CN112835244A - Nonlinear polarization control system and method for circular birefringent optical fiber Mach-Zehnder interference system - Google Patents

Nonlinear polarization control system and method for circular birefringent optical fiber Mach-Zehnder interference system Download PDF

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CN112835244A
CN112835244A CN202110009326.0A CN202110009326A CN112835244A CN 112835244 A CN112835244 A CN 112835244A CN 202110009326 A CN202110009326 A CN 202110009326A CN 112835244 A CN112835244 A CN 112835244A
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optical fiber
polarization
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circular birefringent
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CN112835244B (en
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王春华
宁金星
方捻
高瑞娟
李力
谷祥
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University of Shanghai for Science and Technology
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    • 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
    • G02F1/35—Non-linear optics
    • G02F1/3511—Self-focusing or self-trapping of light; Light-induced birefringence; Induced optical Kerr-effect
    • 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
    • G02F1/35—Non-linear optics
    • G02F1/3515—All-optical modulation, gating, switching, e.g. control of a light beam by another light beam
    • G02F1/3517—All-optical modulation, gating, switching, e.g. control of a light beam by another light beam using an interferometer

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Abstract

The invention discloses a Mach-Zehnder interference structure (MZI) of a circular birefringent optical fiber based on nonlinear polarization control. The invention constructs the circular birefringent fiber MZI by utilizing the nonlinear polarization traction mechanism that the polarization state of the signal light is always pulled to the polarization state of the incident pump light which is transmitted oppositely in the stimulated Brillouin scattering effect of the circular birefringent fiber, and increases the reverse transmission pump light to enable the signal light and the signal light to generate the nonlinear stimulated Brillouin scattering effect in two circular birefringent fiber interference arms of the MZI. Because the polarization states of the incident pump light entering the two interference arms from the output end in opposite directions are the same, the polarization states of the two paths of signal light reaching the output end through the two interference arms are both pulled to the polarization state of the incident pump light, the output light polarization states in the MZI are kept consistent, and the problem of polarization interference noise caused by random stress birefringence introduced by internal and external factors of an optical fiber of the interference arm in a general optical fiber MZI is solved.

Description

Nonlinear polarization control system and method for circular birefringent optical fiber Mach-Zehnder interference system
Technical Field
The invention relates to the field of optical information processing, in particular to a nonlinear polarization control system and method of a circular birefringent optical fiber Mach-Zehnder interference system.
Background
Random polarization perturbations in optical fibers are a major problem that plagues interferometric fiber systems. The problems of polarization phase noise and polarization fading of interference signals caused by polarization disturbance appear in the optical sensing system. In a sensing system of a single-mode optical fiber or a polarization maintaining optical fiber Mach-Zehnder interference structure, especially in a long-distance optical fiber Mach-Zehnder interference system, due to anisotropy of a geometric structure of an optical fiber caused in a drawing process, external stress borne by the optical fiber interference system in laying and installation and bending torsion stress generated by bending and coiling of the optical fiber can introduce random birefringence in the optical fiber, and the stress birefringence changes along with the change of environmental temperature, so that the random disturbance of the polarization states of transmission light of two interference arms in the optical fiber Mach-Zehnder interference system is caused, the random change of the output polarization states of the two interference arms causes output interference fading, namely polarization noise of interference light intensity.
