Nonlinear Fiber Optics for Bio-Imaging

Two-photon excitation (TPE) is a modern technology with applications in microscopy and spectroscopy that has gained a great amount of attention in recent years. This technique is the best suitable to analyze thick tissues and live animals as it works in the near-infrared (NIR) region. In this work we implement and evaluate a two-photon setup that allows the shifting of the working wavelength over a wide range using the soliton self-frequency shift (SSFS) effect. The shifter is implemented using a pulsed fiber laser and a photonic crystal fiber (PCF). We also include a numerical evaluation of the dependency of the fiber shift on the input average power and the fiber length. A semi-analytical model is proposed to investigate the characteristics of the SSFS in optical fibers. SSFS in two different types of fibers were evaluated and the results agree very well with those of numerical simulations. We show that when the frequency shift is small enough, it is inversely proportional to the fourth power of the initial soliton pulse width. However, with large frequency shift, this fourth power rule needs to be modified. We finally show the first two-photon images obtained at the University of Kansas.