The optical schematic of the Thorlabs FT-OSA detailing the dual retro-reflector design.
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Features

• Michelson Interferometer-Based OSA
• Models Available for 350 - 1100 nm, 600 - 1700, or 1100 - 2500 nm
• Spectral Resolution: 7.5 GHz (0.25 cm^-1)
• Includes Laptop with Pre-Installed Software Including Routine Libraries for LabVIEW and Common Programming Languages
• Standalone "Virtual Device" Software Demo Available (See the Software Tab)

Thorlabs' Optical Spectrum Analyzers (OSAs) are general-purpose instruments that measure optical power of the light source as a function of wavelength. This instrument is versatile enough to analyze the spectra of broadband optical signals, the Fabry-Perot modes of a gain chip, or a long-coherence-length single mode external cavity laser.

Commonly available OSAs are typically grating-based monochromators. The Thorlabs OSA is a Fourier Transform Optical Spectrum Analyzer (FT-OSA), which utilizes a scanning Michelson Interferometer in a push/pull configuration as shown in the figure to the right. This approach allows for the design of a full-featured OSA with the additional benefit of a high precision Wavelength Meter.

The Design Principle

Thorlabs' OSAs have an FC/PC-style optical fiber input. Both single mode and step-index multimode fibers up to Ø50 µm can be used. Special designs with other input receptacles are available upon request. The instruments are designed to measure CW light sources but work in some applications where a pulsed light source is used. Please contact Technical Support to discuss pulsed light source applications.

After collimating the 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 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 the 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 delay range. The wavelength range is limited by the bandwidth of the detectors and optical coatings. Furthermore, the accuracy of our system is ensured by including a frequency-stabilized 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.

Additionally, to reduce the effects of water absorption inside the device, the OSA202 and OSA203 feature a purge inlet on the back panel, through which the interferometer can be purged with dry air or nitrogen. Thorlabs' Pure Air Circulator Unit is ideal for this task.

• After a 30 minute warm-up. Single mode fiber with FC/PC connector. Operating temperature 20 - 30 ºC.
• Specified in parts per million. For instance, if the wavelength being measured is 1 µm, the accuracy will be 2 pm. (2 pm of accuracy for every 1,000,000 pm, or 1 µm, of wavelength)
• Spectral Precision is the repeatability with which a spectral feature can be measured using the peak search tool.
• Can be set from 0-9 decimals and have an auto option that estimates the relevant number of decimals.
• Using the same input single mode fiber for all measurements.
• Limited by the damage threshold of the internal components.
• Level accuracy in Absolute Power Mode, Zero Fill = 2, Apodization = Hann. For the range 400 - 1000 nm for OSA201, 600 - 1600 nm for OSA202, and 1000 - 2400 nm for OSA 203. After a 30 minute warm-up. Single mode fiber with FC/PC connector. Operating temperature 20 - 30 ºC.
• Minimum detectable energy per nanometer using highest resolution, highest sensitivity, and Zero Fill=0.

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The Optical Schematic of the Thorlabs FT-OSA Detailing the Dual Retro-Reflector Design

Interferometer Design

Thorlab's optical spectrum analyzer 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. Each OSA model has a spectral resolution of 7.5 GHz, or 0.25 cm^-1. The resolution in 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 HeNe 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. The interferometer utilizes a dispersion compensation plate to nullify the wavelength-dependent optical path length differences for the two arms of the interferometer, which is mainly attributed to the beamsplitter. To reduce the effect of water absorption, the OSA202 and OSA203 have a purge inlet on the back panel, through which the interferometer can be purged with dry air or nitrogen. Thorlabs' Pure Air Circulator Unit is ideal for this task.

Interferogram Data Acquisition

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 16, 32, 64, or 128. At the 128 multiplier setting, the data points are acquired approximately every 5 nm. The multiple PLL filters enable the user to choose system parameters optimized for measurements that range from high speed, reduced sensitivity, reduced resolution to lower speed, and high sensitivity, high resolution.

A high-speed USB 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 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.

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A Typical Interferogram

Interferogram Data Processing

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 both the data processing and user interface operate optimally.

Wavelength Meter Mode

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, m_o is the number of fringes for the HeNe Reference Laser, m is the number of fringes from the unknown input, n_o is the index of refraction of air at the reference laser wavelength, n_λ is the index of refraction of air at the wavelength λ, and λ _o is the vacuum wavelength of the HeNe reference laser.

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 section on 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 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.

Wavelength Calibration and Accuracy

This FT-OSA instrument incorporates 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 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 Laser in air is actively calculated for each measurement using the Eldén 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 RH to be set manually. The effect of the humidity is negligible in the infrared.

