Rep Rate Multiplication of Pseudo-Random Bit Sequences C. Stamatiadis, Ch. Kouloumentas, P. Zakynthinos, and H. Avramopoulos

National Technical University of Athens – School of Electrical and Computer Engineering 9 Heroon Polytechniou Street, Zografou 15773 – Athens, Greece

Email: cstamat@mail.ntua.gr

Abstract: We describe an all-optical method for rate multiplication of pseudo-random bit sequences. We quadruplicate, error-freely to 50 Gb/s, a 27-1, 12.5 Gb/s sequence. The scheme is a feedback loop and comprises of two nonlinear gates. ©2008 Optical Society of America OCIS codes: (200.3760) Logic-based optical processing; (230.3750) Optical logic devices

1. Introduction

One of the recurring difficulties that research groups are faced with is the requirement for renewing test and measurement equipment as data repetition rates are increasing in lightwave transmission systems [1]. At the same time as repetition rates are increasing, all-optical techniques are becoming more relevant [2,3] just as key semiconductor devices, high-nonlinearity materials and high power amplifiers are also improving and are becoming readily available. It is therefore of interest to ask if all-optical concepts may be used to extend the useful life of test and measurement equipment.

Bit error rate (BER) equipment is an example in point for this question. 10 Gb/s BER sets are now becoming obsolescent as repetition rates are moving to 40 Gb/s and beyond. Given the high initial cost of BER equipment, the capability to upgrade its use to higher data rates would be very welcome. In the present communication we develop a concept towards this goal.

We present a novel all-optical method that can be used to increase of the repetition rate of a pseudo-random bit sequence (PRBS) by a factor of 2n. It relies on a regenerative fiber loop consisting of two nonlinear optical loop mirrors (NOLM), the first operating as an exchange-bypass switch and the second as a wavelength converter (WC). We apply this technique and quadruplicate a 12.5 Gb/s 27-1 PRBS to 50 Gb/s. The ensuing 50 Gb/s PRBS was evaluated by demultiplexing its four, interleaved 12.5 Gb/s tributary channels, which were error-free and each, exhibited less than 1.5 dB power penalty compared to the input PRBS signal.

2. Concept and experimental setup

To date, the predominant optical technique to obtain 2n rate multiplication of data signals uses n consecutive stages of conventional “split-shift-and-combine” bit interleavers [4]. This scheme is simple in principle but has the disadvantage that its complexity increases with the multiplication factor, scaling with the number of doubling stages required. This may then result in a cumbersome circuit as each doubling stage requires fine adjustment for bit-wise synchronization, equalization of optical power levels and polarization control.

The concept communicated today uses the same circuit repetitively, a number of times, in feedback configuration to achieve the necessary multiplication factor. This becomes possible by effectively “folding” the n stages of the “split-shift-and-combine” bit interleavers by means of a feedback configuration. The block-diagram of our concept is illustrated in Fig. 1a. It consists of a logical OR-gate and a feedback loop. This loop provides a delay,

Fig. 1: a) PRBS rate multiplication concept, b) Example of rate multiplication of the 23-1 PRBS: the numbers above the pulses indicate the tributary channel of the final high-rate PRBS that the pulses belong to, c)Fiber-based NOLM operating as a 2x2

exchange-bypass switch. Inset: truth table of a 2x2 exchange-bypass in the case of continuous “1s” at In1.

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Fig. 2: Experimental setup

the length of which depends on the multiplication factor and equals to m·T+T/2n, where T is the period of the input PRBS signal and m any integer. Fig. 1b shows as an example, the case of quadruplicating the 23-1 PRBS (1 0 1 1 1 0 0): in the first pass the feedback path is filled in with the input PRBS and in the following 2n -1 passes the content of the feedback path is added to the input PRBS, occupying adjacent bit slots. After a total of 2n passes through the circuit, the rate multiplied PRBS is created, containing 2n tributary channels, each one carrying the low-rate PRBS. From then on the OR-gate continuously reproduces the high-rate PRBS, so that the temporal window that was originally occupied by the low-rate input PRBS, is now occupied by a sequence of 2n PRBS of the same order but with 2n higher repetition rate. In principle this method can by applied to any multiplication factor or PRBS order.

In our demonstration the OR operation was obtained with a 2x2 exchange-bypass switch. Depending on whether the control signal contains a logical “0” or “1”, the switch operates in the bar or cross state, respectively [5], and was realized using a fiber-based NOLM as shown in Fig. 1c. In the absence of a control pulse, both input pulses are mirrored out of the switch to their incoming ports, whereas if a control pulse is present they are switched over.

As shown in the truth table of the inset of Fig. 1c, if a continuous stream of logical “1s” enters In1, the result of the OR-operation between the other input (In2) and the control signal appears at the Out2 port of the NOLM. Therefore the OR operation in our scheme was implemented using an NOLM, in which the optical clock at the final bit rate and the input low-rate PRBS entered ports In1 and In2 at wavelengths λ1 and λ2, respectively, and the stream from the feedback formed the control signal. The truth table shows that the output signal in Out2 consists in general of both wavelengths λ1 and λ2. This was wavelength converted to λ3 before returning as feedback signal.

