Stable photonic microwave generation using semiconductor laser dynamics

Abstract

Photonic generation of stable microwave signals has attracted considerable attention for transmission of microwave signals over optical fibers with low loss, electromagnetic interference immunity, and large bandwidth. The generation techniques have found a range of potential applications including radio-over-fiber communication, photonic microwave signal processing, photonic microwave beamforming, and radar. Among the various techniques for photonic microwave generation, the approaches based on semiconductor lasers are strong competitors due to compactness and potential for integration. In this thesis, the dynamics of both optically injected semiconductor lasers and passively mode-locked semiconductor lasers are investigated for photonic generation of microwave signals. Techniques for improving the frequency stability are explored. Firstly, an optical injection drives the laser into nonlinear dynamical period-one (P1) oscillation at a microwave frequency, which is widely tunable up to 100 GHz. Owing to the laser nonlinearities, the P1 microwave linewidth can even be smaller than the free-running optical linewidth, though it is still affected by intrinsic laser noise. Optical feedback to the injected laser are introduced to further stabilize the P1 frequency fluctuation. Via theoretical analyses and numerical simulations, the P1 microwave linewidth is found to follow an inverse-square dependence on the feedback strength and feedback delay time asymptotically. By modification to a dual-loop feedback, noisy side peaks around the central P1 frequency are effectively suppressed through the Vernier effect. The numerical simulations are in good agreement with the experimental results. Photonic generation at a millimeter-wave frequency of 45.4 GHz is experimentally demonstrated with a clean spectrum and a linewidth below 50 kHz. Secondly, utilizing the optical tunability of the P1 frequency by introducing modulation on the injection power, photonic generation of frequency-modulated continuous-wave (FMCW) microwave signals is also investigated. This generation scheme exhibits superior stability and tunability in terms of central frequency, bandwidth, and chirp rate. The FMCW microwave signal is generated with central frequency, sweep bandwidth, and chirp rate tunable up to 22 GHz, 7.7 GHz, and 0.42 GHz/ns, respectively. Thirdly, monolithic photonic microwave generation is demonstrated by using a passively mode-locked semiconductor laser, where an anticolliding-pulse mode-locking (ACPML) configuration is introduced. The laser consists of a gain section and a saturable absorber (SA) section. With proper DC biasing on the laser, a millimeter-wave signal at around 34.7 GHz with a linewidth on the order of 100 kHz is demonstrated. To realize the ACPML configuration, low-reflection and high-reflection coatings are deposited on the SA section and gain section facets, respectively. After the deposition, the threshold is unchanged, while the modulation of the SA is increased. As a result, simultaneous improvements in pulse peak power, pulse amplitude noise, and timing jitter are achieved. These photonic schemes generate stable microwave signals that offer opportunities in photonic microwave applications. ii

Date of Award	2 Oct 2015
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Sze Chun CHAN (Supervisor)