IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 19, NO. 6, NOVEMBER/DECEMBER 2013 3400307

Low-Crosstalk Balanced Bridge Interferometric-TypeOptical Switch for Optical Signal Routing

Akito Chiba, Tetsuya Kawanishi, Fellow, IEEE, Takahide Sakamoto, Member, IEEE, Kaoru Higuma,Kazumasa Takada, and Masayuki Izutsu, Life Fellow, IEEE

AbstractCrosstalk is one of the significant measures of an op-tical cross-bar switch. In this paper, we describe a configurationfor crosstalk suppression of a balanced-bridge interferometric typeoptical switch and its application to an optical-signal routing de-vice. Since crosstalk of the optical switch originates from imbal-ance in the amplitude of a lightwave propagating within the armof the interferometer, additional MachZehnder structures wereembedded for trimming the amplitude of the light. By adjustingthe transmittance of the trimmers, crosstalk of less than 56 dBis achieved for dc voltage. Also, for high-frequency voltages, acrosstalk estimation procedure was developed, and crosstalk at 10-GHz sinusoidal voltage was evaluated to be 48 dB. Furthermore,based on the transient response measurement, its switching timewas evaluated to be 26 ps. For a demonstration utilizing extremelylow crosstalk and quick response, a guard-time-free optical signalrouting experiment is also described.

Index TermsOptical crosstalk, optical modulation, opticalswitches, routing.

I. INTRODUCTION

THE development of optical switches has been accelerat-ing to satisfy demand in optical communication networks,including quick response, increase of number of optical ports,and low crosstalk. It has been addressed that an optical switchbecomes a bottleneck for increase of transmission rate in anoptical network, if we adopt slow-response optical switch. Forquick response, lithium niobate (LN) is attractive material foroptical devices due to its electro-optical (EO) effect. In additionto the phase of light, the amplitude can be also precisely adjustedby employing unique device structures on an LN substrate [1],[2]. One of the conventional LN devices is an optical inten-sity modulator utilizing the MachZehnder (MZ) waveguidestructure composed of two Y-junctions. Since the junction pos-sesses one coupling port to a radiation mode, Y-junction seemsto be equivalent to an X-junction [3]: i.e., the MZ structure

Manuscript received March 1, 2013; revised April 29, 2013 and April 30,2013; accepted May 1, 2013. Date of publication May 30, 2013; date of currentversion July 23, 2013. This work was supported in part by NEDO (04A12007)and in part by JSPS (20760254, 24760268).

A. Chiba and K. Takada are with the Division of Electronics and Informatics,Faculty of Science and Technology, Gunma University, Gunma 376-0034, Japan(e-mail: akito.chiba@osamember.org; takada@el.gunma-u.ac.jp).

T. Kawanishi and T. Sakamoto are with the Optical Network Institute, Na-tional Institute of Information and Communications Technology (NICT), Tokyo184-8795, Japan (e-mail: kawanish@nict.go.jp; tsaka@nict.go.jp).

K. Higuma is with the New Technology Research Laboratories, Sum-itomo Osaka Cement Company Ltd., Chiba 274-8601, Japan (e-mail:khiguma@soc.co.jp).

M. Izutsu is with Faculty of Science and Engineering, Waseda University,Tokyo 169-8555, Japan (e-mail: izutsu@aoni.waseda.jp).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JSTQE.2013.2263121

P2'

P1'

Fig. 1. Device structure of a 22 optical switch. P1, P2: input ports of light;P1, P2: output ports of light; EA, EB, EM: electrode; MZA, MZB: Sub-MZstructure as an intensity trimmer.

is also applicable to a waveguide-type optical switch. Such adevice is so-called balanced-bridge interferometric (BBI) typeoptical switch [4], [5], whereas optical switches based on a di-rectional coupler [6], an X junction [7], and an asymmetric-Xjunction [8] are also fabricated. Polarization dependence of theEO effect in an LN optical cross-bar switch has been overcomeby the polarization-diversity configuration [9].

Crosstalk is one of the significant measures of an opticalswitch, which induce mixing of optical signals at the switch.Although the optical switch based on micro electromechanicalsystems has low crosstalk, switching speed is limited by itsoperation principle. Similar to the optical extinction ratio ofthe MZ optical modulator, crosstalk of the BBI-type opticalswitch originates from an imbalance in the amplitude betweenlightwaves propagating in each arm of an MZ structure. In orderto achieve a BBI-type optical switch with 40-dB crosstalk,less than 0.1-dB difference is required in the dividing ratio ofdirectional couplers consisting of an interferometer [10], [11].In fabrication of the coupler, this requirement is severe to endurefabrication tolerance, compared with MZ structure based on theY-junctions. Then, recently such a requirement in the differencehas been moderated by employing intensity trimmers [12].

