MRC Compact User manual

Laser Beam Stabilisation System
Compact
User Manual
Manual - Beam Stabilisation System Compact version 14 – 14-March-2022 page 1 of 28

Contents
1 General 4
2 System components 4
3 Specification 5
3 1 Positioning accuracy 6
3 2 Relation between measured voltage and actual position 7
4 Optical components 8
4 1 Steering mirror mounts 8
4 2 Detectors 8
4 2 1 4-quadrant detectors 8
4 2 2 Wide intensity detector - 4-quadrant diode with wide intensity range 9
4 2 3 PSD detector 9
4 3 Vacuum adaptions 10
4 4 Optical filters 10
5 Installation and operation 10
5 1 Set-up of optical components 11
5 2 Connecting the cables 13
5 3 Power supply 13
5 4 Intensity adjustment 14
5 4 1 Adjustment of sensitivity with 4-QDs 14
5 4 2 Adjustment of sensitivity with PSDs 14
5 4 3 How to replace the optical filters in the detector housing 15
5 5 Pre-alignment 15
5 6 Direction coding of detector outputs 15
5 7 Fine-adjustment 15
5 8 Adjustment of the proportional element (P factor) 16
6 Operation and safety features 17
6 1 Power level and position display 17
6 2 Low power switch-off 17
6 3 Switch-on activity delay 17
6 4 Controller status signal (interlock) 17
6 5 Bandwidth limitation switch 18
7 Option: Sample&hold circuit (“ADDA“) 18
7 1 Technical specification 18
7 2 Modes of operation 19
7 3 Configuration and start of operation 19
7 4 Performance 20
8 Option: Serial interface (USB, RS-232 or Ethernet) 23
9 Additional inputs and outputs (Options) 23
9 1 Voltage offset inputs to move the target position on PSDs (“Adjust-in”) 24
9 2 Direct drive of Piezo actuators („Drive Actuator“) 24
9 3 Option: External activation 25
9 4 Intensity outputs at controller 25
9 5 Range outputs for monitoring applied Piezo voltages 25
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10 Drawings 25
11 Cables 26
11 1 Standard cables 26
11 2 Additional cables 26
12 Troubleshooting 26
12 1 No signals on display 26
12 2 No signals on detector 26
12 3 The laser beam is not correctly positioned 26
12 4 The steering mirrors make exceptional noise 27
12 5 Laser position is not stable 27
12 6 Permanently red “Range” signal 27
12 7 “Range” signals jump back and forth when switching the direction coding 27
12 8 System steers the beam away from the centre 27
13 Safety 28
14 Contact 28
Subject to change without prior notice
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1. General
The Compact laser beam stabilisation
compensates for vibrations, shocks, thermal
drift, or other undesired fluctuations of the laser
beam pointing The system should be applied
whenever laser fluctuations or movements of
optical components occur but a high precision
and stability of the beam position and angle is
required
The desired position of the laser beam is
defined by a position detector (4-quadrant-diode
(4-QD) or PSD) For that purpose a small
portion of laser power transmitted through a
high-reflective deflection mirror (“leakage”) is
sufficient
The closed-loop real-time control continuously determines the deviation of the laser beam from the
desired position and drives the fast Piezo actuators in that way that the steering mirrors stabilise the laser
beam in the desired position
The 4-axes system combines two pairs of detectors and steering mirrors in order to stabilise the laser
beam in 4 degrees of freedom (4D, position and angle) The 2-axes system uses only one such pair It
stabilises either the beam position at exactly one point or – if e g the detector is placed into the focus of
a lens – the beam angle
2. System components
The laser beam stabilisation utilizes the control electronics and the optoelectronic components (steering
mirrors, detectors) The following figures show the standard components Beyond these, we offer various
types of steering mirror mounts with Piezo actuators and detectors For more details please check the
specification in sections 3 and 4
Figures 2, 3, and 4 (from left to right): Steering mirror with Piezo dri e ( ersion P2S30), detector
with position and intensity display (horizontal orientation), detector ( ertical orientation)
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Figure 1: Principle of laser beam stabilisation

The system electronics (controller, amplifiers, power supplies) is fully integrated into a single compact
housing It is powered by a standard 12 V wall power supply
Figure 5: Keyboard and connectors on top panel
3. Specification
You can find detailed data sheets of the different components of the beam stabilisation system on our
website We can also send them to you The following table shows an overview:
Optical parameters
Wavelength 320 to 1100 nm, UV and IR detectors are also available
Repetition rate any rate or cw
For lasers with low repetition rates (< 1 kHz), with single pulses or
with laser off-times we offer an additional sample & hold circuit,
see also note 1
Laser beam diameter < 6-8 mm (1/e²), see also note 2
Height of laser beam 40 mm for P2S30 and P4S30,
45 mm for PKS, 39 5 mm for PSH
Mirror diameter P2S30: 1'' (standard)
P4S30: 1'', 1 5'', 2'' and other mirror diameters
Manual - Beam Stabilisation System Compact version 14 – 14-March-2022 page 5 of 28
Figure 7: Input connectors, P factor adjustment
