CN114370928A - Linear type sagnac interferometric optical fiber vibration sensor - Google Patents
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Abstract
The invention provides a linear Sagnac interferometric optical fiber vibration sensor, which comprises a light source module, a sensing module, a modulation module, a light path module and a signal processing module which are sequentially connected; the light source module emits wide-spectrum linearly polarized light, the linearly polarized light passes through the sensing module, the modulation module and the light path module, is reflected by the reflector at the tail end of the light path module, interferes at the polarizer, and finally reaches the signal processing module for demodulation; the optical path module comprises a delay optical fiber ring for generating time delay and a rotating light reflector structure for realizing polarization rotation and reversal; and the signal processing module receives the interference signal and demodulates the signal, so that the vibration is finally measured.
Description
Technical Field
The invention relates to the technical field of optical fiber vibration sensing, in particular to a linear Sagnac interference type optical fiber vibration sensor.
Background
Fiber optic sensors have a number of unique advantages over conventional sensors. The sensor has the advantages of small volume, light weight, electromagnetic interference resistance, corrosion resistance, high sensitivity, wide measurement bandwidth, long distance between detection electronic equipment and a sensor and the like, and can form a sensing network. In particular, the sensitivity of the sensor is several orders of magnitude higher than that of a conventional sensor, and the sensor can measure pressure, temperature, stress (strain), magnetic field, refractive index, deformation, micro-vibration, micro-displacement, sound pressure and the like. Fiber sagnac interferometers have enjoyed an admirable achievement in the practical application of fiber optic gyroscopes and fiber optic current sensors. The sagnac fiber optic interferometer is a rotatable ring interferometer, which splits a beam of light from the same light source into two beams, which converge after they travel in the same loop in opposite directions for a circle, and then generates interference on the screen. The number of fringe movements in the sagnac effect is proportional to the product of the angular velocity of the interferometer and the area enclosed by the loop.
The interferometric optical fiber vibration sensor changes the phase of light waves in the optical fiber based on the effect of the vibration quantity to be detected on the sensing optical fiber, and then converts the phase change into the light intensity change by using the interference technology, so that the vibration signal to be detected is detected and restored. In various interferometer structures, the sagnac interferometer has the characteristic of reciprocity, so that the influence of unstable factors caused by interference of ambient temperature and the like can be avoided. In practical use, fiber optic gyroscopes and fiber optic current sensors based on sagnac interferometer designs have become successful commercial sensing devices, widely used in the measurement of angular velocity, ultrasound, and current.
The distributed optical fiber sensing technology based on the OFDR technology needs a linear sweep frequency light source consisting of a narrow-linewidth single longitudinal mode laser and an electro-optic modulator or an acousto-optic modulator, and has high requirements on the light source. The existing annular Sagnac interferometric vibration sensor adopts a mode of interference of two beams of light which are transmitted clockwise and anticlockwise, is mainly used for positioning vibration signals, and cannot accurately measure the frequency and the size of the vibration signals; other interferometric vibration sensors can only measure vibrations at relatively low frequencies with relatively small variations, and cannot effectively measure vibration signals at high frequencies.
Disclosure of Invention
The purpose of the invention is as follows: the invention provides a linear sagnac interferometric fiber vibration sensor, which adopts a phase modulator to provide a phase generated carrier wave to realize modulation and demodulation of a weak vibration signal, adopts the structure of a linear sagnac interferometer, uses a polarization maintaining fiber as a sensing fiber, and combines a signal processing technology to realize high-precision measurement of the amplitude and frequency of the weak vibration, and can effectively measure high-frequency vibration in particular.
The technical scheme of the invention is as follows: in order to achieve the purpose, the invention adopts the technical scheme that: a linear Sagnac interferometric fiber vibration sensor comprises a light source module, a sensing module, a modulation module, a light path module and a signal processing module; the light source module emits wide-spectrum linearly polarized light, the linearly polarized light passes through the polarization maintaining optical fiber, the sensing module, the modulation module, the rotary light reflector and the light path module, is reflected by the reflector at the tail end of the light path module, interferes at the polarizer, and finally reaches the signal processing module for demodulation.
Further, the light source module is used for generating light waves with wide emission spectrum and high output power.
Further, the sensing module is used for receiving the vibration signal for sensing.
Further, the modulation module is used for generating a phase modulation signal and modulating the phase of the optical wave; and phase modulation under different conditions can be achieved by controlling the drive signal.
