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
  • G02B6/24—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
  • G02B6/278—Controlling polarisation mode dispersion [PMD], e.g. PMD compensation or emulation
• • 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
  • H04B10/2507—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion
  • H04B10/2569—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion due to polarisation mode dispersion [PMD]

Definitions

• the invention relates to an emulator and compensator for polarization mode dispersion, with which the polarization of an electromagnetic, preferably optical wave can be changed as a function of frequency.
• the invention is therefore at the same time a frequency-dependent polarization transformer.
• the PMD compensator should make the polarization transmission behavior of the overall system of transmission link and compensator (or vice versa, ⁇ e according to arrangement) approximately independent of frequency in the approximation of the first and possibly higher order. In wavelength division multiplex operation, it is desirable to use this frequency-independent to achieve the operating wavelengths.
• the invention can be used both as a PMD emulator and as a PMD compensator.
• Requirements for such an assembly are low emfuge vaporization, compatibility with optical fibers and, in many respects, frequency-dependent change in polarization transmission behavior.
• PMD is often described mathematically by many retarders or polarization rotators, which are arranged between strong birefringent fiber optic pieces, that is to say noticeable delay times between the two main polarizations.
• These highly birefringent fiber optic pieces age or preserve two mutually orthogonal main polarizations (p ⁇ ncipal staces-of-polarization), PSP for short, and are therefore polarization-maintaining fiber, PMF for short.
• PMF are highly polarization-dispersive.
• Stucke PMF used which are connected by polarization transformers.
• This reference is important because it describes the connection of a PMD compensator to an optical receiver and the acquisition of a control criterion, and therefore serves as a generic term for this invention. It can be seen that such arrangements can be used both as PMD emulators and as PMD compensators. In practice, the references mentioned are limited to very few pieces of PMF, and the light attenuation that occurs could be quite high due to the necessary splice connections. Functionally similar or equivalent arrangements which allow a large number of such polarization transformers and Cascading PMF pieces in such a way that they have very little steaming is not known.
• a longer piece of polarization-maintaining optical fiber is suitable for producing a PMD
• Emulators or PMD compensators are, as in the case of commercial polarization-maintaining optical fibers, preferably approximately linear in a first exemplary embodiment.
• Spread over the length are torsion sections in which the PMF is twisted, so that polarization transformations result.
• the twist from PMF is already from Applied Optics, Volume 18, No. 13, p. 2241-2251 is known as a means by which polarization transformations can be carried out in linear birefringent LWL, see FIG. 9 there.
• the birefringence of commercially available PMF is so strong that a torsion by the 68 ° specified there the PMF would destroy at least in the long run.
• several torsion sections with an alternating torsion direction are cascaded in accordance with the invention in order to generate the desired transformation.
• a weaker than standard PMF, but much stronger than normal fiber optic birefringent, special PMF should be provided.
• the torsion can be made variable in all of these cases, for example by using stepper motors.
• the torsional range which is naturally limited due to the limited mechanical strength of optical fibers, can impair the ability to compensate, in particular when operating as a PMD compensator.
• Functional inaccuracies can also arise due to the length of the torsion pieces not being determined by the deal and similar influences.
• These possible disadvantages can be eliminated by inserting additional torsion sections. It can be advantageous to arrange several torsion sections or groups of torsion sections that can be twisted independently of one another close to one another. So-called endless polarization regulations are desirable.
• linear birefringent PMF instead of linear birefringent PMF, circular or elliptical birefringent PMF can also be used; however, the polarization transformers have to be modified.
• FIG. 1 shows a PMD emulator or compensator according to the invention in the diagram.
• FIG. 2 shows a phase-adapted mode converter as a polarization transformer.
• FIG. 3 shows an endless polarization transformer with twisted pieces of linearly birefringent optical waveguide.
• Figure 4 shows a movable anchor point linearly birefringent optical fiber m the pierced axis of a stepper motor.
• FIG. 5 shows two individual emulators or compensators, which contain circularly birefringent optical waveguides and movable optical waveguide loops.
