KR20050050063A - Fiber-optic laser interferomtry using zero-crossing detection - Google Patents
Fiber-optic laser interferomtry using zero-crossing detection Download PDFInfo
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- G—PHYSICS
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- G01B11/00—Measuring arrangements characterised by the use of optical techniques
- G01B11/24—Measuring arrangements characterised by the use of optical techniques for measuring contours or curvatures
- G01B11/2441—Measuring arrangements characterised by the use of optical techniques for measuring contours or curvatures using interferometry
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- G01B9/0207—Error reduction by correction of the measurement signal based on independently determined error sources, e.g. using a reference interferometer
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Abstract
광검출기의 신호를 영점 검출을 이용하여 광섬유형 레이저 간섭 시스템의 광간섭무늬 영상취득 동기 신호로 사용하여, 광섬유형 시스템을 구성할경우 발생하는 온도, 진동에 의한 광간섭신호의 변화에도, 빠르고 정확한 위상이동을 가한다. 영점검출 방법은 미소변위 측정, 건축구조물의 크랙검출, 미소 대상체 형상측정 등 레이저 간섭계를 이용하는 다양한 응용의 성능개선에 효과적으로 적용할 수 있다.By using the signal of the photodetector as the optical interference pattern image acquisition synchronization signal of the optical fiber laser interference system by using the zero point detection, it is possible to quickly and accurately change the optical interference signal caused by the temperature and vibration generated when constructing the optical fiber system. Apply phase shift. The zero detection method can be effectively applied to performance improvement of various applications using laser interferometer, such as micro displacement measurement, crack detection of building structures, and micro object shape measurement.
Description
레이저 간섭법을 이용한 계측 방법은 측정 대상체 및 계측 환경에 따라 In-plan ESPI, Out-of-plan ESPI, Shearography로 분류할 수 있다. 이러한 레이저 간섭법은 CCD 카메라같은 촬상소자를 이용하여 측정표면에서 반사된 측정파면과 기준빛의 파면를 빛결합기(beam conbiner)를 통해 광학 간섭무늬 이미지 분석,처리를 통해 측정대상체의 3차원 변형정보나, 대상체의 표면 정보를 추출한다. 이러한 대상체의 표면정보를 추출하기 위해서는 위상이동에 따른 CCD 이미지를 이용한 연산이 필수적인데 이를 위해서 위상변조기를 이용한다. 하지만, 간섭계의 위상이동을 위한 위상변조기의 비선형 동작 구조로 인해, 측정할 때 마다, 광학 시스템의 정렬이 흐트러지고, 위상측정 에러를 발생하기 때문에, self-calibration 알고리즘을 적용한 폐회로 시스템을 이용하여, 위상 변조기의 비선형 동작 특성에 의한 시스템 에러를 줄이는 방법이 제안되었다. 또한, 광섬유형 시스템을 구성할 경우, 환경적인 온도, 진동로 인한 광섬유의 굴절율변화에 따른 간섭출력의 변화량이 크다. 이 같은 외란을 극복하기 위해서 서보, PI제어기 같은 피드백 제어기법이 제안되기도 하였다.Measurement methods using the laser interference method may be classified into In-plan ESPI, Out-of-plan ESPI, and Shearography according to the measurement object and the measurement environment. The laser interference method uses an image pickup device such as a CCD camera to analyze and process optical interference fringe image through a beam conbiner to measure the wavefront reflected from the measurement surface and the reference light. Extract surface information of the object. In order to extract the surface information of the object, a calculation using a CCD image according to phase shift is essential. For this purpose, a phase modulator is used. However, due to the nonlinear operating structure of the phase modulator for phase shifting of the interferometer, since the alignment of the optical system is disturbed every time a measurement is made and a phase measurement error occurs, a closed-loop system employing a self-calibration algorithm is used. A method for reducing system error due to nonlinear operating characteristics of a phase modulator has been proposed. In addition, when the optical fiber type system is configured, the amount of change in the interference output due to the change in the refractive index of the optical fiber due to environmental temperature and vibration is large. In order to overcome this disturbance, a feedback control method such as a servo and a PI controller has been proposed.
