Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/02—Optical fibres with cladding with or without a coating
  • G02B6/024—Optical fibres with cladding with or without a coating with polarisation maintaining properties
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/24—Coupling light guides
  • G02B6/26—Optical coupling means
  • G02B6/27—Optical coupling means with polarisation selective and adjusting means
  • G02B6/2706—Optical coupling means with polarisation selective and adjusting means as bulk elements, i.e. free space arrangements external to a light guide, e.g. polarising beam splitters
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/24—Coupling light guides
  • G02B6/26—Optical coupling means
  • G02B6/27—Optical coupling means with polarisation selective and adjusting means
  • G02B6/2726—Optical coupling means with polarisation selective and adjusting means in or on light guides, e.g. polarisation means assembled in a light guide
  • G02B6/274—Optical coupling means with polarisation selective and adjusting means in or on light guides, e.g. polarisation means assembled in a light guide based on light guide birefringence, e.g. due to coupling between light guides
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
  • H04B10/25—Arrangements specific to fibre transmission
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/24—Coupling light guides
  • G02B6/26—Optical coupling means
  • G02B6/27—Optical coupling means with polarisation selective and adjusting means
  • G02B6/2753—Optical coupling means with polarisation selective and adjusting means characterised by their function or use, i.e. of the complete device
  • G02B6/2766—Manipulating the plane of polarisation from one input polarisation to another output polarisation, e.g. polarisation rotators, linear to circular polarisation converters