In the existing optical fiber Mach-Zehnder interference system, the solution method of the polarization noise mainly comprises the following steps: the polarization diversity receiving method, the polarization state modulation method, the polarization state feedback control method and the full polarization maintaining method. The polarization diversity receiving method adopts multi-polarization state polarization detection receiving at a signal receiving end, and the receiving scheme is complex; the polarization state modulation method mainly carries out polarization disturbance modulation or orthogonal polarization state modulation on the polarization state of the signal light, and carries out weighted average on the influence of the polarization state; the polarization feedback control method monitors, feeds back and controls the light polarization states of the two interference arms, thereby reducing the polarization noise of the interference system; the full polarization-maintaining method is a scheme which is applied more at present, a Mach-Zehnder interference system is constructed by adopting polarization-maintaining optical fibers, the polarization state of input light is strictly controlled to be injected in a linear polarization mode along the main shaft direction of the optical fibers, and a small-diameter optical fiber or a mechanical protection measure is strictly selected, so that the stress action of the two interference arm optical fibers on the outside is reduced, and the polarization state of signal light in the optical fibers is always kept in the main shaft direction. However, since the core size of the optical fiber is relatively fixed (5-10 μm) and the cladding of the optical fiber is much larger than the core of the optical fiber, even if a small diameter optical fiber is used, the birefringence of the optical fiber due to the bending stress of the optical fiber is reduced but not eliminated. Excessive mechanical protection not only increases the environmental adaptation difficulty of system operation, but also weakens the system sensitivity of the optical fiber as the sensing arm. Therefore, the search for an effective polarization control method in the fiber optic interferometric system has been a problem faced by the interferometric fiber optic sensing system.
Therefore, those skilled in the art have been devoted to developing a nonlinear polarization interference noise-resistant method of an interference system constructed by a circular birefringent optical fiber Mach-Zehnder.
Disclosure of Invention
In view of the above-mentioned defects of the prior art, the technical problem to be solved by the present invention is how to reduce the interference polarization noise of signal light caused by the fiber random stress birefringence in various sensing, measuring and other systems using the fiber Mach-Zehnder interference structure.
In order to achieve the above object, the present invention provides a circular birefringent optical fiber Mach-Zehnder interference system based on nonlinear polarization control, which comprises a first isolator, a first polarization controller, a circular birefringent optical fiber Mach-Zehnder interference structure, a second polarization controller, an optical amplifier and a second isolator;
the output end of the first isolator is connected with one end of a first polarization controller, and the other end of the first polarization controller is connected with one side of a circular birefringent optical fiber Mach-Zehnder interference structure; the input end of the second isolator is connected with the output end of the optical amplifier, the output end of the second isolator is connected with one end of a second polarization controller, and the other end of the second polarization controller is connected with the other side of the circular birefringent optical fiber Mach-Zehnder interference structure; frequency vpThe pump light enters from the reverse port of the circular birefringent optical fiber Mach-Zehnder interference structure after passing through the optical amplifier and the second isolator, and the frequency is vsAfter passing through the first isolator, the signal light enters from a forward port of the circular birefringent optical fiber Mach-Zehnder interference structure, in two interference arms of the circular birefringent optical fiber Mach-Zehnder interference structure, a stimulated Brillouin scattering effect occurs between the pump light propagating in the forward direction and the signal light propagating in the reverse direction, and the polarization states of the signal light propagating in the forward direction in the two interference arms are pulledThe position of polarization state of incident pump light; the circular birefringent optical fiber Mach-Zehnder interference structure comprises a first circular birefringent optical fiber coupler, a first circular birefringent optical fiber, a second circular birefringent optical fiber and a second circular birefringent optical fiber coupler, wherein one port on one side of the first circular birefringent optical fiber coupler is connected with a first polarization controller, and two ends on the other side of the first circular birefringent optical fiber coupler are respectively connected with the first circular birefringent optical fiber and the second circular birefringent optical fiber; other ports of the first circular birefringent fiber and the second circular birefringent fiber are connected with two ports on the same side of the second circular refractive fiber coupler; and one port on the other side of the second circular birefringent fiber coupler is connected with the second polarization controller.
Furthermore, the first circular birefringent optical fiber coupler has a splitting ratio of K1∶(1-K1) (wherein 0. ltoreq. K1≦ 1), a 2x2 coupling device in which the polarization state of the coupling region remains unchanged or changes synchronously, including but not limited to a circularly birefringent fiber coupler.
The second circular birefringent fiber coupler has a splitting ratio of K2∶(1-K2) (wherein 0. ltoreq. K2≦ 1), a 2x2 coupling device in which the polarization state of the coupling region remains unchanged or changes synchronously, including but not limited to a circularly birefringent fiber coupler.
