Optical waveguides – With optical coupler – Switch
Reexamination Certificate
2000-04-07
2002-12-10
Patel, Tulsidas (Department: 2839)
Optical waveguides
With optical coupler
Switch
C385S015000, C385S016000, C385S018000
Reexamination Certificate
active
06493479
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a crosspoint switch, and in particular to an apparatus and a method suitable for providing alterative pathways through the switch in the event of a failure of a switching element. These alternative pathways have the potential to be engaged automatically and/or remotely.
BACKGROUND OF THE INVENTION
Communications networks are moving towards becoming all optical (photonic) networks, incorporating photonic (optical) switching in which optical signals are switched directly rather than converted to electrical signals, switched electrically, then converted back to optical signals for re-transmission. Photonic switches may be used to switch wavelength division multiplexed (WDM) signals as a group, or the WDM signals may be demultiplexed outside the switch and switched individually as channels, or as groups of channels as desired. Photonic switches are fabricated using a range of technologies and frequently employ a crosspoint (crossbar) architecture. In such architectures light from an input port may traverse a number of switching elements. At each switching element the light may be switched and directed towards an associated output port or alternatively pass though to the next switching element. Once the light has been directed towards an output port it may traverse more switching elements which, in most implementations, must remain inactive so as not to block or disrupt the light path before it reaches its output port.
For example, a recently developed photonic switch using Micro Electro-Mechanical systems (MEMS) technology is described in “Free-Space Micro Machined Optical Switches for Optical Networking” by L Y Lin et al, IEEE Journal of Selected Topics In Quantum Electronics, Vol. 5 No.1, January/February 1999; which is incorporated herein by reference. Such MEMS switches typically use moveable mirrors to redirect optical paths within the switch in order to complete an optical signal or channel connection across the switch.
FIG. 1
shows a schematic diagram of a typical MEMS photonic switch
100
. The switch
100
is bi-directional, but for simplicity is assumed to comprise
4
inputs in the form of optical fibres
112
,
114
,
116
&
118
, and
4
outputs which are also optical fibres
122
,
124
,
126
&
128
Each input and output has an associated lens
104
which collimates the beams from the inputs and focuses the beam at the outputs. Such a switch is generically referred to as a 4×4 switch (number of inputs×number of outputs).
The switch
100
is a crosspoint (cross bar) switch, having a switching element (here, a mirror,
106
) located at each of the points at which optical signals emitted from the input fibres would cross with optical signals emitted from the output fibres. The switch
100
thus has a four by four array of mirrors
106
mounted on a surface
102
.
In this particular switch, each mirror may be moved between two stable positions.
FIGS. 2
a
and
2
b
illustrate these positions.
FIG. 2
a
shows the mirror in the inactivated position
106
a,
where the mirror is flat i.e. substantially parallel to the surface
102
.
FIG. 2
b
shows the mirror having been raised to the activated or upright position
106
b,
substantially perpendicular to the surface
102
. This activation may be performed by a variety of means e.g. by micro actuators causing the mirror to be rotated about the hinges
108
. The mirrors are typically formed of materials such as polysilicon, the reflectivity of which is increased by providing a reflective coating
107
such as gold. In the inactivated state, it is typical for the relatively non-reflective surface
109
of the mirror to lie adjacent to the surface
102
, so that the reflective coating
107
does not contact the surface
102
.
FIG. 1
shows a typical operation of the switch
100
. By raising the appropriate mirrors, an optical signal from each of the inputs
112
,
114
,
116
&
118
is directed to a respective output
128
,
126
,
122
&
124
. For instance, an optical signal originating from input fibre
112
is formed into a collimated beam
132
by lens
104
. The beam
132
then reflects off the front reflective surface
107
of a raised mirror
106
b
into a further lens
104
which focuses the beam
132
into the output fibre
128
. it will be appreciated that by appropriate control of the array of mirrors
106
, any one of the signals originating from the inputs
112
,
114
,
116
&
118
can be switched into any one of the outputs,
122
,
124
,
126
&
128
.
In any system switching information, it is desirable to provide alternative pathways for the information in the event that the original pathway “fails” and is unable to transmit the signals as desired. Such alternative pathway provision is commonly referred to as “protection” when these pathways may be engaged remotely and/or automatically.
it will be appreciated that a failure in any of the internal switching elements (mirrors
106
) would impair the functionality of the switch. For instance, any of the mirrors could be jammed in either the raised
106
b
or flat
106
a
position, and this would prevent a connection between the input and output corresponding to that mirror. In addition a mirror which is jammed in the raised position has the potential to prevent a connection between the associated input and another output and between another input and the associated output. This is because the raised mirror may act as a block to such light paths.
The present invention aims to address such problems.
SUMMARY OF THE INVENTION
In a first aspect, the present Invention provides a crosspoint switch comprising N primary inputs, M primary outputs and an array of (N+X)×(M+X) switching elements, where M, N and X are all positive Integers, the additional switching elements in said array being arranged to provide alternative connectivity between said inputs and outputs.
A typical crosspoint switch having N inputs and M outputs will have an array of N×M switching elements. By providing the additional switching elements in the array, it becomes possible to provide alternative connectivity between the inputs and outputs to compensate for any failures in the part of the array normally utilised for switching.
Preferably, said switch is a photonic switch. Photonic switches can have switching elements such as reflective surfaces (mirrors), refractive media, or interferometers.
Preferably, said additional switching elements comprise at least one column at an outermost edge of the array, and at least one row at an outermost edge of the array.
Preferably, the additional switching element located at the intersection of each of said row and said column is located in a fixed position so as to redirect incident signals in a predetermined manner. Such a switching element can act to redirect an incident signal from said row along said column, or from said column along said row.
Alternatively the switch can further comprise X additional inputs and X additional outputs, each of said additional outputs being transmissively connected to a respective additional input.
Preferably, said switch is a photonic switch, and said additional outputs are connected to said additional inputs by an optical fibre or other form of optical wave-guide.
Optionally, at least one of said additional outputs is coupled to a tap for the monitoring of signals passing through said output.
Preferably, N=M.
Preferably, X=2. If X=2, or any even number, protection can be provided for one or more switching elements that fail in the active position and act to block signals.
Preferably, said array is substantially rectilinear.
In another aspect the present invention provides a node for a telecommunications network comprising a crosspoint switch comprising N Inputs, M outputs and an array of (N+X)×(M+X) switching elements, where M, N and X are all positive integers, the additional switching elements in said array being arranged to provide alternative connectivity between said inputs and outputs.
I
Lee Mann Smith McWilliams Sweeney & Ohlson
Nortel Networks Limited
Zarroli Michael C.
LandOfFree
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