D.Ing. / This work comprises a study of reflective interferometric fibre optic sensors. The use of Bragg gratings, multilayer quarter-wave stacks, and rugate mirrors for fibre optic sensing applications is discussed in this work. Rugate mirrors are presented in more detail since they form an important part of the research. The control system for an electron cyclotron resonance plasma enhanced chemical vapour deposition reactor was developed for the growth of inhomogeneous dielectric filters. The system is intended to control the growth of films of the required refractive index profile for optical applications on different substrates including fibre ends. The system also includes an automatic microwave tuner. Rugate mirrors deposited directly on optical fibre were used in a dual cavity Fabry- Perot interferometric strain sensor. It was found by computer simulation that reflectances of 40% for the two outer mirrors and 68% for the centre mirror allow the interferometer to have maximum fringe amplitude. The sensor was evaluated experimentally as a strain gauge. The maximum strain applied to the sample was about 0.12% and the corresponding phase change was about 800 radians. The discrepancy between the readings of this sensor and those of a resistive strain gauge, applied to the same structure, did not exceed 5%. Two novel fibre optic sensors were developed during this work: a merged Sagnac- Michelson interferometric sensor for distributed disturbance detection, and a disturbance location sensor using modified Sagnac and Mach-Zehnder interferometers. Both are intended for distributed impact location along the fibre. The magnitude of impact can also be measured with these sensors. The merged Sagnac-Michelson interferometric sensor uses two light sources and a frequency selective mirror to separate the Michelson and Sagnac signals. Birefringence in the fibre was used to bias the Sagnac interferometer to have a sine response. Computer simulations yielded the optimal biasing conditions: the state of linear polarisation of the input light should be rotated by r/4 with respect to the horizontal axis; birefringence in the Sagnac loop must provide retardation of ff/2 with the optical axes rotated by K/4 with respect to the horizontal axis. We verified the sensor concept experimentally. The discrepancy between measured and actual locations of disturbances applied to the fibre did not exceed 2.5 m for a 200 m long sensing loop. The sensor using the modified Sagnac and Mach-Zehnder interferometers makes use of phase modulation and synchronous detection to obtain the sine-biased Sagnac signal. A modified Sagnac interferometer configuration that incorporates an additional coupler and a mirror, allows separation of the Sagnac and Mach-Zehnder signals. Operation of the new configuration was verified experimentally in the system with a sensing fibre of 100 m long. The discrepancy between measured and actual locations of disturbances applied to the fibre did not exceed 2 m.

Thesis. 1975. Ph.D.--Massachusetts Institute of Technology. Dept. of Physics. / Bibliography: leaves 189-194. / Ph.D.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Earth and Planetary Sciences, 1979. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND LINDGREN. / Includes bibliographical references. / by Curtis Alan Knight. / Ph.D.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Earth and Planetary Science, 1982. / Microfiche copy available in Archives and Science / Includes bibliographical references. / by Gaetan Jean Francois Joseph Cuvelier. / M.S.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Earth and Planetary Science, 1979. / Microfiche copy available in Archives and Science. / Bibliography: leaf 26. / by Frank Alan Kreimendahl. / M.S.

The experimental investigation of the application of mechanical interferometry to the construction of influence lines has proven successful. The method is similar in principle to other methods of model analysis. Mechanical interferometry offers the advantages of speed and permanence of record over the other methods in present use. The idea seems to offer commercial possibilities. / Master of Science

Moiré interferometry is a relatively new branch of photomechanics that utilizes a diffraction grating on the specimen to determine surface displacements. To date, it has been used primarily to determine in-plane displacements of the specimen surface. The objective of this work is to demonstrate that the technique is capable of determining out-of-plane displacements as well. A high-frequency phase grating on a specimen surface is illuminated by oblique beams; two diffracted beams are recorded by holographic interferometry. If the wavefront warpages of the two diffracted beams are characterized by fringe orders, Nₐ and N_b, it is demonstrated that the in-plane displacements are proportional to Nₐ - N_b, while out-of-plane displacements are proportional to Nₐ + N_b. Initially, these subtractive and additive parameters are determined numerically, and the resulting displacements are compared to displacements measured by well-accepted prior experimental methods. Excellent agreement with these proven methods is shown. Using this remarkably simple relationship between wavefront warpages, an experimental procedure is developed which is capable of simultaneously determining the in-plane and out-of-plane displacements of a specimen surface. The method requires only one photographic exposure of the deformed specimen, with displacement information being extracted through the use of optical filtering. The fact that the information required to yield these patterns can be obtained from a single photographic recording not only makes this an experimentally simple technique to use, but provides the capability of analyzing dynamic events as well. The sensitivity of measurements made by utilizing this technique are directly proportional to the frequency of the specimen grating. Therefore, it is desirable to obtain specimen gratings with frequencies on the order of 500-2000 lines/mm ( 12, 700 - 50,800 lines/in.). A technique is developed whereby high frequency moire gratings with highly reflective surfaces can be transferred to workpieces made from most engineering materials. Specimen gratings with frequencies as high as 2000 lines/mm (50,800 lines/in.) and exhibiting 10 percent diffraction efficiency in the first diffraction order have been applied to numerous specimens using simple laboratory techniques. / Ph. D.

