A dual beat-frequencies laser Doppler interferometer which can measure high speed back and forth motion is proposed and demonstrated. In this interferometer, the two frequencies, f(1) and f(2), emitted by a Zeeman laser are introduced into the signal beams. They are changed to f(1)' and f(2)' by Doppler shift f(Doppler), which is proportional to the velocity of the object. The use of the two beat signals of |f(1)' - f(2)| and |f(1) - f(2)'| appearing at two photodetectors enables to measure the velocity over the limitation v(c) = λ(f(1) - f(2))/2.

The requirements for evaluating the mechanical characteristics of materials have increased in the various industrial, research and the applications such as materials testing. Therefore, the authors have proposed a method for measuring force acting a material using pendulum [1]. In the Material tester, the mechanical characteristics of material against small force are measured by means of the pendulum mechanism based on the levitation mass method (LMM) [2,3,4].In the LMM, the Doppler shift frequency is measured for measuring the inertial force. The Doppler shift frequency of a laser beam reflected from the mass is accurately measured by using optical interferometer. The velocity, position, acceleration and inertia force of the mass are calculated from the measured time-varying Doppler shift frequency. Thus, the accurate measure of frequency makes that of force.

A testing method for precision force measurement using a small pendulum based on the levitation mass method (LMM) has been developed. In this method, the force acting on the material to be tested is measured using an optical interferometer. In this paper, a collision test of the spring and the pendulum is performed. As the result of the test, the maximum force on the spring was evaluated as approximately 0.533 N. To demonstrate the performance of the developed instrument, the impact responses of the pendulum are accurately evaluated. The possible applications of the developed method are also discussed.