A 1.7 μm band single frequency fiber laser based on a composite glass fiber

Abstract

The application provides a 1.7-micron-band single-frequency fiber laser based on a composite glass optical fiber, which comprises a long linear cavity, and the long linear cavity comprises, in sequence, a high-reflection Bragg grating (3), a composite glass optical fiber (4), a saturable absorber (5) and a low-reflection Bragg grating (6). 3+ The composite glass optical fiber is a multimode optical fiber, has a double-clad structure, and the core is Tb 3+ The inner cladding is Tm 3+ The outer cladding is undoped rare earth composite glass. The 1.7-micron-band single-frequency fiber laser is compact in structure, high in efficiency and narrow in line width, and can be applied to the fields of gas sensing, precision measurement, laser radar and the like.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lasers, in particular to a 1.7 μm waveband single-frequency fiber laser based on a composite glass fiber. BACKGROUND

[0002] With the development of fiber laser technology, single-frequency fiber lasers have been widely used in high-resolution spectral detection, remote sensing, precision measurement, biomedical imaging, and laser radar, etc. due to their excellent coherence, narrow linewidth, high stability, and other characteristics. On the one hand, near-infrared 1.7 μm waveband (1.65~1.75 μm) laser is in the atmospheric window region, with low atmospheric absorption and scattering loss, suitable for long-distance free space transmission. On the other hand, 1.7 μm waveband laser corresponds to important absorption lines of water molecules, methane and other trace gases, and is an important waveband for atmospheric remote sensing monitoring. In addition, compared with traditional 1.55 μm lasers, 1.7 μm waveband lasers have stronger penetration and higher efficiency in some medical and industrial processing scenarios, and have broader application prospects.

[0003] At present, 1.7 μm waveband fiber lasers mainly rely on high-concentration Tm 3+ doped glass fibers, and are realized through cascading energy transfer and nonlinear frequency conversion, etc. Although some research has achieved laser output in this waveband, it still faces technical challenges such as low efficiency, small emission cross section, short upper level lifetime, strong spectral line competition, and poor stability, especially in realizing high-power, high signal-to-noise ratio, and narrow linewidth 1.7 μm waveband single-frequency fiber lasers. For example, Cen et al. used a high-concentration Tm 3+ doped germanate glass fiber to construct a distributed Bragg reflection (DBR) type linear short cavity, and realized single-frequency laser output with a working wavelength of 1727 nm and a power of 12.4 mW

IEEE Photonics Technology Letters, 2021, 33: 350

Optics Express, 2021, 29(14):21409-21417

[0004] On the one hand, Tm 3+The emission cross section of the glass fiber at the 1.7 μm band is obviously lower than that at the long-wave band of 1.75-2.0 μm, so the gain provided by the glass fiber at the 1.7 μm band is extremely low (≤0.1 dB / cm); on the other hand, the laser amplification process at this band is also susceptible to the competition of amplified spontaneous emission (ASE), especially when the gain is insufficient, the ASE will occupy a large proportion of energy, thereby inhibiting the effective growth of signal light; in addition, the Tm 3+ The parasitic oscillation phenomenon commonly existing in the fiber laser system will further occupy the effective gain resources, reduce the system efficiency, and even hinder the generation of the target wavelength laser when it is serious. Therefore, there are still many technical challenges in realizing the high-efficiency and stable single-frequency laser output at the 1.7 μm band.

[0005] The gain fiber is the core material of the fiber laser. The Chinese patent CN114349355A discloses a rare earth doped multi-component oxide glass fiber for generating 1.7 μm band laser, wherein the core is Tm 3+ The cladding is Tb 3+ The core is Tm 3+ The Tm 3+ The long-wave band (1.75-2.0 μm) light emitted has low absorption efficiency, and can only be pumped by a single-mode core, so the pumping efficiency is low, and the high-power cladding pumping technology cannot be used, resulting in low gain of the fiber at the 1.7 μm band, and it is difficult to realize the high-efficiency, high-power, narrow-linewidth and stable single-frequency laser output at the 1.7 μm band. SUMMARY

[0006] In view of the above technical problems, the present application provides a 1.7 μm band single-frequency fiber laser based on a composite glass fiber, and the technical problems to be solved are to overcome the technical defects of the existing single-frequency fiber laser, such as difficult to cover the 1.7 μm band, low conversion efficiency, poor optical signal-to-noise ratio, easy to self-excitation, etc.

[0007] The specific technical scheme adopted by the present application to solve the technical problems is as follows:

[0008] A 1.7 μm band single-frequency fiber laser based on a composite glass fiber, comprising a long linear cavity, wherein the long linear cavity comprises, in sequence, a high-reflection Bragg grating (3), a composite glass fiber (4), a saturable absorber (5) and a low-reflection Bragg grating (6); the composite glass fiber is a multi-mode fiber with a double-clad structure, the core is Tb 3+ doped multi-component glass, and the inner cladding is Tm3+ The doping multi-component glass, the outer cladding is the undoped rare earth multi-component glass, the diameter of the core is 23-27 mu m, the diameter of the inner cladding is 245-255 mu m, the diameter of the outer cladding is 380-420 mu m, the numerical aperture of the inner cladding and the core is 0.06-0.3, and the numerical aperture of the outer cladding and the inner cladding is greater than or equal to 0.45.

