFB fiber laser resonant cavity is:

[0020] The refractive index modulation distribution expression of the obtained grating region obtained by only segmenting the apodization of the phase shift fiber grating is:

[0021]

[0022] Wherein, is the refractive index modulation direct current part, σ(z) is the normalized apodization function, and Λ is the grating period;

[0023] The energy normalized distribution of the focused linear light spot satisfies the expression of the segmented Gaussian apodization function:

[0024]

[0025] Wherein, FWHM1 and FWHM2 are the full width at half maximum of the two sides of the segmented super-Gaussian apodization function, and n is the apodization coefficient;

[0026] Because the fiber birefringence phenomenon will be caused during the writing process, the additional phase shift relative to the initial phase shift π in the x and y orthogonal polarization directions is:

[0027]

[0028] wherein, and are the refractive index modulation depths of the x, y orthogonal polarization directions of the inscription fiber, respectively, Δn x and Δn y are the refractive index modulation profiles of the x, y orthogonal polarization directions of the inscription fiber, respectively, and L is the grating region length.

[0029] The relationship between the gain threshold and the additional phase shift amount in the DFB fiber laser resonator satisfies the following expression:

[0030]

[0031] wherein, Δβ is the wavelength detuning amount, q is the laser mode order, κ is the alternating coupling coefficient, L shift is the phase shift region length, is the grating phase, g th is the gain threshold.

[0032] The above expression can obtain g th , and the closer the phase shift amount is to π, the smaller g th is, so that g is smaller when κ is smaller. The difference between κ and κ' leads to different g th , at this time, the gain thresholds in the x, y orthogonal polarization directions in the DFB fiber laser resonator are different, the size of κ is controlled, that is, the refractive index modulation depth of the grating region is controlled, and a certain intensity range of pump action can make the polarization state with higher gain threshold in the x, y orthogonal polarization directions in the DFB fiber laser resonator unable to oscillate, while the polarization state with lower gain threshold can oscillate, thereby realizing single-polarization single-frequency laser output of the DFB fiber laser resonator.

[0033] The phase shift grating inscription method of the DFB single-polarization single-frequency fiber laser as described above, the gain fiber in the inscription fiber is a non-polarization maintaining gain fiber doped with rare earth ions, the matrix material thereof is one of quartz, phosphate, silicate, tellurite, fluoride or sulfide, or a mixture of any two or more thereof in any ratio, the core of the fiber is uniformly doped with rare earth ions and has ultraviolet light sensitivity, and the diameter thereof is 3-30 μm, the doped rare earth ions are one of Yb 3+ , Er 3+ , Tm 3+ , Nd 3 + , Pr 3+ , Ho 3+ , Eu 3+ , Dy 3+ , or a mixture of any two or more thereof in any ratio.

[0034] The phase shift grating writing method of the DFB single polarization single frequency fiber laser as described above, the passive fiber in the writing fiber is a polarization maintaining or non-polarization maintaining fiber, the matrix material is one of quartz, phosphate, silicate, tellurite, fluoride or sulfide, or a mixture of any two or more thereof in any ratio, the core diameter is 3-30 μm, and the fiber has good fusion matching characteristics with the gain fiber.

[0035] The phase shift grating writing method of the DFB single polarization single frequency fiber laser as described above, the gain wavelength range of the gain fiber in the writing fiber and the working wavelength range of the ASE light source and the spectrometer cover the working wavelength range of the written phase shift grating.

[0036] The advantage of the present application is that the DFB fiber laser resonant cavity is prepared by writing a apodized phase shift fiber grating on the gain fiber, on the one hand, the weak coupling region of the apodized phase shift fiber grating coincides with the high power region of the laser in the cavity, which is beneficial to the gain amplification of the laser in the cavity; on the other hand, the alternating refractive index modulation intensity of the apodized phase shift fiber grating gradually decreases from the center to both sides, which is beneficial to improve the side mode suppression ratio, and the apodization of the phase shift fiber grating without compensation makes the direct current refractive index modulation intensity conform to the apodization function distribution, which can cause the gain threshold difference between the orthogonal polarization states of the DFB fiber laser resonant cavity, and further control the refractive index modulation depth to ensure the stable single polarization single frequency output of the DFB fiber laser; therefore, the present application provides a simple and direct phase shift grating writing method of the DFB single polarization single frequency fiber laser, which is stable and reliable and has high repeatability, and provides a high-quality light source for the fields of nonlinear frequency conversion, gravitational wave detection and coherent polarization beam combination. BRIEF DESCRIPTION OF DRAWINGS

[0037] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or prior art description. Obviously, the drawings in the following description are some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without creative labor.

