An all-fiber amplifier for boosting single-frequency pulse laser energy

Abstract

The present invention discloses an all-fiber amplifier for enhancing the energy of a single-frequency pulsed laser. The amplifier utilizes a low-doped single-mode active fiber, a highly doped multimode active fiber, and a single-mode passive fiber structure connected in sequence as a single-frequency pulsed laser amplification stage, respectively realizing the functions of low-noise-coefficient pre-amplification of a single-frequency seed source, suppression of stimulated Brillouin scattering effects, efficient enhancement of pulse energy, and high-beam-quality laser output, thereby obtaining a high-energy single-frequency pulsed all-fiber laser. The amplifier overcomes the problems of a low stimulated Brillouin scattering effect threshold and deterioration of the signal-to-noise ratio and beam quality caused by a large-mode-field active fiber in a traditional single-frequency fiber laser amplification structure.

Description

Technical Field

[0001] The present invention relates to the field of optical fiber amplifiers, and in particular to an all-optical fiber amplifier for enhancing the energy of single-frequency pulse lasers. Background Art

[0002] As a high-performance laser source, single-frequency pulsed fiber lasers have broad application value in fields such as coherent lidar and atmospheric remote sensing. The laser pulse energy determines the detection range and accuracy of these applications. The high energy output of single-frequency pulsed fiber lasers is crucial for achieving long-range, high-precision coherent detection.

[0003] High-energy single-frequency pulsed fiber lasers are primarily achieved through master oscillator power amplification technology. A single-frequency pulse seed source is then amplified through a multi-stage fiber amplification structure to boost pulse energy. However, due to the narrow linewidth of single-frequency lasers, pulse amplification is often severely constrained by stimulated Brillouin scattering (SBS). Limited by the rare-earth ion doping concentration, traditional single-mode active silica fibers require a long fiber length to provide sufficient laser gain, resulting in a low SBS threshold and limiting the energy increase of single-frequency pulsed lasers. While large-mode-area active fibers used in the power amplification stage can alleviate the SBS constraint to some extent, the low seed energy significantly reduces the signal-to-noise ratio during laser amplification. Furthermore, the few-mode operation characteristics of large-mode-area fibers also degrade laser beam quality. Multicomponent glass fibers with high rare-earth ion doping capacity can effectively reduce fiber length while maintaining laser gain, thereby increasing the SBS threshold and single-pulse energy. However, the low melting point of multicomponent glass fibers makes low-loss splicing with traditional silica fibers challenging, and the mechanical strength of laser systems constructed using them remains a significant challenge. In addition, large-mode-area active photonic crystal fibers are also used in single-frequency pulsed fiber laser amplification systems. Their larger mode area under single-mode operation effectively increases the SBS threshold, enabling millijoule-level single-frequency pulsed lasers. However, the unique structural design of photonic crystal fibers results in poor compatibility with traditional quartz fibers, and optical amplification systems are often constructed using spatial optical structures, which restricts the integration and stability of fiber lasers. Summary of the Invention

[0004] The present invention provides an all-fiber amplifier for enhancing the energy of single-frequency pulsed lasers. The present invention selects a low-doped single-mode active fiber, a highly doped multimode active fiber, and a single-mode passive fiber structure connected in sequence as a single-frequency pulsed laser amplification stage, respectively realizing the functions of low-noise-coefficient pre-amplification of a single-frequency seed source, suppression of stimulated Brillouin scattering effects, efficient enhancement of pulse energy and self-imaging of signal lasers, and high-beam-quality laser output, thereby obtaining a high-energy single-frequency pulsed all-fiber laser. The invention overcomes the problems of low stimulated Brillouin scattering effect threshold and degradation of signal-to-noise ratio and beam quality caused by large-mode-field active fibers in traditional single-frequency fiber laser amplification structures, as described below for details:

[0005] An all-fiber amplifier for increasing the energy of a single-frequency pulsed laser, the all-fiber amplifier comprising:

[0006] The single-frequency pulse seed source is connected to the isolator, and the single-frequency pulse seed source provides a low-power or energy single-frequency pulse laser. The isolator is used to prevent the reverse light of the amplifier from affecting the working stability of the single-frequency pulse seed source;

[0007] The single-frequency pulse laser is injected into the cascade fiber structure through the signal end of the pump and signal combiner for amplification. The pump end of the pump / signal combiner is connected to the multimode pump source to pump the active fiber in the cascade fiber structure. The pump / signal combiner is used to couple the multimode pump source and the single-frequency pulse laser into the cascade fiber structure. The cascade fiber structure is used to amplify the single-frequency pulse laser and ultimately output a single-frequency pulse laser with high beam quality and high energy.

