Lecture Light Source

MKE 1083

Advance Optical Communication

Lecture : LIGHT SOURCE AND TRANSMITTER

Semester II 2009/10

Dr Mohammad Faiz Liew Abdullah

Department of Communication

Faculty of Electrical and Electronic Engineering

University Tun Hussein Onn Malaysia

Dr Mohammad Faiz Liew Abdullah

1

Contents

Properties

Types of Light Source

LED

Laser

Types of Laser Diode

Comparison Modulation

Modulation Bandwidth

Semester II 2009/10 Dr Mohammad Faiz Liew Abdullah

2

Introduction


3

Introduction -

cont


4

Light Sources - Properties i In order for the light sources to function properly and find practical use, the following requirements must be satisfied: i Output wavelength: must coincide with the loss minima of the fibre i Output power: must be high, using lowest possible current and less heat i High output directionality: narrow spectral width i Wide bandwidth i Low distortion


5

Light Sources Types i Every day light sources such as tungsten filament and arc lamps i are suitable, but there exists two types of devices, which are i widely used in optical fibre communication systems:

L ight E mitting D iode (LED)

S emiconductor L aser D iode (SLD or LD) i In both types of device the light emitting region consists of a pn junction constructed of a direct band gap III-V semiconductor, which when forward biased, experiences injected minority carrier recombination, resulting in the generation of photons.


6

Energy states in an atom

E1 : ground state or low energy level

E2 : excited state or high energy level


7

E2

E1

Absorption & Emission of Light


8

Absorption i When a photon with certain energy is incident on an electron in a semiconductor at the ground state(lower energy level E

1

) the electron absorbs the energy and shifts to the higher energy level ( E

2

).

i The energy now acquired by the electron is ( E e

= E

2

– E

1

). Plank's law


9

Spontaneous Emission i

E

2 is unstable and the excited electron(s) will return back to the lower energy level

E

1 i As they fall, they give up the energy acquired during absorption in the form of radiation, which is known as the spontaneous emission process .


10

Stimulated Emission i But before the occurrence of this spontaneous emission process, if external stimulation (photon) is used to strike the excited atom then, it will stimulate the electron to return to the lower state level.

i By doing so it releases its energy as a new photon. The generated photon(s) is in phase and have the same frequency as the incident photon.

i The result is generation of a coherent light composed of two or more photons


11

The Rate Equations


12

LED vs LASER


13

Structural Properties of

Semiconductor


14

Energy Band in Semiconductor


15

P-N Junction


16

P-N Junction


17

P-N Junction


18

P-N Junction


19

p-n junction

Conduction band

E max n p

E c

E f

E v

Junction

Valence band

Electrons diffuse across boundary to fill holes. A built-in field and potential results.


20

Forward biased p-n junction

E max

- E f

E f n eV f p

E c

E v

The depletion region narrows, the barrier potential drops, and majority carriers can diffuse across the junction. This configuration used for light sources.


21

Photon Energy, E

ph

E ph

=

hf

=

E c

−

E v

=

E g

Where h= 6.626 x 10 -34 Js (Plank’s constant) f = frequency of emitted light


22

Semiconductor material

• A direct band gap to produce an adequate level of optical emission.

• The direct band gap materials are made from compounds of a group III element ( e.g Al, Ga or In) and a group V element (e.g P or As), so-called III-V materials.

• The peak emission wavelength, λ can be expressed as:

λ =

E g

1.24

( )

μ m

• The operating wavelength can be chosen for the different materials by varying the proportions of the constituent atoms.


23


24

Direct and Indirect Semiconductors


25

LED - Structure i pn-junction in forward bias, i Injection of minority carriers across the junction gives rise to efficient radiative recombination ( electroluminescence ) of electrons (in Conduction Band) with holes (in Valence Band)


26

LED - Structure


27

Lambertian Source

60

60

I(

θ

)=I o cos

θ

I o

Divergence at

FWHM

Full Wave Half

Maximum

Intensity Distribution

Light from surface of LED is emitted in all directions. This results in an intensity pattern as shown in the figure above.


28

LED - External quantum efficiency η ext

It considers the number of photons actually leaving the LED structure where

F

= Transmission factor of the device-external interface n

= Light coupling medium refractive index n x

= Device material refractive index i

Loss mechanisms that affect the external quantum efficiency:

(1) Absorption within LED

(2) Fresnel losses: part of the light gets reflected back, reflection coefficient:

R

={( n

2

– n

1

) / ( n

2

+ n

1

)}

(3) Critical angle loss: all light gets reflected back if the incident angle is greater than the critical angle.


