Kilowatt-level single-frequency fiber laser

Abstract

The invention relates to a kilowatt-level single-frequency fiber laser which comprises a seed light source module, a gain unit, a forward pumping unit, a backward pumping unit and an output unit, the forward pumping unit, the backward pumping unit and the output unit are connected with the gain unit, and the seed light source module is connected to the gain unit through the forward pumping unit. The gain unit is used for performing power amplification on initial laser based on the first pumping laser and the second pumping laser and then outputting the initial laser to the output unit, the output unit is used for outputting target laser subjected to power amplification, the gain unit comprises a double-cladding active optical fiber beam combiner and a temperature control device, and luminous ions are doped in a fiber core of the optical fiber beam combiner; according to the invention, the scale of the output power of the initial laser is improved through the amplification process of the stimulated radiation light, and meanwhile, a fundamental transverse mode is amplified to suppress the TMI effect by adopting the double-cladding active optical fiber beam combiner of which the inner confinement of the fiber core is doped with the light-emitting ions, so that the quality of the light beam is improved.

Description

Technical Field

[0001] The present application relates to the field of laser technology, and in particular to a kilowatt-class single-frequency optical fiber laser. Background Art

[0002] The high-power single-frequency fiber laser based on the seed source master oscillator power amplifier (MOPA) scheme has the advantages of narrow linewidth, low noise, high power, good beam quality and compact structure. It is widely used in military defense, scientific research, lidar, fiber optic sensing and coherent communications.

[0003] However, the power scale and beam quality of current single-frequency fiber lasers are strongly restricted by stimulated Brillouin scattering (SBS), mode instability (TMI), and spontaneous emission (ASE) effects, and the simultaneous suppression of TMI effect and SBS effect restricts each other, resulting in the problem that the power scale and beam quality of single-frequency fiber lasers cannot be improved at the same time. Summary of the invention

[0004] Based on this, it is necessary to provide a kilowatt-class single-frequency fiber laser that can simultaneously improve the power scale and beam quality of the single-frequency fiber laser in order to address the above-mentioned technical problems.

[0005] In a first aspect, the present application provides a kilowatt-class single-frequency fiber laser, comprising:

[0006] A seed light source module, used to generate initial laser;

[0007] A forward pump unit, connected to the seed light source module, for generating a first pump laser;

[0008] A backward pumping unit, used for generating a second pumping laser;

[0009] A gain unit, connected to the forward pump unit and the backward pump unit respectively, and the gain unit is used to amplify the power of the initial laser based on the first pump laser and the second pump laser;

[0010] An output unit, connected to the gain unit, is used to output a target laser corresponding to the initial laser after power amplification;

[0011] The gain unit includes a double-clad active fiber combiner and a temperature control device, the input end of the double-clad active fiber combiner is connected to the forward pump unit, the pump end of the double-clad active fiber combiner is connected to the backward pump unit, the output end of the double-clad active fiber combiner is connected to the output unit, the core of the double-clad active fiber combiner is doped with luminescent ions, and the luminescent ions include a combination of one or more of rare earth ions, transition metal ions and main group metal ions; the packaging area corresponding to the double-clad active fiber combiner is arranged above the temperature control device.

[0012] In one of the embodiments, a double-clad active fiber combiner is made of a double-clad active fiber using a fused-tapering process, the length of the double-clad active fiber combiner is associated with the wavelength of a first pump laser and the wavelength of a second pump laser, respectively, and the shape of the inner cladding of the double-clad active fiber combiner includes a D-shape, a rectangle, a hexagon, an octagon or a dodecagon.

[0013] In one embodiment, a first circular region within the core of the double-clad active fiber combiner is doped with luminescent ions of a preset concentration.

[0014] In one embodiment, the second circular region in the core of the double-clad active fiber combiner is doped with luminescent ions of a preset concentration, the annular region in the core of the double-clad active fiber combiner is doped with luminescent ions, and the concentration of the luminescent ions in the annular region decreases from the preset concentration to zero along the radial direction from the inside to the outside;

[0015] The second circular area is located in the annular area, and the center of the second circular area overlaps with the center of the annular area.

[0016] In one embodiment, the forward pump unit includes a single-mode pump light source module and a wavelength division multiplexing module, and the wavelength division multiplexing module is respectively connected to the gain unit, the seed light source module and the single-mode pump light source module;

[0017] The seed light source module is used to output the initial laser, the single-mode pump light source module is used to output the first pump laser, and the wavelength division multiplexing module is used to combine the initial laser and the first pump laser into an initial light beam and output it to the gain unit.

[0018] In one embodiment, the seed light source module includes a single-frequency seed source and a pre-amplifier; the single-frequency seed source is connected to the wavelength division multiplexing module through the pre-amplifier;

[0019] The pre-amplifier is doped with luminescent ions, and the wavelength corresponding to the initial laser is within the luminescent wavelength band of the luminescent ions doped in the pre-amplifier.

[0020] In one embodiment, the single-mode pump light source module includes a single-mode pump source, the single-mode pump source is connected to the wavelength division multiplexing module, and the single-mode pump source is a co-band pump source.

[0021] In one embodiment, the wavelength division multiplexing module includes a wavelength division multiplexer and a cladding light filter connected to each other, the pump end of the wavelength division multiplexer is connected to the output end of the single-mode pump source, the signal end of the wavelength division multiplexer is connected to the output end of the pre-amplification device, and the output end of the cladding light filter is connected to the input end of the double-clad active fiber combiner.

[0022] In one embodiment, the backward pumping unit includes a plurality of multi-mode pump sources, and the output end of each multi-mode pump source is connected to the pump end of the double-clad active fiber combiner.

