CN1201901A - Wave-length scanning and interfering system and signal process method therefor - Google Patents

Abstract

A wavelength scan interference system for measuring absolute distance is composed of scan light source, optical fibre coupler, reference interferometer, sensing interferometer, detector, photoelectric converter, amplifier and filter, A/D converter and computerized data processing unit, and features simple structure, less system loss and interference noise and high reliability. A new signal process method is disclosed for higher measuring precision and shorter operating time.

Description

The length scanning interference system and the signal processing method thereof of absolute distance measurement

The invention belongs to the Technology of Precision Measurement field, particularly be used for the optical fibre interference technology of absolute distance.

The measurement of displacement or distance occupies critical role in commercial production and scientific research field, and the particularly measurement of absolute distance is present important developing direction and being studied.Traditional machinery or measuring method can not adapt to the requirement of high precision, automatic measurement.Since the seventies, it is the Technology of Precision Measurement at center that main both at home and abroad development has risen with laser, and measurement range and precision aspect all are greatly improved.Conclude and get up to mainly contain laser interferometry, laser heterodyne interference technology, optical fiber technology, Doppler technology etc.Optical fiber sensing technology is the seventies later stage, along with the development of optical communication technique, fiber optics and integrated optics technique, and the novel sensing technology that emerges rapidly.Comparing Fibre Optical Sensor with other traditional sensor and have little, in light weight, corrosion-resistant, the anti-electromagnetic interference (EMI) of volume, be suitable for inflammable and explosive environment uses and is convenient to advantages such as multiple spot many reference amounts measurement.

Press the effect of optical fiber in sensor, optical fibre displacement sensor can be divided into functional form and NOT-function type.In the functional fiber displacement transducer, the variation of displacement causes that by corresponding construction the relevant physical quantity of optical fiber changes, by the variation displacement as can be known of measuring these physical quantitys.Micro-bending displacement measurement structure as US Naval Research Laboratory N.Lagakis proposition.In the NOT-function type optical fibre displacement sensor, can be divided into two kinds of intensity type and phase types again to the modulator approach difference of light.As the intensity modulation type displacement transducer of propositions such as X.P.Liu, determine displacement by the light intensity that the detection body surface reflects.Interferometric method in the optics is one of the sensitiveest known Detection Techniques.The optical fiber interference type displacement transducer is to utilize extraneous factor to cause that the phase change of light in the optical fiber surveys the variation of displacement or distance, belongs to the phase modulation-type Fibre Optical Sensor, is characterized in highly sensitive.The fibre optic interferometer that is used for displacement sensing at present mainly contains: Mach-Zehnder type, Michelson type and three kinds of fibre optic interferometers of Fobry-Perot type.

The fiber optic interferometric displacement measurement generally is the scheme that adopts traditional interference fringe counting, because its deadly defect is the necessary continuous counter of interference fringe one by one, thereby make its practical application be subjected to very big restriction: or measure to need use guide rail, or response speed is not high, or displacement to be necessary continuous etc.On this basis, recently some interference techniques such as the white light interference technique that develops, the multi-wavelength interference technology of linear frequency modulation difference interference, employing decimal striped etc. make optical fibre displacement sensor, and characteristic, high sensitivity and precision are more noticeable widely with it.The white light interference method requires the stabilized light source of wide spectrum and the machinery of high duplication to move; Frequency modulation heterodyne technical requirement light source center frequency stabilization and strict linear modulation; And the multi-wavelength interference technology of decimal striped not only requires optical source wavelength stable, and measurement range is less.These factors have all been brought very big difficulty to measurement.If be applied to absolute distance measurement, must calibrate as optical source wavelength with reference to yardstick.

For overcoming the shortcoming of conventional art, the present application people has proposed a kind of length scanning interferometer that can be used for absolute distance measurement, and its structure as shown in Figure 1.Scanning light source 11 is divided into two-way by a fiber coupler 131 with scanning light beam, every road links to each other with a reference interferometer 12, sensing interferometer 15 respectively by a fiber coupler 132,133 again, when optical source wavelength scanned, detector 141,142 was connected to the output terminal of fiber coupler 132,133.Detector 142 receives the interference signal V of reference interferometer 12 outputs ₂, detector 141 receives the interference signal V of sensing interferometer 15 outputs ₁, it is handled obtain V respectively ₁, V ₂Phase change Δ φ _bWith Δ φ _a, by formula It is long to obtain chamber to be measured.

