Few-mode fiber Bragg grating and laser system capable of suppressing high-order mode reflection

Abstract

The present invention proposes a few-mode fiber Bragg grating and a laser system that can suppress high-order mode reflection. A circular refractive index modulation zone concentric with the core is arranged in the core of the few-mode fiber. The radius of the refractive index modulation zone is smaller than the radius of the core. The refractive index modulation zone and the optical fiber vector mode are both circularly symmetrical. 01 Module and LP 11 The two modes will not be mutually coupled, and no energy exchange will occur between the two modes. 11 The self-coupling coefficient of the mode is lower than that of LP 01 Mode self-coupling coefficient, LP 11 The reflectivity of the mode is lower than that of LP 01 mode reflectivity, achieving LP 11 The few-mode fiber Bragg grating of the present invention can be used as a cavity mirror in a high-power fiber laser oscillator, which can realize both light reflection and mode control.

Description

Technical Field

[0001] The invention relates to the technical field of optical fiber Bragg gratings, and in particular to a few-mode optical fiber Bragg grating and a laser system capable of suppressing high-order mode reflection. Background Art

[0002] In recent years, with the continuous improvement of power, beam quality and efficiency, high-power fiber lasers have played an increasingly important role in advanced manufacturing, national defense and other fields. In general, there are two structures of high-power fiber laser systems, namely fiber amplifiers and fiber oscillators. Among them, fiber oscillators have the advantages of simple structure, stable system and relatively low cost. They have received more and more attention in recent years, and the output power level has achieved rapid growth. The maximum output power of fiber oscillators reported so far has exceeded 8kW and may soon exceed 10kW. However, stimulated Raman scattering and mode instability effects are key factors limiting the further increase in the power of fiber laser oscillators. Among them, the mode instability effect is the most important limiting factor for the power increase of large mode field fiber laser oscillators. Under high power conditions, the LP in large mode field fibers 01 Module and LP 11 The mode instability effect in high-power fiber laser oscillators is an urgent and challenging task.

[0003] Currently, there are very limited ways to suppress the instability of fiber oscillator modes, usually using a reverse pumping structure or increasing LP by bending the fiber. 11 The transmission loss of the fiber oscillator mode is suppressed, but the suppression effects of these two methods are very limited and cannot meet the demand for further power increase. If the mode instability can be suppressed by optimizing the design of the fiber Bragg grating, the core device in the fiber oscillator, the output power of the oscillator is expected to be further improved. In high-power fiber oscillators, the fiber Bragg grating acts as a cavity mirror to reflect the laser and couple the output laser. Its performance directly determines the output laser power and beam quality. Currently, most fiber Bragg gratings used in fiber oscillators are written by ultraviolet exposure. The fiber Bragg gratings written in this way have low insertion loss and high stability, but their transverse plane refractive index modulation usually presents a non-uniform distribution, which makes the LP 01 Module and LP 11 The modes will be mutually coupled, that is, the mode originally incident on the fiber Bragg grating only exists in the LP 01 mode, but the reflected mode also has LP 01 Module and LP 11 In addition, the self-coupling coefficients of the two modes in the fiber Bragg grating written by UV exposure are relatively close, that is, LP 01 Module and LP11 The self-coupling reflectivity of the modes is similar, which makes the LP 11 The mode is continuously amplified in the oscillator cavity. Both processes will reduce the mode instability threshold of the oscillator, which is not conducive to the increase of output power. Therefore, optimizing the fiber Bragg grating writing process or method is of great significance to suppress mode instability. Summary of the invention

[0004] In view of the deficiencies in the prior art, the present invention proposes a few-mode fiber Bragg grating and a laser system capable of suppressing high-order mode reflection.

