Azevedo, C S

Vasey, F

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A 4-channel parallel analogue optical link for the CMS-TrackerF. Vasey, C. Aguilar, C. Azevedo, V. Arbet-Engels, G. Cervelli, K. Gill, R. Grabit, F. Jensen,C. Mommaert, P. Moreira, G. StefaniniCERN, Geneva, Switzerland   (email: francois.vasey@cern.ch)AbstractA radiation-hard, 4-channel wide parallel optical linkhas been developed for the analogue readout of the CMS-tracker detector.  It is based on edge-emitting lasertransmitters and pin photodiodes aligned to single-modefibres on Si-submounts and housed in custom developed4-way ceramic packages.  The operating wavelength is1310nm.  Experimental results obtained with prototypesclosely resembling the final system are presented.Measurements of dynamic range, linearity andbandwidth are shown, together with first results of inter-channel crosstalk.  The resistance of the laser transmittermodule to neutron irradiation is also reviewed.1.  INTRODUCTIONA radiation-hard, 4-channel wide parallel optical linkhas been developed for the analogue readout of the CMS-tracker detector (Fig. 1). It consists of a monolithic quad-laser driver ASIC followed by a hybrid assembly of 4InGaAsP/InP edge emitting lasers housed in one singlecustom-developed low-mass, non-magnetic package.Single-mode optical fibre ribbons transmit the light at awavelength of l~1310nm over a distance ofapproximately 110m, through 3 patch panels based onangle-polished MT-connectors.  On the receiver side,four InGaAs/InP pin photodiodes housed in a packageidentical to the laser module are followed by 2-stagetransimpedance amplifiers based on commerciallyavailable, single-channel discrete ICs.  Altogether, about50000 optical links will be required to read-out the CMSracker.Results of performance and radiation hardness tests ofindividual components and single-channel optical linkshave been previously published [1].  This paper reportsfor e first time on the performance of a full length 4-chann l parallel link, which closely resembles the finalCMS-tracker optical readout system.  Static and dynamicperformance of the complete link are presented in section2, tog ther with first results of inter-channel rosstalk.The r sistance of the laser module to neutron irradiationis briefly reviewed in section 3, while the evolution pathtowards the final system is sketched in section 4.2.  LINK PERFORMANCE2.1 Link under testMost of the components used to build the paralleloptical link have been described previously [2,3,4] andsingle channel link demonstrators have beencharacterised and distributed to the CMS community [5,6].  However, it is the availability of a 4-way package,custom developed by Italtel Milano [7] which has madethis more realistic evaluation possible.Receiver4 Fibre   ribbonTransmitter4Quad laserdriver ASIC4-laser hybridDIL moduleMT4 angle polishedconnector4-pin hybridDIL moduleDiscretetransimpedanceamplifierI2CControl10m 100m2m 2mFig. 1. 4-channel parallel analogue optical link prototypeOptical fibresSi SubmountOpto chipCeramic packageFig. 2. 4-way Rx module internal configurationThe Italtel 14-pin ceramic DIL package houses up tofour (known good) laser or photodiode assemblies in avolume of approximately 15mm x 10mm x 4mm. Withthe exception of the leadframe (still to be optimised), itcontains no metallic parts and is thus low-mass and nonmagnetic.  A bundle of 4 standard telecom-grade single-mode fibres exit the package through a ceramic ferrule,and are terminated by an angle polished MPO connector.Three pairs of 4-way transmitters and 4-way receivershave been tested.  The lasers have an average thresholdcurrent of 10.3mA and a slope efficiency of 0.059W/A at25°C, while the photodiodes have an averageresponsivity of 0.93A/W.  A diagram and photograph ofan open 4-channel receiver are shown in Fig. 2.The working point and operating range of the testedlinks have been set to closely match the systemspecifications [8]: a laser driver t ansconductance ofapproximately 6.3mS converts the full range input swingof ±400mV into a laser optical output signal ofapproximately 300mW amplitude. This signalcorresponds on the receiver side to an output swing ofalmost 4.2V into a high impedance load.  If one includes2dB of optical loss for the three in-line patch panels, theoutput swing becomes 2.65V, or 1.33V into a 50W l ad.2.2 Experimental set-upThe experimental setup used to characterise theparallel optical link is shown in Fig. 3.  