Absolute Cryogenic Radiometer Research Articles

Study of self-calibrating photodiodes for trap detectors

In accordance with the stated purpose of the study, the measurements and analysis of photodiodes were carried out, and the best alternative for the development of the trap detector was chosen. The paper substantiates the necessity for an alternative to the absolute cryogenic radiometer because of its highly expensive measurements. The need for more accurate measurement data has determined the use of trap detectors, including self-calibrating photodiodes. Such trap detectors can be used independently, without calibration by an absolute cryogenic radiometer. Thus, the cost of the study is reduced. Besides, the given approach does not affect the accuracy of the obtained measurement results.
 When reviewing modern approaches to the use of the above-mentioned trap detectors, it was discovered that trap detectors, together with self-calibrating photodiodes, are widely used both in state measurements of photometric quantities and for laboratory needs in many developed countries worldwide. The given analysis allows us to assert that the issues of developing, studying, and using new trap detectors, considering a significant advantage in their characteristics in comparison with the previously developed ones, make up an urgent scientific task.
 The paper defines selection criteria based on certain characteristics of photodiodes. When investigating the operating spectral range of the selected photodiodes, their internal quantum efficiency, geometrical dimensions of the receiving element of the selected photodiodes, and the coefficient of mirror and diffuse reflection of the receiving surface of the selected photodiodes, measurement results for all the stated characteristics were obtained. Based on these values, a screening of the photodiodes that do not meet the requirements for photodiodes specified for the development of the trap detector was made. Based on experimental studies, photodiodes S1337-1010BQ were selected.

Long-term spectral responsivity stability of predictable quantum efficient detectors

The stability of induced junction silicon photodiodes used to construct the predictable quantum efficient detector (PQED) has been studied over a time period of a decade by measurements of its spectral responsivity against absolute cryogenic radiometers (CR) in two independent laboratories at CMI and PTB. PQEDs operated at room temperature show a long-term temporal stability within 150 ppm over a broad spectral range during a 10-year period, well within the range of the claimed measurement uncertainty values of the CRs. This long term stability fulfills one of the fundamental requirements to establish a new primary standard for the measurement of power of optical radiation.

Calibration and comparison of detection efficiency for free-space single-photon avalanche diodes at 850 nm.

A detection efficiency measurement system for free-space single-photon detectors has been established at the National Research Council (NRC) Canada. This measurement apparatus incorporates an 850 nm fiber laser source and utilizes a double-attenuation and substitution calibration technique. Detection efficiency calibrations of silicon single-photon avalanche photodiodes (SPADs) at incident photon rates in the range of 1.0×105 photon counts per second (Cts/s) (36 fW) to 2.1×106Cts/s (734 fW) are SI-traceable through the substitution configuration with a silicon transfer standard detector, calibrated directly using the NRC absolute cryogenic radiometer. The measurement approach taken by the NRC was compared with the SPAD calibration technique implemented at the National Institute of Standards and Technology (NIST) in the United States. The count-rate-dependent detection efficiency of a silicon SPAD was measured at NIST and compared with results from the same SPAD measured at NRC within the range of incident photon rates from 1×105Cts/s to 5×105Cts/s. Comparison of the calibration results shows agreement between the two laboratories within the combined measurement uncertainties.

Apparatus for Calibration of Photovoltaic Cells at National Institute of Standards

The Egyptian National Institute of Standards (NIS) has established and developed a system for calibrating reference solar cells using photodiode detectors traceable to the absolute cryogenic radiometer (ACR). This research discusses an experimental procedure for measuring spectral responsivity, external quantum efficiency, and internal quantum efficiency of reference solar cells from the range of 340 nm to 930 nm. The associated uncertainty budget is also estimated. Additionally, this paper shows the NIS's calibration facility for pyranometers at different irradiance levels (200 W/m², 800W/m², and 1000W/m²). The measurement of the spectral responsivity and quantum efficiency of reference solar cells using two techniques is converged with each other with an uncertainty of about ±1% (at 900 nm). Also, the sensitivity of the pyranometer at 800W/m² is 15.97µV/W/m² ±1.9%.

Design, calibration, and application of a cryogenic low-background infrared radiometer for spectral irradiance and radiance measurements from 4 μm to 20 μm wavelength.

