A rotatable biprism is introduced in phase mask technique to change the inscribed Bragg of grating.In this system,the gratings are inscribed by the UV interference fringes of 248 nm derived from a rotatable biprism,where the phase mask is used as a beam splitter,and the biprism is rotated to change the intersection angle of two beams.For initializing the reference quantity of Bragg wavelength,the vertex angles of the biprism are determined by the ±1 diffraction angle of phase mask and the refract index of the biprism.Because the change of grating period caused by the asymmetrical rotation of two beams is limited to 5×10-4 nm at a tunability of the Bragg wavelength of ~100 nm,the tilt of grating caused by the rotation of the biprism can be ignorable.As the shift of Bragg wavelength is 1 nm,the maximum rotation angle of the prism is ~1 degree,and the minimum rotation angle is ~3 min.By contrasting with the rotation angle ~23 s/nm of the mirror in Talbot interferometer,the rotation precision of the prisms is decreased by two or three orders of magnitude in this phase mask interferometer.