2-5μm cascaded soft glass fiber mid-infrared wide spectrum laser

Abstract

The present invention provides a 2-5 μm band cascaded soft glass fiber mid-infrared wide spectrum laser, which adopts a passive mode locking method to obtain a 2 μm band seed laser, and then amplifies the laser power through a multi-stage amplifier, and uses a fiber pulse stretcher to perform pulse stretching. By managing the nonlinear effect of each stage of amplification and the thermal effect of the gain fiber, the light conversion efficiency can be improved, and the obtained 2 μm band pump light source further pumps the cascaded soft glass fiber, and the melting point and soft glass fiber heat dissipation treatment are performed by refrigerated nitrogen, which effectively alleviates the heat accumulation at the melting point and can obtain an average power output of tens of watts. The present invention is based on an all-fiber structure, so that the fiber laser has the advantages of compact structure, high integration, good stability, high conversion efficiency, etc., is insensitive to interference factors such as vibration in the working environment, greatly improves the stability and reliability of the laser operation, is suitable for industrial mass production, and is more conducive to the construction of large-scale laser systems.

Description

Technical Field

[0001] The invention belongs to the field of laser technology, and in particular relates to a 2-5 μm band cascaded soft glass optical fiber mid-infrared wide spectrum laser. Background Art

[0002] Mid-infrared ultrashort pulse fiber lasers have the advantages of high light conversion efficiency, good beam quality, simple and compact structure, etc. They are known as the new generation of high-performance industrial lasers, medical lasers and military lasers. In particular, the 2-5μm mid-infrared band is a high-transmittance atmospheric window, and is also a common response spectrum band for various detectors in optoelectronic countermeasures. It has a major strategic demand in the field of national defense and security. In order to produce mid-infrared SC, a 2μm band light source is usually used to pump nonlinear optical fibers to obtain optical fibers with low transmission loss in the mid-infrared region, such as soft glass optical fibers composed of tellurites, sulfides and fluorides. Among all soft glass optical fibers, fluoride glass optical fibers are more mature in technology and they have been commercialized.

[0003] In 2011, the University of Michigan used a 25 m single-mode fiber to transfer the output wavelength of EYDFA from 1.55 μm to 2 μm, and then used a first-stage thulium-doped fiber amplifier to amplify its power, and then pumped ~8.5 m ZBLAN fiber, ultimately increasing the output power of the supercontinuum spectrum to 2.6 W, covering the spectral range of 1.9 μm-4.5 μm. In 2012, the Franco-German Institute used a 2 μm thulium-doped laser pulse with active Q-switched mode-locked output to pump a 15 m single-mode ZBLAN fiber with a core cladding diameter of 8 / 125 μm, achieving a spectral output of 1.08 W with a spectral range of 1.9 μm-3.6 μm. In 2013, the Optoelectronics Research Center of the University of Southampton used a 3.5 μJ, 33 ps, 2008 nm thulium-doped fiber amplifier to pump a 7 m ZBLAN fiber, also achieving a watt-class supercontinuum spectrum output of 750 nm-4000 nm. In the same year, the National University of Defense Technology carried out three-stage power amplification on the SESAM mode-locked 26 ps thulium-doped fiber laser, pumping a 10m ZBLAN fiber with a core cladding diameter of 8 / 130 μm and NA=0.27, and finally achieved a supercontinuum output of 7.11 W with a spectral range of 1.9μm-3.9 μm, and a power of about 3.52W at >2.5 μm.

[34] . In 2014, the power was increased to 13 W, and the spectral range was ~1.9μm-4.3μm. In 2016, Z. Zheng and S. Ruan of Shenzhen University used the fusion splicing technology of quartz fiber and ZBLAN fiber to integrate the high-power 1.95 μm MOPA system and ZBLAN fiber, realizing an all-fiber mid-infrared supercontinuum laser light source with a wavelength coverage of 1.9-4.1μm, with a maximum output power of 10.6 W. However, the average power, spectral width, and spectral flatness of the current mid-infrared ultrashort pulse fiber laser are still limited by the nonlinear soft glass fiber transmission loss and fusion efficiency. At the same time, due to the large difference in glass softening temperature between mid-infrared soft glass fiber and ordinary quartz glass fiber (for example, the glass softening temperature difference between ZBLAN fiber and quartz fiber is nearly 1000℃), the fusion loss between the two is very large, which brings challenges to the realization of high-power all-fiber mid-infrared wide-spectrum laser output. Therefore, it is still of great significance to further study the generation and application of mid-infrared wide-spectrum and improve its performance. Summary of the invention

[0004] To solve the above problems, the present invention provides a 2-5μm band cascaded soft glass fiber mid-infrared wide-spectrum laser, which can obtain high-power, wide-spectrum laser pulse output and has the advantages of compact structure, high integration, good stability, high conversion efficiency, etc.