[0005] In order to achieve the above technical objectives, the specific technical solutions adopted by the present invention are as follows:

[0006] A few-mode fiber Bragg grating that can suppress high-order mode reflection. A circular refractive index modulation zone concentric with the core is arranged in the core of the few-mode fiber. The radius of the refractive index modulation zone is smaller than the radius of the core. The refractive index modulation zone and the fiber vector mode are both circularly symmetrical. 01 Module and LP 11 The modes will not couple with each other, and the LP 11 The self-coupling coefficient of the mode is lower than that of LP 01 Mode self-coupling coefficient, LP 11 The reflectivity of the mode is lower than that of LP 01 The refractive index modulation region and the optical fiber vector mode of the present invention are both circularly symmetric, LP 01 Module and LP 11 The modes will not be mutually coupled, and the two modes will not exchange energy. The few-mode fiber Bragg grating of the present invention completely suppresses the LP 01 Module and LP 11 Mutual coupling between modes. 11 The self-coupling coefficient of the mode is lower than that of LP 01 Mode self-coupling coefficient, LP 11 The reflectivity of the mode is lower than that of LP 01 mode reflectivity, achieving LP 11 Reflection suppression of the model.

[0007] As a preferred solution, the refractive index modulation area is provided in plurality, and the plurality of refractive index modulation areas are periodically distributed in the central area of ​​the core end face at different positions in the length direction of the core, thereby introducing periodic refractive index modulation.

[0008] As a preferred solution, the few-mode fiber Bragg grating LP 11 The degree of suppression of the LP mode decreases as the radius of the refractive index modulation region increases. However, if the radius of the refractive index modulation region is too small, the LP 01 The self-coupling coefficient of the mode cannot be guaranteed, because the LP 11The vector mode of the mode has a phase singularity in the middle of the mode field, and the intensity distribution near this point approaches zero. Therefore, the optimal radius of the refractive index modulation zone is 2 to 3 μm. At this time, LP 11 The overlap between the mode and the refractive index modulation region is lower than that of the LP 01 The degree of overlap of the modes, LP 11 The self-coupling coefficient of the mode is lower than that of LP 01 Mode self-coupling coefficient, LP 11 The reflectivity of the mode should also be lower than that of LP 01 Mode reflectivity, grating to LP 11 The mold has better suppression.

[0009] As a preferred embodiment, the refractive index of the refractive index modulation zone of the present invention is uniformly distributed.

[0010] As a preferred solution, the refractive index modulation region of the present invention is written in the core center area of ​​the few-mode optical fiber by a femtosecond laser surface-by-surface writing method or a femtosecond laser combined with a phase template writing method.

[0011] The laser system comprises a laser oscillator, which comprises a pump light source, a pump light combiner, a high-reflection grating, a doped optical fiber and a low-reflection grating. The high-reflection grating and the low-reflection grating both adopt any of the above-mentioned few-mode fiber Bragg gratings that can suppress high-order mode reflection.

[0012] As a preferred solution, the laser oscillator in the laser system is a forward pumped fiber laser oscillator, a backward pumped fiber laser oscillator or a bidirectionally pumped fiber laser oscillator.

[0013] As a preferred solution, the laser system further comprises one or more laser amplifiers, and the output end of the laser oscillator is connected to one or more laser amplifiers.

[0014] The beneficial effects of the present invention are as follows:

[0015] The refractive index modulation region of the few-mode fiber Bragg grating of the present invention is located in the center of the fiber core, which can conveniently control the mode coupling characteristics therein. The present invention can completely suppress LP 01 Module and LP 11 The mutual coupling between modes can greatly reduce LP 11 mode self-coupling coefficient, realizing the high-order mode (LP 11 The few-mode fiber Bragg grating of the present invention can be used in a high-power fiber laser oscillator as a cavity mirror, which can realize both light reflection and mode control. In addition, the few-mode fiber Bragg grating has the characteristics of simple structure and convenient production. It is a passive device that can effectively suppress the mode instability effect in a high-power fiber laser oscillator without increasing the complexity of the system. It is simple and effective. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the structures shown in these drawings without paying creative work.

