LASERS & ELECTRO-OPTICS SOCIETY...

June 2006Vol. 20, No. 3www.i-LEOS.org IEEE

LASERS & ELECTRO-OPTICS SOCIETY NEWS

Electrical Soliton ModelockingElectrical Soliton Modelocking

SemiconductorTraps for

Laser-CooledAtomic Ions

and ScalableQuantum

Computing

SemiconductorTraps for

Laser-CooledAtomic Ions

and ScalableQuantum

Computing

20leos03.qxd 6/8/06 10:32 AM Page cov1

June 2006 IEEE LEOS NEWSLETTER 1

IEEE

LASERS & ELECTRO-OPTICS SOCIETY NEWSSoliton oscillator topology.Page 6, Figure 3 June 2006 Volume 20, Number 3

FEATURES

Electrical Soliton ModelockingDavid S. Ricketts, Xiaofeng Li, and Donhee Ham . . . . . . . . . . . . . . . . .4

Semiconductor Traps for Laser-Cooled Atomic Ions and Scalable Quantum ComputingD. Stick, W. K. Hensinger, S. Olmschenk, and C. Monroe . . . . . . . . . . .13

COLUMNS

Editor’s Column………………2 President’s Column………………….3

10

4

14

28

DEPARTMENTS

News . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18• Call for 2007 John Tyndall Award Nominations• Nomination form• Announcing: New IEEE/LEOS Young Investigator Award

Careers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20• Focus on the 2005-2006 LEOS Distinguished Lecturers• Announcing the New 2006-07 LEOS Distinguished Lecturers• 2006 IEEE/LEOS Quantum Electronics Award recipient

– Ferenc Krausz • Recovering from Mistakes

Members . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .30Membership in the US and South America• Benefits of IEEE Senior Membership• New Senior Members

Conferences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35• ECOC 2006 Announcement• LEOS 2006 Exhibitor Contract • Conference Calendar

Publications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .42• Call for Papers• IEEE Journal of Selected Topics in Quantum Electronics (JSTQE)• IEEE/OSA Journal of Display Technology (JDT)

20leos03.qxd 6/8/06 10:32 AM Page 1

This issue features two University Research Highlightsarticles. The first article is "Electrical Soliton Modelocking,"by David S. Ricketts, Xiaofeng Li, and Donhee Ham at theHarvard University Electronics and IC Research Laboratory,where Prof. Ham is Assistant Professor of ElectricalEngineering in the Division of Engineering and AppliedSciences. The website for Prof. Ham's Research Group is:h t t p : / / p e o p l e . d e a s . h a r v a r d . e d u / ~ d o n h e e /donheeham.htm

The second article is "Semiconductor Traps for Laser-Cooled Atomic Ions and Scalable QuantumComputing," by D. Stick, W. K. Hensinger, S.Olmschenk, and C. Monroe at the University ofMichigan, where Prof. Monroe is Professor of Physicsand Director of the NSF Focus Physics Frontier Center(Frontiers of Optical Coherent and Ultrafast Science).The webpage for Prof. Monroe's Research Group is:http://iontrap.physics.lsa.umich.edu.

Thank you again for taking the time to read the LEOSnewsletter. We appreciate all of your comments and feed-back! Please send comments to [email protected].

Editor’sColumnM.Y. LANZEROTTI

PresidentAlan WillnerUniversity of Southern CaliforniaDept. of EE-Systems/Rm EEB 538Los Angeles, CA 90089-2565Tel: +1 213 740 4664Fax: +1 213 740 8729Email: [email protected]

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Staff EditorKatrina EdsellIEEE/LEOS445 Hoes LanePiscataway, NJ 08855-1331Tel: +1 732 981 3405Fax: +1 732 562 8434Email: [email protected]

IEEE Lasers and Electro-Optics Society

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AbstractWe recently developed for the first time an electrical mode-locked circuit that self-generates a periodic, stable train ofelectrical solitons. This electrical soliton oscillator was enabledby combining a nonlinear transmission line where solitons canpropagate with an amplifier that “tames” the “unruly” dynam-ics of the electrical soliton. The taming function of the ampli-fier resembles the saturable absorption in optics. Moreover, theoverall oscillator is a direct analog of the soliton modelockedlaser in optics. This paper highlights these recent develop-ments with the exciting possibilities they offer.

