Consider two optical receivers for a digital system. One uses thermal-noise-limited detection and the other uses heterodyne detection. The photodetector in the heterodyne receiver has 2 nA of dark current and 0.5 mA/mW of responsivity. Its operating temperature is 300 K, the load resistance is 50 , and the IF bandwidth is 1 GHz. The received optical signal power is 10 nW. a) Calculate the required local-oscillator power such that the resulting SNR is 3 dB less than the quantum limit. b) Calculate the required signal power to achieve the same SNR as found in a) if the thermalnoise-limited receiver is used. Hint: In b), a bandwidth of 500 MHz is sufficient; the load resistor and the responsivity are same