Selection Guide for Fiber-Coupled Detectors
Wavelength	Element	Bandwidth	Model
400 - 1100 nm	Si	1 GHz	DET02AFC
		2 GHz	DET025AFC
800 - 1700 nm	InGaAs	1.2 GHz	DET01CFC
800 - 1700 nm		5 GHz (Max)	DET08CFC

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PDA200C Benchtop Photodiode Amplifier Connected to a DET02AFC Photodetector Using an SMA-to-BNC Cable

Features

• Four Models Cover Wavelengths from 400 - 1700 nm
• Bandwidths Ranging from 1 to 5 GHz
• Rise Times from 50 ps to 1 ns
• Connect to Single Mode (SM) or Multimode (MM) Fiber
• FC/PC Fiber Input Connector
• SMA Output Connector

Thorlabs offers a variety of fiber-coupled, high-speed, high-bandwidth photodetectors designed to connect to a single mode or multimode fiber with an FC/PC-terminated input. Together, these detectors are sensitive from the visible to the near infrared (400 - 1700 nm); please see the "Selection Guide" table to the right for the exact spectral range covered by each detector. All detectors shown here feature GHz signal bandwidths and 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 free-space radiation, Thorlabs offers high-speed free-space detectors. We also carry a variety of internally biased photodiodes that feature the same ease of use as our fiber-coupled photodetectors but operate at slower speeds. Our biased photodetectors are compatible with our benchtop photodiode amplifier and PMT transimpedance amplifier.

These fiber-coupled 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 fiber-coupled detectors are designed for use in the 400 - 1100 nm wavelength range and provide a bandwidth of 1 GHz (Item # DET02AFC) or 2 GHz (Item # DET025AFC). For applications extending into the near infrared, consider our InGaAs-based fiber-coupled detectors, which provide detection in the 800 - 1700 nm wavelength range and provide a bandwidth of 1.2 GHz (Item # DET01CFC) or 5 GHz (Max) (Item # DET08CFC). 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.

All of these detectors include a replaceable A23 12 VDC Bias battery, providing an extremely low-noise power source. This battery can be replaced by the DET2B power adapter bundle (sold below), which is ideal for 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 recommends using only an Energizer^® battery in our DET series photodetectors.

• Measured with specified bias voltage of 12 V.
• For a 50 Ω Load
• Low battery voltage will result in slower rise times and decreased bandwidth.
• Although this detector has FC/PC input connectors, our testing shows it may be used with FC/APC patch cables with no measurable differences in performance.
• A higher output voltage will decrease the bandwidth.
• Calculated based upon peak responsivity and damage threshold.
• These rise and fall times are tested and guaranteed in our production units. A theoretical rise time of 292 ps can be calculated using the formulas found in chapter 4.6 of the manual or the Photodiode Tutorial tab.
• For a 1 MΩ Load

Battery Lifetime

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 incident 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 DET08CFC 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 DET08CFC. 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 DET08CFC 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.

Photodiode Tutorial

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 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.

Figure 1: Photodiode Model

Photodiode Terminology

Responsivity
The responsivity of a photodiode can be defined as a ratio of generated photocurrent (I_PD) to the incident light power (P) at a given wavelength:

Modes of Operation (Photoconductive vs. Photovoltaic)
A photodiode can be operated in one of two modes: photoconductive (reverse bias) or photovoltaic (zero-bias). Mode selection depends upon the application's speed requirements and the amount of tolerable dark current (leakage current).

Photoconductive
In photoconductive mode, an external reverse bias is applied, which is the basis for our DET series detectors. The current measured through the circuit indicates illumination of the device; the measured output current is linearly proportional to the input optical power. Applying a reverse bias increases the width of the depletion junction producing an increased responsivity with a decrease in junction capacitance and produces a very linear response. Operating under these conditions does tend to produce a larger dark current, but this can be limited based upon the photodiode material. (Note: Our DET detectors are reverse biased and cannot be operated under a forward bias.)

Photovoltaic
In photovoltaic mode the photodiode is zero biased. The flow of current out of the device is restricted and a voltage builds up. This mode of operation exploits the photovoltaic effect, which is the basis for solar cells. The amount of dark current is kept at a minimum when operating in photovoltaic mode.

Dark Current
Dark current is leakage current that flows when a bias voltage is applied to a photodiode. When operating in a photoconductive mode, there tends to be a higher dark current that varies directly with temperature. Dark current approximately doubles for every 10 °C increase in temperature, and shunt resistance tends to double for every 6 °C rise. Of course, applying a higher bias will decrease the junction capacitance but will increase the amount of dark current present.

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, spectral ranges, and costs.

Junction Capacitance
Junction capacitance (C_j) is an important property of a photodiode as this can have a profound impact on the photodiode's bandwidth and response. It should be noted that larger diode areas encompass a greater junction volume with increased charge capacity. In a reverse bias application, the depletion width of the junction is increased, thus effectively reducing the junction capacitance and increasing the response speed.

Bandwidth and Response
A load resistor will react with the photodetector junction capacitance to limit the bandwidth. For best frequency response, a 50 Ω terminator should be used in conjunction with a 50 Ω coaxial cable. The bandwidth (f_BW) and the rise time response (t_r) can be approximated using the junction capacitance (C_j) and the load resistance (R_LOAD):

Noise Equivalent Power
The noise equivalent power (NEP) is the input signal power that results in a signal-to-noise ratio (SNR) of 1 in a 1 Hz output bandwidth. This is useful, as the NEP determines the ability of the detector to detect low level light. In general, the NEP increases with the active area of the detector and is given by the following equation:

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
A load resistance is used to convert the generated photocurrent into a voltage (V_OUT) for viewing on an oscilloscope:

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 R_LOAD. 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
Shunt resistance represents the resistance of the zero-biased photodiode junction. An ideal photodiode will have an infinite shunt resistance, but actual values may range from the order of ten Ω to thousands of MΩ and is dependent on the photodiode material. For example, and InGaAs detector has a shunt resistance on the order of 10 MΩ while a Ge detector is in the kΩ range. This can significantly impact the noise current on the photodiode. For most applications, however, the high resistance produces little effect and can be ignored.

Series Resistance
Series resistance is the resistance of the semiconductor material, and this low resistance can generally be ignored. The series resistance arises from the contacts and the wire bonds of the photodiode and is used to mainly determine the linearity of the photodiode under zero bias conditions.

Common Operating Circuits

Figure 2: Reverse-Biased Circuit (DET Series Detectors)

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.

Figure 3: Amplified Detector Circuit

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:

• Photovoltaic mode: The circuit is held at zero volts across the photodiode, since point A is held at the same potential as point B by the operational amplifier. This eliminates the possibility of dark current.
• Photoconductive mode: The photodiode is reversed biased, thus improving the bandwidth while lowering the junction capacitance. The gain of the detector is dependent on the feedback element (R_f). The bandwidth of the detector can be calculated using the following:

where GBP is the amplifier gain bandwidth product and C_D 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, photoconductive, and pyroelectric detectors. Item numbers in the same row contain the same detector element.