In this paper, we describe a low-crosstalk optical cross-barswitch employing intensity trimmers. In Section II, the devicestructure of the optical switch wherein MZ structures as in-tensity trimmers were embedded is introduced. In Section III,after discussion on crosstalk for dc signal and its suppression,crosstalk for an rf switching signal is estimated based on amodel analysis. In Section IV, the transient response of theswitch is investigated. In Section V, using the switch, extremelylow crosstalk onoff keying (OOK) signal switching withoutany guard time is demonstrated. In Section VI, the dependenceof crosstalk suppression on the carrier wavelength is evaluated.

II. DEVICE STRUCTUREWe integrated an optical 22 switch on an X-cut LiNbO3

substrate with a thickness of 1 mm and an area of about 2 mm90 mm [11]. Fig. 1 shows its configuration, where optical waveg-uides were fabricated by standard 15-h Ti thermal-diffusion pro-cess. Two ports of one optical X-junction coupler (first coupler)

1077-260X 2013 IEEE

3400307 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 19, NO. 6, NOVEMBER/DECEMBER 2013

are used for light input into the switch (P1 and P2 in Fig. 1),while the other ports are connected with each input port of twoMZ structures (MZA and MZB in Fig. 1). Likewise, the out-put ports of the MZ structures are connected to ports of theother coupler (second coupler). For these couplers, the widthof the both waveguides connecting with the MZ structure wasoptimized to be 7 m. On the other hand, the width of the otherwaveguides was slightly modified in a range of few m, in orderto achieve the low crosstalk when the conventional BBI-type op-tical switch is constructed by the couplers. Both of the couplerswere designed to be 3-dB couplers for 1611.3-nm wavelengthlightwave. Actually, the fabrication results were sensitive for pa-rameters such as diffusion time, due to large Ti diffusion alongto horizontal direction of the substrate. In the waveguide struc-ture, optical paths between coupling regions of each coupler arereferred as the main-MZ structure for optical switching.

To choose an output port of light, we deposited a travelling-wave electrode with waveguides between the couplers (mainelectrode, EM in Fig. 1). By applying a voltage to EM, phasedifference of lightwaves propagating in each waveguide occursso that the output port of light can be controlled by the voltage.Similarly, with each MZ structure, we also deposited electrodesto control the balance in optical power propagating in eachwaveguide (electrode A and electrode B, EA and EB in Fig. 1).The halfwave voltages V , required for change in optical inten-sity transmitance from maximum to minimum, were 5.7, 15.6,and 15.5 V for the main electrode, electrode A, and electrodeB, respectively. The 3-dB bandwidth of the main electrode wasevaluated to be 14.1 GHz by a lightwave component analyzer(Agilent, E8361 C and 4373 C).

III. CROSSTALK SUPPRESSION BY INTENSITY TRIMMING

A. Measurement SetupFor evaluating crosstalk of the switch with nested MZ struc-

tures, we used an external-cavity diode laser (Agilent, 81642Aand 81682A) as a light source. Light launched into the inputport 1 of the optical 22 switch and light power was measuredat each output port by an optical power meter (dBm optics,Model 4100, dynamic range: from 10 to 95 dBm). DC voltagewas applied to the EM for evaluating crosstalk for the dc sig-nal, as described in Section III-B. For demonstrating continuousrouting of a lightwave (Section C) and for estimating crosstalkat a high frequency signal (Section D), a square wave and asinusoidal wave with dc offset were applied, respectively.

B. Crosstalk for the DC signalThe effect of intensity trimming in the switch on its crosstalk

was evaluated. The intensity trimming is performed by adjust-ing the intensity transmittance of SMZA and SMZB via a biasvoltage, so as to balance the amplitude of light propagating ineach arm of the interferometer for switching [11], [12]. Forcomparison, crosstalk of the switch whose MZ structures hadmaximum transmittance was also evaluated. The optical switchadjusted to this condition seems to be a 22 MZ optical switchwithout any MZ structures.

Fig. 2. Dependence of bias voltage applied to the main electrode on outputintensity transmittance when light is launched at the input port 1. The solid anddashed lines indicate the case when the intensity in each arm are trimmed ornot, respectively. The thick and thin lines are the transmittance at output ports1 and 2, respectively.

