Gallium arsenide is a promising material for large photoconductor arrays to be operated at submm wavelengths,
where currently small stressed germanium arrays are used. The smaller binding energy of shallow donors in GaAs
compared to Ge results in response at longer wavelengths without having to apply uniaxial stress. Use of n-type
GaAs will greatly simplify the production of detector arrays and therefore allow much larger numbers of pixels.
We have grown n-doped GaAs epitaxial films and demonstrated high absorption coefficients at wavelengths
exceeding 300 μm. Combined with a high purity GaAs layer, a blocked impurity band (BIB) detector can
be formed in order to simultaneously achieve efficient absorption and low dark currents. Recent progress in
GaAs epitaxy technology allows production of such multilayer devices in wafer size. We are presenting the
characterization results of our preliminary GaAs BIB structures.
We are developing a GaAs photoconductive detector for far-infrared (FIR) astronomy. A detector based on GaAs in the blocked impurity band (BIB) con.guration is expected to extend the long wavelegth limit of currently available stressed Ge:Ga photoconductors up to about 330 microns. Without the need of uniaxial stress applied to the crystal, this would furthermore allow the fabrication of single chip arrays with a large number of pixels. We are reporting results of the characterization of preliminary GaAs BIB test structures. The experimental work is supported by numerical modeling that includes all contact and space charge effects.
GaAs photoconductive detectors offer an extended spectral response in the far-infrared (FIR) compared to presently available stressed Ge photoconductors. Furthermore, responsivity at wavelengths up to 330 microns can be reached without having to apply uniaxial stress close to the breaking limit on each pixel. This would greatly simplify the production of detector arrays and therefore allow much larger numbers of pixels. Such arrays are highly demanded for upcoming far-infrared astronomy missions with space and airborne telescopes. However, bulk GaAs photoconductors have only limited sensitivity, due to low absorption and high dark currents. Considerable improvement of the detector performance can be expected from the development of GaAs blocked impurity band (BIB) devices. Our recent crystal growth experiments show that the liquid phase epitaxial (LPE) technique is capable of producing the required purity for the blocking layer. We have also performed far-infrared absorption measurements of doped GaAs layers which demonstrate the spectral range extension to about 330 microns and the enlarged absorption coefficient for the more highly doped absorption layer. Experimental work is supported by numerical modeling of BIB devices done in our group.
Restoration by deconvolution of three-dimensional images that have been contaminated by noise and spatially invariant blur is computationally demanding. We describe efficient parallel implementations of iterative methods for image deconvolution on a distributed memory computing cluster.
With ESO and Onsala Space Observatory as partners, the Max-Planck-Institut for Radioastronomie (MPIfR) is building a submillimeter telescope of 12 m diameter (APEX), to be placed on the ALMA site (Chajnantor) in Chile. The telescope will be a modified copy of that ALMA prototype antenna, which has been designed by Vertex. First light is foreseen for 2003. As a result of the excellent atmospheric conditions of the site, APEX will offer unique opportunities for submm astronomy in the southern hemisphere. Many kinds of astronomical reseach projects benefit from large format bolometer arrays, especially the search for early galaxies and QSOs at very high redshifts. Designed for this purpose, LABOCA, the large bolometer camera, will operate at a wavelength of 870 μm and is planned to be operational soon after first light of APEX.
Total variation-penalized Tikhonov regularization is a popular method for the restoration of images that have been degraded by noise and blur. The method is particularly effective, when the desired noise- and blur-free image has edges between smooth surfaces. The method, however, is computationally expensive. We describe a hybrid regularization method that combines a few steps of the GMRES iterative method with total variation-penalized Tikhonov regularization on a space of small dimension. This hybrid method requires much less computational work than available methods for total variation-penalized Tikhonov regularization and can
produce restorations of similar quality.
The BiCG and QMR methods are well-known Krylov subspace iterative methods for the solution of linear systems of equations with a large nonsymmetric, nonsingular matrix. However, little is known of the performance of these methods when they are applied to the computation of approximate solutions of linear systems of equations with a matrix of ill-determined rank. Such linear systems are known as linear discrete ill-posed problems. We describe an application of the BiCG and QMR methods to the solution of linear discrete ill-posed problems that arise in image restoration, and compare these methods to the conjugate gradient method applied to the associated normal equations and to total variation-penalized Tikhonov regularization.
The GMRES method is a popular iterative method for the solution of linear systems of equations with a large nonsymmetric nonsingular matrix. However, little is known about the performance of the GMRES method when the matrix of the linear system is of ill-determined rank, i.e., when the matrix has many singular values of different orders of magnitude close to the origin. Linear systems with such matrices arise, for instance, in image restoration, when the image to be restored is contaminated by noise and blur. We describe how the GMRES method can be applied to the restoration of such images. The GMRES method is compared to the conjugate gradient method applied to the normal equations associated with the given linear system of equations. The numerical examples show the GMRES method to require less computational work and to give restored images of higher quality than the conjugate gradient method.
A variant of the MINRES method, often referred to as the MR-II method, has in the last few years become a popular iterative scheme for computing approximate solutions of large linear discrete ill- posed problems with a symmetric matrix. It is important to terminate the iterations sufficiently early in order to avoid severe amplification of measurement and round-off errors. We present a new L-curve for determining when to terminate the iterations with the MINRES and MR-II method.
In this paper we compare a new regularizing scheme based on the exponential filter function with two classical regularizing methods: Tikhonov regularization and a variant of truncated singular value regularization. The filter functions for the former methods are smooth, but for the latter discontinuous. These regularization methods are applied to the restoration of images degraded by blur and noise. The norm of the noise is assumed to be known, and this allows application of the Morozov discrepancy principle to determine the amount of regularization. We compare the restored images produced by the three regularization methods with optimal values of the regularization parameter. This comparison sheds light on how these different approaches are related.
Continuum radiometers based on bolometers have a long tradition at the Max-Planck-Institut fur Radioastronomie (MPIfR) in Bonn, Germany. Arrays of bolometers have been under development since the early 90s. A small 7-element system, operating at 300 mK, saw first light in 1992 at the IRAM 30 m- telescope and has been used successfully by numerous observers at that facility since then. While this array had a conventional 'composite' design, it was obvious that larger arrays, especially for higher frequencies, could take advantage of microfabrication technology. The recent MPIfR bolometer arrays employ a hybrid approach. They combine a single-mode horn array with a planar bolometer array on a single crystal Silicon wafer with Silicon-Nitride membranes. With efficient absorbing structures, the bolometers couple to the single mode of the radiation field collected by the horns, without needing integrating cavities. Readout is provided by NTD-Germanium thermistors that are attached to the absorbers. This paper covers the history of this development, the general aspects of the bolometer arrays, including the coupling to the telescope, and the status of work in progress.
We describe new iterative methods for the solution of large ill-conditioned that arise from the discretization of ill-posed problems. In these methods a filter function, which determines the regularization of the problem, is chosen and expanded in terms of orthogonal polynomials. Each iteration yields one new term in this expansion. A variety of iterative methods, which differ in the choice of filter function and orthogonal polynomials, can be derived in this manner.