Phase shift grating inscribing method of DFB single-polarization single-frequency fiber laser

Abstract

The invention discloses a phase shift grating inscribing method of a DFB single-polarization single-frequency fiber laser. The phase shift grating inscribing method comprises the following steps: step (a), carrying out hydrogen carrying treatment on a gain fiber; (b) welding passive optical fibers at the two ends of the gain optical fiber to form inscribing optical fibers; (c) adjusting the relative positions of the amplitude template and the phase mask plate to align the centers of the amplitude template and the phase mask plate, and adjusting the inscribing optical fiber to be positioned near the focal length of the cylindrical lens; (d) setting the repetition frequency and the output power of the ultraviolet laser; (e) straightening the inscribing optical fiber, clamping the inscribing optical fiber on an optical fiber clamping platform, connecting the inscribing optical fiber with an ASE light source and a spectrograph, and monitoring a grating transmission spectrum; and (f) turning on the ultraviolet laser, observing the spectrograph, and completing inscribing when proper grating parameters are achieved. The phase shift grating inscribing method of the DFB single-polarization single-frequency fiber laser is simple, direct, stable, reliable and high in repeatability, and a high-quality light source is provided for the fields of nonlinear frequency conversion, gravitational wave detection, coherent polarization beam combination and the like.

Description

Technical Field

[0001] The present invention belongs to the field of single-frequency fiber lasers, and specifically relates to a method for writing a phase-shifted grating of a DFB single-polarization single-frequency fiber laser. Background Art

[0002] Distributed feedback (DFB) fiber lasers with the aid of phase-shifted fiber gratings are one of the effective ways to achieve single-frequency fiber laser output. The phase-shifted fiber grating has a phase mutation in the refractive index modulation distribution in the grating region, and is usually written on the gain fiber and used as the resonant cavity of the DFB fiber laser. The existence of the phase shift causes the high-order longitudinal modes in the cavity to be damped, thereby ensuring the stability of the laser operating in the single-frequency mode. With the development of laser technology, the demand for single-polarization single-frequency fiber lasers has been put forward in the fields of nonlinear frequency conversion, gravitational wave detection, and coherent polarization beam combination. Single-polarization laser output requires a good polarization extinction ratio, and at the same time, the polarization characteristics are stable and not easy to fluctuate.

[0003] However, the current research and development of high photosensitivity polarization-maintaining gain fibers is seriously insufficient, so phase-shifted fiber gratings are usually fabricated on non-polarization-maintaining gain fibers. At the same time, the current grating writing process mainly uses the lateral ultraviolet exposure technology, and the birefringence phenomenon intensifies continuously with the increase of the refractive index modulation depth during the writing process of non-polarization-maintaining fibers. Therefore, DFB fiber lasers usually start and operate in two orthogonal polarization modes naturally. The two orthogonal polarization modes are in a competitive state in the cavity, which affects the stability of the output laser to a certain extent.

[0004] With the development of fiber and optical device technologies, there have been relevant reports on realizing single-polarization single-frequency laser output using DFB fiber lasers. For example, a phase-shifted fiber grating of a twisted DFB fiber laser obtained single-polarization single-frequency laser with a power of 92 mW [Optics and Laser Technology, 2022, 145: 107519]. B. Yin et al. realized single-polarization single-frequency laser output with a power of 44 mW by using the self-injection ring structure of a DFB fiber laser [IEEE Photonics Journal, 2015, 7(3): 1-9]. In the above achievements, the method of applying lateral stress or torsion to the phase-shifted fiber grating is difficult to determine the theoretical relationship between the output laser polarization state and the grating parameters; self-injection locking, as another effective means of outputting single-polarization single-frequency laser, the phase sensitivity of the injected feedback light easily leads to laser mode jumps. At the same time, the above measures are all based on external control technologies and do not start from the essential attributes of the phase-shifted grating. Therefore, its core mechanism and key technologies have not been effectively broken through. In summary, there is still a lack of a simple, direct, stable and reliable method for realizing a DFB single-polarization single-frequency fiber laser. Summary of the Invention

[0005] The present invention provides a method for writing a phase-shifted grating of a DFB single-polarization single-frequency fiber laser to solve the defects in the prior art.

