Single-frequency polarization-maintaining narrow-linewidth optical fiber amplifier

Abstract

The utility model discloses a single-frequency polarization-maintaining narrow-linewidth optical fiber amplifier, which comprises a seed laser, a pre-amplifier, a first-stage double-clad amplifier and a second-stage double-clad amplifier, the output end of the seed laser is connected with the input end of the pre-amplifier through an optical fiber, the output end of the pre-amplifier is connected with the input end of the first-stage double-clad amplifier through an optical fiber, and the output end of the first-stage double-clad amplifier is connected with the input end of the second-stage double-clad amplifier through an optical fiber; the seed laser is used for generating seed laser; the pre-amplifier is used for performing primary amplification on the seed laser to obtain primary amplified laser with first power and outputting the primary amplified laser; the first-stage double-clad amplifier is used for carrying out second-stage amplification on the first-stage amplified laser to obtain second-stage amplified laser with second power and outputting the second-stage amplified laser; and the second-stage double-clad amplifier is used for performing three-stage amplification on the second-stage amplified laser to obtain and output the amplified laser with the target power. By means of the structure, high-power laser output is achieved, and cost is reduced.

Description

Technical Field

[0001] This utility model relates to the technical field of fiber optic amplifiers, and in particular to a single-frequency polarization-maintaining narrow-linewidth fiber optic amplifier. Background Technology

[0002] In recent years, 1012nm–1020nm single-frequency polarization-maintaining fiber lasers and amplifiers have been widely used in research fields such as nonlinear frequency conversion, cold atom physics experiments, atomic precision measurement, and quantum optics due to their advantages of high beam quality, long coherence length, and narrow spectral linewidth. However, traditional 1012nm–1020nm amplifiers, lacking suitable narrowband filters, exhibit a side-mode rejection ratio (SMSR) of only 25dB after amplification to high power, especially up to 10W, with noise accounting for approximately half of the signal, limiting their applications.

[0003] Existing high-power single-frequency polarization-maintaining fiber amplifiers for the 1012nm–1020nm wavelength range typically employ conventional ytterbium-doped amplifiers or a series of amplification optical paths and narrowband filters to achieve high-power laser output. However, conventional ytterbium-doped amplifiers produce a high signal-to-noise ratio when outputting laser light; when the output power reaches 10W, half of the power is not amplified signal power. Furthermore, without optimized ytterbium-doped fiber length, the spectrum is prone to self-excitation, damaging the optical path. The approach of using a series of amplification optical paths and narrowband filters requires matching a corresponding narrowband filter for each wavelength amplification, necessitating the use of multiple amplifiers for multi-wavelength amplifiers, resulting in excessively high costs. Utility Model Content

[0004] This invention provides a single-frequency polarization-maintaining narrow-linewidth fiber amplifier that reduces the cost of fiber amplifiers while outputting high-power lasers, achieving low cost, high power, high side-mode suppression ratio, high stability, low noise, and high polarization extinction ratio.

[0005] This utility model provides a single-frequency polarization-maintaining narrow-linewidth fiber amplifier, comprising: a seed laser, a preamplifier, a first-stage double-clad amplifier, and a second-stage double-clad amplifier;

[0006] The output of the seed laser is connected to the input of the preamplifier via optical fiber. The output of the preamplifier is connected to the input of the first-stage double-clad amplifier via optical fiber. The output of the first-stage double-clad amplifier and the input of the second-stage double-clad amplifier are connected via optical fiber.

[0007] Seed lasers are used to generate seed lasers with wavelengths of 1012nm-1020nm;

[0008] The preamplifier is used to amplify the seed laser in one stage to obtain a first-stage amplified laser with the first power and output it.

[0009] The first-stage double-clad amplifier is used to perform secondary amplification on the primary amplified laser, resulting in a secondary amplified laser with second power, which is then output.

[0010] The second-stage double-clad amplifier is used to perform a third-stage amplification on the second-stage amplified laser, obtaining the amplified laser with the target power and outputting it.

[0011] Optionally, the preamplifier includes a first pump source, a first wavelength division multiplexer, a first ytterbium-doped fiber, a first cladding pump stripper, and a first tilting grating;

[0012] The input of the first pump source is connected to the input of the first wavelength division multiplexer via optical fiber. The output of the seed laser is connected to the input of the first wavelength division multiplexer via optical fiber. The output of the first wavelength division multiplexer is connected to the input of the first ytterbium-doped fiber via optical fiber. The output of the first ytterbium-doped fiber is connected to the input of the first cladding pump stripper via optical fiber. The output of the first cladding pump stripper is connected to the input of the first tilted grating via optical fiber. The output of the first tilted grating is connected to the input of the first-stage double-clad amplifier via optical fiber.

[0013] Optionally, the preamplifier may also include a first optical coupler, a first photodetector, a second optical coupler, a second photodetector, and a controller;

[0014] The input of the first optical coupler is connected to the output of the seed laser via optical fiber. The first output of the first optical coupler is connected to the input of the first wavelength division multiplexer via optical fiber. The second output of the first optical coupler is connected to the input of the first photodetector via optical fiber. The input of the second optical coupler is connected to the output of the first tilted grating via optical fiber. The first output of the second optical coupler is connected to the input of the first-stage double-clad amplifier via optical fiber. The second output of the second optical coupler is connected to the input of the second photodetector via optical fiber. The outputs of the first and second photodetectors are both electrically connected to the controller. The controller is also electrically connected to the first pump source and the first-stage double-clad amplifier, respectively.

[0015] Optionally, the preamplifier may also include a first optical isolator and a second optical isolator;

[0016] The first optical isolator is located between the seed laser and the first wavelength division multiplexer, and the second optical isolator is located between the first ytterbium-doped fiber and the first cladding pump stripper.

[0017] Optionally, the length of the first ytterbium-doped fiber is 0.8m-1m.

[0018] Optionally, the first-stage double-clad amplifier includes a second pump source, a first combiner, a second ytterbium-doped fiber, a second cladding pump stripper, a third cladding pump stripper, and a second tilting grating.

