Ge Free-Space Amplified Photodetectors, OEM Package

  • Wavelength Range from 800 nm to 1800 nm
  • Bandwidth of 510 kHz
  • Detector on Printed Circuit Board for OEM Applications
  • Switchable Gain Amplification


Switchable Gain
800 - 1800 nm
510 kHz Max Bandwidth

Related Items

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Pinhole Mounted in LMR1 and SM1L40
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The OEM package free space amplified photodetectors can be integrated into a 30 mm cage system. In this example, a LA4148-C plano-convex lens mounted in a CP33 30 mm cage plate focuses light onto a PDAPC8 photodetector mounted with ER4 assembly rods.  
Item # Wavelength Range Bandwidth Noise Equivalent Power (NEP)
Switchable Gain
PDAPC8a 800 - 1800 nm DC - 510 kHzb 4.63 - 176 pW/Hz1/2
  • Switchable with eight user-selectable gain settings in 10 dB steps.
  • This is the maximum possible bandwidth for this amplified photodetector. Bandwidth varies as a function of gain. For more information see the Specs tab


  • Switchable Gain, Low-Noise Amplified Detector for Wavelengths from 800 - 1800 nm
  • 50 Ω and Higher Load Impedances
  • Free-Space Optical Coupling
  • Output Accessible via On-Board Header Pins and Female MMCX Connector

We offer a Germanium (Ge) Free-Space Amplified Photodetector that is sensitive to light in the near infrared. This amplified photodetector features 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.

This photodetector is 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 (0.12") through holes on a 30 mm square grid for compatibility with our 30 mm cage system or mounting in custom housings and devices. 

Power Supply
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 supply should always be powered up using the power switch on the power supply itself. Hot plugging the unit is not recommended.

Performance Specifications
Item # Wavelength Bandwidth Rise Time Peak Responsivity Noise Equivalent Power (NEP)a Active Area Operating
PDAPC8b 800 - 1800 nm DC - 510 kHzc N/Ad 0.85 A/W @ 1550 nm 4.63 - 176 pW/Hz1/2 19.6 mm2 (Ø5 mm) 10 to 40 °C
  • NEP is specified at the peak responsivity wavelength. As NEP changes with the gain setting, an NEP range is given.
  • This detector has a 50 Ω terminator resistor that is in series with the amplifier output. This forms a voltage divider with any load impedance (e.g. 50 Ω load divides signal in half).
  • This is the maximum possible bandwidth for this amplified photodetector. Bandwidth varies as a function of gain. For more information see the table below.
  • Rise times depend on the chosen gain level and wavelength. As one increases the gain of a given optical amplifier, the bandwidth is reduced, and hence, the rise time increases. Please refer to the photodiode tutorial for information on calculating the rise time. Bandwidth specifications may be found in the table below.
Gain Specifications
Item # Gain Step
w/ Hi-Z Load
w/ 50 Ω Load
Bandwidtha Noise
Noise Equivalent Power (NEP)b Offset (±) Output Voltage
w/ Hi-Z Load
Output Voltage
w/ 50 Ω Load
PDAPC8 0 1.51 kV/A ± 2% 0.75 kV/A ± 2% 510 kHz 220 µV 176 pW/Hz1/2 15 mV (20 mV Max) 0 - 10 V 0 - 5 V
10 4.75 kV/A ± 2% 2.38 kV/A ± 2% 270 kHz 233 µV 87.7 pW/Hz1/2
20 15 kV/A ± 2% 7.5 kV/A ± 2% 85 kHz 223 µV 19.5 pW/Hz1/2
30 47.5 kV/A ± 2% 23.8 kV/A ± 2% 22 kHz 230 µV 6.41 pW/Hz1/2
40 151 kV/A ± 2% 75 kV/A ± 2% 6.3 kHz 220 µV 4.63 pW/Hz1/2
50 475 kV/A ± 2% 238 kV/A ± 2% 2 kHz 231 µV 48.3 pW/Hz1/2
60 1.50 MV/A ± 5% 750 kV/A ± 5% 630 Hz 247 µV 6.73 pW/Hz1/2
70 4.75 MV/A ± 5% 2.38 MV/A ± 5% 210 Hz 333 µV 10.4 pW/Hz1/2
  • Tested at 650 nm wavelength. For NIR wavelengths, the rise time of the photodiode element will become slower which may limit the effective bandwidth of the amplified detector.
  • The Noise Equivalent Power is specified at the peak responsivity wavelength.

J2 Header Pins

J2 Header
Pin Description Pin Description
1 +12 VDC 2 GND
3 GND 4 Output
5 -12 VDC 6 No Connection
7 A2a 8 A1a
9 A0a 10 No Connection
  • Pins 7, 8, 9: A0-A2 refers to the three digital pins for the gain adjustment on switchable gain models.

DIP Switches

Switch Function
1 Enable
2 A0
3 A1
4 A2

Gain Settings
(Controllable by J2 Pins or DIP Switches)

A2 A1 A0 Gain
0 0 0 0 dB
0 0 1 10 dB
0 1 0 20 dB
0 1 1 30 dB
1 0 0 40 dB
1 0 1 50 dB
1 1 0 60 dB
1 1 1 70 dB

Female MMCX Connector

SMC Male
0 to 5 V (50 Ω)
0 to 10 V (Hi-Z)

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.

Equation 1
Photodiode Circuit Diagram
Figure 1: Photodiode Model

Photodiode Terminology

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:

Equation 2

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
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
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):

Equation 3

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:

Photoconductor NEP

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 (VOUT) for viewing on an oscilloscope:

Equation 4

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
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

Reverse Biased DET Circuit
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.

