Page 445 - Tunable Lasers Handbook

8 Tunable External-Cavity Semiconductor Lasers 405

€or the unconverted TM-polarized light [96]. The tuning rate of the filter was
-0.05 nm/Y and its FWHM bandwidth was 1.2 nm [97]. The extended cavity did
not provide much feedback, as demonstrated by the fact that its threshold current
wais twice that of the solitary laser diode prior to AR coating. Nevertheless, with
a 1 x 10-3 AR coating on the feedback-coupling facet. the laser could oscillate on
a single extended-cavity mode in 0.4-nm-wide bands around each residual soh-
tary cavity Fabry-Perolt mode for a total wavelength range to -7 nm (limited by
the voltage that could be applied to the electrodes). A linewidth of -60 kHz and
an output of more than 1 mW from the uncoated facet were measured.
Tuning of an 850-nm ECL using a single-stage electro-optically tuned bire-
fringent filter was reported by Schremer and Tang [98]. The extended cavity
comprised a collimating objective. birefringent filter, and external mirror. The
external feedback was estimated to be -30%. The reflectance of the AR-coated
facet was not estimated. The filter consisted of a 38-mm-long piece of 45" y-cur
ADP, with transverse electrodes, oriented so that its fast and slow axes were at
45" to the TE polarization of the laser diode and an ll-mm-long birefringent
quartz plate oriented to cancel the natural birefringence of the ADP crystal. The
tuning rate of the filter was 3.3 nm/kV. Oscillation could be tuned to the residual
Fabry-Perot modes of the gain chip for a total range of 6.9 nm.
Andrew demonstrated tuning of an uncoated 780-nm laser diode in an
extended cavity containing a two-stage birefringent filter controlled with liquid
crystal cells 1991. With the laser diode operated below its solitary threshold cur-
rent, the wavelength could be tuned to each of 12 adjacent diode cavity modes
for a total tuning range of 2.7 nm, for a maximum applied liquid crystal voltage
of 1.7 V. The power consumption of the filter was estimated to be -50 pW. It was
suggested that with optimization this laser might be useful in applications where
voltage and power considerations are paramount.

8.6 Acousto-Optic Tuning
Acousto-optic filters are a very advantageous means for rapid, electronic
wavelength control of ECLs. The wavelength range of an AOTF is typically
much broader than the gain bandwidth of an individual diode laser, so there are
no wavelength range limitations imposed by the filter, in contrast to the case of
electro-optic birefringent tuning. Well-designed AOTFs have high transmittance.
so an acousto-optic external cavity has the potential for providing strong feed-
back. The switching time between random wavelengths is equal to the transit
time of the sound wave across the optical beam, which can be as little as -1 ps.
Multiple control frequencies can also be combined in the rf drive signal to gen-
erate a multiple-wavelength output.
The chief drawback of acousto-optic tuning is that the filter spectral width of
the best filters (-1 nm) is about an order of magnitude greater than the width that
can be readily obtained with a diffraction grating (-0.1 nm). This means that