Optical communications – Transmitter and receiver system – Including polarization
Reexamination Certificate
2000-06-15
2004-11-23
Sedighian, M. R. (Department: 2633)
Optical communications
Transmitter and receiver system
Including polarization
C398S147000, C398S159000, C398S158000, C398S184000, C385S011000, C359S483010, C359S484010, C359S490020
Reexamination Certificate
active
06823142
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a polarization mode dispersion compensating apparatus, and more particularly to an apparatus for compensating polarization mode dispersion that may occur to signal light in an optical transmission line.
BACKGROUND OF THE INVENTION
Along with the spread of the Internet, a demand for larger transmission capacity has been increasing without stopping. As is known in the art, optical fiber communication is suitable for the large capacity transmission. In the optical fiber transmission, it is relatively easy to increase the transmission capacity by employing a wavelength multiplexing transmission system, in which signal lights with different wavelengths propagate on only one optical fiber, so as to increase the number of the wavelengths and accelerate the modulation rate of the signal lights with the respective wavelengths. It has been the mainstream to employ a return to zero (RZ) signal in order to improve the receiving sensitivity and decrease cross phase modulation when the wavelength multiplexing transmission is performed.
Ideally speaking, an optical fiber should be rotationally symmetric about a center axis of a core (i.e. a center axis of the fiber). However, owing to the slight asymmetricity due to fluctuations in the production process, the amount of chromatic dispersion of the signal light differs according to an azimuthal direction around the center axis of the core. This causes the so-called polarization mode dispersion. When the modulation rate of the signal light exceeds 5 Gbit/s, as shown in
FIG. 8
, the RZ signal is divided into two orthogonal polarization components (i.e. the so-called TE and TM components) in the time domain due to the polarization mode dispersion. This causes bit errors in a receiving process at a receiving side. Although the time intervals of the divided orthogonal components depend on the condition of the optical transmission line, they generally vary disorderly with time.
Means for compensating such polarization mode dispersion are disclosed by Fabian Roy et al. in OFC'99IOOC (OFC (Optical Fiber Communication) and the International Conference on Integrated Optics and Optical Fiber Communications (IOOC)), TuS 4-1, pp. 275-278 and by Hiroki Ooi et al. in OFC'99IOOC (OFC (Optical Fiber Communication) and the International Conference on Integrated Optics and Optical Fiber Communications (IOOC) WE 5-1).
A conventional polarization mode dispersion compensating apparatus generally comprises a polarization controller for converting a signal light from an optical transmission line into two orthogonal polarizations, a polarization mode dispersion compensating element for giving a certain time difference between the two orthogonal polarization components of the output light, and a measurer for measuring the intensity or degree of polarization (DOP) of an output light from a polarization mode dispersion compensating element and for controlling the polarization controlling amount or rotational angle of the polarization controller so as to maximize the measured result. In the former reference, the DOP is measured. In the latter reference, a clock component of 20 GHz is measured which frequency is half of a 40 Gbit/s NRZ signal light.
The polarization controller comprises a configuration in which a quarter wave plate and a half wave plate are connected in serial, and the measurer rotates mechanically both wave plates about the optical axis according to the measured result. The polarization of the incident light is, as a result, converted into a linear polarization. The polarization dispersion compensating element generally comprises a polarization maintaining fiber. The polarization maintaining fiber comprises mutually orthogonal slow and fast axes having different chromatic dispersions each other. That is, since the propagation speeds of the signal light differ between the two axes, the polarization maintaining fiber can give the suitable amount of the polarization mode dispersion according to the difference of propagation speeds between the two axes and the propagation length. In the conventional art, the polarization controller is feedback-controlled so as to maximize the optical intensity or DOP of the output light from the polarization maintaining fiber. In this way, the time difference between the orthogonal components given at the optical transmission line is removed by the polarization maintaining fiber and thus the polarization mode dispersion is compensated.
In the standard long-haul optical fiber transmission line, the polarization of the signal light varies every several ten msec at the shortest. However, the response of the mechanical polarization controller having performed per second, the existing polarization mode dispersion compensating apparatus is unable to follow the fast polarization variation.
Also, in the conventional system, the mechanical polarization converter is employed and thus it is difficult to use it over a long period. In other words, it is not very reliable.
Moreover, in the conventional apparatus, the polarization maintaining fiber is employed of having a constant compensation amount for the polarization mode dispersion. Owing to this, when a signal light with a little amount of the polarization mode dispersion is input, the polarization maintaining fiber adds the polarization mode dispersion to the signal instead, conversely increasing the bit error rate.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a polarization mode dispersion compensating apparatus for adapting to any polarization state of input signal light and compensating the polarization mode dispersion.
Another object of the present invention is to provide a polarization mode dispersion compensating apparatus for compensating the polarization mode dispersion of a wider range.
A further object of the present invention is to solve the foregoing inconveniences and to provide a polarization mode dispersion compensating apparatus for responding more rapidly.
A still further object of the present invention is to provide a polarization mode dispersion compensating apparatus for automatically adapting to a polarization state of input signal light and compensating the polarization mode dispersion.
An even further object of the present invention is to provide a polarization mode dispersion compensating apparatus for maintaining high reliability over a long period.
According to the invention, an apparatus for compensating a polarization mode dispersion of an input signal light comprises a polarization converter for converting the polarization of the input signal light into a linear polarization, a polarization extractor for extracting at least one polarization component of two mutually orthogonal components in an output light of the polarization converter, a signal extractor for extracting a signal of a predetermined component from an output light of the polarization extractor, and a controller for controlling the polarization converter so as to increase the output of the signal extractor according to the output of the signal extractor.
With the aforementioned configuration, in the invention, the polarization mode dispersion compensating apparatus automatically adapts to the polarization state of the input signal light and compensates the polarization mode dispersion of the input signal light.
The signal extractor preferably comprises a photodetector for converting the output light of one polarization from the polarization extractor into an electric signal and an extractor for extracting the signal of the predetermined component from the output of the photodetector and applying it to the controller. The extractor comprises either electric filter for extracting the intensity of the clock component of the input signal light or for extracting a mean optical intensity of the input signal light. In this structure, the polarization mode dispersion of the input signal light can be compensated with such simple configuration.
The signal extractor preferably comprises a first photodetector for co
Tanaka Hideaki
Tanaka Shinsuke
Usami Masashi
Christie Parker & Hale LLP
KDDI Corporation
Sedighian M. R.
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