CA1074431A - Method and apparatus for fabricating optical waveguide grating resonators - Google Patents
Method and apparatus for fabricating optical waveguide grating resonatorsInfo
- Publication number
- CA1074431A CA1074431A CA300,250A CA300250A CA1074431A CA 1074431 A CA1074431 A CA 1074431A CA 300250 A CA300250 A CA 300250A CA 1074431 A CA1074431 A CA 1074431A
- Authority
- CA
- Canada
- Prior art keywords
- high index
- sheet
- photoresist layer
- grating
- optical waveguide
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
- 238000002840 optical waveguide grating Methods 0.000 title claims abstract description 12
- 238000000034 method Methods 0.000 title claims abstract description 11
- 230000003287 optical effect Effects 0.000 claims abstract description 47
- 239000011521 glass Substances 0.000 claims abstract description 30
- 229920002120 photoresistant polymer Polymers 0.000 claims abstract description 29
- 239000000758 substrate Substances 0.000 claims abstract description 29
- VYPSYNLAJGMNEJ-UHFFFAOYSA-N silicon dioxide Inorganic materials O=[Si]=O VYPSYNLAJGMNEJ-UHFFFAOYSA-N 0.000 claims abstract description 18
- 230000001427 coherent effect Effects 0.000 claims abstract description 16
- 230000005855 radiation Effects 0.000 claims abstract description 14
- 239000005350 fused silica glass Substances 0.000 claims description 10
- 230000005540 biological transmission Effects 0.000 claims description 8
- 239000003292 glue Substances 0.000 claims description 7
- 238000005459 micromachining Methods 0.000 claims description 4
- 239000011248 coating agent Substances 0.000 claims description 2
- 238000000576 coating method Methods 0.000 claims description 2
- 238000004519 manufacturing process Methods 0.000 claims description 2
- 238000004026 adhesive bonding Methods 0.000 claims 1
- 239000010453 quartz Substances 0.000 abstract description 8
- 238000010884 ion-beam technique Methods 0.000 abstract description 4
- 230000002452 interceptive effect Effects 0.000 abstract description 3
- UIZLQMLDSWKZGC-UHFFFAOYSA-N cadmium helium Chemical compound [He].[Cd] UIZLQMLDSWKZGC-UHFFFAOYSA-N 0.000 abstract description 2
- 238000003801 milling Methods 0.000 abstract 1
- 230000010363 phase shift Effects 0.000 description 11
- 238000010586 diagram Methods 0.000 description 4
- 230000001419 dependent effect Effects 0.000 description 3
- 238000003780 insertion Methods 0.000 description 3
- 230000037431 insertion Effects 0.000 description 3
- 230000003247 decreasing effect Effects 0.000 description 2
- 238000005498 polishing Methods 0.000 description 2
- 238000000926 separation method Methods 0.000 description 2
- SQGYOTSLMSWVJD-UHFFFAOYSA-N silver(1+) nitrate Chemical compound [Ag+].[O-]N(=O)=O SQGYOTSLMSWVJD-UHFFFAOYSA-N 0.000 description 2
- 241001237728 Precis Species 0.000 description 1
- FOIXSVOLVBLSDH-UHFFFAOYSA-N Silver ion Chemical compound [Ag+] FOIXSVOLVBLSDH-UHFFFAOYSA-N 0.000 description 1
- FKNQFGJONOIPTF-UHFFFAOYSA-N Sodium cation Chemical compound [Na+] FKNQFGJONOIPTF-UHFFFAOYSA-N 0.000 description 1
- 238000010276 construction Methods 0.000 description 1
- 238000000151 deposition Methods 0.000 description 1
- 229930192878 garvin Natural products 0.000 description 1
- 238000012625 in-situ measurement Methods 0.000 description 1
- 238000005342 ion exchange Methods 0.000 description 1
- 230000000737 periodic effect Effects 0.000 description 1
- 229910001961 silver nitrate Inorganic materials 0.000 description 1
- 229910001415 sodium ion Inorganic materials 0.000 description 1
Classifications
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B5/00—Optical elements other than lenses
- G02B5/32—Holograms used as optical elements
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
- G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
- G02B6/12007—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind forming wavelength selective elements, e.g. multiplexer, demultiplexer
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
- G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
- G02B6/122—Basic optical elements, e.g. light-guiding paths
- G02B6/124—Geodesic lenses or integrated gratings
Abstract
METHOD AND APPARATUS FOR FABRICATING
OPTICAL WAVEGUIDE GRATING RESONATORS
Abstract of the Disclosure A helium cadmium laser generates an output beam of radiation which is coupled through a spatial filter and beam expander to a beam splitter which in turn provides two enlarged beams of coherent radiation of approximately equal power. Two highly reflective mirrors are used to direct the two beams such that the two beams interfere and create the usual interference pattern in a positive photoresist layer covering an optical substrate. A thin sheet of high index glass is held by two blocks of quartz in a position adjacent to the photoresist layer and oriented such that a part of each beam of coherent radiation is caused to pass through the high index glass before interfering with the other beam on the photoresist layer. As a result, a grating having a quarter wavelength step or discontinuity in the middle of the grating is fabricated in the photoresist. The photoresist pattern is then transferred to the optical waveguide by ion-beam milling. Several high index sheets can be positioned so as to create more than a single quarter wavelength step.
OPTICAL WAVEGUIDE GRATING RESONATORS
Abstract of the Disclosure A helium cadmium laser generates an output beam of radiation which is coupled through a spatial filter and beam expander to a beam splitter which in turn provides two enlarged beams of coherent radiation of approximately equal power. Two highly reflective mirrors are used to direct the two beams such that the two beams interfere and create the usual interference pattern in a positive photoresist layer covering an optical substrate. A thin sheet of high index glass is held by two blocks of quartz in a position adjacent to the photoresist layer and oriented such that a part of each beam of coherent radiation is caused to pass through the high index glass before interfering with the other beam on the photoresist layer. As a result, a grating having a quarter wavelength step or discontinuity in the middle of the grating is fabricated in the photoresist. The photoresist pattern is then transferred to the optical waveguide by ion-beam milling. Several high index sheets can be positioned so as to create more than a single quarter wavelength step.
