Observation of spectral self-imaging by nonlinear parabolic cross-phase modulation.

We report an experimental demonstration of spectral self-imaging on a periodic frequency comb induced by a nonlinear all-optical process, i.e., parabolic cross-phase modulation in a highly nonlinear fiber. The comb free spectral range is reconfigured by simply tuning the temporal period of the pump parabolic pulse train. In particular, undistorted FSR divisions by factors of 2 and 3 are successfully performed on a 10 GHz frequency comb, realizing new frequency combs with an FSR of 5 and 3.3 GHz, respectively. The pump power requirement associated to the SSI phenomena is also shown to be significantly relaxed by the use of dark parabolic pulses.

Self-locked optical parametric oscillation in a CMOS compatible microring resonator: a route to robust optical frequency comb generation on a chip.

We report a novel geometry for OPOs based on nonlinear microcavity resonators. This approach relies on a self-locked scheme that enables OPO emission without the need for thermal locking of the pump laser to the microcavity resonance. By exploiting a CMOS-compatible microring resonator, we achieve oscillation featured by a complete absence of "shutting down", i.e. the self-terminating behavior that is a very common and detrimental occurrence in externally pumped OPOs. Further, our scheme consistently produces very wide bandwidth (>300nm, limited by our experimental set-up) combs that oscillate at a spacing equal to the FSR of the micro cavity resonance.

High-efficiency second-harmonic generation in doubly-resonant χ(²) microring resonators.

By directly simulating Maxwell's equations via the finite-difference time-domain (FDTD) method, we numerically demonstrate the possibility of achieving high-efficiency second harmonic generation (SHG) in a structure consisting of a microscale doubly-resonant ring resonator side-coupled to two adjacent waveguides. We find that ≳ 94% conversion efficiency can be attained at telecom wavelengths, for incident powers in the milliwatts, and for reasonably large bandwidths (Q ∼ 1000s). We demonstrate that in this high efficiency regime, the system also exhibits limit-cycle or bistable behavior for light incident above a threshold power. Our numerical results agree to within a few percent with the predictions of a simple but rigorous coupled-mode theory framework.

Lemming winter habitat choice: a snow-fencing experiment.

The insulative value of early and deep winter snow is thought to enhance winter reproduction and survival by arctic lemmings (Lemmus and Dicrostonyx spp). This leads to the general hypothesis that landscapes with persistently low lemming population densities, or low amplitude population fluctuations, have a low proportion of the land base with deep snow. We experimentally tested a component of this hypothesis, that snow depth influences habitat choice, at three Canadian Arctic sites: Bylot Island, Nunavut; Herschel Island, Yukon; Komakuk Beach, Yukon. We used snow fencing to enhance snow depth on 9-ha tundra habitats, and measured the intensity of winter use of these and control areas by counting rodent winter nests in spring. At all three sites, the density of winter nests increased in treated areas compared to control areas after the treatment, and remained higher on treated areas during the treatment. The treatment was relaxed at one site, and winter nest density returned to pre-treatment levels. The rodents' proportional use of treated areas compared to adjacent control areas increased and remained higher during the treatment. At two of three sites, lemmings and voles showed significant attraction to the areas of deepest snow accumulation closest to the fences. The strength of the treatment effect appeared to depend on how quickly the ground level temperature regime became stable in autumn, coincident with snow depths near the hiemal threshold. Our results provide strong support for the hypothesis that snow depth is a primary determinant of winter habitat choice by tundra lemmings and voles.

Habitat selection, reproduction and predation of wintering lemmings in the Arctic.

Snow cover has dramatic effects on the structure and functioning of Arctic ecosystems in winter. In the tundra, the subnivean space is the primary habitat of wintering small mammals and may be critical for their survival and reproduction. We have investigated the effects of snow cover and habitat features on the distributions of collared lemming (Dicrostonyx groenlandicus) and brown lemming (Lemmus trimucronatus) winter nests, as well as on their probabilities of reproduction and predation by stoats (Mustela erminea) and arctic foxes (Vulpes lagopus). We sampled 193 lemming winter nests and measured habitat features at all of these nests and at random sites at two spatial scales. We also monitored overwinter ground temperature at a subsample of nest and random sites. Our results demonstrate that nests were primarily located in areas with high micro-topography heterogeneity, steep slopes, deep snow cover providing thermal protection (reduced daily temperature fluctuations) and a high abundance of mosses. The probability of reproduction increased in collared lemming nests at low elevation and in brown lemming nests with high availability of some graminoid species. The probability of predation by stoats was density dependent and was higher in nests used by collared lemmings. Snow cover did not affect the probability of predation of lemming nests by stoats, but deep snow cover limited predation attempts by arctic foxes. We conclude that snow cover plays a key role in the spatial structure of wintering lemming populations and potentially in their population dynamics in the Arctic.

