U.S. patent application number 14/322021 was filed with the patent office on 2016-01-07 for making connections through an optical circuit switch.
The applicant listed for this patent is CALIENT Technologies, Inc.. Invention is credited to Mchael Deacon, Jitender Miglani, Sushma Sagaram, Vijayan Thattai, Shifu Yuan.
Application Number | 20160004014 14/322021 |
Document ID | / |
Family ID | 54783132 |
Filed Date | 2016-01-07 |
United States Patent
Application |
20160004014 |
Kind Code |
A1 |
Miglani; Jitender ; et
al. |
January 7, 2016 |
MAKING CONNECTIONS THROUGH AN OPTICAL CIRCUIT SWITCH
Abstract
An optical circuit switch and method. A first mirror element
rotates about orthogonal axes in response to first and second
voltages applied respectively to first and second electrodes. A
second mirror element rotates about orthogonal axes in response to
third and fourth voltages applied respectively to the third and
fourth electrodes. A controller may apply first through fourth
voltages respectively to the first through fourth electrodes to
make a connection between first and second ports. When an initial
insertion loss of the connection does not exceed a predetermined
threshold, the controller may conduct independent single-dimension
searches to determine values for the first through fourth voltages
that minimize the insertion loss. When the initial insertion loss
exceeds the predetermined threshold, the controller may conduct a
four-dimensional search to determine values for the first through
fourth voltages that reduce the insertion loss to not exceed the
predetermined threshold.
Inventors: |
Miglani; Jitender; (Hollis,
NH) ; Deacon; Mchael; (Ventura, CA) ; Thattai;
Vijayan; (Goleta, CA) ; Sagaram; Sushma;
(Camarillo, CA) ; Yuan; Shifu; (Camarillo,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
CALIENT Technologies, Inc. |
Goleta |
CA |
US |
|
|
Family ID: |
54783132 |
Appl. No.: |
14/322021 |
Filed: |
July 2, 2014 |
Current U.S.
Class: |
385/18 |
Current CPC
Class: |
G02B 6/3546 20130101;
G02B 6/3588 20130101; G02B 6/3512 20130101; G02B 6/3586 20130101;
G02B 6/3518 20130101 |
International
Class: |
G02B 6/35 20060101
G02B006/35 |
Claims
1. An optical circuit switch, comprising: a first port associated
with a first mirror element, the first mirror element rotatable
about orthogonal axes in response to first and second voltages
applied respectively to first and second electrodes coupled to the
first mirror element; a second port associated with a second mirror
element, the second mirror element rotatable about orthogonal axes
in response to third and fourth voltages applied respectively to
third and fourth electrodes coupled to the second mirror element;
and a controller that, upon receipt of a command to make a
connection between the first port and the second port, performs
actions comprising: applying nominal values for the first, second,
third, and fourth voltages respectively to the to the first,
second, third, and fourth electrodes to make an optical connection
from the first port to the second port, determining an initial
insertion loss for the optical connection, if the initial insertion
loss is less than or equal to a predetermined threshold, conducting
four independent single-dimension searches to determine values for
the first, second, third, and fourth voltages that minimize the
insertion loss, and when the initial insertion loss is greater than
the predetermined threshold, conducting a four-dimensional search
to determine values for the first, second, third, and fourth
voltages that reduce the insertion loss to less than or equal to
the predetermined threshold.
2. The optical circuit switch of claim 1, wherein the actions
performed by the controller further comprise: after conducting the
four-dimensional search to determine values for the first, second,
third, and fourth voltages that reduce the insertion loss to less
than or equal to the predetermined threshold, conducting four
independent single-dimension searches to determine values for the
first, second, third, and fourth voltages that minimize the
insertion loss.
3. The optical circuit switch of claim 1, wherein the actions
performed by the controller further comprise: after conducting the
four-dimensional search to determine values for the first, second,
third, and fourth voltages that reduce the insertion loss to less
than or equal to the predetermined threshold, conducting a blink
test to determine whether or not the connection is a true
connection.
4. The optical circuit switch of claim 3, wherein the actions
performed by the controller further comprise: when a determination
is made that the connection is not a true connection, resuming the
four-dimensional search.
5. The optical circuit switch of claim 3, wherein the actions
performed by the controller further comprise: when a determination
is made that the connection is a true connection, conducting four
independent single-dimension searches to determine the values for
the first, second, third, and fourth voltages that minimize the
insertion loss.
6. The optical circuit switch of claim 3, wherein the controller
conducting the blink test further comprises: changing at least one
of the first voltage and the second voltage; determining that the
connection is a true connection when the insertion loss changes in
response to the changed voltage; and determining that the
connection is not a true connection when the insertion loss does
not change in response to the changed voltage.
7. The optical circuit switch of claim 1, wherein the controller
conducting a four-dimensional search further comprises: selecting a
first search domain from one or more predetermined search domains,
each search domain including a plurality of search points, each
search point corresponding to particular values for the first,
second, third, and fourth voltages; sequentially selecting search
points within the first search domain in accordance with a
predetermined search pattern; evaluating each selected search
point, by applying the corresponding particular values of the
first, second, third, and fourth voltages respectively to the
first, second, third, and fourth electrodes and determining the
insertion loss of the connection; and terminating the
four-dimensional search if a selected search point results in the
insertion loss being less than or equal to the predetermined
threshold.
8. The optical circuit switch of claim 7, wherein the controller
conducting a four-dimensional search further comprises: when all
search points within the first search domain have been evaluated
without finding a search point that results in the insertion loss
being less than or equal to the predetermined threshold, selecting
a second search domain and sequentially evaluating search points
within the second search domain.
9. The optical circuit switch of claim 1, wherein the first port is
one of a plurality of input ports and the first mirror element is
one of a plurality of input mirror elements, each input port
uniquely associated with a respective one of the input mirror
elements, the second port is one of a plurality of output ports and
the second mirror element is one of a plurality of output mirror
elements, each output port uniquely associated with a respective
one of the output mirror elements, and the actions performed by the
controller further comprise applying respective voltages to
respective electrodes coupled to some input mirror elements in
addition to the first mirror element and some output mirror
elements in addition to the second mirror element to make optical
connections between the associated input ports and output
ports.
10. A method for making a connection from a first port to a second
port in an optical circuit switch, comprising: applying first and
second voltages respectively to first and second electrodes coupled
to a first mirror element associated with the first port, the first
and second voltages applied at respective nominal voltage values;
applying third and fourth baseline voltages respectively to third
and fourth electrodes coupled to a second mirror element associated
with the second port, the third and fourth voltages applied at
respective nominal voltage values; determining an initial insertion
loss for the optical connection; if the initial insertion loss is
less than or equal to a predetermined threshold, conducting four
independent single-dimension searches to determine values for the
first, second, third, and fourth voltages that minimize the
insertion loss; and when the initial insertion loss is greater than
the predetermined threshold, conducting a four-dimensional search
to determine values for the first, second, third, and fourth
voltages that reduce the insertion loss to less than or equal to
the predetermined threshold.
11. The method of claim 10, further comprising: after conducting
the four-dimensional search to determine values for the first,
second, third, and fourth voltages that reduce the insertion loss
to less than or equal to the predetermined threshold, conducting
four independent single-dimension searches to determine values for
the first, second, third, and fourth voltages that minimize the
insertion loss.
