Click to Enlarge
Each detector has an internal SM05 and external SM1 thread and comes with an
Internal SM1 Adapter
Thorlabs' Biased Photodetectors are available in eleven 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. Thorlabs also offers high-speed free-space detectors and high-speed fiber-coupled detectors for wavelengths between 400 - 1700 nm. Our biased photodetectors are compatible with our benchtop photodiode amplifier and PMT transimpedance amplifier.
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. For lower bandwidth applications, our variable terminator or fixed stub-style terminators quickly adjusts the measured voltage. The detectors below do not have amplifiers or built-in gain, which generally allows them to operate at higher speeds than our PDA series of amplified photodetectors; for applications that require gain or switchable filters, a PDA amplified photodetector may be more suitable.
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. 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. See the Mounting Options tab for more details on how to incorporate a DET series photodetector into an optical setup.
Each detector is reverse-biased by an A23 12 VDC battery incorporated into the housing. The housing also includes a red button (pictured to the left) which, when held down, applies the battery's voltage across the external load. For a high Z load, this will output the battery's voltage over BNC, providing an easy way to determine if the battery should be replaced without removing it from the housing. An in-line current-limiting resistor (1.05 kΩ) prevents fast battery drainage if the battery is tested while connected to a 50 Ω load. 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. A battery was chosen for the reverse bias because it provides an extremely low noise source of power. If the finite lifetime of a battery is not acceptable, the battery can be replaced by a DET1B Power Supply Kit. Extra batteries and the DET1B are available for purchase below.
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
0 - 10 V Output, 50 Ω Recommended Termination.
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 DET08 series, it is very important to use a 50 Ω terminating resistor to match the impedance of standard coaxial 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 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 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 Ø1/2" 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 biased photodiode detector with the SM1T1 removed. The external SM1and internal SM05 threading on the detector housing can be seen in this image.
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.
The DET housing can be mounted vertically or horizontally on a Ø1/2" 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).
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.
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.
||Ø1" Adjustable Length Lens Tube
||Broadband Dielectric Laser Mirrors
||SM1 Threaded Lens Tube Iris
||Cage Rods, 4" Long
||30 mm Cage Plate for FiberPorts
||Cage Rods, 6" Long
||Cage System Z-Axis Translation Mount
||Mounted Beamsplitting Cube
||Ø1" Lens Tube, 3" in Length
||Biased Photodiode Detector
||Ø1/2" Post, 2" in Length
||Ø1/2" Post Holder
||Single Mode Fiber Patch Cable
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
The responsivity of a photodiode can be defined as a ratio of generated photocurrent (IPD) 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).
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.)
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 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.
|Material||Dark Current||Speed||Spectral Range||Cost|
||Visible to NIR
|Gallium Phosphide (GaP)
||UV to Visible
|Indium Gallium Arsenide (InGaAs)
|Indium Arsenide Antimonide (InAsSb)
||NIR to MIR
|Extended Range Indium Gallium Arsenide (InGaAs)
|Mercury Cadmium Telluride (MCT, HgCdTe)
||NIR to MIR
Junction capacitance (Cj) 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 (fBW) and the rise time response (tr) can be approximated using the junction capacitance (Cj) and the load resistance (RLOAD):
Noise Equivalent Power
The noise equivalent power (NEP) is the generated RMS signal voltage generated when the signal to noise ratio is equal to one. 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.
A load resistance is used to convert the generated photocurrent into a voltage (VOUT) 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 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 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 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 (Rf). The bandwidth of the detector can be calculated using the following:
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
Dark Current as a Function of Temperature or Reverse-Bias Votage
Measurements of dark current as a function of temperature and dark current as a function of reverse-bias voltage were acquired for several packaged detectors. As is described in the following section, dark current is a relatively small electrical current that flows in p-n junction photodetectors when no light is incident on the detector. For certain applications, it may be necessary to account for the change in dark current as temperature fluctuates and/or as the reverse-bias voltage changes. As a consequence of a battery's supplied voltage decreasing as it drains, the relationship between the reverse-bias voltage and the dark current level may be of particular interest if a battery is used to reverse-bias voltage the photodiode.
