DFB linear polarization single-frequency fiber laser based on polarization traction effect

Abstract

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

Description

Technical Field

[0001] The present invention belongs to the field of single - frequency fiber lasers, and more particularly to a DFB linearly polarized single - frequency fiber laser based on the polarization - pulling effect. Background Art

[0002] At present, there is a high demand for linearly polarized single - frequency fiber laser sources in many important application fields, such as fiber optic gyroscopes, nonlinear frequency conversion, and coherent polarization beam combination. The resonant cavity structures of single - frequency fiber lasers include ring cavities and linear cavities. Among them, the ring cavity needs to introduce additional structures due to the lack of a frequency discrimination mechanism, and has the characteristics of a long cavity, high loss, and complex structure. The linear cavity can be divided into a distributed Bragg reflection (DBR) type and a distributed feedback (DFB) type. 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 in which a phase - shifted fiber grating is written, and the integration of gain and feedback avoids fusion splicing.

[0003] At present, the research and development progress of highly photosensitive polarization - maintaining gain fibers is limited, so the phase - shifted fiber grating is usually written on non - polarization - maintaining gain fibers. Therefore, the output polarization state of DFB fiber lasers is unstable. Applying torsional stress or local lateral stress to the phase - shifted fiber grating of a DFB fiber laser can achieve linearly polarized laser output. However, the current judgment basis for the scale or orientation of the applied stress is still vague, and it is inevitable that the laser is more sensitive to environmental disturbances. Polarization pulling has also been proven to be an effective means to achieve linearly polarized laser output, that is, using polarization feedback to pull the laser resonant cavity to generate laser oscillation of the corresponding polarization state, which has the characteristics of controllability and stability.

[0004] With the progress of grating writing technology and the development of fiber devices, DFB fiber lasers with various structures have achieved linearly polarized single - frequency laser output. For example, a linearly polarized single - frequency laser with a degree of linear polarization of 0.98 was obtained by using the self - injection ring structure of a DFB fiber laser [Laser Physics, 2011, 21(12): 2108 - 2111]. X.H. Li et al. realized the output of linearly polarized single - frequency laser with a power of 9.5 mW by using the composite linear structure of a DFB fiber laser resonant cavity and a fiber loop mirror [Laser Physics Letters, 2010, 7(1): 55 - 59]. The above achievements are all based on polarization controllers, which obtain linearly polarized laser by aligning the polarization direction of the controlled feedback light with the split orthogonal polarization directions in the DFB fiber laser resonant cavity. On the one hand, the uncertainty of the polarization direction limits the connection and use of the DFB fiber laser resonant cavity and polarization - maintaining optical devices. On the other hand, the phase - sensitive characteristics of optical feedback make the laser vulnerable to various disturbances. In summary, there is still a lack of simple, compact, high - degree - of - linear - polarization, and high - stability DFB linearly polarized single - frequency fiber lasers. Summary of the Invention

[0005] The present invention provides a DFB linearly polarized single-frequency fiber laser based on the polarization pulling effect to solve the defects in the prior art.

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

[0007] A DFB linearly polarized single-frequency fiber laser based on the polarization pulling effect includes a polarization-maintaining uniaxial fiber isolator. The input end of the polarization-maintaining uniaxial fiber isolator is connected to the signal end of a polarization-maintaining wavelength division multiplexer. The pump end of the polarization-maintaining wavelength division multiplexer is connected to the output end of a single-mode pump light source. The common end of the polarization-maintaining wavelength division multiplexer is connected to one end of a polarization-maintaining fiber grating. The other end of the polarization-maintaining fiber grating is connected to the output end of a phase-shifted fiber grating through a PZT phase modulator. The phase-shifted fiber grating is inscribed on a gain fiber to form a DFB fiber laser resonator and is fixedly encapsulated in a thermally controlled heat sink. The DFB fiber laser resonator is connected to the input end of a photodetector. The photodetector converts the idle-end laser signal of the DFB fiber laser resonator into an electrical signal. The output end of the photodetector is connected to the input end of a low-pass filter. The output end of the low-pass filter is connected to the positive input end of a differential amplifier. The negative input end of the differential amplifier is connected to a reference electrical signal source. The output end of the differential amplifier is connected to the input end of a PID operational amplifier. The output end of the PID operational amplifier is connected to the PZT phase modulator.

[0008] For a DFB linearly polarized single-frequency fiber laser based on the polarization pulling effect as described above, the operating wavelength range of the polarization-maintaining uniaxial fiber isolator covers the operating wavelength ranges of the polarization-maintaining fiber grating and the phase-shifted fiber grating.

