U.S. patent application number 09/908954 was filed with the patent office on 2002-01-17 for optical devices made from radiation curable fluorinated compositions.
This patent application is currently assigned to Corning Incorporated. Invention is credited to Blomquist, Robert, Eldada, Louay, Norwood, Robert, Xu, Baopei.
Application Number | 20020006586 09/908954 |
Document ID | / |
Family ID | 23320139 |
Filed Date | 2002-01-17 |
United States Patent
Application |
20020006586 |
Kind Code |
A1 |
Xu, Baopei ; et al. |
January 17, 2002 |
Optical devices made from radiation curable fluorinated
compositions
Abstract
The invention provides organic optical waveguide devices which
employ perfluoropolymeric materials having low optical loss and low
birefringence. An optical element has a substrate; a patterned,
light transmissive perfluoropolymer core composition; and a light
reflecting cladding composition on the pattern of the core. Writing
of high-efficiency waveguide gratings is also disclosed.
Inventors: |
Xu, Baopei; (Lake Hiawatha,
NJ) ; Eldada, Louay; (Randolph, NJ) ; Norwood,
Robert; (West Chester, PA) ; Blomquist, Robert;
(Whippany, NJ) |
Correspondence
Address: |
NIXON PEABODY LLP
ATTENTION: DAVID RESNICK
101 FEDERAL STREET
BOSTON
MA
02110
US
|
Assignee: |
Corning Incorporated
|
Family ID: |
23320139 |
Appl. No.: |
09/908954 |
Filed: |
July 19, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09908954 |
Jul 19, 2001 |
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09337337 |
Jun 21, 1999 |
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Current U.S.
Class: |
430/321 ;
385/122; 385/5; 385/50; 430/270.1; 430/281.1; 430/290 |
Current CPC
Class: |
G02B 6/02057 20130101;
C08F 22/20 20130101; G02B 6/1221 20130101; C08F 22/18 20130101;
G02B 2006/12107 20130101; G02B 6/105 20130101; G02F 1/0009
20130101; G02B 1/045 20130101; G02F 2202/023 20130101; G02B 6/124
20130101; C08G 65/007 20130101; G02B 6/13 20130101 |
Class at
Publication: |
430/321 ;
430/290; 430/270.1; 430/281.1; 385/5; 385/50; 385/122 |
International
Class: |
G03C 001/73; G03F
007/30 |
Claims
We claim:
1. A method of making an optical element comprising: a) applying a
core photopolymerizable composition to a support to form a core
photopolymerizable composition layer, said core photopolymerizable
composition including at least one photoinitiator and at least one
core photopolymerizable monomer, oligomer, or polymer having at
least one photopolymerizable group, said core photopolymerizable
monomer, oligomer, or polymer including a perfluorinated
substituent; b) imagewise exposing the core photopolymerizable
composition layer to sufficient actinic radiation to effect the at
least partial polymerization of an imaged portion and to form at
least one non-imaged portion of said core photopolymerizable
composition layer; c) removing said at least one non-imaged portion
without removing said imaged portion, thereby forming a light
transmissive patterned core from said imaged portion; d) applying
an upper cladding polymerizable composition onto the patterned
core; and e) at least partially curing said upper cladding
composition, wherein said upper cladding and the core-interfacing
surface of said support have a lower refractive index than said
core.
2. The method of claim 1 wherein said perfluorinated substituent is
selected from the group consisting of --(CF.sub.2).sub.x--,
--CF.sub.2O--[(CF.sub.2CF.sub.2O).sub.m(CF.sub.2O).sub.n]--CF.sub.2--,
and
--CF(CF.sub.3)O(CF.sub.2).sub.4O[CF(CF.sub.3)CF.sub.2O].sub.pCF(CF.su-
b.3)--, where x is 1-10, m and n designate the number of randomly
distributed perfluoroethyleneoxy and perfluoromethyleneoxy backbone
repeating subunits, respectively, and p designates the number of
--CF(CF.sub.3)CF.sub.2O--backbone repeating subunits.
3. The method of claim 2 wherein said perfluorinated substitutent
is
--CF.sub.2O--[(CF.sub.2CF.sub.2O).sub.m(CF.sub.2O).sub.n]--CF.sub.2--
and the ratio m/n varies from about 0.5 to about 1.4.
4. The method of claim 3 wherein the ratio m/n is about 1 and the
molecular weight of the core photopolymerizable monomer, oligomer,
or polymer lies between about 2000 and about 2800.
5. The method of claim 1 wherein the photopolymerizable group is an
epoxy or ethylenically unsaturated group.
6. The method of claim 5 wherein said epoxy group is selected from
the group consisting of 12
7. The method of claim 5 wherein the ethylenically unsaturated
group is selected from the group consisting of vinyl ethers,
acrylates, and methacrylates.
8. The method of claim 1 wherein the core photopolymerizable
monomer, oligomer, or polymer has the
structureA--R--R.sub.f--R'--Awhere R and R' are divalent or
trivalent connecting groups selected from the group consisting of
alkyl, aromatic, ester, ether, amide, amine, or isocyanate groups;
said photopolymerizable group, A, is selected from the group
consisting of 13CY.sub.2.dbd.C(X)COO--, and CH.sub.2.dbd.CHO--;
where Y=H or D, and X=H, D, F, Cl or CH.sub.3; and said
perfluorinated substitutent, R.sub.f, is selected from the group
consisting of --(CF.sub.2).sub.x,
--CF.sub.2O--[(CF.sub.2CF.sub.2O).sub.m(CF.sub.2O).su-
b.n]--CF.sub.2--, and
--CF(CF3)O(CF.sub.2).sub.4O[CF(CF.sub.3)CF.sub.2O].s-
ub.pCF(CF.sub.3)--, where x is 1-10, m and n designate the number
of randomly distributed perfluoroethyleneoxy and
perfluoromethyleneoxy backbone repeating subunits, respectively,
and p designates the number of --CF(CF.sub.3)CF.sub.2O-- backbone
repeating subunits.
9. The method of claim 8 wherein the connecting group R is
--CH.sub.2-- or --CH.sub.2C(A)HCH.sub.2OCH.sub.2-- and the
connecting group R' is --CH.sub.2-- or
--CH.sub.2OCH.sub.2C(A)HCH.sub.2--.
10. The method of claim 1 wherein said upper cladding polymerizable
composition includes at least one upper cladding photoinitiator and
at least one upper cladding photopolymerizable monomer, oligomer,
or polymer having at least one upper cladding photopolymerizable
group, said upper cladding photopolymerizable monomer, oligomer, or
polymer including an upper cladding perfluorinated substituent.
11. The method of claim 1 wherein said support includes a silicon
wafer substrate.
12. The method of claim 1 where said support is a laminate formed
by: f) applying a coating of a lower cladding polymerizable
composition to a substrate, said lower cladding composition
including at least one lower cladding photoinitiator and at least
one lower cladding photopolymerizable monomer, oligomer, or polymer
having at least one lower cladding photopolymerizable group, said
lower cladding photopolymerizable monomer, oligomer, or polymer
including a lower cladding perfluorinated substituent; and g) at
least partially curing said lower cladding composition to form a
lower cladding layer.
13. The method of claim 12 wherein said at least partial curing
includes exposing said coating of a lower cladding polymerizable
composition to heat and/or actinic radiation.
14. The method of claim 1 wherein said core photopolymerizable
composition includes a first photopolymerizable monomer, oligomer,
or polymer compound and a second photopolymerizable monomer,
oligomer, or polymer compound, both of which compounds include at
least two photopolymerizable groups and a perfluorinated
substituent.
15. The method of claim 14 wherein the difference between the
functionality of said second photopolymerizable compound and said
first photopolymerizable compound is at least one.
16. The method of claim 15 wherein said second photopolymerizable
compound is a tetra-functional or higher functionality compound and
said first photopolymerizable compound is a di-functional or higher
functionality compound.
17. The method of claim 16 wherein said first photopolymerizable
compound is a di-acrylate compound and said second
photopolymerizable compound is a tetra-acrylate compound.
18. The method of claim 15 wherein said core photopolymerizable
composition comprises from about 40 to about 60 wt. % of said first
photopolymerizable compound and from about 40 to about 60 wt. % of
said second photopolymerizable compound based on the weight of said
core photopolymerizable composition.
19. The method of claim 18 wherein said core photopolymerizable
composition comprises about 50 wt. % of said first
photopolymerizable compound and about 50 wt. % of said second
photopolymerizable compound based on the total weight of said first
and second core photopolymerizable compounds.
20. The method of claim 1 further comprising: 1) exposing said at
least partially cured core to light through a phase mask to write a
grating in said core; and 2) thereafter sustantially fully curing
said core with actinic radiation, heat, or both heat and actinic
radiation.
21. A light-guiding optical element comprising: a) an organic upper
cladding layer; b) an organic light transmissive core comprising a
fluoropolymer including at least one perfluorinated substituent; c)
an organic lower cladding layer; and d) a substrate.
22. The optical element of claim 21 wherein said perfluorinated
substituent is selected from the group consisting of
--(CF.sub.2).sub.x,
--CF.sub.2O--[(CF.sub.2CF.sub.2O).sub.m(CF.sub.2O).sub.n]--CF.sub.2--,
and
--CF(CF.sub.3)O(CF.sub.2).sub.4O[CF(CF.sub.3)CF.sub.2O].sub.pCF(CF.su-
b.3)--, where x is 1-10, m and n designate the number of randomly
distributed perfluoroethyleneoxy and perfluoromethyleneoxy backbone
repeating subunits, respectively, and p designates the number of
--CF(CF.sub.3)CF.sub.2O-- backbone repeating subunits.
23. The optical element of claim 22 wherein the fluoropolymer is
produced from a core photopolymerizable composition including the
compound 14where m and n designate the number of randomly
distributed perfluoroethyleneoxy and perfluoromethyleneoxy backbone
repeating subunits, respectively, and the ratio m/n falls within
the range of about 0.5 to about 1.4.
24. The optical element of claim 23 wherein said core
photopolymerizable composition further includes the
compoundCH.sub.2.dbd.CHCO.sub.2CH.sub.2(-
CF.sub.2CF.sub.2O).sub.m(CF.sub.2O).sub.nCF.sub.2CH.sub.2O.sub.2CCH=CH.sub-
.2
25. The optical element of claim 22 wherein the optical loss of
1550 nm light through said light transmissive core is less than
0.75 dB/cm.
26. The optical element of claim 25 wherein the optical loss of
1550 nm light through said light transmissive core is less than 0.5
dB/cm.
27. The optical element of claim 21 wherein the glass transition
point of said upper cladding layer and lower cladding layer is
about 40.degree. C. or less and that of the light transmissive core
is about 50.degree. C. or less.
28. The optical element of claim 27 wherein the glass transition
point of said light transmissive core is less than 0.degree. C.
29. A method of transmitting optical information comprising: a)
providing an information-bearing optical signal; and b) passing
said optical signal through a light-transmissive polymer formed
from a perfluorinated radiation curable monomer, oligomer, or
polymer having at least one radiation curable group selected from
the group consisting of epoxy or ethylenically unsaturated
group.
30. The method of claim 29 wherein said signal is at a wavelength
of about 1550 nm.
31. The method of claim 29 further comprising passing said optical
signal through a diffraction grating written in said
light-transmissive polymer.
32. A method of making an optical element comprising: a) applying a
photopolymerizable composition to a support to form a
photopolymerizable composition layer, said photopolymerizable
composition including an effective amount of at least one
photoinitiator and at least one photopolymerizable monomer,
oligomer, or polymer having at least one photopolymerizable group,
said photopolymerizable monomer, oligomer, or polymer including a
perfluorinated substituent; b) at least partially curing said
layer; c) forming a core by a method selected from the group
consisting of reactive ion etching, micro replication, direct laser
writing, and laser ablation d) applying an upper cladding
polymerizable composition onto said core; and e) at least partially
curing said upper cladding composition to form an upper
cladding.
33. The method of claim 32 wherein said polymerizable composition
is a core polymerizable composition; and forming said core includes
1) protecting a region of said layer with a reactive ion
etching-resistant material; and 2) removing unprotected regions of
said at least partially cured layer to form a raised rib core.
34. The method of claim 32 wherein said polymerizable composition
is a lower cladding polymerizable composition; and forming said
core includes 1) protecting a region of said layer with a reactive
ion etching-resistant material; and 2) removing unprotected regions
of said at least partially cured layer to form a trench in said
lower cladding layer.
35. The method of claim 34 further comprising applying a core
polymerizable composition to said trench and at least partially
curing said core composition.
36. The method of claim 35 further comprising applying an upper
cladding composition to said core and at least partially curing
said upper cladding composition.
37. The method of claim 32 wherein said photopolymerizable
composition is applied to an at least partially cured lower
cladding layer in contact with said support.
38. The method of claim 32 wherein said photopolymerizable
composition is applied in direct contact with said support.
39. The method of claim 32 further comprising applying an electrode
to said upper cladding in alignment with said core.
40. The method of claim 32 wherein said perfluorinated substituent
is selected from the group consisting of --(CF.sub.2).sub.x--,
--CF.sub.2O--[(CF.sub.2CF.sub.2O).sub.m(CF.sub.2O).sub.n]--CF.sub.2--,
and
--CF(CF.sub.3)O(CF.sub.2).sub.4O[CF(CF.sub.3)CF.sub.2O].sub.pCF(CF.su-
b.3)--, where x is 1-10, m and n designate the number of randomly
distributed perfluoroethyleneoxy and perfluoromethyleneoxy backbone
repeating subunits, respectively, and p designates the number of
--CF(CF.sub.3)CF.sub.2O-- backbone repeating subunits.
41. The method of claim 32 wherein the said polymerizable monomer,
oligomer, or polymer has the structureA--R--R.sub.f--R'--Awhere R
and R' are divalent or trivalent connecting groups selected from
the group consisting of alkyl, aromatic, ester, ether, amide,
amine, or isocyanate groups; said polymerizable group, A, is
selected from the group consisting of 15CY.sub.2.dbd.C(X)COO--, and
CH.sub.2.dbd.CHO--; where Y=H or D, and X=H, D, F, Cl or CH.sub.3;
and said perfluorinated substitutent, R.sub.f, is selected from the
group consisting of --(CF.sub.2).sub.x--,
--CF.sub.2O--[(CF.sub.2CF.sub.2O).sub.m(CF.sub.2O).-
sub.n]--CF.sub.2--, and
--CF(CF.sub.3)O(CF.sub.2).sub.4O[CF(CF.sub.3)CF.su-
b.2O].sub.pCF(CF.sub.3)--, where x is 1-10, m and n designate the
number of randomly distributed perfluoroethyleneoxy and
perfluoromethyleneoxy backbone repeating subunits, respectively,
and p designates the number of --CF(CF.sub.3)CF.sub.2O-- backbone
repeating subunits.
42. The method of claim 41 wherein the connecting group R is
--CH.sub.2-- or --CH.sub.2C(A)HCH.sub.2OCH.sub.2-- and the
connecting group R' is --CH.sub.2-- or
--CH.sub.2OCH.sub.2C(A)HCH.sub.2--.
43. A composition comprising: a) a first photocurable
multifunctional perfluorinated compound having a first
functionality; b) a second photocurable multifunctional
perfluorinated compound having a second functionality, wherein the
difference between said second functionality and said first
functionality is at least one; and c) an effective amount of a
photoinitiator.
44. The composition of claim 43 wherein each of said first and
second compounds is an acrylate.
45. The composition of claim 44 wherein from about 40 to about 60
wt. % of said composition is said first compound and from about 40
to about 60 wt. % of said composition is said second compound.
46. The composition of claim 43 wherein said difference is at least
two.
47. The composition of claim 43 wherein said first compound is a
di-acrylate and said second compound is a tetra-acrylate.
48. The composition of claim 43 wherein said first compound is
octafluorohexanediol di-acrylate.
49. The composition of claim 48 wherein said second compound is a
polyether tetra-acrylate.
50. A waveguide grating made from the composition of claim 42.
Description
TECHNICAL FIELD AND INDUSTRIAL APPLICABILITY OF THE INVENTION
[0001] The invention relates to organic optical devices, such as
planar single mode waveguides made from radiation curable
materials. Specifically, the invention relates to low loss, low
polarization dependent, devices made from fluorohydrocarbon
monomers, oligomers, or polymer components end-capped with
radiation curable ethylenically unsaturated groups, such as
acrylate or methacrylate groups. The devices made from these
materials show good long term and short term stability, good
flexibility, and reduced stress or crack induced optical scattering
loss.
BACKGROUND OF THE INVENTION
[0002] In optical communication systems, messages are transmitted
by carrier waves at optical frequencies that are generated by such
sources as lasers and light-emitting diodes. There is interest in
such optical communication systems because they offer several
advantages over conventional communication systems.
[0003] One preferred means for switching or guiding waves of
optical frequencies from one point to another is by an optical
waveguide. The operation of an optical waveguide is based on the
fact that when a light-transmissive medium is surrounded or
otherwise bounded by another medium having a lower refractive
index, light introduced along the inner medium's axis is highly
reflected at the boundary with the surrounding medium, thus
producing a guiding effect.
[0004] A wide variety of optical devices can be made which
incorporate a light guiding structure as the light transmissive
elements. Illustrative of such devices are planar optical slab
waveguides, channel optical waveguides, rib waveguides, optical
couplers, optical splitters, optical switches, optical filters,
variable attenuators, micro-optical elements and the like. These
devices are described in more detail in U.S. Pat. Nos. 4,609,252,
4,877,717, 5,136,672, 5,136,682, 5,481,385, 5,462,700, 5,396,350,
5,428,468, 5,850,498, and U.S. Patent Application Ser. No.
08/838,344 filed Apr. 8, 1997, the disclosures of which are all
incorporated herein by reference.
[0005] It is known in the art to make optical waveguides and other
optical interconnect devices from organic polymeric materials.
Whereas single mode optical devices made from planar glass are
relatively unaffected by temperature, devices made from organic
polymers show a far greater variation with temperature because the
refractive index changes much faster with temperature in polymeric
materials than in glass. This property can be exploited to make
active, thermally tunable or controllable devices incorporating
light transmissive elements made from organic polymers. One type of
thermally tunable devices is a directional coupler activated by a
thermo-optic effect. The thermo-optic effect is a change in the
index of refraction of the optical element that is induced by heat.
Thermo-optic effect devices help to provide less costly routers
when the activation speed of a coupler state is not too high, i.e.,
when the activation speed is in the range of milliseconds.
