This PDF file contains the front matter associated with SPIE Proceedings Volume 7726, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

In traditional electrical sensing applications, multiplexing and interconnecting the different sensing elements is a major challenge. Recently, many optical alternatives have been investigated including optical fiber sensors of which the sensing elements consist of fiber Bragg gratings. Different sensing points can be integrated in one optical fiber solving the interconnection problem and avoiding any electromagnetical interference (EMI). Many new sensing applications also require flexible or stretchable sensing foils which can be attached to or wrapped around irregularly shaped objects such as robot fingers and car bumpers or which can even be applied in biomedical applications where a sensor is fixed on a human body. The use of these optical sensors however always implies the use of a light-source, detectors and electronic circuitry to be coupled and integrated with these sensors. The coupling of these fibers with these light sources and detectors is a critical packaging problem and as it is well-known the costs for packaging, especially with optoelectronic components and fiber alignment issues are huge. The end goal of this embedded sensor is to create a flexible optical sensor integrated with (opto)electronic modules and control circuitry. To obtain this flexibility, one can embed the optical sensors and the driving optoelectronics in a stretchable polymer host material. In this article different embedding techniques for optical fiber sensors are described and characterized. Initial tests based on standard manufacturing processes such as molding and laser structuring are reported as well as a more advanced embedding technique based on soft lithography processing.

In this paper, we report on the strain and pressure testing of highly flexible skins embedded with Bragg grating sensors recorded in either silica or polymer optical fibre. The photonic skins, with a size of 10cm x 10cm and thickness of 1mm, were fabricated by embedding the polymer fibre or silica fibre containing Bragg gratings in Sylgard 184 from Dow Corning. Pressure sensing was studied using a cylindrical metal post placed on an array of points across the skin. The polymer fibre grating exhibits approximately 10 times the pressure sensitivity of the silica fibre and responds to the post even when it is placed a few centimetres away from the sensing fibre. Although the intrinsic strain sensitivities of gratings in the two fibre types are very similar, when embedded in the skin the polymer grating displayed a strain sensitivity approximately 45 times greater than the silica device, which also suffered from considerable hysteresis. The polymer grating displayed a near linear response over wavelength shifts of 9nm for 1% strain. The difference in behaviour we attribute to the much greater Young's modulus of the silica fibre (70 GPa) compared to the polymer fibre (3 GPa).

FBG sensors are particularly suitable for strain measurements on mechanical parts subjected to strong electromagnetic disturbances. This is the case of strain measurements on the pantograph collector strip of underground trains, where, due to arcing phenomena caused by contact loss between the pantograph collector and the contact wire of overhead line during the current collection, high electromagnetic disturbances are present. Moreover, an intrinsic advantage of fiber optic sensors is the ability to electrically insulate the sensor from the conditioning and data-gathering unit. In this work the application of FBG sensors on the pantograph collector of an underground train, instrumented for experimental inline tests, is presented. First, a FEM analysis of the collector to identify the suitable position, in terms of maximum sensitivity and fiber safety, for the strain sensor was performed. The position of a thermal compensating FBG sensor was also selected as a compromise between a place not too far from the main sensor (so that both are subject to the same temperature), and with a small (or negative) strain. The compensated signal is simply computed subtracting the compensator signal from the main one, and this can be considered roughly proportional to the total contact force. A static calibration was conducted in the laboratory with standard masses, and dynamic tests were carried out to dynamically characterize the pantograph. The FBG static measurements were compared with force measurements obtained from traditional sensors positioned on the pantograph collector interface, on each side of the collector, in order to obtain information on the position of vertical contact force. The total vertical contact force from the traditional load cells is compared with the FBG measurement obtained in the center of the collector. The comparison of the total force signals obtained during experimental results for both laboratory and in-line tests showed a good accordance, and these tests can be considered as a validation of the method, so that it can be further developed and used in the future in similar situations.

A novel infrared radiation detector based on a pair of fiber Bragg gratings (FBGs) is described. In the proposed configuration, the two FBGs are distant by a few centimeters and are characterized by Bragg resonances separated by a few nanometers. One FBG of the pair is coated with an IR-absorbing layer which converts the radiation into heat. Therefore, exposure to IR radiations will increase the temperature of the coated FBG, which in turn induces a Bragg wavelength shift to higher values. To take into account the ambient temperature fluctuations, the second grating is protected by an IR-reflecting tube which prevents heating of this grating that can then be used as temperature reference. IR radiation measurements are finally obtained through the monitoring of the differential shift between both Bragg wavelengths. This shift shows a monotonic behavior as a function of the IR radiations. This sensor shows a strong potential for early fire detection as it detects radiation emitted during the fire instead of the temperature increase when the fire is fully developed.

We have successfully created Chemical Composition Gratings (CCGs) into two different types of fiber: standard telecommunications Germanium doped fibers and photosensitive Germanium/Boron co-doped fibers. We will present results on the regeneration process, the sensing properties and also the high temperature decay and maximum operative temperature for the CCGs created in both types of fiber.

Based on the macro-bend induced losses in an optical fiber a linear displacement sensor is developed. The oscillatory bend radius dependent loss is suppressed by coating the bent section with an absorption layer. The sensor is designed as an open-ended fiber probe for easy practical application, where the Fresnel reflection at the cleaved end is utilized. Mechanical modelling is given to optimize the fiber lead length to avoid buckling of the fiber. Resolutions of 30 μm and 0.1 μm, with coefficient of determination of 0.97 and 0.96, respectively over a total measurable displacement of 0 to 30 mm are achieved by utilizing a ratio-metric measurement of the bend loss. Finally a sensor with a smaller footprint is also demonstrated by utilizing a reduced clad fiber with a cladding diameter of 80 μm.