Fig. 3. Time trace of output intensity measured at each output port. Each lineplotted in this figure is under the same conditions as the lines in Fig. 2.

The bias voltage dependence of the intensity transmittanceis shown in Fig. 2. In this plot, the zero in the horizontal axiscorresponds to the balanced state where the switch acts as a3-dB coupler, and the axis is normalized by V of the mainelectrode. The vertical axis is also normalized by the maximumpower emitted at port 1 of the switch without intensity trim-ming. The transmittance of the output port 1, plotted usingthick lines, was minimized at the normalized bias voltage of0.5. This minimum was further decreased from 33.6 to lessthan 58.2 dB after intensity trimming. By trimming, the biasvoltage shift of EA was 2.1 V, which corresponds to a decreaseof 9% in amplitude transmittance by the MZ structure. On theother hand, the decrease in the maximum transmittance is only1.3 dB which is much smaller compared to the enhancement inthe intensity drop. By using the intensity trimming, we eventu-ally obtained a crosstalk of less than 56 dB for the port 1. Sucha crosstalk suppression was also achieved for the output port 2plotted using thin lines. At the normalized bias voltage of 0.5,the intensity drop of 33.0 dB became less than 56.6 dB.

C. Output Port Selection by a Square WaveThe results shown in Fig. 2 also imply that alternate switching

with extremely low-crosstalk can be achieved by a square-wavevoltage with a peak-to-peak amplitude of V . Fig. 3 shows a timetrace of an optical signal passing through the switch driven bythe square wave. The continuous-wave (CW) light launched intoinput port 1 was 5.1 dBm, and the period of the square wave wasset at 200 ms. The solid lines were obtained after the intensitytrimming. The dashed lines were obtained when the intensity

CHIBA et al.: LOW-CROSSTALK BALANCED BRIDGE INTERFEROMETRIC-TYPE OPTICAL SWITCH 3400307

Fig. 4. Analysis model of a balanced-bridge interferometric type opticalswitch with intensity trimmers. Bold lines indicate optical waveguides con-sisting of the optical switch. Dashed arrows indicate propagation directions oflightwave related to each transmission coefficient. 1, 2: Optical input port; 1, 2:Optical output port; 1A , i1B : Amplitude transmittance of 1st coupler; A1 ,iA2 , iB1 , B2 : Amplitude transmittance of the 2nd coupler; A , B : Am-plitude transmittance of nested MZ structures as intensity trimmers. +, :phase shift of lightwaves passing through the optical waveguides.

transmittance of the nested MZ structures was a maximum, sothat the intensity was not trimmed. The thick and thin linesindicate the output intensity at ports 1 and 2, respectively. Asshown in Fig. 3, either the bar state or the cross state is setalternatively with a crosstalk of less than 55 dB. Although thetransient response of the switch seems to be slow, this is dueto the frequency bandwidth of the power meter possessing awide dynamic range. We also adopted an alternative methodfor simultaneous evaluation of the frequency response and thecrosstalk, which is described in the following section.

D. Crosstalk for RF Switching SignalAs described in the previous section, simultaneous evalua-

tion of the rise/fall time and crosstalk of the switch would bedifficult, originating from the finite sensitivity (gain)-bandwidthproduct of the optical-power detector. However, for a monochro-matic rf signal, the crosstalk can be estimated from the intensitypeaks in the optical spectrum. When we feed a sinusoidal sig-nal into the main electrode of the switch, a double sidebandsuppressed-carrier (DSB-SC) optical modulation signal is cre-ated at a drop port, and its carrier component in the opticalspectrum is suppressed together with the even-order sidebands.Since the degree of carrier (and also even-order sidebands) sup-pression in DSB-SC modulation depends on the crosstalk of theswitch, estimation of crosstalk is possible.

We consider the configuration where the CW light, whoseelectric amplitude is E0 , is launched into either input port (1or 2) of the switch, as shown in Fig. 4. is the optical phaseshift induced in one waveguide. In another waveguide, the op-posite phase shift (optical path length difference) is inducedsimultaneously due to the pushpull operation, so that the phasedifference between lightwaves launching into each input port ofthe second coupler is 2 . It should be noted that the conditionof = 0 corresponds to the cross state of the switch.

