Performances of scanning wavelength systems are limited by several factors like the
sweeping range, the mode-hop-free tuning of the wavelength and the nonlinearities in the
sweeping speed. Nonlinearities are probably the last parameter to control to get the least
performances. In our absolute distance interferometer (ADI), we have observed that the
processing technique of the fringes in case of a double target system is very sensitive to
the type of nonlinearities and the final resolution of the ADI depends strongly on their
shape. Although the ideal sweeping speed should be perfectly linear, we have observed
that even in the presence of strong sinusoidal nonlinearities in the sweeping speed,
unexpected good results were obtained and were explained by the fact that these
nonlinearities act like some white noise whose contribution converges to zero as the
number of samples of the processed signal increases. In this paper we focus on the
symmetry of the spectrum, another consequence of these nonlinearities. We show how it
is possible to manipulate the spectrum by changing the sweeping speed. Results of
simulations as well as experimental measurements are presented.
This paper reports the optimization possibilities of some non-linear sources of limitations in the resolution and accuracy
of an Absolute Distance Interferometry setup using an External Cavity Laser Diode for wavelength scanning and a
fibered Mach-Zehnder interferometer as a reference. The system is able to measure one or two simultaneous targets with
a relative uncertainty of some 10<sup>-6</sup> for distances of 1 to 20m. In order to achieve better performances, the experimental
non-linearities in the wavelength sweep are isolated and compared to different simulated sweeping models. This study
leads to the conclusion that accuracy and resolution could be improved by an optimal modulation of the wavelength
sweep. Another sensible point is the drift of the reference Optical Path Difference of the Mach-Zehnder with
temperature variations. This drift can be minimized by using an acrylate-coated fiber and a copper-coated fiber of
different lengths, adjusted by experimental measurements in a climatic chamber for a 10 to 40°C range.
This paper presents new results on a scanning wavelength interferometer for absolute distances measurements. We report its performances on one and two simultaneous targets with a Fourier Transform Technique (FTT) and a regular sampling. Then the limitations of the FTT are discussed, especially with regard to the tunable laser source performances. Finally it concludes on the new results acquired by the use of a non-uniform sampling technique which reduces the non-linearities of the sweeping speed and demonstrates a relative uncertainty of some 10<sup>-6</sup> for distances of 0.25 to 1 m.