CN102322958A - Method for monitoring optical fiber polarization change and optical path system - Google Patents
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Abstract
The invention belongs to the technical field of optical fiber and particularly relates to a method for monitoring optical fiber polarization change and an optical path system. On the basis of the self inherent polarization characteristic of an optical fiber, an optical fiber coupler is adopted to form an interference light path; a monitored optical fiber is connected to the input light path of the interference light path; and the change of the optical fiber polarization state is monitored by detecting the interference light intensity. The invention also provides an optical fiber and optical path forming method equivalent to the effect of a linear polarizer. The method disclosed by the invention can be used for monitoring the physical quantity which causes optical fiber polarization change, and is especially suitable for application, such as perimeter security and the like which adopt the optical fiber to sense outside disturbance.
Description
Technical Field
The invention belongs to the technical field of optical fibers, and particularly relates to an optical fiber polarization change monitoring method and an optical path system.
Background
With the development of optical fiber technology, the application field of optical fiber is wider and wider, and the optical fiber not only is widely applied to optical communication as a transmission medium, but also plays more and more important roles in the field of sensing technology, and particularly in recent years, the optical fiber has a more vigorous development trend.
In an ideal optical fiber, the shape of the cross section of the optical fiber and the refractive index distribution are uniform, the polarization mode is degenerate, the polarization state of light transmitted along the optical fiber can not be changed, however, in practice, the fiber core can not be made into a perfect circle, meanwhile, under the action of the elasto-optical effect, the optical fiber has birefringence, so that the polarization characteristic of light transmitted in the optical fiber is changed, particularly, the optical fiber is easily influenced by the change of the ambient temperature and the state of the optical fiber, and the polarization state of output light has great randomness.
The imperfect characteristics of the optical fiber in practical application limit the exertion of the advantages of the optical fiber to a certain extent, but the polarization characteristics of the optical fiber are applied in some fields, one typical application is that the polarization change of the sensing optical fiber is monitored in the periphery of the optical fiber to judge whether the outside has intrusion behavior, the detection principle is that when the state of the optical fiber is changed due to external disturbance, the polarization characteristics of the optical fiber are changed, the polarization of the light transmitted by the sensing optical fiber is changed, and whether the optical fiber is disturbed or not can be detected by detecting the polarization change of the output light. The patent US 7,693,359B 2 is an optical fiber intrusion detection technology based on detecting polarization changes of optical fibers.
The polarization of the optical fiber is usually detected by obtaining a polarization component by a polarizer or a polarization beam splitter, and observing the polarization component. Fig. 1 shows a scheme mentioned in patent US 7,693,359B 2, in which a polarized light source 1 is injected from one end of a monitored optical fiber 2 (sensing optical fiber), light output from the other end of the monitored optical fiber 2 is injected into a polarizer 3, and the light output from the polarizer 3 is detected by a receiver 4. In this solution, by the action of the polarizer 3, a light intensity is obtained which varies with the polarization.
Disclosure of Invention
The invention aims to provide a method and an optical path system for monitoring the polarization change of an optical fiber, which are simple, convenient and feasible and have lower cost.
The method for monitoring the polarization change of the optical fiber provided by the invention utilizes the inherent polarization characteristic of the optical fiber to construct the optical fiber interference light path, and monitors the polarization change of the monitored optical fiber by detecting the interference light intensity. The invention is suitable for monitoring the physical quantity causing the polarization change of the optical fiber.
The method comprises the following specific steps: firstly, a completely polarized or partially polarized light source is adopted, and an interference light path is constructed by using an optical fiber coupler; then, connecting the monitored optical fiber to an optical input path of the interference optical path; and finally, detecting the interference light intensity, and obtaining the polarization change of the monitored optical fiber from the change of the interference light intensity.
The invention uses the optical fiber coupler to form an optical path system for monitoring the polarization change of the optical fiber. The specific optical path is shown in fig. 2, and includes: monitored optical fiber 6, light source 7, polarization detection component 8; the polarization detection module 8 is composed of aN optical fiber coupler 5, 5a1, 5a2, …, 5aN, 5b1 and 5b2 are ports of the optical fiber coupler 5, 5a1, 5a2, … and 5aN are co-directional ports (N in total), and 5b1 and 5b2 are co-directional ports; 7a is a port of the light source 7. The monitored optical fiber 6 is connected to a transmission path of the light source 7, light output by the light source 7 is transmitted through the monitored optical fiber 6 and then input into the polarization detection assembly 8 through the port 5a1, the port 5b1 of the optical fiber coupler 5 in the polarization detection assembly 8 is connected with the port 5b2 to form aN interference light path, and interference light intensity can be obtained from the ports 5a2, … and 5aN and is used for polarization detection.