Further, the optical path module includes two delay coils (delay fiber rings) for generating time delay, and a spiral mirror structure for realizing polarization rotation and reversal.
Further, the signal processing module converts the optical signal into an electric signal and demodulates the electric signal to obtain a vibration signal.
According to the linear Sagnac interferometric fiber vibration sensing method, light emitted by a light source module is converted into linearly polarized light with a high extinction ratio through a depolarizer and a polarizer, and then is decomposed into two beams of linearly polarized light with mutually orthogonal polarization directions through 45-degree fusion (two polarization-maintaining polarizing fibers deflect for 45-degree re-fusion) and the two beams of linearly polarized light are respectively transmitted along the fast axis and the slow axis of the same polarization-maintaining optical fiber; the two linearly polarized lights are subjected to the vibration transmitted by the polarization-maintaining optical fiber to cause a first vibration phase difference of the two beams of light with the fast axis and the slow axis, and then the first modulation phase difference is introduced through the forward modulation action of the phase modulator of the modulation module; the interchanging of two beams of light of a fast axis and a slow axis is realized through a spiral light reflector structure; when the backward (reflected) light passes through the same phase modulator, backward phase modulation is carried out, and a (second time) modulation phase difference is introduced again; the reverse light is vibrated again to cause a second phase difference between the fast axis light and the slow axis light again; and finally, carrying two beams of polarized light carrying secondary phase difference and secondary modulation phase difference information caused by vibration to generate interference signals at the optical fiber polarizer, wherein the interference signals enter the signal processing module through the circulator to demodulate vibration signals.
Has the advantages that: the invention provides a linear sagnac interferometric fiber vibration sensor, which has low requirements on light sources, can realize frequency modulation of megahertz orders by using a birefringent phase modulator, can carry out good carrier modulation on high-frequency vibration signals, and can also carry out good response on variable vibration signals. In addition, the linear Sagnac interferometer structure is adopted, so that the characteristic of zero optical path difference of the traditional annular Sagnac interferometer is kept, the noise influence caused by a reference arm is avoided, the system structure is simplified, and the complexity of the system is reduced. In addition, the system adopts the polarization maintaining optical fiber as the sensing optical fiber, thereby effectively inhibiting the influence of the intrinsic birefringence of the optical fiber on the measurement and further realizing the measurement of weak signals.
Drawings
FIG. 1 is a schematic structural view of a preferred embodiment of a fiber optic vibration sensor provided by the present invention;
FIG. 2a is a waveform diagram of photocurrent at different modulation voltages;
FIG. 2b is a spectral plot of photocurrent at different modulation voltages;
FIG. 3a is a waveform diagram of a photocurrent carrying a vibration signal at a fixed modulation voltage;
FIG. 3b is a spectral plot of the photocurrent carrying the vibration signal at a fixed modulation voltage;
FIG. 4 is a waveform diagram of a demodulated vibration signal;
FIG. 5 is a basic structure of a linear sagnac interferometric fiber vibration sensor provided by the present invention.
Detailed Description
The present invention will be further described with reference to the accompanying drawings. Reference is made to the accompanying drawings in which:
1-a light source; 2-a circulator; 3-depolarizer; 4-a polarizer; welding at 5-45 deg; 6-sensing optical fiber ring; 7-a delay coil 1; an 8-phase modulator; 9-delay coil 2; 10-a gyrolascope; 11-a photodetector; 12-a signal processing unit; 13-vibration signal.
FIG. 5 shows the basic structure of a linear Sagnac interferometric fiber vibration sensor according to the present invention; FIG. 1 is a schematic structural view of a preferred embodiment of a fiber optic vibration sensor provided by the present invention; the device comprises a light source module, a sensing module, a modulation module, a light path module and a signal processing module; the light source module emits light waves with wide emission spectrum and high output power, the light waves pass through the sensing module, the modulation module and the light path module, are reflected by the reflector at the tail end of the light path module, interfere at the polarizer and finally reach the signal processing module for demodulation.
The light source module comprises a light source (1) for generating light waves with wide emission spectrum and high output power, and can adopt an LD light source, an SLD light source and the like.
The depolarizer (3) changes light emitted by the light source into unpolarized light after passing through the depolarizer so as to eliminate the polarization influence of the light source. The structure of a Loyt optical fiber depolarizer can be adopted, and the ratio of two lengths is l1:l2The optical fiber is formed by welding polarization maintaining optical fibers with the refractive index main shaft rotating 45 degrees relatively, and wedge-shaped crystal, periodic piezoelectric ceramic, and Couple ring-shaped depolarizer structures can also be adopted.
