Coherent combining fiber and all-fiber system based on 45-degree tilted gratings

Abstract

This invention discloses a coherent combining all-fiber system based on 45-degree tilted gratings, comprising N optical fibers corresponding to N sub-beam lasers. Each of the N fibers includes multiple pairs of 45-degree tilted gratings for polarization reflection or combining of n-level pairwise fiber lasers. The first-level reflection or combining comprises N / 2 pairs of 45-degree tilted gratings with identical axial positions and radial symmetry along the x-axis; the second-level reflection or combining comprises N / 4 pairs of 45-degree tilted gratings with identical axial positions and radial symmetry along the y-axis; and so on up to the n-level combining, which comprises N / 2n pairs of 45-degree tilted gratings with identical axial positions. The invention also discloses a corresponding system and method. It solves the problems of dynamic tilt jitter in the spatial optical path of coherent combining, inconsistent collimation size of the sub-beams, coaxiality errors, and decreased combining efficiency. It is easy to build and maintain, has high integration and small size and weight, and exhibits good scalability and robustness.

Description

Technical Field

[0001] This invention belongs to the field of fiber lasers, specifically relating to a coherent synthesizing all-fiber system based on a 45-degree tilted grating. Background Technology

[0002] In the field of high-power laser combining technology research, co-aperture polarization coherent combining is one of the effective means to obtain higher brightness laser output. Currently, co-aperture polarization coherent combining technology is mainly realized through spatial optical paths. The polarization coherent combining elements mainly include fiber collimators, half-wave plates, polarization combiners, and reflecting mirrors. In engineering practice, it is necessary to solve the problems of reduced combining efficiency caused by dynamic tilt jitter of the coherent combining spatial optical path, inconsistent collimation size of sub-beams, coaxiality errors, higher-order aberrations of combining elements, and thermal lensing effects. Moreover, as coherent combining systems develop towards higher power and larger array elements, the above efficiency loss problems will become more significant. In addition, the high peak power density at the end of the combining link places extremely high demands on the damage resistance of combining elements. The increasing number of combining elements will also significantly increase the complexity of the spatial optical path construction, adjustment, and maintenance of the coherent combining system, as well as the size and weight of the spatial optical path. All of these factors greatly limit the application potential of spatial co-aperture polarization coherent combining systems. Summary of the Invention

[0003] To address the aforementioned issues, this invention proposes an all-fiber coherent combining system based on a 45-degree tilted grating. By integrating the spatial coherent combining system into a single special optical fiber, the size of the coherent combining system is significantly reduced, resulting in a simple, compact, and stable system that is easy to build and maintain, and offers excellent scalability in terms of the number of paths.

[0004] To address the aforementioned deficiencies or improvement needs of existing technologies, this invention specifically relates to a coherent combining fiber based on a 45-degree tilted grating, comprising N optical fibers corresponding to N (high-power) sub-beam lasers. Among the N optical fibers are multiple pairs of 45-degree tilted grating groups that realize polarization reflection or combining of n-level pairwise fiber lasers. The first-level reflection or combining comprises N / 2 pairs of 45-degree tilted grating groups with identical axial positions and radial symmetry along the x-axis; the second-level reflection or combining comprises N / 4 pairs of 45-degree tilted grating groups with identical axial positions and radial symmetry along the y-axis; the third-level reflection or combining comprises N / 6 pairs of 45-degree tilted grating groups with identical axial positions and radial symmetry along the y-axis; and so on up to the n-level combining, which comprises N / 2n pairs of 45-degree tilted grating groups with identical axial positions.

[0005] Furthermore, when n is odd, the 45-degree tilted grating groups have the same axial position and are radially symmetrical along the x-axis; when n is even, the 45-degree tilted grating groups have the same axial position and are radially symmetrical along the y-axis.

