CN102589588A - Method for demodulating cavity length of Fabry-Perot cavity by utilizing fiber Bragg gratings - Google Patents

Abstract

The invention discloses a method for demodulating a cavity length of a multi-beam interference cavity with amplitude-splitting interference by utilizing fiber Bragg gratings. The amplitude-splitting multi-beam interference has a typical structure of Fabry-Perot interference (F-P interference for short). The method comprises the following steps of: forming a tunable light source by utilizing a broadband light source, an isolator, a triangular wave voltage generator and a tunable F-P filter controlled by piezoelectric ceramics, taking two fiber Bragg gratings as reference, and measuring a relationship between the reflected light intensity (or transmitted light intensity) of the fiber Bragg gratings and an F-P sensor and time through a photoelectric detector; and calculating the cavity length of the sensed F-P cavity according to the measured relationship between the reflected light intensity (or transmitted light intensity) of the fiber Bragg gratings and the F-P sensor with time. A high-precision piezoelectric transducer (PZT) is not required, and the cost of the system is obviously reduced; and moreover, the demodulation precision of the demodulation method is hardly influenced by the change of ambient temperature, and the demodulation precision is slightly influenced by local distortion of output waveforms of the sensing F-P cavity.

Description

Utilize the long method in chamber, fiber grating demodulation Fabry – P é rot chamber

Technical field

The present invention relates to a kind of long method in (being called for short the F-P chamber) chamber, multiple-beam interference chamber-Fabry-P é rot chamber of utilizing the fiber grating demodulation amplitude-splittine interference, belong to technical field of optical fiber sensing.

Background technology

Optical fiber sensing technology with its anti-electromagnetic interference (EMI), be applicable to that inflammable and explosive environment, corrosion-resistant, high-insulativity, measurement range are wide, highly sensitive, be convenient to be multiplexed into net, advantage such as Miniaturized; Obtain worldwide extensive concern, become one of technology that development is very fast in the sensory field.Obtained using widely in fields such as civil engineering work, Aero-Space, petrochemical complex, electric power, medical treatment, shipping industrys.For example optical fiber Fabry-P é rot (F-P) type pressure transducer is a kind of important method of measuring fluid pressure now.The variation of hydrodynamic pressure can cause the variation that chamber, F-P chamber is long, thereby the transmitted spectrum in F-P chamber or reflectance spectrum are moved.Chamber according to the transmitted spectrum or the reflectance spectrum in F-P chamber, loading back are calculated the F-P chamber is long, just can obtain the size of hydrodynamic pressure.The long demodulation in chamber, F-P chamber is to realize one of gordian technique of optical fiber Fabry-P é rot type sensing.

Present F-P chamber cavity length demodulating method mainly contains intensity demodulation, phase demodulating etc.The intensity demodulation method only needs monochromatic source, and is very simple, direct, with low cost, but this demodulation method is harsh to the manufacture process requirement in F-P chamber, and measuring accuracy is not high, in practical application, adopts less.The phase demodulating method is used wideband light source, adopts spectroanalysis instrument or adjustable light wave-filter to obtain the transmitted spectrum or the reflectance spectrum in F-P chamber.The spectroanalysis instrument volume is big, price is high, and acquisition speed is slow, generally is not suitable for the F-P sensor-based system of actual through engineering approaches.Adjustable light wave-filter is the device commonly used in the optical fiber communication, and its scanning process generally realizes that through piezoelectric ceramics (PZT) device PZT device sweep velocity is very fast, and size is little, and price is lower with respect to spectrometer, in actual engineering system, often adopts.Non-linear and the hysteresis phenomenon of naked piezoelectric ceramics is serious; Influenced the precision of spectral measurement; In order to guarantee measuring accuracy, adopt high accuracy mechanical packaged type PZT device usually, but high-accuracy mechanical packaged type PZT device price is also very high; This just makes the difficult electricity sensor with extensive employing at present of cost of F-P sensor compete mutually, has influenced the popularization of F-P sensor.

