  The routing and wavelength assignment with protection is an important problem
in telecommunications. Given an optical network and incoming connection
requests, a commonly studied variant of the problem aims to grant maximum
number of requests by assigning lightpaths at minimum network resource usage
level, while ensuring the provided services remain functional in case of a
single-link failure through dedicated path protection. We consider a
practically relevant version where alternative lightpaths for requests are
assumed to be given as a precomputed set, and show that it is NP-hard. We
formulate the problem as an integer programming (IP) model, and also use it as
a foundation to develop a novel quadratic unconstrained binary optimization
(QUBO) model, which can be both directly solved by a state-of-the-art solver
like GUROBI. We present necessary and sufficient conditions on objective
function parameters to prioritize request granting objective over
wavelength-link usage for both models, and a sufficient condition to ensure the
exactness of the QUBO model. Moreover, we implement a problem-specific
branch-and-cut algorithm for the IP model, and employ a new quantum-inspired
technology, Digital Annealer (DA), for the QUBO model. We conduct computational
experiments on a large suite of instances that are hard to optimally solve in
order to assess the efficiency and efficacy of all of these approaches as well
as a problem-specific heuristic. The results show that the emerging technology
DA outperforms the considered established techniques coupled with GUROBI, in
finding mostly significantly better or as good solutions in only two minutes
compared to two hours of run time, whereas the problem-specific heuristic fails
to be competitive.
