Reflective Optical Chopper Used in NIST High-Power Laser Measurements

Abstract

For the past ten years, NIST has used high-reflectivity, optical choppers as beamsplitters and attenuators when calibrating the absolute responsivity and response linearity of detectors used with high-power CW lasers. The chopper-based technique has several advantages over the use of wedge-shaped transparent materials (usually crystals) often used as beam splitters in this type of measurement system. We describe the design, operation and calibration of these choppers. A comparison between choppers and transparent wedge beampslitters is also discussed.

Geometric contributions to chopper wheel optical attenuation uncertainty

“…Accordingly, this has made production of a true knife edge slit impractical. Previous development [1] assumed a small edge perpendicular to the reflective face had no appreciable effect on the ratio of transmitted light to incident light. During evaluation of newly fabricated 200 mm diameter chopper wheels having an (absolute) design transmission of 1% and 2%, the effect of a 1 mm thick edge was notable as was displacement in the slit edges due to a subtle fabrication error.…”

“…Optical chopper wheels with a calibrated transmission ratio enable the calibration of power measurement devices over a wider power range than can generally be provided by a laser source alone. Partially transmissive solutions to reduce or divert power such as wedges, absorbers, and partial reflectors suffer detrimental thermal effects in the tens to hundreds of watts [1][2][3]. Purpose-built calibrated optical chopper wheels are used instead for high power applications and see requirements not considered for the more common commercial devices typically used to generate optical pulses for use with lock-in amplification.…”