Mid-infrared distributed bragg reflection single-frequency all-fiber laser and preparation method thereof

Abstract

The application relates to the technical field of lasers, in particular to a mid-infrared distributed Bragg reflection single-frequency all-fiber laser and a preparation method thereof. The mid-infrared distributed Bragg reflection single-frequency all-fiber laser comprises a pump source, a quartz single-mode optical fiber and a holmium-praseodymium co-doped fluorine-zirconium-based glass gain optical fiber connected in sequence. The holmium-praseodymium co-doped fluorine-zirconium-based glass gain optical fiber comprises a high-reflection fiber Bragg grating, an intermediate part and a low-reflection fiber Bragg grating, the high-reflection fiber Bragg grating, the intermediate part and the low-reflection fiber Bragg grating constitute a resonant cavity, the gain spectral line full width at half maximum of the resonant cavity is greater than the adjacent longitudinal mode spacing of the resonant cavity, and the gain spectral line full width at half maximum of the resonant cavity is less than twice the longitudinal mode spacing of the resonant cavity. The application is at least beneficial to realizing a mid-infrared all-fiber single-frequency light source which is simple and compact in structure, stronger in anti-external disturbance capability and more stable in single-frequency output.

Description

Technical Field

[0001] This invention belongs to the field of laser technology, and particularly relates to a mid-infrared distributed Bragg reflection single-frequency all-fiber laser and its fabrication method. Background Technology

[0002] With the rapid development of mid-infrared fiber laser technology in precision machining, coherent detection, spectral measurement, and sensing, the demand for coherent light sources in this band is increasing in both industry and scientific research. The strong absorption band (2.8 μm band), located in the molecular "fingerprint region" and near water molecules, is a highly representative operating window for mid-infrared coherent light sources. In practical applications, conventional fiber lasers in this band often exhibit problems such as high phase noise, significant frequency drift, and insufficient coherence length. This can lead to spectral broadening and resolution degradation in high-resolution spectral measurements, phase jitter and signal-to-noise ratio deterioration in coherent detection, and frequency drift causing calibration failures in precision sensing. Therefore, achieving stable single-frequency fiber laser output is one of the important directions for the engineering of mid-infrared coherent light sources. Single-frequency fiber lasers, due to their narrow linewidth, high coherence, and low intensity noise, are of great value in applications such as high-resolution molecular spectroscopy and trace gas detection, fine ablation of medical and biological tissues, coherent detection and measurement, and precision sensing. However, at present, research on single-frequency fiber lasers and the operating wavelength of corresponding devices are mainly concentrated in the near-infrared band of 1μm to 2μm, while research on the mid-infrared band is relatively limited.

[0003] Existing 2.8μm band continuous fiber lasers typically employ two technical approaches: distributed feedback linear cavity structures and ring cavity structures. Both approaches suffer from high barriers to device and fabrication implementation, difficulties in system engineering, and challenges in achieving a balance between size, cost, and stability. Specifically, in the distributed feedback linear cavity approach, the structure relies on directly writing a long π-phase-shift grating into the gain fiber to achieve single-mode oscillation. This places extremely high demands on the uniformity of the grating coupling coefficient, phase shift accuracy, writing loss, and consistency. However, in the fluoride and other soft glass gain fiber systems commonly used in the 2.8μm band, the grating fabrication process has a narrow window, insufficient processing repeatability, and low yield, resulting in high device manufacturing difficulty and cost. Furthermore, the solutions provided by these technologies are quite sensitive to temperature and disturbances. During laser operation, heat accumulation within the cavity can cause changes in the effective refractive index and Bragg center wavelength of the grating, leading to grating dephase and a decline in laser performance. This results in unstable single-frequency operating points and insufficient long-term operational reliability. For the ring cavity scheme, in order to suppress the spatial hole burning effect and achieve single longitudinal mode oscillation, it is necessary to introduce filter devices such as isolators, circulators, couplers and etalons (e.g., diffraction gratings) into the cavity to complete longitudinal mode selection. This will significantly increase the number of devices and insertion loss, making the system structure longer and more complex. Thus, collimation, alignment and mechanical fixation steps must be performed during construction. The long optical path and complex structure not only increase the system size and weight, but also significantly increase the difficulty of assembly and adjustment and manufacturing cost. Moreover, the maturity of available devices and cost pressures in the mid-infrared band are prominent. The cascading of multiple devices in the cavity is more likely to increase the sensitivity to the external environment, and problems such as power fluctuation, frequency drift and even mode skipping are prone to occur. Therefore, the stability and anti-interference ability of the ring cavity are also poor. Summary of the Invention

[0004] In view of this, the present invention aims to provide a mid-infrared distributed Bragg reflection single-frequency all-fiber laser and its fabrication method, which is at least beneficial to achieving a mid-infrared all-fiber single-frequency light source with simple and compact structure, stronger resistance to external disturbances, and more stable single-frequency output.

