EE5517: Optical EngineeringErbium Doped Fiber Laser

Wenwen Zhao, Shuvan Prashant, Taishi Zhang, Kain Lu Low, Naomi Nandakumar

National University of Singapore (NUS)

Erbium-Doped Fiber Lasers (EDFLs)

Description− Optical Glass Fiber− Glass is doped with the rare-earth

element: Erbium (Er3+ )

Importance of EDFLs− Wavelength output : 1.55μm− Lies in eye-safe region of the spectrum.− Preferred wavelength for high-power

long-distance fiber communications.

1. Pump Source Semiconductor Laser Diode 2. Active Gain Medium Doped Glass Fiber3. Optical Resonator Dielectric Mirrors, Fiber Bragg

Gratings

General – Optical Fiber Lasers

General Laser Optical Fiber Laser

Gain Medium

MirrorMirror

PumpLaser

Erbium Doped Glass Fiber

LaserGain medium

Mirror/Grating

Pump

Isolator Isolator

Mirror/GratingLaser

Diode

Material of Fiber and Rare-Earth doping

Require high doping concentrations.

High-doping leads to Clustering of Er3+ ions in pure Silica.

SiO2 (Silica) + GeO2, P2O5 and Al2O3 (network modifiers) Prevents Clustering

Glass host composition(SiO2)

Solubility of rare earth dopant (Er3+)

Lifetime, absorption, emission, and

excited state of dopant transitions.

Silica LatticeEr3+

Clustering

Network Modifiers No Clustering

Er3+ Energy Levels

5

Pump Laser980 nm

Fast Decay(non-radiative emission)

E1

E2

E3 4I11/2

4I13/2

4I15/2

Spontaneous emission

Stimulated emission

(1520-1570 nm)

Stimulated absorption

Simplified energy levels of Er3+ ions in Erbium doped fibers

Hydrolysis

• React chlorides with Hydrogen flame and collect the silica soot onto a rotating target

• Heat to around 800°C to remove OH

• Sinter to a transparent glass preform.

• Vapor Axial Deposition (VAD)• Outside Vapor Deposition

(OVD)

Oxidation

• React chlorides with Oxygen inside substrate tube that becomes part of cladding.

• Reaction, deposition, sintering simultaneously through the tube.

• Chemical Vapor Deposition• Modified(MCVD)• Plasma (PCVD)• Intrinsic Microwave (IMCVD)

Fabrication of doped Silica Fiber SiCl4, GeCl4, POCl3, SiF4 and BCl3 used

Erbium Dopants ErCl3

Fabrication and Doping Techniques

Materials

Flashback

1961 1965 1979 1985 1986

7

Snitzer sees Nd-doped glass waveguide lasing action

Woodcock Snitzer see Er and Yb doped glass lases at 1.54 um.

Robert Mears, lasing action in Nd and Er-doped fibres.

Mears makesthe first EDFA. Low loss

fiber 0.15 dB

/km at 1.5 µm

How good a laser is ?

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Laser Threshold

PowerInput PowerOutput Efficiency Slope

Laser Output

Pump Power

A

B

Slope A>Slope B

Motivation for EDFL

1.55µm region emission Silica fiber low loss window

9

Initial HurdlesProblem 1: Lasing Transition includes ground state

• Three Level Laser • Half the ion population needs to be excited • High threshold 100 mW Slope efficiency 1%

10

4I11/2

4I13/2

4I15/2

Solution :Co doping with Yb20:1 Threshold 5mW ,Slope Efficiency 8.5%(820 nm pump)

2F7/2

2F5/2

12

4I11/2

4I13/2

4I15/2

3

4

Problem 2 : Excited state

Absorption with a 0.8µm GaAs

laser pump

Excited State

Ground State

Higher Excited State

Evolution of EDFLs

No absorption at 0.98 or 1.48 µm

Semiconductor lasers were developed for this purpose

First Commercial 1.55 µm Lasers

1989 980 pumped EDFL exhibited slope efficiency of 58%

close to Quantum limit of 63%

Tunable Laser

Pump@980nm

Output

Lens Lens

Etalon( 3mm Silica Plate)

Grating

Erbium Doped Fiber

1989 Intracavity etalon between a bare fiber end and

an output mirror Linewidth reduction to 620 MHz Wavelength tuning using grating

Ring Cavity Lasers

UnidirectionalAll-Fiber CavityWavelength division multiplexing couplers60 nm tunability with 15 % slope efficiency

Distributed Bragg Reflector(DBR) Lasers

Fiber Bragg Grating

OutputErbium Doped Fiber

Pump

Fiber Bragg Grating

• Single longitudinal Mode• Narrow Linewidth (<5kHz)• Wavelength tunability by stress or

temperature

Pulsed EDFLs using modelocking

• Laser has lots of longitudinal modes

• If we locks these modes in phase

• Constructive or destructive interference gives rise to pulses.

