光通信系统之光源LED VS LD
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Prof. Z Ghassemlooy 4
LED - Structure
• pn-junction in forward bias, • Injection of minority carriers across the junction gives rise to efficient radiative recombination (electroluminescence) of electrons (in CB) with holes (in VB)
P0 = I
ηint ηhc hf = I q qλ
ηint = Internal quantum efficiency q = Electron charge 1.602 x 10-19 C
P0
Narrowed Depletion region
I
Electron (-)
+
Hole (+)
Prof. Z Ghassemlooy 6
LED Magnitude (dB)
0 -3
ห้องสมุดไป่ตู้LD
550 0-1 130 nm 800 nm
de mo gle Sin ode ltim Mu
1
10
100
1000
10,000
Frequency (MHz)
Prof. Z Ghassemlooy
14
Laser - Characteristics
• The term Laser stands for Light Amplification by Stimulated Emission of Radiation. • Could be mono-chromatic (one colour). • 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 • A pumping source providing power • It had well defined threshold current beyond which lasing occurs • At low operating current it behaves like LED • Most operate in the near-infrared region
v v Light Emitting Diode (LED) Semiconductor Laser Diode (SLD or LD).
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.
Prof. Z Ghassemlooy
2
Light Sources - Properties
In order for the light sources to function properly and find practical use, the following requirements must be satisfied:
R = 0.99
“LED”
coherent light
R = 0.90
Three steps required to generate a laser beam are: • Absorption • Spontaneous Emission • Stimulated Emission
Prof. Z Ghassemlooy 16
LED - Spectral Profile
Intensity
1300-1550 nm
800-900 nm
65 45 15 λ0 15 45 65
Wavelength (nm)
Prof. Z Ghassemlooy
11
LED - Power Vs. Current Characteristics
Prof. Z Ghassemlooy 15
Laser - Basic Operation
Similar to LED, but based on stimulated light emission.
mirror 1 mirror 2
Mirrors used to “re-cycle” phonons”
50
LED - Characteristics
Wavelength • Spectral width (nm) • Output power (mW) • Coupled power (mW)
- 100 um core - 50 um core - Single mode 0.1-2 ELED 0.3-0.4 SLED 0.01-0.05 SLED 0.05-0.15 0.04-0.08 0.03-0.07 0.003-0.04
• GMMF:
ηc = NA2 NA 2 ηc = 2
The optical coupling loss relative to Pe is :
Lc = − 10 log10
Pc Pe
Or the power coupled to the fibre:
Pc (dBm) = Pe (dBm) − Lc (dB)
Prof. Z Ghassemlooy 7
LED - Power Efficiency
• Emitted optical power Pe =
P0 Fn 2 4nx 2
%
External power efficiency
pe ηep = × 100 P
• MMSF:
The coupling efficiency
800-850 nm 30-60 0.4-5
1300 nm 50-150 0.4-1.0
• Drive current (mA) • Modulation bandwidth (MHz)
50-150 80-150
Prof. Z Ghassemlooy
100-150 100-300
13
LED - Frequenct Response
• Output wavelength: must coincide with the loss minima of the fibre • Output power: must be high, using lowest possible current and less heat • High output directionality: narrow spectral width • Wide bandwidth • Low distortion
Power P0 (mW)
5 4 3 2 1
SELED
Temperature
Linear region
ELED
Current I (mA) Since P ∝ I, then LED can be intensity modulated by modulating the I
Prof. Z Ghassemlooy 12
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={(n2-n1)/(n2+n1)} (3) Critical angle loss: all light gets reflected back if the incident angle is greater than the critical angle.
8
Prof. Z Ghassemlooy
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 G Keiser 2000
Prof. Z Ghassemlooy 1
Contents
§ Properties § Types of Light Source § § § §
§ LED § Laser
Types of Laser Diode Comparison Modulation Modulation Bandwidth
Prof. Z Ghassemlooy 3
Light Sources - Types
Every day light sources such as tungsten filament and arc lamps are suitable, but there exists two types of devices, which are widely used in optical fibre communication systems:
Absorption
When a photon with certain energy is incident on an electron in a semiconductor at the ground state(lower energy level E1 ) the electron absorbs the energy and shifts to the higher energy level (E2). The energy now acquired by the electron is Ee = E2 - E1 ). Plank's law E2 E1 E2 E1 E2 E1 Initial state
Prof. Z Ghassemlooy 9
LED- Edge Emitting LED (ELED)
• Higher data rate > 100 Mbps • Multimode and single mode fibres G Keiser 2000
Prof. Z Ghassemlooy 10
Optical Fibre Communication Systems
Lecture 3: Light Sources
Professor Z Ghassemlooy
Electronics & It Division School of Engineering, Sheffield Hallam University U.K. www.shu.ac.uk/ocr
LED - External quantum efficiency ηext
It considers the number of photons actually leaving the LED structure
η ext = Fn 2 4n x 2
Where F = Transmission factor of the device-external interface n = Light coupling medium refractive index nx = Device material refractive index
p
hf ≈ Eg
Hole
n
Electron
--- Fermi levels
hf ≈ Eg
Homojunction LED
Prof. Z Ghassemlooy 5
LED - Structure
• Optical power produced by the Junction:
Pt
Fibre Photons P0 n-type p-type Where
LED - Structure
• pn-junction in forward bias, • Injection of minority carriers across the junction gives rise to efficient radiative recombination (electroluminescence) of electrons (in CB) with holes (in VB)
P0 = I
ηint ηhc hf = I q qλ
ηint = Internal quantum efficiency q = Electron charge 1.602 x 10-19 C
P0
Narrowed Depletion region
I
Electron (-)
+
Hole (+)
Prof. Z Ghassemlooy 6
LED Magnitude (dB)
0 -3
ห้องสมุดไป่ตู้LD
550 0-1 130 nm 800 nm
de mo gle Sin ode ltim Mu
1
10
100
1000
10,000
Frequency (MHz)
Prof. Z Ghassemlooy
14
Laser - Characteristics
• The term Laser stands for Light Amplification by Stimulated Emission of Radiation. • Could be mono-chromatic (one colour). • 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 • A pumping source providing power • It had well defined threshold current beyond which lasing occurs • At low operating current it behaves like LED • Most operate in the near-infrared region
v v Light Emitting Diode (LED) Semiconductor Laser Diode (SLD or LD).