Definitions

• the invention relates to a polarization-maintaining optical fiber transmission system. It is applicable in optical signal transmission techniques using one or more polarization-maintaining fibers. These signals can be laser pulses in power lasers or signals carrying information in telecommunication systems.
• Polarization-maintaining fibers allow, as their name suggests, to transmit a signal while preserving its polarization. They are characterized by two axes called “slow” and “fast”.
• the fiber optic connectors induce stresses on the polarization maintaining fibers which very slightly alter the polarization state of the signal. This modification associated with the difference in speed between the polarization states inside the polarization maintaining fibers causes distortions of the signal. These distortions, very disabling and random, are known as FM-AM conversion in the case of power lasers.
• Polarizing fibers are very difficult to weld and are very sensitive to micro curvatures. Thus, they must be conditioned in a very specific way. - The conventional fibers do not control the polarization. A polarization controller is needed. This solution is very difficult to implement, especially when several controllers have to be stunts in the chain. These controllers are more expensive, generate some additional losses and are not necessarily reliable.
• the invention relates to a system for solving these difficulties.
• the invention therefore relates to a system for transmitting by optical fibers a polarized signal comprising at least one polarization-maintaining optical fiber system making it possible to maintain the polarization of the signal.
• This optical fiber system comprises at least a first and a second section of polarization maintaining fiber each having a slow propagation axis and a fast propagation axis.
• One end of the first fiber section is coupled to one end of the second fiber section so that the slow propagation axis of the first fiber section is in coincidence with the fast propagation axis of the second fiber section and vice versa.
• the fast propagation axis of the first fiber section coincides with the slow propagation axis of the second fiber section.
• the overall differential group time of one or more segments is equal to the overall group time of the other sections so that the time of overall differential group of said optical fiber system is substantially zero.
• the system according to the invention comprises a first and a second fiber section, the two fiber sections having equivalent group differential times.
• the two sections of fibers are made of the same fiber.
• a plurality of pairs of sections may be provided.
• each section being connected in series with the neighboring section so that the slow propagation axis of each fiber section coincides with the axis of rapid propagation.
• the intermediate sections of the series of sections each having a differential group time equal to a determined value, while the first and the last section of the series of sections each have a differential group time equivalent to the half of this determined value.
• the intermediate sections each have a determined length and that the first and the last section each have an equivalent length equal to half of this determined length.
• the end of the first fiber section is coupled to the end of the second fiber section by welding these ends.
• the end of the first fiber section is coupled to the end of the second fiber section by a connector device.
• the system of the invention comprises an input device and an output device as well as a polarization rotator associated with the input device or the output device and making it possible to rotate the polarizations of the signals transmitted to said optical fiber system at an angle corresponding to the sum of the polarization rotations induced by the optical fiber system and in the opposite direction to the sum of these rotations.
• FIGS. 1 and 2a are diagrams explaining the consequences of the rotation of polarizations undergone by an optical signal transmitted in a polarization-maintaining fiber
• FIG. 2b schematically illustrates an exemplary embodiment of the system according to the invention
• FIG. 2c represents the system of FIG. 2b according to the same representation mode as in FIG. 1,
• FIG. 3 schematically illustrates an alternative embodiment of the system according to the invention
• FIGS. 4a and 4b schematically illustrate an example of application of the system of the invention to a telecommunication signal transmission system
• FIGS. 5a and 5b show alternative embodiments of the optical fiber transmission system according to FIG. 'invention.
• the polarization-maintaining fibers allow, as their name indicates, to transmit a signal by preserving its polarization provided that the polarization state of the incident signal is along one of the two axes called clean or main of the fiber to maintain the polarization. polarization. These two axes are called “slow” and “fast” and the difference in arrival time is called differential group time, DGD for Differential Group Delay.
• Any constraint exerted on a polarization-maintaining optical fiber modifies the polarization states of the optical signals transmitted on these fibers.
• Optical fiber connectors in particular, induce stresses on these polarization-maintaining fibers.
• FIG. 1 This spectral image of the phenomenon can be illustrated in FIG. 1.
• a signal SIv is injected on one of the axes of the polarization-maintaining fiber, its polarization state is slightly inclined after the input connector. The rotation is very small (a few degrees) but sufficient to generate the phenomenon.
• the projections S2.v and S2.h of the signal on the two axes propagate at different speeds. For example, it can be seen in FIG. 1 that the signals S3.v and S3.h are shifted by a time ⁇ called differential group delay or DGD (Differential Group Delay). At the output, the signal is again rotated because of the second connector.
• the polarization component S3.v gives rise to two components S4.vv and S4.vh.
• the polarization component S3.h gives rise to the signals S4.hh and S4.hv which in FIG. 1 is represented in two parts because of the time difference between the signals. propagating along the two axes of polarization of the fiber.
• FIG. 2a shows a polarization maintaining optical fiber F in which an optical coupler C1 makes it possible to inject a polarized light signal V.
• the polarizations of the fiber F are symbolized in FIG. 2a by PV and PH.
• the input of the signal V into the fiber by the coupler is subject to a slight rotation of polarization and it is therefore the signal Vr which is transmitted in the fiber.
• This signal Vr can be decomposed into two components V1 and H1 according to the two directions of polarization of the fiber.
• the component H1 propagates faster in the fiber than the component Vl and the component H'1 reaches the output end of the fiber, to the coupler C2, a time ⁇ before the component Vl.
• the fiber in two sections of polarization-maintaining fibers F1 and F2 (FIG. 2b).
• the two sections are of the same length. In comparison with FIG. 2a, they each correspond to half the length of the fiber F.
• the ends E1 and E2 of these two fiber sections are coupled in such a way that the slow and fast propagation axes PV1 and PHl of the section F1 coincide respectively with the fast and slow propagation axes PH2 and PV2 of the section F2.
