CN102322958A - Method for monitoring optical fiber polarization change and optical path system - Google Patents

Abstract

The invention belongs to the field of optical fiber technology, and in particular relates to a method for monitoring the polarization change of an optical fiber and an optical path system. Based on the inherent polarization characteristics of the optical fiber itself, the invention uses a fiber coupler to form an interference optical path, and the monitored optical fiber is connected to the input optical path of the interference optical path, and monitors the change of the polarization state of the optical fiber by detecting the interference light intensity. The invention also provides a method for forming an optical fiber path equivalent to the effect of a linear polarizer. The method of the present invention can be used to monitor the physical quantity causing the optical fiber polarization change, for example, it is especially suitable for applications such as perimeter security where the optical fiber is used to sense external disturbances.

Description

Method and Optical Path System for Monitoring Optical Fiber Polarization Change

technical field

The invention belongs to the field of optical fiber technology, and in particular relates to a method for monitoring the polarization change of an optical fiber and an optical path system.

Background technique

With the development of optical fiber technology, the application field of optical fiber is becoming wider and wider. It is not only widely used in optical communication as a transmission medium, but also plays an increasingly important role in the field of sensing technology, especially in recent years. Come, more booming trends.

In an ideal fiber, the shape and refractive index distribution of the fiber cross-section are uniform, the polarization mode is degenerate, and the polarization state of light traveling along the fiber will not change. However, in practice, due to the impossibility of the core At the same time, the anisotropic stress applied to the fiber is under the action of the elastic-optic effect, and the fiber itself has birefringence, which makes the polarization characteristics of the light transmitted in the fiber change. In particular, the fiber is easy to Affected by changes in ambient temperature and its own state, the polarization state of the output light has great randomness.

The imperfect characteristics of optical fibers in practical applications limit the advantages of optical fibers to a certain extent. However, the polarization characteristics of optical fibers are used in some fields. A typical application is to monitor transmission in the perimeter of optical fibers. The polarization change of the sensing fiber is used to judge whether there is an intrusion from the outside world. The detection principle is to use when the external disturbance causes the state of the fiber to change, resulting in a change in the polarization characteristics of the fiber, resulting in a change in the polarization of the light transmitted through the sensing fiber. Fiber perturbation can be detected by detecting a change in the polarization of the output light. Patent US 7,693,359 B2 is an optical fiber intrusion detection technology based on detecting optical fiber polarization changes.

For the detection of fiber polarization, a polarizer or polarization beam splitter is usually used to obtain a polarization component and observe it. Figure 1 is a scheme mentioned in the patent US 7,693,359 B2. The polarized light source 1 is injected from one end of the monitored fiber 2 (sensing fiber), and the light output from the other end of the monitored fiber 2 is injected into the polarizer 3, and the polarizer 3 The output light is detected by the receiver 4. In this scheme, through the function of the polarizer 3, the light intensity that changes with the polarization is obtained.

Contents of the invention

The purpose of the present invention is to provide a method and an optical system for monitoring the polarization change of an optical fiber which are simple and easy to implement and have low cost.

The method for monitoring the polarization change of the optical fiber proposed by the invention utilizes the inherent polarization characteristics of the optical fiber to construct an optical fiber interference optical path, and monitors the polarization change of the monitored optical fiber by detecting the interference light intensity. The invention is applicable to the monitoring of the physical quantity causing the optical fiber polarization change.

The specific steps of the method of the present invention are as follows: firstly, using a fully polarized or partially polarized light source, using a fiber coupler to construct an interference optical path; then, connecting the monitored optical fiber to the light input path of the interference optical path; finally, detecting the interference light intensity, from The variation of the interference light intensity obtains the polarization variation of the monitored fiber.

