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arXiv · 2409.08764

Quadrature amplitude modulation for electronic sideband Pound-Drever-Hall laser frequency locking

J. Tu·A. Restelli·K. Weber·I. B. Spielman·S. L. Rolston·J. V. Porto·S. Subhankar
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Abstract

The Pound--Drever--Hall (PDH) technique is routinely used to stabilize the frequency of a laser to a reference cavity. Electronic sideband (ESB) locking, a PDH variant, bridges the frequency gap between the discrete cavity resonances and a desired laser frequency. Here we use quadrature amplitude modulation (QAM), a standard technique in digital communications, to generate the high-quality phase-modulated radio-frequency (rf) drive required for ESB locking. We develop a theoretical framework to analyze how in-phase/quadrature-phase (I/Q) impairments distort the ESB error signal and induce frequency offsets relevant to ultranarrow-linewidth lasers. We then design and implement a direct software-defined radio (SDR) on an UltraScale+ RFSoC platform, frequently adopted across modern quantum-computing systems, to digitally compensate QAM I/Q impairments. Using this device, we generate phase-modulated rf signals with a large phase-modulation index of $1.01$ rad and root-mean-square I/Q errors below $0.3\ \%$ over a carrier-frequency range of $350~\mathrm{MHz}$ to $1.75~\mathrm{GHz}$. Finally, we lock a laser to an ultralow expansion (ULE) reference cavity and demonstrate continuous laser-frequency tuning by ramping the carrier frequency while maintaining lock, validating the continuous tunability of our ESB locking instrument.

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J. Tu, A. Restelli, K. Weber, I. B. Spielman, S. L. Rolston, J. V. Porto, S. Subhankar. 2024-09-13. Quadrature amplitude modulation for electronic sideband Pound-Drever-Hall laser frequency locking. https://arxiv.org/abs/2409.08764

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