Abstract

Optical coherence tomography (OCT) is a new technique for micron scale, cross-sectional, tomographic imaging.¹ In biology and medicine, OCT can be a powerful technique for optical biopsy because it permits the direct imaging of cross-sectional tissue architectural morphology without the need to excisionally remove specimens. OCT imaging is somewhat analogous to radar or ultrasound, except that OCT uses a low coherence optical source and interferometry to perform time delay measurement of light which is backreflected or backscattered from optically turbid specimens. Superluminescent diodes can provide 15-20 micron resolution imaging at several fixed wavelengths, including 800 and 1300 nm, but output powers are limited to the order of a milliwatt. Studies at these wavelengths have demonstrated that 1300 nm wavelengths can achieve greater imaging penetration depths in biological tissues because of reduced scattering. Ultrahigh resolution imaging has been demonstrated using the fluorescent emission from solid state laser media² and femtosecond pulses from a mode locked Ti:Al₂O₃ laser.³ OCT using mode locked solid state lasers³ is especially promising because the high output powers available enable higher signal to noise levels as well as faster data acquisition. Using a mode locked source, OCT imaging rates approaching real time can be achieved. In addition, the spectral bandwidth of femtosecond pulses can be broadened by using self phase modulation in an optical fiber thus yielding significant increases in imaging resolution. In this paper we present a Kerr lens mode locked Forsterite laser (at 1.3 micron wavelength) which is spectrally broadened through self-phase modulation in a dispersion shifted fiber. With this source 5.7 micron resolution OCT images with 116 dB signal to noise are demonstrated. Applications of mode locked solid state technology for OCT biomedical imaging is discussed.

© 1996 Optical Society of America