Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2001-05-04
2003-10-14
Bovernick, Rodney (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
Reexamination Certificate
active
06633703
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an arrayed waveguide grating (AWG: Arrayed Waveguide Grating) type optical multiplexer/demultiplexer applicable as a wavelength selection element to wavelength division multiplexing (WDM: Wavelength Division Multiplexing) transmission systems.
2. Related Background Art
The AWG type optical multiplexer/demultiplexers (hereinafter referred to as AWG circuits) are widely applied to the wavelength selection elements in the WDM transmission systems, as wavelength filters enabling extraction or insertion of a specific wavelength by interference. The AWG circuits do not require so precise machining as required by diffraction gratings or so precise multilayer film formation as required by interference films, but they can be constructed by ordinary microprocessing such as lithography, etching, and so on. Therefore, the AWG circuits are expected to develop into dominant optical components in the future WDM transmission systems, also including the possibility of integration with other light waveguide elements.
Such AWG circuits have the structure in which an input waveguide, an input slab waveguide, channel waveguides (phased array) of mutually different lengths, an output slab waveguide, and output waveguides are integrated on a single substrate.
A variety of improvements have been proposed heretofore in the conventional AWG circuits and, for example, in order to decrease loss variation among signal channels (for flattening of passband), M. R. Amersfoort, et al., “Passband broadening of integrated arrayed waveguide filters using multimode interference couplers,” ELECTRONICS LETTERS 29th February, 1996, Vol. 32, No. 5 discloses the AWG circuit provided with a multimode interference coupler (MMI coupler: MultiMode Interference coupler, which will be referred to hereinafter as an MMI coupler) of the shape as illustrated in
FIG. 12A
, in the connection part between the input waveguide and the input slab waveguide (a first conventional example). Japanese Patent Application Laid-Open No. H09-297228 discloses the AWG circuit provided with a parabolic shape waveguide as illustrated in
FIG. 13A
, in the connection part between the input waveguide and the input slab waveguide (a second conventional example). Further, Japanese Patent No. 3039491 discloses the AWG circuit provided with a waveguide formed in the structure of combination of a tapered waveguide with a parabolic waveguide in the connection part between the input waveguide and the input slab waveguide (a third conventional example).
SUMMARY OF THE INVENTION
The inventor investigated the conventional AWG circuits of the structures as described above (the first to third conventional examples) and found out the following problems. Namely, the AWG circuit of the first conventional example is provided with the MMI coupler of the rectangular shape as illustrated in
FIG. 12A
, in the connection part between the input waveguide and the input slab waveguide. In this AWG circuit of the first conventional example, an electric field strength distribution of light having propagated through the input waveguide is badly disturbed during propagation in the MMI coupler, as illustrated in
FIG. 12B
, even if the MMI coupler is precisely processed. This disturbance of the electric field strength distribution is mainly caused by multiple reflection in the MMI coupler and much higher accuracy is required in processing for the width, length, etc. of the MMI coupler in order to achieve stable optical characteristics among the AWG circuits to be fabricated.
On the other hand, every AWG circuit of the above second conventional example is provided with the parabolic waveguide as illustrated in FIG.
13
A. In this AWG circuit, the electric field strength distribution of light propagating in the parabolic waveguide is rarely disturbed, as illustrated in
FIG. 13B
, because the effect of multiple reflection is reduced in the parabolic waveguide. However, the electric field strength distribution of the propagating light becomes broadening according to the propagation in the parabolic waveguide (i.e., the peak-to-peak separation of the electric field strength distribution gradually increases). Since the parabolic waveguide has the small slope dy/dx (where the y-axis is taken along the longitudinal direction of the parabolic waveguide and the x-axis along the normal direction to the longitudinal direction in
FIG. 12A
) of the side faces even near the optical output end face thereof, variation can occur among the optical characteristics of the respective AWG circuits produced without attainment of sufficient processing accuracy or the like even in this second conventional example. This also applies to the AWG circuit of the third conventional example provided with the similar parabolic waveguide.
The present invention has been accomplished in order to solve the problems as described above and an object of the invention is to provide an optical multiplexer/demultiplexer having structure capable of relaxing the processing accuracy or the like required for realizing improvement in transmission wavelength characteristics, such as decrease in the loss variation among the signal channels or the like, i.e., structure capable of realizing higher manufacturing tolerance.
An optical multiplexer/demultiplexer according to the present invention comprises a substrate, at least one input waveguide provided on the substrate, a first slab waveguide, a plurality of channel waveguides, a second slab waveguide, and a plurality of output waveguides provided corresponding to respective signal channels, and is an AWG optical multiplexer/demultiplexer applicable as a wavelength selection element to the WDM transmission systems.
In the optical multiplexer/demultiplexer according to the present invention, each of the first and second slab waveguides has a predetermined slab length. The slab length is normally equivalent to a focal length of an optical input end functioning as a lens surface of each slab waveguide. The input waveguide is a waveguide for guiding signals of channel wavelengths set as signal channels at predetermined wavelength intervals, each to the first slab waveguide, and an optical output end thereof is connected to an optical input end face of the first slab waveguide. The channel waveguides are waveguides of mutually different lengths and are flatly arrayed on the substrate in a state in which optical input ends of the respective channel waveguides are connected to an optical output end face of the first slab waveguide so as to place the first slab waveguide between the input waveguide and the channel waveguides while optical output ends of the respective channel waveguides are connected to an optical input end face of the second slab waveguide so as to place the second slab waveguide between the channel waveguides and the output waveguides. Further, the above output waveguides are waveguides flatly arrayed on the substrate in a state in which optical input ends thereof are connected to an optical output end face of the second slab waveguide and are waveguides for individually taking out the signals of the channel waveguides set at the predetermined wavelength intervals.
Particularly, the optical multiplexer/demultiplexer according to the present invention comprises a waveguide provided between the input waveguide and the first slab waveguide, said waveguide being a free propagation area for coupling part of the fundamental mode of light having propagated through the input waveguide, to a higher order mode. This free propagation area is comprised of a first portion having side faces which extend along predetermined curves so as to increase width from an optical output end of the input waveguide toward an optical input end face of the first slab waveguide, and a second portion provided between the first portion and the first slab waveguide and having width larger than that of the input waveguide. Therefore, the first portion functions to broaden the electric field strength distributio
Bovernick Rodney
McDermott & Will & Emery
Stahl Mike
Sumitomo Electric Industries Ltd.
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