Si Free-Space Amplified Photodetectors, OEM Package
- Wavelength Ranges from 320 nm to 1100 nm
- Maximum Bandwidths up to 12 MHz
- Detector on Printed Circuit Board for OEM Applications
350 - 1100 nm
12 MHz Max Bandwidth
320 - 1100 nm
11 MHz Max Bandwidth
|Item #||Wavelength Range||Bandwidth||NEP|
|PDAPC2a||320 - 1100 nm||DC - 11 MHzb||2.67 - 71.7 pW/Hz1/2|
|PDAPC1a||350 - 1100 nm||DC - 12 MHzb||3.25 - 75.7 pW/Hz1/2|
- Two Models Available for UV, Visible, and NIR:
- PDAPC2: 320 to 1100 nm
- PDAPC1: 350 to 1100 nm
- Low-Noise Amplification with Switchable Gain
- 50 Ω and Higher Load Impedances
- Free-Space Optical Coupling
- Output Accessible via Jumper and MMCX Connector
We offer a selection of Silicon (Si) Free-Space Amplified Photodetectors that are sensitive to light from the UV to near infrared. These amplified photodetectors feature a built-in low-noise transimpedance amplifier (TIA) and switchable gain with eight gain settings. Gain can be adjusted via on-board header pins or manually through four DIP switches. These photodetectors are capable of driving loads from 50 Ω to Hi-Z with the output being accessible through a jumper or an MMCX connector on the rear of the printed circuit board (PCB). Each detector PCB has four Ø3.2 mm through holes on 30 mm spacings for compatibility with our 30 mm cage system or mounting in custom housings and devices.
A ±12 V linear power supply that supports input voltages of 100, 120, and 230 VAC is available for purchase separately below. Before connecting the power supply to mains voltage, ensure that the mains voltage switch on the power supply module is set to the proper voltage range. The power supplies should always be powered up using the power switch on the power supply itself. Hot plugging the unit is not recommended.
|Item #||Wavelength||Bandwidth||Rise Time||Peak Responsivity||Noise Equivalent Power (NEP)a||Active Area||Operating
|PDAPC2b||320 - 1100 nm||DC - 11 MHzc||N/Ad||0.72 A/W @ 960 nm||2.67 - 71.7 pW/Hz1/2||100 mm2 (10 mm x 10 mm)||10 to 40 °C|
|PDAPC1b||350 - 1100 nm||DC - 12 MHzc||N/Ad||0.65 A/W @ 970 nm||3.25 - 75.7 pW/Hz1/2||13 mm2 (3.6 mm x 3.6 mm)||10 to 40 °C|
|Item #||Gain Step
w/ Hi-Z Load
w/ 50 Ω Load
|NEPb||Offset (±)||Output Voltage
w/ Hi-Z Load
w/ 50 Ω Load
|PDAPC2||0||1.51 kV/A ± 2%||0.75 kV/A ± 2%||11 MHz||268 µV||71.7 pW/Hz1/2||8 mV (12 mV Max)||0 - 10 V||0 - 5 V|
|10||4.75 kV/A ± 2%||2.38 kV/A ± 2%||1.4 MHz||195 µV||6.75 pW/Hz1/2|
|20||15 kV/A ± 2%||7.5 kV/A ± 2%||800 kHz||219 µV||3.36 pW/Hz1/2|
|30||47.5 kV/A ± 2%||23.8 kV/A ± 2%||260 kHz||222 µV||2.83 pW/Hz1/2|
|40||151 kV/A ± 2%||75 kV/A ± 2%||90 kHz||229 µV||2.67 pW/Hz1/2|
|50||475 kV/A ± 2%||238 kV/A ± 2%||28 kHz||271 µV||4.2 pW/Hz1/2|
|60||1.5 MV/A ± 5%||750 kV/A ± 5%||9 kHz||423 µV||6.24 pW/Hz1/2|
|70||4.75 MV/A ± 5%||2.38 MV/A ± 5%||3 kHz||1.22 mV||7.88 pW/Hz1/2|
|PDAPC1||0||1.51 kV/A ± 2%||0.75 kV/A ± 2%||12 MHz||258 µV||75.7 pW/Hz1/2||8 mV (12 mV Max)||0 - 10 V||0 - 5 V|
|10||4.75 kV/A ± 2%||2.38 kV/A ± 2%||1.6 MHz||192 µV||5.8 pW/Hz1/2|
|20||15 kV/A ± 2%||7.5 kV/A ± 2%||1 MHz||207 µV||3.4 pW/Hz1/2|
|30||47.5 kV/A ± 2%||23.8 kV/A ± 2%||260 kHz||211 µV||3.4 pW/Hz1/2|
|40||150 kV/A ± 2%||75 kV/A ± 2%||90 kHz||214 µV||3.25 pW/Hz1/2|
|50||475 kV/A ± 2%||238 kV/A ± 2%||28 kHz||234 µV||3.69 pW/Hz1/2|
|60||1.5 MV/A ± 5%||750 kV/A ± 5%||9 kHz||277 µV||4 pW/Hz1/2|
|70||4.75 MV/A ± 5%||2.38 MV/A ± 5%||3 kHz||388 µV||4.29 pW/Hz1/2|
J2 Header Pins
|0 to 5 V (50 Ω)
0 to 10 V (Hi-Z)
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|
|Silicon (Si)||Low||High Speed||Visible to NIR||Low|
|Germanium (Ge)||High||Low Speed||NIR||Low|
|Gallium Phosphide (GaP)||Low||High Speed||UV to Visible||Moderate|
|Indium Gallium Arsenide (InGaAs)||Low||High Speed||NIR||Moderate|
|Indium Arsenide Antimonide (InAsSb)||High||Low Speed||NIR to MIR||High|
|Extended Range Indium Gallium Arsenide (InGaAs)||High||High Speed||NIR||High|
|Mercury Cadmium Telluride (MCT, HgCdTe)||High||Low Speed||NIR to MIR||High|
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
Pulsed Laser Emission: Power and Energy Calculations
Determining whether emission from a pulsed laser is compatible with a device or application can require referencing parameters that are not supplied by the laser's manufacturer. When this is the case, the necessary parameters can typically be calculated from the available information. Calculating peak pulse power, average power, pulse energy, and related parameters can be necessary to achieve desired outcomes including:
- Protecting biological samples from harm.
