Polarisation-Sensitive Optical Coherence Tomography—Sensing Order in the Sub-Wavelength Biological World ^†

Alteration of the polarisation state of light is an appealing source of contrast which has been used in polarised light microscopy of very thin sections for many decades, and has applications in geology, materials, and biology. The polarisation-sensitive (PS) version of optical coherence tomography (OCT), in contrast, probes a sample volume (rather than a thin section) and identifies the presence, relative arrangement, and orientation of sub-wavelength-sized fibrous structures. In biology, PS-OCT is sensitive to structures such as collagen fibres or muscle cells, and the shape of individual scatterers, such as cells. PS-OCT has been in existence since the early 1990s, but it has yielded less to date than its early promise suggested. The received signal is complicated by the cumulative influence of the propagation path on the contrast, and the complex physics of polarised light–hard and soft tissue interactions. Thus, image interpretation has been challenged by the lack of one-to-one correspondence between PS images and OCT structural images. Nonetheless, applications have been pursued, including in glaucoma, tumour and burns assessment, and wound monitoring. In this work, I will describe what has changed recently to overcome the existing challenges, focusing on our recent work, and why we expect polarisation-based contrast to yield great advances over the coming years.