ETTC'2003 - SEE

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Low Phase Noise Optical Links for Reference Frequency
Distribution in Satellite Payloads
Gianandrea QUADRI, Horacio MARTINEZ-REYES,
Benoit BENAZET (1) , Olivier LLOPIS
LAAS-CNRS, 7 Avenue du Colonel Roche, 31077 Toulouse, Cedex 04, France
(1) Alcatel Space, 26 av. J. F. Champollion, B.P. 1187, 31037 Toulouse, France
Abstract — Various circuits dedicated to high spectral
purity signal transmission using fiber optics are presented.
Three application types are investigated : reference
frequency distribution at 10 MHz, IF distribution at
800 MHz and microwave synthesized signals at 3.5 GHz. The
reception circuit is an optically synchronized oscillator,
which provides a good signal conditioning far from the
carrier while maintaining the high input signal quality close
to the carrier.
I. INTRODUCTION
Fiber optics constitutes an attractive alternative to
conventional wiring for numerous analog applications.
Indeed, in addition to its small size and low mass, the fiber
does not interfere with electronic devices and provides an
excellent isolation of the transmitted signal. One of the
targeted application of our work is the reference
frequency signal distribution to various subsystems of a
telecommunications satellite. Another application could
be remote antenna synchronization and control.
However, to meet our system requirements, the fiber
optics link should not degrade the signal quality, and
particularly its phase noise. To this purpose, the optically
synchronized oscillator [1,2] is an efficient approach
which naturally filters the signal far from the carrier, thus
removing the additional noise due to the optical link.
Moreover, an optically controlled oscillator delivers a
constant output power, which is an interesting feature for
a clock or reference frequency delivering network.
Different types of optically synchronized oscillators
may be investigated. The simplest one uses a conventional
photodiode to detect the amplitude modulated optical
signal, and the resulting microwave signal is used in a
classical injection lock of a microwave oscillator, or as a
reference signal in a PLL. In this case, we speak about an
indirect optically synchronized oscillator. Another
possibility is to realize the oscillator with a transistor
sensitive to the optical power (photo-transistor) and thus
to control directly the oscillator amplifying element.
In this study, various solutions are investigated for a
low phase noise optical link at 10 MHz, 800 MHz and
3.5 GHz. Indirect synchronization has been chosen for the
two lower frequencies, and both indirect and direct
synchronization have been investigated at 3.5 GHz using
two types of active devices : an InP based bipolar phototransistor
(photo-HBT) [3], and an InP based HEMT
device [4].
The application goal for the 10 MHz link is the
reference frequency distribution in a telecommunications
satellite. The signal from the ultra stable quartz crystal
oscillator should be transmitted with no degradation of its
phase noise, and this is a very stringent requirement.
The 800 MHz application is also in the field of
telecommunications satellites, and corresponds to an
intermediate frequency in the satellite.
The 3.5 GHz frequency is a first attempt to compare
the different approaches for high spectral purity
microwave signal distribution. The application is not
clearly defined today, but it could be in the field of active
antennas.
II. PHOTO-OSCILLATOR ; THEORY
The indirect optical injection locking can be efficiently
described by combining the classical noise modeling of an
optical link [5,6] to the theory of injection locked
oscillators [7,8]. The only questionable point is probably
in the interaction between the two systems : has the
photodiode (or its associated amplifier) an effect on the
oscillator noise ? But if the two systems are isolated in
some way from one to another, this approach should
apply. The following equation (from Kurokawa’s theory
[8]), is an expression of the output phase noise spectral
density of a synchronized oscillator, versus the input
signal phase noise Sφ input, the free running oscillator phase
noise Sφ free, the locking bandwidth flock and the offset
from carrier fm.
2
⎛ f m ⎞
⎜ ⎟
⎜ f ⎟
1
⎝ lock ⎠
Sϕ
=
⋅ S
S
sync
ϕ +
⋅
2 input
2 ϕ free
⎛ f m ⎞
⎛ f m ⎞
1 + ⎜ ⎟
+ ⎜ ⎟
⎜ f ⎟
1
⎜
lock
f ⎟
⎝ ⎠
⎝ lock ⎠