Electro-Optics - Applied Optics Fundamentals and Device Applications - page 27

illumination depends on the frequency of oscillation of the TRAPATT. At the
low end of the frequency band, the power lever decreases with increasing
illumination, whereas at the higher frequencies, the power level increases
with increasing power level. Power variation of about 9 dB was achieved
when the illumination varied from 10 −2 to 1000 W/cm 2 .
2.2.5 MESFET Oscillator
An experiment on a MESFET oscillator was carried out to determine the
capability to optically injection lock the oscillator [34]. This experiment was
spurred by previous work on MESFETs which showed that the oscillator fre-
quency could be tuned by varying the illumination intensity striking the
active region of the MESFET. The illumination creates free carriers in the
channel region that alters the DC characteristics of the device.
The MESFET was controlled by a GaAs/GaAlAs laser that produced an
output power of 1.5 mW at 850 nm. The laser was modulated by a klystron
oscillator that could be tuned around the MESFET oscillator frequency at
about 2.35 GHz. Comparisons of the FM noise level of the free-running oscil-
lator and the oscillator locked to the klystron were made. Measured in a
1 Hz bandwidth, the RMS FM noise deviation was only a few hertz at 10 kHz
offset from the carrier. This is a significant reduction in the noise spectrum
of the oscillator.
Locking range of the MESFET oscillator was also examined. A maxi-
mum locking range of 5 MHz was obtained. A greater locking range could
be achieved if more efficient coupling existed between the laser and the
FET chip, and also if more of the microwave signal could be coupled to the
laser.
2.2.6 Transistor Oscillators
Transistors can be optically controlled by illuminating the base region with
a light source. The light source generates carriers in the base that affect the
conductivity of the transistor. In other words, the light source acts as an addi-
tional supplier of base current.
Experimental results on optically injection locking a transistor oscillator
will be reviewed [35]. A silicon transistor with its lid partially removed to
allow illumination was used. The illuminating device was GaAlAs laser
operating at 820 nm. The laser output was intensity modulated by a signal
generator with a frequency of 110 MHz. The frequency out of the transistor
oscillator was 330 MHz which demonstrated that subharmonic optical injec-
tion locking took place. Fundamental frequency locking may also occur.
The locking range of the oscillator was limited due to the same reasons
discussed previously. First, the modulation method of the laser was inef-
ficient; second, the coupling efficiency of the laser light into the transistor
base region was low.