Abstract
In this work, we review the viability and precision of the photon-momentum-based optical power measurement method that employs an amplification effect caused by a multi-reflected laser beam trapped in an optical cavity. Measuring the total momentum transfer of the absorbed and re-emitted photons from a highly reflective surface (reflection of the laser beam from an optical mirror) as a force provides the possibility of measuring the optical power with direct traceability to SI units. Trial measurements were performed at two different metrology laboratories: the laboratory for mass/force at the Technical University of Ilmenau, and the clean room laser radiometry laboratory at PTB, with a portable force measurement setup consisting of two electromagnetic force compensation balances. We compared the results of the optical power measurements performed with the force measurement setup, via the photon-momentum-based method, with those performed using a calibrated reference standard detector traceable to PTB’s primary standard for optical power, the cryogenic radiometer. The comparison was carried out for an optical power range between 1 W and 10 W at a wavelength of 532 nm, which corresponds to a force of approximately 2000 nN at the upper limit, yielding approximately 2.3% relative standard uncertainty in the case of 33 reflections. Thus, conflating the high-precision force metrology technique at μN to nN levels with the optical setup required to achieve specular multi-reflection configuration of the laser beam, where a macroscopic optical cavity with ultra-high reflective mirrors (>99.995%) can adjustably be suspended from the force sensors, depending on required geometry of reflections, we show that the uncertainty of the optical power measurements upon further increase of the nominally applied optical power, the number of laser beam reflections, or the reflectivity coefficient of the mirrors can be markedly reduced.
Highlights
The use of photon momentum to determine the optical radiant power or to generate precision/calibration small forces [1,2,3] has made significant progress in mass/force and optical metrology fields in recent years, especially for measurements of the optical power of a laser at kilowatt levels [4]
In the core of the device is a force sensor, which consists of a commercial off-the-shelf electromagnetic force compensation (EMFC) weighing balance and a mirror with high reflectivity (R = 0.9998) attached to it
Vasilyan et al [1, 2] developed a device with two force sensors adapted for differential force measurements, by which the noise level was reduced by one order of magnitude
Summary
The use of photon momentum to determine the optical radiant power or to generate precision/calibration small forces [1,2,3] has made significant progress in mass/force and optical metrology fields in recent years, especially for measurements of the optical power of a laser at kilowatt levels [4]. The measurement of optical power of less than 1 kW using the photon momentum generated by a single reflection is quite challenging, as the force sensor needs to resolve forces at least in the nN-range. There are not traceable reference mass standards for this force range, to date To resolve this problem, Vasilyan et al [1, 2] developed a device with two force sensors adapted for differential force measurements, by which the noise level was reduced by one order of magnitude (from below the 1 μN level for a single sensor to less than 100 nN for a differential signal). Using single- and multi-reflection configurations, a possible standard for the force calibration, or the inverse, a standard for optical (laser) power calibration with direct traceability to the recently renewed SI base units, has already been proposed [2, 5,6,7]
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