CN114200667A - Definable optical filtering method and system based on liquid crystal spatial light modulator - Google Patents

Definable optical filtering method and system based on liquid crystal spatial light modulator Download PDF

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CN114200667A
CN114200667A CN202111313397.6A CN202111313397A CN114200667A CN 114200667 A CN114200667 A CN 114200667A CN 202111313397 A CN202111313397 A CN 202111313397A CN 114200667 A CN114200667 A CN 114200667A
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liquid crystal
core
optical fiber
light modulator
spatial light
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CN114200667B (en
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张伟利
王珊珊
饶云江
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University of Electronic Science and Technology of China
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    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
    • G02B27/0012—Optical design, e.g. procedures, algorithms, optimisation routines
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/0102—Constructional details, not otherwise provided for in this subclass
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells

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Abstract

The invention provides a definable optical filtering method and system based on a liquid crystal spatial light modulator. The method comprises the following steps: the broadband light source sequentially enters the single-mode optical fiber, the multimode optical fiber and the liquid crystal spatial light modulator through the circulator, the gray pattern loaded on the liquid crystal spatial light modulator is continuously adjusted according to the reflection spectrum and the target spectrum by combining an iterative optimization algorithm, so that the output spectrum close to the target spectrum is obtained, and the light source can be filtered in any shape by setting different target spectra. The invention has the characteristics of strong compatibility with a single mode fiber system, large tunable range, high resolution, small loss and the like.

Description

Definable optical filtering method and system based on liquid crystal spatial light modulator
Technical Field
The invention belongs to the technical field of optical filtering, and particularly relates to a definable optical filtering method and system based on a liquid crystal spatial light modulator.
Background
With the increasing number of communication users and the increasing demand for personalization, the demand for increasing the transmission speed and capacity of optical fiber communication networks is increasing. As one of core technologies for expanding the capacity of an optical communication network, the dense wavelength division multiplexing technology greatly increases the bandwidth of an existing optical fiber network system by loading optical signals on optical waves with different wavelengths and simultaneously transmitting the optical signals. The tunable optical filter is used as an important optical communication device in a new generation of dense wavelength division system, so that the capacity, flexibility and expandability of an optical fiber transmission network are greatly enhanced. In addition, the optical filter is also widely applied to the fields of laser and photoelectron technology, aerospace and the like.
The spectral filtering method based on the digital micromirror array (DMD) has the characteristics of high resolution and convenience in adjustment, but has the problem that the resolution and the adjustable bandwidth are mutually restricted due to the fact that the spectral filtering method is based on a grating light splitting principle. Multimode optical fiber, as a special dispersive element, can also realize optical filtering by combining with single-mode optical fiber, but has the disadvantage of single filtering effect. The liquid crystal spatial light modulator is one of important devices for regulating and controlling a light field, can regulate and control the wavefront of incident light, is combined with an iterative optimization algorithm to realize the light field output of a specific target, and is widely applied to the fields of laser processing, beam shaping, adaptive optical imaging and the like. The existing spectral filtering method based on the liquid crystal spatial light modulator and the scattering medium (ground glass) has the problems of large loss and difficulty in being compatible with an optical fiber system. In order to solve the problems in the prior art, the invention combines a spatial light modulator, a large-core-diameter multimode fiber and a single-mode fiber, and realizes a spectral filtering method which has a large tuning range, high resolution and can be defined.
Disclosure of Invention
The invention mainly aims to provide a definable optical filtering method based on a liquid crystal spatial light modulator, which realizes the optical filtering effect of any shape by combining a liquid crystal spatial light modulator, a large-core-diameter multimode optical fiber and a single-mode optical fiber, can solve the problem that the tuning bandwidth range and the resolution ratio based on a DMD filter are mutually restricted, and is compatible with the existing optical fiber system.
In order to achieve the purpose, the invention adopts the following technical scheme:
a broadband light source is vertically incident on a liquid crystal spatial light modulator through a single-mode optical fiber and a large-core-diameter multimode optical fiber, different gray patterns are loaded on the liquid crystal spatial light modulator to perform phase modulation on a light field returned to the multimode optical fiber, and the gray patterns loaded on the liquid crystal spatial light modulator are continuously optimized according to an output spectrum by combining an iterative optimization algorithm, so that a definable spectrum filtering effect is realized.