[0006] The present invention is realized through the following technical solutions:

[0007] A method for writing a phase-shifted grating of a DFB single-polarization single-frequency fiber laser includes the following steps:

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

[0009] Step (b): After cutting the hydrogen-loaded gain fiber to an appropriate length, stripping its coating layer, and fusing passive fibers at both ends to form a writing fiber;

[0010] Step (c): Adjust the centers of the ultraviolet laser, the circular aperture, and the mirror to be on the same horizontal line, and adjust the centers of the rectangular aperture, the beam expander, the cylindrical lens, the phase mask, and the writing fiber to be on another horizontal line rotated by 90° with the center of the mirror, so as to ensure that the ultraviolet light spot can be vertically incident on the beam expander, the cylindrical lens, the phase mask, and the writing fiber in sequence. Adjust the relative positions of the amplitude template and the phase mask to align their centers, and adjust the writing fiber to be near the focal length of the cylindrical lens;

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

[0012] Step (e): Straighten the writing fiber and clamp it on the fiber clamping platform. Next, move the gain fiber part of the writing fiber to the interference region of the phase mask, and at the same time, control the distance between the writing fiber and the phase mask within half of the fiber cladding diameter by observing the microscope image. Finally, connect the writing fiber to the ASE light source and the spectrometer to monitor the grating transmission spectrum;

[0013] Step (f): Turn on the ultraviolet laser, finely adjust the fiber clamping platform to make the writing fiber at the focal length of the cylindrical lens, observe the spectrometer, turn off the ultraviolet laser when the appropriate grating parameters are reached, remove the writing fiber to complete the writing, and finally perform annealing treatment.

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

[0015] In the method for writing a phase-shifted grating of a DFB single-polarization single-frequency fiber laser as described above, the circular aperture, the rectangular aperture, and the amplitude template are made of aluminum alloy and are subjected to black anodizing treatment; the circular aperture and the rectangular aperture are used to filter out the edge diffraction part of the ultraviolet light spot and improve the energy uniformity of the light spot.

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

[0017] A phase shift grating writing method for a DFB single-polarization single-frequency fiber laser as described above. The light-transmitting part of the amplitude template is surrounded by positive and negative apodization functions for refractive index distribution modulation. The phase mask plate is a phase shift phase mask plate with a phase shift of π, located in the middle of the mask area, and its period and the refractive index of the gain fiber jointly determine the grating Bragg center wavelength.

[0018] A phase shift grating writing method for a DFB single-polarization single-frequency fiber laser as described above. The apodization function is used for segmental apodization of the phase shift fiber grating, but it does not compensate for the gain threshold difference between the orthogonally polarized states of the DFB fiber laser resonator. Therefore, it is necessary to further regulate the grating coupling strength to compensate for the gain threshold difference between the orthogonally polarized states of the DFB fiber laser resonator.

[0019] A phase shift grating writing method for a DFB single-polarization single-frequency fiber laser as described above. The specific operation to compensate for the gain threshold difference between the orthogonally polarized states of the DFB fiber laser resonator is as follows:

[0020] The refractive index modulation distribution expression of the grating region obtained by only segmental apodization of the phase shift fiber grating without compensation is:

[0021]

[0022] Where is the DC part of the refractive index modulation, σ(z) is the normalized apodization function, and Λ is the grating period;

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

[0024]

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

[0026] Due to the phenomenon of fiber birefringence during the writing process, the additional phase shift amounts with respect to the initial phase shift amount of π in the x and y orthogonal polarization directions are:

[0027]