[0009] The application constructs a long linear cavity based on a composite glass fiber, wherein the composition of the long linear cavity comprises the composite glass fiber, a saturable absorber and a pair of uniform fiber gratings. The composite glass fiber is a multi-mode fiber with a double-clad structure, the core is doped with Tb 3+ The multi-component glass, the inner cladding is doped with Tm 3+ The multi-mode fiber can utilize multi-mode pumping, the light in the core leaks into the inner cladding, the inner cladding breaks the circular symmetry structure, the high-power cladding pumping technology can be adopted, and the pumping efficiency is significantly higher than that of single-mode core pumping. The Tm 3+ ions absorb the pumping light and emit 1.65-2.0 mu m light, and the Tb 3+ ions can effectively absorb long-waveband 1.75-2.0 mu m ASE, enhance the emission capacity of the composite glass fiber at the 1.7 mu m band (1.65-1.75 mu m), improve the light emission efficiency, the energy concentration and the spectral line selectivity, and thus help to realize high-laser output efficiency and wavelength stability. The outer cladding is an undoped rare earth multi-component glass, provides certain structural support and optical constraint, and the synergistic optimization of the material and the structure can realize high-power, narrow-line-width and high-signal-to-noise ratio 1.7 mu m band single-frequency laser output under the premise of maintaining high beam quality and stability.

[0010] Further optimization of the matrix glass composition (realize high-concentration doping of rare earth ions) and the fiber structure (improve the Tm 3+ absorption efficiency of the pumping light and the Tb 3+ absorption efficiency of long-waveband 1.75-2.0 mu m light) can significantly improve the gain of the composite glass fiber at the 1.7 mu m band.

[0011] Preferably, the cross section of the inner cladding is an eccentric circular structure, a rectangular structure, a hexagonal structure, an octagonal structure, a D-shaped structure or a petal-shaped structure.

[0012] Preferably, the inner cladding of the composite glass fiber is doped with Tm 3+ The multi-component germanate glass can realize high-concentration doping of Tm 3+ , and thus provide high near-infrared band gain.

[0013] Preferably, the core of the composite glass fiber is Tb-doped 3+ The multi-component silicate glass has excellent fiber drawing performance and easily adjustable physical and chemical properties.

[0014] Preferably, the cladding of the composite glass fiber is commercial K9 glass.

[0015] Preferably, the diameter of the core of the composite glass fiber is 23-27 μm, the diameter of the inner cladding is 245-255 μm, the diameter of the outer cladding is 380-420 μm, the numerical aperture of the inner cladding and the core is 0.06-0.3, the numerical aperture of the outer cladding and the inner cladding is ≥0.45, and the fiber use length is 0.5-5 m.

[0016] Preferably, the diameter of the core of the composite glass fiber is 25 μm, the diameter of the inner cladding is 250 μm, the diameter of the outer cladding is 400 μm, the numerical aperture of the inner cladding and the core is 0.12, the numerical aperture of the outer cladding and the inner cladding is 0.45, and the fiber use length is 0.5 m.

[0017] Preferably, the preparation method of the composite glass fiber is as follows:

[0018] 1) High-concentration Tm-doped 3+ Tb-doped germanate glass and Tb-doped 3+ Silicate glass. The raw materials are weighed according to the formula, mixed uniformly after weighing, and then placed in a high-purity platinum crucible, melted at high temperature (1300-1650 ℃) in an electric furnace, continuously stirred during the melting process, and a dry atmosphere or a dehydrating agent is added to remove the hydroxyl groups in the raw materials. After sufficient melting, the glass melt is rapidly poured into a preheated mold for forming, and annealing treatment is carried out at an appropriate temperature to eliminate thermal stress, obtaining high-quality core and inner cladding glass;

[0019] 2) The core glass and the inner cladding glass are respectively mechanically cut, cold worked and precisely polished according to the designed size, and assembled with a commercial K9 cladding glass tube to form a composite glass fiber preform, wherein the inner cladding structure is D-shaped;

[0020] 3) The fiber preform is placed in a commercial fiber drawing tower and heated to draw a fiber, and the fiber preform is vacuumed during the drawing process to remove the interfacial air, obtaining a Tb-doped 3+ Tm-doped multi-component silicate glass core, Tb-doped 3+ Composite glass fiber with Tb-doped multi-component silicate glass core, Tm-doped

[0021] Preferably, the doping concentration of Tb2O3 in the core of the composite glass fiber is 0.5-5 mol.%, and the doping concentration of Tm2O3 in the inner cladding is 1.5-3 mol.%.

[0022] Preferably, the 1.7 μm band single-frequency fiber laser further comprises a pump source (1), a combiner (2) and an isolator (7); the output pigtail of the pump source (1) is connected with the pump input end of the combiner (2), one end of the high-reflection Bragg grating (3) is connected with the combiner output end of the combiner (2); the other end of the high-reflection Bragg grating (3) is connected with the composite glass fiber (4), the other end of the composite glass fiber (4) is connected with the saturable absorber (5), the other end of the saturable absorber (5) is connected with the low-reflection Bragg grating (6), the other end of the low-reflection Bragg grating (6) is connected with the isolator (7), and the other end of the isolator (7) is taken as a single-frequency laser output port.

