![Wavelength range of optical
transmission
15
Wavelength [nm]
Frequency [Hz]2x1014 3x1014 5x1014 1x1015
Infrared
range
Visible
range
Ultraviolet
range
Fiber optic transmission range
Glass Plastic
850 -1630 nm 520-850 nm
1800 1600 1400 1200 1000 800 600 400 200](https://image.slidesharecdn.com/opticalsourceledbysufiyanakhan-150805085618-lva1-app6892/85/Optical-source-LED-by-sufiyan-a-khan-15-320.jpg)
Optical source LED by sufiyan a khan
- 1. OPTICAL SOURCE By Prof.Sufiyan A. Khan ACET, ETC Nagpur
- 2. What Are LED’s? LED is an acronym for Light Emitting Diode Instead of a filament they use a semiconductor diode which emits narrow- spectrum light. Depending on the composition & condition of the semiconducting material used (Silicone, germanium), they come in either Infrared for sensing heat, Visible for every day use, or Near-Ultraviolet for spotting stains at a crime-scene. An LED consists of a chip of semiconducting material that has been “doped” with impurities in order to create a p-n junction. A p-n junction is basically a junction between an anode and a cathode. Current flows easily from the p-side to the n-side, but never in the reverse order. The wavelength and color of the LED depends on the band-gap energy of the materials forming the p-n junction.
- 3. Where do we see LED’s? Everywhere!!
- 4. Current uses of LED’s Status indicators on all sorts of equipment: your cell phone, computer, monitor, stereo Traffic lights Architectural lighting Exit signs Motorcycle and bicycle lights Railroad crossing signals Flashlights Emergency vehicle lighting Message displays at airports, railways, bus stations, trams, trolleys and ferries Military and Tactical missions utilize red and/or yellow lights to retain night vision. Movement sensors LCD backlighting in televisions Christmas Lights Lanterns
- 5. LED’s Vs. Incandescent’s Incandescent Positives Cheap to manufacture & buy Easier to come by Generally stronger light output Better for seeing through Fog and Smoke Negatives VERY breakable Horrible patterns in light Hot burning Heavy on battery consumption Short lifespan Yellowish color filters out anything with yellow in it, IE: White looks yellow, yellow doesn’t show, red looks brown, green looks black. LED Positives Virtually indestructible 100,000 hour lifespan Low energy consumption Symmetrical beam with little-to-no artifacts Cheap to manufacture Available in a multitude of colors without requiring a filter. Pure white light means no color will be filtered out. Low functioning temperature Negatives Less potential output (for now) Slightly more expensive to purchase
- 6. Potential uses in the future LED’s are already being used in tail-lights for cars, and some companies like Lexus are experimenting with LED headlights Home lighting: Imagine a “light-bulb” with 100,000 constant hours of use. In other words: 100,000 hours/24 hours a day = 4,166 days 4,166 days/365 days a year = 11.4 years. Not only will the light bulb last for 11.4 years, but it will also require much less current than a traditional light-bulb. If one LED-light bulb requires half the energy of one Incandescent light-bulb, we may not have to suffer through rolling blackouts ever again! LED’s are already getting brighter. Here is an example of one of the most recent LED’s to hit the market titled the “Luxeon Rebel”. It is both twice as bright, and uses half the current of it’s predecessor of only 2 years. Technology will eventually dictate that LED’s are the light source of the future.
