Si-nanowire optical waveguide devices for optical communications

Abstract

Si-nanowire waveguides are attractive structures for constructing various optical devices that are extremely small, and that can be flexibly connected as devices on a silicon (Si) wafer. The waveguides can be bent with extremely small curvatures of less than a few micrometers of bending radius because the large difference of refractive indices between the Si-core (n = 3.5) and the silica cladding material (n = 1.5) strongly confines the optical power in the waveguide core. Therefore, these waveguides are promising for flexible optical interconnections on Si chips as well as for constructing many extremely small optical devices.We used Si-nanowire waveguides to fabricate optical directional couplers and demonstrated their fundamental characteristics. Their coupling-length was extremely short, several micrometers, because of strong optical coupling between the waveguide cores. Therefore, we could construct ultra compact power combiners/dividers. We have also demonstrated wavelength demultiplexing functions for these devices with a long coupled waveguide. Optical outputs from a device with a 100-µm long coupled waveguide changed reciprocally with 20-nm wavelength spacing between the parallel and cross ports. Ultra small optical add-drop multiplexers (OADMs) with Bragg grating reflectors made of the Si-nanowire waveguides have been also demonstrated. The dropping wavelength bandwidth of the OADMs was less than 0.7 nm, and these dropping wavelengths could be precisely designed by adjusting the grating period, and this indicated the possibility of applying these devices in dense WDM systems.Using the Si-nanowire waveguide, we have also demonstrated nonlinear-optic effects such as the spectral broadening of optical short pulses due to self-phase modulation and nonlinear transmittance based on two-photon absorption. At a 12 W input power level, we observed a 1.5-π nonlinear phase shift as well as strong saturation of optical output power from a 4-mm Si-nanowire waveguide sample, and that indicates the possibility of constructing integrated nonlinearoptic wavelength conversion devices or optical limiters with Si-nanowire waveguides.

Slow light engineering in photonic crystals

“…The dramatic reduction in the transmittance towards the band edge shown in figure 2 is partly a consequence of this decrease in the transmission. Although some structural optimization of the interface structure has been discussed for PC waveguides [23,24], it is generally difficult to completely suppress the loss particularly in the slow light regime. In the case of EIT, an ultra-low temperature of below 1 µK is required to stabilize the energy states of electrons.…”

Nonlinear optical photonic crystal waveguide with TiO2material

“…Recently, integrated optical waveguide devices have been widely applied for the optical communication and biomedical fields [1,2]. Those devices are mainly made of Si or GaAs materials due to the reasons of cost, its high refractive index, or established fabrication process.…”

Novel silicon-on-insulator grating couplers based on CMOS poly-silicon gate layer

“…Besides, the high index contrast between Si and SiO 2 enables it to construct waveguides with submicron cross-sections and very small bending radii, which consequently leads to an ultra-high integration density of photonic devices. Various kinds of optical devices have been demonstrated in recent years based on SOI nanophotonic waveguides [2]. However, the ultra-compact mode size of the waveguides makes light coupling very difficult between the SOI photonic circuits and the outside world (e.g., optical fibers).…”