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All-Optical DPSK Wavelength ConverterBased 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, sartorius@hhi.Jhg.de2: University ofKarlsruhe, 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 conversionofDPSK signals. The functionality is investigated close to 40 Gb/s and error free operationwith less then 1 dB penalty is demonstrated.

OCIS codes: (060.45 10) Optical communications, (130.3 120) Integrated optics devices

IntroductionDifferential Phase Shift Keying (DPSK) is increasingly becoming the communication format of choice for futurenetworks. DPSK offers improved sensitivity and robustness to nonlinear impairments [1], which makes itparticularly 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 currentlyinvestigated. In transparent networks wavelength conversion turns out to be the key to overcome wavelengthcontention issues [2]. In this scenario all-optical conversion appears to be very advantageous - more then forconventional Amplitude Shift Keying (ASK) formats - since DPSK receivers and transmitters are very expensiveand significantly more complex then the corresponding counterparts for ASK. All-optical solutions thus can promisehigh benefits - if they utilize compact devices and simple structures. In this paper we present a semiconductor basedwavelength converter for DPSK signals in the 40 Gb/s range.

Wavelength conversion for DPSK formatsA first option for wavelength conversion of phase encoded formats is to use Four Wave Mixing (FWM) [3]. Thephase 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 systemsshould focus on compact semiconductor devices with low power requirements. They should take into account thedeveloped knowledge and experience in ASK conversion. The best known solutions here exploit SOAs ininterferometer structures. However, control of SOAs requires ASK signals, and presently the interferometeroperation is only optimised for on-off operation. The question therefore is, if there is an easy way to upgradeconventional MZI based all-optical wavelength converters for performing DPSK signal processing. Clearly, theconversion of the DPSK signal to ASK pulses (e.g. using a delay interferometer DI) is needed for controlling theSOAs, and the interferometer has to be operated for obtaining a phase modulated output. A first paper on that waspresented very recently in a post deadline session at the ECOC 2005 [4]. The differential scheme was applied forcontrolling 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 DPSKwavelength conversion in the 40 Gb/s range (31 Gb/s) is demonstrated without applying the differential controlscheme. 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-MZIThe 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 forcontrolling the SOAs. They are injected with exact time correlation into the two control inputs of a Mach ZehnderInterferometer (MZI) with integrated SOAs. The SOAs are controlled either by a high level bit in the ASK or in theinverse ASK signal, very similar to the push-pull operation of electrically controlled MZ modulators in DPSKtransmitters. It needs to be pointed out, that the symmetric scheme provides two ideal DPSK states with identicalamplitude and a perfect 7c-phase shift between the two arms even though there is an uneven gain in the two SOAarms due to carrier depletion.

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All-Optical Wavelength ConverterDPSK )n ASKkn1DPSK Xout

'-Xn inDPSK Dl D

| ~~~ASK X-inFig. 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 phaseinformation is encoded in the MZI via the ASK / inverse ASK pulses on to the clock wavelength. The encoding ofthe data signal is altered in this scheme according to an XOR function between subsequent bits, similar as describedin [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 thisapplication. Several challenges need to be mastered in order to get a high quality DPSK output instead of a simpleon-off function. First, ASK needs a phase shift of l2 TC for on-off modulation, while the DPSK needs twice the phaseshift 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 ASKhas only one state of output signal (and an optimized blocking operation point), DPSK needs two equivalent outputstates, differing only in the phase. In order to balance the operation point of the interferometer independently of theSOA currents we integrated phase tuning sections into the interferometer. The complete device was fabricated usinga polarization insensitive (

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Fig. 3a Eytei t input DPSK Fig. 3b ASK and inverse ASK signal behind Fig. 3c Eye diagram of the converted DPSKsignal. the delay interferometer DI. signal.I [\l I I |-*-~~b-2-;blIBERperformance good syte pe nconvertcde o tAt last the quality of the converted signal wasi3nevaluated by BER measurementscwe g. 42 40 3 - - -32summarizes the results. The BER curve for the | t | i 4 B p o t input ignalconverted DPSK signals shows no error floor down convertdoD gnal.t oa BERof 10-00.A nerrorfree wavelength o D l w l l tconversion dS obtaraed. For comparlson the BERi t g s p o t ccurve of the input DPSK signal is also given. Are ai fe a t o f worksmall negative penalty of s dB at 10- (and even lessfor higher BER) compared to the high quality input K T s i O 2 Atl G USAsignal indicates the good system perfor ance of thew avelengthconverter. Similar results were obtained 10 ,-pp.-48-663 M 2003.also forcLi,eY ong,from other input wavelengths C fo R- SignalI 42 40 -38 -36 _34 ~-32I

Fig. 4 BER performance of the input signaland the converted DPSK signal.

ConclusionsAn 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 31Gb/s. An error free (BER < 10-1) wavelength conversion ofDPSK signals with control power of only 3 dBm (in thefibre) was demonstrated. A low penalty of < ldB indicates the good system performance of the converter. Speedimprovements 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. 21, 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 OpticalWavelength Conversion of 40 Gb/s DPSK Signals Using a Semiconductor Optical Amplifier Mach-Zehnder

[5]~~~~4T.I-- Tekin, C.-Bornoldt DP.-lvk M. -clk B. 'Ptoiu, J. Krisl S. Bauer C. Bobet W. Brnkr IB._