[0006] Furthermore, the N-beam laser employs a two-stage cascaded coherent combining process. The phase control system locks the phase difference between each beam to an integer multiple of π. Simultaneously, the polarization control system enables one p-beam to achieve high transmission in a pair of tilted gratings, while the other s-beam is reflected to another tilted grating in the tilted grating group to achieve coherent combining. These beams are then transmitted to the next stage. This process is repeated to achieve polarization combining of the subsequent two-by-two fiber laser beams.

[0007] As another aspect of the present invention, a coherent synthesizing all-fiber system based on a 45-degree tilted grating is also disclosed, comprising a polarization-maintaining seed source, a polarization control component, a phase control component, a power amplification link component, a coherent synthesizing fiber based on a 45-degree tilted grating, a high lens, a photodetector, a feedback control component, and a power meter.

[0008] The fiber laser output from the polarization-maintaining seed source is split into N sub-beams, which are then sequentially passed through a polarization control component and a phase control component, and subsequently amplified by a power amplification link to obtain high-power, narrow-linewidth, and high-beam-quality sub-beam lasers. These N high-power sub-beam lasers are then combined and integrated into a single fiber. This special fiber contains several pairs of 45-degree tilted gratings, enabling step-by-step polarization reflection or beam combining of the fiber lasers. The combined power output from the target combining fiber at the end of the fiber is coupled out through an end cap and passes through a high-lens optical power meter for power measurement. The low-power reflected light from the high-lens optical power meter is then incident on a photodetector. The output of the photodetector is connected to a controller loaded with an optimization algorithm. The photodetector converts the received optical signal into an electrical signal, and then uses the peak power of the electrical signal as the evaluation function of the controller. The controller loaded with the optimization algorithm includes two functions: polarization control and phase control. The processed polarization control voltage is applied to the polarization controller to regulate the polarization state of the laser sub-beam, and the processed phase control voltage is applied to the phase controller to regulate the phase of the laser sub-beam, forming a closed-loop polarization and phase control system. The ultimate goal of the entire system optimization is to stabilize the laser energy received by the photodetector near its maximum value.

[0009] Furthermore, the polarization-maintaining seed source can be implemented in any way, with no limit on linewidth, center wavelength, or time-domain characteristics.

[0010] Furthermore, the piezoelectric-based active polarization controller converts the incident linearly polarized light into any desired polarization state, and its types include azimuth polarization controllers or delay polarization controllers.

[0011] Furthermore, the power amplification link component enables multi-stage power amplification of the incident laser.

[0012] Furthermore, the feedback control component is loaded with an optimization algorithm, including a machine-parallel gradient descent algorithm, a hill-climbing method, or a genetic algorithm.

[0013] As another aspect of the present invention, it also relates to an all-fiber coherent combining method based on 45-degree tilted gratings, which realizes the polarization reflection or beam combining of two fiber lasers in stages through multiple pairs of 45-degree tilted grating groups; and realizes coherent combining of fiber lasers with an all-fiber structure by controlling the polarization state and phase of the laser sub-beams.

[0014] In summary, compared with the prior art, the above-described technical solutions conceived by this invention can achieve the following beneficial effects:

[0015] (1) The present invention relates to a coherent fiber combining system and method based on a 45-degree tilted grating. The coherent fiber combining system based on the 45-degree tilted grating is formed by combining and pulling the output pigtails of the power amplification link, and includes several pairs of 45-degree tilted grating groups, which can realize the polarization reflection or beam combining of two fiber lasers step by step. By controlling the polarization state and phase of the laser sub-beams through a detection feedback control system, coherent combining of fiber lasers with an all-fiber structure can be realized.

[0016] (2) The coherent synthesis all-fiber system and method based on a 45-degree tilted grating of the present invention avoids the problems of reduced synthesis efficiency caused by dynamic tilt jitter of the spatial optical path, inconsistent collimation size of sub-beams, coaxiality error, higher-order aberrations of synthesizing elements and thermal lensing effect compared with the traditional spatial polarization coherent synthesis system.

[0017] (3) The coherent synthesis all-fiber system and method based on 45-degree tilted grating of the present invention are easier to build and maintain, have high integration and small size and weight, and have good scalability and robustness compared with the traditional spatial polarization coherent synthesis system.

