1. Optical Emitters and Receivers
Emitters basics
LED and LASER
PD Basics
PD parameters

2. Emitters basics Concept Light Emitter = optical electrical converter
Light Radiation: electronic excitation in a semiconductor
Nothing to do with incandescence
Allows very high speed modulation
Better spectrum (narrower, more stable)
Parameters
Operation Wavelength: 0 (nm)
Spectral Width (or BW):  (nm)
Optical Power into Fiber: Po (dBm)
Safe Margin: MS (dB). For the whole system, but assigned to the optical source

3. Optical Emission Fundamentals
Emitters basics
Optical Emission Fundamentals
Emission: Free e- from the conduction band recombine with valence band holes, emitting photons. The wavelength of photons is set by the band gap (E1-E2)
(Planck: h = 6.626·10-34 J·s)
1 J= 6,242e+18 ev

4. Emitters basics

5. Emitters basics
P-N Junction —A forward biased p-n junction

6. Light Amplification by Stimulated Emission of Radiation
LED and LASER
Optical Sources
LED
Light Emitting Diode
Laser Diode (LD)
Light Amplification by Stimulated Emission of Radiation
Opt. Communic. LED
Normal LED
Opt. Communic. LD
Industrial Laser

7. LED and LASER
A= 10−10 m

8. LED and LASER LEDs Characteristics Spectrum width ()
Incoherent emission
Low power
Low cost
Types
Surface LED
Edge LED
More radiation
More directional Δλ

9. LED and LASER LEDs Actual Spectrum FWHM: Full Width at Half
Maximum (Δλ)

10. Characteristics of LEDs

11. LED and LASER
Laser
Operation Principles
An amplifier oscillating
Stimulated emission → avalanche
Diode + resonant cavity (Fabry-Perot) → monochrome

12. Monochromatic spectrum (resonant cavity)
LED and LASER
Laser
Characteristics
Monochromatic spectrum (resonant cavity)
Coherent, more directional (stimulated emission)
High power (avalanche)
Fast modulation
Instability
Expensive
Threshold
Current
Spontaneous
emission = LED
Stimulated
emission = LD

13. LED and LASER
Laser

14. LED and LASER
With L : cavity length
P : mode order
N: group indice

16. LED Versus Laser LED LASER  (nm) Wide (50-100) Narrow (0.5-5)
Po (mW)
Low (1)
High (5-20)
Coupling (dB)
10-13
0-1
BW (GHz)
Small ( )
Huge (0.5-2)
Cost
Cheap
Expensive
Hardware
Easy
Complex
Fiber Type
Multimode
Single mode

17. One absorbed photon creates a pair of free carriers
PD BASICS
Operation Principles
One absorbed photon creates a pair of free carriers
P-n junction, reverse biased
Depletion region (without free carriers)
New photogenerated free carriers are pulled
Depletion Region
Wide: many absorbed
photons
Narrow: high speed

19. PD PARAMETERS Types of PDs (I)
PIN PDs: no avalanche, linear, low sensitivity
Ip = R· po (photocurrent = responsivity · optical power)
Avalanche PDs = APDs: low linearity, high sensitivity
Multiplication Factor: M  40 times
It = Ip· M (total current = photocurrent · avalanche)

20. PD PARAMETERS
Dark current: The leakage current that flows through a photodiode with no light input. Thermally generated.
Transit time: The time it takes a light-induced carrier to travel across the depletion region of a semiconductor. This parameter determines the maximum bit rate possible with a particular photodiode.
Transit time is a function of depletion width and carrier drift velocity
td= w/vd
Light sensitivity: The minimum optical power a light detector can receive and still produce a usable electrical output signal.

21. PD PARAMETERS
Photocurrent

22. PD PARAMETERS
Responsivity (Sensitivity)

23. PD PARAMETERS
PD Cut-Off Wavelength
One photon needs the gap energy to generate a pair

24. Multiplication Coefficient (M)
With
V : polarisation tension.

25. Phototransistor

26. PD PARAMETERS Types of PDs PIN APD Parameter Symbol Unit Si Ge InGaAs
Wavalength λ nm
Responsivity R
A/W
Dark current ID nA
1-10
50-500
Rise time tr ns
0.5-1
Bandwidth B
GHz
0.5-3
1-2
Bias voltage VB V 5
5-10
APD
Parameter
Symbol
Unit Si Ge
InGaAs
Wavalength λ nm
Avalanche M -
20-400
50-200
10-40
Dark current ID nA
0.1-1
50-500
10-50
Rise time tr ns
0.1-2
Gain·Bandwidth
M·B
GHz
2-10
20-250
Bias voltage VB V
20-40
20-30