Note: Descriptions are shown in the official language in which they were submitted.
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OPTICAL WAVEGUIDES, OPTICAL CIRCUITS AND METHOD AND APPARATUS FOR
PRODUCING OPTICAL WAVEGUIDES AND OPTICAL CIRCUITS
FIELD OF THE INVENTION
The invention is in the field of optics, manufacturing methods and
processes for producing optical devices and optical devices so produced
and being useful for performing operations on optical signals,
including but not being limited to filtering, diffracting,
discriminating and sensing.
BACKGROUND OF THE INVENTION
Optical Bragg grating devices are used for performing many operations
on optical signals, such as filtering, light diffraction, and sensing.
Optical waveguide grating, in particular, do theses function while
guiding and confining the light in the waveguide medium as well. A
waveguide grating is normally formed on a waveguide in which at least
one of its parameters is changed almost periodically along the length
of the waveguide. The most commonly perturbed physical parameter in
waveguide grating structures is the refractive index. The waveguide
structure with periodically perturbed refractive index can be used as
an optical filter in which an optical signal is reflected back by the
grating structure at the Bragg wavelength defined by:
g 2ne~5= ~.
where ~g is the Bragg resonance wavelength and ne ~ ~ is the average
effective index of the waveguide, and ~ is the longitudinal period of
refractive index change along the waveguide. A variety of optical
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wavelength band reflection/rejection or transmission filters can be
designed consequently to perform the desired functions. The optical
filter can be designed to have very narrow, that is less than 0.1 nm
line width, or to have relatively wide band filters with desired
transmission reflection wavelength characteristics in the order of few
tens of nm line-width. For instance they can be used for separating
one particular band of the optical signal in wavelength division
multiplexing (WDM) optical transmission system or as dispersion
compensators in long haul transmission systems.
An efficient and popular method of imprinting gratings on waveguides is
to use photosensitive waveguides whose refractive index can be changed
by exposure to radiation of a particular nature, for example
ultraviolet (UV) electromagnetic radiation. Usually a grating is
imprinted by exposing the waveguide under an interferometric pattern of
ultraviolet sources using holographic or phase mask methods.
Imprinting grating by holographic method is described in an article
entitled,"Formation of Bragg Gratings In Optical Fiber By Transverse
Holographic Method", by G. Melts et al., published in 1989 in Optics
letter Ilol. 14, No. 15, at pages 823-825. In the holographic, or
interferometric method, waveguide grating is formed by exposing the
piece of fiber to an interfering pattern of two ultraviolet beams of
light to produce a standing wave to which the waveguide is exposed.
The refractive index of the waveguide is locally and periodically
changed in the exposed area. This grating fabrication approach
requires a laser with high spatial and temporal coherence, and is
highly sensitive to alignment and vibration during production. These
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requirements are more strict in the case of a chirped grating in which
the period of grating pitches must be changed along the waveguide.
Imprinting grating using a phase mask method is described, for example,
in an article entitled, "Bragg Gratings Fabricated In Monomode
Photosensitive Optical Fiber By UV Exposure Through A Phase Mask" by
K.O. Hill et al. published in 1993 in Applied Physics Letters, Volume
62, No. 10, pages 1035-1037. The method is also published in the
United States Patent No. 5,367,588, issued to K. Hill et al on November
22, 1994 and ent i t 1 ed, "Method of Fabr i cat i ng Bragg Grat i ng Us i ng
A
Silica Phase Grating Mask and Mask Used By Same". In this approach, a
phase mask splits the beam into several diffractive orders that
interfere to create the required pattern. The phase mask method is
less sensitive to spatial coherence and alignment. It can also be used
to produce chirped gratings. However it still needs proper optical
alignment, careful control of the space between the phase mask and the
waveguide, with a precise control of waveguide motion under the phase
mask at the same time. In the US Patent No. 5,837,169, "Creation Of
Bragg Reflective Gratings In Waveguides," by H.N. Rourke, Issued No.
