IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 23, NO. 22, NOVEMBER 15, 2011 1649 Phase-Incoherent DQPSK Wavelength Conversion Using a Photonic Integrated Circuit Marios Bougioukos, Thomas Richter, Christos Kouloumentas, Vasilis Katopodis, Robert Harmon, David Rogers, James Harrison, Alistair Poustie, Graeme Maxwell, Colja Schubert, and Hercules Avramopoulos Abstract—We investigate the performance of a large-scale, silica-on-silicon photonic integrated circuit for multiformat signal processing, and we experimentally demonstrate wavelength-con- version of (differential) quadrature phase-shift keying [(D)QPSK] signals. The circuit exploits phase-incoherent techniques to decode the input signal and to phase remodulate two phase-shift-keying components before combining them in a common QPSK output stream. Error-free wavelength conversion with 4-dB power penalty is reported at 44 Gb/s. Index Terms—Differential quadrature phase-shift-keying (DQPSK) wavelength conversion, hybrid integration, Mach–Zehnder, phase-incoherent processing. I. INTRODUCTION A LL-OPTICAL wavelength ( )-conversion will be a key for extended transparency, higher capacity and improved energy efficiency in future networks. Ideally, the -converters should be simple and cost effective, exhibit low consump- tion and small footprint, and be capable of handling different formats, as phase encoded signals, in particular (differential) phase-shift keying ((D)PSK) and quadrature phase-shift keying ((D)QPSK) signals, are going to copropagate and interface with on-off keying (OOK) data streams in modern networks. Compared to the OOK and (D)PSK cases, the efforts on -conversion techniques for (D)QPSK signals are more recent and have resulted in a limited number of demonstrations. These include schemes based on effects inside periodically poled lithium niobate crystals [1], nondegenerated four-wave mixing (FWM)-based schemes inside Kerr media [2], [3] and semiconductor optical amplifiers (SOAs) [4], and schemes that coherently extract the in-phase ( ) and quadrature ( ) compo- nents and generate a new QPSK signal using two SOA-based Manuscript received June 01, 2011; revised July 30, 2011; accepted August 05, 2011. Date of publication August 15, 2011; date of current version October 21, 2011. This work was supported by the EU projects APACHE (Contract 224326) and EURO-FOS (Contract 224402). M. Bougioukos, C. Kouloumentas, V. Katopodis, and H. Avramopoulos are with the Photonics Communications Research Laboratory, National Technical University of Athens, Department of Electrical and Computer Engi- neering, 15773, Zografou, Athens, Greece (e-mail: mpougiou@mail.ntua.gr; ckou@mail.ntua.gr; vkat@mail.ntua.gr; hav@mail.ntua.gr). T. Richter and C. Schubert are with the Fraunhofer Institute for Telecom- munications, Heinrich Hertz Institute, Department of Photonic Networks and Systems, 10587 Berlin, Germany (e-mail: thomas.richter@hhi.fraunhofer.de; Colja.Schubert@hhi.fraunhofer.de). R. Harmon, D. Rogers, J. Harrison, A. Poustie, and G. Maxwell are with The Center for Integrated Photonics Technologies, Suffolk, Ipswich, IP5 3RE, U.K. (e-mail: bob.harmon@ciphotonics.com; dave.rogers@cipho- tonics.com; jim.harrison@ciphotonics.com; alistair.poustie@ciphotonics.com; graeme.maxwell@ciphotonics.com). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2011.2164903 Fig. 1. Operating principle of the (D)QPSK wavelength-converter. Mach–Zehnder interferometers (SOA-MZIs) [5], [6]. The first two types cannot regenerate any signal format and require high input powers. The latter schemes are inherently complex, as they need a locked local oscillator, while their regenerative potential is still to be confirmed for (D)QPSK signals. Recently, we presented a multiformat processing chip (MFPC) with two SOA-MZIs and a linear front end with delay interferometers (DIs) that uses simpler phase-incoherent techniques [7]. Using this device we showed OOK and DPSK regeneration at 22 Gb/s and indicated through simulations the potential for (D)QPSK -conversion [8]. With the present work, we complete the study of the MFPC demonstrating -conversion of 44 Gb/s DQPSK signals with 4 dB power penalty. To our knowledge, this is the first demonstration of (D)QPSK -conversion using phase-incoherent concepts. II. OPERATION PRINCIPLE AND EXPERIMENTAL SETUP Fig. 1 describes the -conversion principle. The (D)QPSK stream at is decoded by two 1-symbol DIs that operate with a relative 90 difference to recover the and components. The complementary OOK outputs of each DI are forwarded to the control ports of the SOA-MZIs with their relative timing preserved. Each SOA-MZI modulates the phase of the clock pulses according to the data of the controls, and provides a PSK signal at [8]. A phase shifter (PS) allows for a 90 differ- ence between the clock pulses entering the upper and the lower SOA-MZI, thus enabling a QPSK signal at the final output after combination of the two PSK signals. The scheme is mostly ap- propriate for return-to-zero (RZ) DQPSK streams, and it is in principle compatible with ultra high-speed signals provided that appropriate push-pull techniques are used. Moreover, it can also facilitate in principle operation with non return-to-zero (NRZ) DQPSK signals at lower symbol rates if the optical clock is re- placed by a continuous wave (CW) source. It is noted that the -converter alters the data of the output signal compared to the input data, as it retrieves the differen- tially encoded data sequences of the input and phase remodu- lates the optical clock without the use of a differential precoder. 1041-1135/$26.00 © 2011 IEEE