TWI407082B - Fiber optic sensing system - Google Patents

Abstract

A fiber sensing system includes a reference wavelength filter, at least one sensing fiber Bragg grating (FBG), an optical amplification resonance cavity and a signal processing computer host, the reference wavelength filter can be used as a reference FBG and provide a reference signal wavelength, the sensing FBG is placed on test sites to measure temperature, pressure, deformation and other physical changes for use as basis to change the reflected wavelength of itself, the optical amplification resonance cavity is regulated by the reflected wavelength difference between the reference signal wavelength and the sensor signal, and the wavelength shift of the sensing FBG is directly converted into a resonant cavity light intensity, and output to the signal processing computer host to calculate the physical changes actually measured by the sensing FBG, thereby enabling the invention to make extensive use of FBG in demodulation of wavelength shift without using expensive spectrometer, it can be used in a variety of physical measurement environments, in addition, its advantage is that the optical signal can be amplified in the optical cavity, and suitable in long-distance sensing systems.

Description

Fiber optic sensing system

The invention relates to a fiber sensing technology, in particular to an optical resonant cavity signal modulation technique for directly converting a displacement of a fiber grating wavelength into a light intensity signal. At the same time, since the light modulation signal can be continuously amplified in the optical cavity, a good signal/noise ratio can be obtained.

Conventional systems that utilize grating elements for sensing primarily include a broadband source, a fiber grating component as a sensing head, and an optical spectrum analizer. When the broadband source is reflected by the grating element, the grating reflects the wavelength shift due to the temperature or stress, which causes the grating to reflect the wavelength shift. Therefore, the displacement of the grating reflection wavelength can be measured by the spectrometer. The accuracy of the measurement method is high, but the disadvantage is that the system is complicated, the spectrometer is expensive, and it is difficult to widely use it. In the actual operation, the spectrometer scans and reads all takes a time, so the instantaneous measurement of the wavelength change of the fiber grating cannot be achieved. .

In view of the fact that the conventional fiber grating must be able to read the wavelength shift of the reflected signal by a professional spectrometer, the spectrometer equipment is expensive and unsuitable for widespread use, and it is difficult to achieve an instantaneous measurement of the wavelength change of the reflected signal, so the main object of the present invention is to provide a The optical fiber sensing system directly converts the wavelength shift difference of the fiber grating reflection signal into a light modulation signal by utilizing the characteristics of the optical vibration cavity, thereby achieving the degree of change of the physical quantity to be measured.

In order to achieve the foregoing objective, the fiber grating sensing system of the present invention comprises: an excitation light source, which is a laser light source capable of generating a gain reversal; a wavelength multiplexer coupled to the excitation light source and the sensing light source; The gain medium is connected to the wavelength multiplexer, and the excitation light source causes the optical gain medium to reach the optical quantity inverse to generate an optical gain effect; a reference wavelength filter has a wavelength filter of a specific bandwidth to provide a reference signal wavelength The wavelength can be adjusted by voltage or current; at least one of the sensing fiber gratings has a reflection wavelength range within a tunable wavelength range of the reference wavelength filter, and senses a change in a physical quantity to cause a displacement of the reflected wavelength itself. The reference wavelength filter is excited by the optical gain medium and the sensing fiber grating forms an optical cavity; a first optical coupler connecting the optical cavity and the sensing fiber grating; and a second optical coupler connecting the optical cavity And a photodetector; a third optocoupler connecting the optical resonant cavity and the reference wavelength filter; and a signal processing computer host connected to the photodetector, The received photodetector converts a signal of current or voltage, and calculates a representative of a physical quantity sensing the actual measurement of the fiber grating measured.

The reference wavelength filter may be constructed by using a fiber grating or other types of filtering components; the number of the sensing fiber gratings may be plural and matched with an optical multiplexing switching device to switch between the sensing gratings to achieve multiple points. Measurement purposes.

