Optical waveguides – Polarization without modulation
Reexamination Certificate
2001-02-12
2003-12-09
Stafira, Michael P. (Department: 2877)
Optical waveguides
Polarization without modulation
C385S031000, C359S490020
Reexamination Certificate
active
06661936
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a method for characterizing polarization transformers and, in particular, polarization transformers traversed by optical signals.
2. Description of the Related Art
International Patent Application PCT/DE 99/03875 discloses polarization transformers and compensators of polarization mode dispersion wherein redundant polarization setting elements that are not required for the actually desired function are utilized for generating control signals that are free of d.c. parts on chronological average. This serves the purpose of avoiding what is referred to as d.c. drift that makes the optical properties of polarization setting elements time-variant and, thus, unpredictable. Despite the described procedure it does not seem certain that d.c. drift can be thereby completely avoided. Moreover, polarization transformers are also subject to other modifications, for example temperature-dependent modifications, of their optical and electro-optical properties. If the properties of a polarization transformer that are present at the moment could be measured during operation, then unwanted variations such as d.c. drift could be neutralized by adapting parameters such as, for example, offset voltages.
In the reference IEEE J. Lightwave Techn. 6(1988)7, pp. 1199-1208, at pages 1205-1206 is disclosed a characterization method for electro-optical wave plates. This and other known characterization methods cannot be utilized during the operation of polarization transformers or PMD (Polarization Mode Dispersion) compensators. The reason for this is that specific operating parameters must be set for the characterization, so that the required degrees of freedom would no longer be available for the actual polarization transformation or PMD compensation. Bit errors due to faulty PMD compensation, for example, would thereby arise in the optical data transmission.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to improve the d.c. drift suppression or other aging phenomena suppression in the form of a method for characterizing polarization transformers.
This object is achieved by a method for characterizing a polarization transformer that is traversed by an optical signal, including the steps of providing a first polarization transformer to be characterized and a further polarization transformer that lies before or after the first transformer in a light path of the optical signal and connected so that both are traversed by the optical signal, and, during the characterization of the first transformer, the further polarization transformer at least approximately assures the implementation of the desired polarization transformation or compensation of polarization mode dispersion of the optical signal.
Advantageous developments are provided by a method further providing that a plurality of polarization transformers are characterized by alternating characterization of one polarization transformer and implementation of a desired polarization transformation by this polarization transformer. Preferably, one polarization transformer acts as an elliptical retarder with a delay. At least a part of this polarization transformer acts as a mode converter with a selectable delay and selectable orientation, i.e. phase difference, between the converted and non-converted signals. In one embodiment, at least a part of this polarization transformer acts as a Soleil-Babinet analog with selectable delay and selectable orientation with mode conversion between the TE and TM modes. According to a development of the invention, at least a part of this polarization transformer acts as a Soleil-Babinet compensator with a selectable delay and selectable orientation with mode conversion between circular polarizations. A mode converter to be characterized may be operated with full mode conversion, i.e. a delay equal to an odd-numbered multiple of the value &pgr; and variable orientation. A mode converter functionally following this mode converter to be characterized at least approximately experiences changes of its orientation that correspond to twice the change of the orientation of the mode converter to be characterized. A polarization transformer functionally preceding this mode converter to be characterized at least approximately experiences no change of its control parameters.
The polarization transformer to be characterized in an embodiment is operated with a delay equal to an even-numbered multiple of the value 2&pgr; and variable orientation. The polarization transformer to be characterized may be operated with variable delay. As a preferred development, changes of the polarization transformation of the optical signal caused by parameter variation of a polarization transformer to be characterized are detected by a detection means. Specifically, a change of other parameters of a polarization transformer to be characterized is undertaken as a function of an output signal of this detection means. This ensues for the purpose of minimizing an output signal of this detection means, or for the purpose of maximizing an output signal of this detection means. The output signal of this detection means can be supplied to a controller and another polarization transformer to at least approximately compensate these changes of the polarization transformation. A change of other parameters of a polarization transformer to be characterized is undertaken as a function of the change of parameters of a further polarization transformer. In a particular embodiment, this ensues for the purpose of minimizing the change of parameters of a further polarization transformer or, alternately, this ensues for the purpose of maximizing the change of parameters of a further polarization transformer.
A polarization transformer to be characterized is part of a polarization transformer that can generate a fixed polarization from every arbitrary polarization of the optical signal or serves for the receiver-side PMD compensation; and potentially existing polarization transformers functionally preceding the polarization transformer to be characterized lie in the beam path preceding the polarization transformer to be characterized, whereas potentially existing polarization transformers that functionally follow lie in the beam path behind the polarization transformer to be characterized.
A polarization transformer to be characterized is part of a polarization transformer that can generate any arbitrary polarization from a fixed polarization of the optical signal or serves for transmission-side PMD compensation; and potentially existing polarization transformers functionally preceding the polarization transformer to be characterized lie in the beam path following the polarization transformer to be characterized, whereas potentially existing polarization transformers that functionally follow lie in the beam path preceding the polarization transformer to be characterized.
In the method according to the invention, the first polarization transformer can be operated with voltages opposite those for the implementation of desired polarization transformation at times wherein the further polarization transformer at least approximately assures the implementation of the desired polarization transformation or compensation of polarization mode dispersion of the optical signal.
The solution of the problem is comprised in the characterization of polarization transformers or PMD compensators during their operation that are to be equipped with redundancy for this purpose. Some of the degrees of freedom thereby serve the desired control function, whereas some other, redundant degrees of freedom serve for the characterization. The architecture of the polarization transformer and the control voltages are selected such that the control function of the polarization transformer is not negatively affected.
In the inventive method, for example, a polarization transformer or PMD compensator is employed for normal control purposes and modifications of the desired polarization transformation d
Bell Boyd & Lloyd LLC
Stafira Michael P.
Valentin II Juan D
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