A 2-μm band ultra-wideband wavelength tunable single-frequency ultra-narrow linewidth fiber laser

Abstract

The present invention discloses a 2-μm band ultra-wideband wavelength tunable single-frequency ultra-narrow linewidth fiber laser, belonging to the field of tunable single-frequency fiber lasers. It includes designing two connection structures, using a MEMS tunable optical filter as a wavelength selection device, using an unpumped thulium-holmium co-doped fiber saturable absorber as a single longitudinal mode filtering device, and using a scattering-enhanced fiber fabricated by inducing random 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 2-μm band ultra-wideband wavelength tunable single-frequency ultra-narrow linewidth fiber laser, the present invention realizes the output of single-frequency ultra-narrow linewidth laser with ultra-wideband wavelength tunability or scanning in the 2-μm band.

Description

Technical Field

[0001] The present invention relates to the technical field of tunable single-frequency fiber lasers, and in particular to a 2μm-band ultra-wideband wavelength tunable single-frequency ultra-narrow linewidth fiber laser. Background Art

[0002] Single-frequency narrow linewidth fiber lasers have advantages such as high coherence, low noise, and strong compatibility with fiber systems, and are very suitable for important applications such as high-capacity long-distance coherent optical communication, high-precision optical metrology and spectroscopy, long-distance high-resolution distributed optical sensing, and coherent lidar. Fiber lasers in the near 2μm band have strong absorbability in animal / human tissues and are safe for the retina, so they are very popular in applications such as laser surgery and biomedicine. In addition, 2μm-band lasers have both high absorption windows and high transmission windows. The spectral region near 1940nm contains many important atmospheric gas absorption spectral lines, such as H2O, SO2, CO2, etc., and is an ideal light source for gas detection lidar. It also has an atmospheric transmittance of more than 80% near 2040nm and is an ideal light source for free-space optical communication and remote wind field lidar. Therefore, a broadband wavelength tunable single-frequency ultra-narrow linewidth fiber laser source covering the working spectral range of 1940 - 2040nm has broad application potential and important research value. However, the biggest problem faced in the development of such light sources is that it is difficult to break through the technical bottleneck of deeply narrowing the laser linewidth for broadband wavelength tunable operation.

[0003] Fiber laser filtering devices that can achieve continuous wavelength tuning mainly include fiber grating stress adjustment filtering devices, Fabry-Perot (F-P) optical filtering devices based on piezoelectric transducers (PZT), all-fiber F-P filtering devices based on PZT, MEMS optical filtering devices, etc. Among them, the first one is difficult to achieve large-range wavelength tuning, the second one is difficult to achieve high-precision narrowband filtering, and the third one has strong temperature and polarization sensitivity. In contrast, the fourth MEMS optical filtering device has advantages such as small volume, high tuning accuracy, and low temperature and polarization sensitivity, but the research on 2μm-band broadband tunable MEMS optical filters is still very rare.

[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 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 and batch manner. More importantly, none of the above technologies can meet the application requirements of broadband wavelength tunable (tuning or scanning) laser linewidth narrowing. In recent years, the laser linewidth narrowing method based on the random distributed feedback of fiber Rayleigh scattering has been widely studied, and it is expected to achieve wavelength adaptive linewidth deep narrowing for normalized applications. However, the random distributed feedback based on the Rayleigh scattering of single-mode fiber itself requires a length of hundreds or even thousands of meters to achieve sufficient feedback accumulation for laser linewidth deep narrowing. This 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. Previously, advanced femtosecond laser direct writing technology was used to induce high-scattering centers in single-mode fibers, and randomly distributed feedback scattering-enhanced fibers were developed. Then, these fibers were introduced into single-longitudinal-mode fiber lasers for linewidth narrowing, and an ultra-narrow linewidth laser output with <150 Hz was obtained, and the overall output performance of the laser was excellent. However, there has been no report on using scattering-enhanced fibers to achieve broadband wavelength tunable fiber laser linewidth deep narrowing, and the application of scattering-enhanced fibers in linewidth narrowing of 2-μm band fiber lasers has not been studied. The present invention has important value for the application of scattering-enhanced fibers in broadband wavelength tunable fiber lasers and their popularization and application in laser linewidth narrowing in different bands. Summary of the Invention

[0005] The object of the present invention is to provide a 2-μm band ultra-broadband wavelength tunable single-frequency ultra-narrow linewidth fiber laser to solve the problems mentioned in the above background technology.