One set of measurements were taken for silicon (Si), germanium (Ge), and indium gallium arsenide (InGaAs) reverse-biased photodiodes over temperatures from 10 °C to 50 °C, and another set of measurements were taken for the same detectors while they were held at 24 °C and the reverse-bias voltage varied from 0 to 10 V. Please click the "More [+]" labels in the following expandable tables to read about the experiments and our measurements.
Current-Voltage Characteristics of p-n Junction Photodiodes
The characteristic current-voltage relationship of p-n junction photodiodes includes a forward-biased and a reverse-biased voltage regime. Operation of p-n juction photodiodes occurs in the reverse-biased voltage regime, in which a potential difference is applied across the diode to resist the flow of current. A convenient feature of some packaged photodiodes is that a battery inserted into the package can supply the reverse-bias voltage. Ideally, if no light is incident on a reverse-biased photodiode, no current flows.
Under real-world conditions, random processes in the semiconductor material of the photodiode always generate current carriers (electrons and holes) that produce current. These current generation processes are not driven by the photogeneration of electrons and holes. Instead, they are largely driven by the thermal energy contained in the semiconductor material. This dark current is generally small, but it is present when the photodiode is reverse biased and not illuminated. Dark current magnitudes vary for photodiodes of different material compositions; the efficiencies of the thermal generation processes depend on the type and crystal quality of the semiconductor used in the detector's sensing head. The magnitude of the dark current can be expected to increase as the temperature of the photodiode increases and as the reverse-bias voltage applied to the photodiode increases.
It is important to note that if the reverse bias voltage is increased beyond a certain threshold, the photodiode will suffer reverse breakdown, in which the magnitude of the current increases exponentially and permanent damage to the diode is likely. For this reason, many of the Thorlabs DET packages include a voltage regulator to prevent the bias voltage from reaching breakdown.
When a photodiode is illuminated, the current generated by the incident light adds to the dark current. The carriers in the photocurrent are generated by the energy contained in the photons of the incident light. Above a certain illumination threshold intensity, the magnitude of the photocurrent exceeds the magnitude of the dark current. When the photocurrent is larger than the dark current, the magnitude of the photocurrent can be calculated by measuring the total current and then subtracting the contribution of the dark current. When the photocurrent is smaller than the noise on the dark current, the photocurrent is undetectable. Because of this, it is desirable to minimize the levels of dark current in photodiodes.
 J. Liu, Photonic Devices. Cambridge University Press, Cambridge, UK, 2005
Dark currents were measured over temperatures of 10 °C to 50 °C for three representative packaged detectors: the Si-based DET100A, the Ge-based DET50B, and the InGaAs-based DET10C.
Click to EnlargeFigure 1:
The Nested Metal Box Test Fixture with Covers Removed and DET10C Installed
Black insulating foam lines the interiors of both boxes.
A: Outer Aluminum Box; B: Inner Aluminum Box; C: Thermistor; D: DET10C; E: BNC-to-Triax Feedthrough; F: BNC-to-BNC Feedthrough
The experimental setup was designed to ensure a constant reverse-bias voltage across the photodiode, control the temperature of the detector, block light from reaching the detector, and establish an electrical path between the detector and the Keithley 6487 ammeter that isolated the measured current from extraneous electromagnetic interference (EMI) noise sources. Dark currents for these detectors can be on the order of nA, which made it important to control these conditions to ensure accurate measurements.
As the dark current depends to some degree on the magnitude of the reverse-bias voltage, care was taken to ensure the reverse-bias voltage remained constant over the duration of each measurement set. For these detectors, the source of the reverse-bias voltage was a battery. Before each data set was acquired, the detector's A23 battery was replaced with a fresh one. The battery's voltage was also measured before and after each test to ensure the bias voltage remained constant over the course of the measurement.
