Polarization-maintaining straight-cavity thulium-doped fiber laser

Abstract

The utility model discloses a polarization-maintaining straight-cavity thulium-doped fiber laser which comprises a first polarization-maintaining grating, a beam combiner, a gain fiber, a second polarization-maintaining grating and an isolator which are connected in sequence. The reflectivity of the first polarization-maintaining grating is greater than that of the second polarization-maintaining grating; the center wavelength of the fast axis of the first polarization-maintaining grating is the same as the center wavelength of the slow axis of the second polarization-maintaining grating and is the output wavelength of the polarization-maintaining straight-cavity thulium-doped fiber laser, and a specific high-reflectivity grating and a specific low-reflectivity grating (including the center wavelength and the bandwidth) are selected to achieve spectrum single-peak output. Stable single-peak output and high-stability polarization extinction ratio and power output of the straight-cavity polarization-maintaining thulium-doped fiber laser are realized, and a basis is provided for subsequent amplification to polarization-maintaining hundred-watt or kilowatt-level power.

Description

Technical Field

[0001] The utility model embodiment relates to the technical field of optical fiber lasers, and in particular to a polarization-maintaining straight-cavity thulium-doped optical fiber laser. Background Art

[0002] At present, there are mainly the following solutions for polarization-maintaining thulium-doped fiber lasers:

[0003] Solution 1: It is achieved through passive mode locking methods, including semiconductor absorption mirror mode locking, nonlinear polarization rotation mode locking, nonlinear loop mirror mode locking, nonlinear amplifying loop mirror mode locking, etc.

[0004] Solution 2: Through active locking method.

[0005] Solution 3: Use a single-frequency distributed feedback (DFB) laser as a seed source and perform subsequent amplification. The DFB seed source can be divided into semiconductor type and all-fiber grating type.

[0006] Solution 4: Use ASE plus filtering solution, add polarization device after filtering for polarization, and then perform subsequent amplification.

[0007] Solution 5: Use a polarization-maintaining fiber Bragg grating as the FP cavity, as a seed source, and perform subsequent high-power amplification.

[0008] Among them, schemes 1 to 4 have complex structures and high costs. Scheme 5 has a simple structure and low cost, but the polarization-maintaining straight cavity is prone to forming double peaks, and the polarization extinction ratio and output power are unstable, which affects the final application effect. Utility Model Content

[0009] The utility model provides a polarization-maintaining straight-cavity thulium-doped fiber laser, which realizes spectral single-peak output by selecting specific high-reflection gratings and low-reflection gratings (including central wavelength and bandwidth), realizes stable single-peak output of the straight-cavity polarization-maintaining straight-cavity thulium-doped fiber laser and high-stability polarization extinction ratio and power output, and provides a basis for subsequent amplification to polarization-maintaining hundreds of watts or kilowatts of power.

[0010] In a first aspect, the present application provides a polarization-maintaining straight-cavity thulium-doped fiber laser, comprising a first polarization-maintaining grating, a beam combiner, a gain fiber, a second polarization-maintaining grating, and an isolator connected in sequence;

[0011] The effective reflectivity of the first polarization-maintaining grating is greater than the effective reflectivity of the second polarization-maintaining grating; the fast-axis center wavelength of the first polarization-maintaining grating is the same as the slow-axis center wavelength of the second polarization-maintaining grating, which is the output wavelength of the polarization-maintaining straight-cavity thulium-doped fiber laser.

[0012] Optionally, the beam combiner includes a polarization-maintaining beam combiner;

[0013] The fusion point between the first polarization-maintaining grating and the beam combiner is a cat's eye orthogonal fusion.

[0014] Optionally, the input end of the beam combiner is connected to the first polarization-maintaining grating through a polarization-maintaining optical fiber, and the output end of the beam combiner is connected to the gain optical fiber through a polarization-maintaining optical fiber.

[0015] Optionally, the optical fiber of the first polarization-maintaining grating, the optical fiber at the input end of the beam combiner, the optical fiber of the second polarization-maintaining grating and the optical fiber of the isolator have the same specification parameters.

[0016] Optionally, the interval between the fast axis center wavelength and the slow axis center wavelength of the first polarization-maintaining grating is 0.25 nm±0.01; the interval between the fast axis center wavelength and the slow axis center wavelength of the second polarization-maintaining grating is 0.25 nm±0.01;

[0017] The reflection bandwidths of the first polarization-maintaining grating and the second polarization-maintaining grating are both less than 0.2 nm.

