All-fiber tunable mid-infrared single-frequency pulsed laser

Abstract

The application relates to an all-fiber tunable mid-infrared single-frequency pulse laser, which comprises a broadband fiber grating, a rare earth ion high-doped fiber, a nonlinear crystal fiber, a narrowband polarization maintaining fiber grating, a first piezoelectric ceramic, a second piezoelectric ceramic, a first temperature control furnace, a second temperature control furnace, a third temperature control furnace, a pump source, a first wavelength division multiplexer, a second wavelength division multiplexer, a third wavelength division multiplexer, a polarization maintaining filter and a polarization maintaining isolator. The laser can improve the tuning precision and tuning rate performance of the mid-infrared laser.

Description

Technical Field

[0001] This application relates to the field of laser technology, and in particular to an all-fiber tunable mid-infrared single-frequency pulse laser. Background Technology

[0002] Mid-infrared single-frequency pulsed lasers operate in the mid-infrared band and have advantages such as narrow output spectral linewidth, low noise, and high peak power, making them widely applicable in fields such as remote sensing, precision measurement, atmospheric environmental monitoring, medical diagnosis and treatment, laser guidance, and military infrared countermeasures.

[0003] Since there is a lack of effective laser working medium in the mid-infrared band, it is difficult to generate lasers directly. Therefore, optical parametric oscillation based on nonlinear frequency conversion technology is the main means of obtaining laser sources in this band.

[0004] However, using the above technology to achieve mid-infrared tunable laser output has problems such as low tuning accuracy and slow tuning rate. Summary of the Invention

[0005] Therefore, it is necessary to provide an all-fiber tunable mid-infrared single-frequency pulse laser that can improve the tuning accuracy and tuning rate performance of mid-infrared lasers, in order to address the above-mentioned technical problems.

[0006] This application provides an all-fiber tunable mid-infrared single-frequency pulse laser, comprising: a broadband fiber grating, a rare-earth ion highly doped fiber, a nonlinear crystal fiber, a narrowband polarization-maintaining fiber grating, a first piezoelectric ceramic, a second piezoelectric ceramic, a first temperature-controlled furnace, a second temperature-controlled furnace, a third temperature-controlled furnace, a pump source, a first wavelength division multiplexer, a second wavelength division multiplexer, a third wavelength division multiplexer, a polarization-maintaining filter, and a polarization-maintaining isolator; the pigtail of the pump source is connected to the pump end of the first wavelength division multiplexer, and the common end of the first wavelength division multiplexer, both ends of the broadband fiber grating, both ends of the rare-earth ion highly doped fiber, and the nonlinear crystal fiber... The two ends of the fiber are sequentially connected to one end of the narrowband polarization-maintaining fiber grating to form a short resonant cavity. The other end of the narrowband polarization-maintaining fiber grating, the common end and signal end of the second wavelength division multiplexer, the common end and signal end of the third wavelength division multiplexer, the two ends of the polarization-maintaining filter and the input end of the polarization-maintaining isolator are sequentially connected. The first piezoelectric ceramic is fixed on the side of the broadband fiber grating, the second piezoelectric ceramic is fixed on the side of the narrowband polarization-maintaining fiber grating, the first temperature-controlled furnace is fixed on the other side of the broadband fiber grating, the second temperature-controlled furnace is fixed on the other side of the narrowband polarization-maintaining fiber grating, and the third temperature-controlled furnace is fixed on the side of the nonlinear crystal fiber.

[0007] The system includes a pump source for outputting pump light with a wavelength of 910–980 nm, and a rare-earth ion-doped fiber generating a fundamental frequency light with a wavelength of 1.06 μm under the action of the pump light. A first temperature-controlled furnace controls the temperature of the broadband fiber grating, a second temperature-controlled furnace controls the temperature of the narrowband polarization-maintaining fiber grating, a first piezoelectric ceramic controls the stress applied to the broadband fiber grating, and a second piezoelectric ceramic controls the stress applied to the narrowband polarization-maintaining fiber grating, thereby achieving tuning and Q-switching of the fundamental frequency light. A nonlinear crystal fiber converts the fundamental frequency light into signal light with a wavelength of 1.5–1.6 μm and idler light with a wavelength of 3–5 μm. A third temperature-controlled furnace controls the operating temperature of the nonlinear crystal fiber to achieve phase matching. The idler light is emitted sequentially through a second wavelength division multiplexer, a third wavelength division multiplexer, a polarization-maintaining filter, and a polarization-maintaining isolator.

[0008] In one embodiment, the broadband fiber grating has a transmittance of pump light greater than or equal to 90% and a reflectance of fundamental frequency light, signal light and idler frequency light greater than or equal to 90%.

[0009] In one embodiment, the rare-earth ion highly doped fiber has a core uniformly doped with a high concentration of ytterbium or neodymium, the rare-earth ion highly doped fiber has a gain coefficient per unit length of more than 1 dB / cm for fundamental frequency light, and the length of the rare-earth ion highly doped fiber is 0.1 to 10 cm.

