Intermediate infrared fiber laser with high polarization slope and adjustable efficiency

Abstract

The invention discloses an intermediate infrared fiber laser with high polarization slope and adjustable efficiency, which adopts an optical coupler to construct a loop reflector as an output port of the fiber laser. The polarization state of an internal light field of the loop reflector is adjusted through the polarization controller to realize 0-100% continuously adjustable reflectivity, so that the maximum output power is realized under all pumping powers, and parabolic slope efficiency is formed; and the other side of the optical fiber oscillator is composed of a polarization-sensitive high-efficiency reflector, so that the laser has high polarization selectivity. According to the invention, the resonant cavity of the non-polarization-maintaining fiber laser can be effectively utilized, linear polarized laser output with the extinction ratio exceeding 99.997% or circular polarized laser output with the same dual-polarization power can be realized along a fast axis or a slow axis, and conditions are provided for remote sensing or coherent laser synthesis.

Description

Technical Field

[0001] The invention belongs to the field of laser technology, and in particular relates to a mid-infrared optical fiber laser with high polarization slope efficiency and adjustable efficiency. Background Art

[0002] In recent years, the development of 2.1-micron fiber lasers has been extensively and deeply studied. With the wide emission band characteristics of Tm3+ ions, on the one hand, Tm-doped all-fiber lasers have successfully covered the wavelength range of 1925 nm to 2200 nm, showing a slope efficiency of more than 42% and an amplified spontaneous emission (ASE) suppression capability of about 40 dB, especially at wavelengths close to 2100 nm. On the other hand, the emission cross section of Ho-doped fiber lasers is more conducive to laser output at longer wavelengths, and laser emission to 2171 nm has been successfully achieved. Among them, Ho-doped fiber lasers show the highest efficiency at a wavelength of 2116 nm, with a slope efficiency of 43% and ASE suppression of more than 50 dB. However, as of now, there is still a lack of efficient lasers that can directly output polarized light from the laser cavity in this wavelength range. In view of the insertion loss, bandwidth limitation (usually about 50 nm near 2 microns) of polarizers, and their low availability as fiberized components, their use should be avoided as much as possible. Instead, high-quality polarized light should be generated directly from the laser cavity. In addition, the all-fiber and simple cavity design is an ideal solution for long-term stability, efficient power handling and compactness.

[0003] Continuous light lasers such as solid-state lasers, gas lasers and fiber lasers can all generate radiation output in the 3.7 to 4.8 micron band. Compared with other technologies, fiber lasers have the advantages of small size, high integration, excellent robustness and significant energy efficiency. In this band, the choices of fiber lasers include cascade Raman lasers based on soft glass, cascade Raman lasers based on antiresonant fibers, antiresonant lasers based on gas radiation, and soft glass fiber lasers based on rare earth element electron inversion mechanisms. In addition, crystal fibers, especially Fe2+-doped ZnSe, are also regarded as a potential excellent gain medium. Compared with the method of relying on nonlinear effects to achieve laser radiation, stimulated emission lasers can accumulate energy at high energy levels due to the long energy level lifetime of the excited state, thereby realizing flexible switching of operating states between continuous light and high-energy pulses. In addition, stimulated emission fiber lasers do not require additional mid-infrared nonlinear fibers, effectively reducing the complexity of the system. However, due to factors such as multi-phonon relaxation of the substrate, fiber drawing process, gain medium concentration, and excited state radiation efficiency, the research and development of efficient and high-energy 4-micron lasers still faces huge challenges worldwide. This band is even known as the "historic wavelength barrier of 4 microns."

