COMMUNICATION LED

LEDs used optical fiber communication

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

2. 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.

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 sm allest 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 Free space communication IR LEDS Optical fiber communication Surface emitting LED (SLED) 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

9. LED for fiber optics communication Mainly there are two types of LEDs Burrus type surface emitting LED (SLED) 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

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

11. Burrus type surface emitting LED (SLED)

12. 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.

13. In SLEDs, the size of the primary active region is limited to a small circular area of 20 m m to 50 m m 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.

14. Edge Emitting LED (ELED)

15. 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. I ts 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.

16. 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.

17. 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

18. 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

19. 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

20. 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

21. VCSEL EDGE EMITTER LASER

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

23. Thank you