- Measuring the pulsed laser emission without damaging photodetectors and other sensors.
- Exciting fluorescence and non-linear effects in materials.
Pulsed laser radiation parameters are illustrated in Figure 1 and described in the table. For quick reference, a list of equations are provided below. The document available for download provides this information, as well as an introduction to pulsed laser emission, an overview of relationships among the different parameters, and guidance for applying the calculations.
Peak power and average power calculated from each other:
|Peak power calculated from average power and duty cycle*:|
|*Duty cycle () is the fraction of time during which there is laser pulse emission.|
Click to Enlarge
Figure 1: Parameters used to describe pulsed laser emission are indicated in the plot (above) and described in the table (below). Pulse energy (E) is the shaded area under the pulse curve. Pulse energy is, equivalently, the area of the diagonally hashed region.
|Pulse Energy||E||Joules [J]||A measure of one pulse's total emission, which is the only light emitted by the laser over the entire period. The pulse energy equals the shaded area, which is equivalent to the area covered by diagonal hash marks.|
|Period||Δt||Seconds [s]||The amount of time between the start of one pulse and the start of the next.|
|Average Power||Pavg||Watts [W]||The height on the optical power axis, if the energy emitted by the pulse were uniformly spread over the entire period.|
|Instantaneous Power||P||Watts [W]||The optical power at a single, specific point in time.|
|Peak Power||Ppeak||Watts [W]||The maximum instantaneous optical power output by the laser.|
|Pulse Width||Seconds [s]||A measure of the time between the beginning and end of the pulse, typically based on the full width half maximum (FWHM) of the pulse shape. Also called pulse duration.|
|Repetition Rate||frep||Hertz [Hz]||The frequency with which pulses are emitted. Equal to the reciprocal of the period.|
Is it safe to use a detector with a specified maximum peak optical input power of 75 mW to measure the following pulsed laser emission?
- Average Power: 1 mW
- Repetition Rate: 85 MHz
- Pulse Width: 10 fs
The energy per pulse:
seems low, but the peak pulse power is:
It is not safe to use the detector to measure this pulsed laser emission, since the peak power of the pulses is >5 orders of magnitude higher than the detector's maximum peak optical input power.
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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.
(Click Link for Image)
|Hi-Z Load||50 Ω Load|
|PDAPC2f||320 - 1100 nm||DC - 11 MHz||1.51 kV/A - 4.75 MV/A||0.75 kV/A - 2.38 MV/A||2.67 -
(10 mm x 10 mm)
|10 to 40 °C|
|PDAPC1g||350 - 1100 nm||DC - 12 MHz||1.51 kV/A - 4.75 MV/A||0.75 kV/A - 2.38 MV/A||3.25 -
||10 to 40 °C|
Click to Enlarge
Pin Diagram for the Included Cable
- Power Supply (Item # LDS12B) and Cable for Use with PDAPC Series Photodetectors
- 3 Pin Cable Output Connects to J2 Jumper on Detector Board
- ±12 VDC Output
- Switchable Input Voltage: 100 V, 120 V, or 230 V
The LD1255-SUPPLY Bundle includes the LDS12B Power Supply and the appropriate cable to connect to PDAPC Series Photo detectors (available above). A switch on the power supply box enables the user to choose a 100 V, 120 V, or 230 V input voltage. The cable is able to transmit ±12 VDC from the power supply to the photodetector. The included cable can be ordered separately from the power supply by contacting Tech Support.
- Optional Cables for Output of PDAPC Series Photodetectors
- MMCX Male Connector to BNC or SMA Male Connector
- DC - 6 GHz Frequency Range
- 50 Ω Normal Impedance
- 170 V Max Operating Voltage
These cables convert the MMCX output of the photodetectors sold above to BNC or SMA male connectors. The CA3339 (1 m long) and CA3272 (1.8 m long) cables feature a male BNC termination while the CA3439 (1 m long) has a male SMA termination.