438 ieee journ al of q u antum electr onics, v ol. 46, no ...vcsel.mntl.illinois.edu/journals/2010...

438 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 46, NO. 4, APRIL 2010

Push-Pull Modulation of a Composite-ResonatorVertical-Cavity Laser

Chen Chen, Student Member, IEEE, Klein L. Johnson, Mary Hibbs-Brenner, and Kent D. Choquette, Fellow, IEEE

Abstract—The two coupled optical cavities within a ver-tical-cavity surface-emitting laser have the unique ability tomodulate the spatial distribution of the longitudinal optical mode,without changing the total photon density in the laser cavities, bysimultaneously directly modulating the two optical cavities exactlyout-of-phase. A rate-equation analysis predicts that this condi-tion, which we term push-pull modulation, exhibits a superiormodulation response than that of conventional direct modulation.The push-pull modulation can enable high-speed operation withlow power consumption, as a large modulation bandwidth canbe achieved independent of the total photon density and/or theinjection dc current. Experimental evidence of spatially changingthe longitudinal mode is presented, and push-pull modulation at2.5 Gb/s is demonstrated for the first time.

Index Terms—Coupled cavity, semiconductor lasers, ver-tical-cavity surface-emitting lasers (VCSEL).

I. INTRODUCTION

T HE vertical-cavity surface-emitting laser (VCSEL) hasemerged as a dominant laser source for short-reach optical

interconnects and optical communication because of its low-power consumption, high-density two-dimensional array inte-gration, and low-cost and high-volume manufacture [1]. Highspeed operation of a VCSEL is needed to satisfy the demand ofincreasing transmission bandwidth of optical networks. Whiledirect modulation of the VCSEL has demonstrated operation upto 35 Gb/s [2]–[6], the VCSEL with two optically coupled cav-ities can enable new modulation concepts and have exhibitedpotential to achieve higher speed modulation [7]–[10]. In partic-ular, the dual cavity VCSEL, which is also known as the com-posite-resonator vertical-cavity laser (CRVCL) [11], [12], hasdemonstrated a unique ability to change the carrier and photondensities in either of the coupled cavities, which can engineerthe modulation characteristics [13] and achieve a larger mod-ulation bandwidth than that of a conventional VCSEL [7]–[9],[14].

Manuscript received June 03, 2009; revised July 17, 2009. Current versionpublished February 15, 2010. A portion of this work was supported by theDefense Advanced Research Projects Agency under Contract W31P4Q-07-C-0284.

C. Chen and K. D. Choquette are with the Department of Electrical andComputer Engineering, University of Illinois at Urbana-Champaign, Urbana,IL 61801 USA (e-mail: [email protected]; [email protected]).

K. L. Johnson and M. Hibbs-Brenner are with Vixar, Plymouth, MN 55447USA (e-mail: [email protected]; [email protected]).

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

Digital Object Identifier 10.1109/JQE.2009.2031119

In this paper, we demonstrate a novel modulation conceptthat will produce light output modulation without varying thetotal photon density inside the laser. In particular, a very largemodulation bandwidth can be achieved independent of the totalphoton density and the relaxation oscillation (RO) frequency.With the total photon density unchanged, light modulation re-sults from spatially varying the longitudinal optical mode dis-tribution by injecting equal current density into both coupledcavities but exactly out-of-phase. We call this push-pull mod-ulation. Note that bandwidth enhancement and chirp reductionhave been also predicted in prior work, by applying the push-pull modulation to a single-cavity distributed feedback (DFB)laser diode [15], [16]. In this paper, we will demonstrate exper-imental evidence of push-pull modulation using a CRVCL.

The paper is organized as follows: in Section II we describethe operation principle of push-pull modulation and the conceptof the longitudinal mode modulation (LMM). In Section III,rate-equation analysis is performed to understand the laser dy-namics under push-pull modulation. The LMM is obtained fromthe difference between the optical field profiles at the “push” and“pull” states, which are calculated using a cold-cavity transmis-sion matrix model for a CRVCL. By combining LMM with therate-equation model, the analysis predicts that push-pull modu-lation has the ability to achieve modulation bandwidth well be-yond the RO frequency. The modulation bandwidth is dictatedby the photon lifetime and is predicted to be on the order of 100GHz, if electrical parasitic effects are neglected. In Section IV,we present the first experimental characterization of push-pullmodulation using a CRVCL, and demonstrate large-signal mod-ulation at a data rate of 2.5 Gb/s.

