We have developped a new kind of all-optical device for measuring electrics fields, based on electric field induced second optical harmonic generation in optically nonlinear polymers (polydia cetylenes). Owing to their centrosymmetry these polymers do not generate spontaneously second optical harmonic. When an external electric field Eo is applied, the centrosymmetry is broken, resulting in second harmonic generation proportional to Et Ieb) . This second harmonic is used as a measure of the field E0. The choice of the polydiacetylene (4BCMU blue form) and the wavelength (1.32 em) is dictated by the large value of x(3) (24);(*),u),0) at this wavelength and low absorption of the second harmonic. The beam of a multimode Q-switch Nd:YAG laser (pulse energy 0.3 mJ duration 1 kis frequency 1 kHz) is focused and guided in a multimode optical fibre (50/125). The film, having a thickness close to the coherence length (C21 em), is deposited under vacuum on a thin silica slab (0.1 mm). This slab is illuminated at normal incidence by the laser radiation emerging from the fibre. Another 50/125 fibre, aligned with the previous one, guides the second harmonic generated in the film. This harmonic is detected by a photomultiplier. As all the elements of the sensor are insulator, there is only a slight distorsion of the electric field. In addition precise field maps are available, owing to the small volume sensor (ne1x1x1 mm3). Measurements performed at 50 Hz show that Iwvaries exactly as E 2 0 . It is possible to measure fields down to 0.5 kV/cm.