Page 460 - Tunable Lasers Handbook

420 Paul Zorabedian

AR coatina Two section LD

- I . f I I lens systei m
tirating
/ Phase-control Active
GRIN lens section section
FIGUR E 42 Extended-cavity laser with two-section pain medium for mode-hopping suppres-
sion. (Reproduced with permission from Notomi er al. [ 1211. Q 1990 IEEE.

completely nonoptical control loop based on the detection of terminal electrical
noise in the gain medium has also been reported [ 1221.

14. MULTIPLE-WAVELENGTH OPERATION

In some applications it is necessary to generate multiple wavelengths simul-
taneously from an ECL. This is a different situation from multimoding because
the multiple wavelengths will in general have a separation greater than the band-
width of the intracavity filter and will be either arbitrarily selected or chosen
from a predetermined set. In this section various types of multiple-wavelength
ECLs are reviewed.

14.1 Multiple-Wavelength A0 ECLs

The AOTF will transmit multiple wavelengths when multiple electrical drive
frequencies are simultaneously applied to the transducer. Dual- and triple-wave-
length output from an A0 ECL has been demonstrated [103]. Because the diode
laser gain is usually strongly homogeneously broadened, the drive levels at the
multiple frequencies must be carefully balanced. In addition, the total acoustic
power must be limited, which leads to reduced feedback at each wavelength. As
a result of these problems the number of simultaneous wavelengths that can be
generated is limited and the power balance between the wavelengths might not
be very stable.

14.2 Multiple-Wavelength Grating ECLs with Conventional
Gain Media
Simultaneous feedback at two wavelengths can be obtained with an extended-
cavity configured as a Michelson interferometer with a Littrow grating in each arm
[ 1231. Dual-wavelength operation has also been obtained in a grazing-incidence
cavity by placing a mask containing a v-shaped double slit in front of the end