Abstract
Symmetry plays a crucial role in explorations of the laws of nature. Parity-time (PT) symmetry phenomena can lead to entirely real spectra in non-Hermitian systems, which attracts considerable attention in the fields of optics and electronics because these phenomena provide a new tool for the manipulation of oscillation modes and non-reciprocal signal transmission. A potential new field of application is microwave photonics, an interdisciplinary field in which the interaction between microwaves and optical signals is exploited. In this article, we report the experimental use of PT symmetry in an optoelectronic oscillator (OEO), a key microwave photonics system that can generate single-frequency sinusoidal signals with high spectral purity. PT symmetry is theoretically analyzed and experimentally observed in an OEO with two mutually coupled active oscillation cavities via a precise manipulation of the interplay between gain and loss in the two oscillation cavities. Stable single-frequency microwave oscillation is achieved without using any optical/electrical filters for oscillation mode selection, which is an indispensable requirement in traditional OEOs. This observation opens new avenues for signal generation and processing based on the PT symmetry principle in microwave photonics.
Highlights
Parity-time (PT) symmetry was first proposed in 1998 by Carl Bender and Steffan Boettcher[1]
We report the experimental application of PT symmetry in an microwave photonics (MWP) system
When the gain and loss are larger than the magnitude of the coupling ratio, PT symmetry will be broken and a single-mode will be selected and enhanced
Summary
Parity-time (PT) symmetry was first proposed in 1998 by Carl Bender and Steffan Boettcher[1]. By increasing the level of nonhermiticity, the symmetry can be broken after passing the so-called transition point, leading to non-real eigenvalues[1,2,3,4] This concept was initially developed and extended in the field of quantum mechanics[4], its application in this field is fundamentally limited because quantum mechanical operators are inherently Hermitian. This is not the case in optics[5], in which PT symmetry can be introduced by providing gain and loss under specific conditions to photonic systems. An experimental verification of this phenomenon in optics was reported in 20108, demonstrating the fruitful utilization of this quantum-induced symmetry in optics[16]
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