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IR Photoconductive Detector

  • PbSe Photoconductive Detector
  • Linear Response for 1.5 - 4.8 µm
  • TO-5 Package Style


Lead Selenide Photoconductor,
1.5 - 4.8 µm

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Mounted and Unmounted Detectors
Unmounted Photodiodes (150 - 2600 nm)
Calibrated Photodiodes (350 - 1800 nm)
Mounted Photodiodes (150 - 1800 nm)
Thermopile Detectors (0.2 - 15 µm)
PbSe Photoconductor (1.5 - 4.8 µm)
Photovoltaic Detectors (2.0 - 10.6 µm)
Pigtailed Photodiodes (320 - 1000 nm)
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The FDPSE2X2 will be retired without replacement when stock is depleted. If you require this item for line production, please contact our OEM Team.

Lead Selenide (PbSe) photoconductors are widely used for the detection of infrared radiation from 1.5 to 4.8 µm. Photoconductors detect light in a broader wavelength range, offer higher detection capability, and provide better linear response in the IR than typical PIN junction photodiodes.

Photoconductors vs. Photodiodes
Unlike PIN junction photodiodes, which generate a photocurrent when light is absorbed in the depleted region of the junction semiconductor, the photoconductive material in the FDPSE2X2 photoconductor exhibits a decrease in electrical resistance when illuminated with IR radiation. Photoconductive detectors typically have a very linear response when illuminated with IR radiation.

Usage Notes
Photoconductors function differently than typical PIN junction photodiodes. We recommend that an optical chopper be employed when using this detector with CW light, due to signal noise issues. PbSe detectors can be used at room temperature. However, temperature fluctuations will affect dark resistance, sensitivity, and response speeds (see the Temperature Considerations section in the Tutorial tab for details).

PbSe Photoconductive Detectors

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. Although PbSe detectors can be used at room temperature, temperature flucturations will affect dark resistance, sensitivity, and response speeds (see Temperature Considerations below).

Photoconductor Basic Model
Photoconductor Basic Schematic
Click to Enlarge

Theory of Operation

For photoconductive materials, incident light will cause 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 hence, photosensitivity is expressed in units of V/W. An example operating circuit is shown to the right. Please note that the circuit depicted 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 by the following equation:

Photoconductor Basic Model

A change ΔVOUT then occurs due to a change ΔRDark in the resistance of the detector when light strikes the active area:

Photoconductor Basic Model

Frequency Response
Photoconductors must be used with a pulsed signal to obtain AC signals. Hence, an optical chopper should be employed when using these detectors with CW light. The detector responsivity (Rf) when using a chopper can be calculated using the equation below:

Photoconductor Responsivity

Here, fc is the chopping frequency, R0 is the response at 0 Hz, and τr is the detector rise time.

Effects of Chopping Frequency
The photoconductor signal will remain constant up to the time constant response limit. 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 for

Photoconductor Chopper Equation

The characteristic curve for Signal vs. Chopping Frequency for each particular detector is provided in chapter 4 of the operating manuals.

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 PbSe bandgaps 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.

Typical Photoconductor 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 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 increase. Provided in chapter 4 of the Operating Manual is a SNR vs. Supply Voltage characteristic curve to help determine best operating condition. The output voltage is derived as the following:

Photoconductor Amp Eq

Photoconductor Basic Amp Model
Amplifier Model

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 of the operating manuals are characteristic curves for SNR vs. Chopping Frequency and SNR vs. Supply Voltage for each particular detector.

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:

Photoconductor NEP

Here, S/N is the Signal to Noise Ratio, Δf is the Noise Bandwidth, and Incident Energy has units of W/cm2. For more information on NEP, please see Thorlabs' Noise Equivalent Power White Paper.

Dark Resistance
Dark Resistance is the resistance of the detector under no illumination. It is important to note that dark resistance will increase or decrease with temperature. Cooling the device will increase the dark resistance. Provided in chapter 4 of the operating manuals is a Dark Resistance vs. Temperature characteristic graph for each particular detector.

Detectivity (D) and Specific Detectivity (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.

Photoconductor Detectivity

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. In the equation below, A is the area of the photosensitive region of the detector and Δf is the effective noise bandwidth.