The invention utilizes the optical fiber coupler to form an optical system for monitoring the polarization change of the optical fiber. The specific optical path is shown in Figure 2, including: monitored optical fiber 6, light source 7, polarization detection component 8; polarization detection component 8 is composed of fiber optic coupler 5, 5a1, 5a2, ..., 5aN, 5b1, 5b2 are fiber optic couplers 5 5a1, 5a2, ..., 5aN are ports in the same direction (a total of N), 5b1 and 5b2 are ports in the same direction; 7a is the port of the light source 7. The monitored optical fiber 6 is connected to the transmission path of the light source 7. After the light output by the light source 7 is transmitted through the monitored optical fiber 6, it is input into the polarization detection component 8 through the port 5a1. The port 5b1 of the fiber coupler 5 in the polarization detection component 8 It is connected with 5b2 to form an interference light path, and interference light intensity can be obtained from ports 5a2,...,5aN for polarization detection. the

The method for connecting the monitored optical fiber 6 to the transmission path of the light source can be shown in FIG. 3 , that is, the monitored optical fiber 6 is directly connected in series between the light source 7 (port 7 a ) and the port 5 a. FIG. 4 shows yet another access mode of the monitored optical fiber 6 . That is, it is connected through the fiber coupler 9 and the reflection device 10 (such as a reflector with a pigtail); let 9a1, 9a2, 5b1 be the ports of the fiber coupler 9, 9a1, 9a2 are the ports in the same direction, and 9b1 is the port in the other direction The port of the light source 7 is connected with the port 9a1 of the coupler; one end of the monitored optical fiber 6 is connected with the port 9b1, and the other end is connected with a reflection device 10; the port 9a2 is connected with the port 5a1, and the transmission path of light is:

7a→9a1→9b1→monitored optical fiber 6→reflection device 10→monitored optical fiber 6→9b1→9a2→5a1.

The working principle of the present invention is analyzed as follows.

，“T”表示转置；由于光纤实际应用的光纤可以看作具有一定双折射的器件，可设从端口5b1到5b2间的传输矩阵为琼斯矩阵J，忽略这段光纤的传输损耗，不影响偏振分析结果，J可表示为： Let the light input from port 5a1 be

, "T" means transpose; since the optical fiber used in the optical fiber can be regarded as a device with a certain birefringence, the transmission matrix from port 5b1 to 5b2 can be set as Jones matrix J, and the transmission loss of this section of optical fiber is ignored, which does not affect The result of polarization analysis, J can be expressed as:

                           （1）

(1)

，

为

的元素，“*”表示共轭。则从端口5aM（M=1, 2, …）输出的光为以下两路光的干涉： in,

,

for

elements, "*" means conjugation. Then the output light from port 5aM (M=1, 2, ...) is the interference of the following two light paths:

5a1→5b1→5b2→5aM, 5a1→5b2→5b1→5aM (M =1, 2, …)

The output vector is:

（M=1, 2, …）    （2）

(M=1, 2, ...) (2)

，

分别为从光输入端口5a1经顺时针和逆时针方向从端口5aM（M in,

,

Respectively from the optical input port 5a1 clockwise and counterclockwise from the port 5aM (M

为耦合器引入的干涉相位差。则，相应的输出光强为： =1, 2, …), the transmission coefficients are real numbers,

The interference phase difference introduced by the coupler. Then, the corresponding output light intensity is:

(3)

，可得： set up

,Available:

     （4）

(4)

in,

， 

,

、

的幅度和相位发生变化，这一项会发生变化，导致输出光强I发生变化。 It can be seen from equation (4) that the first and second terms are constant values, and the third term is related to the polarization input to the port 5aM. When the input polarization changes, the polarization component

,

If the amplitude and phase of λ change, this term will change, resulting in a change in the output light intensity I.

，可以判断从端口5aM输入的光的偏振变化。 Therefore, by detecting the interference light intensity

, the polarization change of the light input from the port 5aM can be judged.

