Includes bibliographical references.The design, fabrication, and experimental demonstration of dynamically reconfigurable binary optical switches based on the integration of GaAs/AlGaAs vertical-cavity surface-emitting lasers and heterojunction phototransistors are reported. These new monolithic optical switches can perform spatial routing and optical amplification functions on input optical data with a fan-out of two, and can be programmed using simple voltage control. The 1 × 2 and 2 × 2 devices provide the basis for a high performance, two-dimensional optical switching fabric with electrical interfaces for optical switching and interconnection networks.The work at UNM was supported by Rome Laboratories (Griffiss AFB), the Advanced Research Projects Agency-MTO, and the Air Force Office of Scientific Research (Bolling AFB). The work performed at Sandia was supported by DOE Contract No. DE-AC04-76DP00789

Adams, A. C.

Chalmers, S. A.

Cheng, Julian

Lear, Kevin L.

Leibenguth, Ronald E.

Lu, Bo

Lu, Yin-Chen

Vawter, G. A.

Zhou, Ping

Zilko, John Leon

Zolper, J. C.

English

The design, fabrication, and experimental demonstration of dynamically reconfigurable binary optical switches based on the integration of GaAs/AlGaAs vertical-cavity surface-emitting lasers and heterojunction phototransistors are reported. These new monolithic optical switches can perform spatial routing and optical amplification functions on input optical data with a fan-out of two, and can be programmed using simple voltage control. The 1 × 2 and 2 × 2 devices provide the basis for a high performance, two-dimensional optical switching fabric with electrical interfaces for optical switching and interconnection networks.The work at UNM was supported by Rome Laboratories (Griffiss AFB), the Advanced Research Projects Agency-MTO, and the Air Force Office of Scientific Research (Bolling AFB). The work performed at Sandia was supported by DOE Contract No. DE-AC04-76DP00789

Vawter, G. A., author

Chalmers, S. A., author

Lear, Kevin L., author

Zolper, J. C., author

Zilko, John Leon, author

Adams, A. C., author

Leibenguth, Ronald E., author

Cheng, Julian, author

Lu, Yin-Chen, author

Lu, Bo, author

Zhou, Ping, author

IEEE, publisher

Mountain Scholar (Digital Collections of Colorado and Wyoming)

