CA2214927A1 - Methods for making phase masks with spatial variable first order efficiency for fiber bragg grating fabrication - Google Patents
Methods for making phase masks with spatial variable first order efficiency for fiber bragg grating fabricationInfo
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- CA2214927A1 CA2214927A1 CA 2214927 CA2214927A CA2214927A1 CA 2214927 A1 CA2214927 A1 CA 2214927A1 CA 2214927 CA2214927 CA 2214927 CA 2214927 A CA2214927 A CA 2214927A CA 2214927 A1 CA2214927 A1 CA 2214927A1
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- grating
- fiber bragg
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- photoresist
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- 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
- G02B6/02—Optical fibres with cladding with or without a coating
- G02B6/02057—Optical fibres with cladding with or without a coating comprising gratings
- G02B6/02076—Refractive index modulation gratings, e.g. Bragg gratings
- G02B6/02123—Refractive index modulation gratings, e.g. Bragg gratings characterised by the method of manufacture of the grating
- G02B6/02133—Refractive index modulation gratings, e.g. Bragg gratings characterised by the method of manufacture of the grating using beam interference
- G02B6/02138—Refractive index modulation gratings, e.g. Bragg gratings characterised by the method of manufacture of the grating using beam interference based on illuminating a phase mask
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B5/00—Optical elements other than lenses
- G02B5/18—Diffraction gratings
- G02B5/1847—Manufacturing methods
- G02B5/1857—Manufacturing methods using exposure or etching means, e.g. holography, photolithography, exposure to electron or ion beams
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- Engineering & Computer Science (AREA)
- Manufacturing & Machinery (AREA)
- General Physics & Mathematics (AREA)
- Optics & Photonics (AREA)
- Diffracting Gratings Or Hologram Optical Elements (AREA)
- Optical Integrated Circuits (AREA)
Description
Patent name: Methods for making phase masks with spatial variable f~lrst order ef~lciency for ~Iber Bragg grating fabrication Background:
UV light can induce a permanent refractive index change in some kind optical fibers and optical wave guides. The photosensitivity of the certain kind optical fiber waveguide can be used to make Bragg gratings, which is a permanent, spatially periodic refractive index modulation along the length of the photosensitive core of the optical fiber or optical wave guide. Fiber Bragg gratings have many applications in optical fiber telecommunication, optical sensor and optical information process. Fiber Bragg gratings can be used as bandpass reflective filters, dense wavelength division multiplexing(DWDM) add and drop filters, dispersion compensation, and optical fiber sensors.
There are two methods of side writing Bragg gratings in optical fiber and optical wave guide, one of them is holography approach and another one is phase mask approach.
Holography approach is using two beam interference to form a periodic light intensity modulation along optical fiber or optical wave guide. The period of the pattern is controlled by controlling the angle between the interfering beams. Holography approach is described in U.S. patent application Ser. No. 4,474,427 invented by K.O.Hill, B.S.Kavassaki, D.C. Johnson, and Yoshimassa Fujii and in U.S. patent application Ser.
No. 4,807,950 invented by Glenn et al. Phase mask method is the most common way to fabricate Bragg grating in optical fiber and optical wave guide. A phase mask is an uniform or chirped grating in fused silica glass substrate. Optical fibers or optical wave guide is placed close or near close under phase mask. When a UV light illuminates the phase mask, +l and -I diffraction orders of the grating in phase mask will interference each other along the fiber or wave guide core and a refractive index change in optical fiber or optical wave guide is formed. Phase mask method is very robust and insensitive to the environment conditions such as vibration. Phase mask method of fabricating such Bragg gratings is described in U.S. patent application Ser. No. 969,774 invented by K.O.Hill, B.Malo, F.Bilodeau and D. Johnson and entitled METHOD OF
FABRICATING BRAGG GRATINGS USING A SILICA GLASS PHASE GRATINGS
MASK.
