(Video Provided by Optica Publishing Group)

 

 

Interview article:

 

Lin Chang. Photonics Research Interview with Professor John Bowers[J]. Photonics Research, 2023, 11(11): 1987

 

Interview transcript

 

Lan Yang: Welcome, everyone. Thanks for attending our special webinar. Today's event is part of a webinar series sponsored by Photonics Research, a Gold Open Access journal jointly published by Chinese Laser Press and Optica Publishing Group. We launched this webinar series last year, as an opportunity to document the experience and insightful advice from great scientists who have made groundbreaking discoveries in the field of optics and photonics. We arrange the webinar in the format of a conversation. In addition to this live interview, we're going to publish the shortened version of the interview transcript later in the Photonics Research journal. I would like to encourage you to share this with others who might find this useful.

 

Lan Yang: Today, it's my great pleasure to introduce our guest of honor. Prof. John Bowers from the University of California, Santa Barbara, in the United States. He's a world-renowned leader in the field of integrated photonics for optics communications. In addition to his career in academia, he's also a very successful entrepreneur. He has founded many start-ups, including Terabit Technology, Calient Networks, Nexus Photonics, some of which have been acquired. On top of these accomplishments, he's also a great educator, giving advice and creating opportunities for many others. So far, more than twenty people from his group have become professors around the world, and more than twenty have started their own companies. In addition to that, he also started the Center for Entrepreneurship and the Engineering Management Program at UCSB, which has evolved into the Department of Technology Management that offers Masters Degrees, Ph.D.'s and certificates in technology management. He also founded a non-profit, Unite to Light, to make low-cost and healthier light sources accessible to the world, specifically for those places with limited resources.

 

Lan Yang: Because of limited time, I wouldn't be able to finish describing the long list of his accomplishments. What I just mentioned is just the tip of the iceberg of what Professor John Bowers has accomplished. We hope to learn from his experience and hear his advice today.

 

Lan Yang: I also want to introduce another important guest tonight, Professor Lin Chang. The interview tonight is special because it's a conversation between a mentor and his former mentee. Prof. Lin pursued a PhD under the supervision of Prof. John Bowers. During this time, he focused on developing the strategies for advanced, heterogeneous photonic integration to achieve new functions for integrated silicon chips. As far as I know, people from industry have started to adopt the strategy developed in their lab. That's really exciting. With all those achievements, Prof. Lin Chang joined Peking University as an assistant professor in the department of electronics. He is now a rising star in the field of integrated photonics.

 

Lan Yang: It's a great honor to have both of them with us today. Also, it's worth mentioning that our interview includes some questions that were collected during the registration process. Thank you to everyone who submitted questions for Prof. John Bowers. There will be no additional questions from the audience today. Now we'll start the live session. Let's welcome Prof. John Bowers and Prof. Lin Chang.

 

Lin Chang: Thanks a lot, Prof. Yang for the introduction. Hi, John. I'm very excited today to have this conversation with you. As you know, I spent eight years at UCSB in your group. Under your guidance, I finished my Ph.D. and during that time, I witnessed how a lot of technologies get developed and then finally change the world.

 

Lin Chang: Your leadership, your great taste in research topics, and your ability to commercialize things are the keys to making all those things happen. I have learned so many things from you and I think many researchers will also benefit a lot from your experience, and also the stories behind your achievements. Let's start. The first question is one I have always been curious about. How did you become interested in silicon photonics?

 

John Bowers: Well, thank you, Lin. It was a great pleasure having you in the group, and you made so many accomplishments. I'm delighted to be here. Well, I got interested in silicon photonics because we were using silicon oxynitride waveguides to dramatically lower the loss of optical waveguides and open up new applications, like optical gyroscopes on chips or optical memory.

 

John Bowers: And this is exciting. But this prospect of integrating lasers, modulators, detectors with those waveguides opened up a lot of new high-volume, low-cost applications, like transceivers. The prospect of skipping five or more generations of process technology and using modern packaging to make scalable devices was very exciting.

 

Lin Chang: Yes, silicon photonics, as you mentioned, has nowadays become a very hot topic in both academia and industry. Could you talk about why it is so important now, and how your work in silicon photonics has been transforming our world for years and will continue to do so in the years ahead?

