De-polarization system of active tuned filter and regulation and control method

Abstract

The invention discloses a depolarization system of an active tuned filter and a regulation and control method, and relates to the field of tuned filters, the depolarization system comprises a main control module, and a tunable laser module, a spectral pattern modulation module, a polarization regulation and control module and an active tuned filter module which are respectively connected with the main control module; the tunable laser module is connected with the spectral pattern modulation module through a line, and the polarization regulation and control module is respectively connected with the spectral pattern modulation module and the active tuning filtering module through lines. According to the invention, the cross-polarization state double-pump light generation technology based on the Faraday rotating mirror is adopted, and the pump light modulation based on the electro-optical modulator is combined, so that the generation of the spectrum type coding double-polarization pump light is realized, and the performance limitation of the polarization state of an optical fiber signal on the active tuned filter is eliminated, thereby constructing the active tuned filter irrelevant to polarization. And core indexes such as filtering spectrum pattern editing, filtering bandwidth, dynamic range, roll-off coefficient and the like of the active tuned filter are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of tuning filters, and in particular to a depolarization system and a control method for an actively tunable filter based on a Faraday rotator mirror. Background Art

[0002] With the development of technology, narrowband tunable filters have become increasingly important and are widely used in applications such as channel selection in dense wavelength division multiplexing systems, optical carrier manipulation in single sideband modulation systems and radio frequency optoelectronic links, coherent optical wave transmission systems, tunable receivers, frequency selection of ASE spectra, and spectral analysis. In these applications, it is required that the tuning filter has characteristics such as low insertion loss, a sharp transition band between the passband and the stopband, a high roll-off coefficient, a high side lobe suppression ability, and a wide tunable range.

[0003] In recent years, actively tunable optical filters constructed based on fiber nonlinear effects such as the fiber stimulated Brillouin effect have good application value in fields such as ultra-high resolution spectral analysis and optical information processing due to their advantages of active tuning, ultra-narrow filtering bandwidth, and high roll-off coefficient. However, the existence of the polarization characteristics of fiber signals has an adverse effect on core indicators such as the filtering bandwidth and dynamic range of such actively tunable filters, affecting performance improvement. Summary of the Invention

[0004] In order to overcome the above problems existing in the prior art, the present invention proposes a depolarization system and a control method for an actively tunable filter based on a Faraday rotator mirror.

[0005] The technical solution adopted by the present invention to solve its technical problems is: a depolarization system for an actively tunable filter, including a main control module, and a tunable laser module, a spectrum modulation module, a polarization control module, and an actively tunable filtering module that are respectively connected to the main control module. The tunable laser module and the spectrum modulation module are connected by a line, and the polarization control module is respectively connected to the spectrum modulation module and the actively tunable filtering module by a line; The main control module is used for the control and working parameter setting of a polarization-independent actively tunable filter; The tunable laser module is used for generating a fixed linearly polarized optical signal with a specific wavelength; The spectrum modulation module is used for generating a modulated encoded pump light required for the spectrum of the actively tunable filter; The polarization control module is used for generating a spectrum-encoded dual-polarization pump light required for constructing a polarization-independent actively tunable filter; The actively tunable filtering module realizes polarization-independent active tuning filtering of fiber signals.

[0006] The above-mentioned depolarization elimination system of an active tuning filter, wherein the spectral modulation module includes an electro-optic modulator and a local oscillator; the electro-optic modulator and the local oscillator are connected by a line, and the electro-optic modulator is respectively connected to the tunable laser module and the polarization control module by lines; The electro-optic modulator is used to modulate the fixed linearly polarized optical signal with a specific wavelength generated by the tunable laser to generate the modulated coded pump light required by the active tuning filter, and the modulated coded pump light directly determines the filtering parameters of the active tuning filter; The local oscillator is used to generate the radio frequency drive signal required for the operation of the electro-optic modulator.

