Shih-Chu Huang

Modified phase-generated carrier demodulation compensated for the propagation delay of the fiber.

In general applications, the output signals of the fiber-optic interferometric sensor (FOIS) need some demodulation techniques to linearly demodulate the sensing phase signal. The common signal demodulation circuit of the FOIS is passive homodyne demodulation using phase-generated carrier [(PGC) demodulation]. Preliminary analysis of the research demonstrates that a variation in the output demodulated signal is related to the length of the fiber, and the output demodulated signal will approach zero at some certain lengths of the fiber. To improve the performance of the FOIS, it is necessary to develop the modified PGC demodulation to compensate for the propagation delay of the fiber.

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.

Method for path imbalance measurement of the two-arm fiber-optic interferometer

The path imbalance (PI) of the two-arm fiber-optic interferometric sensor is a substantial parameter; a precise value of millimeters is required. Currently the precision reflectometry and the millimeter optical time-domain reflectometry are used to measure the tiny optical path difference, but the performances of these measurements are limited from the length and the resolution of the PI. We propose a new method accomplished by interferometer to accurately measure millimeters to within a few decimeters of the PI.

Counting signal processing and counting level normalization techniques of polarization-insensitive fiber-optic Michelson interferometric sensors

A counting signal processing technique of the fiber-optic interferometric sensor is proposed. The technique is capable of counting the numbers of the maximum and minimum of the output interferometric signal in a specific time duration, and it can be used as the basis to distinguish the sensing phase signal. It can also be used as a signal detector on applications such as intrusion detection. All sensors are subject to aging of the optical components and bending loss, and therefore the output signal of each sensor may vary with time. We propose a counting level normalization technique to compensate for these variations and to obtain the correct counting numbers.

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.