KR100622577B1 - Apparatus for improving detection sensitivity of optical fiber sensor and monitoring system using the same - Google Patents

Abstract

Disclosed are a device for improving reception sensitivity of an optical fiber sensor for detecting a change in birefringence of an optical fiber and a change in polarization of traveling light in the optical fiber, and a monitoring system for detecting an abnormal state such as a sudden change in the environment using the optical fiber sensor. The biggest feature of the present invention is that the photodetector is divided into beams of the first polarization and the second polarization that are orthogonal to each other by the polarization beam splitter, and the signal detection is performed separately so as to have a maximum sensitivity to them. It is possible to prevent the phenomenon that the sensitivity of the electrical signal output from the sharp decrease due to the flow of the reference point.

Polarization, beam, splitter, monitoring, sensitivity, fiber optic, sensor

Description

Apparatus for improving detection sensitivity of optical fiber sensor and monitoring system using the same}             

1 is a block diagram showing the principle of the monitoring system disclosed in the prior art Korea Patent Application No. 2001-81444;

FIG. 2 is a view for explaining a phenomenon in which the sensitivity of an electrical signal measured in the photodetector of the monitoring system of FIG. 1 is significantly reduced due to a drift of an offset point;

Figure 3 is a schematic view showing a reception sensitivity improving device of the optical fiber sensor of the present invention; And

Figure 4 is a schematic diagram showing a monitoring system using a reception sensitivity improving device of the optical fiber sensor of the present invention.

The present invention relates to an apparatus for improving reception sensitivity of an optical fiber sensor and a monitoring system using the same, and more particularly, to improve reception sensitivity of an optical fiber sensor for detecting a change in birefringence of an optical fiber and a change in polarization of traveling light in the optical fiber. The present invention relates to an apparatus and a monitoring system that detects an abnormal state such as a sudden change of environment by using the same.

Until now, various optical fiber sensor devices for measuring temperature or strain have been developed. Among them, U.S. Patent No. 5,499,313 discloses a technique for detecting only the ambient temperature, and U.S. Patent No. 5,636,021 consists of a strain sensor using a Sagnac / Michelson interferometer. The monitoring device is disclosed. In particular, the monitoring device of US Pat. No. 5,636,021 interfers with the reference light, which does not reflect the influence of environmental change, and the light reflecting the influence of environmental change, to find out the point where the change occurred and its size. However, such a technique has a problem that it is not suitable for detecting a sudden change in the environment because the structure of the device is complicated and its cost is very expensive because the distribution of temperature or strain is measured very precisely.

Therefore, in order to improve such a problem, some of the inventors of the present invention can detect and monitor when a sudden environmental change, such as a temperature change caused by a strong wind or a fire, occurs in the section to be monitored. While maintaining and repairing is relatively simple, and the structure is simple and economical effect, the environmental monitoring system using the birefringence change of the optical fiber has been developed and filed with Korean Patent Application No. 2001-81444.

1 is a block diagram showing the principle of the monitoring system disclosed in the prior art Korean Patent Application No. 2001-81444. Referring to FIG. 1, after linearly polarized light from the light source 10 travels along the optical fiber 20, it is converted into an electrical signal in the photodetector 40 via a polarizer 30. However, in this process, when the temperature around the optical fiber 20 changes rapidly or when the optical fiber 20 shakes due to the wind or external force, the intensity of the light passing through the polarizer is changed, and thus the photodetector 40 ), The magnitude of the electric signal outputted from is changed. This signal change is interpreted by a signal processing unit 50 so as to monitor the environmental state in the section in which the optical fiber 20 is located. That is, a sudden change in various parameters such as temperature or vibration changes the birefringence inside the optical fiber 20, which in turn changes the polarization of light passing through the optical fiber 20. Since the monitoring system does not aim to accurately and quantitatively measure the parameters of the external environment, it is sufficient to use only the simple polarizer 30 and the photodetector 40 for receiving light.

By the way, in the monitoring system having such a configuration, the sensitivity of the electrical signal output from the photodetector 40 via the polarizer 30 is significantly reduced due to the drift of the offset point, that is, the signal pass. Signal fading may occur. 2 is a diagram for explaining such a phenomenon. In FIG. 2, Pi is the intensity of light passing through the optical fiber 20 of FIG. 1, Po is the intensity of light after passing through the polarizer 30, and θ is the light before entering the polarizer 30 and the polarizer 30. Is the angle between the polarization axes. In this case, the intensity Po of the light passing through the polarizer 30 satisfies the following expression (1).

Referring to FIG. 2, the variation in the intensity of the light output varies according to the angle θ, which is output when θ has values of π / 4 and 3π / 4 and the intensity of incident light changes to a predetermined width C. It can be seen that the change width A of the intensity of light is very large compared to the change width B of the intensity of the output light when θ has a value of π / 2 and π vicinity under the same conditions. That is, when θ has a value near π / 2 and π, the change in intensity of the output light is very small, so that the sensitivity of the electric signal is significantly reduced. Therefore, in order to increase the sensitivity of the electrical signal output from the photodetector 40, it is preferable to adjust θ, which is the angle between the light before entering the polarizer 30 and the polarization axes of the polarizer 30.

