In this work transmission and reflection losses in silicon diffractive THz optical components fabricated by direct laser ablation are investigated. One of the possible sources of reflection/transmission losses in laser-fabricated optics is scattering due to the roughness of laser formed surface. Therefore, influence of laser processing parameters on the transmittance of laser processed silicon wafers was investigated in 0.1 –4.7 THz range. Transmittances of silicon samples ablated in ambient air and argon atmospheres were also compared. Using laser ablation technology MPFLs for 0.6 and 4.7 THz frequency radiation were fabricated and their performance was evaluated.
High-resolution terahertz (THz) heterodyne spectroscopy is an important technique in astronomy. So far frequencies above 2.5 THz could not be accessed by this technique because of the lack of a suitable local oscillator. A novel local oscillator based on a THz quantum-cascade laser allows for the observation of the fine-structure line of neutral atomic oxygen at 4.7448 THz. The local oscillator has been implemented in the GREAT (German REceiver for Astronomy at Terahertz frequencies) spectrometer on SOFIA, the Stratospheric Observatory for Infrared Astronomy. The design and the performance of the local oscillator will be presented.
Heterodyne spectroscopy of molecular rotational lines and atomic fine-structure lines is a powerful tool in astronomy and
planetary research. One example is the OI fine structure line at 4.7 THz. This is a main target for the observation with
GREAT, the German Receiver for Astronomy at Terahertz Frequencies, which will be operated on board of SOFIA. We
report on the development of a compact, easy-to-use source, which combines a quantum-cascade laser (QCL) with a compact,
low-input-power Stirling cooler. This work is part of the local-oscillator development for GREAT/SOFIA. The QCL, which is
based on a two-miniband design, has been developed for high output power and low electrical pump power. Efficient carrier
injection is achieved by resonant longitudinal optical phonon scattering. The amount of generated heat complies with the
cooling capacity of the Stirling cooler. The whole system weighs less than 15 kg including cooler, power supplies etc. The
output power is above 1 mW. With an appropriate optical beam shaping, the emission profile of the laser becomes a
fundamental Gaussian one. Sub-MHz frequency accuracy can be achieved by locking the emission of the QCL to a molecular
resonance.
The terahertz (THz) portion of the electromagnetic spectrum provides specific spectroscopic information for substance
identification. It has been shown that the spectral features of explosive materials might be used for detection and
identification at stand-off distances. We report on the development of a THz spectrometer for explosive detection and
identification. The system is based on THz quantum cascade lasers working at different frequencies. These are used for
illumination of the object under test. The reflected and backscattered radiation from the object under test is detected with
a sensitive heterodyne receiver. As a first step a single frequency, liquid-cryogen free heterodyne receiver operating at
2.5 THz has been developed. In order to realize maximum sensitivity a phonon-cooled NbN hot electron bolometric
mixer with a quantum cascade laser as local oscillator were chosen. The concept of the system and first results will be
presented.
Detection of concealed threats is a key issue in public security. In short range applications, passive imagers operating at
millimeter wavelengths fulfill this task. However, for larger distances, they will suffer from limited spatial resolution.
We will describe the design and performance of 0.8-THz imaging radar that is capable to detect concealed objects at a
distance of more than 20 meter. The radar highlights the target with the built-in cw transmitter and analyses the returned
signal making use of a heterodyne receiver with a single superconducting hot-electron bolometric mixer. With an
integration time of 0.3 sec, the receiver distinguishes a temperature difference of 2 K at the 20 m distance. Both the
transmitter and the receiver use the same modified Gregorian telescope consisting from two offset elliptic mirrors. The
primary mirror defines limits the lateral resolution of the radar to 2 cm at 20 m distance. At this distance, the field of
view of the radar has the diameter 0.5 m. It is sampled with a high-speed conical scanner that allows for a frame time
less than 5 sec. The transmitter delivers to the target power with a density less than ten microwatt per squared centimeter,
which is harmless for human beings. The radar implements a sensor fusion technique that greatly improves the ability to
identify concealed objects.
Suicide bombers and hidden bombs or explosives have become serious threats especially for mass transportation. Until now there exists no established system which can be used against these threats. Therefore new technologies especially for stand-off detection of threats are required. Terahertz (THz) rays offer an alternative inspection method, which can cope with these new challenges. Major advantages of THz radiation as compared to other spectral regions are the possibility to penetrate through clothes and that THz radiation is not harmful for human health. In this report the design and results of a THz stand-off detection system will be presented. The sensor is based on active illumination of the object and sensitive heterodyne detection of reflected and backscattered radiation. The system operates at about 0.8 THz. A THz laser is used for illumination and a superconducting hot-electron bolometric mixer for detection. The local oscillator required for heterodyne detection is a multiplied microwave source. The optical system is designed to allow for stand-off detection at 20 m with a spatial resolution less than 2 cm.
