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Optical Spectrum Analyzers
1.0 - 12.0 µm
600 - 1700 nm
Thorlabs' Optical Spectrum Analyzers (OSAs) perform highly accurate spectral measurements. Compatible with fiber-coupled and free-space light sources, these compact benchtop instruments suit a wide variety of applications, such as analyzing the spectrum of a telecom signal, resolving the Fabry-Perot modes of a gain chip, and identifying gas absorption lines.
Our Optical Spectrum Analyzers acquire the spectrum via Fourier transform, using a scanning Michelson interferometer in a push/pull configuration. This approach enables a high-precision Wavelength Meter mode with seven significant figures and ±1 part-per-million accuracy, allows robust statistical analysis of the acquired spectra, and provides broadband spectral measurements with every scan. Details are provided in the Design tab.
All of Thorlabs' OSAs accept FC/PC-terminated fiber patch cables and collimated free-space beams up to Ø6 mm. Details on compatibility are in the Specs and Free-Space Coupling tabs, respectively. For wavelengths from 2 µm to 5.5 µm, we offer fluoride single mode and fluoride multimode fiber patch cables. Optical Spectrum Analyzers with other fiber input receptacles are available by contacting Tech Support.
These instruments are designed to measure CW light sources. They also work in some applications where a pulsed light source is used; details may be found on the Pulsed Sources tab.
Our stock instruments are not designed for applications where it is necessary to recover small signals, including fluorescence detection and Raman spectroscopy. If your application would benefit from increased detection sensitivity, please refer to the Custom OSAs tab for some of our capabilities.
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To help ensure that our Optical Spectrum Analyzers will meet your application needs, we would be pleased to provide the following:
If you would like to take advantage of any of these services, or if you have feedback or questions, I'd be happy to assist!
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Due to its broad wavelength responsivity, the OSA207C's noise floor is higher than that of our other Optical Spectrum Analyzers, which achieve lower noise floors at the expense of having narrower wavelength ranges. This OSA will easily detect lasers and other narrowband sources, but many broadband sources will not have sufficient power spectral density to be detected. This plot compares the OSA207C's noise floor in Power Density mode to an ideal 1900 K black body and Thorlabs' SLS202L Stabilized Broadband Light Source (which was measured with an OSA205C).
Optical Spectrum Analyzers Comparison
Resolution and Sensitivity Specifications
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The resolution shown here was calculated using the formula explained in the Design tab. Although the formula is valid for all OSA models, the usable wavelength range of each model is limited by the bandwidth of the detectors and optical coatings.
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Absolute Power mode is recommended for narrowband sources. The OSA203C noise floor was measured in low-temperature mode.
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Power Density mode is recommended for broadband sources. The OSA203C noise floor was measured in low-temperature mode.
|Time Between Updates (Update Frequency)|
|Sensitivity||Low Resolution||High Resolution|
|Low||0.5 s (1.9 Hz)||1.8 s (0.6 Hz)|
|Medium Low||0.8 s (1.2 Hz)||2.9 s (0.3 Hz)|
|Medium High||1.5 s (0.7 Hz)||5.2 s (0.2 Hz)|
|High||2.7 s (0.4 Hz)||9.5 s (0.1 Hz)|
The scan sensitivity and resolution are two independent settings controlled from the software. The sensitivity setting modifies the range of detector gain levels, while the resolution setting changes the optical path difference (OPD). For more details, see the Design tab.
This tab describes the key concepts and implementation of the design used in Thorlabs' Optical Spectrum Analyzers.Contents
Thorlabs' Fourier Transform Optical Spectrum Analyzer (FT-OSA) utilizes two retroreflectors, as shown in the figure to the right. These retroreflectors are mounted on a voice-coil-driven platform, which dynamically changes the optical path length of the two arms 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.
After collimating the unknown input, a beamsplitter divides the optical signal into two separate paths. The path length difference between the two paths is varied from 0 to ±40 mm. The collimated light fields then optically interfere as they recombine at the beamsplitter.
The detector assembly shown in the figure to the right records the interference pattern, commonly referred to as an interferogram. This interferogram is the autocorrelation waveform of the input optical spectrum. By applying a Fourier transform to the waveform, the optical spectrum is recovered. The resulting spectrum offers both high resolution and very broad wavelength coverage with a spectral resolution that is related to the optical path difference. The wavelength range is limited by the bandwidth of the detectors and optical coatings. The accuracy of our system is ensured by including a frequency-stabilized (632.991 nm) HeNe reference laser, which acts to provide highly accurate measurements of beam 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.
