We report optimization of the steering efficiency of the 1-D Opto-VLSI processor using direct power measurement method for wavelengths in the near-IR and 632 nm. Highest improvement observed for the signal and interport isolation is 8 dB and 12 dB respectively. This improved performance of the processor is crucial to the realization of low crosstalk reconfigurable optical add/drop multiplexers (ROADM) using Opto-VLSI processors

Alameh, Kamal

Poh, Chung

English

Research Online @ ECU

Edith Cowan University Research Online ECU Publications Pre. 2011 2006 Improving the Steering Efficiency of 1x4096 Opto-VLSI Processor using Direct Power Measurement Method Chung Poh Edith Cowan University Kamal Alameh Edith Cowan University Follow this and additional works at: https://ro.ecu.edu.au/ecuworks  Part of the Engineering Commons This is an Author's Accepted Manuscript of: Poh, C. K., & Alameh, K. (2006). Improving the Steering Efficiency of 1x4096 Opto-VLSI Processor using Direct Power Measurement Method. Proceedings of Third IEEE International Workshop on Electronic Design, Test and Applications DELTA 2006. (pp. 309-314). KL, Malaysia. IEEE Computer Society. Available here © 2006 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. This Conference Proceeding is posted at Research Online. https://ro.ecu.edu.au/ecuworks/2077 Improving the Steering Efficiency of 1x4096 Opto-VLSI Processor Using Direct Power Measurement Method Chung-Kiak Poh and Kamal AlamehCentre of Excellence for MicroPhotonic Systems, Electron Science Research InstituteEdith Cowan University, Joondalup, WA, 6027, Australia.Email: chungp@student.ecu.edu.auAbstractWe report optimization of the steering efficiency ofthe 1-D Opto-VLSI processor using direct powermeasurement method for wavelengths in the near-IR and 632 nm. Highest improvement observed for thesignal and interport isolation is 8 dB and 12 dB respectively. This improved performance of theprocessor is crucial to the realization of lowcrosstalk reconfigurable optical add/dropmultiplexers (ROADM) using Opto-VLSI processors. 1. Introduction Optical beam steering is commonly accomplishedvia mechanical systems like mirrors on gimbals. Assuch systems tend to be large, heavy and complex, a non-mechanical all-electronic beam steeringcomponent would often be preferred [1].  The use of very-large-scale integration (VLSI) technology inconjunction with new mixtures of Liquid Crystals(LC) has recently led to the construction of polarisation-insensitive Opto-VLSI processorscapable of steering, splitting and reshaping opticalbeams [1]. Opto-VLSI processors have severalproperties that are attractive for laser beam steering including a small pixel size, low-switching voltage,low power consumption, and high optical efficiency[2]. From an application point of view non-mechanical beam steering and beam shaping are desired features for many applications includingreconfigurable optical components, free-space optical communications, defence and security technology,3D holographic memories, and laser-based imaging.Our current research has been to harness the unique properties of Opto-VLSI processor to realisesolid-state Reconfigurable Optical Add/DropMultiplexer (ROADM) which involves switching various wavebands between a Thru-port and a Drop-port. In a ROADM, the steering efficiency of theprocessor is crucial as it affects the crosstalk level between the two ports. In this paper, we explore theuse of a real-time power measurement method toimprove the performance of Opto-VLSI processors.An algorithm is developed in LabVIEW to optimisethe command voltage profile of the Opto-VLSIprocessor for maximum diffraction efficiency.2. Opto-VLSI Processor A reconfigurable Opto-VLSI processor comprisesan array of liquid crystal (LC) cells driven by a Very-Large-Scale-Integrated (VLSI) circuit that generates digital holographic diffraction gratings to steer and/orshape optical beams [2,3], as illustrated in Fig. 1.Each pixel is assigned a few memory elements that store a digital value, and a multiplexer that selectsone of the input voltages and applies it to thealuminium mirror plate. Opto-VLSI processors areelectronically controlled, software-configured,polarization independent, cost effective because of the high-volume manufacturing capability of VLSIchips as well as the capability of controlling multiplefiber ports in one compact Opto-VLSI module, and very reliable since beam steering is achieved with nomechanically moving parts.MemoryelementsMirrorD1D2MUXConnection to Mirror2N-phasePixel ArchitectureDNOpto-VLSI ProcessorLC MaterialITOAluminiumMirrorGlassSilicon SubstrateVLSI LayerOpto-VLSI CellStructureData DecoderAddress DecoderQWPFig. 