This section contains examples of wavelength division multiplexing (WDM) circuits. Wavelength division multiplexing is a method of modulating multiple signals at different wavelengths (channels) to transmit them on a single waveguide or fiber.
Introduction
To begin with, we assume that we have the element parameters from a known process design kit (PDK). The goal is to be able to design an 8-channel WDM system using 25 Gbps per channel, using a comb laser as the input [1], cascaded ring modulators with a free spectral range (FSR) of about 1.1 THz to modulate the signal and multiplex, and cascaded ring resonators to demultiplex [2]. The following examples start with a single channel system and works up to 8 channels.
single channel
This example shows the basic operation of a wavelength division multiplexer (WDM) with only one channel. This example uses the ring modulator primitive from the element library, so we are looking at the steady state response of the ring modulator. From the eye diagram, we can see an excellent signal integrity, for a single channel the signal is free from any sources of cross-talk.
Schematic
Eye diagram
2-channel WDM
This example simulates a 2-channel WDM circuit. From the eye diagrams we can see that there is some cross-talk between the two channels, but the eye diagram can be considered open, assuming the signal is deterministic.
Circuit schematic
Signal after first ring modulator
Signal after second ring modulator
Signal at drop port of first ring resonator
Eye diagram of signal at first output
Eye diagram of signal at second output
4-channel WDM
This example simulates a 4-channel WDM circuit. For this system, there is some cross-talk between the channels, and the results can be improved by modifying the ring modulators/resonators to have a larger FSR, and increasing the bandwidth of each channel.
Schematic
Signal after first ring modulator
Signal after third ring modulator
Signal at first filter drop port
Eye diagram of signal at first output
Eye diagram of signal at second output
Eye diagram of signal at third output
Eye diagram of signal at fourth output
8-channel WDM
This example goes through the design of an 8-channel WDM. Our goal is to design an 8-channel WDM system with a comb laser as the input, cascaded ring modulators to modulate and multiplex the signals, and cascaded ring resonators to demultiplex. In the wdm_8channels_1.icp simulation file, we start with all of these components and check the eye diagram at the output. We can see that this circuit as it is is not a suitable system as there is too much noise and cross talk due to frequency detuning effects resulting in a closed eye in the diagram.
Circuit schematic
Spectrum after first ring modulator
Spectrum at drop port of first ring resonator
Eye diagram
Using individual sources and ring resonators
In the wdm_8channels_2.icp file, we replace the comb laser with individual sources in the transmitter. This reduces the amount of cross talk.
Circuit schematic of transmitter
Spectrum after coupler
Spectrum at drop port of first ring resonator
Eye diagram from first output
Using individual sources and receivers
In wdm_8channels_3.icp, we replace the cascaded ring resonators with ideal filters.
Circuit schematic of receiver
Eye diagram
Summary of Results
By replacing the elements with idealized elements, we can see the effect of the different circuit components on signal integrity. A better method for designing would be to start with ideal elements to first see if it's possible to implement your design with the desired requirements, and then replacing each element with realistic elements one at a time.
Related publications
[1]Knights, A.P.; Huante-Ceron, E.; Ackert, J.; Logan, D.; Wojcik, G.; Feng Zhang; Gubenko, A.; Mikhrin, S., "Comb-laser driven WDM for short reach silicon photonic based optical interconnection," Group IV Photonics (GFP), 2012 IEEE 9th International Conference on , vol., no., pp.210,212, 29-31 Aug. 2012.
[2]Fang Q, Phang YT, Tan CW, Liow TY, Yu MB, Lo GQ, Kwong DL, “Multi-channel silicon photonic receiver based on ring-resonators”, Opt Express. 2010 Jun 21;18(13):13510-5. doi: 10.1364/OE.18.013510.