This example illustrates all-optical implementation of wavelength conversion (WC) and optical phase conjugation (OPC) by means of four-wave-mixing (FWM) in a highly nonlinear fiber (HNLF). WC and OPC have been shown to aid traffic grooming  and help compensate fiber nonlinearities , respectively, and thus they are potentially important blocks of future networks. Although both functionalities are easy to implement digitally, this optical approach offers several advantages, e.g. it is unlimited by the bandwidth of electrical components and also introduces only negligible latency due to processing.
The conversion principle is demonstrated for a single DP-16QAM signal at 32 GBd. The signal is generated using a standard transmitter, subjected to the WC/OPC process, and detected using a coherent receiver followed by DSP. In the end, the wavelength-shifted/conjugated copy of the signal is evaluated using EVM to estimate the conversion penalty.
The complete system is demonstrated in , and it is based on a bi-directional loop for polarization insensitive operation . First, DP-16QAM signal is generated around 194.37 THz using a coherent dual-pol. transmitter, and it is mixed with a high-power pump at 194.17 THz. The bands of the signal and the pump are joined together, and the waveforms are upsampled to enable broadband analysis. This is the input to the converter, and it is shown in . Then, the dual-pol. signal and the single-pol. pump are injected into a bi-directional loop via a polarization beam splitter (PBS), where they split into two counter-propagating orthogonally-polarized waves. FWM is induced between co-polarized and co-propagating signal and pump components inside the HNLF, and it can occur in either direction. For polarization-insensitive operation, it is required that the pump is split equally into two halves at the PBS (i.e. it is at 45 deg.), such that the signal exhibits the same conversion efficiency in both directions. At the output of the loop, polarization is re-optimized again, as the waves recombine in the second PBS. Because of FWM inside the HNLF, new waves are generated alongside the signal and the pump, as illustrated in . The positions of the waves are determined by the relative pump-signal separation, while the efficiency of the process depends on the HNLF properties . The wave at 193.97 THz is called the idler, and it is a wavelength-shifted copy of the signal, with its complex field amplitude conjugated. Subsequently, the idler is selected by means of optical filtering, amplified using an EDFA and detected with a coherent receiver followed by a DSP chain and EVM estimation. In the end, the output idler is compared against the input signal, thus it has to be digitally conjugated-back to match.
This is a standard way of implementing all-optical WC and/or OPC both numerically and experimentally. The converter must still be optimized to maximize the quality of the idler, which is typically evaluated with respect to the input signal and pump powers, as illustrated in . There, rising the pump power to higher values is shown to improve the idler performance because it increases the FWM efficiency of the process. On the other hand, increasing the signal power helps early on by limiting the impact of the ASE noise, though it enhances the Kerr-induced distortions later, causing a steep increase in EVM at higher powers. For this system, the converter yields the best performance at the input signal power of around 15 dBm, for which a balance between the linear and the nonlinear penalties is achieved.
Keywords: Wavelength conversion, Optical phase conjugation, Optical signal processing, Kerr effect, Chromatic dispersion, Four-wave-mixing, Highly nonlinear fibers, Coherent communications
Similar demonstrations are available in VPItransmissionMaker Optical Systems and on the VPIphotonics Forum.
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