Quadrant Photodiode (QPD)
www.thorlabs.com
QPD is the device to track laser beam movement for precise displacement
measurement.
 Reference: PDQ80S1 Quadrant Detector System Operating Manual
Introduction
• Also known as quadrant and bi-cell detectors, these devices have
two or four distinct photosensitive elements separated by a
minuscule gap.
• A light spot illuminating just one element only produces
photocurrent in that element. When the spot is translated across
the surface of the detector, the energy becomes distributed
between adjacent elements.
• The ratio between the photocurrent outputs from these elements
determines the relative position of the spot on the surface.
• It's important to note that the detector only provides position
information over a linear distance of the spot diameter. Elsewhere,
it is known to be in a specific segment, but not exactly where.
Because of this, when working with lasers, defocusing may be
required in order to obtain maximum range.
Specifications
[3]
Operating principle
Beam incident angle should be
normal
When the beam is centered on the detector, x, y difference signals come to zero
Trapped Bead Movement Measurement
• Measuring lateral displacement with QPD
– Irrespective of the particle size we can assume the lateral shift of the
particle moves the peak intensity to the cell located in the shift
direction.
QPD
Brownian motion of bead
Lens
Bead
Con’t
– We need to properly set the axial location of the QPD such that the
QPD signal is nulled (all quadrants of equal value) irrespective of the
location of the spherical particle that coincides with the beam waist
center.
– Axial displacement can be measured by the change in the power
caused by defocusing at the QPD.
y
Q2 Q1
Q3 Q4
x
Measurement plane
of QPD
Considerations during QPD calibration
Beam selection
•
•
Beam shape: QPD is optimized for stable circular beam
Beam size: 1mm ~ 3.9 mm. It depends on sensor area of the device. Minimum diameter should be
much larger than the band gap of quadrants. Maximum diameter should be smaller than the half of
sensor area. Use lens to control the beam size
QPD calibration procedure (manual stage, laser source)
1. Position the beam to the center of quadrants
where x and y difference signals are closely zero.
2. From the center, move the beam to the left
until the x difference signal no longer increases
and note the value that is negative x limit.
3. Repeat the above step moving the beam to
the right of center to determine positive x limit.
4. Repeat step 2 and 3 to determine the positive and
negative y limits.
5. Maximum measurement area should not be
inclined. If inclined, check the beam is incident
with normal to the quadrants.
6. Calculate calibration factor (S/nm) by moving the
beam with known distance by using manual stage.
Maximum measurement area
[7]
QPD API
PC
USB communication
API
PDQ.h
USB.h
usb.lib
Function List and Flow chart
•
•
•
•
•
•
•
•
•
USBinitPDQ80S1();
PDQSendScanInterval();
PDQWriteHAlignmentWindow();
PDQWriteVAlignmentWindow();
PDQSendScanInterval();
PDQStartScan();
PDQReadScan();
PDQReadScan();
USBUninit();
USBinitPDQ80S1()
PDQSendScanInterval()
PDQStartScan()
PDQReadScan()
Acquisition
Complete?
Y
PDQStopScan()
N
Example code
QPD (Hamamatsu)
QPD is the device to track laser beam movement for precise displacement
measurement.
Specifications (QPD sensor)
Specifications (NI-DAQ)
Number of Channels
16 SE/8 DI
Sample Rate
1.25 MS/s
Resolution
Simultaneous Sampling
16 bits
No
Maximum Voltage Range
-10..10 V
Range Accuracy
1920 µV
Range Sensitivity
112 µV
Minimum Voltage Range
-100..100 mV
Range Accuracy
52 µV
Range Sensitivity
6 µV
Number of Ranges
7
On-Board Memory
4095 samples
Operating principle
P-polarized beam
S-polarized beam
QPD2
QPD1
IR laser
Beam incident angle should be normal
Dual QPD system (QPD1, QPD2)
When the beam is centered on the detector, x, y difference signals come to zero
Circuit of the QPD module
• Circuit of the QPD module
– D1: QPD sensor
– J1: Voltage input of QPD
(power supply)
• Op-amp (U1~U7)
– voltage amplifier with
differential inputs
– reduce the noise signal
• Each signals are calculated
in the circuit (X, Y, Sum)
– 3 output voltage (J2, J3,
J4)
QPD control with NI DAQ
X - Signal
Y - Signal
Sum - Signal
Input
Voltage
Power Supply
+
QPD sensor and
electrical circuit module
Programming using
DAQmx API
SCB-68 Connector Block
SHC68-68-EPM
Cable
PCI-6250 card
NI DAQ (PCI-6250)
• PCI-6250 is a high-speed multifunction M Series data acquisition
(DAQ) board optimized for superior accuracy at fast sampling
rates
– 16 analog inputs, 1 MS/s (Multichannel)
– improved measurement accuracy, resolution,
and sensitivity by choosing high-accuracy M Series.
• PC-BASED DATA ACQUISITION

Libraries for NI DAQ
- DAQmx driver software interactive data-logging software
NI DAQ (SCB-68)
• The SCB-68 is a shielded I/O connector block for interfacing I/O
signals to plug-in DAQ devices with 68-pin connectors. Combined
with the shielded cables, the SCB-68 provides rugged, very lownoise signal termination
NI DAQ (SCB-68)
• Connecting the SCB-68 with QPD
– AI 0~AI 2 (QPD 1)
• AI 0: x signal
• AI 1: y signal
• AI 2: sum signal
QPD1
– AI 0~AI 5 (QPD 2)
• AI 3: x signal
• AI 4: y signal
• AI 5: sum signal
– AI 8~AI 13
• GND (0 volt)
QPD2
Trapped Bead Movement Measurement
• Freq(Hz) : Setting the sampling rate
• Duration: Measuring time for QPD
QPD2
(S-polarized beam)
• Persistence: Tracking the signal
• Start scan: Starting the QPD scan
• Start save: Generating text file
– Max save count: 6000000
QPD1
(P-polarized beam)
NIDAQmx Functions For QPD Data Aquistion
•
Task Configuration/Control: DAQmxCreateTask (), DAQmxStartTask(),
DAQmxStopTask(), DAQmxClearTask()
•
Channel Creation: DAQmxCreateAIVoltageChan()
•
Timing: DAQmxCfgSampClkTiming()
•
Read: DAQmxReadAnalogF64()
Sample program
In project settings, link, “NIDAQmx.lib”