Optical waveguides – With optical coupler – Plural
Reexamination Certificate
1998-05-08
2001-09-04
Font, Frank G. (Department: 2877)
Optical waveguides
With optical coupler
Plural
C385S024000, C385S003000, C385S008000, C385S009000, C385S037000, C385S046000, C359S199200, C359S199200, C359S199200
Reexamination Certificate
active
06285810
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to optical telecommunication systems functioning in a wavelength division multiplexing (WVDM) mode. More in particular the invention concerns a passive four port optical coupling device for wavelength division multiplexed optical signals which may be used as a filter for injecting or extracting (add/drop) an optical carrier signal or channel of a selected wavelength, from a plurality of WDM channels of distinct wavelengths.
In the transmission of information through an optical fiber network, the wavelength division multiplexing (WDM) technique is employed for incrementing the efficiency of the network system. WDM signals consist of a plurality of optical signals of different wavelength, referred to as carriers or channels through which information may be transmitted on a fiber.
FIG. 1
shows a node N of an optical fiber transmission network that includes an add/drop device to an input port or gate to which a fiber may be coupled, carrying n optical channels corresponding to n optical wavelengths, all contained in a certain “window” of the optical spectrum in correspondence of which state of the art optical fibers show a low attenuation, typically in the so-called III
rd
window and in the amplification band of erbium.
Each channel may be separately modulated in terms of intensity of the light (amplitude) according to a digital or an analog mode of modulation. Each channel is spectrally spaced from adjacent optical channels sufficiently to prevent cross-channel interference.
The usefulness of an ability of readily introducing and/or extracting a selected optical carrier or channel to and from a WDM signal at particular location of the optical fiber transmission network in order to carry out essential communication functions among different stations is evident.
A typical example is represented by the need for a first user, connected to a certain node of the network, to transmit information to a second user connected to another node of the network. The required add/drop devices would normally have two inlet ports and two outlet ports through which simultaneously or separately, extract/introduce a signal of a selected wavelength.
In the diagram of
FIG. 1
, extraction from the output gate d of the device of a signal &lgr;i and a simultaneous injection through the input gate b of the device of a signal &lgr;i so that on the output gate c be present a plurality of n multiplexed (WDM) signals:
1
, . . . &lgr;i . . . n, is assumed. In other words, at the node considered, the extraction of channel &lgr;i and the simultaneous addition of the channel &lgr;i is hypothesized to be required operation.
Optical networks of future generation are expected to integrate two complementary or interlaced architectures: one tape made of Optical Cross-Connect or more briefly OXC units and the other type consisting of ring structures based on the use of optical multiplexers of the add/drop type (OADM). Moreover, local optical networks (optical LAN) are gaining an increasing interest. The ring structure of a local optical network connecting a plurality of nodes through fibers carrying WDM channels rest primarily on the use of efficient and low cost add/drop filters.
Several forms of implementation of a suitable filter or bi-directional selective coupler suited to perform channel-adding as well as channel-dropping functions, have been proposed.
These known forms of implementation of a selective channel add/drop function usually contemplate the use of a certain number of directional couplers (AD) and phase shifters (SF), alternately connected in cascade, as depicted in FIG.
2
.
Notably, a directional coupler is implemented by a pair of waveguides running parallel to each other at a certain distance of separation d (not visible in the figure for graphical reasons), for a certain length l. The separation distance d and the length l are designed so as to determine a transfer of light energy from one guide to the other.
A phase shifter may be conveniently implemented by a pair of waveguides of different length that do not exchange energy among each other and introduce a relative phase shift of a predetermined value in the two optical signals (introduced through respective input ports) as output through respective output ports of the device.
The couplers (AD) are designed through an optimization procedure and usually are different from each other, while the phase shifters (SF), in view of the fact that they determine the periodicity of the transfer function of the filter, are customarily designed to be all substantially identical to each other.
The signal &lgr;i to be extracted is singled out by the sum of the effective lengths l of the single directional couplers. In the design stage, the sum of the guided wavelength equivalent length l of the single directional couplers is made equal to the guided wavelength equivalent coupling length L for the selected wavelength &lgr;i.
In order to better understand the function of the single phase shifting stages, let us consider the transfer function of the device of FIG.
3
.
As will be evident from the observation of
FIG. 3
, the transfer function is not sinusoidal as that of a WDM device, but shows a periodical form in terms of frequency, characterized by the presence of a spectral interval of separation between two successive peaks of the response characteristic of width sufficient to contain all the wavelengths of the WDM signal with the exception of the wavelength to be extracted.
In other words, all the channels that are not the object of the channel drop operation performed by the device, are available on an output gate of the device in a substantially nonattenuated state, while the selected channel is strongly attenuated (bar response characteristics traced with a continuous line). A dual situation occurs on the other output gate (cross response characteristics traced with a dash line) whereas the channel to be extracted is available in a substantially nonattenuated form while all the other (deselected) channels are strongly attenuated.
The known device in question is perfectly symmetric, so that if a channel &lgr;i* is injected through gate b, such a channel &lgr;i will be present on the output gate c together with the other channels
1
. . . n unaffected by a channel drop operation. In other words, in cross function, the channel to be extracted or injected passes from gate a to gate d or from gate b to gate c, while according to the bar function, all other channels (through channels) pass from gate a to gate c or from gate b to gate d.
By modifying the geometry of the phase shift stages, that is by modifying the relative delay of propagation that is introduced by the stages, the response characteristics of the device may be modified. Given the periodical form of the response characteristics of the device, it is possible to widen or narrow the interval of wavelength between two successive strongly attenuated wavelengths. In practice, if a broadening of this interval is desired, the relative delay introduced by the phase shift stages must be reduced, while by increasing said delay the interval is narrowed.
The number of “cells” composing the device, intending as an elementary cell the association of a phase shift stage with a relative directional coupler, that is the number of phase shift stages (SF) composing the device, has a direct influence on the attenuation characteristics of the device. More precisely, the attenuation in the interval comprised between two strongly attenuated wavelengths produced by the device is more uniform the larger the number of cells that compose the selective device.
For a deeper understanding of the functionality of a selective device suitable to implement channel adding and channel dropping functions, reference may be made to the article entitled: “Cascaded Coupler Mach-Zehnder Channel Dropping Filters for Wavelength—Division Multiplexed Optical System” by M. Kuznetsov, Journal of Lightwave Technology, Vol. 12, No. 2, February 1994.
For example, in the perspective of a Euro
Di Mola Domenico
Fincato Antonio
Lenzi Maurizio
Flores Ruiz Delma R.
Font Frank G.
Italtel SpA
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