• a light wave L passes through a PMD emulator or compensator EK within an optical fiber LWL from an input EE to an output AA.
• the optical waveguide LWL consists of polarization-maintaining optical waveguide PMF, which is not interrupted and does not have to be cut during production or spliced apart from input EE and output AA. For this reason, the PMD emulator or compensator EK has a very low emfu vaporization for the light wave L.
• the PMD emulator or compensator EK contains a chain of simple emulators or compensators 1, 2, ... N. Each of these simple emulators or -compensators 1, 2, ...
• N in turn has an input E and an output A, the output A of a single emulator or -ko pensators is each connected to the input E of the following.
• input E is followed by a polarization transformer PT, a polarization-dispersive optical fiber PMF with a differential group delay DGD and output A.
• Input EE and output AA of the arrangement can also be interchanged are so that the light wave L does not pass through the arrangement from the input EE to the output AA, but in the opposite direction.
• the order of polarization transformer PT and differential group delay time DGD having optical waveguide can also be exchanged for one, several or all individual emulators or compensators 1, 2,... N compared to the described order.
• the polarization-dispersive optical waveguide with the group delay DGD is a piece of the polarization-maintaining optical waveguide PMF chosen with a suitable length.
• PMF has beat wavelengths within which a phase delay of 360 ° or a period of time delay of the light wave between the two main polarizations occurs. is of the order of 2 to 4 mm at a wavelength of 1550 nm of the light wave L. This corresponds to a differential delay time DGD of the polarization-dispersive optical fiber of 2.6 to 1.3 ps per meter length.
• the light is also guided in the polarization transformer PT in the polarization-maintaining optical waveguide PMF, which, as described below, is possible by designing a polarization transformer PT according to the invention.
• the total number of simple emulators or compensators can vary widely, between 1 and large, just economically justifiable numbers such as 100 or 200. It is expedient to choose the total number small, but at least so large that similar statistical properties of the Polarization dispersion as in the case of the optical waveguide to be emulated or compensated. Depending on the requirements, around 6 to 50 simple emulators or compensators are required.
• many simple emulators or compensators are particularly useful when PMD emulation or compensation for very broadband signals, i.e. e.g. for data signals with a high transmission bit rate, or for several or many signals in optical wavelength division multiplexing.
• a mode converter for linear birefringent polarization-maintaining optical fibers PMF can be configured as a phase-matched mode converter. verter as shown m Figure 2 are executed.
• torsion anchors FK, BK which here are in the form of combs, are attached, for example by means of epoxy adhesive. These can be rotated against each other around the polarization-maintaining optical waveguide PMF as an axis, external comb parts being able to rotate in the direction of rotation DR.
• Both torsion anchors can be designed to be movable, or one of them is a fixed torsion anchor FK, the other a movable torsion anchor BK.
• Stepper motors SM for example, are suitable as drives for torsion anchors BK, which can be operated in the path-continuous micro-step mode to achieve high resolution.
• Comb teeth ZI The torsion in the torsion pieces TS is a mechanical stress MB.
• comb comb ZI and mechanical loads MB are provided with identifiers.
• the movable torsion anchor or comb. BK has only one prong, so that it degenerates into a rotary lever, this results in one from Applied Optics, Volume 18, No. 13, pp. 2241-2251 (see there Fig. 9) already known mode converter.
• the use of more comb ZI has the advantage that smaller angles of rotation of the two combs BK, FK against each other are required.
• the birefringence of commercially available PMF is so strong that a torsion around the 68 ° indicated in the reference would destroy the PMF at least in the long term.
• the length within which the 68 ° torsion must take place is only 0.7 mm in the case of a 2 mm beat wavelength.
• the solid Tors ⁇ onsan ⁇ er FK has only two prongs here.
• movable torsion anchors BK1, BK2, BK3 are provided.
• the number three is particularly suitable as the number of movable torsion anchors because an endless polarization transformation is possible according to the invention.
