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Byline: A. Melloni, R. Costa, G. Cusmai, F. Morichetti The refractive index contrast plays a fundamental role in determining the characteristics of an optical dielectric waveguide and a higher index contrast permits to move towards larger scales of integration and to access new devices and functionalities. In this paper, the impact of the index contrast on the characteristics of dielectric waveguides such as single mode regime, losses, fibre to waveguide coupling efficiency, minimum bending radius, mode hybridness, birefringence, polarisation effects, repeatability and stability, scale of integration, realisable circuits, technological constraints and available materials are investigated. Both classical step index waveguides and multilayer structures that engineers the contrast are considered.

Sidewall roughness in optical waveguides represents a severe impairment for the proper functionality of photonic integrated circuits. The interaction between the propagating mode and the roughness is responsible for both radiative losses and distributed backscattering. In this paper, a unified vision on these extrinsic loss phenomena is discussed, highlighting the fundamental role played by the sensitivity of the effective index neff of the optical mode to waveguide width variations. The nw model presented applies to both 2D slab waveguides and 3D laterally confined waveguides and is in very good agreement with existing models that individually describe radiative loss or backscattering only. Experimental results are presented, demonstrating the validity of the nw model for arbitrary waveguide geometries and technologies. This approach enables an accurate description of realistic optical waveguides and provides simple design rules for optimization of the waveguide geometry in order to reduce the propagation losses generated by sidewall roughness. [ABSTRACT FROM AUTHOR]

We present an overview of the main achievements obtained in 2013 on the monitoring, stabilization, and feedback loop control of passive and active photonic integrated circuits. Key advances contributed to the evolution of photonic technologies from the current device level toward complex, adaptive, and reconfigurable integrated circuits. [ABSTRACT FROM PUBLISHER]

Extraction of a single channel Brillouin stokes line generated by pumping a length of single mode fibre is demonstrated using a silicon oxynitride microring resonator. The high Q-factor microring resonator has a steep slope transmission profile with an extinction ratio of 20 dB and a free spectral range of 0.4 nm. Experimental results show that extraction of a single Brillouin Stokes line can be achieved with a side mode suppression ratio of more than 12 dB. Additionally, Brillouin Stokes line selection can be achieved by controlling the microring resonance wavelength by thermal tuning. [ABSTRACT FROM AUTHOR]

We present a method to enhance the sensitivity of interferometer based techniques employed for the in-band measurement of optical signal-to-noise ratio (OSNR) in wavelength division multiplexed (WDM) optical transmission systems. With respect to conventional schemes, a narrow band filter (NBF) is added before the variable delay interferometer performing the autocorrelation measurement. The NBF is used to select within the signal spectrum only those frequencies where the signal power spectral density (PSD) is locally lower and more sensitive to noise effects. Performance monitoring of a 10 Gb/s on-off keying (OOK) non-return to zero (NRZ) signal is numerically and experimentally investigated for an OSNR level ranging from 5 dB to 25 dB. Design criteria are pointed out for the bandwidth and the detuning of the NBF with respect to the signal carrier wavelength, as well as for the delay of the interferometer, in order to maximize the system sensitivity to OSNR variations. Experimental results, achieved by using an integrated ring-resonator NBF, demonstrate that presented technique enables to effectively detect small noise variations at high OSNR levels (> 15~\ dB), where conventional interferometer based techniques exhibit a poor sensitivity. Moreover we demonstrate that the NBF can operate directly on the transmitted optical signal without introducing any significant perturbation, this making the presented technique suitable for in-line OSNR measurements. [ABSTRACT FROM PUBLISHER]

A novel class of optical waveguides with a box-shaped cross section consisting of a low-index inner material surrounded by a thin high-index coating layer is presented. This original multilayered structure widens the traditional concept of index contrast for dielectric waveguides toward a more general concept of effective index contrast, which can be artificially tailored over a continuous range by properly choosing the thickness of the outer high-index layers. An electromagnetic analysis is reported, which shows that the transverse electric and transverse magnetic modes are spatially confined in different regions of the cross section and exhibit an almost 90degC rotational symmetry. Such unusual field distribution is demonstrated to open the way to new intriguing properties with respect to conventional waveguides. Design criteria are provided into details, which mainly focus on the polarization dependence of the waveguide on geometrical parameters. The possibility of achieving single-mode waveguides with either zero or high birefringence is discussed, and the bending capabilities are compared to conventional waveguides. The feasibility of the proposed waveguide is demonstrated by the realization of prototypal samples that are fabricated by using the emerging CMOS- compatible Si3N4-SiO2 TriPleX technology. An exhaustive experimental characterization is reported, which shows propagation loss as low as state-of-the-art low-index-contrast waveguides (< 0.1 dB/cm) together with enhanced flexibility in the optimization of polarization sensitivity and confirms the high potentialities of the proposed waveguides for large-scale integrated optics. [ABSTRACT FROM PUBLISHER]

In this paper, polarization rotation due to optical waveguide bending is analytically and numerically investigated. It is shown that the process of polarization conversion in the bend strictly depends on the characteristics of the straight waveguide. The strength of polarization coupling linearly increases with the waveguide refractive index contrast, whereas birefringence is responsible for the phase mismatch that limits the conversion efficiency. Waveguides with refractive index profile symmetrical in the direction orthogonal to the plane of the bend do not exhibit polarization rotation. Inside optical ring resonators, polarization rotation induced by waveguide bending is found to be highly enhanced by the cavity effect, which gives rise to strongly frequency dependent polarization rotation and dramatically alters the ring's spectral response. In the case of single-ring phase shifters, polarization rotation is effectively described by means of an approach based on the theory of poles-zeros loci, which straightforwardly provides conditions for complete polarization conversion, also in the presence of birefringence and losses. The detrimental effects caused by undesired polarization coupling on the spectral response of widespread optical devices based on ring resonators are also discussed. From this point of view, birefringence is demonstrated to be beneficial to make optical components less sensitive to the effects of polarization rotation. [ABSTRACT FROM PUBLISHER]

The effects of backscattering induced by waveguide sidewall roughness in integrated ring resonators (RRs) are experimentally observed. We demonstrate that coherent backscattering, originated by multiple round trips in the RR, increases with the square of the effective group index of the cavity and can dramatically affect the behavior of integrated RRs even at moderate quality factors of 104. From our results backscattering emerges as one of the most severe limiting factors on the performance of RRs fabricated with state-of-the-art silicon-on-insulator nanowaveguides. [ABSTRACT FROM AUTHOR]