[0023] Preferably, the pump source can be a multimode fiber laser with a pigtail output, a semiconductor laser or other types of solid-state lasers, the pump power of which is not less than 100 mW, and the pump wavelength can be selected in the range of 780-820 nm or 1500-1610 nm, and the specific value is matched according to the target working wavelength of the single-frequency laser. The pump mode can be single-wavelength pumping or multi-wavelength hybrid pumping; the pump light injection direction can be forward, backward or bidirectional.

[0024] Preferably, the pump input end and the combiner output end of the combiner are both double-clad fibers, the core size of which is matched with the output pigtail of the pump source; the power bearing capacity of the combiner is greater than 10 W.

[0025] Preferably, the 3 dB bandwidth of the high-reflection Bragg grating is in the range of 1±0.5 nm, the transmission rate of the pump laser wavelength is greater than 95%, and the reflectivity of the signal laser wavelength is greater than 95%.

[0026] Preferably, the saturable absorber has a certain absorption capacity for the signal light wavelength, and is selected from Tm 3+ fiber doped with Ho 3+ fiber doped with Er 3+ fiber doped with Ho 3+ co-doped fiber doped with Er 3+ co-doped fiber doped with Tm 3+ co-doped fiber doped with Yb 3+ co-doped fiber doped with Ho 3+ co-doped fiber doped with Tm 3+ co-doped fiber doped with Ho 3+ co-doped fiber doped with Tm 3+ co-doped fiber doped with Yb 3+ co-doped fiber doped with Tm 3+ co-doped fiber doped with Yb 3+ / Ho 3+ Three-doped optical fiber. Its corresponding length range is 0.5-20 m, only in the interference field formed by two coherent light beams, through the light intensity induced absorption or refractive index modulation can form a periodic dynamic grating structure.

[0027] Preferably, the 3dB bandwidth of the low reflectivity Bragg grating is within 0.2 nm, the transmission to the pump laser wavelength is greater than 95%, and the reflectivity to the signal laser wavelength is 60-95%.

[0028] Preferably, the single-frequency fiber laser has a working wavelength of 1703-1725 nm, a power greater than 50 mW (preferably greater than or equal to 200 mW, more preferably greater than or equal to 500 mW), a line width less than 2 kHz, and an optical signal-to-noise ratio greater than 50 dB.

[0029] Compared with the prior art, the technical effects of the present application are:

[0030] (1) The laser of the present application uses a double-clad composite glass fiber as a gain fiber, the core of which is doped with Tb 3+ Multi-component glass, the inner cladding is doped with Tm 3+ Multi-component glass. The multi-component glass can achieve high concentration doping and obtain high gain in the near-infrared band (1.65-2.0 μm); the multi-mode fiber can use multi-mode pumping, and the light in the core will leak into the inner cladding, and the inner cladding breaks the circular symmetry structure, which is beneficial to the emission of long-waveband (1.75-2.0 μm) light by Tb 3+ in the inner cladding is absorbed by Tb 3+ in the core, so that the composite glass fiber has high gain in the 1.7 μm band (1.65-1.75 μm), and the gain is relatively uniform. The synergistic optimization of such materials and structures helps to suppress the occurrence of multi-longitudinal-mode competition and parasitic oscillation, and realizes high-efficiency, high-power, narrow-line-width 1.7 μm band single-frequency laser output under the premise of maintaining high beam quality and stability.

[0031] (2) The 1.7 μm band single-frequency fiber laser of the present application can use multi-mode cladding pumping technology, which increases the utilization efficiency of pump light, reduces the reabsorption of signal laser and the thermal effect of the system, and improves the output power, stability and reliability of the laser. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 It is a schematic diagram of the cross section of the composite glass fiber in Example 1.

[0033] Figure 2 It is a schematic diagram of the laser device in Example 1. DETAILED DESCRIPTION

[0034] The 1.7 μm band single-frequency fiber laser based on composite glass fiber according to the present application is further described in detail below in conjunction with specific embodiments. The present application can be implemented in many different forms and is not limited to the embodiments described herein. Rather, the purpose of providing these embodiments is to make the disclosure of the present application more thorough and comprehensive.

[0035] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present application belongs. The terminology used in the description of the present application herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present application.

[0036] In the present application, the technical features described in an open form include both the closed technical solution consisting of the listed features and the open technical solution containing the listed features.

[0037] In the present application, when referring to a numerical range, unless otherwise specified, the numerical range is considered to be continuous and includes the minimum value and the maximum value of the range and every value between the minimum value and the maximum value. Further, when the range refers to integers, every integer between the minimum value and the maximum value of the range is included. In addition, when multiple ranges are provided to describe a feature or a property, the ranges can be combined. In other words, unless otherwise indicated, all ranges disclosed herein are to be understood as including any and all sub-ranges subsumed therein.

[0038] In the present application, when referring to a percentage content, unless otherwise specified, it refers to a mass percentage for solid-liquid mixing and solid-solid mixing and a volume percentage for liquid-liquid mixing.

[0039] In the present application, when referring to a percentage concentration, unless otherwise specified, it refers to a final concentration. The final concentration refers to the proportion of the added ingredient in the system after the ingredient is added.

[0040] In the present application, unless otherwise specified, the temperature parameter allows both constant temperature treatment and treatment within a certain temperature range. The constant temperature treatment allows the temperature to fluctuate within the accuracy range controlled by the instrument.

