Optical waveguides – With optical coupler – Switch
Reexamination Certificate
2000-11-28
2003-02-11
Feild, Lynn D. (Department: 2839)
Optical waveguides
With optical coupler
Switch
Reexamination Certificate
active
06519383
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a photonic switch, and in particular to an apparatus and a method suitable for testing the operation of a photonic switch.
BACKGROUND OF THE INVENTION
Communications networks are increasingly becoming all optical networks, incorporating photonic (optical) switching. Photonic switches are typically fabricated using Micro Electro-Mechanical Structures (MEMS) technology. A recently developed photonic switch of this type is described in “Free-Space Micro Machined Optical Switches for Optical Networking” by LY 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 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. MEMS switches typically use moveable mirrors to re-direct 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 bidirectional, 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 beam from each input and focuses the respective beam at each output. Such a switch is generically referred to as a
4
×
4
switch (number of inputs x number of outputs).
The switch
100
is a cross point switch, having a switching device (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
.
Various solutions have been proposed to test the mirror status or switch connection, in order to verify that the mirrors
106
are functioning correctly and are not, for example, jammed in either the raised
106
b
or flat
106
a
position.
One solution is to inject different optical test signals into each input port (i.e.
112
,
114
,
116
,
118
) to the switch
100
via fibre tap couplers (not shown). Such test signals would be distinct from the normal optical signal being switched e.g. of different wave length and/or modulation characteristics. Each output port (i.e.
122
,
124
,
126
,
128
) would then be connected to a further tap coupler, in order that the test signals could be extracted, detected and analysed for verification that the desired input to output connections exist. This solution is true connectivity verification. However, due to the number of components required, it would be both bulky and expensive. For instance, in a N×N switch (where N is an integer) the required components would include 2N couplers, N sources, N detectors, as well as numerous splices and fibre interfaces; additionally there would be the assembly cost.
An alternative solution is to use electrical parameters (e.g. capacitance, inductance or resistance) to monitor the physical position of the mirrors. However, this would double the number of electrical connections to the switch matrix, and is hence impractical for large arrays of mirrors.
Co-pending U.S. application Ser. No. 09/545,545, “Testing Operation of a Photonic Switch”, by the same inventor, describes a method of utilising test optical signals in the plane of the switching mirrors, reflected from the rear of one or more of the mirrors, in order to test whether the mirrors are functioning correctly. This approach has limitations in that it requires the rear of the mirrors to be reflective, and requires an array of mirrors to be in certain predefined configurations for any given mirror to be tested. Simultaneous testing of all mirrors is hence not possible.
The present invention aims to address one or more of the problems of the prior art.
SUMMARY OF THE INVENTION
In a first aspect, the present invention provides a photonic switch having at least one switching means comprising a reflective surface arranged to be moveable between at least a first and a second position, and arranged to switch an incident optical signal by reflectively redirecting the optical path of said signal when in at least one of said positions, the incident and redirected optical paths defining a first plane; the switch further comprising test signal means arranged to provide a test optical signal incident said reflective surface, and measuring means arranged to measure a reflection of the test signal at a predetermined position suitable for determining if said switching means is in said first position from said measurement, the paths of the incident and reflected test signals lying outside of the first plane.
In a further aspect, the present invention provides a telecommunications system comprising a photonic switch having at least one switching means comprising a reflective surface arranged to be moveable between at least a first and a second position, and arranged to switch an incident optical signal by reflectively redirecting the optical path of said signal when in at least one of said positions, the incident and redirected optical path defining a first plane; the switch further comprising test signal means arranged to provide a test optical signal incident said reflective surface, and measuring means arranged to measure a reflection of the test signal at a predetermined position suitable for determining if said switching means is in said first position from said measurement, the paths of the incident and reflected test signals lying outside of the first plane.
In another aspect, the present invention provides a method of testing the status of a photonic switch, the switch having at least one switching means comprising a reflective surface arranged to be moveable between at least a first and a second position, and arranged to switch an incident optical signal by reflectively redirecting the optical path of said signal when in at least one of said positions, the incident and redirected optical paths defining a first plane; the method comprising the steps of providing a test optical signal incident said reflective surface, the optical path of the test s
Feild Lynn D.
Hyeon Hae Moon
Lee Mann Smith McWilliams Sweeney & Ohlson
Nortel Networks Limited
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