Said system is to the hardware requirement height, and device is many, loss is big, and precision, reliability are affected, and the signal processing method operand of this system is big, can not real-time processing data, thereby practical application is restricted.

The objective of the invention is to for overcoming the weak point of prior art, above-mentioned length scanning interference system is improved, make its structure advantages of simple more, the loss of minimizing system and interaction noise, improved the reliability of system, proposed a kind of new signal processing method simultaneously, measuring accuracy is further improved, and shortened operation time greatly, satisfy the needs of practical application better.

The length scanning interference system that the present invention proposes a kind of absolute distance measurement comprises the scanning light source that is fused by single-mode fiber, fiber coupler, reference interferometer and sensing interferometer, the optical signal detector that links to each other with fiber coupler, the opto-electronic conversion that the light signal that detector is detected is changed, the amplification filtering circuit, A/D transducer and the computer data processing unit that the output signal of this converter is handled in real time, said reference interferometer is made of two coaxial 1/4 pitch GRIN Lens that are connected in optic fibre input end respectively, and said sensing interferometer is to be made of GRIN Lens that links to each other with fiber-optic output and determinand surface.It is characterized in that said scanning light source, reference interferometer, sensing interferometer constitute the two-stage series connection structure by two coupling mechanisms and optical fiber.

Principle of work of the present invention is described as follows in conjunction with Fig. 2: scanning light source 21 sends the light beam of the variation of wavelength in a scope, is received by two detectors 241,242 that are connected in coupling mechanism 231 and obtains signal V _A1Produce an interference signal V with the F-P interferometer of forming through two GRIN Lens 22 _A2, treated obtaining with reference to interference signal V _a=V _A1/ V _A2This length scanning light beam is received by two detectors 243,244 that are connected in coupling mechanism 232 and obtains signal V _B1With the sensing F-P interferometer 25 generation sensing interference signal Bs of process by a GRIN Lens and detection thing surface composition _B2, obtain transducing signal V through handling _b=V _B1/ V _B2Set scanning wavelength λ and sweep limit Δ λ, then phase change

So have , l _aFor known reference interferometer chamber long, Δ φ _a, Δ φ _bCan obtain, so can obtain length to be measured (being the long distance in chamber of sensing interferometer) Because Δ φ _b, Δ φ _aSolving precision is limited, l _bThere is error.The following formula gained can only be as the initial value of further accurate solution procedure.

In addition, the frequency of signal is obtained than phase place is easier, therefore asks l _bDuring initial value, can adopt following method: adopt the FFT method to ask V _aAnd V _bFrequency values, the frequency that adopts the conic fitting method to obtain the maximum peak place at spectrum place, peak is respectively f _aAnd f _b, can solve the not high problem of FFT resolution.

, (in the 0-1mm scope, precision is about 1 μ m).After obtaining initial value, further accurately find the solution again.

Interfere light intensity for increasing, reduce the loss of transmission light, two relative two sides of GRIN Lens of said reference interferometer are coated with reflectance coating and form interference cavity; The two-phase back side is coated with anti-reflection film in addition, and forms a little oblique angle with shaft axis of optic fibre.The end face that GRIN Lens in the said sensing interferometer links to each other with fiber-optic output is coated with anti-reflection film, and becomes a little oblique angle with shaft axis of optic fibre, and the other end of this self-focus lens and measured object surface form interference cavity.

The signal processing method of scanning system of the present invention is to utilize the FFT method to ask the long initial value in tested chamber, thereby it is long again scanning center's wavelength and sweep limit to be estimated to obtain tested chamber, finishes the measurement of absolute distance.