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http://en.wikipedia.org/wiki/Mode-locking

Pulsed EDFLs Modelocking

17

State of the Art – Next gen Erbium doped lasers• Compact,• Integrated optics• Highly Stable • Threshold Tunability (μW to mW)• Short Pulse Lasers (< 1ps pulsewidth

and high peak powers 13.5 W)

• EDWAHigh Gain ( 10s of dBs)

• Broad Gain Bandwidth ( ~80 nm)• Lengths ( 1 cm to 10 cms)• Low Pump Requirement (< 10 mW)

EDW

L

EDW

A

Pressure sensorsimple configuration low threshold powerstable output power high SNR

Sensor Applications

Principle: monitor the wavelength shift from FBG with changing variable (pressure)

λB = 2neffΛ

Sensitivity: 0.12 nm/bar

Idris, S.et al Laser Physics 2010, 20 (4), 855-858.

• 11 line optical comb, channel separation at 1.56GHz• Multi-wavelength generated for DWDM system

DWDM Application Optical comb generator

Lamperski, J.,Proc. SPIE 2008, 7120 (1), 71200U.

Challenges

Poor rare earth ion solubility

Limited amplifier bandwidth from 1525-1565, 1570-1610 nm

Low Gain Flatness

Conclusion

• Erbium doped fibers have revolutionized the fiber communications by creating a source for silica low-loss window at 1.55 μm.

• Next generation erbium doped waveguide lasers will be integrated onto single chips enabling compact and efficient communications.

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References• Ainslie, B.J.; , Lightwave Technology, Journal of ,

1991, 9(2),220-227.• Mears, R. J.; Baker, S. R., Optical and Quantum

Electronics 1992, 24 (5), 517-538.• Govind P, A., Chapter 5 - Fiber Lasers. In Applications

of Nonlinear Fiber Optics, Academic Press: San Diego, 2001; pp 201-262.

• Bradley, J. D. B.; Pollnau, M., Laser & Photonics Reviews 2011, 5 (3), 368-403.

• Rare-Earth-Doped Fiber Lasers and Amplifiers, Revised and Expanded, CRC Press: 2001

• http://www.rp-photonics.com 23

Appendix

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• Central element of optical amplifiers: EDFA(in principle, any laser can be used as an amplifier by removing its mirrors)

• Suppliers: Corning, JDSU, etc..

Application (EDFA)

(c) Sergiusz Patela, optical amplifiers

Application (EDFA)

SOA EDFA

High gain (20dB) Even high gain available (30dB-40dB)

Polarization dependent- PMF required Polarization independent

High coupling loss, semiconductor to fiber

Very low coupling loss, all-fiber device

High noise figure Low noise figure

Crosstalk in WDM system WDM compatible, simultaneous amplification

Compact size, and easy integration Bulky, long fibers up to few m or km

ASE , amplified spontaneous emission

Broad operation wavelength (400nm to 2000nm )

C+L band only

Semiconductor Optical Amplifier (SOA) vs. EDFA

Mode locking Ultrafast Region

Chemical Vapor Deposition

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• The chemicals are mixed inside a glass tube that is rotating on a lathe. • They react and extremely fine particles of germano or phosphoro silicate glass are deposited on

the inside of the tube. • A travelling burner moving along the tube causes a reaction to take place and then fuses the

deposited material. • The preform is deposited layer by layer starting first with the cladding layers and followed by the

core layers.• Varying the mixture of chemicals changes the refractive index of the glass.• When the deposition is complete, the tube is collapsed at 2000 C into a preform of the purest

silica with a core of different composition.• The preform is then put into a furnace for drawing.

Outside Vapor Deposition

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• The chemical vapours are oxidised in a flame in a process called hydrolysis.

• The deposition is done on the outside of a silica rod as the torch moves laterally.

• When the deposition is complete, the rod is removed and the resulting tube is thermally collapsed.

Vapor Axial Deposition

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• The deposition occurs on the end of a rotating silica boule as chemical vapours react to form silica.

• Core preforms and very long fibres can be made with this technique.

• Step-index fibres and graded-index fibres can be manufactured this way.