DE19839308A1 - Polarization transformer with adjustable eigenmodes of the polarization elements - Google Patents

Abstract

The inventive polarisation transformer (PT) contains 3 to 8 polarisation elements (W1, W2, ...), at least 3 of which are controlled individually. The intrinsic modes can be regulated in such a way as to produce an at least overall approximate difference in propagation time between the intrinsic modes of pi rad. Any state of polarisation can be achieved by linearly modifying the set values.

Description

The invention relates to a polarization transformer according to the preamble of claim 1.

A polarization transformer is used to change the bottom larization state of an electromagnetic, preferably optical wave.

In optical transmission technology, long light waves are conductor transmission techniques used. The light waves Due to the manufacturing process, conductors are not completely isotropic, but weakly birefringent. Because of the big transfer This results in a frequency-dependent pola polarization mode dispersion (PMD) or shorter polarization dispersion called. This leads to special to broaden transmitted impulses, whereby the Transmission data rate is limited. A compensation will difficult because the polarization mode dispersion by different temperatures or mechanical bean changed. That is why adaptive PMD compen sensors needed, which, in the transmission path adds to compensate for the harmful effects.

Polarization transformers, which are used in PMD compen as well as for other purposes, such as the Setting a desired input polarization state at the input of a polarization-dependent optical amplifier kers that can be used are known in principle and for example in IEEE Journal of Light Wave Technology, Vol. 8, No. 3, March 1990, Page 459-465 or described. Liquid crystal retarders are used to control the polarization used. There is a polarization transformer here from three retarders traversed by a light wave.  

Two retarders work as a unit independently of the third and are controlled by two independent control loops. The The disadvantage of the retarder is that nematic liquid crystals are used. These have a low, for commercial use insufficient response speed speed. In addition, the delay is variable during the egg genmoden are firm. This means that if the delays limits are reached during operation are required, which are difficult to implement and Can cause transmission errors.

The object of the invention is therefore one with few retar specify the realizable polarization transformer, the no reset operations required. Should also be mechanical movable retarders can be implemented.

This task is accomplished through a polarization transformer solved according to claim 1.

It is essential in this polarization transformer that at least three polarization elements independently are controllable. The polarization transformer described can be done with little effort and low internal damping realize. By using a minimal number of The construction volume is also reduced for retarders. The Polarisa tion transformer enables the so-called "endless" polarization regulation. This means that a certain input pole state in any initial polarization stand can be transformed, and that any given ne changes of this initial polarization state to direct ways can be reached, i.e. without detours over long distances re polarization states. The same applies to the Trans formation of any variable transformation state in a certain fixed polarization state. This own shaft is of particular importance since the bit error rate ei optical data transmission system from polarization to stood depends and this in turn on fluctuations of the bottom  Larization transmission behavior of a long light wave conductor path is influenced, a not finite Polarisa short periods with very high bit error rates would cause an endless polarization control can avoid this.

The polarization transformer according to the invention comes theoretically table with three different polarization elements, the but must be adjustable in large areas. The Ver Use of several polarization elements allows larger To tolerances and manages with small setting ranges. Also, the use of ferroelectric liquid crystals stall cells possible, so that moving elements are eliminated.

Embodiments of the invention are based on figures described in more detail.

Show it:

Fig. 1 shows a polarization ellipse,

Fig. 2 is a Poincaré sphere,

Fig. 3 shows a polarization transformer according to the invention,

FIGS. 4 to 8 different Ansteuerdiagramme,

Fig. 9 a transmission device and

Fig. 10 is a compensator for polarization mode dispersion.

The so-called Poincaré sphere has proven itself to represent polarization states. A polarization state is represented by a normalized Stokes vector. Since the length of the vector for fully polarized light is 1, its end points lie on the surface of the sphere. For better understanding, the so-called polarization lens is first shown in FIG. 1. The axes x and y lie in a plane perpendicular to the direction of propagation of the axis z. The electrical field strength components are labeled Ex and Ey. θ denotes the elevation angle of the major major axis c (main direction of polarization) with respect to the x-axis and ε the ellipticity angle. In the three-dimensional data position of the Poincaré sphere, the S1 axis denotes the degree of horizontal or vertical polarization. Since double angles are plotted in the Poin caré sphere, negative values on the S1 axis mean vertical polarization. P1 corresponds to a horizontal polarization state and P2 to a vertical polarization state. The S2 axis stands for + 45 ° / -45 ° linear polarization and the S3 axis for right / left circular polarization.

In the Poincaré sphere are also the equator and that of the plane spanned the axes S2 and S3; the By contrast, the plane spanned by axes S1 and S3 was only indicated.

