Download Dog-Legs Status

Opto-electronics
• Why use opto-electronics
– General advantages
– HEP experiments
• Elements of system
– Emitters
– Fibres
– Receivers
• LHC examples
Detector lectures
T. Weidberg
1
Advantages of Opto-electronics
• General
– Much bigger bandwidth than Cu cables
(bandwidth of a links is speed * distance).
• HEP experiments
– Fibres have lower mass and lower Z than Cu
cables  smaller contribution to the r.l. of
the detector.
– Electrical isolation of the two ends of the
link.
Detector lectures
T. Weidberg
2
Opto-electronic System
Emitter +
driver
Repeater
Receiver
+ amp.
fibre
Detector lectures
T. Weidberg
3
Coding Schemes
• Analogue: optical signal proportional to
signal.
• Digital: digitise data and send binary
signals.
– Non Return to Zero 0
– Bi-Phase Mark
– Others…
1
0
0
Detector lectures
T. Weidberg
0
1
1 0
4
Emitters
• Old emitters were usually LEDs
- power ~ 10 mW, linewidth ~ 50 nm
• Newer emitters are semiconductor lasers
- power ~ few mW, linewidth ~ nm.
-  figures for edge emitters
- advantages of VCSELs  figure.
Detector lectures
T. Weidberg
5
SemiConductor Lasers
Simple homojucntion laser
Very high thresholds.
Hetrojunction
lasers.
Confinement
of carriers and
wave  lower
thresholds.
Detector lectures
T. Weidberg
6
VCSELs
• Very radiation hard
• 850 nm matched to radhard Si PIN diodes.

• Cheap to test and
produce.
• Easy to couple into
fibres.
• Easy to drive.
• Low thresholds (~4 mA).
Detector lectures
T. Weidberg
7
Fibres
• Types of fibres ( figures)
– Step Index Multi-mode (SIMM)
– Graded Index Multi Mode (GIMM)
– Monomode MM
• Pros and Cons
– Dispersion ( figures)
– Launch power
Detector lectures
T. Weidberg
8
SIMM Fibres
• Simplest fibre: Step
Index Multi-mode fibre.
• Light trapped by total
internal reflection.
• Maximum angle
sin( MAX )  ( n12  n22 )1/ 2
• Problem is large modal
dispersion
Detector lectures
T. Weidberg
9
GRIN fibres
Adjust refractive
index profile to
minimise modal
dispersion.
Best way to minimise dispersion is
with single mode fibre
Detector lectures
T. Weidberg
10
Fibre Dispersion and Attenuation
Dispersion is a
minimum ~ 1.3 mm
Attenuation is minimum
~1.5 mm
Detector lectures
T. Weidberg
11
Receivers
• Receivers are usually
PIN diodes.
• Active region is low
doped intrinsic  low
depletion voltages.
• Types of PIN
Si l ~ 850 nm
GaAs l: < ~ 900 nm
InGaAs l: < ~1500 nm
Detector lectures
T. Weidberg
12
ATLAS SCT/Pixel links
• Low mass, low Z package ( figure).
• Very rad-hard
– Spike F doped, pure silica core SIMM fibre
– VCSELs: very rad-hard. Stimulated emission 
short carrier lifetimes  less sensitive to nonradiative processes (caused by radiation induced
defects). Show rapid annealing after irradiation.
– Epitaxial Si PIN diodes. Thin active layer  fully
depleted at low bias voltage (< 10V) even after
radiation damage.
Detector lectures
T. Weidberg
13
2 VCSEL+1 PIN Opto-package
Detector lectures
T. Weidberg
14
VCSEL Array
MT-12 connector
12 way ribbon fibre
Detector lectures
T. Weidberg
15
Liquid Argon Calorimeter Readout
Detector lectures
T. Weidberg
16