CA2184453C - Optical device formed with grating therein, add/drop filter using same, and method of fabricating same - Google Patents
Optical device formed with grating therein, add/drop filter using same, and method of fabricating same Download PDFInfo
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- CA2184453C CA2184453C CA 2184453 CA2184453A CA2184453C CA 2184453 C CA2184453 C CA 2184453C CA 2184453 CA2184453 CA 2184453 CA 2184453 A CA2184453 A CA 2184453A CA 2184453 C CA2184453 C CA 2184453C
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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
- G02B6/10—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
- G02B6/02—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
- G02B6/0208—Refractive index modulation gratings, e.g. Bragg gratings characterised by their structure, wavelength response
- G02B6/02085—Refractive index modulation gratings, e.g. Bragg gratings characterised by their structure, wavelength response characterised by the grating profile, e.g. chirped, apodised, tilted, helical
-
- 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
- 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
-
- 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
- G02B6/24—Coupling light guides
- G02B6/26—Optical coupling means
- G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
- G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
- G02B6/29346—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means operating by wave or beam interference
- G02B6/2935—Mach-Zehnder configuration, i.e. comprising separate splitting and combining means
- G02B6/29352—Mach-Zehnder configuration, i.e. comprising separate splitting and combining means in a light guide
-
- 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
- G02B6/24—Coupling light guides
- G02B6/26—Optical coupling means
- G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
- G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
- G02B6/29379—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means characterised by the function or use of the complete device
- G02B6/2938—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means characterised by the function or use of the complete device for multiplexing or demultiplexing, i.e. combining or separating wavelengths, e.g. 1xN, NxM
- G02B6/29382—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means characterised by the function or use of the complete device for multiplexing or demultiplexing, i.e. combining or separating wavelengths, e.g. 1xN, NxM including at least adding or dropping a signal, i.e. passing the majority of signals
- G02B6/29383—Adding and dropping
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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
- G02B6/0208—Refractive index modulation gratings, e.g. Bragg gratings characterised by their structure, wavelength response
- G02B6/021—Refractive index modulation gratings, e.g. Bragg gratings characterised by their structure, wavelength response characterised by the core or cladding or coating, e.g. materials, radial refractive index profiles, cladding shape
-
- 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
- G02B6/02152—Refractive index modulation gratings, e.g. Bragg gratings characterised by the method of manufacture of the grating involving moving the fibre or a manufacturing element, stretching of the fibre
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- Physics & Mathematics (AREA)
- General Physics & Mathematics (AREA)
- Optics & Photonics (AREA)
- Engineering & Computer Science (AREA)
- Microelectronics & Electronic Packaging (AREA)
- Manufacturing & Machinery (AREA)
- Optical Integrated Circuits (AREA)
- Optical Filters (AREA)
Abstract
A width of a core in an optical waveguide is decreased in accordance with a predetermined value, as a distance of a position of the core to the centre position thereof is decreased. Consequently, a rectangular spectrum property having no ripple is obtained on a lower wavelength side in a reflection grating. In addition, the width of the core is changed at the position Z in the direction of light propagation, while UV light is radiated to the core, so that a periodically-striated distribution of refractive indices having an envelope which is changed in accordance with a value of "¦sin(C~Z)¦/¦C~Z¦" is formed in the core of the optical waveguide, and the phase of light to be propagated in the direction of light propagation is shifted at the position Z as defined below by .pi..
Z=m.pi./C(m=...,-3,-2,-1,1,2,3,...) Thus, a rectangular spectrum property is obtained in a low-reflection grating.
Z=m.pi./C(m=...,-3,-2,-1,1,2,3,...) Thus, a rectangular spectrum property is obtained in a low-reflection grating.
Description
OPTICAL DEVICE FORMED WITH GRATING THEREIN, ADD,/DROP FILTER
USING SAME, AND METHOD OF FABRICATING SAME
The invention relates to an optical device formed with a grating therein, an ADD/DROP filter using the same, and a method of fabricating the same, and more particularly to an optical device such as an optical waveguide in which a core is imprinted with an index grating, an ADD/DROP
filter using such an optical waveguide, and a method of fabricating such an optical device having a core imprinted with an index grating.
A conventional optical fiber having a core imprinted with an index grating is disclosed in U.S. Patent No. 5,367,588, issued on November 22, 1994.
In the conventional optical fiber, the index grating is imprinted in the core using a silica-glass phase-grating mask, which is held in close proximity to an outer periphery thereof. A UV (ultraviolet) light beam emitted from a KrF excimer laser light source is radiated onto a phase-grating mask at a normal incident angle, so that an interference pattern is imprinted in the core of the optical fiber in accordance with the photo-induced effect.
Such an optical fiber having a core imprinted with an index grating is used for the transmission and deflection of light power supplied to the core thereof.
An optical waveguide having a core imprinted with an index grating can be fabricated by replacing the optical fiber with an optical waveguide, and it will be described in more detail prior to the description of preferred embodiments of the invention.
However, the conventional optical waveguide having the core imprinted with the index grating has disadvantages in that a low-reflection spectrum property is of a square law curve which is different from a desired rectangular property, and a high-reflection spectrum property has ripples at a rising portion thereof. Consequently, the reflection power of a light signal fluctuates, even if a small change occurs in the wavelength of the light signal.
Accordingly, it is an object of the invention to provide an optical device formed with a grating, an ADD/DROP
filter using the same, and a method of fabricating the same in which high- and low-reflection spectrum properties are obtained to be rectangular.
