Page 239 - Introduction to Information Optics

124 4. Switching with Optics

Fig. 4.19. Large-signal response of a laser diode, (a) Input current waveform, (b) Laser output
power waveform.

signal modulation. Large-signal modulation can be realized by biasing the laser
close to the threshold and modulation with current pulses of large amplitude.
Large-signal modulation response is generally worse compared to small-signal
response [22,25]. Large-signal modulation can also cause severe frequency
chirping of the laser.
Due to the highly nonlinear optical properties of diode lasers, large-signal
dynamic response can be quite complex. The output optical waveform depends
strongly on the frequency and amplitude of the input current, and harmonic
and intermodulation distortion can be significant [18, 22, 25]. The large-signal
behavior of high-speed lasers has been investigated both theoretically and
experimentally for a variety of modulation schemes, including gain switching
and conventional pulse code modulation [17, 25]. Figure 4.19 shows an
idealized rectangular current drive waveform and the corresponding large-
signal response of a laser [17]. The dc current / 0 can either be slightly below
or above the threshold. The turn-on time of the laser t on is an important
parameter that affects the maximum achievable bit rate in digital systems. The
turn-on time is a function of / 0 that increases while the current increases. It is
clear that turn-on behavior is improved provided the laser is bias above the
threshold. Rate-equations analysis shows that t on can drop by an order of
magnitude from several hundreds of picoseconds to 30 to 60 ps when / 0 is
varied from below the threshold to 30 to 40% above the threshold [18]. Even
though the output optical pulse is not an exact replica of the applied electrical
pulse, deviations are small enough that semiconductor lasers can be used for
data transmission up to bit rates of about 10 Gb/s.
Note that direct current modulation results in the simultaneous amplitude
modulation (AM) and frequency modulation (FM) of the laser emission. This
originates from the refractive index variations in the laser at the same time that