Optical waveguides – With optical coupler – Particular coupling function
Reexamination Certificate
2001-03-21
2003-12-16
Font, Frank G. (Department: 2874)
Optical waveguides
With optical coupler
Particular coupling function
C385S037000, C385S042000, C385S045000
Reexamination Certificate
active
06665467
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical wavelength filter.
2. Description of Related Art
Conventionally, an optical wavelength filter, which selectively outputs light of a specific wavelength from light entered into a Mach-Zehnder optical element having a diffraction grating, was utilized. In Document I (Optical Fiber Communication (OFC'99) TuN 3-1), an optical add-drop element is disclosed as an example thereof.
The optical add-drop element disclosed in Document I has either one or more optical couplers and a diffraction grating in each of a plurality of parallel optical waveguides. The diffraction grating either, for example, is disposed inside one optical coupler like the Bragg reflecting coupler shown in
FIG. 4
of Document I, or, for example, is disposed on two arms between two optical couplers as in the bandpass filter having a Michelson-Bragg diffraction grating shown in
FIG. 1
of Document I.
With such an optical add-drop element, by making light of a specific wavelength incident upon the diffraction grating, it is possible either to drop (to make wavelength division demultiplexing) light of a specific wavelength from the incident light, or to add (to make wavelength division multiplexing) light of a specific wavelength to the incident light.
However, in the optical add-drop element of Document I, the problem is that since the light propagating in the optical coupler ordinarily becomes a propagation light of a high-order mode comprising a plurality of specific (or intrinsic) modes, this gives rise to differences in the propagation constant between the specific modes, and the wavelengths reflected on the diffraction grating will differ for each mode.
That is, when wavelength division multiplexing light (WDM light or WDM optical signal) is made incident on the optical add-drop element, for example, whereas ideally single peak output light having a peak in only one specific wavelength should be selectively outputted, because the reflected wavelength selected in the grating differs for each mode, double peak output light having peaks in a plurality of barely separate wavelengths is outputted.
Further, as disclosed in Document II (Japanese Patent Publication No. 7-89183 (Japanese Patent No. 2053167)), if an optical coupler is constituted such that only one specific mode is excited, this kind of problem does not occur, but, in this case, the optical coupler must be constituted from an asymmetrical branching waveguide.
However, in this asymmetrical branching waveguide, since it is necessary to reduce the branching angle, for example, to around 0.1 &mgr;rad, and to quasi-statically change the structure relative to the direction of propagation of the light, the length of the element inevitably becomes longer, and as a result thereof, it was impossible to realize a more compact optical wavelength filter.
SUMMARY OF THE INVENTION
Therefore, an object of the present invention is to provide an optical wavelength filter, which is capable of suppressing the generation of double peaks caused by differences in the propagation constant between specific modes in an optical coupler.
Another object of the present invention is to provide an optical wavelength filter which, more preferably, is capable of suppressing the generation of double peaks without increasing the length of the filter.
Accordingly, an optical wavelength filter of the present invention comprises a first and second optical coupler and a mode converter. Each of the optical couplers propagate wavelength division multiplexing (WDM) light comprising a plurality of light components with different wavelengths, excite multiple modes of light including high-order mode light for each wavelength of light of the WDM light, and provide a predetermined phase difference between different modes of the multiple modes of light. The mode converter is arranged between the first and second optical coupler, and performs mode conversion between modes of different orders of a specific wavelength selected upon request from among the multiple modes of light excited by the optical couplers.
According to this constitution, because mode conversion is performed between modes of different orders of a specific wavelength selected as desired from among the multiple modes of light excited by the optical couplers, it is possible to adjust the phase difference between modes of light belonging to a specific wavelength of light independently from the phase difference between modes of light belonging to another wavelength of light. Accordingly, it is possible to output only light of a specific wavelength from an output or exit port separate from that for light of another wavelength. Since mode conversion is performed between modes of light belonging to a specific wavelength, it is possible to suppress the generation of double peaks that is caused by the difference in propagation constant for each mode of light in a specific wavelength.
Note that mode conversion in a mode converter signifies mode conversion between at the least modes of different orders. However, in addition thereto, there can also be cases in which mode conversion is performed for reflection and so forth.
Further, in an embodiment of the present invention, preferably a first optical coupler has a first input port for inputting WDM light thereto, and a first optical coupling region for exciting the multiple modes of light of the WDM light inputted from the first input port thereof, and in the optical coupling region, a phase difference of substantially ±&pgr;/2 is provided between the modes of light belonging to each wavelength of light, and a mode converter performs the mode conversion described hereinabove while generally maintaining the phase difference between the modes of light outputted from the first optical coupler, and a second optical coupler has a second optical coupling region for enabling the propagation of multiple modes of light, and a first output port and a second output port, and in this second optical coupling region, once again a phase difference of substantially ±&pgr;/2 is provided between the modes of light belonging to each wavelength of light outputted from the mode converter, and in accordance therewith, the light of a specific wavelength and other light can be outputted from either the first or second output ports, which differ from one another.
By so doing, from among the modes of light constituting WDM light inputted to this optical wavelength filter, a phase difference of either &pgr; or −&pgr; can be provided between the modes of light belonging to a specific wavelength, and a phase difference of either −&pgr; or &pgr; (double signs in same order) can be provided between the modes of light belonging to other wavelengths. That is, it is enabled to output the light of a specific wavelength and the light of other wavelengths from different output ports.
Furthermore, in the first optical coupler, second optical coupler and mode converter, a phase difference, which is either granted or maintained may be considered in a range from −&pgr; to &pgr; in accordance with periodicity of 2&pgr; for each mode of light.
Further, according to an optical wavelength filter of the present invention, because a first and second optical coupler can be formed by a coupler having a symmetrical branching structure, there is no need to utilize such an asymmetrical branching waveguide as disclosed in Document II, and as a result thereof, the element length of the optical wavelength filter can be shortened.
Further, in another preferable embodiment of the present invention, multiple modes of light excited by the first and second optical couplers may be made zero-order mode light and first-order mode light, and in that case, mode conversion in the mode converter can be performed between a zero-order mode light and a first-order mode light each constituting a specific wavelength of light. That is, an optical wavelength filter may be constituted as a 2-mode system, which utilizes two modes of light
Font Frank G.
Mooney Michael P.
Oki Electric Industry Co. Ltd.
Volentine & Francos, PLLC
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