Research On Femtosecond Laser And Pulse Width Measurement Development At The 1700-nm Window

Multiphoton microscopy(MPM)can achieve an imaging depth on the order of millimeter and a spatial resolution on the order of sub-micrond due to its nonlinear and near-infrared excitation characteristics.It has been widely used in research fields such as biology,neuroscience,and oncology.In deep-tissue MPM,imaging depth is further optimized.At present,the main reasons limiting the imaging depth are:(1)the brightness of the fluorescent dye is limited;and(2)the excitation light energy is limited.From the perspective of the light source,the maximum imaging depth of the multiphoton deep brain imaging that has been demonstrated is obtained in the excitation of three-photon imaging at the 1700 nm window.The imaging source used in this experiment was generated in a photonic-crystal(PC)rod pumped by a 1550 nm laser.The 1700-nm high-energy femtosecond pulse was generated by the soliton self-frequency shift(SSFS)effect.Although the existing PC rod can produce higher than 100 nJ of high-energy soliton pulses,there is still the possibility of further improvement.Therefore,the imaging depth can be improved by further increasing the energy of the light source in the 1700 nm window.At present,another problem with the 1700 nm window femtosecond pulse generated by the PC rod source is that the generated soliton has a significant overlap with the 1550 nm pump light,and there is significant spectral modulation.Consequently no clean soliton cannot be filtered through the filter.Furthermore,in order to achieve an accurate measurement of the pulse width,in particular the pulse width on the sample,it is necessary to develop a corresponding pulse width measurement technique.aim of tackling the above questions,in this thesis we demonstrate the following research work:(1)Based on the soliton self-frequency shift(SSFS)effect generated by the femtosecond pulse pumping light source incident on the fiber,a large mode aera(LMA)fiber source with a wavelength tuning range greater than 500 nm is built to study the excitation efficiency of the dye with wavelength;a rod-shaped photonic crystal fiber source with a wavelength tuning range greater than 100 nm is also built,and the soliton energy generated can reach 110 nJ for multiphoton deep-tissue imaging.(2)An interferometric autocorrelator for measuring the pulse width of the solitons is built.Using this device,not only can the fiber output pulse width be accurately measured,but also the pulse width of the excitation light on the sample after the microscope can be measured.This technology lays the foundation for the measurement of fluorescence excitation efficiency of fluorescent dyes and fluorescent proteins in the 1700 nm window.(3)In order to further enhance the soliton pulse energy,a polarization synthesis technique is developed.A polarization splitting and synthesizing device is added to PC rod source,and the filtered soliton single pulse energy is doubled.Comparison of imaging signal intensity of synthesized soliton source and single optical soliton source by multiphoton deep tissue imaging is demonstrated.Our results show that the polarization synthesis device can significantly improve the imaging signal.(4)In order to solve the problem that the PC rod cannot generate clean soliton,a pump pulse compression technique is demonstrated.The pump pulse of the fiber laser output is compressed by the compression device,from 500 fs to 175 fs.The experimental measurement and simulation calculation show that the compressed pulse can produce clean solitons when incident on a PC rod,facilitating subsequent filtering and imaging experiments.The development of both the light source and the pulse width measurement technology will facilitate deep imaging of the 1700 nm window and the three-photon excitation spectrum of the fluorescent marker.