 

John Bowers: Well, there's this very interesting convergence that's happening now between electronics and photonics. Electronics is limited and getting data on and off the chip, and as communication speeds get higher and higher, copper interconnects are really not competitive. They take too much power, require too much equalization and regeneration. So, photonics has moved from simply being used for long-distance communications to connections between racks and now chip-to-chip. The prospect of silicon photonics being used for every high-capacity switching chip, every high-speed GPU or TPU or processor, for high-bandwidth memory is really exciting. Conversely, to make really advanced transceivers, we need electronics intimately integrated with it to lower the power required.

 

John Bowers: It's a very exciting time. For these photonic solutions to be integrated with electronics, they have to be on silicon, right? So, you match the coefficient of thermal expansion of ICs and really take advantage of modern packaging. This exists for iPhones and things. So, no more gold boxes for photonics. I think silicon photonics will transform our world in the future, because it opens up these high-volume, low-cost solutions to not just transceivers and telecommunications, but healthcare, sensors, transportation. Many applications that traditional photonics couldn't address because they were just too expensive.

 

Lin Chang: Sounds great. As everyone knows, your work in 2006 successfully bounding III-V onto silicon waveguides has been widely accepted as a milestone in silicon photonics. It was also the start of laser integration of silicon photonics. Would you share some stories behind this work? What kinds of challenges have you overcome to achieve that?

 

John Bowers: We had developed bonding for different materials to enable high brightness LEDs (which Rajeev Ram, now a Professor at MIT, did) and for high-performance PICs at telecom wavelengths. So, this was successful. I was pretty convinced that we could use bonding to get III-V materials on silicon without the degradation and performance problems you get with epitaxial growing materials, or anything with defects in it. The challenge, of course, is that III-V materials are not in silicon foundries, and there's a lot of concern about that. But they're all dopants, and as long as we exist on the copper side of it, I think it works quite well.

 

Lin Chang: Why do you think laser integration on silicon is so important? Could you also talk about the impact of heterogenous laser integration on silicon photonics?

 

John Bowers: We all learned about the importance of gain when fiber optic networks were transformed by EDFA's (erbium-doped fiber amplifiers). You can make much bigger, much more complex and much cheaper networks by having gain in this optical network. The same thing was obviously true for electronics. Without gain, you really couldn't make an interesting IC. So, while many people around the world focused on silicon germanium for gain regions, it was really clear to me that indirect band gaps would never be competitive. They just wouldn't be as efficient as you'd get with indium phosphide or gallium arsenide technology. To integrate laser and silicon photonics would require direct band-gap material. We originally called the first work "hybrid integration," but now, more properly, "heterogeneous integration" of gallium, phosphide and silicon. And again, gain regions with a lot of defects in it don't typically have good performance, high efficiency, or high reliability. By bonding materials that we grew on pristine materials, on epitaxial wafers, allowed us to get into these large wafer sizes, much larger than gallium arsenide or indium phosphide could do by themselves.

 

Lin Chang: Do you think heterogeneous silicon III-V lasers have reached commercial availability?

 

John Bowers: Yes, it has. Intel has certainly led that and they're doing millions of transceivers a year. And, Juniper (now Openlight) has pursued that. So, I think it solves a lot of problems. If you look at Infinera, a very successful photonic integrated circuit company, they typically have five or six epitaxial growth steps to make lasers, modulators, detectors, or to integrate contacts with waveguides. So those five or six regrowth steps are costly, and yield-limiting. But with heterogeneous integration we can bond all the material at one time and process it all together. So that's a real advantage. Plus, we can get much wider gain widths. We can combine visible devices with infrared, with far infrared devices all in the same PIC. I think heterogeneous has really solved a lot of commercial problems needed for high-volume, low-cost applications.

 

Lin Chang: Sounds great. After the initial work, you continued to lead our group and made many other breakthroughs in silicon photonics. Could you talk about several important ones?