Summary of the invention

The objective of the invention is to propose a kind of tunable optical source that utilizes and combine the long method in chamber, fiber grating demodulation F-P chamber, this method need not high accuracy mechanical packaged type PZT device, can significantly reduce the cost of measuring system, helps the F-P application of sensor and promotes.

In order to realize the foregoing invention purpose, the technical scheme that the present invention adopts is following:

Utilize the long method in chamber, fiber grating demodulation Fabry-P é rot chamber; Comprise the steps: that scan light with the certain frequency scope is coupled in the long sensing Fabry-P é rot chamber, as a reference two fiber gratings and chamber to be measured; Utilize photodetector to measure the output signal in said reference optical fiber grating and sensing Fabry-P é rot chamber; And confirm the periodicity that sensing Fabry-P é rot chamber output intensity changes in certain light frequency scope, long according to the chamber in this periodicity calculating sensing Fabry-P é rot chamber again.

Further scheme of the present invention is: with two fiber gratings as a reference grating be cascaded; The light that wideband light source sends gets into the tunable Fabry-P é rot wave filter by piezoelectric ceramics control through isolator, triangular signal generator basedly applies voltage to said tunable Fabry-P é rot wave filter; The light of said tunable Fabry-P é rot wave filter output is divided into two-way through first coupling mechanism, and second coupling mechanism of leading up to is supplied with two with reference to grating, and sensing Fabry-P é rot chamber is supplied with through the 3rd coupling mechanism in another road; Converted to electric signal output by said two light that reflect with reference to grating through said second coupling mechanism entering photoelectric detector PD 1; The light of being returned by said method sensing Fabry-P é rot cavity reflection converts electric signal output to through said the 3rd coupling mechanism entering photoelectric detector PD 2; At last long by the chamber in the output calculated signals sensing Fabry-P é rot chamber of said photoelectric detector PD 1 and PD2.

Another further technical scheme of the present invention is following: with two fiber gratings grating and be cascaded with sensing Fabry-P é rot chamber as a reference; The light that wideband light source sends gets into the tunable Fabry-P é rot wave filter by piezoelectric ceramics control through isolator, triangular signal generator basedly applies voltage to said tunable Fabry-P é rot wave filter; The light of said tunable Fabry-P é rot wave filter output is supplied with two with reference to grating and sensing Fabry-P é rot chamber through coupling mechanism; Got into photoelectric detector PD by said two light of returning with reference to grating and sensing Fabry-P é rot cavity reflection through said coupling mechanism and convert electric signal output to, long by the chamber in the output calculated signals sensing Fabry-P é rot chamber of said photoelectric detector PD again.

Wherein, said with reference to grating employing Fiber Bragg Grating FBG or long-period gratings.

Method of the present invention can realize the demodulation long to chamber, Fabry-P é rot chamber; Its demodulation accuracy is acted upon by temperature changes hardly; And the local distortion of the sensing F-P chamber output waveform that causes of factor such as wideband light source spectrum unevenness, also less to the demodulation accuracy influence of the inventive method.

Description of drawings

Fig. 1 is the long demodulating system synoptic diagram of F-P sensor cavity of the present invention.

Fig. 2 is the relevant signal waveform synoptic diagram of F-P of the present invention chamber cavity length demodulating system.

Fig. 3 is a F-P of the present invention chamber output waveform local distortion synoptic diagram.

The cavity length demodulating system schematic that Fig. 4 connects with the F-P chamber with reference to grating FBG for the present invention.

Embodiment

Below in conjunction with accompanying drawing and embodiment the present invention is explained further details.

