LEDs for optical source

1. OPTICAL SOURCE
By Prof.Sufiyan A. Khan
ACET, ETC
Nagpur

2. What Are LED’s?
 LED is an acronym for Light Emitting Diode
 Instead of a filament they use a semiconductor diode which emits narrow-
spectrum light.
 Depending on the composition & condition of the semiconducting material
used (Silicone, germanium), they come in either Infrared for sensing heat,
Visible for every day use, or Near-Ultraviolet for spotting stains at a
crime-scene.
 An LED consists of a chip of semiconducting material that has been
“doped” with impurities in order to create a p-n junction.
 A p-n junction is basically a junction between an anode and a cathode.
 Current flows easily from the p-side to the n-side, but never in the reverse
order.
 The wavelength and color of the LED depends on the band-gap energy of
the materials forming the p-n junction.

3. Where do we see LED’s?
Everywhere!!

4. Current uses of LED’s
 Status indicators on all sorts of equipment: your cell phone, computer, monitor, stereo
 Traffic lights
 Architectural lighting
 Exit signs
 Motorcycle and bicycle lights
 Railroad crossing signals
 Flashlights
 Emergency vehicle lighting
 Message displays at airports, railways, bus stations, trams, trolleys and ferries
 Military and Tactical missions utilize red and/or yellow lights to retain night vision.
 Movement sensors
 LCD backlighting in televisions
 Christmas Lights
 Lanterns

5. LED’s Vs. Incandescent’s
Incandescent
Positives
 Cheap to manufacture & buy
 Easier to come by
 Generally stronger light output
 Better for seeing through Fog and Smoke
Negatives
 VERY breakable
 Horrible patterns in light
 Hot burning
 Heavy on battery consumption
 Short lifespan
 Yellowish color filters out anything with yellow in it, IE:
White looks yellow, yellow doesn’t show, red looks brown,
green looks black.
LED
Positives
 Virtually indestructible
 100,000 hour lifespan
 Low energy consumption
 Symmetrical beam with little-to-no artifacts
 Cheap to manufacture
 Available in a multitude of colors without requiring a filter.
 Pure white light means no color will be filtered out.
 Low functioning temperature
Negatives
 Less potential output (for now)
 Slightly more expensive to purchase

6. Potential uses in the future
 LED’s are already being used in tail-lights for cars, and some companies like Lexus are
experimenting with LED headlights
 Home lighting: Imagine a “light-bulb” with 100,000 constant hours of use. In other words:
100,000 hours/24 hours a day = 4,166 days
4,166 days/365 days a year = 11.4 years.
 Not only will the light bulb last for 11.4 years, but it will also require much less current
than a traditional light-bulb. If one LED-light bulb requires half the energy of one
Incandescent light-bulb, we may not have to suffer through rolling blackouts ever again!
 LED’s are already getting brighter. Here is an example of one of the most recent LED’s to
hit the market titled the “Luxeon Rebel”. It is both twice as bright, and uses half the
current of it’s predecessor of only 2 years.
 Technology will eventually dictate that LED’s are the light source of the future.

7. How will this affect the business
world?
 With the horizons of LED technology broadening, many light manufacturer’s are
putting their top scientists to work:
Maglite, for instance, always made Incandescent lights, but have recently begun
creating drop-in LED modules for their incandescent torches.
 If one car company comes out with LED headlights that manage to function at a
higher efficiency and also increase output, it is inevitable that all other car
companies will follow. Nobody likes a burnt out headlight!
 Energy is an expensive commodity! The more money we can save on energy, the
more money we can spend on more important business aspects. One office
building that solely uses LED bulbs could save thousands a year in Energy use
alone.
When the sun explodes we won’t be able to make anymore energy and we will
have to rely on LED’s and their efficiency to find food in the pitch black.
Impact of LED’s on the world of Business

8. Light Emitting Diode: LED

9. What is an LED?
 Light-emitting diode
 Semiconductor
 Has polarity

10. LED: How It Works
 When current flows
across a diode
 Negative electrons move one way
and positive holes move the other
way

11. LED: How It Works
 The wholes exist at a
lower energy level
than the free
electrons
 Therefore when a free electrons
falls it losses energy

12. LED: How It Works
 This energy is
emitted in a form of
a photon, which
causes light
 The color of the light is
determined by the fall of the
electron and hence energy level of
the photon

