Download Transmission of fast signals via optical fibres Richard White Michael Daniel for

Transmission of fast signals
via optical fibres
Michael Daniel for
Richard White
[email protected]
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Digitizing the signal from an IACT camera: why fast signal transmission is needed.
ns
The Cherenkov light front is ~couple nanoseconds in duration.
After passing through Davies-Cotton optics this will be broadened to ~4.5ns rise time
after 100m of co-ax. cable
pulse rise time is ~8-10ns
0 10 20 30 40
[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
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Advantages of an optical fibre system
➢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.
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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.
NEW VCSELS
OLD VCSELS
Mode
Hopping
No Mode
Hopping, Lower
Noise
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The Leeds VCSEL-based transmitter circuit
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)
digital R
RJ45
E2000
laser driver
BNC
inject 0->2.2V pulse
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The Leeds VCSEL-based transmitter circuit
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
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The Leeds VCSEL-based transmitter circuit
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
of temperatures and Ib
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The Leeds VCSEL-based transmitter circuit
frequency response
transmitter alone has bandwidth of ~470MHz
receiver currently limits this to ~250MHz
cf co-axial cable bandwidth of ~150MHz
Ib = 6mA
T = 20C
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The Leeds VCSEL-based transmitter circuit
temperature correction
50C
light output is relative to Ib
As T varies, R varies, thus I varies
-10C
50C
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.
-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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inter-telescope communications
Telescope trigger + image location
16 bits – 1Mb/s – ~100kHz
X
X
X
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inter-telescope communications
Untriggered stream of image characteristics
10Gb/s – ~1GHz
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inter-telescope communications
Untriggered stream of pixel data (2000 pixel camera)
1Tb/s – ~100GHz
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inter-telescope communications
Untriggered stream of pixel data (2000 pixel camera)
1Tb/s – ~100GHz
but just 1 pixel ~100MHz
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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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The Leeds Digital Asynchronous Transceiver (DAT)
Leading edge arrival time
95% of pulses arrive in ±0.265ns
of that channel's average.
Minimum transmittable pulse width is 5ns giving
maximum transmittable data rate of 200MHz
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The Leeds Digital Asynchronous Transceiver (DAT)
T3 Autocorrelations
signal via DAT
Also used in optical signal
correlator tests between
discriminated central pixel
triggers
of
VERITAS
telescopes.
signal via co-ax - 60Hz pickup.
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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).
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