Interferometry Provides Basis for LWIR Remote Thermal Imaging

Prototypes of microelectromechanical systems (MEMS)-based, fixed-cavity Fabry-Pérot interferometers developed by researchers at the University of Western Australia could be used to enable lightweight, field-portable spectroscopic systems for optical remote imaging and sensing in the longwave infrared (LWIR) band.

The interferometers are based on germanium (Ge) barium fluoride (BaF₂) thin-film distributed Bragg reflectors. The researchers selected BaF₂ because it exhibits a low refractive index in the LWIR wavelength range and provides a high refractive index contrast, which can improve the performance of LWIR-based Fabry-Pérot interferometers.

Fabry-Pérot interferometers have an architecture that is compatible with thin-film, surface-micromachined MEMS. When combined with either single-point infrared detectors or focal plane imaging arrays, the interferometers can be used to develop lightweight, portable spectrometers.

The researchers used low-index BaF₂ thin films in combination with high-index Ge thin films to build the interferometers. According to the researchers, this was the first time that BaF₂ and Ge thin films were used for this purpose. The team built flat, free-standing distributed Bragg reflectors using a three-layer Ge/BaF₂/Ge structure of optical thin films. A peak-to-peak flatness was achieved for the free-standing structures within a range of 10 to 20 nm, across spatial dimensions of several hundred microns.

An optical microscopic image of a released α-series Fabry-Pérot interferometer. The crack propagation from the sharp corners of the notches is due to longtime exposure to the oxygen plasma. Courtesy of Gill et al.

Demonstrations showed that the fabricated Fabry-Pérot interferometers exhibited a linewidth of about 110 nm and a peak transmittance value of about 50%, meeting the requirements for use in tunable, MEMS-based LWIR spectroscopic sensing and imaging applications that require spectral discrimination with a narrow linewidth.

The researchers characterized the released fixed air-cavity filters and compared the measured optical performance with the modeled results and those of previous studies. After taking into consideration the impact of fabrication-induced imperfections on the distributed Bragg reflectors, they found that the measured optical characteristics of the Fabry-Pérot interferometers were in good agreement with the modeled optical response.

High-performance wavelength discrimination in the LWIR range (8 to 12 µm) is desirable for both civilian and military applications. Since radiation in the infrared spectral band is emitted by objects based on their temperature, on-chip, fully passive thermal imaging with spectral selectivity would allow remote target recognition without requiring any light source for illumination.

To date, the development of MEMS-based, narrowband Fabry-Pérot interferometers in the LWIR range has been limited by challenges in the fabrication process and a lack of suitable optomechanical materials in the LWIR range. Silicon-based low-index materials are not suitable for the LWIR range because of their high absorption losses. The researchers said the demonstrated on-chip microspectrometer technology could readily be made field-deployable in numerous applications that require mechanical robustness, including robotic vehicles and unmanned aerial vehicles. Remote LWIR imaging and spectroscopic sensing could have particular relevance for target identification and space situation awareness.

“These miniaturized on-chip, lightweight, and small-size devices are being seen as futuristic solutions toward simple and low-cost miniature spectroscopic remote systems operating in the very important thermal infrared emission band of the electromagnetic spectrum, where minimizing weight, size, and power requirements is of most critical importance,” professor Mariusz Martyniuk said.

The research was published in the Journal of Optical Microsystems (www.doi.org/10.1117/1.JOM.2.2.023502).