Multi-Wavelength Setup Based on Lasers for Characterizing Optical Detectors and Materials

Optisia ilmaisimia käytetään monissa sovelluksissa kuvantamislaitteista, turvajärjestelmistä sekä konenäkösovelluksista merkinanto- ja tiedonsiirtojärjestelmiin. Spektrinen tehoherkkyys ja spatiaalivaste ovat ratkaisevan tärkeitä parametreja optisten ilmaisimien karakterisoinnissa. Tässä työssä on suunniteltu ja toteutettu lasereihin perustuva laitteisto kyseisiä mittauksia varten. Mittauslaitteisto ohjaa stabiilin, kollimoidun ja halkaisijaltaan kapean lasersäteen näytepidikkeelle, joka on asennettu korkearesoluutioiselle XY-siirtimelle. Ilmaisimia voidaan vaihtaa keskenään absoluuttisen tehoherkkyyden kalibroimiseksi, tai niiden spatiaalivasteet voidaan skannata. Mittauslaitteistolla voidaan myös karakterisoida materiaalien optisia ominaisuuksia, kuten kellastumista, mittaamalla niiden transmittanssit. Laitteiston suorituskyky karakterisoitiin mittaamalla lasersäteen laatua. Mittausten perusteella laitteiston lasersäteen 8 h stabiilisuuden suhteellinen keskihajonta oli 0.01 % käytettäessä tehosäädintä. Stabiloidun lasersäteen tarkkailu erillisellä ilmaisimella laski suhteellisen keskihajonnan alle 0.007 %:iin. Lisäksi laitteistossa käytetyn spatiaalisuodattimen todettiin tasoittavan lasersäteen profiilia, etenkin diodilaseria käytettäessä. Kehitetty laitteisto parantaa MIKES-Aalto Mittaustekniikassa suoritettavien peräkkäisten vuotuisten ilmaisinkalibrointien tarkkuutta ja toistettavuutta. Laitteisto mahdollistaa spatiaalivasteen rutiiniskannauksen kalibroinnin yhteydessä; sillä voidaan tarkistaa esimerkiksi ilmaisimen kunto. Materiaalitieteen alalla ultravioletti (UV) spektrografin ja kehitetyn mittauslaitteiston yhdistelmä mahdollistaa uuden menetelmän tutkia UV-säteilyn aiheuttamaa materiaalien hajoamista alle 2 nm resoluutiolla, mikä on merkittävä parannus aiempiin menetelmiin verrattuna.Optical detectors are used in various applications ranging from imaging devices, security systems, and robotic vision, to signaling and transmission systems. Spectral power responsivity and spatial uniformity are crucial parameters for characterizing detectors. In this thesis, a multi-wavelength setup based on lasers has been designed and constructed for these measurements. The setup directs a stable, collimated laser beam with a narrow beam diameter to a sample holder unit mounted on a high-resolution XY translation stage. Detectors can be interchanged to calibrate absolute power responsivities, or they can be scanned to obtain spatial uniformities. The measurement setup can also be used to characterize optical properties of materials, such as photoyellowing, by measuring their transmittances. The performance of the setup was characterized by measuring the quality of the laser beam. Based on the measurements, the 8 h stability of the laser when using the laser power controller had a relative standard deviation of 0.01 %. By monitoring the stabilized beam with a monitor detector, the relative standard deviation decreased below 0.007 %. The spatial filter used in the setup was verified to smooth the beam profile, especially with diode lasers. The setup developed improves the accuracy and repeatability of consecutive annual detector calibrations in the Metrology Research Institute. The setup enables routine scanning of the spatial uniformity in pursuance of the calibration; it can be used, e.g., to check the condition of the detector. In the material science, a combination of the ultraviolet (UV) spectrograph and the measurement setup developed enables a novel method for studying the UV radiation-induced degradation of materials with a resolution smaller than 2 nm, which is a significant improvement for earlier methods