[0038] Figure 1 is a top view of the operation structure of the present application;

[0039] Figure 2 is a schematic diagram of the amplitude template in the embodiment of the present application;

[0040] Figure 3 is a schematic diagram of the reflection spectrum and transmission spectrum of the phase shift fiber grating in the embodiment of the present application;

[0041] Figure 4It is a DFB single polarization single frequency fiber laser spectrum and longitudinal mode test schematic diagram in the embodiment of the present application.

[0042] The figure mark: 1, ultraviolet laser; 2, circular diaphragm; 3, mirror; 4, rectangular diaphragm; 5, beam expander; 6, amplitude mask; 7, cylindrical lens; 8, phase mask; 9, writing fiber; 10, fiber clamping platform; 11, microscope; 12, ASE light source; 13, spectrometer. DETAILED DESCRIPTION

[0043] To make the purpose, technical scheme and advantages of the embodiments of the present application more clear, the technical scheme in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the protection scope of the present application.

[0044] As shown in the figure, a phase shift grating writing method of a DFB single polarization single frequency fiber laser comprises the following steps: Figure 1

[0045] Step (a): hydrogenation treatment of gain fiber;

[0046] Step (b): after the hydrogenation of the gain fiber is cut to a proper length, the coating layer is removed, and passive fibers are fused on both ends to form a writing fiber 9;

[0047] Step (c): adjust the centers of the ultraviolet laser 1, circular diaphragm 2 and mirror 3 to be on the same horizontal line, adjust the centers of the rectangular diaphragm 4, beam expander 5, cylindrical lens 7, phase mask 8 and writing fiber 9 to be on another horizontal line rotated by 90°, so as to ensure that the ultraviolet spot can be vertically incident on the beam expander 5, cylindrical lens 7, phase mask 8 and writing fiber 9 in turn, adjust the relative positions of the amplitude mask 6 and phase mask 8 to be aligned in the middle, and adjust the writing fiber 9 to be located near the focal length of the cylindrical lens 7;

[0048] Step (d): set the repetition frequency and output power of the ultraviolet laser 1;

[0049] Step (e): straighten the writing fiber 9 and clamp it on the fiber clamping platform 10, then move the gain fiber part of the writing fiber 9 to the interference area of the phase mask 8, at the same time, observe the image of the microscope 11 to control the distance between the writing fiber 9 and phase mask 8 within half of the fiber cladding diameter, and finally connect the writing fiber 9 with the ASE light source 12 and spectrometer 13 to monitor the grating transmission spectrum;

[0050] ​Step (f): turn on the ultraviolet laser 1, finely adjust the fiber clamping platform 10 to make the inscription fiber 9 located at the focal length of the cylindrical lens 7, observe the spectrometer 13, and turn off the ultraviolet laser 1 when the grating parameters are appropriate, and then take off the inscription fiber 9 to complete the inscription, and finally perform annealing treatment.

[0051] Preferably, the ultraviolet laser 1 of the present application is a KrF or ArF excimer ultraviolet laser; the working wavelength range of the mirror 3, the beam expander 5, and the phase mask 8 covers the working wavelength range of the ultraviolet laser 1.

[0052] Preferably, the circular diaphragm 2, the rectangular diaphragm 4, and the amplitude mask 6 of the present application are made of aluminum alloy and are subjected to black anodic oxidation treatment; the circular diaphragm 2 and the rectangular diaphragm 4 are used to filter out the edge diffraction part of the ultraviolet spot and improve the energy uniformity of the spot.

[0053] Preferably, the mirror 3 of the present application is installed on a two-dimensional mirror frame, and the reflection angle can be adjusted; the beam expander 5, the cylindrical lens 7, the phase mask 8, and the fiber clamping platform 10 are installed on a three-dimensional adjusting frame, and the relative positions can be adjusted to ensure that they are located at the same height and are kept horizontal, and the spot can be made to be vertically incident; the expansion ratio of the beam expander 5 is 5-10 times.

[0054] Preferably, the light-transmitting part of the amplitude mask 6 of the present application is surrounded by positive and negative apodization functions to form refractive index distribution modulation; the phase mask 8 is a phase-shifted phase mask, and the phase shift is π, which is located in the middle of the mask area, and the period and the gain fiber refractive index jointly determine the Bragg center wavelength of the grating.

