Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2001-05-25
2003-12-16
Lester, Evelyn (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S483010, C359S484010, C359S256000, C359S251000, C359S252000, C359S320000, C359S301000, C385S008000, C385S011000
Reexamination Certificate
active
06665106
ABSTRACT:
CLAIM FOR PRIORITY
This application claims priority to Application No. 10026240.6 which was filed in the German language on May 26, 2000.
TECHNICAL FIELD OF THE INVENTION
The invention relates to a method for optical polarization control, and in particular, to endless polarization control of an optical signal.
BACKGROUND OF THE INVENTION
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 Poincaré 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 d
3
=0, d
4
=0, of d
2
′=atan
2
(f
4
(d
2
), f
3
(d
2
)) (atan
2
=arcfunction, known for example from the programming languages Pascal, C, Matlab) of the further actuator which occurs in the case of input polarization P
1
, which is circular here, between this and a proportion of output polarization P
2
(which is orthogonal thereto and is thus oppositely circular here), is also a function of a corresponding phase angle difference d
2
of the original one which is accounted for by a calibration table. Since figure 15 there is a twisted curve, the problem arises that atan
2
(f
4
(d
2
),f
3
(d
2
)) and d
2
can differ, for example by an odd-number multiple of &pgr;. In these cases, the total delay of the polarization transformer formed by the two polarization actuators is obtained by subtracting the delay d
1
of the original one and the delay sqrt (f
3
(d
2
){circumflex over ( )}2+f
4
(d
2
){circumflex over ( )}2) of the further polarization actuator. That is, d
1
−sqrt(f
3
(d
2
){circumflex over ( )}2+f
4
(d
2
){circumflex over ( )}2) for d
1
>sqrt(f
3
(d
2
){circumflex over ( )}2+f
4
(d
2
){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, atan
2
(f
4
(d
2
),f
3
(d
2
)) will change by 4&pgr; or d
2
by 2&pgr; so that the amount |atan
2
(f
4
(d
2
),f
3
(d
2
))−d
2
of the difference atan
2
(f
4
(d
2
),f
3
(d
2
))−d
2
can grow without limits if d
2
grows arbitrarily, e.g. by many times 2&pgr;. In particular, the discussed destructive interplay occurs there, for example with d
2
=0.7*&pgr;, d
2
=1.3*&pgr; and d
2
=1.8*&pgr;.
Since this phase angle difference (d
2
or atan
2
(f
4
(d
2
),f
3
(d
2
))) in each case represents an angle coordinate from an eigenmode of the polarization actuator on a great circle on the Poincaré 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: &agr;*&pgr;/2 between adjacent electrodes, where &agr;=&Lgr;DEV/&Lgr;BEAT), 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.
SUMMARY OF THE INVENTION
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 (|d&zgr;
1
|, |d&zgr;
2
|, |d&zgr;|, |d&zgr;*(2*i−n−1)/(n−1)|) of the variation (d&zgr;
1
, d&zgr;
2
, d&zgr;, d&zgr;*(2*i−n−1)/(n−1)) 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;
1
′>&psgr;
1
, &psgr;′>&psgr;
2
), (&psgr;>&psgr;
1
, &psgr;>&psgr;
2
)) by the limitation of the amount (|d&zgr;
1
|, |d&zgr;
2
|, |d&zgr;|, |d&zgr;*(2*i−n−1)/(n−1)|) of the variation (d&zgr;
1
, d&zgr;
2
, d&zgr;, d&zgr;*(2*i−n−1)/(n−1)).
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
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