1998, there is disclosed a method of writing long fiber grating at
several stages using a number of phase masks that have an alignment
part which is a replicate of the portion of the writing part of the
adjacent mask. Careful motion adjustment must be made to align the
consequent masks and keep the writing conditions the same for each
stage of writing gratings. A number of research papers and patent
disclosures, some of which are listed herein, propose new optical
devices using Bragg grating or disclose improved methods of imprinting
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Bragg grating based on the two above mentioned methods. Nevertheless,
grating fabrication method using these approaches are still time
consuming and unpredictable due to the required mechanical motion
accuracy and stability. This results in low yield in fabrication and
therefore a high manufacturing cost. Therefore there is a need for
alternative methods of manufacturing Bragg grating devices on
waveguides that is suitable for volume manufacturing.
SUhwIARY OF THE INVENTION
In accordance with an aspect of the invention, a method for
manufacturing an optical circuit includes provision of an optical
waveguide being carried by a surface of a substrate, the optical
waveguide having a light transmission property which is alterable in
response to radiation of a predetermined nature. A first mask is
applied to the surface of the substrate, the first mask being non-
transparent to the radiation and includes registration means and a
radiation transparent aperture defined therein for permitting a
predetermined area of the optical waveguide to be exposed to the
radiation. A second mask is affixed over the first mask in alignment
with the registration means. The second mask includes a plurality of
ports defined therein, each of the ports overlapping the aperture and
being transparent to the radiation. A predetermined amount of the
radiation is directed at the second mask, whereby areas of the optical
waveguide are exposed via said ports and the aperture such that a light
transmission property in each of said areas is altered to effect
manufacture of the optical circuit.
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Advantageously the second mask is removed from the optical circuit and
is useful in the fabrication of similar optical circuits.
In accordance with an another aspect of the invention, a process for
effect i ng post manufacture adjustment of an opt i ca 1 c ircu i t hav i ng
been
5 formed in an optical waveguide having a light transmission property
being alterable by exposure to radiation of a predetermined nature,
includes the steps of: a) determining an operating characteristic of
the optical circuit by transmitting light energy in a spectrum in which
the optical circuit is intended to be operable via said optical circuit
and receiving and detecting any of said light energy having traversed
the optical circuit; b) determining if the operating characteristic is
adjustable toward an operating parameter standard by adjusting the
light transmission property of at least one predefined portion of the
optical circuit, and if YES; c) selectively exposing the at least one
predefined portion of the optical circuit to a beam of radiation while
continuing to perform step a) and if the determination in step b)
becomes N0, stopping the exposure in step c).
An apparatus in accordance with an another aspect of the invention,
provides for post manufacture processing of an optical circuit
including an optical waveguide having a light transmission property
which has been altered by exposure to radiation. The apparatus
includes a table for mounting the optical circuit and a beam source for
providing a beam of radiation of a predetermined nature for impinging
upon an optical circuit mounted on the table. A source of light energy
in a spectrum in which the optical circuit is intended to be operable
is connectable for transmitting light to the optical circuit when it is
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mounted on the table. A detector is connectable to receive light
energy from the optical circuit when mounted on the table. The
detector generates indications representative of received light energy.
A controller is dependent upon the indications of received light energy
from the detector and a data base peculiar to a particular design of an
optical circuit, for selection at least one portion of an optical
circuit mounted on the table and directing the beam of electromagnetic
radiation from the beam source means upon a selected portion of the
mounted optical circuit.
In accordance with yet another aspect of the invention an optical
waveguide assembly comprises a circuit substrate having a substantially
planar surface and an optical waveguide being imbedded in the
substantially planar surface of the circuit substrate. The optical
waveguide has a light transmission property being alterable in response
to radiation of a predetermined nature. A mask overlies the
substantially planar surface of the substrate and the optical
waveguide. The mask includes an aperture therein for defining an area
of the waveguide which is accessible for exposure to said
electromagnetic radiation. The mask also includes a registration means
to provide for alignment of the substrate with processing apparatus
useful in the manufacture of an optical circuit.
In one example the optical waveguide assembly includes an optical
circuit being contained along and within a portion of the optical
waveguide in a substrate overlaid with a first mask and having been
manufactured by exposure of areas of the optical waveguide, via a
grating mask registered with the first mask, to electromagnetic
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radiation of an incoherent nature within the ultraviolet spectrum.
