U.S. patent application number 09/853643 was filed with the patent office on 2001-10-11 for cascading of tunable optical filter elements.
Invention is credited to Norwood, Robert A., Rudasill, Meade H., Sossen, David H..
Application Number | 20010028494 09/853643 |
Document ID | / |
Family ID | 22953842 |
Filed Date | 2001-10-11 |
United States Patent
Application |
20010028494 |
Kind Code |
A1 |
Norwood, Robert A. ; et
al. |
October 11, 2001 |
Cascading of tunable optical filter elements
Abstract
A tunable optical signal device and method of using the same
having at least two filter elements, each of said filter elements
being made of a material having an adjustable parameter, wherein
the adjustable parameter is maintained at slightly different values
for adjacent filter elements.
Inventors: |
Norwood, Robert A.;
(Cranford, NJ) ; Sossen, David H.; (Basking Ridge,
NJ) ; Rudasill, Meade H.; (Mendham, NJ) |
Correspondence
Address: |
WATOV & KIPNES, P.C.
P.O. Box 247
Princeton Junction
NJ
08550
US
|
Family ID: |
22953842 |
Appl. No.: |
09/853643 |
Filed: |
May 11, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09853643 |
May 11, 2001 |
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09251893 |
Feb 19, 1999 |
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6256428 |
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Current U.S.
Class: |
359/290 ;
359/291 |
Current CPC
Class: |
G02B 6/29395 20130101;
G02F 2201/06 20130101; G02B 6/29317 20130101; G02F 2201/307
20130101; G02B 6/29398 20130101; H04J 14/02 20130101; G02F 1/212
20210101; G02B 6/29352 20130101; G02F 1/0147 20130101 |
Class at
Publication: |
359/290 ;
359/291 |
International
Class: |
G02B 026/00 |
Claims
What is claimed:
1. A tunable optical signal device comprising at least two filter
elements, each of said filter elements being made of a material
having an adjustable parameter, and means for maintaining said
adjustable parameter at slightly different values for adjacent
filter elements.
2. The tunable optical signal device of claim 1 wherein the
adjustable parameter is selected from temperature and mechanical
stress.
3. The tunable optical signal device of claim 1 wherein the filter
elements are Mach-Zehnder interferometers integrated with tunable
Bragg gratings.
4. The tunable optical signal device of claim 1 wherein the
material is a thermosensitive material.
5. The tunable optical signal device of claim 4 wherein the
thermosensitive material is at least one thermosensitive
polymer.
6. The tunable optical signal device of claim 1 wherein the
adjustable parameter is temperature, said means for maintaining the
adjustable parameter comprising: a) a thermocouple for measuring
the temperature of the material; b) a temperature sensor for
comparing the temperature of the material to a preset temperature;
and c) a heater for applying heat to the material to maintain said
preset temperature.
7. The tunable optical signal device of claim 1 further comprising
a circulator and a tunable Bragg grating.
8. The tunable optical signal device of claim 1 further comprising
a plurality of narrow band mirrors, each mirror segregating a set
of a plurality of wavelength signals and directing said set of
wavelength signals to said filter element.
9. The tunable optical signal device of claim 9 further comprising
GRIN lenses for collimating said set of wavelength signals before
said set of wavelength signals enter the filter element.
10. A method of dropping/adding at least one preselected wavelength
of light from/to an optical signal comprising passing said optical
signal through a tunable optical signal device comprising at least
two filter elements, each of said filter elements being made of a
material having an adjustable parameter, and means for maintaining
said adjustable parameter at slightly different values for adjacent
filter elements, said method comprising adjusting said parameter to
reflect said at least one preselected wavelength of light.
Description
FIELD OF THE INVENTION
[0001] The present invention is generally directed to improved
integrated wavelength division multiplexer/demultiplexer optical
devices in which light of a specific wavelength (or specific
wavelengths) can be added or dropped in an efficient manner. In
particular, the present invention is directed to providing a fine
tune function of such devices by providing the optical elements
with a material having slightly different values for a preselected
variable such as temperature for one or more adjacent optical
elements.
