Electro-Optics - Applied Optics Fundamentals and Device Applications - page 23

is much more difficult to implement. FM modulation was also observed.
Frequency deviations of >4% were seen. Greater deviation could be reached
at the expense of excessive AM modulation.

2.2.3.6 Optical Injection Locking

Optical injection locking is the technique whereby the IMPATT frequency is
controlled by a purer microwave source [30]. This technique is useful when
many IMPATTs must operate in a coherent fashion. Optical injection locking
has a very promising future in the fields of radar and communications.
There are two types of optical injection locking. One is direct injection
locking and the other is indirect injection locking. Direct injection is illus-
trated in Figure 2.13. A laser is used to illuminate the IMPATT diode that
is encased in a microwave cavity. Along the way, the laser light is intensity
modulated by a microwave source feeding an electro-optic modulator. This
intensity modulation triggers the avalanche process, thereby controlling the
frequency of oscillation of the IMPATT. This is referred to as the direct injec-
tion locking method because the optical signal is directly illuminating the
IMPATT. Indirect injection locking is shown in Figure 2.14. Note the differ-
ence between direct and indirect locking. Indirect locking the optical sig-
nal, which is modulated by the master oscillator, does not reach the IMPATT
diode at all. The modulated optical signal controls an intermediate device
that then electrically controls the IMPATT. In Figure 2.14, the intermediate
device is the PIN diode.
Another difference between methods of injection locking is whether the
fundamental frequency of the IMPATT is used as the cleaner microwave
source frequency or whether a subharmonic of the IMPATT frequency is
used as the microwave source frequency. For example, in Figure 2.13 [31],

Modulated
optical beam Circuit

Laser

To load

Electro-optic
modulator

Microwave
source frequency W

Unpackaged
IMPATT

FIGURE 2.13
Optical synchronized microwave oscillator.