CN100439859C - Optical fiber interferometric on-line micro-displacement measurement system using fiber grating - Google Patents

Abstract

The invention discloses a micro-displacement measurement system, in particular to a micro-displacement measurement system suitable for on-line measurement. A fiber composite Michelson interferometer is formed by using the characteristics of fiber gratings and wavelength division multiplexing technology. The interferometer includes two fiber Michelson interferometers with independent mirrors but almost overlapping optical paths. One interferometer is used for measurement, and the other interferometer is controlled by feedback to compensate the influence of measurement due to environmental interference, so that the system is suitable for on-line measurement. The present invention only uses the light emitted by a semiconductor laser with a spectral width of 1.5nm and a central wavelength of 1550nm to act on the two interferometers at the same time, which not only makes the system cost low, but also facilitates automatic measurement, and the feedback control circuit is of great importance to piezoelectric ceramics. The discharge does not affect the measurement, so that the measurement can be carried out continuously.

Description

Optical fiber interferometric on-line micro-displacement measurement system using fiber grating

technical field

The invention relates to a micro-displacement measurement system, in particular to a micro-displacement measurement system suitable for on-line measurement, and belongs to the technical field of optical measurement.

Background technique

Existing report documents that are close to the technology of the present invention have the following two: [1] Dejiao Lin, XiangQian jiang, Fang Xie, Wei Zhang, Lin Zhang, and Ian Bennion, "High stability multiplexed fiber interferometer and its application on absolute displacement measurement and on-line surface metrology”, Optics Express, Vol.12, Issue 23, 2004, P.5729-5734. (Optics Express, 2004, Volume 12, Issue 23, P.5729-5734)[2] Xiangqian Jiang, Dejiao Lin, Liam Blunt, Wei Zhang and Lin Zhang, "Investigation of some critical aspects of on-line surface measurement by wavelength-division-multiplexing technique", Measurement Science and Technology, Vol.17, No.3, 2006 , P.483-487. (Measurement Science and Technology, 2006, Vol. 17, No. 3, P.483-487)

The technical principles of these two documents are the same, and their schematic diagram is shown in Figure 1.

The system consists of two Michelson interferometers with nearly coincident optical paths. One Michelson interferometer is composed of a fiber grating on the measuring arm and a reference mirror as a mirror to complete the stable work; the other Michelson interferometer is composed of a measuring mirror and a reference mirror as a mirror to complete the measurement work . Because the reference arms of the two interferometers share a mirror, the optical paths of the reference arms of the two interferometers are completely coincident, and because the measuring arms of the two interferometers are almost coincident, so when one interferometer is stable, the other interferometer is also stable up.

The light with a wavelength of _λ0 emitted by the semiconductor laser is divided into two paths after passing through two 3dB couplers, one path is reflected by the fiber grating, and the other path is reflected by the reference mirror. The two reflected lights meet again after passing through the 3dB coupler and interfere. The interference signal passes through the gyrator, is reflected by another fiber grating, passes through the gyrator again, and is detected by the detector. The signal detected by the detector passes through the servo circuit. After processing, the piezoelectric ceramic tube (PZT) is driven to adjust the length of the reference arm of the fiber optic interferometer, so that the two interference arms of the stable interferometer are always in an orthogonal state (the phase difference is π/2), thereby achieving a stable interferometer. Purpose.

The wavelength λ _m variable light emitted by the tunable laser is divided into two paths after passing through two 3dB couplers. One path passes through the autocollimation lens and then is reflected by the measuring mirror and returns to the interferometer again. The other path passes through the autocollimation The lens is reflected by the reference mirror and returns to the interferometer again. The two paths of light meet after passing through the 3dB coupler to form an interference signal. Measure the displacement of the mirror.

The problems and deficiencies of this technique are:

1. In the system, the light emitted by two lasers with different wavelengths acts on two Michelson interferometers respectively. These two light sources are semiconductor lasers and tunable lasers. Tunable lasers are very expensive, making the system expensive. Moreover, it is necessary to manually adjust the wavelength of the tunable laser to measure the surface during the measurement process. The principle is shown in Figure 2. This is very labor-intensive and time-consuming in practice, and automatic measurement cannot be realized.

