Note: Descriptions are shown in the official language in which they were submitted.
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WO96/24871 PCTISE96/00121
Arrange~ent and method at an optical fibre
TECHNICAL SPHERE
The present invention relates to an arr~n~m~nt and a method for selective switchin
of wavelength channels to and from an optical fibre.
PRIOR ART
In the sphere of telecommunications there is, in many cases. a need for very high
tran~mi~ion capacity. By ut~ ing optical transmission by way of modulated optical
signals, very rapid data tr~n~mi~ion can be achieved.
In order to L1~L~11IiL a plurality of optical signals o~er a common optical mPAinm
wavelength division multiplexing (WDM) is used. The signals are Ll~ A by
way of independent wavelength ch~nn~l~, which can exist simlllt~n~ously in an
optical fibre.
For selectively ~wiLcl~g a wavelength channel to and from an optical fibre, that is
introducing or removing a wavelength channel on an optical fibre, optical
multiplexers and wavelength-selective ~ mllltirlexers are used.
An optical cl~mllltiplexer cwl~l;~l.lg waveguide coupler and waveguide grating is
known from US-A-5 195 161. A waveguide coupler arran_ed between two parallel
waveguides connects to waveguide gratings, so called Brag~ reflectors, c-~nn~te~ to
two output ports on the coupler.; By means of the said waveguide grating all light
of a desired wavelength can in principle be reflected to an outlet port parallel with
the inlet port by constructively using il~l~r~lcllce. The disadvantage to this multi-
plexer is that all light of a desired wavelength is only fed out if a very precise ratio
exists between the length of waveguide branches between the coupler and the two
reflectors. The dirr~,ellce in optical wavelength must be measured to within fractions
of a light wavelength; otherwise total e~tin~ hing can occur at the outlet. Thistype of precision is not practically feasible, however, in the case of optical fibres, but
can only be achieved in integrated optics. Demultiplexers are consequently both
expensive and diff1cult to use in a fibre-optic network.
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A fibre coupler for selectively switching a wavelength channel to or from an optical
fibre is already known from US-A~} 673 270. This fibre coupler comprises two
optical fibres with di~~ refractive index profiles, which are arran_ed ~ Pnt to
one another in a switching area. In order to connect a certain wavelength channel
5 between the fibres. the connection coefficient must be periodically varied along the
switching area. Ihis addingidropping coupler is difficult to implement since high
precision is required in the switchin~ area between the fibres in order to permit
switching of a certain wavelength channel in the fibre coupler. For this reason this
does not represent an economically or practically feasible solution in many contexts.
DESCRIPTION OF THE INVENTION
The object of the present invention is to provide a wavelength-selective multiplexer
and dPm-lltir~lexer, that is an adding/dropping multiplexer (ADM), by means of which
wavelength ~ h~nnt~l~ can be switched to and from an optical fibre and blocked at
15 ~I,ill~y positions along this in a system with wavelength division multiplexing
(WD~.
Ihis is achieved by the ~n~n~m~nt according to the invention, which c~ mpn.~es two
fibre couplers and one or more fibre ~tin&~ arranged 1X~W~;11 them. Ihe first fibre
20 coupler is ~l~i~l to divide up an incoming optical signal, which co~ " ;~P~ aplurality of wavelength ~h~nn~l~, into a first and a second branch in the fibre
coupler. The strength ratio between the component signals can be varied ~I,iL~ily.
The cclll~oll~ll signal in the first branch is allowed to pass through the fibre grating,
with the exception of one or more wavelength rh~nn~l~ which are reflected in one or
25 more ~tin~ and cnn~eqllPntly prevented from c~ntin--ing on the fibre. Each wave-
length channel which is reflected in a grating is allowed to pass to a third branch in
the first fibre coupler. Other wavelength ~h~nnel~ are allowed to pass further in the
arr~n~ nt Ihe second or third branch of the first fibre coupler is connected to
one or more receivers, preferably int~n~led for the wavelengths blocked in the fibre
30 grating. Ihe second fibre coupler is arranged in a position on the other side of the
fibre grating along the optical fibre. The said fibre coupler is ~ ptecl for introducing
one or more wavelengths removed on the first fibre coupler or for introducing wave-
lengths which are not already present on the fibre.
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The invention also relates to a method for seiectively switching wavelength channels
to and from an optical fibre. In the method according to the invention optical
signals. comprising a plurality of wavelength ~h~nn~-lc, are carried to a first fibre
coupler. The signal is divided up into two component signals, one of which is fed
to a fibre grating, which reflects a certain wavelength channel and prevents this
from continl~ing do~ along the optical fibre. The wavelength channel
corresponding to the blocking area of the fibre grating is taken off to a receiver
conne~teA to the first fibre coupler. One or more wavelength channels are there~er
introduced on the optical fibre in a second fibre coupler.
