Optical: systems and elements – Optical modulator – Light wave directional modulation
Reexamination Certificate
2000-07-26
2001-12-25
Epps, Georgia (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave directional modulation
C385S039000
Reexamination Certificate
active
06333807
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical filter applicable to gain equalizers for equalizing optical signals in optical amplifiers, and the like.
2. Related Background Art
An optical amplifier includes an amplification optical waveguide doped with a fluorescent material which is excitable with pumping light, and a pumping light source for supplying the pumping light to the optical waveguide; and is disposed in a repeater station in an optical transmission system or the like. In particular, it is important for the optical amplifier employed in a wavelength division multiplexing transmission system (WDM transmission system) transmitting a plurality of wavelengths of optical signals to not only collectively amplify a plurality of wavelengths of optical signals with gains identical to each other, but also output the individual optical signals with their power attaining a predetermined target value. Therefore, in order to equalize the amplification gain of optical signals in such an optical amplifier, an optical filter having a loss spectrum with a form identical to that of the gain spectrum in the signal wavelength band has been in use.
For example, a technique aimed at flattening the gain of an optical amplifier by use of an optical filter employing a Mach-Zehnder interferometer is disclosed in the document 1, K. Inoue, et al., “Tunable Gain Equalization Using a Mach-Zehnder Optical Filter in Multistage Fiber Amplifiers,” IEEE Photonics Technology letters, Vol. 3, No. 8, pp. 718-720 (1991). Also, an optical filter in which two optical filters each having the structure described in the above-mentioned document 1 are cascaded to each other is disclosed in the document 2, H. Toba, et al., “Demonstration of Optical FDM Based Self-Healing Ring Network Employing Arrayed-Waveguide-Grating ADM Filters and EDFAs,” Proceedings of ECOC'94, pp. 263-266 (1994). Further, an optical filter comprising a Faraday rotator adapted to alter the amount of rotation of polarizing azimuth of light, a birefringent plate, two birefringent wedge-shaped members, and a lens system is disclosed in the document 3, T. Naito, et al., “Active Gain Slope Compensation in Large-Capacity, Long-Haul WDM Transmission System,” Proceedings of OAA'98, WC5, pp. 36-39 (1999).
SUMMARY OF THE INVENTION
The inventors have studied the conventional techniques mentioned above and, as a result, have found problems as follows. Namely, if the power of optical signals entering an optical amplifier fluctuates due to the fact that the loss in an optical transmission line positioned in front of the optical amplifier fluctuates for some reason in the technique disclosed in the above-mentioned document 1, then the amplification gain for optical signals in the optical amplifier has to be changed in order for the optical signals emitted from the optical amplifier to keep its power constant. If the gain is changed, then the wavelength dependence of gain, i.e., gain slope (slope of a spectrum representing a relationship between wavelength and gain) fluctuates, so that the gain flatness of the optical amplifier is lost, whereby a plurality of wavelengths of optical signals emitted from the optical amplifier yield deviations among their respective powers.
In the technique disclosed in the above-mentioned document 2, for dealing with problems such as those mentioned above, the respective temperatures of the optical couplers and branched optical lines in each Mach-Zehnder interferometer constituting the optical filter are adjusted according to the power of incident optical signals. As a consequence, the slope of loss spectrum (representing a relationship between wavelength and loss) of the optical filter is adjusted, whereby the fluctuation in gain slope accompanying the power fluctuation of incident optical signals is compensated for. However, the slope of loss spectrum in the optical filter is changed according to the power of incident optical signals, and also the loss level of each optical signal in the signal wavelength band fluctuates, whereby the S/N ratio of amplified light outputted from the optical amplifier fluctuates or deteriorates. Also, the number of heaters provided for adjusting the slope of loss spectrum in the optical filter of the document 2 is 6, which is relatively large, whereby the slope control is complicated.
In the technique disclosed in the above-mentioned document 3, the amount of rotation of polarizing azimuth of light in the Faraday rotator is adjusted such that the power deviation among the individual optical signals becomes smaller, whereby the slope of loss spectrum in the optical filter is adjusted. As a result, the power deviation among individual optical signals is lowered. Unlike the technique disclosed in the above-mentioned document 2, the number of components is large in the optical filter of the document 3, whereby not only its configuration is complicated, but also its optical axis adjustment is quite difficult at the time of assembling.
In order to eliminate problems such as those mentioned above, it is an object of the present invention to provide an optical filter comprising a simple structure which can easily realize the slope control of its loss spectrum as a gain equalizer or the like in an optical amplifier.
For achieving the above-mentioned object, the optical filter according to the present invention comprises a main optical line having an entrance end for inputting light in a signal wavelength band and an exit end for emitting the light; and first and second auxiliary optical lines each arranged a long the main optical line. In this optical filter, a first part of the main optical line and the first auxiliary optical line constitute first and second optical couplers, whereas the first part of the main optical line, the first auxiliary optical line, and the first and second optical couplers constitute a first Mach-Zehnder interferometer. Also, a second part of the main optical line and the second auxiliary optical line constitute third and fourth optical couplers, whereas the second part of the main optical line, the second auxiliary optical line, and the third and fourth optical couplers constitute a second Mach-Zehnder interferometer. Further, the optical filter is provided with a first temperature regulating device for regulating the temperature of at least one of the first part of main optical line, which is positioned between the first and second optical couplers, and the first auxiliary optical line, and a second temperature regulating device for regulating the temperature of at least one of the second part of main optical line, which is positioned between the third and fourth optical couplers, and the second auxiliary optical line.
In particular, the optical filter according to the present invention comprises a control system electrically connected to the first and second temperature regulating devices, wherein the control system adjusts, by way of the first and second temperature regulating devices, the temperature of at least one of the first part of the main optical line and first auxiliary optical line and the temperature of at least one of the second part of the main optical line and second auxiliary optical line, thereby regulating the slope of loss spectrum in the optical filter in a state where the amount of loss of light at a reference wavelength in the signal wavelength band is fixed when the light propagates through the main optical line from the entrance end to exit end. Here, the loss spectrum indicates the loss in each wavelength of light in the above-mentioned signal wavelength band when each wavelength of light propagates through the main optical line from the entrance end to exit end.
In this optical filter, as mentioned above, the control system controls the first and second temperature regulating devices, so as to adjust the transmission characteristics of the first and second Mach-Zehnder interferometers, which are cascaded to each other while sharing the main optical line, whereby the slope of loss spe
Hatayama Hitoshi
Sasaoka Eisuke
Epps Georgia
McDermott & Will & Emery
O'Neill Gary
Sumitomo Electric Industries Ltd.
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