[0055] Preferably, the apodization function of the present application is used for segmented apodization of the phase-shifted fiber grating, but does not compensate for the gain threshold difference between the orthogonal polarization states of the DFB fiber laser resonant cavity, so that further regulation of the grating coupling strength is needed to compensate for the gain threshold difference between the orthogonal polarization states of the DFB fiber laser resonant cavity.

[0056] Preferably, the specific operation of the present application for compensating for the gain threshold difference between the orthogonal polarization states of the DFB fiber laser resonant cavity is as follows:

[0057] The obtained grating region refractive index modulation distribution expression of the phase-shifted fiber grating only for segmented apodization without compensation is as follows:

[0058]

[0059] wherein, is the refractive index modulation direct current part, σ(z) is the normalized apodization function, and Λ is the grating period.

[0060] The energy normalized distribution of the focused linear spot satisfies the expression of the piecewise Gaussian apodization function as follows:

[0061]

[0062] Where FWHM1 and FWHM2 are the full width at half maximum (FWHM1) of the two sides of the piecewise super Gaussian apodization function, respectively, and n is the apodization coefficient;

[0063] Since the writing process causes birefringence in the optical fiber, the additional phase shift relative to the initial phase shift π in its x and y orthogonal polarization directions is:

[0064]

[0065] in, and Δn represents the refractive index modulation depth in the x and y orthogonal polarization directions of the fiber 9 being inscribed. x and Δn y These are the refractive index modulation distributions in the x and y orthogonal polarization directions of the fiber 9, respectively, where L is the grating length.

[0066] The relationship between the gain threshold and the additional phase shift within the DFB fiber laser resonator cavity satisfies the following expression:

[0067]

[0068] Where Δβ is the wavelength detuning, q is the laser mode order, κ is the AC coupling coefficient, and L shift Phase shift region length, For grating phase, g th This is the gain threshold;

[0069] The expression for the above relationship can be used to obtain g. th Furthermore, the closer the phase shift is to π, the better g th The smaller, therefore and The differences lead to the formation of different g th At this point, the gain thresholds of the x and y orthogonal polarization directions in the DFB fiber laser resonator are different. By controlling the size of κ, i.e. controlling the refractive index modulation depth of the grating region, a pumping effect within a certain intensity range can prevent the polarization state with a higher gain threshold in the x and y orthogonal polarization directions in the DFB fiber laser resonator from oscillating, while the polarization state with a lower gain threshold can oscillate, thereby realizing single-polarization single-frequency laser output from the DFB fiber laser resonator.

[0070] Preferably, the gain fiber in the inscription fiber 9 of the present application is a non-polarization maintaining gain fiber doped with rare earth ions, whose matrix material is one of silica, phosphate, silicate, tellurite, fluoride or sulfide, or a mixture of any two or more thereof in any ratio, whose core is uniformly doped with rare earth ions and has ultraviolet photosensitivity, and whose diameter is 3-30 μm. The rare earth ions are one of Yb 3+ , Er 3+ , Tm 3+ , Nd 3+ , Pr 3+ , Ho 3+ , Eu 3+ , Dy 3+ , or a mixture of any two or more thereof in any ratio.

[0071] Preferably, the passive fiber in the inscription fiber 9 of the present application is a polarization maintaining or non-polarization maintaining fiber, whose matrix material is one of silica, phosphate, silicate, tellurite, fluoride or sulfide, or a mixture of any two or more thereof in any ratio, whose core diameter is 3-30 μm, and which has good fusion matching characteristics with the gain fiber.

[0072] Preferably, the gain wavelength range of the gain fiber in the inscription fiber 9 of the present application and the working wavelength range of the ASE light source 12 and the spectrometer 13 cover the working wavelength range of the phase shift grating to be inscribed.