[0005] To achieve the above objectives, the technical solution created by this invention is implemented as follows: This invention provides a mid-infrared distributed Bragg reflection single-frequency all-fiber laser, comprising: a pump source, a quartz single-mode fiber, and a holmium-praseodymium co-doped zirconium-based glass gain fiber connected in sequence; wherein the holmium-praseodymium co-doped zirconium-based glass gain fiber includes a high-reflectivity fiber Bragg grating, an intermediate section, and a low-reflectivity fiber Bragg grating, the high-reflectivity fiber Bragg grating being located between the intermediate section and the quartz single-mode fiber, and the low-reflectivity fiber Bragg grating being located on the side of the intermediate section away from the high-reflectivity fiber Bragg grating; the high-reflectivity fiber Bragg grating, the intermediate section, and the low-reflectivity fiber Bragg grating constitute a resonant cavity, the full width at half maximum (FWHM) of the gain spectral line of the resonant cavity being greater than the interval between adjacent longitudinal modes of the resonant cavity, and the full WHM of the gain spectral line of the resonant cavity being less than twice the interval between longitudinal modes of the resonant cavity.

[0006] Furthermore, the total length of the resonant cavity is equal to the sum of the length of the middle section, the effective length of the high-reflectivity fiber Bragg grating, and the effective length of the low-reflectivity fiber Bragg grating, and the total length of the resonant cavity is ≤2.2cm.

[0007] Furthermore, the high-reflectivity fiber Bragg grating is a high-reflectivity narrowband fiber Bragg grating, and the low-reflectivity fiber Bragg grating is a low-reflectivity output-coupled fiber Bragg grating; the reflectivity of the high-reflectivity fiber Bragg grating is greater than that of the low-reflectivity fiber Bragg grating, the transmission bandwidth of the low-reflectivity fiber Bragg grating covers the transmission spectrum of the high-reflectivity fiber Bragg grating, and the center wavelength of the low-reflectivity fiber Bragg grating is the same as that of the high-reflectivity fiber Bragg grating.

[0008] Furthermore, the center wavelength of both the low-reflectivity fiber Bragg grating and the high-reflectivity fiber Bragg grating is 2862 nm. The high-reflectivity fiber Bragg grating has a reflectivity of 98%, a 3dB bandwidth of 0.2 nm, and an actual grating length of 13.6 mm. The low-reflectivity fiber Bragg grating has a reflectivity of 60%, a 3dB bandwidth of 0.59 nm, and an actual grating length of 6.3 mm.

[0009] Furthermore, the effective lengths of the high-reflectivity fiber Bragg grating and the low-reflectivity fiber Bragg grating are obtained using Formula 1, as follows: ; in, This represents the actual length of the corresponding grating. This represents the effective length of the corresponding grating. This represents the reflectivity of the corresponding grating. It is the inverse hyperbolic tangent function.

[0010] Furthermore, the length of the middle section is 10 mm, the effective length of the high-reflectivity fiber Bragg grating is 2.5 mm, and the effective length of the low-reflectivity fiber Bragg grating is 2.4 mm.

[0011] Furthermore, the output laser parameters of the mid-infrared distributed Bragg reflection single-frequency all-fiber laser are as follows: wavelength of 2862nm, slope efficiency of 3.8%, maximum output power of 22.6mW, and output wavelength linewidth of less than 5.98MHz.

[0012] Furthermore, the pump source is a Raman fiber laser with an output wavelength of 1150nm, and the quartz single-mode fiber and the holmium-praseodymium co-doped fluorine-zirconium-based glass gain fiber are connected by heterogeneous fusion splicing.

[0013] Another aspect of this invention provides a method for fabricating a mid-infrared distributed Bragg reflection (DBR) single-frequency all-fiber laser. The method comprises: providing a holmium-praseodymium co-doped zirconium-based glass gain fiber; performing end-face treatment on both ends of the holmium-praseodymium co-doped zirconium-based glass gain fiber; fixing the holmium-praseodymium co-doped zirconium-based glass gain fiber onto a writing fixture; fabricating a high-reflectivity fiber Bragg grating and a low-reflectivity fiber Bragg grating at both ends of the holmium-praseodymium co-doped zirconium-based glass gain fiber respectively using a femtosecond laser line-by-line direct writing method; fusing a quartz single-mode fiber together with the holmium-praseodymium co-doped zirconium-based glass gain fiber using a CO2 laser asymmetric fusion splicing process; and connecting a pump source to the end of the quartz single-mode fiber furthest from the holmium-praseodymium co-doped zirconium-based glass gain fiber.

