THORLABS PDB4 Series User manual

Balance Amplified Photodetectors
PDB4xx Series
Operation Manual
2014

Version:
Date: 2.0 20-Nov-2014
Copyright © 2014 Thorlabs

Foreword
Contents
2
1 General Information 3
31.1 Safety
31.2 Ordering Codes and Accessories
2 Installation 4
42.1 Parts List
42.2 Getting Started
3 Operating Instruction 5
53.1 Operating Principle
63.2 Optical Inputs
83.3 Electrical Outputs
93.4 Mounting
103.5 AC Coupling of the Outputs
113.6 CMRR and Frequency Response
123.7 Recommendations
4 Maintenance and Service 13
5 Appendix 14
155.1 Technical Data
185.1.1 PDB410x Individual Technical Data
215.1.2 PDB420x Individual Technical Data
245.1.3 PDB430x Individual Technical Data
275.1.4 PDB440x Individual Technical Data
305.1.5 PDB450x Individual Technical Data
335.1.6 PDB460x Individual Technical Data
365.2 Dimensions
385.3 Certifications and Compliances
395.4 Warranty
405.5 Copyright and Exclusion of Reliability
415.6 Thorlabs 'End of Life' Policy (WEEE)
425.7 Thorlabs Worldwide Contacts

© 2014 Thorlabs
We aim to develop and produce the best solution for your application
in the field of optical measurement technique. To help us to live up to
your expectations and improve our products permanently we need
your ideas and suggestions. Therefore, please let us know about
possible criticism or ideas. We and our international partners are
looking forward to hearing from you.
Thorlabs GmbH
Warning
Sections marked by this symbol explain dangers that might result in
personal injury or death. Always read the associated information
carefully, before performing the indicated procedure.
Please read these advices carefully!
This manual also contains "NOTES" and "HINTS" written in this form.
Attention
Paragraphs preceeded by this symbol explain hazards that could
damage the instrument and the connected equipment or may cause
loss of data.
Note
2

© 2014 Thorlabs
1 General Information
3
1 General Information
Thorlabs PDB4xx Series Balanced Amplified Photodetectors consist of two well-matched
photodiodes and an ultra-low noise, high-speed transimpedance amplifier that generates an
output voltage (RF OUTPUT) proportional to the difference between the photo currents in the
two photodiodes, i.e. the two optical input signals. Additionally, the unit has two fast monitor
outputs (MONITOR+ and MONITOR-) to measure the individual optical input power level as
well as low frequency modulated signals on each detector separately.
To the PDB4xx Series housing an adapter can be attached to the bottom or side surface for
post mounting. This adapter supports #8-32 as well as M4 post mounts.
The PDB4xx Series is supplied with an external linear power supply.
The “Getting Started” section gives an overview of how to set up the PDB4xx Series
Balanced Amplified Photodetectors. Subsequent sections contain detailed information about
principle of operation, operating suggestions, and technical specifications.
1.1 Safety
Attention
All statements regarding safety of operation and technical data in this instruction manual
will only apply when the unit is operated correctly as it was designed for.
All modules must only be operated with proper shielded connection cables.
Only with written consent from Thorlabs may changes to single components be carried
out or components not supplied by Thorlabs be used.
This precision device is only serviceable if properly packed into the complete original
packaging including the plastic foam sleeves. If necessary, ask for a replacement
package.
1.2 Ordering Codes and Accessories
The following models of the PDB4xx Series are available:
PDB410 100 MHz, fixed gain Balanced Amplified Photodetectors
PDB420 75 MHz, OCT-proved fixed gain Balanced Amplified Photodetectors
PDB430 350 MHz, fixed gain Balanced Amplified Photodetectors
PDB440 15 MHz, OCT-proved fixed gain Balanced Amplified Photodetectors with active
aliasing filter
PDB450 150 MHz, gain switchable Balanced Amplified Photodetectors
PDB460 200 MHz, fixed gain Balanced Amplified Photodetectors
According to Thorlabs general detector part numbering system, the suffix “A” indicates Si
photodiodes while the suffix “C” indicates InGaAs photodiodes.
Thorlabs offers AC-coupled versions. on request Special versions (open detector - cover glass
removed) are available - except PDB430C and PDB460C - as well - please contact Thorlabs
for details.
4
42

