A DFB linearly polarized single-frequency fiber laser based on polarization pulling effect

Abstract

A DFB linearly polarized single-frequency fiber laser based on polarization pulling effect, comprising a polarization maintaining uniaxial fiber isolator, an input end of the polarization maintaining uniaxial fiber isolator is connected with a signal end of a polarization maintaining wavelength division multiplexer, a pump end of the polarization maintaining wavelength division multiplexer is connected with an output end of a single-mode pump light source, a common end of the polarization maintaining wavelength division multiplexer is connected with one end of a polarization maintaining fiber grating, the other end of the polarization maintaining fiber grating is connected with an output end of a phase shift fiber grating through a PZT phase modulator, the phase shift fiber grating is written on a gain fiber to form a DFB fiber laser resonant cavity and is fixedly packaged in a heat sink with automatic temperature control, the DFB fiber laser resonant cavity is connected with an input end of a photoelectric detector, and the photoelectric detector converts a laser signal of an idle end of the DFB fiber laser resonant cavity into an electric signal.The application can provide a linearly polarized single-frequency laser source with excellent performance, high linear polarization degree and high stability for the fields of fiber gyroscopes, nonlinear frequency conversion and coherent polarization beam combination.

Description

Technical Field

[0001] This invention belongs to the field of single-frequency fiber lasers, specifically a DFB linearly polarized single-frequency fiber laser based on the polarization traction effect. Background Technology

[0002] Currently, many important application areas have a high demand for linearly polarized single-frequency fiber laser sources, such as fiber gyroscopes, nonlinear frequency conversion, and coherent polarization beam combinations. The resonant cavity structure of single-frequency fiber lasers includes ring cavities and linear cavities. Ring cavities, lacking a frequency discrimination mechanism, require additional structures and are characterized by long cavities, high losses, and complex structures. Linear cavities can be divided into distributed Bragg reflector (DBR) and distributed feedback (DFB) types. The resonant cavity of a DBR single-frequency fiber laser includes a pair of fiber gratings and a short gain fiber; the resonant cavity of a DFB single-frequency fiber laser only includes the gain fiber with a phase-shifted fiber grating written into it, integrating gain and feedback to avoid fusion splicing.

[0003] Currently, research and development of highly photosensitive polarization-maintaining gain fibers is limited, leading to the common practice of inscribing phase-shifting fiber gratings onto non-polarization-maintaining gain fibers. Consequently, the output polarization state of DFB fiber lasers is unstable. Applying torsional stress or localized transverse stress to the phase-shifting fiber grating of a DFB fiber laser can achieve linearly polarized laser output; however, the criteria for determining the scale or orientation of the applied stress remain unclear, and this inevitably makes the laser more sensitive to environmental disturbances. Polarization pulling has also proven to be an effective means of achieving linearly polarized laser output, i.e., using polarization feedback to pull the laser resonator to generate laser oscillations in the corresponding polarization state, which has the characteristics of controllability and stability.

[0004] With the advancement of grating writing technology and the development of fiber optic devices, DFB fiber lasers of various structures have achieved linearly polarized single-frequency laser output. For example, a linearly polarized single-frequency laser with a degree of polarization of 0.98 was obtained using a self-injected ring structure of a DFB fiber laser [LaserPhysics, 2011, 21(12): 2108-2111]. XHLi et al. achieved a linearly polarized single-frequency laser output with a power of 9.5mW using a composite linear structure of a DFB fiber laser resonator and a fiber loop mirror [LaserPhysicsLetters, 2010, 7(1): 55-59]. All of the above achievements are based on a polarization controller, which aligns the polarization direction of the control feedback light with the orthogonal polarization directions split within the DFB fiber laser resonator to obtain linearly polarized laser. On the one hand, the uncertainty of the polarization direction limits the connection and use of the DFB fiber laser resonator with polarization-maintaining devices. On the other hand, the phase-sensitive characteristics of the optical feedback make the laser susceptible to various disturbances. In summary, there is still a lack of simple, compact, high-linear-polarization, and high-stability DFB linearly polarized single-frequency fiber lasers. Summary of the Invention

[0005] This invention provides a DFB linearly polarized single-frequency fiber laser based on polarization traction effect to overcome the shortcomings of the prior art.