29

LED - Power Efficiency


30

Problem to solve

A planar LED is fabricated from gallium arsenide which has a refractive index of 3.6.

a) Calculate the optical power emitted into air as a percentage of the internal optical power for the device when the transmission factor at the crystal – air interface is 0.68.

b) When the optical power generated internally is 50% of the electrical power supplied, determine the external power efficiency.


31

Types of LED structure

i Planar LED i Dome LED i Surface Emitter LED i Edge emitter LED i Superluminescent LED


32

Planar LED

Cont’d

Dome LED


33

LED- Surface Emitting LED (SLED)

• Data rates less than 20 Mbps

• Short optical links with large NA fibres (poor coupling)

• Coupling lens used to increase efficiency


34

LED- Edge Emitting LED (ELED)

Semester II 2009/10

• Higher data rate > 100 Mbps

• Multimode and single mode fibres

Dr Mohammad Faiz Liew Abdullah

35

LED - Spectral Profile


36

LED - Power Vs. Current Characteristics


37

LED - Characteristics


38

LED - Frequency Response


39

Application of LED


40

Drawbacks of LED

i Large line width (30-40 nm) i Large beam width (Low coupling to the fiber) i Low output power i Low E/O conversion efficiency

Advantages i Robust i Linear


41



42



43

Laser - Characteristics i The term Laser stands for

L ight

A mplification by

S timulated

E mission of

R adiation.

i Could be mono-chromatic (one colour).

i It is coherent in nature. (I.e. all the wavelengths contained within the Laser light have the same phase). One the main advantage of

Laser over other light sources i A pumping source providing power i It had well defined threshold current beyond which lasing occurs i At low operating current it behaves like LED i Most operate in the near-infrared region


44

Examples


45

Laser - Basic Operation i Similar to LED, but based on stimulated light emission

.

Three steps required to generate a laser beam are:

• Absorption

• Spontaneous Emission

• Stimulated Emission


46

Howling Dog Analogy


47

In Stimulated Emission incident and stimulated photons will have i Identical energy Æ Identical wavelength Æ

Narrow linewidth i Identical direction Æ Narrow beam width i Identical phase Æ Coherence and i Identical polarization


48

Stimulated Emission


49

Laser Transition Processes

(Stimulated and Spontaneous Emission)

Energy absorbed from the incoming photon

Random release of energy

Coherent release of energy


50

Stimulated Emission


51

How a Laser Works


52

Population Inversion


53

Population Inversion


54


55

LD - Spectral Profile


56

LD Efficiencies


57

Power Vs. Current Characteristics


58

Laser threshold depends on

Temperature


59

LD - Single Mode i Achieved by reducing the cavity length

L from 250mm to 25 mm i But difficult to fabricate i Low power i Long distance applications

Types: i Fabry-Perot (FP) i Distributed Feedback (DFB) i Distributed Bragg Reflector (DBR) i Distributed Reflector (DR)


60

Optical Feedback


61

Laser - Fabry-Perot i Strong optical feedback in the longitudinal direction i Multiple longitudinal mode spectrum i “Classic” semiconductor laser

– 1st fibre optic links (850 nm or 1300 nm)

– Short & medium range links i Key characteristics

– Wavelength: 850 or 1310 nm

– Total output power: a few mw

– Spectral width: 3 to 20 nm

– Mode spacing: 0.7 to 2 nm

– Highly polarized

– Coherence length: 1 to 100 mm

– Small NA (® good coupling into fiber)


62

Laser - Distributed Feedback (DFB) i No cleaved faces, uses Bragg Reflectors for lasing i Single longitudinal mode spectrum i High performance

– Costly

– Long-haul links & DWDM systems i Key characteristics

– Wavelength: around 1550 nm

– Total power output: 3 to 50 mw

– Spectral width: 10 to 100 MHz (0.08 to 0.8 pm)

– Sidemode suppression ratio (SMSR): > 50 dB

– Coherence length: 1 to 100 m

– Small NA (® good coupling into fiber)


63

Laser - Vertical Cavity Surface

Emitting Lasers (VCSEL) i Distributed Bragg reflector mirrors

– Alternating layers of semiconductor material

– 40 to 60 layers, each λ / 4 thick

– Beam matches optical acceptance needs of fibers more closely i Key properties

– Wavelength range: 780 to 980 nm (gigabit ethernet)

– Spectral width: <1nm

– Total output power: >-10 dBm

– Coherence length:10 cm to10 m

– Numerical aperture: 0.2 to 0.3


64

Laser diode Properties


65

Output Spectrum

Output spectrum for multimode injection laser.

