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An application of a polarization-maintaining photonic crystal fiber (PM-PCF) for torsion sensing is proposed and experimentally demonstrated. The response of the sensor is theoretically validated using the Jones matrix. High normalized sensitivity of ~0.014/[degrees] was measured within a linear twist angle range from 30[degrees] to 70[degrees]. The sensor response was observed to be highly repeatable over a 90[degrees] twist in both the clockwise and counterclockwise directions. The proposed sensor exhibits reduced temperature sensitivity due to the low thermal coefficient of PM-PCF, making it an ideal candidate for torsion sensing. OCIS codes: 060.2370, 060.3735.

A rigorous, much simplified, and accurate analysis of the modal field characteristics such as propagation constants, mode power confinement, delay and dispersion characteristics of a single-mode graded-index elliptical core fiber is presented applying a variational method and super-Gaussian approximation of the fundamental modal field. Normalized propagation constants, a fundamental parameter to evaluate other modal characteristics, obtained through this method showed a greater accuracy over the entire range of practical interest in comparison with other reported methods. The effects of various aspect ratios on these characteristics are analyzed. In addition, the effect of Kerr nonlinearity on these characteristics is investigated using the reported method, and a comparison is made with the linear results. OCIS codes: 060.2310, 060.2270, 060.2400.

A high spectral contrast is expected to be very important when laser-induced fluorescence (LIF) is employed for cancer diagnosis. We developed a LIF optical fiber sensor to achieve a very high spectral contrast between normal and malignant tissues. A comprehensive experimental investigation was carried out to study the role of two critically important parameters for sensor design, namely, the excitation--collection geometry and the excitation wavelength, and their effect on the autofluorescence spectral contrast. An optimum sensing configuration was determined in order to enhance the small, but consistent, spectral difference between the normal and the malignant tissue for improving the accuracy of LIF-based cancer diagnosis. With the optimum sensor configuration, we realized a spectral contrast of more than 22 times between normal and malignant tissue sample spectra. OCIS codes: 060.2370, 060.2310, 300.2530.

A novel intrinsic fiber optic pressure sensor realized with a polarization-maintaining photonic crystal fiber (PM-PCF) based Sagnac interferometer is proposed and demonstrated experimentally. A large wavelength-pressure coefficient of 3.42 nm/MPa was measured using a 58.4 cm long PM-PCF as the sensing element. Owing to the inherently low bending loss and thermal dependence of the PM-PCF, the proposed pressure sensor is very compact and exhibits low temperature sensitivity. OCIS codes: 060.2370, 060.2420, 060.5295.

A spontaneous Raman scattering optical fiber sensor was developed for a specific need of the National Aeronautics and Space Administration (NASA) for long-term detection and monitoring of the purity of liquid oxygen (L[O.sub.2]) in the oxidizer feed line during ground testing of rocket engines. The Raman peak intensity ratios for liquid nitrogen (L[N.sup.2]) and L[O.sub.2] with varied weight ratios (L[N.sup.2]/L[O.sub.2]) were analyzed for their applicability to impurity sensing. The study of the sensor performance with different excitation light sources has helped to design a miniaturized, cost-effective system for this application. The optimal system response time of this miniaturized sensor for L[N.sup.2]/L[O.sub.2] measurement was found to be in the range of a few seconds. It will need to be further reduced to the millisecond range for real-time, quantitative monitoring of the quality of cryogenic fluids in a harsh envioronment. OCIS codes: 0300.6450, 300.6330, 300.0300, 140.3550, 170.5660, 060.2370.

A π-phase-shifted fiber Bragg grating (π-FBG) shows high sensitivity to the ultrasonic (US) wave as compared to the conventional FBG due to the strong slow-light phenomenon at the resonance peak. However, its sensitivity is limited by the interrogation schemes. A combination of π-FBG and unbalanced fiber Mach- Zehnder interferometer (F-MZI) are theoretically analyzed and optimized for the highly sensitive acoustic sensor. The coupled-mode theory (CMT) and transfer matrix method (TMM) are used to establish the numerical modelling of π-FBG. For the optimized grating parameters of π-FBG, the proposed sensing system shows the high strain sensitivity of 1.2 × 108=", the highest dynamic strain resolution of 4:1f"= p Hz, and the highest wavelength shift resolution of 4:9 × 10-9 pm. Further, the proposed sensing system strongly supports both time andwavelength division multiplexing techniques. Therefore, the proposed sensing system shows extreme importance in single as well as quasi-distributed US acoustic wave sensing networks. [ABSTRACT FROM AUTHOR]

This article investigates optical power transmission characteristics with varying launching conditions concentrating on weakly guiding, step-index highly multi-moded optical fibers. A single analytical expression for the fractional power transmitted per mode in a highly multi-moded fiber is obtained. This expression is used to numerically obtain the fractional power per mode, power per modal order, and finally, the total power carried by the fiber. Experiments are carried out to investigate the effect of launching conditions on the total power carried by the fiber and to establish a possible correlation with the theoretical results. Reasons for the observed experimental discrepancies are also discussed. [ABSTRACT FROM AUTHOR]