Ultra-thin Si photodetector for an integrated optical interferometer

A novel optical interferometer based on detecting the standing wave is described. The key device is a newly developed ultra-thin Si photodetector. The active layer of the ultra-thin photodetector is thinner than a half of wavelength of the incident light. Only a small part of the incident light is detected and the rest passes through the ultra-thin photodetector before being absorbed. Being inserted in the optical field, this ultra-thin photodetector can detect the thin intensity profile formed along the propagating direction of the laser beam. Taking advantage of this function, we have realized the vertical construction of a new interferometer detecting standing wave and the waveguide is not used, leading to the possibilities to achieve the integration of the interferometer by stacking planar components layer by layer and to array many interferometers together. The design, fabrication, and operation of this displacement sensor are discussed. The measured interference signal confirms the feasibility of the new sensor system. For one example of the arrayed interferometer, the dual ultra-thin Si photodetector is also fabricated with a phase shifter, and the displacement direction can be detected by comparing the phase of the two signals.