Phase masks can be made by holography approach or by electron beam direct write approach. To make a phase mask by holography approach is very similar as writing a grating on optical fiber. First two interference beams are used to form a sinusoid intensity modulation on a layer of photoresist coated on a fused silica glass substrate. After developing a sinusoid profile on the photoresist layer is formed. Then a dry etching is implemented to transfer the sinusoid profile on the photoresist into the fused silica glass substrate underneath to make a phase mask. The another method to make phase masks is using electron beam lithography machine to write very fine lines on a photoresist layer on the top of a Chromium layer with fused silica glass substrate plate. After developing a very fine pattern on the photoresist layer is formed. The open Chromium on the photoresist pattern layer is removed by a wet etching process and the left photoresist is cleaned to get a Chromium pattern on the fused silica glass substrate. A dry etching process is used to transfer the fine line patterns into fused silica glass substrate to make a phase mask. The electron beam direct write approach is a standard photomask making process in microelectronic industry and t'ollowed by a dry etching process.
Fiber Bragg gratings written by a phase mask with an uniform speed laser scanning will have a main peak in the center reflection spectrum accompanied by a series of side lobes at adjacent wavelengths. It is very important to lower the reflectivity of these side lobes or 'apodise' the reflection spectrum of the fiber Bragg grating in these optical fiber devices where high rejection of the nonresonant light is required. DWDM systems require device that can isolate channels that are spaced by only lOOGHz(0.8nm at1~50nm wavelength). Fiber Bragg gratings are the promise components in the DWDM
systems as optical signal channel adder( input channel ) and optical signal channel dropper( output channel ). For example that one optical wavelength channel will be dropped from optical fiber should have no effect on other optical wavelength channels.
A high main peak in the center wavelength reflection spectrum with very low side lobes at adjacent wavelengths is very important for DWDM fiber Bragg gratings.(reference 1) Another benefit of the reflection spectrum apodisation of fiber Bragg gratings is the improvement of the dispersion compensation characteristics of chirped Bragg gratings.
(reference 2) Both holographic approa~h and phase mask approach can make apodised fiber Bragg gratings. The holographic approach uses two laser beams with intensity variation from beam center to the beam edge (for example Gaussian beam intensity profile) to write fiber Bragg gratings. The two laser beams will make an intensity modulation grating along the fiber cc re so the apodised fiber Bragg grating was made. This kind apodised fiber Bragg grating is not "pure apodisation" since not only the refractive index grating modulation is apodised but also the average induced refractive index is not constant and has a spatial profile along the fiber core. This kind apodisation approach can suppress the side lobes in the spectral response of the fiber Bragg gratings. But the fiber Bragg gratings will have a fine structure on the short wavelength side of their reflection response curve which is particularly strong in high reflectivity gratings.(reference 3) Phase mask approach can also make spodised fiber Bragg gratings by varying exposure time(varying scanning speed) along the length of the fiber Bragg grating. A longer exposure time(slower scanning speed) can induce a larger local refractive index modulation grating and a short exposure time(faster scanning speed) will induce a small local refractive index modulation grating. The apodised fiber Bragg gratings made with this approach will have the same problem as holographic approach does: not only refractive index is modulated but also the average induced refractive index varies along the fiber Bragg grating length. The same problem mentioned above, the fine structure on the short wavelength side of the grating reflection spectrum, will occur with the apodised fiber Bragg gratings made by the phase mask approach.
.1. Albert, K.O.Hill, B.Malo, S. Theriault, F. Bilodeau, D.C.Johnson and L.E.Erickson pror~osed using a phase mask with variable dift'raction efficiency to make a fiber Bragg grating with apodisation of the spectral response. They made a phase mask with variable grating groove size (groove width and or groove depth in the grating period) to varying the relative intensity of the first order diffraction beams. When a UV laser beam is scanning the phase mask with variable local first order diffraction efficiency the interferent pattern modulation varies along the fiber Bragg grating length. Since the exposure time is the same(uniform scanning speed) the average induced refractive index along the fiber Bragg grating length will be constant. This method can make a fiber Bragg grating with "pure apodisation" with only refractive index grating modulation change and the constant average induced refractive index. They used focused ion beam machine to make this kind phase masks. It is not practical to make a long phase mask with focused ion beam machine because it will take very long writing time for focused ion beam machine to make a phase mask with a size of lOmm x 3 mm.
~ I.Singh, and W.W.Morey also made apodised fiber Bragg gratings for DWDM using a variable efficiency phase mask.(reference 4) The basis of their variable efficiency phase mask is by varying the grating groove depth to change the first order diffraction efficiencies, as same as that proposed by J. Albert et al.. But the method to make the variable efficiency phase mask is different from the focused ion beam machine method used by J. Albert et al.. The variable efficiency phase mask is made by holography approach with a interference from two beams that have a spatially variable intensity profile such as Gaussian beam profile. But the holographic approach have some disadvantages such as: not easy to control the interference beam spatial intensity profile, not easy to make chirped phase mask with variable efficiency.