[0018] Optionally, a pump light source is also included; the pump light source is connected to the pump end of the beam combiner.

[0019] Optionally, the pump light source includes a semiconductor pump laser.

[0020] Optionally, the output end of the pump light source is connected to the pump end of the beam combiner through a multimode optical fiber.

[0021] Optionally, the isolator comprises a polarization-maintaining isolator.

[0022] Optionally, the polarization extinction ratio of the isolator is greater than 20 dB.

[0023] In summary, the polarization-maintaining straight-cavity thulium-doped fiber laser provided in the present application includes a first polarization-maintaining grating, a beam combiner, a gain fiber, a second polarization-maintaining grating and an isolator connected in sequence; the reflectivity of the first polarization-maintaining grating is greater than the reflectivity of the second polarization-maintaining grating; the fast-axis center wavelength of the first polarization-maintaining grating is the same as the slow-axis center wavelength of the second polarization-maintaining grating, which is the output wavelength of the polarization-maintaining straight-cavity thulium-doped fiber laser, and specific high-reflection gratings and low-reflection gratings (including center wavelengths and bandwidths) are selected to achieve spectral single-peak output, achieve stable single-peak output of the straight-cavity polarization-maintaining straight-cavity thulium-doped fiber laser, and highly stable polarization extinction ratio and power output, which provide a basis for subsequent amplification to polarization-maintaining power levels of hundreds of watts or kilowatts. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a structural schematic diagram of a polarization-maintaining straight-cavity thulium-doped fiber laser provided by the present application;

[0025] Figure 2 is a cross-sectional schematic diagram of a polarization-maintaining optical fiber provided in the present application;

[0026] Figure 3 This is a test chart of the fast axis and slow axis center wavelengths of a polarization-maintaining straight-cavity thulium-doped fiber laser provided by the present application;

[0027] Figure 4 This is a test chart of the output power of a polarization-maintaining straight-cavity thulium-doped fiber laser provided by the present application;

[0028] Figure 5 This is a test chart of the extinction ratio of a polarization-maintaining straight-cavity thulium-doped fiber laser provided by the present application;

[0029] Figure 6 This is a test chart of the side mode suppression ratio of a polarization-maintaining straight-cavity thulium-doped fiber laser provided in the present application.

[0030] The accompanying drawings are as follows:

[0031] 1-first polarization-maintaining grating; 2-beam combiner; 3-gain fiber; 4-second polarization-maintaining grating; 5-isolator;

[0032] 6-Pump light source. DETAILED DESCRIPTION

[0033] The utility model is further described in detail below in conjunction with the accompanying drawings and examples. It is to be understood that the specific embodiments described herein are only used to explain the present application, rather than to limit the present application. It should also be noted that, for ease of description, only the parts related to the present application rather than all structures are shown in the accompanying drawings. Various modifications and changes can be made in the present application without departing from the spirit or scope of the present application, which is obvious to those skilled in the art. Therefore, the present application is intended to cover the modifications and changes of the present application that fall within the scope of the corresponding claims (technical solutions for protection) and their equivalents. It should be noted that the implementation methods provided in the embodiments of the present application can be combined with each other without contradiction.

[0034] The utility model discloses a polarization-maintaining straight-cavity thulium-doped fiber laser in view of one or more of the above problems existing in the prior art. Figure 1 It is a schematic diagram of a polarization-maintaining straight-cavity thulium-doped fiber laser provided by the present application; Figure 2 is a cross-sectional schematic diagram of a polarization-maintaining optical fiber provided in the present application; Figure 3 This is a test chart of the fast axis and slow axis center wavelengths of a polarization-maintaining straight-cavity thulium-doped fiber laser provided by this application. Figure 1As shown, a polarization-maintaining straight-cavity thulium-doped fiber laser provided in an embodiment of the present application includes a first polarization-maintaining grating 1, a beam combiner 2, a gain fiber 3, a second polarization-maintaining grating 4 and an isolator 5 connected in sequence; the reflectivity of the first polarization-maintaining grating 1 is greater than the reflectivity of the second polarization-maintaining grating 4; the fast-axis center wavelength of the first polarization-maintaining grating 1 is the same as the slow-axis center wavelength of the second polarization-maintaining grating 4, which is the output wavelength of the polarization-maintaining straight-cavity thulium-doped fiber laser.