[0014] Furthermore, when fusion splicing a single-mode quartz fiber with a holmium-praseodymium co-doped zirconium-based glass gain fiber, the center of the high-temperature thermal effect zone is located on the single-mode quartz fiber, and there is a certain distance between the center of the high-temperature thermal effect zone and the holmium-praseodymium co-doped zirconium-based glass gain fiber; the insertion loss of the splice point of the fused single-mode quartz fiber and the holmium-praseodymium co-doped zirconium-based glass gain fiber is ≤0.12dB, and the maximum tensile strength that the splice point can withstand is ≥150gf.

[0015] Compared with existing technologies, the present invention achieves the following beneficial effects: The mid-infrared distributed Bragg reflection single-frequency all-fiber laser provided by the present invention achieves narrowband feedback and longitudinal mode selection within the fiber by employing a more compact distributed Bragg reflection short cavity structure. This solves the problems of high manufacturing difficulty and high cost caused by the high dependence on high-precision fabrication of π phase-shift gratings, narrow process window, and limited repeatability and yield of existing distributed feedback linear cavity schemes. Furthermore, the present invention improves the longitudinal mode selection capability and expands the stable single-frequency operation by optimizing the resonant cavity length and the reflectivity and bandwidth of the fiber Bragg gratings at both ends of the resonant cavity. This invention reduces the frequency drift and mode hopping risk caused by changes in the grating refractive index due to heat accumulation, thereby improving the stability and long-term reliability of single-frequency output. The invention also reduces the need for multiple device cascades and free-space filtering stages required by the ring cavity scheme, such as isolators, circulators, and etalons (diffraction gratings). By reducing the number of intracavity devices and connection points and shortening the optical path to improve the all-fiber integration, the system structure is simpler, smaller, easier to assemble and adjust, has lower insertion loss, and more controllable system cost. Ultimately, it achieves a mid-infrared single-frequency light source with stronger resistance to external disturbances and more stable single-frequency output. Attached Figure Description

[0016] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments and descriptions of the invention are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings: Figure 1 A schematic diagram illustrating the connection between the mid-infrared distributed Bragg reflection single-frequency all-fiber laser, the spectrometer, and the power meter, as described in an embodiment of the present invention. Figure 2 A schematic diagram of the femtosecond laser line-by-line etching trajectory described in the embodiment of the present invention; Figure 3 A schematic diagram of the welding process described in the embodiments of the present invention; Figure 4 A schematic diagram of the transmission peak spectrum of a high-reflectivity narrowband fiber Bragg grating and a low-reflectivity broadband fiber Bragg grating as described in the embodiments of the present invention. Figure 5 The output spectrum of the mid-infrared distributed Bragg reflection single-frequency all-fiber laser described in the embodiment of the present invention; Figure 6 The output longitudinal model characteristic test diagram of the mid-infrared distributed Bragg reflection single-frequency all-fiber laser described in the embodiment of the present invention. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and do not constitute a limitation thereof.

[0018] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0019] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," and "counterclockwise," etc., indicating orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation on this invention. Furthermore, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, features defined with "first," "second," etc., may explicitly or implicitly include one or more of that feature. In the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0020] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art will understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0021] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0022] refer to Figure 1 This invention provides a mid-infrared distributed Bragg reflection single-frequency all-fiber laser, comprising: a pump source 12, a quartz single-mode fiber 11, and a holmium-praseodymium co-doped zirconium-based glass gain fiber 10 connected in sequence; wherein, holmium-praseodymium (Ho... 3+ / Pr 3+The co-doped zirconium-based glass gain fiber includes a high-reflectivity fiber Bragg grating 102, a middle section 103, and a low-reflectivity fiber Bragg grating 104. The high-reflectivity fiber Bragg grating 102 is located between the middle section 103 and the quartz single-mode fiber 11. The low-reflectivity fiber Bragg grating 104 is located on the side of the middle section 103 away from the high-reflectivity fiber Bragg grating 102. The high-reflectivity fiber Bragg grating 102, the middle section 103, and the low-reflectivity fiber Bragg grating 104 constitute a resonant cavity. The full width at half maximum (FWHM) of the gain spectral line of the resonant cavity is greater than the interval between adjacent longitudinal modes of the resonant cavity, and the full WHM of the gain spectral line of the resonant cavity is less than twice the interval between longitudinal modes of the resonant cavity.

[0023] In some embodiments, the end of the holmium-praseodymium co-doped fluorine-zirconium based glass gain fiber 10 away from the pump source 12 is used to output laser light. The operating wavelength of the output laser of the mid-infrared distributed Bragg reflection single-frequency all-fiber laser provided by the present invention is in the range of 2800nm~2900nm.

