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Mid-IR Photovoltaic Detectors, HgCdTe (MCT)![]()
VL5T0 2.7 - 5.0 µm Wavelength Range, VML10T4 2.0 - 10.6 µm Wavelength Range Related Items ![]() Please Wait
![]() Click for Details 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. ![]() Click for Details A standard hemispherical immersion lens forms an image in the plane of the sensor element. Features
These photodiodes operate in photovoltaic mode and provide coverage for Mid-IR wavelengths through 10.6 µm. The detectors are optimized for best performance at a specific wavelength (5.0 µm, 8.0 µm, or 10.6 µm). 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. Our 8.0 µm and 10.6 µm detectors are also available with integrated four-stage thermoelectric coolers (TECs) to maintain the detector elements at -78 °C, as cooling improves detector current responsivity and therefore detectivity. Our MCT detectors with and without TEC are offered in TO-8 or TO-39 packages, respectively. We also incorporate the VML8T0 and VML10T0 detectors in our PDAVJ Series Amplified Photodetectors. Each detector includes a printed test report including measured responsivity and detectivity plots, as well as other electrical characteristics. Click here for a sample test report. Optically Immersed Sensor Element Detectors with TECs Incorrect Heat Sink Placement![]() Click for Details The heat sink should not predominantly contact the mounting screw or the cylindrical housing. Correct Heat Sink Placement![]() Click for Details The heat sink must adequately contact the base of the detector package using thermal grease or heat conductive epoxy. Heat Sink Placement
![]() Click for Details Click Here for Raw Data This graph specifies resistance vs. temperature for the Murata NCP03XM222E05RL thermistor incorporated in our VMLxT4 detectors. The raw data file above also includes resistance values calculated using the β-parameter equation. Temperature Control ![]() MCT Photovoltaic Detector Pin DiagramsBoth views shown below are looking at the connector from the bottom side. ![]() VL5T0, VML8T0, and VML10T0 Detectors (TO-39 Package) ![]() VML8T4 and VML10T4 Detectors with Integrated TEC (TO-8 Package)
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 Thorlabs' selection of photodiodes and photoconductive detectors. Item numbers in the same row contain the same detector element.
![]() ![]() Click to Enlarge VL5T0 Photodiode Detectivity ![]() ![]() Click to Enlarge VML8T0 and VML8T4 Photodiode Detectivity ![]() ![]() Click to Enlarge VML10T0 and VML10T4 Photodiode Detectivity | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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