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

[0010] For a DFB linearly polarized single-frequency fiber laser based on the polarization pulling effect as described above, the operating wavelength range of the polarization-maintaining wavelength division multiplexer covers the operating wavelength ranges of the single-mode pump light source, the polarization-maintaining fiber grating, and the phase-shifted fiber grating. The polarization-maintaining uniaxial 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 spliced by aligning the slow axes or all by aligning the fast axes.

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

[0012] A DFB linearly polarized single-frequency fiber laser based on the polarization pulling effect as described above, wherein the gain fiber is a non-polarization-maintaining gain fiber doped with rare-earth ions, and its matrix material is any one of quartz, phosphate, silicate, tellurite, fluoride or sulfide, or a mixture of any two or more of them 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 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.

[0013] A DFB linearly polarized single-frequency fiber laser based on the polarization pulling effect as described above, wherein the 3dB spectral width of the polarization-maintaining fiber grating reflection spectrum is 0.01 - 0.2 nm, its reflectivity to the laser signal wavelength is 1 - 20%, and the absolute value difference between the central wavelengths of the slow and fast axis reflection spectra is 0.25 - 0.5 nm.

[0014] A DFB linearly polarized single-frequency fiber laser based on the polarization pulling effect as described above, wherein the operating wavelength range of the phase-shifted fiber grating is covered by the gain wavelength range of the gain fiber, and the length of the phase-shifted fiber grating is 2 - 4 cm shorter than that of the gain fiber. The extra gain fiber part is used for laser amplification and feedback amount regulation; the 3dB reflection spectral width of the phase-shifted fiber grating is 0.04 - 0.25 nm, the peak reflectivity is 90 - 100%, and there is a transmission peak within its 3dB reflection spectral width. The absolute value difference between the central wavelength of the transmission peak and the central wavelength of the slow or fast axis reflection spectrum of the polarization-maintaining fiber grating is 0 - 0.02 nm.

[0015] A DFB linearly polarized single-frequency fiber laser based on the polarization pulling effect as described above, wherein the heat sink can control the temperature of the DFB fiber laser resonator to achieve laser wavelength tuning, and the temperature control accuracy is 0.01°C - 0.05°C; the response wavelength range of the photodetector covers the operating wavelength ranges of the polarization-maintaining fiber grating and the phase-shifted fiber grating.

[0016] A DFB linearly polarized single-frequency fiber laser based on the polarization pulling effect as described above, and the calculation formula for the additional gain threshold of the phase-shifted fiber grating inscribed on the gain fiber to form a linear composite cavity with the polarization-maintaining fiber grating and the DFB fiber laser resonator is:

[0017]

[0018] Wherein, d is the effective length of the DFB fiber laser resonator, ω0 is the output angular frequency during free running, ω is the output angular frequency during locking, τ is the round-trip time of the laser in 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 composite effective reflection coefficient r 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 the present invention are as follows: The present invention prepares a DFB fiber laser resonator by writing a phase-shifted fiber grating on a gain fiber. The grating region is only a few centimeters long and is integrated with the gain, avoiding splicing loss and thermal instability, and further ensuring the stable single-frequency performance of the laser. Moreover, in the present invention, the polarization-maintaining fiber grating and the DFB fiber laser resonator form a composite linear cavity, which has a simple and compact structure. It is beneficial to suppress noise and narrow the linewidth, and at the same time, through the polarization pulling effect, the polarization direction of the laser in the cavity is kept consistent with the polarization direction of the slow axis or fast axis of the grating, realizing the output of linearly polarized light. At the same time, in the present invention, the polarization-maintaining optical devices are spliced by aligning the slow axis or fast axis of the optical fiber, and the slow axis or fast axis of the polarization-maintaining single-axis optical fiber isolator works, ensuring a high degree of linear polarization of the laser and reducing the possibility of laser polarization state drift. In addition, the present invention also includes a closed-loop phase-locked control circuit for regulating the phase of the feedback light, further ensuring the long-term stability of the linearly polarized single-frequency laser output. Therefore, the present invention can provide a linearly polarized single-frequency laser source with excellent performance, simple and compact structure, high degree of linear polarization and high stability for fields such as fiber optic gyroscopes, non-linear frequency conversion and coherent polarized beam combination. Description of the Drawings

[0020] 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.

[0021] Figure 1 It is a schematic structural diagram of the present invention.