Fig. 2 shows the experimental setup that consists of the optical signal generation unit, the PRBS multiplier and the evaluation stage. The outputs of two distributed-feedback (DFB) laser diodes at λ1 (1557 nm) and λ2 (1560 nm) are multiplexed in an arrayed-waveguide grating (AWG 1) and are modulated at 12.5 GHz in an electro-absorption modulator (EAM) to generate 7 ps pulse trains at λ1 and λ2 which are next separated in the AWG2. The signal at λ1 enters a fiber Fabry-Pérot filter (FPF) with 50 GHz free spectral range (FSR) and finesse 100 to form the 50 GHz clock, pulse train with 1 dB power variation. The second pulse train at λ2 is modulated by a Li:NbO3 modulator to form a 27-1 PRBS. These signals next enter as inputs to the first NOLM1 (OR-gate). The nonlinear element of the NOLM1 is a 180 m-long HNLF with 10 W-1km-1 γ nonlinear parameter and 1.21 ps/nm/km chromatic dispersion, and the feedback signal at λ3 is amplified to 620 mW. At the transmission port of NOLM1, AWG3 is used to separate λ1 and λ2, whereas subsequently the signals at λ1 and λ2 are wavelength-converted to λ3 (1543 nm) in NOLM2. NOLM2 is used with counter-propagating controls, incorporating 260 m HNLF of the same type as of NOLM1. The control signals (Ctr) at λ1 and λ2 are amplified to 240 mW and 70 mW. The WC output from NOLM2 is carefully delayed in time by m·T+T/4 and provides the signal to close the feedback loop to control NOLM1. The 50 Gb/s PRBS output is obtained from AWG3, as the λ1 output of the OR gate and is evaluated after demultiplexing to 12.5 Gb/s, using an EAM modulator.

4. Results and discussion

Fig. 3 shows a compendium of the PRBS multiplication results. Figs. 3a and 3b show traces and eye-diagrams of the input 12.5 Gb/s and output 50 Gb/s PRBS. Figs. 3c-f, show traces (left column) and eye-diagrams (right column) of

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Fig. 3: Traces and eye-diagrams of a) input PRBS, b) rate multiplied PRBS, c) first, d) second, e) third and f) fourth

tributary channel after demultiplexing, synchronized with rate multiplied PRBS signal. g) BER curves of input PRBS (back-to-back) and the four demultiplexed tributary channels of the rate multiplied PRBS.

the four 12.5 Gb/s interleaved PRBS sequences obtained after demultiplexing with the EAM as well as the complete 50 Gb/s PRBS depicted with the same trigger signal. The four 12.5 Gb/s tributary channels are each equivalent to the input PRBS, and were obtained by delaying the EAM each time by an extra 25 ps. As such and as expected, union of the 10 Gb/s traces/eye-diagrams on the top rows, yields the 50 Gb/s sequences/eye-diagrams in the bottom rows. Finally Fig. 3g demonstrates the BER curves obtained for the initial low rate PRBS as the back-to-back measurement and the four, interleaved 12.5 Gb/s channels after demultiplexing. Error free operation was achieved for all channels with power penalty less than 1.5 dB with respect to the input PRBS.

5. Conclusions

We have described a method to rate multiply a PRBS sequence by a factor of 2n using an all-optical feedback circuit and we have quadruplicated to 50 Gb/s a 12.5 Gb/s 27-1 PRBS. Each of the four interleaved tributaries of the ensuing 50 Gb/s sequence, were error-free with less than 1.5 dB power penalty with respect to the input PRBS.

References [1] H.C.H. Mulvad, E. Tangdiongga, O. Raz, J. Herrera, H. de Waardt, H.J.S. Dorren, “640 Gbit/s OTDM Lab-Transmission and 320 Gbit/s

Field-Transmission with SOA-based Clock Recovery,” in OFC’08 (Conference on Optical Fiber Communications, San Diego, 2008), paper OWS2.

[2] J. Leuthold, J. Jaques, S. Cabot, “All-optical wavelength conversion and regeneration,” in OFC’04 (Conference on Optical Fiber Communications, Los Angeles, 2004), paper WN1.

[3] S. Boscolo and S. K. Turitsyn, “Recent Developments in All-Optical Nonlinear Data Processing,” in ECOC‘08 (European Conference on Optical Communications, Brussels, 2008), paper Th.2.B.1.

[4] Shin Arahira and Yoh Ogawa, “160-Gb/s OTDM Signal Source With 3R Function Utilizing Ultrafast Mode-Locked Laser Diodes and Modified NOLM,” IEEE Photon. Technol. Lett., 17, 992-994 (2005).

[5] D. Tsiokos et al., “10-Gb/s All-Optical Half-Adder With Interferometric SOA Gates,” IEEE Photon. Technol. Lett., 16, 284-286 (2004).

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