Static thermo-optic instability in double-pass fiber amplifiers.

Area of Science:

• Optics and Photonics
• Nonlinear Fiber Optics
• Laser Physics

Background:

• Transverse mode instabilities (TMIs) are a known issue in single-pass optical fiber amplifiers.
• Coupled-mode formalism has been used to analyze TMIs in these systems.
• Understanding mode coupling is crucial for high-power fiber amplifier design.

Purpose of the Study:

• To extend the coupled-mode formalism to double-pass optical fiber amplifiers.
• To investigate the role of thermo-optic nonlinearity in double-pass configurations.
• To identify novel phenomena and their power thresholds in double-pass systems.

Main Methods:

• Adaptation of an existing coupled-mode formalism for double-pass amplifier analysis.
• Theoretical investigation of light-mode coupling induced by thermo-optic effects.
• Analysis of the interaction between counter-propagating light and index perturbations.

Main Results:

• Thermo-optic nonlinearity couples the LP01 and LP11 modes in double-pass amplifiers.
• This coupling leads to a static deformation of the output beam profile.
• The threshold power for this static deformation is significantly lower than for TMIs in single-pass amplifiers.

Conclusions:

• A novel phenomenon of static beam deformation due to thermo-optic nonlinearity is identified in double-pass fiber amplifiers.
• This effect arises from the interaction of counter-propagating light with induced refractive index changes.
• The lower power threshold has implications for the design and operation of high-power double-pass fiber amplifiers.