12. The method of claim 10, further comprising: after conducting
the four-dimensional search to determine values for the first,
second, third, and fourth voltages that reduce the insertion loss
to less than or equal to the predetermined threshold, conducting a
blink test to determine whether or not the connection is a true
connection.
13. The method of claim 12, further comprising: when a
determination is made that the connection is not a true connection,
resuming conducting the four-dimensional search.
14. The method of claim 12, further comprising: when a
determination is made that the connection is a true connection,
conducting four independent single-dimension searches to determine
values for the first, second, third, and fourth voltages that
minimize the insertion loss.
15. The method of claim 12, wherein conducting the blink test
further comprises: changing at least one of the first voltage and
the second voltage; determining that the connection is a true
connection when the insertion loss changes in response to the
changed voltage; and determining that the connection is not a true
connection when the insertion loss does not change in response to
the changed voltage.
16. The method of claim 10, wherein conducting a four-dimensional
search further comprises: selecting a first search domain from one
or more predetermined search domains, each search domain including
a plurality of search points, each search point corresponding to
particular values for the first, second, third, and fourth
voltages; sequentially selecting search points within the first
search domain in accordance with a predetermined search pattern;
evaluating each selected search point, by applying the
corresponding particular values of the first, second, third and
fourth voltages respectively to the first, second, third, and
fourth electrodes and determining the insertion loss of the
connection; and terminating the four-dimensional search if a
selected search point results in the insertion loss being less than
or equal to the predetermined threshold.
17. The method of claim 16, wherein conducting a four-dimensional
search further comprises: when all search points within the first
search domain have been evaluated without finding a search point
that results in the insertion being less than or equal to the
predetermined threshold, selecting a second search domain and
sequentially evaluating the points within the second search
domain.
18. The method of claim 10, wherein the first port is one of a
plurality of input ports and the first mirror element is one of a
plurality of input mirror elements, each input port uniquely
associated with a respective one of the input mirror elements, the
second port is one of a plurality of output ports and the second
mirror element is one of a plurality of output mirror elements,
each output port uniquely associated with a respective one of the
output mirror elements, and the method further comprises applying
respective voltages to respective electrodes coupled to at least
some input mirror elements in addition to the first mirror element
and at least some output mirror elements in addition to the second
mirror element to make optical connections between the associated
input ports and output ports.
Description
NOTICE OF COPYRIGHTS AND TRADE DRESS
[0001] A portion of the disclosure of this patent document contains
material which is subject to copyright protection. This patent
document may show and/or describe matter which is or may become
trade dress of the owner. The copyright and trade dress owner has
no objection to the facsimile reproduction by anyone of the patent
disclosure as it appears in the Patent and Trademark Office patent
files or records, but otherwise reserves all copyright and trade
dress rights whatsoever.
BACKGROUND
[0002] 1. Field
[0003] This disclosure relates to optical communications networks
and more particularly to optical circuit switches using MEMS
(micro-electromechanical system) mirror arrays.
[0004] 2. Description of the Related Art
[0005] Communications networks commonly contain a mesh of
transmission paths which intersect at hubs or nodes. At least some
of the nodes may include a switching device that receives data or
information arriving at the node and retransmits the information
along appropriate outgoing paths.
[0006] Optical fiber links are commonly used to provide high
bandwidth transmission paths between nodes. Such optical fiber
links form the backbone of wide area networks such as the Internet.
Optical fiber links are also applied in high bandwidth local area
networks which may be used, for example, to connect server racks in
large data centers or to connect processors in high performance
computers.
[0007] An optical circuit switch is a switching device that forms
connections between pairs of optical fiber communications paths. A
typical optical circuit switch may have a plurality of ports and be
capable of selectively connecting any port to any other port in
pairs. Since an optical circuit switch does not convert information
flowing over the optical fiber communication paths to electrical
signals, the bandwidth of an optical circuit switch is essentially
the same as the bandwidth of the optical communications paths.
Further, since an optical circuit switch does not convert
information into electrical signals, the power consumption of an
optical circuit switch may be substantially lower than a comparable
conventional (i.e. electronic) switch.
DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is an optical schematic diagram of an optical circuit
switch.
[0009] FIG. 2 is a block diagram of an environment for an optical
circuit switch.
[0010] FIG. 3 is a block diagram of an optical circuit switch.
[0011] FIG. 4 is a flow chart of a process for making a connection
through an optical circuit switch.
[0012] FIG. 5 is an optical schematic diagram of a portion of an
optical circuit switch.
[0013] FIG. 6 is a flow chart of a process for defining a search
protocol.
[0014] FIG. 7 is a graph of insertion loss versus beam angular
error for an exemplary optical circuit switch.
[0015] FIG. 8 is a graphical representation of exemplary search
domains.
[0016] FIG. 9 is a graphical representation of an exemplary search
pattern.
[0017] FIG. 10 is a graphical representation and tabular listing of
another exemplary search pattern.
[0018] FIG. 11 is a flow chart of a process for searching for a
connection through an optical circuit switch.
[0019] Throughout this description, elements appearing in figures
are assigned three-digit reference designators, where the most
significant digit is the figure number where the element is
introduced and the two least significant digits are specific to the
element. An element that is not described in conjunction with a
figure may be presumed to have the same characteristics and
function as a previously-described element having the same
reference designator.
DETAILED DESCRIPTION
[0020] Description of Apparatus
[0021] Referring now to FIG. 1, an exemplary optical circuit switch
100 may be configured to connect a group of n inputs (where n is an
integer greater than 1), labeled In 1 to In n, to a group of n
outputs, labeled Out 1 to Out n. More specifically, the optical
circuit switch 100 may selectively connect up to n pairs of inputs
and outputs.
[0022] Each of the inputs In 1 to In n may include a connector (of
which only the connector 110-1 is identified) to receive an input
optical signal from a optical fiber cable (not shown) external to
the optical circuit switch. Each connector may be coupled by a
respective optical fiber (of which only optical fiber 112-1 is
identified) to a respective tap coupler (of which only tap coupler
114-1 is identified). Each tap coupler may extract an input sample
portion, for example 1% to 11%, of the input optical signal from
the respective optical fiber. Each input sample portion may be
directed to an input optical power meter 170. The remainder of the
input optical signals, other than the input sample portions, may be
conveyed by respective optical fibers to respective collimator
lenses (of which only collimator lens 118-1 is identified). Each
collimator lens may convert the input optical signal from the
respective optical fiber into an input optical beam (of which only
input optical beam 120-1 is identified) in free space. While
lenses, such as the lens 118-1 are commonly referred to as
"collimator" lenses, the input optical beam 120-1 will not be
perfectly collimated due to the finite diameter of the light
exiting the end of the input optical fiber 112-1. Free space
optical beams are shown in FIG. 1 as dashed lines.