[0005] A 2-5 μm band cascaded soft glass fiber mid-infrared wide spectrum laser, comprising a mode-locked fiber laser (1), a preamplifier (2), a fiber stretcher (3), an amplifier (4), a first soft glass fiber (5), a second soft glass fiber (6) and a nitrogen refrigeration device (7) connected in sequence;

[0006] The mode-locked fiber laser (1) is used to generate mode-locked pulsed laser in the 2 micron band;

[0007] The preamplifier (2) is used to preamplify the power of the mode-locked pulse laser for the first time;

[0008] The optical fiber stretcher (3) is used to stretch the pulse of the mode-locked pulse laser after the first pre-amplification;

[0009] The amplifier (4) is used to amplify the power of the mode-locked pulse laser after pulse broadening for a second time;

[0010] The first soft glass optical fiber (5) is used to perform a first spectrum broadening on the mode-locked pulse laser after the second amplification;

[0011] The second soft glass optical fiber (6) is used to perform second…n times of spectrum broadening on the pulsed laser after the first spectrum broadening;

[0012] The nitrogen refrigeration device (7) is used to cool each soft glass optical fiber and its melting point to a temperature of -50 to -20°C.

[0013] Furthermore, the mode-locked fiber laser (1) comprises a saturable absorber (11), a gain fiber (12), a partially reflective fiber Bragg grating (13), a wavelength division multiplexer (14), a semiconductor laser (15), a fiber coupling output device (16) and a first fiber isolator (17); these are connected in sequence through single-mode optical fibers to form a linear cavity structure, and the above devices are all connected by optical fiber fusion splicing, the wavelength division multiplexer (14) and the semiconductor laser (15) are used to provide pump energy, the gain fiber (12) is used to provide gain, the saturable absorber (11) and the partially reflective fiber Bragg grating (13) are used to form a laser resonant cavity, and the saturable absorber (11) is used to achieve mode-locked laser output, the fiber coupling output device (16) comprises two ports, one of which is used to output mode-locked pulse laser, and the other port is used to detect the stability of the mode-locked pulse laser, and the first fiber isolator (17) is used to isolate feedback light formed by a subsequent optical path.

[0014] The mode-locked fiber laser (1) is implemented by passive mode locking; the saturable absorber (11) is a saturable absorber mirror with a modulation depth of 20% and a relaxation time of 10%; the gain fiber (12) is a single-mode thulium-doped fiber; and the laser wavelength band of the semiconductor laser (15) is 1570 nm.

[0015] Furthermore, the preamplifier (2) comprises a 793nm semiconductor laser (21), a fiber combiner (22), a single-mode double-clad thulium-doped optical fiber (23) and a second fiber isolator (24); the 793nm semiconductor laser (21) is used to generate pump light; the fiber combiner (22) is used to couple the pump light and the mode-locked pulse laser into the single-mode double-clad thulium-doped optical fiber (23); the single-mode double-clad thulium-doped optical fiber (23) is used to provide a gain medium for the mode-locked pulse laser under the pumping of the pump light, thereby achieving pre-amplification of the mode-locked pulse laser power; and then the pre-amplified mode-locked pulse laser enters the fiber stretcher (3) through the second fiber isolator (24); the second fiber isolator (24) is used to isolate feedback light formed by a subsequent optical path; and the above devices are all connected by fiber fusion splicing.

[0016] Furthermore, the optical fiber stretcher (3) is arranged between the pre-amplifier (2) and the amplifier (4), and the optical fiber stretcher (3) provides positive dispersion to stretch the pulse width.

[0017] Furthermore, the amplifier (4) comprises a 793nm semiconductor laser, a fiber combiner, a thulium-doped fiber and a fiber isolator. The 793nm semiconductor laser is used to generate pump light. The fiber combiner is used to couple the pump light and the mode-locked pulse laser into the thulium-doped fiber. The thulium-doped fiber is used to provide a gain medium for the mode-locked pulse laser under the pumping of the pump light, thereby further amplifying the pulse laser power. The fiber isolator is used to isolate the feedback light formed by the subsequent optical path. The above devices are all connected by fiber fusion splicing.

[0018] The difference between the amplifier (4) and the preamplifier (2) is that the gain fiber of the preamplifier (4) needs to provide more gain, and the preamplifier (4) needs to provide a higher pump light power, so as to further increase the average power and peak power of the pulse by one to two orders of magnitude, and at the same time provide the necessary power guarantee for the mid-infrared soft glass fiber broadening process of the next stage.

[0019] Furthermore, the light transmission spectral window range of the first soft glass optical fiber (5) is smaller than the light transmission spectral window range of the second soft glass optical fiber (6), and the first soft glass optical fiber (5) and the second soft glass optical fiber (6) are connected to each other in an all-fiber manner by fiber fusion splicing.

[0020] The first soft glass optical fiber (5) and the second soft glass optical fiber (6) include any combination of germanate optical fiber and fluoride optical fiber, fluoride optical fiber and chalcogenide optical fiber, or fluoride optical fiber and tellurite optical fiber;

[0021] The second soft glass optical fiber (6) comprises one or a cascade of two or more soft glass optical fibers with successively increasing light transmission spectral window ranges. Due to the different doping materials of the soft glass optical fibers, the material loss of different soft glass optical fibers in different mid-infrared wavelength regions increases sharply. Therefore, in the selection of soft glass optical fibers, the transmission window range of the second soft glass optical fiber (6) is larger than the transmission window range of the first soft glass optical fiber (5), and the output of a wide spectral range of 2 to 5 microns is achieved by means of "cascade pumping and segmented broadening".

