COMMUNICATION LEDs

Anandhu Thampi2nd sem M.Sc.Physics

Department of physicsCUSAT, COCHIN

Kerala

LIGHT EMITTING DIODE• 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.

PRICIPLE OF LEDDOUBLE HETEROJUNCTIONS

DOUBLE HETEROJUNCTIONHeterojunction is the advanced junction design

to reduce light losses One of the way to increase light output in LEDsThe 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.

Direct and Indirect band gap semiconductor

Direct and Indirect band gap semiconductorDirect band gap Indirect band gapBottom 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 devicesEg) 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

Communication LEDsa. Free space communication

o IR LEDSb. Optical fiber communication

o Surface emitting LED (SLED)o Edge emitting LED (ELED)

LED for free space communicationIR LEDs are commonly used in free space

communicationIt is commonly made with GaAs or GaInAs

active region and GaAS substrateTransmission distance is very short ( less

than 100m)For GaAs active region gives IR ranging from

870nmFor GaInAs it is 950nm

LED for fiber optics communicationMainly 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

Types of LED

The basic LED types used for fiber optic communication systems are

Surface-emitting LED (SLED), Edge-emitting LED (ELED)

Burrus type surface emitting LED (SLED)

Burrus type surface emitting LED This LED was developed in AT & Bell

laboratories in 1971The 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.

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.

Edge Emitting LED (ELED)

Edge emitting LEDThis 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.

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.

Typical semiconductor materials and emission wavelengths LEDs

Materials Typical emission wavelengths

InGaN/GaN, ZnS 450-530nm

GaP:N 565nmAlInGaP 590-620nm

GaAsP,GaAsP:N 610-650nmInGaP 660-680nm

AlGaAs, GaAs 680-860nm

InGaAsP 1000-1700nm

Advantages Simple to fabricateLow costLess 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

Drawbacks of LEDLarge line width (30-40 nm)

Large beam width (Low coupling to the fiber)

Low output power

Low E/O conversion efficiency

The LASERLight 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 communicationSo it is very helpful to send data without noisesTwo types of LASERS used for communication• VCSEL (Vertical Cavity Surface Emitting Laser)

• EDGE EMITTING LASER

VCSEL EDGE EMITTER LASER

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.html5. John M Senior:, Optical Fibre Communication,

second edition, PHI Learning private limited, 1998

Thank you