- 7. How will this affect the business world? With the horizons of LED technology broadening, many light manufacturer’s are putting their top scientists to work: Maglite, for instance, always made Incandescent lights, but have recently begun creating drop-in LED modules for their incandescent torches. If one car company comes out with LED headlights that manage to function at a higher efficiency and also increase output, it is inevitable that all other car companies will follow. Nobody likes a burnt out headlight! Energy is an expensive commodity! The more money we can save on energy, the more money we can spend on more important business aspects. One office building that solely uses LED bulbs could save thousands a year in Energy use alone. When the sun explodes we won’t be able to make anymore energy and we will have to rely on LED’s and their efficiency to find food in the pitch black. Impact of LED’s on the world of Business
- 8. Light Emitting Diode: LED
- 9. What is an LED? Light-emitting diode Semiconductor Has polarity
- 10. LED: How It Works When current flows across a diode Negative electrons move one way and positive holes move the other way
- 11. LED: How It Works The wholes exist at a lower energy level than the free electrons Therefore when a free electrons falls it losses energy
- 12. LED: How It Works This energy is emitted in a form of a photon, which causes light The color of the light is determined by the fall of the electron and hence energy level of the photon
- 13. Inside a Light Emitting Diode 1. Transparent Plastic Case 2. Terminal Pins 3. Diode
- 14. LIGHT EMISSION / DETECTION 14 Transmission channel Tx E O Rx O E ReceiverConverterTransmitter Converter The principle of an optical communication system
- 15. Wavelength range of optical transmission 15 Wavelength [nm] Frequency [Hz]2x1014 3x1014 5x1014 1x1015 Infrared range Visible range Ultraviolet range Fiber optic transmission range Glass Plastic 850 -1630 nm 520-850 nm 1800 1600 1400 1200 1000 800 600 400 200
- 16. From electricity to light 16 Conversion from electricity to light is achieved by a electronic : LED (light emitting diode) VCSEL (Vertical Cavity Surface Emitting Laser) LASERS FP (Fabry - Perot) That: changes modulated electrical signal in light modulated signal inject light into fiber media
- 17. Light emitters characteristics Main characteristics for transmission purposes: 1 Central wavelength (850/1300/1550) 2 Spectrum width (at ½ power) 3 Power 4 Modulation frequency (consequence of slope) 1 Wavelength nm Power dB 3 2Power/2 4
- 18. Spectrum of a LASER or LED source 18 +5 to -10dBm LASER 1-5nm λ LED Density -15 to -25 dBm 60-100nm λ Different frequency = different wavelength = different colors
- 19. Power 19 Is the level of light intensity available for transmission Average power is the mean value of the power during modulation Power available for transmission is also function of: • Fiber core size • Numerical aperture Light entrance cone N.A. (Numerical Aperture)
- 20. Modulated frequency 20 Is the rate at which transmission changes intensity (logical 0 to 1) Rate is function of time Time is function of slope Slope is characteristic of emitter (technology) LED functions at lower frequency (longer time) LASERS at higher (shorter time) TIME influences modal bandwidth
- 21. Emitters comparison 21 Type Cost Wavelength (nm) Spectral width (nm) Modulated frequency Power (dBm) Usage LED $ 850-900 1250-1350 30-60 < 150 < 200 MHz - 10 to -30 F.O. systems Short Wavelength Lasers $$ 780 4 ≥ 1GHz +1 to -5 CD Fiber Ch. VCSEL $$ 850 1300 1 to 6 ≈ 5GHz +1 to -3 F.O. Giga speed Lasers $$$ 1300 1550 1 to 6 ≥ 5GHz +1 to -3 F.O. SM
- 22. Emitter characteristics transmission related effects 22 LED VCSEL LASER Over Filled Launch (OFL) Restricted Mode Launch (RMF) Restricted Mode Launch (RMF) • Emitters inject light into fiber under different conditions (emitter physical characteristic). Modes travel consequently Power is distributed consequently
- 23. Semiconductor Sources for Optical Communications
- 24. Considerations with Optical Sources Physical dimensions to suit the fiber Narrow radiation pattern (beam width) Linearity (output light power proportional to driving current)
- 25. Considerations with Optical Sources Ability to be directly modulated by varying driving current Fast response time (wide band) Adequate output power into the fiber
- 26. Considerations… Narrow spectral width (or line width) Stability and efficiency Driving circuit issues Reliability and cost
- 27. Semiconductor Light Sources 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. In a LASER, the photon is further processed in a resonance cavity to achieve a coherent, highly directional optical beam with narrow linewidth.
