Abstract: The invention relates to polarization mode dispersion; writing index variations in a grating written in an optical fiber generates polarization mode dispersion. The invention proposes to compensate this polarization mode dispersion by mechanical curvature of the fiber. The invention provides a simple method of compensating polarization mode dispersion effectively and improving the performance of a grating written in a fiber; it applies in particular to Bragg gratings.

Abstract: An optical filter has little variation in chromatic dispersion over its passband, the filter being constituted by a Bragg grating whose pitch varies as a function of distance along the filter with a change in the sense of its concavity. The filter also presents apodization, e.g. apodization by a super-Gaussian function or Blackman apodization. The filter of the invention is adapted to high data rate dense WDM transmission systems.

Abstract: The invention proposes a photosensitive optical fiber for inscribing a Bragg grating by UV illumination, in which the photosensitivity is very high either to reduce the exposure time or to increase the contrast of the optically induced refraction index variations, whilst retaining a reasonable exposure time. Thanks to this high contrast, the fiber of the invention, imprinted with a linear chirp, in addition to correcting chromatic dispersion, is also effective in correcting the chromatic dispersion slope of a fiber of ordinary contrast, imprinted with a quadratic chirp. In a preferred embodiment, the fiber of the invention is photosensitive in the cladding. In a particularly advantageous embodiment, a non-photosensitive dopant is added to the core of said fiber to reduce the refractive index in the core, reducing the birefringence of the fiber.

Abstract: The invention relates to the field of optical fiber wavelength division multiplexed transmission, and more particularly to the field of narrow-band optical filters for dense multiplexing. The invention proposes a narrow-band optical filter with a group-delay response that is very flat in the working band, obtained by cascading two Bragg gratings having group delays that are linear as a function of wavelength in the working band, but with their gradients being of opposite signs. The present solution is easier and less costly to implement than the prior art solutions, and it should also offer higher performance.

Abstract: An angled Bragg grating optical waveguide section adjusts the spectral response of a transmission filter in an optical waveguide or in an optical fiber. To smooth the response of the filter, which is subject to modulation due to coupling to the various cladding modes, the pitch of the Bragg grating is caused to vary in a linear fashion along the length of the filter. To adapt the attenuation band to a required band, the cladding of the optical fiber is doped with a photosensitive material so that the grating is also formed in the cladding. The photosensitivity of the cladding is advantageously greater than that of the core, in a ratio in the order of 5:1.

Abstract: It is shown that by modifying the photosensitivity profile it is possible to provide better adjustment of the characteristic curve of a filter, and in particular to eliminate a reflection peak that is present within the working band. The photosensitivity profile is inverted to a considerable extent. In practice, the core of the fiber has only 20% of photosensitive doping material compared with the quantity of photosensitive doping material present in the cladding of the fiber.

Abstract: To adjust the spectral band of an attenuating optical fiber, a section of the optical fiber is formed that has a sloping Bragg grating. To smooth the spectral band of the attenuation, it is shown that the filter section must be short. Independently of selectivity, the smoothing makes it possible to avoid distorting the filtered signal. The length of the filter section is preferably 0.7 mm.

Abstract: To perform selective dropping and adding of channels of wavelength multiplexes without interposing active elements in the main transmission path, the apparatus comprises add-and-drop modules, each of which includes at least one stop filter provided with two opposite ports. An optical switch provided with a drop outlet is provided with drop inlets and with add outlets associated with the modules, and respectively coupled to the opposite ports of the filters. For each module, the switch is organized for selectively coupling one of its drop inlets to one of its add outlets and/or to the drop outlet.

Abstract: A method of manufacturing a multi-core optical fiber, the method including assembling together a plurality of substantially identical single-core optical fiber preforms (2', 2"), referred to as "single-core preforms", each of which includes a core bar (3) surrounded by a layer of optical cladding (4), so as to form a "multi-core preform" (10) and drawing down the multi-core preform (10) so as to obtain the multi-core optical fiber. The assembly step includes securing the single-core preforms (2', 2") to one another by fusing them over their entire lengths or over portions thereof along their tangential lines of contact (T), without inserting the multi-core preform (10) into a holding tube. A vacuum is maintained in the preform during the drawing step, the vacuum being formed before or during the drawing step.

