Optical phase conjugation for high-spectrally efficient transmission

The capabilities of optical communication systems are steadily approaching their limits due to the intrinsic nonlinearity of optical fibers, which are manifested as difficult-tocompensate nonlinear signal distortions. The goal of the project is to compensate these distortions by optical signal processing (OSP) techniques, and to demonstrate improved systems with higher achievable data rates or extended transmission distances.

In particular, this work addresses the impairments with optical phase conjugation (OPC)-based compensation, which has emerged as a strong candidate method for suppressing the fiber nonlinearity. The technique relies on reversing and reapplying the nonlinear distortions throughout the link, though it has remained difficult to efficiently implement in practice.

The thesis is based on theoretical, numerical and experimental results to discuss the requirements of OPC-aided transmission, and it presents optimized system designs that are capable of outperforming standard link configurations. First, means of reversing the nonlinearity by OPC are studied and optimized.

The process is based on four-wave-mixing (FWM) in χ (3) nonlinear materials, namely integrated AlGaAs-on-insulator (OI) platform and strained highly nonlinear fiber (HNLF), whereas the optimization is performed with regard to the input signal parameters, as well as other system characteristics. The presented setups allow for reversing the accumulated nonlinearity with minimum penalty, and thus they are employed throughout the rest of the project.

Second, we focus on engineering the transmission link to induce identical distortions on either side of the OPC, which is generally referred to as the OPC propagation symmetry. On top of the standard methods, the demonstrated systems are optimized using dispersion management, and they exhibit close-to-ideal propagation symmetry.

Third, the OPC-based compensation is encapsulated into scaled-down lumped modules and applied to unrepeatered links at the transmitter- or receiver-side. For such systems, the degree of symmetry is measured by a new metric that is suitable for evaluation of non-homogeneous links, and this approach is applied to multiple setups, all of which demonstrate substantial improvement with the compensation modules included.

Ultimately, the contributions presented in this work introduce novel methods for designing OPC-aided systems that are resilient against the fiber nonlinearity, and they demonstrate that it is a viable technique for improving the performance of optical transmission.