1 / 71

UNIT III SOURCES AND DETECTORS

UNIT III SOURCES AND DETECTORS. Direct Band Gap Semiconductors. Indirect Band Gap Semiconductors. Light-Emitting Diodes (LEDs). For photonic communications requiring data rate 100-200 Mb/s with multimode fiber with tens of microwatts, LEDs are usually the best choice.

danielmorton
Download Presentation

UNIT III SOURCES AND DETECTORS

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. UNIT III SOURCES AND DETECTORS

  2. Direct Band Gap Semiconductors

  3. Indirect Band Gap Semiconductors

  4. Light-Emitting Diodes (LEDs) • For photonic communications requiring data rate 100-200 Mb/s with multimode fiber with tens of microwatts, LEDs are usually the best choice. • LED configurations being used in photonic communications: 1- Surface Emitters (Front Emitters) 2- Edge Emitters

  5. LED PRINCIPLE OF OPERATION • In FOC LEDS must have • high radiance o/p or brightness • Fast emission response time • High Quantum Efficiency RADIANCE Measure of optical power radiated into a unit solid angle per unit area of the emitting surface.(unit is Watts.)

  6. Emission Response Time Time delay between application of a current pulse and respective optical emission. Quantum efficiency Related to fraction of electron hole pairs recombine radiatively.

  7. LED structures • For high radiance and quantum efficiency The LED must have Optical confinement Achieve high level Radiative Recombination in the active region of the device-yield high quantum efficiency Carrier confinement Preventing absorption of the emitted radiation by the material surrounding the PN junction.

  8. HETRO JUNCTION STRUCTURE • Used to achieve Carrier and optical confinement • Consists of two adjoining semiconductor with different band gap energies • Band gap energy difference of adjacent layers confines charge carriers. • RI differences of adjoining layers confines optical field to the central active layer.

  9. Cross-section drawing of a typical GaAlAs double heterostructure light emitter. In this structure, x>y to provide for both carrier confinement and optical guiding. b) Energy-band diagram showing the active region, the electron & hole barriers which confine the charge carriers to the active layer. c) Variations in the refractive index; the lower refractive index of the material in regions 1 and 5 creates an optical barrier around the waveguide because of the higher band-gap energy of this material. [4-3] Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000

  10. LED CONFIGURATION • Surface emitters LED • Data rates above 1000bps • Light emitting region perpendicular to fiber axis • Edge emitters LED • Emit more directional light than SELED • Homo structure • Use an etched well in GaAs substrate in order to prevent heavy absorption. • Double hetero junction structure • Giving increased efficiency from optical and electrical confinement

  11. Surface-Emitting LED • High radiance etched well is 0.8 to 0.9 um • Due to large band gap in confining layer- very low internal absorption. • GaAs is used in well- to avoid heavy absorption of emitted light. • Circular active area of surface emitter is 50 um dia and 2.5um thick. • Emission pattern is isotropic with 120 half power beam width. Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000

  12. LAMBERTIAN PATTERN • Isotropic pattern from surface emitter. • Source equally bright in all directions-view • It decides coupling efficiency • Source equally bright but power dismisses cosᶿ. • Power 50% down in its peak when ᶿ= 60. • So total half power beam width is 120

  13. Coupled power • The power coupled to MMSI is PC =π(1-r)A RD(NA)2 R- fresenel coefficient of fiber surface A- emission area of the source RD - radiance of the source • They allow more power to be coupled- more difficult, expensive.

  14. DH Edge Emitter LED • Emit more directional light • Reduce lose by absorption and more directional –light collected from edge. • Has transparent guiding layers with very thin active layer of 50 um to 100 um – reducing self absorption. • Guiding layer RI < surrounding material(core&cladding)> outer surrounding material. • Form wave guide channel that directs the optical radiation towards fiber. • To match fiber core diameter (50 to 100um) the contact stripes for the edge emitters are 50-70um wide. • In the plane parallel to the junction no waveguide effects. • In the plane highly directional perpendicular to the junction-when half power beam width is 25 to 35 by proper choice of waveguide thickness.

