Page 380 - Fiber Bragg Gratings

8.1 Fiber grating semiconductor lasers: The FGSL 357

potentially increasing the wavelength stability of the laser in a hybrid
configuration. Second, fiber Bragg gratings can be routinely fabricated at
precise wavelengths and are therefore suited to mass production; wave-
length control makes them useful for wavelength division multiplexing
(WDM) applications that require extremely tight tolerances [7], Third,
the longer laser formed by the incorporation of the external cavity reduces
the linewidth in direct relation to the length [8]. Fourth, semiconductor
lasers suffer from chirp as a result of cavity length change during direct
modulation. The longer FGSL dilutes the carrier induced cavity length
change in the semiconductor during modulation, by confining it to the
much smaller fraction of the laser length.
It is usual to quantify the operational sensitivity of a semiconductor
laser by the linewidth enhancement factor a as [9,10]

where A is the lasing wavelength, dn is the change in the refractive index
of the laser, and dg/dN is the differential gain. In a semiconductor laser,
the change in the gain and the refractive index is over the entire length
of the laser L s. With an extended cavity in which only part of the cavity
is active, the influence of both these quantities is limited, with the result
that the effect of the refractive index change (on the cavity length) is
diluted.
The change in the laser wavelength <5A as a function of injected carri-
ers is given by

where dl carrier is the change in the optical length of the semiconductor with
injected current, and A is the lasing wavelength. The inverse dependence of
the shift in the wavelength on the laser cavity length demonstrates the
advantage of the longer cavity [11],
Further, the wide gain bandwidth of the semiconductor chip in con-
junction with a fiber Bragg grating used to define the operating wave-
length can increase the yield, potentially allowing the use of all chips on
a wafer. This becomes especially important with the tight specification
for WDM system wavelengths. Semiconductor production technology of
DFB lasers requires that each laser be measured for wavelength prior to
packaging and consequently has implications for the replacement of faulty