One advantage to the arr~n~emPnt according to the invention is that it is fibre-based
and simple in its structure and therefore inexpensive to implement.
DESCRIPTION OF FIGURES
Figure 1 shows a st h~.m~tic tli~m of a wavelength-selective adding/dropping
multiplexer (ADM) for selective ~wilcl,illg of a wavelength t~h~nn~l;
Figure 2 shows a srh~.m~tic ~ ~m of a second embodiment of a wavelength-
selective ADM for selective ~wi~cl~il,g of a wavelength ch~nn~l;
Figure 3 shows a srl .~ c diagrarn of an ADM for selective switching of four
wavelength ch~nnPl~;
Figure 4 shows a srh~-m~tic diagrarn of a second embodiment of an ADM for
selective switching of four wavelength ~h~nnel~; and
Figure S shows a s~ r..~l~lic fli~m of a third embodiment of an ADM for
selective ~wi~ lg of four ~vavelen~th ( h~nn~l~
PREFERRED EMBODIM~TS
The invention will now be described in detail with reference to the figures for the
various embo~lill Ir- ll~ of adding/dropping multiplexers (AD~.
30 Figure 1 shows an adding/dropping multiplexer for selective ~wilcl~i~lg of a single
specific wavelength ~h~nn~l. A optical signal co~ ;l Ig a plurality of wavelength
h~nn~l~ is carried to a fibre coupler 2, which is adapted to switch an incoming
optical signal, irrespective of wavelength, to a first fibre branch 21 and a second
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fibre branch 22 in accordance with a pre~etr" "in~l switching ratio of the fibrecoupler 2, so that a component signal is obtained on each branch. Each componentsignal comprises all wavelen_th channels: division in the fibre coupler only involves
a strength division of the signal between the two branches 21, 22. The componentS signal in the second branch 22 is allowed to pass to a b~n-lp~ filter 5. through -~
~vhich only the reception wavelength channel is allowed to pass. The component
signal in the first branch 21 is allowed to pass to a band reflex filter, ~vhich is
formed by a fibre ~ating 4. The said fibre grating is d~i~n~ to block and reflect
the reception wavelength rh~nn~l other wavelength ~h~nn~l~ being propagated
10 unrnodified along the optical fibre 1 even downstrearn of the fibre grating 4. The
adding/dropping multiplexer shown in the figure is further provided with a second
fibre coupler 3 with freely selectable ~wil~ g ratio, which is adapted to introduce
wavelength ~h~nnPl~ on the optical fibre 1. Ihe band reflex filter in the form of a
fibre grating f~rilit~t~ the re-intro~--ctir n into the other fibre coupler 3 of the
15 wavelength channel which is removed in the first fibre coupler and blocked in the
gtating 4. In the embodiment shown in the figure, wavelength ~h~nn~l~ can be
removed and blocked indep~nfl~ntly of one another. The ~n~ngernPnt shown in the
figure can ~ Or~ be used to block a wavelength rh~nn~?l on an optical fibre 1, at
the sarne time that a completely di~ wavelength channel is being removed from
20 the fibre. It is also possible to introduce into the other fibre coupler 3 any other
wavelength channel which is not already present on the optical fibre.
Figure 2 shows a second possible embodiment of the adding/~ropping multiplexer.
As in the embodiment shown in Fig.1, the first fibre coupler 2 is adapted to connect
25 an in~min~ optical signal to a first fibre branch 21 and a second fibre branch 22 of
the fibre coupler according to a precl~ . ",;,~ switching ratio. Ihe component
signal which is conn~,t~l to the second branch 22 is not subjected to any further
procP~.sing Ihe component signal in the first branch 21 is allowed to pass to a fibre
grating, which is adapted to entirely reflect a specific wavelength channel, whilst
30 allowing other wavelength ~ h~nn~l~ to pass through the fibre grating 4 and further on
the optical fibre 1. The wavelength channel reflected in the fibre grating is returned
to the fibre coupler 2 and switched to a third fibre branch 23, which in turn connects
to a receiver Rx
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The b~n~1r~cc filter 5 required in the embodiment according to Figure 1 can
consequently be omitted in this embodiment, since filtering out of the wavelength
intended for reception occurs through the fibre grating 4, which reflects the desired
wavelength charmel to the third fibre branch 23 and on to the receiver Rx. This
S wavelength channel is therefore blocked for further tr~nsmi~sion on the fibre 1.
Other wavelen~th rh~nnel.s are allowed to pass through the fibre grating 4, further
on the optical fibre 1. The first fibre coupler 2 and second fibre coupler 3 arearranged, as in the embodiment shown in Fig. 1, on either side of the fibre grating 4
and a wavelength channel removed in the fibre coupler 2 can be reintroduced into the
10 second fibre coupler 3.
