THORLABS Redstone OSA305 User manual

Redstone™ OSA305
Optical Spectrum
Analyzer
User Guide

Redstone™ Optical Spectrum Analyzer
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................................................................................ 5
4.4. Interferogram Data Processing................................................................................ 5
4.5. Wavelength Meter Mode........................................................................................... 6
4.6. Wavelength Calibration and Accuracy.................................................................... 6
4.7. Resolution and Sensitivity ....................................................................................... 6
4.8. Dynamic Range/Optical Rejection Ratio................................................................. 8
4.9. Absolute Power and Power Density........................................................................ 9
4.10. Apodization........................................................................................................ 9
4.11. Zero Fill............................................................................................................. 10
4.12. Software ........................................................................................................... 10
4.13. Trigger.............................................................................................................. 10
4.14. Interlock............................................................................................................ 11
Chapter 5 Using the Free-Space Input ........................................................................................12
5.1. Table Mounting when Using the Free-Space Input.............................................. 12
5.2. Activation of Alignment beam................................................................................ 13
5.3. Free-Space Beam Alignment Example.................................................................. 13
5.3.1. Initial Visual Alignment..................................................................................................13
5.3.2. OSA Software Settings .................................................................................................14
5.3.3. Final Alignment .............................................................................................................15
5.3.4. Error Estimation ............................................................................................................15
Chapter 6 Pure Air..........................................................................................................................16
Chapter 7 Software Setup .............................................................................................................17
7.1. Requirements .......................................................................................................... 17
7.2. Software Installation ............................................................................................... 17
7.3. Connecting to the PC.............................................................................................. 18
7.4. Starting the Application.......................................................................................... 18
Chapter 8 Operation.......................................................................................................................19
8.1. Interface Overview .................................................................................................. 19
8.2. Spectrum View and Interferogram View................................................................ 19
8.2.1. Spectrum View..............................................................................................................19
8.2.2. Interferogram View........................................................................................................20

Redstone™ Optical Spectrum Analyzer
8.3. Customizing the Display......................................................................................... 20
8.3.1. Zooming and Scrolling the Horizontal Axis...................................................................20
8.3.2. Zooming and Scrolling the Vertical Axis .......................................................................21
8.3.3. Setting the Unit of the Horizontal Axis ..........................................................................21
8.3.4. Displaying the Secondary Horizontal Axis....................................................................22
8.3.5. Setting the Unit of the Vertical Axis...............................................................................22
8.3.6. Displaying the Secondary Vertical Axis ........................................................................22
8.3.7. Hiding the Menus and Full Screen Mode......................................................................23
8.3.8. Switching Between Power Density and Absolute Power..............................................24
8.3.9. Displaying a Split View..................................................................................................24
8.3.10.Displaying the Interferogram While Working in Spectrum View ...................................25
8.3.11.Displaying a Legend in the Data Display Area..............................................................25
8.3.12.Dark Room Mode..........................................................................................................26
8.4. Acquiring Data......................................................................................................... 26
8.4.1. Finding the Spectrometer..............................................................................................26
8.4.2. Selecting with Which Instrument to Work .....................................................................26
8.4.3. Acquiring a Single Spectrum/Interferogram..................................................................27
8.4.4. Repeated Measurements..............................................................................................28
8.4.5. Automatic Setup............................................................................................................28
8.4.6. Averaging Spectra.........................................................................................................28
8.4.7. Averaging Interferograms..............................................................................................28
8.4.8. Signal ............................................................................................................................29
8.4.9. Wavelength Calibration.................................................................................................29
8.5. Movable Markers..................................................................................................... 30
8.5.1. Moving Markers.............................................................................................................31
8.5.2. Marker Values...............................................................................................................31
8.5.3. Locking the Markers......................................................................................................31
8.6. Fixed Markers.......................................................................................................... 31
8.6.1. Adding Fixed Markers...................................................................................................31
8.6.2. Moving Fixed Markers...................................................................................................32
8.6.3. Automatically Mark Peaks.............................................................................................32
8.6.4. Value of Markers...........................................................................................................32
8.7. Working with Data in Traces .................................................................................. 33
8.7.1. Active Trace ..................................................................................................................33
8.7.2. Trace Update Options...................................................................................................33
8.7.3. User-Defined Update Options.......................................................................................34
8.7.4. Viewing Properties of a Trace.......................................................................................35
8.7.5. Reordering Traces ........................................................................................................35
8.7.6. Math Operations............................................................................................................35
8.7.7. Functional Traces (Traces Defined by a Function).......................................................37
8.8. Saving and Loading Data ....................................................................................... 37
8.8.1. Saving Data...................................................................................................................38
8.8.2. Loading Data.................................................................................................................38
8.8.3. Automatically Saving Data............................................................................................38
8.8.4. Replaying Automatically Saved Data............................................................................40
8.9. Analyzing Data......................................................................................................... 40
8.9.1. Floating Windows..........................................................................................................41
8.9.2. Displayed Range and Threshold...................................................................................41
8.9.3. Wavelength Meter.........................................................................................................42
8.9.4. Coherence Length.........................................................................................................43
8.9.5. OSNR Analysis .............................................................................................................43
8.9.6. Peak Track....................................................................................................................44
8.9.7. Valley Track...................................................................................................................46
8.9.8. Statistics Analysis .........................................................................................................47
8.9.9. Curve Fit........................................................................................................................48
8.9.10.Power Analysis..............................................................................................................50
8.9.11.Long-Term Analysis......................................................................................................52

