CN113093327A - Preparation method of ultra-flat resonant long-period grating and broadband mode conversion system - Google Patents

Preparation method of ultra-flat resonant long-period grating and broadband mode conversion system Download PDF

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CN113093327A
CN113093327A CN202110418227.8A CN202110418227A CN113093327A CN 113093327 A CN113093327 A CN 113093327A CN 202110418227 A CN202110418227 A CN 202110418227A CN 113093327 A CN113093327 A CN 113093327A
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mode
optical fiber
grating
period
long
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曾祥龙
滕林苹
吕思
陆佳峰
徐江韬
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University of Shanghai for Science and Technology
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University of Shanghai for Science and Technology
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    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/02057—Optical fibres with cladding with or without a coating comprising gratings
    • G02B6/02076—Refractive index modulation gratings, e.g. Bragg gratings
    • G02B6/02123—Refractive index modulation gratings, e.g. Bragg gratings characterised by the method of manufacture of the grating
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/255—Splicing of light guides, e.g. by fusion or bonding
    • G02B6/2551—Splicing of light guides, e.g. by fusion or bonding using thermal methods, e.g. fusion welding by arc discharge, laser beam, plasma torch
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/44—Mechanical structures for providing tensile strength and external protection for fibres, e.g. optical transmission cables
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/02057—Optical fibres with cladding with or without a coating comprising gratings
    • G02B6/02076—Refractive index modulation gratings, e.g. Bragg gratings
    • G02B6/02123—Refractive index modulation gratings, e.g. Bragg gratings characterised by the method of manufacture of the grating
    • G02B2006/02166—Methods of designing the gratings, i.e. calculating the structure, e.g. algorithms, numerical methods

Abstract

The invention discloses a preparation method of an ultra-flat resonant long-period grating and a broadband mode conversion system, wherein the preparation method comprises the following steps: s1, calculating LP of the two-mode optical fiber in the C + L wave band through physical field simulation based on the selected parameters of the two-mode optical fiber01Mode and LP11The correspondence of the mode effective refractive index to the resonant wavelength; s2, obtaining a grating critical period based on the corresponding relation and the phase matching relation of the long-period grating; and S3, preparing the ultra-flat resonant long-period grating on the two-mode optical fiber through a preset preparation device based on the selected critical period of the two-mode optical fiber. The ultra-flat resonant long-period grating prepared by the invention has a resonant wavelength response with flat change, the effective working bandwidth of the long-period grating is increased, and the ultra-flat resonant long-period grating is used as an ultra-wideband modeA converter for realizing LP at multiple wavelengths01Mode to LP11The coupling of modes, and the stable output of the +1/-1 order optical vortex beam.

Description

Preparation method of ultra-flat resonant long-period grating and broadband mode conversion system
Technical Field
The invention relates to the technical field of optical fiber devices and optical modes, in particular to a preparation method of an ultra-flat resonant long-period grating and a broadband mode conversion system.
Background
The core device of the optical fiber mode multiplexing technology is a mode converter, and a high-order mode is effectively excited by the mode converter, so that multiplexing and demultiplexing of different high-order modes in a communication system are realized. Currently, mode converters have been widely used in the fields of large-scale wavelength division multiplexing communication, high-resolution imaging, optical tweezers, material processing, and the like. The conventional mode converters are mainly classified into a space type and an optical fiber type, and the space type mode converter mainly includes: the space add-drop devices are large and heavy, and are not beneficial to integration; the optical fiber mode converter mainly comprises: mode selective couplers, fiber splice, long-period fiber gratings, and the like, wherein the mode selective couplers and the fiber splice mainly have the problem of large insertion loss, and the long-period fiber gratings usually exhibit narrow-band transmission characteristics and change of mode conversion efficiency is not flat enough, so that the application of the mode converter is limited by the problems to be solved.
In summary, it is a problem of interest for researchers to find an all-fiber mode converter with low loss, large bandwidth and flat mode conversion efficiency.
Disclosure of Invention
The invention aims to provide a preparation method of an ultra-flat resonant long-period grating and a broadband mode conversion system, which aim to solve the technical problems in the prior art, the prepared ultra-flat resonant long-period grating has a resonant wavelength response with flat change, the effective working bandwidth of the long-period grating is increased, and the ultra-flat resonant long-period grating is used as an ultra-broadband mode converter to realize LP (low-pass-band) mode conversion under multiple wavelengths01Mode to LP11The coupling of modes, and the stable output of the +1/-1 order optical vortex beam.