It is another object of the invention to provide an optical device formed with a grating, an ADD/DROP filter using the same, and a method of fabricating the same in which the reflection power of a light signal is not changed, even if the wavelength of the light signal varies slightly.
According to a first feature of the invention, an optical device formed with a grating comprises:
an optical waveguide comprising a core of a first refractive index and a cladding layer of a second refractive index lower than the first refractive index, the core being imbedded in the cladding layer;
a grating of a periodically-striated distribution of refractive indices to be formed in the optical waveguide in a direction of light propagation in accordance with a UV
light radiation thereto;
wherein a width of the core is changed in the direction of light propagation.
According to a second feature of the invention, an optical device formed with a low-reflection grating comprises:
an optical waveguide comprising a core of a first refractive index and a cladding layer of a second refractive index lower than the first refractive index, the core being imbedded in the cladding layer;
a grating of a periodically-striated distribution of refractive indices to be formed in the optical waveguide in a direction of light propagation in accordance with a W
light radiation thereto;
wherein a width W of the core is changed at a position Z in the direction of light propagation in accordance with an equation as defined below, W=Wp-OW~Isin(C~Z)~/~C'Z~
where Wo is an inherent and unchanged width of the core, OW is a difference between the unchanged width Wo and a minimum width of the core, and C is a constant; and, an envelope of said periodically-striated distri-bution of refractive indices for the grating is changed in accordance with a value of "~sin(C~Z)~/~C~Z~", whereby light of a specific wavelength to be transmitted through the optical waveguide is shifted in phase at the position Z, meeting an equation as defined below, Z=m~r/C (m=...,-3,-2,-1,1,2,3,...).
According to a third feature of the invention, an ADD/DROP filter using an optical device formed with a grating comprises:
first and second optical waveguides formed on a substrate, such that the first and second optical waveguides are arranged to be proximate to each other at first and second portions to provide first and second optical couplers, each of the first and second optical waveguides comprising a core of a ffirst refractive index and a cladding layer of a second refractive index lower than the first refractive index, the core being embedded in the cladding layer;
a grating of a periodically-striated distribution of refractive indices to be formed in each of the first and second optical waveguides in a direction of light propagation in accordance with a W light radiation thereto;
wherein a width of the core is changed in the direction of the light propagation.
According to a fourth feature of the invention, a method of fabricating an optical device formed with a grating comprises the steps of:
forming a core of a first refractive index having a width W which is changed at a position Z in a direction of light propagation in accordance with an equation as defined below, W=Wo-~W~~sin(C~Z)~/~C~Z~
where Wois an inherent and unchanged width of the core, OW is a difference between the unchanged width Wo and a minimum width of the core, and C is a constant, the core being embedded in a cladding layer of a second refractive index lower than the first refractive index;
positioning a phase-grating mask over the core, the phase-grating mask having a grating striation which is shifted in phase at the position Z as defined below by ~, Z=m~/C (m=...,-3,-2,-1,1,2,3,...);
scanning a W light to be radiated via the phase grating mask to the core in said direction of the light propagation in accordance with a speed determined by a value of "1/(~sin(C~Z)~/~C~Z~)", thereby providing a low reflec tion grating of a periodically-striated distribution of refractive indices.
According to a fifth feature of the invention, a method of fabricating an optical device formed with a low-reflection grating, comprises the steps of:
forming a core of a first refractive index having a width W which is changed at a position Z in a direction of light propagation in accordance with an equation as defined below, W=Wa-OW~~sin(C~Z)~/IC'Z~
where Wo is an inherent and unchanged width of the core, OW is a difference between the unchanged width Wo and a minimum width of the core, and C is a constant, the core being embedded in a cladding layer of a second refractive index lower than the first refractive index;
radiating UV light onto the core to derive an envelope of a periodically-striated distribution of refrac-tive indices, the envelope changing in accordance with a value of "~sin(C~Z)~/~C~Z~"; and, radiating UV light of a fine spot surplusly onto the core at the position Z to shift a phase of a light to be propagated in the direction by ~r as defined below, 5 Z=m~r/C (m=...,-3,-2,-1,1,2,3,...) thereby providing a low-reflection grating of the periodically-striated distribution of refractive indices.
The invention will now be described, by way of example, with reference to the appended drawings, wherein:
Figure lA is an explanatory diagram showing a conventional optical waveguide having a core imprinted with a grating;
Figure 1B is a partial side view showing the conventional optical waveguide as shown in Figure 1A;
Figure iC is a partially-enlarged view showing the conventional optical waveguide as shown in Figure lA;
Figure 1D is an explanatory diagram showing a refractive index along a core of the conventional optical waveguide as shown in Figure lA;
Figure 2 is an explanatory diagram showing a desired high-reflection spectrum property in the conven-tional optical waveguide as shown in Figure lA;
Figure 3 is an explanatory diagram showing a desired envelope for refractive indices of the core in the conventional optical waveguide as shown in Figure lA;
Figure 4 is an explanatory diagram showing a prac-tically realized envelope for refractive indices of the core in the conventional optical waveguide as shown in Figure lA;
Figures 5 and 6 are explanatory diagrams showing normalized reflection power dependent on the wavelength of light signals supplied to the conventional optical waveguide as shown in Figure lA;
Figures 7A and 7B are explanatory diagrams showing an optical device in a preferred embodiment according to the invention;
Figures 8A to 8D are explanatory diagrams showing a configuration of a core and refractive indices thereof in the optical device in the preferred embodiment according to the invention;
Figure 9 is an explanatory diagram showing normalized reflection power dependent on the wavelength of a light signal supplied to the optical device in the preferred embodiment according to the invention;
Figure 10 is an explanatory diagram showing an ADD/DROP filter using an optical device in a preferred embodiment according to the invention;
Figure 11 is an explanatory diagram showing an optical device in a preferred embodiment according to the invention;
Figure 12 is an explanatory diagram showing a phase-grating mask used in the optical device as shown in Figure 11;
Figures 13A to 13D are explanatory diagrams showing a configuration of a core and refractive indices thereof in the optical device as shown in Figure 11;
Figure 14 is an explanatory diagram showing normalized reflection power dependent on the wavelength of a light signal supplied to the optical device as shown in Figure 11; and, Figure 15 is an explanatory diagram showing a method of fabricating a core of an optical waveguide in the optical device as shown in Figure 11.