[0023] Each input optical beam, such as input optical beam 120-1,
may be directed onto a input mirror array 130. The input mirror
array 130 may include n mirrors with a one-to-one correspondence
between input optical beams and mirrors, such that each input
optical beam is directed onto a respective mirror. To improve the
manufacturing yield of the input mirror array, the input mirror
array 130 may include more than n mirrors, in which case the n
input optical beams may be directed to a subset of n mirrors that
are known to be fully functional. Since each of the n input optical
beams originates from a specific port and is directed onto a
specific mirror, each port may be described as "uniquely
associated" with a corresponding mirror. In this patent, "uniquely
associated" means a one-to-one correspondence. To take advantage of
the available fully functional mirrors, the associations between
ports and mirrors may be different in different optical circuit
switches
[0024] Each mirror on the input mirror array 130 may reflect the
respective input optical beam to a selected mirror of a output
mirror array 140. The mirrors of the output mirror array 140 may
reflect the incident beam to form a respective output optical beam
(of which only output optical beam 160-1 is identified). Each
output optical beam may be directed to a corresponding focusing
lens (of which only focusing lens 158-1 is identified). Each
focusing lens may focus the respective output optical beam into an
output optical signal in a respective optical fiber. Each output
optical signal may be conveyed to a respective output tap coupler
(of which only output tap coupler 154-1 is identified). Each output
tap coupler may direct a sample portion (for example 1% to 11%) of
the respective output optical signal to an output optical power
meter 180. The remainder of each output optical signal, other than
the respective sample portion, may be conveyed by a respective
output fiber (of which only output fiber 152-1 is identified) to a
respective output connector (of which only output connector 150-1
is identified).
[0025] The input optical power meter 170 and the output optical
power meter 180 may be a common module. The input optical power
meter 170 and the output optical power meter 180 may measure the
optical power in each of the input sample portions and output
sample portions, respectively. Each of the input optical power
meter 170 and the output optical power meter 180 may include an
optical power detector for each sample portion. Alternatively, each
of the input optical power meter 170 and the output optical power
meter 180 may time-multiplex a single detector or an array of
detectors such that each detector measures the optical power of
sequence of sample portions. For example, each of the input optical
power meter 170 and the output optical power meter 180 may use a
scanning mirror to direct sample portions to a single detector or
an array of detectors as described in U.S. Pat. No. 7,676,125.
[0026] Sample portions may be extracted from the input optical
beams, such as input optical beam 120-1, and/or the output optical
beams, such as output optical beam 160-1, using one or more free
space sampling optical elements. For example, sample portions may
be extracted as described in U.S. Pat. No. 6,597,825 or U.S. Pat.
No. 6,668,118. Input tap couplers, such as input tap coupler 114-1
and/or output tap couplers, such as output tap coupler 154-1, may
not be present when free-space sampling optical elements are used
to extract sample portions.
[0027] The optical circuit switch 100 may include a controller 190.
The controller 190 may control the mirror elements in the input
mirror array 130 and the output mirror array 140 to make desired
optical connections between the input In 1 to In n and the outputs
Out 1 to Out n. For example, as shown in FIG. 1, input In 1 is
connected to output Out 2, input In 2 is connected to output Out n,
and input In n is connected to output Out 1. The controller 190
will be discussed in greater detail subsequently.
[0028] The detail view 105 shows a simplified schematic diagram of
a mirror element from either the input mirror array 130 or the
output mirror array 140. A reflective mirror element 142 is
supported by a pair of torsion bars, of which only a first torsion
bar 144 is visible. The second torsion bar is located on the far
side of the mirror element 142 and axially aligned with the first
torsion bar 144. The mirror element 142 may rotate about the axis
of the torsions bars, with the torsion bars providing a spring
force tending to return the mirror element 142 to a default
position. The mirror element 142 may be coupled to a first
electrode 146 and a second electrode 148. The mirror element 142
may be rotated by electrostatic attraction between the mirror
element and either the first electrode 146 or the second electrode
148.
[0029] For example, applying a voltage between the first electrode
146 and the mirror element 142 will create an attraction between
the mirror element and the first electrode, causing the mirror
element to rotate in a counter-clockwise direction. The mirror will
rotate until the return force of the torsion bars is equal to the
force of the electrostatic attraction. The angular rotation of the
mirror element 142 may be approximately proportional to the square
of the voltage between the first electrode 146 and the mirror
element 142. Similarly, applying a voltage between the second
electrode 148 and the mirror element 142 will cause the mirror to
rotate in a clockwise direction. The first electrode 146 and the
second electrode 148 may be "dedicated" to the mirror element 142,
which is to say the only function of the electrodes 146 and 148 is
to rotate the mirror element 142 and the voltages applied to the
electrodes 146 and 148 have no effect on any mirror element other
than the mirror element 142.
[0030] In the simplified example of FIG. 1, the mirror element 142
rotates about a single axis defined by the torsion bars 144. Either
or both of the input mirror array 130 and the output mirror array
140 may include mirrors configured to independently rotate about
two orthogonal axes. In this case, each mirror element may be
coupled to a first pair of electrodes to cause clockwise and
counter-clockwise rotation about a first axis and a second pair of
electrodes to cause clockwise and counter-clockwise rotation about
a second axis orthogonal to the first axis. The structure of a
mirror array and the associated electrodes may be substantially
more complex than that shown in the simplified schematic detail
view 105. For example, U.S. Pat. No. 6,628,041 describes a MEMS
mirror array having two-axis mirror motion and comb actuators.
[0031] Referring now to FIG. 2, an environment 295 for the
application of an optical circuit switch 200 may include a network
290 and a network controller 210. The optical circuit switch 200
may be the optical circuit switch 100 or may be a compound optical
circuit switch including multiple copies of the optical circuit
switch 100. When the optical circuit switch 200 is a compound
optical circuit switch, the multiple copies of the optical circuit
switch 100 may be collocated or distributed. The optical circuit
switch 200 may be disposed within the network 290 and may function
to switch optical connections between other nodes (not shown)
within the network 290. The network 290 may be, for example, a wide
area network, a local area network, a storage area network, a
private network within a data center or computer cluster, and may
be or include the Internet. While the connections switched by the
optical circuit switch 200 are optical, other connections within
the network 290 may be wired and/or wireless.
[0032] The network controller 210 may be a computing device that
provides a graphic user interface or a command line interface for a
network operator to enter connection commands (i.e. commands to
make or break one or more optical connections) for the optical
circuit switch 200. The network controller 210 may be a computing
device running network management software, in which case
connection commands for the optical circuit switch 200 may be
generated automatically by the network controller 210.
[0033] A communications link 215 between the optical circuit switch
200 and the network controller 210 may be in-band, which is to say
the communications link 215 may be a path within the network 290.
In this case, the optical circuit switch may have a wired,
wireless, or optical connection to the network in addition to the
optical connections being switched. The communications link 215 may
be out-of-band, which is to say the communications link 215 may be
a dedicated connection or a connection via a command network
independent from the network 290. A configuration in which the
network controller 210 executes network management software to
automatically provide connection commands to the optical circuit
switch 200 via an out-of-band communications link 215 is an example
of what is commonly called a "software defined network".
[0034] FIG. 3 is a high-level block diagram of the control and
mirror driver portions of an optical circuit switch 300, which may
be the optical circuit switch 100. The optical circuit switch 300
may include a switch controller 390, an input optical power meter
370, an output optical power meter 380, and a plurality of mirror
driver circuits 350. The optical circuit switch 300 may include one
mirror driver circuit 350 for each mirror in two mirror arrays if
the individual mirror elements are rotatable about a single axis.
The optical circuit switch 300 may include two mirror driver
circuits 350 for each mirror in the mirror arrays if the individual
mirror elements are rotatable about two orthogonal axes. Each
mirror driver circuit 350 may have, for example, two selectable
outputs to drive one or the other of a pair of electrodes, as
described in pending patent application Ser. No. 13/787,621.
[0035] The switch controller 390 may include a command interpreter
320, a port map 322, a mirror calibration table 324, a connection
optimizer 330, a drift compensator 332, and a 4D search engine 334
which may jointly maintain a connection state table 340. The switch
controller 390 may receive connection commands from an external
source such as the network controller 210. The switch controller
390 may receive connection commands from some other source or in
some other manner.
