The attached narrated power point presentation mentions the different types of optical sources used for optical fiber communications, the requirements for light sources for optical fiber communications, direct and indirect bandgap semiconductors and different types of LEDs in use today along with their comparison. The material will be useful for KTU final year B Tech students who prepare for the subject EC 405, Optical Communications.

1. 1
Optical Sources
Light Emitting Diode
MEC

2. 2
Contents
• Introduction.
• Types of Light Sources.
• Source Requirements.
• LED Advantages and Drawbacks.
• Working Principle of LEDs.
• Direct/Indirect Bandgap Semiconductors.
• Types of LEDs.
• Comparison - Surface and Edge Emitters.

3. 3
Introduction
• Active component in an optical fiber
communication system.
• Convert electrical energy (electric current)
into optical energy (light).
• Allows light output to be launched/coupled
into optical fiber.
• Three types of light sources.

4. 4
Types of Light Sources
• Wideband ‘continuous spectra’ sources
(incandescent lamps).
• Monochromatic incoherent sources (light-
emitting diodes, LEDs).
• Monochromatic coherent sources (lasers).
• Gas lasers (helium-neon) used initially - most
powerful narrowband coherent light sources
needed due to severe attenuation & dispersion.
• Later development of semiconductor injection
laser and LED, improvement in the properties of
optical fibers.

5. 5
Source Requirements for Optical
Fiber Communications
• Size and configuration compatible with
launching light into optical fiber - highly
directional.
• Linear - accurately tracking electrical input signal
- minimize distortion and noise.
• Emit light at wavelengths where the fiber has low
losses and low dispersion, where detectors are
efficient.
• Capable of simple signal modulation over wide
bandwidth.

6. 6
Source Requirements for Optical
Fiber Communications
• Couple sufficient optical power to
overcome attenuation and connector
losses, sufficient power to drive the
detector.
• Narrow spectral bandwidth (linewidth),
minimize fiber dispersion.
• Stable optical output, unaffected by
changes in ambient conditions.
• Cheap, highly reliable.

7. 7
Sources for Optical
Communications
• First-generation optical communication sources
designed for 0.8 and 0.9 μm (ideally0.85 μm) -
properties of the semiconductor materials used
permit emission at this wavelength.
• Loss incurred in many fibers near 0.9 μm due to
OH- ion.
• Early systems used multimode step index fibers,
semiconductor lasers for links of reasonable
bandwidth (tens of megahertz) and distances
(several kilometers).

8. 8
Sources for Optical
Communications
• Light Emitting Diodes (LEDs) - lower power
source, little spatial or temporal coherence -
unsuitable for long - distance wideband
transmission, moderate distances.
• Role of LED as optical fiber source enhanced
after development of multimode graded index
fiber - reduced intermodal dispersion.
• LEDs in 0.8 to 0.9 μm wavelength band used for
wide band applications.

9. 9
Light Emitting Diodes
• Second generation optical fiber sources operate
at wavelengths1.1 and 1.6 μm.
• Material losses and dispersion greatly reduced.
• Wideband graded index fiber systems with LED
sources operate over long distances, no need
for intermediate repeaters.
• LEDs - relatively simple construction &
operation, low cost, extended trouble-free life.
• LEDs as multimode sources, acceptable
coupling efficiencies into multimode fiber.

10. 10
Advantages of LEDs
• Simpler fabrication - no mirror facets, no
striped geometry.
• Simpler construction and reduced cost.
• Reliable – no catastrophic degradation,
less sensitive to gradual degradation.
• Immune to self-pulsation and modal noise.
• Less temperature dependence of
characteristics.

11. 11
Advantages of LEDs
• Simpler drive circuitry - lower drive
currents, reduced temperature
dependence - temperature compensation
circuits unnecessary.
• Better Linearity – more linear light output
against current characteristics – useful for
analog modulation.
• Extensively used for optical fiber
communications.

