Transmission of fast signals via optical fibres Richard White - - PowerPoint PPT Presentation

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Transmission of fast signals via optical fibres

Richard White rw@ast.leeds.ac.uk Michael Daniel for

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Digitizing the signal from an IACT camera: why fast signal transmission is needed. The Cherenkov light front is ~couple nanoseconds in duration. After passing through Davies-Cotton optics this will be broadened to ~4.5ns rise time ns want to maintain fast pulse rise structure, for timing information and to integrate signal with as small a time window as possible to enhance S/N ratio against night sky background after 100m of co-ax. cable pulse rise time is ~8-10ns

10 20 30 40 [ns]

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➢Less attenuation/dispersion of signal ➢No cross-talk or electromagnetic pickup between channels ➢Isolated from grounding problems ➢Immune to lightning strikes ➢Reduce weight ➢Reduce costs, e.g. through multiplexing.

Advantages of an optical fibre system

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VCSEL - Vertical Cavity Surface Emitting Laser VCSELs are high-bandwidth and low cost lasers, suitable for transmitting both digital and analogue signals. Analogue – PMT signals are transmitted with virtually no attenuation over fibre. Digital – versatile system for clock/trigger/housekeeping distribution.

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fibre optic solutions used in IACTs

~2000 – Leeds & MPIK develop prototype for use in the outer 111 pixels of the Whipple 10m 490 pixel camera. Problem of VCSEL mode hopping leading to ~50% variations in the optical signal output on minute timescales. ~2004 – MAGIC camera is the first stable, large scale, VCSEL based analogue signal transmission system. Quality control means selecting the best VCSEL for use in the

• camera. VCSEL manufacturing has improved, but still ~30% are rejected to keep a

reasonable ~12% spread in output pulse area and amplitude. An expensive cooling system is required in the camera to keep output constant from temperature fluctuations to 1C, but fibres allow multiplexing of pixel signal into 2GHz FADCs, saving money. ~now – 'a temperature stable optical link for transmission of fast optical signals' NIM A 595, 332 (2008). Further VCSEL manufacturing improvements meant that none were rejected in this prototyping phase.

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The Latest VCSELs

• Apply Bias Current -> Measure Aout, %Noise -> Increase Bias Current.

OLD VCSELS NEW VCSELS Mode Hopping No Mode Hopping, Lower Noise

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BNC inject 0->2.2V pulse 850nm VCSEL has rise/fall times as short as 100ps and can be operated up to 200mA over short duty cycles (large dynamic range for Cherenkov pulses) The Leeds VCSEL-based transmitter circuit E2000 RJ45 digital R laser driver

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signal dispersion

input pulse of 6ns duration & 1.8ns risetime after optical fibre link risetime is 3.2ns and FWHM is 6ns after co-ax. cable risetime is 4.3ns and FWHM is 8.3ns The Leeds VCSEL-based transmitter circuit

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linearity & noise

The linearity directly determines the usable dynamic range of the optical link – with the output pulse being linearly related to the input pulse. The lower limit of the dynamic range is related to the noise in the system, so the amount of noise introduced into a pulse at a given Ib and T is understood 2mV to 2.2V pulse with FWHM of 5 ns and link at 20C Dynamic range of ~1100 with maximum deviation of 12% from linearity signal/noise for a range

• f temperatures and Ib

The Leeds VCSEL-based transmitter circuit

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frequency response

Ib = 6mA T = 20C transmitter alone has bandwidth of ~470MHz receiver currently limits this to ~250MHz cf co-axial cable bandwidth of ~150MHz The Leeds VCSEL-based transmitter circuit

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temperature correction

light output is relative to Ib As T varies, R varies, thus I varies A digital resistor chip is loaded with a lookup of resistance vs T to adjust Ib to maintain a gain equivalent to that at 0C. The Leeds VCSEL-based transmitter circuit

• 10C

50C 50C

• 10C

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inter-telescope communications

• telescopes talking to the central trigger
• telescopes talking to each other

(e.g. when using inter-telescope timing for large impact distance showers).

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inter-telescope communications Telescope trigger only 1 bit – 10kb/s – ~10kHz

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Telescope trigger + image location 16 bits – 1Mb/s – ~100kHz inter-telescope communications

X X X

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Untriggered stream of image characteristics 10Gb/s – ~1GHz inter-telescope communications

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Untriggered stream of pixel data (2000 pixel camera) 1Tb/s – ~100GHz inter-telescope communications

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Untriggered stream of pixel data (2000 pixel camera) 1Tb/s – ~100GHz but just 1 pixel ~100MHz inter-telescope communications

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The Leeds Digital Asynchronous Transceiver (DAT) Used by VERITAS for deadtime free transmission of asynchronous signals from telescope to central trigger & vice versa.

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Leading edge arrival time 95% of pulses arrive in ±0.265ns

• f that channel's average.

Minimum transmittable pulse width is 5ns giving maximum transmittable data rate of 200MHz The Leeds Digital Asynchronous Transceiver (DAT)

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T3 Autocorrelations

signal via DAT signal via co-ax - 60Hz pickup. The Leeds Digital Asynchronous Transceiver (DAT) Also used in optical signal correlator tests between discriminated central pixel triggers

• f

VERITAS telescopes.

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Summary

Signal transmission via optical fibre offers many advantages over traditional co-axial cable. Reliable, high bandwidth solutions for signal transmission are becoming available (though bandwidth is currently receiver limited).