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Electro Optics
Application in Pockels cells
Markus Fegelein
Product Manager Business Unit Crystal Technology
LINOS Photonics GmbH & Co. KG
Author: Frist name Surname, Function │ LASER 2009, Munich
│
Slide 1
Lecture forum
Static birefringence – wave plate
half wave plate
Wave plates are the most common application
of birefringent crystals
Wave plates are used to manipulate the
polarization of light
Incoming light polarized 45°w.r.t. the
optical axis of the crystal
Source 1
Polarization is decomposed into two
normal modes, which see different
refractive indices
Correct crystal length yields a phase shift
of π.
Recombining the two normal modes after
the crystal results in a light wave with
polarization rotated by 90°.
Source 2
Author: Frist name Surname, Function │ LASER 2009, Munich
│
Slide 2
Lecture forum
Electro-optics - Principles
Induced Birefringence:
Several Materials which are optically isotropic under normal conditions
showanisotropyresp. optical birefringence under:
Mechanical stress (Elasto-Optic Effect)
Strong static electric fields (Pockels and Kerr Effect)
Magnetic fields (Faraday Effect)
Kerr and Pockels Effect
Strong electric fields affect the refractive index:
1
n(E) = n + a1E + a2 E 2 + ...
2
Pockelseffect
(Taylor Series disregarding anisotropic effects)
Kerreffect
For most crystals the Pockels effect is dominant while the Kerr effect is negligible.
Author: Frist name Surname, Function │ LASER 2009, Munich
│
Slide 3
Pockels Effect
Lecture forum
Even though the Pockels Effect is the dominating effect, it is really small. For an
electric field of 106 V/m (equals 10 kV applied to a crystal of 1cm thickness) the
induced change of the refractive index is approx. 10-6 - 10-4.
Pockels Cells for Phase Modulation:
After the propagation through a crystal with length L the resulting Phase retardation φ is:
ϕ = n (E )k 0L = 2π n (E )L / λ0
▼Different ways of
constructing a Pockels
Cell.
k0 = vacuum wave vector
▼Intensity modulation for a MachZehnder interferometer with an
electro-optical device.
Source 2
Source 2
Author: Frist name Surname, Function │ LASER 2009, Munich
Source 2
│
Slide 4
Lecture forum
Application: Intensity Modulator
Placing the Pockels Cell between two crossed Polarizers builds an electro optic
intensity modulator
Intensity is controlled by applied voltage
No mechanical parts -> fast on/off, precise switching, high repetition rates
Source 2
Author: Frist name Surname, Function │ LASER 2009, Munich
│
Slide 5
◄The intensity can be
tuned continuously
between 0 and maximum
by the voltage applied to
the crystal.
Lecture forum
Application: Q-switching
The output power of a laser can be increased if all the energy, which can be stored in the
laser media maximally, is emitted in one giant pulse.
Spontaneous laser emission must be suppressed until the media is fully „soaked“
1) Laser media
2) Polarizing Beam Splitter (PBS)
3) Pockels Cell
4) HR mirror
5) Output coupler
Light from the laser media is polarized by the PBS
First passage through the Pockels Cell: light is polarized circularly, reflected by HR-mirror
Second passage through Pockels Cell: Phase shift is increased by another π/2
The plane of polarization of the back coming light is rotated by 90°
PBS deflects the light which causes high cavity losses (= low Q-factor)
The laser cannot start oscillating
Author: Frist name Surname, Function │ LASER 2009, Munich
│
Slide 6
Lecture forum
Application: Q-switching
High losses keep the resonator shut
Meanwhile the media is pumped until the
maximum storage capacity is
reached
Voltage of the Pockels Cell is turned of
rapidly
Resonator losses vanish
instantaneously, Q-factor increases
immediately
A short laser pulse (typ. 10 ns) is emitted
The pulse scoops all the energy stored
in the media
Source 3
Author: Frist name Surname, Function │ LASER 2009, Munich
│
Slide 7
Lecture forum
Application: Pulse Picking
Mode Locked Lasers emit pulses of high quality but of low energy
Repetition rate of typ. 80MHz is too high for direct amplification
EO-System
pulse train
from oscillator
Laser
Oscillator
HVSwitch
FI
repetition
rate: 50 to
100 MHz
pulse train after
pulse picking
PZ
Amplifier
repetition rate:
typ. multi kHz
Pulspicker (PP)
◄Temporal Profile of
electro-optic Pulse Picker
Quelle 5
A Pockels Cell picks selected single pulses from a pulse train, which can be coupled
into an amplifier
Master Laser and Amplifier are decoupled by a Faraday Isolator
Author: Frist name Surname, Function │ LASER 2009, Munich
│
Slide 8
Lecture forum
Electro Optics – Crystal Materials
news.bbc.co.uk
KH2PO4 (KDP, KD*P)
Enables large apertures
Relatively cheap
Wavelength range: approx. 250nm - 1.1µm
Good damage threshold (>500MW/cm2)
Sensitive for piezo ringing
B2BaO4 (BBO)
Expensive, especially for large apertures
High switching voltage
Wavelength range: approx. 190nm - 2 µm
High damage threshold (>600MW/cm2)
Relatively small piezo effects
Author: Frist name Surname, Function │ LASER 2009, Munich
│
Slide 9
Lecture forum
Electro Optics – Crystal Materials
RTiOPO4 (RTP)
Very expensive, difficult to grow
only small apertures
Insensitive for piezo ringing
Very low switching voltage
Wavelength range: approx. 400 nm - 2,5 µm
High damage threshold (>600MW/cm2)
LiNbO3 (Lithiumniobate)
Cheap, enables large Apertures
Wavelength range: approx. 420 nm - 5 µm
Low damage threshold (~200MW/cm2)
Strong piezo effects
Author: Frist name Surname, Function │ LASER 2009, Munich
│
Slide 10
Lecture forum
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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
Source 2: „Fundamentals of photonics“ by Saleh+Teich, Wiley-Interscience
SourceFrist
3: name
„Lasers“
by Siegmann,
University
Science
Author:
Surname,
Function │ LASER
2009, Munich
│ Books
Slide 11