、

变化时，若第三项发生变化，必须有： From equation (4), it can be seen that when the polarization of the input light from port 5ai changes, that is

,

When changing, if the third item changes, there must be:

，即

                      （5）

,Right now

(5)

的干涉光强输出端口。一般来说，当取5a1为光输入端口时，从5a1输出的干涉光的相位差

为0，因此该端口不易作为监测偏振变化的端口。而对于耦合器5为2*2耦合器的结构，光端口5a2输出干涉光的相位差为为

，因此，也不易作为监测端口。这样，5a1和5a2皆不易作为监测端口。虽然在实际使用的器件中，由于2*2耦合器自身的不完美性，使相位差轻微的偏离0（或

），使得输入偏振发生变化时，干涉输出会发生轻微的变化，在该发明采用的方法中，耦合器5不易采用2*2耦合器。 Therefore, it is necessary to choose the phase difference

The output port of the interference light intensity. Generally speaking, when 5a1 is taken as the optical input port, the phase difference of the interference light output from 5a1

is 0, so this port is not easy to be used as a port for monitoring polarization changes. However, for the structure in which the coupler 5 is a 2*2 coupler, the phase difference of the interference light output by the optical port 5a2 is for

, Therefore, it is not easy to be used as a monitoring port. In this way, neither 5a1 nor 5a2 can be easily used as monitoring ports. Although in actual devices, due to the imperfection of the 2*2 coupler itself, the phase difference slightly deviates from 0 (or

), so that when the input polarization changes, the interference output will slightly change. In the method adopted in this invention, the coupler 5 is not easy to use a 2*2 coupler.

发生变化，因此，通过检测干涉光强

的变化，可以监测传输路径上被监测光纤6的偏振是否发生变化。 When the light source 7 is fully or partially polarized light, it is input to the port 5a1 through the optical fiber transmission path (monitored optical fiber 6), and when the polarization of the optical fiber in the transmission path changes due to the change of the fiber state, it will cause the polarization of the light input to the port 5a1 to change. changes, which can lead to light intensity

changes, therefore, by detecting the interference light intensity

can monitor whether the polarization of the monitored optical fiber 6 on the transmission path changes.

It should be noted that, in the access mode of the monitored optical fiber 6 shown in FIG. The light incident on the port 9b1 passes through the monitored optical fiber 6 to the reflection device 10 , and the polarization of the light re-entering the port 9b1 is not affected by the polarization of the monitored optical fiber 6 .

In the present invention, particularly, as shown in Figure 5, before the light enters the port 5a1, it passes through a section of polarization-maintaining fiber 11 (11a is a port of the polarization-maintaining fiber 11, 11b is the other end of the polarization-maintaining fiber, and the port 5a1), the effect similar to that of a linear polarizer can be obtained. Corresponding access modes of the monitored optical fiber 6 are shown in Fig. 6 and Fig. 7 respectively. In FIG. 6, the monitored optical fiber 6 is connected in series with the polarization maintaining optical fiber 11; in FIG. 7, the port 9a2 of the coupler 9 is connected with the tail end 11 of the polarization maintaining optical fiber.

The working principle of Figure 5 is analyzed as follows. Assuming that the length of the polarization-maintaining fiber is much longer than the length of the depolarization, the two polarization modes can be completely separated, making them incoherent. Assuming that the axis of the polarization-maintaining fiber coincides with the coordinate axis of the Jones vector of the light wave at the port 5a1, then,

                                              （6） 

(6)

Right now,

                                          （7）

(7)

Also, due to

                      （8）

(8)

，式（）可表示为， set up

, formula () can be expressed as,

    （9）

(9)

The first three terms of formula (9) are measured, and the fourth term is related to the linear polarization component Proportional amount. Therefore, by filtering out the DC component, we can obtain:

                    （10）

(10)

That is, the monitoring effect is similar to that of a linear polarization device.

The invention provides a new method for polarization detection, the method adopted is simple and easy to implement, and the realization cost is obviously lower than that of the optical fiber polarization device. The present invention can be used not only for the monitoring of optical fiber polarization changes, but also for the monitoring of indirect physical quantities based on the detection of optical fiber polarization changes. The monitored optical cable) is laid on the perimeter that needs to be protected. When external intrusion (disturbance) causes changes in the physical parameters of the sensing optical cable, by detecting polarization changes, it can be judged whether an intrusion has occurred. In practical applications, if the light source and detection light output ports are located at both ends of the monitored optical fiber, the connection methods shown in Figure 3 and Figure 6 can be used; if the light source and detection light output ports need to be located at the same end of the monitored optical fiber cable, the The connections shown in Figure 4 and Figure 7.

