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Title:
Abstract:
Precision optical interferometry enables displacement measurements with sub-picometer sensitivity, but its performance is ultimately limited by a variety of optical, electronic, and environmental noise sources. Among these, parasitic interference arising from unintended optical fields can introduce significant phase errors and become a dominant limitation at low frequencies. This thesis develops a systematic framework for understanding, characterizing, and mitigating parasitic interference in heterodyne interferometers and extends this analysis to laser frequency stabilization using an iodine-filled photonic microcell (PMC).
Parasitic interference is classified into two types with distinct coupling behaviors. A general model of the parasitic phase error is determined by the parasitic-to-main field-amplitude ratio, relative phase, and a coupling coefficient that characterizes the susceptibility of the interferometer configuration. Based on this framework, three principal mitigation strategies are identified: high polarization extinction ratio of the input beams, differential interferometry, and balanced detection. The theoretical predictions are experimentally validated in Mach–Zehnder and differential interferometers. Reductions in the parasitic coupling coefficients by factors of approximately 10^4 and 10 are demonstrated for type-I and type-II parasitic interference, respectively, enabling sub-picometer displacement sensitivity at Fourier frequencies as low as 9 mHz. An interferometer configuration with enhanced rejection of parasitic interference is further developed and is predicted to achieve sub-pm displacement sensitivity down to approximately 0.6 mHz.
The interaction between parasitic interference and laser frequency noise is also investigated. Laser frequency fluctuations couple to the interferometric phase through the optical path difference (OPD), making accurate knowledge of the OPD essential for evaluating and correcting frequency-noise coupling. It is shown that parasitic interference can introduce systematic errors into modulation-based OPD measurements because the applied laser frequency modulation changes both the nominal interferometric phase and the relative phases of parasitic optical fields. As an alternative to conventional laser frequency stabilization, a passive parasitic frequency-noise compensator is developed in which a controlled parasitic field with an engineered optical path difference provides an opposing frequency-to-phase response, thereby suppressing the first-order coupling of laser frequency noise without an additional active frequency-stabilization loop.
Finally, a compact optical frequency reference based on saturated absorption spectroscopy in an iodine-filled PMC is developed. The stabilized laser achieves a fractional frequency stability of 3.5*10^-13 at an integration time of approximately 1000 s. Parasitic interference associated with spurious optical fields and higher-order fiber modes is identified as an important limitation to the frequency stability of the system. Mitigation through parasitic-field amplitude reduction and phase control improves the fractional frequency stability by more than an order of magnitude at integration times longer than several seconds. Together, these results demonstrate the importance of parasitic interference in both displacement interferometry and optical frequency stabilization and provide quantitative methods for predicting and reducing its impact in precision optical measurement systems.
Committee:
Dr. Felipe Guzman (Chair)
Dr. Masud Mansuripur
Dr. Jose Sanjuan