[0041] In the present application, room temperature generally refers to 4℃-30℃, preferably 20±5℃.

[0042] The present application provides a 1.7 μm band single-frequency fiber laser based on composite glass fiber, comprising a long linear cavity, the composition of the long linear cavity comprising a high reflectivity Bragg grating (3), a composite glass fiber (4), a saturable absorber (5) and a low reflectivity Bragg grating (6) connected in sequence; the composite glass fiber is a multimode fiber with a double-clad structure, the core of which is Tb3+ Doped multi-component glass, inner cladding is Tm 3+ Doped multi-component glass, outer cladding is undoped rare earth multi-component glass, the diameter of the core is 23-27 μm, the diameter of the inner cladding is 245-255 μm, the diameter of the outer cladding is 380-420 μm, the numerical aperture of the inner cladding and the core is 0.06-0.3, and the numerical aperture of the outer cladding and the inner cladding is ≥0.45.

[0043] Understandably, in the composite glass optical fiber, the inner cladding and the outer cladding are sequentially coated on the surface of the core.

[0044] Specifically, the diameter of the core includes but is not limited to 23 μm, 24 μm, 25 μm, 26 μm, 27 μm, or a range between any two of the foregoing.

[0045] Specifically, the diameter of the outer cladding includes but is not limited to 380 μm, 390 μm, 400 μm, 410 μm, 420 μm, or a range between any two of the foregoing.

[0046] Specifically, the numerical aperture of the inner cladding and the core includes but is not limited to 0.06, 0.08, 0.1, 0.12, 0.15, 0.2, 0.25, 0.3, or a range between any two of the foregoing.

[0047] Specifically, the numerical aperture of the outer cladding and the inner cladding includes but is not limited to 0.45, 0.48, 0.5, 0.55, 0.6, or a range between any two of the foregoing.

[0048] Further, the cross section of the inner cladding of the composite glass optical fiber is an eccentric circular structure, a rectangular structure, a hexagonal structure, an octagonal structure, a D-shaped structure, or a petal-shaped structure.

[0049] Further, the inner cladding of the composite glass optical fiber is Tm 3+ Multi-component germanate glass, the multi-component germanate glass can realize high-concentration doping of Tm 3+ , thereby providing high near-infrared band gain. As an example, the Tm 3+ doped multi-component germanate glass has a composition including GeO2, PbO, BaO, Al2O3, La2O3, Y2O3, and Tm2O3, with a molar percentage of 65-75%, 10-20%, 5-15%, 1-3%, 0.5-1.5%, 0.5-1.5%, and 1-3%, respectively; or, the Tm 3+The composition of the multi-component germanate glass comprises GeO2, Ga2O3, CaO, La2O3, Y2O3 and Tm2O3, and the molar percentage of each component is 65-75%, 10-20%, 5-15%, 1-3%, 1-3% and 1-3% respectively; or, Tm-doped 3+ The composition of the multi-component germanate glass comprises GeO2, Ga2O3, CaO, BaO, La2O3, Y2O3 and Tm2O3, and the molar percentage of each component is 60-70%, 10-20%, 5-10%, 5-10%, 0.5-1.5%, 0.5-1.5% and 2-5% respectively.

[0050] Further, the core of the composite glass fiber is Tb-doped 3+ The multi-component silicate glass has excellent drawing performance and easily adjustable physicochemical properties. As an example, Tb-doped 3+ The composition of the multi-component silicate glass comprises SiO2, PbO, BaO, Al2O3, B2O3, Na2O and Tb2O3, and the molar percentage of each component is 60-70%, 10-20%, 5-15%, 1-3%, 1-2.5%, 1-2.5% and 3-7% respectively; or, Tb-doped 3+ The composition of the multi-component silicate glass comprises SiO2, Na2O, CaO, Al2O3, B2O3 and Tb2O3, and the molar percentage of each component is 65-75%, 5-15%, 10-20%, 1-3%, 1-3% and 0.5-3% respectively; or, Tb-doped 3+ The composition of the multi-component silicate glass comprises SiO2, Na2O, CaO, Al2O3, B2O3 and Tb2O3, and the molar percentage of each component is 55-65%, 5-15%, 10-20%, 5-15%, 5-15% and 0.3-1% respectively.

[0051] Further, the cladding layer of the composite glass fiber is a commercial K9 glass or a multi-component silicate glass.

[0052] Further, the diameter of the core of the composite glass fiber is 23-27 μm, the diameter of the inner cladding layer is 245-255 μm, the diameter of the outer cladding layer is 380-420 μm, the numerical aperture of the inner cladding layer and the core is 0.06-0.3, the numerical aperture of the outer cladding layer and the inner cladding layer is ≥0.45, and the fiber use length is 0.5-5 m.

[0053] Further, the diameter of the core of the composite glass fiber is 25 μm, the diameter of the inner cladding layer is 250 μm, the diameter of the outer cladding layer is 400 μm, the numerical aperture of the inner cladding layer and the core is 0.12, the numerical aperture of the outer cladding layer and the inner cladding layer is 0.45, and the fiber use length is 0.5 m.