[0007] The above-mentioned depolarization elimination system of an active tuning filter, wherein the polarization control module includes a polarizer, a first Faraday rotator mirror, a first polarization-maintaining fiber circulator, a polarization-maintaining splitter, a first fiber amplifier, a second Faraday rotator mirror, a second polarization-maintaining fiber circulator, a second fiber amplifier, and a polarization-maintaining combiner; the polarizer, the first Faraday rotator mirror, the first polarization-maintaining fiber circulator, and the polarization-maintaining splitter are connected in sequence by lines, the first Faraday rotator mirror is connected to the first polarization-maintaining fiber circulator by a line, the second Faraday rotator mirror is connected to the second polarization-maintaining fiber circulator by a line, and the polarization-maintaining splitter, the second polarization-maintaining fiber circulator, the second fiber amplifier, and the polarization-maintaining combiner are connected in sequence by lines; the first fiber amplifier is respectively connected to the polarization-maintaining splitter and the polarization-maintaining combiner by lines.

[0008] The above-mentioned depolarization elimination system of an active tuning filter, the polarizer is used to adjust the modulated coded pump light input by the spectral modulation module into a fixed linearly polarized modulated coded pump light, and introduce the linearly polarized light into the first polarization-maintaining fiber circulator; The first polarization-maintaining fiber circulator is used to introduce the fixed linearly polarized modulated coded pump light into the first Faraday rotator mirror and introduce the reflected light of the first Faraday rotator mirror into the polarization-maintaining splitter; The first Faraday rotator mirror is used to convert the input specific linearly polarized optical signal into a fixed linearly polarized optical signal with a polarization state orthogonal to it; The polarization-maintaining splitter is used to equally divide the fixed linearly polarized optical signal output by the first polarization-maintaining fiber circulator into two identical fixed linearly polarized lights; The first fiber amplifier amplifies the optical power of the first optical signal of the polarization-maintaining splitter to a specific value; The second polarization-maintaining fiber circulator is used to introduce the second optical signal of the polarization-maintaining splitter into the second Faraday rotator mirror and introduce the reflected light of the second Faraday rotator mirror into the polarization-maintaining splitter; The second Faraday rotator mirror is used to convert the input specific linearly polarized optical signal into a linearly polarized optical signal with a polarization state orthogonal to it, that is, orthogonal to the polarization state of the first optical signal; A second optical fiber amplifier, which is used to amplify the optical power of the optical signal in the second polarization state to the same optical power as that of the optical signal in the first polarization state; A polarization-maintaining beam combiner, which is used to combine the optical signal in the first polarization state and the optical signal in the second polarization state orthogonal to it into a single optical signal, that is, to generate a spectral-encoding dual-polarization pump light, which serves as the depolarization pump light when the active tuning filter works.

[0009] The above depolarization system of an active tuning filter, wherein the active tuning filter module includes an optical isolator, an optical fiber loop, and a single-mode optical fiber circulator; the optical isolator, the optical fiber loop, and the single-mode optical fiber circulator are connected in sequence through lines; An optical isolator, which is used to block any stray light that may interfere with the external signal to be filtered; An optical fiber loop, which is the working medium of the active tuning filter constructed based on the fiber stimulated Brillouin effect; A single-mode optical fiber circulator, which is used to introduce the spectral-encoding dual-polarization pump light into the optical fiber loop and export the filtered signal.

[0010] A control method for the depolarization system of an active tuning filter, based on the above depolarization system of an active tuning filter, includes the following steps: Step 1: Obtain a fixed linearly polarized optical signal with a specific wavelength through a tunable laser module; Step 2: Modulate the fixed linearly polarized optical signal output by the tunable laser module through a spectral modulation module to generate the spectral-encoding pump light required by the active tuning filter; Step 3: Process the spectral-encoding pump light output by the spectral modulation module through a polarization control module to generate the spectral-encoding dual-polarization pump light required for constructing a polarization-independent active tuning filter; Step 4: Through the active tuning filter module, utilize the fiber stimulated Brillouin effect and combine the spectral-encoding dual-polarization pump light to construct a polarization-independent active tuning filter, and realize the polarization-independent active tuning filtering of the optical fiber signal.

[0011] The beneficial effect of the present invention is that the present invention adopts a depolarization and control method for an active tuning filter based on a Faraday rotation mirror, which can meet the requirements of the active tuning filter for polarization-independent spectral-encoding dual-polarization pump light, eliminate the performance limitation of the polarization state of the optical fiber signal on the active tuning filter, thereby constructing a polarization-independent active tuning filter, and realizing the improvement of core indicators such as the filtering spectrum editing, filtering bandwidth, dynamic range, and roll-off coefficient of such active tuning filters.