BACKGROUND OF THE INVENTION
[0002] Devices for adding and dropping wavelength coded signals
(light of a specific wavelength or wavelengths) are known in the
art. Such devices employ optical fibers which are utilized
predominantly in telecommunications in addition to local area
networks, computer networks and the like. The optical fibers are
capable of carrying large amounts of information and it is the
purpose of such devices of the present invention to extract a
selected amount of information from the fiber by segregating the
information carried on different wavelength channels.
[0003] Devices of this type are comprised of a variety of
components which together provide the desired segregation of
wavelength coded signals. Integrated optical couplers and
especially directional couplers have been developed to accomplish
evanescent directional coupling. Optical signals are coupled from
one planar waveguide to another. The signals in the second planar
waveguide propagate in the same direction in which the signals
travel in the first planar waveguide.
[0004] Diffraction gratings (e.g. Bragg gratings) are used to
isolate a narrow band of wavelengths. Such grating reflectors have
made it possible to construct a device for use in adding or
dropping a light signal at a predetermined center wavelength to or
from a fiber optic transmission system without disturbing other
signals at other wavelengths.
[0005] Wavelength division multiplexing systems are being deployed
to greatly increase the band width capacity of existing optical
fiber installations. Key components in these systems are the
wavelength division multiplexers and demultiplexers that serve to
combine and separate the individual wavelength signals at the two
termini of the transmission system. These components include
precision optical filters (e.g. Bragg gratings) that must be
tailored specifically for each wavelength that is being
transmitted. The number of wavelengths and their precise values
vary from system to system and even within a system as a function
of time as wavelength density increases.
[0006] The rapid growth of optical fiber-based telecommunications
systems requires continual improvement in capacity of those systems
to enable the management of increased bandwidth needs. There are
several straightforward ways to increase the capacity of a
system:
[0007] 1. Install more optical fiber--this is the simplest approach
but can be very expensive and time consuming;
[0008] 2. Increase the data rage of the transmitters on the end of
the fiber--this is cheaper and quicker than installing new fiber,
but at high data rates (>5 Gigabits per sec), physical
limitations of the optical fiber begin to be a problem, leading to
unacceptably large dispersion of the optical pulse as it travels
down the fiber;
[0009] 3. Transmit at low data rates at multiple wavelengths--once
again, there is a cost savings over installing new fiber and now
the primary challenge for the optical components is in being able
to provide stable lasers at many wavelengths over the preferred
range of 1530 to 1560 nm, and also providing precise filters that
can segregate a desired wavelength.
[0010] Optical components as mentioned in Item No. 3 above may be
deployed in wavelength division multiplexing (WDM) systems that
carry 4, 8, 16, 32, 40 64, and 80 wavelengths of light
simultaneously. A number of technologies have been used to solve
the filter problem, among them fiber Bragg gratings (FBG) as
disclosed in (No. 1), arrayed waveguide grating (AWG) routers as
disclosed in (No. 2), and thin film dielectric filters as disclosed
in No. 3. All of these approaches result in filter characteristics
of varying quality, with the preferred filter characteristic being
a transmission of 100% at the wavelength of choice +/- some range,
and 0% transmission at all other wavelengths. In terms of dB units,
filters are desired that provide greater than 20 dB and preferably
greater than 30 dB discrimination between the preferred wavelength
band and all other wavelengths.
[0011] With the exception of the AWG, all other filter approaches
rely on a sequential use of discrete filter elements. This places a
high demand on the quality of each filter element. Furthermore,
since approaches such as FBG and thin film dielectric filters are
by their nature fixed filters, and not tunable, each wavelength to
be filtered requires its own, uniquely manufactured filter. As used
herein the term "tunable" means that the filter element can be
adjusted in a manner that will enable optical signals of different
wavelengths to be segregated.
[0012] For example, a FBG suitable for the ITU wavelength 1547.72
nm will be unsuitable for the ITU wavelength 1550.92 nm, where the
ITU wavelengths represent standard communications wavelengths that
have been adopted by telecom system suppliers. This situation
results in a considerable increase in the cost to manufacture the
filters, and also increases cost of ownership because of time
consuming labeling and inventorying of these devices. Thus, there
is a need for a technology that provides for post manufacture
adjustment of the filter wavelength, i.e. a tunable filter. By way
of example, if 40 filter elements were needed, they could all be
made identically and then adjusted, either at a factory or in the
field, to filter the desired wavelength. This provides a greatly
increased modularity to the WDM filter system, reducing cost of
manufacture and ownership.
