Linear polarization narrow linewidth fiber laser based on fast and slow axis orthogonal grating

Abstract

The invention discloses a linear polarization narrow linewidth fiber laser based on fast and slow axis orthogonal gratings, which comprises two polarization-maintaining cladding mode stripping devices, a polarization-maintaining pumping beam combiner, an output head, a polarization-maintaining gain fiber, a polarization-maintaining high-reflectivity grating and a polarization-maintaining low-reflectivity grating, wherein the etching grating surfaces of the polarization-maintaining high-reflectivity grating and the polarization-maintaining low-reflectivity grating are vertically orthogonal; each of the polarization-maintaining high-reflectivity grating and the polarization-maintaining low-reflectivity grating comprises a polarization-maintaining optical fiber and a grating body, the polarization-maintaining optical fiber comprises two stress rods, a fiber core and a cladding, the fiber core is located between the two stress rods, the cladding is used for wrapping the stress rods and the fiber core, and the grating body is arranged in the fiber core. In the invention, due to the application of the orthogonal grating and optical fiber vertical welding mode, the polarization mode of vertical welding is suppressed due to large loss and loss of advantages in competition, otherwise, the laser signal in the other polarization direction obtains advantages in competition and is continuously subjected to gain amplification, so that the laser signal obtains a relatively high linear polarization degree, and the polarization extinction ratio is improved.

Description

Technical Field

[0001] This invention relates to the field of fiber laser technology, and in particular to a linearly polarized narrow-linewidth fiber laser based on a fast-slow axis orthogonal grating. Background Technology

[0002] High-power linearly polarized lasers have significant applications in coherent combining, nonlinear frequency conversion, and gravitational wave detection. Coherent combining is an effective way to overcome the power limitations of single-fiber lasers, increase the output power of fiber lasers, and maintain high beam quality. It achieves coherent combining of multiple lasers by controlling the relative phase of each linearly polarized laser. A stable polarization state is essential for good combining efficiency, and increasing the output power of the single laser used for combining is a crucial means to further improve the combining power. Similarly, in nonlinear frequency conversion, the polarization state and polarization direction of the incident light affect the phase matching of the nonlinear crystal, thus affecting the conversion efficiency. In gravitational wave detection, the high detection sensitivity of gravitational waves demands high laser power and high linear polarization degree from the light source. Therefore, the polarization characteristics of high-power linearly polarized lasers directly affect the efficiency of coherent combining, nonlinear conversion efficiency, and the effectiveness of gravitational wave detection.

[0003] Currently, there are two main methods to achieve linearly polarized laser output in high-power all-fiber narrow-linewidth fiber lasers: The first method is based on a single-frequency seed source and a multi-stage linearly polarized fiber amplifier built with polarization-maintaining fiber devices. The linearly polarized single-frequency seed source is phase-modulated by laser to achieve a narrow linewidth seed, which is then subjected to one or more stages of pre-amplification and main amplification to finally achieve stable linearly polarized narrow-linewidth laser output. This is currently the most mainstream method for realizing narrow-linewidth fiber lasers, featuring high laser output power and good stability (currently, laboratory-scale linearly polarized narrow-linewidth laser outputs exceeding 5kW have been achieved). However, this type of narrow-linewidth laser often requires a multi-stage pre-amplification structure in the single-frequency seed source section, demanding high precision and expensive components. Furthermore, as the power increases further, mode instability effects easily occur in the amplification stage, leading to a decrease in the polarization characteristics of the laser output. It is difficult for the polarization extinction ratio at the output of a kilowatt-level polarization-maintaining amplifier to reach above 15dB.

[0004] The second method employs a linearly polarized narrow-linewidth fiber oscillator structure. This structure uses narrow-linewidth polarization-maintaining fiber gratings and other fully polarization-maintaining fiber devices to build a laser oscillator, achieving linearly polarized narrow-linewidth laser output. Compared to the first method, this approach has advantages such as fewer fiber components, lower cost, and ease of implementation. However, during the fiber oscillation process, phenomena such as spectral broadening and stimulated Raman scattering occur as power is amplified. These phenomena limit further increases in laser power, keeping the output power of the linearly polarized narrow-linewidth laser below 2kW. Furthermore, due to the long fiber and significant spectral broadening, there is a tendency for a decrease in polarization characteristics.

[0005] In summary, regardless of the method used, the polarization characteristics of linearly polarized narrow-linewidth fiber lasers above the kilowatt level will decrease to varying degrees as the fiber laser power increases and the proportion of higher-order modes increases. At the same time, as the laser power increases to a certain level, the thermal effects in the fiber laser will also produce thermally induced depolarization, leading to a further decrease in the laser polarization degree. Summary of the Invention

[0006] The purpose of this invention is to solve the above-mentioned technical problems and provide a linearly polarized narrow-linewidth fiber laser based on a fast and slow axis orthogonal grating.

