High-order mode filter

Abstract

The present invention provides a high-order mode filter, which includes an optical fiber. A cladding waveguide is arranged in the cladding outside the core of the optical fiber. The cladding waveguide has a certain length, and the length direction of the cladding waveguide is consistent with the length direction of the optical fiber. Mode coupling is achieved by using the energy overlap between the evanescent fields of the core and the cladding waveguide respectively, so as to achieve the filtering of the high-order modes in the core. This device has important application value for suppressing the mode instability effect in high-power fiber lasers and improving the output power of the lasers.

Description

Technical Field

[0001] The present invention relates to the technical field of lasers, and particularly relates to a high-order mode filter. Background Art

[0002] Mode instability is an important limiting factor restricting the further improvement of the power of high-power large-core fiber lasers. Since the fiber core used in the laser has a large diameter, it can support the transmission of both LP 01 mode and LP 11 mode. The two modes interfere in the fiber core, and the intensity of the interference field shows a periodic distribution along the axial direction of the fiber. Under certain threshold conditions, due to the photo-thermal effect, the periodic interference field will cause a thermally induced refractive index long-period grating, whose period is the beat length of the interference. When there is noise or other perturbations in the laser, there is a phase difference between the interference field and the thermally induced long-period grating, and then the long-period grating moves. The energy of the LP 01 mode and the LP 11 mode is exchanged, and the power of the output laser fluctuates with a period of milliseconds, and the beam quality deteriorates sharply. Continuing to increase the pump power, the output power of the laser decreases.

[0003] Currently, the methods to suppress mode instability mainly include changing the pumping method, pumping wavelength, and signal light wavelength. These methods change the thermal load of the laser, thereby increasing the threshold of mode instability. However, these methods cannot solve the problem at the root. In addition, designing a large-core fiber that can suppress high-order modes is a long-term task. Although it is theoretically feasible, the fiber drawing process needs to be further explored. Some studies have shown that increasing the photon lifetime of the LP 01 mode or reducing the photon lifetime of the LP 11 mode can increase the mode instability threshold. The specific implementation methods include bending the fiber or writing a fiber Bragg grating with a high reflectivity for the LP 01 mode and a low reflectivity for the LP 11 mode. Bending the fiber can increase the loss of the LP 11 mode, but the loss of the LP 01 mode will also increase to a certain extent, affecting the beam quality of the output laser. And the fiber Bragg grating is mainly used for the cavity mirror of the oscillator, and is limited in the case of the amplifier. Therefore, it is very necessary to design a fiber device that can suppress the LP 11 mode and is applicable to both oscillator and amplifier cases.

[0004] With the development of mode-division multiplexing technology, all-fiber mode multiplexers have become an indispensable part of the system. Common all-fiber mode converters include few-mode long-period fiber gratings, fused taper couplers, and photonic lanterns. Due to the development of femtosecond writing technology, when the phase-matching condition is met, waveguides can be written in the cladding of the fiber to achieve a mode converter, that is, the fundamental mode is injected from the core. As the transmission distance increases, the energy of the fundamental mode gradually converts into the higher-order mode components of the cladding waveguide, and the higher-order modes are output from the cladding waveguide, serving as the energy transmission channel of the mode-division multiplexing system. The optical energy can be converted from the fundamental mode of one waveguide to the higher-order mode of another waveguide. According to the principle of optical path reversibility, the energy of the higher-order mode can also be converted to the fundamental mode. By this method, the filtering of higher-order modes in large-core diameter fibers can be achieved, and different from the bending method, this method will not affect the fundamental mode. Summary of the Invention

[0005] In view of the defects existing in the prior art, the present invention proposes a higher-order mode filter.

[0006] To achieve the above technical objectives, the specific technical solutions adopted by the present invention are as follows:

[0007] The higher-order mode filter includes an optical fiber, and a cladding waveguide is written in the cladding outside the core of the optical fiber. The cladding waveguide has a certain length, and the length direction of the cladding waveguide is the same as the length direction of the optical fiber. The energy overlap between the evanescent fields of the core and the cladding waveguide is used to achieve mode coupling, thereby achieving the filtering of the core higher-order modes. This device has important application value for suppressing the mode instability effect in high-power fiber lasers and improving the output power of the lasers. Further, the optical fiber of the present invention is a large-core diameter fiber with a core diameter of 20 μm, which can support the transmission of two modes at the same time, that is, LP 01 mode and LP 11 mode.

