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High-Speed Free-Space Detectors
Uncoated Ball Lens
Uncoated Ball Lens
Replaces the battery in our DET series detectors and includes the LDS12B power supply and DET2A power adapter, shown connected.
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PDA200C Benchtop Photodiode Amplifier Connected to a DET025A Photodetector Using an SMA-to-BNC Cable
Thorlabs offers a variety of high-speed, high-bandwidth photodetectors designed for free-space input. Together, these detectors are sensitive from the visible to the near infrared (400 - 1700 nm); please see the "Selection Guide" table above for the exact spectral range covered by each detector. All detectors shown here feature GHz signal bandwidths and offer the same ease of use as the rest of our popular DET series. These detectors are designed to perform in test or measurement applications, including research in the fields of data communications, analog microwave, and general high-speed photonics. For comparable detection of fiber-coupled radiation, Thorlabs offers high-speed fiber-coupled detectors. We also have a variety of internally biased free-space photodiodes that operate at slower speeds than the detectors featured here. Our biased photodetectors are compatible with our benchtop photodiode amplifier and PMT transimpedance amplifier.
These free-space detectors are reverse biased and contain an internal bias battery, producing a linear response to the incident input light. To maintain the high signal bandwidth, the signal is output through an SMA connector. Thorlabs offers a complete range of electrical adapters and cables, including SMA cables and SMA-to-BNC adapters, for monitoring the output signal with an oscilloscope or other measurement electronics.
Our Si-based free-space detectors are designed for use in the 400 - 1100 nm (DET025A and DET025AL) wavelength range and provide a bandwidth of 2 GHz. For applications extending into the near infrared, consider our InGaAs-based free-space detectors, which provide detection in the 800 - 1700 nm (DET08C and DET08CL) wavelength range and provide a bandwidth of 5 GHz. When looking at high-speed signals, Thorlabs recommends using a 50 Ω load resistor. For lower bandwidth applications, our variable terminator or fixed stub-style terminators quickly adjusts the measured voltage.
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Uncoated Ball Lens Input on the DET08CL
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Flat, AR-Coated Window Input on the DET025A
Both of the Si-based and InGaAs-based detectors are available with either a flat, AR-coated window or an uncoated ball lens free-space input (see the photos to the right for details). The ball lens captures and focuses incident light onto the relatively small active area of the detector, making it advantagous for use in applications where the signal input needs to be increased. If the input signal is sufficiently large, a detector with a window over the aperture is recommended since focusing a strong input signal can cause saturation, and possibly damage, to the detector. Additionally, we recommend the detectors with the window over those with the lens when working with a pulsed source, as the chromatic dispersion that occurs as the light passes through the lens can artificially elongate the pulse.
All of these detectors include an A23 12 VDC Bias battery; this was chosen because it provides an extremely low noise source of power. This battery is optionally replaceable by the DET2B Power Adapter Bundle (sold below) when the detector is being used in applications where a small increase in the signal noise due to noise in the line voltage is permissible or the finite lifetime of a battery is not acceptable. Please note that due to slight physical variations of the positive terminal from manufacturer to manufacturer, Thorlabs only recommends using an Energizer® battery in our DET series of photodetectors.
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 photodiode output. Thorlabs therefore recommends that the incident light on the photodiode is well centered on the active area. Mounting a focusing lens or pinhole in front of the detector element is recommended.
Note: The data for all graphs above were obtained for their respective detectors with the AR-Coated windows included in the measurement set up.
0 - 10 V w/ 50 Ω
When using a battery-operated photodetector it is important to understand the battery’s lifetime and how this affects the operation of the detector. As a current output device, the output current of the photodetector is directly proportional to the light incidented on the detector. Most users will convert this current to a voltage by using a load-terminating resistor. The resistance value is approximately equal to the circuit gain. For very high speed detectors, such as those sold on this page, it is very important to use a 50 Ω terminating resistor to match the impedance of standard coax cables to reduce cable reflections and improve overall signal performance and integrity. Most high bandwidth scopes come equipped with this termination.
