Electro-Optics - Applied Optics Fundamentals and Device Applications - page 26

of free carriers. This plasma now travels across the diode, but at very low
velocity due to the voltage across the diode dropping significantly after the
avalanche zone passes. Because of the low velocity of the free carriers, the
transit time across the diode can be much longer than if they were travel-
ing at the saturation velocity. This transit time delay provides the needed
phase delay for oscillation. Because the transit time is relatively long, the
frequency of operation can be quite low compared with the IMPATT diode.
2.2.4.1 Illumination Effect on TRAPATT Operation
Three effects of illumination on TRAPATT diodes will now be discussed.
These are not the only effects observed, but are effects that have real system
applications. The effects are reduction of startup jitter, frequency shifting,
and variation of RF power output [33].
2.2.4.2 Experimental Results: Start-Up Jitter Reduction
Start-up jitter in TRAPATT oscillators can be very troublesome. Jitter on
the order of 100 ns is common and this becomes even worse with tempera-
ture. Optical injection of carriers into the diode during the low-current
portion of the RF cycle can stabilize the TRAPATT mode and thus reduce
start-up jitter.
Experiments on a silicon TRAPATT diode operating at 700 MHz with a
power output of 70 W continuous were performed. Illumination was pro-
vided by a GaAs laser diode operating at 904 nm. Without illumination, the
TRAPATT displayed about 100 ns of start-up jitter. When the TRAPATT was
illuminated by a laser pulse positioned in time at the leading edge of the
TRAPATT bias pulse, reduction in jitter occurred. Reduction to as low as
30 ns was observed with a laser output of 0.2 W/cm 2 . This reduction in jitter
was also made less temperature dependent.
2.2.4.3 Frequency Shifting
Experimental results on a silicon TRAPATT diode will now be discussed.
Optical control of the frequency was noticed with laser illumination at lev-
els greater than 10 −2 W/cm 2 from a GaAs laser operating at 904 nm. The
TRAPATT, whose frequency range was from 654 to 1493 MHz, could be
shifted in frequency by about 1% at the low end of the band and by about 7%
at the highest frequency when stimulated by the laser. In most cases, as the
illumination level increased, so did the TRAPATT frequency.
2.2.4.4 Variation of Output Power
Using the same setup as in the previous discussion on frequency shift-
ing, the power level of the TRAPATT could be controlled. The effect of the