[0035] For example, refer to Figure 1 As shown, the first polarization-maintaining grating 1 adopts a polarization-maintaining high-reflection grating with a wavelength range of 2um, a reflection bandwidth of 0.15nm, a central wavelength of the fast axis of 1949.5nm, and a central wavelength of the slow axis of 1949.75nm; the wavelength range of the beam combiner 2 is 2um, and the central wavelength is 793nm; the gain fiber 3 is a thulium-doped fiber with a length of 2m; the second polarization-maintaining grating 4 adopts a polarization-maintaining low-reflection grating with a wavelength range of 2um, a reflection bandwidth of 0.2nm, a central wavelength of the fast axis of 1949.35nm, and a central wavelength of the slow axis of 1949.6nm; the wavelength range of the isolator 5 is 2um.

[0036] Combination Figure 1 and Figure 2 As shown, the first polarization-maintaining grating 1 and the second polarization-maintaining grating 4 can also be called a panda-type polarization-maintaining optical fiber, in which there are two stress regions inside the optical fiber, which present a shape similar to panda eyes on the cross section of the optical fiber. By applying stress in the two stress regions, a birefringence effect is generated in the core of the optical fiber, and the birefringence effect helps to maintain the polarization state of the light wave, thereby achieving the polarization-maintaining function. Among them, the light propagation speed in the direction of the connection between the two panda eyes (as shown in the X direction in the figure) is slow, which is the slow axis light; the light propagation speed in the direction perpendicular to the slow axis light (as shown in the Y direction in the figure) is fast, which is the fast axis light.

[0037] The grating types in the first polarization-maintaining grating 1 and the second polarization-maintaining grating 4 are fiber Bragg Gratings (FBG). In some embodiments, the FBG can be written by a phase mask, wherein the grating formula of the FBG is as follows:

[0038] λB=2neffΛ;

[0039] λB is the Bragg wavelength of the grating, that is, the reflection center wavelength of the grating, neff is the effective refractive index, and Λ is the period of the mask. Figure 3 λB1 in the figure is the reflection center wavelength of the first polarization-maintaining grating 1 ; λB4 in the figure is the reflection center wavelength of the second polarization-maintaining grating 4 .

[0040] Since the doping concentration of the stress area is higher than that of the core, when the reflection center wavelengths of the first polarization-maintaining grating 1 and the second polarization-maintaining grating 4 are engraved on the Panda-type polarization-maintaining optical fiber, the effective refractive index of the stress area is higher. Due to the existence of the stress area, two different effective refractive indices are formed in the first polarization-maintaining grating 1 and the second polarization-maintaining grating 4, that is, there is a refractive index value in the slow axis direction and the fast axis direction, which leads to two values ​​of λB, that is, there are two reflection peaks of the grating, such as Figure 3 As shown, the blue peak λB 11 and purple peak λB 12 The two transmission peaks of the first polarization-maintaining grating 1 are shown in Figure 1. The blue peak is the fast-axis light and the purple peak is the slow-axis light. The green peaks are the two reflection peaks of the second polarization-maintaining grating 4. Similarly, the light with a smaller wavelength is the fast-axis light, corresponding to the green peak λB on the left. 41 , the longer wavelength is the slow axis light, corresponding to the green peak λB on the right 42 .

[0041] Depend on Figure 3 It can be seen that the center wavelengths of the two peaks of the grating are only about 0.25nm apart, which requires that when writing the grating, the fast-axis light wavelength of the high-reflection grating needs to be matched with the slow-axis light of the low-reflection grating. In this application, by setting the effective reflectivity of the first polarization-maintaining grating 1 to be greater than the effective reflectivity of the second polarization-maintaining grating 4, the fast-axis center wavelength of the first polarization-maintaining grating 1 is the same as the slow-axis center wavelength of the second polarization-maintaining grating 4, as the reflection wavelength, that is, the output wavelength of the polarization-maintaining straight-cavity thulium-doped fiber laser. Specifically, as Figure 3 , the transmission peak corresponding to the fast axis light of the first polarization-maintaining grating 1 (high reflection grating) (i.e. the blue peak λB on the left) 11 ), only the reflection peak corresponding to the slow axis light of the second polarization-maintaining grating 4 (low-reflection grating) (i.e. the green peak λB on the right) can be reflected. 42 ), so that the fast-axis light wavelength of the high-reflection grating can be matched with the slow-axis light wavelength of the low-reflection grating, ensuring the single-peak output of the polarization-maintaining straight-cavity thulium-doped fiber laser.

