TEM uAligna User manual

µAligna Manual
TEM Messtechnik GmbH
February 19, 2020
TEM Messtechnik GmbH
Grosser Hillen 38
D-30559 Hannover
Tel: [phone]
Fax: [phone]
E-mail: mailto:[e-mail]
URL: http://www.TEM-Messtechnik.de

µAligna Manual
Contents
1 Introduction 5
2 Hardware Description 6
2.1 µAlignaBlockDiagram ................................ 6
2.2 BackPanelElements.................................. 6
3 Software Installation 8
3.1 Installation of the Kangoo Software . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2 InstallingtheUSBDrivers............................... 9
3.3 UpgradingtheFirmware................................ 9
4 Basic Operation 10
4.1 Using the Kangoo Software .............................. 10
4.1.1 Configuration “Aligna User Menu” . . . . . . . . . . . . . . . . . . . . . . 10
4.1.2 Configuration “BeamLock Basic” . . . . . . . . . . . . . . . . . . . . . . . 10
4.1.3 Configuration “Learn OCL Motors” . . . . . . . . . . . . . . . . . . . . . . 11
4.1.4 Piezo“OCLMatrix”.............................. 14
4.1.5 Enabling the Piezo Regulator . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.1.6 SavingParameters............................... 15
4.2 Running the Plug & Play Kit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5 Advanced Operation 17
5.1 PSD .......................................... 17
5.1.1 BeamSampling ................................ 17
5.1.2 GainsandOffsets ............................... 18
5.1.3 Thresholds................................... 18
5.1.4 Calibration................................... 19
5.1.5 PulsedLasers ................................. 19
5.2 Actuators........................................ 21
5.2.1 MotorOffsets ................................. 22
5.2.2 ReferenceSwitches............................... 22
5.2.3 PiezoActuators ................................ 22
5.3 Tuning the Piezo Regulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
5.4 PCInterface ...................................... 23
6 Connectors and Electrical Specifications 25
6.1 Environmental Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
6.2 µAlignaElectronics .................................. 25
6.3 USBInterface ..................................... 25
6.4 MainsPowerCable................................... 25
6.5 HD-15Connectors................................... 25
7 Safety Instructions 27
Appendices 28
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A Principles of Beam Pointing Stabilization 28
A.1 Reasons for Pointing Instabilities . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
A.1.1 Thermal Properties of the Laser . . . . . . . . . . . . . . . . . . . . . . . . 28
A.1.2 Thermal Movements caused by the Laser Cooling System . . . . . . . . . . 28
A.1.3 Drifts of Alignment and Folding Mirrors . . . . . . . . . . . . . . . . . . . . 28
A.1.4 Air Turbulences and Temperature Gradients in the Air . . . . . . . . . . . . 29
A.1.5 Thermal Effects of Optical Elements . . . . . . . . . . . . . . . . . . . . . 29
A.1.6 Mechanically Moved Optical Elements . . . . . . . . . . . . . . . . . . . . 29
A.2 2Dor4DStabilization................................. 30
A.3 ActuatorPlacement .................................. 32
B Advanced Kangoo Usage 32
8 Customer Service 33
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1 Introduction
TEM Messtechnik’s µAligna is a multi-purpose device for controlling and stabilizing optical elements.
Its ability to drive up to eight stepper motors allows it to control rotational actuators such as po-
larizing filters, as well as linear actuators such as mirror mounts. The µAligna device also features
freely-assignable digital-to-analog converters which can, for instance, provide the input signals for
HV-amplifiers and thus control piezo-actuators.
Input signals can be read by a high-precision analog-to-digital converter and processed for display,
feedback or data logging. Examples of input devices are simple photo detectors to measure the laser
intensity, position-sensitive detectors to measure the beam position and output signals from another
control electronics system. These input and output capabilities can be combined to form closed
regulator loops.
For instance, the µAligna device can act as a laser intensity stabilization system, by continuously
measuring intensity values, comparing these to a specified set point and adjusting an attenuator
accordingly.
In a somewhat more complex application, the µAligna device is able to stabilize the pointing of a
laser beam. This requires two position-sensitive detectors which measure the translation and the
angle of the beam and two motorized mirrors which actively adjust and regulate the beam pointing.
Since the signal processing is exclusively done by a micro-controller, the µAligna device shows great
flexibility. All important parameters and settings are accessible by interfacing the device with a PC,
either using the dedicated program Kangoo or user-supplied software.