  15. Edge-Emitting LED Schematic of an edge-emitting double heterojunction LED. The output beam is lambertian in the plane of junction and highly directional perpendicular to pn junction. They have high quantum efficiency & fast response. Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000

  16. Full Width Half Power Maximum • Width of spectral pattern and its half power point. • Fundamental quantum relation E=hv = h (c/λ) V=c/λ λ= hc/E • Peak emission wavelength λ(um)=1.240/Eg(eV)

  17. Light Source Material • Most of the light sources contain III-V ternary & quaternary compounds. • by varying x it is possible to control the band-gap energy and thereby the emission wavelength over the range of 800 nm to 900 nm. The spectral width is around 20 to 40 nm. • By changing 0<x<0.47; y is approximately 2.2x, the emission wavelength can be controlled over the range of 920 nm to 1600 nm. The spectral width varies from 70 nm to 180 nm when the wavelength changes from 1300 nm to 1600 nm. These materials are lattice matched.

  18. Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000

  19. Spectral width of LED types Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000

  20. Quantum efficiency Intro.. • Excess of electrons and holes in p and n type material created in semiconductor by carrier injection at device contacts. • Excess carrier can be recombine radiatively or non radiatively. • Total carrier generated rate is sum of externally supplied and thermally generated rates. • Externally generated rate = j/qd j – current density q – electron charge • d – thickness of region Themally generation rate = n/Ԏ Ԏ- carrier life time,N – excess carrier

  21. Rate equations, Quantum Efficiency & Power of LEDs • When there is no external carrier injection, the excess density decays exponentially due to electron-hole recombination. • n is the excess carrier density, • Bulk recombination rate R: • Bulk recombination rate (R)=Radiative recombination rate + nonradiative recombination rate [4-4] [4-5]

  22. With an external supplied current density of J the rate equation for the electron-hole recombination is: [4-6] In equilibrium condition:dn/dt=0 [4-7]

  23. INTERNAL QUANTUM EFFICIENCY & OPTICAL POWER • Fraction of electron and holes recombine radiatively • Ratio of radiative recombination to total recombination [4-8] Optical power generated internally in the active region in the LED is: [4-9]

  24. External Quantum Eficiency • In order to calculate the external quantum efficiency, we need to consider the reflection effects at the surface of the LED. If we consider the LED structure as a simple 2D slab waveguide, only light falling within a cone defined by critical angle will be emitted from an LED. [4-10]

  25. [4-11] [4-12] [4-13] [4-14]

  26. Modulation of LED • The frequency response of an LED depends on: 1- Doping level in the active region 2- Injected carrier lifetime in the recombination region, . 3- Parasitic capacitance of the LED • If the drive current of an LED is modulated at a frequency of the output optical power of the device will vary as: • Electrical current is directly proportional to the optical power, thus we can define electrical bandwidth and optical bandwidth, separately. [4-15] [4-16]

  27. [4-17] Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000

  28. LASER (Light Amplification by the Stimulated Emission of Radiation) • Laser is an optical oscillator. It comprises a resonant optical amplifier whose output is fed back into its input with matching phase. Any oscillator contains: 1- An amplifier with a gain-saturated mechanism 2- A feedback system 3- A frequency selection mechanism 4- An output coupling scheme • In laser the amplifier is the pumped active medium, such as biased semiconductor region, feedback can be obtained by placing active medium in an optical resonator, such as Fabry-Perot structure, two mirrors separated by a prescribed distance. Frequency selection is achieved by resonant amplifier and by the resonators, which admits certain modes. Output coupling is accomplished by making one of the resonator mirrors partially transmitting.

  29. LASER • Single wavelength- related to molecular characteristics of material • Lasing medium – gas,liquid,insualting crystal

  30. Principle of operation • Two energy levels E1(lower) and E2(higher) • Three main process for laser action: 1- Photon absorption photon with energy E2-E1 incident on the atom in E1,atom excited into E2 through absorption of photon 2- Spontaneous emission Atom return to E1 by random manner. 3- Stimulated emission Photon have equal energy b/w two states(E1-E2) interact with atoms causing it to lower state with creation of second photon. Give coherent radiation. Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000

  31. Lasing in a pumped active medium • In thermal equilibrium the stimulated emission is essentially negligible, since the density of electrons in the excited state is very small, and optical emission is mainly because of the spontaneous emission. • Stimulated emission will exceed absorption only if the population of the excited states is greater than that of the ground state. This condition is known as Population Inversion. • Population inversion is achieved by various pumpingtechniques. • In a semiconductor laser, population inversion is accomplished by injecting electrons into the material to fill the lower energy states of the conduction band.

