A 2μm single-frequency pulsed laser based on synchronous modulation and solid-state graded doping

Abstract

This 2μm single-frequency pulsed laser, based on synchronous modulation and solid-state graded-doping, comprises a fiber-optic synchronous modulation cascaded amplifier and a solid-state non-uniformly graded-doped two-way amplifier. Synchronous modulation in the fiber section suppresses noise, improving the signal-to-noise ratio and solid-state amplification efficiency. The non-uniformly graded-doped gain crystal group in the solid-state section enables small-signal amplification, high-energy extraction, and high amplification efficiency. This allows for high-efficiency, high-repetition-rate, high-energy, narrow-linewidth pulsed laser output in the tens to hundreds of millijoules range, significantly improving the detection accuracy and capability of radar systems.

Description

Technical Field

[0001] This invention relates to the field of laser technology, and in particular to a 2μm single-frequency pulse laser based on synchronous modulation and solid-state graded doping. Background Technology

[0002] Single-frequency pulsed lasers with wavelengths around 2μm have important applications in coherent lidar, biomedicine, pollutant monitoring, and nonlinear frequency conversion due to their safety for the human eye, high atmospheric transmittance, good coherence, low background noise, and strong CO2 absorption. In recent years, 2μm coherent lidar has been widely used for airport wind shear detection to ensure aviation safety, and can also be applied to wind power site selection, extreme weather warnings, and precision airdrops. CO2 differential absorption radar can monitor greenhouse gases such as CO2 in the atmosphere with high precision and high resolution, contributing to global carbon balance. In 2μm coherent lidar, the 2μm single-frequency pulsed laser source is the core component. To improve the detection capability of the radar system, the source should possess characteristics such as high energy, high repetition rate, high frequency stability, small size, and high environmental stability.

[0003] Traditional methods for realizing 2μm single-frequency pulsed lasers primarily rely on solid-state injected frequency-locked lasers (PZTs) and amplifiers. However, limited by the mechanical motion mechanism of PZTs, the output laser repetition frequency is only in the hundreds of Hertz range, making it difficult to achieve high repetition rates and resulting in poor frequency stability, which affects radar detection accuracy. While 2μm all-fiber direct modulation single-frequency pulsed fiber amplifiers are compact and highly stable, their output energy is limited to the mJ range due to nonlinear effects such as stimulated Brillouin scattering (SBS), making it difficult to meet application requirements. Furthermore, 2μm fiber-solid hybrid single-frequency pulsed fiber lasers suffer from low amplification efficiency, leading to severe thermal effects and hindering further increases in output energy. Summary of the Invention

[0004] To overcome the shortcomings of the prior art, the technical problem to be solved by the present invention is to provide a 2μm single-frequency pulse laser based on synchronous modulation and solid-state graded doping, which can achieve high-efficiency, high-repetition-rate, and high-energy single-frequency pulse laser output.

[0005] The technical solution of the present invention is: a 2μm single-frequency pulse laser based on synchronous modulation and solid graded doping, which includes: a fiber synchronous modulation cascaded amplifier and a solid non-uniform graded doping two-way amplifier.

[0006] In the fiber synchronous modulation cascade amplifier, a fusion coupler, an acousto-optic modulator, and a first fiber isolator are sequentially spliced ​​along the narrow linewidth continuous fiber seed source from start to finish.

[0007] Subsequently, a fiber cascade amplification structure is adopted: the modulated pulsed light enters the first-stage preamplifier, and along the laser transmission direction, a wavelength division multiplexer, a first gain fiber, a first bandpass filter, and a second isolator are fused sequentially from beginning to end. The output fiber of the fiber laser pump source is fused with the pump fiber of the wavelength division multiplexer. Then, it enters the electro-optic modulator, and the acousto-optic modulator and the electro-optic modulator are synchronously modulated by a signal generator. After synchronous modulation, it enters the second-stage fiber preamplifier, and the first fiber combiner, a second gain fiber, a second bandpass filter, and a third fiber isolator are fused sequentially from beginning to end. The output fiber of the first semiconductor laser is fused with the pump fiber of the first fiber combiner. It enters the fiber main amplifier, and the second fiber combiner, a third gain fiber, a third bandpass filter, and a fourth fiber isolator are fused sequentially from beginning to end. The output fiber of the second semiconductor laser is fused with the pump fiber of the second fiber combiner, and the fourth fiber isolator is fused with an output cap with a pigtail.

