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.

Forearm crutches are frequently used in the rehabilitation of an injury to the lower limb. The recovery rate is improved if the patient correctly applies a certain fraction of their body weight (specified by a clinician) through the axis of the crutch, referred to as partial weight bearing (PWB). Incorrect weight bearing has been shown to result in an extended recovery period or even cause further damage to the limb. There is currently no minimally-invasive tool for long-term monitoring of a patient's PWB in a home environment. This paper describes the research and development of an instrumented forearm crutch that has been developed to wirelessly and autonomously monitor a patient's weight bearing over the full period of their recovery, including its potential use in a home environment. A pair of standard forearm crutches are augmented with low-cost off-the-shelf wireless sensor nodes and electronic components to provide indicative measurements of the applied weight, crutch tilt, and hand position on the grip. Data are wirelessly transmitted between crutches and to a remote computer (where they are processed and visualized in LabVIEW), and the patient receives biofeedback by means of an audible signal when they put too much or too little weight through the crutch. The initial results obtained highlight the capability of the instrumented crutch to support physiotherapists and patients in monitoring usage. Keywords assistive healthcare, forearm crutches, instrumented objects, biomedical engineering, patient monitoring, biofeedback IntroductionForearm crutches are used routinely following many operations to the lower limb (including the repair of fractures and the fixation of implants) in order to reduce weight-bearing through the affected limb and optimize the healing conditions for bone and soft tissues. It is widely recognized that excessive loading of the lower limb following certain types of surgery can disrupt the operated tissues and put the healing bones at risk of mal-union, while mobilisation soon after surgery increases the bone turnover metabolism and stimulates bone growth [1]. It has also been recognized that prolonged unloading of the articular cartilage causes the cartilage to become less stiff and less able to tolerate high loads [2]. Therefore, a programme of protective partial weight bearing (PWB) usually begins immediately after certain types of surgery and continues until full weight bearing is achieved at a time when there is sufficient healing in the limb. The level of PWB prescribed by the clinician (ranging from non weight bearing to full weight bearing) is dependent upon the severity and nature of the injury, the method of surgical intervention, and stage in the healing process [3]. It is critical that the patient follows this program in order to expedite the rehabilitation period and avoid further and long-term damage to the affected limb.To ensure that the patient loads their affected limb at the prescribed level, they receive PWB training from a clinician before they are discha...

Abstract:We demonstrate the design, fabrication and characterization of a highly nonlinear graded-index SiGe waveguide for the conversion of midinfrared signals to the near-infrared. Using phase-matched four-wave mixing, we report the conversion of a signal at 2.65 µm to 1.77 µm using a pump at 2.12 µm. "Mid-infrared wavelength conversion in silicon waveguides using ultracompact telecom-band-derived pump source," Nat. Photonics 4(8), 561-564 (2010). ©2014 Optical Society of America

We report the fabrication of a tellurite optical fiber with a suspended core design, formed on a 220 nm wide filament of glass. The fiber was pumped at two different wavelengths (1500 nm and 2400 nm) using femtosecond pulses generated from an optical parametric oscillator (OPO) in order to produce mid-infrared supercontinuum (SC). We observed that SC spectra extending to 3 µm were readily generated. To further optimize the design detailed numerical study was performed which revealed how the fiber structural characteristics dramatically influence the spectral broadening because of the changes in the dispersion profile and in turn, the interplay of nonlinear effects that give rise to SC generation. We found that an accurate control of the core shape can be employed to contain the generated SC spectra within well-defined spectral regions or to provide a broad extension of the continuum to beyond 4 µm.Fiber-based supercontinuum (SC) sources have an attractive combination of high brightness and broad bandwidth, which makes them ideal sources for spectroscopy [1] and if the coherence is high, then also for metrology [2]. Supercontinuum generation (SCG) across the mid-infrared (mid-IR) has enabled the molecular fingerprinting of organic compounds leading to a host of cross-disciplinary applications in the fields of sensing [3], cosmetic product inspection [4], protein structure derivation [5] and detection of biological species [6,7]. To extend the wavelength range beyond the transparency window of silica, mid-IR SCG has often been based on soft glasses [8] such as chalcogenide [9], tellurite (Te) [10,11], lead silicate [12] and fluoride [13]. Microstructuring has been used to engineer the zero-dispersion wavelength (ZDW), which is typically in the mid-IR for soft-glasses, to match the wavelength of widely available near-IR pulsed lasers. However, because of the narrow temperature range of the glass transition in soft glasses, the fibers are challenging to fabricate when compared to the now established procedures used to fabricate silica microstructured fibers. For this reason, as well as for the rather low mechanical robustness of these glasses, the innovation in Te and chalcogenide fiber designs has been quite limited as compared to silica fibers [14][15][16]. In this paper, we present experimental and modelling results showing SCG in a novel Te fiber with an elongated core on a thin filament of glass as shown by the scanning electron microscope (SEM) image in Fig. 1. The preform was fabricated using the extrusion method [17,18] and was then drawn into the final fiber using a conventional fiber drawing tower. The fiber has high birefringence for compatibility with planar waveguides, and the shape of the fiber core is controlled both by the shape of the extrusion die and by surface tension effects during fiber drawing. Hence, there is more design freedom than in socalled wagon-wheel fibers [10,19] where surface tension effects between the core and multiple supporting filaments govern the core shape. Alongside the expe...