  The Pound-Drever-Hall (PDH) cavity-locking scheme has found prevalent uses in
precision optical interferometry and laser frequency stabilization. A form of
frequency modulation spectroscopy, PDH enjoys superior signal-to-noise
recovery, large acquisition dynamic range, wide servo bandwidth, and robust
rejection of spurious effects. However, residual amplitude modulation at the
signal frequency, while significantly suppressed, still presents an important
concern for further advancing the state-of-the-art performances. Here we
present a simplified and improved scheme for PDH using an acousto-optic
modulator to generate digital phase reference sidebands instead of the
traditionally used electro-optic modulator approach. We demonstrate four key
advantages: (1) the carrier and two modulation tones are individually
synthesized and easily reconfigured, (2) robust and orthogonal control of the
modulated optical field is applied directly to the amplitude and phase
quadratures, (3) modulation synthesis, demodulation, and feedback are
implemented in a self-contained and easily reproducible electronic unit, and
(4) superior active and passive control of residual amplitude modulation is
achieved, especially when the carrier power is vanishingly low. These distinct
merits stimulate new ideas on how we optimally enact PDH for a wide range of
applications.