We have constructed, calibrated, and tested a cryogenic low-background infrared radiometer for both spectral radiance and irradiance measurements over the 4 μm to 20 μm wavelength range. The primary purpose of the Missile Defense Transfer Radiometer (MDXR) is to measure absolute irradiance or radiance from cryogenic infrared test chamber sources using a photoconductive Si:As Blocked Impurity Band (BIB) detector and a set of spectral filters. The MDXR also includes an absolute cryogenic radiometer (ACR) and a Fourier transform spectrometer (FTS). For irradiance measurements, the ACR is used to provide the primary power scale for the BIB detector in conjunction with spectral filters, while the FTS/BIB configuration derives its scale from an internal blackbody source. The two measurement scales show agreement for the irradiance of highly collimated (< 1 mrad) infrared beams from 10-13 W/μm/cm2 to 10-8 W/μm/cm2 within the combined relative uncertainties of 2.6 % (coverage factor k = 1.) We have also calibrated the radiometer for radiance measurements by using a large cavity fluid bath blackbody that overfills the spatial and angular extent of the radiometer entrance pupil. The radiometric calibration uncertainty analysis of the radiometer as well as its maintenance and stability are discussed.

Eliminating the middleman: ultraviolet scale realization using a laser-driven plasma light source.

After we replaced the argon mini-arc with a laser-driven light source in the Ultraviolet Spectral Comparator Facility at the National Institute of Standards and Technology (NIST), we realized that the optical power should be sufficient to use the comparator system for absolute-cryogenic radiometry. Calibrating working standard detectors directly against an absolute-cryogenic radiometer in the system used for calibrations would eliminate all uncertainties resulting from the use of transfer standards, which were calibrated in a separate system using a different light source and optics. The transfer standards are the middlemen we refer to in the title. Any uncertainty caused by differences in bandpass, out-off-band radiation, spectral purity, collimation, or data interpolation would be removed. In the end, we successfully set up a twin system resembling the Ultraviolet Spectral Comparator Facility and used this system to perform a primary calibration of several photodiodes, based on an absolute-cryogenic radiometer. Using this system, we were able to reduce relative standard uncertainties at wavelengths below 220 nm from above 1 % (k=1) to below 0.5%. We refer to this system as the Ultraviolet Scale Realization Facility or UV-SRF.

Mid-infrared spectral responsivity scale based on an absolute cryogenic radiometer and a tunable quantum cascade laser

The spectral optical radiant power of a quantum cascade laser (QCL) in the (7.35–10.6) µm spectral range has been measured using a Y-shape absolute cryogenic radiometer (ACR). The spectral responsivity (SR) of a transfer detector based on an integrating-sphere-coupled mercury-cadmium-telluride (IS-MCT) photoconductive detector has been calibrated. The optical radiant power of the QCL can be adjusted and stabilized in the range of (0.035–0.6) mW. The relative uncertainty of the ACR-based optical radiant power measurement was evaluated to be 0.023%–0.04% (k = 1). The relative uncertainty of the SR calibration of the IS-MCT transfer detector was analyzed to be 0.097%–0.16% (k = 1). The spectral range for the SR calibration can be extended to cover (3–12) µm using commercially available QCLs.

Monochromatic Radiation Source Based on a Supercontinuum Laser for Measurement of Spectral Sensitivity of Radiation Detectors in the Range 0.9–1.6 μm with the Use of an Absolute Cryogenic Radiometer

An improved monochromatic radiation source with spectral width 4 nm based on a supercontinuum laser and double monochromator is introduced into a unit based on an absolute cryogenic radiometer in order to increase the precision of measurement of spectral sensitivity in the range 0.9–1.6 μm. A feedback system that supports stabilization of the power of monochromatic radiation with standard deviation 0.025% is developed. The power of optical radiation in the plane of the detector and/or power of the absolute cryogenic radiometer varies from 0.1 to 1.5 mW. The spectral distribution of the power of the newly developed source in different operating regimes of the supercontinuum laser is presented.

Miniaturized integrating sphere light sources based on LEDs for radiance responsivity calibration of optical imaging microscopes.