II. PUSH-PULL MODULATION

Fig. 1 illustrates the operational principle of push-pull mod-ulation in a CRVCL. The forward-bias injection current densityin the top and bottom cavities of a CRVCL will be directly mod-ulated simultaneously but maintained exactly out-of-phase. Asthe carrier density increases in one cavity, the carrier density inthe other cavity equally decreases, maintaining a constant totalcarrier and photon concentration. The laser output modulationoccurs by dynamically varying the longitudinal optical modedistribution in the coupled cavities, which will lead to differentoutput at the facet of the CRVCL, as shown in Fig. 2.

It is known that the spatial distribution of the longitudinal op-tical mode inside a CRVCL is governed by the spectral detuningbetween the coupled cavities; specifically, it allows the longitu-dinal mode to preferentially distribute towards one or the other

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CHEN et al.: COMPOSITE-RESONATOR VERTICAL-CAVITY LASER 439

Fig. 1. Operation principle of the push-pull modulation in a composite-res-onator vertical-cavity laser.

Fig. 2. Calculated refractive index and normalized optical field intensity pro-file for the short-wavelength longitudinal mode along the growth direction of aCRVCL when the top cavity is (a) shorter and (b) longer than the bottom cavity.Insets show the optical field intensity near the laser facet and the percentage oflight at the facet.

cavity [11]. Note that the detuning is unique to the optically cou-pled cavities of the CRVCL, which makes an important distinc-tion from the push-pull modulation of a DFB laser [15], [16].In the CRVCL, the percentage of the optical field overlappingwith the top and bottom cavity is denoted as and , respec-tively. If the two CRVCL cavities are symmetric, both of thelongitudinal modes distribute equally between the two cavities,and thus and are equal to 50%. However, the two cavi-ties can also be detuned from each other, such that one cavitycan have a longer effective cavity length than the other cavity.Fig. 2(a) illustrates the calculated optical field distribution ina CRVCL for the shorter wavelength longitudinal mode, whenthe top cavity is shorter than the bottom cavity. Specifically, theeffective optical path length of the top (bottom) cavity is 0.5%

shorter (longer). The calculation is based on a cold-cavity trans-mission matrix model for a conventional 850 nm CRVCL. TheCRCVL structure used in the calculation consists of 35 periodsof the bottom distributed Bragg reflector (DBR), 12.5 periodsof the middle DBR, and 20 periods of the top DBR, with the topand bottom cavities having an effective thickness of one wave-length if the detuning is zero. The cavity detuning condition canbe achieved dynamically by injecting current in the top cavity,while extracting current out of the bottom cavity during push-pull modulation. The top cavity becomes effectively shorter thanthe bottom cavity due to the free carrier plasma effect [17],[18]. As a result, the shorter wavelength mode tends to shifttowards the output facet, and thus is smaller than as inFig. 2(a). Conversely, Fig. 2(b) illustrates the opposite case,where the bottom (top) cavity becomes 0.5% shorter (longer),and the shorter wavelength longitudinal mode tends to shift to-wards the device substrate and thus is larger than . For thelonger wavelength longitudinal mode, the observation in Fig. 2will be exactly the opposite [11]. The CRVCL will lase predom-inantly with a single longitudinal mode by carefully designingthe spectral overlap between the laser gain and cavity resonance[19], and thus the LMM of the shorter and longer wavelengthlongitudinal modes will not cancel.

Fig. 2 also indicates that the light output at the laser top facetis different for the “push” and “pull” states. The longitudinaloptical mode is either “pushed” toward the laser output facetproducing higher output power, or is “pulled” toward the sub-strate side producing lower output power. The LMM takes placewithout changing the total photon density in the laser cavities,thus the laser modulation is achieved independent of the totalphoton density. The RO phenomenon associated with the dy-namic variation of carriers and photon densities can be avoided,imposing no limitation to the laser bandwidth. The modula-tion bandwidth of the push-pull modulation will depend on thephoton lifetime (on the order of picoseconds), which dictateshow fast the photon population decays from a laser cavity.

III. RATE EQUATION ANALYSIS

A. Rate Equation Small-Signal Analysis

The rate equations for carrier and photon densities are usedby assuming only one longitudinal and transverse optical modeis lasing [7], [20], [21]

(1)

(2)

(3)

for the cavities ,2, where is the carrier densitycm , is the carrier lifetime (s), is the injection current

density A/cm , is the gain region thickness (cm), isthe group velocity of the optical mode in the material (cm/s),

is the material gain cm , is the optical confinementfactor, and is the percentage of the optical standing waveoverlapping with the respective cavity. Additionally, is theelementary charge (C), is the photon lifetime, is the photon



density cm common to both cavities, is the spontaneousemission factor, and is the spontaneous emission rate perunit volume cm s .