[0010] In one embodiment, the nonlinear crystal fiber is a single-period polarized lithium niobate crystal or potassium titanate phosphate crystal, and the polarization period of the nonlinear crystal fiber is 25-35 μm, the diameter is 50-1500 μm, and the length is 0.1-10 cm; both ends of the nonlinear crystal fiber are coated with multilayer antireflection coatings, and the reflectivity of the multilayer antireflection coatings to pump light, fundamental frequency light, signal light and idler light is less than or equal to 5%.

[0011] In one embodiment, the narrowband polarization-maintaining fiber grating has a transmittance of 5-70% for both fundamental frequency light and signal light, and a transmittance of greater than or equal to 90% for both pump light and idler light.

[0012] In one embodiment, the first piezoelectric ceramic is used to apply lateral tension or longitudinal pressure to the broadband fiber grating, and the unit voltage displacement of the first piezoelectric ceramic is greater than or equal to 20 μm / 200V.

[0013] In one embodiment, the second piezoelectric ceramic is used to apply lateral tension or longitudinal pressure to the narrowband polarization-maintaining fiber grating, and the unit voltage displacement of the second piezoelectric ceramic is less than or equal to 1 μm / 200V.

[0014] In one embodiment, the temperature control range of the first, second, and third temperature control furnaces is -20 to 300°C, and the temperature control accuracy is ±0.05°C.

[0015] In one embodiment, the pump end of the second wavelength division multiplexer operates at a wavelength of 910–980 nm, and the signal end of the second wavelength division multiplexer operates at wavelengths of 1.06 μm, 1.5–1.6 μm, and 3–5 μm; the pump end of the third wavelength division multiplexer operates at a wavelength of 1.06 μm, and the signal end of the third wavelength division multiplexer operates at wavelengths of 1.5–1.6 μm and 3–5 μm.

[0016] In one embodiment, the polarization-maintaining filter is a bandpass filter, and the operating wavelength of the polarization-maintaining filter is 3 to 5 μm.

[0017] The aforementioned all-fiber tunable mid-infrared single-frequency pulse laser has at least the following beneficial effects:

[0018] The all-fiber tunable mid-infrared single-frequency pulsed laser of this application includes: a broadband fiber grating, a rare-earth ion highly doped fiber, a nonlinear crystal fiber, a narrowband polarization-maintaining fiber grating, a first piezoelectric ceramic, a second piezoelectric ceramic, a first temperature-controlled furnace, a second temperature-controlled furnace, a third temperature-controlled furnace, a pump source, a first wavelength division multiplexer, a second wavelength division multiplexer, a third wavelength division multiplexer, a polarization-maintaining filter, and a polarization-maintaining isolator. By employing a short resonant cavity active Q-switching, frequency modulation, and an internal cavity optical parametric oscillation structure, stress and temperature applied to the fiber grating are synergistically controlled to obtain linearly polarized fundamental frequency light in tunable and Q-switched pulse states. Placing the crystal fiber within the cavity at high power density improves the tuning performance and conversion efficiency of the 3–5 μm idler light. Furthermore, the short cavity combined with an all-fiber link ensures stable single-frequency operation of the laser, achieving a tuning bandwidth ≥200 GHz, tuning accuracy ≤10 MHz, tuning rate 1 kHz–1 MHz, linewidth ≤10 kHz, and peak power ≥100 W for mid-infrared laser pulses. With the filtering effects of the second wavelength division multiplexer, the third wavelength division multiplexer, the polarization-maintaining filter, and the polarization-maintaining isolator, the output of mid-infrared single-frequency pulsed laser is realized. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of this application or the conventional technology, the drawings used in the description of the embodiments or the conventional technology will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 This is a schematic diagram of the structure of an all-fiber tunable mid-infrared single-frequency pulsed laser in one embodiment;

[0021] Figure 2 This is a diagram showing the relationship between the tuning range of idler light, signal light and fundamental frequency light in one embodiment. Detailed Implementation

[0022] To facilitate understanding of this application, a more complete description will be provided below with reference to the accompanying drawings, which illustrate embodiments of the present application. However, the present application can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided so that the disclosure of this application will be thorough and complete.

[0023] 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 this application belongs. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application.

[0024] It is understood that the terms “first,” “second,” etc., used in this application may be used herein to describe various elements, but these elements are not limited by these terms. These terms are only used to distinguish one element from another.

[0025] Spatial relation terms such as “below,” “under,” “below,” “under,” “above,” “above,” etc., are used herein to describe the relationship between one element or feature shown in the figure and other elements or features. It should be understood that, in addition to the orientation shown in the figure, spatial relation terms also include different orientations of the device in use and operation. For example, if the device in the figure is flipped, the element or feature described as “below,” “under,” or “below” will be oriented “above” the other element or feature. Therefore, the exemplary terms “below” and “under” can include both above and below orientations. Furthermore, the device may also include other orientations (e.g., rotated 90 degrees or other orientations), and the spatial descriptive terms used herein will be interpreted accordingly.

[0026] It should be noted that when one element is considered to be "connected" to another element, it can be directly connected to the other element or connected to the other element through an intermediary element. Furthermore, in the following embodiments, "connection" should be understood as "electrical connection," "communication connection," etc., if there is transmission of electrical signals or data between the connected objects.

