This application claims priority to Application No. 10026240.6 which was filed in the German language on May 26, 2000.
The invention relates to a method for optical polarization control, and in particular, to endless polarization control of an optical signal.
For optical polarization control, polarization transformers are particularly suitable, preferably those of an electro-optical principle of operation. These transformers typically include one or more polarization actuators, the modes of which can be varied endlessly, i.e. without interruption, on a great circle of the Poincarxc3xa9 sphere. Examples of this are known from IEEE J. Quantum Electronics 18(1982)4, pp. 767-711, from IEEE J. Lightwave Techn. 6(1988)7, pp. 1199-1207, from IEEE J. Lightwave Techn. 8 (1990) 3, pp. 438-458, from Electron. Lett. 27(1991)4, pp. 377-379, and from the minutes of the European Conference on Optical Communications 1993, Montreux, Switzerland, pp. 401-404, contribution WeP9.3.
In practice, non-ideal behavior of polarization actuators can impede the capability for endless, uninterrupted polarization tracking which is always the aim. In IEEE J. Lightwave Techn. 8(1990)3, pp. 438-458, it was specified how this problem can be solved purely quantitatively, i.e. by adding another electro-optical polarization actuator. More elaborate tests have shown, however, that only one additional polarization actuator is hardly sufficient in practice.
In IEEE J. Lightwave Techn. 6(1988)7, pp. 1199-1207, a solution to solving this problem is to use a calibration table in a manner which provides the maximum control speed per control step. This can be disadvantageous, however, in that the constructional length of the polarization transformer normally required is distinctly increased by adding a further polarization actuator to an original electro-optical polarization actuator which sufficient in the ideal case, and is even double in this case. As an alternative, the control voltages could be distinctly increased while keeping the total construction length unchanged. Both methods contradict the usual requirements for low supply voltages and short constructional length for the purpose of avoiding DC drift and optical insertion losses. Using this technique, the further polarization actuator is driven as a function of the parameters of the original one. In the nomenclature used there, a phase angle difference, occurring for d3=0, d4=0, of d2xe2x80x2=atan2(f4(d2), f3(d2)) (atan2=arcfunction, known for example from the programming languages Pascal, C, Matlab) of the further actuator which occurs in the case of input polarization P1, which is circular here, between this and a proportion of output polarization P2 (which is orthogonal thereto and is thus oppositely circular here), is also a function of a corresponding phase angle difference d2 of the original one which is accounted for by a calibration table. Since figure 15 there is a twisted curve, the problem arises that atan2(f4(d2),f3(d2)) and d2 can differ, for example by an odd-number multiple of xcfx80. In these cases, the total delay of the polarization transformer formed by the two polarization actuators is obtained by subtracting the delay d1 of the original one and the delay sqrt (f3(d2){circumflex over ( )}2+f4(d2){circumflex over ( )}2) of the further polarization actuator. That is, d1xe2x88x92sqrt(f3(d2){circumflex over ( )}2+f4(d2){circumflex over ( )}2) for d1 greater than sqrt(f3(d2){circumflex over ( )}2+f4(d2){circumflex over ( )}2). This destructive interplay is the reason why a large constructional length and/or high control voltages are needed in accordance with the prior art. If the method specified is properly performed, atan2(f4(d2),f3(d2)) will change by 4xcfx80 or d2 by 2xcfx80 so that the amount |atan2(f4(d2),f3(d2))xe2x88x92d2 of the difference atan2(f4(d2),f3(d2))xe2x88x92d2 can grow without limits if d2 grows arbitrarily, e.g. by many times 2xcfx80. In particular, the discussed destructive interplay occurs there, for example with d2=0.7*xcfx80, d2=1.3*xcfx80 and d2=1.8*xcfx80.
Since this phase angle difference (d2 or atan2(f4(d2),f3(d2))) in each case represents an angle coordinate from an eigenmode of the polarization actuator on a great circle on the Poincarxc3xa9 sphere, the words angle coordinate will be used synonymously with this phase angle difference in the text which follows.