[0006] To achieve the above object, the present invention provides a 2-μm band ultra-wideband wavelength tunable single-frequency ultra-narrow linewidth fiber laser, which includes a pump laser, a first wavelength division multiplexer, a MEMS tunable optical filter, a first fiber optic circulator, a fiber optic coupler, a second wavelength division multiplexer, and a thulium-holmium co-doped single-mode fiber. The output pigtail of the pump laser is connected to the pump light input end of the first wavelength division multiplexer. The signal light input end of the first wavelength division multiplexer is connected to the input end of the MEMS tunable optical filter. The output end of the MEMS tunable optical filter is connected to port 1 of the first fiber optic circulator. Port 3 of the first fiber optic circulator is connected to port 1 of the fiber optic coupler. Port 2 of the fiber optic coupler is connected to the signal light input end of the second wavelength division multiplexer. The output end of the second wavelength division multiplexer is connected to one end of the thulium-holmium co-doped single-mode fiber. The other end of the thulium-holmium co-doped single-mode fiber is connected to the output end of the first wavelength division multiplexer. A FC / APC type fiber optic connector is connected to port 3 of the fiber optic coupler. The MEMS tunable optical filter is also connected to a programmable voltage driver.

[0007] Preferably, the pigtail of the pump light input end of the second wavelength division multiplexer is processed at an 8-degree oblique angle and left idle.

[0008] Preferably, the pump laser is a 1567-nm high-power single-mode fiber laser or a semiconductor laser.

[0009] Preferably, the filtering center wavelength of the MEMS tunable optical filter is 2000 nm. By providing a voltage signal through the programmable voltage driver, the filtering center wavelength is tuned to 1850 nm in the short-wavelength direction and tuned to 2150 nm in the long-wavelength direction.

[0010] Preferably, the filtering center wavelength is adjusted step by step or scanned continuously according to the fixed voltage value or the scanned voltage signal of the programmable voltage driver.

[0011] Preferably, the fiber optic coupler is a 3-port fiber optic coupler with a splitting ratio of 80:20, and the 20% port is used for laser output.

[0012] Preferably, a squeezing type fiber polarization controller is connected to port 2 of the first fiber optic circulator. One end of the squeezing type polarization controller away from the first fiber optic circulator is connected to a thulium-holmium co-doped fiber saturable absorber. One end of the thulium-holmium co-doped fiber saturable absorber away from the squeezing type polarization controller is connected to a first fiber optic mirror. A second fiber optic circulator is arranged between the fiber optic coupler and the second wavelength division multiplexer. Port 2 of the fiber optic coupler is connected to port 1 of the second fiber optic circulator. A scattering-enhanced fiber is connected to port 2 of the second fiber optic circulator. One end of the scattering-enhanced fiber away from the second fiber optic circulator is connected to a second fiber optic mirror. Port 3 of the second fiber optic circulator is connected to the signal light input end of the second wavelength division multiplexer.

[0013] Preferably, port two of the first fiber optic circulator is connected to a scattering-enhanced optical fiber. One end of the scattering-enhanced optical fiber away from the first circulator is connected to a squeeze-type fiber polarization controller. One end of the squeeze-type fiber polarization controller away from the scattering-enhanced optical fiber is connected to a thulium-holmium co-doped fiber saturable absorber. One end of the thulium-holmium co-doped fiber saturable absorber away from the squeeze-type fiber polarization controller is connected to a first fiber optic mirror.

[0014] Preferably, both the first fiber optic mirror and the second fiber optic mirror are made by coating the fiber end faces, and both have a reflectivity greater than 50% in the range of 1800 - 2200 nm.

[0015] Preferably, the scattering-enhanced optical fiber uses the femtosecond laser point-by-point method to randomly distribute high-scattering points in the core region of a single-mode optical fiber, and the intensity of the written high-scattering points is randomly controlled by controlling the random variation of the laser pulse energy within a certain range. The density of the high-scattering points and the length of the scattering-enhanced optical fiber are determined according to actual needs.

