We present a systematic experimental study of the linear and nonlinear optical properties of silicon-germanium (SiGe) waveguides, conducted on samples of varying cross-sectional dimensions and Ge concentrations. The evolution of the various optical properties for waveguide widths in the range 0.3 to 2 µm and Ge concentrations varying between 10 and 30% is considered. Finally, we comment on the comparative performance of the waveguides, when they are considered for nonlinear applications at telecommunications wavelengths.

Abstract:In this paper, we present a review on silicon-based nonlinear devices for all optical nonlinear processing of complex telecommunication signals. We discuss some recent developments achieved by our research group, through extensive collaborations with academic partners across Europe, on optical signal processing using silicon-germanium and amorphous silicon based waveguides as well as novel materials such as silicon rich silicon nitride and tantalum pentoxide. We review the performance of four wave mixing wavelength conversion applied on complex signals such as Differential Phase Shift Keying (DPSK), Quadrature Phase Shift Keying (QPSK), 16-Quadrature Amplitude Modulation (QAM) and 64-QAM that dramatically enhance the telecom signal spectral efficiency, paving the way to next generation terabit all-optical networks.

We demonstrate broadband supercontinuum generation (SCG) in a dispersion-engineered silicongermanium waveguide. The 3-cm long waveguide is pumped by femtosecond pulses at 2.4µm and the generated supercontinuum extends from 1.45µm to 2.79µm (at the -30-dB point). The broadening is mainly driven by the generation of a dispersive wave in the 1.5-1.8µm region and soliton fission. The SCG was modelled numerically and excellent agreement with the experimental results was obtained. © Silicon (Si) photonics has witnessed rapid maturity in recent years, mainly due to its potential for high-yield, low-cost CMOS-compatible fabrication of components. At the same time, the high nonlinear refractive index of silicon (n 2 = 4.5x10 −18 m 2 /W), especially when combined with small-dimension, high refractive-index-contrast waveguide geometries that lead to tight mode confinement, makes Si photonic technologies particularly attractive for nonlinear applications. Si-based devices have already been utilized to demonstrate numerous all-optical signal processing applications. Indeed, nonlinear effects such as four-wave mixing (FWM) [1], self-phase modulation (SPM) [2] and Raman amplification [3] have been demonstrated in silicon-on-insulator (SOI) waveguides and nanowires designed for operation in the near-infrared (IR).Silicon is also an excellent candidate for mid-IR applications, due to its transparency up to 8µm and to the reduced two-photon and free-carrier absorptions at wavelengths beyond 2.2µm. Leveraging on these attributes, moderate to high brightness wide-bandwidth laser sources have been demonstrated based on supercontinuum generation (SCG) in this wavelength region, using waveguides fabricated on either crystalline silicon [4,5], amorphous silicon [6] or silicon nitride [7]. Furthermore, the development of on-chip sources providing short pulses has driven research towards integrating both the pump source and nonlinear element on the same chip [8].We have recently reported the first demonstrations of alloptical signal processing using silicon germanium waveguides both in the near-[9] and mid-IR [10]. These demonstrations, along with a detailed study on the optical properties of SiGe waveguides [11], have highlighted that the addition of germanium to silicon can enhance the nonlinear response in comparison to pure silicon, as well as act as an additional valuable design parameter that can impact a host of optical properties (such as linear loss, two-photon absorption (TPA) and dispersion) of the nonlinear waveguide.In this Letter, we have extended the work presented in [12] where we reported the generation of a broadband supercontinuum (SC) in a dispersion-engineered SiGe waveguide. The waveguide was pumped using femtosecond pump pulses at 2.4µm and the 30-dB bandwidth of the SC extended more than 1330 nm spanning both the mid-IR and the entire telecommunication wavelength window, while maintaining high spectral uniformity. A numerical model was also developed to study the SCG, providing excellent agreement with the exper...

Abstract:We demonstrate four wave mixing (FWM) based wavelength conversion of 40 Gbaud differential phase shift keyed (DPSK) and quadrature phase shift keyed (QPSK) signals in a 2.5 cm long silicon germanium waveguide. For a 290 mW pump power, bit error ratio (BER) measurements show approximately a 2-dB power penalty in both cases of DPSK (measured at a BER of 10 −9 ) and QPSK (at a BER of 10 −3 ) signals that we examined.

Waveguide enhanced Raman spectroscopy (WERS) utilizes simple, robust, high-index contrast dielectric waveguides to generate a strong evanescent field, through which laser light interacts with analytes residing on the surface of the waveguide. It offers a powerful tool for the direct identification and reproducible quantification of biochemical species and an alternative to surface enhanced Raman spectroscopy (SERS) without reliance on fragile noble metal nanostructures. The advent of low-cost laser diodes, compact spectrometers, and recent progress in material engineering, nanofabrication techniques, and software modeling tools have made realizing portable and cheap WERS Raman systems with high sensitivity a realistic possibility. This review highlights the latest progress in WERS technology and summarizes recent demonstrations and applications. Following an introduction to the fundamentals of WERS, the theoretical framework that underpins the WERS principles is presented. The main WERS design considerations are then discussed, and a review of the available approaches for the modification of waveguide surfaces for the attachment of different biorecognition elements is provided. The review concludes by discussing and contrasting the performance of recent WERS implementations, thereby providing a future roadmap of WERS technology where the key opportunities and challenges are highlighted.