2.55%. When slot is cut in the patch, the bandwidth increases to2.78% however there is a slight mismatch (see Fig. 7). Furtherwhen slotted patch is stacked with parasitic elements of the samesize the bandwidth of the patch further improves to 4.07% (see Fig.8) and the matching also improves.

The variation of return loss with frequency for different valueof slot length and slot width is shown in Figures 9 and 10,respectively. It is observed that the resonance frequency dependsinversely on both slot length and width and matching improves.The variation of resonance frequency with substrate thickness h1

and h2 are shown in Figures 11 and 12, respectively. It is foundthat resonance frequency depends directly on substrate thicknessh1 where as it depends inversely on substrate thickness h2 howevermatching improves with increasing value of h1 and h2.

5. CONCLUSION

It may be concluded that resonance frequency of the slot loadedstacked patch is highly dependent on the slot dimensions as well asparasitic element and substrate thickness (h1 and h2).

REFERENCES

1. K.R. Carver and J.W. Mink, Microstrip antenna technology, IEEETrans Antenna Propagat 29 (1981), 2.

2. D.H. Schaubert, D.M. Pozar, and A. Adrian, Effect of microstripantenna substrate thickness and permittivity: comparison of theory andexperiment, IEEE Trans Antennas Propagat 37 (1989), 677–682.

3. J.R. James, P.S. Hall, and C. Wood, Microstrip antenna theory anddesign, Peter Peregrinus, London, 1981, pp. 158–177.

4. D.M. Pozar and D.H. Schaubert, Microstrip antennas, IEEE Press,New York, 1995, pp. 155–166.

5. G. Kumar and K.C. Gupta, Broad-band microstrip antennas usingadditional resonators gap-coupled to the radiating edges, IEEE TransAntennas Propagat 32 (1984), 1375–1379.

6. G. Kumar and K.C. Gupta, None radiating edges and four edges gapcoupled multiple resonator broad-band microstrip antennas, IEEETrans Antennas Propagat 33 (1985),173–178.

7. S.H. Wi, Y. B. Sun, I. S. Song, S. H. Choa, I. S. Koh, Y. S. Lee, andJ. G. Yuok, Package-level integrated antennas based on LTCC tech-nology, IEEE Trans Antenna Propagat 54 (2006), 2190–2197.

8. S.H. Wi, et al., Bow-tie shaped meander slot antenna for 5 GHzapplication, Proc IEEE Int Symp Antenna Propagat 2 (2002), 456–459.

9. H.-S. Tsai and A.Y. Robert, FDTD analysis of CPW fed folded slotand multiple-slot antenna on thin substrates, IEEE Trans AntennasPropagat 44 (1996), 217.

10. I.J. Bahal and P. Bhartia, Microstrip antenna, Artech House, Boston,MA, 1988.

11. L.C. Shen, Resonant frequency of a circular disk printed circuit an-tenna, IEEE Trans Antennas Propagat 25 (1977), 595–596.

12. E.A. Wolf, Antenna analysis, Artech House, Narwood, MA, 1988.13. S. Jiri, Analysis of multilayer microstrip lines by conformal mapping

method, IEEE Trans Microwave Theory Tech 40 (1992), 769.14. F.E. Terman, Electronic and radio engineering, Kogakusha, Tokyo,

Japan, 1995, pp. 15.15. IE3D Zeland software USA version 11.15, 2005.

© 2008 Wiley Periodicals, Inc.