Before describing preferred embodiments according to the invention, a conventional optical device in which a core of an optical waveguide is imprinted with an index grating will be described.
Figure lA shows a fabrication system for the conventional optical device in which a core of an optical waveguide is imprinted with an index grating. The fabri-cation system comprises a KrF excimer laser 1 for emitting UV light 11 having a wavelength of 248nm, a mirror 2 for reflecting the light 11 in a predetermined direction, a lens 3 for focusing the reflected light 11 on a predetermined point, a phase-grating mask 4 having a grating striation 4A
of a pitch A for generating an inference fringe of a pitch A/2, a light source 5 for emitting white light, an optical fiber 7A for transmitting the white light, an optical wave-guide 9 positioned below the grating striation 4A of the phase-grating mask 4 and optically-coupled at an input to the optical fiber 7A, an optical fiber 7B optically-coupled to an output of the optical waveguide 9, and a spectrum analyzer 6 for receiving an output light of the optical waveguide 9 via the optical fiber 7B and analysing the spectrum of the output light.
Figures 1B and 1C show the relation of the phase grating mask 4, the optical waveguide 9, and the optical fibers 7A and 7B, wherein the optical waveguide 9 comprises a substrate 9A, such as Si, quartz, etc. , a core 9B doped with Ge02, P205 or B203, and a buffer layer 9C, and wherein each of the optical fibers 7A and 7B comprises a core 7a and a cladding layer 7b, such that the cores 7a of the optical fibers 7A and 7B are optically-coupled to the input and output ends of the core 9B of the optical waveguide 9.
In operation, the UV light having a wavelength of 248nm emitted from the KrF excimer laser 1 is radiated via the mirror 2 and the lens 3 to the phase-grating mask 4, so that an interference fringe of pitch A/2 is generated in accordance with the interference of plus and minus (positive and negative) first-order diffraction light by the grating striation of the pitch A. The interference fringe thus-generated is radiated to the core 9B of the optical wave-guide, so that the refractive index of the core 9B increases depending on the light intensity of the interference fringe in accordance with the photo-refractive effect.
This is shown in Figure 1D, wherein On indicates the periodically-photo-induced increase of the refractive index along the light-propagation direction of the core 9B
in the optical waveguide 9. Consequently, a grating of a pitch A/2 is imprinted in the core 9B of the optical waveguide 9, as shown in Figure 1C.
Fig 2. shows an ideal spectrum property of the grating imprinted in the core 9B of the optical waveguide 9, wherein light of a Bragg wavelength ~1 which is determined by the below equation is selectively reflected to provide a narrow band-stop filter for an optical communication system.
~i=A/ ( 2 ~ Nef f ) where Neff is an effective refractive index of the waveguide 9.
In an ideal spectrum, a normalized reflection power loss is only as large as -38dB of light power trans-mitted, and is essentially constant in a band-pass wave-length region. In other words, a light propagated through the core 9B of the optical waveguide 9 is transmitted without any substantial reflection, while the normalized reflection power sharply increases as a wavelength of the light transits the band-pass wavelength region to a band-stop wavelength region. In the band-stop wavelength region, the normalized reflection power loss may be as much as all of the light power being transmitted, that is, the light is reflected without any substantial transmission via the core 9B of the optical waveguide 9. The ideal spectrum is considered to be realized by radiating a light beam having an intensity distribution of Gaussian-type to the phase-grating mask 4, thereby providing a modulation degree of the grating imprinted in the core 9B of the optical waveguide 9 which will be a pattern of Gaussian-type. This is shown in Figure 3, wherein S indicates a size of the Gaussian-type light beam which is obtained by placing a spatial-amplitude filter in the path of the light 11.
The conventional optical waveguide having the core imprinted with the index grating, however, has the disadvan-tage that a mean value of refractive indices of the core is increased, as shown in Figure 4, although the modulation degree of a Gaussian-type is obtained for the index grating when the UV light of Gaussian-type is radiated to the optical waveguide 9. Ripples are generated on the lower-wavelength side of the band-stop wavelength region, as shown in Figure 5, resulting in the fluctuation of reflection power in accordance with the slight change of a wavelength of a transmission light. A reflection spectrum property is in the shape of a square curve for a low-reflection grating, as shown in Figure 6, when the W light of Gaussian-type is radiated to the optical waveguide 9.
Next, an optical device formed with a grating therein in a preferred embodiment according to the invention will be explained with reference to Figures 7A and 7B, wherein like parts are indicated by like reference numerals to those used in Figures lA to 1C.
In the preferred embodiment, the optical device is for an optical waveguide 9 having a core 9B which is shown in Figure 7B. As a matter of course, the optical waveguide 9 may be replaced by an optical fiber comprising a core and a cladding layer.
The core 9B of the optical waveguide 9 is of a width W at a position Z in the direction of light propaga-tion as defined by the equation (1).
