Parity-Time Symmetry Enhanced Self-Injection Locking Phonon Laser

Abstract Parity-time (PT) symmetry provides a powerful non-Hermitian approach for manipulating mode competition through engineered gain and loss and has enabled single-mode operation in photonic, electronics and microwave systems. In this paper, we introduce it within a self-injection locking phonon laser to suppress spurious modes. Constructed on a dual-loop optoelectronic cavity with the phonon laser providing optical gain, the injection locking is achieved by tuning the gain and loss of the two loops and the side-mode suppression ratio is improved from 25 to 60 dB at a fiber length of 6 km. The realized single-mode phonon laser delivers ultralow phase noise (−98 dBc/Hz at 10 kHz offset) and suppressed spurious noise (<−78 dBc/Hz). Leveraging the high spectral purity of the phonon laser, we propose an alignment-coupling modulation (ACM) mechanism for ultrasound sensing, in which acoustic vibrations perturb the fiber-to-chip coupling and thereby modulate the phonon lasing frequency. This mechanism enables the detection of the intrinsic mechanical resonances of metal samples, with the enhanced side-mode suppression allowing weak resonance peaks to be clearly resolved. Overall, PT-symmetric self-injection locking is demonstrated as an effective route to high-purity phonon lasers, paving the way for its deployment in nondestructive inspection, material analysis and precision metrology.