Further, the first circularly birefringent optical fiber has a length L1The second circularly birefringent fiber has a length L2Wherein (L)1、L2> 0). Furthermore, the optical fiber of the circular birefringent optical coupler and the optical fiber of the interference arm are the same optical fiber, so as to ensure that the SBS frequency shift of the tail fiber of the splitter is the same as that of the optical fiber of the interference arm, and the circular birefringent optical fiber can be a round birefringent optical fiber or a circular birefringent optical fiber processed by other processes.
The invention also provides a nonlinear polarization control method of the circular birefringent optical fiber Mach-Zehnder interference system based on nonlinear polarization control, which comprises the following steps:
(a) will have a frequency vsSignal light of frequency vpThe pump light is simultaneously input into the circular birefringent optical fiber Mach-Zehnder interference structure through a forward port and a reverse port of the circular birefringent optical fiber Mach-Zehnder interference system based on nonlinear polarization control;
(b) the input pump light and the signal light respectively propagate in opposite directions in the two interference arms, and a stimulated Brillouin scattering effect occurs between the forward propagating signal light and the backward propagating pump light in the two interference arms. The polarization states of the forward propagating signal lights in the two interference arms are all pulled by the polarization of the backward transmitting pump lights with the same incident polarization state, so that the output polarization states of the output signal lights in the two interference arms of the Mach-Zehnder interference structure are all pulled to the position of the polarization state of the incident pump lights.
Further, the frequency v of the pump light in the step (a)pAnd the frequency v of the signal lightsSatisfy the relation: v. ofp-vs＝2vAn/λpWherein λ ispIs the wavelength of the pump light, vAIs the acoustic frequency in the fiber and n is the core index. The pump light power needs to be greater than the stimulated brillouin pump power threshold in the optical fiber.
Further, the pulled direction of the signal light polarization state in step (b) is determined by the incident polarization state position of the pump light, the signal light polarization state is always pulled to the polarization state position of the incident pump light, and since the circular birefringence of the optical fiber is much larger than the random linear birefringence in the launch optical fiber, the evolution equation of the signal light polarization state is as follows:
Figure BDA0002884443920000031
wherein
Figure BDA0002884443920000032
For the signal light polarization state Stokes vector, r0Is the SBS gain coefficient, IpFor pumping light power, the equation shows
Figure BDA0002884443920000033
The equation holds true only in relation to the SBS effect and not in relation to the birefringence of the fiber, as long as the circular birefringence of the fiber is much larger than the random birefringence due to the fiber coiling.
For the existing launch fiber, launch lightA beat length of 10 for the circular birefringence in the fiber-4Of order much greater than the linear birefringence induced by the coiling of the fiber and the applied stress (10)-6～10-8) Magnitude, and therefore the equation holds. The equations theoretically indicate that in a Spun fiber, the signal light polarization will be directed toward the incident pump light polarization regardless of the random birefringence introduced by the fiber internal structural asymmetry or external twist.
Further, the circular birefringent optical fiber Mach-Zehnder interference structure in the step (a) comprises a circular birefringent optical fiber and a splitting ratio K1∶(1-K1) And K2∶(1-K2) The circular birefringent fiber coupler of (1). The light splitting ratio of the coupler, the length of two interference arms of a Mach-Zehnder interference structure and the power of input pump light meet the following relations under the condition of equal-power interference:
Figure BDA0002884443920000034
the directional traction of the pump light to the signal light polarization state is related to the length of a circular birefringent optical fiber interference arm of a Mach-Zehnder interference structure, and the longer the optical fiber is, the longer the action distance of stimulated Brillouin scattering is, and the larger the degree of the polarization traction to the signal light polarization state is.
Technical effects
Compared with the prior art, the invention provides a nonlinear polarization control method based on the nonlinear polarization effect that the polarization state of signal light is directionally pulled to the polarization state of incident pump light in the circular birefringent fiber by stimulated Brillouin scattering, a Mach-Zehnder interference structure is constructed by using the circular birefringent fiber, and the pump light which is transmitted reversely is added to generate the nonlinear stimulated Brillouin scattering effect with the signal light in the interference arms of the two circular birefringent fibers, so that the directional pulling of the polarization state of the pump light to the polarization state of the signal light is realized. The output polarization states of the forward transmitted signal lights in the two interference arms are pulled to the polarization state position of the pump light which is incident reversely, so that the interference polarization noise of the signal lights caused by random stress birefringence is eliminated. Unlike polarization maintaining fibers, their physical mechanism is not consistent. The polarization maintaining fiber is a linear birefringent fiber, and in the polarization maintaining fiber, the polarization state of the signal light is drawn to one of two main axes of the polarization maintaining fiber (specifically determined by the position of the polarization state of the incident pump); in the circular birefringent fiber, the polarization state of the signal light is drawn toward the polarization state of the incident pump light.
