UC Santa Barbara

Optical modulators have been an integral part of fiber optic communication link. Low drive voltage, wide electrical bandwidth, low optical propagation loss, high extinction ratio, easier fabrication and low cost are some of desired properties of an optical modulator. Silicon, III-V and lithium niobate are the most prominent used material systems for modulators. Silicon modulators, being attractive in terms of the substrate cost and mature fabrication process, are limited in using only the free-carrier plasma dispersion effect for designing optical modulators as opposed to Pockel’s effect and Quantum confined stark effect used in III-V based modulators which would enable one to design even more efficient devices.

In the past years, heterogeneous approach enabled to integrate III-V based active photonic devices on to Silicon using bonding. However, the size and the cost of III-V substrate limits scaling the approach over bigger silicon wafers. To overcome that and to integrate efficient III-V based photonic devices on much bigger silicon wafers, in recent years, there has been efforts carried out to grow GaAs/AlGaAs material system directly on Silicon substrates. High performance GaAs material system-based quantum dot lasers @ 1.3 μm have been demonstrated with this technique. This approach can overcome the size limitation associated with III-V based substrates.

GaAs and its lattice matched layers exhibit Pockel’s effect and could be used to design modulators on silicon substrate. In this work, for the first time, experimental proof of Pockel’s effect on GaAs/AlGaAs layers directly grown on silicon substrates is reported. GaAs/AlGaAs MZM’s @ 1.55 μm grown on mis-cut silicon substrates had an electrical design equivalent to NPIN diode. Device demonstrated a low propagation loss of 1.5±0.5 dB/cm and a VπL product of 1.5±0.1 V.cm. Vπ of a 4 mm MZM was 3.6 V.

Challenges in designing high speed MZM’s on silicon substrate is then analyzed and a modified microstrip electrode design with significant improvement in terms of electrical loss

(5 dB/cm @ 67 GHz) and dispersion as opposed to the widely used coplanar electrodes is demonstrated. These electrodes were then integrated with GaAs MZM as a traveling wave design. 3 dB and 6 dB EO bandwidth of 4 mm MZM were 9.6 GHz and 18.5 GHz respectively. A modified modulator design is proposed to improve the electrical bandwidth of the modulator along with the simulation results. Preliminary experimental results on the drive voltage and propagation loss of the modified design on on-axis silicon wafer is presented. Device demonstrated a VπL product of 1.745±0.035 V.cm and propagation loss of less than 3 dB/cm. 8 mm MZM demonstrated a low drive voltage of 2.2 V under push pull. Finally, compact GaAs based waveguide on Silicon using selective oxidation of high Aluminum content AlGaAs layers is reported with a low propagation loss of 4 dB/cm.