Invention In the present invention two new methods of making apodised phase masks, which have spatial variable grating groove depth to get variable first order efficiency along the grating length, are presented.
1. Method one:
A chrome photomask of grating patterns is made first by an electron beam direct write lithography machine. The chrome photomask made on a fused silica plate has many grating patterns inside. Then the chrome photomask is cut into several plates with each one of them contains a grating patterns.(see figure 1) After cleaning the chrome grating plates a photoresist layer is coated on the chrome grating plates.(see figure 2) A gray level mask with spatial variable optical transmission is used in a lithography process to form a spatial variable photoresist thickness on the chrome grating plate along the grating length.(see figure 3) The lithography process can be implemented with a contact mask aligner lithography machine or with a project image reduction lithography machine called "stepper" in microelectronic industry. The gray level mask with the spatial variable optical transmission profile is put on the mask position in the lithography machine and the chrome grating plate coated with a photoresist layer is put in the wafer position in the lithography machine. After the exposed photoresist on the chrome grating plate is developed a spatial variable thickness profile on the photoresist layer is made.(see figure 4) A dry etching process is followed to transform the photoresist profile into the fused silica plate to make a phase mask with spatial groove depth profile.(see figure 5) The chrome grating pattern is used as protect mask during the dry etching process so only open area without chrome in the grating pattern will be etched down into substrate. Since there is a spatial variable photoresist profile on the top of the chrome grating plate, the w~
local grating groove depth on the substrate, where the photoresist is thicker, will be shallow and the local grating groove depth, where the photoresist is thinner, will be deeper.(see figure 6) Since the grating groove depth on the substrate varies along the phase mask length the first order efficiency changes also along the phase mask length.
The fiber Bragg gratings made by the phase mask with spatial variable first order efficiency will have a main reflection peak of the center wavelength with suppressed side lobes: apodised reflection spectrum. By carefully designing the grating groove depth profile along the grating length the phase mask can make fiber Bragg gratings with Gaussian apodisation or other kind apodisation profile in the reflection spectrum.
In the invention the grating patterns in the chrome photomasks made by electron beam lithography approach are uniform gratings or chirped gratings. The grating periods in the chrome photomask can be from 0.5 micro meter to 1.2 micrometers, which are corresponding to working wavelengths from 0.85 micrometer to 1.55 micrometers. To be illuminated by UV laser beam the chrome photomasks are made on fused silica glass plates. One chrome photomask can contain one or several grating patterns. If the chrome photomask contains several grating patterns the chrome photomask can be diced into several plate pieces, each of them contains one grating pattern. After the chrome grating plates are cleaned they are coated with a photoresist layer. A prebake process is followed and the chrome grating plates with the photoresist layer are put into an oven or put on a hot plate for a certain time. Then a lithography process is implemented to transform the spatial variable optical density on a gray level mask into a spatial variable photoresist depth profile on the chrome grating plate.
In the invention the lithography process can be implemented in a contact mask aligner lithography machine or a projection image reduction lithography machine called "stepper" in microelectronic industry. The chrome grating plates are put on the wafer position in the lithography machine. A ~ray level mask with spatial variable optical transmission is used as the mask in the lithography machine. A conventional photomask has only two optical transmission levels: transparent(white) or not transparent(black).
Gray level masks can have more than two optical transmission levels. For example a gray level mask have three optical transmission levels: 100% transparent(white), 50%
transparent and 0~/o transparent(black). Normally a gray level mask can have eight to several hundred optical transmission levels. When a gray level mask with eight transmission levels is illuminated with a uniform optical beam the optical intensity of the output beam will be modulated with eight levels. If a chrome grating plate with a photoresist layer is illuminated under the modulated optical beam a photoresist profile with eight different. depths will form on the chrome grating plate after the exposed photoresist plate is developed. Using a gray level mask with a spatial variable optical transmission profile carefully designed a photoresist layer with the spatial variable depth profile like Gaussian profile or other kind profiles can be obtained on a chrome grating plate.