For small-signal analysis, the equations for the current den-sity, photon density, and carrier density are

(4)

(5)

(6)

(7)

(8)

where for the cavities , , , and are small signalscompared with their corresponding dc values , , and ,respectively. The small-signal modulation is induced through si-multaneous injection current to both the top and bottom cavity,as shown in Fig. 1. Assuming , , and are sinu-soidally varying functions with angular frequency , they canbe described as

(9)

(10)

(11)

(12)

(13)

Solving the rate equations (1) and (2), we can obtain andas follows:

(14)

(15)

where and are the differential gain in the top and bottomcavity, respectively. The rate equation (3) can be expressed as

(16)

Substituting (14) and (15) into (16), the direct modulation re-sponse through both the coupled cavities becomes (17), shownat the bottom of the page. We assume and are re-lated by

(18)

where and are the amplitude and phase difference betweenthe small-signal currents injected in the cavities. Therefore, (17)can be rewritten as (19), also shown at the bottom of the page.If the coupled cavities are identical, and the longitudinal opticalfield is equally distributed between the two cavities, then

(20)

(21)

(22)

(23)

(24)

(25)

(26)

The total modulation response in (19) can be then simplified as

(27)

The push-pull modulation is the special case, where. Under this condition, the total modulation response

(17)

(19)



TABLE IDEVICE PARAMETER VALUES USED DURING CALCULATION,

UNLESS NOTED OTHERWISE

is equal to 0. This means the exact push-pull con-dition of and corresponds to no light modulation.However, the essential component LMM has been neglected forthis analysis.

The LMM is not inherently embedded in the rate equations(1)–(3). In order to account for it, the modulation response in(27) can be modified by adding an additional term

(28)

where is the photon density at the laser facet; 0.15%represents the percentage of the total internal intensity present atthe output laser facet under steady-state (quiescent) condition.The LMM is approximately 10% of the available light at theoutput facet calculated using the parameters in Table I, as shownin the inset of Fig. 2; this estimation will be justified later. Thelast term of (28) includes the frequency dependence of LMM onthe photon lifetime. We model it as a low-pass effect, becausethe amplitude of the LMM decreases as the photon populationdecays out of the laser cavities.

Because both the photon density and the photon spatial distri-bution in the longitudinal direction are varying with the injectioncurrent, the modulation of the photon density at the laser facetis more important than the modulation of the total photon den-sity. This condition is unique for push-pull modulation, in whichlight output modulation from the longitudinal mode variationoccurs without changing the photon density inside the CRVCL.In order to distinguish this from the modulation response de-fined by (19) or (27), the transfer function given by (28) is here-after called the output modulation response.

Fig. 3 illustrates the output modulation responses with fordifferent and values using (28), for two identical cavities.The device parameters used in the calculation are taken from

Fig. 3. Push-pull modulation response for different values of (a) withand (b) with with inclusion of LMM.

[22] and are summarized in Table I. The most pronounced fea-ture of the push-pull modulation is that the relaxation oscilla-tion vanishes, when and which corresponds to thesmall-signal current of both cavities having the same amplitudeand exactly out-of-phase. The condition and mustbe precisely satisfied in order to eliminate the RO peak, whichindicates no change of the total photon density occurs inside theCRVCL, despite the change of carrier densities in both cavities.

In an offset push-pull condition (i.e., or ), themodulation of the total photon density coexists with LMM,causing distortion to the modulation response near the ROfrequency as shown in Fig. 3. The modulation bandwidthimproves with a smaller deviation from the exact push-pullcondition and/or a larger LMM.

In Fig. 3, we assume the two coupled cavities are identical,where the offset push-pull condition results from the amplitudeor phase difference between the modulation currents. In prac-tice, the offset condition can also result from other asymme-tries in the cavity conditions, such as different material gains,differential gains, thermal effects, etc. Therefore, a large LMMis desired to provide sufficient tolerance against the asymmetrybetween the coupled cavities as well as between the modula-tion currents. We will show later that the LMM amplitude canbe designed through the CRVCL epitaxial structure. The mod-ulation bandwidth is ultimately limited by the photon lifetime,if electrical parasitic effects are neglected. This is because the



Fig. 4. Electrical parasitic effect on push-pull modulation response.and is assumed.

amplitude of the LMM decreases as the photon population de-cays out of the laser cavities. It was shown in prior work thatthe beating between the fundamental and higher-order longitu-dinal modes in a DFB laser can produce additional resonancesto the push-pull modulation response of the DFB laser [15],[16]. However, the lack of higher-order longitudinal modes ina CRVCL eliminates this possibility.