[0027] When used herein, the singular forms of “a,” “an,” and “the” may also include the plural forms unless the context clearly indicates otherwise. It should also be understood that the terms “comprising,” “including,” or “having,” etc., specify the presence of the stated feature, whole, step, operation, component, part, or combination thereof, but do not preclude the possibility of the presence or addition of one or more other features, wholes, steps, operations, components, parts, or combinations thereof.

[0028] As described in the background section, 3–5 μm mid-infrared single-frequency pulsed lasers possess advantages such as narrow output spectral linewidth, low noise, and high peak power. This band lies within the atmospheric window and covers the characteristic absorption peaks of many molecules ("molecular fingerprint region"), making it widely applicable in fields such as remote sensing, precision measurement, atmospheric environmental monitoring, medical diagnosis and treatment, laser guidance, and military infrared countermeasures. However, due to the lack of effective laser working media in the 3–5 μm band, it is difficult to directly generate laser light. Therefore, optical parametric oscillation (OPO) based on nonlinear frequency conversion technology is the primary means of obtaining laser sources in this band.

[0029] Current research on tunable mid-infrared lasers mainly focuses on: using external cavity pumping (OPO) technology with broadband (sub-nm linewidth) solid-state lasers or fiber lasers, combined with periodic and temperature tuning measures of nonlinear crystals, to achieve tunable laser output. For example, using a neodymium:YVO4 nanosecond solid-state laser to pump a periodically polarized lithium niobate crystal (PPLN) has achieved mid-infrared pulsed laser output with a tuning range of 2.2–4.8 μm; or using an internal cavity OPO based on a neodymium:YVO4 solid-state laser, and utilizing the periodicity and temperature tuning of multi-period PPLNs, mid-infrared continuous laser output with a tuning range of 2.25–4.79 μm has been achieved. However, the above methods, which rely on mechanical and simple control of the PPLN position (changing the polarization period) and matching temperature, result in low tuning accuracy and slow tuning rate of the laser. Furthermore, the laser is a free-space structure, leading to large size and poor system stability.

[0030] Based on the above reasons, this application provides an all-fiber tunable mid-infrared single-frequency pulse laser to overcome the shortcomings of existing tunable mid-infrared lasers, such as low tuning accuracy, slow tuning rate, wide spectral linewidth, complex device structure, and poor system stability.

[0031] In one exemplary embodiment, such as Figure 1As shown, this application provides an all-fiber tunable mid-infrared single-frequency pulse laser, comprising: a broadband fiber grating 1, a rare-earth ion highly doped fiber 2, a nonlinear crystal fiber 3, a narrowband polarization-maintaining fiber grating 4, a first piezoelectric ceramic 5, a second piezoelectric ceramic 6, a first temperature-controlled furnace 7, a second temperature-controlled furnace 8, a third temperature-controlled furnace 9, a pump source 10, a first wavelength division multiplexer 11, a second wavelength division multiplexer 12, a third wavelength division multiplexer 13, a polarization-maintaining filter 14, and a polarization-maintaining isolator 15; the pigtail of the pump source 10 is connected to the pump end of the first wavelength division multiplexer 11, and the common end of the first wavelength division multiplexer 11, both ends of the broadband fiber grating 1, and both ends of the rare-earth ion highly doped fiber 2 are connected to the pump source 10. The two ends of the nonlinear crystal fiber 3 are sequentially connected to one end of the narrowband polarization-maintaining fiber grating 4 to form a short resonant cavity. The other end of the narrowband polarization-maintaining fiber grating 4, the common end and signal end of the second wavelength division multiplexer 12, the common end and signal end of the third wavelength division multiplexer 13, the two ends of the polarization-maintaining filter 14 and the input end of the polarization-maintaining isolator 15 are sequentially connected. The first piezoelectric ceramic 5 is fixed on the side of the broadband fiber grating 1, the second piezoelectric ceramic 6 is fixed on the side of the narrowband polarization-maintaining fiber grating 4, the first temperature-controlled furnace 7 is fixed on the other side of the broadband fiber grating 1, the second temperature-controlled furnace 8 is fixed on the other side of the narrowband polarization-maintaining fiber grating 4, and the third temperature-controlled furnace 9 is fixed on the side of the nonlinear crystal fiber 3.

[0032] The pump source 10 outputs pump light with a wavelength of 910–980 nm. The rare-earth ion highly doped fiber 2 generates fundamental frequency light with a wavelength of 1.06 μm under the action of the pump light. The first temperature-controlled furnace 7 controls the temperature of the broadband fiber grating 1, the second temperature-controlled furnace 8 controls the temperature of the narrowband polarization-maintaining fiber grating 4, the first piezoelectric ceramic 5 controls the stress loaded on the broadband fiber grating 1, and the second piezoelectric ceramic 6 controls the stress loaded on the narrowband polarization-maintaining fiber grating 4 to achieve tuning and Q-switching of the fundamental frequency light. The nonlinear crystal fiber 3 converts the fundamental frequency light into signal light with a wavelength of 1.5–1.6 μm and idler light with a wavelength of 3–5 μm. The third temperature-controlled furnace 9 controls the operating temperature of the nonlinear crystal fiber 3 to achieve phase matching. The idler light is emitted sequentially through the second wavelength division multiplexer 12, the third wavelength division multiplexer 13, the polarization-maintaining filter 14, and the polarization-maintaining isolator 15.

