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Nonlinear Photonics and High Power Lasers Laboratory, CeNSE, IISc 1
Fiber Lasers: Fundamentals and Applications
Lecture 2
V R Supradeepa
Center for Nano Science and Engineering (CeNSE)
Indian Institute of Science
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Nonlinear Photonics and High Power Lasers Laboratory, CeNSE, IISc 2
Current State of CW Fiber Lasers
Eye Safety, Atmospheric
Transparency
10000W
300W~1000W
~100W
Ytterbium
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Nonlinear Photonics and High Power Lasers Laboratory, CeNSE, IISc 3
Gain from doped fibers
Population inversion achieved in the rare-earth dopants due to
pump absorption.
This is then utilized to amplify signal
Most common doped fibers you would interact with –
• Ytterbium doped (Yb) – Used in most high power lasers
• Erbium doped (Er) – Optical communications, Eye safe
applications
• Thulium (Th) and Holmium doped – Mid IR applications
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Nonlinear Photonics and High Power Lasers Laboratory, CeNSE, IISc 4
Pump Absorption in Double Clad fibers
Net absorption ~ core absorption*(core area)/(cladding area)
(wavelength dependent)
Some numbers – core abs (Yb doped fiber at 975nm)
~700dB/m, core, cladding dimensions – 6/125 micron
Net absorption ~ 1.6dB/m (at 975nm)
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Nonlinear Photonics and High Power Lasers Laboratory, CeNSE, IISc 5
Absorption and Emission in Rare earth Doped
Fibers
Yb doped fiber Er doped fiber
Th doped fiberFundamental Conversion
Efficiency
Pump wavelength/Signal
Wavelength
Reduced quantum
efficiency = high heat load
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Nonlinear Photonics and High Power Lasers Laboratory, CeNSE, IISc 6
Laser operation – Energy levels
Radiative and Non-radiative transitions
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Nonlinear Photonics and High Power Lasers Laboratory, CeNSE, IISc 7
4-level laser systems
From encyclopedia of laser physics and technology
Occupation of state 4
and state 2 zero
Low threshold laser
Efficiencies not as high
Power scaling limited
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Nonlinear Photonics and High Power Lasers Laboratory, CeNSE, IISc 8
3-level laser systems
From encyclopedia of laser physics and technology
Occupation of state 4 is
0
medium threshold laser
Efficiencies medium
Good power scaling
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Nonlinear Photonics and High Power Lasers Laboratory, CeNSE, IISc 9
Quasi 4-level laser systems
From encyclopedia of laser physics and technology
Performance in between
3 level and 4 level
systems
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Nonlinear Photonics and High Power Lasers Laboratory, CeNSE, IISc 10
Quasi 3-level laser system
pump Signal
High threshold laser
Efficiencies high
Excellent power scaling
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Nonlinear Photonics and High Power Lasers Laboratory, CeNSE, IISc 11
2-level system
A two level system cannot become a laser because population inversion
cannot be higher than 50%
pump Signal
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Nonlinear Photonics and High Power Lasers Laboratory, CeNSE, IISc 12
Nd doped fiber
From encyclopedia of laser physics and technology, fibercore
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Nonlinear Photonics and High Power Lasers Laboratory, CeNSE, IISc 13
Yb doped fiber
From encyclopedia of laser physics and technology, fibercore
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Nonlinear Photonics and High Power Lasers Laboratory, CeNSE, IISc 14
Er doped fiber
From encyclopedia of laser physics and technology, fibercore
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Nonlinear Photonics and High Power Lasers Laboratory, CeNSE, IISc 15
Th doped fiber
From encyclopedia of laser physics and technology, thorlabs
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Nonlinear Photonics and High Power Lasers Laboratory, CeNSE, IISc 16
Absorption and Emission Cross-sections
For 3 level and 4 level systems, the absorption and emission cross sections are
measured using experiments
For quasi 3-level system – Mccumber relations
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Nonlinear Photonics and High Power Lasers Laboratory, CeNSE, IISc 17
Rate Equations
(On the white board)
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Nonlinear Photonics and High Power Lasers Laboratory, CeNSE, IISc 18
Pump and Signal Power Equations
(On the white board)
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Nonlinear Photonics and High Power Lasers Laboratory, CeNSE, IISc 19
Gain
From encyclopedia of laser physics and technology
Er doped fiber
Yb doped fiber