To this end, by installing a polarization controller (polarization controller) in front of the polarizer 30 of Figure 1 may be considered to adjust the polarization of the light entering the polarizer 30 to the maximum sensitivity. However, in this case, the system becomes very complicated because a mechanical device that continuously adjusts the polarizer 30 through feedback is added. Also, the implementation of an algorithm that adjusts the polarization controller so that good sensitivity is always achieved is not a simple problem.

Therefore, the technical problem of the present invention, in using a sensor for detecting the polarization change of the light traveling through the optical fiber, to prevent the phenomenon of the signal sensitivity is significantly reduced due to the flow of the reference point, there is no need to always adjust the components In addition, the present invention provides an apparatus for improving reception sensitivity of a fiber optic sensor and a monitoring system using the same.

Receiving sensitivity improving apparatus of the optical fiber sensor of the present invention for solving the above technical problem, is applied to the optical fiber sensor for detecting the polarization change of the light traveling through the optical fiber, and improves the receiving sensitivity,

A polarization beam splitter (PBS) for receiving light that has undergone polarization change through the optical fiber and dividing the light into linearly orthogonal first and second polarized beams;

A first polarizer passing through the first polarization beam and forming an angle of 45 degrees with a polarization axis of the first polarization beam;

A second polarizer passing through the second polarization beam and forming an angle of 45 degrees with a polarization axis of the second polarization beam;

Characterized in having a.

In addition, the monitoring system of the present invention for solving the above technical problem, as detecting the polarization change of the light traveling through the optical fiber to monitor the surrounding environment,

A light source;

An optical fiber passing through the light emitted from the light source and causing a polarization change according to an environment change;

A polarization beam splitter which receives the light that has undergone polarization change through the optical fiber and divides the first polarized light into the first polarized light and the second polarized light beams which are linearly orthogonal to each other;

A first polarizer passing through the first polarization beam and forming an angle of 45 degrees with a polarization axis of the first polarization beam;

A second polarizer passing through the second polarization beam and forming an angle of 45 degrees with a polarization axis of the second polarization beam;

First and second photodetectors for changing an electrical signal of power of light passing through the first and second polarizers, respectively;

Signal processing means for processing the electrical signals converted by the first and second photodetectors;

It characterized by having a.

In addition, in order to block the light emitted from the light source from backscattering toward the light source and re-entering the light source, a light separator may be additionally provided after the light source.

In addition, the signal processing means may further include a remote communication means for informing the monitor at the remote location in real time of the monitoring situation.

Hereinafter, with reference to the accompanying drawings will be described a preferred embodiment of the present invention. In the drawings, the same reference numerals denote the same components, and redundant description thereof will be omitted.

3 is a configuration diagram schematically showing an apparatus for improving reception sensitivity of an optical fiber sensor according to the present invention. Referring to FIG. 3, the passing light Pi after the optical fiber causing the change in polarization is incident on the polarization beam splitter 300. The polarization beam splitter 300 divides the light into a beam of first polarization and a beam of second polarization that are linearly polarized and orthogonal to each other. Subsequently, the first polarizing beam passes through the first polarizer 30a that forms an angle of 45 degrees with the polarization axis of the first polarizing beam, and the second polarizing beam of the second polarizing beam is changed in a state where the optical path is changed by the mirror 310. The second polarizer 30b forms an angle of 45 degrees with the polarization axis, respectively. The beams passing through the first polarizer 30a and the second polarizer 30b are converted into electrical signals by the first and second photodetectors 40a and 40b, respectively.

As shown in FIG. 3, when the reception sensitivity improving apparatus of the optical fiber sensor is configured, when the passing light Pi after the optical fiber is composed of only the first polarized light, the first light passes through the polarized beam splitter 300 and then the first light. The light travels toward the polarizer 30a only and no light travels toward the second polarizer 30b. However, since the polarization axis of the linearly polarized first polarizing beam forms an angle of 45 degrees with the first polarizer 30a, the first photodetector 40a receives a signal with the maximum sensitivity. When the passing light Pi after the optical fiber is composed of only the second polarization, it passes through the polarization beam splitter 300 and then proceeds only toward the second polarizer 30b, and then proceeds toward the first polarizer 30a. There is no light to say. Here too, since the polarization axis of the linearly polarized second polarization beam forms an angle of 45 degrees with the second polarizer 30b, the second photodetector 40b receives the signal with the maximum sensitivity. On the other hand, when the passing light Pi after the optical fiber is in the state where the first polarization and the second polarization are mixed, it passes through the polarization beam splitter 300, and the beam in the first polarization state is the first polarizer ( Towards 30a), the beams of the second polarization state travel toward the second polarizer 30b, respectively. In this case, the first and second photodetectors 40a and 40b respectively receive signals with maximum sensitivity.

Therefore, regardless of what polarization state the passing light Pi undergoes the optical fiber, the signal can be always received at the maximum sensitivity, so that the reception sensitivity of the optical fiber sensor can be improved.