Redstone™ Optical Spectrum Analyzer
8.10. References....................................................................................................... 56
8.10.1.Reference Databases (e.g., HITRAN) ..........................................................................56
8.10.2.Reference Fitting...........................................................................................................58
8.10.3.Baseline Correction.......................................................................................................59
8.11. Keyboard shortcuts......................................................................................... 60
Chapter 9 Settings..........................................................................................................................62
9.1. Basic Settings.......................................................................................................... 62
9.1.1. Active Device Settings ..................................................................................................62
9.1.2. Display Settings ............................................................................................................62
9.1.3. Peak Track Settings......................................................................................................63
9.1.4. System Settings............................................................................................................64
9.1.5. Reset.............................................................................................................................64
9.2. Advanced Settings.................................................................................................. 64
9.2.1. Acquisition Settings.......................................................................................................64
9.2.2. Environment Settings....................................................................................................65
9.2.3. Interferogram Settings...................................................................................................65
9.2.4. Spectrum Settings.........................................................................................................66
9.3. Saving and Loading Settings................................................................................. 67
Chapter 10 Virtual Instruments.......................................................................................................69
Chapter 11 Troubleshooting...........................................................................................................71
Chapter 12 Technical data...............................................................................................................72
12.1. General Specifications.................................................................................... 72
12.2.Update Frequency........................................................................................... 73
12.3. Absolute Maximum Ratings ........................................................................... 73
12.4. Trigger Specifications..................................................................................... 73
12.5. Interlock Specifications.................................................................................. 73
12.6. Internal Sources Specifications..................................................................... 73
Chapter 13 Mechanical Drawings...................................................................................................74
13.1. Dimensions...................................................................................................... 74
13.2. Front and Back Panels.................................................................................... 75
Chapter 14 Certifications and compliances .................................................................................76
Chapter 15 Appendix........................................................................................................................77
15.1. Calculating Absorption Cross Sections........................................................ 77
15.2. Apodization Functions.................................................................................... 78
Chapter 16 Regulatory.....................................................................................................................79
16.1. Waste treatment is your own responsibility................................................. 79
16.2. Ecological background................................................................................... 79
Chapter 17 Thorlabs Worldwide Contacts....................................................................................80

Redstone™ Optical Spectrum Analyzer 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

Redstone™ Optical Spectrum Analyzer Chapter 2: Safety
Rev C, January 21, 2022 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 expost the
unit to any source of excessive moisture.
Only use the instrument standing on its feet.
WARNING
The Redstone OSA305 includes 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.

Redstone™ Optical Spectrum Analyzer Chapter 3: Warranty
Page 3 STN053070-D02
Chapter 3 Warranty
Thorlabs warrants material and production of the Redstone OSA305 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.