[0008] In a solid-state non-uniform graded-doped two-way amplifier, a first lens, a second lens, a first half-wave plate, a spatial optical isolator, a second half-wave plate, a first mirror, a second mirror, an end-face pumped non-uniform graded-doped gain crystal group, a quarter-wave plate, and a 0° mirror are sequentially arranged along the laser transmission direction of the fiber synchronous modulation cascade amplifier. The output laser from the solid-state amplifier pump source passes through the third lens, the fourth lens, the third mirror, the fourth mirror, the first mirror, and the second mirror, respectively, and then enters the non-uniform graded-doped gain crystal group from the end face.

[0009] This invention utilizes synchronous modulation in the optical fiber section to suppress noise and improve the signal-to-noise ratio. In the solid-state section, it employs a non-uniformly graded-doped gain crystal group to achieve small-signal amplification capability and high-energy extraction and amplification efficiency. This enables high-efficiency, high-repetition-rate, high-energy, narrow-linewidth pulsed laser output in the tens to hundreds of millijoules range at 2μm, significantly improving the detection accuracy and capability of radar systems. Attached Figure Description

[0010] Figure 1 This is a schematic diagram of the structure of a 2μm single-frequency pulsed laser based on synchronous modulation and solid-state graded doping according to the present invention.

[0011] Figure 2 This is a schematic diagram of the fiber optic synchronous modulation cascade amplifier according to the present invention.

[0012] Figure 3 This is a schematic diagram of the structure of a solid-state non-uniformly graded-doped two-way amplifier according to the present invention.

[0013] in:

[0014] 1—Narrow linewidth fiber laser; 2—Coupled; 3—Acousto-optic modulator; 4—First fiber isolator; 5—Fiber laser; 6—Wavelength division multiplexer; 7—First gain fiber; 8—First bandpass filter; 9—Second isolator; 10—Electro-optic modulator. 11—First fiber combiner; 12—First semiconductor laser; 13—Second gain fiber; 14—Second bandpass filter; 15—Third fiber isolator; 16—Second fiber combiner; 17—Second semiconductor laser; 18—Third gain fiber; 19—Third bandpass filter; 20—Fourth fiber isolator; 21—Output cap; 22—Signal generator; 23—Fiber optic synchronous modulation cascade amplifier; 24—First lens; 25—Second lens; 26—First half-wave plate; 27—Spatial optical isolator; 28—Second half-wave plate; 29—First mirror; 30—Second mirror; 31—End-face pumped non-uniform graded-doped gain crystal group; 32—Quarter-wave plate; 33—0° mirror; 34—Solid-state amplifier pump source; 35—Third lens; 36—Fourth lens; 37—Third mirror; 38—Fourth mirror. Detailed Implementation

[0015] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0016] It should be noted that the term "comprising" and any variations thereof in the specification, claims and accompanying drawings of this invention are intended to cover non-exclusive inclusion. For example, a process, method, apparatus, product or device that includes a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products or devices.

[0017] like Figure 1-3 As shown, it includes: fiber optic synchronous modulation cascaded amplifier A and solid non-uniform graded doped two-way amplifier B;

[0018] In the fiber synchronous modulation cascade amplifier, the first fiber isolator 4, the coupler 2, the acousto-optic modulator 3, and the first fiber isolator 4 are sequentially spliced ​​along the narrow linewidth continuous fiber seed source 1.

