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Spotlight on Optics


  • August 2012

Optics InfoBase > Spotlight on Optics > Ultrathin fiber probes with extended depth of focus for optical coherence tomography

Ultrathin fiber probes with extended depth of focus for optical coherence tomography

Published in Optics Letters, Vol. 37 Issue 10, pp.1616-1618 (2012)
by Dirk Lorenser, Xiaojie Yang, and David D. Sampson

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Spotlight summary: Optical coherence tomography (OCT) is an interferometric optical tomographic technique, which is widely accepted in ophthalmology, cardiology and other biomedical applications requiring micron-scale axial and lateral resolutions with millimeter penetrations depths. The axial resolution of OCT is a function of the spectral bandwidth of the light source. The problem inherent in OCT, and other optical technologies, is that Gaussian beam optics applies for standard optical elements. As a result, the numerical aperture of the probe determines both the lateral resolution and depth of focus (DOF). Since conventional optics are exclusively used in the construction of OCT probes, achieving a high lateral resolution of a few microns results in a shortened DOF on the order of 100 µm, which under utilizes the potential OCT ranging depth of a several millimeters. This tradeoff between high lateral resolution and extended DOF is further complicated for ultrathin fiber probes by the fact that bulk-optics solutions are difficult to apply due to the size constraints and complicated fabrication processes that are required for nonstandard micro-optical components.

In this paper, the authors present an elegant optical design for achieving an extended depth of focus (EDOF) of ultrathin fiber probes beyond the limit imposed by Gaussian beam optics. The key design element is the use of a phase mask positioned at the distal end of a standard forward-viewing OCT probe. The distal optics assembly consists of a no-core fiber spacer, a GRIN fiber phase mask and a lens, with this assembly fused to the end of the single-mode fiber. For this configuration, optical simulations predict a minimum lateral diameter of 6.5 µm with the beam diameter being less than 10 µm over the entire 1060 µm DOF. Compared to a standard OCT probe, EDOF OCT has an expected DOF gain of 2. Using off-the-shelf optical components and without the need for a complicated fabrication process, the authors have demonstrated an ultrathin fiber EDOF OCT probe having a 980 µm measured DOF, which translates into a DOF gain of 1.55 using standard optical components. For this configuration, the simulated DOF gain is 1.7. The authors have also characterized the noise performance and depth-dependent intensity profile the EDOF probe. Compared to an ideal Gaussian beam of the same minimum diameter, the EDOF probe’s intensity over the DOF is less than 5.2 dB of the peak signal-to-noise ratio and the sidelobe intensity is below 5% of the peak intensity over most of the DOF. Both of these performance characteristics are critical for producing high quality OCT images. For example, axicon-based solutions for achieving EDOF result in a reduction of sensitivity due to power loss and in the production of image artifacts due to the sidelobes of the Bessel beam.

This development has significant implications for clinical imaging with OCT. The ability of this design to significantly extend the focus means that the transverse resolution can be effectively increased over a larger imaging depth. The consequent increase in resolution will result in higher quality images, greater measurement accuracies, and ultimately an increase in diagnostic capabilities. This and future developments in the area of EDOF probes will substantially enhance the utility for medical applications of coherence ranging techniques such as OCT.

--Linbo Liu, Joseph Gardecki, and Gary Tearney

Technical Division: Optics in Biology and Medicine
ToC Category: Medical Optics and Biotechnology
OCIS Codes: (060.2370) Fiber optics and optical communications : Fiber optics sensors
(170.2150) Medical optics and biotechnology : Endoscopic imaging
(170.3880) Medical optics and biotechnology : Medical and biological imaging
(170.3890) Medical optics and biotechnology : Medical optics instrumentation
(170.4500) Medical optics and biotechnology : Optical coherence tomography
(350.3950) Other areas of optics : Micro-optics

Posted on August 03, 2012

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