EE5517: Optical Engineering Erbium Doped Fiber Laser Wenwen Zhao, Shuvan Prashant, Taishi Zhang, Kain Lu Low, Naomi Nandakumar National University of Singapore (NUS)
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 ?
8
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.
22
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.