Integrated Nonlinear Photonics CNF Project # 1552 - 07 Principal Investigator ( s ) : Michal Lipson

A waveguide-coupled multiple-wavelength source can form the backbone of a fully operational on-chip optical network. Such networks can augment the computational capability of multi-core microprocessors [1]. Silicon photonics provides a viable platform for enormous bandwidths on a microelectronic chip by utilizing multiple wavelengths for a wavelength division multiplexing (WDM) scheme. However, a fully integrated multiple wavelength light source has not yet been realized. We demonstrate a multiple wavelength source using a CMOS-compatible silicon nitride ring resonator coupled to a waveguide. The device generates up to 146 new wavelengths from a single wavelength input. Summary of Research: We use four-wave mixing (FWM) parametric amplification in a small cavity to generate new wavelengths. Similar to lasing, optical parametric oscillation occurs when the roundtrip parametric gain exceeds the loss in a cavity. The FWM process typically requires high optical power and a long interaction length, but using a high quality factor (Q) optical resonator reduces the power requirement and device footprint. Due to the energy conservation required by FWM, the generated fields will be resonant only if the cavity modes are equally spaced in frequency. Proper choice of the device cross section enables us to tailor the group-velocity dispersion and achieve constant mode spacing [2]. High-Q micro-toroids [3], and microspheres [4], in silica have shown parametric oscillation. However, since silica possesses a low nonlinearity, the Q needed for operation is extremely high. Therefore, these devices are extremely sensitive to perturbations and not conducive to on-chip integration since their operation requires a purged N2 environment and specially tapered optical fibers for coupling. Silicon nitride, the material of choice here, has recently been shown to have a nonlinear refractive index about an order of magnitude larger than silica [5]. Unlike silicon, silicon nitride does not suffer from nonlinear losses due to two-photon absorption and free-carrier absorption since the band gap is much larger than silicon and carriers are not produced. Silicon nitride is a CMOS-compatible material with a linear refractive index of 1.98 at 1550 nm enabling the design of highindex contrast waveguides with a silicon dioxide cladding. Until recently, the thickness of low-loss silicon nitride waveguides had been restricted to < 250 nm due to tensile stress in the nitride film. Such thin films are poor for nonlinear optics since the waveguide mode is very large and delocalized from the material of interest. Here, by growing thicker films using a thermal cycling process, we achieve thicker waveguides and are able to tailor the waveguide dimensions and optimize them for nonlinear interactions. We demonstrate parametric oscillation in cavities of two different radii. The cavities used are micro-ring resonators (Figure 1) coupled to a channel waveguide. The first measured resonator has a 20 μm radius with a cross-section of 727 × 1580 nm. The quality factor of the device is 100,000, and the free spectral range (FSR) is 9.4 nm. We use a tunable laser initially centered at 1560.7 nm and amplify the emitted light using a high power EDFA. After coupling the pump into the waveguide, we carefully tune the laser into the cavity resonance. We collect the output from the waveguide with a fiber and view the results with an optical spectrum