[0013] There are several tunable filter technologies that have been
developed, chief among them acousto-optic tunable filter (AOTF) as
disclosed in (No. 4) and Fabry-Perot tunable filter (FPTF). AOTF's,
based on the acousto-optic effect present in ferroelectric
materials such as lithium niobate, work by using an acoustic wave,
stimulated by a radio-frequency power supply and transducer, to
induce densification and rarefaction in an optical waveguide
material. In practice, AOTF's usually work by changing the
polarization of light that is at a wavelength that is matched to
the acoustically induced grating. This light may then be separated
from the other wavelength components present. AOTF's have the
advantages of providing very rapid tuning (microseconds) and
complete blanking of the filter (when the radio-frequency power is
removed). However, it is very difficult to achieve the spectral
characteristics desired for WDM by this approach, in terms of
isolation between different wavelength channels, insertion loss at
a given wavelength channel, and, in particular, polarization
independence. FPTF's have been worked both in bulk embodiments as
disclosed in (No. 5), and, more recently, via micromechanical
approaches as disclosed in (No. 6). While FPTF's can achieve
relatively good filter performance, they have the disadvantage of
requiring a physical movement to achieve tuning, which reduces the
overall reliability.
[0014] An ideal tunable filter technology would have both the solid
state tuning of AOTF's coupled with the good filter performance of
FPTF's.
SUMMARY OF THE INVENTION
[0015] The present invention is directed to an optical signal
system including cascading tunable optical filters in which an
externally adjusted parameter such as temperature is maintained for
each filter element at a slightly different value than the same
parameter for an adjacent filter element. The resulting system
provides for the precise segregation of multiple wavelength signals
with less loss of intensity than is obtained with static optical
signal devices employing separate filter elements for segregating
each particular wavelength.
[0016] As used herein the term "tunable" means that the filter
elements of the optical signal device may have its ability to
reflect light varied preferentially for a preselected wavelength.
The term "cascading" means that the optical signal device contains
multiple tunable filter elements.
[0017] In one aspect of the present invention there is provided a
tunable optical signal device comprising at least two filter
elements, each of said filter elements being made of a material
having an adjustable parameter, and means for maintaining the
adjustable parameter.
[0018] In one aspect of the present invention there is provided a
tunable optical signal device including at least two optical filter
elements, each optical filter element comprised of a substrate, a
pair of spaced apart cladding layers and a core layer including a
pair of opposed waveguides, a grating region comprising a filter
means for causing a single wavelength by light of a multiple
wavelength of light source to be segregated therefrom, said core
layer comprised of a material having an adjustable property such
that the adjustable property of one filter element can be
maintained at a first value and the adjustable property of an
adjacent filter element can be maintained at a second value
different from the first value, and means for maintaining the first
and second values of the adjacent filter elements.
[0019] In another aspect of the present invention there is provided
an optical signal device comprised of a plurality of filter
elements in which wavelengths .lambda..sub.1, .lambda..sub.2, . . .
.lambda..sub.N enter a 1.times.4 filter element that has one
optical fiber coming in, four optical fibers with filtered
wavelengths coming out, and one optical fiber with unfiltered
wavelengths coming out. This filter is held at temperature T.sub.1.
This filter removes four of the wavelengths from the stream,
.lambda..sub.1 and three others, determined by the total number of
channels N and the channel spacing. The wavelengths that aren't
removed by this first filter then pass on to the second filter,
held at temperature T.sub.2. This filter is manufactured in exactly
the same way as the first filter, but removes different wavelengths
as it is held at a different temperature. The second filter
performs the same function as the first filter, and the light then
proceeds finally to the N/4.sup.th filter at which point all of the
wavelengths have been filtered out.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] The following drawings in which like reference characters
indicate like parts are illustrative of embodiments of the
invention and are not intended to limit the invention as
encompassed by the claims forming part of the application.
