50-Sample Charge-Coupled Device

A 50-sample, 448 × 448-pixel charge-coupled-device (CCD) imager designed for operation up to 2,000,000 frames per second, with high sensitivity and wide dynamic range, has been developed. A primary application for the high-frame-rate imager is for high-energy explosive shock-front analysis.

The imager performance attributes include
  • Exposure time/frame: 80 ns to 1 ms  
  • Frame rate: 2 MHz to 1 KHz
  • Frames captured: 50
  • Trigger jitter: ≤40 ns
  • Array size: 448 × 448 pixels
  • Pixel size: 40 µm
  • Pixel pitch: 100 µm
  • Dynamic range: 10 bit; full well: 30,000 e-
  • Read noise: 25 e-
  • Anti-blooming: 32× saturation
  • Charge transfer efficiency: 0.9999 @ 2 MHz frame rate
  • Extinction ratio: 106 (see electronic shutter)
448 × 448 pixel array imager. 448 × 448 pixel array imager. 

A set of 50 linear storage registers—one located at every photosite—is used to store 50 consecutive images. The register is a revolving first-in, first-out analog memory, continuously updating and storing the latest frame until triggered to stop, thereby saving the most recent 50 good frames.

This device has several attributes that make it unique among multiple-consecutive-frame storage CCDs:
  • First, it is back-surface illuminated, giving it high sensitivity.
  • Second, it has excellent optical isolation between successive frames. The back surface can contain a metal mask with an aperture at each photosite. Use of this mask and also the use of an electronic shutter (which we describe elsewhere) together yield excellent optical isolation from previous samples should a very bright event happen at the target (up to a million-fold barrier to leakage).  This level of isolation on a front-illuminated device may be difficult to achieve because of possible light-piping laterally in dielectric layers.
  • Third, it has very fast response. Since the device is back-surface illuminated, no space needs to be reserved on the front of the device for light reception. Therefore, very dense metal coverage of the front surface, together with a metal plane on the back surface, allows for very fast signal propagation (and therefore fast shutter speeds) for this device.

A 128 × 128-pixel version of this device has been used in a high-speed camera to show the progress of an explosive event (see movie below). In the foreground above a metal plate are two fiber-optic cables, suspended on a bar, to measure the velocity of fragments. The 50-sample, 128 × 128–pixel camera is looking down on this scene. Below the plate and hidden from view is a layer of highly explosive material.

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

The interval between successive frames of the movie is 640 ns, equivalent to a 1.56 MHz frame rate, and the exposure time for each frame is 200 ns. The particle velocity of the fragments is about 2 km/s. Near the start of the movie, a shock wave is visible moving from the center of the object to the rim. The later part of the movie shows the metal plate expanding and beginning to fragment into pieces because of the stresses from the shock wave. The time from the beginning to the ending of the movie was about 32 microseconds.

 

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