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19 th International Symposium on Space Terahertz Technology 7-1 Invited presentation for ISSTT2008 Pushing the limits of multiplier based local oscillator chains Imran Mehdi, John Ward, Alain Maestrini*, Goutam Chattopadhyay, Erich Schlecht, and John Gill Jet Propulsion Laboratory, California Institute of Technology, Pasadena CA 91109 *Université Pierre et Marie Curie-Paris6, LISIF, Paris, France and Observatoire de Paris, LERMA, France The successful implementation of robust and sufficiently powerful multiplier based sources ranging to 1900 GHz for the Herschel Space Observatory has now made it possible to leverage this technology for future missions and various other applications. Broadband, electronically tunable sources based on Schottky diode frequency multipliers continue to be an ideal solution for a number of proposed applications in the THz range. Fig. 1. 3D schematic view of the bottom half of the power-combined 260- 340 GHz frequency tripler based on two mirror-image integrated circuits. Fig. 2. Close-up view of the power-combined 260-340 GHz frequency tripler showing the two mirror-image GaAs integrated circuits. The E-field vectors in the input and output waveguides are indicated by plain arrows. The E-fields generated by the two sub-circuits are combined in-phase in the output waveguide. This talk will survey the current status of multiplier based sources and focus on the progress that has been made since the delivery of HIFI local oscillator chains. To improve device yields all chips are now fabricated on a membrane regardless of operating frequency. While this has dramatically increased yields and significantly shortened processing times, on-chip thermal management has become extremely important especially for the first stage multipliers. Other objectives of the current effort is to extend output power and frequency range of these sources. A simple in-phase power combining scheme that addresses all of these concerns has been recently demonstrated [1]. A 300 GHz tripler block that utilizes two in-phase planar chips is shown in Figure 1 and 2. Utilizing this simple approach can lead to higher output power without sacrificing bandwidth. Schemes that utilize even more chips in a single block are also being considered. Increased output power in the drive stages allows one to then pump later stages with sufficient power. Using this approach can result in multiplier based sources working up to 3 THz with sufficient power to pump single pixel receivers. [1] Alain Maestrini, John S. Ward, Charlotte Tripon-Canseliet, John J. Gill, Choonsup Lee, Hamid Javadi, Goutam Chattopadhyay, and Imran Mehdi, “In-Phase Power-Combined Frequency Triplers at 300 GHz,” to appear in March 2008 issue of IEEE Microwave and Wireless Components Letters. 62
19 th International Symposium on Space Terahertz Technology Invited presentation 7-2 Experiences with QCL local oscillators H.-W. Hübers German Aerospace Center Institute of Planetary Research Rutherfordstr. 2, 12489 Berlin, Germany In 2001 the first THz quantum cascade laser (QCL) has been demonstrated. Since then rapid progress has been made. Attractive features, which have been demonstrated for lasers operating in cw mode are emission between 1.2 THz and 4.9 THz, operation above the temperature of liquid nitrogen, output power beyond 100 mW, narrow linewidth of less than 10 kHz, and single mode operation by use of a distributed feedback structure. These features make QCLs promising devices for use as local oscillator in heterodyne receivers such as GREAT on SOFIA or future spaceborne heterodyne receivers. Initial experiments with a QCL as local oscillator and a hot electron bolometric mixer have been performed in 2005 for the first time. The noise temperature as well as the Allan time were the same as measured with a gas laser local oscillator. A major issue is the quality of the beam profile. Since the dimensions of the outcoupling facet of a QCL are in the order of or even less than the emission wavelength, diffraction degrades the beam profile. Nevertheless the output profile of a QCL with a surface plasmon waveguide can be transformed into an almost Gaussian profile with appropriate optics. With such a profile it was shown that 10 µW power in front of the cryostat window is sufficient for pumping the HEB mixer. Also, frequency-locking to a gas laser emission line, Doppler limited high resolution molecular spectroscopy, and frequency tuning with an external cavity resonator have been demonstrated. In this presentation the state-of-the-art of QCL local oscillator development will be reviewed and remaining challenges for implementation in a heterodyne receiver will be discussed. 63
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