The BLAST Observatory: A sensitivity study for far-IR balloon-borne polarimeters

Sensitive wide-field observations of polarized thermal emission from interstellar dust grains will allow astronomers to address key outstanding questions about the life cycle of matter and energy driving the formation of stars and the evolution of galaxies. Stratospheric balloon-borne telescopes can map this polarized emission at far-infrared wavelengths near the peak of the dust thermal spectrum—wavelengths that are inaccessible from the ground. In this paper we address the sensitivity achievable by a Super Pressure Balloon polarimetry mission, using as an example the Balloon-borne Large Aperture Submillimeter Telescope (BLAST) Observatory. By launching from Wanaka, New Zealand, the BLAST Observatory can obtain a 30 days flight with excellent sky coverage—overcoming limitations of past experiments that suffered from short flight duration and/or launch sites with poor coverage of nearby star-forming regions. This proposed polarimetry mission will map large regions of the sky at sub-arcminute resolution, with simultaneous observations at 175, 250, and 350 μm, using a total of 8274 microwave kinetic inductance detectors. Here, we describe the scientific motivation for the BLAST Observatory, the proposed implementation, and the forecasting methods used to predict its sensitivity. We also compare our forecasted experiment sensitivity with other facilities

Loistediodien ja ilmakehän otsoniabsorption spektrinen mallintaminen

This thesis focuses on the spectral modelling of light-emitting diodes (LEDs) and developing an uncertainty analysis in atmospheric ozone retrieval algorithms. Attention is also paid to degradation of components used in the atmospheric measurements that brings additional challenge on reliable measurements. Improved spectral models were developed for determining the junction temperatures from the experimental electroluminescence spectra of LEDs. These models consist of the effective joint density of states of conduction band electrons and valence band holes, weighted by the thermal excitation probability. It was noted that the effective joint densities of states of most common LED types made of AlGaInP and InGaN deviate significantly from each other. By fitting the models to experimental spectra, the junction temperatures of red AlGaInP and blue InGaN LEDs were estimated correctly within the standard uncertainty of 4 K. To achieve this uncertainty, the calibration of the model parameters was carried out with one reference spectrum of the same LED specimen at a known junction temperature. A Monte Carlo based uncertainty analysis was developed and integrated into a full spectrum ozone retrieval algorithm to obtain complete uncertainty budget for the total ozone columns. The uncertainty analysis accounts for possible systematic wavelength-dependent deviations in spectral data that significantly increase the uncertainty compared with the assumption that all the spectral uncertainty is white noise. The method was applied to data sets measured with three different spectroradiometers. In the case of array-based spectroradiometers, the results showed that the diurnal cycle of the total ozone columns formed an inverse U-shape arising from the stray light of the instrument. Thus, a new method was developed to reduce the effect of stray light in TOC retrievals. In this thesis, the degradation of white reflectance diffusers, polystyrene, and nickel sulphate filters used in Brewer spectrophotometers were studied. White diffuser targets are used as references in in-orbit calibrations of Earth observation satellite instruments and for characterising the ground reference test sites. In the experiments, fused-silica diffusers were more resistant to hydrocarbon contamination compared with Spectralon diffusers. When the contaminated samples were exposed to ultraviolet radiation in the air, their reflectance returned back to the original level. This is opposite to vacuum tests carried out previously by other researchers, where both hydrocarbon contamination and ultraviolet radiation decreased the reflectance of Spectralon diffusers. In the case of polystyrene, higher energy photons