###### 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. homodyne detection; heterodyne detection; Mach-Zehnder interferometer; acousto-optics modulator; vibrocardiogram; heart period; heart rate; heart rate variability +
Footnote †: journal: Applied system  
monitoring of cardiovascular parameters, such as heart rate (_HR_) and heart rate variability (_HRV_), in scenarios that prevent the use of conventional clinical sensors, normally requiring some sort of direct electrical (resistive) or mechanical coupling. Less variation in _HR_ is not good for the health of a person. Generally, a high variability in _HR_ is an indicator of good health for a person [4; 5]. _HRV_ provides information about the person's physical and emotional response towards any illness and stress [6]. It can provide important information about blood sugar, blood pressure, digestion, and the respiration cycle. Understanding _HRV_ provides vital information about many biological systems and processes. The measurement of _HRV_ is more complex than the measurement of _HR_. As _HRV_ measurement involves changes in the heart period in the order of milliseconds, it requires higher degrees of accuracy than _HR_. Optical interferometer-based detection setups are widely used in many fields, such as medicine, automation, and architecture. Optical coherent detection techniques mainly consist of homodyne and heterodyne type detection methods. [PERSON] first provided the theoretical comparisons of two detection schemes [7], compared the modulation schemes of both techniques, and provided the methods of optical beating inside a Mach-Zehnder interferometer. [PERSON] et. al. presented the results of a hydrophone calibration comparison between reciprocity and interferometry [8]. The same author provided a comparison between heterodyne and homodyne interferometry in order to realize the acoustic pascal unit of acoustic pressure in water [9]. The author directly compared the two techniques used in underwater acoustics and ultrasonics in terms of acoustic pressure estimation. More recently, laser vibrometers have been considered for the non-contact monitoring of cardiac activity from measurements performed on the chest wall. Laser Doppler vibrometer-based methods are used to perform both _HR_ and _HRV_ analysis, with results equivalent to ECG. [PERSON] et al. demonstrated the measurement of pressure waves by means of a laser vibrometer [10]. [PERSON] et al.