Abstract
NIST’s C-series isoperibol calorimeters [1] operating at room temperature have been used as reference instruments in the laser power calibrations at NIST for over 50 years. These calorimeters operate from 100 μW to 300 mW with an expanded uncertainty of 0.86% (k = 2). Recently developed vertically aligned carbon-nanotube (VACNT) absorbers, with spectrally flat and hemispherical absorptance better than 99.95%, have enabled development of planar absolute bolometers with smaller size and significantly faster measurements compared to traditional laser calorimeters.
In this work, we present the latest progress of the Planar Absolute Radiometer for Room Temperature (PARRoT) [2] currently under construction that will replace the old reference C-series calorimeters. PARRoT is based on the electrical power substitution method and it can measure laser powers up to 300 mW with a predicted expanded uncertainty better than 0.1% (k = 2). The radiometer is operated at room temperature and placed in a 15 cm cube vacuum chamber to minimize convection while still providing a compact and sturdy standard. The laser beam is transmitted to the absorber by an uncoated fused silica window with a 0.5° wedge assuring polarization independent laser power detection. PARRoT is background compensated by differential operation where the reference detector chip is driven by a constant DC power and the measuring detector chip is feedback controlled to follow the temperature of the reference detector chip. The closed loop operation makes the radiometer’s response linear across the operational power range. We have optimized the detector chip design by thermal modeling [2]. The modeled electro-optical inequivalence for a centered laser beam is 0.007% and the spatial non-uniformity is ±0.02% within 4 mm radius from the absorber’s center, meaning that PARRoT is not sensitive to small alignment offsets.
PARRoT’s differential background compensation makes it insensitive to variations in background radiation. That combined with a compact design would allow it to be used as a radiance detector standard for field calibrations and atmospheric measurements outdoors or as a transfer standard between laboratories. Similar bolometers will be launched in a CubeSat satellite next year to measure total solar irradiance [3]. By changing a few resistance values in the electronics, by changing the detector chip’s heater resistance, and modifying the thermal conductance of the heat link design, the power range of PARRoT can be modified for different applications without compromising the accuracy. For example, reducing the heater resistance and increasing the heat link’s thermal conductance can extend the power range to 2 W. This enables, for instance, direct calibration of the 1 W laser beam power in LIGO (Laser Interferometer Gravitational-wave Observatory) with an expanded uncertainty of 0.1% (k = 2) which would be an order of magnitude improvement to its current calibration against NIST’s reference calorimeter via an integrating sphere transfer standard.
Acknowledgements: Jenny and Antti Wihuri Foundation, Finland is acknowledged for the financial support of this research.
1. E. D. West et al., “A Reference Calorimeter for Laser Energy Measurements,” J. Res. Natl. Bur. Stand. (U. S.) 76A, 13–26 (1972).
2. A. Vaskuri et al., “Microfabricated bolometer based on a vertically aligned carbon nanotube absorber,” Proc. SPIE 11269, 1–12, Synthesis and Photonics of Nanoscale Materials XVII, 112690L (March 2020).
3. D. Harber et al., “Compact total irradiance monitor flight demonstration,” Proc. SPIE 11131, 1–8, CubeSats and SmallSats for Remote Sensing III, 111310D (August 2019).
Development of 300 mW Background-Compensated Planar Absolute Radiometer Operating at Room Temperature
NIST’s C-series isoperibol calorimeters [1] operating at room temperature have been used as reference instruments in the laser power calibrations at NIST for over 50 years. These calorimeters operate from 100 μW to 300 mW with an expanded uncertainty of 0.86% (k = 2). Recently developed vertically aligned carbon-nanotube (VACNT) absorbers, with spectrally flat and hemispherical absorptance better than 99.95%, have enabled development of planar absolute bolometers with smaller size and significantly faster measurements compared to traditional laser calorimeters.
In this work, we present the latest progress of the Planar Absolute Radiometer for Room Temperature (PARRoT) [2] currently under construction that will replace the old reference C-series calorimeters. PARRoT is based on the electrical power substitution method and it can measure laser powers up to 300 mW with a predicted expanded uncertainty better than 0.1% (k = 2). The radiometer is operated at room temperature and placed in a 15 cm cube vacuum chamber to minimize convection while still providing a compact and sturdy standard. The laser beam is transmitted to the absorber by an uncoated fused silica window with a 0.5° wedge assuring polarization independent laser power detection. PARRoT is background compensated by differential operation where the reference detector chip is driven by a constant DC power and the measuring detector chip is feedback controlled to follow the temperature of the reference detector chip. The closed loop operation makes the radiometer’s response linear across the operational power range. We have optimized the detector chip design by thermal modeling [2]. The modeled electro-optical inequivalence for a centered laser beam is 0.007% and the spatial non-uniformity is ±0.02% within 4 mm radius from the absorber’s center, meaning that PARRoT is not sensitive to small alignment offsets.
PARRoT’s differential background compensation makes it insensitive to variations in background radiation. That combined with a compact design would allow it to be used as a radiance detector standard for field calibrations and atmospheric measurements outdoors or as a transfer standard between laboratories. Similar bolometers will be launched in a CubeSat satellite next year to measure total solar irradiance [3]. By changing a few resistance values in the electronics, by changing the detector chip’s heater resistance, and modifying the thermal conductance of the heat link design, the power range of PARRoT can be modified for different applications without compromising the accuracy. For example, reducing the heater resistance and increasing the heat link’s thermal conductance can extend the power range to 2 W. This enables, for instance, direct calibration of the 1 W laser beam power in LIGO (Laser Interferometer Gravitational-wave Observatory) with an expanded uncertainty of 0.1% (k = 2) which would be an order of magnitude improvement to its current calibration against NIST’s reference calorimeter via an integrating sphere transfer standard.
Acknowledgements: Jenny and Antti Wihuri Foundation, Finland is acknowledged for the financial support of this research.
1. E. D. West et al., “A Reference Calorimeter for Laser Energy Measurements,” J. Res. Natl. Bur. Stand. (U. S.) 76A, 13–26 (1972).
2. A. Vaskuri et al., “Microfabricated bolometer based on a vertically aligned carbon nanotube absorber,” Proc. SPIE 11269, 1–12, Synthesis and Photonics of Nanoscale Materials XVII, 112690L (March 2020).
3. D. Harber et al., “Compact total irradiance monitor flight demonstration,” Proc. SPIE 11131, 1–8, CubeSats and SmallSats for Remote Sensing III, 111310D (August 2019).