[0018] (4) The coherent synthesis all-fiber, system and method based on 45-degree tilted grating of the present invention has wavelength and time domain universality, and the polarization-maintaining seed source used in the invention is not limited. Attached Figure Description

[0019] Figure 1 A schematic diagram of a preferred embodiment of the coherent combining fiber of a 45-degree tilted grating according to the present invention;

[0020] Figure 2 This is a schematic diagram of an embodiment of a four-channel high-power sub-bundle coherent combining system based on a 45-degree tilted grating coherent combining fiber, which is a preferred embodiment of the present invention.

[0021] In all the accompanying drawings, the same reference numerals indicate the same technical features, specifically: 1-coherent synthesizing fiber, 2-polarization-maintaining seed source, 3-polarization control component, 4-phase control component, 5-power amplification link component, 6-high lens, 7-photodetector, 8-feedback control component, 9-power meter. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0023] Given the excellent polarization characteristics and engineering application potential of 45-degree tilted gratings, if a highly integrated set of several pairs of 45-degree tilted gratings is used in a fiber coherent combining system to achieve multi-level polarization coherent combining, replacing the complex spatial coherent combining system, the size of the coherent combining system will be greatly reduced, making the system simple, compact, and stable, easy to build and maintain, and with good scalability. Through further optimization design to reduce energy loss and thermal effects in the coherent combining fiber, it is expected to achieve high-efficiency laser output at higher power levels.

[0024] Example 1

[0025] The core component of this invention is a coherent combining fiber based on a 45-degree tilted grating. N high-power sub-beam lasers are integrated into a single fiber by combining and pulling the beams together. This special fiber contains three pairs of 45-degree tilted grating groups, which can realize the polarization reflection or beam combining of two pairs of fiber lasers in the second stage.

[0026] A coherent combining fiber based on a 45-degree tilted grating includes N fibers corresponding to N high-power sub-beam lasers. Each of the N fibers includes multiple pairs of 45-degree tilted gratings that realize polarization reflection or combining of n-level pairwise fiber lasers. The first-level reflection or combining comprises N / 2 pairs of 45-degree tilted gratings with identical axial positions and radial symmetry along the x-axis; the second-level reflection or combining comprises N / 4 pairs of 45-degree tilted gratings with identical axial positions and radial symmetry along the y-axis; the third-level reflection or combining comprises N / 6 pairs of 45-degree tilted gratings with identical axial positions and radial symmetry along the y-axis; and so on up to the n-level combining, which comprises N / 2n pairs of 45-degree tilted gratings with identical axial positions.

[0027] When n is odd, the 45-degree tilted grating groups have the same axial position and are radially symmetrical along the x-axis; when n is even, the 45-degree tilted grating groups have the same axial position and are radially symmetrical along the y-axis.

[0028] The N-channel sub-beam laser employs a two-stage cascaded coherent combining system. The phase control system locks the phase difference between each beam to an integer multiple of π. Simultaneously, the polarization control system enables one p-beam to achieve high transmission in a pair of tilted gratings, while the other s-beam is reflected to another tilted grating in the same group, achieving coherent combining. These beams are then transmitted to the next stage. This process is repeated to achieve polarization combining of the subsequent two-by-two fiber laser beams.

[0029] The following detailed explanation uses a four-channel high-power sub-bundle coherent combining fiber based on a 45-degree tilted grating as an example. Its structural schematic is shown below. Figure 1 As shown, sub-bundles A1, A2, A3, and A4 correspond to fiber 1-1, fiber 1-2, fiber 1-3, and fiber 1-4 in the coherent combining fiber 1 based on a 45-degree tilted grating, respectively. They are respectively engraved with first-order coherent combining 45-degree tilted grating groups 1-1-1, 1-2-1, 1-3-1, and 1-4-1. The 45-degree tilted grating groups 1-1-1 and 1-3-1 are in the same position along the fiber axis and in the same position along the radial y-direction. The 45-degree tilted grating groups 1-2-1 and 1-4-1 are symmetrical along the x-axis in the radial x-direction of the fiber. They are in the same position in the fiber axial direction, in the same position in the fiber radial y-direction, and symmetrical along the x-axis in the fiber radial x-direction. Sub-bundles 1 and 3 are respectively engraved with second-order coherent combining 45-degree tilted grating groups 1-1-2 and 1-3-2. The grating groups are in the same position in the fiber axial direction, in the same position in the fiber radial x-direction, and symmetrical along the y-axis in the fiber radial y-direction. These tilted grating groups can realize the polarization reflection or beam combining of fiber lasers.

