  Advanced digital signal processing techniques in combination with
ultra-wideband balanced coherent detection have enabled a new generation of
ultra-high speed fiber-optic communication systems, by moving most of the
processing functionalities into digital domain. In this paper, we demonstrate
how digital signal processing techniques, in combination with ultra-wideband
balanced coherent detection can enable optical frequency comb noise
characterization techniques with novel functionalities. We propose a
measurement method based on subspace tracking, in combination with
multi-heterodyne coherent detection, for independent phase noise sources
identification, separation and measurement. Our proposed measurement technique
offers several benefits. First, it enables the separation of the total phase
noise associated with a particular comb-line or -lines into multiple
independent phase noise terms associated with different noise sources. Second,
it facilitates the determination of the scaling of each independent phase noise
term with comb-line number. Our measurement technique can be used to: identify
the most dominant source of phase noise; gain a better understanding of the
physics behind the phase noise accumulation process; and confirm, already
existing, and enable better phase noise models. In general, our measurement
technique provides new insights into noise behavior of optical frequency combs.