Description
~7~
Background of the Invention The invention relates to optical waveguide gratings and more particularly to optical waveguide gratings having a step or discontinuity in the periodi-city of the grating.
As pointed ou-t by ~.A. Haus and C.V. Shank in the article entitled "Antisymmetric Taper of Distributed Feedback Lasers", IEEE Journal of Quantum Electronics, Vol. QE-12, No. 9, December 19, 1976, pages 532-539, a distributed feedback laser with no mode degeneracy can be created by using an optical waveguide grating having a quarter wavelength step or discontinuity in -the middle of the grating. This type of distributed feedback laser is also described in their U.S. Patent No. ~,096,446 entitled "Distributed Feedback Devices with Perturbations Deviating from Uniformity for Removlng Mode Degeneracy", issued June 20, 1978. As pointed out by Messrs. Haus and Shank, this type of distributed feedback laser is preferable in that the output wavelength is entirely predictable unlike the normal distributed feedback laser having a grating with regular periodicity throughout.
One technique presently known to the art for creating an optical waveguide grating is described by G.C. Bjorklund, ~.F. Mullenauer, and W.J. Tomlinson in their U.S. Patent No. 4,093,338, entitIed "Appa~atus ~or Piecewise Generation of Grating-Like Pattern", issued June 6, 1978. This apparatus is also described in their article entitled "Piecewise Interferometric Generation of Precision Gratings", by L.F. Mullenauer and W.J. Tomlinson, Applied Optics, Vol. 16, No. 3, March 1977, pages 555-557.
" . . ' ~ ,':' ',' "
.' , . . ~ ~ '., ~ . ,, ' , : ' . '. ' ,'. ' ' ', -. , , ' ' ,, ", , ~74~3~
In the sjorklund et al apparatus a grating can be generated in a piecewise fashion by moving the interference pattern across an optical substrate having a photoresist layer.
To adopt this technique so as to produce a s-tep or discontinuity in the grating would require the mechanical insertion of a 180 degree phase shifter into the path of one of the coherent beams used to create the grating.
This could only be done ater the interference pattern has been moved so as to not overlap a previously created pattern. The preci'se movement of the two coherent beams and mechanical insertion of the phase shifter are believed to be formidable problems in the creation of an optical waveguide grating having a quarter wavelength step in the structure with minimum separation between the two segments of the grating.
Brief Sum ary of the Inv_ntion A primary object of the present invention is to fabricate an optical waveguide grating having a quarter wavelength step in the grating wlthout requiring mechanical insertion of 180 degree phase shifter during any step of the process.
Another object of the present invention is to create an optical waveguide grating with a minimum separation between the two segments of the grating separated by the quarter wavelength step.
These objects and othexs are provided by the present invention wherein two coherent beams are generated by passing the output from a single laser through an arrangement of a beam splitter and mirrors, and a portion of each one of the two beams is caused to pass through a sheet of high index medium before that portion of the . . , ~ . . .
" :, .
~L~7443~L
beam interferes wi-th the other beam on the photoresist layer of an optical waveguide substrate. In the embodi-ment described in the present specification the sheet of high index medium is a sheet of high index glass which is supported in a position substantially perpendicular to the optical waveguide substrate and is suppor-ted in this position by being sandwiched between two blocks of fused quartz which abut the optical waveguide substrate.
The high index glass introduces a phase shift of +2 or +90 degrees into the radiation passing through it. As a result, the inter~erence pattern created on one side of the thin sheet of high index medium is created by one of the beams interfering with the second beam after the second beam has been phase shifted by 90 degrees, and the interference pattern created on the other side of the high index medium is created by the second beam interfering with the first beam after the first beam has been phase shifted by 90 degrees. The resulting interference pattern which is created in the photoresist layer is developed, and an optical waveguiding grating is created on the substrate through the well known technique of ion beam mlcromachining. This resulting grating has a quarter wavelength step or discontinuity at a location correspondent to the position of the sheet ~-of high index glass.
~.
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' ' :' . :, . .
443~
It is a feature of the present invention that several tandem optical resonators can be created by causing the optical beams to be coupled through a plurality of high index sheets, each one of which is poisitioned substantially perpendicular to the optical waveguide substrate and is oriented such that it is parallel to the adJacent high index sheets.
Brief Description of the Drawings The invention will be more readily understood after .
reading the following detailed description in conjunction with the drawings wherein:
FIG. 1 is a block diagram, partially in pictorial~ of :
an apparatus constructed to utilize the present invention;
FIG. 2 is a diagram of two blocks of fused quartz ~
separated by a sheet of high index glass used in the : :
practice of the present invention; `-FIG. 3 is a magnifled view o~ the interface areas -between the blocks of fused quartz and high index glass; : ;~
FIG. 4 is a pictorial view and ray diagram illustrating the processing of light in the apparatus shown in FIG. l;
FIG. 5 is a plot of transmission versus wavelength for a grating of the type that can be fabricated through use of the present invention; and ~ .
FIG. 6 iS a pictorial view and light ray diagram illustrating the processing of light in an embodiment of the invention wherein a plurality of high index sheets are used to create several tandem optical waveguide gratings.
In accordance with one aspect of the present ~ :~
invention there is provided appparatus for creating an interference pattern in a photoresist layer covering an optical substrate having a waveguiding layer, said ~a~7~L~3~
apparatus comprisinq laser means for developing two coherent beams of radiation that are directed so as to create an interference pattern in a predetermined area of said photoresist layer, and at least one sheet of high refractive index medium positioned adjacent to said predetermined aeea substantially perpendicular to said waveguiding layer and oriented such that only a portion of each of said two beams is caused to pass through said high index sheet before that portion impinges on said photoresist layer, whereby an optical grating resonator can be fabricated in the waveguiding layer of said optical substrate.
