Short-wavelength-band tunable high-power Tm-doped fiber laser

In this work, we realized broadly tunable mode-locking operation from 1730 nm to 1815 nm in normal dispersion regime employing an acousto-optic tunable fiber (AOTF) in a Tm-doped dispersion-managed fiber laser. The AOTF worked as a multifunctional component in laser cavity suppressing undesired wavelength lasing and introducing a frequency shifting, which improved the stability of laser operation. The hybrid mode-locking incorporated by nonlinear polarization rotation (NPR) effect and frequency shifting effect ensured self-starting stable pulsed operation. The pulse spectral widths ranged from 17 nm to 25 nm. The stretching-free direct amplification in two-cascaded fiber amplifier enabled power scaling up to 310 mW and pulse energy of 19 nJ. Pulse duration was compressed down to 282 fs by a pair of gratings. The seed laser is further optimized. The optimized seed laser enhances output power about 5 times. The laser system was designed for multiphoton imaging of bladder cancer in the third biological window to demonstrate the recently discovered nonlinear effect resulting in improvement of signal contrast at the deeper tissue level.