Ring laser having magnetic isolation of counter-propagating light waves

摘要

A ring laser having a nonreciprocal loss element positioned in the aperture of the optical cavity of the laser. The nonreciprocal loss element is adapted to isolate a pair of plane-polarized counter-propagating light waves from each other. The isolation reduces the coupling between the waves and thereby reduces the extent of nonlinear mode locking which typically occurs in ring laser gyroscopes when the difference in frequency between the light in the two waves is small. The isolation is the result of either a spatial or a temporal separation between the counter-propagating light waves. The nonreciprocal loss element of the preferred embodiments comprises a first optical element having reciprocal optical activity and a second optical element having either reversed or reversible nonreciprocal optical activity. The nonreciprocal loss element tends to extinguish either the clockwise or the counterclockwise propagating one of the pair of light waves depending on the direction of magnetization in the nonreciprocal optically active element. In a preferred embodiment, the nonreciprocal optically active element is a film of ferrimagnetic material having a domain wall, or magnetization reversal, so positioned in the aperture of the optical cavity that two regions of the film having opposite magnetizations each occupy approximately half the aperture. Thus, a counterclockwise propagating light wave passing through the region of the film having one magnetization occupies a different portion of the aperture than is occupied by a clockwise propagating light wave passing through the region of the film having the other, opposite, magnetization. In another preferred embodiment, the magnetization of the film is modulated cyclically from magnetic saturation in one direction to magnetic saturation in the opposite direction by an externally applied magnetic field. Only one light wave at a time is thereby permitted to propagate. The counter-propagating light waves are thus alternately turned on and extinguished so that they do not exist at the same time.