STSM report – Aleksandar Danicic

VCSEL arrays in self-interferometry arrangement have the potential to be low cost imaging tool for blood perfusion . The goal of the Action BM1205 is to develop an ultra-compact sensing technology based on the selfmixing interferometer that uses Vertical-Cavity Surface-Emitting Laser (VCSEL) arrays both for the emission and the detection of light. This technique has high sensitivity, high signal to noise ratio, high spatial resolution, simple optical design, low power consumption (portable system), potentially low cost and the significant advantage of possible implementation in massive two-dimensional arrays. Self-mixing interferometry, on which this objective is based, is an acknowledged new technique for detection of small displacements, change in the refractive index of materials, and particle flow. The self-mixing phenomenon occurs when the laser beam is partially reflected from an external target and injected back into the laser cavity. The reflected light interferes or ‘mixes’ with the light inside the laser cavity and produces variations to the threshold gain, emitted power, lasing spectrum and the laser terminal voltage. This phenomenon allows the laser to be used as an interferometric sensor incorporating the light source and the interferometer in one device thus significantly reducing the cost and the complexity of the sensing system. The homodyne (coherent) detection nature of this sensing scheme inherently provides very high sensitivity (at the quantum noise limit) and consequently suffers minimal crosstalk between the channels in a free-space multichannel implementation. Due to its short cavity length of the order of the wavelength, VCSELs operate in a single longitudinal mode, which avoids signal distortions due to mode beatings, have low power consumption and the circular output beam allows simpler optics than for side-emitting lasers. The design of VCSEL devices and arrays has not been expressly pursued for this proposal. Experimental work and commercial products to date have relied on pre-existing devices sourced from manufacturers for communications applications. As such, the performance of any sensor system based on VCSEL devices is ultimately limited. As part of this technical objective, the modelling of the self-mixing effect and related device physics will be required to expand the current single-beam technology to 2D and possibly 3D imaging (depth sectioning) based on two-dimensional VCSEL arrays.