Each OSA model has a spectral resolution of 7.5 GHz, or 0.25 cm-1. The resolution in units of wavelength is dependent on the wavelength of light being measured. For more details, see the Resolution and Sensitivity section below. 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 operating in the Wavelength Meter mode, which is considerably better.
The Thorlabs FT-OSA utilizes a built-in, actively stabilized reference HeNe 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 presence of water absorption lines in the MIR region of the spectrum, our OSAs feature two quick-connect hose connections (1/4" ID) on the back panel, through which the interferometer can be purged with dry air or nitrogen. Our Pure Air Circulator Unit, which uses hosing that can be directly inserted into these connectors, is ideal for this task.
The resolution of this type of instrument depends on the optical path difference (OPD) between the two paths in the interferometer. It is easiest to understand the resolution in terms of wavenumbers (inverse centimeters), as opposed to wavelength (nanometers) or frequency (terahertz).
Assume we have two narrowband sources, such as 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 away 1 cm from the point of zero path difference (ZPD). The OSA can move ±4 cm in OPD, and so it can resolve spectral features 0.25 cm-1 apart. The resolution of the instrument can be calculated as:
where Δλ is the resolution in pm, Δk is the resolution in cm-1 (maximum of 0.25 cm-1 for this instrument) and λ is the wavelength in µm. The resolution in pm as a function of wavelength, converted using this formula, is shown in the graph to the right.
The resolution of the OSA can be set to High or Low in the main window of the 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 OSA software can cut the length of the interferogram that is used in the calculation of the spectrum in order to remove spectral contributions from high-frequency components.
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. The figures below show the dependency of the noise floor on the wavelength and OSA model.
The OSA is also designed so that it samples more points/OPD when the translation of the retroreflector assembly is slower. The data sampling is triggered by the reference signal from the internal stabilized HeNe laser. A phase-locked loop multiplies the HeNe period up to 128X for the highest sensitivity mode. This mode can be very useful when the measured light is weak and broadband, causing only a very short interval in the interferogram at the ZPD to contain all the spectral information. This portion of the interferogram is normally referred to as the zero burst.
The vertical axis of the spectrum can be displayed as Absolute Power or Power Density, both of which can be displayed in either a 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 unit wavelength, where the unit wavelength is based upon a fixed wavelength band and is independent of the resolution setting of the instrument.
The interference pattern of the reference laser is used to clock a 16-bit analog-to-digital converter (ADC) such that samples are taken at a fixed, equidistant optical path length interval. The HeNe reference fringe period is digitized and its frequency multiplied by a phase-locked loop (PLL), leading to an extremely fine sampling resolution. Multiple PLL filters enable frequency multiplication settings of 16X, 32X, 64X, or 128X. At the 128X multiplier setting, data points are acquired approximately every 1 nm of carriage travel. The multiple PLL filters enable the user to balance the system parameters of resolution and sensitivity against the acquisition time and refresh rate.
A high-speed USB 2.0 link transfers the interferogram for the device under test at 6 MB/s with a ping-pong transfer scheme, enabling the streaming of very large data sets. Once the data is captured, the OSA software, which is highly optimized to take full advantage of modern multi-core processors, 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).
A very low noise and low distortion detector amplifier with automatic gain control provides a large dynamic range, allows optimal use of the ADC, and ensures excellent signal-to-noise (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. The balanced detection architecture enhances the SNR of the system by enabling the Thorlabs FT-OSA to use all of the light that enters the interferometer, while also rejecting common mode noise.
The interferograms generated by the instrument vary from 0.5 million to 16 million data points depending on the resolution and sensitivity mode settings employed. The FT-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. Each of our FT-OSA instruments ships complete with a laptop computer that has been carefully selected to ensure that both the data processing and user interface operate optimally.
When narrowband optical signals are analyzed, the FT-OSA automatically calculates the center wavelength of the input, which can be displayed in a window just below the main display that presents the overall spectrum. The central wavelength, λ, is calculated by counting interference fringes (periods in the interferogram) from both the input and reference lasers according to the following formula:
Here, mref is the number of fringes for the reference laser, mmeas is the number of fringes from the input laser, nref is the index of refraction of air at the reference laser wavelength (632.991 nm), and λref,vac is the vacuum wavelength of the reference laser. nmeas is the index of refraction of air at the wavelength λmeas,vac and is determined iteratively from λmeas,air (that is, the measured wavelength in air) using a modified version of the Edlén formula.