[0036] The switch controller 390 may receive data from the input
optical power meter 370 and the output optical power meter 380
indicating the power levels at the inputs and the outputs of the
optical circuit switch 300, respectively. The switch controller 390
may determine or calculate an insertion loss for each connection
through the optical circuit switch based on the data from the input
and output optical power meters.
[0037] The command interpreter 320 may be responsive to a set of
connection commands received by the switch controller 390. The set
of connection commands may include, for example "Break a-b" and
"Make a-b". These commands may respectively instruct the optical
circuit switch 300 to either break an existing connection between
ports a and b (where a and b are identifiers that identify an input
port and an output port, respectively), or to make a new connection
between ports a and b. Each identifier a and b may be an integer
port number, a character string, or some other data that uniquely
identifies the respective ports. The set of connection commands may
include a mass connection command, which may list multiple
connections to be made. The mass connection command may be used,
for example, when the optical circuit switch is initially
integrated into a network or when substantial reconfiguration of
the network or data center is required.
[0038] The command interpreter 320 may include or have access to
the port map 322. As previously described, to allow the use of
mirror arrays with a small number of nonoperational mirror
elements, the number of mirror elements in each mirror array may be
larger than the number of inputs or outputs. Each input and output
may be coupled to a known operational mirror element in the
respective mirror array. The port map 322 may be a table containing
data relating each input to a mirror element in a input mirror
array, and data relating each output to a mirror element in a
output mirror array. The data in the port map 322 may be specific
to the particular input and output mirror arrays used in the
optical circuit switch 300.
[0039] There may be some performance variation from mirror element
to mirror element and/or from mirror array to mirror array. In
particular, there may be some variation in the mirror element
rotation angle versus applied voltage characteristics within and
between mirror arrays. The command interpreter 320 may include or
have access to the mirror calibration table 324 which contains data
describing the performance of each mirror element. For example, the
mirror calibration table 324 may store the rotation angle versus
voltage characteristic of each mirror element. The mirror
calibration table 324 may store, for all possible pairs of input
and output mirror elements, a set of voltages that, when applied to
the appropriate electrodes, will cause the mirror elements to
rotate to make the desired connection. The data in the mirror
calibration table 324 may be specific to the particular mirror
arrays used in the optical circuit switch 300. The data in the
mirror calibration table 324 may be derived, for example, from the
results of tests performed on the particular mirror arrays used in
the optical circuit switch 300.
[0040] The data stored in the mirror calibration table 324 may
indicate nominal voltages required to initially make desired
connections through the optical circuit switch 300. However, once
voltages are applied to electrodes associated with a pair of input
and output mirror elements to initially make a connection, the
positions of the mirror elements may drift over time. The result of
mirror element drift may be failure or degradation (e.g. increased
insertion loss) of the connection. The mirror arrays used in the
optical circuit switch 300 may be fabricated by chemical
micromachining of a silicon substrate. For example, each mirror
element may consist of a reflective coating on a silicon slab that
is connected to the silicon substrate by narrow silicon elements
that function as torsion bars. Each silicon mirror slab may be free
to rotate about the axis or axes defined by the torsion bars.
Mirror element drift may be due to mechanical strain or stress
relief of the torsion bars over time. Further, all or portions of
the silicon surfaces of the mirror array may be coated with SiO2 or
some other dielectric. Electric charge trapped at defects in the
insulators layers may contribute to mirror element drift over time.
Other causes may also contribute to mirror element drift.
[0041] When a new connection in made through the optical circuit
switch, the command interpreter 320 may first determine the mirror
elements to be used to make the connection from the port map 322,
and then retrieve the nominal voltages to be applied to the mirror
arrays from the mirror calibration table 324. The retrieved nominal
voltages may be applied to the appropriate electrodes of the mirror
arrays to attempt to make the new connection. After the nominal
voltages are applied, an insertion loss of the new connection may
be determined based on data from the input and output optical power
meters 370, 380. If the insertion loss of the connection is less
than a predetermined threshold, the connection is presumed to be
successful. In this case, the connection optimizer 330 assumes
control of the connection.
[0042] The connection optimizer 330 may determine the insertion
loss of each active optical connection (i.e. each optical
connection where light is present) from the respective input and
output power levels. The connection optimizer 330 may periodically
adjust the positions of some or all of the mirror elements to
minimize the insertion loss of each optical connection. For
example, to optimize a connection, the connection optimizer 330 may
make incremental changes in the position of one of the mirror
elements used in the connection and observe the resulting effect on
insertion loss. The optimum positions of the mirror elements may
then be found using a hill climbing algorithm or a similar local
area search algorithm. The position of each mirror element may be
optimized periodically. The time interval between successive
optimizations of each mirror element may be short (on the order of
seconds) compared to the time constant of the mirror element drift
(on the order of hours). Periodic optimization of the position of
each mirror element may automatically compensate for mirror element
drift.
[0043] A significant amount of mirror element drift may accumulate
when a connection is made and sustained for a long period of time.
When such a connection is broken, the accumulated mirror element
drift will gradually decay to zero. However, in some case, a new
connection may be attempted using a mirror element before the
accumulated drift of that mirror element has decayed to zero. In
this situation, the remaining residual drift of the mirror element
will cause an error in the mirror element position that may prevent
making the new connection. The switch controller may include a
drift compensator 332 to estimate an amount of residual drift of
each mirror element previously used in a connection. The estimate
of residual drift may be combined with the nominal voltage values
provided by the mirror calibration table 324 to define corrected
voltage values. The corrected voltage values may be applied to the
electrodes associated with a drifted mirror element to compensate,
at least in part, for the residual drift. The use of a drift
compensator to compensate for MEMS mirror element drift is
described in pending patent application Ser. No. 13/958,889.
[0044] In some cases, the application of corrected voltages, or
nominal voltages when drift compensation is not used, may not
result in a connection between particular input and output ports.
Failure to make a connection may be caused, for example, by an
error in the mirror calibration table, by unexpected residual
mirror element drift, by a permanent change in mirror element
characteristics, or another cause. In this situation, the 4D (4
dimensional) search engine 334 may assume control of the mirrors
element used in the failed connection. The 4D search engine may
then conduct a search across four dimensions (2 rotation axes for
each of two mirror elements) to locate a set of voltages that cause
the desired connection to be made. Details of the operation of the
4D search engine 334 will be provided in the subsequent Description
of Processes.
[0045] The command interpreter 320, the connection optimizer 330,
the drift compensator 332, and the 4D search engine 334 may jointly
maintain and share the connection state table 340. The connection
state table 340 may include data indicative of the state or status
of each port of the optical circuit switch 300. Data included in
the connection state table 340 for each port may include a first
flag indicating if the respective port is available or committed to
a connection, and a second flag indicating if the connection has
actually been made. The connection state table 340 may include, for
input ports, a third flag indicating is light is present at the
respective input. For each port that is committed to a connection,
the connection state table 340 may also include the identity of the
port at the other end of the connection, the mirror element
associated with the port, the voltages presently applied to the
electrodes associated with the mirror element, an estimate of the
residual drift of the mirror element associated with the port, and
temporal data such as when the connection was first made and when
the position of the mirror element was most recently optimized.