[0023] In one of the embodiments, the output unit includes a fiber dichroic mirror connected to the output end of the double-clad active fiber combiner;

[0024] The transmittance of the optical fiber dichroic mirror to the target laser is greater than 90%, and the reflectivity of the optical fiber dichroic mirror to the first pump laser and the second pump laser is greater than 90%.

[0025] The above-mentioned kilowatt-class single-frequency fiber laser includes a seed light source module, a gain unit, and a forward pump unit, a backward pump unit and an output unit respectively connected to the gain unit, the seed light source module is connected to the gain unit through the forward pump unit, the gain unit is used to amplify the power of the initial laser based on the first pump laser and the second pump laser and then output it to the output unit, the output unit is used to output the target laser after power amplification, wherein the gain unit includes a double-clad active fiber combiner and a temperature control device, and the core of the double-clad active fiber combiner is doped with luminescent ions; the present application is generally By setting a forward pump unit and a backward pump unit, under the continuous pumping of the first pump laser and the second pump laser, the luminescent ions in the core of the double-clad active fiber combiner show a population inversion, and the stimulated emission light amplification process is completed when the initial laser passes through, thereby achieving an increase in the output power scale. At the same time, by adopting a double-clad active fiber combiner in which a restricted area in the core is doped with a higher concentration of luminescent ions, the fundamental transverse mode is highly overlapped with the doping area of ​​the luminescent ions, thereby efficiently amplifying the fundamental transverse mode to suppress the TMI effect, thereby improving the beam quality of the single-frequency fiber laser. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the conventional technology, the drawings required for use in the embodiments or the conventional technology descriptions are briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0027] FIG1 is a block diagram of a kilowatt-class single-frequency fiber laser according to an embodiment;

[0028] FIG2 is a schematic diagram of the cross-sectional structure of a double-clad active fiber combiner in one embodiment;

[0029] FIG3 is a schematic diagram of the cross-sectional structure of a double-clad active optical fiber combiner in another embodiment;

[0030] FIG4 is a schematic diagram of the structure of a kilowatt-class single-frequency fiber laser in one embodiment;

[0031] FIG5 is a schematic diagram of a power curve and spectrum of a kilowatt-class single-frequency fiber laser in one embodiment. DETAILED DESCRIPTION

[0032] In order to facilitate understanding of the present application, the present application will be described more fully below with reference to the relevant drawings. Embodiments of the present application are provided in the drawings. However, the present application can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the disclosure of the present application more thorough and comprehensive.

[0033] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those generally understood by those skilled in the art of the technical field of the present application. The terms used herein in the specification of the present application are only for the purpose of describing specific embodiments and are not intended to limit the present application. It is understood that the terms "first", "second", etc. used in the present application can be used to describe various elements in this article, but these elements are not limited by these terms. These terms are only used to distinguish a first element from another element.

[0034] When used herein, the singular forms "a", "an", and "said / the" may also include plural forms, unless the context clearly indicates otherwise. It should also be understood that the terms "include / comprise" or "have" and the like specify the presence of stated features, wholes, steps, operations, components, parts, or combinations thereof, but do not exclude the possibility of the presence or addition of one or more other features, wholes, steps, operations, components, parts, or combinations thereof. At the same time, the term "and / or" used in this specification includes any and all combinations of the relevant listed items.

[0035] At present, high-power single-frequency fiber lasers based on the seed source master oscillator power amplifier (MOPA) scheme have the advantages of narrow linewidth, low noise, high power, good beam quality and compact structure, and are widely used in the fields of military defense, scientific research, lidar, fiber optic sensing and coherent communication; however, some special applications have put forward higher requirements on the output power, beam quality, linewidth and other performance of single-frequency fiber lasers. For example, improving the detection sensitivity in ground-based gravitational wave detection applications requires the laser light source to have a kilowatt-level power; improving the harmonic conversion efficiency in nonlinear frequency conversion applications requires the laser light source to have high output power and high beam quality.

[0036] At present, the highest power reports of all-fiber 1.0μm, 1.5μm, and 2.0μm band single-frequency fiber lasers are 700W (beam quality factor is 1.45), 102W, and 316W, respectively. The power scale increase and beam quality optimization or maintenance are strongly restricted by stimulated Brillouin scattering (SBS), mode instability (TMI), spontaneous emission (ASE) and other effects. In order to solve the above-mentioned "bottleneck" problem, the SBS effect is generally suppressed by increasing the mode field area of ​​the active fiber, reducing the fiber length of the single-frequency fiber laser, introducing strain or temperature gradient along the fiber axis, and gain competition strategy. Or the TMI effect is suppressed by bending and coiling the active fiber, optimizing the pump laser wavelength or signal light wavelength (target wavelength), pump structure and other measures. For example, in the prior art, the main amplifier stage adopts an Er with a core diameter of 25μm based on the MOPA scheme. 3+ and Yb 3+ Co-doped double-clad fiber and multimode cladding pumping at a non-peak wavelength of 940nm achieved a 1.56μm single-frequency laser output with a power of 102W, a beam quality factor of 1.1, and a signal-to-noise ratio of 64dB. However, Er 3+ and Yb 3+ The ASE effect and SBS effect caused by the bottleneck of energy transfer limit the further improvement of power.

