Polarization multiplexing in large-mode-area waveguides and its application to signal enhancement in multiphoton microscopy

Soliton self-frequency shift (SSFS) is an efficient technique of generating broadband tunable femtosecond optical pulses. By using large-mode-area (LMA) waveguides such as photonic-crystal (PC) rods or LMA fibers, SSFS is capable of generating solitons with tens of or even >100 nJ pulse energy, enabling deep-tissue multiphoton microscopy (MPM) with the unprecedented imaging depth. MPM signals are proportional to the repetition rate of the laser. Here, we demonstrate an efficient technique of enhancing MPM signals in LMA waveguides, through polarization multiplexing, in both a PC rod with no polarization-maintaining (PM) structure and a PM LMA fiber. The collinear output soliton pulses with orthogonal linear polarizations show similar pulse energy, pulse width, and spectrum. We also demonstrate the application of this polarization multiplexing technique to MPM signal enhancement in biological tissues. Compared with single-polarization soliton excitation, excitation with polarization-multiplexed solitons ca...