Assuming that incident light launched at input port n (n = 1or 2), the electric field of light (normalized by E0) at the outputport m (m = 1 or 2) can be expressed as

Enm

E0= in+m

{(Anm + Bnm )ei

(1)n+m (Anm Bnm )ei}

= 2i[Anm sin

( +

n + m2

)

iBnm cos(

+n + m

2

)]. (1)

Where Anm and Bnm are constants related to the optical pathbetween the ports n and m, depending on the amplitude trans-mittance of the couplers and attenuation by the MZ structure:

Anm =12(nAAAm + nB B Bm ) (2)

Bnm =12(nAAAm nB B Bm ). (3)

Here, nA and nB are the absolute values of amplitudetransmittance of the first coupler between port n and the portconnected to the MZ structures A and B, respectively. SimilarlyAm and Bm are the amplitude transmittance of the secondcoupler. It should be noted that and implicitly include ex-cess losses of the couplers. A and B represent the amplitudetransmittance at the MZ structures A and B used as intensitytrimmers, respectively. In the ideal optical switch wherein lightperfectly disappears at the drop port due to destructive interfer-ence, Bnm equals zero. When only dc bias is applied and swept,the maximum of the normalized amplitude at the through portcorresponds to Anm . In other words, the value |Anm / Bnm |2is the extinction ratio of the switch at the output port m, so that|Bnp / Anm |2 corresponds to crosstalk at the output port m (pequals 1 or 2 and unequal to m).

Applying a dc bias voltage V0 with sinusoidal voltage whoseamplitude and angular frequency are Vm and 0 , respectively,the optical phase shift induced in one waveguide correspondsto

= 0 + sin0t =

2

(V0

V (DC )+

VmV (AC )

sin0t)

(4)

where V (DC) and V (AC) are the halfwave voltages of the elec-trode Em for the dc and rf signals, respectively. Launching lightinto port 1, the normalized amplitudes at the output port 1 canbe expanded with respect to the above :

E11

E0= 2 [B11 cos 0 + iA11 sin 0 ]

p=J2p() exp(i2p0t)

+ 2i [B11 sin 0 iA11 cos 0 ]

p=J2p1() exp(i(2p 1)0t). (5)

Then, even-order and odd-order sideband peaks I2q and I2q1in the normalized optical intensity spectrum at port 1 are derivedas

I2q (B211 cos

2 0 + A211 sin2 0

)J22q () (6)

I2q1 (B211 sin

2 0 + A211 cos2 0

)J22q1() (7)

3400307 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 19, NO. 6, NOVEMBER/DECEMBER 2013

respectively. Similarly, at the output port 2, the normalizedamplitude and sideband peak intensity are:

E21

E0= 2 [iA12 cos 0 B12 sin 0 ]

p=J2p() exp(i2p0t)

+ 2i [iA12 sin 0 + B12 cos 0 ]

p=J2p1() exp(i(2p 1)0t), (8)

I2q (A212 cos

2 0 + B212 sin2 0

)J22q () (9)

I2q1 (A212 sin

2 0 + B212 cos2 0

)J22q1(). (10)

As described previously, crosstalk corresponds to the squareof the ratio between Bnp and Anm , so that crosstalk can beestimated from the peak-intensity ratio of sidebands measuredat each output port. If the degree of intensity trimming by theMZ structure is small to achieve large crosstalk suppression,imbalance in the coupling ratio at each coupler is also small, sothat the difference would be less than the excess-loss difference.For example, if the relative error of the coupling ratio is 10%for both couplers, then roughly 10% trimming in intensity wouldbe required. In this case, the difference between A11 and A21 would be estimated to be 1.4%, corresponding to 0.06 dB. Then,crosstalk is also estimated from the optical spectrum acquiredat one port. From (6) and (7), at the 0 = 0 where port 1 is setto the drop port, the following relation can be derived:

B211

A211 =

I0I1

J21 ()J20 ()

. (11)

Since the argument of the Bessel functions in the order of 0thand first is derived from the peak-intensity ratio for odd-ordersidebands, crosstalk at the cross state of the switch can be esti-mated.

For evaluating the crosstalk for high-frequency voltage, thespectrum of light passing through the optical switch drivenby the rf signal was evaluated [13]. 10 dBm CW lightwavewith 1611.3-nm wavelength is launched into input port 1 of theswitch, wherein 10-GHz sinusoidal voltage with an rf power of21.9 dBm is applied. The bias voltage of the main electrode was5.9 V so as to set the switch to the cross state: i.e. port 1 wasset as a drop port so that odd-order sidebands are dominant inthe lightwave.

