Marek Smid

Predictable quantum efficient detector: II. Characterization and confirmed responsivity

The predictable quantum efficient detector (PQED) is intended to become a new primary standard for radiant power measurements in the wavelength range from 400 nm to 800 nm. Characterization results of custom-made single induced junction photodiodes as they are used in the PQED and of assembled PQEDs are presented. The single photodiodes were tested in terms of linearity and spatial uniformity of the spectral responsivity. The highly uniform photodiodes were proved to be linear over seven orders of magnitude, i.e. in the radiant power range from 100 pW to 400 µW. The assembled PQED has been compared with a cryogenic electrical substitution radiometer with a very low uncertainty of the order of 30 ppm. Experimental results show good agreement with the modelled response of the PQED to optical radiation and prove a near unity external quantum efficiency.

Development of a switched integrator amplifier for high-accuracy optical measurements

In the field of low flux optical measurements, the development and use of large area silicon detectors is becoming more frequent. The current/voltage conversion of their photocurrent presents a set of problems for traditional transimpedance amplifiers. The switched integration principle overcomes these limitations. We describe the development of a fully characterized current-voltage amplifier using the switched integrator technique. Two distinct systems have been developed in parallel at the United Kingdoms National Physical Laboratory (NPL) and Czech Metrology Institute (CMI) laboratories. We present the circuit theory and best practice in the design and construction of switched integrators. In conclusion the results achieved and future developments are discussed.

Low optical power reference detector implemented in the validation of two independent techniques for calibrating photon-counting detectors.

We introduce a technique for measuring detection efficiency that is traceable to the primary standard, the cryogenic radiometer, through a reference silicon photodiode trap detector. The trap detector, used in conjunction with a switched integrator amplifier, can measure signals down to the 0.1 pW (3 x 10⁵ photons second-1) level with 0.1% uncertainty in a total integration time of 300 seconds. This provides a convenient calibration standard for measurements at these levels across the optical spectrum (UV - near IR). A second technique is also described, based on correlated photons produced via parametric down-conversion. This can be used to directly measure detection efficiency in the photon counting regime, and provides a route for expanding the formulation of the candela in terms of photon flux to enable it to address the needs of emerging quantum optical technologies and applications. The two independent techniques were cross-validated by a comparison carried out at 702.2 nm, which showed agreement to within 0.2%.

Metrology for industrial quantum communications: the MIQC project

The ?Metrology for Industrial Quantum Communication Technologies? project (MIQC) is a metrology framework that fosters development and market take-up of quantum communication technologies and is aimed at achieving maximum impact for the European industry in this area.MIQC is focused on quantum key distribution (QKD) technologies, the most advanced quantum-based technology towards practical application. QKD is a way of sending cryptographic keys with absolute security. It does this by exploiting the ability to encode in a photons degree of freedom specific quantum states that are noticeably disturbed if an eavesdropper trying to decode it is present in the communication channel. The MIQC project has started the development of independent measurement standards and definitions for the optical components of QKD system, since one of the perceived barriers to QKD market success is the lack of standardization and quality assurance.

Comparison at the sub-100 fW optical power level of calibrating a single-photon detector using a high-sensitive, low-noise silicon photodiode and the double attenuator technique

A comparison down to sub-100-fW optical power level was carried out between a low-noise Silicon photodiode and a low optical flux measurement facility based on a double attenuator technique. The comparison was carried out via a silicon single-photon avalanche diode (Si-SPAD), which acted as transfer standard. The measurements were performed at a wavelength of 770 nm using an attenuated laser as a radiation source at optical power levels between approximately 86 fW and approximately 1325 fW, corresponding to approximately 330 000 photons s−1 and approximately 5.2 × 106 photons s−1, respectively. The mean relative deviation of the detection efficiencies of the Si-SPAD, determined by the Si-photodiode and the low optical flux measurement facility, i.e. between two completely independent traceability routes, was < 0.2%, thus well within the combined standard uncertainty of the two measurements. To our knowledge, this is the first comparison for the detection efficiency of a single photon detector using a direct optical flux measurement by a conventional Si-photodiode at such low power levels.

