THORLABS EO-AM-NR-C Series User manual
EO-AM-NR-Cx
EO-AM-R-20-Cx
Electro-Optic Amplitude
Modulator
User Guide
Electro-Optic Amplitude Modulator
Page 115956-D02 Rev.B, 1/26/2012
Table of Contents
Chapter 1 Description.......................................................................................................................................2
1.1. Overview........................................................................................................................... 2
1.2. EO Amplitude Modulator Versions................................................................................ 3
1.3. Standard Features ........................................................................................................... 3
1.4. EO-AM Specifications ..................................................................................................... 4
Chapter 2 Setup..................................................................................................................................................5
2.1. Unpacking......................................................................................................................... 5
2.2. Alignment.......................................................................................................................... 5
Chapter 3 Photorefractive Limits...................................................................................................................6
Chapter 4 Cleaning and Maintenance ..........................................................................................................6
4.1. Cleaning the Optics......................................................................................................... 6
4.2. Cleaning the Compensator Housing............................................................................. 6
Chapter 5 Mechanical Drawings....................................................................................................................7
Chapter 6 Warranty Information ....................................................................................................................8
6.1. General Product Warranty.............................................................................................. 8
6.2. Opto-Mechanics............................................................................................................... 8
6.3. Optical Tables and Breadboards ................................................................................... 8
6.4. Lasers and Imaging Systems......................................................................................... 8
6.5. Opto-Electronics, Control Electronics, Optics, and Non-Positioning Product Lines
8
6.6. Warranty Exclusions ....................................................................................................... 8
Chapter 7 Regulatory........................................................................................................................................9
7.1. Waste Treatment is Your Own Responsibility ............................................................. 9
7.2. Ecological Background................................................................................................... 9
Chapter 8 Thorlabs Worldwide Contacts ..................................................................................................10
Chapter 1: Description
Page 2
Figure 1 Output Polarization
Chapter 1 Description
1.1. Overview
The electro-optic amplitude modulator (EO-AM) is a Pockels cell type modulator consisting of two lithium niobate
crystals packaged in a compact housing with an RF input connector. Voltage applied across the crystal structure
induces change in the indices of
refraction (both ordinary and
extraordinary), leading to an electric field
dependent birefringence. An optical wave
(with polarization components on both the
ordinary and extraordinary axes) will
experience a change in polarization state
after traversing the crystal, from the
relative phase delay between the
orthogonal components. The electro-optic
crystal acts as a variable waveplate with
retardance linearly dependent on the
applied voltage. Figure 1, below, shows
the polarization state emerging from the
electro-optic crystal as a function of
applied voltage when the input
polarization state is linear at 45°relative
to the ordinary and extraordinary axes.
The voltage required to produce a
retardance of πradians is called the
halfwave voltage or simply Vπ. For an
optical input linearly polarized at 45°,
applying a halfwave voltage rotates the
polarization by 90°. When the output
beam is then passed through a linear
polarizer, the resulting intensity can be
rapidly modulated from maximum
intensity (output linearly polarized along the polarizer transmission axis) to minimum intensity (output polarized
perpendicular to the polarizer transmission axis) by changing the voltage applied to the crystal from 0 volts to Vπ.
Note: Regarding static birefringence, lithium nobiate is birefringent even without an electric field applied. This
static birefringence will induce a change in polarization without an electric field applied. In addition, this static
birefringence is temperature dependent leading to variations in the output polarization with slight changes in
temperature. To minimize this effect, the modulator is constructed of two matched lithium niobate crystals rotated
90°with respect to each other. The temperature dependent static birefringence in the first crystal is canceled by
the temperature dependent static birefringence in the second. In this manner, the temperature sensitivity of the
modulator is generally less than 1 mrad retardance per °C.
