THORLABS DET36A2 User manual

DET36A2
Si Biased Detector
User Guide

Si Biased Detector
Table of Contents
Chapter 1Warning Symbol Definitions ............................................. 1
Chapter 2Description .........................................................................2
Chapter 3Setup ...................................................................................3
Chapter 4Operation ............................................................................4
4.1.TheoryofOperation...................................................4
4.2.Responsivity...............................................................4
4.3.ModesofOperation....................................................4
4.4.DarkCurrent...............................................................5
4.5.JunctionCapacitance..................................................6
4.6.BandwidthandResponse............................................6
4.7.TerminatingResistance..............................................7
4.8.ShuntResistance.........................................................7
4.9.SeriesResistance........................................................7
4.10.BatteryCheck.............................................................7
4.11.BatteryReplacement..................................................8
Chapter 5Common Operating Circuits ............................................. 9
Chapter 6Troubleshooting...............................................................11
Chapter 7Specifications................................................................... 12
7.1.ResponseCurve........................................................13
7.2.MechanicalDrawing.................................................14
Chapter 8Certificate of Conformance............................................. 15
Chapter 9Regulatory ........................................................................16
Chapter 10Thorlabs Worldwide Contacts ........................................17

Si Biased Detector Chapter 1: Warning Symbol Definitions
Rev A, December 27, 2017 Page 1
Chapter 1 Warning Symbol Definitions
Below is a list of warning symbols you may encounter in this manual or on your
device.
Symbol Description
Direct Current
Alternating Current
Both Direct and Alternating Current
Earth Ground Terminal
Protective Conductor Terminal
Frame or chassis Terminal
Equipotentiality
On (Supply)
Off (Supply)
In Position of a Bi-Stable Push Control
Out Position of a Bi-Stable Push Control
Caution, Risk of Electric Shock
Caution, Hot Surface
Caution, Risk of Danger
Warning, Laser Radiation
Caution: ESD Sensitive Components

Si Biased Detector Chapter 2: Description
Page 2 TTN134189-D02
Chapter 2 Description
The Thorlabs DET36A2 is a biased, Silicon (Si) detector designed for detection
of light signals ranging from 350 to 1100 nm. The unit comes complete with a
photodiode and internal 12 V bias battery enclosed in rugged aluminum housing.
The DET36A2 includes a removable 1" optical coupler (SM1T1), providing easy
mounting of ND filters, spectral filters, fiber adapters (SMA, FC and ST style),
and other 1" stackable lens mount accessories offered by Thorlabs.
ESD Caution
The components inside this instrument are ESD sensitive. Take all appropriate
precautions to discharge personnel and equipment before making any electrical
connections to the unit.

Si Biased Detector Chapter 3: Setup
Rev A, December 27, 2017 Page 3
Chapter 3 Setup
The detector can be set up in many different ways using our extensive line of
adapters. However, the detector should always be mounted and secured for best
operation. Step 1 in the setup instructions below outline how to mount the
detector onto a post.
1. Unpack the optical head, install either an imperial or metric optical post into
one of the mounting holes located on the bottom and side of the sensor, and
mount using a post holder. Note that these detectors feature tapped holes
that accept both 8-32 and M4 threads, so using either imperial or metric TR
posts is possible.
2. Attach a 50 Ωcoax cable (i.e. RG-58U) to the output of the DET. Select and
install a terminating resistor to the remaining end of the cable and connect to
a voltage measurement device. See Chapter 4, page 7 to determine resistor
values. Thorlabs sells a 50 Ωterminator (T4119) for best frequency
performance and a variable terminator (VT2) for output voltage flexibility.
Note the input impedance of your measurement device since this will act as
a terminating resistor. A load resistor is not necessary when using current
measurement devices.
3. Power the DET on using the power switch. To check battery voltage, see the
Battery Check section on page 7.
4. Install any desired filters, optics, adapters, or fiber adapters to the input
aperture.
Caution!
The DET36A2 was designed to allow maximum accessibility to the
photodetector by having the active area surface of the diode flush with the
outside of the DET housing. When using fiber adapters, make sure that the
fiber ferrule does not crash into the detector. Failure to do so may cause
damage to the diode and/or the fiber.
5. During alignment, take appropriate precautions, such as using reduced
radiation power, or other precautions, and use proper eye and/or skin
protection as recommended by the radiation source manufacturer.
6. Apply a light source to the detector.
!
!

Si Biased Detector Chapter 4: Operation
Page 4 TTN134189-D02
Chapter 4 Operation
4.1. Theory of Operation
A junction photodiode is an intrinsic device which 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. 
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Figure 1 Photodiode Model
4.2. 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:
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4.3. Modes of Operation
A photodiode can be operated in one of two modes: photoconductive (reverse
bias) or photovoltaic (zero-bias). Mode selection depends upon the applications
speed requirements and the amount of tolerable dark current (leakage current).

Si Biased Detector Chapter 4: Operation
Rev A, December 27, 2017 Page 5
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
The DET detectors are reverse biased and cannot be operated under
forward bias conditions.
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.
4.4. Dark Current
Dark current is leakage current which flows when a bias voltage is applied to a
photodiode. Photoconductive mode tends to generate a dark current that varies
directly with temperature. It can be inferred that dark current can approximately
double for every 10 °C increase in temperature, and shunt resistance can
decrease to approximately half 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. Please note that sensitivity values in the table are typical values;
Thorlabs offers photodetectors with sensitivity ranges that vary from those shown
below.