Because the LMM depends on direct current modulation,the electrical parasitics associated with the current modulationshould be considered in the analysis. Fig. 4 illustrates thepush-pull modulation response when both the top and bottomcavities are limited to 20 GHz electrical bandwidth due todevice parasitics. The electrical parasitics introduce a dominantpole to the push-pull modulation response, which reduces theoverall modulation bandwidth. Push-pull modulation does notbenefit from RO peak enhancement, as is the case for conven-tional direct modulation in a laser.

B. Longitudinal Mode Modulation Calculation

Additional calculations are performed in order to justify theLMM condition that is included in (28). Fig. 5(a) illustrates thecalculated LMM as a function of the carrier-induced change ofrefractive index for different middle DBR periods in a CRVCLwith 35 (22) bottom (top) DBR periods. The LMM is calculatedfrom the cold-cavity optical field profiles similar to those shownin Fig. 2. The LMM is defined as , whereis the light output intensity at the CRVCL facet without modu-lation, and and are the light output intensity at highand low level, respectively, owing to the change of the effec-tive cavity length. The LMM increases with larger change ofrefractive index and thus the effective cavity length. Assuminga linear relationship between the change of current density andrefractive index [17], [18], a 1.08% change in refractive indexcorresponds to a 0.75 mA current change into a current apertureof m and produces 13% LMM for a CRVCL with a10 period middle DBR. Therefore, the value of 10% LMM usedin (28) should not be difficult to achieve. The change of refrac-tive index is calculated using the average change of the carrierdensity in the laser cavity.

Fig. 5. Calculated (a) LMM and (b) extinction ratio as a function of thechange in refractive index in the CRVCL cavities and the number of middleDBR periods.

Apparent from Fig. 5(a), the LMM scales with the numberof middle DBR periods. This is because the longitudinal modeis more confined into the cavities for increasing number of themiddle DBR periods, creating a higher contrast for switching.The extinction ratio, defined as , is plottedin Fig. 5(b). The extinction ratio scales with the change in re-fractive index as expected, and an extinction ratio greater 3 dBcan be achieved by using an optimized CRVCL structure.

C. Large Signal Analysis

Fig. 6 illustrates the large-signal response of a CRVCL underpush-pull modulation, assuming identical coupled cavities. Thelarge-signal response is calculated by solving the rate equa-tions (1)–(3) numerically in the time domain [23]. The inputsquare pulses into both cavities are modulated at 20 GHz withthe same amplitude but exactly out-of-phase. We assume theLMM follows the input pulses instantaneously. The carrier den-sities in the top and bottom cavity vary out-of-phase as expected,which is not the case for direct modulation into one cavity [seeFig. 6(a)]. Note that for direct modulation only into the topcavity, the same square pulse signal at 20 GHz is used. Thevariation of the carrier density over a 50 ps modulation periodin Fig. 6(a) is about cm , corresponding to 0.17%change of refractive index. A larger change of refractive indexand corresponding larger LMM can be attained by increasing themodulation signal amplitude. Fig. 6(b) shows there is no change



Fig. 6. Large-signal response for a CRVCL under the push-pull modulation.(a) The carrier density in the top and bottom cavity, (b) total photon density, and(c) photon density at the laser facet as a function of time.

in the total photon density averaging over the entire CRVCL,consistent with the small-signal result shown in Fig. 3. A sharpsquare-wave optical pulse can be observed from the laser facetas a result of LMM in Fig. 6(c), neglecting electrical parasiticeffects. The output photon density is obtained by multiplyingthe total photon density by the LMM. This calculation con-firms that push-pull modulation has the ability to achieve lightoutput modulation without changing the photon density insidethe CRVCL, which cannot be accomplished using direct modu-lation.

Since the LMM is a direct function of the injection currentfrom the electro-optical effect, the light modulation is no longerlimited by the photon-carrier interaction. In Fig. 6(b) and (c),even though it is difficult for the photons to follow the 20 GHz

Fig. 7. Scanning electron micrograph of a fabricated CRVCL with a GSSGcoplanar contact.

input pulse using direct modulation (see solid lines), the lightmodulation from the push-pull modulation has a sharp squarepulse shape (see dash line). With a larger injection current andthus a larger photon density, direct modulation can also obtain alarger modulation bandwidth, producing a sharper pulse. How-ever, Fig. 6(c) here is to emphasize the idea that the push-pullmodulation bandwidth is not limited by the RO frequency andthus is independent of the injection dc current, if the LMM issufficiently large.