[0046] The functional elements of the switch controller 390 may be
implemented by a collection of hardware, which may be augmented by
firmware and/or software. The collection of hardware may include
analog circuits, digital circuits, and one or more processors such
as micro controllers, microprocessors, and/or digital signal
processors. All or portions of the functional elements of the
switch controller 390 may be implemented by a processor executing
stored software instructions. All or portions of the functional
elements of the switch controller 390 may be implemented in one or
more application specific integrated circuits (ASICs) and/or one or
more programmable circuit devices such as programmable logic arrays
and field programmable gate arrays (FPGAs).
[0047] Configuration data for programming a programmable circuit
device may be stored in a machine readable storage medium and used
to configure a programmable circuit device upon power-up of a test
system. Software instruction for execution by a processor may also
be stored in a machine readable storage medium. In this patent, the
term "machine readable storage medium" means a physical medium for
storing digital data. Examples of machine readable storage media
include optical discs such as CD-ROM, CD-RW, and DVD discs;
magnetic medium such as hard and flexible magnetic discs and
magnetic tape; and nonvolatile semiconductor devices such as
read-only and flash memories. The term "machine readable storage
medium" does not encompass transitory media such as signals and
waveforms.
[0048] The depiction of the switch controller 390 as a plurality of
functional elements does not imply a corresponding physical
division of the hardware constituting the scan controller. Any
functional element may be divided between two or more modules,
circuit cards, programmable circuit devices, ASICs, or other
circuit devices. All or portions of two or more functional element
may be collocated within a common programmable circuit device,
ASIC, or other circuit device.
[0049] Description of Processes
[0050] FIG. 4 is a flow chart of a process 400 for making a
connection through an optical circuit switch such as the optical
circuit switches 100 and 300. The process 400 may start at 405 when
a connection command is received by a switch controller such as the
switch controller 390. The process 400 may end at 490 if the
desired connection cannot be made, or may end at 495 if the
connection is made and subsequently broken.
[0051] Multiple instantiations of the process 400 may proceed
sequentially and/or concurrently to make multiple connections,
involving multiple mirrors, through the compound optical circuit
switch. Note that two instantiations of the process 400 are
required to make both directions of a full-duplex connection.
[0052] As previously described, a connection optimizer 330 and a
drift compensator 332 with the optical circuit switch may rely upon
measurements of the input and output optical power of each
connection. Dark connections, or connections without any input
light, cannot be optimized and the drift of mirror elements
involved in dark connections cannot be determined or compensated.
Thus, after a connection command is received at 405, a
determination may be made at 410 whether or not light is present at
the input port identified in the received connection command. For
example, the input optical power ("Pin") at the input port may be
measured by the input optical power meter 370 and compared to a
threshold minimum optical power ("Pin.sub.min"). Pin.sub.min may
be, for example, equal to a minimum anticipated input optical
signal. When Pin.gtoreq.Pin.sub.min a determination may be made
("yes" at 410) that input light is present, and the process may
continue to 415. When Pin<Pin.sub.min a determination may be
made ("no" at 410) that input light is not present, and the process
may idle at 410 until input light is provided.
[0053] At 415, an attempt to make the requested connection may be
made. First, the mirror elements associated with the ports
specified in the connection command may be identified. For example,
the mirror elements may be identified by consulting the port map
322. Nominal voltages to be applied to electrodes associated with
the identified mirror elements may then be retrieved from the
mirror calibration. The drift compensator 332 may then adjust the
nominal voltages as required to compensate for residual drift of
the identified mirror elements to provide corrected voltages. The
corrected voltages may then be applied to the electrodes associated
with the identified mirror elements to attempt to make the
requested connection.
[0054] At 420, a determination may be made whether or not a
connection has resulted from the action at 415. For example, the
output optical power ("Pout") at the output port may be measured by
the output optical power meter 380 and compared to a threshold
minimum output optical power ("Pout.sub.min"). In order to optimize
a connection through the optical circuit switch, the output optical
power meter must be able to detect small changes in Pout. To allow
detection of small changes, Pout needs to be sufficiently above the
noise floor, or the bottom of the dynamic range, of the output
optical power meter. Thus Pout.sub.min may be set to equal the
noise floor of the output optical power meter plus a margin of, for
example, 2 or 3 dB. When Pout.gtoreq.Pout.sub.min a determination
may be made ("yes" at 420) that a connection has been made, and the
process may continue to 425. When Pout<Pout.sub.min a
determination may be made ("no" at 420) that a connection has not
been made, and the process may continue at 430.
[0055] Alternatively, the output optical power Pout at the output
port may be measured by the output optical power meter 380 and the
insertion loss IL of the connection may be calculated. The
insertion loss ("IL") may be compared to a threshold insertion loss
("IL.sub.th"). The threshold insertion loss may be set based on the
minimum expected input signal power and the noise floor of the
output monitoring module. For example, if the minimum expected
optical signal power is -20 dBm and the noise floor of the output
optical power meter is -32 dBm, the insertion loss threshold may be
set to 10 dB. When IL.ltoreq.IL.sub.th a determination may be made
("yes" at 420) that a connection has been made, and the process may
continue to 425. When IL>IL.sub.th a determination may be made
("no" at 420) that a connection has not been made, and the process
may continue at 430.
[0056] When a determination is made at 420 that a connection has
been made, the connection may be optimized at 425 by a connection
optimizer such as the connection optimizer 330. The connection
optimizer may optimize the connection be performing separate
searches on each of four axes (two rotation axes for each of two
mirrors) using a hill-climbing algorithm or other local area search
algorithm. The connection optimizer may optimize each axis in
sequence and repeat the sequence two or more times.
[0057] After completion of the connection optimization at 425, a
determination may be made at 450 whether or not the connection is
acceptable. For example, the input optical power Pin and the output
optical power Pout may be measured and the insertion loss of the
connection may be calculated and compared to a maximum acceptable
insertion loss ("IL.sub.max"). When IL.ltoreq.IL.sub.max a
determination may be made ("yes" at 450) that an acceptable
connection has been made, and the process may continue to 460. When
IL>IL.sub.max a determination may be made ("no" at 450) that the
connection is not acceptable, and the process may return to
430.
[0058] When a determination is made at 450 that an acceptable
connection has been made, the process 400 may enter a connection
maintenance loop at 460. At 462, the connection optimizer may
optimize the mirror element positions as previously described with
respect to 425. Optimizing the mirror element positions at 462 may
be performed periodically to compensate for any mirror element
drift that may occur. Optimizing the mirror element positions at
462 may be performed repeatedly until either a determination is
made at 464 that a command to break the connection has been
received, or until a determination is made at 466 that the
connection has been lost.
[0059] When a determination is made at 464 that a break command has
been received ("yes" at 464), the mirror elements may be placed in
respective parked positions and the process 400 may end at 495.
[0060] A connection through the optical circuit switch can be lost
if the input light is removed from the connection. In this case,
the ongoing drift of the mirror elements used in making the
connection cannot be compensated. If the input light is removed for
a sufficiently long time period, the cumulative uncompensated
mirror element drift may result in the connection being lost. A
connection may also be lost due to a failure or error within the
optical circuit switch. Additionally, in extraordinary
circumstances, the connection made at 415 may be a false connection
between the desired output port and an incorrect input port. This
can occur if the mirror element associated with the incorrect input
port is in a position that reflects light from the incorrect input
port to the mirror element associated with the desired output port.
A false connection will be broken when the mirror associated with
the incorrect input port is either used to make a different
connection or is placed in its parked position. Thus, false
connections are usually temporary.
