Single-channel Mach-Zehnder interferometric biochemical sensor based on two-lateral-mode spiral waveguide.

We propose and demonstrate a single-channel Mach-Zehnder interferometric (MZI) biochemical sensor consisting of two single-mode waveguides connected by a two-lateral-mode spiral sensing waveguide through two discontinuous junctions. The use of a two-lateral-mode waveguide offers the advantage of simple fabrication using single-step lithography and etching process. Meanwhile, the two-mode waveguide folded in a spiral layout can achieve high sensitivity of a long sensing waveguide while providing a compact sensing area compatible with commercial spotting machine and requiring small volume of sample. The sensor is demonstrated in silicon waveguides and the effect of the discontinuity offset distance on the interference visibility is studied. The bulk and surface sensitivity of a fabricated sensor with a 4582-μm-long two-mode spiral waveguide folded within a 185 μm diameter spot are characterized to be 461.6 π/RIU (refractive index unit) and 1.135 π/ng mm(-2), respectively. The biosensing capability of the sensor is verified by the measurement of biotin-streptavidin interaction of different concentrations.