The battery usage lifetime directly correlates to the current used by the detector. Most battery manufacturers provide a battery lifetime in terms of mA hr. For example, the battery supplied with the DET08CL detectors is rated for 40 mA hrs. This means that it will reliably operate for 40 hr at a current draw of 1.0 mA. This battery will be used in the following example on how to determine battery lifetime based on usage.
For this example we have a 780 nm light source with an average 1 mW power is applied to an DET08CL. The responsivity of a biased photodetector based on the response curve at this wavelength is 0.5 A/W. The photocurrent can be calculated as:
Given the battery has a rated lifetime of 40 mA hr, the battery will last:
or 3.3 days of continuous use. By reducing the average incident power of the light to 10 µW, the same battery would last for about 333 days when used continuously. When using the recommended 50 Ω terminating load, the 0.5 mA photocurrent will be converted into a voltage of:
If the incident power level is reduced to 10 µW, the output voltage becomes 0.25 mV. For some measurement devices this signal level may be too low and a compromise between battery life and measurement accuracy will need to be made.
When using a battery-powered, biased photodetector, it is desirable to use as low a light intensity as is possible, keeping in mind the minimum voltage levels required. It is also important to remember that a battery will not immediately cease producing a current as it nears the end of its lifetime. Instead, the voltage of the battery will drop, and the electric potential being applied to the photodiode will decrease. This in turn will increase the response time of the detector and lower its bandwidth. As a result, it is important to make sure the battery has sufficient voltage (as given in the Troubleshooting chapter of the detector's manual) for the detector to operate within its specified parameters. The voltage can be checked with a multimeter.
Another suggestion to increase the battery lifetime is to remove, or power down the light source illuminating the sensor. Without the light source, the photodetector will continue to draw current proportional to the photodetector’s dark current, but this current will be significantly smaller. For example, the DET08CL has a dark current less than 1.5 nA.
For applications where a DET series photodetector is being continuously illuminated with a relatively high-power light source or if having to change the battery is not acceptable, we offer the DET2B power adapter bundle, which includes the power adapter and power supply (sold below). The drawback to this option is the noise in the line voltage will add to the noise in the output signal and could cause more measurement uncertainty.
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.
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SMA Output on the DET025A Detector
The DET025A(/M) and DET025AL(/M) high-speed, free-space detectors are designed for use in the 400 - 1100 nm spectral range. The DET025A(/M) has a flat, AR coated window at the input aperture, while the DET025AL(/M) uses the FDS025 photodiode which has an uncoated ball lens at the input aperture that focuses light onto the detector.
Both detectors have a 2 GHz bandwidth. An 8-32 tapped mounting hole (M4 for the metric version) allows easy mounting to our Ø1/2" posts.
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SMA Output on the DET08C Detector
The DET08C(/M) and DET08CL(/M) high-speed, free-space detectors are designed for use in the 800 - 1700 nm spectral range. The DET08C(/M) detector has a flat, AR-coated window at the input aperture, while the DET08CL(/M) detector has an uncoated ball lens at the input aperture that focuses the light onto the detector.
These detectors use an InGaAs detector element and feature a 5 GHz bandwidth. An 8-32 tapped mounting hole (M4 for the metric version) allows easy mounting to our series of Ø1/2" posts.
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Exploded View of SBP12 Battery Pack
A23 and T505 Alkaline Batteries
SBP12 Battery Pack
As shown by the photo to the right, the SBP12 consists of an A23 battery in a newly designed housing. You may already own this housing if you purchased your SV2-FC or SIR5-FC in or after October 2013, or if you have already purchased an SBP12. If you do own this housing, then it is necessary to purchase only the A23 battery.
Customers who own an SV2-FC or SIR5-FC detector purchased before October 2013 will need to bend two pins to ensure that the SBP12 battery pack makes electrical contact. The procedure is illustrated in the spec sheet of the battery, which can be downloaded here.
DET2B Installation Procedure
DET2A Power Adapter
LDS12B Power Supply
DET2B Power Adapter Bundle