Raman Lasing and Transverse Mode Selection in a Multimode Graded-Index Fiber with a Thin-Film Mirror on Its End Face.

Area of Science:

• Optics and Photonics
• Laser Physics
• Materials Science

Background:

• Multimode fibers offer potential for high-power lasers but require transverse mode control.
• Efficient mode control is crucial for maintaining beam quality in high-power fiber laser systems.

Purpose of the Study:

• To investigate the use of a micro-fabricated dielectric mirror for transverse mode control in a Raman fiber laser.
• To optimize mirror design for efficient fundamental mode selection and power scaling.

Main Methods:

• Fabrication of a dielectric thin-film mirror on a multimode fiber end face.
• Experimental testing of the mirror as an output coupler in a diode-pumped Raman laser cavity.
• Comparison of experimental results with COMSOL simulations to determine optimal mirror diameter.
• Characterization of output beam quality (M²), power, spectrum, and stability.

Main Results:

• Raman lasing at 976 nm achieved with a threshold pump power of ~80 W.
• Optimal mirror diameter for mode selectivity determined to be ~12 µm.
• Output beam quality of M²~2 near threshold, indicating fundamental mode selection.
• Demonstrated ~35 W output power with ~60% slope efficiency and narrow spectrum.
• Achieved high intensity (>30 MW/cm²) without mirror degradation, with beam quality degrading slightly to M²~3 at higher power.

Conclusions:

• Micro-fabricated dielectric mirrors effectively control transverse modes in multimode fiber lasers.
• The developed technique enables high-power Raman laser operation with good beam quality.
• The approach shows promise for power scaling in fiber lasers and other high-power applications.