A ring-locked, phenyltetraene-based, second-order nonlinear optical chromophore (CLD-1) was synthesized. It shows no drop in decomposition temperature compared to its phenyltriene analogue, while the molecular nonlinearity increases by a factor of 2.25 from 6252 × 10-48 to 14065 × 10-48 esu at 1.9 μm. Thin films and waveguide devices were prepared from CLD-1/PMMA and CLD-1/APC (an amorphous polycarbonate) composites to study its electrooptic (EO) activity, optical loss, and photostability. An average EO coefficient of 92 pm/V at 1.06 μm was achieved in a 25 wt % CLD-1/APC composite. Mach−Zender (MZ) modulators fabricated from CLD-1/APC showed good thermal stability, low optical loss, low modulation voltage, and high extinction ratio. The photochemical stability of the CLD-1/APC modulator can be dramatically improved by operating in an inert gas atmosphere. Evidence shows that the predominant photochemical degradation mechanism of the material in air at 1.55 μm is the reaction between oxygen and photoexcited polyene-based chromophores.