Ashish M. Vengsarkar

Quadrature phase-shifted, extrinsic Fabry-Perot optical fiber sensors

We demonstrate the operation of a quadrature phase-shifted extrinsic Fabry-Perot fiber-optic sensor for the detection of the amplitude and the relative polarity of dynamically varying strain. Two laterally displaced single-mode fibers inserted within a hollow silica tube form the 90 degrees phase-shifted sensing system. A multimode fiber, placed in the tube facing the two fibers, acts as a reflector, thereby creating an air gap that acts as a Fabry-Perot cavity. A theoretical description of the sensor is given, and its operation as a dynamically varying strain sensor is described. Strain sensitivities of 5.54 degrees phase shift/microstrain cm(-1) are obtained.

Broad-band erbium-doped fiber amplifier flattened beyond 40 nm using long-period grating filter

Broad-bandwidth amplification is essential for the construction of high-capacity multichannel communication systems. We describe a silica-based erbium doped fiber amplifier (EDFA) with a flat gain bandwidth exceeding 40 nm. The dual-stage EDFA includes a precisely designed inter-stage long-period fiber grating filter with more than 14-dB peak attenuation. By careful choice of the filter spectrum and fiber lengths, this EDFA is flat to within 1 dB over 40 nm while producing a noise figure below 4.0 dB and nearly +15-dBm output power.

Optical fiber-based dispersion compensation using higher order modes near cutoff

Higher order spatial modes in optical fibers exhibit large, negative chromatic dispersion when operated near their cutoff wavelength. By using a spatial mode-converter to selectively excite a higher order mode in specially designed multimode fiber, this dispersion can be used to compensate the positive dispersion in conventional single-mode fiber spans. In this paper, issues related to compensating fiber and mode-converter design are explored. Experimental measurements in specially designed two-mode fibers operated in LP/sub 11/ mode show negative dispersion as large as -70 ps/nm/spl middot/km at 1555 nm. Pulse propagation and system experiments employing spatial mode-converters to excite LP/sub 11/ mode in a two-mode fiber demonstrate the feasibility of this technique for dispersion compensation in lightwave systems. >

Elliptical-core two mode optical-fiber sensor implementation methods

Experimental methods for the practical implementation of few-mode elliptical-core sensors are described. Techniques for desensitizing the lead-in and lead-out fibers are discussed, and results of a vibration sensor embedded in a graphite-epoxy composite are presented. A scheme using a single-mode elliptical-core fiber as the lead-in fiber and an offset circular-core single-mode fiber as the lead-out fiber is successfully implemented. Detection techniques for few-mode fiber sensors are reviewed, and a novel fringe-counting method to unambiguously detect the amplitude and direction of dynamic strain is reported. >

Fiber-optic dual-technique sensor for simultaneous measurement of strain and temperature

Polarimetric and two-mode differential interferometric schemes incorporated in an elliptical-core fiber are able to resolve strain and temperature simultaneously with resolutions of 10 /spl mu/m/m and 5/spl deg/C, respectively. A technique, based on the evaluation of the condition number of a matrix, is shown to be useful in evaluating comparative merits of multiparameter sensing schemes. The determinant of the beat length matrix is expressed in terms of mode propagation constants, and a method for designing specialized fibers suitable for simultaneous measurement of strain and temperature is proposed. Experimental results for four fibers are presented and cross-sensitivity issues are discussed. >

Fabry–Perot fiber-optic sensors in full-scale fatigue testing on an F-15 aircraft

We report results from fiber-optic-sensor field tests on an F-15 aircraft mounted within a full-scale test frame for the purpose of fatigue testing. Strain sensitivities of the order of 0.01 μm/m have been obtained.

Photoinduced refractive-index changes in two-mode, elliptical-core fibers: sensing applications

Photoinduced refractive-index changes in two-mode, elliptical-core optical fibers are shown to affect the differential phase modulation between the LP(01) and the LP(11)(even) modes. This change in beat length is dependent on the amount of strain induced in the fiber while the grating is being formed. We present experimental results that agree with conventional coupled-mode theory and propose the use of such sensors for weighted and distributed applications.

Splicer-based long-period fiber gratings

Summary form only given. Presents a one-step process for the fabrication of efficient, compact long-period gratings. The technique makes use ofa controlled arc to periodically modify an optical fiber by a pair of electrodes. The setup comprises a commercially available splicer whose fiber holding mechanism is replaced by a motorized stage, which allows the translation of the fiber between the electrodes. The motion of the fiber and the input current to the electrodes is controlled by a computer and an optical spectrum analyzer constantly monitors the transmission spectrum of the grating being fabricated.

Highly efficient single-mode fiber for broadband dispersion compensation

The advent of erbium-doped optical fiber amplifiers which operate at 1.55 µm has triggered considerable interest in schemes that will permit the upgrade of installed systems operating at 1.31 µm to operate in the 1.55 µm window. Since installed systems use standard single mode fibers with a dispersion-zero at 1.31 µm, any upgrade scheme must incorporate a method of compensating for the positive dispersion of the standard (non-dispersion-shifted) fiber at 1.55 µm if high transmission rates are to be achieved. An equalization technique that uses two-mode fibers was suggested by Poole et al..1 While high compensation ratios (defined as the ratio of the length of the communications fiber to the length of the compensating fiber) of 30:1 have been achieved, the higher-order mode technique requires intermodal convertors and polarization rotators, thus adding to the system cost and complexity. Dugan et al. 2 and Izadpanah et al.3 demonstrated the use of single-mode fiber compensators (using the fundamental LP01 mode). However, the single-mode fiber-based compensating schemes could not compensate for the second-order dispersion and absolute values of the dispersion were typically less than 100 ps/nm-km. In this paper, we present a multiclad dispersion compensating fiber (DCF) operated in the single-mode regime that has a dispersion of -212 ps/nm-km and is capable of compensating dispersion over a broad wavelength range.

Extrinsic Fabry-Perot Optical Fiber Sensor

Phase-modulated fiber optic sensors have been shown to possess high sensitivities for the measurement of strain, temperature, vibration, pressure and other parameters.1 Fabry-Perot (FP) sensors that are based on multiple beam interference eliminate the need for a reference arm and do not require sophisticated stabilization techniques as in the case of Mach-Zehnder and Michelson interferometers.2 Several techniques to create intrinsic optical fiber Fabry-Perot interferometers have been described in the past.3-5 In a recent paper, we described an optical fiber extrinsic FP interferometer and used it as a sensor of microdisplacements and thermally- induced strain.6 The fiber interferometer was classified as extrinsic because the FP cavity was an air-gap between two fiber ends and the sensor output was immune to perturbations in the input/output fiber.