[0028] Among them, and are the refractive index modulation depths in the x- and y-orthogonal polarization directions of the inscribed optical fiber, respectively, and Δn x and Δn y are the refractive index modulation distributions in the x- and y-orthogonal polarization directions of the inscribed optical 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] Among them, Δβ is the wavelength detuning amount, q is the laser mode order, κ is the AC coupling coefficient, and L shift is the phase shift region length, is the grating phase, and g th is the gain threshold;

[0032] Through the expression of the above relationship, g th can be obtained, and the closer the phase shift amount is to π, the smaller g th is. Therefore, it is and that the difference leads to the formation of different g th . At this time, the gain thresholds in the x- and y-orthogonal polarization directions in the DFB fiber laser resonator are different. By controlling the magnitude of κ, that is, controlling the refractive index modulation depth of the grating region, a pumping action 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 and single-frequency laser output of the DFB fiber laser resonator.

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

[0034] A method for writing a phase-shifted grating of a DFB single-polarization single-frequency fiber laser as described above, wherein the passive fiber in the written fiber is a polarization-maintaining or non-polarization-maintaining fiber, and its matrix material is one of quartz, phosphate, silicate, tellurite, fluoride or sulfide, or a mixture of any two or more of them in any proportion. Its core diameter is 3-30 μm, and it has good fusion splicing matching characteristics with the gain fiber.

[0035] A method for writing a phase-shifted grating of a DFB single-polarization single-frequency fiber laser as described above, wherein the gain wavelength range of the gain fiber in the written fiber, the working wavelength ranges of the ASE light source and the spectrometer cover the working wavelength range of the written phase-shifted grating.

[0036] The advantages of the present invention are as follows: The present invention prepares a DFB fiber laser resonator by writing a apodized phase-shifted fiber grating on a gain fiber. On the one hand, the weak coupling region of the apodized phase-shifted fiber grating coincides with the high-power region of the intracavity laser, which is beneficial to the gain amplification of the intracavity laser. On the other hand, the refractive index ac modulation intensity of the apodized phase-shifted fiber grating gradually decreases from the center to both sides, which is beneficial to improving the side mode suppression ratio. And the phase-shifted fiber grating is only apodized without compensation, so that its dc refractive index modulation intensity conforms to the apodization function distribution, which can cause the gain threshold difference between the orthogonal polarization states of the DFB fiber laser resonator. Further controlling the refractive index modulation depth can ensure the stable single-polarization single-frequency output of the DFB fiber laser. Therefore, the present invention provides a simple, direct, stable, reliable and highly repeatable method for writing a phase-shifted grating of a DFB single-polarization single-frequency fiber laser, providing a high-quality light source for fields such as nonlinear frequency conversion, gravitational wave detection and coherent polarization beam combination. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

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

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

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

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

[0042] Reference numerals: 1, ultraviolet laser; 2, circular aperture; 3, mirror; 4, rectangular aperture; 5, beam expander; 6, amplitude template; 7, cylindrical lens; 8, phase mask; 9, writing fiber; 10, fiber clamping platform; 11, microscope; 12, ASE light source; 13, spectrometer. Detailed implementation manners

[0043] To make the objectives, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are some but not all of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0044] As Figure 1 shown, a phase-shifted grating writing method for a DFB single-polarization single-frequency fiber laser includes the following steps:

[0045] Step (a): Hydrogen loading treatment of the gain fiber.

[0046] Step (b): Cut the gain fiber after hydrogen loading to an appropriate length, strip its coating, and fuse passive fibers to both ends thereof to form the writing fiber 9.

[0047] Step (c): Adjust the centers of the ultraviolet laser 1, the circular aperture 2, and the mirror 3 to be on the same horizontal line, and adjust the centers of the rectangular aperture 4, the beam expander 5, the cylindrical lens 7, the phase mask 8, and the writing fiber 9 to be on another horizontal line rotated 90° with the center of the mirror 3, so as to ensure that the ultraviolet light spot can vertically enter the beam expander 5, the cylindrical lens 7, the phase mask 8, and the writing fiber 9 in sequence. Adjust the relative positions of the amplitude template 6 and the phase mask 8 to align their centers, and adjust the writing fiber 9 to be 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. Next, move the gain fiber part of the writing fiber 9 to the interference region of the phase mask 8, and at the same time observe the image of the microscope 11 to control the distance between the writing fiber 9 and the phase mask 8 within half of the fiber cladding diameter. Finally, connect the writing fiber 9 to the ASE light source 12 and the spectrometer 13 to monitor the grating transmission spectrum.

