The photoelasticity of the C a 3 T a G a 3 S i 2 O 14 (CTGS) crystal was studied by an interferometric method based on a single-pass Mach-Zehnder interferometer. The maximum number of sample orientations for the piezo-optic experiments was applied to prove accuracy in the determination of the piezo-optic coefficients. Based on the matrices of the piezo-optic coefficients and the elastic stiffness coefficients, all the coefficients p ik of the elastic-optic matrix are calculated. For the highest p ik coefficient, the acousto-optic efficiency is evaluated. The results obtained for CTGS are compared with the corresponding results for L a 3 G a 5 S i O 14 (langasite) crystals. The highest acousto-optic figure of merit of CTGS M 2 = 1.66 ⋅ 10 − 15 s 3 / k g is two and three times higher, compared with langasite and strontium borate, respectively, which are often used for acousto-optic modulation of light in the ultraviolet spectral range.