Analysis and mitigation of single mode emission limiting factors in high-power RW-DFB laser diodes

This thesis analyzes the spectral and tuning behavior of high-power GaAs-based DFB laser diodes, with special focus on mode hops. Two types of 780 nm DFB lasers were studied: (1) devices with a uniform Bragg grating, HR rear facet, and AR front facet, and (2) devices with both facets AR-coated and a built-in phase shift. Because the facet position relative to the Bragg grating varies after cleaving, certain rear-facet phase conditions cause mode hops during current tuning, especially in configuration (1). Numerical simulations identified these critical phase conditions and revealed the mechanisms behind mode hops—mainly carrier accumulation and temperature gradients. Experiments confirmed the simulation results, including tests with devices featuring an adjustable rear-facet phase via a micro-heater. The study also found that mechanical strain from mounting strongly influences the lasers’ spectral properties; low-temperature bonding successfully prevents this issue. Phase-shift DFB lasers (configuration (2)) at 780 nm and 1064 nm were demonstrated, offering wide mode-hop-free tuning and record-high output powers of around 100 mW from a single facet. Additionally, tilted ridge waveguides were used to greatly reduce facet reflectivity, easing AR-coating requirements and improving device consistency.