Large-aperture Fabry-Perot filters based on silicon/silicon carbonitride distributed Bragg reflectors for the near-infrared
This contribution presents silicon carbonitride as a new low refractive index material for integration together with silicon in suspended distributed Bragg reflector membranes. We show how its benign mechanical stress properties can be used to fabricate inherently flat mirror membranes of up to 5 mm diameter. Due to the large refractive index contrast to silicon such mirror membranes exhibit high reflectance over a wide spectral range. Using these mirrors, we demonstrate a proof of concept for a large aperture Fabry-Perot filter with narrow bandwidth that could help miniaturize hyperspectral imaging sensors.
MEMS Fabry–Pérot Interferometers With Double Membrane Mirrors for Improved Mirror Parallelism
This paper presents surface micromachined MEMS Fabry–Pérot interferometers (FPIs) with two released membrane mirrors. Fabrication of such FPIs with large-area membrane reflectors of up to 5 mm in diameter for first-order operation in the near infrared is shown. The mirrors are distributed Bragg reflectors based on silicon and silicon carbonitride. Single mirror membrane FPIs with the same mirror design are used as a benchmark in order to demonstrate the improvements achievable by the double membrane approach. To that end, the FPIs are characterized with respect to their top surface flatness by white light interferometry as well as to the variations of the optical gap width over the aperture area by spatially resolved transmittance measurements. It is shown that the upper mirror membranes are flat within the nanometer range for both approaches. Nevertheless, single membrane FPIs exhibit strongly increasing gap variations for increasing membrane diameters due to a bow in the substrate wafer, thus decreasing the filter resolution over the full aperture. On the other hand, for double-membrane FPIs, the mirrors are highly parallel with a standard deviation of the gap as low as 2 nm. [2018-0096]
High-frequency hearing thresholds: effects of age, occupational ultrasound and noise exposure
AbstractPurpose
It has been suggested that high-frequency audiometry (HFA) could represent a useful preventive measure in exposed workers. The aim was to investigate the effects of age, ultrasound and noise on high-frequency hearing thresholds.MethodsWe tested 24 industrial ultrasound-exposed subjects, 113 industrial noise-exposed subjects and 148 non-exposed subjects. Each subject was tested with both conventional-frequency (0.125–8 kHz) and high-frequency (9–18 kHz) audiometry.ResultsThe hearing threshold at high frequency deteriorated as a function of age, especially in subjects more than 30 years old. The ultrasound-exposed subjects had significantly higher hearing thresholds than the non-exposed ones at the high frequencies, being greatest from 10 to 14 kHz. This hearing loss was already significantly evident in subjects with exposure <5 years and increased with years of exposure and advancing age. The noise exposure group had significantly higher hearing thresholds than the non-exposed group at the conventional frequencies 4 and 6 kHz and at the high frequency of 14 kHz. After stratification for age, there was a significant difference between the two groups at 9–10 and 14–15 kHz only for those under 30 years of age.ConclusionMultivariate analysis indicated that age was the primary predictor, and noise and ultrasound exposure the secondary predictors of hearing thresholds in the high-frequency range. The results suggest that HFA could be useful in the early diagnosis of noise-induced hearing loss in younger groups of workers (under 30 years of age).