Abstract: A band-pass filter written into an optical waveguide includes at least one Bragg grating creating coupling between two propagation modes. In one version the pitch of the grating is equal to the length of beating of these two modes, to create codirectional coupling for the LP01 and LP0i modes. In another version the grating couples the LP01 mode to any contradirectional radiation mode. The filter includes, at one point at least of the grating, an area in which one half-period of the grating is omitted to create a phase-shift of .pi. between the two coupled modes.

Abstract: An optical filter is formed by writing at least one long-period Bragg array into an optical fiber which is tapered to define two substantially adiabatic transition areas delimiting an intermediate area in which the long-period Bragg grating is written to produce codirectional coupling between two guided modes in the intermediate area at a wavelength that is a function of the period of the grating.

Abstract: A method of manufacturing a multi-core optical fiber, the method including assembling together a plurality of substantially identical polished single-core optical fiber preforms (2', 2"), referred to as "single-core preforms", each of which includes a core bar (3) surrounded by a layer of optical cladding (4), so as to form a "multi-core preform" (10), and drawing down the multi-core preform (10) so as to obtain the multi-core optical fiber. The assembly step includes securing the single-core preforms (2', 2") to one another by fusing them over their entire lengths or over portions thereof along their tangential lines of contact (T), without inserting the multi-core preform (10) into a holding tube. A vacuum is maintained in the preform during the drawing step, the vacuum being formed before or during the drawing step.

Abstract: A system for writing a Bragg grating point-by-point in an optical waveguide includes an illumination system for producing a primary beam. It further includes a diffraction grating to receive the primary beam and to produce a zero order secondary beam and two symmetrical secondary beams of higher order and a focusing optical device for focusing the secondary beams onto a writing area of the waveguide. The diffraction grating and the focusing optical device define an intensity function of writing in the area of the guide including a central peak and two secondary peaks spaced from the central peak by a distance equal to the pitch of the grating.

Abstract: An all-optical time slot interchange device comprises at least two wavelength selective delay lines disposed in series and each comprising a delay line for delaying the propagation of an optical signal by increasing the length of the optical path it travels. An optical signal at a given wavelength is extracted from a guide structure and re-injected into the guide structure via the delay line so as to apply a time-delay to the optical signal at a given wavelength. Applications include wavelength-division multiplexing optical telecommunication systems.

Abstract: The invention provides a Fabry-Perot filter for guided light, the filter comprising two Bragg gratings that follow each other along a common light guide, a gap being left between the two gratings, wherein the two gratings have respective different lengths. The invention is particularly applicable to optical fiber telecommunications systems.

Abstract: A single-mode optical waveguide comprises an optical core and a cladding surrounding the optical core having a predetermined refractive index. The core comprises a central area that has a refractive index lower than the predetermined refractive index and annular area around the central area that has a refractive index higher than the predetermined refractive index. Between the peripheral annular area and the cladding is at least one composite annular area made up of two successive annular areas respectively having refractive indices lower than and higher than the predetermined refractive index.

Abstract: Dispersion-shifted monomode optical fibers have an effective mode surface area greater than 65 .mu.m.sup.2 by optimization of the geometrical characteristics that characterize the fibers.

Abstract: Dispersion-shifted monomode optical fibers have an effective mode surface area greater than 65 .mu.m.sup.2 by optimization of the geometrical characteristics that characterize the fibers. The fibers have substantially zero chromatic dispersion in the vicinity of 1.55 .mu.m, and they include an optical core having a central portion, a first layer having an index lower than the index of the central portion, and a second layer having an index higher than the index of the first layer and higher than the index of the optical cladding.

Abstract: A laser source supplies a primary laser beam. A phase grating generates two secondary beams from the primary beam so as to form interference fringes that are inscribed in an optical fiber by photo-refraction. An optical system of variable focal length varies the gap between the interference fringes so as to vary the pitch of the Bragg grating discretely for successive segments of the optical fiber. Means displace the primary laser beam and the optical fiber relative to each other by successive segments of said fiber. These means are synchronized with the variation of the focal length of the optical system. Application to telecommunications.

Abstract: Dispersion-shifted monomode optical fibers have an effective mode surface area greater than 65 &mgr;m2 by optimization of the geometrical characteristics that characterize the fibers.