The signal processing method of this scanning interferometer system may further comprise the steps:

1) four output and treated sequential value V that become digital signal of detector in the acquisition scans interference system _A1i, V _A2i, V _B1i, V _B2iAnd the lock chamber long value l of reference interferometer _a

2) will go up the sequence of steps value and obtain cosine signal values V through the normalization signal Processing _AiAnd V _Bi

3) adopt the FFT method to obtain V _aAnd V _bCentre frequency f _aAnd f _b

4) according to l _b'=l _aF _b/ f _aObtain the long approximate value l in chamber of tested sensing interferometer _b';

5) initial value of the central wavelength lambda c of setting scanning process obtains the initial value λ of sweep limit Δ λ for λ co ₀

6) adopt recurrence method to obtain the exact value of λ c and Δ λ;

7) utilize sequence V _Bi, λ c and Δ λ obtain l _bExact value.

The method of the invention is utilized following algorithm:

Step 1:V _a'=V _A1/ V _A2, V _b'=V _B1/ V _B2, press least square method to V _b' and V _b' normalization is handled and is obtained V _aAnd V _bAdopt the FFT method to ask its centre frequency f _aAnd f _b, l _bInitial estimated value be l _b＇=l _aF _b/ f _a(1) precision of lb ' only is 1 μ m.

If length scanning from

Arrive , λ c is a centre wavelength, Δ λ is a sweep limit, is normal value in the single pass process, but in each time scanning, may be different.If f ₀Be signal center frequency, then have $\mathrm{\&Delta;\&lambda;}=\frac{{\mathrm{\&lambda;c}}^2}{2l}\&CenterDot;{\mathrm{<figure-callout\; id="0"\; label="f"\; filenames="A9810103400093.png"\; state="\{\{state\}\}">f</figure-callout>}}_0\&CenterDot;N/f_s---\left(2\right)$ N is a sampling number, f _sBe sample frequency.

Step 2: establishing λ c initial value is λ _C0, (2) formula of utilization is obtained the initial value of Δ λ

Adopt recurrence method to ask λ c and Δ λ exact value below: we utilize

Approach Va, , note $F_1\left(\mathrm{\&lambda;c}\right)=\underset{i=1}{\overset{N}{\mathrm{\&Sigma;}}}[\cos \frac{{4\mathrm{\&pi;l}}_a}{\mathrm{\&lambda;c}+{\mathrm{\&eta;}}_j}-V_a\left(i\right){]}^2---\left(3\right)$ η _j(i=1 2......N) exists

In value uniformly-spaced.For example get ε _λ=2nm, λ c is at [λ _C0-ε λ, λ _C0+ ε λ] ask F1 (F in the scope ₁(λ _c)) minimum value, this moment, corresponding λ c was λ _C1, with λ _C1Substitution (2) gets Δ λ, and the pairing λ of its minimum value is obtained in substitution (3) again _C2, and utilize (2) to ask Δ λ ₂, if | λ _C2-λ _C1|＜0.2nm, recursion stops, λ _C2With Δ λ ₂Satisfy accuracy requirement.Otherwise, continue to calculate.

Usually, at Δ λ ₁Obtain under the situation, utilize (3) to ask F1 (λ c) minimum value, corresponding λ _Ci+1, if | λ _Ci+1-λ _Ci|＜0.2nm then stops to calculate, otherwise asks λ again _Ci+2... general recursion can be above 5 times.So far λ c and Δ λ have been obtained.

Step 3: utilize sequence Vb, λ c and Δ λ ask the exact value of lb.Note $F_2\left(l\right)=\underset{i}{\mathrm{\&Sigma;}}(\cos \frac{4\mathrm{\&pi;l}}{{\mathrm{\&lambda;}}_c+{\mathrm{\&eta;}}_j}-V_b\left(i\right){)}^2----\left(4\right)$ , for example get ε _b=1 μ m, l [l ' _b-ε _b, l ' _b+ ε _b] in the scope, ask F ₂(l) minimum value, this moment, corresponding l was the l that asks _b

The present invention has following characteristics:

The first, reduced by a coupling mechanism than prior art, thereby reduced loss, simple in structure, compact, volume is little.

The second, native system has adopted the antistructure that disappears, and has reduced interaction noise.

The 3rd, the signal processing method that the present invention adopts has the precision height, the characteristics that operation time is short.