The resolution of the FT-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 (see the Interferogram Data Acquisition section above). 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 HeNe, interferometer alignment, and other systematic errors. In Wavelength Meter mode, 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 the Specs tab 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.
The FT-OSA instruments incorporate a stabilized HeNe reference laser with a vacuum wavelength of 632.991 nm. The use of a stabilized HeNe ensures long-term wavelength accuracy as the dynamics of the stabilized HeNe are well-known and controlled. The instrument is factory-aligned so that the reference HeNe 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. The wavelength of the reference HeNe in air is actively calculated for each measurement using the Edlén formula with temperature and pressure data collected by sensors internal to the instrument.
For customers operating in the visible spectrum, the influence of relative humidity (RH) on the refractive index of air can affect the accuracy of the measurements. To compensate for this, the software allows the assumed RH value to be set manually. The effect of the humidity is negligible in the infrared.
|Distance from 1550 nm Peak||Optical Rejection Ratio|
|0.2 nm (25 GHz)||30 dB|
|0.4 nm (50 GHz)||30 dB|
|0.8 nm (100 GHz)||30 dB|
|4 nm (500 GHz)||39 dB|
|8 nm (1000 GHz)||43 dB|
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 between the power at a given distance from the peak and 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 be limited by the OSA's response, rather than reflecting a true property of the tested source. The table to the right provides an example.
Thorlabs' OSAs each include a free-space optical input aperture, allowing them to directly accept collimated beams up to a maximum beam size of Ø6 mm. To align the input light source with respect to the OSA's internal interferometer, a red, Class 1 alignment beam, which is activated by a rotating switch, is emitted from the aperture. The input source should be made collinear to the alignment beam for the OSA to provide optimal measurement accuracy. Four 4-40 taps around the input aperture enable compatibility with our 30 mm cage system; use cage rods no shorter than 1.5" to prevent attached cage components from clashing with the door.
The interferometer assembly normally "floats" on gel bushings inside the case. When using the free-space input, it may be desirable to lock the interferometer to the optical table surface. This can be accomplished by using the provided mounting feet (see the Shipping List tab) to secure the OSA using two of Thorlabs' CF175C(/M) clamping forks, as illustrated by the images below.
When the interferometer is locked to an optical table, the beam height is 61 mm (2.4") from the table surface. To adjust the input beam height to that of the OSA's input, we recommend using Thorlabs' RS99(/M) Periscope Assembly or a periscope constructed with our DP14A(/M) Damped Post.
We recommend only using the posts supplied with the OSA to secure it to the optical table. Other posts, such as our Ø1/2" optical posts, should not be used to secure the OSA; they will not provide adequate support because the OSA weighs ~20 lbs (~10 kg). We also do not recommend using long optical posts to raise the OSA off of the optical table surface.
Each Optical Spectrum Analyzer includes a Windows® laptop with our OSA software suite pre-installed. This software features a straightforward, intuitive, responsive interface that exposes all functions in one or two clicks. We regularly update this software to add significant new features and make improvements suggested by our users. Several key functions are explained in the Tutorial Videos tab.
The software download page also offers programming reference notes for interfacing with our Optical Spectrum Analyzers using LabVIEW™, Visual C++, Visual C#, and Visual Basic. Please see the Programming Reference tab on the software download page for more information and download links.
This software package is also compatible with Thorlabs' Compact CCD Spectrometers.
The text below summarizes several key features of the OSA software suite. Complete details on the software are available from the manual (PDF link).
Built-In Tools for Simple and Complex Analysis
The OSA software displays either the fast-Fourier-transformed spectrum or the raw interferogram obtained by the instrument. In the main window, it is possible to average multiple spectra; display the X axis in units of nm, cm-1, THz, or eV; compare the live spectrum to previously saved traces; perform algebraic manipulations on data; and calculate common quantities such as transmittance and absorbance.
Robust graph manipulation tools include automatic and manual scaling of the displayed portion of the trace and markers for determining exact data values and visualizing data boundaries. Automated peak and valley tracking modules (see the screenshot to the right) identify up to 2048 peaks or valleys within a user-defined wavelength range and follow them over a long period of time. Statistical parameters of traces such as standard deviations, RMS values, and weighted averages are available, and a curve fit module fits polynomials, Gaussians, and Lorentzians to the spectrum or interferogram.