[0022] Furthermore, the nitrogen refrigeration device (7) comprises high-purity nitrogen (71), a pressure valve (72), a first drying tube (73), a liquid nitrogen refrigeration device (74), a second drying tube (75) and a calcium fluoride window screen (76) in sequence; the purity of the high-purity nitrogen (71) is above 99.9999%, which is to prevent water vapor from causing deliquesce of the first soft glass optical fiber (5) and the second soft glass optical fiber (6); the pressure valve (72) is used to control the nitrogen flow rate; since soft glass optical fiber is very easy to deliquesce when it comes into contact with water, a drying tube device is added, the first drying tube (73) is used to dry the high-purity nitrogen for the first time; the liquid nitrogen refrigeration device (74) is used to refrigerate the high-purity nitrogen; the second drying tube (75) is used to dry the refrigerated high-purity nitrogen for the second time; the calcium fluoride window screen (76) is used to seal the cooled nitrogen in a certain space environment, so that the first soft glass optical fiber (5) and the second soft glass optical fiber (6) can effectively dissipate heat over a large area. Beneficial Effects

[0023] The present invention provides a 2-5 μm band cascaded soft glass fiber mid-infrared wide spectrum laser. Based on an all-fiber structure, the fiber laser has the advantages of compact structure, high integration, good stability, high conversion efficiency, etc., is insensitive to interference factors such as vibration in a working environment, greatly improves the stability and reliability of the laser operation, and is suitable for industrial mass production; at the same time, the present invention uses a cascaded soft glass fiber method to broaden the spectrum, and through the cascade pump spectrum through the window gradient of the soft glass fiber with different nonlinear coefficients, the spectrum range is gradually extended to the long wavelength direction, and the pressure of soft glass fiber power enhancement and spectrum broadening is also relieved, and finally high-power, wide-spectrum laser pulse output can be obtained, which is more conducive to gas detection, laser medical treatment and military applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 A schematic diagram of the structure of a 2-5 μm band cascaded soft glass fiber mid-infrared wide spectrum laser provided by the present invention;

[0025] 1-mode-locked fiber laser, 2-preamplifier, 3-fiber stretcher, 4-amplifier, 5-first soft glass fiber, 6-second soft glass fiber, 7-nitrogen refrigeration.

[0026] Figure 2 A schematic diagram of the structure of a 1-mode-locked fiber laser of a 2-5 μm band cascaded soft glass fiber mid-infrared wide spectrum laser provided by the present invention;

[0027] 11-saturable absorber, 12-gain fiber, 13-partially reflective fiber Bragg grating, 14-wavelength division multiplexer, 15-semiconductor laser, 16-fiber coupler output device, 17-fiber isolator;

[0028] Figure 3 A schematic diagram of the structure of a 2-preamplifier for a 2-5 μm band cascaded soft glass fiber mid-infrared wide spectrum laser provided by the present invention;

[0029] 21-793nm semiconductor laser, 22-fiber combiner, 23-single-mode double-clad thulium-doped fiber, 24-fiber isolator;

[0030] Figure 4 A schematic diagram of the structure of a 7-nitrogen refrigeration device for a 2-5 μm band cascaded soft glass fiber mid-infrared wide spectrum laser provided by the present invention;

[0031] 71 - high purity nitrogen, 72 - pressure valve, 73 - first drying tube, 74 - liquid nitrogen refrigeration device, 75 - second drying tube, 76 - calcium fluoride window screen. DETAILED DESCRIPTION

[0032] In order to enable those skilled in the art to better understand the solution of the present application, the technical solution of the embodiment of the present application is clearly and completely described below in conjunction with the drawings in the embodiment of the present application.

[0033] See also Figure 1 , which is a schematic diagram of the structure of the 2-5 μm band cascaded soft glass fiber mid-infrared wide spectrum laser provided by the present invention. The 2-5 μm band cascaded soft glass fiber mid-infrared wide spectrum laser comprises a mode-locked fiber laser (1), a preamplifier (2), a fiber stretcher (3), an amplifier (4), a first soft glass fiber (5), a second soft glass fiber (6) and a nitrogen refrigeration device (7);

[0034] The mode-locked fiber laser (1) is used to generate mode-locked pulsed laser in the 2 micron band;

[0035] The preamplifier (2) is used to preamplify the power of the mode-locked pulse laser for the first time;

[0036] The optical fiber stretcher (3) is used to stretch the pulse of the mode-locked pulse laser after the first pre-amplification;

[0037] The amplifier (4) is used to amplify the power of the mode-locked pulse laser after pulse broadening for a second time;

[0038] The first soft glass optical fiber (5) is used to perform a first spectrum broadening on the mode-locked pulse laser after the second amplification;

[0039] The second soft glass optical fiber (6) is used to perform second…n times of spectrum broadening on the pulsed laser after the first spectrum broadening;

[0040] The nitrogen refrigeration device (7) is used to perform low-temperature cooling on each soft glass optical fiber and its melting point.

