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GaP Transimpedance Amplified Photodetector![]()
Detector with Ø1" Lens PDA25K2 Power Supply Included with Detector ![]() Please Wait ![]() Click to Enlarge PDA25K2 Shown with Included Power Supply ![]() Click to Enlarge Each detector has internal SM05 and external SM1 threads and includes an SM1T1 Internal SM1 Adapter and SM1RR Retaining Ring. Features
The GaP-Based Transimpedance Amplified Photodetector is sensitive to light in the UV to Visible region from 150 nm to 550 nm. This switchable-gain detector is housed in a compact, low-profile package that is ideal for use in light paths with space constraints. All connections and controls are located perpendicular to the light path to provide easy accessibility when integrating the detector into a setup. Amplification is provided by a low-noise transimpedance amplifier that is capable of driving 50 Ω loads. Signal output is via a BNC connector. This photodetector can be used with Thorlabs' passive low-pass filters, which 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. This housing of the PDA25K2 features external SM1 (1.035"-40) threading, internal SM05 (0.535"-40) threading, and two universal mounting holes that accept both 8-32 and M4 threads, allowing for either vertical or horizontal post mounting. Addtionally, an SM1T1 internally threaded SM1 coupler and SM1RR retaining ring are included, allowing convenient mounting of SM1-compatible accessories, optics, and cage assembly accessories. 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. 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 because it collides with the photodiode. 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. The active area of the detector is flush with the front of the housing, simplifying alignments within optomechanical systems. 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 be well-centered on the active area. Performance Specifications
Gain Specifications
![]() Click to Enlarge The power input and BNC connectors are located at the top of the housing. ![]() Click to Enlarge The detector has internal SM05 and external SM1 threads and includes an SM1T1 Internal SM1 Adapter and SM1RR Retaining Ring. Housing Features of the Amplified GaP PhotodetectorsThorlabs' PDA25K2 is a compact GaP amplified photodetector with a switchable gain. This detector's housing features internal SM05 (0.535"-40) threading and external SM1 (1.035"-40) threading. It includes an SM1T1 internally SM1-threaded adapter and an SM1RR retaining ring, as shown to the right. The SM1T1 can hold up to 0.1" (2.8 mm) thick optics. This detector can be mounted using a 1/2" Post, as shown in the images below. The PDA25K2 housing features the active area flush with the front of the housing, simplifying alignments within optomechanical systems. This design also has two universal mounting holes that accept both 8-32 and M4 threads. An eight-position rotary gain switch is 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. As a convenience, the back panel is engraved with the responsivity curve of the photodiode. Lens Tube Compatibility Cage System Compatibility Post Mounting ![]() Click to Enlarge PDA Photodetector Mounted Horizontally ![]() Click to Enlarge PDA Photodetector Connected to an SM1 Lens Tube in a 30 mm Cage System ![]() Click to Enlarge PDA Photodetector Integrated into a 30 mm Cage System Using the External SM1 Threads ![]() Click to Enlarge PDA Photodetector Integrated into a 30 mm Cage System Using the SM1T1 (included) and SM1T2 Adapter BNC Female Output (Photodetector)0 - 10 V Output PDA Male (Power Cables)![]() PDA Female (Photodetector)Photodiode TutorialTheory of OperationA junction photodiode is an intrinsic device that behaves similarly to an ordinary signal diode, but it generates a photocurrent when light is absorbed in the depleted region of the junction semiconductor. A photodiode is a fast, highly linear device that exhibits high quantum efficiency based upon the application and may be used in a variety of different applications. It is necessary to be able to correctly determine the level of the output current to expect and the responsivity based upon the incident light. Depicted in Figure 1 is a junction photodiode model with basic discrete components to help visualize the main characteristics and gain a better understanding of the operation of Thorlabs' photodiodes.
Photodiode TerminologyResponsivity Modes of Operation (Photoconductive vs. Photovoltaic) Photoconductive Photovoltaic Dark Current The dark current present is also affected by the photodiode material and the size of the active area. Silicon devices generally produce low dark current compared to germanium devices which have high dark currents. The table below lists several photodiode materials and their relative dark currents, speeds, sensitivity, and costs.
Junction Capacitance Bandwidth and Response 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. Terminating Resistance Depending on the type of the photodiode, load resistance can affect the response speed. For maximum bandwidth, we recommend using a 50 Ω coaxial cable with a 50 Ω terminating resistor at the opposite end of the cable. This will minimize ringing by matching the cable with its characteristic impedance. If bandwidth is not important, you may increase the amount of voltage for a given light level by increasing RLOAD. In an unmatched termination, the length of the coaxial cable can have a profound impact on the response, so it is recommended to keep the cable as short as possible. Shunt Resistance Series Resistance Common Operating Circuits
The DET series detectors are modeled with the circuit depicted above. The detector is reverse biased to produce a linear response to the applied input light. The amount of photocurrent generated is based upon the incident light and wavelength and can be viewed on an oscilloscope by attaching a load resistance on the output. The function of the RC filter is to filter any high-frequency noise from the input supply that may contribute to a noisy output.
One can also use a photodetector with an amplifier for the purpose of achieving high gain. The user can choose whether to operate in Photovoltaic of Photoconductive modes. There are a few benefits of choosing this active circuit:
where GBP is the amplifier gain bandwidth product and CD is the sum of the junction capacitance and amplifier capacitance. Effects of Chopping FrequencyThe photoconductor signal will remain constant up to the time constant response limit. Many detectors, including PbS, PbSe, HgCdTe (MCT), and InAsSb, 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 The following table lists Thorlab's selection of previous and current generation PDA, PDF, and DET detectors.
Pulsed Laser Emission: Power and Energy CalculationsDetermining whether emission from a pulsed laser is compatible with a device or application can require referencing parameters that are not supplied by the laser's manufacturer. When this is the case, the necessary parameters can typically be calculated from the available information. Calculating peak pulse power, average power, pulse energy, and related parameters can be necessary to achieve desired outcomes including:
Pulsed laser radiation parameters are illustrated in Figure 1 and described in the table. For quick reference, a list of equations are provided below. The document available for download provides this information, as well as an introduction to pulsed laser emission, an overview of relationships among the different parameters, and guidance for applying the calculations.
![]() Click to Enlarge Figure 1: Parameters used to describe pulsed laser emission are indicated in the plot (above) and described in the table (below). Pulse energy (E) is the shaded area under the pulse curve. Pulse energy is, equivalently, the area of the diagonally hashed region.
Example Calculation: Is it safe to use a detector with a specified maximum peak optical input power of 75 mW to measure the following pulsed laser emission?
The energy per pulse: seems low, but the peak pulse power is: It is not safe to use the detector to measure this pulsed laser emission, since the peak power of the pulses is >5 orders of magnitude higher than the detector's maximum peak optical input power.
The following table lists Thorlabs' selection of photodiodes and photoconductive detectors. Item numbers in the same row contain the same detector element.
![]() ![]() 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. ![]()
Note: The APC adapters have two dimples in the front surface that allow them 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. FC/PC and FC/APC adapters are available with either narrow (2.0 mm) or wide (2.2 mm) key connectors; for more details on narrow versus wide key connectors, please see our Intro to Fiber tutorial. ![]()
Note: 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. FC/PC and FC/APC adapters are available with either narrow (2.0 mm) or wide (2.2 mm) key connectors; for more details on narrow versus wide key connectors, please see our Intro to Fiber tutorial. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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