[0054] Further, the preparation method of the composite glass fiber is as follows:

[0055] 1) High concentration Tm 3+ doped germanate glass and Tb 3+ doped silicate glass. The raw materials are weighed according to the formula, and the weighed raw materials are mixed uniformly and placed in a high-purity platinum crucible. The raw materials are melted at a high temperature (1300-1650 ℃) in an electric furnace, and the stirring is continued during the melting process and a dry atmosphere or a dehydrating agent is added to remove the hydroxyl groups in the raw materials. After sufficient melting, the glass melt is rapidly poured into a preheated mold for forming, and annealing treatment is carried out at an appropriate temperature to eliminate thermal stress, so as to obtain high-quality core and inner cladding glasses;

[0056] 2) The core glass and the inner cladding glass are respectively mechanically cut, cold processed and precisely polished according to the designed size, and are assembled with a commercial K9 cladding glass tube to form a composite glass fiber preform, wherein the inner cladding structure is D-shaped;

[0057] 3) The fiber preform is placed in a commercial fiber drawing tower and drawn into a fiber at a high temperature. The fiber preform is vacuumed during the drawing process to remove the interfacial air, so as to obtain a Tb 3+ doped multi-component silicate glass core, Tm 3+ doped multi-component germanate glass inner cladding and silicate glass outer cladding structure composite glass fiber.

[0058] Further, the doping concentration of Tb2O3 in the core of the composite glass fiber is 0.5-5 mol.%. Specifically, the doping concentration of Tb2O3 includes but is not limited to: 0.5 mol.%, 1 mol.%, 1.5 mol.%, 2 mol.%, 2.5 mol.%, 3 mol.%, 3.5 mol.%, 4 mol.%, 4.5 mol.%, 5 mol.% or a range between any two of the foregoing.

[0059] Further, the doping concentration of Tm2O3 in the inner cladding of the composite glass fiber is 1.5-3 mol.%. Specifically, the doping concentration of Tm2O3 includes but is not limited to: 1.5 mol.%, 2 mol.%, 2.5 mol.%, 3 mol.% or a range between any two of the foregoing.

[0060] Further, the 1.7 μm band single-frequency fiber laser further comprises: a pump source (1), a beam combiner (2) and an isolator (7); the output pigtail of the pump source (1) is connected with the pump input end of the beam combiner (2), one end of the high-reflection Bragg grating (3) is connected with the beam-combining output end of the beam combiner (2); the other end of the high-reflection Bragg grating (3) is connected with the composite glass fiber (4), the other end of the composite glass fiber (4) is connected with the saturable absorber (5), the other end of the saturable absorber (5) is connected with the low-reflection Bragg grating (6), the other end of the low-reflection Bragg grating (6) is connected with the isolator (7), and the other end of the isolator (7) is taken as a single-frequency laser output port.

[0061] Further, the pump source can be a multimode fiber laser with a pigtail output, a semiconductor laser or other types of solid-state lasers, the pump power of which is not less than 100 mW, and the pump wavelength can be selected in the range of 780-820 nm or 1500-1610 nm, and the specific value is matched according to the target working wavelength of the single-frequency laser. The pump mode can be single-wavelength pumping or multi-wavelength hybrid pumping, and the pump light injection direction can be forward, backward or bidirectional.

[0062] Further, the pump input end and the beam-combining output end of the beam combiner are both double-clad fibers, the core size of which is matched with the output pigtail of the pump source; the beam combiner can withstand a power greater than 10 W.

[0063] Further, the 3 dB bandwidth of the high-reflection Bragg grating is in the range of 1±0.5 nm, the transmission rate of the pump laser wavelength is greater than 95%, and the reflectivity of the signal laser wavelength is greater than 95%.

[0064] Further, the saturable absorber has a certain absorption capacity for the signal light wavelength, and is selected from Tm 3+ doped fiber, Ho 3+ doped fiber, Er 3+ / Ho 3+ co-doped fiber, Er 3+ / Tm 3+ co-doped fiber, Yb 3+ / Ho 3+ co-doped fiber, Tm 3+ / Ho 3+ co-doped fiber, Tm 3+ / Yb 3+ co-doped fiber or Tm 3+ / Yb 3+ / Ho 3+Three-doped fiber. Its corresponding length range is 0.5-20 m, only in the interference field formed by two coherent beams, through the light intensity induced absorption or refractive index modulation can form a periodic dynamic grating structure.

[0065] Further, the 3 dB bandwidth of the low reflectivity Bragg grating is within 0.2 nm, the high transmission to the pump laser wavelength is greater than 95%, and the reflectivity to the signal laser wavelength is 60-95%.

[0066] Further, the single-frequency fiber laser has a working wavelength of 1703 nm-1725 nm, a power greater than 50 mW (preferably ≥200 mW, more preferably ≥500 mW), a line width less than 2 kHz, and an optical signal-to-noise ratio greater than 50 dB.

[0067] In the following specific examples, the experimental parameters not written in the specific examples are preferably referred to the guidance given in the present application file, and can also be referred to the experimental manual in the art or other experimental methods known in the art, or to the experimental conditions recommended by the manufacturer.

[0068] The raw materials and reagents involved in the following specific examples can be obtained commercially or prepared by those skilled in the art according to known means.

