Performance of the CMS Tracker Optical Links and Future Upgrade Using Bandwidth Efficient Digital Modulation

The Compact Muon Solenoid (CMS) experiment at the Large Hadron Collider (LHC) particle accelerator will begin operation in 2007. The innermost CMS subdetector, the Tracker, comprises ~10 million detector channels read out by ~40 000 analog optical links. The optoelectronic components have been designed to meet the stringent requirements of a high energy physics (HEP) experiment in terms of radiation hardness, low mass and low power. Extensive testing has been performed on the components and on complete optical links in test systems. Their functionality and performance in terms of gain, noise, linearity, bandwidth and radiation hardness is detailed. Particular emphasis is placed on the gain, which directly affects the dynamic range of the detector data. It has been possible to accurately predict the variation in gain that will be observed throughout the system. A simulation based on production test data showed that the average gain would be ~38% higher than the design target at the Tracker operating temperature of -10°C. Corrective action was taken to reduce the gains and recover the lost dynamic range by lowering the optical receiver's load resistor value from 100Ω to 62Ω. All links will have gains between 0.64 and 0.96V/V. The future iteration of CMS will be operated in an upgraded LHC requiring faster data readout. In order to preserve the large investments made for the current readout system, an upgrade path that involves reusing the existing optoelectronic components is considered. The applicability of Quadrature Amplitude Modulation (QAM) in a HEP readout system is examined. The method for calculating the data rate is presented, along with laboratory tests where QAM signals were transmitted over a Tracker optical link. The results show that 3-4Gbit/s would be possible if such a design can be implemented (over 10 times the equivalent data rate of the current analog links, 320Mbits/s).(Abridged version) The CMS experiment at the LHC will begin operation in 2007. The CMS Tracker sub-detector, comprises ~10 million detector channels read out by ~40 000 analog optical links. The optoelectronic components have been designed to meet the stringent requirements of a HEP experiment in terms of radiation hardness, low mass and low power. Extensive testing has been performed on the components and on complete optical links in test systems. Their functionality and performance in terms of gain, noise, linearity, bandwidth and radiation hardness is detailed. Particular emphasis is placed on the gain, which directly affects the dynamic range of the detector data. It has been possible to accurately predict the variation in gain that will be observed throughout the system. A simulation based on production test data showed that the average gain would be ~38% higher than the design target at the Tracker operating temperature of -10{\deg}C. Corrective action was taken to reduce the gains and recover the lost dynamic range by lowering the optical receiver's load resistor value from 100{\Omega} to 62{\Omega}. All links will have gains between 0.64 and 0.96V/V. The future iteration of CMS will be operated in an upgraded LHC requiring faster data readout. In order to preserve the large investments made for the current readout system, an upgrade path that involves reusing the existing optoelectronic components is considered. The applicability of Quadrature Amplitude Modulation (QAM) in a HEP readout system is examined. The method for calculating the data rate is presented, along with laboratory tests where QAM signals were transmitted over a Tracker optical link. The results show that 3-4Gbit/s would be possible if such a design can be implemented (over 10 times the equivalent data rate of the current analog links, 320Mbits/s)

Predicting the Gain Spread of the CMS Tracker Analog Readout Optical Links

Approximately 40 000 analog optical links will read out the data from 10 million silicon microstrips in the CMS Tracker. In an analog system, the overall gain directly determines the dynamic range and resolution of the data being read out. Production is sufficiently advanced to allow the extraction of the real distribution of gain for each component making up the complete optical link. The purpose of this study is to examine the aggregate effect of the individual component gain distributions on the readout system's dynamic range, and its uniformity throughout the thousands of deployed links in the CMS Tracker. To this end, a Monte Carlo simulation based on production test data, and augmented with results from deployed links in real test systems, has been carried out. The results give an estimate of the spread in gain and dynamic range that can be expected in the final system, running at -10 degree C

Maximizing the Bandwidth Efficiency of the CMS Tracker Analog Optical Links

The feasibility of achieving faster data transmission using advanced digital modulation techniques over the current CMS Tracker analog optical link is explored. The spectral efficiency of Quadrature Amplitude Modulation -Orthogonal Frequency Division Multiplexing (QAM-OFDM) makes it an attractive option for a future implementation of the readout link. An analytical method for estimating the data-rate that can be achieved using OFDM over the current optical links is described and the first theoretical results are presented

Potential Upgrade of the CMS Tracker Analog Readout Optical Links Using Bandwidth Efficient Digital Modulation

The potential application of advanced digital communication schemes in a future upgrade of the CMS Tracker readout optical links is currently being investigated at CERN. We show experimentally that multi-Gbit/s data rates are possible over the current 40 MSamples/s analog optical links by employing techniques similar to those used in ADSL. The concept involves using one or more digitally-modulated sinusoidal carriers in order to make efficient use of the available bandwidth.Comment: Presented at LECC 2006, Valencia, Spain. 5 pages, 11 figures

Feasibility of Using Bandwidth Efficient Modulation to Upgrade the CMS Tracker Readout Optical Links

Plans to upgrade the LHC after approximately 10 years of operation are currently being considered at CERN. A tenfold increase in luminosity delivered to the experiments is envisaged in the so-called Super LHC (SLHC). This will undoubtedly give rise to significantly larger data volumes from the detectors, requiring faster data readout. The possibility of upgrading the CMS Tracker analog readout optical links using a bandwidth efficient digital modulation scheme for deployment in the SLHC has been extensively explored at CERN. Previous theoretical and experimental studies determined the achievable data rate using a system based on Quadrature Amplitude Modulation (QAM) to be ~3-4Gbit/s (assuming no error correction is used and for an error rate of ~10-9). In this note we attempt to quantify the feasibility of such an upgrade in terms of hardware implementation complexity, applicability to the high energy physics (HEP) environment, technological feasibility and R&D effort required.Comment: CERN CMS Note. 16 pages, 10 figure

A programmable, multi-format photonic transceiver platform enabling flexible optical networks

Development of programmable photonic devices for future flexible optical networks is ongoing. To this end, an innovative, multi-format QAM transmitter design is presented. It comprises a segmented-electrode InP IQ-MZM to be fabricated in InP, which can be directly driven by low-power CMOS logic. Arbitrary optical QAM format generation is made possible using only binary electrical signals, without the need for high-performance DACs and high-swing linear drivers. The concept enables a host of Tx-side DSP functionality, including the spectral shaping needed for Nyquist-WDM system concepts. In addition, we report on the development of an optical channel MUX/DEMUX, based on arrays of microresonator filters with reconfigurable bandwidths and center wavelengths. The device is intended for operation with multi-format flexible transceivers, enabling Dense (D)WDM superchannel aggregation and arbitrary spectral slicing in the context of a flexible grid environment

Segmented optical transmitter comprising a CMOS driver array and an InP IQ-MZM for advanced modulation formats

Segmented Mach-Zehnder modulators are promising solutions to generate complex modulation schemes in the migration towards optical links with a higher-spectral efficiency. We present an optical transmitter comprising a segmented-electrode InP IQ-MZM, capable of multilevel optical signal generation (5-bit per I/Q arm) by employing direct digital drive from integrated, low-power (1W) CMOS binary drivers. We discuss the advantages and design tradeoffs of the segmented driver structure and the implementation in a 40 nm CMOS technology. Multilevel operation with combined phase and amplitude modulation is demonstrated experimentally on a single MZM of the device for 2-ASK-2PSK and 4-ASK-2-PSK, showing potential for respectively 16-QAM and 64-QAM modulation in future assemblies