VCSEL array senses focus using optical feedback
An array of vertical-cavity surface-emitting lasers (VCSELs) is at the heart of a confocal autofocus sensor demonstrated by researchers at the University of Colorado (Boulder, CO). The sensor has been incorporated into a high-speed image-acquisition system used in the detection of cervical cancer cells. For analysis, the specimen cells--which are placed in a single layer on a microscope slide and imaged onto a CCD--must be sharply in focus or all relevant detail is lost.
John Korah and Kristina Johnson fold the light from a slightly tilted 8 ¥ 8 VCSEL array into the optics of the image-acquisition system so that the image of the array coincides with the slide surface. When the image of a particular VCSEL falls precisely at the surface, light from the laser feeds back into itself, reducing the electrical resistance of the VCSEL in proportion to the optical feedback. Measuring the change in resistance provides a focus signal. Although each VCSEL responds over only a few microns of focal shift, the entire tilted array spans 25 µm of slide focus at submicron accuracy. The sensor works four times faster than an image-processing-based autofocus system.