Methods and apparatus for feedback control of mode MUX and DEMUX

A method comprising:

• spatially demultiplexing, by a mode demultiplexer (DEMUX), a multi-mode signal into a set of wavelength division multiplexing (WDM) signals, wherein each WDM signal is in fundamental mode and is modulated with a pilot tone of different frequency;
• detecting the pilot tones from the set of WDM signals based on the pilot tone applied to each WDM signal;
• and
• tuning the mode DEMUX based on the detected pilot tones from the set of WDM signals;
• wherein the detecting comprises:
• demultiplexing, by a wavelength DEMUX, each WDM signal into the set of wavelength channels;
• tapping part of light comprising the pilot tone at each wavelength channel of the set of wavelength channels;
• and
• detecting, by a set of photodetectors (PDs), the tapped light;
• or
• wherein the detecting comprises:
• demultiplexing, by a wavelength DEMUX, each WDM signal into the set of wavelength channels,
• detecting, by a set of coherent receivers, the pilot tones at each wavelength channel;
• processing, by the set of coherent receivers, power levels of the detected pilot tones;
• and
• sending, by the set of coherent receivers to a digital signal processing (DSP), the processed power levels.

An apparatus comprising:

• a receiver;
• a processor;
• and
• a non-transitory memory storing machine executable instructions, which when executed by the processor, configure the apparatus for:
• for each input port of a mode multiplexer (MUX), modulating a wavelength division multiplexing (WDM) signal of a first set of WDM signals with a pilot tone of different frequency, wherein each WDM signal is in fundamental mode;
• spatially multiplexing, by the mode MUX, the first set of WDM signals to generate a multi-mode signal, the multimode signal comprising the pilot tones;
• detecting the pilot tones from the multi-mode signal, the detected pilot tones corresponding to the first set of WDM signals;
• and
• tuning the mode MUX based on the detecting:
• wherein the detecting comprises:
• tapping part of the multi-mode signal comprising the pilot tones;
• spatially demultiplexing, by a mode demultiplexer (DEMUX), the tapped part of the multi-mode signal into a second set of WDM signals, wherein each WDM signal of the second set of WDM signals is in fundamental mode;
• filtering, by a set of optical bandpass filters (OBPF), the second set of WDM signals;
• and
• detecting, by a set of photodetectors (PDs), the filtered set of WDM signals.