In accordance with another aspect of the present invention there is provided a method of fabricating an ~ ~
optical waveguide grating resonator comprising the steps -.
of coating an optical waveguide substrate with a photoresist layer, mechanically positioning and holding at least one sheet of high refractive index medium in a plane substantially perpendicular to said optical waveguide substrater generating two coherent beams of radiation from a single source that are oriented with respect to each other such that an interference pattern can be created in a predetermined area of a predetermined spatial plane, mechanically positioning the integral structure including said optical waveguide substrate and said at least one sheet of high index medium such that said photoresist layer is in said predetermined spatial plane and said . :
predetermined area is on both sides of said sheet of high index medium, developing said photoresist layer, and micromachining a grating in said optical waveguide in a pattern corresponding to said interference pattern.
- ~a - :
. .
~ 6:37~4~
:e~=}l~ ~ ption In FIG. 1, a helium cadmium laser 10 generates a coherent beam 3 of radiation (of approximately ten milliwatts) at a wavelength of 3250 ~. The coherent beam of radiation is coupled to a spatial filter 11 which filters the beam so as to provide at its output a single Gaussian mode of radiation. This single mode of radiation is coupled to a beam expander 12 which enlarges the beam to a diameter of one to ten mm., the particular diameter chosen being dependent on the size of grating to be fabricated. The enlarged beam from beam expander 12 is coupled to a beam splitter 13 which reflects a portion of the power so as to generate a coherent beam 1 and transmits the remainder of the power so as to generate a second coherent beam 2.
Coherent beams 1 and 2 are reflected by mirrors 14 and 15, respectively, such that beams 1 and 2 are directed so as to interfere in a predetermined spatial plane. This predetermined spatial plane corresponds to the plane in which beams 1 and 2 will generate the usual lDterference patte~n necessary to create an optical grating in a photoresist layer. The plane in which beams 1 and 2 - interfere is not simply determined in FIG. 1 by the line - of sight intersection, since beams 1 and 2 are coupled to fused quartz blocks 21 and 20, respectively. In passing into this more dense medium, beams 1 and 2 are refracted and after refraction interfere in a predetermined spatial plane which is coincident with a positive photoresist layer (e.g., Shipley AZ1350B) deposited on the surface of an optical substrate 40.
The predetermined spatial plane at which the interference pattern is created is dependent on the 5 _ 7443~
orien-tation of mirrors 14 and 15. Fach of these mirrors is oriented so as to present an incident angle ~ to their respective incoming beams of radiation. The angle can be changed for each of the mirrors 14 and 15 in order to change the periodicity of the grating being fabricated.
As is well known to those skilled in the art, this change of periodicity also requires a repositioning of the optical substrate 40 so as to ensure that the interference pattern is created in the photoresist layer. The integral i-structure including blocks 20 and 21 and optical substrate 40 can also be rotated about an axis which is perpendicular to beams 1 and 2 in a direction indicated by the angle in FIG. 1. This structure is rotated for reasons to be ~
given hereinafter in connection with the discussion of ~ -the construction of the block structure using optical quartz blocks 20 and 21 in FIG. 1. ;
A separate pictoria] view of the optical quartz blocks 20 and 21 is shown in FIG. 2. In the present embodiment, each of the blocks has height H and width W
equal to one inch and a length equal to two inches.
slOcks 20 and 21 are cemented to a thin in-between high~
index medium to be described hereinafter, thereby creating ~ ~
an overall integrated block length L equal to about four ;
inches. This integrated block is made by first taking - ---individual quartz hlocks and polishing the optical surfaces designated as 22 through 27 in FIG. 2. These highly polished surfaces must be as flat as possible to minimize beam wavefront distortion.
A thin sheet of high index glass measuring one inch by one inch should be optically polished on both one inch by one inch surfaces with a minimum wedge : .... .. . .: . .. . .
~ ~ ~7~43~L
angle. Schott glass types LAK 8 or LA~ 10 are acceptable for this high index glass. Subsec~uent to polishing, the thin sheet of high index glass is cemented using ultra-violet (uv) transmitting glue to the highly polished one inch by one inch surface on either one of the blocks 20 or 21. This sheet of high index glass is then ground using lens grinding techniques until the sheet has a thickness D in the range of 10 to 25 microns. A layer of uv transmitting glue is then applied to the exposed surface of the sheet of high index glass and the other block has its highly polished one inch by one inch surEace cemented to this surface of the high index glass. It is to be understood that the invention is not limited to the use of this particular high index glass. In fact, an organic film such as a plastic could easily provide the necessary high index of refraction and could also be a more rugged self-supporting structure.
The entire preceding operation of creating the integrated block results in a sandwich layer of the type illustrated in FIG. ~, which is an enlarged cutaway portion of the integrated block shown in FIG. 2. As indicated in FIG. 3, the highly polished surface 24 of block 20 is separated from the sheet of high index glass 30 ~ -by an ultraviolet transmitting glue 31. Similarly, optical surface 25 of block 21 is also separated from the high index glass sheet 30 by a layer of ultraviolet transmitting glue 32. The transmitting glue layers 31 and 32 must be m~de as thin as posslble, preferably less than 25 microns, and the glue should be chosen to have an index of lefraction that is as close as posoible to the index : `' "
; ' ':
~ 4~31 of the quartz.
lhe positioning of the optical beams l and 2 with respect to the integrated block structure and the optical substrate is illustrated in FIG. 4. As indlcated in FIG. 4, the block structure using quartz blocks 21 and 20 is positioned such that beams 1 and 2 impinge upon the photoresist layer 41 on both sides of the line along which the sheet of hlgh index glass abuts the photoresist layer. Optical substrate 40 can be made from a simple glass slide that has been immersed in silver nitrate solution. In this process the silver ion exchanges place with the sodium ion of the glass and in so doing Eorms a waveguiding layer on the surface of the glass - slide. A photoresist layer 41 in FIG. 4 has been exaggerated in its dimension in order to illustrate its presence between the optical substrate 40 and the integrated block structure using blocks 20 and 21 and high index sheet 30.
