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PTB Terahertz calibration satisfies US laser manufacturer

November 06, 2009

Terahertz radiation still lies in a metrological no man's land - a metrology gap. The Physikalisch-Technische Bundesanstalt (PTB) can now close this gap. For the first time, a commercial Terahertz laser was traced back to the international system of units (SI) by measuring its output power absolutely. PTB achieved this success with a power meter which had been calibrated beforehand against a cryogenic radiometer, Germany's primary standard for power measurement of electromagnetic radiation. Therefore, this laser, namely a SIFIR-50 manufactured by the US company Coherent Inc., is the first THz laser in the field with a reliably proven output power enabled by a novel calibration capability set up at the PTB.

Terahertz radiation refers to frequencies from 0.3 THz to 10 THz, which are located in between microwaves and infrared radiation at long wavelengths. These are the limits of two different mechanisms for generating radiation: microwaves are produced electronically by semiconductors and infrared radiation in contrast, optically by means of lasers. This Terahertz gap has only recently been penetrated, but it shows to be promising for a variety of applications. Meanwhile, Terahertz radiation is used for spectroscopy, analytical science and astronomy. As technological innovations have noticeably improved the generation and detection of THz radiation, more and more areas have been added: testing of materials, security checks at airports, biological and medical science, quality inspection of foodstuffs and agricultural goods, global environmental monitoring and information and communication techniques, as well. All of these applications benefit from reliable power measurement in the THz region.

That is why a world leader in laser technology, the company Coherent Inc., and a leading institute of metrology, the Physikalisch-Technische Bundesanstalt, have joined forces to improve the measurement of the power of Terahertz lasers and to trace it back to the SI units by comparing it to a national standard.

The Coherent SIFIR-50 laser which was operated at 2.5 THz for the first power measurement is the core instrument of a new facility for the determination of the THz radiant power in SI units and for the calibration of THz detectors. It is a far-infrared molecular gas laser pumped by an integrated, frequency stabilized, tunable 50 W CO2 laser. Dr. Andreas Steiger of PTB Berlin explains: "The unique feature of the system is its Fabry-Perot lock used to control the operating frequency of the CO2 laser which is optimal for pumping the vibrational transitions of the gas molecules". Combined with its thermally-compensated THz resonator and pump optics design, the SIFIR-50 delivers stable THz radiation, tunable from 1 THz to 7 THz, to several rotational transitions of the molecules in use. The wide tuning range is four times larger than the whole visible spectrum and contains a variety of discrete THz lines in many different gases. Steiger is confident: "The performance and beam quality of the laser enables PTB to take a large step in bridging the metrology gap between ultra high-frequency electronics and far-infrared optics in the near future."

Physikalisch-Technische Bundesanstalt (PTB)