THORLABS INT-COM-1300 User manual

Operation Manual
Thorlabs Instrumentation
INT-COM-1300
Common Path OCT Interferometer

V
ersion: 1.0
Date: 30-11-2007
Copyright
©
2007, Thorlabs GmbH

Contents Page
1Overview 1
1.1 Ordering Codes and Customization 1
1.2 Safety 2
2Getting Started Quickly 3
2.1 Unpacking 3
2.2 Setup 3
2.3 First Operation 4
3Detailed Description 5
3.1 General Principle of Operation 5
3.2 Optical Input and Output, Fiber Network 6
3.3 Electrical Output 9
3.4 Detector Responsivity 11
3.5 Mounting the INT-COM-1300 12
3.6 Basic Measurement Setup 13
3.7 Recommendations 15
4Maintenance and Repair 16
4.1 General Care 16
4.2 Cleaning 16
4.3 Repair 17
5Appendix 18
5.1 Technical Data 18
5.2 Mechanical Drawing 19
5.3 Warranty 20
5.4 Certifications and Compliances 21
5.5 Thorlabs “End of Life” Policy (WEEE) 22
5.5.1 Waste treatment on your own responsibility 22
5.5.2 Ecological background 23
5.6 List of Figures 24
5.7 List of Acronyms 24
5.8 Addresses 25

We aim to develop and produce the best solution for your application in the
field of optical measurement technique. To help us to come up to your
expectations and develop 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 hear from you.
Thorlabs GmbH
This part of the instruction manual contains every specific information on how to
handle and use the INT-COM-1300 Common Path OCT Interferometer. A general
description is followed by explanations of how to operate the unit.
Attention
This manual contains “WARNINGS” and “ATTENTION” label in this form, to
indicate danger for persons or possible damage of equipment.
Please read these advises carefully!
NOTE
This manual also contains “NOTES” and “HINTS” written in this form.

1Overview
1 Overview
The Thorlabs INT-COM-1300 Common Path OCT Interferometer is a subassembly to
be used in swept source Fourier domain OCT systems and other applications. It
contains a fusion coupler network to operate with an external Common Path
interferometer probe. The generated interference fringes are detected by an
integrated balanced detector. The used fusion couplers are optimized for flat
wavelength response and very low PDCR (polarization dependant coupling ratio
change) to make the signal nearly independent of input polarization changes. An
additional aiming laser input simplifies the optical alignment of the probe.
The “Getting Started Quickly” section below gives an overview of how to set up the
INT-COM-1300 Common Path OCT Interferometer. Subsequent sections contain
detailed information about principle of operation, operating suggestions and technical
specifications.
1.1 Ordering Codes and Customization
Ordering code::
INT-COM-1300 Common Path OCT Interferometer, 1250 - 1350nm
Different wavelength ranges can be ordered on request.
For the standard version, mounting holes are located on the small edges of the
enclosure. An alternative cover with four mounting holes as well as a customized
cover can be ordered on request.
Please refer to www.thorlabs.com for new models.
INT-COM-1300 / page 1

1 Overview
1.2 Safety
Attention 
All statements regarding safety of operation and technical data in
this instruction manual will only apply when the unit is operated
correctly.
Before connecting the power supply to the mains make sure that the
line voltage range marked on the power supply agrees with your
local supply.
The unit must not be operated in explosion endangered environ-
ments!
Only with written consent from Thorlabs GmbH may changes to
single components be carried out or components not supplied by
Thorlabs GmbH be used.
This precision device is only dispatchable if duly packed into the
complete original packaging including the plastic form parts. If
necessary, ask for a replacement package.
Attention 
Mobile telephones, handy phones or other radio transmitters are not
to be used within the range of three meters of this unit since the
electromagnetic field intensity may then exceed the maximum
allowed disturbance values according to EN 50 082-1.
INT-COM-1300 / page 2

2Getting Started Quickly
2 Getting Started Quickly
This section is intended to provide information how to set up quickly the
INT-COM-1300. More details and advanced features are described in further
sections.
2.1 Unpacking
The INT-COM-1300 Common Path OCT Interferometer consists of the following
items:
•INT-COM-1300 Common Path OCT Interferometer
•Power supply (±12V, 0.2A), 110V or 230V line voltage
•Operation manual
NOTE
Please check prior to operation, if the indicated line voltage range on the
power supply matches with your local mains voltage!
NOTE
If you want to use your own power supply, you can ask Thorlabs for an
appropriate power connector cable.
2.2 Setup
•Carefully unpack the unit and accessories. If any damage is noticed, do not
use the unit. Call Thorlabs and have us replace the defective unit.
•If necessary, mount the unit on your optical table or application. The unit has
four tapped 6-32 mounting holes (see section 3.5 for details).
•Plug the power connector cable into the DC INPUT.
•Plug the power supply into a 50-60Hz, 100-120VAC outlet (220V-240VAC for
EC version).
•Use coaxial cables with SMA connectors for OUTPUT.
INT-COM-1300 / page 3

