CN202267808U - Digital demodulation device for interferometric fiber optic sensor - Google Patents

Digital demodulation device for interferometric fiber optic sensor Download PDF

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CN202267808U
CN202267808U CN2011202099672U CN201120209967U CN202267808U CN 202267808 U CN202267808 U CN 202267808U CN 2011202099672 U CN2011202099672 U CN 2011202099672U CN 201120209967 U CN201120209967 U CN 201120209967U CN 202267808 U CN202267808 U CN 202267808U
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optical fiber
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phase
interferometer
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张亦慧
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Shenzhen Polytechnic
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The utility model provides a digital demodulation device for interferometric fiber optic sensor, which comprises modules such as a laser source, an interferometer, a photoelectric detector, an analog filter circuit, a driving circuit, a microprocessor and the like. Each of a detecting arm and a reference arm of the interferometer comprises a sensing optical fiber, a compensated optical fiber and a phase modulator. Carrier driving voltage with frequency being omega C is added to the phase modulators, and the optical fiber length of the reference arm is increased by means of the compensated optical fiber to compensate the optical fiber length difference between the reference arm and the detecting arm so as to balance the interferometer. Interference light total intensity signals I (t) is subjected to discretization sampling with sampling period being Ts to obtain signal sequence I (nTS), the I (nTS) is multiplied with a digital local oscillation signal cos (2 omega CnTS) with frequency being 2 omega C and a digital local oscillation signal cos (omega CnTS) with frequency being omega C respectively to perform digital down conversion and independently obtain in-phase component II (nTS) and quadrature component IQ (nTS) of a sensing signal, and then the sensing signal can be worked out through the II (nTS) and the IQ (nTS) to complete digital modulation of the interferometric fiber optic sensor. Compared with the analogue technique, the digital demodulation device is simple in structure and high in maneuverability and stability.

Description

The interferometric optical fiber sensor digital demodulating apparatus
[technical field]
The utility model relates to the technology of interferometric optical fiber sensor, particularly the digital demodulation techniques of interferometric optical fiber sensor.
[background technology]
Along with development of fiber technology, the research of Fibre Optical Sensor receives extensive attention.Different to the modulation principle of photon physical parameter according to external physical quantity (like temperature, pressure, displacement, vibration), Fibre Optical Sensor is divided into several types such as intensity modulation, phase modulation (PM), Polarization Modulation and wavelength-modulated.Wherein the phase modulation (PM) Fibre Optical Sensor is called interferometric optical fiber sensor again, is that measurement sensitivity is the highest, most widely used a kind of in the above-mentioned several types.It utilizes interferometer, to light phase modulation information, converts external physical quantity the modulation intelligence of light intensity into; Through measuring the Strength Changes of interference light, realize the sensing of physical quantity.Yet, because the nonlinear relationship between optical phase information and the light intensity can not directly be worth optical phase information by luminous intensity measurement.Because the influence of the factors such as phase noise of temperature drift, external shock and light source, interference light intensity can random fluctuation again.Special when interferometer work least sensitive when regional, signal is understood complete blanking, so-called phase fading phenomenon occurs.
Therefore, practical interferometric optical fiber sensor must adopt the signal demodulation techniques, eliminates influence of environmental noise, from the Strength Changes information of interference light, recovers the modulation intelligence of external physical quantity to the photon phase place, thereby realizes the physical quantity sensing exactly.
Avoid occurring the phase fading phenomenon for making interferometric optical fiber sensor; The technological means of Chinese patent " digital closed-loop method of interferometric optical fiber sensor and Control work point " (application number 200910077387.X) is: interfere at two of interferometer and introduce the square wave phase modulation signal between the light wave; Make the output signal of interferometer become the square wave error signal by cosine response; Generate compensating phase shift according to the square wave error signal; And compensating phase shift fed back in the light path, be used to offset phase drift, make interferometer be stabilized in the quadrature working point.Because this Technology Need adopts the complicated devices such as sequential circuit, phase-modulator, D/A converter and driver of a cover to form a closed-loop control system, this system architecture is loaded down with trivial details, and process is complicated, and sensor array is formed in inconvenience, is difficult to make the multipoint mode measuring system.