Fig. 5 shows the optical spectra measured at output port 1,using an optical spectrum analyzer (Advantest, Q8386). Thesolid curve indicates the case when the light within the switchwas trimmed by the nested MZ structures in order to suppresscrosstalk, while the dotted line was obtained from the switch inwhich transmittance of both MZ structures is maximum. As canbe seen, for both curves, there are few differences in the peakpower of the odd-order sidebands, indicating that the effect ofintensity trimming is negligible for the desired components inthe lightwave. For the first-order sideband, the peak intensitywas 29.45 dBm, which was almost the same as the first-ordersidebands at port 2 of the switch set as the BAR state. On the

Fig. 5. Optical modulation spectrum acquired at port 1 of the switch. The solidand dashed lines were obtained from the switch where the intensity trimmer wasactivated or not activated, respectively.

other hand, suppression of the carrier and even-order sidebandwere different: the peak power at carrier wavelength was sup-pressed, from 50.1 to 68.1 dBm, by the intensity trimming.Since obtained from the peak-intensity ratio between thefirst sideband and the third sideband is 0.201, crosstalk foreach condition is derived as 30.2 and 48.2 dB, respectively,i.e., a crosstalk suppression of 18 dB was achieved for the rfsignal. Actually, in this case, crosstalk at port 2 was slightlydepressed due to the imbalance in the amplitude transmittance,though it was also confirmed that such suppression can be pos-sible for both ports in the order of 10 dB. These results indicatethat, in addition to low crosstalk, rapid switching would be ex-pected without accumulation of undesired lightwave signals.

IV. TRANSIENT RESPONSE OF OPTICAL SWITCHINGThe aforementioned results imply the possibility of the 22

switch as an ideal port-switching device, possessing both rapidresponse and extremely low crosstalk, so that a transient re-sponse of the optical switch was evaluated [14]. As the input intoport 1 of the switch, the CW light generated from an external-cavity diode laser (wavelength: 1609 nm) followed by an opticalamplifier, and a polarization controller, were employed. The biasvoltage of the main electrode EM for output port selection wasadjusted to be equal in intensity between the output ports, anda 50% duty ratio, 77-MHz rectangular signal was superposedon the bias. For generating the rectangular signal with 20-Gb/ssignal rate, a conventional pulse-pattern generator (PPG), whichcan generate a binary signal of up to 44-Gb/s, was used. Therectangular signal was amplified by a broadband amplifier witha bandwidth of 38 GHz and 22-dBm saturation power, and ap-plied to the electrode EM via attenuators. Its waveform is shownin Fig. 6(a). Fig. 6(b) shows the output light signal detected byinverting-output photodiode having a bandwidth of 50 GHz,after amplification by an EDFA followed by a Gaussian-shapeoptical filter (3 dB bandwidth: 0.6 nm). In this figure, a substep-like response due to the broadband amplifier characteristic isobserved before the signal reaches its high-level state, and therise time for 20%80% in peak-to-peak voltage is evaluatedto be 26 ps. Some fluctuation observed at both signal level onFig. 6(b) would be mainly due to photodiode itself, since sucha trace acquired by an optical port on the sampling oscilloscopeshow less fluctuation.

CHIBA et al.: LOW-CROSSTALK BALANCED BRIDGE INTERFEROMETRIC-TYPE OPTICAL SWITCH 3400307

Fig. 6. Time trace of (a) an electric signal applied to the main electrode of the22 optical switch and the intensity at the output port of the optical switch. Therange is 20 ps/div for the horizontal axis and (a) 1 V/div. and (b) 0.01 V/div. forthe vertical axis, respectively.

Fig. 7. Experimental setup for the (a) Tx side, (b) switching side, and (c) Rxside. Solid lines and dashed lines indicate an optical signal and an electric signalfor the data signal, respectively. LD: External-cavity laser diode; P.C.: Polar-ization controller; Mod.: Optical Intensity modulator; Amp: Optical Amplifier;BPF: Optical band-pass filter; VOA: Variable optical attenuator; 1, 2: optical sig-nal input port of the optical switch; 1,2: optical signal output port of the opticalswitch; PM: Inline power meter; PD: Photodiode; rfAmp: broadband Amplifier;PPG: Pulse-pattern generator; BERT: Bit-error rate tester; OSC: oscilloscope.

