Optical frequency comb locked signal synthesis

Abstract

Highly stable optical frequency combs (OFCs), particularly those generated by modelocked lasers, have become important tools for frequency and time metrology, and spectroscopy. This is due to their ability to span wide bandwidths, to act as highly accurate frequency references, and to provide a direct link between the optical and radio frequencies. However, the narrow comb spacing of most mode-locked OFCs makes it difficult to access their individual modes for a wide range of other potential applications.
This thesis investigates comb mode extraction from a 250 MHz spaced OFC by phase locking semiconductor lasers (slave lasers) to individual comb modes. This was achieved using optical injection locking in combination with a low bandwidth electronic feedback loop. The locking process forced a slave laser to emit at the same frequency as the comb mode it was locked to, but at its natural output power. Hence a locked slave laser effectively behaved as a ultra-narrowband filter with active gain.
The locking process was characterised in terms of its long-term frequency stability over a period of 8 hours (minimum Allan deviation of less than 10-18) and its short term phase noise across a bandwidth from 100 Hz to 500 MHz (minimum integrated phase noise of 0.02 rad2). Amplification of the residual comb modes was measured and found to have a dependence on the master-slave frequency detuning. The results from numerical modelling found that this was due to phase modulation induced in the slave laser by the injected OFC and could always be suppressed by controlling the frequency detuning.
Fourier synthesis of high repetition rate waveforms was explored as one of the potential applications of this phase locking technique. Multiple lasers were made coherent with one another by locking them to different modes of a common OFC. This enabled them to behave as different frequency components of a Fourier series to generate various waveforms. This was achieved by independently controlling the relative amplitude and phase of each slave laser, and combining them together. The generation of stable waveforms with at-top, triangular, parabolic, and sawtooth intensity profiles was demonstrated at a repetition rate of 100GHz.

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ORCID for D.J. Richardson: orcid.org/0000-0002-7751-1058

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