CN104535094A - Optical fiber Brillouin spectrum fitting method for pulse light - Google Patents

Abstract

The invention provides an optical fiber Brillouin spectrum fitting method for pulse light. A Brillouin time domain reflectometer sensing measurement system is adopted for measuring a Brillouin spectrum frequency domain sampling value of sensing optical fiber. Fitting is carried out on the measured Brillouin spectrum frequency domain sampling value by adopting functions in the following formula (1), and please see the formula in the specification. The method can improve the detection precision of Brillouin frequency shift, and three parameters of phonon attenuation coefficients in the optical fiber, Brillouin frequency shift of the optical fiber and coherence time of the scattered pulse light of the optical fiber can be obtained at the same time.

Description

A kind of optical fiber Brillouin spectrum approximating method of pulsed light

Technical field

The present invention relates to a kind of optical fiber Brillouin spectrum approximating method of pulsed light, belong to technical field of optical fiber sensing.

Background technology

Owing to having, anti-electromagnetic interference capability is strong, equipment is easy to installation, good insulating, sensitivity advantages of higher for optical fiber sensing technology, receives increasing attention in fields such as electric power, building, civil engineering works.Brillouin light domain reflectometer (Brillouin Optical Time DomainReflectometry, BOTDR) carries out the measurement of temperature and strain by the centre frequency shift measuring incident light pulse spontaneous brillouin scattering spectrum in a fiber.Spontaneous brillouin scattering light in optical fiber is divided into stokes light and anti-Stokes light, and Brillouin spectrum described in the present invention represents the power spectrum signal of any one and light source Heterodyne detect in stokes light or anti-Stokes light.Under the incidence of continuous single-frequency light, the stokes spectrum of spontaneous brillouin scattering and anti-Stokes spectrum all distribute in Lorenz function, the low f of center frequency ratio incident light frequency of stokes spectrum _b, the center frequency ratio incident light frequency height f of anti-Stokes spectrum _b, f _bbe called as Brillouin shift, the temperature at itself and fibre scattering point place and strain all linear, therefore, measure f _b, temperature or the strain at fibre scattering point place can be measured.BOTDR technology adopts pulsed light to be incident light, and pulsed light is in a fiber in traveling process, and constantly produce backward spontaneous brillouin scattering, BOTDR utilizes optical time domain reflection principle, by detecting Brillouin shift f _balong the distribution of optical fiber, the temperature along optical fiber or Strain Distribution can be measured.Therefore Brillouin shift f is detected _bfor one of the gordian technique of BOTDR.

At present, to optical fiber Brillouin frequency displacement f _bdetection, obtain by carrying out Lorenz function matching to Brillouin spectrum.Namely pass through frequency sweeping at equal intervals, measure Brillouin spectrum sampled value, then carry out matching with Lorenz function to the frequency domain sample value measured, the frequency that the Lorenz function central point simulated is corresponding is Brillouin shift f _b.The method is simple, is easy to realize, and is therefore widely used.But when the method is applied to the optical fiber Brillouin spectrum matching of pulsed light, there is the problem of fitting precision difference, although the spontaneous Brillouin spectrum of single-frequency (or narrow linewidth) continuous light is Lorenz function distribution, but the Brillouin spectrum of pulsed light is not Lorenz function distribution, reason is that the frequency spectrum of pulsed light is relative to broadened continuous light, and the actual Brillouin spectrum obtained is also just corresponding broadened.Therefore adopt Lorentz lorentz's matching can not obtain high-precision fitting result simply, also just can not obtain Brillouin shift f _band the high precision test result of fiber optic temperature or strain.Lorentz fit only can obtain Brillouin shift in addition, can not obtain parameter coherence time of acoustic attenuation coefficient in optical fiber and scattering pulse light.

Summary of the invention

The object of the invention is to the drawback for prior art, a kind of optical fiber Brillouin for pulsed light spectrum approximating method is proposed, when improving the fitting precision of Brillouin spectrum, three parameters coherence time of the acoustic attenuation coefficient in optical fiber, Brillouin shift and scattering pulse light can be obtained simultaneously.

Problem of the present invention solves by with following technical proposals:

1. adopt Brillouin light domain reflectometer (BOTDR) sensing measurement system to record Brillouin spectrum frequency domain sample the value { (f of sensor fibre _k, S _k), k=1,2 ... N}, N are sampling number, f _kfor the Scanning Detction frequency of Brillouin spectrum, S _kfor corresponding frequency f _kthe Brillouin spectrum sampled value at place, sample frequency scope (Δ f=f _n-f ₁) be at least the Brillouin spectrum half value overall with (i.e. three dB bandwidth) of 3 times, and make Brillouin spectrum crest frequency be positioned at sample frequency scope central area, guarantee that sampled value includes a complete three dB bandwidth Brillouin spectrum sample value.

