Fully Integrated Hollow Waveguide Laser Heterodyne Radiometer

STFC Rutherford Appleton Laboratory, supported by Hollowguide Ltd

Pathfinder Project

Continuing the work undertaken in an earlier project in high resolution spectroscopy, this project addresses the design, manufacture and demonstration of a fully integrated, miniaturised Quantum Cascade Laser Heterodyne Radiometer for Earth observation. It represents the combination of highly successful work on laser heterodyne radiometry with novel hollow waveguide optical integration techniques. The work will culminate in demonstrating the integration, ruggedisation and miniaturisation of a laboratory bench based system which originally had a footprint of 75 x 75 cm and will be reduced to something that can be held in the palm of the hand, e.g. ~10 x 10 cm. Commensurate reductions in mass and optical robustness will also be achieved. These will be coupled with significant improvements in heterodyne mixing efficiency leading to a fundamental advance in laser heterodyne radiometer instruments for Earth observation applications and beyond.

The scope of the project encompasses the design, realization, demonstration, and assessment of a fully integrated system comprising both active (quantum cascade laser, optical detector) and passive (polarizer and beam splitter/combiner) components. The components will be integrated into alignment features in a common ceramic substrate which also incorporates a hollow waveguide mixing circuit. In this manner the aim will be to design, build and assess the very first fully integrated, miniaturised and ruggedized, laser heterodyne radiometer. The proposed work represents a critical step towards the development of an in-orbit demonstrator instrument.

The fully integrated instrument will be tested in the laboratory using blackbody sources and absorption spectroscopy to establish noise equivalent spectral radiance figures and compare them with ideal ones.

The lead project organisation is STFC Rutherford Appleton Laboratory, supported by Hollowguide Ltd.

Previous Phase - High Resolution Spectroscopy

The Rutherford Appleton Laboratory is leading work to develop a new concept in very high resolution spectroscopy, by developing a Laser Heterodyne Radiometer (LHR). This is a passive radiometer which uses a low-power, highly-stable quantum cascade laser to modulate the incoming optical beam. The most recent work approved for funding is for a study to investigate use of hollow waveguide technology developed by QinetiQ Ltd for a space-qualified instrument. QinetiQ have developed an approach to optical and laser systems manufacture which is the optical equivalent of the electronic printed circuit board (PCB). In this approach, hollow waveguides formed in the surface of a dielectric substrate are used to guide light through a circuit of discrete optical components. The waveguides and the alignment slots for the components are created in the substrate using precision computer controlled milling techniques. The technology provides a fundamentally new way of manufacturing compact, low mass, low cost optical systems which have excellent performance and are robust to misalignment in harsh vibrational and thermal environments. For this seedcorn project, the focus is on demonstrating and studying the concept of hollow-waveguide-based heterodyne optical mixing. This demonstration is the first and crucial step toward the full optical integration of the radiometer. Three activities will be undertaken:

  • the development of a hollow waveguide board for heterodyne mixing
  • the assessment of mixing performance using quantum cascade lasers
  • the implementation and demonstration of a full laser heterodyne radiometer based on hollow waveguide mixing board.

The LHR represents a truly new technology with high potential for EO applications, and which has never been deployed in space. The proposed work will be a significant step forwards towards a fully integrated laser heterodyne radiometer.Contact point for further information: Dr Damien Weidmann, STFC/RAL