Electro-Optics - Applied Optics Fundamentals and Device Applications - page 22

2.2.3.4 Noise Reduction by Optical Means
One source of noise in an IMPATT device is the noise generated during the
avalanche multiplication process [28]. This noise is due to the variation in suc-
cessive cycles of the injected avalanche current. This variation is due to the
small amount of carriers present during the start-up of the avalanche cycle
that leads to jitter in the avalanche cycle. This causes high oscillator noise
levels. By increasing the reverse saturation current in the IMPATT, noise can
be reduced. Optical illumination can be used to increase the reverse satura-
tion current by photogenerating carriers in the diode.
Experiments were performed on both Si and GaAs IMPATT diodes. The
Si oscillator had an RF output power of 260 mW at 10.47 GHz, and the GaAs
oscillator power was 420 mW at 10.44 GHz. Both diodes were illuminated
with a HeNe laser at 632.8 nm. The FM noise output from 0 to 50 kHz offset
was measured for both diodes.
At very low illumination levels, no improvement in noise performance
was measured. This is because the optically produced current is much less
than the thermally generated saturation current that is greater for Si than for
GaAs since the bandgap for Si is less than that for GaAs.
Increasing the illumination resulted in a decreasing FM noise level. The
GaAs diode showed a 5 dB improvement in carrier/noise level at <7.5 kHz
offset from carrier. The Si diode showed only a 2 dB improvement at <7.5 kHz
offset because of the greater reverse saturation current already present in
the Si diode. Results from this experiment also showed that if laser sources
with greater amplitude stability were available, the noise performance of the
IMPATT diode could be improved even further.
2.2.3.5 Optically Induced AM/FM Modulation
Optically illumination can be used to modulate an IMPATT oscillator [29].
AM modulation can be attained by two methods—quenching or enhancing.
Quenching occurs when illumination of the diode alters the Q factor of the
oscillator circuit enough to stop all oscillations of the diode. Enhancement
of the oscillator output can occur with illumination of the diode, but only
within a limited optical power range; therefore, quenching is the dominant
effect used to AM modulate an IMPATT diode oscillator. FM modulation is
produced by the change in susceptance of the device due to optical illumina-
tion. This change in susceptance leads to significant shifts in frequency.
Experimental results on X-band silicon IMPATT diodes that were modu-
lated by optical means will now be discussed. The laser used was a 10 W
900 nm GaAlAs source. The laser was pulsed for 0.1 μs at a repetition rate
of 200 Hz. AM modulation was observed in both the quenched and the
enhancement modes. In the quenched mode, the ratio of RF power during
laser-on and laser-off states was about 10 dB. A total of 20 dB was obtained in
the enhancement mode; however, AM modulation in the enhancement mode