MRC XY4QD User manual

Real-time Laser Beam Position Detector
„XY4QD”
User Manual
XY-4QD – User Manual version 4 – 25.07.2014 page 1 of 7

1. Introduction
The XY4QD is a 4- uadrant-diode with readout electronics and outputs for x and y position. The position
of the laser beam on the diode can be displayed without relevant time delay on a scope. Thus, even
fastest fluctuations can be resolved. This is the main difference between this detector and beam profilers
that can not measure a position in real-time and integrate over several pulses or fast photodiodes that can
not measure any position. In addition to the x- and y-position there is also an output for the measured
intensity level on the detector.
The XY4QD displays the information about the intensity level and the position directly on the housing.
For this purpose an LED line and an LED cross are used, respectively.
The following figure shows the backside with the displays:
. System components
A typical delivery includes the following components:
•Detector with integrated 4- uadrant-diode (4-QD), readout electronics, and optical filter
•3 measurement cables MCX↔BNC
•Wall power supply 12V
•User manual
XY-4QD – User Manual version 4 – 25.07.2014 page 2 of 7
Figure 1: XY4QD with displays for intensity level
and position
LED cross for
position display
LED line for
intensity level
display
Intensity
output
x and y
position output
Power supply
(12V)

3. Specification
3.1 Technical data
Opto-electronic properties
Bandwidth up to 10 kHz (without intensity scaling > 100 kHz)
(The bandwidth is optimized to a customized value in our lab.
For the resolution of single pulses one should choose:
bandwidth ≤ laser repetition rate)
Sensor area 10.0 x 10.0 mm2
Typical spatial resolution < 1 µm (depending on the beam diameter and profile)
Spectral sensitivity 320 – 1100 nm
Mechanical dimensions
Housing 49 x 40 x 23.9 m3
Thread M4
Optical filter (can be adapted on laser power and wavelength)
Dimensions 11.9 x 11.9 mm2
Fastening screws M2
Connectors
x, y, I / Connectors analog x, y: ± 5 V / I: 0 – 7V / MCX Coax connectors, standard
oscilloscope input (high impedance, ≥ 4,7kOhm)
Power supply / Connector 12V, < 200mA / DC pin-and-socket connector 1.1mm
Measurement cables MCX↔BNC, standard length1,8m (other lenghts on re uest)
3. Housing
Figure 2: Housing
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3.3 Position accuracy
The position accuracy depends on several parameters:
•Beam diameter: Having the same absolute change of laser beam position, a smaller diameter
leads to stronger power differences on the uadrants and therefore a steeper control signal. That
is why laser beams with smaller diameter can be measured with higher accuracy.
•Intensity: The resolution of the detectors further depends on the intensity hitting the sensitive
area. This can be varied by an appropriate choice of optical filters and optimised electronically
(see also section 3.5).
In figure 3 typical resolutions of the detector are displayed. The example shows that a resolution of better
than 100 nm on the detector can be achieved with an appropriate choice of parameters.
Figure 3: Resolution of a 4 quadrant diode irradiated by a red
He Ne laser with different beam diameters and laser intensities
3.4 Displays
The XY4QD is delivered with a LED line for the intensity level and a LED cross for position display.
3.5 Intensity adjustment
To make sure that the detector operates in the linear range, the power level can be adjusted by tuning the
potentiometer for intensity variation (see figure 4). For that purpose adjust the laser beam onto the
detector 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 you do not find an appropriate adjustment you have to exchange the optical filters in front of the 4-QD.
If the re uired filters are not available please contact the manufacturer or distributor.
XY-4QD – User Manual version 4 – 25.07.2014 page 4 of 7

Figure 4: 4 quadrant diode. The arrow points to the
potentiometer for intensity variation (Please use a screwdriver)
Notes:
•In a standard delivery we integrate two optical filters in front of the sensor area. These are filter s
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 area is uite sensitive. If you want to clean it you should do this
carefully with a wet lint-free cotton swab.
If you want to exchange the filters you can detach the plastic screws which fix the filters in the housing.
With a tilt of the detector housing it should be possible to release the filters. Once you put in new filters
please be careful so that you do not damage the detector. Finally you can fix them with the plastic
screws.
3.6 Intensity scaling
The standard XY4QD is e uipped with an intensity scaling. Here, a divider is integrated into the readout
electronics that sets the intensity of each uadrant in relation to the total intensity on all four uadrants.
In that way the measurement is independent of the actual laser power.
The intensity scaling can be omitted on re uest in order to reach even higher bandwidths and get access
to the raw data.
4. Installation and operation
The intensity on the 4-QD should be chosen with values that lead to illumination of at least 2 and not
more than 9 LEDs. This corresponds to voltages of 0.5 to 6 V. You can achieve the correct values by
means of appropriate filters in front of the 4-QD. The XY4QD should be aligned to the laser beam in that
position where it hits the center of the 4-QD. In this case you can detect the widest range of position
changes.
You can get a comparable information regarding the position by using a scope. In order to get this you
can connect the x and y signals to two channels of the scope and choose the scope's x-y-display mode.
XY-4QD – User Manual version 4 – 25.07.2014 page 5 of 7

4.1 LED cross for position display
If the laser beam hits the center of the 4-QD 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 5.
a) c)
b) d)
Figure 5: Examples for laser beams hitting the 4 QD (orange spots) and the corresponding position display. The
left pictures are shown from the direction of the position display, i.e. from the rear side of the housing.
If only green and yellow LEDs shine the sensor electronics is in the linear range where a direct
correlation between measured signal and position exists (see section “Calibration”). If a red LED shines
too, the correlation is no more possible due to the principle of 4-QDs.
4. Calibration
The absolute position in x and y depends on the laser beam's diameter and (without the intensity-scaling
feature) intensity. We recommend to perform a calibration of the detector for the actual laser beam
parameters by moving the XY4QD relative to the laser beam by means of a micro-positioner and
measuring the voltages for different positions.
4.3 Sham fluctuations at low intensities
Depending on the laser beam diameter and the chosen bandwidth sham fluctuations of the beam position
can occur at low intensities although the beam position might be stable. These fluctuations are due to
small intensity deviations of the laser and the noise of the diodes. Please take care for an appropriate
intensity on the 4-QD.
If the intensity on the 4-QD is too small, the central green LED will not shine.
XY-4QD – User Manual version 4 – 25.07.2014 page 6 of 7

5. Safety
The detector has left our factory in a faultless state. Please only store and operate it in dry environments
in order to maintain this state.
The device was designed and manufactured according to DIN EN 61000-3-2 and satisfies
the re uirements of the European EMC Directive 89/336/EWG.
Label
6. Contact
MRC Systems GmbH
Hans-Bunte-Strasse 10
D-69123 Heidelberg
Germany
Phone: [phone]
Fax: [phone]
Web: www.mrc-systems.de
E-mail: [e-mail]
XY-4QD – User Manual version 4 – 25.07.2014 page 7 of 7
SN: 100208DBA036
Model: XY4QD 1kHz
Supply: 12VDC 0,2A
M C Systems GmbH
Germany