W=Wo-~W~exp(-Z2/S2) (1) where Wo is an inherent and unchanged width of the core 9B, OW is a decreased width of the core 9B, and S is a diameter of a Gaussian-type light beam that is radiated onto a phase-grating mask 4.
In Figure 8A, the width of the core 9B is decreased, as a distance to the centre point 0 is decreased.
As shown therein, the core 9B is imprinted with an index grating of a pitch A/2.
In Figure 8B, an effective refractive index Neff of the optical waveguide 9 is changed proportionally to the width of the core 9B to derive a Gaussian-distribution.
In Figure 8C, a mean value of refractive indices to be increased by the radiation of the UV light is maximum at the centre point 0.
In Figure 8D, the decreased width OW of the core 9B is optimized to derive a flat mean value of refractive indices of the core 9B in the direction of the light propa-5 gation in accordance with the cancellation of the increase and decrease of the refractive indices as shown in Figures 8B and 8C.
In an experiment, the core 9B of the optical waveguide 9 was formed by doping Ge02 into a predetermined 10 region of a Si substrate 9A. The core 9B was structured to have the width W of 5~m at the centre point (Z=0) and the unchanged width Wo of 6~Cm, thereby deriving the decreased width amount AW of l~Cm. W light of Gaussian-distribution emitted from the KrF excimer laser 1 was radiated onto the phase-grating mask 4 in accordance with a beam spot S of 0.8mm, so that the maximum value of photo-induced refractive indices ~n was 0.1. In this optical waveguide 9, a specific refractive index of the core 9B was 0.5, and a pitch A/2 of the grating imprinted in the core 9B was 0.5373~,m. The width of the core 9B was changed with l~,m for a light propagation distance of 400~,m, so that a core width-changing rate was obtained to be 0. 0025 rad ( l~Cm/400~Cm) , while a length of the core 9B representing a taper of Gaussian-type is 0.8mm which is the same as the beam spot size S.
Figure 9 shows a spectrum property obtained in the optical waveguide 9 having the core 9B imprinted with the index grating in the preferred embodiment, wherein this spectrum property is similar to a rectangular spectrum property.
Figure 10 shows an ADD/DROP filter using an optical device formed with a grating therein in a preferred embodiment according to the invention. The ADD/DROP filter is a Mach-Zehnder interferometer, and comprises first and second 3dB couplers 82 and 87, a pair of optical waveguide arms 88 for connecting the first and second 3dB couplers 82 and 87, and optical waveguides for connecting an input port 83 and a drop port 84 to the first 3dB coupler 82, and an add port 85 and an output port 86 to the second 3dB coupler 87, wherein a light-radiation grating 81 which is discussed in the aforementioned preferred embodiment is imprinted in the optical waveguide arms 88.
In operation, a wavelength division-multiplexed (WDM) light signal Eli is supplied to the input port 83, and is equally divided at the first 3dB coupler 82 to be propagated through the optical waveguide arms 88, so that light signals of a specific wavelength ~, among the WDM
light signals ~ thus divided, are reflected to be combined at the first 3dB coupler 82 by the light radiation grating 81. Then, the combined light signal of the wavelength ~1 is radiated from the drop port 84. On the other hand, the WDM
light signals Eli excluding the reflected light signals of the wavelength ~1 are transmitted through the optical waveguide arms 88 to be combined at the second 3dB coupler 87. Then, the combined WDM light signal Eli is radiated from the output port 86.
On the contrary, when a light signal of the specific wavelength ~I is supplied to the add port 85, the supplied light signal is equally divided to be propagated through the optical waveguide arms 88, and is reflected to be propagated through the optical waveguide arms 88 back to the second 3dB coupler 87 by the light-radiation grating 81.
At the second 3dB coupler 87, the reflected light signals are combined to be radiated from the output port 86.
In this ADD/DROP filter, a pitch A/2 of the light radiation grating 81 may be changed dependently on the wavelength of a light signal to be added or dropped, and a light signal of a specific wavelength can be added or dropped with low loss and suppressed cross-talk with adjacent channels.
Another optical device formed with a grating therein in another preferred embodiment according to the invention will be explained with reference to Figure 11, wherein like parts are indicated by like reference numerals as used in Figures 1A to 1C, and 7A and 7B.
In this preferred embodiment, the optical device is for an optical waveguide 9 having a core which is shown in the former preferred embodiment. As explained before, the optical waveguide 9 may be replaced by an optical fiber comprising a core and a cladding layer.
The optical waveguide 9 is placed on a fine movement table 71 which is finely moved in the direction Z
of light propagation by a driver 73 which is controlled by a computer (not shown), and a phase-grating mask 4 has a grating striation 4A formed with phase shift portions 4B of ~r as shown in Figure 12. The phase shift portions 4B of ~r are provided at positions which are determined by the equation (2).
Y=~sin(C~Z)~/~C'Z~ (2) where C is a constant.
The positions correspond to a zero value (y=0) of the equation (2), that is, a phase of a transmission light of a specific wavelength is shifted by n at points meeting the equation (3).
Z=mgr / C ( 3 ) where m is a positive or negative integer (m=..., -3,-2, -1, 1, 2, 3, ...).
In order to fabricate a low-reflection grating, it is derived from detailed calculations that a coupling coefficient which is proportional to the increase of the grating striation refractive index is required to be pro-portional to "sin(Z)/(Z)" which is a value of the equation (2), assuming that the constant C is one (C=1). The value of equation (2) can be positive or negative. In practice, however, it is impossible to fabricate a light-radiation grating having a negative value when the refractive index is changed.
