Photonic Engineering Group of the University of Cantabria: Recent R&D Contributions in Photonic Sensing Technology

The recent research and development results of the Photonic Engineering Group in the photonic sensing field are reported. This article includes a sample of contributions in several of the ongoing R&D lines: fiber sensing using Bragg grating technology; gas sensing, high temperature monitoring; optoelectronic instrumentation for laser welding monitoring; fiber active devices; and, finally, on sensing technology using plastic optical fiber.     

Polarized He spin-filters using MEOP for wide-angle polarization analysis

Polarized ³He spin-filters are currently employed on a wide range of neutron instruments at the ILL, primarily for diffraction, reflectometry and fundamental physics. A wide range of recent and ongoing improvements are enabling the implementation of this technique for wide-angle polarization analysis for inelastic measurements. These include

• Progress in metastability-exchange optical pumping (MEOP), resulting in on-beam polarization levels of up to 80%.

• 1st generation “Pastis-1” coils for rotating the neutron polarization at the sample position, allowing for “XYZ” polarization analysis.

• 2nd generation “Pastis-2” coils with no blind angles in the equatorial plane.

• Spin-filter cells with glued silicon windows, allowing for wide-angle “banana” cells with very low background scattering.

• Polarization-preserving capillaries for transferring polarized ³He gas into the cell without manual access.

The development of capillary transfer also allows for a completely new way of working with ³He spin-filters: connecting the cells on the instruments directly to the MEOP filling station several tens of meters away and allowing for quasi-continuous operation.     

基于Lab VIEW的半导体光源P-I特性测试系统

采用PCI-6251数据采集卡和LahVIEW平台。设计了一套半导体光源的P-I特性测试系统。该系统通过软件驱动数据采集卡为发光器件提供合适的驱动电压,采集发光器件的光功率和驱动电流,实现了P-I特性曲线的测量、分析和存储等功能。测试结果表明该系统设计成本低、测量效率高、人机界面友好、操作简单、扩展性强,适用于教学、实验及科研等领域。     

Determination of the OH radical in atmospheric pressure dielectric barrier discharge plasmas using near infrared cavity ring-down spectroscopy

The hydroxyl radical (OH) plays an important role in combustion systems, atmospheric chemistry and the removal of air pollutants by non-thermal plasmas. The present work reports the determination of the hydroxyl radicals in atmospheric dielectric barrier discharge plasmas via near infrared continuous wave cavity ring-down spectroscopy. The P-branches of OH X²Π_i (ν' = 2 ←ν^′′ = 0) bands were used for its number density measurements. The minimum measurable absorption coefficient is about 3 × 10^-8 cm^-1 in DBD plasmas. At certain experimental conditions (a.c. frequency of 70 kHz, 6700 ppm H₂O in He, 1 atm), when the peak-to-peak discharge voltage varied from 6 kV to 10.4 kV, the determined OH radical concentration increased from (2.1 ± 0.1) × 10¹³ molecules cm^-3 to (3.7 ± 0.1) × 10¹³ molecules cm^-3. The plasma gas temperature, derived from the Boltzmann plots of OH rotational population distributions, ranged from 312 ± 10 K to 363 ± 10 K when the discharge voltage was raised in the above range. The influences of O₂ and N₂ addition on the production of OH radicals have been also investigated.     

A twelve‐analyzer detector system for high‐resolution powder diffraction

A dedicated high‐resolution high‐throughput X‐ray powder diffraction beamline has been constructed at the Advanced Photon Source (APS). In order to achieve the goals of both high resolution and high throughput in a powder instrument, a multi‐analyzer detector system is required. The design and performance of the 12‐analyzer detector system installed on the powder diffractometer at the 11‐BM beamline of APS are presented.     

Characterization of Wall Paintings of the Harem Court in the Alhambra Monumental Ensemble: Advantages and Limitations of In Situ Analysis

