Linewidth enhancement factor in semiconductor lasers subject to various external optical feedback conditions

Area of Science:

• Optics and Photonics
• Semiconductor Physics
• Laser Engineering

Background:

• The linewidth enhancement factor (α) is a critical parameter in semiconductor lasers, influencing their spectral properties and stability.
• Optical feedback can significantly alter laser behavior, leading to complex dynamics and potential instabilities.
• Understanding α under various feedback conditions is crucial for designing stable and high-performance semiconductor lasers.

Purpose of the Study:

• To experimentally and theoretically investigate the linewidth enhancement factor (α) in semiconductor lasers subjected to optical feedback.
• To explore the influence of feedback strength, external cavity length, and feedback phase on α.
• To demonstrate the continuous tunability of α, particularly in the short external cavity regime.

Main Methods:

• Experimental setup utilizing a pellicle beamsplitter on a PZT stage for precise control of external cavity length and phase.
• Theoretical analysis based on the minimum of the Hopf bifurcation curve under optical feedback and injection.
• Systematic variation of feedback strength, external cavity length (long to short regimes), and feedback phase.

Main Results:

• The linewidth enhancement factor (α) increases with feedback strength in moderate regimes.
• α is largely insensitive to feedback phase in the long cavity regime.
• Continuous tuning of α with feedback phase is achieved in the short cavity regime, with a normalized variation range of 21.59% at 1.5 cm external cavity length.
• The tunability of α is enhanced by further shortening the external cavity.

Conclusions:

• This work presents the first detailed study of α across long and short cavity regimes in semiconductor lasers with optical feedback.
• Continuous tuning of α via feedback phase variation in short cavities is demonstrated for the first time.
• The findings offer new possibilities for controlling and optimizing semiconductor laser characteristics through optical feedback manipulation.