[0033] Among them, the broadband fiber grating 1 refers to a fiber structure with a relatively wide bandwidth and periodic refractive index variation, used to introduce frequency-selective coupling effect of light in the fiber, enabling wavelength selection. The rare-earth ion highly doped fiber 2 is a gain fiber material doped with rare-earth luminescent ions, which can emit light based on stimulated emission, thereby achieving light gain amplification. The nonlinear crystal fiber 3 is a crystal material with nonlinear optical properties and a fiber waveguide structure. When an optical signal is transmitted within it, nonlinear effects such as frequency doubling, optical parametric oscillation, and optical parametric amplification occur, which can be used to generate new optical frequency components, i.e., frequency conversion. The narrowband polarization-maintaining fiber grating 4 is a fiber grating with narrow bandwidth and polarization-maintaining performance, used to introduce frequency-selective coupling effect of light in the fiber while maintaining the polarization state of the light. The first piezoelectric ceramic 5 and the second piezoelectric ceramic 6 can change the refractive index distribution and reflection peak in the fiber grating by altering the stress applied to it, thereby achieving frequency selection of light. The first temperature-controlled furnace 7, the second temperature-controlled furnace 8, and the third temperature-controlled furnace 9 refer to a device used to control the temperature of a fiber optic grating or nonlinear crystal fiber 3. By adjusting the temperature of the fiber optic grating and the nonlinear crystal fiber 3, their refractive index (reflection peak) and the operating temperature of the crystal are changed, thereby achieving frequency selection of light and phase matching of the crystal. The temperature-controlled furnace can control the temperature by heating or cooling to meet specific application requirements. The pump source 10 can refer to a single-mode semiconductor laser using forward pumping, with an operating wavelength of 980nm and an output power of 750mW. The first wavelength division multiplexer 11 refers to a device that combines pump light and signal light of different wavelengths into a single beam for transmission along a single optical fiber. The first wavelength division multiplexer 11 can effectively couple the pump light generated by the pump source 10 into a short resonant cavity, minimizing pump light loss and reflection, thereby improving coupling efficiency and maintaining the stability and quality of the pump light. The second wavelength division multiplexer 12 and the third wavelength division multiplexer 13 can refer to a filter device used to filter out light within a preset frequency band. The polarization-maintaining filter 14 is a birefringent filter that allows only one type of polarized light to pass through, perpendicular to the direction of light propagation. For example, in lasers, it can be used to filter noise and stray light, increase side-mode suppression ratio, improve beam quality, and extend laser bandwidth. The polarization-maintaining isolator 15 is an optical device consisting of a polarization beam splitter and an optical isolator, used to maintain the polarization state of the optical signal and prevent the back propagation of reflected light signals.

[0034] For example, a short linear cavity and internal cavity OPO structure is adopted, consisting of a rare-earth ion highly doped fiber 2, a fiber grating pair (broadband fiber grating 1 and narrowband polarization-maintaining fiber grating 4), and a nonlinear crystal fiber 3. The centimeter-long rare-earth ion highly doped fiber 2 and nonlinear crystal fiber 3 are used as the laser working medium and nonlinear frequency conversion medium, respectively. The broadband fiber grating 1 and narrowband polarization-maintaining fiber grating 4, which simultaneously incorporate piezoelectric ceramics and a temperature-controlled furnace, serve as the front and rear cavity mirrors, respectively. The fiber grating pair, piezoelectric ceramics, and temperature-controlled furnace constitute the active Q-switching and frequency modulation functional parts. Specifically, firstly, under the continuous pumping of pump source 10 and the frequency selection of fiber grating pairs, rare earth luminescent ions in the core of rare earth ion highly doped fiber 2 exhibit population inversion, generating stimulated emission fundamental frequency light. Under the feedback of the front and rear cavity mirrors, the 1.06μm fundamental frequency light oscillates back and forth multiple times and is amplified multiple times. Due to the short linear cavity structure, the longitudinal mode spacing in the cavity can reach the GHz level. When the 3dB reflection spectrum of the narrowband polarization-maintaining fiber grating 4 is narrow to tens of picometers, that is, only a single longitudinal mode exists in the resonant cavity, thus realizing the operation of high-power single-frequency 1.06μm fundamental frequency light.

