689 OCR Output

elements associated with each group of front-end electronics charmels.collision vertex to individual detector elements, must be provided by programmable delaydelays due to drift times and cable propagation times, and particle flight times from theadditional variable delays, such as electronics delays of the detector element signals, detectorautomatically compensated for variable phase delays up to that point. Compensation forthe front-end electronics modules, which is synchronized to the LHC RF system andsingle location in the vicinity of the detector, within a cable path length of 50 — 100 m from allProven techniques [1] developed at CERN are available to generate a master timing signal at a

considerable challenges in several areas of technology.electronics cheap enough to be implemented in such a huge number of channels posesAchieving this order of precision in a somewhat unfavourable EMC environment withof 500 ps, while for precision voltage sampling even higher accuracy could be desired.However, drift time measurement in tracking detectors could require an accuracy of the ordersignal accuracy of a few ns relative to the bunch crossing period of 15 ns will be adequate.For front·end bunch-counter clocks, gating signals or analog memory switching, a timing

system integration with the iirst—1evel trigger.while in RD-27 additional studies are being undertaken of high-power laser transmitters andelements of the system are currently being developed in the context of DRDC Project RD·12,proton·proton operation), can be broadcast to all the front-end electronics modules. Basicperiodic clock signal, synchronized to the bunch crossing frequency (66.8 MHz forplanned to provide them with timing signal distribution networks by which a high-precisionrange 1200 — 2500 mz and would be equipped with about 107 electronics channels. It isThe proposed ATLAS and CMS general—purpose LHC detectors have surface areas in the

Introduction

integration with the level-] central trigger processor.couplers, low-cost optoelectronic receivers, programmable delay technologies and systemhigh-power fibre-coupled laser transmitters and their modulators, coding, passive optical treeprojects are currently exploring various aspects of this concept, including studies of appropriateaddressed digital data such as deskew parameters and virtual pipeline address offsets. Two DRDCexploited for the broadcasting of trigger-accept and control signals and the transmission ofA multichannel optical fibre network for timing signal distribution could simultaneously be

particle flight times and delays due to detector processes, electronics and cable propagation.different groups of channels must be phase-compensated with sub·nanosecond precision fordestinations located within a detector volume of up to 9500 m3. The signals delivered tosignals derived from the machine's radio frequency system must be distributed to thousands ofTo synchronize the front·end electronics of experiments at the future Large Hadron Collider, clock

Abstract

ECP Division, CERN

B.G. Taylor

Timing and Control Signals

Multichannel Optical Fibre Distribution System for LHC Detector

691 OCR Output

sequential phase detector or ‘digital’ implementations.PLL architectures with inherently low phase jitter are suitable, which precludes certain(PLL) to control a local VCO which may incorporate a SAW filter or other resonator. Onlyserial/parallel conversion functions. Such circuits generally employ a phase-locked loopimplement the required modulation/demodulation, clock recovery, byte framing andmuch attention is being given to the development of hybrid or monolithic ICs whichAs a result of the growing demand for high bit-rate optical fibre data communications links,

CATV, which favours the second window, could alter this ranking in the foreseeable future.few hundred MHz. Larger volume production of InGaAs devices for applications such asthey are biased at inconveniently high reverse voltages, Si diode bandwidths are limited to aless expensive than the InGaAs photodetectors required for 1300 nm operation. But unlessthe Si PlN photodiodes which are appropriate for short-wavelength reception are substantiallyinterface towards the periphery of the distribution tree, their unit cost is critical. At present,In view of the large number of receivers which would be required to deploy the fibre/copper

Optoelectronic receivers

step-index Hbre would be preferable in regions of high radiation intensity.low-NA graded-index fibres incur less multimode dispersion and short runs of pure silica corefavour laser diode coupling efficiency and power-handling capacity, whereas smaller diametercharacteristics is necessarily a compromise. Large diameters and high numerical aperturesAs with the selection of operating wavelength and coupler technology, the choice of fibre

to the requirements of this application.point, classical fused biconic taper products currently offer the best price/performance matchuniformity and achromaticity, and GRIN lens fibre splitters permit larger fanouts at a singledestinations). While planar silica waveguide coupler technology [8] offers improvedto the large fanout, which can exceed 50 dB through three levels of 1:32 coupler (32Krequired optical fibre, connectors and splices is negligible compared with the splitting loss dueall—glass signal distribution network composed of optical tree couplers. The attenuation of theReliability and electromagnetic compatibility are attractive features of an entirely passive