The use of optical fibers as sensing element is increasing in clinical, pharmaceutical and industrial applications. Excellent light delivery, long interaction length, low cost and ability not only to excite the target molecules but also to capture the emitted light from the targets are the hallmarks of optical fiber as biosensors. In biosensors based on fiber optics the interaction with the analyte can occur within an element of the optical fiber. One of the techniques for this kind of biosensors is to remove the fiber optic cladding and substitute it for biological coatings that will interact with the parameter to sensorize. The deposition of these layers can be made by sol-gel technology. The sol-gel technology is being increasingly used mainly due to the high versatility to tailor their optical features. Incorporation of suitable chemical and biochemical sensing agents have allowed determining pH, gases, and biochemical species, among others. Nonetheless, the relatively high processing temperatures and short lifetime values mean severe drawbacks for a successful exploitation of sol-gel based coated optical fibres. With regard to the latter, herein we present the design, preparation and characterization of novel sol-gel coated optical fibres. Low temperature and UV curable coating formulations were optimized to achieve a good adhesion and optical performance. The UV photopolymerizable formulation was comprised by glycidoxypropyltrimethoxysilane (GLYMO), Tetraethylorthosilicate (TEOS) and an initiator. While the thermoset coating was prepared by using 3-aminopropyltrimethoxysilane, GLYMO, and TEOS as main reagents. Both curable sol-gel coated fibres were analysed by FTIR, SEM and optical characterization. Furthermore, in the present work a new technique for silica cladding removal has been developed by ultra-short pulses laser processing, getting good dimensional accuracy and surface integrity.

Spatially resolved sensing of molecular oxygen is important for many biological and environmental applications. For this purpose, time-resolved fluorescence measurements were combined with optical time domain reflectometry (OTDR), a technique which was primarily developed for inspections of optical fiber lines. To achieve spatial resolution of some meters, which are typical for commercial OTDR instruments, the lifetimes of the sensor dyes must be within the range of some nanoseconds, which is much shorter than the decay times of common phosphorescent oxygen probes. Therefore, the measurements were performed with a novel fluorescent triangular-[4]phenylene sensor dye. The fluorescence decay times are around 80 ns in absence of oxygen and around 20 ns in the presence of air. The [4]phenylene sensor dye was applied in toluene solution as well as immobilized in a polymer film. Using a branched model fiber line, oxygen measurements were carried out in a micro- to millimolar concentration range. Oxygen-dependent fluorescence decay times measured with OTDR in toluene were verified by use of time-correlated single photon counting (TCSPC). The Stern-Volmer plots obtained for fluorescence quenching of sensor dyes dissolved in toluene solution and polymer-based sensor spots show good linearity.

Electro-optic measurement (EO) constitutes an efficient technique to characterize electrical (E) fields : indeed, the Pockel's effect properties (linear modification of refractive indices of some non-centrosymetric crystals induced by the E-field)1 leads to a vectorial measurement. Thus, it allows to map the E-field vector and its transient evolution, either in free space or inside guiding structures. Pigtailed EO sensors are naturally becoming a reliable and consistent mean of characterization for many applications, e.g. high power microwaves (HPM), electromagnetic interference (EMI), on chip diagnostic, bio-electromagnetism (e.g. influence of mobile phones on the human body). Even if these non-invasive sensors provide a greater temporal and spatial resolution (femtosecond and sub-millimeter, respectively) than commonly used sensors (antennas, bolometers), it remains temperature dependant and quite low sensitive. EO probes are based on the modification of a laser beam (either its polarization, phase or amplitude) crossing an EO crystal. We demonstrate here the last developments and improvements for EO probes as well as for whole EO setups, exploiting polarization state or amplitude modulation. The sensor is constituted by a polarization maintaining (PM) fiber carrying the beam to the crystal and taking it back once modulated, gradient index lense(s) managing the shape of the beam, half or quarter wave plate controlling the input and output polarizations and a crystal (either anisotropic: LiTaO3, LiNb03, DAST, KTP or isotropic : ZnTe, InP) converting the E-field into a modulation. Our probes are fully dielectric and cylindrically shaped (length ~ 1 cm and diameter ~ 2-3 mm). The setup is made of a 1.5 μm DFB laser, some photodiodes (low and high speed) added with a polarization state analyser arrangement in case of EO probes based on polarization state modulation scheme. The measurement bench is fully automated and compensate/measure the temperature deviation simultaneously. Sensitivity of our EO probe reaches 0.7 V.m^-1Hz^-1/2, the bandwidth covers an ultra wide frequency band (kHz - and more than 20 GHz), the selectivity (orthogonal E-field components rejection) is about 25 dB, and a spatial resolution greater than 100 μm is achieved. Transient and frequency measurements and 2D E-field mapping will be presented during the conference.

In recent years, optical microresonators have been extensively investigated for possible applications in many different areas of research. In optical communications such resonators can be used for switching, filtering or multiplexing devices. Due to its high quality factors, spherical microresonators are of great interest for optical sensing. Here we will discuss the use of a microparticle array as a spectral sensing device. Especially the accuracy in wavelength determination for broad light sources are in the focus of this work. Beside this, results for two light sources with different wavelengths are given.

In this publication optical spectroscopy is considered to be a supplementary technique to study ancient colored glass. It results from a systematic study of the UV-VIS-NIR transmission spectra of intentionally colored glass fragments from various archaeological and historical sites and dated from the Roman period to the 21^th century AD. The main goal consists of defining optical sensing parameters for this type of material. The considered colorants are iron, cobalt, manganese, copper and chromium. It is proved that many cases exist where optical spectroscopy can be seen as a straightforward, non-destructive, low-cost and in-situ applicable technique in identifying authentic material or to obtain information about the origin of the material. Possible sensing parameters are defined as the absence/presence of absorption bands characteristic for a specific coloring metal oxide and the spectral position of these bands. These parameters could reveal information about the applied furnace conditions and/or to the composition of the glass matrix. It is shown that the cobalt absorption band situated around 535 nm for soda rich glasses (Roman and industrial times) is shifted towards 526 nm for potash rich glasses (medieval and post-medieval times).

Optical spectroscopy has been utilized in various fields of science, industry and medicine, since each substance is discernible from all others by its spectral properties. However, optical spectroscopy traditionally generates information on the bulk properties of the whole sample, and mainly in the agri-food industry some product properties result from the heterogeneity in its composition. This monitoring is considerably more challenging and can be successfully achieved by the so-called hyperspectral imaging technology, which allows the simultaneous determination of the optical spectrum and the spatial location of an object in a surface. In addition, it is a nonintrusive and non-contact technique which gives rise to a great potential for industrial applications and it does not require any particular preparation of the samples, which is a primary concern in food monitoring. This work illustrates an overview of approaches based on this technology to address different problems in agri-food and industrial sectors. The hyperspectral system was originally designed and tested for raw material on-line discrimination, which is a key factor in the input stages of many industrial sectors. The combination of the acquisition of the spectral information across transversal lines while materials are being transported on a conveyor belt, and appropriate image analyses have been successfully validated in the tobacco industry. Lastly, the use of imaging spectroscopy applied to online welding quality monitoring is discussed and compared with traditional spectroscopic approaches in this regard.