The invention relates to a definable optical filtering method based on a liquid crystal spatial light modulator, which comprises the following steps:
step S1: the broadband light source enters from the first port of the optical fiber circulator through the single-mode optical fiber and exits from the second port of the optical fiber circulator, and then is output through the large-core-diameter multimode optical fiber;
step S2: the output light field is collimated by a collimating convex lens, expanded by two convex lenses, polarized by a polarizing film and then incident on a liquid crystal spatial light modulator;
step S3: the light modulated by the liquid crystal spatial light modulator is reflected back to the large-core-diameter multimode optical fiber and is input into the spectrometer through a third port of the optical fiber circulator;
step S4: the computer optimizes the gray pattern loaded on the liquid crystal spatial light modulator by using an iterative optimization algorithm according to the target spectrum based on the obtained spectrum data;
step S5: judging whether the filtered spectrum meets the filtering requirement according to an iterative optimization algorithm, and if so, stopping optimization; if not, step S4 is repeated.
The broadband light source can be ASE, super-continuum spectrum or SLED light source, and the central wavelength of the broadband light source is suitable for any wave band of visible light, near infrared and middle infrared.
Further, the large-core multimode fiber comprises a large-core step-index multimode fiber, a large-core coreless fiber and a large-core hollow fiber, and can also be replaced by a strong-coupling multi-core fiber;
further, the realized resolution can be adjusted by adjusting the length, the refractive index and the core diameter of the large-core-diameter multimode optical fiber;
further, the iterative optimization algorithm is a hybrid non-dominated sorting genetic algorithm, and comprises the following steps:
step A1: initializing a parameter and an adaptive value function, setting k to be 0, the population size to be NP, the genetic algebra to be NG, the number of input modes to be N, and the cross probability to be PcThe mutation probability is PmMixing factor H, the number M of the wave lengths of filtering, randomly initializing the population and calculating the adaptive value corresponding to each population:
Figure BDA0003342706010000021
wherein, ImIntensity value corresponding to the m-th wavelength, I, read from the spectrometerrefIs the background average;
step A2: carrying out non-dominant sorting on population individuals, and calculating a crowding distance according to the adaptive value;
step A3: NP parents were selected using the binary tournament method. Firstly, randomly selecting two individuals from NP parent individuals, comparing the grades of the two individuals, and winning with low grade, if the grades are the same, then comparing the crowding distance, and winning with larger crowding distance; repeating the steps NP times to select NP parent individuals;
step A4: NP offspring individuals were generated by uniform crossover and single point variation. Dividing NP parent individuals into NP/2 groups randomly, and the parent individuals in each group are called P1And P2(ii) a Generating a random binary mask S, passing C1＝S·P1+(1-S)·P2，C2＝S·P2+(1-S)·P1Calculating to obtain two new masks, and performing single-point mutation on the two masks to obtain two offspring C1And C2(ii) a Repeating the step NP/2 times to obtain NP filial generation individuals;
step A5: calculating the obtained adaptive value of the NP sub-generation individuals, if k<H or M ═ 1, then F1＝-f1，F2100; otherwise F1＝-f1，F2＝f2；
Step A6: mixing NP parent individuals and NP child individuals, and executing the step A2 to obtain 2. NP mixed individuals;
step A7: executing an elite saving strategy, and reserving NP sub-generation individuals with lower sequence and larger crowding distance in the 2. NP individuals obtained in the step A6; setting k as k + 1;
step A8: if k is NG, go to step a9, otherwise go back to step A3;
step A9: and obtaining the population individuals most consistent with the target filtering effect according to the adaptive values of the NP sub-generation individuals.
Further, the filtering resolution of the definable optical filtering method based on the liquid crystal spatial light modulator is determined by the correlation between the output speckles of the large-core multimode optical fiber at different incident wavelengths:
c (Δ λ, x) < I (λ, x) I (λ + Δ λ, x) >/[ < I (λ, x) > < I (λ + Δ λ, x) > ] -1, where I (λ, x) is the light intensity value at the output terminal x when the input light wavelength is λ, and < … > represents the average value. Further, when a bundle of monochromatic light with wavelength λ is input to the large-core multimode optical fiber with length L, the electric field distribution at the output end is:
Figure BDA0003342706010000031
wherein A ismAnd
Figure BDA0003342706010000032
respectively amplitude and initial phase, Ψ, of the mth modemAnd betamThe spatial distribution and the transmission constant of the mth mode, respectively.
