Origin and Control of Sinusoidal Nonlinearities in Wavelength-Tuned Littman External Cavity Laser Diodes

In a recent paper, we presented the influence of sinusoidal nonlinearities in the wavelength sweeping speed on the performance of an absolute distance interferometry setup. Nonlinearities in an optical frequency modulation system are generally the source of degradation in performance. The interest of this study comes from the experimental observation of the good behavior of an absolute distance measurement system with the presence of sinusoidal nonlinearities in the wavelength sweeping speed obtained from an external cavity laser diode (ECLD). This paper presents experimental results obtained with a custom ECLD based on a Littman-Metcalf setup. We link the sinusoidal nonlinear sweep to some mechanical properties of the cavity. The main source of these nonlinearities comes from the digitization of the piezoelectric transducer excitation. Different types of excitation like super-Gaussian and ramp, including or not sinusoidal modulations, were implemented and analyzed. By this way, we are able to reduce the spectral width by a factor 3 without adding linearization electronics such as a servo-loop control on injection current or nonuniform samplers, and our system achieves a 3 times 10^{- 6} relative uncertainty on distance measurement with a number of samples reduced by a factor 8.