Figure 1
From: High Speed All Optical Nyquist Signal Generation and Full-band Coherent Detection

The principle.
All optical Nyquist pulse can be generated by a frequency-locked comb source cascaded with a band-pass filter. The high-order formats modulation is realized by inphase and quadrature (I and Q) signal modulation; Nyquist signal generation can be realized by time-domain multiplexing; Full-band signal coherent detection and digital signal processing can be realized by a common coherent detection setup.