[0004] As an effective means to improve laser efficiency, dual-wavelength pumping technology can reduce quantum loss and enhance the output power of the laser. The output wavelength of the erbium-doped fluoride fiber laser using the dual-wavelength pumping scheme can be adjusted in the range of 3.4 to 3.8 microns, and an output power of 5.6 watts and a slope efficiency of 26% can be achieved. By optimizing the resonant cavity structure, fiber length and doping concentration, the maximum laser efficiency of this scheme can reach 38%. If the output power needs to be further increased, the core pumping scheme needs to be converted to a cladding pumping scheme to avoid damage to the core. However, due to the resonance frequency limitation of the CH bond and the -OH bond, the 2-micron pump light can only propagate in the core, which limits the further increase of the laser power. In addition, given the current technical maturity of doped fluoride fibers and related devices, fiber lasers with high polarization extinction ratios in the 4-micron band still face great challenges. It is worth noting that high polarization extinction ratio is an important prerequisite for the coherent beam combining technology of continuous lasers; however, the current technology level limits the output power of 4-micron band continuous lasers to less than 10 watts, which seriously hinders the application of 4-micron band lasers in military and civilian fields. Therefore, it is necessary to provide a new concept of high slope efficiency and high polarization degree mid-infrared fiber laser. Summary of the invention

[0005] In order to overcome the shortcomings of the prior art, the present invention provides a mid-infrared fiber laser with high polarization slope efficiency and adjustable efficiency, and uses an optical coupler to construct a loop reflector as the output port of the fiber laser. The polarization state of the light field inside the loop reflector is adjusted by a polarization controller to achieve a continuously adjustable reflectivity from 0 to 100%, thereby achieving the maximum output power under all pump powers and forming a parabolic slope efficiency; the other side of the fiber oscillator is composed of a polarization-sensitive high-efficiency reflector, so that the laser has a high degree of polarization selectivity. The present invention can effectively utilize the resonant cavity of a non-polarization-maintaining fiber laser to achieve linear polarization laser output with an extinction ratio of more than 99.997% along the fast axis or slow axis, or circular polarization output with the same dual polarization power, providing conditions for remote sensing or coherent laser synthesis.

[0006] The technical solution adopted by the present invention to solve the technical problem is as follows:

[0007] A mid-infrared fiber laser with high polarization slope efficiency and adjustable efficiency, comprising a front pump laser FL, a first wavelength division multiplexer WDM1, a laser oscillator and a laser working state monitor and controller;

[0008] The laser oscillator comprises a polarization-sensitive reflector, a gain fiber AF, a second wavelength division multiplexer WDM2, a first optical splitter, a second polarization controller PC2, a first isolator ISO1, a first polarization controller PC1 and a post-pump laser BL;

[0009] The laser working state monitoring and controller includes a second isolator ISO2, a second optical splitter OC2, a spectrometer OSA, a first power meter PM1, a second power meter PM2, a host computer DC and a polarization beam splitter PBS;

[0010] The front pump laser FL is a gain fiber AF, which provides a single-mode or multi-mode pump laser, passes through the optical fiber P11, passes through the polarization-sensitive reflector, and is absorbed by the gain fiber AF;

[0011] The gain fiber AF generates amplified spontaneous radiation along two different directions P14 and P21; wherein, the spontaneous radiation in the direction of P21 is introduced into the first optical splitter through the second wavelength division multiplexer WDM2; the first optical splitter divides the spontaneous radiation energy into P23 and P24 loops, and the two loop light fields circulate clockwise and counterclockwise respectively and return to the first optical splitter, and the light field generates an interference effect, and the polarization of the clockwise and counterclockwise light fields is adjusted through the second polarizer PC2, and the light field interference intensity is changed, thereby realizing continuous adjustment of the reflectivity of the ring mirror from 0 to a%;

[0012] The reflected light from the first beam splitter after interference returns to the laser oscillator and enters the gain fiber AF again. At this time, the spontaneous radiation becomes stimulated radiation. The stimulated radiation enters the first polarization controller through the direction of P14. The polarization of the stimulated radiation is adjusted by the first polarization controller.

