Eight-Wavelength Switchable Ultra-Narrow Linewidth Fiber Laser Based on Scattering-Enhanced Fiber

Abstract

The present invention discloses an eight-wavelength switchable ultra-narrow linewidth fiber laser based on scattering-enhanced fiber, belonging to the field of single-frequency fiber lasers. It includes a switchable polarization-dependent eight-channel filter fabricated by combining parallel FBGs written in a single-mode fiber using femtosecond laser direct writing technology with an electro-optic switch as a wavelength selection device, and a scattering-enhanced fiber fabricated by inducing randomly distributed and randomly intense high-scattering centers in a single-mode fiber using femtosecond laser direct writing technology as a laser linewidth narrowing device. By adopting the above-mentioned eight-wavelength switchable ultra-narrow linewidth fiber laser based on scattering-enhanced fiber, the present invention realizes the output of single-frequency ultra-narrow linewidth laser with eight switchable wavelengths.

Description

Technical Field

[0001] The present invention relates to the technical field of single-frequency fiber lasers, and in particular to an eight-wavelength switchable ultra-narrow linewidth fiber laser based on scattering-enhanced fiber. Background Art

[0002] Single-frequency narrow linewidth fiber lasers have unique properties of high coherence, low noise, and strong compatibility with fiber systems, and can be used in high-capacity ultra-long-distance coherent optical communication, high-precision optical metrology and spectroscopy, long-distance high-resolution distributed fiber sensing, coherent Doppler lidar, gravitational wave measurement, and other applications related to optical atomic clocks, fundamental constant measurement, and physics. With the continuous improvement of the performance of related applications, it is required that single-frequency narrow linewidth fiber lasers simultaneously have performance such as wavelength tuning / scanning / switching, dual / multi-wavelength operation, and moreover, have ultra-narrow linewidth output in various modes, which poses high requirements for linewidth narrowing technology.

[0003] For the realization of the multi-wavelength switchable laser output function, a typical method is to introduce a multi-channel fiber Bragg grating (FBG) filter in the laser system for wavelength selection and filtering. Most of the FBG devices proposed at present are fabricated in single-mode fiber or polarization-maintaining fiber based on the phase mask method. Among them, the device with polarization-dependent characteristics, combined with a polarization controller, is the most effective method to achieve wavelength switching. In recent years, femtosecond laser direct writing technology has been widely used in FBG fabrication, greatly increasing the fabrication flexibility of new FBG multi-channel filters, and moreover, functional filtering devices that cannot be realized by the phase mask method at all can be fabricated. Using femtosecond laser direct writing to fabricate a new type of multi-channel polarization-dependent FBG filter can achieve higher-performance laser output.

[0004] Linewidth narrowing is the core technology of ultra-narrow linewidth fiber lasers. The methods that have been proposed include slow light effect, self-injection locking, saturable absorber, and Pound-Drever-Hall (PDH) frequency stabilization, etc. It is very difficult to compress the linewidth to the order of hundreds of Hz by the first three methods. The PDH technology can compress the linewidth to the order of millihertz (mHz), but it requires an ultra-stable cavity and a complex optoelectronic auxiliary servo system, and the working conditions are very harsh, making it impossible to be applied in a normalized batch manner. In recent years, the laser linewidth narrowing method based on the random distributed feedback of Rayleigh scattering in optical fibers has been widely studied, and it is expected to achieve a deep narrowing of the linewidth for normalized applications. However, the random distributed feedback based on the Rayleigh scattering of a single-mode fiber itself requires a length of hundreds or even thousands of meters to achieve sufficient feedback accumulation, which will not only reduce the anti-environmental disturbance ability of the fiber laser but also be unfavorable for the miniaturization and integration of the system. Our research group previously induced high-scattering centers in single-mode fibers using advanced femtosecond laser direct writing technology, developed randomly distributed feedback scattering-enhanced fibers, and introduced them into single-longitudinal-mode fiber lasers for linewidth narrowing, obtaining an ultra-narrow linewidth laser output with <150 Hz, and the overall output performance of the laser is excellent. However, there is no report on achieving a deep narrowing of the linewidth of a large-range multi-wavelength switchable fiber laser by using a scattering-enhanced fiber in combination with a novel multi-channel FBG filter fabricated by femtosecond laser. This is of great value for the development of a large-range tunable ultra-narrow linewidth fiber laser, the application of the scattering-enhanced fiber in a broadband wavelength tunable / scanning fiber laser, and the popularization and application of laser linewidth narrowing in different wavelength bands. Summary of the Invention

