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Optical Emitters and Receivers

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Presentation on theme: "Optical Emitters and Receivers"— Presentation transcript:

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

15

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

18

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


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