The technological means that Chinese patent " orthogonal demodulation device of heterodyne phase interference fiber sensor " (application number 201010039599.1) adopts heterodyne measurement to combine with fibre delay line; Reference arm at interferometer adds heterodyne modulator; Make reference arm produce frequency displacement; Two arms form frequency difference, thereby make the output of interferometer comprise frequency difference signal; Interference light is divided into two, and one road light directly links to each other with a photodetector, and another road links to each other with another photodetector through behind an adjustable optic fibre lag line; After the output signal of two photodetectors is done analog down through two independent mixting circuit and heterodyne modulation signal source respectively; The phase differential that makes this two-way light when the adjustable optic fibre lag line can obtain the in-phase component and the quadrature component of signal during for
Figure BSA00000521659300021
; The analog demodulator arithmetic system of forming through a cover complicated circuit again obtains the phase place transducing signal at last.Therefore; The accurate wavelength of light source must be known by this system; According to wavelength value; Could control the adjustable optic fibre lag line, realize that phase differential adopts analog device for
Figure BSA00000521659300022
owing to this system mostly, in the actual mechanical process; Because of the otherness and the stability of device, certainly will cause the poor effect of signal demodulation.Used parts such as light frequency-shift modulator in the heterodyne measurement of other this system, it costs an arm and a leg, and cost is high.
[utility model content]
In order to solve the problems of the prior art, the utility model provides a kind of interferometric optical fiber sensor digital demodulating apparatus, and it comprises LASER Light Source, interferometer, photodetector, simulation amplification filtering circuit, driving circuit, microprocessor; Described interferometer comprises first fiber coupler, sensor fibre, compensated optical fiber, optical fiber type phase-modulator, second fiber coupler, and its sensor fibre constitutes the feeler arm of interferometer, and compensated optical fiber and phase-modulator constitute the reference arm of interferometer; From the light wave of LASER Light Source, at first get into first fiber coupler, be divided into two-way by it; One road light gets into sensor fibre; Form flashlight, another road gets into compensated optical fiber, phase-modulator, forms reference light; The built-in modulus converter A/D of described microprocessor, digital to analog converter D/A; Microprocessor is through digital to analog converter D/A, driving circuit, and to this phase-modulator, in addition frequency is ω CThe cosine carrier driving voltage, make additional variation of phase place of reference light
Figure BSA00000521659300023
M wherein CBe the phase modulation (PM) degree of depth, by feeler arm flashlight that returns and the reference light that returns by reference arm, again through second fiber coupler; Merge together,, received by photodetector because of the formed interference light of interference effect; This photodetector is converted into electric signal to the interference light intensity signal, and this electric signal comprises the fundamental frequency and the higher hamonic wave signal of transducing signal, carrier wave, and this electric signal is behind a simulation amplifilter; This simulating signal I (t) externally exports through test port, and simultaneously, I (t) is sent to the modulus converter A/D of microprocessor; Carry out the time discretization sampling, obtain its digital signal sequences I (nT S), n is a positive integer, and is kept at the storage unit RAM of microprocessor, microprocessor is through its each algorithm software module of operation, to this digital signal sequences I (nT S) handle calculating, obtain the phase place transducing signal, and pass through the I/O mouth of microprocessor, phase place heat transfer agent correspondence with foreign country is exported.
Further improvement as the utility model; In the reference arm of described interferometer, adopt one section compensated optical fiber to increase the fiber lengths of reference arm, poor with the fiber lengths that compensates between itself and the feeler arm; Feeler arm is equated with the reference arm light path as far as possible; Keep equilibrium state, can reduce noise dramatically like this, improve the signal to noise ratio (S/N ratio) of interferometer.
As the further improvement of the utility model, to the phase-modulator of said interferometer, frequency is ω in addition CThe cosine carrier driving voltage, get carrier frequency ω CFar above transducing signal
Figure BSA00000521659300031
Frequencies omega SensorLike this, help the differentiation of transducing signal and carrier signal, be convenient to follow-up Digital Down Convert and handle.And
Figure BSA00000521659300032
The influence that changed by external physical quantity,
Figure BSA00000521659300033
Generally not high to time rate of change; As carrier frequency ω CWhen electing tens KHz as,
Figure BSA00000521659300034
Signal frequency ω Sensor, relative carrier frequency ω CEnough little.