[0035] Furthermore, by applying a DC bias voltage to the first piezoelectric ceramic 5 and the second piezoelectric ceramic 6, the transverse tension or longitudinal pressure on the fiber grating pair is controlled, and the operating temperature of the fiber grating pair is controlled by the first temperature-controlled furnace 7 and the second temperature-controlled furnace 8. When the reflection peak corresponding to the slow axis of the narrowband polarization-maintaining fiber grating 4 overlaps with the reflection peak of the broadband fiber grating 1 at any position λ1, the corresponding output laser operating wavelength is λ1. Based on the above adjustment method, the overlapping positions of the reflection peaks can be λ2, λ3, ... λ by adjusting the operating temperatures of the first temperature-controlled furnace 7 and the second temperature-controlled furnace 8. n This corresponds to the output laser operating wavelengths of λ2, λ3, ... λ n By continuously and synchronously changing the overlap position of the reflection peaks of the fiber grating pair, tunable linearly polarized 1.06μm fundamental frequency light can be achieved. Considering the effect of the first piezoelectric ceramic 5 and the second piezoelectric ceramic 6 on the cavity length of the resonant cavity (i.e., the cavity structure composed of rare-earth ion highly doped fiber 2 and fiber grating pair), it can be seen that the laser resonant frequency and the cavity length have a certain corresponding relationship. The change of the longitudinal mode spacing Δν of the laser resonant frequency and the cavity length ΔL of the resonant cavity satisfy a simple relationship: Δν / ν=k×ΔL / L, where k is a proportionality coefficient, and its value is generally close to 1. At this time, the frequency (wavelength) tuning bandwidth is equal to the longitudinal mode spacing of the resonant cavity. In a specific embodiment, the cavity length of the short resonant cavity is generally controlled at about 2cm, with a value of L=2×10 -2 m and k = 1, assuming the fundamental wavelength is 1064 nm (ν = 2.82 × 10⁻⁶). 5When ΔL = 14μm (i.e., a cavity length change of 14μm), the fundamental frequency light can achieve a frequency tuning bandwidth of 200GHz, and the corresponding 3-5μm idler light can also achieve a frequency tuning bandwidth of approximately 200GHz. By adjusting the cavity length of the resonant cavity, the frequency tuning of the laser can be achieved. Furthermore, by selecting high-precision first temperature-controlled furnace 7 and second temperature-controlled furnace 8, a wide bandwidth, high precision, and high-rate tunable fundamental frequency light with linear polarization of 1.06μm can be realized.

[0036] In addition, the aforementioned all-fiber tunable mid-infrared single-frequency pulse laser can also achieve loss modulation within the resonant cavity. Specifically, when the reflection peak corresponding to the slow axis of the narrowband polarization-maintaining fiber grating 4 is misaligned with the reflection peak of the broadband fiber grating 1, the resonant cavity is in a high-loss state, and the laser does not oscillate within the resonant cavity. By using a temperature-controlled furnace and piezoelectric ceramics, the reflection peak corresponding to the slow axis of the narrowband polarization-maintaining fiber grating 4 is changed, causing the reflection peaks (center reflection wavelength) of the fiber grating pair to rematch, reducing the loss of the short resonant cavity (increasing the Q value). That is, by applying a periodic voltage signal to the piezoelectric ceramics, the overlap of the reflection peaks of the fiber grating pair can be controlled, achieving a high-power-density Q-switched pulse state for linearly polarized 1.06μm fundamental frequency light.

[0037] Based on the above, the nonlinear crystal fiber 3 is placed in a Q-switched resonant cavity operating at high power density for optical parametric oscillation. Through the optical parametric oscillation process in the nonlinear crystal fiber 3, the 1.06μm fundamental frequency light is converted into 1.5–1.6μm signal light. Due to the reflection effect of the front and rear cavity mirrors, the 1.5–1.6μm signal light passes through the nonlinear crystal fiber 3 multiple times, forming resonant enhanced near-infrared parametric light, thereby effectively improving the conversion efficiency and power level of the 3–5μm idler light. Simultaneously, under the action of the highly tuned 1.06μm fundamental frequency light, combined with the precise control of the operating temperature of the nonlinear crystal fiber 3 by a high-precision temperature-controlled furnace, the optimal temperature-acceptable bandwidth and phase matching state are obtained, thereby achieving the wide tuning bandwidth, high tuning accuracy, and fast tuning rate of the 3–5μm idler light. Under the action of the second wavelength division multiplexer 12, the third wavelength division multiplexer 13, the polarization-maintaining filter 14, and the polarization-maintaining isolator 15, a mid-infrared single-frequency pulse laser is output.

[0038] The all-fiber tunable mid-infrared single-frequency pulse laser in the above embodiments includes: a broadband fiber grating 1, a rare-earth ion highly doped fiber 2, a nonlinear crystal fiber 3, a narrowband polarization-maintaining fiber grating 4, a first piezoelectric ceramic 5, a second piezoelectric ceramic 6, a first temperature-controlled furnace 7, a second temperature-controlled furnace 8, a third temperature-controlled furnace 9, a pump source 10, a first wavelength division multiplexer 11, a second wavelength division multiplexer 12, a third wavelength division multiplexer 13, a polarization-maintaining filter 14, and a polarization-maintaining isolator 15. By employing a short resonant cavity active Q-switching, frequency modulation, and internal cavity optical parametric oscillation structure, stress and temperature applied to the fiber grating are synergistically controlled to obtain linearly polarized fundamental frequency light in tunable and Q-switched pulse states. Placing the crystal fiber within the cavity at high power density improves the tuning performance and conversion efficiency of the 3–5 μm idler light. Furthermore, the short cavity combined with an all-fiber link ensures stable single-frequency operation of the laser, achieving a tuning bandwidth ≥200 GHz, tuning accuracy ≤10 MHz, tuning rate 1 kHz–1 MHz, linewidth ≤10 kHz, and peak power ≥100 W for mid-infrared laser pulses. Under the filtering effects of the second wavelength division multiplexer 12, the third wavelength division multiplexer 13, the polarization-maintaining filter 14, and the polarization-maintaining isolator 15, the output of mid-infrared single-frequency pulsed laser is realized.

