THORLABS FDPS3X3 User manual

FDPS3X3
Lead Sulfide Photoconductor
User Guide

PbS Detector
Page 1 Rev B, September 8
th
, 2014
Table of Contents
Chapter 1 Warning Symbol Definitions ............................................. 2
Chapter 2 Description ......................................................................... 3
Chapter 3 Operation ............................................................................ 3
3.1. Theory of Operation ................................................... 3
3.2. Frequency Response ................................................... 4
3.3. Effects of Chopping Frequency .................................... 4
3.4. Temperature Considerations ...................................... 5
3.5. ignal to Noise Ratio .................................................. 5
3.6. Noise Equivalent Power .............................................. 5
3.7. Dark Resistance ......................................................... 6
3.8. Detectivity (D) and D* ................................................. 6
3.9. Typical Amplifier Circuit ............................................. 6
Chapter 4 Specifications..................................................................... 8
4.1. Response Curve .......................................................... 9
4.2. NR vs. Chopping Frequency ....................................... 9
4.3. NR vs. upply Voltage ............................................. 10
4.4. Dark Resistance vs. Temperature .............................. 10
4.5. Temperature vs. ensitivity ...................................... 11
Chapter 5 Mechanical Drawing ........................................................12
Chapter 6 Regulatory ........................................................................ 13
6.1. Waste Treatment is Your Own Responsibility ............ 13
6.2. Ecological Background ............................................. 13
Chapter 7 Thorlabs Worldwide Contacts ........................................14

Chapter 1: Warning Symbol Definitions
24700-D02 Page 2
Chapter 1 Warning Symbol Definitions
Below is a list of warning symbols you may encounter in this manual or on your
device.
Symbol
Description
Direct Current
Alternating Current
Both Direct and Alternating Current
Earth Ground Terminal
Protective Conductor Terminal
Frame or chassis Terminal
Equipotentiality
On (Supply)
Off (Supply)
In Position of a Bi-Stable Push Control
Out Position of a Bi-Stable Push Control
Caution, Risk of Electric Shock
Caution, Hot Surface
Caution, Risk of Danger
Warning, Laser Radiation
Caution, Spinning Blades May Cause Harm

PbS Detector
Page 3 Rev B, September 8
th
, 2014
Chapter 2 Description
The Thorlabs FDPS3X3 photoconductor is a Lead Sulfide Detector (PbS), which
is ideal for measuring both pulse and chopped infrared light sources. The
photoconductor is housed conveniently in a TO-5 package which offers easy
integration into existing setup and/or systems. The Lead Sulfide detector is
modeled as a resistor: as the detector area is illuminated with IR radiation the
effective resistance of the photoconductor is reduced.
Chapter 3 Operation
3.1. Theory of Operation
Lead Sulfide (PbS) and Lead Selenide (PbSe) photoconductive detectors are
widely used in detection of infrared radiation from 1000 to 4800 nm. Unlike
standard photodiodes, which produce a current when exposed to light, the
electrical resistance of the photoconductive material is reduced when illuminated
with light. PbS and PbSe detectors can be used at room temperature. However,
temperature changes affect dark resistance, sensitivity, and response speeds
(see Temperature Considerations below).
Incident light causes the number of charge carriers in the active area to increase,
thus decreasing the resistance of the detector. This change in resistance leads to
a change in measured voltage, and so photosensitivity is expressed in V/W. An
example operating circuit is shown to the right. Please note that circuit depicted
below is not recommended for practical purposes since low frequency noise will
be present.
The detection mechanism is based upon the conductivity of the thin film of the
active area. The output signal of the detector with no incident light is defined as
the following:
 
  
A change ∆VOUT then occurs due to a change ∆RDark in the resistance of the
detector when light strikes the active area:
   
 

Chapter 3: Operation
24700-D02 Page 4
Figure 1: Photodiode Model
3.2. Frequency Response
Photoconductors must be used with a pulsed signal, to obtain AC signals. An
optical chopper, such as the Thorlabs MC2000, is recommended for use with CW
light. The detector responsivity (Rf) when using a chopper can be calculated
using the equation below:
 


 !"#$$%&'()*+)&!,
 ()-$#&-)./012
 3)/)!/#((%-)/%4)
3.3. Effects of Chopping Frequency
The photoconductor signal will remain constant up to the time constant response
limit. PbS and PbSe detectors have a typical 1/f noise spectrum (i.e., the noise
decreases as chopping frequency increases , which has a profound impact on the
time constant at lower frequencies.
The detector will exhibit lower responsivity at lower chopping frequencies. Frequency
response and detectivity are maximized at:
Ground
Load
Resistance
RLOAD
Output
Signal
VOUT
+
-
Active Area
Dark Resistance RDARK
Bias Voltage
VBIAS

PbS Detector
Page 5 Rev B, September 8
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, 2014

