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

Amplifier Types and Applications Fiber Optics Communication Technology-Mynbaev & Scheiner 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

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

Comparison of Real and Ideal Amplifier

Inhomogeneous Gain Broadening Lasers-Siegman 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

Interaction of Atoms with Light

Rate Equations and Populations

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 R 2 while that of level 1 is decreased at the rate R 1 For large  N or N o (also called inversion density) We want  2 long, but  21 not too small,  1 and R 1 large Idealy  21 ~  sp <<  20 so  2 ~  sp

Population densities with a strong resonant signal

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

Amplified Spontaneous Emission

Noise Figure Measurement Fiber Optics Communication Technology-Mynbaev & Scheiner

Noise Figure

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

Laser Amplifiers

Semiconductor Optical Amplifiers Fiber Optics Communication Technology-Mynbaev & Scheiner

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

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

Traveling Wave SOA Fiber Optics Communication Technology-Mynbaev & Scheiner 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

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

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

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 (cm 3 )  =recombination lifetime (s)  =confinement factor (unitless)  =photon density (cm -3 ) a=gain coefficient (cm -1 )

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.

Reduction of Polarization Dependence Fiber Optics Communication Technology-Mynbaev & Scheiner 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

Undesired effects in an SOA Fiber Optics Communication Technology-Mynbaev & Scheiner 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

Short Pulse Amplification in SOAs

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

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