[0007] The technical solution of this invention: A linearly polarized narrow-linewidth fiber laser based on orthogonal fast and slow axis gratings, employing an all-fiber structure, includes two polarization-maintaining cladding mode strippers, a polarization-maintaining pump combiner, and an output head. It also includes a polarization-maintaining gain fiber and a pair of polarization-maintaining high-reflection gratings and a polarization-maintaining low-reflection grating with perpendicularly orthogonal etched grating surfaces. Both the high-reflection and low-reflection gratings include a polarization-maintaining fiber and a grating body. The polarization-maintaining fiber includes two stress bars, a core, and a cladding. The core is located between the two stress bars, and the cladding encloses the stress bars and the core. The core contains... The grating body is configured such that the direction of the line connecting the centers of the two stress bars is the slow axis direction of the polarization-maintaining fiber, and the direction perpendicular to both the slow axis direction and the fiber core axis is the fast axis direction of the polarization-maintaining fiber. The etched grating surface of the high-reflection polarization-maintaining grating is perpendicular to the etched grating surface of the low-reflection polarization-maintaining grating. When the etched grating surface of the high-reflection polarization-maintaining grating is parallel to the fast axis direction, the etched grating surface of the low-reflection polarization-maintaining grating is parallel to the slow axis direction. When the etched grating surface of the high-reflection polarization-maintaining grating is parallel to the slow axis direction, the etched grating surface of the low-reflection polarization-maintaining grating is parallel to the fast axis direction and perpendicular to the fiber axis.

[0008] Preferably, both the high-reflectivity grating and the low-reflectivity grating have a reflection bandwidth, wherein the high-reflectivity grating has a reflection bandwidth of less than 0.2 nm and the low-reflectivity grating has a reflection bandwidth of less than 0.05 nm.

[0009] Preferably, the fusion splicing of the polarization-maintaining high-reflection grating and the polarization-maintaining gain fiber is a fast-axis to slow-axis vertical fusion splicing, and the fusion splicing of the polarization-maintaining gain fiber and the polarization-maintaining low-reflection grating is a fast-axis to fast-axis or slow-axis to slow-axis parallel fusion splicing.

[0010] Preferably, the polarization-maintaining pump combiner is placed in the laser resonant cavity between the polarization-maintaining high-reflectivity grating and the polarization-maintaining low-reflectivity grating.

[0011] Preferably, both the polarization-maintaining high-reflectivity grating and the polarization-maintaining low-reflectivity grating are fiber gratings etched using grating lasers and phase masks.

[0012] Preferably, the grating laser is an ultraviolet laser or a femtosecond laser.

[0013] The beneficial effects of this invention are as follows: Due to the application of orthogonal gratings and vertical fiber fusion splicing, the laser signals in two polarization directions within the laser resonant cavity compete for modes. The vertically fused polarization mode, with its higher loss, loses its advantage in this competition and is suppressed. Conversely, the laser signal in the other polarization direction gains an advantage in the competition and is continuously amplified, resulting in a higher degree of linear polarization and improved polarization extinction ratio. Simultaneously, the increased degree of laser polarization also helps to raise the TMI threshold, thereby increasing the laser's output power. Therefore, it can improve the polarization extinction ratio and laser output power of narrow-linewidth fiber lasers. Attached Figure Description

[0014] Figure 1 This is a schematic diagram of the linearly polarized narrow-linewidth fiber laser based on a fast-slow axis orthogonal grating according to the present invention. Figure 2 This is a schematic diagram of a polarization-maintaining fiber structure provided in an embodiment of the present invention; Figure 3 A structural diagram of the grating body of a polarization-maintaining high-reflection grating and a polarization-maintaining low-reflection grating with mutually perpendicular grating surfaces, provided in an embodiment of the present invention; Figure 4 This is another exemplary structural diagram of a linearly polarized narrow-linewidth fiber laser based on a fast-slow axis orthogonal grating, provided as an embodiment of the present invention.

[0015] Figure reference numerals: 1. Polarization-maintaining cladding mode stripper; 2. Polarization-maintaining pump combiner; 3. Polarization-maintaining high-reflection grating; 4. Polarization-maintaining gain fiber; 5. Polarization-maintaining low-reflection grating; 6. Output head; 7. Stress bar; 8. Fiber core; and 9. Cladding. Detailed Implementation

[0016] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. 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 are within the scope of protection of the present invention.

