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

Precision improvement in frequency-scanning interferometry based on suppressing nonlinear optical frequency sweeping

“…2), and t p1 À t v0 and t p4 À t v3 are the half periods of the sinusoidal signal; the ratios of ðt p1 À t s Þ=2ðt p1 À t v0 Þ and ðt e À t p3 Þ=2ðt p4 À t v3 Þ refer to the fractional parts of the interference fringe number. However, when the driving signal of the ECLD is linear, the optical frequency tuning rate is not a constant because the ECLD exhibits nonlinear tunability [10][11][12]. The optical frequency tðtÞ can be expressed in a nonlinear form by Taylor expansion as:…”

“…The linearity of the optical frequency curve is changed by adjusting the parameters in Eq. (10). Accordingly, different L sim values can be obtained by using different linearities of optical frequency for the same true value L true .…”

“…However, an ECLD exhibits nonlinearity in optical frequency tuning because of hysteresis and creep of its piezoelectric actuator (PZT) [4,[10][11][12], when the driving signal of the laser is linear. Because of this nonlinearity, extracting the phase of the interference signal may cause errors and make the FSI less precise.…”