Optical distribution network

claimed is appropriate for low·cost mass production.1 mm square dice of gain medium and a simple and material—saving process which it isvolume at much lower cost [7]. These lasers, which can generate 100 mW at 1319 nm, usemicrochip or cube lasers suggests that in the future it could be possible to manufacture them inand high quality coated optical components, the recent announcement of tiny Nd:YAGWhile today’s DPSS products are expensive assemblies of precision mechanical structures

optical amplifier of the modulated output of a second laser [6].4-6 dB. This problem can be circumvented at a cost by operating the DPSS laser as anmust be used in arrays to support this power level, and they introduce an insertion loss ofdirectly at high frequencies, cxtemal modulation is necessary. Mach-Zehnder modulatorschromatic dispersion wavelength of 1300 nm. Since a DPSS laser cannot be modulatedthermal defocusing) can generate over 1 Watt of CW optical power close to the fibre zeroDiode-pumped solid state (DPSS) lasers using Nd:YAG (or Nd:YLF, which suffers from less

appropriate compensation techniques are the subjects of parts of the current study.sensitivity mechanisms which contribute to the systematic jitter. These characteristics andunder modulation. They also suffer from relaxation oscillations, tum-on delay and pattemindex-guided, operate in multiple longitudinal modes and exhibit mode-hopping and chirpingdiodes can be modulated directly at over 1 GHz. The highest power devices, althoughOCR OutputAlthough package inductances of several nH are a major problem, some high power laser

692

the 2048 Kbit/s Hierarchy, CCI'I'I` Blue Book, vol. III, Geneva, 1989.[9] Recommendation G.823, The Control of Jitter and Wander within Digital Networks which are based on

France Composants Opto-elecuoniques, 1992.[8] An Integrated Optics Technology for Producing Coming Couplers, Technical Report TR-70, Corning[7] M. Fahey, Low-cost Lasers could power Cable TV Systems, Lightwave, July 1992, pp. 1/16/18.

Resonator Configuration, IEEE J. Quantum Electron., vol. 28, pp. 1131-1138, 1992.[6] T.M. Baer et al, Performance of Diode-Pumped Nd:YAG and Nd:YLF Lasers in a Tightly Folded[5] Semiconductor Laser Breakthrough, IEE Review, vol. 38, p. 242, July/August 1992.

Publishing Co. Inc., September 1991.[4] G. Rogers and M.A. Fitch, Lens Coupling Boosts Light Launching, Photonics Spectra, Laurin[3] Low Cost Gigabit Fiber Optic Links, Finisar Corp., 1992.[2] OLC—266 & OLC-266D 266 Mb/s Optical Link Cards, IBM Specification PN 86F1058, January 1992.

Fibre~optic Link, Electron. Lett., vol. 22, pp. 868-869, 1986.[1] E. Peschardt and J.P.H. Sladen, Transmission of a Stabilised RF Phase Reference over a Monomode

References

achieved during the period of LHC detector preparadon.telecommunications infrasuucture, gives grounds for cautious optimism that this will beintroduction of broadband FITI-I services and the planned renovation of the East Europeanpotential market ranging from optical disks, laser printers, cheap LANs and CATV, to theThe rate of innovation in these technologies, combined with the expected demands of a vast

becoming available at affordable costcouplers, optoelectronic receivers and programmable delays leads to these critical componentswill be necessary that increased volume production of high-power laser sources, opticalTo implement the proposed muldchannel fibre optic timing and conuol disuibution system, it

Conclusion

receiver.

serial/parallel conversion, address decoding and data storage functions of a complete timinglayer phase shifters. The ideal ASIC would integrate this function with the demodulation,delay lines, gate cascades, ACT (acoustic charge transfer) devices and GaAs SAW depletionOther candidate technologies include fast state machines or shift registers, tapped or hybrid

must have a low inherent trigger delay and reset interval.applied in a timing disuibution system which generates periodic clock signals, such devicesthis application which can be programmed with 10 ps resolution over an 8-bit range. To bemultiple device probes, and integrated ramp/DAC/comparator DTCs have been developed forATE systems for the precision deskewing of the test signals which must be generated atphases delivered to the front—end electronics. Similar problems are encountered in analog LSIimplementation of the programmable delay function required for adjustment of the clockA wide range of technologies at different stages of maturity appear promising for the

Programmable delays

group codes, Miller code or scrambled NRZ.sideband energy, such as biphase-level or biphase-mark, are to be preferred to those based onTechniques which result in a modulated signal spectrum with a high ratio of synchronizing toideally have zero DC offset characteristics over the bunch crossing period of 15 ns.risetimes must be preserved throughout the system and the coding method employed shouldinformation is a secondary consideration. In order to minimize systematic jitter [9] fastimportance, while eficiency of channel utilization for the transmission of the digital controlIn this timing distribution application thc extracted clock phase stability is of primary