In the structure of the present invention, in order to meet the needs of practical applications, ordinary optical fibers can be connected in series between each device to extend the transmission distance of light.

Description of drawings

Figure 1 is an optical fiber intrusion detection scheme based on polarization monitoring mentioned in the patent US 7,693,359 B2.

Fig. 2 is a method for realizing polarization detection by using a fiber coupler to form an interference optical path.

Figure 3 shows how the optical fiber to be monitored is connected in series to the input path of the light source.

Figure 4 shows how the optical fiber to be monitored is connected to the light source path through a coupler and a reflection device.

Fig. 5 is an optical path connection method using a polarization-maintaining fiber to achieve an effect equivalent to that of a polarizing device.

Fig. 6 is an access mode of the monitored optical fiber in the optical path connection mode using the polarization maintaining optical fiber to realize the effect of the equivalent polarization device—serial access.

Fig. 7 is an access mode of the optical fiber to be monitored in the optical path connection mode in which the polarization-maintaining optical fiber is used to realize the effect of an equivalent polarization device—access through a coupler and a reflection device.

Fig. 8 is the optical path adopted in the embodiment.

FIG. 9 is an optical path established to verify the linear polarization effect of the embodiment.

In the figure, 1 is a polarized light source, 2 is a monitored optical fiber (sensing optical fiber), 3 is a polarizer, and 4 is a receiver. 5 is a fiber optic coupler, 5a1, 5a2, ..., 5aN, 5b1, 5b2 are ports of the coupler 5, 5a1, 5a2, ..., 5aN are ports in the same direction, 5b1 and 5b2 are ports in the same direction; 8 is a fiber optic coupler 5 6 is the monitored fiber (sensing fiber); 7 is the light source, 7a is the port of the light source; 9 is the fiber coupler, 9a1, 9a29, 5b1 are the ports of the fiber coupler 9, 9a1, 9a2 are Ports in the same direction, 9b1 is the port in the other direction; 10 is a reflection device; 11 is a polarization maintaining fiber; 12 is a 1*2 polarization maintaining coupler, 12a, 12b1, 12b2 are ports of the 1*2 polarization maintaining coupler 12, Among them, 12b1 and 12b2 are ports in the same direction; 13 is a polarization beam splitter, 13a, 13b1, and 13b2 are ports of the polarization beam splitter 13, and 13a is a polarization-maintaining optical fiber. The light input from 13a is divided into two polarization modes, Output via 13b1 and 13b2 respectively. the

Detailed ways

The present invention is further described below by way of examples.

、

为±120°，所以这两个端口都可以作为光检测端。输出的光信号输入到光电探测电路中进行检测，光电探测器为44所生产的型号为GT322C500的InGaAs光电探测器；被监测光纤6为一段美国“康宁”生产的G652型单模光纤。 In this embodiment, a structure as shown in FIG. 8 is adopted. The light source is the SO3-B superluminescent diode (SLD) stable light source produced by the 44 Research Institute of the Electronics Group Corporation, with an extinction ratio of 3dB; the coupler 5 uses an evenly divided 3*3 single-mode fiber coupler , the fibers of the two ports 5b1 and 5b2 of 3*3 are fused together; a section of Fujikura Panda polarization-maintaining fiber is used at the optical input end of the coupler. In order to ensure that the two polarization modes output from it are incoherent, the length used is 5m . The interference phase difference between ports 5a2 and 5a3 due to the use of a 3*3 split coupler

,

It is ±120°, so these two ports can be used as light detection ports. The output optical signal is input to the photoelectric detection circuit for detection. The photodetector is an InGaAs photodetector of model GT322C500 produced by 44; the monitored optical fiber 6 is a section of G652 single-mode optical fiber produced by "Corning" in the United States.

