2 [mu] m single-frequency pulse laser based on synchronous modulation and solid gradient doping

Abstract

The invention relates to a 2 [mu] m single-frequency pulse laser based on synchronous modulation and solid gradient doping. The laser comprises an optical fiber synchronous modulation cascade amplifier and a solid non-uniform gradient doping two-way amplifier. Synchronous modulation is utilized in the optical fiber part, noise suppression is achieved, and the signal-to-noise ratio and the solid amplification efficiency are improved. The non-uniform gradient doping gain crystal group is utilized in the solid part, the small signal amplification capability, the large energy extraction capability and the high amplification efficiency capability are realized, the high-efficiency 2-micron high-repetition-frequency large-energy narrow-linewidth pulse laser output of tens to hundreds of millijoule magnitude can be realized, and the detection precision and the detection capability of a radar system are greatly improved.

Description

Technical Field

[0001] The present invention relates to the field of laser technology, and in particular, to a 2μm single-frequency pulsed laser based on synchronous modulation and solid gradient doping. Background Art

[0002] Single-frequency pulsed lasers with wavelengths near the 2μm band have important applications in coherent lidar, biomedical, pollutant monitoring, and nonlinear frequency conversion due to their eye-safe band, high atmospheric transmittance, good coherence, low background noise, and strong CO2 absorption. In recent years, 2μm coherent lidar has been widely used in airport wind shear detection to ensure aviation safety, and can also be applied to the site selection of wind power generation, extreme weather warning, precise airdrop and airborne operations, etc. CO2 differential absorption lidar can monitor greenhouse gases such as CO2 in the atmosphere with high precision and high resolution, which is conducive to achieving global carbon balance. In 2μm coherent lidar, a 2μm single-frequency pulsed laser source is the core device. To improve the detection ability of the radar system, the light source should have characteristics such as high energy, high repetition frequency, high frequency stability, small volume, and high environmental stability.

[0003] Traditional methods for realizing 2μm single-frequency pulsed lasers mainly rely on solid-state injection-locked lasers and amplifiers. Limited by the mechanical movement mechanism of PZT, the output laser repetition frequency is only in the order of hundreds of hertz, making it difficult to achieve high repetition frequency output, and the frequency stability is poor, which affects the radar detection accuracy. The 2μm all-fiber directly modulated single-frequency pulsed fiber amplifier has a compact structure and high stability, but due to the influence of the nonlinear effect stimulated Brillouin scattering (SBS), it is difficult to increase the output energy to the mJ level, making it difficult to meet the application requirements. The 2μm fiber-solid hybrid single-frequency pulsed fiber laser has a low amplification efficiency, resulting in serious thermal effects and difficult to further increase the output energy. Summary of the Invention

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

[0005] The technical solution of the present invention is as follows: This 2μm single-frequency pulsed laser based on synchronous modulation and solid gradient doping includes: a fiber synchronous modulation cascaded amplifier and a solid non-uniform gradient doping double-pass amplifier; In the fiber synchronous modulation cascaded amplifier, a coupler, an acousto-optic modulator, and a first fiber isolator are sequentially fusion-spliced along the narrow-linewidth continuous fiber seed source from head to tail; Subsequently, a fiber cascaded amplification structure is adopted: the modulated pulsed light enters the first-stage pre-amplifier, and a wavelength division multiplexer, a first gain fiber, a first band-pass filter, and a second isolator are sequentially fusion-spliced along the laser transmission direction from head to tail. The output fiber of the fiber laser pump source is fusion-spliced 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 through a signal generator; after synchronous modulation, it enters the second-stage fiber pre-amplifier, and a first fiber combiner, a second gain fiber, a second band-pass filter, and a third fiber isolator are sequentially fusion-spliced from head to tail. The output fiber of the first semiconductor laser is fusion-spliced with the pump fiber of the first fiber combiner; it enters the fiber main amplifier, and a second fiber combiner, a third gain fiber, a third band-pass filter, and a fourth fiber isolator are sequentially fusion-spliced from head to tail. The output fiber of the second semiconductor laser is fusion-spliced with the pump fiber of the second fiber combiner, and the fourth fiber isolator is fusion-spliced with the output end cap with a tail fiber; In the solid non-uniformly and gradually doped double-pass 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-pumped non-uniformly and gradually 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 cascaded amplifier. The output laser of the solid amplifier pump source respectively passes through a third lens, a fourth lens, a third mirror, a fourth mirror, a first mirror, and a second mirror and then enters the non-uniformly and gradually doped gain crystal group from the end face.

