CN212364642U

CN212364642U - High-order mode filter

Info

Publication number: CN212364642U
Application number: CN202021591143.1U
Authority: CN
Inventors: 王泽锋; 李宏业; 王蒙; 饶斌裕; 田鑫; 赵晓帆; 胡琪浩; 奚小明; 陈子伦; 潘志勇; 王小林; 许晓军; 陈金宝
Original assignee: National University of Defense Technology
Current assignee: National University of Defense Technology
Priority date: 2020-08-04
Filing date: 2020-08-04
Publication date: 2021-01-15
Anticipated expiration: 2030-08-04

Abstract

The utility model provides a high order mode filter, including optic fibre, be provided with the cladding waveguide in the cladding in the optic fibre core outside, the cladding waveguide has certain length, the length direction of cladding waveguide is unanimous with the length direction of optic fibre. Mode coupling is realized by utilizing energy overlapping between evanescent fields of a fiber core and a cladding waveguide, so that filtering of a fiber core high-order mode is realized, and the device has important application value for inhibiting mode instability effect in a high-power optical fiber laser and improving the output power of the laser.

Description

High-order mode filter

Technical Field

The utility model relates to a laser instrument technical field, concretely relates to high order mode filter.

Background

Mode instability is an important limiting factor for limiting the further increase of the power of the high-power large-core fiber laser. Because of the large core diameter of the optical fiber used by the laser, the laser can simultaneously support LP₀₁Mold and LP₁₁The two modes interfere in the fiber core, the intensity of an interference field presents periodic distribution along the axial direction of the optical fiber, under the condition of a certain threshold value, due to photo-thermal effect, the periodic interference field can cause a thermotropic refractive index long-period grating, the period of the grating is beat length of interference, when the laser has noise or other disturbance, phase difference exists between the interference field and the thermotropic long-period grating, the long-period grating moves, and LP (Long period) grating is moved₀₁Mold and LP₁₁The energy of the mode is exchanged, the power generation period of the output laser is the fluctuation of millisecond level, the quality of the light beam is rapidly deteriorated, the pumping power is continuously improved, and the output power of the laser is reduced.

The current method for suppressing the mode instability is mainly realized by changing the pumping mode, the pumping wavelength and the wavelength of the signal lightThese methods change the thermal load of the laser, thereby raising the threshold for mode instability. These methods, however, do not solve the problem at its root. In addition, designing optical fibers with large core diameters that suppress higher order modes is a long-standing task, and while theoretically feasible, the fiber drawing process is yet to be explored. It has been shown that the improvement of LP₀₁Photon lifetime of modes or reduction of LP₁₁The photon lifetime of the mode can improve the mode instability threshold, and the specific implementation method comprises bending the optical fiber or writing LP₀₁High mode reflectivity and LP₁₁Fiber Bragg grating with low mode reflectivity, bending the fiber to improve LP₁₁Loss of mode, but LP₀₁The loss of mode will be improved to a certain extent, and the quality of output laser beam is affected, and the fiber Bragg grating is mainly used for cavity mirror of oscillator, and the condition of amplifier is limited, so that it designs a kind of suppressing LP which is applicable to oscillator and amplifier condition at the same time₁₁A fiber optic device of the mode is necessary.

With the development of the mode division multiplexing technology, the all-fiber mode multiplexer becomes an indispensable part of the system. Common all-fiber mode converters include few-mode long-period fiber gratings, fused biconical tapered couplers, photonic lanterns, and the like. Due to the development of the femtosecond writing technology, under the condition of meeting the phase matching condition, writing the waveguide in the cladding of the optical fiber can realize the mode converter, namely, the fundamental mode is injected from the fiber core, along with the increase of the transmission distance, 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 and is used as the energy transmission channel of the mode division multiplexing system. The light energy can be converted from a basic mode of one waveguide to a high-order mode of the other waveguide, the energy of the high-order mode can also be converted into the basic mode according to the principle that the light path is reversible, the method can be used for filtering the high-order mode in the large-core optical fiber, and the method is different from a bending method and cannot influence the basic mode.

