106 CHAPTER 3. OPTICAL TRANSMITTERS
Figure 3.19: Schematic of a 1.55-μm VCSEL made by using the wafer-bonding technique.
(After Ref. [58];©c2000 IEEE; reprinted with permission.)
In recent years, the VCSEL technology have advanced enough that VCSELs can be
designed to operate in a wide wavelength range extending from 650 to 1600 nm [55].
Their applications in the 1.3- and 1.55-μm wavelength windows require that VCSELs
operate in a single transverse mode. By 2001, several techniques had emerged for
controlling the transverse modes of a VCSEL, the most common being the oxide-
confinement technique in which an insulating aluminum-oxide layer, acting as a di-
electric aperture, confines both the current and the optical mode to a<3-μm-diameter
region. Such VCSELs operate in a single mode with narrow linewidth and can replace
a DFB laser in many lightwave applications as long as their low output power is accept-
able. They are especially useful for data transfer and local-loop applications because
of their low-cost packaging. VCSELs are also well suited for WDM applications for
two reasons. First, their wavelengths can be tuned over a wide range (>50 nm) using
the micro-electro-mechanical system (MEMS) technology [56]. Second, one can make
two-dimensional VCSELS arrays such that each laser operates at a different wave-
length [60]. WDM sources, containing multiple monolithically integrated lasers, are
required for modern lightwave systems.
3.5 Laser Characteristics
The operating characteristics of semiconductor lasers are well described by a set of
rate equations that govern the interaction of photons and electrons inside the active re-
gion. In this section we use the rate equations to discuss first both the continuous-wave
(CW) properties. We then consider small- and large-signal modulation characteristics
of single-mode semiconductor lasers. The last two subsections focus on the intensity
noise and spectral bandwidth of semiconductor lasers.