In a previous paper a new approach was explored where the output parameters of a welding monitoring system based on plasma spectroscopy were the participation profiles of plasma ions and neutral atoms. They were obtained by the generation of synthetic spectra and the use of an optimization algorithm, showing correlation to the appearance of defects on the seams. In this work a feature selection algorithm is included in the model to determine the most discriminant wavelengths in terms of defect detection, thus allowing to reduce the spectral range where the synthetic spectra are generated. This should also give rise to an improvement in the overall computational performance of the algorithm. Alternatives to the use of controlled randomn search algorithms will be also explored, and the resulting model will be checked by means of experimental and field tests of arc-welding processes.

The use of laser-induced breakdown spectroscopy (LIBS) for the sorting of polymers containing heavy metal impurity is investigated. Our main attention is directed towards the detection of cadmium and lead in real-life waste materials and certified reference polymer materials. UV (193 nm ArF) and IR (1064 nm Nd:YAG) laser radiation is employed for ablation and plasma generation in air. The LIBS spectra are measured in the UV / VIS range by using an Echelle spectrometer equipped with an ICCD camera. Spectra are compared for the different lasers. Sorted polymer materials are investigated by reference analysis.

Silicon MEMS cantilever-based photoacoustic technology allows for the sensing of ultra low gas concentrations with very wide dynamic range. The sensitivity enhancement is achieved with a cantilever microphone system in which the cantilever displacement is probed with an optical interferometer providing a pico-meter resolution. In the gas sensor, the silicon cantilever microphone is placed in a two-chamber differential gas cell. By monitoring differential pressure changes between the two chambers, the differential cell operates as a differential infra-red detector for optical absorption signals through a measurement and reference path. The differential pressure signal is proportional to gas concentration in the optical measurement path. We have designed, implemented and tested a differential photo acoustic gas cell based on Low Temperature Co-fired Ceramic (LTCC) multilayer substrate technology. Standard LTCC technology enables implementation of 2.5D structures including holes, cavities and channels into the electronic substrate. The implemented differential photoacoustic gas cell structure includes two 10 mm long cylindrical cells, diameter of 2.4 mm. Reflectance measurements of the cell showed that reflectivity of the substrate material can be improved by a factor 15 - 90 in the 3 - 8 μm spectral region using gold or silver paste coatings. A transparent window is required in the differential gas cell structure in order to probe the displacement of the silicon cantilever. The transparent sapphire window was sealed to the LTCC substrate using two methods: screen printed Au80/Sn20 solder paste and pre-attached glass solder paste (Diemat DM2700P/H848). Both methods were shown to provide hermetic sealing of sapphire windows to LTCC substrate. The measured He-leak rate for the 10 sealed test samples implemented using glass paste were less than 2.0 ×10^-9 atm×cm³/s, which meets the requirement for the leak rate according to MIL-STD 883. The achieved hermetic level suggests that the proof-of-principle packaging demonstrator paves the way for implementing a novel differential photoacoustic gas cell for a future miniature gas sensor module. The future module consisting of a sample gas cell and immersion lens IR-LEDs together with interferometric probing of the cantilever microphone is expected to be capable of measuring ultra low concentrations of a wide range of gases with their fundamental absorption bands at 3 - 7 μm wavelength, such as CO, CO2 and CH₄.

Au and Pt nanoparticles are prepared with colloidal techniques in order to achieve high morphological quality, capped with a polymer and then embedded inside a TiO₂ sol-gel matrix, resulting in a homogeneous dispersion of both metal colloids, confirmed by TEM analyses. Refractive index values measured with ellipsometry increase with the annealing temperature, with quite a linear trend, and at the same time the Au surface plasmon resonance peak undergoes a red shift: the refractive index evaluated from the Au plasmon band is slightly lower than the measured value, indicating that the refractive index just around metal particles is different from the average of the matrix, likely because of the polymeric capping agent. Optical gas sensing tests towards CO and H₂ are presented as one of the possible applications of these nanocomposites.

Every gas (e.g. CO₂) absorbs IR-radiation at individual gas specific IR-wavelengths. Non-dispersive infrared (NDIR) gas sensors exploit this property for gas monitoring. Such sensors are used in various applications, e.g. for control of air quality in office buildings or cars. This is a big market for low cost sensors. A NDIR sensor consists basically of three components: an IR-emitter, a chamber containing the sample gas, and an IR-detector with a filter for the observed wavelength. Commercially available systems use broadband IR-emitters (e.g.: micro-lamps) in combination with thermopile or pyroelectric detectors fabricated with a narrowband gas-specific IR-filter, e.g., an interference filter. We devised a concept for a simple and cost-effective NDIR-gas sensor based on two non-symmetric Fabry-Perot absorberstructures as IR-emitter and as IR-detector where no additional interference filter is needed. The presented sensor combines thin layer technology with optical sensing techniques. The system was first analyzed using ray tracing models based on a Monte Carlo method in order to model the response function of the system's sample chamber. For our results, the sample gas is CO₂ where the major absorption is centered around 4.26μm.

In this paper we introduce a multi gas sensor system based on refractive index changes in a 2D slab photonic crystal. The sensor is formed by a L3 resonant cavity sandwiched between two W1.06 waveguides in the photonic crystal. The sensor configuration is similar to an Add-Drop filter structure. The transmission spectrums of the sensor with different ambient refractive indices ranging from n = 1.0 to n = 1.1 are calculated. The simulation results show that a change in ambient RI of Δn = 0.0008 is apparent with a corresponding change in output wavelength of the sensor of 2.4 nm. The properties of the sensor are simulated using the 3D finite-difference time-domain (FDTD) method. The Q-factor of the sensor is also optimized, with highest values reaching over 30,000. The sensor system is hybrid integrated with a wireless RF chip which processes the sensor data and transmits them in effect turning the entire system into a wireless sensor mote.

