Multiframe-based non-local means denoising for Raman spectra.

A method for denoising Raman spectra is presented in this paper. The approach is based on the principle that the original signal can be restored by averaging pixels based on structure similarity. Similarity searching and averaging are not limited to the neighbouring pixels but extended throughout the entire signal range across different frames. This approach is distinguished from the conventional single-frame neighbour pixel-based filtering. The effectiveness and robustness of the proposed method are demonstrated through denoising simulated and experimental Raman data sets with fixed denoising parameters. Several denoised results and statistical indicators are presented for the simulated data. Recovery of the experimental Raman spectrum from our newly developed cost-effective waveguide-enhanced Raman spectroscopy system is also presented and compared to the spectrum from a conventional expensive Raman microscope for the same analyte.

Grating-incoupled waveguide-enhanced Raman sensor.

We report a waveguide-enhanced Raman spectroscopy (WERS) platform with alignment-tolerant under-chip grating input coupling. The demonstration is based on a 100-nm thick planar (slab) tantalum pentoxide (Ta2O5) waveguide and the use of benzyl alcohol (BnOH) and its deuterated form (d7- BnOH) as reference analytes. The use of grating couplers simplifies the WERS system by providing improved translational alignment tolerance, important for disposable chips, as well as contributing to improved Raman conversion efficiency. The use of non-volatile, non-toxic BnOH and d7-BnOH as chemical analytes results in easily observable shifts in the Raman vibration lines between the two forms, making them good candidates for calibrating Raman systems. The design and fabrication of the waveguide and grating couplers are described, and a discussion of further potential improvements in performance is presented.

Waveguide Enhanced Raman Spectroscopy for Biosensing: A Review.

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.

Optimized design for grating-coupled waveguide-enhanced Raman spectroscopy.

We report a new design optimization process for planar photonic waveguides applied to waveguide-enhanced Raman spectroscopy (WERS) that combines the optimization of both the surface intensity performance and the grating coupling efficiency. We consider the impact of film thickness on the grating coupling efficiency of two materials with different refractive indices, namely tantalum pentoxide (Ta2O5) and silicon (Si). We propose a new figure-of-merit (FOM) that takes into account both the coupling efficiency and surface intensity dependence for Raman excitation on the film thickness. Our study shows that the optimum surface-sensitive waveguide thickness is thinner than the optimum coupling efficiency thickness for both material systems. As an example, for a tantalum pentoxide waveguide operating at 785 nm, our optimization strategy proposes a 20% increase in waveguide core thickness relative to the optimum surface-sensitive thickness to achieve the best performance in WERS applications.

Broadband telecom to mid-infrared supercontinuum generation in a dispersion-engineered silicon germanium waveguide.

We demonstrate broadband supercontinuum generation (SCG) in a dispersion-engineered silicon-germanium waveguide. The 3 cm long waveguide is pumped by femtosecond pulses at 2.4 µm, and the generated supercontinuum extends from 1.45 to 2.79 µm (at the -30 dB point). The broadening is mainly driven by the generation of a dispersive wave in the 1.5-1.8 µm region and soliton fission. The SCG was modeled numerically, and excellent agreement with the experimental results was obtained.

Towards nonlinear conversion from mid- to near-infrared wavelengths using Silicon Germanium waveguides.

We demonstrate the design, fabrication and characterization of a highly nonlinear graded-index SiGe waveguide for the conversion of mid-infrared 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.

FWM-based wavelength conversion of 40 Gbaud PSK signals in a silicon germanium waveguide.

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.

Optical properties of silicon germanium waveguides at telecommunication wavelengths.

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.

Phase regeneration of DPSK signals in a highly nonlinear lead-silicate W-type fiber.

We experimentally demonstrate phase regeneration of a 40-Gb/s differential phase shift keying (DPSK) signal in a 1.7-m long highly nonlinear lead silicate W-type fiber using a degenerate two-pump phase-sensitive amplifier (PSA). Results show an improvement in the Error Vector Magnitude (EVM) and a reduction of almost a factor of 2 in the phase noise of the signal after regeneration for various noise levels at the input.

Phase sensitive amplification in a highly nonlinear lead-silicate fiber.

We experimentally demonstrate phase-sensitive amplification in a highly nonlinear and low-dispersion lead-silicate W-type fiber. A phase-sensitive gain variation of 6 dB was observed in a 1.56-m sample of the fiber for a total input pump power of 27.7 dBm.