8 Tunable External-Cavity Semiconductor Lasers 42 |
mirror . The average wavelength and the wavelength separation of the feed-
back are controlled, respectively, by tangential and sagittal translation of the mask.
This scheme was actually implemented using a 10-stripe 0.8-gm laser diode array,
but the array was phase locked and functionally it was essentially the same as a
high-power single-stripe element. The minimum and maximum wavelength sepa-
rations obtained were 3.52 and 11.29 nm, respectively. Because of the need to
provide equal gain to each wavelength, operation was restricted to wavelengths
symmetrically displaced with respect to the gain peak.
To avoid the problems associated with multiple wavelengths competing for
gain in a single active stripe, a multistripe laser diode array can be coupled to a
grating extended cavity to form a
multichannel grating cavity
extended cavity can be configured so that each of a set of discrete wavelengths
resonates between a different gain stripe and a common collection waveguide,
which can either be itself a separate gain stripe in the array  or an optical
fiber [ 127]. A monolithically integrated version, called a
multistripe array grat-
ing integrated cavity
(MAGIC) laser, has also been developed [128-130].
Another scheme in which a laser diode array is coupled to a grazing-incidence
grating external cavity to generate multiple output wavelengths with nearly con-
stant offsets and single-knob tuning has also been proposed [131 ].
15. WAVELENGTH STABILIZATION
Narrow intrinsic linewidths of <100 kHz have been demonstrated in ECLs at
all the major wavelengths (0.67, 0.78, 0.85, 1.33, and 1.55 lam). Despite these
narrow instantaneous linewidths, ECLs generally display much larger center-
frequency jitter or drift (collectively known as
noise) due to thermal,
mechanical, and acoustic disturbances. For many applications, it is important that
the residual FM be reduced by active stabilization.
There are two requirements for an ECL frequency stabilization system: (1) a
fine-tuning mechanism by which the laser's frequency can be servoed and (2) a
frequency reference is needed with respect to which frequency drift can be
sensed. Current modulation or cavity-length variation can be used to fine-tune
the frequency of an ECL. Laser diode temperature can also be used, but it has a
slower response time. The transmission peaks of Fabry-Perot etalons and fiber
resonators and the absorption lines of atomic or molecular vapors are commonly
used frequency references. Frequency locking is typically implemented by
applying a small frequency dither to the laser to generate the first derivative of
the transmission or absorption peak. By applying negative feedback, the laser
can then be locked to the zero of the derivative signal.