CCD 카메라를 이용하여 얻은 간섭패턴에서 각 픽셀의 출력은 간섭이론으로부터 아래 식(1)과 같이 표현된다. In the interference pattern obtained using the CCD camera, the output of each pixel is expressed from the interference theory as shown in Equation (1) below.
........(1) ........(One)
여기서, 는 i번째 간섭출력의 DC, 는 i번째 간섭출력의 가시도, 는 측정 대상체의 표면 위상정보, 는 위상변조기를 이용한 위상이동 변화량이다.here, Is the DC of the i th interference output, Is the visibility of the i th interference output, Is the surface phase information of the measurement object, Is a change amount of phase shift using a phase modulator.
위상변조기로 연속하는 π/2의 배수에 해당하는 위상을 가하여, 4개의 간섭패턴을 얻으면 식(2)과 같이 표현할 수 있다. If a phase corresponding to a multiple of consecutive pi / 2 is added with a phase modulator and four interference patterns are obtained, it can be expressed as in Equation (2).
.......(2) .......(2)
식(2)에서 얻어진 4개의 간섭출력을 아래의 연산과정에 대입하여 측정 대상체의 위상 변형 값과 간섭패턴의 가시도를 추출하게 된다. The four interference outputs obtained in Equation (2) are substituted into the following calculation process to extract the phase distortion value of the measurement object and the visibility of the interference pattern.
........(3) ........ (3)
........(4) ........(4)
도 1에서 레이저 간섭계의 양 끝단에서 반사된 광신호는 마이켈슨 간섭계의 팔길이 창의 2배에 해당하는 광경로차에 해당하는 동일한 경로를 왕복한 후 광검출기에서 간섭을 일으키기 때문에, 광검출기 출력신호의 위상은 CCD카메라에서 간섭하는 광간섭의 위상의 2배가 된다. 따라서, 광검출기에서 얻는 간섭출력으로 CCD카메라의 간섭무늬의 위상변화를 감시할 수 있고, π배 위상이동점에 동기된 신호를 생성하면 π/2의 정수배에 해당하는 위상차를 가지는 CCD 카메라 영상취득 신호로 사용할 수 있다. In FIG. 1, since the optical signal reflected at both ends of the laser interferometer reciprocates the same path corresponding to the optical path difference corresponding to twice the arm length window of the Michelson interferometer, the photodetector causes interference, thus the photodetector output signal The phase of is twice the phase of the optical interference which is interfered by the CCD camera. Therefore, the interference output obtained from the photodetector can monitor the phase change of the interference fringe of the CCD camera, and if the signal synchronized with the π times phase shift point is generated, the CCD camera image having a phase difference corresponding to an integer multiple of π / 2 is acquired. Can be used as a signal.
본 발명의 목적은 광섬유형 레이저 간섭 시스템의 영상 알고리즘을 통한 3차원 입체정보를 빠르고, 쉽고, 정확하게 취득하기 위해 CCD 카메라에서 간섭위상을 광검출기로 모니터 하여, 광검출기의 신호를 영점검출을 통한 위상이동 시스템을 제공하는 것이다. SUMMARY OF THE INVENTION An object of the present invention is to monitor the interference phase with a photodetector in a CCD camera to acquire three-dimensional stereoscopic information through an image algorithm of a fiber-optic laser interference system with a photodetector, and to phase the signal of the photodetector through zero detection To provide a mobile system.
또한, 본 발명은 환경적 변화(온도, 진동)에 따른 광섬유 시스템의 오차, 위상이동 변조기의 비선형성 오차등을 극복하고, π/2 위상이동을 가하기 위하여 고안되었다. In addition, the present invention is designed to overcome the error of the optical fiber system due to environmental changes (temperature, vibration), the nonlinearity error of the phase shift modulator, and to apply π / 2 phase shift.
도 1은 본 발명에 의하여 제조된 광섬유 레이저 간섭 시스템의 개략도이다. 1 is a schematic diagram of a fiber laser interference system manufactured according to the present invention.