[0017] Figure 1 Schematic diagram of few-mode fiber Bragg grating using femtosecond writing to suppress high-order mode reflection;

[0018] Figure 2 For LP 01 Module and LP 11 Schematic diagram of the vector mode intensity distribution of the mode;

[0019] Figure 3 This is a schematic diagram of the refractive index distribution of the transverse end face of the few-mode fiber Bragg grating;

[0020] Figure 4 Schematic diagram of the overlap between each mode and the refractive index modulation region when the radius of the refractive index modulation region is 2 μm;

[0021] Figure 5 For LP 01 Module and LP 11 Schematic diagram of self-coupling overlap integral of each vector mode of the mode at different refractive index modulation zone radii;

[0022] Figure 6 For LP 01 Schematic diagram of the self-coupling overlap integral of the mode at different refractive index modulation zone radii;

[0023] Figure 7 For LP 01 Module and LP 11 Schematic diagram of the ratio of self-coupling overlap integrals of modes at different refractive index modulation zone radii;

[0024] Figure 8 Schematic diagram of the optical path of a laser oscillator built using a fiber Bragg grating that can suppress high-order mode reflection. DETAILED DESCRIPTION

[0025] In order to make the technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0026] The present invention utilizes the overlap degree of the transverse end face refractive index modulation area of ​​the few-mode fiber Bragg grating and different modes to adjust the self-coupling coefficient of each mode and the mutual coupling coefficient between modes, thereby realizing the few-mode fiber Bragg grating to LP 01 The mode has a higher reflection while the LP 11 The reflectivity of the mode is low. This few-mode fiber Bragg grating that can filter out high-order modes can be realized by femtosecond laser surface-by-surface writing or dynamic writing combining femtosecond laser and phase template.

[0027] like Figure 1 The figure shows a schematic diagram of the optical path of a few-mode fiber Bragg grating that can suppress high-order mode reflection by femtosecond laser writing. The few-mode fiber includes a cladding 3 and a core 1. The femtosecond laser 7 is focused at the core 1 of the few-mode fiber through a cylindrical lens 4 and a phase template 5 in sequence. The phase template 5 and the few-mode fiber are fixed on the same three-dimensional displacement platform 6. Since the diameter of the light spot focused by the cylindrical lens 4 is small, the refractive index modulation area brought by a single light spot is too small to meet the requirements of mode control. In order to obtain a larger refractive index modulation area, the three-dimensional displacement platform 6 that fixes the phase template 5 and the few-mode fiber needs to be moved two-dimensionally along the plane where the fiber cross section is located. In this way, the radius of the refractive index modulation area 2 can be changed so that the refractive index modulation area 2 presents the desired shape.

[0028] In the case of femtosecond laser writing on each surface, there is no phase template in the system for diffraction splitting. After a single refractive index modulation zone is written in the exact center area of ​​the core end face corresponding to a certain length position in the length direction of the core, and a single grating plane is completed, the three-dimensional displacement platform 6 needs to move along the axial direction of the optical fiber to the next length position, and continue to write the refractive index modulation zone in the exact center area of ​​the core end face corresponding to the next length position to complete the writing of the next grating plane, and so on, introducing periodic refractive index modulation.

[0029] like Figure 2 The figure shows the LP in few-mode fiber. 01 Module and LP 11 The vector mode intensity distribution of the mode, where LP 01 The modulus shows a Gaussian distribution, while LP 11 There is a phase singularity at the center of the mode, near which the intensity of the mode field approaches 0. In addition, both modes show circular symmetry, and the vector modes are orthogonal to each other. Designing the transverse refractive index distribution of the few-mode fiber Bragg grating based on the symmetry and intensity distribution characteristics of the mode field can effectively achieve mode field regulation.