Applied Voltage
π/2
π
π/4
3π/4
Retardance
Output
Polarization
n
n
E
V
Electro-Optic Amplitude Modulator
Page 315956-D02 Rev.B, 1/26/2012
The halfwave voltage of lithium niobate is dependent on the optical wavelength and is given by:
L
d
rnrn
V
oe 13
3
33
3
−
=
λ
π
,
Where λ= optical wavelength
d= electrode spacing
L= optical path length
rij = electro-optic coefficients
noe = ordinary and extraordinary indices of refraction
The indices of refraction themselves are also functions of wavelength and temperature. While the above formula for the
halfwave voltage is relatively complicated, a reasonably close approximation for this modulator is obtained using a linear fit
over the typical operating range,
844.23361.0 −=
λ
π
V
where λis the optical wavelength in nm.
1.2. EO Amplitude Modulator Versions
Thorlabs offers the EO amplitude modulators in two basic versions:
1. DC-coupled Broadband Version: The electro-optic crystal is directly coupled to the RF modulation input
connector allowing the full bandwidth of the crystal to be utilized. The broadband version can be driven
from DC up to the frequency limit of the RF driver. (Note: The external driver must have sufficient slew
rate, and output current level for driving the capacitive load of the crystal. Driver limitations and crystal
capacitance typically determine the limit for maximum drive frequency)
2. High Q Resonant Mode Modulator: A high Q resonant circuit is used to boost the RF input voltage up to
the high voltage needed by the EO crystal. The required peak to peak input voltage (as given in the
formula above) is then reduced by the Q-factor of the tank circuit, when driven sinusoidally at the
resonant frequency. An impedance matching network transforms the reactive crystal load to a 50 Ωinput
to allow for easy matching to standard RF drivers and function generations.
Note: By definition, high Q circuits have a narrow operating bandwidth. The resonant EO modulators have a
typical operating bandwidth that is 5% of the center frequency. Therefore, the operating frequency must be
specified at time of purchase.
1.3. Standard Features
•Broadband DC coupled and High Q Resonant models
•MgO doped Lithium Nioabate crystal
•Standard Broadband AR coatings cover 400 to 1650 nm range
•2 mm diameter clear aperture
•SMA RF modulation input connector
•DC to 100 MHz
•Flange with thru holes on housing for secure mounting
•#1/4-20 and #8-32 tapped mounting holes on bottom
Chapter 1: Description
Page 4
•Custom OEM versions available
1.4. EO-AM Specifications
Specification
Description
Modulator Crystal
MgO-Doped Lithium Niobate
(LiNbO3)
Wavelength Range
C1
600 to 900 nm
C2
900 to 1250 nm
C3
1250 to 1650 nm
C4
400 to 600 nm
Clear Aperture
2 mm diameter
Input Connector
SMA Female
Halfwave Voltage, V
π
, Non-
Resonant
205 V @ 633 nm (See Fig. 2)
Halfwave Voltage, Vπ, Resonant
15V @ 633 nm
Extinction Ratio
>10 dB
Input Capacitance, Non-Resonant
14 pF (typical)
Input Impedance, Resonant
50 ohms
Maximum Optical Power Density 2 W/mm
2
@ 532 nm
4 W/mm
2
@ 1064 nm
.
Figure 2 Halfwave Voltage
Electro-Optic Amplitude Modulator
Page 515956-D02 Rev.B, 1/26/2012
Chapter 2 Setup
WARNING
Do not remove cover. High Voltages are used to drive the crystals. Contact with these high voltages
may be harmful to the user.
2.1. Unpacking
The modulator is shipped form the factory ready to be placed into your setup. Carefully unpack the modulator and inspect the
optics by looking through the aperture. If the modulator is shipped with protective tape over the optical apertures, it will need to
be removed prior to operation.
WARNING
Do not drop or bang modulator. While Thorlabs supplies a rugged enclosure, excessive mechanical
shock will damage the LiNbO3 crystal.
