ChinYi Liaw

A precision fiber optic displacement sensor based on reciprocal interferometry

A fiber optic nanometer range displacement sensor has been developed based on reciprocal interferometry, a concept widely used in fiber optic gyroscopes. Its configuration is similar to a Michelson interferometer but with only one of the two arms used. The principle of operation is the interference between the reflected light wave from the fiber end and that from a reflective object. As both the reference and the sensing light waves travel through the same optical path except for the air gap which is the distance to be sensed, the system is thus reciprocal and insensitive to perturbations introduced to the fiber path. While the system is very simple, it has demonstrated substantially improved immunity against environmental perturbations over the conventional Michelson interferometer. Our experimental results have shown that the interference behavior agrees well with its mathematical model. The system has demonstrated a resolution of 5 nm. The influence of temperature change and PZT induced phase shift to the fiber has been studied and the results have shown that the system is indeed insensitive to these perturbations.

Development and applications of a laser writing lithography system for maskless patterning

A laser writing lithography system is developed based on the 325-nm UV radiation of a helium-cadmium laser. A custom-built optical system with a modified microscope and computerized exposure and motion stages are the other main components of the system. The lithography system is able to process data input from industry-standard Caltech intermediate form (CIF) graphic files. We achieve a minimum linewidth resolution of 1.6 μm on a positive photoresist-coated silicon substrate written with a 10× UV objective lens of numerical aperture 0.20. Other processing parameters, such as the optimal writing speed and the amount of overlap for pattern generation, are also determined. Application of the laser lithography system is also demonstrated in the fabrication of a UV-detecting metal-semiconductor-metal diamond thin film photodetector and in the direct delineation of polythiophene polymer film.

Cladding-Mode Resonance in Polarization-Maintaining Photonic-Crystal-Fiber-Based Sagnac Interferometer and Its Application for Fiber Sensor

A novel cladding-mode-resonance-based polarization-maintaining photonic-crystal-fiber Sagnac interferometer (PMPCF-SI) and its application for fiber sensor are demonstrated. By introducing a mode field diameter mismatch at a splicing joint of an inbuilt single-mode fiber (SMF)-PMPCF-SMF structure connected in an SI, an intermodal interference is constructed by the excited cladding modes and the polarization modes of the PMPCF. With such a PMPCF-SI, simultaneous strain and temperature measurements are demonstrated. The SI behaves as a sensor and a filter simultaneously. By measuring the wavelength variation and transmissivity difference between two almost symmetrical resonance peaks in the spectral response of the SI, simultaneous strain and temperature measurements are achieved.

Theory of an amplified closed-Sagnac-loop interferometric fiber-optic gyroscope

An amplified closed-Sagnac loop interferometric fiber optic gyroscope (AC-IFOG) has been proposed and demonstrated in our earlier work. This paper reports the theoretical analysis of its operation. The analysis has been focused on the effect of the erbium-doped fiber amplifier (EDFA) inserted within the reentrant path. The analysis shows that the sensitivity can be improved by at least two orders of magnitude as compared to a conventional IFOG if the EDFA is an ideal device. The simulated open-loop response coincides well with that of the experimental data. Further analysis shows that the detection limit is restricted by the beat noise that build ups rapidly in successive circulations. The rate of noise build-up depends greatly on the optical bandwidth of the closed-Sagnac loop.

Characterization of an open-loop interferometric fiber-optic gyroscope with the Sagnac coil closed by an erbium-doped fiber amplifier

This paper reports the performance of an open-loop interferometric fiber-optic gyroscope with an erbium-doped fiber amplifier (EDFA) that closes the sensing coil into a reentrant configuration. Preliminary results have shown a 6.56-fold increase in rotation response factor at a rate of 5/spl deg//s over a conventional gyroscope using the same fiber length and light source. The dynamic range is tunable by changing the gain of the EDFA.

Fabrication of a low-operating voltage diamond thin film metal–semiconductor–metal photodetector by laser writing lithography

An ultraviolet-sensitive photodetector based on the metal-semiconductor-metal structure has been fabricated on a chemical-vapor-deposited diamond thin film grown on a silicon substrate. Device processing techniques employed include maskless laser writing lithography, image reversal processing and sacrificial layer inclusion, which resulted in devices with an electrode finger separation of only 2.5 μm. This allows for a low operating voltage of only 5 V, which is very much lower than what has been achieved so far.

Development of a laser-writing system for the fabrication of micro-optical elements

A laser writing system using a 325-nm UV laser beam for the fabrication of micro-optical elements is developed. The system consists essentially of a laser source, an optical subsystem, a moveable stage, a computer, and an observation and alignment subsystem. The laser writing system is characterized for the resolution of the system and optimal writing conditions, based on the objective of fabricating micro-optical elements. The resolution of the laser writing systems is 1.2 micrometers , obtained with the 40x UV objective lens at a writing speed of 100 micrometers /s, writing height of 6 micrometers and laser intensity of 1 (mu) W. This agrees fairly well with the calculated theoretical value. We have shown that the laser intensity, writing speed, writing height are three major parameters that determine the optical performance of the micro-optical elements fabricated by the laser writing system. The variation of the thickness and linewidth of the written pattern with the different laser intensities, writing speeds and writing heights are discussed.

Fabrication of composite sol-gel optical channel waveguides by laser writing lithography

We report the preparation of high optical quality sol-gel waveguide films made from high titanium content organically modified silane (ORMOSIL) and the fabrication of optical channel waveguides in the deposited sol-gel films. The waveguide films were deposited on a solid substrate (such as silicon) by spin-coating and low temperature baking, and the channel waveguides were fabricated using laser writing and reactive ion etching (RIE). The properties of the sol-gel waveguide films were characterized using atomic force microscopy (AFM), ellipsometry, and UV-visible spectroscopy (UV-VIS). AFM and ellipsometry results showed that a dense and porous-free waveguide film could be obtained at the heat treatment temperature of 100 degrees celsius. The influence of the RIE parameters including O2 content, rf power, and pressure on etching rate of the sol-gel waveguide films have been investigated. After a number of exploratory experiments, suitable etching parameters for the case of photoresist mask layer have been obtained.

Theory of an amplified closed-Sagnac-loop interferometric fiber optic gyroscope

An Amplified Closed-Sagnac Loop Interferometric Fiber Optic Gyroscope (AC-IFOG) has been proposed. It is constructed from an open-loop interferometric fiber-optic gyroscope with an erbium-doped fiber amplifier (EDFA) that closes the sensing coil into a re-entrant configuration. Earlier experiments have shown a 6.56 fold increase in rotation response factor at a rate of 5 degrees over a conventional gyroscope using the same fiber length and light source. This paper reports the theoretical analysis of its operation. The analysis has been focused on the effect of the EDFA inserted within the re-entrant path. The analysis shows that the rotation detection factor can be increased by at least two orders as compared to a conventional IFOG when the EDFA is an ideal device. A mathematical expression for the open-loop response has been derived and it agrees well with the experiment result.

Planar optical system design and fabrication

The study of planar imaging system is an interesting topic because its compact structure and lightweight are consistent with the trend of production miniaturization. This paper presents our study of designing and fabricating planar imaging elements. Theoretical analysis of grating and off- axis imaging diffractive elements was conducted. A low- aberration symmetrical imaging optical system was designed and evaluated with CODE V. Using a variable intensity laser- writing system, diffractive surface relief patterns were written on photoresist and were later transferred to a fused quartz plate.