Non-invasive techniques (X-ray fluorescence, XRF, and Raman spectroscopy) were used for the study of the Hispano Muslim wall paintings. Principal component analysis (PCA) was performed on the semi-quantitative XRF results directly provided by the in-built factory calibrations with minimum user manipulation. The results obtained were satisfactory and highlighted differences and similarities among the measurement points. In this way, it was possible to differentiate the decorations carried out on gypsum plasterwork and the wall paintings over lime plaster. The color palette, revealed by combining the results from XRF and Raman spectroscopies, comprised the pigments hematite, lapis lazuli, cinnabar (in poor conservation state), and possibly, carbon. Evidence of past interventions was also provided by PCA on XRF data, which detected the presence of Pb, Ba, and Zn in some areas. Furthermore, the preparation layers have been studied in detail on cross-sections of two microsamples. Several layers of lime plaster with a compact microstructure have been observed. The characteristic of the pictorial layer and the identification of calcium oxalate point to the use of a secco-technique. The main alteration identified was a gypsum surface layer covering the painting and signs of plaster deterioration due to gypsum migration to more internal areas. Finally, the comparison with the observations made by restorers in previous interventions on these paintings revealed the importance of the representativeness gained with the in situ study, which enabled the analysis of a high number of areas.     

离轴抛物镜准直特性的研究

由于反射式离轴抛物镜不产生色差并具有较大的有效孔径,已经广泛应用于辐射校准、宽波段的目标模拟和能量聚焦工作中。影响离轴抛物镜准直特性的主要因素有离轴量,有效孔径和焦面出射点的位置。采用矢量的方法,分析了离轴量等参量和准直性能之间的关系。给出了焦面出射点的位置从0~0.05f,相对孔径D/f从0~0.40,相对离轴量h/f从0.1~0.5变化时,离轴抛物镜准直仪的准直性能曲线。运用CODE V对矢量法的分析结果进行了验证,在条件一致的情形,两者结果吻合。这些结果有助于准直仪的设计。     

Proposal for Fourier-Transform Phase-Modulation Fluorometer

I propose a concept of a novel Fourier-transform phase-modulation fluorometer by which a fluorescence decay waveform can be obtained. In the fluorometer, the modulation frequency of the excitation light source is swept continuously from a start frequency f_min to an end frequency f_max with a time duration T. The resultant fluorescence signal waveform is Fourier-transformed to obtain amplitude and phase spectra. The ratio of the amplitude spectrum and the difference of the phase spectrum over those of the reference spectra that are obtained from a non-fluorescent material are calculated, respectively, and the pair of both spectral data is inverse-Fourier-transformed again to obtain the fluorescence decay waveform. To verify and demonstrate the effectiveness of the concept, I carried out (1) numerical simulations, (2) determination of a time constant of a passive resistor-capacitor (RC) differential circuit, and (3) measurement of a fluorescent decay waveform of YAG materials packed in Nichia’s white LED.     

Observation of disruptions in tokamak plasma under the influence of resonant helical magnetic fields

Summary Disruptive instabilities were investigated in the small-tokamak TBR-1 during the application of resonant helical magnetic fields created by external helical windings. Indications were found that the main triggering mechanism of the disruption was the rapid increase of them=2/n=1 mode which, apparently after reaching a certain amplitude, interacts with other resistive modes: the internal 1/1 mode in the case of a major disruption, or with higherm components, as the 3/1 or 4/1, in the case of minor disruptions. After the coupling, the growth of the associated islands would create a chaotic field line distribution in the region between the corresponding rational magnetic surfaces which caused the gross particle transport and, finally, destroyed the confinement. In addition, investigations on higherZ _eff discharges in which a mixture of helium and hydrogen was used resulted in much more unstable plasmas but apparently did not alter the basic characteristics of the disruptions. The authors of this paper have agreed to not receive the proofs for correction.     

The FIGARO II experiment: a general outline of the mission and the principal scientific results

Summary The FIGARO II (French Italian Gamma-Ray Observatory) experiment has been launched successfully three times: in July 1986 from Milo (Trapani), in November 1988 from Charleville (Australia) and in July 1990 again from Milo. In the first flight the observational program was limited to the Crab pulsar PSR0531+21 only because of a telemetry failure: the high sensitivity of FIGARO II allowed an accurate study of the pulse shape as well as a phase-resolved spectroscopy. It was also possible to evaluate the dispersion measure of the Crab pulsar at the flight date from the time delay between gamma-ray and radio pulses. The major results of the second flight were a stringent upper limit to the low-energy gamma-ray flux from PSR 0833-45 (Vela pulsar)—well below the detection claimed by the UCR group —and the observation of a strong emission in the 0.511 MeV annihilation line from the inner region of our Galaxy. The data acquired in the third flight are still under analysis, but preliminary results suggest changes in the shape of the pulse profile. Paper presented at the V Cosmic Physics National Conference, S. Miniato, November 27–30, 1990.