222 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 6, NO.2, FEBRUARY 1994Reconfigurable Binary Optical RoutingSwitches with Fan-Out Based on theIntegration of GaAs/AIGaAs Surface-EmittingLasers and Heterojunction PhototransistorsBo Lu, Ping Zhou, Yin-Chen Lu, Julian Cheng, Senior Member, IEEE, R. E. Leibenguth, A. C. Adams, J. L.Zilko, J. C. Zolper, Member, IEEE, K. L. Lear, Member, IEEE, S. A. Chalmers and G. A. VawterAbstract- The design, fabrication, and experimental demon-stration of dynamically reconfigurable binary optical switchesbased on the integration of GaAslAIGaAs vertical-cavity surface-emitting: lasers and heterojunction phototransistors are reported.These new monolithic optical switches can perform spatial rout-ing and optical amplification functions on input optical datawith a fan-out of two, and can be programmed using simplevoltage control. The 1 x 2 and 2 x 2 devices provide the basisfor a high performance, two-dimensional optical switching fabricwith electrical interfaces for optical switching and interconnectionnetworks.I. INTRODUCTIONMONOLITHIC arrays of optical switches based on theintegration of GaAs/AIGaAs vertical-cavity surface-emitting lasers (VeSELs) and heterojunction phototransis-tors (HPT) represent a promising two-dimensional approachto high-throughput optical data processing [1] and opticalswitching networks. [2] VeSEL-based optical switches withalternatively non-latching or latching switching characteristicshave been demonstrated by integrating VeSELs with HPTs orphotothyristors (PNPNs), respectively. Both of these switchescan perform digital optical logic processing functions. [1]They c,m also be further combined monolithically into morecomplex binary switch nodes that can spatially reconfigure thesurface-normal optical signal paths. [3] These binary opticalswitches can perform a variety of spatial switching functions,including bypass and exchange, which provide the basis for acompact, multi-stage optical switching network. In additionto forming a switching fabric capable of spatially routinghigh speed optical data amongst a large number of paralleloptical channels, each switch also provides a high speedoptical-to-electrical interface between the optical switchingnetwork and its electronic control functions. [4] To be apractical switching technology, these switching nodes mustbe dynamically reconfigurable, [5 - 7] and must be able toManuscript received October 29, 1993.B. Lu, P. Zhou, Y.-c. Lu and J. Cheng are with the University of NewMexico, Center for High Technology Materials, Albuquerque, NM 87131.R. E. Leibenguth, A. C. Adams and J. L. Zilko are with AT&T BellLaboratories. Breinigsville, PA 18031.J. C. Zolper, K. L. Lear, S. A. Chalmers and G. A. Vawter asre with SandiaNational Laboratory, Albuquerque, NM 87185.IEEE Log Number 9215892accommodate a variety of different network topologies (e.g.,Omega or Banyan), and some degree of optical fan-out mustbe provided for multi-cast and broadcast operations. Althoughour earlier binary 2 x 2 optical switch design [2] is capableof performing optical bypass-exchange operations, it lacks afan-out capability, and was experimentally demonstrated usingprincipally PNPNNeSEL switches. In this paper, we report anew and improved device design for the monolithic, 1 x 2 and2 x 2, HPTNeSEL binary optical switches, and demonstratetheir operation as dynamically reconfigurable optical signalrouting switches with a variable fan-out capability. We willshow that packets of optical data can be routed rapidly to oneor more destinations by means of simple voltage control.The circuit design and the operation principles of the binaryoptical switches are shown in Fig. 1. The 1 x 2 switch (Fig.l(a» contains a segmented HPT optical input port, each halfof which is controlled by a separate bias voltage (VI or V2 )and is serially connected to a different VeSEL output port.The optical input data impinges on both HPT segments inequal proportions, and depending on the state (VI, V2 ) ofthe control voltages, the photoinduced and amplified currentfrom the HPT is routed alternatively to VeSEL #1 (1,0),to veSEL #2 (0,1), or to both VeSELs (1,1), where thedata is regenerated as an amplified optical output (Fig. I(c».Thus alternate routing as well as an optical fan-out of 2 can beachieved. A 2 x 2 switch can be realized by combining two1 x 2 switch nodes, which may (as in Fig. I(b» share the sametwo input and output ports. In this case, routing is controlled bythe