V. DEMONSTRATION OF OOK SIGNAL SWITCHING

A. SetupFor evaluating switching performance for a nonreturn-to zero

OOK (NRZ-OOK) signal, an experimental setup shown in Fig. 7was constructed. The light source described in Section III-A wasalso used in the setup. By using another 10-GHz PPG, a PRBSsequence (length: 271) followed by a 0 was created as the datasignal, and the sequence was amplified by another broadbandamplifier with a bandwidth of 18 GHz and saturation power of24 dBm for intensity modulation of the light generated by thesource. The NRZ-modulation light was launched into the opti-

Fig. 8. Time trace (2 ns / div.) of intensity (0.1 mW/div) acquired at (a) theport 1 and (b)(c) the port 2 of the switch, respectively. The switch was (a)(b)alternatively driven by a rectangular voltage signal and (c) acts as a 3-dB opticalcoupler by static dc bias voltage.

cal switch driven by the same rectangular signal as describedpreviously for port selection via an optical attenuator. The lightof one output port was detected in the same manner describedpreviously, except for the following point: after detection, thesignal was amplified by a 38-GHz bandwidth broadband am-plifier and divided into two paths for simultaneous observationof time trace and bit-error rate (BER). During the experiment,the two PPGs and the BER tester were synchronized by a 10-GHz signal generator. It should be noted that, for simplicity, thedc bias and 10-GHz signal generator for synchronizing pulse-pattern generators and BER tester are omitted in Fig. 7.

B. Experimental ResultsThe time-traced intensity of the NRZ-OOK signal at output

port 1 and 2 are shown in Fig. 8(a) and (b), respectively. Forcomparison, in Fig. 8(c), the optical signal at port 2 of theswitch where no port-selection signal is applied (i.e., the opticalswitch acted as a 3-dB optical coupler) is also shown. As canbe seen, when each output port is at its drop state as determinedby the control signal, the output intensities of the switch aresufficiently dropped due to sufficient crosstalk enhanced by theintensity trimmers.

BER characteristics for both output ports when the opticaldata were launched into port 1 and 2 are shown in Fig. 9. Here,the triangles and reversed triangles indicate the BERs measuredat port 1 and port 2, respectively. For reference, the BER char-acteristics were also measured for optical signals which did notpass through the LN optical switch, indicated by white circlesin Fig. 9(a). The dashed line is the 3 dB shifted fitting line ofthe white circles, which indicates effective reference due to theaverage power difference from the optical data passing throughthe LN optical switch. A power penalty of 1.2 dB is observedfor optical signals passing through the switch from the input

3400307 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 19, NO. 6, NOVEMBER/DECEMBER 2013

Fig. 9. BER curves evaluated at the output port 1 (Triangles) and 2 (reversedtriangles), respectively. The optical signal launched at (a) port 1 and (b) port 2,respectively. Open circles are obtained from the signal not passing through theoptical switch.

Fig. 10. Extinction ratio at port 1 of the switch when light was launched atinput port 1. Open circles are obtained from the switch in which intensity istrimmed. Solid circles are obtained from the switch, bias of whose MZ structuresis set at maximum-transmittance bias point.

port 1 to the output port 1. When we changed the timing ofport selection against the optical data signal launched into port1, 0.3 and 1.0 dB power penalties were observed for BERs ofthe signals emitted at the output ports 1 and 2 respectively, sothat the dependence on the switching timing for the data wouldbe negligible. This implies that the guard time of the packet-like optical data can be decreased further by using an adequateoptical switch and developing port-selection control protocols.

VI. WAVELENGTH DEPENDENCE OF CROSSTALKWe investigated the effectiveness of the intensity trimmer for

other carrier wavelengths, by adopting the extinction ratio atoutput port 1 as a measure. Fig. 10 shows the dependence ofthe extinction ratio on the wavelength ranging from 1525.0 to1635.0 nm. In this figure, the open circles are obtained from theswitch, where intensity trimming is performed at each wave-length. For comparison, extinction ratio of the switch actingas a conventional BBI cross-bar switch, i.e., transmittance ofnested MachZehnder structures are set to its maximum, is alsoplotted by the filled circle. By controlling the MZ bias voltages,at the drop port (port 1) the extinction ratio was enhanced overthe wavelength range of 110 nm which almost covers the rangenear the C- and L-band. The maximum increase in extinctionratio at port 1 was 37.1 dB at 1562.5-nm wavelength, which

reached 46 dB in the extinction ratio. In this case, when the ex-tinction ratio at one port was initially poor before the intensitytrimming, the intensity trimming has a smaller influence on theextinction ratio at the other port. Actually, amplitude transmit-tance of the two couplers deviate from ideal for the wavelengthfar from 1611.3 nm, and such dependence on wavelength is sym-metric with respect to inputoutput port combination. Then, forcrosstalk suppression in wide wavelength at both ports, more de-tailed optimization of the coupler for wavelength-independentoperation would be unavoidable. By combination with the in-tensity trimmer, BBI-type optical switch with extremely lowcrosstalk would be endured for lightwave without dependenceon the wavelength.

