Efficient optical packet-generationand -compression scheme

Paul R. Prucnal and Seung-Woo Seo

An efficient optical packet-generation and -compression scheme is proposed. Packet compression isachieved when the packet is sent through a series of semiconductor optical amplifiers, which have eithera transmitting or an absorbing state. The proposed scheme requires no fast electronics and usesexceptionally simple devices such as a tapped series of D flip-flops and frequency dividers. A detailedperformance analysis on the system size limitations is also provided by the consideration of pulse-spreading effects and semiconductor optical-amplifier noise.Key words: Optical packet generation, optical packet compression, asynchronous transfer mode,

semiconductor optical amplifiers. r 1996 Optical Society of America

1. Introduction

Optical fibers have made it feasible to transmit dataat rates above 100 Gbits@s on a single fiber inpoint-to-point communication as well as in net-works.1,2 In future broadband integrated networks,e.g., the broadband integrated-services digital net-work, each of the nodes will generate data at morethan 1 Gbits@s. An asynchronous transfer mode1ATM2 has been proposed as an efficient packet-switching technique for the broadband integrated-services digital network. In the ATM, multiplexingis performed statistically at the cell level. It usesfixed-size 53-byte packets, in which each packet iscomposed of a 5-byte header followed by a 48-bytepayload.In an optical time-division multiplexing system,

the conversion of electronic data to an optical formatmay be performed with an electro-optic 1E–O2 modu-lator.3 The E–O modulator is nothing more than ahigh-speed on–off switch that is driven by the incom-ing electronic data, controlling the amplitude of theoutgoing optical pulses. For a high aggregate net-work throughput to be achieved, a very-high-speedlaser producing short pulses is required as an E–Omodulator source, which has been a limiting factor todate. Consequently, there is often a significantmismatch in speed between the incoming electronic

The authors are with the Department of Electrical Engineering,Princeton University, Princeton, New Jersey 08544.Received 18August 1995.0003-6935@96@203815-04$10.00@0r 1996 Optical Society of America

data packets 1for example, 155Mbits@s or 622Mbits@sin commercial ATM switches2 and the outgoing opti-cal pulses 1.100 Gbits@s2 that are to be sent throughthe optical fiber.Because of the speed limits in electronics, the

direct modulation of non-return-to-zero electronicsignals into return-to-zero 1RZ2 optical pulses maynot be possible at more than a few gigabits persecond with the current technology. As an alterna-tive, some research efforts have been made to inter-leave the optical pulses at the bit or packet level.Lu et al.4 proposed a compact multiplexing scheme inwhich individually modulated optical pulses aretime compressed, time delayed, and interleaved toform a high-speed data stream. However, theirscheme requires redundant hardware to produce thehigh-speed data stream, and as the number of nodesincreases, the complexity of the system increasessignificantly.In this paper a simple packet-compression scheme

is proposed for the individually modulated low-speedoptical pulses. Each of the ATM packets are indi-vidually compressed before being sent into the net-work. Interleaving of the time-compressed ATMpackets is done at the packet 1cell2 level in a statisti-cal manner. High-speed transmission of the datapackets through an optical network can be accom-plished if the packet is compressed just before enter-ing the switching network and then expanded at thereceiving node. Large-scale compression permitsmany more packets to flow simultaneously throughthe network, thus increasing its capacity. Alterna-tively, for a fixed throughput, compression can re-

10 July 1996 @ Vol. 35, No. 20 @ APPLIED OPTICS 3815

duce the probability of collision and therefore reducethe delay inATM networks.The remainder of the paper is organized as follows.

In Section 2 the packet-generation and -compressionscheme is described. Aperformance analysis on thesystem size limitations is provided by a consider-ation of pulse-spreading effects and semiconductoroptical amplifier 1SOA2 noise in Section 3. Resultsare summarized in Section 4.