12. 12
Drawbacks of LEDs
• Lower optical power coupled into a fiber
(microwatts).
• Lower modulation bandwidth.
• Harmonic distortion.
• Incoherency, emitted photons have random
phases.
• Greater spectral line width, supports many
optical modes.
• Stimulated emission not encouraged – no optical
amplification through stimulated emission.

13. 13
Spontaneous Emission
• Forward biased p–n junction - increased
concentration of minority carriers in the opposite
type region leads to the recombination of
carriers across the bandgap.
• Normally empty electron states in conduction
band of p-type material and normally empty hole
states in valence band of n-type material
populated by injected carriers, recombine across
the bandgap.
• Energy released by recombination
approximately equal to bandgap energy (Eg).

14. 14
Spontaneous Emission
electroluminescence

15. 15
Spontaneous Emission
Carrier recombination gives spontaneous emission of light in a p–n junction

16. 16
Spontaneous Emission
• Excess carrier population decreased by
recombination, radiative or nonradiative.
• Nonradiative recombination - energy
released dissipated in the form of lattice
vibrations and thus heat.
• In band-to-band radiative recombination,
energy released with creation of a photon
of energy
,

17. 17
Direct Bandgap Semiconductors
• Direct bandgap semiconductors - electrons and
holes on either side of the forbidden energy gap
have same value of crystal momentum, direct
recombination possible.
• Crystal momentum p = 2πhk, k – wave vector.
• Energy maximum of the valence band occurs at
very nearly the same value of electron crystal
momentum as the energy minimum of the
conduction band.
• Electron Momentum virtually constant, energy
released corresponds to band gap energy Eg,
emitted as light.

18. 18
Direct Bandgap Semiconductors
• Minority carrier lifetime - average time the
minority carrier remains in free state
before recombination- relatively short (10−8
to 10−10 s).
• Examples – GaAs (∆E = 1.43), InAs (∆E =
0.35).

19. 19
Indirect Bandgap Semiconductors
• Maximum and minimum energies occur at different
values of crystal momentum.
• Electron lose momentum, has momentum
corresponding to the maximum energy of the
valence band.
• Conservation of momentum requires emission or
absorption of a third particle, a phonon.
• Recombination in indirect band gap semiconductors
relatively slow (10−2 to 10−4 s) - longer minority
carrier lifetime, more probability of non-radiative
transitions.
• Examples : Si (∆E = 1.12), Ge (∆E = 0.67).

20. 20
Energy – Momentum Diagrams
three-particle recombination
Non- radiative recombination involves lattice
defects and impurities

21. 21
Recombination Coefficient
• Recombination coefficient
obtained from measured
absorption coefficient of
the semiconductor.
• For low injected minority
carrier density relative to
majority carriers it is
related approximately to
radiative minority carrier
lifetime
N, P - majority carrier
concentrations in n- and
p-type regions.
Direct bandgap materials for electroluminescent sources.

22. 22
Internal Quantum Efficiency
• Internal quantum efficiency - ratio of the number
of radiative recombinations (photons produced
within the structure) to the number of injected
carriers, as a percentage.
• Higher for direct bandgap semiconductors.
• Internal quantum efficiency of 50% for simple
homojunction devices, 60 to 80% for double-
heterojunction (DH) structures.
• LED internal quantum efficiency - ratio of
radiative recombination rate to total
recombination rate.

23. 23
External Quantum Efficiency and
Modulation Bandwidth
• External quantum efficiency - ratio of photons
emitted from the device to the photons internally
generated, also the ratio of the number of photons
emitted to the total number of carrier recombinations
(radiative and nonradiative).
• Modulation bandwidth defined in either electrical or
optical terms.
• Electrical bandwidth - ratio of electric output power
to electric input power in dB, electrical 3 dB point/
frequency at which output electric power reduced by
3 dB.
• Optical bandwidth - ratio of optical output power to
optical input power in dB, frequencies at which the
output current has dropped to 0.5 of the input
current to the system.

