Advanced Fiber Soliton Sources for Nonlinear Deep Tissue Imaging in Biophotonics

Optical imaging plays a major role in both basic biological research and clinical diagnostics, providing noninvasive or minimally invasive microscopic imaging capability to investigate biological tissues. Optical image acquisition through significant depths of biological tissues, however, presents a major challenge since tissue is extremely heterogeneous and the strong scattering of the various tissue components has restricted high-resolution optical imaging to superficial layers. Multiphoton microscopy (MPM) has significantly extended the penetration depth of high-resolution optical imaging, particularly for in vivo applications. Multiphoton imaging critically depends on ultrafast technologies, particularly pulsed excitation sources. In this paper, the basics of deep tissue MPM and its improvements utilizing soliton self-frequency shift (SSFS) are reviewed. Wavelength tunable, high-energy soliton generation through SSFS in large-mode-area (LMA) fibers and photonic crystal rods is presented. The application of these solitons to MPM enables noninvasive imaging in biological tissues with unprecedented depth. The main characteristics of the excitation source for deep tissue MPM, such as wavelength, pulse energy, and repetition rate, are discussed.