 

John Bowers: Well, probably the most important one is the one that we did together, that you really led, namely making initially highly confined waveguides and lithium niobate for second harmonic generation, and then moving on to gallium arsenide. And again, the fact that you have this strong confinement makes the lithium niobate devices work better than our native substrate. And same thing when you put gallium arsenide on top of silicon with SiO₂ all around—then you get very efficient second harmonic generation, very efficient comb generation, much better than you would get on a native substrate. So heterogeneous on silicon works better than on gallium arsenide, or on lithium niobate.

 

John Bowers: So, I think it's very exciting. Once we integrate the laser with these new low-loss devices, then you get interesting comb sources, and get very narrow lines with lasers, right? The Lorentzian linewidth was at less than a tenth of a hertz already, and I think we can get to a millihertz. And so again, combine the very low loss of what you can get with waveguides and silicon, with now gain or nonlinear devices, and it makes much better devices. We started trying to make lasers that were as good on silicon as native substrate. But now we can make better lasers on silicon than native substrate.

 

John Bowers: We always think of semiconductor lasers as being noisy, much worse than a gas or solid-state laser, but now they could be lower noise, and that'll open up a lot of new applications.

 

Lin Chang: Yeah, indeed. You mentioned hybrid integration in one of the previous questions. There have been several different integration strategies. What is your reading of different integration strategies like monolithic, hybrid, and heterogeneous integration for the future of silicon photonic systems?

 

John Bowers: Certainly, most people use hybrid integration. The lasers may be fiber-coupled to the PIC, or they may be adjacent to each other, and indeed that works well. For making narrow linewidth lasers, that works very well, but it's not a very scalable, high-volume approach. On the other hand, when you go to heterogeneous, when you bond III-V materials onto there, you can do all this processing in parallel, right? The laser, modulator, detector integration all happens, processed at one time. The contacts are made at one time. And you get all the advantages of wafer scale testing. I think that's really key for scalable low-cost systems.

 

John Bowers: Eventually I think monolithic will become important. So far, you give up performance and you give up reliability, but it's moving very quickly. Recently, we have been working on epitaxially growing quantum dot lasers on 300 mm wafers, and it works quite well. We can get over 100 mW output and pretty good reliability, but not yet as advanced as heterogeneous integration.

 

Lin Chang: You have mentioned that there has been a lot of progress in the area of integrated narrow linewidth lasers. Why are narrow linewidth lasers very important for silicon photonics?

 

John Bowers: The obvious applications are coherent communications, particularly as we make higher levels of QAM. And in general, optical gyroscopes need narrow linewidth lasers, spectroscopy needs narrow linewidth lasers. Linewidth is sort of a metric for lower frequency or phase noise. What we've seen by integrating DFB (distributed feedback) lasers with resonators is that the frequency noise, and phase noise comes down by literally 70 dB or more. That has a lot of advantages for a variety of sensors, certainly spectroscopic devices and others. Microphotonics works much better by pushing down that noise, and I think it's really important.

 

Lin Chang: You have also mentioned microcombs. As we know, microcombs over the last few years have become a very hot topic, and you are one of the leaders in this direction. What do microcombs bring to silicon photonics?

 

John Bowers: The first is certainly just as a comb source. By having literally hundreds or a thousand wavelengths that are very precisely spaced from each other, I think is really important. I think you can do things like dual-comb spectroscopy by having two combs that are slightly different, and the repetition rate opens. It's much better than traditional spectroscopic measurement systems, and so they can be compact, low cost. And I think we'll find a lot of applications. It's a really exciting area.

 

Lin Chang: Could you talk a little bit about our recent demonstration of the optical frequency synthesizer and atomic clock by using silicon photonics? Do you think that in the future we can integrate entire such systems on chips?

 

John Bowers: Sure. Most optical systems today use one laser or a couple of lasers. In that case, you have different options, but the point of that recent paper was that using a comb source with tens of hundreds of tightly controlled wavelengths combined with fairly complex silicon photonic PICs allows us to demonstrate very high-capacity communications—two terabytes in the case of the Nature paper—or also very complex microwave photonics, which was also there. So, Lin, you led that effort and made it successful. I'm glad I was able to support it.

