MEMS-Actuated Silicon Tunable Optical Microresonator

Researchers: Jin Yao, David Leuenberger, M.-C. Mark Lee* (*now a faculty at NTHU, Taiwan)

Funding Agency: CONSRT, DARPA through CS-WDM

The goal of this project is to study high quality factor (Q), tunable, integrated microresoantors. Semiconductor optical microresonators are building blocks for many wavelength-division-multiplexing (WDM) photonic integrated circuits. Their applications include wavelength-division-multiplexing (WDM) photonic integrated circuits such as on-chip channel add-drop filters and wavelength-selective switches, compact nonlinear optical devices, and optical sensors. Adding a tuning mechanism is desired to implement a dynamically reconfigurable function.

Depending on the coupling regime, i.e. critical coupling or over-coupling, the microresonator can be operated as an optical add-drop filter or tunable optical dispersion compensator, respectively (see Fig. 1).

Fig. 1: (a) Schematics of add-drop filter. The resonant wavelength (red) couples to the microresonator and is dropped to the drop port. (b) Schematics of a variable dispersion compensator.

Although thermo-optics, electro-absorption, and free-carrier-injection have been demonstrated to manipulate the optical signal transmission, tuning the power coupling ratio between microresonators and waveguides is attractive for signal processing both in transmission and phase engineering. For most of devices, the coupling is usually fixed. In our study, tunable microresonators integrated with MEMS actuators are proposed for the first time. Variable power coupling ratio between a microresonator and a waveguide can be adjusted by gap spacing, which is controlled by deforming the waveguides, as showed in Fig. 2. Theoretical analysis shows a large tuning range by controlling the gap spacing between the microresonator and the waveguide within 1┬Ám. A novel hydrogen annealing process leads to microdisks with quality factors (Q) in excess of 300,000. With the fabricated devices, we have demonstrated a dynamic add-drop filter with 20 dB extinction ratio and a tunable dispersion compensator with tunable dispersion from 185 ps/nm to 1200 ps/nm.

Figure 2: SEM image of the device. Two waveguides are suspended at the edge of a microdisk. By applying voltage on the electrode, the waveguides deform to vary the gap spacing between the disk and waveguide.

Another application area for microresonators is nonlinear optics. The high quality factor enhances the pump power inside the disk by several orders of magnitude. The high pump power in combination with the small modal area of the whispering gallery modes (0.2 square micron) make microresonators a promising candidate for efficient and compact Raman wave generation (see Fig. 3).

Fig. 3: The depicted microresonator configuration can be used to study nonlinear optical effects at moderate input powers.