The present invention proposes a kind of length scanning interference system embodiment of absolute distance measurement, is described in detail as follows in conjunction with Fig. 2-Fig. 7:

Native system embodiment general structure as shown in Figure 2.Comprise scanning light source 21, coupling mechanism 231,232, reference interferometer 22, sensing interferometer 25, photo-detector 241-244, above-mentioned each parts are fused by optical fiber, constitute the optical measuring system of present embodiment, by conversion of signals, filtering circuit, the conversion of signals data handling system that A/D transducer and microcomputer are formed.

Each several part detailed structure and function are respectively described below:

Scanning light source adopts Tunable External Cavity Semiconductor Laser in the present embodiment, and its structure as shown in Figure 3.The left side of semiconductor laser 31 constitutes exocoel between self-focusing 32 and grating 34, its length 3-4cm.The emergent light of right side enters optical fiber 37 by coupled lens 33.The angle of laser axis and grating planar is θ, and when motor 35 back and forth rotated by cam 34 drive gratings, θ changed, and realizes the continuous sweep of output optical maser wavelength.For the LD of λ=1.3 μ m, adopt the grating of d=1/1200mm, press grating feedback formula:

44dsin θ=λ (5) can obtain 23 ° of θ ≈.When 1 ° of grating rotational angle, can make the output wavelength sweep limit surpass the laser spectrum width 0.001A of 40nm.LD output, output power surpasses 0.5mw.Motor speed is 3000-4800 rev/min, and then the sweep frequency of wavelength is 100-160Hz.

Coupled lens 33 is made up of GRIN Lens, contains optoisolator (genus mature technology), coupling efficiency 20-50%.

Adopted two fiber couplers in the system, the used here single-mode fiber that is, (prevent the modal dispersion in the multimode optical fiber, improve measuring accuracy), and coupling mechanism is 2 * 2 types (X types), and splitting ratio is 1: 1, and each arm lengths of optical fiber is that 1.5m-2m is long.

The reference F-P interferometer structure of present embodiment as shown in Figure 4.Connect 1/4 pitch self-focus lens 421,422 respectively at input optical fibre 411 and output optical fibre 412 ends, its overcoat is straight connector 423 surely, changes the outgoing diverging light of optical fiber into parallel beam, interferes in the F-P chamber.421,422 and fiber end face contact position plating anti-reflection film, and adopt 8 ° of oblique angle structures, for the reflected light that reduces this surface of contact enters detector, and the loss (Fresnel loss) that reduces transmission light, two GRIN Lens 421,422 opposing end surfaces plating reflectance coating (reflection coefficient r ₁And r ₂), purpose is to increase to interfere light intensity, present embodiment is got 0.2≤r ₁≤ 0.3,0.3≤r ₂≤ 0.4.The long la in chamber gets 0.9-1mm.

The sensor F-P interferometer structure of present embodiment as shown in Figure 5, probe 52 and measured object surface composition that the GRIN Lens that is linked to each other by single-mode fiber 51 output terminals constitutes, reflecting surface can be pasted in the measured object surface, also can be directly with its surface as reflecting surface.The end face of lens 52 is coated with reflectance coating, and the face that the other end links to each other with optical fiber is coated with anti-reflection film, and becomes 8 ° of oblique angles with shaft axis of optic fibre.

Present embodiment connects two photo-detector 241-244 respectively at two photo-coupler output terminals, obtains the interference signal V of two interferometers respectively _A1, V _A2, V _B1, V _B2, each signal is through conversion, and the amplification filtering circuit is handled, and its circuit is as shown in Figure 6.

Send into microcomputer through the signal of conversion process and carry out data processing, the data processor of present embodiment as shown in Figure 7, treatment step is:

1) output signal of detector 241-244 becomes digital signal sequences V after treatment _A1, V _A2, V _B1, V _B2, to import in this handling procedure, long la is known for lock chamber.

2) through the normalization signal Processing, obtain cosine signal V _a' and V _b'.V _a'=V _A1/ V _A2, V _b'=V _B1/ V _B2, V _a', V _b' after the amplitude normalization is handled, be V _aAnd V _b

3) adopt the FFT method to ask V _aAnd V _bCentre frequency f _aAnd f _b

4) press l _b'=l _aF _b/ f _a, obtain a long initial value of chamber to be measured.