The conception, the specific structure and the technical effects of the present invention will be further described with reference to the accompanying drawings to fully understand the objects, the features and the effects of the present invention.
Drawings
FIG. 1 is a diagram showing a nonlinear polarization controlled circular birefringent optical fiber Mach-Zehnder interference system of example 1 of this invention;
fig. 2 is a diagram showing a nonlinear polarization controlled circular birefringent optical fiber Mach-Zehnder interference system of example 2 of the present invention.
Detailed Description
The technical contents of the preferred embodiments of the present invention will be more clearly and easily understood by referring to the drawings attached to the specification. The present invention may be embodied in many different forms of embodiments and the scope of the invention is not limited to the embodiments set forth herein.
The "circular birefringent optical fiber Mach-Zehnder interference structure" referred to herein is in simplified form as a "Mach-Zehnder interference structure"; the "SBS effect" refers to the stimulated brillouin scattering effect. The directional words "forward" and "reverse" herein refer to: the direction in which the signal light enters the Mach-Zehnder interference structure is referred to as "forward direction", and then a port for receiving the signal light in the Mach-Zehnder interference structure is a "forward direction port"; the direction in which the pump light enters the Mach-Zehnder interference structure is referred to as "reverse", and then the port for receiving the entry of the pump light in the Mach-Zehnder interference structure is the "reverse port".
The term "connected" and the like are to be understood broadly, e.g., as being fixedly connected, detachably connected, or integrated; can be mechanically or electrically connected; either directly or indirectly through intervening media, or through an internal connection or relationship between two elements. Unless otherwise specifically defined, specific meanings of the above terms in the present invention can be understood as specific conditions by those of ordinary skill in the art.
The principle of the invention is as follows: the method is characterized in that a circular birefringent optical fiber Mach-Zehnder interference structure is constructed based on a polarization directional traction effect of a nonlinear stimulated Brillouin scattering effect in a circular birefringent optical fiber, namely, the polarization state of signal light in the stimulated Brillouin scattering effect is always evolved towards the position of the polarization state of incident pump light under the action of the pump light, and the signal light are subjected to stimulated Brillouin scattering in an optical fiber ring through an additional reverse pump light source, so that the traction of the polarization state of the signal light is realized. The light passes through the circular double-refraction optical fiber coupler, so that the initial polarization states of the pump light entering the two interference arms are ensured to be the same, the polarization states of the output signal light of the two interference arms are pulled to the same polarization state position, and the interference polarization noise of the signal light caused by random stress double refraction in the optical fiber is weakened.
When the frequency v of the pump lightpSum signal light frequency vsSatisfy the relation: v. ofp-vs＝2vAn/λp(wherein λ)pIs the wavelength of the pump light, vAIs the acoustic frequency in the optical fiber, n is the refractive index of the fiber core), and when the pump light power is greater than the SBS threshold power, in two circular birefringent optical fiber interference arms of the Mach-Zehnder interference system, the nonlinear SBS effect occurs between the signal light transmitted in the forward direction and the pump light transmitted in the reverse direction, and the evolution equation of the polarization state of the signal light can be deduced as follows:
Figure BDA0002884443920000051
wherein
Figure BDA0002884443920000052
For the signal light polarization state Stokes vector, r0Is the SBS gain coefficient, IpIn order to pump the optical power of the light,
Figure BDA00028844439200000511
and pumping light polarization states Stokes vectors for input ends.