[0013] The polarization-changed stimulated radiation is incident on the polarization-sensitive reflector, and by adjusting the polarization-sensitive reflector, the stimulated radiation near the high reflection wavelength of the polarization-sensitive reflector corresponding to the laser target working wavelength W1 is made to correspond to the strong reflection polarization ne of the polarization-sensitive reflector, thereby enhancing the reflection of W1;

[0014] Then W1 is reflected back to the gain fiber AF, and the further amplified W1 stimulated radiation passes through the second wavelength division multiplexer WDM2, enters the first optical splitter, and is returned to the laser oscillator again; after multiple oscillations, W1 passes through the first isolator ISO1 in the form of narrow linewidth laser and is sent to the subsequent optical path or application;

[0015] A small part of the laser power passes through the first wavelength division multiplexer WDM1 and enters the second optical splitter OC2 through the second isolator ISO2; OC2 adopts non-equiproportional splitting, and a small part of the power enters the spectrometer OSA to provide the host computer with the spectrum wavelength, noise compression ratio, and laser line width parameters of the mid-infrared fiber laser. The remaining power is split by the polarization beam splitter PBS according to two mutually perpendicular polarizations and enters the power meters PM1 and PM2 respectively; the readings of the two power meters are fed back to the host computer DC to obtain the polarization state and polarization extinction ratio of the laser; at the same time, the total power of the power meters PM1 and PM2, combined with the reflectivity of the polarization-sensitive reflector, can be used to deduce the output power of the laser;

[0016] According to the parameters obtained in the previous section, the host computer connects the first polarization controller PC1 and the second polarization controller PC2, adjusts the two polarization controllers, and optimizes or controls the laser output state.

[0017] Preferably, the pump wavelength of the single-mode or multi-mode pump laser is 790 nm, 1950 nm or 1550-1650 nm.

[0018] Preferably, the laser is oriented towards the mid-infrared band, i.e., wavelengths of 2000 nm and above.

[0019] Preferably, if the splitting ratio of the beam splitter is 50 / 50, then a is at most 100%.

[0020] Preferably, the small portion of laser power refers to less than 1% of laser power.

[0021] Preferably, the second optical splitter OC2 adopts non-uniform ratio splitting, with a ratio of 90 / 10.

[0022] The beneficial effects of the present invention are as follows:

[0023] The present invention uses an optical coupler to construct a loop reflector as the output port of the fiber laser. The polarization state of the light field inside the loop reflector is adjusted by a polarization controller to achieve a continuously adjustable reflectivity from 0 to 100%, thereby achieving the maximum output power at all pump powers and forming a parabolic slope efficiency: that is, when the power is low, the reflectivity is increased, the laser threshold is reduced, and the output power is increased; when the pump power is low, the reflectivity is synchronously reduced, the slope efficiency is improved, and the output power is increased. The other side of the fiber oscillator is composed of a polarization-sensitive high-efficiency reflector, such as a Bragg grating based on a polarization-maintaining fiber, so that the laser has a high degree of polarization selectivity. At this time, due to the characteristics of stimulated radiation, a high degree of polarization is spontaneously maintained in the laser oscillator, and no polarization-maintaining fiber is required. This function can greatly reduce the difficulty of making optical fibers in the laser cavity, especially fluoride fibers used in the 3 to 5 micron mid-infrared window. This unique resonant cavity structure can effectively utilize the non-polarization-maintaining fiber laser resonant cavity to achieve linearly polarized laser output with an extinction ratio of more than 99.997% along the fast axis or slow axis, or circularly polarized output with the same dual polarization power, providing conditions for remote sensing or coherent laser synthesis. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a schematic diagram of the principle of the mid-infrared fiber laser of the present invention;

[0025] Figure 2 This is a schematic diagram of the structure of the mid-infrared fiber laser of the present invention (single-clad gain fiber low-noise laser design);

[0026] Figure 3This is a schematic diagram of the structure of the mid-infrared fiber laser of the present invention (double-clad gain fiber high-power laser design);

[0027] Figure 4 It is a slope efficiency optimization diagram generated by the adjustable output of an embodiment of the present invention;

