(The) Optical Low Angle Pass Filter for High Signal-to-noise Ratio Photoplethysmography Monitoring

Abstract

The photoplethysmography (PPG) signal exhibits the human vascular activity using the parameters extracted from the PPG waveform. However, the reflectance type PPG signal is liable to be distorted by noise. In this work, we present the optical low angle pass filter (OLAPF) for high signal-to-noise ratio photoplethysmography (PPG) monitoring. The highly scattered light, which loses their directionality under the human tissue act as a noise in PPG signal. The OLAPF can cut-off these scattered light using their non-directionality. The OLAPF was designed by the ray-optics analysis and numerical analysis. The fabrication of the OLAPF was done by using guided resist reflow and soft lithography method. To figure out the difference between with and without OLAPF, we made the conventional PPG sensor, which can be changed the glass on the photodetector to the OLAPF. As a result, the in-band noise overlapped the signal bandwidth of 0.5 Hz to 4 Hz with OLAPF is 2 times smaller than without the OLAPF. The low-frequency power spectral density, which has bandwidth of 0 Hz to 0.4 Hz of PPG signal with OLAPF is 3 times smaller than without OLAPF. This device can be applied not only PPG signal but also NIRS to improve the signal resolution.