Thermally Reflowed Die-Attached Linear Variable Optical Filter for Mid-Infrared Volatile Organic Compounds Detection

Accidental or deliberate discharge of industrial chemicals into the environment has to be identified rapidly. Given its ease of fabrication and possibly low-cost operation, linear variable optical filter (LVOF), have been explored as on-chip mid-infrared optical engines. In this work, highly precise and replicable LVOF taper was demonstrated by sandwiching photoresist pillars between two Si-SiO
₂
dielectric Bragg reflector deposited Si wafers to form a LVOF. By varying the pillar height, different wedge angles were achieved. It was shown that as wedge angle varied from 0.0786° to 0.0205°, full width half maximum measured at 3.70 μm improved from 320 nm to 220 nm while transmission intensity increased by 35%. LVOF operating range was revealed to be inversely proportional to the wedge angle. Furthermore, the temperature insensitive filter was demonstrated to qualify the presence of vapor ethanol. Finally, LVOF design principles with respect to integrated linear array detectors were discussed.