© 2014 Thorlabs4
PDB4xx Series
2 Installation
This section is intended to provide information that explains how to quickly set up the PDB4xx
Series Balanced Amplified Photodetectors. More details and advanced features are described
in further sections.
2.1 Parts List
Inspect the shipping container for damage.
If the shipping container seems to be damaged, keep it until you have inspected the contents
and you have inspected the item mechanically and electrically.
Verify that you have received the following items within the package:
1. PDB4xx Balanced Amplified Photodetector
2. Adapter Plate with four M2x8 screws and a hex key 1.5, for post-mounting the unit on a
optical table
3. Power supply (±12V, 0.2A), switchable to 115V or 230V line voltage
4. Operation manual
2.2 Getting Started
Note
Prior to operation, please check, if the indicated line voltage range on the power supply
matches with your local mains voltage! If you want use your own power supply, Thorlabs offers
an appropriate power connector cable.
Carefully unpack the unit and accessories. If any damage is noticed, do not use the unit.
Contact Thorlabs and have us replace the defective unit.
If required, mount the unit on your optical table or application. Therefore, mount the adapter
plate on bottom or side wall using the four M2x8 screws first. The adapter plate has two
mounting holes, M4 and #8-32. The M4 thread is marked. These threads can be used for
mounting onto Thorlabs posts.
If required, mount external optics, filters, apertures or fiber adapters.
Set the power supply to your local mains voltage (115 or 230VAC):
Connect the DC output cable of the power supply to the POWER IN jack.
Connect the power supply to the AC outlet, turn power supply on
Connect RF OUTPUT with coaxial cable to the data acquisition device.
If necessary, connect monitor outputs (MONITOR+, MONITOR-) to measure the optical
input power for each channel individually.
42

© 2014 Thorlabs
3 Operating Instruction
5
3 Operating Instruction
Turn the power switch of the power supply to I. The green LED next to the DC input
connector indicates that the power supply is operating correctly.
Connect the optical source(s) to the optical input(s). The FC adapter will accommodate
multi-mode as well as single-mode fiber.
Attention
Be very careful about the force when connecting the fibers to the optical inputs! The
photodiodes are mounted in such way, that the gap between the protective diode glass and the
fiber tip is as small as possible. For this reason, do not tighten the fiber connector extremely in
order to avoid damages to the glass window that protects the photodiode.
For free-space beam applications, the FC adapters can be removed in order to have direct
access to the photodetectors (not for PDB430C and PDB460C - see section "Optical
Inputs" for details).
The MONITOR outputs can be used to conveniently check the detected signal from each
free space input beam. The maximum output voltage swing of the MONITOR outputs
is10V for high impedance loads (1.5V into 50 loads). Saturation of the MONITOR
outputs will occur at optical input power greater than 1mW.
The RF output signal must not exceed the maximum RF OUTPUT voltage swing (see
Technical Data) to avoid saturation.
For balanced operation illuminate both photodetectors simultaneously and use either the
RF OUTPUT or the MONITOR outputs to fine-tune the optical power balance by observing
voltage on a digital voltmeter or other low-frequency measurement device.
Do not exceed a maximum power density of 4W/cm2 for maximum linearity performance
when measuring focused beams, fiber outputs, or small diameter beams.
After finishing measurements, turn power off.
Note
To prevent saturation of the balanced amplifier make sure that the power difference between
the optical inputs remains less than the saturation power level (see "RF OUT CW Saturation
Power" in the Technical Data section).
Attention
The damage threshold of the photo diodes is 20 mW! Exceeding this value will permanently
destroy the detector!
3.1 Operating Principle
Thorlabs PDB4xx Series Balanced Amplified Photodetectors consist of two well-
matchedphotodiodes and an ultra-low noise, high-speed transimpedance amplifier that
generates an output voltage (RF OUTPUT) proportional to the difference between the photo
currents of the two photodiodes, i.e. the difference of between the two optical input signals.
Additionally, the unit has two monitor outputs (MONITOR+ and MONITOR-) to observe the
optical input power level on each photodiode separately. Due to their increased cut-off
frequency, these outputs can also be used to measure low frequency modulated signals on
each detector separately.
The PDB4xx Series is powered by an external linear power supply ( 12V, 200mA - included)
via a PICO M8 power connector.
6