Acquired data can be saved as a spectrum file that can be loaded quickly into the main window. Data can also be exported into Matlab, Galactic SPC, CSV, and text formats.
Adjustable Sensitivity and Resolution Settings
The scan sensitivity and resolution can be adjusted by the user to balance the needs of the experiment against the data acquisition rate. These settings vary the number of data points per interferogram from 0.5 million to 16 million. The sensitivity setting modifies the range of detector gain levels, while the resolution setting controls the optical path difference (OPD). The table in the Specs tab shows how the data acquisition rate depends upon the chosen settings.
Wavelength Meter Module for Narrowband Sources
For sources with <10 GHz linewidth, the Wavelength Meter module enables extremely accurate determinations of the center wavelength (±1 ppm accuracy, 0.2 ppm precision, and 0.1 ppm resolution). This mode allows the system to resolve a fraction of a fringe in the interferogram, using the phase-locked loop that is generated by the internal stabilized reference HeNe laser (see Interferogram Data Acquisition in the Design tab for details). The uncertainty in the measurement is continuously determined and displayed as gray numbers.
As shown in the image to the right, a built-in module plots the output of the wavelength meter measurement as a function of time. If the software determines that the wavelength meter will give inaccurate results (as it would for broadband sources), it is automatically disabled.
Coherence Length Module for Broadband Sources
Because Thorlabs' OSAs obtain the raw interferogram of the unknown source (as opposed to grating-based spectrum analyzers, which cannot offer this capability), the software is able to calculate the coherence length of the input signal, as shown by the screenshot to the right. The Coherence Length module considers the envelope of the interferogram and reports the optical path length over which the envelope's amplitude decays to 1/e of its maximum value on both sides.
The ability to view the raw interferogram in real time allows the user to confirm the coherence length reported by the software and adjust the signal amplitude to avoid saturation. The maximum coherence length measurable by the OSA is limited by the maximum optical path difference of ±4 cm in high-resolution mode, making this module best suited for broadband sources.
Apodization and Interferogram Truncation
Since the resolution of any Fourier-transformed spectrum is intrinsically constrained by the finite path length over which the interferogram is measured, the software implements several functions to account for the effect of the finite path length on the spectrum that is obtained. The user may select from a number of apodization methods (dampening functions), including cosine, triangular, Blackman-Harris, Gaussian, Hamming, Hann, and Norton-Beer functions, and the effective optical path length can also be shortened to eliminate contributions from high-frequency spectral components.
Libraries for LabVIEW, C, C++, C#, and Java
Device interface libraries containing a multitude of routines for data acquisition, instrument control, and spectral processing and manipulation are also provided with the instrument. These libraries can be used to develop customized software using LabVIEW, C, C++, C#, Java, or other programming languages. We also provide a set of LabVIEW routines to assist with writing your own applications.
Spectroscopic Analysis from HITRAN Reference Database
In environmental sensing and telecom applications, it is often useful to identify atmospheric compounds (such as water vapor, carbon dioxide, and acetylene) whose absorption lines overlap with that of the unknown source being measured. Some example measurements are shown below. The OSA software includes built-in support for HITRAN line-by-line references, which can be used to calculate absorption cross sections as a function of vapor pressure and temperature. The predictions can be fit to the measured trace for comparison, and fits using mixtures of gases are supported. See the Gas Spectroscopy tab for an example setup.
To help customers learn about, use, and understand the Optical Spectrum Analyzer software, we have prepared several short narrated videos that describe the basic aspects of the software and the optimal settings for common types of measurements. Although the OSA model shown in the videos has been discontinued, the principles of operation have not changed.
Full Screen, 720p Resolution Recommended
In order to be able to read the text in the videos, we strongly recommend viewing these videos at full screen, 720p resolution. To expand the video to full screen, click on the button shown in the screenshot above. Pressing the Escape key will restore the video to its original size. To choose 720p resolution, use the Quality menu, which appears after clicking on the gear icon, as shown by the screenshot to the right.
