All-optical microwave oscillator assisted by parity-time-symmetry breaking

Parity-time ($\mathcal{PT}$) symmetry provides a new technique for the manipulation of oscillation modes in the field of microwave photonics. In this paper, we conceive and experimentally demonstrate a simple $\mathcal{PT}$-symmetric all-optical microwave oscillator (AOMO) based on a single polarimetric loop, overcoming the challenges of manipulating gain and loss and mutual coupling ratio and perfect control of the geometry in the dual-loop configurations. We also uncover the analytics of the $\mathcal{PT}$-symmetry condition in an AOMO with a detailed nonquantum description. By controlling the polarization rotation angle of the $\mathcal{PT}$-symmetric structure, two mutually coupled loops with balanced gain and loss are easily conceived, enabling the proposed AOMO to operate in the single mode. The optical-optical feedback modulation and oscillatory frequency selection are implemented using a semiconductor optical amplifier (SOA) and the stimulated Brillouin scattering (SBS) effect, respectively. Benefiting from the SOA's cross-gain modulation, the SBS-based frequency selection, and the $\mathcal{PT}$-symmetric breaking, dual-wavelength optical signals can be achieved with wideband tunability, very low beating phase noise, and high stability. In the experiment, we successfully generated an oscillating signal tuning from 3 to 30 GHz, and the measured phase noise reached \ensuremath{-}103.2 dBc/Hz at 10 GHz and \ensuremath{-}90.69 dBc/Hz at 30 GHz at 10-kHz offset frequency.