R Goebel

Polarization dependence of trap detectors

The sensitivity of a series of silicon trap detectors has been measured as a function of beam polarization. Measurements and numerical simulation show that very small departures from the ideal orientation of the photodiodes mounted in the trap induces a significant sensitivity to the state of polarization of the beam. Consequences of this polarization dependence for the use of trap detectors as transfer detectors in high-accuracy applications, particularly in cryogenic radiometry, are discussed.

Results of an international comparison of spectral responsivity of silicon photodetectors

The spectral responsivity scales of eighteen national standards laboratories have been compared within the wavelength range 250 nm to 1000 nm using two different types of silicon photodetector. Overall, good agreement was found between the scales of the participating laboratories, although some large deviations were also observed, especially in the ultraviolet.

First results of measurements with the BIPM cryogenic radiometer and comparison with the INM cryogenic radiometer

An Oxford Instruments Radiox cryogenic radiometer was recently installed at the Bureau International des Poids et Mesures (BIPM). The first measurements were aimed at characterizing the performance of the system. The Radiox was then compared with the cryogenic radiometer of the French Institut National de Metrologie (INM), a CRI LaseRad, at the BIPM by successive measurements of the power in a laser beam at two wavelengths: 647 nm and 476 nm. The radiometers differed in these measurements by 1,9 parts in 104, the uncertainties in each being smaller than 1 part in 104.

Some effects of low-power ultraviolet radiation on silicon photodiodes

A short-term effect of low-power ultraviolet (UV) radiation on the widely used Hamamatsu S1337 photodiode is investigated. Experiments show that such photodiodes exposed to 248 nm radiation exhibit a 0,6% increase in responsivity at this wavelength, and a smaller increase at longer wavelengths. The increase is produced during the first tens of seconds of exposure and the new value reached by the responsivity remains stable even after further irradiation. The effect strongly depends on the wavelength of the radiation but is independent of power level within the measurement range of 8 µW to 50 µW. The surface reflectivity of the photodiode is not modified, indicating that radiation directly affects the internal quantum efficiency. It thus seems advisable to age this type of photodiode before calibration or simply to avoid any exposure to radiation below 300 nm.

Nonlinearity and polarization effects in silicon trap detectors

The study reported here was carried out as part of an international comparison of cryogenic radiometers, using silicon trap detectors as transfer instruments. Nonlinearity measurements are part of a more general characterization process of these detectors, including, among other parameters, spatial uniformity, temperature coefficient, sensitivity to beam polarization, and stability. When calibrated against cryogenic radiometers, the uncertainty in their calibration is of the order of 1 part in 104. The accuracy in the nonlinearity measurements should therefore be better than this value.

Comparison of the BIPM cryogenic radiometer with a mechanically cooled cryogenic radiometer from the NPL

The direct comparison at the Bureau International des Poids et Mesures (BIPM) of the BIPM cryogenic radiometer (Radiox) with a mechanically cooled cryogenic radiometer (MCR) from the National Physical Laboratory (NPL) is described. The radiometers were compared using a laser beam at λ = 647 nm with power levels of 400 μW and 650 μW. They were found to be in good agreement, as the measured difference in responsivity was found to be 1,5 parts in 105, with a combined standard uncertainty of 8 parts in 105.

Results of an international comparison of water triple-point cells

The reference temperatures of a large number of triple point of water cells collected from manufacturing sources around the world have been compared at the BIPM and in twelve national laboratories. This allowed an assessment to be made of the reproducibility of the realizations of the triple point of water and the equivalence of the experimental procedures. The results show that, in most cases, the temperatures of the cells agree to within ±0,1 mK. In some cases, however, much larger differences and temperature changes were observed, for which no obvious explanation has so far been found. The results also provide a comparison of a large number of national reference cells.

Influence of the beam shape on aperture measurements with the laser beam scanning technique

The realization of a number of radiometric and photometric units requires one to measure accurately the area of precision apertures. The BIPM employs the laser-beam scanning technique which is based on the generation of a uniform irradiance by a two-dimensional superposition of many regularly spaced laser beams. The existing literature describes the formation of the uniform irradiance as a superposition of Gaussian laser beams and derives a condition for the largest allowable lattice spacing to obtain the necessary uniformity. In this paper we analyse the effect of the use of real laser beams which are almost, but not completely, Gaussian by applying Fourier analysis. Experimental results of aperture measurements with different spacings of the laser beams are also shown. The results of the calculations and the measurements show a characteristic dependence of the measured area on the grid spacing, even for spacings sufficiently small according to the condition based on Gaussian theory. This can be explained by very small deviations of the actual beam shape from a perfect Gaussian profile.

Direct comparison of cryogenic radiometers from the BIPM and the PTB

For the first time, the cryogenic radiometer of the Bureau International des Poids et Mesures (BIPM) has been taken to a national metrology institute for direct comparison with another cryogenic radiometer. This paper describes a comparison at the Physikalisch-Technische Bundesanstalt (PTB, Braunschweig, Germany) with the PTB cryogenic radiometer. The two systems have been shown to differ, in relative terms, by 2,3 × 10-4 in the measurement of optical power at λ = 632,8 nm with a power level of about 700 μW. The standard uncertainty in the determination of this relative difference is estimated to be 5,7 × 10-5. As the source of this discrepancy, which is larger than the uncertainties, has not yet been found, a closer investigation of the two systems is necessary.

Experimental procedures for the comparison of cryogenic radiometers at the highest accuracy

Cryogenic radiometers are recognized as by far the most accurate radiometers for visible and nearinfrared radiation. The now widespread use of such instruments around the world as the basis for optical radiation measurement scales makes it important to carry out a comparison. This will ensure not only that the instruments perform to the uncertainties claimed, a few parts in 104 or less, but also that their operators can disseminate such uncertainties. In this paper results of some pilot comparisons, both direct and indirect using transfer standards, are given. The experimental techniques used at the Bureau International des Poids et Mesures (BIPM) to achieve the highest accuracy are described in detail.