도 1의 영점검출의 측정 원리는 다음과 같다. 2×2 광섬유 커플러(single mode fiber coupler)를 이용하여 마이켈슨 간섭계를 구성하여 한쪽팔을 위상변조기인 PZT(piezoelectric transducer)에 감고, 위상변조기는 4π의 주기를 가지는 위상이동을 가지도록 25Hz의 톱니파를 신호발생기로 생성하여 구동하였다. 레이저의 출력은 3dB 광섬유 커플(fiber coupler)에 통하여, 50:50으로 나뉜 후, 그중 한쪽 팔은 측정대상체를 위한 광원(object beam)으로 조사되고, 다른 하나는 기준광원(reference beam)을 위해, 조사된다. 각각의 광섬유 끝단에서 반사된 광신호는 다시 광섬유 커플러를 거친 후, 광검출기에서 결합하여 간섭신호를 발생한다. 광검출기에서 검출되는 간섭신호는 저 대역 필터를 거치고, DAQ(data aqusition)보드를 이용하여 디지털화한 후 컴퓨터로 읽어 들이며, AC커플링을 통하여 DC 성분을 제거한다. 프로그램을 통해서, 취득한 신호를 톱니파에 동기화된 TTL신호 주기 동안 영점 검출을 이용하여, 4개의 영점 위치를 얻는다. 컴퓨터로 DAQ보드를 통해, 4개의 영점 위치에 해당하는 시간에 연속하는 위상계단 신호를 생성하여, CCD 카메라의 영상취득 신호로 사용한다. The measuring principle of the zero detection of FIG. 1 is as follows. A Michelson interferometer is constructed using a 2 × 2 single mode fiber coupler, and one arm is wound around a PZT (piezoelectric transducer), which is a phase modulator. Was generated by a signal generator and driven. The output of the laser is divided into 50:50 through a 3dB fiber coupler, one arm of which is irradiated with an object beam for the measurement object, and the other for a reference beam. Is investigated. The optical signal reflected at each end of the optical fiber passes through the optical fiber coupler, and then combines in the photodetector to generate an interference signal. The interference signal detected by the photodetector goes through a low band filter, digitized using a data acquisition board, and then read by a computer, and removes the DC component through AC coupling. Through the program, four zero positions are obtained using zero detection during the TTL signal period synchronized with the acquired signal. Through the DAQ board, the computer generates a phase-sequence signal that is continuous at the time corresponding to the four zero positions and uses it as the image acquisition signal of the CCD camera.
도 2는 25Hz의 톱니파와 이를 위상변조기의 구동신호를 사용한 후, 간섭계의 검출신호이다. 불연속적인 환경변화(온도, 진동)에 따른 위상변화는 톱니파에 의한 4π위상주기를 빠르게, 혹은 느리게 한다. 이때, 영점검출을 사용하면, 환경변화에 따른 외란에도, 정확한 4개의 π위상 이동점을 찾을 수가 있다. 또한, 함수 발생기에서 PZT 구동신호인 톱니파에 동기된 TTL신호를 시스템의 트리거 신호로 사용하면, 순차적으로 4개의 π배수에 해당하는 위상 이동점을 취득할 수 있다. 2 is a detection signal of an interferometer after using a sawtooth wave of 25 Hz and a drive signal of a phase modulator. The phase change caused by discontinuous environmental changes (temperature, vibration) causes the 4π phase period caused by the sawtooth wave to be slow or fast. At this time, if zero detection is used, accurate four pi-phase moving points can be found despite disturbance caused by environmental changes. In addition, when the function generator uses a TTL signal synchronized with a sawtooth wave, which is a PZT driving signal, as a trigger signal of the system, phase shift points corresponding to four? Multiples can be obtained sequentially.
도 3은 영점 검출 실험 결과 파형이다. a신호는 저대역(120Hz)필터를 거친 광검출기의 간섭신호의 파형이고, b신호는 위상변조기를 구동하는 톱니파의 동기 TTL신호로 레이저간섭 시스템의 동작 신호이다. c신호는 a신호를 AC 커플링 한 후 영점 검출의 결과 파형이다. 3 is a waveform of a zero detection test result. The a signal is the waveform of the interference signal of the photodetector passed through the low band (120 Hz) filter, and the b signal is the synchronous TTL signal of the sawtooth wave driving the phase modulator. The c signal is a waveform of zero detection after AC coupling of a signal.