When pump depletion is not considered, the solution of differential equation (1) is:
Figure BDA0002884443920000053
wherein:
Figure BDA0002884443920000054
Figure BDA0002884443920000055
for the polarization states of the incident signal light and the pump light, Ip0Is reversely input pump light power, MTIs a transposed mueller matrix of fibers from output to input. Signal light gain of
Figure BDA0002884443920000056
L is the fiber length. Adjusting the polarization state of the incident signal light in relation to the polarization state of the incident signal light
Figure BDA0002884443920000057
Then, the maximum SBS gain obtained is:
Figure BDA0002884443920000058
(2) the formula shows that, in the circularly birefringent optical fiber, along the signal light propagation direction, when the optical fiber is sufficiently long,
Figure BDA0002884443920000059
i.e. the signal light of any incident polarization state and the incident polarization state are
Figure BDA00028844439200000510
The SBS effect is generated between the counter-propagating pump lights, and the polarization state of the signal light is pulledTo the location where the pump light is in the incident polarization state. In practical systems, the birefringence of random lines introduced by coiling of optical fibers is generally 10-6～10-8Much smaller than a circularly birefringent fiber (e.g., round birefringence of a round fiber of the order of 10-4Left and right). Therefore, the polarization evolution of the signal light in the propagation process is mainly determined by the circular birefringence of the optical fiber and the SBS polarization pulling, and the polarization state of the signal light is pulled and bound to the polarization state of the input pump light at the output end. In the circular birefringent optical fiber Mach-Zehnder interference structure, as long as the incident polarization states of the reversely transmitted pump light at the output ends of the two interference arms are ensured to be the same, the signal light transmitted in the forward direction in the two interference arms is simultaneously pulled to the incident pump light polarization state. The above formula also shows that the larger the incident pump light power, the larger the traction force; the longer the active fiber, the stronger the pulling effect.
Example 1
As shown in figure 1, the circular birefringent optical fiber Mach-Zehnder interference structure has a light splitting ratio of K1∶(1-K1) A circular birefringent optical fiber coupler 4 having a length L1Has a length L of the first circularly birefringent fiber 52And a second circularly birefringent optical fiber 6, and a splitting ratio of K2∶(1-K2) And a circular birefringent optical fiber coupler 7. Frequency vsAfter passing through the first isolator 2, the signal light 1 is subjected to polarization state adjustment through the first polarization controller 3, so that the SBS gain in the interference arm reaches the maximum, enters from the port I of the circular birefringent optical fiber coupler 4, is output from the port III and the port IV, and the output power distribution ratio is K1∶(1-K1) And respectively enter two circular birefringent fiber interference arms. Frequency vpThe pump light 11 is amplified in power by the optical amplifier 10, passes through the second isolator 9, is adjusted in polarization state by the second polarization controller 8, and is input from the port of the circular birefringent fiber coupler 8, is output from the ports of the fifth and sixth, and reversely enters the two interference arms of the Mach-Zehnder interference structure. The polarization state of the output pump light of the fifth port and the sixth port is the same, and the optical power is respectively K2Ip0And (1-K)2)Ip0. Pump for forward transmission of signal light and backward transmission in two interference armsThe light undergoes the SBS effect. The signal lights of the two interference arms are converged by the sixth port and the sixth port of the second circular birefringent fiber coupler 8. The net gains of the two interference signals output by the eight ports are respectively
Figure BDA0002884443920000061
And
Figure BDA0002884443920000062
where α is the fiber loss. Selecting two circular birefringent couplers to satisfy the relation:
Figure BDA0002884443920000063
the output optical power of the signal light of the two interference arms can be equal. Due to the SBS effect in the circular birefringent fiber, the polarization state of the signal light is dragged by the SBS polarization of the reversely transmitted pump light, so that the output polarization state of the signal light in the two interference arms is constrained to the polarization state position of the incident pump light, and the polarization interference noise can be eliminated.
Example 2
As shown in fig. 2, the key of the circular birefringent optical fiber Mach-Zehnder interference system based on nonlinear polarization control is to ensure that the initial polarization states of the signal light injected into the Mach-Zehnder interference system in the forward direction are the same in the two interference arms, and the initial polarization states of the pump light injected into the Mach-Zehnder interference system in the reverse direction are the same in the two interference arms, so that any space or waveguide structure can be adopted, and a 2x2 coupling device capable of ensuring that the polarization states in the coupling region are maintained or changed synchronously can be applied to the system.