SUMMERY OF THE UTILITY MODEL

To the defect that exists among the prior art, the utility model provides a high order mode filter.

In order to achieve the technical purpose, the utility model discloses a specific technical scheme as follows:

the high-order mode filter comprises an optical fiber, wherein a cladding waveguide is inscribed in a cladding on the outer side of a fiber core of the optical fiber, the cladding waveguide has a certain length, the length direction of the cladding waveguide is consistent with the length direction of the optical fiber, and mode coupling is realized by energy overlapping between evanescent fields of the fiber core and the cladding waveguide. The utility model discloses can realize the filtering of fibre core high order mode. The device has important application value for inhibiting mode instability effect in a high-power optical fiber laser and improving the output power of the laser. Furthermore, the optical fiber of the present invention is a large core diameter optical fiber with a core diameter of 20 μm, which can simultaneously support two mode transmission, i.e. LP₀₁Mold and LP₁₁And (5) molding.

As the preferred scheme of the utility model, the cladding waveguide adopts femto second laser to write and form. And (3) writing cladding waveguides with different refractive indexes, different lengths and different intervals with the fiber core by femtosecond laser. Wherein the femtosecond laser is focused in the cladding outside the fiber core through a lens in the femtosecond laser writing device to realize refractive index change. The distance between the cladding waveguide and the fiber core, namely the distribution of the cladding waveguide in the transverse plane is realized by the movement of a displacement platform in a 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. The change of the refractive index of the cladding waveguide can be realized by changing the single pulse energy of the femtosecond laser.

As the preferred scheme of the utility model, in order to realize the coupling with ordinary single mode fiber, the radius of cladding waveguide sets up to 4 μm. In addition, to ensure coupling of the core to the cladding waveguide mode, i.e. the core LP₁₁The evanescent field of the mode and the evanescent field of the cladding waveguide fundamental mode are overlapped, the distance between the fiber core and the cladding waveguide is not too large, and the distance between the two structures is within 20 mu m. Further, for better effect, the refractive index of the clad waveguide needs to satisfy: effective refractive index of cladding waveguide fundamental mode and fiber core LP₁₁The effective indices of the modes are close to satisfy the phase matching condition. In order to make the fiber core LP₁₁Mode coupling with cladding waveguide fundamental modeThe following phase matching conditions must be satisfied:

wherein

Is a fiber core LP₁₁Propagation constant of the mode, beta_waveguideIs the propagation constant of the fundamental mode of the cladding waveguide, i.e. only the effective refractive index of the fundamental mode of the cladding waveguide and the LP of the fiber core₁₁The effective refractive indices of the modes are equal or close. The relationship between the propagation constant of the cladding waveguide fundamental mode and the effective refractive index of the cladding waveguide fundamental mode is:

wherein λ is the wavelength of the laser, n_effIs the effective refractive index of the cladding waveguide fundamental mode.

As the preferred scheme of the utility model, all write the cladding waveguide of a certain length in the outside cladding of the x direction of optic fibre core and y direction. LP in the core_11aMode coupling to fundamental mode of cladding waveguide in x-direction, LP in core_11bThe mode is coupled with the basic mode of the cladding waveguide in the y direction, and LP in the fiber core is realized to the maximum extent₁₁And (4) filtering out the die.

The utility model discloses utilize the coupled mode theory, carve the single mode waveguide in the cladding of big core footpath optic fibre through femto second laser to realize LP in the fibre core₁₁The mode is coupled to the fundamental mode of the cladding waveguide, so that filtering of medium and high order modes of the fiber core is realized, the unstable threshold of the high-power fiber laser mode is further improved, and the device has the advantages of high integration level, easiness in processing and the like, and cannot bring negative influence to the laser.

