Timothy J. Finn

Comparison of near-field measurements and electromagnetic simulations of the focal plane unit of the Heterodyne Instrument for the Far-Infrared

The Heterodyne Instrument for the Far-Infrared (HIFI) of the ESA cornerstone mission Herschel is required to operate at wavelengths between 157 and 625 μm. Because of the long-wavelength character, and the complexity and modularity of the optical design, there is a clear need for accurate electromagnetic simulations supported by experimental verification. The need for a compact layout in order to reduce mass and volume as far as possible has important optical consequences. Several mirrors are illuminated in the propagating near-field rather than in the far-field. Consequently the classical geometrical design and analysis approach is inadequate. The long-wavelength character of the system can not be ignored and the associated diffraction effects inevitably become important. In this paper we describe the results of electromagnetic simulations of the optical system for band 1 of HIFI at a wavelength of 625 μm. In order to verify the results of the front-to-end coherent propagation of the detector beams, near-field facilities capable of measuring both amplitude and phase of the electromagnetic field have been developed. A unique feature of these facilities is that the absolute coordinates of the measured field components are known within a fraction of a wavelength. Therefore a true comparison with theoretical predictions can be made. We compare measurement data taken at 625 μm with simulations and discuss to what extent measured and simulated results may be expected to agree. We conclude by presenting the consequences of our observations in terms of system performance.

Modeling of millimetre-wave and terahertz imaging systems

In order to improve the design and analyse the performance of efficient terahertz optical systems, novel quasi-optical components along with dedicated software tools are required. At sub-millimetre wavelengths, diffraction dominates the propagation of radiation within quasi-optical systems and conventional geometrical optics techniques are not adequate to accurately guide the beams or assess optical efficiency. In fact, in general Optical design in the terahertz waveband suffers from a lack of dedicated commercial software packages for modelling the range of electromagnetic propagation regimes that are important in such systems. In this paper we describe the physical basis for efficient CAD software tools we are developing to specifically model long wavelength systems. The goal is the creation of a user-friendly package for optical engineers allowing potential systems to be quickly simulated as well as also providing an analytical tool for verification of existing optical systems. The basic approach to modelling such optical trains is the application of modal analysis e.g. [1][2], which we have extended to include scattering at common off-axis conic reflectors. Other analytical techniques are also ncluded within the CAD software framework such as plane wave decomposition and full physical optics. We also present preliminary analytical methods for characterising standing waves that can occur in terahertz systems and report on novel binary optical components for this wavelength range. Much of this development work has been applied to space instrumentation but is relevant for all Terahertz Imaging systems.

Gaussian-beam mode analysis of reflection and transmission in multilayer dielectrics

The analysis of reflections from thin films or dielectric materials can be approached by a matrix method that treats any thin-layer device as a cascade of sequential, zero-thickness reflecting thin-layer surfaces [J. Opt. Soc. Am. A2, 1363 (1985)]. Our paper presents an alternative method for predicting the reflection/transmission characteristics of such dielectric films in a Fabry-Perot interferometer configuration based on a Gaussian-beam modal analysis within a scattering-matrix framework [in Proceedings of IEE 7th International Conference on Antennas and Propagation (IEE, 1991), Issue 15, p. 201.] We present and validate a scalar Gaussian-beam modal scattering-matrix approach using long-wavelength examples, where diffraction effects are important to model total transmission and reflection characteristics that also include a waveguide modal description of a corrugated horn. For optical beams the same technique is equally applicable, but diffraction is less severe within this framework. This approach is flexible and has many applications within laser optics and in far-infrared or submillimeter-instrumentation optical analysis, where it is possible to incorporate reflections in both waveguide and free space within the description of a whole system. To conclude and verify the accuracy of the technique, experimental measurements taken at 94 GHz are compared with theoretical predictions for a dielectric cavity of polyethylene sheets between corrugated source and detector antennas.

Quasi-optical verification of the focal plane optics of the heterodyne instrument for the far-infrared (HIFI)

HIFI is one of the three instruments for the Herschel Space Observatory, an ESA cornerstone mission. HIFI is a high resolution spectrometer operating at wavelengths between 157 and 625 μm. The need for a compact layout reducing the volume and mass as much as possible has important consequences for the optical design. Many mirrors are located in the near-field of the propagating beam. Especially in the long wavelength limit diffraction effects might therefore introduce significant amplitude and phase distortions. A classical geometrical optical approach is consequently inadequate. In this paper we present a rigorous quasi-optical analysis of the entire optical system including the signal path, local oscillator path and onboard calibration source optical layout. In order to verify the results of the front-to-end coherent propagation of the detector beams, near-field measurement facilities capable of measuring both amplitude and phase have beam developed. A remarkable feature of these facilities is that the absolute coordinates of the measured field components are known to within fractions of a wavelength. Both measured and simulated fields can therefore compared directly since they are referenced to one single absolute position. We present a comparison of experimental data with software predictions obtained from the following packages: GRASP (Physical Optics Analysis) and GLAD (Plane Wave Decomposition). We also present preliminary results for a method to correct for phase aberrations and optimize the mirror surfaces without changing the predesigned mechanical layout of the optical system.

Gaussian beam mode analysis of standing waves in submillimeter telescope and receiver systems

In this paper, we report on extending a theoretical framework based on Gaussian Beam Mode Analysis for modelling standing waves in receiver systems coupled to submillimetre wave telescopes. This analytical technique includes a full electromagnetic description of corrugated detector horns, used as a standard feed horn in the THz frequency range. In previous papers we reported on the underlining theory and described some important examples including reflections between a feed horn and telescope secondary mirror and also reflections between a horn and a plano-convex lens. As the theory uses a full multi-moded scattering matrix description within the horn, which can then be transformed to equivalent free space modes, mulitple reflections between the detector, located at the back of the horn, and any arbitrary surface in the optical path can be accurately analysed. We present an experimental validation of the model, comparing predicted standing wave patterns occuring between two corrugated horns to laboratory measurements, owrking in a frequency range around 0.1THz.