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All-Optical DPSK Wavelength Converter
Based on MZI with Integrated SOAs and Phase Shifters
B. Sartorius (1), C. Bornholdt (1), J. Slovak (1), M. Schlak (1), Ch. Schmidt (1),
A. Marculescu (2), P. Vorreau (2), S. Tsadka (3), W. Freude (2), J. Leuthold (2),
1: FhG - Heinrich-Hertz-Institut, Einsteinufer 37, 10587 Berlin, Germany, [email protected]
2: University of Karlsruhe, Institute ofHigh-Frequency and Quantum Electronics, D-76131 Karlsruhe
3 : Kailigth Photonics, I- 76124 Revhot, Israel
Abstract: An integrated MZI-SOA device is developed for all-optical wavelength conversion
of DPSK signals. The functionality is investigated close to 40 Gb/s and error free operation
with less then 1 dB penalty is demonstrated.
OCIS codes: (060.45 10) Optical communications, (130.3 120) Integrated optics devices
Introduction
Differential Phase Shift Keying (DPSK) is increasingly becoming the communication format of choice for future
networks. DPSK offers improved sensitivity and robustness to nonlinear impairments [1], which makes it
particularly attractive for ultra-long haul applications. To further reduce cost in long-haul transmission networks,
transparent system approaches comprising reconfigurable optical add-drop multiplexers (ROADMs) are currently
investigated. In transparent networks wavelength conversion turns out to be the key to overcome wavelength
contention issues [2]. In this scenario all-optical conversion appears to be very advantageous - more then for
conventional Amplitude Shift Keying (ASK) formats - since DPSK receivers and transmitters are very expensive
and significantly more complex then the corresponding counterparts for ASK. All-optical solutions thus can promise
high benefits - if they utilize compact devices and simple structures. In this paper we present a semiconductor based
wavelength converter for DPSK signals in the 40 Gb/s range.
Wavelength conversion for DPSK formats
A first option for wavelength conversion of phase encoded formats is to use Four Wave Mixing (FWM) [3]. The
phase information is preserved using FWM, and thus the technique can be used for conversion of DPSK signals.
However, FWM needs very high power, and it not really provides regeneration. Concepts for real world systems
should focus on compact semiconductor devices with low power requirements. They should take into account the
developed knowledge and experience in ASK conversion. The best known solutions here exploit SOAs in
interferometer structures. However, control of SOAs requires ASK signals, and presently the interferometer
operation is only optimised for on-off operation. The question therefore is, if there is an easy way to upgrade
conventional MZI based all-optical wavelength converters for performing DPSK signal processing. Clearly, the
conversion of the DPSK signal to ASK pulses (e.g. using a delay interferometer DI) is needed for controlling the
SOAs, and the interferometer has to be operated for obtaining a phase modulated output. A first paper on that was
presented very recently in a post deadline session at the ECOC 2005 [4]. The differential scheme was applied for
controlling a hybrid interferometer, and conversion to a cw signal was demonstrated.
In this paper, we present an integrated MZI-SOA device developed especially for DPSK processing. DPSK to DPSK
wavelength conversion in the 40 Gb/s range (31 Gb/s) is demonstrated without applying the differential control
scheme. Conversion to an optical clock signal is chosen in order to include a 3R regenerative functionality.
Structure of the wavelength converter based on integrated SOA-MZI
The structure of the developed DPSK converter is shown in Fig. 1. In the input stage we use a Delay Interferometer
(DI) to convert the DPSK signal into ASK and inverted ASK signals. These on-off signals can be used for
controlling the SOAs. They are injected with exact time correlation into the two control inputs of a Mach Zehnder
Interferometer (MZI) with integrated SOAs. The SOAs are controlled either by a high level bit in the ASK or in the
inverse ASK signal, very similar to the push-pull operation of electrically controlled MZ modulators in DPSK
transmitters. It needs to be pointed out, that the symmetric scheme provides two ideal DPSK states with identical
amplitude and a perfect 7c-phase shift between the two arms even though there is an uneven gain in the two SOA
arms due to carrier depletion.
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All-Optical Wavelength Converter
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Fig. 1 Scheme of the DPSK wavelength converter.
