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Current optical transport networks use optical channel carriers (wavelengths) that are defined and constrained by a fixed ITU-T dense wavelength division multiplexing (DWDM) grid. Such a grid is not adapted to high data rates (beyond 100 Gb/s) and is inefficient when a wavelength is assigned to a low-rate optical signal. Consequently, the ITU-T is updating the set of DWDM reference frequencies with the inclusion of a smaller channel spacing (e.g., 6.25 GHz) while allowing the allocation of frequency slots, that is, variable-sized frequency ranges composed of a number of slices. In this paper, we propose the design, implementation, and experimental validation of a Generalized Multi-Protocol Label Switching/path computation element (GMPLS/PCE) control plane for such flexible optical networks, using optical orthogonal frequency division multiplexing transmission technology, given its unique flexibility, bit-rate/bandwidth scalability, and subwavelength granularity. The control plane uses a distance-adaptive and PCE-based routing and modulation assignment, combined with distributed frequency slot (spectrum) selection. A comparative analysis of path computation algorithms is carried out, highlighting the benefits of extending the path computation function with the knowledge of the status of the slices and the spectral efficiency of the modulation formats.