[0037] Generally, the simultaneous suppression of TMI effect and SBS effect is mutually restricted. For example, the SBS threshold can be effectively increased by increasing the mode field area of ​​the double-clad active fiber (core diameter: 30μm, 35μm, etc.), but the large core diameter will lead to the generation of a large number of high-order transverse modes, reducing the TMI threshold and beam quality of the single-frequency fiber laser. In addition, the double-clad active fiber can be tapered at both ends or partially doped with the core to increase the mode loss of the high-order transverse mode, which can effectively suppress the SBS effect and optimize the beam quality. For example, based on the MOPA scheme, the main amplifier stage uses partially doped Yb 3+ Double-clad fiber, by simulating and discussing the impact of uniform doping design of the core on beam quality, achieved 1.0μm single-frequency laser output with a power of 322W, a beam quality factor of 1.29, and a signal-to-noise ratio of 48dB. However, the residual pump light degraded the signal-to-noise ratio of the laser. Furthermore, by drawing a low-NA double-clad active fiber with a core numerical aperture (NA) less than 0.06, near-single-mode output can be achieved in a larger core diameter. However, the manufacturing process of tapered, low-NA active fibers is very difficult, which has brought limitations to the construction of single-frequency fiber laser systems and the expansion of their application areas.

[0038] The kilowatt-class single-frequency fiber laser provided in the embodiment of the present application adopts a double-clad active fiber combiner that is confinedly doped with a relatively high concentration of luminescent ions, so that the fundamental transverse mode is highly overlapped with the doped region, while the overlap of the high-order transverse modes is relatively weak, so that the fundamental transverse mode is efficiently amplified, and the beam quality is optimized or maintained to suppress the TMI effect; furthermore, by providing a forward pumping unit and a backward pumping unit, that is, by adopting hybrid pumping, under the continuous pumping of the first pumping laser and the second pumping laser, the luminescent ions in the core of the double-clad active fiber combiner present a population inversion, and the stimulated emission light amplification process is completed when the initial laser passes through, thereby achieving a substantial increase in the output power scale.

[0039] In an exemplary embodiment, as shown in FIG1 , a kilowatt-class single-frequency fiber laser 100 is provided, comprising:

[0040] A seed light source module 110 is used to generate initial laser light;

[0041] A forward pump unit 120, connected to the seed light source module, for generating a first pump laser;

[0042] A backward pumping unit 130, used to generate a second pumping laser;

[0043] A gain unit 140, connected to the forward pump unit 120 and the backward pump unit 130 respectively, and the gain unit 140 is used to amplify the power of the initial laser based on the first pump laser and the second pump laser;

[0044] The output unit 150 is connected to the gain unit 140, and is used to output the target laser corresponding to the initial laser after the power is amplified;

[0045] Among them, the gain unit 140 includes a double-clad active fiber combiner 142 and a temperature control device 144, the input end of the double-clad active fiber combiner 142 is connected to the forward pump unit 120, the pump end of the double-clad active fiber combiner 142 is connected to the backward pump unit 130, the output end of the double-clad active fiber combiner 142 is connected to the output unit 150, and the core of the double-clad active fiber combiner 142 is doped with luminescent ions, and the luminescent ions include a combination of one or more of rare earth ions, transition metal ions and main group metal ions; the packaging area corresponding to the double-clad active fiber combiner 142 is arranged above the temperature control device 144.

[0046] Among them, rare earth ions may include Nd 3+ , Yb 3+ , Er 3+ and Tm 3+ , transition metal ions may include Fe 2+ and Cr 2+, the main group metal ion can be Bi, and the doping method can be high concentration, large mode field area, and restricted area doping; it can be understood that the above-mentioned luminescent ions can also use other ions, and are not limited to the ions mentioned in the above embodiments, as long as they can achieve the function of absorbing the first pump laser and the second pump laser (pump energy), presenting a population inversion and amplifying the power of the initial laser; the concentration of the luminescent ions doped in the core of the double-clad active fiber combiner can be set according to actual conditions and is not limited in the embodiments of the present application.

[0047] Specifically, as shown in FIG1 , the seed light source module outputs an initial laser, the forward pump unit 120 outputs a first pump laser, and the initial light beam (initial laser and first pump laser) are coupled together from the forward direction into the core of the double-clad active fiber combiner 142; the second pump laser generated by the backward pump unit 130 is coupled from the backward direction into the inner cladding of the double-clad active fiber combiner 142. Under the continuous pumping of the first pump laser and the second pump laser, the luminescent ions in the core of the double-clad active fiber combiner 142 are inverted in number, and the stimulated emission light amplification process is completed when the initial laser passes through, thereby achieving a significant increase in the output power scale, and the output unit 150 outputs the target laser that has completed power amplification; it should be noted that the packaging area corresponding to the double-clad active fiber combiner 142 will generate heat due to loss, and the temperature control device is arranged below the packaging area corresponding to the double-clad active fiber combiner to achieve heat dissipation of the packaging area.

[0048] Exemplarily, the wavelengths corresponding to the initial laser, the target laser, the first pump laser and the second pump laser can be set according to actual conditions, which is not limited in the embodiments of the present application; the temperature control device can be composed of a TEC (ThermoElectric Cooler), the temperature control accuracy is less than or equal to 0.1°C, and the temperature setting range is -20°C to 40°C; it can be understood that the above-mentioned temperature control device can also be other devices, not limited to the devices mentioned in the above embodiments, as long as it can achieve the function of heat dissipation of the packaging area corresponding to the double-clad active fiber combiner.