IntroductionWe recently introduced the first electrical modelocked circuitthat self-generates a periodic, stable train of electrical solitons[1-3]. This electrical soliton oscillator, which is a direct ana-logue of the soliton modelocked laser in optics [4], was madepossible by connecting a nonlinear transmission line (NLTL),which supports electrical soliton propagation, around a uniquetransistor-based amplifier in a circular topology [Fig. 1].

Electrical soliton generation per se is not new: over the past40 years, the NLTL has been extensively used to create elec-trical solitons for sharp pulse generation applications [5]. Thistraditional approach, however, almost exclusively uses theNLTL as a “2-port (input + output)” device that requires anexternal high-frequency input to produce the soliton output.What distinguishes our circuit is that it is an oscillator [“1-port (output-only)” system] that self-generates electrical soli-

tons without requiring an external high-frequency input, justas is done in the optical soliton laser. This 1-port oscillator isa self-contained, self-regulated system that provides muchimproved pulse control and quality over the 2-port NLTL.

By nature, electrical solitons are “unruly,” which had ren-dered previous attempts to build the electrical soliton oscilla-tor unsuccessful, usually resulting in chaotic oscillations [2,6].The key to our success was finding a way to “tame” the elec-trical solitons: the transistor amplifier in Fig. 1 was uniquelydesigned to stabilize soliton oscillation, in addition to provid-ing gain. The stabilizing functionality of this amplifier muchresembles the saturable absorber widely used in optical mode-locking [4]. Our three prototypes attested to the validity ofthe electrical soliton oscillator concept, with our latest chip-scale prototype producing a 293-ps soliton pulse width. Nowwith the concept firmly demonstrated, the soliton oscillator,especially its NLTL, can be quickly scaled and optimized toprovide a much narrower pulse width close to 1 ps [7].

As the ultrashort light pulses generated by modelockedlasers have found numerous applications, so too will thepicosecond-duration electrical pulses generated by the electri-cal soliton oscillator. Applications include all-electricalpicosecond-resolved metrology [5, 8], time-domain reflectom-etry (TDR) [9], ranging radars, microwave imaging, andpulse-based wireless or chip-to-chip communications. Withthis work, the electrical soliton is no longer a specialized lab-oratory exercise, but can now be engineered and utilized in amedium that touches our everyday lives, modern electronics,for the wide array of applications. This paper highlights theseexciting new developments.

NLTL & Electrical Solitons - A ReviewThis section briefly reviews electrical solitons on the NLTL toprovide the necessary background for our electrical solitonoscillator work presented later. Solitons are a unique class ofpulse-shaped waves that propagate in nonlinear dispersivemedia [10]. They maintain spatial confinement of wave ener-gy in a pulse shape over the course of propagation and exhib-it singular nonlinear dynamics. Balance between nonlinearityand dispersion creates the soliton phenomena. Common inNature, solitons are found in various nonlinear dispersivemedia, e.g., hydrodynamic solitons in shallow water and opti-cal solitons in fibers [10].

In the electrical domain, the nonlinear transmission line(NLTL), a 1D ladder network of inductors and varactors [Fig.2(a), top] or alternatively a linear transmission line periodical-ly loaded with varactors [Fig. 2(a), bottom], serves as a non-linear dispersive medium. In the NLTL, the nonlinearity orig-inates from the varactors while the dispersion arises from thestructural periodicity. For certain pulse-shaped voltage waveson the NLTL, the nonlinearity balances out the dispersion, andthese pulses propagate on the NLTL maintaining their exact

4 IEEE LEOS NEWSLETTER June 2006

Electrical Soliton ModelockingDavid S. Ricketts, Xiaofeng Li and Donhee Ham

University Research Highlights

DIVISION OF ENGINEERING AND APPLIED SCIENCES, HARVARD UNIVERSITY, CAMBRIDGE, MA 02138.

