Pseudo-random binary sequence phase modulation for narrow linewidth, kilowatt, monolithic fiber amplifiers

Area of Science:

• Optics and Photonics
• Laser Physics

Background:

• High-power monolithic fiber amplifiers are crucial for various laser applications.
• Stimulated Brillouin scattering (SBS) limits the performance of these amplifiers by reducing the SBS threshold.
• Narrow-linewidth output is essential for applications like beam combining.

Purpose of the Study:

• To investigate the effectiveness of pseudo-random binary sequence (PRBS) phase modulation for suppressing SBS in kilowatt-class fiber amplifiers.
• To analyze the impact of PRBS modulation on the SBS threshold and linewidth.
• To demonstrate a high-power, narrow-linewidth fiber amplifier utilizing PRBS modulation.

Main Methods:

• Experimental analysis of SBS threshold enhancement factors for different PRBS modulation patterns.
• Investigation of SBS seeding by phase-modulated signals near the Brillouin shift frequency.
• Demonstration of a 1.17 kW fiber amplifier with 3 GHz PRBS modulation.

Main Results:

• PRBS phase modulation significantly enhances the SBS threshold, aligning well with theoretical predictions.
• Approximately 30% variations in SBS power thresholds were observed with small modulation frequency tunings.
• A 1.17 kW fiber amplifier achieved near diffraction-limited beam quality (M2 = 1.2) and 83% optical pump efficiency.

Conclusions:

• PRBS phase modulation is an effective method for SBS suppression in high-power fiber amplifiers.
• The achieved narrow linewidth and improved SBS suppression are beneficial for beam combining of kilowatt-class fiber lasers.
• This technique offers a pathway to higher-power, high-quality fiber laser systems.