[0021] FIG. 1 is a schematic elevational view of one embodiment of
a filter element employed in the optical signal devices of the
present invention;
[0022] FIG. 2 is a graph showing the change in the wavelength of
light reflected by an embodiment of a filter element employed in
the present invention as a function of increasing temperature;
[0023] FIG. 3 is a schematic view of one embodiment of the optical
signal device of the present invention employing multiple filter
elements;
[0024] FIG. 4 is a schematic view of another embodiment of the
invention employing a circulator for forwarding a segregated
wavelength signal to a detector;
[0025] FIG. 5 is a schematic view of a still further embodiment of
the present invention showing multiple filter elements for
segregating an optical signal comprised of wavelengths
.lambda..sub.1-.lambda..sub.n;
[0026] FIG. 6 is a schematic view of another embodiment of the
present invention in which narrow band mirrors are used to
selectively target a preset wavelength for each optical filter
element; and
[0027] FIG. 7 is a schematic view of a temperature control system
employed in the present invention to maintain respective elements
at a desirable temperature.
DETAILED DESCRIPTION OF THE INVENTION
[0028] The present invention is directed to an optical signal
device having a plurality of optical filter elements in which
adjacent optical filter elements have different property values
(e.g. are set at different temperatures).
[0029] In a preferred form of the invention, Mach-Zehnder type
couplers or directional couplers are employed having two planar
waveguides which are aligned with each other in two coupling
regions or filter elements. Between the coupling regions or filter
elements is a grating region comprised of a grating system (e.g.
Bragg gratings). The waveguides are typically spaced apart from
each other so that evanescent coupling does not occur in the
grating region.
[0030] In accordance with the present invention, the optical signal
device employs at least two such optical filter elements set at
slightly different property values (e.g. slightly different
temperatures) to enable the reflection of different wavelength
signals.
[0031] The grating region is provided with a heater (such as an
electrode of specified resistance) or other means of inducing a
change of temperature of the polymer. The heaters of adjacent
filler elements are controlled such that the respective optical
filter elements are maintained at slightly different temperatures
and thereby reflect different wavelengths of light (i.e. different
wavelength optical signals). Alternatively, the mechanical stress
value of adjacent optical filter elements may be set at different
values. This will also result in the reflection of different
wavelength signals.
[0032] Referring to FIG. 1 there is shown a typical construction of
an optical filter element of the optical signal device of the
present invention and particularly the grating region. The filter
element 2 includes a core region 4 having on each side thereof
respective cladding layers 6A and 6B. Above the cladding layer 6A
in the specific embodiment shown in FIG. 1 is a heater 8. Beneath
the undercladding layer 6B there is provided a substrate 10. The
overcladding layer 6A and undercladding layer 6B are made of
thermosensitive polymers as described hereinafter. The core layer
is typically made of the same type of material as the cladding
layers although the refractive index of the respective layers will
differ as discussed hereinafter.
[0033] In accordance with the embodiment shown in FIG. 1, a heater
is provided in proximity to the filter element to heat the
thermosensitive polymers. As shown in FIG. 2, as the temperature of
the filter element is increased, the wavelength of the reflected
light will decrease, typically in a linear slope. As shown
specifically in FIG. 2, the wavelength of the reflected light will
decrease 0.256 nm per degree centigrade within the range of 20 to
100.degree. C. The wavelength of the reflected light will vary
linearly by about 20 nm within this temperature range. The
embodiment of the present invention shown in FIG. 1 therefore
changes the wavelength of the reflected light of a filter element
of an optical signal device by raising or lowering the temperature
of the material used to construct the filter element and by
maintaining the temperature thereof at a different value than the
temperature of an adjacent optical filter element.
[0034] An embodiment of an optical signal device in accordance with
the present invention is shown in FIG. 3. Each of the optical
filter elements 2A and 2B, having the same construction and made of
the same thermosensitive materials as described in connection with
FIG. 1, includes a heating system as described hereinafter
designated by the numerals 8A and 8B, respectively. The heating
system 8A is set to maintain the optical filter element 2A at a
temperature T.sub.1 different than the temperature maintained by
the heater 8B for the optical element 2B.
