Optical waveguides – With optical coupler – Plural
Reexamination Certificate
2000-02-11
2001-09-18
Lee, John D. (Department: 2874)
Optical waveguides
With optical coupler
Plural
C385S016000, C359S199200
Reexamination Certificate
active
06292605
ABSTRACT:
The invention lies in the field of optical transmission using wavelength division multiplexing (WDM). It relates to wavelength filtering apparatus and more particularly to reconfigurable filters for selectively removing one or more channels of the multiplex. The invention also relates to apparatus that makes it possible selectively to drop and/or to insert at least some of said channels. Such apparatus may commonly be referred to as a“drop-and-insert” multiplexer and may advantageously use such reconfigurable filters.
BACKGROUND OF THE INVENTION
In WDM transmission systems, a plurality of channels are conveyed simultaneously over the same fiber, each of the channels being carried by an associated wave having a determined center wavelength and a determined bandwidth. In particular, for performing routing or switching functions, it is often necessary to drop certain channels and to modify the multiplex, e.g. so as to modify the information conveyed by one or more channels, or so as to replace the wavelength carrying the information with some other wavelength.
As shown diagrammatically in
FIG. 1
, in order to implement a drop-and-insert multiplexer, it is possible to use a stop filter
1
associated with drop coupling means C
1
and with insert coupling means C
2
. The filter is organized to prevent transmission of at least one of the wavelengths &lgr;k of the multiplex, and also to reflect it/them towards the drop coupling means C
1
. The drop-and-insert multiplexer becomes reconfigurable if the filter is itself reconfigurable as a function of a channel selection command CD.
In the example shown, the multiplexer is essentially constituted by the reconfigurable filter
1
provided with two opposite access ports Pa, Pb. The ports Pa and Pb are coupled respectively to the inlet coupling means C
1
and to the outlet coupling means C
2
. In this example, the coupling means are three-port circulators. The inlet circulator C
1
is provided with an inlet port P
12
suitable for receiving an input multiplex We. A second port P
2
is coupled to the first access port Pa of the filter
1
. A third port P
3
constitutes a drop port. The circulator C
1
is disposed in a manner such as to enable the input multiplex We to be fed into the filter
1
via the first access port Pa, while the wave Wd reflected by the filter
1
and exiting via the same port Pa is received by the port P
2
so as to be taken via the drop port P
3
.
In the same way, the outlet circulator C
2
is provided with a first port P
5
coupled to the filter
1
via the second access port Pb. A second port P
4
of the circulator C
2
makes it possible to take the wave exiting from the filter via the port Pb. The port P
4
constitutes an outlet port making it possible to take an output multiplex Ws. Finally, the circulator C
2
is provided with a third port P
6
which constitutes an insert port.
As a function of the command CD that is applied, the filter
1
is configured to have a stop comb or band set to one or more determined wavelengths &lgr;k of the multiplex.
In operation, an input multiplex We is fed in via the inlet port P
1
. The channels Wd of the multiplex that are carried by the wavelengths stopped by the filter are than dropped and accessible via the drop port P
3
. In addition, by injecting new signals Wa carried by the same wavelengths via the insert port P
6
, corresponding channels are inserted into the output multiplex Ws.
Conventional solutions for making a reconfigurable filter use fixed elementary filters associated with 2-to-2 optical switches of the“crossbar” type.
FIG. 2
shows a first conceivable example of such a solution.
The filter includes n 2-to-2 switches X
1
, X
2
, Xk, Xn connected in cascade so that one of the outlets of each switch is connected to one of the inlets of the next switch. In addition, the second inlet of each switch is connected to its second outlet via an associated stop filter F
1
, F
2
, Fk, Fn. Generally, each of the filters is organized to reflect a single one of the wavelengths &lgr;1, &lgr;k, &lgr;n of the inlet multiplex We, but for certain applications, it is also possible to use stop filters that stop combs of wavelengths corresponding respectively to determined subsets of channels.
For example, the 2-to-2 optical switches include moving optical waveguides, such as fiber segments, that can be actuated electro-mechanically, e.g. by means of electric micromotors or piezoelectric elements. The coupling between switches uses optical fibers and the stop filters can be made in the form of Bragg gratings photo-inscribed in the coupling fibers.
FIG. 3
shows another example based on a 1-to-2 optical switch X
1
, 2-to-2 optical switches Xk, Xn, and a 2-to-1 optical switch SW. In this example, the second outlet of each of the
n
first switches is connected to the second inlet of the next switch via a stop filter.
As explained in detail below, those solutions suffer from the drawback of not being optimal with respect to the insertion losses generated by the switches.
OBJECTS AND SUMMARY OF THE INVENTION
An object of the invention is to remedy that drawback.
To this end, the invention provides optical filtering apparatus including
n
stop filters set to different wavelengths or to different combs of wavelengths, and optical switching means having a plurality of states and suitable for coupling an input optical signal to any one of said filters or to a plurality of said filters selectively coupled in cascade, wherein said switching means are provided with n+l inlet ports of ranks 0 to n, and n+1 outlet ports of ranks 1 to n+1, as well as a set of waveguides making it possible to couple each outlet port selectively to one of said inlet ports of lower rank, wherein, with each of said filters being provided with first and second opposite ports, the first port and the second port of each filter are coupled respectively to an associated outlet port and to an associated inlet port of the same rank of said switching means, and wherein the inlet port of rank 0 is organized to receive said input optical signal, and the outlet port of rank n+1 is organized to deliver an output optical signal.
The switching means may be constituted by an (n+1)-to-(n+1) optical switch of the usual“crossbar” type. However, such a solution is not optimum as regards complexity and cost. A crossbar switch has (n+1)! possible states and must make it possible to couple any inlet port to any outlet port, which corresponds to (n+1)
2
possibilities for optical links between inlet ports and outlet ports. For the application considered, 2
n
states suffice because the order in which the selected filters are cascaded does not matter. That observation leads to possibilities for simplification as regards implementing the switching means in which it suffices to provide (n+1)(n+2)/2 possible optical links.
Thus, with a usual static crossbar switch constituted by n+1 1-to-(n+1) couplers interconnected with n+1 (n+1)-to-1 couplers via waveguides provided with controlled optical gates, the number of waveguides used is in fact only (n+1)(n+2)/2. It is thus possible to reduce the number of waveguides and the number of gates accordingly.
More precisely, in an advantageous configuration of the invention, said switching means are not organized to be capable of coupling each outlet port to said inlet ports having ranks higher than or equal to the rank of said outlet port.
Similarly, if an optical switch is used that is implemented by means of moving optical waveguides that are actuated electro-mechanically, an (n+1)-to-(n+1) switch of the usual crossbar type requires the switch to have n+1 waveguides in respective distinct planes so as to enable the waveguides to cross over one another. The reduction in the number of possible optical links to be provided may also be used advantageously to avoid this constraint.
Thus, in another aspect of the invention, the couplings between said filters and said switching means are organized such that
Bisson Arnaud
Bruyere Franck
Noirie Ludovic
Alcatel
Lee John D.
Rahll Jerry T
Sughrue Mion Zinn Macpeak & Seas, PLLC
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