and switches on right side
Figure 6: Power input and output connectors
on left side

Mirror thickness 1/4" or 1/8" (recommended)
Controller housing imensions
w x h x d 166 x 106 x 56 mm3
Control features
Power level display LED line with 10 elements on the backside of the detector
Position display LED cross on the backside of the detector
Variable intensity gain Continuous, adjustable with potentiometer (1:6)
Low power switch-off Power level falls below 10% of saturation power
Switch on activity delay 300 ms
Computer interface
Options USB, RS-232 or Ethernet
Connectors at controller unit
Actuator LEMO 0S series
Detector LEMO 0B series
Controller status signal (interlock) LEMO 00 series
x, y position output LEMO 00 series
P factor setting and read-out LEMO 00 series
Power supply 12 V / DC pin-and-socket connector
Notes:
(1) A description of the sample & hold circuit is given in section 7 of this user manual
(2) If the beam diameter is larger than 8 mm, a lens in front of the detector can be used Please refer
to the description “Optimisation of the setup with lenses” on our website for details
3.1. Positioning accuracy
The positioning accuracy depends on several parameters:
•Optical distance between steering mirror and detector: The accuracy is higher for larger
distances Therefore a large distance should be chosen
•Beam diameter: Having the same absolute change of laser beam position, a smaller diameter
leads to stronger power differences on the quadrants of a 4-QD and therefore a steeper control
signal That is why laser beams with smaller diameter can be positioned with higher accuracy
•Intensity: The resolution of the detectors further depends on the intensity hitting the sensitive
area However, this can be varied by optical filters and optimised electronically (see section 5 4)
•Repetition rate and pulse duration: The controller bandwidth can be optimised for different laser
parameters Higher bandwidths lead to a faster reaction and therefore higher accuracy in case of
fast fluctuations
Note: The system uses the centre of the transversal laser beam profile It does not reduce fluctuations of
the laser beam profile itself
The position signals of the detectors can be read out at the front panel of the controller
Position outputs x, y
Description 4 outputs: beam position (stage 1 and stage 2)
Signal Analog, - 5 V … + 5 V
Connectors LEMO 00 series
Figure 8 shows the typical resolutions of the 4-quadrant detectors The example demonstrates that a
resolution of better than 100 nm on the detectors can be achieved with an appropriate choice of
Manual - Beam Stabilisation System Compact version 14 – 14-March-2022 page 6 of 28

parameters The angular resolution can be determined from these data with respect to the respective arm
lengths
Figure 8: Resolution of a 4-quadrant diode irradiated by a red He-Ne laser
with different beam diameters and laser intensities
By the use of the material Invar with a very low coefficient of thermal expansion the detectors are
stabilised against temperature variations which ensures that the accuracy is maintained over long term
The actuators are controlled with an analog signal so that the positioning is not restricted to separate
steps The positioning accuracy of the Piezo elements is in the range of a few nrads
3.2. Relation between measure voltage an actual position
The position signals are given as voltages The following formulas allow to convert the voltages into
actual positions
4-qua rant etector
For the calculation, the beam diameter must be determined first Then the deviations of the positions in
µm can be approximated with the following formula which is valid as long as the beam is near to the
centre of the 4-quadrant diode:
x[µm] = D[µm]
π
⋅x[V]
I[V]
Where x is the x position signal measured in volts or calculated in µm The same calculation can be
made for y D is the Gaussian beam diameter (1/e2) and I is the measured intensity signal For a more
precise calculation or if the beam is further away from the centre, the following formula can be used
Here erfinv() is the inverse error function:
x[µm] = D[µm]
2⋅
√
2⋅erfinv (x[V]
I[V])
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In case of non-Gaussian beams or to obtain the exact relation, you have to perform a calibration by
means of a micrometer stage
PSD etector
In case of PSDs the relation between voltage and position is almost linear Here you can use the
following ratio (similarly for y):
x[µm] = x[mV ]
(1.2 ±0.03)
You can find further information on the calculations in the description “Position and angular accuracy”
on our website
4. Optical components
In this chapter we summarize some essential properties of the optical components of the beam
stabilisation More detailed information can be found in the respective data sheets
4.1. Steering mirror mounts
Specification P2S30 P4S30
Tilting range 2 mrad (± 1 mrad) mechanical,
4 mrad optical
4 mrad (± 2 mrad) mechanical,
8 mrad optical
Coarse adjustment (manually) ± 4 5° ± 4 5°
Piezo stacks 2 integrated Piezo stacks 4 integrated Piezo stacks
High resonant frequencies up to 1,200 Hz > 1,200 Hz (with 1'' mirror)
~ 300 Hz (with 2'' mirror)
