With the development of optical techniques, scanning laser vibrometers have been applied successfully in measuring particle velocities and distributions in ultrasonic fields. In this paper, to develop the optical interferometry in measuring focused fields with small amplitude, the "effective" refractive index used for plane waves and extended for spherical waves is presented, the piezo-optic effect as a function of the incident angle of the laser beam is simulated, and the ultrasonic field produced by a concave spherical transducer is calculated numerically around its focal region. To verify the feasibility of the optical method in detecting focused ultrasonic fields, a measurement system was set up that utilized both a scanning laser vibrometer and a membrane hydrophone. Measurements were made in different zones of a focusing transducer, and good results were acquired from the optical interferometry in regions where acoustic waves travel in plane form or spherical form. The data obtained from the optical method are used to reconstruct acoustic fields, and it is found that the focal plane, the maximum pressure, and the beamwidth of the transducer can be forecasted accurately.