Page 173 - Tunable Lasers Handbook

152 Charles Freed

In the third-generation design careful choices of materials and techniques are
employed for enhancing the open-loop stability of the optical cavity. However, in
spite of the rigid structure, the laser design is entirely modular and can be rapidly
disassembled and reassembled; mirrors can be interchanged, and mirror holders can
be replaced by piezoelectric and grating-controlled tuners. The stainless steel end-
plates and the eight differential-alignment screws of the first- and second-generation
designs were replaced by much more stable black diabase endplates and a novel
internal mirror-alignment mechanism that is not accessible from the outside. The
third-generation lasers are not only more stable, but also much easier to align and
less costly to manufacture compared to the older designs.
In the simplest configuration the laser has two mirrors, one of which is piezo-
electrically tunable. Two-mirror lasers come in various lengths, depending on the
output power requirements, and are used primarily in CO, optical radars as local
and power oscillators. However, for applications in spectroscopy, grating-con-
trolled lasers are much more suitable than the simpler two-mirror lasers.
Figure 22 is a close-up photograph of a grating-controlled stable TEM,,
mode laser. Many variants of this basic design exist both at Lincoln Laboratory
and elsewhere. This particular unit was built for a relatively high-power applica-
tion such as optical pumping and frequency shifting. In the laser shown in Fig. 22
the first-order reflection of the grating was coupled through a partially reflecting
output mirror. For heterodyne spectroscopy, purely zero-order output coupling
from the grating is preferable because many more laser transitions can be obtained
with such lasers.
Three grating-controlled lasers with zero-order output coupling are con-
tained in Fig. 23, a photograph of the two-channel heterodyne measurement sys-
tem, the block diagram of which was previously shown in Fig. 13. The two
external frequency-stabilization cells, used for the individual line-center locking
of lasers in pairs, are also clearly visible in Fig. 23.
Some of the lasers have short intracavity absorption cells that can be used
either for frequency stabilization or for very stable high-repetition-rate passive Q-
switching. Such a laser was previously illustrated in Fig. 9, which shows a 50-cm
two-mirror laser with a short (3-cm) internal absorption cell. This laser was the

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FIGURE 22 Basic grating-controlled stable 'E% mode CO, laser. (Reprinted with permission
from Freed [75]. 0 1982 IEEE.)