Progress on Embedded Radiation Pressure Sensors for Absolute Power Measurement

High-accuracy power measurement of high-power lasers has traditionally involved the absorption of (ideally) all the laser energy by a large thermal mass resulting in the temperature change of that sensor. Though the measurement uncertainty of high powers can be ~1%, this thermal process can be slow (with cooling times on the order of tens of minutes for multi-kilowatt calorimeters) and excludes the laser from any operational use during the course of a measurement. Radiation pressure sensors, on the other hand, offer a means for in situ laser power measurement at much faster measurement speeds (seconds and below). The first realization of a multi-kilowatt radiation pressure power meter (RPPM) recognized a new primary standard for optical power measurement that traces the optical watt directly to the kilogram by measuring the force of reflection on a highly reflective mirror [1]. This technology now boasts measurement accuracy equal to that of thermal methods with the aforementioned benefits of speed and nonexclusive measurement of the laser beam. However, the device is sensitive to tilt with respect to gravity and to environmental sources of vibration, is relatively bulky compared to other optical components, and is slower than what some high-power laser applications require. Therefore, we are currently developing a micromachined capacitor-based RPPM that is faster than its bulk counterpart and is small enough to be embedded directly within laser processing systems. In this presentation, we will discuss the principle of radiation pressure power measurements and highlight our current progress in developing the small-package RPPM. We will present the results of open loop measurements with a prototype device that show significant improvements. In particular, we demonstrate the ability to measure both optical (1070 nm) and RF (15 GHz) powers with the use of different mirror types. In both cases, we see a noise floor on the order of 1 W/√Hz. This agrees well with theoretical predictions of noise equivalent power at 0.44 W/√Hz, where environmental noise is not considered. We additionally measure the response time of the prototype to be less than 100 ms when operated in open loop. Furthermore, the sensor design suppresses signal noise from orientation with respect to gravity and low-frequency vibrations by using symmetric springs to reject common-mode inertial forces. We additionally carry out careful characterization of the electromechanical system to properly estimate the system responsivity, noting the nonlinear behavior of the parallel plate capacitor in open loop. From multiple overlapping tests, we characterize the spring stiffness, plate spacing, natural resonance, quality factor, and parasitic capacitance of our instrument [2,3] obtaining not only the necessary information to predict responsivity, but also demonstrating different techniques that will be used to calibrate the device. These tests demonstrate improved measurement capability for fast, sensitive detection of optical and RF power that we predict will meet the growing demand for embedded, NIST traceable, absolute power metering. [1] P. Williams, J. Hadler, F. Maring, R. Lee, K. Rogers, B. Simonds, M. Spidell, M. Stephens, A. Feldman, and J. Lehman, Optics Express, 25(4), p.4382, 2017. [2] I. Ryger, A. Artusio-Glimpse, P. Williams, N. Tomlin, M. Stephens, M. Spidell, and J. Lehman, Proceedings of 13th International Conference on New Developments and Applications in Optical Radiometry, Tokyo, Japan, 13-16 June 2017. [3] I. Ryger, A. Artusio-Glimpse, P. Williams, N. Tomlin, M. Stephens, K. Rogers, M. Spidell, and J. Lehman, “Micromachined force balance for optical power measurement by radiation pressure sensing,” submitted