Monolithically integrated photodetectors for optical signal monitoring in silicon waveguides

The development of monolithic silicon photonic systems has been the subject of intense research over the last decade. In addition to passive waveguiding structures suitable for DWDM applications, integration of electrical and optical functionality has yielded devices with the ability to dynamically attenuate, switch and modulate optical signals. However, for silicon to dominate as the substrate of choice for the fabrication of photonic circuits, the development of a full range of monolithically integrated functionality is required including detectors capable of signal monitoring at a wavelength around 1550nm. Photodetectors integrated with silicon-on-insulator rib waveguides are here demonstrated. Significant response at infrared wavelengths is shown to be mediated via deliberately introduced deep band-gap levels. This paper describes in detail the device fabrication and the performance of the waveguide photodetectors with regard to photoresponse, bandwidth, polarization sensitivity and thermal stability. Currently typical devices tap between 10-20% of an optical signal from an SOI waveguide and generate a photocurrent of several micro-amps. The most efficient device extracted 19% of the optical signal while exhibiting a responsivity of 3mA/W. We also describe results from the operation of an integrated photonic circuit consisting of a variable optical attenuator (VOA) and a photodetector. The detector monitors the optical signal as it is modulated using the VOA, however there exists a small, systematic offset in response as compared to measurements made with an external detector.