[0049] The above-mentioned kilowatt-class single-frequency fiber laser is provided with a forward pumping unit and a backward pumping unit. The initial laser and the first pumping laser (initial light beam) are coupled into the core of the double-clad active fiber combiner from the forward direction, and the second pumping laser is coupled into the inner cladding of the double-clad active fiber combiner from the backward direction. Under the continuous pumping of the first pumping laser and the second pumping laser, the luminescent ions in the core of the double-clad active fiber combiner are inverted in number, and the stimulated emission light amplification process is completed when the initial laser passes through, thereby improving the scale of the output power of the single-frequency fiber laser. At the same time, the core of the double-clad active fiber combiner is doped with luminescent ions so that the fundamental transverse mode is highly overlapped with the doping region, thereby efficiently amplifying the fundamental transverse mode to suppress the TMI effect, thereby improving the beam quality of the single-frequency fiber laser.

[0050] In one of the embodiments, a double-clad active fiber combiner is made of a double-clad active fiber using a fused-tapering process, the length of the double-clad active fiber combiner is associated with the wavelength of a first pump laser and the wavelength of a second pump laser, respectively, and the shape of the inner cladding of the double-clad active fiber combiner includes a D-shape, a rectangle, a hexagon, an octagon or a dodecagon.

[0051] Specifically, the double-clad active fiber combiner can be formed by drawing a (confined doped) double-clad active fiber using a fused taper process, and a preset length (set according to actual conditions, generally less than 10 cm) is reserved for use as an output end; the port type of the double-clad active fiber combiner is (n+1)×1, where n is the number of pump ports (n is greater than or equal to 2), and the specific use length of the double-clad active fiber combiner can be determined based on the wavelength of the initial laser, the wavelength of the first pump laser, the wavelength of the second pump laser, and the doping concentration of the luminescent ions.

[0052] Exemplarily, the active optical fiber core diameter of the double-clad active optical fiber combiner can range from 30μm to 90μm, and the core numerical aperture ranges from 0.02 to 0.2; the shape of the active optical fiber inner cladding is a non-circular structure such as D-shape, rectangle, hexagon, octagon, dodecagon, etc., and the diameter of the inscribed circle of the active optical fiber inner cladding ranges from 125μm to 600μm; it should be noted that the shape of the inner cladding of the double-clad active optical fiber combiner can also be other non-circularly symmetrical shapes, and is not limited to the shapes mentioned in the embodiments of the present application. The circular inner cladding has low absorption of the second pump laser, and it is difficult to achieve an increase in the output power scale of the subsequent single-frequency fiber laser.

[0053] In the embodiment of the present application, by directly drawing a double-clad active fiber into a double-clad active fiber combiner, compared with a single-frequency fiber laser system constructed by a passive fiber combiner, the connection loss and the fiber usage length are reduced, the reliability and stability of the single-frequency fiber laser are improved, and the SBS effect is effectively suppressed; at the same time, the high gain and large mode area characteristics of the active fiber are combined to further suppress the SBS effect, thereby improving the output power level of the single-frequency fiber laser.

[0054] In one embodiment, a first circular region within the core of the double-clad active fiber combiner is doped with luminescent ions of a preset concentration.

[0055] Among them, the preset concentration can be set according to actual conditions and is not limited in the embodiments of the present application. In the embodiments of the present application, the preset concentration is taken as a concentration greater than 1wt% as an example for illustration; the diameter corresponding to the first circular area can be set according to actual conditions. In the embodiments of the present application, the diameter range of the doped area (first circular area) is taken as 20μm to 50μm as an example for illustration.

[0056] Specifically, Figure 2 exemplarily shows a schematic diagram of the cross-sectional structure of a part of a double-clad active fiber combiner, (a) in Figure 2 represents a double-clad active fiber combiner with a D-shaped inner cladding, (b) in Figure 2 represents a double-clad active fiber combiner with a rectangular inner cladding, (c) in Figure 2 represents a double-clad active fiber combiner with a hexagonal inner cladding, 61 in Figure 2 represents the inner cladding, 62 represents the fiber core, 63 represents the first circular area doped with a preset concentration of luminescent ions, and the remaining areas are undoped.

[0057] In the embodiment of the present application, by concentrating the luminescent ions at a uniform and high concentration in a partial area at the center of the fiber core (restricted area doping), that is, the first circular area, the fundamental transverse mode is highly overlapped with the doped area, and the overlap of the high-order transverse mode with the doped area is weakened, thereby efficiently amplifying the fundamental transverse mode to suppress the TMI effect, thereby optimizing or maintaining the beam quality of the single-frequency laser.

[0058] In one embodiment, the second circular region in the core of the double-clad active fiber combiner is doped with luminescent ions of a preset concentration, the annular region in the core of the double-clad active fiber combiner is doped with luminescent ions, and the concentration of the luminescent ions in the annular region decreases from the preset concentration to zero along the radial direction from the inside to the outside;

[0059] The second circular area is located in the annular area, and the center of the second circular area overlaps with the center of the annular area.

[0060] Among them, the preset concentration can be set according to actual conditions and is not limited in the embodiments of the present application. In the embodiments of the present application, the preset concentration is taken as a concentration greater than 1wt% as an example for explanation; the diameter corresponding to the second circular area can be set according to actual conditions. In the embodiments of the present application, the diameter range of the doped area (second circular area) is 20μm to 35μm as an example for explanation; the ring width corresponding to the annular area can be set according to actual conditions. In the embodiments of the present application, the ring width range of the doped ring (annular area) is 5μm to 27.5μm as an example for explanation.

[0061] Specifically, as shown in Figure 3, Figure 3 exemplarily shows a schematic diagram of the cross-sectional structure of another part of a double-clad active fiber combiner, (a) in Figure 3 represents a double-clad active fiber combiner with a D-shaped inner cladding shape, (b) in Figure 3 represents a double-clad active fiber combiner with a rectangular inner cladding shape, and (c) in Figure 3 represents a double-clad active fiber combiner with a hexagonal inner cladding shape. 61 in Figure 3 represents the inner cladding, 62 represents the fiber core, 63 represents the second circular region doped with luminescent ions of a preset concentration, and 64 represents the non-uniform doping region (annular region) of luminescent ions.