As the further improvement of the utility model, described simulation amplifilter is made up of with low-pass filter circuit the signal amplification, except that the signal amplification, can guarantee that its output signal I (t) does not contain carrier frequency ω CHarmonic component more than 10 times, its purpose further limits the signal bandwidth of I (t), for the sampling of follow-up discretize creates favorable conditions.
As the further improvement of the utility model, described modulus converter A/D, to interference light total intensity signal, carrying out the sampling period is T sTime discretization sampling, sampling period T wherein s, satisfy:
T S = 1 f S = 1 20 ω C 2 π = π 10 ω C ,
Because under the effect of simulation amplifilter, filtered signal I (t) does not contain carrier frequency ω CHarmonic component more than 10 times, so the highest frequency of I (t) is no more than 10 ω CAccording to the Nyquist sampling theory, as long as SF f S ( f S = 1 T S ) , Satisfy: f S = 2 * 10 ω C 2 π = 10 ω C π ,
By the discrete series signal I (nT after the sampling S), can undistorted recovery original analog I (t).
As the further improvement of the utility model, the algorithm software of said microprocessor comprises with lower module, Digital Down Convert, phase place heat transfer agent, digital local oscillation signal generating module.
As the further improvement of the utility model, said Digital Down Convert algorithm comprises the following steps:
Step 1: will interfere digital signal sequences I (nT S) with frequency be 2 ω CDigital local oscillation signal NCO1 multiply each other NCO1=cos (2 ω wherein CNT S),
Step 2: with step 1 gained result, be wave digital lowpass filter LPF and handle, and multiplication by constants C1, after, C1=1/J 2(M c), J 2(M C) be the 2nd grade of Bessel function, obtain the in-phase component I of signal I(nT S):
Figure BSA00000521659300041
Step 3: will interfere digital signal sequences I (nT S) and frequencies omega CDigital local oscillation signal NCO2 multiply each other, wherein, NCO2=cos (ω CNT S),
Step 4: with step 3 gained result, be wave digital lowpass filter LPF and handle, and behind the multiplication by constants C2, C2=-1/J 1(M c), J 1(M C) be the 1st grade of Bessel function, obtain the quadrature component I of signal Q(nT S):
Figure BSA00000521659300042
Therefore, the essence of Digital Down Convert is to interfere digital signal sequences I (nT S) do mixing with digital local oscillation signal NCO1, NCO2 respectively, utilize transducing signal, carrier signal otherness in frequency domain, through low-pass filter, the in-phase component that independently obtains
Figure BSA00000521659300043
Quadrature component
Figure BSA00000521659300044
Further improvement as the utility model; After in-phase component
Figure BSA00000521659300045
quadrature component
Figure BSA00000521659300046
that independently obtains; Said microprocessor calculates phase place transducing signal calculating
Figure BSA00000521659300047
by following formula
(1) works as I I(nT S)＞0, I Q(nT S)＞0 o'clock,
Figure BSA00000521659300048
(2) work as I I(nT S)＜0, I Q(nT S)＞0 o'clock,
Figure BSA00000521659300049
(3) work as I I(nT S)＜0, I Q(nT S)＜0 o'clock,
Figure BSA000005216593000410
(4) work as I I(nT S)＞0, I Q(nT S)＜0 o'clock,
Figure BSA00000521659300051
As the further improvement of the utility model, said frequencies omega CDigital local oscillation signal
NCO2=cos (ω CNT S), by the ROM of the microprocessor generation of tabling look-up, its frequency 2 ω CThe numeral this
Signal NCO1=cos (2 ω shake CNT S) then by digital local oscillation signal NCO2, be calculated as follows: NCO1=cos (2 ω CNT S)=2cos 2(ω CNT S)-1
；
＝2*NCO2*NCO2-1
The benefit of doing like this is, both saved the storage space that guarantees the ROM of microprocessor, can guarantee the phase place absolute synchronization between digital local oscillation signal NCO2 and the NCO1 again.
