October 2007

EEEL Develops Calibration Source for Power in the Millimeter-Wave/Terahertz Range

Researchers in EEEL’s Optoelectronics Division have developed and characterized the first broadband calibrated blackbody source as part of a new metrology infrastructure for the millimeter-wave/terahertz frequency range of the spectrum. The techniques and tools available for the measurement of radiometric power and related quantities in this frequency band are far less advanced than they are at optical and infrared frequencies. However, applications are rapidly emerging, including millimeter-wave remote sensing for atmospheric science, for which the shortcomings of present calibration targets are widely known, and millimeter-wave/terahertz detector arrays for security systems, for which noise-equivalent power and noise-equivalent temperature difference are usually the most important factors in determining overall effectiveness.

Charles Dietlein and Erich Grossman, in collaboration with the University of Colorado, developed and characterized the novel, aqueous blackbody calibration (“ABC”) source. The blackbody is a body of liquid water (extremely absorptive in this frequency range) held at a uniform and known temperature in a specially designed container made of expanded polystyrene. The problem of non-zero reflectance at interfaces is solved by a four-reflection, optical trap geometry, the same idea behind trap detectors used in the calibration of optical fiber power meters. Uncertainty in radiometric temperature due to the undesirable reflectance present at a normal water-EPS interface is minimized by the trap geometry, which ensures that all radiation incident on the entrance aperture encounters two transverse-electric- and two transverse-magnetic-field reflections at 45°. The measured reflectance of less than 1.5% (effective emissivity of > 98.5 %) agrees with the predicted values up to a frequency of 260 GHz. The ABC source is generally applicable for calibration and measurement needs from 100 GHz to 1 THz. So far, ten institutions have requested the source for their calibration needs, and eight are currently participating in “beta” testing. The ABC source is easy to fabricate, inexpensive to manufacture and operate, reproducible, scalable, and easily adaptable to a variety of measurement scenarios.

Contact:  Erich Grossman, phone 303-497-5102