None of the existing spectrometer technologies currently fulfil the demands for larger IF-bandwidth up to 10 GHz for future THz heterodyne systems without using hybrid-solutions.
At KOSMA (Koelner Observatorium fuer Sub-Millimeter Astronomie) we are investigating the idea of a laser side-band-spectrometer.
We use laser modulation method to generate optical sidebands near of the
laser frequency, which can then be frequency analyzed by standard optical
methods like a high finesse Fabry-Perot (FP) etalon.
First laboratory results of a prototype setup with 9 GHz Bandwidth will be presented.
The present status of AOS development at KOSMA is discussed. A study of a new generation of AOS using the new Bragg-cell material "Rutil" is on the way, which is supposed to lead to spectrometers in the range of 4 GHz total bandwidth at an resolution of 2-3 MHz. A second alternative for a 4 GHz bandwidth spectrometer has been developed as an engineering model for the HIFI instrument aboard the ESA cornerstone mission "Herschel". It consists of an array-AOS with 1 GHz bandwidth of each of the four AOS bands at a resolution of 1 MHz. A hybrid system for an input between 4 and 8 GHz is setup, and various laboratory tests have demonstrated that this system is well suited for large bandwidth applications like with HIFI. For eventual future demand of even larger bandwidth, details of a new optical method for Rf-analysis are discussed. It consists of a modulated laser with one or two Fabry-Perot etalons to analyze the frequency distribution of the resulting laser sidebands. A bandwidth of several 10 GHz at moderate resolution can be achieved.
The wideband acousto-optical spectrometer (WBS) for HIFI- FIRST is comprised of two array-AOS with 4 times 1 GHz bands each. There are some advantages to this design, the most important one is that relative frequency and amplitude variations between the 4 bands are rather unlikely. This is demonstrated by laboratory tests, which verify also that fairly slow beam-switching at 0.5 Hz may be a sufficient chop speed for HIFI. The performance of array-AOS has also been demonstrated during measurements at ground-based observatories. WBS consists of three independent units, one IF-, one optics-, and one electronics-unit. Some of the details of the WBS design are described, and the present performance estimates are given.
Large bandwidth acousto-optical spectrometers have now reached a very high level of maturity. They achieve very compatible results in comparison with other spectrometer types like filterbanks, autocorrelators, and chirp transform spectrometers. In addition, AOS are rather simple in design, have little complexity and can be designed for space applications very easily. A new generation of broad-band AOS, the array-AOS, consists of four parallel 1 GHz spectrometers built into one optical unit. Tests results in the laboratory as well at a radio-observatory are very promising. For example, the Allan variance minimum time has been found above 1000 seconds. In comparison it can be shown that the AOS spectra are less affected by instrumental noise or baseline distortions due to platforming effects as they are visible with most hybrid autocorrelators. For future applications of acousto-optics the development of cross-correlators seems to be feasible. First steps in this new direction are on the way.
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