Dynamic Range

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 be defined as the ratio between the power at a given distance from the 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. The table to the right provides some example values for the optical rejection ratio of the OSA203 at 1550 nm with the following settings: High Resolution, Low Sensitivity, Average 4, Apodization Hann. All OSA models show similar behavior if the distance from the peak is measured in GHz.

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 unit wavelength where the unit wavelength is based upon a fixed wavelength band and is independent of the resolution setting of the instrument.

Resolution and Sensitivity

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OSA Resolution vs. Wavelength

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Noise Floor in Power Density Mode

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 narrow-band 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 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  0.25cm^-1 apart. The resolution of the instrument can be calculated as:

where Δλ is the resolution in pm, Δk is the OPD in cm^-1 (maximum 0.25 for this instrument) and λ is the wavelength in µm.

The resolution in the OSA can be set to High and Low in this instrument. In high resolution mode, the retroreflectors move the maximum ±1cm mechanically (±4 cm in OPD) while in low resolution mode, the retroreflectors move ±0.25 cm mechanically (±1 cm in OPD). During setup of the Thorlabs OSA Software, the length of the interferogram that is used in the calculation of the spectrum can be cut to reduce the resolution to the level the user wishes.

The sensitivity of the instrument depends on the electronic gain used in the sensor electronics. Since the increased gain reduces the bandwidth of the detectors, the instrument will run slower when higher gain settings are used. The figure to the right shows 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 it runs 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 128 times for the highest sensitivity mode. This mode can be very useful when the measured light is weak and broadband, hence only a very short interval in the interferogram at the ZPD contains all the spectral information. This is normally referred to as the zero burst.

Free-Space Coupling

To utilize our optical spectrum analyzer in free-space applications, we recommend using a reflective collimator and a Ø50 µm core, 0.22 NA multimode fiber to collect light and transport it to the instrument, as shown below. A single mode patch cable can also be employed to collect the light. This may provide more precise results, but the alignment procedure is far more difficult (see Data Acquisition with a Single Mode FIber Patch Cable, below, for details).

When using the OSA203 with wavelengths above 2 µm, we recommend using Fluoride Single Mode and Multimode Fibers as they offer very good transmission in the mid IR. Our standard silica fibers can also be used, but since their attenuation is high above 2 µm, it is important to minimize attenuation by choosing a fiber with the shortest feasible length (1 m is preferred). For more information about fiber transmission above 2 µm, please contact techsupport@thorlabs.com.

A list of parts used in this setup is available in the tables below. Mouse over the photo to see the corresponding part highlighted in the tables.

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Basic Setup

Thorlabs’ OSA spectrum analyzer can be used to study free-space light sources using a folding mirror pair and the RC08FC-P01 reflective collimator. In this example, a 1532 nm HeNe laser is coupled into the OSA202 Optical Spectrum Analyzer.

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Coupling Equipment

The 1523 nm HeNe laser can be attached to the optical table using a P6 Ø1.5” post and a C1503 Kinematic V-Clamp Mount. The folding mirror pair consists of two Ø1/2” PF05-03-P01 silver mirrors mounted in POLARIS-K05 mirror mounts. The Polaris mounts should be mounted on RS3P8E Ø1”, 3” long posts held to the table with CF125 clamping forks. Mount the RC08FC-P01 reflective collimator using an LMR05 fixed mount, a TR2 Ø1/2” post, and a UPH1.5 post holder. The beam height should be kept as low as possible in order to provide the best alignment stability.

In this example, two output fibers were used: an M42L01 Ø50 µm core multimode FC/PC-to-FC/PC patch cable and a P1-SMF28E-FC-1 single mode FC/PC-to-FC/PC patch cable. Initial coupling alignment should be conducted using the multimode fiber. Once the system is aligned for good coupling efficiency using the multimode fiber, the MM patch cable can be replaced by an SM patch cable, if desired. The system will then need to be tweaked for optimal coupling efficiency.

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Alignment Procedure

Our HLS635 635 nm, 1 mW portable alignment laser, which is a battery-powered 635 nm laser source, can be used to rough-in the alignment of the system. At the start of the alignment, place both the HeNe laser and the reflective collimator at the same optical height as the folding mirror pair; this will minimize the amount of vertical adjustment of the beam path needed.

Mount the collimator and laser parallel to the hole pattern in the table. Plug the laser into the output fiber to run the light backwards through the system. Place the first mirror onto the table so that the laser beam exiting the reflective collimator is incident on it at a 45 degree angle, with the beam exiting the mirror parallel to the optical table's holes. Then, place the second mirror similarly, so that the beam is indicent on the output aperture of the laser. At this point, the clamping forks can be used to secure the each mirror mount's posts to the table, and the system should be close to proper alignment.