The polarizer (3) converts non-polarized light into linearly polarized light for being decomposed into two beams of light of an X axis (a slow axis) and a Y axis (a fast axis) through 45-degree welding (5). The polarizer can be formed by structures such as an online optical fiber polarizer, a crystal polarizer, a polaroid, a Nicole prism and the like.
The sensing module adopts a polarization maintaining optical fiber to receive the vibration signal for sensing.
The modulation module adopts a phase modulator (8) to generate a phase modulation signal and modulates the phase of the optical wave; the phase modulation under different conditions can be realized by changing the frequency and amplitude of the driving signal, and the applied voltage signal is in direct proportion to the phase change of the light wave. When the frequency of a signal to be measured is low, a phase modulator can be manufactured by winding a polarization maintaining optical fiber on piezoelectric ceramic (such as PZT) with applied voltage, the piezoelectric ceramic can generate periodic deformation under the condition of the applied voltage, so that the polarization maintaining optical fiber wound on the piezoelectric ceramic can also generate deformation, and periodic phase change is generated, and phase modulation is realized; when the frequency of the vibration signal to be measured is high, the birefringence phase modulator can be used, and the birefringence phase modulator is required to have the characteristics of high modulation frequency, low insertion loss, low half-wave voltage, low back scattering, wide working wavelength, excellent electro-optic response and the like.
The optical path module comprises a first delay coil 1(7), a second delay coil 2(9) and a rotary mirror (10) which are arranged in front of and behind the phase modulator, and are used for generating time delay and reflecting the light waves back to the optical path in a rotating mode. The total length of the first delay coil 1 and the second delay coil 2 is used for matching the frequency of the vibration signal, the length of the delay coil 2 is used for matching the modulation frequency of the phase modulator, and the time delay tau and the frequency omega generated by the delay coilsmNeed to satisfy
In relation to (3), a delay coil of about 850m is required for a vibration frequency of 60 kHz. And (3) performing 45-degree axial welding on the delay coil 2(9) and a section of polarization-maintaining optical fiber, cutting the quarter beat length of the polarization-maintaining optical fiber by using a movable clamp to obtain a quarter wave plate, and plating a layer of metal with high reflectivity on the end surface of the quarter wave plate to obtain the rotary light reflector (10). The second delay coil end is welded with a spiral reflector (10), and a Faraday rotator can also be used as the spiral reflector.
The signal processing module comprises a photoelectric detector (11) and a signal processing unit (12) and is used for receiving interference signals, converting optical signals into electric signals and demodulating vibration signals.
The Sagnac interferometric fiber vibration sensor based on the phase generated carrier selects an all-fiber optimal structure, and the working method specifically comprises the following processes:
the light source emits wide-spectrum high-power light waves, and unpolarized light is generated by the optical fiber depolarizer to eliminate the polarization influence of the light source; the unpolarized light is converted into linearly polarized light with high extinction ratio through an optical fiber polarizer, the linearly polarized light is decomposed into two beams of linearly polarized light with mutually orthogonal polarization directions through a 45-degree welding point, and the two beams of linearly polarized light are respectively transmitted along an X axis (slow axis) and a Y axis (fast axis) of the polarization-maintaining optical fiber; when vibration acts on the sensing optical fiber, the phase difference of two beams of light of an X axis (a slow axis) and a Y axis (a fast axis) is caused, and then the two beams of linearly polarized light are subjected to the forward modulation action of the phase modulator to introduce the modulation phase difference; when the two beams of light pass through the quarter-wave plate welded at 45 degrees, the two beams of light are respectively converted into left-handed circularly polarized light and right-handed circularly polarized light, and after the two beams of light are reflected by the end reflecting mirror, the left-handed circularly polarized light is converted into right-handed circularly polarized light, and the right-handed circularly polarized light is converted into left-handed circularly polarized light; the two reflected circularly polarized lights return along the original optical path, and when the circularly polarized lights pass through the quarter-wave plate again, the circularly polarized lights are changed into linearly polarized lights, and the polarization directions of the reverse (reflected) lights and the forward lights are interchanged, namely X polarized input light → Y polarized output light, and Y polarized input light → X polarized output light; when backward light passes through the phase modulator, the backward phase modulation is carried out to generate a non-reciprocal phase difference; the reverse light is subjected to the action of vibration again to cause the phase difference of two beams of light of an X axis (slow axis) and a Y axis (fast axis) again; finally, two beams of polarized light carrying phase difference caused by vibration and phase difference modulation information interfere at the optical fiber polarizer and enter the photoelectric detector, and the photoelectric detector converts the optical signal into an electric signal; the photocurrent signal is input to a signal processing unit, so that a vibration signal is demodulated.