[0028] Figure 5 This is a polarization-adjustable spectrum diagram of the laser output according to an embodiment of the present invention. DETAILED DESCRIPTION

[0029] The present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0030] The main transparency window of the atmosphere is located in the range of 3 to 5 microns, while there is a low-loss window in the atmosphere in the range of 2.09 to 2.1 microns. Therefore, compact and robust fiber lasers in this wavelength range are of great interest for coherent light detection and ranging (LIDAR) technology and are expected to become potential light sources for new optical communication windows. In this wavelength range, laser modulation is indispensable in both application areas, whether for information transmission in hollow-core optical fibers or for improving the performance of lidar. The modulator requires stable, linearly polarized input light with narrow linewidth. Therefore, the development of long-term stable laser sources with linear polarization characteristics is crucial for optical systems in this wavelength range. A highly polarized laser can provide additional degrees of freedom for data retrieval, a technology that has long been used in polarized backscatter lidar. In addition, the optical system should also have a high and stable degree of polarization (DOP) to meet the long-term stability requirements of remote sensing or coherent synthesis.

[0031] The electric fields propagating clockwise and counterclockwise interfere when they recombine at the coupler. If the two paths are exactly the same, the 50 / 50 coupler will act as a perfect mirror. However, if the two polarizations are placed on different axes, interference can be completely avoided or the loop mirror can be made to have zero reflection. When used inside a laser cavity, it can be used as an adjustable reflector and has a strong dependence on polarization. Therefore, FLM-based laser cavities can have a low laser threshold while having high slope efficiency. In addition, the loop mirror has a strong polarization dependence and therefore acts as a polarization-selective element. With proper cavity design, highly polarization-sensitive laser reflection can be achieved, while at the other end is another polarization-sensitive high-reflectivity device (such as PM-FBG), further improving the polarization selectivity of the oscillator. Among them, PM-FBG is manufactured on polarization-maintaining fiber, which helps to ensure the all-fiber characteristics and mechanical robustness of the laser. Its fast and slow axis Bragg wavelengths are slightly different in center (<10nm). The bandwidth and reflectivity of the two axes are similar. The polarization controller can use a squeeze-type polarization controller to minimize insertion loss. At the same time, the polarization controller can be driven by a motor and controlled by a computer or a single-chip microcomputer to achieve automatic adjustment.

[0032] The ideal coupling ratio of the coupler is 50 / 50 and it is used as a loop mirror at the output end, but other splitting ratios (such as 85 / 15) can also be used depending on the actual situation. The second polarization controller is used to adjust the reflectivity of the loop mirror to maximize ASE suppression and slope efficiency. The coupler can be composed of fiber or free space devices, but fiber devices are generally used to improve system integration and robustness.

[0033] For 2.1-micron lasers, 1950nm wavelength thulium-doped fiber lasers can be used as pump lasers. At this time, for lasers within 10W, the single-clad fiber / core pumping design can still be used.

[0034] When higher output power is required or a dual-wavelength pumping solution is needed, a double-clad fiber / cladding pumping design is generally used due to the low mode field quality of high-power pump lasers and the high power density of high-power lasers.

[0035] The leakage light from the high reflectivity port can be used to adjust the output / monitor the working condition of the laser. Connect the high reflectivity end to a 90 / 10 coupler. An optical spectrum analyzer (OSA) is connected to the 10% port to record the spectrum. The 90% coupler port is connected to a polarization controller or directly adopts a polarization-maintaining structure, and then a polarization beam splitter (PBS) is used to monitor the power on the two polarization axes. The power of the two ports can provide polarization information and output power information to the host computer, and the spectrum of the OSA can provide the host computer with specific operating wavelength and linewidth information.