[0008] The cascaded optical fiber structure is composed of a section of single-mode active optical fiber with low rare earth ion doping concentration, a section of multi-mode active optical fiber with high rare earth ion doping concentration, and a section of single-mode passive optical fiber connected in sequence.

[0009] The single-mode active optical fiber, multi-mode active optical fiber, and single-mode passive optical fiber are all double-clad optical fibers, and the inner cladding sizes are the same.

[0010] Furthermore, the single-mode active optical fiber has a relatively low gain coefficient, thereby preventing ASE caused by a relatively low power level of the seed light from causing a reduction in spectral purity.

[0011] Furthermore, the length of the multimode active optical fiber satisfies that its output end is a self-imaging point of multimode interference of the signal light, thereby ensuring that the signal light energy is coupled into the core of the single-mode passive optical fiber.

[0012] Furthermore, the single-mode passive optical fiber is shortened in length while satisfying the function of filtering out the remaining pump light, thereby avoiding the accumulation of SBS gain during the transmission of high-energy single-frequency laser light in the single-mode passive optical fiber.

[0013] The beneficial effects of the technical solution provided by the present invention are:

[0014] 1. In the cascaded fiber amplifier structure, the present invention uses a double-clad single-mode active fiber with a low rare earth ion doping concentration to amplify the single-frequency pulse seed source, effectively avoiding the degradation of spectral purity and reduction of signal light amplification efficiency caused by the low power of the seed source in the traditional fiber amplifier structure;

[0015] 2. The present invention utilizes the large core size of multimode active optical fiber in a cascaded optical fiber amplifier structure to successfully suppress the SBS effect of single-frequency laser power amplification. In addition, combined with the high energy storage advantage of multimode active optical fiber, high-energy amplification of single-frequency pulses can be successfully achieved.

[0016] 3. The cascaded fiber amplification structure used in the present invention itself constitutes a multimode interference structure. By selecting a fiber length that satisfies the self-imaging of the signal light multimode interference, high-efficiency coupling of high-energy signal light to the core of the single-mode passive fiber is achieved. Combined with the cascaded single-mode passive fiber, high-quality beam output of high-energy single-frequency laser is ensured.

[0017] 4. The single-frequency pulse laser amplifier proposed in this invention still uses an all-quartz fiber structure, which can achieve low-loss, high-mechanical-strength fusion splicing. Compared with high-energy single-frequency pulse laser amplification technologies based on multi-component glass fibers and photonic crystal fibers, this invention has the advantages of low technical barriers and easy application of all-fiber system integration.

[0018] 5. The single-frequency pulse laser all-fiber amplifier proposed in the present invention has broken through the energy level of traditional single-frequency pulse amplifiers based on uniform doping concentration and uniform size optical fibers. It can be used as a laser source with high coherence, high energy and high integration to support the application of coherent laser radar and remote sensing in the field of long-distance detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 A schematic diagram of the optical path of an all-fiber amplifier used to enhance the energy of single-frequency pulsed lasers;

[0020] Figure 2 Schematic diagram of the cascaded optical fiber structure.

[0021] In the accompanying drawings, the components represented by the reference numerals are as follows:

[0022] 1: Single-frequency pulse seed source; 2: Isolator;

[0023] 3: Multimode pump source; 4: Pump / signal combiner;

[0024] 5: Cascaded fiber structure; 5-1: Single-mode active fiber;

[0025] 5-2: Multimode active optical fiber; 5-3: Single-mode passive optical fiber. DETAILED DESCRIPTION

[0026] In order to make the objectives, technical solutions and advantages of the present invention more clear, the embodiments of the present invention are described in further detail below.

[0027] Example 1

[0028] An all-fiber amplifier for boosting the energy of single-frequency pulsed lasers, see Figure 1 The amplifier includes: a single-frequency pulse seed source 1, an isolator 2, a multi-mode pump source 3, a pump / signal combiner 4, and a cascaded optical fiber structure 5.

[0029] The single-frequency pulse seed source 1 is connected to an isolator 2, which provides low-power / energy single-frequency pulsed laser light. The isolator 2 is used to prevent the reverse light from the amplifier from affecting the operating stability of the single-frequency pulse seed source 1. The single-frequency pulsed laser light is injected into the cascade fiber structure 5 via the signal end of the pump / signal combiner 4 for amplification. The multimode pump source 3 is connected to the pump end of the pump / signal combiner 4 to pump the active fiber within the cascade fiber structure 5. The pump / signal combiner 4 is used to couple the multimode pump source laser light and the single-frequency pulsed laser light into the cascade fiber structure 5, which is used to efficiently amplify the single-frequency pulsed laser light.

