THORLABS DET Series User manual

DET08C(/M)
Free-Space Window Input
InGaAs Biased Detector
User Guide

DET08C(/M) 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
4.3.1. Photoconductive..........................................................................5
4.3.2. Photovoltaic.................................................................................5
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. Damage Threshold......................................................... 7
4.11. Battery Replacement ..................................................... 8
Chapter 5 Common Operating Circuits ...........................................9
Chapter 6 Specifications.................................................................11
6.1. Response Curve............................................................ 12
6.2. Typical Response ......................................................... 12
6.3. Mechanical Drawing.................................................... 13
Chapter 7 Troubleshooting.............................................................14
Chapter 8 Certificate of Conformance...........................................15
Chapter 9 Regulatory.......................................................................16
Chapter 10 Thorlabs Worldwide Contacts.......................................17

DET08C(/M) Biased Detector Chapter 1: Warning Symbol Definitions
Rev F, May 15, 2019 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

DET08C(/M) Biased Detector Chapter 2: Description
Page 2 TTN020104-D02
Chapter 2 Description
The DET08C(/M) is a ready-to-use, high-speed InGaAs photodetector for use
with NIR, free-space optical systems. This photodetector unit features a free-
space lens input and includes 12 V bias battery, all in a compact aluminum
housing. The output uses an SMA jack to minimize size and maximize frequency
response. The maximum bandwidth is 5 GHz and will operate over the spectral
range of 800 - 1700 nm. A visible version, the DET025A(/M), is also available for
operation in the 400 - 1100 nm spectral range.
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.

DET08C(/M) Biased Detector Chapter 3: Setup
Rev F, May 15, 2019 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 8-32 (M4 on the /M version) tapped holes located on the
bottom and side of the housing, and mount using post holder.
2. Attach a 50 Ω SMA-to-coaxial cable (i.e., Item # CA28xx) to the output
of the detector. Select and provide a terminating resistor to the
remaining end of the cable. See Chapter 4, page 7, to determine
resistor values. Thorlabs sells a 50 Ω terminator (Item # T4119) for best
frequency performance and a variable terminator (Item # 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-measuring devices.
Note: For fastest response, terminate with a 50 Ω load.
3. 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.
4. Apply a light source to the detector.

DET08C(/M) Biased Detector Chapter 4: Operation
Page 4 TTN020104-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, linear
device that exhibits high quantum efficiency based upon the application used in a
variety of different applications.
It is necessary to be able to determine correctly the expected level of the output
current 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 definition of photodiode responsivity is the ratio of generated photocurrent
(IPD) to the incident light power (P) at a given wavelength:
󰇛󰇜
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4.3. Modes of Operation
The photodiode can operate in one of two modes: photoconductive (reverse bias)
or photovoltaic (zero-bias). Mode selection depends upon the speed
requirements of, and the amount of tolerable dark current (leakage current)
within, each individual application.

DET08C(/M) Biased Detector Chapter 4: Operation
Rev F, May 15, 2019 Page 5
4.3.1. Photoconductive
In photoconductive mode, a reverse external 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 and a decrease in junction
capacitance: a linear response. Operating under these conditions tends to
produce a larger dark current, but this can be limited by selecting an appropriate
photodiode material. Note: This detector is reverse biased and cannot be
operated under a forward bias.
4.3.2. Photovoltaic
In photovoltaic mode, the photodiode is zero biased. The flow of current out of
the device is restricted causing a buildup of voltage. This mode of operation
exploits the photovoltaic effect, which is the basis for solar cells. When operating
in photovoltaic mode, the amount of dark current is at a minimum setting.
4.4. Dark Current
When a bias voltage is applied to a photodiode, a leakage current, called dark
current, is produced. Photoconductive mode tends to generate a higher dark
current that varies directly with temperature. Dark current approximately doubles
every 10 °C increase in temperature, and shunt resistance doubles every 6 °C
rise. Applying a higher bias will decrease the junction capacitance but will also
increase the amount of dark current present.
The photodiode material, and the size of the active area, also affect the amount
of dark current present. Silicon devices generally produce low dark current
compared to germanium devices, which have high dark currents. The table on
the next page lists several photodiode materials and their relative dark currents,
speeds, sensitivities, and costs.

DET08C(/M) Biased Detector Chapter 4: Operation
Page 6 TTN020104-D02
The table below compares five common types of detector materials.
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 it can have
a profound impact on the photodiode’s bandwidth and response. It reaffirms that
larger diode areas encompass a greater junction volume with increased charge
capacity. In a reverse bias application, the depletion width of the junction
increases; 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 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):  
󰇛


1
Approximate values, actual wavelength values will vary from unit to unit.

DET08C(/M) Biased Detector Chapter 4: Operation
Rev F, May 15, 2019 Page 7
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 photodiode, the 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; thus, we recommend
the cable length to be as short as possible.
4.8. Shunt Resistance
Shunt resistance represents the resistance of the zero-biased photodiode
junction. An ideal photodiode has an infinite shunt resistance, but actual values
may range from the order of 10 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 affect 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. This
resistance is typically very low and can be ignored. The series resistance arises
from the contacts and the wire bonds of the photodiode. It mainly determines the
linearity of the photodiode under zero bias conditions.
4.10. Damage Threshold
Exposure to an intense light source can easily damage a photodiode. One of the
main characteristics of a damaged photodiode is the presence of increased dark
current, along with burn spots on the detector active area. The damage threshold
may vary from photodiode to photodiode, as this is generally dependent on
material. Silicon devices tend to be more durable than InGaAs and can handle
higher energy levels.
The formula below calculates the energy of each pulse, using the average power
and the repetition rate. If the pulse width is given, the peak power can also be
determined.  



DET08C(/M) Biased Detector Chapter 4: Operation
Page 8 TTN020104-D02
4.11. Battery Replacement
Thorlabs delivers each detector with an A23 12 V
battery installed. This battery is readily available at
most retail stores, as well as through Thorlabs. The
supplied battery will deliver about 40 hours of operation
with a 1 mA load, which is roughly equivalent to a
continuous 1.5 mW light source at peak wavelength.
When no light is applied, the supply current is very
small and the battery hardly degrades.
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
the cap back into the detector. Be careful not to cross
thread the cap into the housing. This detector does not
include a protection diode to prevent damage if the
battery is installed backwards. The correct battery
direction is also indicated on the housing.