Mao-Hsiung Chen

Crosstalk analysis and system design of time-division multiplexing of polarization-insensitive fiber optic Michelson interferometric sensors

A system of time-division multiplexing of polarization-insensitive fiber-optic Michelson interferometric sensors that uses Faraday rotator mirror elements is demonstrated. This system is constructed with conventional low-birefringence single-mode fiber and is able to solve the polarization-fading problem by a combination of Faraday rotator mirrors with unbalanced Michelson interferometers. The system is lead-fiber insensitive and has potentials for practical field applications.

Phase sensitivity normalization in time‐division multiplexing of polarization‐insensitive interferometric sensors using phase‐generated carrier demodulation

The time-division multiplexing of a polarization-insensitive fi- ber optic Michelson interferometric sensor (TDM-PIFOMI) using phase- generated carrier (PGC) demodulation is presented. A new automatic gain control (AGC) process for the phase sensitivity normalization of each sensor in the TDM-PIFOMI system using the PGC demodulation is described and demonstrated. In the TDM-PIFOMI system, this process can eliminate effects including the fluctuation of the laser source intensity and the variation of the light pulse intensity distribution in all fibers. The advantage of the AGC process is to implement the process under the optimum condition of the PGC demodulation.

Modified in-line Sagnac interferometer with passive demodulation technique for environmental immunity of a fiber-optic current sensor

A modified in-line Sagnac interferometer (MISI) with passive demodulation Technique (PDT) was proposed to immunize the fiber-optic current sensor (FOCS) from environmental perturbations. A large vibration to simulate the environmental perturbations with acceleration up to 12 g was applied to the lead fiber of the FOCS. The noise floor could be significantly suppressed (20 dB) by the MISI better than by a conventional interferometer. In the same dynamic environments, the PDT could make the FOCS achieve a good linear demodulation with average distortion rates always lower than 0.9%. In addition, all the sensitivities measured in both static and dynamic environments are all approximately 4.5 microrad/(A(rms) turns), which is close to the literatural data measured in static environments. These considerable achievements of high sensitivity, environmental immunity, and free electric shock concerns may lead FOCSs to field-monitoring applications of power delivery lines.

Cross-talk analysis of time-division multiplexing ofpolarization-insensitive fiber-optic Michelson interferometric sensors with a3 × 3 directional coupler

A time division multiplexing of polarization-insensitive fiber-optic Michelson interferometric sensors (TDM-PIMIs) with a 3 x 3 directional coupler is presented. The elimination of polarization-induced fading and the output intensities of the TDM-PIMI system are described and demonstrated. The output intensity of each sensor of the system can be demodulated by a passive homodyne method to increase the sensor bandwidth significantly. The sensor cross talk of the system having an optical gate with a finite extinction ratio is analyzed. The use of a laser source with an adequate coherence length to reduce the sensor cross talk is suggested. The delay-fiber cross talk of the system by Rayleigh backscattering is analyzed and demonstrated. We further suggest some methods that could possibly reduce the effect of the Rayleigh backscattered light. Finally a sophisticated design of a TDM-PIMI system with a 3 x 3 directional coupler is described.

Fiber optic microampere dc current sensor

A microampere dc current is measured by a fiber optic sen- sor. The phase change of the guided light induced by ohms heat over a golden-coated fiber is detected by an interferometer. The thermal con- duction of the optical fiber and the relation of the temperature-induced phase change of the guided light are explored. A Michelson interferom- eter plus a Faraday rotator mirror enable system immunity from the polarization-induced signal fading due to the environmental disturbance, especially by vibration and temperature. A passive phase-generated car- rier (PGC) demodulation scheme enables the system to demodulate the interference signal stably by shifting the signal away from the high-noise, low-frequency band through a carrier. The sensitivity of the sensing sys- tem is improved by a biased method. The minimum measurable dc cur- rent is 3.416310 27 A with a dynamic range of 89.33 dB.

Vibration insensitive optical fiber current sensor with a modified reciprocal reflection interferometer

A modified reciprocal reflection interferometer (MRRI) is proposed for vibration insensitive optical fiber current sensors (OFCSs). The verifications of experiments are in good agreement with our theoretical analyses. A large vibrational perturbation applied to a segment of lead fiber to simulate the disturbance to the OFCS can be suppressed over 20 dB compared with a conventional reciprocal reflection interferometer (RRI). The noise floor of the OFCS with an MRRI under perturbations is about 1.36 Armsturns/Hz1/2 near the shot noise limit. A good linear demodulation result is obtained and the evaluation of the Verdet constant 1.1?rad/Arms turns approximates the literatural data measured in static environment.

Fiber-Optic Current Sensor Using Passive Demodulation Interferometric Scheme

An interferometer using a Faraday rotator with 22.5 rotation bias is proposed for the use of a fiber-optic current sensor (FOCS) to demodulate phase signals passively. High sensitivities and extra low distortions are obtained by this passive demodulation interferometric scheme (PDIS). The Verdet constant of optical fibers at 1300 nm is evaluated to be 4.5 murad (A rms.turns), which is in good agreement with the literature. With signal distortions always below 0.8 %, the proposed scheme should be significantly effective for the demodulation needs of an FOCS. The PDIS also could enable a FOCS to directly connect with fiber-optic communication networks for the implementation of long-distance remote and multiplexing sensor systems. Moreover, electric shock concerns on current intensity measurements of high-voltage power delivery systems are greatly reduced.

Analysis of distortion of a fiber optic Michelson interferometric sensor caused by the imperfect properties of its 3×3 coupler

We investigate the distortion of demodulated output signal in a fiber optic Michelson interferometric sensor in relationship to the imperfect properties of its 3 X 3 coupler. A general guide rule in choosing a 3 X 3 coupler and its connection type in the interferometer to reduce this kind of signal distortion is also suggested.

Transfer function of the thin-shell PZT ceramic cylinder as a phase modulator in fiber optic sensors

An optical fiber PZT cylinder phase modulator is frequently used in an optical fiber interferometric sensor for demodulating sensing signals. In this paper, we combine the piezoelectric equations and the fiber phase shift relations to deliver a thorough description of the phase response of the optical fiber PZT cylinder phase modulator. We also design an experiment to verify our derivation. The result is quite compatible with published data.

Environmental immunity of time-division multiplexing optical fiber current sensor employing a modified reciprocal reflection interferometer with a passive demodulation scheme

A modified reciprocal reflection interferometer (MRRI) with passive demodulation scheme (PDS) has been proposed for the implementations of time-division multiplexing optical fiber current senor (OFCS) system. The PDS to simplify the demodulating systems of OFCS has been developed. Descriptions of theory as well as experimental verification are presented. A good linear demodulation result is obtained. The Verdet constant measured experimentally is 4.2 (mu) rad/A*turns, in good agreement with the literatural data. By using the proposed modified reciprocal reflection interferometer, an OFCS is able to become insensitive to environment perturbations. The OFCSs noises that arise from its lead fiber disturbed by vibrating perturbations can be retrained over 20dB compared with the noises of an OFCS using a conventional configuration. It is believed that this work could significantly lead an OFCS not only to the developments of a multiplexing current sensor system, but also to a long-distance remote monitoring of HV-lines because of both the geometric feasibility of fiber optic mechanism of the MRRI and the effectively demodulated circuit reduction of the PDS.