Tiny MID- and long-wave infrared spectrometer module with a MEMS dual-band Fabry-Pérot filter

Cost effective and highly miniaturized spectrometer modules for infrared absorption spectroscopy in the mid wave infrared (MWIR) based on the hybrid integration of pyroelectric detectors and MEMS Fabry-Pérot filters (FPF) have been developed and are commercially available. The filters, based on bulk micromachining and Si-SiO2 Bragg reflectors exhibit a high finesse up to 60 over the entire aperture of Ø 1.9 mm. However, many substances have strong and more specifiable absorption bands in the long wave infrared (LWIR) from 8 to 12 ?m (fingerprint region). This paper reports on new Dual-Band FP-Filters, which can be operated simultaneously in the mid and long wave infrared by using the first and second interference order. For the LWIR new materials have to be chosen to create the reflectors. Using three material components instead of two gives additional degrees of freedom to optimize reflectance and phase dispersion and - most important - to achieve stress compensation within the dielectric layer stack. A conventional Bragg reflector consisting of high (H) and low (L) refracting quarter-wave stacks cannot cover the whole range of 3 - 12 µm. The new design approach with a |HLH| base period offers two stop bands within the desired regions. First samples of Dual-Band reflectors, optimized for 4 - 4.8 µm and 8 - 10.5 µm, have been fabricated and tested in tunable FP-Filters. The reflectors exhibit a very low curvature of <= 10 nm. A revised chip layout was made completely symmetrical, which significantly reduces reflector tilting and gives rise to a remarkable high defect finesse. Spectral measurements show a high peak transmittance of about 80 % in both ranges and low bandwidths of < 120 nm in the long wave band and < 60 nm in the short wave band. This corresponds to an effective finesse of about 56. Further optimized designs are under development to improve the finesse combined with a wider tuning range. The spectrometer module is based on a dual channel pyroelectric detector, one channel for each band. The spectral separation of the two bands is realized by means of a dichroic beamsplitter and two broad bandpass filters. Compared to conventional 50:50 beamsplitters this solution minimizes signal losses in the optical path. The complete spectrometer module fits in a TO-8 package. First prototypes were fabricated and tested