After manufacture the grating mask is removed and an aperture in the
first mask defines an area of said portion of the waveguide which is
available for post manufacture modification by controlled exposure to
said electromagnetic radiation.
BRIEF DESCRIPTION OF THE ~tAWINGS
Example embodiments of the invention are described with reference to
the accompanying drawings in which:
Figures 1, 2 and 3 are exemplary of some arrangements of optical
waveguide assemblies which are suitable for use in a manufacturing
process in accordance with the invention;
Figure 4 illustrates a first stage of manufacture of an optical circuit
using an optical waveguide assembly, for example like that shown in
figure 2;
Figure 5 illustrates a grating mask useful for processing an optical
waveguide assembly similar to that shown in figure 4;
Figure 6 illustrates the optical waveguide assembly of figure 4 and the
grating mask of figure 5 in vertical alignment preparatory to a process
assembly;
Figure 7 illustrates elements of figures 5 and 6 in a process assembly
suitable for radiation with non-coherent ultraviolet light to provide
the optical circuit;
Figure 8 illustrates an apparatus for post manufacture processing of an
optical circuit having been manufactured by controlled exposure to
ultraviolet light as exemplified in figure 7; and
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Figure 9 is a flow chart illustration being exemplary of functions by
which the post manufacture processing apparatus shown in figure 8 is
operable.
DETAILED DESCRIPTION
The optical waveguide assembly in Figure 1 is shown as having a
substrate 11 with a planar surface 12 having several optical waveguide
elements 13 lodged therein. Such an optical waveguide assembly is
fabricated by any of several conventional methods such as flame
hydrolysis, sol-gel deposition and the like.
The optical waveguide assembly in Figure 2 is shown as having a
substrate 14 with a planar surface 15 having several optical waveguide
elements 16 lodged therein. In one example such an optical waveguide
assembly is fabricated by embedding optical fibers into grooves having
been formed in the planar surface 15. Each of the optical fibers
includes a core 16a shrouded by a layer of cladding 16b.
The optical waveguide assembly in Figure 3 is shown as having a
substrate 17 with a planar surface 18 carrying an embedded optical
fiber 19 in the form of a spiral.
In these waveguide assemblies one or more of the waveguides or optical
fibers contains a photosensitive material in at least one of the core
and the cladding. Photosensitive materials are often photo-refractive
to a particular spectrum of an electromagnetic radiation.
Photosensitive materials which exhibit a noticeable change of
refractive index upon an exposure to ultraviolet light have been found
to be convenient, however other materials showing photosensitivity to
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electromagnetic radiation in other spectrums are envisaged as being
useful. One or more of the waveguides 13 contain at least one
photosensitive area and are fabricated for instance by depositing a
layer of photosensitive sol-gel glass as the cladding on the waveguide
circuits, made of a variety of materials, as disclosed in a Canadian
patent application number 2190-886 by H. Hatami-Hanza et al. filed Nov.
21, 1996. The waweguide 13 can also be made entirely by sol-gel glass
method as described for example in an article, "Fabrication and
Characterization of Low-Loss, Sol-Gel Planar Waveguides, published in
1994 volume 66 of Anal. Chem, pp. 1254-1263 and in another article,
"Ultraviolet light imprinted sol-gel silica glass channel waveguides on
silicon, Authored by Najafi et al., published in 1996 in vol. 2695 of
the SPIE, pp. 38-41. The waveguide 13 with at least one photosensitive
area can also be fabricated by a flame hydrolysis method using
germanium doped silica glass deposition as described in an article by
Jorg Huber et al., entitled, "U11-Written Y-Splitter in Ge Doped
Silica," published in 1996 in vol. 2695 of the SPIE, pp. 98-105.
