CN1831485A - Cavity length demodulating algorithm of fibre-optical F-P sensor - Google Patents

Abstract

A demodulation - algorithm of optical fiber F-P transducer cavity length includes calculating out a group of ideal spectrum sequence for reasonable estimation element according to transfer function of optical fiber F-P sensing head, calculating out a group of mean - square error value according to calculated out ideal spectrum sequence and measured spectrum sequence, using estimation element corresponding to minimum mean - square error as optimum estimation element of actual cavity length.

Description

A kind of cavity length demodulating algorithm of fibre-optical F-P sensor

Technical field

The invention belongs to optical fiber communication and technical field of optical fiber sensing.Relate to a kind of cavity length demodulating algorithm general, high-precision Wavelength demodulation type optical fibre Fabry-perot (F-P) sensor, specially refer to the demodulating algorithm of the F-P chamber type Fibre Optical Sensor in the technical field of optical fiber sensing.

Background technology

1988, people such as Lee successfully made the sensor based on optical fiber Intrinsical F-P first; 1991, people such as Murphy were developed into the fiber optic sensing device based on extrinsic type F-P chamber.Because it is little that the fiber F-P sensing head has a volume, simple in structure, highly sensitive, make flexibly, be not subjected to advantage such as electromagnetic interference (EMI) and obtained development fast, and be widely used in sensing measurement under the rugged surroundings such as high temperature, high pressure and strong electromagnetic, such as the health monitoring of intellectual material and heavy construction structure, high temperature, high-tension measurement under the dark oil well, the infrasonic wave of nuclear explosion test is measured, the temperature of large scale electrical power unit/electric breakdown monitoring, the pressure of petrochemical complex industry, temperature survey etc.

In the fiber F-P sensing measurement system, cavity length demodulating is the important component part of whole measuring system, and demodulating algorithm directly affects the resolution of system, stability and accuracy of measurement.At present the long demodulation method in fibre-optical F-P sensor chamber is comprised intensity demodulation, phase demodulating and Wavelength demodulation.The intensity demodulation method has fast, the simple in structure and low cost and other advantages of response, but, the mechanism of intensity demodulation changes because being the light intensity that detects a certain sideband of interference fringe, therefore it is limited to measure dynamic range, manufacture craft to sensing head is strict, be subjected to simultaneously the influence of light source power fluctuation and fiber transmission attenuation easily, must take indemnifying measure.Peak Search Method in the Wavelength demodulation then is that the method by the bimodal or multimodal of following the tracks of F-P cavity reflection spectrum realizes demodulation, and demodulation accuracy depends on the accuracy to the interference peak peak-seeking.Fourier transform is a kind of method that often adopts in the signal analysis, adopts the fast Fourier transform (FFT) method can eliminate the gross error that exists in the peak-peak algorithm, but has the low problem of resolution equally.The long conversion of discrete cavities (DGT) can be eliminated the hypothesis to the even equal interval sampling of light frequency in the FFT demodulating algorithm, so demodulation accuracy is higher than the FFT method, but because the envelope broad of its peak value, so the peak-seeking error is big, operand is also very big simultaneously.People such as ClaudeBelleville in 1993 have proposed the simple crosscorrelation demodulating algorithm of fibre-optical F-P sensor, the cross-correlation coefficient of the desirable F-P spectrum that the measurement spectrum by calculating actual fiber F-P sensor and a series of trials chamber are long, searching is long as actual chamber corresponding to the peaked trial of cross-correlation coefficient chamber length, can realize long measurement the in absolute chamber of high-resolution.

Summary of the invention

The objective of the invention is, a kind of cavity length demodulating algorithm of fibre-optical F-P sensor is provided, can realize high-resolution absolute cavity length demodulating, thereby obtain high-resolution measurand information.

Technical scheme of the present invention is, adopts square error that the quality of the long estimator of fiber F-P sensing head cavity is estimated, and the chamber that obtains actual fiber F-P chamber sensing head measure spectrum is long.

In the parametric statistics theory, we generally wish the estimator of true value θ Be no inclined to one side, i.e. its deviation $\mathrm{bias}\left(\widehat{\mathrm{\θ}}\right)=0,$ Wish its variance simultaneously

As much as possible little.Square error then be defined as estimator and true value error square mathematical expectation, promptly

$\mathrm{mse}=E\left\{{(\widehat{\mathrm{\θ}}-\mathrm{\θ})}^2\right\}=\mathrm{var}\left(\widehat{\mathrm{\θ}}\right)+{\mathrm{bias}}^2\left(\widehat{\mathrm{\θ}}\right)---\left(1\right)$

As can be seen, square error has comprised deviation and the caused error of variance, so square error is to estimate extraordinary means of an estimator quality.If a series of estimator is arranged for some given true value, then we think estimator with least mean-square error than other estimator more near true value.The present invention adopts square error that the quality of the long estimator of fiber F-P sensing head cavity is estimated, and can realize the long accurate demodulation in chamber of fibre-optical F-P sensor.Below be example with the reflectance spectrum of the extrinsic fiber F-P sensing head of low fineness, algorithm of the present invention is elaborated.

The reflectance spectrum of the enhanced extrinsic F-P optical fiber sensing head of low fineness has the transport function of following form:

$\mathrm{FP}\left(\mathrm{\λ}\right)=2R\·(1+v\·\cos (\frac{4\mathrm{\πd}}{\mathrm{\λ}}+\mathrm{\π}))---\left(2\right)$

Wherein, R is the Fresnel reflection rate of air and optical fibre interface, and ν is the contrast of interference fringe, and d is that the chamber in sensor F-P chamber is long, and π is because the half-wave loss that light is produced by the optically denser medium reflex time from optically thinner medium under the situation of normal incidence.