In this regard, the phase-grating mask 4 is formed with the phase-shift portions 4B of ~r in the grating stria-tion 4A at the positions where the value of "sin(Z)/(Z)"
changes between plus and minus values. This provides the same effect as a monotonous grating having a negative value when the refractive index is changed.
In Figure 13A, the core 9B of the optical wave-guide 9 is wider at positions corresponding to the phase-shift portions 4B of the grating striation 4A of the phase-grating mask 4. In more detail, the width of the core 9B is defined by the equation (4).
to w=wo-ow~ (sin(c~z) I/I (c~z> I (4) In operation, UV light 11 having a beam spot of approximately 5o ~cm is scanned over the phase-grating mask 4 and the core 9B of the optical waveguide 9 with a scanning speed proportional to "1/(Isin(C~Z)I/I(C~Z)I". This scan ning of the W light 11 is carried out by the fine movement table 71 which is finely moved in the direction Z of the light propagation by the driver 73.
In Figure 13B, an envelope of the increase of refractive indices is obtained to be approximately propor tional to "Isin(C~Z)I/IC~ZI", because the increase of a refractive index is proportional to the radiation time of the UV light 11.
In Figure 13C, an effective refractive index Neff of the optical waveguide 9 is changed depending on the width of the core 9B. The change of the effective refractive index Neff of the optical waveguide 9 is determined to com-pensate the mean value of the increased refractive indices (Figure 13B) by optimizing the width of the core 9B in accordance with the equation (4).
In Figure 13D, the mean value of the refractive indices (Figure 13B) and the effective refractive index Neff are cancelled to derive an index grating of a low-reflec-tion-spectrum property.
In an experiment, an excimer laser light of Gaussian-distribution was scanned to radiate, via the phase grating mask 4, to the core 9B of the optical waveguide 9.
USING SAME, AND METHOD OF FABRICATING SAME
The invention relates to an optical device formed with a grating therein, an ADD/DROP filter using the same, and a method of fabricating the same, and more particularly to an optical device such as an optical waveguide in which a core is imprinted with an index grating, an ADD/DROP
filter using such an optical waveguide, and a method of fabricating such an optical device having a core imprinted with an index grating.
A conventional optical fiber having a core imprinted with an index grating is disclosed in U.S. Patent No. 5,367,588, issued on November 22, 1994.
In the conventional optical fiber, the index grating is imprinted in the core using a silica-glass phase-grating mask, which is held in close proximity to an outer periphery thereof. A UV (ultraviolet) light beam emitted from a KrF excimer laser light source is radiated onto a phase-grating mask at a normal incident angle, so that an interference pattern is imprinted in the core of the optical fiber in accordance with the photo-induced effect.
Such an optical fiber having a core imprinted with an index grating is used for the transmission and deflection of light power supplied to the core thereof.
An optical waveguide having a core imprinted with an index grating can be fabricated by replacing the optical fiber with an optical waveguide, and it will be described in more detail prior to the description of preferred embodiments of the invention.
However, the conventional optical waveguide having the core imprinted with the index grating has disadvantages in that a low-reflection spectrum property is of a square law curve which is different from a desired rectangular property, and a high-reflection spectrum property has ripples at a rising portion thereof. Consequently, the reflection power of a light signal fluctuates, even if a small change occurs in the wavelength of the light signal.
Accordingly, it is an object of the invention to provide an optical device formed with a grating, an ADD/DROP
filter using the same, and a method of fabricating the same in which high- and low-reflection spectrum properties are obtained to be rectangular.
It is another object of the invention to provide an optical device formed with a grating, an ADD/DROP filter using the same, and a method of fabricating the same in which the reflection power of a light signal is not changed, even if the wavelength of the light signal varies slightly.
According to a first feature of the invention, an optical device formed with a grating comprises:
an optical waveguide comprising a core of a first refractive index and a cladding layer of a second refractive index lower than the first refractive index, the core being imbedded in the cladding layer;
a grating of a periodically-striated distribution of refractive indices to be formed in the optical waveguide in a direction of light propagation in accordance with a UV
light radiation thereto;
wherein a width of the core is changed in the direction of light propagation.
According to a second feature of the invention, an optical device formed with a low-reflection grating comprises:
an optical waveguide comprising a core of a first refractive index and a cladding layer of a second refractive index lower than the first refractive index, the core being imbedded in the cladding layer;
a grating of a periodically-striated distribution of refractive indices to be formed in the optical waveguide in a direction of light propagation in accordance with a W
light radiation thereto;
wherein a width W of the core is changed at a position Z in the direction of light propagation in accordance with an equation as defined below, W=Wp-OW~Isin(C~Z)~/~C'Z~
where Wo is an inherent and unchanged width of the core, OW is a difference between the unchanged width Wo and a minimum width of the core, and C is a constant; and, an envelope of said periodically-striated distri-bution of refractive indices for the grating is changed in accordance with a value of "~sin(C~Z)~/~C~Z~", whereby light of a specific wavelength to be transmitted through the optical waveguide is shifted in phase at the position Z, meeting an equation as defined below, Z=m~r/C (m=...,-3,-2,-1,1,2,3,...).
According to a third feature of the invention, an ADD/DROP filter using an optical device formed with a grating comprises:
first and second optical waveguides formed on a substrate, such that the first and second optical waveguides are arranged to be proximate to each other at first and second portions to provide first and second optical couplers, each of the first and second optical waveguides comprising a core of a ffirst refractive index and a cladding layer of a second refractive index lower than the first refractive index, the core being embedded in the cladding layer;
a grating of a periodically-striated distribution of refractive indices to be formed in each of the first and second optical waveguides in a direction of light propagation in accordance with a W light radiation thereto;
wherein a width of the core is changed in the direction of the light propagation.
