## 3 Experimental Setup of Optical Homodyne and Heterodyne Detection
### Experiment of Heart Rate Detection with Self Mixing Optical Heterodyne Technique  
The AOM used for this experiment was ISOMET AOM with model number 1205C-2 [35]. Table 1 shows the specifications of ISOMET AOM.  
An RF signal source was used to modulate the reference frequency of the actual laser beam. RF signal generator tunes the laser beam given to AOM and modulates it with various frequency range. Further, an RF amplifier was used in series with RF generator to provide high gain. RF amplifier was needed because AOM requires a minimum 25 dBm RF power as an input in order to provide modulation. The maximum gain of 32 dB can be obtained with RF amplifier. Table 2 shows the specifications of the RF generator and the RF amplifier circuit.  
The optical signal produced multiple order side bands with frequency 454.5 THz \(\pm\) 60 MHz. These side bands are shown in Figure 11a,b. Figure 12 shows the modulation of 660 nm laser beam with 60 MHz RF signal, where 660 nm laser corresponds to 454.5 THz frequency. Light signal was modulated by the RF signal of 60 MHz after passing through AOM [36].  
\begin{table}
\begin{tabular}{c c} \hline
**Parameter** & **Specification** \\ \hline Material & Lead molybdate \\ Active aperture & 2 mm \\ Center frequency & 60 MHz \\ Center wavelength & 660 nm \\ \hline \end{tabular}
\end{table}
Table 1: AOM specification.  
Figure 11: (**a**) Working of AOM; (**b**) observation of higher order modes after modulation of laser from AOM.  
\begin{table}
\begin{tabular}{c c} \hline
**Parameter** & **Specification** \\ \hline RF signal generator voltage & 12 V \\ RF signal generator output frequency & 70 MHz–200 MHz \\ Output power RF generator & 10 dBm \\ RF amplifier supply voltage & 12 V DC \\ RF amplifier gain & 32 dB \\ \hline \end{tabular}
\end{table}
Table 2: Specification of RF generator and amplifier.  
Figure 13a,b shows the optical heterodyne detection setup and measurement of heart frequency at the wrist position of the test subject, respectively. The optical signal is divided into two parts with beam splitter. One part of the optical signal travels in test arm of interferometer which consists of an AOM, while the other part of the optical signal travels in the reference arm of the interferometer. The bio-modulated optical signal reflected from the subject is combined with AOM modulated optical signal at a beam splitter in the test arm. The optical beat signal of test arm and reference arm is obtained at the 3 dB coupler. The measurement procedure on human skin for optical heterodyne detection was the same as that of the optical homodyne detection setup.  
Figure 14 shows the response of the photodetector for intermediate frequency \(f_{IF}\). Figure 15 shows the detection of heart frequency \(f_{H}\). The heart frequency \(f_{H}\) was obtained by demodulation of intermediate frequency \(f_{IF}\) with modulation frequency \(f_{m}\). A frequency mixer was used for the purpose of demodulation and extraction of heart frequency from intermediate frequency.  
Figure 12: First order modulation of 660 nm laser with AOM with central frequency 60 MHz.  
Figure 13: (a) Experimental setup for optical heterodyne detection; (b) measurement on human wrist position.