With the block structure positioned as shown in FIG. 4, approximately one-half of each of the beams 1 and 2 passes through the high index sheet 30 before it interferes with the other beam on the photoresist layer. -In~passing through this sheet of high index glass, the ;
light is subjected to a net phase change of approxlmately - '90 degrees, the exact phase shift being dependent on the thickness of the high index sheet. As a result, there is a phase change of about 180 degrees between the interference pattern created on one side of the high index sheet as compared with the interference pattern created on the other side of the high index sheet.
After exposure to the interference pattern, .. ~',"' .
,. . . , , ~, .. . .
~07~3~
the photoresist layer is developed in the usual fashion and an optical grating is created on the surface of the - optical substrate 40 by using the technique of ion beam micromachining. See the article entitled "Ion Beam Micromachining of Integrated Optics Components" by ~oL~ Garvin et al in Applied Optics, Vol. 12, No. 3, March 1973, pages 455-459.
If the high index sheet presents a phase shift of substantially 90 degrees to each beam, the resulting grating will have a perfectly symmetrical transmission characteristic around the resonant wavelength of the grating. The resulting transmission grating can be checked for symmetry by connecting an optical source to one end of the waveguiding layer created in optical substrate 40 and measuring the output provided at the other end of substrate 40. If the desired one-quarter : wavelength step has been created in the grating, a transmission characteristic of the type shown in FIG. 5 will result from this me~surement. As shown in FIG. 5, the grating with the one-quarter wavelength step provides a narrow bandpass characteristic around a single ~avelength ~0.
The characteris~ic is only achieved with : perfect symmetry if the high index sheet.30 provides a phase shif* of +90 degrees to both of the coherent beams 1 .~
and 2. A sheet thickness which provides phase shift of .
other than 90 degrees can still be caused to produce the :~ ;
symmetrical characteristic by rotating the sheet 30 ~
about an axis perpendicular to beams 1 and 2 in the -direction illustrated by the angle ~ in FIG. 1. Such a rotation will present more of the high index medium to .' ~
.. ...
~7~L4~
one of the beams while decreasing the amount of high index mediurn presented to the other beam. A rotation of several degrees in ~ will change the amount of phase shift presented by 180 degrees. In addition, the phase shift introduced by rotation is a nonlinear function of the change in ~ and therefore the amount of phase shift added to one beam does not equal the amount of decreased phase shift in the other beam. Consequently, sheet 30 and optical substrate 40 can be rotated about an a~is perpendicular to beams 1 and 2 to achieve a 180 degree phase shift in the created interference pattern with a rather large range of permitted thickness (D) for high index sheet 30. Sheet 30 should, of course, be kept as small in thickness (D) as possible to minimize the dimension of the introduced quarter wavelength step. -Inasmuch as it is extremely difficult to provide -precisely the right thickness of high index sheet to achieve a 90 degree phase shift, the usual procedure of obtaining a symmetrical transmission characteristic is to expose several optical substrates at different values for the angle designated as ~ in FIG. 1. A plot can then be made of the position of the bandpass peak 51 with respect to the angle ~ and this plot can then be utilized to determine the precise angle at which the substrate 40 must be positioned in order to obtain a perfectly symmetrical transmission characteristic.
The type of gratiny which has been described thus far is useful in connection with a distributed feedback laser of the type described in the above-identified ;
30 U.S. Patent No. 4,096,446. This type grating has a single ~
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,. , , . . . . . ~
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step or discontinuity in the middle of the grating. A
grating with several one-quarter wavelength steps separated from each other by a predetermined interval of regular periodic grating is useful in providing a series of tandem resonant structures at optical frequencies. This latter type grating with multiple one~quarter wavelength steps can be fabricated by using an optical block structure of the type illustrated in FIG. 6.
In FIG. 6, blocks 65 and 66 are similar to blocks 20 and 21 in FIG. 4, but are separated by a sandwich structure using three sheets of high index medium 60, 61 and 62 which are in turn separated from each other by smaller blocks of fused quartz designated as 63 and 64 in FIG. 6. As illustrated in FIG. 6, beams 1 and 2 in this case encounter several phase shifts of 90 degrees in -passing through the multiple layers of high index medium.
The resulting grating will have one-~uarter wavelength steps at positions in the grating corresponding to the lines along which sheets oE high index medium 60, 61 and 62 are adjacent to the photoresist layer 41.
In the structure of FIG. 6, which uses multiple ~sheets of high index medium, strict control must be main-tained over the thickness of the high index sheets.
Consequently, mediums other than glass are preferred in this embodiment. The sheets of high index medium having accurately controlled thickness can be established on -~
blocks o~ quartz by using a deposition technique~with in situ measurement of the deposited film thickness. ~-It should be apparent to those skilled in the art that the invention is in no way limited as to - 11 - . ., , .
,, . : :, .
7q~43~L
the number of one-quarter wavelength steps that can be created in the grating. Additional steps simply require -the placement of additional sheets of high index glass in the transmission paths encountered by beams l and 2.
1~7hat has been described hereinabove is an lllustrative embodimen~ of the present invention.
I~umerous departures may be made by those skilled in the art without departing from the spirit and scope of the present invention. For example, means other than the fused quartz blocks can be utilized to support the sheets of high index glass. In fact, the sheets of high index glass can be made self-supporting with the proper mechanical structure. The optical quartz blocks are simply used in the present embodiment to provide a supporting structure for the high index glass.
., :
, .
Background of the Invention The invention relates to optical waveguide gratings and more particularly to optical waveguide gratings having a step or discontinuity in the periodi-city of the grating.
As pointed ou-t by ~.A. Haus and C.V. Shank in the article entitled "Antisymmetric Taper of Distributed Feedback Lasers", IEEE Journal of Quantum Electronics, Vol. QE-12, No. 9, December 19, 1976, pages 532-539, a distributed feedback laser with no mode degeneracy can be created by using an optical waveguide grating having a quarter wavelength step or discontinuity in -the middle of the grating. This type of distributed feedback laser is also described in their U.S. Patent No. ~,096,446 entitled "Distributed Feedback Devices with Perturbations Deviating from Uniformity for Removlng Mode Degeneracy", issued June 20, 1978. As pointed out by Messrs. Haus and Shank, this type of distributed feedback laser is preferable in that the output wavelength is entirely predictable unlike the normal distributed feedback laser having a grating with regular periodicity throughout.
