Optimization of multi-beam silicon photonics based laser Doppler vibrometry for measuring cardiovascular signals on bare skin

Non-invasive monitoring of cardiovascular diseases has been explored by means of laser Doppler vibrometry (LDV). In previous work, we have developed a handheld 6-beam on-chip LDV-device based on silicon photonics that can simultaneously measure the skin vibrations induced by cardiac action in multiple positions. This allows for the estimation of the pulse wave velocity (PWV), which is the current gold standard for evaluating arterial stiffness. The demonstrator has been used in a series of clinical feasibility studies. However, the system required the application of a retro-reflective (RR) patch to the skin prior to the measurement in order to enhance skin reflection. The use of the RR patch reduces the device usability and may also impact the measurement results. In this work, we bring the concept one step further by eliminating the need for the RR patch during the measurement. The diffuse reflection from the skin leads to the low intensity of the back-reflected light detected by the interferometric readout system of the LDV. In order to increase the reflection signal level, we propose to operate the LDV at 1310nm where skin reflection is relatively strong while still being insensitive to skin tones. Furthermore, the optical imaging system between LDV-chip and skin has been designed for optimal signal strength in combination with sufficient depth of focus. We report on LDV measurements without using RR patch, and on the details of the optimized optical system.