[0073] Embodiment

[0074] One of the inscription methods for the phase shift grating of a DFB single-polarization single-frequency fiber laser in the embodiment of the present application is used to realize stable single-polarization single-frequency output of the DFB fiber laser, and the specific implementation process is as follows:

[0075] Step (a): hydrogen loading treatment of the gain fiber;

[0076] Step (b): after the hydrogen-loaded gain fiber is cut to a proper length, the coating layer is removed, and passive fibers are fused to the two ends to form the inscription fiber 9;

[0077] Step (c): adjust the centers of the ultraviolet laser 1, the circular diaphragm 2 and the reflecting mirror 3 to be on the same horizontal line, adjust the centers of the rectangular diaphragm 4, the beam expander 5, the cylindrical lens 7, the phase mask 8 and the inscription fiber 9 to be on another horizontal line rotated by 90° from the center of the reflecting mirror 3, adjust the reflecting mirror 3, the beam expander 5, the cylindrical lens 7 and the phase mask 8 to ensure that the ultraviolet spot can be vertically incident on the beam expander 5, the cylindrical lens 7, the phase mask 8 and the inscription fiber 9 in turn, adjust the relative positions of the amplitude mask 6 and the phase mask 8 to align them, and adjust the inscription fiber 9 to be near the focal length of the cylindrical lens 7;

[0078] Step (d): Set the repetition rate and voltage of the UV laser 1.

[0079] Step (e): Straighten the inscription fiber 9 and clamp it on the fiber clamping platform 10, then move the gain fiber part of the inscription fiber 9 to the interference region of the phase mask plate 8, control the distance between the inscription fiber 9 and the phase mask plate 8 within half of the fiber cladding diameter by observing the image of the microscope 11, and finally connect the inscription fiber 9 with the ASE light source 12 and the spectrometer 13 to monitor the grating transmission spectrum.

[0080] Step (f): Turn on the UV laser 1, fine-tune the fiber clamping platform 10 to make the inscription fiber 9 located at the focal length of the cylindrical lens 7, observe the spectrometer 13, and turn off the UV laser 1 when the appropriate grating parameters are reached and remove the inscription fiber 9 to complete inscription, and finally perform annealing treatment.

[0081] In this embodiment, the UV laser 1 is a KrF excimer UV laser with a central working wavelength of 248 nm, an output spot size of 6*3 mm, and a divergence angle of 2*1 mrad.

[0082] The working wavelengths of the mirrors 3, beam expander 5, and phase mask plate 8 in this embodiment all cover 248 nm.

[0083] In this embodiment, the circular aperture 2, rectangular aperture 4, and amplitude template 6 are made of aluminum alloy and treated with black anodic oxidation. The circular aperture 2 and rectangular aperture 4 are used to filter out the edge diffraction part of the UV spot and improve the uniformity of the spot energy, and the transparent part of the amplitude template 6 is formed by positive and negative segmented super-Gaussian apodization functions: overall width 20 mm, height 12 mm, both sides of the super-Gaussian apodization function are 10 mm, and the full width at half maximum is 5 mm and 6 mm respectively, which is used for refractive index distribution modulation.

[0084] In this embodiment, the mirror 3 is installed on a two-dimensional mirror frame, which can adjust the reflection angle. The beam expander 5, cylindrical lens 7, phase mask plate 8, and fiber clamping platform are installed on a three-dimensional adjustment frame, which can adjust the relative position to ensure that they are at the same height and remain horizontal, and can make the spot perpendicular to the incident. The magnification of the beam expander 5 is 10 times.

[0085] In this embodiment, the phase mask plate is a phase shift phase mask plate 8 with an interference region of 45*5 mm and a phase shift of π, located in the middle of the mask region.

[0086] In this example, a uniform Yb3+ doped single-mode non-polarization maintaining quartz optical fiber with a cladding diameter of 125 μm, a core diameter of 3 μm, and a core numerical aperture of 0.2 is selected as the gain fiber, and the core has photosensitivity. HI1060 is selected as the passive optical fiber for fusion splicing.

[0087] The phase-shifted fiber grating written in the example has a total length of 20 mm, a 3dB reflection spectrum width of 0.1-0.2 nm, a center wavelength of 1063.5-1063.7 nm, a transmission spectrum depth of -15 to -12 dB, and the control parameter range can ensure single-polarization single-frequency laser output.

[0088] In the example, the working wavelength ranges of the ASE light source 12 and the spectrometer 13 used for monitoring the grating writing parameters both cover the working wavelength range of the phase-shifted fiber grating.