Redstone™ Optical Spectrum Analyzer Chapter 4: Description
Rev C, January 21, 2022 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 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 [email protected]m 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. 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 OSA305 model. The
Redstone OSA305 is a more advanced instrument 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 Redstone FT-OSA has an FC/PC-style optical fiber input and a free-space input. After collimating
the input light, 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 160 mm. The collimated light fields then optically
interfere as they recombine at the beamsplitter. The detector units shown in Figure 1 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 bandwidth of the
detectors and the optical coatings. Furthermore, the accuracy of our system is ensured by including an infrared
frequency-locked reference laser. 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.
4.2. Interferometer Design
The Redstone OSA uses a standard arrangement of one fixed and one movable retroreflector as shown in
Figure 1. 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
optical path difference (OPD) between the two branches by twice the mechanical movement of the platform.
The Redstone OSA collects single-sided interferograms, which are asymmetrical in terms of the location of the
point of zero path difference (ZPD), which is located significantly closer to one end of the stroke.
The Redstone OSA305 has a 80 mm stroke length capacity, and thus a maximum OPD of 160 mm and a
spectral resolution of 2.0 GHz or 0.0625 cm-1. The wavelength resolution is dependent on the wavelength of
light being measured. For more details see Section 4.7 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. These spectral resolution numbers should not
be confused with the resolution when using the Wavelength Meter or Peak Track tools, which is considerably
better.
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.

Redstone™ Optical Spectrum Analyzer Chapter 4: Description
Page 5 STN053070-D02
Figure 1. Optical Schematic of the Thorlabs Redstone OSA305
4.3. Interferogram Data Acquisition
The Redstone OSA samples 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 OSA 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 to10 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.
4.4. 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 OSA software 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.

Redstone™ Optical Spectrum Analyzer Chapter 4: Description
Rev C, January 21, 2022 Page 6
4.5. 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 (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 wavelength in 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 sampling frequency (see Section 4.3 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 13 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.
4.6. Wavelength Calibration and Accuracy
4.6.1. TheRedstoneOSA305incorporatesafrequency-lockedreferencelaserwithavacuumwavelengthof
1532.8323nm.Becauseofthefrequency-lock,long-termwavelengthaccuracyisensured,andthereference
laserisconnectedtoanFC/APCoutputandcanbefedintotheinstrumentfiberinputforadditionalcalibration
if so desired; see Section 8.4.9 Measurements of Pulsed Sources
While a Michelson interferometer depends on the input light not changing properties during a measurement, it
is possible to measure pulsed sources with a Thorlabs OSA under certain circumstances. The pulses will
introduce artifacts –so called spectral ghosts –in the spectrum, but if the repetition rate of the input light is
higher than 1 to 7 kHz, depending on the wavelength, the spectral ghosts will be pushed out from the vicinity of
the true peak and the spectrum of the pulsed light will be close to that of a corresponding CW light source, even
using a normal Single or Repeat measurement.
For those cases, in which the spectral ghosts are interfering with the true spectrum, the Pulsed Source
Measurement mode can help improve the result. To start this routine, click the “Pulsed” button located under
the Instrument menu, noting thatthis routine will overwrite the currentsettings for and data in traces A through D.
In order to perform a Pulsed Source Measurement, the software switches to Spectrum view, and sets the
number of averaged traces to 1000. The OSA is then cycled through all available sensitivity modes and rolling
averages of the spectra in each sensitivity mode are stored in traces A through C. Trace D, which represents
the final measured spectrum of the pulsed source, is generated by comparing the three preceding traces and
retaining the minimum measured value in each data point.
The pulsed measurement will run repeatedly until the “Stop” button is clicked. Note that there can be a delay
after the “Stop” button is clicked until the measurement is actually stopped.
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.

Redstone™ Optical Spectrum Analyzer Chapter 4: Description
Page 7 STN053070-D02
Figure 2833. Pulsed Source Measurement Dialog
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 4.5, 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.
4.7. Resolution and Sensitivity
The resolution of this type of instrument depends on the optical path difference (OPD) between the two paths
in the interferometer. In this case it is easier to work with wavenumbers (inverse centimeters) than wavelength
(nanometers) or frequency (terahertz).
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, we would need to move 1 cm away from
the point of zero path difference (ZPD). The Redstone OSA305 can move 16 cm in OPD and thus spectral
features that are 0.0625 cm-1 apart can be resolved. The resolution of the instrument can be calculated as
follows: 
Here, is the resolution in pm, is the OPD in cm-1 and is the wavelength in µm.
The Redstone OSA305 instrument 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 2 for a
visual comparison of the different resolutions for the OSA20xC and Redstone OSA305 instruments.

