Arrayed Waveguide Grating for Wavelength Division Multiplexing

Arrayed Waveguide Grating (or AWG) is a method for wavelength division multiplexing or demultiplexing. The approach for multiplexing is to use unequal path lengths to generate a phase delay and constructive interference for each wavelength at an output port of the AWG. Demultiplexing is done with the same process, but reversed.

Arrayed Waveguide Gratings are commonly used in photonic integrated circuits. While Ring Resonators are also used for WDM, ring resonators see other uses, such tunable or static filters. Further, a ring resonator selects a single wavelength to be removed from the input. In the case of AWGs, light is separated according to wavelength. For many applications, this is a more superior WDM, as it offers great capability for encoding and modulating a large amount of information according to a wavelength.

Both the design of the star coupler and the path length difference according to the designed wavelength division make up the significant amount of complexity of this component. RSoft by Synopsys includes an AWG Utility for designing arrayed waveguide gratings.

RSoft AWG Utility Guide

Using this utility, a star coupler is created below:

Star Coupler for AWG designed in RSoft using AWG Utility

Methods of Optical Coupling

An optical coupler is necessary for transferring optical energy into or out of a waveguide. Optical couplers are used for both free-space to waveguide optical energy transmission as well as a transmission from one waveguide to another waveguide, although the methods of coupling for these scenarios are different. Some couplers selectively couple energy to a specific waveguide mode and others are multimode. For the PIC designer, both the coupling efficiency and the mode selectivity are important to consider for optical couplers.

Where the coupling efficiency η is equal to the power transmitted into the waveguide divided by the total incident power, the coupling loss (units: dB) is equal to
L = 10*log(1/η).

Methods of optical coupling include:

  • Direct Focusing
  • End-Butt Coupling
  • Prism Coupling
  • Grating Coupling
  • Tapered Coupling (and Tapered Mode Size Converters)
  • Fiber to Waveguide Butt Coupling

Direct Focusing for Optical Coupling

Direct focusing of a beam to a waveguide using a lens in free space is termed direct focusing. The beam is angled parallel with the waveguide. This is also one type of transverse coupling. This method is generally deemed impractical outside of precision laboratory application. This is also sometimes referred to as end-fire coupling.

End-Butt Coupling

A prime example of end-butt coupling is for a case where a laser is fixated to a waveguide. The waveguide is placed in front of the laser at the light-emitting layer.

Prism Couplers

Prism coupling is used to direct a beam onto a waveguide when the beam is at an oblique incidence. A prism is used to match the phase velocities of the incident beam and the waveguide.

Prism Coupling

Grating Couplers

Similar to the prism coupler, the grating coupler also functions to produce a phase match between a waveguide mode and an oblique incident beam. Gratings perturb the waveguide modes in the region below the grating, producing a set of spatial harmonics. It is through gratings that an incident beam can be coupled into the waveguide with a selective mode.

Grating Coupler in RSoft

Tapered Couplers

Explained in one way, a tapered coupler intentionally disturbs the conditions of total internal reflection by tapering or narrowing the waveguide. Light thereby leaves the waveguide in a predictable manner, based on the tapering of the waveguide.

Tapered Mode Size Converters

Mode size converters exist to transfer light from one waveguide to another with a different cross-sectional dimension.

Butt Coupling

The procedure of placing the waveguide region of a fiber directly to a waveguide is termed butt coupling.

RF Spectrum Analyzers

A Spectrum analyzer (whether in the RF Domain or optical) is a tool that is dual of the oscilloscope. An oscilloscope displays a waveform in time domain. When this is represented as a function, a Fourier transform can be used on it to obtain its spectrum. A spectrum analyzer displays this content.

Spectrum analyzers are very similar to radio receivers. A radio receiver could be classified into many types: (Super)heterodyne, crystal video, etc. Similar to a heterodyne receiver, which features a bandpass filter, mixer and low pass filter, a spectrum analyzer must tune over a specific range. This range must be very narrow, which requires a high Quality factor bandpass filter to operate. This is where the YIG (Yttrium Iron Garnet) filter comes into play. YIG has a very high quality factor and resonates when exposed to a DC magnetic field. This is what determines the spectrum analyzers “resolution bandwidth”. Of course, a narrow RBW means a less noisy display and better resolution. The tradeoff for this is increased sweep time. The sweep time is inversely proportional to the RBW squared.

A sweep generator is used to repetitively scan over the frequency band. The oscillator sweeps and repetitively mixes/multiples with the input signal and is filtered with a low pass filter. The low pass filter determines the spectrum analyzer’s “video bandwidth”.