Si Biased Detector Chapter 4: Operation
Page 6 TTN134189-D02
Material Dark
Current Speed Sensitivity1 (nm) Cost
Silicon (Si) Low High 400 – 1000 Low
Germanium (Ge) High Low 900 – 1600 Low
Gallium Phosphide (GaP) Low High 150 – 550 Med
Indium Gallium Arsenide
(InGaAs) Low High 800 – 1800 Med
Extended Range: Indium
Gallium Arsenide (InGaAs) High High 1200 – 2600 High
4.5. Junction Capacitance
Junction capacitance (CJ) is an important property of a photodiode as this can
have a profound impact on the bandwidth and the response of a photodiode. 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.
4.6. Bandwidth and Response
The response time of a photodiode is a function the speed at which these
charges move inside the semiconductor as well as the external circuitry
connected to the photodiode. The three main contributions to the response time
are drift time, diffusion, and rise time of the RC circuit.
Drift time (tdf): Time for electron-hole pairs to move from the depletion zone to the
electrodes. The depletion zone has an intrinsic electric field that accelerates the
charges so that they are collected quickly. Hence, the drift time is generally in the
ps time regime.
Diffusion (tdiff): Diffusion time for electron-hole pairs generated outside of the
depletion zone to be collected. Depending on the wavelength used, a larger
proportion of electron-hole pairs are generated in the N and P layers of the
semiconductor, which do not have an electric field to accelerate the charges.
Therefore, the charges are only collected after slowly diffusing through the
semiconductor.
RC (tRC): Rise time of the RC circuit formed by the junction capacitance of the
photodiode, its series resistance (generally negligible), and the load resistance.
This is generally the dominating factor on the bandwidth of the PDs and
detectors and is the reason why the bandwidth is specified for a particular load
resistance. For best frequency response, a 50 Ωterminator should be used in
conjunction with a 50 Ωcoaxial cable. The bandwidth (fBW) and the rise time
response (tr) can be approximated using the junction capacitance and the load
resistance (RLOAD):
1Approximate values, actual wavelength values will vary.

Si Biased Detector Chapter 4: Operation
Rev A, December 27, 2017 Page 7
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4.7. Terminating Resistance
A load resistance is used to convert the generated photocurrent into a voltage
(V
OUT
) for viewing on a voltage reading device, e.g., an oscilloscope.
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Depending on the type of the photodiode, load resistance can affect the
response speed. For maximum bandwidth, we recommend using a 50 Ωcoaxial
cable with a 50 Ωterminating 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.
4.8. 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 Ωto thousands of MΩand is dependent
on the photodiode material. For example, an InGaAs detector has a shunt
resistance on the order of 10 MΩwhile a Ge detector is in the kΩrange. This can
significantly impact the noise current on the photodiode. For most applications,
however, the high resistance produces little effect and can be ignored.
4.9. Series Resistance
Series resistance models the resistance of the semiconductor material plus the
contacts and the wire bonds, and this low resistance can generally be ignored.
The series resistance is used to determine the linearity of the photodiode under
zero bias conditions.
4.10. Battery Check
The DET series includes a battery check feature that will allow the user to
monitor the bias voltage on the output BNC. Simply hold down the “Battery
Check” button located on the bottom edge of the unit. The bias voltage will be
output to the BNC. If a high impedance load is used (>100 kΩ), the output will be
equal to the bias voltage. This feature includes a 2.49 kΩcurrent limiting resistor
(RCL) to prevent excessive loading of the battery if using small terminating
resistors. For example, a 50 Ωload resistor with a 10 V bias will produce a

Si Biased Detector Chapter 4: Operation
Page 8 TTN134189-D02
200 mA current without this resistor. This will significantly decrease lifetime of the
battery.
The output bias voltage will be dependent on the load resistor as described
below. The A23 battery voltage characteristics show that the charge level is
almost depleted as the voltage drops below 10 V. For this calculation we assume
9.6 V since V
BAT
= low battery voltage – one diode drop (0.6 V) = ~9 V. The
detector will continue to operate until the battery charge is completely drained,
however these numbers provide a reference point at which the battery should be
replaced.
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Note that V
out
is a theoretical value. If one would like to reverse calculate the bias
voltage more precisely using this feature, then a high impedance load is
suggested.
4.11. Battery Replacement
Thorlabs delivers each DET with an A23 12 V battery.
This battery is readily available at most retail stores, as
well as through Thorlabs. The battery supplied will deliver
about 40 hours with a 1 mA load, roughly equivalent to a
continuous 1.5 mW light source with responsivity 0.6 A/W
The supply current when the unit is on and no light is
applied is very small and should not significantly degrade
the battery.
Locate the battery cap directly above the output BNC.
Unthread the cap and remove the battery. Install the new
battery into the cap, negative side in, and thread back into
the DET. Be careful not to cross thread the cap into the
housing. The DET includes a protection diode to prevent
damage if the battery is installed backwards. The battery
direction is located on the housing.