Patel, Rikesh Ramesh
(2014)
Ultrastable heterodyne interferometry using a modulated light camera.
PhD thesis, University of Nottingham.
Abstract
Interferometry is used in a wide variety of fields for the instrumentation and analysis of subjects and the environment. When light beams interfere, an interference fringe pattern is generated. Captured widefield interference patterns can be used to determine changes in the optical path length of interfering beams across a 2D area. Two interferometer schemes regularly implemented in modern systems include the homodyne interferometer, where light with the same optical frequency in used to generate static intensity fringe patterns, and the heterodyne interferometer, where light with different optical frequencies are used to generate a fringe pattern that is modulated at a frequency equal to the optical frequency difference (beat frequency). A widefield heterodyne system is not straightforward to bring into practice, however, it does offer some benefits over a comparable homodyne interferometer, such as direct phase interpretation and the suppression of low frequency background light in interferograms.
In this thesis, a widefield heterodyne interferometer system is presented. A custom prototype modulated light camera (MLC) chip was used to capture both homodyne and heterodyne fringe patterns. The 32x32 pixel camera is capable of continuously demodulating incident modulated light at frequencies between 100kHz and 17MHz. In the presented system, an error in the interferogram phase was determined to be Δφ = ±0.16radians (~9.1º). Comparisons between homodyne and heterodyne interferograms, captured using the MLC, are also presented.
With modifications to the system, an ultrastable widefield heterodyne interferometer system was implemented. The intention of this system was to eliminate the contribution of piston phase to a captured interferogram without the need for common path optics. In contrast to the standard heterodyne setup, the reference signal used in the demodulation process was derived from one of the pixels on-board the MLC, rather than from an external source. This new local reference signal tracks the common changes in the temporal phase detected by all the MLC's pixels, eliminating piston phase and substantially reducing the contributions of unwanted vibrations and microphonics from interferograms. To demonstrate this ultrastable system, it is incorporated into a Mach-Zehnder interferometer, where a vibration is induced onto an object arm mirror (using a mounted speaker) at various frequencies. Stable interferograms are captured with the mirror moving at up to 85mm/s at 62Hz (an optical path length of 220μm, or 350 wavelengths for λ = 633nm), however, this limit was the result of the complex motion in the mirror mount rather than the stability limit of the system. The system is shown to be insensitive to pure piston phase variations equivalent to an object velocity of over 3m/s.
As an application of the ultrastable system, a novel interferometer has been developed that captures the widefield fringe patterns generated by interfering two independent light sources, rather than by a single split source. The two separately stabilised HeNe lasers, constructed in the laboratory, produce light with a reasonably stable output frequency. Interfering two of these sources produce a heterodyne interference pattern with an unknown beat frequency. The beat frequency continuously varies because of the variation in the output frequency of each laser, but these stabilised lasers produce a beat frequency that drift by as little as 3MHz over 30 minutes. As the ultrastable system tracks changes in the temporal phase and instantaneous frequency of an incident fringe pattern, it can be used to track the variations in the modulation frequency generated by the fluctuations in the two separate lasers. The separation between the two lasers with regards to the images presented was about 35cm, but they can be separated by much larger amounts.
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