The method for accessing the optical fiber 6 to the optical source transmission path can be as shown in fig. 3, that is, the optical fiber 6 to be monitored is directly connected in series between the optical source 7 (port 7 a) and the port 5 a. Fig. 4 shows another access method for the monitored optical fiber 6. I.e. via the fiber coupler 9 and the reflecting device 10 (e.g. a mirror with a pigtail); let 9a1, 9a2, 5b1 be the ports of the fiber coupler 9, 9a1, 9a2 be the same-direction ports, 9b1 be the other-direction ports; the port of the light source 7 is connected to the port 9a1 of the coupler; one end of the monitored optical fiber 6 is connected to the port 9b1, and the other end is connected to a reflection device 10; the port 9a2 is connected to the port 5a1, and the optical transmission path is:
7a → 9a1 → 9b1 → monitored optical fiber 6 → reflecting device 10 → monitored optical fiber 6 → 9b1 → 9a2 → 5a 1.
The working principle of the present invention is analyzed as follows.
Let the light input from the port 5a1 be
"T" denotes transpose; since the optical fiber in practical application can be regarded as a device with certain birefringence, the transmission matrix from the port 5b1 to the port 5b2 can be set as jones matrix J, the transmission loss of the optical fiber is neglected, the polarization analysis result is not affected, and J can be expressed as:
wherein,
，
is composed of
The element of (c), (d), (. The light output from the port 5aM (M =1, 2, …) is interference of the following two lights:
5a1→5b1→5b2→5aM，5a1→5b2→5b1→5aM （M =1, 2, …）
the output vector is:
wherein,
，
from the optical input port 5a1 via the port 5aM (M) in clockwise and counterclockwise directions, respectively
=1, 2, …), is a real number,
the interference phase difference introduced for the coupler. Then, the corresponding output light intensity is:
（3）
is provided with
The following can be obtained:
wherein,
as can be seen from equation (4), the first term and the second term are constant values, and the third term is related to the polarization input to the port 5aM, and causes a polarization component when the input polarization is changed
、
Changes in amplitude and phase, which changes, resulting in a change in the output intensity I.
Therefore, by detecting the intensity of the interference light
The polarization change of the light input from the port 5aM can be judged.
As can be seen from equation (4), when the polarization of the light input from the port 5ai changes, that is
、
When the third term is changed, the following items are required:
Therefore, the phase difference must be selected
The interference light intensity output port. In general, when 5a1 is taken as an optical input port, the phase difference of the interference light output from 5a1
Is 0 and therefore this port is not readily available as a port for monitoring polarization changes. In the case of the configuration in which the coupler 5 is a 2-by-2 coupler, the optical port 5a2 outputs the interference light with a phase difference ofIs composed of
Therefore, it is not easy to be used as a monitoring port. Thus, neither 5a1 nor 5a2 are readily available as monitoring ports. Although in a practical device the phase difference slightly deviates from 0 (or 0) due to imperfections in the 2 x 2 coupler itself
) So that the interference output will change slightly when the input polarization changes, in the method adopted by the invention, the coupler 5 is not easy to adopt a2 x 2 coupler.
When the light source 7 is completely or partially polarized light and is input to the port 5a1 via the optical fiber transmission path (monitored optical fiber 6), and the polarization of the light input to the port 5a1 is changed due to a change in the state of the optical fiber in the transmission path, resulting in a change in the polarization of the light input to the port 5a1, which leads to a change in the light intensity
Change is made so that the intensity of light is detected by detecting the interference
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. 4, the reflection device 10 cannot be a faraday rotator, because the faraday rotator eliminates the polarization effect between the port 9b1 and the reflection device 10, so that the polarization of the light entering from the port 9b1 and re-entering the port 9b1 through the monitored optical fiber 6 to the reflection device 10 is not affected by the polarization of the monitored optical fiber 6.
In the present invention, in particular, as shown in fig. 5, before light enters the port 5a1, it passes through a section of polarization maintaining fiber 11 (11 a is one port of the polarization maintaining fiber 11, and 11b is the other end of the polarization maintaining fiber, and is connected to the port 5a 1), and an effect similar to that of a linear polarizer can be obtained. The corresponding access modes of the monitored optical fiber 6 are shown in fig. 6 and 7, respectively. In fig. 6, the monitored optical fiber 6 is connected in series with the polarization maintaining optical fiber 11; in fig. 7, port 9a2 of coupler 9 is connected to tail end 11 of the polarization maintaining fiber.
The working principle of fig. 5 is analyzed as follows. Assuming that the length of the polarization maintaining fiber is much greater than the depolarization length, the two polarization modes can be completely separated so that they do not have coherence. If the axis of the polarization maintaining fiber is exactly aligned with the coordinate axis of the jones vector of the light wave at the port 5a1, then,
that is to say that the first and second electrodes,
and, due to
Is provided with
The formula () can be represented as,
the first three terms of equation (9) are measured, and the fourth term is measured with the linear polarization componentA proportional amount. Thus, by filtering out the direct current component, one obtains:
i.e. a monitoring effect similar to that of a linear polarizer device.