[0050] Step (f): Turn on the ultraviolet laser 1, finely adjust the fiber clamping platform 10 so that the writing fiber 9 is at the focal length of the cylindrical lens 7, observe the spectrometer 13, turn off the ultraviolet laser 1 when appropriate grating parameters are achieved, remove the writing fiber 9 to complete the writing, and finally perform an annealing process.

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

[0052] Preferably, the circular aperture 2, the rectangular aperture 4, and the amplitude template 6 in the present invention are made of aluminum alloy and are subjected to black anodizing treatment; the circular aperture 2 and the rectangular aperture 4 are used to filter out the edge diffraction part of the ultraviolet light spot and improve the energy uniformity of the light spot.

[0053] Preferably, the reflecting mirror 3 in the present invention is installed on a two-dimensional mirror mount 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 mount, and the relative positions can be adjusted to ensure that they are at the same height and remain horizontal, and can make the light spot vertically incident; the magnification of the beam expander 5 is 5 to 10 times.

[0054] Preferably, the light-transmitting part of the amplitude template 6 in the present invention 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, the phase shift amount is π, and it is located in the middle of the mask area. Its period and the refractive index of the gain fiber jointly determine the grating Bragg center wavelength.

[0055] Preferably, the apodization function in the present invention is used for segmented apodization of the phase-shifted fiber grating, but it does not compensate for the gain threshold difference between the orthogonally polarized states of the DFB fiber laser resonator. Therefore, it is necessary to further regulate the grating coupling strength to compensate for the gain threshold difference between the orthogonally polarized states of the DFB fiber laser resonator.

[0056] Preferably, the specific operation for compensating the gain threshold difference between the orthogonally polarized states of the DFB fiber laser resonator in the present invention is as follows:

[0057] The expression of the refractive index modulation distribution in the grating region obtained by only performing segmented apodization on the phase-shifted fiber grating without compensation is:

[0058]

[0059] Among them, is the DC part of the refractive index modulation, σ(z) is the normalized apodization function, and Λ is the grating period;

[0060] The energy normalization distribution of the focused linear light 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 of the two sides of the piecewise super-Gaussian apodization function respectively, and n is the apodization coefficient;

[0063] Due to the phenomenon of fiber birefringence caused during the inscription process, the additional phase shift amounts relative to the initial phase shift of π in the x and y orthogonal polarization directions are as follows:

[0064]

[0065] where, and are the refractive index modulation depths in the x and y orthogonal polarization directions of the inscribed fiber 9 respectively, and Δn x and Δn y are the refractive index modulation distributions in the x and y orthogonal polarization directions of the inscribed fiber 9 respectively, and L is the grating region length;

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

[0067]

[0068] where Δβ is the wavelength detuning amount, q is the laser mode order, κ is the ac coupling coefficient, L shift is the phase shift region length, is the grating phase, and g th is the gain threshold;

[0069] From the expression of the above relationship, g th can be obtained, and the closer the phase shift amount is to π, the smaller g th is. Therefore, it is the difference between that leads to the formation of different g th At this time, the gain thresholds in the x and y orthogonal polarization directions in the DFB fiber laser resonator are different. By controlling the magnitude of κ, that is, controlling the refractive index modulation depth of the grating region, a pump action 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 and single-frequency laser output of the DFB fiber laser resonator.

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

[0071] Preferably, the passive fiber in the writing fiber 9 of the present invention is a polarization-maintaining or non-polarization-maintaining fiber, and its matrix material is one of quartz, phosphate, silicate, tellurite, fluoride or sulfide, or a mixture of any two or more of them in any proportion. Its core diameter is 3 to 30 μm, and it has good fusion splicing matching characteristics with the gain fiber.