[0005] The purpose of the present invention is to provide an eight-wavelength switchable ultra-narrow linewidth fiber laser based on a scattering-enhanced fiber to solve the problems mentioned in the above background technology.

[0006] To achieve the above object, the present invention provides an eight-wavelength switchable ultra-narrow linewidth fiber laser based on a scattering-enhanced optical fiber, which includes a pump source, a wavelength division multiplexer, a polarization controller for coiling an erbium-doped fiber, an optical fiber polarizer, a squeezing type polarization controller, a fiber coupler group, an optical fiber circulator, a scattering-enhanced optical fiber, an electro-optic switch, a voltage driver, a parallel fiber grating group, a fiber coupler five, and an FC / APC type optical fiber connector three. The output pigtail of the pump source is connected to the pump light input end of the wavelength division multiplexer. The output end of the wavelength division multiplexer is connected to one end of the optical fiber pigtail of the polarization controller for coiling the erbium-doped fiber. The signal light input end of the wavelength division multiplexer is connected to the input end of the optical fiber polarizer. The output end of the optical fiber polarizer is connected to one end of the squeezing type polarization controller. The other end of the squeezing type polarization controller is connected to the fiber coupler group. The end of the fiber coupler group far from the squeezing type polarization controller is connected to port one of the optical fiber circulator. Port two of the optical fiber circulator is connected to one end of the scattering-enhanced optical fiber. The other end of the scattering-enhanced optical fiber is connected to port one of the electro-optic switch. Port two and port three of the electro-optic switch are connected to the parallel fiber grating group. Port three of the optical fiber circulator is connected to port one of the fiber coupler five. Port two of the fiber coupler five is connected to the other end of the polarization controller for coiling the erbium-doped fiber. Port three of the fiber coupler five is connected to the pigtail of the FC / APC type optical fiber connector three.

[0007] The electro-optic switch is provided with a voltage drive signal by the voltage driver and is connected through an electric wire.

[0008] Preferably, the fiber coupler group includes a fiber coupler one, a fiber coupler two, a fiber coupler three, and a fiber coupler four. The squeezing type polarization controller is connected to port one of the fiber coupler one. Port two of the fiber coupler one is connected to port four of the fiber coupler two. Port three of the fiber coupler one is connected to port one of the fiber coupler two. Port three of the fiber coupler two is connected to port one of the fiber coupler three. Port two of the fiber coupler three is connected to port four of the fiber coupler four. Port three of the fiber coupler three is connected to port one of the fiber coupler four. Port three of the fiber coupler four is connected to port one of the optical fiber circulator.

[0009] Preferably, the pigtails of port four of the fiber coupler one, port two of the fiber coupler two, port four of the fiber coupler three, and port two of the fiber coupler four are all processed at an 8-degree angle.

[0010] Preferably, the parallel fiber grating group includes a parallel fiber grating one and a parallel fiber grating two. Port two of the electro-optic switch is connected to one end of the parallel fiber grating one. The other end of the parallel fiber grating one is connected to an FC / APC type optical fiber connector one. Port three of the electro-optic switch is connected to one end of the parallel fiber grating two. The other end of the parallel fiber grating two is connected to an FC / APC type optical fiber connector two.

[0011] Preferably, the pump source is a 980 nm semiconductor laser.