In order to achieve the purpose, the invention provides the following scheme: the invention provides a preparation method of an ultra-flat resonant long-period grating, which comprises the following steps:
s1, calculating the LP of the two-mode optical fiber in the C + L wave band through physical field simulation based on the selected parameters of the two-mode optical fiber01Mode and LP11The correspondence of the mode effective refractive index to the resonant wavelength;
s2 LP in C + L wave band based on two-mode optical fiber01Mode and LP11Obtaining the critical period of the grating by the corresponding relation between the mode effective refractive index and the resonance wavelength and the phase matching relation of the long-period grating;
s3, preparing a super-flat resonant long-period grating on the two-mode optical fiber through a preset preparation device based on the critical period of the selected two-mode optical fiber; wherein the preparation apparatus comprises: the device comprises a super-continuous light source, a first single-mode fiber, a mode stripper, a second single-mode fiber, a fiber spectrometer, two motor motion systems, a laser system and a program control system.
Preferably, in S1, the parameters of the two-mode optical fiber include a core refractive index, a core radius, a cladding refractive index, and a cladding radius; the two-mode optical fiber supports LP in C + L wave band01Mode and LP11Transmission of a pattern; the two-mode fiber slave LP01Mode to LP11The grating period required for mode conversion varies as a non-monotonic concave-up parabola with the resonant wavelength.
Preferably, in S2, the method for obtaining the critical period of the grating includes:
the phase matching relationship of the long period grating is shown as follows:
A＝λres/n01-n11
in which Λ represents the grating period, λresFor the resonant wavelength, n01And n11Are respectively LP01Mode and LP11The effective refractive index of the mode;
and obtaining a phase matching curve of the grating period and the resonant wavelength based on the phase matching relation of the long-period grating, wherein the phase matching curve is an upper concave parabola, and the grating period corresponding to the inflection point at the bottom of the phase matching curve is the grating critical period.
Preferably, in S3, the method specifically includes the following steps:
s3.1, connecting the two-mode optical fiber to the preparation device; the method specifically comprises the following steps:
the two-mode optical fiber is connected with the supercontinuum light source through the first single-mode optical fiber; the two-mode optical fiber is connected with the optical fiber spectrometer through the second single-mode optical fiber; the mode stripper is arranged on the two-mode optical fiber, the two-mode optical fiber is fixed in the two motor motion systems, the laser system is arranged between the two motor motion systems, and the program control system is respectively connected with the laser system and the motor motion systems;
s3.2, setting a program for the program control system based on the grating critical period obtained in the step S2;
s3.3, based on the program set for the program control system in the step S3.2, exposing the two-mode optical fiber through the laser system to obtain the ultra-flat resonant long-period grating; and in the preparation process of the ultra-flat resonant long-period grating, the motor motion system and the laser system are multiplexed in a time-sharing mode.
Preferably, in step S3.1, the first single-mode fiber and the two-mode fiber, and the two-mode fiber and the second single-mode fiber are connected by a fusion splicer; wherein the electrodes of the welding machine generate arc discharge.
Preferably, in step S3.2, the setting the program of the program control system specifically includes:
the first step is as follows: setting a starting point, a moving direction, a moving speed and a moving time of the motor moving system through the program control system, wherein the product of the moving speed and the moving time is a grating critical period;
the second step is that: setting a laser exposure starting point, exposure intensity and exposure time of the laser system through the program control system;
the third step: setting the number of times that the motor motion system and the laser system work in sequence through the program control system; wherein the motor motion system is time-multiplexed with the laser system.
Preferably, in step S3.3, the light source of the laser system (7) is a carbon dioxide laser.
Preferably, the ultra-flat resonant long-period grating prepared in step S3.3 is packaged by a U-shaped groove and a heat shrink tube.