[0022] Reference numerals: 1, polarization-maintaining single-axis optical fiber isolator; 2, single-mode pump light source; 3, polarization-maintaining wavelength division multiplexer; 4, polarization-maintaining fiber grating; 5, PZT phase modulator; 6, phase-shifted 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 Embodiments

[0023] 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. Apparently, 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.

[0024] A DFB linearly polarized single-frequency fiber laser based on the polarization pulling effect includes a polarization-maintaining uniaxial fiber isolator 1. The input end of the polarization-maintaining uniaxial 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 light 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-shifted fiber grating 6 through a PZT phase modulator 5. The phase-shifted fiber grating 6 is written on a gain fiber 7 to form a DFB fiber laser resonator and is fixedly encapsulated in a thermally controlled heat sink 8. The DFB fiber laser resonator is connected to the input end of a photodetector 9. The photodetector 9 converts the idle-end 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 a low-pass filter 10. The output end of the low-pass filter 10 is connected to the positive input end of a differential amplifier 12. The negative input end of the differential amplifier 12 is connected to a reference electrical signal source 11. The output end of the differential amplifier 12 is connected to the input end of a PID operational amplifier 13. The output end of the PID operational amplifier 13 is connected to the PZT phase modulator 5. The phase-shifted fiber grating 6 written on the gain fiber 7 to form the DFB fiber laser resonator, the photodetector 9, the low-pass filter 10, the differential amplifier 12, the PID operational amplifier 13, and the PZT phase modulator 5 can form a closed-loop phase-locked control circuit. The photodetector 9 converts the idle-end laser signal of the DFB fiber laser resonator formed by writing the phase-shifted fiber grating 6 on the gain fiber 7 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 to achieve feedback optical phase control. The output laser of the DFB fiber laser resonator formed by writing the phase-shifted fiber grating 6 on the gain fiber 7 is fed back and injected back by the polarization-maintaining fiber grating 4 after passing through the PZT phase modulator 5. The laser in the cavity is sequentially passed through the PZT phase modulator 5, the polarization-maintaining fiber grating 4, the polarization-maintaining wavelength division multiplexer 3, and the polarization-maintaining uniaxial fiber isolator 1 under the traction of the slow-axis or fast-axis polarization feedback light to form a stable linearly polarized single-frequency laser output.

[0025] Preferably, the operating wavelength range of the polarization-maintaining uniaxial optical fiber isolator 1 of the present invention covers the operating wavelength ranges of the polarization-maintaining fiber grating 4 and the phase-shifted fiber grating 6, and its slow axis operates while the fast axis is cutoff or the fast axis operates while the slow axis is 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 light source 2 of the present invention is any one of a semiconductor laser single-mode pump light source, a fiber laser single-mode pump light source, a gas laser single-mode pump light source, or a solid laser single-mode pump light source.

[0027] Preferably, the operating wavelength range of the polarization-maintaining wavelength division multiplexer 3 of the present invention covers the operating wavelength ranges of the single-mode pump light source 2, the polarization-maintaining fiber grating 4, and the phase-shifted fiber grating 6. The polarization-maintaining uniaxial optical 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 spliced by aligning the slow axes or all by aligning the fast axes.

[0028] Preferably, the PZT phase modulator 5 of the present invention is used to control the phase of the feedback light 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 of quartz, phosphate, silicate, tellurite, fluoride, or sulfide, or a mixture of any two or more of them in any proportion; the core of the gain fiber 7 is uniformly doped with rare earth ions and has ultraviolet photosensitivity, and its diameter is 3 - 10 μm. The doped rare earth ions are 3+ Yb 3+ Er 3+ Tm 3+ Nd 3+ Pr 3+ Eu 3+ Dy 3+ Any one of them or a mixture of any two or more of them in any proportion.

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

[0031] Preferably, the operating wavelength range of the phase-shifted 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-shifted fiber grating 6 is 2-4 cm shorter than that of the gain fiber 7. The extra gain fiber part is used for laser amplification and feedback amount regulation; the 3dB reflection spectrum width of the phase-shifted fiber grating 6 is 0.04-0.25 nm, the peak reflectivity is 90-100%, and there is a transmission peak within the 3dB reflection spectrum width. The absolute value difference between the central wavelength of the transmission peak and the central wavelength of the reflection spectrum of the slow axis or fast axis 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, and the temperature control accuracy is 0.01°C to 0.05°C; the response wavelength range of the photodetector 9 covers the operating wavelength ranges of the polarization-maintaining fiber grating 4 and the phase-shifted 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 calculation formula for the additional gain threshold of the DFB fiber laser resonator formed by inscribing the phase-shifted fiber grating 6 on the gain fiber 7 and the linear composite cavity formed by the polarization-maintaining fiber grating 4 in this embodiment is:

[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 external cavity time, r2 is the reflection coefficient at ω of the grating at the output end of the DFB fiber laser resonator, and the composite effective reflection coefficient r 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.