[0012] Preferably, the rare earth doped optical fibers coiled around the three fiber disks in the polarization controller for the coiled erbium-doped optical fiber are erbium-doped optical fibers or erbium-ytterbium co-doped optical fibers.

[0013] Preferably, the difference in the loop lengths between the optical fiber loop formed by optical fiber coupler one and optical fiber coupler two and the optical fiber loop formed by optical fiber coupler three and optical fiber coupler four is 1 cm.

[0014] Preferably, both the parallel fiber grating one and the parallel fiber grating two belong to narrowband four-channel fiber grating filters; both the parallel fiber grating one and the parallel fiber grating two use the femtosecond laser point-by-point method to symmetrically write two uniform fiber gratings with different periods on both sides of the optical fiber center line; the switching use of the parallel fiber grating one and the parallel fiber grating two in the fiber laser system is controlled by an electro-optic switch.

[0015] Preferably, the wavelength regions where the reflection filtering channels of the parallel fiber grating one and the parallel fiber grating two are concentrated do not overlap.

[0016] Preferably, the optical fiber coupler five is a 3-port optical fiber coupler with a splitting ratio of 90:10, and the 10% port is used for laser output.

[0017] Therefore, the present invention adopts the above-mentioned eight-wavelength switchable ultra-narrow linewidth fiber laser based on scattering-enhanced optical fiber, and has the following beneficial effects:

[0018] (1) When the fiber laser operates, only the output power of the 980 nm pump source needs to be adjusted above the laser threshold. By switching the working channels of the electro-optic switch and simply adjusting the polarization controller for the coiled erbium-doped optical fiber and the extrusion-type polarization controller, single-frequency ultra-narrow linewidth laser output with eight switchable wavelengths can be obtained.

[0019] (2) Utilizing the flexible and adjustable characteristics of the femtosecond laser direct technology, the reflection channels of the parallel fiber grating one and two can be arbitrarily adjusted, and the random distributed feedback scattering characteristics of the scattering-enhanced optical fiber can also be arbitrarily adjusted. It is easy to achieve ultra-narrow linewidth switchable operation of any eight wavelengths, and has potential application value in the fields of space optical communication, fiber sensing, etc.

[0020] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Description of the Drawings

[0021] Figure 1 It is a schematic structural diagram of an eight-wavelength switchable ultra-narrow linewidth fiber laser based on scattering-enhanced optical fiber according to an embodiment of the present invention;

[0022] Figure 2Schematic diagram of the femtosecond laser fabrication method for parallel fiber grating 1 in the embodiment of the present invention;

[0023] Figure 3 Transmission and reflection spectra of parallel fiber grating 1 in the embodiment of the present invention;

[0024] Figure 4 Transmission and reflection spectra of parallel fiber grating 2 in the embodiment of the present invention;

[0025] Figure 5 Backscattering measurement results of the scattering enhanced fiber in the embodiment of the present invention;

[0026] Figure 6 Spectral diagram of the switching operation of eight-wavelength lasers output by the fiber laser in the embodiment of the present invention;

[0027] Figure 7 Test result diagram of the linewidth narrowing effect of the eight-wavelength single-frequency laser by the scattering enhanced fiber in the embodiment of the present invention. Among them, Figure a is the measurement result of the laser λ1 linewidth using the scattering enhanced fiber, Figure b is the measurement result of the laser λ1 linewidth without using the scattering enhanced fiber, Figure c is the measurement result of the laser λ1 linewidth using a single-mode fiber instead of the scattering enhanced fiber, and Figure d is the comparison result of the linewidths of the eight-wavelength lasers measured with and without using the scattering enhanced fiber;

[0028] Figure 8 Schematic diagram of the backscattering Rayleigh distribution of the 11.5 m scattering enhanced fiber measured by the optical frequency domain reflectometer in Embodiment 2 of the present invention;

[0029] Figure 9 Schematic diagram of measuring the output linewidth of laser λ1 using the delayed self-heterodyne system in Embodiment 2 of the present invention;