[0019] Subsequently, a fiber cascade amplification structure is adopted: the modulated pulsed light enters the first-stage preamplifier I, and wavelength division multiplexer 6, first gain fiber 7, first bandpass filter 8, and second isolator 9 are fused sequentially along the laser transmission direction. The output fiber of fiber laser pump source 5 is fused with the pump fiber of wavelength division multiplexer 6. Then it enters electro-optic modulator 10, and acousto-optic modulator 3 and electro-optic modulator 10 are synchronously modulated by signal generator 22. After synchronous modulation, it enters the second-stage fiber preamplifier II, and the first fiber is fused sequentially at both ends. The fiber bundler 11, the second gain fiber 13, the second bandpass filter 14, the third fiber isolator 15, and the output fiber of the first semiconductor laser 12 are fused with the pump fiber of the first fiber bundler 11; then they enter the fiber main amplifier III, where the second fiber bundler 16, the third gain fiber 18, the third bandpass filter 19, and the fourth fiber isolator 20 are fused sequentially from end to end; the output fiber of the second semiconductor laser 17 is fused with the pump fiber of the second fiber bundler 16, and the fourth fiber isolator 20 is fused with the output end cap 21 with a pigtail;

[0020] In a solid-state non-uniform graded-doped two-way amplifier, a first lens 24, a second lens 25, a first half-wave plate 26, a spatial optical isolator 27, a second half-wave plate 28, a first reflector 29, a second reflector 30, an end-face pumped non-uniform graded-doped gain crystal group 31, a quarter-wave plate 32, and a 0° reflector 33 are arranged sequentially along the laser transmission direction of the fiber synchronous modulation cascade amplifier 23. The output laser from the solid-state amplifier pump source 34 passes through the third lens 35, the fourth lens 36, the third reflector 37, the fourth reflector 38, the first reflector 29, and the second reflector 30, and then enters the non-uniform graded-doped gain crystal group 31 from the end face.

[0021] This invention utilizes synchronous modulation in the optical fiber section to suppress noise and improve the signal-to-noise ratio. In the solid-state section, it employs a non-uniformly graded-doped gain crystal group to achieve small-signal amplification capability and high-energy extraction and amplification efficiency. This enables high-efficiency, high-repetition-rate, high-energy, narrow-linewidth pulsed laser output in the tens to hundreds of millijoules range at 2μm, significantly improving the detection accuracy and capability of radar systems.

[0022] Preferably, the output of the narrow linewidth continuous fiber seed source is a high-stability, low-noise, narrow linewidth linearly polarized continuous fiber laser. The acousto-optic modulator and electro-optic modulator modulate the continuous light into pulsed light, and the pulse waveform, repetition frequency, and pulse width are adjustable.

[0023] Preferably, the first gain fiber, the second gain fiber, and the third gain fiber are thulium-doped, holmium-doped, or thulium-holmium co-doped gain fibers.

[0024] Preferably, the first bandpass filter, the second bandpass filter, and the third bandpass filter filter out pump light and amplify spontaneous emission, while narrowing the laser pulse linewidth.

[0025] Preferably, the output cap is a quartz cap with a pigtail.

[0026] Preferably, the first half-wave plate and the second half-wave plate adjust the polarization direction of the fiber seed source to ensure consistency with the non-uniformly graded-doped gain crystal group; the quarter-wave plate adjusts the polarization state of the laser after passing through the non-uniformly graded-doped gain crystal group so that the returned laser is perpendicular to the polarization direction of the non-uniformly graded-doped gain crystal group.

[0027] Preferably, the non-uniform gradient doped gain crystal group is a series connection of a highly doped short-length crystal and a low-doped long-length crystal, employing a two-pass amplification structure.

[0028] Preferably, the 0° reflector reflects the signal light, causing the signal light to pass through the gain crystal again; the 0° reflector is placed at the focal point of the thermal lens generated by the gain crystal, so that the laser emitted the first time returns to the gain crystal with the same light distribution.

[0029] Preferably, the fiber optic synchronous modulation cascade amplifier uses fully polarization-maintaining fiber and devices, and the output after solid-state amplification is linearly polarized laser.