[0089] The specific principle of the phase-shifted fiber grating writing method for the DFB single-polarization single-frequency fiber laser according to the embodiment of the application is as follows:

[0090] In the writing light path system, the output laser spot of the ultraviolet laser 1 sequentially passes through the circular diaphragm 2, the reflecting mirror 3, the rectangular diaphragm 4, the beam expander 5, the amplitude mask 6, and the cylindrical lens 7 to be expanded, shaped, and focused to form a linear spot with a segmented super-Gaussian distribution of energy. The spot acts on the optical fiber after passing through the phase mask plate 8 to form a diffraction interference pattern, thereby realizing the writing of the phase-shifted fiber grating. Meanwhile, the monitoring system composed of the ASE light source 12, the spectrometer 13, and the writing fiber 9 is connected to feedback the refractive index modulation depth of the grating region. The phase-shifted fiber grating is only segmented apodized without compensation, which causes the gain threshold difference between the orthogonal polarization states of the DFB fiber laser resonant cavity. Further regulating the grating coupling strength can obtain the DFB single-polarization single-frequency fiber laser.

[0091] The refractive index modulation distribution of the grating region obtained by only segmenting apodizing the phase-shifted fiber grating without compensation is as follows:

[0092]

[0093] wherein, is the refractive index modulation direct current part, σ(z) is a normalized apodization function, and Λ is the grating period.

[0094] The energy normalized distribution of the focused linear spot satisfies a segmented Gaussian apodization function as follows:

[0095]

[0096] wherein, FWHM1 and FWHM2 are the full width at half maximum of the two sides of the segmented super-Gaussian apodization function, and n is the apodization coefficient.

[0097] Since the fiber birefringence phenomenon is caused in the writing process, the additional phase shift relative to the initial phase shift π in the x and y orthogonal polarization directions is as follows:

[0098]

[0099] wherein, and respectively are the refractive index modulation depths of the x, y orthogonal polarization directions of the inscription fiber 9 x and Δn y respectively are the refractive index modulation profiles of the x, y orthogonal polarization directions of the inscription fiber 9, and L is the grating length.

[0100] The relationship between the gain threshold and the additional phase shift in the DFB fiber laser cavity satisfies:

[0101]

[0102] where Δβ is the wavelength detuning, q is the laser mode order, κ is the ac coupling coefficient, L shift is the phase shift length, is the grating phase, g th is the gain threshold.

[0103] At this time, solving the above formula can obtain g th , obviously, the closer the phase shift is to π, the smaller g th is. Therefore, and the difference between results in different g th , at this time, the gain thresholds in the x, y orthogonal polarization directions of the DFB fiber laser cavity are different, controlling the size of κ, that is, controlling the refractive index modulation depth of the grating, a certain intensity range of pump action can make the polarization state with a higher gain threshold in the x, y orthogonal polarization directions of the DFB fiber laser cavity unable to oscillate, while the polarization state with a lower gain threshold can oscillate, thereby realizing the single polarization and single frequency laser output of the DFB fiber laser cavity.

[0104] In summary, through the direct apodization effect of the apodized amplitude template in Figure 2 , the single polarization and single frequency laser output of the DFB fiber laser cavity can be realized as Figure 4The single-frequency laser output is shown, simple operation and remarkable effect are achieved. Compared with the prior art, the DFB fiber laser resonator is prepared by inscribing a segmented apodized phase-shift fiber grating on a gain fiber, on the one hand, the weak coupling region of the segmented apodized phase-shift fiber grating is coincided with the high power region of the cavity laser, which is beneficial to the gain amplification of the cavity laser; on the other hand, the alternating refractive index modulation intensity of the segmented apodized phase-shift fiber grating gradually decreases from the center to both sides, which is beneficial to improve the side mode suppression ratio, and the direct current refractive index modulation intensity of the phase-shift fiber grating only apodizes and does not compensate, so that the distribution of the direct current refractive index modulation intensity conforms to the segmented apodization function, which can cause the gain threshold difference between the orthogonal polarization states of the DFB fiber laser resonator, and further control the refractive index modulation depth can ensure the stable single polarization single-frequency output of the DFB fiber laser. Therefore, the present application provides a segmented apodized phase-shift grating inscribing method of the DFB single polarization single-frequency fiber laser, which is simple, direct, stable, reliable and has high repeatability, and provides a high-quality light source for the fields of nonlinear frequency conversion, gravitational wave detection and coherent polarization beam combination.

