1-MHz Mode-Locked All-Fiber Linear-Cavity Laser Based on Two SESAMs at $1.7\ \mu\mathrm{m}$ for Multiphoton Microscopy

As a powerful optical tool, multiphoton microscopy (MPM) has played an essential role in deep-tissue imaging and in vivo imaging [1]. Among the laser sources for MPM,
$1.7\ \mu\mathrm{m}$
laser sources with a low repetition rate have recently become a hot topic for the abilities of 3-photon and 4-photon excitation of most existing fluorescent probes, such as green fluorescent protein and red fluorescent protein [2], which is necessary for high-contrast imaging deep in scattering biological tissues [3]. Although there are few reports on 3-photon microscopy with
$1.7-\mu\mathrm{m}$
laser systems utilizing soliton self-frequency shift [4], which are usually intricate, studies on
$1.7-\mu\mathrm{m}$
low repetition rate fiber lasers are rare. In this work, for the first time, we present a
$1.7-\mu\mathrm{m}$
mode-locked (ML) all-fiber laser with a repetition rate of about 1 MHz. It is mode-locked by two semiconductor saturable absorber mirrors (SESAM) and employs a simple and compact linear cavity, opening up broad prospects for multiphoton microscopy at
$1.7\ \mu\mathrm{m}$
region.