• One or both of the fixed torsion anchors FK can also be replaced by movable torsion anchors BKO, BK4.
• this offers additional variability in the polarization transformation, so that deviations in the lengths of the torsion sections TS1, TS2, TS3, TS4 from the desired value, inhomogeneities in the birefringence of the optical waveguide PMF and similar inaccuracies cannot impair the ability to control polarization endlessly .
• the anchor points are negligibly short, so that the polarization transformer PT consists essentially of twistable optical fibers.
• these lengths can also be varied;
• the torsion sections TS1, TS4 can each be selected so long that they have phase delays of approximately 1.6 rad, and torsion sections TS2, TS3 can each be selected so long that they have phase delays of approximately 1.8 rad.
• torsion anchor BK2 should be at least about ⁇ 73 °, and torsion anchor BK1,
• BK3 can be rotated by at least about ⁇ 35 ° relative to the torsion-free position. Since the torsion of the optical fiber PMF not only rotates the main axes geometrically, but also the polarization ellipse of the optical wave rotates to a small extent, about 5 to 10% of the geometrical rotation, the torsion range of the torsion anchor is one factor in practice Multiply F from about 1.05 to 1.1, so that you get about ⁇ 79 ° for torsion anchor BK2, and about ⁇ 38 ° for torsion anchor BK1, BK3. Depending on the type of optical fiber, these values can fluctuate by approximately ⁇ 10%.
• Endless polarization transformation is always possible if certain common periodic rotation changes of the movable torsion anchors BK1, BK2, BK3 can be specified in such a way that a main polarization of the polarization-maintaining optical fiber PMF m each phase of these rotation changes at least approximately m the other, orthogonal to the first Main polation is transformed. With less than the rotation angles specified in this way, all others can then be used achieve possible and necessary polarization transformations.
• the columns - TS1, TS4 mean the differential decelerations m radians of the torsion sections TS1, TS4,
• Circular birefringence components of the torsion sections in m radians which, as explained below, in a direct manner
• CA, CB, CC specify the specified periodic changes in rotation of the movable torsion anchors BK1, BK2, BK3, which before a main polarization of the polarization-maintaining optical waveguide PMF of each phase of these rotation changes, at least approximately, leads to the other orthogonal main polarization.
• the actual turning radians are as follows:
• TS1, TS4 TS2, TS3 CA CB CC W 1, 6822 1.7063 0.3838 2.5547 1.2078 0 1.2671 1.5748 0.3495 2.3959 1.1854 0.2618 1.4455 1.1221 0.5732 2, 6100 1.0970 0.5236 1.2025 0.8886 0, 6022 2.5738 1.0694 0.7854 1.3073 0.4848 0.8161 2, 8904 1.0063 1.0472 1, 0198 0.2692 0.8146 2.8355 0.99983 1.3090 0.8165 0.0201 0.8617 2.8883 0.9779 1.5708 0.8589 -0.3367 1.0935 3.3401 0.9318 1.8326 2.7641 1.4520 0.7614 2.7293 1, 8135 2.0944 2.5026 1.3362 0.7189 2, 6072 1, 6315 2.3562 1.8341 1.4392 0.8738 2, 8254 1.2814 2, 6180 0.9181 1.5792 1.1139 3.3008 1.0629 2.8798 1.1501 1.1127 0.8317 2.7393 1.12
• torsion sections TS1, TS2, TS3, TS4 which are longer than specified by a whole number of beat wave lengths of the light wave.
• the torsion angles are to be changed in such a way that an unchanged torsion rate, i.e. Changes in length per unit length result.
• the examples given can also be implemented with negative lengths of practical examples.
• column W This can easily be seen by comparing the first and last table rows; these lines differ only by the value 2 * p ⁇ radiant m of column W.
• angles CA, CB, CC given in the table are only to be understood as reference values, because it can be useful to achieve greater variability in polarization transformations be significantly larger, for example by a factor of 1.5 or even 2 enlarged swivel CA, CB, CC.
