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IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 64, NO. 6, JUNE 2015
Realization of Optical Power Scale Based on Cryogenic Radiometry and Trap Detectors Thiago Menegotto, Thiago Ferreira da Silva, Maurício Simões, Willian A. T. de Sousa, and Giovanna Borghi
Abstract— We report the realization of the Brazilian optical power scale based on cryogenic radiometry. The electricalsubstitution cryogenic radiometer (ESCR) national primary standard is characterized and used for measurement of the optical power at specific laser lines. The scale is transferred to silicon detectors arranged in a reflection-trap configuration through the direct calibration of their spectral power responsivity. These devices are then characterized and used as standards for radiometry and photometry. The experimental results for the spatial nonuniformity, polarization dependence, and spectral responsivity are shown and discussed. The external quantum efficiency of the detectors is modeled to extend the optical power scale over the visible spectral range. The results are validated by comparing the modeled scale with a calibration certificate for the spectral power responsivity of one of the standards. A bilateral international comparison of ESCRs using the characterized trap detectors was additionally performed to link the optical power scale to the Consultative Committee for Photometry and Radiometry intercomparison of cryogenic radiometers. The set of results establishes the Brazilian optical power scale traced to the cryogenic radiometer. Index Terms— Electromagnetic measurements, measurement techniques, metrology, radiometers, radiometry.
I. I NTRODUCTION
R
ADIOMETRY is a very important subject for several fields, such as optical technology, thermometry, and remote sensing [1], [2]. The measurement of optical power traceable to the International System of Units (SI) is essential for wide ranging applications in basic metrology, including the realization of the SI base unit of luminous intensity, the candela [3], [4]. At many National Metrology Institutes (NMIs), the optical power is measured with an electrical-substitution cryogenic radiometer (ESCR), where it is determined by the comparison with the electrical watt [5], [6]. The ESCR is the primary standard in radiometry due its low uncertainty value for the optical power measurement. From the ESCR, the traceability chain is transferred to optical detectors, usually through
Manuscript received August 26, 2014; revised October 29, 2014; accepted December 1, 2014. Date of publication January 8, 2015; date of current version May 8, 2015. The work of M. Simões was supported by the National Council for Scientific and Technological Development through the grant PROMETRO 563198/2010-9. The Associate Editor coordinating the review process was Dr. Branislav Djokic. The authors are with the Optical Metrology Division, National Institute of Metrology, Quality and Technology—Inmetro, Duque de Caxias 25250-020, Brazil (e-mail:
[email protected];
[email protected];
[email protected];
[email protected]; gbalmeida@ inmetro.gov.br). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TIM.2014.2383072
a measurement of spectral power responsivity. The ESCR measurements trace back to electrical standards. We present here the characterization of the ESCR and its uncertainty budget. The traceability chain for radiometry is mainly based on silicon photodiodes, the typical devices used in the visible spectral range [7]. The spectral power responsivity of these transfer standards is calibrated directly with the ESCR. Mounting the devices in a transmittance or reflectance trap configuration minimizes light losses due to reflection and reduces dependence on polarization of light [8]. The trap detectors are calibrated against the ESCR, in spectral power responsivity, with typical uncertainties reaching a few parts in 104 [6]. In addition to the spectral power responsivity, some parameters as the spatial nonuniformity (NU) and the polarization dependence of these devices must be characterized for using as standards for radiometry [9], [10]. We present the main steps of the characterization of these transfer standards. The operation of the ESCR at high vacuum and at the liquid helium temperature is expensive and time consuming, so the optical power scale is usually transferred to the transfer standards at some discrete laser wavelengths. The scale is then extended over the visible spectral region by interpolating the detector response to fill the gap between the calibration wavelengths. As long as the scale is extended, the transfer detectors can be used to calibrate work standards by the comparison method in a noncryogenic facility. A straightforward approach to interpolate the spectral power responsivity of trap detectors is based on directly fitting the external quantum efficiency (EQE) model by means of phenomenological functions. B-spline, high-order polynomials, and others mathematical functions can be used [11], [12]. Here, we reconstruct the optical power scale by fitting the EQE model to the calibrated points of one of the transfer standards [13]. The results are validated by comparing the modeled scale to a calibration certificate for its spectral power responsivity. The metrological compatibility of the measurement results are assessed by checking the agreement within the normalized error. A final step in the validation of the realization of the optical watt is the comparison with other NMIs, which have participated in international comparisons of the Consultative Committee for Photometry and Radiometry (CCPR), as the CCPR.S3 key comparison [14]. Two of the transfer standards previously characterized were used in an international bilateral comparison of ESCRs, with agreement of the results within the normalized error criterion [15].
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MENEGOTTO et al.: REALIZATION OF OPTICAL POWER SCALE
Fig. 1. Schematic of the ESCR showing the cavity and the temperature controller.
