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Big Berthas Used To Measure Laser Power

WRIGHT-PATTERSON AFB, Ohio (AFLCMC) – The Air Force Metrology and Calibration Division is using some unique mobile ‘Big Bertha’ series High Energy Laser primary reference calorimeters to support Air Force Research Laboratory’s national defense programs work on lasers.

The HEL calibration supports the Laser Hardened Materials Evaluation Laboratory (LHMEL) of the Air Force Research Laboratory’s Materials and Manufacturing Directorate at Wright Patterson AFB and the AFRL Directed Energy Directorate at Kirtland AFB in Albuquerque, NM. For over 35 years, AFRL’s LHMEL has provided the aerospace community with a comprehensive source for high temperature characterization of current and emerging materials using a variety of infrared laser sources and environmental simulation capabilities.

The calibrations performed by AFMETCAL enable LHMEL to accurately simulate high service life temperatures experienced by parts within a jet engine.

These calorimeters, designated BB1 and BB2 (Big Bertha 1 and 2), which are the national standards for HEL measurements very precisely measure the energy in a laser shot.  AFMETCAL is the division of the Air Force Life Cycle Management Center responsible to ensure Air Force systems and equipment are accurate, uniform, reliable and traceable to National Institute of Standards and Technology (NIST) or other approved sources for SI units of power and energy for optical radiation measurements.

Typically, calibration work of this type is done at the AFMETCAL facilities in Heath, Ohio. In this instance, the AFRL Directed Energy Directorate at Kirtland AFB, NM needed on-site High Energy Laser calibration support using some very specialized pieces of equipment. AFMETCAL calibration services for laser power and energy meters have been provided by use of calorimeters that were electrically calibrated and directly traceable to SI units through electrical standards.

“There are only two of them that actually exist in the in the world,” said Jennifer Landry, AFMETCAL Electrical Engineer and project lead.

Landry with AFMETCAL engineers, Thomas Jenkins, Tyler Youngman, and Tesfatsion Sereke teamed with Air Force Primary Standards Laboratory (AFPSL) personnel transporting primary reference standards to the Kirtland on-site location.

Upon request, these BB series calorimeter standards are used to perform NIST SI traceable calibrations at DoD research and development facilities. “These calibrations support materials testing by Air Force Systems Command laboratories and are also used to provide accurate measurement for Strategic Defense Initiative programs on laser weaponry,” said Sereke, AFMETCAL Senior Metrology and Calibration Engineer.

These calorimeters were developed through a collaborative research and development effort between NIST, which is a part of the U.S. Department of Commerce, and the Air Force. Each calorimeter system weighs more than 1000 pounds, and both calorimeter systems are mounted on a specialized HEL Mobile Calibration Van to go on site for customer support

“This van was prepared specifically for this purpose. So it is reinforced to be capable of carrying the weight and it has all the necessary electrical hookups in it,” Landry said. “It also has physical tie downs that provide stability so we’re not worried about shipping problems in the entire transportation process.”

During the test, the device actually captures energy from the laser source to measure it.

“It is a large, roughly 30 inches wide by 72 inches long by 48 inches high, metallic rectangle with a cavity about 10 inches in the middle,” Landry said. “The laser shot gets directed into that cavity and on the inside of it is absorbed by some sensitive electronic gear that precisely measures the energy.”

AFPSL technicians configure the measurement set-up where a laser is shot through a “chopper wheel” and into the BB, according to Landry.  Through this process AFMETCAL is able to accurately calibrate these calorimeters and determine the energy levels these items can support.  This enables LHMEL to perform laser/materials interaction testing that determines the amount of energy a material can absorb before damage is incurred, and Kirtland to be able to accurately support the Directed Energy efforts.

The request to support the research presented a challenge, especially during the pandemic.  In preparation for the onsite calibration deployment, the AFMETCAL team of engineers provided pre-calibration support, updated software and went onsite to provide troubleshooting as needed to ensure mission success, according to Youngman.

“The calibration support effort also includes testing of the Air Force’s new HEL standard Radiation Pressure Power Meter (RPPM), which will eventually be replacing the bulky BB standards mentioned above,” Landry said. “The RPPM is the product of a RDT&E project developed by NIST. The footprint of the RPPM is much smaller and will alleviate the Air Force’s reliance on a HEL van.” 

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