Optical PAM4 transceiver

In this example, we use INTERCONNECT solutions to study the 4-Pulse Amplitude Modulation (PAM) format. In this example, you will learn how to:

• Generate 4PAM electrical signals
• Do signal modulation
• Do signal transmission and detection
• Measure signal qualities
• Map signals

Problem Definition: More Details

The system in this example contains the following elements:

• 2 Pseudo-random Bit Stream (PRBS) block
• 2 NRZ Pulse Generator (NRZ)
• 1 CW Laser (CWL)
• 3 1x2 Fork (FORK)
• 2 Electrical Not Gate (NOT)
• 1 Optical Phase Shift (PHS)
• 2 Waveguide Coupler (C)
• 4 Optical Modulator Measured (OM)
• 1 Optical Attenuator (ATT)
• 1 Electrical DC Source (DC)
• 2 PIN Photodetector (PIN)
• Oscilloscopes (OSC)
• 1 Low Pass Filter (LPF)
• 1 Eye Diagram Analyzer (EYE), and
• 1 Vector Signal Analyzer (VSA)

Modeling instruction

This page contains 2 sections. The simulation can be set up from a new simulation, starting at the Setup model section below. Otherwise, the attached file can be used..

Set up Model

• Start a new INTERCONNECT project. You can start a new project by pressing Ctrl+N, or by selecting New in the File menu.
• To generate 4-PAM modulated signal, from the Element Library drag and drop 2 PRBS Generators (Element Library\ Sequence Generator), two NRZ Pulse Generators (Element Library\ Pulse Generators\ Electrical) and two 1xN Forks (Element Library\ Tools), set the fork number of port to be 2, then connect the elements as follows:

Click on the schematic background and set the global properties according to the following table:

• Drag and drop a CW Laser from the Element Library (Element Library\ Sources\ Optical) and set the power to be 0.01W. Drag and drop a Waveguide Coupler (Element Library\ Waveguides\ Couplers) and an Optical Phase Shift (Element Library\ Passive\ Optical), set the phase shift to be pi/4. Connect the elements as following:

• Drag and drop four Optical Modulator Measured (Element Library\ Modulators\ Optical) and another Waveguide coupler, connect the elements as following:

• Add two Electrical Not (Element Library\ Logic\ Electrical), another 1x2 Fork splitter, two PIN Photodetector (Element Library\ Photodetectors) and one Low-pass Bessel Filter (Element Library\ Filters\ Electrical), connect all the elements as following and add visualizers to view data.

• Set the VSA "modulation type" to be 4PAM with the build in "symbol map table" as following:

Run Simulation, Visualize Results

• To run the simulation, click on the run button

  

  on the tool bar. When a simulation is running, the calculation process will appear at the top of the analyzer.

• When the simulation finishes running, the Results Window of the Analyzers will be populated with results. Users can simply right-click on each result to visualize this in the Visualizer window. For example, to look at the eye diagram, select the Eye Diagram analyzer, go to the results window and right-click on the "eye diagram" result and select "Visualize".

Discussion and results

This system simulates the 4-PAM transceiver with an EOE process. There are three steps associated with the whole process. Signal integrity analysis is done by special elements, the analyzers. Analyzers allows for post-processing of data stored in monitors. The results of each step could be shown by analyzers.

Step 1: Generating Electrical Signal

After this step, the NRZ electrical signal is generated, the fork splitter will duplicate the signal and put them to drive the modulator.  

Step 2: Modulation

The two cascaded phase modulator in each branch modulates the NRZ electrical signal to a four phase fixed power optical signal; when combined by the coupler, the output signal is with four different phases and four different amplitudes.  

Step 3: Photo Detect

The optical output signal is duplicated again and detects by two PIN photodetectors. The lower branch is then degraded by a low-pass filter and the upper branch output is used as the reference for the eye diagram to estimate and compensate for propagation delays (clock recovery) between the transmitter output and receiver input.

The modulator modulates the pseudo-random bit sequence (which is a random bit stream with combination of 1s and 0s) to the pulse amplitude (four different levels around 0.001a.u., 0.000824a.u., 0.00065a.u. and 0.00049a.u. ). The reference inputs to VSA generates the decision points of the data and then the received data can be mapped according to the decision regions.

See also

• Optical QPSK
• Optical DPSK
• Optical QPSK + Electrical QAM-16