E-MAILS: [email protected], [email protected]@DEAS.HARVARD.EDU

Figure 1: Electrical soliton modelocked oscillator.

NonlinearTransmission Line

Amplifier

VX

VY

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shape (in the absence of loss). These are electrical solitonsdescribed by the Korteweg-DeVries (KdV) equation [10]. Thegeneral soliton propagation solution on the NLTL is a period-ic train of soliton pulses [Fig. 2(b)]. In the presence of loss, thesolitons cannot maintain their exact shape in the course ofpropagation since they have to lose energy, but they still main-tain spatial confinement of wave energy in a pulse shapethrough a unique soliton damping process [11]. See the insetfor comparison between electrical and optical solitons.

In addition to their ability to maintain spatial confinementof wave energy, the electrical solitons on the NLTL possessother unique properties [10]. To begin with, a taller soliton

travels faster than a shorter one on the NLTL. Due to thisamplitude-dependent speed, if a taller soliton is placed behinda shorter one as shown at the top of Fig. 2(c), the taller one willcatch up with the shorter one and move ahead of it after a col-lision [Fig. 2(c)]. When two solitons collide [middle of Fig.2(c)], they do not linearly superpose, but rather experiencesignificant amplitude modulations (nonlinear collision). Afterthe collision [bottom of Fig. 2(c)], the two solitons that havereturned to their original shapes have however acquired a per-manent time (phase) shift, shown by the difference in d1 andd2 in Fig. 2(c). The three soliton properties above, i.e., 1)amplitude-dependent speed, 2) amplitude modulation during

Figure 2: (a) Nonlinear transmission line (NLTL). (b) A general soliton waveform on the NLTL. (c) Solitons’ amplitude-dependent speedand nonlinear collision on an NLTL. (d) Hypothetical transient, soliton-forming processes on the NLTL.

NLTL(a)

(b)

(c)

Time Elapse (∆t)NLTL

NLTL

NLTL

Time Elapse (∆t)

NLTL

NLTL

(d)

d2

ZV0

V

W

C(ν)

d1

u + 6u u + u = 0∂ ∂ ∂3

∂t ∂z ∂z3i A − A − 2|A|2 A = 0

∂ ∂2

∂z ∂t2

Electrical Soliton Optical Soliton

KdV Equation NS Equation

vg ∝ |u| vg ∝ ω0

2nd Order Soliton (N = 2)tt zz

|A|2u

u(z,0) = N (N + 1) sech2 z

2nd Order Soliton (N = 2)A(z,0) = N sech z

Comparison of the electrical soliton andthe optical soliton: Top: The electricalsoliton is a baseband pulse with nounderlying carrier signal. Middle: Theelectrical soliton is based on KdVdynamics, while the optical soliton isderived form the nonlinear Schrödingerequation (NSE). Electrical soliton veloc-ity is amplitude dependent. Opticalsoliton velocity is carrier frequencydependent. Bottom: Shown here is thetime evolution of a second order electri-cal and optical soliton. The second orderelectrical soliton separates into two soli-tons, while the second order optical soli-ton changes shape, or “breathes,” as itpropagates [10].”

20leos03.qxd 6/8/06 10:32 AM Page 6


the collision, and 3) phase modulationafter the collision, are the key obstacles toconstructing a stable soliton oscillator, aswill be seen shortly.

Non-soliton waves can also travel onthe NLTL, but only by changing theirshape to form into a soliton or solitons. Anon-soliton pulse close to soliton shapewill be sharpened into a soliton [Fig. 2(d),top]. A non-soliton pulse that is signifi-cantly different from soliton shape willbreak up into multiple solitons of differ-ent amplitudes [Fig. 2(d), bottom].

It should be finally noted that one canvary the values of the two components(inductors and varactors) comprising theNLTL across a wide range to obtain a spe-cific soliton pulse width over manydecades. This is in contrast with the opti-cal fiber whose properties, predeterminedby materials and geometry, are notadjustable by the designer.