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Next, turn on the HeNe laser and view the two laser beams along the optical path using a VRC4 IR viewing card. Adjust the mirrors so that the beams are incident on the same spot on the card at each point along the optical path. In the photo to the right, the small bright beam is from the HeNe laser, while the large red beam is from the alignment laser, incident on the backside of the card.

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Next, measure the power of the free-space beam using the PM200 touch screen power meter and S122C sensor head, mounted on TR2 Ø1/2” posts, a UPH1.5 post holder, and an RA90 angle clamp. In this example, the free-space power of the laser was measured to be 1.55 mW.

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Next, set up the PM200 power meter with the S155C fiber-coupled sensor to measure the output power in the fiber while the alignment of the system is fine tuned.

First, use the tip/tilt controls on one of the folding mirrors to find a maximum signal level. Next, turn the vertical adjustment screw on that mirror mount a quarter-turn, and then use the other folding mirror to find the new maximum. If this power level is higher than the original maximum, then continue this process until an absolute maximum is reached. If the power level was lower than the original level, repeat the same process, but turn the adjustment screw on the first mirror mount in the opposite direction.

Repeat this process for the horizontal adjustment, and then iterate between horizontal and vertical adjustments until an absolute maximum power level is reached. As shown above, in this example, a maximum coupling efficiency of ~80% was reached.

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Data Acquisition

Finally, plug the M42L01 patch cable into the OSA to take data.

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Data Acquisition with a Single Mode Fiber Patch Cable

In some circumstances, using a single mode fiber patch cable may increase the accuracy of the OSA wavelength meter, due to less variations in the optical path length inside the fiber. The alignment procedure is similar with single mode fiber, except that single mode fiber is much more sensitive to errors in alignment. The system should be fully aligned using multimode fiber before switching to single mode fiber. Much smaller adjustment should be made with the folding mirror pair during single mode alignment, and a lower coupling efficiency should be expected.

Here, a P1-SMF28E-FC-1 is being used to take data.

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Results

Here is a screenshot of the OSA software taking data for this experiment. It shows the spectrum of the laser (top), as well as the OSA's wavelength meter.

The 1523 nm HeNe laser line corresponds to the 2s² → 2p¹ transition in Ne I, which has an energy corresponding to a vacuum wavelength of 1523.48765 nm*. In this example, the OSA202 measured a center vacuum wavelength of 1523.488 nm, which is within the specified ±2 pm accuracy of the OSA wavelength meter.

* Information from the NIST Atomic Spectra Database.

Software for the Optical Spectrum Analyzer

Software

Version 1.96

Includes GUI for control of the OSA, as well as a "virtual device" mode ideal for evaluating the software prior to purchase.

Operation

A GUI allows easy operation from a PC connected via USB port to the FT-OSA. The PC records the interferometric signal from the FT-OSA, which is then fast Fourier transformed (FFT) to yield the resulting spectra.

Monochromatic light may be viewed with sub-picometer resolution by utilizing the wavelength meter mode of the FT-OSA. Broadband emission can also be viewed through the OSA's software, which has built-in zoom and peak analysis features. A peak discriminator can select bands that exceed a user-defined intensity and display them according to their wavelength (nm), wavenumber (cm^-1), or frequency (GHz). The instrument has a spectral resolution of 7.5 GHz and a wavelength accuracy better than 2 ppm. In the wavelength meter modes the resolution is 0.1 ppm.

Software

The FT-OSA is shipped with the software package pre-installed on the laptop computer that is included with the purchase of this instrument.

The software has a customizable graphical user interface for acquiring, inspecting, manipulating, and analyzing spectra and interferograms. The software makes it easy to locate and track spectral peaks or valleys, measure the optical input power over any wavelength range, calculate an absorption spectrum in real-time, or track a large number of parameters over time. In addition, the software features a virtual device function that lets users manipulate virtual spectra and test out the features of the software.

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 the user's own application using Labview, C, C++, C#, Java, or other programming languages. Each OSA ships with a set of Labview routines to assist with writing your own applications.

The screen shots below were taken using the included software. Each trace utilized a 1550 nm laser diode and demonstrates some of the various measurements that are possible with the optical spectrum analyzer.

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Figure 1: The peak and total optical power of a 1550 nm gain chip operating well below threshold.

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Figure 2: The ASE spectrum of the same 1550 nm gain chip as in Figure 1. The ripple is caused by Fabry Perot modes in the chip.

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Figure 3: 1550 nm gain chip in an external cavity laser. The software is set up to display the spectrum and the optical power. The wavelength meter mode window is also activated.

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Figure 4: A trace of the acetylene absorption spectrum. The 1550 nm gain chip was used in ASE mode as the source, with the valley search function activated.