[0035] The particular wavelength of light which is dropped from the
light source can be precisely selected in accordance with the
present invention by adjusting the heater 8A in accordance with
FIG. 2. In particular, for each .degree. C. that the temperature of
the grating region is raised, the wavelength reflected will be
reduced by 0.256 nm.
[0036] The remaining wavelengths of light which pass the filter
element 2A may be further processed in a second filter element 2B
which is heated by a heater 8B and maintained at a temperature
different than the optical filter element 2A. As a consequence a
second, different wavelength of light .lambda..sub.2 is dropped
from the second optical filter element 2B.
[0037] In another embodiment of an optical signal device of the
present invention a circulator which is a three port device that
delivers light entering port 1 to port 2 and light entering port 2
to port 3 is employed in conjunction with each optical filter
element to segregate a one or more single wavelengths of light
while allowing the remaining optical signal to pass through for
eventual segregation of a different single or multiple band of
wavelengths in an adjacent filter element.
[0038] Referring to FIG. 4 there is shown an optical signal
comprising wavelengths .lambda..sub.1 . . . X.sub.n entering a
filter element 40 including a grating system 42. The filter element
is heated by a heater 44 to a temperature T.sub.1 which results in
the reflection of optical signal .lambda..sub.R to a circulator 46A
as defined above resulting in the transmission of .lambda..sub.R to
a detector 48A.
[0039] The optical signal .lambda..sub.1 . . . .lambda..sub.n
absent .lambda..sub.R is passed through the filter element 40 and
enters a filter element 50 manufactured to the same specifications
as filter element 40. The filter element 40 includes a grating
system 52 and is connected to a heater 54 which heats the filter
element 50 to a temperature T.sub.2, different than the temperature
T.sub.1. As a result a different wavelength signal or set of
wavelengths represented by .lambda..sub.T is reflected to a
circulator 46B where the wavelength signal .lambda..sub.T is sent
to a detector 48B.
[0040] An embodiment of the present invention showing multiple
filter elements is shown in FIG. 5. Wavelengths .lambda..sub.1,
.lambda..sub.2, . . . .lambda..sub.N enter, for example, a
1.times.4 filter element 60 that has one optical fiber 62 coming
in, four optical fibers collectively shown as numeral 64 and
filtered wavelengths coming out, and one optical fiber 66 with
unfiltered wavelengths coming out. The filter element 60 is held at
temperature T.sub.1. This filter removes four of the wavelengths
from the stream, .lambda..sub.1 and three others, determined by the
total number of channels N and the channel spacing. The wavelengths
that aren't removed by this first filter then pass on to the second
filter element 70, held at temperature T.sub.2. This filter is
manufactured in exactly the same way as the first filter, but
removes different wavelengths through optical fibers 72 as it is
held at a different temperature.
[0041] The unfiltered wavelengths pass through an optical fiber 76
into a filter element 80 held at a temperature T.sub.N/4-1. The
reflected wavelengths signal .lambda..sub.N/4-1 passes out through
optical fiber 82. Eventually the unfiltered wavelengths pass
through a filter element 90 heated to a temperature T.sub.N/4 and
thereby reflects a corresponding wavelength signal through optical
fiber 92 to complete the segregation of all wavelengths contained
with the optical signal .lambda..sub.1 . . . A.sub.n.
[0042] A necessary condition for the embodiment shown in FIG. 5 is
that the temperature required to achieve a channel spacing shift in
wavelength not be so large as to be unfeasible. The second filter
performs the same function as the first filter, and the light then
proceeds finally to the N/4.sup.th filter at which point all of the
wavelengths have been filtered out.
[0043] An alternative approach that makes use of the common module
architecture of FIG. 5 is shown in FIG. 6. FIG. 6 employs narrow
band mirrors to segregate sets of wavelength signals (e.g.
.lambda..sub.1-.lambda..sub.4) for an optical signal having
multiple wavelengths .lambda..sub.1-.lambda..sub.16 (i.e.
N=.lambda..sub.16). It will be understood that the value of N may
be larger or smaller and the number of filter elements may vary
from that shown specifically in FIG. 6.