2. pair survey Brillouin spectrum sampled value { (f _k, S _k), k=1,2 ... N} adopts following (1) formula function to carry out matching,

$\begin{array}{c}{S}_{\mathrm{SP}}\left(f\right)A\frac{\mathrm{\Γ}}{{(\mathrm{\Γ}/2)}^2+{4\mathrm{\π}}^2{\left({f-f}_B\right)}^2}+A\frac{{2e}^{-\mathrm{\Γ}\·/2}[{(\mathrm{\Γ}/2)}^2-{-4\mathrm{\π}}^2{\left({f-f}_B\right)}^2]\cos \left[2\mathrm{\π}\left({f-f}_B\right)T\right]}{{T[{(\mathrm{\Γ}/2)}^2+{4\mathrm{\π}}^2{\left({f-f}_B\right)}^2]}^2}\\ -A\frac{{4\mathrm{\πe}}^{-\mathrm{\Γ}\·T/2}\mathrm{\Γ}\left({f-f}_B\right)\sin \left[2\mathrm{\π}\left({f-f}_B\right)T\right]+2[{(\mathrm{\Γ}/2)}^2-{4\mathrm{\π}}^2{\left({f-f}_B\right)}^2]}{{T[{(\mathrm{\Γ}/2)}^2+{4\mathrm{\π}}^2{\left({f-f}_B\right)}^2]}^2}\\ ---\left(1\right)\end{array}$

Wherein A, Γ, f _bwith T for treating Fitted parameter, A is range coefficient, Γ is phonon damping coefficient, f _bfor Brillouin shift, T is the coherence time of scattered light pulse, and f is the stokes light of Brillouin scattering or the difference of anti-Stokes light and light source frequency.

(1) on the right of formula, Section 1 is Lorenz function, second, third spectrum widening part brought for pulsed light of the right.Levenberg – Marquardt algorithm can be applied or other numerical fitting algorithm carries out matching.

3. A, Γ, f of obtaining after matching being completed _bsubstitute into (1) formula with the value of T, this formula is the Brillouin spectrum of pulsed light, Γ, f _bthe coherence time of phonon damping coefficient, Brillouin shift and scattering pulse light in measured optical fiber is respectively with the value of T.

Above-mentioned Brillouin spectrum sampled value { (f _k, S _k), k=1,2 ... N} adopts infradyne manner to detect by BOTDR system.The spontaneous brillouin scattering stokes light of first paired pulses light or anti-Stokes light and light source carry out Heterodyne detect, then to f each in the electric signal after Heterodyne detect _kthe power of frequency component carries out detection and obtains Brillouin spectrum sampled value { (f _k, S _k), k=1,2 ... N}.

The present invention is owing to taking above technical scheme, and it has the following advantages and useful effect: the error that the error ratio that (1) adopts approximating method of the present invention to produce adopts Lorentz fit to produce is less; (2) method of the present invention can simulate three parameters coherence time of phonon damping coefficient, Brillouin shift and scattering pulse light in optical fiber simultaneously.

Accompanying drawing explanation

Fig. 1 is the Brillouin spectrum sampled value at certain some place on the optical fiber that records of BOTDR system;

Fig. 2 adopts the inventive method to the fitting result of Brillouin spectrum sampled value;

Fig. 3 adopts Lorentzian to the fitting result of Brillouin spectrum sampled value.

Embodiment

Below in conjunction with drawings and Examples, the present invention is described in more detail.

First 1, Brillouin light domain reflectometer (BOTDR) sensing measurement system running parameter be set, such as, incident pulse optical width is 10ns, and the frequency domain measurement scope of Brillouin spectrum is 10.72GHz-11GHz, records Brillouin spectrum 57 frequency domain sample value { (f of sensor fibre _k, S _k), k=1,2 ... 57}, sample frequency interval delta f _k=f _k+1-f _k=5MHz, initial frequency f ₁=10.72GHz, stops frequency f ₅₇=11GHz, sampled value includes a complete three dB bandwidth Brillouin spectrum collection of samples.Brillouin spectrum sampling as shown in Figure 1.

2, to surveyed Brillouin spectrum sampled value { (f _k, S _k), k=1,2 ... 57}, adopts (1) formula function to carry out matching, A, Γ, f _bwith T for treating Fitted parameter, A is range coefficient, Γ is phonon damping coefficient, f _bfor Brillouin shift, T is the coherence time of scattering pulse light, and f is the stokes light of Brillouin scattering or the difference of anti-Stokes light and light source frequency.Application Levenberg – Marquardt algorithm carries out Function Fitting, and result is: A=6.2636 × 10 ⁵, Γ=1.91742 × 10 ⁸, f _b=10.8547GHz, T=11.02ns, error of fitting is 1.496 × 10 ^-6.