[0030] A few-mode fiber Bragg grating capable of suppressing high-order mode reflection, wherein a circular refractive index modulation region 2 concentric with the core is arranged in the core 1 of the few-mode fiber, the radius of the refractive index modulation region 2 is smaller than the radius of the core 1, and the refractive index modulation region 2 and the optical fiber vector mode are both circularly symmetric, LP 01 Module and LP 11 The modes will not couple with each other, and the LP 11 The self-coupling coefficient of the mode is lower than that of LP 01 Mode self-coupling coefficient, LP 11 The reflectivity of the mode is lower than that of LP 01 The reflectivity of the mode. Figure 3 As shown, it is a schematic diagram of the refractive index distribution of the transverse end face of the few-mode fiber Bragg grating. The refractive index modulation area 2 is set as a circular area with a radius R and the same center as the core 1, and the radius R of the refractive index modulation area 2 should be smaller than the radius of the core. The refractive index of the refractive index modulation area 2 is uniformly distributed. Δn is the change of the refractive index modulation area relative to the refractive index of the core, according to the overlap integral formula:

[0031]

[0032] in, is the mode field distribution, m and n are the mode numbers. 01 Module and LP 11 The two modes are orthogonal to each other and present a centrally symmetrical distribution. As long as the refractive index modulation area is set to be circular and located at the exact center of the fiber core, mutual coupling suppression can be achieved. The radius R determines the LP 11 The degree of suppression of mode self-coupling.

[0033] like Figure 4 is the LP calculated by formula (1) when the radius R of the refractive index modulation region is 2 μm. 01 Model and LP 11 The self-coupling and mutual coupling overlap integrals of each vector mode of the mode, where the mutual coupling is completely suppressed, and the LP 01 The degree of self-coupling of the mode is much greater than that of LP 11 mode self-coupling, so LP can be achieved 11 Reflection suppression of the model.

[0034] like Figure 5 For LP 01 Module and LP 11 The self-coupling overlap integral of each vector mode of the mode under different refractive index modulation zone radius R, due to LP 11 There is a phase singularity at the center of the module, LP 11 The self-coupling overlap integral of the mode basically tends to 0 when R is a small value, while LP 01The mode is strongest at the center, and at smaller radii there will be a greater degree of self-coupling.

[0035] like Figure 6 For LP 01 The self-coupling overlap integral of the mode at different refractive index modulation zone radii is used to suppress the LP 11 While mode reflection, LP must also be ensured 01 The self-coupling of the mode is at a high level, LP 01 The self-coupling overlap integral of the mode is an important reference indicator. 01 When the self-coupling coefficient of the mode is too low, the self-coupling coefficient can also be increased by increasing Δn.

[0036] like Figure 7 For LP 01 Module and LP 11 The ratio of the self-coupling overlap integral of the mode under different refractive index modulation zone radii R can reflect the LP under different refractive index modulation zone radii. 11 The suppression of mode reflection, the larger the value, the better the LP 11 The better the suppression effect of mode reflection.

[0037] comprehensive Figure 6 and Figure 7 , the smaller the radius, the better the LP 11 The better the suppression effect of mode reflection, the better the LP 01 The reflection efficiency of the mode is also at a low level. Although the self-coupling coefficient can be increased by increasing the refractive index change, the size of the refractive index modulation area is close to the wavelength of the reflected light, which is prone to Mie scattering, resulting in an increase in the insertion loss of the few-mode fiber Bragg grating. Therefore, it is appropriate to select a radius of 2 to 3 μm, which can ensure that no mutual coupling occurs and at the same time ensure LP 01 The reflection intensity of the mode and the LP 11 The suppression of mode reflection can also avoid additional insertion loss caused by the small size of the refractive index modulation area.

[0038] like Figure 8 The optical path of the laser oscillator constructed by using the few-mode fiber Bragg grating that can suppress high-order mode reflection. This embodiment is a bidirectional pumped fiber laser oscillator, including 1# pump light source 100, 2# pump light source 101, 1# pump light combiner 102, 2# pump light combiner 103, high-reflection grating 104, ytterbium-doped fiber 106, low-reflection grating 105, pump dump 107 and energy transfer fiber 108, wherein the high-reflection grating 104 and the low-reflection grating 105 both adopt the few-mode fiber Bragg grating that can suppress high-order mode reflection, wherein the high-reflection grating 104 is a high-reflection few-mode fiber Bragg grating, and the low-reflection grating 105 is a low-reflection few-mode fiber Bragg grating.