[0062] It should be noted that the doping concentration of the luminescent ions in the circular ring region decreases in a gradient or gradual manner to zero.

[0063] In the embodiments of the present application, the luminescent ions are doped in a restricted area, that is, the luminescent ions are uniformly and highly concentrated in a partial area of ​​the center of the fiber core (the second circular area, the annular area), so that the fundamental transverse mode is highly overlapped with the doped area, while the overlap of the high-order transverse modes is weaker, so that the fundamental transverse mode is efficiently amplified, and the beam quality is optimized or maintained to suppress the TMI effect.

[0064] In one embodiment, the forward pump unit includes a single-mode pump light source module and a wavelength division multiplexing module, and the wavelength division multiplexing module is respectively connected to the gain unit, the seed light source module and the single-mode pump light source module;

[0065] The seed light source module is used to output the initial laser, the single-mode pump light source module is used to output the first pump laser, and the wavelength division multiplexing module is used to combine the initial laser and the first pump laser into an initial light beam and output it to the gain unit.

[0066] Specifically, the single-mode pump light source module can be a pump light source to output a first pump laser. The wavelength division multiplexing module combines the initial laser and the first pump laser, and then couples them into the core of the confined doped double-clad active optical fiber (double-clad active optical fiber combiner) from the forward direction to facilitate subsequent power amplification of the initial laser.

[0067] In one embodiment, as shown in FIG4 , the seed light source module includes a single-frequency seed source 1 and a pre-amplifier 2 ; the single-frequency seed source 1 is connected to the wavelength division multiplexing module through the pre-amplifier 2 ;

[0068] The pre-amplifier 2 is doped with luminescent ions, and the wavelength corresponding to the initial laser is within the luminescent wavelength band of the luminescent ions doped in the pre-amplifier 2 .

[0069] Among them, the wavelength of the signal light output by the single-frequency seed source 1 can be set according to actual conditions and is not limited in the embodiment of the present application; the pre-amplification device 2 can be a device for pre-amplifying the signal light power. In the embodiment of the present application, the pre-amplification device 2 is taken as a rare earth ion-doped optical fiber amplifier as an example for explanation.

[0070] Specifically, as shown in Figure 4, the single-frequency seed source 1 outputs signal light to the pre-amplifier 2, the pre-amplifier 2 is doped with rare earth ions, and the wavelength corresponding to the initial laser is within the luminous band of the rare earth ions doped in the pre-amplifier 2. The power of the signal light output by the single-frequency seed source 1 can be pre-amplified to a certain power level (set according to actual conditions), that is, the initial laser is output.

[0071] It can be understood that the luminescent ions doped in the pre-amplification device can be set according to actual conditions and are not limited in the embodiments of the present application.

[0072] It should be noted that 7 in FIG. 4 is a temperature control device.

[0073] Exemplarily, the single-frequency seed source can be a single-frequency (single longitudinal mode) fiber laser, a semiconductor laser or a non-planar ring cavity laser with a pigtail-coupled output, and the signal power of the single-frequency seed source is greater than 1 mW and the line width is less than 10 kHz.

[0074] In an embodiment of the present application, a pre-amplification device doped with luminescent ions is provided to achieve preliminary amplification of the signal light output by a single-frequency seed source to obtain an initial laser, which facilitates further power amplification of the initial laser by a subsequent double-clad active fiber combiner.

[0075] In one embodiment, as shown in FIG. 4 , the single-mode pump light source module includes a single-mode pump source 3 , which is connected to the wavelength division multiplexing module and is a co-band pump source.

[0076] Among them, the same-band pump source (same-band single-mode fiber core pump) means that the absorption wavelength of the single-mode pump source and the emission wavelength of the laser (initial laser) are in the same band (within the band), and the two wavelengths are closely spaced to reduce quantum loss (less waste heat is generated).

[0077] Specifically, as shown in FIG. 4 , by adopting a single-mode pump source 3 (same-band single-mode core pumping), the quantum loss in the amplification process is reduced and the energy conversion efficiency is improved.

[0078] Exemplarily, the single-mode pump source may be a single-mode (single transverse mode) fiber laser with pigtail-coupled output; the pump power of the single-mode pump source is greater than 10W.

[0079] In one embodiment, as shown in FIG. 4 , the wavelength division multiplexing module includes a wavelength division multiplexer 4 and a cladding light filter 5 connected to each other, the pump end of the wavelength division multiplexer 4 is connected to the output end of the single-mode pump source 3, the signal end of the wavelength division multiplexer 4 is connected to the output end of the pre-amplifier 2, and the output end of the cladding light filter 5 is connected to the input end of the double-clad active fiber combiner 6.

[0080] The wavelength division multiplexer 4 can be a fused-taper optical device, and the pigtail types of the signal end, common end and pump end of the wavelength division multiplexer 4 are all single-mode optical fibers; the power handling power of the wavelength division multiplexer 4 is greater than 10W.

[0081] Specifically, the signal light emitted by the single-frequency seed source 1 is boosted in power by the pre-amplifier 2 (initial laser), and then combined with the pump light (first pump laser) generated by the single-mode pump source 3 by the wavelength division multiplexer 4 and the cladding light is filtered out by the cladding light filter, and then coupled from the forward direction into the core of the confined doped double-clad active optical fiber (double-clad active optical fiber combiner).

