Yi-Hao Chen

Yi-Hao Chen in the lab.
  • Hometown: Chiayi city, Taiwan
  • Applied and Engineering Physics

“I can’t remember the exact moment when I decided I wanted to be a physicist,” mused Yi-Hao Chen, a 6th-year Ph.D. student in AEP studying with Professor Frank Wise. “I’ve been interested in physics since I was a kid back in high school, and I’ve enjoyed the feeling of over-running my brain on tough questions for as long as I can remember.” His interest in tackling the toughest questions has paid off: in October, Chen will be awarded the “Emerging Researcher Best Paper Prize” for his 2021 paper, “Starting dynamics of a linear-cavity femtosecond Mamyshev oscillator,” published by The Journal of the Optical Society of America B (JOSA B). According to JOSA B, this new prize recognizes a student or early-career researcher who is the first author of a paper that a committee of JOSAB editors judges to be outstanding. The committee chose Chen’s paper because it felt that “the thoroughness of the study, the clarity of presentation, and the significance of the work for both fundamental laser science and state-of-the-art applications of fiber lasers were particularly impressive.”

Chen received a B.S.E in materials science and a M.S. in physics from National Taiwan University. His interest in physics was solidified in high school, spurred on by teacher who encouraged his interest, and also by receiving 3rd-place in a national science competition when he was a senior. However, he first became interested in optics while at a conference in Japan during his master’s degree. There, he learned about the serial time-encoded amplified microscopy (STEAM) camera, the world’s fastest all-optical camera. Because photons can move faster than electrons, the STEAM camera is able to overcome the speed limitation set by conventional electronics with laser pulses. “I knew that I wanted to pursue the coolest research and solve the hardest problems in the world, ones that use light and fiber lasers,” Chen said, “so I decided to pursue my PhD at Cornell.”

Upon joining the Wise group, he learned that a key problem in laser optics had still not been solved: creating a simple, reliable source of strong fiber-laser-based pulses. Among all types of fiber lasers, Mamyshev oscillators are relatively new lasers that exhibit environmental stability and can deliver remarkably strong and short (40-fs, or 40-femtosecond) pulses. These are useful for work in bioimaging and protein crystallography, among others. However, the use of two offset filters in Mamyshev oscillators prevents low-intensity noise from surviving this system and later evolving into the desired strong pulses. This has been described as a “starting problem,” and means that the oscillators do not output a pulse if they are simply turned on. Prior solutions to the starting problem come with trade-offs such as losing environmental stability or requiring mechanical intervention that isn’t reliable enough. Chen decided to solve the starting problem as part of his PhD so that Mamyshev oscillators could begin to be used for real applications, beyond research in labs. In his research, Chen was able to find a way to use only electronics to trigger noise that is strong enough to survive two filters and later evolve into a mode-locked state (i.e., a pulse). Furthermore, he also found that Faraday rotators are necessary to prevent damage to the laser.

“In my follow-up work, this type of pulse has been proven to efficiently generate 1,300-nm pulses through a nonlinear, frequency-conversion process, or soliton self-frequency shift,” he said. “This wavelength—1,300 nm—is sought after in three-photon bioimaging and can produce high-resolution, deep imaging. An all-electronic control of a Mamyshev oscillator can pave the way for future reliable laser sources important in applications that require not only high pulse energy but also short pulse duration, which are typically difficult to achieve simultaneously.”

Besides Mamyshev oscillators, Chen is working on other projects that focus on generating high-energy, short pulses. “I’m on my way to pushing the limit of what a fiber-laser-based pulse can be; it’s an endless quest and I’m interested to see where this path will lead me,” he said. Although he finds it interesting to pursue academically unresolved problems, he is interested in the practical applications of his work. “In the future, I’d like to go into industry to see what is most important to the non-academic part of the world. I’d like to take a step out of this ivory tower—as amazing as it is— to see if I can contribute directly to real-world problems. I’d like to contribute to pushing my work further beyond electronics and into optics, and into a whole new territory to be discovered.”

Figure, which is described in the caption, is from Chen's 2021 paper.
Starting dynamics with pump modulation, from Chen's 2021 paper. (a) Illustration of the concept; scale is not quantitative. (I) Temporary self-𝑄-switched state appears after turning on the 80-kHz modulated pump, and then (II) it spontaneously evolves into a periodically modulated mode-locked state. (III) A stable mode-locked state is reached after the pump modulation is turned off. (b)–(d) DFT measurements of the transitions of states. Note that the time axis in (b) is in units of the modulation cycle, instead of the round-trip time of the cavity, to better reveal the self-𝑄-switching. We attribute the spikes at the end of the modulation to the pump laser overshooting as the modulation is turned off.


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