24. 24
Modulation Bandwidth
Optical bandwidth is higher than electrical bandwidth.
corresponds to an electric power attenuation of 6 dB

25. 25
Other radiative recombination
processes
(electron)

26. 26
Filled-in Electron States for Intrinsic
Direct Band Gap Semiconductors

27. 27
Radiation Geometry for LEDs
• Radiation geometry for a
planar LED is Lambertian.
• Surface radiance - power
radiated from a unit area into
a unit solid angle constant in
all directions.
• Maximum intensity I0 is
perpendicular to the planar
surface, reduced on the
sides in proportion to cosine
of viewing angle θ.
external power efficiency
I0 - radiant intensity along θ = 0.

28. 28
Coupling Light into Fiber
• Light incident on the exposed core end within
the acceptance angle θa is coupled.
• Incident light at angles greater than θa not
coupled.
I0 -radiant intensity along θ = 0.
• Coupling efficiency allows estimates for
percentage of optical power coupled into the
step index fiber relative to the amount of optical
power emitted from the LED.

29. 29
Coupling Efficiency
• Consider a source smaller than, close to,
fiber core, assume cylindrical symmetry,
coupling efficiency
since
Device must exhibit very high radiance.

30. 30
Double-Heterojunction LED

31. 31
Double-Heterojunction LED
• Forward bias – electrons from n-type layer injected
through the p–n junction into p-type GaAs layer
where they become minority carriers.
• Minority carriers diffuse away from the junction
recombine with majority carriers (holes).
• Photons produced with energy corresponding to the
bandgap energy of the p-type GaAs layer.
• Injected electrons inhibited from diffusing into p-type
AlGaAs layer due to potential barrier offered by p–p
heterojunction.
• Electroluminescence occurs in the GaAs junction
layer.

32. 32
Double-Heterojunction LED
• Light emitted from the device without
reabsorption - bandgap energy in AlGaAs
layer is large.
• Efficient incoherent sources for application
within optical fiber communications.
• Good internal quantum efficiency and
high-radiance emission.

33. 33
Planar LED
• Forward current flow
through the junction
gives Lambertian
spontaneous emission.
• Device emits light from
all surfaces.
• Limited amount of light
escapes the structure
due to total internal
reflection.
• Low Radiance.
p-type diffusion into n-type substrate

34. 34
Dome LED
• Diameter of the dome
chosen to maximize the
amount of internal
emission reaching the
surface.
• Higher external power
efficiency than planar
LED.
• Dome far larger than
active recombination
area, greater effective
emission area, reduced
radiance.

35. 35
Surface Emitter LED
Pioneered by Burrus and Dawson
restricts emission to a small active region.

36. 36
Edge emitter LED
High effective radiance
Beam Width

37. 37
Surface Emitters vs Edge Emitters
• Surface emitters radiate more power into air (2.5
- 3 times) than edge emitters, emitted light less
affected by reabsorption & interfacial
recombination.
• Edge emitters couple more optical power into
low NA (< 0.3) than surface emitters, opposite is
true for large NA (> 0.3).
• Enhanced waveguiding of edge emitter enables
it to couple 7.5 times more power into low-NA
fiber than surface emitter.

38. 38
Surface Emitters vs Edge Emitters
• Similar coupling efficiencies achieved into low-
NA fiber with surface emitters using a lens. Lens
coupling with edge emitters may increase
coupling efficiencies by around five times.
• Edge emitters have better modulation bandwidth
of the order of hundreds of megahertz than
comparable surface-emitting structures.
• Possible to construct edge-emitting LEDs with
narrower linewidth than surface emitters.

39. 39
Lens Coupling into Optical Fiber

40. 40
Surface Emitters vs Edge Emitters
• Stripe geometry of the edge emitter allows
very high carrier injection densities for
given drive currents.
• Possible to couple a milliwatt of optical
power into low-NA (0.14) multimode step
index fiber with edge-emitting LEDs
operating at high drive currents (500 mA).

41. 41
LED Characteristics

42. 42
Thank You