[0039] Figure 8 In the figure, the 976nm pump light generated by the 1# pump light source 100 and the 2# pump light source 101 enters the oscillator through the 1# pump combiner 102 and the 2# pump combiner 103 respectively. The output end of the 1# pump combiner 102 as forward pump is connected to the high reflection grating 104, and the other end of the high reflection grating 104 is connected to the ytterbium-doped fiber 106. After the light of different modes is amplified by the ytterbium-doped fiber 106, it passes through the low reflection grating 105. Part of the light energy is reflected back into the cavity as feedback for further amplification, and the other part of the light is output to the energy transmission fiber 108 through the 2# pump combiner 103 for output detection. The 2# pump combiner 103 realizes the function of reverse pumping. The pump dump 107 on the energy transmission fiber 108 absorbs the excess pump energy remaining in the cladding, thereby avoiding the influence of the pump light on the output signal light and improving the beam quality.

[0040] Furthermore, the laser oscillator can also be a forward pumped fiber laser oscillator or a backward pumped fiber laser oscillator. The output end of the laser oscillator can also be connected to a laser amplifier of one level or more. In the laser system, the few-mode fiber Bragg grating that can suppress the reflection of the high-order mode is used to form a fiber Bragg grating pair, which can increase the threshold of the high-order mode oscillation, making the high-order mode at a disadvantage in the competition, thereby increasing the threshold of mode instability and increasing the power of the output signal light.

[0041] In summary, although the present invention has been disclosed as above with preferred embodiments, it is not intended to limit the present invention. Any ordinary technician in the field can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention shall be based on the scope defined in the claims.

Claims

1. A few-mode fiber Bragg grating capable of suppressing high-order mode reflection, characterized in that: A circular refractive index modulation zone concentric with the core is arranged in the core of the few-mode optical fiber. The refractive index modulation zone is arranged in multiple, and the multiple refractive index modulation zones are periodically distributed in the central area of ​​the core end face at different positions in the length direction of the core. The radius of the refractive index modulation zone is smaller than the radius of the core. The refractive index modulation zone and the optical fiber vector mode are both circularly symmetrical. Model and The modes do not couple with each other, and The self-coupling coefficient of the mode is lower than The self-coupling coefficient of the mode, The reflectivity of the mode is lower than The reflectivity of the model.

2. The few-mode fiber Bragg grating capable of suppressing high-order mode reflection according to claim 1, characterized in that: The few-mode fiber Bragg grating pair The degree of mode suppression decreases as the radius of the refractive index modulation region increases.

3. The few-mode fiber Bragg grating capable of suppressing high-order mode reflection according to claim 1, characterized in that: The radius of the refractive index modulation region is .

4. The few-mode fiber Bragg grating capable of suppressing high-order mode reflection according to claim 1, characterized in that: The refractive index of the refractive index modulation zone is uniformly distributed.

5. The few-mode fiber Bragg grating capable of suppressing high-order mode reflection according to any one of claims 1 to 4, characterized in that: The refractive index modulation zone is inscribed in the core center of the few-mode optical fiber by femtosecond laser surface-by-surface inscription.

6. The few-mode fiber Bragg grating capable of suppressing high-order mode reflection according to any one of claims 1 to 4, characterized in that: The refractive index modulation zone is inscribed in the core center of the few-mode optical fiber by combining femtosecond laser and phase template.

7. Laser system, characterized in that: The invention comprises a laser oscillator, which comprises a pump light source, a pump light combiner, a high-reflection grating, a doped optical fiber and a low-reflection grating. The high-reflection grating and the low-reflection grating both adopt the few-mode fiber Bragg grating capable of suppressing high-order mode reflection as described in claim 1.

8. The laser system according to claim 7, characterized in that: The laser oscillator is a forward pumped fiber laser oscillator, a backward pumped fiber laser oscillator or a bidirectional pumped fiber laser oscillator.

9. The laser system according to claim 7, characterized in that: It also includes one or more laser amplifiers, and the output end of the laser oscillator is connected to one or more laser amplifiers.

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