By using the jones matrix, the expression of the photocurrent can be calculated as follows:
using Faraday rotator mirrors
In the form of (1). Wherein E is0For the light source to output light intensity, phimModulating the carrier depth, omega, of a phase modulatormThe carrier frequency is modulated for the phase modulator,
a phase difference caused by the vibration signal.
Will be provided with
Expansion using Bessel function:
when the vibration signal
When the average molecular weight is 0, the average molecular weight,
fig. 2a shows the photocurrent signal waveforms at different modulation driving voltages without the vibration signal. When the driving voltage is continuously increased, the modulation depth of the phase modulator is also continuously increased, and the modulation depth and the driving voltage are in a direct proportion relation.
FIG. 2b is a graph of the spectra of the signals in FIG. 2a, the second harmonic and the fourth harmonic, I, being the stronger signals2h＝2J2(φm)cos(2ωmt) and I4h＝2J4(φm)cos(4ωmt)。
The signal processing unit can use a phase-locked amplifier to demodulate signals. Meanwhile, the driving signal of the phase modulator adopts a sinusoidal signal provided by a phase-locked amplifier, and the amplitude of the sinusoidal signal can be changed within the working voltage range of the phase modulator, so that the phase modulator works at the optimal modulation depth.
Fig. 3a shows the waveform of the photocurrent signal in the presence of the vibration signal, and fig. 3b is a spectrum diagram of the signal in fig. 3 a.
As can be seen from FIG. 3b, the frequency is ωmCarrier signal of 100kHz and frequency omegavThe vibration signal of 48kHz is mixed, and the frequency of the mixed signal is omegam+ωvAnd ωm-ωvThe frequency sweep function of the phase-locked amplifier is utilized to read the corresponding frequency, so that the frequency omega of the vibration signal is demodulatedv. Will be provided with
Developed in the same way, when the second harmonic and the fourth harmonic are I2h＝2J0(φv)J2(φm)cos(2ωmt) and I4h＝2J0(φv)J4(φm)cos(4ωmt). The amplitude is measured by a phase-locked amplifier and divided to obtain
So that phi can be solved according to the Bessel functionmIs substituted back into | I2hI or I4hIn |, J can be obtained0(φv) So as to demodulate the amplitude phi of the vibration signalvFinally, the vibration signal can be demodulated
As shown in fig. 4.
In addition, a closed-loop feedback system can be used for signal demodulation, and a square wave + step wave closed-loop signal modulation scheme can be generally used for digital signal processing. The square wave modulation needs to be calculated according to a specific modulator, so that the phase modulator is generated
Such that the interferometer operates near the point on the cosine response curve where the sensitivity is highest, the photocurrent can be expressed as
The digital step wave signal is loaded on a phase modulator to generate nonreciprocal compensation phase shift phi (t) to counteract the phase difference generated by vibration
The interferometer is always operated near the position with the highest sensitivity, so that the vibration signal is demodulated by detecting the compensation phase shift phi (t).
In summary, the invention provides a novel sagnac interferometric fiber optic vibration sensor based on phase generated carrier. The realization approach is to modulate the weak vibration signal by generating a high-frequency phase carrier signal through a birefringence phase modulator; the polarization maintaining optical fiber is used as the sensing optical fiber, so that the influence of the intrinsic birefringence of the optical fiber on the measurement is effectively inhibited; the linear Sagnac interference type structure is adopted, the zero-optical-path-difference characteristic is achieved, noise influence caused by a reference arm is avoided, measurement of a phase position is converted into measurement of light intensity, and the system is more convenient to measure while the system complexity is simplified.
The above description is only of the preferred embodiments of the present invention, and it should be noted that: it will be apparent to those skilled in the art that various modifications and adaptations can be made without departing from the principles of the invention and these are intended to be within the scope of the invention.
Claims (10)
1. A linear Sagnac interferometric optical fiber vibration sensor is characterized by comprising a light source module, a sensing module, a modulation module, an optical path module and a signal processing module which are sequentially connected; the light source module emits wide-spectrum linearly polarized light, the linearly polarized light passes through the sensing module, the modulation module and the light path module, is reflected by the reflector at the tail end of the light path module, interferes at the polarizer, and finally reaches the signal processing module for demodulation; the optical path module comprises a delay coil and a delay coil for generating time delay, and a rotating light reflector structure for realizing polarization rotation and reverse.
