Polarization multiplexed 2200-nm soliton generation and its application to 3-photon microscopy.

The 2200-nm window has recently been demonstrated as the longest excitation window for deep-tissue multiphoton microscopy (MPM). So far, MPM at this window exclusively uses a soliton laser source based on soliton self-frequency shift (SSFS). In order to boost the multiphoton signal level at this window, here we demonstrate a polarization multiplexed soliton source based on orthogonal polarized SSFS in a polarization maintaining large mode area (PM LMA) fiber. Using this technology, the effective pulse repetition rate, i.e., the pulse number per unit time, is twice higher than the 1550-nm pump laser with a fixed repetition rate. We further demonstrate comparative third harmonic generation (THG) and 3-photon fluorescence (3PF) imaging with this polarization multiplexed soliton source and single polarization soliton source. The ∼2 times higher 3-photon signal of the former indicates that this soliton multiplexing technology is promising for MPM at the 2200-nm window.