5
The characteristic curve for Signal vs. Chopping Frequency for this particular detector
is provided in chapter 4.
3.4. Temperature Considerations
These detectors consist of a thin film on a glass substrate. The effective shape and
active area of the photoconductive surface varies considerably based upon the
operating conditions, thus changing performance characteristics. Specifically,
responsivity of the detector will change based upon the operating temperature.
Temperature characteristics of PbS and PbSe band gaps have a negative coefficient,
so cooling the detector shifts its spectral response range to longer wavelengths. For
best results, operate the photodiode in a stable controlled environment. See the
Operating Manuals for characteristic curves of Temperature vs. Sensitivity for a
particular detector.
3.5. Signal to Noise Ratio
Since the detector noise is inversely proportional to the chopping frequency, the noise
will be greater at low frequencies. The detector output signal is linear to increased
bias voltage, but the noise shows little dependence on the bias at low levels. When a
set bias voltage is reached, the detector noise will increase linearly with applied
voltage. At high voltage levels, noise tends to increase exponentially, thus degrading
the signal to noise ratio (SNR further. To yield the best SNR, adjust the chopping
frequency and bias voltage to an acceptable level. Provided in chapter 4 are
characteristic curves for SNR vs. Chopping Frequency and SNR vs. Supply Voltage.
3.6. Noise Equivalent Power
The noise equivalent power (NEP) is the generated RMS signal voltage
generated when the signal to noise ratio is equal to one. This is useful as the
NEP determines the ability of the detector to detect low level light. In general, the
NEP increases with the active area of the detector and is given by the following
equation:
678  9&!%3)&/7&)(',:().
;
6<

= ;%'&.>/#6#%-)./%#
<  6#%-)?.&3@%3/"
9&!%3)&/7&)(',  8#@)(A:().

Chapter 3: Operation
24700-D02 Page 6
3.7. Dark Resistance
Dark Resistance is the resistance of the detector under no illumination. It is
important to note the dark resistance will increase or decrease with temperature.
Cooling the device will increase the dark resistance. Provided in chapter 4, is a
Dark Resistance vs. Temperature characteristic graph for the particular detector.
3.8. Detectivity (D) and D*
Detectivity (D) is another criteria used to evaluate the performance of the
photodetector. Detectivity is a measure of sensitivity and is the reciprocal of NEP.
B  
678
Higher values of detectivity indicate higher sensitivity, making the detector more
suitable for detecting low light signals. Detectivity varies with the wavelength of
the incident photon.
NEP of a detector depends upon the active area of the detector, which in
essence will also affect detectivity. This makes it hard to compare the intrinsic
properties of two detectors. To remove the dependence, Specific Detectivity (D*),
which is not dependent on detector area, is used to evaluate the performance of
the photodetector.
B:
678
Where A is the area of the photosensitive region of the detector, ∆f is the
effective noise bandwidth, and NEP is the noise equivalent power.
3.9. Typical Amplifier Circuit
Due to the noise characteristic of a photoconductor, it is generally suited for AC
coupled operation. The DC noise present with the applied bias will be too great at
high bias levels, thus limiting the practicality of the detector. For this reason, IR
detectors are normally AC coupled to limit the noise. A pre-amplifier is required to
help maintain the stability and provide a large gain for the generated current
signal.
Based on the schematic below, the op-amp will try to maintain point A to the
input at B via the use of feedback. The difference between the two input voltages
is amplified and provided at the output. It is also important to note the high pass
filter that AC couples the input of the amplifier blocks any DC signal. In addition,
the resistance of the load resistor (RLOAD) should be equal to the dark resistance
(RD) of the detector to ensure maximum signal can be acquired. The supply
voltage (+V) should be at a level where the SNR is acceptable and near unity.
Some applications require higher voltage levels; as a result the noise will

PbS Detector
Page 7 Rev B, September 8
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, 2014
increase. Provided in chapter 4 is a SNR vs. Supply Voltage characteristic curve
to help determine best operating condition. The output voltage is derived as the
following:
CDE  F
GH9
Figure 2: Amplifier Model
B
A
Out
Feedback Resistor
Rf
+V
Detector
RD
R filter
C filter
RLOAD
Ri
IS
RC Filter
Optical
Chopper

Chapter 4: Specifications
24700-D02 Page 8
Chapter 4 Specifications
All measurements performed with a 50 Ωload unless stated otherwise.
Electrical Specifications
Detector PbS
Active Area 3.0 x 3.0 mm
2
(9 mm
2
)
Wavelength Range λ1000 to 2900 nm
Peak Wavelength λ
p
2200 nm (Typ.)
Peak Sensitivity
1
ℜ(λ) 2 x 10
4
V/W (Min.)
5 x 10
4
V/W (Typ.)
Rise Time
2
(0 to 63%)

200 µs
Detectivity
3
(
I
P
, 600, 1) D
*
1 x 10
11

J

K
LM
N
(Typ.)
Dark Resistance R
D
0.25 to 2.5 MΩ
Bias Voltage V
B
100 V
General
Package TO-5
Operating Temperature -30 to 65 °C
Storage Temperature -55 to 65 °C
1Measured at chopping frequency of 600 Hz, Bias Voltage of 15 V, RD= RL.
2Rise Time is measured from 0 to 63% of final value.
3Measured at chopping frequency of 600 Hz, Peak Wavelength.

PbS Detector
Page 9 Rev B, September 8
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, 2014
4.1. Response Curve
4.2. SNR vs. Chopping Frequency
0
10,000
20,000
30,000
40,000
50,000
1 1.25 1.5 1.75 2 2.25 2.5 2.75 3
Photo Sensitivity (V/W)
Wavelength (
O
P
PbS Photoconductor Responsivity
0.1
1
10
10 100 1000
Relative S/N
Chopping Frequency
SNR vs Chopping Frequency
Supply Voltage: 15 V, Rise Time: 200 μs
S/N
Signal
Noise