[0006] Unfortunately, most polymeric materials contain
carbon-to-hydrogen chemical bonds which absorb strongly at the 1550
nm wavelength that is commonly used in telecommunication
applications. It has long been known that fluoropolymers, for
example, have significantly reduced absorption at 1550 nm. While
planar waveguides made from fluorinated polyimide and deuterated
polyfluoromethacrylate have achieved single mode losses of as
little as 0.10 db/cm at 1300 nm, it is relatively difficult to make
optical devices from these materials. Specifically, the
photolithographic process by which they have been made includes a
reactive ion etching step. Fluorinated polyimide and deuterated
polyfluoromethacrylate also have higher losses at 1550 nm,
typically on the order of 0.6 dB/cm.
[0007] Photopolymers have been of particular interest for optical
interconnect applications because they can be patterned using
standard photolithographic techniques. As is well known,
photolithography involves patternwise exposure of a light-sensitive
polymeric layer deposited on a chosen substrate followed by
development of the pattern. Development may be accomplished, for
example, by removal of the unexposed portion of the photopolymeric
layer by an appropriate solvent.
[0008] U.S. Pat. No. 4,609,252 teaches one method of
lithographically forming optical elements using an acrylic
photoreactive composition which is capable of forming a waveguide
material upon polymerization. This patent teaches one to utilize
polymers with as high a glass transition temperature as possible,
i.e., 90.degree. C.-220.degree. C., in order to provide for the
greatest operating temperatures. U.S. Pat. No. 5,136,682 teaches
the production of waveguides using photopolymerizable compositions
such as acrylics having a glass transition point, T.sub.g, of at
least 100.degree. C. The foregoing waveguides, however, suffer from
undesirably high optical loss and are not sufficiently
flexible.
[0009] Among the many known photopolymers, acrylate materials have
been widely studied as waveguide materials because of their optical
clarity, low birefringence and ready availability of a wide range
of monomers. However, the performance of optical devices made from
many acrylate materials has been poor, due to high optical losses,
poor resistance to aging and yellowing, and thermal instability of
the polymerized material.
[0010] There continues to be a need for low loss radiation curable
materials that can be used to make optical devices by a more direct
process having fewer manufacturing steps. Specifically, a process
is desired that does not require a reactive ion etching (RIE) step
to develop the pattern of the optical element core. Such materials
could be used to make optical devices by a relatively simple and
more direct lithographic procedure.
[0011] It is also important that these materials have little or no
birefringence. As is well known in this art, birefringence is the
difference between the refractive index of the transverse electric
or TE polarization (parallel to the substrate surface) and the
transverse magnetic or TM polarization (perpendicular to the
substrate surface). Such birefringence is undesirable in that it
can lead to devices with large polarization dependant losses and
increased bit error rates in telecommunication systems.
[0012] Another type of useful optical device is a waveguide
grating. Diffraction gratings, e.g., Bragg gratings, are used in
the telecommunications industry to isolate a narrow band of
wavelengths from a broader telecommunications signal. Polymeric
planar waveguide gratings have a number of advantages in terms of
their relative ease of manufacture and their ability to be tuned
over a wide range of frequencies by temperature or induced stress.
In addition, such devices have the advantage of being easily
incorporated into integrated devices. Unfortunately, such gratings
in polymeric materials typically are of relatively low efficiency.
This drawback can result in poor signals with increased bit error
rates. It would, therefore, be beneficial to find a method of
making polymeric planar waveguide gratings with improved
efficiency.
[0013] Dense Wavelength Division Multiplexing (DWDM) systems have
recently attracted a lot of interest because they address the need
for increased bandwidth in telecommunication networks. The use of
DWDM systems allows the already installed point-to-point networks
to greatly multiply their capacity without the expensive
installation of additional optical fiber. DWDM systems can send
multiple wavelengths (signals) over the same fiber by using passive
optical components to multiplex the signals on the one end of the
line and demultiplex them on the other end of the line. Polymeric
materials provide a low-cost, alternative solution to a variety of
optical components for DWDM.
[0014] WDM devices can be designed by using planar waveguides with
gratings that can reflect a single wavelength or channel as a
building block. These devices can be fabricated with low
temperature processes and high throughput. In this disclosure, we
focus on the properties of this fundamental building block, the
fabrication of a grating in a waveguide structure, outline what we
believe is the basic mechanism responsible for the grating
formation, and its environmental, humidity and temperature
performance.
[0015] Prior approaches to meeting these needs have not been
completely satisfactory, and the present invention provides
significant and unexpected improvements applicable to this
technology in order to satisfy the materials, process, and device
application requirements noted above.
BRIEF SUMMARY OF THE INVENTION
[0016] According to one aspect of the invention, there is provided
a photolithographic method of making optical elements
comprising:
[0017] a) applying a core photopolymerizable composition to a
support to form a core photopolymerizable composition layer, said
core photopolymerizable composition including at least one
photoinitiator and at least one core photopolymerizable monomer,
oligomer, or polymer having at least one photopolymerizable group,
said core photopolymerizable monomer, oligomer, or polymer
including a perfluorinated substituent;
[0018] b) imagewise exposing the core photopolymerizable
composition layer to sufficient actinic radiation to effect the at
least partial polymerization of an imaged portion and to form at
least one non-imaged portion of said core photopolymerizable
composition layer;
[0019] c) removing said at least one non-imaged portion without
removing said imaged portion, thereby forming a light transmissive
patterned core from said imaged portion;
[0020] d) applying an upper cladding polymerizable composition onto
the patterned core; and
[0021] e) curing said upper cladding composition, wherein said
upper cladding and the core-interfacing surface of said support are
each made from materials having a lower refractive index than said
core.
[0022] According to another aspect of the invention, there is
provided a reactive ion etching method of making optical elements
comprising:
[0023] a) applying a photopolymerizable composition to a support to
form a photopolymerizable composition layer, said
photopolymerizable composition including an effective amount of at
least one photoinitiator and at least one photopolymerizable
monomer, oligomer, or polymer having at least one
photopolymerizable group, said photopolymerizable monomer,
oligomer, or polymer including a perfluorinated substituent;
[0024] b) at least partially curing said layer;
[0025] c) forming a core by reactive ion etching;
[0026] d) applying an upper cladding polymerizable composition onto
said core; and
[0027] e) at least partially curing said upper cladding composition
to form an upper cladding.
[0028] According to another aspect of the invention, a
light-guiding optical element is provided which includes:
[0029] a) an organic upper cladding layer;
[0030] b) an organic light transmissive core comprising a
fluoropolymer including at least one perfluorinated
substituent;
[0031] c) an organic lower cladding layer; and
[0032] d) a substrate.
[0033] According to another aspect of the invention, a method of
transmitting optical information is provided, the method
comprising:
[0034] a) providing an information-bearing optical signal; and
[0035] b) passing the optical signal through a light-transmissive
polymer formed from a perfluorinated radiation curable monomer,
oligomer, or polymer having at least one radiation curable group
selected from the group consisting of epoxy or ethylenically
unsaturated group.
[0036] According to another aspect of the invention, a composition
is provided, the composition comprising:
[0037] a) a first photocurable multifunctional perfluorinated
compound having a first functionality;
[0038] b) a second photocurable multifunctional perfluorinated
compound having a second functionality, wherein the difference
between said second functionality and said first functionality is
at least one; and
[0039] c) an effective amount of a photoinitiator.
[0040] According to another aspect of the invention, a waveguide
grating is provided, the grating being made from the composition
listed above.
[0041] Polymerizable compositions for making waveguides in which
diffraction gratings can be written are preferably combinations of
multifunctional halogenated acrylate monomers, oligomers, or
polymers. Ideally, the comonomers are fluorinated species to reduce
optical losses through the cured composition . Mixtures of these
monomers can form highly cross-linked networks while allowing at
the same time the precise formulation of the refractive index. The
ability to control the refractive index to 10.sup.-4 accuracy makes
possible the fabrication of single mode waveguide structures with
well-defined numerical apertures (NA).
[0042] One particular combination of comonomers described in this
patent application is especially well-suited for writing
diffraction gratings in the waveguides made according to the
fabrication methods taught here. Using this material, a single mode
channel waveguide has been found to have a loss of 0.24 dB/cm as
determined by the cleave-back method. This material exhibits low
dispersion (on the order of 10.sup.-6 at 1550 nm), low
birefringenve (.ident.-10-4), and high environmental stability. It
also allows formation of waveguide gratings with excellent filter
characteristics. In a 2 cm grating, reflectivity over 99.997% and a
0.2 nm width in the reflection peak at the 3 dB point in
reflectivity has been measured. Furthermore, no side lobes have
been observed in the reflection spectrum.
[0043] It has also been discovered that a good system-candidate for
strong gratings is a mixture of two monomers with different
polymerization rates each of which forms a polymer when fully cured
having different indices of refraction. Comonomers differing in
reactive group functionality are also preferred for making gratings
in waveguides. Such systems perform well when roughly equal weight
proportions of each comonomer is present in the polymerizable
system. More specifically, the preferred systems includes a
photocurable tetra-functional monomer, an approximately equal
weight proportion of a photocurable di-functional monomer, and an
effective amount of a photoinitiator.
[0044] Preferred photopolymerizable monomers, oligomers, and
polymers have the structure
A--R--R.sub.f--R'--A
[0045] where
[0046] R and R' are divalent or trivalent connecting groups
selected from the group consisting of alkyl, aromatic, ester,
ether, amide, amine, or isocyanate groups; said polymerizable
group, A, is selected from the group consisting of 1
[0047] CY.sub.2.dbd.C(X)COO--, and
[0048] CH.sub.2.dbd.CHO--;
[0049] where
[0050] Y=H or D, and
[0051] X=H, D, F, Cl or CH.sub.3; and
[0052] said perfluorinated substitutent, R.sub.f, is selected from
the group consisting of
[0053] --(CF.sub.2).sub.x--,
[0054]
--CF.sub.2O--[(CF.sub.2CF.sub.2O).sub.m(CF.sub.2O).sub.n]--CF.sub.2-
--, and
[0055]
--CF(CF.sub.3)O(CF.sub.2).sub.4O[CF(CF.sub.3)CF.sub.2O].sub.pCF(CF.-
sub.3)--,
[0056] where x is 1-10, m and n designate the number of randomly
distributed perfluoroethyleneoxy and perfluoromethyleneoxy backbone
repeating subunits, respectively, and p designates the number of
--CF(CF.sub.3)CF.sub.2O-- backbone repeating subunits.
[0057] These and other aspects of the invention will become
apparent from the detailed description of the invention set forth
below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0058] FIG. 1 is a section view of a layer of uncured lower
cladding polymerizable composition on a substrate.
[0059] FIG. 2 is a section view of the lower cladding polymerizable
composition of FIG. 1 being cured to form the lower cladding
layer.
[0060] FIG. 3 is a section view of a layer of uncured core
polymerizable composition on the lower cladding layer of FIG.
2.
[0061] FIG. 4 is a section view of the imagewise actinic radiation
exposure of the core polymerizable composition of FIG. 3.
[0062] FIG. 5 is a section view of the core on the lower cladding
layer.
[0063] FIG. 6 is a section view of a layer of uncured upper
cladding polymerizable composition covering the core and lower
cladding.
[0064] FIG. 7A is a section view of the imagewise actinic radiation
exposure of the upper cladding polymerizable composition of FIG.
6.
[0065] FIG. 7B is a section view of an optical device resulting
from development of the upper cladding layer shown in FIG. 7A.
[0066] FIG. 8A is a section view of the blanket exposure of the
upper cladding polymerizable composition of FIG. 6 with actinic
radiation to form the upper cladding layer.
[0067] FIG. 8B is a section view of an optical device resulting
from curing of the upper cladding layer shown in FIG. 8A.
[0068] FIG. 9 is a section view of a layer of uncured core
polymerizable composition on a substrate.
[0069] FIG. 10 is a section view of the imagewise actinic radiation
exposure of the core polymerizable composition of FIG. 9.
[0070] FIG. 11 is a section view of the cured and developed core in
contact with the substrate.
[0071] FIG. 12 is a section view of a layer of uncured upper
cladding polymerizable composition covering the core and
substrate.
[0072] FIG. 13 is a section view of an optical device resulting
from imagewise exposure to actinic radiation and development of the
layer of upper cladding polymerizable composition of FIG. 12.
[0073] FIG. 14 is a section view of an optical device resulting
from blanket of the layer of upper cladding polymerizable
composition of FIG. 12 exposure to actinic radiation.
[0074] FIG. 15 is a section view of a layer of uncured lower
cladding polymerizable composition on a substrate.
[0075] FIG. 16 is a section view of the lower cladding
polymerizable composition of FIG. 15 being cured to form the lower
cladding layer.
[0076] FIG. 17 is a section view of a layer of uncured core
polymerizable composition on the lower cladding layer of FIG.
16.
[0077] FIG. 18 is a section view of the at least partial curing of
the core layer.
[0078] FIG. 19 shows the patterned reaction ion etching-resistant
layer on the upper cladding layer.
[0079] FIG. 20 is a section view of the reaction ion-etching
step.
[0080] FIG. 21 is a section view of the device after removal of the
RIE-resistant layer.
[0081] FIG. 22 is a section view of the uniform curing of the upper
cladding.
[0082] FIG. 23 is a section view of an alternative pattern of the
RIE-resistant material suitable for forming a trench.
[0083] FIG. 24 is a section view of the reaction ion-etching step
forming a trench.
[0084] FIG. 25 is a section view showing uncured core polymerizable
material in the trench.
[0085] FIG. 26 is a section view of the at least partial curing of
the core.
[0086] FIG. 27 is a section view of the application of an uncured
coating.
[0087] FIG. 28 is a section view of the uniform curing of the upper
cladding layer.
[0088] FIG. 29 is a section view of a waveguide device having an
electrode aligned with the core.
[0089] FIG. 30 is a graph showing the dependence of signal level on
waveguide length for an optical waveguide made in accordance with
the invention.
[0090] FIG. 31 shows absorption spectra for uncured liquid samples
of hexanediol diacrylate and octafluorohexanediol diacrylate.
[0091] FIG. 32 shows absorption spectra for uncured liquid
octafluorohexanediol diacrylate and cured octafluorohexanediol
diacrylate.
[0092] FIG. 33A is a schematic representation of the distribution
of monomers before grating writing.
[0093] FIG. 33B is a graph of the sinusoidal intensity of light
passing through a grating writing phase mask.
[0094] FIG. 33C-FIG. 33D are schematic representations of monomer
diffusion and creation of a polymer concentration gradient during
the writing of a grating in a waveguide.
[0095] FIG. 33E is a schematic representation of the polymer
concentration gradient "locked in" after the full cure step of
grating writing.
[0096] FIG. 33F is a graph of modulation of the refractive index in
the waveguide following writing of the grating.
[0097] FIG. 34 shows writing of a grating using a phase mask.
[0098] FIG. 35 shows writing of a grating using a two-beam
interference set-up.
[0099] FIG. 36 is a photo-differential scanning calorimetry plot of
extent of polymerization versus time for two comonomers that can be
used in the invention.
[0100] FIG. 37 is a plot of transmitted power versus wavelength
near 1550 nm for a reflection waveguide grating made in accordance
with the invention.
[0101] FIG. 38 is a plot demonstrating the strong linear dependence
of the reflected wavelength of a grating made in accordance with
the invention with temperature.
[0102] FIG. 39 is a plot of the dependence of the change in the
Bragg wavelength of a grating made in accordance with the invention
with temperature (d.lambda..sub.B/dt) on the coefficient of thermal
expansion of the waveguide substrate.
[0103] FIG. 40 is the flowsheet for an algorithm useful in
screening comonomer system candidates for use as a grating
material.
[0104] FIG. 41 is a plot generated by a computer program
implementing the flowsheet of FIG. 40 which shows the fraction of a
monomer formed into a polymer for four comonomer system candidates
under evaluation.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
[0105] The invention will now be described in more detail by way of
example with reference to the embodiments shown in the accompanying
figures. It should be kept in mind that the following described
embodiments are only presented by way of example and should not be
construed as limiting the inventive concept to any particular
physical configuration.
[0106] According to a preferred embodiment of the invention, a film
of a lower cladding polymerizable composition 1 is applied to the
surface of a substrate 4, as shown in FIG. 1. The film may be
applied in a number of different ways known in the art, such as
spin coating, dip coating, slot coating, roller coating, doctor
blading, liquid casting or the like. Generally, the lower cladding
polymerizable composition is applied at a thickness of from at
least about 0.01 microns, preferably at least about 1 micron, to
about 10 microns or more.
[0107] While the lower cladding can be made from any material
having a refractive index lower than the core, the most preferred
lower cladding material is a fluoropolymeric composition as
described below. A low loss cladding material, such as a
fluorinated polymer, is preferred in part because while the
majority of the optical signal is transmitted through the core, a
portion of the signal is transmitted through the cladding
material.
[0108] Preferably, the lower cladding polymerizable composition is
curable by heat and/or actinic radiation. More preferably, the
lower cladding polymerizable composition is photocurable by actinic
radiation. Upon exposure to an appropriate source of radiation 5
effective to at least partially cure the lower cladding
polymerizable composition, as shown in FIG. 2, a lower cladding 6
is formed on the substrate 4. Preferably, the radiation 5 is a
blanket or overall, non-imagewise exposure of ultraviolet
radiation.
[0109] To form the light transmissive region or core, a thick or
thin film of a core polymerizable composition 2 is applied to the
lower cladding 6, as shown in FIG. 3. Generally, the core
polymerizable composition is applied at a thickness of from about 1
micron to about 1 mm, preferably from about 5 microns to about 500
microns. Preferably, the core polymerizable composition is
photopolymerizable, i.e., curable by exposure to actinic radiation.
As described more fully below, the preferred core polymerizable
compositions is a low loss fluorinated material.
[0110] In one embodiment of the invention, the core polymerizable
composition layer is imagewise exposed to a suitable form of curing
radiation 5 that is effective to at least partially cure the
exposed, image portion of the core polymerizable composition layer
without substantially curing the unexposed, non-image areas of the
core polymerizable composition layer, as shown in FIG. 4.
Preferably, the curing radiation 5 is actinic radiation, more
preferably ultraviolet radiation, exposed through a core photomask
7. The position and dimensions of the light transmissive core is
determined by the pattern of the actinic radiation upon the surface
of the film. The radiation pattern preferably is chosen so that the
polymerizable composition is polymerized in the desired pattern and
so that other regions of the core polymerizable film remain
substantially unreacted. If, as in a preferred embodiment, the
polymerizable composition is photocurable, the photopolymer is
conventionally prepared by exposing the core polymerizable
composition to actinic radiation of the required wavelength and
intensity for the required duration to effect the at least partial
curing of the photopolymer.
[0111] In one preferred embodiment, the core polymerizable
composition is not fully cured, but is only partially polymerized
prior to applying the upper cladding polymerizable composition.