[description of drawings]
Fig. 1 is the utility model interferometric optical fiber sensor digital demodulating apparatus structural drawing;
Fig. 2 is the utility model Digital Down Convert procedure chart;
Fig. 3 is the front panel figure of the utility model interferometric optical fiber sensor digital demodulating apparatus;
Fig. 4 is the rear panel figure of the utility model interferometric optical fiber sensor digital demodulating apparatus.
[embodiment]
Below in conjunction with description of drawings and embodiment the utility model is further specified.
The principle of work of this device is, when the sense light and the reference light of interferometric optical fiber sensor interferes, the interference light total intensity of its output is I (t), and I (t) satisfies following cosine function relational expression:
Figure BSA00000521659300052
Wherein, A and B represent the direct current context parameter and the contrast level parameter of interference light respectively; is the phase differential between interferometer reference arm and the signal arm, and
Figure BSA00000521659300054
is the phase place transducing signal.If introduce phase-modulator in the interferometer reference path, to this phase-modulator, in addition frequency is ω CThe cosine carrier driving voltage, make additional variation of phase place of reference light M CThe phase modulation (PM) degree of depth.Then (1) formula changes into:
Figure BSA00000521659300056
(2) formula can be used the Bessel functional expansion:
Figure BSA00000521659300061
Adopt photodetector to measure above interference light total intensity I (t), and with the signal of photodetector output through an analogue low pass filtering, be used for filtering I T(t) the carrier frequency harmonic component in more than 10 times.The I after analogue low pass filtering then T(t), available following formula is represented:
Figure BSA00000521659300062
Above signal I T(t) the contained highest frequency of frequency spectrum is 10 ω C
To above total intensity signal I (t), carrying out the sampling period is T with A/D converter sDiscretize sampling, obtain its digital signal sequences I (nT S):
Figure BSA00000521659300063
Digital signal sequences I (nT with the expression of (4) formula S) and frequency 2 ω CDigital local oscillation signal NCO1=cos (2 ω CNT S) multiply each other:
Figure BSA00000521659300064
Figure BSA00000521659300065
Figure BSA00000521659300066
Because get carrier frequency ω CFar above the transducing signal frequency, so signal
Figure BSA00000521659300067
Relative cos (2k ω CNT s), cos ((2k+1) ω CNT sIt is low frequency signal.
Then following formula obtains through wave digital lowpass filter LPF:
Figure BSA00000521659300071
Here [] LPFExpression is carried out LPF to the amount in the bracket;
In like manner, I T(nT S) and frequencies omega CDigital local oscillation signal NCO2=cos (ω CNT S) multiply each other:
Figure BSA00000521659300072
Figure BSA00000521659300074
Following formula obtains through wave digital lowpass filter LPF:
Figure BSA00000521659300075
Again respectively with the right multiplication by constants C1=1/J of (5), (6) formula 2(M c), constant C 2=-1/J 1(M c) after, obtain the in-phase component I of signal I(nT S), quadrature component I Q(nT S):
Figure BSA00000521659300076
(7)
Figure BSA00000521659300077
So far, the work that obtains below in-phase component and the quadrature component
Figure BSA00000521659300079
of phase place transducing signal is exactly to calculate the following four kinds of situations of phase place
Figure BSA000005216593000710
branch according to the mathematics triangle formula:
(1) works as I I(nT S)＞0, I Q(nT S)＞0 o'clock,
Figure BSA000005216593000711
Be complex plane coordinate first quartile:
Figure BSA000005216593000712
(2) work as I I(nT S)＜0, I Q(nT S)＞0 o'clock,
Figure BSA000005216593000713
Be complex plane coordinate second quadrant:
Figure BSA000005216593000714
(3) work as I I(nT S)＜0, I Q(nT S)＜0 o'clock,
Figure BSA000005216593000715
Be complex plane coordinate third quadrant:
Figure BSA000005216593000716
(4) work as I I(nT S)＞0, I Q(nT S)＜0 o'clock, Be complex plane coordinate four-quadrant:
Figure BSA00000521659300081
The operation of this device may further comprise the steps:
1,, selects the length of sensor fibre at first according to application scenarios;
2, according to the length of sensor fibre, select the length of compensated optical fiber, the feeler arm of interferometer is equated with the reference arm light path as far as possible, keep equilibrium state;
3, use device button is roughly selected carrier frequency ω C, phase modulation (PM) degree of depth M CReach parameters such as traffic rate;
4, by the external simulating signal I (t) of output of device to test port, adopt oscillograph observation I (t), according to waveform and the sharpness of simulating signal I (t), can differentiate laser instrument, whether interferometer is in proper working order, and roughly selecting carrier frequency ω C, phase modulation (PM) degree of depth M COn the basis, fine tuning ω C, M CMake the signal to noise ratio (S/N ratio) of interferometer best;
5, in a single day setting finishes, but button writes down set parameter;
6, button gets into normal operating conditions, and its communication port will send always, heat transfer agent.