High stabilisation bandwidths ~ 400 Hz (with 1'' mirror) > 400 Hz (with 1'' mirror)
> 100 Hz (with 2'' mirror)
Mirror sizes 1 inch 1, 1 5, 2 and 3 inches
Free aperture for beam transmission 12 x 12 mm2-
Notes:
•The movable top plate of the Piezo elements is sensitive to mechanical forces Please avoid the
impact of strong forces or torsional moments on it The Piezo stacks are attached to this plate
•If you intend to remove a mirror adapter you should be especially careful We can provide a
specific instruction and a tool for this purpose
4.2. Detectors
4.2.1. 4-qua rant etectors
Specification vis 4-QD
(silicon)
UV 4-QD 3x3
(enhanced silicon)
I InGaAs 4-QD I Germanium
4-QD
Wavelength range 320 – 1 100 nm 190 - 1 000 nm 900 - 1 700 nm 800 - 2 000 nm
Sensitivity range 10 x 10 mm23 x 3 mm2Ø = 3 mm 5 x 5 mm2
Element gap 30 µm 100 µm 45 µm 20 µm
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Sensitivity range 10–165 μW / 3-55 nJ @ 532 nm cw
(adjustable with gain potentiometer and optical filters)
Damage threshold limited by optical filters - typical values:
Max absorbed power: 0 05 – 0 1 W for Ø=1mm, 0 2 – 0 4 W for Ø=3mm
Max absorbed energy: 1-5 µJ for Ø=1mm, 5-20 µJ for Ø=3mm
Dimensions
Housing (w x h x d) 40 x 49 5 x 23 9 mm3
Optical filter 11 9 x 11 9 mm2
Further functions
Power indication LED line with 10 elements on the backside
Position display LED cross on the backside
Connectors
x, y, intensity outputs MCX
Power supply 12 V / MCX
4.2.2. Wi e intensity etector - 4-qua rant io e with wi e intensity range
Specification
Dynamics / Intensity range 3 decades
Bandwidth < 10 kHz
Signal scaling 9 mV / µm (typical for 1 mm beam diameter)
Sensitivity range ~ 5 µW – 5 mW (@ 532 nm, cw)
Reproducibility over the complete intensity range 10 mV (with 1 mm beam diameter ~ ± 1 µm)
All other specifications are the same as those of the standard 4-quadrant detectors
Notes:
•For the wide intensity detector, the function of the intensity display is unchanged It can still
support the selection of the filters The potentiometer described in section 5 4, however, is
omitted
•Due to the wide intensity range it is possible to detect even lowest laser powers That is why, de -
pending on the selection of the optical filters, the detection can be affected by ambient light
4.2.3. PSD etector
Specification VIS PSD
Wavelength range 320 - 1,100 nm
Sensitive area 9 x 9 mm2
In contrast to the 4-quadrant diode the PSD has a continuous measurement area
Notes:
•If we equip the beam stabilisation with PSDs but no further measures, we use the electronic
centre (defined by a voltage of 0V for x and y position) as the target position
•The position vs voltage characteristics of a PSD is usually not linear This means that a
pincushion distortion occurs when the beam is sweeping the complete sensor area I e the ex-
pected position value can deviate minimally from the voltage value Therefore, we recommend
performing a calibration if the target shall be moved along a defined path The absolute accuracy
at a stabilized position is not affected
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4.3. Vacuum a aptions
Both, the detectors and the actuators, can be adapted for use in vacuum In case of the actuators, this is
possible for vacuum pressures down to 10-11 mbar But this is an extreme value In case you intend to
place some components in vacuum please let us know the conditions so that we can discuss and suggest
the required measures Some measures (choice of materials, cables, sealing) are mainly focussed to avoid
degassing and depend on the pressure Others are important to protect the components themselves
Notes:
•The controller itself should not be placed in vacuum
•The vacuum compatible detector does not have the LED displays on the backside That is why
additional intensity outputs are integrated into the control box
4.4. Optical filters
We usually integrate a pair of optical filters in front of each sensor They have a size of 11 9 x 11 9 mm2
and fit into the provided slot in the detector housing The filter which is further inside is optically denser
5. Installation an operation
A quick installation guide is part of the scope of delivery It explains how to start up the beam
stabilisation If you no longer have this guide, you can download it from our website or ask us to send it
In the following sections, we explain individual steps in more detail
The system operation can be described best with reference to figures 5 to 7 The top panel in figure 5
shows the keyboard and the position signal outputs for two pairs of detectors and actuators (stage 1 and
stage 2) Each stage can be started and stopped independently by pressing the Start/Stop button When
the stage is started the small LED in the top right corner of the button is shining The Range display
shows whether or not the steering mirrors are within the available capture range The Acti e LED is
shining whenever the control stage is active This is the case whenever the Start/Stop button has been
pressed and the laser power on the detectors has the right level
The Position outputs on the top panel can be used to read out the current position of the laser beam on
each detector (x and y)
Notes:
