Schweber Lasers

NIST Refining Laser-Power Measurement Over 20 Orders of Magnitude

May 26, 2020
To meet the challenge of measuring laser power ranging from a few photons to thousands of kilowatts, and tie the measurement to the new SI definition of the kilogram, NIST is examining its entire portfolio of sensing and measuring techniques.

Engineers know that measuring dc or low-frequency electrical power is fairly straightforward, but such measurements become more difficult as you go up the spectrum into the tens-of-gigahertz range. Still, that’s only the beginning of the truly challenging part of the spectrum range. Laser-sourced optical power is even more difficult to measure, due to its wide spectrum (IR, visible, and UV), sensors with limited spectral sensitivity, and wide dynamic range. Nonetheless, as optical components and optoelectronics become increasingly critical parts of system, assessing basic parameters such as optical power becomes even more important.

The dynamic range of optical power in applications from advanced research projects to industrial use is truly impressive, beginning at low levels of “photon counting” at just 103 photons/s (~10-15 W). These little quanta of energy are skittish, elusive, and don’t like to have their pictures taken (so to speak), while reaching out to touch them, or be touched by them, also changes them. At the other end of the power range are 100-kW lasers that can burn through steel (~1023 photons/s at 1070 nm) and thus will easily destroy any power sensor.

The problems of precise optical-power measurements are well known to the science and engineering communities, while the need for them is dramatically increasing. For that reason, researchers at the National Institute of Standards and Technology (NIST) are seeking to unify and clarify their disparate techniques. The new study was done in the context of NIST’s laser power measurement and calibration services, first offered in 1974.

Their analysis began as an overview of the many methods used for both wavelength and power level (see table).

Laser-power measurement services supported at NIST. The relative expanded uncertainty 𝑈𝒫̂𝑎 represents a coverage factor k=2 defining an interval having a level of confidence of approximately 95%. The numbers in [brackets] refer to references in the published paper. ††Relative expanded uncertainty of secondary standards is reported from traceability to the laser-optimized cryogenic radiometer (LOCR). *The mid-power bolometer “Next Generation C” is still under development but is included here for completeness. **For the radiation pressure power meter (RPPM), the upper power range is listed as 50 kW only because it’s the highest power for which it has been rigorously tested—full operability is expected at higher powers.

In the widely used thermal method, the laser’s output is focused on a specially coated detector, and the change in detector temperature (however minuscule) is measured. This change is then compared to the amount of electrical power needed to generate an equivalent amount of heat (Fig. 1). This method can be indirectly traced to the SI units through the volt and the ohm, which are “derived units” based on NIST-developed equations using multiple SI units. The corresponding equipment for measuring the optical watt this way is large and not portable.

1. In the electrical-substitution approach to measuring input laser power, the optical absorber is instrumented with a thermometer (measuring temperature T), a current meter (measuring current I), and a voltmeter (measuring voltage V). Laser light incident on the optical absorber causes an increase in the absorber’s temperature. Alternatively, a direct-current (dc) voltage V0 applied to the resistive heater can be used to cause an equal temperature rise for an accurately known electrical power VI. Measurements to characterize the equivalence between optical and electrical heating allow for an accurate measure of laser power as a function of absorber temperature.

Instead, NIST wants to focus on laser-power measurement accomplished by comparing the optical power to the force of gravity on a reference mass or an equivalent force, following the recent redefinition of mass (kilogram) in terms of absolute units and a reproducible standard rather than a discrete, primary physical artifact.1 The result is measured in either milligrams (mass) or micronewtons (force) and can be used to calculate optical power (Fig. 2).

2. NIST is looking to transition its fundamental techniques for measuring laser optical power from a thermal approach to a radiation-pressure approach, over the entire span of power level.

Radiation pressure-based measurement of laser power would allow NIST to fully define and measure the optical watt directly through force metrology, which has the potential to reduce measurement uncertainty for laser-power radiometry.2

NIST’s primary standard-power meters are the laser-optimized cryogenic radiometer (LOCR), which currently has the lowest measurement uncertainty, and the radiation pressure power meter (RPPM), which has a unique traceability path through the kilogram. Therefore, the NIST paper compared their other power meters to these two primary standards (Fig. 3).

3. Illustration of the direct comparisons performed between the various power meters indicating the power and wavelength at which each was carried out. The solid-fill boxes indicate primary standards, while the outline boxes denote secondary (transfer) standards.

NIST researchers are also looking to link their work to developing a set of primary standards that are “synchronized” with the recent redefinitions from SI (Fig. 4). Therefore, it would offer more reliable measurements based on presumably unchanging fundamental properties of nature.

4. Traceability map for the NIST primary and secondary laser-power measurement standards. The secondary (transfer) standards are abbreviated as “OFPM” (optical fiber power meter), “Trap” (silicon/germanium trap-based photodiode detectors), SPAD (single-photon avalanche photodiode), and “Thermopile” (thermopile based thermal detector). The primary standards are as described as “OFCR” (optical fiber cryogenic radiometer), “LOCR” (laser-optimized cryogenic radiometer), “C” (mid-power range isoperibol calorimeter), ““FWOPM” (flowing water optical power meter), “K” (high-power range isoperibol calorimeter),and RPPM (radiation pressure power meter).The relevant defining constants are e (electronic charge), h (Planck’s constant), Dn (cesium hyperfine splitting frequency), and c (the speed of light in vacuum).

The NIST effort is detailed in the lengthy but readable paper “Meta-study of laser power calibrations ranging 20 orders of magnitude with traceability to the kilogram,”published in Metrologia, with 14 authors representing different NIST specialties and supported by 63 references.The paper is a good review of the state of the art with respect to precision and uncertainty of laser-power measurement techniques.

References

1. Electronic Design, “Sorry, Primary Kilogram, But We Are Dumping You

2. NIST, “Gauging Multi-Kilowatt Laser Power with the Pressure of Light

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