The present invention also provides a broadband mode conversion system, comprising: the tunable optical fiber grating comprises a tunable light source, a first single-mode optical fiber, a two-mode optical fiber, an ultra-flat resonant long-period grating and an image acquisition system; the tunable light source is connected with the two-mode optical fiber through the first single-mode optical fiber, one end of the two-mode optical fiber is connected with the first single-mode optical fiber, the other end of the two-mode optical fiber is connected with the image acquisition system, the two-mode optical fiber is provided with an ultra-flat resonant long-period grating, the two-mode optical fiber and the ultra-flat resonant long-period grating are of an integrated structure, and the ultra-flat resonant long-period grating is prepared through the preparation method of the ultra-flat resonant long-period grating;
the two-mode optical fiber is further provided with a mode stripper and a polarization controller, the mode stripper is arranged between the first single-mode optical fiber and the ultra-flat resonant long-period grating, and the polarization controller is arranged between the ultra-flat resonant long-period grating and the image acquisition system.
Preferably, the tunable light source is capable of generating light beams of different wavelengths in the C + L band.
The invention discloses the following technical effects:
(1) the ultra-flat resonant long-period grating provided by the invention is formed by periodic exposure of laser, the whole manufacturing process is simple and easy to implement, the cost is low, and the accuracy is high; the formed grating has compact structure, low insertion loss, ultra-wide and ultra-flat working bandwidth, and ensures the stability of the long-period grating;
(2) the broadband mode conversion system based on the ultra-flat resonant long-period grating can realize LP under different wavelengths of the whole C + L waveband01Mode to LP11Mode conversion and +1/-1 order optical vorticesOutputting the swirling light beams;
(3) the invention is of all-fiber structure, easy to be compatible with fiber communication system, small volume, light weight, easy to be integrated and commercialized.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings needed in the embodiments will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings without creative efforts.
FIG. 1 is a flow chart of a method for fabricating an ultra-flat resonant long-period grating according to the present invention;
FIG. 2 is a drawing of a two-mode fiber slave LP in accordance with one embodiment of the present invention01Mode to LP11A phase matching curve of the mode transition;
FIG. 3 is a schematic structural diagram of an apparatus for manufacturing an ultra-flat resonant long-period grating according to an embodiment of the present invention;
FIG. 4 is a transmission spectrum of the ultra-flat resonant long-period grating in the first embodiment of the present invention when the grating periods are 366 μm, 368 μm, and 370 μm, respectively;
fig. 5 is a schematic structural diagram of a broadband mode conversion system based on an ultra-flat resonant long-period grating according to a second embodiment of the present invention;
FIG. 6 shows the LP at 1490nm, 1510nm, 1530nm, 1550nm, 1570nm, 1590nm for the broadband mode conversion system according to a second embodiment of the present invention11Mode and +1/-1 order vortex beam output results;
in the figure, 1 is a super-continuous light source, 2 is a first single-mode fiber, 3 is a two-mode fiber, 4 is a mode stripper, 5 is a fiber spectrometer, 6 is a motor motion system, 7 is a laser system, 8 is a program control system, 9 is a second single-mode fiber, 10 is a super-flat resonant long-period grating, 11 is a polarization controller, 12 is an image acquisition system, and 13 is a tunable light source.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
In order to make the aforementioned objects, features and advantages of the present invention comprehensible, embodiments accompanied with figures are described in further detail below.
The first embodiment is as follows:
referring to fig. 1, this embodiment provides a method for preparing an ultra-flat resonant long-period grating, where the period of the long-period grating is in the order of microns, and a fiber core LP capable of performing homodromous transmission01Mode to cladding mode or core LP01Mode to core high order mode conversion. The long period grating period provided in this embodiment is several hundred microns, and the 3dB bandwidth range of the ultra-flat resonance is 1460-1620 nm. The preparation method specifically comprises the following steps:
s1, calculating the LP of the two-mode optical fiber 3 in the C + L waveband through physical field simulation based on the selected parameters of the two-mode optical fiber 301Mode and LP11The correspondence of the mode effective refractive index to the resonant wavelength; wherein, the parameters of the two-mode optical fiber 3 comprise the refractive index of the fiber core, the radius of the fiber core, the refractive index of the cladding and the radius of the cladding, and the physical field simulation software is utilized to calculate the LP of the two-mode optical fiber 3 in the C + L waveband01Mode and LP11The correspondence of the mode effective refractive index to the resonant wavelength; the two-mode optical fiber 3 can support LP in a C + L waveband01And LP11For the transmission of modes, the two-mode optical fiber 3 satisfies the following characteristics: LP01To LP11The grating period required for mode conversion varies with the resonant wavelength as a non-monotonic concave-up parabola, i.e., the Phase Matching Curve (PMC) is in the form of a concave-up parabola.
