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On the Mitigation of RIN Transfer and Transmission Performance Improvement in Bidirectional Distributed Raman Amplifiers | IEEE Journals & Magazine | IEEE Xplore

On the Mitigation of RIN Transfer and Transmission Performance Improvement in Bidirectional Distributed Raman Amplifiers


Abstract:

We develop a novel broadband first-order Raman pump for use as a forward pump in transmission experiments. Our results show significant signal relative intensity noise (R...Show More

Abstract:

We develop a novel broadband first-order Raman pump for use as a forward pump in transmission experiments. Our results show significant signal relative intensity noise (RIN) reduction, to a level comparable to backward only pumping. The corresponding optical signal to noise ratio can be improved in dual and first-order forward pumped 83.32 km bidirectional distributed Raman amplifiers by using the proposed broadband pump as a first-order pump. A detailed experimental characterization of RIN, signal power evolution and performance of a 10 × 120 Gb/s DP-QPSK coherent wavelength division multiplexing transmission system are presented. We report ~10 dB RIN reduction and 0.7 dB Q factor improvement, which allows a 1250 km transmission distance increase compared with conventional low RIN and narrowband first-order pump sources. We also demonstrate that, bidirectional pumping with only broadband first-order forward pumping at 50 mW shows the lowest RIN transfer from pump to signal. This extends the transmission reach up to 8332 km with maximum distances increased by 1250 and 1667 km compared with conventional backward only and first-order semiconductor forward pumped bidirectional pumping, respectively.
Published in: Journal of Lightwave Technology ( Volume: 36, Issue: 13, 01 July 2018)
Page(s): 2611 - 2618
Date of Publication: 23 March 2018

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I. Introduction

Distributed Raman amplifier (DRA) has many advantages in modern long-haul coherent dense wavelength division multiplexed (DWDM) transmission systems such as improved optical signal to noise ratio (OSNR) essential for higher order and spectrally efficient modulation formats [1], [2], flexible signal power profile along transmission span enabling efficient nonlinear compensation techniques i.e., optical phase conjugation (OPC) [3], [4] or nonlinear Fourier transform (NFT) [5] . Although DRA with bidirectional higher order Raman pumping [6], [7] is the best choice to maintain a quasi-lossless signal power profile with maximum reduction of amplified spontaneous emission (ASE) noise compared with conventional backward only pumping, RIN transfer [8], [9] from forward pump to the signal is a major drawback in these systems, which may counteract the benefit of improved OSNR. The stochastic intensity fluctuations in the Raman pump also induce relative phase noise through pump-signal cross phase modulation [10] and cross-polarization interactions in polarization division multiplexed coherent transmission systems. RIN transfer mainly depends on the amplitude noise level in the pump(s), pumping configurations and chromatic dispersion of the fibre which determine the walk-off between the pump and signal. There have already been many efforts in reducing the pump to signal RIN transfer such as: introducing high dispersion fibre [11], intensity modulation in dual order Raman pumping [9], forward pumping with incoherent broadband pump [12], [13] and dual order forward pumping with optimized low reflectivity fibre Bragg grating (FBG) at 1st order pump wavelength [14]. Recently we have proposed a RIN mitigation technique in a random distributed feedback (DFB) lasing based bidirectional, dual order DRA without any 1st order forward seed [15]. Although this technique shows improved transmission performance, it requires very high 2nd order forward pump power to transfer the gain efficiently to the signals which are two Stokes shift away from the pump. Using a low RIN, narrowband conventional 1st order forward pump seed (i.e., semiconductor laser diode) improves the overall pump efficiency but transmission performance may still be limited by the RIN transfer from high power 2nd order pump.

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