We present a precise electric-field-temperature phase diagram of an antiferroelectric liquid crystal with a short pitch Sm-Calpha* phase. This was obtained by using a photoelastic modulator. A unique field-induced phase was found inside the Sm-Calpha* 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.