Highlighted are novel innovations in hybrid optical design physical sensors for extreme environments. Various hybrid design compositions are proposed that are suited for a particular sensor application. Examples includes combining freespace (wireless) and fiber-optics (wired) for gas turbine sensing and combining single crystal and sintered Silicon Carbide (SiC) materials for robust extreme environment Coefficent of Thermal Expansion (CTE) matched frontend probe design. Sensor signal processing also includes the hybrid theme where for example Black-Body radiation thermometry (pyrometry) is combined with laser interferometry to provide extreme temperature measurements. The hybrid theme also operates on the optical device level where a digital optical device such as a Digital Micromirror Device (DMD) is combined with an analog optical device such as an Electronically Controlled Variable Focal Length Lens (ECVFL) to deliver a smart and compressive Three Dimensional (3-D) imaging sensor for remote scene and object shape capture including both ambient light (passive) mode and active laser targeting and receive processing. Within a device level, the hybrid theme also operates via combined analog and digital control such as within a wavelength-coded variable optical delay line. These powerful hybrid design optical sensors have numerous applications in engineering and science applications from the military to the commercial/industrial sectors.

The chemical and physical condition of oils in marine engines must be monitored to ensure optimum performance of the engine and to avoid damage by degraded oil not adequately lubricating the engine. Routine monitoring requires expensive laboratory testing and highly skilled analysts. This work describes the adaptation and implementation of a mid infrared (MIR) sensor module for continued oil condition monitoring in two-stroke and four-stroke diesel engines. The developed sensor module will help to reduce costs in oil analysis by eliminating the need to collect and send samples to a laboratory for analysis. The online MIR-Sensor module measures the contamination of oil with water, soot, as well as the degradation indicated by the TBN (Total Base Number) value. For the analysis of water, TBN, and soot in marine engine oils, four spectral regions of interest have been identified. The optical absorption in these bands correlating with the contaminations is measured simultaneously by using a four-field thermopile detector, combined with appropriate bandpass filters. Recording of the MIR-absorption was performed in a transmission mode using a flow-through cell with appropriate path length. Since in this case no spectrometer is required, the sensor including the light source, the flowthrough- cell, and the detector can be realised at low cost and in a very compact manner. The optical configuration of the sensor with minimal component number and signal intensity optimisation at the four-field detector was implemented by using non-sequential ray tracing simulation. The used calibration model was robust enough to predict accurately the value for soot, water, and TBN concentration for two-stroke and four-stroke engine oils. The sensor device is designed for direct installation on the host engine or machine and, therefore, becoming an integral part of the lubrication system. It can also be used as a portable stand-alone system for machine fluid analysis in the field.

Leak detection and monitoring on subsea structures is an area of increasing interest for the detection and monitoring of production and control fluids for the oil and gas industry. Current techniques such as capacitive (dielectric) based measurement or passive acoustic systems have limitations and we report here an optoelectronic solution based upon fluorescence spectroscopy to provide a permanent monitoring solution. We report here a new class of optoelectronic subsea sensor for permanent, real time monitoring of hydrocarbon production systems. The system is capable of detecting small leaks of production or hydraulic fluid (ppm levels) over distances of 4-5 meters in a subsea environment. Ideally systems designed for such applications should be capable of working at depths of up to 3000m unattended for periods of 20+ years. The system uses advanced single emitter LED technology to meet the challenges of lifetime, power consumption, spatial coverage and delivery of a cost effective solution. The system is designed for permanent deployment on Christmas tree (XT), subsea processing systems (SPS) and associated equipment to provide enhanced leak detection capability.

The correct alignment and setting of the components inside subsea wellheads is critical to ensure that all seals perform at the high pressures found in subsea hydrocarbon production systems. Of particular importance is the alignment of the tubing hanger and how it engages with the subsea trees or wellhead. The tubing hanger supports the production tubing string with carries the hydrocarbon from the reservoir and experiences pressures up to 15 kPsi. Full and correct engagement of the locking and sealing mechanisms of the tubing hanger is critical in containing the production fluids. Here we present the results of a trial of a laser profiler to assess the interior surfaces of a wellhead. The scanner has been shown to be capable of effectively inspecting the interior surface of the wellhead for damage and measuring the interior structures to allow the tubing hanger to be installed correctly.

When addressing optical sensors for use in e.g. industry, compactness, robustness and performance are essentials. Adhering to these demands, we have developed a suit of compact optical sensors for the specific purposes of measuring angular velocity and linear translations of rigid objects. The technology is based on compact and low-cost laser sources such as Vertical Cavity Surface Emitting Lasers (VCSELs). The methods characterise the object motion by speckle translation in the near field (imaging) or far field (optical Fourier transform) by optical spatial filtering velocimetry. The volume of the two optical solutions is less than 1 cm³, including the application specific integrated circuit (ASIC), which processes the data and interfaces a PC/Laptop directly via a USB driver. The sensors are designed for working distances of 2 and 12 mm for near field and far field, respectively. We will consider the requirements for the optical designs in order to optimize the two sensor concepts for their respective purpose. For the angular velocity sensor the phase curvature of the illuminating beam is important in order to avoid parasitic contributions from any linear (transverse, in-plane) translations. The linear translation sensor is based on an imaging system. Therefore, the optical solution requires some kind of a beam-combining device because the VCSEL and the photodetectors being located in separate areas on the ASIC. We will present these two optical sensor designs and measurements for evaluation of their performance.

Polymer photonic crystal fibres combine two relatively recent developments in fibre technology. On the one hand, polymer optical fibre has very different physical and chemical properties to silica. In particular, polymer fibre has a much smaller Young's modulus than silica, can survive higher strains, is amenable to organic chemical processing and, depending on the constituent polymer, may absorb water. All of these features can be utilised to extend the range of applications of optical fibre sensors. On the other hand, the photonic crystal - or microstructured - geometry also offers advantages: flexibility in the fibre design including control of the dispersion properties of core and cladding modes, the possibility of introducing minute quantities of analyte directly into the electric field of the guided light and enhanced pressure sensitivity. When brought together these two technologies provide interesting possibilities for fibre sensors, particularly when combined with fibre Bragg or long period gratings. This paper discusses the features of polymer photonic crystal fibre relevant to sensing and provides examples of the applications demonstrated to date.

The usage of Nematic Liquid Crystal (NLC) infiltrated Photonic Crystal Fibers (PCFs) for a low cost, all-fiber, high voltage sensing device is experimentally demonstrated. The infiltrated PCF which transmit light based on the photonic bandgap mechanism is employed in an intensity based all-fiber voltage measurement scheme, by applying the voltage perpendicular to the fiber axis. The output transmittance of the PCF varies linearly with increasing voltage. The PCF voltage sensor is capable of measuring high voltages in the range from 100 V to 850 V. The voltage sensing device is compact and exhibits accurate voltage measurement owing to its intrinsic nature with the NLC material infiltrated within the air-holes of the PCF microstructure.

