Testing operation of a photonic switch

Details Testing operation of a photonic switch Testing operation of a photonic switch

2000-04-07

2003-06-03

Juba, Jr., John (Department: 2872)

Optical waveguides

With optical coupler

Switch

C385S017000, C385S018000

Reexamination Certificate

active

06574384

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 systems (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 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 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×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 activated 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.
The present invention aims to address such problems.
SUMMARY OF THE INVENTION
In a first aspect, the present invention provides a method of testing the operation of a photonic switch, said switch comprising switching means arranged to be movable between at least a first and a second position, and arranged to switch an incident optical signal by redirection of the optical path of said signal, the method comprising the steps of providing a test optical signal arranged to be incident upon said switching means when in said first position, along a path distinct from the switched optical signal path; and measuring the test signal at a predetermined position suitable for determining if said switching means is in said first position from the measurement. By the term distinct, it is understood that at least a portion of the test optical signal path is different from the switched optical signal path.
Preferably, the method further includes the step of the switching means, when in said first position, redirecting the test optical signal path. Alternatively, the switching means could act to either block (prevent the optical signal reaching a detector) or pass the test signal when in said first position.
Preferably, the redirection occurs as a consequence of at least one of reflection and refraction. Hence a reflective surface such as a mirror or a refractive medium such as glass could be utilised to redirect the signal.
Preferably, the switch comprises a plurality of said switching means, the method steps each being performed a predetermined number of times. For instance, a cross point switch having N inputs and M outputs would have N×M switching means, and it could be desirable to check the operation of some or all of the switching means.
Preferably, the method steps are sequentially repeated.
Preferably, the method steps are performed prior to the switch being utilised to switch live optical signals, the method further comprising the steps of sequentially switching said switching means between said first and said second position. For instance, the steps could be performed in order to test the operation of each of the switching means in a recently installed or manufactured switch. Equally, the steps could be performed in order to test the switch operation after a storage or transportation period, prior to the switch being deployed/installed in a system.
Preferably, the switch further comprises a plurality of further switching means arranged to switch a test optical signal along a plurality of paths, each path being incident upon a switching means when in said first position, the method further comprising the step of utilising said further switching means to sequentially provide a test optical signal incident upon the switching means in a predetermined sequence.
Preferably, the method steps are performed while said switch is carrying live optical signals.
In a further aspect, the present invention provides a computer programme arranged to perform a method of t

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