Performance Considerations For Optical Modulation Analysis

Figure 1: Error-vector magnitude defined

In recent posts, we've covered the fundamentals of coherent signals and the basics of optical modulation analyzers. Let's now turn to the operational parameters of OMAs, in particular system bandwidth, and how that figure of merit in an OMA can determine how far your measurement system can take you in terms of meaningful analysis.

What Is An Optical Modulation Analyzer?

Figure 1: A representative block diagram of a coherent
transmitter and receiver

In an earlier post, we looked at some of the fundamentals of coherent signals: what comprises a coherent signal, how and why they're used, and a bit about how they're represented visually on an oscilloscope. The advent of coherent signals has brought about the rise of a new class of test instrumentation, known as an optical modulation analyzer (OMA). In this post, we'll examine what an OMA is and what it brings to the party above and beyond a stand-alone oscilloscope.

The Fundamentals of Coherent Signals

Figure 1: These two coherent light waves have
a constant phase offset

Optical communications have come a long way from the simple direct detection of amplitude-modulated transmissions. Today's long-haul optical networks make use of coherent detection, and for good reason. Using coherent detection, receivers can track the phase of the incoming signal and thereby extract any information conveyed using phase and/or frequency modulation. Thus, coherent detection facilitates much higher capacity without using more bandwidth.