[0036] Example:

[0037] The present application provides a mid-infrared fiber laser oscillator based on a reflectivity-adjustable ring mirror and a polarization-sensitive high-reflection mirror. Figure 1 The schematic diagram of the system principle of the mid-infrared fiber laser oscillator based on the reflectivity adjustable ring mirror and the polarization sensitive high reflective mirror is shown. Figure 1 As shown, the mid-infrared fiber laser oscillator based on the reflectivity adjustable ring mirror and the polarization-sensitive high-reflection mirror includes a front pump laser FL, a first wavelength division multiplexer WDM1, a laser oscillator 101, and a laser working state monitoring and controller 102. Specifically, the pump laser FL passes through the optical fiber P11, passes through the polarization-sensitive reflector 101-1, and is absorbed by the gain fiber AF. The gain fiber AF generates amplified spontaneous radiation along the directions of P14 and P21. Among them, the spontaneous radiation in the direction of P21 is introduced into the first optical splitter 101-2 through the second wavelength division multiplexer WDM2. The first optical splitter divides the spontaneous radiation energy into P23 and P24 loops. Its light field circulates clockwise and counterclockwise respectively, and returns to the first optical splitter 101-2. At this time, the light field produces an interference effect, and the polarization of the clockwise and counterclockwise light fields is adjusted through the second polarizer PC2, and the light field interference intensity is changed, thereby realizing the continuous adjustment of the ring mirror reflectivity from 0 to a%. In particular, if the splitting ratio of the beam splitter is 50 / 50, the maximum value of a is 100%. After the interference is completed, the reflected light returns to the laser oscillator via P22 and enters the gain fiber AF again. At this time, the spontaneous radiation becomes stimulated radiation, but the wavelength and mode are not yet stable. The stimulated radiation enters the first polarization control via P14, and the polarization of the stimulated radiation is adjusted by the first polarization controller. It should be noted that at this time, the stimulated radiation has not yet shown a strong polarization characteristic, that is, the power of W1 and W2 is close, and the polarizations are perpendicular to each other. The stimulated radiation after polarization change passes through P13 and enters the polarization-sensitive reflector 101-1. By adjusting 101-1, the stimulated radiation near the wavelength of W1 corresponds to the strong reflection polarization (ne) of 101-1, and the reflection of W1 is slightly enhanced. At the same time, the gain radiation of other parts is discarded. At this time, W1 is reflected back to the gain fiber. Since the polarization regulation of the first polarization controller PC1 is a reciprocal operation, the polarization of W1 remains unchanged at this time. After entering the gain fiber AF, the stimulated radiation with the same wavelength and polarization as W1 is further enhanced. At the same time, due to the gain competition effect, the radiation energy of other polarizations and wavelengths is reduced. The further amplified W1 stimulated radiation passes through the second wavelength division multiplexer WDM2, enters the first optical splitter 101-2, and is returned to the oscillator again. When the power provided by the pump lasers FL and BL can compensate for the oscillator loss, after multiple oscillations in the cavity, W1 is output from P25 in the form of narrow linewidth laser, and is sent to the subsequent optical path or application through the first isolator ISO1.

[0038] Since the reflector 101-1 cannot satisfy 100% complete reflection, its leakage light can be used for monitoring and automatic operation of the laser, as shown in 102. A small part (<1%) of the laser power passes through WDM1 and enters the second optical splitter OC2 along P32 from the second isolator ISO2. OC2 adopts non-uniform splitting, usually 90 / 10. Among them, 10% of the power enters the spectrometer OSA through P33, providing the upper computer with parameters such as the laser's spectral wavelength, noise compression ratio, and laser line width. The remaining 90% of the power is split along P35 and P36 according to two mutually perpendicular polarizations by the polarization beam splitter PBS, and enters the power meter PM1 and PM2. The power meter reading is fed back to the upper computer DC, and the polarization state and polarization extinction ratio of the laser can be obtained. At the same time, the total power of PM1 and PM2, combined with the reflectivity of the reflector 101-1, can be used to deduce the output power of the laser. Using the above parameters, the upper computer can adjust the polarization controller through E11 and E12 to optimize or control the laser output state.

[0039] Lasers are mainly used in the mid-infrared band, that is, wavelengths of 2000nm and above, but the same principle can also be applied to communications or shorter laser wavelengths, such as 1000nm, etc.

