THORLABS Redstone OSA302 User manual

Redstone®OSA302,
Redstone OSA305,
OSA201C, OSA202C,
OSA203C, OSA205C,
OSA207C Optical
Spectrum Analyzers
Manual

Optical Spectrum Analyzers
Table of Contents
Chapter 1 Warning Symbol Definitions....................................................................................... 1
Chapter 2 Safety............................................................................................................................. 2
Chapter 3 Warranty........................................................................................................................ 3
3.1. Restriction of Warranty...........................................................................................3
Chapter 4 Description ................................................................................................................... 4
4.1. Introduction..............................................................................................................4
4.2. Interferometer Design .............................................................................................4
4.3. Interferogram Data Acquisition ..............................................................................7
4.4. Detector Temperature Mode (OSA203C only)........................................................7
4.5. Interferogram Data Processing...............................................................................8
4.6. Wavelength Meter Mode..........................................................................................8
4.7. Wavelength Calibration and Accuracy...................................................................8
4.8. Resolution and Sensitivity......................................................................................9
4.9. Dynamic Range/Optical Rejection Ratio..............................................................11
4.10. Apodization ....................................................................................................12
4.11. Zero Fill...........................................................................................................12
4.12. Software..........................................................................................................13
4.13. Trigger (Redstone OSA30x only)..................................................................13
4.14. Interlock (Redstone OSA30x only)................................................................13
Chapter 5 Using the Free-Space Input...................................................................................... 15
5.1. Table Mounting when Using the Free-Space Input .............................................15
5.2. Activation of Alignment Beam..............................................................................16
5.3. Free-Space Beam Alignment Example.................................................................18
5.3.1. Initial Visual Alignment..................................................................................................18
5.3.2. Adjust Software Settings...............................................................................................19
5.3.3. Final Alignment .............................................................................................................20
5.3.4. Error Estimation ............................................................................................................20
Chapter 6 Pure Air....................................................................................................................... 21
Chapter 7 Software...................................................................................................................... 22
7.1. Spectrum View and Interferogram View...............................................................22
7.1.1. Spectrum View..............................................................................................................22
7.1.2. Interferogram View........................................................................................................22
7.1.3. Setting the Unit of the Horizontal Axis ..........................................................................22
7.1.4. Setting the Unit of the Vertical Axis...............................................................................23
7.1.5. Displaying the Interferogram While Working in Spectrum View ...................................23
7.2. Acquiring Data.......................................................................................................23
7.2.1. Acquiring a Single Spectrum/Interferogram..................................................................23
7.2.2. Repeated Measurements..............................................................................................23
7.2.3. Automatic Setup............................................................................................................23
7.2.4. Averaging Interferograms (Redstone OSA30x only) ....................................................24
7.2.5. Signal ............................................................................................................................24
7.2.6. Measurements of Pulsed Sources................................................................................24

Optical Spectrum Analyzers
7.2.7. Wavelength Calibration.................................................................................................25
7.3. Working with Data in Traces.................................................................................26
7.3.1. Trace Update Options...................................................................................................26
7.4. Saving and Loading Data......................................................................................27
7.4.1. Saving Data...................................................................................................................27
7.4.2. Loading Data.................................................................................................................27
7.5. Analyzing Data.......................................................................................................27
7.5.1. Wavelength Meter.........................................................................................................27
7.5.2. Coherence Length.........................................................................................................28
7.6. Scripting the ThorSpectra Software (OSA20xC only) .........................................29
Chapter 8 Settings....................................................................................................................... 30
8.1. Basic Settings........................................................................................................30
8.1.1. Active Device Settings ..................................................................................................30
8.2. Advanced Settings ................................................................................................30
8.2.1. Acquisition Settings.......................................................................................................30
8.2.2. Environment Settings....................................................................................................31
8.2.3. Interferogram Settings...................................................................................................31
8.2.4. Spectrum Settings.........................................................................................................32
Chapter 9 LabVIEW®Drivers...................................................................................................... 34
Chapter 10 Troubleshooting......................................................................................................... 35
Chapter 11 Technical data............................................................................................................ 36
11.1. Common Specifications ................................................................................36
11.2. Model-Specific Specifications.......................................................................37
11.3. Update Frequency..........................................................................................37
11.3.1.OSA20xC ......................................................................................................................37
11.3.2.Redstone OSA30x ........................................................................................................38
11.4. Absolute Maximum Ratings..........................................................................38
11.5. Trigger Specifications (Redstone OSA30x only) .........................................38
11.6. Interlock Specifications (Redstone OSA30x only).......................................38
11.7. Internal Sources Specifications....................................................................38
Chapter 12 Mechanical Drawings................................................................................................ 39
12.1. Redstone OSA30x..........................................................................................39
12.1.1.Dimensions....................................................................................................................39
12.1.2.Front and Back Panels..................................................................................................40
12.2. OSA20xC ........................................................................................................41
12.2.1.Dimensions....................................................................................................................41
12.2.2.Front and Back Panels..................................................................................................42
Chapter 13 Certifications and compliances ............................................................................... 43
13.1. Redstone OSA30x..........................................................................................43
13.2. OSA20xC ........................................................................................................44
Chapter 14 Appendix..................................................................................................................... 45
14.1. Calculating AbsoLrption Cross Sections.....................................................45
14.2. Apodization Functions ..................................................................................46
Chapter 15 Regulatory .................................................................................................................. 47

