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Realization of long wave infrared spectrum sensing by large area array narrow band F-P interferometer

Researchers at the University of Western Australia realized optical remote control imaging and sensing in the long wave infrared (LWIR) band by using a fixed cavity Fabry Perot (F-P) interferometer based on MEMS, and completed the lightweight and portable spectral system.

F-P interferometer is based on germanium (GE) barium fluoride (BaF2) thin film distributed Bragg reflector. Researchers chose BaF2 because it exhibits low refractive index and provides high refractive index contrast in the LWIR wavelength range, which is conducive to improving the performance of the device. The interferometer has an architecture compatible with thin film and surface micromachining MEMS. When combined with single point infrared detector or focal plane imaging array, it can be used to develop portable spectrometer.

According to the researchers, this is the first time to combine the low index BaF 2 film with the high index Ge film to build an interferometer. The team built a flat, independent distributed Bragg reflector using a three-layer ge/baf2/ge optical film structure. In the range of 10 to 20nm, across hundreds of microns, the independent structure achieves the flatness between peaks.

The experimental results show that the linewidth of the F-P interferometer is about 110Nm and the peak transmittance is about 50%, which meets the requirements of tunable, MEMS based LWIR spectral sensing and imaging applications requiring narrow linewidth.

The fixed cavity filter was characterized, and the measured optical properties were compared with the modeling results and the results of previous studies. After considering the influence of manufacturing defects on distributed Bragg reflectors, they found that the measured optical characteristics of F-P interferometer were very consistent with the simulated optical response.

Professor Mariusz martyniuk said:”these miniaturized on-chip, light-weight and small-size devices are regarded as solutions for simple and low-cost micro spectral remote systems in the future, and the requirements of lightweight, small-size and low-power for thermal infrared emission bands are crucial.”

The study was published in the Journal of optical microsystems under the title”large area narrowband Fabry – P é rot investigators for long-wavelength infrared spectral sensing”.