[0069] Example 1:

[0070] In this embodiment, the outer cladding of the composite glass fiber uses commercial K9 optical glass, the core is Tb 3+ Doped silicate glass, the molar composition is 65 mol.% SiO2-15 mol.% PbO-10 mol.% BaO-2 mol.% Al2O3-1.5 mol.% B2O3-1.5 mol.% Na2O-5 mol.% Tb2O3, the inner cladding is high-concentration Tm 3+ Doped germanate glass, the molar composition is 70 mol.% GeO2-14 mol.% PbO-10 mol.% BaO-2 mol.% Al2O3-1 mol.% La2O3-1 mol.% Y2O3-2 mol.% Tm2O3, the core and inner cladding glass melted are assembled into a composite glass fiber preform rod after processing, and are drawn into a composite glass fiber on a commercial drawing tower, and the cross-sectional structure is as shown in Figure 1 The inner cladding 200 and the outer cladding 300 are sequentially coated on the surface of the core 100. The core diameter of the fiber is 25 μm, the inner cladding diameter is 250 μm, and the outer cladding diameter is 400 μm. The inner cladding has a D-shaped structure, and the numerical aperture of the inner cladding and the core is 0.12. The numerical aperture of the outer cladding and the inner cladding is 0.45.

[0071] A 1.7 μm band single-frequency fiber laser is constructed by using the above-mentioned 0.5 m long composite glass fiber, and a device diagram thereof is shown in Figure 2 The connection relationship between the components is as follows: one end of the pump source (1) is connected with one end of the beam combiner (2), one end of the high reflectivity Bragg grating (3) is connected with the output end of the beam combiner (2); the other end of the high reflectivity Bragg grating (3) is connected with the composite glass fiber (4), the other end of the composite glass fiber (4) is connected with the saturable absorber (5), the other end of the saturable absorber (5) is connected with the low reflectivity Bragg grating (6), the other end of the low reflectivity Bragg grating (6) is connected with the isolator (7), and the other end of the isolator (7) is used as a single-frequency laser output port. In this embodiment, the center wavelength of the high reflectivity Bragg grating (3) is 1703 nm, the center wavelength reflectivity is 99%, the transmittance for the pump laser wavelength 793 nm is 99.9%, and the 3 dB bandwidth is 0.5 nm; the center wavelength of the low reflectivity Bragg grating (6) is 1703 nm, the center wavelength reflectivity is 60%, and the 3 dB bandwidth is 0.12 nm. The pump input end fiber of the beam combiner (2) is matched with the output fiber of the 793 nm pump source (1), and the output end of the beam combiner is matched with the high reflectivity Bragg grating fiber, which can withstand a power of more than 10 W. The saturable absorber (5) is a commercial Tm 3+ The double-clad fiber has a core diameter of 24 μm and an inner cladding diameter of 248 μm, and the length of the fiber used is 1.5 m. The pump source (1) is a multimode fiber laser with a tail fiber coupling output, the working wavelength of the pump source is 793 nm, the power is 12 W, and the pump direction is backward pumping.

[0072] In this embodiment, the 793 nm pump light generated by the pump source enters the inner cladding of the composite glass fiber through the beam combiner. Under the continuous excitation of the pump light, the population of Tm 3+ ions is inverted, and 1703 nm stimulated radiation signal laser is generated. The laser is oscillated back and forth in the high reflectivity Bragg grating and the low reflectivity Bragg grating, strengthened, and subjected to a continuous stimulated radiation process. In this embodiment, a 0.5 m long composite glass fiber is used as a gain fiber, so that the longitudinal mode spacing is much smaller than the required interval for single longitudinal mode output. Therefore, the absorber can generate optical nonlinear effects under the action of strong laser, form periodic refractive index modulation, and constitute a narrowband dynamic grating. The narrowband dynamic grating reflects light of a specific wavelength, and its reflection efficiency saturates with the change of incident light intensity, thereby realizing saturable absorption effect, and finally realizing stable single longitudinal mode laser output. Finally, a single-frequency fiber laser with a working wavelength of 1703 nm, a power of 200 mW, a line width of 1.9 kHz, and an optical signal-to-noise ratio of 52 dB is obtained.

[0073] Example 2:

[0074] In this embodiment, the outer cladding of the composite glass fiber is made of commercial K9 optical glass, and the core is Tb 3+ The doped silicate glass has a molar composition of 70 mol.% SiO2-10 mol.% Na2O-15 mol.% CaO-2 mol.% Al2O3-2 mol.% B2O3-1 mol.% Tb2O3, and the inner cladding is high-concentration Tm 3+ The doped germanate glass has a molar composition of 68 mol.% GeO2-15 mol.% Ga2O3-12 mol.% CaO -1.5 mol.% La2O3-2 mol.% Y2O3-1.5 mol.% Tm2O3. The molten core and inner cladding glasses are processed and assembled with a K9 cladding glass tube to form a composite glass fiber preform, which is drawn into a composite glass fiber on a commercial drawing tower. The fiber has a core diameter of 23 μm, an inner cladding diameter of 255 μm, and an outer cladding diameter of 420 μm. The inner cladding has a hexagonal structure, and the numerical aperture of the inner cladding and the core is 0.06. The numerical aperture of the outer cladding and the inner cladding is 0.5.