[0041] See also Figure 2 , which is a schematic structural diagram of a mode-locked fiber laser (1) of a 2-5 μm band cascaded soft glass fiber mid-infrared wide spectrum laser provided by the present invention. The mode-locked fiber laser (1) comprises a saturable absorber (11), a gain fiber (12), a partially reflective fiber Bragg grating (13), a wavelength division multiplexer (14), a semiconductor laser (15), a fiber coupling output device (16) and a first fiber isolator (17); they are connected in sequence through single-mode optical fibers to form a linear cavity structure; the above devices are all connected by optical fiber fusion splicing; the wavelength division multiplexer (14) and the semiconductor laser (15) are used to provide pump energy; the gain fiber (12) is used to provide gain; the saturable absorber (11) and the partially reflective fiber Bragg grating (13) are used to form a laser resonant cavity; the saturable absorber (11) is used to achieve mode-locked laser output; the fiber coupling output device (16) comprises two ports, one of which is used to output mode-locked pulse laser and the other is used to detect the stability of the mode-locked pulse laser; the first fiber isolator (17) is used to isolate feedback light formed by a subsequent optical path;

[0042] It can be seen that the connection relationship and working principle of the optical components of the mode-locked fiber laser (1) are as follows: a saturable absorber (11), a gain fiber (12), a partially reflective fiber Bragg grating (13), a wavelength division multiplexer (14), a fiber coupling output device (16) and a first fiber isolator (17) are sequentially arranged along the optical path, and a semiconductor laser (15) is connected to the wavelength division multiplexer (14) through an optical fiber. In the linear cavity, the fully reflective saturable absorber (11) serves as a cavity mirror of the resonant cavity, and the partially reflective fiber Bragg grating (13) serves as another cavity mirror of the resonant cavity, which plays a role in wavelength selection and can also serve as a beam splitter to keep a part of the signal light in the resonant cavity to continue oscillating, and output the other part of the signal light to the next stage. The packaging method adopts a full-fiber packaging method, and is feasible in experiments. The semiconductor laser (15) is coupled into the cavity through a wavelength division multiplexer (14) by means of core pumping to pump the gain optical fiber (12), thereby generating a stable seed mode-locked laser pulse. The output end of a portion of the mode-locked seed laser pulse is connected to a photoelectric probe via an optical fiber coupler output device (16) to monitor the stability of the seed oscillator. The first optical fiber isolator (17) is used to prevent the backward feedback light from damaging the oscillator.

[0043] It should be noted that the length of the gain fiber (12) of the mode-locked fiber laser (1), the reflectivity of the partially reflecting fiber Bragg grating (13), the splitting ratio of the fiber coupling output (16), etc. can be adjusted according to actual conditions and the pulse characteristics of the seed laser, the pump power, etc. Further, the mode-locked fiber laser (1) is implemented by passive mode locking; the saturable absorber (11) is a saturable absorber mirror with a modulation depth of 20% and a relaxation time of 10%; the gain fiber (12) is a single-mode thulium-doped fiber; and the laser wavelength of the semiconductor laser (15) is 1570 nm.

[0044] See also Figure 3 , which is a schematic structural diagram of a preamplifier (2) for a 2-5 μm band cascaded soft glass fiber mid-infrared wide spectrum laser provided by the present invention. The preamplifier (2) comprises a 793 nm semiconductor laser (21), a fiber combiner (22), a single-mode double-clad thulium-doped fiber (23) and a second fiber isolator (24). The 793 nm semiconductor laser (21) is used to generate pump light. The fiber combiner (22) is used to couple the pump light and the mode-locked pulse laser into the single-mode double-clad thulium-doped fiber (23). The single-mode double-clad thulium-doped fiber (23) is used to provide a gain medium for the mode-locked pulse laser under the pumping of the pump light, thereby realizing the pre-amplification of the mode-locked pulse laser power. Then, the pre-amplified mode-locked pulse laser enters the fiber stretcher (3) through the second fiber isolator (24). The second fiber isolator (24) is used to isolate the feedback light formed by the subsequent optical path. The above devices are all connected by fiber fusion splicing.

[0045] It should be noted that if the power amplified by each stage of the pre-amplifier module is too high, the nonlinear effect will be stronger. Therefore, the power amplified by the pre-amplifier module can be reduced. Through multi-stage amplification, the nonlinear effect can be reduced. Furthermore, the pre-amplifier (2) may include one or more stages of pre-amplifiers.

[0046] The pulse laser generated by the mode-locked fiber laser (1) is amplified by a pre-amplifier (2), thereby obtaining a mode-locked pulse laser with a power increased by one hundred times.