[0039] In an exemplary embodiment, the broadband fiber grating 1 has a transmittance of pump light greater than or equal to 90%, and a reflectance of fundamental frequency light, signal light, and idler frequency light greater than or equal to 90%.

[0040] For example, the broadband fiber grating 1 has a transmittance of 99.9% for pump light and a reflectance of 99.9% for fundamental frequency light, signal light and idler frequency light.

[0041] In the above embodiments, the broadband fiber grating 1 has high transmittance for pump light, ensuring that most of the pump light can pass through effectively, while it has high reflectance for fundamental frequency light, signal light, and idler light, ensuring that most of the fundamental frequency light, signal light, and idler light can be effectively reflected back, avoiding light loss. This helps to improve the optical conversion efficiency of the laser, reduce energy loss, and improve the performance of the laser. Secondly, the high transmittance and reflectance of the fiber grating can reduce the influence of external environmental factors (such as temperature, pressure, etc.) on the grating characteristics, thereby improving the stability of the grating.

[0042] In an exemplary embodiment, the rare earth ion highly doped optical fiber 2 has a core uniformly doped with a high concentration of ytterbium or neodymium, the rare earth ion highly doped optical fiber 2 has a gain coefficient per unit length of more than 1 dB / cm for fundamental frequency light, and the length of the rare earth ion highly doped optical fiber 2 is 0.1 to 10 cm.

[0043] For example, the rare earth ion highly doped fiber 2 has a core uniformly highly doped with ytterbium ions, and its gain coefficient per unit length at 1.06 μm is 10 dB / cm, with a length of 1 cm.

[0044] In the above embodiments, the rare-earth ion highly doped fiber 2 has a gain coefficient per unit length greater than 1 dB / cm for fundamental frequency light, enabling the optical signal to achieve high amplification and oscillation effects even within a short operating length. The relatively short length of the rare-earth ion highly doped fiber 2, ranging from 0.1 to 10 cm, shortens the resonant cavity length, increases the spacing between adjacent longitudinal modes, and improves the stability and high-efficiency output of single-longitudinal-mode (single-frequency) operation. Furthermore, the shorter fiber length reduces the system's length, volume, and weight, making it suitable for applications with high space and weight requirements.

[0045] In an exemplary embodiment, the nonlinear crystal fiber 3 is a single-period polarized lithium niobate crystal or potassium titanate phosphate crystal. The polarization period of the nonlinear crystal fiber 3 is 25-35 μm, the diameter is 50-1500 μm, and the length is 0.1-10 cm. Both ends of the nonlinear crystal fiber 3 are coated with multilayer antireflection films. The reflectivity of the multilayer antireflection films for pump light, fundamental frequency light, signal light, and idler light is less than or equal to 5%.

[0046] For example, the nonlinear crystal fiber 3 is a single-period polarized lithium niobate crystal with a polarization period of 30 μm, a diameter of 800 μm, and a length of 2 cm. Both ends of the nonlinear crystal fiber 3 are coated with antireflection films, with a reflectivity of 2% for both pump light and fundamental frequency light, and a reflectivity of 4% for both signal light and idler frequency light.

[0047] In the above embodiments, the single-period polarized lithium niobate crystal or potassium titanate oxyphosphate crystal has a large nonlinear coefficient and a high anti-optical damage threshold. When placed in a cavity under high power density operation of fundamental frequency light, it can improve the tuning performance and conversion efficiency of 3-5 μm idler light. Both ends of the nonlinear crystal fiber 3 are coated with multilayer antireflection films, and the reflectivity of pump light, fundamental frequency light, signal light and idler light is less than or equal to 5%, which can reduce the attenuation and loss of laser during transmission and further improve the conversion efficiency.

[0048] In an exemplary embodiment, the narrowband polarization-maintaining fiber grating 4 has a transmittance of 5-70% for both fundamental frequency light and signal light, and a transmittance of greater than or equal to 90% for both pump light and idler light.

[0049] For example, the narrowband polarization-maintaining fiber grating 4 has a transmittance of 30% at the center wavelength of the fundamental frequency light, a transmittance of 10% at the center wavelength of the signal light, and a transmittance of 99.9% for the pump light and idler light.

[0050] In the above embodiments, the narrowband polarization-maintaining fiber grating 4 has low transmittance (high reflectance) for both fundamental and signal light, ensuring that the fundamental and signal light can be fully reflected and utilized for resonance enhancement within the front and rear cavity mirrors, thereby increasing the intracavity laser power density and improving the conversion efficiency of the idler light. Meanwhile, the narrowband polarization-maintaining fiber grating 4 has high transmittance for both pump and idler light, achieving high transmission efficiency for the idler light and ensuring its output power.

[0051] In an exemplary embodiment, the first piezoelectric ceramic 5 is used to apply lateral tension or longitudinal pressure to the broadband fiber grating 1, and the unit voltage displacement of the first piezoelectric ceramic 5 is greater than or equal to 20μm / 200V.

