Femtosecond-pulse lasers are useful for observing the nonlinear optical-tissue interaction and commonly used in biology and medicine both for experimental and clinical uses. For therapies, it is commonly used for selective ablation (removal) of the tissue component effectively without extensive collateral damage. Recently, therapeutic femtosecond laser system was introduced for corneal reshaping as a replacement for LASIK procedure which safety issue is under scrutiny nowadays. Its applications in medicine expand to various types of other microscopic modification of biological tissues.
In imaging area, the femtosecond laser pulses are used to generate nonlinear optical phenomenon such as two-photon fluorescence, higher harmonic generation, and broad-band source generation (for high-resolution OCT). Using two-photon fluorescence, it is possible to observe deeper regions and to induce lower damage to the target sample. The higher harmonic imaging of cells and tissues is drawing much attention recently since the higher harmonic signals originate from specific structures. We measured the second harmonic generation (SHG) from collagen-rich tissues such as disks, skin, cartilage, etc. Since the even number harmonic generation in tissue volume occur only from non-symmetric molecular structure such as collagen, this technique may be useful for studies related to aging, cartilage regeneration, skin disorders, and cancers.