|Click to Enlarge
The Optical Schematic of the Thorlabs FT-OSA Detailing the Dual Retro-Reflector 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.
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, mo is the number of fringes for the HeNe Reference Laser, m is the number of fringes from the unknown input, no is the index of refraction of air at the reference laser wavelength, nλ 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.
|From Peak||Dynamic Range|
|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 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
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.