Angle-selective optical filter for highly sensitive reflection photoplethysmogram.

Chan-Sol Hwang,Kyung-Won Jang,Ki-Hun Jeong,Jung-Woo Park,Sung-Pyo Yang

doi:10.1364/boe.8.004361

Chan-Sol Hwang, Kyung-Won Jang + Show 3 more

Open Access

https://doi.org/10.1364/boe.8.004361

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Abstract

We report an angle-selective optical filter (ASOF) for highly sensitive reflection photoplethysmography (PPG) sensors. The ASOF features slanted aluminum (Al) micromirror arrays embedded in transparent polymer resin, which effectively block scattered light under human tissue. The device microfabrication was done by using geometry-guided resist reflow of polymer micropatterns, polydimethylsiloxane replica molding, and oblique angle deposition of thin Al film. The angular transmittance through the ASOF is precisely controlled by the angle of micromirrors. For the mirror angle of 30 degrees, the ASOF accepts an incident light between - 90 to + 50 degrees and the maximum transmittance at - 55 degrees. The ASOF exhibits the substantial reduction of both the in-band noise of PPG signals over a factor of two and the low-frequency noise by three times. Consequently, this filter allows distinguishing the diastolic peak that allows miscellaneous parameters with diverse vascular information. This optical filter provides a new opportunity for highly sensitive PPG monitoring or miscellaneous optical tomography.

Highlights

Optical photoplethysmography (PPG) exhibits human vascular activity in a non-invasive and compact manner [1,2]
The measured peak-to-peak PPG signals are hardly affected by the angle-selective optical filter (ASOF) and they are well matched with the calculated results based on a ray-tracing software ASAP, where the pulsatile components of PPG signals were considered as the variation in blood volume fraction (BVF) of upper dermis [21]
The experimental results clearly show that optical transmittance through the ASOF changes with the filter angle, which was determined by the inclined angle of Al micromirror arrays

Summary

Introduction

Optical photoplethysmography (PPG) exhibits human vascular activity in a non-invasive and compact manner [1,2]. The diastolic peaks become readily distorted due to the low signal-to-noise ratio (SNR) of reflection PPG sensors [12], which results from highly scattered light in turbid human tissue. Conventional noise reduction of reflection PPG signals can be achieved by either increasing the number of optoelectronic modules or employing the signal processing techniques Both two green and red light emitting diodes (LEDs) simultaneously provide different spectral signals to reduce low frequency motion artifacts [13]. The ASOF exhibits the controlled angular acceptance for backscattered LED light within human tissue following the Snell's law and reduces both the non-directional and the undesired noise signals prior to a photodetector.

Design and fabrication

Findings

Conclusions