Article Development of an Optical System for Non-Contact Type Measurement of Heart Rate and Heart Rate Variability Jyoti Gondane * and Meena S. Panse   Citation: Gondane, J.; Panse, M.S. Development of an Optical System for Non-Contact Type Measurement of Heart Rate and Heart Rate Variability. Appl. Syst. Innov. 2021, 4, 48. https://doi.org/10.3390/ asi4030048 Academic Editor: Christos Douligeris Received: 16 June 2021 Accepted: 23 July 2021 Published: 28 July 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Department of Electrical Engineering, Veermata Jeejabai Technological Institute (VJTI), Mumbai 400019, India; mspanse@el.vjti.ac.in * Correspondence: jagondane@el.vjti.ac.in Abstract: Self-mixing optical coherent detection is a non-contact measurement technique which provides accurate information about the vibration frequency of any test subject. In this research, novel designs of optical homodyne and heterodyne detection techniques are explained. Homodyne and heterodyne setups are used for measuring the frequency of the modulated optical signal. This technique works on the principle of the optical interferometer, which provides a coherent detection of two self-mixing beams. In the optical homodyne technique, one of the two beams receives direct modulation from the vibration frequency of the test subject. In the optical heterodyne detection technique, one of the two optical beams is subjected to modulation by an acousto-optics modulator before becoming further modulated by the vibration frequency of the test subject. These two optical signals form an interference pattern that contains the information of the vibration frequency. The measurement of cardiovascular signals, such as heart rate and heart rate variability, are performed with both homodyne and heterodyne techniques. The optical coherent detection technique provides a high accuracy for the measurement of heart period and heart rate variability. The vibrocardiogram output obtained from both techniques are compared for different heart rate values. Results obtained from both optical homodyne and heterodyne detection techniques are compared and found to be within 1% of deviation value. The results obtained from both the optical techniques have a deviation of less than 1 beat per minute from their corresponding ECG values. Keywords: homodyne detection; heterodyne detection; Mach–Zehnder interferometer; acousto- optics modulator; vibrocardiogram; heart period; heart rate; heart rate variability 1. Introduction The maximum critical cases across the world are related to chronic cardiorespiratory conditions. Demographic changes are expected to cause home monitoring approaches to take a leading role in the future treatment of such patients [1,2]. Remote monitoring technologies have gained a significant importance in the COVID-19 era. During the pandemic, the use of non-contact type techniques for the measurement of bio-parameters have increased rapidly. Although fixed-on-body electrodes are reliable and give good signal quality, there are several disadvantages of this method. The major demerit of this technique is the direct fixing of electrodes on skin, as it leads to discomfort among patients. Direct measurement on skin becomes very critical, especially in the case of infants and people with burn injuries. Therefore, the interpretation of cardiovascular signals through unobtrusive means has gained importance in recent years. A vast amount of research is available for the measurement of cardiovascular parameters, but most of the research is related to contact type measurement. There is much less clinical awareness of non-contact type optical measurement of cardiovascular parameters. There are several reasons contributing to this fact, such as the absence of a specific therapy for prognosis improvement. Furthermore, there is a lack of standardized methodology for parameter assessment due to the variability of factors, such as gender, age, medical history for illness, and drug interferences [3]. The common intention behind this development is to enable the Appl. Syst. Innov. 2021, 4, 48. https://doi.org/10.3390/asi4030048 https://www.mdpi.com/journal/asi