communication led
TRANSCRIPT
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