- Open Access
Autocorrelation noise free Optical Coherence Tomography using the novel concept of resonant OCT (ROCT)
© The Author(s) 2016
- Received: 21 January 2016
- Accepted: 31 May 2016
- Published: 25 July 2016
Optical Coherence Tomography OCT is a noninvasive imaging technique that takes pictures of cross sections of human body tissues with a great resolution compared to other techniques. Fourier Domain OCT method provides significant improvement of imaging speed and detection sensitivity but suffers from autocorrelation noise arising as interference signals from reflections of sample layers that tends to obscure some of sample structure details.
We present in this paper a new implementation of Common Path Optical Coherence Tomography, based on a resonant structure. The structure employs a semiconductor optical amplifier SOA and uses two mirrors, one coated fiber end and the other is the sample under test. Amplified multiple reflections between the laser cavity high reflection mirror and the sample layers along with SOA gain behavior results in the reduction of autocorrelation noise.
Autocorrelation noise is greatly reduced by a factor of 5 dB compared to an ordinary FDOCT system.
This new structure, with the absence of autocorrelation noise that covers some of the details of the sample under test in OCT setups, is capable practically of attaining images with higher resolution.
- Optical Coherence Tomography
- Laser Cavity
- Semiconductor Optical Amplifier
- Sample Layer
- Axial Resolution
Optical Coherence Tomography has become a powerful imaging technique which started in ophthalmological domain in the 1990’s. Since then it is widely applied in many other medical areas where it is used in diagnosis of diseases, and in technical fields. Nowadays there are many diseases as cancer which require a resolution in the micron and sub-micron range, so improvements in resolution are required to detect such diseases.
There are two variants of OCT techniques depending on the detection system: Time-domain (TDOCT) and Frequency-domain (FDOCT). TDOCT was proposed by Huang et al. in 1991  and is based on a scanning optical delay line (mechanical displacement of a reference arm). FDOCT provides significant improvement of imaging speed and detection sensitivity as compared to TDOCT [2, 3]. FDOCT is based on analyzing a signal caused by interference of light beams and can be performed in two ways: The first technique is called Spectral OCT (SOCT) where a light source with broad spectral bandwidth (~100 nm) is used in combination with a spectrometer and a line or array of photo-sensitive detectors [4, 5].
The second technique is called Swept source OCT (SSOCT) where a tunable laser is used in conjunction with a photodetector [8, 9]. This second method usually operates at speeds comparable to SOCT employing a rapidly tunable laser [10, 11].
The axial resolution of most SS-OCT systems is on the order of 10 μm in tissue and doesn’t match high resolution SOCT systems. Due to the high imaging speed, FDOCT systems enable the acquisition of three dimensional image data in-vivo which is especially beneficial for numerous ophthalmic imaging applications .
Despite its superiority over TD-OCT, FD-OCT implementation exhibit drawbacks in terms of autocorrelation noise artifacts, which obscure details of the image and degrade the system sensitivity. The autocorrelation terms arise from the interference occurring between different sample reflectors within the target. Jun Ai, et.al proposed the elimination of autocorrelation noise through asynchronous acquisition of two interferograms using an optical switch and attaining an axial resolution of 15 μm in air .
FD-OCT bases itself upon low coherence interferometry. Optical Coherence interferometry combines two or more light waves in an optical instrument in such a way that interference occurs between them.
Where λo is the central wavelength of the light and Δλ is its spectral full width at half maximum (FWHM). Thus, if we increase BW of light used (using a source of shorter coherence length), we can increase axial resolution. The advantage of increased resolution is lost by the autocorrelation noise that covers required signals of the sample structure.
In this paper we present a novel implementation of a common path OCT setup that is free from autocorrelation noise signals that may overlap the desired image of the object layers. This is achieved through establishing a laser cavity composed of a semiconductor optical amplifier SOA, a 90:10 fiber coupler, and two mirrors at the two ends. One mirror has a reflection coefficient of 99.9 % and is attained by coating the fiber end with a multilayered structure. The other mirror is simply a cleaved fiber end facing the sample under test at a very small distance. The output from this low finesse and low quality factor laser cavity represents the measured interferogram. The Fast Fourier Transform of the obtained interferogram gives the detailed structure of the sample layers. The obtained axial resolution, due to the absence of undesired signals, shows an enhancement over the OCT technique employing a wide band source. The reduction in autocorrelation noise is attributed to amplified multiple reflections between the laser cavity high reflection mirror and the sample layers.
Experimental setup of a resonant OCT
Modeling of the system
Where l2 is the distance between the cavity right mirror and the first surface of the sample.
Signals reflected of sample layers undergo multiple round trips inside the laser cavity. Repeated amplification of these signals play a major role in reducing autocorrelation noise as we will show.
Experimental results of ROCT vs. OCT
We analyzed and implemented experimentally a resonant common path OCT setup. The new proposed scheme shows an enhancement over the ordinary OCT system where the level of autocorrelation noise signal is greatly reduced. These signals obscure some of the details of the sample under test in ordinary OCT setups. Therefore, this promising result is expected to increase the system axial resolution.
The authors would like to thank the Deanship of Scientific Research at Al Imam Mohammad Ibn Saud Islamic University for the financial support of the project: No 351403/1435H, and for the continuous help during this work by providing the space and equipment required to carry out the experimental measurements.
Both authors contributed equally in all the sections of this work.
Both authors contributed equally in all the sections of this work.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
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