Francois Vasey

Optical readout and control systems for the CMS tracker

The Compact Muon Solenoid (CMS) Experiment will be installed at the CERN Large Hadron Collider (LHC) in 2007. The readout system for the CMS Tracker consists of /spl sim/10 million individual detector channels that are time-multiplexed onto /spl sim/40000 uni-directional analogue (40MS/s) optical links for transmission between the detector and the /spl sim/65m distant counting room. The corresponding control system consists of /spl sim/2500 bi-directional digital (40Mb/s) optical links based upon the same components as far as possible. The on-detector elements (lasers and photodiodes) of both readout and control links will be distributed throughout the detector volume in close proximity to the silicon detector elements. For this reason, strict requirements are placed on minimal package size, mass, power dissipation, immunity to magnetic field and radiation hardness. It has been possible to meet the requirements with the extensive use of commercially available components with a minimum of customization. The project has now entered its volume production phase after successful completion of technical feasibility. Components have been identified that meet both the stringent analogue performance targets and are sufficiently radiation-hard for use in the CMS Tracker, where lifetime radiation exposure is expected to reach /spl sim/3.4/spl times/10/sup 14//cm/sup 2/ fluence and /spl sim/150kGy dose. Analogue and digital system performance, as well as the component radiation hardness and quality assurance procedures, are reviewed this paper.

A Radiation Tolerant Laser Driver Array for Optical Transmission in the LHC Experiments

A 3-way Laser Driver ASIC has been implemented in deep-submicron CMOS technology, according to the CMS Tracker performance and rad-tolerance requirements. While being optimised for analogue operation, the full-custom IC is also compatible with LVDS digital signalling. It will be deployed for analogue and digital transmission in the 50.000 fibre link of the Tracker. A combination of linearization methods allows achieving good analogue performance (8-bit equivalent dynamic range, with 250 MHz bandwidth), while maintaining wide input common-mode range (±350 mV) and power dissipation of 10 mW/channel. The linearly amplified signals are superposed to a DC-current, programmable over a wide range (0-55 mA). The latter capability allows tracking of changes in laser threshold due to ageing or radiation damage. The driver gain and laser bias-current are programmable via a SEU-robust serial interface. The results of ASIC qualification are discussed in the paper.

Development of radiation-hard optical links for the CMS tracker at CERN

A radiation-hard optical link is under development for readout and control of the tracking detector in the future CMS experiment at the CERN Large hadron Collider. We present the optical system architecture based on edge-emitting InGaAsP laser-diode transmitters operating at a wavelength of 1.3 /spl mu/m, single mode fiber ribbons, multi-way connectors and InGaAs Pin photodiode receivers. We report on radiation hardness tests of lasers, photodiodes, fibers and connectors. Increases of laser threshold and pin leakage currents with hadron fluence have been observed together with decreases in laser slope-efficiency and photodiode responsivity. Short lengths of single-mode optical fiber and multi-way connectors have been found to be little affected by radiation damage. We analyze the analog and digital performance of prototype optical links transmitting data generated at a 40 MSamples/s rate. Distortion, settling time, bandwidth, noise, dynamic range and bit-error-rate results are discussed.

Gamma and neutron radiation damage studies of optical fibres

An optical data link is under development at CERN for readout of the tracking detectors in the future compact muon solenoid experiment (CMS) at the CERN large hadron collider (LHC). Radiation doses inside the experiment will be ∼ 20 Mrad and ∼ 1014 n/cm2 over a 10 year period of operation. The effects of neutron and gamma irradiation on the attenuation at 1550 nm wavelength have been studied in both germanium-doped and pure-silica core single-mode optical fibres. The induced attenuation was greater in the Ge-doped fibre than in the pure-silica core fibres for both neutron and gamma damage. Pure-silica core fibres will be therefore be used in the CMS tracker. The gamma induced attenuation in the pure-silica core fibres was dependent on total dose, allowing a prediction to be made for the transmission losses during LHC operation.

