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January, 2005 EEEL Measures Diode Laser Efficiencies |
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EEEL researchers, Chris Cromer and Xiaoyu Li, have been tasked by DARPA to provide wall plug efficiency and spectral measurements of high-power high-efficiency laser diodes and arrays for DARPA's Super High Efficiency Diode Sources (SHEDS) program. These laser diode arrays present a significant measurement challenge. Edge emitters have a high degree of divergence on the fast axis, which makes complete beam capture with the optical radiometer an issue. In addition, the high power levels and the large size of the arrays can challenge the best commercially available optical radiometers. Measurements of laser power commonly use commercially available, thermopile-based or integrating-sphere radiometers, but these instruments have shortcomings when applied to the diode lasers and arrays we are dealing with here. In order to capture the multiple beams from the diode array, the radiometer must be very close to the array. Unfortunately with large area disk-type radiometers, this close proximity can disrupt the thermal equilibrium of the absorber disk, which distorts the temperature measured with the thermopile. In addition, most high-power detector coatings reflect a significant portion of the incoming radiation, but instead of being lost this reflected radiation can be reflected back to the detector by hardware near the radiometer. On the other hand, integrating-sphere based radiometers are notoriously sensitive to port conditions. Again, since the laser diode or diode array being measured must be placed directly in the input port, significant errors can occur. In addition, baffle design and thermal effects can influence the accuracy of sphere measurements, especially for high-power, highly diverging sources. To avoid these problems, we have developed an optical radiometer based on measuring the temperature increase of water cooling the device. A copper cavity that has a black coating on the inside captures and converts the laser output to heat. The cavity is cooled with water flowing through channels on the outer surface of the copper. The power is calculated from the measured increase T of the cooling water, the water flow, and heat capacity. The effective heat capacity of the cooling water can be measured by introducing an electrical heater into the water flow making the optical measurements traceable to SI units. If demand
for these measurements is sufficient at the conclusion of the SHEDS project,
we intend to continue offering these measurements as part of the NIST
Calibration Services for optical radiation measurements, which are available
to anyone for a fee.
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