Optical amplifiers and their applications. Ref: Optical Fiber Communications by: G. Keiser; 3 rd edition

Optical amplifiers and their applications Ref: Optical Fiber Communications by: G. Keiser; 3 rd edition

Optical Amplifiers Two main classes of optical amplifiers include: Semiconductor Optical Amplifiers (SOA) and Doped Fiber Amplifiers (DFA)

Basic operation of optical amplifiers

Semiconductor Optical Amplifiers Ref: Optical Fiber Communications by: G. Keiser; 3 rd edition There are two types of SOAs: --- Fabry- erot amplifiers (FA) When the light enters FA it gets amplified as it reflects back and forth between the mirrors until emitted at a higher intensity. It is sensitive to temperature and input optical frequency. ---Non-resonant traveling-wave amplifiers (TWA) It is the same as FA except that the end facets are either antireflection coated or cleaved at an angle so that internal reflection does not take place and the input signal gets amplified only once during a single pass through the device. They widely used because they have a large optical bandwidth, and low polarization sensitivity.

Non-resonant traveling-wave amplifiers (TWA) External umping External pumping injection creates population inversion similar to LASERs. The rate equations can be defined as: n( t) t R p ( t) R st ( t) n( t) τ r J ( t) R p ( t) qd is the external pumping rate, J(t) is the current density, d is the active layer thickness, and τ r is the combined time constant coming from spontaneous-carrier recombination mechanism. Rst(t) is the stimulated emission and it is equal to: R st ( t) Γav ( n n ) N g th ph gv g N ph

External umping (Cont ) where vg is the group velocity of the incident light, Г, optical confinement factor, a is the gain constant, nth is threshold carrier density, Nph is the photon density and g is the overall gain per unit of length. N ph v g s ( hv )( wd ) where s is the power of optical signal, w and d are width and the thickness of active area respectively. Under steady state condition, variation of n vs time is zero, therefore: R p R st n + τ r

External umping (cont ) Substituting for Rp and Rst and solving for g yields: g v g J nth qd τ r N + 1 /( Γaτ ) 1+ ph r N ph g 0 / N ph; sat where N ph; sat 1 Γav τ g r and g 0 Γaτ r J qd n τ th r where g o is the zero or small-signal gain per unit of length (in the absence of the signal input)

Amplifier Gain Amplifier gain or signal gain G is defined as: G s, out s, in Or as we saw in the case of laser: _ G exp Γ g m α L exp [ g( z) L] where, gm, α, and L are the material gain coefficient, the effective absorption coefficient of the material and amplifier length respectively. g(z) is the overall gain per unit of length. g(z) can written as: g ( z ) 1 + g 0 s ( z ) amp, sat go, s(z), and amp,sat are the unsaturated medium gain per unit of length in the absence of signal input, internal signal power at z, and amplifier saturation power.

Amplifier Gain The increase in the light power in incremental length of dz can be expressed as: d g( z) s ( z) dz Which can show: now L 0 g 0 dz s, out 1 ( and finally one can see that: z) + 1 s, in s amp, sat g 0 dz d 1 + s ( z) G 1+ 1 amp, sat s, in d G ln G amp, sat 0 where G o exp (g o L) is the single-pass gain in the absence of light.

Amplifier gain versus power

Erbium-Doped Fiber Optic Amplifiers Erbium energy-level diagram and amplification mechanism

EDFA configurations

EDFA ower-conversion Efficiency (CE) and Gain The input and output power of an EDFA can be expressed: λ p s, out s, in + p, in λs The ower Conversion Efficiency (CE) is defined as (always less than unity) CE s, out p, in s, in s, out p, in λ λ p s 1 We can also write the amplifier gain as: G s, out s, in 1 + λ λ p s p, in s, in

Optical Amplifiers EDFA ower-conversion Efficiency (CE) and Gain In order to achieve a specific maximum gain G, the input signal power can NOT exceed a value given by s, in λ λ p s G p, in 1

Optical Amplifiers EDFA ower-conversion Efficiency (CE) and Gain The maximum gain in a 3 level laser medium of length L can also be given as follow (in addition to pump power, the gain depends on the filter length) ( ρσ ) Gmax exp el where ρ is the rare-earth element concentration and σe is the signalemission cross section. Therefore the maximum gain or power will be defined as: G min exp ( ρσ L ) e,1 + λ λ p s p, in s, in min s ( ρσ L) p s, out, in exp e, s, in + p, in λs λ

Gain versus EDFA length

Absorption and Emission Cross-Sections in EDFA The effect of absorption and emission efficiencies in external pumping in EDFA are realized by defining new parameters called Absorption Cross-Section, σ and a Emssion Cross-Section, σ e respectively. σ a determines the pumping rate. If the pumping power is p and Er ground state population is N 0, the pumping rate is WpN0 where, σ e determines the medium gain, g σ e N2. N2 is metastable (inversion layer) population>n1 Stimulated emission rate, Rs is: Where s-in is the incident light power. Therefore the pumping gain will be: L is the length of the pump. G R p s W p V g σ ap hνa gn p out p in ph σ e 2 e s inn hνa ( σ e N 2 σ a N 0 ) L

Components for Optical Communications Materials for self-studies assive Components Couplers, Attenuators Equalizers, Isolators WDM Active Components Modulators, Diodes, Switches, Routers

Some applications of Light polarization

Optical Diode Rutile Half Wave late Faraday Rotator Calcite, Rutile Rutile Half wave plate Faraday Rotator

Transmission windows

WDM

Optical Interference

Fiber Bragg grating

Fiber Bragg grating fabrication hase Mask: Direct Imprinting 248 nm Laser hase Mask Λ M Ge doped Fiber Diffraction m -1 0 th order (Suppressed) Diffraction m +1

Reflection grating

Typical WDM Network

Simple demultiplexer function

Extended add/drop multiplexer

Tunable optical filter