Real-time all-optical performance monitoring using …anderson/QoSRealTime.pdfReal-time all-optical...

Real-time all-optical performance monitoring usingoptical bit-shape correlation

Feras Abou-Galala1 and Betty Lise Anderson2,*1Currently with Department of Electrical Engineering, University of California, Riverside,

900 University Avenue, Riverside, California 92521, USA2Department of Electrical and Computer Engineering, Ohio State University, 205 Dreese Laboratory,

2015 Neil Avenue, Columbus, Ohio 43210, USA

*Corresponding author: [email protected]

Received 22 October 2008; accepted 16 December 2008;posted 5 January 2009 (Doc. ID 103103); published 0 MONTH 0000

A new optical performance monitoring technique of an optical link in real time is experimentally demon-strated. Rather than comparing bit streams or analyzing eye diagrams, we use a novel optical correlatorto compare the shapes of the individual received bits to a standard. The all-optical correlator outputs apulse whose strength directly measures the degradation of the bit during transmission. Results are pro-duced within three bit periods in real time instead of requiring statistical analysis of long data streams.The correlator is based on a simplified White cell-based true-time delay device. © 2009 Optical Societyof America

OCIS codes: 060.2330, 070.4550, 200.4560.

1. Introduction

Currently the increasing demand on the Internetnetwork for real-time multimedia data traffic withhigh quality of service (QoS) is pushing the limitsof existing network structures [1–3]. Next-generationnetworks call on a new paradigm of all-optical dyna-mically routed networks, where network links arefully transparent to the data bit rate and format. Op-tical performance monitors are expected to be an in-tegral part of densewavelength divisionmultiplexinglinks in next-generation Internet networks, where re-liability and high QoS of high-bandwidth optical net-works are essential aspects of the network’s design.Well-established techniques for quality monitoring

at the physical or channel layer include bit-error-rate(BER) testing and eye-diagram analysis. Both rely oncollecting a long stream of data and can take fromseconds to minutes, and both require optical-to-electronic (OE) conversion. More recently, otherelectrical techniques such as amplitude histograms

[4,5] and subcarrier multiplexing [3] have beenproposed.

All-optical techniques avoid OE conversion andprovide real-time measurements by keeping thesignals in the optical domain, for example, usingoptical logic gates to measure the BER [6] or non-linear optical loop mirrors to measure the opticalsignal-to-noise ratio [7].

We propose a quality-of-signal measurementmethod that is all optical and provides results in realtime [8]. The approach is based on correlating theshape of each received data bit with a standard bitrepresenting the ideal shape of a sent bit. The corre-lation output produces an optical pulse for each bit,whose amplitude and shape are related to the degreeof correlation. As the bits accumulate attenuation,dispersion, noise, and jitter, they will resemble thesent bits less and produce a smaller correlationenergy per bit. Note that this is different from ap-proaches that correlate an entire bit stream to detectwhether the receive sequence matches the sent se-quence, for example [9]. By correlating each receivedbit with a standard, one can immediately measurewhether the accumulated impairments have caused

0003-6935/09/070001-08$15.00/0© 2009 Optical Society of America

1 March 2009 / Vol. 48, No. 7 / APPLIED OPTICS 1

the performance to fall below some specified level.Since degradation is instantly detected, the networkcould be reconfigured on the fly to maintain servicewhile the link is diagnosed and fixed. The output ofthe correlator is an optical pulse, which can be usedwith an optical thresholder to control other systemsor be converted to an electrical signal for the samepurpose.Figure 1 briefly reviews the principle. Let us as-

sume the correlator’s input to be square pulses, forsimplicity, with bit period T. The correlator consistsof a tapped delay line (TDL), a set of weights that re-present the reference or standard pulse shape, and asummer. For every recognizable pulse that arrives,an optical pulse is created. The output pulses arethen thresholded as shown in Fig. 2, using, for exam-ple, a saturable absorber, and only those correlationoutputs that exceed the threshold and would pass tothe next system.In [8] it was shown that the area above the thresh-