[0019] The input of the second pump source is connected to the input of the first combiner via optical fiber. The output of the preamplifier is connected to the input of the first combiner via optical fiber. The output of the first combiner is connected to the input of the second ytterbium-doped fiber via optical fiber. The output of the second ytterbium-doped fiber is connected to the input of the second cladding pump stripper via optical fiber. The output of the second cladding pump stripper is connected to the input of the third cladding pump stripper via optical fiber. The output of the third cladding pump stripper is connected to the input of the second tilted grating via optical fiber. The output of the second tilted grating is connected to the input of the second-stage double-clad amplifier via optical fiber.

[0020] Optionally, the first-stage double-clad amplifier may also include a third optical isolator;

[0021] The third optical isolator is located between the second cladding pump stripper and the third cladding pump stripper.

[0022] Optionally, the first-stage double-clad amplifier may also include a third optical coupler, a third photodetector, and a controller;

[0023] The input of the third optical coupler is connected to the output of the second tilted grating via optical fiber. The first output of the third optical coupler is connected to the input of the second-stage double-clad amplifier via optical fiber. The second output of the third optical coupler is connected to the input of the third photodetector via optical fiber. The output of the third photodetector is electrically connected to the controller. The controller is also electrically connected to the second pump source and the second-stage double-clad amplifier.

[0024] Optionally, the length of the second ytterbium-doped fiber is 0.6m-0.8m.

[0025] Optionally, the second-stage double-clad amplifier includes a third pump source, a second bundler, a third ytterbium-doped fiber, and a fourth clad pump stripper.

[0026] The input of the third pump source is connected to the input of the second combiner via optical fiber. The output of the first-stage double-clad amplifier is connected to the input of the second combiner via optical fiber. The output of the second combiner is connected to the input of the third ytterbium-doped fiber via optical fiber. The output of the third ytterbium-doped fiber is connected to the input of the fourth clad pump stripper via optical fiber.

[0027] Optionally, the second-stage double-clad amplifier may also include a fourth optical isolator, a fourth optical coupler, a fourth photodetector, and a feedback circuit;

[0028] The input of the fourth optical isolator is connected to the output of the fourth cladding pump stripper via optical fiber. The output of the fourth optical isolator is connected to the input of the fourth optical coupler via optical fiber. The second output of the fourth optical coupler is connected to the input of the fourth photodetector via optical fiber. The output of the fourth photodetector is electrically connected to the feedback circuit. The feedback circuit is also electrically connected to the third pump source.

[0029] Optionally, the length of the third ytterbium-doped fiber is 0.6m-0.7m.

[0030] The technical solution of this utility model involves setting up a pre-amplifier, a first-stage double-clad amplifier, and a second-stage double-clad amplifier. The seed laser outputs a seed laser with a working wavelength of 1012nm-1020nm and a power of approximately 10mW. This seed laser is incident on the pre-amplifier, which amplifies it in one stage to obtain a first-stage amplified laser with the first power, which is then output to the first-stage double-clad amplifier. Next, the first-stage amplified laser is incident on the first-stage double-clad amplifier, which amplifies it in two stages to obtain a second-stage amplified laser with the second power, which is then output to the second-stage double-clad amplifier. Finally, a 1.5W second-stage amplified laser is incident on the second-stage double-clad amplifier, which amplifies it in three stages to obtain the amplified laser with the target power, which is then output. Thus, a 10W high-power amplified laser output is achieved through stage-by-stage amplification. Using the above structure, high-power amplified laser output in the 1012nm-1020nm range was achieved through a step-by-step amplification structure, improving the side-mode suppression ratio and polarization extinction ratio, as well as the signal-to-noise ratio and cost.

[0031] It should be understood that the description in this section is not intended to identify key or essential features of the embodiments of this utility model, nor is it intended to limit the scope of this utility model. Other features of this utility model will become readily apparent from the following description. Attached Figure Description

[0032] To more clearly illustrate the technical solutions in the embodiments of this utility model, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this utility model. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0033] Figure 1 A schematic diagram of the structure of a single-frequency polarization-maintaining narrow-linewidth fiber amplifier provided for an embodiment of this utility model;

[0034] Figure 2A spectrum of amplified laser output from a single-frequency polarization-maintaining narrow-linewidth fiber amplifier with a wavelength of 1012 nm, provided for an embodiment of this utility model;

[0035] Figure 3 A spectrum of amplified laser output from a single-frequency polarization-maintaining narrow-linewidth fiber amplifier with a wavelength of 1018 nm, provided for an embodiment of this utility model;

[0036] Figure 4 A curve showing the stability of the amplified laser output from a single-frequency polarization-maintaining narrow-linewidth fiber amplifier, provided for an embodiment of this utility model.

[0037] Figure 5 A graph showing the relationship between frequency and signal-to-noise ratio for a single-frequency polarization-maintaining narrow-linewidth fiber amplifier provided for an embodiment of this utility model. Detailed Implementation

[0038] 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. 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 protection scope of the present invention.

[0039] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this utility model are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of the utility model described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises 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 apparatus.

[0040] In one embodiment, Figure 1 This is a schematic diagram of the structure of a single-frequency polarization-maintaining narrow-linewidth fiber amplifier provided in an embodiment of the present invention. Figure 2 The spectrum of the amplified laser output by a single-frequency polarization-maintaining narrow-linewidth fiber amplifier with a wavelength of 1012 nm is provided in this embodiment of the present invention. Figure 3 The spectrum of the amplified laser output by a single-frequency polarization-maintaining narrow-linewidth fiber amplifier with a wavelength of 1018 nm, provided as an embodiment of this utility model. Figure 4A curve illustrating the stability of the amplified laser output from a single-frequency polarization-maintaining narrow-linewidth fiber amplifier, provided as an embodiment of this utility model. Figure 5 This embodiment of the invention provides a curve showing the relationship between frequency and signal-to-noise ratio for a single-frequency polarization-maintaining narrow-linewidth fiber amplifier. This embodiment is applicable to situations where high-power lasers are generated and output in the 1012-1020nm band, while simultaneously reducing costs and improving side-mode rejection ratio. Figures 1 to 5 As shown, the single-frequency polarization-maintaining narrow-linewidth fiber amplifier includes a seed laser 10, a preamplifier 20, a first-stage double-clad amplifier 30, and a second-stage double-clad amplifier 40. The output of the seed laser 10 is connected to the input of the preamplifier 20 via fiber 1, the output of the preamplifier 20 is connected to the input of the first-stage double-clad amplifier 30 via fiber 1, and the output of the first-stage double-clad amplifier 30 and the input of the second-stage double-clad amplifier 40 are connected via fiber 1. The seed laser 10 is used to generate a seed laser with a wavelength of 1012nm-1020nm. The preamplifier 20 is used to amplify the seed laser in one stage to obtain a first-stage amplified laser with a first power and output it. The first-stage double-clad amplifier 30 is used to amplify the first-stage amplified laser in two stages to obtain a second-stage amplified laser with a second power and output it. The second-stage double-clad amplifier 40 is used to amplify the second-stage amplified laser in three stages to obtain an amplified laser with the target power and output it.

