Abstrct-An acoustooptic ultrasonic sensor using a single-mode fiber is discussed. The sensor is based on acoustically induced modal birefringence which alters the polarization state of the optical beam.NTEREST in acoustooptic sensors has continued to increase since the first report on those devices appeared a few years ago [ I ] , [2] . Several types of devices have been proposed based on phase modulation [ l ] , microbending loss [3], [4], polarization rotation [5], and evanescent field coupling [6] ; however, with the exception of the work of Kingsley [7] most of the research and development for those devices has been done at sonic or near sonic frequencies, where the acoustic wavelength is much larger than the fiber diameter. The present study examines an acoustooptic sensing technique at ultrasonic frequencies, where the acoustic wavelength is comparable to or smaller than the fiber diameter.This acoustooptic sensor is based on the evolution of a polarization state in a single-mode fiber caused by ultrasonically induced modal birefringence. This theoretical and experimental investigation shows that ultrasonic waves propagating in a fluid and directly incident upon a single-mode fiber induce anisotropic strains in the fiber of sufficient magnitude to be readily detected.Single-mode fibers are in fact bimodal. They can propagate two nearly degenerate orthogonal polarizations of the HE,,, mode [8] . The ultrasonic wave breaks the near degeneracy by causing an unequal variation in the phase velocity of each eigenmode (modal birefringence) of the optical beam propagating through the fiber. Detection of the induced birefringence with a circular polariscope [ 9 ] , [ 101 can thus form the basis of a sensing system.We have calculated the modal birefringence induced in a single-mode optical fiber of radius "a" by an ultrasonic wave propagating in a fluid by solving the displacement wave equa-where U is the displacement vector, @ the scalar, and *the vector potential. It is necessary to apply four boundary conditions which hold at the surface of the fiber. The displacement and normal stress must be continuous and the tangential stress must be zero at the boundary. For ultrasonic waves of normal incidence, with a propagation constant k = 27r/h, where X is the acoustic wavelength, the induced principal strains are as follows:where err, EBB, and ere are the strains in cylindrical coordinates, and w, is the frequency of the ultrasonic wave.The optical phase shift due to the induced anisotropic strains can be written from the concept of index ellipsoid for the polarized modes as [ 131 The induced linear birefringence is given by [ 131where APlland Ap, are the induced phase shifts in the parallel and perpendicular directions. The elastooptic coefficients are Pll, P12, and P44 = (Pll -P12)/2. The principal strains e l and e2 are shown in Fig. 1 as a function of ka, where ka is a nondimensional frequency constant. The free-space optical wavenumber is k,, the refractive index is no, and I is the acoustooptic interaction ...