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PbSe Transimpedance Amplified Photodetector
Detector with Ø1" Lens
Power Supply Included with Detector
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Each detector has an internal SM05 and external SM1 thread and comes with an
attached SM1T1 Internal SM1 Adapter
and SM1RR Retaining Ring.
Thorlabs' PDA20H(-EC) Amplified Detector, based on a photoconductive lead selenide (PbSe) detector element, is sensitive to mid-IR radiation (MIR) in the 1.5 - 4.8 µm spectral range. It detects light in a broader wavelength range, offers higher sensitivity, and provides better linear response in the MIR than typical PIN junction photodiodes.
The slim profile housing enables use in light paths with space constraints. All connections and controls are located perpendicular to the light path, providing increased accessibility. Amplification is provided by low noise transimpedance or voltage amplifiers that are capable of driving 50 Ω loads. Signal output is via a BNC connector. This photodetector is ideal for use with Thorlabs' passive low-pass filters; these filters have a 50 Ω input and a high-impedance output that allows them to be directly attached to high-impedance measurement devices such as an oscilloscope. Thorlabs offers a wide variety of BNC, BNC-to-SMA, and SMC cables, as well as a variety of BNC, SMA, and SMC adapters.
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The power supply is included with all of the detectors on this page.
The housing provides two 8-32 tapped mounting holes (M4 for the -EC model) for vertical or horizontal post mounting. The housing also features external SM1 (1.035"-40) threading and internal SM05 (0.535"-40) threading that are compatible with most Thorlabs SM1- and SM05-threaded accessories. Additionally, an internally threaded SM1 coupler is included with each detector. This allows convenient mounting of SM1 compatible accessories, optics, and cage assembly accessories. The internal SM05 threading is only suitable for mating to an externally threaded SM05 lens tube (components such as fiber adapters cannot be threaded onto the SM05 threading). Most SM1-threaded fiber adapters are compatible with this detector. However, the S120-FC internally SM1-threaded fiber adapter is not compatible with this detector. Externally SM1-threaded adapters should be mated to the included internally SM1-threaded adapter, while internally SM1-threaded adapters can be mated directly to the housing.
A ±12 V linear power supply that supports input voltages of 100, 120, and 230 VAC is included with each amplified photodetector. Replacement power supplies are available separately below. Due to limitations in the IC, the high-speed amplifier used in this device may become unstable, exhibiting oscillations or negative output if the linear power supply voltage is applied when the module is on. The unit should always be powered up using the power switch on the power supply or the unit itself. Hot plugging the unit is not recommended. Additionally, inhomogeneities at the edges of the active area of the detector can generate unwanted capacitance and resistance effects that distort the time-domain response of the photodetector output. Thorlabs therefore recommends that the incident light on the photodetector is well centered on the active area. The SM1 (1.035"-40) threading on the housing is ideally suited for mounting a Ø1" focusing lens or pinhole in front of the detector element.
Photoconductors vs. Photodiodes
PDA Series Design, scale in inches [mm].
Compact PDA & PDF Series Design
Thorlabs' Amplified Photodiode series features a slim design, which allows the detector access to the light path even between closely spaced optical elements.
The power supply input and the BNC output are located on the same outer edge of the package, further reducing the device thickness and allowing easier integration into tight optic arrangements. The PDA and PDF series detectors can fit into spaces as thin as 0.83" (21.1 mm) when the SM1 coupler is removed. With the SM1 coupler attached, the smallest width the detector can fit into is 1.03" (26.2 mm).
Additionally, the detectors have two tapped mounting holes perpendicular to each other so that the unit can be mounted in a horizontal or vertical orientation. This dual mounting feature offsets the fact that the cables protrude out the side of the package, thus requiring more free space above or alongside your beam path.
The switchable gain detectors feature an eight-position rotary gain switch (pictured below right) mounted on an outside edge perpendicular to the power supply and BNC output connections. The location of the gain switch allows for easy adjustments while the detector is mounted.
BNC Female Output (Photodetector)
0 - 10 V Output
PDA Male (Power Cables)
PDA Female (Photodetector)
PDA Series Mounting Options
The PDA series of amplified photodetectors are compatible with our entire line of lens tubes, TR series posts, and cage mounting systems. Because of the wide range of mounting options, the best method for mounting the housing in a given optical setup is not always obvious. The pictures and text in this tab will discuss some of the common mounting solutions. As always, our technical support staff is available for individual consultation.
TR Series Post (Ø1/2" Posts) System
The PDA housing can be mounted vertically or horizontally on a TR Series Post using the threaded holes for 8-32 (M4 on metric versions). Select PDA housings feature universally threaded holes for both 8-32 and M4 threads.
