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On-chip Imaging Spectrometer Closer Than You Think - WA Node

June 2012

Over the last 15 years the Micro-electronics Research Group (MRG) at the ANFF WA Node has established itself as a leader in MEMS based infrared sensors. The technologies under development are of significant interest to the industry sector with MRG recently securing a $1.5M research grant in this area with the majority of the funding originating with a Fortune 500 defense company in the USA. Another, similarly co-funded, grant of $1.5M has received excellent reviews, and is awaiting final decision by the funding body. This success is made possible by access to the world-class fabrication facilities at the WA node of the ANFF, as well as access to facilities at other ANFF nodes.

A large part of that work has been focused on the MEMS microspectrometer device depicted in Figures 1a and 1b below. The MRG microspectrometer consists of a tunable MEMS Fabry-Perot optical filter fabricated optically ahead of an infrared detector. The filter consists of a fixed dielectric bottom mirror, and a moveable dielectric top mirror. Actuation of the top mirror is achieved electrostatically by applying a voltage or charge to the top mirror. Since mirror separation is directly related to the transmitted wavelength of the Fabry Perot filter, actuating the top mirror scans ("tunes") the micro-spectrometer wavelength. Figure 2 shows that the short-wave infrared (SWIR) microspectrometer devices possess a 50 nm spectral line-width at a centre wavelength of 2 μm, and tune from 1.6 μm to 2.5 μm with a low drive voltage of 23V. Figure 2 also demonstrates that the spectral tuning range can be shifted anywhere in the SWIR and mid-wave infrared (MWIR) range. One of the aims of recent industry co-funded projects is the extension of the operational range of this on-chip spectro-meter well into the visible spectrum.

This technology is now being developed for imaging in remote sensing applications, with the implementation of the tunable optical filter at the focal plane array (FPA) level. Remote sensing plays an increasingly important role in today's world for many applications including, crop monitoring, oil/mineral exploration, disaster monitoring, and surveillance for national security. Conventional imaging in remote sensing employs either grey-scale or three-colour cameras, with occasional use of monochrome infrared imaging. True spectral imaging, in the infrared and visible spectral regions would provide a host of extra information, tremendously assisting the target identification process in these applications. While some true-colour thermal imaging solutions do exist, they rely on a spectrometer external to the FPA, resulting in very bulky, fragile, and power hungry systems, which are ill suited to remote sensing purposes. Another intractable feature of spectral imaging is the sheer volume of information that is collected, much of which is not relevant to the application. This large volume of information renders it impossible to undertake real-time processing or decision-making.

The thrust of this research is to realise, for the first time, an FPA-independent high-performance infrared spectrometer technology, allowing mating to any type of imaging FPA: effectively, an on-chip imaging spectrometer. The proposed imaging spectrometer will be capable of adaptively imaging at any given wavelength, without the need to scan through the entire spectrum.

The key advantages of such a paradigm shift include:

It is anticipated that this enabling technology will dramatically lower the entry barriers to spectral imaging applications for numerous diverse users, both governmental and commercial. In turn this can be expected to fuel more innovation in sensor architectures, hyperspectral algorithms and information products.

Story courtesy of Prof. Dilusha Silva, Engineering Manager MRG and Res. Prof. Mariusz Martinyuk, Facility Manager ANFF WA Node.