Multiple Beat-Noise Suppression in Polarization-Multiplexed Pump Light for Forward-Pumped Raman Amplifier

In this paper, we discuss the beat-noise induced by a distributed Raman amplifier system with forward pumping and propose a technique to suppress it. To construct a Raman amplifier, several techniques are required. First, the pump light must be depolarized due to the polarization dependence of the Raman gain. Second, the intensity noise of the pump light must be suppressed to stabilize the Raman gain. Due to the fast response of Raman amplifiers, especially forward-pumped, the instability of the pump power induces relative intensity noise (RIN) in the amplified signal light. To achieve depolarization, an orthogonally polarized pump light emitted from two different laser sources was widely used in conventional Raman pump units. These sources often have a fiber Bragg grating (FBG) to lock their wavelengths. In this paper, we show that a FBG can induce fluctuations in Raman gain. We also show that an orthogonally polarized pump laser light can induce beat noise in the amplified signal. We propose a pump unit that can suppress this beat noise. We measure the RIN of the amplified light using our proposed technique and demonstrate its advantages over a conventional Raman pump unit. Finally, we present experimental results of an optical data transmission with a forward-pumped Raman amplifier. Utilizing our proposed noise suppression technique, the signal-to-noise ratio of the probabilistically-shaped 36-QAM signal after a 1,920-km transmission is improved compared with the results using a conventional Raman pump unit.