Mode locked solid state laser sources for optical coherence tomography

A high speed technique for performing 'optical biopsies,' or optical diagnostic imaging of in vivo tissue architectural morphology, would greatly enhance the diagnosis and clinical management of many diseases. Optical coherence tomography (OCT) is a novel optical imaging technique that uses low coherence interferometry to obtain micron scale, cross- sectional images of biological systems. OCT was initially applied in ophthalmology to provide high resolution, cross sectional, tomographic images of the transparent structures in the eye and clinical studies show that OCT has considerable promise for the diagnosis of a wide range of retinal macular diseases. OCT imaging in other human tissues is more difficult due to optical scattering. However, recent in vitro studies have shown that OCT can image architectural morphology in highly optically scattering tissues. One of the key technological issues for OCT in optical biopsy is the development of low coherence laser sources. Essential attributes of a clinically viable light source for OCT include high single-transverse-mode power, short coherence length, and a central wavelength optimal for deep penetration within human tissue. Passively mode locked solid state lasers based on Ti:Al₂O₃ and Cr:Mg ₂SiO₄ are capable of providing hundreds of milliwatts of single-transverse mode light with coherence lengths as short as 1.8 microns. We present recent developments in the optimization of mode locked solid state lasers for application to OCT and demonstrate the resulting capability to enable fast acquisition of high resolution tomographic images.