© 2014 Thorlabs6
PDB4xx Series
Below is a functional block diagram of the PDB4xx Series Balanced Amplified Photodetectors:
3.2 Optical Inputs
For all models except PDB430C and PDB460C, the fiber inputs are coupled to the photodiodes
using two removable FC adapters. These FC receptacles accommodate either single-mode or
multi-mode fiber with FC/PC or FC/APC connectors as well. PDB430C and PDB460C have
also FC adapters, but they are not removable.
Attention
Be very careful about the force when connecting the fibers to the optical inputs! The
photodiodes are mounted in such way, that the gap between the protective diode glass and the
fiber tip is as small as possible. For this reason, do not tighten the fiber connector extremely in
order to avoid damages to the glass window that protects the photodiode.
For all PDB4xxC models the FC adapters are aligned for Corning SMF28TM single mode fiber
with PC connectors. When using FC/APC connectors, minimal alignment errors may occur due
to the small detector size, which will result in a reduced output signal. In such case, FC
receptacle can be rotated from its original position in steps of 90° to check for an improved
alignment. For this process use an optical input power below the saturation level while
observing RF OUTPUT voltage on a digital voltmeter or other low-frequency measurement
device. If you have an AC-coupled version, use either MONITOR output (CW signal) or RF
OUTPUT (modulated optical signal) with connected oscilloscope for measurement.
In general, multi-mode fiber at the input can be used, but in this case the light beam spot
diameter exceeds detector’s active area, which results in a reduced output signal as well.

© 2014 Thorlabs
3 Operating Instruction
7
For free-space beam applications it is recommended to remove FC adapter (receptacle) to
have direct access to the photodiodes as shown below:
Note
FC adapters (receptacles) are not removable from PDB430C and PDB460C enclosures!
Note
Do not exceed a maximum power density of 4W/cm2 for maximum linearity performance when
measuring focused or small diameter beams. Always try to illuminate the whole detector active
area to prevent nonlinearities. Equal power densities on both detectors are important for
maximum common mode noise suppression (CMRR).
The PDB4xx Series can be used in balanced mode (both inputs are illuminated) as well as in
single detector mode. In single detector mode, the RF OUTPUT swing depends on which
INPUT is used: it is positive for INPUT+ and negative for INPUT-.
In single detector mode, the optical input power should be below the specified CW saturation
power (see Technical Data ) to avoid saturation of the RF OUTPUT amplifier.
In balanced mode the power difference between the optical inputs should be less than the CW
Saturation Power. If necessary, use external neutral density filters or attenuators to reduce the
input light level.
Attention
The optical damage threshold is 20mW. Exceeding this value will permanently damage the
photodiodes!
15

