THz Local Oscillator Research Articles

Bandwidth of a 4.7 THz beam multiplexer based on Fourier grating

We present an analysis of the bandwidth of an asymmetric 8-beam Fourier grating as the beam multiplexer for a 4.7 THz local oscillator used in a heterodyne receiver. We take the grating designed for NASA GUSTO balloon observatory as an example to address the bandwidth question although it does not need to operate over a wide frequency range. By illuminating the grating at different frequencies from 4.445 to 5.045 THz, we simulated the changes of its performance in three aspects using COMSOL Multiphysics: diffraction efficiency, power uniformity, and the angular distribution of the output beams. These parameters can affect the coupling efficiency between the output beams of the grating and the beams of a mixer array. The bandwidth of the grating is found to be 230 GHz, corresponding to 4.9% of the operating frequency, which is sufficient for many applications.

4.7 THz asymmetric beam multiplexer for GUSTO.

A full demonstration of the Fourier phase grating used as 4.7 THz local oscillator (LO) multiplexer for Galactic/Extragalactic ULDB Spectroscopic Terahertz Observatory (GUSTO) is presented in this paper, including its design, modeling, tolerance analysis, and experimental characterizations of the angular and intensity distributions among 2 × 4 output beams and the power efficiency. A quantum cascade laser (QCL) is used to generate the input beam for evaluation of the grating performance in its all relevant aspects with an accuracy level never reported before, where good agreements with modeling results are found. This is the first asymmetric-profile grating fully modelled and characterized at a THz frequency, that further confirms the versatility of this technology for providing an intermediate optical element for feeding multiple array detectors with a single radiation source at such a scientifically interesting frequency regime.

Technology and Performance of THz Hot-Electron Bolometer Mixers

Hot-electron bolometer (HEB) mixers are a complex multi-layer thin film structure containing an ultra-thin superconducting film of NbN as a detecting element and a thick normal metal layer as an antenna structure. We have optimized the fabrication process starting with ultra-thin NbN films, Au films for antenna structures and their patterning using e-beam lithography and lift-off. The coupling between normal conducting antenna and NbN detector has been improved by introducing an intermediate NbN film to reduce proximity suppression of superconductivity in the detecting element. A critical temperature of about 9.5 K is reached for NbN films with a thickness between 5 nm and 6 nm. A twofold increase of the film thickness increases the critical temperature to 12 K. We have shown that a 20 nm thick buffer layer of NbN under a much thicker Au layer is sufficient to ensure a critical temperature of the bi-layer of 9 K. This value is close to the critical temperature of 5.5 nm thick HEB devices. The noise temperature of HEB mixer made using improved technology is about 800 K and was measured in a liquid cryogen free system with a quantum cascade laser as 2.5 THz local oscillator.

Hot-electron bolometer terahertz mixers for the Herschel Space Observatory

We report on low noise terahertz mixers (1.4-1.9 THz) developed for the heterodyne spectrometer onboard the Herschel Space Observatory. The mixers employ double slot antenna integrated superconducting hot-electron bolometers (HEBs) made of thin NbN films. The mixer performance was characterized in terms of detection sensitivity across the entire rf band by using a Fourier transform spectrometer (from 0.5 to 2.5 THz, with 30 GHz resolution) and also by measuring the mixer noise temperature at a limited number of discrete frequencies. The lowest mixer noise temperature recorded was 750 K [double sideband (DSB)] at 1.6 THz and 950 K DSB at 1.9 THz local oscillator (LO) frequencies. Averaged across the intermediate frequency band of 2.4-4.8 GHz, the mixer noise temperature was 1100 K DSB at 1.6 THz and 1450 K DSB at 1.9 THz LO frequencies. The HEB heterodyne receiver stability has been analyzed and compared to the HEB stability in the direct detection mode. The optimal local oscillator power was determined and found to be in a 200-500 nW range.

Compact 1.9 THz BWO local-oscillator for the GREAT heterodyne receiver

The 1.9THz local-oscillator (LO) of the GREAT heterodyne receiver is presented. The LO is based on a frequency tripled backward-wave oscillator source. The frequency stabilization system is described and an astigmatic imaging system, developed for improved beam coupling, is presented. Allan variances and temperature dependent power drifts are analyzed. The LO is designed as a stand-alone system and fits into GREAT’s local-oscillator compartments. It produces more than 1.5μW of stable output power to pump the hot electron bolometer mixers of the GREAT instrument.

A simple method to design astigmatic off-axis mirrors

In Gaussian Optics, beams with a circular cross section can easily be matched by using off-axis mirrors. Since the E-field distribution of a Gaussian beam mode is entirely independent in the two directions perpendicular to its axis of propagation, it is also possible to employ the formalism for asymmetric beams. Conventional off-axis mirrors, however, are not suitable to properly match these elliptical beams. In this case, astigmatic optics are required. This paper presents a method of designing astigmatic off-axis mirrors for Gaussian beam modes starting from a conventional off-axis mirror. A numerical simulation and an application for KOSMA’s 1.9 THz local oscillator for the GREAT instrument on SOFIA gives evidence supporting the theory.

Distributed sources on coplanar waveguides: Application to photomixers for THz local oscillators

AbstractAn electromagnetic model and a design of a photomixer‐based local oscillator for THz frequencies is presented. The theoretical investigation is on the nature of the fields excited in the coplanar waveguide (CPW) by impressed distributed sources. On this basis a CPW and antenna design is presented to optimize the radiated output power given the photoinduced rf currents. © 2002 Wiley Periodicals, Inc. Microwave Opt Technol Lett 33: 430–435, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.10342

Heterodyne spectrometers

An overview is given on current heterodyne receiver technology and on heterodyne receivers for astronomical and atmospheric science applications. The principle of heterodyne receivers is explained. Technological developments in the fields of THz mixer, local oscillators (LOs) and wideband spectrometers are described. A review on existing airborne and spaceborne heterodyne receivers is given. Finally, possible future THz heterodyne receivers for the ESA-cornerstone FIRST (far-infrared and submillimeter telescope) and the astroplane SOFIA (Stratospheric Observatory for Infrared Astronomy) are outlined.