2. The wavelength of the light source used to stabilize the interferometer and the light source used to measure the interferometer in the system are different; moreover, the reference arms of the measuring interferometer and the stabilizing interferometer use the same mirror, so the reference arms of the two interferometers The light paths are completely overlapped. In order to enable the piezoelectric ceramics to complete the tracking work continuously, when the driving voltage on the piezoelectric ceramics reaches the saturation value (maximum is the power supply voltage value), it is necessary to discharge the piezoelectric ceramics. When the feedback control system discharges the piezoelectric ceramics , the measurement data before discharge is not continuous with the measurement data after discharge, so the measurement must be completed between two discharges. Under laboratory conditions, the discharge frequency of piezoelectric ceramics is about 0.2 Hz, which will limit the practical application of this technology.

Contents of the invention

The present invention proposes aiming at the problems and deficiencies existing in the prior art. Only a cheap semiconductor laser with a spectral width of 1.5nm is used as a light source. The light emitted by this light source passes through wavelength division multiplexing technology, using fiber gratings and chirped fiber gratings, and simultaneously acts on the stable Michelson interferometer and measures the Michaelson interferometer. In the Michelson interferometer; at the ends of the two interference arms of the Michelson interferometer, two fiber gratings with the same parameters are respectively written as the two mirrors of the stable Michelson interferometer, so that the measurement Michelson interferometer and the stable Michelson interferometers have independent mirrors, and the optical paths of the measuring arm and reference arm of the two interferometers are almost coincident but not identical. The cost of the whole system is low, the measurement process does not require human intervention, and automatic measurement can be realized; the discharge of the piezoelectric ceramic by the feedback system no longer affects the measurement process, which can not only ensure the continuous stability of the interferometer, but also enable continuous measurement.

The present invention is achieved through the following technical solutions.

Using a semiconductor laser with a spectral width of 1.5nm, and using the characteristics of fiber gratings and chirped fiber gratings and wavelength division multiplexing technology to form two fiber Michelson interferometers with two independent mirrors but almost overlapping optical paths. Two fiber gratings with the same parameters are respectively located at the ends of the two interference arms of the fiber Michelson interferometer used for measurement, forming a composite fiber Michelson interferometer with two optical paths almost overlapping. The piezoelectric ceramic tube is driven by the feedback control system to adjust the length of the fiber interference arm so that the two interference arms of the Michelson interferometer composed of fiber gratings as mirrors are in an orthogonal state (the phase difference between the two arms is π/2), so that , which compensates for the influence of vibration and ambient temperature drift on the interferometer with the fiber grating as the mirror, so that the interferometer can also be kept stable in the field environment. Since the optical path of this interferometer almost coincides with the optical fiber Michelson interferometer used for measurement, the fiber Michelson interferometer used for measurement is also stabilized, thus enabling the system to be used for on-line measurement.

所以初始相位差的变化量与被测位移的变化量成正比。This system uses another piezoelectric ceramic to linearly modulate the optical path of the reference arm of the measuring fiber Michelson interferometer, so that the phase difference of the two arms increases linearly from zero to 2π, and the initial phase difference of the two arms is zero . When the optical path of the measuring arm changes Δd due to the change of the measured displacement, the initial phase difference of the two arms will become

Therefore, the variation of the initial phase difference is proportional to the variation of the measured displacement.

The displacement is measured by measuring the variation of the initial phase difference.

The beneficial effects of the present invention mainly contain two:

1. In the measurement system of the present invention, only a cheap semiconductor laser with a spectral width of 1.5nm and a center wavelength of 1550nm is used. The light emitted by the laser is simultaneously applied to the stable interferometer and the measurement interferometer by the wavelength division multiplexing technology. This not only reduces the cost of the measurement system by more than 50%, but also facilitates the automatic measurement of the system.

2. The present invention utilizes a pair of optical fiber gratings with the same parameters as the reflectors of the stable interferometer, and the stable Michelson interferometer and the measuring Michelson interferometer have independent reflectors; since the working wavelengths of the two interferometers are from the same One laser and two interferometers have almost the same working wavelength, so the piezoelectric ceramic discharge will not affect the measurement process, and the measurement data before discharge is continuous with the measurement data after discharge, which enables continuous measurement and makes the measurement system practical. value.

Description of drawings

Fig. 1 is the general schematic diagram of prior art;

Fig. 2 is the realization surface measurement schematic diagram of prior art;

Fig. 3 is a schematic diagram of the present invention;

Fig. 4 is an explanatory diagram showing that the discharge of piezoelectric ceramics does not affect the continuity of measurement in the present invention.

Detailed ways

The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments.

