Abstract
We describe a simple and low-cost technique for resolving the complex conjugate ambiguity in Fourier domain optical coherence tomography (OCT) that is applicable to many swept source OCT (SSOCT) systems. First, we review the principles of coherence revival, wherein an interferometer illuminated by an external cavity tunable laser (ECTL) exhibits interference fringes when the two arms of the interferometer are mismatched by an integer multiple of the laser cavity length. Second, we report observations that the spectral interferogram obtained from SSOCT systems employing certain ECTLs are automatically phase modulated when the arm lengths are mismatched this way. This phase modulation results in a frequency-shifted interferogram, effectively creating an extended-depth heterodyne SSOCT system without the use of acousto-optic or electro-optic modulators. We suggest that this phase modulation may be caused by the ECTL cavity optical pathlength varying slightly over the laser sweep, and support this hypothesis with numerical simulations. We also report on the successful implementation of this technique with two commercial swept source lasers operating at 840nm and 1040nm, with sweep rates of 8kHz and 100kHz respectively. The extended imaging depth afforded by this technique was demonstrated by measuring the sensitivity fall-off profiles of each laser with matched and mismatched interferometer arms. The feasibility of this technique for clinical systems is demonstrated by imaging the ocular anterior segments of healthy human volunteers.
Highlights
Optical coherence tomography [1] enables non-invasive, micrometer scale imaging of biological tissues over depth ranges of a few millimeters, and has found widespread use in several biomedical imaging applications, especially ophthalmic [2] and cardiovascular imaging [3]
This technique exploits the fact that some external cavity tunable lasers (ECTLs) used for Swept source optical coherence tomography (SSOCT) automatically produce a phase modulated signal when used in an interferometer whose arms are mismatched by an integer multiple of the laser’s cavity length
Coherence revival is an attractive implementation of heterodyne SSOCT, and carries with it a number of advantages over traditional methods employing acousto-optic modulators (AOMs)
Summary
Optical coherence tomography [1] enables non-invasive, micrometer scale imaging of biological tissues over depth ranges of a few millimeters, and has found widespread use in several biomedical imaging applications, especially ophthalmic [2] and cardiovascular imaging [3]. We present a novel and extremely simple method of realizing heterodyne SSOCT using coherence revival This technique exploits the fact that some external cavity tunable lasers (ECTLs) used for SSOCT automatically produce a phase modulated signal when used in an interferometer whose arms are mismatched by an integer multiple of the laser’s cavity length. This technique has a number of advantages over traditional, AOM-based heterodyne SSOCT in that it is simple to implement, causes no reduction of axial resolution, and requires no additional hardware beyond a traditional SSOCT system. The only additional processing step required is the use of a numerical dispersion compensation algorithm, which is an ordinary processing step in many SSOCT systems
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