A reference cavity is a passive optical resonator (resonant cavity) which is used as a kind of fly-wheel oscillator (short-term frequency reference) in an optical frequency standard. The optical frequency of a single-frequency laser (or of a single spectral line of the output of a mode-locked laser) can be stabilized to the frequency of a resonance of the reference cavity, effectively transferring the higher frequency stability of the cavity to the laser. Such stabilization (frequency locking) can be achieved e.g. with an electronic feedback system based on the Pound–Drever–Hall method or the Hänsch–Couillaud method.
Note that the linewidth of a laser oscillator which is stabilized to a reference cavity can be far below the resonator bandwidth, except if the latter is already very small. Therefore, the laser's Q factor may be higher than that of the reference cavity.
Achieving High Frequency Stability
Compared with a laser resonator, a passive reference cavity can be significantly more stable, as it does not have the disturbing influences introduced by a laser gain medium. Also, it can have a very high finesse and Q factor, as the round-trip power losses can be minimized; that leads to a small resonator bandwidth, so that the resonance frequencies can be precisely determined. Further measures can be used to achieve extraordinarily high stability:
- using a rugged construction for the cavity setup, possibly with vibration-absorbing damping elements
- using a spacer material (fixing the spacing of the cavity mirrors) which has a low thermal expansion coefficient, e.g. Invar steel, Zerodur glass or ULE (ultra-low expansion) glass
- isolating the cavity from thermal influences, e.g. by placing it within a temperature-stabilized housing
- placing the cavity in an evacuated chamber, thus removing various influences introduced by the air
- limiting the circulating optical power level, and stabilization of the circulating power, to eliminate or stabilize heating effects
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