Electro-Optics - Applied Optics Fundamentals and Device Applications - page 30

the elements must be controlled rather precisely. In the past, these elements,
which are actually oscillators, were controlled electrically either by coaxial
cable or by waveguide. Because of the weight, volume, and expense involved
in running >10,000 waveguides to a phased array radar, many radars that
could have been phased array in the past were implemented in some other
fashion. With optically controlled oscillators, however, these radars can now
be implemented as phase arrays, because the control signal to each oscilla-
tor can be fed through small, lightweight, and inexpensive fiber-optic cable.
Thousands of oscillators can be injection locked by a single master oscil-
lator whose output is modulating a laser. Megawatts of output power can
be achieved if IMPATT or TRAPATT diode oscillators are used. Millimeter
wave operation is quite feasible.
Another radar application using optically controlled devices is in Doppler
type radars. Depending on the frequency of radar operation and the velocity
of the expected targets, the Doppler return could be very close to carrier fre-
quency. Doppler offsets of 1 Hz are commonplace. In order for the Doppler
to be detected, the system must be coherent in frequency and phase, and the
noise level close to the carrier must be relatively low. Using the optical tech-
nique described in the IMPATT section, one could optically injection lock
several oscillators in a system that would maintain frequency and reduce
phase noise.
High-resolution radar applications could benefit from optically controlled
devices. With the shorter pulse durations and the reduction in leading edge
jitter (start-up jitter) afforded by optical control, the ambiguity in target range
is reduced dramatically.
Frequency agile communications and radar systems are also candidates
for optically controlled devices. The faster a system can “hop” from one fre-
quency to another, the better. Subnanosecond frequency switching times
exhibited by the optically controlled IMPATT diode oscillator mentioned
earlier would be ideal.
Systems requiring immunity to EMI could use optical technology.
Shipboard systems, for example, have to withstand extreme EMI environ-
ments because of the radar and communication gear on board. Since the
optical signal would run through fiber-optic cable, the control signal would
be inherently immune to outside EMI. Also, if there is information being
transmitted through the cable that must be protected, the information is
more secure than with electrical conductors.
Still another application is in airborne or portable systems where size and
weight must be kept to a minimum. Using fiber cable reduces both of these
critical parameters and also allows more complicated systems not possible
without optical control. Finally, laboratory test setups are in need of devices
and systems that are optically controlled. Laboratory work usually involves
more precise control of parameters than in fielded systems. Narrower pulses
and faster tuning can be achieved with optical control.