[0008] As a preferred embodiment of the present invention, the cladding waveguide is written by femtosecond laser. Cladding waveguides with different refractive indices, different lengths, and different distances from the core are written by femtosecond laser. Among them, the femtosecond laser is focused by the lens in the femtosecond laser writing device in the cladding outside the core to achieve refractive index change. The distance between the cladding waveguide and the core, that is, the distribution of the cladding waveguide in the transverse plane, is achieved by the movement of the displacement platform in the femtosecond laser writing device in the transverse plane. The length of the cladding waveguide is determined by the movement of the displacement platform along the axial direction of the optical fiber. By changing the single-pulse energy of the femtosecond laser, the refractive index change of the cladding waveguide can be achieved.

[0009] As a preferred embodiment of the present invention, in order to achieve coupling with a common single-mode fiber, the radius of the cladding waveguide of the present invention is set to 4 μm. In addition, in order to ensure the coupling of the core and the cladding waveguide modes, that is, the core LP 11The evanescent field of the mold overlaps with the evanescent field of the fundamental mode of the cladding waveguide. The distance between the core and the cladding waveguide should not be too large, and the distance between the two structures is within 20 μm. Further, for better results, the refractive index of the cladding waveguide needs to satisfy: the effective refractive index of the fundamental mode of the cladding waveguide is close to the effective refractive index of the core LP 11 mode to meet the phase matching condition. In order to couple the core LP 11 mode with the fundamental mode of the cladding waveguide, the following phase matching condition must be satisfied:

[0010]

[0011] where is the propagation constant of the core LP 11 mode, and β waveguide is the propagation constant of the fundamental mode of the cladding waveguide, that is, it only needs to ensure that the effective refractive index of the fundamental mode of the cladding waveguide is equal to or close to the effective refractive index of the core LP 11 mode. The relationship between the propagation constant of the fundamental mode of the cladding waveguide and the effective refractive index of the fundamental mode of the cladding waveguide is:

[0012]

[0013] where λ is the laser wavelength and n eff is the effective refractive index of the fundamental mode of the cladding waveguide.

[0014] As a preferred embodiment of the present invention, the outer cladding in the x-direction and y-direction of the fiber core of the present invention is inscribed with a cladding waveguide of a certain length. The LP 11a mode in the core is coupled with the fundamental mode of the cladding waveguide in the x-direction, and the LP 11b mode in the core is coupled with the fundamental mode of the cladding waveguide in the y-direction, so as to maximally filter the LP 11 mode in the core.

[0015] The present invention utilizes the coupled mode theory to inscribe a single-mode waveguide in the cladding of a large-core fiber by femtosecond laser, so as to realize the coupling of the LP 11 mode in the core to the fundamental mode of the cladding waveguide, thereby realizing the filtering of the high-order modes in the core, and further increasing the threshold of the mode instability of the high-power fiber laser. This device has the advantages of high integration and easy processing, and will not bring negative effects to the laser.

[0016] The present invention provides a design method for a high-order mode filter, including:

[0017] (1) Given a large-core fiber, the core diameter, cladding radius, and refractive index of the large-core fiber are known;

[0018] (2) Determine each parameter of the cladding waveguide to be inscribed, including the number of cladding waveguides, the length of each cladding waveguide, the refractive index of each cladding waveguide, the radius of each cladding waveguide, and the distance between each cladding waveguide and the core.

[0019] (3) According to the parameters determined in step (2), use femtosecond laser inscription on the large-core fiber given in step (1) to obtain the corresponding high-order mode filter.