By means of an "endless" polarization control, a be correct input polarization state, for example P1 (S1 = 1) transformed into any initial state and also any given change in this output polarisa tion state can be achieved directly. Be before this is discussed in more detail, the method of operation of the polarization transformer are explained in more detail.

In Fig. 3 an embodiment of the polarization transformer PT is shown. The light wave LW arriving via an optical waveguide is fanned out in a gradient lens L1 and passes through various polarization elements or retarders W1 to W6 in order to be combined in a second lens L2 and via a polarization-maintaining optical waveguide PMF (with a length that a group delay difference between the two obtained polarizations). Suitable retarder for the W1 to W6 are wave plates and particularly ferro-electric liquid crystal cells that are provided with electrodes and act as polarization elements by applying control voltages U1 to U6. Each retarder has two mutually orthogonal eigenmodes and an approximately constant transit time difference, called delay, between "fast" and "slow" eigenmodes. By individually controlling the retarders with different voltages, starting from a certain polarization state, for example P1 (S1 = 1), all points on the surface of the Poincaré sphere, ie all polarization states, can be reached.

A mental intermediate step is useful to facilitate understanding. A prerequisite for the desired operation is that the polarization transformer has at least approximately a total delay between its orthogonal eigenmodes corresponding to π rad, that is to say can cause a polarization change of 90 ° in the case of horizontal or vertical input polarization. In the representation on the Poincaré sphere, this means that the polarization state P1 on the axis S1 can be converted into the polarization state P2 (S1 = -1), which is shown opposite the point P1. The polarization transformer, viewed as a homogeneous element, naturally also has orthogonal eigenmodes in addition to the delay mentioned. The polarization transformation caused by it can therefore be specified in the form of a single rotation. The axis of rotation runs through the two eigenmodes. In the example given, it lies in the plane formed by the axes S2 and S3 and the angle 2 γ is specified between the positive S2 axis and the axis of the great circle. The transfer of one polarization state into the other polarization state was carried out via a large circle GK (from any number of large circles).

In Fig. 4 for a polarizer with 3 retarders W1, W2, and W3 the functions of the setting values, here the respective double elevation angle 2 xθ1, 2 xθ2 and 2 xθ3 of the individual retarders, are shown, for which for the most different values of 2 γ ( 2 γ corresponds to the pivoting of the natural modes or the axis of a great circle around the S1 axis and thus indicates any large circles going through S = 1 and S = -1) a linear input polarization state P1 (S1 = 1) with an elevation angle of 0 ° on the output side becomes a polarization state with an elevation angle of π / 2 (S1 = -1), ie it becomes a perpendicular polarization state.

However, if one does not want to achieve this maximum change in the state of polarization, but rather a “operational polarization state” PB required for compensation, the elevation angles indicated in FIG. 4 are to be reduced uniformly linearly in order to achieve a large circle determined by 2 γ to get to the desired state of polarization. Since with the elevation angle in general the eigenmodes of the entire polarization transformer also change a little, for example, emerge from the plane formed by the axes S2 and S3, the group of elevation angles, which is read at the associated value 2 γ, is not exactly decisive , but a value group of elevation angles that can be read in the neighborhood and that can be reduced linearly evenly. In this case, the desired polarization state PB is nevertheless achieved.

Additional functionality, which provides robustness against fluctuations of all types, for example temperature fluctuations, is obtained if, in the case of a set delay of at least approximately π, the eigenmodes of the polarization transformer can not only be set endlessly on a large circle of the Poincaré sphere, but instead also in the neighborhood of this great circle. A corresponding tolerance band TB, within which the eigenmodes of the polarization transformer can move with a set delay of π, is shown schematically in FIG. 2. This additional functionality arises automatically, especially in cases in which the polarization transformer is composed of four or more individual retarders, through the individual control options for these retarders.

It can be seen that the function profiles of the elevation angles as a function of 2 γ are continuous and periodic (same start and end values). For example, to generate endlessly variable output polarizations that represent the states (a) circular (S3 = 1), (b) linear with elevation angle - π / 4 (S2 = -1), (c) opposite circular (S3 = -1) , (d) linearly with elevation angle π / 4 (S2 = 1) and again (e) linearly with elevation angle π / 4 (S2 = 1) successively and with constant change, are continuous value groups, which with continuous values the horizontal axis can be read, using linear reduction.

The value groups for the individual polarization states cannot therefore be taken directly from the diagram shown in FIG. 4 - except for the points P1 and P2 - but rather approximately halved for the points lying in the plane S2, S3.