[0030] Preferably, the first, second, third, and fourth reflectors are all placed at a 45° angle to the optical path.

[0031] The specific working process of this invention is as follows:

[0032] like Figure 1 As shown, this invention comprises two parts: a fiber optic synchronous modulation cascaded amplifier and a solid-state non-uniform graded-doped two-way amplifier. The fiber optic synchronous modulation cascaded amplifier uses a high-stability continuous narrow-linewidth fiber laser as the seed source. An acousto-optic modulator modulates the continuous light into pulsed light. The waveform, pulse width, and repetition frequency of the output pulsed laser are adjusted by the acousto-optic modulator. After a first-stage pre-amplification, the modulated pulsed light enters an electro-optic modulator for synchronous modulation using a pulsed square wave, filtering out residual continuous light components from single modulation. It then undergoes preliminary amplification through a polarization-maintaining fiber pre-amplifier and a fiber main amplifier. The solid-state amplifier uses a two-way amplification structure with a non-uniform graded-doped gain crystal group for further amplification. The pump source output wavelength is located at the strongest absorption peak of the solid, ensuring high-efficiency amplification and ultimately achieving high-efficiency 2μm high-repetition-rate, high-energy single-frequency pulsed laser output. All modules in the front-end fiber section are fully fiber-fused, resulting in a compact structure and stable performance. The rear-end solid-state amplification section facilitates the generation of high-efficiency, high-energy, and high-peak-power pulsed laser output. The entire laser structure is compact, the system is stable, and the pulse width and repetition rate are adjustable.

[0033] like Figure 2 As shown, the fiber synchronous modulation cascade amplifier of the present invention includes a high-stability narrow-linewidth fiber laser 1, with a coupler 2, an acousto-optic modulator 3, and a first fiber isolator 4 fused sequentially along the seed laser. Then, a fiber cascade amplification structure is adopted: the modulated pulsed light enters the first-stage preamplifier, and a wavelength division multiplexer 6 is fused sequentially along the laser transmission direction; the output fiber of the fiber laser pump source 5 is fused to the pump fiber of the wavelength division multiplexer 6; the output fiber of the wavelength division multiplexer 6 is fused sequentially to a first gain fiber 7, a first bandpass filter 8, and a second isolator 9; then it enters the electro-optic modulator 10, and the signal generator 22 connects the acousto-optic modulator 3 and the electro-optic modulator 10. Synchronous modulation is performed; after synchronous modulation, the signal enters the second-stage fiber preamplifier, where it is fused to the first fiber combiner 11. The output fiber of the first semiconductor laser 12 is fused to the pump fiber of the first fiber combiner 11. The output fiber of the first fiber combiner 11 is then fused to the second gain fiber 13, the second bandpass filter 14, and the third fiber isolator 15 in sequence. The signal then enters the fiber main amplifier, where it is fused to the second fiber combiner 16. The output fiber of the second semiconductor laser 17 is fused to the pump fiber of the second fiber combiner 16. The output fiber of the second fiber combiner 16 is then fused to the third gain fiber 18, the third bandpass filter 19, and the fourth fiber isolator 20 in sequence. Finally, an output cap 21 with a pigtail is fused to the output. This fiber synchronous modulation cascaded amplifier achieves an output energy in the hundreds of μJ range at a repetition frequency of kHz.