Reverse Biased DET Circuit
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:

Equation 5

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

Photoconductor Chopper Equation

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. 



Period and repetition rate are reciprocal:    and 
Pulse energy calculated from average power:       
Average power calculated from pulse energy:        
Peak pulse power estimated from pulse energy:            

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.
Pulsed Laser Emission Parameters
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. 

Parameter Symbol Units Description
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.

Example Calculation:

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.

Posted Comments:
No Comments Posted

The following table lists Thorlabs' selection of photodiodes, photoconductive, and pyroelectric detectors. Item numbers in the same row contain the same detector element.

Photodetector Cross Reference
Wavelength Material Unmounted
Amplified Detector,
OEM Package
200 - 1100 nm Si FDS010 SM05PD2A
Si - SM1PD2A - - -
240 - 1170 nm B-Si - - DET20X2 - -
320 - 1000 nm Si - - - PDA8A2 -
320 - 1100 nm Si FD11A SM05PD3A - PDF10A2 -
Si - a - DET100A2 a PDA100A2 a PDAPC2 a
340 - 1100 nm Si FDS10X10 - - - -
350 - 1100 nm Si FDS100
FDS100-CAL b
Si FDS1010
FDS1010-CAL b
- - -
400 - 1000 nm Si - - - PDA015A2
400 - 1100 nm Si FDS015 c - - - -
Si FDS025 c
FDS02 d
- DET02AFC(/M)
- -
400 - 1700 nm Si & InGaAs DSD2 - - - -
500 - 1700 nm InGaAs - - DET10N2 - -
0.6 - 16 µm LiTaO3 - - - PDA13L2e -
750 - 1650 nm InGaAs - - - PDA8GS -
800 - 1700 nm InGaAs FGA015 - - PDA015C2 -
InGaAs FGA21
InGaAs FGA01 c
- DET01CFC(/M) - -
InGaAs FDGA05 c - - PDA05CF2 PDAPC6
InGaAs - - DET08CFC(/M)
- -
InGaAs - - - PDF10C2 -
800 - 1800 nm Ge FDG03
SM05PD6A DET30B2 PDA30B2 -
Ge FDG05 - - - -
900 - 1700 nm InGaAs FGA10 SM05PD4A DET10C2 PDA10CS2 -
900 - 2600 nm InGaAs FD05D - DET05D2 - -
950 - 1650 nm InGaAs - - - FPD310-FC-NIR
1.0 - 5.8 µm InAsSb - - - PDA10PT(-EC) -
2.0 - 8.0 µm HgCdTe (MCT) VML8T0
VML8T4 f
- - PDAVJ8 -
2.0 - 10.6 µm HgCdTe (MCT) VML10T0
VML10T4 f
- - PDAVJ10 -
2.7 - 5.0 µm HgCdTe (MCT) VL5T0 - - PDAVJ5 -
2.7 - 5.3 µm InAsSb - - - PDA07P2 PDAPC9
  • If you are interested in purchasing the bare photodiode incorporated in these detectors without the printed circuit board, please contact Tech Support.
  • Calibrated Unmounted Photodiode
  • Unmounted TO-46 Can Photodiode
  • Unmounted TO-46 Can Photodiode with FC/PC Bulkhead
  • Pyroelectric Detector
  • Photovoltaic Detector with Thermoelectric Cooler
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Ge Free-Space Switchable Gain Amplified Photodetector, OEM Package

Item #a Package
Gain Noise Equivalent Power (NEP)d Typical
Active Area
(Click Link for Image)
Hi-Z Load 50 Ω Load
PDAPC8e 800 - 1800 nm DC - 510 kHz 1.51 kV/A - 4.75 MV/A 0.75 kV/A - 2.38 MV/A 4.63 - 176 pW/Hz1/2 info 19.6 mm2
(Ø5 mm)
10 to 40 °C
  • Click on the link to view a photo of the item.
  • Click the icon for details.
  • Switchable with eight user-selectable gain settings in 10 dB steps. Bandwidth varies inversely with gain.
  • NEP is specified at the peak responsivity wavelength. As NEP changes with the gain setting, an NEP range is given.
  • Also available packaged in a housing with connectors, see the Cross Reference tab.
Based on your currency / country selection, your order will ship from Newton, New Jersey  
+1 Qty Docs Part Number - Universal Price Available
PDAPC8 Support Documentation
PDAPC8NEW!Ge Switchable Gain Detector on PCB, 800 - 1800 nm, 510 kHz BW, 19.6 mm²
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Power Supply with Cable for PDAPC Series Photodetectors

LD3000R SmartPack Packaging
Click to Enlarge

Pin Diagram for the Included Cable
Pin # Assignment
1 +12 V
3 -12 V
  • 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.

Based on your currency / country selection, your order will ship from Newton, New Jersey  
+1 Qty Docs Part Number - Universal Price Available
LD1255-SUPPLY Support Documentation
LD1255-SUPPLYCable and LDS12B Power Supply Bundle for LD1255R Driver and PDAPC Series Photodetectors
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Coaxial Cables with MMCX Connectors

  • 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.

Based on your currency / country selection, your order will ship from Newton, New Jersey  
+1 Qty Docs Part Number - Universal Price Available
CA3339 Support Documentation
CA3339Customer Inspired! RG-174 Coaxial Cable, MMCX Male to BNC Male, 1 m (39")
CA3272 Support Documentation
CA3272RG-174 Coaxial Cable, MMCX Male to BNC Male, 1.8 m (72")
CA3439 Support Documentation
CA3439Customer Inspired! RG-174 Coaxial Cable, MMCX Male to SMA Male, 1 m (39")