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EE 230: Optical Fiber Communication Lecture 7

EE 230: Optical Fiber Communication Lecture 7. Optical Amplifiers-the Basics. From the movie Warriors of the Net. Amplifier Types and Applications. Amplifiers are used to overcome fiber loss They are used in 4 basic applications: In-line amplifiers for periodic power boosting

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EE 230: Optical Fiber Communication Lecture 7

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  1. EE 230: Optical Fiber Communication Lecture 7 Optical Amplifiers-the Basics From the movie Warriors of the Net

  2. Amplifier Types and Applications Amplifiers are used to overcome fiber loss They are used in 4 basic applications: In-line amplifiers for periodic power boosting Power Amplifier to increase the power to greater levels than possible from the source Pre-amplifier to increase the received power sensitivity Distribution loss compensation in local area or cable networks Fiber Optics Communication Technology-Mynbaev & Scheiner

  3. Characteristics of all amplifiers • They operate by creating a population inversion, where there are more individuals in a high energy state than in a lower one • The incoming pulses of signal on the fiber induce stimulated emission • They saturate above a certain signal power • They add noise to the signal

  4. Comparison of Real and Ideal Amplifier

  5. Inhomogeneous Gain Broadening Inhomogeneous broadening The individual atomic responses within and inhomogeneously broadened transition all add up to yield the measured lineshape A Gaussian inhomogeneously broadened atomic lineshape such as produced by doppler broadening in atoms Lasers-Siegman

  6. Interaction of Atoms with Light

  7. Rate Equations and Populations

  8. Unstimulated Population densities in 2 ‘ level atom Energy levels 1 and 2 and their decay times. By means of pumping, the population density of level 2 is increased at the rate R2 while that of level 1 is decreased at the rate R1 Idealy t21~tsp<<t20 so t2~tsp For large DN or No (also called inversion density) We want t2 long, but t21 not too small, t1 and R1 large

  9. Population densities with a strong resonant signal

  10. Ideal Amplifier System Third excited state with very short lifetime, no fluorescence Second excited state with very long lifetime and high cross section for stimulated emission Pump process with large cross section Energy gap between first and second excited states matches telecommunication frequencies First excited state with very short lifetime

  11. Amplified Spontaneous Emission

  12. Noise Figure Measurement Fiber Optics Communication Technology-Mynbaev & Scheiner

  13. Noise Figure

  14. 3 main types and 3 Big Ideas • The main types of optical amplifiers are: • Semiconductor amplifiers (lasers that aren’t lasing) • Doped fiber amplifiers • Raman and Brillouin Amplifiers • The three big ideas • Gain and gain bandwidth • Gain saturation • Noise and noise figure

  15. Laser Amplifiers

  16. Semiconductor Optical Amplifiers Fiber Optics Communication Technology-Mynbaev & Scheiner

  17. Types of SOA • Fabry-Perot Amplifier • High gain but non-uniform gain spectrum • Traveling wave amplifier • Broadband but very low facet reflectivities are needed • Gain as a function of frequency • Ripples are caused by the cavity modes • The overall gain curve is due to the width of the atomic transition in the semi-conductor Fundamentals fo Multiaccess Optical Fiber Networks Dennis J. G. Mestgagh

  18. Amplifier Bandwidths Comparison of the bandwidths of Fabry Perot and Traveling wave amplifiers Fiber Optics Communication Technology-Mynbaev & Scheiner

  19. Traveling Wave SOA To make a traveling wave Semiconductor Optical Amplifier the Fabry-Perot cavity resonances must be supressed. To accomplish this the reflectivity must be reduced. Three approaches are commonly used: Anti-reflection coating Tilted Active Region Use of transparent window regions Fiber Optics Communication Technology-Mynbaev & Scheiner

  20. Saturation Power Semiconductor Optical amplifiers saturate silmilarly to a 2 level atom The typical saturation output power for SOAs is around 5-10 mW Gain saturation and saturation power Fiber Optics Communication Technology-Mynbaev & Scheiner

  21. Crosstalk in Semiconductor Amplifiers Rate equation for pump current If Φ suddenly goes to zero, as in 1-0 sequence, Time constant is (ns) If Φ suddenly turns on, which is smaller

  22. Parameters on previous slide • N=carrier density (cm-3) • I=pump current (amp=coul/s) • q=charge on electron (coul) • L,w,d=cavity dimensions (cm3) • =recombination lifetime (s) • =confinement factor (unitless) • =photon density (cm-3) • a=gain coefficient (cm-1)

  23. Crosstalk in semiconductor amplifiers If time constant for spontaneous decay of excited state is shorter than the bit duration, the population of the excited state will vary sharply with the optical power in the fiber, and gain will depend on the fraction of 1s and 0s in the data stream. If time constant is long, then the population in the excited state will be constant, dependent upon the pump power but not the signal power.

  24. Reduction of Polarization Dependence • Three main approaches • Connect the amplifiers in series • Residual facet reflectivity • can cause undesired coupling between amplifiers resulting in poor noise and dynamic performance • Connect them in parallel • Good solution but complex • Double pass with polarizaion • rotation • Automatic 6 db loss due to coupler Fiber Optics Communication Technology-Mynbaev & Scheiner

  25. Undesired effects in an SOA • Cross saturation can cause undesired coupling between channels • This can be used for wave length conversion and “controlling light with light” • If used for multiple channels in a switched network gain must be adjusted as channels are added and dropped • Four wave mixing is also quite pronounced in SOAs • Causes undesired coupling of light between channels • Can however also be used to advantage in wavelength converters. • High coupling loss • Polarization sensitive gain Fiber Optics Communication Technology-Mynbaev & Scheiner

  26. Short Pulse Amplification in SOAs

  27. Semiconductor amplifier advantages • Are the right size to be integrated with waveguide photonic devices (short path length requirement) • Can easily be integrated as preamplifiers at the receiver end • Use same technology as diode lasers • Gain relatively independent of wavelength • Are pumped with current, not another laser

  28. Semiconductor amplifier disadvantages • Polarization dependence • Self-phase modulation leading to chirp • Cross-phase modulation • Four-wave mixing and crosstalk • Extremely short (ns) excited state lifetimes

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