[0044] Referring to FIG. 6, the sixteen wavelengths enter via an
optical fiber 100 that terminates in a GRIN collimating lens (not
shown). The collimated light then proceeds to a narrowband mirror
102A that removes four of the wavelengths (.lambda..sub.1 through
.lambda..sub.4). These four wavelengths are then collected with a
GRIN lens 104A into an optical fiber 106 and are separated out by a
1.times.4 filter element 108A similar to that described in FIG.
5.
[0045] Wavelengths .lambda..sub.5-.lambda..sub.16 then proceed to
the next narrowband mirror 102B which reflects the next four
wavelengths .lambda..sub.5-.lambda..sub.8 which enter a GRIN lens
104B, an optical fiber 106B and a 1.times.4 filter element 108B.
The process is repeated to remove wavelength signals
.lambda..sub.g-.lambda..sub.12 and .lambda..sub.13-.lambda..sub.16
through additional series of GRIN lenses, optical fibers and
1.times.4 filter elements represented by numerals 102C through 108D
until all of the wavelengths are segregated. While this embodiment
is more complex than that shown in FIG. 5, it has the advantage of
having better uniformity in the outputs. In the case of FIG. 5, the
wavelengths exiting from the last module will be significantly more
attenuated than those exiting from the first module owning to
unavoidable coupling and propagation losses in the devices. For the
embodiment shown in FIG. 6, it is possible to reduce the additional
losses from the GRIN lenses and the mirrors to negligible levels,
whereby all of the filtered signals are roughly equal in intensity
(assuming they are equal in intensity at the input).
[0046] The tunable filter elements of the present invention are
preferably manufactured as 1.times.N Mach-Zehnder or directional
coupler integrated optical circuits incorporating Bragg gratings in
a temperature sensitive optical material. Optical signal devices
produced in this manner typically have opposed waveguides which
comprise a core layer and upper and lower cladding layers as
previously described in which the respective layers are preferably
made of a photosensitive material which enables the application of
a refractive grating system by photolithography.
[0047] In general, the optical signal device comprises a substrate
having thereon a pair of spaced apart cladding layers have a core
layer therebetween with the core layer including a pair of opposed
waveguides. The waveguides are preferably applied to the core layer
by direct photolithography. The filter is preferably in the form of
a Bragg reflection grating system which preferably extends through
the core and cladding layers to enable the single wavelength
channel of light to be segregated from an input light source.
[0048] The substrates employed for fabrication of the optical
signal device can be selected from a variety of materials including
glass, silicon, plastics (e.g. polyurethane and polycarbonate) and
the like. The undercladding layer and the overcladding layer are
preferably made from photosensitive materials, preferably polymeric
materials which have a lower refractive index value than the core
layer. Such photosensitive materials include ethoxylated bisphenol
diacrylate and chloroflourodiacrylate and are of the type of
materials which can be treated with a source of energy to
differentiate one region of the material (e.g. where the waveguides
are imprinted) from another region of the material through the use
of, for example, a photomask and the like. Tunable optical elements
of this type are capable of being formed into single mode optical
waveguide structures such as directional couplers by direct
photolithography. Bragg gratings which are used as the filter
elements can be formed through holographic illumination. The
cross-linked, UV curable acrylate copolymers which are preferred
for fabrication of the optical signal devices possess a large
thermo-optic effect in that there is a measurable change in the
refractive index with temperature, and in some cases a large
photoelastic effect results in a change in the refractive index
with applied mechanical stress when mechanical stress is to be used
as the externally controlled variable.
[0049] The desirable properties for the thermosensitive materials
include a large thermo-optic coefficient. The thermo-optic
coefficient is defined as the change in refractive index with
temperature, dn/dT, where n is the refractive index and T is the
temperature. For typical glasses and inorganic dielectrics, the
dn/dT is on the order of 1.times.10.sup.-5/.degree. C., while for
polymers it is about -2.times.10.sup.-4/.degree. C. This means that
to effect a given change in refractive index, the temperature of a
typical polymer need be changed only one-twentieth the amount that
a typical glass would have to be changed. This results in thermally
tuned filters being straightforward to implement in polymers.