[0030] A schematic diagram of the coherent combining system based on a 45-degree tilted grating is shown below. Figure 2As shown, the system includes a seed source 1, a polarization control component 3, a phase control component 4, a power amplification link component 5, a coherent combining fiber 1 based on a 45-degree tilted grating, a high lens 6, a photodetector 7, a feedback control component 8, and a power meter 9. The fiber laser output from the seed source 1 is split into four sub-beams, which are then sequentially passed through the polarization control component 3 and the phase control component 4, and subsequently amplified by the power amplification link 5 to obtain high-power, narrow-linewidth, and high-beam-quality sub-beam lasers. These four high-power sub-beam lasers are then combined and integrated into the aforementioned coherent combining fiber 1 with the 45-degree tilted grating. The combined power output from the target combining fiber at the end of the coherent combining fiber 1 enters the optical power meter 9 through the high lens 6 for power testing. The low-power reflected light from the high lens 6 is incident on the photodetector 7. The output of the photodetector 7 is connected to the controller 8 loaded with the optimization algorithm. The optical signal received by the photodetector 7 contains polarization and phase information. After converting the optical signal into an electrical signal, it is fed back to the controller 8 loaded with the optimization algorithm. The controller 8 loaded with the optimization algorithm has two functions: polarization control and phase control. The processed polarization control voltage is applied to the polarization controller 3 to regulate the polarization state of the laser sub-beam, and the processed phase control voltage is applied to the phase controller 4 to regulate the phase of the laser sub-beam, forming a closed-loop polarization and phase control system.

[0031] The four sub-beams employ a two-stage cascaded coherent combining process. A phase control system locks the phase difference between each beam to an integer multiple of π. Simultaneously, a polarization control system ensures that beam 1 entering fiber 1 is p-ray with high transmission at point 1-1-1, while beam 2 entering fiber 2 is s-ray reflected at point 1-2-1 to the 45-degree tilted grating at point 1-1-1. Beams 1 and 2 are then coherently combined at this grating. Similarly, beam 3 entering fiber 3 is p-ray with high transmission at point 1-3-1, and beam 4 entering fiber 4 is s-ray reflected at point 1-4-1 to the 45-degree tilted grating at point 1-3-1. Beams 3 and 4 are then coherently combined at this grating. Thus, beams 1 and 2, and beams 3 and 4, complete the first stage of coherent combining and are then transmitted to the next stage via fiber 1 and fiber 3, respectively. The phase difference between the second-order combined beams is locked to an integer multiple of π by the phase control system. At the same time, the polarization control system enables the first beam to be p-light with high transmission at position 1-1-2, and the third beam to be s-light reflected at position 1-3-2 to the 45-degree tilted grating at position 1-1-2. The first beam and the third beam are coherently combined at the 45-degree tilted grating at position 1-1-2 and transmitted through the first optical fiber.

[0032] Example 2

[0033] After the coherent combining system of the M groups in Example 1 is built, the output fibers of the M groups of coherent combining systems can be integrated using a single coherent combining fiber based on a 45-degree tilted grating, as described in Example 1. This enables rapid combining and separating of high-power output modules, facilitates efficient expansion of combining modules, and allows for flexible application in practical applications.

