Introduction
Smart pixels, the integration of photodetector arrays and processing electronics on a single semiconductor chip, have been driven by its capability to perform parallel processing of large pixelated images and in real-time reduce a complex image into a manageable stream of signals that can be brought off-chip.1-2 In recent years, optical modulators and emitters have been integrated with photodetectors and on-chip electronics.3-5 The driving applications for this have been highly parallel optical interconnects and optical switching networks.
For sensor applications, photodetector arrays require the objects they sense to be illuminated by some means. The simplest rely on ambient light and suffer from the effects of spatial, spectral, and temporal variations in the illumination. Built-in illumination can either flood the object or be structured in the form of an array that matches the spacing of the photodetectors. Built-in illumination sources reduce the deleterious effects of ambient illumination, but typically require bulky beamsplitters and tight alignment tolerances on uniformity, rotation, translation, and focus, especially if the illumination is structured. The tight tolerances lead to high design and fabrication costs. High optical efficiency (greater than 25% efficiency) can require polarization optics that is expensive, bulky, and impose limits on the objects polarization properties. If the built-illumination is spatially flat, the illumination level must be set to keep the majority of the photodetectors within their dynamic range. Bright and dull areas of the object can become saturated or lost in the noise, respectively. Power consumption and dissipation constraints often limit the cumulative illumination. In a uniform illuminator, where the peak and average illumination are the same, the illumination per pixel is severely limited.
If an emitter array and detector array were integrated on a single chip, a compact optical sensor design could be constructed. Using an integrated emitter and detector sensor array requires an optical system that images the emitter array onto a remote object and then efficiently and accurately images the illuminated spots onto the detector array. A conventional imaging system would image the illuminated spots back onto the emitters. Various schemes employing bulk image shifters can be envision, but are not practicable. In this paper, we propose a compact and practical optical design for enabling an integrated array of emitters and detectors to illuminate and sense a remote object. The design allows each pixel to control its illumination through feedback from the detectors. The smart pixels with smart illumination (SPSI) concept has several potential applications in sensor array technology.