Further, after different modes are transmitted by the large-core-diameter multimode optical fiber, different phase delays beta can be accumulatedmL, wherein the phase difference between the fundamental mode and the highest-order mode of the maximum-phase-difference fiber
Figure BDA0003342706010000041
When the input wavelength is changed by delta lambda,
Figure BDA0003342706010000042
the change of pi should be necessary to cause a significant change in the optical field intensity distribution at the output of the fiber, i.e.
Figure BDA0003342706010000043
Further obtaining delta lambda-lambda2/[nL(NA)2]δ λ is inversely proportional to the fiber length.
In addition, the application also relates to a definable optical filtering system based on the liquid crystal spatial light modulator, which comprises a liquid crystal spatial light modulator, a large-core-diameter multimode optical fiber and a single-mode optical fiber mixed structure, wherein one end of the large-core-diameter multimode optical fiber is connected with the single-mode optical fiber, an optical field output by the other end is vertically incident on the liquid crystal spatial light modulator after collimation, beam expansion and polarization, and the large-core-diameter multimode optical fiber serves as a dispersion element; the single mode fiber plays a role in wavelength selection; the liquid crystal spatial light modulator plays a role in controlling the dispersion effect of the multimode optical fiber.
Further, the large-core-diameter multimode optical fiber comprises a large-core-diameter step-index multimode optical fiber, a large-core-diameter coreless optical fiber and a large-core hollow optical fiber.
Further, the large-core multimode fiber is replaced by a strongly coupled multi-core fiber.
The features mentioned above can be combined in various suitable ways or replaced by equivalent features as long as the object of the invention is achieved.
Compared with the prior art, the definable optical filtering method and system based on the liquid crystal spatial light modulator provided by the invention at least have the following beneficial effects: the optical filtering method provided by the invention has no mutual restriction relation in the aspects of filtering resolution and spectral range. By setting different target spectrums, filtering with any shape can be carried out on the light source. The filtering device is output by a single-mode optical fiber end, so that the filtering device has the characteristics of good compatibility with an optical fiber system, large tunable range, high resolution, small loss and the like.
Drawings
FIG. 1 is a flow chart of a definable optical filtering method of the present invention based on a liquid crystal spatial light modulator;
FIG. 2 is a schematic diagram of a definable optical filter arrangement based on a liquid crystal spatial light modulator of the present invention;
FIG. 3 shows a schematic diagram of tunable single-wavelength filtering implemented by the filtering structure of the present invention;
FIG. 4 shows a schematic diagram of multi-wavelength filtering implemented by the filtering structure of the present invention;
Detailed Description
Hereinafter, a detailed description will be given of embodiments of the present invention. While the invention will be described and illustrated in connection with certain specific embodiments thereof, it should be understood that the invention is not limited to those embodiments. Rather, modifications and equivalents of the invention are intended to be included within the scope of the claims.
Furthermore, in the following detailed description, numerous specific details are set forth in order to provide a better understanding of the present invention. It will be understood by those skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, components and circuits have not been described in detail so as not to obscure the present invention.
As shown in fig. 1, the embodiment of the present invention discloses a definable optical filtering method based on a liquid crystal spatial light modulator, which includes:
step S1: the broadband light source enters from the first port of the optical fiber circulator through the single-mode optical fiber and exits from the second port of the optical fiber circulator, and then is output through the large-core-diameter multimode optical fiber;
step S2: the output light field is collimated by a collimating convex lens, expanded by two convex lenses, polarized by a polarizing film and then incident on a liquid crystal spatial light modulator;
step S3: the light modulated by the liquid crystal spatial light modulator is reflected back to the large-core-diameter multimode optical fiber and is input into the spectrometer through a third port of the optical fiber circulator;
step S4: and optimizing the gray pattern loaded on the liquid crystal spatial light modulator by the computer based on the obtained spectrum data and according to the target spectrum by utilizing an iterative optimization algorithm.
