Higher-Order Core-Like Modes in Double-Clad Fiber Contribute to Multipath Artifacts in Optical Coherence Tomography

Double-clad fiber (DCF) has enabled the combination of endoscopic optical coherence tomography (OCT) with secondary optical modalities. While DCF offers an additional optical channel, it is widely understood that its use reduces the quality of OCT owing to the introduction of multipath artifacts. We show here that an unexpected higher-order mode (HOM) with its energy confined to the DCF core can contribute to these artifacts. The existence of this HOM is confirmed using the spatially and spectrally (S
²
) resolved imaging method. The group delay difference of the HOM is shown to be consistent with the delay of the diffuse ghost artifact in DCF-acquired OCT images.