The full characterisation of photon counting detection systems is important because it allows the identification and
subsequent adoption of the system with the optimum performance. It also allows the uncertainty contributions
introduced by a particular detection system to be calculated and used in the estimation of the combined uncertainty of the
measurement in which that detection system is being used. The Optical Metrology Group at the National Physical
Laboratory (NPL) has assembled dedicated facilities, which are able to characterise the critical operating parameters of
photon counting systems anywhere in the 250 nm to 1600 nm wavelength region. These include the absolute and relative
spectral responsivity over the wavelength range of interest, the spatial uniformity of response at the wavelengths of
interest, the deviation from a true linear response as a function of incident radiant power/irradiance and the stability of
response as a function of time or ageing. Using these facilities, the performance of a number of photon counting systems
has been evaluated in an effort to identify the most appropriate detector technologies for the various radiometric
applications NPL is currently addressing. This document describes the dedicated facilities which exist at NPL and
highlights how they are being used to provide traceable measurements of the key performance parameters of photon
counting systems. Examples of characterisations of photon counting systems are presented.
Low photon flux measurements are widely used in the fields of biology, nuclear physics, medical physics and
astrophysics. This paper will highlight the key requirements and considerations needed for accurate, traceable
measurement at these low light levels. A new driver for these techniques is the rapidly advancing field of optical
quantum information processing<sup>1</sup> which requires the development of single photon counting detectors, in addition to the
wider use of optical technologies in the photon counting regime. The paper will present the results of the measurement of
the quantum efficiency of a channel photomultiplier detector using an absolute radiometric technique based on correlated
photons produced in non-linear crystals. Case studies will also be presented to illustrate this work.
Many of the schemes utilizing photon states under investigation for Quantum Information Processing (QIP) technology involve active and passive optical components. In order to be able to establish fidelity levels for these schemes, the performance of these optical components and their coupling efficiencies require careful and accurate characterization. Correlated photons, the basis of entangled photon states, offer a direct means of measuring detector quantum efficiency and source radiance in the photon counting regime. Detector and source calibration by correlated photon techniques therefore address some of the key factors critical to QIP technology and the developing techniques of correlated/entangled photon metrology. Work is being undertaken at NPL to establish the accuracy limitations of the correlated photon technique for detector and source calibration. This paper will report on investigations concerning the characterization of silicon avalanche photodiode detectors using the correlated photon technique.
Many of the schemes under study for Quantum Information Processing technology based on photon states involve active and passive optical components as well as detectors. In order to able to establish fidelity levels for these schemes, the performance of the optical components and the quantum efficiency (q.e.) of the detectors require careful and accurate characterization. Correlated photons produced from spontaneous parametric downconversion, which are also the basis of entangled photon states, conveniently offer a direct means of measuring detector q.e. in the photon counting regime, while stimulated parametric downconversion can be used to measure source radiance. Detector and source calibration using correlated photon techniques therefore address some of the key issues critical to the development of QIP technology and the development of correlated/entangled photon metrology. This paper reports work being undertaken at NPL to establish the accuracy limitations of these correlated photon techniques. Significant sources of uncertainty are the need to measure losses due to any optical components used and the requirement to obtain and maintain good geometrical and spectral alignment.
Thermal barrier coatings are widely used in heat engines for improving efficiency by allowing higher operating temperatures; yttria stablised zirconia is the most widely used material. Their use has been extended to rotating parts, in particular to gas turbine engine blades, and any loss of coating would represent a major problem. During deposition of the coating, a thin (< 1μm) alumina layer grows due to oxidation of the bondcoat, and it is this alumina layer which promotes bonding between the coating and the coated substrate. The spectral shape and position of the R-line fluorescence of Cr<sup>3+</sup> ions normally present in small amounts in the alumina is sensitive to stress, temperature and other
environmental effects. Stress is the key factor determining spallation, and piezospectroscopy refers to the use of spectroscopic measurement to determine stress within a material. Measurements have been carried out as a function of various ageing treatments in order to evaluate the potential of the technique to be a non-destructive probe for determining the onset of spallation. Interpreting the changes in the fluorescence spectra requires the use of sophisticated curve-fitting techniques and therefore requires reliable and accurate measurements. This paper will discuss these measurement requirements and their potential for development into a non-destructive tool for lifetime prediction of these structures.