Basic Features of OSA Software
Tips for Choosing the Best Acquisition Settings
Measuring a Narrowband Source
Measuring Optical Input Power
Performing a Filter Measurement
Every OSA order includes the following:
Introduction and Summary of Results
While Thorlabs' Optical Spectrum Analyzers (OSAs) have been designed for analysis of CW signals, it is possible to measure pulsed spectra under certain situations. Measurement of pulsed spectra suffers from several issues that must be overcome for accurate measurements; for instance, "spectral ghosts" arise due to the pulsed nature of the source as well as the varying optical path difference (OPD) of the OSA. In addition, the noise floor for pulsed sources is much higher than that for CW sources. One method for measuring pulsed sources with the OSA involves taking several successive measurements at the four different sensitivity levels; the minimum at each wavelength of these four traces is used to form a combined spectrum, which suppresses the spectral ghosts. This technique is implemented in the OSA software by choosing "Pulsed" under the "Sweep" tab. The following tutorial explains the rationale of this technique and the pulsed sources for which it is useful.
In summary, for pulse rates over 30 kHz, standard mode can be used because the repetition rate is greater than the detectors' bandwidth. For broadband signals with low repetition rates, care must be taken to ensure that the "zero burst" of the interferogram coincides with one of the pulses. Also, when using a pulsed source "Automatic Gain" does not work properly, so the user must monitor the interferogram and manually set the gain so that a strong, but not saturated, signal is obtained.
Impact of a Pulsed Source on the Interferogram and Spectrum
As the Optical Path Difference (OPD) continuously changes during an interferogram measurement, a pulsed light source effectively modulates the interferogram. In the case of 100% modulation (i.e. on-off pulsation), the resulting interferogram will contain repetitive regions (slots) with no information. These slots correspond to OPDs when no light can be measured by the detector assembly. The resulting interferogram in this case is the true interferogram masked with the pulsed signal. Figure 1 shows measured interferograms and the corresponding spectra for a light source in CW and pulsed operation. Although the spectrum of the light source is expected to be the same for CW and pulsed operation (ignoring small changes in the peak shape and position due to, for example, a decreased LD chip temperature resulting from the pulsed drive), additional frequency artifacts appear symmetrically about the expected peak due to the modulation in the pulsed interferogram. These "spectral ghosts" are a result of the temporal, rather than the spectral, behavior of the source. To measure the true spectrum of the light source, it is crucial to make the spectral ghosts sufficiently small or force the spectral ghosts to fall outside the frequency / wavelength range of interest.
Figure 1: Measured interferograms and spectra for a narrowband light source in CW (Top) and pulsed at 20 kHz (Bottom)
operation. The square wave modulation of the interferogram induces the spectral ghosts shown in the bottom left plot.
Mathematically, the resultant spectrum of a pulsed source can be described by a convolution between the spectrum of the light source and the spectrum corresponding to the pulses. As a result, the impact of these artifacts will vary with the pulse repetition rate and the modulation depth of the light source as well as the OPD sample rate (cm/s) of the OSA. The modulation depth of the light source determines the amplitude of the spectral ghosts; a weak modulation yields weak spectral ghosts while a modulation of 100% (on-off pulsation) yields the strongest spectral ghosts.
Figure 2 shows how the behavior of the spectral ghosts as a function of the pulse repetition rate for a narrowband source. In the figure, the spectra were measured for 55 pulse repetition rates between 100 Hz and 100 kHz for a 1550 nm DFB laser diode. We have offset the y-axis such that the true peak (the light gray horizontal line) has been centered at a relative frequency of 0 THz. The figure can be divided into three regions: fp ≤ 3 kHz, 3 kHz < fp ≤ 30 kHz and fp >30 kHz. For fp ≤ 3 kHz, the spectral ghosts are clearly observed symmetrically about the true peak within the resultant spectrum, and move farther and farther away from the true peak as the repetition rate increases. The second region starts above 3 kHz, when the first spectral ghosts have moved beyond the spectral range of the OSA. However, aliasing / folding create higher order spectral ghosts that appear within the spectral range of the OSA. In the third region, fp > 30 kHz, the resulting spectrum agrees very well with the CW spectrum because the repetition rate of the source has extended beyond the bandwidth limit of the detectors. As a result, the pulsed source appears like a CW source to the OSA electronics.