In order to verify the effect of the optical path with the linear polarization device, connect the optical path as shown in Figure 9. In the figure, 12 is a 1*2 polarization maintaining coupler, 12a, 12b1, 12b2 are its ports, among which 12b1, 12b2 are ports in the same direction; 13 is a polarization beam splitter, 13a, 13b1, 13b2 are its ports, 13a is In the polarization-maintaining fiber, the light input from 13a is divided into two polarization modes, which are output through 13b1 and 13b2 respectively. Before the monitored optical fiber 6 enters the polarization-maintaining optical fiber, it is connected to the port 12a of the polarization-maintaining coupler 12, the optical fiber at port 12b1 is polarization-maintaining fused with the optical fiber at port 13a, and the optical fiber at port 12b2 is fused with the end 11a of the polarization-maintaining optical fiber 11. The light output by the ports 13b1, 13b2, 5a2, 5a3 is observed by the electrical signal output by the photoelectric detection circuit at the same time, and it can be found that when the monitored optical fiber is disturbed, the electrical signal changes corresponding to the ports 5a2, 5a3 correspond to the ports 13b1, 13b2 The electrical signal changes are consistent, indicating that the connection method between the polarization maintaining optical fiber 11 and the polarization monitoring component 8 can achieve the effect of a linear polarization device.

Claims (5)

1. A method for monitoring optical fiber polarization changes, characterized in that, at first, using a fully polarized or partially polarized light source, utilizing a fiber coupler to construct an interference optical path; then, the monitored optical fiber is connected to the optical input path of the interference optical path; finally , detect the interference light intensity, and obtain the polarization change of the monitored optical fiber from the change of the interference light intensity. 2. An optical path system for monitoring optical fiber polarization changes, characterized in that it includes: monitored optical fiber 6, light source 7, polarization detection assembly 8; polarization detection assembly 8 is formed by the first optical fiber coupler 5, 5a1, 5a2, ..., 5aN , 5b1, 5b2 are the ports of the fiber coupler 5, 5a1, 5a2, ..., 5aN are ports in the same direction, a total of N, 5b1 and 5b2 are ports in the same direction; 7a is the port of the light source 7; the monitored optical fiber 6 is connected to the light source 7, the light output by the light source 7 is transmitted through the monitored optical fiber 6, and then input into the polarization detection component 8 through the port 5a1, and the ports 5b1 and 5b2 of the first optical fiber coupler 5 in the polarization detection component 8 are connected. An interference light path is formed, and interference light intensity can be obtained from ports 5a2, ..., 5aN for polarization detection. the 3. The optical system for monitoring optical fiber polarization changes according to claim 2, characterized in that the way the monitored optical fiber 6 is connected to the transmission path of the light source is: the monitored optical fiber 6 is directly connected in series with the port 7a of the light source 7 and the first Between the ports 5a1 of the fiber coupler 5. 4. the optical path system of monitoring optical fiber polarization variation according to claim 2, it is characterized in that the mode that monitored optical fiber 6 inserts light source transmission path is: insert by second optical fiber coupler 9 and reflector 10, set 9a1, 9a2 and 5b1 are the ports of the fiber coupler 9, 9a1 and 9a2 are ports in the same direction, and 9b1 is the port in the other direction; the port of the light source 7 is connected to the port 9a1 of the fiber coupler 9; one end of the monitored optical fiber 6 is connected to the fiber coupler The port 9b1 of 9 is connected, and the other end is connected with a reflection device 10; the port 9a2 of the fiber coupler 9 is connected with the port 5a1 of the fiber coupler 5, and the transmission path of light is: 7a→9a1→9b1→monitored optical fiber 6→reflection device 10→monitored optical fiber 6→9b1→9a2→5a1. 5. The optical path system for monitoring optical fiber polarization changes according to claim 3, characterized in that the way the monitored optical fiber 6 is connected to the light source transmission path is: before the light enters the port 5a1, it first passes through a section of polarization-maintaining optical fiber 11, assuming 11a is a port of the polarization maintaining fiber 11, 11b is the other end of the polarization maintaining fiber, and 11b is connected to the port 5a1.

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