Moving to the next higher data speed class of #400 Gbps or# 600 Gbps, the question is again how transmission efficiency can be further increased.
In the #optical communications world, capacity gains come essentially from three variables: more carriers through techniques such as polarization and multi-carrier #OFDM modulation, better spectrum efficiency through higher modulation density and higher symbol rate (Figure 1).
Figure 1. Factors contributing to optical system capacity.
Testing these higher order systems with data rates close to #1 Tbps requires test equipment capable of clean signal generation and analysis and a measurement bandwidth of at least 20 GHz, to be sure the measurements represent system performance, not the limitations of the test equipment. The instruments must offer the flexibility to address many different #modulation schemes on 4 synchronized channels for a dual-polarization I/Q signal. Traditionally, receiver tests such as #phase noise, observed #signal to noise ratio and polarization tests have been performed using a “gold” transmitter, giving a view of the device but lacking completely deterministic knowledge (Figure 2).
Figure 2. Traditional optical receiver test setup.
Using an arbitrary waveform generator (AWG) such as the Keysight M8196A allows the creation of test signals in the electrical domain, including both clean signals and signals with specific, known, impairments. For transmitter test, these can be fed directly to a transmitter and the resultant error rate can be measured directly. For receiver test, they can be used directly to test #DSP stages and be translated to the optical domain to create both clean and stressed deterministic optical signals for full receiver test. (Figure 3)
Figure 3 . Optical transmitter and receiver test scenarios using an arbitrary waveform generator.
The key challenges in making measurements on #coherent optical systems lie in providing known, repeatable clean and distorted test signals at data rates in excess of 32 GBaud and with the flexibility to support diverse modulation formats. Test system calibration should be possible, not only at the front panel of the test signal generator and measurement equipment, but at any point in the signal chain through embedding and de-embedding techniques using the transmission system’s S-parameters.
Figure 4. Cumulative optical modulation analysis using a Keysight N4391A or N4392A optical modulation analyzer.