Tarun Kumar Gangopadhyay

Non-contact vibration measurement based on an extrinsic Fabry–Perot interferometer implemented using arrays of single-mode fibres

This paper presents a novel non-contact vibration-monitoring technique based on transient measurements from a Fabry–Perot interferometric displacement sensor implemented using single-mode fibre. Two different extrinsic sensor configurations are demonstrated using one-fibre and two-fibre arrays of 4/125 µm single-mode fibre butted with a gradient-index lens. The design concept, mathematical modelling and results are demonstrated. The sensor configurations exploit fringe discrimination over multiple orders. A working range up to 14 µm is demonstrated for the measurement of vibration amplitude at 1 kHz excitation using the one-fibre configuration. Similar tests using the two-fibre configuration indicated a displacement measurement up to 65 µm and vibration measurement up to 7 µm at 2.5 kHz. The two-fibre configuration is also assessed in conjunction with an absolute scheme for the measurement of vibration using dual-wavelength signal processing. In this scheme, two wavelength signals are captured from the sensing interferometer to provide unambiguous measurement of vibration direction.

Vibration: history and measurement with an extrinsic Fabry–Perot sensor with solid-state laser interferometry

We have studied the history of vibration and demonstrate a laser-based noncontact interferometric vibration sensor. The sensor promises the measurement of microdisplacement by using a Fabry-Perot cavity formed between a partially coated gradient-index lens and a movable reflector. Displacement is determined by the detection of interference fringes caused by phase modulation within the cavity. The sensor was tested in conjunction with both multimode and single-mode fiber transmission. Calibration with multimode fiber produced a fringe-contrast function that decreased monotonically with displacement. This calibration allowed at least 30 fringes to be discriminated, giving a displacement resolution of 0.034 microm across a range of 10.2 microm. Dynamic tests demonstrated a working range of at least 3.74 microm at frequencies as high as 2 kHz. Similar tests in which single-mode fiber was used indicated a dynamic working range of at least 4.29 microm.

Wavelet analysis of optical signal extracted from a non-contact fibre-optic vibration sensor using an extrinsic Fabry–Perot interferometer

Interferometric optical fibre sensors have proved to be many orders of magnitude more sensitive than their electrical counterparts, but they suffer from limitations in signal demodulation caused by phase ambiguity and complex fringe counting when the output phase difference exceeds one fringe period and for multiple fringes. This paper presents a novel signal decoding technique based on the wavelet transform of optical data extracted from a non-contact vibration sensor using an extrinsic Fabry-Perot interferometer (EFPI) implemented using single-mode fibre. The EFPI cavity has been used to generate an optical interference signal between two parallel, highly reflective surfaces separated by a variable distance. Firstly, a few recorded experimental results of the interference fringe formation due to vibration are presented in this paper. Then the wavelet transform is used for decoding the vibration signature for three major purposes of the data analyses, namely elimination of noise from the optical signals collected in real time, identification of the frequency breakdown points of the signal efficiently and automatic counting of the interference fringes. In turn, the wavelet transform is successfully employed to decode the vibration signature from the non-stationary output signal of an EFPI sensor.

Modeling and analysis of an extrinsic Fabry–Perot interferometer cavity

A schematic representation of optical feedback between two resonator mirrors undergoing a phase shift each round trip as a function of the separation of the mirrors is studied. A transfer function modeling of the extrinsic Fabry-Perot interferometer (EFPI) is presented. Nyquist analysis has been used to forecast the operational stability and possibility of interference in an EFPI. The analysis with two perfectly parallel surfaces of the cavity shows efficient interference. The performance when there is some tilt between the two mirrors in the cavity is also studied and is presented. In this case some restricted interference is found.

Optical Fiber Sensing Based on Reflection Laser Spectroscopy

An overview on high-resolution and fast interrogation of optical-fiber sensors relying on laser reflection spectroscopy is given. Fiber Bragg-gratings (FBGs) and FBG resonators built in fibers of different types are used for strain, temperature and acceleration measurements using heterodyne-detection and optical frequency-locking techniques. Silica fiber-ring cavities are used for chemical sensing based on evanescent-wave spectroscopy. Various arrangements for signal recovery and noise reduction, as an extension of most typical spectroscopic techniques, are illustrated and results on detection performances are presented.

