Bridging the THz Gap in Radiometry

By Dr. Ephraim Greenfield, Chief Technology Officer, Ophir Photonics

The Renowned German standards laboratory Physikalisch-­‐Technische Bundesanstalt – PTB, has now developed a highly accurate calibration standard for calibrating Terahertz radiation based on a modified Ophir 3A-­‐P meter.

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 uncertainly 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).

The meter is identical to the standard Ophir 3A-­‐P-­‐ THz meter that is used for measuring THz radiation except that the absorber in the meter has been somewhat modified as explained in the PTB news release. The modified absorber has a specular surface on the front and gold coating on the back so that both absorbed and reflected power can be accounted for, thereby enabling precise calibration at the THz wavelengths.

With the Ophir meter, PTB has been able to obtain calibration accuracy of +/-­‐2% at THz wavelengths between 1 and 5THz and has incorporated the meter as its working calibration standard. Due to the very low noise level of the 3A-­‐P, PTB has been easily able to accurately measure powers as low as 10μW as can also be done by users of the standard Ophir 3A-­‐P-­‐THz sensor.

Since the Ophir 3A-­‐P-­‐THz is very similar to the PTB standard, users can have good confidence that with the Ophir 3A-­‐P-­‐THz they will get an accurately calibrated reading.

Bridging the THz Gap in Radiometry

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.

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