[0075] A 1.7 μm band single-frequency fiber laser is constructed using the above-mentioned 0.8 m long composite glass fiber. The device diagram is similar to that of Embodiment 1. Among them, the high-reflectivity Bragg grating (3) works at a center wavelength of 1710 nm, the center wavelength reflectivity is 99.5%, the transmittance for the pump laser wavelength 1610 nm is 95.5%, and the 3 dB bandwidth is 1.5 nm. The low-reflectivity Bragg grating (6) works at a center wavelength of 1710 nm, the center wavelength reflectivity is 75%, the pump wavelength transmittance is 98%, and the 3 dB bandwidth is 0.15 nm. The pump input end fiber of the combiner (2) is matched with the output fiber of the 1610 nm pump source (1), and the combiner output end is matched with the high-reflectivity Bragg grating fiber, which can withstand a power of more than 10 W. The saturable absorber (5) is a commercial Ho 3+ The double-clad fiber has a core diameter of 25 μm, an inner cladding diameter of 255 μm, and an outer cladding diameter of 420 μm, and the length is 1.8 m. The pump source (1) is a multimode fiber laser with a tail fiber coupled output, and the working wavelength of the pump source is 1610 nm, the power is 15 W, and the pump direction is backward pumping.

[0076] In this embodiment, the 1610 nm pump light generated by the pump source enters the inner cladding of the composite glass fiber through the combiner. Under the continuous excitation of the pump light, the Tm 3+The number of particles in the ions is reversed, generating 1710 nm stimulated emission signal laser, back and forth in high reflectivity Bragg grating and low reflectivity Bragg grating, strengthening and experiencing continuous stimulated emission process; In this embodiment, a 0.8 m long composite glass fiber is used as a gain fiber, so that the longitudinal mode spacing is much smaller than the single longitudinal mode output requirement interval, and therefore can be combined with the absorber to produce optical nonlinear effects under the action of strong laser, forming a periodic refractive index modulation, constituting a narrowband dynamic grating. The narrowband dynamic grating reflects light of a specific wavelength, and its reflection efficiency saturates with the change of incident light intensity, thereby realizing saturable absorption effect, and finally realizing stable single longitudinal mode laser output. Finally, a single-frequency fiber laser with a working wavelength of 1710 nm, a power of 500 mW, a line width of 1.5 kHz, and an optical signal-to-noise ratio of 60 dB is obtained.

[0077] Embodiment 3:

[0078] In this embodiment, the outer cladding of the composite glass fiber uses a multi-component silicate glass with a molar composition of 60 mol.% SiO2-8 mol.% Na2O-12 mol.% BaO-10 mol.% Al2O3-10 mol.% Y2O3; the core is Tb 3+ doped silicate glass with a molar composition of 59.5 mol.% SiO2-8 mol.% Na2O-12 mol.% CaO-10 mol.% Al2O3-10 mol.% B2O3-0.5 mol.% Tb2O3; the inner cladding is high-concentration Tm 3+ doped germanate glass with a molar composition of 65 mol.% GeO2-15 mol.% Ga2O3-7.5 mol.% CaO-7.5 mol.% BaO-1 mol.% La2O3-1 mol.% Y2O3-3 mol.% Tm2O3; the melted glass is processed and assembled into a composite glass fiber preform, and then drawn into a composite glass fiber on a commercial drawing tower. The fiber has a core diameter of 27 μm, an inner cladding diameter of 245 μm, and an outer cladding diameter of 380 μm, the inner cladding is a square structure, and the numerical aperture of the inner cladding and the core is 0.3, and the numerical aperture of the outer cladding and the inner cladding is 0.48.

[0079] A 1.7 μm band single frequency fiber laser is constructed by using the above 2 m long composite glass fiber. The device diagram is similar to that of Example 1. Among them, the high reflectivity Bragg grating (3) works at a center wavelength of 1725 nm, the center wavelength reflectivity is 99.9%, the transmittance for the pump laser wavelength 820 nm is 99%, and the 3 dB bandwidth is 1 nm. The low reflectivity Bragg grating (6) works at a center wavelength of 1725 nm, the center wavelength reflectivity is 95%, the pump wavelength transmittance is 97%, and the 3 dB bandwidth is 0.1 nm. The pump input end of the combiner (2) is matched with the output end fiber of the 820 nm pump source (1), and the combiner output end is matched with the high reflectivity Bragg grating tail fiber, which can withstand a power of more than 10 W. The saturable absorber (5) is a commercial Tm 3+ / Ho 3+ The co-doped double-clad fiber has a core diameter of 25 μm, an inner cladding diameter of 250 μm, and an outer cladding diameter of 400 μm, and the length used is 2.5 m. The pump source (1) is a solid-state laser with a tail fiber coupled output, the working wavelength of the pump source is 820 nm, the power is 30 W, and the pump direction is forward pumping.

[0080] In this embodiment, the 820 nm pump light generated by the pump source enters the composite glass fiber core through the combiner. Under the continuous excitation of the pump light, the population inversion occurs in the core, and the 1725 nm stimulated radiation signal laser is generated. The laser is oscillated back and forth in the high reflectivity Bragg grating and the low reflectivity Bragg grating, strengthened, and experienced a continuous stimulated radiation process. In this embodiment, a 2 m long composite glass fiber is used as a gain fiber, so that the longitudinal mode spacing is much smaller than the required interval for single longitudinal mode output. Therefore, the optical nonlinear effect can be generated by the interaction of the absorber and the strong laser, forming a periodic refractive index modulation, which constitutes a narrowband dynamic grating. The narrowband dynamic grating reflects light of a specific wavelength, and its reflection efficiency saturates with the change of incident light intensity, thereby realizing the saturable absorption effect, and finally realizing stable single longitudinal mode laser output. Finally, a single frequency fiber laser with a working wavelength of 1725 nm, a power of 550 mW, a line width of 1 kHz, and an optical signal-to-noise ratio of 65 dB is obtained.

