Abstract: The present invention relates to a new model for interpreting interferometric measurements, able to lead to a new method, system (10) and device (27) for measuring the PMD (Polarisation Mode Dispersion) of an optical fibre (11). The model, which is based on the theory of propagation of the optical signal as can be determined according to the socalled Prindpal States of Polarisation (PSPs) of the fibre, leads to a new interpretation of the interferogram generated with the interferometric measurements and is characterized in that it identifies in said interferogram a central peak and two side lobes. Thanks to this new model, it is possible to determine the PMD of the fibre based on the information content associated to at least one of the two side lobes identified in the interferogram.

Abstract: A multi-stage optical amplifier for a fiber-optic transmission system, in particular, an amplifier having a multi-stage configuration that allows an optimization of its performance with respect to noise, gain and bandwidth. The multi-stage amplifier has a first amplifying stage including a rare-earth doped optical active fiber; a second amplifying stage connected to the first amplifying stage, the second amplifying stage including a tellurite-based active fiber doped with a rare earth element; and a third amplifying stage connected with the second amplifying stage, the third amplifying stage including a silica-based fiber.

Abstract: The present invention relates to a new model for interpreting interferometric measurements, able to lead to a new method, system (10) and device (27) for measuring the PMD (Polarisation Mode Dispersion) of an optical fibre (11). The model, which is based on the theory of propagation of the optical signal as can be determined according to the socalled Prindpal States of Polarisation (PSPs) of the fibre, leads to a new interpretation of the interferognram generated with the interferometric measurements and is characterised in that it identifies in said interferogramn a central peak and two side lobes. Thanks to this new model, it is possible to determine the PMD of the fibre based on the information content associated to at least one of the two side lobes identified in the interferogram.

Abstract: Transform-limited ultrashort optical pulses are obtained by sending into a dispersion-shifted optical fibre pulses of such duration and intensity as to produce self-phase modulation. The pulses are also made to travel through a conventional fibre with suitable dispersion and are subjected to optical filtering, so as to compensate, for a particular peak power, the phase effect due to the chirp. Pulse peak power is selected so as to coincide with the vertex of the parabola plotting the square of the duration of the outgoing pulse versus the peak power of the incoming signal.

Abstract: A train of transform-limited optical pulses with wavelength close to the zero-dispersion wavelength of the fiber and high and variable peak power, such as to give rise to self-phase modulation, is sent into a fiber. The spectral broadening of the signal exiting the fiber is measured for a number of values of the peak power of the pulses and the nonlinear refractive index is obtained from the angular coefficient of the straight line representing spectral broadening versus peak power.

Abstract: Ultra-short transform-limited optical pulses, to be utilized for high bit-rate transmission systems, are obtained through direct modulation of a semiconductor laser by pulses of such duration as to excite only the first peak of the relaxation oscillations of the cavity of laser, tuned to such a length that the pulse portions that overlap inside the cavity interfere constructively. The pulses emitted by the laser then pass upon a fiber with high negative dispersion to compensate for the phase effect due to the chirp.

Abstract: A coherent optical communications system with continuous polarization scrambling, wherein such scrambling is obtained by combining orthogonally polarized radiations at different frequencies emitted by two independent sources. The two sources may be employed to build the transmitted or the local oscillator of the system.

Abstract: A coherent optical communications system using polarisation modulation, wherein the polarisation modulation of the optical signals is obtained by using two lasers (1, 2) emitting orthogonally polarised radiations at different frequencies, which are combined without losses prior to transmission over the line (FIG. 1).

Abstract: In order to determine the nonlinear refractive index coefficient of a material which, once inserted into a Fabry-Perot cavity (1), has an optically bistable behavior, the output power (12; 13) of the cavity (1) is measured in function of the power (1) of an amplitude-modulated optical input signal to obtain the material hysteresis loop; the hysteresis loop saturation region is detected, where the phase (.phi.) of the electromagnetic field associated with the signal within the cavity is a linear function of the output power; the values of the phase (.phi.) in said region are obtained from the output and input power values (12; 13; 11) and the nonlinear refractive index coefficient (n2) is obtained from the angular coefficient of the straight line representing said phase.

Abstract: The modulator comprises an integrated optical switch (6) receiving at an input a linearly-polarized optical carrier and transferring same to a first or a second output (P3, P4), according to the logic values of the bits of a modulating binary data signal. The radiations outgoing from the switch are sent to a polarizing beam splitter (13) with their original polarization or with a polarization rotated by 90.degree. depending on the switch output (P3, P4) from which they come. A signal with the original polarization or with the polarization rotated by 90.degree. is present at the splitter output, depending on the logic values of the bits of the modulating signal.

Abstract: The characteristic parameters of a semiconductor laser (1) acting as an amplifier and brought to bistable operating conditions are determined. The output power (I2) of laser (1) is measured as a function of the power (I1) of an amplitude-modulated optical input signal to determine the laser hysteresis loop; the switching points (P1, P2) between the two stable states of the laser (1) are identified, the input and output power values relevant to such points are memorized, and at least the value of the non-linear refractive index coefficient (n.sub.2) of the material used to fabricate the laser (1) as determined starting from the power values relevant to at least one of said points (P1, P2). By exploiting the power values relevant to both switching points (P1, P2) also the amplification factor (A), the finesse parameter (F) of the passive cavity of the laser (1) and the wavelength difference (.lambda.1-.lambda.

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: The interferometer makes use of an acousto-optic device to perform a frequency shift of the beam sent along one of the interferometer branches, so as to allow the determination of the state of polarization by heterodyne radiofrequency detection. The acousto-optic device can be inserted downstream of the device for splitting the beam emitted by the source into the two beams sent along the two interferometer branches, or it can also act as the beam splitter.

Abstract: Optical-fiber transmission system with heterodyne coherent detection in which, at the transmitting side, the polarization of an optical carrier is modulated, and at the receiving side the beam resulting from a combination of the modulated beam and a beam emitted from a local oscillator is split into two orthogonally-polarized components. The two optical signals are detected and sent to an electronic mixer which receives the two components and carries out a synchronous demodulation eliminating the effects of the linewidth of the source and of the local oscillator.

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: Method and device for realtime measurement of the state of polarization at a quasi-monochromatic light beam. Two quasi-monochromatic radiations with slightly different optical frequencies are generated. A radiation is converted into a 45.degree. linearly polarized radiation, while the other presents the polarization state imposed by a body under test. In each radiation, horizontal and vertical polarization components are separated, and then recombined into two different beams comprising radiations at both frequencies, respectively polarized in the same plane. Beatings between the two components of each beam are originated and from the two electrical signals in the radiofrequency range thus obtained the information is extracted on the relative phase and the amplitude of the two components of the radiation with polarization imposed by the body under test.