Page 497 - Tunable Lasers Handbook

9 Tunable Free-Electron Lasers 457

inside the vacuum chamber, the mirror change can take anywhere from a couple
of hours to a couple of days depending on the quality of vacuum desired. Many
user facilities are considering the use of in vacuo turntables so that the vacuum
does not have to be broken to change a mirror.

3.2 Unstable Resonators
If one has an optical cavity with all metal optics the obvious question
arises of how to couple the laser power out of the cavity. One common method
to accomplish this is to use an unstable resonator [40]. Because the gain
medium is unidirectional. an obvious design is the negative branch ring unsta-
ble resonator [41]. No FEL has been operated in an unstable mode in order to
outcouple the laser light. A couple of lasers have been operated in an unstable
configuration either by accident or with dielectric mirror output coupling
[42,43]. Due to the small gain volume of FELs and their relatively small satu-
ration gain, the only possible unstable resonator designs are negative branch
nearly confocal cavities. This allows the mode to be quite small in the gain
region while keeping the mode large on the output coupler. One can have a
cylindrically symmetric cavity or have one axis of the cavity stable and one
unstable as proposed bj7 Siegman [14]. The stable/unstabie cavity has the
advantage of a slower change in the optical mode size as the wavelength is
changed. Shih has studied how to configure the stable/unstable cavity in order
to optimize the output mode quality 1451.

3.3 Brewster Plate Output Coupling
Because the light from a FEL is usually strongly linearly polarized one can
install a Brewster plate in the cavity without adding to the cavity losses (with the
exception of scatter and absorption, which can be kept quite small). If one then
rotates the plate by a few degrees. one can increase the losses by a calculated
amount. The light can then be deflected out of the laser mode and through an
output window. There will be four reflections from the plate, two in each direc-
tion. Because the Brewster angle is insensitive to wavelength. the cavity is quite
broadband. The optical mode quality is quite good for each of the four reflec-
tions. One also has Ehhe interesting possibility of continuously variable output
coupling. There are, however. many disadvantages of this method.

1. If one uses a parallel plate. one gets two almost overlapping spots in the
output beam separated in time. This is bad for any applications that require indi-
vidual pulses or diffraction-limited spots. If one wedges the plate. one can elimi-
nate the extra spot with the penalty of decreased output coupling efficiency. One
can use a separate mirror to recover at least one of the two backwards reflec-
tions, but this also reduces the mode quality.