International round-robin experiment for angle-resolved light scattering measurement.

An international round-robin experiment has been conducted to test procedures and methods for the measurement of angle-resolved light scattering. ASTM E2387-05 has been used as the main guide, while the experience gained should also contribute to the new ISO standard of angle-resolved scattering currently under development (ISO/WD 19986:2016). Seven laboratories from Europe and the United States measured the angle-resolved scattering from Al/SiO2-coated substrates, transparent substrates, volume diffusors, quasi-volume diffusors, white calibration standards, and grating samples at laser wavelengths in the UV, VIS, and NIR spectra. Results were sent to Fraunhofer IOF, which coordinated the experiments and analyzed the data, while ESA-ESTEC, as the project donor, defined conditions and parameters. Depending mainly on the sample type, overall good to reasonable agreements were observed, with largest deviations at scattering angles very close to the specular beam. Volume diffusor characterization unexpectedly turned out to be challenging. Not all participants provided measurement uncertainty ranges according to the Guide to the Expression of Uncertainty in Measurement; often, a single general scatterometer-related measurement uncertainty value was stated. Although relative instrument measurement uncertainties close to 1% are sometimes claimed, the comparison results did not support these claims for specular scattering samples as mirrors, substrates, or gratings.

Light trimming of a narrow bandpass filter based on a photosensitive chalcogenide spacer.

We present an experimental study of the photosensitive properties of a narrow bandpass filter based on a Ge(15)Sb(20)S(65) spacer fabricated by electron beam deposition. For a single layer, near the optical bandgap of this chalcogenide material, the efficiency of the photo-bleaching increases as the central wavelength of the light source for exposure decreases. The maximum relative photo-induced change of the optical thickness reaches about 1%. By using controlled light exposure around 480 nm of a photosensitive narrow bandpass filter centered at 1550 nm, we obtained a spatially localized shift of its peak wavelength up to 5.4 nm. This property is used to perform, for the first time at our knowledge, the post trimming of a narrow bandpass filter with a light beam. A 5 x 5 mm(2) ultra uniform area in which the relative spatial variation of its peak wavelength remains below 0.004% is demonstrated.

An alternative scattering method to characterize surface roughness from transparent substrates.

An alternative scattering method is developed to characterize surface roughness from the two faces of transparent substrates. Specific weights are attributed to each surface in the scattering process, due to the large substrate thickness. The resulting roughness spectra are shown to quasi-overlap those of near field microscopy.

Improvement of pyroanalyzer sensitivity for infrared and far-infrared studies.

By using a [(signal) + (noise)] - (noise) digital averaging technique in order to reduce the offset drift, an improvement of about a factor 100 has been obtained in the sensitivity of a pyroelectric detector array (PDA). Different applications of the apparatus for measurement of infrared and far-infrared beam characteristics are also presented.

A high accuracy femto-/picosecond laser damage test facility dedicated to the study of optical thin films.

A laser damage test facility delivering pulses from 100 fs to 3 ps and designed to operate at 1030 nm is presented. The different details of its implementation and performances are given. The originality of this system relies the online damage detection system based on Nomarski microscopy and the use of a non-conventional energy detection method based on the utilization of a cooled CCD that offers the possibility to obtain the laser induced damage threshold (LIDT) with high accuracy. Applications of this instrument to study thin films under laser irradiation are presented. Particularly the deterministic behavior of the sub-picosecond damage is investigated in the case of fused silica and oxide films. It is demonstrated that the transition of 0-1 damage probability is very sharp and the LIDT is perfectly deterministic at few hundreds of femtoseconds. The damage process in dielectric materials being the results of electronic processes, specific information such as the material bandgap is needed for the interpretation of results and applications of scaling laws. A review of the different approaches for the estimation of the absorption gap of optical dielectric coatings is conducted and the results given by the different methods are compared and discussed. The LIDT and gap of several oxide materials are then measured with the presented instrument: Al(2)O(3), Nb(2)O(5), HfO(2), SiO(2), Ta(2)O(5), and ZrO(2). The obtained relation between the LIDT and gap at 1030 nm confirms the linear evolution of the threshold with the bandgap that exists at 800 nm, and our work expands the number of tested materials.

Interferometric measurement of the phase of diffracted waves near the plasmon resonances of metallic gratings.

We propose a method for analyzing both theoretically and experimentally the behavior of the phase of the waves diffracted by gratings. The method is applied to the study of resonance phenomena. It is used for determining the optogeometrical parameters of a metallic grating. We show that the experimental setup, which is insensitive to mechanical drifts or thermal fluctuations, can be used for sensing purposes.

Influence of chromatic dispersion on a dual-wavelength passive-homodyne detection method for fiber-coupled interferometers.

The passive-homodyne method is very attractive for demodulating optical signals available at the output of interferometric sensors by the use of coherence multiplexing. Phase measurement and the use of two broadband sources with different central wavelengths permit a resolution of λ/10(5) to be achieved with a 20-µ range-sensor optical path difference. However, dispersive birefringent elements are required in this technique, which has some disturbing effects on the correlation signal position, on its envelope form, and on the measured phase. An analytical treatment of the problem and a numerical validation are described. Experimental evidence of the effects predicted by theory is presented.