We report the multiplexing of photonic crystal fiber interferometric sensors that exhibit sinusoidal interference pattern. The frequency division multiplexing technique combined with a simple fast Fourier transform demodulation method was used to multiplex several devices in a single channel. To avoid crosstalk between the sensors, we calculate the optimal relationship between their periods. The technique is validated experimentally by multiplexing two-mode PCF interferometers but it can be adapted to multiplex any other optical fiber sensors that exhibit sinusoidal patterns. The technique and results reported here may allow the development of PCFI-based sensor networks.

A new low-cost superheterodyne configuration, without acousto-optic modulator, is applied to the two-wavelength interferometry for absolute distance measurement. The principle relies on a synchronized frequency sweep of two optical signals, but with different frequency excursions. The frequency difference between the two optical waves is highly accurate. This is realized by injecting a frequency modulated laser signal in an intensity modulator that is biased at halfwave voltage and driven by a digitally swept radio-frequency signal between 13 and 15 GHz. This latter is a continuous up and down ramp. The two synchronized optical signals emerging from the modulator produce in a Michelson interferometer a distance dependent superheterodyne signal, with a variable synthetic wavelength of about 10 mm. The superheterodyne frequency depends linearly on distance and on the radio-frequency excursion. The integration time for a distance measurement point corresponds to the duration of single sweep (i.e. one millisecond in our case). Absolute distance measurements from 1 to 15 meters yield an accuracy of ±50 μm, showing the validity of the technique.

We present a new optical tomography technique based on phase-shifting schlieren deflectometry. The principle is that of computerized tomography. The three-dimensional profile is reconstructed from the deflection angles of rays passing through the tested object. We have investigated optical phantoms chosen in view of the characterization of dendritic growth in a solidification process. Promising results have been obtained with a homogeneous sphere and a bundle of 200μm fibers. The deviation angles exceed two degrees with a variation of the refractive index ▵n=0.025.

With deflectometric measurement methods there are powerful systems available today that are capable of measuring the geometry of specular surfaces or the power distribution of refractive optics in a fast and flexible manner without influencing the measurement object by tactile probing. They are based on the general principle to image a known spatially coded reference structure via the unknown measurement object onto an optically calibrated camera. As a representation of the reference structure LC-displays are very suitable as they provide a high flexibility in the generation of spatial coding patterns like sinusoidal fringes. As the characteristics of the reference structure have a huge impact on the resolution, the accuracy and the measurement range of the whole system, in this work two displays with different LCD technologies are analysed, compared and evaluated especially for deflectometric applications. The main focus is on the quality of gray-value rendering and the dependency between the characteristic curve and the observation angle. The experimental results corroborate the theoretical finding that IPS-technology is superior to TN- and MVA-displays in terms of an observation-angle independent shape of the grayscale-characteristic curve. So IPS should be the technologyof- choice when selecting a LC-display for a deflectometric measurement system.

The pyramid wavefront sensor (PWS) was initially proposed by astronomers to measure aberrations introduced by the atmosphere. More recently it has been used to measure aberrations of the human eye, and has been successfully incorporated into an adaptive optics loop to correct those aberrations. The raw sensor signal can be used as feedback to control a wavefront correcting device, or with appropriate scaling, to reconstruct the wavefront map in the pupil. In practice, use of dynamic modulation allows one to tune the sensitivity and range of the sensor to best suit the particular application. We describe a PWS primarily designed to perform in-vivo measurements of human eyes. The sensor is calibrated over a wide range of settings allowing one to choose those best suited to a specific task. For example, enhanced-sensitivity measurements of very small aberrations require small range (closed loop adaptive optics). Alternatively, if one wants to measure the aberrations of the eye without any correction, the range required is subject-dependent and can be large; the price paid is in reduced sensitivity . We present in-vivo measurements of human eyes taken at a number of experimental settings and compare the performance of the PWS at each.

This contribution investigates the influence of phase shift on the measured displacement error in interferometers based on quadrature detection. This error was experimentally investigated using a two-detector homodyne quadrature laser interferometer (HQLI) with two orthogonally polarized signals. Here, the phase shift can be continuously varied by rotating a wave plate. However, the rotation of the wave plate also produces unequal signal amplitudes and different zero offsets, both of which can be corrected with an appropriate signal processing. The measured phase-shift error perfectly agrees with the theoretically determined phase-shift error region. This error is systematic, periodic and severely asymmetrical around the nominal displacement value. For the robust realization of a HQLI, a slight deviation from the aligned angle of the wave plate should not shift the phase significantly from the ideal 90°. This may pose a problem if an additional phase shift originates from the polarization-sensitive light reflections, such as the reflection at the nonpolarizing beam splitter.

This paper proposes a highly sensitive and compact optofluidics-based refractometer. Our key idea is based on the use of a two-channel microfluidic chip in a free-space Young interferometer structure that provides an inherent advantage in cancelling the unwanted optical phase noise induced from the surrounding environment during optical beam propagation. By using a 655-nm monochromatic light that travels in free space for 57.5 cm and a microfluidic chip designed to have two fluidic channels in parallel with a distance between channels of 900 μm and a channel depth of 100 μm, our simulation indicates that analysis of the movement of the interference fringe offers a sensitivity of better than 1.31×10^-4 in measuring the change of the refractive index. A better than thousand-fold improvement can also be accomplished by investigating the amplitude change of the interference fringe via a > 10-bit digitalization. Our experiment in analyzing the change of the refractive index of the sucrose solution will be discussed. Polarization insensitivity and simplicity in design and implementation are also additional key advantages.

Far infrared (FIR) is becoming more widely accepted within the automotive industry as a powerful sensor to detect Vulnerable Road Users like pedestrians and bicyclist as well as animals. The main focus of FIR system development lies in reducing the cost of their components, and this will involve optimizing all aspects of the system. Decreased pixel size, improved 3D process integration technologies and improved manufacturing yields will produce the necessary cost reduction on the sensor to enable high market penetration. The improved 3D process integration allows a higher fill factor and improved transmission/absorption properties. Together with the high Thermal Coefficient of Resistance (TCR) and low 1/f noise properties provided by monocrystalline silicon germanium SiGe thermistor material, they lead to bolometer performances beyond those of existing devices. The thermistor material is deposited and optimized on an IR wafer separated from the read-out integrated circuit (ROIC) wafer. The IR wafer is transferred to the ROIC using CMOS compatible processes and materials, utilizing a low temperature wafer bonding process. Long term vacuum sealing obtained by wafer scale packaging enables further cost reductions and improved quality. The approach allows independent optimization of ROIC and thermistor material processing and is compatible with existing MEMS-foundries, allowing fast time to market.