Step S5: judging whether the filtered spectrum meets the filtering requirement according to an iterative optimization algorithm, and if so, stopping optimization; if not, step S4 is repeated.
A definable optical filtering device based on a liquid crystal spatial light modulator is shown in fig. 2, and comprises a broadband light source 1, a single-mode optical fiber 2, an optical fiber type circulator 3, a large-core-diameter multimode optical fiber 4, a collimating convex lens 5, a convex lens 6, a convex lens 7, a polarizing plate 8, a liquid crystal spatial light modulator 9, a spectrometer 10 and a computer 11. The center wavelength of the broadband light source is 1560nm, and the 3dB bandwidth is about 60 nm; the large-core-diameter multimode fiber comprises a large-core-diameter step-index multimode fiber, a large-core-diameter coreless fiber and a large-core-diameter hollow fiber, and can also be replaced by a strong-coupling multi-core fiber, wherein the large-core-diameter step-index multimode fiber in the embodiment of the invention is exemplified by the large-core-diameter step-index multimode fiber with the fiber core and the cladding of which the diameters are respectively 105 and 125 mu m, the numerical aperture is 0.22 and the length is 1 m. One end of the large-core multimode optical fiber is connected with the port of the circulator 2, and the other end of the large-core multimode optical fiber passes through the collimating convex lens. The focal length f of the collimating convex lens is 6.2mm, and the collimating convex lens is used for collimating the light field output by the large-core-diameter multimode optical fiber. The pixels of the liquid crystal spatial modulator are 1920 × 1152; the polaroid is a vertical polaroid, and the polaroid is used for selecting the polarization state of incident light because the liquid crystal spatial light modulator only modulates the incident light in the vertical polarization state; the spectrometer is used for monitoring the filtered spectrum; the computer is used for reading spectrometer data, running an iterative optimization algorithm and generating a gray pattern loaded on the liquid crystal spatial light modulator.
The iterative optimization algorithm takes a mixed non-dominated sorting genetic algorithm as an example, and comprises the following specific steps:
step A1: initialization parameters and fitness functions: let k be 0. The population size is NP, the genetic algebra is NG, the number of input modes is N, and the cross probability is PcThe mutation probability is PmMixing factor H, the number M of the wave lengths of filtering, randomly initializing the population and calculating the adaptive value corresponding to each population:
Figure BDA0003342706010000061
wherein, ImIntensity value corresponding to the m-th wavelength, I, read from the spectrometerrefIs the background average.
Step A2: carrying out non-dominant sorting on population individuals, and calculating a crowding distance according to the adaptive value;
step A3: NP parents were selected using the binary tournament method. First, two individuals are randomly selected from NP parent individuals, the grades of the two individuals are compared, winning is carried out at a low grade, and if the grades are the same, the crowding distance is compared, and winning is carried out at a large crowding distance. And repeating the steps NP times to select NP parents.
Step A4: NP offspring individuals were generated by uniform crossover and single point variation. Dividing NP parent individuals into NP/2 groups randomly, and the parent individuals in each group are called P1And P2. Generating a random binary mask S, passing C1＝S·P1+(1-S)·P2，C2＝S·P2+(1-S)·P1Calculating to obtain two new masks, and performing single-point mutation on the two masks to obtain two offspring C1And C2. Repeating the steps NP/2 times to obtain NP progeny individuals.
Step A5: calculating the adaptive value (if k) of the obtained NP sub-generation individuals<H or M ═ 1, then F1＝-f1，F2100; otherwise F1＝-f1，F2＝f2)。
Step A6: NP parent individuals and NP child individuals were pooled, and step a2 was performed to obtain 2. NP pooled individuals.
Step A7: executing an elite preservation strategy: reserving NP sub-generation individuals with lower rank and larger crowding distance in the 2. NP individuals obtained in the step A6; let k be k + 1.
Step A8: if k is NG, go to step a9, otherwise go back to step A3.
Step A9: and obtaining the population individuals most consistent with the target filtering effect according to the adaptive values of the NP sub-generation individuals.
When M is 1, that is, the intensity value of only a single wavelength is optimized, the optimization effect is achieved as shown in fig. 3, and wavelength tuning can be achieved by optimizing the intensity values of different wavelengths; when M is 8, i.e. the intensity values of eight wavelengths are optimized simultaneously, the eight-wavelength filtering effect is achieved as shown in fig. 4.