One technique presently known to the art for creating an optical waveguide grating is described by G.C. Bjorklund, ~.F. Mullenauer, and W.J. Tomlinson in their U.S. Patent No. 4,093,338, entitIed "Appa~atus ~or Piecewise Generation of Grating-Like Pattern", issued June 6, 1978. This apparatus is also described in their article entitled "Piecewise Interferometric Generation of Precision Gratings", by L.F. Mullenauer and W.J. Tomlinson, Applied Optics, Vol. 16, No. 3, March 1977, pages 555-557.
" . . ' ~ ,':' ',' "
.' , . . ~ ~ '., ~ . ,, ' , : ' . '. ' ,'. ' ' ', -. , , ' ' ,, ", , ~74~3~
In the sjorklund et al apparatus a grating can be generated in a piecewise fashion by moving the interference pattern across an optical substrate having a photoresist layer.
To adopt this technique so as to produce a s-tep or discontinuity in the grating would require the mechanical insertion of a 180 degree phase shifter into the path of one of the coherent beams used to create the grating.
This could only be done ater the interference pattern has been moved so as to not overlap a previously created pattern. The preci'se movement of the two coherent beams and mechanical insertion of the phase shifter are believed to be formidable problems in the creation of an optical waveguide grating having a quarter wavelength step in the structure with minimum separation between the two segments of the grating.
Brief Sum ary of the Inv_ntion A primary object of the present invention is to fabricate an optical waveguide grating having a quarter wavelength step in the grating wlthout requiring mechanical insertion of 180 degree phase shifter during any step of the process.
Another object of the present invention is to create an optical waveguide grating with a minimum separation between the two segments of the grating separated by the quarter wavelength step.
These objects and othexs are provided by the present invention wherein two coherent beams are generated by passing the output from a single laser through an arrangement of a beam splitter and mirrors, and a portion of each one of the two beams is caused to pass through a sheet of high index medium before that portion of the . . , ~ . . .
" :, .
~L~7443~L
beam interferes wi-th the other beam on the photoresist layer of an optical waveguide substrate. In the embodi-ment described in the present specification the sheet of high index medium is a sheet of high index glass which is supported in a position substantially perpendicular to the optical waveguide substrate and is suppor-ted in this position by being sandwiched between two blocks of fused quartz which abut the optical waveguide substrate.
The high index glass introduces a phase shift of +2 or +90 degrees into the radiation passing through it. As a result, the inter~erence pattern created on one side of the thin sheet of high index medium is created by one of the beams interfering with the second beam after the second beam has been phase shifted by 90 degrees, and the interference pattern created on the other side of the high index medium is created by the second beam interfering with the first beam after the first beam has been phase shifted by 90 degrees. The resulting interference pattern which is created in the photoresist layer is developed, and an optical waveguiding grating is created on the substrate through the well known technique of ion beam mlcromachining. This resulting grating has a quarter wavelength step or discontinuity at a location correspondent to the position of the sheet ~-of high index glass.
~.
: ', ' ..
' ' :' . :, . .
443~
It is a feature of the present invention that several tandem optical resonators can be created by causing the optical beams to be coupled through a plurality of high index sheets, each one of which is poisitioned substantially perpendicular to the optical waveguide substrate and is oriented such that it is parallel to the adJacent high index sheets.
Brief Description of the Drawings The invention will be more readily understood after .
reading the following detailed description in conjunction with the drawings wherein:
FIG. 1 is a block diagram, partially in pictorial~ of :
an apparatus constructed to utilize the present invention;
FIG. 2 is a diagram of two blocks of fused quartz ~
separated by a sheet of high index glass used in the : :
practice of the present invention; `-FIG. 3 is a magnifled view o~ the interface areas -between the blocks of fused quartz and high index glass; : ;~
FIG. 4 is a pictorial view and ray diagram illustrating the processing of light in the apparatus shown in FIG. l;
FIG. 5 is a plot of transmission versus wavelength for a grating of the type that can be fabricated through use of the present invention; and ~ .
FIG. 6 iS a pictorial view and light ray diagram illustrating the processing of light in an embodiment of the invention wherein a plurality of high index sheets are used to create several tandem optical waveguide gratings.
In accordance with one aspect of the present ~ :~
invention there is provided appparatus for creating an interference pattern in a photoresist layer covering an optical substrate having a waveguiding layer, said ~a~7~L~3~
apparatus comprisinq laser means for developing two coherent beams of radiation that are directed so as to create an interference pattern in a predetermined area of said photoresist layer, and at least one sheet of high refractive index medium positioned adjacent to said predetermined aeea substantially perpendicular to said waveguiding layer and oriented such that only a portion of each of said two beams is caused to pass through said high index sheet before that portion impinges on said photoresist layer, whereby an optical grating resonator can be fabricated in the waveguiding layer of said optical substrate.
In accordance with another aspect of the present invention there is provided a method of fabricating an ~ ~
optical waveguide grating resonator comprising the steps -.
of coating an optical waveguide substrate with a photoresist layer, mechanically positioning and holding at least one sheet of high refractive index medium in a plane substantially perpendicular to said optical waveguide substrater generating two coherent beams of radiation from a single source that are oriented with respect to each other such that an interference pattern can be created in a predetermined area of a predetermined spatial plane, mechanically positioning the integral structure including said optical waveguide substrate and said at least one sheet of high index medium such that said photoresist layer is in said predetermined spatial plane and said . :
predetermined area is on both sides of said sheet of high index medium, developing said photoresist layer, and micromachining a grating in said optical waveguide in a pattern corresponding to said interference pattern.
- ~a - :
. .