© 2014 Thorlabs8
PDB4xx Series
3.3 Electrical Outputs
The Thorlabs PDB4xx Series has three SMA output connectors:
MONITOR +
MONITOR -
RF OUTPUT
RF OUTPUT delivers an output voltage proportional to the difference between the photo
currents of the two photodiodes This voltage can by calculated to:
with: ( ) - responsivity of the photo diode at given wavelength
Popt,1 and Popt,2 - optical input power
G - transimpedance gain of the RF output
The responsivity ( ) for a given wavelength can be read from the individual curves in section
Technical Data to estimate the RF OUTPUT voltage. Please note that the given responsivity
curves represent typical values - individual responsivity may deviate.
The maximum output voltage swing of the RF OUTPUT can be found in the Technical Data
section.
The optical input saturation power (see the PDB4xx Series individual technical data in the
appendix) of the balanced detector is the minimum value, as it is given for the wavelength with
the detector highest responsivity. At other wavelengths, saturation will be reached at higher
input power levels. The output signal should not exceed the maximum output voltage to avoid
saturation. Therefore the optical input power (or the power difference between the optical
inputs) should not exceed CW Saturation Power listed in Specifications.
MONITOR Outputs
The signal monitor outputs (MONITOR+ and MONITOR-) allow to observe the input power level
and can be used as individual power indicators. These outputs can also be used to measure
low frequency modulated signals on each detector separately. The maximum output voltage
swing of the MONITOR output is +10V for high impedance loads (+1.5V into 50 ). Saturation
of MONITOR outputs will occur at optical input power level greater than 1mW, depending on
the detector's wavelength response.
MONITOR outputs can be used to roughly adjust equal input power levels on each detector for
balanced operation. While the DC component of the RF OUTPUT in balanced mode is zero,
the MONITOR outputs provide capability to independently observe the individual optical input
power. MONITOR outputs of the unit are also convenient to use for free-space beam
alignment.
The amplifier offset voltage is factory set to zero at 23°C ambient temperature. A small drift
during a short warm-up period (~5min) may occur. For exact DC light level measurements a
constant temperature environment is recommended.
15

© 2014 Thorlabs
3 Operating Instruction
9
3.4 Mounting
The PDB4xx Series is housed in a rugged shielded aluminum enclosure.
For post mounting an adapter can be attached to the bottom or side surface using four M2x8
screws (see below). This adapter supports #8-32 as well as M4 post mounts. The M4 tread is
marked.

© 2014 Thorlabs10
PDB4xx Series
3.5 AC Coupling of the Outputs
Beside the standard DC coupling of the RF OUTPUT, AC coupled versions for any model of
PDB4xx Series are available on request. AC coupling blocks the CW component (the
unmodulated part) of the optical input signal. However, large CW components of the optical
input signal will decrease linearity of the detectors.
AC coupling helps to improve the measurement capabilities in applications, where a
comparably weak frequency modulated signal shall be measured on a strong CW background
signal, which could saturate the amplifier. With AC coupling, equalizing of CW power levels on
both inputs is not mandatory for noise cancellation. However, for optimal noise suppression the
signal of interest (e.g. the modulated part) should be well balanced. AC coupling is also
recommended when using the balanced detector in combination with a chopper and lock-in
amplifier to further decrease noise level.
The lower cut-off frequency of the AC coupled versions is typically below 5 Hz.
Please note, that AC coupling slightly increases noise figures at lower frequencies. The
measurement bandwidth of the RF OUTPUT is not affected by AC coupling.
The figure below shows the comparison of AC and DC coupled RF Output signals when
modulating the input signal with a mechanical chopper at a frequency of 500 Hz.
Note: Input signal for AC coupling was increased by factor 2 to allow direct waveform
comparison

© 2014 Thorlabs
3 Operating Instruction
11
3.6 CMRR and Frequency Response
An important specification for balanced amplifiers is the Common Mode Rejection Ratio
(CMRR) that reflects the ability to suppress common mode noise.
In the setup as described below, the Device under Test (DuT) - here a PDB4xx Series balanced
detector - is tested for CMRR. A common mode signal is generated, which is canceled out
when the amplifier is in balanced mode.
A network analyzer is used as signal generator (output A) and receiver (input B) The receiver is
synchronized with the signal generator and measures selectively at the same frequency. A
laser light source is modulated by the signal generator (port A) and acts as transmitter. To the
laser output a 3 dB fusion coupler is connected, splitting the modulated light signal into two
paths.Depending on the measurement task, one or both coupler outputs are connected to the
inputs of the DuT, for example using S120-FC adapters. One of the DuT's outputs is connected
to the network analyzers Port B.
Frequency response measurements
The frequency response of each signal path can be measured by connecting only one coupler
output to the appropriate input. This way, the frequency response curves of the RF OUTPUT
from INPUT + and INPUT- can be measured, as well as the frequency responses of the
MONITOR outputs, as shown in the individual technical data.
CMRR measurement
For Common Mode Rejection measurement, both outputs of the fusion coupler are connected
to both inputs of the DuT. The optical power level at both inputs must be well matched
("balanced") in order to achieve the optimal common mode suppression. Now the common
mode rejection can be measured as a function of frequency. The frequency response of the RF
OUTPUT must be considered when calculating the CMRR - it is the difference between the RF
OUTPUT signal at a given frequency and the measured common mode or balanced output
signal - at the same frequency. Typical measurement curves can be found in the individual
technical data.

