13
waveforms per second). The highest scan rates (e.g. thousands of waveforms per
second) can be achieved using a piezo-electric device, but with a limited (tens of
picosecond) scan range. At these higher rates, the image acquisition time may no
longer be limited by the time to measure a THz waveform, but instead by the rate at
which the object (or the THz beam) can be raster scanned. However, the more limited
scan range does limit the information contained in each waveform. As noted above,
a shorter scan range limits the spectral resolution of the measurement. A shorter
scan range also limits the range of depths to which the THz pulse can penetrate
through a material and still be detected, since a larger optical depth could delay the
pulse outside of the temporal window of the measurement. This latter effect will be
illustrated more clearly in the discussion of time-of-flight imaging (see below). Other
types of mechanical scanning devices (e.g. spinning mirror devices) can generate
several hundreds of picoseconds of delay range with a scan rate in the vicinity of 100
Hz [30]. In any event, the motion of the scanning delay line must be synchronized to
the raster scan of the object, so that it is possible to determine the location of the
object at the moment each waveform is acquired.
Recently, several groups have demonstrated that it is possible to dispense with
the mechanical scanning delay line entirely, and instead make use of asynchronous
optical sampling [31,32]. In this approach, two femtosecond lasers are used instead
of one. The repetition rate of one laser is locked to that of the other, with a fixed
frequency offset. One laser is used to generate the THz pulse, and the second to gate
the detector. In this way, the delay of the THz pulse sweeps automatically, relative
to the gating of the detector, at a rate which is determined by the frequency offset
between the two lasers. This eliminates the moving parts, at the expense of a second
laser and feedback electronics.