NASA's Langley Research Center and Science Applications International Corporation have developed a method of processing data from Fourier transform spectroscopy (FTS) measurements that improves upon existing methods. This method is simpler, more accurate, faster, and less expensive than previous methods. It uses less hardware and can be used with all wavelengths.
In conventional measurements, a reference laser signal runs through the device, is guided to a separate detector, and triggers capture of the spectral signal. This old method restricts usable wavelengths to less than half the frequency of the laser. As part of a modification that does away with this limitation, a mirror slides along the device at a constant speed during the scan. Unavoidable velocity variations require linearization or resampling with respect to a known reference for which a metrology laser is employed. The laser is guided as before, but the signal is sent to a different detector where the timing information is stored and used to mathematically correct the velocity of the original signal in postprocessing. The problem with this approach is that extra hardware, post-processing, and tuning are required, and the process can be somewhat difficult to perform.
Langley's method digitizes the laser signal in a separate channel along with spectra data, which eliminates the hardware required in previous methods. It then demodulates the laser signal with a synthetic quadrature phase detector combined with phase tracking to derive the proper slide position for each data point. This method only requires inexpensive 24-bit audio digitizers, rather than the more expensive event counters of the previous method. The new method does not require tuning, and high-resolution data can be obtained at any wavelength.