Thermistors were used to monitor the temperatures of each photodiode continuously during the experiment. A thermistor was held in contact with the outer surface of the TO can housing using a piece of thermal tape. With the thermistor in place it was not possible to use the detector cap to block light from reaching the photodiode.
The detector enclosure consisted of a pair of nested aluminum metal boxes, which are shown with their covers off in Figure 1. Only one box would be required to create a light-tight environment for the photodiode, but an outer box was included in the setup to shield the inner box from EMI related noise. The detector was placed inside the inner box, which was equipped with a BNC-to-triax adapter feedthrough; the BNC end was accessible from inside of the box, and the triax end was accessible from outside of the box. The BNC connector on the detector was attached directly to the BNC end of the adapter, which advantageously eliminated the need to use a BNC cable. BNC cables are poorly shielded, and using one could introduce noise from EMI sources to the signal read from the detector. This inner metal box was placed inside of an outer metal box, which possessed a triax-to-triax feedthrough. A triax cable was used to route the detector signal between the feedthroughs on the inner and outer boxes. A triax cable was also used to connect the Keithley 6487 ammeter to the end of the Triax-to-Triax feedthrough accessible on the outside of the outer box. The outer metal box served to isolate the inner metal box from EMI, and the triax cables shielded the detector signal from EMI. After covering two boxes with their respective lids, the nested box set was placed inside an ESPEC ESX-3CW temperature chamber. The ammeter was located outside of the chamber.
The electrical connection between the thermistor and TSP01 temperature logger was performed using BNC cables, BNC-to-BNC bulkhead feedthroughs to route the signal out of the nested boxes, and a custom BNC-to-phono jack cable to connect to the temperature logger.
The data curves plotted in Figure 2 are the dark currents measured for the Si-based DET100A, the Ge-based DET50B, and the InGaAs-based DET10C. Data were acquired continuously while the temperature was within the specified operating range of the photodiodes: between 10 °C and 50 °C (shaded region of Figure 3). Figure 3 is representative of the temperature of the photodiode's TO can measured by the thermistor during the experiment. The temperature profile of the chamber intentionally included higher and lower temperatures to ensure that the measurements could be taken over the full temperature operating range of the photodiodes. The data plotted in Figure 2 include dark current measurements acquired as the temperature was both increasing and decreasing; the two data sets overlay one another for each diode.
The data curves plotted in Figure 2 show the Si-based detector exhibited the least amount of dark current and the Ge-based detector exhibited the most, with the levels of the latter being approximately 5 orders of magnitude higher. In all cases, the dark current increased with the temperature of the photodiode, as expected. The individual points on this graph, plotted as diamonds, are the values of the dark current specified for each detector at 25 °C. These points specify a maximum value of dark current at 25 °C; each diode's dark current must be equal to or less than this value at 25 °C, but the dark current may exceed this specification at higher temperatures, as is the case for the measured DET50B detector.
Click to EnlargeFigure 2:
Dark Current Data Measured for Three Packaged Photodiodes
The discrete data points, plotted as diamonds, are values of the dark current specified for each detector at 25 °C.
Click to EnlargeFigure 3:
Temperature of a Representative Photodiode Controlled by the Conditions in the Environmental Chamber
Dark current measurements were acquired between 10 °C and 50 °C (indicated by the shaded region) and are plotted in Figure 2 .