Optical Spectrum Analyzers
15.1. Waste treatment is your own responsibility.................................................47
15.2. Ecological background..................................................................................47
Chapter 16 Thorlabs Worldwide Contacts.................................................................................. 48

Optical Spectrum Analyzers Chapter 1: Warning Symbol Definitions
Page 1 STN053070-D02
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

Optical Spectrum Analyzers Chapter 2: Safety
Rev F, January 2, 2024 Page 2
Chapter 2 Safety
All statements regarding safety of operation and technical data in this instruction manual will only apply when
the unit is operated correctly.
SHOCK WARNING
High voltage inside. To avoid electrical shock, prior to powering on the unit, make sure that the
protective conductor of the 3-conductor power cord is correctly connected to the protective
earth contact of the socket outlet. Improper grounding can cause electric shock, resulting in
severe injury or even death.
Do not operate without the cover installed.
CAUTION
This unit must not be operated in explosive environments.
If purging gas other than dry air is used, make sure that the environment for the operator is well
ventilated since the gas will leak out from the instrument.
This product is for indoor use only. To prevent potential fire or shock hazard, do not expose the
unit to any source of excessive moisture.
Only use the instrument standing on its feet.
WARNING
The OSA20xC instruments include a He-Ne laser.
The Redstone OSA302 and Redstone OSA305 include a red laser and an infrared laser.
The classification of these lasers follows the European Standard IEC 60825-1:2014.
Viewing the laser output with telescopic optical instruments (for example, telescopes and
binoculars) close to the aperture may pose an eye hazard! Using the instrument in combination
with collimating optics may have an impact on the assigned laser classification. The safety of
any system incorporating the equipment is the responsibility of the assembler of the system.
Use of controls or adjustments or performance of procedures other than those specified herein
may result in hazardous radiation exposure.

Optical Spectrum Analyzers Chapter 3: Warranty
Page 3 STN053070-D02
Chapter 3 Warranty
Thorlabs warrants material and production of the Redstone OSA302, Redstone OSA305, OSA201C, OSA202C,
OSA203C, OSA205C, and OSA207C for a period of 24 months starting with the date of shipment. During this
warranty period, Thorlabs will see to defaults by repair or exchange if the product is entitled to warranty. For
warranty repairs or service, the unit must be sent back to Thorlabs or to a place determined by Thorlabs. The
customer will carry the shipping costs to Thorlabs; in case of warranty repairs Thorlabs will carry the shipping
costs back to the customer. If no warranty repair is applicable, the customer also has to carry the costs for return
shipment. In case of shipment from outside, EU duties, taxes, etc., that may arise have to be carried by the
customer.
Thorlabs warrants the hardware and software determined by Thorlabs for this unit to operate fault-free provided
that they are handled according to our requirements. However, Thorlabs does not warrant a fault-free and
uninterrupted operation of the unit, of the software or firmware for special applications, or this instruction manual
to be error-free. Thorlabs is not liable for consequential damages.
Please visit the below link for Thorlabs’ General Terms and Conditions:
https://www.thorlabs.com/Images/PDF/LG-PO-001_Thorlabs_terms_and_%20agreements.pdf
3.1. Restriction of Warranty
The aforementioned warranty does not cover errors and defects as a result of improper treatment, software or
interface not supplied by Thorlabs, modification of product, unauthorized maintenance, or misuse or operation
outside the defined ambient stated by Thorlabs.
Further claims will not be consented to and will not be acknowledged. Thorlabs does explicitly not warrant the
usability or the economical use for certain cases of application.
Thorlabs reserves the right to change this instruction manual or the technical data of the described unit without
notice.