13. Inside a Light Emitting Diode
1. Transparent
Plastic Case
2. Terminal Pins
3. Diode

14. LIGHT EMISSION / DETECTION
14
Transmission
channel
Tx
E
O
Rx
O
E
ReceiverConverterTransmitter Converter
The principle of an optical communication system

15. Wavelength range of optical
transmission
15
Wavelength [nm]
Frequency [Hz]2x1014 3x1014 5x1014 1x1015
Infrared
range
Visible
range
Ultraviolet
range
Fiber optic transmission range
Glass Plastic
850 -1630 nm 520-850 nm
1800 1600 1400 1200 1000 800 600 400 200

16. From electricity to light
16
Conversion from electricity to light is achieved by a electronic :
 LED (light emitting diode)
 VCSEL (Vertical Cavity Surface Emitting Laser)
 LASERS FP (Fabry - Perot)
That:
 changes modulated electrical signal in light modulated signal
 inject light into fiber media

17. Light emitters characteristics
Main characteristics for transmission purposes:
1 Central wavelength
(850/1300/1550)
2 Spectrum width (at ½ power)
3 Power
4 Modulation frequency
(consequence of slope)
1 Wavelength nm
Power dB
3
2Power/2
4

18. Spectrum of a LASER or LED source
18
+5 to -10dBm
LASER
1-5nm
λ
LED
Density
-15 to -25 dBm
60-100nm
λ
Different frequency = different wavelength = different colors

19. Power
19
Is the level of light intensity available for transmission
Average power is the mean value of the power during modulation
Power available for transmission is also function of:
• Fiber core size
• Numerical aperture
Light entrance cone
N.A.
(Numerical Aperture)

20. Modulated frequency
20
 Is the rate at which transmission changes intensity
(logical 0 to 1)
 Rate is function of time
 Time is function of slope
 Slope is characteristic of emitter (technology)
LED functions at lower frequency (longer time)
LASERS at higher (shorter time)
TIME influences modal bandwidth

21. Emitters comparison
21
Type Cost Wavelength
(nm)
Spectral
width (nm)
Modulated
frequency
Power
(dBm)
Usage
LED $ 850-900
1250-1350
30-60
< 150
< 200 MHz - 10 to -30 F.O.
systems
Short
Wavelength
Lasers
$$ 780 4 ≥ 1GHz +1 to -5 CD
Fiber Ch.
VCSEL $$ 850
1300
1 to 6 ≈ 5GHz +1 to -3 F.O.
Giga speed
Lasers $$$ 1300
1550
1 to 6 ≥ 5GHz +1 to -3 F.O. SM

22. Emitter characteristics
transmission related effects
22
LED
VCSEL
LASER
Over Filled Launch (OFL)
Restricted Mode Launch (RMF)
Restricted Mode Launch (RMF)
• Emitters inject light into fiber under different conditions (emitter
physical characteristic).
 Modes travel consequently Power is distributed consequently

23. Semiconductor Sources for
Optical Communications

24. Considerations with Optical Sources
 Physical dimensions to suit the fiber
 Narrow radiation pattern (beam width)
 Linearity (output light power proportional to driving current)

25. Considerations with Optical Sources
 Ability to be directly modulated by varying driving current
 Fast response time (wide band)
 Adequate output power into the fiber

26. Considerations…
 Narrow spectral width (or line width)
 Stability and efficiency
 Driving circuit issues
 Reliability and cost

27. Semiconductor Light Sources
 A PN junction (that consists of direct band gap semiconductor
materials) acts as the active or recombination region.
 When the PN junction is forward biased, electrons and holes
recombine either radiatively (emitting photons) or non-
radiatively (emitting heat). This is simple LED operation.
 In a LASER, the photon is further processed in a resonance
cavity to achieve a coherent, highly directional optical beam
with narrow linewidth.