[0006] In the fiber part of the present invention, synchronous modulation is utilized to achieve noise suppression and improve the signal-to-noise ratio. In the solid part, a non-uniformly and gradually doped gain crystal group is utilized to achieve the ability of small-signal amplification, large-energy extraction, and amplification efficiency, and can achieve high-efficiency 2μm high-repetition-frequency large-energy narrow-linewidth pulsed laser output in the order of dozens to hundreds of millijoules, greatly improving the detection accuracy and detection ability of the radar system. Description of the Drawings

[0007] Figure 1 It is a schematic structural diagram of a 2μm single-frequency pulsed laser based on synchronous modulation and solid gradient doping according to the present invention.

[0008] Figure 2 It is a schematic structural diagram of a fiber synchronous modulation cascaded amplifier according to the present invention.

[0009] Figure 3 It is a schematic structural diagram of a solid non-uniformly and gradually doped double-pass amplifier according to the present invention.

[0010] Wherein: 1 - narrow linewidth fiber laser, 2 - coupler, 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 end cap, 22 - signal generator, 23 - fiber synchronous modulation cascaded amplifier, 24 - first lens, 25 - second lens, 26 - first half-wave plate, 27 - spatial light isolator, 28 - second half-wave plate, 29 - first mirror, 30 - second mirror, 31 - end-pumped non-uniformly graded doped gain crystal group, 32 - quarter-wave plate, 33 - 0° mirror, 34 - solid amplifier pump source, 35 - third lens, 36 - fourth lens, 37 - third mirror, 38 - fourth mirror. Detailed implementation mode

[0011] In order to enable those skilled in the art to better understand the solution of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0012] It should be noted that the term "including" and any variation thereof in the specification and claims of the present invention and the above drawings are intended to cover non-exclusive inclusion. For example, a process, method, device, product or equipment that includes a series of steps or units does not necessarily have to be limited to those steps or units clearly listed, but may include other steps or units not clearly listed or inherent to these processes, methods, products or equipment.

[0013] As Figures 1-3 shown, it includes: a fiber synchronous modulation cascaded amplifier A and a solid non-uniformly graded doped double-pass amplifier B; In the fiber synchronous modulation cascaded amplifier, a coupler 2, an acousto-optic modulator 3, and a first fiber isolator 4 are sequentially fusion-spliced along the head and tail of the narrow linewidth continuous fiber seed source 1; Subsequently, a fiber cascade amplification structure is adopted: the modulated pulsed light enters the first-stage pre-amplifier I, and a wavelength division multiplexer 6, a first gain fiber 7, a first band-pass filter 8, and a second isolator 9 are sequentially fusion-spliced along the laser transmission direction from head to tail. The output fiber of the fiber laser pump source 5 is fusion-spliced with the pump fiber of the wavelength division multiplexer 6; then it enters the electro-optic modulator 10, and the acousto-optic modulator 3 and the electro-optic modulator 10 are synchronously modulated through the signal generator 22; after synchronous modulation, it enters the second-stage fiber pre-amplifier II, and a first fiber combiner 11, a second gain fiber 13, a second band-pass filter 14, and a third fiber isolator 15 are sequentially fusion-spliced from head to tail. The output fiber of the first semiconductor laser 12 is fusion-spliced with the pump fiber of the first fiber combiner 11; it enters the fiber main amplifier III, and a second fiber combiner 16, a third gain fiber 18, a third band-pass filter 19, and a fourth fiber isolator 20 are sequentially fusion-spliced from head to tail. The output fiber of the second semiconductor laser 17 is fusion-spliced with the pump fiber of the second fiber combiner 16, and the fourth fiber isolator 20 is fusion-spliced with the output end cap 21 with a tail fiber; In the solid non-uniformly and gradually doped two-pass 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-pumped non-uniformly and gradually doped gain crystal group 31, a quarter-wave plate 32, and a 0° reflector 33 are sequentially arranged along the laser transmission direction of the fiber synchronous modulation cascade amplifier 23. The output laser of the solid amplifier pump source 34 respectively passes through a third lens 35, a fourth lens 36, a third reflector 37, a fourth reflector 38, the first reflector 29, and the second reflector 30 and then enters the non-uniformly and gradually doped gain crystal group 31 from the end face.

