CASIMIR, the Caltech Airborne Submillimeter Interstellar Medium Investigations Receiver, is a far-infrared
and submillimeter heterodyne spectrometer, being developed for the Stratospheric Observatory For Infrared Astronomy,
SOFIA. CASIMIR will use newly developed superconducting-insulating-superconducting (SIS) mixers.
Combined with the 2.5 m mirror of SOFIA, these detectors will allow observations with high sensitivity to be
made in the frequency range from 500 GHz up to 1.4 THz. Initially, at least 5 frequency bands in this range
are planned, each with a 4-8 GHz IF passband. Up to 4 frequency bands will be available on each flight and
bands may be swapped readily between flights. The local oscillators for all bands are synthesized and tuner-less,
using solid state multipliers. CASIMIR also uses a novel, commercial, field-programmable gate array (FPGA)
based, fast Fourier transform spectrometer, with extremely high resolution, 22000 (268 kHz at 6 GHz), yielding
a system resolution > 10<sup>6</sup>. CASIMIR is extremely well suited to observe the warm, ≈ 100K, interstellar medium,
particularly hydrides and water lines, in both galactic and extragalactic sources. We present an overview of the
instrument, its capabilities and systems. We also describe recent progress in development of the local oscillators
and present our first astronomical observations obtained with the new type of spectrometer.
We summarize the development and the delivery of two SIS mixers for the 1.1-1.25 THz band of the heterodyne
spectrometer of Herschel Observatory (HSO). The quasi-optical SIS mixer has two Nb/AlN/NbTiN junctions with
the area of 0.25 um<sup>2</sup>. The Josephson critical current density in the junction is 30-50 kA/cm<sup>2</sup>. The tuning circuit
integrated with SIS junction has the base electrode of Nb and a gold wire layer.
With the new SIS mixers the test receiver maximum Y factor is 1.41. The minimum receiver uncorrected DSB
noise temperature is 450 K. The SIS receiver noise corrected for the loss in the optics is 350-450 K across the
1100-1250 GHz band. The receiver has a uniform sensitivity in the full IF range of 4-8 GHz. The sub-micron
sized SIS junction shape is optimized to ease the suppression of the Josephson current, and the receiver operation
is stable. The measured mixer beam pattern is symmetrical and, in a good agreement with the requirements, has
the f/d =4.25 at the central frequency of the operation band. The minimum DSB SIS receiver noise is close to
6 hv/k, the lowest value achieved thus far in the THz frequencies range.
We present a low noise SIS mixer developed for the 1.2 THz band of the heterodyne spectrometer of the Herschel Space Observatory. With the launch of the Herschel SO in 2007, this device will be among the first SIS mixers flown in space. This SIS mixer has a quasi-optical design, with a double slot planar antenna and an extended spherical lens made of pure Si. The SIS junctions are Nb/AlN/NbTiN with a critical current density of about 30 KA/cm2 and with the junction area of a quarter of a micron square. Our mixer circuit uses two SIS junctions biased in parallel. To improve the simultaneous suppression of the Josephson current in each of them, we use diamond-shaped junctions. A low loss Nb/Au micro-strip transmission line is used for the first time in the mixer circuit well above the gap frequency of Nb. The minimum uncorrected Double Sideband receiver noise is 550 K (Y=1.34). The minimum receiver noise corrected for the local oscillator beam splitter and for the cryostat window is 340 K, about 6 hv/k, the lowest value achieved thus far in the THz frequencies range.
We present some detail of the waveguide probe and SIS mixer chip designs for a low-noise 180-300 GHz double-sideband receiver with an instantaneous RF bandwidth of 24 GHz. The receiver's single SIS junction is excited by a broadband, fixed-tuned waveguide probe on a silicon substrate. The IF output is coupled to a 6-18 GHz MMIC low-noise preamplifier. Following further amplification, the output is processed by an array of 4 GHz, 128-channel analog autocorrelation spectrometers (WASP II). The single-sideband receiver noise temperature goal of 70 Kelvin will provide a prototype instrument capable of rapid line surveys and of relatively efficient carbon monoxide (CO) emission line searches of distant, dusty galaxies. The latter application's goal is to determine redshifts by measuring the frequencies of CO line emissions from the star-forming regions dominating the submillimeter brightness of these galaxies. Construction of the receiver has begun; lab testing should begin in the fall. Demonstration of the receiver on the Caltech Submillimeter Observatory (CSO) telescope should begin in spring 2003.