state of the voltages (VI, V2 , Vi, V{), which provide a largernumber of routing configurations, including the importantbypass (1,0,0,1) and exchange (0,1,1,0) operations (Fig.I(c». Although it is less readily apparent, each 1x 2 switch canalso be interconnected with other 1 x 2 switches with which itshares only one common input (segmented HPT) or output port(VeSEL), but not both, to form a different 2 x 2 switch con-figuration (2 inputs, 2 outputs) that can be used to implementan Omega network topology. Every HPT is connected to twoVeSELs (at least one of which is non-local), and the VeSELat each node is similarly connected to two HPTs at differentsites. It is the different pairing of switch nodes, or sites, thatfunctionally defines a specific routing network topology.1041-1135/94$04.00 © 1994 IEEELU et ai: RECONFIGURABLE BINARY OPTICAL ROUTING SWITCHES 223OUTPUT 1 T OUTPUT 2AINPUT 1OUTPUT 1viveSEL 2T'OUTPUT 2(a) 1 x 2 SWITCHING NODE (b) 2 X 2 SWITCHING NODE(1,0)BA(0,1)BB\. IAI \.AA(1,0)AB~ (0,0)AA(1,1)(0,0)AB~A(0,1)ROUTING FAN-OUT(c)BYPASS EXCHANGEFig.!. Circuit design of the monolithic (a) 1 x 2 and (b) 2 x 2 binary optical switches, and (c) some of their reconfigurable modes of switching operation.A cross-sectional view of the new epilayer structure fora 2 x 2 binary HPTNeSEL optical switch and its devicedesign are shown in Fig. 2(a), while a photomicrograph of themonolithic switch is shown in Fig. 2(b). The VeSEL and HPTepilayers are vertically stacked together and are electricallyisolated by a thick, low-doped GaAs layer, and an emitter-up configuration is chosen for the HPT. The detailed epilayerstructure is described in Ref. 4, and is grown by molecularbearn epitaxy (MBE) on an n-GaAs substrate. The 2 x 2switch contains two segmented HPTs and two VeSELs. EachGaAs/AIGaAs VeSEL has a nominal active area diameterof 20 Mm, which is defined by proton-implantation (335keV, 4 x 1014/cmz dose). Each HPT input port contains twonearly contiguous but electrically-isolated 40 ust: by 80 Mmsegments. The VeSELs and HPTs are individually isolated bya multiple-energy, multiple-dose proton-implant. The devicefabrication started with the etching of two mesas using a(1:1:40) NH40H:HzOz:HzO solution to define the areas ofthe emitter segments and of the collector (70 ust: by 115Mm), stopping at the AlAs stop-etch layer. The latter is thenremoved, followed by metallization of the n-type emitter andcollector contacts. Next, separate proton implants are used todefine the current confinement in the VeSEL and to isolate theindividual VeSELs and HPTs. The upper (p-type) and lower(n-type) contacts of the VeSEL are then deposited, and theVeSEL implant and the contacts are simultaneously annealed.A photosensitive polyimide layer is then deposited to provideelectrical isolation for the metal interconnection layer, whichconnects the HPT emitter to the VeSEL p-contact. Finallyholes are opened to provide access to the various contacts.The threshold current of the VeSELs is between 4 to 6 rnA,the differential slope efficiency is TJs ~ 30% - 60%, andtheir series resistance is typically ~ lOOn (due to a non-optimum p-contact). The small signal current gain {3 of theHPTs varies from 60 to 100 across the range of operatingcurrent densities. The external quantum efficiency TJe of eachHPT segment is estimated to be ~ 25%, taking into accountthe input partition (each segment of an HPT receives less thanhalf the incident light), the Fresnel reflection loss at the topsurface (~ 30%), the absorption in the GaAs cap layer of the224(B)IEEE PHOTONICS TECHNOLOGY LETTERS. VOL. 6. NO.2. FEBRUARY 1994INPl1J' CHANNF1. 1 OR 2n-AIAs/n-AlGaAs MLR2n-GaAs BlJIItIERT(A)Fig. 2. 'a) The epitaxial layer structure and schematic device design of a binary optical switch containing two VeSEL optical output ports and two segmentedHPT optical input ports (only one is shown). (b) Photo micrograph showing the monolithic circuit layout of an array of 2 x 2 switches.HPT, and the internal quantum efficiency of the collector-basejunction.To demonstrate the operation of the HPTNeSEL switch,the input light from an external laser source containing botha de optical bias and a modulated ac component.is incidenton a segmented HPT through a butt-coupled multi-modeoptical fiber. The size of the segmented HPT was chosen tocapture most of the fiber's optical output. The HPTNeSELoptical switch is a non-latching threshold switch, in whichthe photocurrent induced by the optical input is multipliedby