[0014] In the fiber part of the present invention, synchronous modulation is utilized to achieve noise suppression and improve the signal-to-noise ratio. In the solid part, a non-uniformly and gradually doped gain crystal group is utilized to achieve the small-signal amplification ability, the large-energy extraction and amplification efficiency ability, and can achieve high-efficiency 2μm high-repetition-frequency large-energy narrow-linewidth pulsed laser output in the order of dozens to hundreds of millijoules, greatly improving the detection accuracy and detection ability of the radar system.

[0015] Preferably, the output of the narrow-linewidth continuous fiber seed source is a high-stability low-noise narrow-linewidth linearly polarized continuous fiber laser, and the acousto-optic modulator and the electro-optic modulator modulate the continuous light into pulsed light, and the pulse waveform, repetition frequency, and pulse width are adjustable.

[0016] Preferably, the first gain fiber, the second gain fiber, and the third gain fiber are gain fibers doped with thulium, holmium, or co-doped with thulium and holmium.

[0017] Preferably, 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 at the same time narrow the laser pulse linewidth.

[0018] Preferably, the output end cap is a quartz end cap with a pigtail fiber.

[0019] 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 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.

[0020] Preferably, the non-uniformly graded doped gain crystal group is a series group of high-doped short-length and low-doped long-length crystals, adopting a double-pass amplification structure.

[0021] Preferably, the 0° mirror reflects the signal light, enabling the signal light to pass through the gain crystal again; the 0° mirror is placed at the focal point of the thermal lens generated by the gain crystal, so that the laser emitted for the first time returns to the gain crystal with the same light distribution.

[0022] Preferably, the fiber synchronous modulation cascaded amplifier uses all polarization-maintaining fibers and devices, and the output after solid-state amplification is linearly polarized laser.

[0023] Preferably, the first mirror, the second mirror, the third mirror, and the fourth mirror are all placed at 45° to the respective optical paths.

[0024] The specific working process of the present invention is as follows: As Figure 1 shown, the present invention includes two parts: a fiber synchronous modulation cascaded amplifier and a solid non-uniformly graded doped double-pass amplifier. The fiber synchronous modulation cascaded amplifier uses a high-stability continuous narrow-linewidth fiber laser as the seed source, and uses an acousto-optic modulator to modulate the continuous light into pulsed light. The waveform, pulse width, and repetition frequency of the output pulsed laser are adjusted through the acousto-optic modulator. The modulated pulsed light enters an electro-optic modulator for synchronous modulation using a pulsed square wave to filter out the continuous light component remaining from a single modulation; it is preliminarily amplified through a polarization-maintaining fiber pre-amplifier and a fiber main amplifier; the solid-state amplifier further amplifies using a double-pass amplification structure of a non-uniformly graded doped gain crystal group, and the output wavelength of the pump source is at the strongest absorption peak of the solid, ensuring high-efficiency amplification, and finally achieving high-efficiency 2μm high-repetition-rate, high-energy single-frequency pulsed laser output. Each module in the front-stage fiber part adopts a fully fiberized fusion splicing method, with a compact structure and stable performance; the rear-stage solid-state amplification part is conducive to generating pulsed laser output with high efficiency, high energy, and high peak power. The entire laser has a compact structure, a stable system, and adjustable pulse width and repetition frequency.

[0025] As Figure 2As shown in the figure, the fiber optic synchronous modulation cascaded amplifier of the present invention includes a high-stability narrow linewidth fiber laser 1. A coupler 2, an acousto-optic modulator 3 are sequentially fusion spliced along the head and tail of the seed laser, and finally a first fiber isolator 4 is fusion spliced. Subsequently, a fiber optic cascaded amplification structure is adopted: the modulated pulsed light enters the first-stage pre-amplifier, and a wavelength division multiplexer 6, the output fiber of the fiber laser pump source 5 is fusion spliced with the pump fiber of the wavelength division multiplexer 6 along the head and tail in the laser transmission direction, and the output fiber of the wavelength division multiplexer 6 is sequentially fusion spliced with a first gain fiber 7, a first band-pass filter 8, and a second isolator 9. Subsequently, it enters an electro-optic modulator 10, and the acousto-optic modulator 3 and the electro-optic modulator 10 are synchronously modulated through a signal generator 22. After synchronous modulation, it enters the second-stage fiber pre-amplifier, and a first fiber combiner 11 is fusion spliced. The output fiber of the first semiconductor laser 12 is fusion spliced with the pump fiber of the first fiber combiner 11, and the output fiber of the first fiber combiner 11 is sequentially fusion spliced with a second gain fiber 13, a second band-pass filter 14, and a third fiber isolator 15. It enters the fiber main amplifier and a second fiber combiner 16 is fusion spliced. The output fiber of the second semiconductor laser 17 is fusion spliced with the pump fiber of the second fiber combiner 16, and the output fiber of the second fiber combiner 16 is sequentially fusion spliced with a third gain fiber 18, a third band-pass filter 19, and a fourth fiber isolator 20, and finally a tail-fiber output end cap 21 is fusion spliced. For this fiber optic synchronous modulation cascaded amplifier, at a repetition frequency of kHz, the output energy reaches the order of several hundred μJ.