[0016] Therefore, the present invention adopts the above-mentioned 2μm-band ultra-wideband wavelength-tunable single-frequency ultra-narrow linewidth fiber laser, which has the following beneficial effects:

[0017] (1) When the fiber laser operates, only need to adjust the output power of the 1567nm pump laser above the laser threshold, and make the output single longitudinal mode stable by appropriately adjusting the squeeze-type polarization controller, then the MEMS tunable optical filter can be driven by giving different fixed voltages or continuously scanning voltages to achieve 2μm-band ultra-wideband wavelength-tunable single-frequency ultra-narrow linewidth laser output;

[0018] (2) Utilizing the flexible and adjustable characteristics of the femtosecond laser direct writing technology, the random distributed feedback scattering characteristics of the scattering-enhanced optical fiber can be arbitrarily adjusted, thereby realizing the ultra-wideband tunable wavelength self-adaptive laser linewidth narrowing, and can also realize the dynamic laser linewidth narrowing of wavelength continuous scanning tuning, which has important application value in the fields of free space optical communication, fiber optic sensing, etc.

[0019] Next, through the drawings and embodiments, the technical solutions of the present invention will be further described in detail. Description of the Drawings

[0020] Figure 1 is the structural schematic diagram of Embodiment 1 of the present invention;

[0021] Figure 2 is the tuning and filtering spectrogram of the MEMS tunable optical filter of the embodiment of the present invention;

[0022] Figure 3 is the reflectivity spectrogram of the first fiber optic mirror of the embodiment of the present invention;

[0023] Figure 4 Reflectivity spectrum diagram of the second fiber optic mirror according to an embodiment of the present invention;

[0024] Figure 5 Backscattering distributed measurement results of the scattering-enhanced optical fiber according to an embodiment of the present invention;

[0025] Figure 6 Broadband wavelength stepped adjustment laser output spectrum diagram of the fiber laser according to an embodiment of the present invention;

[0026] Figure 7 Broadband wavelength scanning laser output spectrum diagram of the fiber laser according to an embodiment of the present invention;

[0027] Figure 8 Single-frequency laser operation measurement results at different output wavelengths of the fiber laser according to an embodiment of the present invention;

[0028] Figure 9 Frequency noise comparison measurement results of the fiber laser with and without the scattering-enhanced optical fiber according to an embodiment of the present invention;

[0029] Figure 10 Intrinsic linewidth measurement results of the fiber laser at ultra-broadband wavelength tunable output according to an embodiment of the present invention;

[0030] Figure 11 Structural schematic diagram of Embodiment 2 of the present invention;

[0031] Reference numerals

[0032] 1. Pump laser; 2. First wavelength division multiplexer; 3. Thulium-holmium co-doped single-mode fiber; 4. Second wavelength division multiplexer; 5. MEMS tunable optical filter; 6. Programmable voltage driver; 7. First fiber optic circulator; 8. Extrusion-type fiber polarization controller; 9. Thulium-holmium co-doped fiber saturable absorber; 10. First fiber optic mirror; 11. Fiber optic coupler; 12. FC / APC type fiber optic connector; 13. Second fiber optic circulator; 14. Scattering-enhanced optical fiber; 15. Second fiber optic mirror. Detailed implementation manners

[0033] 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.

[0034] Embodiment 1:

[0035] Refer to Figure 1, the present invention discloses a 2μm-band ultra-wideband wavelength tunable single-frequency ultra-narrow linewidth fiber laser, which includes a pump laser 1, a first wavelength division multiplexer 2, a MEMS tunable optical filter 5, a first fiber circulator 7, a fiber coupler 11, a second fiber circulator 13, a second wavelength division multiplexer 4, and a thulium-holmium co-doped single-mode fiber 3. The output pigtail of the pump laser 1 is connected to the pump light input end of the first wavelength division multiplexer 2. The signal light input end of the first wavelength division multiplexer 2 is connected to the input end of the MEMS tunable optical filter 5. The output end of the MEMS tunable optical filter 5 is connected to port 1 of the first fiber circulator 7. A squeezing-type fiber polarization controller 8 is connected to port 2 of the first fiber circulator 7. One end of the squeezing-type polarization controller away from the first fiber circulator 7 is connected to a thulium-holmium co-doped fiber saturable absorber 9. One end of the thulium-holmium co-doped fiber saturable absorber 9 away from the squeezing-type polarization controller is connected to a first fiber mirror 10. Port 3 of the first fiber circulator 7 is connected to port 1 of the fiber coupler 11. A second fiber circulator 13 is arranged between the fiber coupler 11 and the second wavelength division multiplexer 4. Port 2 of the fiber coupler 11 is connected to port 1 of the second fiber circulator 13. A scattering-enhanced fiber 14 is connected to port 2 of the second fiber circulator 13. One end of the scattering-enhanced fiber 14 away from the second fiber circulator 13 is connected to a second fiber mirror 15. Port 3 of the second fiber circulator 13 is connected to the signal light input end of the second wavelength division multiplexer 4. The output end of the second wavelength division multiplexer 4 is connected to one end of the thulium-holmium co-doped single-mode fiber 3. The other end of the thulium-holmium co-doped single-mode fiber 3 is connected to the output end of the first wavelength division multiplexer 2. Port 3 of the fiber coupler 11 is connected to an FC / APC type fiber connector 12 for laser output. The MEMS tunable optical filter 5 is also connected to a programmable voltage driver 6 through an electric wire.