"Pulsed Mode" Operation
To help remove some of these frequency artifacts, the OSA software contains a "Pulsed Mode" measurement (Figure 3). The "slot period" of the interferogram, determined by the pulse repetition rate of the light source and the OPD rate of the OSA, affects the positions of the spectral ghosts. A shorter slot period yields a larger spectral distance between the true peak and the first order ghost peaks. In Thorlabs' OSAs, the OPD sample rate is given by the speed of the moving carriage which can be controlled by the user indirectly through the sensitivity setting. The higher the sensitivity setting is, the speed of the moving carriage will be slower. Thus, the use of the "High" sensitivity mode of the OSA will provide the shortest slot period (i.e. the largest spacing between the feature of interest and the frequency artifacts). In pulsed mode, the software acquires four spectra with different sensitivity settings (or OPD sample rates) and filters out the changing spectral features. The sensitivity is first set to low, followed by Medium-Low, Medium-High, and High before it again is set to Low yielding a periodically changing sensitivity. The captured spectra are then combined using the minimum hold function. The spectral ghosts (Figure 4), whose positions depend on the sensitivity setting (the OPD rate), can then be reduced in the measurement as shown in Figure 4. It is important to note that the Pulse Mode button is found under the "Sweep" menu and can be started only after the current sweep has been completely stopped.
Narrowband Light Source
A DFB laser diode emitting at 1550 nm (193.7 THz) was used as a narrowband light source and measured with an OSA203C in both CW and pulsed operation. The laser diode was modulated (using Thorlabs' ITC4001 controller) with repetition rates between fp = 20 Hz and 100 kHz. Five averaged spectra were captured for each light source setting; the CW spectra were acquired in high sensitivity mode, and the pulsed spectra were recorded in both high sensitivity and pulsed mode. It is important to note that the pulsed mode does not allow averaging. Instead the minimum hold function was used for 5 sets of spectra from the four different sensitivity settings.
Figure 5 shows the resultant spectra for the source in CW mode as well as four different pulse repetition rates between 100 Hz and 100 kHz. As the pulse rate increases, the spectral ghosts (as recorded in the high sensitivity mode) move further and further away from the true laser peak until nearly identical spectra are obtained at 100 kHz.
Figure 5: Spectra from measurements of a 1550 nm (193.7 THz) pulsed narrowband source. Pulse repetition rates shown (left to right): 100 Hz, 1 kHz, 13 kHz, and 100 kHz. Black line: CW measurement; blue line: pulsed source measured with high sensitivity; red line: pulsed source measured using the pulsed mode. The lower plots are the same data set as the upper plots only on a shorter frequency scale.
Broadband Light Source
A gain chip was driven in amplified spontaneous emission (ASE) mode to create a broadband light source centered at 850 nm (352.9 THz) with a FWHM of 36.4 nm (15.2 THz). An OSA201C was used to measure the spectrum for CW and pulsed operation with pulse repetition rates from fp = 100 Hz to 100 kHz. The ASE diode was modulated (using Thorlabs' ITC4001 controller) with a 50% duty cycle square wave. A total of 10 averaged spectra were acquired using high sensitivity (CW and pulsed sources) and the pulsed mode (pulsed source). Because pulsed mode does not allow averaging, the minimum hold function was used to acquire five sets of the four different sensitivity settings.
In general, the spectral ghosts are less visible for the broadband peak compared to a narrowband peak. However, the noise floor is higher and the spectral ghosts are clearly seen for a repetition rate of 1 kHz and 13 kHz in Figure 6. Similar to the narrowband source, the spectral ghosts move farther and farther away from the true peak with increasing repetition rate. For a repetition rate of 100 kHz both the measurement using high sensitivity and pulsed mode agree well with the CW measurement. As seen, the shape of the peak is slightly different for the CW spectrum compared to the pulsed spectrum. This is not related to the behavior of the OSA but due to a true change in the peak during pulsed operation, e.g., a lower chip temperature.
Figure 6: Measured spectra from a pulsed broadband source with a center wavelength (frequency) of 850 nm (352.9 THz). The pulse repetition rates shown are 100 Hz, 1 kHz, 13 kHz, and 100 kHz. Top and bottom rows show the full spectrum and the ±50 THz range surrounding the peak, respectively. Black line: CW; blue line: pulsed source measured using high sensitivity; red line: Pulsed Mode.
It is extremely important to note that in general, one has to be careful when measuring broadband peaks at low repetition rates. Since most of the information in the interferogram is located about the zero burst, the peak can be completely missed if the zero burst coincides with no light falling on the detector as shown in Figure 7.
Figure 7: Measured interferograms (left) and spectra (right) obtained when the zero burst resulting from a broadband
source coincides with a pulse (blue curves) and is missed if no light reaches the detector at OBD ~ 0 (red curves).