In IEEE J. Lightwave Techn. 6(1988)7, pp. 1199-1207, an electro-optical polarization transformer for transforming a particular one into any arbitrary polarization state or conversely has been described; such polarization transformers could be abbreviated by GSBA (Generalized Soleil Babinet Analogue). Polarization transformers for transforming any arbitrary polarization state into any arbitrary one will be abbreviated by ER (Elliptical Retarder) in the text which follows. ERs with calibration table(s) for compensating for non-ideal component behavior have not previously been known. This is especially true of ERs that necessitate a short constructional length since, compared with GSBAs, ERs already need twice the constructional length in the ideal case.
In Proc. Fourth European Conference on Integrated Optics ECIO 87, Glasgow, Scotland, pp. 115-118, a GSBA is specified with many sections and fixed angle coordinate differences (For example: xcex1*xcfx80/2 between adjacent electrodes, where xcex1=xcex9DEV/xcex9BEAT), which are only dependent on the wavelength but not on the desired polarization transformation, of different sections. Correspondingly, this GSBA also does not have a calibration table for compensating for non-ideal behavior.
In one embodiment of the invention, there is a method for endless polarization control of an optical signal. The method includes, for example, converting a polarization of the optical signal using polarization acutuators, with arbitrary, freely and interruption free selectable individual phase angle differences as individual angle coordinates, at least partially into a respective orthogonal, which together form a polarization transformer with a total delay and a total phase angle difference as a total angle coordinate between the polarization and a respective orthogonal, in which an individual angle coordinate is varied as a function of another angle coordinate, wherein the amount (|dxcex61|, |dxcex62|, |dxcex6|, |dxcex6*(2*ixe2x88x92nxe2x88x921)/(nxe2x88x921)|) of the variation (dxcex61, dxcex62, dxcex6, dxcex6*(2*ixe2x88x92nxe2x88x921)/(nxe2x88x921)) of an individual angle coordinate is limited for sequences of desired polarization transformations.
In another aspect of the invention, individual delays support each other in their effects (("psgr"1xe2x80x2 greater than "psgr"1, "psgr"xe2x80x2 greater than "psgr"2), ("psgr" greater than "psgr"1, "psgr" greater than "psgr"2)) by the limitation of the amount (|dxcex61|, |dxcex62|, |dxcex6|, |dxcex6*(2*ixe2x88x92nxe2x88x921)/(nxe2x88x921)|) of the variation (dxcex61, dxcex62, dxcex6, dxcex6*(2*ixe2x88x92nxe2x88x921)/(nxe2x88x921)).
In another aspect of the invention, the another angle coordinate is the actual or attempted total angle coordinate or an attempted individual angle coordinate.
In yet another aspect of the invention, a number of individual angle coordinates are varied as a function of the another angle coordinate.
In another aspect of the invention, at least in the case of equal attempted individual angle coordinates, one of the variations of individual angle coordinates is equal to the negative of another one of the variations or equal to zero.
In another aspect of the invention, at least one of the individual delays is varied as the function of another angle coordinate.
In still another aspect of the invention, one of the variations of the individual delays is equal to another one of the variations or equal to zero, at least in the case of equal attempted individual delays.
In another aspect of the invention, the method includes applying, to at least two additional polarization actuators, which are located before or after the polarization actuators in the beam path to produce another polarization transformer and form an elliptical retarder.
In another aspect of the invention, one of the polarization actuators operates as an electrooptical Soleil Babinet compensator which can convert circular polarizations at least partially into one another.
In yet another aspect of the invention, at least two additional polarization actuators producing an additional polarization transformer form an elliptical retarder with the polarization transformer.
In another aspect of the invention, at least one of the polarization actuators operates as an electrooptical Soleil Babinet compensator, the polarizations which are at least partially converted being circular polarizations.
In another aspect of the invention, at least one of the polarization actuators operates as an electrooptical Soleil Babinet analog, the polarizations which are at least partially converted being TE and TM polarizations.