Quantum cascade lasers (QCLs) operating at 2.5 THz have been used for gas phase spectroscopy and as local oscillator
in a heterodyne receiver. One QCL has a Fabry-Perot resonator while the other has a distributed feedback resonator. The
linewidth and frequency tunability of both QCLs have been investigated by either mixing two modes of the QCL or by
mixing the emission from the QCL with the emission from a 2.5 THz gas laser. The frequency tunability as well as the
linewidth is sufficient for Doppler limited spectroscopy of methanol gas. The QCLs have been used successfully as local
oscillators in a heterodyne receiver. Noise temperature measurements with a hot electron bolometer and a QCL yielded
the same result as with a gas laser as local oscillator.
Heterodyne receivers for applications in astronomy and planetary research need quantum limited sensitivity. In instruments which are currently build for SOFIA and Herschel superconducting hot electron bolometers (HEB) will be used to achieve this goal at frequencies above 1.4 THz. The local oscillator and the mixer are the most critical components for a heterodyne receiver operating at 3-5 THz. The design and performance of an optically pumped THz gas laser optimized for this frequency band will be presented. In order to optimize the performance for this frequency hot electron bolometer mixers with different in-plane dimensions and logarithmic-spiral feed antennas have been investigated. Their noise temperatures and beam patterns were measured. Above 3 THz the best performance was achieved with a superconducting bridge of 2.0 x 0.2 μm2 incorporated in a logarithmic spiral antenna. The DSB noise temperatures were 2700 K, 4700 K and 6400 K at 3.1 THz, 4.3 THz and 5.2 THz, respectively. The results demonstrate that the NbN HEB is very well suited as a mixer for THz heterodyne receivers up to at least 5 THz.
We present a design concept for a new state-of-the-art balloon borne atmospheric monitor that will allow enhanced limb sounding of the Earth's atmosphere within the submillimeter and far-infrared wavelength spectral range: TELIS, TErahertz and submm LImb Sounder. The instrument is being developed by a consortium of major European institutes that includes the Space Research Organization of the Netherlands (SRON), the Rutherford Appleton Laboratory (RAL) will utilize state-of-the-art superconducting heterodyne technology and is designed to be a compact, lightweight instrument cpaable of providing broad spectral coverage, high spectral resolution and long flight duration (~24 hours duration during a single flight campaign). The combination of high sensitivity and extensive flight duration will allow evaluation of the diurnal variation of key atmospheric constitutenets sucyh as OH, HO2, ClO, BrO togehter will onger lived constituents such as O3, HCL and N2O. Furthermore, TELIS will share a common balloon platform to that of the MIPAS-B Fourier Transform Spectrometer, developed by the Institute of Meteorology and Climate research of the over an extended spectral range. The combination of the TELIS and MIPAS instruments will provide atmospheric scientists with a very powerful observational tool. TELIS will serve as a testbed for new cryogenic heterodyne detection techniques, and as such it will act as a prelude to future spaceborne instruments planned by the European Space Agency (ESA).
NbN hot- electron bolometer mixers have reached the level of 10hv/k in terms of the input noise temperature with the noise bandwidth of 4-6 GHz from subMM band up to 2.5 THz. In this paper we discuss the major characteristics of this kind of receiver, i.e. the gain and the noise bandwidth, the noise temperature in a wide RF band, bias regimes and optimisation of RF coupling to the quasioptical mixer. We present the status of the development of the mixer for Band 6 Low for Herschel Telescope.
Heterodyne receivers for applications in astronomy need quantum limited sensitivity. In instruments which are currently under development for SOFIA or Herschel superconducting hot electron bolometers (HEB) will be used to achieve this goal at frequencies above 1.4 THz. We present results of the development of a phonon-cooled NbN HEB mixer for GREAT, the German Receiver for Astronomy at Terahertz Frequencies, which will be flown aboard SOFIA. The mixer is a small superconducting bridge incorporated in a planar feed antenna and a hyperhemispherical lens. Mixers with logarithmic-spiral and double-slot feed antennas have been investigated with respect to their noise temperature, conversion loss, linearity and beam pattern. At 2.5 THz a double sideband noise temperature of 2200 K was achieved. The conversion loss was 17 dB. The response of the mixer was linear up to 400 K load temperature. The performance was verified by measuring an emission line of methanol at 2.5 THz. The measured linewidth is in good agreement with the linewidth deduced from pressure broadening measurements at millimeter wavelength. The results demonstrate that the NbN HEB is very well suited as a mixer for far-infrared heterodyne receivers.
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