the average current gain ((3) of the HPT to switch onthe VeSEL. The switching threshold is thus the input opticalpower I~ 350 /-lW) required to bias the VeSEL at its lasingthreshold (6 rnA). From this, the value of Tie ((3) is determinedto be 25. By prebiasing the VeSEL near threshold, the opticalswitchmg energy and tum-on delay are both reduced, andthe switch possesses linear optical amplification characteristicsabove threshold. Switching is produced by the modulatedoptical input data pulses (~ 150 /-lW), which should producean amplified optical output with a differential optical gainG = TleTls{3. With Tie (/3) ~ 25, and Tis ~ 30%, an opticalgain of about 8 is expected. However, the observed opticalgain is much lower (~ 1-2) due to gain compression in theHPT at the current densities above threshold (> 2 kA/cm 2 ) ,where the overall current gain (/3)(~ 100) far exceeds thedifferential current gain (3("5. 20).The modes of operation of the 1 x 2 switch have beendemonstrated using the 2 x 2 switch (Fig. I(b)) by settingV{ and V; equal to 0, i.e., operating in the (VI, V2,0,0) state.In the upper traces of Fig. 3, the control voltages VI and V2 areshown as synchronous, complementary gating pulses (~ 5.5V), while the optical input incident on HPT #1 is a train of 850nm optical pulses (~ 50 ns wide, ~ I MHz frequency) with ade optical prebias. Since the VeSEL threshold voltage is 2.3V, while the saturation voltage of the HPT is only about 2 V, asignificant part of the 5.5 V routing voltage is dropped acrossthe series resistance of the VeSEL. As the control voltagetoggles between VI and V2 , the regenerated optical input isseen to emerge alternately from either channel I (VeSEL#1), i.e., the (1, 0) state, or from channel 2 (VeSEL #2),LU et al. RECONFIGURABLE BINARY OPTICAL ROUTING SWITCHES 225V1V2Complementary Routing Control V~ltageSCH1(c)(b)s us/dlv,Switched Ouput 2Data Input 1ICH2I '................,. _.- .,FANOUT, .I____Switched Out.p.':I~!.._._ (a).-------I_V1~ "=bV2Routing Control Voltages with Overlapt-CH 1....._..~.-+l._. .J,. _ _.~ _ .•. _ __-_: FANOUTSwitched putputs (b)5Ils/div.Fig; 3. The experimental demonstration of the operations of a 1 x 2 binaryoptical sWitch, showing the complementary routing control voltages andthe optical output channels for the cases of (a) non-overlapping.and (b)overlapping control voltages, with the latter showing an optical fan-out of 2.i.e., the (0, 1) state. In the upper output traces in Fig. 3(a),the control voltages VI and V2 do not overlap in time, whilefor the lower output traces they partially overlap. In the lattercase, the overlapping of VI and V2 produces the (1, 1) statewith a fan-out of 2, in which the input data emerges from bothVCSEL output ports. Thus the optical input data can be routedin packet form to either or both output ports in a completelycontrollable and reconfigurable manner.Fig. 4 illustrates the operation of the 2 x 2 optical switchin the bypass (1,0,0,1) and exchange (0,1, 1,0) modes, inwhich the complementary voltage pairs VI = V{ and V2 = V;are alternately set equal to 1 and O. Here optical INPUT 1and INPUT 2 are two optical pulse trains characterized bydifferent frequencies (0.6 MHz and 1.2 MHz) and amplitudes,which are incident on the segmented HPT # 1 and HPT#2, respectively. As the control voltages (~ 6.5 V) togglebetween the (1,0,0,1) and the (0,1,1,0) states, the outputtraces show that the regenerated channel #1 input data isswitched alternately between the channel #1 and channel #2output ports, while at the same time, the channel #2 input datais switched between the channel #2 and channel #1 outputports, respectively. The former thus represents the bypass (orstraight-through) configuration, and the latter the exchange (orcrossover) configuration. Other 2 x 2 routing configurationshave also been experimentally demonstrated.Fig. 4. Experimental demonstration of a 2 x 2 binary optical switch,showing (a) the control voltages, (b) the input optical channels, and (c) theoutput optical channels, indicating the optical bypass and exchange operations.The optical pulse response of the switch shows a rise timeof ~ 3 ns. However, it has a longer decay time, which islimited by the large size (40 ttrn by 80 Mm) and large effectivecapacitance of the HPT, which limit the minimum optical pulsewidth used in this experiment to about 20 ns. Although boththe HPT and the VCSEL can be optimized to respond morerapidly to an input optical signal, the current design