A comparison between use of metasurface and conventional refractive axicons in OCT systems

Optical Coherence Tomography (OCT) is a non-destructive imaging technique capable of performing subsurface imaging and reconstructing a 3D volumetric image of the sample being measured. Initially developed for bio-medical applications, OCT is now finding uses such as evaluation and characterization of optically compliant non-biological materials and inspection in manufacturing. The use of axicons in the illumination and measurement paths extends the depth range achievable in a single measurement; however, it also modifies how the detected signal is generated. In conventional OCT, which utilizes a spherical lens, signal generation can be viewed as that from a Michelson interferometer. The more complex signal produced when using an axicon should therefore be considered when interpreting measurement results. In addition, the specific form of the axicon also affectsthe generated signal. We compare the use of a metasurface axicon as the imaging optic in an extended-range Fourier domain OCT system with a refractive axicon. In both cases, a model is constructed to propagate light through the system, from source to scatterer to spectrometer, and used to examine the signals generated for scatterers at a range of locations after the axicon. In a traditional axicon, light is refracted at both the front and back surfaces. This is not the case for a metasurface axicon due to its planar form, so the measurement signal differs between the two systems. Considerations of signal formation in these cases leads to suggestions about how extended-range OCT systems can be modified to optimize the signal produced, particularly regarding spectrometer location.