High-Q distributed-Bragg-grating laser cavities

Applying Bragg gratings in Al2O3 channel waveguides, we demonstrate distributed Bragg reflectors with Q-factors of 1.02x10e6. An integrated Al2O3:Yb3+ waveguide laser with 67% slope efficiency and 47 mW output power is achieved with such cavities

The diurnal evolution of the urban heat island of Paris: a model-based case study during Summer 2006

The urban heat island (UHI) over Paris during summer 2006 was simulated using the Advanced Regional Prediction System (ARPS) updated with a simple urban parametrization at a horizontal resolution of 1 km. Two integrations were performed, one with the urban land cover of Paris and another in which Paris was replaced by cropland. The focus is on a five-day clear-sky period, for which the UHI intensity reaches its maximum. The diurnal evolution of the UHI intensity was found to be adequately simulated for this five day period. The maximum difference at night in 2 m temperature between urban and rural areas stemming from the urban heating is reproduced with a relative error of less than 10%. The UHI has an ellipsoidal shape and stretches along the prevailing wind direction. The maximum UHI intensity of 6.1 K occurs at 23:00 UTC located 6 km downstream of the city centre and this largely remains during the whole night. An idealized one-column model study demonstrates that the nocturnal differential sensible heat flux, even though much smaller than its daytime value, is mainly responsible for the maximum UHI intensity. The reason for this nighttime maximum is that additional heat is only affecting a shallow layer of 150 m. An air uplift is explained by the synoptic east wind and a ramp upwind of the city centre, which leads to a considerable nocturnal adiabatic cooling over cropland. The idealized study demonstrates that the reduced vertical adiabatic cooling over the city compared to cropland induces an additional UHI build-up of 25%. The UHI and its vertical extent is affected by the boundary-layer stability, nocturnal low-level jet as well as radiative cooling. Therefore, improvements of representing these boundary-layer features in atmospheric models are important for UHI studies

Alignment issues in photonic crystal device fabrication

An important requirement in the fabrication of photonic crystal structures is the correct relative alignment of structural elements. Accuracy should be in the order of some tens of nanometres. Some of the options for providing such accuracy are discussed. Examples are given of aligning defects with respect to a predefined 2D lattice, aligning access waveguides with respect to a small local photonic crystal structure, and the alignment of successive periodically structured layers in a 3D "woodpile" structure

Effect of fruit-to-leaf area ratio on fruit quality and vegetative growth of 'bing' sweet cherry trees at optimal leaf area index

Fruit yield and quality determine grower income from commercial sweet cherry orchards. The objective of this study was to determine the effect of Fruit Number to Leaf Area Ratio (FNLAR, fruit m-2 LA) on Mean Fruit Weight (MFW), firmness (F), soluble solids content (SSC), titratable acidity (TA) and SSC:TA ratio of `Bing¿ sweet cherries trees of near-optimal leaf area index (LAI). The effect of FNLAR on Mean Shoot Growth (MSG) and trunk cross-sectional area increment (TCSAI) also was analysed to determine possible competition between reproductive and vegetative growth. Regression analysis was used with FNLAR as the independent variable. While SSC:TA, MSG and TCSAI were not significantly correlated to FNLAR (P>0.05), MFW, TA and SSC decreased linearly with increasing FNLAR (

Towards spectral-domain optical coherence tomography on a silicon chip

Optical coherence tomography (OCT) is a widely used optical imaging technology, particularly in the medical field, since it can provide non-invasive, sub-micrometer resolution diagnostic images of tissue. Current OCT systems contain optical fibers and free-space optical components which make these instruments bulky and costly. A significant decrease in the size and cost of an OCT system is possible through the use of integrated optics, allowing for compact and low-cost OCT systems, especially suited for applications in which instrument size may play an important role. In this work, we present a miniaturized spectral-domain OCT (SD-OCT) system. We design an arrayed waveguide grating (AWG) spectrometer in silicon oxynitride for the 1300-nm spectral range. The spectral range of the SD-OCT system near 1300 nm is specifically selected for skin imaging. We aim at 18-μm depth resolution (determined by the full width at half maximum values of the transmission spectrum of the AWG) and a 1-mm depth range (determined by the wavelength spacing per output waveguide). The free spectral range of 78 nm and wavelength resolution of 0.4 nm of the AWG are determined to meet these requirements. We use ahe fiber-based SD-OCT system with AWG spectrometer. The Michelson interferometer is illuminated using a superluminescent diode which has a Gaussian-like spectrum with a bandwidth of 40 nm and a central wavelength of 1300 nm. Via a circulator the light is coupled into a 90/10 beamsplitter. Polarization controllers are placed into both, sample and reference arm. The backreflected light is redirected through the optical circulator to the AWG spectrometer. The collimated beam is imaged with a camera lens onto a 46 kHz CCD linescan camera. The acquired spectra are processed by first subtracting the reference arm spectrum, then compensating the dispersion, and finally resampling to k-space. We achieve a depth range of 1mm. The measured signal-to-noise ratio (SNR) is 75 dB. The axial resolution (FWHM) is determined from a Gaussian fit to the point spread function in amplitude at various depths. A slight decrease in depth resolution is observed at higher depth ranges, which we attribute to misalignment and lens aberrations. As a demonstration of OCT imaging using the AWG spectrometer, an image of a layered phantom is recorded. The phantom consists of three layers of scattering medium (µs = 4 mm-1, refractive index n = 1.41) interleaved with non-scattering tape. We can observe all three scattering layers up to the maximum imaging depth of 1 mm

Laser interference lithography with highly accurate interferometric alignment

Three dimensional photonic crystals, e.g. for obtaining the so-called woodpile structure, can, among others, be fabricated by vertical stacking of multiple gratings. One of the requirements for obtaining a full photonic bandgap in such a photonic crystal is an accurate angular and lateral alignment of the successive gratings. Using laser interference lithography at 266 nm wavelength, we fabricated gratings in silicon with periods down to 300 nm. We present a method for aligning further grating exposures with respect to this grating with a 0.001 degree angular and a few nanometers lateral resolution

Laser interference lithography with highly accurate interferometric alignment

It is shown experimentally that in laser interference lithography, by using a reference grating, respective grating layers can be positioned with high relative accuracy. A 0.001 degree angular and a few nanometers lateral resolution have been demonstrated

Modeling and experimental verification of the dynamic interaction of an AFM-tip with a photonic crystal microcavity

We present a transmission model for estimating the effect of the atomic-force microscopy tapping tip height on a photonic crystal microcavity (MC). This model uses a fit of the measured tip-height-dependent transmission above a “hot spot” in the MC. The predicted transmission versus average tapping height is in good agreement with the values obtained from tapping mode experiments. Furthermore, we show that for the existing, nonoptimized structure, the transmission coefficient can be tuned between 0.32 and 0.8 by varying the average tapping height from 26 to 265 nm. A transmission larger than that of the undisturbed cavity at resonance was observed at specific tip locations just outside the cavity-terminating holes