Broad bandwidth laser and nonlinear optical sources for OCT

Abstract

OCT achieves very high axial image resolutions independent of focusing conditions because the axial and transverse resolutions are determined independently by different physical mechanisms. This implies that axial OCT resolution can be enhanced using broad bandwidth, low coherence length light sources. The light source not only determines axial OCT resolution via its bandwidth and central emission wavelength but also determines the penetration in the sample (biological tissue), the contrast of the tomogram, and the OCT transverse resolution. A minimum output power with low amplitude noise is also necessary to enable high sensitivity and high–speed “ real time ” OCT imaging. Hence, it is obvious that the light source is the key technological parameter for an OCT system, and proper choice is imperative. Ultrabroad bandwidth light source technology enables ultrahigh–resolution OCT in the visible and near–infrared wavelength region. Kerr-lens mode–locked solid–state lasers can generate broad bandwidth spectra spanning up to one optical octave. Nonetheless they are restricted to the fluorescence bands of the laser crystal and have a complex architecture making them expensive and preventing widespread industrial use. Spectra far broader than one optical octave can be produced via nonlinear propagation of laser pulses having only moderate energies of a few nJ in microstructured fibers. Complex fibers with one, two, or even no zero–dispersion wavelength can be designed and fabricated to fulfill special requirements as large optical bandwidth and low noise.