  We propose a transmitter skew calibration based on direct detection of
coherent signals with estimation errors of +/-0.2ps, providing a reliable,
accurate and low-cost scheme to calibrate skew for coherent transceivers.
  In October 2019, this work was submitted / was exposed to Optical Fiber
Communication Conference 2020 but an acceptance was not granted. We claimed the
first time to use a direct detection-based feedback method for coherent
transmitter calibration.