[0047] The optical fiber stretcher (3) is arranged between the preamplifier (2) and the amplifier (4), and the optical fiber stretcher (3) provides positive dispersion for stretching the pulse width after passing through the preamplifier (2). The mode-locked pulse laser after pulse stretching is amplified for the second time by the amplifier (4) to obtain a mode-locked pulse laser with further increased power.

[0048] The amplifier (4) comprises a 793nm semiconductor laser, a fiber combiner, a thulium-doped fiber and a fiber isolator. The 793nm semiconductor laser is used to generate pump light. The fiber combiner is used to couple the pump light and the mode-locked pulse laser into the thulium-doped fiber. The thulium-doped fiber is used to provide a gain medium for the mode-locked pulse laser under the pumping of the pump light, thereby further amplifying the pulse laser power. The fiber isolator is used to isolate feedback light formed by a subsequent optical path. The above devices are all connected by fiber fusion splicing.

[0049] The difference between the amplifier (4) and the preamplifier (2) is that the gain fiber of the preamplifier (4) needs to provide more gain, and the preamplifier (4) needs to provide a higher pump light power, so the power value of the 793nm semiconductor laser used by the amplifier (4) will be higher, thereby achieving a higher average power output after the amplifier (4). This is to further increase the average power and peak power of the pulse by one to two orders of magnitude, while providing the necessary power guarantee for the mid-infrared soft glass fiber broadening process of the next stage.

[0050] Therefore, the working principle of the pump light source of the 2-5 μm band cascaded soft glass fiber mid-infrared wide spectrum laser of the present invention is as follows: the mode-locked fiber laser (1) is used to generate a stable mode-locked pulse seed laser, the preamplifier (2) is used to perform a first pre-amplification on the seed laser, and output the laser after the first pre-amplification to the fiber stretcher (3), the fiber stretcher (3) is used to stretch the laser pulse after the first pre-amplification to prevent the device from being damaged by excessive peak power; the amplifier (4) is used to amplify the laser pulse after the stretching, and output a 2-micron band high-power laser after the amplifier (4), which is used as the pump light source of the first soft glass fiber (5) and the second soft glass fiber (6).

[0051] The light transmission spectrum window range of the first soft glass optical fiber (5) is smaller than the light transmission spectrum window range of the second soft glass optical fiber (6), and the first soft glass optical fiber (5) and the second soft glass optical fiber (6) are connected to each other in an all-fiber manner by fiber fusion splicing.

[0052] It should be noted that the first soft glass optical fiber (5) and the second soft glass optical fiber (6) include, for example, a combination of a germanate optical fiber and a fluoride optical fiber, a fluoride optical fiber and a chalcogenide optical fiber, or a fluoride optical fiber and a tellurite optical fiber; the second soft glass optical fiber (6) includes one or a cascade of two or more soft glass optical fibers with successively increasing light transmission spectrum window ranges. Due to the different doping materials of the soft glass optical fibers, the material loss of different soft glass optical fibers in different mid-infrared wavelength regions increases sharply. Therefore, in the selection of soft glass optical fibers, the transmission window range of the second soft glass optical fiber (6) is larger than the transmission window range of the first soft glass optical fiber (5), and the output of a wide spectrum range of 2 to 5 microns is achieved by means of "cascade pumping and segmented broadening".

[0053] Therefore, the working principle of the spectrum broadening light source of the 2-5 μm band cascaded soft glass fiber mid-infrared wide spectrum laser of the present invention is as follows: a 2-micron band high-power laser generated by a pump light source of the 2-5 μm band cascaded soft glass fiber mid-infrared wide spectrum laser is sequentially pumped to generate a spectrum laser of a certain wavelength range in the first soft glass fiber (5), and then enters the second soft glass fiber (6) for nonlinear broadening again. The dispersion and nonlinearity in the soft glass fiber cause the pulse to undergo self-phase modulation, modulation instability, soliton self-frequency shift, dispersion wave and other nonlinear effects in the soft glass fiber, so that the spectrum is further broadened in the long wavelength direction in the cascaded second soft glass fiber (6) to obtain a laser output. An octave angle is cut at the end of the final output fiber to prevent Fresnel reflection, thereby obtaining a high-power, wide spectrum laser in the 2-5 μm band.

[0054] See also Figure 4, which is a schematic structural diagram of a nitrogen refrigeration device (7) for a 2-5 μm band cascaded soft glass fiber mid-infrared wide spectrum laser provided by the present invention. The nitrogen refrigeration device (7) comprises, in sequence, high-purity nitrogen (71), a pressure valve (72), a first drying tube (73), a liquid nitrogen refrigeration device (74), a second drying tube (75) and a calcium fluoride window screen (76); the purity of the high-purity nitrogen (71) is above 99.9999% to prevent water vapor from causing deliquesce of the first soft glass optical fiber (5) and the second soft glass optical fiber (6); the pressure valve (72) is used to control the nitrogen flow rate; since soft glass optical fibers are very likely to deliquesce when in contact with water, a drying tube device is added, wherein the first drying tube (73) is used to dry the high-purity nitrogen for the first time; the liquid nitrogen refrigeration device (74) is used to refrigerate the high-purity nitrogen; the second drying tube (75) is used to dry the refrigerated high-purity nitrogen for the second time; and the calcium fluoride window screen (76) is used to seal the cooled nitrogen in a certain space environment so that the first soft glass optical fiber (5) and the second soft glass optical fiber (6) can dissipate heat effectively and over a large area.

