Characterization and control of the electro-optic phase dispersion in lithium niobate modulators for wide spectral band interferometry applications in the mid-infrared
Résumé
Mid-infrared wideband modulation (3.2–3.7 μm) is achieved in an electro-optic Y-junction using lithium niobate
waveguides in TE polarized light. Comparison between external (scanning mirror) and internal (electro-optical)
modulation allows studying the chromatic polynomial dependence of the relative phase. Internal modulation
consists on a V AC ramp up to 370 V at 0.25 Hz, applied over 14 mm long electrodes with 14 μm separation.
The overall V πLπ obtained is 17.5 V·cm, meaning that using a 300 V generator we can actively scan and track the
whole L-band (3.4–4.1 μm) wideband fringes. We observe a dramatic reduction of the coherence length under
electro-optic modulation, which is attributed to a strong nonlinear dependence of the electro-optic effect on the
wavelength upon application of such high voltages. We study the effect of applying a V DC offset, from −50 V to
200 V (50 V step). We characterize this dispersion and propose an improved dispersion model that is used to show
active dispersion compensation in wideband fringe modulation in the mid-infrared. This can be useful for long
baseline interferometry or pulse compression applications when light propagates along fibers, in order to compensate
for chromatic effects that induce differential dispersion or pulse spreading, respectively.