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HgCdTe (MCT) Amplified Photodetectors
2.0 - 8.0 µm,
2.0 - 10.6 µm,
2.7 - 5.0 µm,
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A hyperhemispherical lens forms an image on a virtual plane behind the sensor element, resulting in an effective optical area greater than the physical sensor area.
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A standard hemispherical immersion lens forms an image in the plane of the sensor element.
Thorlabs' Amplified HgCdTe (mercury cadmium telluride, MCT) Photovoltaic Detectors are sensitive to MIR light. A rotary switch controls the gain amplifier (shown in the photo below), allowing performance to be optimized for a variety of applications. The gain switch features eight discrete steps from 0 to 30 dB (Item #s PDAVJ8 and PDAVJ10) or 0 to 42 dB (Item # PDAVJ5). For best results, we recommend connecting the output cable (not included) to a 50 Ω termination.
Each HgCdTe (MCT) sensor element is integrated with a hyperhemispherical GaAs lens to achieve optical immersion, and each detector is provided with a wedged ZnSe window that is AR-coated for the 2 - 13 µm wavelength range.
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Side View Showing Gain Adjuster
The detector package incorporates many of the same mechanical features as our other mounted photodetectors. External SM1 (1.035"-40) and internal SM05 (0.535"-40) threading allows Ø1" lens tubes or Ø1/2" lens tubes to be mounted in front of the detector element. Universal 8-32 & M4 tapped holes connect a Ø1/2" post to the housing in one of two perpendicular orientations. A location-specific power supply is included with each detector.
Optically Immersed Sensor Element
All specifications are valid at 23 ± 5°C and 45 ± 15% relative humidity.
0 - 1.0 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.