In this embodiment, the filtering resolution achieved is determined by the correlation between the output speckles of the large core multimode fiber at different incident wavelengths:
c (Δ λ, x) < I (λ, x) I (λ + Δ λ, x) >/[ < I (λ, x) > < I (λ + Δ λ, x) > ] -1, where I (λ, x) is the light intensity value at the output terminal x when the input light wavelength is λ, and … represents the average value. When a beam of monochromatic light with the wavelength of lambda is input into the large-core-diameter multimode optical fiber with the length of L, the electric field distribution of the output end is as follows:
Figure BDA0003342706010000071
wherein A ismAnd
Figure BDA0003342706010000072
respectively amplitude and initial phase, Ψ, of the mth modemAnd betamThe spatial distribution and the transmission constant of the mth mode, respectively. After different modes are transmitted by the large-core-diameter multimode optical fiber, different phase delays beta can be accumulatedmL, wherein the phase difference between the fundamental mode and the highest-order mode of the maximum-phase-difference fiber
Figure BDA0003342706010000073
When the input wavelength is changed by delta lambda,
Figure BDA0003342706010000074
the change of pi should be necessary to cause a significant change in the optical field intensity distribution at the output of the fiber, i.e.
Figure BDA0003342706010000075
When the core diameter of the large-core-diameter multimode optical fiber is large enough and the Numerical Aperture (NA) is small, delta lambda-lambda can be obtained2/[nL(NA)2]I.e., δ λ is inversely proportional to the fiber length.Therefore, the longer the large core multimode fiber length, the more easily the output speckles at different incident wavelengths are uncorrelated, and the higher the resolution that can be achieved. The length of the large-core-diameter multimode optical fiber does not influence the spectral range of optical filtering, so that the optical filtering method provided by the invention has no mutual restriction relation in the aspects of filtering resolution and spectral range. In addition, the filtering device is output by a single-mode optical fiber end, so that the filtering device has good compatibility with an optical fiber system and low loss.
The foregoing detailed description and drawings are merely representative of the typical embodiments of the invention. It will be apparent that various additions, modifications and substitutions are possible without departing from the spirit and scope of the invention as defined in the accompanying claims. It will be appreciated by those skilled in the art that the present invention may be varied in form, structure, arrangement, proportions, materials, elements, components and otherwise, used in the practice of the invention, depending upon specific environments and operating requirements, without departing from the principles of the present invention. Accordingly, the presently disclosed embodiments are meant to be illustrative only and not limiting, the scope of the invention being indicated by the appended claims and their legal equivalents, rather than by the foregoing description.

Claims (7)

1. A definable optical filtering method based on a liquid crystal spatial light modulator is characterized in that,
step S1: the broadband light source enters from the first port of the optical fiber circulator through the single-mode optical fiber and exits from the second port of the optical fiber circulator, and then is output through the large-core-diameter multimode optical fiber;
step S2: the output light field is collimated by a collimating convex lens, expanded by two convex lenses, polarized by a polarizing film and then incident on a liquid crystal spatial light modulator;
step S3: the light modulated by the liquid crystal spatial light modulator is reflected back to the large-core-diameter multimode optical fiber and is input into the spectrometer through a third port of the optical fiber circulator;
step S4: the computer optimizes the gray pattern loaded on the liquid crystal spatial light modulator by using an iterative optimization algorithm according to the target spectrum based on the obtained spectrum data;
step S5: judging whether the filtered spectrum meets the filtering requirement according to an iterative optimization algorithm, and if so, stopping optimization; if not, step S4 is repeated.