[0042] It should be noted that the interval between the fast-axis center wavelength and the slow-axis center wavelength of the first polarization-maintaining grating 1 is 0.25nm±0.01; the interval between the fast-axis center wavelength and the slow-axis center wavelength of the second polarization-maintaining grating 4 is 0.25nm±0.01; the reflection bandwidths of the first polarization-maintaining grating 1 and the second polarization-maintaining grating 4 are both less than 0.2nm. This setting can avoid the reflection bandwidth being too large, avoid all the fast and slow axis light wavelengths of the low-reflection grating from reflecting to form double peaks, and ensure single-peak output.

[0043] Based on the above embodiments, Figure 1 and Figure 2 As shown, the beam combiner 2 includes a polarization-maintaining beam combiner, and the fusion point between the first polarization-maintaining grating 1 and the beam combiner 2 is a cat's eye orthogonal fusion.

[0044] Specifically, during the welding process, the polarization-maintaining high-reflection grating and the polarization-maintaining beam combiner are orthogonally welded, that is, when the panda eyes are connected, one panda eye is kept horizontal and the other panda eye is kept vertical. All other points need to ensure that the panda eyes on both sides are aligned and welded to ensure that the slow-axis light maintains its polarization state in all subsequent optical paths.

[0045] Based on the above embodiments, continue to refer to Figure 1 As shown, the optical fiber of the first polarization-maintaining grating 1, the optical fiber at the input end of the beam combiner 2, the optical fiber of the second polarization-maintaining grating 4 and the optical fiber of the isolator 5 have the same specification parameters;

[0046] Exemplarily, the optical fiber of the first polarization-maintaining grating 1, the optical fiber at the input end of the combiner 2, the second polarization-maintaining grating 4 and the optical fiber of the isolator 5 all adopt polarization-maintaining single-mode optical fiber with a core diameter of 12um, a cladding diameter of 130um and a wavelength range of 2μm to achieve stable linearly polarized single-mode output.

[0047] Figure 4 This is a test chart of the output power of a polarization-maintaining straight-cavity thulium-doped fiber laser provided in the present application.

[0048] Based on the above embodiments, continue to refer to Figure 1 As shown, the polarization-maintaining straight-cavity thulium-doped fiber laser further includes a pump light source 6 ; the pump light source 6 is connected to the pump end of the beam combiner 2 .

[0049] Optionally, the pump light source 6 includes a semiconductor pump laser.

[0050] Combination Figure 4 As shown, for example, by pumping with a 10W / 793nm semiconductor pump laser, a 1949.6nm laser with an output power of more than 5W is obtained, and its output power stability is 2%, which has good stable power output.

[0051] Based on the above embodiments, continue to refer to Figure 1 As shown, the pump end of the beam combiner 2 is connected to the pump light source 6 via a multimode optical fiber.

[0052] Exemplarily, the pump end optical fiber of the combiner 2 is a multimode optical fiber, and the optical fiber type is MM-S105 / 125-22A, wherein MM stands for multimode, the optical fiber core diameter is 105 μm, the cladding diameter is 125 μm, and the numerical aperture NA is 0.22.

[0053] Figure 5 This is a test chart of the extinction ratio of a polarization-maintaining straight-cavity thulium-doped fiber laser provided in the present application.

[0054] Based on the above embodiments, Figure 1 and Figure 5As shown, the isolator 5 comprises a polarization-maintaining isolator, and the polarization extinction ratio of the isolator 5 is greater than 20 dB.

[0055] Exemplarily, the isolator 5 can adopt a polarization-maintaining isolator with a wavelength range of 2um. Tests show that the polarization extinction ratio of the polarization-maintaining straight-cavity thulium-doped fiber laser is greater than 24dB. Thus, the polarization extinction ratio of the final laser output can be further improved by adopting a polarization-maintaining isolator.

[0056] Based on the above embodiments, continue to refer to Figure 1 As shown, the input end of the beam combiner 2 is connected to the first polarization-maintaining grating 1 through a polarization-maintaining optical fiber, and the output end of the beam combiner 2 is connected to the gain optical fiber 3 through a polarization-maintaining optical fiber.