Redstone™ Optical Spectrum Analyzer Chapter 4: Description
Rev C, January 21, 2022 Page 8
Figure 2. Resolution as a Function of Wavelength
The sensitivity of the instrument depends on the electronic gain used in the sensor electronics. Since an
increased gain setting reduces the bandwidth of the detectors, the instrument will run slower when higher gain
settings are used. Figure 3 and Figure 4 show the dependence of the noise floor on the wavelength and OSA
model.
Figure 3. Noise Floor in Absolute Power Mode
0 2 4 6 8 10 12
100
101
102
103
104
105
OSA20xC, High
OSA20xC, Low
OSA305, Low
OSA305, Medium Low
OSA305, Medium High
OSA305, High
Resolution in Spectrometer Mode
Resolution (pm)
Wavelength (µm)
110
-105
-95
-85
-75
-65
-55
-45
OSA305
OSA201C
OSA205C
OSA203C
OSA207C
OSA202C
OSA Noise Floor in Absolute Power Mode
Noise Floor (dBm)
Wavelength (µm)
0.3 20

Redstone™ Optical Spectrum Analyzer Chapter 4: Description
Page 9 STN053070-D02
Figure 4. Noise Floor in Power Density Mode
4.8. 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 a
given distance from an interesting peak to the power at the peak.
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 5 and the table below provide some example
values for the optical rejection ratio of the 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 = 2.
Values for the OSA205C are included for comparison. All OSA20xC models and the Redstone OSA305 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
46 dB
0.8 nm (100 GHz)
37 dB
52 dB
6.2 nm (800 GHz)
44 dB
58 dB
7.8 nm (1000 GHz)
44 dB
61 dB
110
-90
-80
-70
-60
-50
-40
-30
OSA305
OSA201C
OSA205C
OSA203C
OSA207C
OSA202C
200.3
OSA Noise Floor in Power Density Mode
Noise Floor (dBm/nm)
Wavelength (µm)

Redstone™ Optical Spectrum Analyzer Chapter 4: Description
Rev C, January 21, 2022 Page 10
Figure 5. Spectrum of a 1532 nm Frequency-Locked Laser measured by a Redstone OSA305. The
vertical lines represent where the ORR is measured.
4.9. Absolute Power and Power Density
The vertical axis of the spectrum can be displayed as absolute power or power density, both of which can be
represented in either linear or logarithmic scale. In Absolute Power mode, the total power displayed is based
on the actual instrument resolution for that specific wavelength; this setting is recommended to be used only
with narrow spectrum input light. For broadband devices, it is recommended that the Power Density mode is
used. Here the vertical axis is displayed in units of power per x-axis unit, where the x-axis unit is based upon a
fixed range and is independent of the resolution setting of the instrument.
4.10. 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 OSA software to account for the effect of
the finite path length on the measured interferogram.
The measured, finiteinterferogram can be thought of as an ideal, infinite interferogram thatconsists 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:󰇛󰇜󰇛󰇜. A sinc function is a periodic 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
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)

Redstone™ Optical Spectrum Analyzer Chapter 4: Description
Page 11 STN053070-D02
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 OSA software, please see Section
16.2 Apodization Functions.
4.11. 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
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 OSA software, a zero fill factor (ZFF) of 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.
4.12. 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. Thesoftware makes it easy to locate and trackspectral
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
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.
4.13. Trigger
At the back panel of the Redstone OSA305 instrument, there are four BNC contacts which can be used to
externally trigger a measurement. To do this, the Trigger Type combo box in the Settings menu’s Acquisition
tab must be set to “External.”
•Trigg: When 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 13.4.
4.14. Interlock
The Redstone OSA305 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

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Rev C, January 21, 2022 Page 12
back panel’s interlock jack in place of the shorting device. The electrical specifications of the interlock jack are
listed in Section 13.5.
Note that the Redstone OSA305 requires 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.

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Chapter 5 Using the Free-Space Input
Figure 6. Front Panel of the Redstone OSA305
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 7. Free-Space Input (Left) and FC/PC Input (Right) on the Redstone OSA305 front
5.1. Table Mounting when Using the Free-Space Input
Place the OSA on a flat and stable surface, preferably an optical table. If the free-space input is to be used, it
is recommended to use two Thorlabs CF175 clamping forks on the mounting feet to secure the OSA to the
optical table and reduce any vibrations that might interfere with the measurements.
Figure 8. Using two CF175 Clamping Forks, the Interferometer Assembly can be Locked to the Table