An important concept with regards to bandwidth is thermal noise. Thermal noise is the single greatest source of noise in systems under 100 GHz. Past 100 GHz and into optics, shot noise becomes more apparent. However, bandwidth is the greatest contributor to thermal noise, as noise power is given as kTB. Since k is a constant and T has a relatively negligible effect on thermal noise (the main thing is that T is nonzero. At absolute zero, you have no thermal noise. Anything above that, you have thermal noise. The difference between a pretty cold device and a scorching hot one is only maybe 10 dBm or so. Just ballparking), this means that bandwidth has a huge effect on noise. A higher RBW increases the spectrum’s noise floor and makes it harder for closely spaced frequency components to be seen, as more frequency components are passed through the envelope detector.

Video bandwidth, on the other hand, typically determines resolution between power levels and smooths the display. It is important to note that the VBW contribution happens after data has been collected and does not affect the measurement results, whereas the RBW dictates the minimum measurable bandwidth.

Phase noise is also present in a spectrum analyzer and can affect measurements near the center frequency and results from phase jitter. Since this is pretty much a phase modulation, sidebands are produced near the center frequency which can interfere with measurement. Jitter refers to deviation from periodicity of a signal.

Programs for PIC (photonic Integrated Circuit) Design

For building PICs or Photonic Integrated Circuits, there are a number of platforms that are used in industry today. Lumerical Suite is a major player for instance with built in simulators. Cadence has a platform that can simulate both photonic and electronic circuits together, which for certain applications provides a major advantage. There are two platforms that I’ve become familiar with, which are the Synopsys PIC Design Suite (available for students with an agreement, underwritten by a professor at your university to ensure it’s use is for only educational purposes) and Klayout using Nazca Design packages.

Synopsys is another great company with advanced programs for photonic simulation and PIC design. Synopsys Photonic Design Suite can include components that are designed using Rsoft. OptoDesigner is the program in the PIC design suite where PICs are designed, yet the learning curve may not be what you were hoping. The 3,000+ page manual let’s the user dive into the scripting language PheoniX, which is necessary to learn for PIC design using Synopsys. Using a scripting language means that designing your PIC can be automated, thereby eliminating repetitive designing. There also comes other advantages to this such as being able to fine tune one’s design without needing to click and drag components. Coding for PIC design might sound tedious, but if you start to use it, I think you’ll realize that it’s really not and that it’s a very powerful way of designing PICs. If you’d like to use PheoniX scripting language using the Synopsys PIC design suite, note that the scripting language is similar to C.

Synopsys PIC Design Suite, Tutorial Program for Ring Resonator

One of the greatest aspects of OptoDesigner and the PIC Design Suite is the simulation capabilities. Much like the simulations that can be run in Rsoft, these are available in OptoDesigner.

Running FDTD in OptoDesigner

The downside of Synopsys PIC Design Suite is in the difficulty of obtaining a legal copy that can be used for any and all purposes, even commercial. I mentioned that I obtained a student version. This is great for learning the software, to a certain extent. The learning stops when I would like to build something that could be sent out to a foundry for manufacture. Let’s be honest though, there is a lot to learn before getting to that point. Still, if we would even like to use a Process Design Kit (PDK) which contains the real component models for a real foundry so that you can submit your design to be built on a wafer, you will need to convince Synopsys that the PDK is only used for educational purposes and not only for learning, but as part of an education curriculum. If your university let’s you get your hands on a PDK with Synopsys Student version, you will essentially have free range to design PICs to your hearts content. If you have a student version, you’ll still have to buy a professional version if you want to design a PIC using a foundy PDK, submit it for a wafer run and sell it. I’ll let you look up the cost for that. The best way to use Synopsys is to work for a company that has already paid for the profession version, in conclusion.

Now, if you find yourself in the situation where all the simulation benefits of using OptoDesigner are outweighed by the issue of needing to perform a wafer run, you might just want to use Klayout with Nazca Design photonic integrated circuit design packages. These are both open source. Game changer? Possibly. Suddenly, you picture yourself working as an independent contractor for PIC design someday and you’ll have Klayout to thank.

Klayout and the Nazca Design packages are based on the very popular Python programming language. Coding can be done in Spyder, Notepad or even Command Prompt (lol?). If you aren’t familiar with how Python works, PIC design might move you to learn. Python takes the place of PheoniX scripting language as is used in OptoDesigner, so you still have the automation and big brain possibilities that a scripting language gives you for designing PICs. As for simulations, you’ll have to go with your gut, but you could use discrete components to design your circuit and evaluate that.

Klayout doesn’t come with a 3,000+ page manual, but you’ll likely find that it is a simpler to use than OptoDesigner. Below is a Python script, which generates a .gds file and then the file opened in Klayout.

Python Script for PIC Design in Klayout using Nazca Design packages
.gds file opened in Klayout