[0030] Reference numerals

[0031] 1. Pump source; 2. Wavelength division multiplexer; 3. Polarization controller for coiled erbium-doped fiber; 4. Fiber polarizer; 5. Squeezing type polarization controller; 6. Fiber coupler 1; 7. Fiber coupler 2; 8. Fiber coupler 3; 9. Fiber coupler 4; 10. Fiber circulator; 11. Scattering enhanced fiber; 12. Electro-optic switch; 13. Voltage driver; 14. Parallel fiber grating 1; 15. FC / APC type fiber connector 1; 16. Parallel fiber grating 2; 17. FC / APC type fiber connector 2; 18. Fiber coupler 5; 19. FC / APC type fiber connector 3; 20. Femtosecond laser; 21. Focusing lens; 22. Single-mode fiber; 23. Core. Detailed implementation manners

[0032] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. 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.

[0033] Embodiment 1:

[0034] Referring to Figure 1 , the present invention discloses an eight-wavelength switchable ultra-narrow linewidth fiber laser based on scattering-enhanced fiber, including a pump source 1, a wavelength division multiplexer 2, a polarization controller 3 for coiling erbium-doped fiber, an optical fiber polarizer 4, a squeezing type polarization controller 5, a fiber coupler group, an optical fiber circulator 10, a scattering-enhanced fiber 11, an electro-optic switch 12, a voltage driver 13, a parallel fiber grating group, a fiber coupler five 18, and an FC / APC type fiber connector three 19. The output pigtail of the pump source 1 is connected to the pump light input end of the wavelength division multiplexer 2. The output end of the wavelength division multiplexer 2 is connected to one end of the fiber pigtail of the polarization controller 3 for coiling erbium-doped fiber. The signal light input end of the wavelength division multiplexer 2 is connected to the input end of the optical fiber polarizer 4. The output end of the optical fiber polarizer 4 is connected to one end of the squeezing type polarization controller 5. The other end of the squeezing type polarization controller 5 is connected to the fiber coupler group. The fiber coupler group includes a fiber coupler one 6, a fiber coupler two 7, a fiber coupler three 8, and a fiber coupler four 9. The squeezing type polarization controller 5 is connected to port one of the fiber coupler one 6. Port two of the fiber coupler one 6 is connected to port four of the fiber coupler two 7. Port three of the fiber coupler one 6 is connected to port one of the fiber coupler two 7. Port three of the fiber coupler two 7 is connected to port one of the fiber coupler three 8. Port two of the fiber coupler three 8 is connected to port four of the fiber coupler four 9. Port three of the fiber coupler three 8 is connected to port one of the fiber coupler four 9. Port three of the fiber coupler four 9 is connected to port one of the optical fiber circulator 10.

[0035] The pigtails of port four of the fiber coupler one 6, the pigtails of port two of the fiber coupler two 7, the pigtails of port four of the fiber coupler three 8, and the pigtails of port two of the fiber coupler four 9 are all processed at an 8-degree angle.

[0036] Port two of the optical fiber circulator 10 is connected to one end of the scattering-enhanced optical fiber 11. The other end of the scattering-enhanced optical fiber 11 is connected to port one of the electro-optic switch 12. Port two and port three of the electro-optic switch 12 are connected to the parallel fiber grating group. The parallel fiber grating group includes the parallel fiber grating one 14 and the parallel fiber grating two 16. Port two of the electro-optic switch 12 is connected to one end of the parallel fiber grating one 14. The other end of the parallel fiber grating one 14 is connected with an FC / APC type optical fiber connector one 15. Port three of the electro-optic switch 12 is connected to one end of the parallel fiber grating two 16. The other end of the parallel fiber grating two 16 is connected with an FC / APC type optical fiber connector two 17. Port three of the optical fiber circulator 10 is connected to port one of the optical fiber coupler five 18. Port two of the optical fiber coupler five 18 is connected to the other end of the polarization controller 3 coiled with the erbium-doped optical fiber. Port three of the optical fiber coupler five 18 is connected to the pigtail of the FC / APC type optical fiber connector three 19. The electro-optic switch 12 is provided with a voltage drive signal by the voltage driver 13 and is connected through an electric wire. All the above connections are fusion splicing connections using an optical fiber fusion splicer, and partial or all of them can also be connected using optical fiber connectors.