[0024] The holmium-praseodymium co-doped zirconium-fluorine based glass gain fiber 10, composed of a high-reflectivity fiber Bragg grating 102, a middle section 103, and a low-reflectivity fiber Bragg grating 104, is equivalent to a Fabry-Perot resonant cavity. The pump light emitted from the pump source 12 is injected into one end of the high-reflectivity fiber Bragg grating 102 of the holmium-praseodymium co-doped zirconium-fluorine based glass gain fiber 10 through a quartz single-mode fiber 11. The signal light is amplified back and forth in the cavity under the narrowband feedback formed by the high-reflectivity fiber Bragg grating 102 and the low-reflectivity fiber Bragg grating 104 to achieve laser oscillation. Thus, the narrowband grating frequency selection combined with the short cavity increases the longitudinal mode spacing, thereby realizing single longitudinal mode (single frequency) output. This avoids the strong dependence of the distributed feedback scheme on the high-precision writing of the π phase shift grating and the refractive index change, frequency drift, and mode hopping problems caused by its thermal effect.

[0025] It should be noted that, to ensure that the mid-infrared distributed Bragg reflection single-frequency all-fiber laser with a short-cavity distributed Bragg reflection structure meets the single-longitudinal-mode oscillation condition, this invention limits the cavity length by matching the longitudinal-mode spacing with the grating resonant peak bandwidth. Specifically, according to laser mode theory, the spacing between adjacent longitudinal modes in the resonant cavity is related to the cavity length; when the cavity length is constant, the longitudinal-mode spacing remains unchanged. To achieve single-longitudinal-mode output, in addition to meeting the oscillation condition that the cavity gain is greater than the loss, the full width at half maximum (FWHM) of the cavity gain spectral line must also be greater than the adjacent longitudinal-mode spacing and less than twice the longitudinal-mode spacing. The relevant formulas are expressed as follows: , Represents the laser frequency. Represents the longitudinal mode number (positive integer) within the resonant cavity. At the speed of light, The effective refractive index of the fiber core is the gain fiber. Let be the total length of the resonant cavity. Therefore, we can obtain: ,in, If the interval between adjacent longitudinal modules is [missing information], then ,in, The full width at half maximum (FWHM) of the gain spectral line is represented. The full width at half maximum (FWHM) of the resonance peak of a high-reflectivity narrowband fiber Bragg grating is represented. Represents wavelength, therefore ,according to The theoretical length range of the resonant cavity can be calculated. Furthermore, the effective length of the grating can be estimated based on the group velocity delay model to complete the cavity parameter design and optimization.

[0026] Furthermore, the total length of the resonant cavity is equal to the sum of the length of the middle part 103, the effective length of the high-reflectivity fiber Bragg grating 102, and the effective length of the low-reflectivity fiber Bragg grating 104. Through relevant analysis and calculation, it can be clearly determined that the total length of the resonant cavity is ≤2.2cm.

[0027] Furthermore, the high-reflectivity fiber Bragg grating 102 is a high-reflectivity narrowband fiber Bragg grating (HR-FBG), and the low-reflectivity fiber Bragg grating 104 is a low-reflectivity output-coupled fiber Bragg grating (OC-FBG). The reflectivity of the high-reflectivity fiber Bragg grating 102 is greater than that of the low-reflectivity fiber Bragg grating 104. The 3dB bandwidth of the low-reflectivity fiber Bragg grating 104 is greater than that of the high-reflectivity fiber Bragg grating 102. The transmission bandwidth of the low-reflectivity fiber Bragg grating 104 covers the transmission spectrum of the high-reflectivity fiber Bragg grating 102, and the center wavelength of the low-reflectivity fiber Bragg grating 104 is the same as that of the high-reflectivity fiber Bragg grating 102.

[0028] Furthermore, the center wavelength of the low-reflectivity fiber Bragg grating 104 and the center wavelength of the high-reflectivity fiber Bragg grating 102 are both 2862 nm. The high-reflectivity fiber Bragg grating 102 has a reflectivity of 98%, a 3dB bandwidth of 0.2 nm, and an actual grating length of 13.6 mm. The low-reflectivity fiber Bragg grating 104 has a reflectivity of 60%, a 3dB bandwidth of 0.59 nm, and an actual grating length of 6.3 mm.

[0029] Furthermore, the effective lengths of the high-reflectivity fiber Bragg grating 102 and the low-reflectivity fiber Bragg grating 104 are obtained using Formula 1, as follows: ; in, This represents the actual length of the corresponding grating. This represents the effective length of the corresponding grating. This represents the reflectivity of the corresponding grating. It is the inverse hyperbolic tangent function.