3, A, Γ, f of matching being obtained _bsubstitute into (1) formula with the value of T, this formula is the Brillouin spectrum of pulsed light, and the Brillouin spectrum frequency curve that matching obtains as shown in Figure 2.

4, in order to contrast, Lorenz function is adopted $S_l\left(f\right)=A\frac{\mathrm{\Γ}}{{(\mathrm{\Γ}/2)}^2+{4\mathrm{\π}}^2{\left({f-f}_B\right)}^2}$ Carry out matching to same sampled point application Levenberg – Marquardt algorithm, obtaining result is: A=6.8439 × 10 ⁵, Γ=5.1381 × 10 ⁸, f _b=10.8551GHz, error of fitting is 4.519 × 10 ^-6, the Brillouin spectrum S obtained _lf () as shown in Figure 3, compared with Fig. 2, error of fitting increases greatly.

(1) formula proposed due to the inventive method not only contains Lorenz function item, the spectrum widening item that coherence time simultaneously containing scattering pulse light brings, more truly reflecting the frequency domain characteristic of Brillouin spectrum, therefore adopting (1) formula can obtain better fitting precision than only adopting Lorenz function.

Claims (2)

1. an optical fiber Brillouin spectrum approximating method for pulsed light, is characterized in that, said method comprising the steps of:

Steps A: adopt Brillouin light domain reflectometer (BOTDR) sensing measurement system to record Brillouin spectrum frequency domain sample the value { (f of sensor fibre _k, S _k), k=1,2 ... N}, N are sampling number, f _kfor the Scanning Detction frequency of Brillouin spectrum, S _kfor corresponding frequency f _kthe Brillouin spectrum sample value at place, sample frequency scope (Δ f=f _n-f ₁) be at least the Brillouin spectrum half value overall with (i.e. three dB bandwidth) of 3 times, and make Brillouin spectrum crest frequency be positioned at sample frequency scope central area, guarantee that sampled value includes a complete three dB bandwidth Brillouin spectrum sample value;

Step B: to surveyed Brillouin spectrum sampled value { (f _k, S _k), k=1,2 ... N} adopts following formula function to carry out matching,

$\begin{array}{c}{S}_{\mathrm{SP}}\left(f\right)=A\frac{\mathrm{\Γ}}{{(\mathrm{\Γ}/2)}^2+4{\mathrm{\π}}^2{(f-f_B)}^2}+A\frac{2e^{-\mathrm{\Γ}\·T/2}[{(\mathrm{\Γ}/2)}^2-4{\mathrm{\π}}^2{(f-f_B)}^2]\cos \left[2\mathrm{\π}(f-f_B)T\right]}{T{[{(\mathrm{\Γ}/2)}^2+4{\mathrm{\π}}^2{(f-f_B)}^2]}^2}\\ -A\frac{4\mathrm{\π}{e}^{-\mathrm{\Γ}\·T/2}\mathrm{\Γ}(f-f_B)\sin \left[2\mathrm{\π}(f-f_B)T\right]+2[{(\mathrm{\Γ}/2)}^2-4{\mathrm{\π}}^2{(f-f_B)}^2]}{T{[{(\mathrm{\Γ}/2)}^2+4{\mathrm{\π}}^2{(f-f_B)}^2]}^2}\end{array}$

Wherein, A, Γ, f _bwith T for treating Fitted parameter, A is range coefficient, Γ is phonon damping coefficient, f _bfor Brillouin shift, T is the coherence time of scattering pulse light, and f is the stokes light of Brillouin scattering or the difference of anti-Stokes light and light source frequency; On the right of above formula, Section 1 is Lorenz function, the spectrum widening part that on the right of above formula, second and third brings for pulsed light; Levenberg-Marquardt algorithm can be applied or other numerical fitting algorithm carries out matching;

Step C: A, Γ, f of obtaining after matching is completed _bsubstitute into above formula with the value of T, this formula is the Brillouin spectrum of pulsed light, Γ, f _bthe coherence time of phonon damping coefficient, Brillouin shift and scattering pulse light in measured optical fiber is respectively with the value of T.

2. a kind of optical fiber Brillouin of pulsed light composes approximating method according to claim 1, it is characterized in that, described Brillouin spectrum sampled value { (f _k, S _k), k=1,2 ... N} adopts infradyne manner to detect by Brillouin light domain reflectometer (BOTDR) system; During detection, first the spontaneous brillouin scattering stokes light of paired pulses light or anti-Stokes light and light source carry out Heterodyne detect, then to f each in the electric signal after Heterodyne detect _kthe power of frequency component detects, and obtains Brillouin spectrum sampled value { (f _k, S _k), k=1,2 ... N}.