Experimental Investigation on the Effect of Central Wavelength Tuning of FBG-Based Phase Shifter for Raman-Assisted Phase Sensitive Amplifier

A FBG-based pump-phase-shifter is used in the Raman-assisted PSA. By actively tuning the FBG central-wavelength to enable pump phase optimization, up-to-5.6dB signal gain is observed. An improvement of ~6% EVM and ~4dB system sensitivity is observed by 20/25-Gbaud QPSK signals. Introduction Capacity upgrade utilizing higher level modulation formats such as 64QAM and 256QAM has been studied aggressively. Such higher level formats need sufficient OSNR to maintain the transmission reach. Phase sensitive amplifier (PSA) is one of potential candidates due to the below 3dB noise figure 1,2 . In order to achieve the optimal performance of PSA, an optical phase shifter is necessary to ensure phase matching among PSA pump, signal and idler, which could be realized by a waveshaper. However, the insertion loss of a typical waveshaper might decrease the PSA gain by reducing the power of both signal and pump. On the other hand, Bragg grating has many applications such as tunable filter 3 , chromatic dispersion compensator 4 and sensor 57 . In addition, a Fiber Bragg grating (FBG) with 0.4dB insertion loss has been used as a pumpphase-shifter in a Raman-assisted PSA system, where more than 20dB signal net gain has been experimentally demosntrated 8 . However, in the previous research, the FBG central wavelength is fixed. Therefore, once the pump wavelength is determined, the FBG induced phase shift is also fixed. This is a potential limitation for the practical implementation, in which phase matching condition may degrade due to some reasons such as system component aging. Therefore, it might be valuable to actively tune the FBG central wavelength, enabling more flexible pump phase adjustment. In this paper, the temperature of a FBG-based phase shifter is tuned to investigate the effect of FBG central wavelength shift on the phase matching condition in the Raman-assisted PSA system. A FBG central wavelength shift up to 0.66nm is observed when increasing the temperature by 60 o C. With FBG central wavelength tuning, up to 5.6dB signal gain improvement is observed. For a 20 Gbaud QPSK signal, the EVM is decreased by ~6%. In addition, ~4dB system sensitivity improvement is experimentally demonstrated for both 20/25 Gbaud QPSK signals. Concept and Experimental Setup Figure 1 shows the conceptual diagram. In order to suppress the noise on the signal wavelength at the idler generation stage, the PSA pump power is adjusted to have moderate conversion efficiency (~-10dB). The power imbalance is compensated by the higher Raman gain on the idler (I) which is achieved by placing signal (S) away from Raman gain profile. By locating PSA pump wavelength close to the FBG central (Bragg) wavelength but detuning from FBG bandwidth, only the phase of PSA pump can be affected by the FBG. Based on the thermoelectric effect, current injection changes the temperature of FBG, which in turn shifts the FBG central wavelength to optimize the phase of PSA pump. Therefore, the phase matching condition is achieved in the following PSA stages, enabling optimized system performance. Fig. 1: Concept of Raman-assisted PSA enabled by the FBG-based phase shifter. The phase adjustment of PSA pump is achieved by FBG central wavelength tuning based on thermoelectric effect. The power imbalance between signal and idler is compensated by placing the signal away from Raman gain profile. Figure 2(a) shows the experimental setup. At the transmitter, the signal modulates a laser at the wavelength of 1569.8nm through a QPSK modulator. An attenuator (ATT-1) is placed afterwards to change the input signal power to the system. A PSA pump at the wavelength of PSA Pump S I HNLF HNLF PSA Pump S I Phase Adjustment of PSA Pump PSA Pump