The functions shown for the heights to be set Practice angles fluctuate around mean values at which the input polarization pas the polarization transformer unchanged siert. This ensures that everyone between the possible starting polarizations mentioned possible polarization states by a corresponding Appropriate value group can be achieved with low amplitudes can.

The elevation angle shown in Fig. 4 must be converted into control voltages taking into account non-linearities and non-constant delays between the eigenmodes of the retarders and are dependent on the transit time differences of the eigenmodes. In the example given, the retarders W1 and W3 have a transit time difference (delay) between the eigenmodes corresponding to the angle of ϕ1 = ϕ3 = 1.8947375 rad (radians), and the retarders W2 have a transit time difference of 1.9427 rad.

The advantage of this embodiment is that it is very small number of wave plates required; is disadvantageous the relatively large maximum required setting angle of the Eigenmode. Ferroelectric liquid crystal cells, the are particularly suitable for realization currently only setting ranges of +/- 45 ° or less.

The large elevation angles or setting ranges can be significantly reduced by more than 3 retarders are provided. Fig. 5 shows this when using four retarders, the transit time differences for the eigenmodes ϕ1 = ϕ2 = -ϕ3 = -ϕ4 = 2,123 rad. This polarization transformer has the advantage that only one type of wave plate or liquid crystal element is required, of which the latter are rotated by 90 ° in the last. By rotating by 90 °, the slow and fast eigenmodes are interchanged, so that, based on the original eigenmodes, the sign of the delay changes. As can be seen from FIG. 5, the maximum adjustment ranges required are twice the elevation angle at about +/- 1.1 rad. This can already be achieved with ferroelectric liquid crystal cells (FLC). Instead of the deceleration mentioned, slightly different values (0.96... 1.1) x 2.123 rad can be used.

In Fig. 6 the corresponding values diagram is shown for 5 retarder. The delays are for ϕ1 = ϕ5 = 1.2860 rad, ϕ2 = ϕ4 = 2.1823 rad, ϕ3 = 1.4939 rad. In Fig. 7 the value diagram of a different polarization compensator is shown, the delays are ϕ1 = ϕ2 = ϕ3 = ϕ4 = ϕ5 = 1.27 rad. Because of the existing degrees of freedom, more than three polarization elements can always specify multiple sets of continuous functional courses.

Fig. 8 shows the values for 6 retarder. The delays are for ϕ1 = ϕ3 = -ϕ4 = -ϕ6 = 1.8652 rad and for ϕ2 = -ϕ5 = 2.631 rad. The rear three shaft plates are rotated by 90 ° in relation to the three front ones in order to be able to realize negative deceleration values with the same components. There are only three function profiles specified, each of which applies to an elevation angle of a wave plate and its negative function for another wave plate.

For embodiments with an odd number of elements, it is favorable to make the delay values symmetrical with respect to the middle element, since in this way the smallest possible required maximum setting ranges of the double elevation angles 2 xθ1, 2 xθ2,. . . surrender.

While wave plates have linear eigenmodes, this is not absolutely true for ferroelectric liquid crystal cells, since they have chiral molecules and a helical molecule superstructure. Nevertheless, the invention can also be used without restriction in these cases, provided that the eigenmodes move on a great circle of the Poincaré sphere when a cell voltage is applied. In practice, this will not always be the case. Nevertheless, a polarization transformer according to the invention can also be easily implemented for such cases, for example with ferroelectric liquid crystal cells, if four or more liquid crystal cells are provided. Even if the delays do not remain constant when the eigenmodes change, the additional degrees of freedom make it manageable. With reference to FIG. 2, it may be necessary in such cases, in particular, that the points P1 and P2 on the poin caré ball are not or not exactly opposite. As can be seen from the depictions of the value groups, the selectable function profiles of the adjustable eigenmodes increasingly change into sine profiles with an increasing number of retarders. Exact sine waveforms on all wave plates can only be achieved with an infinite number of wave plates.

A polarization transformer according to the invention can replace the previous polarization transformers, as described, for example, in patent application DE 36 31 798.5. As an alternative to this use in a compensator, the use of the invention in the transmission-side compensation or, more precisely, the avoidance of first-order polarization dispersion is possible by the transmission polarization of a polarization of the transmission path not subject to the polarization mode dispersion, a so-called "Principal state of polarization" corresponds. For this purpose, ferroelectric liquid cells, for example, can be installed in the transmitter laser module. This is shown in principle in FIG. 9, in which a laser LA and a polarization transformer PT are combined.