Specific embodiment:
A kind of interferometric optical fiber sensor digital demodulating apparatus; It comprises LASER Light Source, interferometer, photodetector, simulation amplification filtering circuit, driving circuit, microprocessor; Described interferometer comprises first fiber coupler, sensor fibre, optical fiber type phase-modulator, second fiber coupler; Said sensor fibre constitutes the feeler arm of interferometer; The optical fiber type phase-modulator constitutes the reference arm of interferometer, also comprises compensated optical fiber in the reference arm of said interferometer, and compensated optical fiber and optical fiber type phase-modulator constitute the reference arm of interferometer.Described simulation amplifilter is made up of with low-pass filter circuit the signal amplification.From the light wave of LASER Light Source, at first get into first fiber coupler, be divided into two-way by it; One road light gets into sensor fibre, forms flashlight, and another road gets into the optical fiber type phase-modulator; Form reference light, the built-in modulus converter A/D of described microprocessor, digital to analog converter D/A, microprocessor is through digital to analog converter D/A, driving circuit; To said optical fiber type phase-modulator, frequency is ω in addition CThe cosine carrier driving voltage, make additional variation of phase place of reference light
Figure BSA00000521659300082
ω wherein CBe the phase modulation (PM) degree of depth, the reference light after flashlight and the phase change is again through second fiber coupler; Merge together,, received by photodetector because of the formed interference light of interference effect; This photodetector is converted into electric signal to the interference light intensity signal, and electric signal comprises the fundamental frequency and the higher hamonic wave signal of transducing signal, carrier wave, and this electric signal is behind a simulation amplifilter; This simulating signal I (t) externally exports through test port, and simultaneously, I (t) is sent to the modulus converter A/D of microprocessor; Carry out the time discretization sampling, obtain its digital signal sequences I (nT S), n is a positive integer, and is kept at the storage unit RAM of microprocessor, microprocessor is through its each algorithm software module of operation, to this digital signal sequences I (nT S) handle calculating, obtain the phase place transducing signal, and pass through the I/O mouth of microprocessor, phase place heat transfer agent correspondence with foreign country is exported.The algorithm software of said microprocessor comprises with lower module, Digital Down Convert, and the phase place transducing signal calculates, digital local oscillation signal generating module.
To phase-modulator, frequency is ω in addition CThe cosine carrier driving voltage, get carrier frequency ω CFar above the transducing signal frequencies omega Sensor
Described modulus converter A/D, it is T that interference light total intensity signal is carried out the sampling period STime discretization sampling, sampling period T wherein S, satisfy:
T S = 1 f S = 1 20 ω C 2 π = π 10 ω C
Said Digital Down Convert algorithm comprises the following steps:
Step 1: will interfere digital signal sequences I (nT S) with frequency be 2 ω CDigital local oscillation signal NCO1 multiply each other NCO1=cos (2 ω wherein CNT S),
Step 2: with step 1 gained result, be wave digital lowpass filter LPF and handle, and multiplication by constants C1, after, C1=1/J 2(M c), J 2(M C) be the 2nd grade of Bessel function, obtain the in-phase component I of signal I(nT S):
Figure BSA00000521659300092
Step 3: will interfere digital signal sequences I (nT S) and frequencies omega CDigital local oscillation signal NCO2 multiply each other, wherein, NCO2=cos (ω CNT S),
Step 4: with step 3 gained result, be wave digital lowpass filter LPF and handle, and behind the multiplication by constants C2, C2=-1/J 1(M c), J 1(M C) be the 1st grade of Bessel function, obtain the quadrature component I of signal Q(nT S):
Figure BSA00000521659300093
Calculate phase place transducing signal calculating
Figure BSA00000521659300094
(1) works as I I(nT S)＞0, I Q(nT S)＞0 o'clock,
(2) work as I I(nT S)＜0, I Q(nT S)＞0 o'clock,
(3) work as I I(nT S)＜0, I Q(nT S)＜0 o'clock,
Figure BSA00000521659300103
(4) work as I I(nT S)＞0, I Q(nT S)＜0 o'clock,
Figure BSA00000521659300104
Said frequencies omega CDigital local oscillation signal NCO2=cos (ω CNT S), by the ROM of the microprocessor generation of tabling look-up, its frequency 2 ω CDigital local oscillation signal NCO1=cos (2 ω CNT S) then by digital local oscillation signal NCO2, be calculated as follows:
NCO1＝cos(2ω CnT S)＝2cos 2(ω CnT S)-1
＝2*NCO2*NCO2-1
Above content is the further explain that combines concrete preferred implementation that the utility model is done, and can not assert that the practical implementation of the utility model is confined to these explanations.For the those of ordinary skill of technical field under the utility model, under the prerequisite that does not break away from the utility model design, can also make some simple deduction or replace, all should be regarded as belonging to the protection domain of the utility model.