[0105] Finally, it should be noted that: the above examples are only used to illustrate the technical solutions of the present application, and not to limit them; although the present application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing examples, or make equivalent replacement for part of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A phase-shift grating writing method of a DFB single-polarization single-frequency fiber laser, characterized in that: It comprises the following steps: Step (a): hydrogen loading treatment of gain fiber; Step (b): after the gain fiber is cut to a proper length, the coating is removed, and passive fibers are fused at both ends to form a writing fiber (9); Step (c): the centers of the ultraviolet laser (1), the circular aperture (2), and the mirror (3) are adjusted to be on the same horizontal line, the centers of the mirror (3), the rectangular aperture (4), the beam expander (5), and the cylindrical lens (7) are adjusted to be on the same vertical line, so as to ensure that the ultraviolet spot can be vertically incident on the beam expander (5), the cylindrical lens (7), the phase mask (8), and the writing fiber (9) in turn, and the relative positions of the amplitude mask (6) and the phase mask (8) are adjusted to be aligned in the middle, the light-transmitting part of the amplitude mask (6) is a super-Gaussian cut function, which can control the refractive index modulation depth of the grating region to make one polarization direction unable to oscillate and the orthogonal polarization direction oscillate to realize a single-polarization single-frequency fiber laser, and the writing fiber (9) is adjusted to be located near the focal length of the cylindrical lens (7); Step (d): the repetition frequency and output power of the ultraviolet laser (1) are set; Step (e): the writing fiber (9) is straightened and clamped on the fiber clamping platform (10), then the gain fiber part of the writing fiber (9) is moved to the interference region of the phase mask (8), the distance between the writing fiber (9) and the phase mask (8) is controlled within half of the fiber cladding diameter by observing the image of the microscope (11), and finally the writing fiber (9) is connected with the ASE light source (12) and the spectrometer (13) to monitor the grating transmission spectrum; Step (f): the ultraviolet laser (1) is turned on, the fiber clamping platform (10) is finely adjusted so that the writing fiber (9) is located at the focal length of the cylindrical lens (7), the spectrometer (13) is observed, and when the appropriate grating parameters are reached, the ultraviolet laser (1) is turned off and the writing fiber (9) is removed to complete the writing, and finally annealing treatment is performed.

2. The phase-shifted grating writing method of a DFB single-polarization single-frequency fiber laser according to claim 1, characterized in that: The ultraviolet laser (1) is a KrF or ArF excimer ultraviolet laser; the working wavelength range of the mirror (3), the beam expander (5), and the phase mask (8) covers the working wavelength range of the ultraviolet laser (1).

3. The phase-shifted grating writing method of a DFB single-polarization single-frequency fiber laser according to claim 1, characterized in that: The circular aperture (2), the rectangular aperture (4), and the amplitude mask (6) are made of aluminum alloy and are treated with black anodic oxidation; the circular aperture (2) and the rectangular aperture (4) are used to filter out the edge diffraction part of the ultraviolet spot and improve the energy uniformity of the spot.

4. The phase-shifted grating writing method of a DFB single-polarization single-frequency fiber laser according to claim 1, characterized in that: The mirror (3) is installed on a two-dimensional mirror frame and can adjust the reflection angle; the beam expander (5), the cylindrical lens (7), the phase mask (8), and the fiber clamping platform (10) are installed on a three-dimensional adjustment frame, which can adjust the relative positions to ensure that they are at the same height and remain horizontal, and can make the spot vertically incident; the magnification of the beam expander (5) is 5-10 times.

5. The phase-shifted grating writing method of a DFB single-polarization single-frequency fiber laser according to claim 1, characterized in that: The transparent part of the amplitude mask (6) is surrounded by positive and negative apodization functions, and is used for refractive index distribution modulation; the phase mask (8) is a phase-shifted phase mask, and the phase shift is π, which is located in the middle of the mask area, and the period and gain fiber refractive index jointly determine the Bragg center wavelength of the grating.

6. The phase-shifted grating writing method of a DFB single-polarization single-frequency fiber laser according to claim 5, characterized in that: The apodization function is used for segment apodization of the phase-shifted fiber grating, but does not compensate for the gain threshold difference between the orthogonal polarization states of the DFB fiber laser resonant cavity, so that the grating coupling strength needs to be further regulated to compensate for the gain threshold difference between the orthogonal polarization states of the DFB fiber laser resonant cavity.