[0006] This invention is achieved through the following technical solution:

[0007] A DFB linearly polarized single-frequency fiber laser based on polarization traction effect includes a polarization-maintaining single-axis fiber isolator. The input end of the polarization-maintaining single-axis fiber isolator is connected to the signal end of a polarization-maintaining wavelength division multiplexer (PWM), the pump end of the PWM isolator is connected to the output end of a single-mode pump source, the common end of the PWM isolator is connected to one end of a polarization-maintaining fiber grating (PSG), and the other end of the PSG is connected to the output end of a phase-shifting fiber grating (PSGF) via a PZT phase modulator. The PGF is etched onto a gain fiber to form a DFB fiber laser resonant cavity and is fixedly sealed. The DFB fiber laser resonator is housed in an automatically temperature-controlled heat sink. It is connected to the input of a photodetector, which converts the idle laser signal from the DFB fiber laser resonator into an electrical signal. The output of the photodetector is connected to the input of a low-pass filter, which is connected to the positive input of a differential amplifier. The negative input of the differential amplifier is connected to a reference electrical signal source, and its output is connected to the input of a PID operational amplifier. The output of the PID operational amplifier is connected to a PZT phase modulator.

[0008] As described above, in a DFB linearly polarized single-frequency fiber laser based on polarization traction effect, the operating wavelength range of the polarization-maintaining single-axis fiber isolator covers the operating wavelength range of polarization-maintaining fiber gratings and phase-shifting fiber gratings.

[0009] As described above, in a DFB linearly polarized single-frequency fiber laser based on polarization traction effect, the single-mode pump source is any one of a semiconductor laser single-mode pump source, a fiber laser single-mode pump source, a gas laser single-mode pump source, or a solid-state laser single-mode pump source.

[0010] As described above, in a DFB linearly polarized single-frequency fiber laser based on polarization traction effect, the operating wavelength range of the polarization-maintaining wavelength division multiplexer covers the operating wavelength range of the single-mode pump source, the polarization-maintaining fiber grating, and the phase-shifting fiber grating. The polarization-maintaining single-axis fiber isolator and the polarization-maintaining wavelength division multiplexer, as well as the polarization-maintaining wavelength division multiplexer and the polarization-maintaining fiber grating, are all fused together using either slow-axis alignment or fast-axis alignment.

[0011] As described above, in a DFB linearly polarized single-frequency fiber laser based on polarization traction effect, the PZT phase modulator is used to control the feedback light phase injected back into the DFB fiber laser resonator by the polarization-maintaining fiber grating.

[0012] As described above, a DFB linearly polarized single-frequency fiber laser based on polarization traction effect is described, wherein the gain fiber is a non-polarization-maintaining gain fiber doped with rare-earth ions, and its matrix material is any one or a mixture of any two or more of quartz, phosphate, silicate, tellurate, fluoride, or sulfide in any proportion; the core of the gain fiber is uniformly doped with rare-earth ions and has ultraviolet photosensitivity, with a diameter of 3-10 μm, and the doped rare-earth ion is Yb. 3+ Er 3+ Tm 3+ 、Nd 3+ Pr 3+ Ho 3+ Eu 3+ Dy 3+ A mixture of any one or more of them in any proportion.

[0013] As described above, the DFB linearly polarized single-frequency fiber laser based on polarization traction effect has a 3dB spectral width of 0.01–0.2 nm for the reflection spectrum of the polarization-maintaining fiber grating, a reflectivity of 1–20% for the laser signal wavelength, and an absolute difference of 0.25–0.5 nm between the center wavelengths of its fast and slow axis reflection spectra.

[0014] As described above, in a DFB linearly polarized single-frequency fiber laser based on polarization traction effect, the operating wavelength range of the phase-shifting fiber grating is covered by the gain wavelength range of the gain fiber, and the length of the phase-shifting fiber grating is 2-4 cm shorter than the length of the gain fiber. The excess gain fiber portion is used for laser amplification and feedback modulation. The 3dB reflection spectrum of the phase-shifting fiber grating is 0.04-0.25 nm, the peak reflectivity is 90-100%, and there is a transmission peak within its 3dB reflection spectrum. The absolute difference between the center wavelength of the transmission peak and the center wavelength of the slow-axis or fast-axis reflection spectrum of the polarization-maintaining fiber grating is 0-0.02 nm.