[0036] The connections between the above-mentioned fiber devices are all fusion connections made using a fiber fusion splicer, and fiber connectors can also be used partially or entirely.

[0037] The pigtail of the thulium-holmium co-doped fiber saturable absorber 9 is connected to the first fiber mirror 10, and the pigtail of the scattering-enhanced fiber 14 is connected to the second fiber mirror 15.

[0038] The pump laser 1 is developed from a 1567nm semiconductor seed laser plus a high-power fiber laser amplifier. The output pigtail is a common single-mode fiber for communication, and the maximum laser output power is 5W.

[0039] The first wavelength division multiplexer 2 and the second wavelength division multiplexer 4 are 1567 / 2000 nm type all-fiber wavelength division multiplexers fabricated by the fused biconical taper method. The pump laser is input from the 1567 nm pump light input end of the first wavelength division multiplexer 2 to excite the thulium-ho doped single-mode fiber 3 to generate spontaneous emission and stimulated emission. The pump light that is not completely absorbed is output through the 1567 nm pump light input end of the second wavelength division multiplexer 4 to avoid introducing unnecessary thermal effects to other optical devices in the laser resonator. The pigtail of the pump light input end of the second wavelength division multiplexer 4 is processed at an 8-degree angle to prevent strong fiber end-face reflection from feedback into the laser cavity and affecting the lasing characteristics of the gain fiber.

[0040] The length of the thulium-ho doped single-mode fiber 3 is 4 m, and this length is more conducive to the lasing of lasers with wavelengths above 2000 nm.

[0041] The MEMS tunable optical filter 5 is a voltage-controlled ultra-wideband tunable filter developed based on microelectromechanical system (MEMS) technology and diffraction grating technology. The designed filtering center wavelength is 2000 nm. The MEMS tunable optical filter 5 has three voltage connection pins: one ground (pin 2) and two positive (pins 1 and 3). Pin 2 is grounded permanently. When a positive voltage is applied to pin 1 (referred to as the left arm), the filtering center wavelength of the filter is adjusted towards the short wavelength direction. When a positive voltage is applied to pin 3 (referred to as the right arm), the filtering center wavelength of the filter is adjusted towards the long wavelength direction. Moreover, when voltages are applied to the left and right arms respectively, there is a monotonic one-to-one correspondence between the voltage value and the filtering wavelength value. By adjusting the voltage signal provided by the programmable voltage driver 6, the filtering center wavelength can be tuned to 1850 nm towards the short wavelength direction and tuned to 2150 nm towards the long wavelength direction. And by controlling the given voltage signal mode as a fixed voltage or a continuous scanning voltage, the step adjustment or scanning tuning of the filtering wavelength of the MEMS tunable optical filter 5 can be achieved.

[0042] As Figure 2 shown, the filtering tuning characteristics of the MEMS tunable optical filter 5 were measured using a supercontinuum white light source. It can be seen within the allowable range of the light source output power and wavelength that the MEMS tunable optical filter 5 has ultra-wideband tunable narrowband filtering characteristics. The filtering bandwidth is less than 1.4 nm and the insertion loss is less than 8 dB within the full tuning range.

