Design and fabrication of silicon-based mid infrared multi-lenses for gas sensing applications

To improve the sensitivity of a non-dispersive infrared optical gas sensor, diffractive Fresnel lenses have been designed, fabricated with silicon microtechnologies, and tested. The target gases (for fruit storage applications) determine the wavelengths for the lens design: 10.6 μm, 9.7 μm, 3.5 μm, and 3.9 μm for ethylene, ammonia, ethanol, and the reference band, respectively. Four lenses are fabricated on the same silicon substrate in a combined multi-lens. In order to reduce the number of photolithographic steps, a new design based on sharing up to sixteen quantization steps by the four lenses is done. Due to the high reflection losses at the silicon–air surfaces, some multi-lenses have been coated with zinc sulphide antireflection layers. The difference between the measured and the target focal length is smaller than 5%. Alignment fixtures have been fabricated to assemble the Fresnel lenses chip on the detector lid in the correct orientation.