VII. CONCLUSIONCrosstalk of the BBI-type optical cross-bar switch is highly

suppressed, by adjusting embedded MZ structures as intensitytrimmers in each arm. By adjusting transmittance of the MZstructures, intensity imbalance in the interferometer is highlysuppressed, so that the crosstalk of less than 55 dB has beenachieved. And, for a high-frequency voltage signal, the deviationof the crosstalk from an optical spectrum is introduced. Accord-ing to the procedure, it is shown that the trimmer can also beuseful for quick optical switching. Utilizing the extremely highcrosstalk and rapid response of the switch, OOK signal routinghas also been demonstrated without any guard time. Nested MZstructures as intensity trimmers would be a useful configurationto improve the performance of the BBI-type optical switch.

ACKNOWLEDGMENT

The authors would like to thank Dr. G. -W. Lu of NICT forhis encouragement and fruitful discussions.

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[11] A. Chiba, T. Kawanishi, T. Sakamoto, K. Higuma, and M. Izutsu,Crosstalk suppression of a balanced bridge interferometric-type LiNbO3optical switch by using MachZehnder structures, IEEE Photon. Technol.Lett., vol. 20, no. 10, pp. 872874, May 2008.

[12] T. Kawanishi, T. Sakamoto, M. Tsuchiya, M. Izutsu, S. Mori, andK. Higuma, 70 dB extinction-ratio LiNbO3 optical intensity modula-tor for two-tone lightwave generation, presented at the Tech. Dig. Opt.Fiber Commun. Conf. 2006, Anaheim, CA, USA, 2006, Paper OWC4.

[13] A. Chiba, T. Kawanishi, T. Sakamoto, K. Higuma, and M. Izutsu, High-extinction-ratio (>55 dB) port selection by using a high-speed LiNbO3optical switch with intensity trimmers, in Proc. Tech. Dig. IEEE-LEOSPhotonics in Switching, Aug. 2007, pp. 5152.

[14] A. Chiba, T. Kawanishi, T. Sakamoto, K. Higuma, and M. Izutsu, Guard-time-free port selection by a LiNbO3 optical switch embedded intensitytrimmers for optical packet switching, presented at the Opt. Fiber Com-mun. Conf., San Diego, CA, Mar. 2009, Paper JThA35.

Akito Chiba received the B.E. degree in electric andprecision engineering, and the M.E. and Ph.D. de-grees in the field of electronics and information engi-neering from Hokkaido University, Sapporo, Japan,in 2000, 2002, and 2005, respectively.

From 20052010, he was with Lightwave DevicesProject in New-Generation Network Research Cen-ter, National Institute of Information and Communi-cations Technology (NICT), Koganei, Tokyo, Japan,where he was engaged in lithium niobate electroopticdevices and their applications to optical communica-

tion. From 20102011, he joined the Faculty of Engineering, Shizuoka Univer-sity, Hamamatsu, Shizuoka, Japan, as a Postdoctoral Fellow for CREST Projectsupported by Japan Science and Technology Agency, where he was involvedin the development of cathodoluminescent thin film for electron-beam-assistedhigh-resolution optical imaging. Since 2011, he has been an Assistant professorin the Division of Electronics and Informatics, Faculty of Science and Tech-nology, Gunma University, Kiryu, Gunma, Japan. His current research interestsinclude the field of applied optics and fiber optics utilizing modulation and de-modulations for optical communication and measurement.

Dr. Chiba is a member of the Optical Society, the Japan Society of AppliedPhysics, and the Institute of Electronics, Information, and Communication En-gineering of Japan.