[0072] Preferably, the gain wavelength range of the gain fiber in the writing fiber 9 of the present invention, the working wavelength ranges of the ASE light source 12 and the spectrometer 13 cover the working wavelength range of the written phase-shift grating.

[0073] Embodiment

[0074] A method for writing a phase-shift grating for a DFB single-polarization single-frequency fiber laser in an embodiment of the present invention is used to achieve stable single-polarization single-frequency output of a DFB fiber laser. The specific implementation process of this method is as follows:

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

[0076] Step (b): After cutting out an appropriate length of the hydrogen-loaded gain fiber, strip its coating layer, and fuse passive fibers at both ends thereof to form the writing fiber 9;

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

[0078] Step (d): Set the repetition frequency and voltage of the ultraviolet laser 1;

[0079] Step (e): Straighten and clamp the writing fiber 9 on the fiber clamping platform 10. Next, move the gain fiber part of the writing fiber 9 to the interference region of the phase mask 8, and at the same time observe the image of the microscope 11 to control the distance between the writing fiber 9 and the phase mask 8 within half of the fiber cladding diameter. Finally, connect the writing fiber 9 to the ASE light source 12 and the spectrometer 13 to monitor the grating transmission spectrum;

[0080] Step (f): Turn on the ultraviolet laser 1, finely adjust the fiber clamping platform 10 so that the writing fiber 9 is at the focal length of the cylindrical lens 7. Observe the spectrometer 13, turn off the ultraviolet laser 1 when the appropriate grating parameters are reached, remove the writing fiber 9 to complete the writing, and finally perform annealing treatment.

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

[0082] In this embodiment, the operating wavelengths of the reflecting mirror 3, the beam expander 5, and the phase mask 8 all cover 248 nm.

[0083] In this embodiment, the circular aperture 2, the rectangular aperture 4, and the amplitude template 6 are made of aluminum alloy and are subjected to black anodizing treatment. Among them, 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. The light-transmitting part of the amplitude template 6 is surrounded by positive and negative segmented super-Gaussian apodization functions: the overall width is 20 mm, the height is 12 mm, the super-Gaussian apodization functions on both sides are 10 mm each, and their full widths at half maximum are 5 mm and 6 mm respectively, which are used for refractive index distribution modulation.

[0084] In this embodiment, the reflecting mirror 3 is installed on a two-dimensional mirror mount and the reflection angle can be adjusted. The beam expander 5, the cylindrical lens 7, the phase mask 8, and the fiber clamping platform are installed on a three-dimensional adjustment mount, and the relative positions can be adjusted to ensure that they are at the same height and horizontal, and can make the spot vertically incident. Among them, the magnification of the beam expander 5 is 10 times.

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

[0086] In this example, a uniformly doped Yb3+ single-mode non-polarization-maintaining silica 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 its core has photosensitivity. HI1060 is selected as the passive fiber for fusion matching.

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

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

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

[0090] In the writing optical path system, the output laser spot of the ultraviolet laser 1 sequentially passes through the circular aperture 2, the mirror 3, the rectangular aperture 4, the beam expander 5, the amplitude mask 6, and the cylindrical lens 7 for beam expansion, shaping, and focusing to form a linear spot with energy satisfying a segmented super-Gaussian distribution. This spot acts on the optical fiber through the phase mask plate 8 to form a diffraction interference pattern, thereby realizing the writing of the phase-shifted fiber grating. At the same time, the monitoring system composed of the ASE light source 12, the spectrometer 13, and the written optical fiber 9 feeds back the refractive index modulation depth of the grating region. The phase-shifted fiber grating only performs segmented apodization without compensation, which causes a gain threshold difference between the orthogonally polarized states of the DFB fiber laser resonator. Further adjusting the grating coupling strength can obtain a DFB single-polarization and single-frequency fiber laser.

