## 5 Experimental and Results  
A CMOS sensor for imaging and spatial optical communication is designed and implemented using 0.18 \(\upmu\)m CIS technology. Figure 18 shows a microphotograph of the chip. It has 640 \(\times\) 240 pixel image array and 640 \(\times\) 240 LPR cell array with analog/digital readout circuits. The pixel and LPR cell occupy 7.5 \(\times\) 7.5 \(\upmu\)m\({}^{2}\) with 20% fill factor. The parameters of the fabricated sensor are summarized in Table 2.  
Figure 19 shows typical waveforms of the communication signal outputs observed using test channel outputs. The communication cell outputs (pixel source follower output, readout amplifier output and pulsed (digital) output) are successfully read out at the frequency of 1 MHz and photo current amplitude of 5 nA. \begin{table}
\begin{tabular}{|c|c|} \hline Process & 0.18 \(\upmu\)m CIS \\ \hline Chip Size & 8 mm \(\times\)7 mm \\ \hline \# of Imaging Pixels & 640(H) \(\times\)240(V) \\ \hline \# of LPR Cells & 640(H) \(\times\)240(V) \\ \hline Pixel/LPR Cell Size & 7.5 \(\times\)7.5 \(\upmu\)m\({}^{2}\) \\ \hline Fill Factor & 20\% (Imaging Pixel) 20\% (LPR Cell) \\ \hline Power Supply & 3.3 V Analog 1.8 V Digital \\ \hline Frame Rate & 60 fps \\ \hline Sensitivity & 8.3 V/lx sec \\ \hline \end{tabular}
\end{table} 
Table 2: Parameters of fabricated sensor. Figure 18: The micrograph of CMOS image sensor chip. Figure 20 shows the measurement results of signal amplitude of the LPR cell output (source follower output) as a function of pulse frequency. As explained in the analysis of the pixel response, the amplitude has a linear response to the logarithm of the frequency. At the frequency of 1 MHz, amplitude of more than 10 mV is obtained for the photocurrent of 1 nA. From Figure 20, Equation (9), and Equation (16), the time constant and sensing node capacitance are estimated as a function of photo current as shown in Figure 21. The time constant is inversely proportional to the photocurrent as predicted by Equation (9). The sensing node capacitance is almost constant at around 9 fF to the photocurrent except for very low photocurrent of 100 pA at which a measurement error due to the poor SNR occurs. This capacitance is larger than the value estimated by the device simulations, where C\({}_{\text{CS}}\)= 6.8-6.9 fF. The reason of the difference between the simulation and the experimental result is mainly due to capacitance of the source flower transistor and wiring. Figure 19: Waveformes of communication signals. Figure 20: Amplitude as a function of Pulse Frequency (LED: \(\lambda\) = 870 nm, IE = 80 mW/sr, Lens: FN = 1.4, f = 6 mm, Distance: 2 m). Figure 22 shows an eye diagram of the communication signal output at the carrier frequency of 300 kHz and the photo current level of 2.2 nA. The received optical signal is modulated by random baseband signal and Manchester coding. At this frequency and photo current level, a sufficiently large eye opening is realized. Figure 23 shows the BER (Bit Error Rate) as a function of optical power measured at 500 kbps. An acceptable BER for spatial optical communication is larger than 1E-5, which corresponds to the optical power per pixel of 50 nW. With the measured radiant sensitivity of the photo diode of 0.07 A/W at 870 nm, the photo current level is 3.5 nA. From Figure 13, this photo current level is available at the communication distance of 65 m. The same signal voltage amplitude with a smaller photo current level can be obtained if the communication data rate is slower. Theoretically, the photo current level of 1.3 nA which corresponds to the available photo current level at 100 m provides the same signal voltage amplitude at the data rate of 186 kbps from Equations (9) and (16). Figure 21: Estimated time constant and charge sensing capacitance as a function of photo current. Figure 22: Eye Diagram. As a basic testing of the implemented ISC chip, a spatial optical communication between non-moving object (sender: a LED light source) and the non-moving camera (receiver: ISC chip) has been carried out.