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
Manual - Beam Stabilisation System Compact version 14 – 14-March-2022 page 3 of 28

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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