[0043] The thulium-ho co-doped fiber saturable absorber 9 is specifically a section of thulium-ho co-doped single-mode fiber 3 with a length of 1.3 m, having the same parameters as the thulium-ho co-doped single-mode fiber 3. The thulium-ho co-doped fiber saturable absorber 9 is combined with the first fiber mirror 10 to form a dynamic self-tracking narrowband filtering device when the laser is input, thereby realizing the selection of single longitudinal mode of the laser. Due to the holmium ions (Ho 3+) The introduction of thulium and holmium co-doped fiber enhances the absorption cross-section at long wavelengths. The formed saturable absorber type narrowband filter has both an extremely narrow filtering bandwidth and an ultra-wideband wavelength self-tracking function;

[0044] As Figure 3 shown, the reflectivity spectrum of the first fiber mirror 10 measured has a reflectivity greater than 60% in the range of 1800 - 2200 nm, and a reflectivity greater than 80% in the range of 1880 - 2060 nm;

[0045] As Figure 4 shown, the reflectivity spectrum of the second fiber mirror 15 measured has a reflectivity greater than 60% in the range of 1800 - 2200 nm, and a reflectivity greater than 80% in the range of 1880 - 2060 nm;

[0046] The scattering enhanced fiber 14 (SEF) is made by randomly writing high scattering points in the core region of a single-mode fiber using the femtosecond laser point-by-point method. The specific method is to randomly control the femtosecond laser pulse energy to vary within a certain range (355 - 390 nJ), irradiate with a single laser pulse and induce micro refractive index change regions (high scattering points) point by point in the single-mode core, randomly control the spatial position of the high scattering points in the core region, control the distance between adjacent high scattering points to be 2 - 3 cm, the length of the fabricated scattering enhanced fiber 14 is about 3 m, and the total number of high scattering points is 150;

[0047] As Figure 5 shown, the random Rayleigh scattering distribution of the scattering enhanced fiber 14 measured using an optical frequency domain reflectometer shows that the scattering intensity of the high scattering points introduced by the femtosecond laser is increased by about 45 dB compared with the intrinsic scattering of the single-mode fiber as a whole, and the scattering characteristics have strong random distribution characteristics;

[0048] The fiber coupler 11 has 3 ports with a splitting ratio of 80:20, and 20% of the ports are used for laser output.

[0049] The output power of the pump laser 1 is set to 2.8 W, and by setting the output voltage of the programmable voltage driver 6 to independent voltage values or continuously scanning periodic voltage signals, laser outputs with wavelength step adjustment and continuous wavelength scanning are respectively obtained;

[0050] As Figure 6 shown, using the spectral analyzer in the repeat scan mode, the spectra of the laser output are measured every about 5 nm and named λ1 - λ 37 , the wavelength adjustment range is about 1885 - 2055 nm, a total of about 170 nm, and within the overall range, the signal-to-noise ratio of the laser output is greater than 60 dB;

[0051] As Figure 7As shown, using the maximum hold mode of the spectral analyzer, the scanned spectrum of the laser output was measured. The wavelength adjustment range is approximately 1885 - 2055 nm, totaling approximately 170 nm. To make the scanning speed of the spectral analyzer fast enough, a lower resolution was used, which deteriorated the measurement signal-to-noise ratio. The actual instantaneous signal-to-noise ratio of the laser output cannot be directly obtained by the spectrometer.

[0052] As Figure 8 shown, the self-homodyne method was used to test the longitudinal mode characteristics of the laser output from λ1 to λ 37 . No beat frequency noise was captured under all wavelength operating modes, proving that the laser is in a stable single longitudinal mode operating state.

[0053] The Michelson interferometer type frequency noise test system based on the 3×3 fiber coupler 11 was used to test the frequency noise spectra of the lasers at each wavelength before and after removing the scattering enhancement fiber 14. The measurement results of the laser at the typical wavelength of 2000 nm are given. As Figure 9 shown, it can be seen that the introduction of the scattering enhancement fiber 14 reduces the frequency noise of the laser output by approximately 18 dB, and the white noise floor S0 characterizing the intrinsic linewidth of the laser reaches 11 Hz 2 / Hz. According to theoretical calculations, the intrinsic linewidth of the laser output is S0×π≈35 Hz.

[0054] As Figure 10 shown, the intrinsic linewidth of the laser in the wavelength adjustment range of 1885 - 2055 nm was measured and calculated. The linewidth of the laser output is greater than 100 Hz only at some wavelengths, and the intrinsic linewidth at other wavelengths is below 100 Hz, proving the ability of the scattering enhancement fiber to achieve ultra-wideband wavelength adaptive linewidth narrowing.

[0055] This embodiment shows that the linewidth narrowing by the scattering enhancement fiber 14 has weak wavelength correlation, can achieve wavelength adaptive linewidth narrowing within a wideband wavelength tunable range, and has higher stability and stronger potential for miniaturized integration compared with single-mode fibers.