In the system, the circular birefringent optical fiber Mach-Zehnder interference structure has a light splitting ratio of K1∶(1-K1) A first waveguide splitter 4 of length L1Has a length L of the first circularly birefringent fiber 52And a second circularly birefringent optical fiber 6, and a splitting ratio of K2∶(1-K2) And a second waveguide splitter 7. Wherein the length L of the interference arm1，L2Optical fiber loss coefficient alpha and light splitting coefficient of optical fiber/waveguide splitterSatisfy the relation:
Figure BDA0002884443920000064
frequency vsThe signal light 1 passes through the first isolator 2, the polarization state is adjusted by the first polarization controller 2, the SBS gain reaches the maximum, and then the signal light enters the circular birefringent optical fiber Mach-Zehnder interference structure from the port I of the first waveguide splitter 4 in the positive direction; frequency vpAfter the pump light 11 is subjected to power amplification through the optical amplifier 10 and is isolated by the second isolator 9, the polarization state is adjusted through the second polarization controller 8, so that the SBS gain is maximized; and then the optical fiber is branched by a port of the waveguide branching device 7, output by ports of the fifth and sixth, and reversely enter two interference arms of a Mach-Zehnder interference structure. The output pump light polarization states of the fifth port and the sixth port are the same, and the light power is respectively K2Ip0And (1-K)2)Ip0. In the two interference arms, the forward transmission signal light and the backward transmission pump light generate SBS effect. The signal lights of the two interference arms are converged via the # and # ports of the waveguide splitter 7. The polarization state of the signal light is dragged by SBS polarization of the reversely transmitted pump light, so that the output polarization state of the signal light in the two interference arms is constrained to the polarization state position of the incident pump light, the polarization states of the two paths of interference light are kept consistent, and the polarization interference noise is eliminated. Selecting the length L of the interference arm1，L2The optical fiber loss coefficient alpha, the light splitting coefficient of the optical fiber/waveguide splitter and the incident pump light power satisfy the relational expression
Figure BDA0002884443920000065
The optical power of the two interference signals can be kept the same.
The foregoing detailed description of the preferred embodiments of the invention has been presented. It should be understood that numerous modifications and variations could be devised by those skilled in the art in light of the present teachings without departing from the inventive concepts. Therefore, the technical solutions available to those skilled in the art through logic analysis, reasoning and limited experiments based on the prior art according to the concept of the present invention should be within the scope of protection defined by the claims.

Claims (8)

1. A circular birefringent optical fiber Mach-Zehnder interference system based on nonlinear polarization control is characterized by comprising a first isolator, a first polarization controller, a circular birefringent optical fiber Mach-Zehnder interference structure, a second polarization controller, an optical amplifier and a second isolator,
the output end of the first isolator is connected with one end of the first polarization controller, and the other end of the first polarization controller is connected with one side of the circular birefringent optical fiber Mach-Zehnder interference structure; the input end of the second isolator is connected with the output end of the optical amplifier, the output end of the second isolator is connected with one end of the second polarization controller, and the other end of the second polarization controller is connected with the other side of the circular birefringent optical fiber Mach-Zehnder interference structure;
frequency vpThe pump light enters from the reverse port of the circular birefringent optical fiber Mach-Zehnder interference structure after passing through the optical amplifier and the second isolator, and the frequency is vsAfter passing through the first isolator, the signal light enters from a positive port of the circular birefringent optical fiber Mach-Zehnder interference structure, in two interference arms of the circular birefringent optical fiber Mach-Zehnder interference structure, a stimulated Brillouin scattering effect occurs between the pump light propagating in the positive direction and the signal light propagating in the reverse direction, and the polarization state of the signal light propagating in the positive direction in the two interference arms is pulled to the position of the polarization state of the incident pump light;
the circular birefringent optical fiber Mach-Zehnder interference structure comprises a first circular birefringent optical fiber coupler, a first circular birefringent optical fiber, a second circular birefringent optical fiber and a second circular birefringent optical fiber coupler, wherein one port on one side of the first circular birefringent optical fiber coupler is connected with the first polarization controller, and two ends on the other side of the first circular birefringent optical fiber coupler are respectively connected with the first circular birefringent optical fiber and the second circular birefringent optical fiber; the other ports of the first circular birefringent fiber and the second circular birefringent fiber are connected with the two ports on the same side of the second circular refractive fiber coupler; and one port on the other side of the second circular birefringent fiber coupler is connected with the second polarization controller.