© 2014 Thorlabs12
PDB4xx Series
3.7 Recommendations
Thorlabs PDB4xx Series Balanced Amplified Photodetectors can eliminate noise sources to
allow precise measurements. The PDB4xx Series is designed to be used in a dual beam setup:
one optical path for measurement and one invariant reference path. If set up properly, the
PDB4xx Series can reduce common mode noise for more than 35 dB over the specified
frequency range. Below are given some recommendations to achieve an optimal common
mode suppression:
To obtain the maximum possible common mode rejection (common mode noise
suppression), equal power levels on each photodetector are essential. Any power imbalance
will be amplified and hence decrease the possible noise reduction
Equal power densities on both detectors are important as well to obtain maximum possible
common mode rejection. Always try to illuminate the whole active area of the detectors to
prevent nonlinearities. Focused high power beams may lead to frequency response
degradation, resulting in dramatically reduced common mode rejection.
Equal optical path lengths are very important for common mode noise suppression especially
at higher frequencies. Any path length difference will introduce a phase difference between
the two signals, which will decrease the noise reduction capability of the balanced detector.
The figure on next page shows the maximum allowed path length difference in air to obtain a
desired CMRR. For fiber based application the maximum path length difference must divided
by 1.5.
Avoid etalon effects (interference fringes caused between two optical surfaces) in optical
paths. Using angle polished optical connectors will greatly reduce etalon effects in a fiber
based setup. Effects like residual frequency modulation, polarization noise, polarization
wiggle or spatial modulation can also degrade common mode noise suppression. For further
details contact Thorlabs. In general, reducing sources of differential losses in the optical
paths (other than the measurement itself) will improve the common mode noise reduction.
Another critical point can be electrostatic coupling of electrical noise associated with ground
loops. In most cases an electrically isolated post (see Thorlabs parts TRE or TRE/M) will
suppress electrical noise coupling. Always try to identify the electrical noise sources and
increase the distance to the PDB4xx Series Balanced Detector. Different common ground
points can also be tested.

© 2014 Thorlabs
4 Maintenance and Service
13
4 Maintenance and Service
Protect the PDB4xx Series from adverse weather conditions. The PDB4xx Series is not water
resistant.
Attention
Be very careful about the force when connecting the fibers to the optical inputs! The
photodiodes are mounted in such way, that the gap between the protective diode glass
and the fiber tip is as small as possible. For this reason, do not tighten the fiber
connector extremely in order to avoid damages to the glass window that protects the
photodiode.
Attention
To avoid damage to the instrument, do not expose it to spray, liquids or solvents!
The unit does not need a regular maintenance by the user. It does not contain any modules
and/or components that could be repaired by the user himself. If a malfunction occurs, please
contact Thorlabs for return instructions.
Do not remove covers!
To clean the PDB4xx Series series housing, use a mild detergent and damp cloth. Do not soak
the unit in water or use solvent based cleaners.
When cleaning the windows of the photodetectors, please remember that is a sensitive optical
device. Gently blow off any debris using compressed air and wipe gently with an optic tissue
wetted with isopropanol or alcohol.
42