In another embodiment of samples, as shown in Figures 2 and 3, the
optical fibers have been embedded in a substrate such as glass or a
silicon. A method of embedding optical fibers in a substrate has been
described in a patent application by H. Hatami-Hanza and V. Benham,
filed in Canada, and entitled, "An Integrated Optical Board Comprising
Integrated Optic Waveguide Circuit Modules". Accordingly optical
fib~~s are first embedded in the substrate in grooves with the desired
shape. The embedded fibers are then affixed and perhaps covered by an
adhesive and annealed to solidify the substrate with the fibers
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embedded therein to provided an assemble similar to that shown in
figure 2. The substrate with embedded optical fibers is then polished
to achieve almost optically fiat surface wherein the cladding or core
of each of the optical fibers is exposed and becomes a constituent of
5 an optically flattened surface. In Figure 2 optical fibers 16 are
shown embedded in parallel grooves on the substrate and in Figure 3 a
long piece of optical fiber 19 has been embedded in the form of a
spiral in a the substrate 18. The assembly in Figure 3 is particularly
useful for writing a long grating on the optical fiber to perform
10 various functions, one such function being dispersion compensation, for
example. Waveguide assemblies as in any of figures 1, 2 and 3 may be
written or otherwise provided with gratings or other elements to
produce a desired optical circuit. The range of sizes of such
waveguide assemblies is practically unlimited. Waveguide assemblies on
substrates of dimensions ranging from a few millimetre to 20
centimetres or so, in rectangular and other forms, are contemplated.
Referring to Figure 4, a waveguide assembly is provided by a substrate
21 and embedded optical fibers having areas 22, 23, 24 and 25, each of
which includes photosensitive material as yet unexposed to ultraviolet
light. The waveguide assembly is shown to be masked by a first mask
wh i ch def i nes a pattern with areas under apertures 28 where opt i ca 1
circuits are to be formed and also defines alignment marks or targets
29 as registering means. The first mask is produced by depositing a
layer 27 of a material which is not transparent to ultraviolet light
upon the substrate 21. The deposited layer can be a metal, such as
chromium or aluminum, or any other suitable material which is not
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transparent to ultraviolet light, for example polymer resists. Next a
pattern is transformed from a pattern mask (not shown) onto the
deposited layer, for example by conventional photolithography or by
electron-beam lithography. The fundamentals of the lithography and the
associated process are described for example in a monograph entitled
"Eximer Laser Photography," by Kanti Jain, published in SPIE, 1989 and
in a book entitled "Introduction to Microelectronics Fabrication,
Molecular Series on Solid State Devices", volume 5, authored by Richard
Jaeger, editored by Gerold W. Neubeck, and Robert F. Pierret, and
published in 1993 by the Addison-Wesley Publishing Company. The
pattern being so transformed provides the desired apertures 28 and
alignment marks 29 within the first mask.
Referring to Figure 5, the grating mask includes a silica substrate 31
with phase gratings 33 and targets or alignment marks 39 so located
within the substrate 31 as to correspond with the apertures 28 and
alignment marks 29 within the first mask. The grating mask is
fabricated for example by direct electron-beam writing over a silica
substrate, masked with a layer of resist or metal, followed by an
associated etching method to create an etched phase grating over the
silica for the desired wavelengths. The fundamentals of electron beam
lithography and processes are described in chapter 5 of "Electron-based
Technology in Microelectronics Fabrication", edited by Goarge Barnere
and published in 1980 by the Academic Press. The grating mask can have
as many phase gratings as can be fabricated over the areas
corresponding to the apertures 28 mask and may include phase gratings
with chirped pitches.
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Figure 6 illustrates the optical waveguide assembly of figure 4 and the
grating mask of figure 5 in vertical alignment preparatory to the
process for writing of grating on the designated areas to form the
desired optical circuits. Gaskets 41, 42, 43, and 44 rest on the first
mask adjacent the four edges of the planar substrate 21, the gasket 44
being obscured by the silica substrate 31. The gaskets 41, 42, 43, and
44 may by made of strips of a compressible material such as Teflon and
support the grating mask spaced closely adjacent the substrate 21.