[0041] The seed laser 10 is the laser that generates the initial optical signal, providing the fiber optic amplifier with the initial optical signal, which serves as the "seed" for the amplification process. The seed laser 10 can be any type of laser, such as a semiconductor laser or a solid-state laser, and its output characteristics (such as wavelength, power, and beam quality) directly affect the quality of the final output optical signal. In this embodiment, the seed laser 10 is used to generate a seed laser with a wavelength of 1012nm-1020nm. The preamplifier 20 is the first amplification stage in the fiber optic amplifier system. It amplifies the optical signal generated by the seed laser 10 to a sufficiently high power level for subsequent processing or transmission. The preamplifier 20 is typically designed with a low noise figure and high gain to ensure good quality of the optical signal during amplification. The first-stage double-clad amplifier 30 is a specially designed fiber amplifier in which the fiber has two cladding layers: an inner cladding for transmitting pump light and an outer cladding for transmitting signal light. The first-stage double-clad amplifier 30 further amplifies the optical signal output from the preamplifier 20. This design allows the pump light to propagate more efficiently within the fiber, resulting in higher gain and better beam quality. The first-stage double-clad amplifier 30 is typically designed with high gain to ensure the signal reaches the required power level. The second-stage double-clad amplifier 40 is the second amplification stage in the fiber amplifier system, used to further amplify the optical signal output from the first-stage double-clad amplifier 30 to achieve the final required output power. This stage amplifier may have higher pump power and greater gain to support higher output power and a wider power adjustment range.

[0042] Specifically, the emission wavelength of ytterbium-doped fiber is typically concentrated in the 1030nm–1080nm range. Therefore, achieving a high-power, narrow-linewidth single-frequency fiber amplifier with a laser wavelength <1020nm is very difficult. This is because in ytterbium-doped gain media, lasers with wavelengths shorter than 1020nm have low inherent gain and face strong gain competition from the amplified laser beam (ASE) with a center wavelength near 1030nm, requiring a specially designed optical path structure. Therefore, to ensure that the laser with an emission wavelength of 1012nm–1020nm ultimately outputs a high-power amplified laser, the initial laser needs to be amplified stage by stage. In this embodiment, a preamplifier 20, a first-stage double-clad amplifier 30, and a second-stage double-clad amplifier 40 are provided. When the seed laser 10 is working, it outputs a seed laser with a working wavelength of 1012nm–1020nm, and the power of this seed laser is typically around 10mW. A 10mW seed laser is incident on preamplifier 20, which amplifies the seed laser in one stage, resulting in a first-stage amplified laser with a first power of 150mW, which is then output to the first-stage double-clad amplifier 30. Next, the 150mW first-stage amplified laser is incident on the first-stage double-clad amplifier 30, which amplifies it in two stages, resulting in a second-stage amplified laser with a second power of 1.5W, which is then output to the second-stage double-clad amplifier 40. In other words, the first-stage double-clad amplifier 30 amplifies the input 150mW first-stage amplified laser into a 1.5W second-stage amplified laser. Finally, the 1.5W secondary amplified laser is incident on the second-stage double-clad amplifier 40, which performs tertiary amplification of the secondary amplified laser to obtain the amplified laser with the target power of 10W, which is then output. Thus, a 10W high-power amplified laser output is achieved through staged amplification.

[0043] Additionally, refer to Figure 2 and Figure 3 As shown, when the seed laser wavelength output by seed laser 10 is 1012 nm, the corresponding side-mode rejection ratio (SMR) is 49 dB; when the seed laser wavelength output by seed laser 10 is 1018 nm, the corresponding SMR is improved to 64 dB. (Reference) Figure 4 The x-axis represents time, and the y-axis represents output power. In this embodiment, the amplified laser output from the fiber optic amplifier ranges from a minimum of 10W to a maximum of 10.46W within one hour, exhibiting minimal fluctuation, indicating excellent stability of the amplified laser. The RMS stability is 0.08%, and the PP stability is 0.57 within one hour. (Reference) Figure 5 The horizontal axis represents frequency (Hz), and the vertical axis represents signal-to-noise ratio (RIN). Figure 5The top image is the original test image, and the bottom image is the curve obtained by integrating the signal-to-noise ratio data tested in the top image. It can be seen that the maximum signal-to-noise ratio of the fiber optic amplifier is 3.9×10^-7dBc / Hz, which greatly reduces the signal-to-noise ratio of the fiber optic amplifier.

[0044] The technical solution of this embodiment of the invention, by setting up a pre-amplifier, a first-stage double-clad amplifier, and a second-stage double-clad amplifier, allows the seed laser to output a seed laser with a working wavelength of 1012nm-1020nm and a power of approximately 10mW. The seed laser is incident on the pre-amplifier, which amplifies it in one stage to obtain a first-stage amplified laser with a first power, which is then output to the first-stage double-clad amplifier. Next, the first-stage amplified laser is incident on the first-stage double-clad amplifier, which amplifies it in two stages to obtain a second-stage amplified laser with a second power, which is then output to the second-stage double-clad amplifier. Finally, a 1.5W second-stage amplified laser is incident on the second-stage double-clad amplifier, which amplifies it in three stages to obtain the amplified laser with the target power, which is then output. Thus, a 10W high-power amplified laser output is achieved through stage-by-stage amplification. Using the above structure, high-power amplified laser output in the 1012nm-1020nm range was achieved through a step-by-step amplification structure, improving the side-mode suppression ratio and polarization extinction ratio, as well as the signal-to-noise ratio and cost.

