Single-mode operation of a laser-diode array with frequency-selective phase-conjugate feedback.

Area of Science:

• Optics and Photonics
• Semiconductor Lasers

Background:

• Laser diode arrays typically exhibit low spatial and spectral coherence, limiting their single-mode operation.
• Achieving single-mode output from multimode laser systems is crucial for applications requiring high spectral purity and beam quality.

Purpose of the Study:

• To present a new technique for achieving stable single-mode operation in gain-guided Gallium Aluminum Arsenide (GaAlAs) laser diode arrays.
• To demonstrate the effectiveness of a frequency-selective phase-conjugate feedback system in controlling laser array modes.

Main Methods:

• Coupling a GaAlAs laser diode array to an external feedback system.
• Incorporating a photorefractive barium titanate crystal, a Fabry-Perot etalon, and a spatial filter within the feedback loop.
• Utilizing the Fabry-Perot etalon as the key component for mode selection.

Main Results:

• The laser diode array successfully operated in a single spatial and single longitudinal mode.
• Far-field pattern was reduced to 1.4 times the diffraction limit at twice the threshold current.
• Spectral bandwidth was narrowed to less than 0.02 nm, and coherence length increased by 70 times.

Conclusions:

• The developed phase-conjugate feedback technique effectively forces multimode laser arrays into single-mode operation.
• The method shows broad applicability to various other multimode laser systems.
• This advancement offers a pathway to improved laser performance and new applications.