[0015] As described above, in a DFB linearly polarized single-frequency fiber laser based on polarization traction effect, the heat sink can control the temperature of the DFB fiber laser resonator to achieve laser wavelength tuning, with a temperature control accuracy of 0.01℃~0.05℃; the response wavelength range of the photodetector covers the operating wavelength range of polarization-maintaining fiber gratings and phase-shifting fiber gratings.

[0016] The formula for calculating the additional gain threshold of the DFB linearly polarized single-frequency fiber laser based on the polarization pulling effect described above, where the phase-shifting fiber grating is inscribed on the gain fiber to form the DFB fiber laser resonant cavity and the polarization-maintaining fiber grating forms the linear composite cavity, is as follows:

[0017]

[0018] Where d is the effective length of the DFB fiber laser resonator, ω0 is the output angular frequency during free operation, ω is the output angular frequency during locking, τ is the laser round-trip time to the external cavity, r2 is the reflection coefficient at ω of the grating at the output end of the DFB fiber laser resonator, and the combined effective reflection coefficient at ω of the external cavity is r = r3e gL Where r3 is the reflection coefficient at ω of the polarization-maintaining fiber grating, g is the external cavity gain coefficient, and L is the external cavity length.

[0019] The advantages of this invention are as follows: This invention fabricates a DFB fiber laser resonator by inscribing a phase-shifting fiber grating on the gain fiber. The grating region is only a few centimeters long and is integrated with the gain, avoiding splicing loss and thermal instability, further ensuring stable single-frequency laser performance. Furthermore, the polarization-maintaining fiber grating and the DFB fiber laser resonator in this invention form a composite linear cavity, with a simple and compact structure. This not only helps suppress noise and narrow linewidth but also ensures that the laser polarization direction within the cavity is consistent with the slow or fast axis polarization direction of the grating through the polarization pulling effect, achieving linearly polarized light output. Simultaneously, the polarization-maintaining optical devices in this invention are spliced ​​by aligning the slow or fast axes of the fiber, and the slow or fast axis operation of the polarization-maintaining single-axis fiber isolator ensures high linear polarization of the laser and reduces the possibility of laser polarization state drift. In addition, this invention includes a closed-loop phase-locked loop control circuit for regulating the feedback light phase, further ensuring the long-term stability of the linearly polarized single-frequency laser output. Therefore, this invention can provide a high-performance, simple, compact, high-linear-polarization-degree, and highly stable linearly polarized single-frequency laser source for fields such as fiber optic gyroscopes, nonlinear frequency conversion, and coherent polarization beam combinations. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 This is a schematic diagram of the structure of the present invention.

[0022] Figure reference numerals: 1. Polarization-maintaining single-axis fiber isolator; 2. Single-mode pump source; 3. Polarization-maintaining wavelength division multiplexer; 4. Polarization-maintaining fiber grating; 5. PZT phase modulator; 6. Phase-shifting fiber grating; 7. Gain fiber; 8. Heat sink; 9. Photodetector; 10. Low-pass filter; 11. Reference electrical signal source; 12. Differential amplifier; 13. PID operational amplifier. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0024] A DFB linearly polarized single-frequency fiber laser based on polarization traction effect includes a polarization-maintaining single-axis fiber isolator 1. The input end of the polarization-maintaining single-axis fiber isolator 1 is connected to the signal end of a polarization-maintaining wavelength division multiplexer 3. The pump end of the polarization-maintaining wavelength division multiplexer 3 is connected to the output end of a single-mode pump source 2. The common end of the polarization-maintaining wavelength division multiplexer 3 is connected to one end of a polarization-maintaining fiber grating 4. The other end of the polarization-maintaining fiber grating 4 is connected to the output end of a phase-shifting fiber grating 6 through a PZT phase modulator 5. The phase-shifting fiber grating 6 is etched on a gain fiber 7 to form a DFB fiber laser harmonic. The resonant cavity is fixedly encapsulated in an automatically temperature-controlled heat sink 8. The DFB fiber laser resonant cavity is connected to the input terminal of a photodetector 9. The photodetector 9 converts the idle laser signal of the DFB fiber laser resonant cavity into an electrical signal. The output terminal of the photodetector 9 is connected to the input terminal of a low-pass filter 10. The output terminal of the low-pass filter 10 is connected to the positive input terminal of a differential amplifier 12. The negative input terminal of the differential amplifier 12 is connected to a reference electrical signal source 11. The output terminal of the differential amplifier 12 is connected to the input terminal of a PID operational amplifier 13. The output of amplifier 13 is connected to PZT phase modulator 5. The phase-shifting fiber grating 6 is etched on gain fiber 7 to form a DFB fiber laser resonator. The photodetector 9, low-pass filter 10, differential amplifier 12, PID operational amplifier 13, and PZT phase modulator 5 can form a closed-loop phase-locked control circuit. The photodetector 9 converts the laser signal at the idle end of the DFB fiber laser resonator formed by the phase-shifting fiber grating 6 etched on gain fiber 7 into an electrical signal. The low-pass filter 10 filters the electrical signal. The differential amplifier 12 compares the filtered electrical signal with a reference electrical signal source. The input signal of 11 is differentially amplified, and the PID operational amplifier 13 performs operational amplification on the differential electrical signal and inputs it into the PZT phase modulator 5 to realize feedback optical phase control. The phase-shifting fiber grating 6 is written on the gain fiber 7 to form the output laser of the DFB fiber laser resonator. After passing through the PZT phase modulator 5, it is fed back by the polarization-maintaining fiber grating 4. The laser in the cavity is pulled by the slow-axis or fast-axis polarization feedback light and passes through the PZT phase modulator 5, the polarization-maintaining fiber grating 4, the polarization-maintaining wavelength division multiplexer 3 and the polarization-maintaining single-axis fiber isolator 1 in sequence to form a stable linearly polarized single-frequency laser output.