As shown in Figure 3, the light emitted by the semiconductor laser LD with a spectral width of 1.5nm and a center wavelength of 1550nm passes through the single-mode fiber 3dB-coupler 1, gyrator 1 and 3dB-coupler 2 and is divided into two paths, respectively reaching the optical fiber The gratings FBG1 and FBG2, the parameters of FBG1 and FBG2 are the same, and the Bragg wavelengths they reflect are also the same, and they will reflect back light with a spectral width of 0.1nm and a center wavelength of 1549nm. The light of the remaining spectrum will pass through FBG1 and FBG2, and after being collimated by the self-collimating lens (GRIN), it will become a parallel beam, reach the measuring mirror and the reference mirror respectively, and then be reflected by the measuring mirror and the reference mirror, and re-enter the In the interferometer, the coupler 2 meets and interferes. The interference signal is detected by the detector PD3 through the gyrator 3 and FBG3 (the reflection wavelength of FBG3 is the same as that of FBG1 and FBG2). When the measured object is displaced, the phase of the interference signal will change. If the measured object is displaced by Δd in the longitudinal direction (perpendicular to the measured surface), then the phase change of the corresponding interference signal is:

经过数据处理，即可测出被测物体在纵向方向位移Δd。为此，系统对参考光路中的压电陶瓷PZT2加周期性的锯齿波电压，周期性地线性调节参考光路的光程。调节锯齿波电压的幅值及参考光路光程，使得周期性的锯齿波电压与干涉信号同周期同相位。当被测物体在纵向方向有位移时，锯齿波电压的相位和该干涉信号的相位就不同，测出二者之间的相位差，经过数据处理后，即得到被测物体在纵向方向的位移值。图3所示，把测量干涉仪中测量臂的自准直透镜改为自聚焦透镜(GRIN)，即可对表面进行测量，其横向位移用步进电机实现。where λ is the wavelength of the incident light. It can be seen from equation (1) that as long as the phase change of the interference signal is demodulated

After data processing, the displacement Δd of the measured object in the longitudinal direction can be measured. For this reason, the system applies a periodic sawtooth wave voltage to the piezoelectric ceramic PZT2 in the reference optical path, and periodically adjusts the optical path of the reference optical path linearly. The amplitude of the sawtooth wave voltage and the optical path of the reference optical path are adjusted so that the periodic sawtooth wave voltage and the interference signal have the same period and phase. When the measured object has a displacement in the longitudinal direction, the phase of the sawtooth wave voltage and the phase of the interference signal are different, and the phase difference between the two is measured. After data processing, the displacement of the measured object in the longitudinal direction is obtained. value. As shown in Figure 3, the surface can be measured by changing the self-collimating lens of the measuring arm in the measuring interferometer to a self-focusing lens (GRIN), and its lateral displacement is realized by a stepping motor.

According to the characteristics of fiber grating, when a broadband spectrum is incident on a fiber grating, the fiber grating will reflect back the wavelength satisfying the Bragg condition, and transmit other wavelengths. The wavelength satisfying the Bragg condition is:

λ _Bragg = 2n _eff Λ(2)

where n _eff is the effective refractive index of the fiber, and Λ is the grating period of the fiber grating.

The parameters of the three fiber gratings (FBG1, FBG2, FBG3) used in the measurement system are the same, and they reflect the same wavelength. The fiber grating in the system will reflect the 1549nm wavelength. Write FBG1 and FBG2 as close as possible to the fiber self-collimating lens (GRIN), so that the optical paths of the fiber Michelson interferometer composed of FBG1 and FBG2 as mirrors and the Michelson interferometer in the measurement circuit overlap as much as possible.

The light reflected by FBG1 and FBG2 meets and interferes in coupler 2, and the interference signal of coupler 2 passes through gyrator 1, gyrator 2 and chirped fiber Bragg grating CFBG4, and is detected by detector PD1; it comes out of coupler 2 Another path of interference signal passes through the gyrator 3, is reflected by the FBG3, passes through the gyrator 3 again, and is detected by the detector PD2. The signals detected by the detectors PD1 and PD2 are processed by the feedback control system and act on the piezoelectric ceramic tube PZT1 as a feedback signal. One arm of the optical fiber interferometer is wound on the PZT1, and the length of the optical fiber is adjusted according to the size of the feedback signal PZT1 to This adjusts the optical path of the optical path so that the two arms of the fiber grating Michelson interferometer are always in the orthogonal state (the phase of the interference signal is always π/2). This realizes real-time compensation for the additional optical path brought by the fiber grating Michelson interferometer due to temperature drift and environmental vibration equal disturbance, so that the fiber grating Michelson interferometer has strong anti-interference ability. The optical path of the fiber grating Michelson interferometer and the main part of the optical path of the measuring Michelson interferometer in the measurement circuit almost overlap, as long as the fiber grating Michelson interferometer is stable, the Michelson interferometer in the measurement circuit is also stable. The operating frequency range of the feedback control system is 0-5kHz, that is to say, the feedback control system can correct and compensate the interference signal of 0-5kHz, so that the Michelson interferometer in the measurement circuit can accurately perform measurement work, which is suitable for for online measurement.