[0012] The core polarization control module of the present invention adopts a Faraday rotator, a passive optical fiber device, which does not require fine control of the polarization state of the pump light. By adopting a dual-Faraday rotator configuration, the crosstalk problem in the process of pump light control is solved, and the problem of pump light dispersion introduced by the traditional method that requires a delay module is eliminated, greatly improving the accuracy of the filtering spectrum in the application of the filter. The polarization elimination method for the actively tunable filter proposed by the present invention can ensure that the actively tunable filter achieves a dynamic range improvement of more than 15 dB. The polarization-independent actively tunable filtering method proposed by the present invention can ensure that the actively tunable filter achieves a configurable bandwidth of 10 MHz to 5 GHz, and configurable filtering spectra such as Gaussian and Lorentz. The polarization elimination method for the actively tunable filter proposed by the present invention can effectively eliminate the random spectral curve distortion introduced by the polarization state mismatch between the pump light and the light to be filtered, and can ensure the repeatability and filtering purity of the filtered signal of the actively tunable filter. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 is a schematic diagram of an embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0014] To enable those skilled in the art to better understand the technical solutions of the present invention, the present invention will be described in detail below with reference to the drawings and specific embodiments.

[0015] As Figure 1 shown, a polarization elimination device for an actively tunable filter based on a Faraday rotator includes a main control module, a tunable laser module, a spectrum modulation module, a polarization control module, and an actively tunable filtering module, which are respectively connected to the main control module. The tunable laser module and the spectrum modulation module are connected by a line, and the polarization control module is respectively connected to the spectrum modulation module and the actively tunable filtering module by a line; The main control module is configured to control and set the working parameters of the polarization-independent actively tunable filter; The tunable laser module is configured to generate a fixed linearly polarized light signal with a specific wavelength; The spectrum modulation module is configured to generate a modulated encoded pump light required for the actively tunable filter; The polarization control module is configured to generate a spectrum-encoded dual-polarization pump light required for constructing a polarization-independent actively tunable filter; The actively tunable filtering module is configured to achieve polarization-independent actively tunable filtering of optical fiber signals; The spectrum modulation module includes an electro-optic modulator and a local oscillator; the electro-optic modulator and the local oscillator are connected by a line, and the electro-optic modulator is respectively connected to the tunable laser module and the polarization control module by a line; An electro-optic modulator, configured to modulate an optical signal with a fixed linear polarization state at a specific wavelength generated by a tunable laser, to generate a modulated encoded pump light required for an active tuning filter. The modulated encoded pump light directly determines filtering parameters such as the spectral pattern, bandwidth, and roll-off coefficient of the active tuning filter; A local oscillator, configured to generate a radio frequency drive signal required for the operation of the electro-optic modulator; A polarization control module, including a polarizer, a first Faraday rotator mirror, a first polarization-maintaining fiber circulator, a polarization-maintaining splitter, a first fiber amplifier, a second Faraday rotator mirror, a second polarization-maintaining fiber circulator, a second fiber amplifier, and a polarization-maintaining combiner; The polarizer, the first Faraday rotator mirror, the first polarization-maintaining fiber circulator, and the polarization-maintaining splitter are connected in sequence by lines. The first Faraday rotator mirror is connected to the first polarization-maintaining fiber circulator by a line. The second Faraday rotator mirror is connected to the second polarization-maintaining fiber circulator by a line. The polarization-maintaining splitter, the second polarization-maintaining fiber circulator, the second fiber amplifier, and the polarization-maintaining combiner are connected in sequence by lines; The first fiber amplifier is connected to the polarization-maintaining splitter and the polarization-maintaining combiner respectively by lines; The polarizer is configured to adjust the modulated encoded pump light input by the spectral modulation module into a modulated encoded pump light with a fixed linear polarization state, and then introduce the linearly polarized light into the first polarization-maintaining fiber circulator; The first polarization-maintaining fiber circulator is configured to introduce the modulated encoded pump light with a fixed linear polarization state into the first Faraday rotator mirror, and introduce the reflected light of the first Faraday rotator mirror into the polarization-maintaining splitter; The first Faraday rotator mirror is configured to convert an input optical signal with a specific linear polarization state into an optical signal with a fixed linear polarization state orthogonal to its polarization state; The polarization-maintaining splitter is configured to equally divide the optical signal with a fixed linear polarization state output by the first polarization-maintaining fiber circulator into two identical optical signals with a fixed linear polarization state; The first fiber amplifier is configured to amplify the optical power of the first path of the optical signal of the polarization-maintaining splitter to a specific value; The second polarization-maintaining fiber circulator is configured to introduce the second path of the optical signal of the polarization-maintaining splitter into the second Faraday rotator mirror, and introduce the reflected light of the second Faraday rotator mirror into the polarization-maintaining splitter; The second Faraday rotator mirror is configured to convert an input optical signal with a specific linear polarization state into an optical signal with a linear polarization state orthogonal to its polarization state, that is, orthogonal to the polarization state of the first path of the optical signal; The second fiber amplifier is configured to amplify the optical power of the second path of the polarized optical signal to the same optical power as the first path of the polarized optical signal; A polarization-maintaining beam combiner, configured to combine a first polarization-state optical signal and a second polarization-state optical signal orthogonal to the first one into a single optical signal, that is, to generate a spectrally encoded dual-polarization pump light, which serves as the depolarization pump light when the active tuning filter is operating; The polarization states of the optical signals output by the polarization control module are orthogonal and incoherent.

