Cancellation of all Kerr nonlinearities in long fiber spans using a LiNbO/sub 3/ phase conjugator and Raman amplification

Abstract

Kerr nonlinearities can cause severe distortions in fiber transmission systems. The penalties introduced by these nonlinearities rise as the transmission capacity is pushed to its limits. Ken nonlinearities can in principle be cancelled using a device with negative n2 or by mid-span optical phase conjugation (OPC) at alternate repeater sites 1, 2. The latter has been used only in limited instances because of a fundamental issue: OPC can compensate for Kerr nonlinearities only in the absence of dispersion slope and when the OPC device is followed by a fiber with negative absorption 3. The last requirement is obviously unphysical, and therefore the only choice was to reduce absorption by using short fiber spans. A more severe limitation is met when long dispersive fiber spans are used with relatively higher bit rates and when one attempts to cancel cross phase modulation (XPM). Under these conditions, dispersion effects combined with a lack of symmetric power distribution with respect to the OPC device makes the distortion introduced by Kerr-nonlinearities irreversible. that compensates both for Kerr nonlinearities and fiber dispersion. The technique is based on a compact LiNb03 phase conjugator combined with Raman amplification. The demonstration is z Z performed up to at a data rate of 100 Gb/s, which In this paper we demonstrate a novel scheme P is the highest reported data rate for any transmis-~ LINb03 OPC sion experiment with OPC. We show cancella-F, tion of all Kerr-nonlinearities, four-wave mixing, self-phase modulation (SPM) and cross-phase D,a,n, (XPM), in long fiber 'pans Fig. 1: Our Ke,-cancellation scheme: Raman counter-pumping in the second fiber span provides aOur technique uses counterpropagating Raman decreasing absorption needed for canceling any nonpumping in all fiber spans or at alternate fiber linear phase using phase conjugation. spans in conjunction with OPC. This creates a good approximation to a symmetric power distribution profile that allows taking full advantage of the time reversal property of OPC. Fig. 1 shows this arrangement; this unit can then be repeated many times in order to reach the total desired transmission length. The nonlinear phase accumulated over the absorption length (shaded area) in the first fiber span will be compensated by the nonlinear phase accumulated over a comparable length towards the end of the second span (where the bits in the WDM channels achieve their original relative timing too).We first test the proposed scheme by canceling FWM in Tmewave fiber. The experimental setup is the same as shown in Fig. 1. We use two 75 km long fiber spans and with zero dispersion wavelengths (Lo) chosen such that the channels and their phase conjugated counterparts always overlap the region of zero .

Spectral Efficiency Limits and Modulation/Detection Techniques for DWDM Systems

“…The impact of Kerr nonlinearity can be reduced or canceled using phase conjugation [41], [42]. In DWDM systems with many channels, Kerr effect compensation reduces or cancels the nonlinear terms originating from other channels.…”

Optical phase conjugation for ultra long-haul phase-shift-keyed transmission

“…It has been shown in [17] that the compensation of the Kerr effect is optimal when a powersymmetric transmission link is realized. The average net Raman gain in this experiment is −10.3 dB.…”

“…Impairments experienced in the first part of the link are then cancelled out with the impairments in the second part of the transmission link. It has been experimentally shown that OPC can compensate for chromatic dispersion [11]- [15], self-phase modulation (SPM), [16] and also intrachannel nonlinear impairments [17]- [19]. The key advantages of OPC are that simultaneous processing of multiple channels is possible [20] and that an OPC is transparent to modulation format and data rate [21].…”

Integrated Devices for Terabit per Second 1.3 and 1.5 Micron WDM/TDM Network Applications

“…Secondly, most components are commercially available making the system cost effective. Thirdly, the idler pulses, instead of being filtered out, could be utilized for mid-span wavelength conversion to extend the transmission distance [31,32], which is an advantage of the present scheme over the XPM-induced mode locking based clock recovery scheme [22].…”