THORLABS PSDM2 User manual

PSDM2, PSDM4, and PSDM5
High-Resolution, Non-Contact,
Position-Sensing Detector
and Mount
Operating Manual

PSDM Series Position Sensing Systems
12400-D02, 10/26/2007 Page 2 www.thorlabs.com
Table of Contents
Part 1. Features and Applications................................................................................................................ 4
Part 2. Introduction....................................................................................................................................... 6
Part 3. Output Signals................................................................................................................................... 7
Part 4. Technical Data.................................................................................................................................. 8
4.1. Spectral Response...........................................................................................................................................8
Lateral Detectors (PSDM2 and PSDM4)..........................................................................................................................8
Quadrant Detector (PSDM 5) ...........................................................................................................................................8
4.2. Standard Silicon Lateral Detectors (PSDM2 and PSDM4)........................................................................9
4.3. InGaAs Quadrant Detector (PSDM5)........................................................................................................11
Part 5. PSDM Sensing Principle................................................................................................................ 12
5.1. PSDM Lateral Detector...............................................................................................................................12
Normalize Position Independent of Power Variations....................................................................................................13
5.2. PSDM Quadrant Detector...........................................................................................................................14
Position Response ...........................................................................................................................................................15
Part 6. Application Information................................................................................................................. 16
6.1. Resolution .....................................................................................................................................................16
Beam Spot Power Noise .................................................................................................................................................18
6.2. Lateral Detector Linearity ..........................................................................................................................18
6.3. Lateral Detector Saturation........................................................................................................................19
6.4. Beam Power Modulation.............................................................................................................................19
6.5. Sampling Techniques...................................................................................................................................20
Part 7. Warranty.......................................................................................................................................... 21
General Product Warranty .............................................................................................................................................. 21
Optomechanics................................................................................................................................................................21
Optical Tables and Breadboards .....................................................................................................................................21
Lasers and Imaging Systems...........................................................................................................................................21
Opto-Electronics, Control Electronics, Optics, and Nano-Positioning Product Lines....................................................21
Warranty Exclusions.......................................................................................................................................................21
Part 8. Regulatory ....................................................................................................................................... 22
8.1. Waste Treatment is Your Responsibility...................................................................................................22
8.2. Ecological Background................................................................................................................................22
Part 9. Thorlabs, Inc. Worldwide Contacts................................................................................................ 23

PSDM Series Position Sensing Systems
12400-D02, 10/26/2007 Page 3 www.thorlabs.com
Table of Figures
Figure 1: PSDM Connections, Mounting Holes, and Switches ............................................................ 5
Figure 2: PSDM Optical Position Detectors ......................................................................................... 6
Figure 3: 16-Pin Header Viewed from the Top.....................................................................................7
Figure 4: Lateral Detector Spectral Response ....................................................................................... 8
Figure 5: Quadrant Detector Spectral Response ...................................................................................8
Figure 6: One-Dimensional Lateral Detector ...................................................................................... 12
Figure 7: PSDM Amplifying Principle................................................................................................ 13
Figure 8: Lateral Detector Position Responce..................................................................................... 13
Figure 9: Quadrant Detector Photodiode Elements............................................................................. 14
Figure 10: Normalized Quadrant X Position Signal as a Function of Actual Position ......................... 15
Figure 11: Resolution vs. Beam Spot Power at Several Wavelengths .................................................. 17
Figure 12: Beam Spot Power Noise Influence on Difference ............................................................... 18
Figure 13: Electrical Model of a One-Dimensional Lateral Detector ................................................... 19
Figure 14: Recommended Connection to Data Acquisition Card......................................................... 20