Optical Spectrum Analyzers Chapter 4: Description
Rev F, January 2, 2024 Page 4
Chapter 4 Description
The Thorlabs Optical Spectrum Analyzer (OSA) is a general-purpose spectrum analyzer for optical research
and production applications. It has a user-friendly Graphical User Interface (GUI) and can be controlled from a
standard PC via a High-Speed USB port. Each instrument offers an FC/PC fiber input receptacle and a free-
space input for collimated beams. Special designs with other input receptacles are available on request. The
instrument is designed for measurements of continuous wave (CW) light sources, but works in some
applications where pulsed light is used. Please contact Technical support at [e-mail] to
discuss your application if you have a pulsed source.
4.1. Introduction
Thorlabs’ Optical Spectrum Analyzers are general-purpose instruments that measure optical power as a
function of wavelength. These OSA instruments are versatile enough to analyze broadband optical signals,
resolve the Fabry-Perot modes of a gain chip, and identify gas absorption lines in a spectral measurement.
Whereas commonly available OSAs are typically grating-based monochromators, the Thorlabs instruments are
Fourier Transform Optical Spectrum Analyzers (FT-OSAs), which utilize a scanning Michelson interferometer
design, as shown in Figure 1 and Figure 2 on pages 6 and 7. This approach allows for the design of a full-
featured OSA with the additional benefit of a high-precision wavelength meter.
Thorlabs offers two lines of analyzers, including the OSA20xC models and the Redstone OSA30x models. The
Redstone OSA302 and OSA305 are more advanced instruments for demanding applications that require a
higher optical resolution or sensitivity whereas the OSA20xC instruments are general-purpose analyzers with
moderate resolution and high update rate.
The Thorlabs FT-OSA has an FC/PC-style optical fiber input and a free-space input. After the input light is
collimated, a beamsplitter divides the optical signal into two separate branches. The path length difference
between the two branches is varied up to a maximum of 40 mm for the OSA20xC instruments or 160 mm for
the Redstone OSA30x instruments. The collimated light fields then optically interfere as they recombine at the
beamsplitter. The detector units shown in Figures 1 and 2 record the interference pattern, which is commonly
referred to as an interferogram. This interferogram is the autocorrelation waveform of the input optical spectrum.
By applying Fourier transform to the waveform, the optical spectrum is recovered. The resulting spectrum offers
both a high resolution and a very broad wavelength coverage with a spectral resolution that is related to the
optical delay range. The wavelength range is limited by the optical bandwidth of the detectors and of the optical
coatings. The two main detectors of the Redstone OSA302 and OSA305 instruments are responsive over
different wavelength ranges, and the two separate spectra can be stitched together to provide a single spectrum
covering the entire device wavelength range.
Furthermore, the accuracy of our system is ensured by including a frequency-stabilized HeNe reference laser
in the OSA20xC models and an infrared frequency-locked reference laser in the Redstone OSA30x models.
The reference lasers provide highly accurate measurements of optical path length changes, allowing the system
to continuously self-calibrate. This process ensures accurate optical analysis well beyond what is possible with
a grating-based OSA. The Redstone OSA30x instruments also include an additional detector and broadband
source, which is used to accurately determine the location of the point of zero path difference (ZPD).
4.2. Package Contents
On unpacking the OSA20xC, the package should include the below items:
•Optical Spectrum Analyzer
•Windows® Laptop
oIncludes Pre-Installed OSA Software (Version 3.30) and a Mouse
oU.S. English Configuration
•Factory Calibration Report
•OSA and Laptop Power Supplies with Region-Specific Power Cords
•High-Speed USB 2.0 Cable (Replacement Item # USB-A-79)
oConnects the OSA to the Laptop
•Three Mounting Feet