1. Typical multi-phase Opto-VLSI processor and LC cell structure design.Proceedings of the Third IEEE International Workshop on Electronic Design, Test and Applications (DELTA’06) 0-7695-2500-8/05 $20.00 © 2005 IEEE Fig. 1 also shows a typical layout and a cell designof a multi-phase Opto-VLSI processor. Indium-TinOxide (ITO) is used as the transparent electrode, and evaporated aluminium is used as the reflective electrode. The ITO layer is generally grounded and avoltage is applied at the reflective electrode by theVLSI circuit below the LC layer.Fig. 2 illustrates the steering capability of Opto-VLSI processors. The way the processor steers an incident light beam is similar to the way a prismdeflects an incident light beam via the prism action.In other words, we can think of the processor as aprogrammable prism as far as beam steering isconcerned. The application of a periodic sequence of voltage ramps of period / across the Opto-VLSI array aperture steers the incident light beam to anangle T  (relative to the normal) given by the generalgrating equation [8,9]»¼º«¬ª /  )sin(sin 1 DOT o                                       (1) where oO  is the incident wavelength in free space, Dis the angle of incidence, is the grating’s period = , is the number of pixels within the periodof the programmable grating, and  is the interpixeldistance./dqu qdSteering Phase HologramTqdIncidentBeamD DDiffractedBeamOpto-VLSIProcessorFig. 2. Steering capability of an Opto-VLSI processor. Liquid crystal is known to be birefringent and thatthe orientation of the molecules can be influenced byan applied external electric field. By changing the voltage across an LC cell, it is possible to rotate theorientation of these molecules and thus change theindex of refraction for a given polarization.The major challenge with LC is that switching isthat the switching speed of a nematic liquid crystal phase shifter is generally inversely proportional tothe square of the thickness of the nematic liquidcrystal layer. Therefore, for fast response, it isdesired to keep the liquid crystal layer thin. This canbe achieved by driving the LC with modulo-2Ssawtooth ramp [4]. This modulo-2S sawtooth rampstill steers an optical beam with unity efficiency (nosidelobes), but only for the designed wavelength forwhich the phase resets exactly equal 2S radians. Fig.3 illustrates the equivalence of the phase retardationof a linear phase ramp and a blazed grating profile (both steer the light with unity efficiency). Liquid crystals suffer from fringing fields and resistance to rapid change in orientation, whichmakes it difficult to electronically address a liquidcrystal array at sub-wavelength spacing. Thisdifficulty does not manifest during the uniformphase-ramp portion of the addressing process [5].Instead, the difficulty comes at the reset point whereabruptly drop in phase is desired.Phase change S6420Horizontal distance (a.u.)Linear phaserampBlazed gratingprofileSSFig. 3. Phase retardation of a prism. The linear phase ramp can be replaced by a blazed grating profile which still steers light withunity efficiency.During the phase ramp, fringing field effectssmooth the phase profile. At the reset point, thefringing field creates a spatial region with oppositephase profile as shown in Fig. 4. The flyback steersthe portion of an incident optical beam to an oppositeangular position thus reducing the diffractionefficiency. The length of the flyback region is approximately equal to the thickness of the LC layer [6]. For small steering angles (i.e. small ramp slope), the flyback region is a small percentage of the phase ramp and has a small impact on the diffractionProceedings of the Third IEEE International Workshop on Electronic Design, Test and Applications (DELTA’06) 0-7695-2500-8/05 $20.00 © 2005 IEEE efficiency. However, as the steering angle gets larger,a significant portion of the wavefront is coupled intohigher diffraction orders, resulting in a significantreduction in diffraction efficiency and highercrosstalk.Greatestpossible phasegradientResulting phase in liquid crystalPhase imposed by electrodesFlyback results in non-zero phase.Fig. 4. Phase profile due to pixilation of electrodes and resistanceof liquid crystal material to rapid change in orientation.The overall diffraction efficiency due to quantization as well as the flyback effects can be approximated by [4]221/)/sin(¸¸¹·¨¨©§/»¼º«¬ª dqqq Ftotal SSK (2)where/F = width of the flyback region/ = length of the grating period = qǜdAnother contribution of loss comes from thenonlinear phase response of the LC as a function ofthe applied voltage. This effect happens in the linearramp region that contributes to the steering of thelight beam. Therefore, the amount of loss due to non-linear effect becomes significant when the number of pixels per grating period is large (small steeringangle). In short, at large steering angle, the loss is primarily due to pixilation and flyback, while at smallsteering angle, the majority of the loss is due tononlinear phase change. Note that the ‘nonlinearloss’ is not fundamental and it could be removedsignificantly by taking the phase—voltage intoaccount. Fig. 5 shows the steering efficiency versusthe number of pixels for an Opto-VLSI processorwith and without the effects of flyback. It is obviousthat the flyback significantly reduces the steering efficiency when the number of pixels is low. Spacing at the half-wavelength level would allowlarge-angle beam steering without creating thedetrimental flyback effect [7]. For optimalperformance, the beam-steering capabilities of liquidcrystal devices are currently limited to about 4q ineither direction in order to maintain high efficiencybeam steering. 