The invention can also be used as in a compensator for polarization mode dispersion. For this purpose, a polarization transformer PT1, PT2, PT3,. . . and a delay element which has an approximately frequency-dependent large delay time between its princi pal states of polarization, for example a piece of polarization-maintaining fiber PMF1, PMF2, PMF3,. . run through by a light wave LW. Such a compensator is preferably set on the receiver side at the end of the transmission link. In cases where any and generally several polarizations are present at the input of a polarization transformer at the same time, the input polarization is no longer equal to point P1 ( FIG. 1). However, the requirements for the polarization transformers are identical to those described above.

Claims (16)

1. polarization transformer (PT) with a plurality of adjustable retarders (W1-W6) passing through a light wave, which have two mutually orthogonal eigenmodes and at least an approximately constant transit time difference between these eigenmodes,
characterized by
that at least three of the retarders (W1, W2, W3) are controlled individually,
that at least one retarder can have intrinsic limits due to electrostatic fields, i.e. not endlessly adjustable eigenmodes,
that the eigenmodes of the retarders (W1, W2, W3) can be set so that there is at least approximately a running time difference between the eigenmodes corresponding to a phase difference of π rad,
that the transition of the first polarization state (P1) into the second polarization state (P2) takes place at least approximately over any large circles (GK) of a Poincaré sphere,
different groups of setting values ( 2 xθ1, 2 xθ2, 2 xθ3,...) of the individual retarders (W1, W2, W3,...) are assigned to them, and
that between the first polarization state (P1) and the second polarization state (P2), at least approximately along all great circles, changes in the polarization state can be carried out by at least approximately linear changes in the setting values ( 2 xθ1, 2 xθ2, 2 xθ3,...). 2. polarization transformer according to claim 1, characterized, that the transfer of the first polarization state (P1) in the second polarization state (P2) over any large circles (GK) of a Poincaré ball.   3. polarization transformer according to claim 1 or 2, characterized, that when setting a delay of π eigenmodes turned on can be set up on a large circle closing tolerance band (TB) of the Poincaré sphere endlessly above all let riar. 4. Polarization transformer according to claim 2 or 3, characterized in that at least one of the setting values ( 2 xθ1, 2 xθ2, 2 xθ3,... ) Of the retarders (W1, W2, W3,...) Functions not Si-shaped . 5. Polarization transformer according to claim 4, characterized in that the setting values ( 2 xθ1, 2 xθ2, 2 xθ3,... ) Are varied by means of mean values at which the input polarization remains unchanged. 6. Polarization transformer according to one of the preceding Expectations, characterized, that 3 to 8 retarders (W1, W2, W3,...) are provided. 7. polarization transformer according to claim 5 or 6, characterized, that retarders (W1 to W6) with at least approximately the same Setting options for their own modes are provided. 8. polarization transformer according to claim 6 or 7, characterized, that wave plates are provided as retarders (W1 to W8).   9. polarization transformer according to claim 6 or 7, characterized, that as a retarder (W1 to W8) ferroelectric liquid crystal l elements are provided. 10. polarization transformer according to claim 9, characterized, that four to eight liquid crystal elements are provided. 11. polarization transformer according to claim 6, characterized, that 3 retarders (W1, W2, W3) are provided, the runtime dif differences between the eigenmodes of at least approximately ϕ1 = 3 = 1.89947375 rad and ϕ2 = 1.9427 rad. 12. Polarization transformer according to claim 6, characterized, that four retarders (W1, W2, W3, W4) are provided, the run time differences between the eigenmodes of at least approx nd1 = ϕ2 = -ϕ3 = -ϕ4 = 2.123 rad. 13. polarization transformer according to claim 6, characterized, that five retarders (W1, W2, W3, W4, W5) are provided, the Runtime differences between the eigenmodes from at least approaching ϕ1 = ϕ5 = 1.2860 rad, ϕ2 = ϕ4 = 2.1823 rad and ϕ3 = 1.4939 rad. 14. polarization transformer according to claim 6, characterized, that 6 polarization elements (W1, W2, W3, W4, W5, W6) are provided hen, the transit time differences between the eigenmodes of at least approximately ϕ1 = ϕ3 = -ϕ4 = -ϕ5 = 1.8652 rad and, ϕ2 = -ϕ5 = 2.6310 rad.   15. Polarization transformer according to one of the preceding Expectations, characterized, that he has an optical transmission link for control the transmission polarization is provided. 16. Polarization transformer (PT1, PT2,...) According to one of the previous claims, characterized, that he is in series with a differential term (PMF1, PMF2, PMF3,...) Close to the receiver for controlling Po larization mode dispersion is provided.