Andrew Powell UK LHCb Workshop - Tuesday, 23 rd August 2005

1. The RICH Optical Data Link A Summary of Optical Tests • Link Overview • Attenuation Results • Bit Error Rate Results • Eye Diagram Investigations* *Measurements made by Paolo Ciambrone, Frascati Andrew Powell UK LHCb Workshop - Tuesday, 23rd August 2005

2. Requirements of Optical Link ~ 100m Transport the 1.6 Gigabit/s data flow from the on-detector Level 0 front-end to Level 1 optical receiver High Rad. Environment (Detector Hall) Non Rad. Environment (Counting Room) Plan view of optical fibre path for RICH 1

3. Data Flow From HPD to Level 1 HPD Data accepted by the L0 trigger is formatted into an ‘event-block’ of 32bits x 35 + V 32bits G O L VCSEL 0 1 Encoding (8B/10B) Laser Driver 32bits Serializer 33 34 VCSEL (Vertical Cavity Surface Emitting Laser Diode) G O L (Gigabit Optical Link) Event Block from 1 HPD CERN developed ASIC to mux. and serialize data words New Technology – Low lasing threshold, and high operating speeds

4. RICH Optical Link Scheme On Detector 80m 12-way MTP-MTP Counting Room Multiple interfaces along optical fibre link Many Potential Power Losses

5. Characterisation of Optical Link 1) Optical Power Margins How does the light intensity out of the fibre link compare with the dynamic range of the optical receiver? i.e. Are we near saturation or sensitivity thresholds for the optical receiver? Required Test: Attenuation studies of each type of interface and entire link 2) Rate of Erroneous Bits Detected Is the number of erroneous bits within a given number of transferred data words under an acceptable limit? i.e. Is the Bit Error Rate (BER) sufficiently small? …and if yes (hopefully)… What is the actual optical power safety margin w.r.t an acceptable BER? Required Test: BER tests performed with variable optical attenuator in link 3) Quality of Optical Data What is the quality of the optical signal at the beginning and end of the optical link? Required Test: Assessment of “Eye” diagrams using high speed ‘scope

6. Theoretical Extremities Maximum Theoretical Power Profile Minimum Theoretical Power Profile = DC Driving Current of VCSEL

7. Comparison with Actual Measured Values Theory and Data in good agreement

8. Comparison with Optical Receiver Specs 0.631 mW 0.501 mW 0.025 mW So, in THEORY we are safe. What about in practice?

9. Bit Error Rate (BER) Tests Absolute requirement that RICH link BER < 10-12 How to quantify the optical power margin available beforea BER > 10-12 is obtained? Insert aVariable Optical Attenuatorinto link, transfer 1013 bitsand count the number of errors seen. Find the additional attenuation need to generate 10 or more errors. L0 L1 How to count the errors at L1? Optical Attenuator

10. a b . . . . i j 0 . . . . 7 8B/10B Encoding 8B/10B Encoding & Error Detection 10 bits 8 bits Why Encode? • Efficiency; Data are at a constant DC offset • Ensure sufficient transitions between “1” and “0” occur to enable clock recovery at the receiver • Enables a high degree of Error Detection 8B/10B encoding enables a receiver to compare incoming data packets with a series of error conditions. If any of these conditions are met then invalid data has been received, i.e. an error has occurred to the data since it had been encoded e.g. The 4 least significant bits can not be equal (a=b=c=d) The Cambridge L1 receiver fully integrates this 8B/10B error detection

11. BER Test Results Additional Attenuation (dB) Number of Errors within 1013 bits Transferred *All results taken with a GOL bias current of 1.8mA -12.2 -12.0 -11.6 -11.4 -11.0 -10.0 1591 237 7 1 1 0 P1 = Input Power (W) P2 = Output Power (W) Results demonstrate we have a huge margin of safety (-11dB) w.r.t a BER of 10-12 In good agreement with attenuation results Assuming, Number of Error Bits

12. 0 1 0 0 0 0 0 1 1 1 0 1 1 0 1 Eye Diagram Investigations A quick and reliable method of assessing the quality of a digital signal Super-imposing all possibilities gives us … 1 0 0 t But, in reality we have finite rise and fall times… 1 1 0 1 0 1 t “Eye” By using a sufficiently fast ‘scope, eye diagrams for our 1.6 Gbit/s data flow can be obtained and any “jitter” can be observed

13. 3 Metres The eye is very clean with little jitter

14. 100 Metres Little difference seen over 100m of fibre

15. Conclusions The RICH Optical Data Link has now been thoroughly tested: Attenuation tests on the individual link interfaces have demonstrated there can be a large variation in the optical power seen between different fibre channels at the L1 receiver. However, this spread is adequately within the acceptance ranges of the optical receivers. BER tests have confirmed the large margin of safety we have w.r.t. the sensitivity threshold of the optical receiver. In normal operation, we can expect BERs <<10-14 Eye diagram investigations have demonstrated that the optical transmission circuitry (GOL / VCSEL) generate clean data with very little jitter. The addition of 100m of fibre appears to have minimal effect on the eye diagram. Overall, we are very confident in the link’s performance