This document discusses double heterojunction LEDs and their use in optical fiber communication. It explains that a double heterojunction structure sandwiches a small bandgap semiconductor between two large bandgap semiconductors. This reduces light losses by emitting photons from the small bandgap region. Surface emitting LEDs and edge emitting LEDs are described as the main types used for fiber optic communication over short distances and low data rates. Common semiconductor materials and their emission wavelengths are also listed.

1. Anandhu Thampi
2nd sem M.Sc.Physics
Department of physics
CUSAT, COCHIN
Kerala

2. • A Light emitting diode is an
optoelectronic device
• It generates light via electroluminescence
•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.

3. PRICIPLE OF LED
DOUBLE HETEROJUNCTIONS

4. DOUBLE HETEROJUNCTION
 Heterojunction is the advanced junction design to
reduce light losses
 One of the way to increase light output in LEDs
 The Heterojunction region is realized by sandwiching a
smallest band gap semiconductor between two largest
band gap semiconductor
 Photons are emitted having energy equal to the smallest
band gap active region
 The light emitted doesnot reabsorb because there are
different band gap semiconductors used.

5. Direct and Indirect band gap
semiconductor

6. Direct and Indirect band gap
semiconductor
Direct band gap Indirect band gap
 Bottom of conduction
band(CB) lies directly above
top of the valance band(VB)
 Electron recombines with
holes gives photon
 The photon have energy
equal to the band gap
 It is radiative recombination
 It is used to build light
emitting devices
 Eg) GaAs
 Bottom of CB not lies directly
above the top of VB
 So for the conservation of
momentum electron losses its
energy by interacting with
phonons
 The electron and hole
recombines getting energy as
form of heat
 It is Non radiative
recombination
 Eg) Si,Ge

7. Communication LEDs
a. Free space communication
o IR LEDS
b. Optical fiber communication
o Surface emitting LED (SLED)
o Edge emitting LED (ELED)

8. LED for free space communication
 IR LEDs are commonly used in free space
communication
 It is commonly made with GaAs or GaInAs active
region and GaAS substrate
 Transmission distance is very short ( less than 100m)
 For GaAs active region gives IR ranging from 870nm
 For GaInAs it is 950nm

10. LED for fiber optics communication
 Mainly there are two types of LEDs
1. Burrus type surface emitting LED (SLED)
2. Edge emitting LED (ELED)
 It is widely used in local area low and medium bit
rate optical communication
 Light emitting spot should be smaller than core
diameter of optical fiber
 Circular emission region with
• 20-50 µm – multimode fiber
• 50-100µm – silica multimode fiber
• 1mm- plastic fiber

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

12. Burrus type surface emitting LED
(SLED)

13. Burrus type surface emitting LED
 This LED was developed in AT & Bell laboratories in
1971
 The surface-emitting LED is also known as the Burrus
LED in honor of C. A. Burrus, its developer.
 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 , they are almost exclusively used in
multimode systems.

14.  In SLEDs, the size of the primary active region is limited
to a small circular area of 20 mm to 50 mm in diameter.
 The active region is the portion of the LED where
photons are emitted.
 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.

15. Edge Emitting LED (ELED)

16. Edge emitting LED
This type of LEDs make use of the transparent guiding
layers with a very thin active layer (50 to 100 μm) in order
that the light produced in the active layer spreads into the
transparent guiding layers.
Majority of the propagating light are emitted at one end
face with the light reflected back from the other end face.
Its coupling efficiency is higher than the surface emitter
LEDs for smaller NA fiber.
The edge emitter LEDs radiate less power to the air
compared to the surface emitter LEDs
The edge emitter LEDs can transfer higher data rate, as
much as 100 MHz than the surface emitter LEDs.

17.  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.

20. Typical semiconductor materials
and emission wavelengths LEDs
Materials Typical emission wavelengths
InGaN/GaN, ZnS 450-530nm
GaP:N 565nm
AlInGaP 590-620nm
GaAsP,GaAsP:N 610-650nm
InGaP 660-680nm
AlGaAs, GaAs 680-860nm
InGaAsP 1000-1700nm

21. Advantages
 Simple to fabricate
 Low cost
 Less temperature dependence(The light output against
current characteristic is less affected)
 ELED can be used for both multimode and single
mode fibers
 Simpler drive circuitry

22. 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

23. The LASER
 Light Amplification by ‘Stimulated Emission’ of
Radiation (L A S E R)
 Coherent light (stimulated emission)
 Narrow beam width (very focused beam)
 High output power (amplification)
 Useful in long distant communication
 So it is very helpful to send data without noises
 Two types of LASERS used for communication
• VCSEL (Vertical Cavity Surface Emitting Laser)
• EDGE EMITTING LASER

24.  VCSEL  EDGE EMITTER LASER

25. Reference
1. E . Fred Schubert:, Light-Emitting Diode, first
edition, The press syndicate of the university of
Cambridge, 2003
2. S . C Gupta:, Optoelectronics Devices and systems,
second edition, PHI Learning private limited, 2015
3. E . W Williams & R . Hall:, Luminescence and the
Light Emitting Diode, first edition, Pergamon press,
1978
4. www.rp-photonics.com/encylopedia.html
5. John M Senior:, Optical Fibre Communication,
second edition, PHI Learning private limited, 1998

26. Thank you