[0082] In the embodiment of the present application, a wavelength division multiplexer and a cladding light filter are provided to combine the first pump laser and the initial laser, and the cladding light of the initial light beam and the second pump laser is filtered out, so as to facilitate the acquisition of the subsequent target laser and improve the beam quality of the target laser.

[0083] In one embodiment, as shown in FIG. 4 , the backward pumping unit includes a plurality of multimode pump sources 8 , and the output end of each multimode pump source 8 is connected to the pump end of the double-clad active fiber combiner.

[0084] Among them, the number of multi-mode pump sources 8 can be set according to actual conditions and is not limited in the embodiment of the present application.

[0085] Specifically, as shown in FIG4 , the multimode pump source 8 can be non-peak pumping, that is, non-peak means that the pump wavelength is not pumped using the strongest pump absorption peak wavelength, so as to avoid excessive absorption of the pump light (second pump laser) by the gain fiber (the optical fiber in the double-clad active fiber combiner); for example: doping Yb 3+ The gain fiber has the strongest absorption for the pump wavelength of 976nm (where the absorption peak is), so a flatter wavelength of 940nm with low pump absorption is used.

[0086] Exemplarily, the multi-mode pump source is a multi-mode (multi-transverse mode) semiconductor laser, a fiber laser, or a solid laser with pigtail-coupled output; the pump power of the multi-mode pump source is greater than 10W.

[0087] In the embodiment of the present application, by utilizing the single-mode core and multi-mode cladding mixed pumping of the same-band pump source (single-mode pump source) and the non-peak pump (multi-mode pump source), the same-band single-mode core pump (single-mode pump source) reduces the quantum loss in the amplification process and improves the energy conversion efficiency. At the same time, the non-peak multi-mode cladding pump (multi-mode pump source) alleviates the energy transfer bottleneck between the multi-doping of the luminescent ions to suppress the TMI effect and the ASE effect, and finally a single-frequency fiber laser with a kilowatt-level power and a near-diffraction-limited beam quality can be obtained.

[0088] In one of the embodiments, the output unit includes a fiber dichroic mirror connected to the output end of the double-clad active fiber combiner;

[0089] The transmittance of the optical fiber dichroic mirror to the target laser is greater than 90%, and the reflectivity of the optical fiber dichroic mirror to the first pump laser and the second pump laser is greater than 90%.

[0090] Specifically, the optical fiber dichroic mirror can be a dichroic mirror with pigtail coupling input and free space output, and an internally tilted dichroic mirror; the dichroic mirror is highly transparent to the signal light wavelength (target laser), with a transmittance greater than 90%; and highly reflective to the pump light wavelength (the first pump laser and the second pump laser), with a reflectivity greater than 90%, that is, the initial laser is power amplified by a double-clad active fiber combiner to output the target laser, and the target laser can be output through the optical fiber dichroic mirror.

[0091] To facilitate the understanding of those skilled in the art, a kilowatt-class single-frequency fiber laser is described below with reference to a specific example:

[0092] Taking the cross-sectional structure of a double-clad active fiber combiner as shown in Figure 2 (c) as an example, the inner cladding of the double-clad active fiber combiner is hexagonal in shape. Both the core and the inner cladding of the double-clad active fiber combiner can be made of quartz glass. The core of the double-clad active fiber combiner is uniformly co-doped with 4wt% of Er. 3+ and 8wt% Yb 3+ , the core diameter is 50μm, the luminescent ion Er 3+ and Yb 3+ The diameter of the doped region (first circular region) is 37 μm, and the core numerical aperture is 0.06; the diameter of the inscribed circle of the inner cladding is 350 μm, and the cladding numerical aperture is 0.46; the diameter of the outer cladding is 800 μm.

[0093] The double-clad active fiber combiner is made directly on the confined doped double-clad active fiber by fusion taper. A 5cm fiber is reserved at one end of the double-clad active fiber combiner for use as the output end. The port type of the confined doped double-clad active fiber combiner is (6+1)×1, and the usable length is 2.5 meters. The single-frequency seed source is a fiber laser with a signal light (initial laser) wavelength of 1550nm, a power of 50mW, and a line width of 2kHz. The preamplifier (pre-amplification device) is Er 3+ / Yb 3+ The co-doped fiber amplifier can pre-amplify the signal light (initial laser) power to 15W; the single-mode pump source is a fiber laser with a wavelength of 1018nm and a power of 350W; the multi-mode pump source is a semiconductor laser with a wavelength of 940nm and a power of 450W, and the number used is 5; the wavelength division multiplexer is made by melting and pulling a cone, and its withstand power is 350W; the temperature control device is composed of a thermoelectric cooler TEC, with a temperature control accuracy of 0.05℃ and a temperature setting of 20℃. Among them, the fiber dichroic mirror is internally tilted, with a transmittance of 99.9% for the wavelength of the signal light (target laser) and a reflectivity of 99.9% for the wavelength of the pump light (first pump laser and second pump laser). It should be noted that the temperature controlled by the temperature control device can also be other temperatures, and is not limited to the 20℃ given in the embodiment of the present application.

[0094] Among them, the signal light (initial laser) emitted by the 1550nm single-frequency laser seed source, after the power is increased to 15W by the pre-amplifier, is combined with the 1018nm pump light (first pump laser) generated by the single-mode pump source through the wavelength division multiplexer and the cladding light is filtered out by the cladding light filter, and then enters the core of the confined doped double-clad active fiber (double-clad active fiber combiner) from the forward direction; the 940nm pump light (second pump laser) generated by the multimode pump source passes through the (6+1)×1 double-clad active fiber combiner and enters the inner cladding of the confined doped double-clad active fiber. Under the continuous pumping of the two types of pump sources (single-mode pump source and multi-mode pump source), the Yb in the core of the double-clad active fiber combiner is 3+ As a sensitizing ion, it absorbs pump energy and transfers the energy to Er through cross relaxation. 3+ , showing a particle number inversion; when the 1550nm signal light (initial laser) enters the main amplifier stage, the power amplification of the single-frequency signal light (initial laser) is achieved through the stimulated emission light amplification process.