Claims (6)

1. interferometric optical fiber sensor digital demodulating apparatus, it is characterized in that: it comprises LASER Light Source, interferometer, photodetector, simulation amplification filtering circuit, driving circuit, microprocessor, and described interferometer comprises first fiber coupler, sensor fibre, optical fiber type phase-modulator, second fiber coupler; Said sensor fibre constitutes the feeler arm of interferometer, and the optical fiber type phase-modulator constitutes the reference arm of interferometer, from the light wave of LASER Light Source; At first get into first fiber coupler; Be divided into two-way by it, one road light gets into sensor fibre, forms flashlight; Another road gets into the optical fiber type phase-modulator; Form reference light, the built-in modulus converter A/D of described microprocessor, digital to analog converter D/A, microprocessor is through digital to analog converter D/A, driving circuit; To said optical fiber type phase-modulator, frequency is ω in addition CThe cosine carrier driving voltage, make additional variation of phase place of reference light
Figure DEST_PATH_FSB00000718916900011
M wherein CBe the phase modulation (PM) degree of depth, the reference light after flashlight and the phase change is again through second fiber coupler; Merge together,, received by photodetector because of the formed interference light of interference effect; This photodetector is converted into electric signal to the interference light intensity signal, and this electric signal is behind a simulation amplifilter, and this simulating signal I (t) externally exports through test port; Simultaneously; I (t) is sent to the modulus converter A/D of microprocessor, carries out the time discretization sampling, obtains its digital signal sequences I (nT S), n is a positive integer, and is kept at the storage unit RAM of microprocessor, microprocessor is through its each algorithm software module of operation, to this digital signal sequences I (nT S) handle calculating, obtain the phase place transducing signal, and pass through the I/O mouth of microprocessor, phase place heat transfer agent correspondence with foreign country is exported.
2. interferometric optical fiber sensor digital demodulating apparatus according to claim 1 is characterized in that, also comprises compensated optical fiber in the reference arm of said interferometer, and compensated optical fiber and optical fiber type phase-modulator constitute the reference arm of interferometer.
3. interferometric optical fiber sensor digital demodulating apparatus according to claim 1 is characterized in that, to phase-modulator, frequency is ω in addition CThe cosine carrier driving voltage, get carrier frequency ω CFar above the transducing signal frequencies omega Sensor
4. interferometric optical fiber sensor digital demodulating apparatus according to claim 1 is characterized in that, described simulation amplifilter is made up of with low-pass filter circuit the signal amplification.
5. interferometric optical fiber sensor digital demodulating apparatus according to claim 1 is characterized in that: described modulus converter A/D, it is T that interference light total intensity signal is carried out the sampling period STime discretization sampling, sampling period T wherein S, satisfy:
Figure DEST_PATH_FSB00000718916900021
6. interferometric optical fiber sensor digital demodulating apparatus according to claim 1 is characterized in that: said electric signal comprises the fundamental frequency and the higher hamonic wave signal of transducing signal, carrier wave.
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