[0016] The active tuning filter module includes an optical isolator, an optical fiber loop, and a single-mode fiber circulator; the optical isolator, the optical fiber loop, and the single-mode fiber circulator are connected in sequence by lines; The optical isolator is configured to block any stray light that may interfere with the external signal to be filtered; The optical fiber loop is configured as the working medium of the active tuning filter based on the stimulated Brillouin effect of the optical fiber; The single-mode fiber circulator is configured to introduce the spectrally encoded dual-polarization pump light into the optical fiber loop and export the filtered signal.

[0017] In this embodiment, the tunable laser can be a polarization-maintaining output tunable laser such as the TLM 8700 of Newport Corporation.

[0018] The spectral modulation module includes an electro-optic modulator and a local oscillator, and the following factors need to be considered: (1) For the electro-optic modulator, an amplitude, phase, or IQ-type electro-optic modulator can be selected according to the filtering parameter requirements such as the spectrum type, bandwidth, and roll-off coefficient of the active tuning filter; (2) For the local oscillator, a radio frequency driver with arbitrary waveform output and a frequency band of 10 MHz to 18 GHz is used; The polarization control module includes a polarizer, a first Faraday rotator, a first polarization-maintaining fiber circulator, a polarization-maintaining splitter, a first fiber amplifier, a second Faraday rotator, a second polarization-maintaining fiber circulator, a second fiber amplifier, and a polarization-maintaining beam combiner, and the following factors need to be considered: (1) The polarization direction of the polarizer should match the polarization state of the light output by the tunable laser; (2) The first Faraday rotator preferably uses a 180° phase-shifted Faraday rotator; (3) The first polarization-maintaining fiber circulator preferably uses a polarization-maintaining fiber circulator in the operating band of the active tuning filter; (4) The polarization-maintaining splitter preferably uses a 1×2 polarization-maintaining fiber splitter; (5) The first fiber amplifier preferably uses an erbium-doped fiber amplifier, which can generate an amplified light output of more than ten dBm, fully meeting the requirements of the active tuning filter for the pump light power; (6) The second Faraday rotator preferably uses a 180° phase-shifted Faraday rotator; (7) The second polarization-maintaining optical fiber circulator is preferably a polarization-maintaining optical fiber circulator that actively tunes the operating band of the filter; (8) The second optical fiber amplifier is preferably an erbium-doped optical fiber amplifier, which can generate an amplified optical output of more than ten dBm, fully meeting the requirements of the active tuning filter for the pump optical power; (9) The polarization-maintaining beam combiner is preferably a 2×1 polarization-maintaining optical fiber beam combiner.

[0019] The active tuning filter module includes an optical isolator, an optical fiber loop, and a single-mode optical fiber circulator. The following factors need to be considered: (1) A conventional optical fiber isolator can be used for the optical isolator; (2) The optical fiber loop is preferably made of bismuth-doped optical fiber or chalcogenide glass optical fiber. Compared with other optical fibers, the Brillouin effect of bismuth-doped optical fiber or chalcogenide glass optical fiber is better; (3) The single-mode optical fiber circulator is preferably a single-mode optical fiber circulator in the operating band of the active tuning filter.

