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Brillouin amplification in phase coherent transfer of optical frequencies over 480 km fiber

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Abstract

We describe the use of fiber Brillouin amplification (FBA) for the coherent transmission of optical frequencies over a 480 km long optical fiber link. FBA uses the transmission fiber itself for efficient, bi-directional coherent amplification of weak signals with pump powers around 30 mW. In a test setup we measured the gain and the achievable signal-to-noise ratio (SNR) of FBA and compared it to that of the widely used uni-directional Erbium doped fiber amplifiers (EDFA) and to our recently built bi-directional EDFA. We measured also the phase noise introduced by the FBA and used a new and simple technique to stabilize the frequency of the FBA pump laser. We then transferred a stabilized laser frequency over a wide area network with a total fiber length of 480 km using only one intermediate FBA station. After compensating the noise induced by the fiber, the frequency is delivered to the user end with an uncertainty below 2 × 10−18 and an instability σy(τ) = 2 × 10−14 /(τ/s).

©2010 Optical Society of America

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Figures (8)

Fig. 1
Fig. 1 Set-up to measure the gain of FBA or EDFA: VA: variable attenuator, PD: photodetector, signal: signal laser with isolator, pump: pump laser, AOM: acousto-optic modulator, a 25 km fiber is used as gain medium for FBA.
Fig. 2
Fig. 2 FBA in comparison to EDFAs (uni-directional and bi-directional) for different signal powers received at the output of a 25 km fiber: (a) gain (b) SNR. The spectrum analyzer bandwidth is 100 kHz.
Fig. 3
Fig. 3 Detected heterodyne beat power (RF) obtained with FBA and with EDFA, when 5 mW signal power are injected into (a) 148 km fiber (P pump = 20 mW, νB = 10.974 GHz) and (b) 332 km fiber (P pump = 40 mW, νB = 10.970 GHz).
Fig. 4
Fig. 4 (a) Phase noise of the free-running interferometer without (black line) and with a bi-directional EDFA (red ο). (b) Phase noise of 25 km spooled SMF28 fiber without (black line) and with using FBA (blue ο).
Fig. 5
Fig. 5 (a) Measurement setup for the scattered and transmitted pump power changes. The pump is injected in the opposite direction to the signal laser. CIR: circulator and OPM: optical power meter. (signal power is about 1 μW after 148 km fiber) (b).The change of the scattered pump power when the pump frequency is swept around sig + νB ) and a Gaussian fit.
Fig. 6
Fig. 6 Brillouin amplification in a frequency transfer system, AOM: Acousto-optic modulator, PD: photodetector, FM: Faraday mirror, C1: 30/70, C2: 40/60, C3:40/60, C4:30/70 simple fiber couplers (for details see text).
Fig. 7
Fig. 7 Phase noise of the out-of-loop signal (OL) before (black dashed), and after (red solid) compensation. The green curve (ο) gives the theoretical compensation limit according to [18].
Fig. 8
Fig. 8 Out-of-loop signal (OL) with compensated (red ο) and uncompensated phase noise (black ■).

Equations (2)

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P c r i t = 21 A γ L e f f ( 1 + Δ ν l a s e r Δ ν B )
g = γ L e f f P p u m p A
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