Semester II 2009/10

Output spectrum for a single

Dr Mohammad Faiz Liew Abdullah mode injection laser.

66

Comparison i

LED i Low efficiency i Slow response time i Lower data transmission rate i Broad output spectrum i In-coherent beam i Low launch power i Higher distortion level at the output i Suitable for shorter transmission distances. i Higher dispersion i Less temperature dependent i Simple construction i Life time 10 7 hours


Laser Diode

High efficiency

Fast response time

Higher data transmission rate

Narrow output spectrum

Coherent output beam

Higher bit rate

High launch power

Less distortion

Suitable for longer transmission distances

Lower dispersion

More temperature dependent

Construction is complicated

Life time 10 7 hours

67

Modulation of Optical Sources i Optical sources can be modulated either directly or externally .

i Direct modulation is done by modulating the driving current according to the message signal (digital or analog) i In external modulation, the laser is emits continuous wave (CW) light and the modulation is done in the fiber


68

Why Modulation

i A communication link is established by transmission of information reliably i Optical modulation is embedding the information on the optical carrier for this purpose i The information can be digital (1,0) or analog (a continuous waveform) i The bit error rate (BER) is the performance measure in digital systems i The signal to noise ratio (SNR) is the performance measure in analog systems


69

Types of Optical Modulation

i Direct modulation is done by superimposing the modulating (message) signal on the driving current i External modulation , is done after the light is generated; the laser is driven by a dc current and the modulation is done after that separately i Both these schemes can be done with either digital or analog modulating signals


70

Direct Modulation

Bias Tee

Bias Current

RF in

Laser

Diode

F ib re

L ink

Photo

Detector

RF out i The message signal (ac) is superimposed on the bias current

(dc) which modulates the laser i Robust and simple, hence widely used i Issues: laser resonance frequency, chirp, turn on delay, clipping and laser nonlinearity


71

Direct Modulation - Analogue

LED LASER

I

'

B

=

I

B

I

'

B

=

I

B

−

I th

Modulation index (depth) m

= Δ

I


I

'

B

72

Direct Intensity Modulation- Digital


73

Turn on Delay (lasers)

i When the driving current suddenly jumps from low ( I

1

< I th

) to high (

I

2

> I th

) , (step input), there is a finite time before the laser will turn on i This delay limits bit rate in digital systems i Can you think of any solution? t d

=

τ

sp ln

⎡

⎢

I

I

2

2

−

−

I

I th

1

⎤

⎥


74

Limitations of Direct Modulation i Turn on delay and resonance frequency are the two major factors that limit the speed of digital laser modulation i Saturation and clipping introduces nonlinear distortion with analog modulation (especially in multi carrier systems) i Nonlinear distortions introduce second and third order intermodulation products i Chirp: Laser output wavelength drift with modulating current is also another issue


75

External Modulation i For high frequencies 2.5 Gbps - 60 Gbps i Modulation and light generation are separated i More expensive and complex i Used in high end systems


76

Modulation : 3-dB bandwidths

P

( f

) =

P o

/ 1 + ( 2

π

f

τ

) 2

Optical Power ∝ I(f); Electrical Power ∝ I 2 (f)

Electrical Loss = 2 x Optical Loss


77

Lecture Light Source

Introduction

Introduction -

Energy states in an atom

Absorption & Emission of Light

LED vs LASER

Energy Band in Semiconductor

P-N Junction

P-N Junction

P-N Junction

P-N Junction

p-n junction

Forward biased p-n junction

Photon Energy, E

=

=

−

=

Semiconductor material

Lambertian Source

Problem to solve

Types of LED structure

Cont’d

Application of LED

Drawbacks of LED

Application of LED

Application of LED

Examples

Stimulated Emission

Stimulated Emission

How a Laser Works

Population Inversion

Population Inversion

Laser threshold depends on

Temperature

Optical Feedback

Output Spectrum

Why Modulation

Types of Optical Modulation

Direct Modulation

Direct Modulation - Analogue

=

−

= Δ

Turn on Delay (lasers)

τ

π

τ

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