Abstract: The value of a transverse moment of any order of the electromagnetic field associated with an optical beam (and in particular the spot-size of such a beam) is determined by two successive measurements of the optical power of the beam: one measurement is a direct measurement, the other is effected after spatially modulating the transverse distribution of the electromagnetic field by a factor proportional to the n-th power of the transverse coordinate of the beam, n being the moment order. The moment can be obtained from the ratio between the two measurements. The invention includes also the apparatus for carrying out the method.

Abstract: A constant force perpendicular to the fiber axis is applied to the fiber so as to cause a power coupling from the fundamental mode, which is guided, to a secondary mode which is irradiated, and the intensity of the scattered radiation associated with that secondary mode is measured. Such intensity depends on the local state of polarization. By dispacing the force application point step by step along the fiber axis and by measuring for each point the intensity of the scattered radiation, beat length is obtained as the distance between two consecutive points where the scattered radiation has the same intensity.

Abstract: A constant force perpendicular to the fiber axis is applied to the fiber and is displaced step by step along the fiber axis. The state of polarization of a radiation outgoing from the fiber is determined at each step and the beat-length values are obtained from both the amplitude ratio and the phase difference between the components, along the two birefringence axes, of the electrical field associated with a radiation launched into the fiber.

Abstract: Mode field radius in single-mode optical fibers is measured by the modulation of an optical beam by means of a spatial filter with radial-distribution of transmittivity of Gaussian type. Modulated beam intensity is measured in correspondence with the beam axis for different widths of the Gaussian curve, and the mode field radius is derived from both such intensity values and a parameter dependent on the Gaussian width.

Abstract: An optical beam is injected into a single-mode fiber of a given length and outgoes from an end face of the fiber, is reflected by a reflecting optical system into the fiber; the optical power of the reflected beam guided backward by the fiber is measured. The end face of the fiber is placed in the optical center of the reflecting system to obtain the maximum reflected power, and is then deviated from the position detecting the occurring reductions of the reflected optical power.

Abstract: The refractive-index value is obtained by radially scanning a near-field image of the fiber output face and by computing, for each image point, the ratio between a first intensity value, obtained in the presence of a mask, having a transmissivity proportional to the fourth power of the transversal beam coordinate and located in a far-field plane, and a second intensity value, obtained in the absence of such a mask. An automated measurement apparatus for implementing the method is also provided.

Abstract: A method and an apparatus wherein a light beam is passed through a slit oscillating with respect to the beam, and a periodic signal is obtained from the beam portion traversing the slit. The periodic signal is split into its spectral components. Moment values are derived from the values of the amplitudes of such components.

Abstract: The cut-off wavelength of the first higher order mode in optical fibers for telecommunication is measured based on the fact that the introduction of perturbations in an optical fiber can generate greater losses on higher order modes than on the lower order modes. A spectral scanning of the fiber output power is effected under a certain number of different perturbations conditions and the values obtained are combined so as to determine the fraction of power guided in the fundamental mode. The cut-off wavelength is that where this fraction exceeds a certain value.

Abstract: A body transparent to laser radiation, such as an optical fiber or a preform thereof in the process of manufacture, is transluminated by a monochromatic beam of a frequency in the THz range split off from a composite laser beam with two closely spaced frequencies produced by the Zeeman effect. Another monochromatic beam of the second laser frequency bypasses the transparent body and is photoelectrically recombined with the first beam to provide an electrical measuring wave at a beat frequency in the MHz range differing in phase from an electrical reference wave of the same beat frequency, similarly derived from the original laser beam, to an extent determined by the refractive index and the thickness of the transluminated body. Two or more phase comparisons are made with different angles of incidence of the transluminating beam and their results are compared for an arithmetic determination of refractive index and thickness.