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Magneto Optics
Application in Faraday isolators
Markus Fegelein
Product Manager Business Unit Crystal Technology
LINOS Photonics GmbH & Co. KG
Author: Markus Fegelein, Product Manager Crystal Technology │ LASER 2009, Munich
│
Slide 1
Lecture forum
Faraday Isolator: Principle Of Operation
Faraday Isolators are optical components,
which allow light to travel in only one
direction.
In principle the function of an optical isolator
is analogous to that of an electrical diode.
However the principle is completely different
to that of a semi-transparent mirror.
Laser
Faraday
Isolator
Laser
Faraday
Isolator
Quelle 5
Author: Markus Fegelein, Product Manager Crystal Technology │ LASER 2009, Munich
│
Slide 2
Lecture forum
Physical Principle – The Faraday Effect
Magneto optic effect
Michael Faraday discovered the MO effect in
1845.
Rotation of polarization by magnetic field
The plane of polarization of light is rotated when
passing through a transparent material which is
subjected to a strong magnetic field parallel to
the direction of propagation.
Source 1
Circular birefringence
The Faraday effect causes waves to be
decomposed into two circularly polarized rays
which propagate at different speeds.
β = V ⋅B ⋅d
V = Verdet constant
B = Magnetic field
d = length of the faraday media
Author: Markus Fegelein, Product Manager Crystal Technology │ LASER 2009, Munich
│
Slide 3
Lecture forum
Physical Principle – The Faraday Effect
Parallel propagation
For light propagating parallel to the magnetic field, a positive Verdet constant corresponds to
a counter clockwise rotation.
Non-reciprocal
Unlike optical activity in crystals (i.e. wave retarders) this effect is non-reciprocal.
Anti-parallel propagation
For light propagating anti parallel to the magnetic field, a positive Verdet constant
corresponds to a clockwise rotation.
Accumulation of rotation angle
If light is passed through the Faraday media and reflected back through it, the rotation
doubles.
Source 2
Author: Markus Fegelein, Product Manager Crystal Technology │ LASER 2009, Munich
│
Slide 4
Lecture forum
Faraday Isolator – Principle Of Construction
Correct dimensioning of all components
Choose crystal material, crystal length and magnetic field to give the polarization a 45°rotation.
Adjustment of output polarizer
Polarizer B (tilted by 45°) enables the light with rotated polarization to pass.
Polarizing refelcted light
Reflected light passing coming in through the “output polarizer” becomes polarized at 45°.
Accumulation of rotation angle
The Faraday Rotator again
rotates the polarization by 45°.
Blocking back travelling light
back travelling light is polarized
horizontally when leaving the
faraday rotator and blocked by
“input polarizer A”.
Source 2
Author: Markus Fegelein, Product Manager Crystal Technology │ LASER 2009, Munich
│
Slide 5
Lecture forum
Faraday Isolators – Technical Realization
The faraday media (mostly TGG) is placed in a rare earth ring magnet.
Inside the magnet the magnetic field is very strong and homogeneous.
Thus the crystals can be kept short and the rotation is uniform over the whole aperture.
Polarizing beam splitting cubes deflect the blocked light.
This ensures thermal stable operation even at higher laser power levels.
Blocked light can be used for other applications: i.e. feedback control circuits.
polarizer
compact design
design
compact
small insertion
insertion loss
loss
small
S
N
polarizer
polarizer
maximum
maximum
extinction
extinction
maximum
maximum
all
t
transmission
a
t
s
transmission
ys
-ccrry S
G
S
GG
TG
T
N
N
Faraday-rotator
Faraday-rotator
large aperture
aperture
large
high isolation
isolation
high
Author: Markus Fegelein, Product Manager Crystal Technology │ LASER 2009, Munich
│
Slide 6
Lecture forum
Faraday Isolators - Applications
Protection of solid state laser resonators from
+ feedback
+ damage
Protection of diode lasers from
+ feedback
+ damage (low damage threshold of diodes!)
Prevention of parasitic oscillation in multistage solid state amplifiers
+ Decoupling master laser from amplifier
+ Incoupler for injection lock-in
Author: Markus Fegelein, Product Manager Crystal Technology │ LASER 2009, Munich
│
Slide 7
Lecture forum
Faraday Isolators – Engineering Challanges
Verdet Constant
Materials with high Verdet constant keep isolators
small and enable good optical properties.
TGG offers highest Verdet constant for most laser
lines.
Verdet constant decreases with increasing
wavelength.
IR-Isolators need long crystals and large magnets.
V [°/(T*mm)]
TGG Transmission Spectrum (dmr: 5mm, L: 20mm)
380
580
100
90
transmission [a.u.]
80
980
1180
The wavelength range of TGG prevents
isolators for far IR, UV light, and @490nm.
Substitutes can be magneto optical
glasses and B.I.G. crystals films.
Thermal stability requires low absorption,
good thermal conductivity and short optical
path lengths.
70
60
50
40
30
20
10
0
300
780
wavelength [nm]
500
700
900
1100
1300
1500
1700
wavelength [nm]
Author: Markus Fegelein, Product Manager Crystal Technology │ LASER 2009, Munich
│
Slide 8
Lecture forum
InnovationPoint
Thank you for listening!
Markus Fegelein
Product Manager Business Unit Crystal Technology
LINOS Photonics GmbH & Co. KG
Sources of pictures:
Source 1: www.wikipedia.com
SourceMarkus
2: „Fundamentals
photonics“
by Saleh+Teich,
Wiley-Interscience
Author:
Fegelein, Product of
Manager
Crystal Technology
│ LASER
2009, Munich │ Slide 9