3.4. CONTROL OF LONGITUDINAL MODES 101
Figure 3.16: DFB and DBR laser structures. The shaded area shows the active region and the
wavy line indicates the presence of a Bragg gratin.
Bragg reflector(DBR) lasers. Figure 3.16 shows two kinds of laser structures. Though
the feedback occurs throughout the cavity length in DFB lasers, it does not take place
inside the active region of a DBR laser. In effect, the end regions of a DBR laser act
as mirrors whose reflectivity is maximum for a wavelengthλBsatisfying Eq. (3.4.2).
The cavity losses are therefore minimum for the longitudinal mode closest toλBand
increase substantially for other longitudinal modes (see Fig. 3.15). The MSR is deter-
mined by the gain margin defined as the excess gain required by the most dominant
side mode to reach threshold. A gain margin of 3–5 cm−^1 is generally enough to re-
alize an MSR>30 dB for DFB lasers operating continuously [39]. However, a larger
gain margin is needed (>10 cm−^1 ) when DFB lasers are modulated directly.Phase-
shifted DFB lasers[38], in which the grating is shifted byλB/4 in the middle of the
laser to produce aπ/2 phase shift, are often used, since they are capable of provid-
ing much larger gain margin than that of conventional DFB lasers. Another design
that has led to improvements in the device performance is known as thegain-coupled
DFB laser[42]–[44]. In these lasers, both the optical gain and the mode index vary
periodically along the cavity length.
Fabrication of DFB semiconductor lasers requires advanced technology with mul-
tiple epitaxial growths [41]. The principal difference from FP lasers is that a grating
is etched onto one of the cladding layers surrounding the active layer. A thinn-type
waveguide layer with a refractive index intermediate to that of active layer and the
substrate acts as a grating. The periodic variation of the thickness of the waveguide
layer translates into a periodic variation of the mode index ̄nalong the cavity length
and leads to a coupling between the forward and backward propagating waves through
Bragg diffraction.