LED-based integrating sphere light sources (LED-ISLSs) in the size of typical microscope slides were developed to calibrate the radiance responsivity of optical imaging microscopes. Each LED-ISLS consists of a miniaturized integrating sphere with a diameter of 4 mm, an LED chip integrated on a printed circuit board, and a thin circular aperture with a diameter of 1 mm as the exit port. The non-uniformity of the radiant exitance of the LED-ISLSs was evaluated to be 0.8%. The normal radiance of the LED-ISLSs in the range of (5∼69) W m-2 sr-1 was measured with a standard uncertainty of 1.3% using two precision apertures and a standard silicon photodetector whose spectral responsivity is traceable to an absolute cryogenic radiometer. The LED-ISLSs were applied to calibrate the radiance responsivity of a home-built optical imaging microscope with a standard uncertainty of 2.6∼2.9%. The LED-ISLSs offer a practical way to calibrate the radiance responsivity of various optical imaging microscopes for results comparison and information exchange.

Instrument Development: Chinese Radiometric Benchmark of Reflected Solar Band Based on Space Cryogenic Absolute Radiometer

Low uncertainty and long-term stability remote data are urgently needed for researching climate and meteorology variability and trends. Meeting these requirements is difficult with in-orbit calibration accuracy due to the lack of radiometric satellite benchmark. The radiometric benchmark on the reflected solar band has been under development since 2015 to overcome the on-board traceability problem of hyperspectral remote sensing satellites. This paper introduces the development progress of the Chinese radiometric benchmark of the reflected solar band based on the Space Cryogenic Absolute Radiometer (SCAR). The goal of the SCAR is to calibrate the Earth–Moon Imaging Spectrometer (EMIS) on-satellite using the benchmark transfer chain (BTC) and to transfer the traceable radiometric scale to other remote sensors via cross-calibration. The SCAR, which is an electrical substitution absolute radiometer and works at 20 K, is used to realize highly accurate radiometry with an uncertainty level that is lower than 0.03%. The EMIS, which is used to measure the spectrum radiance on the reflected solar band, is designed to optimize the signal-to-noise ratio and polarization. The radiometric scale of the SCAR is converted and transferred to the EMIS by the BTC to improve the measurement accuracy and long-term stability. The payload of the radiometric benchmark on the reflected solar band has been under development since 2018. The investigation results provide the theoretical and experimental basis for the development of the reflected solar band benchmark payload. It is important to improve the measurement accuracy and long-term stability of space remote sensing and provide key data for climate change and earth radiation studies.

Development of the Chinese Space-Based Radiometric Benchmark Mission LIBRA

Climate observations and their applications require measurements with high stability and low uncertainty in order to detect and assess climate variability and trends. The difficulty with space-based observations is that it is generally not possible to trace them to standard calibration references when in orbit. In order to overcome this problem, it has been proposed to deploy space-based radiometric reference systems which intercalibrate measurements from multiple satellite platforms. Such reference systems have been strongly recommended by international expert teams. This paper describes the Chinese Space-based Radiometric Benchmark (CSRB) project which has been under development since 2014. The goal of CSRB is to launch a reference-type satellite named LIBRA in around 2025. We present the roadmap for CSRB as well as requirements and specifications for LIBRA. Key technologies of the system include miniature phase-change cells providing fixed-temperature points, a cryogenic absolute radiometer, and a spontaneous parametric down-conversion detector. LIBRA will offer measurements with SI traceability for the outgoing radiation from the Earth and the incoming radiation from the Sun with high spectral resolution. The system will be realized with four payloads, i.e., the Infrared Spectrometer (IRS), the Earth-Moon Imaging Spectrometer (EMIS), the Total Solar Irradiance (TSI), and the Solar spectral Irradiance Traceable to Quantum benchmark (SITQ). An on-orbit mode for radiometric calibration traceability and a balloon-based demonstration system for LIBRA are introduced as well in the last part of this paper. As a complementary project to the Climate Absolute Radiance and Refractivity Observatory (CLARREO) and the Traceable Radiometry Underpinning Terrestrial- and Helio- Studies (TRUTHS), LIBRA is expected to join the Earth observation satellite constellation and intends to contribute to space-based climate studies via publicly available data.

Measurement of the distance between two parallel precision apertures using time-of-flight method for cryogenic radiometry.

Time-of-flight method was adopted to measure the distance between two parallel precision apertures utilized in a vacuum chamber for cryogenic radiometry. The diameters of the apertures are 9 mm and 8 mm, respectively. A 1550-nm femtosecond pulse laser, a 70-GHz photodetector, and a 30-GHz oscilloscope were used to measure the round-trip flight time difference between the flat front surfaces of the two precision apertures. The distance between the apertures was analyzed to be 0.36423 m with a relative standard uncertainty of 0.004%. The non-contact distance measurement method is useful for applications such as low background infrared radiance measurement system based on an absolute cryogenic radiometer.

