Optimization of frequency-doubled Er-doped fiber laser for miniature multiphoton endoscopy.

Frequency-doubled femtosecond Er-doped fiber laser is a low-cost and portable excitation source suitable for multiphoton endoscopy. The frequency-doubled wavelength at 780 nm is used to excite the intrinsic fluorescence signal. The frequency-doubling with a periodically poled MgO  :  LiNbO3 (PPLN) is integrated in the distal end of the imaging head to achieve fiber connection. The imaging speed is further improved by optimizing the excitation laser source. A 0.3-mm length of PPLN crystal is selected and the Er-doped fiber laser is manipulated to match its bandwidth with the acceptance bandwidth of the PPLN. Through this optimization, a reduced pulsewidth of 80 fs of the frequency-doubled pulse is achieved. All-fiber dispersion compensation and pulse compression by single mode fiber is conducted, which makes the fiber laser directly fiber-coupled to the imaging head. An imaging speed of 4  frames  /  s is demonstrated on ex vivo imaging of unstained biological tissues, which is 10 times faster than our previous study using a 1-mm-long PPLN. The results show that miniature multiphoton endoscopy using frequency-doubled Er-doped fiber laser has great potential for clinical applications.