S2 LP in C + L waveband based on two-mode optical fiber 301Mode and LP11Correspondence of mode effective index to resonant wavelengthObtaining the critical period of the grating according to the relationship and the phase matching relationship of the long-period grating; the phase matching relationship of the long-period grating is shown as the following formula:
A＝λres/n01-n11
in which Λ represents the grating period, λresFor the resonant wavelength, n01And n11Are respectively LP01Mode and LP11The effective refractive index of the mode;
a phase matching curve of the grating period and the resonant wavelength is obtained based on a phase matching relationship of the long-period grating, the phase matching curve is an upwardly concave parabola, and the grating period corresponding to an inflection point at the bottom of the phase matching curve is a grating critical period, which is specifically shown in fig. 2.
S3, preparing the ultra-flat resonant long-period grating 10 on the two-mode optical fiber 3 through a preset preparation device based on the selected critical period of the two-mode optical fiber 3; the method specifically comprises the following steps:
s3.1, connecting the two-mode optical fiber 3 into the preparation device;
the preparation device comprises: the system comprises a supercontinuum light source 1, a first single-mode fiber 2, a mode stripper 4, a second single-mode fiber 9, a fiber spectrometer 5, two motor motion systems 6, a laser system 7 and a program control system 8;
firstly, the two-mode optical fiber 3 is respectively connected with the supercontinuum light source 1 and the fiber spectrometer 5 through the first single-mode optical fiber 2 and the second single-mode optical fiber 9, and the mode stripper 4 is arranged on the two-mode optical fiber 3; the method specifically comprises the following steps: the first single-mode optical fiber 2 is connected with the two-mode optical fiber 3, and the two-mode optical fiber 3 is connected with the second single-mode optical fiber 9 through a fusion splicer; wherein the electrodes of the welding machine generate arc discharge.
Secondly, removing 3-4 cm of the coating layer of the two-mode optical fiber 3 by using wire stripping pliers, and wiping the coating layer with alcohol;
thirdly, straightening and fixing the two-mode optical fiber 3 in two motor motion systems 6, and enabling the coating removing area of the two-mode optical fiber 3 to be located between the two motor motion systems 6, wherein the two motor motion systems 6 are arranged between the mode stripper 4 and the optical fiber spectrometer 5; the laser system 7 is disposed between the two motor motion systems 6, and the program control system 8 is connected to the motor motion systems 6 and the laser system 7, as shown in fig. 3.
Wherein the first single-mode fiber 2 and the second single-mode fiber 9 only support one mode (LP) in the C + L waveband01Mode(s).
The wavelength detection range of the fiber spectrometer 5 is as follows: 1200nm-2400 nm.
The wavelength range of the super-continuous light source 1 is as follows: 400nm-2400 nm.
The mode stripper 4 is a cylindrical glass rod, and the two-mode optical fiber 3 is wound on the cylindrical glass rod for 3-4 circles to filter out a fusion point A of the first single-mode optical fiber 2, the second single-mode optical fiber 9 and the two-mode optical fiber 31、A2A small number of high order modes are generated.
S3.2, setting a program for the program control system 8 based on the grating critical period obtained in the step S2; the method specifically comprises the following steps:
the first step is as follows: setting a starting point, a moving direction, a moving speed and a moving time of a motor moving system 6 in the program control system 8, wherein the product of the moving speed and the moving time is an applied grating period which is a grating critical period of a phase matching curve;
the second step is that: setting a laser exposure starting point, exposure intensity and exposure time of the laser system 7 in the program control system 8;
the third step: setting the number of times of the first step and the second step which are executed in sequence in the program control system 8, namely the number of the periods of the long-period grating; the motor motion system 6 and the laser system 7 are multiplexed in a time-sharing mode, so that the two-mode optical fiber 3 is static during exposure, and the two-mode optical fiber 3 is not exposed during movement, so that periodic refractive index modulation of the ultra-flat resonant long-period grating is formed.
And S3.3, based on the program set for the program control system 8 in the step S3.2, exposing the area of the two-mode optical fiber 3 without the coating layer through the laser system 7 to obtain the ultra-flat resonant long-period grating 10.
Wherein, the light source of the laser system 7 is carbon dioxide laser. In the preparation process of the ultra-flat resonant long-period grating 10, the transmission spectrum of the ultra-flat resonant long-period grating 10 is monitored through the fiber spectrometer 5, and the critical period of the grating is adjusted based on the transmission spectrum, so that the prepared ultra-flat resonant long-period grating 10 can meet the performance requirement.