[0026] As Figure 3As shown in the figure, in the solid non-uniformly graded doped two-pass amplifier of the 2μm hybrid high-efficiency amplification technology based on fiber synchronous modulation and solid non-uniform gradient doping of the present invention, 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 mirror 29 placed at 45° to the optical path, a second mirror 30, an end-pumped non-uniformly graded 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 fiber synchronous modulation cascaded amplifier 23. The output laser of the solid amplifier pump source 34 passes through the third lens 35, the fourth lens 36, the third mirror 37 placed at 45° to the optical path, the fourth mirror 38, the first mirror 29, and the second mirror 30 respectively, and then enters the non-uniformly graded doped gain crystal group 31 from the end face. Among them, the first half-wave plate 26 and the second half-wave plate 28 are used to adjust the polarization direction of the laser output after the fiber synchronous modulation amplifier to be consistent with the axis of the end-pumped non-uniformly graded doped gain crystal group 31. The non-uniformly graded doped gain crystal group 31 is composed of a series connection of a high-doped short-length crystal and a low-doped long-length crystal. 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, effectively avoiding the problems of severe reabsorption and thermal effects, and realizing high-efficiency small-signal amplification. The quarter-wave plate 32 adjusts the polarization state of the laser passing through the non-uniformly graded doped gain crystal group 31, so that the returned laser is perpendicular to the polarization direction of the end-pumped non-uniformly graded doped gain crystal group 31, and the two are combined to achieve high-efficiency amplification. The first lens 24, the second lens 25, the third lens 35, and the fourth lens 36 perform beam transformation on the signal light and the pump light respectively, ensuring that the beam waist sizes and positions of the two beams of light are at the 0° mirror 33, realizing high-efficiency amplification. The 0° mirror 33 is placed at the focus of the thermal lens generated by the gain crystal group, so that the laser emitted for the first time returns to the gain crystal group with the same light distribution, thereby realizing two-pass high-efficiency amplification. At a repetition frequency of kHz, the output energy reaches the order of dozens to hundreds of millijoules.

[0027] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. The present invention adopts the fiber-solid hybrid amplification method, which makes full use of the high stability of the fiber and the energy extraction ability of the solid. A large-mode-field, high-doped, short-length fiber is used to suppress the stimulated Brillouin scattering effect for preliminary amplification; the solid amplifier part consists only of an amplifier, which improves the energy while reducing the use of spatial components and avoids introducing the stimulated Brillouin scattering effect. The entire laser system has a compact structure and stable performance.

[0028] 2. The present invention innovatively adopts the synchronous modulation method, and performs acousto-optic and electro-optic synchronous modulation on the fiber amplifier. This method can filter out the continuous light components remaining from single modulation, suppress noise, and improve the signal-to-noise ratio. It can provide high-quality signal light for solid-state amplification, greatly improving the amplification efficiency of the solid-state amplifier.

[0029] 3. The present invention innovatively adopts the structure of a solid non-uniformly graded doped gain crystal group, connecting high-doped short-length and low-doped long-length gain crystals in series, which is beneficial to small-signal amplification and the control of thermal effects, greatly improving the laser amplification efficiency and output energy. Finally, based on a high repetition rate, high-efficiency amplification of high-energy 2-μm single-frequency pulsed laser pulses is achieved.

[0030] The above are only the preferred embodiments of the present invention, and do not impose any form of limitation on the present invention. Any simple modification, equivalent change, and modification made to the above embodiments based on the technical essence of the present invention still fall within the protection scope of the technical solution of the present invention.

