Page 46 - Tunable Lasers Handbook

2 Narrow-tinewidth Laser Oscillators 27

A topic of considerable interest in grating tuned cavities is long-range wave-
length tuning. One approach utilized in cavities incorporating gratings in a LiT-
trow configuration, intracavity beam expansion, and an intracavity etalon, 1s to
synchronize the motion of the grating and the etalon. Using this technique. sev-
eral authors hake demonstrated extended frequency scanning ranges up to sev-
eral tens of inverse centimeters [63-64].
An elegant approach to long-range wavelength scanning in single-longitudinal-
mode oscillators is the use of synchronous scanning methods. This involves
simultaneous adjustment of the cavity length and the feedback angle of the tun-
ing element. This is necessary to suppress mode hopping. The approach intro-
duced bj Liu and Littman [65] and Littman [ 171 for grazing-incidence cavities is
to rotate the tuning mirror about an axis defined uppro.~imnrely by the intersec-
tion of the surface planes (perpendicular to the propagation plane) of the output
coupler mirror, the tuning mirror. and the grating. The word appro.x-ii.iwteiy \vas
used because the gain region makes the physical length of the cavity slightly dif-
ferent from the optical length of the cavity. As discussed by Littman [ 17,661 the
optical length of the gain region alters slightly the optimum position of the pivot.
McNicholl and Metcalf [67] provide a scalar diffraction analysis for Littrow and
grazing-incidence cavities. For the case of the Littrow cavity, in the absence of
intracavity beam expansion, these authors have determined that the optimum
position for the rotational axis of the grating is defined by the intersection of the
surface plane of the optical origin of the cavity (which is close and parallel to the
surface plane of the output coupler) and the surface plane of the Littrow grating.
Although Littman 1171 reports scanning ranges of up to 15 cm-1, in a grazing-
incidence configuration, McNicholl and Metcalf [67] predict considerable exten-
sions in the tuning range.
For a cavity where the dispersion is provided by a chain of prisms in an
additive configuration. tuning is performed by rotating a mirror at the exit of the
prismatic assembly [68] since the exit angle at the with prism varies according

n(h) sin

where is related geometrically to the exit angle of the previous prism $2.fnlpl,,
and a,nis the apex angle of the nzth prism. The sign in this equation is reversed if
am<y1,, [2]. For an array of r identical prisms deployed in an additive configura-
tion and with identical angles of incidence. the cumulative angular spread at the
end prism can be significant since the overall single-pass dispersion is [2]