Millimetre and Submillimetre Wave Components for Future Diagnostic Instrumentation
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Millimetre and Submillimetre Wave Components for Future Diagnostic Instrumentation. Chris Mann The Rutherford Appleton Laboratory, Chilton, Oxfordshire, OX11OQX, U.K [email protected] Abstract Contrary to common belief the technology associated with components and systems for operation in the millimetre and submillimetre or Terahertz (THz) region of the electromagnetic spectrum is mature. However, it has largely been developed for use in the fields of radio astronomy and remote sensing, two areas where size and cost have not been the driving issues. This is changing, both fields now have ambitious plans to put large complex systems into space in experiments such as NASA’s EOS MLS and ESA’s FIRST amongst others. Also, ground based systems such as ALMA are planned where the total number of receivers exceeds 1000. For such instruments cost and size now becomes important. Taking these factors into account for the last decade or so there has been a concerted drive towards smaller and cheaper instrumentation. Initially, the use of quasi-optical systems was implemented in place of waveguide mainly because of its prohibitive manufacturing cost. Quas-ioptical systems are still being developed but waveguide has recently seen a revival due to the use of new micromachining techniques for its fabrication. Planar or integrated circuitry mounted in waveguide has now demonstrated state of the art performance to 2.5THz. Combining these factors means that for the first time the cost of manufacturing high performance RF electronics in the THz region has finally become affordable thereby making it available to new areas of research, material science is one of many. This paper describes how new devices and systems may be implemented to realise the basic building blocks of such instrumentation, namely, detectors and sources. Introduction There has recently been a lot of excitement generated in the media by the use of THz imaging for medical purposes1. The ability to form such images, in this instance of teeth can largely be attributed to the arrival of femto-second electro-optical sampling techniques. The basic mode of operation of this technique is to generate a very short electrical pulse via an optical laser. The resulting radiation which is rich in THz components is then passed through the sample before being coherently detected and formed into an image. The main advantage of the technique is that a very broad spectrum of RF is generated and detected. In addition, complete images can be formed simultaneously without the need for scanning. Limitations include relatively low signal to noise and poor spectral resolution. Undoubtedly these will be improved as the technology matures. In the meantime, measurements such as these can be achieved using an all-electronic system. An electronic system will have orders of magnitude more signal to noise mainly due to the increased sensitivity of the detectors and availability of much higher source powers. In addition, the spectral resolution can be as high as 1 part in 1011. For these reasons, mil
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