[0050] Other desirable properties for the thermosensitive materials
are high coefficient of thermal expansion, typically at least 50
ppm/.degree. C., preferably from about 100 to 200 ppm/.degree. C.
and a low thermal conductivity, typically no more than about 0.5
W/m/.degree. C., preferably from about 0.1 to 0.3 W/m/.degree.
C.
[0051] The heating systems employed in the present invention to
heat and maintain the temperature of the optical filter element at
a desired temperature include resistive film heaters,
thermoelectric devices, ceramic heaters, thin film heaters and the
like. It is important that the heating system have a control means
to control the temperature of the thermosensitive materials and
maintain the temperature at the desired temperature selected fro
the particular optical filter element.
[0052] One such heating system is shown in FIG. 7. The requirements
for a thermally tuned filter with closed loop temperature control
are shown in FIG. 7. The temperature is measured with a
thermocouple; as the temperature varies from its setpoint
(determined by the filtered wavelength desired) the power from the
power supply to a resistive heating element is altered to maintain
the desired setpoint temperature. Usually, the ambient temperature
of the filter will be raised to a level 5 to 10.degree. C. beyond
the expected environmental variation in temperature. With polymer
waveguide Bragg grating filters, the change in wavelength
.DELTA..lambda. is related to the change in temperature .DELTA.T
via .DELTA..lambda.=-0.2 .DELTA.T nm/.degree. C. An additional
advantage of certain polymer Bragg grating filters is their low
optical loss and low birefringence. To realize a system such as is
shown in FIG. 5, each stage would differ from the next stage by a
channel spacing. If a typical channel spacing is 0.8 nm, then
T.sub.2=T.sub.1-4, and thus T.sub.i=T.sub.1-4*i, where T.sub.i is
the temperature of the ith stage. For a system such as that shown
in FIG. 6, each stage will be shifted from the adjacent stage by 4
channel spacings, so we have T.sub.2=T.sub.1-16, and, in general
T.sub.i=T.sub.1-16*i. Thus, the insertion loss advantage of the
system in FIG. 6 is traded off against the need to hold the stages
at larger differences in temperature.
EXAMPLE 1
[0053] A four channel tunable demultiplexer based on polymer
waveguide gratings is used as defined in FIG. 5. The channel
spacing of the demultiplexer is 400 GHz or 3.2 nm. The specific
wavelengths filtered when the device is held at 60.degree. C. are
1547.72, 1550.92, 1554.12, and 1557.32 nm. There is one input
single-mode fiber, four output single-mode fibers and a throughport
single mode fiber. All fibers are Corning SMF-28. It is preferred
that the input fiber be connectorized with an angle polished
connector to reduce back reflection. The four output fibers are
terminated with FC/PC connectors. The throughport has an angle
polished connector or comes out to a pigtailed fiber which is
connected to the next demultiplexer which is identical to the first
one except that it is held at temperature T.sub.2=64.degree. C.,
and filters wavelengths 1548.52, 1551.72, 1554.92 and 1558.12 nm.
Two subsequent demultiplexers held at temperatures 68.degree. C.
and 72.degree. C. filter wavelengths 1549.32, 1552.52, 1555.72 and
1559.96, and 1550.12, 1553.32, 1556.52, and 1560.72 nm,
respectively. Interchannel crosstalk for the filtered channels is
<-30 dB. The cumulative loss of light experienced at the final
throughport for nonfiltered wavelengths is on the order of 15-20
dB.
EXAMPLE 2
[0054] A four channel tunable demultiplexer based on polymer
waveguide gratings is used as defined in FIG. 6. The channel
spacing of the demultiplexer is 100 GHz or 0.8 nm. The specific
wavelengths filtered are 1547.72,1548.52, 1549.32 and 1550.12 nm
when the device is held at 60.degree. C. There is one input
single-mode fiber connectorized with an angle polished connector.
The light from the fiber is launched into a GRIN lens provided by
NGK that collimates the light from the fiber. This light is then
incident on a thin filter interference filter made by OCLI that
reflects at greater than 95% wavelengths from 1547.5 to 1550.3 nm.
