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Fibre-Brillouin-Amplifiers

Fibre-Brillouin-Amplifiers

Brillouin amplification for long-distance optical frequency transfer has been investigated at PTB since 2010 [1]. It is an enabling technology for achieving world-record distances of metrological optical fiber links [2,3]. In these cases, fibre Brillouin amplification was realized in a laboratory setup.

 

Schematics of Brillouin amplification Schematics of Brillouin amplification

Stimulated Brillouin amplification is a distributed process and relies on the interaction of intense light with stimulated acoustic phonons: Depending on their frequency offset, the interference of counter-propagating pump light and signal light gives rise to a moving density grating. The density grating coherently scatters pump photons into the signal beam, which thereby is amplified. For silica fiber, the required frequency offset typically is around 11 GHz.

 

Gain curve of a fibre Brillouin amplifier (laboratory measurement). Gain curve of a fibre Brillouin amplifier (laboratory measurement).

Characteristics of fibre Brillouin amplification are a narrow bandwidth and a high small-signal gain. With a bandwidth of around 10 MHz, Brillouin amplification has a “built-in” narrow optical (and directional) filter. This allows to specifically amplify e.g. the frequency-shifted return signal in fiber links, as opposed to backscatter from the outgoing signal. In contrast to broad-band amplifiers, s.a. Erbium doped fiber amplifiers (EDFA), this allows taking advantage of a single stage gain of up to 50 dB or even more.

 

Results from optical frequency transfer employing a remote fibre Brillouin amplifier. Shown here is the short-term performance of the transfer instability (derived from [4]). Results from optical frequency transfer employing a remote fibre Brillouin amplifier. Shown here is the short-term performance of the transfer instability (derived from [4]).

Drawing on the experience gained in previous laboratory setups, field-able Brillouin amplifiers are currently developed within working group 4.34 as part of the European project “NEAT-FT” (EMRP programme). Recently, we were able to demonstrate the first optical link worldwide which employs a largely autonomous fibre Brillouin amplifier [4], located remotely in a server room. We were able to bridge distances of up to 250 km without intermediate amplification and realized continuous measurements of days. Over a distance of 660 km we achieved an accuracy and instability of the frequency transfer of 10-19. This will allow comparing the most advanced frequency standards and clocks, e.g. for the purpose of relativistic geodesy.

 

  1. O. Terra, G. Grosche, and H. Schnatz, “Brillouin amplification in phase coherent transfer of optical frequencies over 480 km fiber,” Opt. Exp. 18, 16102 (2010).
  2. K. Predehl, G. Grosche, S. M. F. Raupach, S. Droste, O. Terra, J. Alnis, Th. Legero, T. W. Hänsch, Th. Udem, R. Holzwarth, and H. Schnatz, “A 920-Kilometer Optical Fiber Link for Frequency Metrology at the 19th Decimal Place,” Science 336, 441 (2012).
  3. S. Droste, F. Ozimek, Th. Udem, K. Predehl, T. W. Hänsch, H. Schnatz, G. Grosche, and R. Holzwarth, “Optical-Frequency Transfer over a Single-Span 1840 km Fiber Link,“ Phys. Rev. Lett. 111, 110801 (2013).
  4. S. M. F. Raupach, A. Koczwara, and G. Grosche, "Optical frequency transfer via a 660 km underground fiber link using a remote Brillouin amplifier," Opt. Express 22, 26537-26547 (2014).