An apparatus and method for determining an expanded diameter of a catheter balloon. An optical sensor includes an elongated optical fiber with at least one diffraction grating formed in a core of a distal portion thereof. The fiber distal portion is coupled to the inflatable balloon. Broadband light is transmitted to the diffraction grating by an optical interrogator. A portion of the light is reflected from the diffraction grating and is received by a wavelength detector. The wavelength of the reflected portion of light is dependent on the strain applied to the diffraction grating. The wavelength detector determines the strain in the balloon and correlates the strain to the diameter of the balloon.
