Coherent instabilities in thulium-based fiber amplifiers induced by laser frequency modulation

Area of Science:

• Fiber optics
• Laser physics
• Nonlinear optics

Background:

• Frequency modulation (FM) of narrow-linewidth lasers can induce coherent backscattering.
• This phenomenon is particularly problematic in cladding-pumped fiber amplifiers, especially those based on Thulium (Tm).
• Coherent backscattering presents a significant limitation for power scaling in fiber amplifier applications.

Purpose of the Study:

• To investigate instabilities caused by frequency modulation in Thulium (Tm)- and Thulium/Holmium (Tm/Ho)-doped fiber amplifiers.
• To analyze the impact of various design parameters and operational regimes on coherent backscattering.
• To understand the fundamental mechanisms behind the backward-propagating (BP) signal generation.

Main Methods:

• Experimental investigation of Tm- and Tm/Ho-doped fiber amplifiers.
• Systematic variation of active fiber length, pumping scheme, and dopant type.
• Analysis of amplifier operation across a range of laser frequency tuning rates and amplifier gains.

Main Results:

• Coherent backscattering was observed in both Tm- and Tm/Ho-doped fiber amplifiers.
• The backward-propagating (BP) signal exhibited a peak at a specific laser frequency tuning rate for each amplifier configuration.
• BP signal amplitude and frequency increased with higher amplifier gain and longer active fiber lengths.

Conclusions:

• Frequency modulation-induced coherent backscattering is a critical instability in Tm-based fiber amplifiers.
• Amplifier gain and fiber length are key parameters that exacerbate this detrimental effect.
• Understanding these instabilities is crucial for advancing power scaling in fiber laser and amplifier technologies.