Page 218 - Introduction to Information Optics

4.2. All-Optical Switches 203

to the eventual error rate of the transmission system. An ideal switch would
have an infinite on-off ratio (Imin = 0).
Switching time (z) measures how fast the switch can perform, and is defined
as the time required for switching the output intensity from 10% to 90% of
I,,,. It is related to the - 3 dB bandwidth (Av)

AV = 0.35/~ Hz. (4.2)
Insertion loss (L) describes the fraction of power lost when the switch is placed
in the system. The insertion loss does not include the additional loss during
switching, and is defined as
L(dB) = 10 log Pout/Pin. (4.3)

where Pout is the transmission power when the switch is not in the system, and
Pin is the transmitted power when the switch is in the system and adjusted to
provide the maximum transmission.
Power consumption is defined as the power consumed by the switch during
operation. The consumed power will eventually turn into heat, and limit the
number of switches or other devices can be put on a system unit. It will also
set a demand on the power supply.
While the above parameters measure the performance of both on-off and
routing switches, the final parameter, cross talk, only applies to the routing
switches. It describes how effective a signal is isolated between two unconnec-
ted channels. Consider that a routing switch has one input and two outputs.
When the input is connected to output channel 1 of the output, cross talk in
this case describes how much of the input signal appears on channel 2. It is
defined as

Cross talk (dB) = 10 10g(12/11), (4.4)
where I, is the output intensity in the connected channel, and I, is the intensity
in the unselected channel. Ideally, I, should be zero.
The above parameters depend on the material and configurations used in
switching devices. In some switching configurations, wavelengths and polariz-
ation states of the signal and control beams may also affect these parameters.
In these cases, the switches are called wavelength and polarization dependent.

4.2. ALL-OPTICAL SWITCHES

All-optical switches are nonlinear optical devices the output characteristics
of which are controlled by the intensity of the input signal or by a separate
optical signal for self-switching and controlled switching, respectively (Fig. 4.2).