Design and calibration of electrical substitution radiometer traced to absolute cryogenic radiometer

The quantification of remote sensing information requires spectral radiometric calibration technology support with high accuracy, and this technology can ensure the comparability, accuracy, and long-term stability of data acquisition for sensor. Now, the calibration technique traced to the absolute cryogenic radiometer (ACR) is the trend of development. To improve the accuracy of infrared absolute spectral responsivity, a gold-plated hemisphere reflector is added to a thermopile detector with electrical substitution pins. So an infrared electrical substitution radiometer (ESR) is developed as the standard transfer detector. The spectral response linearity, uniformity, and stability of ESR are tested by electric substitution technology. We also have established a radiometric calibration system based on ACR. The combined uncertainty in the spectral responsivity of this detector from 0.7 to 20 μm is about 0.99%. The application of infrared ESR to radiometric calibration can improve the calibration accuracy of detectors through its shorter calibration chain.

Supercontinuum laser-based monochromatic radiant source for detector spectral responsivity characterization in the range 0.9–1.6 μm using absolute cryogenic radiometer

An improved monochromatic radiant source with spectral bandwidth of 4 nm based on supercontinuum laser and a double monochromator was included in absolute cryogenic radiometer-based facility to improve the accuracy of spectral responsivity measurement in the range 0.9–1.6 μm. The developed feedback system ensures stabilization of monochromatic radiant power with standard deviation up to 0.025 %. Radiant power that proceeds detector under test or absolute cryogenic radiometer varies from 0.1 to 1.5 mW in dependence of wavelength. The spectral power distribution of its monochromatic source for various operating mode is presented.

Design and analysis of thermo-structure for cryogenic radiometer

Absolute cryogenic radiometer is built based on a new theory of electrical-substitution measurement, which is for measuring the radiant power by using the equivalent electrical power and has recently served as a primary standard for radiant power measurements. This study aims to design and implement a cryogenic radiometers to measure the optical power in a range from &lt;inline-formula&gt;&lt;tex-math id="M1"&gt;\begin{document}$0.1\;{\text{μ}}{\rm{W}}$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20181880_M1.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20181880_M1.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; to 2 mW, which can substitute for the imported products. Intensive experiments are performed to study the thermal circuit of cryogenic radiometer, and systematically analyze the influences of cavity assembly and heat link materials on the responsivity and thermal time constant of cryogenic radiometer. On this basis, the thermo-structure mechanical parts are developed, which are comprised of a blackbody cavity, heat link and heat sink. Both the heat sink and the blackbody cavity are made of OFHC copper that is plated with gold. All surfaces are highly polished and reflective to reduce any radiative effects. The absorptance of the cavity can reach up to 0.999995 at 633 nm. And then, a characteristic parameters’ test system of cryogenic radiometer is built. Through optimizing the temperature control system and improving the design of the heat sink, the standard deviation of the heat sink can be kept under 0.2 mK for 30 min. By using that test system, the responsivity and thermal time constant of cryogenic radiometer with four different kinds of heat link materials (OHFC copper, 6061 Al, SS304 stainless steel, and polyimide) are tested experimentally. The experimental results show that the responsivity and thermal time constant are 35.5 K/W and 23 s for OHFC copper, 318.9 K/W and 106 s for 6061 Al, 434.8 K/W and 297 s for SS304 stainless steel, 714.8 K/W and 506 s for polyimide. As the thermal conductivity of heat link material changes, the two parameters of responsivity and thermal time constant will simultaneously change significantly. The responsivity and thermal time constant are a pair of mutually constrained parameters, and temperature stability is an important parameter for designing the thermo-structure. After increasing the responsivity, it will not only significantly increase the measurement time and resource consumption, but also affect the temperature control stability, and hence limiting the measurement accuracy. All the test data indicate that the characteristic parameter of cryogenic radiometer can be adjusted by changing the material and structure of heat link. The obtained results will have a certain reference value for the index distribution of cryogenic radiometer characteristic parameters and designing the next generation of absolute cryogenic radiometers.