2. Packet Generation and Compression

Packet generation and compression can be accom-plished by the scheme explained below. On the leftin Fig. 1 is a mode-locked 1ML2 laser that suppliespulses every T seconds, where T 5 1@f0 and f0 is theoptical pulse-repetition rate. This pulse train isencoded into an optical ATM packet after passingthrough an E–O modulator, which is driven by anelectronic pulse train containing the ATM data withthe same rate as the laser source. The resultingsingle uncompressed packet consists of m bits ofinformation, which is usually 424 bits.Packet compression is achieved when the packet is

sent through a series of 2 3 2 crossbar switches, or afunctionally equivalent arrangement, such as SOA’s,which have either a transmitting or an absorbingstate. Because crossbar switches are lossy, thesimple concatenation of 2 3 2 switches into multiplestages substantially attenuates the optical signal.5In contrast, using SOA’s can reduce or eliminateoptical loss through the packet compressor. Severalstages of the packet compressor are illustrated inFig. 1, and the jth stage is shown in detail in Fig. 2.Each of the stages consists of two optical paths that

Fig. 1. Diagram of the proposed packet-compression scheme1Gb@s, gigabits per second2.

Fig. 2. Detail of the jth stage of packet compression 1DF@F, Dflip-flop; CLK, clock2.

3816 APPLIED OPTICS @ Vol. 35, No. 20 @ 10 July 1996

traverse SOA gates, with the upper path containingan optical delay in excess of the lower path. Thetwo SOA’s are driven by an electronic clock into anamplifying–absorbing cycle, but with a p phasedifference. For what follows we define T as theoriginal bit separation, t as the compressed-bit sepa-ration, k 5 T@t as the compression factor, and m asthe number of bits in a packet.For a sequence of bits separated by T, the packet

compressor works as follows. At stage 1 the SOA’sare driven at a frequency f0@2, causing alternate bitsto emerge from the upper and lower SOA’s. Becauseof the phase difference p between the clocks, whichare fed into the two SOA’s, they have a positive gainduring nonoverlapping time intervals. Because theupper path contains a T 2 t second delay in excess ofthe lower path, if the upper SOA is on 1the bottomSOA is off2, the pulse will experience a T 2 t seconddelay. After t seconds, the bottom SOA is on, andthe second pulse immediately follows the first pulse.Therefore, when these two pulse trains are multi-plexed together, bits 1 and 2 are separated by t, asare bits 3 and 4, and so on. Each bit pair 1forexample, bit pair 1 and 2 and the next bit pair 3 and42 is now separated by 2T. The process is repeatedat stage 2, in which the SOA’s are now driven at afrequency f0@4, causing alternate bit pairs to emergefrom the upper and lower SOA’s. The passive delayin the upper path is now 21T 2 t2 seconds, so that,when these two pulse trains are multiplexed to-gether, bits 1, 2, 3, and 4 are separated by t, as arebits 5, 6, 7, and 8, and so on. Now each set of fourbits is separated by 4T.More formally, if we denote each of the pulses as

pi1t2, where 1 # i # m, then the scheme is equivalentto dividing the original pulse train, P0 5 53pi1t246, eachof which is separated by T, into groups of pulses ateach stage. For example, at the second stage, thereare m@2 groups of pulses, P1 5 53pi1t2, pi111t246, wherethe index i is an integer in 31, m4 that satisfies thecondition 31i mod 22 ; 14. Because of the binary na-ture of each stage, the total number of stages re-quired is log2m810N2, wherem8 is the smallest powerof 2 that is larger thanm. In general, the jth stagewill have a clock frequency of fj 5 f0@2 j and a passivedelay of 2 j211T 2 t2 seconds in the upper path. Them@2 j pulse groups Pj, each of which is separated by2 jT, are created at the output of the jth stage:

Pj 5 53pi1t2, pi111t2, . . . , pi112 j2121t246,

where 1 # j # N and the index i is in 31, m4,satisfying the condition 31i mod 2 j2 ; 14. At the in-put to the last stage, there are two compressedgroups of m@2 bits that are separated by 2N21T.The output of this stage of course is one group ofm-compressed bits. The compression factor k isdependent on the size of t used in the passive delays.Figure 3 shows an example for the above packet-

compression scheme. The frequency of the opticalpulses from a laser source is f0 5 1.25 GHz 1T 5 800ps2, and the electronic ATM data packets are also

