Ultrastable optical cavity
A high-finesse Fabry-Perot resonator engineered so its length, and therefore its resonance frequency, is as constant as materials physics allows. Lasers locked to such cavities reach sub-hertz linewidths and fractional frequency instabilities down to a few parts in 10^17.
A free-running laser wanders by kilohertz to megahertz over a second as its cavity length breathes with temperature, vibration, and current noise. An ultrastable optical cavity is the fix: an isolated Fabry-Perot resonator built to be a better frequency reference than any laser, so that a servo (almost always Pound-Drever-Hall locking) can force the laser to track one cavity resonance. The laser then inherits the stability of a carefully engineered block of glass.
The engineering stack. The spacer is typically ultra-low-expansion (ULE) glass, operated in vacuum at the temperature where its thermal expansion coefficient crosses zero, inside several nested thermal shields. Mirrors with parts-per-million losses are optically contacted to the spacer, giving finesse of a few hundred thousand. The mounting geometry is chosen so that residual vibrations compress the cavity symmetrically and leave the optical length nearly unchanged. Done well, nothing ordinary moves the mirrors anymore, and what remains is fundamental.
The thermal noise floor. What remains is Brownian motion: thermally driven jitter of the mirror coatings, substrates, and spacer. For room-temperature ULE cavities this floors the fractional frequency instability around to . Crystalline AlGaAs coatings and cryogenic single-crystal silicon spacers push lower; a 21 cm silicon cavity at 124 K reached , with laser linewidths in the tens of millihertz. The scale is worth pausing on: of a 10 cm cavity is 10 attometers, well under one percent of the diameter of a proton, held for the duration of a measurement.
What they are for:
- Optical clocks: the interrogation laser must hold its phase during the atomic probe time, and cavity noise, aliased into the measurement, often sets the clock's short-term instability.
- Ultralow-noise microwaves: dividing a cavity-stabilized optical frequency down through a frequency comb produces microwave signals with phase noise beyond any electronic oscillator.
- Precision measurement: spectroscopy, tests of relativity, and pre-stabilization for gravitational-wave interferometers all start from a cavity-locked laser.
Limits in practice. ULE creeps: the glass relaxes, dragging the resonance by tens of kilohertz per year, so long experiments steer the cavity against an atomic line. Residual vibration sensitivity, temperature gradients, and pressure fluctuations in an imperfect vacuum all sit one engineering mistake away from spoiling the thermal-noise floor, which is why a working cavity says as much about the lab as about the design.
References: Kessler et al., Nat. Photonics 6, 687 (2012); Matei et al., Phys. Rev. Lett. 118, 263202 (2017).