[0045] Optionally, the preamplifier 20 includes a first pump source 21, a first wavelength division multiplexer 22, a first ytterbium-doped fiber 23, a first cladding pump stripper 24, and a first tilted grating 25; the input end of the first pump source 21 is connected to the input end of the first wavelength division multiplexer 22 via fiber 1, the output end of the seed laser 10 is connected to the input end of the first wavelength division multiplexer 22 via fiber 1, the output end of the first wavelength division multiplexer 22 is connected to the input end of the first ytterbium-doped fiber 23 via fiber 1, the output end of the first ytterbium-doped fiber 23 is connected to the input end of the first cladding pump stripper 24 via fiber 1, the output end of the first cladding pump stripper 24 is connected to the input end of the first tilted grating 25 via fiber 1, and the output end of the first tilted grating 25 is connected to the input end of the first-stage double-clad amplifier 30 via fiber 1.

[0046] The first pump source 21 is the light source that provides energy to the first ytterbium-doped fiber 23. The photons it emits pump the ytterbium ions in the first ytterbium-doped fiber 23 from the ground state to a higher energy level, achieving population inversion. The first pump source 21 is generally a high-power semiconductor laser (LD), whose main function is to convert electrical energy into optical energy. In this embodiment, the first pump source 21 is a 974nm single-mode laser with an output power of 800mW. The first wavelength division multiplexer 22 is an optical component that can simultaneously transmit multiple optical signals of different wavelengths into a single optical fiber. These optical signals are transmitted independently in the fiber without interfering with each other. Its main function is to combine optical signals of different wavelengths from multiple transmitters into a single wavelength-multiplexed optical signal for transmission through a single optical fiber. The first ytterbium-doped fiber 23 refers to fiber preforms with ytterbium doped into the core portion. 3+ Ions, Yb doped into optical fibers 3+ The purpose of ionization is to transform ordinary passive transmission optical fibers into active optical fibers with amplification capabilities. In this embodiment, the first ytterbium-doped fiber 23 has an absorption of 250 dB / m at 976 nm. The first cladding pump stripper 24 is an optical fiber device used to strip the pump light from the inner cladding of a double-clad fiber, allowing only the signal light to pass through; this design can improve the utilization efficiency of the pump light and reduce its waste. The first tilted grating 25 is a specially designed fiber grating whose grating period is tilted along the fiber axis, enabling selective reflection or transmission of optical signals at specific wavelengths. In this embodiment, the first wavelength division multiplexer 22, the first ytterbium-doped fiber 23, and the first tilted grating 25 are all fabricated using polarization-maintaining fiber. The first wavelength division multiplexer 22 operates at wavelengths of 980 nm and 1064 nm, with an output power of 2 W. The first tilted grating 25 is a wide bandwidth grating with a center wavelength of 1021nm-1050nm and a reflectivity greater than 99%. It is made of PLMA-GDF-10 / 125 optical fiber.

[0047] Specifically, during the first-stage amplification of the seed laser, both the pump light generated by the first pump source 21 and the seed laser generated by the seed laser 10 are incident on the first wavelength division multiplexer 22. The first wavelength division multiplexer 22 combines the pump light and the seed laser to generate a single laser beam, which is then output to the first ytterbium-doped fiber 23. The first ytterbium-doped fiber 23 serves as the gain medium, and under the action of the pump light, it can amplify the laser in the first stage, outputting a 150mW amplified laser beam. The amplified laser beam is incident on the first tilted grating 25. Because the grating surface of the first tilted grating 21 has a certain angle with the radial direction of the fiber 1, the stimulated amplification spontaneous emission (ASE) light in the 1021nm-1050nm range (approximately 1μm) is reflected and enters the cladding from the fiber core, no longer propagating in the fiber core. Light in other wavelength ranges does not reflect and directly passes through the first tilted grating 25 to continue propagating, achieving an effective filtering effect on the forward-propagating 1μm ASE. The first-stage amplified laser reflected onto the cladding is incident on the first cladding pump stripper 24. The first cladding pump stripper 24 can remove the 1μm ASE reflected into the cladding from the first tilted grating 25, and also remove unused pump light during laser amplification, thus ensuring the purity of the preamplifier output spectrum, which is beneficial for subsequent signal amplification. Finally, the first-stage amplified laser with ASE filtered out is output from the preamplifier 20 to the first-stage double-clad amplifier 30. Experiments show that without the first tilted grating 21, the spectral SMSR is only 45dB; with the first tilted grating 21, the spectral SMSR can be increased to 61dB, significantly improving the gain of the fiber amplifier.

[0048] Optional, continue to refer to Figure 1 The preamplifier 20 also includes a first optical coupler 26, a first photodetector 27, a second optical coupler 28, a second photodetector 29, and a controller (not shown in the figure). The input end of the first optical coupler 26 is connected to the output end of the seed laser 10 via optical fiber 1. The first output end of the first optical coupler 26 is connected to the input end of the first wavelength division multiplexer 22 via optical fiber 1. The second output end of the first optical coupler 26 is connected to the input end of the first photodetector 27 via optical fiber 1. The input end of the second optical coupler 28 is connected to the output end of the first tilted grating 25 via optical fiber 1. The first output end of the second optical coupler 28 is connected to the input end of the first-stage double-clad amplifier 30 via optical fiber 1. The second output end of the second optical coupler 28 is connected to the input end of the second photodetector 29 via optical fiber 1. The output ends of the first photodetector 27 and the second photodetector 29 are both electrically connected to the controller. The controller is also electrically connected to the first pump source 21 and the first-stage double-clad amplifier 30, respectively.