[0091] Among them, the refractive index modulation distribution obtained by only performing segmented apodization on the phase-shifted fiber grating without compensation is:

[0092]

[0093] Among them, is the DC part of the refractive index modulation, σ(z) is the normalized apodization function, and Λ is the grating period.

[0094] Among them, the energy normalized distribution of the focused linear spot satisfies the segmented Gaussian apodization function:

[0095]

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

[0097] Due to the occurrence of the fiber birefringence phenomenon during the writing process, the additional phase shift amounts with respect to the initial phase shift amount π in the x and y orthogonal polarization directions are:

[0098]

[0099] Among them, and are the refractive index modulation depths in the x and y orthogonal polarization directions of the inscribed optical fiber 9, respectively, and Δn x and Δn y are the refractive index modulation distributions in the x and y orthogonal polarization directions of the inscribed optical fiber 9, respectively, and L is the grating region length.

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

[0101]

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

[0103] At this time, solving the above equation can obtain g th , obviously, the closer the phase shift amount is to π, the smaller g th is. Therefore, The difference between results in different gs th , at this time, the gain thresholds in the x and y orthogonal polarization directions in the DFB fiber laser resonator are different. By controlling the magnitude of κ, that is, controlling the refractive index modulation depth of the grating region, a pump action 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 of the DFB fiber laser resonator.

[0104] In summary, through Figure 2 the direct apodization effect of the apodization amplitude template in Figure 4The single-frequency laser output shown is simple to operate and has remarkable effects. Compared with the prior art, in the present invention, a DFB fiber laser resonator is prepared by writing a segmented apodized phase-shifted fiber grating on a gain fiber. On the one hand, the weak coupling region of the segmented apodized phase-shifted fiber grating coincides with the high-power region of the intracavity laser, which is beneficial to the gain amplification of the intracavity laser. On the other hand, the refractive index alternating modulation intensity of the segmented apodized phase-shifted fiber grating gradually decreases from the center to both sides, which is beneficial to improving the side-mode suppression ratio. And the phase-shifted fiber grating is only apodized without compensation, making its DC refractive index modulation intensity conform to the segmented apodized function distribution, which can cause the gain threshold difference between the orthogonal polarization states of the DFB fiber laser resonator. Further controlling the refractive index modulation depth can ensure the stable single-polarization and single-frequency output of the DFB fiber laser. Therefore, the present invention provides a phase-shifted grating writing method for a DFB single-polarization and single-frequency fiber laser that is simple, direct, stable, reliable, and has a high degree of repeatability, providing a high-quality light source for fields such as nonlinear frequency conversion, gravitational wave detection, and coherent polarization beam combination.

[0105] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some 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 various embodiments of the present invention.

Claims

1. A method for writing a phase-shifted grating of a DFB single-polarization single-frequency fiber laser, characterized in that: It includes the following steps: Step (a): Hydrogen loading treatment of the gain fiber; Step (b): After cutting the appropriate length of the hydrogen-loaded gain fiber, strip its coating layer, and fuse passive fibers at both ends to form the inscription fiber (9); Step (c): Adjust the centers of the ultraviolet laser (1), circular aperture (2), and mirror (3) to be on the same horizontal line, and adjust the centers of the rectangular aperture (4), beam expander (5), cylindrical lens (7), phase mask (8), and inscription fiber (9) to be on another horizontal line rotated 90° with the center of the mirror (3), so as to ensure that the ultraviolet light spot can be vertically incident on the beam expander (5), cylindrical lens (7), phase mask (8), and inscription fiber (9) in sequence. Adjust the relative positions of the amplitude mask (6) and the phase mask (8) to align their centers, and adjust the inscription fiber (9) to be near the focal length of the cylindrical lens (7); Step (d): Set the repetition frequency and output power of the ultraviolet laser (1); Step (e): Straighten the inscription fiber (9) and clamp it on the fiber clamping platform (10). Next, move the gain fiber part of the inscription fiber (9) to the interference region of the phase mask (8), and at the same time, control the distance between the inscription fiber (9) and the phase mask (8) within half of the fiber cladding diameter by observing the image of the microscope (11). Finally, connect the inscription fiber (9) to the ASE light source (12) and the spectrometer (13) to monitor the grating transmission spectrum; Step (f): Turn on the ultraviolet laser (1), finely adjust the fiber clamping platform (10) so that the inscription fiber (9) is at the focal length of the cylindrical lens (7), observe the spectrometer (13), turn off the ultraviolet laser (1) when the appropriate grating parameters are reached, remove the inscription fiber (9) to complete the inscription, and finally perform annealing treatment.

