  Superchannels leverage the flexibility of elastic optical networks and pave
the way to higher capacity channels in space division multiplexing (SDM)
networks. A superchannel consists of subchannels to which continuous spectral
grid slots are assigned. To guarantee superchannel operation, we need to
account for soft failures, e.g., laser drifts causing interference between
subchannels, wavelength-dependent performance variations, and filter
misalignments affecting the edge subchannels. This is achieved by reserving
spectral guardband between subchannels or by employing a lower modulation
format. We propose a process that dynamically retunes the subchannel
transmitter (TX) lasers to compensate for soft failures during operation and
optimizes the total capacity or the minimum subchannel quality of transmission
(QoT) performance. We use an iterative stochastic subgradient method that at
each iteration probes the network and leverages monitoring information,
particularly subchannels signal-to-noise ratio (SNR) values, to optimize the TX
frequencies. Our results indicate that our proposed method always approaches
the optima found with an exhaustive search technique, unsuitable for operating
networks, irrespective of the subchannel number, modulation format, roll-off
factor, filters bandwidth, and starting frequencies. Considering a
four-subchannel superchannel, the proposed method achieves 2.47 dB and 3.73 dB
improvements for a typical soft failure of +/- 2 GHz subchannel frequency
drifts around the optimum, for the two examined objectives.