2 Getting Started Quickly
2.3 First Operation
•Please note, that all fiber connectors must have a FC/APC connector!
•Connect a swept laser source to 1300nm IN.
•If necessary, connect a 660nm aiming laser to 660nm IN.
•Connect your Common Path interferometer probe to PROBE.
•Connect an external variable optical attenuator (VOA) to VOA IN and
VOA OUT.
•Adjust the external VOA to suppress any DC component in the interference
signal.
•The maximum OUTPUT voltage swing is ±3.6 V for high impedance loads
(±1.8V for 50Ωloads). The OUTPUT signal should not exceed this maximum
output voltage to avoid saturation.
NOTE
Clean all connectors before plug-in into the module. Please make sure,
that used connector type is FC/APC.
NOTE
To prevent saturation of the amplifier keep the optical input powers less
than the saturation power listed in specification.
Attention 
Refer to the specification and pay attention to the optical damage
threshold!
Exceeding these values will permanently destroy the detectors!
INT-COM-1300 / page 4

3Detailed Description
3 Detailed Description
3.1 General Principle of Operation
The Thorlabs INT-COM-1300 Common Path OCT Interferometer is a subassembly to
be used in swept source Fourier domain OCT systems and other applications. It
contains a fusion coupler / circulator network to operate with an external Common
Path interferometer probe. The integrated balanced detector similar to Thorlabs
PDB145C is used for detection of the generated interference fringes. The used fusion
couplers are optimized for flat wavelength response and very low PDCR (polarization
dependent coupling ratio change) to make the signal nearly independent of input
polarization changes. An additional aiming laser input simplifies the optical alignment
of the probe.
The INT-COM-1300 is powered by the included external power supply (±12V,
200mA) via a PICO M8 power connector.
Figure 1 shows a functional block diagram of the INT-COM-1300 Common Path OCT
Interferometer.
Figure 1:Functional block diagram of the INT-COM-1300
WDM
PROBE
VOA IN
Circulator
Slope Compensation 95/5
660 nm IN
OUTPUT
1300 nm IN
WDM Circulator
DC INPUT
VOA OUT
PROBE
VOA IN
Slope Compensation 95/5
660 nm IN
1
2
1300 nm IN
3
VOA OUT
OUTPUT
DC INPUT
INT-COM-1300 / page 5

3 Detailed Description
3.2 Optical Input and Output, Fiber Network
The INT-COM-1300 Common Path OCT Interferometer subassembly is equipped
with five FC/APC adaptors to be connected to your application. Corning SMF28
TM
single mode fiber is used for fusion couplers and internal fiber network.
NOTE
FC/APC connectors must be used for all INPUT and OUTPUT connectors.
Carefully clean the connectors prior to connection.
The internally fiber network is designed for swept source Fourier domain OCT
systems that require a common path interferometer configuration. The probe
interfaced with this module should have a reference arm and a sample arm. The
reflections from both arms are combined to produce the interference fringes. This
signal is detected by the balanced detector integrated into the INT-COM-1300
subassembly. The second input of the balanced detector is used to compensate the
DC component in the interference signal. This requires a variable optical power at the
second input of the balanced detector. An external VOA is used for this purpose.
The input signal from OCT source is split using a 95/5 TAP coupler. 95% of the light
is transmitted to a circulator (port 1). Port 2 of the circulator is connected to a WDM
coupler which combines the signals of the OCT source and the aiming laser. The
output of the WDM coupler is connected to PROBE port. Back reflected light passes
the WDM coupler again and leaves the circulator at port 3, which is connected to one
input of the balanced detector. The tapped 5% of the OCT input signal is passed
through a slope compensation coupler to the VOA. This additional coupler is
necessary to compensate the wavelength dependent coupling ratio change of the
95/5 TAP coupler. TAP coupler and slope compensation coupler are selected in pairs
in order to get a VOA IN signal nearly independent from OCT laser wavelength. This
provides a broadband DC offset compensation.
Typical insertion loss from 1300nm IN to PROBE is less than 1.5dB including
connector losses. Insertion loss from 1300nm IN to VOA IN is about 17dB. The
typical insertion loss for the aiming laser signal between 660nm IN and PROBE is
about 2dB including connector losses. Please note that SMF 28 is not single-mode at
aiming laser wavelength. The coupling of the aiming laser into the module can be
improved by changing the mode distribution moving the aiming laser input fiber.
Specially designed fusion couplers are used to achieve flat wavelength response for
both the PROBE and the VOA IN signals. Furthermore, fusion couplers are selected
INT-COM-1300 / page 6