• the signal V entering the fiber section F1 gives rise to two components V1 and H1 which propagate at different speeds.
• the component H2 propagates along the fast axis and reaches the other end of section Fl before the component V2 propagates along the slow axis. Having previously provided that the lengths of the fiber sections Fl and F2 are equal to half the length of the fiber F, the component H2 reaches the end of the section Fl a time ⁇ / 2 before the component V2. Since the propagation axes PV1 and PH1 of the fiber section F1 are respectively coupled to the propagation axes PH2 and PV2 of the section F2, it can be considered that the signal corresponding to the component V2 in the section F1 is found in the section F2 in the form an H3 component. Similarly, the H2 component is found in the form of the V3 component.
• the H3 component now propagates along the fast axis and the V3 component along the slow axis. It follows that the H3 component will catch up with its ⁇ / 2 delay relative to the V3 component. The two components H4 and V4 thus arrive at the same moment at the output end of the fiber section F2. This assumes of course that the two fiber sections Fl and F2 have the same characteristics.
• FIG. 2c represents the system of FIG. 2b according to the same representation mode as in FIG. 1. It can thus be seen that at the output of the fiber section F2, the components H4 and V4 are in phase. If a coupler is provided at the output of the section F2, the signals are again subjected to a slight polarization rotation. The component H4 gives rise to the components H5 and V6 and the component V4 gives rise to the components V5 and H6. After crossing a polarizer, we thus obtain the components H5 and H6 which are in phase.
• a polarization-maintaining optical fiber transmission system for a fiber connection made in at least two sections, as just described, between two couplers or between two stress zones of the fiber, or between a stress zone and a coupler.
• FIG. 3 shows an exemplary embodiment in which the fiber sections TF1 and TF2 are coupled with the slow axes PV1 and PV2 of the two coincident sections and the fast axes PH1 and PH2 coincidentally.
• the fiber section TF3 is coupled to the section TF2 with its fast axis PH3 in coincidence with the slow axis PV2 of the section TF2 and its slow axis PV3 coinciding with the fast axis PH2 of the section TF2.
• the fiber section TF4 is oriented in the same way as the sections TF1 and TF2 and is coupled to the section TF3 with its slow axis PV4 coinciding with the fast axis PH3 of the section TF4 and its fast axis PH4 coinciding with the axis slow PV3 of section TF3.
• the fiber sections may have different lengths and may not be arranged regularly.
• the essential point is that, in a given transmission system, the overall differential time (propagation difference along the slow and fast axes) of one or more sections is compensated by the differential time of the other sections.
• the total differential time of the sections TF1, TF2 and TF4 is compensated by the differential time of the section TF3.
• FIGS. 5a and 5b show alternative embodiments of a transmission system in which there are three or more sections and the length of one of the sections situated in the intermediate position is a length d 0 imposed by constraints that are outside the scope of the 'invention. According to the invention, it is then expected that the lengths of the intermediate sections are equal to dO and that the end sections have lengths dO / 2 halves of this length.
• FIG. 5a represents, by way of example, a system comprising an odd number of fiber sections, for example seven sections TF1 to TF7.
• Intermediate sections TF2 and TF6 each have a determined differential group time.
• the end sections TF1 and TF7 are of similar constitutions and each have a differential group time which is half that of the intermediate sections TF2 to TF6.
• the differential group time of the fiber section TF2 is compensated, for example, by the differential group time of the section TF3. That of the fiber section TF4 is compensated by the group time of the section TF5 and that of the section TF6 is compensated by the sum of the differential times of the sections TF1 and TF7.
• FIG. 5b represents a system comprising an even number of fiber sections, six sections TF1 to TF6, for example.
• the intermediate sections TF2 to TF5 each have a determined differential group time.
• the end sections TF1 and TF6 are of similar constitutions and each have a group time which is half that of the intermediate sections TF2 to TF5.
• the sum of the differential group times of the fiber sections TF2 and TF4 is compensated by the sum of the differential group times of the sections TF3 and TF5.
• the differential group time of the fiber section TF1 is compensated by the group time differential of the TF6 section.
• the invention consists in inverting the axes of the polarization-maintaining fibers to compensate for the differences in speed of the polarization components of a signal. These inversions may be random but we can also provide alternating sections of the same length.
• sections of the same length it will be possible advantageously to provide sections of half lengths at both ends of the system.
• constraints may be imposed, such as total length the fiber optic system or the number of sections. This will be the analytical model or a numerical simulation that will determine the number of fiber sections and their lengths. Even more advantageously, the sections will be welded unrestrained two by two and not connected by connectors.
• the object of the invention is to maintain the polarization and not to make a physical process independent of the polarization state of the signal. It was not obvious a priori that crossing the axes of a fiber would be useful for the simple transport of a signal.
• a polarization or rotator controller of the directions of polarizations RO whose function will be to rotate the polarizations of the signals transmitted by an angle corresponding to the sum of the rotations of polarizations that these signals will undergo in the transmission system. Rotation induced by the RO rotator is will reverse the overall rotation induced in the transmission system.
• the polarization rotator RO has been placed at the output of the system and associated, for example, with the coupler C2.
• Figure 4b it was placed at the entrance of the system.

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• Electromagnetism (AREA)
• Engineering & Computer Science (AREA)
• Computer Networks & Wireless Communication (AREA)
• Signal Processing (AREA)
• Optical Communication System (AREA)

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• 2004
  • 2004-10-21 FR FR0452395A patent/FR2874294B1/fr not_active Expired - Fee Related
• 2005
  • 2005-08-10 EP EP05797573A patent/EP1779559A1/de not_active Withdrawn
  • 2005-08-10 WO PCT/FR2005/050666 patent/WO2006021731A1/fr active Application Filing
  • 2005-08-10 CA CA002577189A patent/CA2577189A1/en not_active Abandoned
  • 2005-08-10 JP JP2007526522A patent/JP2008510402A/ja active Pending
  • 2005-08-10 US US11/660,228 patent/US20080144991A1/en not_active Abandoned

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