[0037] The wavelength division multiplexer 2 is of the 2×1 type and includes a pump light input end, a signal light input end, and an output end.

[0038] The pump source 1 is a 980nm semiconductor laser with a maximum output power of 550mW. The rare earth doped optical fiber coiled in the three optical fiber coils of the polarization controller 3 coiled with the erbium-doped optical fiber is erbium-doped optical fiber or erbium-ytterbium co-doped optical fiber, and the number of coiled fiber turns is 2 turns, 4 turns, and 2 turns in sequence.

[0039] The ring length of the optical fiber loop formed by the optical fiber coupler one 6 and the optical fiber coupler two 7 is 60cm. The ring length of the optical fiber loop formed by the optical fiber coupler three 8 and the optical fiber coupler four 9 is 61cm. The cross-coupling ratio of the four optical fiber couplers is all 90:10. The four optical fiber couplers are combined into a double-ring composite resonator (DCR-CC) filter for single longitudinal mode selection of an optical fiber laser. After calculation, the free spectral range (FSR) of the DCR-CC is about 20.46GHz, and the filtering bandwidth (FWHM) is about 8.75MHz.

[0040] As Figure 2As shown, the parallel fiber Bragg grating 14 (P-FBG-1) is fabricated using the femtosecond laser direct writing technique. Specifically, the femtosecond laser 20 is focused into the core 23 of a single-mode fiber 22 (SMF) 203 through a focusing lens 21. Uniform fiber Bragg gratings (grating 1 and grating 2) with periods Λ1 = 1.0409 μm and Λ2 = 1.0422 μm are symmetrically written on both sides of the fiber centerline using the point-by-point writing method. The distances of the two fiber Bragg gratings from the centerline are both 1 μm. Due to the structural asymmetry of the distribution of the two fiber Bragg gratings in the fiber core 23 region, high birefringence characteristics are exhibited.

[0041] As Figure 3 shown, the transmission and reflection spectra of the parallel fiber Bragg grating 14 are measured using linearly polarized light with perpendicular polarization directions (X-pol. and Y-pol.). It can be seen that the parallel fiber Bragg grating 14 is a narrowband four-channel fiber Bragg grating filter. The central wavelengths of the four reflection channels are 1528.111 nm, 1528.233 nm, 1530.122 nm, and 1530.241 nm, respectively, concentrated around 1530 nm. The reflection bandwidths are all less than 0.280 nm, and the calculated reflectivities are all greater than 90%.

[0042] The fabrication process of the parallel fiber Bragg grating 16 (P-FBG-2) is the same as that of the parallel fiber Bragg grating 14, but the periods of the written uniform fiber Bragg gratings (grating 3 and grating 4) are Λ3 = 1.0555 μm and Λ4 = 1.0559 μm, respectively.

[0043] As Figure 4 shown, the transmission and reflection spectra of the parallel fiber Bragg grating 16 are measured using linearly polarized light with perpendicular polarization directions (X-pol. and Y-pol.). It can be seen that the parallel fiber Bragg grating 16 is a narrowband four-channel fiber Bragg grating filter. The central wavelengths of the four reflection channels are 1547.956 nm, 1548.163 nm, 1549.917 nm, and 1550.131 nm, respectively, concentrated around 1550 nm. The reflection bandwidths are all less than 0.280 nm, and the calculated reflectivities are all greater than 90%.

[0044] The parallel fiber Bragg grating 14 and the parallel fiber Bragg grating 16 are combined with the electro-optic switch 12 to form a switchable polarization-dependent eight-channel filter (S-PDECF). The electro-optic switch 12 can switch between using the 1→2 channel and the 1→3 channel, thereby realizing the switching operation of the parallel fiber Bragg grating 14 and the parallel fiber Bragg grating 16.

