Harmeet Singh

Simultaneously measuring temperature and strain using optical fiber microcavities

Presents an optical fiber sensor capable of simultaneously measuring both axial strain and temperature. The sensor configuration is composed of a 70 /spl mu/m in-line fiber etalon (ILFE) cascaded with a 170 /spl mu/m intrinsic Fabry-Perot (IFP) sensor to form a lead in-sensitive, localized, 240 /spl mu/m gauge-length dual parameter sensor. The sensor response from this configuration is interrogated using coherence division multiplexing based on white light interferometry, and using pseudoheterodyning demodulation. The results obtained in a simultaneously varying temperature and strain field by the fiber sensor are compared with a semiconductor temperature gauge and a resistance strain gauge. The correlation is very good.

Evaluation of integrated optic modulator-based detection schemes for in-line fiber etalon sensors

This paper compares three classes of demodulation schemes for interpreting the static and dynamic response of in-line fiber etalon (ILFE) sensors. Each demodulation scheme employs path matched differential interferometry and an unbalanced integrated optic Mach-Zehnder interferometer as a readout interferometer. Active homodyne, synthetic-heterodyne and two types of pseudo-heterodyne based demodulators are evaluated. In the results presented here, signals generated by the ILFE ranging from quasistatic to about 100 Hz were recorded and compared with the signals generated by a resistance strain gauge, and were found to be in good agreement. >

Direct extraction of phase gradients from Fourier-transform and phase-step fringe patterns

An approach to computing pixel-by-pixel gradients of optical phase directly from digitally encoded Fourier-transform or phase-stepped fringe patterns is described. This approach can be classified as a phase unwrapping but is really a sine-cosine demodulation technique that finds its roots in the differential cross-multiplier phase-demodulation technique commonly used by the optical fiber sensor community. This technique is algorithmically simple, does not rely on a computation of the arctangent, and therefore is not subject to some of the limitations of the standard phase-unwrapping methodologies. The proposed phase-gradient technique is demonstrated by the calculation of strain fields from moir6 interferometric fringe patterns.

Narrow-band rejection filters with negligible backreflection using tilted photoinduced gratings in single-mode fibers

Suppression of the forward propagating LP/sub 01/ core mode of >17 dB with <-30-dB backreflection over a narrow wavelength band is demonstrated utilizing a tilted photoinduced Bragg grating in a deep depressed inner cladding single-mode fiber (SMF). Theoretical and experimental results detailing the tilted grating filter performance in matched cladding, depressed inner cladding, and photosensitive cladding SMFs are presented.

Narrow-depressed cladding fiber design for minimization of cladding mode losses in azimuthally asymmetric fiber Bragg gratings

The cladding and radiation mode loss characteristics of photo-induced gratings written in fibers with a narrow-depressed inner cladding are compared to the properties of gratings written in matched clad, photosensitive clad, and wide-depressed cladding fibers under varying grating azimuthal asymmetry conditions. Fibers with narrow-depressed claddings are found to have lower cladding mode losses for small degrees of asymmetry than previously proposed fiber designs for cladding-mode reduction.

Simultaneous measurement of temperature and strain using a single Bragg grating

A fiber optic sensor using the first and the second order fiber Bragg grating spectra to simultaneously measure temperature and strain is investigated. A theoretical model for estimating the wavelength and the reflectivity of the second order Bragg resonance in a fiber grating is developed, and the results are compared with the experimental results in Corning SMF 28 fiber. Moreover, technical issues regarding the sensor design such as the spectral characteristics of the Bragg gratings, bending loss in optical fibers and the higher order propagation modes are investigated. Experiments are conducted to measure the strain and temperature response of the grating sensors and compared to conventional sensors with encouraging results.

Dual-parameter optical fiber sensor

In this paper we have presented a sensor system capable of measuring two parameters on a surface mounted beam. We have used an In-Line Fiber Etalon (ILFE) and an Intrinsic Fabry- Perot (IFP) sensor to capture two parameters at a single point. This sensor with gage length of about 200 micrometers can be used to determine the axial strain and temperature on a beam or can be embedded in composite material to determine two strain components. The two sensor signals which travel in the same fiber are discerned using coherence based multiplexing schemes. Three tests are performed with the sensor mounted on a cantilevered beam to demonstrate the concept. In the first test, the beam is subjected to simple vibration and the phase strain model is used to calculate the strains from the two sensors. In the second test the beams is given only thermal loading, and in the third test, the beam is subjected to a combination of both thermal and mechanical loading.

Cross-talk and noise issues in coherence multiplexed in-line Fabry-Perot etalon (ILFE) strain sensor

In this paper we have investigated the cross-talk and noise issues that arise in serial multiplexing of In-Line Fiber Etalon (ILFE) strain sensors using three different topologies. First, the broadband source is numerically modeled to preserve all the artifacts in the line structure. Second, the effect of beam expansion in the non-guiding ILFE cavity is investigated. The two models are combined to calculate the fringe contrast, signal and background power, the shot-noise and cross-talk noise in the system. Also the signal extinction is studied as a function of mismatch between the ILFE gagelength and path-matching interferometer gagelength. Finally, suggestions are made to enhance the fringe contrast in ILFE based systems.

Micromechanical analysis of thick composites with embedded optical fibers

This paper addresses the micro-mechanical issues associated with embedding optical fiber sensors in a thick composite material. An optical fiber embedded in a thick composite creates a perturbation in the micro-structure of the composite, which in turn alters the strain state of composite in the region surrounding the optical fiber. Moire Interferometry combined with Fourier transform based digital fringe processing is used to measure and analyze the displacement field in the composite material.