THORLABS DET100A User manual

DET100A
Large Area Si Biased Detector
User Guide

Si Biased Detector
Page 1 Rev F, December 8, 2014
Table of Contents
Chapter 1 Warning Symbol Definitions .............................................2
Chapter 2 Description .........................................................................3
Chapter 3 Setup ...................................................................................4
Chapter 4 Operation ............................................................................5
4.1. Theory of Operation ................................................... 5
4.2. Responsivity ............................................................... 5
4.3. Modes of Operation.................................................... 5
4.4. Dark Current............................................................... 6
4.5. Junction Capacitance .................................................. 7
4.6. Bandwidth and Response............................................ 7
4.7. Terminating Resistance .............................................. 8
4.8. Shunt Resistance......................................................... 8
4.9. Series Resistance ........................................................ 8
4.10. Battery Check ............................................................. 8
4.11. Battery Replacement .................................................. 9
Chapter 5 Common Operating Circuits ........................................... 10
Chapter 6 Troubleshooting............................................................... 12
Chapter 7 Specifications................................................................... 13
7.1. Response Curve ........................................................ 14
7.2. Mechanical Drawing................................................. 15
Chapter 8 Certificate of Conformance.............................................16
Chapter 9 Regulatory ........................................................................17
Chapter 10 Thorlabs Worldwide Contacts ........................................ 18

Chapter 1: Warning Symbol Definitions
13056-S01 Page 2
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, Spinning Blades May Cause Harm

Si Biased Detector
Page 3 Rev F, December 8, 2014
Chapter 2 Description
The Thorlabs DET100A is a biased, Silicon (Si) detector designed for detection
of light signals over 400 to 1100 nm range. The unit comes complete with a
photodiode and internal 12 V bias battery enclosed in rugged aluminum housing.
The DET100A includes a removable 1" optical coupler (SM1T1), providing easy
mounting of ND filters, spectral filters, fiber adapters (SMA, FC and ST style),
and other Thorlabs 1" stackable lens mount accessories.

Chapter 3: Setup
13056-S01 Page 4
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 a Thorlabs TR-series ½" diameter post into
one of the #8-32 (M4 on /M version) tapped holes, located on the bottom
and side of the sensor, and mount into a PH-series post holder.
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 5to determine resistor
values. Thorlabs sells a 50 Ωterminator (T4119) for best frequency
performance and a variable terminator (VT1) 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 8.
4. Install any desired filters, optics, adapters, or fiber adapters to the input
aperture.
Caution!
The DET100A was designed to allow maximum accessibility to the
photodetector by having the front 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. An easy way to accomplish this is to
install a SM1RR retaining ring (included with the DET10C) inside the 1"
threaded coupler before installing the fiber adapter
5. Apply a light source to the detector.
!!

Si Biased Detector
Page 5 Rev F, December 8, 2014
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. =+
Figure 1 Photodiode Model
4.2. Responsivity
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:
()=

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).
Photodetector
Diode Junction
Capacitance
Shunt
Resistance
Series
Resistance
External
Load
Resistance
IPD ID
IOUT

Chapter 4: Operation
13056-S01 Page 6
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 a forward bias.)
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. The amount of dark current is
kept at a minimum when operating in photovoltaic mode.
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 higher 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 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.

Si Biased Detector
Page 7 Rev F, December 8, 2014
The table below gives some advantages to each common type of detector
material.
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
A load resistor will react with the photodetector junction capacitance to limit the
bandwidth. 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):
= 1
(2×)
=0.35

1Approximate values, actual wavelength values will vary from unit to unit.

Chapter 4: Operation
13056-S01 Page 8
4.7. Terminating Resistance
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 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 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.
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, and 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, 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.
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 “V Bias Out”
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 (>10 kΩ), the output will be equal to
the bias voltage. This feature includes a 1.05 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 200 mA current
without this resistor. This will significantly decrease lifetime of the battery.

Si Biased Detector
Page 9 Rev F, December 8, 2014
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 VBAT = 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.
 = ∗ 
+
: =9, =50, =1050
=
4.11.Battery Replacement
Thorlabs delivers each DET with an A23 12 V battery installed. 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 at peak wavelength. 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.