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2 Hardware Description
2.1 µAligna Block Diagram
At the heart of the µAligna device lies a micro-controller, which handles all communications over
the USB interface and over the I2C or SPI bus. It also does most of the signal processing, such
as the implementation of PID-regulators, the generation of control signals for micro-stepper motors
or digital filtering. For reading input signals or acquiring data, the µAligna is equipped with an
8-channel, 16-bit ADC with an input range of ±10V. Signal output is provided in the form of two
8-channel, 16-bit digital-to-analog converter (DAC). The output range of these DACs is ±10V.
Furthermore, the µAligna device features up to eight power drivers (H-bridge drivers with PWM
voltage control) capable of controlling eight micro-stepper motors simultaneously.
For beam stabilization systems with piezo actuators, the µAligna has a four-channel piezo driver,
operating at 150V.
Figure 1: Block diagram. Optional components shown in gray.
The standard configuration of the µAligna electronics features one 8-channel input connector and
two output connectors, configured to drive four stepper motors.
All input sockets (HD-15 plugs) have access to an I2C/SPI data bus and to supply voltages of ±15V.
2.2 Back Panel Elements
Figure 2 shows the back panel of the µAligna device with all connectors and UI-elements.
NOTE: The PSD detector and the actuators have identical connector cables. Please make sure that
they are not interchanged before applying power to µAligna electronics. Plugging the detector into
a motor output will most likely damage the detector.
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PSD
active
status
www.TEM-Messtechnik.de
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overheat current clip com
TEM Messtechnik GmbH, +49 (0)511 / 510896 30
Grosser Hillen 38, 30559 Hannover (Germany)
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Figure 2: Back panel elements.
1 USB USB communication with a PC, configured as a virtual COM port
2 Power 12V power jack
3 Key switches the regulator on or off
4 LED status LED, slow breathing: inactive, fast breathing: regulator on
5 M1 HD-15 connector (output) for motorized actuator 1
6 M2 HD-15 connector (output) for motorized actuator 2
7 PSD HD-15 connector (input) for a PSD-4D detector
8 A1, A2 HD-15 connector (output) for piezo actuators
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3 Software Installation
3.1 Installation of the Kangoo Software
To install the Kangoo software, start the program “Install.exe” in the root directory of the installation
CD. The installer will show a welcome screen with several options.
The default options should work fine, with the possible exception of the section “Destination Path”,
where the destination directory is specified. The standard directory is “TEM” in the “Program
Files” folder. On Windows Visa or Windows 7 systems, please avoid the “Program Files” folder and
choose and a different path, for example “C:/TEM”. The button “OK, install now!” starts copying
all required files from the source path to the destination path. During the installation procedure,
the installation program checks all required files.
The program then creates a list of file copy commands. When this list is complete, you can check
the list and start the copy procedure.
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3.2 Installing the USB Drivers
Typically, when the USB connection between the micro-controller and a PC is first made, Windows
will open the Found New Hardware Wizard. Here, choose to install drivers from a user-specified
location. The necessary driver file is located in the directory “TEM/Service/USB Driver” in the
Kangoo installation directory (or on the install CD). The Hardware Wizard will now finish the
installation and no further configuration will be necessary.
Once the installation is complete, Windows will assign a COM-port. To find out which COM-port
has been assigned, check for a new entry in the section “Ports (COM & LPT)” of the device
manager.
3.3 Upgrading the Firmware
Please contact TEM Messtechnik for details about firmware upgrades.
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4 Basic Operation
4.1 Using the Kangoo Software
The alignment of the parameters of a µAligna system is most easily done with the supplied software
package Kangoo. Since the µAligna device is controlled by plain-text commands, it can be config-
ured with any control software like LabView, TestPoint, or other programs written in VisualBasic,
C++,C#, etc. However, to become acquainted with the system it is highly recommended to use Kan-
goo and benefit from the multitude of built-in functions and algorithms specifically targeted at laser
beam stabilization using µAligna. Note that the Kangoo software and the configurations applicable
to pointing stabilization are compatible with both the µAligna and the more extensive Aligna system.
Kangoo is a comprehensive measurement and controlling software system, which can be adapted
to control completely different applications (Controlling a µAligna or Aligna system is just one of
a long list). A detailed description of Kangoo’s functionalities can be found in Appendix B. Here,
only a short guide of the software’s basic use is given.
