A technology of absolute distance measurement based on the multi-wavelength self-mixing interferometry of a threewavelength optical fiber laser is presented and experimented. The optical fiber laser which can emit three wavelengths simultaneously is composed of a single fiber ring and three fiber branches. Each fiber branch includes a length of erbium-doped fiber and a fiber Bragg grating (FBG). The erbium-doped fiber is used as the gain medium while the FBG is used as a laser cavity reflective mirror and wavelength selector. Three independent laser cavities have been constructed in the single laser. As there is no laser mode competition, three wavelengths with stable power can be emitted simultaneously and the frequency stability of each wavelength can reach 10-6 . Absolute distance measurement can be performed using multiple self-mixing interferometry of the three wavelengths. Based on the idea that the calculated decimal phase of the self-mixing interferometric signal of each wavelength should equal the measured decimal phase of the self-mixing interferometric signal of the same wavelength, absolute distance measurement can be realized. The nominal length of a gauge block has been used to prove the correctness of the measurement results by experiments. The standard deviation of ten times repeated measurements for a distance of 7 mm is 4.4 nm.
Two kinds of tunable double-wavelength erbium-doped fiber (EDF) ring lasers were developed and their application to step height measurement using two-wavelength self-mixing interferometry (SMI) was demonstrated. The fiber lasers can emit two different wavelengths without any laser mode competition. Each wavelength has its own gain medium which is a length of erbium-doped fiber. Fiber Bragg gratings are used to determine the value of the emitted wavelengths. Large step heights can be measured using multiple self-mixing interference of the two wavelengths. The maximum height that can be measured is half synthetic wavelength of the two wavelengths. A step height of 2mm constructed with two gauge blocks has been measured. The standard deviation of measurement results is 2.5nm.
In this paper, an optical fiber multiplexing interferometric system including a Fizeau interferometer and a Michelson interferometer is designed for remote and high precision step height measurement. The Fizeau interferometer which is inserted in the remote sensing field is used for sensing the measurand, while the Michelson interferometer which is stabilized by a feedback loop works in both modes of low coherence interferometry and high coherence interferometry to demodulate the measurand. The range of the step height is determined by the low coherence interferometry and the value of it is measured precisely by the high coherence interferometry. High precision has been obtained by using the symmetrical peak-searching method to address the peak of the low coherence interferogram precisely and stabilizing the Michelson interferometer with a feedback loop. The maximum step height that could be measured is 6 mm while the measurement resolution is less than 1 nm. The standard deviation of 10 times measurement results of a step height of 1 mm configurated with two gauge blocks is 0.5 nm.
The paper researches on a high precision displacement measurement system mixing double-wavelength interferometry and single-wavelength interferometry by waveform transforming based on Fast Fourier Transform (FFT) technology. The signal of double-wavelength interferometry is used for determining the amplitude of the measurand which makes the measurement range be as large as half a synthetic-wavelength, while that of single-wavelength interferometry is for measuring the value of the measurand precisely which endows the measurement resolution to be as high as less than 1nm, for the amount of the interference fringes of the signal of single-wavelength interferometry during the shifting range of the peak of the signal of double-wavelength interferometry demonstrates the value of the measurand. However, as the signal of double-wavelength interferometry is cosine amplitude modulated, the peak area of it is flatten and the peak position is difficult to be determined, which will influence the measurement precision directly. In order to address the peak position of the double-wavelength interferometric signal accurately, we transform using FFT technology the cosine amplitude modulated signal of double-wavelength interferometry into a triangle-wave amplitude modulated signal to make the peak position prominent. It is very easy to determine the peak position accurately and the amount of the interference fringes of the signal of single-wavelength interferometry during the shifting range of the peak will also be determined precisely. High precision displacement measurement with large range and high resolution could be realized.
A vibration-displacement measurement system by tracking the phase variation of an optical fiber Michelson
interferometer with an electronic feedback loop is presented. The measurement system includes an electronic feedback
loop which is used to track the phase variation induced by the measured vibration-displacement and provides a sense of
direction of the displacement simultaneously. The measurement system is designed to be capable of measuring
vibration-displacement with frequencies up to 200Hz and the measurement resolution can reach 13nm.
A sensor which exploits dual-interferometric technique for absolute displacement measurement is proposed. The interferometer performing the measurement task operates low coherent-optical interferometry and highly coherent-optical interferometry simultaneously. The absolute measurement range, which is determined by the bandwidth of the Bragg wavelength reflected by the Fiber Bragg Grating (FBG) in the sensor, is larger than 6mm. And the measurement resolution, which is determined by the highly coherent-optical interferometry, is less than 1nm.