[0061] As previously described, a the positions of mirror elements
used in making each connection are periodically optimized to
minimize the insertion loss of the connection, which is to say to
maximize the output power form the connection. This optimization
can only be performed when the output power from the connection is
at a usable level. At 466, a determination may be made whether or
not the connection has been lost. The connection may be considered
as lost if the output power falls below a threshold level, which
may be the threshold minimum output optical power Pout.sub.min from
415. The connection may be considered as lost if the output power
falls below a threshold level for more than a predetermined time
interval. The connection may be considered as lost at 466 if the
insertion loss of the connection increases by more than a
predetermined amount or if the output power of the connection
decreases by more than a predetermined amount.
[0062] When a determination is made at 466 that the connection has
been lost ("yes" at 466), the mirror elements may be returned to
their respective parked positions, and the process 400 may return
to 410 to either wait for input light or to attempt to remake the
connection.
[0063] Although the actions 462, 464, and 466 within the
maintenance loop 460 are shown as sequential for ease of
discussion, optimizing the mirror element positions at 462 may be
performed periodically, and the actions at 464 and 466 may be
performed continuously and simultaneously.
[0064] When a determination is made at 420 that a connection has
not been made ("no" at 420) or when a determination is made at 450
that a connection is not acceptable ("no" at 450), a 4D search may
be performed at 430. The 4D search process will be described in
more detail subsequently. The 4D search 430 may end in either
success (i.e. establish a connection) or failure (i.e. not find a
connection). When the 4D search 430 results in a connection
("success" at 430), the process 400 may continue at 425. When the
4D search 430 does not find a connection ("failure" at 430), the
process 400 may continue at 435.
[0065] At 435, a determination may be made whether or not to retry
to make the requested connection. For example, the process 400 may
be defined to never retry, to retry a predetermined number of
times, or to retry repeatedly without limit until the connection
command is cancelled or a connection is made. When a determination
made that a retry will not be attempted ("no" at 435), the mirror
elements may be placed in their respective parked positions and the
process 400 may end in failure at 490. When a determination is made
that a retry will be attempted ("yes" at 435), the mirror elements
may be placed in their respective parked positions at 465 and the
process 400 may repeat from 410.
[0066] FIG. 5 is an optical schematic diagram of a portion of an
optical circuit switch such as the optical circuit switch 100. An
input signal is conveyed through an input optical fiber 512 to a
lens 518 which converts the input signal into an input beam 520.
The input beam 520 may have a Gaussian energy profile. The input
beam 520 may have a finite diameter at the lens 518 and may be
slightly converging rather than precisely collimated.
[0067] The input beam 520 may reflect from a first mirror element
530a which may be part of an input mirror array, such as the input
mirror array 130, including a plurality of input mirror elements.
The first mirror element 530a may be independently rotatable about
an x-axis 534 and a y-axis (not identified) orthogonal to the plane
of the drawing. A rotation angle about the x-axis 534 may be
controlled by a first voltage V1 provided by a driver circuit 532
and applied to an electrode (not shown) coupled to the first mirror
element 530a. Similarly, a rotation angle about the y-axis may be
controlled by a second voltage V2 provided by a driver circuit 536
and applied to an electrode (not shown) coupled to the first mirror
element 530a.
[0068] The beam 525a (dashed line) reflected from the first mirror
element 530a may be directed to a second mirror element 540a which
may be part of an output mirror array, such as the output mirror
array 140, including a plurality of output mirror elements. The
second mirror element 540a may be independently rotatable about an
x-axis 544 and a y-axis (not identified) orthogonal to the plane of
the drawing. A rotation angle about the x-axis 544 may be
controlled by a third voltage V3 provided by a driver circuit 542
and applied to an electrode (not shown) coupled to the second
mirror element 540a. Similarly, a rotation angle about the y-axis
may be controlled by a fourth voltage V4 provided by a driver
circuit 546 and applied to an electrode (not shown) coupled to the
second mirror element 540a.
[0069] The beam 560 reflected from the second mirror element 540a
may be directed to a lens 558 that converts the beam 560 into an
output signal in an output optical fiber 552. In order to form an
efficient optical connection, an image of an exit face of the input
optical fiber 512 must be precisely imaged onto an entrance face of
the output optical fiber 552. Further, the entire diameter of the
beams 520 and 525 must reflect from the first mirror element 530a
and the second mirror element 540b, respectively, without
vignetting. Any error in the rotation of either the first mirror
element 530a or the second mirror element 540a may result in
increased insertion loss of the optical connection or, if the error
is sufficient, failure to form the optical connection. For the
purpose of discussion, mirror element rotation errors may be
quantified in terms of an "inter-mirror angle", which is the angle
between the optical beam 525a directed from the first mirror
element 530a to the second mirror element 540a and an erroneous
beam 525b (dash-dot line) directed from the first mirror element
530a to an incorrect output mirror element 540b adjacent to the
second mirror element 540a.
[0070] As previously described, a false connection between the
desired output port and an incorrect input port can occur in some
circumstances. In a false connection, an optical beam 527
(dash-dot-dot line) from an incorrect port (i.e. a port other than
the port associated with the first mirror element 530a) is
reflected from an associated input mirror element 530b to the
second mirror element 540a. The second mirror element 540a may
reflect the optical beam 527 into the lens 558. The lens 558 may
convert the beam 527 into a false output signal in the output
optical fiber 552.
[0071] When an attempt to make a connection through an optical
circuit switch has been unsuccessful, one or both of the input
mirror element and the output mirror element may be rotated to an
incorrect angle on one or both of the respect x and y rotation
axes. In this event, a 4D search may be performed. For example, a
4D search may be initiated after either 420 or 450 in the process
400. The 4D search may be performed by progressively changing the
first, second, third, and fourth voltages (V1, V2, V3, V4
respectively) applied by the driver circuits 532, 536, 542, 546 to
the respective electrodes coupled to the first mirror element 530a
and the second mirror element 540a. The drive voltages may be
varied to cause the mirror elements 530a, 540a to rotate in
predetermined angular steps about the respective x and y rotation
axes. A large number of different combinations of the first,
second, third, and fourth voltages may be evaluated to find a
combination that makes the desired connection.
[0072] Prior to performing a 4D search, a search protocol may be
defined. The search protocol may include an angular step size for
each of the four axes, which may be the same or different for each
axis. The search protocol may also include definition of one or
more search domains. In this context, a "domain" is a set of points
in four-dimensional space over which a search will be conducted.
Each search domain may have a respective extent, which is to say
the number of angular steps that will be taken along each of the
four axes during the search. Multiple search domains of differing
extent may be defined, such as, for example, a small search domain
to be used initially with the hope of quickly establishing a
connection and one or more larger search domains to be used if the
search over the smaller search domain is not successful. The search
protocol may also include one or more search patterns or orders in
which the points within the search domains will be attempted.
[0073] A 4D search may try or evaluate all of the points within a
search domain, and then select the point that provided the best
result against one or more criteria. For example, a 4D search for a
connection in an optical circuit switch may measure the insertion
loss for each point (i.e. each combination of mirror drive voltages
or angles) and then select the point with the lowest insertion
loss. Alternatively, a 4D search for a connection in an optical
circuit switch may measure the insertion loss for a series of
points but stop searching when a point with "good enough" insertion
loss (i.e. insertion loss less than or equal to a predetermined
threshold) is achieved.