3Detailed Description
to get the OUTPUT signal nearly independent to input polarization changes. Figure 2
and Figure 3show the typical spectral behavior of the INT-COM-1300 measured
from 1300nm IN to PROBE port and from 1300nm IN to VOA IN port.
1200 1250 1300 1350 1400
0
20
40
60
80
100
1300nm IN --> PROBE (%)
Wavelength (nm)
Figure 2: INT-COM-1300 coupling ratio from 1300nm IN to PROBE
1200 1250 1300 1350 1400
0
1
2
3
4
1300nm IN --> VOA IN (%)
Wavelength (nm)
Figure 3: INT-COM-1300 coupling ratio from 1300 IN to VOA IN
For optimal operation of the balanced detector the optical path length of the two input
channels should be matched. Therefore the optical path length from VOA IN to
VOA OUT port (including the VOA) should be twice as long compared to the optical
INT-COM-1300 / page 7

3 Detailed Description
path length from PROBE port to sample. An additional optical delay line may be
inserted between VOA IN and VOA OUT for fine tuning the path length difference.
Please note, that the INT-COM-1300 can also operate outside the specified
wavelength range. It will generate expedient output signals, but specification cannot
be guaranteed.
INT-COM-1300 / page 8

3Detailed Description
3.3 Electrical Output
The INT-COM-1300 has an OUTPUT SMA connector carrying the detected OCT
fringe signal.
This OUTPUT delivers an output voltage proportional to the difference between the
balanced detector’s two optical input signals (i.e. difference between the photo
currents of both photo detectors), which is amplified by an ultra low noise, high-speed
transimpedance amplifier. The amplifier’s transimpedance gain is 51x10
3
V/A (into
high impedance loads) resp. 25.5x10
3
V/A into a 50Ωload. The maximum output
voltage swing of OUTPUT is ±3.6V (high impedance load) resp. ±1.8V into 50Ω.
The output signal must not exceed the maximum output voltage to avoid saturation.
Therefore, the difference between the optical input power at PROBE and VOA OUT
must not exceed 70µW. It can be estimated from the OCT laser power (1300nm IN)
with respect to external probe losses and sample reflectivity.
The 3dB bandwidth of the OUTPUT signal is limited to 15MHz. Figure 4 shows a
typical frequency response curve of INT-COM-1300 OUTPUT signal. For this
measurement a test signal was applied to VOA OUT port.
0 102030405060
-30
-20
-10
0
10
20
30
40
Amplitude (dB)
Frequency (MHz)
Figure 4: INT-COM-1300 OUTPUT frequency response
The balanced detector includes an active filter section to suppress the generation of
aliasing frequencies in the digitized fringe signal, which would degrade the quality of
the OCT image. The active filter is designed for a sample rate of 50Msample/sec.
Signal frequencies beyond 25MHz are suppressed by more than 20dB. For different
INT-COM-1300 / page 9

3 Detailed Description
sample rates, the filter design can be modified accordingly - please contact Thorlabs
GmbH for details.
Figure 5 shows typical noise spectrum at OUTPUT of INT-COM-1300 measured
using an electrical spectrum analyzer (resolution bandwidth 10 kHz, video BW 3kHz).
The optical inputs of the INT-COM-1300 were darkened during measurement. The
lower curve is measured with INT-COM-1300 switched off , i.e., it represents the
measurement system’s noise floor.
10 20 30 40 50 60
-100
-95
-90
-85
-80
-75
-70
Reference
INT-COM-1300
Power (dBm)
Frequency (MHz)
Figure 5: INT-COM-1300 spectral noise measurement
INT-COM-1300 has a minimum noise-equivalent power (NEP) of 3.2pW/√Hz (DC-
15MHz). The integrated noise over this frequency range (DC-15MHz) is 13nW
RMS
.
This input optical noise level is approximately the minimum optical signal that can be
detected. For INT-COM-1300, the overall output voltage noise (V
RMS
) across a
50Ohm load is 0.32mV
RMS
.
INT-COM-1300 / page 10

