Answer Happy

Proble m 2. Direct Detection \& Heterodyne Detection If an electro-optic modulator is biased at the quadrature point and the amplitude of the input AC signal is small, the modulator acts as a classical amplitude modulator with an output optical spectrum as shown in Figure 1. Assume the optical frequency of the laser, vs​ is 193THz and the RF modulation frequency, fm​ is 10MHz. Figure 1. Output optical spectrum of MZM. a) For direct detection, the output of the MZM is connected directly to a photodetector as shown in Figure 2. Draw the output photocurrent power spectrum including the DC and AC components and show proportionality of these components to the electric field, ES​
Figure L. Uirect-detection recerver. b) For heterodyne detection, a local oscillator (i.e. laser) with frequency vLO​=vs​−Δv and amplitude ELO​ is mixed with the output of the MZM onto a photodiode. In this example, let Δv=10GHz. The optical spectrum incident onto the photodiode is shown in Figure 3. For heterodyne detection, the MZM signal and local oscillator are combined and coupled to a photodetector as shown in Figure 4. Draw the output photocurrent power spectrum including the DC and AC components and show proportionality of these components to the electric fields, ES​ and ELO​. Figure 3. Optical spectrum of local oscillator and MZM output that are combined on photodetector for heterodyne detection.
Figure 4. Heterodyne-detection receiver.