[0020] The working principle of the depolarization device of the active tuning filter based on a Faraday rotator mirror proposed in this embodiment is as follows: Under the control of the main control module, the fixed linearly polarized optical signal of a specific wavelength generated by the tunable laser module generates the modulated coded pump light required by the active tuning filter through the spectral modulation module, and then enters the polarization control module to generate the spectral coded double-polarization pump light containing two orthogonal polarization states, a wide wavelength tuning range, and non-coherence required for constructing a polarization-independent active tuning filter, which is used as the polarization-independent pump light of the active tuning filter module, and then actively tunes and filters the optical fiber signal in the active tuning filter module.

[0021] The depolarization device of the active tuning filter based on a Faraday rotator mirror proposed in this embodiment adopts a method for generating orthogonal polarization state double-pump light with a double Faraday rotator mirror, which can meet the requirements of the active tuning filter for polarization-independent spectral coded double-polarization pump light, eliminates the performance limitation of the polarization state on the active tuning filter, thereby constructs a polarization-independent active tuning filter, and realizes the improvement of core indicators such as the filtering spectral pattern editing, filtering bandwidth, dynamic range, and roll-off coefficient of such active tuning filters.

[0022] Based on the above depolarization system of the active tuning filter based on a Faraday rotator mirror, this embodiment also discloses a depolarization method, including the following steps: Step 1: The tunable laser module outputs a signal light with a wavelength of 1550 nm, a line width of 100 kHz, and a p polarization state; Step 2: The tunable laser module outputs a signal light with a wavelength of 1550 nm, a linewidth of 100 kHz, and a p-polarization state, which enters the spectral modulation module. Driven by a multi-carrier cosine local oscillator signal output by an RF driver with a frequency band of 10 MHz to 1 GHz and a carrier spacing of 10 MHz, it is modulated by an 18 GHz amplitude electro-optic modulator to generate a multi-carrier spectral encoding pump light with a carrier width of 1 GHz and a carrier spacing of 10 MHz. Step 3: The multi-carrier spectral encoding pump light with a carrier width of 1 GHz and a carrier spacing of 10 MHz passes through the polarization control module to process the spectral encoding pump light output by the spectral modulation module, generating a spectral encoding dual-polarization pump light that includes p and s polarization states, has an optical power of 10 dBm, and is incoherent. Step 4: The spectral encoding dual-polarization pump light that includes p and s polarization states, has an optical power of 10 dBm, and is incoherent serves as the polarization-independent pump light of the active tuning filter module, thereby constructing a polarization-independent active tuning filter with a 1 GHz bandwidth and a Gaussian line shape, and further realizing the polarization-independent active tuning filtering of the optical fiber signal.

[0023] The above embodiments are only exemplary embodiments of the present invention and are not used to limit the present invention. Those skilled in the art can make various modifications or equivalent replacements to the present invention within the essence and protection scope of the present invention, and such modifications or equivalent replacements should also be regarded as falling within the protection scope of the present invention.

Claims

1. A depolarization system for an actively tuned filter, characterized in that: It includes a main control module and a tunable laser module, a spectrum modulation module, a polarization control module, and an active tuning filter module respectively connected to the main control module, wherein the tunable laser module and the spectrum modulation module are connected through lines, and the polarization control module is respectively connected to the spectrum modulation module and the active tuning filter module through lines; Main control module, used for controlling and setting working parameters of polarization-independent active tuning filter; A tunable laser module for generating a fixed linear polarization state optical signal of a specific wavelength; A spectrum modulation module, used to generate modulation coded pump light required for actively tuning the spectrum of the filter; A polarization control module, used to generate spectrally coded dual-polarization pump light required to construct a polarization-independent active tuning filter; Active tuning and filtering module realizes polarization-independent active tuning and filtering of optical fiber signals.

2. The depolarization system of an active tuning filter according to claim 1, characterized in that: The spectrum modulation module includes an electro-optic modulator and a local oscillator; The electro-optic modulator and the local oscillator are connected via a line, and the electro-optic modulator is connected to the tunable laser module and the polarization control module via lines respectively; An electro-optic modulator is used to modulate a fixed linear polarization state optical signal of a specific wavelength generated by a tunable laser to generate a modulation coded pump light required for an active tuning filter. The modulation coded pump light directly determines the filtering parameters of the active tuning filter. The local oscillator is used to generate the RF drive signal required for the operation of the electro-optic modulator.

