Sang Jun Choi

Microdisk lasers vertically coupled to output waveguides

Semiconductor microdisk resonator lasers vertically coupled to bus waveguides are demonstrated for the first time. These structures have many of the characteristics required for a light source in photonic integrated circuits because they can be coupled to other optical elements (switches, routers, filters, etc.) through common bus lines. A continuous-wave single-mode laser at 1579 nm with a side-mode suppression ratio greater than 30 dB has been achieved for an 8-/spl mu/m-radius microdisk. For larger disks, mode competition between modes near the maximum gain wavelength is observed from the light output-current (L-I) characteristics. Improved heat sink design is required for future devices.

CH4-based dry etching of high Q InP microdisks

CH4-based InP dry etching techniques have been investigated by using electron cyclotron resonance etching and reactive ion etching (RIE) methods to obtain microdisk and ring structures having smooth, vertical sidewalls, and specular etched surfaces. The RIE method is chosen for the device etch process owing to the higher perfection of the surfaces generated by this process. Excess CH4 introduced in the InP RIE process was found to generate excessive polymers and resulted in sloped, rough sidewalls. A multistep RIE process involving a high-pressure (75 mTorr) condition, followed by a lower pressure (15 mTorr) etching to the completion of the structure was developed that leads to very smooth sidewalls. This process was successfully utilized in the fabrication of vertically coupled microdisk resonators.

Eight-channel microdisk CW laser arrays vertically coupled to common output bus waveguides

1.6-nm spectrally spaced eight-channel semiconductor microdisk laser arrays are presented, where high-Q disk lasing modes are vertically coupled out through a common bus waveguide. The spectral channel spacing is achieved by varying the disk resonator radii from 10.6 to 10.95 /spl mu/m. Typical linewidth of 0.25 nm and side-mode suppression ratio of -20dB are observed under continuous-wave lasing operation near /spl lambda/=1.51 /spl mu/m. This is the first demonstration of integrated microresonator laser arrays.

Optical Modulators based on depletion width translation in semiconductor microdisk resonators

Optical modulation using a depletion width translation mechanism in microdisk devices is investigated. The devices are designed to operate at low drive voltages and have high bandwidths. Analog modulators that operate at drive voltages below 1 V with a 3-dB bandwidth of over 8 GHz were achieved. Eye diagrams showed that the device is capable of operating as a digital switch with speeds up to 10 Gb/s. Since the speed in the case of depletion width modulation is essentially determined by the junction area of the device, it can be improved by further reduction in the active area.

Tunable microdisk resonators vertically coupled to bus waveguides using epitaxial regrowth and wafer bonding techniques

We demonstrate two different types of tunable microdisk resonators vertically coupled to bus waveguides: a microdisk resonator coupled to air-guided rib bus waveguides and the other one built on buried heterostructure bus lines, respectively. The latter type contacts the substrate without relying on an extra supporting structure, which affords more reliable conductive paths toward the substrate. A planarized epitaxial regrowth technology combined with wafer bonding is used to fabricate the structure. The measured transmission spectra indicate improved tuning capability of buried-bus-coupled microdisks, but the overall device performance is limited by optical leakage through the substrate.

High-speed, low-voltage modulation in circular WGM microresonators

Novel approaches to obtain compact, high-speed, low-voltage modulators using circular microresonators are presented. Two different mechanisms, free carrier injection and depletion width modulation are considered. Frequency 3 dB bandwidths of over 8 GHz are achieved at modulation voltages less than 1V.

High-Q vertically-coupled microresonators built by wafer-bonding technique

Summary form only given. We demonstrate high Q vertically coupled passive microdisk resonators based on the InP material system as a first step towards utilizing active devices, and to understand the parameters limiting their performance. Vertical coupling is chosen because it exhibits two major advantages compared to the lateral geometry, namely: i) precise control of the coupling coefficient by epitaxial growth; ii) the material composition of the waveguides and resonator can be optimized and grown independently, which facilitates the design of active microdisk devices - ON/OFF switches, modulators and microdisk lasers.

8-channel tunable MUX/DEMUX using vertically coupled active microdisk resonators

1.6-nm-spaced, 8-channel tunable microdisk demultiplexers are presented. Spectral channel spacing adjustment is achieved by free carrier injection in active microdisk resonators. High Qs (/spl sim/6000) and extinctions (/spl sim/7 dB) are observed in the transmission spectra.

Microresonator devices for DWDM systems

Filters and switches for integrated DWDM subsystems using microresonators vertically coupled to waveguides are described. High Q resonator/waveguides components are fabricated by double-sided processing using wafer bonding. Tunable laser and detector designs are described.

Fabrication of vertically coupled InP microdisk resonators by using smooth, CH/sub 4/-based reactive ion etching methods

Obtaining smooth sidewalls is crucial to suppress the scattering loss of whispering gallery modes along the disk edges and to achieve a high quality factor in microdisk resonators. CH/sub 4/-based reactive ion etching (RIE) is a preferred choice for InP disk formation, due to the sidewall protection afforded by polymers formed in the etching process and the excellent etching selectivity achieved with common dielectric masking materials. We developed a multi-step process RIE involving high-pressure (75 mTorr) RIE conditions for 10 min., followed by a lower pressure (15 mTorr) etch to the completion of the structure. The higher pressure etch yields higher etch rate with less sensitivity to surface conditions but results in severe undercutting during extended etching. The overall etching rate of this process is approximately 0.03 /spl mu/m/min.