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THz Gap in Radiometry Bridged by PTB


Figure 1: PTB’s new "gold standard" for detector-based THz radiometry: The cutaway view exhibits a schematic cross-section of the THz radiation absorber with gold (Au) coating on its back side. Part of the incoming (1) radiation is reflected (2) at the front side. The lack of interference modulation of this radiation indicates no remarkable residual (3) radiation after two passes through the absorbing disc enabled by the gold coating.

First performed at 2.52 THz only, PTB has now extended its unique detector-based radiometric calibration service to the full tuning range of a Terahertz laser spanning from 1 THz to 5 THz. As published in Optics Express, the standard measurement uncertainty of the THz power scale is reduced simultaneously to below 2 % by a new way to link the scale to the International System of Units (SI).

In detail, an optically polished neutral density glass disc is coated with a thin gold layer on the back side and mounted as a special broad band THz radiation absorber inside a commercial laser power meter as shown in the figure. In the case of incomplete absorption during the first pass through the disc, the remaining radiation is reflected by the gold mirror to experience a second absorption process. By this, the radiation loss is dominated by the front surface reflection. Moreover, the reflected radiation has been measured precisely at all spectral emission lines of a THz molecular gas laser and in the visible spectrum as well. As a consequence, the power responsivity of such a modified laser power meter can be determined with the lowest possible uncertainty by means of a HeNe laser in the visible and extrapolated into the THz regime.

Hence the question "How much THz power do I have?" is solved because reliable THz power measurements with known accuracy are possible now. As of late, the traceable calibration of suitable THz detectors in the wide spectral range from 1 THz up to 5 THz has become part of the official services of PTB offered world-wide to customers in research and industry.

A. Steiger, M. Kehrt, C. Monte, and R. Müller , Optics Express 21, 14466 (2013)


A. Steiger, 7.34, e-mail:  Andreas.Steiger(at)ptb.de