Ways to Put Lasers on Silicon | Hacker News Reader

Ways to Put Lasers on Silicon

spectrum.ieee.org

67 pointsmfiguiere7 comments

Does someone have the background to describe what's happening in the top banner illustration on that page? (https://spectrum.ieee.org/media-library/an-array-of-gray-and...) There's a repeated grid of chips, each with an integrated laser in the upper left. But what are the other components? Are the arrow shaped objects on top of the repeated squares representing light, or are they electrical lead wires? And the lower rectangular ones with 4 such arrows? And what are the brighter raised cubes on some of those squares?

I have a strange reason for asking. My wife did most of the illustrations for this article, but not that one. It was sent along as part of the information packet for her to work from, but we had trouble figuring out what it was actually showing. She asked the art director, but I wasn't sure if he was answering the right question. She eventually did a version of it, but they didn't put it in, at least yet. So I thought maybe someone here would know!

Hard to tell but they are likely couplers for light from the lasers and electrical interconnects you can sort of see some grating at the ends. The 4x ones are likely for 4x laser assemblies like in QSFP (Quad) or for cases where you want to TX rx over multiple fibers.

The bright cubes are the laser chips that are bonded on top of the quartz, since native silicon lasers aren't practical.

That whole illustration looks like a bunch of chips in a wafer maybe with testing and monitoring patterns included.

Looks like some other stuff like modulators in the top right of the square. Maybe a test run for process control?

Edit: yeah the cubes are definitely laser dies, fig 2 shows the process. The patterns are the couplers and such to get the light in without losses and at the right direction.

Hmm, if this is right then some of the illustrations might not be showing quite what is intended. We assumed that the rectangular gray structure in the upper left of each square chip was the laser. All the other illustrations (lower in the article and in color) show different ways these can be attached, one per chip. If the bright yellow blobs on the smallest squares are also lasers, something is off.

In some ways it's fascinating that illustrations for technical articles work at all. Usually there are two layers of people who may not understand the tech between the artist and the scientists, and no direct communication allowed. Choices often come down to Googling to find earlier publications online by the same group, and hoping those earlier illustrations were accurate.

The tiny yellow blobs are the bonded lasers, so the locations of the bonded lasers in the subsequent figures are incorrect but still get the point across. This page from IMEC has a video actually showing some of the process. (The image at the top is from a press release from them about successfully doing flip chop bonding of lasers.) https://www.imec-int.com/en/silicon-photonic-ICs-prototyping

I think it still gets the point across, namely that you have to stick one chip on top of another, but it's a little wild they didn't at least give the artist labeled notes and legends

I took Lukas Chrostowski's silicon photonics design course back when I was studying at UBC and it was one of the most interesting courses I ever participated in. We designed and actually did tape out for multiple projects like DBR resonator cavities, which was an incredible experience. For anyone intetested in the field I recommend his book Silicon Photonics Design: From Devices to Systems.

For low volume applications, hybrid integration is a less challenging way for integrating a laser with passive photonic integrated circuits.

The idea is simply place a laser die and the passive die next to each other during packaging, and the laser power is transferred between the dies by butt-coupling (or photonic wire bonding). The R&D cost can be much lower but the tedious (and usually required to be active) alignment procedure prevents mass-production.

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