Optical Fiber-Based Multi-Directional Fabry-Perot Interferometry Prpbe
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Abstract
The motivation of this thesis is to create an optical probe for the measurement of surfaces in confined spaces. This thesis presents design and operation of an optical fiber-based probe to measure a surface profile or relative displacement of external surfaces, with the vertical range of 3 nanometers to 300 micrometers and resolution of 3.25 nm. Three different probe designs have been fabricated and tested. These three probes are classified as; a single-axis, dual-axis non-simultaneous and dual-axis simultaneous measurements. The main common components of the probe system are; Graded index lens (GRIN lens) for focusing light onto the surface, two by one fiber couplers/ splitters, photodetectors, single mode optical fibers, piezo electric actuators, and single-mode laser diode coherent sources. Displacement is measured by mechanically modulating the optical cavity formed by an internal surface and the external surfaces being measured, each cavity of which comprises a Fabry-Perot interferometer. To modulate the phase for each of the designed probe models, a piezo electric actuator is used to oscillate the GRIN lens sinusoidally along the optical axis and perpendicular to the external test surface, with a desired frequency of modulation. For harmonics extraction, quadrature detection and phase unwrapping, a LabVIEW FPGA program has been implemented. For the single-axis surface measurement probe, a closed loop-controlled scanning stage was designed and fabricated using a voice coil and mechanical flexures. A specimen attached to the scanning stage is translated under the vertical probe to measure the surface profile. The range of this scan is 1.2 mm with a resolution of traverse 17.5 µm and can scan the surface profile with speed of 1 mm/sec. The working distance of the probe is a flexible number depends on the assembly of GRIN lens and fiber tip and can change between 0 to 20 mm. For the dual-axis displacement measurement probe, two plain mirrors were attached to separate piezoelectric translation stages with their translation axis along each probe axis. Movement of each stage is measured using capacitance displacement gauges (Lion Precision CPL-190) and will mimic the effect of scanning. During experiments, each of the piezoelectric actuators is energized using a slowly, sinusoidally varying voltage resulting in a peak to valley motion of the surfaces typically of around two micrometers amplitude (corresponding to between six to ten optical fringes). The results of both non-simultaneously and simultaneously independent displacement measurements of plain mirrors is presented in this thesis. Uncertainty measurements is calculated for each probe axis and a result of 8.1 nm rms noise was measured for them. The reason for using optical fibers and a GRIN lens in this study is to make the probe as compact and flexible as possible to be appropriate for scanning inaccessible, hard to reach surfaces such as inside of a hole or barrel or any other hard to reach areas which is not possible to scan with commercial lens-based microscopes. Also, the multi-directional optical fiber probe design, increase the flexibility of surfaces scan substantially relative to the available surface measurement fiber-based products.