Experimental study of spatial and temporal coherence in a laser diode with optical feedback

Area of Science:

• Laser Physics
• Optical Engineering

Background:

• Optical feedback significantly alters semiconductor laser linewidth and temporal coherence.
• The impact of optical feedback on spatial coherence remains less understood.
• Distinguishing spatial and temporal coherence effects is crucial for laser applications.

Purpose of the Study:

• To develop and demonstrate an experimental technique for differentiating the effects of optical feedback on temporal and spatial coherence.
• To analyze how optical feedback influences spatial mode excitation and its impact on laser beam quality.
• To investigate the role of fiber type (multimode vs. single mode) in characterizing feedback-induced coherence changes.

Main Methods:

• Experimental analysis of a commercial edge-emitting laser diode with optical feedback.
• Comparison of speckle image contrast recorded using multimode (MM) and single mode (SM) fibers with an optical diffuser.
• Analysis of optical spectra to assess feedback-induced spectral broadening and line narrowing.

Main Results:

• Optical spectra confirmed feedback-induced linewidth broadening.
• Speckle contrast analysis revealed reduced spatial coherence due to feedback-excited spatial modes.
• A significant reduction (up to 50%) in speckle contrast was observed with MM fiber, indicating decreased spatial coherence.
• SM fiber effectively filtered out feedback-excited spatial modes, preserving speckle contrast.

Conclusions:

• The presented technique successfully discriminates between feedback effects on temporal and spatial coherence.
• Optical feedback excites spatial modes, leading to reduced spatial coherence in semiconductor lasers.
• The use of SM fiber is effective in isolating the effects of feedback on temporal coherence by filtering spatial modes.