In the invention the gray level mask, which has a spatial variable optical transmission profile specially designed, can be made on the high energy beam sensitive glass with electron beam lithograr)hy machine(reference 5). The gray level mask can also be made on a conventional sio2 by mean of evaporation of metal alloy such as Inconel(a nickel chromium iron alloy) and a following liftoff step.(reference 6) The gray level mask can also be made by changing the number or area of openings in a chrome photomask.(reference 7) The gray level mask can also be made on the conventionalcommercial slide imagers and low-contrast film(reference 8-9).
In the invention after a spatial variable photoresist thickness layer is formed on the chrome grating plate with a lithography process a dry etching process is implemented to transfer the spatial photoresist profile into the fused silica glass substrate. The dry etching process can be reactive ion etching. The dry etching process can also be a chemical assistant ion beam etching. The etching ratio between photoresist and substrate can be different. The photoresist etching ratio can be quicker than the substrate etching ratio.
The photoresist etching ratio can be slower than the substrate etching ratio. For different etching ratio between the photoresist and the substrate different spatial photoresist profiles are required to get final spatial grating groove depth profiles in the substrate.
In the invention the chrome grating pattern is used as a protection mask during the dry etching process. Only the open area without chrome will be etched down into fused silica substrate with the dry etching process. If there is no photoresist layer on the top of the chrome grating pattern a grating with the same grating groove depth will be made by the dry etching process. If there is no chrome grating pattern under the spatial variable photoresist layer a spatial variable profile in the substrate will be made with the dry etching process. Since there is a spatial variable photoresist profile on the top of the chrome grating plate the etched grating groove depth will have a spatial variable profile along the grating length. After cleaning the chrome and photoresist finally a phase mask with a spatial variable grating groove depth profile along the grating length on the fused silica glass substrate has been made. A phase mask with a spatial variable grating groove depth profile will have a spatial variable first order diffraction efficiencies along the grating length. The phase mask with a spatial variable first order efficiencies along the grating length can make apodised fiber Bragg gratings with the main center reflective peak and suppressed side lobes, which is very important for DWDM system to reduce the interchannel interference.
In the invention the Bragg grating can be made in optical fiber or optical waveguide.
UV light can induce a permanent refractive index change in some kind optical fibers and optical wave guides. The photosensitivity of the certain kind optical fiber waveguide can be used to make Bragg gratings, which is a permanent, spatially periodic refractive index modulation along the length of the photosensitive core of the optical fiber or optical wave guide. Fiber Bragg gratings have many applications in optical fiber telecommunication, optical sensor and optical information process. Fiber Bragg gratings can be used as bandpass reflective filters, dense wavelength division multiplexing(DWDM) add and drop filters, dispersion compensation, and optical fiber sensors.
There are two methods of side writing Bragg gratings in optical fiber and optical wave guide, one of them is holography approach and another one is phase mask approach.
Holography approach is using two beam interference to form a periodic light intensity modulation along optical fiber or optical wave guide. The period of the pattern is controlled by controlling the angle between the interfering beams. Holography approach is described in U.S. patent application Ser. No. 4,474,427 invented by K.O.Hill, B.S.Kavassaki, D.C. Johnson, and Yoshimassa Fujii and in U.S. patent application Ser.
No. 4,807,950 invented by Glenn et al. Phase mask method is the most common way to fabricate Bragg grating in optical fiber and optical wave guide. A phase mask is an uniform or chirped grating in fused silica glass substrate. Optical fibers or optical wave guide is placed close or near close under phase mask. When a UV light illuminates the phase mask, +l and -I diffraction orders of the grating in phase mask will interference each other along the fiber or wave guide core and a refractive index change in optical fiber or optical wave guide is formed. Phase mask method is very robust and insensitive to the environment conditions such as vibration. Phase mask method of fabricating such Bragg gratings is described in U.S. patent application Ser. No. 969,774 invented by K.O.Hill, B.Malo, F.Bilodeau and D. Johnson and entitled METHOD OF
FABRICATING BRAGG GRATINGS USING A SILICA GLASS PHASE GRATINGS
MASK.
