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