Interferogram Data Processing; Wavelength Meter Mode - THORLABS Redstone OSA305 Manual

Optical Spectrum Analyzers
a higher sensitivity. However, the low temperature also makes the detector insensitive to wavelengths longer
than 2500 nm.
In the high temperature mode, the temperature of the detectors is stabilized at near room temperature. The
thermal noise of the detector is higher in this mode than in the low temperature mode, but the higher temperature
makes the detector sensitive to wavelengths longer than 2500 nm, effectively extending the operating range of
the OSA203C up to 2600 nm.
When using the low temperature mode, the set point of the detector temperature control is –30°C. It is the
default operating mode of the OSA203C and is enabled by default at startup. Also note that the power level
calibration of the OSA203C is only guaranteed between 1000 and 2400 nm.
4.5.

Interferogram Data Processing

The interferograms generated by the instrument vary from 50 thousand to 16 million data points depending on
the resolution and sensitivity settings employed. The ThorSpectra software applies the user-specified
apodization function, analyzes the input data and intelligently selects the optimal FFT algorithm from our internal
library.
Additional software performance is realized by utilizing an asynchronous, multi-threaded approach to collecting
and handling interferogram data through the multitude of processing stages required to yield spectrum
information. The software's multi-threaded architecture manages several operational tasks in parallel by actively
adapting to the PC's capabilities, thus ensuring maximum processor bandwidth utilization. Furthermore, a
specialized GPU is tasked to perform the most advanced calculations. Each of our OSA instruments ships with
a laptop computer that has been carefully selected to ensure optimum data processing and user interface
operation.
4.6.

Wavelength Meter Mode

When narrowband optical signals are analyzed, the OSA automatically calculates the center wavelength of the
input, which can be displayed in a floating window or docked just below the main data display, presenting the
overall spectrum. The central wavelength, ��, is calculated by counting interference fringes (periods in the
interferogram) from both the input and reference lasers according to the following formula:
Here, �� is the number of fringes for the reference laser, ��
�� is the vacuum wavelength of the reference laser (632.9918 nm or 1532.8323 nm), and ��
,
of refraction of air at the reference laser wavelength. ��
�� and is determined iteratively from ��
,
formula as given by NIST
The resolution of the OSA operating as a wavelength meter is substantially higher than the system when it
operates as a broadband spectrometer because the system can resolve a fraction of a fringe, up to the limit set
by the phase-locked loop multiplier or sampling frequency (see Section 4.3 Interferogram Data Acquisition). In
practice, the resolution of the system is limited by the bandwidth and structure of the unknown input, noise in
the detectors, drift in the reference laser, interferometer alignment, and other systematic errors. The system has
been found to offer reliable results as low as 0.1 pm in the visible spectrum and 0.2 pm in the NIR/IR (see
Chapter 11 Technical data for details).
The software evaluates the spectrum of the unknown input in order to determine an appropriate display
resolution. If the data is unreliable, as would be the case for a multiple peak spectrum, the software disables
the wavelength meter mode so it does not provide misleading results.
1
See NIST's Engineering Metrology Toolbox (https://emtoolbox.nist.gov/Wavelength/Documentation.asp,
collected 2021-08-17) for details. Even though the suggested validity range for the formula is 300 - 1700 nm, it
is used by the software for the entire operation wavelength range. As suggested by NIST we assume the CO
concentration to be 450 µmol/mol. Older versions of the Thorlabs OSA software (v2.90 and older) use a modified
version of Edlén's formula, which agrees with Ciddor's formula to less than 10
Page 7
��
��
,
��
,
1 .
��
∙ ∙ ��
,
��
is the number of fringes from the input laser,
is the index of refraction of air at the wavelength
— that is, the measured wavelength in air — using Ciddor's
Chapter 4: Description
is the index
-6 .
STN053070-D02
2