IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 19, NO. 10, MAY 15, 2007 783 All-Optical Asynchronous Serial-to-Parallel Converter Circuit for DPSK Optical Packets Nicola Calabretta, Marco Presi, Giampiero Contestabile, and Ernesto Ciaramella, Member, IEEE Abstract—We propose a novel asynchronous all-optical circuit for extraction and serial-to-parallel conversion of label bits from differential phase-shift keying (DPSK) packets. The circuit re- quires only two optical switches regardless of the number of bits to be extracted and parallelized from the packet. Experimental evidence of practical use of the circuit to four bit labels at 10 Gb/s is provided. The circuit is scalable with the number of bits, oper- ates at low input power, and is suitable for photonic integration. The asynchronous nature of the circuit allows us to efficiently extract/read one specific label field of variable length without processing the entire label, leading to a simplified architecture of the label processing circuit. Index Terms—Differential phase-shift keying, header/label processor, optical packet switching (OPS), self-synchronization, serial-to-parallel conversion (SPC), wavelength conversion. I. INTRODUCTION I NTERNET-BASED data traffic generated by advanced telecommunication services and applications boosted the photonic community to develop high-capacity systems. Dif- ferential phase-shift-keying (DPSK) wavelength-division- multiplexing transmission systems, featuring higher robustness to transmission impairments, recorded capacity of tens of terabits per second [1]. Optical packet switched (OPS) node based on all-optical circuits, with higher speed operation and lower power consumption than electronic circuits, is viewed as a viable solution to route these high-speed data packets. In devising an OPS node, a key function is the all-optical label processing system (AOLS) that provides the forwarding infor- mation for routing the packets. Generally, the AOLS includes a label/payload separator (LPS), which extracts the label from the packet, and a serial-to-parallel conversion (SPC) circuit, which parallelizes the label bits. The parallelized bits can be then processed by fast complementary metal–oxide–semiconductor electronics [2]–[5] or by self-routing all-optical circuits [6]–[8]. Several solutions for the all-optically label extractor were pre- sented in [5], [9]. Those solutions require quite involved schemes including nontrivial devices per packet clock-recovery and switches. Moreover, the solution presented in [9] does not simply allow for extracting only a well-defined field within the label bits. All-optical solutions for parallelization of the label Manuscript received December 28, 2006; revised February 20, 2007. This work was supported in part by Ericsson under a grant. The authors are with Scuola Superiore Sant’Anna, 56124 Pisa, Italy (e-mail: nicola.calabretta@cnit.it). 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.2007.895895 bits were presented in [4], [5], and [10]. In [4] and [10], the schemes require an external synchronous sampling pulse. In [5], the system is based on splitting and delaying the input label in copies ( number of bits in a label), which are then sampled by a synchronous control pulse. However, the splitting losses and the required number of optical switches linearly increase with , which makes the system bulky and difficult to scale. Most of all, the SPC requires a complex preprocessing circuit to retrieve the synchronous sampling pulse. Moreover, some of these approaches do not operate in the -band, are not suitable for photonic integration, depend on the state of polarization, and require high pump power. Furthermore, none has been operated with DPSK packets. Here we demonstrate a novel all-optical circuit that simul- taneously extracts and parallelizes the label bits from DPSK packets. The technique is based on the conversion of the serial bits to parallel bits at distinct wavelengths. The main advan- tages are the scalability with increasing and the asynchronous operation. Indeed, the all-optical circuit is realized by using only two optical switches (regardless of the number of bits), without the need for large splitting losses and additional switches. The intrinsic asynchronous operation of the circuit eases the OPS node architecture eliminating also the need of the LPS or prepro- cessing circuit providing the sampling pulse. Finally, the circuit is polarization-independent, has low power consumption, and could be made compact by photonic integration. II. PRINCIPLE OF OPERATION The all-optical circuit, shown in Fig. 1, consists of two all-optical blocks: a compact all-optical function (AOF) and an optical AND logic function. The AOF receives at its input the DPSK packets and continuous-wave (CW) lightwaves. It produces two outputs: the DPSK demodulated data packet (label “A” in Fig. 1) and a sequence of colored pulses, with repetition rate equal to the bit time of the label (label “B”). The demodulated label bits and the colored pulses are thus fed into the logic AND. The AND logic function between the two signal is used to transfer the pattern information contained in the label bits to the sequence of colored pulses. By using an arrayed waveguide grating (AWG), the resulting colored pulses are separated at different spatial outputs (label “C”), representing the parallelized copy of the label pattern. These parallel bits are thus ready to be processed by either electrical or all-optical means. The AOF circuit was realized using a linear optical amplifier (LOA) and a one-bit delay interferometer (DI), followed by a wavelength-dependent delay line (WDL) [7]. The incoming DPSK packets are coupled by an AWG with local CW 1041-1135/$25.00 © 2007 IEEE