1. Field of the Invention
The present invention relates to an optical filter which is suitably employable as a gain equalizer for equalizing the gain of optical amplification of signal light, or the like in an optical amplifier.
2. Related Background Art
An optical amplifier includes an optical waveguide, doped with a fluorescent material excitable with pumping light, for optically amplifying signal light; and pumping means for supplying pumping light to the optical waveguide. Such an optical amplifier is provided in a repeating 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 for transmitting signal light of a plurality of channels to optically amplify the plurality of channels in a collective manner at respective gains equal to each other, and to output each channel of signal light with a power held at a constant target value. Hence, for equalizing the optical amplification of signal light in such an optical amplifier, an optical filter having a loss spectrum in a form similar to that of the gain spectrum in the amplifying optical waveguide is used as a gain equalizer.
For example, literature 1xe2x80x94K. Inoue, et al., xe2x80x9cTunable Gain Equalization Using a Mach-Zehnder Optical Filter in Multistage Fiber Amplifiers,xe2x80x9d IEEE Photonics Technology Letters, Vol. 3, No. 8, pp. 718-720 (1991)xe2x80x94and literature 2xe2x80x94G. H. B. Thompson, et al., xe2x80x9cPlanar Waveguide Filters for Dynamic Equalization of EDFA Spectra,xe2x80x9d ECOC""99 (1999)xe2x80x94disclose techniques for flattening the gain of an optical amplifier by means of an optical filter using a Mach-Zehnder interferometer. The techniques disclosed in these literatures aim at adjusting the respective temperatures of individual optical couplers and individual branching optical paths in a Mach-Zehnder interferometer according to the input signal light power, so as to regulate the slope of transmission loss to wavelengths in optical filter, thereby compensating for the fluctuation in the slope of gain to wavelengths accompanying the fluctuation in input signal light power.
In the above-mentioned conventional techniques, if the power of signal light outputted from the optical amplifier is to be kept constant when the loss in an optical transmission line in front of the optical amplifier fluctuates due to some reason and thereby the signal light fed into the optical amplifier alters its power, it will be necessary for the optical amplification of signal light in the optical amplifier to change its gain. When the gain is changed, the wavelength dependence of gain, i.e., the slope of gain to wavelengths (gain slope), may fluctuate, thereby deteriorating the gain flatness of optical amplifier, which causes the respective powers of a plurality of channels of signal light outputted from the optical amplifier to deviate from each other. Therefore, the respective temperatures of individual optical couplers and branching optical paths in each Mach-Zehnder interferometer constituting the optical filter are adjusted according to the input signal light power, so as to adjust the slope of loss to wavelengths (loss slope) in optical filter, thereby compensating for the fluctuation in gain slope accompanying the fluctuation in gain slope. When the loss slope in optical filter is changed according to the input signal light power, however, the loss level in the signal light wavelength band may fluctuate, whereby the signal light outputted from the optical amplifier after being optically amplified may fluctuate and deteriorate its S/N ratio. Also, the number of heaters provided for adjusting the loss slope in this optical filter is 3 or 6, which is so large that the control of loss slope is complicated.
In order to overcome the problems mentioned above, it is an object of the present invention to provide an optical filter which is suitably employable as a gain equalizer or the like in an optical amplifier, whereas its loss slope is easy to control.
For achieving the above-mentioned object, the optical filter in accordance with the present invention comprises: (1) a main optical path for guiding light from an input end to an output end; (2) an auxiliary optical path, optically coupled to the main optical path with first, second, and third optical couplers, wherein the optical path length between said first and second optical couplers and an optical path length between said second and third optical couplers are different from the main optical path; (3) first temperature adjusting means, disposed in at least one of the main and auxiliary optical paths between the first and second optical couplers, for adjusting the temperature of the main and/or auxiliary optical paths; and (4) second temperature adjusting means, disposed in at least one of the main and auxiliary optical paths between the second and third optical couplers, for adjusting the temperature of the main and/or auxiliary optical paths.
In the optical filter in accordance with the present invention, the main and auxiliary optical paths and the first and second optical couplers constitute a first Mach-Zehnder interferometer, the main and auxiliary optical paths have respective optical path lengths different from each other between the first and second optical couplers, and the temperature of one or both of the optical paths is regulated by the first temperature adjusting means. Further, the main and auxiliary optical paths and the second and third optical couplers constitute a second Mach-Zehnder interferometer, the main and auxiliary optical paths have respective optical path lengths different from each other between the second and third optical couplers, and the temperature of one or both of the optical paths is regulated by the second temperature adjusting means. The temperature adjustment effected by each of the first and second temperature adjusting means makes it possible to set the slope of loss to wavelengths in a given wavelength band. Thus, this optical filter has a simple configuration and is easy to control the loss slope, whereby it can suitably be used as a gain equalizer in an optical amplifier, or the like, for example.
When the first and second temperature adjusting means are disposed in one of the main and auxiliary optical paths alone, it will be sufficient if only two pieces of heaters, Peltier devices, and the like employable as temperature adjusting means are provided, whereby it becomes further easier to control the loss slope.
Preferably, the optical filter further comprises control means for controlling the first and second temperature adjusting means so as to set a slope of transmission loss across the main optical path to wavelengths in a given wavelength band.
Preferably, between the first and second optical couplers and between the second and third optical couplers, the optical path length differences between the main and auxiliary optical paths are not longer than 42 xcexcm. This configuration is suitable for setting the loss slope deviation from a given straight line to 1 dB or less.
Preferably, the transmission loss characteristics across the main optical path may be substantially independent of wavelength in a given wavelength band by adjusting the first and second temperature adjusting means.
The optical filter in accordance with the present invention may be configured such that the bandwidth of the given wavelength band is 25 nm or wider, the absolute value of the slope of transmission loss across the main optical path to wavelengths in the given wavelength is variable at least within the range of 0 to 5 dB/125 nm, a deviation from a given straight line is 1 dB or less, and the minimum value of the transmission loss across the main optical path in the given wavelength band is 2 dB or less. Alternatively, it may be configured such that the bandwidth of the given wavelength band is 36 nm or wider, the absolute value of the slope of transmission loss across the main optical path to wavelengths in the given wavelength is variable at least within the range of 0 to 5 dB/36 nm, a deviation from a given straight line is 1 dB or less, and the minimum value of the transmission loss across the main optical path in the given wavelength band is 2 dB or less. In these cases, the optical filter maintains sufficient linearity and sufficiently low loss, while the loss slope is variable in a sufficient range, in the wavelength band from 1535 nm to 1565 nm, which is a signal light wavelength band, for example.