•Whenever the Start/Stop button is pressed (and the Acti e LED is on) the actuators start to move
from the zero position and then respond to the controller input
•If a Range LED is shining red, this does not automatically mean that the beam is not stable But
it indicates that no further tilt of the respective steering mirror is possible although it might be
necessary
•If the power on the detectors is too low the actuators are driven to the zero position (and the
Acti e LED is off) This is due to the low power switch off that was implemented for safety
reasons (see section 6 2)
Figures 6 and 7 show both sides of the control box with the connectors, the P factor adjustment and the
switches for the Directions and the Bandwidth selection The cables to the actuators are connected on the
left side The cables coming from the detectors are connected on the right side
The description of the adjustment and read-out of the P factor is given in section 5 8 The Directions
switches enable a coding of the x and y directions of each control stage They are connected with Det1
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and Det2, respectively The performance is further described in section 5 6 The function of the
bandwidth limitation switch is explained in section 6 5
The Status signal output can be used as an interlock or to drive a shutter (see section 6 4)
Note: The Piezo elements have a large electrical capacity That is why the cables should not be
disconnected as long as the Piezo elements are charged I e you should always switch off the power of
the stabilisation system on the left side of the panel and then wait for a few seconds before you
disconnect the actuator cables
5.1. Set-up of optical components
The steering mirrors and detectors can be set up in variable arrangements for different applications
The detectors can be placed behind high-reflection mirrors They are very sensitive and can work with
the leakage behind the mirrors This has the advantage, that no additional components are required in the
beam path Alternatively, it is possible to use the reflection of a glass plate or a beam splitter The latter
can be necessary for lasers with larger beam sizes where the actuator would constrain the transmission
In any case, the centres of the detectors should be positioned in that way, that they define the desired
laser beam direction The target positions on the PSD detectors can be different from their centres For
further information please refer to section 4 2 3 The first actuator should be placed close to the laser or
the last source of interference The last detector should be placed close to the target
Note: Take care for a robust mechanical mounting of the optical components If possible, the delivered
components should be directly tightened to an optical table without further positioning equipment (like
height adjustment) If there are oscillating components with resonance frequencies within the control
bandwidth in the set-up, such resonances can provoke oscillations of the system at those frequencies
The following figures 9-13 show a selection of possible arrangements These examples are demonstrated
with the 4-axes system with two detectors However, they can be applied in similar configurations for the
2-axes system with only one actuator and one detector
•Figure 9 shows a typical 4-axes set-up of the system where the laser beam hits the optical
components in the following sequence: steering mirror, combination of steering mirror and
detector, mirror with detector
•Figure 10 shows a similar set-up where additional lenses are placed in front of the detectors
Further, a beam splitter is integrated in the beam path This set-up might be better for lasers with
large beam diameters
•In figure 11 a lens is placed in front of detector 2 in order to improve the angular resolution In
this case, the distance between lens and detector should be the focal length of the lens The focal
length itself should be chosen in that way – depending on the beam diameter – that the focal spot
is not too small In case of 4-QDs the beam should still have a diameter on the sensor area of
> 50 µm, so that it hits all quadrants of the diode (The gap between the quadrants is 30 µm for
our standard 4-QD, and even more for other 4QDs )
•Figure 12 shows a variation of 11 where both detectors are placed behind the same mirror In
order to measure both, the beam position and the direction at the same point, a lens is placed in
front of detector 2
•Figure 13 finally shows an arrangement where the 4-axes system is used as two 2-axes systems,
i e the two stages of the controller are used to separately stabilise two independent beam lines
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Notes:
•In some cases in set-ups where the distance between the actuators 1 and 2 is rather small, a
positioning error can occur This is the case if detector 1 is not placed sufficiently close behind
actuator 2 A lens in front of detector 1 can eliminate this positioning error The lens and the
distances should be chosen in a way that the front surface of the mirror is imaged on the detector
The distances and the focal length f of the lens can be calculated with the lens equation 1/f = 1/