Phase masks can be made by holography approach or by electron beam direct write approach. To make a phase mask by holography approach is very similar as writing a grating on optical fiber. First two interference beams are used to form a sinusoid intensity modulation on a layer of photoresist coated on a fused silica glass substrate. After developing a sinusoid profile on the photoresist layer is formed. Then a dry etching is implemented to transfer the sinusoid profile on the photoresist into the fused silica glass substrate underneath to make a phase mask. The another method to make phase masks is using electron beam lithography machine to write very fine lines on a photoresist layer on the top of a Chromium layer with fused silica glass substrate plate. After developing a very fine pattern on the photoresist layer is formed. The open Chromium on the photoresist pattern layer is removed by a wet etching process and the left photoresist is cleaned to get a Chromium pattern on the fused silica glass substrate. A dry etching process is used to transfer the fine line patterns into fused silica glass substrate to make a phase mask. The electron beam direct write approach is a standard photomask making process in microelectronic industry and t'ollowed by a dry etching process.
Fiber Bragg gratings written by a phase mask with an uniform speed laser scanning will have a main peak in the center reflection spectrum accompanied by a series of side lobes at adjacent wavelengths. It is very important to lower the reflectivity of these side lobes or 'apodise' the reflection spectrum of the fiber Bragg grating in these optical fiber devices where high rejection of the nonresonant light is required. DWDM systems require device that can isolate channels that are spaced by only lOOGHz(0.8nm at1~50nm wavelength). Fiber Bragg gratings are the promise components in the DWDM
systems as optical signal channel adder( input channel ) and optical signal channel dropper( output channel ). For example that one optical wavelength channel will be dropped from optical fiber should have no effect on other optical wavelength channels.
A high main peak in the center wavelength reflection spectrum with very low side lobes at adjacent wavelengths is very important for DWDM fiber Bragg gratings.(reference 1) Another benefit of the reflection spectrum apodisation of fiber Bragg gratings is the improvement of the dispersion compensation characteristics of chirped Bragg gratings.
(reference 2) Both holographic approa~h and phase mask approach can make apodised fiber Bragg gratings. The holographic approach uses two laser beams with intensity variation from beam center to the beam edge (for example Gaussian beam intensity profile) to write fiber Bragg gratings. The two laser beams will make an intensity modulation grating along the fiber cc re so the apodised fiber Bragg grating was made. This kind apodised fiber Bragg grating is not "pure apodisation" since not only the refractive index grating modulation is apodised but also the average induced refractive index is not constant and has a spatial profile along the fiber core. This kind apodisation approach can suppress the side lobes in the spectral response of the fiber Bragg gratings. But the fiber Bragg gratings will have a fine structure on the short wavelength side of their reflection response curve which is particularly strong in high reflectivity gratings.(reference 3) Phase mask approach can also make spodised fiber Bragg gratings by varying exposure time(varying scanning speed) along the length of the fiber Bragg grating. A longer exposure time(slower scanning speed) can induce a larger local refractive index modulation grating and a short exposure time(faster scanning speed) will induce a small local refractive index modulation grating. The apodised fiber Bragg gratings made with this approach will have the same problem as holographic approach does: not only refractive index is modulated but also the average induced refractive index varies along the fiber Bragg grating length. The same problem mentioned above, the fine structure on the short wavelength side of the grating reflection spectrum, will occur with the apodised fiber Bragg gratings made by the phase mask approach.
.1. Albert, K.O.Hill, B.Malo, S. Theriault, F. Bilodeau, D.C.Johnson and L.E.Erickson pror~osed using a phase mask with variable dift'raction efficiency to make a fiber Bragg grating with apodisation of the spectral response. They made a phase mask with variable grating groove size (groove width and or groove depth in the grating period) to varying the relative intensity of the first order diffraction beams. When a UV laser beam is scanning the phase mask with variable local first order diffraction efficiency the interferent pattern modulation varies along the fiber Bragg grating length. Since the exposure time is the same(uniform scanning speed) the average induced refractive index along the fiber Bragg grating length will be constant. This method can make a fiber Bragg grating with "pure apodisation" with only refractive index grating modulation change and the constant average induced refractive index. They used focused ion beam machine to make this kind phase masks. It is not practical to make a long phase mask with focused ion beam machine because it will take very long writing time for focused ion beam machine to make a phase mask with a size of lOmm x 3 mm.
~ I.Singh, and W.W.Morey also made apodised fiber Bragg gratings for DWDM using a variable efficiency phase mask.(reference 4) The basis of their variable efficiency phase mask is by varying the grating groove depth to change the first order diffraction efficiencies, as same as that proposed by J. Albert et al.. But the method to make the variable efficiency phase mask is different from the focused ion beam machine method used by J. Albert et al.. The variable efficiency phase mask is made by holography approach with a interference from two beams that have a spatially variable intensity profile such as Gaussian beam profile. But the holographic approach have some disadvantages such as: not easy to control the interference beam spatial intensity profile, not easy to make chirped phase mask with variable efficiency.