The adding/dropping mllltirle~Pr shown in Figure 1 can easily be ~ pte~l for
introducing or removing a plurality of ~h~nnPlc Fig 3 shows an example for 4
~h~.".~lc Since each fibre grating c~nstih tP-s a band reflex filter for a wavelength
15 range or a certain special wavçlPn~h, a plu~lity of fibre ~tin~s muct be arranged
in an adding/dropping multiplexer for multiple wav~lPn~c. In the example shown
in Figure 3 for four cll~nnplc~ four di~ fibre ~tin~c 4a, b, c, d must be
arranged between fibre coupler 2 to receiver Rx and fibre coupler 3 for ~e
Tx. The embodiment shown in the figL~e Ps~Pnti~lly corresponds to the
20 embodiment in Figure 1. The fimction of a first fibre coupler 2 is to deflect a part of
the signal to an optical fibre branch, which is ~ l to a number of b~nflp~cc
filters 5. Each of these b~n~p~cc filters 5 is adapted to allow a certain wavelength
rh~nnPl to pass to a receiver Rx, provided for this wavelength ~h~nnPI, whilst the
other wavelength rh~nnPlc are prevented from passing A second branch 22 of the
25 first fibre coupler 2 is conn~cte-l to the receivers. Ihis branch 22 is further divided
by the use of a star coupler or cascade-coupled 2 x 2 coupler to four branches. The
four fibre gratings 4a! b, c, d between the first fibre coupler and the second fibre
coupler 3 are adapted to prevent the wavelengths which it is int~nrle~ to remove,
from progressing to the second fibre coupler 3, in which the same wavelengths are
30 ~lcrcl~ly reintroduced from the optical fibre 1 with a new content. In the
embodiment shown in the figure, wavelength ch~nnPlc can be removed and blocked
indepPn-lPntly of one another, it being possible, Lh~l~rc~lc, to remove ~h~nnelc other
than those which are blocked in tne respective fibre gratings. On the ~ eI side
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the si~nals from four ~ lers Tx are combined and carlied to a fibre branch in
the second fibre coupler 3. The wavelength rh~nnt-ls are then carried to the optical
fibre 1 by switching from the fibre coupler 3.
S In the same way the embodiment shown in Figure 2 can be adapted to switching a s~
plurality of wavelength channels to or from an optical fibre 1. Figure 4 shows this
adding/dropping multiplexer ~pted to four wavelength ~~h~nn~ls Four fibre
gratings 4a, b, c, d are designed so that a first fibre coupler 2 reflects four specific
wavelengths. A second branch 22 of the fibre coupler 2, to which the reflected
signal is switched, contains a node in which the reflected signal is distributed on the
same nurnber of fibre branches as the number of rh~nn~lc that is four in this case.
Each branch comprises a b~ntlp~ss filter Sa, b, c, d, by means of which wavelength
rh~nn~ls to be received are allowed to pass to a receiver Rx adapted to the wave-
length. Since the adding/dropping multiplexer shown in Figure 2 is adapted to a
plurality of rll~nn~ls, the b~n~p~s filters Sa, b, c, d can not be avoided in c~nnrction
with removal of the signal, but the double filtrrinE~ which is obtained gives a better
filtered signal to the receivers. As in earlier e~mpl~, the second fibre coupler 3,
that is that for c ~nnrctin~ the ~ .";ll~,x Tx, is ~n~n~l along the optical fibre 1,
at a position in the tl~ inn direction of the signal on the optical fibre 1 which is
located d~wl~LLcalll of the fibre grating
The embodirnent shown in Figure S finally shows a third way of using an adding/
dropping multiplexer for ~ cl~illg a plurality of wavelength rh~nnt-ls to and from an
optical fibre 1. In this embodiment four separate first fibre couplers 2a, b, c, d are
used for taking off four wavelength rh:~nnrls to four di~lc~ll receivers R~ Fibre
gratings 4 a, b, c, d are arranged after each of the said fibre couplers~ in thedo~l~ll~ll direction along the optical fibre 1. The said gratings are (1PS;~1 toreflect a selected signal, in the same way as in the embodiment shown in Figure 2,
down to a receiver Rx for the wavelength channel corresponding to the signal.
Optical isolators 6 a, b, c are arranged between res~;li~e fibre gratings 4a, b, c,
and on fibre couplers 2b, c, d following these, in order to prevent the reflected signal
from cnntim]ing ~LLca~ in the fibre. The introduction of four ne~v wavelength
r.h~nnel.s is achieved in the same way as in the embodiment shown in rl-nnrctiQn
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with Figure 3 and Figure 4. Since a large number of first fibre couplers 2a! b. c. d is
used in the embodiment shown in this figure, the switching ratio between the various
branches of the fibre couplers is of great irnportance.
~ =