[0030] Furthermore, the length of the middle section 103 is 10 mm, the effective length of the high-reflectivity fiber Bragg grating 102 is 2.5 mm, and the effective length of the low-reflectivity fiber Bragg grating 104 is 2.4 mm. The calculated resonant cavity length is approximately 1.5 cm, which is less than the theoretically calculated maximum cavity length of 2.2 cm. Therefore, the conditions for single-frequency output are met.

[0031] Furthermore, the output laser parameters of the mid-infrared distributed Bragg reflection single-frequency all-fiber laser are as follows: wavelength of 2862nm, slope efficiency (the slope of the straight segment above the threshold is the slope efficiency when plotting a curve with laser output power as the vertical axis and injected pump power as the horizontal axis), maximum output power of 22.6mW, and output wavelength linewidth of less than 5.98MHz.

[0032] The output laser light of a mid-infrared distributed Bragg reflection single-frequency all-fiber laser can be measured using a spectrometer (measurement range 1500nm~3400nm) to obtain the following results: Figure 5 The spectrum is shown, and the characteristics of the output laser are characterized using a Fabry-Poisson interferometer measurement system, as shown below. Figure 6 The longitudinal model sex test chart shown is based on Figure 5 and Figure 6 It can be seen that there are only two main peak signals under a slope signal, which can determine that the output laser of the mid-infrared distributed Bragg reflection single-frequency all-fiber laser is a single-frequency laser. Then, the power of the output laser is tested with a power meter, and finally a laser output with an output wavelength of 2862nm, a slope efficiency of 3.8%, and a maximum output power of 22.6mW is obtained. The calculated output wavelength linewidth is less than 5.98MHz.

[0033] Furthermore, the pump source 12 is a Raman fiber laser with an output wavelength of 1150nm, and the quartz single-mode fiber 11 and the holmium-praseodymium co-doped fluorine-zirconium-based glass gain fiber 10 are connected by heterogeneous fusion splicing.

[0034] Another aspect of this invention provides a method for fabricating a mid-infrared distributed Bragg reflection (DBR) single-frequency all-fiber laser. The method comprises: providing a holmium-praseodymium co-doped zirconium-based glass gain fiber 10; performing end-face treatment on both ends of the holmium-praseodymium co-doped zirconium-based glass gain fiber 10; fixing the holmium-praseodymium co-doped zirconium-based glass gain fiber 10 onto a writing fixture; fabricating a high-reflectivity fiber Bragg grating 102 and a low-reflectivity fiber Bragg grating 104 at both ends of the holmium-praseodymium co-doped zirconium-based glass gain fiber 10 using a femtosecond laser line-by-line direct writing method; fusing a quartz single-mode fiber 11 to the holmium-praseodymium co-doped zirconium-based glass gain fiber 10 using a CO2 laser asymmetric fusion splicing process; and connecting a pump source 12 to the end of the quartz single-mode fiber 11 furthest from the holmium-praseodymium co-doped zirconium-based glass gain fiber 10.

[0035] Furthermore, when fusion splicing the quartz single-mode fiber 11 with the holmium-praseodymium co-doped zirconium-based glass gain fiber 10, the center of the high-temperature thermal effect zone is located on the quartz single-mode fiber 11, and there is a certain distance between the center of the high-temperature thermal effect zone and the holmium-praseodymium co-doped zirconium-based glass gain fiber 10; the insertion loss of the splice 101 of the quartz single-mode fiber 11 and the holmium-praseodymium co-doped zirconium-based glass gain fiber 10 after fusion splicing is ≤0.12dB, and the maximum tensile strength that the splice 101 can withstand is ≥150gf.

[0036] In some embodiments, the distance between the center of the high-temperature thermally active region and the holmium-praseodymium co-doped fluorine-zirconium-based glass gain fiber is in the range of 35µm to 45µm.

[0037] This invention utilizes femtosecond laser direct writing to fabricate fiber Bragg gratings on fluorine-zirconium based fibers, preferably employing a transverse line-by-line direct writing method to obtain the fiber Bragg gratings. The writing trajectory can be as follows: Figure 2 As shown. Figure 2 In the holmium-praseodymium co-doped zirconium-based glass gain fiber 10, there are a cladding 111 and a core 112. The arrow direction represents the direction of laser movement relative to the holmium-praseodymium co-doped zirconium-based glass gain fiber 10. The dense horizontal and vertical lines are the writing path, which completely covers the core 112.

[0038] from Figure 4 As can be seen, the reflectivity of a narrowband fiber Bragg grating can reach 98%, with a bandwidth of 0.2nm; while the reflectivity of a low-reflectivity broadband fiber Bragg grating is 60%, with a bandwidth of 0.59nm.

