Ms6127Bwithfigures

2. Experimental set-up
The schematic of the optical link and compensation system is displayed on Figure 1. The 
link is obtained by cascading two 43-km twin fibers connecting the laboratories LPL and LNE-
SYRTE. Both ends are collocated at LPL, one labelled as local and the other as remote. 
The performance of our previous optical link was limited by the signal-to-noise ratio at detection 
and the phase noise and long-term instability of the laser source which induced a parasitic noise due 
to the fiber dispersion [11]. Thus two main changes have been implemented. The reference carrier 
frequency has been increased from 1 GHz to 9.15 GHz inducing a complete change of the 
compensation system, laser sources and detection set-up. Moreover a section of negative dispersion 
fiber has been added to the link to compensate for its dispersion. 
At each end two very low phase noise microwave synthesis systems generate all the signals 
necessary for the compensation system. At the local end, a 9.15 GHz Yttrium Iron Garnet 
microwave oscillator (YIG) is phase-locked to a low phase noise 100 MHz voltage controlled 
quartz oscillator (VCXO) with a double stage RF down conversion approach using a 200 MHz 
sampling mixer [19, 20]. At the remote end a similar technique is used to phase lock a second 
microwave oscillator operating at 9.25 GHz by harmonic sampling to a low phase noise 1 GHz 
Surface Acoustic Wave oscillator (SAW). Each microwave signal directly modulates the beam 
intensity of a 6 mW distributed feedback laser diode (DFB-LD) at 1.55 µm using an Electro-
Absorption Modulator (EAM), the DFB-LD and the EAM being integrated on the same chip 
(Mitsubishi FU-653SEA). The modulation power of about 5 dBm is injected in the EAM using a 
bias-tee to optimize the EAM operation point with a DC bias of a few Volts. The modulation is 
detected with a fast photodiode (10 GHz bandwidth Discovery Semiconductors DSC50S) at the 
other end of the fiber. We measured the phase noise of the emitter/receiver part (DFB-LD, 
photodiode and microwave amplifier) to be −105 dBrad
2
/Hz at 1 Hz offset, rolling down with a 
close to 1/f slope up to 1 kHz. This residual noise is compatible with a frequency instability better 
than 10
-15
at 1 s. At the remote end the 9.25 GHz microwave signal is mixed with the incoming 
9.15 GHz signal detected by the photodiode and the resulting beat-note signal at 100 MHz is mixed 
to DC with the SAW oscillator output digitally divided by ten. This error signal is used to phase 
lock the SAW oscillator to the incoming signal and consequently the 9.25 GHz with a bandwidth of 
a few kHz. This way the 9.25 GHz YIG oscillator signal reproduces the one-way phase 
perturbations of the link. This signal modulates a second laser diode which is injected back into the 
link using an optical circulator. At the local end the 9.25 GHz backward signal is detected with a 
second fast photodiode (DSC50S). This signal carries the round-trip phase perturbations 
accumulated over the link. In order to compensate for these perturbations the phase is revealed by 

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successive mixing processes with the input 9.15 GHz signal and the 100 MHz from the VCXO. 
This generates an error signal which is processed by a simple low frequency loop filter and applied 
to two variable delay lines at the local input in order to cancel out the link phase perturbations. 
One variable delay line is home made with a 15-meter long optical fiber wrapped around a 
20-mm diameter cylindrical piezoelectric transducer (PZT) to correct the fast and small phase 
perturbations [17]. Voltage applied to the PZT stretches the fiber with a dynamic range of 15 ps and 
a bandwidth of about 1 kHz. The second variable delay line consists of a 2.5-km long fiber spool in 
an enclosure which is temperature controlled by thermoelectric modules. This device corrects slow 
and large phase perturbations with a dynamic range of about 4 ns corresponding to a temperature 
range of 50°C (88 ps/°C). It enables the correction of a global temperature change of 1.5°C of the 
optical link, whereas the temperature change is typically less than 0.5 °C per week. 
The use of two different microwave frequencies for the forward and backward signals 
(9.15 GHz and 9.25 GHz) prevents interferences between the main signal, Brillouin backscattering 
and parasitic reflections from connectors and splices along the link.
The polarization mode dispersion (PMD) and the chromatic dispersion are two detrimental 
fiber propagation effects that limit the performance of the correction system as discussed in [11, 
21]. Both phenomena induce an asymmetry of the phase perturbation in forward and backward 
propagation directions along the link with the result that the round-trip phase fluctuations do not 
exactly correspond to twice the forward phase fluctuations. To minimize PMD, the laser beam 
polarization is directly scrambled at each emitted source by a 3 axes polarization scrambler at 
frequencies higher than the inverse of the light propagation delay in the overall round-trip (0.5 ms). 
Each scrambler acts as three cascaded variable retardation waveplates excited at different resonant 
frequencies (approximately 60 kHz, 100 kHz and 130 kHz). This enables the exploration of all 
polarization states. The chromatic dispersion of the fiber (D~-17 ps/km/nm) converts the laser 
diodes’ frequency noise into an excess phase noise of the optical link [11]. To reduce this effect 
11 km of highly negative dispersion fiber is inserted at the local end. Thus the total dispersion is 
reduced to less than 5% of the original 86-km fiber dispersion. This also prevents periodic signal 
fading and extinction along the link. Without this compensation the dispersion creates a differential 
phase shift between the microwave sidebands of the optical signal. It amounts to 
∆φ
=2
π
cDL(
Ω/ω
)
2
with D the fiber dispersion, L the fiber length, and 
Ω
and 
ω
the microwave and optical angular 
frequencies respectively. This leads to a modulation of the detected microwave signal amplitude 
with the fiber length with a first complete extinction at around 22 km [11]. 
Two optical amplifiers (EDFA) are used to optimize the signal-to-noise ratio at detection 
and to compensate for the 11 dB additional optical losses introduced by the negative dispersion 

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fiber. Despite these EDFAs and additional attenuation, the signal-to-noise ratio has not been 
degraded thanks to the higher modulation frequency compared to a 1 GHz system [11].