2.2. Alignment
The EO-AM can be used as a variable waveplate without additional optics. Maximum amplitude modulation
requires both polarized input and a polarizer/analyzer at the output. The optical input may be polarized either
vertically or horizontally. The output polarizer may be aligned either parallel or perpendicular to the input
polarization. Parallel alignment of the output polarizer will give maximum transmission at low applied voltage and
minimum transmission at high applied voltage. Conversely, perpendicular alignment of the output polarizer will
give minimum transmission at low applied voltage and maximum transmission at high applied voltage. Best
results are obtained when the optical propagation is precisely aligned along the principal axis of the crystals. With
the input beam polarized either vertically or horizontally, direct the beam through the center of the modulator
aperture. Place a card several inches from the output of the modulator and mark the location where the beam
strikes the card. Insert a polarizer with polarization perpendicular to the input polarization between the card and
the modulator. Cover the input aperture with a piece of frosted adhesive tape (ScotchTM Magic Mending Table No.
810 or similar material).
As shown in Figure 3 on page 7, a geometric pattern (isogyre) similar to Isogyre “A” will be projected on the
viewing card. Adjust the modulator angle of incidence such that the center of the geometric pattern is coincident
with the mark previously made on the card indicating the beam location.
Figure 3 Geometric Pattern (Isogyre “A” and Isogyre “B”)
If voltage is applied to the modulator in this configuration, the pattern will be observed to change from Isogyre “A”
to Isogyre “B” as the voltage increases. If the isogyre is not fully closed with zero applied voltage (i.e.,
transmission of a centered beam through the analyzer is not at a minimum) the pattern can be adjusted using a
bias voltage (for broadband modulators only) or by using a quarter-waveplate at the modulator entrance.
Alternatively, the modulator can be tilted slightly so that the location of one of the dark lines corresponds to the
mark on the card denoting the position of the beam without the tape (or alternative diffuser).
Chapter 3: Photorefractive Limits
Page 6
Chapter 3 Photorefractive Limits
Lithium Niobate, LiNbO3, has an optical transparency range from 320 nm to over 5 µm and can withstand pulsed
power densities up to 1 MW/cm2. However, short wavelength operation is limited by photorefractive effects which
lead to color center formation and progressively increasing absorption. Photorefractive damage is noted by
increase scattering and absorption in the optical path along with gray appearance in the crystal (gray tracking).
Photorefractive limits are a concern in the visible region (400 – 700 nm) particularly with frequency- doubled
lasers.
The lithium niobate crystals in Thorlabs’ EO modulators are MgO-doped for increased resistance to
photorefractive damage.
Chapter 4 Cleaning and Maintenance
Under normal operating conditions, the modulator needs very little, if any maintenance.
4.1. Cleaning the Optics
The modulator crystal is recessed inside the housing and under normal conditions, the crystal faces should not
need cleaning. If necessary, use a gentle stream of compressed air (dry nitrogen under low pressure or a can of
compressed air made specifically for cleaning optics).
WARNING
Damage may result from excessive cleaning air pressure of if condensation is allowed to build up from using a stream of
compressed air from an inverted can.
4.2. Cleaning the Compensator Housing
The housing can be wiped clean with a lint free rag wetted with propanol or a commercial window cleaner. Do not
soak the housing – it is not watertight and any moisture introduced inside the modulator may cause condensation
problems.
Electro-Optic Amplitude Modulator
Page 715956-D02 Rev.B, 1/26/2012
Chapter 5 Mechanical Drawings
Chapter 6: Warranty Information
Page 8
Chapter 6 Warranty Information
6.1. General Product Warranty
Thorlabs warrants that all products sold will be free from defects in material and workmanship, and will conform to
the published specifications under normal use and service when correctly installed and maintained.
6.2. Opto-Mechanics
Lifetime Warranty: Thorlabs offers a lifetime warranty on all opto-mechanical components. Thorlabs will repair or
replace any opto-mechanical product which after evaluation has failed to perform in the above conditions.
6.3. Optical Tables and Breadboards
Lifetime Warranty: We provide a lifetime guarantee that all of our passively damped optical tables and
breadboards will meet all originally stated performance specifications under normal use and proper handling. We
additionally guarantee that all our table tops and breadboards, both active and passive, will be free from defects in
workmanship, including de-lamination of the skins under normal use and handling.
