Periodic Dynamics in a Semiconductor Laser with Optical Feedback: Theory and Applications

Rich laser dynamics have potential for various applications, ranging from cryptography, microwave photonics to sensing instrumentation. Laser dynamics can be induced by external perturbation including optical injection or external optical feedback (EOF). Laser sensing systems utilizing a semiconductor laser (SL) with EOF are highly regarded for their cost-effective implementation, reduced component requirements, and straightforward optical alignment, making them more attractive in practical applications.

This research introduces innovative methods to produce dual-frequency lasers and optical frequency combs (OFCs) by utilizing SLs under external optical feedback (EOF) and their nonlinear dynamics. It explores the application of these techniques in: 1. High-resolution velocity measurements using Dual-frequency Doppler LiDAR (DFDL). 2. High-precision distance measurements using dual-comb interferometry.

First, a new method for generating dual-frequency lasers using a single-frequency SL with EOF operating in period-one (PI) oscillation is proposed and demonstrated. By adjusting both the EOF strength and external cavity length, the laser output spectrum can be tuned to exhibit two dominant frequencies with nearly equal amplitudes and controllable frequency separation. The characteristics and tunability of the dual-frequency spectra are investigated in detail.

In Chapter 2, the proposed dual-frequency laser is used to build a novel DFDL system for velocity measurement. Compared to conventional single-frequency Doppler LiDAR, DFDL offers a different approach by using the microwave beat frequency generated between two optical frequencies to measure the velocity of a moving target. This measurement approach significantly reduces errors that typically arise from atmospheric turbulence and surface scattering. The proposed DFDL system, employing the PI dual-frequency laser with a simple and cost-effective setup, demonstrates a velocity resolution of 55.5μm/s and 58.3μm/s, a significant improvement over existing methods.

Chapter 3 presents a novel method for generating OFCs with adjustable repetition rates. This is achieved by operating the laser in specific periodic dynamical states that are induced by EOF. Periodic windows embedded in the chaotic regimes of the laser dynamics are identified and utilized. An investigation of four distinct types of periodic windows reveals their unique laser dynamics and the corresponding characteristics of the generated OFCs. By precisely controlling the external cavity length, a tunable OFC repetition rate with a range up to 18.07 MHz is achieved. As an application example, a dual-comb system using the proposed OFC generation technique is developed for distance measurement. By interfering two OFCs with slightly different repetition rates and extracting the phase information from the resulting microwave beat spectrum, the target distance can be determined with high precision. Results demonstrate a measurement error within 31/μm for distances up to 2.6m, highlighting the system’s accuracy and potential for practical use.

In summary, both theory and application studied in this thesis are expected to advance the field of laser dynamics and its application by introducing innovative techniques for dual-frequency laser and OFC generation based on the nonlinear dynamics of semiconductor lasers with EOF. The demonstration of high-resolution velocity and distance measurements using these techniques opens up new possibilities for optical sensing and metrology.