Optical Coherence Tomography (OCT) allows for fast, sensitive, and high resolution imaging. Light scattered from refractive index changes in a sample (such as the eye’s retina) is amplified by interference with a reference beam. It is a non-invasive technique because no contrasting or labelling agents are required and is widely used in ophtalmology and cardio-vascular imaging as well as dermatology. Regarding resolution and penetration depth, OCT bridges the gap between confocal microscopy (high resolution) and ultrasound (high penetration depth). High numerical apertures allow for high resolution in the lateral plane, but limit the depth of field. This dilemma can be solved by an Extended Focus with a Bessel beam.
Bessel beams are sometimes also called diffraction-less beams: They have a thin central lobe which exhibits a propagation-invariant lateral profile over an extended range. Behind obstructions, the Bessel beam forms anew by constructive interference from its side lobes. This is interesting for instance for Optical Tweezers and Extended Focus microscopy. Bessel beams can be created by use of an axicon lens, an annular aperture in the back focal plane of an objective lens or with the help of a spatial light modulator (SLM). We have developped a method of creating such a Bessel illumination for OCT and other broadband applications.One drawback of this type of light beams is that a large portion of their energy is contained in the ring system around the central lobes and that they have to be scanned across the sample. Spatial light modulators phase-modulate and reflect (or transmit) a light beam. They usually act as a second screen to a computer, thus the applied phase modulation can conveniently be controlled by displaying grey-level images. The ability to control and change a wavefront is highly useful in beam shaping.
Another way to improve both lateral resolution and imaging speed is to combine OCT with Digital Holography (“Holoscopy”) and to record full-field holograms at many wavelengths with a tunable laser and a fast sCMOS camera, thus eliminating the need for scanning. Such a holography-based OCT system is currently under development and promises to acquire three-dimensional widefield data of biological tissues at micrometer resolution within a second. Acquired holograms have to be reconstructed offline with a custom-written algorithm.