Phase regeneration of optical signals

Abstract

We present recent advances in phase-sensitive amplification technology and its application to the regeneration of phase-encoded signals. Using a combination of parametric effects in fibers and optical injection locking of lasers, it is possible to observe phase regeneration in signals with multiple levels of phase encoding.

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Phase regeneration of optical signals

Periklis Petropoulos1,*, Joseph Kakande1, Radan Slavík1, Francesca Parmigiani1, Adonis Bogris2, 3, Dimitris Syvridis2,
and David J. Richardson1
1 Optoelectronics Research Centre, University of Southampton, Southampton SO17 1BJ, United Kingdom
2 National and Kapodistrian University of Athens, Panepistimiopolis, Illisia, Athens, GR-15784, Greece
3 Technological Educational Institute of Athens, Department of Informatics, Aghiou Spiridonos, 12210, Egaleo, Greece
* pp@orc.soton.ac.uk

Abstract- We present recent advances in phase-sensitive
amplification technology and its application to the regeneration of
phase-encoded signals. Using a combination of parametric effects
in fibers and optical injection locking of lasers, it is possible to
observe phase regeneration in signals with multiple levels of phase
encoding.
I.
INTRODUCTION
Signals travelling over long transmission distances in an
optical fiber generally suffer from noise, which is introduced
both directly from the optical amplifiers included in the
transmission link, as well as from the nonlinear interaction
between signals co-propagating in the same fiber. Signal
regenerators are therefore required to clear the transmitted
signals from the accumulated noise and condition them before
they can be re-transmitted. Signal regeneration is currently
performed in commercial systems using optical-electrical-
optical (O/E/O) conversion; such regenerators include a full
receiver and transmitter per optical channel. However,
considerations relating to the energy and cost efficiency of
O/E/O systems often make all-optical solutions attractive. The
use of optical nonlinearities in fibers has long been considered
for these purposes. A notable example is the Mamyshev-
regenerator which employs self-phase modulation in an optical
fiber followed by spectral filtering in order to regenerate
intensity modulated signals [1]. However, it is the regeneration
of phase-encoded signals that is becoming ever more
interesting in modern high-speed transmission systems, where
the use of such modulation formats is favored for the resilience
they offer to transmission-induced impairments and their
prospect for achieving a high spectral efficiency [2]. In this
case, phase noise represents a significant source for errors,
since it can directly distort the value of the transmitted
symbols.
Parametric effects in fibers, such as four-wave mixing, are
particularly interesting in this respect, since unlike self-/cross-
phase modulation, they can support phase modulation formats.
In addition, parametric effects can allow phase-sensitive
operation, i.e. it is possible to control the efficiency of the
mixing process between two or more waves by adjusting the
relation between their optical phases. In the work described
here, we demonstrate the regeneration of phase-encoded

This research was funded from the European Communities Seventh
Framework Programme FP/2007-2013 under grant agreement 224547
(PHASORS)
signals by making use of phase-sensitive amplification (PSA)
based on parametric processes in highly nonlinear optical
fibers (HNLFs). Suitable combinations of such processes allow
even the regeneration of signals with multiple levels of phase
encoding, by effectively quantizing the phase at the output of
the system to a corresponding number of levels.
II. BASIC PRINCIPLE
The difference between conventional (PI) and phase-
sensitive amplification can be conceptually understood by the
diagram shown in Fig.1. Whereas in a PI amplifier the intensity
of a signal experiences amplification regardless of its phase, in
a PSA only those components whose phase is aligned to a
certain reference value experience amplification. Any out-of-
phase components can be made instead to experience de-
amplification by the same amount. The significance of this
effect, e.g. for the regeneration of (differential) phase shift-
keyed ((D)PSK) signals becomes apparent in the diagram,
where only those signals with a phase aligned along the 0-π
axis are amplified, whereas any departure from this axis
(representing phase noise here) results in de-amplification.
Figure 1. Phase-insensitive (PI) versus phase-sensitive
amplification (taken from [3]).
978-1-61284-318-6/11/$26.00 ©2011 IEEE