~ 6:37~4~
:e~=}l~ ~ ption In FIG. 1, a helium cadmium laser 10 generates a coherent beam 3 of radiation (of approximately ten milliwatts) at a wavelength of 3250 ~. The coherent beam of radiation is coupled to a spatial filter 11 which filters the beam so as to provide at its output a single Gaussian mode of radiation. This single mode of radiation is coupled to a beam expander 12 which enlarges the beam to a diameter of one to ten mm., the particular diameter chosen being dependent on the size of grating to be fabricated. The enlarged beam from beam expander 12 is coupled to a beam splitter 13 which reflects a portion of the power so as to generate a coherent beam 1 and transmits the remainder of the power so as to generate a second coherent beam 2.
Coherent beams 1 and 2 are reflected by mirrors 14 and 15, respectively, such that beams 1 and 2 are directed so as to interfere in a predetermined spatial plane. This predetermined spatial plane corresponds to the plane in which beams 1 and 2 will generate the usual lDterference patte~n necessary to create an optical grating in a photoresist layer. The plane in which beams 1 and 2 - interfere is not simply determined in FIG. 1 by the line - of sight intersection, since beams 1 and 2 are coupled to fused quartz blocks 21 and 20, respectively. In passing into this more dense medium, beams 1 and 2 are refracted and after refraction interfere in a predetermined spatial plane which is coincident with a positive photoresist layer (e.g., Shipley AZ1350B) deposited on the surface of an optical substrate 40.
The predetermined spatial plane at which the interference pattern is created is dependent on the 5 _ 7443~
orien-tation of mirrors 14 and 15. Fach of these mirrors is oriented so as to present an incident angle ~ to their respective incoming beams of radiation. The angle can be changed for each of the mirrors 14 and 15 in order to change the periodicity of the grating being fabricated.
As is well known to those skilled in the art, this change of periodicity also requires a repositioning of the optical substrate 40 so as to ensure that the interference pattern is created in the photoresist layer. The integral i-structure including blocks 20 and 21 and optical substrate 40 can also be rotated about an axis which is perpendicular to beams 1 and 2 in a direction indicated by the angle in FIG. 1. This structure is rotated for reasons to be ~
given hereinafter in connection with the discussion of ~ -the construction of the block structure using optical quartz blocks 20 and 21 in FIG. 1. ;
A separate pictoria] view of the optical quartz blocks 20 and 21 is shown in FIG. 2. In the present embodiment, each of the blocks has height H and width W
equal to one inch and a length equal to two inches.
slOcks 20 and 21 are cemented to a thin in-between high~
index medium to be described hereinafter, thereby creating ~ ~
an overall integrated block length L equal to about four ;
inches. This integrated block is made by first taking - ---individual quartz hlocks and polishing the optical surfaces designated as 22 through 27 in FIG. 2. These highly polished surfaces must be as flat as possible to minimize beam wavefront distortion.
A thin sheet of high index glass measuring one inch by one inch should be optically polished on both one inch by one inch surfaces with a minimum wedge : .... .. . .: . .. . .
~ ~ ~7~43~L
angle. Schott glass types LAK 8 or LA~ 10 are acceptable for this high index glass. Subsec~uent to polishing, the thin sheet of high index glass is cemented using ultra-violet (uv) transmitting glue to the highly polished one inch by one inch surface on either one of the blocks 20 or 21. This sheet of high index glass is then ground using lens grinding techniques until the sheet has a thickness D in the range of 10 to 25 microns. A layer of uv transmitting glue is then applied to the exposed surface of the sheet of high index glass and the other block has its highly polished one inch by one inch surEace cemented to this surface of the high index glass. It is to be understood that the invention is not limited to the use of this particular high index glass. In fact, an organic film such as a plastic could easily provide the necessary high index of refraction and could also be a more rugged self-supporting structure.
The entire preceding operation of creating the integrated block results in a sandwich layer of the type illustrated in FIG. ~, which is an enlarged cutaway portion of the integrated block shown in FIG. 2. As indicated in FIG. 3, the highly polished surface 24 of block 20 is separated from the sheet of high index glass 30 ~ -by an ultraviolet transmitting glue 31. Similarly, optical surface 25 of block 21 is also separated from the high index glass sheet 30 by a layer of ultraviolet transmitting glue 32. The transmitting glue layers 31 and 32 must be m~de as thin as posslble, preferably less than 25 microns, and the glue should be chosen to have an index of lefraction that is as close as posoible to the index : `' "
; ' ':
~ 4~31 of the quartz.
lhe positioning of the optical beams l and 2 with respect to the integrated block structure and the optical substrate is illustrated in FIG. 4. As indlcated in FIG. 4, the block structure using quartz blocks 21 and 20 is positioned such that beams 1 and 2 impinge upon the photoresist layer 41 on both sides of the line along which the sheet of hlgh index glass abuts the photoresist layer. Optical substrate 40 can be made from a simple glass slide that has been immersed in silver nitrate solution. In this process the silver ion exchanges place with the sodium ion of the glass and in so doing Eorms a waveguiding layer on the surface of the glass - slide. A photoresist layer 41 in FIG. 4 has been exaggerated in its dimension in order to illustrate its presence between the optical substrate 40 and the integrated block structure using blocks 20 and 21 and high index sheet 30.
With the block structure positioned as shown in FIG. 4, approximately one-half of each of the beams 1 and 2 passes through the high index sheet 30 before it interferes with the other beam on the photoresist layer. -In~passing through this sheet of high index glass, the ;
light is subjected to a net phase change of approxlmately - '90 degrees, the exact phase shift being dependent on the thickness of the high index sheet. As a result, there is a phase change of about 180 degrees between the interference pattern created on one side of the high index sheet as compared with the interference pattern created on the other side of the high index sheet.
After exposure to the interference pattern, .. ~',"' .
,. . . , , ~, .. . .
~07~3~
the photoresist layer is developed in the usual fashion and an optical grating is created on the surface of the - optical substrate 40 by using the technique of ion beam micromachining. See the article entitled "Ion Beam Micromachining of Integrated Optics Components" by ~oL~ Garvin et al in Applied Optics, Vol. 12, No. 3, March 1973, pages 455-459.