Invention In the present invention two new methods of making apodised phase masks, which have spatial variable grating groove depth to get variable first order efficiency along the grating length, are presented.
1. Method one:
A chrome photomask of grating patterns is made first by an electron beam direct write lithography machine. The chrome photomask made on a fused silica plate has many grating patterns inside. Then the chrome photomask is cut into several plates with each one of them contains a grating patterns.(see figure 1) After cleaning the chrome grating plates a photoresist layer is coated on the chrome grating plates.(see figure 2) A gray level mask with spatial variable optical transmission is used in a lithography process to form a spatial variable photoresist thickness on the chrome grating plate along the grating length.(see figure 3) The lithography process can be implemented with a contact mask aligner lithography machine or with a project image reduction lithography machine called "stepper" in microelectronic industry. The gray level mask with the spatial variable optical transmission profile is put on the mask position in the lithography machine and the chrome grating plate coated with a photoresist layer is put in the wafer position in the lithography machine. After the exposed photoresist on the chrome grating plate is developed a spatial variable thickness profile on the photoresist layer is made.(see figure 4) A dry etching process is followed to transform the photoresist profile into the fused silica plate to make a phase mask with spatial groove depth profile.(see figure 5) The chrome grating pattern is used as protect mask during the dry etching process so only open area without chrome in the grating pattern will be etched down into substrate. Since there is a spatial variable photoresist profile on the top of the chrome grating plate, the w~
local grating groove depth on the substrate, where the photoresist is thicker, will be shallow and the local grating groove depth, where the photoresist is thinner, will be deeper.(see figure 6) Since the grating groove depth on the substrate varies along the phase mask length the first order efficiency changes also along the phase mask length.
The fiber Bragg gratings made by the phase mask with spatial variable first order efficiency will have a main reflection peak of the center wavelength with suppressed side lobes: apodised reflection spectrum. By carefully designing the grating groove depth profile along the grating length the phase mask can make fiber Bragg gratings with Gaussian apodisation or other kind apodisation profile in the reflection spectrum.
In the invention the grating patterns in the chrome photomasks made by electron beam lithography approach are uniform gratings or chirped gratings. The grating periods in the chrome photomask can be from 0.5 micro meter to 1.2 micrometers, which are corresponding to working wavelengths from 0.85 micrometer to 1.55 micrometers. To be illuminated by UV laser beam the chrome photomasks are made on fused silica glass plates. One chrome photomask can contain one or several grating patterns. If the chrome photomask contains several grating patterns the chrome photomask can be diced into several plate pieces, each of them contains one grating pattern. After the chrome grating plates are cleaned they are coated with a photoresist layer. A prebake process is followed and the chrome grating plates with the photoresist layer are put into an oven or put on a hot plate for a certain time. Then a lithography process is implemented to transform the spatial variable optical density on a gray level mask into a spatial variable photoresist depth profile on the chrome grating plate.
In the invention the lithography process can be implemented in a contact mask aligner lithography machine or a projection image reduction lithography machine called "stepper" in microelectronic industry. The chrome grating plates are put on the wafer position in the lithography machine. A ~ray level mask with spatial variable optical transmission is used as the mask in the lithography machine. A conventional photomask has only two optical transmission levels: transparent(white) or not transparent(black).
Gray level masks can have more than two optical transmission levels. For example a gray level mask have three optical transmission levels: 100% transparent(white), 50%
transparent and 0~/o transparent(black). Normally a gray level mask can have eight to several hundred optical transmission levels. When a gray level mask with eight transmission levels is illuminated with a uniform optical beam the optical intensity of the output beam will be modulated with eight levels. If a chrome grating plate with a photoresist layer is illuminated under the modulated optical beam a photoresist profile with eight different. depths will form on the chrome grating plate after the exposed photoresist plate is developed. Using a gray level mask with a spatial variable optical transmission profile carefully designed a photoresist layer with the spatial variable depth profile like Gaussian profile or other kind profiles can be obtained on a chrome grating plate.