A highly stabilized dynamic displacement measurement system, which employs fiber Bragg gratings to interleave two fiber Michelson interferometers that share the common interferometric optical path, is presented. The phase change in the interferometric signals of the two fiber Michelson interferometers is tracked respectively by maintaining the phase difference in quadrature with two electronic feedback loops. One of the fiber interferometers is used to stabilize the system by the use of an electronic feedback loop to eliminate the influences that result from the environmental disturbances, while the other fiber interferometer is used for the measurement by employing another electronic feedback loop to track the phase change in the interferometric signal. The system is able to measure dynamic displacement and provide a sense of direction of the displacement at the same time. The dynamic displacement with frequencies ranging from 0.1 Hz to 200 Hz and with a maximum amplitude of 60 µm can be measured, and the measurement resolution can reach 10 nm.
The stabilization of a multiplexed optical fiber interferometer system for on-line displacement precision measurement
with a simple electric feedback loop is presented. Based on the characteristics of fiber Bragg gratings, a multiplexed
optical fiber interferometer system includes two independent optical fiber Michelson interferometers, of which the
optical path is almost overlapped. One interferometer is used for stabilization while other interferometer is used for
measurement. A feed back signal from the feedback loop drives tube PZT, on which one arm of the fiber interferometer
is wounded. The phase-shift in two arms of the interferometer resulting from the temperature fluctuations and other types
of environmental disturbances is compensated. The bandwidth of the feedback loop is 5kHz. This makes the multiplexed
fiber interferometer system stable enough for on-line precision measurement.
A vibration-displacement measurement system based on tracking the phase variation of an optical fiber Michelson interferometer with electronic feedback loops is presented. The measurement system includes two sets of electronic feedback loops. One electronic feedback loop is used to compensate for the low frequency drifts in the phase of the interferometric signal that results from environmental disturbances, while the other one is used to track the phase variation induced by the measured vibration displacement, and thus realize the measurement of the vibration displacement and provide a sense of direction of the displacement simultaneously. The measurement system is designed to be capable of measuring vibration displacement with frequencies ranging from 1.5 to 200 Hz, with measurement resolution reaching 13 nm.
The stabilization of a multiplexed optical fiber interferometer system for on-line displacement precision measurement
with a simple electric feedback loop is presented. Based on the characteristics of fiber Bragg gratings, the multiplexed
optical fiber interferometer system includes two independent optical fiber Michelson interferometers of which the optical
path is almost overlapped. One interferometer is used for the stabilization while the other interferometer is used for the
measurement. A feed back signal from the feedback loop is driving a tube PZT on which one arm of the fiber
interferometer is wounded. The phase-shift in the two arms of the interferometer resulting from the temperature
fluctuations and other types of environmental disturbances is compensated. The bandwidth of the feedback loop is 5kHz.
This makes the multiplexed fiber interferometer system stable enough for the on-line precision measurement. An active
phase tracking technique is applied for signal processing to achieve high resolution. The measurement resolution of the
system is less than 2nm.
A multiplexed optical fiber Michelson interferometer system that is self-referenced with a stabilizing feedback loop is presented. This system employs fiber Bragg gratings and wavelength division multiplexing technique to combine two optical fiber interferometers that share the same optical path in the main part of the optical system. When one Michelson interferometer, which uses the fiber Bragg gratings as reflective mirrors and is used as reference interferometer, is stabilized by an electric feedback loop, the other interferometer, which is used for the measurement, is also stabilized. This system is therefore suitable for online precision measurement. An active phase-tracking technique is applied for signal processing to achieve high resolution.
In this paper we report the study of a single mode fiber grin lens tunable Fabry-Perot filter. The Fabry-Perot filter consists of two grin lens on which the multi-layer film with high reflectivity is deposited. The cavity is tuned continuously over the whole wavelength range of 40nm by a piezoelectric transducer (PZT). The device has a free spectrum of 30nm and 3dB peak bandwidth is less than 1dB. The filter can be applied to wavelength interrogation in fiber Bragg grating (FBG) sensing system to detect the drift of the Fiber Bragg wavelength.
A novel technology for simultaneous and independent measurement of dual parameters is proposed and experimented. The length of a single fibre Bragg grating (FBG) is divided into two parts. The temperature variation and the strain are able to be measured independently and simultaneously, and the thermal effect can be erased. A metrological grating is employed to read out the output of the measurement system. The resolution of the measurement of strain can reach to 0.5μ strain, which corresponds to 0.04°C of temperature variation.
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