old related very well to eye-diagram measurementsof attenuation, dispersion, noise, and jitter and tocombinations of these effects. It is not the intent hereto diagnose what is wrong with the link, but rather tomonitor whether it currently meets some minimumstandard due to combined impairments. Note thatthis information is obtained in just three bit periods,which requires only tens of picoseconds, dependingon the bit rate. Once a fault is detected, data canbe rerouted to a healthier path in real time.In this work, we achieve two goals. One is experi-

mentally demonstrating optical performance moni-toring (OPM) using bit-shape correlation. Thesecond is conducting a proof-of-concept demonstra-tion for a new optical correlator that is scalable tovery high resolutions (thousands of samples). Thecorrelator’s TDL is based on a White cell true-timedelay device, which was developed elsewhere forsteering phased array antennas [10–13]. An inherentadvantage of using the White cell technology in thecorrelator’s design is the fact that the delays in alltaps are fixed. Therefore the White cell is completelypassive, hence simple and inexpensive to implement.In Section 2 we briefly review the principle of

operation of the bit-shape correlator. Section 3 de-

scribes the experimental apparatus, and the resultsare covered in Section 4. Section 5 provides a sum-mary and discussion.

2. White Cell-Based Optical Correlator Principlesof Operation

The correlation function for discrete signals isgiven by

ψðtÞ ¼XN�1

k¼0

skðtÞrðt − kτkÞ; ð1Þ

where skðtÞ represents the reference functionsampled atN intervals of time τ, and rðt − kτkÞ repre-sents the kth of N replicas of the signal. Physicallythe correlator is implemented using a TDL, a setof weighting elements or reference elements, andan optical summer. The correlation takes place be-tween the received signal rðtÞ after going throughthe optical link and a reference signal representinga copy of the original transmitted signal. The refer-ence signal is represented by the weighting elementssðtÞ. The weights could be amplitude weights orphase weights, and we chose to use amplitudeweights in our approach. This choice allows us tosum the beams incoherently on a single photodetec-tor or a photodetector array, which simplifies the con-trol and stability requirements on the summingoptics. The TDL is the key element of the correlator,and the more taps implemented, the higher the reso-lution of the correlation output.

Optical TDLs have been implemented in the pastusing fiber ladders [14–16] or trees [17–20]. Super-structured fiber-Bragg-grating (FBG) TDLs with asmany as 63 taps have been reported [21]. In laddersand FBGs, each splitter or grating has to have a dif-ferent splitting ratio if all replicas are to have thesame output optical power. Those TDLs implemen-ted using tree structures require the N replicas tobe made first using splitters, and then each replicagets delayed separately via an independent path.For high-performance optical signal processing, avery large number of taps (hundreds, even thou-sands) may be required.

A high-resolution TDL can be implemented using aWhite cell. The White cell, originally invented forspectroscopy [22], is a free-space system that bounces

Fig. 1. Correlating a degraded bit (input) with a reference repre-sented by the weights results in an optical output pulse whoseheight and width directly measure the degree of degradation ofthe pulse during transmission.

Fig. 2. Resulting correlation output for a single 010bit sequence,comparing the ideal case (no degradation) to a signal with noise,dispersion, and timing jitter. The more degraded the signal, theless energy exceeds the threshold.

2 APPLIED OPTICS / Vol. 48, No. 7 / 1 March 2009

a light beam back and forth between various spheri-cal mirrors to create a long optical path. We havepreviously adapted White cells to develop program-mable delay lines. We did so by (1) introducing arraysof light beams instead of a single beam as the input tothe White cell [Fig. 3(a)], (2) replacing one of theWhite cell mirrors, the field mirror, with a micromir-ror array to switch beams independently on everybounce [Fig. 3(b)], and (3) adding additional Whitecell paths of different lengths to vary the time delay.The operation of the White cell is explained in detailin [13,23,24]. For the present application, the opticalcorrelator, multiple input beams are introduced suchthat there is one beam for each delay tap. Each beamgets a different fixed delay, hence there is no needfor programmability, making the entire device pas-sive. That is, for a programmable delay line, themicromirror array would be replaced by a microelec-tromechanical system (MEMS), but for a correlator,it is simply an array of fixed micromirrors built on aflat substrate, each tipped permanently to the appro-priate angle [Fig. 3(c)]. Such an array could be made,for example, by active ion etching or slow-tooldiamond turning.There are different styles of White cell, depending

on the number of taps needed. For example, a “quad-ratic” cell, such as that shown in Fig. 3(b), can pro-vide 15 taps in 13 bounces [24], and an “octic” cellcan provide 6399 taps in 11 bounces [12].