caused more damage and the photodegradation was non-linear with respect to radiant exposure. The spatial uniformities of old nickel sulphate solar blind filters were tens of percents worse compared with a new one.Tässä väitöskirjassa kehitettiin loistediodien eli LEDien spektrimalleja ja analyysimenetelmä ilmakehän otsonikerroksen paksuuden ja siihen liittyvien epävarmuuksien määrittämiseen. Lisäksi väitöskirjassa tutkittiin ilmakehän ominaisuuksien mittaamisessa käytettävien komponenttien ikääntymistä. Komponenttien ikääntyminen tuo lisähaasteen luotettavien mittausten tekemiseen. Työssä kehitettiin parempia malleja LEDien liitoslämpötilojen määrittämiseen mitatuista elektroluminesenssispektreistä. Nämä mallit koostuvat johtavuusvyön elektronien ja valenssivyön aukkojen efektiivisestä yhdistetystä tilatiheydestä, jota painotetaan lämpötilan aiheuttaman virittymisen todennäköisyysjakaumalla. Työssä osoitettiin, että yleisimmin käytettyjen AlGaInP ja InGaN LEDien efektiiviset tilatiheydet poikkeavat huomattavasti toisistaan. Sovittamalla mallit mitattuihin spektreihin, punaisten AlGaInP ja sinisten InGaN LEDien liitoslämpötilat ennustettiin oikein alle 4 K keskihajonnalla. Tämän epävarmuuden saavuttamiseksi, malliparametrit määritettiin käyttäen saman LEDin yhtä mitattua spektriä tunnetussa liitoslämpötilassa. Monte Carlo -menetelmään perustuva epävarmuusanalyysi kehitettiin ja integroitiin osaksi otsonikerroksen paksuuden määrittämiseen käytettyä ilmakehämallia, otsonikolumnin täydellisen epävarmuusbudjetin määrittämistä varten. Epävarmuusanalyysi huomioi mahdolliset aallonpituusriippuvat systemaattiset poikkeamat spektridatassa, jotka nostavat merkittävästi otsonikolumnien epävarmuuksia verrattuna oletukseen, että spektrit sisältävät vain valkoista kohinaa. Menetelmää sovellettiin kolmen eri spektroradiometrin mittaamiin spektreihin. Kennopohjaisten spektroradiometrien mittaamissa päivittäisissä otsonikolumneissa näkyi käänteisen U:n muotoinen mittausvirhe johtuen spektroradiometrin hajavalosta. Hajavalon vaikutuksen pienentämiseksi kehitettiin uusi analysointimenetelmä otsonikolumnin määritykseen. Työssä tutkittiin heijastavien valkoisten diffuuserien, polystyreenin ja Brewer-spektrofotometreissä käytettävien nikkelisulfaatti-suodattimien ikääntymistä. Valkoisia diffuusereita käytetään referensseinä kiertoradalla tapahtuviin satelliittien optisten mittalaitteiden kalibrointiin ja maanpäällisten referenssikohteiden karakterisointiin. Kokeissa kvartsilasi-diffuuserit olivat kestävämpiä hiilivetykontaminaatiolle kuin Spectralon-diffuuserit. Kun kontaminoidut diffuuserit altistettiin ultraviolettisäteilylle huoneilmassa, niiden heijastavuus palasi alkuperäiselle tasolle diffuuserityypistä riippumatta. Tulos on päinvastainen verrattuna aiempiin vakuumitesteihin, joissa sekä hiilivedyt että ultraviolettisäteily heikensivät Spectralon-diffuuserien heijastavuutta. Polystyreenin tapauksessa huomattiin, että korkeampienergiset fotonit vaurioittivat polystyreeniä enemmän ja ikääntyminen oli epälineaarista säteilyannoksen funktiona. Vanhojen nikkelisulfaatti-hajavalosuodattimien spatiaalitasaisuudet olivat kymmeniä prosentteja huonompia uuteen verrattuna

Facility for Determining Action Spectra of UV Photodegradation

We have developed an ultraviolet (UV) ageing facility consisting of a spectrograph which can be used to expose material samples with spectrally dispersed UV radiation at the wavelength range 273 – 423 nm, and an automated, high-resolution laser-based setup for measuring the induced color changes in the transmittances of the exposed samples. The samples are scanned using a suitable laser wavelength to obtain the wavelength dependence on the color change. The locations on the samples are converted to exposure wavelengths with the known dispersion of the spectrograph. We demonstrate the facility by exposing and measuring a set of atactic polystyrene sheets. By studying samples aged for different time periods we derive a complex action spectrum for the photoyellowing that is nonlinear with respect to the radiant exposure. We further derive linear activation spectra for the photoyellowing demonstrating how they depend on the selected threshold level.Peer reviewe