00.20.40.60.811.20 10 20 30 40 50 60 70 80 90 100Number of phase-shifter within reset, qTheoretical efficiencyDue to 'flyback'effectDue to step-quantisationFig. 5. Steering efficiency of the first order beam versus number ofpixels due to step-quantization of Opto-VLSI processor and flyback effect.3. Opto-VLSI based ROADM architecture A schematic setup to realize ROADM using Opto-VLSI processor is as shown in Fig. 6. The inputwavelengths launched at the Input-port are routed,through a circulator, to a fiber collimator thatconverts the WDM signal into a collimated opticalbeam, which is demultiplexed by a dispersive gratingalong different directions. This causes differentwavelengths to be mapped onto different regions on the Opto-VLSI active window. Depending on thehologram loaded into the Opto-VLSI processor, each wavelength can either be steered to the Thru-port, ordirected to the Drop-port for Drop-operation. As anillustrative example shown in Fig. 6, where only thewavelength O1 is steered to the Drop-port while theother channels are steered to the Thru-port. If thesame wavelength O1* (carrying different data) islaunched into the Add-port, it follows the sameoptical path of the dropped wavelength but inopposite direction, and hence it is coupled into theThru-port and multiplexed with the WDM channels.Note that because each wavelength resides on aunique position on the active window, there is one appropriate steering angle that couples thatwavelength into the desired port with the highestpossible power. Proceedings of the Third IEEE International Workshop on Electronic Design, Test and Applications (DELTA’06) 0-7695-2500-8/05 $20.00 © 2005 IEEE INPUT: O1, O2,…, OnTHRU: O2, …, OnADD: O1*DROP: O1Opto-VLSIprocessorBlazedgratingCollimatorsOpticalFibersCirculatorsOnO1O2computerFig. 6. Experimental setup for the reconfigurable optical add/dropmultiplexer (ROADM). 4. Experimental results Two experiments were performed to validate thesteering efficiency improvement. The primaryexperiment involved the above reconfigurable opticaladd/drop multiplexer (ROADM) setup and the use of a near-infra-red (NIR) tunable laser source. In the second experiment, we focused on the optimizationof the steering efficiency of the Opto-VLSI processor illuminated using a He-Ne laser source (632 nm) in free-space.A. Optimization results for NIR (ROADM setup)In this setup, the waveband 1545 nm was launched into the input port of the ROADM, and theappropriate phase hologram with linear blazed grating profile was uploaded into the processor so as to couple the waveband into the Thru-port withhighest coupling efficiency and the optical power onthe spectrum analyzer was recorded. Next, the grating profile was allowed to evolve to maximizethe power of the first order beam. This approach (i)minimized the flyback effect and (ii) helped tolinearise the LC phase-voltage relationship takinginto account the fringing field due to the pixilatedelectrode array and the LC viscosity. The sameprocedure was repeated for other wavebands namely1550, 1555, 1560, 1565, and 1570 nm which requireddifferent grating periods ranging from 36 to 166 Pmrespectively, for optimum coupling. The outputpower levels for these wavebands are plotted in Fig.7 for the linear as well as the optimized voltageprofiles.Fig. 7 shows that the power output of the linearseries begins to drop off as the steering anglebecomes smaller (larger grating period). This is inagreement with the hypothesis put forward in theintroduction (Section 1). That is, the non-linear LCphase-voltage effect becomes more pronounced as the period becomes larger and more optical power isdistributed to other higher diffraction orders when linear voltage ramp is applied. On the other hand, inthe short period region the non-linear LC phase–voltage becomes negligible while the flyback effectbegins to dominate.In the ROADM setup, light from higherdiffraction orders can potentially couple intoundesired port leading to poor inter-port isolation.Optimization can increase the power of the first order (the steered order) while reduce powers of the higherorders. Table 1 shows measurements of twooptimised Opto-VLSI voltage profiles for steeringangles of 0.27° and 2.7°. It was noticed that for a steering angle of 0.27° the optimized nonlinearvoltage profile exhibits 8dB improvement in couplingefficiency and 12dB enhancement in port isolation.For a steering angle of 2.7°, the fly-back effectdominates over the nonlinear effect, leading tolimited improvements in diffraction efficiency and port isolation of 2dB and 3dB, respectively. Notethat, despite the fact that these improvements at highsteering angles are limited to a few dB, the signal andinter-port isolation levels are adequate for practicalROADMs.