[0045] The scattering enhanced fiber 11 (SEF) introduces high scattering points randomly in the core 23 region of the single-mode fiber 22 by using the femtosecond laser 20 point-by-point method. The specific method is to randomly control the average output power of the femtosecond laser 20 to vary within a certain appropriate range, irradiate with a single laser pulse and induce a micro refractive index change region point-by-point in the single-mode core 23, randomly control the spatial position of the high scattering points in the core 23 region, and control the spacing between adjacent high scattering points to be 8 - 10 cm. The length of the fabricated scattering enhanced fiber 11 is 22.3 m;

[0046] As Figure 5 shown, the random Rayleigh scattering distribution of the scattering enhanced fiber 11 measured by the optical frequency domain reflectometer shows that the scattering intensity of the high scattering points introduced by the femtosecond laser 20 is increased by about 45 dB overall compared to the intrinsic scattering of the single-mode fiber 22, and the scattering characteristics have a strong random distribution characteristic;

[0047] The fiber coupler five 18 is a 3-port fiber coupler with a splitting ratio of 90:10, and the 10% port is used for laser output.

[0048] Set the output power of the pump source 1 to 180 mW. By switching the working channels of the electro-optic switch 12 and carefully adjusting the states of the polarization controller 3 coiling the erbium-doped fiber and the extrusion-type polarization controller 5, laser outputs at eight wavelengths are successfully obtained;

[0049] As Figure 6 shown, the output spectra of the 8 laser wavelengths measured by the spectral analyzer. The 8 laser wavelengths λ1, λ2, λ3, λ4, λ5, λ6, λ7 and λ8 are 1528.111 nm, 1528.233 nm, 1530.122 nm, 1530.241 nm, 1547.956 nm, 1548.163 nm, 1549.917 nm and 1550.131 nm respectively. The optical signal-to-noise ratio of the laser in each wavelength output mode is ≥ 71 dB, showing excellent beam quality;

[0050] As Figure 7As shown in the figure, the output laser linewidth of a fiber laser is measured using the delayed self-heterodyne method. Figure a shows the measurement result of the laser λ1 linewidth using the scattering-enhanced fiber 11, Figure b shows the measurement result of the laser λ1 linewidth without using the scattering-enhanced fiber 11, Figure c shows the measurement result of the laser λ1 linewidth using a single-mode fiber 22 instead of the scattering-enhanced fiber 11, and Figure d shows the comparison result of the linewidths of 8-wavelength lasers measured with and without using the scattering-enhanced fiber 11. It can be seen that the scattering-enhanced fiber 11 can narrow the linewidths of all lasers with a wavelength range span greater than 20 nm to the same level (<250 Hz), indicating that the scattering-enhanced fiber 11 has weak wavelength correlation in linewidth narrowing, can achieve wavelength-adaptive linewidth narrowing within a broadband wavelength tunable range, and has higher stability and stronger potential for miniaturized integration compared to the single-mode fiber 22.

[0051] Example 2:

[0052] The femtosecond laser 20 direct writing technology for fabricating the scattering-enhanced fiber 11 has very flexible adjustable characteristics. The overall cavity length of the fiber laser and the output mode stability must be inversely proportional. The longer the laser cavity length, the weaker the anti-environmental perturbation ability. Therefore, the shorter the scattering-enhanced fiber 11 used under the condition of achieving the same linewidth narrowing effect, the better. While keeping the high number of scattering points fabricated in the scattering-enhanced fiber 11 unchanged, the adjacent point spacing was shortened to 5 - 6 cm, and a scattering-enhanced fiber 11 with a length of 11.5 m was fabricated and introduced into the eight-wavelength switchable single-frequency fiber laser of this example for linewidth narrowing.