[0055] Therefore, the working principle of the nitrogen refrigeration device (7) of the 2-5 μm band cascaded soft glass fiber mid-infrared wide spectrum laser of the present invention is as follows: high-purity nitrogen is first dried by a first drying tube to remove moisture in the air, then passed through a liquid nitrogen refrigeration device to obtain low-temperature cooled high-purity nitrogen, then passed through a drying tube again to further remove water to prevent frosting of the pipeline, and then the refrigerated nitrogen is sent to the calcium fluoride window screen. The calcium fluoride window screen is used on the one hand to allow the soft glass optical fiber to dissipate heat over a larger area, and on the other hand to prevent the screen from frosting and affecting the monitoring of the temperature of the soft glass optical fiber.

[0056] The mode-locked fiber laser (1) is used to generate a stable mode-locked pulse seed laser; the preamplifier (2) is used to perform a first pre-amplification on the seed laser and output the laser after the first pre-amplification to the fiber stretcher (3); the fiber stretcher (3) is used to stretch the laser pulse after the first pre-amplification to prevent excessive peak power from damaging the device; the amplifier (4) is used to amplify the laser pulse after the stretching and output a 2 micron band high-power laser after the amplifier (4) to serve as a pump light source for the first soft glass optical fiber (5) and the second soft glass optical fiber (6). A 2-micron band high-power laser generated by a pump light source of a 2-5 μm band cascaded soft glass fiber mid-infrared wide-spectrum laser is sequentially pumped on a first soft glass fiber (5) and a second soft glass fiber (6), firstly generating a spectrum laser in a certain wavelength range in the first soft glass fiber (5), and then entering the second soft glass fiber (6) for nonlinear broadening again. Dispersion and nonlinearity in the soft glass fiber are utilized to cause the pulse to undergo self-phase modulation, modulation instability, soliton self-frequency shift, dispersion waves and other nonlinear effects in the soft glass fiber, so that the spectrum is further broadened in the long wavelength direction in the cascaded second soft glass fiber (6) to obtain a laser output. An octave angle is cut at the end of the final output fiber to prevent Fresnel reflection, thereby obtaining a 2-5 μm band high-power, wide-spectrum laser.

[0057] The mode-locked fiber laser (1) is used to generate a mode-locked pulse laser in the 2 micron band. The mode-locked fiber laser (1) comprises a saturable absorber (11), a gain fiber (12), a partially reflective fiber Bragg grating (13), a wavelength division multiplexer (14), a semiconductor laser (15), a fiber coupling output device (16) and a first fiber isolator (17); they are sequentially connected through a single-mode optical fiber to form a linear cavity structure, and the above devices are all connected by fiber fusion splicing. The wavelength division multiplexer (14) and the semiconductor laser (15) are used to provide pump energy, the gain fiber (12) is used to provide gain, the saturable absorber (11) and the partially reflective fiber Bragg grating (13) are used to form a laser resonant cavity, and the saturable absorber (11) is used to achieve mode-locked laser output. The fiber coupling output device (16) comprises two ports, one of which is used to output the mode-locked pulse laser, and the other port is used to detect the stability of the mode-locked pulse laser. The first fiber isolator (17) is used to isolate the feedback light formed by the subsequent optical path.

[0058] The preamplifier (2) is used to preamplify the power of the mode-locked pulse laser for the first time. The preamplifier (2) comprises a 793nm semiconductor laser (21), a fiber combiner (22), a single-mode double-clad single-mode double-clad thulium-doped optical fiber (23) and a second optical fiber isolator (24). The 793nm semiconductor laser (21) is used to generate pump light. The fiber combiner (22) is used to couple the pump light and the mode-locked pulse laser into the single-mode double-clad thulium-doped optical fiber (23). The single-mode double-clad thulium-doped optical fiber (23) is used to provide a gain medium for the mode-locked pulse laser under the pumping of the pump light, so as to achieve preamplification of the power of the mode-locked pulse laser. Then, the preamplified mode-locked pulse laser enters the optical fiber stretcher (3) through the second optical fiber isolator (24). The second optical fiber isolator (24) is used to isolate feedback light formed by a subsequent optical path. The above devices are all connected by optical fiber fusion splicing.

[0059] The optical fiber stretcher (3) is used to stretch the mode-locked pulse laser after the first pre-amplification, so as to reduce the peak power and increase the threshold of the nonlinear effect.

[0060] The amplifier (4) is used to amplify the power of the mode-locked pulse laser after pulse broadening for the second time. The amplifier (4) includes a 793nm semiconductor laser, a fiber combiner, a thulium-doped fiber, and a fiber isolator. The 793nm semiconductor laser is used to generate pump light. The fiber combiner is used to couple the pump light and the mode-locked pulse laser into the thulium-doped fiber. The thulium-doped fiber is used to provide a gain medium for the mode-locked pulse laser under the pumping of the pump light, thereby further amplifying the pulse laser power. The fiber isolator is used to isolate the feedback light formed by the subsequent optical path. The above devices are all connected by fiber fusion splicing.

