Characterization and application of chirped photonic crystal fiber in multiphoton imaging

Fiber delivery of ultrashort pulses is important for multiphoton endoscopy. A chirped photonic crystal fiber (CPCF) is first characterized for its transmission bandwidth, propagation loss, and dispersion properties. Its extremely low dispersion (~150 fs(2)/m) enables the delivery of sub-30 fs pulses through a ~1 m-long CPCF. The CPCF is then incorporated into a multiphoton imaging system and its performance is demonstrated by imaging various biological samples including yew leaf, mouse tendon, and human skin. The imaging quality is further compared with images acquired by a multiphoton imaging system with free-space or hollow-core photonic band-gap fiber (PBF) delivery of pulses. Compared with free-space system, the CPCF delivered system maintains the same ultrashort pulsewidth and the image qualities are comparable. Compared with the PBF delivery, CPCF provides a 35 times shorter pulsewidth at the sample location, which results in a ~12 and 50 times improvement in two-photon excitation fluorescence (TPEF) and second harmonic generation (SHG) signals respectively. Our results show that CPCF has great potential for fiber delivery of ultrashort pulses for multiphoton endoscopy.