A method of modifying a uÌ ber optic fused biconical tapered coupler to produce a relatively temperature insensitive Michelson interferometer is presented. The modification was accomplished by cleaving the coupler after the minimum taper region and polishing, perpendicular to the endface, to a point just short of the interaction region. This allows one of the two fiber cores, which are within micrometers of each other with their claddings fused together, to be coated at its endface with a reflecting material. This reflecting core serves as the reference arm, while the other core serves as the sensing arm. Variations in the method of fabrication of the sensor are described. Because of the close proximity and short length of the reference and sensing arms, the interferometer is temperature insensitive. The miniaturized Michelson interferometer is characterized and its limitations are discussed. A surface acoustic wave detection scheme is successfully demonstrated. Results of a magnetic field sensor using the miniaturized Michelson interferometer and a magnetostrictive material are presented. / Master of Science

Fiber optic interferometric sensors have been used to measure a number of physical measurands like strain and temperature. Among the different interferometric sensor configurations in use today, the Fabry-Perot interferometer has been shown to have a high sensitivity. The extrinsic Fabry-Perot sensor (EFPI) is a very good candidate for smart structure applications. In this thesis, a new configuration of the extrinsic Fabry-Perot interferometer is developed and demonstrated. This new configuration, based on four-beam interference, utilizes two EFPI sensor heads on a single directional coupler in a Split-cavity Cross-coupled Extrinsic Fiber Interferometer (SCEFI) arrangement. This sensor configuration eliminates the need for biasing the phase difference at the quadrature point. The spectrum analysis scheme, devised for signal demodulation of the SCEFI output allows a linear readout of the phase shift. Both dynamic and static phase shifts are considered. The spectrum analysis scheme developed in this thesis, also allows independent measurements of the phase shift in the two sensor heads. This leads to the multiplexing of two sensors, using a single source and a single coupler. The application of the SCEFI to two-sensor multiplexing is demonstrated experimentally in this thesis. Other significant applications of this new sensor configuration are also discussed. / Master of Science

This thesis describes the creation of a metrology system based upon an actively controlled diffraction grating interferometer, which measures relative linear distances. The dynamics of this sensor are estimated based on experimental testing, and a suitable controller is designed to maintain the position of the sensor in the most sensitive operating region. This controller is implemented on a field programmable gate array (FPGA) processor, which allows for flexible programming and real-time control. The sample under test is mounted atop a three axis linear stage system, which allows the diffraction grating interferometer to scan across the surface of the device, creating maps of the static and dynamic measurements. The controller is shown to maintain the sensitivity of the sensor during this operation. This insures all data are taken on the same scale, creating more accurate results. The controller increases the signal to noise ratio as compared to the system without the controller. The specifications of the entire metrology system are detailed including the sensor and controller bandwidth, the vertical and horizontal resolution, and the signal to noise ratio. A case study utilizing a capacitive micromachined ultrasonic transducer (cMUT) is presented. The sensor generates static and dynamic displacement maps of the surface of this MEMS device. The controller improves these measurements by maintaining a position of high sensitivity during operation. Finally, the preliminary results of a miniaturized version of this system are presented including the implementation of two fully independent parallel sensors. This allows for array implementation of these sensors, which is crucial for the batch fabrication photolithography techniques used to create many MEMS devices. Recommendations on the future work needed to complete the array implementation are given in conjunction with methods for increasing the resolution and robustness of the macroscale system described in this thesis.