96 CHAPTER 3. OPTICAL TRANSMITTERS
Figure 3.11: Gain and loss profiles in semiconductor lasers. Vertical bars show the location
of longitudinal modes. The laser threshold is reached when the gain of the longitudinal mode
closest to the gain peak equals loss.
optical mode extends beyond the active layer while the gain exists only inside it. As
a result,g=Γgm, whereΓis the confinement factor of the active region with typical
values<0.4.
The phase condition in Eq. (3.3.5) shows that the laser frequencyνmust match one
of the frequencies in the setνm, wheremis an integer. These frequencies correspond to
thelongitudinal modesand are determined by the optical lengthnL. The spacing∆νL
between the longitudinal modes is constant (∆νL=c/ 2 nL) if the frequency dependence
ofnis ignored. It is given by∆νL=c/ 2 ngLwhen material dispersion is included [2].
Here thegroup index ngis defined asng=n+ω(dn/dω). Typically,∆νL=100–
200 GHz forL=200–400μm.
A FP semiconductor laser generally emits light in several longitudinal modes of
the cavity. As seen in Fig. 3.11, the gain spectrumg(ω)of semiconductor lasers is
wide enough (bandwidth∼10 THz) that many longitudinal modes of the FP cavity
experience gain simultaneously. The mode closest to the gain peak becomes the dom-
inant mode. Under ideal conditions, the other modes should not reach threshold since
their gain always remains less than that of the main mode. In practice, the difference is
extremely small (∼ 0 .1cm−^1 ) and one or two neighboring modes on each side of the
main mode carry a significant portion of the laser power together with the main mode.
Such lasers are called multimode semiconductor lasers. Since each mode propagates
inside the fiber at a slightly different speed because of group-velocity dispersion, the
multimode nature of semiconductor lasers limits the bit-rate–distance productBLto
values below 10 (Gb/s)-km for systems operating near 1.55μm (see Fig. 2.13). The
BLproduct can be increased by designing lasers oscillating in a single longitudinal
mode. Such lasers are discussed in Section 3.4.
3.3.3 Laser Structures
The simplest structure of a semiconductor laser consists of a thin active layer (thickness
∼ 0. 1 μm) sandwiched betweenp-type andn-type cladding layers of another semi-