b + 1/g While g is the distance between the mirror's surface and the lens, b is the distance
between the lens and the detector's surface
•For setups with lenses, please also refer to the description "Optimisation of the setup with
lenses" on our website
Manual - Beam Stabilisation System Compact version 14 – 14-March-2022 page 12 of 28
Figure 9: Typical sequence of components for
the 4-axes stabilisation: Detector 1 stabilises the
beam position on actuator 2. Detector 2 then
defines the beam position at a separate point and
hence the direction.
Figure 10: Set-up as in 9, with an additional
beam splitter and a lens in front of detector 1
and an additional lens in front of detector 2
(Often used for lasers with larger beam
diameters)
Figure 11: Set-up as in 9, but a lens is used to
discriminate the angle by means of detector 2.
This can be of ad antage in case of restricted
space with small distances between the optical
components. Detector 2 must be placed in the
focal plane of the lens.
Figure 12: This set-up shows a ariation of figure
11. Both detectors are placed behind the same
mirror in order to measure both, the beam position
and the direction at the same point. A lens is
placed in front of detector 2 which discriminates
for the angle.

Figure 13: Set-up of a 4-axes system used as two 2-axes systems. With this set-up the position of two independent
lasers can be stabilised with one controller.
5.2. Connecting the cables
The first steering mirror is connected to the Actuator 1 output The second steering mirror is connected
to Actuator 2
The detectors are connected to the control box with a LEMO cable with a length of 4 m and an adapter
cable that splits the LEMO cable into four separate cables These cables are connected to the detectors
according to the following rules: The x and y lines have to be connected in accordance to the orientation
of the detector housing If the detector is oriented in vertical orientation as shown in figure 4, the x line
has to be connected to the x output and the y line to the y output If the detector is turned by 90° to a
horizontal orientation as shown in figure 3, the x line has to be connected to the y output and the y line to
the x output At the other end, the LEMO cables of the detectors are connected to the respective detector
inputs at the controller module
Note: In case of the 2-axes system you can either use the first or the second stage for stabilisation
5.3. Power supply
The power supply is provided by the delivered plug-in power supply The system is switched on via the
switch on the left side of the housing
Note: The power supply is specified with 12V and 3 8A The 3 8A is not needed during the operation
However, due to the loading of the buffer capacitors and the ramp-up of the high voltage module, the
peak currents are quite high when the system is switched on If at least 3A are not available during the
switch-on phase, the high-voltage modules do not reach their rated voltage and there is a risk of damage
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5.4. Intensity a justment
5.4.1. A justment of sensitivity with 4-QDs
To make sure that the detectors operate in the linear range, the power level can be adjusted by tuning the
potentiometer for intensity variation (see figure 14) For that purpose, switch on the system (Power on)
and inactivate the closed-loop control (Start/Stop button switched off, green Acti e-LED and LED on
button off) Then adjust the laser beam onto the detectors in that way that at least 3 but not more than 9
elements of the power level display are shining The amplification increases by counter-clockwise
rotation If the intensity on a detector is too high the sensor gets saturated In this case all LEDs of the
power level display are blinking
If you do not find an appropriate adjustment you have to exchange the optical filters in front of the 4-
QDs (see section 5 4 3) If the required filters are not available please contact us
Notes:
•In a standard delivery we integrate two optical filters in front of the sensor These are filters with
a high and a low density for coarse and fine adjustment, respectively Usually the filter which is
the first to be reached is the low density one
•Please be aware that the sensor is quite sensitive If you want to clean it you should do this
carefully with a lint-free cloth
Figure 14: 4-quadrant-diode. The arrow points to the potentiometer
for intensity ariation (Please use a screwdri er)
5.4.2. A justment of sensitivity with PSDs
The PSDs have small push-buttons instead of the potentiometer The adjustment of this digital
potentiometer can be carried out with the delivered metal pin by means of soft pushing There is a small
push-button for each direction behind the bores in the housing (see the arrows in figure 15) With the
upper push-button you can increase the gain step by step, with the lower push-button you can decrease it
There are 64 steps between the highest and the lowest gain This corresponds to a change of the
sensitivity by a factor of 20
If you do not find a fitting adjustment, you can exchange the optical filters in front of the PSD sensors