[0025] Preferably, the operating wavelength range of the polarization-maintaining single-axis fiber isolator 1 of the present invention covers the operating wavelength range of the polarization-maintaining fiber grating 4 and the phase-shifting fiber grating 6, and its slow axis operation is fast axis cutoff or fast axis operation is slow axis cutoff, which is used to improve the linear polarization degree of the output laser and isolate the return light.

[0026] Preferably, the single-mode pump source 2 of the present invention is any one of a semiconductor laser single-mode pump source, a fiber laser single-mode pump source, a gas laser single-mode pump source, or a solid-state laser single-mode pump source.

[0027] Preferably, the operating wavelength range of the polarization-maintaining wavelength division multiplexer 3 of the present invention covers the operating wavelength range of the single-mode pump light source 2, the polarization-maintaining fiber grating 4, and the phase-shifting fiber grating 6. The polarization-maintaining single-axis fiber isolator 1 and the polarization-maintaining wavelength division multiplexer 3, and the polarization-maintaining wavelength division multiplexer 3 and the polarization-maintaining fiber grating 4 are all fused together by slow axis alignment or by fast axis alignment.

[0028] Preferably, the PZT phase modulator 5 of the present invention is used to control the feedback light phase injected back into the DFB fiber laser resonator by the polarization-maintaining fiber grating 4.

[0029] Preferably, the gain fiber 7 of the present invention is a non-polarization-maintaining gain fiber doped with rare earth ions, and its matrix material is any one or a mixture of any two or more of quartz, phosphate, silicate, tellurate, fluoride or sulfide in any proportion; the core of the gain fiber 7 is uniformly doped with rare earth ions and has ultraviolet photosensitivity, with a diameter of 3-10 μm, and the doped rare earth ion is Yb. 3+ Er 3+ Tm 3+ 、Nd 3+ Pr 3+ Ho 3+ Eu 3+ Dy 3+ A mixture of any one or more of them in any proportion.

[0030] Preferably, the polarization-maintaining fiber grating 4 of the present invention has a 3dB spectral width of 0.01 to 0.2 nm, a reflectivity of 1 to 20% for laser signal wavelength, and an absolute difference of 0.25 to 0.5 nm between the center wavelengths of its fast and slow axis reflection spectra.

[0031] Preferably, the operating wavelength range of the phase-shifting fiber grating 6 of the present invention is covered by the gain wavelength range of the gain fiber 7, and the length of the phase-shifting fiber grating 6 is 2-4 cm shorter than the length of the gain fiber 7. The excess gain fiber portion is used for laser amplification and feedback modulation. The 3dB reflection spectrum of the phase-shifting fiber grating 6 is 0.04-0.25 nm, the peak reflectivity is 90-100%, and there is a transmission peak within its 3dB reflection spectrum. The absolute difference between the center wavelength of the transmission peak and the center wavelength of the slow-axis or fast-axis reflection spectrum of the polarization-maintaining fiber grating 4 is 0-0.02 nm.

