  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.
