One of the biggest hurdles for implementation of the NICE-OHMS technique is indisputably the locking of the frequency of the laser to that of a cavity mode.

Without an etalon, a laser will generally produce light over a wavelength range corresponding to a number of cavity modes, which are similar to Fabry-Pérot modes.

Inserting an etalon into the laser cavity, with well-chosen finesse and free-spectral range, can suppress all cavity modes except for one, thus changing the operation of the laser from multi-mode to single-mode.

The last term models coupling between the cavity mode and a classical field, i.e. a laser.

Single-mode lasers employ etalons to suppress all optical cavity modes except the single one of interest.

For excessive net group delay dispersion (GDD) of the laser cavity, the phase of the cavity modes can not be locked over a large bandwidth, and it will be difficult to obtain very short pulses.

When the RF frequency fed by the antenna is the same as that of a cavity mode, the resonant fields build to high amplitudes.

When the laser is in resonance with a cavity mode, intensity builds up in the cavity due to constructive interference.

However, for high finesse cavities the ratio of "on" and "off" a cavity mode is small, given by the inverse of the finesse, whereby the transmission as well as the integrated absorption becomes small.

This formulation requires one equation for the carrier density, and one equation for the photon density in each of the optical cavity modes: