105 research outputs found
Skyglow relieves a crepuscular bird from visual constraints on being active
No full textArtificial light at night significantly alters the predictability of the natural light cycles that most animals use as an essential Zeitgeber for daily activity. Direct light has well-documented local impacts on activity patterns of diurnal and nocturnal organisms. However, artificial light at night also contributes to an indirect illumination of the night sky, called skyglow, which is rapidly increasing. The consequences of this wide-spread form of artificial night light on the behaviour of animals remain poorly understood, with only a few studies performed under controlled (laboratory) conditions. Using animal-borne activity loggers, we investigated daily and seasonal flight activity of a free-living crepuscular bird species in response to nocturnal light conditions at sites differing dramatically in exposure to skyglow. We find that flight activity of European Nightjars (Caprimulgus europaeus) during moonless periods of the night is four times higher in Belgium (high skyglow exposure) than in sub-tropical Africa and two times higher than in Mongolia (near-pristine skies). Moreover, clouds darken the sky under natural conditions, but skyglow can strongly increase local sky brightness on overcast nights. As a result, we find that nightjars' response to cloud cover is reversed between Belgium and sub-tropical Africa and between Belgium and Mongolia. This supports the hypothesis that cloudy nights reduce individual flight activity in a pristine environment, but increase it when the sky is artificially lit. Our study shows that in the absence of direct light pollution, anthropogenic changes in sky brightness relieve nightjars from visual constraints on being active. Individuals adapt daily activities to artificial night-sky brightness, allowing them more time to fly than conspecifics living under natural light cycles. This modification of the nocturnal timescape likely affects behavioural processes of most crepuscular and nocturnal species, but its implications for population dynamics and interspecific interactions remain to be investigated.</p
Assessing effects of nocturnal LED illumination on aquatic primary producers
No full textRivers are increasingly illuminated at night, due to the continuous expansion of artificial lighting. As light-emitting diodes (LED) are increasingly used in outdoor illumination, rivers are more and more frequently illuminated by LEDs. Exposure to broad-band LED light at night is expected to increase ecological effects of nocturnal illumination. While LED was found to affect numerous organisms and processes in a range of ecosystems, its effects on aquatic ecosystems remain understudied. In small and mid-sized rivers and streams, primary production is dominated by benthic primary producers in periphyton. Primary producers use light as a key source of energy for photosynthesis and a source of information to regulate their circadian rhythms. Light emitted by LED lamps has a significant amount of photosynthetically active radiation and typically a high content of blue light that is known to entrain circadian clock in almost all living beings, including primary producers. Nocturnal LED illumination is therefore likely to have complex impacts on the physiology of periphyton, and net effects are hard to predict. We studied effects of nocturnal blue-rich white LED light on periphyton in field and laboratory experiments. In field experiments, we found that exposure to nocturnal LED light of an intensity commonly found in urban and suburban waters (20 lux) can decrease periphyton biomass and alter its community composition by changing relative proportions of diatoms and cyanobacteria. To identify critical thresholds of impacts, we performed controlled laboratory experiments where we exposed ALAN-naïve periphyton to a range of LED intensities as found in urban and suburban waters at night (from 1 to 40 lux) and assessed its effects on biomass and photosynthetic efficiency relative to communities that experienced no light at night. Our results contribute to our understanding of ecological impacts of artificial illumination on aquatic ecosystems. As periphyton forms the base of the food web and plays an important role in nutrient and carbon cycling in aquatic ecosystems, the changes induced by nocturnal LED illumination may cascade to higher trophic levels and influence important ecosystem functions. Our results can help inform lighting policies to minimize ecological impacts of nocturnal illumination on riverine ecosystems
Maßgeschneiderte Emission aus Breitstreifenlasern mit externen Resonatoren zur Ermöglichung nichtlinearer Frequenzkonversion
No full textA huge number of applications require coherent radiation in the visible spectral range. Since diode lasers are very compact and efficient light sources, there exists a great interest to cover these applications with diode laser emission. Despite modern band gap engineering not all wavelengths can be accessed with diode laser radiation. Especially in the visible spectral range between 480 nm and 630 nm no emission from diode lasers is available, yet. Nonlinear frequency conversion of near-infrared radiation is a common way to generate coherent emission in the visible spectral range. However, radiation with extraordinary spatial temporal and spectral quality is required to pump frequency conversion. Broad area (BA) diode lasers are reliable high power light sources in the near-infrared spectral range. They belong to the most efficient coherent light sources with electro-optical efficiencies of more than 70%. Standard BA lasers are not suitable as pump lasers for frequency conversion because of their poor beam quality and spectral properties. For this purpose, tapered lasers and diode lasers with Bragg gratings are utilized. However, these new diode laser structures demand for additional manufacturing and assembling steps that makes their processing challenging and expensive. An alternative to BA diode lasers is the stripe-array architecture. The emitting area of a stripe-array diode laser is comparable to a BA device and the manufacturing of these arrays requires only one additional process step. Such a stripe-array consists of several narrow striped emitters realized with close proximity. Due to the overlap of the fields of neighboring emitters or the presence of leaky waves, a strong coupling between the emitters exists. As a consequence, the emission of such an array is characterized by a so called supermode. However, for the free running stripe-array mode competition between several supermodes occurs because of the lack of wavelength stabilization. This leads to power fluctuations, spectral instabilities and poor beam quality. Thus, it was necessary to study the emission properties of those stripe-arrays to find new concepts to realize an external synchronization of the emitters. The aim was to achieve stable longitudinal and transversal single mode operation with high output powers giving a brightness sufficient for efficient nonlinear frequency conversion. For this purpose a comprehensive analysis of the stripe-array devices was done here. The physical effects that are the origin of the emission characteristics were investigated theoretically and experimentally. In this context numerical models could be verified and extended. A good agreement between simulation and experiment was observed. One way to stabilize a specific supermode of an array is to operate it in an external cavity. Based on mathematical simulations and experimental work, it was possible to design novel external cavities to select a specific supermode and stabilize all emitters of the array at the same wavelength. This resulted in stable emission with 1 W output power, a narrow bandwidth in the range of 2 MHz and a very good beam quality with M²<1.5. This is a new level of brightness and brilliance compared to other BA and stripe-array diode laser systems. The emission from this external cavity diode laser (ECDL) satisfied the requirements for nonlinear frequency conversion. Furthermore, a huge improvement to existing concepts was made. In the next step newly available periodically poled crystals were used for second harmonic generation (SHG) in single pass setups. With the stripe-array ECDL as pump source, more than 140 mW of coherent radiation at 488 nm could be generated with a very high opto-optical conversion efficiency. The generated blue light had very good transversal and longitudinal properties and could be used to generate biphotons by parametric down-conversion. This was feasible because of the improvement made with the infrared stripe-array diode lasers due to the development of new physical concepts.Für eine Vielzahl von interessanten Anwendungen z.B. in den Lebenswissenschaften werden kohärente Strahlquellen im sichtbaren Spektralbereich benötigt. Diese Strahlquellen sollen sich durch eine hohe Effizienz (d.h. Sparsamkeit), Mobilität und eine hohe Güte des emittierten Lichtes auszeichnen. Im Idealfall passt die Lichtquelle in die Hosentasche und kann mit herkömmlichen Batterien betrieben werden. Diodenlaser sind solche kleinen und sehr effizienten Strahlquellen. Sie sind heutzutage allgegenwärtig, begegnen uns in CD-Playern, Laserdruckern oder an Supermarktkassen im täglichen Leben. Diodenlaser zeichnen sich durch ihren extrem hohen Wirkungsgrad aus, da hier elektrischer Strom direkt in Licht umgewandelt wird. Jedoch können bisher noch nicht alle Wellenlängen im sichtbaren Bereich mit diesen Lasern realisiert werden. Eine Möglichkeit, diesen Wellenlängenbereich über einen Umweg zu erreichen, ist Frequenzkonversion von infrarotem in sichtbares Licht mit sogenannten nichtlinearen optischen Kristallen. Dies ist im Prinzip auch mit Diodenlasern möglich, konnte bisher jedoch nur sehr ineffizient oder mit erheblichem Aufwand umgesetzt werden. Allerdings kann mit Hilfe von externen Resonatoren die Emission solcher Standard-Laserdioden maßgeblich beeinflusst und die Qualität des Lichtes erheblich verbessert werden. Hier setzt die Zielsetzung dieser Arbeit an: Das Licht von infraroten Hochleistungslaserdioden, sogenannten „Streifen-Arrays“, sollte durch einen externen Resonator stabilisiert und für die Frequenzverdopplung erschlossen werden. Diese Arrays bestehen aus mehreren dicht nebeneinander angeordneten Einzelemittern und zeichnen sich dadurch aus, dass eine Kopplung dieser Emitter von außen möglich ist. Im ersten Schritt sollte eine solche Synchronisation der Emitter erreicht werden. In einem zweiten Schritt soll das von außen beeinflusste Licht des Arrays mit einer hohen Effizienz in sichtbares (blaues) Licht konvertiert werden um den Wirkungsgrad der Diodenlaser voll auszunutzen. Dafür war es notwendig die Physik der Streifen-Arrays sorgfältig zu untersuchen. Es mussten Methoden entwickelt werden, durch die eine gezielte Beeinflussung der Emitter möglich ist, damit es zu einer globalen Kopplung und Synchronisation der Array Emitter kommt. Dafür wurden mit Hilfe von mathematischen Modellierungen und Experimenten verschiedene Resonatorkonzepte entwickelt und realisiert. Schlussendlich war es möglich, die Emissionseigenschaften der Arrays um mehrere Größenordnungen zu verbessern und sehr effizient kohärentes blaues Licht sehr hoher Güte zu erzeugen. In einem weiteren Experiment ist es zusätzlich gelungen nichtklassisches Licht bzw. Paarphotonen zu generieren, die ebenfalls interessant für die Lebenswissenschaften sind
High cw power using an external cavity for spectral beam combining of diode laser-bar emission
No full textIn extension to known concepts of wavelength-multiplexing diode laser arrays, a new external cavity is presented. The setup simultaneously improves the beam quality of each single emitter of a standard 25 emitter broad-area stripe laser bar and spectrally superimposes the 25 beams into one. By using this external resonator in an "off-axis" arrangement, beam qualities of M-slow(2) < 14 and M-fast(2) < 3 with optical powers in excess of 10 W in cw operation are obtained
Second-order coherence properties of amplified spontaneous emission from a high-power tapered superluminescent diode
No full textWe study the degree of second-order coherence of the emission of a high-power multi-quantum well superluminescent diode with a lateral tapered amplifier section with and without optical feedback. When operated in an external cavity, the degree of second-order coherence changed from the almost thermal case of g((2))(0)approximate to 1.9 towards the mostly coherent case of g((2)) (0) approximate to 1.2 when the injection current at the tapered section was increased. We found good agreement with semi-classical laser theory near and below threshold while above laser threshold a slightly higher g((2))(0) was observed. As a free running device, the superluminescent diode yielded more than 400 mW of optical output power with good spatial beam quality of M-slow(2) < 1.6. In this case, the degree of second-order coherence dropped only slightly from 1.9 at low powers to 1.6 at the maximum output power. To our knowledge, this is the first investigation of a high-power tapered superluminescent diode concerning the degree of second-order coherence. Such a device might be useful for real-world applications probing the second order coherence function, such as ghost imaging
Water isotope values from a spatial campaign conducted in four northeastern German lakes in September 2020
No full textWater isotopes (δ2H and δ18O) were analyzed in samples collected in lakes associated to major riverine systems in northeastern Germany throughout 2020. This sub-dataset is derived from water samples taken at multiple spatially distributed spots in four selected lakes. A Limnos water sampler was used to obtain samples from 1 m below water surface on 29th and 30th September 2020. Isotope analysis was conducted at IGB Berlin, using a Picarro L2130-i cavity ring-down spectrometer
Spectral characterization of storage phosphor imaging plates
No full textThis work presents attenuation and sensitivity measurements of radiographic imaging plates (IPs) with quasimonoenergetic X-rays in the 8–60 kiloelectronvolt range. The measurements are used to validate theoretical predictions. A short overview of the theoretical model is given. The model can be used to describe the sensitivity of different detector types to a wide range of X-ray energies
Concepts for evaluation of image quality in digital radiology
No full textConcepts for digital image evaluation are presented for Computed Radiography (CR) and Digital Detector Arrays (DDAs) used for weld inspection. The precise DDA calibration yields an extra ordinary increase of contrast sensitivity up to 10 times in relation to film radiography. Restrictions in spatial resolution caused by pixel size of the DDA are compensated by increased contrast sensitivity. First CR standards were published in 2005 to support the application of phosphor imaging plates in lieu of X-ray film, but they need already a revision based on experiences reported by many users. One of the key concepts is the usage of signal-to-noise (SNR) measurements as equivalent to the optical density of film and film system class. The contrast sensitivity, measured by IQI visibility, depends on three essential parameters: The basic spatial resolution (SRb) of the radiographic image, the achieved signal-to-noise ratio (SNR) and the specific contrast (µeff - effective attenuation coefficient). Knowing these 3 parameters for the given exposure condition, inspected material and monitor viewing condition permits the calculation of the just visible IQI element. Furthermore, this enables the optimization of exposure conditions. The new ISO/FDIS 17636-2 describes the practice for digital radiography with CR and DDAs. It considers the first time compensation principles, derived from the three essential parameters. The consequences are described
Multi-wavelength, high spatial brightness operation of a phase-locked stripe-array diode laser
No full textStable continuous wave multi-wavelength operation of a stripe-array diode laser with an externalcavity spectral beam combining geometry is presented. In this setup each emitter of the stripe-array is forced to operate at a different wavelength, which leads to a decoupling between the usually phase-locked emitters. With a reflective diffraction grating with a period of 300 lines per mm, 33 equidistant laser lines around a center wavelength of 978 nm were realized, spanning a spectral range of 26 nm. With this novel approach near-diffraction limited emission with a beam quality of M (2) < 1.2 and an output power of 450 mW was achieved. This laser light source can be used for applications requiring low temporal but high spatial coherence
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