In-field Raman amplification on coherent optical fiber links for frequency metrology.

C Clivati,F Levi,S Faralli,D Calonico,A Mura,G Bolognini

doi:10.1364/oe.23.010604

Abstract

Distributed Raman amplification (DRA) is widely exploited for the transmission of broadband, modulated signals used in data links, but not yet in coherent optical links for frequency metrology, where the requirements are rather different. After preliminary tests on fiber spools, in this paper we deeper investigate Raman amplification on deployed in-field optical metrological links. We actually test a Doppler-stabilized optical link both on a 94 km-long metro-network implementation with multiplexed ITU data channels and on a 180 km-long dedicated fiber haul connecting two cities, where DRA is employed in combination with Erbium-doped fiber amplification (EDFA). The performance of DRA is detailed in both experiments, indicating that it does not introduce noticeable penalties for the metrological signal or for the ITU data channels. We hence show that Raman amplification of metrological signals can be compatible with a wavelength division multiplexing architecture and that it can be used as an alternative or in combination with dedicated bidirectional EDFAs. No deterioration is noticed in the coherence properties of the delivered signal, which attains frequency instability at the 10(-19) level in both cases. This study can be of interest also in view of the undergoing deployment of continental fiber networks for frequency metrology.

Highlights

In the last twenty years, frequency metrology has progressively enabled the investigation of fundamental physics at new orders of magnitude
Our experiment was based on 200 km of fiber spools in a laboratory environment [32]. We further investigate this technique applied to real metrological links based both on a dark channel architecture where the fiber is shared with other data channels, and on a dark fiber architecture where only the metrological signal is present on the fiber
In this work, we investigate the use of distributed Raman amplification in a coherent frequency link based on two experimental sessions, one involving a 94 km shared link with International Telecommunication Union (ITU) channels carrying data traffic, the other one on a dedicated 180 km-long inter-city fiber link in combination with b-Erbium-doped fiber amplification (EDFA)

Summary

Introduction

In the last twenty years, frequency metrology has progressively enabled the investigation of fundamental physics at new orders of magnitude. Attention has been paid to increasing the backbones robustness of metrological optical links and to ultra-long hauls bridging The compatibility of this technique with a Wavelength Division Multiplexing (WDM) approach has been explored, autonomous and compact optical lasers systems have been developed [19], alternative techniques for the remote phase-comparisons [29,30] are under study and other amplification schemes are being investigated as an alternative to b-EDFA. The experimental apparatus, the technical issues and the experimental results are described, demonstrating that DRA can be a viable alternative to other coherent amplification techniques and can be reliably adopted in long-haul metrological optical links, in WDM networks and in hybrid schemes in combination with bi-directional Erbium-doped fiber amplifiers

Common experimental set-up

Conclusions