Radiation damage studies of optoelectronic components for the CMS tracker optical links

Optical links are being developed to transfer analogue tracking data and digital timing and control signals in the future Compact Muon Solenoid (CMS) experiment at CERN. The radiation environment inside the CMS tracker will be extreme, with hadron fluences up to /spl sim/10/sup 14//cm/sup 2/ and ionising doses of /spl sim/100 kGy over the experimental lifetime. Prototype link elements, consisting of commercially available 1310 nm multi-quantum-well InGaAsP lasers and InGaAs p-i-n photodiodes, have been irradiated in a fully packaged form with /spl sim/6 MeV neutrons to 10/sup 15/ n/cm/sup 2/, 24 GeV protons to 4/spl times/10/sup 14/ p/cm/sup 2/ and /sup 60/Co-gammas to 100 kGy. Three types of single-mode optical fiber, two pure-silica core and one Ge-doped core, were irradiated in several stages with /sup 60/Co-gammas to a total dose of /spl sim/90 kGy. Neutron and proton damage induced large increases in laser threshold and significant decreases in light output efficiency. P-i-n leakage current increased by up to 6-7 orders of magnitude for neutron and proton damage. P-i-n response was relatively unaffected until /spl sim/2/spl times/10/sup 14/ n/cm/sup 2/, or /spl sim/4/spl times/10/sup 13/ p/cm, after which the photocurrent decreased rapidly. Gamma damage after 100 kGy was minor in comparison to hadron damage in both the lasers and p-i-n photodiodes. The radiation induced attenuation at 1300 nm in the optical fibers was dependent upon the fiber type, with losses of 0.08 dB/m for the pure-silica core fiber and 0.12 dB/m in the Ge-doped core fiber, after /spl sim/90 kGy. The annealing in one of the pure-silica core fibers was found to be temporary in nature.

Single event upset tests of an 80-Mb/s optical receiver

This paper studies the single event upset (SEU) sensitivity of a radiation-tolerant 80-Mb/s receiver developed for the CMS Tracker digital optical link. Bit error rate (BER) measurements were made while irradiating the receiver with protons and neutrons at different beam energies and incident angles and for a wide range of optical power levels in the link. Monte Carlo simulations have also been used to assist in the interpretation of the experimental results. As expected, the photodiode is the most sensitive element to SEU. The fake signal induced by direct ionization dominates the bit-error cross-section only for protons incident on the photodiode at large angles and low levels of optical power. Comparison of the neutron and proton bit-error cross-sections demonstrates that nuclear interactions contribute significantly to the proton-induced SEU errors and that they will dominate the radiation-induced error rate in the real Tracker application.

Radiation hardness qualification of InGaAsP/InP 1310 nm lasers for the CMS Tracker optical links

The series of validation tests for radiation hardness qualification of lasers for use in 46000 optical links of the CMS Tracker detector at CERN, Geneva, Switzerland, are presented. These tests included accelerated radiation damage, annealing, and aging studies, simulating the effect of doses and fluences, up to 2/spl times/10/sup 14/ particles/cm/sup 2/ and 100 kGy, accumulated over a ten-year operating lifetime. The worst-case damage effect, in lasers operating closest to the beam-collision point, is expected to be a threshold current increase of under 6 mA. The lasers tested therefore qualify as being sufficiently radiation hard. The qualification tests also form the basis of future radiation hardness assurance of lasers during final production. An advance validation test of lasers from candidate wafers is defined that will confirm the radiation hardness of lasers before a large number of transmitters are assembled from these wafers.

Degradation of carrier lifetime in irradiated lasers

The effect of radiation damage on carrier lifetime in 1310 nm InGaAsP/InP multi-quantum-well lasers irradiated with 0.8 MeV neutrons, was investigated for fluences up to 6.9 X 1014 n/cm2. The damage to the carrier lifetime was studied by measuring the transient response of irradiated lasers to incident optical pulses of 1064 nm and 532 nm wavelength, and by relative intensity noise measurements. The carrier lifetime was determined to be degraded to a similar extent in both the InGaAsP laser cavity and the surrounding InP material following radiation damage.

ANALOGUE OPTICAL LINKS FOR THE CMS TRACKER READOUT SYSTEM

The optical fibre links for the analogue readout of the CMS tracker are based on the direct modulation of edge-emitting lasers located inside the detector volume. The front-end signals are transmitted by single-mode optical fibres and detected in the counting room using PIN photodiodes. One channel link demonstrators consisting of commercially available optoelectronic devices have been fully characterized. The lasers, fibres, and PIN diodes are representative of the devices intended to be used in the final system. The link overall performance is discussed in terms of its static and dynamic responses and is shown to meet the technical specifications of the full readout chain .

Modal Dispersion Mitigation in Standard Single-Mode Fibers at 850 nm With Fiber Mode Filters

We investigate the possibility of using a fiber which supports only the fundamental LP01 mode at 850 nm as a mode filter to overcome modal dispersion in standard single-mode fiber links that employ either multitransverse or single-transverse-mode vertical-cavity surface-emitting lasers. The individual power penalties due to modal noise and modal crosstalk effects in the link are investigated.