Embedded Fiber Bragg Grating Sensors for Monitoring Temperature and Thermo-Elastic Deformations in a Carbon Fiber Optical Bench.

A composite optical bench made up of Carbon Fiber Reinforced Polymer (CFRP) skin and aluminum honeycomb has been developed for the Tunable Magnetograph instrument (TuMag) for the SUNRISE III mission within the NASA Long Duration Balloon Program. This optical bench has been designed to meet lightweight and low sensitivity to thermal gradient requirements, resulting in a low Coefficient of Thermal Expansion (CTE). In addition to the flight model, a breadboard model identical to the flight one has been manufactured, including embedded fiber Bragg temperature and strain sensors. The aim of this is to explore if the use of distributed fiber Bragg gratings (FBGs) can provide valuable information for strain and temperature mapping of an optical instrument on board a space mission during its operation as well as its on-ground testing. Furthermore, surface-mounted strain FBG sensors and thermocouples have been installed in the optical bench for intercomparison purposes. This paper presents the results obtained from a thermal vacuum test consisting of three thermal cycles with stabilization steps at 100 °C, 60 °C, 20 °C and -20 °C. Experimental results provide information about how FBG embedded temperature sensors can provide a proper and quick response to the temperature changes of the optical bench and that embedded FBG strain sensors are able to measure micro-deformation induced in a close-to-zero CTE optical bench.

Effect of Low-Doses of Gamma Radiation on Electric Arc-Induced Long Period Fiber Gratings.

This work presents an experimental study on the effects of gamma radiation on Long Period Fiber Gratings (LPFGs) in a low-dose test campaign to evaluate their eventual degradation. The study was carried out with standard single-mode fibers where the grating was inscribed using the Electric-Arc Discharge (EAD) technique. Before the gamma campaign, a detailed optical characterization was performed with repeatability tests to verify the accuracy of the setup and the associated error sources. The gamma-induced changes up to a dose of 200 krad and the recovery after radiation were monitored with the Dip Wavelength Shift (DWS). The results show that the gamma sensitivity for a total dose of 200 krad is 11 pm/krad and a total DWS of 2.3 nm has been observed with no linear dependence. Post-radiation study shows that recovery from radiation-induced wavelength shift is nearly complete in about 4000 h. Experimental results show that the changes suffered under gamma irradiation of these LPFGs are temporary making them a good choice as sensors in space applications.

The Complex Molecules Detector (CMOLD): A Fluidic-Based Instrument Suite to Search for (Bio)chemical Complexity on Mars and Icy Moons.

Organic chemistry is ubiquitous in the Solar System, and both Mars and a number of icy satellites of the outer Solar System show substantial promise for having hosted or hosting life. Here, we propose a novel astrobiologically focused instrument suite that could be included as scientific payload in future missions to Mars or the icy moons: the Complex Molecules Detector, or CMOLD. CMOLD is devoted to determining different levels of prebiotic/biotic chemical and structural targets following a chemically general approach (i.e., valid for both terrestrial and nonterrestrial life), as well as their compatibility with terrestrial life. CMOLD is based on a microfluidic block that distributes a liquid suspension sample to three instruments by using complementary technologies: (1) novel microscopic techniques for identifying ultrastructures and cell-like morphologies, (2) Raman spectroscopy for detecting universal intramolecular complexity that leads to biochemical functionality, and (3) bioaffinity-based systems (including antibodies and aptamers as capture probes) for finding life-related and nonlife-related molecular structures. We highlight our current developments to make this type of instruments flight-ready for upcoming Mars missions: the Raman spectrometer included in the science payload of the ESAs Rosalind Franklin rover (Raman Laser Spectrometer instrument) to be launched in 2022, and the biomarker detector that was included as payload in the NASA Icebreaker lander mission proposal (SOLID instrument). CMOLD is a robust solution that builds on the combination of three complementary, existing techniques to cover a wide spectrum of targets in the search for (bio)chemical complexity in the Solar System.

Optical sensing of the fatigue damage state of CFRP under realistic aeronautical load sequences.

We present an optical sensing methodology to estimate the fatigue damage state of structures made of carbon fiber reinforced polymer (CFRP), by measuring variations on the surface roughness. Variable amplitude loads (VAL), which represent realistic loads during aeronautical missions of fighter aircraft (FALSTAFF) have been applied to coupons until failure. Stiffness degradation and surface roughness variations have been measured during the life of the coupons obtaining a Pearson correlation of 0.75 between both variables. The data were compared with a previous study for Constant Amplitude Load (CAL) obtaining similar results. Conclusions suggest that the surface roughness measured in strategic zones is a useful technique for structural health monitoring of CFRP structures, and that it is independent of the type of load applied. Surface roughness can be measured in the field by optical techniques such as speckle, confocal perfilometers and interferometry, among others.

Surface Irregularity Factor as a Parameter to Evaluate the Fatigue Damage State of CFRP.