© 2014 Thorlabs14
PDB4xx Series
5 Appendix
Comments and explanations to the individual specifications
-Typical max. responsivity is the peak responsivity max of the photo diode.
-Transimpedance and conversion gain [V/A] is the ratio of output voltage to photo
current, it is wavelength independent. Values to RF output are always given into a high
impedance load. For 50 loads these values are divided by two.
Conversion gain [V/W] is the ratio of output voltage to input optical power, by other words
GCONV = GTRANS ×
This formula shows, that the conversion gain is dependent on the actual wavelength. In
specifications, it is given only for the peak responsivity wavelength of the photo diode.
Values to RF output are always given into a high impedance load. For 50 loads these
values are divided by two.
-NEP (Noise Equivalent Power) is commonly stated for DC to 10 MHz frequency range, this
is a usual criteria for balanced detectors. If different, the frequency range is stated
individually.
-Integrated noise is always given from DC to the actual detector’s bandwidth, measured as
RMS power into a 50 load.
-Overall output noise voltage [VRMS] is the value which can be measured across a 50
load at large bandwidth, e.g., if connect the RF output to a 50 terminated scope input.
-Max. input power is the damage threshold of the photo diode.
-Typical noise spectra (diagrams): These spectra were measured using an electrical
spectrum analyzer (resolution bandwidth 100 kHz, video bandwidth 10 kHz unless
otherwise noted). The INPUTs of the balanced detectors under test were blocked. The
lower curve in the diagram was measured with the same setup and the balanced detectors
under test switched off , i.e., it represents the measurement system’s noise floor.
-Monitor outputs are designed for use with high impedance loads (e.g., high-Z scope input
etc.), but can also drive 50 loads. Monitor outputs conversion gain is 10V/mW, given at
the detectors peak responsivity and high impedance load.
-Typical frequency response curves are measured using the setup described in section
"CMRR and Frequency Response"
11

© 2014 Thorlabs
5 Appendix
15
5.1 Technical Data
Common
Optical Inputs
FC, removable
(except PDB430C and PDB460C: NOT removable)
Max. Input Power (Damage Threshold)
20 mW
Electrical Outputs
SMA
RF OUTPUT
Impedance
50
Voltage Swing, max.
(PDB410 / 420 / 430 / 440 / 460)
± 3.6 V 1)
Voltage Swing, max.
PDB450, gain - 103 to 106
PDB450, gain - 107
± 4.6V 1)
± 10V 1)
DC Offset
< ± 3 mV
MONITOR Outputs
Impedance
220
Voltage Swing, max.
10 V (high impedance load)
1.55 V (50 load)
Bandwidth
DC to 1 MHz
Conversion Gain
10 V/mW @ peak responsivity
Voltage Noise
<180 µVRMS
DC Offset
< ± 2 mV
General
Power Supply
±12 V, 200mA
Dimensions
85 x 80x 30 mm3
Operating Temperature Range 2)
0 - 40 °C
Storage Temperature Range
-40 to 70 °C
Weight
0.35 kg (w/o power supply)
1) Values are given for high impedance load. For a 50 load, values to be divided by 2.
2) non-condensing

© 2014 Thorlabs16
PDB4xx Series
Typical Detector responsivity curves
Typical InGaAs Detector responsivity (PDA4xxA)
Typical Si Detector responsivity (PDA4xxC)

© 2014 Thorlabs
5 Appendix
17
Typical Monitor Output Frequency Response
PDB4xx Series Monitor Output Frequency Response

© 2014 Thorlabs18
PDB4xx Series
5.1.1 PDB410x Individual Technical Data
PDB410A
PDB410C
Detector Material / Type
Si / PIN
InGaAs / PIN
Operating Wavelength Range
320 - 1000 nm
800 - 1700 nm
Max. Responsivity, typ.
0.53 A/W
1.0 A/W
Detector Active Area Ø
0.8 mm
0.3 mm
RF OUTPUT Bandwidth (-3 dB)
DC - 100 MHz
CMRR
>25 dB (typ. >35 dB)
RF OUTPUT Transimpedance Gain 1)
50 × 103 V/A
RF OUTPUT Conversion Gain 2)
26.5 × 103 V/W
50 × 103 V/W
CW Saturation Power
135 µW @ 820 nm
72 µW @ 1550 nm
Minimum NEP (DC to 10 MHz)
7 pW/√Hz
3.8 pW/√Hz
Integrated Noise (DC to 100 MHz)
100 nWRMS
65 nWRMS
Overall Output Voltage Noise
2 mVRMS
2.2 mVRMS
1) Values are given for high impedance load. For a 50 load, values to be divided by 2.
2) Values are given at peak responsivity of the detector, for high impedance load. For a 50 load, values to be
divided by 2.