In order to advance the safety of some important structures that will be required to perform under increasingly stringent conditions, it will be necessary to use materials that are lighter weight, have superior strength, and have the ability to change such parameters as shape, degree of stiffness, and electrical and mechanical properties as needed. These future materials had been named 'smart' material. It is important to measure strain within the materials. A fiber optic sensor may be used to support the necessary sensing functions of these 'smart' materials and has several compelling advantages with respect to electrical alternatives, which are tolerant to corrosion, resisted to disturvance of electric and magnetic, easily to be embedded into composites. Unless the (pi) /2 nonreciprocal phase bias and the phase shifting become a problem, interferometers using single mode fibers as optical propagation paths will be superior. It is well known that the oscillating frequency of coupled cavity semiconductor laser can be modulated by induced current. In this paper we studied the coherent frequency modulated heterodyne interferometer, (used to overcome the problem of (pi) /2 nonreciprocal phase bias, and the phase shifting problem without help of active control by ceramics), and the measuring of strain of smart structures when embedded into materials. The maximum defection range is limited by the coherent length and the dependence of the semiconductor laser.