The structure (is example with the optical fiber bragg grating FBG) as shown in Figure 1 of the demodulating system that the chamber, demodulation F-P chamber that the present invention proposes is long, the wideband light source among the figure, isolator, tunable F-P wave filter triangular signal generator based and that controlled by PZT constitute a Wavelength tunable light source.Optical fiber bragg grating FBG 1 and FBG2 be grating as a reference, and the chamber length of F-P sensor is to be measured.Tunable optical source output light is divided into two-way through coupling mechanism Coupler1, and one the road supplies with reference to grating FBG1 and FBG2, and one the road supplies with the F-P sensor.Converted to electric signal output by the light that reflects with reference to grating FBG1 and FBG2 through photoelectric detector PD 1, the light that is reflected by the F-P sensor converts electric signal to through photoelectric detector PD 2 and exports.Start the triangle wave voltage generator; Apply triangle wave voltage to adjustable light wave-filter; Gather FBG and the time dependent curve of light intensity (not providing among data acquisition processing circuit Fig. 1) of sensing F-P cavity reflection, the i.e. I among Fig. 2 of PD1 and PD2 output simultaneously with data acquisition system (DAS) ₁～t and I ₂～t curve.

As long as the chamber of tunable optic filter is considerable enough little, its free spectrum width FSR just can so that in whole tuning range, only have the light output of a wavelength greater than the bandwidth of wideband light source.Satisfy between the chamber of output light wavelength and tunable F-P wave filter is long

λ＝2nd/m，m＝1，2，3，…… (1)

Wherein n is a refractive index, and d is that the chamber is long.If do not consider sluggishness and non-linear (it influences later analysis) of PZT; Displacement-voltage relationship of promptly supposing PZT is linear fully; Then as the input voltage v of PZT in time during the t linear change; Its displacement is linear change in time also, and the output wavelength λ of tunable optical source is t linear change in time also, and is as shown in Figure 2.The light intensity I of photoelectric detector PD 1 and PD2 ₁, I ₂With the relation of time t also shown in Fig. 2, λ wherein _G1And λ _G2It is respectively the reflection wavelength of FBG1 and FBG2.

Also can know by formula (1), to sensing F-P chamber, its I ₂The difference of the light frequency between the adjacent peak value (or valley) of～t curve does

$\mathrm{\&Delta;f}=\frac{\mathrm{<figure-callout\; id="2"\; label="c"\; filenames="HDA0000136309990000011.png,HDA0000136309990000012.png"\; state="\{\{state\}\}">c</figure-callout>}}{{2\mathrm{nd}}_s}---\left(2\right)$

Adjacent peak value and the optical frequency rate variance between the valley are Δ f/2, and wherein c is the speed of light wave, d _sFor the chamber in sensing F-P chamber long.

If the catoptrical frequency with reference to grating FBG1 and FBG2 is f _G1And f _G2, λ _G1And the time interval Δ t ' between adjacent trough ₁Pairing difference on the frequency is Δ f ₁, λ _G2And the time interval Δ t ' between adjacent trough ₂Pairing difference on the frequency is Δ f ₂, see Fig. 2.

Because the output wavelength that tunable optical source has been supposed in the front is linear change in time, and Δ f usually ₁＜＜f _G1Total energy satisfies, so

${\mathrm{\&Delta;f}}_1=\frac{\mathrm{\&Delta;f}}{2}\&CenterDot;\frac{{\mathrm{\&Delta;t}}_1^{\&prime;}}{{\mathrm{\&Delta;t}}_1}---\left(3\right)$

Δ t wherein ₁Be λ _G1The time interval between near adjacent peaks trough.

In like manner ${\mathrm{\&Delta;\; f}}_2=\frac{\mathrm{\&Delta;\; <figure-callout\; id="2"\; label="f"\; filenames="HDA0000136309990000011.png,HDA0000136309990000012.png"\; state="\{\{state\}\}">f</figure-callout>}}{2}\&CenterDot;\frac{{\mathrm{\&Delta;\; <figure-callout\; id="2"\; label="t"\; filenames="HDA0000136309990000011.png,HDA0000136309990000012.png"\; state="\{\{state\}\}">t</figure-callout>}}_2^{\&prime;}}{{\mathrm{\&Delta;\; t}}_2}---\left(4\right)$

Δ t wherein ₂Be λ _G2The time interval between near adjacent peaks trough.