3. Experiment

We set out to demonstrate two things experimen-tally: (1) the use of optical correlation to measurethe effects of optical impairments on a link and(2) an optical correlator using a White cell asthe TDL.

A. Impairment Measurement

The effect of dispersion and attenuation were mea-sured using optical correlation. We did not haveaccess to an operating fiber link, so we introducedthese impairments artificially by electrically modify-ing the shape of the pulses before modulating thelaser beam. Figure 4 shows a block diagram of theapparatus. A 1550nm laser beam is modulated usinga Mach–Zehnder modulator. The electrical signalinto the modulator consists of pulses with varyingrise and fall times, to simulate dispersion, and vary-ing amplitude, to simulate attenuation. The electri-cal signal is generated using a pulse generator with apulse width of 41:16ns, a rise/fall time of 5ns, and ata repetition rate of 8:09MHz followed by a circuitthat adds the “impairments.”Figure 5 shows the design of a circuit that can ad-

just the rise/fall times and the amplitude of a givensignal [25]. The circuit uses a pair of metal-oxide-semiconductor field-effect transistors and acouple of 10ΩK digital potentiometers with standard0:25W resistors and ceramic capacitors. The resis-tor–capacitor time constant controls the rise and falltime of the output signal. Attenuation can be simply

modeled by adjusting the amplitude of the modu-lated signal from the function generator.

B. White Cell-Based Correlator

The correlator consists of four elements: 1 ×N split-ter, TDL, N weighting elements, and a summer.

Fig. 3. (a) Array of spotsmakesmultiple bounces in a regular pat-tern on the field mirror. (b) Original White cell is adapted to timedelays by replacing the field mirror with an array of micromirrorsthat switch individual beams among paths of different lengths.(c) Fixed micromirror array can be used in the correlator to controlthe paths of the beams in the TDL.


We begin by discussing the weights since thatdrove the rest of the design. Note that the weightscould be applied before or after delaying the signals.In our case, we chose to correlate our artificially im-paired pulses with a perfect square wave so theweights were zeros and ones. Amplitude weightscould be realized using shutters, and since the “zero”beams are in effect turned off, we simply did notimplement them in the first place.The number of samples N is further limited by the

number and pitch of pixels or mirrors in the micro-mirror array. We did not have the means to fabricatea fixed, passive micromirror array, but we did have aMEMS mirror array left over from another project,which we used by fixing the mirror tip angles. TheMEMS used was an analog MEMS originally de-signed for optical routing (Calient Networks Dia-mondwave Half Switch). The MEMS pixels can betipped to any angle between −10° and 10° in an ana-log fashion in both the x and the y directions. The tipangle is controlled by the voltage applied to twoelectrodes, one in x and the other in y. The pixelsare organized in a hexagonal array comprising 24rows and 17 columns, resulting in a total of 384 pix-els. Some of these were not functioning at the time ofthe experiment, which imposed an additional limita-tion on the number of taps that could be implemen-ted. Each beam required 10 pixels in a specificarrangement to complete its bounce pattern; itturned out however that, to avoid the bad pixels, onlysix beams could circulate in the White cell. Thusthere were 6 beams weighted “one” and 12 more ima-ginary beams weighted “zero,” for a total sample re-solution of N ¼ 18. Figure 6 shows the expectedcorrelation output function for an unimpaired beam.Since N was so small, we chose the “linear” style

White cell [12,13] for the TDL, meaning that the

number of possible delays is linearly proportionalto the number of bounces. This type of White cell,shown in Fig. 7, has two arms of the same lengthterminating in Mirrors A and B, which constitutea “null” cell, and one longer delay arm that uses alens train and terminates in Mirror C. The delayproduced is a function of the length of the delayarm. The delay element is set to be τ ¼ 6:86ns perround trip, which translates to a total length ofthe delay arm of 2:06m (6:75 ft) longer than the nullpath.