For both the small- and large-signal analysis, the LMM am-plitude is determined a priori from a cold-cavity optical modecalculation. A self-consistent model incorporating dynamiccavity detuning and the longitudinal mode distribution arisingfrom varying cavity conditions, such as laser gain, index sup-pressed by charge carriers, etc., would provide a more accuratedescription of push-pull modulation.

IV. EXPERIMENTAL RESULTS

Fig. 1 illustrates the device structure of the CRVCL studiedin this work. The CRVCL is fabricated from an epitaxial waferconsisting of a monolithic bottom p-type distributed Bragg re-flector (DBR) with 35 periods, a middle n-type DBR with 12.5periods, and an upper p-type DBR with 22 periods. The middleDBR mirrors are doped n-type to improve the uniformity of cur-rent injection [12]. The middle DBR mirrors separate two op-tical cavities, each of which contains five GaAs/Al Ga Asquantum wells nominally lasing at 850 nm. A m topmesa through the top cavity and a m bottom mesathrough the bottom cavity are formed using inductively coupledplasma reactive ion etching. An m ion implantation aper-ture and a m oxide aperture are formed in the top andbottom mesa, respectively [1], [13]. The CRVCL is planarizedwith polyimide and then a ground-signal-signal-ground (GSSG)coplanar contact is deposited to facilitate high speed signalinginto both optical cavities, as depicted in Fig. 7.

Large-signal modulation is performed using a pattern gener-ator and an oscilloscope. A broadband power divider is used tosplit the modulation signal into both cavities of the CRVCL, andvariable attenuators are used to control the relative amplitudebetween the modulation signals into both cavities. A cleaved62.5 m core graded-index multimode fiber and a high speed



photodetector are used to collect output light from the CRVCLunder test.

It is difficult in practice to determine the exact push-pullcondition, as mismatch between the top and bottom cavity canarise from many different, yet correlated device parameterssuch as the longitudinal mode distribution, series resistanceand capacitance, device heating, current aperture size, currentinjection efficiency, and modal gain in the active region. Itis, thus, a challenge to observe how an optical output signalcan be decomposed into the sum of the modulation responsesfrom both cavities as well as LMM. Therefore, the goal of theCRVCL characterization is twofold. The first objective is todemonstrate the physics of the push-pull modulation or LMM,which is light modulation without changing the total photonnumber in a CRVCL. Second, if the pull-push modulationcondition can be determined, we wish to determine whetherthe bandwidth from push-pull modulation can exceed that fromdirect modulation of a single cavity.

The challenge for demonstrating push-pull modulation is de-termining the dc bias for modulation. In order to accomplishthis, the continuous wave relative intensity noise (RIN) is mea-sured at different current injection levels into the two cavities todetermine the dc points for the push-pull modulation. The RIN isused to characterize the laser noise as a function of frequency ata dc current level, from which an unchanging RO frequency andthus the photon number can be maintained. Therefore, if thereexist two dc states, which give the same RO frequency in theRIN spectra but produce different light output, then switchingbetween these two states maintains constant total photon den-sity and the light modulation is a result of LMM.

Fig. 8 illustrates light–current (LI) characteristics of aCRVCL, indicating the push and pull dc states used in thelarge-signal modulation. The push dc state (top cavity current:5.6 mA, bottom cavity current: 4.4 mA) and the pull dc state(top cavity current: 4.4 mA, bottom current: 5.6 mA) aredetermined from RIN measurements. Fig. 9 compares the RINspectra when the CRVCL is operating at the push, pull, andquiescence state. The RO frequencies are identical for these dcstates, indicating no change in the total photon density insidethe CRVCL. The RIN for a nonpush-pull operation point (topcavity current: 4.4 mA, bottom current: 5 mA), which is neitherthe push nor the pull state, is also plotted for comparison.The RO peak in this case shifts to a lower frequency due todecreasing total photon density. Fig. 10 shows the optical spec-trum taken at the quiescent state (5 mA into both cavities). Thequiescent state is chosen so that the CRVCL maintains singletransverse (side-mode suppression ratio 20 dB) and singlelongitudinal mode operation during the push-pull modulation.

After determining the push and pull dc states, push-pull mod-ulation at 2.5 Gb/s is performed. The maximum peak-to-peakinput voltage from the pattern generator is 2 V. A variable at-tenuator is used to vary the modulation signal amplitude for thetop cavity. Note the input modulation signals are out-of-phasefrom each other. The quiescent state corresponds to both cavi-ties of the CRVCL biased at 5 mA.

