PhD defence of Yixiang Hu – Advancing Next-generation Fiber-Optic Transmission Systems for Enhanced Fiber Sensing and Optical Communication Performance

Fiber-optic communication systems form the backbone of modern telecommunication networks and datacenter interconnects. As traffic demand continues to grow, next-generation optical networks must provide higher transmission capacity and enhanced operational intelligence. This thesis develops and experimentally demonstrates several advanced technologies that improve the performance and expand the functionality of optical infrastructure.

First, we develop techniques for integrating distributed vibration sensing into coherent transmission systems without dedicated sensing hardware. Point-source vibrations are detected and localized using carrier-phase estimates obtained through digital signal processing at both ends of a bidirectional link. A laser phase-noise (PN) suppression technique is proposed to improve detection sensitivity and localization accuracy. A relative delay between two pilot tones enables retrieval and suppression of the transmit-laser PN at the coherent receiver. Over an 80-km bidirectional single-mode fiber link, the laser-induced detection noise floor is reduced by an order of magnitude around the vibration frequency. Error-free 200-Gbps transmission is achieved while simultaneously detecting and localizing a 3-kHz vibration. The localization error is reduced from 102 m to 43 m, and the standard deviation from 355 m to 67 m.

To eliminate dedicated tones, we then propose a method that introduces a delay element on one polarization tributary of a conventional dual-polarization in-phase/quadrature modulator (DP-IQM). The method retrieves the far-end transmit-laser PN directly from the modulated data. Over a 100-km link, bidirectional PN retrieval and suppression improve the vibration-detection signal-to-noise ratio by more than 10 dB from 10 to 15 kHz. Simulations further evaluate the effects of chromatic dispersion and amplified spontaneous emission noise.

To further enhance sensing at kilohertz vibration frequencies, we investigate an ultra-stable, frequency-locked semiconductor distributed-feedback laser. Its impact on vibration detection and localization is characterized in a 60-km bidirectional coherent system. A localization error of 23 m is achieved for a 2-kHz vibration without digital denoising filters. Performance is compared with that of three other narrow-linewidth semiconductor lasers across different vibration frequencies.

Finally, we demonstrate and systematically characterize a spectrally efficient self-coherent transmission system for short-reach optical communications. It employs an integrated silicon-photonic phase-diverse direct-detection receiver based on asymmetric self-coherent detection (ASCD) and a Mach–Zehnder interferometer. The receiver directly recovers the complex optical field without a local-oscillator laser and uses only two single-ended photodiodes, one interferometer, and two analog-to-digital converters. Over 40 km, a record electrical spectral efficiency of 5.86-bit/s/Hz per wavelength per polarization is achieved with a net 200-Gbps single-polarization 16-QAM signal. Transmission performance is evaluated in terms of optical signal-to-noise ratio, carrier-to-signal power ratio, signal-signal beating interference mitigation, and receiver calibration to clarify the limitations and implementation trade-offs of self-coherent detection."