Optical Spectrum Analyzers Chapter 4: Description
Page 5 STN053070-D02
oUsed to Secure the Interferometer to an Optical Table (See 5.1)
•SPW603 Spanner Wrench and VP10C Vacuum Pickup Tool
oUsed to Remove Protective Window in Front of Free-Space Input Cases When Etalons are
Visible in the Spectrum
On unpacking the Redstone OSA30x, the package should include the below items:
•Optical Spectrum Analyzer
•Windows® Laptop
oIncludes Pre-Installed OSA Software (Version 3.30) and a Mouse
oU.S. English Configuration
•Factory Calibration Report
•OSA and Laptop Power Supplies with Region-Specific Power Cords
•USB 3.0 Cable (Replacement Item # CABU31)
oConnects the OSA to the Laptop
•SPW801 Spanner Wrench and VP10C Vacuum Pickup Tool
oUsed to Remove Protective Window in Front of Free-Space Input Cases When Etalons are
Visible in the Spectrum
•Lens Tube Adapter for Free-Space Port
•Calibration Patch Cable (Replacement Item # P5-SMF28E-FC-2)
4.3. Interferometer Design
The OSA20xC models use an arrangement of two retroreflectors as shown in Figure 1. These retroreflectors
are mounted on a voice-coil-driven platform, which dynamically changes the optical path length of the two
branches of the interferometer simultaneously and in opposite directions. The advantage of this layout is that it
changes the optical path difference (OPD) of the interferometer by four times the mechanical movement of the
platform; the longer the change in OPD, the finer the spectral detail the FT-OSA can resolve.
The Redstone OSA models use a standard arrangement of one fixed and one movable retroreflector as shown
in Figure 2. The mobile retroreflector is mounted on a voice-coil-driven platform, which changes the optical path
length of the branch. The fixed retroreflector is mounted on a piezo actuator that translates the retroreflector in
the two dimensions orthogonal to the beam path in a self-alignment algorithm. This configuration changes the
OPD between the two branches by twice the mechanical movement of the platform. The Redstone OSA collects
asymmetrical interferograms in terms of the ZPD, which is located significantly closer to one end of the stroke.
These interferograms are usually called single-sided whereas the OSA20xC instruments collect symmetrical,
or double-sided, interferograms.
The maximum physical movement of the OSA20xC platform is 10 mm from the ZPD, which translates to a
maximum OPD of 40 mm and a spectral resolution of 7.5 GHz or 0.25 cm-1. The Redstone OSA30x series has
an 80 mm stroke length capacity, and thus a maximum OPD of 160 mm and a spectral resolution of 1.9 GHz or
0.063 cm-1. The wavelength resolution is dependent on the wavelength of light being measured. For more details
see Section 4.9 Resolution and Sensitivity. In this context, the spectral resolution is defined according to the
Rayleigh criterion and is the minimum separation required between two spectral features in order to resolve
them as two separate lines. Note that when using the Wavelength Meter or Peak Track tools, the number of
significant figures can far exceed the Rayleigh spectral resolution as those tools use several data points and
curve fitting techniques.
The Thorlabs OSAs utilize a built-in, actively stabilized reference laser to interferometrically record the variation
of the optical path length. This reference laser is inserted into the interferometer and closely follows the same
path traversed by the unknown input light field. To reduce the effects of water absorption on the acquired
spectrum, our OSAs have two 1/4" inner-diameter quick-connection hose fittings on the back panel, through
which the interferometer can be purged with dry air or nitrogen, as detailed in Chapter 6.

Optical Spectrum Analyzers Chapter 4: Description
Rev F, January 2, 2024 Page 6
Figure 1. Optical Schematic of the Thorlabs OSA20xC Detailing the Dual Retroreflector Design
Note: In the OSA20xC interferometer design, both retroreflectors are attached to a common platform that is
moved via a voice-coil motor. This configuration provides an optical delay that is four times the displacement of
the carriage.

Optical Spectrum Analyzers Chapter 4: Description
Page 7 STN053070-D02
Figure 2. Optical Schematic of the Thorlabs Redstone OSA30x
Note: The Redstone OSA302 does not include the dispersion compensation plate in the interferometer.
4.4. Interferogram Data Acquisition
The OSA20xC instruments use the interference pattern of the HeNe reference laser to clock a 16-bit Analog-to-
Digital Converter (ADC) such that samples are taken at fixed, equidistant optical path length intervals. The HeNe
reference fringe period is digitized, and its frequency is multiplied by a phase locked loop (PLL), which leads to
an extremely fine sampling resolution. Multiple PLL filters enable frequency multiplication settings of 16X, 32X,
64X, or 128X. At the 128X multiplier setting, the data points are acquired approximately every 1 nm of carriage
travel. The multiple PLL filters enable the user to choose system parameters optimized for measurements that
range from high speed, reduced sensitivity, and reduced resolution to lower speed, higher sensitivity, and higher
resolution.
Redstone OSA30x models instead sample the interferogram at a fixed frequency of up to 1 MHz with an 18-bit
ADC while the retroreflector platform moves at a constant speed controlled by a PID loop. The data is resampled
to fit up to 128 data points per reference laser period.
A high-speed USB link transfers the interferograms to the ThorSpectra software, which is highly optimized to
take full advantage of modern multi-core processors, as well as top-performing graphics processing units
(GPUs). The software performs a number of calculations to analyze and condition the input waveform in order
to obtain the highest possible resolution and signal-to-noise ratio (SNR) at the output of the Fast Fourier
transform (FFT).
Very low-noise and low-distortion detector-amplifiers with automatic gain control provide a large dynamic range,
allow optimal use of the ADC, and ensure excellent SNR for up to 10 mW of input power. For low-power signals,
the system can typically detect less than 100 pW from narrowband sources for mid-wavelength models and as
low as femtowatts for short-wavelength models.
4.5. Detector Temperature Mode (OSA203C only)
The OSA203C features a temperature controller for the detector, which can be set to a low or a high
temperature. The detector is cooled in low temperature mode, which leads to reduced thermal noise and thereby
a higher sensitivity. However, the low temperature also makes the detector insensitive to wavelengths longer
than 2500 nm.