[0017] Reference Figures 1 to 4A linearly polarized narrow-linewidth fiber laser based on orthogonal fast and slow axis gratings is disclosed. Employing an all-fiber structure, it includes two polarization-maintaining cladding mode strippers 1, a polarization-maintaining pump combiner 2, and an output head 6. It also includes a polarization-maintaining gain fiber 4 and a pair of polarization-maintaining high-reflection gratings 3 and low-reflection gratings 5 ​​with perpendicularly orthogonal etched grating surfaces. Both the high-reflection grating 3 and the low-reflection grating 5 comprise a polarization-maintaining fiber and a grating body. The polarization-maintaining fiber includes two stress bars 7, a core 8, and a cladding 9. The core 8 is located between the two stress bars 7, and the cladding 9 encloses the stress bars 7 and the core 8. The core 8 contains... The grating body is configured such that the direction of the line connecting the centers of the two stress bars 7 is the slow axis direction of the polarization-maintaining fiber, and the direction perpendicular to both the slow axis direction and the fiber core 8 axis is the fast axis direction of the polarization-maintaining fiber. The etched grating surface of the high-reflection polarization-maintaining grating 3 is perpendicular to the etched grating surface of the low-reflection polarization-maintaining grating 5. When the etched grating surface of the high-reflection polarization-maintaining grating 3 is parallel to the fast axis direction, the etched grating surface of the low-reflection polarization-maintaining grating 5 is parallel to the slow axis direction. When the etched grating surface of the high-reflection polarization-maintaining grating 3 is parallel to the slow axis direction, the etched grating surface of the low-reflection polarization-maintaining grating 5 is parallel to the fast axis direction and perpendicular to the fiber axis. In this invention, the polarization-maintaining pump combiner 2 is placed outside the laser resonant cavity. Pump light is injected from either the forward or reverse direction. The etched grating surface of the polarization-maintaining high-reflection grating 3 is parallel to the fast axis direction, and the etching direction mainly modulates the change in refractive index along the fast axis. The polarized light modulated by the grating is output along the fast axis. Conversely, the etched grating surface of the polarization-maintaining low-reflection grating 5 is parallel to the slow axis direction, and the etching direction mainly modulates the change in refractive index along the slow axis. The polarized light modulated by the grating is output along the slow axis. The polarization directions of the two gratings are perpendicular to each other and also perpendicular to the fiber axis. The laser signals excited in the polarization-maintaining gain fiber 4, which are parallel to and perpendicular to the fast axis direction, are transmitted to the end of the polarization-maintaining high-reflection grating 3. Because the polarization-maintaining high-reflection grating 3 and the polarization-maintaining gain fiber 4 are vertically fused... In this method, polarized light parallel to the fast axis is modulated into the cladding by the refractive index, resulting in greater loss. Conversely, polarized light parallel to the slow axis has a higher reflectivity and thus has an advantage in polarization mode competition. It is continuously amplified and then transmitted to the end of the polarization-maintaining low-reflection grating 5. Its output direction is consistent with that of the polarization-maintaining low-reflection grating 5. The competitive advantage of the slow-axis polarized light against the fast-axis polarized light is further expanded, and the fast-axis polarized light continues to be suppressed. This process is repeated until the signal of the slow-axis polarized light exceeds the laser threshold and finally the laser is output from the end of the polarization-maintaining low-reflection grating 5. Furthermore, the reflection bandwidth of the polarization-maintaining high-reflection grating 3 and the polarization-maintaining low-reflection grating 5 is relatively narrow, so the laser linewidth selected by the grating is narrow, resulting in a laser output with a narrow linewidth and high polarization degree.

[0018] As a preferred embodiment of the present invention, it may also have the following additional technical features: In this embodiment, both the polarization-maintaining high-reflectivity grating 3 and the polarization-maintaining low-reflectivity grating 5 have a reflection bandwidth, wherein the reflection bandwidth of the polarization-maintaining high-reflectivity grating 3 is less than 0.2 nm, and the reflection bandwidth of the polarization-maintaining low-reflectivity grating 5 is less than 0.05 nm.