Partial polymerization of the core polymerizable composition layer
prior to application of the upper cladding polymerizable
composition layer allows the two compositions to intermingle at
their interface. This improves adhesion of the two layers and also
reduces optical loss by reducing scattering at the interface of the
core and cladding. Additionally, by not fully polymerizing the core
at this point in the process allows for the writing of diffraction
gratings in the core layer in a subsequent step, if desired, as
described more fully below. The same partial polymerization
technique can be used at the lower cladding/core interface as well
by not fully curing the lower cladding polymerizable composition
layer before applying the core polymerization composition
layer.
[0112] After the core polymerizable composition has been at least
partially polymerized to form the predetermined pattern of the
polymer on the surface of the lower cladding, the pattern is
developed by removing the nonimage areas and leaving intact the
predetermined pattern of core 8, as shown in FIG. 5. Any
conventional development method can be used, for example, flushing
with a solvent for the unirradiated composition. Such solvents
include polar solvents, such as alcohols and ketones. The most
preferred solvents are acetone, methanol, propanol, tetrahydrofuran
and ethyl acetate. For highly fluorinated materials, the preferred
solvent is Galden.RTM. HT-110, a perfluorinated ether available
from Ausimont USA.
[0113] Although FIG. 4-FIG. 5 show the formation of just one core
using a photomask having one transparent image-forming region, the
skilled artisan will appreciate that multiple spaced-apart cores
could be formed on the lower cladding simultaneously using a
photomask having multiple transparent image-forming regions or
similar devices capable of causing the exposure of multiple image
areas.
[0114] Two alternative methods of forming the upper cladding will
now be described. In each case, a film of upper cladding
polymerizable composition 3 is applied over the lower cladding 6
and core 8, as shown in FIG. 6. Like the lower cladding layer,
while the upper cladding can be made from any material having a
refractive index lower than the core, the most preferred upper
cladding material is a fluoropolymeric composition as described
below. As noted above, a low loss cladding material is preferred in
part because a portion of the optical signal is transmitted through
the cladding material.
[0115] Preferably, the upper cladding polymerizable composition is
curable by heat and/or actinic radiation. More preferably, the
upper cladding polymerizable composition is photocurable by actinic
radiation. The preferred form of actinic radiation is ultraviolet
radiation.
[0116] The upper cladding polymerizable composition layer is at
least partially cured by an appropriate form of curing radiation 5.
In one method shown in FIG. 7A-FIG. 7B, actinic radiation is
exposed through an imaging cladding photomask 11 to form an imaged,
at least partially cured region and unexposed, uncured regions. The
upper cladding 9 is developed by removal of the unexposed, uncured
regions by an appropriate solvent, for example. The resulting core
8 and upper cladding 9 form a ridge-like structure extending above
the plane of the lower cladding 6 and substrate 4. Upper cladding 9
covers the top and sides of the core 8. This type of upper cladding
9 is advantageous since its core 8 exhibits low internal stresses.
Preferably, the core 8 is entirely enveloped by the lower cladding
6 and upper cladding 9. The upper and lower claddings may, of
course, be referred to collectively as simply the cladding.
[0117] In an alternative method shown in FIG. 8A-FIG. 8B, the upper
cladding polymerizable composition layer 3 is simply blanket,
overall, or non-imagewise exposed to a suitable form of curing
radiation 5 effective to at least partially cure the upper cladding
polymerizable composition, as shown in FIG. 8A, to form a planar
upper cladding layer 10, as shown in FIG. 8B. Preferably, the core
8 is entirely enveloped by the lower cladding 6 and upper cladding
10.
[0118] So that the resulting structure functions as a waveguide by
guiding light through the core, the polymerizable compositions are
selected so that the refractive index of the lower cladding (fully
cured) and the refractive index of the upper cladding (fully cured)
are both less than the refractive index of the core (fully cured).
The refractive indices of the lower and upper cladding layers can
be the same or different. Preferably, the lower cladding has a
similar T.sub.g property as that of the upper cladding, but it need
not be made from the identical composition. The lower cladding
polymerizable composition and processing conditions are selected
such that the T.sub.g of the polymerized lower cladding layer
preferably ranges from about 60.degree. C. or less, more preferably
about 40.degree. C. or less and even more preferably about
25.degree. C. or less. Preferably, the refractive index of the
upper cladding will be the same as that of the lower cladding. The
lower cladding polymerizable composition and the upper cladding
polymerizable composition may be the same material.
[0119] If diffraction gratings are not to be written in the
waveguide, after application of the upper cladding polymerizable
composition, any unpolymerized or not fully polymerized portions of
the upper cladding, lower cladding or core layers may be subjected
to a hard curing by a blanket or overall exposure to actinic
radiation such that they are substantially fully polymerized. In
this manner, the core and cladding compositions intermix at their
interface and can be mixed in any desired proportions to fine tune
the refractive indices of the cladding, core and the overall device
and insure good adhesion between the layers by covalent
bonding.
[0120] If diffraction gratings are to be written in the partially
cured waveguide, reasonable measures should be taken to protect the
waveguide laminate from further polymerization, such as that
induced by actinic radiation or heat, until the grating writing
step.
[0121] In some cases, for example, when the refractive index of the
substrate is less than that of the core, a lower cladding will not
be necessary. One process of making a light-guiding optical device
without a lower cladding is illustrated in FIG. 9-FIG. 14. To form
the core 8, a film of a core polymerizable composition 2 is applied
to the substrate 4, as shown in FIG. 9. The core polymerizable
composition layer 2 is imagewise exposed, e.g., through core
photomask 7, to a suitable form of curing radiation 5, e.g.,
ultraviolet radiation, that is effective to at least partially cure
the exposed, image portion of the core polymerizable composition
layer without substantially curing the unexposed, non-image areas
of the core polymerizable composition, as shown in FIG. 10. Upon
development of the imaged area by removal of the uncured non-image
area, as by an appropriate solvent for the uncured non-imaged area
but not for the cured image area, a core 8 is formed on the
substrate 4 without an intervening lower cladding layer between the
core and substrate, as shown in FIG. 11.
[0122] The upper cladding layers 9, 10 can be formed in accordance
with the description above. That is, an upper cladding
polymerizable composition 3 is applied over the substrate 4 and
core 8, as shown in FIG. 12. The upper cladding polymerizable
composition layer 3 may then be cured by an appropriate form of
curing radiation to form an at least partially cured upper cladding
layer. In one variation of this method similar to that shown in
FIG. 7A, an upper cladding photomask, an appropriately selected
curing radiation effective to at least partially cure the upper
cladding polymerization composition, and development of the imaged
area can be used to form the upper cladding layer 9 to produce the
lower cladding-free ridge-like optical device 13 shown in FIG. 13.
Alternatively, the upper cladding polymerizable composition layer
is simply blanket-, overall-, or non-imagewise-exposed to a
suitable form of curing radiation, such as ultraviolet radiation,
by a method similar to that shown in FIG. 8A, to form planar upper
cladding 10, as shown in FIG. 14.
[0123] In addition to using these materials for making planar
waveguides by the lithographic method described above, reactive ion
etching (RIE) may also be used to make planar waveguides in a
manner similar to that described in the Journal of Lightwave
Technology, Vol. 16, June 1998, page 1024.
[0124] A representative procedure for making waveguides by a RIE
method is shown in FIG. 15-22. A uniform polymerized core layer 12
is provided on top of a polymerized lower cladding layer 6 atop
substrate 4 using actinic radiation 5 as described previously and
as shown in FIG. 15-FIG. 18. Preferably, the lower cladding and/or
core layers are partially rather than fully polymerized to improve
interlayer adhesion, and to allow for subsequent writing of a
grating in the waveguide, if desired. A patterned RIE resistant
layer (mask) 13 could then be applied on top of the core layer 12
by procedures known in the art, such as conventional
photolithographic or other type patterning methods, as shown in
FIG. 19. The patterning preferably would be selected such that the
RIE resistant layer 13 would lie above the area where the waveguide
core is desired. Such an RIE resistant layer could be composed of a
photoresist, a dielectric layer, or a metal as is familiar to those
skilled in the art. Reactive ion etching would then be employed
using ion beams 14 to remove the core material down to the level of
the lower cladding, as shown in FIG. 20. The area of the core
protected from the ion beams by the RIE resistant layer would
remain after removal of the RIE resistant layer by conventional
techniques, as indicated by core 8 at FIG. 21, thereby producing a
raised rib structure of waveguide core 8 made of the core material.
A top coat of upper cladding material could be applied and cured
using actinic radiation 5 to form upper cladding layer 10 to
complete the waveguide, as shown in FIG. 22.
[0125] As mentioned previously, partial polymerization of the
layers could be used to improve the interlayer adhesion, reduce
optical losses, and allow for writing of a grating in the waveguide
in a subsequent step. It is especially advantageous to leave the
lower cladding layer only partly polymerized before the core layer
is applied. In this case the subsequent radiation dose applied to
the core, as shown in FIG. 18, also acts to further polymerize the
lower cladding and strengthens the bond between the layers.
[0126] Another method of making waveguides by RIE also begins by at
least partially polymerizing a lower cladding coating layer 1
applied to a substrate 4 with actinic radiation 5 to form a lower
cladding layer 6, as previously described and shown in FIG. 15 and
FIG. 16. An RIE resistant layer 13 could then be patterned on top
of the lower cladding layer 6, as shown in FIG. 23. The lower
cladding layer 6 in FIG. 23 is relatively thicker than the lower
cladding layer 6 shown in FIG. 16 for clarity in describing the
method involving a RIE step. The figures are not drawn to
scale.
[0127] The resistant layer 13 is preferably applied in vertical
registration with the portions of the lower cladding layer that
will remain after formation of the waveguide core. Reactive ion
etching could then be performed using ion beams 14 to remove the
unprotected portions of lower cladding layer 6 down to a desired
depth, i.e., to remove the lower cladding layer except where the
RIE resistant layer was patterned, to produce a trench 15, as shown
in FIG. 24. In cases where the index of refraction of the substrate
is higher than that of the cured core material, a residual portion
16 of the lower cladding is not removed during the ion etching
step. In cases where the substrate has a lower refractive index
than the cured core, the lower cladding layer may be removed down
to the level of the substrate, if desired (not shown). The trench
15 could then be at least partially filled with core material 1, as
shown in FIG. 25. The uncured core material could then be at least
partially cured by actinic radiation 5 to form a waveguide core 8,
as shown in FIG. 26. Subsequently, an upper cladding coating layer
2 can be applied by methods previously described, for example, as
shown at FIG. 27. As described previously, by only partially
polymerizing the layers, the interlayer adhesion and the optical
losses can be improved, and gratings can later be written in the
waveguide, if desired. The upper cladding coating layer 2 may then
be uniformly cured by actinic radiation to form an upper cladding
12, as shown in FIG. 28.
[0128] Further techniques that may be used include micro
replication as exemplified in U.S. Pat. No. 5,343,544, the
disclosure of which is incorporated herein by reference, direct
laser writing similar to that described in the Journal of Lightwave
Technology, Vol. 14, No. 7, July 1996, page 1704, and laser
ablation similar to that described in U.S. Pat. No. 5,106,211, the
disclosure of which is incorporated herein by reference.
[0129] Insofar as the combined lower cladding/substrate of FIG. 5
or the substrate of FIG. 11 each serves to support the core, either
structure may be referred to as a core support.
[0130] Regardless of the specific manner of making the waveguide
device, i.e., with or without a RIE step, optional additional
layers may also be employed above or below the upper cladding or
lower cladding, respectively. For example, one or more conductive
layers, such as electrode 17 shown in FIG. 29, could be applied
above the upper cladding layer for use in thermo-optic applications
using patterning or other method known to those skilled in the art.
Preferably, the electrode 17 is aligned in registration with the
core. The conductive layer may be made of metal or a conductive
polymer, for example.
[0131] If the core has a refractive index that is lower than the
substrate material, it is necessary to first form a layer of
material having a refractive index lower than the refractive index
of the core. Such a layer may be referred to as a buffer layer and
may be comprised of, for example, a semiconductor oxide, a lower
refractive index polymer (as in the method shown by FIG. 1-FIG. 6),
or a spin-on silicon dioxide glass material.
[0132] The substrate may be any material on which it is desired to
establish a waveguide. The substrate material may, for example, be
selected from glass, quartz, plastics, ceramics, crystalline
materials and semiconductor materials, such as silicon, silicon
oxide, gallium arsenide, and silicon nitride. Formation of the
optical elements on wafers made of silicon or other compositions
are specifically contemplated. Silicon wafers are preferred
substrates in part due to their high surface quality and excellent
heat sink properties. To improve adhesion of the photopolymer to
the silicon wafer, the wafer may be cleaned and treated with silane
or other adhesion promoter, if desired. The substrate may or may
not contain other devices, either topographical features such as
grooves or electrical circuits, or electro-optic devices such as
laser diodes.
[0133] A preferred plastic substrate is a urethane-coated
polycarbonate substrate which is described in provisional patent
application Ser. No. 60/121,259 filed on Feb. 23, 1999, for
"Control of Temperature Dependent Planar Polymeric Waveguide
Devices through the use of Substrate and Suprastrate Layers with
Specific Coefficients of Thermal Expansion," the disclosure of
which is incorporated herein by reference.
[0134] The terms "lower cladding" and "upper cladding" refer to
cladding layers positioned on opposite sides of a core.
Accordingly, the terms "lower cladding" and "upper cladding" are
used here without regard to their position relative to any
gravitational field.
[0135] The terms "lower cladding polymerizable composition," "upper
cladding polymerizable composition," and "core polymerizable
composition" correspond to the third, second, and first
compositions, respectively, of co-pending patent application Ser.
No. 08/838,344 filed Apr. 8, 1997. Compositions suitable for use as
a lower cladding, upper cladding, or core polymerizable composition
are not limited, however, to the compositions described in the
081838,344 application.
[0136] The polymerizable compositions suitable for use in this
invention include a polymerizable compound or mixture of two or
more polymerizable compounds and other additives, such as
photoinitiators. The polymerizable compounds which can be used to
form the cladding and core may be monomers, oligomers, or polymers
which are addition polymerizable, nongaseous (boiling temperature
above 30.degree. C. at atmospheric pressure) compounds containing
at least one and preferably two, three, four, or more polymerizable
groups, e.g., an epoxy or ethylenically unsaturated group, and are
capable of forming high molecular weight polymers by radical cation
initiated or free radical initiated, chain propagating addition
polymerization. Such compounds are well known in the art. The
polymerizable compounds may be polymerized by the action of actinic
radiation, heat, or both. The polymerizable compounds that can be
polymerized by the action of actinic radiation may be referred to
as being photopolymerizable, photocuring, photocurable, radiation
curable, or the like. In one preferred embodiment, at least one of
the polymerizable compounds contains at least two polymerizable
groups per polymerizable monomer, oligomer, or polymer, e.g., at
least two epoxy or ethylenically unsaturated groups. Accordingly,
the preferred polymerizable compounds are multi-functional, i.e.,
di-functional, tri-functional, tetra-functional, etc., in that they
include at least two polymerizable functional groups. At least one
of the polymerizable compounds may contain, for example, four
polymerizable groups, in particular, four epoxy or four
ethylenically unsaturated groups. The polymerizable compounds
preferably are selected so that after exposure, they yield the
below described T.sub.g and refractive index.
[0137] A preferred polymerizable composition includes at least one
multi-functional polymerizable compound and at least one other
higher-order multi-functional polymerizable compound. For example,
one polymerizable compound in the polymerizable composition may be
a di-functional polymerizable compound while another polymerizable
compound in the composition may be a tri-functional,
tetra-functional, penta-functional, or higher functionality
polymerizable compound. Preferably, the difference in functionality
between at least one of the polymerizable compounds and at least
one other polymerizable compound in the polymerizable composition
is at least two, e.g., a di-functional compound and a
tetra-functional compound, a tri-functional compound and a
penta-functional compound, etc., or a mono-functional compound and
a tri-functional or higher functionality compound.
[0138] In order to form cross-linked polymers, at least one
polymerizable compound in the polymerizable composition must be at
least di-functional. Monofunctional halogenated or non-halogenated
monomers can also be used, but there may be some long-term
outgassing or material migration of any non-reacted monomers of
this type. By using monomers that are at least di-functional, the
likelihood of a monomer not having at least partially reacted is
dramatically reduced.
[0139] In polymerizable compositions including more than one
polymerizable compound, the compounds are preferably present in
roughly equal weight proportions. For example, in a two
polymerizable-compound composition, the composition preferably
includes from about 40 to about 60 wt. % of one compound and from
about 40 to about 60 wt. % of the other compound, based on the
total weight of the polymerizable compounds in the composition.
More preferably, the composition includes from about 45 to about 55
wt. % of one compound and from about 45 to about 55 wt. % of the
other compound, based on the total weight of the polymerizable
compounds in the composition. Most preferably, the composition
includes about 50 wt. % of each of the two polymerizable compounds
based on the total weight of the polymerizable compounds.
Similarly, in a three polymerizable-compound composition, the
composition preferably includes from about 25 to about 40 wt. % of
each of the three compounds based on the total weight of the
polymerizable compounds in the composition. More preferably, the
composition includes about 33 wt. % of each of the three
polymerizable compounds based on the total weight of the
polymerizable compounds in the polymerizable composition. Four or
more polymerizable compounds may be formulated in a polymerizable
composition, if desired.
[0140] An especially preferred polymerizable composition for making
waveguide laminates is one including roughly equal weight
proportions of two or more multi-functional polymerizable compounds
at least two of which compounds differ in functionality by at least
two. Such a polymerizable composition would preferably include an
effective amount of one or more polymerization initiators. More
preferably, the multi-functional polymerizable compounds differing
in functionality would be photopolymerizable in the presence of an
effective amount of one or more photoinitiators and an effective
dosage of actinic radiation, such as ultraviolet light.
Furthermore, the multi-functional polymerizable compounds in the
composition would preferably polymerize at different rates.
[0141] The photopolymerizable compositions may be used to make
partially cured waveguide laminates according to the methods
described above. Diffraction gratings, e.g., Bragg diffraction
gratings, can then be written in these partially cured waveguide
laminates using a light source, such as a laser, and a phase mask
or two-beam interference set-up. One such composition suitable for
use in making Bragg diffraction gratings in planar polymeric
waveguides is described at Example G below. Methods for writing
gratings in the waveguide laminates will be disclosed in greater
detail after describing the polymerizable compositions.
[0142] Photopolymerizable compounds are preferred for use in the
polymerizable compositions. In particular, multifunctional acrylate
monomers are preferred. The generalized structure of the
multifunctional acrylates is given by structure (I): 2
[0143] For the core, m preferably ranges from 1 to about 6; R.sub.2
is H or CH.sub.3, and R.sub.1 may be a linkage of aliphatic,
aromatic or aliphatic and aromatic mixed organic molecular
segments. Preferably R.sub.1 is an alkylene, alkylene oxide,
arylene oxide, aliphatic polyether or polyester moiety and R.sub.2
is H. To ensure solvent resistance of the cured film and high
contrast photolithography, crosslinked polymers are preferred, so
multifunctional acrylate monomers (m.gtoreq.2) are preferred.