4.1.1 Configuration “Aligna User Menu”
The configuration Aligna / BeamLock Menu leads to different user configurations. A short explana-
tion of each button and the corresponding configuration is given in the status line when the mouse
pointer hovers over the respective button. Some of the individual configurations will be described
in following sub-chapters.
 BeamLock Basic Use: General use of µAligna or Aligna systems or even PSD systems (without
actuator control or servo loops)
 measure OCL Matrix: This configuration is used to teach the system the optical setup and to
tune the regulator performance.
 Gray buttons are pointing to a configuration which is not enabled with your software or
hardware version.
4.1.2 Configuration “BeamLock Basic”
BeamLock Basic is the main configuration for controlling the µAligna system. It provides access to
the most commonly used functionalities while retaining enough simplicity for everyday use. The most
important section is the “PSD input” section in the top-lefthand corner. It displays the intensity
of the two PSD-detector chips on two bar-indicators and the current beam pointing in the large
oscilloscope screen. Here, the red color represents the angle detector and green corresponds to
the position detector. The “active” LED allows the user to disable requests of the current beam
pointing. During normal operation, these should always be active. Below that LED, the “zoom”
factor scales the oscilloscope display. Set this to 10 in order to see the full range of the PSD signals.
Zoom in to see small beam movements or pointing fluctuations. The current beam pointing is
displayed as digital numbers on the right, in the section “phys. units”.
The section “PSD” controls settings inside the PSD detector. In particular, each PSD chip has a gain
and electronic offsets associated with it. The gain allows the system to accommodate different laser
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Figure 3: Kangoo configuration “Aligna User Menu”
intensities. “PSD A gain” and “PSD B gain” should always be adjusted such that the two intensity
meters show about five volts. The button “AutoGain” below the intensity meters automatically
adjusts both gains. Similar to the gains, there are offsets associated to each detector chips. With
these offsets, the beam can be stabilized to positions other than the detectors’ centers. Again, the
buttons “AutoZeroA” and “AutoZeroB” automatically adjust the offsets to produce a (0,0,0,0)
pointing display. Please note that these gains and offsets are not stored in the detector, but in the
µAligna electronics, so changing the controller does not retain the offset values.
The section “Thresholds” contains a number of threshold values for the intensity, the pointing and
the regulator. Factory values should work well in most setups. For more detail, please refer to
section 5.1.3.
4.1.3 Configuration “Learn OCL Motors”
The configuration Learn OCL Motors is used for the initial setup of your µAligna system. Here,
OCL stands for “Output Crosslink Matrix”. This matrix allows the actuators to achieve a nearly
pure angle or position movement of the beam. A parallel beam movement, for example, is achieved
by an angle change of the first actuator and a corresponding angle change of the second actuator
by exactly the negative value of the first. Thus the resulting angle movement will be compensated
for and a pure parallel beam translation results. So a position movement requires a combination
movement of both actuator mirrors. By extension, a combination movement of both mirrors can act
as a beam angle movement around any arbitrary point of the beam. Due to properties of the opto-
mechanical setup, a movement of one actuator (say the X- direction of actuator 1) may cause an
unwanted movement also in the Y-direction. This can be compensated for by suitable combination
movement of all four actuators.
The OCL matrix translates the four detector signals (angle X,Y and position X,Y) into the corre-
sponding actuator combination movements. In other words, this matrix stores the particular opto-
mechanical setup, i.e., the positions and orientations of both actuators and the detector. Hence,
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Figure 4: Kangoo configuration “BeamLock Basic”
when a µAligna system is first set up in a new optical layout, a new OCL matrix must be determined.
Figure 5: Kangoo configuration “measure OCL matrix”
The Kangoo software can perform a measurement cycle to automatically determine the correct
matrix. The basic steps for this are as follows:
 Swtich on the laser and align the beam such that both detector chips (A and B) are hit.
 Run the learning procedure
 Switch on the regulator to check whether the learning procedure was successful. If not, re-run
the procedure.
Rough manual alignment: Please switch on the laser and use the knobs on the actuators to bring
it into alignment. If the PSD-detector is positioned correctly, both PSD-A and PSD-B should be
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hit by the laser. Use the two intensity indicators to the left of the scope to verify this. Hit the
“AutoGain” button, which should adjust both intensity indicators to about 5V.
Next, try to roughly align both angle and position of the beam to the detectors’ centers (say to
within ±2V).
Running the learning procedure: Choose “Learn Program: Motor OCL”. The system will learn the
OCL for the motor servos and the “Motors Output Cross Link Matrix” will become visible.