• Exemplary embodiments with more than three movable torsion anchors can, for example, be designed such that at least approximately the same torsion profiles can be achieved as a function of the location in the polarization-maintaining optical waveguide PMF as in the examples given.
• the beat wavelength is chosen to be sufficiently large.
• beat wave lengths of 10 to 200 mm, preferably those in the range between 30 and 100 mm.
• the differential delay time is approximately 0.1 ps per meter.
• DGD differential group transit time
• a total of approximately 2.5 km of optical fibers are then required.
• Polarization-maintaining optical waveguides with linear birefringence of the order of magnitude mentioned can easily be produced according to the prior art by means of an elliptical core cross section or by targeted installation of mechanical stresses.
• the former does not cause particularly high attenuation losses due to the very low attenuation of quartz glass optical fibers, and the latter can be taken into account in the design and commissioning of the polarization transformers PT by more anchor points FK, BK or by specifying deviations in the angle of rotation ranges from the values given above, see table above.
• the attenuation of a length of 2.5 km can be very low, down to about 0.5 dB in total.
• the polarization-maintaining optical waveguide PMF can simultaneously be designed for compensation or emulation of chromatic optical waveguide dispersion.
• turnstiles can be used
• FIG. 4 shows a stepper motor SM with a schematically drawn stator ST and an axis AX, which is drilled through with a bore BO.
• the optical fiber LWL is guided concentrically in the axis diameter and stator, which in this exemplary embodiment is a polarization-maintaining optical fiber PMF.
• the optical fiber LWL, PMF is firmly glued to the axis AX, so that the axis AX simultaneously represents a movable torsion anchor BK, BKO, BK1, BK2, BK3, BK4.
• This arrangement is extremely compact, particularly when using a stepping motor with a disc-shaped (flat) special design, so that the entire polarization transformer PT of FIG. 3 can also be constructed compactly.
• the distances between the movable anchor points BK, BKO, BK1, BK2, BK3, BK4 can be so small that birefringent optical waveguides PMF with a beat length of moderate size can be used.
• FIG. 5 shows, by way of example, two individual emulators or compensators 1, 2, which contain circularly birefringent optical waveguides and movable optical waveguide loops or fiber loops FS1, FS2.
• Optical fiber LWL is clamped in front of and behind the polarization transformers PT at breakpoints H. Between two adjacent breakpoints H, which enclose a polarization transformer PT, it acts as a normal optical fiber LWL, which is almost non-birefringent without additional bending. At least in certain positions of the rotatable fiber loops, it is undistorted, with free m the light wave conductor loops FSl, FS2 movable optical fiber LWL even in any position of these optical fiber loops FSl, FS2.
• the optical waveguide is heavily twisted between two adjacent breakpoints, between which there is no polarization transformer PT, so that it acts as a circular birefringent and circular polarization-maintaining optical waveguide PMF with a differential group delay DGD between the two circular main polarizations.
• a stranding machine is suitable for production.
• the polarization transformers PT contain fiber loops FS1, FS2.
• the fiber loops FS1, FS2 are designed to be rotatable, and moreover the optical fiber LWL is bent there in principle, so that there is a mechanical stress MB of the optical fiber LWL as in the first exemplary embodiment.
• the PMD compensator EK according to the invention can be m
• Electron. Lett., Feb. 17 1994, volume 30, no. 4, pp. 348-349 can be used in an optical receiver for PMD compensation.

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• 1998
  • 1998-04-14 DE DE19816178A patent/DE19816178A1/de not_active Withdrawn
• 1999
  • 1999-04-07 JP JP2000543870A patent/JP3648453B2/ja not_active Expired - Fee Related
  • 1999-04-07 EP EP99945694A patent/EP1080388A2/de not_active Withdrawn
  • 1999-04-07 CN CNB998073660A patent/CN1241043C/zh not_active Expired - Fee Related
  • 1999-04-07 US US09/673,501 patent/US6529648B1/en not_active Expired - Fee Related
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