This work presents an optical non-contact technique to evaluate the fatigue damage state of CFRP structures measuring the irregularity factor of the surface. This factor includes information about surface topology and can be measured easily on field, by techniques such as optical perfilometers. The surface irregularity factor has been correlated with stiffness degradation, which is a well-accepted parameter for the evaluation of the fatigue damage state of composite materials. Constant amplitude fatigue loads (CAL) and realistic variable amplitude loads (VAL), representative of real in- flight conditions, have been applied to "dog bone" shaped tensile specimens. It has been shown that the measurement of the surface irregularity parameters can be applied to evaluate the damage state of a structure, and that it is independent of the type of fatigue load that has caused the damage. As a result, this measurement technique is applicable for a wide range of inspections of composite material structures, from pressurized tanks with constant amplitude loads, to variable amplitude loaded aeronautical structures such as wings and empennages, up to automotive and other industrial applications.

Electronic speckle pattern interferometry using vortex beams.

We show that it is possible to perform electronic speckle pattern interferometry (ESPI) using, for the first time to our knowledge, vortex beams as the reference beam. The technique we propose is easy to implement, and the advantages obtained are, among others, environmental stability, lower processing time, and the possibility to switch between traditional ESPI and spiral ESPI. The experimental results clearly show the advantages of using the proposed technique for deformation studies of complex structures.

A comprehensive approach to deal with instrumental optical aberrations effects in high-accuracy photon's orbital angular momentum spectrum measurements.

With the current and upcoming applications of beams carrying orbital angular momentum (OAM), there will be the need to generate beams and measure their OAM spectrum with high accuracy. The instrumental OAM spectrum distortion is connected to the effect of its optical aberrations on the OAM content of the beams that the instrument creates or measures. Until now, the effect of the well-known Zernike aberrations has been studied partially, assuming vortex beams with trivial radial phase components. However, the traditional Zernike polynomials are not best suitable when dealing with vortex beams, as their OAM spectrum is highly sensitive to some Zernike terms, and completely insensitive to others. We propose the use of a new basis, the OAM-Zernike basis, which consists of the radial aberrations as described by radial Zernike polynomials and of the azimuthal aberrations described in the OAM basis. The traditional tools for the characterization of aberrations of optical instruments can be used, and the results translated to the new basis. This permits the straightforward calculation of the effect of any optical system, such as an OAM detection stage, on the OAM spectrum of an incoming beam. This knowledge permits to correct, a posteriori, the effect of instrumental OAM spectrum distortion on the measured spectra. We also found that the knowledge of the radial aberrations is important, as they affect the efficiency of the detection, and in some cases its accuracy. In this new framework, we study the effect of aberrations in common OAM detection methods, and encourage the characterization of those systems using this approach.

Optical inspection of liquid crystal variable retarder inhomogeneities.

Liquid crystal variable retarders (LCVRs) are starting to be widely used in optical systems because of their capacity to provide a controlled variable optical retardance between two orthogonal components of incident polarized light or to introduce a known phase shifting (PS) between coherent waves, both by means of an applied voltage. Typically, the retardance or PS introduced by an LCVR is not homogeneous across the aperture. On the one hand, the LCVR glass substrates present a global bend that causes an overall variation of the retardance or PS. On the other hand, in the manufacturing process of an LCVR, there sometimes appears a set of micro-air bubbles that causes local retardance or PS inhomogeneities. In this work, we present an interferometric technique based on a Mach-Zehnder interferometer that is insensitive to vibrations and capable of inspecting and characterizing the LCVR's retardance or PS inhomogeneities. The feasibility of the proposed method is demonstrated in the experimental results, where the LCVR retardance is measured with an error of about 0.2 rad. The thickness of possible micro-air bubbles is obtained with a resolution of about 50 nm.

A highly photoconductive poly(vinylcarbazole)/2,4,7-trinitro-9-fluorenone Sol-Gel material that follows a classical charge-generation model.

Highly photoconductive properties are reported for organic-inorganic hybrid sol-gel thin film materials composed of a classical poly(vinylcarbazole)/2,4,7-trinitro-9-fluorenone (PVK/TNF) polymeric mixture, entrapped in a SiO(2) matrix, whose pores have been chemically modified by organic functional groups. The highest photosensitivity obtained, 3.4 x 10(-10) cm Omega(-1) W(-1) at E 22 V microm(-)1, at the optimum molar ratio between the active components, TNF, PVK, and SiO(2), is in the range of the highest values ever reported for any PVK/TNF-based classical photoconductive material. It is demonstrated that the PVK/TNF-based sol-gel films follow Onsager's classical charge-generation model. The analysis of the photocurrent efficiency (Phi) of PVK/TNF-based sol-gel films by such a model provides the primary quantum yield of thermalized pair formation and the initial thermalized pair distance, phi(0) = 0.12 and r(0) = 66.1 Angstrom, respectively, for the optimized sample. As a result of Onsager's analysis, a notorious improvement of the photocurrent generation process was achieved for low TNF concentrations.

Shrinkage control in a photopolymerizable hybrid solgel material for holographic recording.

We report the correction of the shrinkage observed during UV postrecording curing in a holographic solgel material that was recently achieved by the use of various chemical formulations for the composition of the hybrid supporting matrix. We found that a chemical modification of the matrix noticeably attenuates the shrinkage (from 1.3% to 0.4% of the material's initial thickness with the inclusion of just 20% tetramethylorthosilicate), providing a material with improved stability for permanent data storage applications. The holographic properties of samples with different binders are also reported. In addition, a theoretical study has revealed the way by which to compensate for angular deviation in the Bragg condition during UV postrecording by tailoring the binder shrinkage (s), the maximum refractive-index modulation capability of the photosensitive mixture (deltan), or both.