Grating coupled optical waveguide interferometer for label-free biosensing

A phase shifting optical waveguide interferometer is demonstrated for label-free evanescent wave biosensing in aqueous solutions. In the proposed configuration, the reference and measurement arms of the interferometer are combined inside an integrated optical Ta2O5 waveguide using an ion implanted grating. Biomolecules adsorbing on the waveguide surface shift the phase in the measurement arm, which is modulated at the same time by a periodically relaxing liquid crystal phase modulator. It is demonstrated that by analyzing the periodic intensity response at the end facet of the waveguide, the phase shifts in the measurement arm can be monitored in real-time with a precision of 10−4 rad. The high phase resolution allows the detection of surface adsorbed molecule densities below 1 pg/mm2 without using any labeling or on-chip referencing. The instrument performance is demonstrated by monitoring in situ protein adsorption and affinity binding of low molecular weight pure biotin to an immobilized avidin layer.