Electro-Optics - Applied Optics Fundamentals and Device Applications - page 21

p +

n

n +

Optical
fiber

GaAs
IMPATT
diode

50 µm

50 µm Diam. core

Microwave
output

Microwave
cavity

FIGURE 2.12
Optically synchronized microwave oscillator.

frequency of about 92 GHz. Most previous experimentation on optical
control had been done on IMPATTs that operated below 18 GHz.
In this experiment, the p + -n-n + diode was illuminated by a GaAs/GaAlAs
laser operating at 850 nm. Figure 2.12 shows the basic method used to illu-
minate the IMPATT diode. The output frequency was first measured with
the free-running oscillator. The frequency was 91.83 GHz with a bias current
of 100 mA. The diode was then illuminated with 3 mW of laser power that
optically generated 20.5 μA of current. This resulted in a 9 MHz shift in fre-
quency. Less than 10 MHz frequency shift was achieved because of the poor
coupling between the laser and the IMPATT diode. If the coupling efficiency
was improved to the point that typical high-speed photodiodes have per-
formed, a frequency shift of 600 MHz could be obtained.
In addition to the tuning range, tuning speed is another important param-
eter. Ninety percent of the 9.4 MHz frequency change occurred in 55 ps, or
just <20,000 changes in frequency in about 1 μs. Optical control certainly
allows fast response.