Page 238 - Introduction to Information Optics

4.3. Fast Electro-optic Switches: Modulators 223

the maximum possible modulation bandwidth in lasers is in excess of 23 to 46
GHz [20,21]. In practice, this bandwidth is usually limited by RC parasitics,
device heating, and maximum power-handling capability of the laser facets.
(b) Bandwidth limits due to parasitics: The dynamic response of the intrinsic-
laser is reduced by parasitics. The parasitic element limitation has been
comprehensively discussed previously [18,20,21].
To discuss the effect of parasitics, the laser is treated as an electrical element
and an equivalent circuit is established for the intrinsic laser in conjunction
with the bond wire, the mount elements, and the transmission line used for
impedance matching. Then, practical equivalent circuits can be developed for
diode lasers with different designs. A number of low-parasitic devices for
high-speed operation have been demonstrated [22]. One scheme uses a
semi-insulating substrate for decreasing shunt capacitance. Another type of
low-parasitic device is called the constricted mesa or mushroom stripe laser.
Several features in these devices help minimize shunt capacitance. The 3-dB
bandwidth due to parasitics in properly designed laser diodes can be as high
as 20 to 25 GHz [20, 21, 22].
In general, the effect of parasitics can be represented by using the electrical
response R (, of the diode laser in the following simple form [21]:

, ___ (4 37.)
1 + (o)/'w RCY I + (CO/ID ph)" '

where m represents low-frequency modulation efficiency, and CD RC and o) pn
describe the high-frequency roll-off caused by the series resistance and the
diffusion capacitance of the p-n junction, respectively. The overall laser
frequency response, therefore, should be R = R;R e. However, detailed analyses
of any particular laser structure will produce more complicated equations [18,
21, 23].
The 3-dB direct modulation bandwidth is mainly limited by the parasitics
in practical devices. For instance, a 25-GHz modulation bandwidth has been
measured in long-wavelength MQW lasers, whereas the maximum possible
intrinsic bandwidth determined by the K factor of the laser has been obtained
over 40 GHz [24].

4.3.1.2. Large-Signal Effect
The results presented in the previous sections are concerned with the
small-signal modulation response of diode lasers. However, in high-bit-rate
digital optical fiber communication systems, the on-off ratio in the laser
output often should be as large as possible to reduce the bit-error rate and
avoid excessive power penalty [22]. This leads to the requirement of large-