The resolution of grating spectrometers is insufficient to resolve many
features present in the emission spectra of solid-state quantum
emitters. Spectral resolution may be improved by inserting a
Fabry–Perot interferometer (FPI) whose length is tuned by a
piezoelectric actuator. Yet piezo creep and thermal fluctuations cause
instability that makes this solution unsuitable for measurement times
longer than tens of seconds. To overcome this challenge, we employ
active feedback derived from the Newton’s rings interference pattern
formed by the reflection of a single-frequency laser from the FPI
cavity. The resulting FPI transmission frequency is stable to within
50 MHz over several hours.