As mentioned earlier, the two coupled cavities are oftenasymmetric in practice. It is thus nontrivial to achieve the exactpush and pull states shown in Fig. 8 with the modulation signals

Fig. 8. LI characteristics of the CRVCL with the push, pull, and quiescent state.

Fig. 9. RIN spectra when the CRVCL is at the push, pull, and quiescent state.

Fig. 10. Optical spectrum taken at the quiescent dc state.

from the pattern generator. Fig. 11 illustrates a calibrationprocedure to determine the modulation signals for the precisepush-pull condition. The quiescent points in Fig. 11(a) and (b)are the same bias point. The calibration can be done in thefollowing two steps. First, the CRVCL is directly modulatedonly from the top cavity. The modulation signal amplitude intothe top cavity is adjusted, until the same extinction ratio fromthe optical waveform is achieved as that expected from the LIcurve (e.g., Fig. 8). Similarly, the modulation signal for thebottom cavity can be calibrated, by applying direct modulationonly to the bottom cavity and achieving the correspondingextinction ratio. Note that when the top and bottom cavityare at “pull” and “push” states [see Fig. 11(a) and (b)], thetotal photon population is approximately equal. Therefore,the calibration process ensures that the CRVCL is modulatedwithin the push and pull dc states shown in Fig. 8. In practice,



Fig. 11. Calibration concept to achieve the exact push-pull condition using twomodulation signals from a signal generator.

laser driver circuits can be incorporated with the CRVCL tomaintain the calibrated push and pull states.

This procedure was followed at a modulation rate of 2.5 Gb/s.Fig. 12(a) and (b) illustrates the observed calibrated opticaloutput signal, when only the top and bottom cavity is underdirect modulation, respectively. The optical signal from topcavity modulation has larger peak–peak amplitude, which isconsistent with a larger extinction ratio expected from theLI characteristics. Fig. 12(c) shows the optical signal whenboth cavities are under direct modulation simultaneously but180 out-of-phase, which is push-pull modulation. The phasedifference between the two modulation signals is adjustedusing the pattern generator. In the absence of push-pull opera-tion, the signal in Fig. 12(c) would be expected to not exhibitany modulation. However, modulation is observed with anextinction ratio of 1.27 dB. The push-pull modulation speed islimited by the test equipment in this experiment. The push-pullmodulation in Fig. 12(c) does not show significant reductionof rise/fall time, due to the device parasitics of this particularCRVCL.

V. CONCLUSION

In this paper, we have proposed and experimentallydemonstrated a new modulation concept for the CRVCL,namely push-pull modulation. With push-pull modulation, aCRVCL can produce light modulation by spatially varying thelongitudinal optical mode distribution, or the LMM, withoutchanging the total carrier and photon densities inside the lasercavities. From a simple rate-equation analysis, we predictthat the push-pull modulation exhibits a superior modulationresponse than that of conventional direct modulation. Specif-ically, push-pull modulation can produce a larger modulationbandwidth and a more linear response by suppression of theRO phenomenon. Moreover, push-pull modulation can achievehigh-speed operation with low power consumption, as a largebandwidth can be achieved independent of the total photondensity and/or the dc injection current. Last, we have foundexperimental evidence for push-pull modulation from thelarge-signal modulation and RIN characteristics, and push-pull

Fig. 12. Optical output signal at 2.5 Gb/s, when the (a) top and (b) bottomcavity is under direct modulation, and (c) when both cavities are under directmodulation simultaneously at the push-pull condition.

modulation at 2.5 Gb/s is demonstrated for the first time. Witha dynamic rate-equation model and a laser structure that isoptimized to reduce device parasitics and improve the opticaloutput contrast ratio (or LMM), the advantages of push-pullmodulation can be fully exploited to enable energy-efficienthigh-speed applications in the future.

ACKNOWLEDGMENT

The authors thank A. Allerman at Sandia National Laborato-ries for epitaxial materials and P. Crump for technical discus-sions.

REFERENCES[1] K. D. Choquette and K. M. Geib, “Fabrication and performance of ver-

tical-cavity surface-emitting lasers,” in Vertical-Cavity Surface-Emit-ting Lasers, C. Wilmsen, H. Temkin, and L. Coldren, Eds. New York:Cambridge Univ. Press, 1999, pp. 193–232.

[2] Y. C. Chang, C. S. Wang, and L. A. Coldren, “High-efficiency, high-speed VCSELs with 35 Gb/s error-free operation,” Electron. Lett., vol.43, no. 19, pp. 1022–1023, 2007.

