High-Speed Imagers

High-speed imaging applications require short effective shutter times and are challenging from several points of view. First, very few photons may be collected since, by definition, the image exposure is very short. This fact means that the device sensitivity must be optimized, that is, the imager must have high fill factor, high quantum efficiency, and low read noise. These requirements are at odds with the industry-standard front-illuminated charge-coupled device (CCD) or active-pixel-sensor devices normally used for short-exposure applications. The shutter mechanism for such commercial devices normally requires the photocharge to be hidden behind opaque (metal) lines after the shutter is closed so image charge collection can be halted. These metal lines limit the pixel fill factor of the device and therefore its sensitivity.

Lincoln Laboratory has developed an electronic shutter that can be used with a high-quantum-efficiency and 100%-fill-factor CCD to enable devices with both high sensitivity and high-speed sample rate.

Sixteen-port high-speed, low noise CCDFigure 1. Sixteen-port high-speed, low-noise CCD

For real-time high-speed imaging applications, such as adaptive optics, we have successfully designed and built a number of small CCD devices, typically with multiple readout ports for low read noise, both with and without the electronic shutter. An example of such an imager—a 160 × 160-pixel, 16-port device—is shown in Figure 1. This device is capable of very fast effective frame rates with a read noise of less than 2 electrons. To optimize frame rate while preserving low noise, the device is equipped with a fast line-dumping drain, so that lines that contain no useful information may be disposed of quickly and noiselessly. 

Low-noise, high-speed cameraFigure 2. Low-noise, high-speed camera.

Figure 2 shows this device mounted in a low-noise high-speed camera system also developed at Lincoln Laboratory.

A second approach to high-speed imaging technology is to take and store a number of fast samples of a scene during an event. After the event is over, the stored samples are read out at low rate and with low noise. One recent implementation of this concept is a four-sample high-burst-rate CCD imager developed at Lincoln Laboratory. This device is capable of taking a series of four images in less than 500 ns apart. This 100-%-fill-factor imager makes use of the electronic shutter technology mentioned above to store four samples in a single pixel and to ensure good isolation between samples taken at different times.

Movie of exploding plate taken at Lawrence Livermore National Laboratory with Lincoln Laboratory 50-sample high-speed CCD imager. (Movie: Lawrence Livermore National Laboratory)

A 50-sample device is a second example of this type of imager. This device is capable of capturing 50 images at an 80 ns exposure time and 2 MHz frame rate and has excellent blooming protection so that one extremely bright frame will not contaminate previously stored frames.

For very high-speed applications, we have developed a very fast-shutter-speed subnanosecond (100 ps) imager composed of a silicon diode array bonded to a CMOS readout circuit.


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