Optical Spectrum Analyzers Chapter 4: Description
Rev F, January 2, 2024 Page 8
In the high temperature mode, the temperature of the detectors is stabilized at near room temperature. The
thermal noise of the detector is higher in this mode than in the low temperature mode, but the higher temperature
makes the detector sensitive to wavelengths longer than 2500 nm, effectively extending the operating range of
the OSA203C up to 2600 nm.
When using the low temperature mode, the set point of the detector temperature control is –30°C. It is the
default operating mode of the OSA203C and is enabled by default at startup. Also note that the power level
calibration of the OSA203C is only guaranteed between 1000 and 2400 nm.
4.6. Interferogram Data Processing
The interferograms generated by the instrument vary from 50 thousand to 16 million data points depending on
the resolution and sensitivity settings employed. The ThorSpectra software applies the user-specified
apodization function, analyzes the input data and intelligently selects the optimal FFT algorithm from our internal
library.
Additional software performance is realized by utilizing an asynchronous, multi-threaded approach to collecting
and handling interferogram data through the multitude of processing stages required to yield spectrum
information. The software’s multi-threaded architecture manages several operational tasks in parallel by actively
adapting to the PC’s capabilities, thus ensuring maximum processor bandwidth utilization. Furthermore, a
specialized GPU is tasked to perform the most advanced calculations. Each of our OSA instruments ships with
a laptop computer that has been carefully selected to ensure optimum data processing and user interface
operation.
4.7. Wavelength Meter Mode
When narrowband optical signals are analyzed, the OSA automatically calculates the center wavelength of the
input, which can be displayed in a floating window or docked just below the main data display, presenting the
overall spectrum. The central wavelength, , is calculated by counting interference fringes (periods in the
interferogram) from both the input and reference lasers according to the following formula:
 
  
Here,  is the number of fringes for the reference laser,  is the number of fringes from the input laser,
 is the vacuum wavelength of the reference laser (632.9918 nm or 1532.8323 nm), and  is the index
of refraction of air at the reference laser wavelength.  is the index of refraction of air at the wavelength
 and is determined iteratively from  —that is, the measured wavelengthin air —using Ciddor’s
formula as given by NIST
1
.
The resolution of the OSA operating as a wavelength meter is substantially higher than the system when it
operates as a broadband spectrometer because the system can resolve a fraction of a fringe, up to the limit set
by the phase-locked loop multiplier or sampling frequency (see Section 4.4 Interferogram Data Acquisition). In
practice, the resolution of the system is limited by the bandwidth and structure of the unknown input, noise in
the detectors, drift in the reference laser, interferometer alignment, and other systematic errors. The system has
been found to offer reliable results as low as 0.1 pm in the visible spectrum and 0.2 pm in the NIR/IR (see
Chapter 11 Technical data for details).
The software evaluates the spectrum of the unknown input in order to determine an appropriate display
resolution. If the data is unreliable, as would be the case for a multiple peak spectrum, the software disables
the wavelength meter mode so it does not provide misleading results.
4.8. Wavelength Calibration and Accuracy
The OSA20xC instruments use a built-in stabilized HeNe reference laser with a vacuum wavelength of
632.9918 nm. The use of a stabilized HeNe ensures long-term wavelength accuracy as the dynamics of the
1
See NIST’s Engineering Metrology Toolbox (https://emtoolbox.nist.gov/Wavelength/Documentation.asp,
collected 2021-08-17) for details. Even though the suggested validity range for the formula is 300 - 1700 nm, it
is used by the software for the entire operation wavelength range. As suggested by NIST we assume the CO2
concentration to be 450 µmol/mol. Older versions of the Thorlabs OSA software (v2.90 and older) use a modified
version of Edlén’s formula, which agrees with Ciddor’s formula to less than 10-6.

Optical Spectrum Analyzers Chapter 4: Description
Page 9 STN053070-D02
stabilized HeNe are well-known and controlled. The Redstone OSA30x platform incorporates a frequency-
locked reference laser with a vacuum wavelength of 1532.8323 nm. Because of the frequency-lock, long-term
wavelength accuracy is ensured, and the reference laser is connected to an FC/APC output and can be fed into
the instrument fiber input for additional calibration if so desired; see Section 7.2.7 Wavelength Calibration.
The instrument is factory-aligned so that the reference and unknown input beams experience the same optical
path length change as the interferometer is scanned. The effect of any residual alignment error on wavelength
measurements is less than 0.5 ppm; the input beam pointing accuracy is ensured by a high-precision ceramic
receptacle and a robust interferometer cavity design. No optical fibers are used within the scanning
interferometer. Similarly, as described in Section 4.7, each measured wavelength in the spectrum is corrected
for the difference in refractive indices for the reference and measured wavelength. The refractive indices are
calculated using Ciddor’s formula as given by NIST with the temperature, pressure, and relative humidity
measured by sensors internal to the instrument.
4.9. Resolution and Sensitivity
The resolution of this type of instrument depends on the optical path difference (OPD) between the two branches
in the interferometer. In this case it is easier to work with wavenumbers (inverse centimeters, ) than
wavelength (nanometers) or frequency (terahertz). From the Fourier transform, the relation between spectral
resolution in cm-1 and OPD in cm is simply:
 

