Page 175 - Tunable Lasers Handbook

154 Charles Freed

with line-center-stabilized lasers just began in 1994 [87,88] even though they were
first identified in 1973 [94] and extensively studied from 1976 on [89,90].
Most of the sequence band and many of the hot band lasing transitions are
very close to the frequencies of those of the much higher gain regular band laser
lines. Thus if the laser cavity does not have sufficient frequency discrimination,
the regular band laser transitions will dominate as a result of gain competition.
As an initial approach to overcome this problem, one can use higher resolution
gratings than the 80 line/mm gratings used in the measurements of regular band
lasing transitions at MIT Lincoln Laboratory. Indeed, groove densities as high as
171 line/mm were employed in some of the recent work carried out at NIST
[80,8 1,831.
A more effective way of suppressing the oscillation of regular band lasing
transitions was achieved by the addition of an intracavity hot CO, absorption
cell to prevent the buildup of radiation at the regular band transition frequencies.
This technique was first used by Reid and Siemsen [89,90] in their comprehen-
sive study of sequence band laser transitions in CO,. An additional improvement
was introduced only very recently by Evenson et aj. by the addition of a ribbed
tube to inhibit the waveguide (or wall-bounce) modes of regular band lasing
transitions [80,81].

13. SPANNING THE FREQUENCY RANGE BETWEEN LINE-CENTER
STABILIZED CO, LASER TRANSITIONS

This section briefly outlines three methods that can provide continuously
tunable cw signal sources to either partially or completely span the frequency
ranges between adjacent line-center-stabilized isotopic CO, laser transitions.
The first of these methods uses small-bore (1- to 2.5-mm circular or rectan-
gular cross section) relatively high-pressure (100- to 400-Torr) CO, lasers that
could (theoretically at least) provide a tuning range of a few hundred megahertz
with relative ease and perhaps as much as 2 to 3 GHz with a great deal of diffi-
culty. Such lasers would have to be relatively long (for a small-bore tube) in
order to provide adequate gain to operate in other than the highest gain lasing
transitions. Thus they would have to operate in a waveguide mode and their cav-
ity design would be rather complex to provide single axial mode selectivity. An
excellent comprehensive review of multimirror (interferometric) laser cavities
and other optical resonator mode control methods was published by Smith in
1972 [131,18,19]. The development of waveguide mode CO, lasers has taken
great strides during the past decade or so. and nowadays probably the majority
of small commercially produced CO, lasers are waveguide mode lasers. How-
ever, at the present at least. I am not aware of a commercially available, high-
pressure, single-mode CO, laser that could provide more than a few hundred
megahertz tuning range in other than the most powerful laser transitions.