[0144] One of the embodiments of this invention decreases stress
induced scattering optical loss of the final waveguiding device by
using flexible, low glass transition temperature (T.sub.g)
polymers. It is known in the art that the glass transition
temperature (T.sub.g) of a crosslinked polymer depends on the
crosslinking density and the structure of the linkage between
crosslinking points. It is also known that both low crosslinking
density and flexible linkage require a low T.sub.g. To ensure low
crosslinking density, monomers with 1.ltoreq.m.ltoreq.3, preferably
m=2, and long linkage segments between two ethylenically
unsaturated functionalities are preferred. For this invention, long
linkage segments are those which have an average molecular chain
length of at least about 4 carbon atoms or larger and preferably 6
or larger. Suitable flexible linkage structures include alkylenes
with chain length larger than about 3 carbon atoms, poly(ethylene
oxide), poly(propylene oxide), ethoxylated bisphenol A, polyethers,
thioethers, aliphatic and aromatic hydrocarbons, ethers, esters and
polysiloxanes, etc. These may optionally be substituted with any
pendant group which does not substantially detract from the ability
of the polymerizable compound to photopolymerize. Suitable
substituents nonexclusively include alkyl, aryl, alkoxy and
sulfoxide groups, etc. To ensure high resistance to thermal
degradation and discoloration, thermally stable molecular
structures of R.sub.1 are preferred. Such R.sub.1 segments are
preferably substantially free of thermally susceptible moieties
such as aromatic urethane and amide groups. To ensure low
birefringence, R.sub.1 linkages with low stress optic coefficient
and optical polarizability are preferred.
[0145] For the cladding, the acrylate is also as described above,
however, the average molecular chain length between ethylenically
unsaturated functionalities is preferably about 6 carbon atoms or
longer, preferably 8 or longer and more preferably 12 or longer.
Suitable flexible linkage structures include alkylenes with chain
length larger than 6 carbon atoms, poly(ethyleneoxide),
poly(propylene oxide) and ethoxylated bisphenol A.
[0146] Preferred polymerizable components for both the cladding and
the core are esters and partial esters of acrylic acid and of
aromatic and aliphatic polyols containing preferably 2 to 30 carbon
atoms. The partial esters and esters of polyoxyalkylene glycols are
also suitable. Examples are ethylene glycol diacrylate, diethylene
glycol diacrylate, triethylene glycol diacrylate, tetraethylene
glycol diacrylate, polyethylene glycol diacrylates and
polypropylene glycol diacrylates having an average molecular weight
in the range from 200 to 2000, propylene glycol diacrylate,
dipropylene glycol diacrylate, (C.sub.2 to C.sub.40) alkane diol
diacrylates such as hexanediol diacrylate, and butanediol
diacrylate, tripropylene glycol diacrylate, trimethylolpropane
triacrylates, ethoxylated trimethylolpropane triacrylates having an
average molecular weight in the range from 500 to 1500,
pentaerythritol diacrylate, pentaerythritol triacrylate,
pentaerythritol tetraacrylate, dipentaerythritol diacrylate,
dipentaerythritol triacrylate, dipentaerythritol tetraacrylate,
dipentaerythritol pentaacrylate, dipentaerythritol hexaacrylate,
tripentaerythritol octaacrylate, sorbitol triacrylate, sorbitol
tetraacrylate, sorbitol pentaacrylate, sorbitol hexaacrylate,
oligoester acrylates, glycerol di- and triacryiate, 1,4-cyclohexane
diacrylate, bisacrylates of polyethylene glycols having an average
molecular weight from 100 to 1500, and mixtures of the above
compounds. Preferred multifunctional acrylate oligomers include,
but are not limited to acrylated epoxies, acrylated polyurethanes
and acrylated polyesters. Preferred photopolymerizable compounds
are aryl acrylates. Illustrative of such aryl acrylate monomers are
aryl diacrylates, triacrylates and tetraacrylates as, for example,
di, tri and tetraacrylates based on benzene, naphthalene,
bisphenol-A, biphenylene, methane biphenylene, trifluoromethane
biphenylene, phenoxyphenylene, and the like. The preferred aryl
acrylate monomers are multifunctional aryl acrylates and more
preferred aryl acrylate monomers are di, tri and tetra acrylates
based on the bisphenol-A structure. Most preferred aryl acrylate
monomers are alkoxylated bisphenol-A diacrylates such as
ethoxylated bisphenol-A di-acrylate, propoxylated bisphenol A
diacrylates and ethoxylated hexafluorobisphenol-A diacrylates. The
aryl acrylate monomers of choice are ethoxylated bisphenol-A
diacrylates. Preferred polymerizable components are monomers having
the structure (II): 3
[0147] In a preferred embodiment, for the core, n is about 10 or
less, preferably about 4 or less and most preferably about 2 or
less. In one preferred embodiment, for the cladding, n is about 2
or more, preferably about 4 or more and most preferably about 10 or
more. Also useful are acrylate-containing copolymers which are well
known in the art. In one preferred embodiment, the cladding layer
comprises a polymerizable component which has the ethoxylated
bisphenol-A diacrylate structure (II) shown above wherein
1.ltoreq.n.ltoreq.20, preferably 4.ltoreq.n.ltoreq.15, and more
preferably 8.ltoreq.n.ltoreq.12. In the most preferred embodiment
of the invention, the second photosensitive composition is miscible
with the polymerized first photosensitive composition at their
interface.
[0148] Preferred polymerizable components for making low loss
waveguides are multifunctional monomers having the structure
(III):
A--R--R.sub.f--R'--A (III)
[0149] where
[0150] R and R' are divalent or trivalent connecting groups
selected from the group consisting of alkyl, aromatic, ester,
ether, amide, amine, or isocyanate groups;
[0151] A is a polymerizable group, such as
[0152] CY.sub.2.dbd.C(X)COO--
[0153] or
[0154] CH.sub.2.dbd.CHO--
[0155] or 4
[0156] where
[0157] Y=H or D, and
[0158] X=H, D, F, Cl or CH.sub.3; and
[0159] R.sub.f is a perfluorinated substitutent, such as
[0160] --(CF.sub.2).sub.x--, where x is 1-10,
[0161]
--CF.sub.2O--[(CF.sub.2CF.sub.2O).sub.m(CF.sub.2O).sub.n]--CF.sub.2-
--, or
[0162]
--CF(CF.sub.3)O(CF.sub.2).sub.4O[CF(CF.sub.3)CF.sub.2O].sub.pCF(CF.-
sub.3)--,
[0163] where m and n designate the number of randomly distributed
perfluoroethyleneoxy and perfluoromethyleneoxy backbone repeating
subunits, respectively, and p designates the number of
--CF(CF.sub.3)CF.sub.2O-- backbone repeating subunits, where m, n,
and p are integers 0, 1, 2, 3, . . . Preferably, x is 4-6.
[0164] Accordingly, the polymerizable compounds suitable for use in
the invention include, for example, polydifluoromethylene
diacrylates, perfluoropolyether diacrylates, perfluoropolyether
tetraacrylates, and chloroflurodiacrylates. One suitable
chlorofluoroduacrylate is the compound
CH.sub.2.dbd.CHCO.sub.2CH.sub.2CF.sub.2(CFClCF.sub.2).sub.nCH.sub.2O.sub.2-
CCH.dbd.CH.sub.2.
[0165] One purpose in incorporating chlorine atoms in the structure
is to raise the refractive index to that of a fully fluorinated
compound without increasing the optical loss values.
[0166] In addition to the groups listed above, the polymerizable
group A may also be a thiol group. Thiol-polyene UV curable systems
can also be used. Without intending to be bound to any particular
explanation for this curing system, the mechanism for the
thiol-polyene reaction is generally understood as follows:
PI.+RSH.fwdarw.PI--H+RS.
RS.+H.sub.2C.dbd.CHR'.fwdarw.RSCH.sub.2--{dot over (C)}HR'
RSCH.sub.2--{dot over
(C)}HR'+RSH.fwdarw.RSCH.sub.2--CH.sub.2R'+RS.multido- t.
[0167] In the first step of this reaction, a
photoinitiator-generated free radical removes a proton from a thiol
group to create a thiol radical. This thiol radical then reacts
with a carbon double bond to create a radical intermediate. The
radical intermediate then abstracts a proton from another thiol
forming a thiol ether and another thiol radical. In this reaction,
one thiol reacts with one carbon double bond. Also, for a polymer
to develop, both the thiol and the alkene must be at least
di-functional. In order to get a cross-linked polymer, it is
necessary that at least one of the components be at least
tri-functional.
[0168] The polymers generated by this reaction generally have good
physical properties. Their shrinkage is also likely to be low.
Unlike acrylates, this reaction is fairly insensitive to oxygen,
but does have termination steps that occur when two radicals come
together. These properties suggest that these materials may be able
to produce reasonable lithographic resolution. The main problem
with this approach is the availability of low-loss starting
materials. Since these materials preferably formulated on a 1:1
thiol:alkene basis, varying refractive index requires at least
three different compounds instead of two as exemplified elsewhere
in this application.
[0169] When the perfluorinated substitutent group R.sub.f is
--CF.sub.2O--[(CF.sub.2CF.sub.2O).sub.m(CF.sub.2O).sub.n]--CF.sub.2--,
[0170] the ratio m/n preferably varies from about 0.5 to about 1.4.
A sample of these materials will include a distribution of
molecules having different numbers of repeating subunits. In such a
sample, the average value of m preferably falls within the range of
from about 6.45 to about 18.34, and the sample average value of n
preferably falls within the range of from about 5.94 to about
13.93. Most preferably, the ratio m/n is about 1 and the sample
average values of m and n are each about 10.3.
[0171] Preferably, the connecting group R is --CH.sub.2-- or
--CH.sub.2C(A)HCH.sub.2OCH.sub.2-- and the connecting group R' is
--CH.sub.2-- or --CH.sub.2OCH.sub.2C(A)HCH.sub.2--, where A is
defined as above. In light of this disclosure, the skilled artisan
will recognize that a wide variety of connecting groups R and R'
could be used in addition to those listed here.
[0172] A particularly preferred polymerizable compound for use in
the invention has the structure 5
[0173] Preferably, the ratio m/n is about 1 and the molecular
weight is between about 2000 and 2800.
[0174] When selecting the polymerizable compounds to be used in
each of the core and the cladding, it is important that the core
which results after full polymerization has a higher refractive
index than that of the cladding after polymerization. Preferably
the core has a refractive index in the range of from about 1.3 to
about 1.6, or more preferably from about 1.35 to about 1.56.
Preferably the cladding has a refractive index in the range of from
about 1.29 to about 1.58, or more preferably from about 1.34 to
about 1.55. Although the cladding and core may be comprised of
structurally similar compositions, it is clear that in order for
the cladding to have a refractive index which is lower than the
refractive index of the core, they must have different chemical
compositions for any individual application. In addition, as noted
above, if the chosen substrate has a refractive index which is
greater than that of the core, then a buffer layer is required and
the buffer must have a refractive index which is lower than that of
the core.
[0175] In selecting other monomers and oligomers that may be
suitable for forming planar light guiding devices, the following
observations should be considered. For high purity fluorinated
acrylates, the majority of the absorbance at 1550 nm is a result of
carbon-to-hydrogen bonds. The absorption spectra for the
non-fluorinated compound hexanediol diacrylate (HDDA) and the
fluorinated compound octafluorohexanediol diacrylate (OFHDDA), in
which eight hydrogen atoms are replaced by fluorine, as shown in
FIG. 31, illustrate this point. The small peaks around the 1550 nm
and 1310 nm regions of the spectra are characteristic of uncured
liquids. After cure, virtually all of these fluctuations are
eliminated, as shown in the spectrum of cured octafluorohexanediol
diacrylate appearing at FIG. 32. Most of the elimination is
probably due to the conversion of the carbon double bonds to carbon
single bonds as the acrylate cures. Further, differences in the
baseline absorbance values are believed to be the result of the
higher level of scattering in the solid sample. Such scattering is
an artifact of the way in which the sample was made and the
thickness variation in the sample. Actual waveguide losses for this
material would be substantially lower than indicated in FIG.
32.
[0176] In evaluating the relative merits of a particular acrylate
based on its structure, it is useful to determine the molar
concentration of hydrogen bonds for a particular candidate
material. Since the absorption loss (in dB/cm) is determined by the
relation 1 Absorption loss = 10 A b = 10 c ,
[0177] where A is the absorbance, .epsilon. is the molar
absorptivity, b is the path length in centimeters, and c is the
molar concentration, the lower the molar concentration, the lower
the absorption loss. Since almost all of the loss comes from
carbon-to-hydrogen bonds, the molar concentration of hydrogen
(C.sub.H) for a particular monomer can be calculated using the
number of hydrogens per molecule (H), the molecular weight of the
monomer (Mw), and its density (.rho.), as shown by the equation: 2
C H = H 1000 Mw
[0178] While an exact relationship between C.sub.H and the loss
measurement in a particular waveguide is unlikely, this relation
gives a first indication of which materials may be useful in
lowering loss values. When making these calculations, it is most
appropriate to use the sensitivity of a cured film of the monomer
since it is the loss of the cured film that is of greatest
interest. However, since the measure of density of such films is
difficult, the density of the liquid could be used with the
understanding that it does introduce some error.
[0179] Preferably, the photopolymerizable compounds to be used in
the waveguide core produce a core which after polymerization has a
glass transition temperature of about 80.degree. C. or less and
more preferably about 50.degree. C. or less. Furthermore, it is
preferred that the polymerizable compounds to be used in the
waveguide cladding produce a cladding which after polymerization
has a glass transition temperature of about 60.degree. C. or less,
more preferably about 40.degree. C. or less and most preferably
about 25.degree. C. or less. Preferably, the polymerizable
compounds included in the cladding polymerizable compositions are
also photopolymerizable. The particular T.sub.g may be easily
obtained by the skilled artisan by characterization and selection
of the polymerizable component. This depends on such factors as the
molecular weight, number of sites of unsaturation, and crosslink
density of the polymerizable component. A single polymerized
component may itself have the desired T.sub.g, or the polymerizable
component may be tailored by blending mixtures of polymerizable
monomer, oligomers and/or polymers having the desired T.sub.g. The
T.sub.g may also be controlled by varying the irradiation exposure
time and temperatures at which polymerization is conducted.
[0180] The polymerizable compound is present in each polymerizable
composition in an amount sufficient to polymerize upon exposure to
sufficient heat and/or actinic radiation. The amount of the
photopolymerizable compound in the composition may vary widely and
amounts normally used in photopolymerizable compositions for use in
the preparation of photopolymers for use as the light transmissive
element of light transmissive devices may be used. The amount of
photopolymerizable compound is generally used in an amount of from
about 35 to about 99.9% by weight of the composition. In the
preferred embodiment, one or more photopolymerizable compounds in
the overall photopolymerizable composition account for from about
80% to about 99.5% by weight, most preferably from about 95 to
about 99.5% based on the weight of the overall composition.
[0181] Each light sensitive composition further comprises at least
one photoinitiator. The photoinitiator can be a free radical
generating addition polymerization initiator activated by actinic
light and is preferably thermally inactive near room temperature,
e.g., from about 20.degree. C. to about 80.degree. C. Any
photoinitiator which is known to photopolymerize acrylates can be
used. Preferred photoinitiators nonexclusively include those
described in U.S. Pat. No. 4,942,112; quinoxaline compounds as
described in U.S. Pat. No. 3,765,898; the vicinal polyketaldonyl
compounds in U.S. Pat. No. 2,367,660; the alpha-carbonyls in U.S.
Pat. Nos. 2,367,661 and 2,367,670; the acyloin ethers in U.S. Pat.
No. 2,448,828; the triarylimidazolyl dimers in U.S. Pat. No.
3,479,185; the alpha-hydrocarbon substituted aromatic acyloins in
U.S. Pat. No. 2,722,512; polynuclear quinones in U.S. Pat. Nos.
2,951,758 and 3,046,127; and s-triazines in U.S. Pat. No.
4,656,272. These patents are incorporated herein by reference.
[0182] Photopolymerizable compounds end-capped with at least one
epoxy, acrylate, or methacrylate group can be initiated by a free
radical type photoinitiator. Suitable free radical initiated type
photoinitiators include aromatic ketones such as benzophenone,
acrylated benzophenone, 2-ethylanthraquinone, phenanthraquinone,
2-tert-butylanthraquinone, 1,2-benzanthraquinone,
2,3-benzanthraquinone, 2,3-dichloronaphthoquinone, benzyl dimethyl
ketal and other aromatic ketones, e.g., benzoin, benzoin ethers
such as benzoin methyl ether, benzoin ethyl ether, benzoin isobutyl
ether and benzoin phenyl ether, methyl benzoin, ethyl benzoin and
other benzoins. Preferred photoinitiators are
1-hydroxy-cyclohexyl-ph- enyl ketone (Irgacure 184), benzoin,
benzoin ethyl ether, benzoin isopropyl ether, benzophenone,
2,2-dimethoxy-2-phenylacetophenone (commercially available from
CIBA-GEIGY Corp. as Irgacure 651), .alpha.,.alpha.-diethyloxy
acetophenone, .alpha., .alpha.-dimethyloxy-.al- pha.-hydroxy
acetophenone (Darocur 1173), 1-[4-(2-hydroxyethoxy)phenyl]-2--
hydroxy-2-methyl-propan-1-one (Darocur 2959),
2-methyl-1-[4-methylthio)phe- nyl]-2-morpholino-propan-1-one
(Irgacure 907), 2-benzyl-2-dimethylamino-1--
(4-morpholinophenyl)-butan-1-one (Irgacure 369),
poly{1-[4-(1-methylvinyl)- phenyl]-2-hydroxy-2-methyl-propan-1-one}
(Esacure KIP), [4-(4-methylphenylthio)-phenyl]phenylmethanone
(Quantacure BMS), di-campherquinone. The most preferred
photoinitiators are those which tend not to yellow upon
irradiation. Such photoinitiators include benzodimethyl ketal
(Irgacure 651), 2-hydroxy-2-methyl-1-phenyl-propan-1-- one
(commercially available from Ciba-Geigy Corporation under the name
Darocur 1173), 1-hydroxy-cyclohexyl-phenyl ketone (Irgacure-184),
and 1-[4-(2-hydroxyethoxy)phenyl]-2-hydroxy-2-methyl-propan-1-one
(Darocur 2959).
