The name is absent

11

duration of the temporal scan, which in most cases is limited by the length of a scan-
ning delay line as in a conventional Fourier transform spectrometer. With 15 cm of
travel, a typical mechanical delay line will provide up to 1 nanosecond of delay range,
corresponding to f = 1 GHz. This value is inadequate for high-resolution applica-
tions such as gas-phase spectroscopy [18]. Also, experiments which require radiation
in the higher frequency range may require a source other than THz-TDS. The high
dynamic range typically quoted for THz-TDS measurements is a frequency-dependent
quantity, which decreases exponentially with increasing frequency as shown in Fig-
ure 2.2 [27]. Thus, a THz-TDS system may compare very favorably to an electronic
cw system based on a Gunn diode operating below 1 THz, for example [25], but
would perform less well in comparison to a quantum cascade laser operating at 4.9
THz [3]. Finally, one practical disadvantage is the requirement for a femtosecond
optical source. Recent dramatic advances in femtosecond fiber laser technology are
beginning to overcome this problem, but the laser is still the most expensive and
sophisticated piece of equipment in the spectrometer.

2.1.2 Imaging with a time-domain spectrometer

The first TDS imaging system, reported in 1995, implemented an operational method
which has subsequently been replicated many times [16, 17]. A typical system di-
agram is shown in Figure 2.3. This shows a time-domain spectrometer based on
photoconductive antennas electro-optic generation and detection are also commonly
used [28,29]. In order to be suitable for image formation, a second set of focusing
optics are inserted into the THz beam to form an intermediate focal spot halfway
between the THz transmitter and THz detector.

For image acquisition, one of the key considerations is the rate at which THz

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