3.4. CONTROL OF LONGITUDINAL MODES 105
over a wavelength range exceeding 100 nm. By controlling the current in the phase-
control section, a quasicontinuous tuning range of 40 nm was realized in 1995 with a
superstructure grating [48]. The tuning range can be extended considerably by using a
four-section device in which another DBR section is added to the left side of the device
shown in Fig. 3.18(c). Each DBR section supports its own comb of wavelengths but
the spacing in each comb is not the same. The coinciding wavelength in the two combs
becomes the output wavelength that can be tuned over a wide range (analogous to the
Vernier effect).
In a related approach, the fourth section in Fig. 3.18(c) is added between the gain
and phase sections: It consist of a grating-assisted codirectional coupler with a super-
structure grating. The coupler has two vertically separated waveguides and selects a
single wavelength from the wavelength comb supported by the DBR section with a su-
perstructure grating. The largest tuning range of 114 nm was produced in 1995 by this
kind of device [49]. Such widely tunable DBR lasers are likely to find applications in
many WDM lightwave systems.
3.4.4 Vertical-Cavity Surface-Emitting Lasers
A new class of semiconductor lasers, known asvertical-cavity surface-emitting lasers
(VCSELs), has emerged during the 1990s with many potential applications [53]–[60].
VCSELs operate in a single longitudinal mode by virtue of an extremely small cav-
ity length (∼ 1 μm), for which the mode spacing exceeds the gain bandwidth (see
Fig. 3.11). They emit light in a direction normal to the active-layer plane in a manner
analogous to that of a surface-emitting LED (see Fig. 3.8). Moreover, the emitted light
is in the form of a circular beam that can be coupled into a single-node fiber with high
efficiency. These properties result in a number of advantages that are leading to rapid
adoption of VCSELs for lightwave communications.
As seen in Fig. 3.19, fabrication of VCSELs requires growth of multiple thin lay-
ers on a substrate. The active region, in the form of one or several quantum wells, is
surrounded by two high-reflectivity (> 99 .5%) DBR mirrors that are grown epitaxi-
ally on both sides of the active region to form a high-Q microcavity [55]. Each DBR
mirror is made by growing many pairs of alternating GaAs and AlAs layers, eachλ/4
thick, whereλis the wavelength emitted by the VCSEL. A wafer-bonding technique is
sometimes used for VCSELs operating in the 1.55-μm wavelength region to accommo-
date the InGaAsP active region [58]. Chemical etching or a related technique is used
to form individual circular disks (each corresponding to one VCSEL) whose diameter
can be varied over a wide range (typically 5–20μm). The entire two-dimensional array
of VCSELs can be tested without requiring separation of lasers because of the vertical
nature of light emission. As a result, the cost of a VCSEL can be much lower than that
of an edge-emitting laser. VCSELs also exhibit a relatively low threshold (∼1mAor
less). Their only disadvantage is that they cannot emit more than a few milliwatts of
power because of a small active volume. For this reason, they are mostly used in local-
area and metropolitan-area networks and have virtually replaced LEDs. Early VCSELs
were designed to emit near 0.8μm and operated in multiple transverse modes because
of their relatively large diameters (∼ 10 μm).