FIG. 4 is a transmission spectrum diagram of the ultra-flat resonant long-period grating of the present embodiment with grating periods of 366 μm, 368 μm and 370 μm, respectively. As can be seen from FIG. 4, in the two-mode fiber, when the applied grating period is 370 μm, the resonance peak in the transmission spectrum shows two distant single peaks; the grating period is reduced to 368 mu m, and the distance between two resonance peaks is shortened to present an adjacent double-peak effect; when the grating period is reduced to 366 mu m, the double peaks are combined into a single peak with an ultra-flat bottom, the bottom width of a resonance peak is widened, and the working area of the ultra-flat resonance long-period grating is increased. In fact, according to the phase matching condition, the long period grating with ultra-flat resonance can be formed macroscopically only when the grating period is in the critical period, and microscopically shows LP inside the two-mode optical fiber01Mode and LP11The group velocities of the modes are equal.
In order to isolate the influence of external environmental factors, the prepared ultra-flat resonant long-period grating 10 is packaged by a U-shaped groove and a heat-shrinkable tube.
Referring to fig. 5, this embodiment further provides a broadband mode conversion system based on an ultra-flat resonant long-period grating, including: the tunable optical fiber grating comprises a tunable light source 13, a first single-mode optical fiber 2, a two-mode optical fiber 3, an ultra-flat resonant long-period grating 10 and an image acquisition system 12; the tunable light source 13 is connected with the two-mode optical fiber 3 through the first single-mode optical fiber 2, one end of the two-mode optical fiber 3 is connected with the first single-mode optical fiber 2, the other end of the two-mode optical fiber is connected with the image acquisition system 12, the two-mode optical fiber 3 is provided with an ultra-flat resonant long-period grating 10, and the two-mode optical fiber 3 and the ultra-flat resonant long-period grating 10 are of an integrated structure; the two-mode optical fiber 3 is further provided with a mode stripper 4 and a polarization controller 11, and the mode stripper 4 is arranged between the first single-mode optical fiber 2 and the ultra-flat resonant long-period grating 10 to filter a small number of high-order modes generated by a fusion point B of the first single-mode optical fiber 2 and the two-mode optical fiber 3; the polarization controller 11 is arranged between the ultra-flat resonant long-period grating 10 and the image acquisition system 12.
Injection of LP through the tunable light source 1301Mode light beam, LP01The light beams of the modes reach the ultra-flat resonant long-period grating 10 through the first single-mode fiber 2, the two-mode fiber 3 and the mode stripper 4, and LP is converted through the ultra-flat resonant long-period grating 10 when the phase matching condition is achieved01Conversion of a patterned beam to LP11Linearly polarized light of a mode; the polarization controller 11 twists and extrudes the two-mode optical fiber 3 between the ultra-flat resonant long-period grating 10 and the image acquisition system 12, and can output +1 or-1 order optical vortex light beams, and the image acquisition system 12 detects the +1 or-1 order optical vortex light beams.
The tunable light source 13 is capable of generating light beams of different wavelengths in the C + L band.
The ultra-flat resonant long-period grating 10 is the ultra-flat resonant long-period grating prepared in the first embodiment.
FIG. 6 shows the LP of the broadband mode conversion system based on the ultra-flat resonant long-period grating provided in this embodiment at the wavelengths of 1490nm, 1510nm, 1530nm, 1550nm, 1570nm and 1590nm11Mode and output result of +1/-1 order optical vortex light beam, the tunable light source 13 is tuned to different wavelengths of C + L band to prove the broadband performance of the broadband mode conversion system. As shown in the first row of pictures in FIG. 6, this broadband mode conversion system converts LP at the six selected wavelengths described above01Mode conversion to LP with two lobe profiles11A mode; the second row of pictures in FIG. 6 shows the twisting or squeezing of the two-mode fiber between the SSG 10 and the image acquisition system 12 using the polarization controller 11, but in a different wayThe output of the ring beam can be realized at the wave band, and further +1 order and-1 order optical vortex beam outputs can be obtained, and the third row and the fourth row of pictures show interference patterns with spiral shapes for distinguishing the-1 order and the +1 order optical vortex beams respectively.