Then $f_{G2}-f_{G1}=(k+\frac{{\mathrm{\&Delta;\; t}}_1^{\&prime;}}{{\mathrm{\&Delta;\; t}}_1}+\frac{{\mathrm{\&Delta;\; <figure-callout\; id="2"\; label="t"\; filenames="HDA0000136309990000011.png,HDA0000136309990000012.png"\; state="\{\{state\}\}">t</figure-callout>}}_2^{\&prime;}}{{\mathrm{\&Delta;\; t}}_2})\&CenterDot;\frac{\mathrm{\&Delta;\; <figure-callout\; id="2"\; label="f"\; filenames="HDA0000136309990000011.png,HDA0000136309990000012.png"\; state="\{\{state\}\}">f</figure-callout>}}{2}$

$\mathrm{\&Delta;f}=\frac{2c}{k+\frac{{\mathrm{\&Delta;t}}_1^{\&prime;}}{{\mathrm{\&Delta;t}}_1}+\frac{{\mathrm{\&Delta;<figure-callout\; id="2"\; label="t"\; filenames="HDA0000136309990000011.png,HDA0000136309990000012.png"\; state="\{\{state\}\}">t</figure-callout>}}_2^{\&prime;}}{{\mathrm{\&Delta;<figure-callout\; id="2"\; label="t"\; filenames="HDA0000136309990000011.png,HDA0000136309990000012.png"\; state="\{\{state\}\}">t</figure-callout>}}_2}}\&CenterDot;(\frac{1}{{\mathrm{\&lambda;}}_{G2}}-\frac{1}{{\mathrm{\&lambda;}}_{G1}})---\left(5\right)$

Wherein k is that sensing F-P chamber is in wavelength X _G1And λ _G2Between the number of semiperiod of changing of light intensity, it is long to obtain the chamber in sensing F-P chamber by formula (2) again

$d_s=\frac{k+\frac{{\mathrm{\&Delta;t}}_1^{\&prime;}}{{\mathrm{\&Delta;t}}_1}+\frac{{\mathrm{\&Delta;<figure-callout\; id="2"\; label="t"\; filenames="HDA0000136309990000011.png,HDA0000136309990000012.png"\; state="\{\{state\}\}">t</figure-callout>}}_2^{\&prime;}}{{\mathrm{\&Delta;<figure-callout\; id="2"\; label="t"\; filenames="HDA0000136309990000011.png,HDA0000136309990000012.png"\; state="\{\{state\}\}">t</figure-callout>}}_2}}{4n}\&CenterDot;\frac{{\mathrm{\&lambda;}}_{G1}{\mathrm{\&lambda;}}_{\mathrm{<figure-callout\; id="2"\; label="G"\; filenames="HDA0000136309990000011.png,HDA0000136309990000012.png"\; state="\{\{state\}\}">G</figure-callout>}2}}{{\mathrm{\&lambda;}}_{G1}-{\mathrm{\&lambda;}}_{G2}}---\left(6\right)$

Can know that by formula (6) the cavity length demodulating result of this kind method and light intensity are irrelevant, have eliminated the influence of the light-intensity variation of light source to demodulation result, also belong to the phase demodulating method.

Discussed above is that wavelength reduces the long method in chamber, demodulation F-P chamber in (descending) process in time, long to the also available similar method demodulation of the up process of wavelength chamber.

