100 CHAPTER 3. OPTICAL TRANSMITTERS
LFigure 3.15: Gain and loss profiles for semiconductor lasers oscillating predominantly in a single
longitudinal mode.
and becomes the dominant mode. Other neighboring modes are discriminated by their
higher losses, which prevent their buildup from spontaneous emission. The power
carried by these side modes is usually a small fraction (<1%) of the total emitted
power. The performance of a SLM laser is often characterized by themode-suppression
ratio(MSR), defined as [39]
MSR=Pmm/Psm, (3.4.1)
wherePmmis the main-mode power andPsmis the power of the most dominant side
mode. The MSR should exceed 1000 (or 30 dB) for a good SLM laser.
3.4.1 Distributed Feedback Lasers
Distributed feedback (DFB) semiconductor lasers were developed during the 1980s
and are used routinely for WDM lightwave systems [10]–[12]. The feedback in DFB
lasers, as the name implies, is not localized at the facets but is distributed throughout
the cavity length [41]. This is achieved through an internal built-in grating that leads
to a periodic variation of the mode index. Feedback occurs by means ofBragg diffrac-
tion, a phenomenon that couples the waves propagating in the forward and backward
directions. Mode selectivity of the DFB mechanism results from theBragg condition:
the coupling occurs only for wavelengthsλBsatisfying
Λ=m(λB/2 ̄n), (3.4.2)whereΛis the grating period, ̄nis the average mode index, and the integermrepresents
the order of Bragg diffraction. The coupling between the forward and backward waves
is strongest for the first-order Bragg diffraction (m=1). For a DFB laser operating at
λB= 1. 55 μm,Λis about 235 nm if we usem=1 and ̄n= 3 .3 in Eq. (3.4.2). Such
gratings can be made by using a holographic technique [2].
From the standpoint of device operation, semiconductor lasers employing the DFB
mechanism can be classified into two broad categories: DFB lasers anddistributed