The invention provides a novel means for detecting polarization, the adopted method is simple and easy to implement, and the realization cost is obviously lower than that of an optical fiber polarization device. The invention can be used for monitoring the polarization change of the optical fiber and can also be used for monitoring indirect physical quantity based on the polarization change detection of the optical fiber, for example, the invention is suitable for the practical application of perimeter security protection and the like which adopt the optical fiber to sense external disturbance: the sensing optical cable (the monitored optical cable) is arranged on the perimeter needing to be prevented, when external intrusion (disturbance) causes the change of physical parameters of the sensing optical cable, whether intrusion behaviors occur or not can be judged by detecting polarization change. In practical application, if the light source and the detection light output end are located at two ends of the monitored optical fiber, the connection mode shown in fig. 3 and fig. 6 can be adopted; if the light source and the detection light output end need to be located at the same end of the monitored optical cable, the connection mode shown in fig. 4 and 7 can be adopted.
In the structure of the invention, in order to adapt to the practical application requirement, common optical fibers can be connected in series among all devices to prolong the transmission distance of light.
Drawings
Fig. 1 shows a fiber intrusion detection scheme based on polarization monitoring as mentioned in patent US 7,693,359B 2.
FIG. 2 is a method for polarization detection by using a fiber coupler to form an interference optical path.
Fig. 3 shows the manner in which the monitored optical fibers are connected in series into the input path of the light source.
Fig. 4 is a diagram of the manner in which the monitored fiber is coupled into the path of the light source via a coupler and a reflecting device.
Fig. 5 shows an optical path connection method using a polarization maintaining fiber to achieve an effect equivalent to that of a polarization device.
FIG. 6 is a series access, which is an access mode of a monitored optical fiber in an optical path connection mode for achieving an equivalent polarization device effect by using a polarization maintaining optical fiber.
FIG. 7 is a monitored optical fiber accessing mode in the optical path connecting mode of using polarization maintaining optical fiber to realize the effect of equivalent polarization device, which accesses through a coupler and a reflecting device.
Fig. 8 shows the optical path used in the embodiment.
Fig. 9 is a light path established by verifying 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 coupler, 5a1, 5a2, …, 5aN, 5b1 and 5b2 are ports of the coupler 5, 5a1, 5a2, … and 5aN are homodromous ports, and 5b1 and 5b2 are homodromous ports; 8 is a polarization detection component formed by the optical fiber coupler 5; 6 is monitored optical fiber (sensing optical fiber); 7 is a light source, and 7a is a port of the light source; 9 is an optical fiber coupler, 9a1, 9a29 and 5b1 are ports of the optical fiber coupler 9, 9a1 and 9a2 are homodromous ports, and 9b1 is another directional port; 10 is a reflection device; 11 is a polarization maintaining optical fiber; 12 is a1 x 2 polarization maintaining coupler, 12a, 12b1, 12b2 are ports of the 1 x 2 polarization maintaining coupler 12, wherein 12b1, 12b2 are co-directional ports; reference numeral 13 denotes a polarization beam splitter, 13a, 13b1, and 13b2 denote ports of the polarization beam splitter 13, 13a denotes a polarization maintaining fiber, and light input from 13a is divided into two polarization modes and output through 13b1 and 13b2, respectively.
Detailed Description
The invention is further described below by way of examples.
In the present embodiment, the structure shown in fig. 8 is adopted. The light source adopts SO3-B type super-radiation light emitting tube (SLD) type stable light source produced by institute of electronic group headquarters 44, and the extinction ratio is 3 dB; the coupler 5 uses an equipartition 3 x 3 single-mode fiber coupler, and the two ports 5b1 and 5b2 of 3 x 3 are fused together; using a section of panda in rattan house at the optical input end of the coupler to maintain the polarizationThe optical fiber, in order to ensure that the two polarization modes output therefrom are incoherent, is used with a length of 5 m. The interference phase difference of ports 5a2 and 5a3 due to the use of a3 x 3 equal division coupler
、
Is 120 deg., so both ports can be used as light detection ports. The output optical signal is input into a photoelectric detection circuit for detection, and the photoelectric detector is an InGaAs photoelectric detector which is produced by 44 and has the model number of GT322C 500; the monitored optical fibre 6 is a length of single mode fibre of the G652 model produced by corning, usa.