Femtosecond Pulsed Laser
We measured the spectrum of a broadband femtosecond laser (Thorlabs' OCTAVIUS-85M-HP) using an OSA201C. This laser has a repetition rate of 85 MHz, a pulse width of 10 fs, and an average power of about 300 µW into the fiber. The OSA was set to Low Resolution, High Sensitivity, 5 spectral averages, and no apodization. Light output from the laser was collected with a fiber patch cable (SM600 fiber; 0.12 NA, 4.6 µm mode field diameter at 680 nm) connected to the OSA.
Figure 8 shows the interferogram collected during acquisition, which does not contain any empty slots. This was expected as the 85 MHz repetition rate of the laser is well beyond the 40 kHz bandwidth of the OSA's detectors. Furthermore, the spectrum measured by the OSA agrees very well with the reference spectrum captured using a grating-based OSA that is scanned slowly enough to provide adequate signal for each wavelength measured.
|Item #||Frequency Range||Level Sensitivity
(Click for Graph)a
|OSA207C||833 - 10 000 cm-1
(12.0 - 1.0 µm)
|OSA205C||1786 - 10 000 cm-1
(5.6 - 1.0 µm)
|OSA203C||3846 - 10 000 cm-1
(2.6 - 1.0 µm)
As shown in the table to the right, many of Thorlabs' Optical Spectrum Analyzers (OSAs) offer detection extending into the mid-infrared (MIR) region of the spectrum, where many gaseous species characteristically absorb. Moreover, the software included with all OSA models supports files from the HITRAN database, a spectroscopic reference standard. These files can be fit to measured traces to identify unknown gases. With the ability to fit multiple analytes simultaneously and built-in hose connections (compatible with Thorlabs' Pure Air Circulator Unit) for purging the interferometer's cavity of trace gases, these OSAs are ideal for use in home-built gas detection setups.
A sample detection setup is shown below. Broadband MIR light generated by a Stabilized Light Source is emitted from a zirconium fluoride fiber (), collimated, then sent into a multipass cell () containing the gas analyte in a sample chamber. Each end of the chamber is sealed by an airtight, transparent window. Gold mirrors on each side of the chamber provide multiple reflections that increase the sensitivity of the measurement; the mirror closer to the light source has a center hole to allow the optical path to enter and exit the chamber. Light exiting the detection setup is collimated by a long-focal-length lens and reflected by a D-shaped mirror into the free-space port of the OSA203C (). The temperature inside the chamber is elevated and held constant in order to prevent the gas's absorption lines from shifting during the measurement.
|Parts Used in Sample Setup
(Click Here for a Metric Item List)
|SLS202L||1||Stabilized Fiber-Coupled Light Source, 450 nm - 5.5 µm
|FB2000-500||1||Ø1" Bandpass Filter, 2.0 µm CWL, 0.5 µm FWHM (Not Shown)|
|MZ21L1||1||ZrF4 Multimode Fiber Patch Cable, SMA905 Connectors|
|F028SMA-2000||1||SMA905 Fiber Collimator, AR Coated: 1.8 - 3.0 µm|
|POLARIS-K1||1||Polaris™ Ø1" Kinematic Mirror Mount|
|AD11NT||1||Unthreaded Adapter for Ø11 mm Cylindrical Components|
|OSA203C||1||Optical Spectrum Analyzer, 1.0 - 2.6 µm|
|MB1218||1||12" x 18" Aluminum Breadboard|
|CF125C||3||Clamping Fork with Captive Screw|
|RS2||6||Ø1" Pillar Post, Length = 2"|
|RS3||1||Ø1" Pillar Post, Length = 3"|
|RS4||2||Ø1" Pillar Post, Length = 4"|
|BA2F||9||Flexure Clamping Base|
|Parts Used in Sample Setup (Continued)
(Click Here for a Metric Item List)
|Beam Path Into and Out of Multipass Cell|
|LB4374||1||Uncoated, Ø1", f = 1000 mm Bi-Convex UV Fused Silica Lens|
|CP33||1||Post-Mountable, SM1-Threaded Cage Plate for Ø1" Optics|
|CM750-200-M01||2||Ø75 mm, f = 200 mm Protected Gold Concave Mirror
(One Mirror Contains a Center Hole, Similar
to Our Herriott Cell Mirrors)
|KS3||2||Kinematic Mount for Ø3" Mirrors|
|VPCH512||2||Ø2.75" CF Flange with CaF2 Window, 180 nm - 8.0 µm|
|C1513||1||Kinematic V-Clamp Mount|
(One Clamping Arm is Included with Each C1513 Mount)
|P6||1||Ø1.5" Mounting Post, Length = 6"|
|PB2||1||Base for Ø1.5" Mounting Posts|
|PFD10-03-M01||1||1" Protected Gold D-Shaped Pickoff Mirror|
|KM100D||1||Kinematic Mount for 1" D-Shaped Pickoff Mirrors|
|MB624||1||6" x 24" Aluminum Breadboard|
Assigning Peaks in an Unknown Spectrum
Once the experimental spectrum is obtained, the user chooses a gas or gas mixture that is believed to be present inside the sample chamber, as shown in the figure below to the left. There is no limit to how many species can be considered in the fit, but the fit is more likely to converge when fewer species are chosen. The OSA software ships with HITRAN line-by-line references for acetylene (C2H2), water vapor (H2O), and carbon dioxide (CO2), and can import additional references downloaded from the HITRAN database. Previously saved spectra in the OSA file format can also be used as references. See the References section of the OSA manual for details.