[0034] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A 45-degree tilted grating based coherent combining optical fiber, characterized in that, Including those corresponding to N-channel sub-beam lasers N optical fibers, each containing multiple pairs of 45-degree tilted gratings for polarization reflection or combining n-level pairwise fiber lasers; the first-level reflection or combining consists of N / 2 pairs of 45-degree tilted gratings with the same axial position and radial symmetry along the x-axis; the second-level reflection or combining consists of N / 4 pairs of 45-degree tilted gratings with the same axial position and radial symmetry along the y-axis; the third-level reflection or combining consists of N / 6 pairs of 45-degree tilted gratings with the same axial position and radial symmetry along the y-axis; and so on up to the n-level combining which consists of N / 2n pairs of 45-degree tilted gratings with the same axial position, where N is a multiple of 12.

2. The 45-degree tilted grating-based coherent combining fiber of claim 1, wherein, When n is odd, the 45-degree tilted grating groups have the same axial position and are radially symmetrical along the x-axis; when n is even, the 45-degree tilted grating groups have the same axial position and are radially symmetrical along the y-axis.

3. The 45-degree tilted grating-based coherent combining fiber of claim 2, wherein, The N-channel sub-beam laser employs a two-stage cascaded coherent combining system. The phase control system locks the phase difference between each beam to an integer multiple of π. Simultaneously, the polarization control system enables one p-beam to achieve high transmission in a pair of tilted gratings, while the other s-beam is reflected to another tilted grating in the same group, achieving coherent combining. These beams are then transmitted to the next stage. This process is repeated to achieve polarization combining of the subsequent two-by-two fiber laser beams.

4. The 45-degree tilted grating-based coherent combining optical fiber coherent combining all-fiber system of claim 1, wherein, include: Polarization-maintaining seed source, polarization control component, phase control component, power amplification link component, coherent synthesizing all-fiber based on 45-degree tilted grating, high lens, photodetector, feedback control component and power meter; The fiber laser output from the polarization-maintaining seed source is split into N sub-beams, which are then sequentially passed through a polarization control component and a phase control component, and subsequently amplified by a power amplification link to obtain high-power, narrow-linewidth, and high-beam-quality sub-beam lasers. These N high-power sub-beam lasers are then combined and integrated into a single fiber. This special fiber contains several pairs of 45-degree tilted gratings, achieving polarization reflection or beam combining of the fiber lasers at each stage. The combined power output from the target combining fiber at the end of the fiber is coupled out through an end cap, passes through a high-lens optical sensor, and enters an optical power meter for power testing. The low-power reflected light from the high-lens optical sensor is incident on a photodetector. The output of the photodetector is connected to a controller loaded with an optimization algorithm. The photodetector converts the received optical signal into an electrical signal, and then uses the peak power of the electrical signal as the evaluation function of the controller. The controller loaded with the optimization algorithm includes two functions: polarization control and phase control. The processed polarization control voltage is applied to the polarization controller to regulate the polarization state of the laser sub-beam, and the processed phase control voltage is applied to the phase controller to regulate the phase of the laser sub-beam, forming a closed-loop polarization and phase control system. The ultimate goal of the entire system optimization is to stabilize the laser energy received by the photodetector near its maximum value.

5. The 45-degree tilted grating-based coherent combining fiber-optical coherent combining all-fiber system of claim 4, wherein, The polarization-preserving seed source can be implemented in any way, with no limit on linewidth, center wavelength, or time-domain characteristics.

6. The 45-degree tilted grating based coherent combining fiber system of claim 4, wherein: The polarization control component converts incident linearly polarized light into any desired polarization state, and its types include azimuth polarization controllers or delay polarization controllers.

7. The 45-degree tilted grating-based coherent combining fiber-optical coherent combining all-fiber system of claim 4, wherein, The power amplification link component enables multi-stage power amplification of the incident laser.

8. A 45-degree tilted grating based coherent combining fiber system according to any of claims 4-7, characterized in that: The feedback control component is loaded with an optimization algorithm, which includes a machine-parallel gradient descent algorithm, a hill-climbing method, or a genetic algorithm.

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