PhD Defense: "Heterogeneous Silicon/III-V Mode-Locked Lasers"

Michael Davenport

May 26th (Friday), 9:00am
Engineering Science Building (ESB), Room 2001

The mode-locked laser is a specialized laser that is used to produce repetitive ultra-short optical pulses at microwave and millimeter wave frequencies. The resulting spectral profile of the laser light is a phase-locked comb of regularly spaced longitudinal modes, and the timing of the pulse train is highly stable. These pulses are among the shortest man-made events, and the temporal concentration of optical energy can be extremely high. These properties are useful in a wide range of applications. Surgical instruments take advantage of highly spatially and temporally focused pulses for precision tissue removal. The broad-band optical comb emission can be used for chemical spectroscopy, or in specialized optical clocks for extremely accurate frequency counting. The stable radio-frequency train of short pulses can provide difference frequency generation and optical sampling signals for microwave photonic applications such as high frequency radar.

The most common mode-locked lasers are optically pumped solid state lasers, utilizing an electronic transition in an ionized impurity, like neodymium, erbium, or titanium, embedded in a transparent host material, which can be a bulk crystal like sapphire or a silica fiber; however, the size, cost, and power consumption of these devices limit their widespread application.
By contrast, the mode-locked laser diode uses an electrically pumped semiconductor as a medium for optical amplification. Diode lasers are hundreds to thousands of time smaller than solid state lasers, and the laser diode is an extremely efficient light source, with power consumption usually several hundred milliwatts, compared to tens to thousands of watts for solid state lasers. They have similarly reduced output power, unfortunately, and so have not been able to compete with solid state lasers in commercial applications.

Mode-locked laser diodes also bring the promise of photonic integration: the combination of multiple optical and electronic functions, manufactured together on a single chip. While this allows production at high volume and lower cost, the true potential of integration is to open applications for mode-locked laser diodes where solid state lasers cannot fit, either due to size and power consumption constraints, or where small optical or electrical paths are needed for high bandwidth. Unfortunately, most high power and highly stable mode-locked laser diode demonstrations in scientific literature are based on the Fabry-Perot resonator design and are unsuitable for use in integrated circuits.

Silicon photonics and heterogeneous integration with III-V gain material is used to produce the most powerful and lowest noise fully integrated mode-locked laser diode in the 20 GHz frequency range, and if low noise and high peak power are required, it is arguable the best performing fully integrated mode-locked laser ever demonstrated.

This thesis will present the design methodology and experimental pathway to realize a fully integrated mode-locked laser diode. The construction of the device, beginning with the selection of an integration platform, and proceeding through the fabrication process to final optimization, will be presented in detail.

Applications for integrated circuit mode-locked lasers will also be proposed, as well as proposed methods for using integration to improve mode-locking performance to beyond the current state of the art.

About Michael Davenport:

Michael L. Davenport received his Undergraduate degree in optical engineering from the University of Alabama Huntsville, in 2007, and his Masters degree in electrical engineering from the University of California Santa Barbara, in 2009, where he is currently working toward the Ph.D. degree in electrical engineering. His research interests center around silicon photonic integrated low noise single wavelength and mode-locked lasers for microwave photonics applications.

Hosted by: John Bowers, ECE Professor