- 28. LED vs. laser spectral width Single-frequency laser (<0.04 nm) Standard laser (1-3 nm wide) LED (30-50 nm wide) Wavelength Laser output is many times higher than LED output; they would not show on same scale
- 29. Light Emission Basic LED operation: When an electron jumps from a higher energy state (Ec) to a lower energy state (Ev) the difference in energy Ec- Ev is released either as a photon of energy E = h (radiative recombination) as heat (non-radiative recombination)
- 30. Energy-Bands In a pure Gp. IV material, equal number of holes and electrons exist at different energy levels.
- 31. n-type material Adding group V impurity will create an n- type material
- 32. p-type material Adding group III impurity will create a p-type material
- 33. The Light Emitting Diode (LED) For fiber-optics, the LED should have a high radiance (light intensity), fast response time and a high quantum efficiency Double or single hetero-structure devices Surface emitting (diffused radiation) Vs Edge emitting (more directional) LED’s Emitted wavelength depends on bandgap energy /hchEg
- 34. Heterojunction Heterojunction is the advanced junction design to reduce diffraction loss in the optical cavity. This is accomplished by modification of the laser material to control the index of refraction of the cavity and the width of the junction.
- 35. The p-n junction of the basic GaAs LED/laser described before is called a homojunction because only one type of semiconductor material is used in the junction with different dopants to produce the junction itself. The index of refraction of the material depends upon the impurity used and the doping level.
- 36. The Heterojunction region is actually lightly doped with p-type material and has the highest index of refraction. The n-type material and the more heavily doped p-type material both have lower indices of refraction. This produces a light pipe effect that helps to confine the laser light to the active junction region. In the homojunction, however, this index difference is low and much light is lost.
- 37. Gallium Arsenide-Aluminum Gallium Arsenide Heterojunction Structure and index of refraction n for various types of junctions in gallium arsenide with a junction width d. (a) is for a homojunction. (b) is for a gallium arsenide-aluminum gallium arsenide single heterojunction. (c) is for a gallium arsenide-aluminum gallium arsenide double heterojunction with improved optical confinement. (d) is for a double heterojunction with a large optical cavity of width w.
- 39. Structure of a Generic Light Emitter: Double-Heterostructure Device
- 40. OPERATING WAVELENGTH Fiber optic communication systems operate in the 850-nm, 1300-nm, and 1550-nm wavelength windows. Semiconductor sources are designed to operate at wavelengths that minimize optical fiber absorption and maximize system bandwidth
- 41. LED Wavelength (eV) 2399.1 m)( E = hc/E(eV) = wavelength in microns H = Planks constant C = speed of light E = Photon energy in eV
- 42. Bandgap Energy and Possible Wavelength Ranges in Various Materials
- 43. SEMICONDUCTOR LIGHT-EMITTING DIODES Semiconductor LEDs emit incoherent light. Spontaneous emission of light in semiconductor LEDs produces light waves that lack a fixed-phase relationship. Light waves that lack a fixed-phase relationship are referred to as incoherent light
- 44. SEMICONDUCTOR LIGHT-EMITTING DIODES Cont… The use of LEDs in single mode systems is severely limited because they emit unfocused incoherent light. Even LEDs developed for single mode systems are unable to launch sufficient optical power into single mode fibers for many applications. LEDs are the preferred optical source for multimode systems because they can launch sufficient power at a lower cost than semiconductor LDs.
- 45. Semiconductor LDs Semiconductor LDs emit coherent light. LDs produce light waves with a fixed-phase relationship (both spatial and temporal) between points on the electromagnetic wave. Light waves having a fixed-phase relationship are referred to as coherent light.