1.25 Gbits@s. Compressing 424 bits in an ATMpacket requires log2 424 , log2 512 5 9 stages. Ifthe compressed-bit separation t 5 10 ps, as in 100Gbits@s, the compression factor k is 800@10 5 80.Figure 4 shows a more detailed illustration of packetcompression at each stage and with clock signals.Ahigh compression rate is limited only by an array ofdelay lines and the available pulse width from MLlasers, not by the switching speed of the SOA’s. It isimportant to note that this scheme requires no fastelectronics: the first clock in the delay stage runs athalf the electronic data rate, and the clock frequencyof each successive stage decreases by a factor of 2.Decreasing the clock by halves can be done simply bythe use of a frequency divider or a counter. Theclock electronics are therefore exceptionally simpleand consist of just a tapped series of D flip-flops 1seeFig. 22. The switching speed of a typical SOA is ,1GHz 1T 5 1 ns2, and the input data into the first-stage SOA should be slower than this speed 1other-stage SOA’s can be clocked at a slower rate than thefirst-stage SOA2.

3. System Size Limitations

Here the proposed packet-compression scheme isevaluated by the investigation of the limitations onthe allowable compression rate. We also examinethe maximum number of allowable compression

Fig. 3. Illustration of generating 100-Gbit@s optical pulses.

1a2

1b2

1c2

Fig. 4. Example of the proposed packet-compression scheme,with inputs of 1a2 the first compression stage, 1b2 the secondcompression stage, and 1c2 the third compression stage.

stages 1i.e., the allowable number of bits in a packet2in conjunction with the noise in the SOA’s.

A. Maximum Compression Rate

The limitation on the maximum compression rate isimposed by the optical pulse-spreading effect. Itdetermines the value of t in delay lines at each stage.As before, let T denote the original bit separation, lett be the compressed-bit separation, and let k 5 T@tbe the compression factor. Also let d representpulse spreading 1e.g., caused by material dispersion,etc.2 in time. We assume that the allowable pulsespread is equal to Sm percent of the pulse duration.If the pulse duration is denoted as D, then d 5 SmD.The minimum allowable bit interval after compres-sion, tmin, is the sum of the bit duration and the pulsespread, i.e., tmin 5 D 1 d 5 D 1 SmD. Hence, themaximum allowable compression rate, kmax, is givenas follows:

kmax 5T

tmin5

T

11 1 Sm2D.

In particular, in the case of the RZ pulse, the pulseduration is half the interval tmin, i.e., D 1 d 5 tmin@2.As an illustration, if D 5 1 ps, T 5 800 ps, and Sm 530% for the previous example, kmax would be 307with the RZ encoding.

B. Effects of Noise in Semiconductor Optical Amplifiers

Although the maximum compression rate kmax isdetermined by the pulse-spreading effects in a fiber,the maximum number of packet-compression stagesN is limited by the inherent noisy characteristics ofSOA’s. Because each optical pulse passes throughone of the two SOA’s at each stage, the scheme can bemodeled as a series of SOA’s, as in Ref. 6. Thismaximum number of permissible SOA’s to be concat-enated determines the total number of bits in apacket to be compressed at a time. The spontaneous-emission noise power Psp at the output from a SOA isgiven as7

Psp 5 nsp1G 2 12hnB0,

where nsp is the spontaneous-emission factor, G isthe SOA gain, hn is the photon energy, and B0 is theSOA bandwidth. The factor nsp ranges from 1 to 5,depending on the pumping current and operatingwavelength. If SOA’s are concatenated in N stages,the signal power gain GN and a spontaneous-emission gain Gsp from the Nth stage are given asfollows6:

