Self-phase-modulation and group velocity dispersion of near IR femtosecond pulses in fibers restrict their use in two-photon fluorescence microscopy (TPFM). Here we demonstrate a hollow-core photonic crystal fiber based two-photon fluorescence microscope with low nonlinearity and dispersion effects. We use this fiber-based TPFM system to take two-photon fluorescence (chlorophyll) images of mesophyll tissue in the leaf of Rhaphidophora aurea. With less than 2mW average power exposure on the leaf at 755nm, the near zero-dispersion wavelength, chloroplasts distribution inside the mesophyll cells can be identified with a sub-micron spatial resolution. The acquired image quality is comparable to that acquired by the conventional fiber-free TPFM system, due to the negligible temporal pulse broadening effect.

Since the first demonstration in 1990, two-photon fluorescence microscopy (TPFM) has made a great impact on biomedical researches. With its high penetration ability, low out-of-focus photodamage, and intrinsic three-dimensional (3D) sectioning capability, TPFM has been widely applied to various medical diagnosis and genome researches. Recently, single-mode optical fibers were introduced into the TPFM systems for remote optical pulse delivery. Fiberbased TPFM has advantages including isolating the vibration from laser and electronic devices, flexible system design, and low cross-talks. It is also the first step toward an all-fiber based two-photon endoscope. However, due to serious temporal broadening when conventional Ti:sapphire based femtosecond pulses propagate through the fiber, the twophoton excitation efficiency of the fiber-optic TPFM is much lower than the conventional one. The temporal broadening effect mainly comes from group velocity dispersion (GVD) and self-phase modulation (SPM), which also leads to significant spectral broadening. To reduce the temporal broadening effect, here we present a hollow-core photonic-bandgap fiber based TPFM. By replacing the conventional single-mode fiber with the hollow core photonic bandgap fiber, the GVD and SPM effects can be greatly reduced for high intensity, ultra-short pulse delivery. Femtosecond Ti:sapphire pulses passing through the fiber with negligible GVD and SPM effects is demonstrated in this paper. Much improvement of two-photon fluorescence excitation efficiency is thus achieved with the hollow-core photonic-bandgap fiber based TPFM.

By measuring the spectral loss characteristics of subwavelength-diameter terahertz fibers, our study supports the recent theory proposed by M. Sumetsky [Opt. Lett. 31, 870 (2006)] that diameter-variation-induced radiation is a dominant loss mechanism for subwavelength fibers in the low- (<1%) core-fraction-power regime. This physical mechanism limits the lowest guidable frequency in a subwavelength fiber.

A simple terahertz (THz) Fourier transform spectrometer is theoretically and experimentally demonstrated with a low-reflectivity Fabry-Perot interferometer. Composed with only two parallel low reflectivity surfaces, this simple spectrometer has the ability to measure the spectrum over more than one octave with a controllable resolution. The emission spectra of a wavelength-tunable photonic transmitter excited by an optical coherent control system are determined by the spectrometer. With a simple algorithm for the spectral reconstruction and a compact structure with easy alignment, the demonstrated spectrometer will meet the needs of many important applications where a compact and convenient spectrometer is required in the frequency range from sub-THz wave up to even visible light.