On the other hand, as the polarizing beam splitter 300 described above, the input end and the output end may be pigtailed with an optical fiber. However, when the output optical fiber is too long, the polarization change caused by the optical fiber may occur with respect to the light incident on the first polarizer 30a or the second polarizer 30b. It is preferable to use an optical fiber capable of suppressing polarization change, such as a polarization maintaining fiber.

Compared with the prior art Korean Patent Application No. 2001-81444, the obvious difference is that in the prior art, since the passing light Pi is directly incident on the polarizer, the polarization of the passing light Pi after the optical fiber is advanced. In the present invention, there is a problem in that the signal sensitivity decreases depending on the state, but in the present invention, since the light having the polarization state determined by the polarization beam splitter 300 is incident on the polarizer, the signal sensitivity may be increased.

Figure 4 is a schematic diagram showing a monitoring system using a reception sensitivity improving device of the optical fiber sensor of the present invention. The present monitoring system is installed between the transmission towers 90 and 91 to monitor a sudden environmental change applied to the optical fiber 20. Referring to FIG. 4, the monitoring system is largely comprised of an optical transmitter 80, an optical fiber 20, and an optical receiver / signal processor 81, so that light emitted from the optical transmitter 80 passes through the optical fiber 20, and thus, optically. It is transmitted to the reception / signal processing unit 81. The optical transmitter 80 serves to emit linearly polarized light, and the light source 10 included therein may employ a laser diode or the like that outputs linearly polarized light. Alternatively, a wavelength filter and a polarizer may be used in combination with a light source that outputs unpolarized light having a broad spectrum, such as an LED. Next, if a laser is used as the light source 10, an optical separator 11 is provided to prevent the light from being reflected back to the light source in the optical connector or the like on the light propagation path. This is because when the light exiting the laser enters the laser, the operation of the laser is often unstable.

On the other hand, the light from the optical transmitter 80 is transmitted through the optical fiber 20 in the composite processing line along the optical fiber 20. Therefore, when a sudden environmental change occurs between the transmission towers 90 and 91 where the optical fiber 20 is hypothesized, the polarization of the light traveling through this section is also greatly changed, and the light reaching the transmission tower 91 causes the optical fiber 20 to Therefore, the optical receiver / signal processor 81 detects light and interprets a signal. Since the operation of the light receiving / signal processing unit 81 has been described in detail through the description of FIG. 3, redundant description thereof will be avoided. As a result, the monitoring system of the present invention employs the reception sensitivity improving device of the optical fiber sensor described above, whereby a significant reduction in the sensitivity of the output electrical signal occurs when the polarization axis of the light incident on the polarizer does not match the angle of the polarizer. Rather, it exhibits high reception sensitivity. Although not shown in the drawing, the light source 10, the photodetectors 40a and 40b, and a power supply for supplying power to the signal processing unit 50 are separately provided, and the communication unit 320 is provided in the signal processing unit 50. It is possible to notify the remote monitor of the environmental situation of the transmission tower (90, 91) section.

As described above, according to the present invention, the signal sensitivity is significantly reduced due to the flow of the reference point, but since the components do not need to be adjusted at any time, the reception sensitivity of the optical fiber sensor can be improved at a simple and low cost. In addition, environmental monitoring can be easily performed.

Claims (4)

Apparatus for applying to the optical fiber sensor for detecting the change in polarization of the light traveling through the optical fiber to improve the reception sensitivity, A polarization beam splitter which receives the light that has undergone polarization change through the optical fiber and divides the first polarized light into the first polarized light and the second polarized light beams which are linearly orthogonal to each other; A first polarizer passing through the first polarization beam and forming an angle of 45 degrees with a polarization axis of the first polarization beam; A second polarizer passing through the second polarization beam and forming an angle of 45 degrees with a polarization axis of the second polarization beam; Receiving sensitivity improving apparatus of the optical fiber sensor characterized in that it comprises a. In the system for monitoring the surrounding environment by detecting the polarization change of the light traveling through the optical fiber, A light source; An optical fiber passing through the light emitted from the light source and causing a polarization change according to an environment change; A polarization beam splitter which receives the light that has undergone polarization change through the optical fiber and divides the first polarized light into the first polarized light and the second polarized light beams which are linearly orthogonal to each other; A first polarizer passing through the first polarization beam and forming an angle of 45 degrees with a polarization axis of the first polarization beam; A second polarizer passing through the second polarization beam and forming an angle of 45 degrees with a polarization axis of the second polarization beam; First and second photodetectors for changing an electrical signal of power of light passing through the first and second polarizers, respectively; Signal processing means for processing the electrical signals converted by the first and second photodetectors; Monitoring system comprising a. 3. A monitoring system according to claim 2, further comprising an optical splitter following said light source to prevent light from said light source from backscattering in the direction of the light source and re-entering said light source. The monitoring system according to claim 2, wherein the signal processing means further comprises a remote communication means for informing a monitor at a remote location in real time of the monitoring situation.

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