Absolute cryogenic radiometer for high accuracy optical radiant power measurement in a wide spectral range

An absolute cryogenic radiometer (ACR) with a detachable optical window was designed and built for high accuracy optical radiant power measurement and photodetector spectral responsivity calibration. The ACR receiver is an electroplated pure copper cavity with a 50-μm-thick wall and inner surface coated with a specular black polymer material mixed with highly dispersible carbon nanotubes. The absorptivity of the cavity receivers was evaluated to be ≥0.9999 in the 250 nm–16 μm wavelength range and ≥0.99995 in 500 nm–16 μm. The cavity receiver works at the temperature of ∼5.2 K with nanowatt-level noise-equivalent power. The relative standard uncertainty is 0.041% for the measurement of ∼100 μW optical radiant power (250 nm–16 μm) and 0.015% for ∼1 mW (500 nm–16 μm).

Design and investigation of absolute radiance calibration primary radiometer

In order to satisfy the requirement of calibration precision and traceability for space remote sensor, the absolute radiance calibration primary radiometer (ARCPR) is investigated for establishing the irradiance calibration standard in orbit. ARCPR is a mechanical cryogenic absolute radiometer for space application, and works at 20 K. The cryogenic environment is obtained by the Stryn-type pulse tube cryocooler. The environment temperature is stable at 23.9658 K by high-precision temperature controllers. The detector of ARCPR is designed and investigated based on the experimental prototype. The experimental results of detector characteristics measurement illustrate that the sensitivity is 3.9526 K/mW, and the thermal time constant is 134 s. The procedure of optical power measurement is optimised for satisfying the requirements of in orbit measurement. ARCPR achieves a relative standard uncertainty of 210 ppm at an optical power level of 0.4 mW. Therefore, ARCPR could be the radiometric standard for optical measurement. Furthermore, the investigation result provides the theoretical and experimental basis for the establishment of radiometric calibration standard in orbit.

Invited Article: Advances in tunable laser-based radiometric calibration applications at the National Institute of Standards and Technology, USA.

Recent developments at the National Institute of Standards and Technology's facility for Spectral Irradiance and Radiance responsivity Calibrations using Uniform Sources (SIRCUS) are presented. The facility is predicated on the use of broadly tunable narrow-band lasers as light sources in two key radiometric calibration applications. In the first application, the tunable lasers are used to calibrate the spectral power responsivities of primary standard detectors against an absolute cryogenic radiometer (ACR). The second function is to calibrate the absolute radiance and irradiance responsivities of detectors with uniform light sources, typically generated by coupling the laser light into integrating spheres. The radiant flux from the uniform sources is determined by the ACR-calibrated primary standard detectors. Together these sources and detectors are used to transfer radiometric scales to a variety of optical instruments with low uncertainties. We describe methods for obtaining the stable, uniform light sources required for low uncertainty measurements along with advances in laser sources that facilitate tuning over broader wavelength ranges. Example applications include the development of a detector-based thermodynamic temperature scale, the calibration and characterization of spectrographs, and the use of a traveling version of SIRCUS (T-SIRCUS) to calibrate large aperture Earth observing instruments and astronomical telescopes.

Mid-infrared absolute spectral responsivity scale based on an absolute cryogenic radiometer and an optical parametric oscillator laser

We are reporting on a laser-based absolute spectral responsivity scale in the mid-infrared spectral range. By using a mid-infrared tunable optical parametric oscillator as the laser source, the absolute responsivity scale has been established by calibrating thin-film thermopile detectors against an absolute cryogenic radiometer. The thin-film thermopile detectors can be then used as transfer standard detectors. The extended uncertainty of the absolute spectral responsivity measurement has been analyzed to be 0.58%–0.68% (k = 2).

Realization of the spectral responsivity scale based on an absolute cryogenic radiometer

We present the concept of and the procedure for realizing the primary scale of the spectral responsivity based on an absolute cryogenic radiometer at the Korea Research Institute of Standards and Science (KRISS). A detailed error and uncertainty analysis is performed by calibrating the standard detectors in the wavelength range from 300 nm to 1000 nm. The relative expanded uncertainty of the spectral responsivity calibration for a Si trap detector is 0.06% at 632.8 nm and 0.4% for other wavelengths (k = 2). The calibration results are validated by comparing with the results from a theoretical model of quantum efficiency for a transmission-type trap detector.