[0183] Photopolymerizable compounds end-capped with at least one
vinyl ether group can be initiated by a radical cation type
photoinitiator. Suitable radical cation type photoinitiators
include various compounds which respond to irradiation by producing
acid species capable of catalyzing cationic polymerization. See
Crivello, Advances in Polymer Science, 62, p. 1-48 (1984). Onium
salts of Group V, VI and VII elements are stated to be the most
efficient and versatile of the cationic photoinitiators. They
generate strong Lewis acids which can promote cationic
polymerization. Curing of vinyl ether compositions is not limited
to a particular class of such photoinitiators, although certain
types are preferred, including onium salts based on halogens and
sulfur. More specifically, the onium salt photoinitiators described
in Crivello's U.S. Pat. No. 4,058,400 and in particular iodonium
and sulfonium salts of BF.sub.4.sup.-, PF.sub.6.sup.-,
SbF.sub.6.sup.-, and SO.sub.3CF.sub.3.sup.-. Preferred
photoinitiators are triarylsulfonium salts, and diaryliodonium
salts. Preferred anions are hexafluorophosphate and
hexafluoroantimony. They are usually required in amounts from about
0.1 to about 5 wt. %. Preferred initiators include: 6
[0184] where X is SbF.sub.6.sup.- or PF.sub.6.sup.-. Commercially
available initiators include UVI-6974 (a SbF.sub.6.sup.- salt) and
UVI-6990 (a PF.sub.6.sup.- salt) supplied by Union Carbide. Other
cationic photoinitiators are defined by the formulas 7
[0185] where y is 1 to 18.
[0186] The free radical or radical cation generating photoinitiator
is present in each photopolymerizable composition in an amount
sufficient to effect photopolymerization of the photopolymerizable
compound upon exposure to sufficient actinic radiation. The
photoinitiator is generally present in an amount of from about
0.01% to about 10% by weight of the overall composition, or more
preferably from about 0. 1% to about 6% and most preferably from
about 0.5% to about 4% by weight based on the total weight of the
composition.
[0187] Photopolymerizable compositions may include mixtures of
polymerizable compounds end-capped with at least one actinic
radiation curable group, such as the above-described epoxy or
ethylenically unsaturated groups, specifically acrylate,
methacrylate, and vinyl ether. Vinyl ethers can react with
acrylates. Although acrylates and vinyl ethers do not ordinarily
react with epoxies, mixed systems of vinyl ethers, acrylates, and
epoxies can form interpenetrating networks if suitable
photoinitiators are used. Accordingly, mixed systems can be used in
making optical devices by the methods described here.
Photoinitiators that are suitable for use in such mixed systems are
described in U.S. Pat. No. 5,510,226, the disclosure of which is
incorporated herein by reference.
[0188] For more highly fluorinated multifunctional acrylates, such
as the fluorinated compound L-9367 available from 3M Specialty
Chemicals Division, St. Paul, Minn., the structure of which is
shown below, a preferred photoinitiator is a fluorinated
photoinitiator such as those described in U.S. Pat. Nos. Re. 35,060
and 5,391,587, the disclosures of which are incorporated herein by
reference. In particular, a fluorinated photoinitiator having the
structure (IV) 8
[0189] and described at Example 1 of Re. 35,060, may be used. It is
also possible to cure the fluorinated materials of Examples A
through D without photoinitiators through the use of electron beam
curing.
[0190] It is possible to readily cure the polymerizable compounds,
such as those described in the examples below, by heating them in
the presence of a thermal type free radical polymerization
initiator. While actinic radiation curing is preferred for the
imagewise exposure steps described above, thermal curing could be
used for any non-imagewise curing step. Suitable known thermal
initiators include, but are not limited to, substituted or
unsubstituted organic peroxides, azo compounds, pinacols, thiurams,
and mixtures thereof. Examples of operable organic peroxides
include, but are not limited to benzoyl peroxide, p-chlorobenzoyl
peroxide and like diacyl peroxides; methyl ethyl ketone peroxide,
cyclohexanone peroxide and like ketone peroxides; tert-butyl
perbenzoate, tert-butyl peroxy-2-ethylhexoate and like peresters;
tert-butyl hydroperoxide, cumene hydroperoxide and like
hydroperoxides; di-tert-butyl peroxide, di-sec-butyl peroxide,
dicumyl peroxide and like dialkyl peroxides; and diary peroxides.
Other suitable organic peroxide include
2,5-dimethyl-2,5-di(t-butylperoxy)-hexane,
1,3-bis(t-butylperoxyisopropyl)benzene,
1,3-bis-(cumylperoxyisopropyl)ben- zene, 2,4-dichlorobenzoyl
peroxide, caprylyl peroxide, lauroyl peroxide, t-butyl
peroxyisobutyrate, hydroxyheptyl peroxide, di-t-butyl
diperphthalate, t-butyl peracetate, and
1,1-di(t-butylperoxy)-3,3,5-trime- thylcyclohexane. The organic
peroxide is added to the composition in an amount ranging from
0.01-10%, preferably 0.1-5%, by weight based on the weight of the
acrylate or methacrylate.
[0191] Suitable azo-type thermal curing initiators include
2,2'-azobisisobutyronitrile,
2,2'-azobis(2,4-dimethylvaleronitrile),
(1-phenylethyl)azodiphenylmethane,
2,2'-azobis(4-methoxy-2,4-dimethylvale- ronitrile),
dimethyl-2,2'-azobis(1-cyclohexanecarbonitrile),
2-(carbamoylazo)-isobutyronitrile,
2,2'-azobis(2,4,4-trimethylpentane),
2-phenylazo-2,4-dimethyl-4-methoxyvaleronitrile,
2,2'-azobis(2-methylprop- ane) and like azo compounds.
[0192] Other additives may also be added to the photosensitive
compositions depending on the purpose and the end use of the light
sensitive compositions. Examples of these include antioxidants,
photostabilizers, volume expanders, free radical scavengers,
contrast enhancers, nitrones and UV absorbers. Antioxidants include
such compounds as phenols and particularly hindered phenols
including tetrakis[methylene
(3,5-di-tert-butyl-4-hydroxyhydrocinnamate)] methane (commercially
available under the name Irganox 1010 from CIBA-GEIGY Corporation);
sulfides; organoboron compounds; organophosphorous compounds;
N,N'-hexamethylenebis(3,5-di-tert-butyl-4-hydroxyhydrocinnamamide)
(available from Ciba-Geigy under the tradename Irganox 1098).
Photostabilizers and more particularly hindered amine light
stabilizers that can be used include, but are not limited to,
poly[(6-morpholino-s-tr-
iazine-2,4-diyl)[2,2,6,6,-tetramethyl-4-piperidyl)imino]-hexamethylene[2,2-
,6,6,-tetramethyl-4-piperidyl)imino)] available from Cytec
Industries under the tradename Cyasorb UV3346. Volume expanding
compounds include such materials as the spiral monomers known as
Bailey's monomer. Suitable free radical scavengers include oxygen,
hindered amine light stabilizers, hindered phenols,
2,2,6,6-tetramethyl-1-piperidinyloxy free radical (TEMPO), and the
like. Suitable contrast enhancers include other free radical
scavengers such as nitrones. UV absorbers include benzotriazole,
hydroxybenzophenone, and the like. These additives may be included
in quantities, based upon the total weight of the composition, from
about 0% to about 6%, and preferably from about 0.1% to about 1%.
Preferably all components of the overall composition are in
admixture with one another, and most preferably in a substantially
uniform admixture.
[0193] When the radiation curable compounds described above are
cured by ultraviolet radiation, it is possible to shorten the
curing time by adding a photosensitizer, such as benzoin, benzoin
methyl ether, benzoin ethyl ether, benzoin isopropyl ether, benzil
(dibenzoyl), diphenyl disulfide, tetramethyl thiuram monosulfide,
diacetyl, azobisisobutyronitrile, 2-methyl-anthraquinone,
2-ethyl-anthraquinone or 2-tertbutyl-anthraquinone, to the monomer,
oligomer, or polymer component or its solution. The proportion of
the photosensitizer is preferably at most 5% by weight based on the
weight of the curable compound.
[0194] As used herein "actinic radiation" is defined as light in
the visible, ultraviolet or infrared regions of the spectrum, as
well as electron beam, ion or neutron beam or X-ray radiation.
Actinic radiation may be in the form of incoherent light or
coherent light, for example, light from a laser. Sources of actinic
light, and exposure procedures, times, wavelengths and intensities
may vary widely depending on the desired degree of polymerization,
the index of refraction of the photopolymer and other factors known
to those of ordinary skill in the art. Such conventional
photopolymerization processes and their operational parameters are
well known in the art. Sources of actinic radiation and the
wavelength of the radiation may vary widely, and any conventional
wavelength and source can be used. It is preferable that the
photochemical excitation be carried out with relatively short
wavelength (or high energy) radiation so that exposure to radiation
normally encountered before processing, e.g., room lights will not
prematurely polymerize the polymerizable material. Alternatively,
the processing can utilize a multiphoton process initiated by a
high intensity source of actinic radiation such as a laser. Thus,
exposure to ultraviolet light (300-400 nm wavelength) is
convenient. Also, exposure by deep ultraviolet light (190-300 nm
wavelength) is useful. Convenient sources are high pressure xenon
or mercury-xenon arc lamps fitted with appropriate optical filters
to select the desired wavelengths for processing. Also, short
wavelength coherent radiation is useful for the practice of this
invention. An argon ion laser operating in the UV mode at several
wavelengths near 350 nm is desirable. Also, a frequency-doubled
argon ion laser with output near 257 nm wavelength is highly
desirable. Electron beam or ion beam excitation may also be
utilized. Typical exposure times normally vary from a few tenths of
seconds to about several minutes depending on the actinic source.
Temperatures usually range from about 10.degree. C. to about
60.degree. C., however, room temperature is preferred.
[0195] Control of the spatial profile of the actinic radiation,
that is, where it falls on the layer of photopolymerizable material
may be achieved by conventional methods. For example, in one
conventional method, a mask bearing the desired light transmissive
pattern is placed between the source of actinic radiation and the
photopolymerizable composition film. The mask has transparent and
opaque regions which allow the radiation to fall only on the
desired regions of the film surface. Masked exposure of thin films
is well known in the art and may include contact, proximity and
projection techniques for printing the light transmissive pattern
onto the film. Another conventional method of spatial control is to
use a source of actinic radiation which comprises a directed or
focused beam such as a laser or electron beam. Such a beam
intersects only a small area of the photo-polymerizable material
film surface. The pattern of the desired light transmissive regions
is achieved by moving this small intersection point around on the
film surface either by scanning the beam in space or by moving the
substrate so that the intersection point is changed relative to a
stationary beam. These types of exposure using a beam source are
known in the art as direct-write methods. By choosing the spatial
characteristics of irradiation, it is possible to create light
transmissive regions on the surface of the substrate and produce
slab and channel waveguides. A slab waveguide is one in which the
optical wave is confined only to the plane of the film. A channel
waveguide is one in which the optical wave is also confined
laterally within the film. A channel structure is necessary for
many nonlinear and electro-optic devices because it allows the
light to be directed to certain areas of the substrate as well as
providing a mechanism for splitting, combining optical waves,
coupling light from the waveguide to optical fibers, and
maintaining the high intensity available in an optical fiber.
[0196] The method of this invention can be used for making a wide
variety of optical elements. By using a suitable mask and by
controlling the degree of collimation of the actinic radiation used
for exposure, it is also possible to create arrays of micro-optical
elements such as lenses or prisms which can be designed to transmit
light in a direction roughly orthogonal to the substrate. Such
optical element arrays find utility in application to backlights,
e.g., for liquid crystal displays, projection systems, front or
rear projection screens, diffusers, collimators, liquid crystal
viewing screens, light directing arrays for collimators and
lighting fixtures, exit signs, displays, viewing screens, displays
for projection systems, and the like. For such applications it is
important to create an essentially cosmetically perfect device
composed of individual elements which have sharp definition and
smooth walls. The composition of the current invention can be used
to enhance the critical aspects of definition and wall smoothness.
For some applications, the substrate may optionally be removed from
the waveguide core and cladding.
[0197] The optical elements produced by the instant invention
preferably have an optical loss at 1550 nm of about 0.1 dB/cm or
less to about 0.5 dB/cm, more preferably less than about 0.3 dB/cm,
even more preferably less than about 0.25 dB/cm, and most
preferably less than about 0.20 dB/cm. In addition, the polymerized
cladding, core and buffer layers preferably have a Gardner index as
described by ASTM D 1544-80 of about 3 or less, more preferably
about 2 or less and most preferably about 1 or less.
[0198] Device testing and modeling suggest a device lifetime (time
for 0.1 dB/cm loss) of more than 10 years at 120.degree. C.
(operation temperature) and more than 1 hour at 250.degree. C. (a
typical device packaging temperature), thus allowing for use of
devices made in accordance with this disclosure applicable in the
aerospace, military, and telecommunications industries. Flexibility
of the materials allows for fabrication of devices with desired
bending angles. Cracking is also avoided even when the device is
exposed to very high or very low temperatures. Good adhesion of the
materials permits fabrication of robust devices on a variety of
substrates without delamination even in some harsh environments
such as high temperature and high humidity. Compatibility of device
fabrication techniques with those of the semiconductor industry
allows for development of hybrid optoelectronic circuitry.
[0199] The following non-limiting examples serve to illustrate the
invention. It will be appreciated that variations in proportions
and alternatives in elements of the components of the
photosensitive coating composition will be apparent to those
skilled in the art and are within the scope of the present
invention.
EXAMPLES
[0200] To synthesize the crosslinked photopolymers, the monomers or
the oligomers were mixed with the photoinitiators and the
antioxidant and well stirred. The solutions obtained were coated
into thin liquid films by spin coating, slot coating or direct
liquid casting with appropriate spacers. The thickness of the film
was controlled by spinning speed or spacer thickness. The thickness
of the films below 50 .mu.m was measured with a Sloan Dektak IIA
profilometer and the thickness of the thicker films were measured
with a microscope.
[0201] Some of the fluorinated acrylates and methacrylates used in
the examples of this invention are commercially available. For
example, the fluorinated acrylates used in Examples C and D are
available from 3M Specialty Chemicals Division, St. Paul, Minn.
Alternatively, the fluorinated acrylates useful in this invention
can be made from commercially available fluorinated polyols using
methods generally known to those skilled in the art. The
fluorinated polyol used in Example A, for example, is available
from Ausimont USA, Inc., of Thorofare, N.J. Fluorinated acrylates
can also be prepared from the polyol
2,2,3,3,4,4,5,5,-octafluoro-1,6-hexanediol available from Lancaster
Synthesis, Inc., of Windham, N.H.
[0202] If the polymerizable compounds, such as acrylates, are
synthesized from polyols, care should be taken to remove as much as
practicable any residual alcohols or other hydroxyl group-bearing
impurities since the hydroxyl group absorbs strongly in the
spectral region of interest in telecommunications device
applications, namely, in the 1300 to 1550 nm region. A preferred
product purification technique is described in Example A.
Example A
[0203] A three-neck glass flask was fitted with a condenser and
stirrer. Fluorolink.RTM. T brand fluorinated polyol (compound V,
900 g) and p-methoxyphenol (0.5 g) were added to the flask. The
fluorinated polyol used in this example is a compound that can be
described as having structure (V): 9
[0204] where the ratio m/n preferably varies from about 0.5 to
about 1.4, m (average) varies from about 6.45 to about 18.34, and n
(average) varies from about 5.94 to about 13.93. Most preferably,
the ratio m/n is about 1 and m (average) and n (average) are each
about 10.3.
[0205] Acryloyl chloride (170 g) was then added and the mixture was
vigorously stirred. The resulting exotherm brought the temperature
up to 70.degree. C. The temperature was then raised to 90.degree.
C. and the reaction was run for three hours. The system was then
placed under vacuum to remove the HCl generated by the reaction and
the excess acryloyl chloride. The mixture was then cooled to room
temperature. The infrared spectrum of the batch confirmed the
disappearance of the broad absorbence at 3500 cm.sup.-1, which is
attributed to hydroxyl groups on the polyol. Triethylamine (124 g)
was then slowly added to the reaction flask over a 1/2-hour period.
The sample was then filtered to remove triethyl amine hydrochloride
which formed. The sample was then washed twice with water. The
resulting tetraacrylate was isolated. The tetraacrylate product is
a compound that can be described as having structure (VI): 10
[0206] where the ratio m/n preferably varies from about 0.5 to
about 1.4, m (average) varies from about 6.45 to about 18.34, and n
(average) varies from about 5.94 to about 13.93. Most preferably,
the ratio m/n is about 1 and m (average) and n (average) are each
about 10.3.
[0207] Such compounds having structure (VI) are perfluoropolyether
tetraacrylates. Because they are tetra-functional, they can also be
useful in adjusting the crosslink density of the cured film to vary
its physical properties. High molecular weight versions of this
material can also be very low in loss while tending to have better
solubility than some other materials described in this disclosure.
Physical properties for one of these materials are shown in the
table below.
1 Liquid Cured Molecular Refractive Refractive # of Weight
Index.sup.a Index.sup.b Density Hydrogens C.sub.H.sup.c 2400 1.3362
1.335 1.663 26 18.0 .sup.an.sub.D.sup.20 .sup.bMetricon 2010 prism
coupler reading at 1550 nm for a cured film made using 1%
photoinitiator. .sup.cMolar concentration of hydrogen atoms in
compound (described above)
Example B
[0208] Suitable monomers for use in this invention include
polydifluoromethylene diacrylates having the generic structure:
CH.sub.2.dbd.CHCO.sub.2CH.sub.2(CF.sub.2).sub.nCH.sub.2O.sub.2CCH.dbd.CH.-
sub.2 where n is preferably 1-10. For this class of materials, the
higher the value of n, the lower the refractive index, the lower
the crosslink density, and the lower the absorbance. These
materials tend to produce relatively hard films of high cross-link
density. They also have excellent adhesive properties but have
higher absorption losses than some of the other materials described
in this application. The table below shows selected physical
property values of two of these materials.
2 # of Liquid Cured Molecu- Repeat Refractive Refractive # of lar
Units (n) Index.sup.a Index.sup.b Density Hydrogens Weight
C.sub.H.sup.c 4 1.3920 1.4180 1.433 10 370 38.7 6 1.3797 1.4061
1.510 10 370 32.1 .sup.an.sub.D.sup.20 .sup.bMetricon 2010 prism
coupler reading at 1550 nm for a cured film made using 1%
photoinitiator. .sup.cMolar concentration of hydrogen atoms in
compound (described above)
[0209] The compound octafluorohexanediol diacrylate was made as
follows. A three-neck glass flask was fitted with a condenser. The
polyol 2,2,3,3,4,4,5,5-octafluoro-1,6-hexanediol (OFHD, 300 g)
obtained from Lancaster Synthesis of Windham, N.H., and
p-methoxyphenol (0.5 g) were added to the flask. The flask was
heated to 70.degree. C. to melt the OFHD. Acrylol chloride (228 g)
was then added and the mixture was vigorously stirred. The
resulting exotherm brought the temperature up to 90.degree. C. The
temperature was then held at 90.degree. C. and the reaction was run
for three hours. The system was then placed under vacuum to remove
the HCl generated by the reaction and the excess acryloyl chloride.