If the high index sheet presents a phase shift of substantially 90 degrees to each beam, the resulting grating will have a perfectly symmetrical transmission characteristic around the resonant wavelength of the grating. The resulting transmission grating can be checked for symmetry by connecting an optical source to one end of the waveguiding layer created in optical substrate 40 and measuring the output provided at the other end of substrate 40. If the desired one-quarter : wavelength step has been created in the grating, a transmission characteristic of the type shown in FIG. 5 will result from this me~surement. As shown in FIG. 5, the grating with the one-quarter wavelength step provides a narrow bandpass characteristic around a single ~avelength ~0.
The characteris~ic is only achieved with : perfect symmetry if the high index sheet.30 provides a phase shif* of +90 degrees to both of the coherent beams 1 .~
and 2. A sheet thickness which provides phase shift of .
other than 90 degrees can still be caused to produce the :~ ;
symmetrical characteristic by rotating the sheet 30 ~
about an axis perpendicular to beams 1 and 2 in the -direction illustrated by the angle ~ in FIG. 1. Such a rotation will present more of the high index medium to .' ~
.. ...
~7~L4~
one of the beams while decreasing the amount of high index mediurn presented to the other beam. A rotation of several degrees in ~ will change the amount of phase shift presented by 180 degrees. In addition, the phase shift introduced by rotation is a nonlinear function of the change in ~ and therefore the amount of phase shift added to one beam does not equal the amount of decreased phase shift in the other beam. Consequently, sheet 30 and optical substrate 40 can be rotated about an a~is perpendicular to beams 1 and 2 to achieve a 180 degree phase shift in the created interference pattern with a rather large range of permitted thickness (D) for high index sheet 30. Sheet 30 should, of course, be kept as small in thickness (D) as possible to minimize the dimension of the introduced quarter wavelength step. -Inasmuch as it is extremely difficult to provide -precisely the right thickness of high index sheet to achieve a 90 degree phase shift, the usual procedure of obtaining a symmetrical transmission characteristic is to expose several optical substrates at different values for the angle designated as ~ in FIG. 1. A plot can then be made of the position of the bandpass peak 51 with respect to the angle ~ and this plot can then be utilized to determine the precise angle at which the substrate 40 must be positioned in order to obtain a perfectly symmetrical transmission characteristic.
The type of gratiny which has been described thus far is useful in connection with a distributed feedback laser of the type described in the above-identified ;
30 U.S. Patent No. 4,096,446. This type grating has a single ~
- 10- ,,,``
, ,:. ' ' ~
,. , , . . . . . ~
: . ., ~ , : . .
.:
~4~3~ :
step or discontinuity in the middle of the grating. A
grating with several one-quarter wavelength steps separated from each other by a predetermined interval of regular periodic grating is useful in providing a series of tandem resonant structures at optical frequencies. This latter type grating with multiple one~quarter wavelength steps can be fabricated by using an optical block structure of the type illustrated in FIG. 6.
In FIG. 6, blocks 65 and 66 are similar to blocks 20 and 21 in FIG. 4, but are separated by a sandwich structure using three sheets of high index medium 60, 61 and 62 which are in turn separated from each other by smaller blocks of fused quartz designated as 63 and 64 in FIG. 6. As illustrated in FIG. 6, beams 1 and 2 in this case encounter several phase shifts of 90 degrees in -passing through the multiple layers of high index medium.
The resulting grating will have one-~uarter wavelength steps at positions in the grating corresponding to the lines along which sheets oE high index medium 60, 61 and 62 are adjacent to the photoresist layer 41.
In the structure of FIG. 6, which uses multiple ~sheets of high index medium, strict control must be main-tained over the thickness of the high index sheets.
Consequently, mediums other than glass are preferred in this embodiment. The sheets of high index medium having accurately controlled thickness can be established on -~
blocks o~ quartz by using a deposition technique~with in situ measurement of the deposited film thickness. ~-It should be apparent to those skilled in the art that the invention is in no way limited as to - 11 - . ., , .
,, . : :, .
7q~43~L
the number of one-quarter wavelength steps that can be created in the grating. Additional steps simply require -the placement of additional sheets of high index glass in the transmission paths encountered by beams l and 2.
1~7hat has been described hereinabove is an lllustrative embodimen~ of the present invention.
I~umerous departures may be made by those skilled in the art without departing from the spirit and scope of the present invention. For example, means other than the fused quartz blocks can be utilized to support the sheets of high index glass. In fact, the sheets of high index glass can be made self-supporting with the proper mechanical structure. The optical quartz blocks are simply used in the present embodiment to provide a supporting structure for the high index glass.
., :
, .
Claims (7)
1. Apparatus for creating an interference pattern in a photoresist layer covering an optical substrate having a waveguiding layer, said apparatus comprising laser means for developing two coherent beams of radiation that are directed so as to create an interference pattern in a predetermined area of said photoresist layer, and at least one sheet of high refractive index medium positioned adjacent to said predetermined area substantially perpendicular to said waveguiding layer and oriented such that only a portion of each of said two beams is caused to pass through said high index sheet before that portion impinges on said photoresist layer, whereby an optical grating resonator can be fabricated in the waveguiding layer of said optical substrate.
2. Apparatus as defined in claim 1 wherein said apparatus further includes two blocks of fused quartz, and said at least one sheet of high index medium is a sheet of high index glass mechanically held in its position by being sandwiched between said two blocks of fused quartz.
3. Apparatus as defined in claim 2 wherein each one of said two blocks of fused quartz is adhered to said sheet of high index glass by a layer of ultraviolet transmitting glue.
4. Apparatus as defined in claim 1 wherein said optical substrate and said at least one sheet of high index medium are part of an integral unit that can be rotated about an axis that is perpendicular to the transmission paths of both of said two beams of coherent radiation.
5. Apparatus for creating an interference pattern as defined in claim 1 wherein said apparatus includes several sheets of high index medium positioned adjacent to said predetermined area substantially perpendicular to said waveguiding layer.
