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.

Detector lectures T. Weidberg 14

2 VCSEL+1 PIN Opto-package

Detector lectures T. Weidberg 15

VCSEL Array

MT-12 connector

12 way ribbon fibre

Detector lectures T. Weidberg 16

Liquid Argon Calorimeter Readout