[0057] Figure 6 This is a test chart of the side mode suppression ratio of a polarization-maintaining straight-cavity thulium-doped fiber laser provided in the present application.

[0058] Specifically, refer to Figure 6 As shown, the polarization-maintaining straight-cavity thulium-doped fiber laser provided in the embodiment of the present application has a spectrally stable single-peak output and a side mode suppression ratio of 61.6 dB.

[0059] In summary, the polarization-maintaining straight-cavity thulium-doped fiber laser provided in the present application has a simple optical path structure, low cost, and easy operation, and can achieve stable single-peak output of the straight-cavity polarization-maintaining straight-cavity thulium-doped fiber laser as well as high-stability polarization extinction ratio and power output, providing a basis for subsequent amplification to polarization-maintaining power levels of hundreds of watts or kilowatts.

[0060] Note that the above are only preferred embodiments of the present invention and the technical principles used. Those skilled in the art will understand that the present invention is not limited to the specific embodiments described herein, and that various obvious changes, readjustments, combinations and substitutions can be made by those skilled in the art without departing from the scope of protection of the present invention. Therefore, although the present invention is described in more detail through the above embodiments, the present invention is not limited to the above embodiments, and may include more other equivalent embodiments without departing from the concept of the present invention, and the scope of the present invention is determined by the scope of the appended claims.

Claims

1. A polarization-maintaining straight-cavity thulium-doped fiber laser, characterized in that: It comprises a first polarization-maintaining grating (1), a beam combiner (2), a gain optical fiber (3), a second polarization-maintaining grating (4) and an isolator (5) which are connected in sequence; The effective reflectivity of the first polarization-maintaining grating (1) is greater than the effective reflectivity of the second polarization-maintaining grating (4); the fast-axis center wavelength of the first polarization-maintaining grating (1) is the same as the slow-axis center wavelength of the second polarization-maintaining grating (4), which wavelength is the output wavelength of the polarization-maintaining straight-cavity thulium-doped fiber laser.

2. The polarization-maintaining straight-cavity thulium-doped fiber laser according to claim 1, characterized in that: The beam combiner (2) comprises a polarization-maintaining beam combiner; The fusion point between the first polarization-maintaining grating (1) and the beam combiner (2) is a cat's eye orthogonal fusion.

3. The polarization-maintaining straight-cavity thulium-doped fiber laser according to claim 2, characterized in that: The input end of the beam combiner (2) is connected to the first polarization-maintaining grating (1) via a polarization-maintaining optical fiber, and the output end of the beam combiner (2) is connected to the gain optical fiber (3) via a polarization-maintaining optical fiber.

4. The polarization-maintaining straight-cavity thulium-doped fiber laser according to claim 2, characterized in that: The optical fiber of the first polarization-maintaining grating (1), the optical fiber at the input end of the beam combiner (2), the optical fiber of the second polarization-maintaining grating (4) and the optical fiber of the isolator (5) have the same specification parameters.

5. The polarization-maintaining straight-cavity thulium-doped fiber laser according to claim 1, characterized in that: The interval between the fast axis center wavelength and the slow axis center wavelength of the first polarization-maintaining grating (1) is 0.25 nm±0.01; the interval between the fast axis center wavelength and the slow axis center wavelength of the second polarization-maintaining grating (4) is 0.25 nm±0.01; The reflection bandwidths of the first polarization-maintaining grating (1) and the second polarization-maintaining grating (4) are both less than 0.2 nm.

6. The polarization-maintaining straight-cavity thulium-doped fiber laser according to claim 1, characterized in that: It also comprises a pump light source (6); the pump light source (6) is connected to the pump end of the beam combiner (2).

7. The polarization-maintaining straight-cavity thulium-doped fiber laser according to claim 6, characterized in that: The pump light source (6) comprises a semiconductor pump laser.

8. The polarization-maintaining straight-cavity thulium-doped fiber laser according to claim 6, characterized in that: The output end of the pump light source (6) is connected to the pump end of the beam combiner (2) via a multimode optical fiber.

9. The polarization-maintaining straight-cavity thulium-doped fiber laser according to claim 1, characterized in that: The isolator (5) comprises a polarization-maintaining isolator.

10. The polarization-maintaining straight-cavity thulium-doped fiber laser according to claim 9, characterized in that: The polarization extinction ratio of the isolator (5) is greater than 20 dB.

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