 

Lin Chang: Thank you. There has been another work, a paper you've just published in Nature, in collaboration with Nexus Photonics, which has also overcome a longstanding bottleneck in silicon photonics. Could you talk about this work, and how it will affect silicon photonics in the future?

 

John Bowers: Sure. Most silicon photonic works use silicon waveguides, and all the datacom and telecom work at 1.3 and 1.5 micron. But silicon is absorbing as you get into the visible region. The Nature paper with Nexus Photonics is about silicon nitride waveguides and integrating shorter wavelength gain regions. I think what we've done is just the beginning and now with Nexus pushing 980 nm wavelengths, and then shorter wavelengths in the future, it opens up a lot of new applications. It's essential then to have gain coupled to nitride waveguides, not silicon waveguides. In a sense it's a much harder problem, because III-V and silicon indices are about the same in index. So, coupling between them is fairly easy. But silicon nitride waveguides, particularly a thin nitride, for most of the energy is in the oxide, which is how you get the lowest loss, is an index of 1.5. So, coupling from this 3.5 index to a 1.5 index is a much harder problem. As that paper showed, you can get very efficient coupling (less than a dB loss) and very little reflection from those interfaces. So, for the tunable lasers that were demonstrated there, that's absolutely key to get smooth tuning, not to have reflections at those interfaces. And the really exciting thing about that paper was the lasers blazed up to CW, up to 185°C. That allows us to now make tunable lasers that are not limited to room temperature operation, but don't need temperature cooling. With wavelength measurement on a chip, you can again have this laser uncooled and for almost any application. Once you have visible wavelengths, then there's a lot of applications for AR/VR and for atomic clocks and a host of new, important fields. So, I think that was an important paper.

 

Lin Chang: Indeed. You have published many works in top journals over the last few years, so could you talk about how the publishing process has benefitted your career?

 

John Bowers: Well, you know, there's a famous phrase which is "not to keep your light under a bushel basket," so it is important to find prestigious places like Photonics Research that many people read to get the word out on what one has done. I think I have published eight papers in Photonics Research and they've been highly cited, so I appreciate the widespread readership. A story I've told many students about publishing involves Herb Kroemer, who won the Nobel Prize at UCSB for inventing the double heterostructure laser. The interesting part is that his original paper was rejected. I always tell students, don't get too discouraged if your paper is rejected. Herb was good enough for a Nobel Prize. Fortunately, he didn't give up easily.

 

John Bowers: The key is to not give up easily, but have your own strong opinion about the importance of the research you're doing. Herb did resubmit his paper. It was published. He did submit a patent which was issued. So, he got the recognition he deserved. But again, people doing some of the lasers at the time—this is 1962-1963—were not aware of his work, and so it's unfortunate because he could have impacted the field a lot sooner if he had worked in one of the groups that was in that area. But he was working at a different company that didn't do some of the lasers, and so it's important to make sure people learn about your work. Don't keep your light under a bushel basket.

 

Lin Chang: Indeed, that's also a very important lesson for me and I will also teach this to my students, too. Besides your outstanding scientific achievements, you are also famous for your great success in technology transfer from academia to industry. Could you share your experience on how to commercialize technologies?

 

John Bowers: I don't know that I know how to do it. Many people do it much better than I do, but if you can get a big company like Intel to take up your research and commercialize it, that's obviously the best outcome. Going and giving talks at companies and conferences is really important, but many times you do good research, and no one picks it up. In many cases, it's only the person doing the inventing of the development, like the student or whoever. And then, if no one picks it up, it'll get lost. It doesn't cross that valley of death, and then you have to start a company to get it commercialized. Many times, only the inventors see the value. I've been lucky to work with a lot of brilliant students who took the ideas they developed here and developed them into companies.

 

John Bowers: Fortunately, UC is very receptive to licensing the technology to inventors, and they've been very supportive to me. I've taken I think three leaves of absence to look at starting companies. And UC has been very supportive.

 

Lin Chang: Yeah, great. Besides those questions, there are also many questions that we received from the that audience. Many of them are from young researchers seeking advice for their career. As everyone knows, you are a very good supervisor. As Prof. Yang mentioned, there are many people from our group who started their own companies and have become very successful. What advice do you have for researchers in terms of launching start-ups in silicon photonics?