[0039] The softening temperature of the holmium-praseodymium co-doped fluorine-zirconium based glass gain fiber 10 is lower than that of the silica single-mode fiber 11. Therefore, the reference... Figure 3The fusion splice position, where the dashed box is located, needs to be offset towards the quartz single-mode fiber 11 so that the center of the high-temperature heat-affected zone (the dashed line represents the central axis of the high-temperature heat-affected zone) is located on the quartz single-mode fiber 11. Only in this way can a better fusion splice effect be achieved.

[0040] In some embodiments, during the writing process, the single-pulse energy, repetition frequency, writing speed, grating period, and grating length need to be controlled to form a periodic refractive index modulation structure within the fiber core, thereby fabricating a high-reflectivity fiber Bragg grating 102 (narrowband) with a center wavelength of 2862 nm, a reflectivity of approximately 98%, and a 3dB bandwidth of approximately 0.2 nm. Simultaneously, a low-reflectivity fiber Bragg grating 104 (wideband) with a center wavelength of 2862 nm, a reflectivity of approximately 60%, a 3dB bandwidth of approximately 0.59 nm, and a grating length of 6.3 mm is fabricated.

[0041] To achieve "all-fiber" pump injection and device integration and reduce the difficulty of spatial coupling alignment, this invention uses a CO2 laser to fusion splice single-mode silica fiber with zirconium fluoride-based soft glass gain fiber, such as... Figure 3 As shown, the key to fusion splicing lies in using an asymmetric heat load distribution approach to alleviate the differences in softening temperature, thermal conductivity, and thermal stress between the two materials, thereby achieving a better fusion splicing effect. After optimizing fusion parameters such as pre-fusion power, main fusion power, overlap distance, spacing position, and light transmission time, heterogeneous optical fibers can be fused together, ensuring that the insertion loss at splice point 101 is approximately 0.12 dB and the maximum tensile strength is approximately 150 gf, thus providing a foundation for achieving a compact all-fiber structure and long-term stable operation.

[0042] In some embodiments, when a high-reflectivity fiber Bragg grating 102 and a low-reflectivity fiber Bragg grating 104 are fabricated at both ends of a holmium-praseodymium co-doped fluorine-zirconium-based glass gain fiber 10, the reflectivity and bandwidth of the corresponding gratings can be monitored in real time via online spectral analysis to determine whether they meet the requirements for short-cavity single-frequency mode selection.

[0043] In some embodiments, after a single-mode silica fiber is fused with a zirconium fluoride-based soft glass gain fiber using a CO2 laser, the fusion splice 101 can be mechanically protected, for example, by forming a protective cladding layer on the fusion splice.

[0044] After the mid-infrared distributed Bragg reflection single-frequency all-fiber laser is fabricated, the spectrum of the output laser can be detected by a spectrometer, and the longitudinal mode state can be verified by a scanning Fabry-Porost interferometer to confirm the single longitudinal mode (single frequency) output and complete the power-related tests.

[0045] In one specific embodiment, a Raman fiber laser with a maximum output power of 20W and a working wavelength of 1150nm can be selected as the pump source 12. The pump light is output from a single-mode quartz fiber and injected into Ho³ through the heterojunction fusion joint 101. + / Pr³ + Fluorine-zirconium-based glass gain fiber is used to achieve laser output in the target wavelength band. Both the high-reflectivity fiber Bragg grating 102 and the low-reflectivity fiber Bragg grating 104 are fabricated using a femtosecond laser line-by-line direct writing process. The high-reflectivity fiber Bragg grating 102, with a grating length of 13.6 mm and a center wavelength of 2862 nm, is preferably fabricated using a femtosecond laser line-by-line direct writing process with a single pulse energy of 0.62 µJ, a repetition rate of 1 kHz, and a writing speed of 100 µm / s. The high-reflectivity fiber Bragg grating 102 is fabricated using a single pulse energy of 0.62 µJ and a repetition rate of 0.5 µJ / s. A low-reflectivity fiber Bragg grating 104 with a grating length of 6.3 mm and a center wavelength of 2862 nm was fabricated using a femtosecond laser line-by-line direct writing process with a kHz writing speed of 100 µm / s. The grating period of both the low-reflectivity fiber Bragg grating 104 and the high-reflectivity fiber Bragg grating 102 is 1.947 µm. The insertion loss of the splice 101 between the single-mode silica fiber and the zirconium fluoride-based soft glass gain fiber is 0.12 dB after process optimization, and it can withstand a tensile strength of 150 gf, meeting the requirements for the construction of an all-fiber single-frequency cavity.