Vibration monitoring by using a dynamic proximity sensor with interferometric encoding

An optical sensor for the dynamic measurement of proximity is presented. The sensor combines an extrinsic geometric transducer with interferometric encoding for high vibration sensitivity. Static calibration showed a unique variation in interference contrast over at least 60 fringes, leading to a measurement range of 20 mum and a resolution of at least 0.033mum. Dynamic excitation by using low-amplitude vibrations at 3.6 kHz showed a similar contrast variation, verifying fringe discrimination up to the sixth order. With verification of dynamic performance over all 60 fringes, the sensor should offer a low-cost approach to vibration monitoring in electrical switchgear.

Fiber-optic sensor for real-time monitoring of temperature on high voltage (400KV) power transmission lines

On-line monitoring of temperature and sag in 400KV power transmission line has successfully been implemented by a novel device using fibre Bragg grating (FBG) sensors. The complete device has been fabricated with aluminum mount connected via fibre-optic cable and installed on ACSR power conductor for continuous two years measurement. This paper presents the excellent results and experience of the tests in controlled indoor environments conducted in Norway and real-field application on installed power conductor in India. Thus, better surveillance of the thermal and mechanical loads on power lines can be possible using this FBG sensor system.

Eigenvalue equation and core-mode cutoff of weakly guiding tapered fiber as three layer optical waveguide and used as biochemical sensor.

The core-mode cutoff plays a major role in evanescent field absorption based sensors. A method has been proposed to calculate the core-mode cutoff by solving the eigenvalue equations of a weakly guiding three layer optical waveguide graphically. The variation of normalized waveguide parameter (V) is also calculated with different wavelengths at core-mode cutoff. At the first step, theoretical analysis of tapered fiber parameters has been performed for core-mode cutoff. The taper angle of an adiabatic tapered fiber is also analyzed using the length-scale criterion. Secondly, single-mode tapered fiber has been developed to make a precision sensor element suitable for chemical detection. Finally, the sensor element has been used to detect absorption peak of ethylenediamine. Results are presented in which an absorption peak at 1540 nm is observed.

Time-frequency analysis of multiple fringe and nonsinusoidal signals obtained from a fiber-optic vibration sensor using an extrinsic Fabry-Pe/spl acute/rot interferometer

In the case of multiple fringes and complex frequency measurements, the frequency of the output signal changes rapidly when the vibration changes and frequency breakdown takes place at the turning point. For a particular vibration signature containing many frequency components at different time intervals, it is often difficult to trace the direction of the vibration as well as individual frequency peaks. In such cases, advanced signal-processing scheme is necessary to decode the vibration signature. This paper investigates the data interrogation technique for multifrequency and complex signals of surface vibration obtained from an extrinsic Fabry-Pe/spl acute/rot interferometric sensor. In this paper, wavelet transform (WT)-based signal processing methodology has been employed to count of optical fringes with special reference to signals having subfringes. A WT-based tool has also been developed for unambiguous identification of frequency components from a nonsinusoidal vibration. The results of such WT-based analyses are presented, and merits as well as demerits of the proposed methods are discussed.

Fabrication of tapered single mode fiber by chemical etching and used as a chemical sensor based on evanescent field absorption

Single mode tapered fiber (SMTF) has been fabricated with core diameter of 8 μm and reduced cladding diameter up to 11 μm by hydrofluoric acid (HF) etching technique. To obtain the required cladding diameter, the time of etching has been optimized by using different HF concentrations. The mechanism as well as kinetics path of etching reaction on standard optical fiber is discussed. This study is related to surface catalyzed dissociation of HF followed by direct reaction with adsorbate molecules and the surface silicon oxide molecules. The etched tapered fibers are then packaged on quartz substrate to use as sensor element. Finally, the etched fiber is used as an element within chemical sensor based on evanescent field absorption. In this experiment, a 419-ppm cobalt nitrate solution is used for sensing.