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Streak Cameras

Definition: devices for measuring the power of an optical pulse versus time

German: Schmierbild-Kamera

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Streak cameras are devices for measuring the temporal evolution of the optical power or intensity of optical pulses. The basic principle of operation is that the incident light generates a spot which is rapidly deflected during some short time interval. The spatial distribution resulting from the moving spot (called the streak image) then reflects the temporal evolution of the optical power, since each temporal position is mapped to a spatial position. The streak image is a linear structure with varying light intensity, unlike an oscilloscope image, which is two-dimensional. It is often used for measuring the pulse duration and/or the peak power.

The simplest kind of streak camera is based on some means for deflecting an incident light beam – for example, a rotating mirror. Such devices are fairly limited in terms of temporal resolution, because the speed of moving parts is limited.

The most common type of streak camera is an optoelectronic device, where the incident light hits a photocathode within a vacuum tube. Electrons emitted from that cathode are accelerated by a high-voltage, forming a pulsed electron beam, where the beam intensity is approximately linearly dependent on the optical intensity. That electron beam is deflected by an additional pair of electrodes, to which a rapidly varying electric voltage is applied. The electron beam can finally hit a phosphor screen for generating a visible streak image. Alternatively, the beam can hit a charge-coupled device (CCD) in order to obtain an electronic signal, which may be processed by a computer as in an digital oscilloscope; such a device may display a curve on a computer screen just as a digital oscilloscope does. The fastest optoelectronic streak cameras reach temporal resolutions of the order of 100 femtoseconds – however, only for rather weak input signals. For stronger signals, as needed for a higher signal-to-noise ratio, the temporal resolution is decreased due to the mutual repulsion of the electrons.

As in an oscilloscope, the deflection mechanism in a streak camera needs to be precisely triggered at a suitable time, e.g. just before a short optical pulse hits the photocathode. Both single-shot operation and repetitive operation can be realized; the latter is often desired for use with a mode-locked laser, for example.

For pulses with shorter durations, other devices are used for pulse characterization – for example, autocorrelators and setups for frequency-resolved optical gating (FROG) or spectral interferometry (SPIDER). Even for longer optical pulses, such methods have become more common, since they are easier to make (particularly autocorrelators) and partially allow for complete pulse characterization including the spectral phase (FROG and SPIDER), which cannot be measured with a streak camera. However, streak cameras have the advantage that their photocathodes can work in a very range of optical wavelengths; some streak cameras are even used for ultraviolet light or for X-ray pulses.

See also: pulse characterization, pulse duration

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