An analyte-detection system has an optical waveguide with first and second cladding layers adjacent a core; a light source coupled to provide light to the waveguide; a photodetector such as a metal-semiconductor-metal, vertical PIN, or horizontal PIN photodetectors, the photodetector having an absorber configured to detect light escaping from the waveguide through the first cladding layer; multiple, separate, photocurrent collectors, where each photocurrent collector collects current from a separate portion of the photodetector absorber; and at least one current-sensing amplifier for receiving photocurrent. The photodetector absorber is an undivided absorber region for multiple photocurrent collectors. Either separate amplifiers are provided for each of the multiple photocurrent collection lines, or multiplexing logic couples selected photocurrent collectors to amplifiers, while coupling unselected photocurrent collectors to a bias generator.