Redstone™ Optical Spectrum Analyzer Chapter 5: Using the Free-Space Input
Rev C, January 21, 2022 Page 14
The Redstone OSA305 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 Redstone OSA305 includes a red alignment laser, which is activated with a button in the “Free-space
Alignment” window (see Figure 9). When the alignment beam is activated, one optical arm of the interferometer
is blocked and no interferometric measurements can be performed, and a warning message will appear in the
OSA software. However, the second optical arm is not blocked and light from this arm is incident on the
detectors. This “DC signal”of the detectors is available when the alignment beam is activated and can be of
assistance in the alignment process.
Note that when the “Free-space Alignment” window is open, the detector offset is disabled and the saturation
level is decreased. When clicking the “Done” button to close the window, the autogain algorithm will try to find
gain and offset settings that optimize the interferogram signal.
Figure 9. Alignment Beam Activation for the Redstone OSA305
The alignment beam should be collinear to the unknown input for optimal measurement accuracy.
To begin the alignment procedure, turn on the alignment beam and proceed with the alignment, following the
instructions in Section 5.3.1. Clicking the “Help” button in the “Free-space Alignment” window in Figure 9 will
open a PDF file with a tutorial with further details and examples.
5.3. Free-Space Beam Alignment Example
5.3.1. Initial Visual Alignment
Figure 10 below shows an example alignment setup, the most important part of which is to include two mirrors
in kinematic mounts. When two mirrors rather than one can be tipped and tilted, the conditions on the position
of the light source are much less stringent. Use the mirror closest to the test source to steer the input source
into the OSA, using a viewing card (such as Thorlabs Item # VRC2) and/or an alignment plate (such as Thorlabs

Redstone™ Optical Spectrum Analyzer Chapter 5: Using the Free-Space Input
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Item # CPA2) to aid in the alignment. Then, use the mirror closest to the OSA to point the alignment beam
emitted by the OSA at the center of the input source. Repeat these steps until the test source beam and
alignment beam overlap over the entire beam path.
When activating the alignment beam on the Redstone OSA, the DC signal of the detectors will be available, the
optimization of which will most often lead to good interferometric signals. Since the DC signals don’t require any
motor motion, the update rate is several Hz and the manual alignment process can thus be completed in a
shorter amount of time.
A portion of the red alignment beam light is coupled out from the Calibration output port on the Redstone OSA
front panel, which can aid in the alignment if the light source is fiber-coupled and emits outside of the visible
range. The test source fiber can then be replaced by a fiber connected to the Calibration output, and the beam
path of the test source is visualized.
If the input test source is sensitive to feedback from the alignment beam, position a suitable dichroic mirror as
close as possible to the test source.
Figure 10. Example Setup for Aligning the Free-Space Input
5.3.2. OSA Software Settings
Once the system is roughly aligned, disable the alignment beam in order to see a spectrum or interferogram in
the main software window. Change the acquisition settings to Low Resolution and Low Sensitivity. Adjust the
gain level(s) such that a strong signal is achieved while not saturating the detector(s); a signal value in the range
of 10% to 80% is suggested. It is recommended to start a time series measurement of the signal and/or power
via the Long-Term functionality (see Section 8.9.11 Long-Term Analysis) to get a reading of how the signal
value and detected input power changes as the two mirrors are adjusted.

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Rev C, January 21, 2022 Page 16
Figure 11. An Example of a Helpful Windows Layout for Free-Space Alignment
5.3.3. Final Alignment
Repeat the alignment process in Section 5.3.1, but utilize the OSA readings instead of the viewing card to
optimize the alignment. Remember to decrease the gain level if any detector gets saturated. The Redstone
OSA305 includes an attenuation filter in front of each detector that can be activated and deactivated with buttons
in the ”Free-space Alignment” window.
5.3.4. Error Estimation
When the signal cannot be optimized further, the wavelength error caused by any remaining misalignment can
be calculated. Measure the offset () between the source exit point and the alignment beam. The wavelength
error is the path difference between the alignment beam and the source beam, and can be estimated as
󰇡󰇡
󰇢󰇢, where is the distance between the source and the detector. Note: this is the total distance,
i.e. including the distance inside the OSA between the free-space aperture and the detector; 64 cm for Redstone
OSA305. It should be possible to have better than 1 mm for = 1000 mm, which yields an error of less than
5 x 10-7 (about 1 - 3 pm depending on the source wavelength range).