With the foregoing architecture, the optical gain medium excited by the reference wavelength filter and the sensing grating form an optical resonant cavity, and the reference wavelength filter can provide a reference signal adjustment by voltage control when the sensing fiber grating is used for the designated location. When measuring physical quantities such as temperature and pressure, the filter wavelengths of both the reference wavelength filter and the sensing fiber grating may produce a displacement difference, and the difference amount of the wavelength may provide optical gain amplification modulation of the optical amplification cavity, the light modulation signal The optical modulation signal is converted into a current or voltage signal by a photodetector, and the signal processing computer host performs raster sensing demodulation operation, that is, the physical quantity actually measured by the sensing fiber grating can be quickly obtained.

Please refer to the first figure, which is a system architecture diagram of the present invention, comprising: an excitation light source (2), which is a laser source capable of generating a gain-reducing inverse, the wavelength of the light source is 980 nm; a wavelength multiplexer ( WDM) (4), connecting the excitation light source (2) and the sensing laser light source; an optical gain medium (6), such as an erbium-doped fiber or other optical gain medium, is connected to the wavelength multiplexer (4) from The laser of the excitation light source (2) generates a sensing grating light modulation effect in the optical cavity through the optical gain medium (6), and the wavelength range thereof is between 1515 nm and 1575 nm; a reference wavelength filter (wave length filter) (20), having a specific bandwidth filter performance, this embodiment uses a tunable wave length filter as an example to illustrate that the wavelength range can be between 1540 nm and 1580 nm, the reference wavelength filter ( 20) modulating the voltage signal, the filter wavelength of the reference wavelength filter (20) can be changed by a piezoelectric effect; at least one sensing fiber Bragg grating (30) whose reflection wavelength is at a reference wavelength Within the adjustable range of the filter (20), the sensing fiber grating (3 0) sensing a change in a physical quantity to cause a displacement of a self-reflecting wavelength, including temperature, deformation, stress, and a sensing physical quantity to which it is applied. In this embodiment, a plurality of sensing fiber gratings (30) are used. An optical multiplexing switching device (40) performs multi-point signal measurement, and the optical multiplexing switching device (40) can switch or fixed one of the sensing fiber gratings (30) as long-term measurement or dynamics arbitrarily or sequentially. a source of signal scanning data; a first optical coupler (11) connected to the optical gain medium (6), which is an erbium doped fiber and transmitted through the optical multiplexing switching device (11) and connected to the sensing fiber grating (30), the first optical coupler (11) has a coupling ratio adjusted according to the system, and the example is 10/90; a second optical coupler (12) functions as a part of the optical cavity The light intensity output is adjusted to a photodetector (14), and the photodetector (14) converts the optical signal into a current or voltage signal and outputs the signal to a signal processing computer host (50) for raster sensing signal adjustment. The coupling ratio of the second optical coupler (12) can be set according to system requirements, and the embodiment is 10/90; a three-optical coupler (13) connected to the reference wavelength filter (20), the wavelength multiplexer (4) and the second optical coupler (12) for providing a reference wavelength of the optical resonant cavity, the third optical coupling The coupling ratio of the device (13) can be set according to the system requirements, and the embodiment is 10/90.

Based on the hardware architecture of the grating sensing signal demodulation system described above, the reference wavelength filter (20) of the present invention can provide a reference wavelength signal by adjusting the voltage, and the sensing fiber grating (30) is treated as a Measuring component for sensing a physical quantity such as temperature or pressure, such that the reference wavelength filter (20), A ring-shaped optical cavity is formed between the optical gain medium (6) and the sensing fiber grating (30). When the sensing fiber grating (30) is displaced due to temperature or pressure difference, the optical cavity is caused to be displaced. Magnification intensity adjustment.

The light intensity signal generated by the sensing grating is placed in the optical amplifying cavity to repeatedly obtain the optical gain amplification, thereby facilitating signal detection of the long-distance fiber sensing, and the amplified signal portion is passed through the first The two optocoupler (12) outputs to a photodetector (14), and the photodetector (14) can directly convert the optical modulation signal into a current or voltage signal, and the signal is processed by a computer processing host (50) The operation is converted to data representative of the actual temperature or pressure strain of the sensing fiber grating (30).

