Continuously apodized fiber-to-chip surface grating coupler with refractive index engineered subwavelength structure Halir, R.; Cheben, P.; Schmid, J. H.; Ma, R.; Bedard, D.; Janz, S.; Xu, D.-X.; Densmore, A.; Lapointe, J.; Molina-Fernández, Í. We demonstrate a fully etched, continuously apodized fiber-to-chip surface grating coupler for the first time (to our knowledge). The device is fabricated in a single-etch step and operates with TM-polarized light, achieving a coupling efficiency of 3:7 dB and a 3 dB bandwidth of 60 nm. A subwavelength microstructure is employed to generate an effective medium engineered to vary the strength of the grating and thereby maximize coupling efficiency, while mitigating backreflections at the same time. Minimum feature size is 100 nm for compatibility with deep-UV 193 nm lithography.

This paper presents a detailed analysis of the CanX-1 engineering model spectral reflectance measurements that were collected during the model's spectrometric characterization. Over 1000 low-resolution spectra spanning a range between 350 and 2500 nm were gathered over a wide range of observational geometries in a controlled laboratory environment. As part of the data analysis, a physics-based reflectance model was implemented in a numerical computing environment to allow for a better understanding of the measurements. In the end, the experiment led to two unexpected results. First, the spectrometric measurements of the triple-junction photovoltaic cells showed significant interference fringes at wavelengths greater than 800 nm, which, at the time of this experiment, did not correspond to any previously published spectra for this type of material. Second, these empirical results also showed that the spectral reflectance of photovoltaic cells varied significantly as a function of the illumination geometry. The modeling results confirmed this finding and showed that this was also the case for an aluminum surface with an rms surface roughness different from zero. These results invalidate a widely held assumption among those who study the spectrometric characterization of artificial space objects that the spectral reflectance of homogeneous materials is invariant. The outcomes of this study not only serve to improve the laboratory characterization of spacecraft in controlled environments before their launch but also provide new insight in the interpretation of spectrometric measurements of artificial space objects, namely spacecraft and space debris, in Earth orbit.

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This paper presents laboratory measurements and analysis of optical spectral bidirectional reflectance distribution functions of materials commonly found on the surfaces of spacecraft. A goniospectrometer sensitive to light with wavelengths between 350 and 1100 nm was constructed, and four classes of materials were studied in a variety of illumination and observational geometries: triple-junction photovoltaic cells, aluminum, white-coated metallic surface, and aluminized polyimide films. The measurements were analyzed to study how specific materials could be characterized based on their spectral bidirectional reflectance distribution function and how it varies with changing illumination and observational geometry. In the end, the experiment yielded two significant outcomes. First, the results provide a better understanding of how the spectral energy distribution of light reflected from these materials behaves as the illumination and observational geometry is varied. This result will be particularly useful in the interpretation of spectra and color photometric measurements of artificial objects in Earth orbit. Second, and more important, this experiment convincingly demonstrates that any spectral library of materials commonly found on spacecraft must include measurements taken throughout the widest range of observational geometries. With this in mind, the measurements collected during this experiment now serve as the first entries in an open-access spectral library of spacecraft material.

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