본 발명에 의한 π/2 위상이동 시스템은 도 4에 도시된 바와 같이 검출기 신호를 영점 검출을 이용하여 영점위치에 따라 CCD 카메라에서 간섭무늬 위상이 π/2 위상이동 따른 영상취득 신호를 생성하였다. 100Hz 속도의 CCD 카메라 이미지 취득 동기 신호를 얻었다. In the π / 2 phase shifting system according to the present invention, as illustrated in FIG. 4, a detector signal is generated using an zero point detection to generate an image acquisition signal in which an interference fringe phase is shifted π / 2 in a CCD camera according to a zero position. A CCD camera image acquisition synchronization signal with a 100 Hz speed was obtained.
전술한 바와 같이, 영점검출을 이용하여, 환경적 외란에 따른 광간섭신호의 변화와 위상변조기의 비선형 특성에도 빠르고 정확하게 간섭계의 π/2 위상변조를 얻을 수 있는 광섬유형 시스템을 구성하였다. 이것은 벌크 형태 시스템을 제작할 경우에 비하여, 제작상의 경제성을 현격하게 증가할 수 있으며, 소형화하여, 휴대가능한 시스템으로 제작할 수 있는 장점이 있다. As described above, the zero point detection was used to construct an optical fiber type system capable of quickly and accurately obtaining π / 2 phase modulation of an interferometer even with changes in an optical interference signal due to environmental disturbance and nonlinear characteristics of a phase modulator. This can significantly increase the manufacturing economics, compared to the case of manufacturing a bulk type system, there is an advantage that can be manufactured in a compact, portable system.
도 1은 본 발명에 의한 영점검출을 이용한 광섬유형 레이저 간섭I시스템의 개략도이다. 1 is a schematic diagram of an optical fiber type laser interference I system using zero detection according to the present invention.
도 2는 환경적인 외란(온도, 진동)에 의한 영향으로 간섭신호의 파형변화를 도시한다. Figure 2 shows the waveform change of the interference signal due to environmental disturbances (temperature, vibration).
도 3은 본 발명에 의한 간섭 위상이동에 따른 영점검출 파형을 도시 한다. Figure 3 shows a zero detection waveform according to the interference phase shift in accordance with the present invention.
도 4는 본 발명에 영점검출에 따른 CCD카메라를 위한 위상이동 계단 파형을 도시한다. Figure 4 shows a phase shift step waveform for a CCD camera according to the zero point detection in the present invention.
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Cited By (3)
Publication number | Priority date | Publication date | Assignee | Title |
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KR100810145B1 (en) * | 2006-09-29 | 2008-03-06 | 전북대학교산학협력단 | Strain measurement system using double-pass mach-zehnder interferometer and fiber grating sensor |
CN104236453A (en) * | 2014-10-16 | 2014-12-24 | 国家电网公司 | Anti-interference automatic counting type michelson interferometer and interference fringe counting method |
CN105953725A (en) * | 2016-05-06 | 2016-09-21 | 哈尔滨工程大学 | Phase carrier type laser interference signal closed-loop demodulation method |
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2005
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Cited By (4)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
KR100810145B1 (en) * | 2006-09-29 | 2008-03-06 | 전북대학교산학협력단 | Strain measurement system using double-pass mach-zehnder interferometer and fiber grating sensor |
CN104236453A (en) * | 2014-10-16 | 2014-12-24 | 国家电网公司 | Anti-interference automatic counting type michelson interferometer and interference fringe counting method |
CN104236453B (en) * | 2014-10-16 | 2017-02-08 | 国家电网公司 | Anti-interference automatic counting type michelson interferometer and interference fringe counting method |
CN105953725A (en) * | 2016-05-06 | 2016-09-21 | 哈尔滨工程大学 | Phase carrier type laser interference signal closed-loop demodulation method |
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