We present a precise electric-field-temperature phase diagram of an antiferroelectric liquid crystal with a short pitch Sm-C ␣ * phase. This was obtained by using a photoelastic modulator. A unique field-induced phase was found inside the Sm-C ␣ * phase, which displayed low birefringence. Two tricritical points related to the phase were also observed. In addition, numerical calculations were made based on the discrete phenomenological model. The numerical results reproduced the experimental ones and it was clarified that the phase has a three-layer structure without spatial modulation. Many phases have been found in antiferroelectric smectic liquid crystals. The variety can be ascribed to the interlayer interactions between the tilts of molecules. The Sm-C ␣ * phase with a short pitch helix arises from competition between nearest-neighbor ͑NN͒ and next-nearest-neighbor ͑NNN͒ interactions ͓1͔, and the distorted four-layer phase ͑Sm-C FI2 * ͒ and the distorted three-layer phase ͑Sm-C FI1 * ͒ from NN nonlinear interaction ͓2͔. A phase diagram in a parameter space of coefficients in the free energy was obtained on the basis of a discrete phenomenological model. As the temperature changes, the interactions change so that sequential transitions take place. Consequently, by changing the temperature one can obtain information on the interactions. In addition, using an electric field can also be a good method for investigating the interactions because in chiral smectic liquid crystals the electric field couples with the order parameter representing the amplitude and phase of the molecular tilt in each layer. The electric-field effect has been extensively studied, mainly by using D-E hysteresis loops, optical transmission measurements under ac electric fields, and conoscopic observations ͓3͔. However, these experiments are not detailed enough to precisely detect small orientational changes in molecules over a wide range of temperatures and electric fields.We adopted an optical system using a photoelastic modulator ͑PEM͒, by means of which we could simplify the si-

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