 

John Bowers: Well, I think as Wayne Gretzky famously said, you should skate to where the puck is going, not to where it is now. If you're going to start a company, you can't do something that's immediately applicable. You need to leapfrog existing companies and technologies and make a breakthrough. While you're building the company, everybody else is progressing, so make sure it is a breakthrough, and then pursue it vigorously. Great ideas are important, but you have to assume that other people have thought of it too, and so you have to move very quickly to implement it and do it tenaciously. You can't give up. There'll be a lot of down moments, and so anticipate that and just pursue it very vigorously.

 

Lin Chang: Yeah. There are more than twenty people from our group who have become professors. If you were a graduate student today, given today's support for R&D, would you opt for an academia or industry career?

 

John Bowers: Well, I've been lucky to do both, right? I started at the laboratories and really learned a lot there and worked with a lot of great people. And I've been lucky to be at UCSB, and they've given me the chance to kind of cross the line, and take probably a total of about four years leave of absence. Both directions are ways to change the world, and that's what's important. The choice of academia or a company really depends on what you enjoy doing. I enjoy teaching. I get a lot of pleasure of seeing students become successful, and I'm very proud of what you've accomplished. I take pleasure in that, so an academic career works really well for me.

 

Lin Chang: In terms of academia, do you have any suggestions for people who want to pursue a faculty position?

 

John Bowers: Well, I think one thing is certainly to do a good post-doc. In many cases, you can go directly from a Ph.D. to a university, but it's difficult to get a really good academic position. You could be with a lot of other people, right? A good post-doc can help you get a better position at a better university than you might otherwise get. But in general, just throughout life, just keep learning, right? Don't get stuck in a rut, basically. If you stay and do a post-doc where you did your Ph.D., make sure it's on something a little different and something you can learn from.

 

I was a grad student working in solid state physics, and in the zinc oxide world, again on silicon—taking existed electrical materials and putting them on silicon. But as a postdoc, I was working in the fiber optic field, and so it was a big change for me. I learned a lot, and that really gave me a leg up when I went to Bell Laboratories.

 

Lin Chang: Indeed. Another interesting question is that, in addition to being a great scientist, you are also a great bicycle rider and snow skier. I recall that you went across the whole U.S. riding a bicycle. How did you become interested in these hobbies and how do you maintain work-life balance?

 

John Bowers: Well, in that case, I saw a movie about riding the Tour Divide, which goes from Canada down to Mexico along the Continental Divide, so that got me interested in trying that. I think the point is that to sustain yourself over the long run, you have to remain healthy, and not allow work to run you down, not let your health degrade. Exercise is really important to maintaining vitality. When I was younger, I used to run and play soccer and tennis, but my knees are not in such good shape anymore. So, biking is a way to stay in shape. I do remember when I was at Bell Labs with Tom Wood and others, we used to run at lunchtime every day. We were coming back after that, there's about ten of us, and our boss was kind of irritated that we were all off at lunchtime running. They asked us where we ran, and Tom Wood was smart, and he said, "Well, we only run as far as you can get to in an hour, so we keep it during our lunch break." But yeah, whatever you choose is fine, I think.

 

Lin Chang: Yeah, that's a very interesting story. And finally, do you have any other advice for young researchers today?

 

John Bowers: Find something you're passionate about and pursue it relentlessly. Become the very best at something. That's certainly the expectation for anyone when they get a Ph.D. is by the time they graduate they know more about that topic than anyone in the world. It takes hard work, and you have to expect that you're going to be frustrated and have a lot of downturns. But just have the vision that what you're doing is the right thing, getting a Ph.D. or pursuing a particular idea. So, change the world. You can work on something very broad, or it could be very narrow, but do something that will have an impact. Sometimes, when students ask about a topic, I'll say, okay, fine, maybe you're right that will work and it will be successful, but is anybody going to care? Make sure you choose something that really will impact the world. And we all only have a limited amount of time on this planet, so it's important to make the world a better place with the time we have.

