Optics: measuring and testing – By light interference – Using fiber or waveguide interferometer
Reexamination Certificate
1999-06-18
2001-08-07
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
Using fiber or waveguide interferometer
C356S484000, C356S073100
Reexamination Certificate
active
06271922
ABSTRACT:
This application is based on and claims the benefit of German Patent Application No. 198 27 638.9 filed Jun. 20, 1998, which is incorporated by reference herein.
BACKGROUND OF THE INVENTION
The invention is based on a method for measuring interference effects of a glass fiber transmission link as well as a transmission system according to the generic class of the independent claims. In Proceedings II, NOC '97, Antwerp, H. Bülow, pp. 65-72, the effects of polarization mode dispersion are described. Polarization mode dispersion occurs due to the birefringent effects in the glass fibers used for the transmission links. The light signal is divided into two components corresponding to the fast and the slow axes of the polarization states, whereby these two components are transmitted at different group velocities over the glass fiber link. The different group velocities of the two signal components cause interference and, in the general case, disturbance of the data information. The polarization mode dispersion effect is a statistical effect since the behavior of glass fibers varies due to their temperature, their state of strain, as well as due to aging effects. The change of the polarization mode dispersion is generally a slow process that takes place over longer periods. Consequently, it is not necessary to continuously measure the effects of polarization mode dispersion for a transmission link.
U.S. Pat. No. 5,473,457 discloses a method for measuring polarization mode dispersion. According to this method, the light signal is transmitted via a fiber and the polarization state is subsequently measured in a polarization controller. The signal is then supplied to a polarization maintaining fiber. The two polarization planes of the light are then separated in a device and coherently superimposed. The signal serves to calibrate the polarization controller. In this manner, the polarization state of the signal is actively influenced.
SUMMARY OF THE INVENTION
The inventive method for measuring interference effects caused by polarization mode dispersion with the characteristic features of the independent claim has the advantage, in contrast, that a simple measurement of an output signal comprising two frequency bands at a coherent receiver supplies a measure for the effects, which is available for further analysis. This method completely dispenses with the use of polarization controllers, as well as polarization measurements, and polarization maintaining fibers, all of which are optical components that cause great complexity and great costs. The method according to the invention makes possible a simple measurement of the characteristics of the transmission link by purely electric means.
The procedures set forth in the subclaims permit advantageous further embodiments and improvements of the method defined in the independent claim. Using this measuring method and using a high frequency, e.g. of 60 GHz, one can determine the polarization mode dispersion (PMD) at the receiver by means of a coherent measurement method. Advantageously, the method according to the invention is not used for the entire transmission period since polarization mode dispersion involves slow changes of the properties of the transmission link. It is therefore advantageous to use only one modulator, which as a measuring unit produces a side band modulated signal at least from time to time and thus delivers a signal that can be evaluated for the measurement.
A further advantage of the method is that it can be used online and thus permits a continuous transmission of the side band modulated signals for statistical recording and analysis.
The transmission system according to the invention, comprising an optical sender, a transmission link, and an optical receiver, has the advantage that one modulator produces the side bands for the measuring signal, while a second modulator processes the signals for the data. A further advantageous embodiment uses one modulator for both producing the side bands and for data modulation.
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“Polarization-mode dispersion mandate computation and control” Lightwave, Sep. 1996, pp. 43-47.
Bulow Henning
Schmuck Harald
Alcatel
Sughrue Mion Zinn Macpeak & Seas, PLLC
Turner Samuel A.
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