Electrical Soliton Oscillator - Topology and Operating PrinciplesAs pointed out earlier, the essence of our work is the con-struction of an oscillator that self-generates a periodic train ofelectrical solitons without requiring an external high-frequen-cy input. The starting idea to build our soliton oscillator wasto combine a ring NLTL with a non-inverting amplifierinserted in the ring [Fig.3 (a)] [1,2]. The ring NLTL supportscertain soliton circulation modes determined by the periodicboundary condition, l = n�(n=1,2,3,…) (l: circumference ofthe ring NLTL, �: spacing between two adjacent solitons)[Fig.3(b)]. The intention of the amplifier is to provide gain toinitiate startup and to compensate loss in steady state. Theultimate goal of this topology is to self-generate and self-sus-tain one of the soliton circulation modes of Fig. 3(b).

The topology does indeed lead to oscillations, self-startingfrom noise. However, when standard amplifiers are used in thetopology, the oscillations tend to be plagued with instabilityproblems, exhibiting significant variations in pulse amplitudeand repetition rate [2,6]. See Fig. 4

The oscillation instabilities arise because the circular looptopology of Fig. 3(a) not only generates the desired soliton circu-lation mode, but can also excite other parasitic solitons [1,2]. Thedesired and parasitic solitons continually collide while circulatingin the loop due to their generally different amplitudes and result-ant speed difference (due to previously mentioned solitons’ ampli-tude-dependent speed). It is these soliton collision events thatcause the significant modulations in the pulse amplitude and rep-etition rate (these undesirable effects of the soliton collision weredescribed earlier), leading to the oscillation instabilities.

We overcame the instability problems and obtained a sta-ble soliton oscillator in [1-3] by developing a special amplifi-er, which not only provides gain but also incorporates threestability mechanisms to prevent the soliton collision events insteady state. The three stability mechanisms are:

Reduced signal saturation: If the amplifier saturates its out-put significantly in Fig. 3(a), the amplifier output will beclose to a square pulse. As explained with Fig. 2(d), bottom,

Figure 3: (a) Soliton oscillator topology. (b) Ring NLTL. Mode 1 (l = �), Mode 2 (l = 2�),Mode 3 (l = 3�).

RingNLTL

Mode 1

Mode 3Mode 2

Amplifier

NLTL

VY

VX

(a) (b)

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this square pulse will break apart into multiple solitons of dif-fering amplitudes, traveling down the NLTL. These multiplesolitons will circulate around the loop at different speeds (dueto the amplitude-dependent speed), and be again distorted bythe amplifier, creating even more solitons of different ampli-tudes and speeds. This process repeats itself, and the solitonscontinue to circulate in the loop at different speeds, continu-ally colliding with one another, causing oscillation instabili-ties. It is therefore necessary to minimize signal saturation.

Perturbation rejection: In steady-state oscillation the ampli-fier should attenuate any small ambient perturbation (e.g.,noise) that could otherwise grow into parasitic solitons. Unlessthis is achieved, the desired soliton circulation mode and par-

asitic solitons will propagate at different speeds due to theirgenerally different amplitudes, colliding and building uposcillation instabilities.

Single mode selection: The amplifier should select a singlesoliton circulation mode in steady-state oscillation among themany possible modes [Fig. 3(b)]. If this is not achieved, vari-ous modes with generally different amplitudes will circulatein the loop at different speeds, leading to soliton collisionevents and hence unstable oscillations.

In [1-3], we achieved these three stability mechanisms byincorporating an adaptive bias control in a standard saturatingamplifier. Figure 5, showing the input-output transfer curveof the saturating amplifier, explains how this is achieved. Thetransfer curve is divided into the attenuation, gain, and satu-ration regions based on the curve’s tangential slopes. At start-up the amplifier is biased at point A in the gain region so thatambient noise can be amplified to initiate the oscillation start-up. As the oscillation grows and forms into a pulse train, thedc component of the amplifier output increases. This increasein the dc component is used to adaptively lower the amplifierbias (dashed arrow in Fig. 5). The reduced bias corresponds toan overall gain reduction, since a portion of the pulse entersthe attenuation region. The bias point continues to movedown on the curve until the overall gain becomes equal to thesystem loss, settling at the steady-state bias B.

