Static and dynamic mode coupling in a double-pass rod-type fiber amplifier.

Area of Science:

• Laser Physics and Photonics
• Fiber Optics and Amplification

Background:

• High-power fiber amplifiers are crucial for various scientific and industrial applications.
• Rod-type fiber amplifiers offer potential for high-gain and excellent beam quality.
• Understanding limitations like mode instability is key for further development.

Purpose of the Study:

• To experimentally realize and characterize a double-pass amplifier utilizing rod-type fiber.
• To investigate the amplifier's performance, including gain, efficiency, beam quality, and polarization.
• To analyze the onset and cause of transverse mode instability at high output powers.

Main Methods:

• Experimental setup of a double-pass amplifier employing rod-type fiber.
• Amplification of a 300 mW seed signal to high output power (120 W).
• Characterization of gain, optical-to-optical efficiency, beam quality, and polarization extinction ratio.
• Analysis of mode quality and degree of polarization degradation under high power load.

Main Results:

• Achieved a gain of 26 dB, amplifying a seed signal to 120 W.
• Demonstrated high optical-to-optical efficiency of 50% and excellent beam quality.
• Obtained a polarization extinction ratio of 33 dB.
• Observed marginal degradation of mode quality and polarization due to transverse mode instability at 120 W.

Conclusions:

• The rod-type fiber double-pass amplifier shows promising high-power performance.
• Transverse mode instability, initiated by static thermal long-period gratings, limits performance at 120 W.
• Understanding mode coupling mechanisms is essential for mitigating instability in high-power fiber amplifiers.