 Click “create out data” (Triangle data ramps are created by the micro-controller, and displayed
on the PC)
 Click “start measurement!”, which starts the learning procedure
If “autoGZ” is set to a non-zero value, the system will first perform a “PSD AutoGain” procedure.
Then all four channels (Ax,Ay,Bx,By) will scan all actuators, using the current OCL matrix.
You can watch the red and the green dots. The better the OCL fits to the real setup, the more
independently the dots will move (to the right, to the left, up, and down, back again to the middle
position, both for channels A and B). When the procedure has finished, a pop-up window asks the
user whether the OCL matrix should be automatically corrected. Please hit “Yes, correct”.
Figure 6: Measurement results with a good OCL matrix
Figure 6 shows the results of the learning procedure when the OCL matrix is well-adjusted. The
four triangular curves show the detector outputs (Ax,Ay,Bx,By) while the actuators perform
combination movements. Ideally, the results are as shown, meaning that an Axcombination move-
ment only influences that detector channel. If a wrong OCL matrix is set, e.g. the unit matrix, the
measurement results will look similar to Figure 7.
Figure 7: Measurement results with an unsuitable OCL matrix
In more complicated optical setups, it is possible that the learning procedure produces somewhat
intermediate results, as shown in Figure 8. Here, the regulation will work, but not at its peak
performance.
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Figure 8: Measurement results with a roughly correct OCL matrix
It is recommended to re-run the learning procedure in order to improve the OCL matrix. Typically,
the learning procedure should be run three times: Once to determine a good OCL matrix, a second
time to improve residual errors and a third time just to verify the results (there is no need to correct
the OCL when the results look good). If the automatic learning fails to produce good results, check
whether the beam leaves the PSD chips during the procedure (i.e. check if the intensities fail). In
this case, reduce the learning amplitude.
4.1.4 Piezo “OCL Matrix”
If your µAligna system has piezo actuators, those have their own dedicated OCL matrix. Please
switch to the configuration “Learn OCL Piezo”.
Then click on “start measurement!” to start the same learning procedure as before.
One speciality of the piezo learning procedure is the fact that the entries of the piezo OCL matrix
will be clipped if their values exceed the range −1.5. . . 1.5. In this case, please increase the amplifi-
cation factors “PiezoAmplifA” or “PiezoAmplifB”, which will scale down the corresponding matrix
elements.
Adjust “PsdPiezoAmplifA” if clipped values appear in the first two rows of the matrix and “Psd-
PiezoAmplifB” if clipping appears in the last two rows. Conversely, if all entries of the piezo OCL
matrix are small (significantly below 1), then decrease these amplification factors. Next, repeat the
automatic learning procedure.
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4.1.5 Enabling the Piezo Regulator
After a satisfactory learning procedure, it is recommended to test the regulation. This is done by
pressing the large “RegOn” indicator and the small Piezo enable indicator to the right of it. If your
system has motor and piezo actuators, please also enable the motor regulator before saving the
parameters to ensure that motors and piezos are both active. This way, the regulation system is
able to compensate for both fast fluctuations and larger-amplitude slow drifts.
4.1.6 Saving Parameters
All micro-controller parameters are saved in the controller’s RAM, which means that they will be
lost after a reset or power cycle. It is however possible to save the current parameter settings
and have them automatically loaded at start-up. This is done from the menu Programming uC →
Commands to uC →Command >VarSave<, or by entering the command VarSave at the terminal.
This procedure can be repeated at any time.
4.2 Running the Plug & Play Kit
Some µAligna systems are delivered with a Plug & Play kit, as show in Fig. 9. The purpose of
this kit is to familiarize the user with the µAligna system and its parameters before transferring the
opto-mechanical components into the machine or experiment.
Figure 9: The µAligna Plug & Play Kit
The Plug & Play system includes
 some acrylic base plates
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 a test laser
 the mirror mounts with actuators and mirrors (with aluminum coating for testing purposes
only)
 the 4D detector
 a beam sampler plate (for testing only)
 two apertures, representing your experimental setup
The acrylic glass parts of the Plug & Play Kit connect magnetically and each connection is in-
dividually numbered, which makes the setup easy and fast. Once assembled , please connect all
cables (they are also uniquely labelled), including the USB connection to a PC. Then install the
USB drivers and the Kangoo software. Power up the µAligna electronics, start the software and
open the configuration “BeamLock Basic”. Finally, press the button “AutoGain” and activate the
regulator. The µAligna is parameterized for the Plug & Play kit, so that the regulation will work
straight away.