In steady state with the bias at B, the three stability mech-anisms are simultaneously satisfied. First, the reduced biasensures that the peak portions of the input pulses do not enterthe saturation region, reducing distortion (reduced signal satu-ration). Second, with the reduced bias, the steady-state inputsoliton train is placed across the attenuation and gain regions,causing small perturbations around the bias to be attenuated(perturbation rejection). Note that perturbation rejection isaccomplished while maintaining gain for the main portions ofthe input soliton train to compensate loss. This threshold-dependent gain-attenuation mechanism is a technique widelyemployed in modelocked lasers in optics, where it is known assaturable absorption [4], but was originally introduced in elec-tronics domain by Cutler for his linear pulse oscillator [12].Third, the dependence of the steady-state bias on the dc com-ponent of the output leads to a mode-dependant gain sinceeach mode has a different dc component. This can be used toselect one particular mode (single mode selection).

Electrical Soliton Oscillator -Experimental ResultsThree soliton oscillator prototypes have confirmed the con-cepts and operating principles of our soliton oscillator. Thefirst two prototypes [1, 2] were built using discrete compo-nents (measured pulse widths: 43 ns and 827 ps) in order toexplicitly examine the detailed dynamics of the soliton oscil-lator. The third prototype [3] was implemented on a CMOSintegrated circuit (measured pulse width: 293 ps). Figure 6shows the measured steady-state soliton oscillations from eachprototype. Note that the pulse widths of 43 ns to 293 ps wereengineered by our choice of circuit components.

Figure 5: Transfer curve of a saturating amplifier. Startup bias Ais in the gain region. As the dc component of the amplifier outputincreases in initial transient, the bias is adaptively lowered(dashed arrow) towards steady-state bias B.

VY

Attenuation Gain Saturation

A

B

0 VX

Figure 4: Unstable oscillations that can result from Fig.3(a).

V(t)

t

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The most fascinating dynamics of the soliton oscillator canbe observed by following the pulse around the oscillator loopin steady state. Figure 7 shows such spatial dynamics meas-ured from our first prototype [1,2]. At the output of the

amplifier the pulse (width: 100 ns) is not exactly a soliton and,hence, sharpens into a soliton while propagating down theNLTL. Once the soliton is formed (width: 43 ns), it does notfurther sharpen since it is now a soliton. Instead, the loss on

Figure 7: Measured spatial dynamics of the soliton oscillator in steady state.

4

2

0

4

2

0

4

2

0

Pulse Width~ 100 ns

Pulse Width~ 43 ns

Pulse Width~ 110 nsV (t)V (t)VY(t)

[400 ns/div] t t t

VX VYNLTL

AMP

Figure 6: (Left) First soliton oscillator (pulse width: 43 ns, pulse repetition rate: 1.4 MHz). (Center) Second soliton oscillator (pulse width:827 ps, pulse repetition rate: 130 MHz). (Right) Third, chip-scale soliton oscillator (pulse width: 293 ps, pulse repetition rate: 1.14 GHz)

IC

4

3

2

1

0

V(t) [ volt]

43ns

V(t) [volt]7.7 ns

827ps

V(t) [volt] 293 ps

[500 ps/div][10 ns/div]

t

8

6

4

2 t

20 cm7.5 mm

Glass Substrate

10 cm

[1 µs/div]

0.7 µs

20leos03.qxd 6/8/06 10:32 AM Page 10


the NLTL becomes the dominant process, and the solitonexhibits a unique soliton damping [11] as it further travelsdown the NLTL, reducing its amplitude and velocity whileincreasing its width (note that in the NLTL the loss is signif-icantly higher than in optical fiber). At the end of the NLTL,the pulse width has increased to 110 ns. It is this clear exis-tence of the transition point between two distinctively differ-ent processes, the pulse sharpening and widening, thatunequivocally confirms the formation of the soliton at thattransition point.

