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Design of an electrostatic balance to measure optical power of 100 kW



Lorenz Keck, Gordon A. Shaw, Renee Theska, Stephan Schlamminger


To accommodate the need for increased portability and accuracy in laser power measurement instrumentation above 100 Watt at the National Institute of Standards and Technology (NIST), a new instrument is required. This instrument is intended to create a new class of laser power measurement that measures photon pressure force with built-in traceability to the redefined International System of Units (SI) with uncertainty of 1E-3. Reflection and absorption of laser light provide a measurable force from photon momentum exchange that is directly proportional to laser power. This force can be directly compared to an SI-traceable electrostatic force using a suitable instrument. Previously, the NIST electrostatic force balance (EFB) was used to measure 3 W laser power, but is limited in its power handling capability due to its inability to support the required high-reflectivity mirror, and its portability due to the requirement of vacuum operation. The new balance mechanism must be portable and capable of supporting the components necessary to measure \SI100}\kilo\watt} laser power in an ambient environment at the desired uncertainty. Therefore, a monolithic parallelogram linkage containing notch flexure hinges is designed. An inverted pendulum reduces the stiffness of the compliant system close to zero without compromising dominating uncertainty contribution to the force measurement due to increasing tilt sensitivity. Both analytical and numerical methods are used to demonstrate functionality of the mechanism. Additionally, a method to lock the mechanism during transportation and also protect the hinges from over-load while assembling the balance without over-constraining the structure is introduced. Based on the main factors driving uncertainty, the design was executed with attention to force sensitivity, corner loading errors as well as mechanical and thermal robustness.
IEEE Transactions on Instrumentation and Measurement


electrostatic balance, measurement of optical power, new SI, mechanical design


Keck, L. , Shaw, G. , Theska, R. and Schlamminger, S. (2021), Design of an electrostatic balance to measure optical power of 100 kW, IEEE Transactions on Instrumentation and Measurement, [online],, (Accessed June 14, 2024)


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Created February 19, 2021, Updated October 12, 2021