Observation of PT-symmetry in a fiber ring laser.

A wavelength-tunable single-mode laser with a sub-kilohertz linewidth based on parity-time (PT)-symmetry is proposed and experimentally demonstrated. The proposed PT-symmetric laser is implemented based on a hybrid use of an optical fiber loop and a thermally tunable integrated microdisk resonator (MDR). The MDR, implemented based on the silicon-on-insulator, operates with the optical fiber loop to form two mutually coupled cavities with an identical geometry. By controlling two light waves passing through the two cavities, with one having a gain coefficient and the other a loss coefficient but with an identical magnitude, a PT-symmetric laser is implemented. Thanks to an ultranarrow passband of the cavity due to PT-symmetry, single-longitudinal mode lasing is achieved. The tuning of the wavelength is implemented by thermally tuning the MDR. The proposed PT-symmetric laser is demonstrated experimentally. Single-longitudinal mode lasing at a wavelength of around 1555 nm with a sub-kilohertz linewidth of 433 Hz is implemented. The lasing wavelength is continuously tunable from 1555.135 to 1555.887 nm with a tuning slope of 75.24 pm/°C.