-13-12-11-10-9-8-7-6-5-425 45 65 85 105 125 145 165Optical power(dBm)Variable grating period (μm)Optical power(dBm) With ‘optimised’ voltage rampWith linear voltage rampFig. 7. Optimisation results using the near-IR source for linearvoltage ramp input and ‘optimised’ (nonlinear) voltage ramp input.Proceedings of the Third IEEE International Workshop on Electronic Design, Test and Applications (DELTA’06) 0-7695-2500-8/05 $20.00 © 2005 IEEE TABLE 1: COMMAND VOLTAGE  PROFILES FOR STEERING ANGLE OF 0.27 DEGREE  AND 2.7 DEGREES.Steeringangle[degree]Measured optical power [dBm] ImprovementLinear Non-linear (Optimized)Commandvoltageprofile0 50 100 150 200050100150200250Electrode numberVoltage level0 50 100 150 200050100150200250Electrode numberVoltage levelSignal -18 -10 +8 dB0.27Inter-portisolation13 25 +12dBCommandvoltageprofile0 5 10 15 20050100150200250Electrode numberVoltage level0 5 10 15 20050100150200250Electrode numberVoltage levelSignal -13 -11 +2 dB2.7Inter-portisolation34 37 +3dBB. Optimization results for He-Ne laser (632nm) in free-space A linear voltage ramp of period 128 electrodes per grating period was uploaded into the Opto-VLSIprocessor to generate a diffraction pattern in whichthe first order corresponds to the steered beam. The diffraction pattern was intercepted by a screen placed 3 m away from the Opto-VLSI processor. The optical power of the first order beam was measured using a power meter (Newport 1830-C). The diffractionpattern generated by both the linear and ‘optimized’voltage ramps are as shown in Fig. 8. Optimization ofthe voltage ramp yielded a power improvement of 7.5 dB.1.51 PW8.40 PWHigher orders0th order blocked1st orderWith linear voltage ramp grating period = 128 electrodeWith non-linear voltage rampFig. 8. Optimization result for the He-Ne laser in free space using asteering grating period of 128 electrodes per period (128 Pm).From Fig. 8, we can also observe that (i) theposition of the first and other higher orders remain atthe same position for a given grating period, and (ii)the shape of the voltage profile ramp affects the exactdistribution of optical power among the diffractionorders. The optimized voltage ramp shifted the powerProceedings of the Third IEEE International Workshop on Electronic Design, Test and Applications (DELTA’06) 0-7695-2500-8/05 $20.00 © 2005 IEEE distribution from the higher orders into the first order (the steered beam). This increases the signal while reduces the ‘noise’. In other words, it improves the signal-to-noise ratio significantly, and this significant improvement is consistent with the near-IR results tabulated in table 1 whereby the improvement in signal and inter-port isolation is 8 dB and 12 dB respectively.The general optimized voltage profile is largely in agreement with the LC phase-voltage plot if not for the ‘hump’ that seems to prevail for both the near-IR and 632 nm.  5. Conclusion The impact of non-linearity of LC materials and the flyback-effect on the steering efficiency and inter-port isolation has been investigated in this paper. A computer algorithm has been developed and experimentally demonstrated, which optimizes the voltage profile to maximize the steering efficiency and the inter-port isolation. Improvements in signal and inter-port isolation of 8 dB and 12 dB respectively, have been demonstrated.  6. References [1] Emil Hällstig, “Nematic Liquid Crystal Spatial Light Modulators for Laser Beam Steering”, Dissertations, Acta Universitas Upsaliensis, 2004. [2] Xinghua Wang, “Liquid Crystal on Silicon (LCOS) Wavefront Corrector and Beam Steerer”, SPIE Proc., Vol. 5162, pp. 139-146, Dec 2003. [3] Mehrdad Raisi, Selam Ahderom, Kamal Alameh, Kamran Eshraghian, “Dynamic MicroPhotonic WDM Equalizer”, Second IEEE International Workshop on Electronic Design, Test and Applications, IEEE, 2004. [4] Xinghua Wang, “Performance evaluation of liquid crystal on silicon spatial light Modulator”, Optical Engineering, Vol. 43(11), pp. 2769-2774, November 2004.[5] D. Dayton, S. Restaino, J.Gonglewski, “Novel Spatial Light Modulators for Active and Adaptive Optics”, SPIE Proc., Vol. 4124, November 2000. [6] Paul McManamon, “Optical Phased Array”, IEEE, Vol. 84, No. 2, pp. 268-298, February 1996. [7] Paul McManamon, “Putting on the Shift”, SPIE’s oemagazine, pp. 15-17, April 2003. Proceedings of the Third IEEE International Workshop on Electronic Design, Test and Applications (DELTA’06) 0-7695-2500-8/05 $20.00 © 2005 IEEE 

Improving the Steering Efficiency of 1x4096 Opto-VLSI Processor using Direct Power Measurement Method

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Improving the Steering Efficiency of 1x4096 Opto-VLSI Processor using Direct Power Measurement Method

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