Low light optical detection applications require very sensitive optical detectors with relatively low noise in order to produce discernible signal outputs. In recent years, a popular type of imaging camera is based on the use of a so-called “electron multiplying charge coupled device” (EMCCD). A charge coupled device (CCD) is an analog shift register that allows electrical charges accumulated at individual locations, or pixels, in the CCD to be shifted from one charge position to an adjacent position. Such a device may be used in an optical imaging application by converting light arriving at each pixel location to photoelectrons. The accumulated electron charges at each of the pixel locations may then be “read out” of the CCD by progressive shifting from one position to the next. For a two-dimensional detector, the shifting is done in each of two perpendicular directions (referred to as “horizontal” and “vertical”).
Conventional CCDs have limitations in low light situations because of the relatively high noise band of these devices. EMCCDs addressed this problem by providing an additional shift register through which the pixel charges are shifted after being shifted out of the conventional CCD structure. This additional register, referred to as a gain register, is driven by switching voltages that create a relatively high voltage drop between two register positions. This high energy transition results in impact ionization and the corresponding generation of additional electrons. Thus, the original pixel charges experience gain due to an electron multiplication effect. This gain may be adjusted by a user to a level exceeding the noise band of the detector, thus allowing the accurate detection of images in low light situations.
While EMCCDs are successful in boosting signal gain substantially for low-light optical detection, they still suffer from various noise problems. The effect of these additional noise sources limits the extreme low-light sensing capabilities of the device. Further reduction of these additional noise sources would improve performance and extend the capabilities of EMCCD based detectors.