Each beam visits an independent set of micromir-rors that control the number of times each beam issent to the delay arm and hence the total delay ac-cumulated. One beam makes all its bounces in thenull cell; it comes out the soonest. The next beamis sent to Arm C one time but spends the rest ofits bounces in the null cell and comes out with a delayof τ relative to the first. The third beam is delayed by2τ and so on. The choice of τ was based on the slowspeed of the pulse generator and the availability ofoff-the-shelf lenses.

In Fig. 8 we show the general configuration of theexperimental setup as assembled on the optical ta-ble. The figure is to scale and describes the locationof all three arms of the White cell in addition to theinput and output arms.

The final element of the correlator is a summer.Any type of optical summer would work, but we choseto simply demagnify the output spot array to fit allthe beams on a single integrating detector. Thiswas possible because the slow signal rate allowedfor a comparatively large area photodiode; we usedan InGaAs infrared detector with a PIN structure

Fig. 4. Block diagram of the apparatus. MZ, Mach–Zehndermodulator; PD, photodetector.

Fig. 5. Circuit schematic for dispersion generation circuitry [25].

Fig. 6. Expected correlator output for an unimpaired beam. HereN ¼ 18, with 6 beams weighted “one” and 12 beams weighted“zero” (not generated).


with a rise/fall time of 7:0ns (typical) with a 5V biasvoltage. The active area was 0:8mm2. The detector’soutput was connected to a low-noise high-frequencyphotocurrent amplifier to amplify the photocurrentand increase the signal-to-noise ratio of the output.The transimpedance gain of the amplifier was6250V=A, with an output root mean square noiseof 3:2mV.We attempted to add noise as well but found that

the low bandwidth detector filtered out all the high-frequency noise and negated the result. This could beavoided in future by using a fiber combiner or othertype of summer that allows for a smaller area detec-tor with higher bandwidth to be used.

4. Experimental Results

To help with aligning and focusing the White cell, weused pellicle beam splitters in Arms A and B, or-iented such that they directed a fraction of the beamsreturning from those mirrors to infrared cameras.The cameras recorded an image of the micromirrorplane or any conjugate plane of the micromirror ar-ray. In this White cell, every beam visits Arm A oneven-numbered bounces but will visit either Arm Bor Arm C on odd-numbered bounces. Thus, inFig. 9(a), the beams returning from Mirror A areshown, corresponding to the six beams’ bounces 0,2, 4, 6, 8, and 10. The intensity drops off with higherbounce numbers because of the pellicle, which was

Fig. 7. “Linear”White cell TDL. Each light beam visits the longer path (Mirror C) a successive number of times to produce the sequence ofdelayed beams.

Fig. 8. Experimental apparatus.


removed before taking correlation measurements.Figure 9(b) shows the odd-numbered bounces return-ing from Arm B, and it can be seen that the top beamvisits Arm B once (and therefore visits Arm C theother four bounces), the second beam visits Arm Btwice, and the last beam is delayed on every oddbounce, so it never goes to Arm B, and its spotsare not seen on this camera. The third camera doesthe same thing as the second but with highermagnification for alignment purposes (resultsnot shown).Figure 10(a) shows the measured correlator output

of an unimpaired signal. At left is the input pulse,and at right is the measured correlation function.The amplitude weights were not all the same dueto variations in splitting ratio of the fiber splitterand the different path traveled through the Whitecell for each beam, which explains why the measuredcorrelation pulse is more curved than triangular(compare Fig. 8). Table 1 shows the relative strengthsof the individual beams. The output pulse is twice thepulse width, as expected from Fig. 6. A third pulsewidth was left as a guard band between correlations.Figure 10(b) shows the correlation function expectedfor the measured weights. The match is not exact, be-cause the weights were measured for each individualbeam with optimal coupling to the detector; duringactual correlation, all the beams are incident onthe detector at once, and so the outer ones (1 and6) may be slightly truncated.Figure 11(a) shows the effect on the correlator out-