[0056] Example 2:

[0057] As Figure 11As shown in the figure, it includes a pump laser 1, a first wavelength division multiplexer 2, a MEMS tunable optical filter 5, a first fiber optic circulator 7, an optical fiber coupler 11, a second wavelength division multiplexer 4, and a thulium-holmium co-doped single-mode optical fiber 3. The output pigtail of the pump laser 1 is connected to the pump light input end of the first wavelength division multiplexer 2. The signal light input end of the first wavelength division multiplexer 2 is connected to the input end of the MEMS tunable optical filter 5. The output end of the MEMS tunable optical filter 5 is connected to port one of the first fiber optic circulator 7. A scattering enhancement optical fiber 14 is connected to port two of the first fiber optic circulator 7. One end of the scattering enhancement optical fiber 14 far from the first circulator is connected to a squeezing type fiber optic polarization controller 8. One end of the squeezing type fiber optic polarization controller 8 far from the scattering enhancement optical fiber 14 is connected to a thulium-holmium co-doped fiber optic saturable absorber 9. One end of the thulium-holmium co-doped fiber optic saturable absorber 9 far from the squeezing type fiber optic polarization controller 8 is connected to a first fiber optic mirror 10. Port three of the first fiber optic circulator 7 is connected to port one of the optical fiber coupler 11. Port two of the optical fiber coupler 11 is connected to the signal light input end of the second wavelength division multiplexer 4. The output end of the second wavelength division multiplexer 4 is connected to one end of the thulium-holmium co-doped single-mode optical fiber 3. The other end of the thulium-holmium co-doped single-mode optical fiber 3 is connected to the output end of the first wavelength division multiplexer 2. Port three of the optical fiber coupler 11 is connected to an FC / APC type fiber optic connector 12 for laser output. The MEMS tunable optical filter 5 is also connected to a programmable voltage driver 6. The connections between the above-mentioned optical fiber devices are all fusion connections using an optical fiber fusion splicer, and partial or all of them can also be connected using optical fiber connectors.

[0058] The parameter selection of all devices and components in this embodiment is the same as that described in Embodiment 1, and others can also be selected according to actual needs.

[0059] Therefore, the present invention adopts the above-mentioned 2μm band ultra-wideband wavelength tunable single-frequency ultra-narrow linewidth fiber laser, which can drive the MEMS tunable optical filter by giving different fixed voltages or continuously scanning voltages, realize the output of 2μm band ultra-wideband wavelength tunable single-frequency ultra-narrow linewidth laser, realize the ultra-wideband adjustable wavelength self-adaptive laser linewidth narrowing, and can also realize the dynamic laser linewidth narrowing of wavelength continuous scanning tuning, which has important application value in the fields of space optical communication, fiber optic sensing, etc.

[0060] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than 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. A 2-μm band ultra-wideband wavelength tunable single-frequency ultra-narrow linewidth fiber laser, characterized in that: It includes a pump laser, a first wavelength division multiplexer, a MEMS tunable optical filter, a first fiber optic circulator, an optical fiber coupler, a second wavelength division multiplexer, and a thulium-holmium co-doped single-mode optical fiber. The output pigtail of the pump laser is connected to the pump light input end of the first wavelength division multiplexer. The signal light input end of the first wavelength division multiplexer is connected to the input end of the MEMS tunable optical filter. The output end of the MEMS tunable optical filter is connected to port 1 of the first fiber optic circulator. Port 3 of the first fiber optic circulator is connected to port 1 of the optical fiber coupler. Port 2 of the optical fiber coupler is connected to the signal light input end of the second wavelength division multiplexer. The output end of the second wavelength division multiplexer is connected to one end of the thulium-holmium co-doped single-mode optical fiber. The other end of the thulium-holmium co-doped single-mode optical fiber is connected to the output end of the first wavelength division multiplexer. An FC / APC type optical fiber connector is connected to port 3 of the optical fiber coupler. The MEMS tunable optical filter is also connected to a programmable voltage driver; The pigtail of the pump light input end of the second wavelength division multiplexer is processed at an 8-degree angle. Port 2 of the first fiber optic circulator is connected to a squeeze type fiber optic polarization controller. One end of the squeeze type polarization controller away from the first fiber optic circulator is connected to a thulium-holmium co-doped fiber saturable absorber. One end of the thulium-holmium co-doped fiber saturable absorber away from the squeeze type polarization controller is connected to a first fiber optic mirror. A second fiber optic circulator is arranged between the optical fiber coupler and the second wavelength division multiplexer. Port 2 of the optical fiber coupler is connected to port 1 of the second fiber optic circulator. Port 2 of the second fiber optic circulator is connected to a scattering enhanced optical fiber. One end of the scattering enhanced optical fiber away from the second fiber optic circulator is connected to a second fiber optic mirror. Port 3 of the second fiber optic circulator is connected to the signal light input end of the second wavelength division multiplexer.