2. The linear sagnac interferometric fiber optic vibration sensor of claim 1, wherein the light source module comprises a broad spectrum light source, a depolarizer, and a polarizer for generating linearly polarized light with broad emission spectrum and high output power.
3. The linear sagnac interferometric fiber optic vibration sensor of claim 1, wherein polarization maintaining fibers are used as the optical fiber and the sensing fiber loop, the delay coil, the sensing fiber loop is used for receiving vibration signal for sensing.
4. The linear sagnac interferometric fiber optic vibration sensor of claim 1, wherein the modulation module is configured to generate a phase modulation signal for modulating the phase of the optical wave; phase modulation under different conditions can be realized by controlling the driving signal; the optical path module comprises two delay coils for generating time delay and a rotating light reflector structure for realizing polarization rotation and reverse; the modulation module uses piezoelectric ceramics to wind a polarization maintaining optical fiber or a birefringence phase modulator to generate a phase modulation signal and modulates the phase of an optical wave; and phase modulation under different conditions can be achieved by controlling the drive signal.
5. The linear sagnac interferometric fiber optic vibration sensor of claim 1, wherein the signal processing module converts an optical signal into an electrical signal and demodulates the vibration signal using a lock-in amplifier open loop or closed loop feedback system.
6. The method for sensing the vibration of the optical fiber vibration sensor according to any one of claims 1 to 5, wherein the light emitted by the light source module is converted into linearly polarized light with high extinction ratio through the depolarizer and the polarizer, and then is decomposed into two linearly polarized light beams with mutually orthogonal polarization directions through 45-degree welding, and the two linearly polarized light beams are respectively transmitted along the fast axis and the slow axis of the same polarization-maintaining optical fiber; the two linearly polarized lights are subjected to the vibration transmitted by the polarization-maintaining optical fiber to cause a first vibration phase difference of the two beams of light with the fast axis and the slow axis, and then the first modulation phase difference is introduced through the forward modulation action of the phase modulator of the modulation module; the interchanging of two beams of light of a fast axis and a slow axis is realized through a spiral light reflector structure; when the backward (reflected) light passes through the same phase modulator, backward phase modulation is carried out, and a (second time) modulation phase difference is introduced again; the reverse light is vibrated again to cause a second vibration phase difference of the fast axis light and the slow axis light; and finally, carrying two beams of polarized light carrying secondary phase difference and secondary modulation phase difference information caused by vibration to generate interference signals at the optical fiber polarizer, wherein the interference signals enter the signal processing module through the circulator to demodulate vibration signals.
7. The fiber optic vibration sensing method of claim 6 wherein the phase modulation under different conditions is achieved by varying the frequency and amplitude of the drive signal. When the frequency of a signal to be measured is lower, a phase modulator is manufactured in a mode of winding a polarization maintaining optical fiber on piezoelectric ceramic applied with voltage; when the frequency of the vibration signal to be measured is higher, the birefringence phase modulator is used.
8. The optical fiber vibration sensing method according to claim 6, wherein the optical path module comprises a first delay coil (7), a second delay coil (9) and a rotating mirror (10) in front of and behind the phase modulator for generating a time delay and reflecting the light wave back to the optical path. The total length of the first delay coil and the second delay coil is used for matching the frequency of the vibration signal, the length of the delay coil 2 is used for matching the modulation frequency of the phase modulator, and the time delay tau and the frequency omega generated by the delay coilmNeed to satisfy
A delay coil of about 850m is required for a vibration frequency of 60 kHz; and (3) performing 45-degree axial welding on the delay coil 2(9) and a section of polarization-maintaining optical fiber, cutting the quarter beat length of the polarization-maintaining optical fiber by using a movable clamp to obtain a quarter wave plate, and plating a layer of metal with high reflectivity on the end surface of the quarter wave plate to obtain the rotary light reflector (10).
9. The optical fiber vibration sensing method according to claim 6, wherein the second delay coil end is welded with a spin mirror (10), or a Faraday rotator mirror is used as the spin mirror.
10. The optical fiber vibration sensing method according to claim 6, wherein the signal processing unit performs signal demodulation using a lock-in amplifier; meanwhile, the driving signal of the phase modulator adopts a sinusoidal signal provided by a phase-locked amplifier, and the amplitude of the sinusoidal signal changes within the working voltage range of the phase modulator.
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