Single-mode resonator incorporating an internal multimode optical fiber and a phase-conjugate reflector

Phase-conjugate mirrors (PCM’s) can be used to correct for phase distortion introduced by optical elements included within laser resonators, enabling diffraction-limited output beams to be produced. A severe example of a phase-distorting medium that could be included within a resonator is a multimode optical fiber. We describe the operation of a resonator that contains such a fiber and uses a PCM to restrict the output from the fiber to the lowest-order transverse mode. The system thereby enables the output from a laser to be transported to a remote location through the multimode fiber without loss of mode quality. The PCM was a high-gain passive PCM made from a barium titanate crystal pumped by a 60-mW single-mode argon-ion laser.