28. LED vs. laser spectral
width
Single-frequency laser
(<0.04 nm)
Standard laser
(1-3 nm wide)
LED (30-50 nm wide)
Wavelength
Laser output is many times
higher than LED output; they
would not show on same scale

29. Light Emission
 Basic LED operation: When an electron jumps from a higher energy
state (Ec) to a lower energy state (Ev) the difference in energy Ec- Ev
is released either
 as a photon of energy E = h (radiative recombination)
 as heat (non-radiative recombination)

30. Energy-Bands
In a pure Gp. IV material, equal number of holes and electrons
exist at different energy levels.

31. n-type material
Adding group V impurity will create an n- type material

32. p-type material
Adding group III impurity will create a p-type material

33. The Light Emitting Diode (LED)
 For fiber-optics, the LED should have a high radiance (light intensity),
fast response time and a high quantum efficiency
 Double or single hetero-structure devices
 Surface emitting (diffused radiation) Vs Edge emitting (more
directional) LED’s
 Emitted wavelength depends on bandgap energy
 /hchEg 

34. Heterojunction
 Heterojunction is the advanced junction design to reduce diffraction loss in
the optical cavity.
 This is accomplished by modification of the laser material to control the index
of refraction of the cavity and the width of the junction.

35.  The p-n junction of the basic GaAs LED/laser described before is called a
homojunction because only one type of semiconductor material is used in
the junction with different dopants to produce the junction itself.
 The index of refraction of the material depends upon the impurity used
and the doping level.

36.  The Heterojunction region is actually lightly doped with p-type material and
has the highest index of refraction.
 The n-type material and the more heavily doped p-type material both have
lower indices of refraction.
 This produces a light pipe effect that helps to confine the laser light to the
active junction region. In the homojunction, however, this index difference is
low and much light is lost.

37. Gallium Arsenide-Aluminum Gallium
Arsenide Heterojunction
 Structure and index of refraction n for various types of junctions in gallium
arsenide with a junction width d.
 (a) is for a homojunction.
 (b) is for a gallium arsenide-aluminum gallium arsenide single heterojunction.
 (c) is for a gallium arsenide-aluminum gallium arsenide double heterojunction
with improved optical confinement.
 (d) is for a double heterojunction with a large optical cavity of width w.

38. Double-
heterostructure
configuration

39. Structure of a Generic Light Emitter:
Double-Heterostructure Device

40. OPERATING WAVELENGTH
Fiber optic communication systems operate in the
 850-nm,
 1300-nm, and
 1550-nm wavelength windows.
 Semiconductor sources are designed to operate at wavelengths that
minimize optical fiber absorption and maximize system bandwidth

41. LED Wavelength
(eV)
2399.1
m)(
E

 = hc/E(eV)
 = wavelength in microns
H = Planks constant
C = speed of light
E = Photon energy in eV

42. Bandgap Energy and Possible Wavelength
Ranges in Various Materials

43. SEMICONDUCTOR LIGHT-EMITTING
DIODES
 Semiconductor LEDs emit incoherent light.
 Spontaneous emission of light in semiconductor LEDs produces light
waves that lack a fixed-phase relationship. Light waves that lack a
fixed-phase relationship are referred to as incoherent light

44. SEMICONDUCTOR LIGHT-EMITTING DIODES Cont…
 The use of LEDs in single mode systems is severely limited because they
emit unfocused incoherent light.
 Even LEDs developed for single mode systems are unable to launch
sufficient optical power into single mode fibers for many applications.
 LEDs are the preferred optical source for multimode systems because
they can launch sufficient power at a lower cost than semiconductor LDs.

45. Semiconductor LDs
 Semiconductor LDs emit coherent light.
 LDs produce light waves with a fixed-phase relationship (both spatial and
temporal) between points on the electromagnetic wave.
 Light waves having a fixed-phase relationship are referred to as coherent
light.

46. Semiconductor LDs Cont..
 Semiconductor LDs emit more focused light than LEDs, they are able to
launch optical power into both single mode and multimode optical fibers.
 LDs are usually used only in single mode fiber systems because they
require more complex driver circuitry and cost more than LEDs.

47. Produced Optical Power
Optical power produced by optical
sources can range from microwatts
(W) for LEDs to tens of milliwatts
(mW) for semiconductor LDs.
However, it is not possible to
effectively couple all the available
optical power into the optical fiber
for transmission.