[0049] In this embodiment, both the first optical coupler (OC1) 26 and the second optical coupler (OC2) 28 are used to distribute the input optical signal to multiple output ports according to a certain ratio. In this embodiment, both the first optical coupler 26 and the second optical coupler 28 are 1:99 optical couplers, meaning that after the seed laser is incident on the first optical coupler 26, 99% of the laser signal is normally transmitted to the first wavelength division multiplexer 22, and 1% of the laser signal is transmitted to the first photodetector 27. Similarly, after the first-stage amplified laser is incident on the second optical coupler 28, 99% of the laser signal is normally transmitted to the first-stage double-clad amplifier 30, and 1% of the laser signal is transmitted to the second photodetector 29. The first photodetector 27 is used to monitor the laser intensity signal entering the preamplifier 20 to determine whether light has entered. The second photodetector 29 is used to monitor the optical power of the first-stage amplified laser output from the preamplifier 20 to determine whether the optical power output by the preamplifier 20 reaches the first power level. The controller is electrically connected to the first photodetector 27 and the second photodetector 29. It receives the light intensity signal monitored by the first photodetector 27 and the light power monitored by the second photodetector 29. When the light intensity signal is less than a preset light intensity, it indicates that there is no laser input to the preamplifier 20 or the input laser intensity is too low, posing a risk of damaging the optical path. Therefore, the controller will shut down the first pump source 21. Additionally, when the received light power is less than a first power, it indicates an abnormal amplification of the seed laser in the preamplifier 20. Therefore, the controller will shut down the first-stage double-clad amplifier 30 to prevent damage to the first-stage double-clad amplifier 30.

[0050] Optional, continue to refer to Figure 1 The preamplifier 20 also includes a first optical isolator 211 and a second optical isolator 212; the first optical isolator 211 is located between the seed laser 10 and the first wavelength division multiplexer 22, and the second optical isolator 212 is located between the first ytterbium-doped fiber 23 and the first cladding pump stripper 24.

[0051] Both the first optical isolator 211 and the second optical isolator 212 are optical devices whose core function is to allow optical signals to propagate in one direction while blocking the propagation of reverse light, i.e., they have unidirectional transmission capability. In this embodiment, the first optical isolator 211 is set between the seed laser 10 and the first wavelength division multiplexer 22, and the second optical isolator 212 is set between the first ytterbium-doped fiber 23 and the first cladding pump stripper 24. Both the first optical isolator 211 and the second optical isolator 212 are used to isolate the 1μm ASE propagating in the reverse direction of the optical path. If the second optical isolator 212 malfunctions and fails to completely isolate the ASE, the first optical isolator 211 can still isolate the reverse-propagating ASE, preventing the incompletely stripped 1μm ASE from damaging or burning the seed laser 10, thus providing double-layer protection for the seed laser 10. The first optical isolator 211 has a wavelength of 1020nm, a power of 50mW, and is formed using polarization-maintaining fiber. The second optical isolator 212 has a wavelength of 1020nm and a power of 5W, and is made of polarization-maintaining fiber.

[0052] Optional, continue to refer to Figure 1 The length of the first ytterbium-doped optical fiber 23 is 0.8m-1m.

[0053] Specifically, the preamplifier 20 amplifies the power of the seed laser to 150mW. Since the emission wavelength of the first ytterbium-doped fiber 23 is mainly concentrated in the 1030nm-1080nm range, amplifying the laser with a wavelength of 1012nm-1020nm requires a very short first ytterbium-doped fiber 23. Moreover, there will be high ASE energy in the 1021nm-1050nm band, so the length of the first ytterbium-doped fiber 23 must be shortened and the structure of the preamplification optical path 20 must be optimized. In this embodiment, the length of the first ytterbium-doped fiber 23 is 0.8m-1m, preferably 0.8m. Even with such a short length, the first-stage amplified laser in the 1021nm-1050nm range will still have ASE rise. If this wavelength energy is not processed, it will be easier to obtain high gain in subsequent amplification stages than the signal light. Therefore, in this embodiment, a first tilted grating 25 is added after the second optical isolator 212 to couple the ASE of 1021nm to 1050nm into the cladding and remove it through the first cladding pump stripper 24, thereby ensuring the purity of the preamplifier output spectrum, which is beneficial to the subsequent signal amplification.

[0054] Optional, continue to refer to Figure 1The first-stage double-clad amplifier 30 includes a second pump source 31, a first combiner 32, a second ytterbium-doped fiber 33, a second cladding pump stripper 34, a third cladding pump stripper 35, and a second tilted grating 36. The input end of the second pump source 31 is connected to the input end of the first combiner 32 via fiber 1. The output end of the preamplifier 20 is connected to the input end of the first combiner 32 via fiber 1. The output end of the first combiner 32 is connected to the input end of the second ytterbium-doped fiber 33 via fiber 1. The output end of the second ytterbium-doped fiber 33 is connected to the input end of the second cladding pump stripper 34 via fiber 1. The output end of the second cladding pump stripper 34 is connected to the input end of the third cladding pump stripper 35 via fiber 1. The output end of the third cladding pump stripper 35 is connected to the input end of the second tilted grating 36 via fiber 1. The output end of the second tilted grating 36 is connected to the input end of the second-stage double-clad amplifier 40 via fiber 1.

[0055] Optional, continue to refer to Figure 1 The length of the second ytterbium-doped fiber 33 is 0.6m-0.8m.

[0056] The second pump source 31 has the same meaning as the first pump source 21, as described above, and will not be repeated here. The first beam combiner 32 is an optical or radio frequency device used to combine the outputs of two or more signal sources into a single channel. That is, the first beam combiner 32 can combine light waves from different light sources into a single beam for transmission through the same optical fiber. The second ytterbium-doped fiber 33 has the same meaning as the first ytterbium-doped fiber 23, and will not be repeated here. The second cladding pump stripper 34 and the third cladding pump stripper 35 have the same meaning as the first cladding pump stripper 24, and will not be repeated here. The second tilted grating 36 has the same meaning as the first tilted grating 25, and will not be repeated here.

