###### Abstract
used a commercial laser Doppler vibrometer for vibratory measurements at long operative distances (tens of meters) [11; 12]. [PERSON] et al. proposed a protocol for monitoring cardiac activity by measuring the skin surface vibrations of the main neck vessels caused by vascular wall motion in the carotid artery [13]. The author proposed the measurement of the velocity of displacement of the skin in correspondence to the chest wall. The method is based on the optical recording of the movements of the neck by means of laser Doppler interferometry. [PERSON] et al. proposed a vibratory signal measurement from the chest wall and carotid [14]. Many studies have demonstrated displacement measurements with \(\frac{\lambda}{2}\) (or better resolution). In this approach, scattered light from a vibrating surface is used to detect the amplitude and frequency of surface vibrations [15; 16; 17]. Many studies have demonstrated the absolute distance measurement with a self-mixing method [18; 19; 20; 21; 22]. The heart and lungs work together to transport oxygenated blood throughout the body. Deformation of the heart results in the generation of vibration frequencies on human skin, especially on the chest wall and the wrist. Therefore, the analysis of vibrations on the skin surface provides crucial information about the type of disorder. The movement and deformation of the heart, as well as the pulse wave traveling through the body, results in displacement and vibration of the body surface [23]. The human chest wall consists of independent mechanical components that contribute to chest wall mechanics. Optical coherent detection is one of the important techniques for measuring the frequency of vibration on a test subject. The aim of the optical sensing method proposed here is to observe the activity of the heart with an approach based on a custom-made low-cost self-mixing laser interferometer. The optical interferometer-based system directly measures the vibration frequency on human skin with a very high accuracy and temporal resolution. The coherent detection technique involves the vibration measurement by two independent optical setups, namely homodyne and heterodyne. It works on the principle of the modulation of light signal by mechanical vibrations. Measurements carried out with these techniques are non-intrusive and non-destructive. These techniques are completely contactless and are relatively precise and sensitive [24]. The detection method is calledself-mixing because the two interfering beams are obtained from the same optical source from the beam splitter [25]. The optical coherent detection technique is useful for the monitoring of the burnt patients or preterm infants. The main contributions of this research are:  
1. A novel design of a self-mixing optical homodyne and heterodyne detection setup for measuring the vibrations of chest wall or wrist position is proposed;
2. A comparison of results of homodyne and heterodyne techniques with available contact type measurement techniques, such as an electrocardiogram (ECG), is studied;
3. The feature extraction of the vibocardiogram (VCG) signal for the monitoring of \(HR\) and \(HRV\) is obtained. In this research paper, Section 2 describes the modulation scheme for the design of optical homodyne detection and optical heterodyne detection. Section 3 provides details about the experimental setup for the measurement of vibration on the chest wall and wrist. Section 4 analyses the results of the \(HR\) and \(HRV\) measurement. In this section, feature extraction of vibocardiogram (VCG) waves is performed and its mapping with ECG signal is explained. Further, the results of optical homodyne and heterodyne techniques are compared based on the results. The research also provides details of the construction of the proposed optical homodyne and heterodyne setups.