PHOTONIC TIME-SPACE CONVERTER FOR DIGITAL COMMUNICATION SIGNALS Pierpaolo Boffi, Silvia Pietralunga*, Davide Piccinin and Mario Martinelli* CORECOM - Consorzio Ricerche Elaborazione e Commutazione Ottica Milano * Dipartimento di Elettronica e Informazione, Politecnico di Milano Pza Leonardo da Vinci, 32, 20133 Milano, ITALY e-mail:
[email protected] Abstract A photonic sub-system which converts the bit time-coded sequence of a 1550 nm communication signal into an equivalent space-coded pattern has been realized in a completely optical way in free-propagation. Architecture and experimental results are reported. Free-space optical processing appears today very promising to manage high fluxes of digital information. The parallelism represents one of the most important feature of the future switching fabric and it can be fully deployed by means of free-space architectures (1). Since the communication signals are time-coded, the conversion operation between a time-coded sequence of bits and an equivalent space-coded pattern is basic for any hypothesis of freespace architecture. The aim of this communication is to present a time-space converter realized by means of CdTe switching gates operating on 1550 nm gaussian-limited signals in free-propagation architecture. The principle of the whole converter is shown in fig.1. The n-bit word is cloned in n parallel streams. Each stream is subjected to a specific delay which is a multiple of the time slot of the bit flow. Hence, a gating operation allows to sample a single bit from each flow and to cut the bit frame which contains the whole n-bit word in a parallel way. Monocrystals of CdTe, Indium doped, are used to realize the optically controlled gates (2). The gate operates on the polarization state of the 1550 nm signal beam by means of a combined electro-optic and photoconductive effect. The electro-optic effect, induced by an electric field applied to the crystal, causes a rotation to the linear polarization of the input light. As a consequence, the signal beam is absorbed by an analyzer set on the exit face of the crystal. A pulsed 1064 nm control beam quenches the electro-optic effect and induces the transmission of the signal beam. The switching speed is mainly limited by the pulse duration of the control beam. In previous experiments, a transaction of the order of the Q-switched Nd:YAG pulse width was measured (rise-time 3 nsec). Switching time of the order of picosecond has been also pointed out (3). The fall-time remains instead orders longer. In order to develop a real "optical gate" for 1550 nm signals two CdTe switching cells are added sequentially. The second cell followed by a crossed analyzer causes the interruption of the signal with the same velocity. The window of the optical gate corresponds to the temporal delay between the two control pulses operating on the two crystals. Fig. 2 shows the optical gate (one bit-time slot in duration) which allows to sample only one bit (at 140 Mbit/sec). In the practical realization of the converter the n-bit signal beam is split in n flows by a 1xn coupler and the n streams are delayed by fiber-loop delay lines. A set of fiber-made polarization controllers allows to set the right polarization at the input of the elementary optical gates. The whole switching area is illuminated by two control beams transmitted by a
couple of optical fibers. The different length of the fibers allows to set the window of the optical gate. Different configurations for 4-bit and 8-bit words, generated in the RTZ modality at different bit-rates, are tested. The bit length limitation is due only to the number of adopted switching cells. In fig. 3 the result of the conversion for the configuration (1010) is shown. In summary, a first example of a completely optical conversion from a time-coded word to a space-coded signal is shown. The experiment constitutes the premise of further optical manipulation of the optical bits. The goal of the experiment was to demonstrate the potential of a photonic manipulation of communication signals in free-space architecture. By using source in the regime of the psec a similar experiment in the domain of the Gbits is foreseen. References: (1) H. Scott Hinton, An introduction to Photonic Switching Fabric, Plenum Press ( 1993) (3) P. Boffi, M. Martinelli, Optics Letters, 20, 641, (1995) (2) I. Rukmann et al., Optics Letters A, 55, 30, (1992)
OUT
control pulses
IN
4 th bit 3 th bit 2 nd bit 1 st bit time-coded n-bits word
n x word
delay lines
space-coded n-bits word
n x optical gate t
Fig. 1. The scheme of the converter. The n-bit word is cloned in n streams and each one is separately delayed. The switching area is illuminated by optical control pulses and converts the signal in a space-coded configuration.
Fig. 2. The answer of the all-optical gate. Upper trace: a continuous stream of bits at 140 Mbit/sec. Medium trace: the optical gate window (7 nsec in duration). Lower trace: the single sampled bit.
10 nsec/div
input word Fig. 3. The measured response of the time-space converter. The upper trace shows the time-coded word at 33 Mbit/sec; in the other traces the gated data streams producing the space pattern in output are reported.
4th bit: 0 3th bit: 1
2th bit: 0 1th bit: 1
50 nsec/div
