Optical Diagnostics and Imaging - Applied Optics Fundamentals and Device Applications - page 296

6.2 BandwidthMeasurement
A key characteristic of optical switches and modulators is their switching
speed and modulation depth. For switches, the bandwidth is related to the
switching speed by the equation

= 2π
Δ

T (6.1)

f

A modulator's bandwidth is often expressed as the frequency range over
which it can maintain a modulation depth of 50%. Various expressions are
used to express the modulation depth, depending on whether the applied
electrical signal acts to increase or decrease the intensity of the transmitted
light. The respective expressions are given as Equation 6.2:

( l − l ) ( l − l )

0 0

n = or n = (6.2)

l l

0 m

where
l is the transmitted optical intensity
l 0 is the quiescent optical intensity
l m is the optical intensity under maximum electrical signal

To test the active devices for modulation depth and speed, high-speed detec-
tors and storage oscilloscopes are used.

6.3 Stability:TemperatureandTimeEffects
The most significant problem of the stability of waveguide building blocks
and devices is introduced by the temperature dependence of the index of
refraction resulting from change in the energy bandgap with temperature.
Propagation and attenuation of optical energy in a semiconductor can be
described by a complex index of refraction

n

c = −

n ik (6.3)

where the real part of the index is related to the propagation constant β by

n = 2 πβ
λ 0

(6.4)