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Figure 15: PSD detector. The arrows point to the push-buttons of the digital potentiometers
for the gain adjustment (which can be carried out with the deli ered metal pin)
5.4.3. How to replace the optical filters in the etector housing
In some cases it can be necessary to exchange the optical filters The filters are fixed to the housing with
two plastic screws To replace the filters carefully open the plastic screws You can use forceps to hold
the screws during the fixation Usually, the filter with the higher optical density is the one which is
deeper in the slot
5.5. Pre-alignment
For pre-alignment of the laser onto the detectors you should at first not activate the control (Acti e-LEDs
off) However, the electronics must be switched on (supplied with power) so that the Piezo actuators
drive into their zero positions The laser should be aligned onto the detectors so that only the green LEDs
in the centre of the LED position display are shining If you use the software, you can also observe the
positions there
5.6. Direction co ing of etector outputs
Each control stage makes use of a steering mirror and a detector as described in sections 5 1 For any
deviation of the laser beam position on a detector the respective steering mirror is tilted in that way that
it aligns the laser beam back to the desired position The components that are working together are
identically coloured in figures 9-13 The direction in which the steering mirror must be tilted depends on
the arrangement of detector and steering mirror It can be changed during the adjustment process
described in section 5 7 in the following way:
There are four switches on the right side of the controller module (see figure 7) These switches stand for
the x and y directions of the control stages Stage 1 and Stage 2 To turn them into the correct position
just activate the respective stage If the laser beam is then deflected into an extreme x (horizontal) and/or
y (vertical) position instead of the centre of the detector, you have to toggle the belonging switch
5.7. Fine-a justment
The fine-adjustment should also be performed with inactivated control stages The better the correlation
of desired position and zero position of the Piezo actuators, the smaller the position shift once the closed-
loop control is started
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Adjust the laser beam by means of manually tilting the steering mirrors or any other mirrors in the setup
in that way, that it hits the centres of the detectors This can be done by observing the displays in the
software or by reading out the x and y position outputs of the controller which deliver voltages that
directly depend on the deviation from the target position You can easily display these signals on an
oscilloscope
A helpful indication for a good adjustment are also the Piezo voltages If you use the software, you can
continue to manually adjust the laser beam onto the detectors with the control-loop activated ( Start
button switched on, green Acti e-LED and LED on button shining) for a final adjustment until all four
Range displays in the software are close to 0 V Now the steering mirrors are operating in their linear
range
After these adjustments the system should show no fluctuations of the laser beam position after the last
mirror with detector when the controller is activated
5.8. A justment of the proportional element (P factor)
Usually the factory settings of the proportional and integral elements of the control loop lead to a very
stable performance of the beam stabilisation system with desired bandwidths That is why no user
interactions are required to adjust the control loop However, in specific cases the user might wish to
adjust the control loop for his application Such cases can e g be setups with rather long arm lengths
Since the control loop is mainly influenced by the proportional element, the system offers a direct access
to the P factors of both control stages by means of potentiometers or via the (optional) serial interface
The potentiometers P1 and P2 are located at the side panel of the control box (figure 7) The adjustment
can be done separately for each stage An increase of the P factor usually leads to an increase of the
overall bandwidth In order to optimize the performance, we recommend to start with a small P factor
and operate the system in this stable configuration Then you can increase the P factor by simply turning
the potentiometer in clockwise direction or by increasing the values in the software, until the system
reaches its stabilisation limits and starts to oscillate The potentiometers or values should then be turned
back to a level, where an operation without oscillations is guaranteed
Notes:
•The optimal P factors of stages 1 and 2 can differ
•If the distances of the optical components, the beam diameter, the laser intensity, or other laser
data change, the P factor of the overall system might also change
The system is also equipped with analog inputs for a remote setting of the P factors The remote
adjustment connectors are integrated into the control box in addition to the potentiometers They are