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Parametric effects in optical fibers have long been studied
for their application in PSA operation (see e.g. [4]), and indeed
the proof-of-principle of regeneration of DPSK signals based
on these effects has been demonstrated [5]. The complexity in
the operation of PSA-based regenerators lies mainly on the fact
that all of the waves involved in the process need to be phase-
locked relative to each other. This requirement has usually
been addressed in laboratory demonstrations by generating all
of the signals together, e.g. in a separate PI optical parametric
amplifier (OPA). In contrast, the configurations described
below allow the pump sources to be independent of the signal,
therefore the PSA-based system can be located at a remote
node, as required in regeneration applications.
III. EXPERIMENTS
A generic block diagram of our regenerator is shown in
Fig.2. The PSA in the regenerator is a dual-pump fiber OPA.
The two waves acting as the pumps are generated locally, and
are phase-locked relative to each other and the incoming signal
by means of an additional PI-OPA stage. This mixes the light
from Pump laser 1 and the signal and generates a tone which is
phase-locked to these two and has the same wavelength as
Pump laser 2. The parametrically generated tone is then used to
optically injection-lock (OIL) Pump laser 2, which is
consequently also phase-locked to the other two waves [3, 6].
A programmable phase filter controls the phase of the signal at
the PSA input relative to that of the two pumps, thereby
controlling the gain and phase response of the output signal.
Gain saturation of the PSA ensures that both amplitude- as well
as phase-regeneration is achieved. The technique benefits
crucially by the use of dispersion-shifted HNLFs with a high
stimulated Brillouin scattering threshold as well as from
narrow linewidth laser sources that are used as the low-noise
pumps for the PSA.
By selecting the suitable harmonic from the parametric
mixing process (Fig.2) to be used as the tone to be injected to
Pump laser 2, it is possible to control the number of phase
levels allowed at the regenerator output, thereby adjusting the
system operation for different modulation formats [7]. This is
demonstrated in Fig.3, which shows example constellation
diagrams for operation either with DPSK or differential
quaternary phase shift keying (DQPSK) signals. The diagrams
show that even large amounts of phase noise can be mitigated.
IV. CONCLUSIONS
PSA techniques represent a powerful signal processing tool
for phase-encoded signals. Important developments in fiber
and laser technologies allow for the implementation of
practical systems.
ACKNOWLEDGMENT
The authors are grateful to Dr L. Gruner-Nielsen (OFS,
Denmark) and Dr R. Phelan (Eblana Photonics, Ireland) for
providing respectively the HNLFs and narrow linewidth
sources used in these experiments.
REFERENCES
[1] P. V. Mamyshev, "All-optical data regeneration based on self-phase
modulation effect," in ECOC'98, Madrid, pp. 475-476, 1998.
[2] A. H. Gnauck and P.J. Winzer, “Optical phase-shift-keyed transmission,”
J. Lightwave Technol., vol. 23, pp. 115-130, 2005.
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Andrekson, R. Weerasuriya, S. Sygletos, A. D. Ellis, L. Gruner-Nielsen,
D. Jakobsen, S. Herstrom, R. Phelan, J. O'Gorman, A. Bogris, D.
Syvridis, S. Dasgupta, P. Petropoulos, and D. J. Richardson, "All-optical
phase and amplitude regenerator for next-generation telecommunications
systems," Nature Photonics, vol. 4, pp. 690-695, 2010.
[4] C. McKinstrie and S. Radic, "Phase-sensitive amplification in a fiber,"
Opt. Express, vol. 12, pp. 4973-4979, 2004.
[5] K. Croussore, I. Kim, Ch. Kim, Y. Han, and G. Li, "Phase-and-amplitude
regeneration of differential phase-shift keyed signals using a phase-
sensitive amplifier," Opt. Express, vol. 14, pp. 2085-2094, 2006.
[6] S. Sygletos et al., “Phase locking and carrier extraction scheme for phase
sensitive amplification,” ICTON’10, paper Mo.C1.3, Munich, June 2010.
[7] J. Kakande, A. Bogris, R. Slavik, F. Parmigiani, D. Syvridis, P.
Petropoulos, and D. J. Richardson, "First Demonstration of All-Optical
QPSK Signal Regeneration in a Novel Multi-Format Phase Sensitive
Amplifier," in ECOC'2010, Turin, paper PD3.3 (Postdeadline), 2010.
Figure 2. Block diagram of the PSA-based regenerator.
Figure 3. Constellation diagrams at the input (top) and the output (bottom)
of the regenerator for DPSK (left) and DQPSK (right) signals.