Mart Noorma

Characterization of filter radiometers with a wavelength-tunable laser source

Results of filter radiometer characterization with a wavelength-tunable Ti : sapphire laser in the wavelength band around 900 nm are presented. The effect of interference between the reflections from filter surfaces in the case of coherent laser light was studied and reduced with a special filter design with antireflection coatings. Measuring the responsivity as a function of wavelength over a very narrow band was used to reveal the remaining interference effects. Uncertainty analysis and test results indicate that filter radiometers can be characterized with a relative standard uncertainty of 9×10−4 using the scanning method. The results agree well with more conventional monochromator-based measurements.

MEASUREMENT OF APERTURE AREAS USING AN OPTICAL COORDINATE MEASURING MACHINE

The use of an optical coordinate measuring machine (CMM) for the diameter measurement of optical apertures is described. The traceability and mechanical stability of the aperture areas are of importance for accurate photometric and radiometric measurements. Detailed evaluation of the measurement uncertainty for the aperture diameter is presented. High-accuracy mechanical CMM was used to confirm the validity of the optical CMM results. The difference between the contact and non-contact measurement was 0.1 µm for the mean diameter result. If the required standard uncertainty for the mean diameter is of the order of 1 µm, the optical CMM provides an efficient method for aperture area measurements.

Comparison of absolute spectral irradiance responsivity measurement techniques using wavelength-tunable lasers

Independent methods for measuring the absolute spectral irradiance responsivity of detectors have been compared between the calibration facilities at two national metrology institutes, the Helsinki University of Technology (TKK), Finland, and the National Institute of Standards and Technology (NIST). The emphasis is on the comparison of two different techniques for generating a uniform irradiance at a reference plane using wavelength-tunable lasers. At TKKs Laser Scanning Facility (LSF) the irradiance is generated by raster scanning a single collimated laser beam, while at the NIST facility for Spectral Irradiance and Radiance Responsivity Calibrations with Uniform Sources (SIRCUS), lasers are introduced into integrating spheres to generate a uniform irradiance at a reference plane. The laser-based irradiance responsivity results are compared to a traditional lamp-monochromator-based irradiance responsivity calibration obtained at the NIST Spectral Comparator Facility (SCF). A narrowband filter radiometer with a 24 nm bandwidth and an effective band-center wavelength of 801 nm was used as the artifact. The results of the comparison between the different facilities, reported for the first time in the near-infrared wavelength range, demonstrate agreement at the uncertainty level of less than 0.1%. This result has significant implications in radiation thermometry and in photometry as well as in radiometry.

Characterization of germanium photodiodes and trap detector

We have developed and characterized new detectors based on germanium (Ge) photodiodes to be used in the wavelength region between 900 and 1650 nm. The effects of spatial uniformity, temperature and low shunt resistance on the spectral responsivity measurements are studied. Our results for the spatial uniformities show improvements as compared with earlier studies. The spectral reflectances of a Ge photodiode and trap detector are also studied, and the trap reflectance is found to be less than 10−4 at wavelengths longer than 900 nm. The results of this study show that with careful use, the large area Ge photodiodes can offer a cost-effective alternative as compared with InGaAs photodiodes of similar diameters.

Characterization of GaAsP trap detector for radiometric measurements in ultraviolet wavelength region

A trap detector was constructed of three Schottky-type 10×10mm2 GaAsP photodiodes. The spectral reflectance of the trap detector was calculated from the measured spectral reflectances of a single GaAsP photodiode in the wavelength range between 240 and 600nm, and compared to the measured spectral reflectance of the trap detector at three laser wavelengths. The absolute spectral responsivity of the trap detector was measured. The internal quantum efficiencies (IQE) of the trap detector and a single photodiode were calculated in the wavelength region between 250 and 400nm from the spectral reflectances and responsivities. The comparison revealed reduction of the apparent IQE of the trap detector as compared to the single photodiode at the level of 10%. The spatial uniformity of the responsivity of the trap detector was measured, and the corresponding uncertainty component at 325nm was calculated to be 4×10−4. The effect of moderate ultraviolet exposure at the level of 50mJ∕cm2 on the stability of the respon...