 

Lin Chang: Thanks, John. We really appreciate that you're sharing all those valuable experiences and insights. I'm sure that many people today will benefit from them, and also in the future. Thanks again for spending your time for this interview. I look forward to more exciting news of silicon photonics from our group in the future. Thanks, John.

 

John Bowers: Thank you. It was very nice. Thank you, Lin.

 

Lan Yang: Great. That's wonderful. Thanks a lot for being with us today. I really learned a lot from the conversation. There's a proverb I want to quote whenever I hear a great conversation. This is something I learned when I was young. It says a single conversation with a wise person is better than ten years of studying books. Prof. Bowers, seeing the dynamics between you and Lin, who was smiling all the time during this interview; now I know why there are so many people from your group having successful careers. They learned from a great mind, and that is why I feel very grateful that today we had the chance to learn from you. As a world-renowed scientist, your advice is insightful and useful for many of us. Thanks for sharing your experiences with us.

 

Lan Yang: Now, what I'd like to share some slides with you about Photonics Research, which is sponsoring this webinar. Photonics Research is a relatively new journal compared with other journals associated with Optica Publishing Group. It's a partnered journal co-published by Chinese Laser Press and Optica Publishing Group. Currently, the Impact Factor is 7.254, ranked at #12 out of 102 journals in the Optics category. It publishes both fundamental and applied research. We encourage papers in all subjects in photonics. As a partner journal, we have certain advantages. For example, we can promote research in different social media platforms associated with both Chinese Laser Press and Optica. It's our great pleasure to share with a broad audience about the research progress in your group, if you submit and publish your work in Photonics Research.

 

Lan Yang: In addition to regular papers, each year we have feature issues led by researchers with strong reputation in specific fields. We have the feature issues to keep the community up to date on subjects in fast moving areas. For example, in 2021, we had a feature issue on deep learning in photonics and some of the papers have received a lot of attention from the community. This year we will have a feature issue about integrated photonics, which is a subject just mentioned by Prof. John Bowers and Prof. Lin Chang. It is a fast-moving, exciting topic happening in both academia and industry. Next year we will have another feature issue coming out on optical metamaterials fundamentals and applications. We can have feature issues in two ways. One is solicited by the editors of Photonics Research, or we can get proposals from the community. So, if you have an idea about an exciting topic that is of broad interest to the community, I would like to encourage you to submit a proposal. Reach out to us, and we would love to talk to you about a feature issue related to your expertise.

 

Lan Yang: In addition to feature issues, I also want to mention something interesting, the Editor-in-Chief Choice Award which comes with ¥10,000. The money is by no means reflective of the value of the work. The research discovery is invaluable. What are the criteria to choose the papers that can get this award? It's very simple. It's all about quality, impact and significance. It doesn't matter where the paper comes from. Recipients of this award can be in a small group, a big group, a senior group, or a young group. It can be a group focused on theoretical research or fundamental, or application-driven research. I encourage you to submit your best work to Photonics Research, and I wish you the best to be the recipient of the next Editor-in-Chief Choice Award.

 

Lan Yang: As we end, I wanted to emphasize again, to make Photonics Research a great journal, we count on authors. I do think the criteria to evaluate whether a journal is good or not is not about the Impact Factor. It's about the reputation, the reputation in the heart of authors. So, I would like to encourage you to send your papers to Photonics Research, because, as I mentioned, we have a lot of ways to promote your research, and we also have a special editorial review process. All papers submitted to Photonics Research will go through a two-step process. When a paper is submitted to the journal, the Editor-in-Chief or Deputy Editor will assign the paper to some associate editors for initial screening. Papers that pass this step will go through an external review. Through this rigorous progress we control the quality of papers published in Photonics Research. If you have any ideas to improve our journal, such as suggestions for the next scientist we should interview, please feel free to send an email to us at prjournal@optica.org or prjournal@siom.ac.cn

 

Lan Yang: I want to thank you again for being with us today. We would not have a successful event without your participation. Thanks to Prof. John Bowers and Prof. Lin Chang for being with us tonight. I wish you the best and look forward to meeting you in the future. I wish everyone a great day.

 

John Bowers: Thank you.

 

Lin Chang: Thank you.