Double-clad photonic crystal fibre for two-photon microscopy

Miniaturized and head-mountable scanning nonlinear fibre-optic microscopy offers an exciting opportunity for the visualization of brain activity on freely walking mice. One challenge in the creation of such miniaturized microscopes is the design of optical fibres capable of delivering ultrashort pulses without distortion and collecting fluorescence simultaneously and efficiently. A double-clad photonic crystal fibre (DCPCF) with 37-cell air holes arranged in hexagonal lattice in core is proposed. The full vector finite element method (FEM) is used for numerical analysis. Nearly zero dispersion of -0.34 ps/nm/km at 920 nm wavelength is finally achieved. The single-mode core with an NA of 0.53 at 920 nm wavelength is used to deliver the femtosecond excitation beam and the inner cladding with an NA of 0.54 at 532 nm is used to collect the two-photon fluorescence signal. The nearly zero dispersion, high NA and low nonlinearity will be beneficial to promote its application in two-photon microscopy.