The utility model has the advantages as follows:

the utility model discloses a high order mode filter both can apply to high power fiber oscillator, can apply to high power amplifier again.

The utility model discloses a high-order mode filter is direct to LP₁₁The mold acts on LP₀₁The mode has an effect that the device can avoid the quality degradation of the output beam compared to the bending method which loses the higher order mode.

The utility model discloses a high order mode filter directly writes through femto second laser, and its nature can not change under the high power environment, and has very high thermal stability.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings needed to be used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the structures shown in the drawings without creative efforts.

FIG. 1 is a schematic view of the structure of example 1 (high-order mode filter having one clad waveguide).

FIG. 2 is a graph of the effect of the cladding waveguide index on the effective index of its fundamental mode.

Fig. 3 is a mode field diagram with a high-order mode filter for different transmission lengths, where the transmission length L in (a) is 0; (b) the middle transmission length L is 0.9835 mm; (c) the medium transmission length L is 1.967 mm.

FIG. 4 is the effect of the spacing between the core and cladding waveguides on the coupling length with a high order mode filter.

FIG. 5 is a schematic view of the structure of example 2 (high-order mode filter with two clad waveguides).

FIG. 6 is a schematic diagram of a high order mode filter fabrication process.

Illustration of the drawings:

C₁: a core of the large-core optical fiber; c₂: an x-axis cladding waveguide; c₃: a y-axis cladding waveguide; r is₁: the radius of an inner cladding of the large-core optical fiber; r is₂: the core radius of the large-core optical fiber; r is₃: the radius of the cladding waveguide;n₁: a cladding index of refraction value 1.4584; n is₂: a core index of 1.4598; n is₃: the refractive index of the cladding waveguide is adjusted to make LP in the core₁₁The mode and the basic mode of the cladding waveguide meet the phase matching condition; d, the distance between the fiber core and the cladding waveguide; l: the length of the cladding waveguide.

Detailed Description

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 with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the invention.

Example 1:

as shown in fig. 1, it is a schematic structural view of embodiment 1. The high-order mode filter comprises a large-core-diameter optical fiber and a fiber core C on the large-core-diameter optical fiber₁An x-axis cladding waveguide C is inscribed in the outer cladding₂Said x-axis cladding waveguide C₂Having a length, said x-axis cladding waveguide C₂The longitudinal direction of (a) is identical to the longitudinal direction of the large core optical fiber. Core C using large core diameter optical fiber₁And x-axis cladding waveguide C₂Mode coupling is realized by energy overlapping between the respective evanescent fields, so that the filtering of a fiber core high-order mode is realized. The radius of the inner cladding of the large-core optical fiber is r₁In this embodiment, r₁200 μm. The core radius of the large-core optical fiber is r₂In this embodiment, r ₂10 μm. x-axis cladding waveguide C₂Has a radius of r₃In this embodiment, r₃＝4μm。

In order to couple with a common single mode fiber, the cladding waveguide in this embodiment, i.e. the x-axis cladding waveguide C₂Is set to be 4 μm. Further, to ensure coupling of the core to the cladding waveguide mode, i.e. the core LP₁₁The evanescent field of the mode and the evanescent field of the cladding waveguide fundamental mode are overlapped, the distance between the fiber core and the cladding waveguide is not 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 and the fiber core LP (Low-refractive index)₁₁The effective refractive indices of the modes are close to each other,to satisfy the phase matching condition.

The effect of the cladding waveguide index on the effective index of its fundamental mode is shown in figure 2. In order to make the fiber core LP₁₁The mode is coupled with the waveguide fundamental mode, and the phase matching condition must be met:

wherein

Is a fiber core LP₁₁Propagation constant of the mode, beta_waveguideIs the propagation constant of the cladding waveguide fundamental mode, i.e. only the effective refractive index of the waveguide fundamental mode and the fiber core LP need to be ensured₁₁The effective refractive index of the mode. As shown in FIG. 3, as the refractive index of the clad waveguide increases, the effective refractive index of the fundamental mode increases, and the refractive index n in the clad waveguide increases₃1.46017, the effective refractive index of the cladding waveguide fundamental mode and the core LP₁₁The effective refractive indices of the modes are similar.

