The National Institute of Advanced Industrial Science and Technology (AIST) (President: Hiroyuki Yoshikawa) and Nippon Telegraph and Telephone Corporation (NTT) (President: Satoshi Miura) have succeeded in detecting weak terahertz waves over a wide frequency range using only one highly sensitive receiver. In a contract research project sponsored by the National Institute of Information and Communications Technology (NICT) (President: Hideo Miyahara) and promoted by the Ministry of Internal Affairs and Communications, AIST and NTT developed a low noise receiver which covers a frequency range that previously required three or more receivers for coverage. Based on this technique, it is expected that one can realize high-precision, general-purpose measuring instruments for the terahertz range, and remote dangerous gas detectors for use in disaster sites. Details of the technology will be exhibited at the AIST Open Lab to be held at AIST Tsukuba on October 20 and 21, 2008.

Studies into terahertz waves, electromagnetic waves on the border between light and radio waves, are very active these days.  As toxic gases such as carbon monoxide, hydrogen chloride, hydrogen cyanide and sulfur dioxide have their specific absorption spectra in the terahertz frequency region, it is expected that the concentration of these dangerous gases contained in the atmosphere can be measured using terahertz waves.  However, terahertz waves are known as "the unexplored frequency," and it is difficult to accurately and directly measure their power or frequency.  The heterodyne technique accurately converts the input terahertz signal into the microwave output which general-purpose measurement hardware commercially available can process. This technique is vitally important in utilizing terahertz waves for the purposes as described above.  The performance and usability of a heterodyne receiver are dependant on two key components: a frequency converter (mixer) and a reference signal generator (local oscillator), which jointly constitute a receiver.  To date, due to the lack of suitable mixers and local oscillators, there have been no terahertz receivers available that possess both performance and usability comparable to receivers in other frequency ranges. 

The collaborative research group has developed a low-noise, wide-band mixer based on superconducting technology, as well as a wide-band, high-output local oscillator based on photonic technology.  Integrating these technologies (Fig. 2), we have developed the world's first heterodyne receiver which satisfies the following conditions simultaneously: First, the receiver covers a wide frequency range corresponding to 86% of the central frequency, while three or more conventional heterodyne receivers share this frequency band (Fig. 1). Next, in the 74% of the central frequency, the receiver noise temperature is very low, i.e., less than 20 times of the quantum limit. Furthermore, it is verified that this receiver is applicable to spectral radiometers for the weak emission of gases, which provides a useful approach to identify the gaseous species and their concentration (Fig. 3). 

In the future, we will improve the frequency resolution to apply this receiver to the spectrum analysis of terahertz oscillators under the test with high signal purity. Unlike visible and infrared light, terahertz waves can transmit smoke fog and flames. Thus, this receiver is expected to apply to the remote sensing of dangerous gases, even when one cannot easily or safely approach a disaster site. Aiming at the realization of such systems, we will continue to develop key components and technologies including optics.

Fig. 1. Conventional and newly developed heterodyne receivers

Fig. 2. Spectral radiometer for gases based on our receiver

Fig. 3. Measured (red) and calculated (blue) spectrum of N2O gas