[0046] Based on an ultrashort linear cavity, this invention uses a femtosecond laser direct writing method to fabricate a pair of fiber Bragg gratings at both ends of the gain fiber of the laser. The fiber Bragg grating on the pump source 12 side is required to have high reflectivity and narrow linewidth (3dB bandwidth), while the fiber Bragg grating on the laser output end has low reflectivity and wide linewidth. The single-frequency mode selection of the laser is achieved by the combination of narrowband grating and short cavity structure.

[0047] In other embodiments, femtosecond laser surface-by-surface modification can also be used to form high-reflectivity fiber Bragg grating 102 and low-reflectivity fiber Bragg grating 104.

[0048] Compared to the distributed feedback resonant cavity single-frequency scheme, this invention employs a distributed Bragg reflection short-cavity structure. A linear resonant cavity is formed by a high-reflectivity narrowband fiber Bragg grating (i.e., high-reflectivity fiber Bragg grating 102) and an output coupling grating (low-reflectivity fiber Bragg grating 104). Single-mode oscillation is achieved through a combination of narrowband feedback and increased longitudinal mode spacing in the short cavity. This significantly reduces the dependence on writing long π phase-shift gratings and solves the problem of phase-shift gratings being susceptible to phase loss due to intracavity heat accumulation, leading to laser performance degradation. Compared to ring cavity / composite cavity schemes that require isolators, circulators, couplers, and filtering devices such as etalons or diffraction gratings for mode selection, the mode selection function of this invention is mainly completed within the fiber by the high-reflectivity and low-reflectivity fiber Bragg gratings. This greatly reduces the number of intracavity components, significantly simplifies the system structure and setup process, reduces insertion loss and the number of failure points, and results in a smaller, lighter system that is easier for field deployment and subsequent applications. Furthermore, this invention employs a femtosecond laser line-by-line direct writing method to fabricate fiber Bragg gratings in fluoride glass fibers, obtaining a parameter combination of a high-reflectivity narrowband grating and a low-reflectivity broadband grating. This allows for controllable reflectivity, bandwidth, and center wavelength of the fiber Bragg grating, significantly improving the fabrication efficiency and fundamentally enhancing the cavity mirror quality and mode selection capability. Through center wavelength matching and bandwidth coverage design between the high-reflectivity narrowband fiber Bragg grating (high-reflectivity fiber Bragg grating 102) and the output-coupled fiber Bragg grating (low-reflectivity fiber Bragg grating 104), the output coupling end achieves power output while further limiting the oscillation spectrum, suppressing stray mode oscillation within the reflection bandwidth, reducing power fluctuations caused by mode competition, and thus improving the purity and power stability of single-frequency output. In addition, the system utilizes fusion splicing technology to achieve an all-fiber structure, ensuring fewer components, shorter optical paths, fewer alignment steps, lower splice losses, and higher mechanical strength, which is beneficial for improving the overall efficiency and anti-interference capability of the laser.

[0049] It should be understood that the various forms of processes shown above can be used to reorder, add, or delete steps. For example, the steps described in this invention disclosure can be executed in parallel, sequentially, or in different orders, as long as the desired result of the technical solution disclosed in this invention can be achieved, and this is not limited herein.

[0050] The specific embodiments described above do not constitute a limitation on the scope of protection of this invention. Those skilled in the art should understand that various modifications, combinations, sub-combinations, and substitutions can be made according to design requirements and other factors. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this invention should be included within the scope of protection of this invention.

Claims

1. A mid-infrared distributed Bragg reflection single-frequency all-fiber laser, characterized in that, include: The pump source, the single-mode quartz fiber, and the holmium-praseodymium co-doped fluorine-zirconium-based glass gain fiber are connected in sequence. The holmium-praseodymium co-doped zirconium-based glass gain fiber comprises a high-reflectivity fiber Bragg grating, an intermediate section, and a low-reflectivity fiber Bragg grating. The high-reflectivity fiber Bragg grating is located between the intermediate section and the quartz single-mode fiber, and the low-reflectivity fiber Bragg grating is located on the side of the intermediate section away from the high-reflectivity fiber Bragg grating. The high-reflectivity fiber Bragg grating, the intermediate section, and the low-reflectivity fiber Bragg grating constitute a resonant cavity. The full width at half maximum (FWHM) of the gain spectral line of the resonant cavity is greater than the interval between adjacent longitudinal modes of the resonant cavity, and the full WHM of the gain spectral line of the resonant cavity is less than twice the interval between the longitudinal modes of the resonant cavity.

2. The mid-infrared distributed Bragg reflection single-frequency all-fiber laser according to claim 1, characterized in that, The total length of the resonant cavity is equal to the sum of the length of the middle section, the effective length of the high-reflectivity fiber Bragg grating, and the effective length of the low-reflectivity fiber Bragg grating, and the total length of the resonant cavity is ≤2.2cm.