7. The phase-shifted grating writing method of a DFB single-polarization single-frequency fiber laser according to claim 6, characterized in that: The specific operation of compensating for the gain threshold difference between the orthogonal polarization states of the DFB fiber laser resonant cavity is: The obtained grating area refractive index modulation distribution expression of the phase-shifted fiber grating only segment apodization without compensation is: wherein is the refractive index modulation DC part, is the normalized apodization function, is the grating period; The energy normalized distribution of the focused linear spot satisfies the expression of the segment Gaussian apodization function: wherein, and FWHM are the full width at half maximum of the two sides of the piecewise super-Gaussian tail function, respectively, is the tailing coefficient; Since the birefringence of the optical fiber is induced during the writing process, the additional phase shift in the direction of the orthogonal polarization with respect to the initial phase shift amount is: The additional phase shift amount is: ​ in, and They are respectively for writing optical fibers (9) Refractive index modulation depth in orthogonal polarization directions and The optical fiber (9) is respectively inscribed with Refractive index modulation distribution in orthogonal polarization directions This is the length of the gate area; The relationship between the gain threshold of the DFB fiber laser resonant cavity and the additional phase shift satisfies the following expression: wherein, is a wavelength detuning amount, is a laser mode order, is an AC coupling coefficient, is a phase shift region length, is a grating phase; The expression of the above relationship can be obtained , and the phase shift is closer to , then is smaller, so is different from , which leads to different , at this time, the gain threshold of the DFB fiber laser resonator in the orthogonal polarization direction is different, and the size, that is, the refractive index modulation depth of the grating region, is controlled, and a certain intensity range of pumping action can make the polarization state with a higher gain threshold in the orthogonal polarization direction in the DFB fiber laser resonator unable to oscillate, while the polarization state with a lower gain threshold can oscillate, thereby realizing single-polarization single-frequency laser output of the DFB fiber laser resonator.

8. The phase-shifted grating writing method of a DFB single-polarization single-frequency fiber laser according to claim 1, characterized in that: The gain fiber in the inscription fiber (9) is a non-polarization maintaining gain fiber doped with rare earth ions, the matrix material of which is one of quartz, phosphate, silicate, tellurite, fluoride or sulfide, or a mixture of any two or more thereof in any ratio, the core of which is uniformly doped with rare earth ions and has ultraviolet photosensitivity, and the diameter of which is 3-30 μm. The rare earth ions doped therein are one of Yb 3+ , Er 3+ , Tm 3+ , Nd 3+ , Pr 3+ , Ho 3+ , Eu 3+ , Dy 3+ , or a mixture of any two or more thereof in any ratio.

9. The phase-shifted grating writing method of a DFB single-polarization single-frequency fiber laser according to claim 1, characterized in that: The passive fiber in the inscription fiber (9) is a polarization maintaining or non-polarization maintaining fiber, the matrix material of which is one of quartz, phosphate, silicate, tellurite, fluoride or sulfide, or a mixture of any two or more thereof in any proportion, the core diameter of which is 3-30 μm, and which has good fusion matching characteristics with the gain fiber.

10. The phase-shifted grating writing method of a DFB single-polarization single-frequency fiber laser according to claim 1, characterized in that: The gain wavelength range of the gain fiber in the inscription fiber (9) and the working wavelength range of the ASE light source (12) and the spectrometer (13) cover the working wavelength range of the inscription phase-shifted grating. The apodization function is used for segment apodization of the phase-shifted fiber grating, but does not compensate for the gain threshold difference between the orthogonal polarization states of the DFB fiber laser resonant cavity, so that the grating coupling strength needs to be further regulated to compensate for the gain threshold difference between the orthogonal polarization states of the DFB fiber laser resonant cavity. The specific operation of compensating for the gain threshold difference between the orthogonal polarization states of the DFB fiber laser resonant cavity is: The obtained grating area refractive index modulation distribution expression of the phase-shifted fiber grating only segment apodization without compensation is: The energy normalized distribution of the focused linear spot satisfies the expression of the segment Gaussian apodization function: The relationship between the gain threshold of the DFB fiber laser resonant cavity and the additional phase shift satisfies the following expression: The passive fiber in the inscription fiber (9) is a polarization maintaining or non-polarization maintaining fiber, the matrix material of which is one of quartz, phosphate, silicate, tellurite, fluoride or sulfide, or a mixture of any two or more thereof in any proportion, the core diameter of which is 3-30 μm, and which has good fusion matching characteristics with the gain fiber. The gain wavelength range of the gain fiber in the inscription fiber (9) and the working wavelength range of the ASE light source (12) and the spectrometer (13) cover the working wavelength range of the inscription phase-shifted grating.

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