Patent of the Month: June 2021

Patent Description

NIST has invented an instrument that permits full and accurate characterization of a multi-kilowatt laser beam without attenuating it or perturbing its direction. This radiometer has been tested for laser powers up to 10 kW, but the maximum power is limited only by the irradiance allowable on the mirrors and as such, is expected to operate at much higher power levels. The unique three-mirror design allows the absolute power to be measured via radiation pressure (from the force on one mirror) while the negligibly-small fraction of light that leaks through the other two mirrors can be measured concurrently for a variety of laser beam parameters such as transverse profile, temporal response, spectral signature, etc.

Providing a vertically directed force simplifies the force calibration process and permits lower overall measurement uncertainties. Because this design redirects the laser beam to impinge on a horizontal mirror for force measurement, this allows the use of a variety of "off-the-shelf" scale technologies and force-sensors which reduces cost.

For laser beams in the anticipated power range of this instrument, there is currently no means of measuring the beam parameters in an absolute (fully-calibrated) sense without perturbing (absorbing or scattering) it. By pairing a radiation-pressure power measurement with a parasitic beam profilometer located behind one of the turning mirrors, we enable the absolute power to be measured as well as the relative beam profile. The relative beam profile can then be calibrated with the absolute power for an absolute measurement of intensity distribution. This same approach can be taken for other relative measurements such as fast power response, spectral content, etc.

Benefits

1. Our design accepts a horizontally-directed laser beam and re-directs it to measure its force in a vertical direction and again redirects it to its original horizontal direction.  This allows the use of a variety of commercial or custom-designed force-sensing technologies (balances) which were not previously usable for this application.
2. No other technique has been demonstrated to simultaneously measure absolute laser power and a variety of other laser beam parameters (such as beam profile, spectral content, beam divergence, beam quality factor, fast temporal response) non-perturbatively. This design also permits such relative parameters (beam profile, temporal response, etc.) to be calibrated to the radiation-pressure-based laser power measurement for each laser injection.
3. The previous radiation pressure approach is very sensitive to azimuthal alignment about a vertical axis. This new invention swaps the azimuthal sensitivity to an elevation angle sensitivity which can be significantly mitigated by aligning the beam to be horizontal. Further, this design permits a simple, non-perturbing optical sensor to be used to indicate proper alignment of the laser beam into the radiometer.
4. The previous radiation pressure approach redirected the laser beam by 90 degrees. This technique provides an exit beam that is colinear with the input. This allows the power meter to be inserted into or removed from a laser operation setup without modifying the beam alignment.

Licensing Inquiries may be sent to TPO [at] NIST.GOV

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