The operating range of heavy duty gas turbines that feature lean premixed combustion to achieve low NO, emissions is limited by thermoacoustic oscillations. T o extend the operational envelope of the gas turbine, passive means have to be developed to suppress thermoacoustic instabilities. In order to develop passive means the complex interaction between acoustics and thermal heat release has to be taken into account. A new stability chart applicable to the qualification of industrial design has been developed that accounts for the acoustic properties of the combustion system including its boundary conditions and the flame response data. The method has been validated using detailed measurements of the eigenmodes in an operating gas turbine as well as experimental data from component test rigs. An explanation is given of the significant extension of the operation envelope of the gas turbine a s an effect of cylindrical extensions to the burner nozzle.

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We present a THz investigation of histo-pathological samples including the larynx of a pig and a human liver with metastasis. Our measurements show that different types of tissue can be clearly distinguished in THz transmission images, either within a single image or by a comparison of images obtained for different frequency windows. This leads to the problem that images obtained for different frequencies inherently have a different spatial resolution. An image obtained from two such images by a simple mathematical operation may contain artefacts. We discuss measures to deal with this problem. Furthermore, we investigate the possibility of improving the spatial resolution of THz images. Finally, we present a cw THz imaging system based on a photomixer and an external cavity semiconductor laser that allows for simultaneous two-mode operation. The cw system is less expensive and more compact than conventional time-domain imaging systems.

We have observed four-wave mixing in a semiconductor laser configured to emit on two wavelengths simultaneously. The four-wave mixing sidebands exist up to 4 THz stemming from a modulation of the carrier plasma at the difference frequency of the two laser modes. In addition, we were able to generate and detect tunable THz radiation at this difference frequency from the laser device itself suggesting a scheme for a tunable THz source.

Abstract:We discuss and analyze concepts for the generation of tuneable continuous wave terahertz (THz) radiation with two color diode lasers. First, different geometries of two color lasers are reviewed. We show that the THz power of two color lasers in combination with external photomixers becomes sufficient for scanning THz imaging applications when optical amplification with a tapered amplifier is implemented. Then, the concept of direct emission of THz radiation out of a two-color semiconductor laser is reviewed and the potential of this concept with respect to THz bandwidth and achievable THz power is critically analyzed.Scheme of the FTECAL-laser cavity consisting of collimated laser diode, grating lens and mirror.

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Direct emission of terahertz (THz) radiation out of a two-colour semiconductor laser is reported and analysed. The geometry of the tunable two-colour laser is described together with the physical mechanisms responsible for the emission at the THz difference frequency of the two pump colours used. On the basis of this analysis, different possibilities are suggested to increase the emitted THz intensity towards application relevant levels.