[0020] Furthermore, step (2) of the present invention includes:

[0021] (2.1) Given the cladding radius and refractive index of the optical fiber, establish a single waveguide model, and use finite element software to calculate and determine the range of the radius and refractive index of the cladding waveguide so that it can only transmit the fundamental mode. It is necessary to satisfy that the effective refractive index of the fundamental mode of the cladding waveguide is close to the effective refractive index of the LP 11 mode of the core to satisfy the following phase matching condition:

[0022]

[0023] where is the propagation constant of the LP 11 mode of the core, and β waveguide is the propagation constant of the fundamental mode of the cladding waveguide. The relationship between the propagation constant of the fundamental mode of the cladding waveguide and the effective refractive index of the fundamental mode of the cladding waveguide is:

[0024]

[0025] where λ is the laser wavelength, and n eff is the effective refractive index of the fundamental mode of the cladding waveguide.

[0026] (2.2) According to the known core radius and refractive index of the optical fiber and the radius and refractive index of the cladding waveguide determined in (2.1), establish a transverse plane model of the mode filter. Using finite element software, the distance between the cladding waveguide and the core can be determined, and the length of the cladding waveguide can be obtained according to the following coupling length formula:

[0027]

[0028] where, in the case of coupling between the core and the cladding waveguide, the two waveguides are regarded as a whole, and the mode field at this time is considered to be formed by the superposition of odd-symmetric and even-symmetric supermodes. β e and β o are the propagation constants of the even-symmetric and odd-symmetric supermodes respectively.

[0029] Under the condition of satisfying the phase matching condition, writing a waveguide in the cladding of an optical fiber can achieve mode conversion, that is, the fundamental mode is injected from the core. As the transmission distance increases, the energy of the fundamental mode is gradually converted into the high-order mode component of the cladding waveguide, and the high-order mode is output from the cladding waveguide. The optical energy can be converted from the fundamental mode of one waveguide to the high-order mode of another waveguide. According to the principle of optical path reversibility, the energy of the high-order mode can also be converted to the fundamental mode. Through this method, the filtering of high-order modes in large-core-diameter optical fibers can be achieved, and different from the bending method, this method will not affect the fundamental mode.

[0030] The beneficial effects of the present invention are as follows:

[0031] The high-order mode filter of the present invention can be applied to both high-power fiber oscillators and high-power amplifiers.

[0032] The high-order mode filter of the present invention directly acts on the LP 11 mode and will not affect the LP 01 mode. Compared with the bending method of losing high-order modes, this device can avoid the degradation of the output beam quality.

[0033] The high-order mode filter of the invention is written by femtosecond laser direct writing, and its properties will not change under high-power environments and has extremely high thermal stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on the structures shown in these drawings.

[0035] Figure 1 is a schematic structural diagram of Embodiment 1 (a high-order mode filter with one cladding waveguide).

[0036] Figure 2 is a diagram showing the influence of the refractive index of the cladding waveguide on its effective refractive index of the fundamental mode.

[0037] Figure 3 is a mode field diagram with a high-order mode filter at different transmission lengths, where (a) has a transmission length L = 0; (b) has a transmission length L = 0.9835 mm; (c) has a transmission length L = 1.967 mm.

[0038] Figure 4 is the influence of the distance between the core and the cladding waveguide on the coupling length of the high-order mode filter.

[0039] Figure 5It is a schematic structural diagram of Embodiment 2 (a high-order mode filter with two cladding waveguides).

[0040] Figure 6 It is a schematic diagram of the processing process of the high-order mode filter.

[0041] Legend Explanation:

[0042] C1: Core of the large-core diameter optical fiber; C2: X-axis cladding waveguide; C3: Y-axis cladding waveguide; r1: Inner cladding radius of the large-core diameter optical fiber; r2: Core radius of the large-core diameter optical fiber; r3: Radius of the cladding waveguide; n1: Cladding refractive index, with a value of 1.4584; n2: Core refractive index, with a value of 1.4598; n3: Cladding waveguide refractive index, by adjusting this value, the LP in the core 11 mode and the fundamental mode of the cladding waveguide satisfy the phase matching condition; D: Spacing between the core and the cladding waveguide; L: Length of the cladding waveguide. Specific Embodiments

[0043] In order to make the technical solutions and advantages of the present invention clearer, the present 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 only used to explain the present invention and are not used to limit the present invention.

