Advanced Materials for Integrated Optical Waveguides by Xingcun Colin Tong Ph.D

Author:Xingcun Colin Tong Ph.D
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

6.8.3 Linear Applications

Using low-loss SOI waveguides, passive devices including wavelength filters, MMI couplers, and WDM filters have been developed, which can be in the configuration of Mach–Zehnder interferometers, Bragg gratings, ring resonators and photonic-crystal waveguides. Other than passive devices, active devices have also been developed to control the flow of light, including modulators and switches. The basic idea is to change the transmission properties of the device through a change in the refractive index. Modulation of the refractive index of silicon can be done using either the thermo-optic effect or the electro-optic effect. However, the thermo-optic effect is relatively slow and can only be used for up to 1-MHz modulation frequencies (Lipson 2005). For high-speed modulation, electro-optic devices are required. However, pure crystalline silicon does not exhibit a linear electro-optic effect. An effective mechanism for changing the refractive index in silicon at a high speed is the free-carrier plasma-dispersion effect. The refractive index of silicon varies with the free-carrier concentration, which can be manipulated through injecting or depleting carriers by applying an electric field to the device. However, FCA is a disadvantage associated with this technique. For instance, an MOS configuration embedded in silicon waveguides is used in a Mach–Zehnder geometry and able to demonstrate high-frequency modulation with 3 dB bandwidth of 20 GHz and data transmission up to 30 Gb/s (Liu et al. 2007). This performance is limited by the free-carrier’s lifetime. The high speed is the result of a unique device design with a traveling-wave drive scheme allowing electrical and optical signal co-propagation along the waveguide. The traveling-wave electrode which is based on a coplanar waveguide was designed to match the velocity for both optical and electrical signals. The speed was pushed even further up to 40 Gb/s by optimizing the device packaging (Yin 2009).

Although a lot of optical devices have been developed with the silicon platform, a room-temperature, electrically pumped, silicon laser is still the missing piece for monolithic integration because silicon has a poor stimulated emission cross-section due to its indirect band gap structure. There have been many efforts toward the goal of silicon lasers. Silicon light-emitting diodes have also been demonstrated. Another effort was carried out on the hybrid AlGaInAs-silicon evanescent laser. The device is comprised of a multiple-quantum-well structure bonded to a silicon waveguide on the SOI wafer. The optical mode overlaps both the III-V material and the silicon waveguide so that the mode can obtain electrically pumped gain from the III-V region while being guided by the silicon waveguide region. The device showed room temperature continuous-wave (CW) lasing with 65 mA threshold, 1.8 mW output power and overall differential efficiency of 12.7 %. Meanwhile, mode-locked silicon evanescent lasers have also been demonstrated. Besides silicon-based light sources, an integrated germanium-on-silicon detector has also been developed, which can operate at the speed of 40 Gb/s (Yin et al. 2007).

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