Photoconductor D*

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The following table lists Thorlabs' selection of photodiodes and photoconductive detectors. Item numbers in the same row contain the same detector element.

Photodetector Cross Reference
Wavelength Material Unmounted
150 - 550 nm GaP FGAP71 - SM05PD7A DET25K2 PDA25K2
200 - 1100 nm Si FDS010 - SM05PD2A
Si - - SM1PD2A - -
320 - 1000 nm Si - - - - PDA8A2
320 - 1100 nm Si FD11A SM05PD3A PDF10A2
Si - - - DET100A2 PDA100A2
340 - 1100 nm Si FDS10X10 - - - -
350 - 1100 nm Si FDS100
FDS100-CAL a
- SM05PD1A
Si FDS1010
FDS1010-CAL a
- -
400 - 1000 nm Si - - - - PDA015A(/M)
400 - 1100 nm Si FDS015 b - - - -
Si FDS025 b
FDS02 c
- - DET02AFC(/M)
400 - 1700 nm Si & InGaAs DSD2 - - - -
500 - 1700 nm InGaAs - - - DET10N2 -
750 - 1650 nm InGaAs - - - - PDA8GS
800 - 1700 nm InGaAs FGA015 - - - PDA015C(/M)
InGaAs FGA21
- SM05PD5A DET20C2 PDA20C2
InGaAs FGA01 b
- - DET01CFC(/M) -
InGaAs FDGA05 b - - - PDA05CF2
InGaAs - - - DET08CFC(/M)
800 - 1800 nm Ge FDG03
- SM05PD6A DET30B2 PDA30B2
Ge FDG50 - - DET50B2 PDA50B2
Ge FDG05 - - - -
900 - 1700 nm InGaAs FGA10 - SM05PD4A DET10C2 PDA10CS2
900 - 2600 nm InGaAs FD05D - - DET05D2 -
FD10D - - DET10D2 PDA10D2
950 - 1650 nm InGaAs - - - - FPD310-FC-NIR
1.0 - 5.8 µm InAsSb - - - - PDA10PT(-EC)
1.5 - 4.8 µm PbSe - FDPSE2X2 - - -
2.0 - 5.4 µm HgCdTe (MCT) - - - - PDA10JT(-EC)
2.0 - 8.0 µm HgCdTe (MCT) VML8T0
VML8T4 d
- - - PDAVJ8
2.0 - 10.6 µm HgCdTe (MCT) VML10T0
VML10T4 d
- - - PDAVJ10
2.7 - 5.0 µm HgCdTe (MCT) VL5T0 - - - PDAVJ5
2.7 - 5.3 µm InAsSb - - - - PDA07P2
  • Calibrated Unmounted Photodiode
  • Unmounted TO-46 Can Photodiode
  • Unmounted TO-46 Can Photodiode with FC/PC Bulkhead
  • Photovoltaic Detector with Thermoelectric Cooler

PbSe Photoconductor: 1.5 - 4.8 µm

  • Good Performance from 1.5 - 4.8 µm
  • For Detection of CW Light We Recommend an Optical Chopper
Item #a Info Wavelength
FDPSE2X2 info 1.5 - 4.8 µm 4 mm2 TO-5 10 µs 4 µm (Typ.) 1.5 x 103 V/W (Min)
3.0 x 103 V/W (Typ.)
2.5 x 109
cm•Hz1/2/W (Typ.)
0.1 - 3.0 MOhm STO5S
  • All measurements performed with 25 °C element temperature unless stated otherwise.
  • Rise Time is measured from 0 - 63% of final value.
  • Measured at Peak Wavelength, Chopping Frequency of 600Hz, and Bias Voltage of 15 V, RDARK = RLOAD
  • Measured at Peak Wavelength and a Chopping Frequency of 600 Hz
Based on your currency / country selection, your order will ship from Newton, New Jersey  
+1 Qty Docs Part Number - Universal Price Available
FDPSE2X2 Support Documentation
FDPSE2X2PbSe Photoconductor, 2 mm x 2 mm Active Area, 10 µs Rise Time, 1.5 - 4.8 µm
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