[0044] Embodiment 1:

[0045] As Figure 1 shown, it is a schematic structural diagram of Embodiment 1. The high-order mode filter includes a large-core diameter optical fiber, and an X-axis cladding waveguide C2 is inscribed in the cladding outside the core C1 of the large-core diameter optical fiber. The X-axis cladding waveguide C2 has a certain length, and the length direction of the X-axis cladding waveguide C2 is consistent with the length direction of the large-core diameter optical fiber. Mode coupling is achieved by using the energy overlap between the evanescent fields of the core C1 of the large-core diameter optical fiber and the X-axis cladding waveguide C2 respectively, thereby realizing the filtering of the core high-order mode. The inner cladding radius of the large-core diameter optical fiber is r1, and in this embodiment, r1 = 200 μm. The core radius of the large-core diameter optical fiber is r2, and in this embodiment, r2 = 10 μm. The radius of the X-axis cladding waveguide C2 is r3, and in this embodiment, r3 = 4 μm.

[0046] In order to achieve coupling with a common single-mode optical fiber, the radius of the cladding waveguide, that is, the X-axis cladding waveguide C2 in this embodiment, is set to 4 μm. Further, in order to ensure that the core and the cladding waveguide modes are coupled, that is, the evanescent field of the core LP 11 mode and the evanescent field of the fundamental mode of the cladding waveguide overlap, the distance between the core and the cladding waveguide should not be too large, and the refractive index of the cladding waveguide needs to be reasonably designed so that the effective refractive index of the fundamental mode of the cladding waveguide is close to the effective refractive index of the core LP 11 mode to meet the phase matching condition.

[0047] As Figure 2 shown is the influence of the cladding waveguide refractive index on its fundamental mode effective refractive index. In order to couple the core LP 11 mode with the waveguide fundamental mode, the phase matching condition must be satisfied:

[0048]

[0049] where is the propagation constant of the core LP 11 mode, and β waveguide is the propagation constant of the cladding waveguide fundamental mode, that is, it only needs to ensure that the effective refractive index of the waveguide fundamental mode is the same as that of the core LP 11 mode. As Figure 3 shown, as the refractive index of the cladding waveguide increases, the effective refractive index of its fundamental mode also increases. When the refractive index of the cladding waveguide n3 = 1.46017, the effective refractive index of the cladding waveguide fundamental mode is close to that of the core LP 11 mode.

[0050] As Figure 3 shown, when the distance between the core and the cladding waveguide C2 on the x-axis is 15 μm, the mode field distribution under different transmission distances is considered here. Only the case where the LP 11 mode is incident from the core is considered. The LP 01 mode does not satisfy the phase matching condition and will not be coupled into the cladding waveguide. In the case where the core and the cladding waveguide are coupled, the two waveguides can be regarded as a whole. At this time, the mode field can be considered to be formed by the superposition of odd-symmetric and even-symmetric supermodes:

[0051]

[0052] where A is the amplitude, z represents the transmission distance, E e (x,y) and E o (x,y) are the vector distributions of the even-symmetric and odd-symmetric supermodes respectively, and β e and β o are the propagation constants of the even-symmetric and odd-symmetric supermodes respectively. When the transmission length is 0, only the core LP 11 mode exists. As the transmission length increases, the LP 11 mode gradually transforms into the fundamental mode of the cladding waveguide. When the length is 0.9835 mm, half of the energy of the LP 11 mode is transferred into the cladding waveguide. According to the supermode theory, the calculation formula for the coupling length is:

[0053]

[0054] When the distance between the core and the cladding waveguide is 15 μm, the coupling length is 1.967 mm. At this time, LP11 The mode conversion is maximized, and only a small part of the energy of the LP mode remains in the core, which is the crosstalk of the device. Due to the symmetry of the device, the energy conversion only occurs between the LP mode and the fundamental mode of the cladding waveguide. If LP mode filtering is required, another cladding waveguide needs to be set on the y-axis of 11 the 11a mode and the fundamental mode of the cladding waveguide. For example, if LP mode filtering is required, another cladding waveguide needs to be set on the y-axis of 11b the Figure 2 device.