  32. ILD ADVANTAGES • Coherent light • Less coupling loss • High o/p power • Used High bit rates • Monochromatic light • Good spatial coherence DISADVANTAGES • 10 times expensive than LED • Shorter life time due to high power operation • Temperature dependent

  33. SEMICONDUCTOR INJECTION LASER • BW Greater than 200Mhz. • Its have response time less than 1ns • HOptical BW 2nm or less • igh coupling efficiency • Multilayered • Smaller temp dependence

  34. Fabry-Perot Resonator [4-18] R:reflectance of the optical intensity, k: optical wavenumber

  35. Hetero junctions • Semiconductor lasers cleaved facets are used instead of external mirrors • Reflectivity Rm=(n-1/n+1)^2 • In DH structures-carrier and optical confinement reduces threshold currents for lasing by a factor 100 • DH forward bias applied by +ve to p & -ve to n. • Voltage correspond to band gap energy applied –large no of es are injected into active layer lasing commences. • Small amount of energy required for operation of laser. • Realized only by laser pumped above threshold • Threshold current- current needed to reach threshold

  36. Fabry Perot resonator cavity for Laser Diode • Laser diode is an improved LED, in the sense that uses stimulated emission in semiconductor from optical transitions between distribution energy states of the valence and conduction bands with optical resonator structure such as Fabry-Perot resonator with both optical and carrier confinements. Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000

  37. Cavity much smaller • 250 – 500 um long • 5 – 15 um wide • 0.1 – 0.2 um Thick

  38. Laser Diode Characteristics • Nanosecond & even picosecond response time (GHz BW) • Spectral width of the order of nm or less • High output power (tens of mW) • Narrow beam (good coupling to single mode fibers) • Laser diodes have three distinct radiation modes namely, longitudinal, lateral and transverse modes. • In laser diodes, end mirrors provide strong optical feedback in longitudinal direction, so by roughening the edges and cleaving the facets, the radiation can be achieved in longitudinal direction rather than lateral direction.

  39. Modes of cavity • The radiation with in the resonance cavity of a laser diode set up a pattern of electric and magnetic lines • Modes • TE modes • TM modes Each modes described in lateral, longitudinal and transverse.

  40. Longitudinal modes • Related to length L of the cavity. • Much larger then lasing wavelength approximately 1um • Longitudinal modes • Lin in the plane of p-n junction. • Depends on side wall preparation, width of cavity. • Transverse modes • Associated with electromagnetic field and beam profile perpendicular to the plane of p-n junction.

  41. Laser Operation & Lasing Condition • To determine the lasing condition and resonant frequencies, we should focus on the optical wave propagation along the longitudinal direction, z-axis. The optical field intensity, I, can be written as: • Lasing is the condition at which light amplification becomes possible by virtue of population inversion. Then, stimulated emission rate into a given EM mode is proportional to the intensity of the optical radiation in that mode. In this case, the loss and gain of the optical field in the optical path determine the lasing condition. The radiation intensity of a photon at energy varies exponentially with a distance z amplified by factor g, and attenuated by factor according to the following relationship: [4-19] LASING – Light amplification possible in laser diode

  42. [4-20] Z=0 Z=L [4-21] Lasing Conditions: [4-22]

  43. Optical output vs. drive current Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000

  44. Threshold gain & current density [4-23] For laser structure with strong carrier confinement, the threshold current Density for stimulated emission can be well approximated by: [4-24]

  45. External quantum efficiency • Number of photons emitted per radiative electron-hole pair recombination above threshold, gives us the external quantum efficiency. • Note that: [4-29]

  46. Threshold current Density & excess electron density • At the threshold of lasing: • The threshold current needed to maintain a steady state threshold concentration of the excess electron, is found from electron rate equation under steady state condition dn/dt=0 when the laser is just about to lase: [4-26] [4-27]

  47. Semiconductor laser rate equations • Rate equations relate the optical output power, or # of photons per unit volume, , to the diode drive current or # of injected electrons per unit volume, n. For active (carrier confinement) region of depth d, the rate equations are: [4-25]

  48. DFB(Distributed FeedBack) Lasers • In DFB lasers, the optical resonator structure is due to the incorporation of Bragg grating or periodic variations of the refractive index into multilayer structure along the length of the diode. Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000

More Related