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Pump-phase-noise-free optical wavelength data exchange between QAM signals with 50-GHz channel-spacing using coherent DFB pump

Lu, G.-W. ; Drummond, M. V. ; Albuquerque, A. ; Puttnam, B. J. ; Sakamoto, T. ; Nogueira, R.N. ; Kanno, A. ; Shinada, S. ; Wada, N. ; Kawanishi, T.

Optics Express Vol. 24, Nº 4, pp. 3702 - 3713, February, 2016.

ISSN (print): 1094-4087
ISSN (online):

Journal Impact Factor: 3,488 (in 2014)

Digital Object Identifier: 10.1364/OE.24.003702

Abstract
An important challenge for implementing optical signal
processing functions such as wavelength conversion or wavelength data
exchange (WDE) is to avoid the introduction of linear and nonlinear phase
noise in the subsystem. This is particularly important for phase noise
sensitive, high-order quadrature-amplitude modulation (QAM) signals. In
this paper, we propose and experimentally demonstrate an optical data
exchange scheme through cascaded 2nd-order nonlinearities in periodicallypoled
lithium niobate (PPLN) waveguides using coherent pumping. The
proposed coherent pumping scheme enables noise from the coherent pumps
to be cancelled out in the swapped data after WDE, even with broad
linewidth distributed feedback (DFB) pump lasers. Hence, this scheme
allows phase noise tolerant processing functions, enabling the low-cost
implementation of WDE for high-order QAM signals. We experimentally
demonstrate WDEs between 10-Gbaud 4QAM (4QAM) signal and 12.5-
Gbaud 4QAM (16QAM) signal with 3.5-MHz linewidth DFB pump lasers
and 50-GHz channel spacing. Error-free operation is observed for the
swapped QAM signals with coherent DFB pumping whilst use of freerunning
DFB pumps leads to visible error floors and unrecoverable phase
errors. The phase noise cancellation in the coherent pump scheme is further
confirmed by study of the recovered carrier phase of the converted signals.
In addition to pump phase noise, the influence of crosstalk caused by the
finite extinction ratio in WDE is also experimentally investigated for the
swapped QAM signals.