According to a fourth feature of the invention, a method of fabricating an optical device formed with a grating comprises the steps of:
forming a core of a first refractive index having a width W which is changed at a position Z in a direction of light propagation in accordance with an equation as defined below, W=Wo-~W~~sin(C~Z)~/~C~Z~
where Wois an inherent and unchanged width of the core, OW is a difference between the unchanged width Wo and a minimum width of the core, and C is a constant, the core being embedded in a cladding layer of a second refractive index lower than the first refractive index;
positioning a phase-grating mask over the core, the phase-grating mask having a grating striation which is shifted in phase at the position Z as defined below by ~, Z=m~/C (m=...,-3,-2,-1,1,2,3,...);
scanning a W light to be radiated via the phase grating mask to the core in said direction of the light propagation in accordance with a speed determined by a value of "1/(~sin(C~Z)~/~C~Z~)", thereby providing a low reflec tion grating of a periodically-striated distribution of refractive indices.
According to a fifth feature of the invention, a method of fabricating an optical device formed with a low-reflection grating, comprises the steps of:
forming a core of a first refractive index having a width W which is changed at a position Z in a direction of light propagation in accordance with an equation as defined below, W=Wa-OW~~sin(C~Z)~/IC'Z~
where Wo is an inherent and unchanged width of the core, OW is a difference between the unchanged width Wo and a minimum width of the core, and C is a constant, the core being embedded in a cladding layer of a second refractive index lower than the first refractive index;
radiating UV light onto the core to derive an envelope of a periodically-striated distribution of refrac-tive indices, the envelope changing in accordance with a value of "~sin(C~Z)~/~C~Z~"; and, radiating UV light of a fine spot surplusly onto the core at the position Z to shift a phase of a light to be propagated in the direction by ~r as defined below, 5 Z=m~r/C (m=...,-3,-2,-1,1,2,3,...) thereby providing a low-reflection grating of the periodically-striated distribution of refractive indices.
The invention will now be described, by way of example, with reference to the appended drawings, wherein:
Figure lA is an explanatory diagram showing a conventional optical waveguide having a core imprinted with a grating;
Figure 1B is a partial side view showing the conventional optical waveguide as shown in Figure 1A;
Figure iC is a partially-enlarged view showing the conventional optical waveguide as shown in Figure lA;
Figure 1D is an explanatory diagram showing a refractive index along a core of the conventional optical waveguide as shown in Figure lA;
Figure 2 is an explanatory diagram showing a desired high-reflection spectrum property in the conven-tional optical waveguide as shown in Figure lA;
Figure 3 is an explanatory diagram showing a desired envelope for refractive indices of the core in the conventional optical waveguide as shown in Figure lA;
Figure 4 is an explanatory diagram showing a prac-tically realized envelope for refractive indices of the core in the conventional optical waveguide as shown in Figure lA;
Figures 5 and 6 are explanatory diagrams showing normalized reflection power dependent on the wavelength of light signals supplied to the conventional optical waveguide as shown in Figure lA;
Figures 7A and 7B are explanatory diagrams showing an optical device in a preferred embodiment according to the invention;
Figures 8A to 8D are explanatory diagrams showing a configuration of a core and refractive indices thereof in the optical device in the preferred embodiment according to the invention;
Figure 9 is an explanatory diagram showing normalized reflection power dependent on the wavelength of a light signal supplied to the optical device in the preferred embodiment according to the invention;
Figure 10 is an explanatory diagram showing an ADD/DROP filter using an optical device in a preferred embodiment according to the invention;
Figure 11 is an explanatory diagram showing an optical device in a preferred embodiment according to the invention;
Figure 12 is an explanatory diagram showing a phase-grating mask used in the optical device as shown in Figure 11;
Figures 13A to 13D are explanatory diagrams showing a configuration of a core and refractive indices thereof in the optical device as shown in Figure 11;
Figure 14 is an explanatory diagram showing normalized reflection power dependent on the wavelength of a light signal supplied to the optical device as shown in Figure 11; and, Figure 15 is an explanatory diagram showing a method of fabricating a core of an optical waveguide in the optical device as shown in Figure 11.
Before describing preferred embodiments according to the invention, a conventional optical device in which a core of an optical waveguide is imprinted with an index grating will be described.
Figure lA shows a fabrication system for the conventional optical device in which a core of an optical waveguide is imprinted with an index grating. The fabri-cation system comprises a KrF excimer laser 1 for emitting UV light 11 having a wavelength of 248nm, a mirror 2 for reflecting the light 11 in a predetermined direction, a lens 3 for focusing the reflected light 11 on a predetermined point, a phase-grating mask 4 having a grating striation 4A
of a pitch A for generating an inference fringe of a pitch A/2, a light source 5 for emitting white light, an optical fiber 7A for transmitting the white light, an optical wave-guide 9 positioned below the grating striation 4A of the phase-grating mask 4 and optically-coupled at an input to the optical fiber 7A, an optical fiber 7B optically-coupled to an output of the optical waveguide 9, and a spectrum analyzer 6 for receiving an output light of the optical waveguide 9 via the optical fiber 7B and analysing the spectrum of the output light.
Figures 1B and 1C show the relation of the phase grating mask 4, the optical waveguide 9, and the optical fibers 7A and 7B, wherein the optical waveguide 9 comprises a substrate 9A, such as Si, quartz, etc. , a core 9B doped with Ge02, P205 or B203, and a buffer layer 9C, and wherein each of the optical fibers 7A and 7B comprises a core 7a and a cladding layer 7b, such that the cores 7a of the optical fibers 7A and 7B are optically-coupled to the input and output ends of the core 9B of the optical waveguide 9.
