The name is absent

10

Another unique characteristic of THz-TDS is the broad bandwidth of the radia-
tion, broader than any other source (other than a thermal source) in the terahertz
range. Typically, the bandwidth can span more than an order of magnitude in wave-
length. Broadband coverage is valuable for spectroscopic measurements, which can
be used to identify the chemical composition of unknown materials in an image or to
locate materials according to their terahertz absorption signatures.

It is also worth noting some of the limitations of working with THz-TDS systems.
A primary difficulty is the power in the THz beam, which is quite low (typically less
than 1 μW average power) because of the inefficiency of non-linear optical mixing.
However, the dynamic range can be quite high in THz-TDS, even though the THz
power is low [26]. This is a result of the coherent detection which effectively rejects
many common sources of noise. This high dynamic range permits measurements even
in situations where only a tiny fraction of the generated radiation reaches the detector.
Nevertheless, it is important to note that existing commercially available focal plane
detectors (such as, for example, pyroelectric cameras) require much more power to
operate than the THz-TDS systems generally produce (e.g., a minimum power level
of 100 μW per illuminated pixel). As a result, the majority of time-domain imaging
systems rely on raster scanning of either the THz beam or the object, so that images
can be assembled serially using a single detector or perhaps a few operating in parallel.
This places a significant limitation on the image acquisition rate. Concerns about
power may also play a role in experiments which require long distance atmospheric
propagation or which seek to study material nonlinearities at terahertz frequencies.

Other difficulties are inherent in the nature of the time-domain system. For in-
stance, the time-domain scanning puts a practical upper limit on the spectral resolu-
tion which can be achieved. The spectral resolution δf is given by the inverse of the

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