The mixture was then cooled to room temperature. The infrared
spectrum of the batch confirmed the disappearance of the broad
absorbance at 3500 cm.sup.-1, which is attributed to hydroxyl
groups on the polyol. Triethylamine (189 g) was then slowly added
to the reaction flask over a 1/2-hour period. The sample was then
filtered to remove the triethyl amine hydrochloride which formed.
The sample was then washed twice with water. The remaining water
was then stripped away under vacuum.
[0210] The reaction forming the octafluorohexanediol diacrylate
compound (VIII) from the polyol
2,2,3,3,4,4,5,5-octafluoro-1,6-hexanediol (compound VII) is
depicted below: 11
Example C
[0211] Another multifunctional acrylate that can be used in this
invention include the fluorinated acrylate
CH.sub.2.dbd.CHCO.sub.2CH.sub.2CF(CF.sub.3)O(CF.sub.2).sub.4O[CF(CF.sub.3)-
CF.sub.2O].sub.pCF(CF.sub.3)CH.sub.2O.sub.2CCH.dbd.CH.sub.2
[0212] having the trade name L-1 2043 available from 3M Specialty
Chemicals Division.
Example D
[0213] Another multifunctional acrylate that can be used in this
invention include the fluorinated acrylate
CH.sub.2.dbd.CHCO.sub.2CH.sub.2(CF.sub.2CF.sub.2O).sub.m(CF.sub.2O).sub.nC-
F.sub.2CH.sub.2O.sub.2CCH.dbd.CH.sub.2
[0214] having the trade name L-9367 also available from 3M
Specialty Chemicals Division.
[0215] Polymerizable monomers useful in practicing the invention
can also be made from amino-terminated poly(perfluoroalkylene
oxides), such as structure IX,
HOCH.sub.2CH.sub.2(CH.sub.3)NCO--CF.sub.2O--(CF.sub.2CF.sub.2O).sub.m(CF.s-
ub.2O).sub.n--CF.sub.2--CON(CH.sub.3)CH.sub.2CH.sub.2OH (IX)
[0216] or from the diamine of structure X,
H.sub.2NCH.sub.2CF.sub.2O--(CF.sub.2CF.sub.2O).sub.m(CF.sub.2O).sub.n--CF.-
sub.2--CH.sub.2NH.sub.2 (X)
[0217] by reaction with an acrylic acid halide or anhydride in the
presence of a tertiary amine.
[0218] In order to make suitable planar polymeric optical
waveguides, it is preferred to finely control the refractive index
of various core and cladding layers. While this can theoretically
be achieved by tailoring the structure of a single monomer,
oligomer, or polymer component used in a particular coating layer
to achieve the desired refractive index, in practice, it is
oftentimes easier to blend several monomers, oligomers, or polymer
components of different refractive indices together to obtain the
desired composite refractive index.
[0219] The refractive index of each of the polymerizable compounds
made in Example A-B, or described above at Examples C-D, was
measured by mixing each with 1% by weight of an appropriate
photoinitiator. The mixtures were then spin coated onto a silicon
wafer at a thickness of 5 to 10 microns. The samples were purged
with nitrogen and cured to a hardened film with UV light. The
refractive index of the films was then measured using a Metricon
2010 testing apparatus with a 1550 nm laser source in the TE mode.
The results are tabulated in Table 2.
3 TABLE 2 Sample Refractive index at 1550 nm A 1.3519 B 1.4183 C
1.3454 D 1.3079
[0220] The samples were purged with nitrogen to remove oxygen, a
known photopolymerization inhibitor, from the samples before
photoinitiation. Alternatively, the container holding the samples
can be evacuated to remove oxygen. Oxygen inhibition is generally
not desired so that the polymerizable materials are substantially
fully cured to produce cured materials having refractive index
values that do not drift significantly over time or upon possible
subsequent exposure to additional radiation. If desired, however,
layers may be partially cured and, once the entire multi-layer
structure is built, some or all layers may be cured further in a
post-cure exposure step, as discussed above.
[0221] Using various mixtures of the Example A-D materials, it is
possible to achieve a layer with a controlled refractive index
lying between 1.3079 and 1.4183. It is also possible to extend this
range further by using other materials that meet the chemical
structure (III) defined above. Structures with R.sub.f groups that
are larger or smaller than those in Examples A-D defined by Table 2
are likely to have refractive index values outside the range.
[0222] It is also possible to blend the monomers satisfying generic
formula (III) with other monomers, such as the non-fluorinated
compounds described above. Conventional (meth)acrylates, including
non-fluorinated compounds, can have refractive index values ranging
from about 1.4346 to about 1.5577, as shown in Table 3. The table
lists refractive index values of various acrylate and methacrylate
monomers provided by the Sartomer Company, of Exton, Pa. It is
likely, however, that mixed systems including non-fluorinated
monomers will be higher in loss than fully fluorinated systems.
4TABLE 3 Re- Sartomer fractive Chemical Name Product Index Isooctyl
Acrylate SR-440 1.4346 2-2(Ethoxyethoxy)ethyl Acrylate SR-256
1.4355 2 (2-Ethoxyethoxy) Ethylacrylate SR-256 1.4366 Triethylene
Glycol Diacetate SR-322 1.4370 Isodecyl Acrylate SR-395 1.4395
Isodecyl Methacrylate SR-242 1.4414 Lauryl Acrylate SR-335 1.4416
Lauryl Methacrylate SR-313 1.4420 Isodecyl Acrylate SR-395 1.4431
Propoxylated Neopentyl Glycol Diacrylate SR-9003 1.4464 Alkoxylated
Difunctional Acrylate Ester SR-9040 1.4470 Glycidyl Methacrylate
SR-379 1.4470 Glycidyl Methacrylate SR-379 1.4470 Propoxylated
Neopentyl Glycol Diacrylate SR-9003 1.4470 Alkoxylated Difunctional
Acrylate Ester SR-9040 1.4470 Tridecyl Methacrylate SR-493 1.4472
Tridecyl Acrylate SR-489 1.4474 Caprolactone Acrylate SR-495 1.4483
Tripropylene Glycol Diacrylate SR-306 1.4485 Stearyl Methacrylate
SR-324 1.4485 Tris (2-Hydroxy Ethyl) Isocyanurate Triacrylate
SR-368 1.4489 1,3-Butylene Glycol Dimethacrylate SR-297 1.4489
1,3-Butylene Glycol Diacrylate SR-212 1.4501 Neopentyl Glycol
Diacrylate SR-247 1.4503 Neopentyl Glycol Dimethacrylate SR-248
1.4510 Adhesion Promoting Monofunctional Acid Ester CD-9050 1.4513
Ethylene Glycol Dimethacrylate SR-206 1 .4522 Alkoxylated Aliphatic
Diacrylate Ester SR-9209 1.4533 1,4-Butanediol Diacrylate SR-213
1.4535 1,4-Butanediol Dimethacrylate SR-214 1.4545 C14-C15 Acrylate
Terminated Monomer SR-2000 1.4548 1,4-Butanediol Dimethacrylate
SR-214 1.4548 Tetrahydrofurfuryl Methacrylate SR-203 1.4553
Hexanediol Diacrylate SR-238 1.4553 1,6-Hexanediol Dimethacrylate
SR-239 1.4556 1,6-Hexanediol Diacrylate SR-238 1.4560
Tetrahydrofurfuryl Acrylate SR-285 1.4563 Hexanediol Dimethacrylate
SR-239 1.4565 Propoxylated Trimethylolpropane Triacrylate SR-501
1.4567 Cyclohexyl Acrylate SR-208 1.4567 Highly Propoxylated
Glyceryl Triacrylate SR-9021 1.4575 Tetrahydrofurfuryl Acrylate
SR-203 1.4575 Cyclohexyl Methacrylate SR-220 1.4575
Tetrahydrofurfuryl Acrylate SR-285 1.4577 Triethylene Glycol
Dimethacrylate SR-205 1.4580 C14-C15 Methacrylate Terminated
Monomer SR-2100 1.4585 Tetraethylene Glycol Dimethacrylate SR-209
1.4587 Propoxylated.sub.3 Trimethylolpropane Triacrylate SR-492
1.4590 Diethylene Glycol Diacrylate SR-230 1.4590 Polyethylene
Glycol Dimethacrylate SR-210 1.4598 Propoxylated Glyceryl
Triacrylate SR-9020 1.4605 Triethylene Glycol Diacrylate SR-272
1.4606 Diethylene Glycol Dimethacrylate SR-231 1.4607 Highly
Propoxylated Glyceryl Triacrylate SR-9021 1.4610 Propoxylated
Glyceryl Triacrylate SR-9020 1.4612 Tetraethylene Glycol Diacrylate
SR-268 1.4621 Caprolactone Acrylate SR-495 1.4637 Polyethylene
Glycol (200) Diacrylate SR-259 1.4639 Polyethylene Glycol (400)
Dimethacrylate SR-603 1.4645 Di-trimethylolpropane Tetraacrylate
SR-355 1.4654 Polyethylene Glycol (600) Dimethacrylate SR-252
1.4655 Polyethylene Glycol (400) Diacrylate SR-344 1.4655
Polyethylene Glycol (600) Dimethacrytate SR-252 1.4666 Polyethylene
Glycol (600) Diacrylate SR-610 1.4676 Ethoxylated
Trimethylolpropane Triacrylate SR-454 1.4686 Ethoxylated.sub.3
Trimethyolopropane Triacrylate SR-454 1.4689 Ethoxylated.sub.6
Trimethylolpropane Triacrylate SR-499 1.4691 Ethoxylated.sub.9
Trimethylolpropane Triacrylate SR-502 1.4692 Adhesion Promoting
Trifunctional Acid Ester CD-9051 1.4692 Ethoxylated.sub.15
Trimethylolpropane Triacrylate SR-9035 1.4695 Alkoxylated
Trifunctional Acrylate Ester SR-9008 1.4696 Ethoxylated
Trimethylolpropane Triacrylate SR-9035 1.4697 Ethoxylated.sub.20
Trimethylolpropane Triacrylate SR-415 1.4699 Trimethylolpropane
Trimethacrylate SR-350 1.4701 Ethoxylated Trimethylolpropane
Triacrylate SR-415 1.4705 Ethoxylated Pentaerythritol Triacrylate
SR-494 1.4711 Isobornyl Acrylate SR-506 1.4722 Trimethylolpropane
Triacrylate SR-351 1.4723 Trifunctional Methacrylate Ester SR-9010
1.4723 Trifunctional Methacrylate Ester SR-9010 1.4723
Trifunctional Methacrylate Ester SR-9011 1.4724 Isobornyl Acrylate
SR-506 1.4738 Isobornyl Methacrylate SR-423 1 .4738 Isobornyl
Methacrylate SR-423 1.4740 Saret Crosslinking Agent (Trifunctional)
SARET 500 1.4751 Sarit Crosslinking Agent (Trifunctional) SR-500
1.4751 Di-Trimethylolpropane Tetraacrylate SR-355 1.4758 Aromatic
Acid Methacrylate Half Ester SB-600 1.4767 In Trifunctional
Methacrylate Monomer Pentaerythritol Triacrylate SR-444 1.4790
Aliphatic Urethane Acrylate CN-965 1.4800 Pentaerythritol
Triacrylate SR-444 1.4801 Aromatic Urethane Acrylate CN-972 1.4810
Aliphatic Urethane Acrylate CN-962 1.4812 Low Viscosity Aliphatic
Diacrylate Oligomer CN-132 1.4817 Epoxidized Soy Bean Oil Acrylate
CN-111 1.4821 Pentaerythritol Tetraacrylate SR-295 1.4823
Pentaerythritol Tetraacrylate SR-295 1.4847 Dipentaerythritol
Pentaacrylate SR-399 1.4885 Pentaacrylate Ester SR-9041 1.4887
Pentaerythritol Pentaacrylate SR-399 1.4889 Low Viscosity Aliphatic
Triacrylate Oligomer CN-133 1.4896 Pentaacrylate Ester SR-9041
1.4899 Aromatic Acid Methacrylate Half Ester In SB-401 1.4905 EEP
Ester Solvent Highly Ethoxylated.sub.30 Bisphenol A Dimethacrylate
CD-9036 1.4906 Aliphatic Urethane Acrylate CN-981 1.4916 Aromatic
Acid Methacrylate Half Ester in SB-400 1.4921 PM Alcohol Solvent
Aliphatic Urethane Acrylate CN-980 1.4931 Ethoxylated Nonylphenol
Acrylate SR-504 1.4936 Aromatic Acid Methacrylate Half Ester In
SR454 SB-500E50 1.5010 Aromatic Acid Acrylate Half Ester in SR454
SB-520E35 1.5022 Aromatic Acid Methacrylate Half Ester in SR344
SB-500K60 1.5029 Phenoxyethyl Methacrylate SR-340 1.5100
2-Phenoxyethyl Methacrylate SR-340 1.5109 Highly Ethoxylated.sub.10
Bisphenol A Dimethacrylate SR-480 1.5112 Ethoxylated.sub.10
Bisphenol A Diacrylate SR-602 1.5142 Phenoxyethyl Acrylate SR-339
1.5151 2-Phenoxyethyl Acrylate SR-339 1.5160 Ethoxylated.sub.6
Bisphenol A Dimethacrylate CD-541 1.5227 Low Viscosity Aromatic
Monoacrylate Oligomer CN-131 1.5259 Stearyl Acrylate SR-257 1.5312
Ethoxylated.sub.4 Bisphenol A Dimethacrylate CD-540 1.5315
Ethoxylated.sub.4 Bisphenol A Diacrylate SR-601 1.5340 Ethoxylated
Bisphenol A Dimethacrylate SR-348 1.5389 Ethoxylated.sub.2
Bisphenol A Dimethacrylate SR-348 1.5424 Ethoxylated Bisphenol A
Diacrylate SR-349 1.5424 Ethoxylated.sub.2 Bisphenol A Diacrylate
SR-349 1.5425 Epoxy Acrylate CN-120 1.5558 Epoxy Acrylate CN-104
1.5577
[0223] In addition, it is also possible to include the use of
dissolved thermoplastic materials in these formulations. The use of
either alternative monomers and/or polymers is limited strictly by
their compatibility with the cured materials of this invention.
Comparative Example 1
[0224] A straight waveguide was made using the following
procedeure. A clean silicon wafer was silane treated by spin
coating to provide an adhesive tie layer for acrylate formulations.
The treated wafer was spin coated with a lower cladding
polymerization composition including the amounts indicated of the
polymerizable compounds, photoinitiator, and antioxidant listed on
the table below. The thickness of the lower cladding layer was
equal to or greater than about 10 .mu.m thick. The assembly was
then cured with UV light while blanketed with nitrogen. A core
polymerizable composition was formulated including the amounts
indicated of the polymerizable compounds, photoinitiator, and
antioxidant set forth in the table below. The core polymerizable
composition was then spin coated on top of the lower cladding
layer. The core polymerizable composition was formulated such that
it would have a higher refractive index than the lower cladding
layer. The thickness of the core layer depended on the desired
height of the waveguide, which typically ranged from about 5 to
about 9 microns for single mode guides. The core polymerizable
composition was then exposed to UV light through a photomask. The
unexposed material was then removed by solvent. An upper cladding
layer, which was typically made from the same material used in the
lower cladding layer, was then coated on top of the core layer. The
preferred method of coating was spin coating. The upper cladding
composition was then cured.
[0225] Comparative Example 1
5 Comparative Example 1 Ingredient or Property Core Cladding wt %
Sartomer SR349 10.0 wt. % -- Sartomer SR238 5.0 wt. % -- Sartomer
SR610 27.6 wt. % 32.6 wt. % Sartomer SR306 55.1 wt. % 65.2 wt. %
Irgacure 651 photoinitiator 1.0 wt. % 1.0 wt. % Irganox 1010
antioxidant 0.3 wt. % 0.3 wt. % Refractive Index (at 1550 nm)
1.4980 1.4928 T.sub.g(.degree. C.) 11 --
Example E
[0226] The procedure used for making the Comparative Example 1
optical element was repeated using the formulations listed in the
following table:
[0227] Example E
6 Cladding Ingredient or Property Core wt % Product made in Example
B 13 wt. % -- L-12043 available from 3M 86 wt. % 99 wt. % Specialty
Chemicals Division Photoinitiator (compound IV) 1.0 wt. % 1.0 wt. %
Refractive Index (at 1550 nm) 1.3562 1.3471 T.sub.g (.degree. C.)
32 (see note 1) Note 1: The T.sub.g values of the core layers were
determined by dynamic mechanical analysis. The T.sub.g values of
the cladding layers were not determined, but they are expected to
be nearly the same as that of the core.
Example F
[0228] The procedure used for making the Comparative Example 1
optical element was repeated using the formulations listed in the
following table:
[0229] Example F
7 Ingredient Core Cladding Product made in Example A 60 wt % 30 wt.
% L-9367 (available from 3M 38 wt. % 68 wt. % Specialty Chemicals
Division) Compound IV photoinitiator 2.0 wt. % 2.0 wt. % Refractive
Index (at 1550 nm) 1.3249 1.3188 T.sub.g(.degree. C.) 8 (see note
1) Note 1: The T.sub.g g values of the core layers were determined
by dynamic mechanical analysis. The T.sub.g values of the cladding
layers were not determined, but they are expected to be nearly the
same as that of the core.
Example G
[0230] A straight waveguide was made using the following procedure.
Unoxidized silicon wafers were cleaned by the Standard Clean 1
(SC1) process. Standard Clean 1 is a well-known chemical
combination that is used to clean bare silicon or a silicon wafer
with thermally grown or deposited oxide. The cleaning process
entailed dipping the wafers into a 1:5:1 solution of ammonium
hydroxide:water:30% hydrogen peroxide. The temperature of the
solution was then raised to 70.degree. C. for 1/2-hour. The wafers
were then rinsed in deionized water. The wafer was then treated
with 3acryloxypropyltrichloro silane (Gelest Inc., Tullytown, Pa.)
by applying it onto the wafer using a clean room swab. Excess
3-acryloxypropyltrichloro silane was rinsed off with ethanol
followed by a light wiping with a clean room cloth to remove
particles. The wafer was then dried on a hot plate set at a surface
temperature of 70.degree. C.
[0231] The lower cladding polymerizable composition was formulated
per the table below, and filtered at 0.1 microns. A quantity (1.0
ml) of this composition was applied to the wafer while it sat
centered on the chuck of a spin coater (available from Cost
Effective Equipment division of Brewer Science, Inc., Rolla, Mo.,
USA). The material was spun to obtain a 10 micron thick layer. This
entailed a 100 rpm spread for 30 seconds followed by a ramp at 100
rpm/sec to 750 rpm for 60 seconds. The sample was then placed in a
purge box and flooded with nitrogen for two minutes at a flow of
7.1 liters per minute. The sample was then exposed at 10.4
W/cm.sup.2 through a 3.degree. diffuser using a Tamarack light
source. The sample was then reloaded onto the spin coater. The core
polymerizable composition formulated according to the table below
was then filtered as above and 1.5 ml was dispensed onto the wafer.