48. Dependence of coupled power
The amount of optical power coupled into the fiber is the relevant optical power.
It depends on the following factors:
 The angles over which the light is emitted
 The size of the source's light-emitting area relative to the fiber core size
 The alignment of the source and fiber
 The coupling characteristics of the fiber (such as the NA and the
refractive index profile)

49.  Typically, semiconductor lasers emit light spread out over
an angle of 10 to 15 degrees.
 Semiconductor LEDs emit light spread out at even larger
angles.
 Coupling losses of several decibels can easily occur when
coupling light from an optical source to a fiber, especially
with LEDs.
 Source-to-fiber coupling efficiency is a measure of the
relevant optical power.
 The coupling efficiency depends on the type of fiber that is
attached to the optical source.
 Coupling efficiency also depends on the coupling
technique.

50.  Current flowing through a semiconductor optical source causes it to
produce light.
 LEDs generally produce light through spontaneous emission when a
current is passed through them.

51. Spontaneous Emission
 Spontaneous emission is the random generation of photons within the
active layer of the LED. The emitted photons move in random directions.
Only a certain percentage of the photons exit the semiconductor and are
coupled into the fiber. Many of the photons are absorbed by the LED
materials and the energy dissipated as heat.

52. LIGHT-EMITTING DIODES
 A light-emitting diode (LED) is a semiconductor device that emits
incoherent light, through spontaneous emission, when a current is
passed through it. Typically LEDs for the 850-nm region are fabricated
using GaAs and AlGaAs. LEDs for the 1300-nm and 1550-nm regions are
fabricated using InGaAsP and InP.

53. Types of LED
The basic LED types used for fiber
optic communication systems are
Surface-emitting LED (SLED),
Edge-emitting LED (ELED), and

54. LED performance differences (1)
 LED performance differences help link designers decide which device is
appropriate for the intended application.
 For short-distance (0 to 3 km), low-data-rate fiber optic systems, SLEDs and
ELEDs are the preferred optical source.
 Typically, SLEDs operate efficiently for bit rates up to 250 megabits per second
(Mb/s). Because SLEDs emit light over a wide area (wide far-field angle), they
are almost exclusively used in multimode systems.

55. LED performance differences (2)
 For medium-distance, medium-data-rate systems,
ELEDs are preferred.
 ELEDs may be modulated at rates up to 400 Mb/s.
ELEDs may be used for both single mode and
multimode fiber systems.
 Both SLDs and ELEDs are used in long-distance, high-
data-rate systems. SLDs are ELED-based diodes
designed to operate in the superluminescence mode.
 SLDs may be modulated at bit rates of over 400 Mb/s.

56. Surface-Emitting LEDs
 The surface-emitting LED is also known as the Burrus
LED in honor of C. A. Burrus, its developer.
 In SLEDs, the size of the primary active region is limited
to a small circular area of 20 m to 50 m in diameter.
 The active region is the portion of the LED where
photons are emitted. The primary active region is below
the surface of the semiconductor substrate perpendicular
to the axis of the fiber.
 A well is etched into the substrate to allow direct
coupling of the emitted light to the optical fiber. The
etched well allows the optical fiber to come into close
contact with the emitting surface.

57. Surface-emitting LED

58. Edge-emitting LED

59. LED Spectral Width
Edge emitting LED’s have slightly narrow line width

60. Quantum Efficiency
 Internal quantum efficiency is the ratio between the radiative
recombination rate and the sum of radiative and nonradiative
recombination rates
 For exponential decay of excess carriers, the radiative recombination
lifetime is n/Rr and the nonradiative recombination lifetime is n/Rnr
)/(int nrrr RRR 

61. Internal Efficiency
If the current injected into the LED is I, then the total number of
recombination per second is, Rr+Rnr = I/q where, q is the charge of an
electron.
That is, Rr = intI/q.
Since Rr is the total number of photons generated per second, the optical
power generated internal to the LED depends on the internal quantum
efficiency

62. External Efficiency
Fresnel Transmission Coefficient
 24)0(
21
21
nn
nnT


External Efficiency for air
n2=1, n1 = n
2
)1(
1


nnext
n1
n2
Light
emission
cone

63. 3-dB bandwidths
Optical Power  I(f); Electrical Power  I2(f)
2
)2(1/)( fPfP o 
Electrical Loss = 2 x Optical Loss

64. Drawbacks of LED
 Large line width (30-40 nm)
 Large beam width (Low coupling to the fiber)
 Low output power
 Low E/O conversion efficiency
Advantages
 Robust
 Linear