Silicon-based integrated circuits have long been used as a platform for microelectronic applications. For example, microprocessors in computers, automobiles, avionics, mobile devices, control and display systems and in all manner of consumer and industrial electronics products are all traditionally based on a silicon platform that facilitates and directs the flow of electricity. As processing requirements have increased, the design of silicon-based integrated circuits has adapted to accommodate for faster processing times and increased communication bandwidths. Primarily, such performance gains have been the result of improvements in feature density, meaning that technologies have been developed to crowd ever-increasing numbers of features such as transistors onto a silicon chip. While efforts to increase feature density continue, alternative methods for increasing processing speeds and bandwidth on silicon-based platforms are also being developed. One such method is known as silicon photonics.
The term “silicon photonics” relates to the study and application of photonic systems that use silicon as an optical medium. Thus, instead of or in addition to using silicon to facilitate the flow of electricity, silicon is used to direct the flow of photons or light. While the speed of electricity and the speed of light are the same, light is able to carry data over a wider range of frequencies than electricity, meaning that the bandwidth of light is greater than that of electricity. Thus, a stream of light can carry more data than a comparable stream of electricity can during the same period of time. Accordingly, there are significant advantages to using light as a data carrier. Furthermore, using silicon as a preferred optical medium allows for application of and tight integration with existing silicon integrated circuit technologies. Silicon is transparent to infrared light with wavelengths above about 1.1 micrometers. Silicon also has a high refractive index of about 3.5. The tight optical confinement provided by this high index allows for microscopic optical waveguides, which may have cross-sectional dimensions of only a few hundred nanometers, thus facilitating integration with current nanoscale semiconductor technologies. Thus, silicon photonic devices can be made using existing semiconductor fabrication techniques, and because silicon is already used as the substrate for most integrated circuits, it is possible to create hybrid devices in which the optical and electronic components are integrated onto a single microchip.
In practice, silicon photonics are implemented using silicon-on-insulator, or SOI, technology. In order for the silicon photonic components to remain optically independent from the bulk silicon of the wafer on which they are fabricated, it is necessary to have an intervening material. This is usually silica, which has a much lower refractive index of about 1.44 in the wavelength region of interest. This results in total internal reflection of light at the silicon-silica interface and thus transmitted light remains in the silicon.
A typical example of data propagation using light is illustrated in FIG. 1. FIG. 1 illustrates an optical transmission system 100 that includes, for example, a silicon waveguide 110. The silicon waveguide may make up the entirety of the optical transmission system 100 or just one or more portions of the system 100. The system includes multiple data input channels 120, where each channel 120 transmits data in the form of pulses of light. In order to simultaneously transmit the data carried on the multiple data channels 120, the light in each channel 120 is modulated by a frequency modulator 130. The modulated light from each channel 120 is then combined into a single transmission channel 150 using an optical multiplexer 140. The multiplexed light is then transmitted along the single transmission channel 150 to an endpoint (not shown) where the light is de-multiplexed and demodulated before being used by an endpoint device.
Transmission of light in an optical waveguide is, however, affected by temperature. In general, changes in temperature can result in changes in the device dimensions (due to thermal expansion) and refractive indices of the materials used in the optical waveguide. More particularly, changes in temperature can affect the operation of the optical frequency modulators 130 illustrated in FIG. 1. Resonant photonic modulators are designed to only modulate received frequencies that are at or close to specific known frequencies. To only allow the modulation of the specific known frequencies, the modulators include resonant structures that act to filter out all but the known frequencies which are to be modulated by the modulators. Thus, the known frequencies are resonant frequencies of the resonant structures. Unfortunately, because the refractive indices of the resonant structures tend to change according to temperature, the specific frequencies that are modulated (i.e., the resonant frequencies) tend to deviate from the known frequencies as the temperature changes. Therefore, there is a need for silicon optical waveguides with modulator circuits that are tolerant of changes in temperature.