3. The mid-infrared distributed Bragg reflection single-frequency all-fiber laser according to claim 1 or 2, characterized in that, The high reflectivity fiber Bragg grating is a high reflectivity narrowband fiber Bragg grating, and the low reflectivity fiber Bragg grating is a low reflectivity output-coupled fiber Bragg grating. The high-reflectivity fiber Bragg grating has a higher reflectivity than the low-reflectivity fiber Bragg grating. The transmission bandwidth of the low-reflectivity fiber Bragg grating covers the transmission spectrum of the high-reflectivity fiber Bragg grating, and the center wavelength of the low-reflectivity fiber Bragg grating is the same as that of the high-reflectivity fiber Bragg grating.

4. The mid-infrared distributed Bragg reflection single-frequency all-fiber laser according to claim 3, characterized in that, The center wavelength of the low-reflectivity fiber Bragg grating and the center wavelength of the high-reflectivity fiber Bragg grating are both 2862nm. The reflectivity of the high-reflectivity fiber Bragg grating is 98%, the 3dB bandwidth of the high-reflectivity fiber Bragg grating is 0.2nm, and the actual grating length of the high-reflectivity fiber Bragg grating is 13.6mm. The low-reflectivity fiber Bragg grating has a reflectivity of 60%, a 3dB bandwidth of 0.59nm, and an actual grating length of 6.3mm.

5. The mid-infrared distributed Bragg reflection single-frequency all-fiber laser according to claim 2, characterized in that, The effective lengths of the high-reflectivity fiber Bragg grating and the low-reflectivity fiber Bragg grating are obtained by formula 1, which is as follows: ； in, This represents the actual length of the corresponding grating. This represents the effective length of the corresponding grating. This represents the reflectivity of the corresponding grating. It is the inverse hyperbolic tangent function.

6. The mid-infrared distributed Bragg reflection single-frequency all-fiber laser according to claim 5, characterized in that, The length of the middle section is 10mm, the effective length of the high reflectivity fiber Bragg grating is 2.5mm, and the effective length of the low reflectivity fiber Bragg grating is 2.4mm.

7. The mid-infrared distributed Bragg reflection single-frequency all-fiber laser according to claim 6, characterized in that, The output laser parameters of the mid-infrared distributed Bragg reflection single-frequency all-fiber laser are as follows: wavelength of 2862nm, slope efficiency of 3.8%, maximum output power of 22.6mW, and output wavelength linewidth of less than 5.98MHz.

8. The mid-infrared distributed Bragg reflection single-frequency all-fiber laser according to claim 1, characterized in that, The pump source is a Raman fiber laser with an output wavelength of 1150nm, and the quartz single-mode fiber and the holmium-praseodymium co-doped fluorine-zirconium-based glass gain fiber are connected by heterogeneous fusion splicing.

9. A method for fabricating a mid-infrared distributed Bragg reflection single-frequency all-fiber laser, characterized in that, The method for fabricating the mid-infrared distributed Bragg reflection single-frequency all-fiber laser according to any one of claims 1 to 8 comprises: A holmium-praseodymium co-doped zirconium-based glass gain fiber is provided. After end face treatment of both ends of the holmium-praseodymium co-doped zirconium-based glass gain fiber, the holmium-praseodymium co-doped zirconium-based glass gain fiber is fixed on a writing fixture. High-reflectivity fiber Bragg gratings and low-reflectivity fiber Bragg gratings were fabricated at both ends of the holmium-praseodymium co-doped fluorine-zirconium-based glass gain fiber using a femtosecond laser line-by-line direct writing method. A CO2 laser asymmetric fusion splicing process was used to fuse a quartz single-mode fiber with the holmium-praseodymium co-doped fluorine-zirconium-based glass gain fiber. A pump source is connected to the end of a quartz single-mode fiber that is away from the holmium-praseodymium co-doped zirconium-based glass gain fiber.

10. The method for fabricating a mid-infrared distributed Bragg reflection single-frequency all-fiber laser according to claim 9, characterized in that, When splicing a single-mode quartz fiber to the holmium-praseodymium co-doped zirconium-based glass gain fiber, the center of the high-temperature thermal effect zone is located on the single-mode quartz fiber, and there is a certain distance between the center of the high-temperature thermal effect zone and the holmium-praseodymium co-doped zirconium-based glass gain fiber. The insertion loss of the splice between the quartz single-mode fiber and the holmium-praseodymium co-doped fluorine-zirconium-based glass gain fiber is ≤0.12dB, and the maximum tensile strength of the splice is ≥150gf.

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