[0040] Figure 2 Shown based on Figure 1 Schematic diagram of the optical structure of the single-clad laser principle, which is mainly aimed at lasers below 10W, with high signal-to-noise ratio and low line width as the main features. Figure 2As shown, the single-clad laser includes single-mode pump lasers 201 and 210, which are used to generate pump light for driving the laser. Wavelength division multiplexers 202 and 206 are used to send pump light into the laser and separate the laser output wavelength from the pump light. Fiber Bragg grating 203 uses polarization-maintaining fiber to achieve high polarization selectivity and sensitivity, and provides high selectivity of wavelength and polarization for stimulated radiation generated by gain fiber 205. Squeezing polarization controller 204 is used to change the polarization of stimulated radiation from 205 or 203, enhance the polarization extinction ratio, and can also adjust the laser to another working wavelength and polarization. Note that polarization controllers 203 and 208 can also be composed of free-space paddles or paddle polarization controllers. Here, squeezing is used to improve the integration. Optical coupler 207 usually adopts a fiber coupler, in which the output ports on one side are connected to each other and connected to polarization controller 208 to form a fiber ring mirror; one of the two ports on the other side is connected to wavelength division multiplexer 206, and the stimulated radiation is reflected to gain fiber 205 to complete the closure of the laser oscillator. Through the polarization controller 208, the reflection efficiency of the ring mirror is reduced, the output power of the laser is maximized, and the polarization extinction ratio of the laser is also improved. The other is connected to the isolator 209 to output the laser. The laser leaked from 203 is separated by the wavelength division multiplexer 202 and enters the isolator 211. Using the fiber coupler 211, the leaked light is unequally divided into the polarization combiner 214 and the spectrometer 213. Usually, the splitting ratio of 212 is 90 / 10. The polarization combiner decomposes 90% of the power orthogonally and sends two mutually perpendicular polarized lights into the power meter 215 and the power meter 216.

[0041] Figure 3 Shown based on Figure 1 Schematic diagram of the optical structure of the principle double / multi-cladding laser, which is mainly aimed at lasers above 10W or mid-infrared fiber lasers in the band above 2.2 microns, with high power, long wavelength, and high atmospheric permeability as the main features. To adapt to high-power operation, the ring mirror formed by the coupler 301 is combined in the form of a combination of three wave plates 316, 317, and 318. Here, the polarization rotation combination of the paddles is not unique, as long as any polarized light can be adjusted to linear polarization on the Poincare sphere, such as a combination of 1 / 4 wave plate 316, 1 / 2 wave plate 317, and 1 / 4 wave plate 315. For the dual-wavelength pumping scheme required for 3.4 to 3.8 micron lasers, a multi-wavelength wavelength division multiplexer 305 is required, which can be made by a fiber-coupled multi-layer dielectric coated mirror. 306 and 307 are multi-mode high-power optical fibers, and the wavelength ratio must be the same depending on the application. After the pump light passes through 305, it is absorbed by the gain fiber 303. To reduce the thermal effect in the cavity, the remaining pump light is stripped by the pump stripper 302. Figure 1After being adjusted by the polarization controller 309, the stimulated radiation enters the high-reflection fiber grating 304 composed of polarization-maintaining fiber, and is reflected again to form a laser oscillator. After multiple oscillations between 301 and 304, a stable laser output is generated, and finally, the mid-infrared laser is output through the isolator 319. Similarly, Figure 1 , the leakage light of 304 is separated by the wavelength division multiplexer 305, and enters the coupler 311 through the isolator 308. Due to the high power, the splitting ratio of 309 is usually 1 / 99, and 1% of the power enters the spectrometer 310; the remaining 99% is orthogonally decomposed by the polarization beam combiner 312, and the two mutually perpendicular polarizations are sent to the power meters 313 and 314.