[0095] It should be noted that the confined doped double-clad active fiber combiner reduces the fusion point and the fiber length used in the single-frequency fiber laser system to suppress the SBS effect; at the same time, combined with the high gain, large mode area and restricted area doping characteristics of the confined doped double-clad active fiber, the SBS effect is further suppressed; the 1550nm signal light (initial laser) passes through the restricted doping area to efficiently amplify the fundamental transverse mode, while suppressing the amplification of the higher-order transverse mode, that is, optimizing or maintaining the beam quality of the laser to suppress the TMI effect. Furthermore, the 1018nm single-mode core pump (single-mode pump source) has lower quantum loss and higher energy conversion efficiency for the 1550nm signal light, that is, it reduces the thermal effect to further suppress the TMI effect; combined with the 940nm non-peak multimode cladding pump (multimode pump source), the excited Yb 3+ Quantity, relieved Er 3+ and Yb 3+ The energy transfer bottleneck between the two is eliminated, and the TMI effect and ASE effect are further suppressed. Finally, a 1550nm single-frequency fiber laser with a power of 1030W, a beam quality factor of 1.18, a linewidth of 2kHz, and an optical signal-to-noise ratio of 75dB is obtained.

[0096] To facilitate the understanding of those skilled in the art, a single-frequency fiber laser is described below in conjunction with another specific example:

[0097] Taking the cross-sectional structure of a double-clad active fiber combiner as shown in Figure 3 (a) as an example, the inner cladding shape of the double-clad active fiber combiner is D-shaped, the core and inner cladding of the double-clad active fiber combiner can both be composed of quartz glass, and the core of the double-clad active fiber combiner is uniformly co-doped with 7.5wt% Tm 3+ and 3wt% / Ho 3+ , core diameter is 55μm, Tm 3+ and Ho 3+ The diameter of the doped area (second circular area) is 25μm, and the doping concentration in the remaining area (ring area) decreases in a gradient manner (along the radial direction). The ring width of the doped ring (ring area) is 15μm, and the core numerical aperture is 0.055; the diameter of the inscribed circle of the inner cladding is 400μm, and the cladding numerical aperture is 0.46; the diameter of the outer cladding is 800μm; the double-clad active fiber combiner is directly made on the confined doped double-clad active fiber by fusion taper, and a 5cm fiber length is reserved at one end of the double-clad active fiber combiner as the output end; the port type of the confined doped double-clad active fiber combiner is (6+1)×1, and the usable length is 2.5 meters; the single-frequency seed source is a fiber laser with a signal light (initial laser) wavelength of 2015nm, a power of 20mW, and a line width of 3kHz; the preamplifier (pre-amplification device) is a Tm doped 3+The fiber amplifier pre-amplifies the signal light (initial laser) power to 20W; the single-mode pump source is a fiber laser with a wavelength of 1610nm and a power of 200W; the multi-mode pump source is a semiconductor laser with a wavelength of 800nm ​​and a power of 350W, and the number of used lasers is 5. The wavelength division multiplexer is made by fused taper, and its withstand power is 200W; the temperature control device is composed of a thermoelectric cooler TEC, with a temperature control accuracy of 0.05℃ and a temperature setting of 18℃; the fiber dichroic mirror is internally tilted, with a transmittance of 99.9% for the wavelength of the signal light (target laser) and a reflectivity of 99.9% for the wavelength of the pump light (the first pump laser and the second pump laser).

[0098] Among them, the signal light (initial laser) emitted by the 2015nm single-frequency laser seed source, after the power is increased to 20W by the pre-amplifier, is combined with the 1610nm pump light (first pump laser) generated by the single-mode pump source through the wavelength division multiplexer and enters the core of the confined doped double-clad active fiber from the forward direction; the 800nm ​​pump light (second pump laser) generated by the multi-mode pump source enters the inner cladding of the confined doped double-clad active fiber through the (6+1)×1 double-clad active fiber combiner. Under the continuous pumping of the two types of pump sources (single-mode pump source and multi-mode pump source), the Tm in the core of the double-clad active fiber combiner 3+ As a sensitizing ion, it absorbs pump energy and transfers the energy to Ho by cross relaxation. 3+ , showing a population inversion; when the 2015nm signal light (initial laser) enters the main amplifier stage, the power amplification of the single-frequency signal light (initial laser) is achieved through the stimulated emission light amplification process; the confined doped double-clad active fiber combiner reduces the fusion point and the fiber length used in the single-frequency fiber laser system to suppress the SBS effect; at the same time, combined with the high gain, large mode area and restricted area doping characteristics of the confined doped double-clad active fiber, the SBS effect is further suppressed; the 2015nm signal light (initial laser) is efficiently amplified through the restricted doping area to achieve the fundamental transverse mode, while suppressing the amplification of the higher-order transverse mode, that is, optimizing or maintaining the beam quality of the laser to suppress the TMI effect. Furthermore, the 1610nm single-mode core pump (single-mode pump source) has lower quantum loss and higher energy conversion efficiency for the 2015nm signal light, that is, it reduces the thermal effect to further suppress TMI; combined with the 800nm ​​non-peak multimode cladding pump (multimode pump source), the excited Tm 3+ Quantity, relieved Tm 3+ and Ho 3+The energy transfer bottleneck between them is eliminated, and the TMI effect and ASE effect are further suppressed. Finally, a 2015nm single-frequency fiber laser with a power of 1050W, a beam quality factor of 1.16, a linewidth of 3kHz, and an optical signal-to-noise ratio of 72dB is obtained. The output results of the power curve and spectrum of the single-frequency fiber laser are shown in Figure 5 (a) and (b), respectively.