The working principle of the invention is as follows:
a preparation method of an ultra-flat resonant long-period grating is realized according to a phase matching theory, and the ultra-flat resonant long-period grating is essentially a long-period grating with matched group velocity. Periodic, transmitted LP of long period gratings according to phase matching conditions01Mode effective index, LP11The mode effective refractive index and the resonant wavelength need to satisfy a matching relationship, and the variation of the grating period with the resonant wavelength is called a Phase Matching Curve (PMC). Generally, the PMC exhibits a monotonous change characteristic, that is, a single grating period corresponds to a single resonant wavelength, which means that a spectrum of a conventional long-period grating is a single resonant peak; however, PMC calculated in some two-mode optical fibers is a non-monotonic concave-up parabola, that is, a proper single grating period can correspond to two resonant wavelengths, and in the process that the grating period gradually decreases to the bottom inflection point (critical period) of the parabola, the double resonant peaks gradually approach to merge to form a resonant peak with an ultra-wide and ultra-flat bottom. In practice the microscopic behaviour at the critical period is: LP01Mode and LP11The group velocities of the modes are equal; the macroscopic appearance is as follows: the double peaks merge into a single peak of ultra-wide, ultra-flat resonance, and the modal coupling bandwidth is increased. Based on the broadband coupling characteristic of the ultra-flat resonant long-period grating, an ultra-wideband mode converter can be obtained, and the invention can be utilized to realize LP in an ultra-wide wavelength range11Mode output, and further can realize the output of the +1/-1 order optical vortex light beam.
The above-described embodiments are merely illustrative of the preferred embodiments of the present invention, and do not limit the scope of the present invention, and various modifications and improvements of the technical solutions of the present invention can be made by those skilled in the art without departing from the spirit of the present invention, and the technical solutions of the present invention are within the scope of the present invention defined by the claims.

Claims (10)

1. The preparation method of the ultra-flat resonant long-period grating is characterized by comprising the following steps:
s1, calculating the LP of the two-mode optical fiber (3) in the C + L waveband through physical field simulation based on the selected parameters of the two-mode optical fiber (3)01Mode and LP11The correspondence of the mode effective refractive index to the resonant wavelength;
s2, based on the two-mode optical fiber (3), LP in the C + L waveband01Mode and LP11Obtaining the critical period of the grating by the corresponding relation between the mode effective refractive index and the resonance wavelength and the phase matching relation of the long-period grating;
s3, preparing the ultra-flat resonant long-period grating (10) on the two-mode optical fiber (3) through a preset preparation device based on the critical period of the selected two-mode optical fiber (3); wherein the preparation apparatus comprises: the device comprises a super-continuous light source (1), a first single-mode fiber (2), a mode stripper (4), a second single-mode fiber (9), a fiber spectrometer (5), two motor motion systems (6), a laser system (7) and a program control system (8).
2. The method of claim 1, wherein in S1, the parameters of the two-mode fiber (3) include core refractive index, core radius, cladding refractive index, and cladding radius; the two-mode optical fiber (3) supports LP in C + L wave band01Mode and LP11Transmission of a pattern; the two-mode optical fiber (3) is from LP01Mode to LP11The grating period required for mode conversion varies as a non-monotonic concave-up parabola with the resonant wavelength.
3. The method for preparing an ultra-flat resonant long-period grating as claimed in claim 1, wherein in S2, the method for obtaining the critical period of the grating includes:
the phase matching relationship of the long period grating is shown as follows:
Λ＝λres/n01-n11
in which Λ represents the grating period, λresFor the resonant wavelength, n01And n11Are respectively LP01Mode and LP11The effective refractive index of the mode;
and obtaining a phase matching curve of the grating period and the resonant wavelength based on the phase matching relation of the long-period grating, wherein the phase matching curve is an upper concave parabola, and the grating period corresponding to the inflection point at the bottom of the phase matching curve is the grating critical period.
