The next test solution would comprise an oscilloscope with an optical/electrical (O/E) converter (such as Teledyne LeCroy's OE695G), which converts optical intensity levels into voltage levels, or the opposite of the linear optical modulator. This setup would work well for those interested in the output side of the linear optical modulator in the middle application example shown in the figure.
Finally, for the 16-QAM, long-haul signals at the output of a coherent optical modulator, the test setup of choice would be an oscilloscope in tandem with a coherent optical receiver (an example would be Teledyne LeCroy's Optical Modulation Analyzer). Here, two signals are encoded into one light stream to modulate both amplitude and phase.
Next time, we'll look at the test challenges associated with PAM4 encoding.