2. The non-linear polarization control-based circularly birefringent fiber Mach-Zehnder interference system of claim 1, wherein the first circularly birefringent fiber coupler splitting ratio is K1:(1-K1) (wherein 0. ltoreq. K1≦ 1) 2 × 2 coupling devices in which the polarization state of the coupling region remains unchanged or changes synchronously, including but not limited to circular birefringent fiber couplers.
3. The non-linear polarization control-based circularly birefringent fiber Mach-Zehnder interference system of claim 1, wherein the second circularly birefringent fiber coupler split ratio is K2:(1-K2) (wherein 0. ltoreq. K2≦ 1) 2 × 2 coupling devices in which the polarization state of the coupling region remains unchanged or changes synchronously, including but not limited to circular birefringent fiber couplers.
4. The non-linear polarization control-based circular birefringent fiber Mach-Zehnder interference system of claim 1, wherein the first circular birefringent fiber has a length L1The second circular birefringent fiber has a length L2Wherein (L)1、L2＞0)。
5. A nonlinear polarization control method of a circular birefringent optical fiber Mach-Zehnder interferometer system based on nonlinear polarization control as defined in any of claims 1-4, comprising the steps of:
(a) frequency is nusSignal light and frequency of vpThe pump light is simultaneously input into the circular birefringent optical fiber Mach-Zehnder interference structure through a forward port and a reverse port of the circular birefringent optical fiber Mach-Zehnder interference system based on the nonlinear polarization control;
(b) the input pump light and the signal light are respectively transmitted in opposite directions in the two interference arms, a stimulated Brillouin scattering effect is generated between the signal light transmitted in the forward direction and the pump light transmitted in the reverse direction, and the polarization state of the signal light transmitted in the forward direction is pulled by the polarization of the pump light transmitted in the reverse direction, so that the output polarization state of the output signal light in the two interference arms of the Mach-Zehnder interference structure is simultaneously pulled to the same incident pump light polarization state position.
6. The nonlinear polarization control method of claim 5, wherein the frequency v of the pump light in the step (a) ispAnd a frequency v of the signal lightsSatisfy the relation: v isp-νs＝2νAn/λpWherein λ ispIs the wavelength of the pump light, vAIs the acoustic frequency in the fiber and n is the core index.
7. The nonlinear polarization control method of claim 5, wherein the pulled direction of the polarization state of the signal light in step (b) is determined by the incident polarization state position of the pump light, the polarization state of the signal light is always pulled to the polarization state position of the incident pump light, and in the Spun fiber, since the circular birefringence of the fiber is much larger than the random linear birefringence, the evolution equation of the polarization state of the signal light is:
Figure FDA0002884443910000021
wherein
Figure FDA0002884443910000022
Normalized polarization Stokes vectors, r, of the signal light and the pump light, respectively0Is the SBS gain coefficient, IpFor pumping light power, the equation shows
Figure FDA0002884443910000023
The equation holds true only in relation to the SBS effect and not in relation to the birefringence of the fiber, as long as the circular birefringence of the fiber is much larger than the random birefringence due to fiber coiling and perturbation.
8. The non-wire of claim 5The linear polarization control method is characterized in that the circular birefringent optical fiber Mach-Zehnder interference structure in the step (a) comprises the lengths L1 and L2 of interference arms of two circular birefringent optical fibers and the light splitting coefficient K of the first and second optical circular birefringent optical fiber couplers1、K2Optical fiber loss coefficient alpha, SBS gain coefficient r0Input pump light power Ip0Obeying the following relation according to the equal interference light intensity condition:
Figure FDA0002884443910000024
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