THE INFLUENCE OF THE ASE NOISEON THE CASCADABILITY OF ACTIVEMACH-ZEHNDER INTERFEROMETERSWITCHES

Jose Manuel Martinez, Javier Herrera, Francisco Ramos, andJavier MartiNanophotonics Technology Centre, Universidad Politecnica deValencia, Camino de Vera, s/n. 46022 Valencia, Spain;Corresponding author: framos@upvnet.upv.es

Received 4 February 2008

ABSTRACT: The influence of the ASE noise on the cascadability ofSOA-MZI structures is investigated. The experimental results show thatsaturation effects in the SOAs are the main cause of signal degradation,so counter-propagation pump schemes as well as lower bias currentsand optical powers are desirable when cascading several SOA-MZI op-tical switches. © 2008 Wiley Periodicals, Inc. Microwave OptTechnol Lett 50: 2529 –2531, 2008; Published online in Wiley Inter-Science (www.interscience.wiley.com). DOI 10.1002/mop.23737

Key words: optical switches; optical performance; semiconductor opti-cal amplifiers; ASE noise

1. INTRODUCTION

The integrated active Mach-Zehnder interferometer based on semi-conductor optical amplifiers (SOAs), commonly known as SOA-MZI, is a flexible and powerful device in all-optical signal pro-

5 5.2 5.4 5.6 5.8 6-25

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Figure 11 Variation of return loss with frequency for stacked diskmicrostrip antenna for different thickness (h1) (h2 � 2.5 mm, L � 10 mm,w � 0.5 mm). [Color figure can be viewed in the online issue, which isavailable at www.interscience.wiley.com]

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Figure 12 Variation of return loss with frequency for stacked diskmicrostrip antenna for different thickness (h2) (h1 � 2.5 mm, L � 10 mm,w � 0.5 mm). [Color figure can be viewed in the online issue, which isavailable at www.interscience.wiley.com]

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 50, No. 10, October 2008 2629

cessing techniques [1]. To date, all-optical wavelength converters,demultiplexers, regenerators, switches, logic gates and flip-flops[2–6], among others, have been implemented employing the SOA-MZI as the basic functional device. In all these applications, theSOA-MZI operation relies on nonlinear cross-phase modulation(XPM) effects in the SOAs induced by strong CW or pulsedoptical signals. Depending on the presence or absence of suchcontrol signals, the input data signal interferes either destructivelyor constructively at the output of the interferometer due to theXPM-induced phase shifts. The main advantages of SOA-MZIdevices for all-optical signal processing applications are their lowswitching energies, their photonic integration potential and theircapability of high-speed operation. For example, operation at 40Gb/s bitrates and beyond was demonstrated employing a differen-tial scheme at the SOA-MZI input [7].

However, an important drawback of SOA-MZI devices is theASE noise introduced by the SOAs [2] which considerably limitsthe maximum number of devices that a signal can sequentiallytransverse without optical regeneration. Therefore, those applica-tions in which several SOA-MZI devices need to be cascaded willbe severely affected, such as the implementation of all-opticalpacket header processors [8] or the use of SOA-MZI-based 1 � 2switches to build 1 � N and N � N switching matrices [9, 10]. Toour knowledge, experiments with three or more cascaded SOA-MZI devices without signal regeneration have not been reportedyet. On the other hand, optical regeneration based on two cascadedSOA-MZIs has been demonstrated in a recirculating loop config-uration [4]. In this experiment, up to 100 error-free circulations ina 40-km-long loop were demonstrated employing SOA-MZI-basedoptical 3R regenerators.

In this paper, the influence of the ASE noise on the cascadabil-ity of SOA-MZI structures is investigated. To this end, severalSOA-MZI switches are serially interconnected and their operationparameters tuned to check the influence of the accumulated ASEnoise levels on the signal performance. Conversely to the experi-mental set-up in [4], there is no signal recirculation here (“truecascading”) and the SOA-MZI switches do not provide signalregeneration. The experimental results show that the ASE noiselevels severely limit the number of SOA-MZI stages which can becascaded. As an example, around 1-dB Q-factor penalty is intro-duced by each SOA-MZI switch. So the maximum number ofSOA-MZI devices which can be cascaded for error-free operationis limited and some guidelines must be considered when usingthese devices in a cascaded configuration.