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.
Prof. Z Ghassemlooy
2
Light Sources - Properties
In order for the light sources to function properly and find practical use, the following requirements must be satisfied:
R = 0.99
“LED”
coherent light
R = 0.90
Three steps required to generate a laser beam are: • Absorption • Spontaneous Emission • Stimulated Emission
Prof. Z Ghassemlooy 16
LED - Spectral Profile
Intensity
1300-1550 nm
800-900 nm
65 45 15 λ0 15 45 65
Wavelength (nm)
Prof. Z Ghassemlooy
11
LED - Power Vs. Current Characteristics
Prof. Z Ghassemlooy 15
Laser - Basic Operation
Similar to LED, but based on stimulated light emission.
mirror 1 mirror 2
Mirrors used to “re-cycle” phonons”
50
LED - Characteristics
Wavelength • Spectral width (nm) • Output power (mW) • Coupled power (mW)
- 100 um core - 50 um core - Single mode 0.1-2 ELED 0.3-0.4 SLED 0.01-0.05 SLED 0.05-0.15 0.04-0.08 0.03-0.07 0.003-0.04
• GMMF:
ηc = NA2 NA 2 ηc = 2
The optical coupling loss relative to Pe is :
Lc = − 10 log10
Pc Pe
Or the power coupled to the fibre:
Pc (dBm) = Pe (dBm) − Lc (dB)
Prof. Z Ghassemlooy 7
LED - Power Efficiency
• Emitted optical power Pe =
P0 Fn 2 4nx 2
%
External power efficiency
pe ηep = × 100 P
• MMSF:
The coupling efficiency
800-850 nm 30-60 0.4-5
1300 nm 50-150 0.4-1.0
• Drive current (mA) • Modulation bandwidth (MHz)
50-150 80-150
Prof. Z Ghassemlooy
100-150 100-300
13
LED - Frequenct Response
• Output wavelength: must coincide with the loss minima of the fibre • Output power: must be high, using lowest possible current and less heat • High output directionality: narrow spectral width • Wide bandwidth • Low distortion
Power P0 (mW)
5 4 3 2 1
SELED
Temperature
Linear region
ELED
Current I (mA) Since P ∝ I, then LED can be intensity modulated by modulating the I
Prof. Z Ghassemlooy 12
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={(n2-n1)/(n2+n1)} (3) Critical angle loss: all light gets reflected back if the incident angle is greater than the critical angle.
8
Prof. Z Ghassemlooy
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 G Keiser 2000
Prof. Z Ghassemlooy 1
Contents
§ Properties § Types of Light Source § § § §
§ LED § Laser
Types of Laser Diode Comparison Modulation Modulation Bandwidth
Prof. Z Ghassemlooy 3
Light Sources - Types
Every day light sources such as tungsten filament and arc lamps are suitable, but there exists two types of devices, which are widely used in optical fibre communication systems:
Absorption
When a photon with certain energy is incident on an electron in a semiconductor at the ground state(lower energy level E1 ) the electron absorbs the energy and shifts to the higher energy level (E2). The energy now acquired by the electron is Ee = E2 - E1 ). Plank's law E2 E1 E2 E1 E2 E1 Initial state
Prof. Z Ghassemlooy 9
LED- Edge Emitting LED (ELED)
• Higher data rate > 100 Mbps • Multimode and single mode fibres G Keiser 2000
Prof. Z Ghassemlooy 10
Optical Fibre Communication Systems
Lecture 3: Light Sources
Professor Z Ghassemlooy
Electronics & It Division School of Engineering, Sheffield Hallam University U.K. www.shu.ac.uk/ocr
LED - External quantum efficiency ηext
It considers the number of photons actually leaving the LED structure
η ext = Fn 2 4n x 2
Where F = Transmission factor of the device-external interface n = Light coupling medium refractive index nx = Device material refractive index
p
hf ≈ Eg
Hole
n
Electron
--- Fermi levels
hf ≈ Eg
Homojunction LED
Prof. Z Ghassemlooy 5
LED - Structure
• Optical power produced by the Junction:
Pt
Fibre Photons P0 n-type p-type Where