Coherence-length isolation for balanced detection in common-path swept-source OCT

For handheld, endoscopic, and other mobile-probe optical coherence tomography (OCT), probe-coupled optical fiber motion inevitably leads to unstable optical paths and phase fluctuations, substantially degrading image quality. Although common-path OCT can effectively mitigate the motion artifacts, it is inherently incompatible with balanced detection which is critical for suppressing relative intensity noise, eliminating autocorrelation artifacts, and maximizing signal-to-noise ratio in swept-source OCT. Here, we propose a novel coherence-length isolation scheme that integrates the stability of common-path OCT with the performance advantages of balanced detection. This method allows the sample and reference light to propagate collinearly within the probe without premature interference by ensuring the path difference exceeds the source coherence length. Interference is subsequently enabled through proximal beam splitting and optical-path-difference compensation. We implemented this scheme in a 10-MHz ultra-high-speed swept-source OCT system with a polarization-isolated stretched-pulse mode-locked laser. Experimental results demonstrate an 11.2-dB signal-to-noise ratio enhancement over non-balanced configuration, along with effective elimination of autocorrelation artifacts. Furthermore, comparative tests under fiber disturbance confirm that the proposed system exhibits superior axial and phase stability compared to non-common-path architectures. This approach provides a robust solution for achieving high-quality and stable OCT imaging in mobile-probe scenarios.