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Physical Review Applied

Cavity-Free Circulator with Low Insertion Loss Using Hot Atoms

Shicheng Zhang, Gongwei Lin, Yiqi Hu, Yihong Qi, Yueping Niu, Shangqing Gong

Published Aug 13, 2020• 36 citations
Publisher

Abstract

Nonreciprocal optical devices, such as isolators and circulators, provide highly desirable functionalities in routing signals. This motivates the active research on high-efficiency nonreciprocal devices, especially for miniaturized and integrable magnetic-free schemes. All the present experimental demonstrations of circulators in optical domains are based on the high-$Q$ factor cavity. In this work, we demonstrate a cavity-free optical circulator with low insertion loss by making use of a far-detuned nonlinear Raman process with the assistance of the atomic Doppler effect. Due to the large detuning, the excited state can be adiabatically eliminated, resulting in nearly vanishing absorption. An optical circulator with average transmission of more than $85\mathrm{%}$ and fidelity above $0.98$ is well achieved in an experiment with hot atoms. Our scheme shows the possibility of cavity-free nonreciprocal optical devices with low insertion loss.

DOI
10.1103/physrevapplied.14.024032

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Nonreciprocal optical devices, such as isolators and circulators, provide highly desirable functionalities in routing signals. This motivates the active research on high-efficiency nonreciprocal devices, especially for miniaturized and integrable magnetic-free schemes. All the present experimental demonstrations of circulators in optical domains are based on the high-QQQ factor cavity. In this work, we demonstrate a cavity-free optical circulator with low insertion loss by making use of a far-detuned nonlinear Raman process with the assistance of the atomic Doppler effect. Due to the large detuning, the excited state can be adiabatically eliminated, resulting in nearly vanishing absorption. An optical circulator with average transmission of more than 85\mathrm{%} and fidelity above 0.980.980.98 is well achieved in an experiment with hot atoms. Our scheme shows the possibility of cavity-free nonreciprocal optical devices with low insertion loss.
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Nonreciprocal optical devices are important in wide areas, ranging from optical telecommunications to signal processing. Here, we experimentally realize the magnet-free and cavity-free optical nonreciprocity based on phase-matched four-wave mixing (FWM) in hot atoms. Specifically, the isolation of more than 20 dB and insertion loss less than 1 dB without using any optical cavity can be achieved simultaneously. Our results provide general insights into a broad class of parametrically modulated nonreciprocal behavior, paving the way toward optimal nonreciprocal devices based on the nonlinear FWM process.
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Optical nonreciprocity plays an important role in optical communication and signal processing. Here, we propose and demonstrate a magnetic-free optical nonreciprocal amplification based on a far-detuned four-wave mixing process in hot atoms. The absorption of the probe field can be significantly reduced by the large detuning. In our experiment, with just one pump field, an ultra-strong forward gain of 45 dB is achieved while the backward one keeps a near unit transmission. The experimental realization of ultra-strong nonreciprocal amplification may have applications for the generation of strong nonreciprocal quantum correlations.
Atomic absorption-induced insertion loss poses a fundamental limitation on signal transmission in optical nonreciprocity (ONR) systems. We propose a lossless and magnetic-field-free ONR scheme based on dual-Raman interference in two far-detuned Λ-type three-level systems. In the forward direction, destructive interference cancels the imaginary part of the susceptibility while retaining a large phase shift, enabling absorption-free transmission. In contrast, Doppler shifts arising from atomic thermal motion disrupt the Raman resonances in the backward direction, suppressing both absorption and phase modulation. This direction-dependent optical response yields nonreciprocal phase shifts which, when implemented in a Mach-Zehnder interferometer, enable an optical isolator with 100% forward transmittance and an isolation of up to 37 dB.