[0074] FIG. 6 is a flow chart of an exemplary process 600 for
defining a search protocol. The process 600 may start at 610 when
the requirements for and design of a particular optical circuit
switch are known. The process may end at 690 when all of the
parameters of a search protocol have been defined. The actions at
620, 630, and 640 may be performed in some other order.
[0075] At 620, an angular step size may be defined for each of the
four axes (two axes for each of two mirror elements). The angular
step sizes are critical elements of a 4D search protocol. With an
excessively small step size, a 4D search may require an inordinate
number of steps to find a connection. With an excessively large
steps size, the search may inadvertently "step over" or miss the
connection. Note that a 4D search does not have to find an optimum
connection--the 4D search only needs to find mirror element drive
voltages that result in a connection that is "close enough" to be
optimized at 425 in the process 400. To this end, a 4D search only
needs to find mirror element drive voltages that result in a true
connection with an output power level sufficiently above a noise
floor of the output optical power meter. The optimum step size may
depend on the diameter of the optical beams, the surface area and
spacing of the mirror elements in the mirror arrays, and the
sensitivity of the output optical power meter. The optimum step
size may be defined based upon an analysis of the optical system of
the optical circuit switch.
[0076] Referring now to FIG. 7, a chart 700 shows added insertion
loss as a function of x-axis and y-axis mirror angular errors for
an input mirror element within a particular optical circuit switch,
with the assumption that the alignment of the output mirror element
is perfect. In this context "added insertion loss" is insertion
loss in addition to the expected insertion loss of the other
elements of an optical circuit switch (e.g. losses due to input and
output tap couplers, reflection losses, etc.). The near-circular
shapes in the chart 700 are contours of equal added insertion loss.
The respective added insertion loss is given by a parameter
superimposed over each contour. The mirror element angular errors
are expressed as fractions of the respective inter-mirror angles,
which may not be the same for the x-axis and the y-axis.
[0077] The chart 700 is an example based upon a particular design
for an input mirror array and an output mirror array and a
particular optical design. Further, the chart 700 was generated
using the assumption that the optical beam reflected from the input
mirror element has a Gaussian profile and a particular beam waist
diameter. A graph such as the chart 700 may be generated, for
example, by modeling the optical system of FIG. 5 using an optical
modeling software tool and causing the software tool to
systematically vary the rotation angles of the input and output
mirror elements. A graph similar to the chart 700 may be generated
for any optical circuit switch, but the relationship between added
insertion loss and mirror angular errors may be significantly
different from that shown in the chart 700. Graphs such as the
chart 700 may be used to select an angular step size for each axis
of a 4D search.
[0078] An angular step size may be selected from the chart 700
given a maximum allowable insertion loss that can be attributed to
the angular errors of a single mirror. For the purpose of
explanation, assume that the optimization process 425 can optimize
a connection so long as the initial added insertion loss is less
than or equal to 10 dB. In this case, the maximum added assertion
loss that can be attributed to each of the two mirror elements is 5
dB. This assumption is exemplary, and a different optical circuit
switch may be able to optimize connections stating with higher or
lower added insertion loss.
[0079] In FIG. 7, a dashed rectangle 710 defines a suitable search
step size of 0.5 (relative to the x axis inter-mirror angle) on the
x axis and 0.25 (relative to the y axis inter-mirror angle) on the
y axis. The corner points 712, 714, 716, 718 of the rectangle 710
represent combinations of mirror angular errors that result in just
less than 5 dB insertion loss. Importantly, if the rectangle 710 is
shifted in any direction with respect to the chart 700, the
insertion loss for at least one of the corner points will decrease
to less than 5 dB. At least one of the corner points 712, 714, 716,
718 will have less than 5 dB insertion loss for any possible
alignment of the rectangle 710 on the chart 700. Thus a 4D search
performed by stepping the input mirror element by 0.5 inter-mirror
angle on the x axis and 0.25 inter-mirror angle on the y axis will
not "step over" a connection having less that 5 dB insertion loss.
A similar graph may be generated for the output mirror element and
angular step sizes for the output mirror element may be similarly
defined.
[0080] Referring back to FIG. 6, the extent of one or more search
domains may be defined at 630. The extent of each search domain may
be defined as a range of angle or a number of points on each of the
four axes. For ease of visualization, a search domain may be
depicted, as shown in FIG. 8, as a two-dimensional grid of angular
positions for the input (first) mirror element and a
two-dimensional grid of angular positions of the output (second)
mirror element. For example, the two dimensional input mirror
element search grid 810 contains 7.times.11=77 combinations of
angular positions for the input mirror element. Similarly, the two
dimensional output mirror element search grid 820 contains
7.times.11=77 combinations of angular positions for the output
mirror element. These two search grids define a search domain in
which each possible point in the input search grid is paired with
each possible point in the output search grid, for a total of 5,929
points in the search domain. An input or output search grid may
have more or fewer than 7.times.11 points, the input and output
search grids may not necessarily be the same.
[0081] As previously described, more than one search domain may be
defined at 630. For example, small search domain defined by
7.times.11 point input and output mirror search grids 810-820 may
be searched first to attempt to quickly find the connection. One or
more larger search domains 812-822, 814-824, 816-826 may be defined
and searched if a search of a smaller search domain was not
successful. Larger search domains include more points and thus take
longer to search. Additionally, when searching a larger search
domain after searching a smaller search domain, it may be necessary
to search only the new points (i.e. points not included in the
smaller search domain). The table in FIG. 8 defines the extent of
the search domains and the number of new points (i.e. points not
included in the next smaller search domain) in each search
domain.
[0082] Returning to FIG. 6, one or more search domains may be
defined at 630 based on known or anticipated error distribution for
the input and output mirror elements. For example, if the input and
output mirror elements are susceptible to larger angular errors on
one axis than the other axis, the extent of a search domain may be
greater along the axis with larger errors. For further example, if
the simultaneous presence of large angular errors on both the
x-axis and the y-axis is very improbable, a search domain may be
defined with a cruciform shape with arms extending along both axes.
Conversely, if the angular errors on the x-axis and the y-axis are
not independent, such that a large error on one axis increases the
probability of a large error on the other axis a search domain may
be defined with a cruciform shape with arms extending at 45 degree
angles to the axes.
[0083] After one or more search domains are defined at 630, one or
more search patterns may be defined at 640. In this context, a
"search pattern" defines the order in which the points within a
search domain will be tried. A search pattern may be an ordered
list of the points with a search domain, or an algorithm that
determines an order in which the points within the domain will be
tried.
[0084] When angular errors on the four axes are random and
independent, the point that produces the lowest insertion loss is
more like to be near the center of the search domain than at an
extreme of the search domain. In this case, a spiral search pattern
proceeding from the center of the search domain outward may
minimize the search time require to arrive at a "good enough"
point. FIG. 9 shows an exemplary spiral search pattern 900 for a
7.times.7 element grid. The search pattern 900 may define, for
example, the order in which the angles on the two rotation axes of
the input mirror are varied during a search. A second search grid,
which may be the same or different, may define the order in which
the angles of the output mirror are varied. For example, the output
mirror may be set to a first point and the input mirror may stepped
from point 1 to point 49 in the order shown in the search pattern
900. The output mirror may then be set to a second point and the
input mirror may stepped from point 1 to point 49 in the order
shown in the search pattern 900 (or from point 49 to point 1 in
reverse order). In this manner all 49 points in the search pattern
900 may be tried with all of the points in the output mirror search
grid until a point is found with "good enough" insertion loss.