The reflection wavelength of fiber grating can change along with variation of temperature, its temperature coefficient K _TApproximately be 7.55 * 10 ^-6/ ℃ (" theory of the fiber optical and application ", Wu Chaoxia, National Defense Industry Press, in March, 2011, first published P42), established that the reflection wavelength with reference to grating FBG1 and FBG2 is λ ' behind the temperature variation Δ T _G1And λ ' _G2, then

$\frac{{\mathrm{\&lambda;}}_{\mathrm{<figure-callout\; id="2"\; label="G"\; filenames="HDA0000136309990000011.png,HDA0000136309990000012.png"\; state="\{\{state\}\}">G</figure-callout>}2}^{\&prime;}{\mathrm{\&lambda;}}_{\mathrm{<figure-callout\; id="2"\; label="G"\; filenames="HDA0000136309990000011.png,HDA0000136309990000012.png"\; state="\{\{state\}\}">G</figure-callout>}2}^{\&prime;}}{{\mathrm{\&lambda;}}_{G1}^{\&prime;}-{\mathrm{\&lambda;}}_{\mathrm{<figure-callout\; id="2"\; label="G"\; filenames="HDA0000136309990000011.png,HDA0000136309990000012.png"\; state="\{\{state\}\}">G</figure-callout>}2}^{\&prime;}}=\frac{{\mathrm{\&lambda;}}_{G1}{\mathrm{\&lambda;}}_{\mathrm{<figure-callout\; id="2"\; label="G"\; filenames="HDA0000136309990000011.png,HDA0000136309990000012.png"\; state="\{\{state\}\}">G</figure-callout>}2}}{{\mathrm{\&lambda;}}_{G1}-{\mathrm{\&lambda;}}_{G2}}(1+K_T\mathrm{\&Delta;T})---\left(7\right)$

Because K _TMinimum, even Δ T=50 ℃, the cavity length demodulating error that causes is also less than 0.04%, and visible variation of ambient temperature is very little to cavity length demodulating result influence.

Do not consider in the derivation of front that PZT's is non-linear and sluggish; But non-linear and sluggish is the build-in attribute of PZT; Long change non-linear and sluggish of the non-linear and sluggish tunable F-P filter cavity that causes by PZT; The monochromatic wavelength linear change in time not that will inevitably make tunable optical source output, and up curve and descending curve do not overlap yet.

Obviously, wavelength nonlinearities change in time can not influence sensing F-P chamber curve of output I ₂The precision of complete semiperiod number k among the～t, but can influence the precision of formula (3), formula (4).For long F-P chamber,

Generally total energy satisfies, so PZT's is non-linear less to the measuring accuracy influence, and at Δ t ₁With Δ t ₂Because the non-linear error that causes is little more a lot of than the nonlinearity erron in the whole sweep limit, sensing F-P chamber is long more, Δ t in the time interval ₁With Δ t ₂Just more little, at Δ t ₁With Δ t ₂Be approximated to linear change in time to the output wavelength of tunable optical source in the time interval, the error that causes is just more little.So in order to improve measuring accuracy, the chamber that in design process, should increase sensing F-P chamber as far as possible is long, just can reduce the requirement to the PZT linearity.

Owing in above-mentioned measuring process, do not need to know in advance displacement-voltage curve of PZT, as long as displacement-voltage curve of PZT linearity as far as possible, so the sluggishness of PZT does not influence the precision of cavity length demodulating.

Because the spectrum of wideband light source can not be very smooth; Will inevitably cause local distortion's (see figure 3) of sensing F-P chamber output waveform; As can beappreciated from fig. 2 the local distortion of F-P chamber output waveform can not cause the variation of k in the formula (6); Only possibly cause the variation of and

; But, just there be

to make the local distortion of F-P chamber output waveform can ignore to the influence of demodulation accuracy as long as the F-P chamber is longer.

If be connected on (see figure 4) on the light path to the long sensing F-P chamber of two Fiber Bragg Grating FBGs and its chamber to be measured as a reference; Measure the reflectance spectrum in two Fiber Bragg Grating FBGs and sensing F-P chamber through photodetector after, it is long also can to demodulate the chamber in sensing F-P chamber with similar algorithms and formula (6).

In Fig. 1,4; If replace optical fiber bragg grating FBG as a reference with long-period gratings; Measure the transmission spectrum (rather than measuring emission spectrum) in the sensing F-P chamber of long-period gratings and its chamber length to be measured through photodetector after, also can use the chamber in formula (6) demodulation sensing F-P chamber long.