[0053] As Figure 8 shown, the backward Rayleigh scattering distribution of the 11.5 m scattering-enhanced fiber 11 was measured using an optical frequency domain reflectometer. It can be seen that its overall scattering enhancement amplitude is basically the same as that of the scattering-enhanced fiber 11 used in Example 1.

[0054] As Figure 9 shown, the output linewidth of the laser λ1 was measured using a delayed self-heterodyne system. Still taking the laser λ1 as an example, it can be seen that the 11.5 m scattering-enhanced fiber 11 still has an obvious linewidth narrowing effect, with a linewidth value of 239 Hz, and the linewidth value after narrowing is similar to the result of linewidth narrowing using the 22.3 m scattering-enhanced fiber 11 in Example 1, and the narrowing effect is almost the same. This result proves that the scattering-enhanced fiber 11 is expected to achieve miniaturization of devices in deep laser linewidth narrowing, and is more conducive to the optimal design and development of ultra-narrow linewidth fiber lasers.

[0055] Therefore, the present invention adopts the above-mentioned eight-wavelength switchable ultra-narrow linewidth fiber laser based on scattering-enhanced fiber, uses the parallel FBG inscribed in the single-mode fiber by femtosecond laser direct writing technology and an electro-optic switch to fabricate a switchable polarization-dependent eight-channel filter as a wavelength selection device, and uses the scattering-enhanced fiber fabricated by inducing randomly distributed and randomly intense high-scattering centers in the single-mode fiber by femtosecond laser direct writing technology as a laser linewidth narrowing device to achieve single-frequency ultra-narrow linewidth laser output with eight switchable wavelengths.

[0056] 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 preferred embodiments, those of ordinary skill in the art should understand that they can still modify or equivalently replace the technical solutions of the present invention, and these modifications or equivalent replacements cannot make the modified technical solutions deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. An eight-wavelength switchable ultra-narrow linewidth fiber laser based on scattering-enhanced fiber, comprising a pump source, a wavelength division multiplexer, a polarization controller of a coiled erbium-doped fiber, a fiber polarizer, a squeezed polarization controller, a fiber coupler group, a fiber circulator, a scattering-enhanced fiber, an electrically controlled optical switch, a voltage driver, a parallel fiber Bragg grating group, a fiber coupler five, and a FC / APC type fiber connector three, characterized in that: The output pigtail of the pump source is connected to the pump light input end of the wavelength division multiplexer, the output end of the wavelength division multiplexer is connected to one end of the fiber pigtail of the polarization controller of the coiled erbium-doped fiber, the signal light input end of the wavelength division multiplexer is connected to the input end of the fiber polarizer, the output end of the fiber polarizer is connected to one end of the squeeze-type polarization controller, the other end of the squeeze-type polarization controller is connected to the fiber coupler group, the end of the fiber coupler group away from the squeeze-type polarization controller is connected to port one of the fiber circulator, port two of the fiber circulator is connected to one end of the scattering enhanced fiber, the other end of the scattering enhanced fiber is connected to port one of the electrically controlled optical switch, ports two and three of the electrically controlled optical switch are connected to the parallel fiber grating group, port three of the fiber circulator is connected to port one of the fiber coupler five, port two of the fiber coupler five is connected to the other end of the polarization controller of the coiled erbium-doped fiber, and port three of the fiber coupler five is connected to the pigtail of the FC / APC type fiber connector three; The electrically controlled optical switch is provided with a voltage drive signal by a voltage driver and connected via wires: The fiber coupler group includes a fiber coupler 1, a fiber coupler 2, a fiber coupler 3 and a fiber coupler 4, a squeeze-type polarization controller is connected to port 1 of the fiber coupler 1, port 2 of the fiber coupler 1 is connected to port 4 of the fiber coupler 2, port 3 of the fiber coupler 1 is connected to port 1 of the fiber coupler 2, port 3 of the fiber coupler 2 is connected to port 1 of the fiber coupler 3, port 2 of the fiber coupler 3 is connected to port 4 of the fiber coupler 4, port 3 of the fiber coupler 3 is connected to port 1 of the fiber coupler 4, and port 3 of the fiber coupler 4 is connected to port 1 of the fiber circulator; the length of the fiber ring formed by the fiber coupler 1 and the fiber coupler 2 is 60 cm, the length of the fiber ring formed by the fiber coupler 3 and the fiber coupler 4 is 61 cm, the cross-coupling ratios of the four fiber couplers are all 90:10, and the four fiber couplers are combined to form a dual-ring composite resonant cavity DCR-CC filter for single longitudinal mode selection of fiber lasers; The parallel fiber Bragg grating group includes a parallel fiber Bragg grating 1 and a parallel fiber Bragg grating 2. Port 2 of the electric-controlled optical switch is connected to one end of the parallel fiber Bragg grating 1, and the other end of the parallel fiber Bragg grating 1 is connected to an FC / APC type optical fiber connector 1; port 3 of the electric-controlled optical switch is connected to one end of the parallel fiber Bragg grating 2, and the other end of the parallel fiber Bragg grating 2 is connected to an FC / APC type optical fiber connector 2; the parallel fiber Bragg grating 1 and the parallel fiber Bragg grating 2 are combined with the electric-controlled optical switch to form a switchable polarization-dependent eight-channel filter S-PDECF, and the electric-controlled optical switch switches between channels 1→2 and 1→3, thereby realizing the switching of the parallel fiber Bragg grating 1 and the parallel fiber Bragg grating 2.

