Tapered quasi-planar germanium waveguides for mid-IR chemical and biochemical sensing

We have developed an efficient means of coupling light through a thin mid-IR-transmitting Ge waveguide. A commercial tool for grinding concave cylindrical lenses is used to grind and polish the waveguide, allowing it to be tapered from a thickness of 1 mm at the ends to a minimum of 20-100 μm at the center. This tapering improves the efficiency of the optical coupling both into the waveguide from an FTIR spectrometer, and out of the waveguide onto a small-area IR detector. The tapering makes it possible to dispense with using an IR microscope couple light through the waveguide. Instead, it is possible to obtain extremely efficient coupling with an detector directly coupled to an immersion lens. This optical arrangement makes such thin supported waveguides more useful as sensors, because they can be made quite long (e.g. 50 mm) and mounted horizontally. Furthermore, even with a 20-μm×1-mm cross section, sufficient throughput is obtained to give signal/noise ratios in excess of 1000 over most of the 1000-5000 cm^-1 range, with just 2 min of scanning at 8 cm^-1 resolution. The small (0.02 mm²) cross section of the waveguide nevertheless yields great sensitivity to small numbers of IR-absorbing molecules near its surface.