6. A method of fabricating an optical waveguide grating resonator comprising the steps of coating an optical waveguide substrate with a photoresist layer, mechanically positioning and holding at least one sheet of high refractive index medium in a plane substantially perpendicular to said optical waveguide substrate, generating two coherent beams of radiation from a single source that are oriented with respect to each other such that an interference pattern can be created in a predetermined area of a predetermined spatial plane, mechanically positioning the integral structure including said optical waveguide substrate and said at least one sheet of high index medium such that said photoresist layer is in said predetermined spatial plane and said predetermined area is on both sides of said sheet of high index medium, developing said photoresist layer, and micromachining a grating in said optical waveguide in a pattern corresponding to said interference pattern.
7. A method as defined in claim 6 wherein the step of mechanically holding and positioning said at least one sheet of high index medium includes a step of gluing a block of optical fused quartz to opposite surfaces of a sheet of high index glass.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US05/800,651 US4093339A (en) | 1977-05-26 | 1977-05-26 | Method and apparatus for fabricating optical waveguide grating resonators |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1074431A true CA1074431A (en) | 1980-03-25 |
Family
ID=25178976
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA300,250A Expired CA1074431A (en) | 1977-05-26 | 1978-03-31 | Method and apparatus for fabricating optical waveguide grating resonators |
Country Status (10)
| Country | Link |
|---|---|
| US (1) | US4093339A (en) |
| JP (1) | JPS6060042B2 (en) |
| BE (1) | BE867225A (en) |
| CA (1) | CA1074431A (en) |
| DE (1) | DE2822571A1 (en) |
| FR (1) | FR2392404A1 (en) |
| GB (1) | GB1602667A (en) |
| IT (1) | IT1095903B (en) |
| NL (1) | NL7805590A (en) |
| SE (1) | SE431033B (en) |
Families Citing this family (14)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR2426922A1 (en) * | 1978-05-26 | 1979-12-21 | Thomson Csf | COMPACT OPTICAL STRUCTURE WITH INTEGRATED SOURCE |
| US4412719A (en) * | 1981-04-10 | 1983-11-01 | Environmental Research Institute Of Michigan | Method and article having predetermined net reflectance characteristics |
| JPS6011802A (en) * | 1983-06-30 | 1985-01-22 | Ricoh Co Ltd | Transmission type surface relief diffraction grating |
| WO1986001303A1 (en) * | 1984-08-13 | 1986-02-27 | United Technologies Corporation | Method for impressing grating within fiber optics |
| GB2209408B (en) * | 1987-09-04 | 1991-08-21 | Plessey Co Plc | Optical waveguide device having surface relief diffraction grating |
| GB2212938A (en) * | 1987-12-10 | 1989-08-02 | Le I Yadernoi Fiz Im B P Konst | Apparatus for recording a radial diffraction grating |
| GB8904320D0 (en) * | 1989-02-24 | 1989-07-05 | Pilkington Plc | Holographic exposure method and apparatus |
| US5066133A (en) * | 1990-10-18 | 1991-11-19 | United Technologies Corporation | Extended length embedded Bragg grating manufacturing method and arrangement |
| US5598300A (en) * | 1995-06-05 | 1997-01-28 | Board Of Regents, The University Of Texas System | Efficient bandpass reflection and transmission filters with low sidebands based on guided-mode resonance effects |
| DE69822362T2 (en) * | 1997-05-29 | 2005-02-17 | Corning Inc. | Space filter for a high power laser beam |
| CA2217806A1 (en) * | 1997-10-07 | 1999-04-07 | Mark Farries | Grating and method of providing a grating in an ion diffused waveguide |
| TW538258B (en) * | 2002-07-16 | 2003-06-21 | Ind Tech Res Inst | Grating interferometer device with tunable resolution |
| US6904202B1 (en) * | 2002-07-31 | 2005-06-07 | Intel Corporation | Writing waveguides with an arbitrary Bragg wavelength |
| US7508132B2 (en) * | 2003-10-20 | 2009-03-24 | Hewlett-Packard Development Company, L.P. | Device having a getter structure and a photomask |
Family Cites Families (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB1088838A (en) * | 1964-11-19 | 1967-10-25 | Nat Res Dev | Diffraction grating |
| US3622220A (en) * | 1969-10-08 | 1971-11-23 | Bell Telephone Labor Inc | Holographic optical polarizers and beam splitters |
| US3977763A (en) * | 1972-06-07 | 1976-08-31 | Thomson-Csf | Coupling device for an optical waveguide and a method for manufacturing such a device |
-
1977
- 1977-05-26 US US05/800,651 patent/US4093339A/en not_active Expired - Lifetime
-
1978
- 1978-03-31 CA CA300,250A patent/CA1074431A/en not_active Expired
- 1978-05-18 BE BE187823A patent/BE867225A/en not_active IP Right Cessation
- 1978-05-22 GB GB21140/78A patent/GB1602667A/en not_active Expired
- 1978-05-22 FR FR7815048A patent/FR2392404A1/en active Granted
- 1978-05-23 IT IT23702/78A patent/IT1095903B/en active
- 1978-05-23 NL NL7805590A patent/NL7805590A/en not_active Application Discontinuation
- 1978-05-24 DE DE19782822571 patent/DE2822571A1/en not_active Withdrawn
- 1978-05-24 SE SE7805934A patent/SE431033B/en unknown
- 1978-05-26 JP JP53062394A patent/JPS6060042B2/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| IT1095903B (en) | 1985-08-17 |
| SE7805934L (en) | 1978-11-27 |
| JPS6060042B2 (en) | 1985-12-27 |
| IT7823702A0 (en) | 1978-05-23 |
| GB1602667A (en) | 1981-11-11 |
| JPS53147555A (en) | 1978-12-22 |
| DE2822571A1 (en) | 1978-11-30 |
| FR2392404A1 (en) | 1978-12-22 |
| SE431033B (en) | 1983-12-27 |
| FR2392404B1 (en) | 1980-10-31 |
| NL7805590A (en) | 1978-11-28 |
| BE867225A (en) | 1978-09-18 |
| US4093339A (en) | 1978-06-06 |
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