The next generation of automotive Night Vision Enhancement systems offers automatic pedestrian recognition with a performance beyond current Night Vision systems at a lower cost. This will allow high market penetration, covering the luxury as well as compact car segments. Improved performance can be achieved by fusing a Far Infrared (FIR) sensor with a Near Infrared (NIR) sensor. However, fusing with today's FIR systems will be too costly to get a high market penetration. The main cost drivers of the FIR system are its resolution and its sensitivity. Sensor cost is largely determined by sensor die size. Fewer and smaller pixels will reduce die size but also resolution and sensitivity. Sensitivity limits are mainly determined by inclement weather performance. Sensitivity requirements should be matched to the possibilities of low cost FIR optics, especially implications of molding of highly complex optical surfaces. As a FIR sensor specified for fusion can have lower resolution as well as lower sensitivity, fusing FIR and NIR can solve performance and cost problems. To allow compensation of FIR-sensor degradation on the pedestrian detection capabilities, a fusion approach called MultiSensorBoosting is presented that produces a classifier holding highly discriminative sub-pixel features from both sensors at once. The algorithm is applied on data with different resolution and on data obtained from cameras with varying optics to incorporate various sensor sensitivities. As it is not feasible to record representative data with all different sensor configurations, transformation routines on existing high resolution data recorded with high sensitivity cameras are investigated in order to determine the effects of lower resolution and lower sensitivity to the overall detection performance. This paper also gives an overview of the first results showing that a reduction of FIR sensor resolution can be compensated using fusion techniques and a reduction of sensitivity can be compensated.

Cost efficient integration technologies and materials for manufacturing of uncooled infrared bolometer focal plane arrays (FPA) are presented. The technology platform enables 320x240 pixel resolution with a pitch down to 20 μm and very low NETD. A heterogeneous 3D MEMS integration technology called SOIC (Silicon-On-Integrated-Circuit) is used to combine high performance Si/SiGe bolometers with state-of-the-art electronic read-out-integrated-circuits. The SOIC integration process consists of: (a) Separate fabrication of the CMOS wafer and the MEMS wafer. (b) Adhesive wafer bonding. (c) Sacrificial removal of the MEMS handle wafer. (d) Via-hole etching. (e) Via formation and MEMS device definition. (f) Sacrificial etching of the polymer adhesive. We will present an optimized process flow that only contains dry etch processes for the critical process steps. Thus, extremely small, sub-micrometer feature sizes and vias can be implemented for the infrared bolometer arrays. The Si/SiGe thermistor is grown epitaxially, forming a mono-crystalline multi layer structure. The temperature coefficient of resistance (TCR) is primarily controlled by the concentration of Ge present in the strained SiGe layers. TCR values of more than 3%/K can be achieved with a low signal-to-noise ratio due to the mono-crystalline nature of the material. In addition to its excellent electrical properties, the thermistor material is thermally stable up to temperatures above 600 °C, thus enabling the novel integration and packaging techniques described in this paper. Vacuum sealing at the wafer level reduces the overall costs compared to encapsulation after die singulation. Wafer bonding is performed using a Cu-Sn based metallic bonding process followed by getter activation at ≥350 °C achieving a pressure in the 0.001 mbar range. After assembling, the final metal phases are stable and fully compatible with hightemperature processes. Hermeticity over the product lifetime is accomplished by well-controlled electro-deposition of metal layers, optimized bonding parameters and a suitable bond frame design.

A thermopile-based detector array for use in a miniaturized Infrared (IR) spectrometer has been designed and fabricated using CMOS compatible MEMS technology. The emphasis is on the optimal of the detector array at the system level, while considering the thermal design, the dimensional constraints of a design on a chip and the CMOS compatibility. The resolving power is maximized by spacing the Thermo-Electric (TE) elements at an as narrow as possible pitch, which is limited by processing constraints. The large aspect ratio of the TE elements implies a large cross-sectional area between adjacent elements within the array and results in a relatively large lateral heat exchange between micromachined elements by thermal diffusion. This thermal cross-talk is about 10% in case of a gap spacing of 10 μm between elements. Therefore, the detector array should be packaged (and operated) in vacuum in order to reduce the cross-talk due to the air conduction through the gap. Thin film packaging is a solution to achieve an operating air pressure at 1.3 mBar, which reduces the cross-talk to 0.4%. One of other advantages of having low operating pressure is the increased sensitivity of single TE element. An absorber based on an optical interference filter design is also designed and fabricated as an IC compatible post-process on top the detector array. The combination of the use of CMOS compatible materials and processing with high absorbance in 1.5 - 5 μm wavelength range makes a complete on-chip microspectrometer possible.

This paper presents an architecture for the implementation of vision chips in 3-D integration technologies. This architecture employs the multi-functional pixel concept to achieve full parallel processing of the information and hence high processing speed. The top layer includes an array of optical sensors which are parallel-connected to the second layer, consisting of an array of mixed-signal read-out and pre-processing cells. Multiplexing is employed so that each mixedsignal cell handles several optical sensors. The two remaining layer are respectively a memory (used to store different multi-scale images obtained at the mixed-signal layer) and an array of digital processors. A prototype of this architecture has been implemented in a FDSOI CMOS-3D technology with Through-Silicon-Vias of 5μm x 5μm pitch.

In this paper a Time-Of-Flight range camera based on Current Assisted Photonic Demodulators is presented. The sensor, fabricated in a 0.18 μm CMOS technology, features 120x160 pixel resolution with 10μm pixel pitch and 24% fill factor. Pixel, camera and system architectures are described highlighting the most important design issues, and a selection of experimental results is presented. The chip has a power consumption of 200mW, mainly due to the contribution of modulation current. A range camera system was realized using the proposed sensor, a focusing optics providing a 23°x30° field of view, and a 3-LED illumination module delivering 140mW optical power on the target. The system is capable of acquiring a stream of 7 3D frames/s with a maximum non-linearity of 3.3% in the range 1.2m-3.7m and a precision better than 10 cm at 2m and 20 cm at 3m.