2. The eight-wavelength switchable ultra-narrow linewidth fiber laser based on scattering-enhanced fiber according to claim 1, characterized in that: The pigtail of port four of optical fiber coupler one, the pigtail of port two of optical fiber coupler two, the pigtail of port four of optical fiber coupler three, and the pigtail of port two of optical fiber coupler four are all processed at an angle of 8 degrees.

3. The eight-wavelength switchable ultra-narrow linewidth fiber laser based on scattering-enhanced fiber according to claim 1, characterized in that: The pump source is a 980nm semiconductor laser.

4. The eight-wavelength switchable ultra-narrow linewidth fiber laser based on scattering-enhanced fiber according to claim 1, characterized in that: The rare-earth-doped optical fibers wound in three optical fiber coils in the polarization controller of the coiled erbium-doped optical fiber are erbium-doped optical fibers or erbium-ytterbium co-doped optical fibers.

5. The eight-wavelength switchable ultra-narrow linewidth fiber laser based on scattering-enhanced fiber according to claim 2, characterized in that: The difference in ring length between the optical fiber ring formed by the optical fiber coupler 1 and the optical fiber coupler 2 and the optical fiber ring formed by the optical fiber coupler 3 and the optical fiber coupler 4 is 1 cm.

6. The eight-wavelength switchable ultra-narrow linewidth fiber laser based on scattering-enhanced fiber according to claim 1, characterized in that: Both parallel fiber Bragg grating 1 and parallel fiber Bragg grating 2 are narrow-band four-channel fiber Bragg grating filters; both parallel fiber Bragg grating 1 and parallel fiber Bragg grating 2 use femtosecond laser point-by-point method to symmetrically write two uniform fiber Bragg gratings with different periods on both sides of the center line of the optical fiber; the switching of parallel fiber Bragg grating 1 and parallel fiber Bragg grating 2 in the fiber laser system is controlled by an electrically controlled optical switch.

7. The eight-wavelength switchable ultra-narrow linewidth fiber laser based on scattering-enhanced fiber according to claim 6, characterized in that: The wavelength regions concentrated in the reflection filter channels of the parallel fiber grating 1 and the parallel fiber grating 2 do not cross or overlap.

8. The eight-wavelength switchable ultra-narrow linewidth fiber laser based on scattering-enhanced fiber according to claim 1, characterized in that: The fiber coupler five is a 3-port fiber coupler with a splitting ratio of 90:10, wherein 10% of the ports are used for laser output.

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