PSDM Series Position Sensing Systems
12400-D02, 10/26/2007 Page 4 www.thorlabs.com
Part 1. Features and Applications
Features
•High resolution (nm scale) non-contact position sensing in one or two dimensions
•Sensitive to wavelengths from 400 to 1100 nm (lateral silicon photodiode detector) or 900 to
1700 nm (quadrant InGaAs detector)
•Gain-adjustable X and Y position signals
•SUM signal for source power indication, source power control feedback, or setup alignment
•Stable mechanics enable long-term low drift measurements
•Mounting holes for Thorlabs’ cage system and SM1 threaded components facilitates lens
mounting
•16-pin header enables sampling of low level signals for fast external signal processing
•LEDs on the front panel indicate on/off, position signal limitations, and power signal limitations
•The PSDM can be mounted to optical tables, posts, or other mechanics using three M4 or three
#8-32 threaded mounting holes
Applications
•General Position, Angle, and Motion Measurements
•Distance Measurements Through Triangulation
•Surface Measurements
•Spectral and Color Analysis

PSDM Series Position Sensing Systems
12400-D02, 10/26/2007 Page 5 www.thorlabs.com
Figure 1: PSDM Connections, Mounting Holes, and Switches

PSDM Series Position Sensing Systems
12400-D02, 10/26/2007 Page 6 www.thorlabs.com
Part 2. Introduction
A radiation or light spot on an optical position-sensing detector generates currents that relate to spot
position. A PSDM hosts a position-sensing detector and associated electronics that convert detector
currents into signals proportional to optical spot position.
The detectors come in two types: lateral silicon photodiodes and quadrant InGaAs photodiodes. The
lateral type consists of a single photodiode for continuous (no pixels), highly linear position sensing
whereas the quadrant type consists of four photodiodes, each shaped as a circle quadrant, that together
form a circular detection area. The quadrant detector is especially useful in alignment applications where
the alignment target is to be centered on the detector.
PSDM2 and PSDM4 have 1D and 2D lateral detectors, respectively, while PSDM5 has a quadrant
detector. Upon request, PSDM2 and PSDM4 can be equipped with a UV enhanced lateral detector,
enabling response down to 200 nm.
Figure 2: PSDM Optical Position Detectors

PSDM Series Position Sensing Systems
12400-D02, 10/26/2007 Page 7 www.thorlabs.com
Part 3. Output Signals
The X and Y signals are proportional to spot position.
The SUM signal is proportional to the optical beam spot power. In order to make position signals
independent of spot power variations, the SUM signal can be used to normalize position with respect to
power. Alternatively, the SUM signal can be used as a power source control feedback to achieve
constant beam power.
X:A, X:B, Y:A, and Y:B CURRENT are voltage signals proportional to detector currents generated by
current to voltage converters (transimpedance amplifiers). In order to cancel out offset errors, which are
mainly caused by detector dark current, these signals can be sampled directly for external computation
of position (dark current offsets alone can be pre-measured with zero radiation).
Signals can be connected through either BNC connectors (X, Y, and SUM only) or through a 16-pin
male header.
Figure 3: 16-Pin Header Viewed from the Top
Pin Signal
2, 4, 6…Reference/GND
1 X:A current
3 X:B current
5 Y:A current
7 Y:B current
9 SUM
13 X
15 Y

PSDM Series Position Sensing Systems
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Part 4. Technical Data
4.1. Spectral Response
Lateral Detectors (PSDM2 and PSDM4)
Figure 4: Lateral Detector Spectral Response
Quadrant Detector (PSDM 5)
Figure 5: Quadrant Detector Spectral Response

PSDM Series Position Sensing Systems
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4.2. Standard Silicon Lateral Detectors (PSDM2 and PSDM4)
Parameter Units Min Typical Max Conditions
Housing Dimensions
Length mm 98
Height mm 98
Width mm 34
Bottom to Detector Center mm 44
Power Consumption W <3
Wavelength Range nm 400 640 1100
Detector Dimensions
PSDM 2 mm 10
PSDM 4 mm 10 x 10
Resolution†
BW = 0.1 to 300 Hz >100,000
Resolves change in position:
s
olution
L
pos DETECTOR
Re
<Δ
Single mode beam (TEM 00)
Beam spot near mid position
5 V < SUM < 10 V
Detector Nonlinearity
PSDM 2 % 0.1 0.2
PSDM 4 % 0.3 0.8
Within 80% of Detector
Dimension
Detector Sensitivity‡A/W 0.10 0.40 0.69
Detector Dark Current
PSDM 2 nA 8 50
PSDM 4 nA 100 500
Transimpedance Gain, TIA V/A 50,000
Transimpedance Balance % 0.2
TIA Noise V rms < 30 E-6 0.1 Hz to 10 kHz with 10 mm
detector
†See section 6.1( page 16) for more information
‡Min at 440 nm, typical at 640 nm, and max at 950 nm