Luminance meter for photopic and scotopic measurements in the mesopic range

This paper presents a design and realization of a dual-channel luminance meter for simultaneous measurement of luminance with photopic and scotopic weightings. Such measurements are useful in mesopic conditions, i.e. when the luminance is in the range of 0.005–5 cd m−2. The instrument is a spot luminance meter with two spectrally weighted channels. The collected light is detected with silicon detectors and a computer-controlled dual-channel switched-integration amplifier. The instrument is characterized for relative spectral responsivity against a calibrated spectroradiometer using a radiance source based on an integrating sphere with input from a monochromator. An absolute luminance responsivity calibration is made against a sphere-based luminance standard at a level close to the high end of the mesopic range. The standard uncertainty in luminance responsivity calibration is 0.3% for the photopic channel and 0.6% for the scotopic channel. In addition, characterization measurements were carried out for the instruments linearity, stray light sensitivity and polarization dependence. The results show very good noise performance, allowing fast measurements over the whole mesopic range. The noise equivalent power was measured to be approximately 20 fW Hz−1/2, equal to a noise equivalent luminance of 30 µcd m−2 Hz−1/2. Estimated uncertainty of measurements for typical light sources is 2.2% (k = 2) at the lowest luminance levels of the mesopic range.

Diode-array UV solar spectroradiometer implementing a digital micromirror device

The solar ultraviolet spectrum captured by commercially available diode-array spectroradiometers is dominated by stray light from longer wavelengths with higher intensity. The implementation of a digital micromirror device in an array spectroradiometer has the potential to enable the precise selection of desired wavelengths as well as the ability to reduce spectral intensity of some wavelengths via selective mirror modulation, both reducing long wavelength stray light. A prototype consisting of off-the-shelf components has been assembled to verify the validity of the base concept, and initial measurements have been performed to confirm the throughput and image qualities such as spectral resolution and astigmatism.

Realization of an accurate and repeatable wavelength scale for double subtractive monochromators

We present the results and discussion of two methods for achieving a higher degree of wavelength accuracy of double grating monochromators. In particular, we assessed the benefits of using differential evolution curve fitting techniques to minimize wavelength uncertainties associated with the manufacture of lead screws in sine-bar driven monochromators. Absolute wavelength-scale uncertainties better than ?0.1?nm can be realized using this technique.We also report the realization of an accurate and repeatable wavelength scale, using a novel calibration technique, that is applicable to a wide range of monochromators. We present results demonstrating the robustness of the technique by realizing an absolute wavelength scale, across four grating sets from 250?nm to 1600?nm, with scale uncertainties within ?0.02?nm and repeatable to ?0.005?nm.

Liquid nitrogen cryostat for predictable quantum efficient detectors

We present the design and testing of a cryostat to be used with induced junction photodiodes of the Predictable Quantum Efficient Detector (PQED). Long-term reflectance measurements indicate that possible ice growth on the photodiodes at the temperature of liquid nitrogen (LN) is significantly reduced from earlier PQED cryostat designs.

Carbon nanotube planar absorber cryogenic bolometer as a novel primary absolute standard detector for optical power measurements for fibre optics and photonics (Conference Presentation)

Primary standards of optical radiation total radiant flux are traditionally realized by absolute cryogenic radiometers [1] working on the principle of electrical substitution with a relative total uncertainty of 1e-4 in the power measurement. The current cryogenic radiometers though operate over a limited spectral range, usually from 350 nm to 800 nm and working with free space beam. For fibre optics telecom spectral range 1300 nm - 1650 nm this scale is then extended in several steps, typically via application of other standard detector systems such as spectrally flat room temperature pyro detectors [2] and spectrally dependent temperature stabilized solid state detectors [3], which adversely affects the scale accuracy by a factor of approximately one order of magnitude. The typical relative total uncertainty of state-of-the-art transfer standard fibre coupled detectors reaches 0.5 %. Recently published results on planar electrical-substitution carbon nanotube cryogenic radiometer (PCBR) [4] brought the opportunities for using these systems as new absolute primary standards in telecom spectral range directly in fibre coupled configuration. This shortens the traceability chain, with a potential improvement in the total uncertainty to below 0.1 %. CMI in collaboration with NIST are developing the first prototypes of fibre coupled PCBR systems. First both free space and fibre coupled measurements have confirmed radiometric The paper will present both the core physical parameters of these PCBR electrical-substitution systems and initial results including the currently achieved agreement of traditional transfer standards with the PCBR at the level of 0.2 %. The work reported in this abstract was partially funded by project EMPIR 14IND13 PhotInd. This project has received funding from the EMPIR programme co-financed by the Participating States and from the European Union’s Horizon 2020 research and innovation programme. References: [1] Martin J E, Fox N P and Key P J 1985 Metrologia 21, 147 [2] Lehman J., Theocharous E., Eppeldauer G., and Pannel C., “Gold-black coatings for freestanding pyroelectric detectors” Measurement Science and Technology, 14, 916-922, 2003 [3] E. Theocharous, M. Smid, T. Ward, N. Fox, “The establishment of an absolute infrared scale using cavity pyroelectric detectors”, in preparation. [4] N A Tomlin, M White, I Vayshenker, S I Woods and J H Lehman, Planar electrical-substitution carbon nanotube cryogenic radiometer 2015 Metrologia 52 376