put as the attenuation is varied. Figure 11(b), thetheoretical case for an ideal square pulse, is repeatedhere from [8]. The amplitude of the correlationfunction reduces in a linear fashion, as expected.Figures 11(c) and 11(d) show the case for dispersion.The percent dispersion is defined as the fraction ofthe actual bit period occupied by the transition,either rise or fall. When the dispersion reaches50%, the rising and falling transitions meet in themiddle of the bit. In this case, energy is moved fromthe center of the correlation pulse to the wings, butthe sloping sides stay the same. Experimental mea-surements were found to follow our simulations with

a root mean square error of no more than 5%, whichis within typical experimental error.

5. Summary and Discussion

We have demonstrated the first all-optical correlatorbased on a White cell TDL and applied that correla-tor to a proof-of-concept measurement of opticalsignal quality using bit-shape correlation.

The quality-of-signal detection is based on corre-lating the shapes of the individual bits as they arereceived, one bit at a time, with the ideal unimpairedbit shape. A pulse that is little degraded produces astrong and sharp correlation pulse for every bit se-quence 010. As impairments accumulate, the corre-lation pulse gets more spread out and weaker. An

Fig. 9. (a) Light beams visit Arm A on every even-numberedbounce and (b) visit Arm B a varying number of times on theodd-numbered bounces. One beam is sent to the delay Arm Con every odd bounce and never visits Arm B.

Fig. 10. Correlator output for an unimpaired input signal.(a) Measured correlation. (b) Expected correlation using theweights of Table 1.

Table 1. Relative Weights in the Experiment of the Six LightBeams

BeamNumber

Output Power(μW)

Factorized WeightðPi=PtotalÞ%

1 16 3.592 42 9.433 91 20.444 93 20.895 188 42.246 15 3.37


optical thresholding device can be used to determinewhether the correlated pulse meets some predeter-mined standard for quality. When the combinedeffects of all the impairments are sufficiently large,the correlation pulses will fail to meet the threshold,and loss of signal quality is detected. Thus if a teststream 010 is sent, the quality is measured in threebit periods, which could be tens of picoseconds, com-pared to many seconds or minutes for techniquesthat use BER or eye diagrams. In earlier work [8],we showed in simulation that the correlation pulseshape relates well to eye-diagram opening.The optical correlator we used is a novel type based

on a passive, White cell-based TDL. It uses a simpli-fied version of the White cell true-time delay devicepreviously developed for steering phased array an-tennas. While we used a linear White cell (numberof taps is proportional to the number of bouncesthe beams make in the White cell) with six tapsfor this demonstration, White cells with 81 taps havebeen demonstrated [11], and White cells with 6399taps have been proposed [12]. The other simplifica-tion is the ability to use a fixed, passive, micromirrorarray instead of a MEMS, since programmability isnot required for a simple TDL.The White cell correlator worked as predicted, pro-

ducing the expected output for the particular weightsused. By introducing impairments onto the inputpulses, here achieved by artificially distorting thepulses before modulating the laser, good agreementwas obtained between the experiment and thetheory.

The White cell was unnecessarily large here, be-cause the electronics available were quite slow, solong delays were needed that required long opticalpaths, nearly 12m. For realistic signal rates, thepaths can be much shorter, and the White cell canbe just a few inches on a side, even while providinghundreds of taps. Such high resolution may not beneeded for quality-of-signal monitoring but wouldbe useful for other optical signal processing functionssuch as optical code-division multiplexing and en-cryption. To simplify alignment, the objective mir-rors can be cut from a single substrate using, forexample, slow-tool diamond turning. Thus the Whitecell correlator can be a compact and robust tool foreven highly complex optical signal processing.

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