2. The phase shift 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 ranges of the mirror (3), beam expander (5), and phase mask (8) cover the working wavelength range of the ultraviolet laser (1).

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

4. A method for writing a phase shift grating 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 mount and can adjust the reflection angle; the beam expander (5), cylindrical lens (7), phase mask (8), and fiber clamping platform (10) are installed on a three-dimensional adjustment mount, and 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 magnification of the beam expander (5) is 5 to 10 times.

5. A method for writing a phase-shifted grating of a DFB single-polarization single-frequency fiber laser according to claim 1, characterized in that: The light-transmitting 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 with a phase shift of π, located in the middle of the mask area, and its period and the refractive index of the gain fiber jointly determine the grating Bragg center wavelength.

6. A method for fabricating a phase-shifted grating of a DFB single-polarization single-frequency fiber laser according to claim 5, characterized in that: The apodization function is used for segmental apodization of the phase-shifted fiber grating, but it does not compensate for the gain threshold difference between the orthogonally polarized states in the DFB fiber laser resonator. Therefore, it is necessary to further adjust the grating coupling strength to compensate for the gain threshold difference between the orthogonally polarized states in the DFB fiber laser resonator.

7. A method for writing a phase shift grating of a DFB single-polarization single-frequency fiber laser according to claim 6, characterized in that: The specific operation to compensate for the gain threshold difference between the orthogonally polarized states in the DFB fiber laser resonator is as follows: The expression of the refractive index modulation distribution in the grating region obtained when the phase-shifted fiber grating only undergoes segmental apodization without compensation is: wherein, is the DC part of the refractive index modulation, σ(z) is the normalized apodization function, and Λ is the grating period; The energy normalization distribution of the focused linear light spot satisfies the expression of the segmented Gaussian apodization function as: where FWHM1 and FWHM2 are respectively the full width at half maximum of the two sides of the segmented super-Gaussian apodization function, and n is the apodization coefficient; Due to the phenomenon of fiber birefringence during the writing process, the additional phase shift amounts with respect to the initial phase shift of π in its x and y orthogonal polarization directions are: Among them, and are respectively the refractive index modulation depths in the x and y orthogonal polarization directions of the inscribed optical fiber (9), Δn x and Δn y are respectively the refractive index modulation distributions in the x and y orthogonal polarization directions of the inscribed optical fiber (9), and L is the grating region length; The relationship between the gain threshold in the DFB fiber laser resonator and the additional phase shift amount satisfies the following expression: where Δβ is the wavelength detuning, q is the laser mode order, κ is the ac coupling coefficient, L shift is the length of the phase shift region, is the grating phase, and g th is the gain threshold; The expression of the above relationship can be used to obtain g th , and the closer the phase shift amount is to π, the smaller g th . Therefore, it is and that the difference leads to the formation of different g th . At this time, the gain thresholds in the x and y orthogonally polarized directions in the DFB fiber laser resonator are different. By controlling the magnitude of κ, that is, controlling the refractive index modulation depth of the grating region, the pumping action within a certain intensity range can prevent the polarization state with a higher gain threshold in the x and y orthogonally polarized directions in the DFB fiber laser resonator from oscillating, 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 resonator.

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

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

10. A phase shift grating writing method for 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 writing fiber (9), and the working wavelength ranges of the ASE light source (12) and the spectrometer (13) cover the working wavelength range of the written phase-shifted grating.

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