In the invention the gray level mask, which has a spatial variable optical transmission profile specially designed, can be made on the high energy beam sensitive glass with electron beam lithograr)hy machine(reference 5). The gray level mask can also be made on a conventional sio2 by mean of evaporation of metal alloy such as Inconel(a nickel chromium iron alloy) and a following liftoff step.(reference 6) The gray level mask can also be made by changing the number or area of openings in a chrome photomask.(reference 7) The gray level mask can also be made on the conventionalcommercial slide imagers and low-contrast film(reference 8-9).
In the invention after a spatial variable photoresist thickness layer is formed on the chrome grating plate with a lithography process a dry etching process is implemented to transfer the spatial photoresist profile into the fused silica glass substrate. The dry etching process can be reactive ion etching. The dry etching process can also be a chemical assistant ion beam etching. The etching ratio between photoresist and substrate can be different. The photoresist etching ratio can be quicker than the substrate etching ratio.
The photoresist etching ratio can be slower than the substrate etching ratio. For different etching ratio between the photoresist and the substrate different spatial photoresist profiles are required to get final spatial grating groove depth profiles in the substrate.
In the invention the chrome grating pattern is used as a protection mask during the dry etching process. Only the open area without chrome will be etched down into fused silica substrate with the dry etching process. If there is no photoresist layer on the top of the chrome grating pattern a grating with the same grating groove depth will be made by the dry etching process. If there is no chrome grating pattern under the spatial variable photoresist layer a spatial variable profile in the substrate will be made with the dry etching process. Since there is a spatial variable photoresist profile on the top of the chrome grating plate the etched grating groove depth will have a spatial variable profile along the grating length. After cleaning the chrome and photoresist finally a phase mask with a spatial variable grating groove depth profile along the grating length on the fused silica glass substrate has been made. A phase mask with a spatial variable grating groove depth profile will have a spatial variable first order diffraction efficiencies along the grating length. The phase mask with a spatial variable first order efficiencies along the grating length can make apodised fiber Bragg gratings with the main center reflective peak and suppressed side lobes, which is very important for DWDM system to reduce the interchannel interference.
In the invention the Bragg grating can be made in optical fiber or optical waveguide.
2. Method two:
A phase mask with uniform grating groove depth is made first. (see figure 7) Then the phase mask is coated with a photoresist layer.(see figure 8) A gray level mask with spatial variable optical transmission is used in a lithography process to form a spatial variable photoresist thickness on the chrome grating plate along the grating length.(see figure 9) The lithography process can be implemented with a contact mask alignerlithography machine or with a project image reduction lithography machine called"stepper" in microelectronic industry. The gray level mask with the spatial variable optical transmission profile is put on the mask position in the lithography machine and the phase mask with the uniform grating groove depth coated with a photoresist layer is put in the wafer position in the lithography machine. After the exposed photoresist on the phase mask is developed a spatial variable thickness profile on the photoresist layer is made.(see figure 10) A dry etching process is followed to transform the photoresist profile into the fused silica plate to make a phase mask with spatial groove depth profile.(see figure 11) Since there is a spatial variable photoresist profile on the top of the phase mask, the local grating groove depth on the substrate, where the photoresist is thicker, will be shallow and the local grating groove depth, where the photoresist is thinner, will be deeper.(see figure 12) Since the grating groove depth on the substrate varies along the phase mask length the first order efficiency changes also along the phase mask length. The fiber Bragg gratings made by the phase mask with spatial variable first order ef'ficiency will have a main reflection peak of the center wavelength withsuppressed side lobes: apodised reflection spectrum. By carefully designing the grating groove depth profile along the grating length the phase mask can make fiber Bragg gratings with Gaussian apodisation or other kind apodisation profile in the reflection spectrum.
References:
1). Matschara M. and Hill K.O., "Optical-waveguide band-rejection filters: design", Appl. Opt., vol. l 3, pp.2886-2888(1974).