[0052] In an exemplary embodiment, the second piezoelectric ceramic 6 is used to apply lateral tension or longitudinal pressure to the narrowband polarization-maintaining fiber grating 4, and the unit voltage displacement of the second piezoelectric ceramic 6 is less than or equal to 1 μm / 200V.

[0053] For example, the first piezoelectric ceramic 5 has a unit voltage displacement of 30μm / 200V, which is used to apply lateral tension to the broadband fiber grating 1; the second piezoelectric ceramic 6 has a unit voltage displacement of 0.5μm / 200V, which is used to apply lateral tension to the narrowband polarization-maintaining fiber grating 4. The voltage resolution of the power supply to the piezoelectric ceramic is 0.1V, which can achieve a frequency tuning accuracy of less than 10MHz, thereby realizing wide bandwidth, high precision and fast rate tunability of linearly polarized fundamental frequency light.

[0054] In an exemplary embodiment, the temperature control range of the first temperature-controlled furnace 7, the second temperature-controlled furnace 8, and the third temperature-controlled furnace 9 are all -20 to 300°C, and the temperature control accuracy is ±0.05°C.

[0055] For example, the first temperature-controlled furnace 7 and the second temperature-controlled furnace 8 respectively perform precise temperature control on the broadband fiber grating 1 and the narrowband polarization-maintaining fiber grating 4, with a control range of 10–150°C and a temperature control accuracy of ±0.05°C. The third temperature-controlled furnace 9 performs precise temperature control on the nonlinear crystal fiber 3, with a control temperature of 60°C and a temperature control accuracy of ±0.05°C. Based on the above temperature control, when the nonlinear crystal fiber 3 is a periodically polarized lithium niobate crystal with a polarization period of 30 μm and a control temperature of 60°C, the fundamental frequency tuning range is 1064.00–1064.75 nm, the signal light tuning range is 1557.02–1556.93 nm, and the idler frequency tuning range is 3360.25–3368.14 nm. The relationship between the idler frequency and signal light tuning ranges and the fundamental frequency tuning range is as follows: Figure 2 As shown.

[0056] In an exemplary embodiment, the pump end of the second wavelength division multiplexer 12 operates at a wavelength of 910–980 nm, and the signal end of the second wavelength division multiplexer 12 operates at wavelengths of 1.06 μm, 1.5–1.6 μm, and 3–5 μm; the pump end of the third wavelength division multiplexer 13 operates at a wavelength of 1.06 μm, and the signal end of the third wavelength division multiplexer 13 operates at wavelengths of 1.5–1.6 μm and 3–5 μm.

[0057] For example, the pump end of the second wavelength division multiplexer 12 operates at a wavelength of 980 nm, and the signal end operates at wavelengths of 1.06 μm, 1.5–1.6 μm, and 3–5 μm, filtering out residual 980 nm pump light through the pump end. The pump end of the third wavelength division multiplexer 13 operates at a wavelength of 1.06 μm, and the signal end operates at wavelengths of 1.5–1.6 μm and 3–5 μm, filtering out residual 1.06 μm fundamental frequency light through the pump end.

[0058] In the above embodiments, the second wavelength division multiplexer 12 and the third wavelength division multiplexer 13 in the laser function to filter out pump light and fundamental frequency light in a specific frequency range and retain 3-5μm idler light. By selectively transmitting or filtering out signal laser of a specific wavelength, the frequency of laser output can be controlled to meet specific application requirements.

[0059] In one exemplary embodiment, the polarization-maintaining filter 14 is a bandpass filter, and the operating wavelength of the polarization-maintaining filter 14 is 3 to 5 μm.

[0060] In the above embodiments, the polarization-maintaining filter 14 can filter out the 1.5-1.6μm signal light and keep the polarization state of the 3-5μm idler light unchanged. By selectively transmitting or filtering out signal lasers in a specific wavelength range, the frequency and polarization of the output laser can be controlled.

[0061] In one specific embodiment, based on the selection of the device parameters in the above embodiments, the all-fiber tunable mid-infrared single-frequency pulsed laser outputs a mid-infrared pulsed laser with a tuning bandwidth of 200 GHz, a tuning accuracy of 5 MHz, a tuning rate of 0.1 MHz, a linewidth of 3 kHz, and a peak power of 150 W through the output terminal of the polarization-maintaining isolator 15.

[0062] In the description of this specification, references to terms such as "some embodiments," "other embodiments," and "ideal embodiments" indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, the illustrative descriptions of the above terms do not necessarily refer to the same embodiments or examples.

[0063] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above 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.

[0064] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims.

