Development of a broadband visible to infrared astrocomb
Cheng, Yuk Shan
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Motivated by the applications of high-resolution spectroscopy in astrophotonics, this thesis presents the experimental work and development of a broadband wide-mode-spacing astrocomb system as part of the UK consortium for the High Resolution Spectrograph (HIRES) in the forthcoming Extremely Large Telescope (ELT). A 1-GHz Ti:sapphire laser served as the master source comb of the system, pumping a visible broadband supercontinuum and a phase-coherent PPKTP-based degenerate optical parametric oscillator (OPO), spectrally broadened in highly non-linear fibre. The system was fully stabilised, and provided an atomically-traceable 1-GHz astrocomb across nearly two octaves from 500–2200 nm. Residual noise from the dither-locked OPO hindered subsequent modal filtering in a Fabry Perot cavity motivating development of a dither-free technique exploiting the always-present parasitic sum-frequency light. This locking protocol provided a sixfold lower relative intensity noise and nearly five times less power fluctuation than the dither-locked version, leading to successful modal filtering from 1150–1800 nm. The spectra of the filtered output were recovered using Fourier transform spectroscopy, with the 10-GHz comb mode spacings directly resolved optically. Through linear regression, fCEO was found to be 565.7 MHz ± 64.3 MHz and frep was calculated as 992.1 MHz ± 352 Hz. Individual comb mode values were identified with high accuracy using the comb equation. This technique was verified with a heterdyne experiment between the Ti:sapphire comb source and a Rb-referenced CW laser to which the FTS was referenced. An alternative approach for visible wavelength generation was investigated using a PPLN waveguide. Using a sample optimised for 1560 nm, 1.5 % of our coupled degenerate OPO input was up-converted to second harmonic generation as well as sum-frequency mixing and showed a good agreement with modelled results, providing confidence in the ability to use such simulations to design fully grating-engineered PPLN waveguide for broadband conversion.