Assume we have two narrowband sources, such as two lasers with a 1 cm-1 energy difference: 6500 cm-1 and
6501 cm-1. To distinguish between these signals in the interferogram and thus the spectrum, we would need to
move the voice-coil platform from the point of zero path difference (ZPD) such that we have introduced at least
1 cm OPD. The OSA20xC can move ±4 cm in OPD and thus spectral features 0.25 cm-1 apart can be resolved,
while the Redstone OSA30x can move 16 cm in OPD and resolve spectral features that are 0.063 cm-1 apart.
The resolution of the instrument can be calculated as follows:

Here, is the resolution in pm, is the resolution in cm-1, and is the wavelength in µm.
The resolution of OSA20xC instruments can be set to High or Low in the main window of the ThorSpectra
software. In High resolution mode, the retroreflectors translate by the maximum of ±1 cm (±4 cm in OPD), while
in Low resolution mode, the retroreflectors translate by ±0.25 cm (±1 cm in OPD). The Redstone OSA30x
instruments can be set to one of four resolution modes: High, Medium High, Medium Low, or Low, which
correspond to OPDs of 16 cm, 4 cm, 1 cm, and 2.5 mm, respectively. See Figure 3 for a visual comparison of
the different resolutions for the OSA20xC and Redstone OSA30x instruments. In the Interferogram settings
section of the ThorSpectra software, the length of the interferogram used in the calculation of the spectrum can
be cut to remove spectral contributions from high-frequency components (see Section 8.2.3 Interferogram
Settings).

Optical Spectrum Analyzers Chapter 4: Description
Rev F, January 2, 2024 Page 10
Figure 3. Resolution as a Function of Wavelength
Note that the resolution shown is valid over the specified wavelength range of the individual instrument (ie.
OSA305 is specified from 1.0 µm to 5.6 µm).
The sensitivity of the instrument depends on the electronic gain used in the sensor electronics. Since an
increased gain setting reduces the electrical bandwidth of the detectors, the instrument will run slower when
higher gain settings are used. Figure 4 and Figure 5 show the dependence of the noise floor on the wavelength
and OSA model.
Figure 4. Noise Floor in Absolute Power Mode, i.e., the mode for measuring narrowband light sources
0 1 2 3 4 5 6 7 8 9 10 11 12
10-1
100
101
102
103
104
105
OSA20xC, Low
OSA20xC, High
OSA30x, Low
OSA30x, Medium Low
OSA30x, Medium High
OSA30x, High
Resolution in Spectrometer Mode
Resolution (pm)
Wavelength (µm)
0.2 1 10 20
-105
-95
-85
-75
-65
-55
-45 OSA302
OSA201C
OSA202C
OSA203C
OSA205C
OSA305
OSA207C
OSA Noise Floor in Absolute Power Mode
Noise Floor (dBm)
Wavelength (µm)

Optical Spectrum Analyzers Chapter 4: Description
Page 11 STN053070-D02
Figure 5. Noise Floor in Power Density Mode, i.e., the mode for measuring broadband light sources
4.10. Dynamic Range/Optical Rejection Ratio
The ability to measure low-level signals close to a peak is determined by the optical rejection ratio (ORR) of the
instrument. It can be seen as the filter response of the OSA and can be defined as the ratio of the power at an
interesting peak to the power at a given distance from said peak. For the Thorlabs OSAs the ORR is defined
with a stricter condition: instead of just considering the power at a single data point, e.g., 25 GHz from the peak,
the power of the highest noise in the wavelength range from 25 GHz to 2 THz is related to the power of the
peak. This way, any additional limiting peaks that are not located at the test distances will still be included in the
ORR value:
󰇛󰇜
󰇛󰇟󰇠󰇜
If the ORR is not higher than the optical signal-to-noise ratio of the source to be tested, the measurement will
indicate the limit of the OSA rather than the tested source. Figure 6 and the table below provide some example
values for the optical rejection ratio of the OSA205C and Redstone OSA305 for a narrowband source at
1532 nm with the following settings: High Resolution, Low Sensitivity, average of 5 traces, Hann apodization,
and Zero Fill = 0. All OSA20xC models and the Redstone OSA30x instruments show similar behavior if the
distance from the peak is measured in frequency, e.g., GHz.
Distance from
1532 nm Peak
Optical Rejection Ratio
OSA205C
Redstone OSA305
0.2 nm (25 GHz)
30 dB
40 dB
0.8 nm (100 GHz)
37 dB
42 dB
6.2 nm (800 GHz)
44 dB
45 dB
7.8 nm (1000 GHz)
44 dB
45 dB
0.2 1 10 20
-90
-80
-70
-60
-50
-40
-30
OSA302
OSA201C
OSA202C
OSA203C
OSA205C
OSA305
OSA207C
OSA Noise Floor in Power Density Mode
Noise Floor (dBm/nm)
Wavelength (nm)

