A paper recently published by Wei et al. showcases one application of Castor Optics’ double-clad fiber coupler (DCFC): confocal imaging in a single-path configuration. The study follows a lineup of new developments using DCFCs for confocal imaging encompassing applications in endoscopy, multimodality imaging, retinal imaging, and OCT imaging.
This last paper presents a confocal Scanning Light Ophthalmoscopy (SLO) system with a novel, efficient scan pattern that allows fast retinal imaging for pathology screening. The authors' development will lead to more rapid and robust in vivo imaging, and its handheld packaging will increase accessibility to the technology, particularly for screening applications, compared to current tabletop commercial systems. And since optical fiber alignment is critical in confocal systems, the robustness of the device is granted using one of Castor Optic's DCFCs.
DCFs for Single-path Confocal Imaging
Single-path confocal imaging uses the double-clad fiber's core for illumination and the inner cladding for signal collection. Using the multimode inner cladding for signal collection instead of another single-mode fiber was first done by Yelin et al. The study showed a reduced speckle contrast, increased depth of field, and increased signal intensity granted by the larger diameter of the fiber while preserving lateral resolution.
Unlike the conventional geometry, which uses two single-mode optical fibers for illumination and detection, this proposed geometry is laterally self-aligned, making it more resistant to misalignment. This is a considerable advantage for handheld systems, which are subject to repeated mechanical shocks, as well as for applications sensitive to vibrations (e.g., motorized vehicles, LiDAR), or systems with complex optical realignment (e.g., remote access) or limited space (e.g., endoscopy).
Maximizing Transfer and Collection for Improved SNR
Wei's paper also highlights a critical aspect of fiber-based components: transmission efficiency. To maximize signal-to-noise ratio (SNR), one should give special attention to technical specifications of insertion loss, transfer ratio, and return loss.
Castor Optics’ DCFC efficiently separates or combines light from the double-clad fiber's core and inner cladding, ideally suited for optimized SNR.
Transmission from Port A to Port S (associated with insertion loss) and Port S to Port B (associated with transfer ratio) will dictate signal intensity. Castor's couplers offer quasi-lossless transmission from port A to port S and ≥70% transfer from port S to port B, allowing performant signal collection.
The signal transmitted from Port A to Port B (i.e., crosstalk related to return loss) contributes to noise in the collected signal. This crosstalk originates from back-reflections of the core signal at the fiber optic end interfaces of Port S and R, being coupled to the inner cladding and transferred to Port B. Solutions to reduce this effect are designed at Castor, lowering back-reflections from Port R to -50 dB. Our team can also develop customized solutions to minimize back-reflections coming from Port S.
Maintaining the Confocality Criterion
It has been demonstrated that an ideal pinhole size ratio, i.e., the ratio between the illumination and collection pinhole sizes, results in an increased detected signal intensity and decreased speckle contrast with minimal effect on axial resolution. In the case of DCF-based confocal imaging, the core and inner cladding diameters correspond to illumination and collection pinholes, respectively. The core size is primarily dictated by propagation constraints of the fiber’s fundamental mode for a given operational wavelength range. To maintain an ideal pinhole size ratio, one has to optimize the inner cladding size. Castor Optics’ small-cladding DCFCs use dedicated double-clad fibers to respect this confocality criterion.
Different coupler versions are available, each optimized for a specific imaging modality and wavelength range.
The DCFC series is available on Thorlabs.com. To learn more about how Castor Optics' innovative fiber optics technology can enable faster, farther, and brighter sensing, contact our team at sales@castoroptics.com.
Written by Audrey Laurence
Applications scientist