Next, the user is encouraged to use the Plug & Play Kit to “play” with different settings and
parameters. In particular, we recommended trying
 the OCL matrix learning procedure
 manual motor offsets and reference switches
 PSD offsets with and without regulation
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5 Advanced Operation
5.1 PSD
The detector delivered with a µAligna system measures the beam pointing in four dimensions,
namely beam angle (Axand Ay) and beam position (Bxand By). To this end, it contains two
separate detector chips, Aand B. In addition to these four pointing values, the detector measures
the intensities at both chips, labelled SumAand SumB. These are used for normalization of the
pointing signals, i.e., to make them independent of intensity fluctuations. The sum signals also
indicate whether the laser hits both detector chips, and they allow the PSD-4D to be used to
monitor the optical power. The detector returns its signals in the form of analog voltages to the
µAligna electronics, with ranges of ±10 V for the pointing signals and 0−10 V for the sum signals.
These signals can be monitored on an oscilloscope without disturbing the normal operation of the
system (use the add-on “Aligna Con”, which breaks out the signals to BNC connectors).
5.1.1 Beam Sampling
In almost all applications, the PSD-4D is placed behind a beam sampler which splits a sample beam
off the main beam. The sample beam then enters the PSD-4D and the main beam continues to
the target. The optical power required for good detector signals is about 100 µ W for the visible
spectrum and about 500 µ W for near UV (say 355 nm) and near IR (say 1064 nm).
Clearly, for the Aligna system to work well, the sample beam’s pointing must be a faithful rep-
resentation of the main beam’s pointing. Large intensity fluctuations in the sample beam can be
mis-interpreted as pointing drifts by the Aligna system. Therefore, the beam sampling is crucial to
the performance. In most applications, beam sampler glass plates with parallel surfaces or beam
splitter cubes are not recommended, since they can suffer from interference effects. It is also very
important to mount the glass plates without mechanical stress to avoid birefringence and deforma-
tion of the surfaces. Similarly, very thin glass plates (less than 1mm) are not recommended. The
following general points should be observed:
 A beam sampling glass plate or beam splitter cube located in the main beam path may influence
the beam quality if the flatness, the transmission properties or the polishing is non-perfect.
High-quality elements have to be used.
 A glass plate may cause interference effects due to multiple reflections between surfaces. Glass
plates with a small angle (wedge plates) are preferred. However, they cause an angle deviation
in the main beam.
 In femto-second laser applications, the glass in the beam path may cause unwanted dispersion
effects. However, the problem of interference will not appear with fs laser applications, since
the pulses are too short to interfere with reflected (and therefore delayed) pulses. Here it is
not necessary to use wedge plates. In this case thin plates (1...2mm) are preferred. In most
applications, dispersion effects can then be neglected.
 An uncoated glass plate at 45◦splits approximately 1% of the beam for one polarization
direction (p-light) and 8 % for the other polarization direction (s-light). These values differ
by nearly one order of magnitude, which leads to a strong unwanted polarization dependence
of the test beam intensity.
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 In most applications, both values (1% and 8%) lead to test beam intensities far above the
necessary intensities. These test beam intensities have to be reduced by strong optical filters,
and they are lost for the main beam. With horizontally polarized laser light, it is possible to
get very low reflections by using a reflection angle around Brewster’s angle, ≈57◦. This angle
will be slightly more difficult to align than a 45◦angle. Alternatively, one (or both) surfaces
can be AR (anti-reflex coated) for the target wavelength at 45◦deflection. However, it is not
easy (and not cheap) to find high quality broadband AR coatings with well-defined reflection
grades.
 The transmission of a high reflecting mirror can be used as a sample beam. These transmissions
are typically between 1% and 0.01 %. However, the polarization dependence of the transmitted
beam can be large. Especially high-bred mirrors for high-power or high energy fs pulse lasers
may have a polarization difference between s and p light by factor of 100 or even 1000.
5.1.2 Gains and Offsets
The PSD-4D detector has integrated pre-amplifiers, which transform the generated photo currents
into voltages. These amplifiers have digitally controllable gains, which serve two purposes: Firstly,
they can be adjusted to guarantee a good range of the PSD-4D’s output voltages. In particular,
the signals SumAand SumBshould be kept at 5V. Secondly, the gains allow the user to dial
in detector offsets, which allows to stabilize the beam to positions other than the detector chips’
centers. This is extremely useful, since it removes the need for exact mechanical placement of the
PSD-4D. Changing these offsets while the regulator is active has the effect of moving the target
beam in a controlled manner. In this way, the beam pointing at the experiment or target can be
optimized fast and conveniently. Figure 10 shows the Kangoo section for setting offsets and gains.