In operation, the UV light having a wavelength of 248nm emitted from the KrF excimer laser 1 is radiated via the mirror 2 and the lens 3 to the phase-grating mask 4, so that an interference fringe of pitch A/2 is generated in accordance with the interference of plus and minus (positive and negative) first-order diffraction light by the grating striation of the pitch A. The interference fringe thus-generated is radiated to the core 9B of the optical wave-guide, so that the refractive index of the core 9B increases depending on the light intensity of the interference fringe in accordance with the photo-refractive effect.
This is shown in Figure 1D, wherein On indicates the periodically-photo-induced increase of the refractive index along the light-propagation direction of the core 9B
in the optical waveguide 9. Consequently, a grating of a pitch A/2 is imprinted in the core 9B of the optical waveguide 9, as shown in Figure 1C.
Fig 2. shows an ideal spectrum property of the grating imprinted in the core 9B of the optical waveguide 9, wherein light of a Bragg wavelength ~1 which is determined by the below equation is selectively reflected to provide a narrow band-stop filter for an optical communication system.
~i=A/ ( 2 ~ Nef f ) where Neff is an effective refractive index of the waveguide 9.
In an ideal spectrum, a normalized reflection power loss is only as large as -38dB of light power trans-mitted, and is essentially constant in a band-pass wave-length region. In other words, a light propagated through the core 9B of the optical waveguide 9 is transmitted without any substantial reflection, while the normalized reflection power sharply increases as a wavelength of the light transits the band-pass wavelength region to a band-stop wavelength region. In the band-stop wavelength region, the normalized reflection power loss may be as much as all of the light power being transmitted, that is, the light is reflected without any substantial transmission via the core 9B of the optical waveguide 9. The ideal spectrum is considered to be realized by radiating a light beam having an intensity distribution of Gaussian-type to the phase-grating mask 4, thereby providing a modulation degree of the grating imprinted in the core 9B of the optical waveguide 9 which will be a pattern of Gaussian-type. This is shown in Figure 3, wherein S indicates a size of the Gaussian-type light beam which is obtained by placing a spatial-amplitude filter in the path of the light 11.
The conventional optical waveguide having the core imprinted with the index grating, however, has the disadvan-tage that a mean value of refractive indices of the core is increased, as shown in Figure 4, although the modulation degree of a Gaussian-type is obtained for the index grating when the UV light of Gaussian-type is radiated to the optical waveguide 9. Ripples are generated on the lower-wavelength side of the band-stop wavelength region, as shown in Figure 5, resulting in the fluctuation of reflection power in accordance with the slight change of a wavelength of a transmission light. A reflection spectrum property is in the shape of a square curve for a low-reflection grating, as shown in Figure 6, when the W light of Gaussian-type is radiated to the optical waveguide 9.
Next, an optical device formed with a grating therein in a preferred embodiment according to the invention will be explained with reference to Figures 7A and 7B, wherein like parts are indicated by like reference numerals to those used in Figures lA to 1C.
In the preferred embodiment, the optical device is for an optical waveguide 9 having a core 9B which is shown in Figure 7B. As a matter of course, the optical waveguide 9 may be replaced by an optical fiber comprising a core and a cladding layer.
The core 9B of the optical waveguide 9 is of a width W at a position Z in the direction of light propaga-tion as defined by the equation (1).
W=Wo-~W~exp(-Z2/S2) (1) where Wo is an inherent and unchanged width of the core 9B, OW is a decreased width of the core 9B, and S is a diameter of a Gaussian-type light beam that is radiated onto a phase-grating mask 4.
In Figure 8A, the width of the core 9B is decreased, as a distance to the centre point 0 is decreased.
As shown therein, the core 9B is imprinted with an index grating of a pitch A/2.
In Figure 8B, an effective refractive index Neff of the optical waveguide 9 is changed proportionally to the width of the core 9B to derive a Gaussian-distribution.
In Figure 8C, a mean value of refractive indices to be increased by the radiation of the UV light is maximum at the centre point 0.
In Figure 8D, the decreased width OW of the core 9B is optimized to derive a flat mean value of refractive indices of the core 9B in the direction of the light propa-5 gation in accordance with the cancellation of the increase and decrease of the refractive indices as shown in Figures 8B and 8C.
In an experiment, the core 9B of the optical waveguide 9 was formed by doping Ge02 into a predetermined 10 region of a Si substrate 9A. The core 9B was structured to have the width W of 5~m at the centre point (Z=0) and the unchanged width Wo of 6~Cm, thereby deriving the decreased width amount AW of l~Cm. W light of Gaussian-distribution emitted from the KrF excimer laser 1 was radiated onto the phase-grating mask 4 in accordance with a beam spot S of 0.8mm, so that the maximum value of photo-induced refractive indices ~n was 0.1. In this optical waveguide 9, a specific refractive index of the core 9B was 0.5, and a pitch A/2 of the grating imprinted in the core 9B was 0.5373~,m. The width of the core 9B was changed with l~,m for a light propagation distance of 400~,m, so that a core width-changing rate was obtained to be 0. 0025 rad ( l~Cm/400~Cm) , while a length of the core 9B representing a taper of Gaussian-type is 0.8mm which is the same as the beam spot size S.
Figure 9 shows a spectrum property obtained in the optical waveguide 9 having the core 9B imprinted with the index grating in the preferred embodiment, wherein this spectrum property is similar to a rectangular spectrum property.