2. The definable optical filtering method based on a liquid crystal spatial light modulator according to claim 1, characterized in that the iterative optimization algorithm is a hybrid non-dominated sorting genetic algorithm comprising the steps of:
step A1: initializing a parameter and an adaptive value function, setting k to be 0, the population size to be NP, the genetic algebra to be NG, the number of input modes to be N, and the cross probability to be PcThe mutation probability is PmMixing factor H, the number M of the wave lengths of filtering, randomly initializing the population and calculating the adaptive value corresponding to each population:
Figure FDA0003342705000000011
wherein, ImIntensity value corresponding to the m-th wavelength, I, read from the spectrometerrefIs the background average;
step A2: carrying out non-dominant sorting on population individuals, and calculating a crowding distance according to the adaptive value;
step A3: NP parents were selected using the binary tournament method. Firstly, randomly selecting two individuals from NP parent individuals, comparing the grades of the two individuals, and winning with low grade, if the grades are the same, then comparing the crowding distance, and winning with larger crowding distance; repeating the steps NP times to select NP parent individuals;
step A4: NP offspring individuals were generated by uniform crossover and single point variation. Dividing NP parent individuals into NP/2 groups randomly, and the parent individuals in each group are called P1And P2(ii) a Generating a random binary mask S, passing C1＝S·P1+(1-S)·P2，C2＝S·P2+(1-S)·P1Calculating to obtain two new masks, and performing single-point mutation on the two masks to obtain two offspring C1And C2(ii) a Repeating the step NP/2 times to obtain NP filial generation individuals;
step A5: calculating the adaptive value of the obtained NP sub-generations, and if k is less than H or M is 1, F1＝-f1，F2100; otherwise F1＝-f1，F2＝f2；
Step A6: mixing NP parent individuals and NP child individuals, and executing the step A2 to obtain 2. NP mixed individuals;
step A7: executing an elite saving strategy, and reserving NP sub-generation individuals with lower sequence and larger crowding distance in the 2. NP individuals obtained in the step A6; setting k as k + 1;
step A8: if k is NG, go to step a9, otherwise go back to step A3;
step A9: and obtaining the population individuals most consistent with the target filtering effect according to the adaptive values of the NP sub-generation individuals.
3. The definable optical filtering method based on liquid crystal spatial light modulator according to claim 2, wherein the filtering resolution of the definable optical filtering method based on liquid crystal spatial light modulator is determined by the correlation between the output speckles of the large core multimode fiber at different incident wavelengths:
C(Δλ，x)＝<I(λ，x)I(λ+Δλ，x)>/[<I(λ，x)><I(λ+Δλ，x)>]-1，
where I (λ, x) is the light intensity at the output end x when the wavelength of the input light is λ, < … > represents the average value.
4. A definable optical filtering method based on a liquid crystal spatial light modulator as claimed in claim 3 characterised in that when a beam of monochromatic light of wavelength λ is input to a large core multimode fibre of length L, the electric field distribution at the output end is:
Figure FDA0003342705000000021
wherein A ismAnd
Figure FDA0003342705000000022
respectively amplitude and initial phase, Ψ, of the mth modemAnd betamSpatial distribution and transmission constant of the mth mode respectively; after different modes are transmitted by the large-core-diameter multimode optical fiber, different phase delays beta can be accumulatedmL, wherein the phase difference between the fundamental mode and the highest-order mode of the maximum-phase-difference fiber
Figure FDA0003342705000000023
When the input wavelength is changed by delta lambda,
Figure FDA0003342705000000024
the change of pi should be necessary to cause a significant change in the optical field intensity distribution at the output of the fiber, i.e.
Figure FDA0003342705000000025
Further obtaining delta lambda-lambda2/[nL(NA)2]I.e., δ λ is inversely proportional to the fiber length.
5. A definable optical filtering system based on a liquid crystal spatial light modulator is characterized by comprising a liquid crystal spatial light modulator, a large-core-diameter multimode fiber and a single-mode fiber mixed structure, wherein one end of the large-core-diameter multimode fiber is connected with the single-mode fiber, an optical field output by the other end of the large-core-diameter multimode fiber is vertically incident on the liquid crystal spatial light modulator after collimation, beam expansion and polarization, and the large-core-diameter multimode fiber serves as a dispersion element; the single mode fiber plays a role in wavelength selection; the liquid crystal spatial light modulator plays a role in controlling the dispersion effect of the multimode optical fiber.
6. The definable optical filter system of claim 5, wherein the large-core multimode optical fiber comprises a large-core step-index multimode optical fiber, a large-core coreless optical fiber, and a large-core hollow optical fiber.
7. The liquid crystal spatial light modulator-based definable optical filter system of claims 5-6, wherein the large core multimode optical fiber is replaced by a strongly coupled multi-core optical fiber.
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