[0030] The cascaded optical fiber structure 5 is composed of a section of single-mode active optical fiber with a low rare earth ion doping concentration, a section of multi-mode active optical fiber with a high rare earth ion doping concentration, and a section of single-mode passive optical fiber connected in sequence.

[0031] Furthermore, the single-mode active optical fiber, the multi-mode active optical fiber, and the single-mode passive optical fiber are all double-clad optical fibers, and the inner cladding sizes are the same.

[0032] Among them, single-mode active fiber has a low laser gain coefficient. While ensuring the effective amplification of the low-power single-frequency pulse seed source, it avoids ASE caused by insufficient extraction of the inverted particle number, which leads to a decrease in spectral purity, thereby realizing low-noise coefficient pre-amplification of the seed source.

[0033] Furthermore, multimode active optical fiber has a high laser gain coefficient, which can provide sufficient laser gain for high-energy amplification of pulsed lasers, and the large core size can effectively improve the SBS threshold of single-frequency pulsed lasers.

[0034] Among them, the single-mode passive optical fiber is used to filter out the remaining pump light and ensure high beam quality signal light output.

[0035] Furthermore, the cascaded optical fiber structure 5 constitutes a multimode interference structure. The length of the multimode active optical fiber satisfies that its output end is the self-imaging point of the multimode interference of the signal light, ensuring that most of the signal light energy is coupled into the core of the single-mode passive optical fiber.

[0036] Among them, the length of the single-mode passive optical fiber should be shortened as much as possible on the basis of satisfying the function of filtering out the remaining pump light to avoid the accumulation of SBS gain during the transmission of high-energy single-frequency laser in the single-mode passive optical fiber.

[0037] In summary, the embodiments of the present invention utilize a cascaded low-doped single-mode active fiber, a highly doped multimode active fiber, and a single-mode passive fiber structure as a single-frequency pulse laser amplification stage to respectively realize the functions of low-noise coefficient pre-amplification of a single-frequency seed source, suppression of the stimulated Brillouin scattering effect, efficient enhancement of pulse energy and signal laser self-imaging, and high-beam-quality laser output, thereby obtaining a single-frequency pulse all-fiber laser with high energy and high beam quality, which can be used as a high-performance coherent light source in the field of long-distance, high-resolution target detection by lidar.

[0038] Example 2

[0039] The embodiment of the present invention provides a high energy 1064nm single frequency pulse laser all-fiber amplifier, the structure of which is as follows: Figure 1 As shown, the laser all-fiber amplifier includes: a single-frequency pulse seed source 1, an isolator 2, a multi-mode pump source 3, a pump / signal combiner 4, and a cascade fiber structure 5.

[0040] Among them, the single-frequency pulse seed source 1 is a single-frequency fiber laser with a continuous wave power of 300mW and a central wavelength of 1064nm. It is modulated outside the cavity by an acousto-optic modulator to achieve single-frequency lasers with different pulse parameters, and is output by a single-mode HI1060 optical fiber; the isolator 2 has an operating center wavelength of 1064nm and an isolation of 30dB, and the device pigtail is a HI1060 optical fiber; the multimode pump source 3 is a 976nm semiconductor laser with a continuous wave maximum output power of 60W. The output pigtail is a multimode fiber with a core / cladding diameter of 105 / 125 μm and a numerical aperture of 0.22; the pump / signal combiner 4 is a (2+1)×1 combiner, the pump end pigtail matches the pigtail of the multimode pump source 3, the input end signal light pigtail is an HI1060 fiber, and the output end signal light pigtail is a double-clad fiber with a core / inner cladding diameter of 15 / 400 μm and a core / inner cladding numerical aperture of 0.06 / 0.46; the cascaded fiber structure 5 is as follows: Figure 2As shown, it is composed of a single-mode active optical fiber 5-1, a multi-mode active optical fiber 5-2 and a single-mode passive optical fiber 5-3 which are fused in sequence. Among them, the single-mode active optical fiber 5-1 is a double-clad ytterbium-doped optical fiber with a core / inner cladding diameter of 15 / 400μm and a core / inner cladding numerical aperture of 0.06 / 0.46. The absorption coefficient at 976nm is 0.5dB / m, and the optical fiber length is 40cm; the multimode active optical fiber 5-2 is a double-clad ytterbium-doped optical fiber with a core / inner cladding diameter of 100 / 400μm and a core / inner cladding numerical aperture of 0.09 / 0.46. The absorption coefficient at 976nm is 26dB / m. The optical fiber length of 30.6cm is the high-order self-imaging point of the multimode interference effect; the single-mode passive optical fiber 5-3 is a double-clad optical fiber with a core / inner cladding diameter of 15 / 400μm and a core / inner cladding numerical aperture of 0.06 / 0.46. The optical fiber length is 15cm, and the output end is beveled at an 8° angle.