Long to a chamber is d ₀The actual measurement reflectance spectrum according to the wavelength sequential sampling after, the spectral sequence that obtains dispersing, element in the sequence is with x (n, d ₀) expression, wherein n is the ordinal number of sampling element.Suppose that d is d ₀An estimator, then the ideally-reflecting in (2) formula spectrum is also carried out same discrete processes, the element in this sequence with FP (n, d) expression:

$\mathrm{FP}(n,d)=2R\·(1+v\·\cos (\frac{4\mathrm{\πd}}{{\mathrm{\λ}}_0+n\·\mathrm{\Δ\λ}}+\mathrm{\π}))---\left(3\right)$

Then we can obtain the long estimator d in chamber and its true value d ₀Square error:

$\mathrm{mse}\left(d\right)=\frac{1}{N}\·\underset{n=0}{\overset{N-1}{\mathrm{\Σ}}}{(x(n,d_0)-\mathrm{FP}(n,d))}^2---\left(4\right)$

Can be in the reality according to needed cavity length demodulating precision and arithmetic speed, select the scope and the step-length of a long estimator in rational chamber, can obtain a series of square mean error amount { mse (d) } by following formula, wherein the pairing estimator d of least mean-square error is exactly the long d in actual chamber ₀Optimum estimate.

30 minutes the long resolution test result in chamber that accompanying drawing 1 has provided the fiber F-P sensing head under free state, its average cavity is long to be 303.92102um, variance is 0.08859nm, if the twice that adopts standard deviation is as the long resolution in chamber, then the long resolution in the chamber of this fiber F-P sensing head can reach 0.18nm.Test condition: light spectrum detecting apparatus is SI720, and spectral range is 80nm, and sequence length N is 2000, and the step-length of the long estimator in chamber is 0.075nm.

Adopt cavity length demodulating resolution of the present invention not only to depend on the step-length of estimator, also improve simultaneously along with the broadening of the effective spectral range of light source.

Effect of the present invention and benefit are, adopt the sensor-based system of demodulating algorithm of the present invention to have the long resolution height in chamber, dynamic range is big and advantage such as absolute measurement, can realize simultaneously the high resolving power demodulation of the series/parallel multiplex system of fibre-optical F-P sensor, can realize the multiplexing demodulation of connecting of fibre-optical F-P sensor and bragg grating (FBG) or long period fiber grating (LPFG), in the health monitoring of heavy construction structure, aspects such as high temperature, high-tension measurement and large scale electrical power unit monitoring have important application value under the oil well.

Description of drawings

Accompanying drawing 1 is the fiber F-P sensing head under free state 30 minutes the long resolution test in chamber figure as a result.

Accompanying drawing 2 has provided an optical fiber F-P pressure sensor embodiment synoptic diagram of the present invention.

1 is spectra collection equipment among the figure, and for the present embodiment employing is the SI720 of U.S. Micron Optics Inc..The 2nd, extrinsic fiber F-P sensing head, the 3rd, single-mode fiber, the 4th, comprise the computing machine of demodulation operation program.

Accompanying drawing 3 is based on the optical fiber F-P pressure sensor calibration curve figure of demodulating algorithm of the present invention.

Embodiment

Describe most preferred embodiment of the present invention in detail below in conjunction with technical scheme and accompanying drawing.

Accompanying drawing 2 has provided the fiber F-P pressure-measuring system that is applicable to high-tension measurement under the oil well.Spectra collection equipment in 1 adopts the SI720 of Micron Optics Inc. herein, and its wavelength coverage is 1510～1590nm.The narrow band light of scanned laser output is transferred to enhanced extrinsic F-P sensing head 2 by single-mode fiber 3 from SI720, and its reflectance spectrum sends computing machine to by communication interface after being received by SI720, carries out the long computing in chamber by demodulation process of the present invention.

The DPI610 pressure calibration instrument that adopts Druck company in the experiment is in the fine F-P sensing head calibration of the scope interior focusing of 0～31MPa.Accompanying drawing 3 has provided the pressure calibration curve of this sensing head.In the pressure range of 31MPa, the chamber length of fiber F-P sensing head has changed 1.84um.The degree of correlation of the linear fit of this calibration curve can reach 0.99999, and pressure sensitivity is 59.19nm/MPa, calculates according to the long resolution in the chamber of 0.18nm, and then the pressure resolution of this sensing head can reach 3.04kPa, less than 0.01% of full scale.If adopt the bigger light source of effective spectral width, perhaps reduce the step-length of the long estimator in chamber, then the long resolution in the chamber of system can also further improve.

Claims (1)

1. the cavity length demodulating algorithm of a fibre-optical F-P sensor, be to adopt square error to estimate the spectrum that receives from the fiber F-P sensing head is carried out high-precision absolute cavity length demodulating, it is characterized in that adopting square error that the quality of the long estimator of fiber F-P sensing head cavity is estimated, at first according to the transport function of fiber F-P sensing head, obtain the desired light spectral sequence of one group of long estimator of actual chamber, spectral sequence with the fiber F-P sensing head of measuring calculates then, draw one group of square mean error amount, wherein the long estimator in the pairing chamber of Zui Xiao square error is exactly long optimum estimate in measure spectrum chamber.

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