A clock signal at the new output wavelength is injected into the centre arm of the interferometer. The phase
information is encoded in the MZI via the ASK / inverse ASK pulses on to the clock wavelength. The encoding of
the data signal is altered in this scheme according to an XOR function between subsequent bits, similar as described
in [4]. That has to be addressed in the precoding of the DPSK signal or by a post processing in the receiver.
The key device of the converter is the integrated SOA-MZI module (Fig.2) which was developed in view of this
application. Several challenges need to be mastered in order to get a high quality DPSK output instead of a simple
on-off function. First, ASK needs a phase shift of l2 TC for on-off modulation, while the DPSK needs twice the phase
shift for on-on modulation with phase inversion.
Fig. 2 Photograph of the integrated MZI module.
In order to improve the phase conversion efficiency, long SOAs (2 mm) are integrated into the device. While ASK
has only one state of output signal (and an optimized blocking operation point), DPSK needs two equivalent output
states, differing only in the phase. In order to balance the operation point of the interferometer independently of the
SOA currents we integrated phase tuning sections into the interferometer. The complete device was fabricated using
a polarization insensitive (<2dB) strained bulk buried heterostructure and butt coupling between SOAs and
waveguide [3]. The chip with tilted, AR coated facets was packaged into a module.
Functionality of the DPSK wavelength converter
To test the functionality of the DPSK converter, a DPSK signal was generated (eye diagram in Fig. 3a) and injected
into the converter. Fig. 3b shows the ASK and inverse ASK patterns behind the DI. The inverse nature of the
sequences can clearly be noticed. These signals were injected at a power of 3 dBm (in the fibre) into the MZI. The
quality of the wavelength converted DPSK signal behind the SOA-MZI was detected using a one bit DI / balanced
receiver module (provided by u2t) and analysed by a sampling oscilloscope and a BER receiver. In our experiments
we noticed speed limitations, caused by imperfections of the present device generation. The proof-of-concept
experiments were thus performed at 31 Gb/s.
Fig. 3c shows the eye diagram of the DPSK signal wavelength converted from 1545 rnm to 1560 nm. The eyes are
open, indicating an error free wavelength conversion. Similar results were obtained for other input wavelengths
within the C-band.
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Conclusions
An all-optical DPSK waverino40G/rter has been developed, applying a DI and compact semiconductors only.
Key device is an MZI with integrated long SOAs and phase shifters. Evaluation in this paper was perfo.93ed at 31
Gb/s. An error free (BER < 10-1°) wavelength conversion of DPSK signals with control power of only 3 dBm (in the
fibre) was demonstrated. A low penalty of < ldB indicates the good system performance of the converter. Speed
improvements on the device and investigations on regenerative features are topic of further work.
References
[M]A. Gnauck, "40 Gb/s RZ-Differential Phase Shift Keyed Transmission", OFC 2003, Atlanta, Georgia, USA
invited paper ThEC, Proc. vol.2, pp. 450 - 45 p.
[2] C. Nuzman, J. Leuthold, R. Ryf, S. Chandrasekhar, C.R. Giles, D.T. Neilson, " Design and Implementation of
Wavelength-Flexible Network Nodes", J. Lightwave Technol., vol. 2 1, pp. 648-663, March 2003.
[3] Z. Li, Y. Dong, J. Mo, Y. Wang, C. Lu, "Cascaded All-Optical Wavelength Conversion for R_Z-DPSK Signal
Based on Four-Wave Mixing in Semiconductor Optical Amplifier", Z. Li, Y. Dong, J. Mo, Y. Wang, C. Lu,
IEEE Photon. Technol. Lett. , vol. l16, pp. l1685-1l697, July 2004.
[4] I. Kang, C. Dorrer, L. Zhang, M. Rasras, L. Buhl, A. Bhardwaj, S. Cabot, M. Dinu, X. Liu, M. Capuzzo, L.
Gomez, A. Wong-Foy, Y.F. Chen, S. Patel, D.T. Neilson, J. Jaques, and C.R. Giles, " Regenerative All Optical
Wavelength Conversion of 40 Gb/s DPSK Signals Using a Semiconductor Optical Amplifier Mach-Zehnder
[5]~~~~4
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