Stimulated Raman generation in two-mode long-length fibers pumped by a mode-locked laser

Area of Science:

• Nonlinear Optics
• Fiber Optics
• Laser Physics

Background:

• Stimulated Raman scattering (SRS) is a key nonlinear optical process in optical fibers.
• Mode-dependent propagation characteristics significantly influence nonlinear effects in multimode fibers.
• Nd:YAG lasers are widely used as pump sources in various spectroscopic and nonlinear applications.

Purpose of the Study:

• To investigate single-pass Raman generation in long two-mode fibers.
• To analyze the influence of input power on stimulated Raman scattering characteristics.
• To understand the role of mode group delay in Raman generation.

Main Methods:

• Utilizing a mode-locked Nd:YAG laser for pumping.
• Employing two-mode fibers, each 35 km in length.
• Analyzing input power dependence of stimulated Raman scattering.
• Measuring the peak wavelength and output power of the first Stokes signal.

Main Results:

• A dramatic jump in the first Stokes peak wavelength from 1.114 to 1.12 micrometers was observed at 1.65-W input power.
• Total output power of the first Stokes increased rapidly at this critical input power.
• These results were obtained for a fiber with a 1.27-micrometer cutoff wavelength under LP(11) mode excitation.
• Experimental findings were explained by the group delay differences between LP(01) and LP(11) modes.

Conclusions:

• Group delay significantly impacts stimulated Raman scattering in two-mode fibers.
• The observed wavelength jump and power enhancement are directly related to mode group delay.
• Maximum interaction length and threshold power for Raman generation can be predicted using group delay data.