[0036] Example:

[0037] This embodiment includes a polarization-maintaining single-axis optical 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-shifted 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 optical fiber isolator 1 and the output end of the single-mode pump light source 2 are respectively connected to the signal end and the pump end of the polarization-maintaining wavelength division multiplexer 3. 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-shifted fiber grating 6 through the PZT phase modulator 5. The phase-shifted fiber grating 6 is inscribed on the gain fiber 7 to form a DFB fiber laser resonator and is fixedly encapsulated in the automatically temperature-controlled heat sink 8 to ensure the stable operation of 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 optical fiber isolator 1 serves as the output end of the DFB linearly polarized single-frequency fiber laser. At the same time, it also includes a closed-loop phase-locked control circuit. The photodetector 9 converts the laser signal at the idle end 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 adopts a polarization feedback and closed-loop phase-locked control scheme using a polarization-maintaining fiber grating. The polarization-maintaining fiber grating 4 reflects the forward output light at the output end of the phase-shifted fiber grating 6. The single-mode pump light source 2 provides backward pumping for the DFB fiber laser resonator through the polarization-maintaining wavelength division multiplexer 3. The closed-loop phase-locked control circuit converts and differentiates the laser signal at the idle end of the DFB fiber laser resonator and the reference signal to form an error signal, and controls the PZT phase modulator 5 to match the optical feedback phase and the laser frequency to achieve locking.

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

[0040] In this example, the central working wavelength of the polarization-maintaining single-axis optical fiber isolator 1 is 1064 nm, and it operates on the slow axis while the fast axis is cutoff.

[0041] In this example, the central working wavelength of the polarization-maintaining wavelength division multiplexer 3 is 974 / 1064 nm, 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 is selected as the gain fiber 7, and its core has photosensitivity.

[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 to 5% - 15%, expanding the range and degree of influence of the polarization pulling effect.

[0044] In this example, the 4-cm phase-shifted fiber grating 6 is inscribed on the 6 - 8-cm gain fiber 7. The gain margin can be used for laser amplification, and at the same time, by changing the pump laser power, its gain coefficient can be affected, thereby regulating the feedback amount. The phase shift amount of the phase-shifted fiber grating 6 is π. There is a transmission peak near the center position of the reflection spectrum. Its 3-dB reflection spectral width is 0.1 - 0.15 nm, and the peak reflectivity is 97% - 98%. During the ultraviolet exposure inscription process, a segmented super-Gaussian apodization amplitude plate is used to apodize and compensate 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 a low 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 1064 nm, and the absolute difference is 0 - 0.02 nm.

[0045] In this example, the pigtails of the polarization-maintaining single-axis optical fiber isolator 1, the polarization-maintaining wavelength division multiplexer 3, and the polarization-maintaining fiber grating 4 are all single-mode polarization-maintaining fibers, and they are fusion-spliced by aligning the slow axes, with good mode matching and no splicing errors.

[0046] In this example, the response wavelength range of the photodetector 9 covers 1064 nm.

[0047] In this example, the determination method of the output electrical signal of the reference electrical signal source 11 is as follows: A sawtooth wave signal is applied to the PZT phase modulator 5 to make the feedback light phase 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 wave peaks within a single period. Appropriately select the corresponding electrical signal peak value as the reference electrical signal value.

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

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

[0050] The closed-loop phase-locked control circuit converts and differentiates the laser signal at the idle end of the DFB fiber laser and the reference signal to form an error signal, and controls the PZT phase modulator 5 to match the optical feedback phase and the laser frequency to achieve locking.

[0051] Among them, 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] Among them, d is the effective length of the DFB fiber laser resonator, ω0 is the output angular frequency during free running, ω is the output angular frequency during locking, τ is the laser round-trip external cavity time, r2 is the reflection coefficient at ω of the grating at the output end of the DFB fiber laser resonator, and the composite effective reflection coefficient r 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.

[0054] The additional gain threshold G EXCESS is a periodic function of the phase difference ω0τ:

[0055]

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

[0057]

[0058] At this time, G EXCESS ′(r) < 0, and the additional gain coefficient G EXCESS decreases as the composite effective reflection coefficient r increases. When the laser polarization direction in the DFB fiber laser resonator is aligned with the slow axis direction of the polarization-maintaining fiber grating 4, r is the largest. At this time, this polarization mode is most likely to oscillate. The pump laser input power can be controlled to change the external cavity gain coefficient g and regulate the composite effective reflection coefficient r to achieve a more effective polarization pulling effect, generating and controlling linearly polarized single-frequency laser with high linear polarization degree and high stability.