2. The ultra-wideband wavelength tunable single-frequency ultra-narrow linewidth fiber laser in the 2μm band according to claim 1, wherein: The pump laser is a 1567nm high-power single-mode fiber laser or a semiconductor laser.

3. A 2-μm band ultra-wideband wavelength tunable single-frequency ultra-narrow linewidth fiber laser according to claim 2, characterized in that: The filtering center wavelength of the MEMS tunable optical filter is 2000nm. By providing a voltage signal through the programmable voltage driver, the filtering center wavelength is tuned to 1850nm in the short wavelength direction and tuned to 2150nm in the long wavelength direction.

4. A 2-μm band ultra-wideband wavelength tunable single-frequency ultra-narrow linewidth fiber laser according to claim 3, characterized in that: The filtering center wavelength realizes stepwise adjustment or continuous scanning according to the fixed voltage value or the scanned voltage signal of the programmable voltage driver.

5. A 2-μm band ultra-wideband wavelength tunable single-frequency ultra-narrow linewidth fiber laser according to claim 4, characterized in that: The optical fiber coupler is a 3-port optical fiber coupler with a splitting ratio of 80:20, and the 20% port is used for laser output.

6. The ultra-wideband wavelength tunable single-frequency ultra-narrow linewidth fiber laser in the 2μm band according to claim 1, characterized in that: Both the first fiber optic mirror and the second fiber optic mirror are made by coating the fiber end face, and both have a reflectivity greater than 50% in the range of 1800 - 2200nm.

7. A 2-μm band ultra-wideband wavelength tunable single-frequency ultra-narrow linewidth fiber laser according to claim 1, characterized in that: The scattering enhanced optical fiber randomly writes high-scattering points in the single-mode fiber core region by the femtosecond laser point-by-point method, and randomly controls the intensity of the written high-scattering points by controlling the random change of the laser pulse energy.

8. A 2μm band ultra-wideband wavelength tunable single-frequency ultra-narrow linewidth fiber laser, characterized in that: It includes a pump laser, a first wavelength division multiplexer, a MEMS tunable optical filter, a first fiber optic circulator, an optical fiber coupler, a second wavelength division multiplexer, and a thulium-holmium co-doped single-mode optical fiber. The output pigtail of the pump laser is connected to the pump light input end of the first wavelength division multiplexer. The signal light input end of the first wavelength division multiplexer is connected to the input end of the MEMS tunable optical filter. The output end of the MEMS tunable optical filter is connected to port 1 of the first fiber optic circulator. Port 3 of the first fiber optic circulator is connected to port 1 of the optical fiber coupler. Port 2 of the optical fiber coupler is connected to the signal light input end of the second wavelength division multiplexer. The output end of the second wavelength division multiplexer is connected to one end of the thulium-holmium co-doped single-mode optical fiber. The other end of the thulium-holmium co-doped single-mode optical fiber is connected to the output end of the first wavelength division multiplexer. An FC / APC type fiber optic connector is connected to port 3 of the optical fiber coupler. The MEMS tunable optical filter is also connected to a programmable voltage driver; The pigtail of the pump light input end of the second wavelength division multiplexer is processed at an 8-degree inclined angle; A scattering-enhanced optical fiber is connected to port 2 of the first fiber optic circulator. One end of the scattering-enhanced optical fiber away from the first circulator is connected to a squeeze-type fiber optic polarization controller. One end of the squeeze-type fiber optic polarization controller away from the scattering-enhanced optical fiber is connected to a thulium-holmium co-doped fiber saturable absorber. One end of the thulium-holmium co-doped fiber saturable absorber away from the squeeze-type fiber optic polarization controller is connected to a first fiber optic mirror. Port 2 of the optical fiber coupler is connected to the signal light input end of the second wavelength division multiplexer.

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