As shown in fig. 3, is a core and x-axis cladding waveguide C₂The mode field distribution at different transmission distances at a pitch of 15 μm is considered here only for LP₁₁Case of mode incidence from the core, LP₀₁The mode does not satisfy the phase matching condition and does not couple into the cladding waveguide. Under the condition that the fiber core and the cladding waveguide are coupled, the two waveguides can be regarded as a whole, and the mode field can be regarded as formed by overlapping of odd-symmetric super modes and even-symmetric super modes:

wherein A is amplitude, z represents transmission distance, E_e(x, y) and E_o(x, y) are vector distributions of even-symmetric and odd-symmetric supermodes, beta_eAnd beta_oThe propagation constants of the even symmetric and odd symmetric supermodes are respectively. At transmission length 0, only the core LP₁₁Presence of modes, with length of transmissionIncrease of (2), LP₁₁The mode is gradually converted to the fundamental mode of the cladding waveguide, and half of the LP is obtained at a length of 0.9835mm₁₁The energy of the mode is converted into the cladding waveguide, and according to the theory of the supermode, the calculation formula of the coupling length is as follows:

when the core and cladding waveguides are spaced 15 μm apart, the coupling length is 1.967mm, at which time LP₁₁The mode realizes maximum conversion, and only a small part of LP is left in the fiber core₁₁The energy of the mode, which is the crosstalk of the device. Due to the symmetry of the device, energy conversion only occurs at LP_11aBetween mode and cladding-waveguide fundamental mode, if desired to achieve LP_11bThe mode filtering requires a further cladding waveguide on the y-axis of figure 2.

As shown in fig. 4, the coupling lengths corresponding to different core-to-cladding waveguide spacings are longer as the spacing increases. The coupling length is adjusted by controlling the distance, so that the heat dissipation characteristic of the device is regulated and controlled, which is very important in the actual laser.

Example 2:

based on embodiment 1, the high-order mode filter provided in this embodiment is applied to the core C of the large-core optical fiber₁An x-axis cladding waveguide C is inscribed in the outer cladding₂And y-axis clad waveguide C₃. x-axis cladding waveguide C₂And y-axis clad waveguide C₃Is identical in length, refractive index and spacing from the core.

As shown in fig. 5, the core and the x-axis cladding waveguide C₂And y-axis clad waveguide C₃Under the condition that the spacing is 15 mu m, a single-mode waveguide with the length of 1.967mm is engraved on the cladding in the x direction and the y direction of the fiber core of the large-core-diameter fiber by using the femtosecond laser direct writing technology. x-axis cladding waveguide C₂And y-axis clad waveguide C₃Must be reasonably designed, the length of the core, the interval between the core and the fiber, the refractive index and other parameters_11aCladding of mode in x directionFundamental mode coupling of waveguides, LP in the core_11bThe fundamental mode coupling of the cladding waveguide with the mode in the y direction can realize LP in the fiber core to the maximum extent₁₁And (4) filtering out the die.

The utility model discloses utilize femto second laser to directly write the technique, carve the waveguide of writing certain length near the cladding of big core footpath optic fibre core, realize the LP of fibre core₁₁The mode is coupled to the cladding waveguide, so that the filtering of a fiber core high-order mode is realized, and the device has important application value for inhibiting the mode instability effect in a high-power fiber laser and improving the output power of the laser.

As shown in fig. 6, a single-mode waveguide is written on the cladding layer near the core of the large-core optical fiber by using the femtosecond laser direct writing technology, and mode coupling is realized by using energy overlapping between evanescent fields of the core and the cladding waveguide. The femtosecond laser is focused in a cladding layer near a fiber core through a lens to realize refractive index change, the distribution of the waveguide in a transverse plane is realized by the movement of the displacement platform in the plane, the length of the waveguide is determined by the movement of the displacement platform along the axial direction of the optical fiber, and the refractive index change of the waveguide can be realized by changing the single pulse energy of the femtosecond laser.