[3] T. Anan, N. Suzuki, K. Yashiki, K. Fukatsu, H. Hatakeyama, T. Aka-gawa, and M. Tsuji, “High-speed 1.1- m-range InGaAs VCSELs,” inProc. Opt. Fiber Commun. Conf., San Diego, CA, Mar. 2008.

[4] R. H. Johnson and D. M. Kutcha, “30 Gb/s directly modulated 850 nmdatacom VCSELs,” in Proc. Conf. Lasers Electro Opt., San Jose, CA,May 2008.

[5] P. Westbergh, J. S. Gustavsson, A. Haglund, H. Sunnerud, and A.Larsson, “Large aperture 850 nm VCSELs operating at bit rates up to25 Gbit/s,” Electron. Lett., vol. 44, no. 15, pp. 7–8, 2008.

[6] C. Chen, P. O. Leisher, D. M. Kuchta, and K. D. Choquette, “Highspeed modulation of index-guided implant-confined vertical cavity sur-face-emitting lasers,” IEEE J. Sel. Topics Quantum Electron., vol. 15,no. 3, pp. 673–678, 2009.

[7] D. M. Grasso, D. K. Serkland, G. M. Peake, K. M. Geib, and K. D.Choquette, “Direct modulation characteristics of composite resonatorvertical-cavity lasers,” IEEE J. Quantum Electron., vol. 42, no. 12, pp.1248–1254, 2006.

[8] V. A. Shchukin, N. N. Ledentsov, J. A. Lott, H. Quast, F. Hopfer, L.Y. Karachinsky, M. Kuntz, P. Moser, A. Mutig, A. Strittmatter, V. P.Kalosha, and D. Bimberg, “Ultra high-speed electro-optically mod-ulated VCSELs: Modeling and experimental results,” in Proc. SPIE,2008, vol. 6889.



[9] J. van Eisden, M. Yakimov, V. Tokranov, M. Varanasi, E. M. Mo-hammed, I. Young, and S. Ortyabrsky, “Optical decoupled loss modu-lation in a duo-cavity VCSEL,” IEEE Photon. Technol. Lett., vol. 20,no. 1, pp. 42–44, 2008.

[10] C. Chen, P. O. Leisher, D. M. Grasso, C. Long, and K. D. Choquette,“High-speed electroabsorption modulation of composite-resonator ver-tical-cavity lasers,” IET Optoelectron., vol. 3, no. 2, pp. 93–99, 2009.

[11] P. Pellandini, R. P. Stanley, R. Houdre, U. Oesterle, M. Ilegems, andC. Weisbuch, “Dual wavelength laser emission from a coupled semi-conductor microcavity,” Appl. Phys. Lett., vol. 71, no. 7, pp. 864–866,1997.

[12] A. J. Fischer, K. D. Choquette, W. W. Chow, H. Q. Hou, and K. M.Geib, “Coupled-resonator vertical-cavity laser diode,” Appl. Phys.Lett., vol. 75, no. 19, pp. 3020–3022, 1999.

[13] C. Chen and K. D. Choquette, “Multilevel amplitude modulation usinga composite-resonator vertical-cavity lasers,” IEEE Photon. Technol.Lett., vol. 15, no. 21, pp. 30–32, 2009.

[14] C. Chen et al., “Coupled-cavity surface-emitting lasers: modulationconcepts, performance and applications,” Ph.D. dissertation, Univ. Illi-nois at Urbana-Champaign, Urbana, IL, 2009.

[15] M. C. Nowell, J. E. Carrol, R. G. S. Plumb, D. D. Marcenac, M. J.Robertson, H. Wickes, and L. M. Zhang, “Low chirp and enhanced-res-onant frequency by direct push-pull modulation of DFB lasers,” IEEEJ. Sel. Topics Quantum Electron., vol. 1, no. 2, pp. 433–441, 1995.

[16] J. Chen, R. Maciejko, and T. Makino, “Dynamic properties of push-pullDFB semiconductor lasers,” IEEE J. Quantum Electron., vol. 32, no.12, pp. 2156–2165, 1996.

[17] J. Manning, R. Olshansky, and C. B. Su, “The carrier-induced indexchange in AIGaAs and 1.3 m InGaAsP diode lasers,” IEEE J.Quantum Electron., vol. 19, no. 10, pp. 1525–1530, 1983.

[18] B. R. Bennett, R. A. Soref, and J. A. Del Alamo, “Carrier inducedchange in refractive index of InP, GaAs, and InGaAsP,” IEEE J.Quantum Electron., vol. 26, no. 1, pp. 113–122, 1990.