Measurements were performed using a single representative of each detector type, as these data were intended to illustrate general trends. These data should not be taken as specific for a particular diode. The measured dark current is a function of voltage bias, temperature dependence of resistive loads, and other effects. Efforts were made to suppress their influence on these measurements, including replacing each detector's battery with a fresh one prior to each measurement and using an ammeter to measure the dark current. Using the ammeter removed the need to use a load resisitor, which may exhibit its own temperature dependence. The thermistor was placed as close to the semiconductor sensor as was possible, but it was not placed in direct contact with the sensor. Because of this, there may have been a difference between the measured temperature and the temperature of the semiconductor material. The dark current was measured while the temperature of the environmental chamber was continuously varied. Measurements were not made during steady-state temperature conditions, but rather during a slow temperature ramp. Temperatures below 10 °C and above 50 °C were not measured, and the effects of quickly-varying changes in temperature were not investigated. Humidity was not controlled during this experiment.
Dark currents were measured while three representative packaged detectors, the Si-based DET100A, the Ge-based DET50B, and the InGaAs-based DET10C, were held at 24 °C and their reverse-bias voltages were varied between 0 and 10 V. Either a battery or an external power supply, such as the DET1B Power Adapter, can be used to provide the required reverse-bias voltage. The measurements described below were made to demonstrate how the dark current changes as the voltage supplied by the battery decreases as the battery drains.
Click to EnlargeFigure 1:
The Nested Metal Box Test Fixture with Covers Removed and DET10C Installed
Black insulating foam lines the interiors of both boxes.
A: Outer Aluminum Box; B: Inner Aluminum Box; C: Thermistor; D: DET10C; E: Power Adapter of the DET1B Power Supply; F: BNC-to-BNC Feedthrough for the DET1B, G: BNC-to-Triax Feedthrough for the Detector Signal; H: BNC-to-BNC Feedthrough for the Thermistor
The experimental setup was designed to measure the dark current of the photodiodes as a function of the reverse bias voltage, while mitigating the contributions from noise sources. The setup controlled the temperature of the detector, blocked light from reaching the detector, and established an electrical path between the detector and the Keithley 6487 ammeter, which isolated the measured current from extraneous electromagnetic interference (EMI) noise sources. Dark currents for these detectors can be on the order of nA, which made it important to control these conditions to ensure accurate measurements.
The detector enclosure consisted of a pair of nested aluminum metal boxes, which are shown with their covers off in Figure 1. Only one box would be required to create a light-tight environment for the photodiode, but an outer box was included in the setup to shield the inner box from EMI related noise. The detector was placed inside the inner box, and electrical connections were made through it using BNC and a BNC-to-triax adapter feedthroughs.
The measurement signal from the detector was routed through the BNC-to-triax adapter feedthrough. The BNC connector on the detector was attached directly to the BNC end of the adapter, which advantageously eliminated the need to use a BNC cable. BNC cables are poorly shielded, and using one could introduce noise from EMI sources to the signal read from the detector. This inner metal box was placed inside of an outer metal box, which possessed a triax-to-triax feedthrough. A triax cable was used to route the detector signal between the feedthroughs on the inner and outer boxes. A triax cable was also used to connect the Keithley 6487 ammeter to the end of the Triax-to-Triax feedthrough accessible on the outside of the outer box. The outer metal box served to isolate the inner metal box from EMI, and the triax cables shielded the detector signal from EMI. After covering the two boxes with their respective lids, the nested box set was placed inside an ESPEC ESX-3CW temperature chamber. The ammeter was located outside of the chamber.
A DET1B, which includes a power supply and a power adapter designed to be inserted into the battery port of the DET series detectors, was used to supply the reverse-bias voltage, which was routed through BNC feedthroughs. The power adapter was inserted into the detector, and the opposite end of the adapter was connected to a BNC feedthough fitted into the wall of the inner box. A BNC cable and another feedthrough was used extend the electrical connection to the outside of the outer box, and the voltage output of the Keithley 6487 was connected to that. A computer was interfaced with the Keithley, which allowed the computer to simultaneously control the reverse-bias voltage while measuring the dark current.