[0032] Preferably, the heat sink 8 of the present invention can control the temperature of the DFB fiber laser resonator to achieve laser wavelength tuning, with a temperature control accuracy of 0.01℃~0.05℃; the response wavelength range of the photodetector 9 covers the operating wavelength range of the polarization-maintaining fiber grating 4 and the phase-shifting fiber grating 6; the output electrical signal of the reference electrical signal source 11 is determined by the optical path parameters of the DFB linearly polarized single-frequency fiber laser.

[0033] Preferably, the formula for calculating the additional gain threshold of the DFB fiber laser resonator formed by the phase-shifting fiber grating 6 inscribed on the gain fiber 7 and the linear composite cavity formed by the polarization-maintaining fiber grating 4 in this embodiment is as follows:

[0034]

[0035] Where d is the effective length of the DFB fiber laser resonator, ω0 is the output angular frequency during free operation, ω is the output angular frequency during locking, τ is the laser round-trip time to the external cavity, r2 is the reflection coefficient at ω of the grating at the output end of the DFB fiber laser resonator, and the combined effective reflection coefficient at ω of the external cavity is r = r3e gL Where r3 is the reflection coefficient at ω of polarization-maintaining fiber grating 4, g is the external cavity gain coefficient, and L is the external cavity length.

[0036] Example:

[0037] This embodiment includes a polarization-maintaining single-axis fiber isolator 1, a single-mode pump light source 2, a polarization-maintaining wavelength division multiplexer 3, a polarization-maintaining fiber grating 4, a PZT phase modulator 5, a phase-shifting fiber grating 6, a gain fiber 7, a heat sink 8, a photodetector 9, a low-pass filter 10, a reference electrical signal source 11, a differential amplifier 12, and a PID operational amplifier 13. The input end of the polarization-maintaining single-axis fiber isolator 1 and the output end of the single-mode pump light source 2 are connected to the signal end and pump end of the polarization-maintaining wavelength division multiplexer 3, respectively. The common end of the polarization-maintaining wavelength division multiplexer 3 is connected to one end of the polarization-maintaining fiber grating 4. The other end of the polarization-maintaining fiber grating 4 is connected to the output end of the phase-shifting fiber grating 6 through the PZT phase modulator 5. The phase-shifting fiber grating 6 is etched onto the gain fiber 7 to form a polarization-maintaining single-axis fiber isolator 1, a single-mode pump light source 2, a reference electrical signal source 11, a differential amplifier 12, and a PID operational amplifier 13. The DFB fiber laser resonator is fixedly encapsulated in an automatically temperature-controlled heat sink 8 to ensure stable operation of the linearly polarized single-frequency laser. The polarization-maintaining fiber grating 4 and the DFB fiber laser resonator form a composite linear cavity structure. The output end of the polarization-maintaining single-axis fiber isolator 1 serves as the output end of the DFB linearly polarized single-frequency fiber laser. It also includes a closed-loop phase-locked control circuit. The photodetector 9 converts the idle laser signal of the DFB fiber laser resonator into an electrical signal. The low-pass filter 10 filters the electrical signal. The differential amplifier 12 differentially amplifies the filtered electrical signal and the input signal of the reference electrical signal source 11. The PID operational amplifier 13 performs operational amplification on the differential electrical signal and inputs it into the PZT phase modulator 5.

[0038] This embodiment employs a polarization-maintaining fiber grating polarization feedback and closed-loop phase-locked control scheme. The polarization-maintaining fiber grating 4 reflects the forward output light at the output end of the phase-shifting fiber grating 6. The single-mode pump source 2 provides backward pumping to the DFB fiber laser resonator through the polarization-maintaining wavelength division multiplexer 3. The closed-loop phase-locked control circuit converts the laser signal at the idle end of the DFB fiber laser resonator with the reference signal to form an error signal, and controls the PZT phase modulator 5 to match the optical feedback phase with the laser frequency to achieve locking.