GN 5 1LSOALinLoutG2N 5 1LtotalG2N, 112

Gsp 5 1 GN1@N

LSOALin2 Lout2 o

i50

N21

1GN2i@N, 122

respectively, where LSOA is the internal SOA loss andLin and Lout are the input and output coupling losses,respectively. From Eqs. 152 and 162 in Ref. 6, the

10 July 1996 @ Vol. 35, No. 20 @ APPLIED OPTICS 3817

maximum allowable spontaneous gain Gsp,max andthe optimum input-signal level can be calculated fora given signal gain GN by consideration of thesignal-to-noise ratio and the SOA saturation.Usually, Gsp,max has relatively large values, which

gives enough of a margin in the number of cascad-able SOA’s. For example, letGN 5 10 after compen-sating all losses, e.g., LSOA 5 Lin 5 Lout 5 0.5, andsplitting the 0.5 loss at each stage. Then, by Eq. 162in Ref. 6, Gsp,max can be as large as 6.536 3 102 fortypical values of input parameters, i.e., l 5 1.3 µm,GN 5 10, nsp 5 5, peff 5 2, B0 5 1 THz, Psat 5 50 mW,and Pin 5 2.4 mW, where peff is the effective polariza-tion-dependent factor, Psat is the SOA’s saturationpower, and Pin is the input signal power. Althougha closed-form solution of Eq. 122 does not exist, themaximum number of allowable compression stagesN can be calculated numerically. With this largevalue of Gsp,max, N can be as large as 46. Theseresults indicate that it is sufficient to design anine-stage packet compressor for ATM packets with424 bits without much effect from the SOAnoises.

C. Gain-Saturation Effect

When SOA’s are operated in an unsaturated condi-tion, they provide a wide range of gain bandwidth.However, if the pulse energy is high enough tosaturate the SOA, the output pulse shapes aredistorted and gains are decreased. Saitoh andMukai8 showed that the saturation characteristics ofthe pulse-energy gain versus the average outputintensity in high-repetition-rate pulse amplificationcoincides with that of the signal gain in cw amplifica-tion 1i.e., in the case in which the gain recovery timetc is much larger than the pulse-repetition period Tr2.Here the pulse-energy gain is defined as the ratio ofthe output to the input pulse energy per unit area.This is clear if we compare Figs. 1 and 12 in Ref. 8.This result implies that, for extremely high-repeti-tion-rate pulses, saturation of the pulse-energy gaincan be determined by the average signal power,which is the product of the input pulse energy andthe repetition rate. Referring to Fig. 13 in Ref. 8,we see that, if the ratio of tc to Tr, i.e., tc@Tr, becomesmore than 50 as it does in the 100-Gbits@s data rate,the normalized effective saturation energy 1Us8@Us2reaches 220 dB 1510222, where Us is the saturationenergy per unit area in single-pulse amplificationand Us8 is the effective saturation energy per unitarea in repetitive-pulse amplification. This resultimplies that only ,1% of the saturation energy insingle-pulse amplification can cause the SOA to besaturated at 100 Gbits@s. Therefore, for example, ifthe saturation energy of the InGaAsP SOA is 5

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pJ@µm2, then each of the pulse energies, which willnot saturate the last-stage SOA’s in the packetcompressor, should be less than 5@100 pJ@µm2.

4. Conclusions

In this paper we proposed a simple optical packet-generation and -compression scheme. A high com-pression rate can be achieved by the use of SOA’s andfiber delay lines. We showed that the maximumcompression rate is limited by pulse-spreading ef-fects, and the maximum number of allowable stagesis limited by the noise in the SOA’s. The precisionof fiber delay lengths is the major challenge in thisscheme. Burnett and Jones9 reported on an experi-mental demonstration in which the precision ofcutting fibers can be within 10 µm. In addition, thelength of the fibers in each arm can be further tuned1i.e., fine length tuning2 by adjustable delay lines orby the fiber-stretching technique.

This work was supported by the Advanced Re-search Project Agency under contract F19628-94-C-0045.

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