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Thorlabs stocks a wide selection of discrete PIN junction photodiodes (PD) in various active area sizes and packages. These include indium gallium arsenide (InGaAs), gallium phosphide (GaP), silicon (Si), and germanium (Ge) photodiodes. We also offer four photodiode packages with enhanced performance characteristics: DSD2, FD05D, FD10D, and FGAP71. The DSD2 is a dual band photodiode, which incorporates two photodetectors sandwiched on top of each other (Si substrate on top of an InGaAs substrate), offering a combined wavelength range of 400 to 1700 nm. The FD10D and FD05D are InGaAs photodiodes with high responsivity from 800 to 2600 nm, allowing detection of wavelengths beyond the normal 1800 nm range of typical InGaAs photodiodes. We also offer the FGAP71, a gallium phosphide (GaP) photodiode, which is useful for detection of UV light sources from 150 to 550 nm.
To complement our photodiode product line, we also offer a range of compatible mounts and accessories. Please note that the PDs sold below are not calibrated, and specifications may differ slightly from lot to lot. We also offer calibrated photodiodes, which come with with NIST-traceable calibration, as well as a selection of mounted photodiodes. Our photodiodes can be reverse voltage biased using the PBM42 DC Bias Module for higher optical power detection; for more information on voltage biasing as well as the noise floor, please see the Lab Facts tab.
Inhomogeneity on the edge of an active area of the detector can generate unwanted capacitance and resistance that distorts the time-domain response of a photodiode. Thorlabs therefore recommends that the incident light on the photodiode is well centered on the active area. This can be accomplished by placing a focusing lens or pinhole in front of the detector element.
The responsivity of a particular photodiode varies from lot to lot. Due to this, the photodiode you receive may have a slightly different response than what is represented below. For example, to the right, a graph for the FDS1010 photodiode shows the extent that the response may vary. This data was collected from 104 photodiodes. Minimum, Average, and Maximum responsivity was calculated at each data point and has been plotted.
To view typical responsivity vs. wavelength data for each individual photodiode, please click the buttons in the product specifications tables below.
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
Click to Enlarge
Figure 1. Overview of the Photodiode's Response Curve,
Highlighting the Saturation Limit and the Noise Floor
Thorlabs Lab Fact: Photodiode Saturation Limit and Noise Floor
We present laboratory measurements of the saturation limit and noise floor of a Thorlabs silicon photodiode. While all photodiodes function similarly, there are a number of parameters that affect the noise floor and saturation limit of a photodiode including the sensor temperature, resistivity, reverse bias voltage, responsivity, and system bandwidth. Here we investigated the effect of reverse bias voltage and load resistance within a silicon-based photodiode detection system. Increasing the reverse bias increased the saturation limit and had minimal effect on the noise floor. Decreasing the load resistance decreased the noise floor until reaching the noise of the measurement system, but also decreased the saturation limit. These results demonstrate some of the considerations necessary for choosing the reverse bias voltage and load resistance, and emphasize that noise sources within all of the components must be considered when creating a detection system.
For our experiment we used the FDS100 Si Photodiode as the photodiode under investigation. The collimated output of a fiber-pigtailed laser diode was used as the light source with output power from 0 to 50 mW. The collimated beam was incident upon a beamsplitter that transmitted the majority of the light to the photodiode under investigation and reflected the rest towards a reference power sensor. The photodiode response was then evaluated under various resistive loads and with different reverse bias voltages.
The plots to the right and below summarize the measured results for the various tested configurations. From these graphs the changes to the photodiode's linear response, noise floor, and saturation limit can be observed under different reverse voltage biases and load resistances. Figure 1 provides an overview of the photodiode response with a reverse voltage bias of 5 V and resistive load of 10 kΩ. The photodiode saturated at the upper limit of the response when the output photovoltage approached the reverse bias voltage. The noise floor at the lower limit of the response was a result of dark current and the thermal noise from the resistive load (Johnson noise). Figure 2 summarizes the results obtained using the photodiode with a 1 kΩ resistive load and various reverse bias voltages. It illustrates that the saturation limit can be raised by increasing the reverse bias voltage (within specification). Figure 3 summarizes the results from using the photodiode with a 5 V reverse bias voltage and various resistive loads. It illustrates that the slope of the photovoltage response increased as the load resistance was increased. Figure 4 summarizes the noise floor results obtained using a 0 V reverse bias voltage and various resistive loads. The noise floor increased when larger load resistances were used. It is important to note that the 1 kΩ data was measured above the theoretical Johnson noise due to the voltage noise within the measurement system. Minimal change in the overall noise floor was seen when using a 5 V reverse bias voltage. For details on the experimental setup employed and these summarized results, please click here.
The following table lists Thorlabs' selection of photodiodes and photoconductive detectors. Item numbers in the same row contain the same detector element.