[0085] FIG. 10 defines, in tabular form, a double spiral search
pattern for a 3.times.3.times.3.times.3 4D search domain. The 4D
search domain may be represented graphically by a 3.times.3 input
grid 1010 representing 9 possible angular positions of an input
mirror element and a 3.times.3 output grid 1020 representing 9
possible positions of an output mirror element. The 4D search
starts with the first point in both the input and output grids.
Subsequently, a new point is added to either the input or output
grid. Each time a new point is selected, all combination of the new
point and all previously added points in the other of the input or
output search grid. Examination of the table shows that each step
in the 4D search requires moving only one of the input and output
mirror elements. Further each step requires a mirror element to
move only the distance between adjacent points in the corresponding
grid. The search pattern of FIG. 10 can be scaled to arbitrarily
large search spaces.
[0086] Referring now to FIG. 11, a process 1100 for performing a 4D
search may be suitable for use at 430 in the process 400. The
process 1100 may start at 1110, after an attempt to make a
connection through an optical circuit switch has been unsuccessful.
When the process 1100 succeeds in making a connection, the process
may end at 1195. When a connection cannot be made, the process 1100
may end in failure at 1190. In either case, the process 400 may
continue after the process 1100 has ended.
[0087] The process 1100 may be performed in accordance with a
search protocol, which may be defined using the process 600 of FIG.
6. The search protocol may define, for example, one or more search
domains, a step size between search points along each of the four
axes, and one or more search patterns. At 1120 a search domain may
be selected. If the search protocol only defines a single search
domain, that domain may be selected at 1120 by default. When a
search protocol defines more than one search domain, a smallest
domain may be initially selected at 1120. If a connection is not
found within the smallest domain, one or more progressively larger
domains may be selected at 1120 during subsequent iterations of the
process.
[0088] At 1130, a point within the search domain may be selected.
For example, the initial point selected may be at the center of the
search domain (i.e. at the center of the extent of the search along
each of the four axes). Some other initial point may be selected. A
different point within the search domain may be selected during
each repetition of the actions 1130-1170. Points within the search
domain may be selected in the order specified by a search pattern
defined at 1105.
[0089] After a point within the search domain is selected, the
point may be evaluated at 1140. The search domain selected at 1120
and the associated search grid may be conveniently defined in terms
of angular positions for the input and output mirror elements. To
evaluate a point, the corresponding angular positions may be
converted into particular voltage values (i.e. particular voltage
values for the first, second, third, and fourth voltages described
in conjunction with FIG. 5) to be applied to electrodes coupled to
the input mirror element and the output mirror element. For
example, a mirror calibration table, such as the mirror calibration
table 324, may list a set of nominal voltages to make each possible
connection through the optical circuit switch and a search voltage
step size for each of the first through fourth voltages. The
particular voltages corresponding to a search point may be
determined in some other manner.
[0090] The particular voltages may then be applied to the first
through fourth electrodes and the insertion loss of the connection
may be determined at 1140. A brief delay (not shown) may be
provided between changing the applied voltages and determining the
insertion loss to allow the mirror elements to settle in position.
The insertion loss may be determined at 1140, for example, using an
input optical power meter to measure input optical power at the
input side of the connection and an output optical power meter to
measure output optical power at the output side of the connection.
The insertion loss may be determined as the ratio of the output
optical power to the input optical power. The input and output
optical power may commonly be expressed in dBm, in which case the
insertion loss may be determined as the input optical power minus
the output optical power (in which case insertion loss is a
positive number).
[0091] At 1150, the insertion loss determined at 1140 may be
compared to a predetermined stop-search-loss (SSL) threshold.
Continuing the example of FIG. 7, SSL may be, for example, 10 dB.
SSL may be larger or smaller than 10 dB. In general, setting a
smaller SSL value may dictate a smaller search step size on some or
all of the search axes. Setting a larger SSL value may dictate a
larger search step size on some or all of the search axes.
[0092] When a determination is made at 1150 that the insertion loss
of the connection is greater than SSL ("no" at 1150), the process
1100 may proceed to 1170. At 1170, a determination may be made
whether or not there are additional points to try within the search
domain. When there are more points available ("yes" at 1170),
another point may be selected at 1130 and the actions from
1130-1170 may be repeated until either a tentative connection is
found (i.e. a point is found with insertion loss less than SSL at
1150) or all points within the search domain have been tried.
[0093] When a determination is made at 1170 that all points within
a search domain have been tried ("no" at 1170), a determination may
be made at 1180 whether or not one or more additional search
domains were defined at 1105. If a determination is made that there
is another domain to search ("yes" at 1180), the process 1100 may
return to 1120 to select a new search domain. The actions from 1120
to 1180 may be repeated until either a tentative connection is
found (i.e. a point is found with insertion loss less than SSL at
1150) or all points within all search domains have been tried.
[0094] When a determination is made at 1180 that all points within
all search domains have been attempted without finding a
connection, the process 1100 may end in failure at 1190.
[0095] When a determination is made at 1150 that the insertion loss
of the connection is less than SSL ("yes" at 1150), a blink test
may be performed at 1160. As previously described, a false
connection may occasionally occur between an output port and an
incorrect input port. In this case, light from the incorrect input
port is reflected by a corresponding input mirror element different
from the input mirror element associated with the correct input
port. To detect a false connection, the correct input mirror
element may be deflected from its last position at 1162 by changing
one or both of the first and second voltages. The insertion loss of
the connection may be determined at 1164. The input mirror element
may then be returned to its last position (not shown). The actions
at 1162 and 1164 may be repeated two or more times. A determination
may then be made at 1166 if the connection is a true connection or
a false connection. If the connection is a true connection, the
insertion loss measured at 1164 will change in response to the
changed voltage at 1162. If the connection is a false connection,
the insertion loss measured at 1164 will not change in response to
the changed voltage at 1162.
[0096] When a determination is made at 1166 that the connection is
a true connection, the process 1100 may end in success at 1195.
When a determination is made at 1166 that the connection is a false
connection, the process 1100 may return to 1170 to continue to
search for a true connection.
[0097] Closing Comments
[0098] Throughout this description, the embodiments and examples
shown should be considered as exemplars, rather than limitations on
the apparatus and procedures disclosed or claimed. Although many of
the examples presented herein involve specific combinations of
method acts or system elements, it should be understood that those
acts and those elements may be combined in other ways to accomplish
the same objectives. With regard to flowcharts, additional and
fewer steps may be taken, and the steps as shown may be combined or
further refined to achieve the methods described herein. Acts,
elements and features discussed only in connection with one
embodiment are not intended to be excluded from a similar role in
other embodiments.
[0099] As used herein, "plurality" means two or more. As used
herein, a "set" of items may include one or more of such items. As
used herein, whether in the written description or the claims, the
terms "comprising", "including", "carrying", "having",
"containing", "involving", and the like are to be understood to be
open-ended, i.e., to mean including but not limited to. Only the
transitional phrases "consisting of" and "consisting essentially
of", respectively, are closed or semi-closed transitional phrases
with respect to claims. Use of ordinal terms such as "first",
"second", "third", etc., in the claims to modify a claim element
does not by itself connote any priority, precedence, or order of
one claim element over another or the temporal order in which acts
of a method are performed, but are used merely as labels to
distinguish one claim element having a certain name from another
element having a same name (but for use of the ordinal term) to
distinguish the claim elements. As used herein, "and/or" means that
the listed items are alternatives, but the alternatives also
include any combination of the listed items.
* * * * *