It consists ofpairs of generating and measuring instruments linked viaGPIB to a computer running labview software. For thestatic characterisation, an arbitrary waveform generatorand an oscilloscope are used; the dynamiccharacterisation is performed with a tracking generatorcoupled to a spectrum analyser; a pulse generator,together with an oscilloscope, are used to evaluatecrosstalk.  For high resolution measurements such aslinearity, a 12bit A to D converter on a VME board isused.  Also hooked on the VME bus is an I2C interfaceboard which controls the laser driver and allowsindividual setting and adjustment of the lasers workingpoints. The I2C bus remains idle during linkmeasurements.The laser driver ASIC features differential inputs.However, to ease the interface with laboratoryinstruments, only one input is active in this test, whilethe other one remains grounded.On e receiving side, all outputs are terminated with50W to ground, resulting in a 50% reduction of link netgain.  It has been experimentally demonstrated that whenintegrating the receiving amplifiers close to the A to Donverters on the Front End Drivers, this termination canbe removed, thus eventually allowing to recover the fullgain of the readout chain.AWGPulse GEN.Tracking GEN.SCOPESpectrum ANAL.I2C ADCCOMPUTERGPIBVMEFig. 3. Experimental setup for link performanceevaluation2.3 Static characterisationThe measurement procedure of the optical link statictransfer characteristic consists in scanning the laserdriver input voltage and, for about 100 input points,measuring the DC value and standard deviation of therec iver output voltageThe static transfer characteristics of the 12 testedoptical channels are shown in Fig. 4 for an input voltagerange slightly exceeding the nominal value of ±400mV.Th  kink at very low input voltages is due to the laserb ing operated below threshold.  Even though this couldbe e sily avoided by adjusting the laser bias point, it isuseful in the characterisation phase to have an absolutereference point indicating the operating conditions of thesystem.  The measured link gains (outputs terminatedwith 50W resistors) range from 0.98 to 1.7 V/V, with anaverage value of 1.2 V/V.  This spread is essentiallycaused by the inhomogeneity in the insertion losses of thethree in-line connectors.Fig. 4.  Optical link static transfer characteristicsFor each static measurement point, the standarddeviation of the output voltage is also measured.  Theresulting noise characteristics are shown in Fig. 5 as afunction of input voltage for the 12 tested channels.  Thebandwidth of the measuring oscilloscope largely exceedsthat of the optical link (~120MHz, see section 2.5) sothat the values shown truly represent the link rms noise.Since different channels have different gains, it isdifficult to precisely compare noise figures at this point.A signal to noise ratio calculation is described in thefollowing section.Fig. 5.  Optical link output noise characteristics2.4 Linearity and dynamic range evaluationBased on the static characterisation data presentedabove, the analogue optical link performance is evaluatedoff-line with the method described in [9]: a workingpoint is chosen above threshold and the signals sentthrough the link are modelled as pulses superimposed tothis DC working point.  In the present analysis, aworking point of -0.4V has been chosen for all inputchannels.To evaluate the system linearity, a regression line isdrawn between the working point and a calibration pointselected on the static transfer characteristic, within theoperating range of the link.  The relative deviation fromlinearity is defined as the normalised error between theoutput signals extracted from the transfer characteristicand those predicted by the regression line.  In the presentevaluation, the calibration point has been chosen to be250mV above the working point (input voltage = -0.15V)for all channels.  The calculated relative deviation fromlinearity is plotted in Fig. 6 as a function of inputvoltage.  It is zero at the calibration point, and practicallydoes not exceed 2.5% in the link full operating range.This figure is however dependent on the specific choiceof the calibration point, and has not been finelyoptimised in this example.  