A linear array of 128 Active Pixel Sensors has been developed in standard CMOS technology and a Linear Variable Optical Filter (LVOF) is added using CMOS-compatible post-process, resulting in a single chip highly-integrated highresolution microspectrometer. The optical requirements imposed by the LVOF result in photodetectors with small pitch and large length in the direction normal to the dispersed spectrum (7.2μ;m×300μm). The specific characteristics of the readout are the small pitch, low optical signals (typically a photocurrent of 100fA~1pA) and a much longer integration time as compared to regular video (typically 100μs~63s). These characteristics enable a very different trade-off between SNR and integration time and IC-compatibility. The system discussed in this paper operates in the visible part of the spectrum. The prototype is fabricated in the AMIS 0.35μm A/D CMOS technology.

We report on the fabrication and characterization of solar blind Metal-Semiconductor-Metal (MSM) based photodetectors for use in the extreme ultraviolet (EUV) wavelength range. The devices were fabricated in the AlGaN-on- Si material system, with Aluminum Gallium Nitride (AlGaN) epitaxial layers grown on Si(111) by means of Molecular Beam Epitaxy. The detectors' IV characteristics and photoresponse were measured between 200 and 400 nm. Spectral responsivity was calculated for comparison with the state-of-the-art ultraviolet photodetectors. It reaches the order of 0.1 A/W at the cut-off wavelength of 360 nm, for devices with Au fingers of 3 μm width and spacing of 3 μm. The rejection ratio of visible radiation (400 nm) was more than 3 orders of magnitude. In the additional post-processing step, the Si substrate was removed locally under the active area of the MSM photodetectors using SF₆-based Reactive Ion Etching (RIE). In such scheme, the backside illumination is allowed and there is no shadowing of the active layer by the metal electrodes, which is advantageous for the EUV sensitivity. Completed devices were assembled and wire-bonded in customized TO-8 packages with an opening. The sensitivity at EUV was verified at the wavelengths of 30.4 and 58.4 nm using a He-based beamline. AlGaN photodetectors are a promising alternative for highly demanding applications such as space science or modern EUV lithography. The backside illumination approach is suited in particular for large, 2D focal plane arrays.

The large interest shown in the field of terahertz detectors research by the scientific community brought to the development of several kinds of devices based on different principles. Many, however, have some peculiar characteristics that prevent their use in low-cost or compact equipment, because of cryogenic cooling, or the use of exotic materials, etc. Recently, approaches using field-effect transistors exploiting the oscillation of electron density (plasma waves) or the phenomena known as "self-mixing" in RF modulators, allowed to foresee the possibility to employ standard CMOS technologies to build such sensors. In this work we analyze the behavior of this kind of detectors in order to understand how to design an optimized device and then how to exploit it by proper readout. Optimized electromagnetic coupling to the detector has been implemented using a dipole antenna. Electromagnetic simulations together with the developed model allowed calculating a projected noise figure of the detector of 38ρW / √Hz. Two dedicated readout circuits have been designed, one intended to read the detector as a current generator, while the other reads it as a voltage generator. The developed circuits have been designed and sent for fabrication in a standard 0.35μm CMOS technology.

Commercially available instruments for measuring and monitoring surface temperature of metal parts are very limited and often unsuitable for applications at harsh environment conditions. Another major challenge is to measure temperature of a rotating surface, as it is very difficult to take an electrical signal out from rotating parts. A novel optical reflectance based non-contact temperature measurement technique is discussed which can be used for temperature measurement on metal surfaces. The optical reflectivity of metals is known to depend on metal temperature and wavelength of the incident light. An increase in metal temperature resulted in the change (increase or decrease depending on particular metal properties) of reflected laser power from the metal surface. This also depends on the surface geometries of the metal surface being measured. We have shown that the sensitivity of the temperature measured depends on the angle of incidence, surface topology and surface properties of the object.

One of the key issues concerning the measurement of size and density of dust grains based on light scattering system is the compensation of the stray light due to the optical components misalignment and to the possible contamination of these components by the dust particles during the measurement runs. This paper focuses on the case study of MEDUSA (Martian Environmental DUst Systematic Analyzer), one of the experiments initially selected for the ExoMars mission, planned by the European Space Agency (ESA), with the scientific objective to study water and dust in Mars atmosphere. The MEDUSA experiment foresees an Optical System (OS) aimed at measuring atmospheric dust content and size distribution. One pump assures that the proper gas and dust flow circulates inside the instrument. This paper reports the description and trade off analysis of several techniques for the stray-light compensation implemented on the MEDUSA OS Proximity Electronics (PE) Test Board (2006), designed and manufactured by INAF-Osservatorio Astronomico di Capodimonte, in the frame of the MEDUSA Bread Board (B/B) activities. The PE Test Board can implement more than one compensation mode, such as: AC coupling, DC coupling with offset compensation via external loop and DC coupling with offset compensation via on board HW loop. The choice among the mentioned compensation modes shall be done also according to the configuration of the overall acquisition system, implemented by the Main Electronics (ME), as explained in the reported trade-off analysis. For the architecture configuration of the industrial breadboard (2008) the preferred solution was the one based on the DC coupling with on board HW loop, for which some test results are reported.

Since the deployment of the credit card, the number of credit card fraud cases has grown rapidly with a huge amount of loss in millions of US dollars. Instead of asking more information from the credit card's holder or taking risk through payment approval, a nondestructive and data-non-intrusive credit card verifier is highly desirable before transaction begins. In this paper, we review optical techniques that have been proposed and invented in order to make the genuine credit card more distinguishable than the counterfeit credit card. Several optical approaches for the implementation of credit card verifiers are also included. In particular, we highlight our invention on a hyperspectral-imaging based portable credit card verifier structure that offers a very low false error rate of 0.79%. Other key features include low cost, simplicity in design and implementation, no moving part, no need of an additional decoding key, and adaptive learning.

