CN102003971B - Method for eliminating backscattering light influence in optical fiber sensor - Google Patents

Abstract

The invention belongs to the technical field of optical fiber sensing, in particular to a method for eliminating backscattering light influence in an optical fiber sensor. By arranging a phase modulator at the tail end of a sensing optical fiber (cable) and using modulation and demodulation technology for phase generated carrier, a useful interference signal is extracted from a signal interfered by optical fiber scattering light. The method can make a great breakthrough in monitoring distance of the traditional optical fiber sensing equipment, and the method also represents new application of the phase generated carrier technology.

Description

A Method to Eliminate the Effect of Backscattered Light in Optical Fiber Sensor

technical field

The invention belongs to the technical field of optical fiber sensing, and in particular relates to a method for eliminating the influence of backscattered light in an optical fiber sensor.

Background technique

Optical fiber sensing technology is often used in large-scale and long-distance monitoring, such as in the safety monitoring of oil pipelines, high-voltage power grids, gas pipelines, communication optical cables and other infrastructures. It uses optical fibers as sensors to collect relevant information in real time. Disturbance signal, through the analysis of the characteristics to determine the location of the disturbance. The single-core feedback optical path structure uses a single optical fiber in the sensing section, and the optical fiber itself does not need to be closed. Only a feedback device, such as a mirror, is added at the end of the optical fiber to form an interference optical path. In practical application, this structure is convenient and flexible to lay. The characteristic of this type of monitoring system is that the light carrying the disturbance information is the light fed back by the feedback device after being transmitted to the end of the optical fiber.

The following is a positioning technology adopted by the single-core feedback positioning system.

，当光先后两次经过扰动点D，相位受到的调制为： Figure 1 shows a sensing section of optical fiber (optical cable), 1 is the starting point of the optical fiber (optical cable), and there is a feedback device 2 at the end of the sensing section, such as a mirror. Assuming that there is a disturbance at the external point D, the modulation on the optical phase is

, when the light passes through the disturbance point D twice successively, the modulation of the phase is:

，L为扰动点D距反馈装置2的距离，c为真空中的光速，

为光纤的等效折射率。 in,

, L is the distance between the disturbance point D and the feedback device 2, c is the speed of light in vacuum,

is the equivalent refractive index of the fiber.

Construct the interference light path, as shown in Figure 2.

The interference optical route is composed of N*M (N, M are integers) coupler 3, P*Q (P, Q are integers) coupler 4, optical fiber delayer 5, the delay is τ, optical fiber (optical cable) 6 and feedback device 2 . 3a1, 3a2, ..., 3aN, 3b1, 3b2 are ports of the coupler 3, 3a1, 3a2, ..., 3aN are ports in the same direction, a total of N, 3b1, 3b2 are another group of ports in the same direction of the coupler 3 (a total of M) of the two ports. 4a1, 4a2, 4b1 are the ports of coupler 4, 4a1, 4a2 are two ports in a group of co-directional ports (total P) of coupler 2, 4b1 is another group of co-directional ports of coupler 4 (total Q) of the two ports. Optical fiber 6 is an induction optical fiber. The feedback device 2 makes the light transmitted along the optical fiber reenter the optical fiber 6 and return to the coupler 4 . The light source is input through the port 3a1 of the coupler 3, and then output through the ports 3b1 and 3b2 respectively after being split by the coupler 3, two paths of light:

I: 3b1→5→4a1→4b1→6→2→6→4b1→4a2→3b2

II: 3b2→4a2→4b1→6→2→6→4b1→4a1→5→3b1

They meet again at the coupler 3 and interfere with each other, and the interference signals are output through ports 3a1, 3a2, . . . , 3aN.

In the interference optical path, the light that first passes through the retarder 5 and enters the optical cable 6 undergoes phase modulation as follows:

The phase difference of two coherent interference lights is:

In the frequency spectrum of the phase difference, there is a frequency sink point, that is, a "notch point". According to the position of the notch point, the location where the disturbance occurs can be determined. The "notch point" is shown in Figure 3. In this amplitude-frequency diagram obtained through time-frequency transformation, the position marked by "○" is the frequency notch point. The relationship between the notch point and the disturbance position is:

 

 

为k阶陷波点的频率。 in,

is the frequency of the k-order notch point.

It can be seen from the above principles that the coherent light must go through the process of being transmitted from the end point 1 of the sensing fiber 6 to 2 and then returning to the sensing fiber 6 in order to carry the position "L" information. However, in practice, due to the structural characteristics of the optical fiber and the defects of the optical fiber itself, scattered light exists in the optical fiber, such as Rayleigh scattered light.