The invention utilizes a ring-shaped optical amplifying resonant cavity, wherein one of the fiber gratings (30) senses the displacement of the wavelength caused by the change of the external environment, thereby causing the optical modulation of the annular optical amplifying resonant cavity to re-amplify the reflected signal of the grating The light detector (14) directly converts the wavelength shift of the sensing grating into a current or voltage signal, thereby increasing the signal noise ratio and monitoring the instantaneous environmental change.

The invention can be widely applied to safety monitoring of public buildings, such as bridges, buildings, dikes, tunnels, railways, oil storage tanks, oil pipelines, etc., and can also be applied to other environmental monitoring related to people's livelihood, such as earthquakes, fires, earth and rock flows, etc. Natural disaster warnings, even general household anti-theft and hydro-electric security alarm systems, can be applied at a wide range of applications, because the use of expensive spectrum analyzers is not required, the cost of the fiber-optic sensing system can be reduced, and the fiber-optic grating signal can be improved. Demodulation time is measured to achieve high frequency vibration or change measurement.

(2). . . Excitation source

(4). . . Wavelength multiplexer

(6). . . Optical gain medium

(11). . . First optocoupler

(12). . . Second optocoupler

(13). . . Third optical coupler

(14). . . Light detector

(20). . . Reference wavelength filter

(30). . . Sense fiber grating

(40). . . Optical multiplexing switching device

(50). . . Signal processing computer host

First Figure: System architecture diagram of the present invention.

(2). . . Excitation source

(4). . . Wavelength multiplexer

(6). . . Optical gain medium

(11). . . First optocoupler

(12). . . Second optocoupler

(13). . . Third optical coupler

(14). . . Light detector

(20). . . Reference wavelength filter

(30). . . Sense fiber grating

(40). . . Optical multiplexing switching device

(50). . . Signal processing computer host

Claims (7)

An optical fiber sensing system comprises: an excitation light source, which is a laser light source capable of generating a gain reversal; a wavelength multiplexer connected to the excitation light source and the sensing light source; and an optical gain medium connected to the wavelength The excitation light source causes the optical gain medium to be inverted to generate an optical gain effect; a reference wavelength filter having a filtering performance of a specific bandwidth provides an adjustable filtering wavelength; at least one sensing The fiber grating has a reflection wavelength range within a tunable wavelength range of the reference wavelength filter, and the sensing fiber grating senses a change in a physical quantity to cause a displacement of a self-reflecting wavelength, and the reference wavelength filter is excited by the optical gain. The medium and the sensing fiber grating form an optical resonant cavity; a first optical coupler connecting the optical resonant cavity and the sensing fiber grating; a second optical coupler connecting the optical resonant cavity and a photodetector; a three-optical coupler connecting the optical resonant cavity and the reference wavelength filter; a signal processing computer host connected to the optical detector to receive the current or voltage signal converted by the optical detector And the sensing operation became represent physical measurement of the actually measured fiber grating. The optical fiber sensing system of claim 1, comprising a plurality of sensing fiber gratings and an optical multiplexing switching device, wherein the optical multiplexing switching device is connected to the plurality of sensing fiber gratings and Between a coupler. The optical fiber sensing system of claim 1 or 2, wherein the filter wavelength of the reference wavelength filter can be adjusted by voltage or current. The optical fiber sensing system of claim 3, wherein the optical gain medium is an erbium doped fiber. The optical fiber sensing system of claim 4, wherein the reference wavelength filter is a fiber grating. The optical fiber sensing system of claim 5, wherein the excitation light source generates laser light having a wavelength of 980 nm, and the laser light is amplified by the optical gain medium, and the wavelength ranges from 1515 nm to 1575 nm. The optical fiber sensing system of claim 6, wherein the reference wavelength filter has a filtering wavelength between 1540 and 1580 nm.