The method for manufacturing the high-order mode filter in the embodiment comprises the following steps:

(1) given a large-core optical fiber, the core diameter, the cladding radius and the refractive index of the large-core optical fiber are known;

(2) various parameters of the cladding waveguides to be inscribed are determined, including the number of cladding waveguides and the length of each cladding waveguide, the refractive index of each cladding waveguide, the radius of each cladding waveguide, and the distance of each cladding waveguide from the core.

(2.1) knowing the cladding radius and the refractive index of the optical fiber, establishing a single waveguide model, and calculating and determining the radius and the refractive index range of the cladding waveguide through finite element software to ensure that the cladding waveguide can only transmit a fundamental mode and the effective refractive index of the cladding waveguide fundamental mode and the fiber core LP are required to be met₁₁The effective indices of the modes are close to satisfy the following phase matching conditions:

wherein

Is a fiber core LP₁₁Propagation constant of the mode, beta_waveguideIs the propagation constant of the cladding waveguide fundamental mode, wherein the relationship between the propagation constant of the cladding waveguide fundamental mode and the effective refractive index of the cladding waveguide fundamental mode is:

(2.2) establishing a mode filter transverse plane model according to the known radius and refractive index of the fiber core of the optical fiber and the radius and refractive index of the cladding waveguide determined in (2.1), determining the distance between the cladding waveguide and the fiber core by using finite element software, and obtaining the length of the cladding waveguide according to the following coupling length formula:

wherein, under the condition that the fiber core and the cladding waveguide are coupled, the two waveguides are regarded as a whole, and the mode field is formed by overlapping odd-symmetric super modes and even-symmetric super modes, namely beta_eAnd beta_oThe propagation constants of the even symmetric and odd symmetric supermodes are respectively.

(3) And (3) according to the parameters determined in the step (2), writing on the large-core-diameter optical fiber given in the step (1) by adopting femtosecond laser to obtain a corresponding high-order mode filter.

In summary, although the present invention has been described with reference to the preferred embodiments, it should be understood that the present invention is not limited thereto, and those skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention.

Claims

1. High order mode filter, its characterized in that: comprising optical fibres whose core is capable of supporting two modes simultaneously, i.e. LP₀₁Mold and LP₁₁Molding; the cladding waveguide is arranged in the cladding outside the fiber core of the optical fiber, the cladding waveguide has a certain length, the length direction of the cladding waveguide is consistent with the length direction of the optical fiber, and mode coupling is realized by energy overlapping between evanescent fields of the fiber core and the cladding waveguide.

2. The high order mode filter according to claim 1, wherein: the optical fiber is a large-core optical fiber, and the diameter of the core of the optical fiber is 20 mu m.

3. The high order mode filter according to claim 1, wherein: the cladding waveguide is formed by femtosecond laser writing.

4. The high order mode filter according to claim 1, wherein: the radius of the clad waveguide was set to 4 μm.

5. The high order mode filter according to claim 1, wherein: the distance between the core and the cladding waveguide is within 20 μm.

6. The high order mode filter according to claim 1, wherein: the refractive index of the cladding waveguide satisfies the following phase matching conditions:

wherein

Is a fiber core LP₁₁Propagation constant of the mode, beta_waveguideIs a cladding waveA propagation constant of the fundamental mode, wherein 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:

7. The high order mode filter according to any one of claims 1 to 6, wherein: cladding waveguides with certain length are respectively inscribed in the outer claddings in the x direction and the y direction of the fiber core of the optical fiber, and LP in the fiber core_11aMode coupling to fundamental mode of cladding waveguide in x-direction, LP in core_11bThe mode couples with the fundamental mode of the cladding waveguide in the y-direction.