[0039] In this example, the single-mode pump source 2 is a semiconductor laser single-mode pump source with a working wavelength of 974nm, an output pigtail core diameter of 5μm, and a maximum output power of 360mW.

[0040] In this example, the center operating wavelength of the polarization-maintaining single-axis fiber isolator 1 is 1064nm, and its slow axis is in operation while its fast axis is cut off.

[0041] In this example, the center operating wavelength of polarization-maintaining wavelength division multiplexer 3 is 974 / 1064nm, and it operates on both axes.

[0042] 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 was selected as the gain fiber 7, and its core is photosensitive.

[0043] In this example, the polarization-maintaining fiber grating 4 is 1.5 cm long, and its 3 dB slow axis reflection spectral width is 0.04–0.06 nm. As needed, its reflectivity to the laser signal wavelength can be changed from 5% to 15% to expand the range and degree of the polarization pulling effect.

[0044] In this example, a 4cm phase-shifted fiber grating 6 is etched onto a 6-8cm gain fiber 7. The gain margin can be used for laser amplification, and its gain coefficient can be affected by changes in the pump laser power, thus controlling the feedback quantity. The phase shift of the phase-shifted fiber grating 6 is π. There is a transmission peak near the center of the reflection spectrum, with a 3dB reflection spectrum width of 0.1-0.15nm and a peak reflectivity of 97%-98%. During the ultraviolet exposure etching process, a segmented super-Gaussian apodized amplitude plate is used to apodize the grating, realizing an asymmetric output mode where the laser power at the output end of the DFB fiber laser resonator is greater than that at the idle end. The end with the lower refractive index modulation depth of the grating is used as the output end, and the other end is used as the idle end. The center wavelength of the slow-axis reflection spectrum of the polarization-maintaining fiber grating 4 and the center wavelength of the transmission peak of the phase-shifted fiber grating 6 are both 1064nm, with an absolute difference of 0-0.02nm.

[0045] In this example, the pigtails of polarization-maintaining single-axis fiber isolator 1, polarization-maintaining wavelength division multiplexer 3, and polarization-maintaining fiber grating 4 are all single-mode polarization-maintaining fibers. They are fused together by slow axis alignment, with good mode matching and no fusion errors.

[0046] In this example, the photodetector 9 has a response wavelength range covering 1064 nm.

[0047] In this example, the output electrical signal of the reference electrical signal source 11 is determined as follows: a sawtooth wave signal is applied to the PZT phase modulator 5 to make the phase of the feedback light change periodically. At this time, the response electrical signal of the laser signal at the idle end of the DFB fiber laser resonator also changes periodically and forms multiple peaks in a single period. The corresponding electrical signal peak value is appropriately selected as the reference electrical signal value.

[0048] The specific working method and principle of this embodiment are as follows:

[0049] The single-mode pump source 2 emits a 974nm single-mode pump laser, which enters the 974nm pump port of the polarization-maintaining wavelength division multiplexer 3. The back pump light is provided to the DFB fiber laser resonator through the 974 / 1064nm common port of the polarization-maintaining wavelength division multiplexer 3. Under the combined effect of fiber gain and grating feedback, the resonator outputs a 1064nm single-frequency laser. Part of the emitted laser is reflected by the polarization-maintaining fiber grating 4. The laser inside the cavity is pulled by the slow-axis polarization feedback light and then transmitted out of the polarization-maintaining fiber grating 4 to form a 1064nm linearly polarized (slow-axis direction) single-frequency laser, which is then output after passing through the polarization-maintaining wavelength division multiplexer 3 and the polarization-maintaining single-axis fiber isolator 1 in sequence.

[0050] The closed-loop phase-locked loop control circuit converts the laser signal at the idle end of the DFB fiber laser into a differential error signal by comparing it with the reference signal. This error signal is then used to control the PZT phase modulator 5 to match the optical feedback phase with the laser frequency and achieve locking.

[0051] The DFB fiber laser resonator and the polarization-maintaining fiber grating 4 form a linear composite cavity, and its additional gain threshold compared to the DFB fiber laser resonator is:

[0052]

[0053] Where d is the effective length of the DFB fiber laser resonator, ω0 is the output angular frequency during free operation, ω is the output angular frequency during locking, τ is the laser round-trip time to the external cavity, r2 is the reflection coefficient at ω of the grating at the output end of the DFB fiber laser resonator, and the combined effective reflection coefficient at ω of the external cavity is r = r3e gL Where r3 is the reflection coefficient at ω of polarization-maintaining fiber grating 4, g is the external cavity gain coefficient, and L is the external cavity length.