Lens Tube System
Each PDA housing includes a detachable Ø1" Optic Mount (SM1T1) that allows for Ø1" (Ø25.4 mm) optical components, such as optical filters and lenses, to be mounted along the axis perpendicular to the center of the photosensitive region. The maximum thickness of an optic that can be mounted in the SM1T1 is 0.1" (2.8 mm). For thicker Ø1" (Ø25.4 mm) optics or for any thickness of Ø0.5" (Ø12.7 mm) optics, remove the SM1T1 from the front of the detector and place (must be purchased separately) an SM1 or SM05 series lens tube, respectively, on the front of the detector.
The SM1 and SM05 threadings on the PDA photodetector housing make it compatible with our SM lens tube system and accessories. Two particularly useful accessories include the SM-threaded irises and the SM-compatible IR and visible alignment tools. Also available are fiber optic adapters for use with connectorized fibers.
The simplest method for attaching the PDA photodetector housing to a cage plate is to remove the SM1T1 that is attached to the front of the PDA and use the external SM1 threads. A cage plate, such as the CP33 30 mm cage plate, can be directly attached to the SM1 threads. Then the retaining ring, included with the SM1T1, can be threaded using a spanner wrench into the CP33 to ensure the cage plate is tightened to the desired location and square with the photodetector housing.
This method for attaching the PDA photodetector housing to a cage plate does not allow much freedom in determining the orientation of the photodetector; however, it has the benefit of not needing an adapter piece, and it allows the diode to be as close as possible to the cage plate, which can be important in setups where the light is divergent. As a side note, Thorlabs sells the SM05PD and SM1PD series of photodiodes that can be threaded into a cage plate so that the diode is flush with the front surface of the cage plate; however, the photodiode is unbiased.
For more freedom in choosing the orientation of the PDA photodetector housing when attaching it, an SM1T2 lens tube coupler can be purchased. In this configuration the SM1T1 is left on the detector and the SM1T2 is threaded into it. The exposed external SM1 threading is now deep enough to secure the detector to a CP33 cage plate in any orientation and lock it into place using one of the two locking rings on the ST1T2.
Although not pictured here, the PDA photodetector housing can be connected to a 16 mm cage system by purchasing an SM05T2. It can be used to connect the PDA photodetector housing to an SP02 cage plate.
The image below shows a Michelson Interferometer built entirely from parts available from Thorlabs. This application demonstrates the ease with which an optical system can be constructed using our lens tube, TR series post, and cage systems.
The table below contains a part list for the Michelson Interferometer for use in the visible range. Follow the links to the pages for more information about the individual parts.
PbS and PbSe Photoconductive Detectors
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. Although PbS and 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
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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:
A change ΔVOUT then occurs due to a change ΔRDark in the resistance of the detector when light strikes the active area:
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 detector will exhibit lower responsivity at lower chopping frequencies. Frequency response and detectivity are maximized for
See Chapter 5 of the manuals for detector rise time values.
Temperature characteristics of PbS and 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.
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. The output voltage is derived as the following:
Signal to Noise Ratio
Noise Equivalent Power
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.
Detectivity (D) and Specific Detectivity (D*)
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.
The following table lists Thorlabs' selection of photodiodes and photoconductive detectors. Item numbers in the same row contain the same detector element.
These items will be retired without replacement when stock is depleted. If you require these parts for line production, please contact our OEM Team.
The PDA-C-72 power cord is offered for the PDA line of amplified photodetectors when using with a power supply other than the one included with the detector. The cord has tinned leads on one end and a PDA-compatible 3-pin connector on the other end. It can be used to power the PDA series of amplified photodetectors with any power supply that provides a DC voltage. The pin descriptions are shown to the right.
The LDS12B ±12 VDC Regulated Linear Power Supply is intended as a replacement for the supply that comes with our PDA and PDF line of amplified photodetectors sold on this page. The cord has three pins: one for ground, one for +12 V, and one for -12 V (see diagram above). A region-specific power cord is shipped with the LDS12B power supply based on your location. This power supply can also be used with the PDB series of balanced photodetectors, PMM series of photomultiplier modules,APD series of avalanche photodetectors, and the FSAC autocorrelator for femtosecond lasers.
These internally SM1-threaded (1.035"-40) adapters mate terminated fiber to any of our externally SM1-threaded components, including our photodiode power sensors, our thermal power sensors, and our photodetectors. These adapters are compatible with the housing of the photodetectors on this page.
The APC adapter has two dimples in the front surface that allow it to be tightened with the SPW909 or SPW801 spanner wrench. The dimples do not go all the way through the disk so that the adapter can be used in light-tight applications when paired with SM1 lens tubes.
Each disk has four dimples, two in the front surface and two in the back surface, that allow it to be tightened from either side with the SPW909 or SPW801 spanner wrench. The dimples do not go all the way through the disk so that the adapters can be used in light-tight applications when paired with SM1 lens tubes. Once the adapter is at the desired position, use an SM1RR retaining ring to secure it in place.