As shown in Figure 4, point 7 is a scattering point, and the backscattered light returns to the interference structure along the optical cable, so there are two beams of light:

I: 3b1→5→4a1→4b1→6→7→6→4b1→4a2→3b2

II: 3b2→4a2→4b1→6→7→6→4b1→4a1→5→3b1

Due to the similar spectral characteristics, when there is no disturbance, the optical paths are equal, so recombination at the coupler 3 will also cause interference. Obviously, the information of the disturbance point carried by the two beams of interference light is the length L ₇ from the point 7 to the disturbance point D. Let point 8 be another scattering point, and the length information carried by the interference formed by backscattering at this point is the length L ₈ from point 8 to the disturbance point D. Obviously, , because these interferences are mixed together at the output end, the interference light generated by Brillouin backscattered light or Raman backscattered light can be filtered out by optical filters, but for the interference generated by Rayleigh scattering Light, or the interference light generated by the reflection of the joints on the optical path, cannot be eliminated by optical filtering, which will inevitably affect the purity of useful interference signals and directly affect the accuracy of the disturbance point position L. Under normal circumstances, the interference intensity generated by backscattered light and contact reflected light is significantly smaller than the interference intensity (effective interference signal) generated by reflected light, and will not have a significant impact on the effective interference signal. The accuracy of L can meet the needs of actual use. However, when the monitored line reaches a certain length, the comprehensive influence of the scattered light of the entire line will be obvious. At this time, it can be observed that the interference signal has undergone obvious distortion, so the system cannot normally obtain an effective interference signal, and the monitoring distance of the system is also due to the background It is significantly limited due to the reason of scattered light.

Similarly, reflections from junctions in the optical path can have the same detrimental effect on interference signals.

Contents of the invention

The purpose of the present invention is to provide a method capable of effectively eliminating the influence of backscattered light in an optical fiber sensor.

The method proposed by the present invention that can eliminate the influence of backscattered light in the optical fiber sensor is to use the Phase Generated Carrier (Phase Generated Carrier) technology to output the effective interference phase information from the optical output mixed with backscattered light and contact reflected light interference interference signal Separation, that is, to obtain a pure signal containing effective disturbance position information, so as to achieve the purpose of eliminating the influence of backscattered light and so on.

Phase-generated carrier technology is a commonly used technology in optical fiber sensing. It is usually used to overcome the phase fading phenomenon. In the interference of light, when the phase difference of the two interfering beams of light is 0 (or Integer multiples), the interference is in the least sensitive state to phase changes, that is, phase fading occurs, and phase generation carrier technology is usually used to overcome this phase fading phenomenon.

In the present invention, the main purpose of using the phase generation carrier technology is not to overcome the phase fading, but to use the phase generation carrier to mark the effective optical signal, so that the useful interference signal can be finally extracted from the output of the optical path. In other words, It is used to suppress the interference caused by the backscattered light in the optical path to the measurement.

The method of the present invention is specifically shown in FIG. 5 . At the end of the sensing fiber (optical cable) 6 , a phase modulator 9 is connected close to the feedback device 2 . Applying a modulation signal to the phase modulator 9 will change the phase of the light passing through the phase modulator 9 . Let the interference phase difference introduced by the phase modulator be , the interference signal generated by the light returning to the optical cable after arriving at the feedback device 2 reflected by the phase modulator can be expressed as:

                         （1）

(1)

是干涉结构的初始相位，

是由扰动引起的干涉相位差。 in, is a constant coefficient related to the system parameters,

is the initial phase of the interference structure,

is the interference phase difference caused by the disturbance.

For the backscattered light caused by the path before the phase modulator 9 in the optical cable, etc., such as the backscattered light caused by points 7 and 8, as can be seen from Figure 5, since the phase modulator 9 will not pass through during transmission, its The phase change is not affected by the signal applied to the phase modulator 9 . This part of the optical interference signal can be expressed as:

                          （2）

(2)

是光纤6上第i个散射点引起的干涉的系数，

是与第i个散射点相应的初始相位，

是由扰动引起的相应于第i个散射点的干涉相位差。

表示沿着相位调制器9之前的感应光纤6上的所有散射点求和。 in,

is the coefficient of interference caused by the ith scattering point on the fiber 6,

is the initial phase corresponding to the ith scatter point,

is the interference phase difference corresponding to the ith scattering point caused by the disturbance.

represents the summation of all scattering points along the sensing fiber 6 before the phase modulator 9 .

The total output signal change part can be expressed as:

                                           （3）

(3)

Assume that the phase signal applied on the phase modulator 9 is a sinusoidal transform signal with frequency f _m , and set,

                                         （4）

(4)

Then formula (1) can be expressed as:

（5）

(5)

In the above formula, the selection of f _m is similar to the carrier frequency determination method when the traditional phase generation carrier technology is used, and it should be much larger than the bandwidth of the interference signal caused by the external signal when the wave signal is not loaded.

