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This tab describes the key concepts and implementation of the design used in Thorlabs' Fourier Transform Optical Spectrum Analyzers.
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Schematic of the optical path in Thorlabs' Redstone OSA. Note that the OSA302 does not include the dispersion compensation plate in the interferometer.
Schematic of the optical path in Thorlabs' OSA20xC OSAs, detailing the dual retroreflector design.
Thorlabs' OSA20xC Fourier Transform Optical Spectrum Analyzers (FT-OSA) utilize two retroreflectors, as shown in Figure 1. These retroreflectors are mounted on a voice-coil-driven platform, which dynamically changes the optical path length of the two 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.
The Redstone® OSA30x FT-OSA uses a standard arrangement of one fixed and one movable retroreflector as shown in Figure 2. The mobile retroreflector is mounted on Thorlabs' patented voice-coil-driven platform*, which changes the optical path length of the branch. The fixed retroreflector is mounted on a piezo actuator that translates the retroreflector in the two dimensions orthogonal to the beam path in a self-alignment algorithm. This configuration changes the OPD between the two branches by twice the mechanical movement of the platform.
*The Redstone OSA30x is constructed using Thorlabs' patented voice-coil-driven platform design.
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 up to 40 mm for the OSA20xC and 160 mm for the Redstone. The collimated light fields then optically interfere as they recombine at the beamsplitter.
The detector assemblies shown in Figures 1 and 2 record 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. Note that the Redstone OSA collects asymmetrical interferograms in terms of the location of the point of zero path difference (ZPD), and these interferograms are usually called single-sided; the OSA20xC instruments collect double-sided interferograms. 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 systems is ensured by including a frequency-stabilized 632. nm HeNe reference laser in the OSA20xC models and a frequency-locked . nm infrared reference laser in the Redstone OSA30x models. The reference laser is inserted into the interferometer and closely follows the same path traversed by the unknown input light field. This reference laser provides highly accurate, interferometric measurements of beam path length changes, allowing the systems to continuously self-calibrate and ensure accurate optical analysis well beyond what is possible with a grating based OSA.
OSA Resolution vs. Wavelength of the Unknown InputThe resolution shown here was calculated using the formula to the left. 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. Note that for the OSA20xC instruments, Δk = 1 cmfor Low Resolution Mode and Δk = 0.25 cmfor High Resolution Mode. For the Redstone OSA30x, Low, Medium Low, Medium High, and High Resolutions correspond to Δk's of 4.0, 1.0, 0.25, and 0.063 cm, respectively. Note that the resolution is valid only for the specified wavelength range of the instrument.
The OSA20xC and Redstone OSAs have spectral resolutions of 7.5 GHz (0.25 cm-1) and 1.9 GHz (0.063 cm-1), respectively. 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.
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 usually easier 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, cm-1 and 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 OSA20xC can move ±4 cm in OPD and thus spectral features 0.25 cm-1 apart can be resolved, while the Redstone OSA30x can move 16 cm in OPD and resolve spectral features that are 0.063 cm-1 apart. The resolution of the instrument can be calculated as:
where Δλ is the resolution in pm, Δk is the resolution in cm-1, and λ is the wavelength in µm. The resolution in pm as a function of wavelength, converted using this formula, is shown in Figure 3.
The resolution of the OSA20xC instruments 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 Redstone OSA30x instruments can be set to one of four resolution modes: High, Medium High, Medium Low, or Low. These settings correspond to OPDs of 16 cm, 4 cm, 1 cm, and 2.5 mm, respectively. 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.
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Noise Floor in Absolute Power ModeAbsolute Power mode is recommended for narrowband sources. The OSA203C noise floor was measured in low-temperature mode.
Noise Floor in Power Density ModePower Density mode is recommended for broadband sources. The OSA203C noise floor was measured in low-temperature mode.
The vertical axis of the spectrum can be displayed as Absolute Power (Figure 4) or Power Density (Figure 5), 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.
For the OSA20xC models, 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.
The Redstone OSA30x instruments instead sample the interferogram at a fixed frequency of up to 1 MHz with an 18-bit ADC while the retroreflector platform moves at a constant speed controlled by a PID loop. At the highest sensitivity setting, there is one data point collected about every 1 nm of travel. The collected data is resampled to fit up to 128 data points per reference laser period.
The fine sampling 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 center burst.
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).
The OSA20xC devices make use of 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 OSA20xC to use all of the light that enters the interferometer, while also rejecting common mode noise.
The Redstone OSA detector modules contain high gain-bandwidth product (GBP) amplifiers, which fully exploit the detectors' dynamic range, combined with high-current low-noise buffers driving a high-bandwidth 18-bit ADC. Automatic amplifier adjustments optimize the number of used ADC bits (filling ratio). Switchable optical attenuators provide for a wide input power range even with early saturating IR-detectors. Differential amplifiers, as well as balanced transmission architectures with effective screening and filtering techniques, assure a good signal integrity for a high spurious-free dynamic range.
A Typical Interferogram
The interferograms (see Figure 6) generated by the instrument vary from 50 thousand 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. nm or . 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 (OSA software version 2.90 or lower) or Ciddor's formula (OSA software version 3.0 and higher).
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 (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 laser, 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 on the main OSA presentation 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 OSA20xC instruments use a built-in stabilized HeNe reference laser with a vacuum wavelength of 632. 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 Redstone OSA30x models incorporate a frequency-locked reference laser with a vacuum wavelength of . nm. Because of the frequency-lock, long-term wavelength accuracy is ensured, and the reference laser is connected to an FC/APC output and can be fed into the instrument fiber input for additional calibration if so desired.
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 Edlén formula (OSA software version 2.90 or lower) or Ciddor's formula (OSA software version 3.0 and higher) with temperature and pressure data collected by sensors internal to the instrument.
Optical Rejection Ratio Distance from nm Peak OSA205C OSA305 0.2 nm (25 GHz) 30 dB 40 dB 0.8 nm (100 GHz) 37 dB 42 dB 6.2 nm (800 GHz) 44 dB 45 dB 7.8 nm ( GHz) 44 dB 45 dBThe 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.
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