Features

• 10 Models Cover the 150 nm to 2.6 μm Wavelength Range
• Rise Times as Fast as 1 ns
• Thin Profile [3/4" (19.1 mm)] Allows Measurements in Tight Spaces
• SM05 Lens Tube, SM1 Lens Tube, Cage System, and Ø1/2" Post Compatible
• Internal A23 Bias Battery (Included)

Thorlabs' biased photodetectors are available in ten models that cover the wavelength range from the UV to the mid-IR (150 nm to 2.6 µm) with improved bandwidth and NEP performance over previous models. The slim housing allows the optical detector to slip into tight setups. Each model comes complete with a fast PIN photodiode and an internal bias battery packaged in a rugged aluminum housing.

With a wide bandwidth DC-coupled output, these detectors are ideal for monitoring fast pulsed lasers as well as DC optical sources. The direct photodiode anode current is provided on a side panel BNC. This output is easily converted to a positive voltage using a terminating resistor. When looking at high-speed signals, Thorlabs recommends using a 50 Ω load resistor (sold below). For lower bandwidth applications, our variable terminator (also sold below) quickly adjusts the measured voltage.

All connections and controls are located away from the light path, which simplifies integration of our detectors in enclosed spaces. The SM1 (1.035"-40), SM05 (0.535"-40), and 8-32 (M4 for items ending in /M) threadings on the DET detector housing allow it to be mounted in a cage system, lens tube system, or on a Ø1/2" optical post. See the Mounting Options tab for more details on how to incorporate a DET series photodetector into an optical setup. Thorlabs also offers fiber-coupled biased photodetectors for wavelengths between 320 - 1700 nm.

Each DET includes an A23 12 VDC Bias battery. A battery was chosen because it provides an extremely low noise source of power. The battery can be replaced with a DET1B power adapter (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 permissable 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. The SM1 (1.035"-40) threading on the housing is ideally suited for mounting a Ø1" focusing lens or pinhole in front of the detector element.

Output Voltage Signal

BNC Female

0 - 10 V Output, 50 Ω Recommended Termination.

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 amount of incident light on the detector. Most users will convert this current to a voltage by using a terminating load resistor. The resistance value is approximately equal to the circuit gain. For very high speed detectors, such as the SIR5, 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 mAh (milliamp hours). For example, if a battery is rated for 190 mA hrs, it will reliably operate for 190 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 a detector. 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 190 mA hr, the battery will last:

or 16 days of continuous use. By reducing the average incident power of the light to 10 µW, the same battery would last for about 4 years 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 40 µW, the output voltage becomes 1 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 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 (lowering the bandwidth of the detector). As a result, it is important to make sure that the battery is operating within its specified parameters in order to ensure the proper functioning of the biased photodetector. The battery can be tested by following the procedure described in the specifications sheet or manual for the detector.

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 applications where a DET series photodetector is continuously illuminated with a relatively high-power light source, or if having to change the battery is not acceptable, we offer the DET1B 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.

The DET series biased photodiode detector housing is compatible with our line of lens tubes, TR series Ø1" posts, and cage systems. Because of the flexibility, the best method for mounting the housing in a given optical setup is not always obvious. The pictures and text in this tab will discuss some of the common mounting solutions. As always, our technical support staff is available for individual consultation.

Picture of a DET series biased photodiode detector as it will look when unpackaged.	Picture of a DET series biased photodiode detector with the included SM1T1 and its retaining ring removed from the front of the housing.	A close up picture of the front of a DET series biasedphotodiode detector with the SM1T1 removed. The external SM1and internal SM05 threading on the detector housing can be seen in this image.

Lens Tube System

Each DET housing includes a detachable Ø1" Optic Mount (SM1T1) that allows for Ø1" (Ø25.4 mm) optical components, such as optical filters and lenses, to be mounted along the axis perpendicular to the center of the photosensitive region. The maximum thickness of an optic that can be mounted in the SM1T1 is 0.1" (2.8 mm). For thicker Ø1" (Ø25.4 mm) optics or for any thickness of Ø0.5" (Ø12.7 mm) optics, remove the SM1T1 from the front of the detector and place (must be purchased separately) an SM1 or SM05 series lens tube, respectively, on the front of the detector.

The SM1 and SM05 threading on the DET biased photodiode detector housing make it compatible with our SM lens tube system and accessories. Two particularly useful accessories include the SM threaded irises and the SM compatible IR and visible alignment tools. Also available are fiber optic adapters for use with connectorized fibers; please see the Accessories tab above.

TR Series Post (Ø1/2" Posts) System

The DET housing can be mounted vertically or horizontally on a TR Series Post using the 8-32 (M4) threaded holes.

DET series detector mounted horizontally on a TR series post. Notice how the on/off switch is easily accessible from the top and the electrical connection comes in perpendicular to the beam path.	DET series detector mounted vertically on a TR series post. This image shows the VBIAS OUT button that can be pressed and held to check the battery's charge (this process is described in the manual).

Cage System

The simplest method for attaching the DET biased photodiode detector housing to a cage plate is to remove the SM1T1 that is attached to the front of the DET when it is shipped. This will expose external SM1 threading that is deep enough to thread the detector directly to a CP02 30 mm cage plate. When the CP02 cage plate is tightened down onto the DET biased photodiode detector housing the cage plate will not necessarily be square with the detector. To fix this, back off the cage plate until it is square with the detector and then use the retaining ring included with the SM1T1 to lock the DET detector into the desired location. This method for attaching the DET biased photodiode detector housing to a cage plate does not allow for much freedom in determining the orientation of the biased photodiode detector; however, it has the benefit of not needing an adapter piece and it allows the photodiode to be as close as possible to the cage plate, which can be important in setups where the light is divergent. On a side note, Thorlabs sells the SM05PD and SM1PD series of photodiodes that can be threaded into a cage plate so that the diode is flush with the front surface of the cage plate; however, the photodiode is unbiased.

For more freedom in choosing the orientation of the DET biased photodiode detector housing when attaching it, a SM1T2 lens tube coupler can be purchased. In this configuration the SM1T1 is left on the detector and the SM1T2 is threaded into it. The exposed external SM1 threading is now deep enough to secure the biased photodiode detector to a CP02 cage plate in any orientation and lock it into place using one of the two locking rings on the ST1T2.

This picture shows a DET series detector attached to a CP02 cage plate after removing the SM1T1. The retaining ring from the SM1T1 was used to make the orientation of the detector square with the cage plate.	This picture shows a DET series detector attached to a CP02 cage plate using an SM1T2 adapter in addition to the SM1T1 that comes with the DET series detector.

Although not pictured here, the DET detector housing can be connected to a 16 mm cage system by purchasing a SM05T2. It can be used to connect the DET detector housing to a SP02 cage plate.

Application

The image below shows a Michelson Interferometer built entirely from parts available from Thorlabs. This application demonstrates the ease with which an optical system can be constructed using our lens tube, TR series post, and cage systems.

The table contains a part list for the Michelson Interferometer with links to the pages that contain information about the individual parts. 

The following table lists the biased detectors found on this page, along with the unmounted and mounted photodiodes and amplified detectors which use the same internal photodiode.

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 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.

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, sensitivity, 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):

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 ofphotocurrent 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