[0059] In summary, compared with the prior art, the present invention prepares a DFB fiber laser resonator by writing a phase-shifted fiber grating 6 on a gain fiber 7. The grating region is only a few centimeters long and is integrated with the gain, avoiding fusion loss and thermal instability, further ensuring the stable single-frequency performance of the laser. Moreover, the polarization-maintaining fiber grating 4 and the DFB fiber laser resonator form a composite linear cavity, which is simple and compact. It is beneficial to suppress noise and narrow the linewidth, and through the polarization pulling effect, the polarization direction of the laser in the cavity is kept consistent with the polarization direction of the slow axis of the grating, realizing the output of slow-axis polarized light. At the same time, the polarization-maintaining optical devices are fused by aligning the slow axes of the fibers, and the slow axis of the polarization-maintaining uniaxial fiber isolator 1 works, ensuring a high degree of linear polarization of the laser and reducing the possibility of laser polarization state drift. This laser is suitable for large-scale commercial production.

[0060] In addition, the present invention also includes a closed-loop phase-locked control circuit for regulating the phase of the feedback light, further ensuring the long-term stability of the output of linearly polarized single-frequency laser. Therefore, the present invention can provide a linearly polarized single-frequency laser source with excellent performance, simple and compact structure, high degree of linear polarization and high stability for fields such as fiber optic gyroscopes, nonlinear frequency conversion and coherent polarized beam combination.

[0061] 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 recorded in the foregoing embodiments, or perform equivalent replacements for some of the technical features. However, such modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A DFB linearly polarized single-frequency fiber laser based on polarization pulling effect, characterized in that: The invention comprises a polarization-maintaining single-axis optical fiber isolator (1), wherein the input end of the polarization-maintaining single-axis optical 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 light source (2), the common end of the polarization-maintaining wavelength division multiplexer (3) is connected to one end of a polarization-maintaining optical fiber grating (4), the other end of the polarization-maintaining optical fiber grating (4) is connected to the output end of a phase-shifting optical fiber grating (6) through a PZT phase modulator (5), the phase-shifting optical fiber grating (6) is inscribed on a gain optical fiber (7) to form a DFB optical fiber laser resonant cavity and is fixedly packaged in an automatic temperature-controlled heat sink (8), the DF The B fiber laser resonant cavity is connected to the input end of a photodetector (9), the photodetector (9) converts the idle end laser signal of the DFB fiber laser resonant cavity into an electrical signal, the output end of the photodetector (9) is connected to the input end of a low-pass filter (10), the output end of the low-pass filter (10) is connected to the positive input end of a differential amplifier (12), the negative input end of the differential amplifier (12) is connected to a reference electrical signal source (11), the output end of the differential amplifier (12) is connected to the input end of a PID operational amplifier (13), and the output end of the PID operational amplifier (13) is connected to a 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 optical fiber isolator (1) covers the operating wavelength ranges of the polarization-maintaining optical fiber grating (4) and the phase-shifting optical fiber grating (6).

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

4. 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 wavelength division multiplexer (3) covers the operating wavelength ranges 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 optical 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 by slow-axis alignment or fast-axis alignment.

5. The 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 phase of feedback light injected from the polarization-maintaining fiber grating (4) back into the DFB fiber laser resonant cavity.

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

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

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

9. The 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 resonant cavity to achieve laser wavelength tuning, and the temperature control accuracy is 0.01° C. to 0.05° C.; the response wavelength range of the photodetector (9) covers the operating wavelength range of the polarization-maintaining fiber grating (4) and the phase-shifted fiber grating (6).

10. The DFB linearly polarized single-frequency fiber laser based on polarization pulling effect according to claim 1, characterized in that: The calculation formula for the additional gain threshold of the phase-shifted fiber grating (6) written on the gain fiber (7) to form a DFB fiber laser resonant cavity and the polarization-maintaining fiber grating (4) to form a linear composite cavity is: Where d is the effective length of the DFB fiber laser resonator, ω0 is the output angular frequency when in free operation, ω is the output angular frequency when locked, τ is the time it takes for the laser to travel back and forth to the external cavity, r2 is the reflection coefficient of the grating at the output end of the DFB fiber laser resonator at ω, and the composite effective reflection coefficient at the external cavity ω 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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