[0042] When different pump modes are used to amplify the stretched pulse S5, the amplification logic is slightly different. Specifically:

[0043] In an exemplary embodiment of the present application, the optical devices inside the oscillator 101 can be replaced in whole or in part by polarization-maintaining optical fibers; similarly, the optical devices inside the oscillator 101 can also be replaced in part or in whole by spatial optical devices with similar functions. In an embodiment of the present application, lasers in the 2.1-micron band can be generated by using thulium-doped, holmium-doped or thulium-holmium co-doped optical fibers; similarly, the optical fiber can be replaced by a doped bulk gain medium, such as Ho:YAG, Tm:YAG and other gain media. Similarly, for the 3-micron to 5-micron band, the oscillator scheme is also applicable to fluoride optical fibers or soft glass materials co-doped with holmium, erbium, thulium and dysprosium alone or with multiple elements. Therefore, in the present application, it is only necessary to adjust the wavelengths of the pump lasers BL and FL to meet the excitation light quantum energy of the corresponding rare earth elements, and combine with the reflectors of the corresponding wavelengths to generate lasers at the corresponding wavelengths. Furthermore, in an embodiment of the present application, even for single-clad optical fibers, single-phase pumping can be used, and laser output with high polarization and adjustable slope efficiency can also be obtained.

[0044] In order to make the technical solution of the present application clearer, the working mechanism and characteristics of the mid-infrared fiber laser oscillator based on the reflectivity adjustable ring mirror and the polarization-sensitive high-reflection mirror in the present application are explained with a specific embodiment.

[0045] The holmium-doped fiber 205 is pumped by 1.95 micron pump light through wavelength division multiplexers 202 and 206, thereby generating spontaneous radiation with a 3 dB bandwidth of about 300 nanometers at around 2.1 microns.

[0046] Spontaneous radiation propagates to both sides of the gain fiber, reflects at the ring mirror formed by the reflection grating 203 and the coupler 207, and returns to the laser oscillator. At this time, maintain a low laser pump (<1W) to keep the laser below the light output power threshold. The laser output state is monitored by power meters 215 and 216 and spectrometer 213, and the polarization controller is adjusted by the host computer to make the polarization of 207 efficiently reflected coincide with the wavelength and polarization of 203 efficiently reflected, reduce the loss in the laser cavity, and gradually meet the laser light output conditions.

[0047] After the laser reaches the threshold, the pump power is maintained, and the polarization controller 204 and the polarization controller 208 are fine-tuned to optimize the output efficiency to achieve Figure 4 Asymptotes for the “high Q, low pump” condition.

[0048] At this time, the pump power can be increased to the maximum value, but limited by the output ratio of the ring mirror formed by the coupler 207, the power can only be increased along the asymptote of "high Q value, low pump", and the slope efficiency is not optimized.

[0049] According to the sequence of "polarization controller 208 - polarization controller 204", the oscillator output efficiency is repeatedly adjusted and polarization changes are compensated to increase the laser output power and make its slope efficiency approach the asymptote of "low Q value, high pumping", thereby achieving the maximum output power and obtaining the highest slope efficiency.

[0050] This process can be combined with machine learning to achieve high-speed, fully automatic adjustment.

[0051] The laser output spectrum is as follows Figure 5 As shown, the polarizations of the two wavelengths output by the laser are orthogonal to each other, and the power of each wavelength can be modulated by more than 30 dB. By adjusting the polarization controller 204 and the polarization controller 208, the laser can output linear polarization, circular polarization or elliptically polarized light at any angle under the same output power.