Future ExtensionsThe minimum pulse width of 293 ps achieved in our latestprototype is not a record number as compared to the state-of-the-art 2-port GaAs NLTL (480-fs rise time) [7]. The value ofour work so far, rather, lies in the clear demonstration of thesoliton oscillator concept. Now with the concept firmlydemonstrated, the soliton oscillator can be quickly extendedto a significantly higher speed (shorter pulse width). Forinstance, the ultrafast GaAs NLTL in [7] can be incorporatedin our soliton oscillator to substantially reduce the solitonpulse width down close to 1 ps.

Placing such an ultrafast NLTL in the electrical soliton oscil-lator raises an important question on the impact of the amplifi-er bandwidth on the minimum soliton pulse width. While thepropagation of a 1-ps wide pulse on the stand-alone NLTL isfeasible [7], amplifiers, even in the state-of-the-art solid-statetechnologies, cannot provide bandwidth for such a sharp pulse.The experimental results in Fig. 7 clearly suggest, however, thatthe soliton compression on the NLTL may be able to overcomethe bandwidth limitation of the amplifier, and hence, it may befeasible to achieve a 1-ps pulse width using the NLTL of [7]despite the relatively slower amplifier. The explicit demonstra-tion of this interesting possibility remains an open question,and would be a natural future extension of this work.

Such picosecond electrical soliton oscillators will offer a newplatform for all-electrical ultrafast time-domain metrology.This is because the short pulse duration directly translates tohigh temporal resolution in time-domain measurements: thenarrow electrical pulses can be used to sample, or take “snap-shots” of, rapidly varying electrical signals with picosecondtemporal resolution [5,8]. Similarly, the picosecond electricalpulses can be used as probe signals for high-precision time-domain reflectometry (TDR) [9]. While the electrical solitonoscillator with the picosecond pulses will never compete withthe optical modelocked laser with its femtosecond pulses, itwill allow wide applications in an all-electrical medium.

References[1] D. S. Ricketts, X. Li, M. DePetro, and D. Ham, “A self-

sustained electrical soliton oscillator,” IEEE MTT-S Int.Microwave Symp. Dig., June 2005. pp 1365-1368.

[2] D. S. Ricketts, X. Li, and D. Ham, “Electrical solitonoscillator,” IEEE Trans. Microw. Theory Tech., Jan. 2006.vol. 54, no. 1, pp 373-382

[3] D. S. Ricketts and D. Ham, “A chip-scale electrical soli-ton oscillator,” IEEE Int. Solid-State Circ. Conf. Dig.Tech. Papers, vol. 49, pp. 432-433, Feb. 2006.

[4] H. A. Haus, “Mode-locking of lasers,” IEEE. J. Sel.Top. Quant. Elect., vol. 6, no. 6, pp. 1173-1185,Nov/Dec 2000.

[5] M. J. Rodwell et al, “Active and nonlinear wave propa-gation devices in ultrafast electronics and optoelectron-ics,” Proc. IEEE, vol. 82, no. 7, pp. 1037-1059, July1994.

[6] G. J. Ballantyne, Periodically Amplified SolitonSystems, PhD Dissertation, U. of Canterbury, NewZealand, 1994.

[7] D. W. van der Weide, “Delta-doped Schottky diode non-linear transmission lines for 480-fs, 3.5-V transients,”Appl. Phy. Lett., vol. 65, no. 7, pp. 881-883, August1994.

[8] M. Kahrs, “50 years of RF and microwave sampling,”IEEE Trans. Microw. Theory Tech., vol. 51, no. 6, pp.1787-1805, June 2003.

[9] B. Elliot, “High-sensitivity picosecond time-domainreflectometry,” IEEE Trans. Instrum. Meas., vol. IM-25,Dec. 1976.

[10] M. Remoissenet, Waves Called Solitons: Concepts andExperiments, New York, NY: Springer, 1999.

[11] E. Ott and R. N. Sudan, “Damping of solitary waves,”The Physics of Fluids, vol. 13, no. 6, pp. 1432-1434,June 1970.

[12] C. C. Cutler, “The regenerative pulse generator,” Proc.IRE, vol. 43, no. 2, pp. 140-148, Feb. 1955.

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