The gain values should be in the range 10 −1000. Lower values indicate that too much laser power
Figure 10: Kangoo section for setting gains and offsets.
reaches the detectors, and an optical filter or a similar method should be used to reduce the power.
If the gain exceeds 1000, there is too little laser power (or the chip is not hit properly). When using
the “AutoGain” button, it is advisable to check the gain levels afterwards for consistency.
5.1.3 Thresholds
The Kangoo section “Thresholds” allows to further control the behavior of the micro-controller. The
three values minIntens,maxIntens and setIntens are connected to the intensity display: If either of
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Figure 11: Kangoo section for setting thresholds.
the current intensity values lies outside the minimum or the maximum threshold, the regulator
stops working. This is a safety feature, to ensure for example that the regulator does not react to
environment light when the laser is blocked.
The MotorWrk threshold affects the motor regulator: If all pointing values are within this threshold,
the regulator remains inactive. The MotorWrk value can be set to zero to ensure continuous
regulation.
The threshold PosOK affects the reaction when the regulator is switched off. If, at that moment, all
pointing values are within this threshold, the actuators remain at their current position. If however,
the pointing lies outside the threshold when the regulator is switched off, the micro-controller
assumes that the regulator did not work properly. It then re-sets the actuators to the position where
the regulator was activated. This means that, if the regulator ever “runs away” (maybe because of
an unsuitable OCL matrix), the best course of action is to simply switch off the regulator again.
5.1.4 Calibration
For a standard µAligna system , the displayed pointing values are already calibrated to physical
units. The angle is shown in milli-radiants and the position in millimeters. In addition, the motor-
ized actuators are calibrated to milli-radiants, so that an offset of ”1” on any of the motors will
cause the beam angle to change by one milli-radiant.
5.1.5 Pulsed Lasers
For the operation of the µAligna, one needs to distinguish between pulsed and continuous wave
(CW) lasers. We will subdivide the lasers into the following categories:
1. CW lasers: These lasers are the easiest to handle, because the detectors receive continuous
information about the momentary beam pointing.
2. Pulsed lasers with high repetition rates (15 kHz . . . >1 GHz): The µAligna electronics contains
low-pass input filters which average over multiple pulses and flatten the electronic pointing
signals before sampling. The operation is largely the same as for CW lasers, but the signal-
to-noise ratio of the detector signals may be slightly worse.
3. Pulsed lasers with slow repetition rates (1 Hz . . . 15 kHz): To regulate the pointing of slowly
repeating lasers requires special sample & hold electronics to process the detector signals. To
activate these, change the setting cw/pulsed from “CW” to “pulsed”.
The µAligna is able to generate an internal trigger signal from the PSD-signal. If the user
wants to scan the beam and is thus leaving the detector one needs to feed in an external
trigger signal. Internally, each detected laser pulse is processed and used for a single cycle of
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Figure 12: Location of the cw/pulsed device
the regulator. After that, the regulator goes into a hold state until the next pulse is detected.
Therefore, the µAligna electronics work with arbitrarily low repetition rates. At low repetition
rates, it may be necessary to tune the regulator gains for optimal performance.
Setting up the pulsed detection
To understand how to set up the pulsed detection of the µAligna we explain the detection principle
of pulsed lasers used for our pointing stabilization. Usually a short pulse saturates any detecting
device (photodiode, PSD) and the pointing information is corrupted. Thus the electric signal of the
photodiode is stretched by a low pass filter to spread the energy over a bigger time section. This
stretched pulse is processed by the µ C.
The µAligna distinguishes between two variants to sample the signal. Below 4 kHz the beam is
sampled twice. One sample is taken at the peak and on at idle level. The difference between these
two samples is the photodetector value. This way DC offsets like environmental light or 50 Hz hum
will be ignored. Figure 13 shows the principle of differential pulse sampling. After getting the pulse
trigger the µ C waits a defined Pulse-Delay takes the first sample and, after a second Pulse-Delay,
takes the sample in idle state.
Figure 13: Differential sampling below 4 kHz
At higher repetition rates the pulses begin to overlap each other and differential sampling will lead
to wrong readings. The normalization will not work properly and the autogain procedure may fail.
µAligna Manual
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