3.6. TRANSMITTER DESIGN 123
grated on the same chip. Low-chirp transmission at a bit rate of 5 Gb/s was demon-
strated as early as 1994 by integrating an electroabsorption modulator with a DBR
laser [114]. By 1999, 10-Gb/s optical transmitters with an integrated electroabsorption
modulator were available commercially and were used routinely for WDM lightwave
systems [119]. By 2001, such integrated modulators exhibited a bandwidth of more
than 50 GHz and had the potential of operating at bit rates of up to 100 Gb/s [120].
An electroabsorption modulator can also be used to generate ultrashort pulses suitable
for optical time-division multiplexing (OTDM). A DFB laser, integrated monolithi-
cally with a MQW modulator, was used as early as 1993 to generate a 20-GHz pulse
train [113]. The 7-ps output pulses were nearly transform-limited because of an ex-
tremely low chirp associated with the modulator. A 40-GHz train of 1.6 ps pulses was
produced in 1999 using an electroabsorption modulator; such pulses can be used for
OTDM systems operating at a bit rate of 160 Gb/s [116].
The second category of optical modulators makes use of the LiNbO 3 material
and a Mach–Zehnder (MZ) interferometer for intensity modulation [121]–[126]. Two
titanium-diffused LiNbO 3 waveguides form the two arms of a MZ interferometer (see
Fig. 3.27). The refractive index of electro-optic materials such as LiNbO 3 can be
changed by applying an external voltage. In the absence of external voltage, the optical
fields in the two arms of the MZ interferometer experience identical phase shifts and in-
terfere constructively. The additional phase shift introduced in one of the arms through
voltage-induced index changes destroys the constructive nature of the interference and
reduces the transmitted intensity. In particular, no light is transmitted when the phase
difference between the two arms equalsπ, because of destructive interference occur-
ring in that case. As a result, the electrical bit stream applied to the modulator produces
an optical replica of the bit stream.
The performance of an external modulator is quantified through the on–off ratio
(also called extinction ratio) and the modulation bandwidth. Modern LiNbO 3 mod-
ulators provide an on–off ratio in excess of 20 and can be modulated at speeds up
to 75 GHz [122]. The driving voltage is typically 5 V but can be reduced to below
3 V with a suitable design [125]. LiNbO 3 modulators with a bandwidth of 10 GHz
were available commercially by 1998, and the bandwidth increased to 40 GHz by
2000 [126].
Other materials can also be used to make external modulators. For example, mod-
ulators have been fabricated using electro-optic polymers. Already in 1995 such a
modulator exhibited a modulation bandwidth of up to 60 GHz [127]. In a 2001 ex-
periment, a polymeric electro-optic MZ modulator required only 1.8 V for shifting the
phase of a 1.55-μm signal byπin one of the arms of the MZ interferometer [128].
The device was only 3 cm long and exhibited about 5-dB chip losses. With further
development, such modulators may find applications in lightwave systems.
3.6.4 Optoelectronic Integration
The electrical components used in the driving circuit determine the rate at which the
transmitter output can be modulated. For lightwave transmitters operating at bit rates
above 1 Gb/s, electrical parasitics associated with various transistors and other compo-
nents often limit the transmitter performance. The performance of high-speed trans-