Date of Award

Summer 1993

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Electrical & Computer Engineering

Program/Concentration

Electrical Engineering

Committee Director

Oscar R. Gonzalez

Committee Member

Sacharia Albin

Committee Member

Ravindra P. Joshi

Call Number for Print

Special Collections LD4331.E55N58

Abstract

Fiber optic sensors are widely used in today's technological applications, providing many advantages that conventional sensors lack. In particular, there is a demand in hypersonic applications to develop microphones to measure acoustic pressure that performs in high temperature and noisy surroundings. This thesis analyzes fiber optic lever microphones (FOLM), since they perform at high temperatures. An FOLM system is an intensity modulated sensor, which is simple to implement and is small in size. Two models of the FOLM have been fabricated and calibrated. The first model uses a seven-element fiber array while the second uses a single-fiber to transmit and receive light. The second model was found to be more stable at the expense of some reduced sensitivity. This microphone is able to detect acoustic pressures at temperatures of 1000° F with good sensitivity. The operating frequency range extends to 25 KHz, and the dynamic range is 190 dB. The microphone is also stable with temperature cycling, and the sensitivity remains within a tolerance of ± 3 dB.

This thesis details the design of the single-fiber FOLM system, analysis of the transfer function of the system, laboratory calibration, noise analysis and the results of a field test. Computational algorithms are presented to predict the theoretical frequency response, determined primarily by the membrane parameters, and the gap between fiber and mirror to obtain maximum response. Calibration methods and instrumentation are explored to provide suitable procedures to calibrate the single-fiber FOLM system at room and high temperatures, which is believed to be done for the first time. A new data acquisition system was built to extend the frequency of an existing spectrum analyzer, and to serve as a portable system for field test purposes. The calibration data is analyzed to justify the theory of the system and to determine specifications. The noise analysis reveals the noise sources in the FOLM system and their contribution to the total background noise floor, which provides ideas to obtain better signal-to-noise ratio, that is, higher sensitivity for the microphone. This thesis also includes a field test result to show the ability of a single-fiber FOLM system in a high temperature and harsh environment, such as, in the Thermal Acoustic Fatigue Apparatus at NASA Langley Research Center. The results show stable response from the system in a high acoustic intensity, high temperature environment.

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DOI

10.25777/akgy-z992

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