Dual-correlated pumping scheme for phase-noise retention in FWM

Abstract

Summary form only given. Four-wave mixing (FWM) is a nonlinear process which is widely proposed to become the technique to be employed for all-optical wavelength conversion, because of its transparency to modulation format and bit rate, and its high efficiency. Transparency to modulation format is becoming increasingly important as the demand for capacity is pushing the optical communications industry towards the use of advanced modulation formats [1]. When FWM is used for all-optical wavelength conversion, it is important to understand the change in signal quality, from the signal (probe) to the converted wavelength (conjugate). In advanced modulation formats where information is encoded onto both the phase and amplitude of the optical carrier, the phase noise is a determining factor that influences the system performance as indicated by the bit error rate (BER). It has been shown that when two frequencies (w Pump and w Probe ) undergo FWM in a nonlinear medium to generate a new frequency (conjugate, w Stokes at the lower frequency and w anti-Stokes at the higher frequency), the phase noise relationship is given by [2], Δω Stokes/anti-Stokes = 4Δω Probe/Pump + Δω Pump/Probe (1) when ω Probe Pump . Δω represents the phase noise. Thus, the phase noise of the generated conjugate is always larger than that of the probe signal in the commonly used schemes. In this work, we propose a scheme, involving the FWM of two pumps with correlated phase noise with an uncorrelated probe signal and show that the conjugates satisfy the following phase noise relations, Δω Stokes = 4Δω Pump + Δω Probe (2a) Δω anti-Stokes = Δω Probe (2b) Thus, the generated conjugate frequency in the anti-Stokes side is found to retain the phase noise of the probe, which is ideal for the wavelength conversion of phase modulated data signals without any performance degradation. The experimental setup in Fig. 1(a) is used to validate the proposed scheme. Light from a laser source (Laser-1) is amplitude modulated in carrier suppressed configuration at 25 GHz to generate two correlated pumps with 50 GHz separation. This signal is combined with the probe (Laser-2) using a 3 dB coupler and passed through an SOA where it undergoes FWM. The FWM components are filtered using a filtering stage and analyzed using the linewidth measurement system detailed in [3]. Fig. 1(b) represents the FM noise spectrum for different FWM components. It is observed that the anti-Stokes component retains the phase noise of the probe (eqn (2a)) and Stokes component has a phase noise that follows the relationship detailed in eqn (2b).ΔωStokes/anti-Stokes = 4ΔωProbe/Pump + ΔωPump/Probe (1) when ωProbe<;ωPump. Δω represents the phase noise. Thus, the phase noise of the generated conjugate is always larger than that of the probe signal in the commonly used schemes. In this work, we propose a scheme, involving the FWM of two pumps with correlated phase noise with an uncorrelated probe signal and show that the conjugates satisfy the following phase noise relations, ΔωStokes = 4ΔωPump + ΔωProbe (2a) Δωanti-Stokes = ΔωProbe (2b) Thus, the generated conjugate frequency in the anti-Stokes side is found to retain the phase noise of the probe, which is ideal for the wavelength conversion of phase modulated data signals without any performance degradation. The experimental setup in Fig. 1(a) is used to validate the proposed scheme. Light from a laser source (Laser-1) is amplitude modulated in carrier suppressed configuration at 25 GHz to generate two correlated pumps with 50 GHz separation. This signal is combined with the probe (Laser-2) using a 3 dB coupler and passed through an SOA where it undergoes FWM. The FWM components are filtered using a filtering stage and analyzed using the linewidth measurement system detailed in [3]. Fig. 1(b) represents the FM noise spectrum for different FWM components. It is observed that the anti-Stokes component retains the phase noise of the probe (eqn (2a)) and Stokes component has a phase noise that follows the relationship detailed in eqn (2b).Even though white noise (frequency independent phase noise represented by flat region of FM noise spectrum) is more detrimental to coherent optical communication systems [4], we used the technique detailed in [3], to show that this phase noise relationship (better represented by phase-error variance) is true irrespective of the type of the phase noise (frequency independent/dependent). The detailed theoretical model describing the retention of phase noise of the signal will be presented, with a comparison of this scheme with the other FWM schemes.