Acousto Optic Modulator Based En Face OCT
The classical scanning scheme of optical coherence tomography (OCT)  is based on A-scans, i.e., the fast scanning direction is the axial direction perpendicular to the sample surface. While this method has the advantage of directly providing a carrier frequency for the interferometric signal (the Doppler frequency shift of the reference beam caused by the moving reference mirror ), it also has a considerable disadvantage: it is not well adapted to the geometry of samples and tissues usually imaged by OCT. These samples typically have a much larger transversal extension (several millimeter) than achievable imaging depth (a few 100μm to 1–2mm). If a cross sectional (2D) image or a full 3D data set is to be recorded, very many short A-scans have to be recorded to address every point in the sample. For every A-scan the reference mirror has to be accelerated and decelerated, thus the achievable imaging speed is limited by mechanical inertia problems.
The idea of transversal scanning (TS) or en face OCT is to set the fast scanning direction along one of the longer transversal extensions of the sample. This requires fewer scans for recording 2D or 3D data sets, alleviating the mechanical demands and allowing for higher imaging speeds. The drawback of TS-OCT is that the generation of a suitable carrier frequency requires some additional consideration. The first TS-OCT scheme reported generated the carrier frequency by the path length modulation induced by the transversal galvo scanners , which, however, has the drawback of a varying carrier frequency. We developed an alternate method of carrier frequency generation based on acousto optic modulators (AOMs) . The advantage of this method is a very stable carrier frequency which can be very high, allowing high speed imaging. This was the initial motivation for the development of our TS-OCT technique. With the advent of the more sensitive and even faster spectral domain (SD) OCT technique [5–7], this initial motivation lost importance.
KeywordsOptical Coherence Tomography Retinal Pigment Epithelium Stratum Corneum Axis Orientation Optical Coherence Tomography Image
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