- 46. Semiconductor LDs Cont.. Semiconductor LDs emit more focused light than LEDs, they are able to launch optical power into both single mode and multimode optical fibers. LDs are usually used only in single mode fiber systems because they require more complex driver circuitry and cost more than LEDs.
- 47. Produced Optical Power Optical power produced by optical sources can range from microwatts (W) for LEDs to tens of milliwatts (mW) for semiconductor LDs. However, it is not possible to effectively couple all the available optical power into the optical fiber for transmission.
- 48. Dependence of coupled power The amount of optical power coupled into the fiber is the relevant optical power. It depends on the following factors: The angles over which the light is emitted The size of the source's light-emitting area relative to the fiber core size The alignment of the source and fiber The coupling characteristics of the fiber (such as the NA and the refractive index profile)
- 49. Typically, semiconductor lasers emit light spread out over an angle of 10 to 15 degrees. Semiconductor LEDs emit light spread out at even larger angles. Coupling losses of several decibels can easily occur when coupling light from an optical source to a fiber, especially with LEDs. Source-to-fiber coupling efficiency is a measure of the relevant optical power. The coupling efficiency depends on the type of fiber that is attached to the optical source. Coupling efficiency also depends on the coupling technique.
- 50. Current flowing through a semiconductor optical source causes it to produce light. LEDs generally produce light through spontaneous emission when a current is passed through them.
- 51. Spontaneous Emission Spontaneous emission is the random generation of photons within the active layer of the LED. The emitted photons move in random directions. Only a certain percentage of the photons exit the semiconductor and are coupled into the fiber. Many of the photons are absorbed by the LED materials and the energy dissipated as heat.
- 52. LIGHT-EMITTING DIODES A light-emitting diode (LED) is a semiconductor device that emits incoherent light, through spontaneous emission, when a current is passed through it. Typically LEDs for the 850-nm region are fabricated using GaAs and AlGaAs. LEDs for the 1300-nm and 1550-nm regions are fabricated using InGaAsP and InP.
- 53. Types of LED The basic LED types used for fiber optic communication systems are Surface-emitting LED (SLED), Edge-emitting LED (ELED), and
- 54. LED performance differences (1) LED performance differences help link designers decide which device is appropriate for the intended application. 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 (wide far-field angle), they are almost exclusively used in multimode systems.
- 55. LED performance differences (2) 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. SLDs are ELED-based diodes designed to operate in the superluminescence mode. SLDs may be modulated at bit rates of over 400 Mb/s.
- 56. Surface-Emitting LEDs The surface-emitting LED is also known as the Burrus LED in honor of C. A. Burrus, its developer. In SLEDs, the size of the primary active region is limited to a small circular area of 20 m to 50 m in diameter. The active region is the portion of the LED where photons are emitted. The primary active region is below the surface of the semiconductor substrate perpendicular to the axis of the fiber. 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.
- 59. LED Spectral Width Edge emitting LED’s have slightly narrow line width
- 60. Quantum Efficiency Internal quantum efficiency is the ratio between the radiative recombination rate and the sum of radiative and nonradiative recombination rates For exponential decay of excess carriers, the radiative recombination lifetime is n/Rr and the nonradiative recombination lifetime is n/Rnr )/(int nrrr RRR
- 61. Internal Efficiency If the current injected into the LED is I, then the total number of recombination per second is, Rr+Rnr = I/q where, q is the charge of an electron. That is, Rr = intI/q. Since Rr is the total number of photons generated per second, the optical power generated internal to the LED depends on the internal quantum efficiency
- 62. External Efficiency Fresnel Transmission Coefficient 24)0( 21 21 nn nnT External Efficiency for air n2=1, n1 = n 2 )1( 1 nnext n1 n2 Light emission cone
- 63. 3-dB bandwidths Optical Power I(f); Electrical Power I2(f) 2 )2(1/)( fPfP o Electrical Loss = 2 x Optical Loss
- 64. 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 Advantages Robust Linear