Coaxial bright and dark field adaptive optics optical coherence tomography

We introduce a novel coaxial bright-field and dark-field adaptive optical coherence tomography (CBAD-AO-OCT) system that, when operating at a central wavelength of 850 nm, partially alleviates the trade-off between imaging depth and resolution in highly scattering biological tissues. Unlike traditional OCT systems, our dual-channel coaxial design enables the simultaneous capture of bright-field and dark-field information. The 45° rod mirror spatially separates the bright-field and dark-field signals, while the adaptive optics system integrated into the dark-field optical path corrects for inherent static aberrations in the system, thereby maintaining collection efficiency and image fidelity when collecting photons over a wider scattering angle in dark-field. Compared to conventional OCT, CBAD-AO-OCT offers superior signal retention at deeper depths and improved contrast-to-noise ratio in soft and hard tissues. Lateral resolution improves from 22.07 µm in the bright field to 8.7 µm in the dark field, while axial resolution remains consistent at 3.55 µm. The system achieves a sensitivity of 97.3 dB and a 3 dB roll-off depth of 1.5 mm. Experimental validation using multilayer phantoms, microsphere models, avian wing tissue, and avian bone confirms its versatility across diverse scattering regimes. This study demonstrates that, at a wavelength of 850 nm, the combination of adaptive optics with a coaxial bright-field and dark-field design can enhance the imaging depth and contrast of scattered samples while maintaining resolution. Consequently, CBAD-AO-OCT provides a validated platform for collecting biologically scattered photons, and its application can be extended to longer wavelengths in the future to further improve penetration depth.