To verify the effect of the optical path with the linearly polarizing device, the optical path connection as in fig. 9 was made. In the figure, 12 is a1 × 2 polarization maintaining coupler, 12a, 12b1 and 12b2 are ports thereof, wherein 12b1 and 12b2 are co-directional ports; a polarization beam splitter 13, ports 13a, 13b1, and 13b2, and a polarization maintaining fiber 13a, and the light input from 13a is divided into two polarization modes and output through 13b1 and 13b2, respectively. Before the monitored optical fiber 6 enters the polarization-maintaining optical fiber, the monitored optical fiber is connected with a port 12a of the polarization-maintaining coupler 12, an optical fiber at a port 12b1 is fused with an optical fiber at a port 13a in a polarization-maintaining mode, and an optical fiber at a port 12b2 is fused with an optical fiber at a port 11a in a polarization-maintaining mode. The light output by the ports 13b1, 13b2, 5a2 and 5a3 is observed by an oscilloscope at the same time through the electrical signals output by the photoelectric detection circuit, and it can be found that when the monitored optical fiber is disturbed, the electrical signal changes corresponding to the ports 5a2 and 5a3 are consistent with the electrical signal changes corresponding to the ports 13b1 and 13b2, which indicates that the connection method of the polarization maintaining optical fiber 11 and the polarization monitoring assembly 8 can achieve the effect of a linear polarization device.
Claims (5)
1. A method for monitoring polarization change of an optical fiber is characterized in that firstly, a completely polarized or partially polarized light source is adopted, and an interference light path is constructed by using an optical fiber coupler; then, connecting the monitored optical fiber to an optical input path of the interference optical path; and finally, detecting the interference light intensity, and obtaining the polarization change of the monitored optical fiber from the change of the interference light intensity.
2. An optical path system for monitoring polarization changes of an optical fiber, comprising: monitored optical fiber 6, light source 7, polarization detection component 8; the polarization detection assembly 8 is composed of a first optical fiber coupler 5, 5a1, 5a2, …, 5aN, 5b1 and 5b2 are ports of the optical fiber coupler 5, 5a1, 5a2, … and 5aN are homodromous ports, the total number of the ports is N, and 5b1 and 5b2 are homodromous ports; 7a is the port of the light source 7; the monitored optical fiber 6 is connected to a transmission path of the light source 7, light output by the light source 7 is transmitted through the monitored optical fiber 6 and then input into the polarization detection assembly 8 through the port 5a1, the port 5b1 of the first optical fiber coupler 5 in the polarization detection assembly 8 is connected with the port 5b2 to form aN interference light path, and interference light intensity can be obtained from the ports 5a2, … and 5aN and is used for polarization detection.
3. The optical path system for monitoring polarization change of optical fiber according to claim 2, wherein the monitored optical fiber 6 is connected to the transmission path of the light source by: the monitored optical fiber 6 is directly connected in series between the port 7a of the light source 7 and the port 5a1 of the first optical fiber coupler 5.
4. The optical path system for monitoring polarization change of optical fiber according to claim 2, wherein the monitored optical fiber 6 is connected to the transmission path of the light source by: the optical fiber coupler 9 and the reflecting device 10 are connected, and 9a1, 9a2 and 5b1 are ports of the optical fiber coupler 9, 9a1 and 9a2 are homodromous ports, and 9b1 is another directional port; the port of the light source 7 is connected with the port 9a1 of the optical fiber coupler 9; one end of the monitored optical fiber 6 is connected with the port 9b1 of the optical fiber coupler 9, and the other end is connected with a reflecting device 10; the port 9a2 of the optical fiber coupler 9 is connected to the port 5a1 of the optical fiber coupler 5, and the optical transmission path is:
7a → 9a1 → 9b1 → monitored optical fiber 6 → reflecting device 10 → monitored optical fiber 6 → 9b1 → 9a2 → 5a 1.
5. The optical path system for monitoring polarization change of optical fiber according to claim 3, wherein the monitored optical fiber 6 is connected to the transmission path of the light source by: before entering port 5a1, light passes through a length of polarization maintaining fiber 11, where 11a is one end of polarization maintaining fiber 11, 11b is the other end of polarization maintaining fiber, and 11b is connected to port 5a 1.
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| CN114112313A (en) * | 2021-12-02 | 2022-03-01 | 中国电子科技集团公司第四十六研究所 | Single-polarization optical fiber transmission performance testing device and testing method |
| CN114112313B (en) * | 2021-12-02 | 2024-04-19 | 中国电子科技集团公司第四十六研究所 | Single polarization optical fiber transmission performance testing device and testing method |
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Application publication date: 20120118 Assignee: Dongguan advanced optical fiber Application Technology Research Institute Co Ltd Assignor: Fudan University Contract record no.: 2018310000016 Denomination of invention: Method for monitoring optical fiber polarization change and optical path system Granted publication date: 20140709 License type: Exclusive License Record date: 20180529 |
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