[0099] For example, the structure of the single-frequency fiber laser provided in the embodiment of the present application is also applicable to different numbers of pump sources, output powers, pump wavelengths, pumping modes, and other parameters such as different gain fiber doping ion types, doping concentrations, fiber core numbers, fiber core diameters, and use lengths. As mentioned above, the above embodiments are only preferred embodiments of the present invention and are not intended to limit the scope of implementation thereof.

[0100] In the description of this specification, the description with reference to the terms "some embodiments", "other embodiments", etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present application. In this specification, the schematic description of the above terms does not necessarily refer to the same embodiment or example.

[0101] The technical features of the above-described embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0102] The above-described embodiments only express several implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the present application. It should be pointed out that, for a person of ordinary skill in the art, several variations and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the present application shall be subject to the attached claims.

Claims

1. A kilowatt-class single-frequency fiber laser, characterized in that: include: A seed light source module, used to generate initial laser; A forward pumping unit, connected to the seed light source module, for generating a first pumping laser; A backward pumping unit, used for generating a second pumping laser; a gain unit, connected to the forward pumping unit and the backward pumping unit respectively, and configured to amplify the power of the initial laser based on the first pumping laser and the second pumping laser; An output unit, connected to the gain unit, and configured to output a target laser corresponding to the initial laser after power amplification; The gain unit comprises a double-clad active fiber combiner and a temperature control device, the input end of the double-clad active fiber combiner is connected to the forward pump unit, the pump end of the double-clad active fiber combiner is connected to the backward pump unit, the output end of the double-clad active fiber combiner is connected to the output unit, the core of the double-clad active fiber combiner is doped with luminescent ions, and the luminescent ions include a combination of one or more of rare earth ions, transition metal ions and main group metal ions; the packaging area corresponding to the double-clad active fiber combiner is arranged above the temperature control device.

2. The kilowatt-class single-frequency fiber laser according to claim 1, characterized in that: The double-clad active fiber combiner is made of double-clad active optical fiber using a fused taper process. The length of the double-clad active fiber combiner is associated with the wavelength of the first pump laser and the wavelength of the second pump laser, respectively. The shape of the inner cladding of the double-clad active fiber combiner includes a D shape, a rectangle, a hexagon, an octagon or a dodecagon.

3. The kilowatt-class single-frequency fiber laser according to claim 2, characterized in that: The first circular region in the core of the double-clad active optical fiber combiner is doped with the luminescent ions at a preset concentration.

4. The kilowatt-class single-frequency fiber laser according to claim 2, characterized in that: The second circular region in the core of the double-clad active fiber combiner is doped with the luminescent ions at a preset concentration, the annular region in the core of the double-clad active fiber combiner is doped with the luminescent ions, and the concentration of the luminescent ions in the annular region decreases from the preset concentration to zero from the inside to the outside along the radial direction; The second circular area is located in the annular area, and the center of the second circular area overlaps with the center of the annular area.

5. The kilowatt-class single-frequency fiber laser according to claim 1, characterized in that: The forward pump unit includes a single-mode pump light source module and a wavelength division multiplexing module, and the wavelength division multiplexing module is respectively connected to the gain unit, the seed light source module and the single-mode pump light source module; The seed light source module is used to output the initial laser, the single-mode pump light source module is used to output the first pump laser, and the wavelength division multiplexing module is used to combine the initial laser and the first pump laser into an initial light beam and output it to the gain unit.

6. The kilowatt-class single-frequency fiber laser according to claim 5, characterized in that: The seed light source module comprises a single-frequency seed source and a pre-amplifier; the single-frequency seed source is connected to the wavelength division multiplexing module through the pre-amplifier; The luminescent ions are doped in the pre-amplifier, and the wavelength corresponding to the initial laser is within the luminescent wavelength band of the luminescent ions doped in the pre-amplifier.

7. The kilowatt-class single-frequency fiber laser according to claim 6, characterized in that: The single-mode pump light source module comprises a single-mode pump source, the single-mode pump source is connected to the wavelength division multiplexing module, and the single-mode pump source is a same-band pump source.

8. The kilowatt-class single-frequency fiber laser according to claim 7, characterized in that: The wavelength division multiplexing module includes a wavelength division multiplexer and a cladding light filter connected to each other, the pump end of the wavelength division multiplexer is connected to the output end of the single-mode pump source, the signal end of the wavelength division multiplexer is connected to the output end of the pre-amplification device, and the output end of the cladding light filter is connected to the input end of the double-cladding active fiber combiner.

9. The kilowatt-class single-frequency fiber laser according to claim 1, characterized in that: The backward pumping unit comprises a plurality of multi-mode pumping sources, and the output end of each of the multi-mode pumping sources is connected to the pumping end of the double-clad active optical fiber combiner.

10. The kilowatt-class single-frequency fiber laser according to claim 1, characterized in that: The output unit comprises a fiber dichroic mirror, and the fiber dichroic mirror is connected to the output end of the double-clad active fiber combiner; The transmittance of the optical fiber dichroic mirror to the target laser is greater than 90%, and the reflectivity of the optical fiber dichroic mirror to the first pump laser and the second pump laser is greater than 90%.

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