Page 503 - Tunable Lasers Handbook

9 Tunable Free-Electron Lasers 46

length. Recently they have succeeded in operating in a mode with very short
pulses of only 500 fs [521.
There are four user rooms serviced by the purged optical beamlines. The
users are supplied with basic optical equipment such as optical benches, detec-
tors, and monochromators, and. because LUKE is a synchrotron light user facil-
ity, there are many user services such as machine shops and electronics shops.
As with most facilities there is always interest in extending the wavelength
range and improving the performance in other ways. There is only one laser on
the accelerator and they have no room to add another. They therefore must
modify the laser they have to improve the performance. One simple upgrade is
to increase the repetition rate of the electron beam in order to increase the a\Ter-
age power. They expect to be able to operate with up to 10 W of output power
in the mid-infrared region. The second upgrade, already complete, is to replace
the wiggler chamber bore to reduce diffraction and allow operation at longer
wavelengths,

5.2 Duke University
Duke University runs a small user facility using an infrared FEL similar 10
the one used at Vanderbilt University (see Sec, 5.7). The so-called Mark I14
IRFEL [26] operates in the range of 2 to 9 pm on the fundamental and has opcr-
ated at the third harmonic at wavelengths between 1.3 and 1.7 pm. As in the
CLIO device, the long-wavelength end is determined by the diffraction in the
wiggler bore, and power drops rapidly at wavelengths longer than 8 pm. The
maximum power is a.*railable between 3 and 5 pm. The peak power is approxi-
mately 1 to 3 MW delivered to the optical bench. The a1:erage power during the
macropulse varies from 2 to 30 kW depending on the wavelength and the output
coupler used. The wavelength can be tuned over a factor of 1.7 by gap tuning the
wiggler. Changing the energy can take several hours so user time is scheduled to
make use of similar wavelengths on any given day.
The output coupling is via a Brewster plate output coupler. They have
recently tried out hole coupling with mixed results [53]. The time structure has
micropulses arriving at 2.857 GHz and 1- to 3-ys macropulses occurring at a
repetition rate of up to 30 Hz. The spectral bandwidth is typically 0.5% FVVHM
in the 2- to 4-pm wavelength range. increasing to approximately 1 % at the long-
wavelength end of the range.
The user area is quite small with one user table available, and the user time
available for outside use is less than at other infrared facilities in the United
States but the lab has an active machine physics program. which results in a
machine with great flexibility. Researchers at Duke have phase locked the
microbunches in the laser, thus increasing the mode spacing in the frequency
domain to 2.857 GHz [53]. One of the modes was then filtered out and high-
resolution spectroscopy was carried out with it. The lab is also working on plse