6.4. Lasers and Imaging Systems
Thorlabs offers a one year warranty on all lasers and imaging systems, with the exceptions of laser diodes. Some
products are warranted for the number of hours specified in the operating manual of each laser.
6.5. Opto-Electronics, Control Electronics, Optics, and Non-Positioning Product Lines
Thorlabs offers a two year warranty on the above mentioned product lines, providing normal use and
maintenance of the products and when properly handled and correctly installed.
Thorlabs shall repair or replace any defective or nonconforming product as detailed above. We ask that the buyer
contact Thorlabs for a Return Material Authorization number (RMA#) from our Customer Service/Returns
department in order to most efficiently process the return and/or repair.
Products returned for repair that are not covered under warranty, a Thorlabs standard repair charge shall be
applicable in addition to all shipping expenses. This repair charge will be quoted to the customer before the work
is performed.
6.6. Warranty Exclusions
The stated warranty does not apply to products which are (a) specials, modifications, or customized items
(including custom patch cables) meeting the specifications you provide; (b) ESD sensitive items whose static
protection packaging has been opened; (c) items repaired, modified, or altered by any party other than Thorlabs;
(d) items used in conjunction with equipment not provided by, or acknowledged as compatible by Thorlabs; (e)
subjected to unusual physical, thermal, or electrical stress; (f) damaged due to improper installation, misuse,
abuse, or storage; (g) damaged due to accident or negligence in use, storage, transportation, or handling.
Electro-Optic Amplitude Modulator
Page 915956-D02 Rev.B, 1/26/2012
Chapter 7 Regulatory
As required by the WEEE (Waste Electrical and Electronic Equipment Directive) of the European Community and
the corresponding national laws, Thorlabs offers all end users in the EC the possibility to return “end of life” units
without incurring disposal charges.
•This offer is valid for Thorlabs electrical and electronic equipment:
•Sold after August 13, 2005
•Marked correspondingly with the crossed out “wheelie bin” logo (see right)
•Sold to a company or institute within the EC
•Currently owned by a company or institute within the EC
•Still complete, not disassembled and not contaminated
As the WEEE directive applies to self-contained operational electrical and electronic
products, this end of life take back service does not refer to other Thorlabs products,
such as:
•Pure OEM products, that means assemblies to be built into a unit by the user (e.g. OEM laser driver
cards)
•Components
•Mechanics and optics
•Left over parts of units disassembled by the user (PCB’s, housings etc.).
If you wish to return a Thorlabs unit for waste recovery, please contact Thorlabs or your nearest dealer for further
information.
7.1. Waste Treatment is Your Own Responsibility
If you do not return an “end of life” unit to Thorlabs, you must hand it to a company specialized in waste recovery.
Do not dispose of the unit in a litter bin or at a public waste disposal site.
7.2. Ecological Background
It is well known that WEEE pollutes the environment by releasing toxic products during decomposition. The aim of
the European RoHS directive is to reduce the content of toxic substances in electronic products in the future.
The intent of the WEEE directive is to enforce the recycling of WEEE. A controlled recycling of end of life products
will thereby avoid negative impacts on the environment.
Wheelie Bin Logo
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Page 10 '5HY%
Chapter Thorlabs Worldwide Contacts
For technical support or sales inquiries, please visit us at www.thorlabs.com/contact for our most up-to-
date contact information.
USA, Canada, and South America
Thorlabs, Inc.
techsupport@thorlabs.com
Europe
Thorlabs GmbHGm
France
Thorlabs SAS
Japan
Thorlabs Japan, Inc.
UK and Ireland
Thorlabs Ltd.
techsupport.uk@thorlabs.com
Scandinavia
Thorlabs Sweden AB
Brazil
Thorlabs Vendas de Fotônicos Ltda.
China
Thorlabs China
www.thorlabs.com
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