Around the working point(Xwork=-0.4V), the relative deviation from linearitydiverges as the signal reaches zero.  Below the workingpoint, the linearity is degraded by the laser approachingthreshold; above the working point, it is essentiallydetermined by the laser driver static transfercharacteristic.Fig. 6. Relative deviation from linearity computed fromdata in Fig. 4 (Xwork=-0.4V, DXcal=250mV)The system signal to noise ratio is obtained by dividingthe output signal (extracted from the static transfercharacteristic) by the output noise (from the output noisecharacteristic).  As can be seen in Fig. 7, peak signal tonoise ratios in excess of 350:1 are achieved for allmeasured channels (at Xpeak=+400mV), with the best linkreaching a value close to 600:1.  As was the case for thedeviation from linearity analysis, the signal to noiseratios shown here are dependent on choice of workingpoint, and could be further optimised on a channel bychannel basis.In order to assess the impact of the optical link on thetotal noise of the CMS-tracker readout chain, one mustma  the output range of the front-end chip into the inputrange of the optical link.  Assuming an 8MIP dynamicrange would correspond to the 800mV input range of thesystem described here, then the equivalent input noisegenerated by the optical link would for instance be lessthan 0.023MIPs for all channels.Fig. 7. Signal to noise ratio computed from data infigures 4 and 5 (Xwork=-0.4V)2.5 Dynamic characterisationThe optical link bandwidth is essentially limited by thereceiving electronics.  The frequency transfer function, asmeasured for one pair of 4-way modules is plotted in Fig.8.  It features a flat response with a -3dB bandwidth ofabout 120MHz.  In the time domain, this value wouldcorrespond to a settling time shorter than 10ns [2].Fig. 8.  Optical link frequency transfer functionIn the final readout system, the optical link bandwidthwill have to be optimised, together with the front-endand back-end responses, so as to minimise the amount ofnoise induced for a given readout system speed.Due to the pulsed nature of the signal transfer schemeus d for the CMS tracker, the crosstalk (or relativefeedthrough from one channel to its neighbour) isdefined at sampling time s³20ns for a step input injectedat t=0s.  Even though some feedthrough is apparentduring the few nanoseconds after the step edge, very littleeffect remains visible after 20ns.  We measure typicalcrosstalk values of about -63dB, in no cases exceeding -55dB.2.6 Summary of test measurementsWe compare in Table 1 the results presented in thissection with the system specifications [8]. The 12 testedprototype channels all meet the objectives of the CMStracker readout system.3. RADIATION HARDNESSWith the exception of the laser-driver ASIC, which isstill integrated in a radiation-soft technology, all opto-electronic front-end components have been validated forradiation hardness.  Single-channel laser-transmitters[10], standard telecom-grade optical fibres [11] andmulti-way MT connectors [12] have shown goodresistance to levels in excess of the CMS-trackerrequirements.  Figure 9 shows the normalisedegradation of laser threshold current and efficiency as afunction of neutron fluence for a 4-channel module.  Asexpected, the results are similar to the ones obtained withsingle channel devices.  Fluences (integrated over thelifetime of the CMS experiment) to be expected at theinnermost Si-tracker layers are ~1014neutrons/cm2(1MeV) and 1.6x1014 charged hadrons/cm2 [13] (mainlycharged pions, 102 - 104 MeV, found to be almost 4 timesmore damaging than neutrons (6MeV) [10]).1.61.20.80.40.086420neutron fluence (x1014n/cm2)1.000.950.900.850.80 threshold (I thr (0)~12mA) efficiency (E(0)~60µW/mA)Fig. 9. Degradation of a 4-channel laser moduleirradiated with 6MeV neutrons (annealing behaviour notshown)Table 1.  Comparison of specified [8] and measured optical link performanceSpecified Measured# specification min typ max unit note2.1 Length 60 100 120 114 m2.2 Gain 1.4 2.85 5.7 2.4 V/V Into high impedance2.3 Peak signal to noise ratio 48 >50.9 dB2.4 Relative linearity deviation 2.5 <2.5 % in output range2.5 Bandwidth 70 120 MHz DC coupled2.6 Settling time 18 <10 ns2.9 Crosstalk -48 <-55 dB ts ³ 20ns4. CONCLUSIONWith the demonstration of a functional and radiation-hard 4-channel parallel analogue optical link, thebaseline for the CMS-tracker optical readout system isset:¨ Assemblies of edge-emitting lasers, pin photodiodesand optical fibres on silicon submounts are the basicbuilding blocks of the optical link transmitters andreceivers. They can be individually tested forperformance and radiation hardness, packaged into one-way or multi-way housings, or possibly even assembleddirectly onto hybrids or printed circuit boards.  