[0034] like Figure 3As shown, the solid-state non-uniform graded-doped two-way amplifier in the 2μm hybrid high-efficiency amplification technology based on fiber synchronous modulation and solid-state non-uniform graded doping of the present invention consists of a first lens 24, a second lens 25, a first half-wave plate 26, a spatial optical isolator 27, a second half-wave plate 28, a first reflector 29 and a second reflector 30 placed at 45° to the optical path, an end-face pumped non-uniform graded-doped gain crystal group 31, a quarter-wave plate 32, and a 0° reflector 33. The output laser of the solid-state amplifier pump source 34 passes through the third lens 35, the fourth lens 36, the third reflector 37, the fourth reflector 38 placed at 45° to the optical path, the first reflector 29, and the second reflector 30, and then enters the non-uniform graded-doped gain crystal group 31 from the end face. The first half-wave plate 26 and the second half-wave plate 28 are used to adjust the polarization direction of the laser output from the fiber synchronous modulation amplifier to be consistent with the axis of the end-pumped non-uniform graded-doped gain crystal group 31. The non-uniform graded-doped gain crystal group 31 is composed of a high-doped short-length crystal and a low-doped long-length crystal connected in series. The laser first passes through the high-doped crystal, which is beneficial for small signal amplification, and then passes through the low-doped long-length crystal, which effectively avoids the problems of reabsorption and severe thermal effects, and can achieve high-efficiency amplification of small signals. The quarter-wave plate 32 adjusts the polarization state of the laser after passing through the non-uniform graded-doped gain crystal group 31, so that the returning laser is perpendicular to the polarization direction of the end-pumped non-uniform graded-doped gain crystal group 31. The combination of the two achieves high-efficiency amplification. The first lens 24, the second lens 25, the third lens 35, and the fourth lens 36 respectively perform beam transformation on the signal light and the pump light, ensuring that the beam waist size and position of the two beams are at 0° of the reflector 33, so as to achieve high-efficiency amplification. A 0° reflector 33 is placed at the focal point of the thermal lens generated by the gain crystal group, ensuring that the first emitted laser beam returns to the gain crystal group with the same light distribution, thus achieving efficient two-way amplification. At a repetition frequency of kHz, the output energy reaches tens to hundreds of millijoules.

[0035] Compared with the prior art, the beneficial effects of this invention are as follows:

[0036] 1. This invention employs a hybrid fiber-solid amplification method, fully utilizing the high stability of optical fibers and the energy extraction capabilities of solid-state components. Large-mode-field, highly doped, and short-length optical fibers are used to suppress stimulated Brillouin scattering (SBS) for initial amplification; the solid-state amplifier section consists only of amplifiers, increasing energy while reducing the use of spatial components and avoiding the introduction of SBS. The entire laser system is compact and exhibits stable performance.

[0037] 2. This invention innovatively adopts synchronous modulation, using acousto-optic and electro-optic synchronous modulation for the fiber optic amplifier. This method can filter out the continuous light component remaining from single modulation, suppress noise, and improve the signal-to-noise ratio, providing high-quality signal light for solid-state amplification and greatly improving the amplification efficiency of the solid-state amplifier.

[0038] 3. This invention innovatively employs a solid-state non-uniformly graded-doped gain crystal group structure, connecting highly doped short-length and low-doped long-length gain crystals in series. This facilitates small-signal amplification and thermal effect control, significantly improving laser amplification efficiency and output energy. Ultimately, based on a high repetition rate, high-efficiency amplification of high-energy 2μm single-frequency pulsed laser pulses is achieved.