Window glass fragments from four Belgian sites were studied and for a set of eighty-five samples the UV-VIS-NIR transmission spectra were analyzed. This collection contains historical and archaeological finds originating from religious buildings namely the Basilica of Our Lady of Hanswijk in Mechelen (17^th-20^thc) and the Church of Our Lady in Bruges (16^th-20^thc) as well as from secular buildings as a private house/Antwerp (18^th-1948) and the castle of Middelburg-in-Flanders (1448-17thc). All sites contain material on the hinge point between the medieval and the industrial tradition. The variation in composition of the analyzed samples can be explained by the use of different glassmaking recipes, more specifically the use of different raw materials. The composition of window glass differs essentially in the type of flux, using a potash rich fluxing agent until the post-medieval times and industrial soda from the 19^th century onwards. A second difference concerns the iron impurities in the glass. For all fragments a clear compositional classification could be made based on the iron concentration. These conclusions were based on archaeological research and drawn after submitting samples to expensive, complex, time-consuming and destructive chemical analyzing methods. Our study indicates that similar conclusions could be made applying the proposed optical based methodology for plain window glass. As a whole, the obtained results make it possible to cluster the fragments for a particular site based on three different sensing parameters: the UV absorption edge, the color and the presence of characteristic absorption bands. This information helps in identifying trends to date window glass collections and indicating the use of different raw materials, production technologies and/or provenance.

First results for luminescence-based dual sensing of environmental parameters with a multicolor LED are presented. With this method, cross-sensitivities of the luminescence material between such environmental properties can be eliminated. A very compact and inexpensive sensor setup is possible by use of an LED as excitation source and another LED simultaneously as luminescence detector. The parametrization of the cross-sensitivity will be discussed and possible further simplifications of the sensor will be pointed out.

An acoustic emission sensor was designed for partial discharges (PD), constructed, calibrated, and tested. The device is based on an interferometric fiber-optic probe and is able to detect PD ultrasound emission at 150 kHz. It may be placed next to the discharge source, thus overcoming the difficulties of acoustic attenuation. The device works in the appropriate bandwidth for narrowband acoustic detection of PD activity, like the PZT transducers mounted on the exterior of the transformer tank. It represents a simple and cheap alternative for detecting acoustic emission, susceptible of being used in a multi-channel optical configuration and able to provide information for locating the source. The sensor is an optical fiber coil exposed to the ultrasonic waves that is interrogated with an all-fiber Mach-Zehnder interferometer. We report first the calibration at the natural frequency of the coil (20 kHz) and at the main frequency of the application (150 kHz), and compared with the response of PZT transducers. The sensitivity decays with the frequency, but it is comparable with the PZT sensitivity by placing the sensor next to the source, which is possible with the immersed approach (or embedded). A certain range of compensation is obtained at low frequencies with a feed-back loop. A second feed-back loop with electronic resonance around 150 kHz is used in order to improve the sensitivity. Thus, the conditioning circuit provides directly the amplified optical phase signal. Results of acoustic emission with both frequencies simultaneously are presented.

This paper presents the simulation modelling of a typical experimental setup for time-resolved fluorescence measurement. The developed model takes into account the setup geometry, characteristics of light source, detector and fluorescent sample as well as the adopted measurement technique. A qualitative verification of the model has been reported before. In this paper, we present a quantitative analysis and verification of the system versatility. For this we conducted time-resolved fluorescence measurements using a two-chip based micro-system, including a blue micro-LED array as a light source and a CMOS SPAD array as a detector. The sample of interest (CdSe/ZnS quantum dots in toluene) in a micro-cavity slide and an excitation filter were placed in the gap between the excitation and detection planes. A time-correlated single photon counting module was used to build fluorescence decay curves. A range of experiments with different excitation light pulse widths and using several setups have been performed. The simulated data are in good agreement with measured results and the model proves to be flexible enough to simulate different light sources and detector quenching/recharging circuits. This model can be used to predict qualitative and quantitative results for specific experimental setups, supporting the explanations of observed effects and allowing the realisation of virtual experiments.

Synchrotron radiation is emitted whenever a beam of charged particles passes though a magnetic field. The power emitted is strongly dependent on the relativistic Lorentz factor of the particles, which itself is proportional to the beam energy and inversely proportional to the particle rest mass. Thus, synchrotron radiation is usually associated with electron accelerators, which are commonly used as light sources. However the largest proton machines reach sufficiently high energies to make synchrotron light useful for diagnostic purposes. The Large Hadron Collider at CERN will accelerate protons up to an energy of 7TeV. An optical arrangement has been made which focuses synchrotron light from two LHC magnets to image the cross-section of the beam. It is also planned to use this setup to produce a longitudinal profile of the beam by use of fast Single Photon Counting. This is complicated by the bunched nature of the beam which needs to be measured with a very large dynamic range. In this contribution we present early experimental data of the transverse LHC beam profile together with a scheme for measuring the longitudinal profile with a time resolution of 50 ps. It includes the use of a gating regime to increase the dynamic range of the photon counter and a three-stage correction algorithm to compensate for the detector's deadtime, afterpulsing and pile-up effects.

We report on a refractive index sensor based on a planar Bragg grating (PBG) capable to online monitor the water content in Biodiesel and the amount of ethanol admixture to conventional fuels, respectively. Our results demonstrate the capability of the sensor to distinguish the transition between about 190 and 500 ppm water in Biodiesel, enabling to monitor the production process of Biodiesel in the relevant range according to industrial standards. The ethanol content in petrol has been investigated in the range of 0-100%, covering the entire standardized range of E-5 to E-85 fuel mixing ratios. These experiments reveal a sensitivity of 112 nm/riu allowing the measurement of the ethanol content with a resolution of 8.9·10^-6.

This paper presents a simultaneous measurement technique for strain and temperature with dynamic temperature compensation using two SMS structures with one acting as a temperature compensating element and another SMS structure acting as a strain sensor. Experimental results show that this technique offers a resolution of better than 2 με for strain measurements in the range from 0 to 1000 με and the temperature induced error is as low as 7.5 με in the temperature range from 15 to 45 °C. The temperature can be measured simultaneously with an experimentally demonstrated resolution better than 0.84 °C.

InGaAsN is a promising material system to enable low-cost GaAs-based detectors to operate in the telecommunication spectrum, despite the problems posed by the low growth temperature required for nitrogen incorporation. We demonstrate that InGaAsN p⁺-i-n⁺ structures with nominal In and N fraction of 10% and 3.8%, grown by molecular beam epitaxy (MBE) under non-optimal growth conditions, can be optimized by post growth thermal annealing to match the performance of optimally grown structures. We report the findings of an annealing study by comparing the photoluminescence spectra, dark current and background concentration of the as-grown and annealed samples. The dark current of the optimally annealed sample is approximately 2 μA/cm² at an electric field of 100 kV/cm, and is the lowest reported to date for InGaAsN photodetectors with a cut-off wavelength of 1.3 μm. Evidence of lower unintentional background concentration after annealing at a sufficiently high temperature, is also presented.