4. The method for preparing an ultra-flat resonant long-period grating as claimed in claim 1, wherein said step S3 specifically comprises the steps of:
s3.1, connecting the two-mode optical fiber (3) into the preparation device; the method specifically comprises the following steps:
the two-mode optical fiber (3) is connected with the supercontinuum light source (1) through the first single-mode optical fiber (2); the two-mode optical fiber (3) is connected with the optical fiber spectrometer (5) through the second single-mode optical fiber (9); the mode stripper (4) is arranged on the two-mode optical fiber (3), the two-mode optical fiber (3) is fixed in the two motor motion systems (6), the laser system (7) is arranged between the two motor motion systems (6), and the program control system (8) is respectively connected with the laser system (7) and the motor motion systems (6);
s3.2, setting a program for the program control system (8) based on the grating critical period obtained in the step S2;
s3.3, based on the program set in the step S3.2 for the program control system (8), exposing the two-mode optical fiber (3) through the laser system (7) to obtain the ultra-flat resonant long-period grating (10); and in the preparation process of the ultra-flat resonant long-period grating (10), the motor motion system (6) and the laser system (7) are multiplexed in a time-sharing mode.
5. The method according to claim 4, wherein in step S3.1, the first single-mode fiber (2) and the two-mode fiber (3), and the two-mode fiber (3) and the second single-mode fiber (9) are connected by a fusion splicer; wherein the electrodes of the welding machine generate arc discharge.
6. The method for preparing an ultra-flat resonant long-period grating as claimed in claim 4, wherein in step S3.2, the programming the program control system (8) specifically comprises:
the first step is as follows: setting a starting point, a moving direction, a moving speed and a moving time of the motor moving system (6) through the program control system (8), wherein the product of the moving speed and the moving time is a grating critical period;
the second step is that: setting a laser exposure starting point, exposure intensity and exposure time of the laser system (7) through the program control system (8);
the third step: setting the number of times that the motor motion system (6) and the laser system (7) work in sequence through the program control system (8); wherein the motor motion system (6) is time-multiplexed with the laser system (7).
7. The method according to claim 4, wherein in step S3.3, the light source of the laser system (7) is a carbon dioxide laser.
8. The method of claim 4, wherein the ultra-flat resonant long-period grating (10) prepared in step S3.3 is encapsulated by a U-shaped groove and a heat-shrink tube.
9. A broadband mode conversion system, comprising: the tunable optical fiber grating comprises a tunable light source (13), a first single-mode optical fiber (2), a two-mode optical fiber (3), an ultra-flat resonant long-period grating (10) and an image acquisition system (12); the tunable light source (13) is connected with the two-mode optical fiber (3) through the first single-mode optical fiber (2), one end of the two-mode optical fiber (3) is connected with the first single-mode optical fiber (2), the other end of the two-mode optical fiber is connected with the image acquisition system (12), the two-mode optical fiber (3) is provided with a super-flat resonant long-period grating (10), the two-mode optical fiber (3) and the super-flat resonant long-period grating (10) are of an integrated structure, and the super-flat resonant long-period grating (10) is prepared by the preparation method of the super-flat resonant long-period grating according to any one of claims 1 to 8;
still be equipped with mode stripper (4), polarization controller (11) on two mode optic fibre (3), mode stripper (4) are located first single mode optical fibre (2) with between super flat resonance long period grating (10), polarization controller (11) are located super flat resonance long period grating (10) with between image acquisition system (12).
10. A broadband mode conversion system according to claim 9, wherein the tunable light source (13) is capable of generating light beams of different wavelengths in the C + L band.
CN202110418227.8A 2021-04-19 2021-04-19 Preparation method of ultra-flat resonant long-period grating and broadband mode conversion system Pending CN113093327A (en)

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20040042714A1 (en) * 2002-09-04 2004-03-04 Siddharth Ramachandran Tunable mode-converters using few mode fibers
US20040081383A1 (en) * 2002-10-23 2004-04-29 Siddharth Ramachandran Tunable polarizers
CN103033883A (en) * 2013-01-04 2013-04-10 北京遥测技术研究所 Long period fiber grating machining method and long period fiber grating
CN112505823A (en) * 2020-11-24 2021-03-16 南京师范大学 Optical fiber type tunable broadband mode converter and preparation method thereof

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20040042714A1 (en) * 2002-09-04 2004-03-04 Siddharth Ramachandran Tunable mode-converters using few mode fibers
US20040081383A1 (en) * 2002-10-23 2004-04-29 Siddharth Ramachandran Tunable polarizers
CN103033883A (en) * 2013-01-04 2013-04-10 北京遥测技术研究所 Long period fiber grating machining method and long period fiber grating
CN112505823A (en) * 2020-11-24 2021-03-16 南京师范大学 Optical fiber type tunable broadband mode converter and preparation method thereof

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Application publication date: 20210709