Thisflexibility in the packaging scheme allows testing andvalidation of opto-electronic components in submountform before the full details of the system architecture andimplementation are known, and before a final package orhybrid has been selected and designed.¨ Standard telecom-grade single mode fibre ribbonslink front-end to back-end.  They are connected in threepoints with MT-compatible connectors to easeinstallation, test and maintenance of the detector.Three 4-way parallel analogue optical links closelyresembling the final CMS system have demonstrated aperformance complying with the specifications.  Thecustom developed 4-way package used both for thetransmitter and receiver modules has been successfullytested for mechanical robustness [7], and will besubjected to magnetic field tests in the coming months.Irradiated and non-irradiated lasers and pins arecurrently undergoing accelerated ageing tests.  Up to thisday, no significant degradation effects have beenobserved in more than 1200 hours of operation at 80°C.Work is in progress to better adapt the link modularityand density to the needs of the CMS tracker.  Inparticular, projects with industrial partners have beenlaunched to increase the density of the optical patch-panel, multi-ribbon cable and receiver modules.5.  REFERENCES[1] F. Vasey, V. Arbet-Engels, J. Batten, G. Cervelli, K.Gill, R. Grabit, C. Mommaert, G. Stefanini, J.Troska, "Development of radiation-hard optical linksfor the CMS tracker at CERN", IEEE transactionson nuclear science, Vol. 45, No. 3, pp. 331-7, 1998.[2] V. Arbet-Engels, G. Cervelli, K. Gill, R. Grabit, C.Mommaert, P. Moreira, G. Stefanini, F. Vasey, 4papers published in proceedings of the thirdworkshop on electronics for LHC experiments,CERN/LHCC/97-60, pp. 270-298, 1997.[3] G. Chiaretti, "Optical link technology: silicon opticalbench technology", proceedings of the thirdworkshop on electronics for LHC experiments,CERN/LHCC/97-60, pp. 115-119, 1997.[4] A. Marchioro, P. Moreira, T. Toifl, T. Vaaraniemi,"A  integrated laser driver array for analogue datatransmission in the LHC experiments", proceedingsof the third workshop on electronics for LHCexperiments, CERN/LHCC/97-60, pp 282-286,1997.[5] V. Arbet-Engels, G. Cervelli, K. Gill, R. Grabit, C.Mommaert, G. Stefanini, F. Vasey, "Analogueoptical links for the CMS tracker readout system",Nuclear instruments and methods in physicsresearch A409, pp. 634-8, 1998.[6] Report on the tracker readout milestone, CMStracker technical design report, CERN/LHCC 98-6,pp. 319-23, 1998.[7] M.Magliocco, P. Nugent, "Four channel fibre optictransmitter and receiver modules for CMS detectortracker readout links", these proceedings.[8] F. Vasey, "CMS tracker optical readout linkspecification", Vers. 2.1, June 17, 1998.  Availableon:http//www.cern.ch/CERN/Divisions/ECP/CME/OpticalLinks/[9] G. Cervelli, V. Arbet-Engels, K. Gill, R. Grabit, C.Mommaert, G. Stefanini, F. Vasey, "A method forthe static characterisation of the CMS trackeranalogue optical links", Technical note CMS Note1998/043.[10]K. Gill, V. Arbet-Engels, G. Cervelli, R. Grabit, C.Mommaert, G. Stefanini, J. Troska, F. Vasey, "Piondamage in semiconductor lasers”, Technical noteCMS Note 1998/046.[11]J. Troska, J. Batten, K. Gill, F. Vasey, "Radiationeffects in commercial off-the-shelf single-modeoptical fibres", proceedings of the SPIE, Vol. 3440(in print), 1998.[12]J. Batten, K. Gill, J. Troska, F. Vasey, "Radiationtolerance of MT multi-way single mode fibre-opticconnectors to gamma and neutron irradiation",Workshop record of the data workshop, RADECSconference on radiations and their effects on devicesand systems, Cannes, Sept 17, 1997, pp 75-80.[13]Report on the tracker radiation environment, CMStracker technical design report, CERN/LHCC 98-6,pp. A1-A22, 1998.

A 4-channel parallel analogue optical link for the CMS-tracker

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