The set of results presented establishes the Brazilian optical power scale at the visible spectral region traced through silicon trap detectors to the primary-standard cryogenic radiometer. II. C RYOGENIC R ADIOMETRY AS P RIMARY S TANDARD The primary standard for the optical power scale is an ESCR [5]—Cryorad II, Cambridge Research Instruments.1 The optical power measurement is obtained through the measurement of electrical power. By knowing the amount of electric power fed into the system, incident optical power levels ranging from 1 μW to 1 mW can be determined. The ESCR achieves an uncertainty level ∼0.01%. A. Technical Description The ESCR is composed by a cooled cavity attached to a heat sink and by a temperature controller [germanium resistance thermometer (GRT) and electrical heater], as observed in Fig. 1. The copper cavity is placed in high vacuum and operated just above the transition temperature of liquid-to-gas helium. The high-thermally conductive cavity absorbs optical radiation and constantly dissipates heat to the heat sink. The system quantifies the amount of electrical power necessary to reach thermodynamic stability relative to the temperature value measured by the GRT. As the electric power fed into the receiver by the electrical heater can be measured, the optical power is directly determined by equivalence. B. Characterization The optical power measured by the ESCR should be corrected by the transmittance of the Brewster-angled input window and cavity absorptance. The Brewster angle is adjusted for each wavelength, prior to the measurements. The transmittance is measured by detaching the window mount from the vacuum chamber. A detector is placed behind the window mount and the ratio of transmitted to incident light (measured removing the windows from the beam path) is obtained. As the Brewster angle is polarization sensitive, the laser beam must be linearly polarized parallel to the 1 Certain commercial equipment, instruments, or materials are identified in this paper to foster understanding. Such identification does not imply recommendation or endorsement by the National Institute of Metrology, Quality and Technology, nor does it imply that the material or equipment are necessarily the best available for the purpose.
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plane of incidence. The transmittance of the Brewster-angled window is characterized at one wavelength (HeNe laser line at 632.8 nm) and this value is used for correcting the measurements performed with the ESCR. The measured transmittance value is 0.99994(13). The wavelength dependence on the transmittance of the Brewster-angled window was checked in other wavelengths (458, 488, and 514 nm) and the values are covered by the reported uncertainty. The cavity is geometrically designed to reduce the wavelength dependence of light losses. The absorptance value of 0.999840(33) is used to correct the optical power measurements. This value was measured by the manufacturer at 632.8 nm. The electrical power dissipated by the heater is obtained from measurements performed by internal current and voltage meters. A calibrated digital voltmeter (DVM) can be attached externally to the ESCR. A correction factor to the electrical power measurement can be determined by comparing the reference values calculated from the calibrated DVM measurements with the power measured by the ESCR. Nonequivalence between the optical and electrical power in ESCR can arise due to the distinct places where the electrical heater and the laser are heating the cavity. Considering this geometrical dependence of the nonequivalence, an upper estimate of its magnitude is obtained using a spare electrical heater placed in the opposite side of the cavity, near its entrance. Both heaters are alternated keeping a given set value for the GRT. The difference between the powers dissipated by the heaters is assigned to the nonequivalence. III. T RANSFERRING THE O PTICAL P OWER S CALE The optical power scale is transferred from the primary standard—the ESCR—to a transfer standard—a photodetector—through the direct measurement of its spectral power responsivity. The detector is submitted to an optical beam whose optical power was determined by the ESCR. The realization of the optical power scale and the traceability transfer are performed at discrete laser lines. The ESCR absorbing cavity and the transfer standard are positioned at the same distances from the optical source. The devices are alternately submitted to the same power stabilized, p-polarized optical beam, and the optical power (at the ESCR) and the photocurrent (at the transfer standard) are acquired at each wavelength to determine the spectral power responsivity of the device. The spectral responsivity of a detector is defined at a given wavelength as the ratio of the electric signal measured on the detector (i ) to the optical power () incident on its active area s(λ) = i (λ)/.
(1)
In the case of the photodiodes, the electric signal is the photocurrent, so the spectral responsivity is reported as [ampere/watt]. A. Transfer Standards: Silicon Trap Detectors Our transfer standards are composed by three photodiodes each (Hamamatsu S1337), disposed in a reflective-trap configuration [8]. The first photodiode is placed at an angle
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Fig. 2.
IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 64, NO. 6, JUNE 2015
Setup for measurement of spectral responsivity with the ESCR.
of 45° relative to the propagation axis of the optical beam. The reflected intensity, ∼30% of the input value, is directed to the second photodiode, also placed at an angle of 45°. The reflected beam orthogonally reaches the third photodiode, and the reflected beam traces back to the input. The fivefold reflection is sufficient to reduce the trap detector insertion loss to values