PSDM Series Position Sensing Systems
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Parameter Units Min Typical Max Conditions
Optical Power
Damage Threshold W/cm23
Detector Current, Position
Any μA See 6.1 100
Detector Current, Position
Mid μA See 6.1 200
@ 440 nm μW See 6.1 2,000 Beam Spot at Mid Position
@ 640 nm μW See 6.1 500 Beam Spot at Mid Position
@ 780 nm μW See 6.1 350 Beam Spot at Mid Position
Signal Output Range
X:A and B Current V -10 -5 >10 kΩ, Low Capacitance Load
Y:A and B Current V 5 10 >10 kΩ, Low Capacitance Load
DIFF X and Y V -10 10 >10 kΩ, Low Capacitance Load
SUM V 0 10
>10 kΩ, Low Capacitance Load
Signal Output Bandwidth
X and Y Current kHz 10 -3 dB
DIFF X, DIFF Y, SUM Hz 300 -3 dB

PSDM Series Position Sensing Systems
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4.3. InGaAs Quadrant Detector (PSDM5)
Parameter Unit Min Typ Max Condition
Wavelength Range (nm) nm 900 1550 1700
Dimension Ø mm 2
Distance Between Quadrants mm 0.1
Resolution†
BW = 0.1 to 300 Hz >20,000
Resolves change in position:
s
olution
L
pos BEAMSPOT
Re
<Δ
Beam spot near mid position
Single mode beam with Gaussian
beam spot intensity profile.
Optical sensitivity (A/W) Detector Sensitivity
@1300 nm 0.90
@1550 nm 0.95
Detector Dark Current nA 5
Transimpedance Gain, TIA‡V/A 10,000
Transimpedance Balance % 0.2
TIA Noise V rms < 10 E-6 0.1 Hz – 10 kHz with detector
Optical Power
Detector Current, Position
Any μA 0.002 500 Min Power: 0.1 Hz to 10 kHz
Detector Current, Position
Mid μA 0.008 2000
SUM Signal: 1000 μA Max
@ 1300 nm 9 nW 2.2 mW Beam at Mid Position
@ 1500 nm 8 nW 2.1 mW Beam at Mid Position
Signal Output Range
X:A and B Current V -10 0
Y:A and B Current V -10 0
DIFF X and Y V -10 10
SUM V 0 10
>10 kΩ, Low Capacitance Load
Signal Output Bandwidth
X and Y Current
Diff X, Y, and SUM
kHz
Hz
10
300
-3 dB
-3 dB
†See section 6.1 (page 16) for more information
‡See section 5.2 (page 14) for more information

PSDM Series Position Sensing Systems
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Part 5. PSDM Sensing Principle
5.1. PSDM Lateral Detector
The PIN layers of a lateral effect photodiode constitute three parallel planes, where the P and N layers
have uniform resistance. One-dimensional detectors have two output terminals situated at the edges on
opposite sides of the P-layer and duo-lateral detectors have terminals on both outer layers for dual axis
response (i.e. two terminals at each layer). The photogenerated current at a given spot position is then
split towards the two output terminals, and due to the uniform resistance within the P and N layers, the
output currents are proportional to spot movements perpendicular to the terminal sides.
Figure 6: One-Dimensional Lateral Detector
The difference between currents through X:A and X:B is proportional to beam spot position:
()
det
2
,L
pos
vityresponsitiPposPXBXA beam
pdbeambeambeam
⋅
⋅⋅=− ⎭
⎬
⎫
⎩
⎨
⎧−= 2
...
2
detdet LL
posbeam XBXAIrP atedPhotogenerpdbeam
+
==
⋅
The first amplifier stage converts detector currents to corresponding voltages through transimpedance
amplifiers (TIA):
XARCurrentXA TIA
⋅
=
_
The X and SUM output signals are difference and sum signals, respectively, determined from the
voltage signals X:A CURRENT and X:B CURRENT.

