Modeling, Optimization, and Validation of an Extended-Depth-of-field Optical Coherence Tomography Probe Based on a Mirror Tunnel

The diagnostic capability of high-resolution optical coherence tomography (OCT) may be enhanced by using extended depth-of-field (EDOF) imaging that retains high transverse resolution over long depths. A recently developed mirror-tunnel optical probe design (single-mode fiber to multimode fiber to lens structure) that generates coaxially focused modes has been previously shown to enable EDOF for endoscopic OCT applications. Here, we present ray-tracing optical modeling of this optical configuration, which has the potential to guide performance improvement through optimization. The Huygens wave propagation of the field was traced through probe components with initial lengths. The irradiance along the x - z plane was analyzed, yielding an average full width at half-maximum (FWHM) of 9 µm over a 640 µm DOF (defined as the axial range over which the beam’s transverse FWHM is maintained). A custom merit function was then defined, based on the focal region illumination intensity profile that yielded the maximum possible depth having a constrained FWHM size. An orthogonal gradient descent optimization algorithm was then applied using this merit function, using the multimode fiber, spacer, and lens lengths as variables. Optimization resulted in a modeled mean 6 µm FWHM spot diameter over an EDOF of 1 mm. Following optimization, a probe was fabricated, and was validated using a custom-built near-field scanning pinhole beam profiler. The experimental results (6 µm mean FWHM over 800 µm EDOF) showed reasonable correspondence to the simulated predictions, demonstrating the potential utility of optical modeling and optimization for improving EDOF performance in mirror-tunnel endoscopic OCT probes.