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Microwave Photonic Filters for Adaptive
Phased Array Antenna Systems

Adviser: Paul R. Prucnal

John Chang

Electrical Engineering

“Making discoveries, however small, always gave me the excitement of scientific discovery—the feeling that I had developed something that was distinctly mine and belonged only to me.”

chang-john

By the time my senior year rolled around, I had already worked with my adviser, Professor Paul Prucnal, for nearly two and a half years—ever since the summer of my freshman year. I had worked on three separate projects prior to my senior thesis under the supervision of a graduate student mentor, Zhenxing Wang. My project topics had been quite diverse, from volumetric holographic storage and implementing all-optical XOR operation to a method for encrypting messages using optical steganography. Even though I had plenty of experience in the lab, I was still mentally unprepared for the task of my senior thesis.

The project was as intimidating as the title sounds, and, to be honest, I tucked away the mounds of background papers I had to read for a good few weeks. The “PAA project,” as we called it, was still in its infancy, and Professor Prucnal was considering whether or not to make it a full-blown project. A phased array antenna is a type of radar that uses multiple antennas arranged in some kind of formation and uses digital post-processing in order to “steer” the direction of the radar without moving parts. While systems such as these had been created before, Professor Prucnal wanted to design a way to perform the digital postprocessing component completely in the optical domain, something that had never been attempted before. My task was to take a look at the underlying theory and then make some preliminary experimental measurements to make sure that the project was feasible and should be pursued. In the end, my senior thesis consisted both of a theoretical analysis of the postprocessing algorithm and an experimental component demonstrating a proof-of-concept of the system.

While I was already familiar with working on the lab equipment, I was less prepared for the theoretical analysis of the system. In our lab, the Lightwave Communications Laboratory, we have a distinct mentoring system where each veteran graduate student takes on a younger student with less experience. It was through this system that I first approached my independent research, working closely with Zhenxing. Zhenxing not only taught me how to use the myriad of lasers, fiber-optic cables, and detectors that our research dealt with, but also showed me proper scientific method for conducting research. With familiarity, he let me branch out and explore on my own. However, my previous projects were already clearly defined, and little theoretical pre-or post-analysis was really needed.

My first task was to review the background literature for the phased array antenna, present it to the lab group, and offer some thoughts on the feasibility of applying optics to the system. None of the other lab members would be helping me look through the papers, and this sudden new independence had me quaking. One thing important to conducting research is to remember that there’s no answer waiting to be found in some textbook or article; unlike taking classes, one cannot simply expect the professor to release solutions sets to the problems—indeed, there often isn’t one “correct” answer; there can be many different and viable solutions, and it’s your job as researchers to choose the one that best fits the situation at hand. The other important thing to remember is that Princeton actually prepares its students quite well to approach research. For all the courses we begrudgingly take, the information we learn can be quite useful. Going through all my courses, I found that Princeton has taught us, through exams and the endless problem sets, the proper way to approaching a difficult problem—that is, slowly and methodically, starting with the information that we already know and applying it in a meaningful way.

I started my reading slowly, and I made sure I understood the underlying methodology behind each theory, which allowed me to more easily grasp advanced concepts. Luckily for me, I found that a large chunk of the theory was covered in some of the operations research and financial engineering classes I took only to fulfill distribution requirements and didn’t think I would ever use again. Applying concepts old and new allowed me to fully understand algorithms and to make novel generalizations to the optical domain of my own—and keep Professor Prucnal happy.

For me, I think the part I enjoyed more were the experiments. Experimenting allowed me to get into the lab and test and verify the theories we had on paper. It provided proof that the mess of jumbled symbols in our equations were actually physically meaningful and could be practically implemented. It is always exciting to be able to apply some theory learned in class to make an experiment work. Making discoveries, however small, always gave me the excitement of scientific discovery—the feeling that I had developed something that was distinctly mine and belonged only to me. It reminded me of the feelings Nikola Tesla or Joseph Henry, innovators extraordinaire, must have had in their heydays. I remember that for most of my senior spring, I actually looked forward to research in the lab. It provided a break from the monotony of my classes, where I was just counting down the days until graduation. I could always expect something new and unexpected to happen in the lab.

I also liked research because it allowed me to dictate my own pace. I could come in when I wanted and leave when I was tired. However, this method lends itself to a lot of potential procrastination, and the friends I knew who finished their thesis in a few weeks never seemed to enjoy their thesis as much as I did mine. Having a set of strong fundamentals lends itself to having a much better time with your thesis. Even laying down some basic foundations as having a good topic prepared, a list of relevant background literature, and an outline of tasks to be completed makes the thesis experience significantly more enjoyable.

I would love to say that I finished my senior thesis without a hitch, but in the middle of the spring semester I ran into significant problems with my experiments. Unfortunately, research never goes the way you expect, and a good researcher always has to be prepared to deal with running into an unexpected problem. Unlike working with equations on paper, you can always count on the experiment to fail on its first try. The majority of my time was spent debugging my experiments, making sure every piece of equipment was used correctly, or even going back to the basics and scrapping the entire setup in favor of a new one. In these cases, it is most important to find out exactly what went wrong and why—and not to become frustrated. In fact, most research and meaningful results are defined by how a researcher approaches solving problems. Theory can only tell you half the story, and experiments fill in the plot holes.

