Optical Coherence Tomography
Subsurface imaging
Among imaging systems, optical coherence tomography (OCT) is especially useful for probing subsurface tissue microstructure. OCT measurements map backscattered light as a function of depth, enabling optical scanning of the tissue through low-coherence interferometry. By repeating these measurements at different transverse positions, cross-sectional images of the tissue can be reconstructed. OCT systems typically achieve axial resolutions on the order of 10 µm and can image tissue to a depth of a few millimeters. Their fiber-based architectures also enable integration into catheters or endoscopes.
Multimodal imaging
Adding Functional Contrast
Double-clad fiber couplers (DCFCs) enable efficient integration of OCT with a second modality within a compact, single-fiber architecture compatible with endoscopic or catheter-based systems. Compared with alternative free-space beam splitter approaches, they provide a more robust and efficient solution. The OCT signal can be transmitted to and from the sample with minimal loss through the core, while the inner cladding enables efficient collection of isotropic incoherent light. This architecture is well-suited to multimodal systems combining OCT with fluorescence, spectroscopy, or hyperspectral imaging.
Displayed is an in vivo OCT and autofluorescence image of human peripheral airways obtained using Castor's Double-clad fiber coupler.
Understanding crosstalk-induced ghost artifacts
Practical Considerations
Typical issues encountered when using DCFCs include crosstalk-induced ghost artifacts and back reflections, both of which affect the signal-to-noise ratio. Ghost artifacts arise when a portion of the light couples from the core into the inner cladding and then back into the core after traveling along a different optical path. Because this parasitic signal acquires a delay relative to the main signal guided in the core, it can interfere with the reference arm at an incorrect depth, producing ghost images. In an OCT system, such crosstalk may occur at interfaces such as connectors' mating, fusion splices, or transitions between propagation media, including the air–tissue interface.
In the image on the left, such an artifact is visible in the human skin image, highlighted by the yellow bracket (a-b). It appears at a shallower depth than the main image because light traveling through the inner cladding experiences a shorter optical path than light guided in the core, owing to the lower refractive index of the inner cladding. As reported in this study, the authors used index-matching gel at the fiber–air interface to reduce specular reflections into the inner cladding, which were the main contributor to crosstalk in the system (c-d). However, depending on the system configuration and the sample being imaged, such a strategy may not always be practical.
Shifting Ghost Artifacts with Fiber Length Optimization
The appearance of ghost artifacts depends on the delay between light propagating in the inner cladding and the fundamental mode in the core. As defined by Han and Kang, this delay is proportional to the optical path difference, ΔL:
ΔL = Δn · L with Δn = nfm − neff
where Δn is the refractive index difference between the fundamental mode, nfm, and the inner cladding mode, neff, and L is the physical distance traveled between coupling sites.
To shift the ghost artifact outside the imaging range, ΔL must exceed that range, sometimes by many folds depending on the system's sensitivity. One possible strategy is therefore to increase the fiber length between coupling sites. In general, increasing L can help reduce the impact of ghost artifacts, even when pushing them fully outside the imaging window is not practical.
The optimal fiber length, however, remains system-specific. It depends on parameters such as the coherence length and sensitivity of the OCT system, as well as the sample being imaged. For example, the most reflective structure may occur near the surface in some tissues, such as skin, while in other applications, such as full-eye imaging, deeper structures may dominate the signal. These differences influence how ghost displacement should be optimized in practice.
The Castor Optics team can help evaluate this trade-off and define a DCFC configuration adapted to your system and imaging conditions.
Managing Incoherent Noise and Back-Reflections
Back-reflections at fiber–air interfaces are a significant source of incoherent noise in DCFC-based systems. This issue is especially important in systems that combine OCT with a reflectance modality, where unwanted reflections can degrade the reflectance signal, in addition to contributing to the OCT background. A major source of this noise is the reflection generated at the fiber facet of Port S (see figure below). Because the inner cladding has a higher numerical aperture than that of standard single-mode fibers, the angle of an APC termination is not sufficient to fully suppress this reflection. Another contributor is the reflection at Port R, the unused port, where light can be reflected back directly toward the detector associated with the second modality at Port B.
One effective strategy is to use a high-return-loss termination on the unused Port R to reduce the amount of light reflected back into the detection path. In addition, minimizing reflectivity at the fiber facet of Port S can further reduce incoherent noise. To address this, Castor Optics offers solutions such as anti-reflection coatings and high-angle polishing, which can significantly improve return loss depending on the DCFC configuration.
DCFCs offer a practical and efficient path to multimodal OCT system integration, enabling compact fiber-based architectures while addressing the demanding photon-budget constraints of advanced imaging systems. Because performance depends strongly on system architecture, sample type, and interface conditions, component selection and implementation should be tailored to the application. The DCFC series is available on Thorlabs.com. To discuss the most suitable configuration for your system, contact the Castor Optics team at sales@castoroptics.com.