[0057] Specifically, during the secondary amplification of the primary amplified laser, both the pump light generated by the second pump source 31 and the primary amplified laser generated by the preamplifier 20 are incident on the first combiner 32. The first combiner 32 combines the pump light and the primary amplified laser to generate a single laser beam, which is then output to the second ytterbium-doped fiber 33. The second ytterbium-doped fiber 33, acting as the gain medium, can perform secondary amplification of the primary amplified laser under the action of the pump light, and outputs a secondary amplified laser with a power of 1.5W. The secondary amplified laser is incident on the second tilted grating 36. Due to the certain angle between the grating surface of the second tilted grating 36 and the radial direction of the fiber 1, the stimulated amplification spontaneous emission (ASE) light in the 1021nm-1050nm (approximately 1μm) range is reflected and enters the cladding from the fiber core, no longer propagating in the fiber core. Light in other wavelength ranges does not undergo reflection and directly passes through the second tilted grating 36 to continue propagating, achieving an effective filtering effect on the forward-propagating 1μm ASE. The secondary amplified laser reflected onto the cladding is incident on the third cladding pump stripper 35 and the second cladding pump stripper 34. The third cladding pump stripper 35 can remove the 1μm ASE reflected from the second tilted grating 36 into the cladding, and can also remove the pump light that was not used during laser amplification. If the third cladding pump stripper 35 does not remove all ASE, it can be removed a second time by the second cladding pump stripper 34, thus ensuring the purity of the secondary amplified laser output from the first-stage double-clad amplifier 30, which is beneficial for subsequent signal amplification. Finally, the secondary amplified laser with ASE filtered out is output from the first-stage double-clad amplifier 30 to the second-stage double-clad amplifier 40.

[0058] It should be noted that in the first-stage double-clad amplifier 30, the length of the second ytterbium-doped fiber 33 also needs to be optimized. Experiments show that the shorter the gain fiber length, the lower the ASE intensity in the long-wavelength band and the more it shifts towards the short-wavelength direction, which is more conducive to short-wavelength laser lasing. Conversely, the longer the gain fiber, the higher the ASE intensity in the long-wavelength band, which limits short-wavelength laser lasing. While long-wavelength lasers can achieve more gain oscillation, parasitic oscillations are very likely to occur. As the fiber length gradually increases, the ASE intensity in the long-wavelength band also continuously increases, and the peak ASE intensity shifts towards the long-wavelength direction. Therefore, to achieve short-wavelength laser oscillation, the length of the gain fiber, i.e., the length of the second ytterbium-doped fiber 33, needs to be shortened. In this embodiment, the length of the second ytterbium-doped fiber 33 is 0.6m-0.8m. Experiments have verified that the length of the second ytterbium-doped fiber 33 in this embodiment is 0.7m. Secondly, the second pump source 31 employs a 976nm multimode pump. Compared to a 915nm multimode pump, choosing a 976nm multimode pump for amplification of the second ytterbium-doped fiber 33 reduces quantum loss, thereby achieving higher amplification efficiency. Simultaneously, the absorption of 976nm by the second ytterbium-doped fiber 33 is three times that of 915nm, thus allowing for a shorter length of the second ytterbium-doped fiber 33. Thirdly, this embodiment optimizes the fiber type of the second ytterbium-doped fiber 33 by increasing its core-cladding ratio. Experiments have shown that a larger core-cladding ratio results in stronger ASE suppression capability, meaning that with the cladding size remaining constant, increasing the fiber core diameter is beneficial for ASE suppression, necessitating an appropriate increase in fiber core diameter. In this embodiment, the core-cladding parameters of the second ytterbium-doped fiber 33 are selected as 14 / 125. Furthermore, the second cladding pump stripper 34 employs a self-made fiber etching process, effectively stripping away excess 975nm pump light not absorbed by the second ytterbium-doped fiber 33, as well as a small portion of the signal light in the cladding. When the power of the second pump source 31 is 13W, the output power of the second-stage amplified laser after amplification by the first-stage double-clad amplifier 30 is 1.5W, and the spectral SMSR is greater than 56dB, which is 13dB higher than the corresponding spectral SMSR of 43dB without the second tilting grating 36.

[0059] Optional, continue to refer to Figure 1 The first-stage double-clad amplifier 30 also includes a third optical isolator 37; the third optical isolator 37 is located between the second clad pump stripper 34 and the third clad pump stripper 35.

[0060] The third optical isolator 37 has the same meaning as the first optical isolator 211 and the second optical isolator 212, and will not be described again here. In this embodiment, a third optical isolator 37 is provided between the second cladding pump stripper 34 and the third cladding pump stripper 35. The third optical isolator 37 is used to isolate the 1μm ASE traveling in the reverse direction along the optical path, preventing the seed laser 10 from being damaged or burned by the incompletely stripped 1μm ASE, thus protecting the seed laser 10. The third optical isolator 37 has a wavelength of 1020nm, a power of 5W, and is formed using polarization-maintaining fiber.

[0061] Optional, continue to refer to Figure 1 The first-stage double-clad amplifier 30 also includes a third optical coupler 38, a third photodetector 39, and a controller (not shown in the figure); the input terminal of the third optical coupler 38 is connected to the output terminal of the second tilted grating 36 via optical fiber 1, the first output terminal of the third optical coupler 38 is connected to the input terminal of the second-stage double-clad amplifier 40 via optical fiber 1, and the second output terminal of the third optical coupler 38 is connected to the input terminal of the third photodetector 39 via optical fiber 1; the output terminal of the third photodetector 39 is electrically connected to the controller, and the controller is also electrically connected to the second pump source 31 and the second-stage double-clad amplifier 40 respectively.

[0062] In this embodiment, the third optical coupler 38 has the same meaning as the first optical coupler 26 and the second optical coupler 28, and will not be described again here. In this embodiment, the third optical coupler 38 is a 0.1:99.9 optical coupler, meaning that after the secondary amplified laser is incident on the third optical coupler 38, 99.9% of the laser signal is normally transmitted to the second-stage double-clad amplifier 40, and 0.1% of the laser signal is transmitted to the third photodetector 39. The third photodetector 39 is used to monitor the optical power of the secondary amplified laser to determine whether the optical power of the secondary amplified laser has reached the second power. The controller is electrically connected to the third photodetector 39 and is used to receive the optical power monitored by the third photodetector 39. When the received optical power is less than the second power, it indicates that the amplification power of the first-stage double-clad amplifier 30 for the primary amplified laser is abnormal. Therefore, the controller will control the second-stage double-clad amplifier 40 to shut down to avoid damage to the second-stage double-clad amplifier 40. Furthermore, when the controller controls the first-stage double-clad amplifier 30 to shut down, it essentially controls the second pump source 31 to shut down, thereby stopping the output of pump light.

