ampli laser
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Fiber lasers and amplifiersFiber lasers and amplifiers
Optical fibers - losses
3 low loss windows for telecommunication850 nm, 1300 nm, 1550 nm.
Optical fibers - dispersion
Motivation
• The development of optical fibers doped with rare earth ions has been initially pushed by the optical telecommunication field
in-line optical amplification
Transmitter
Receiver
A ATX RX
DL AM
1 1 0 1
Electro-optical regeneration
• Until years ‘80: electrical regeneration
LASER S.C.
ELECTRICSIGNAL
PHOTODETECTOR
ELECTRICSIGNAL
L
OPTICAL FIBER
E/O CONV. O/E CONV.
TX RX
TX RX TX RX TX RXCONV. E/OCONV. O/E
Long distance trasmission:
Electro-Optical regeneration
Drawbacks:• expensive• limited bit-rate• the optical link cannot be upgraded in terms of:
wavelengthbit ratesignal format
Optical amplification
• Semiconductor optical amplifier (SOA):
– low compatibility with optical fibers– performances not optimized for in-line amplification
Doped Fibers
• Doped fibers (‘80) with rare earths ions
– Erbium (Er3+) 1550 nm
– Praseodimium (Pr3+)
– Neodimium (Nd3+)1300 nm
•Silica matrix completely compatible with in-line optical fibers
•Low cost
Rare earths
• Electronic configuration[Xe] 6s2 4f N with 0 < N < 14
ion 3+: the dopant looses 2 electrons s and 1 electron f
Optical transitions involve levels of the f orbital:• Internal orbitals : the influence of the external local
fields is minimised
Table of the elements
Doping
dopant concentration: ~100 ppmImportance of mantaining the properties of passive optical fibers• Low loss• NA• Compatibility of the silica matrices
PROBLEM: Low solubility - clustering
Trade off:Trade off:concentration low amplificationconcentration crystallization (loss)
clustering - gain quenching (nonradiative disexcitation)
• First optimized doped fiber (1985): ErbiumErbiumUniv. Southampton + Pirelli Cables (Milan)
Erbium doped fiber
ground level
amplificationupper level ( τ = 1-3 ms)
pumping level
980 nm(InGaAs)
1480 nm(InGaAsP) 1550 nm
non radiativedecay
4I11/2
4I13/2
4I15/2
Intensity Gain α(ν) = σ21N2-σ12N1
Φ (z)= Φοe α(ν) z
Gain
σ21σ12
α(ν) ∝ σ21N2-σ12N1 Gain shape depends on population inversion
n2 = N2 /Nt
Gain bandwidth
Wavelength (nm)
Gai
n (d
B)
30 nm bandwidth
the broadening is due to the host field
Problem:gain flattening
Amplification bandwidth co-dopant
Codopants lead to a spectral change of the amplification band
Aluminum doping leads to gain broadening and flattening
Amplification in the fiber
pump
signal
amplifiedsignal
Optimal length depends on doping level and pump power
Pump and signal monomode: good spatial overlap
standard fibers, Pump 150-200 mW , Lopt 10-15 m
pump
signal
amplifiedsignal
too long too short
Exponential decay
NO INVERSION
Optical amplifiers (EDFA) scheme
PUMPLASER
980 nm
1550 nmopticalsignal 980+1550 nm
WDM(980-1550 nm)
Doped fiber Er3+
(10-20 m)opticalisolator
splices
opticalisolator amplified
optical signal
Alternative schemes: co- and counter propagating pump
PUMPLASER
WDM(980-1550 nm)
Counter propagating scheme
Optical components
WDM wavelength divisionmultiplexer
• fused fibers• micro-optics
λ1 , λ2
λ2
λ1
Isolator light can betransmitted
only in one direction
Amplification regime
PP
Ps Pout=α PP= Pase + GPsG
•quantum efficiency η = hνs/hνp•absorption cross section
When Ps increasesGAIN SATURATION
G = Go/(1+Ps/Psat)
ASEAmplified
spontaneousemission
Performances
• Gain 30-40 dB (103-104) with pump 100-400 mW
• Output power up to 23 dBm• Bandwidth 30-40 nm
The in-line amplifiers usually work in the saturated regime: • the output power is almost constant independently from the input power• the ASE power is reduced
EDFA applications
• in line amplifier
• booster
• optical preamplifier
• loss compensation
TX RX
TX
RX
N fibers
New generation fibers
New optical amplifiers
RAMAN amplification
C- band
L-bandS-band
C - band 1530 – 1570 nmL - band 1570 –1635S – band
L-band amplifiers
The long wavelength band (L-band) is defined approximately as 1570 nm to 1605 nm. This wavelength range encompasses only the tail of the erbium gain band
Gain/loss spectra at different inversion levels
Very low gain can be obtained.Long spans of fibers are needed
Prototype optical amplifiers
Tellurium: TeO2Thulium commonly a compound ofthulium and fluoride TmF3
Standard optical amplifier
Bench top amplifier
High power amplifiers
WDM systems Splitter 1× N
A
High power amplifiers
Issues: higher power levels for optical communicationhigh power systems (lasers)
Outline:-Yb3+ and Er3+ Yb3+ doped fibers-Novel fiber structures
Yb3+ fiber
• Highly efficient amplification system in silica host (no concentration quenching)
• 2 level systemNon radiative decay
900 nm 980 nm1100 nm
emissionabsorbtion
Cross-sections
Er Yb fiber
Yb Er
resonant transfer
Yb: high absorbtion cross sectionimproved pumping efficiency
Clustering reduction
Fiber length reduction!
