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HgCdTe (MCT) Amplified Photodetector
Post Not Included
Thorlabs' PDA10JT(-EC) Amplified Detector is a thermoelectrically cooled photoconductive HgCdTe (mercury cadmium telluride, MCT) detector. It is sensitive to light in the mid-IR spectral range from 2.0 to 5.4 µm. Two rotary switches control the gain amplifier and detector package bandwidth, allowing performance to be optimized for a variety of applications. The gain switch features eight discrete steps from 0 - 40 dB, while the bandwidth switch provides eight discrete steps from 1.25 - 160 kHz. The thermoelectric cooler (TEC) uses a thermistor feedback loop to hold the temperature of the detector element at -30 °C, minimizing thermal contributions to the output signal.
For best results, we recommend connecting the output cable (not included) to a 50 Ω termination. Because the detector is AC coupled, it requires a pulsed or chopped input signal. AC-coupled detectors will not see unchopped CW light because they are only sensitive to intensity changes, not absolute intensity.
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Side View Showing Gain and Bandwidth Adjusters
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Top View Showing Signal Output and Power Input
The detector package incorporates many of the same mechanical features as our other mounted photodetectors. An internal SM1 (1.035"-40) threading allows Ø1" lens tubes to be mounted in front of the detector element. Two 8-32 (M4 in the -EC version) tapped holes connect a Ø1/2" post to the housing in one of two perpendicular orientations, as shown in the image at the top of the page. The PDA10JT includes a 100 - 120 VAC power adapter, while the PDA10JT-EC ships with a 220 - 240 VAC power adapter. If you require a different adapter plug, please contact Tech Support prior to ordering. An SM1RR Retaining Ring is also included.
This detector's output signal depends nonlinearly on the optical input power; for a plot of this, see the Graphs tab. Please note that 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 output. Thorlabs therefore recommends that the incident light is well centered on the active area. The SM1 (1.035"-40) threading on the housing can be connected to an SM1 lens tube; the lens tube can be used to mount an iris or pinhole in front of the detector element. Because the detector package protrudes 3.9 mm beyond the front of the threading, optics and optomechanics cannot be attached directly to the housing.
In addition to the HgCdTe detector sold here, Thorlabs manufactures a InAsSb detector with broader wavelength sensitivity and higher bandwidth at the expense of a higher NEP. If a more compact detector housing is desired, we also offer room-temperature amplified photodetectors.
All values given below are for a 50 Ω load, unless otherwise stated.
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These traces compare the noise level for the lowest gain and bandwidth settings to the noise level for the highest gain and bandwidth settings.
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This trace shows that the responsivity varies with input power. For example, increasing the power from 2.5 mW to 5 mW (a difference of 2.5 mW) produces a greater signal change than increasing the power from 25 mW to 27.5 mW (also a difference of 2.5 mW).
Detectivity, D*, is defined as:
where A is the area of the photosensitive region of the detector, Δf is the effective noise bandwidth, and NEP is the noise-equivalent power.
0 - 5 V at 50 Ω
Theory of Operation
A 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.
Modes of Operation (Photoconductive vs. Photovoltaic)
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.
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.
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.
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 Frequency
The 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 Thorlabs' selection of photodiodes and photoconductive detectors. Item numbers in the same row contain the same detector element.