[0063] Optional, continue to refer to Figure 1The second-stage double-clad amplifier 40 includes a third pump source 41, a second combiner 42, a third ytterbium-doped fiber 43, and a fourth cladding pump stripper 44. The input end of the third pump source 41 is connected to the input end of the second combiner 42 via optical fiber 1. The output end of the first-stage double-clad amplifier 30 is connected to the input end of the second combiner 42 via optical fiber 1. The output end of the second combiner 42 is connected to the input end of the third ytterbium-doped fiber 43 via optical fiber 1. The output end of the third ytterbium-doped fiber 43 is connected to the input end of the fourth cladding pump stripper 44 via optical fiber 1.

[0064] Optional, continue to refer to Figure 1 The length of the third ytterbium-doped fiber 43 is 0.6m-0.7m.

[0065] The meaning of the third pump source 41 is the same as that of the second pump source 31 and the first pump source 21, as described above, and will not be repeated here. The meaning of the second combiner 42 is the same as that of the first combiner 32, and will not be repeated here. The meaning of the third ytterbium-doped fiber 43 is the same as that of the second ytterbium-doped fiber 33, and will not be repeated here. The meaning of the fourth cladding pump stripper 44 is the same as that of the third cladding pump stripper 35, and will not be repeated here.

[0066] Specifically, during the third-stage amplification of the second-stage amplified laser, both the pump light generated by the third pump source 41 and the second-stage amplified laser are incident on the second beam combiner 42. The second beam combiner 42 combines the pump light and the second-stage amplified laser to generate a single laser beam, which is then output to the third ytterbium-doped fiber 43. The third ytterbium-doped fiber 43 serves as the gain medium and, under the action of the pump light, can perform third-stage amplification of the second-stage amplified laser, outputting a 10W amplified laser beam after the third-stage amplification.

[0067] It should be noted that in the second-stage double-clad amplifier 40, since the pre-stage injection power is 1.5W and the target output power of the second-stage double-clad amplifier 40 is 10W, the gain of this stage amplifier is less than 10dB. Therefore, only the length of the third ytterbium-doped fiber 43 needs to be optimized, and there is no need to set a tilting grating. By continuously shortening the length of the third ytterbium-doped fiber 43, experiments have verified that when the length of the third ytterbium-doped fiber 43 is 0.6m-0.7m, the efficiency and output spectrum of the second-stage double-clad amplifier 40 are optimal when the length of the third ytterbium-doped fiber 43 is preferably 0.6m. This is because when the length of the third ytterbium-doped fiber 43 is too short, the efficiency of the second-stage double-clad amplifier 40 drops significantly, and the output spectrum SMSR is improved by less than 1dB; when the length of the third ytterbium-doped fiber 43 is too long, the efficiency of the second-stage double-clad amplifier 40 will improve, but the output spectrum SMSR will decrease. Furthermore, since the third ytterbium-doped fiber 43 is only 0.6m long, a large amount of pump light remains unabsorbed. This unabsorbed pump light needs to be removed by the fourth cladding pump stripper 44. Similar to the second cladding pump stripper 34, the fourth cladding pump stripper 44 employs a self-made fiber etching process, effectively removing the excess 976nm pump light not absorbed by the third ytterbium-doped fiber 43, as well as a small portion of the signal light in the cladding. It should be noted that when the output power of the third pump source 41 is 30W, the final output power of the amplified laser from the second-stage double-clad amplifier 40 can reach 10W, with a spectral SMSR > 49dB.

[0068] Optional, continue to refer to Figure 1 The second-stage double-clad amplifier 40 also includes a fourth optical isolator 45, a fourth optical coupler 46, a fourth photodetector 47, and a feedback circuit (not shown in the figure). The input terminal of the fourth optical isolator 45 is connected to the output terminal of the fourth cladding pump stripper 44 via optical fiber 1. The output terminal of the fourth optical isolator 45 is connected to the input terminal of the fourth optical coupler 46 via optical fiber 1. The second output terminal of the fourth optical coupler 46 is connected to the input terminal of the fourth photodetector 47 via optical fiber 1. The output terminal of the fourth photodetector 47 is electrically connected to the feedback circuit. The feedback circuit is also electrically connected to the third pump source 41.

[0069] The fourth optical isolator 45 has the same meaning as the third optical isolator 37, and will not be described again here. The fourth optical coupler 46 has the same meaning as the third optical coupler 38, and will not be described again here. The fourth photodetector 47 has the same meaning as the third photodetector 39, and will not be described again here. The feedback circuit is used to monitor the output power of the amplified laser and fine-tune the output power of the third pump source 41 according to the output power to ensure that the final output power of the fiber amplifier remains at a stable value.

[0070] Specifically, the fourth optical coupler 46 is a 0.1:99.9 optical coupler, meaning that after the amplified laser is incident on the fourth optical coupler 46, 99.9% of the amplified laser signal is output normally, while 0.1% of the amplified laser signal is transmitted to the fourth photodetector 47. The fourth photodetector 47 is used to monitor the optical power of the amplified laser to determine whether the optical power of the amplified laser reaches the target power of 10W. The feedback circuit is electrically connected to the fourth photodetector 47 and is used to receive the optical power of the amplified laser monitored by the fourth photodetector 47. When the received optical power is not the target power, it indicates that the stability of the fiber amplifier is not good. Then, the output power of the third pump source 41 is controlled and adjusted to ensure that the final output power of the fiber amplifier remains at a stable value. Through the APC (constant power output) function of the feedback circuit, the stability of the output power of the fiber amplifier is greatly improved. Furthermore, a fourth optical isolator 45 is provided between the second cladding pump stripper 34 and the third cladding pump stripper 35. This fourth optical isolator 45 isolates the 1μm ASE (associated ester) propagating along the optical path, preventing incomplete stripping of the 1μm ASE from damaging or burning the fiber amplifier, thus providing protection. The fourth optical isolator 45 has a wavelength of 1020nm, a power of 10W, and is fabricated using polarization-maintaining fiber.

[0071] It should be understood that the various forms of the process shown above can be used to rearrange, add, or delete steps. For example, the steps described in this utility model can be executed in parallel, sequentially, or in different orders, as long as the desired result of the technical solution of this utility model can be achieved, and this is not limited herein.

[0072] The specific embodiments described above do not constitute a limitation on the scope of protection of this utility model. Those skilled in the art should understand that various modifications, combinations, sub-combinations, and substitutions can be made according to design requirements and other factors. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this utility model should be included within the scope of protection of this utility model.