Due to the effect of chirped fiber Bragg grating CFBG4 (its reflection wavelength is 1.549.2 ~ 1552nm.), the measurement interference signal cannot reach the detector PD1, and the stable interference signal cannot reach PD3 due to the reflection of FBG3, so the measurement interferometer signal and that of the stabilization interferometer are separated.

Further illustrate below that the discharge of piezoelectric ceramics in the present invention has no influence on the measurement, and the two arm lengths of the measuring interferometer as shown in Figure 4 are respectively:

L ₁ =l ₁₁ +l ₁₂

L ₂ =l ₂₁ +l ₂₂

The optical path difference of the two interference arms of the measuring interferometer is:

Δ _m =L ₂ -L ₁ =(l ₂₁ -l ₁₁ )+(l ₂₂ -l ₁₂ )(3)

The corresponding phase difference is:

It can be seen from equation (4) that the effective value of the phase difference is determined by the second item of equation (4), but has nothing to do with the first item, so the piezoelectric ceramic discharge will not affect the measurement results.

The foregoing specific examples have been described for the purpose of illustrating the practice of the invention. However, other changes and modifications of the present invention are obvious to those skilled in the art, and any modification/change or imitation transformation within the essence and basic principles of the present invention without disclosure all belong to the protection scope of the claims of the present invention.

Claims (2)

1, a kind of interferometric on-line micro-displacement measuring system utilizing fiber grating, is characterized in that: it comprises measurement loop and feedback control loop, and it is made of a semiconductor laser LD, 3dB-coupler, fiber gyrator, fiber grating (FBG ), chirped fiber grating, detector, self-collimating lens or self-focusing lens (GRIN), measurement mirror and reference mirror, piezoelectric ceramic (PZT), feedback control circuit, oscilloscope, A/D conversion card, signal Composed of a generator and a computer; using a semiconductor laser with a spectral width of 1.5nm, using fiber grating characteristics and wavelength division multiplexing technology to form two fiber-optic Michelson interferometers with independent mirrors and almost overlapping but not identical optical paths ; Two fiber gratings with the same parameters are respectively located at the ends of two interference arms of a fiber Michelson interferometer for measurement, and the two fiber gratings are used as mirrors to form another fiber Michelson interference for stabilization The optical paths of the two interferometers are almost coincident but not identical; the piezoelectric ceramics are driven by the feedback control circuit to adjust the length of the reference arm of the interferometer, so that the two optical fiber Michelson interferometers with fiber gratings as mirrors The interference arms are in the orthogonal state, that is, the phase difference between the two arms is π/2, so that the impact of environmental vibration and temperature drift on the Michelson interferometer with the fiber grating as the mirror is compensated, and the stability of the The purpose of the interferometer; and because the optical paths of the two interferometers are almost coincident, the other optical fiber Michelson interferometer used for measurement is also stable, so that the system can be used for online measurement; through the piezoelectric ceramic pair to measure Michelson The reference arm of the interferometer is modulated so that the phase difference of the two arms of the interferometer increases linearly from zero to 2π, and the initial phase difference of the two arms is zero; when the optical path of the measuring arm changes due to the measured displacement When causing a change in Δd, the initial phase difference of the two arms changes as

λ is the luminous wavelength of the laser, and the variation of the initial phase difference is proportional to the variation of the measured displacement. The measurement of the displacement is realized by measuring the variation of the initial phase difference. 2. An interferometric on-line micro-displacement measurement system using fiber gratings according to claim 1, characterized in that: using wavelength division multiplexing technology and using the characteristics of fiber gratings to reflect Bragg wavelengths to form two composite One fiber optic Michelson interferometer, one interferometer completes the stabilization work, and the other interferometer completes the measurement work, so that the system is suitable for on-line measurement.

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