Date of Award
Doctor of Philosophy (PhD)
Optical Science and Engineering
Optical fiber detectors., Mode-locked lasers.
Ultralow-noise and ultra-stable sources of reference signals in the optical domain have become indispensable for applications such as precision measurements, optical sensing, optical frequency metrology, coherent optical communications, and microwave photonics. Such sources possess not only very narrow linewidth but also absolute frequency stability. Attaining high frequency stability in the optical domain requires innovative approaches to analyze and mitigate frequency noises and drifts. In this dissertation, a novel scheme of laser frequency analysis with improved specificity is proposed and demonstrated. The proposed method is based on power spectral density (PSD) measurement in the radio-frequency (RF) range aided by electronic frequency dividers (EFD). It is capable of differentiating modulation scenarios such as wideband frequency modulation (FM) and broadband phase modulation without ambiguity. Moreover, it allows for quantitative assessment of wideband FM parameters such as modulation frequency, modulation index and frequency deviation, which is not possible with any conventional spectral analysis methods. To mitigate the slow frequency drift of a diode laser, an optical phase-locked loop (OPLL) has been developed to stabilize it in long-term. An optical frequency comb (OFC) laser is used as a reference in the OPLL for leveraging its very high long-term stability. The beat note between an OFC and a DL has been stabilized for over 8 hours while the loop is in the locked state. Finally, an application of such frequency-stabilized laser in fiber-optic sensing with improved performance is demonstrated. Specifically, a high-resolution dynamic strain sensor is built using a Bragg grating written in a polarization maintaining fiber. Using a polarization-assisted detection scheme, an improvement of 28 dB in signal level, 18 dB in signal-to-noise ratio, and 20× in strain resolution have been demonstrated in comparison with the conventional unpolarized FBG strain sensors. Moreover, this new sensing concept does not require complicated frequency locking systems like Pound-Drever-hall locking scheme, making it much easier to scale up.
Barot, Dipen, "Analysis and mitigation of laser frequency drift and its application in fiber-optic sensing" (2019). Dissertations. 195.