Building Of Polarization-multiplexed Light Source And Two-color Soliton Light Source Based On Large-mode-area Waveguides And Their Applications To Microscopy At The 1700-nm Window

Combining high-energy femtosecond fiber laser with the nonlinear optical effect of soliton self-frequency shift(SSFS)in large-mode-area(LMA)waveguides,such as photonic-crystal(PC)rods or LMA fibers,we can generate broadband wavelength tunable femtosecond pulses with ultrashort pulse width and high pulse energy.Such excitation sources are ideal for multiphoton microscopy(MPM),and so far,they have been successfully applied in deep-tissue MPM and multi-color MPM.However,there remains much room to further optimize these soliton sources for MPM.Specifically,in deep-tissue MPM,it has been reported that the 1700-nm window is the optimal excitation window for imaging in deep biological tissues.However,the signal level deep in the tissue is depleted,which is the main limit for the imaging depth achievable so far.From the perspective of light source,it is due to the fact that the output energy of the existing PC rod has reached the maximum.In multi-color microscopy,the wavelength separation between multi-color solitons cannot be continuously tuned,preventing multi-color imaging of different fluorescent labels.In the aim of solving these problems and further optimizing the soliton sources for these applications,in this thesis we demonstrate the following technological advancements:(1)In order to increase the output power of soliton source,we demonstrate a polarization-multiplexed light source system.The collinear soliton pulses output with orthogonal polarizations show the same wavelength,pulse width and pulse energy,equivalent to a femtosecond pulse light source with a doubled repetition frequency due to the well-separating in time between the two orthogonally-polarized pulses.Compared with single-polarization soliton excitation,our polarization-multiplexed soliton source can efficiently boost MPM signals in different modalities of MPM.(2)Here we propose a 2-color soliton light source system to generate wavelength separation tunable 2-color solitons,based on our recently proposed theoretical scheme of prechirping to manipulate the soliton order of the excitation pulse.Using this method,we demonstrate a maximum soliton separation tuning range of 190 nm,with 1550-nm excitation.As an experimental demonstration of the applicability of this technique,we further show MPM results using this 2-color soliton source for 2-color,3-photon fluorescence imaging of fluorescent beads.The above technologies we have developed will provide novel and effective light source for deep-tissue and multi-color microscopy.