[0081] The technical features of the above-described embodiments can be combined in any manner. In order to make the description concise, not all possible combinations of the technical features in the above-described embodiments are described, but as long as the combinations of the technical features do not exist, they should be considered as the scope of the present disclosure.

[0082] The above-described embodiments only express several implementation manners of the present application, facilitate specific and detailed understanding of the technical solutions of the present application, but cannot be understood as a limitation on the patent protection scope of the application. It should be noted that, for ordinary skilled persons in the art, several modifications 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. It should be understood that, on the basis of the technical solutions provided by the present application, the technical solutions obtained by logical analysis, reasoning or limited experiments by those skilled in the art all belong to the protection scope of the appended claims of the present application. Therefore, the protection scope of the patent of the present application should be subject to the content of the appended claims, and the description can be used to explain the content of the claims.

Claims

1. A 1.7 pm band single frequency fiber laser based on a composite glass fiber, characterized in that, The long linear cavity comprises, in sequence, a high reflectivity Bragg grating (3), a composite glass fiber (4), a saturable absorber (5) and a low reflectivity Bragg grating (6). The composite glass optical fiber is a multimode optical fiber, has a double-clad structure, the core is Tb 3+ The doped multi-component glass is Tm 3+ The doped multi-component glass is Tm The diameter of the core is 23-27 μm, the diameter of the inner cladding is 245-255 μm, the diameter of the outer cladding is 380-420 μm, the numerical aperture of the inner cladding and the core is 0.06-0.3, and the numerical aperture of the outer cladding and the inner cladding is ≥0.

45. The inner cladding has a cross section in the shape of an eccentric circle, a rectangle, a hexagon, an octagon, a D letter or a petal.

2. The 1.7 pm band single-frequency fiber laser based on composite glass fiber according to claim 1, characterized in that, The doping concentration of Tb2O3 in the core is 0.5-5 mol.% and the doping concentration of Tm2O3 in the inner cladding is 1.5-3 mol.%.

3. The 1.7 pm band single-frequency fiber laser based on composite glass fiber according to claim 1 or 2, characterized in that, Further comprising: a pump source (1), a combiner (2) and an isolator (7); the output pigtail of the pump source (1) is connected to the pump input end of the combiner (2), one end of the high reflectivity Bragg grating (3) is connected to the combiner output end of the combiner (2); the other end of the high reflectivity Bragg grating (3) is connected to the composite glass fiber (4), the other end of the composite glass fiber (4) is connected to the saturable absorber (5), the other end of the saturable absorber (5) is connected to the low reflectivity Bragg grating (6), the other end of the low reflectivity Bragg grating (6) is connected to the isolator (7), and the other end of the isolator (7) is used as a single-frequency laser output port.

4. The 1.7 pm band single-frequency fiber laser based on composite glass fiber according to claim 3, characterized in that, The pump source (1) is a solid-state laser with a pigtail output, and the pump wavelength ranges from 780 nm to 820 nm or from 1500 nm to 1610 nm.

5. The 1.7 pm band single frequency fiber laser based on composite glass fiber according to claim 3, characterized in that, The pump input end and the combiner output end of the combiner (2) are both double-clad fibers, the core size of which matches the output pigtail of the pump source (1); the combiner (2) can withstand a power greater than 10 W.

6. The 1.7 pm band single frequency fiber laser based on composite glass fiber according to claim 1 or 2, characterized in that, The 3 dB bandwidth of the high reflectivity Bragg grating (3) ranges from 1 nm to 0.5 nm, it has high transmittance to the pump laser wavelength, the transmittance being greater than 95%, and it has a reflectivity greater than 95% to the signal laser wavelength.

7. The 1.7 pm band single frequency fiber laser based on composite glass fiber according to claim 1 or 2, characterized in that, The saturable absorber (5) is selected from Tm-doped 3+ Fiber, Ho-doped 3+ Fiber, Er-doped 3+ / Ho 3+ Co-doped fiber, Er 3+ / Tm 3+ Co-doped fiber, Yb 3+ / Ho 3+ Co-doped fiber, Tm 3+ / Ho 3+ Co-doped fiber, Tm 3+ / Yb 3+ Co-doped fiber or Tm 3+ / Yb 3+ / Ho 3+ Triply-doped fiber.

8. The 1.7 pm band single frequency fiber laser based on composite glass fiber according to claim 1 or 2, characterized in that, The 3 dB bandwidth of the low reflectivity Bragg grating (6) is ≤0.2 nm, it has high transmittance to the pump laser wavelength, the transmittance being greater than 95%, and it has a reflectivity of 60-95% to the signal laser wavelength.

9. The 1.7 pm band single frequency fiber laser based on composite glass fiber according to claim 1 or 2, characterized in that, The 1.7 μm band single-frequency fiber laser has a power greater than 50 mW, a line width less than 2 kHz and an optical signal-to-noise ratio greater than 50 dB.

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