Extremely high aspect ratio GaAs and GaAs/AlGaAs nanowaveguides fabricated using chlorine ICP etching with N2-promoted passivation.

Semiconductor nanowaveguides are the key structure for light-guiding nanophotonics applications. Efficient guiding and confinement of single-mode light in these waveguides require high aspect ratio geometries. In these conditions, sidewall verticality becomes crucial. We fabricated such structures using a top-down process combining electron beam lithography and inductively coupled plasma (ICP) etching of hard masks and GaAs/AlGaAs semiconductors with Al concentrations varying from 0 to 100%. The GaAs/AlGaAs plasma etching was a single-step process using a Cl(2)/BCl(3)/Ar gas mixture with various fractions of N(2). Scanning electron microscope (SEM) observations showed that the presence of nitrogen generated the deposition of a passivation layer, which had a significant effect on sidewall slope. Near-ideal vertical sidewalls were obtained over a very narrow range of N(2), allowing the production of extremely high aspect ratios (>32) for 80 nm wide nanowaveguides.

Supercontinuum generation in a high index doped silica glass spiral waveguide.

We demonstrate supercontinuum (SC) generation at both 1550 nm and 1288 nm in a compact (< 5mm(2)) 45 cm spiral waveguide composed of CMOS-compatible doped high-index glass. While both wavelengths have weak dispersion and are near zero dispersion points, they present different symmetries. At 1550 nm, the normal dispersion regime takes place at longer wavelengths, whereas at 1290 nm it is at shorter wavelengths, and we observe features in the SC spectra that clearly reflect this. In particular, the spectrum at 1550 nm is more than 300 nm wide (limited by detection) and is well reproduced by simulations based on the measured dispersion. This work represents a practical on-chip broadband wavelength source with potential use in many important applications.

Hybrid integration of Ca 0.28 Ba 0.72 Nb2O6 thin film electro-optic waveguides with silica/silicon substrates.

Ca(0.28)Ba(0.72)Nb(2)O(6) (CBN-28) waveguides based on thin film technology were fabricated on SiO(2)/(100) Si substrates. By using X-ray diffraction, we confirmed the preferential c-axis orientation of the CBN structures. An effective unclamped electro-optic r33 coefficient of 12 pm/V was measured in CBN thin films by using an ellipsometric technique in reflection geometry. In addition, by means of a Fabry-Perot technique, the propagation losses of our strip loaded waveguides were estimated to be as low as 4.8 dB/cm and 6.5 dB/cm at telecommunication wavelengths for the fundamental TE and TM modes, respectively.

Efficient self-phase modulation in low loss, high index doped silica glass integrated waveguides.

We demonstrate efficient self phase modulation, as well as negligible nonlinear absorption, in low loss (<0.06 dB/cm), high index silica glass-based waveguides. Using approximately 1 ps pulses near 1560 nm we achieve a 1.5pi nonlinear phase shift in an integrated 45 cm long spiral waveguide with <60 W of peak input power, corresponding to a large nonlinearity (gamma) of 220 W(-1)km(-1). Further, we observe negligible nonlinear absorption for input intensities > 25 GW/cm(2). The high nonlinearity and low linear and nonlinear losses of these waveguides make them promising for nonlinear all-optical signal processing applications.

Picosecond linear optical pulse shapers based on integrated waveguide Bragg gratings.

We demonstrate a general linear pulse-shaping technique based on integrated III-V Bragg gratings (BGs). Such a technique allows for the synthesizing of complex waveforms with picosecond resolution using a compact single-waveguide design. This approach is experimentally demonstrated by fabricating and testing a series of integrated ultrafast optical pulse shapers based on BG geometries acting as time-domain code generators operating at 500 Gbits/s.