[0019] In this embodiment, the polarization-maintaining high-reflection grating 3 and the polarization-maintaining gain fiber 4 are fused together in a fast-axis to slow-axis perpendicular fusion, and the polarization-maintaining gain fiber 4 and the polarization-maintaining low-reflection grating 5 are fused together in a fast-axis to fast-axis or slow-axis to slow-axis parallel fusion. Selecting a pair of gratings with mutually perpendicular polarization output directions and narrow reflection bandwidth to form a laser oscillator, and employing both perpendicular and parallel fusion methods for the polarization-maintaining gratings at both ends of the oscillator, aims to ensure sufficient mode competition between the slow-axis output polarized light and the fast-axis output polarized light laser signals within the laser resonant cavity. During this competition, the fast-axis output polarized light loses its advantage due to higher losses and is suppressed; conversely, the slow-axis output polarized light gains an advantage, is continuously amplified, and eventually exceeds the laser threshold to form laser output. In this fast-slow axis output polarization mode competition, the signal light in one polarization direction is continuously suppressed, while the laser signal in the other polarization direction is continuously amplified, continuously increasing the purity of the polarized signal light, ultimately achieving a higher degree of linear polarization and improving the polarization degree of the laser output.

[0020] In this embodiment, the polarization-maintaining pump combiner 2 is placed within the laser resonant cavity between the polarization-maintaining high-reflectivity grating 3 and the polarization-maintaining low-reflectivity grating 5. Pump light is injected from the end closest to the low-reflectivity grating, resulting in a higher Raman threshold for the laser and enabling higher power laser output.

[0021] In this embodiment, both the polarization-maintaining high-reflectivity grating 3 and the polarization-maintaining low-reflectivity grating 5 are fiber gratings etched using grating lasers and phase masks.

[0022] In this embodiment, the grating laser is an ultraviolet laser or a femtosecond laser.

[0023] It should be noted that the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0024] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A linearly polarized narrow-linewidth fiber laser based on a fast-slow axis orthogonal grating, employing an all-fiber structure, comprising two polarization-maintaining cladding mode strippers (1), a polarization-maintaining pump combiner (2), and an output head (6), characterized in that, It also includes a polarization-maintaining gain fiber (4) and a pair of polarization-maintaining high-reflection gratings (3) and low-reflection gratings (5) with perpendicularly orthogonal etched grating surfaces. Both the high-reflection grating (3) and the low-reflection grating (5) include a polarization-maintaining fiber and a grating body. The polarization-maintaining fiber includes two stress bars (7), a core (8), and a cladding (9). The core (8) is located between the two stress bars (7). The cladding (9) is used to wrap the stress bars (7) and the core (8). The grating body is disposed inside the core (8). The center line connecting the two stress bars (7) is... The direction is the slow axis direction of the polarization-maintaining fiber, and the direction perpendicular to both the slow axis direction and the fiber core (8) axis is the fast axis direction of the polarization-maintaining fiber. The etched grating surface of the polarization-maintaining high-reflection grating (3) is perpendicular to the etched grating surface of the polarization-maintaining low-reflection grating (5). When the etched grating surface of the polarization-maintaining high-reflection grating (3) is parallel to the fast axis direction, the etched grating surface of the polarization-maintaining low-reflection grating (5) is parallel to the slow axis direction. When the etched grating surface of the polarization-maintaining high-reflection grating (3) is parallel to the slow axis direction, the etched grating surface of the polarization-maintaining low-reflection grating (5) is parallel to the fast axis direction and perpendicular to the fiber axis.

2. A linearly polarized narrow-linewidth fiber laser based on a fast-slow axis orthogonal grating according to claim 1, characterized in that: Both the high-reflectivity grating (3) and the low-reflectivity grating (5) have a reflection bandwidth, wherein the high-reflectivity grating (3) has a reflection bandwidth of less than 0.2 nm and the low-reflectivity grating (5) has a reflection bandwidth of less than 0.05 nm.

3. A linearly polarized narrow-linewidth fiber laser based on a fast-slow axis orthogonal grating according to claim 1, characterized in that: The polarization-maintaining high-reflection grating (3) and the polarization-maintaining gain fiber (4) are fused together as a fast-axis to slow-axis vertical fusion splice, and the polarization-maintaining gain fiber (4) and the polarization-maintaining low-reflection grating (5) are fused together as a fast-axis to fast-axis or a slow-axis to slow-axis parallel fusion splice.

4. A linearly polarized narrow-linewidth fiber laser based on a fast-slow axis orthogonal grating according to claim 1, characterized in that: The polarization-maintaining pump combiner (2) is placed in the laser resonant cavity between the polarization-maintaining high-reflection grating (3) and the polarization-maintaining low-reflection grating (5).

5. A linearly polarized narrow-linewidth fiber laser based on a fast-slow axis orthogonal grating according to claim 1, characterized in that: Both the polarization-maintaining high-reflectivity grating (3) and the polarization-maintaining low-reflectivity grating (5) are fiber gratings etched using grating lasers and phase masks.

6. A linearly polarized narrow-linewidth fiber laser based on a fast-slow axis orthogonal grating according to claim 5, characterized in that: The grating laser is an ultraviolet laser or a femtosecond laser.

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