[0061] The glass optical fiber 1 (5) is used to perform a first spectrum broadening on the mode-locked pulse laser after the second amplification; the second soft glass optical fiber (6) is used to perform a second ... nth spectrum broadening on the pulse laser after the first spectrum broadening. The light transmission spectrum window range of the first soft glass optical fiber (5) is smaller than the light transmission spectrum window range of the second soft glass optical fiber (6), and the first soft glass optical fiber (5) and the second soft glass optical fiber (6) are connected to each other in full fiber connection by fiber fusion splicing.

[0062] The nitrogen refrigeration device (7) is used to cool each soft glass optical fiber and its melting point at low temperature. The nitrogen refrigeration device (7) comprises high-purity nitrogen (71), a pressure valve (72), a first drying tube (73), a liquid nitrogen refrigeration device (74), a second drying tube (75) and a calcium fluoride window screen (76) in sequence; the purity of the high-purity nitrogen (71) is above 99.9999% to prevent water vapor from causing deliquesce of the first soft glass optical fiber (5) and the second soft glass optical fiber (6); the pressure valve (72) is used to control the nitrogen flow rate; since soft glass optical fiber is very likely to deliquesce when in contact with water, a drying tube device is added, the first drying tube (73) is used to dry the high-purity nitrogen for the first time; the liquid nitrogen refrigeration device (74) is used to refrigerate the high-purity nitrogen; the second drying tube (75) is used to dry the refrigerated high-purity nitrogen for the second time; the calcium fluoride window screen (76) is used to seal the cooled nitrogen in a certain space environment so that the first soft glass optical fiber (5) and the second soft glass optical fiber (6) can dissipate heat effectively and over a large area.

[0063] Mid-infrared 2-5μm band high-power wide-spectrum fiber lasers are mostly used in gas detection, biomedicine, military defense and other fields. They usually have very high requirements for the power of laser pulses and the wavelength range covered by the spectrum. In order to obtain high-power pulse output, chirped pulse amplification technology is often used to obtain it. By widening the pulse in the time domain, the peak power is first reduced, and then the power is effectively amplified, and finally compressed back to the original pulse width to achieve high-power pulse output. The present invention is based on an all-fiber structure, adopts chirped pulse amplification technology, provides gain through gain fiber, uses widening fiber to widen the pulse, reduces the peak power of the pulse, and can achieve high-power power output of hundreds of watts after step-by-step amplification. It can be used as a pump light source for a high-power, wide-spectrum supercontinuum light source. By cascading pumping soft glass fibers with different dispersions, nonlinear parameters, and different transmission window ranges, high-power laser output covering the 2-5μm band can be obtained. The present invention adopts an all-fiber structure, so that the laser is small and compact, and has the advantages of compact structure, high integration, good stability and high conversion efficiency, and can obtain high-power and wide-spectrum laser output.

[0064] Of course, the present invention may have many other embodiments. Without departing from the spirit and essence of the present invention, those skilled in the art may make various corresponding changes and modifications based on the present invention, but these corresponding changes and modifications should all fall within the scope of protection of the claims attached to the present invention.

Claims

1. 2-5μm band cascaded soft glass fiber mid-infrared wide spectrum laser, characterized by: The invention comprises a mode-locked fiber laser (1), a preamplifier (2), a fiber stretcher (3), an amplifier (4), a first soft glass fiber (5), and a second soft glass fiber (6) which are connected in sequence, wherein the first soft glass fiber (5) and the second soft glass fiber (6) are fusion-connected, and the melting points of the first soft glass fiber (5) and the second soft glass fiber (6) and the first soft glass fiber (5) and the second soft glass fiber (6) are refrigerated by a nitrogen refrigeration device (7); The mode-locked fiber laser (1) is used to generate mode-locked pulsed laser in the 2 micron band; The preamplifier (2) is used to preamplify the power of the mode-locked pulse laser for the first time; The optical fiber stretcher (3) is used to stretch the pulse of the mode-locked pulse laser after the first pre-amplification; The amplifier (4) is used to amplify the power of the mode-locked pulse laser after pulse broadening for a second time; the mode-locked pulse laser after the second amplification is used as a pump light source for the first soft glass optical fiber (5) and the second soft glass optical fiber (6); The first soft glass optical fiber (5) is used to perform a first spectrum broadening on the mode-locked pulse laser after the second amplification; The second soft glass optical fiber (6) is used to perform second…n times of spectrum broadening on the pulsed laser after the first spectrum broadening; The nitrogen refrigeration device (7) is used to perform low-temperature cooling on each soft glass optical fiber and its melting point.

