& _- __ l!!B 15 October 1997 OPTICS COMMUNICATIONS ELSEVIER Optics Communications 142 (1997) 203-207 An optical coherence microscope with enhanced resolving power in thick tissue J.M. Schmitt *, S.L. Lee, KM. Yung zyxwvutsrqponmlkjihgfedcbaZYXWVU Department zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA qf zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Electrical and Electronic Engineering, Hong Kong Unir~ersi@ of Science and Technology Clear W ater Bay. Kouloon. Hong Kong Received 17 March 1997; accepted 21 May 1997 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQ Abstract A new configuration of an optical coherence microscope for high-resolution imaging of biological tissues and other scattering materials is described. The microscope employs multiple sources and detectors combined with high-numerical- aperture optics to suppress speckle fluctuations and enhance resolution. Results of experiments on living tissue and tissue phantoms are presented to demonstrate the design principles of the microscope and to compare its performance to that of a conventional optical coherence microscope. 0 1997 Elsevier Science B.V. Kewvords: Microscopy: Tomography: Interferometry: Biomedical optics 1. Introduction Optical coherence tomography (OCT) has emerged as a powerful technique for probing microscopic structures be- neath the surface of optically dense materials [1.2]. Since its potential as a non-invasive diagnostic tool was first demonstrated several years ago, OCT has been investi- gated for application in ophthalmology, cardiology, derma- tology, and other medical disciplines [l-6]. Despite im- provements in scanning speed [7] and resolution [8,9], the basic design of the OCT scanner has changed little since its conception. As presently configured, OCT scanners provide remarkably clear images of microscopic structures in transparent tissues, but rather poor images of skin and other highly scattering tissues [6]. Better optics and new signal-processing strategies are needed to expand the range of applications of OCT. * E-mail: eeschmit@ee.ust.hk. This Communication describes an optical coherence microscope that is designed to alleviate three major defi- ciencies which limit the performance of OCT in optically thick tissue: (i) small numerical aperture (NA), (ii) low axial resolution and (iii) sensitivity to speckle fluctuations. In conventional OCT systems, which employ a fixed ob- jective and a scanning mirror in the reference arm, the first of these deficiencies arises from the need to keep the depth of the focal zone long enough to encompass the entire axial scan range (usually l-2 mm). An additional compli- cation arises from the need to avoid the focal walk-off caused by the refractive-index mismatch between air and tissue. The second deficiency is related to the spectra1 bandwidth of the source. To attain a coherence time short enough to reduce the thickness of an optical section below IO km, the width of the nucleus of a typical biological cell, a light source emitting at 1300 nm would need to have a FWHM bandwidth of more than 100 nm. Commer- cially available light-emitting diodes (LEDs) with band- widths this wide have irradiances too low for probing deep in weakly backscattering tissue. The third deficiency, which 0030-401 E/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII s0030-4018(97)00280-0