# Experimental Study on Partially Coherent Optical Coherent Detection
## 3 Experimental Study
### Partial Coherent Polarization Measurement and the Coherent Detection Experiment  
This experiment used optical components like quarter wave plates and polarizers to create signal light and local oscillator light with various polarization states. To achieve an ideal mixing efficiency, good polarization matching must be maintained between the signal light and the local oscillator light. Local oscillator light with linearly polarized, circularly polarized, and elliptically polarized states was generated using a linearly polarized plate and a quarter wave plate, when the polarization state of the signal light was linear. Using a BOSA 400C spectrometer(Zaragoza, Spain), the various polarization states of signal light and local oscillator light were measured, as indicated in Figure 8.  
The data points for the measured signal light and the local oscillator light are both close to the equator of the Poincare sphere, as shown in Figure 8a,b, demonstrating the distribution of linear polarization. As Figure 8c shows that when the polarization characteristics of the beam are controlled with a quarter wave plate, the data point of the local oscillator is close to the south pole of the Banga sphere. The distribution of circular polarization is consistent with this observation. This indicates that the quarter wave plate was successful in changing the polarization state of the beam and creating a circularly \begin{table}
\begin{tabular}{c c} \hline \hline
**Experimental Equipment** & **Performance Parameter** \\ \hline Signal laser & Wavelength 1550 nm, line width 0.1 kHz, output power 0–20 mW \\ Local oscillator laser & Wavelength 1550 nm, line width 10 kHz, output power 20 mW \\ Transmitting antenna & Effective aperture 105 mm, obstruction ratio 0.2 \\ Receiving antenna & Effective aperture 105 mm, obstruction ratio 0.2 \\
90\({}^{\circ}\) mixer & Wavelength range 1520–1570 nm, operating temperature 0–70 \({}^{\circ}\)C \\ Balanced detector & Wavelength range 1260–1650 nm, response bandwidth 200 MHz \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Experimental Equipment. Figure 7: Experimental setup for generating E GSM local oscillator light. When the signal light's EGSM beam is linearly polarized, the output power of the signal light laser was adjusted to about 1.3 \(\upmu\)W, and the output power of the local oscillator laser was fixed at 1.5 mW. The intermediate frequency signal waveform obtained using the Tektronix DPO5204B oscilloscope is displayed in Figure 9.  
From the interface of the oscilloscope, it can be seen that when the signal light is in a linearly polarized state and the local oscillator light is linearly polarized, the peak-to-peak value of the intermediate frequency signal is 369.6 mV; when the signal light is linearly polarized and the local oscillator light is circularly polarized, the peak-to-peak value of the intermediate frequency signal is 146.6 mV; and when the signal light is in a linearly polarized state and the local oscillator light is elliptically polarized, the peak-to-peak value of the intermediate frequency signal is 92.1 mV. Applying the same technique can produce local oscillator light with varying polarization states when the EGSM beam signal is circularly polarized. From Figure 10a it can be seen that the data point of the signal light is close to the south pole of the Banga sphere, which agrees with the distribution of circularly polarized beams on the Banga sphere. Similarly, from Figure 10b-d, it can be seen that circularly polarized local oscillator light, linearly polarized local oscillator light are, respectively, generated. Figure 8: Linearly polarized signal light and three polarization states of local oscillator light: linear polarization, circular polarization, and elliptical polarization. (**a**) Linearly polarized signal light; (**b**) linearly polarized local oscillator light; (**c**) circularly polarized local oscillator light; and (**d**) elliptically polarized local oscillator light.