[19] A. C. Lehman and K. D. Choquette, “Threshold gain temperature de-pendence of composite resonator vertical cavity lasers,” IEEE J. Sel.Topics Quantum Electron., vol. 11, no. 5, pp. 962–967, 2005.

[20] S. L. Chuang, Physics of Optoelectronic Devices. New York: Wiley,1995.

[21] V. Badilita, J.-F. Carlin, M. Ilegems, and K. Panajotov, “Rate-equationmodel for coupled-cavity surface-emitting lasers,” IEEE J. QuantumElectron., vol. 40, no. 12, pp. 1646–1656, 2004.

[22] L. A. Coldren and S. W. Corzine, Diode Lasers and Photonic Inte-grated Circuits. New York: Wiley, 1995.

[23] Z. Toffano, A. Gholami, M. Fez, A. Destrez, and M. Marec, “VCSELshort reach communications: Behavioural modeling of high speed op-toelectronic modules,” in Proc. 4th Int. Conf. Numer. Simul. Optoelec-tron. Devices, 2004, pp. 49–50.

Chen Chen (S’07) received the B.S., M.S., and Ph.D.degrees in electrical and computer engineering fromthe University of Illinois at Urbana-Champaign, in2004, 2006, and 2009, respectively.

He is currently a Postdoctoral Fellow in electricaland computer engineering at McGill University,Montreal, Canada. His research interests includedesign, fabrication, and characterization of ver-tical cavity surface emitting lasers (VCSELs) andother novel optoelectronics devices, and VCSELapplications for data communications, microwave

photonics, and green photonics. He is the author/coauthor of more than 30papers and conference presentations.

Klein L. Johnson received the B.A. degree in mathe-matics and physics from St. Olaf College, Northfield,MN, and the Ph.D. degree in electrical engineeringfrom the University of Minnesota, Minneapolis.

He is a cofounder of Vixar and, since 2005,has been its Chief Technology Officer5. Prior tocofounding Vixar, he spent 12 years with Honeywellas a Research Scientist and Project Leader. His workspanned a broad range of research and developmentin optoelectronic device and packaging, includingthe development of polymer and glass waveguides,

a very high bandwidth array transceiver, optoelectronic integrated circuits, andrugged packaging for military applications. He has three patents, seven patentapplications, and is the author/coauthor of multiple papers and conferencepresentations.

Mary Hibbs-Brenner received the B.A. degree inphysics from Carleton College, Ottawa, Canada,the M.B.A. degree from the Carlson School ofBusiness, University of Minnesota, and the Ph.D.degree in materials science from Stanford University,Stanford, CA.

She has been a Cofounder and Chief ExecutiveOfficer of Vixar since 2005, which offers uniqueVCSEL components to the industrial, medical,consumer, and office equipment markets. Prior tocofounding Vixar, she held business development,

technical management, and research positions at Honeywell. She was respon-sible for initiating and developing the company’s VCSEL business, whichoffered VCSEL products for fiber optic links within Local Area Networksand Storage Area Networks. She also served as a business development leadwithin the Honeywell Technology organization, developing business plans forbringing new technologies to market. She spent two years doing Postdoctoralresearch with the University of Linköping, Linköping, Sweden. She has sevenpatents, and more than 50 publications and invited presentations.

Kent D. Choquette (M’97–F’03) received the B.S.degrees in engineering physics and applied mathe-matics from the University of Colorado, Boulder, andthe M.S. and Ph.D. degrees in materials science fromthe University of Wisconsin, Madison.

From 1990 to 1992, he held a Postdoctoral ap-pointment with AT&T Bell Laboratories, MurrayHill, NJ. He joined Sandia National Laboratories,Albuquerque, NM, from 1993 to 2000. He becamea Professor with the Electrical and Computer En-gineering Department, University of Illinois, in

2000. His Photonic Device Research Group pursues the design, fabrication,characterization, and applications of vertical cavity surface-emitting lasers(VCSELs), photonic crystal light sources, nanofabrication technologies, andhybrid integration techniques for photonic devices. He has authored morethan 200 technical publications and three book chapters, and has presentednumerous invited talks and tutorials.

Dr. Choquette has served as an Associate Editor of the IEEE JOURNAL OFQUANTUM ELECTRONICS, IEEE PHOTONIC TECHNOLOGY LETTERS, and theJOURNAL OF LIGHTWAVE TECHNOLOGY, as well as a Guest Editor of the IEEEJOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS. He was awardedthe 2008 IEEE/LEOS Engineering Achievement Award. He is a Fellow of theOptical Society of America and SPIE.


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