Soliton Self-frequency Shift In Air-core Fiber And Its Application In Multiphoton Microscopy

Multiphoton microscopy(MPM)is receiving much attention for notable advantages of subcellular spatial resolution(on the order of submicron),deep-tissue penetration capability(typically millimeter),3D imaging capability,dynamic tracking,functional imaging and non-invasive imaging.MPM has been applied to a variety of imaging tasks,especially in deep-tissue biological samples and in brain research in vivo.One reason for inadequate fluorescence signal intensity generation is the depletion of the excitation energy as the soliton energy of photonic crystal rod(PC rod)has reached its limit.Therefore,it is necessary to build more energetic excitation source.Air-core fiber can support more energetic femtosecond soliton pulses due to its low nonlinearity.This thesis demonstrates soliton self-frequency shift(SSFS)in an air-core fiber from both theoretical and experimental perspective,and comparative 3-photon microscopy(3PM)excited with both air-core and PC rod solitons.Through these works,we propose the more suitable source for deep-tissue 3PM at the 1700 nm window.We have demonstrated the following results in this thesis:(1)We build and optimize a tunable Air-core fiber source,with improved stability of air-core fiber solitons.Besides,we perform experimental research on soliton self-frequency shift in air-core fiber and detailed experimental characterization of soliton pulse parameters.Through SSFS in an air-core fiber,we generate solitons from 1563 nm to1605 nm,with a 42 nm wavelength tunability.Furtheromore,we can generate the most energetic femtosecond soliton pulses at the 1700 nm window,with an energy of 503 nJ,5.2 times higher than that from PC rod,while suffers long pulse width.(2)We perform comparative 3PM excited with both air-core and PC rod solitons.Using a sulforhodamine 101 sample solution for comparative 3-photon fluorescence(3PF)imaging and an excised mouse skull for comparative third-harmonic generation(THG)imaging.Our results show that at similar excitation power,3-photon signals excited by air-core fiber solitons are an order of magnitude smaller than those excited by the PC rod solitons.In order to generate comparable 3-photon signals,3 times more air-core fiber soliton power is needed than PC rod soliton power.Our research is the first demonstration of the application of air-core fiber soliton self-frequency shifting technology to the third optical transparent window imaging.(3)We carry out numerical simulation research on self-frequency shift of air-core fiber solitons.The theoretical calculation shows qualitative agreement with experimental results especially in predicting soliton wavelength and energy,it also indicates 9.2 m length is needed to shift the soliton to 1600 nm.We further elucidate the challenge of generating shorter soliton pulses from air-core fibers through using input pulse width of 100 fs,200 fs,500 fs in numerical simulation.Despite the difference in input pulse width,the output soliton pulse widths are similar.Together with comparative 3PM results and damage power of biological samples,we propose PC rod soliton a more suitable source for 3PM at the 1700 nm window.