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Optics Letters

Nonreciprocity of counterpropagating signals in a monolithically integrated Sagnac interferometer

Venon M. Menon, Wilson Tong, Fengnian Xia, Chianqiang Li, Stephen R. Forrest

Published Mar 1, 2004• 20 citations
Publisher

Abstract

We demonstrate nonlinearly induced nonreciprocity of counterpropagating waves in a monolithically integrated Sagnac interferometer that employs a semiconductor optical amplifier as the nonlinear element. We show that the dependence of the linewidth enhancement factor on charge injection can influence the third-order nonlinearity in the semiconductor gain medium to a surprisingly large degree. This effect is utilized to control the phases of the counterpropagating signals in the interferometer. A theoretical model is used to explain the experimental observations. We show that these effects have significant practical implications by demonstrating an all-optical controlled-NOT gate.

DOI
10.1364/ol.29.000513

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We demonstrate nonlinearly induced nonreciprocity of counterpropagating waves in a monolithically integrated Sagnac interferometer that employs a semiconductor optical amplifier as the nonlinear element. We show that the dependence of the linewidth enhancement factor on charge injection can influence the third-order nonlinearity in the semiconductor gain medium to a surprisingly large degree. This effect is utilized to control the phases of the counterpropagating signals in the interferometer. A theoretical model is used to explain the experimental observations. We show that these effects have significant practical implications by demonstrating an all-optical controlled-NOT gate.
Abstract : A substantial enhancement (by orders of magnitude) of the Sagnac effect in a passive ring resonator can be attained by using a nonreciprocal feedback. This feedback is based on the nonreciprocal element controlled by the signal proportional to the difference between intensities of counterpropagating waves, and is an opto-electronic analog of nonlinear nonreciprocity proposed by us early. Under some critical conditions, this system can exhibit directional bistability and directional switching of the counterpropagating waves. (Author)
A substantial enhancement (by orders of magnitude) of the Sagnac effect in a passive ring resonator can be attained by using a nonreciprocal feedback. This feedback is based on the nonreciprocal element controlled by the signal proportional to the difference between intensities of counterpropagating waves, and is an optoelectronic analog of nonlinear nonreciprocity proposed by us earlier. Under some critical conditions, this system can exhibit directional bistability and directional switching of the counterpropagating waves.
In our recent research1, it was suggested that counter-propagating laser beams in a Kerr-nonlinear medium can induce intensity-dependent nonreciprocity which consists in different changes of refractive indecies for both of the waves. Under some conditions, this can result in such directionally asymmetrical effects as (i) directional bistability in a nonlinear ring resonator which allows one to switch the direction of propagation of a dominant mode and (ii) enhancement of the Sagnac effect in a rotating ring resonator.
We report the observation of optical stability for a plane parallel semiconductor crystal which forms a Fabry-Perot interferometer using only the natural reflectivity of its surfaces. Nonlinear transmission is observed for cw laser intensities above ∼ 100 W/cm2 for radiation at 1895 cm−1 near the energy gap of InSb at 5 K. The effect is interpreted in terms of a very large intensity-dependent refractive index giving a 5λ/2 optical thickness change for an intensity of ∼2 kW/cm2. Clear bistability is seen in fifth-order interference, the first such observation above first order in an intrinsic, one-element system, in addition to regions exhibiting signal amplification. The same crystal also shows strong modulation of the transmission of one laser beam induced by a second, with real signal gain, thus demonstrating an ’’optical transistor.’’
In order to improve the phase sensitivity of nonlinear Sagnac interferometer, a nonlinear Sagnac fiber optic interferometer based on feedback control is designed to improve the phase sensitivity by introducing a feedback control loop into the nonlinear Sagnac interferometer. This design adopts the composite feedback control structure of Nonlinear Beam Splitter (NBS) and Linear Beam Splitter (BS). By adjusting the reflectivity of the Linear Beam Splitter, the intensity of the feedback signal can be effectively changed to regulate the phase sensitivity of the interferometer. The feedback amplified signal light passes through the Sagnac fiber optic ring before being processed by a second NBS for quantum enhancement to further improve the phase sensitivity of the interferometer. The simulation results show that the phase sensitivity of the interferometer with certain feedback strength is better than that without feedback. Especially when the phase difference is π, the phase sensitivity of the interferometer with low and medium feedback strengths is improved by 33.3% and 28.9% respectively; In addition, the gain coefficient of the NBS is a key parameter affecting the performance of the interferometer, and a suitable gain coefficient can further optimize the phase sensitivity of the interferometer.