FEATURE TERAHERTZ Though terahertz
sources have been around for some time, there is still an issue with measurement standards
Even though terahertz sources have been around for some time, there is still an issue with measurement standards. ‘On the sensor side of things, calibration is a big issue,’ said Dooley. ‘For other laser types there are many detector calibration standards that cover the UV through the far IR. However, for the terahertz spectrum, there are very few calibration standards. ‘When terahertz laser manufacturers come to us, they want absolute power or energy
measurements and we have to say “here is the state of the art in terahertz calibration, we can offer reasonably low uncertainty (+/- 15 per cent) in the 30 to 0.7THz range, but beyond that range the best you can hope for is a relative measurement”.’
The Physikalisch-Technische Bundesanstalt (PTB) in Germany offers calibration of terahertz sensors and has done so for around two years. But as Dooley explained, it is the only one in the world to offer this. He added: ‘PTB has recently expanded the spectrum to cover 1THz to 5THz. However, this is discrete wavelength calibration.’ In the USA, a team at the National Institute of Standards and Technology (NIST) is developing a terahertz calibration standard at the moment and plans to offer calibration at one or two wavelengths. NIST has
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The University of Leeds team’s 1W terahertz laser cell
now developed several terahertz calibration standards and is in the process of determining their uncertainty. Once this work is complete, which Dooley predicts could be later this year, NIST will offer calibration to industry. Gentec-EO and others are trying
to reduce calibration uncertainty. One way to improve calibration is by coating the surface of the detector with a non-reflective material. New coatings are being
tested which reduce the reflection of terahertz radiation from the detector surface, and so improve the accuracy of a power reading. Dooley explained that ‘what you are shooting for is something that has a very flat absorption over the widest spectrum. This is so you can calibrate at a signal wavelength, and then offer correction factors that have low uncertainty over a good part of the terahertz spectrum.’ l
TeraHertz Detectors
These detectors incorporate pyroelectric-based sensors and are easy to use, at room temperature. Choose either a standalone head with on-board electronics or Gentec's T-Rad meter and a separate sensor. Up to 3 W of continuous power is now available with the NEW THZ12D model, the highest in Gentec's THz range of products.
Contact Gentec’s exclusive UK distributor:
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