12
scanning
optical
delay line
femtosecond laser
Figure 2.3 : A schematic of a typical transmission-mode raster-scan time-domain
imaging system.
waveforms can be acquired, since this often determines the time for forming an image.
Typically, a motorized scanning stage is used to raster the object to be imaged through
the terahertz beam focus, so that image data is acquired one pixel at a time. In the
earliest example, THz waveforms were measured at a rate of 20 waveforms per second,
so acquiring a 100 × 100 pixel image took close to 10 min. This rate was determined
by the scan rate of the optical delay line, a galvanometric scanner with a corner
cube mirror. Modern THz imaging systems which use this raster-scanning method
employ more sophisticated methods for generating optical delay, and thus can run at
substantially higher speeds. In such systems, however, there is frequently a trade-off
between the scan range (as measured in picoseconds) and the scan rate (number of
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