Optical coherence tomography (OCT) is a well-established technique for biological imaging, mainly used in the field of ophthalmology for routine corneal and retinal clinical examinations. The conventional OCT system produces longitudinal images, by performing an axial scan at each x or y point on the object surface, and builds a two-dimensional xz or yz images.
A variant of OCT called full-field optical coherence tomography (FF-OCT) is an emerging non-invasive, label-free, interferometric technique that has the inherent ability of providing rapid en face (xy) images of the object by using a detector array (CCD or CMOS), thus avoiding the necessity for using the instrumentationally complex, lateral point scanning scheme. In addition, the use of a spatially incoherent broadband light source and high numerical aperture objective in FF-OCT also enables speckle-free, high-resolution 3D imaging.
In most FF-OCT systems, en face OCT images are constructed by using a conventional phase-shifting technique that involves shifting of the reference beam phase with a piezoelectric translator. However, with the use of a broadband source in FF-OCT, the phase shifts of different spectral components are not the same, resulting in systematic errors for reconstruction of tomographic images. To solve the problem, an achromatic phase shifter based on the geometric phase principle has been proposed. An achromatic phase shift can be realized by cyclic change of the polarization state of the light beam through rotating a wave plate or polarizer using a stepper motor. However, in this case, due to the slow response, real-time biomedical imaging is not attainable.
We present a prototype full-field OCT system based on geometric phase that incorporates a fast switchable ferroelectric liquid-crystal (FLC) phase modulator. It has a fast response time and can accurately map and produce 3D images of complex biological samples.