Page 451 - Tunable Lasers Handbook

8 Tunable External-Cavity Semiconductor Lasers 41 1

changing the cavity length. 4s will be shown later, this can be accomplished by
AOTFs without the use of moving parts in the cavity.

10.2 Embodiments of Phase-Continuous Tuning
Phase-continuous tuning over a 15-nm range was obtained without senlocon-
trol from a 1.26-pm Liatrow-grating-tuned extended-cavity laser using a rather
complex mechanical arrangement in which the grating was mounted on a bar
whose ends were pinned to a pair of orthogonal sliding stages [107]. It is also
possible to achieve continuous tuning with a single mechanical degree of free-
doni. Phase-continuous tuning over a range of 1000 GHz wa5 obtained by push-
ing with a PZT on a pivot arm that simultaneously rotates and translates the grat-
ing [lo81 (Fig. 36). Up to 82 nm (more than 10.000 GHz) of phase-continuous
tuning has been reported [ 1091. The grating is mounted on a pivot arm, which is
rotated about an axis lying near the intersection of the grating surface and the
plane of the laser diode output facet. The mode-hop-free tuning range is quite
sensitive to the precise location of the rotation axis (Fig. 37). There have been
several studies of the optimum location of this "magic" pivot point, taking into
account chromatic dispersion and phase shifts introduced by the gain medium
and various intracavity optical elements [ 110-1 131.
Phase-continuous tuning can also be realized with an acousto-optically con-
trolled ECL. Introducing a slight offset (e.g.. -1 Hz) between the drive frequen-
cies of the two intracavity filters will generate a net chirp of the oscillating mode
on each round-trip. If the passbands of the filters are simultaneously slewed so
as to match the induced chirp. phase-continuous tuning will result [114]. The
key to the success of this method is to obtain single-mode oscillation in the first
place, which is difficult because of the broad bandwidth of AOTFs.

Axis of

output'
coupling lens k=2cmi
FIGURE 3 '6 Phase-continuous tuned grating extended-cavity laser with PZT actuator. (Repro-
duced with permission from Schremer and Tang [lo@. 0 1990 IEEE.)