A fiber Bragg grating sensor technique is employed to measure two critical properties of elastomers for residual stress evaluation (total effective chemical shrinkage and equilibrium modulus). A novel experimental procedure is developed to cope with technical challenges associated with the extremely low modulus of elastomers. A constraint-free mold is first developed to ensure "free" movement of the elastomer during curing. Then, a special optical fiber with a diameter smaller than the standard fiber is employed in the experiment to make a test configuration unaffected by the free-edge effect. Two critical assumptions made for measurements are verified by an extra specimen with the standard fiber and a conventional tension test. The measured properties are used as input variables to analyze the stress distribution in a potted electrolytic capacitor system using FEM.