Hsiang-Yu Chung

Energetic ultrafast fiber laser sources tunable in 1030–1215 nm for deep tissue multi-photon microscopy

We demonstrate an energy scalable approach to implement ultrafast fiber laser sources suitable for deep tissue multi-photon microscopy imaging. Enabled by fiber-optic nonlinearities (dominated by self-phase modulation), these unique ultrafast sources produce nearly transform-limited pulses of 50-90 fs in duration with the center wavelength tunable in the wavelength range of 1030-1215 nm. The resulting pulse energy can be scaled up to 20 nJ by optimizing fiber dispersion, shortening fiber length, and using large-mode-area fibers. We applied such an energetic source to a proof-of-principle study of ex vivo human skin based on harmonics (i.e., second-harmonic generation and third-harmonic generation) imaging. This new type of fiber-format energetic ultrafast source provides a robust solution for multiphoton microscopy applications.

Megawatt peak power tunable femtosecond source based on self-phase modulation enabled spectral selection

Wavelength widely tunable femtosecond sources can be implemented by optically filtering the leftmost/rightmost spectral lobes of a broadened spectrum due to self-phase modulation (SPM) dominated fiber-optic nonlinearities. We numerically and experimentally investigate the feasibility of implementing such a tunable source inside optical fibers with negative group-velocity dispersion (GVD). We show that the spectral broadening prior to soliton fission is dominated by SPM and generates well-isolated spectral lobes; filtering the leftmost/rightmost spectral lobes results in energetic femtosecond pulses with the wavelength tuning range more than 400 nm. Employing an ultrafast Er-fiber laser and a dispersion-shifted fiber with negative GVD, we implement an energetic tunable source that produces ~100-fs pulses tunable between 1.3 µm and 1.7 µm with up to ~16-nJ pulse energy. Further energy scaling is achieved by increasing the input pulse energy to ~1-μJ and reducing the fiber length to 1.3 cm. The resulting source can produce >100-nJ femtosecond pulses at 1.3 µm and 1.7 µm with MW level peak power, representing an order of magnitude improvement of our previous results. Such a powerful source covers the 2nd and the 3rd biological transmission window and can facilitate multiphoton deep-tissue imaging.

Er-Fiber laser enabled, energy scalable femtosecond source tunable from 1.3 to 1.7

We demonstrate that energetic femtosecond pulses tunable from 1.3 to 1.7 µm can be achieved using self-phase modulation enabled spectral broadening followed by spectral lobe filtering. Based on a home-built 5-W Er-fiber laser system operating at 31-MHz repetition rate, we obtain femtosecond pulses that can be continuously tuned from 1.3 to 1.7 µm with >4.5 nJ pulse energy. We further optimize the spectral broadening process using a fiber with larger mode area and scale up the pulse energy to >10 nJ; the resulting pulse duration is as short as ~50 fs. Such a widely tunable, energetic femtosecond source is well suited for driving a laser scanning microscope to perform deep tissue multiphoton microscopy.

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We demonstrate an Er-fiber laser based source that produces 100–200 fs pulses widely tunable from 1300 to 1700 nm, constituting an ideal driving source for three-photon excitation nonlinear optical microscopy.