High-accuracy room temperature planar absolute radiometer based on vertically aligned carbon nanotubes.

We have developed a planar absolute radiometer for room temperature (PARRoT) that will replace the legacy C-series calorimeter as the free-space continuous-wave laser power detector standard at the National Institute of Standards and Technology (NIST). This instrument will lower the combined relative expanded measurement uncertainty (k = 2) from 0.84 % to 0.13 %. PARRoT's performance was validated by comparing its response against a transfer standard silicon trap detector traceable to NIST's primary standard laser optimized cryogenic radiometer (LOCR) and against the C-series calorimeter. On average, these comparisons agreed to better than 0.008 % and 0.05 %, respectively.

Optical Power Scale Realization by Laser Calorimeter after 45 Years of Operation.

To calibrate laser power and energy meters, the National Institute of Standards and Technology (NIST) uses several detector-based realizations of the scale for optical radiant flux; these realizations are appropriate for specific laser power/energy ranges and optical coupling configurations. Calibrations from 1 µW to 2 W are currently based upon calorimeters. Validation by comparisons against other primary representations of the optical watt over the last two decades suggests the instruments operate well within their typical reported uncertainty level of 0.86 % with 95 % confidence. The dominant uncertainty contribution in the instrument is attributable to light scattered by the legacy window, which was not previously recognized. The inherent electro-optical inequivalence in the calorimeter's response was reassessed by thermal modeling to be 0.03 %. The principal contributions to the overall inequivalence were corrected, yielding a shift in scale representation under 0.2 % for typical calibrations. With updates in several uncertainty contributions resulting from this reassessment, the resulting combined expanded uncertainty (k = 2) is 0.84 %, which is essentially unchanged from the previous result provided to calibration customers.

Spectral, spatial, and survivability evaluation of a flash-dried plasma-etched nanotube spray coating.

We demonstrate improved manufacturability of spectrally flat detectors for visible to mid-infrared wavelengths by characterizing a carbon nanotube spray coating compatible with lithium tantalate and other thermal sensors. Compared against previous spray coatings, it demonstrated the highest responsivity yet attained due to both higher absorptivity and thermal diffusivity, while also being matured to a commercially available product. It demonstrated spectral nonuniformity from 300 nm to 12 µm less than 1% with uncertainty (k=2) under 0.4%. The spatial nonuniformity of the assembled sensor was less than 0.5% over the central half (4 mm) of the absorber. As with previous developments employing isotropic carbon nanotube coatings, the absorber surface was sufficiently robust to withstand cleaning by compressed air blast and survived repeated vacuum cycling without measurable impact upon responsivity.

Portable, high-accuracy, non-absorbing laser power measurement at kilowatt levels by means of radiation pressure.

We describe a non-traditional optical power meter which measures radiation pressure to accurately determine a laser's optical power output. This approach traces its calibration of the optical watt to the kilogram. Our power meter is designed for high-accuracy and portability with the capability of multi-kilowatt measurements whose upper power limit is constrained only by the mirror quality. We provide detailed uncertainty evaluation and validate experimentally an average expanded relative uncertainty of 0.016 from 1 kW to 10 kW. Radiation pressure as a power measurement tool is unique to the extent that it does not rely on absorption of the light to produce a high-accuracy result. This permits fast measurements, simplifies power scalability, and allows high-accuracy measurements to be made during use of the laser for other applications.

Noise characteristics of thermistors: Measurement methods and results of selected devices.

As part of the development of a spectrally uniform room-temperature absolute radiometer, we have studied the electrical noise of several bulk chip thermistors in order to estimate the noise floor and optical dynamic range. Understanding the fundamental limits of the temperature sensitivity leads inevitably to studying the noise background of the complex electro-thermal system. To this end, we employ a measurement technique based on alternating current synchronous demodulation. Results of our analysis show that the combination of a low-current noise Junction Field Effect Transistor (JFET) preamplifier together with chip thermistors is optimal for our purpose, yielding a root mean square noise temperature of 2.8 µK in the frequency range of 0.01 Hz to 1 Hz.