Claims

1. A fully fiber-optic tunable mid-infrared single-frequency pulse laser, characterized in that, include: The system comprises a broadband fiber grating, a rare-earth ion-doped fiber, a nonlinear crystal fiber, a narrowband polarization-maintaining fiber grating, a first piezoelectric ceramic, a second piezoelectric ceramic, a first temperature-controlled furnace, a second temperature-controlled furnace, a third temperature-controlled furnace, a pump source, a first wavelength division multiplexer, a second wavelength division multiplexer, a third wavelength division multiplexer, a polarization-maintaining filter, and a polarization-maintaining isolator. The pigtail of the pump source is connected to the pump end of the first wavelength division multiplexer. The common end of the first wavelength division multiplexer, both ends of the broadband fiber grating, both ends of the rare-earth ion-doped fiber, both ends of the nonlinear crystal fiber, and one end of the narrowband polarization-maintaining fiber grating are sequentially connected to form the system. A short resonant cavity is provided. The other end of the narrowband polarization-maintaining fiber grating, the common end and signal end of the second wavelength division multiplexer, the common end and signal end of the third wavelength division multiplexer, the two ends of the polarization-maintaining filter, and the input end of the polarization-maintaining isolator are connected in sequence. The first piezoelectric ceramic is fixed to the side of the broadband fiber grating, the second piezoelectric ceramic is fixed to the side of the narrowband polarization-maintaining fiber grating, the first temperature-controlled furnace is fixed to the other side of the broadband fiber grating, the second temperature-controlled furnace is fixed to the other side of the narrowband polarization-maintaining fiber grating, and the third temperature-controlled furnace is fixed to the side of the nonlinear crystal fiber. The pump source outputs pump light with a wavelength of 910–980 nm, and the rare-earth ion highly doped fiber generates fundamental frequency light with a wavelength of 1.06 μm under the action of the pump light. The first temperature-controlled furnace controls the temperature of the broadband fiber grating, the second temperature-controlled furnace controls the temperature of the narrowband polarization-maintaining fiber grating, the first piezoelectric ceramic controls the stress applied to the broadband fiber grating, and the second piezoelectric ceramic controls the stress applied to the narrowband polarization-maintaining fiber grating to achieve tuning and Q-switching of the fundamental frequency light. The nonlinear crystal fiber converts the fundamental frequency light into signal light with a wavelength of 1.5–1.6 μm and idler light with a wavelength of 3–5 μm. The third temperature-controlled furnace controls the operating temperature of the nonlinear crystal fiber to achieve phase matching. The idler light is emitted sequentially through the second wavelength division multiplexer, the third wavelength division multiplexer, the polarization-maintaining filter, and the polarization-maintaining isolator.

2. The all-fiber tunable mid-infrared single-frequency pulse laser according to claim 1, characterized in that, The broadband fiber grating has a transmittance of 90% or more for the pump light and a reflectance of 90% or more for the fundamental frequency light, the signal light, and the idler frequency light.

3. The all-fiber tunable mid-infrared single-frequency pulse laser according to claim 1, characterized in that, The rare-earth ion highly doped optical fiber has a core uniformly doped with a high concentration of ytterbium or neodymium. The rare-earth ion highly doped optical fiber has a gain coefficient per unit length greater than 1 dB / cm for the fundamental frequency light. The length of the rare-earth ion highly doped optical fiber is 0.1 to 10 cm.

4. The all-fiber tunable mid-infrared single-frequency pulse laser according to claim 1, characterized in that, The nonlinear crystal fiber is a single-period polarized lithium niobate crystal or potassium titanate phosphate crystal. The polarization period of the nonlinear crystal fiber is 25-35 μm, the diameter is 50-1500 μm, and the length is 0.1-10 cm. Both ends of the nonlinear crystal fiber are coated with multilayer antireflection films. The reflectivity of the multilayer antireflection films for the pump light, the fundamental frequency light, the signal light, and the idler light is less than or equal to 5%.

5. The all-fiber tunable mid-infrared single-frequency pulse laser according to claim 1, characterized in that, The narrowband polarization-maintaining fiber grating has a transmittance of 5-70% for both the fundamental frequency light and the signal light; and a transmittance of greater than or equal to 90% for both the pump light and the idler frequency light.

6. The all-fiber tunable mid-infrared single-frequency pulse laser according to claim 1, characterized in that, The first piezoelectric ceramic is used to apply lateral tension or longitudinal pressure to the broadband fiber grating, and the unit voltage displacement of the first piezoelectric ceramic is greater than or equal to 20μm / 200V.

7. The all-fiber tunable mid-infrared single-frequency pulse laser according to claim 1, characterized in that, The second piezoelectric ceramic is used to apply lateral tension or longitudinal pressure to the narrowband polarization-maintaining fiber grating, and the unit voltage displacement of the second piezoelectric ceramic is less than or equal to 1 μm / 200V.

8. The all-fiber tunable mid-infrared single-frequency pulse laser according to claim 1, characterized in that, The temperature control range of the first temperature control furnace, the second temperature control furnace, and the third temperature control furnace is -20 to 300℃, and the temperature control accuracy is ±0.05℃.

9. The all-fiber tunable mid-infrared single-frequency pulse laser according to claim 1, characterized in that, The pump end of the second wavelength division multiplexer operates at a wavelength of 910–980 nm, and the signal end of the second wavelength division multiplexer operates at wavelengths of 1.06 μm, 1.5–1.6 μm, and 3–5 μm; the pump end of the third wavelength division multiplexer operates at a wavelength of 1.06 μm, and the signal end of the third wavelength division multiplexer operates at wavelengths of 1.5–1.6 μm and 3–5 μm.

10. The all-fiber tunable mid-infrared single-frequency pulse laser according to claim 1, characterized in that, The polarization-maintaining filter is a bandpass filter, and the operating wavelength of the polarization-maintaining filter is 3 to 5 μm.

Images