Optical waveguides – With optical coupler – Plural
Reexamination Certificate
1998-01-29
2001-04-10
Lee, John D. (Department: 2874)
Optical waveguides
With optical coupler
Plural
C385S015000, C385S037000
Reexamination Certificate
active
06215922
ABSTRACT:
BACKGROUND TO THE INVENTION
1. Field of the Invention
The present invention relates to a lightwave filter which allows light signals with a plurality of lightwaves to be selected and extracted from a multiplexed light signal, and to a lightwave selective router which makes use of this.
2. Description of the Related Art
There are lightwave filters which allow light signals with a plurality of lightwaves to be selected and extracted from a multiplexed light signal. One example of a conventional lightwave filter of this sort is to be found in the preliminary collection of papers read at the International Conference on Optical Fiber Communication 1996 (Reference Material 1). The lightwave selective filter which is disclosed in this paper is configured by connecting a plurality of reflected lightwave variable gratings rectilinearly. The wavelength of the light signal which each grating reflects basically corresponds one by one to the wavelengths of the multiplexed light signal, and this reflection wavelength is known as the basic reflection wavelength. Since light signals can be allocated to them, each multiplexed wavelength is also referred to as a wavelength channel, irrespective of whether a light signal is actually allocated to it or not.
The reflected lightwave variable gratings are capable of shifting the wavelength of light signals which they reflect from the basic reflection wavelength to other new reflection wavelengths. A wavelength shifted in this manner is known as a shift wavelength.
When selecting a wavelength, a lightwave filter is able, by causing the reflection wavelength of the grating to shift from the basic reflection wavelength to a shift wavelength, to allow light signals of a specific wavelength through, while reflecting those of other wavelengths.
Moreover, in optical communications using multiplexed optical signals it is desirable, when a light signal of a specific wavelength has been isolated and extracted from a multiplexed optical signal, to ensure the effective use of wavelengths by inserting a new optical signal into the vacant wavelength channel. An example of a lightwave selective router which interchanges light signals of specific wavelengths from among multiplexed light signals of this sort is disclosed in the
Journal of Lightwave Technology
14: 1320-40, 1996 (Reference Material 2). In the lightwave selective router illustrated in
FIG. 17
of Reference Material 2 in particular, a wave-dividing element, whereby the multiplexed light signal is divided by wavelength channel, is connected along with a wave-joining element to each wavelength channel by means of optical fibers. A 2×2 optical switch is provided on the light propagation path of each optical fiber, enabling each wavelength channel's light signals to be inserted or isolated by the operation of these 2×2 switches.
However, in the case of high-density multiplexed light signals, the wavelength interval between neighboring wavelength channels is short. Thus, use of the lightwave filter described in the above-mentioned paper with, for instance, a high-density multiplex where the wavelength interval is about 1 nm would result in the shift wavelength from the basic reflection wavelength of one grating overlapping with the wavelength of the wavelength channel neighboring this basic reflection wavelength. In such a case, for instance, it becomes difficult to extract light signals of a plurality of neighboring wavelengths by allowing them to pass selectively through a lightwave filter. Otherwise, if a wavelength is to be prevented from overlapping neighboring wavelengths, it is necessary to shift the reflection wavelength into a wavelength band outside the multiplexed band.
For this reason, it has been hoped that a lightwave filter would materialize which would be capable of extracting light signals of a plurality of wavelengths even in the case of high-density multiplexing.
Moreover, the lightwave selective router which is described in the second paper mentioned above makes use of a 2×2 optical switch, but it has hitherto been impossible to obtain a 2×2 optical switch with a sufficiently high switching speed. What is more, 2×2 optical switches in general require a large amount of motive power, it is difficult to reduce crosstalk sufficiently to improve the accuracy of the selected wavelength, and they are dependent on polarization. Thus, the speed of switching wavelengths and the accuracy of the selected wavelength in conventional lightwave selective routers have been restricted by 2×2 optical switches. For this reason, it has been hoped that a new lightwave selective router would materialize which would not need to make use of a 2×2 optical switch.
SUMMARY OF THE INVENTION
A first aspect of the present invention provides a lightwave filter having an input port, an output port, an optical router unit connected to these input and output ports, and a plurality of rows of gratings each connected individually to this optical router unit. This lightwave filter has the following characteristics.
Each of the rows of gratings has a fixed reflection wavelength grating whereof the reflection wavelength is fixed, and a variable reflection wavelength grating whereby it is possible to shift the reflection wavelength from a basic reflection wavelength to another wavelength. These are connected to each other rectilinearly, and wavelengths are selected and reflected by both the fixed and variable gratings. The new reflection wavelength which results from shifting is known as the shift wavelength.
The optical router unit guides the input multiplexed optical signal from the input port to the first row of gratings. Optical signals which are reflected by the first row of gratings are guided to subsequent gratings. Those which are reflected by the last row are output towards the output port.
In each row of gratings the fixed and basic reflection wavelengths differ from each other. Each wavelength of an input optical signal matches in each row of gratings with either the fixed or the basic reflection wavelength. Each wavelength of an input optical signal also matches in each row of gratings with the basic reflection wavelength of one row. The shift wavelength of a given variable reflection wavelength grating is always a wavelength other than the one which is to be selected and reflected by that row of gratings.
In this manner, each row of gratings in this configuration of the lightwave filter comprises a variable reflection wavelength grating (hereinafter referred to also as a ‘variable grating’) and a fixed reflection wavelength grating (hereinafter referred to also as a ‘fixed grating’). Thus, when selecting a wavelength it is possible to shift the reflection wavelength of the variable grating to a wavelength other than the basic reflection wavelength of the other variable gratings, which is to say to a shift wavelength (non-basic reflection wavelength). If this shift wavelength is, for instance, the reflection wavelength of a fixed grating, it is possible by shifting the reflection wavelength of the variable grating to ensure even with a high-density multiplexed optical signal that the optical signal of the desired specific wavelength is selected and reflected. Consequently, this selection and reflection is feasible, for instance, for all the multiplexed optical signals even with high-density multiplexing.
Moreover, the reflection wavelength of a variable grating during selection of wavelengths may be shifted, for instance, to a reflection wavelength of a fixed grating in the immediate vicinity of the basic reflection wavelength, so that there is no need to shift the reflection wavelength outside the multiplex band.
In the preferred embodiments, the lightwave filter to which the present invention pertains has an optical circulator as the optical router unit. This optical circulator has an input terminal, an output terminal and a plurality of input/output terminals, the input terminal being connected to the input port, the output terminal being connected to the output p
Lee John D.
OKI Electric Industry Co., Ltd.
Wenderoth , Lind & Ponack, L.L.P.
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