The wafer was then spun at a 100 rpm spread for 30 seconds followed
by a ramp at 100 rpm/sec to 1350 rpm for 60 seconds to yield a 6
micron thick layer. The sample was then placed in a vacuum bell jar
and evacuated to 0.2 torr to remove bubbles. The photomask was then
brought in contact with the sample under vacuum and held for 1
minute. The vacuum was then released and the sample was placed in a
purge box as above and exposed at 11.9 mW/cm.sup.2 for 20 seconds.
The mask was removed and the wafer was placed again on the spinner.
The sample was spun at 1100 rpm and was developed for 90 seconds
using 8 ml of Galden.RTM. HT110 perfluorinated ether solvent
obtained from Ausimont USA. The sample was then coated with an
upper layer of cladding material in the same manner as the lower
cladding layer except that the cure was for 60 seconds at 9.3
mW/cm.sup.2.
[0232] Example G
8 Ingredient or Property Core Cladding Product of Example A 49.5
wt. % 55.9 wt. % Product of Example B 49.5 wt. % 43.1 wt. % Darocur
1173 photoinitiator 1.0 wt. % 1.0 wt. % Refractive Index (at 1550
nm) 1.3786 1.3723 T.sub.g(.degree. C.) 30 (see note 1) Note 1: The
T.sub.g values of the core layers were determined by dynamic
mechanical analysis. The T.sub.g values of the cladding layers were
not determined, but they are expected to be nearly the same as that
of the core.
[0233] The cured composition Example G material exhibits low
dispersion, i.e., on the order 10.sup.-6 at 1550 nm, low
birefringence (.ident.-10.sup.4), and high environmental
stability.
[0234] The total loss through single mode waveguides made from
different materials was measured as a function of the length of the
waveguide. Using these results, it was possible to determine the
loss through the material.
[0235] Loss measurements of a waveguide made using the Example E
core and cladding are shown in FIG. 30. The loss was measured
through a 20 mm long waveguide. The guide was then cleaved to
produce a 15 mm guide and the loss was re-measured. The guide was
then finally cleaved again to produce a 10 mm guide. An
extrapolated point of zero loss at zero length was then added to
the graph. The slope of the line was determined and recorded in
decibels per centimeter (dB/cm). Table 4 tabulates the results for
each of Comparative Example 1 and Examples E-G.
9 TABLE 4 Sample dB/cm @ 1550 nm Comparative Example 1 0.75 Example
E 0.29 Example F 0.19 Example G 0.24
[0236] As can be seen from the loss results for Example E, F, and
G, the use of fluorinated alkyl or fluorinated ether acrylates is
capable of producing waveguides with very low propagation losses
compared to those of conventional materials.
[0237] The materials from Examples E, F and G also exhibited no
measurable polarization dependence when tested using a Metricon
2010 prism coupling refractive index measuring device in both the
TE and TM modes at 1550 nm. The results observed imply a refractive
index difference between the TE and TM polarizations of less than
0.0001, the measurement sensitivity of the testing instrument. The
results for the invention compare to differences of 0.008 (at 1.3
.mu.m wavelength light) for high T.sub.g fluorinated polyimides, as
reported in U.S. Pat. No. 5,598,501. While fluorinated polyimides
exhibit low loss, their birefringence is a clear disadvantage to
their use. As is known in the art, a birefringent material has
different refractive indices depending on orientation of the
material. Since the operation of devices, such as thermo-optic
switches, directional couplers, and the like depends on small
refractive index differences, the operation may be different for TE
and TM polarizations in highly birefringent materials. This is
generally unacceptable since the light coming into the device will
have an unknown state of polarization. The virtual absence of
polarization dependence in Examples E. F, and G indicates that
these materials are capable of low loss and can produce waveguides
with minimal polarization losses and shifts.
Example H
[0238] The following procedure was performed to test the assumption
that a liquid material undergoing a rapid curing process is less
likely to result in physical stress than a dried thermoplastic.
[0239] A UV-coating made solely of ethoxylated bisphenol A
diacrylate (EBDA, Sartomer 349 from Sartomer Company, Exton, Pa.)
with 1% photoinitiator was spin coated on a silicon wafer and fully
cured with UV light to produce a 10 micron thick layer. Another
silicon wafer was coated with Joncryl 130 (S.C. Johnson Polymer,
Racine, Wis.), an aqueous styrenated acrylic copolymer and dried
for 10 minutes at 70.degree. C. Both materials have a glass
transition temperature of 62.degree. C. Both materials also possess
both aromatic and aliphatic chemical groups. The cured film of the
EBDA is highly cross-linked, while the dried film of the Joncryl
130 is thermoplastic. One of ordinary skill would normally assume
that a polymer that is highly cross-linked would be under a lot
more stress than a thermoplastic polymer. This should result in a
greater difference between TE and TM refractive index measurements.
In fact, the opposite is true as shown below:
10 Before Annealing After Annealing EBDA Joncryl 130 EBDA Joncryl
130 Avg. TE 1.54518 1.53968 1.54562 1.53397 Avg. TM 1.54486 1.54020
1.54542 1.5405 Difference -0.00032 0.00052 -0.0001 0.0008 ANOVA
P-Value 0.32223 0.02602 0.2565 0.0008
[0240] The table above shows the average of 10 readings for TE and
TM for both materials using a Metricon 2010 Prism Coupler. The
difference between the average TE and TM readings was determined
and an analysis of variance (ANOVA) was performed to determine if
the difference was statistically significant. Before annealing, the
EBDA sample had a difference between TE and TM of -0.00032,
however, the high P-value indicates that this result is not
statistically significant. It is essentially below the error limits
of what the experiment could measure. The Joncryl 130 material had
a difference of 0.00052. Unlike the EBDA sample, this difference
was highly statistically significant. After annealing for two hours
at 70.degree. C., the difference of TE and TM for EBDA decreased
slightly and remained statistically insignificant. The Joncryl 130
material, however, actually increased in difference between TE and
TM and remained statistically significant. As noted above, the
Joncryl 130 is a thermoplastic that does have any of the additional
stress that would be associated with a subsequent cross-linking
step. When this experiment was repeated with a cross-linkable,
solid epoxy novalac resin (Epon SU-8, Shell Chemical, Houston
Tex.), which has been used to make optical waveguides, as disclosed
in U.S. Pat. No. 5,054,872, the difference between TE and TM was
found to be greater than 0.001 regardless of annealing
conditions.
[0241] As a result of this test, liquid photocurable compositions
are preferred over solid thermoplastic photocurable polymers
dissolved in solvents.
Example I
[0242] Perfluoropolyether diacrylates, such as those described by
the generic formula
CH.sub.2.dbd.CHCO.sub.2CH.sub.2CF.sub.2O(CF.sub.2CF.sub.2O).sub.m(CF.sub.2-
O).sub.nCF.sub.2CH.sub.2O.sub.2CCH.dbd.CH.sub.2
[0243] may be used in practicing the invention. For these
materials, the values of both m and n can vary considerably. Final
molecular weights of these materials can vary between about 500 and
4000. The higher the values for m and n, the lower the refractive
index, the lower the crosslink density, and the lower the
absorption loss. As can be seen from the refractive indexes and the
CH values given in the table below, these materials can be very
highly fluorinated. While it is desirable to use as much
fluorination as possible for loss purposes, such highly fluorinated
materials can cause difficulty in adhesion when applying subsequent
layers, such as electrodes. In addition, these materials have
relatively limited solubility with other less fluorinated
materials. For the higher molecular weight varieties, fluorinated
photoinitiators, such as those described in U.S. Pat. No. 5,391,587
and Reissue Pat. No. 35,060, should be used. These materials also
produce extremely soft films. Glass transition temperatures for
these materials can be as low as -90.degree. C.
11 Liquid Cured Molecular Refractive Refractive # of Weight
Index.sup.a Index.sup.b Density Hydrogens C.sub.H.sup.c 1100 1.3239
1.3389 1.649 10 15.0 2100 1.3090 1.3079 1.749 10 8.3
.sup.an.sub.D.sup.20 .sup.bMetricon 2010 prism coupler reading at
1550 nm for a cured film made using 1% photoinitiator. .sup.cMolar
concentration of hydrogen atoms in compound (described above)
Example J
[0244] A chlorofluorodiacrylate compound having the structure
CH.sub.2.dbd.CHCO.sub.2CH.sub.2CF.sub.2(CFClCF.sub.2).sub.4CH.sub.2O.sub.2-
CCH.dbd.CH.sub.2
[0245] can be used in practicing the invention. The compound has
the properties listed in the table below.
12 Liquid Cured Refractive Refractive # of Molecular Index.sup.a
Index.sup.b Density Hydrogens Weight C.sub.H.sup.c 1.4216 1.4416
1.580 10 684 23.1 .sup.an.sub.D.sup.20 .sup.bMetricon 2010 prism
coupler reading at 1550 nm for a cured film made using 1%
photoinitiator. .sup.cMolar concentration of hydrogen atoms in
compound (described above)
Example K
[0246] Monofunctional fluorinated acrylates having the generic
structure
CF.sub.3(CF).sub.n(CH.sub.2).sub.mO.sub.2CCH.dbd.CH.sub.2
[0247] where m is typically 1 or 2 and n can range from 0 to 10 or
higher, may be used to practice the invention. Typical property
values for the material where n=8 and m=2 are shown in the table
below. For this material, the higher the value of n, the lower the
refractive index, glass transition temperature, and absorption
loss. As noted above, while monofunctional monomers can be used in
the invention, there may be some long-term outgassing or material
migration of any non-reacted monomers of this type. To avoid the
possibility of a monofunctional monomer not having at least
partially reacted, higher radiation dosages for longer periods of
time may be required to assure sufficient cure of these materials.
Such efforts are generally not required using multi-functional
monomers.
13 Liquid Cured Refractive Refractive # of Molecular Index.sup.a
Index.sup.b Density Hydrogens Weight C.sub.H.sup.c 1.3387 1.3325
1.6 7 569 19.7 .sup.an.sub.D.sup.20 .sup.bMetricon 2010 prism
coupler reading at 1550 nm for a cured film made using 1%
photoinitiator. .sup.cMolar concentration of hydrogen atoms in
compound (described above)
[0248] Diffraction gratings, e.g., Bragg diffraction gratings, may
be written in partially cured planar waveguide laminates, i.e., one
that is not fully cured. Such partially cured waveguide laminates
may be fabricated using the photolithographic or reactive ion
etching techniques described in this disclosure, or by any other
method that is compatible with the preferred polymerizable
compositions disclosed here. The grating is written in at least a
partially cured waveguide core, but the grating should extend into
the core-adjacent cladding as well.
[0249] In general, the partially cured waveguide device in which a
grating can be written should be fabricated from materials using
methods that produce a low-loss, low-birefringence,
high-performance waveguide, such as one made in accordance with the
disclosure set forth above. That is, apart from any additional
factors discussed below which may be considered in selecting
materials especially suitable for making efficient gratings in the
waveguide device, the considerations noted above for making low
loss waveguides generally should not be disregarded if possible.
For example, the preferred polymerizable core and/or cladding
compositions are photopolymerizable and contain at least one
photoinitiator effective for initiating the photopolymerization of
each preferably perfluorinated photopolymerizable compound in the
compositions upon exposure to a dosage of actinic radiation
effective to partially cure them.
[0250] If gratings are to be written in the waveguide, especially
preferred materials for use in fabricating at least the core and,
preferably, the cladding as well, are partially cured
photopolymerizable compositions containing roughly equal weight
proportions of at least two photopolymerizable compounds of
differing refractive index (when fully cured) and characterized
further by one or more of the following properties: Differing
functionality, polymerization rates, and molecular diffusion rates
within the partially cured polymer matrix. As explained below,
these properties are advantageous in writing efficient gratings in
partially cured waveguides.
[0251] A method of writing diffraction gratings in polymeric
waveguides is described in patent application Ser. No. 09/026,764
for "Fabrication of Diffraction Gratings for Optical Signal Devices
and Optical Signal Devices Containing the Same," filed on Feb. 20,
1998, attorney docket no. 30-4466(4290), the disclosure of which is
incorporated herein by reference. In that disclosure, core and
cladding waveguide structures are described as being formed in
partially cured UV curable materials. The curable compositions
include at least two photopolymerizable comonomers. The partially
cured waveguide structure is then exposed with additional UV light
through a photomask that generates light and dark regions in both
the core and cladding. In the light regions, the UV radiation
causes additional polymerization of the monomers to occur. Because
the monomers are chosen so as to have different polymerization and
diffusion rates, the polymer formed in the light areas during the
phase mask exposure, or "writing," step has a different composition
than the polymer in the dark areas. After exposure through the mask
is complete, there remains unreacted monomer.
[0252] Without intending to be bound by or limited to any
theoretical explanation for the mechanism at work in the invention,
it is believed that this unreacted monomer will diffuse to
establish a uniform monomer composition throughout the partially
cured portions of the device. When the device is subsequently
uniformly exposed without a mask, all of the remaining monomer is
converted to polymer. This full-cure exposure step locks in the
polymeric compositional differences between the light and dark
regions and results in a permanent grating. Modulation of the
refractive index in the fully cured diffraction grating arises from
this difference in composition. As mentioned above, this process
works because the polymers resulting from photopolymerizable of the
monomers, oligomers, or polymers selected for use in the core
composition and, preferably, the cladding composition as well,
differ in refractive index and the selected monomers, oligomers,
and polymers differ in cure rate and diffusion rate. It is believed
that these differences cause the composition at a selected point in
the device to vary as a function of exposure time and radiation
dosage. If the composition did not vary with exposure, regions that
received more exposure through the phase mask would be expected to
have the same percentage of each monomer as the dark areas.
Consequently, no diffusion would be expected to take place between
the light and dark regions. When subsequently uniformly exposed
again to achieve full cure, both the light and dark regions would
have the same refractive index and no grating would result.
[0253] A model for explaining the creation of modulations in the
refractive index of a planar waveguide device is shown in FIG. 33A
to FIG. 33F. For the purposes of illustration, the simplified case
of a binderless two monomer (A* and B*) photopolymerizable system
in which the polymerization reaction rate of monomer A* is higher
than that of monomer B* is shown. Before exposure to the grating
writing radiation, there are both species of unreacted monomer A*
and monomer B* in the partially polymerized waveguide, as shown in
FIG. 33A. For simplicity, polymer A and polymer B already formed
during the waveguide fabrication process are not shown.
[0254] The sinusoidal pattern 18 of the grating writing radiation
intensity, I(x), including intensity maxima and intensity minima,
is shown adjacent to the brighter regions and darker regions of the
partially polymerized waveguide material in FIG. 33C. The grating
writing radiation intensity pattern may be produced using a phase
mask 19, as shown in FIG. 34, by a two-beam interference set-up 20,
as shown in FIG. 35, or by any other method.
[0255] Bearing in mind that the waveguide is already partially
polymerized from the waveguide fabrication process, further
polymerization of monomer A* is initiated in the brighter regions
of the writing pattern. Since the polymerization rate of monomer A*
is faster than that of monomer B*, with time, the brighter regions
contain primarily polymer A while the darker regions have mainly
polymer B even after removal of the interference pattern, as shown
in FIG. 33D and FIG. 33E.
[0256] The brighter regions 21 are expected to become enriched in
the more quickly formed polymer (polymer A) and depleted of the
more quickly consumed monomer (monomer A*), as shown in FIG. 33C
and FIG. 33D. Due to the resulting concentration gradients of
monomer A*, monomer A* is expected to diffuse from the darker
region 22 to the brighter region in order to establish a uniform
concentration, as shown in FIG. 33D. As in any diffusion process,
temperature, concentration difference, and mobility of the monomers
will affect the overall diffusion rate.
[0257] After some enrichment by diffusion of the faster reacting
monomer A* into the light regions and enrichment of the darker
regions by the slower reacting monomer B*, the waveguide is flood
exposed to react all unreacted monomer to "lock in" the
concentration gradients of polymer A and polymer B. The flood
exposure taking place in FIG. 33E may be accomplished using any
fast-acting radiation source, such as an actinic radiation source
suitable for the polymerizable compositions selected, such as a
ultraviolet (UV) radiation source (not shown). While heat could be
applied to effect the final uniform curing step, actinic radiation
is preferred due to its fast cure time in light transmissive
systems. Optionally, both a final full actinic radiation cure and a
final full heat cure can be carried out. During this step,
unreacted monomer B* is polymerized. Assuming that the refractive
indices of polymer A and polymer B are different, a steady state or
"permanent" modulation of the refractive index, i.e., a grating, is
thereby formed in the waveguide. The grating has the same period as
the light pattern created by the phase mask, two interfering beams,
or other form of writing radiation. The maximum modulation depth is
given by the difference of the indices of the individual
components, as shown in FIG. 33F.
[0258] While differences in refractive index, diffusion, and cure
rate can produce gratings, the need for very high grating
efficiency is typically not achieved by these differences alone. In
order to achieve an even higher compositional change with exposure,
it has been discovered that choosing monomers of differing
functionality can substantially boost the performance of these
gratings. Functionality in this case is defined as the number of
actinic radiation curable functional groups per monomer molecule. A
wide variety of monomers having actinic radiation curable (ARC)
groups could be selected. Preferred ARC groups include epoxies and
ethylenically unsaturated groups, such as acrylates,
(meth)acrylates, and vinyl ethers, to name just a few. Other
suitable reactive groups are described above.
[0259] To introduce how the functionality of the monomers can
effect composition, several conceptual Examples 1-3 will first be
discussed followed by presentation of a preferred comonomeric
composition (Example 4). In each of these examples, it is assumed
that the relative reaction and diffusion rates of the monomers are
the same.
Example 1
[0260] A formulation of two monomers with the characteristics shown
in Table 1 is provided. As noted above, "functionality" is defined
as the number of actinic radiation curable groups per monomer
molecule.
14 TABLE 1 Monomer Molecular Wt. Functionality Wt. % A 100 2 50 B
100 2 50
[0261] For a 100 g quantity of the above mixture, the values
tabulated in Table 2 result.
15TABLE 2 Initial Relative Initial Wt. % Equiva- Equivalents Weight
of of Monomer Moles lents % Equivalents Equivalents A 0.5 1.0 50 50
50 B 0.5 1.0 50 50 50
[0262] The values shown for the number of moles and the number of
equivalents are the typical values familiar to chemists and
physicists. The number of moles is merely the weight of the monomer
divided by its molecular weight. The number of equivalents is the
number of moles of the monomer multiplied by its functionality.