However, the most important key to success is the simplest: finding a good adviser and a good topic. It is best to have an adviser who gently steers your research without dominating the topic. Having an adviser who respects you and your ideas is vital.

This way, you’ll have the freedom to explore the area of research while having a support system to bounce ideas off your adviser. No matter how much preparation or work you put into your thesis, without good chemistry between you and your adviser, your senior thesis will neither be enjoyable nor instructive.

Keep in mind that your thesis is not only a culmination of all your work done at Princeton, but it also is a valuable learning experience as well. Working through a senior thesis teaches you worthwhile skills and experimental techniques while providing necessary experience as well. It teaches you how to approach problems in totally new subjects with just a basic set of fundamentals, how to manage your time effectively, how to make your own timetable to complete projects, and how to debug experiments in order to make new discoveries. In the end, I had such a good time with my senior thesis that I decided to come back to Princeton for my Ph.D., and these are important skills I need to use every day as a graduate student now in Professor Prucnal’s lab, working on a continuation of my senior thesis, which Professor Prucnal has now made a full-fledged project. Don’t think for a minute that your thesis is just some exercise to be completed for graduation—it represents a significant contribution to your field and to your lab, if you take the time and effort to make it count.

Microwave Photonic Filters for Adaptive
Phased Array Antenna Systems

John Chang

Paul R. Prucnal

Professor of Electrical Engineering

“The applications of John’s work are far-reaching, ranging from commercial air-traffic control systems to jamming-resistant antennas on aircraft carriers.”

The Lightwave Communications Laboratory, where John Chang did his senior thesis work, is an exciting and dynamic place that typically integrates the work of half a dozen undergrads, an equal number of grad students, a postdoc, and several scientists from commercial and government labs who visit weekly. One of the lab’s greatest strengths is that everyone is excited about his or her research and thrives on interacting with others. During the academic year, the undergrads usually do independent study work for academic credit, and in the summer, an internship. Though some undergrads wait until their senior year to begin their research, most get involved much earlier. Each week, students take turns presenting their work to the rest of the group, and frequent discussions take place at all hours of the day and night during the rest of the week.

The lab is structured in a way that provides students as much support as possible. In addition to working directly with me, we have a mentoring system in which “younger” students work as apprentices to “older” students. This provides undergrads with early exposure to our research and training with our technically advanced lab equipment. Because most of our experiments involve ultrafast (picosecond) infrared pulses in optical fiber, the light that students measure is invisible to the human eye, is too short to measure with conventional electronics, and requires alignment with nanometer precision. Developing the skills needed to do these experiments requires both patience and hard work, so the undergrad apprentices benefit from getting an early start. Once they have command of the experimental techniques, undergrads are able to work independently in the lab, just like the graduate students. A large fraction of the undergrads in our group carry out independent research and publish in archival research journals by the time they graduate.

For me, working with undergrads on research is a vitalizing experience that is completely unlike teaching in the classroom. Though interacting with students in small classes can be very rewarding, it doesn’t necessarily foster the same kind of relationship that can be developed in the lab over the course of a summer, a year, or even longer. I think the students can find the excitement of scientific discovery to be unparalleled by any experience in the classroom. In my group, this process often involves sitting around a conference table all afternoon on Fridays (sometimes while eating pizza) and casually knocking around ideas.

Most of my grad students already have considerable expertise in optics before they arrive at Princeton and approach their research in a professional way. Their discoveries are usually made in a methodical, deliberate fashion. By contrast, undergraduates usually start out knowing next to nothing about optics, but learn at an incredibly fast pace. They fear nothing and risk nothing by making mistakes, usually attacking problems without bias and with an unfettered enthusiasm. They never stop digging into problems and asking questions, well past the point when I run out of answers. They constantly challenge my assumptions, forcing me to reconsider ideas I already may have discarded. Working with undergrads has been both an exhausting and exhilarating experience.

To me, John has been the quintessential Princeton undergraduate researcher: inquisitive, engaging, and scary-smart. John started working with me three years ago during the summer after his freshman year and has relentlessly pursued research ever since. By senior year, John had worked through almost every project in my lab, had published several papers, and was already defining his own research direction. His appetite for knowledge and thirst for doing experiments has been insatiable. Often his progress was so fast that it was hard for me to keep up with him. The centerpiece of John’s senior research was designing an intelligent radar antenna that could coordinate its own actions, very much like the human eye and brain cooperate, but in less than a billionth of a second. For example, if the antenna notices motion at its periphery, then it is able to quickly turn and focus its attention in that specific direction.

This project was originally motivated by our collaborators at the Lockheed Martin Advanced Technology Labs in Cherry Hill, who developed radar control systems based on neural computing, but found electronic instantiations of neural networks to be too slow. My group started thinking about faster “neurons” that could use photonic technology to replace electronics, and John became responsible for the part of the neuron that performs pattern recognition. He designed “matched filters” using a series of laser amplifiers and time delays, built them in the lab, and compared his experimental results to theory. He also investigated using supervised learning to allow his filters to adapt to changing environments, and designed architectures that could be scaled to hundreds or thousands of antennas, acting more like a bug’s eye than a human eye.

The applications of John’s work are far-reaching, ranging from commercial air-traffic control systems to jamming-resistant antennas on aircraft carriers. Though I have worked with many extraordinary undergrads over the years, John’s research, and the experience of working with him, has been among the best. He epitomizes why Princeton students make teaching here such a pleasure.