Tetsuya Kawanishi (M06SM06F13) receivedthe B.E., M.E., and Ph.D. degrees in electronicsfrom Kyoto University, Kyoto, Japan, in 1992, 1994,and 1997, respectively. From 1994 to 1995, he waswith the Production Engineering Laboratory, Mat-sushita Electric Industrial (Panasonic) Company, Ltd.In 1997, he was with Venture Business Laboratory ofKyoto University, where he was engaged in researchon electromagnetic scattering and on near-field op-tics. He joined the Communications Research Lab-oratory, Ministry of Posts and Telecommunications

(now the National Institute of Information and Communications Technology,NICT), Koganei, Tokyo, Japan, in 1998. In 2004, he was a Visiting Scholarwith the Department of Electrical and Computer Engineering, University ofCalifornia, San Diego, USA. He is currently a Leader of Lightwave DevicesLaboratory, Photonic Network Research Institute in NICT, and is working onhigh-speed optical modulators and on RF photonics. Dr. Kawanishi receivedthe URSI Young Scientists Award in 1999, an award for young scientists in thefield of science and technology in 2006, from ministry of Education, Culture,Sports, Science, and Technology, Japan.

Takahide Sakamoto (S98M03) was born inHyogo, Japan, in 1975. He received the B.S., M.S.,and Ph.D. degrees in electronic engineering from theUniversity of Tokyo, Tokyo, Japan, in 1998, 2000,and 2003, respectively.

Since 2003, he has been with the CommunicationsResearch Laboratory (now National Institute of In-formation and Communications Technology, NICT),Tokyo, Japan, where he is engaged in the area ofoptical-fiber communications. In 20102012, he wasa Visiting Scholar with the Department of Electrical

and Computer Engineering, University of California, Davis, supported by JapanSociety for the Promotion of Science. He is currently a Senior Researcher ofLightwave Devices Laboratory, Photonic Network Research Institute in NICT.His current research interests include fiber-optic devices and subsystems foroptical modulation/demodulation and signal processing.

Dr. Sakamoto is a member of the IEEE Photonics Society and the Instituteof Electronics, Information and Communication Engineering (IEICE) of Japan.

Kaoru Higuma received the B.E. and M.E. degreesin physics from Waseda University, Tokyo, Japan, in1994 and 1996, respectively.

In 1996, he joined the Opto-Electronics ResearchDivision, New Technology Research Laboratories,Sumitomo Osaka Cement Company, Ltd., Chiba,Japan. He has been engaged in the research and de-velopment of LN optical modulators.

Kazumasa Takada was born in Saitama Prefecture, Japan, on May 14, 1955.He received the B.S. degree in physics from Saitama University, Japan, in 1979,and the M.S. degree in physics, and the Ph.D. degree in electronics engineeringfrom the University of Tokyo, Japan, in 1981 and 1993, respectively.

In 1981, he joined the Ibaraki Electrical Communication Laboratory, NipponTelegraph, and Telephone Public Corporation, Ibaraki, Japan, where he workedon characterization of birefringent polarization-maintaining fibers and devel-opment of optical fiber gyroscopes using the polarization-maintaining fibers.From 1989 to 1992, he was engaged in research on photonic switching systems.Since 1992, he has been involved in the field of optical low coherence interfer-ometry for planar lightwave circuits. Since 2002, he has been a Professor in theDepartment of Electronic Engineering, Graduate School of Engineering (nowDivision of Electronics and Informatics, Faculty of Science and Technology),Gunma University, Kiryu, Gunma, Japan.

Dr. Takada is a member of the Optical Society (OSA).

Masayuki Izutsu (S70M75SM90F04LF13) received the B.E., M.E., and D.Eng. degreesin electrical engineering from Osaka University,Osaka, Japan, in 1970, 1972, and 1975, respectively.

He joined the Department of Electrical Engi-neering, Faculty of Engineering Science, OsakaUniversity, in 1975, where he was involved in thefield of guided-wave optoelectronics. From 1983to 1984, he was a Senior Visiting Research Fellowat the Department of Electronics and ElectricalEngineering, University of Glasgow, Glasgow, U.K.

In 1996, he joined the Communications Research Laboratory, Ministry ofPosts and Telecommunications (now the National Institute of Information andCommunications Technology, NICT), where he served as a DistinguishedResearcher, and was in-charge of its New Generation Network Research Centeras a special duty as well. After he retired from NICT, in 2008, he served as aProfessor, Tokyo Institute of Technology, Tokyo, Japan, till 2011, and has alsobeen a Guest Professor, Waseda University, Tokyo, Japan.

Dr. Izutsuis serving as a cooperate member of the Science Council ofJapan, a member of Japanese National Committee for International Unionof Radio Science (URSI), Editor-in-chief, Electronics Express publishedfrom the Institute of Electronics, Information, and Communication Engineers(IEICE).He is a Fellow IEICE, Senior Member OSA, and member JSAP. Hereceived the Best Paper Award and the Award for Significant Achievement in1981 and 1988, respectively, from IEICE.

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