Setup for studying speckle noise of spectroradiometer diffusers in Earth observation applications

Diffusers in in-orbit spectroradiometers cause speckle under partially coherent solar radiation. A speckle pattern entering a spectroradiometer through a small slit creates systematic spectral deviations in measured spectra. We have developed a setup to characterise the spatial speckle of diffusers and the related spectral features. The decorrelation angles measured at 532 nm for Spectralon, Diffusil, and Heraeus diffusers were 0.021, 0.014, and 0.005 respectively. This information can be used for compensating speckle-related spectral features from the radiometric satellite measurements by averaging over multiple decorrelated spectra.Peer reviewe

Aluminium-Stabilized High-Temperature Superconducting Cable for Particle Detector Magnets

Within the context of EP R&D;, CERN is developing a high-temperature superconducting (HTS) conductor for future superconducting detector magnet projects. The conductor features a Rare-Earth Barium Copper Oxide (REBCO) tapes soldered to a copper-coated high-purity aluminium stabilizer. Critical currents of the 200 mm long straight cable samples measured with various numbers of REBCO tapes at 77 K are consistent with empirical scaling formulas for critical current within 10%. A few percent deviations are expected to arise from the critical current variation along the length of the HTS tape. No degradation was observed after multiple soldering and de-soldering cycles at 165 $^\mathrm{\circ}$C with Bi–Sn based solder and after thermal cycling between 77 K and room temperature. However, extreme bending (of 100 mm radius) of the already soldered HTS cable leads to failure of the cable. We repeated the critical current measurements with a 650 mm long cable loop sample with 85 mm bending radius, where the HTS tapes were soldered after the aluminium profile was bent. The critical current of the HTS cable loop was 7% lower than the prediction. Based on the first critical current measurements, the HTS cable preparation method presented in this work results in repeatable quality aluminium-stabilized HTS cable

High-resolution setup for measuring wavelength sensitivity of photoyellowing of translucent materials

VK: MittaustekniikkaPeer reviewe

Internal quantum efficiency of silicon photodetectors at ultraviolet wavelengths

Funding Information: Projects 18SIB10 Chipscale and 22IEM06 ScaleUp leading to this publication have received funding from the EMPIR program and from the European Partnership on Metrology, respectively, co-financed by the participating states and the European Union’s Horizon 2020 and Horizon Europe research and innovation programmes. We also acknowledge support by the Academy of Finland Flagship Programme, Photonics Research and Innovation (PREIN), Decision Number 320167. Publisher Copyright: © 2023 The Author(s). Published on behalf of BIPM by IOP Publishing Ltd.We determine experimentally the internal quantum efficiency of a 3-element trap detector made of Hamamatsu S1337 photodiodes and of a predictable quantum efficient detector (PQED) over the wavelength range of 250-500 nm using an electrically calibrated pyroelectric radiometer as reference detector. The PQED is made of specially designed induced junction photodiodes, whose charge-carrier recombination losses are minimized. The determined internal quantum efficiency of PQED is always 1 or larger, whereas the 3-element trap detector has internal quantum efficiency smaller than 1 in the spectral range of 330-450 nm. This finding demonstrates the advantages of PQED photodiodes for studying the quantum yield due to impact ionization by charge carriers in the silicon lattice. For this purpose, we develop an extrapolation model for the charge-carrier recombination losses of the PQED, which allows us to separate the quantum yield from the measured internal quantum efficiency. Measurements of PQED spectral responsivitythus allow to determine the quantum yield in silicon, which can be further used for quantifying the charge-carrier recombination losses in the 3-element trap detector. Numerical values of the latter are from 6% to 2% in the spectral range from 250 nm to 380 nm. Finally, our results are encouraging for the aim of developing the PQED to a primary detector standard also at ultraviolet wavelengths.Peer reviewe