The light reflected is directed at another GRIN lens that focuses
the light back down into the input fiber of the demultiplexer which
then separates out the four wavelengths. Light not reflected by the
first thin film interference filter passes to the next one,
F.sub.2, which reflects wavelengths from 1550.5 to 1553.5 nm with
greater than 95% efficiency. Light is then directed through a GRIN
lens into another four channel demultiplexer, identical to the
first one but held at temperature T.sub.2=76.degree. C., and
filtering specific wavelengths 1550.92,1551.72,1552.52 and 1553.32
nm. Thin film interference filters F.sub.3 and F.sub.4 act
similarly, reflecting wavelength bands 1553.5 to 1556.7 and 1557.0
to 1560.2 nm, respectively. Four channel demultiplexers at
temperatures 92.degree. C. and 108.degree. C. then capture
wavelengths 1554.12, 1554.92, 1555.72 and 1556.52 nm and 1557.32,
1558.12, 1558.92 and 1559.72 nm, respectively. The interchannel
crosstalk for the filtered channels is <-20 dB. The cumulative
loss experienced by light that has not been filtered is on the
order of 5 dB.
[0055] Although the present invention has been specifically
described with reference to temperature as the variable which
distinguishes adjacent filter elements, it is within the scope of
the present invention to employ other variables such as materials
whose mechanical stress can be varied. More specifically for a
mechanically tuned polymer Bragg grating cascaded tunable filter
system of the type shown in FIG. 3, the stages will be maintained
in a different state of mechanical stress, such that the lowest
wavelength in one of the stages is at .lambda..sub.0 and the
difference in strain between each successive stage is give by
.DELTA..epsilon.=.DELTA..lambda..sub.WDM/(d.lambda..sub.B/d.epsilon.).
The derivative, d.lambda..sub.B/d.epsilon., is related to the
photoelastic constants of the material, which depend on the
refractive indices, the Poisson ratio, and generalized Pockel's
coefficients, in general.
1 EXHIBIT A (Listed by U.S. Pat. No. or AlliedSignal Case No.
corresponding to U.S. Pat. Application filing and includes all
foreign counterparts and related cases) PATENTS APPL. NO. PAT. NO.
MDC NO. TITLE ISSUE DATE 45619 5036142 28280082 Process for Making
Electro-Optically Active Polymers July 30, 1991 456420 5061404
28130082 Electro-Optical Materials and Light Modulator Oct. 29,1991
Devices Contianing Same 664248 (Patent) 5176983 28610082 Polymeric
Nitrones Having an Acrylic Backbone Chain Jan. 5, 1993 770373
5186865 28130082 Electro-Optical Materials and Light Modulator Feb.
16, 1993 Devices Containing Same 944383 (Div.) 5273863 28610082
Polymeric Nitrones Having an Acrylic Backbone Chain Dec. 28, 1993
08/043318 5274179 31040082 Fluorinated Photoinitiators and Their
Applications In UV Dec. 28, 1993 Curing of Fluorinated Monomers
983065 5354511 31500082 Unsymetrically Substituted Fluorenes For
Nonlinear Oct. 11, 1994 Optical Applications J8/111254 5359687
1870082 Polymers Microstructures Which Facilitate Fiber Optic Oct.
25, 1994 To Waveguide Coupling 08/054607 5391587 31040082
Fluorinated Photoinitiators and Their Applications in UV Feb. 21,
1995 08/252873 RE35060 31040082 Curing of Fluorinated Monomers Oct.
10, 1995 08/342399 5541039 25240030 Method For Forming Optically
Active Waveguides July 30, 1996 08/028921 5670603 32680082 Polymers
Exhibiting Nonlinear Optical Properties Sept. 23, 1997 08/838342
5850498 40210030 Low Stress optical Waveguide Having Conformal Dec.
15, 1998 Cladding and Fixture For Precision Optical Interconnects
08/838343 5974214 39600030 Raised Rib Waveguide Ribbon For
Precision Optical Oct. 26, 1999 Interconnects 09/026764 6023545
44660030 Fabrication of Diffraction Gratings for Optical Signals
Feb. 8, 2000 Devices and Optical Signal Devices Containing the
Same
* * * * *