As the dark current depends strongly on temperature, thermistors were used to monitor the temperatures of each photodiode continuously during the experiment. A thermistor was held in contact with the outer surface of the TO can housing using a piece of thermal tape. With the thermistor in place, it was not possible to use the detector cap to block light from reaching the photodiode. The electrical connection between the thermistor and TSP01 temperature logger was performed using BNC cables, BNC-to-BNC bulkhead feedthroughs to route the signal out of the nested boxes, and a custom BNC-to-phono jack cable to connect to the temperature logger.
The data curves plotted in Figures 2 through 4 are the dark currents measured for the Si-based DET100A, the Ge-based DET50B, and the InGaAs-based DET10C, respectively.
The Si-based DET100A does not contain an integrated voltage regulator, while both the Ge-based DET50B and the InGaAs-based DET10C do. Voltage regulators integrated into photodiodes are used to maintain a stable reverse bias voltage across the photodiode.
In general, voltage regulators are applied in a range of applications to automatically maintain a stable voltage level: the regulator ensures that the output voltage level is approximately constant as long as the voltage input falls within a specified range. When the input voltage decreases below the voltage regulator's dropout voltage, which marks the lower end of the range, the regulator is no longer able to regulate the output voltage. As the input voltage continues to decrease from the dropout voltage, the output voltage decreases until a threshold is reached and the regulator becomes non-functional. While in this off state, the output voltage is negligible. In the case of the Ge-based DET50B and the InGaAs-based DET10C, the output voltage provided by the voltage regulator is used to reverse bias the photodiode.
The absence of a voltage regulator in the Si-based DET100A results in its measured dark current, plotted in Figure 2, continuing to increase as the voltage supplied by the Keithley 6487 increases. The dark current measured for the DET100A increases exponentially up to a reverse-bias voltage of 0.5 V, and then it increases linearly for reverse-bias voltages between approximately 0.5 V and 10 V. A fresh battery supplies ≥10 V.
Both the Ge-based DET50B, shown in Figure 3, and the InGaAs-based DET10C, shown in Figure 4, exhibit no dark current until the reverse-bias voltage reaches a threshold (approximately 1 V for the DET50B and 1.5 V for the DET10C). The voltage range up to this threshold value corresponds to the off state of the voltage regulator. After this threshold is exceeded, the dark currents measured for these detectors increase, approximately exponentially, until the voltage supplied by the Keithley 6487 reaches the dropout regulator voltage of 5.5 V. For supplied voltages between 5.5 V and 10 V, the voltage regulator maintains a constant 5.5 V reverse-bias across the photodiodes, which results in an approximately constant values of dark current for supplied voltages over this range. As a fresh battery supplies ≥10 V, the voltage regulator has the effect of ensuring the dark current level remains approximately constant over much of the battery's lifetime, as well as protecting the photodiode from exposure to excessive reverse-bias voltages in general.
Click to EnlargeFigure 2:
Dark current data measured for the Si-based DET100A packaged photodiode, which does not include an integrated voltage regulator, is plotted as a function of the voltage supplied by the Keithley 6487.
Click to EnlargeFigure 3:
Dark current data measured for the Ge-based DET50B packaged photodiode, which includes an integrated voltage regulator, is plotted as a function of the voltage supplied by the Keithley 6487.
Click to EnlargeFigure 4:
Dark current data measured for the InGaAs-Based DET10C packaged photodiode, which includes an integrated voltage regulator, is plotted as a function of the voltage supplied by the Keithley 6487.
Measurements were performed using a single representative of each detector type, as these data were intended to illustrate general trends. These data should not be taken as specific for a particular diode. The measured dark current is a function of voltage bias, temperature dependence of resistive loads, and other effects. Efforts were made to suppress their influence on these measurements, including performing the measurement in a temperature controlled chamber and using an ammeter to measure the dark current. Using the ammeter removed the need to use a load resistor, which may exhibit its own temperature dependence. Humidity was not controlled during this experiment.
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The following table lists Thorlabs' selection of photodiodes and photoconductive detectors. Item numbers in the same row contain the same detector element.