PSDM Series Position Sensing Systems
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Figure 7: PSDM Amplifying Principle
Normalize Position Independent of Power Variations
A power normalized position (i.e. position independent of beam spot power level) can be computed
externally with either an analogue division circuit or, if sampled for example by a data acquisition card
(PC AD-converter card), by software.
2⋅
=
+
−
=SUM
X
XBXA XBXA
POSNORM 1
Figure 8: Lateral Detector Position Responce
1The lateral detector models does not reference low level signals to ground (see section 6.2).

PSDM Series Position Sensing Systems
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The plot shows the highly linear position response typical of lateral detectors, where nonlinearity at the
edges originates mainly from truncation of beam spot.
A signal directly corresponding to distance from mid position can be obtained by
XBXA XBXA
L
POS ector
NORM +
−
=2
det
,
which for a 10 mm detector would yield an output range from -5 to 5.
5.2. PSDM Quadrant Detector
PSDM positions signals are processed according to
(
)
(
)
CADBX
+
−
+
=
,
(
)
(
)
DCBAY
+
−
+
=
,
and
DCBASUM
+
+
+
=
.
where A, B, C, and D are signals that are proportional to currents from each of the photodiode elements
(see transimpedance).
Figure 9: Quadrant Detector Photodiode Elements
Normalized position independent of power level can be computed externally via
SUM
Y
OR
SUM
X
SUM
DIFF
POSNORM == .
If using low level signals (X:A CURRENT, etc.), the normalized position can be computed from

PSDM Series Position Sensing Systems
12400-D02, 10/26/2007 Page 15 www.thorlabs.com
(
)
(
)
YBYAXBXA YBXBYAXA
XPOS NORM +++
+
−
+
=_
and
(
)
(
)
YBYAXBXA XAYBXBYA
YPOS NORM +++
+
−
+
=_.
Position Response
Quadrant detectors for telecom wavelengths (1300 and 1500 nm) are often used in applications where
beams emerge from single mode fibers with a Gaussian intensity distribution. The plots below show
how output response depends on beam spot diameter, here defined as the point where 2
max
e
I
I=.
Figure 10: Normalized Quadrant X Position Signal as a Function of Actual Position
A rectangular spot with sides LX and LY that has uniform intensity gives a linear position response (no
spot truncation and distance between quadrants disregarded).
SUM
X
LX
POS BEAMSPOT
XNORM 2
_=

PSDM Series Position Sensing Systems
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Part 6. Application Information
6.1. Resolution
PSDM position signal resolution is in most cases restricted by beam spot quality (i.e. spot position noise
and spot power noise) and environment. To maximize the capabilities of the PSDM, the optical setup
should have a non-interfering level of ambient light. Also, the beam direction is sensitive to air currents
because they create a graded index of refraction in the beam path. If measuring with a spot at mid
position, set the maximum PSDM gain to overcome measuring equipment noise (digital oscilloscopes
can seldom resolve signals below 1 mV).
Noise can be attributed to detector shot noise, thermal noise (PSDM2 and PSDM4 only), electronics
transimpedance amplifier noise, and beam spot noise (i.e. position and power noise).
For a sample lateral detector resolution computation, please refer to the PSDM web page.
The lateral detector signal to noise ratio S/N is given by
()
BIIq
RkT
I
I
I
NS
PHdark
DET
PH
N
PH
⋅
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛++
==
2
4
/
where
(
)
optSourcePH PysensitivitI
⋅
=
λ
.
The lateral detector and electronics signal to noise ratio is given by
()
2
_
2
2
_
2
det_
2
2
/
TIANTIAN
TIAPH
TIANN
PH
URI
RI
UU
U
NS
⋅+⋅
⋅
=
=
⋅+
=
As can be seen from the S/N ratio above, resolution improves with beam spot power and detector
sensitivity.
The plots below show the worst-case scenario for resolution as a function of power level at several
different incident wavelengths. Better resolution can be achieved (>50dB or >105) with a higher power
level but then the position range will be smaller (position range < detector length).

