Page 66 - Tunable Lasers Handbook

3 Tunable Excimer Lasers 47

a

i

b
OUTPUT

C

FIGURE 9 Grazing-incidence oscillator configurations incorporating (a) a Michelson interferom-
eter, (b) a multipass grating interferometer, and (c) a Fox-Smith interferometer (from Sze et al. [ 151).

a long-pulse excimer discharge laser. Incorporation of a Michelson interferometer
arm narrowed the linewidth further to %oth of a wave number. This configuration
can be altered to a high-Q Fox-Smith cavity [65] by turning the beamsplitter by
90" and making it a high reflector. In principle, this can give a large reduction in
linewidth but the mirror spacing must be kept very small because the resonance
condition is for the sum of the path lengths for the two arms.
Figure 9b tunes the grating angle so that the first order is normal to the grat-
ing. This configuration [18] allows the first order to be reflected back to the inci-
dent beam with its zero order reflected straight back on itself and therefore set-
ting up a cavity with additional resonance conditions. Armandillo et al. [I81
report obtaining single-longitudinal-mode lasing in XeF using this technique.
This was, however, done at very low gains. We had a great deal of trouble using
this technique in systems with reasonable gain. The difficulty arises from the
fact that when the first order of the grating is tuned normal to the grating, the
second order is in the Littrow condition. Thus, the second order often controls
the oscillator, making the first-order resonant technique useless.
Figure 9c attempts to improve the grazing incidence of Fig. 9(b) by reflect-
ing back the loss from zeroth order of the first-order diffracted signal. Again the