labelled with P1-Sig and P2-Sig (figure 7) Whenever a voltage signal is applied to the remote
adjustment, the potentiometers are ineffective The input voltages can be set between 0 and 5 V The
interface can also be used to read out the current voltages as set by the potentiometers
Specification
Input/output voltage range 0 … +5 V
Connector LEMO 00 series
Cable (optional) LEMO 00 → BNC for each stage, length 2 m, 2 units
Note: The remote adjustment has to be driven with a low impedance voltage source (≤ 1 kOhm),
whereas the read-out drives only high impedance terminations (≥ 1 MOhm)
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6. Operation an safety features
6.1. Power level an position isplay
The total power on each connected detector is displayed by means of a LED line on the backside of the
detector housing Furthermore, a LED cross on the detector housing displays the current laser beam
position If the laser beam hits the centre of the detector only the green LED of the position display will
shine In other cases also yellow and red LEDs will shine according to the examples in figure 16
a) c)
b) d)
Figure 16: Examples for laser beams hitting the detector (orange spots) and the corresponding position display.
The left pictures are shown in a iew from the rear side of the housing to the sensor area.
If only green and yellow LEDs are shining the sensor electronics is in the linear range where a direct
correlation between measured signal and position exists If a red LED is shining too, the correlation is no
more possible due to the principle of 4-QDs In case of the PSDs, if a red LED is shining, the beam
probably hits an edge of the sensor Please check if the full diameter of the beam hits the sensor area
6.2. Low power switch-off
If the total power falls below 10% of the saturation power (only two LEDs of the LED line are on) the
controller automatically drives the mirrors into their zero positions This leads to the advantage that the
closed-loop control can start from the zero position even if the laser was switched off or blocked
6.3. Switch-on activity elay
The integrated switch-on activity delay starts the control only some time after sufficient intensity hits the
detector again This ensures that the control does not start until a reliable control signal is present and the
steering mirrors have reached the zero positions The Acti e LED will not shine during this delay
6.4. Controller status signal (interlock)
If the system is completely switched off (power off), the Piezo actuators of the P2S30 tilt the steering
mirror into an extreme position This is about 1 mrad from the zero position (The P4S30 does not show
this behaviour due to its design with 4 Piezo stacks ) However, the system is equipped with a TTL output
that can be used to block or electronically switch off the laser in order to avoid damage by the
misaligned beam The level is HIGH when the controller is active and the steering mirrors are in the
correct range or in zero position It is LOW if the controller is active and one of the actuators is out of
range (If the controller is not active, the level is always HIGH )
Manual - Beam Stabilisation System Compact version 14 – 14-March-2022 page 17 of 28

Note: The criterion for the actuators being "out of range" is that the Piezo voltage reaches 95% of its
maximum or minimum value
Status signal
Description 1 output for both stages
Signal TTL, LOW if Piezo is out of range
Connector
Cable (optional)
LEMO 00
LEMO 00 → BNC, length 2 m
6.5. Ban wi th limitation switch
The controller bandwidth directly influences the quality of the stabilisation The system can be operated
with two different controller bandwidths The default setting is the high bandwidth However, especially
in case of unstable mechanical set-ups or if a mutual interference of the control stages occurs it can be of
advantage to choose the low bandwidth Therefore a bandwidth limitation switch is integrated in the
controller module (Bandwidth, see figure 7, H = HIGH, L = LOW bandwidth) The bandwidth can be
chosen independently for both stages
7. Option: Sample&hol circuit (“ADDA“)
The additional circuit is used to fix the laser beam in the last position during laser off times With this
add-on, which is integrated into the control box, the positions of the steering mirrors can be fixed for an
arbitrarily long time interval without control signal or laser intensity on the detectors In that way it is
possible to start the control-loop after switching on the laser not from the zero position but from that
latest stabilised position You can find the detailed description "Sample&Hold circuit ("ADDA")" on our
website which explains the various applications of this add-on
The name “ADDA” is derived from the functional aspect that the actuators' drive signals are first AD
converted and digitally stored before they are subsequently DA converted again and fed to the amplifiers
of the mirror actuators
7.1. Technical specification
Sample & Hol circuit
Storage principle Digital storage of position data
Sampling time 64 µs per event
Freezing interval unlimited
Requirement for automatic triggering Minimal laser on time: > 100 ms
External triggering
Signal levels TTL, HIGH for laser on, LOW for laser off