was chosento facilitate the ease of optical coupling rather than maximizingthe switching speed. The rise time of the switched output islimited by the time needed to charge the HPT's junctions toa level that would permit the collector current to bias theVCSEL above its lasing threshold. The time for bringing theseparticular HPTs from a zero bias to threshold is about 1 to 2tLS. By superimposing a de optical or electrical threshold pre-bias on the optical data pulses, the precharge time has beengreatly reduced to ~ 3 ns. To further reduce the decay time,the active area of the HPTs must be scaled down, and severalother different approaches are under investigation.II. CONCLUSIONIn conclusion, we have described the design and fabricationof monolithic, 1 x 2 and 2 x 2 HPTNCSEL binary optical226switches. We have experimentally demonstrated their opera-tion, for the first time, as a flexible spatial routing switch withfan-out, which can be programmed simply and rapidly to routeoptical data packets amongst parallel optical data channelsthrough a number of reconfigurable paths in the switchingfabric. High performance switching has been achieved usingswitches that are capable of accommodating a variety ofdifferent switching network topolgies, while providing multi-cast capabilities. The present devices are capable of routing 50Mb/s data packets (20 ns clock period), but the next generationof switch designs under study will be able to push this intothe hundreds of Mb/s regime. The switching pulse energy,currently at 100-150 JL W, can be lowered by increasing theHPT current gain f3 to >500, and improving the input powerefficiency from 25% to 40-50%, thereby reducing the inputswitching power to a few tens of JLW. Improving the slopeefficiency of the VCSEL by reducing its self-heating and modemisalignment would also increase the optical output power.An optical gain of over 100 is potentially achievable with abinary optical switch.ACKNOWLEDGMENTThe work at UNM was supported by Rome Laboratories(Griffis, AFB), the Advanced Research Projects Agency-MTO, and the Air Force Office of Scientific Research (BollingAFB). The work performed at Sandia was supported by DOEIEEE PHOTONICS TECHNOLOGY LETTERS. VOL. 6, NO.2, FEBRUARY 1994Contract No. DE-AC04-76DP00789. We gratefully acknowl-edge the technical assistance of J. Escobedo of Sandia NationalLaboratory .REFERENCES[I] J. Cheng and P. Zhou, "Integrable surface-emitting laser-based opticalswitches and logic gates for parallel digital optical computing," Ap-plied Optics, Special Issue on Optical Computing, vol. 31, no. 26, pp.5592-5603, 1992.[2] J. Cheng, P. Zhou, S. Z. Sun and D. R. Myers, "Surface-emitting laser-based Smart Pixels for two-dimensional optical logic and reconfigurableoptical interconnections," IEEE J. of Quantum Electron., vol. 29, no. 2,pp. 741-756., Special issue on Smart Pixels.[3] J. Cheng and P. Zhou, "Smart Pixels for two-dimensional arrays," IEEECircuits and Devices, vol. 9, no. 2, pp. 19-27, March, 1993.[4] P. Zhou, J. Cheng, J. C. Zolper, K. L. Lear, S. A. Chalmers, G. A.Vawter,R.,E. Leibenguth and A. C. Adams, "Monolithic optotelectronic switchbased on the integration of a GaAs/AIGaAs heterojunction bipolartransistor and a GaAs Vertical-Cavity Surface-Emitting Laser," IEEEPhotons Techno!. Lett., vol. 5, no. 9, pp., Sept. 1993.[5] H. S. Hinton and A. L. Lentine, "Multiple quantum-well technologytakes SEED," IEEE Circuits and Devices, vol. 9, no. 2, pp. 12-18,March 1993.[6] F. B. McCormick, F. A. P. Tooley, T. J. Cloonan, J. L. Brubaker,A. L. Lentine, et aI., "Experimental investigation of a free-spaceoptical switching network using S-SEEDs," Applied Optics, vol. 31,pp. 5431-5446, 1992.[7] K. Noguchi, T. Sakano and T. Matsumoto, "A Multichannel free-spaceoptical switch based on multistage network configuration," J. LightwaveTechno!. vol. 9, 1726-1732, 1991.[8] T. Sakano, K. Noguchi and T. Matsumoto, "Multiprocessor system usingan automatically rearrangeable free-space multichannel optical switch:COSINE-2," Appl. Optics vol. 32, pp. 3690-3699, 1993.

Reconfigurable binary optical routing switches with fan-out based on the integration of GaAs/AlGaAs surface-emitting lasers and heterojunction phototransistors

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Reconfigurable binary optical routing switches with fan-out based on the integration of GaAs/AlGaAs surface-emitting lasers and heterojunction phototransistors

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