Rate-equation theory of a feedback insensitive unidirectional semiconductor ring laser

Area of Science:

• Physics
• Optics
• Laser Technology

Background:

• Ring lasers are susceptible to back-scattering, leading to coupled-mode or bistable operation.
• Intra-cavity isolation is crucial for achieving stable unidirectional operation in ring lasers.
• Understanding mode coupling mechanisms is essential for laser design and performance optimization.

Purpose of the Study:

• To formulate a rate-equation theory for continuous-wave, single-frequency ring lasers with intra-cavity isolators.
• To analyze the impact of back-scattering sources (side-wall irregularities, inversion-grating spatial hole burning) on laser operation.
• To investigate the robustness of unidirectional operation against back-reflection and grating-induced mode coupling.

Main Methods:

• Development of a rate-equation theory incorporating two mutual back-scattering sources.
• Theoretical analysis of ring laser operation with and without intra-cavity isolation.
• Derivation of an explicit expression for directionality in the presence of external optical feedback and isolation.

Main Results:

• Confirmed that inversion-grating-induced bistable operation overrules back-reflection-induced coupled-mode operation in non-isolated ring lasers at high pumping strengths.
• Demonstrated that intra-cavity isolation effectively mitigates back-reflection and grating-induced mode coupling.
• Derived an expression showing that predictions remain unaltered with strong isolation, even with mode coupling mechanisms present.

Conclusions:

• The developed rate-equation theory accurately describes complex mode dynamics in isolated ring lasers.
• Intra-cavity isolation is a robust method for achieving stable unidirectional operation.
• The findings provide a theoretical basis for designing high-performance, single-frequency ring lasers.