Claims

1. A single frequency polarization maintaining narrow line width fiber amplifier, characterized by, include: Seed laser, preamplifier, first-stage double-clad amplifier, and second-stage double-clad amplifier; The output of the seed laser is connected to the input of the preamplifier via an optical fiber, the output of the preamplifier is connected to the input of the first-stage double-clad amplifier via the optical fiber, and the output of the first-stage double-clad amplifier and the input of the second-stage double-clad amplifier are connected via the optical fiber. The seed laser is used to generate a seed laser with a wavelength of 1012nm-1020nm; The preamplifier is used to amplify the seed laser in one stage to obtain a first-stage amplified laser with a first power and output it. The first-stage double-clad amplifier is used to perform secondary amplification on the first-stage amplified laser to obtain a second-stage amplified laser with second power and output it. The second-stage double-clad amplifier is used to perform a third-stage amplification on the second-stage amplified laser to obtain the amplified laser with the target power and output it.

2. The fiber amplifier of claim 1, wherein, The preamplifier includes a first pump source, a first wavelength division multiplexer, a first ytterbium-doped fiber, a first cladding pump stripper, and a first tilting grating; The input end of the first pump source is connected to the input end of the first wavelength division multiplexer via the optical fiber. The output end of the seed laser is connected to the input end of the first wavelength division multiplexer via the optical fiber. The output end of the first wavelength division multiplexer is connected to the input end of the first ytterbium-doped fiber via the optical fiber. The output end of the first ytterbium-doped fiber is connected to the input end of the first cladding pump stripper via the optical fiber. The output end of the first cladding pump stripper is connected to the input end of the first tilted grating via the optical fiber. The output end of the first tilted grating is connected to the input end of the first-stage double-clad amplifier via the optical fiber.

3. The fiber amplifier of claim 2, wherein, The preamplifier also includes a first optical coupler, a first photodetector, a second optical coupler, a second photodetector, and a controller; The input terminal of the first optical coupler is connected to the output terminal of the seed laser via the optical fiber; the first output terminal of the first optical coupler is connected to the input terminal of the first wavelength division multiplexer via the optical fiber; the second output terminal of the first optical coupler is connected to the input terminal of the first photodetector via the optical fiber; the input terminal of the second optical coupler is connected to the output terminal of the first tilted grating via the optical fiber; the first output terminal of the second optical coupler is connected to the input terminal of the first-stage double-clad amplifier via the optical fiber; the second output terminal of the second optical coupler is connected to the input terminal of the second photodetector via the optical fiber; the output terminals of the first and second photodetectors are both electrically connected to the controller; the controller is also electrically connected to the first pump source and the first-stage double-clad amplifier, respectively.

4. The fiber amplifier of claim 2, wherein, The preamplifier also includes a first optical isolator and a second optical isolator; The first optical isolator is located between the seed laser and the first wavelength division multiplexer, and the second optical isolator is located between the first ytterbium-doped fiber and the first cladding pump stripper.

5. The fiber amplifier of claim 2, wherein, The length of the first ytterbium-doped optical fiber is 0.8m-1m.

6. The fiber amplifier of claim 1, wherein, The first-stage double-clad amplifier includes a second pump source, a first combiner, a second ytterbium-doped fiber, a second cladding pump stripper, a third cladding pump stripper, and a second tilting grating; The input end of the second pump source is connected to the input end of the first combiner via the optical fiber. The output end of the preamplifier is connected to the input end of the first combiner via the optical fiber. The output end of the first combiner is connected to the input end of the second ytterbium-doped fiber via the optical fiber. The output end of the second ytterbium-doped fiber is connected to the input end of the second cladding pump stripper via the optical fiber. The output end of the second cladding pump stripper is connected to the input end of the third cladding pump stripper via the optical fiber. The output end of the third cladding pump stripper is connected to the input end of the second tilted grating via the optical fiber. The output end of the second tilted grating is connected to the input end of the second-stage double-clad amplifier via the optical fiber.

7. The fiber amplifier of claim 6, wherein, The first-stage double-clad amplifier also includes a third optical isolator; The third optical isolator is located between the second cladding pump stripper and the third cladding pump stripper.

8. The fiber amplifier of claim 6, wherein, The first-stage double-clad amplifier also includes a third optical coupler, a third photodetector, and a controller; The input terminal of the third optical coupler is connected to the output terminal of the second tilted grating via the optical fiber; the first output terminal of the third optical coupler is connected to the input terminal of the second-stage double-clad amplifier via the optical fiber; the second output terminal of the third optical coupler is connected to the input terminal of the third photodetector via the optical fiber; the output terminal of the third photodetector is electrically connected to the controller; the controller is also electrically connected to the second pump source and the second-stage double-clad amplifier, respectively.

9. The fiber amplifier of claim 6, wherein, The length of the second ytterbium-doped optical fiber is 0.6m-0.8m.

10. The fiber amplifier of claim 1, wherein, The second-stage double-clad amplifier includes a third pump source, a second bundler, a third ytterbium-doped fiber, and a fourth clad pump stripper; The input end of the third pump source is connected to the input end of the second combiner via the optical fiber. The output end of the first-stage double-clad amplifier is connected to the input end of the second combiner via the optical fiber. The output end of the second combiner is connected to the input end of the third ytterbium-doped fiber via the optical fiber. The output end of the third ytterbium-doped fiber is connected to the input end of the fourth clad pump stripper via the optical fiber.

11. The fiber amplifier of claim 10, wherein, The second-stage double-clad amplifier also includes a fourth optical isolator, a fourth optical coupler, a fourth photodetector, and a feedback circuit; The input terminal of the fourth optical isolator is connected to the output terminal of the fourth cladding pump stripper via the optical fiber. The output terminal of the fourth optical isolator is connected to the input terminal of the fourth optical coupler via the optical fiber. The second output terminal of the fourth optical coupler is connected to the input terminal of the fourth photodetector via the optical fiber. The output terminal of the fourth photodetector is electrically connected to the feedback circuit. The feedback circuit is also electrically connected to the third pump source.

12. The fiber amplifier of claim 10, wherein, The length of the third ytterbium-doped optical fiber is 0.6m-0.7m.

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