[0054] Additional gain threshold G EXCESS Let ω0τ be a periodic function of phase difference:

[0055]

[0056] When cos(ω0τ)=0, G EXCESS (ω0τ) < 0 and takes its minimum value. When the closed-loop phase-locked loop control circuit controls the PZT phase modulator 5 to match the optical feedback phase with the laser frequency, that is, when the phase difference is zero:

[0057]

[0058] At this time, G EXCESS ′(r)<0, additional gain coefficient G EXCESS The effective composite reflection coefficient r decreases as the composite reflection coefficient r increases. When the laser polarization direction in the DFB fiber laser resonator is aligned with the slow axis of the polarization-maintaining fiber grating 4, r is at its maximum. At this point, the polarization mode is most easily oscillated. The external cavity gain coefficient g can be changed by controlling the pump laser input power, thereby regulating the effective composite reflection coefficient r and achieving a more effective polarization pulling effect. This generates and controls a linearly polarized single-frequency laser with high linear polarization degree and high stability.

[0059] In summary, compared with the prior art, this invention fabricates a DFB fiber laser resonator by inscribing a phase-shifting fiber grating 6 on the gain fiber 7. The grating region is only a few centimeters long and is integrated with the gain, avoiding splicing loss and thermal instability, further ensuring the stable single-frequency performance of the laser. Furthermore, the polarization-maintaining fiber grating 4 and the DFB fiber laser resonator form a composite linear cavity with a simple and compact structure, which is beneficial for suppressing noise and narrowing linewidth. It also ensures that the laser polarization direction in the cavity is consistent with the slow-axis polarization direction of the grating through the polarization pulling effect, realizing slow-axis polarized light output. At the same time, the polarization-maintaining optical devices are spliced ​​by aligning the slow axis of the optical fibers, and the slow-axis operation of the polarization-maintaining single-axis fiber isolator 1 ensures high linear polarization of the laser and reduces the possibility of laser polarization state drift. This laser is suitable for large-scale commercial production.

[0060] Furthermore, this invention includes a closed-loop phase-locked control circuit for regulating the feedback light phase, further ensuring the long-term stability of the linearly polarized single-frequency laser output. Therefore, this invention can provide a high-performance, simple, compact, highly linearly polarized, and highly stable linearly polarized single-frequency laser source for fields such as fiber optic gyroscopes, nonlinear frequency conversion, and coherent polarization beam combinations.

[0061] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A DFB linearly polarized single-frequency fiber laser based on polarization pulling effect, characterized in that: The system includes a polarization-maintaining single-axis fiber isolator (1), the input end of which is connected to the signal end of a polarization-maintaining wavelength division multiplexer (3), the pump end of which is connected to the output end of a single-mode pump light source (2), the common end of which is connected to one end of a polarization-maintaining fiber grating (4), and the other end of which is connected to the output end of a phase-shifting fiber grating (6) via a PZT phase modulator (5). The phase-shifting fiber grating (6) is etched onto a gain fiber (7) to form a DFB fiber laser resonator and is fixedly encapsulated in an automatically temperature-controlled heat sink (8). The B fiber laser resonator is connected to the input end of the photodetector (9). The photodetector (9) converts the idle laser signal of the DFB fiber laser resonator into an electrical signal. The output end of the photodetector (9) is connected to the input end of the low-pass filter (10). The output end of the low-pass filter (10) is connected to the positive input end of the differential amplifier (12). The negative input end of the differential amplifier (12) is connected to the reference electrical signal source (11). The output end of the differential amplifier (12) is connected to the input end of the PID operational amplifier (13). The output end of the PID operational amplifier (13) is connected to the PZT phase modulator (5).

2. The DFB linearly polarized single-frequency fiber laser based on polarization pulling effect according to claim 1, characterized in that: The operating wavelength range of the polarization-maintaining single-axis fiber isolator (1) covers the operating wavelength range of the polarization-maintaining fiber grating (4) and the phase-shifting fiber grating (6).