When polymerization occurs, a reactive group from one of the
monomers adds to the growing polymer chain. The likelihood that a
particular free monomer will react is dependent on the
concentration of the reactive groups for the monomers. To determine
this concentration at the start of the reaction, the relative
amount of equivalents of each monomer was determined as a
percentage of the total number of equivalents and reported in the
tables as Equivalents %. These values are multiplied by the
molecular weight of the respective monomers to arrive at the
initial relative weight of equivalents of each of the monomers. The
initial wt. % of the equivalents of the monomers is then
calculated. As can be seen in Table 2, the initial wt. % of the
equivalents of the monomers in this example is the same as the wt.
% of the monomers. Because the final wt. % of a monomer in a
polymer is equal to the wt. % of the monomers, the fully
polymerized polymer will in this case be composed of 50% of monomer
A and 50% of monomer B. Based on the initial wt. % of equivalents
of the monomers, when the polymer first begins to form, it will
also be composed of 50% of monomer A and 50% of monomer B. Since
the reaction and diffusion rates are assumed to be the same, this
suggests that the concentration of the monomers will not vary as
the polymerization proceeds. This means that this idealized
material will not likely form a grating by the process previously
described. Accordingly, such a component of monomer A and B would
not be preferred for use in making photopolymerized diffraction
gratings.
Example 2
[0263] The properties of interest for two monomers A and B which
differ in equivalent weight are shown in Table 3 below:
16 TABLE 3 Monomer Molecular Wt. Functionality Wt. % A 100 2 50 B
200 2 50
[0264] For a 100 g quantity of the above mixture, the values
tabulated in Table 4 will result.
17TABLE 4 Initial Relative Initial Wt. % Equiva- Equivalents Weight
of of Monomer Moles lents % Equivalents Equivalents A 0.5 1.0 66.67
66.67 50 B 0.25 0.5 33.33 66.67 50
[0265] As can be seen in Table 4, the initial wt. % of equivalents
is equal to the wt. % of the monomers. Accordingly, no
concentration gradient during cure will be expected and no grating
is expected to result.
Example 3
[0266] Monomers A and B have the same molecular weights, but they
have different functionalities as shown in Table 5:
18 TABLE 5 Monomer Molecular Wt. Functionality Wt. % A 100 2 50 B
100 3 50
[0267] For a 100 g quantity of the above mixture, the values shown
in Table 6 will result.
19TABLE 6 Initial Relative Initial Wt. % Equiva- Equivalents Weight
of of Monomer Moles lents % Equivalents Equivalents A 0.5 1.0 40 40
40 B 0.5 1.5 60 60 60
[0268] As shown in Tables 5 and 6, the initial wt. % of equivalents
is different than the wt. % of the monomers. This implies that
there will be a concentration gradient as polymerization proceeds.
Accordingly, such a combination of monomers could be expected to
form a grating even if the reaction and diffusion rates of the
monomer are the same.
[0269] When the reaction first begins, there are equal numbers of
molecules of both monomers A and B. Since monomer B has one and
one-half as many reactive groups as monomer A, the polymerization
will initially use more molecules of B then monomer A. As the
reaction proceeds, the concentration of unreacted monomer B
molecules will begin to decrease and the likelihood of monomer A
molecules polymerizing will increase. Once the polymerization is
complete, an equal number of both monomeric molecules will have
been consumed by the reaction and the concentration by weight of
monomers in the polymer will be equal.
Example 4
[0270] Monomer A is octafluorohexanediol diacrylate obtained
commercially. Monomer B is the tetra-acrylate of Fluorolink.RTM. T
brand tetra-functional fluorinated polyether polyol from Ausimont
Corporation.
20 TABLE 7 Monomer Molecular Wt. Functionality Wt. % A 370 2 50 B
2416 4 50
[0271] For a 100 g quantity of the above mixture of the monomers A
and B, the values shown in Table 8 are expected.
21TABLE 8 Initial Relative Initial Equiva- Weight of Wt. % of
Monomer Moles Equivalents lents % Equivalents Equivalents A 0.135
0.2703 76.55 283.2 33.33 B 0.021 0.0828 23.45 566.5 66.67
[0272] This set of monomers A and B should produce a grating since
the values for the weight percent of Table 7 and the initial weight
percent of equivalents of Table 8 for each monomer are unequal.
[0273] A Monte Carlo calculation was performed for each of the
above examples. The calculation was performed using a computer
program based on the flow chart shown in FIG. 40. The algorithm can
be used to evaluate the potential of a selected pair of monomers
characterized in terms of molecular weight, functionality, and
initial weight proportions in the composition to form a diffraction
grating in waveguides.
[0274] The program begins by simulating 10,000 theoretical
molecules, e.g., monomers A and B, based on the starting
formulation. Since each of the monomers in the above examples is
present at the 50 wt. % level, there are 5000 unreacted molecules
each monomer at the start of the calculation. The fraction of end
groups for the monomer A is calculated. A random number between 0
and 1 is then chosen. If the random number is less than the
fraction of end groups for the monomer A, then one molecule of A is
considered to have been added to the forming polymer and the number
of unreacted molecules of A is decreased by one. If the random
number is greater than the fraction of end groups of A, then a
molecule of B is considered to have been added to the forming
polymer and the number of free molecules of B is decreased by one.
The weight % of A in the forming polymer is then calculated and
recorded. The fraction of end groups for A in the remaining free
monomer is then recalculated. The process is repeated until all of
the molecules are converted to polymer.
[0275] FIG. 41 shows the results of these calculations for each of
the above examples. Examples 1 and 2 initially show some deviation
from the 50% level as a result of the random nature of this
process. However, they quickly approach the 50% level after only
about a 1000 molecules have been added to the polymer. Since the
actual number of molecules used in making a grating is much larger,
such random fluctuations would have little impact on making an
actual grating. In both Examples 3 and 4, there is some early
fluctuation in the values as a result of this random approach, but
both curves approach the 0.5 level until virtually all of the
molecules are consumed. This calculated result demonstrates the
effectiveness of using monomers having different functionalities in
producing effective gratings.
[0276] Accordingly, a method of making diffraction gratings in a
planar polymeric waveguide laminate will now be described. A
waveguide is provided which includes a polymeric light guiding core
surrounded by a lower refractive index material. As noted above,
the lower refractive index material may be a substrate, a buffer
layer of a support including a substrate, or a lower cladding layer
on a substrate.
[0277] The light guiding core in which the grating is to be written
should not be fully cured prior to the grating writing step.
Preferably, the core and at least that portion of the cladding
surrounding the core in which the grating will be written is only
partially cured prior to the grating writing step. More preferably,
the extent of cure in the waveguide formation step is minimized to
allow for a maximum of extent of further polymerization during the
grating formation step. Doing so increases the potential difference
between the maximum and minimum refractive index in the final
grating for a given polymerizable composition.
[0278] Especially preferred polymerizable compositions for
fabricating the core and, if desired, the cladding layers as well,
of waveguide laminates intended for subsequent grating writing are
those that include roughly equal weight proportions of two or more
multi-functional photopolymerizable monomers, oligomers, or
polymeric compounds ("comonomers") which differ in polymerization
reaction rate and functionality. It is preferred that the
functionality of the at least two comonomers of the composition
differ by at least one, and, preferably, by at least two. The
photopolymerizable composition should also include an effective
amount of a suitable photoinitiator or mixture of suitable
photoinitiators.
[0279] Polymerizable compositions having, say, two comonomers of
differing functionality should be able to form efficient
diffraction gratings even if the polymerization reaction rates of
the individual monomers and their respective diffusion rates are
the same. The increased performance of the resulting diffraction
grating is especially pronounced, however, if a monomer with a
higher functionality also polymerizes at a faster rate than a
monomer with a lower functionality. If a monomer with a higher
functionality polymerizes at a slower rate than a monomer with a
lower functionality, then the advantage produced by the higher
functionality will be expected to be offset somewhat.
[0280] One such suitable core composition includes roughly equal
weight proportions of the low-loss low-birefringence perfluorinated
photopolymerizable tetra-acrylate compound having structure (VI)
(synthesized from Fluorolink.RTM. T brand fluorinated polyether
polyol from Ausimont USA) and the perfluorinated photopolymerizable
di-functional octafluorohexanediol diacrylate compound having
structure (VIII). Synthesis of the tetra-acrylate is exemplified by
Example A while that of the di-acrylate is exemplified by Example
B. A composition of the two compounds together with a
photoinitiator is exemplified by Example G.
[0281] Photo-differential scanning calorimetry studies confirm that
the higher functionality comonomer of this system, i.e., the
tetra-acrylate of the Fluorolink.RTM. T fluorinated polyether
polyol (curve 24), reacts at a higher rate than the lower
functionality octafluorohexanediol diacrylate (curve 23), as shown
at FIG. 36.
[0282] Once the partially polymerized waveguide is made, the
grating is "written" in the waveguide. This step is accomplished by
exposing the inside the partially polymerized waveguide to an
interference pattern of sufficient intensity to effect additional
polymerization. The interference pattern can be established, for
example, using a conventional phase mask 19 designed for writing
gratings, such as that shown in FIG. 34, or by using a conventional
two-beam interference setup 20, as shown in FIG. 35.
[0283] The fabrication of gratings in a planar waveguide using a
phase mask is shown schematically in FIG. 34. Light of wavelength
.lambda. illuminates the phase mask of period .LAMBDA.. The writing
light is diffracted by the phase mask. The intensity distribution
resulting from the interference pattern created by the phase mask
at the waveguide initiates further photochemical reaction in the
partially cured photopolymerizable composition of the waveguide.
The result is the creation of a phase grating written in the
waveguide with period .LAMBDA..sub.g. For a phase mask that is
designed to diffract in the +1 and -1 orders, the grating period is
one-half the phase mask period. Light travelling inside the
waveguide grating is reflected when its wavelength is equal to
.lambda..sub.B=2 n.sub.eff .LAMBDA..sub.g where n.sub.eff is the
waveguide's effective refractive index and .lambda..sub.B is the
Bragg wavelength.
[0284] For the creation of a purely sinusoidal pattern, it is
necessary to use a phase mask with a 50% diffraction efficiency in
the +1 and -1 diffraction orders and 0% efficiency in the 0.sup.th
and all higher orders. In reality, due to phase mask fabrication
errors, there is always some small percentage of light diffracted
in unwanted orders. If the phase mask has as little as, say, 5%
diffraction efficiency in the 0.sup.th order, the grating will
still have a period of .LAMBDA./2, but the interference maxima are
not all at the same intensity level.
[0285] A phase mask for writing gratings is itself a grating,
typically etched in a silica substrate, with an etching depth such
that it diffracts most of the light in the +1 and -1 orders. Beams
corresponding to the +1 and -1 diffraction orders are interfered
inside the material where they create a sinusoidal interference
pattern. This diffraction pattern is very important for the quality
of the grating that is formed in the material. Typical measured
diffraction efficiencies for commercially available phase masks are
0.sup.th order (.eta..sub.o) 7.7%, 1.sup.st order (.eta..sub.1)
42%, -1.sup.st order (.eta..sub.-1) 39.6%, 2.sup.nd order
(.eta..sub.2) 6% and -2.sup.nd order (.eta..sub.-2) 4%.
[0286] Preferably, the waveguide sample is exactly positioned under
the phase mask such that the spacing between the phase mask and the
waveguide is substantially constant across the waveguide.
[0287] Although writing using a phase mask is desirable in a
manufacturing setting, as noted above, a two-beam interference
set-up can also be used to write the grating in the partially
polymerized waveguide. The fabrication of gratings in a planar
waveguide using a two-beam interference set-up is shown
schematically in FIG. 35. Light beam 23 from light source 24
preferably passes through beam splitter 25 so that two interfering
beams 26, 27, separated by angle 2.theta., interfere at the
partially polymerized optical waveguide device 28. Mirrors can be
used to position the beams. The light source can be a UV laser or
other source of actinic radiation.
[0288] One advantage of the two-beam interference approach is that
a sinusoidal intensity pattern in the polymerizable material is
more likely than in the phase mask approach. Another advantage is
that the period of the grating can be changed simply by changing
the angle between the interfering beams. Since each phase mask is
designed for a specific illuminating wavelength and grating period,
a new mask is required every time a change in the grating period is
desired.
[0289] Gratings have been written in planar waveguiding optical
devices according to the invention using both the phase mask and
interfering beam approach.
[0290] Following the grating writing step, the waveguide with the
grating is flood exposed with actinic radiation to fully cure the
photopolymerizable layers thereby "locking in" the periodic
refractive index variations, and prevent further material
diffusion.
Example L
[0291] A grating was written in a single mode straight waveguide
according to the procedure described in patent application Ser. No.
09/026,764 referred to above. The waveguide was made using a
photopolymerizable composition including about 50 wt. % of the
structure (VI) tetra-acrylate obtained from the Fluorolink.RTM. T
fluorinated polyether polyol material from Ausimont USA and about
50 wt. % of octafluorohexanediol di-acrylate (structure VIII) based
on the total weight of these two compounds, and including about 1
wt. % photoinitiator. The period of the phase mask was selected to
product a reflection at 1550 nm. The transmission spectrum of this
grating is shown in FIG. 37. The intensity of the transmitted
signal at this wavelength decreased by over 45 dB, the limit of the
detection equipment used. As demonstrated by this data, a highly
efficient grating was made using these materials and fabrication
methods.
Example L
[0292] A clean silicon wafer is used as a substrate. A liquid
negative-tone photopolymerizable composition is formulated to
include 55.9 wt. % of compound (VI) (the tetra-acrylate of the
Fluorolink.RTM. T brand fluorinated polyether polyol made according
to the procedure of Example A), 43.1 wt. % of octafluorohexanediol
diacrylate compound (VIII) made according to the procedure of
Example B, and 1 wt. % Darocur 1173 photoinitiator to form a
cladding polymerizable composition. The cladding composition is
spin-coated on the substrate to form a lower cladding coating that
is 10 microns thick. The lower cladding coating is then uniformly
exposed to ultraviolet light under a mercury lamp (Hg line
wavelength=365 nm) to form a solid thin film of refractive index
1.3723 (at 1550 nm when fully cured) as a lower cladding layer. The
exposure time is kept short (1 sec.) at this point to obtain a
layer that is only partially polymerized.
[0293] A liquid negative-tone photopolymerizable composition is
formulated to include 49.5 wt. % of compound (VI), 49.5 wt. % of
compound (VIII) made according to the procedure of Example B, and 1
wt. % Darocur 1173 photoinitiator to form a core polymerizable
composition. The core composition has a refractive index of 1.3786
(at 1550 nm when fully cured). The core composition is spin-coated
on the lower cladding layer to form a core coating that is 6
microns thick. The core coating is placed in contact with a
photoimaging mask where the waveguiding circuit (a cascaded
4-channel add/drop device where each of the four add/drop elements
in the cascade is a Mach-Zehnder interferometer) is clear (the
width of the waveguides in the mask is 6 microns). The core coating
is selectively UV-cured through the mask under the mercury lamp for
a short time of 3 sec. to ensure only partial polymerization. The
mask is removed and the unexposed sections are developed away using
an appropriate solvent.
[0294] Additional cladding composition as listed above is
formulated and spin-coated onto the core structure so as to form a
conformal layer that is 10 microns thick and that layer is
subsequently blanket UV-exposed under the mercury lamp to form a
solid conformal film of refractive index 1.3723 (at 1550 nm when
fully cured) as an overcladding layer. This layer is also exposed
for a short time (1 sec.) to ensure only partial polymerization at
this stage. A phase mask with a grating is used to print (using an
Argon ion laser operating at 363.8 nm) a grating across the core in
each of the four Mach-Zehnder devices. The sample with the planar
waveguiding circuit is held parallel to the phase masks at 50
microns from the mask. The laser beam is directed perpendicularly
to the mask and the sample. The laser beam diameter is 3 mm (at
1/e.sup.2 intensity). The laser is scanned 3 mm across the center
of the 6-mm-long Mach-Zehnder arms, creating gratings in the three
partially cured waveguide layers. The sample is finally subjected
to a final UV cure in a nitrogen ambient atmosphere under the
mercury lamp (60 sec.) and a final thermal cure (90 deg. C. for 1
h) is carried out to effect a full polymerization of all three
layers. Testing of the sample reveals that all the gratings are
reflecting the desired wavelength channels.
[0295] Compositions made from the same two comonomers in
approximately the same proportions as that made in Example G and
Example L have very desirable thermo-optic properties after curing.
The rate of change in the refractive index of the cured composition
with temperature, dn/dt, is approximately
-3.times.10.sup.-4/.degree. C. This property results in a tuning
rate of about -0.256 nm/.degree. C. for gratings made from this
material, as shown by the graph appearing in FIG. 38. Importantly,
the curve is remarkably linear which permits highly predictable and
reproducible tuning of the reflected wavelength.
[0296] While this property of linear tunability is a highly desired
property in making thermo-optic devices, it also useful in making
gratings which are stable to temperature changes. This can be
accomplished by changing the substrate on which the grating is
made. By choosing substrates with different coefficients of thermal
expansion (CTE), the expansion rate of the Bragg grating can be
altered. The change in the Bragg wavelength of the grating with
temperature (d.lambda..sub.B/dt), as shown in FIG. 39, can be
altered by using substrates with different CTEs. Substrates that
produce a value of d.lambda..sub.B/dt as little as -0.06
nm/.degree. C. have been developed. Datum 30 refers to the
urethane-coated polycarbonate substrate noted above.
[0297] Gratings made from the octafluorohexanediol
di-acrylate/tetra-acryl- ate of Fluorolink.RTM. T material in
accordance with the invention showed a Bragg wavelength shift of
just 0.2 nm when the ambient relative humidity was changed by 90%
at a constant temperature of 50.degree. C. This result was
favorably much smaller than the result obtained using gratings made
from other materials where the shift was 3.7 nm. This unexpected
benefit may allow optical devices made in accordance with the
invention to be packaged without having to be hermetically
sealed.
[0298] It will be apparent to one skilled in the art that the
manner of making and using the claimed invention has been
adequately disclosed in the above-written description of the
preferred embodiment(s) taken together with the drawings; and that
the above described preferred embodiment(s) of the present
invention are susceptible to various modifications, changes, and
adaptations, and the same are intended to be comprehended within
the meaning and range of equivalents of the appended claims.
[0299] Further, although a number of equivalent components may have
been mentioned herein which could be used in place of the
components illustrated and described with reference to the
preferred embodiment(s), this is not meant to be an exhaustive
treatment of all the possible equivalents, nor to limit the
invention defined by the claims to any particular equivalent or
combination thereof. A person skilled in the art would realize that
there may be other equivalent components presently known, or to be
developed, which could be used within the spirit and scope of the
invention defined by the claims.
* * * * *