The two blocks in the middle with red dotted lines around them are QPSK transmitter and receiver PICs. On the left of the design area, there’s an off-chip laser emitting light at 1550nm, a standard telecom C-band wavelength. Two decorrelated data sources with NRZ drivers and low pass filters provide push-pull RF drives at 32-Gbd/s speed to the modulators, while bias signals choose the bias points for the modulators.
The transmitter and receiver PICs are connected back to back. Another off-chip laser at the receiver serves as a local oscillator. To provide homodyne detection, the laser wavelength is identical to that of the received signal.
On the right of the receiver PIC, the balanced post-detection electronics extract in-phase and quadrature components of the received QPSK signal. The blocks in yellow are measurement components for visualization of signals, spectra, eye diagrams, BER, etc.
Figure 8 shows waveforms at the transmitter PIC.