PSDM Series Position Sensing Systems
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Figure 11: Resolution vs. Beam Spot Power at Several Wavelengths
The PSDM2 and PSDM4 beam power normalized position signal is shown versus beam spot power.
Each curve represents a different wavelength (refer to the PSDM web page and the S/N ratio above).
Please note that the beam spot noise as well as the position and power noise has been disregarded. The
best resolution occurs for the maximum power. Notice that this resolution corresponds to the detector
length divided by 105.

PSDM Series Position Sensing Systems
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Beam Spot Power Noise
A change in power will lead to a change in difference signals X and Y as long as spot position is not
zero (i.e. located at mid position). Consequently, power noise will show up as position noise, which
increases as the distance to the detector’s mid position increases. If your measuring resolution is not
satisfactory, try to lower the power noise, position the spot in mid range, or regulate beam spot power
with sum X:A + X:B as beam power control feedback.
Figure 12: Beam Spot Power Noise Influence on Difference
Beam spot power noise influence on resolution (PSDM2 and PSDM4). The current noise at detector
terminals/electrodes X:A and X:B originating from beam power noise have equal magnitude at mid
position (correlated).
6.2. Lateral Detector Linearity
Linearity is influenced by spot truncation, detector nonlinearity, electronics transimpedance balance, and
(at very low levels of photogenerated current) by op-amplifier offsets. Detector dark current is split
evenly to terminals X:A and X:B, regardless of spot position and will have no impact on the linearity of
the difference X and Y output signals. However, dark current will affect the SUM signal and degrades
spot-power normalized position signal linearity. Detector dark current and the TIA offsets can be
compensated for by premeasuring X:A, X:B, Y:A, and Y:B signals at zero radiation. Detector
terminal/electrode resistance degrades linearity near detector edges, which is why detector nonlinearity
data is only valid within 80% of a detector’s length.

PSDM Series Position Sensing Systems
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6.3. Lateral Detector Saturation
The lateral detector divides the photocurrent into two resistor sheets. Consequently the bias voltage
across the PN-junction decreases with increasing photocurrent. As the bias decreases, photogenerated
charges will tend to worsen linearity because they will modulate/affect the resistor sheets (charge build
up). If measuring over a long time period, a position signal will tend to drift under such circumstances.
Figure 13: Electrical Model of a One-Dimensional Lateral Detector
The PSDM transimpedance gain (TIA) was chosen to secure a certain amount of bias by not allowing
more than 200 μA of detector current, as signals limit when exceeding this amount. A lower beam
source power or on/off modulation of beam power might, however, improve further on long-term
stability.
6.4. Beam Power Modulation
On/off modulation of beam power enables PSDM setups to make measurements under normal indoor
ambient light conditions.
Preferably, use electrical modulation of source power (laser or LED current modulation). In applications
where beam path length in air is limited or where high resolution is not required, an optical chopper can
be used such as the Thorlabs MC1000. In this case, use the photocurrent signals X:A and X:B2and a
modulation frequency of 500 to 1000 Hz.
2See chapter Part 5, page 2.

PSDM Series Position Sensing Systems
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6.5. Sampling Techniques
Multimeters, oscilloscopes, and data acquisition cards can all be used as sampling equipment (modern
multimeters often have store and retrieve capabilities).
When measuring several signals with a data acquisition card, it is preferable that the card preamplifiers
be configured to measure differentially with no coupling to card ground. Connect PSDM GND to the
card GND through the PSDM GND 4 mm socket as shown in the figure below.
Figure 14: Recommended Connection to Data Acquisition Card