Detector lectures T. Weidberg 1
Opto-electronics
• Why use opto-electronics– General advantages– HEP experiments
• Elements of system– Emitters– Fibres– Receivers
• LHC examples
Detector lectures T. Weidberg 2
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 3
Opto-electronic System
Emitter + driver
fibre
Repeater Receiver + amp.
Detector lectures T. Weidberg 4
Coding Schemes
• Analogue: optical signal proportional to signal.
• Digital: digitise data and send binary signals.– Non Return to Zero – Bi-Phase Mark– Others…
0 1 0
0 011 0
Detector lectures T. Weidberg 5
Emitters
• Old emitters were usually LEDs- power ~ 10 W, 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 6
SemiConductor Lasers
Simple homojucntion laser
Very high thresholds.
Hetrojunction lasers. Confinement of carriers and wave lower thresholds.
Detector lectures T. Weidberg 7
VCSELs
• Very radiation hard
• 850 nm matched to rad-hard Si PIN diodes.
• Cheap to test and produce.
• Easy to couple into fibres.
• Easy to drive.
• Low thresholds (~4 mA).
Detector lectures T. Weidberg 8
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 9
SIMM Fibres
• Simplest fibre: Step Index Multi-mode fibre.
• Light trapped by total internal reflection.
• Maximum angle
• Problem is large modal dispersion
2 2 1/ 21 2sin( ) ( )MAX n n
Detector lectures T. Weidberg 10
GRIN fibres
Adjust refractive index profile to minimise modal dispersion.
Best way to minimise dispersion is with single mode fibre
Detector lectures T. Weidberg 11
Fibre Dispersion and Attenuation
Dispersion is a minimum ~ 1.3 m
Attenuation is minimum ~1.5 m
Detector lectures T. Weidberg 12
Receivers
• Receivers are usually PIN diodes.
• Active region is low doped intrinsic low depletion voltages.
• Types of PINSi ~ 850 nmGaAs : < ~ 900 nmInGaAs : < ~1500 nm
Detector lectures T. Weidberg 13
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 non-radiative 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.