The demand for higher data rates, bandwidth and wider operational ranges will continue to be the reliable driver of the communication market for the next few years. Big data analysis has not tapped its full potential yet. The number and size of data centers is growing tremendously. On this side we seem to be prepared for the avalanche of structured and unstructured data. However, the infrastructure outside needs to adjust in order to be able to provide higher data capacities. A major question that needs to be answered here is how data rates of 400 Gb/s and 1Tb/s can be reached on existing networks. The research for suitable signal formats comes along with new test needs for validating the concepts and new device designs with realistic signal scenarios.
Characterizing High-Speed Coherent Optical Transmission Systems
To accommodate the higher data rates into the existing fiber optical infrastructure and their allocated frequency bands, new modulation formats have to be considered for increasing spectral efficiency.
The industry has already largely adopted coherent optical transmission communication systems, which use advanced modulation schemes, to address the explosive demand for data transportation. This solution comes with some new requirements, which need to be understood along the whole communication link from the transmitter, through fiber cable and network elements to the receiver. The network design needs to be robust against signal distortions and link impairments. The RF community in the mobile industry has solved similar problems in the last two decades. So the optical community is able to leverage these technologies from the RF world and adopt them to the special optical requirements. The solution is to use modulation schemes that transmit more information per bandwidth than the traditional on- and off-switching of the light (RZ or NRZ modulation). Thus, the spectral efficiency and the transmission rate can be increased at the cost of more complex hardware and additional digital signal processing. This demands for a high-speed, wider-bandwidth and high-frequency test system which can generate the desired modulation schemes as needed to test devices under the real world scenarios. The latest developments in 100 G+ coherent optical transmission systems and sub-systems also require more and more flexibility to generate clean modulated signals as well as distorted test signals.
The M8196A arbitrary waveform generator gives the versatility to create the signals needed for dual polarization coherent transmission (BPSK, QPSK, PAM4, PAM8, QAM up to QAM256), orthogonal frequency division multiplexing (OFDM), time-domain pulse shaping and more with data rates of 64 GBaud and beyond. Also linear and non-linear impairments can be added to the signal. With sample rates of up to 92 GSa/s with 32 GHz analog bandwidth and up to 4 channels simultaneously in one single AXIe module for emulating 2 independent I/Q signals, the M8196A AWG provides an excellent platform for testing high-speed coherent optical transmission systems. Figure 1 shows the typical test setup for high-speed coherent optical transmission systems.
The M8196A AWG is used here to generate the complex modulated signal. For each of the two polarization planes one I/Q pair is needed. That means that digital-to-analog converter 1 (DAC1) and DAC2 drive the modulators for the x- (or horizontal) polarization, DAC3 and DAC4 are used to stimulate the modulators for the y- (or vertical) polarization. Additional linear driver amplifiers are required to boost the output level of the AWG depending on the Mach-Zehnder modulators (MZM) used for the setup. An Erbium doped fiber amplifier (EDFA) is used to boost the signal before it is sent into a longer fiber. Depending on the length of the fiber an additional fiber amplifier may be used at the end of the fiber to boost the signal and compensate for the fiber attenuation. A coherent receiver is used to receive the signal. The second tunable laser source is used as a local oscillator for the coherent receiver.
As devices and interfaces become faster and more complex, the M8196A AWG gives you the versatility to create the signals you need for digital applications in optical and electrical communication. Out-of-the-box and in-situ calibration and signal pre-distortion methods allow generation of exceptionally clean signals even at the highest data rates. This unique functionality allows engineers to make reliable, repeatable measurements when they are working on binary and multilevel, multichannel digital interfaces as well as coherent optical and wideband communication applications.
Flexibility and features available on this platform enable it to use it for a wider range of applications. In conjunction with the 81195A optical modulation generator software, emulation of signal parameters and distortions is further simplified.
