1. Field of the Invention
The present invention relates generally to semiconductor devices, and, more specifically, to loss modulated silicon evanescent lasers.
2. Description of the Related Art
Semiconductor chip level bonded devices have found uses in several consumer and commercial applications. Typically, semiconductor devices are made from a single type of material, or different types of material are grown onto a substrate based on lattice matching and compatible crystalline structures. As such, devices manufactured from semiconductor materials from Groups III and V of the periodic table materials) are typically grown on gallium arsenide or other compound semiconductor substrates, while silicon devices are grown or fabricated on silicon substrates. III-V material-based devices are difficult to integrate with electronic devices fabricated on silicon because of lattice mismatches and incompatible crystalline structures between silicon and III-V materials.
Optical transmitters are one of the most important components of any optical communication system. Typically, optical transmitters are fabricated with semiconductor materials from Groups III and V (III-V) of the periodic table, e.g., Gallium Arsenide (GaAs). Such materials are typically used because Silicon (Si), typically used for electronic communication systems, has an indirect bandgap which makes silicon a poor photon (light) emitter, and thus silicon does not perform well in optical transmitter applications. The indirect bandgap and resultant poor light emission of silicon has limited the realization of an electrically pumped Si-based laser, which is one of the key elements for optical transmitters and Si-based optical communications systems.
Silicon is a preferred semiconductor material, because silicon is easily processed in a variety of ways, is readily available at high quality for reasonable cost, and complex supporting electronic circuits for communications systems are readily available in silicon. In recent years, silicon photonic devices (e.g., silicon devices that emit photons) have been extensively studied because of the potential for low-cost optoelectronic solutions. Silicon photonic devices would reduce device cost over III-V materials because the fabrication is compatible with silicon-based electronics, especially Complementary Metal-Oxide Semiconductor (CMOS) processing.
Recently, hybrid structures combining III-V active region and silicon optical waveguides have been demonstrated as a solution for electrically pumped Si-based lasers. Such devices have many desirable properties, including continuous wave laser output (lasing) at temperatures up to 105° C., continuous wave output powers up to 30 mW, and mode locking at 40 Gbit/s. Such hybrid III-V silicon “evanescent” structures comprise a III-V quantum-well region bonded to a Silicon-On-Insulator (SOI) wafer, with optical waveguides defined by trenches at the Si layer. In this way, the hybrid structure behaves like an inverse ridge waveguide. Such devices are called “evanescent” in that the transition between the III-V structure and the silicon structure within the device tends to vanish after bonding, as the optical mode of the device crosses the III-V/silicon boundary.
Although such lasers are now possible, for an optical transmitter to be realized with such devices, high-speed modulation of the optical signal is also desirable. Direct modulation of the injected pump current is a simple approach to such a problem, however, as the injection current is increased, the extinction ratio decreases, thus limiting the amount of injection current that can be applied. Further, direct modulation is limited in speed to typically less than 10 GHz, and the transmission distance of directly modulated signals is limited because of the wavelength “chirp” induced by the direct modulation. Direct modulation of the current also changes the gain of the laser, which causes the light output of the laser to vary, all of which are not desirable device characteristics in communications systems.
External modulators have also been evaluated to determine the extension of the modulation rate and transmission distance. Mach-Zender Interferometric (MZI) modulators, which are large (millimeter size) devices have been shown to increase modulation rate and transmission distance through carrier depletion in the waveguide, which introduces a fast refractive index modulation; however, such modulators are difficult to integrate with silicon and/or hybrid devices because of their large relative size.
It can be seen, then, that there is a need in the art for a silicon-based laser that can be used in optical transmission systems. It can also be seen that there is a need in the art for integration between a silicon-based laser and a modulator. It can also be seen that there is a need in the art for modulation techniques that can be more easily integrated with semiconductor laser devices.