Claims

1. A mid-infrared fiber laser with high polarization slope efficiency and adjustable efficiency, characterized in that: It includes a front pump laser FL, a first wavelength division multiplexer WDM1, a laser oscillator and a laser working state monitor and controller; The laser oscillator comprises a polarization-sensitive reflector, a gain fiber AF, a second wavelength division multiplexer WDM2, a first optical splitter, a second polarization controller PC2, a first isolator ISO1, a first polarization controller PC1 and a post-pump laser BL; The laser working state monitoring and controller includes a second isolator ISO2, a second optical splitter OC2, a spectrometer OSA, a first power meter PM1, a second power meter PM2, a host computer DC and a polarization beam splitter PBS; The front pump laser FL is a gain fiber AF, which provides a single-mode or multi-mode pump laser, passes through the optical fiber P11, passes through the polarization-sensitive reflector, and is absorbed by the gain fiber AF; The gain fiber AF generates amplified spontaneous radiation along two different directions P14 and P21; wherein, the spontaneous radiation in the direction of P21 is introduced into the first optical splitter through the second wavelength division multiplexer WDM2; the first optical splitter divides the spontaneous radiation energy into P23 and P24 loops, and the two loop light fields circulate clockwise and counterclockwise respectively and return to the first optical splitter, and the light field generates an interference effect, and the polarization of the clockwise and counterclockwise light fields is adjusted through the second polarizer PC2, and the light field interference intensity is changed, thereby realizing continuous adjustment of the reflectivity of the ring mirror from 0 to a%; The reflected light from the first beam splitter after interference returns to the laser oscillator and enters the gain fiber AF again. At this time, the spontaneous radiation becomes stimulated radiation. The stimulated radiation enters the first polarization controller through the direction of P14. The polarization of the stimulated radiation is adjusted by the first polarization controller. The polarization-changed stimulated radiation is incident on the polarization-sensitive reflector, and by adjusting the polarization-sensitive reflector, the stimulated radiation near the high reflection wavelength of the polarization-sensitive reflector corresponding to the laser target working wavelength W1 is made to correspond to the strong reflection polarization ne of the polarization-sensitive reflector, thereby enhancing the reflection of W1; Then W1 is reflected back to the gain fiber AF, and the further amplified W1 stimulated radiation passes through the second wavelength division multiplexer WDM2, enters the first optical splitter, and is returned to the laser oscillator again; after multiple oscillations, W1 passes through the first isolator ISO1 in the form of narrow linewidth laser and is sent to the subsequent optical path or application; A small part of the laser power passes through the first wavelength division multiplexer WDM1 and enters the second optical splitter OC2 through the second isolator ISO2; OC2 adopts non-equiproportional splitting, and a small part of the power enters the spectrometer OSA to provide the host computer with the spectrum wavelength, noise compression ratio, and laser line width parameters of the mid-infrared fiber laser. The remaining power is split by the polarization beam splitter PBS according to two mutually perpendicular polarizations and enters the power meters PM1 and PM2 respectively; the readings of the two power meters are fed back to the host computer DC to obtain the polarization state and polarization extinction ratio of the laser; at the same time, the total power of the power meters PM1 and PM2, combined with the reflectivity of the polarization-sensitive reflector, can be used to deduce the output power of the laser; According to the parameters obtained in the previous section, the host computer connects the first polarization controller PC1 and the second polarization controller PC2, adjusts the two polarization controllers, and optimizes or controls the laser output state.

2. A mid-infrared fiber laser with high polarization slope efficiency and adjustable efficiency according to claim 1, characterized in that: The pump wavelength of the single-mode or multi-mode pump laser is 790nm, 1950nm or 1550-1650nm.

3. A mid-infrared fiber laser with high polarization slope efficiency and adjustable efficiency according to claim 1, characterized in that: The laser is oriented towards the mid-infrared band, i.e., wavelengths of 2000 nm and above.

4. The mid-infrared fiber laser with high polarization slope efficiency and adjustable efficiency according to claim 1, characterized in that: If the splitting ratio of the beam splitter is 50 / 50, then a is at most 100%.

5. The mid-infrared fiber laser with high polarization slope efficiency and adjustable efficiency according to claim 1, characterized in that: The small portion of laser power refers to less than 1% of laser power.

6. The mid-infrared fiber laser with high polarization slope efficiency and adjustable efficiency according to claim 1, characterized in that: The second optical splitter OC2 adopts non-uniform ratio splitting, with a ratio of 90 / 10.

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