| Summary: | Multiphoton Microscopy (MPM) is a widely used imaging technique in the biomedicine field. Fiber-based multiphoton endoscopes are important for in vivo clinical application because they enable minimally invasive imaging. One major challenge for these endoscopes is the efficient delivery of ultrashort pulses in the near infrared region through the optical fibers. MPM requires ultrashort pulses to obtain high peak power for the nonlinear excitation. However, the optical fibers, can introduce dispersion which can severely broaden the pulses and reduce the peak power. The purpose of this study is to find a good candidate of optical fibers that can propagate sub-30 fs pulses while maintaining high peak power for the MPM excitation.
In this project, I investigate the feasibility of applying a specially-designed chirped photonic crystal fiber (CPCF) for MPM imaging because CPCF has unique cell-size radial chirp can achieve low dispersion in a broadband transmission window. The key features of the CPCF are characterized, such as spectra, mode profile, and dispersion parameter, etc. A fiber-delivered MPM system is developed by adding the fiber coupling to a multimodal microscope. A prism-based dispersion pre-compensation unit is optimized to compensate the dispersion from all the optical components in the CPCF-delivered system. After pre-compensation, the appealing performance of CPCF applied in MPM is demonstrated by imaging various biological samples. Additionally, traditional hollow core fiber (HCF) is used as a comparison. The HCF consists of several identical reflective layers in the cladding. I pre-compensate the laser pulses after the HCF propagation and MPM images for similar samples are acquired. Large improvement in image contrast is observed in all samples for the system using the CPCF for light delivery compared with the system using the conventional HCF. The enhancement in second harmonic generation (SHG) is more significant than that in two photon excited fluorescence (TPEF).
Our study shows that CPCF can successfully deliver sub-30 fs pulses with significantly increased excitation efficiency of MPM for the broad-band laser. These properties are highly sought after in MPM endoscopy. With the fiber delivery of femtosecond pulses, MPM can be developed into a portable system for in vivo imaging.
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