3Detailed Description
3.4 Detector Responsivity
INT-COM-1300 uses InGaAs PIN photodiodes. Figure 6 shows a typical responsivity
curve for this photodiode type.
800 1000 1200 1400 1600 1800
0,0
0,2
0,4
0,6
0,8
1,0
Responsivity (A/W)
Wavelength (nm)
Figure 6: INT-COM-1300 detector responsivity
This wavelength dependency affects the output signal amplitude. In order to
compensate this responsivity, the measured electrical signal must be multiplied by
the normalized wavelength dependent responsivity.
INT-COM-1300 / page 11

3 Detailed Description
3.5 Mounting the INT-COM-1300
The INT-COM-1300 is housed in a rugged 120x80x21mm shielded aluminum
enclosure. The unit can be mounted to small edges by 6-32 screws, see Figure 7.
Threads are of 6 mm depth.
Figure 7: Possible mounting points for INT-COM-1300
An alternative cover with four mounting holes or a customized cover can be ordered
on request, see Figure 8.
Figure 8: INT-COM-1300 with alternative cover
INT-COM-1300 / page 12

3Detailed Description
3.6 Basic Measurement Setup
This section is dedicated to INT-COM-1300 Common Path OCT Interferometer basic
setup in a swept source Fourier domain OCT system, see Figure 9 . A swept laser is
connected to 1300nm IN while an red aiming laser is connected to 660nm IN. OCT
fringe signals are recorded to a Data Acquisition Device connected to OUTPUT. A
common path interferometer probe is connected to PROBE and a variable optical
attenuator (VOA) is connected between VOA IN and VOA OUT.
Probe Circulator
Slope Compensation
WDM
95/5
Aiming
laser
OCT
laser
Data
Acquisition
Device
VOA
Sample
INT-COM-1300
Probe Circulator
Slope Compensation
WDM
95/5
Aiming
laser
OCT
laser
Data
Acquisition
Device
Data
Acquisition
Device
VOAVOA
Sample
INT-COM-1300
Figure 9: Basic measurement setup
The VOA is used to adjust for power balance at the balanced detector’s inputs to
remove any DC component in the interference signal. Furthermore in balanced
condition maximum common mode rejection (maximum noise suppression) is
achieved. The VOA setting mainly depends on external Common Path probe loss
and sample reflectivity. The INT-COM-1300 is designed for low reflectivity samples.
For high reflectivity samples it could be necessary to attenuate the back reflected
probe light to achieve power balance at the balanced detector.
To obtain maximum noise suppression across the entire bandwidth of the balanced
detector, equal optical path lengths are also essential. Any path length imbalance will
introduce a phase shift between the two optical signals detected by the balanced
detector which will decrease noise reduction performance. The 95/5 TAP coupler
acts as signal divider for the balanced detection.
INT-COM-1300 / page 13

3 Detailed Description
One optical path is formed from the TAP coupler 95% output through the circulator
and WDM coupler to PROBE port, passing the external Common Path probe towards
the sample, from which the optical signal is back reflected to PROBE port and
passes then the WDM coupler and circulator to one input of the balanced detector.
The other optical path leads from TAP coupler 5% output via the slope compensation
coupler and the external VOA to the second input of the balanced detector. While the
inner path lengths are nearly identical the external path length has to be matched. As
a rule of thumb, the PROBE-to-sample optical path length shall be half of the optical
path length between VOA IN and VOA OUT. The reason is that the probe signal
passes the PROBE-sample distance two times. For fine adjustment of the optical
path length, an additional optical delay line between VOA IN and VOA OUT should
be inserted, alternatively, the distance between probe and sample should be
adjusted.
In perfect balance condition, i.e. equal input powers and equal optical path lengths, a
common mode rejection ratio (CMRR) > 35dB across the entire bandwidth of the
balanced detector is achievable. With path length imbalances high CMRR values are
achievable in the low frequency region only, while the CMRR value decrease with
increasing frequency. Figure 10 shows the common mode rejection capability in
balanced condition.
0 102030405060
-40
-30
-20
-10
0
10
20
30
40
CMRR
VOA OUT
PROBE
Balance Mode
Amplitude (dB)
Frequency (MHz)
Figure 10: INT-COM-1300 frequency response and common mode rejection ability
INT-COM-1300 / page 14