2). Hill K.O., Theriallt S., Malo B., Bilodeau F., Kitigawa T., Johnson D.C., Albert J., Takiguchi K., Kataoka T., and Hagimoto K., "Chirped in-fiber Bragg grting dispersion compensators: Linearisation of dispersion characteristic and demonstration of dispersion compensation in 100km, 10Gbit/s optical fibre link", Electron. Lett., vol. 30, ppl755-1756( l 994)
A phase mask with uniform grating groove depth is made first. (see figure 7) Then the phase mask is coated with a photoresist layer.(see figure 8) A gray level mask with spatial variable optical transmission is used in a lithography process to form a spatial variable photoresist thickness on the chrome grating plate along the grating length.(see figure 9) The lithography process can be implemented with a contact mask alignerlithography machine or with a project image reduction lithography machine called"stepper" in microelectronic industry. The gray level mask with the spatial variable optical transmission profile is put on the mask position in the lithography machine and the phase mask with the uniform grating groove depth coated with a photoresist layer is put in the wafer position in the lithography machine. After the exposed photoresist on the phase mask is developed a spatial variable thickness profile on the photoresist layer is made.(see figure 10) A dry etching process is followed to transform the photoresist profile into the fused silica plate to make a phase mask with spatial groove depth profile.(see figure 11) Since there is a spatial variable photoresist profile on the top of the phase mask, the local grating groove depth on the substrate, where the photoresist is thicker, will be shallow and the local grating groove depth, where the photoresist is thinner, will be deeper.(see figure 12) Since the grating groove depth on the substrate varies along the phase mask length the first order efficiency changes also along the phase mask length. The fiber Bragg gratings made by the phase mask with spatial variable first order ef'ficiency will have a main reflection peak of the center wavelength withsuppressed side lobes: apodised reflection spectrum. By carefully designing the grating groove depth profile along the grating length the phase mask can make fiber Bragg gratings with Gaussian apodisation or other kind apodisation profile in the reflection spectrum.
References:
1). Matschara M. and Hill K.O., "Optical-waveguide band-rejection filters: design", Appl. Opt., vol. l 3, pp.2886-2888(1974).
2). Hill K.O., Theriallt S., Malo B., Bilodeau F., Kitigawa T., Johnson D.C., Albert J., Takiguchi K., Kataoka T., and Hagimoto K., "Chirped in-fiber Bragg grting dispersion compensators: Linearisation of dispersion characteristic and demonstration of dispersion compensation in 100km, 10Gbit/s optical fibre link", Electron. Lett., vol. 30, ppl755-1756( l 994)
3). Mizrahi V., and Sipe J.E., "Optical properties of photosensitive gratings", Journal of Lightwave Technology, LT-Il, pp.823-825(1993).
4). H. Singh, and W.W.Morey, "Apodized Fiber Gratings for DWDM Using Variable Efficiency Phase Masks", IEEE/LEOS Summer Topical Meetings, 1997, Montreal, Canada.
5). Walter Daschner, Pin Long, Robert Stein, Chuck Wu and S.H.Lee, " General aspheric refractive micro-optics fabricated by optical lithography using a high energy beam sensitive glass gray-level mask", Journal of Vacuum Science and Technology B, vol.
14(6), pp. 3730-3733(1996).
14(6), pp. 3730-3733(1996).
6). Walter Daschner, Pin Long, Michael Larsson, and S.H.Lee, " Fabrication of diffractive optical elements using a single optical exposure with a gray level mask", Journal of Vacuum Science and Technology B, vol. 13(6), pp.2729-2731 (1995).
7). Y.Opplinger, P.Sixt, J.M.Stauffer, J.M.Mayor, P.Regnault and G.Voirin"One-step 3D
Shaping Using a Gray-Tone Mask for Optical and Microelectronic Applications", Microelectronic Engineering, Vol. 23, pp.449-454(1994).
Shaping Using a Gray-Tone Mask for Optical and Microelectronic Applications", Microelectronic Engineering, Vol. 23, pp.449-454(1994).
8). Thomas J. Suleski and Donald C. O'Shea, " Gray-scale masks for diffractive-optics fabrication: I. Commercial slide imagers", Applied Optics, Vol. 34, pp.7507-7517(1995).
9). Donald C. O'Shea and Willie S. Rockward, "Gray-scale masks for diffractive-optics fabrication: lI. Spatially filtered halftone screens", Applied Optics, Vol. 34, pp.7518-7526(1995).
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| CA 2214927 CA2214927A1 (en) | 1997-10-31 | 1997-10-31 | Methods for making phase masks with spatial variable first order efficiency for fiber bragg grating fabrication |
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| CA 2214927 CA2214927A1 (en) | 1997-10-31 | 1997-10-31 | Methods for making phase masks with spatial variable first order efficiency for fiber bragg grating fabrication |
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