A Photonics Playground - Optics & Photonics News July/August 2020

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Southampton Nanofabrication Centre.

Courtesy of University of Southampton

EDUCATION

A Photonics Playground Silicon photonics is revolutionizing the Datacom industry, compelling educators to modernize the way this technology is taught to the next generation of engineers.

Goran Z. Mashanovich, Milos Nedeljkovic, Callum G. Littlejohns and Milan M. Milosevic

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hotonics—“the electronics of the 21st century”—has become a ubiquitous technology in modern society, impacting many fields and addressing several grand challenges. Silicon photonics, in particular, is an important field since it leverages CMOS technology and is being increasingly implemented in data centers, lidar systems and sensing applications. As photonics’ star continues to rise, there is a growing demand for employees with a broad skillset—encompassing everything from photonics and electronics to process and software engineering.

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Therefore, it’s imperative that we provide students with opportunities to acquire skills in modeling, design, fabrication, characterization and analysis of photonic devices and circuits.

Since the University of Southampton has one of the best electronics departments in the U.K. and a worldleading Optoelectronics Research Centre (ORC) with extensive academic cleanroom and laboratory facilities, we in the Silicon Photonics Group were motivated to take advantage of such an environment and introduce a

photonics pathway in undergraduate education.

Teaching photonics Southampton’s Electronics and Computer Science (ECS) department recently introduced two modules, Photonics I in year two and Photonics II in year three, that offer hands-on experience in design and characterization of silicon photonics circuits. In Photonics I, optical fibers and passive silicon photonic devices (waveguides, couplers, splitters, ring resonators and interferometers) are taught, while Photonics II covers active devices, such as detectors, modulators and lasers.

Our students first complete simulation labs in which they learn how to design waveguides, bends, directional couplers, Mach-Zehnder interferometers and optical modulators in silicon using the Lumerical for Education software package. Their main coursework assignment is to simulate and draw a layout mask design of a silicon photonic circuit (passive circuit in year two, and active circuit in year three).

We fabricate their designs in the ORC cleanroom, through the CORNERSTONE rapid-prototyping platform, so that they are able to measure their own devices in the characterization lab sessions, which we believe is a more rewarding learning experience for the students than measuring generic devices.

Real and virtual labs The teaching laboratories include several experimental setups for measuring near-infrared light transmission through photonic integrated circuits, and for simultaneously applying low-frequency electrical signals to the chips. Each setup includes a tunable laser, detector,

Silicon photonics experimental setup.

Mimi Lee

For many of the students, this is the first time that they are really able to picture the abstract concepts from the lectures and simulations. fibers, polarization controller, stages for manipulating the fiber positions, and magnifying cameras for viewing the chip during alignment (see setup above).

For many of the students, this is the first time that they are really able to picture the abstract concepts from the lectures and simulations. “Oh, so this is what a chip looks like,” some students have commented. In our experience with teaching these modules, being able to visualize these concepts inspires new realizations (“Those lines on its surface are the waveguides”), as well as new questions (“How do I get the light in and out?”). Students also notice how the actual device behavior is

different from their simulations, and they begin to understand the practical effects of fabrication errors and measurement inaccuracies.

In Photonics I, students learn how to align fibers to grating couplers, to run wavelength-versus-transmission sweeps, and to analyze the results obtained from the characterization of passive silicon photonic circuits.

In Photonics II, they go a step further and measure the low-frequency behavior of thermo-optic modulators and switches. They measure the modulation efficiency and bandwidth—and the adventurous are tasked with applying a PAM-4 modulated signal to the light wave.

To enhance the laboratory experience, we are developing a virtual lab environment to enable the students to familiarize themselves with the equipment and characterization steps prior to the lab. With the support of the Silicon Photonics Group, Southampton’s Digital Learning Team used the e-learning program Articulate Storyline to create the Virtual Silicon Photonics Experimental Lab (V-SPELL). V-SPELL consists of six sections. Students work through the steps linearly, as they would when

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The Virtual Silicon Photonics Experimental Lab (V-SPELL) progresses step by step to familiarize students with the components of a photonics measurement setup before they enter a lab space. Students are introduced to the equipment, and they learn to place the sample, tune the laser wavelength and take measurements. Courtesy of the University of Southampton

characterizing their chips in the lab space. Instructions, with help buttons, are provided to guide and test them throughout the process, while videos are embedded into the resource to reinforce and support learning.

Due to the COVID-19 pandemic, Photonics I students were not able to measure their chips this year, making V-SPELL an even more valuable resource since it was the only opportunity for our students to learn about characterization of silicon photonics chips this past semester.

The student experience The students’ feedback, we’re happy to report, has been overwhelmingly positive. Students commented that “it was an excellent module overall,” and that the “integrated photonics section was the most interesting part in the whole semester.” Compared with other “theory-heavy modules,” some students felt that the practical assignment tasks of Photonics I and II were “a great relief.”

“It was one of the most interesting courseworks so far,” said one student, “and showed clear application of topics studied.” This, we feel, is a testament to the importance of

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Blended-learning approaches and remote teaching are becoming increasingly important in today’s reality. including practical labs in photonics syllabuses and providing hands-on experience to students. The students seemed to connect with this teaching style, and we believe that is a result of placing pedagogy in the center of the education process, thereby encouraging peer learning and improving student self-assessment skills.

Future photonics Blended learning approaches and remote teaching are becoming increasingly important in today’s reality, so we will continue to improve V-SPELL by including characterization of active photonic circuits. We also plan to develop a virtual lab for training students how to fabricate photonic devices.

Many academic institutions, we realize, may be interested in introducing experimental

characterization of silicon photonic circuits in their courses, but may not have adequate cleanroom facilities to fabricate their students’ designs. Therefore, the University of Southampton offers chip fabrication as a teaching service at a significant educational discount. Chips are typically fabricated and shipped within two weeks of the design submission. (For more information, email cornerstone@soton.ac.uk.)

We hope that this service, and the success of our modules, will encourage more universities to include practical labs in their photonics syllabuses as institutions train the next generation of engineers. OPN We acknowledge the support of the Digital Learning Team (Sarah Fielding, Mimi Lee and Sofia Bazzini) at the University of Southampton, and funding through the CORNERSTONE EPSRC project (EP/L021129/1).

Goran Z. Mashanovich (g.mashanovich@ soton.ac.uk) is a photonics professor at the Optoelectronics Research Centre, University of Southampton, U.K. Milos Nedeljkovic (Royal Academy of Engineering Research Fellow), Callum Littlejohns and Milan Milosevic (Fellow of the Higher Education Academy) are members of the Silicon Photonics Group at the University of Southampton, U.K.