5) centre wavelength of establishing this scanning process is λ _C0=1310nm (initial value), (2) formula of utilization is obtained the initial value Δ λ of sweep limit ₀,

6) suppose at known Δ λ _iSituation under, i=0,1,2 ... ask the minimal value of formula (3), corresponding λ _cBe designated as λ _Ci+1, and utilize formula (2) to obtain Δ λ _I+1When minimizing, λ _cAt λ _Ci-ε λ is to λ _Ci+ ε λ, ε λ gets 2nm.λ _cStepping is less than 0.1nm.

7) hunting zone, judge | λ _Ci+1-λ _Ci| whether less than 0.2nm, if do not satisfy, then with Δ λ _I+1Substitution formula (2) repeats process 6.

8) if satisfy, then with gained Δ λ _I+1And λ _Ci+1(algorithm has been carried out i+1 time), substitution (4) formula is asked the minimal value of function, and this moment, the hunting zone of l was [l _b'-ε _b, ε _b+ l _b'], l _b' in process 4, try to achieve.The corresponding l of function minimal value is the long l in chamber to be measured _b

9) output l _b

Said procedure gives in the memory that is stored in computing machine earlier, and this program execution time on 586 machines is less than 100ms, for the general outside cavity gas laser of scan performance, and precision 0.2 μ m, and for the light source of better performance, precision can be better than 0.05 μ m.Algorithm resolution depends on the search stepping of formula (4), is generally 0.02 μ m, and in the present embodiment, precision is at 0.1 μ m.

Claims (4)

1, a kind of length scanning interference system of absolute distance measurement comprises the scanning light source that is fused by single-mode fiber, fiber coupler, reference interferometer and sensing interferometer, the optical signal detector that links to each other with fiber coupler, the opto-electronic conversion that the light signal that detector is detected is changed, the amplification filtering circuit, A/D transducer and the computer data processing unit that the output signal of this converter is handled in real time, said reference interferometer is made of two coaxial 1/4 pitch GRIN Lens that are connected in optic fibre input end respectively, and said sensing interferometer is to be made of GRIN Lens that links to each other with fiber-optic output and determinand surface.It is characterized in that said scanning light source, reference interferometer, sensing interferometer constitute the two-stage series connection structure by two coupling mechanisms and optical fiber.

2, interference system as claimed in claim 1 is characterized in that two relative two sides of GRIN Lens of said reference interferometer are coated with reflectance coating formation interference cavity, and the two-phase back side is coated with anti-reflection film in addition, and forms a little oblique angle with shaft axis of optic fibre; The end face that GRIN Lens in the said sensing interferometer links to each other with fiber-optic output is coated with anti-reflection film, and becomes a little oblique angle with shaft axis of optic fibre, and the other end of this self-focus lens and measured object surface form interference cavity.

3, the system as claimed in claim 1 signal processing method is characterized in that, utilizes the FFT method to ask the long initial value in tested chamber, thereby it is long that the centre wavelength of scanning and sweep limit are estimated to obtain tested chamber, finishes the measurement of absolute distance.

4, a kind of signal processing method that is used for scanning interferometer as claimed in claim 3 system comprises following concrete steps:

1) four output and treated digital signal sequences value V that become of detector in the acquisition scans interference system _A1i, V _A2i, V _B1i, B _B2iAnd the lock chamber length value l of reference interferometer _a

2) will go up the described value of step and obtain cosine signal values V through the normalization signal Processing _AiAnd V _Bi

3) adopt the FFT method to obtain V _aAnd V _bCentre frequency f _aAnd f _b

4) according to l _b'=l _aF _b/ f _aObtain the long approximate value l in chamber of tested sensing interferometer _b';

5) initial value of the central wavelength lambda c of setting scanning process obtains the initial value Δ λ 0 of sweep limit Δ λ for λ co;

6) adopt recurrence method to obtain the exact value of λ c and Δ λ;

7) utilize sequence V _Bi, λ c and Δ λ obtain l _bExact value.

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