Han Cheng Seat

Robust Detection of Non-Regular Interferometric Fringes From a Self-Mixing Displacement Sensor Using Bi-Wavelet Transform

An innovative signal processing method based on custom-made wavelet transform (WT) is presented for robust detection of fringes contained in the interferometric signal of self-mixing (SM) laser diode sensors. It enables the measurement of arbitrarily-shaped vibrations even in the corruptive presence of speckle. Our algorithm is based on the pattern recognition capability of bespoke WTs for detecting SM fringes. Once the fringes have been correctly detected, phase unwrapping methods can be applied to retrieve the complete instantaneous phase of the SM signals. Here, the novelty consists in using two distinct mother wavelets Ψr(t) and Ψd(t) specifically designed to distinguish SM patterns as well as the displacement direction. The peaks, i.e. the maxima modulus of WT, then allow the detection of the fringes.

Dual-modulation fiber Fabry-Perot interferometer with double reflection for slowly-varying displacements.

This Letter describes a dual-amplitude modulation technique incorporated into a double reflection extrinsic-type fiber Fabry-Perot interferometer to measure periodic, nonperiodic as well as quasi-static displacements. The modulation scheme simultaneously maintains the interference signal pair in quadrature and provides a reference signal for displacements inferior to a quarter of the source wavelength. The control and phase demodulation of the interferometer carried out via software enable quasi-real-time measurement and facilitates sensor alignment. The sensor system can be exploited in the low frequency range from 10(-3) to ∼500 Hz and has a resolution better than 2.2 nm, targeting applications in geophysics.

Amplitude and Phase Drift Correction of EFPI Sensor Systems Using Both Adaptive Kalman Filter and Temperature Compensation for Nanometric Displacement Estimation

Nanometric displacement measurements by Extrinsic Fiber Fabry-Perot interferometers (EFPI) is extremely susceptible to external environmental changes. Temperature, in particular, has a remarkable influence on the optical power and wavelength of the laser diode in use, in addition to the thermal expansion of the mechanical structure. In this paper we propose an optimization of the EFPI sensor in order to use it for very long-term (more than one year) and for high-precision displacement measurements. For this purpose, a real time and adaptive estimation procedure based on a homodyne technique and a Kalman filter is established. During a sinusoidal laser diode current modulation, the Kalman filter provides a correction of the amplitude drift caused by the resultant optical power modulation and external perturbations. Besides, stationary temperature transfer operators are estimated via experimental measurements to reduce the additive thermal noise induced in the optical phase and mechanical components.

Compact System for Photometric Characterization of Automotive Headlamps

We present a system allowing the determination of the distribution of light at any given distance (e.g. 25m) from a light source, by measuring the intensity and direction of the illumination distribution at a closer distance (e.g. near 20 cm). The measurement principle is based on the acquisition of images taken by a CCD matrix-sensor through an optical system focused at infinity. A sequence of images is obtained by moving the camera at different positions in front of the source. The process involves the accumulation of shifted images and corrections for optics (geometrical distortion and vignetting) and electronic shutter (exposure time). We describe the developed experimental set-up and the calibration procedures needed to obtain absolute photometric values. We present results obtained for automotive headlamps: computed relative illumination distribution is very similar to that obtained using a reference given by a measurement tool using a lightmeter. Absolute calibration of the system is performed from the set of measured points

Robust fringe detection based on bi-wavelet transform for self-mixing displacement sensor

A novel signal processing method based on custom-made Wavelet Transform (WT) is presented for robust detection of fringes contained in the interferometric signal of Self-Mixing (SM) laser diode sensors. It enables the measurement of arbitrarily-shaped vibrations even in the corruptive presence of speckle. Our algorithm is based on the pattern recognition capability of the customized WT for identifying SM fringes. Once the fringes have been correctly detected, phase unwrapping methods can be applied to retrieve the complete instantaneous phase of the SM signals. Here, the novelty consists in using two distinct mother wavelets ψr(t) and ψd(t) particularly designed to distinguish SM patterns as well as the displacement direction. The peaks, i.e. maxima of WT, then allow the detection of the fringes.

Comprehensive modeling of multimode fiber sensors for refractive index measurement and experimental validation

We propose and develop a comprehensive model for estimating the refractive index (RI) response over three potential sensing zones in a multimode fiber. The model has been developed based on a combined ray optics, Gaussian beam, and wave optics analysis coupled to the consideration of the injected interrogating lightwave characteristics and validated experimentally through the realization of three sensors with different lengths of stripped cladding sections as the sensing region. The experimental results highly corroborate and validate the simulation output from the model for the three RI sensing zones. The sensors can be employed over a very wide dynamic RI range from 1.316 to over 1.608 at a wavelength of 1550 nm, with the best resolution of 2.2447 × 10−5 RI unit (RIU) obtained in Zone II for a 1-cm sensor length.

A novel FBG interrogation method for potential structural health monitoring applications

We propose an improved innovative interrogation scheme for fiber Bragg grating-based strain sensors using the optical feedback coupled into the cavity of a DFB laser diode. Strain-induced shifts of the Bragg grating peak reflectivity, which are back-reflected into the laser cavity when both the Bragg and laser wavelengths are in resonance, result in sawtooth-type fringes typical of optical feedback interference being produced. By introducing a modulation scheme to the laser drive current, the laser wavelength is scanned slowly across the extremities of the FBG spectrum, allowing the dynamic strain range to be extended by a factor of approximately half the width of the FBG spectrum. A resolution of ∼6 µe is currently obtained via basic fringe counting of the detected sawtooth interference signals.

Self-mixing sensing under strong feedback using multimode semiconductor lasers

The use of bi-mode semiconductor lasers is demonstrated as a candidate to solve the problems of self-mixing sensing under strong feedback. Experimental results show the recovery of fringes when the laser is in bi-mode regime.

A phase-tracking fiber interferometer for seismologic applications

A fiber Fabry-Pe acuterot interferometer is proposed for high precision seismic wave detection. Simple polarization optics are employed in the sensor head design to generate two inter-dependent quadrature phase-shifted interference fringes, with the added advantage of reducing/suppressing signal attenuation due to polarization scrambling in the fiber system. Interference signal demodulation exploits an algorithm which optimizes the phase calculation on the alternatively evolving quadrature pair as well as enables phase compensation for out-of-quadrature signals. Vibration measurements carried out concurrently with a seismometer under free oscillating conditions show excellent agreement. Differences obtained from both instruments in displacement amplitude and velocity amounted to ~50 nm and 5 nm/ms, respectively.

A multimode fiber refractive index sensor

We present a simple refractive index sensor based on multimode fibers realized by stripping three different lengths of cladding. We theoretically explain and experimentally validate three mechanisms occurring in the multimode fiber for sensing surrounding refractive index. This sensor has been demonstrated at 1550 nm for a wide range of refractive index variation from 1.3164 to 1.608. Our sensor is very sensitive for measuring refractive index values lower than but close to the refractive index value of the fiber core.