[0041] A single-frequency pulsed laser passes through an isolator 2 and a beam combiner 4 and is injected into a cascaded fiber structure 5. The single-frequency pulsed laser is first pre-amplified with a low-noise coefficient in a low-doped single-mode active fiber 5-1 to prevent the generation of severe ASE components from low-power laser amplification. It is then amplified at high energy in a highly doped multimode active fiber 5-2. The large core size of the multimode fiber effectively suppresses the SBS effect of the single-frequency laser. By controlling the length of the multimode active fiber 5-2 to ensure self-imaging of the laser, the high-energy single-frequency pulsed laser is efficiently coupled into the single-mode passive fiber 5-3. The residual pump light in the single-mode fiber is stripped away, resulting in a high-quality, high-energy 1064nm single-frequency pulsed laser output. The fiber amplifier proposed in this embodiment can achieve a breakthrough in the energy level of 1064nm single-frequency pulsed fiber lasers in the millijoule range, enabling applications in high-resolution coherent lidar for detecting target speed and distance characteristics at distances up to 10km.

[0042] In summary, the advantages of the embodiments of the present invention are that the single-frequency pulsed laser is first amplified at low power in a low-doped active fiber to avoid severe ASE generation; it is then amplified at high energy in a highly doped, large-core multimode fiber, effectively avoiding the constraints of the SBS effect; and by fusing a section of single-mode passive fiber, the dual effects of stripping residual pump light and ensuring high beam quality are achieved. The all-fiber laser of the embodiments of the present invention has a compact design and the fiber fusion technology is relatively simple, which is conducive to the rapid promotion and application of high-energy single-frequency pulsed fiber lasers.

[0043] Unless otherwise specified, the embodiments of the present invention do not limit the models of the components. Any component that can perform the above functions may be used.

[0044] Those skilled in the art will understand that the accompanying drawings are only a schematic diagram of a preferred embodiment, and the serial numbers of the embodiments of the present invention are only for description and do not represent the advantages or disadvantages of the embodiments.

[0045] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. An all-fiber amplifier for enhancing the energy of single-frequency pulsed lasers, characterized in that: The all-fiber amplifier comprises: The single-frequency pulse seed source is connected to the isolator, and the single-frequency pulse seed source provides a low-power or energy single-frequency pulse laser. The isolator is used to prevent the reverse light of the amplifier from affecting the working stability of the single-frequency pulse seed source; The single-frequency pulse laser is injected into the cascade fiber structure through the signal end of the pump and signal combiner for amplification. The pump end of the pump / signal combiner is connected to the multimode pump source to pump the active fiber in the cascade fiber structure. The pump / signal combiner is used to couple the multimode pump source and the single-frequency pulse laser into the cascade fiber structure. The cascade fiber structure is used to amplify the single-frequency pulse laser and ultimately output a single-frequency pulse laser with high beam quality and high energy. The cascaded optical fiber structure is composed of a section of single-mode active optical fiber with low rare earth ion doping concentration, a section of multi-mode active optical fiber with high rare earth ion doping concentration, and a section of single-mode passive optical fiber connected in sequence.

2. The all-fiber amplifier for enhancing the energy of a single-frequency pulsed laser according to claim 1, characterized in that: The single-mode active optical fiber, multi-mode active optical fiber, and single-mode passive optical fiber are all double-clad optical fibers, and the inner cladding sizes are the same.

3. An all-fiber amplifier for enhancing the energy of a single-frequency pulsed laser according to claim 1 or 2, characterized in that: The single-mode active optical fiber has a relatively low gain coefficient, thereby preventing the reduction of spectral purity caused by amplified spontaneous radiation caused by a relatively low power level of the seed light.

4. An all-fiber amplifier for enhancing the energy of a single-frequency pulsed laser according to claim 1 or 2, characterized in that: The length of the multimode active optical fiber satisfies that the output end thereof is a self-imaging point of multimode interference of the signal light, thereby ensuring that the signal light energy is coupled into the core of the single-mode passive optical fiber.

5. The all-fiber amplifier for enhancing the energy of a single-frequency pulsed laser according to claim 1 or 2, characterized in that: The single-mode passive optical fiber is shortened in length while meeting the function of filtering out residual pump light, thereby avoiding the accumulation of SBS gain during the transmission of high-energy single-frequency laser light in the single-mode passive optical fiber.

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