High Spectral Efficiency Fiber-Optic Transmission Systems Using Pilot Tones

Licentiatavhandling, 2018

Modern fiber-optic communication systems combine state-of-the-art components with powerful digital signal processing (DSP) to maximize the system spectral efficiency (SE). Systems rely on wavelength-division multiplexing, including superchannel transmission, to enable transmission over the available bandwidth which reaches about 10 THz when accounting for the so-called C and L bands. A superchannel is a set of densely packed wavelength channels viewed as a single unit. By treating the channels together, they can be packed more closely than what is normally feasible and sharing of resources among the channels within the superchannel can be considered.

In this thesis we focus on the special case of superchannels formed using coherent optical frequency combs. A frequency comb is a multi-wavelength light source and comb-based superchannels consists of channels which are modulated on lines originating from a common comb. Frequency combs have phase-locked carriers, meaning that in contrast to the standard case of independent lasers, the channels within a comb-based superchannel are locked on a frequency grid. Moreover, it implies that the carrier offsets originating from a non-ideal laser source are shared among all comb lines.

Shared carrier offsets can be exploited to reduce the complexity of the DSP used to effectively recover the data. A frequency comb is fully characterized by knowing the state of two of its lines, meaning that if this information is transferred to the receiver, one could compensate carrier offsets for all wavelength channels within the superchannel. By transmission of optical pilot tones, self-homodyne detection of a 50x20Gbaud PM-64QAM superchannel is demonstrated with 4% spectral overhead.

While two tones are required to fully phase-lock two combs, a single tone is enough to enable significant relaxation of the DSP-requirements while at the same time requiring minimal additional complexity compared to standard intradyne systems. Superchannel transmission using a single shared pilot tone is demonstrated by transmission of a 51x24Gbaud PM-128QAM superchannel with a resulting SE of 10.3bits/s/Hz. The single pilot scheme is also evaluated for distances up to 1000km showing high robustness to both noise and fiber nonlinearities. Finally, the high gain low overhead combination of the single pilot-tone scheme was used in a record demonstration reaching a SE of 11.5bits/s/Hz for fully loaded C-band transmission.