[0055] As Figure 4 shown in the figure, the coupling length corresponding to different core-cladding waveguide spacings is shown. As the spacing increases, the coupling length also becomes longer. By controlling the spacing, the coupling length can be adjusted, and then the heat dissipation characteristics of the device can be regulated, which is very important in actual lasers.

[0056] Embodiment 2:

[0057] Based on Embodiment 1, the high-order mode filter provided in this embodiment has an x-axis cladding waveguide C2 and a y-axis cladding waveguide C3 inscribed in the cladding outside the core C1 of the large-core fiber. The lengths, refractive indices, and spacings from the core of the x-axis cladding waveguide C2 and the y-axis cladding waveguide C3 are exactly the same.

[0058] As Figure 5 shown, when the spacings between the core and the x-axis cladding waveguide C2 and the y-axis cladding waveguide C3 are both 15 μm, a single-mode waveguide with a length of 1.967 mm is inscribed in the cladding in the x-direction and y-direction of the core of the large-core fiber by using femtosecond laser direct writing technology. The parameters such as the lengths, spacings from the core, and refractive indices of the x-axis cladding waveguide C2 and the y-axis cladding waveguide C3 must be reasonably designed, so that the LP 11a mode in the core is coupled to the fundamental mode of the x-direction cladding waveguide, and the LP 11b mode in the core is coupled to the fundamental mode of the y-direction cladding waveguide. This design can maximize the filtering of the LP 11 mode in the core.

[0059] The present invention uses femtosecond laser direct writing technology to inscribe a waveguide with a certain length in the cladding near the core of the large-core fiber, realizing the coupling of the LP 11 mode in the core to the cladding waveguide, thereby realizing the filtering of the high-order mode in the core. This device has important application value for suppressing the mode instability effect in high-power fiber lasers and improving the output power of lasers.

[0060] As Figure 6As shown, a single-mode waveguide is inscribed in the cladding near the core of a large-core fiber by using femtosecond laser direct writing technology, and mode coupling is achieved by the energy overlap between the evanescent fields of the core and the cladding waveguide respectively. The femtosecond laser is focused by a lens in the cladding near the core to achieve a refractive index change. The distribution of the waveguide in the transverse plane is realized by the movement of the displacement platform in this plane, and the length of the waveguide is determined by the movement of the displacement platform along the fiber axis. By changing the single-pulse energy of the femtosecond laser, the refractive index of the waveguide can be changed.

[0061] Example 3:

[0062] This embodiment provides a method for fabricating a high-order mode filter, including:

[0063] (1) Given a large-core fiber, the core diameter, cladding radius, and refractive index of the large-core fiber are known;

[0064] (2) Determine the parameters of the cladding waveguide to be inscribed, including the number of cladding waveguides, the length of each cladding waveguide, the refractive index of each cladding waveguide, the radius of each cladding waveguide, and the distance between each cladding waveguide and the core.

[0065] (2.1) Given the cladding radius and refractive index of the fiber, establish a single waveguide model, and calculate and determine the range of the radius and refractive index of the cladding waveguide through finite element software so that it can only transmit the fundamental mode, and it is necessary to satisfy that the effective refractive index of the fundamental mode of the cladding waveguide is close to the effective refractive index of the core LP 11 mode to meet the following phase matching condition:

[0066]

[0067] where is the propagation constant of the core LP 11 mode, β waveguide is the propagation constant of the fundamental mode of the cladding waveguide, and the relationship between the propagation constant of the fundamental mode of the cladding waveguide and the effective refractive index of the fundamental mode of the cladding waveguide is:

[0068]

[0069] where λ is the laser wavelength, and n eff is the effective refractive index of the fundamental mode of the cladding waveguide.

[0070] (2.2) According to the known core radius and refractive index of the fiber and the radius and refractive index of the cladding waveguide determined in (2.1), establish a transverse plane model of the mode filter, and use finite element software to determine the distance between the cladding waveguide and the core, and obtain the length of the cladding waveguide according to the following coupling length formula:

[0071]

[0072] Among them, in the case where the core and cladding waveguides are coupled, the two waveguides are regarded as a whole, and the mode field at this time is considered to be formed by the superposition of odd-symmetric and even-symmetric supermodes, β e and β o are the propagation constants of the even-symmetric and odd-symmetric supermodes, respectively.