2. EXPERIMENT

The experimental set-up shown in Figure 1 was used to assess theinfluence of the ASE noise introduced by a cascade of SOA-MZI-based switches. A fiber-based pulsed laser at 1553 nm with arepetition rate of 10 GHz was externally modulated with a 10

Gbit/s PRBS using a Mach-Zehnder modulator (MZM). The op-tical signal at the output of the MZM was then amplified, filteredto reduce the ASE noise introduced by the EDFA and its polar-ization state controlled before being applied to one of the commoninput ports of the first SOA-MZI switch. The peak power of theinput pulses was adjusted employing a variable optical attenuator.An optional CW pump was also applied to one of the SOA-MZIbranches to switch the optical signal between both common outputports. Several SOA-MZI switches were cascaded to account for theinfluence of the accumulated ASE noise introduced by the SOAson the signal performance passing through them. The output fromeach SOA-MZI-based switching stage was interconnected to theinput of the next SOA-MZI switch through an optical filter toreduce the ASE noise and a variable optical attenuator to set aconstant input power at the successive stages. The SOA-MZIs usedin the experiments were provided by The Centre for IntegratedPhotonics. They have eight input/output ports: two common inputports on one side, two common output ports on the other side andtwo additional input ports on each side for introducing co- andcounter-propagating signals in the upper and lower SOA-MZIbranches. Although Figure 1 shows a counter-propagating pumpconfiguration, an scheme with co-propagating signals was alsoconsidered. The SOAs in the SOA-MZI switches are characterizedby a length of 2 mm and a noise figure of 8–10 dB. The measure-ment points in Figure 1 are marked as #b-b, #1, . . . , #n�1 and #n.A total of n � 4 SOA-MZIs were used in the experiments. Anoscilloscope with optical input was connected to each of thesepoints to measure the signal quality by means of the Q-factor.

3. RESULTS AND DISCUSSION

The experimental results for the Q-factor as a function of thenumber of SOA-MZI stages is shown in Figure 2 for differentSOA bias currents and for an input peak power of 1 mW. Theback-to-back measurement is represented by the zero abscissa inFigure 2 (Q � 8.40 dB). Initially, no pump signal was used toswitch the input signal between the SOA-MZI output ports. Ob-viously, the signal performance is degraded by increasing thenumber of SOA-MZI switches that the signal goes through be-cause they do not provide signal regeneration. Slightly betterperformance is obtained for higher bias currents up to an specificnumber of switching stages, from which an opposite behavior isfound. For example, a 150-mA bias current provides better per-formance at the output of the first SOA-MZI, but worse resultswhen cascading two or more SOA-MZIs, whereas a 125 mA biasFigure 1 Experimental set-up

Figure 2 Signal performance as a function of the number of switchingstages for different SOA bias currents: 100 mA (triangles), 125 mA(squares), and 150 mA (circles)

2630 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 50, No. 10, October 2008 DOI 10.1002/mop

current provides better results than 100 mA up to four switchingstages. The main reason for this behavior is that the accumulatedASE noise level grows with the number of SOA-MZI switches, soit is better to reduce the bias current as the number of switchingstages increases to avoid saturation.

On the other hand, the performance as a function of the numberof SOA-MZI stages is shown in Figure 3 for different input peakpowers and for a constant SOA bias current of 125 mA. In asimilar way, better results are obtained for higher input peakpowers, but the behavior changes when cascading more SOA-MZIs. For example, the better performance is obtained at theoutput of the first SOA-MZI and for back-to-back measurementsfor a 2-mW input peak power, whereas it becomes the worst whencascading two or more SOA-MZIs. This behavior may be ex-plained in a similar way to the previous one.

Finally, some measurements were carried out to see the influ-ence of using an external 0 dBm CW pump to switch the inputsignal between the SOA-MZI output ports. In this case, the per-formance results are shown in Figure 4 for two different config-urations: co- and counter-propagating pump signal. As it can bededuced from the results of Figure 4, generally the use of a pumpsignal enhances the system performance. Furthermore, the signalquality is much better when using a counter-propagation scheme.For example, Q values higher than 7 dB are obtained for up to 4SOA-MZI switching stages, which assures error-free operation.The better results for the counter-propagation scheme may beexplained by the fact that a lower signal gain is obtained for thisconfiguration, and therefore less saturation is obtained.

