<title>Digital all-optical switching in a ytterbium-doped two-mode fiber interferometer</title>

Abstract

A resonance-enhanced nonlinearity is used to demonstrate a digital switching response in a two-mode fibre interferometer constructed from ytterbium(III)-doped optical fibre. The fibre used was single-mode at the pump wavelength (980 nm) and two-moded at the signal wavelength (514 nm). Interferometer-based optical switches are generally sensitive to fluctuations in pump power, because of the sinusoidal dependence of the output state on pump power. However by allowing the doped fibre to lase, thus clamping the phase shift, this sensitivity can be eliminated to yield a bi-stable, digital switching response. In our interferometer configuration, a laser cavity was defined in the ytterbium-doped fibre by a matched pair of Bragg gratings, and the magnitude of the clamped phase shift was controlled by stretching one of the gratings to adjust the laser threshold. Digital switching of a 514 nm signal with 11 dB isolation and a power-length product of 1.3 mW.m was demonstrated. The relaxation time of the switch was determined by the ytterbium(III) excited state lifetime, while the rise time could be reduced by increasing the pump power without compromising the isolation. The clamping technique demonstrated in this paper should be applicable to any optical switch incorporating a gain medium.