Linear readout of integrated interferometric biosensors using a periodic wavelength modulation

An all-optical phase modulation method for the linear readout of integrated interferometric biosensors is demonstrated, merging simple intensity detection with the advantages offered by spectral interrogation. The phase modulation is introduced in a simple and cost-effective way by tuning a few nanometers the emission wavelength of commercial laser diodes, taking advantage of their well-known drawback of power-wavelength dependence. The method is applied to the case of a bimodal waveguide (BiMW) interferometric biosensor, fabricated with standard silicon technology and operated at visible wavelengths, rendering a detection limit of 4x10-7 refractive index units for bulk sensing. The biosensing capabilities of the phase-linearized BiMW device are assessed through the quantitative immunoassay of C-reactive protein, a key protein in inflammatory processes. This method can be applied to any modal interferometer.