，由于散射光引起的干涉没有相位载波的参与，因而其信号不会出现在这些基频和倍频边带附近，因此，不会干扰到这些边带中所含的信息，利用相位生成载波方法中常用的解调技术，便可以将纯净的、扰动引起的相位信息解调出来，因而可以获得更加准确的扰动位置信息，提高监测的距离。 It can be seen from formula (5) that the sidebands of the carrier fundamental frequency and multiplied frequency contain the phase change information caused by the disturbance

, because the interference caused by scattered light does not have the participation of phase carrier, so its signal will not appear near these fundamental frequency and double frequency sidebands, therefore, it will not interfere with the information contained in these sidebands, using the phase generation carrier method The demodulation technology commonly used in the field can demodulate the pure phase information caused by the disturbance, so that more accurate disturbance position information can be obtained and the monitoring distance can be improved.

The advantage of the present invention is that it can effectively eliminate the influence of backscattered light in the single-core feedback optical fiber sensing device (similarly, it is also applicable to the reflected light of the joint). The characteristic of this single-core feedback sensor is that the sensing part is composed of a feedback device connected to the end of a single optical fiber. The feedback device can be directly connected to the optical fiber, or it can be a reflective surface that is not connected to the optical fiber and has a space distance from the optical fiber. .

From the perspective of monitoring distance, the present invention can significantly improve the monitoring distance. This invention is also another breakthrough new application of phase-generated carrier technology, which is used to extract useful information from severely jammed signals.

Description of drawings

Figure 1 is the positioning principle of a single-core feedback sensor.

Figure 2 is a single-core feedback interference structure. 3 is an N*M (N, M is an integer) coupler, 4 is a P*Q (P, Q is an integer) coupler, 5 is a fiber optic delay, the delay is τ, 6 is a sensing fiber (optical cable) and 2 Constituted as a feedback device. 3a1, 3a2, ..., 3aN, 3b1, 3b2 are ports of the coupler 3, 3a1, 3a2, ..., 3aN are ports in the same direction, a total of N, 3b1, 3b2 are another group of ports in the same direction of the coupler 3 (total M) of the two ports. 4a1, 4a2, 4b1 are the ports of coupler 4, 4a1, 4a2 are two ports in a group of co-directional ports (total P) of coupler 2, 4b1 is another group of co-directional ports of coupler 4 (total Q) of the two ports.

Fig. 3 is the frequency spectrum of the phase signal demodulated from the interference signal, "○" is the frequency "notch point".

FIG. 4 is a schematic diagram of the influence of backscattered light. 7 and 8 are scattering points in the optical fiber.

Fig. 5 is an implementation method of eliminating the influence of backscatter by using the phase generation carrier technology. 9 is a phase modulator.

Detailed ways

Example

This embodiment adopts the interference structure as shown in FIG. 2 . The light source used is the SO3-B super-superluminant diode (SLD) produced by the 44 Research Institute of the Electronics Group Corporation, with a working wavelength of 1310nm. Coupler 3 uses a 3*3 optical fiber fused tapered single-mode coupler that is equally divided, and coupler 4 uses a 2*2 optical fiber fused tapered single-mode coupler that is equally divided, both of which are produced by Wuhan Institute of Posts and Telecommunications. The optical fiber used in the optical fiber delayer is G652 type single-mode optical fiber. The photoelectric conversion device used in photoelectric conversion and information processing is an InGaAs photodetector model GT322C500 produced by 44. The feedback device 2 is made of evaporated aluminum film at the end of the optical fiber, and the reflectivity is greater than 95%. The phase modulator 9 connected in series at the tail end is made by winding an optical fiber on a piezoelectric ceramic. The frequency of the sinusoidal signal loaded on the phase modulator is 60kHz.

When the phase generation carrier technology is not used, the maximum monitoring distance of the line is 40km. After the phase generation carrier technology is used, the maximum monitoring distance can reach 60km.

Claims (2)

1. A method for eliminating the influence of backscattered light in an optical fiber sensor, characterized in that the effective interferometric phase information is separated from the light output mixed with backscattered light and joint reflected light interference signal by utilizing phase generation carrier technology, That is to obtain a pure signal containing effective disturbance position information, thereby eliminating the influence of backscattered light; In a single-core feedback interference structure, a phase modulator (9) is connected to the end of the sensing fiber (6) close to the feedback device (2), and a modulation signal is applied to the phase modulator (9); In the single-core feedback interference structure, the sensing part is composed of a feedback device connected to the end of a single optical fiber, and the feedback device is directly connected to the optical fiber, or a reflective surface that is not connected to the optical fiber and has a space distance from the optical fiber; The interference optical path in the single-core feedback interference structure is composed of N*M coupler (3), P*Q coupler (4), optical fiber delayer (5), induction fiber (6) and feedback device (2), The delay of the optical fiber delayer (5) is τ, and the feedback device (2) is arranged at the end of the sensing optical fiber (6); N, M, P, and Q are integers. 2. The method for eliminating the influence of backscattered light in an optical fiber sensor according to claim 1, characterized in that the modulation signal applied to the phase modulator (9) is a sinusoidal transformation signal with frequency f _m .

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