3. The depolarization system of an active tuning filter according to claim 1, characterized in that: The polarization control module comprises a polarizer, a first Faraday rotator, a first polarization-maintaining fiber circulator, a polarization-maintaining splitter, a first fiber amplifier, a second Faraday rotator, a second polarization-maintaining fiber circulator, a second fiber amplifier, and a polarization-maintaining combiner; the polarizer, the first Faraday rotator, the first polarization-maintaining fiber circulator, and the polarization-maintaining splitter are connected in sequence through lines, the first Faraday rotator is connected to the first polarization-maintaining fiber circulator through lines, the second Faraday rotator is connected to the second polarization-maintaining fiber circulator through lines, the polarization-maintaining splitter, the second polarization-maintaining fiber circulator, the second fiber amplifier, and the polarization-maintaining combiner are connected in sequence through lines; the first fiber amplifier is respectively connected to the polarization-maintaining splitter and the polarization-maintaining combiner through lines.

4. The depolarization system of an active tuning filter according to claim 3, characterized in that: A polarizer, used for adjusting the modulation and coding pump light input by the spectrum modulation module into the modulation and coding pump light with a fixed linear polarization state, and introducing the linear polarization light into the first polarization-maintaining optical fiber circulator; A first polarization-maintaining fiber circulator is used to introduce the modulated coded pump light with a fixed linear polarization state into the first Faraday rotator, and to introduce the reflected light of the first Faraday rotator into the polarization-maintaining splitter; A first Faraday rotator is used to convert an input optical signal of a specific linear polarization state into an optical signal of a fixed linear polarization state orthogonal to the polarization state; A polarization-maintaining splitter, used for equally splitting the fixed linear polarization state optical signal output by the first polarization-maintaining optical fiber circulator into two paths of light with the same fixed linear polarization state; A first optical fiber amplifier is used to amplify the optical power of the first optical signal of the polarization-maintaining splitter to a specific value; A second polarization-maintaining optical fiber circulator, used for introducing a second optical signal of the polarization-maintaining splitter into the second Faraday rotator, and introducing reflected light of the second Faraday rotator into the polarization-maintaining splitter; The second Faraday rotator is used to convert the input optical signal of a specific linear polarization state into an optical signal of a linear polarization state orthogonal to the polarization state thereof, that is, orthogonal to the polarization state of the first optical signal; A second optical fiber amplifier is used to amplify the optical power of the second polarization state optical signal to the same optical power as the first polarization state optical signal; The polarization-maintaining beam combiner is used to combine a first polarization state optical signal and a second polarization state optical signal orthogonal to the first polarization state optical signal into one optical signal, that is, to generate a spectrally coded dual-polarization pump light beam as a depolarized pump light when the active tuning filter works.

5. The depolarization system of an active tuning filter according to claim 1, characterized in that: The active tuning filter module includes an optical isolator, an optical fiber ring, and a single-mode optical fiber circulator; the optical isolator, the optical fiber ring, and the single-mode optical fiber circulator are connected in sequence through a line; Optical isolators to block any stray light that could interfere with the external signal to be filtered; Fiber ring, the working medium of the active tuning filter based on the stimulated Brillouin effect of optical fiber; The single-mode fiber circulator is used to introduce spectrally coded dual-polarization pump light into the fiber ring and export the filtered signal.

6. A method for controlling a depolarization system of an active tuning filter, characterized in that: A depolarization system of an active tuning filter according to any one of claims 1 to 5, comprising the following steps: Step 1: Obtain a fixed linear polarization state optical signal of a specific wavelength through a tunable laser module; Step 2: modulate the fixed linear polarization state optical signal output by the tunable laser module through the spectrum modulation module to generate the spectrum coded pump light required by the active tuning filter; Step 3: Processing the spectral coded pump light output by the spectral modulation module through the polarization control module to generate the spectral coded dual-polarization pump light required for constructing the polarization-independent active tuning filter; Step 4: Through the active tuning filter module, using the stimulated Brillouin effect of the optical fiber and combining the spectrally coded dual-polarization pump light, a polarization-independent active tuning filter is constructed to achieve polarization-independent active tuning filtering of the optical fiber signal.

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