ETTC'2003 - SEE

Photo-oscillator
active device
Optical power at the
end of the fiber
Oscillator dynamic
synchronization
bandwidth
Free running
oscillator phase
noise at 100 kHz
InP
HEMT
8 dBm
20 kHz
-108 dBc/Hz
InP
Photo-HBT
-2 dBm
80 kHz
-105 dBc/Hz
InGaAs
Photodiode +
SiGe HBT
oscillator
-2 dBm
80 kHz
-127 dBc/Hz
Table 2
Synchronization bandwidth of the 3.5 GHz optically synchronized
oscillators, and phase noise of the free running oscillator at 100 kHz
offset. The amplitude modulation factor of the optical signal is about
0.25.
Phase Noise (dBc/Hz)
20
0
-20
-40
-60
-80
-100
-120
-140
Residual phase noise of the optical link
Free running oscillator
-160
1 10 100 1000 10000 100000
Frequency (Hz)
Optical transmission of a synthesized
signal (Anristu 69147A)
Figure 3
Phase noise of a 3.5 GHz optical link realized with a photo-diode + an
SiGe HBT synchronized oscillator, and featuring 10 dB optical losses.
Three different spectra are plotted : 1) free running oscillator phase noise
2) residual phase noise of the optical link 3) transmitted spectrum of an
Anritsu synthesizer
Concerning the phase noise, the optical power does
not too much modify the phase noise of the free running
oscillators, even if a strong change is observed on the DC
current. The best results far from the carrier are obtained
with the photodiode + SiGe HBT oscillator configuration,
because of the low 1/f noise properties of the SiGe device.
However, at higher frequencies (millimeter wave range)
the silicon devices may be difficult to use and the
compacity of the photo-HBT approach is particularly
interesting.
VI. CONCLUSION
Different RF and microwave optical links using
optically synchronized oscillators as receivers have been
presented. The advantage of this approach for the
transmission of pure sinusoidal signals (reference signals)
is clearly underlined. At 10 MHz, a strong improvement
of the optical link phase noise has been obtained. At
higher frequencies, the improvement is much based on the
filtering of the signal far from the carrier and on the
constant output power imposed by the oscillator. At
microwave frequencies, the photodiode has been replaced
successively by two different types of light sensitive
devices. Good results have been obtained with the photo-
HBT approach, while the illumination of the gate region
of an InP HEMT device has been found to be less
efficient.
ACKNOWLEDGEMENT
This work has been partly supported by the French
National Space Agency (CNES).
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