[0073] (3) According to the parameters determined in step (2), a corresponding high-order mode filter is written on the large-core fiber given in step (1) by using femtosecond laser writing.

[0074] In summary, although the present invention has been disclosed above with preferred embodiments, it is not intended to limit the present invention. Any person of ordinary skill in the art can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the scope defined by the claims.

Claims

1. Method for manufacturing a high-order mode filter, characterized in that, Including: (1) Given a large-core fiber, the core diameter, cladding radius, and refractive index of the large-core fiber are known; (2) Determine the parameters of the cladding waveguide to be inscribed, including the number of cladding waveguides, the length of each cladding waveguide, the refractive index of each cladding waveguide, the radius of each cladding waveguide, and the distance between each cladding waveguide and the core; according to the known core radius and refractive index of the optical fiber, the determined radius and refractive index of the cladding waveguide, establish a transverse plane model of the mode filter, use finite element software to determine the distance between the cladding waveguide and the core, and obtain the length of the cladding waveguide according to the following coupling length formula: ； Among them, and are the propagation constants of the even-symmetric and odd-symmetric supermodes, respectively; (3) According to the parameters determined in step (2), use femtosecond laser inscription on the large-core fiber given in step (1) to obtain the corresponding high-order mode filter.

2. The manufacturing method of the high-order mode filter according to claim 1, wherein In step (2), according to the cladding radius and refractive index of the optical fiber, a single waveguide model is established. By calculating with finite element software, the range of the radius and refractive index of the cladding waveguide is determined so that it can only transmit the fundamental mode. It is necessary to satisfy that the effective refractive index of the fundamental mode of the cladding waveguide is close to that of the core mode to meet the following phase matching condition: ； wherein is the propagation constant of the core is the propagation constant of the fundamental mode, is the propagation constant of the fundamental mode of the cladding waveguide, and the relationship between the propagation constant of the fundamental mode of the cladding waveguide and the effective refractive index of the fundamental mode of the cladding waveguide is as follows: ； Among them, is the laser wavelength, is the effective refractive index of the fundamental mode of the cladding waveguide.

3. The high-order mode filter manufactured by the manufacturing method of the high-order mode filter as described in claim 1, wherein: including an optical fiber, the core of the optical fiber being capable of simultaneously supporting two-mode transmission, namely mode and mode; a cladding waveguide is disposed in the cladding outside the core of the optical fiber, the cladding waveguide having a certain length, the length direction of the cladding waveguide being consistent with the length direction of the optical fiber, and energy overlap between the respective evanescent fields of the core and the cladding waveguide realizes mode coupling.

4. The high-order mode filter according to claim 3, wherein: The optical fiber is a large-core optical fiber with a core diameter of .

5. The high-order mode filter according to claim 3, characterized in that: The cladding waveguide is formed by femtosecond laser inscription.

6. The high-order mode filter according to claim 3, wherein: The radius of the said cladding waveguide is set to .

7. The high-order mode filter according to claim 3, wherein: The spacing between the core and the cladding waveguide is within inward.

8. The high-order mode filter according to claim 3, wherein: The refractive index of the cladding waveguide satisfies the following phase matching condition: ； wherein is the propagation constant of the core mode, is the propagation constant of the fundamental mode of the cladding waveguide, and the relationship between the propagation constant of the fundamental mode of the cladding waveguide and the effective refractive index of the fundamental mode of the cladding waveguide is: ； Among them, is the laser wavelength, is the effective refractive index of the fundamental mode of the cladding waveguide.

9. The high-order mode filter according to any one of claims 3 to 8, characterized in that: The x direction of the optical fiber core and the y direction of the outer cladding are both inscribed with a certain length of cladding waveguides. The mode in the core is coupled with the fundamental mode of the x direction of the cladding waveguide, and the mode in the core is coupled with the fundamental mode of the y direction of the cladding waveguide.

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