Elsevier

Optics Communications

Volume 202, Issues 1–3, 1 February 2002, Pages 29-35
Optics Communications

Parallel optical coherence tomography in scattering samples using a two-dimensional smart-pixel detector array

https://doi.org/10.1016/S0030-4018(02)01073-8Get rights and content

Abstract

Parallel optical coherence tomography in scattering samples is demonstrated using a 58×58 smart-pixel detector array. A femtosecond mode-locked Ti:Sapphire laser in combination with a free space Michelson interferometer was employed to achieve 4μm longitudinal resolution and 9μm transverse resolution on a 260×260μm2 field of view. We imaged a resolution target covered by an intralipid solution with different scattering coefficients as well as onion cells.

Introduction

Optical coherence tomography (OCT) allows acquisition of spatially resolved maps of reflectivity in scattering samples. In most common OCT systems, depth scanning is achieved by the longitudinal translation of a reference mirror, and lateral scanning is obtained by the lateral translation of a focused probe beam using scanning mirrors [1], [2], [3]. To increase the acquisition speed and eliminate the need for lateral scanning, parallel detection schemes have been investigated [4], [5], [6], [7], [8]. Parallel OCT systems previously developed consist of a free-space Michelson interferometer illuminated by a short temporal coherence source. The sample under study is placed in one arm of the interferometer, illuminated with a uniform extended beam and imaged on an array of photodetectors. Charge coupled devices (CCD) cameras are the most commonly used imaging devices for parallel detection schemes. However, CCD cameras suffer from two drawbacks when used in parallel OCT systems: (1) the high optical DC intensity reflected by the reference mirror reduces the dynamic range available for AC interferometric signal detection, (2) the CCD frame rate (typically ∼100 Hz for 512×512 pixels) is much lower than the interferometric signal frequency (typically >1 kHz). In this case a lock-in detection or synchronous illumination scheme has to be employed [5], which limits the image acquisition speed. A different photodetector array based on CMOS technology was developed for parallel OCT [8], [9]. Besides transducing light signals into electrical signals, CMOS detectors offer the additional functionality of customized, integrated signal processing for each pixel. Such smart-pixel detector arrays (SPDAs) have been developed for OCT interferometric signal demodulation. The feasibility of using one- and two-dimensional SPDAs for OCT was demonstrated on reflective surfaces [7], [8].

In the present work we demonstrate the feasibility of employing an SPDA to obtain parallel OCT images in scattering samples.

Section snippets

Optical set-up

The optical set-up is illustrated in Fig. 1. The output of a femtosecond mode-locked Ti:Sapphire laser (MLTS) is coupled into 100 m of single-mode optical fiber in order to reduce peak pulse powers by pulse stretching. Dispersion in the fiber increases the temporal width of the laser pulses, which reduces dramatically the peak power. This precaution is taken to avoid damage to the sample and the optics. The fiber output is collimated by lens L2 and linearly polarized by polarizer P. An average

System performances

We measured the system longitudinal response on a mirror in air to be 4μm. The depth resolution measurement was repeated on the reflective bars of the USAF resolution target placed behind a layer of scattering intralipid solution (sample 1) and the same result was obtained taking into account the index of refraction of intralipid (1.33). Thus, neither scattering nor dispersion degrades the system longitudinal resolution as measured in sample 1. Transverse resolution in air was determined by

Conclusion and outlook

These results show the performances of our parallel OCT system using an SPDA for imaging scattering samples. To our knowledge it is the first demonstration of the use of CMOS smart-pixel technology for parallel OCT in scattering phantoms and biological samples. Volumetric image acquisition is achieved at a relatively high speed (2.5×106pixels/s) compared to OCT systems using transverse point-by-point scanning mechanisms (up to 2×106pixels/s [18]). However, imaging depth penetration is currently

Acknowledgements

The authors would like to thank Prof. A.F. Fercher from the Institute of Medical Physics at the University of Vienna for his contribution in terms of interesting and stimulating scientific discussions during his stay as a visiting professor at the Swiss Federal Institute of Technology in Lausanne, Switzerland.

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