3. A DFB linearly polarized single-frequency fiber laser based on polarization pulling effect according to claim 1, characterized in that: The single-mode pump source (2) is any one of a semiconductor laser single-mode pump source, a fiber laser single-mode pump source, a gas laser single-mode pump source, or a solid-state laser single-mode pump source.

4. A DFB linearly polarized single-frequency fiber laser based on polarization pulling effect according to claim 1, characterized in that: The operating wavelength range of the polarization-maintaining wavelength division multiplexer (3) covers the operating wavelength range of the single-mode pump light source (2), the polarization-maintaining fiber grating (4), and the phase-shifting fiber grating (6). The polarization-maintaining single-axis fiber isolator (1) and the polarization-maintaining wavelength division multiplexer (3), and the polarization-maintaining wavelength division multiplexer (3) and the polarization-maintaining fiber grating (4) are all fused together by slow axis alignment or by fast axis alignment.

5. A DFB linearly polarized single-frequency fiber laser based on polarization pulling effect according to claim 1, characterized in that: The PZT phase modulator (5) is used to control the feedback phase of the polarization-maintaining fiber grating (4) injected back into the DFB fiber laser resonator.

6. A DFB linearly polarized single-frequency fiber laser based on polarization pulling effect according to claim 1, characterized in that: The gain fiber (7) is a non-polarity-maintaining gain fiber doped with rare earth ions. Its matrix material is any one or a mixture of any two or more of the following in any proportion: quartz, phosphate, silicate, tellurate, fluoride, or sulfide. The core of the gain fiber (7) is uniformly doped with rare earth ions and has ultraviolet photosensitivity. Its diameter is 3-10 μm. The doped rare earth ion is Yb. 3+ Er 3+ Tm 3+ 、Nd 3+ Pr 3+ Ho 3+ Eu 3+ Dy 3+ A mixture of any one or more of them in any proportion.

7. A DFB linearly polarized single-frequency fiber laser based on polarization pulling effect according to claim 1, characterized in that: The polarization-maintaining fiber grating (4) has a 3dB spectral width of 0.01 to 0.2 nm, a reflectivity of 1 to 20% for laser signal wavelengths, and an absolute difference of 0.25 to 0.5 nm between the center wavelengths of its fast and slow axis reflection spectra.

8. A DFB linearly polarized single-frequency fiber laser based on polarization pulling effect according to claim 1, characterized in that: The operating wavelength range of the phase-shifting fiber grating (6) is covered by the gain wavelength range of the gain fiber (7), and the length of the phase-shifting fiber grating (6) is 2-4 cm shorter than the length of the gain fiber (7). The excess gain fiber portion is used for laser amplification and feedback modulation. The 3dB reflection spectrum of the phase-shifting fiber grating (6) is 0.04-0.25 nm, the peak reflectivity is 90-100%, and there is a transmission peak within its 3dB reflection spectrum. The absolute difference between the center wavelength of the transmission peak and the center wavelength of the slow-axis or fast-axis reflection spectrum of the polarization-maintaining fiber grating (4) is 0-0.02 nm.

9. A DFB linearly polarized single-frequency fiber laser based on polarization pulling effect according to claim 1, characterized in that: The heat sink (8) can control the temperature of the DFB fiber laser resonator to achieve laser wavelength tuning, with a temperature control accuracy of 0.01℃~0.05℃; the response wavelength range of the photodetector (9) covers the working wavelength range of the polarization-maintaining fiber grating (4) and the phase-shifting fiber grating (6).

10. A DFB linearly polarized single-frequency fiber laser based on polarization pulling effect according to claim 1, characterized in that: The formula for calculating the additional gain threshold of the DFB fiber laser resonator formed by the phase-shifting fiber grating (6) inscribed on the gain fiber (7) and the linear composite cavity formed by the polarization-maintaining fiber grating (4) is as follows: Where d is the effective length of the DFB fiber laser resonator, ω0 is the output angular frequency during free operation, ω is the output angular frequency during locking, τ is the laser round-trip time to the external cavity, r2 is the reflection coefficient at ω of the grating at the output end of the DFB fiber laser resonator, and the combined effective reflection coefficient at ω of the external cavity is r = r3e gL , where r3 is the reflection coefficient at ω of the polarization-maintaining fiber grating (4), g is the external cavity gain coefficient, and L is the external cavity length.

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