Referring to Figure 7, elements of figures 4 and 5 are shown in a
process assembly suitable for irradiation with non-coherent ultraviolet
light to provide the optical circuit. The grating mask is brought into
close proximity with the waveguide assembly and placed over the gaskets
in such a way that targets 29 and 39 are in precise alignment ensuring
the satisfactory matching of the phase grating on the corresponding
apertures over the waveguide assembly. The silica substrate 31 is
removably attached to the substrate 21 by resilient wire clips 45 -48
to maintain the aligned relationship. Alternately the substrates may
be attached perhaps with some temporary adhesive or the like.
Thickness of the gaskets 41 - 44 should be substantially uniform and
may be selected to be between 10 to 100 micron depending on the
flatness of the layer 27. The placement of gaskets ensures that the
substrate 31 is affixed spaced from the surface of the layer 27 at a
predetermined distance and that the substrate 21 and the silica
substrate 31 do not move relative to each other once attached in
compression by the wire clips 45 -48. The resulting structure is
relatively impervious to vibration such that the vibration free
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condition over the time for writing grating on the photosensitive
waveguide is greatly relaxed. Exposure can be done in a clean
environment with a broad band ultraviolet light source such as
ultraviolet lamps. The source of the electromagnetic radiation is
advantageously an ultraviolet lamp with large aperture area, so that
the radiation covers at least the whole area of the phase gratings 33
and waveguides therebeneath. After exposer to the ultraviolet light
the grating mask is detached from the resulting optical circuits, and
it is available for reuse in the manufacture of more optical circuits
of substantially similar operating parameters. The optical circuits
remain in the substrate 21 but the layer 27 may be removed or
alternately the layer 27 may be imprinted with one or more additional
patterns by the lithography process before packaging.
Referring to Figures 8 and 9, the processing apparatus illustrated is
useful for tuning the operating parameters of an optical circuit
manufacture as previously disclosed. An optical circuit so
manufactured has the benefit of having the spacial locations of its
physical circuit elements being precisely defined relative to the
targets 29. Thus these circuit elements are easily located for
individual and virtually isolated exposure to a modulated ultraviolet
beam in a post manufacturing process. The apparatus show in figure 8
includes an X - Y motion table 51 for mounting an optical circuit 200
carried in a substrate 210. A beam source of ultraviolet radiation 61
provides a beam via a variable attenuator 62 for modulating the
intensity of the radiation. Thereafter the beam is divided by a beam
splitter 63 so that a portion of the beam is directed toward the X -Y
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table 51 and the remainder of the beam is detected by a detection power
meter 64. An optical source 55 provides light energy in a spectrum in
which the optical circuit 200 is intended to be operable and is shown
connected thereto. A test and wavelength measurement device 56
provides a detection means for receiving light energy from the optical
circuit and generates indications representative of received light
energy. A controller 70 operates dependent upon the indictions
representative of received light energy from the test and wavelength
measurement device 56, beam power detected by the detection power meter
64, and a predetermined data base peculiar to a particular design of
the optical circuit. In accordance with the flow chart in Figure 9,
the controller 70 determines if further radiation of any particular one
of the circuit elements in the optical circuit 200 would favourably
affect the operating characteristics of the optical circuit 200. If
YES, the controller generates X and Y drive signals to manipulate the
X -Y motion table 51 to place the circuit element in the path of the
beam. Then the controller 70 reduces the attenuation of the attenuator
62 to let a predetermined amount of ultraviolet light energy impinge
upon the circuit element. As the operating characteristics are sensed
via the test and wavelength measurement device 56 the controller
adjusts the intensity of radiation in the beam path and eventually
terminates the exposure. Subsequently the controller 70 determines if
another circuit element is adjustable and if such adjustment would
favourably affect the operating characteristics of the optical circuit
200. Dependent upon the data base the controller 70 may move the X -Y
motion table 51 to select another circuit element for similar treatment
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and so on until no further circuit elements are identified as being
adjustable for the intended effect. By this function an individual
optical circuit is finely tuned or adjusted for an intended
application. This final step of adjustment can be performed before or
5 after the packaging and it is envisaged that it may even be performed
by the consumer or user of the optical circuit.
Persons of typical skill in this art and with knowledge of the
foregoing disclosure will become aware of modifications, and variations
which are within the spirit of the invention as defined in the appended
10 claims.