Optical Spectrum Analyzers Chapter 4: Description
Rev F, January 2, 2024 Page 12
Figure 6. Spectrum of a 1532 nm Frequency-Locked Laser measured by a Redstone OSA305. The
vertical lines represent where the ORR is measured.
4.11. Apodization
According to Fourier theory, the spectrum of the unknown input light can be exactly determined from the Fourier
transform of the measured interferogram, but only if the interferogram is acquired over an optical path difference
extending from zero to infinity. Of course, the mirrors in the OSA can only translate through a finite distance.
We have therefore implemented several apodization functions in the ThorSpectra software to account for the
effect of the finite path length on the measured interferogram.
The measured, finite interferogram can be thought of as an ideal, infinite interferogram that consists of measured
values over a short interval (say, to ) and is equal to zero everywhere else. This treatment is equivalent
to multiplying the measured interferogram (which has measured values from to ) by a boxcar function that
is equal to 1 from to and is equal to zero everywhere else. Mathematically, the Fourier transform of the
product of two functions (i.e., the interferogram and the boxcar function) is equal to the convolution of the Fourier
transform of the first function (i.e., the interferogram) with the Fourier transform of the second function (i.e., the
boxcar). That is, if 󰇛󰇜is defined as the Fourier transform of , then
󰇛󰇜󰇛󰇜󰇛󰇜
The Fourier transform of the interferogram is the spectrum, and the Fourier transform of a boxcar function is the
sinc function:󰇛󰇜󰇛󰇜. The sinc function is an oscillating function with diminishing amplitude away
from zero. Therefore, the Fourier transform of a measured, finite interferogram yields the exact spectrum, but it
is convolved with additional periodic structures that are not representative of the unknown input light.
The effect of the additional periodic structures is strongest when measuring a narrowband source, because the
amplitude of the interferogram of a narrowband source is high over the entire measurement range. To remove
the structures, the interferogram is multiplied by apodizing (dampening) functions that decrease the amplitude
of the interferogram close to the edges of the measurement range. This reduces or removes the artificial ringing
at the expense of reducing the resolution of the final spectrum.
For a detailed description of the apodization functions implemented in the ThorSpectra software, please see
Section 14.2 Apodization Functions.
4.12. Zero Fill
Zero filling is the process of extending the interferogram with zeros. This effectively interpolates the spectrum
by adding more data points between the measured data. It is especially interesting for spectra that contain sharp
1524 1526 1528 1530 1532 1534 1536 1538 1540 1542
-70
-60
-50
-40
-30
-20
-10
0
10
1532 nm Frequency-Locked Laser Spectrum
Normalized Power (dB)
Wavelength (nm)