920 nm
1550 nm
Yb3+
Yb3+
Yb3+
Yb3+
Yb3+
Yb3+
Yb3+
Er3+
Problem: high power pumping
• To obtain high power output (several Watts) high pump powers are needed
Available lasers: solid state lasers low efficiencyvery cumbersomenot tunable
semiconductor lasers or arrayHigh efficiencyWavelength selectableHigh output power (> 100 W)
Problem : coupling to a single mode fiber
Coupling
• Coupling is optimized when the beam is spatially single mode, has gaussian shape and divergence similar to the fiber NA
High power semiconductor lasers:spatially multimodal, high divergence,astigmatic beam.Coupling can be of theorder of a few %
Good optical couplingGood matching beetween the beam and the mode profile
Cladding pumped fibers
0.075
0.060
Δn
8 μm
300 μm
doped core
internalcladding
externalcladding
doped core
externalcladding
internalcladding
The pump is efficiently coupled in the multimodal internalcladding and it progressively transferred in the doped core
Optimized geometries
• Maximum super position between the core and the pump fields
low pump absorption Fiber stretching: optical modes scrambling
High power Er Yb amplifiers
High power Er Yb amplifiers
1300 nm amplifiers
• Interesting materials :NeodimiumDisprosiumPraseodimium best results in ZBLAN
Silica host is not suitable: phononic interaction: non radiative disexcitation
New hosts : Fluoride glasses (ZBLAN Zirconium Barium, Lanthanum, Alluminium, Nitrogen)
Chalcogenide glasses LaGaS, GeGaS.
Fiber lasers
• First motivation:Transmitter for optical communication
CW
ML
AM
AM
Transmissionline
NOW:•CW high power lasers•pulsed laser for laboratory applications
Fiber cavities
Output coupler
pump
WDM
ErEr
Bragg gratings
Bragg gratings
pumpOutputsignal
RING CAVITY Fabry Perot CAVITY
Advantages with respect to solid state lasers:Compact, low cost, high efficiency
Mode locked ring cavities
• Active and passive mode locked laser are commercially available for laboratory applications
• Too expensive for TLC• The stability is not sufficiently optimised
• very compact, efficient
Fiber Bragg gratings
• Fundamental break-throughEr
External mirror
External mirror
lenslens
Modulation of the core refractive index
n1 n1 n1n2n2
Λ The light is reflected whenconstructive interference occurs
λ = 2neffΛwavelength selective mirrors
Bandwidth 0.1 - 2 nmR up to 100%
Bragg gratings fabrication
UV laser (200 nm)
Phase mask
fiber
•Refractive index of germanium-doped silica core can be changed by exposure to UV light
•A refractive index change Δn 10-3 - 10-6 can be obtained
•The physical phenomena is not completely understood - phorefractivity
Excimer laser or doubled frequency Ar laser
zΛ
Fibre sensitisation
• Low refractive index change in standard silica fibers.• To enhance photosensitivity:
– increase of germanium concentration. The fibre become incompatible with standard ones -
• losses - different field diameters- lossy splices– different co-dopants have been tried with varying success– best results: diffusion of molecular hydrogen into the fiber
core at high pressure. Δn > 10-2 has been obtained
High power fiber lasers
• Fiber gratings + cladding pumped fibers• Most efficient laser Ytterbium doped fiber• Pumping with diode array diode
Big break- through:• high efficiency (low power consumption)• 70-80 % against a few % in solid state lasers• air cooling: very high value of the ratio surface/volume
Amplification in Yb fiber
3 level system 4 level system
High quantum efficiencyBroad pumping band (800 - 1064) - Semiconductor lasers
920 nm 980 nm 920 nm 1020 nm
Yb fiber laser
• Prototype models : up to 100 W power output.• Commercially available up to 15 W
• Important feature: the output is spatially single mode
Cladding pumpedYb fiber
Bragg gratings
Bragg gratings
pumpOutputsignal
The laser is pumped at 920 nmby semiconductor array
diode lasers
The output wavelength is selected by the Bragg gratings
Yb fiber lasers
Yb fiber lasers
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