Key character of the present invention is to confirm the periodicity that sensing F-P chamber output intensity changes in certain light frequency scope as a reference with two fiber gratings, and then the chamber in demodulation sensing F-P chamber is long.This instructions has only provided Fig. 1 and two kinds of connected modes of Fig. 4 for convenience's sake.If light path is improved, adopt two fiber gratings to come the chamber in demodulation sensing F-P chamber long as a reference but remain on the principle, also belong to protection scope of the present invention.

The formula (6) that the chamber, calculating F-P sensing chamber that more than provides is long; Be not with two fiber gratings long unique algorithm of demodulation F-P sensor cavity as a reference; If only improve to calculating the long formula (6) in chamber, F-P sensing chamber; Adopt two fiber gratings to come the chamber in demodulation F-P sensing chamber long as a reference but remain on the principle, also belong to protection scope of the present invention.

In order to verify the feasibility of said method, with matlab software it is carried out simulating, verifying below.If two reflection wavelengths with reference to grating FBG are 1520nm and 1580nm; The refractive index of sensing F-P chamber medium is 1; Chamber length is 300 δ μ m; Airy function F is 3, and the result of calculation of matlab shows that the chamber length that k=29,

substitution formula (6) calculate is 299.7 μ m.It is thus clear that theory of the present invention and method are correct, and the cavity length demodulating precision is very high.

Claims (3)

1. utilize the long method in chamber, fiber grating demodulation Fabry-P é rot chamber; It is characterized in that comprising the steps: that scan light with the certain frequency scope is coupled in the long sensing Fabry-P é rot chamber, as a reference two fiber gratings and chamber to be measured; Utilize photodetector to measure the output signal in said reference optical fiber grating and sensing Fabry-P é rot chamber; And confirm the periodicity that sensing Fabry-P é rot chamber output intensity changes in certain light frequency scope, long according to the chamber in this periodicity calculating sensing Fabry-P é rot chamber again.

2. the long method in chamber, fiber grating demodulation Fabry-P é rot chamber of utilizing according to claim 1; It is characterized in that; These method concrete steps are following: with two fiber gratings as a reference grating be cascaded; The light that wideband light source sends gets into the tunable Fabry-P é rot wave filter by piezoelectric ceramics control through isolator, triangular signal generator basedly applies voltage to said tunable Fabry-P é rot wave filter; The light of said tunable Fabry-P é rot wave filter output is divided into two-way through first coupling mechanism, and second coupling mechanism of leading up to is supplied with two with reference to grating, and sensing Fabry-P é rot chamber is supplied with through the 3rd coupling mechanism in another road; Converted to electric signal output by said two light that reflect with reference to grating through said second coupling mechanism entering photoelectric detector PD 1; The light of being returned by said sensing Fabry-P é rot cavity reflection converts electric signal output to through said the 3rd coupling mechanism entering photoelectric detector PD 2; At last long by the chamber in the output calculated signals sensing Fabry-P é rot chamber of said photoelectric detector PD 1 and PD2.

3. the long method in chamber, fiber grating demodulation Fabry-P é rot chamber of utilizing according to claim 1; It is characterized in that; These method concrete steps are following: with two fiber gratings grating and be cascaded with sensing Fabry-P é rot chamber as a reference; The light that wideband light source sends gets into the tunable Fabry-P é rot wave filter by piezoelectric ceramics control through isolator, triangular signal generator basedly applies voltage to said tunable Fabry-P é rot wave filter; The light of said tunable Fabry-P é rot wave filter output is supplied with two with reference to grating and sensing Fabry-P é rot chamber through coupling mechanism; Got into photoelectric detector PD by said two light of returning with reference to grating and sensing Fabry-P é rot cavity reflection through said coupling mechanism and convert electric signal output to, long by the chamber in the output calculated signals sensing Fabry-P é rot chamber of said photoelectric detector PD again.

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