[0039] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A 2 μm single frequency pulsed laser based on synchronous modulation and solid-state graded-doping, characterized in that: It includes: Optical fiber synchronous modulation cascade amplifier (A) and solid non-uniform gradually doped double-pass amplifier (B); In the optical fiber synchronous modulation cascade amplifier, a narrow linewidth continuous optical fiber seed source (1) is sequentially connected by a coupler (2), an acousto-optic modulator (3), and a first optical fiber isolator (4) at both ends; Then, the modulated pulse light enters a first-stage preamplifier (I), and a wavelength division multiplexer (6), a first gain optical fiber (7), a first band-pass filter (8), and a second isolator (9) are sequentially connected at both ends along the laser transmission direction; the output fiber of the fiber laser pump source (5) is fused with the pump fiber of the wavelength division multiplexer (6); then, the light enters an electro-optic modulator (10), and the acousto-optic modulator (3) and the electro-optic modulator (10) are synchronously modulated by a signal generator (22); After synchronous modulation, the light enters a second-stage optical fiber preamplifier (II), and a first optical fiber combiner (11), a second gain optical fiber (13), a second band-pass filter (14), and a third optical fiber isolator (15) are sequentially connected at both ends; the output fiber of the first semiconductor laser (12) is fused with the pump fiber of the first optical fiber combiner (11); The light enters a fiber main amplifier (III), and a second optical fiber combiner (16), a third gain optical fiber (18), a third band-pass filter (19), and a fourth optical fiber isolator (20) are sequentially connected at both ends; the output fiber of the second semiconductor laser (17) is fused with the pump fiber of the second optical fiber combiner (16), and the fourth optical fiber isolator (20) is fused with the output end cap (21) of the tail fiber; In the solid non-uniform gradually doped double-pass amplifier, a first lens (24), a second lens (25), a first half-wave plate (26), a spatial light isolator (27), a second half-wave plate (28), a first mirror (29), a second mirror (30), an end-pumped non-uniform gradually doped gain crystal group (31), a quarter-wave plate (32), and a 0° mirror (33) are sequentially arranged along the laser transmission direction of the optical fiber synchronous modulation cascade amplifier (23); the output laser of the solid amplifier pump source (34) enters the non-uniform gradually doped gain crystal group (31) from the end face after passing through a third lens (35), a fourth lens (36), a third mirror (37), a fourth mirror (38), the first mirror (29), and the second mirror (30); the non-uniform gradually doped gain crystal group is composed of a high-doped short-length crystal and a low-doped long-length crystal connected in series, and the laser first passes through the high-doped short-length crystal and then passes through the low-doped long-length crystal.

2. The single frequency pulsed laser at 2 pm based on synchronous modulation and solid-state graded-doping according to claim 1, characterized in that: The output of the narrow linewidth continuous optical fiber seed source is a high-stability low-noise narrow linewidth linearly polarized continuous optical fiber laser; the acousto-optic modulator and the electro-optic modulator modulate the continuous light into pulse light, and the pulse waveform, repetition frequency, and pulse width are adjustable.

3. The single frequency pulsed laser at 2 pm based on synchronous modulation and solid-state gradient-doping according to claim 1, characterized in that: The first gain optical fiber, the second gain optical fiber, and the third gain optical fiber are thulium-doped, holmium-doped, or thulium-holmium co-doped gain optical fibers.

4. The single frequency pulsed laser at 2 pm based on synchronous modulation and solid-state gradient-doping according to claim 1, characterized in that: The first band-pass filter, the second band-pass filter, and the third band-pass filter filter out the pump light and amplified spontaneous emission, and simultaneously narrow the laser pulse linewidth.

5. The synchronous modulation based and solid-state gradient-doped 2 pm single-frequency pulsed laser according to claim 1, characterized in that: The output end cap is a pigtail quartz end cap.

6. The synchronous modulation based and solid-state gradient-doped 2 pm single-frequency pulsed laser according to claim 1, characterized in that: The first and second half-wave plates adjust the polarization direction of the fiber seed source, and ensure consistency with the non-uniformly doped gain crystal group; The quarter-wave plate adjusts the polarization state of the laser passing through the non-uniformly doped gain crystal group, so that the returned laser is perpendicular to the polarization direction of the non-uniformly doped gain crystal group.

7. The synchronous modulation based and solid-state gradient-doped 2 pm single-frequency pulsed laser according to claim 1, characterized in that: The 0° reflector reflects the signal light, so that the signal light passes through the gain crystal again; the 0° reflector is placed at the focal point of the thermal lens generated by the gain crystal, so that the first emitted laser returns to the gain crystal with the same light distribution.

8. The single frequency pulsed laser at 2 pm based on synchronous modulation and solid-state gradient-doping according to claim 1, characterized in that: The fiber synchronous modulation cascade amplifier uses a full polarization-maintaining fiber and device, and the output after solid amplification is linearly polarized laser.

9. The synchronous modulation based and solid-state gradient-doped 2 pm single-frequency pulsed laser according to claim 1, characterized in that: The first reflector, the second reflector, the third reflector and the fourth reflector are placed at 45° with the light path.

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