We present the analysis of an unorthodox technique for locking a laser to a resonant optical cavity. Error signals are derived from the interference between the fundamental cavity mode and higher-order spatial modes of order two excited by mode mismatch. This scheme is simple, inexpensive, and, in contrast to similar techniques, first-order insensitive to beam jitter. After mitigating sources of technical noise, performance is fundamentally limited by quantum shot noise.
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