4. CONCLUSION

The ASE noise introduced by SOA-MZI-based switches severelydegrades the signal quality when they are interconnected in cas-caded configurations. The experimental results show the perfor-mance degradation as a function of the number of cascaded de-vices for up to four devices. Although there is a clear dependenceon system parameters such as bias currents or optical powers, inour experiments around 1-dB Q-factor degradation per SOA-MZIstage was found. The experimental results show that it is veryimportant to avoid saturation when trying to reduce the influenceof ASE noise and optimize the signal performance. Therefore,counter-propagation pump configurations, as well as low biascurrents and optical powers are desirable when working with ahigh number of SOA-MZI switching stages.

ACKNOWLEDGMENT

The European Commission is gratefully acknowledged for partialfunding of the IST-2004–507509 LASAGNE project in the 6thFramework programme.

REFERENCES

1. B. Dagens, A. Labrousse, R. Brenot, B. Lavigne, and M. Renaud,SOA-based devices for all-optical signal processing, Proc. Conf. Op-tical Communications (OFC’2003), 2003, Vol. 2, pp. 582–583, Invitedpaper ThX1.

2. S.-C. Cao and J.C. Cartledge, Time- and frequency-domain character-ization of the modulated ASE noise in SOA-MZI wavelength convert-ers, IEEE Photon Technol Lett 14 (2002), 962–964.

3. M. Heid, S.L. Jansen, S. Spalter, E. Meissner, W. Vogt, and H.Melchior, 160-Gbit/s demultiplexing to base rates of 10 and 40 Gbit/swith a monolithically integrated SOA-Mach-Zehnder interferometer,Proc. European Conf. Optical Communications (ECOC’2001), Vol. 6,2001, pp. 82–83, paper 8.4.3.

4. O. Leclerc, B. Lavigne, E. Balmefrezol, P. Brindel, L. Pierre, D.Rouvillain, and F. Seguineau, Optical regeneration at 40 Gb/s andbeyond, J Lightwave Technol 21 (2003), 2779–2790.

5. T. Fjelde, D. Wolfson, A. Kloch, B. Dagens, A. Coquelin, I. Guil-lemot, F. Gaborit, F. Poingt, and M. Renaud, Demonstration of 20Gbit/s all-optical logic XOR in integrated SOA-based interferometricwavelength converter, Electron Lett 36 (2000), 1863–1864.

6. R. Clavero, F. Ramos, and J. Marti, All-optical flip-flop based on anactive Mach-Zehnder interferometer with a feedback loop, Opt Lett 30(2005), 2861–2863.

7. H. Chen, G. Zhu, Q. Wang, J. Jaques, J. Leuthold, A.B. Piccirilli, andN.K. Dutta, All-optical logic XOR using differential scheme andMach-Zehnder interferometer, Electron Lett 38 (2002), pp. 1271–1272.

8. J.M. Martinez, F. Ramos, and J. Marti, All-optical packet headerprocessor based on cascaded SOA-MZIs, Electron Lett 40 (2004), pp.894–895.

9. R.A. Spanke, Architectures for guided-wave optical space switchingsystems, IEEE Commun Mag 25 (1987), 42–48.

10. R. Clavero, F. Ramos, and J. Marti, All-optical self-routing latchingswitch based on active Mach-Zehnder interferometer, IEEE PhotonTechnol Lett 18 (2006), 2475–2477.

© 2008 Wiley Periodicals, Inc.

Figure 3 Signal performance as a function of the number of switchingstages for different input peak powers: 0.5 mW (triangles), 1 mW(squares), and 2 mW (circles)

Figure 4 Signal performance as a function of the number of switchingstages using a co-propagating (squares) or counter-propagating (circles)pump as the control signal. The dashed line indicates the scheme with nopump signal

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 50, No. 10, October 2008 2631