The user may optionally allow the software to shift the reference spectrum in wavelength in order to account for measurement effects related to the sample environment. In the case of gas mixtures (i.e., fits performed using more than one reference spectrum), the software scales the intensity of each reference as needed to reproduce the measured spectrum. As shown in the figure below to the right, the output of the fit operation is a graph comparing the measured spectrum, each scaled (and possibly also shifted) reference spectrum, and the sum of the scaled reference spectra.
Thorlabs' in-stock OSA models offer a number of detection options for various experimental situations. We invite customers whose needs are not addressed by these models to tailor an OSA to a specific application by working with our engineering and manufacturing team.
In the past, we have built OSAs with user-specified optical inputs, such as FC/APC and SMA905 fiber receptacles, and we have incorporated optical bandpass and notch filters directly into the optical path to reduce light source noise. For customers who use these instruments for sample characterization, our software team has implemented user-designed data analysis modules within the standard OSA software suite.
We have also worked with our customers to choose detector elements targeted at specific light sources and analytes. The graphs below were obtained from custom-built OSAs that were designed for especially high detection sensitivity. Our engineers are well-versed in the tradeoffs between detection bandwidth, sensitivity, and linearity, and can make recommendations based upon the needs of the application and prior customers' experiences. By constraining the OSA's design for a particular use case, additional performance enhancements for that application can be realized.
If you would like to discuss a custom OSA, please contact us with your experimental requirements.
Thorlabs' OSAs measure the optical power of both narrowband and broadband sources as a function of wavelength. The maximum spectral resolution of 7.5 GHz (0.25 cm-1) is set by the maximum optical path length difference of ±4 cm, as explained in the Design tab, while the high spectral accuracy of ±2 ppm (parts per million) is ensured by simultaneously measuring the interferogram of a stabilized 632.991 nm HeNe laser. For sources with linewidth < 10 GHz, the Wavelength Meter mode provides center wavelength measurements with 0.1 ppm resolution and ±1 ppm accuracy.
Fiber-Coupled and Free-Space Inputs
All of our OSAs directly accept fiber-coupled or free-space optical inputs. The fiber-coupled input is compatible with single mode and multimode FC/PC patch cables. For multimode patch cables made from standard silica glass, cores up to Ø50 µm and NA up to 0.22 are recommended; for those made from fluoride glass, cores up to Ø100 µm and NA up to 0.26 are recommended. Single mode patch cables provide the highest contrast. OSAs with other fiber input receptacles are available by contacting Tech Support.
The free-space input aperture accepts collimated input beams up to a maximum beam size of Ø6 mm. To align the input light with respect to the OSA's internal interferometer, a red, Class 1 alignment beam, which is activated by a rotating switch, is emitted from the aperture. (See 2:47 in the video above for a demonstration.) The input beam should be made collinear to the alignment beam for the OSA to provide optimal measurement accuracy. Four 4-40 taps around the input aperture enable compatibility with our 30 mm cage system; use cage rods no shorter than 1.5" to prevent attached cage components from clashing with the door.
Hose Inlets for Optional Purging
To reduce the presence of water absorption lines in the measured spectrum, our OSAs offer two 1/4" ID quick-connect hose connections on the back panel, through which the interferometer can be purged with dry air. Thorlabs' Pure Air Circulator Unit is ideal for this task. Purging the OSA is not generally necessary, since none of the optics are made from hygroscopic materials. An example spectroscopy setup is described in the Gas Spectroscopy tab above.