Claims

1. A 2-μm single-frequency pulsed laser based on synchronous modulation and solid graded doping, characterized in that: It includes: An optical fiber synchronous modulation cascaded amplifier (A) and a solid non-uniformly graded doped double-pass amplifier (B); In the optical fiber synchronous modulation cascaded amplifier, a coupler (2), an acousto-optic modulator (3), and a first optical fiber isolator (4) are sequentially fusion-spliced to the head and tail of a narrow linewidth continuous optical fiber seed source (1). Subsequently, an optical fiber cascaded amplification structure is adopted: the modulated pulsed light enters the first-stage pre-amplifier (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 fusion-spliced to the head and tail along the laser transmission direction. The output fiber of the optical fiber laser pump source (5) is fusion-spliced to the pump fiber of the wavelength division multiplexer (6); then it enters the electro-optic modulator (10), and the acousto-optic modulator (3) and the electro-optic modulator (10) are synchronously modulated through a signal generator (22). After synchronous modulation, it enters the second-stage optical fiber pre-amplifier (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 fusion-spliced to the head and tail. The output fiber of the first semiconductor laser (12) is fusion-spliced to the pump fiber of the first optical fiber combiner (11). It enters the optical 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 fusion-spliced to the head and tail. The output fiber of the second semiconductor laser (17) is fusion-spliced to the pump fiber of the second optical fiber combiner (16), and the fourth optical fiber isolator (20) is fusion-spliced to the output end cap (21) with a tail fiber. In the solid non-uniformly graded doped double-pass 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-pumped non-uniformly graded doped gain crystal group (31), a quarter-wave plate (32), and a 0° reflector (33) are sequentially arranged along the laser transmission direction of the optical fiber synchronous modulation cascaded amplifier (23). The output laser of the solid amplifier pump source (34) respectively passes through a third lens (35), a fourth lens (36), a third reflector (37), a fourth reflector (38), a first reflector (29), and a second reflector (30) and then enters the non-uniformly graded doped gain crystal group (31) from the end face.

2. The 2-μm single-frequency pulsed laser based on synchronous modulation and solid gradual doping according to claim 1, wherein: 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 pulsed light, and the pulse waveform, repetition frequency, and pulse width are adjustable.

3. The 2μm single-frequency pulsed laser based on synchronous modulation and solid gradual 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 gain optical fibers doped with thulium, holmium, or co-doped with thulium and holmium.

4. The 2μm single-frequency pulsed laser based on synchronous modulation and solid gradual doping according to claim 1, wherein: 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 at the same time narrow the laser pulse linewidth.

5. The 2μm single-frequency pulsed laser based on synchronous modulation and solid gradual doping according to claim 1, characterized in that: The output end cap is a quartz end cap with a tail fiber.

6. The 2μm single-frequency pulsed laser based on synchronous modulation and solid-state graded doping according to claim 1, wherein: The first half-wave plate and the second half-wave plate adjust the polarization direction of the optical 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 passing through the non-uniformly and gradually doped gain crystal group, so that the returned laser is perpendicular to the polarization direction of the non-uniformly and gradually doped gain crystal group.

7. The 2μm single-frequency pulsed laser based on synchronous modulation and solid gradual doping according to claim 1, wherein: The non-uniformly and gradually doped gain crystal group is a series group of a highly doped short-length crystal and a low-doped long-length crystal, and adopts a double-pass amplification structure.

8. The 2μm single-frequency pulsed laser based on synchronous modulation and solid-state graded doping according to claim 1, wherein: The 0° mirror reflects the signal light, so that the signal light passes through the gain crystal again; the 0° mirror is placed at the focal point of the thermal lens generated by the gain crystal, so that the laser emitted for the first time returns to the gain crystal with the same light distribution.

9. The 2μm single-frequency pulsed laser based on synchronous modulation and solid-state graded doping according to claim 1, characterized in that: The fiber synchronous modulation cascade amplifier uses all polarization-maintaining fibers and devices, and the output after solid-state amplification is linearly polarized laser.

10. The 2-μm single-frequency pulsed laser based on synchronous modulation and solid gradual doping according to claim 1, wherein: The first mirror, the second mirror, the third mirror, and the fourth mirror are all placed at 45° to the optical path where they are located.

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