2. The 2-5 μm band cascaded soft glass fiber mid-infrared wide spectrum laser according to claim 1, characterized in that: The nitrogen refrigeration device (7) comprises high-purity nitrogen (71), a pressure valve (72), a first drying tube (73), a liquid nitrogen refrigeration device (74), a second drying tube (75) and a calcium fluoride window screen (76) which are arranged in sequence; The purity of the high-purity nitrogen (71) is above 99.9999%; the pressure valve (72) is used to control the nitrogen flow rate; the first drying tube (73) is used to dry the high-purity nitrogen for the first time; the liquid nitrogen refrigeration device (74) is used to refrigerate the high-purity nitrogen; the second drying tube (75) is used to dry the refrigerated high-purity nitrogen for the second time; and the calcium fluoride window screen (76) is used to seal the cooled nitrogen in a certain space environment.

3. The 2-5 μm band cascaded soft glass fiber mid-infrared wide spectrum laser according to claim 1, characterized in that: The mode-locked fiber laser (1) comprises a saturable absorber (11), a gain fiber (12), a partially reflective fiber Bragg grating (13), a wavelength division multiplexer (14), a semiconductor laser (15), a fiber coupling output device (16) and a first fiber isolator (17); they are connected in sequence through single-mode optical fibers to form a linear cavity structure, and are all connected by fiber fusion splicing; the wavelength division multiplexer (14) and the semiconductor laser (15) are used to provide pump energy, the gain fiber (12) is used to provide gain, the saturable absorber (11) and the partially reflective fiber Bragg grating (13) are used to form a laser resonant cavity, and the saturable absorber (11) is used to achieve mode-locked laser output; the fiber coupling output device (16) comprises two ports, one of which is used to output mode-locked pulse laser, and the other port is used to detect the stability of the mode-locked pulse laser; the first fiber isolator (17) is used to isolate feedback light formed by a subsequent optical path; The mode-locked fiber laser (1) is implemented by passive mode locking; the saturable absorber (11) is a saturable absorber mirror with a modulation depth of 20% and a relaxation time of 10%; the gain fiber (12) is a single-mode thulium-doped fiber; and the laser wavelength band of the semiconductor laser (15) is 1570 nm.

4. The 2-5 μm band cascaded soft glass fiber mid-infrared wide spectrum laser according to claim 1, characterized in that: The preamplifier (2) comprises a first 793nm semiconductor laser (21), a first optical fiber combiner (22), a single-mode double-clad thulium-doped optical fiber (23) and a second optical fiber isolator (24), all of which are connected by optical fiber fusion splicing; the first 793nm semiconductor laser (21) is used to generate pump light, the first optical fiber combiner (22) is used to couple the pump light and the mode-locked pulse laser into the single-mode double-clad thulium-doped optical fiber (23), the single-mode double-clad thulium-doped optical fiber (23) is used to provide a gain medium for the mode-locked pulse laser under the pumping of the pump light, so as to achieve pre-amplification of the mode-locked pulse laser power, and then the pre-amplified mode-locked pulse laser enters the optical fiber stretcher (3) through the second optical fiber isolator (24), and the second optical fiber isolator (24) is used to isolate feedback light formed by a subsequent optical path.

5. The 2-5 μm band cascaded soft glass fiber mid-infrared wide spectrum laser according to claim 1, characterized in that: The optical fiber stretcher (3) is arranged between the pre-amplifier (2) and the amplifier (4), and the optical fiber stretcher (3) provides positive dispersion to stretch the pulse width.

6. The 2-5 μm band cascaded soft glass fiber mid-infrared wide spectrum laser according to claim 1, characterized in that: The amplifier (4) comprises a second 793nm semiconductor laser, a second fiber combiner, a thulium-doped fiber and a third fiber isolator, all of which are connected by fiber fusion splicing; the second 793nm semiconductor laser is used to generate pump light, the second fiber combiner is used to couple the pump light and the mode-locked pulse laser into the thulium-doped fiber, the thulium-doped fiber is used to provide a gain medium for the mode-locked pulse laser under the pumping of the pump light, thereby further amplifying the pulse laser power, and the third fiber isolator is used to isolate feedback light formed by a subsequent optical path, all of which are connected by fiber fusion splicing.

7. The 2-5 μm band cascaded soft glass fiber mid-infrared wide spectrum laser according to claim 1, characterized in that: The light transmission spectrum window range of the first soft glass optical fiber (5) is smaller than the light transmission spectrum window range of the second soft glass optical fiber (6), and the first soft glass optical fiber (5) and the second soft glass optical fiber (6) are connected to each other in an all-fiber manner by fiber fusion splicing; The first soft glass optical fiber (5) and the second soft glass optical fiber (6) include a combination of germanate optical fiber and fluoride optical fiber, fluoride optical fiber and chalcogenide optical fiber, or fluoride optical fiber and tellurite optical fiber; The second soft glass optical fiber (6) comprises one or a cascade of two or more soft glass optical fibers whose light transmission spectral window ranges increase successively.

8. The 2-5 μm band cascaded soft glass fiber mid-infrared wide spectrum laser according to claim 1, characterized in that: The temperature range of the low temperature cooling is -50°C to -20°C.

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