Optical Spectrum Analyzers Chapter 4: Description
Page 13 STN053070-D02
features. More data points close to a sharp spectral feature can make it easier to determine a peak’s center
frequency or intensity.
In the ThorSpectra software, a zero fill factor (ZFF) of 0 does nothing to the interferogram, whereas ZFF = 1
means that the interferogram is made twice as long (equally many zeroes as true data points) and ZFF = 2
means that the interferogram is made four times as long (three times more zeroes than data points).
It is important to note that since no real data is added, the resolution of the final spectrum does not change. This
means that, for example, two peaks located close to each other in frequency would not be more resolvable if
ZFF = 2 were used, rather than ZFF = 0.
4.13. Software
The OSA is shipped with the software package pre-installed on the laptop computer that is included with the
purchase of this instrument. The software has a customizable graphical user interface for acquiring, inspecting,
manipulating, and analyzing spectra and interferograms. The software makes it easy to locate and track spectral
peaks or valleys, measure the optical input power over any wavelength range, calculate an absorption spectrum
in real-time, or track a large number of parameters over time. A device interface library containing a multitude
of routines for data acquisition, instrument control, and spectral processing and manipulation is also provided
with the instrument. The library can be used to develop customized software for your own application using
python, LabVIEW®, C/C++, C#, or other programming languages. Each OSA ships with a set of LabVIEW®
routines to assist with writing user defined applications. More information on how to operate the software can
be found in Chapters 8 and 9.
4.14. Trigger (Redstone OSA30x only)
At the back panel of the Redstone OSA30x instruments, there are four BNC connectors, some of which can be
used to externally trigger a measurement. The four BNC connectors are labeled on the instrument back panel
and provide the following functionality:
•Trigg: When the negative flank of a signal is registered by this input, the Redstone OSA will start an
acquisition with the current settings. Note that the trigger will be ignored unless the output Aux out is
low, due to the fact that the retroreflector first needs to move to the correct starting position.
•Acquiring data: This output will be high while the data acquisition is active.
•Aux in: Currently not used.
•Aux out: When this output is low, the instrument is ready to receive an external trigger via the Trigg
input. When high, the instrument is busy.
More detailed specifications for these connectors can be found in Section 11.5 Trigger Specifications (Redstone
OSA30x only).
To perform a triggered measurement, the following steps should be followed:
1. The Trigger Type drop-down menu in the Settings menu’s Acquisition tab must be set to “External.”
2. Click the “Single” or “Repeat” button.
3. Check that Aux out is low and send trigger to the Trigg input.
4.15. Interlock (Redstone OSA30x only)
The Redstone OSA30x devices are equipped with a phono-type interlock jack located on the back panel. This
is a laser safety feature for the internal reference laser source. To enable the reference laser source, a short
circuit (< 430 Ω) must be applied across the terminals of the interlock connector. The shorting device (interlock
pin) installed in all units shipped from Thorlabs performs this function. Leave the shorting device installed unless
using an external safety circuit or other type of remotely controlled switch to enable laser output. Making use of
the interlock feature requires the appropriate 2.5 mm phono-type plug, which is readily available through most
electronics retailers. The plug should be wired to the external safety circuit or switch and then plugged into the

Optical Spectrum Analyzers Chapter 4: Description
Rev F, January 2, 2024 Page 14
back panel’s interlock jack in place of the shorting device. The electrical specifications of the interlock jack are
listed in Section 11.6 Interlock Specifications (Redstone OSA30x only).
Note that the Redstone OSA30x instruments require the reference laser source to collect accurate
interferograms; hence, with the Interlock unplugged, the instrument is disabled.
Further, the sole output for the reference laser is the Calibration output fiber port on the front of the instrument.

Optical Spectrum Analyzers Chapter 5: Using the Free-Space Input
Page 15 STN053070-D02
Chapter 5 Using the Free-Space Input
Figure 7. Front Panels of the OSA201C (Left) and Redstone OSA305 (Right)
These instruments have two input options: fiber coupled and free-space. It is not possible to use both input
options at the same time. By default, we provide an FC/PC receptacle for the fiber input, but other receptacles
can be requested at the time of the order.
Figure 8. Free-Space Input (Left) and FC/PC Input (Right) on the OSA20xC Front
Figure 9. Free-Space Input (Left) and FC/PC Input (Right) on the Redstone OSA30x Front
5.1. Table Mounting when Using the Free-Space Input
The OSA20xC interferometer assembly normally “floats” on gel bushings inside the case. When coupling an
external light source to the free-space input, it may be desirable to lock the interferometer to a stable surface
such as an optical table. This can be accomplished by using the provided mounting feet to secure the OSA to
the breadboard using two or more of Thorlabs’ CF175 clamping forks, as shown in Figure 10. Likewise, even

Optical Spectrum Analyzers Chapter 5: Using the Free-Space Input
Rev F, January 2, 2024 Page 16
though the Redstone OSA30x instruments are fixed within the instrument case, it is recommended to use two
or more clamping forks when using the free-space input.
Figure 10. Three mounting feet are included with the OSA20xC. Using two CF175 clamping forks, the
interferometer assembly can be locked to the table.
When the OSA20xC interferometer is locked to an optical table with the provided posts, the beam height is
61 mm (2.4’’) above the table surface. These fixed-height posts are long enough to extend to the table surface
while also being short enough to prevent the OSA from being lifted off the table.
The Redstone OSA30x free-space input accepts a beam height of 117 mm (4.6’’) above the table surface.
If the beam height of your experimental setup is not consistent with the free-space input, we recommend using
a periscope assembly, such as Thorlabs’ RS99(/M) periscope, to adjust the beam height to that of the OSA’s
input. Do not use longer posts to elevate the OSA.
5.2. Activation of Alignment Beam
The OSA20xC models include a red alignment beam, which can be activated by opening the free-space door
and rotating the switch on the front panel to the ON position, as shown in Figure 11. The red alignment beam
will then exit the free-space input port. When the alignment beam is enabled, no measurements can be
performed, and the ThorSpectra software will display the message shown in Figure 11.