Inputs 1 input for each stage
Connector
Cable (optional)
2x LEMO 00, separate connectors for stage 1 and stage 2
LEMO 00 → BNC for each stage, length 2 m, 2 units
Minimal length of trigger signal “high“ tmin ≥ 10 µs
Trigger start 10 µs before until 50 µs after start of laser pulse
Trigger end max 1 ms after laser pulse end
Trigger (digital)
via serial interface Commands: “SetTriggerFreeze”, “ClearTriggerFreeze“
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7.2. Mo es of operation
Automatic control of sample & hol elements
The beam stabilisation with additional S&H circuit includes an automatic recognition of laser on and off
states This is done by sampling the intensity on the position detectors The automatic operation controls
the S&H elements in order to store the signals during laser on times and fix the position of the steering
mirrors during intervals with no intensity
For this mode of operation the laser on intervals or the respective duration of pulse packages must be
longer than 100 ms In case of the automatic control you do not need to provide any trigger signals
Note: When using WID detectors (see section 4 2 2), the automatic control does on principle not work
External triggering of the sample & hol elements
For single laser pulses or lasers with very low repetition rates, modulated cw lasers or pulse trains
< 100 ms the automatic control can not release the stored beam position in due time In such cases it is
necessary to control the S&H elements by means of external triggering The requirements for the trigger
signals are described in section 7 3
7.3. Configuration an start of operation
Cabling
In the operation mode of automatic control there is no need for additional cabling For external triggering
the trigger signals have to be fed into the control box via the respective LEMO connectors marked with
“Trig” (see figure 17) The left connector controls the S&H function for stage 1 / steering mirror 1 The
right connector controls it for stage 2 / steering mirror 2
Figure 17: Left panel of control box with trigger inputs
External triggering
The external triggering enables an accurate timely assignment when the system shall store the position of
the steering mirrors and when the position shall be fixed This assignment is especially important in case
of single laser pulses For an optimal function of the S&H circuit there are time restrictions for the
trigger signal which should be met Figure 18 illustrates the respective tolerances of the trigger signal
Manual - Beam Stabilisation System Compact version 14 – 14-March-2022 page 19 of 28

Start of operation
Whenever the stabilisation is de-activated (i e the Start/Stop button is in off-state) the stored position of
the steering mirrors is reset In this state the steering mirrors are in their zero position In this way it is
guaranteed that the system can be adjusted as described in this user manual
Note: Please note that the last position of the steering mirrors is lost whenever the stabilisation is de-
activated As soon as the system is started again it starts from the zero position of the steering mirrors In
case of large distances between steering mirrors and detectors there is a risk that the beam will not hit the
detector without a prior re-adjustment
7.4. Performance
The performance of the additional S&H circuit shall be explained in the following sections with the help
of some examples In figure 19 a sequence of pulse trains with a repetition rate of 1 kHz and a duration
of about 300 ms was applied The pulse trains are displayed with green colour The violet curve shows
the position signal of the laser on the detector
During the first pulse train the stabilisation was de-activated You can see that the pulse does not hit the
detector in the centre During the second pulse train the stabilisation was started You can see an initial
spike of the position (enlarged view is shown in figure 20) and then a stable position signal which is also
stable during the third and fourth pulse train Without the S&H circuit the spiking of the steering mirror
would occur again and again in the second and all following pulse trains
At the time the beam stabilisation is started the steering mirrors are in their zero position Since this
position usually differs from the desired position the system recognises a strong control amplitude
immediately after its activation This leads to the described spike In normal use cases where the laser
provides a continuous control signal this is not a problem since the controller always gets a signal
However, in case of some applications, there are time intervals without a control signal In these cases
the additional S&H circuit becomes effective: After time intervals without laser intensity the stabilised
operation is re-activated for the next pulse train without a larger spike This will be demonstrated in the
following sections “Automatic control” and “Operation with external trigger” Without the S&H circuit
it would have started from the upper position and would have produced a spike
Manual - Beam Stabilisation System Compact version 14 – 14-March-2022 page 20 of 28
Figure 18: Timing of trigger signal