Figure 10 shows an ADD/DROP filter using an optical device formed with a grating therein in a preferred embodiment according to the invention. The ADD/DROP filter is a Mach-Zehnder interferometer, and comprises first and second 3dB couplers 82 and 87, a pair of optical waveguide arms 88 for connecting the first and second 3dB couplers 82 and 87, and optical waveguides for connecting an input port 83 and a drop port 84 to the first 3dB coupler 82, and an add port 85 and an output port 86 to the second 3dB coupler 87, wherein a light-radiation grating 81 which is discussed in the aforementioned preferred embodiment is imprinted in the optical waveguide arms 88.
In operation, a wavelength division-multiplexed (WDM) light signal Eli is supplied to the input port 83, and is equally divided at the first 3dB coupler 82 to be propagated through the optical waveguide arms 88, so that light signals of a specific wavelength ~, among the WDM
light signals ~ thus divided, are reflected to be combined at the first 3dB coupler 82 by the light radiation grating 81. Then, the combined light signal of the wavelength ~1 is radiated from the drop port 84. On the other hand, the WDM
light signals Eli excluding the reflected light signals of the wavelength ~1 are transmitted through the optical waveguide arms 88 to be combined at the second 3dB coupler 87. Then, the combined WDM light signal Eli is radiated from the output port 86.
On the contrary, when a light signal of the specific wavelength ~I is supplied to the add port 85, the supplied light signal is equally divided to be propagated through the optical waveguide arms 88, and is reflected to be propagated through the optical waveguide arms 88 back to the second 3dB coupler 87 by the light-radiation grating 81.
At the second 3dB coupler 87, the reflected light signals are combined to be radiated from the output port 86.
In this ADD/DROP filter, a pitch A/2 of the light radiation grating 81 may be changed dependently on the wavelength of a light signal to be added or dropped, and a light signal of a specific wavelength can be added or dropped with low loss and suppressed cross-talk with adjacent channels.
Another optical device formed with a grating therein in another preferred embodiment according to the invention will be explained with reference to Figure 11, wherein like parts are indicated by like reference numerals as used in Figures 1A to 1C, and 7A and 7B.
In this preferred embodiment, the optical device is for an optical waveguide 9 having a core which is shown in the former preferred embodiment. As explained before, the optical waveguide 9 may be replaced by an optical fiber comprising a core and a cladding layer.
The optical waveguide 9 is placed on a fine movement table 71 which is finely moved in the direction Z
of light propagation by a driver 73 which is controlled by a computer (not shown), and a phase-grating mask 4 has a grating striation 4A formed with phase shift portions 4B of ~r as shown in Figure 12. The phase shift portions 4B of ~r are provided at positions which are determined by the equation (2).
Y=~sin(C~Z)~/~C'Z~ (2) where C is a constant.
The positions correspond to a zero value (y=0) of the equation (2), that is, a phase of a transmission light of a specific wavelength is shifted by n at points meeting the equation (3).
Z=mgr / C ( 3 ) where m is a positive or negative integer (m=..., -3,-2, -1, 1, 2, 3, ...).
In order to fabricate a low-reflection grating, it is derived from detailed calculations that a coupling coefficient which is proportional to the increase of the grating striation refractive index is required to be pro-portional to "sin(Z)/(Z)" which is a value of the equation (2), assuming that the constant C is one (C=1). The value of equation (2) can be positive or negative. In practice, however, it is impossible to fabricate a light-radiation grating having a negative value when the refractive index is changed.
In this regard, the phase-grating mask 4 is formed with the phase-shift portions 4B of ~r in the grating stria-tion 4A at the positions where the value of "sin(Z)/(Z)"
changes between plus and minus values. This provides the same effect as a monotonous grating having a negative value when the refractive index is changed.
In Figure 13A, the core 9B of the optical wave-guide 9 is wider at positions corresponding to the phase-shift portions 4B of the grating striation 4A of the phase-grating mask 4. In more detail, the width of the core 9B is defined by the equation (4).
to w=wo-ow~ (sin(c~z) I/I (c~z> I (4) In operation, UV light 11 having a beam spot of approximately 5o ~cm is scanned over the phase-grating mask 4 and the core 9B of the optical waveguide 9 with a scanning speed proportional to "1/(Isin(C~Z)I/I(C~Z)I". This scan ning of the W light 11 is carried out by the fine movement table 71 which is finely moved in the direction Z of the light propagation by the driver 73.
In Figure 13B, an envelope of the increase of refractive indices is obtained to be approximately propor tional to "Isin(C~Z)I/IC~ZI", because the increase of a refractive index is proportional to the radiation time of the UV light 11.
In Figure 13C, an effective refractive index Neff of the optical waveguide 9 is changed depending on the width of the core 9B. The change of the effective refractive index Neff of the optical waveguide 9 is determined to com-pensate the mean value of the increased refractive indices (Figure 13B) by optimizing the width of the core 9B in accordance with the equation (4).
In Figure 13D, the mean value of the refractive indices (Figure 13B) and the effective refractive index Neff are cancelled to derive an index grating of a low-reflec-tion-spectrum property.
In an experiment, an excimer laser light of Gaussian-distribution was scanned to radiate, via the phase grating mask 4, to the core 9B of the optical waveguide 9.
A maximum value of 0.023 was derived for a photo-induced refractive index Vin, so that a specific refractive index of the core 9B was 0.5, and a pitch A/2 and a length of the imprinted index grating were 0.5373~,m, and 40mm, respec-tively. The width of the core 9B was set to meet the following parameters.
Wo=6~M, AW=l~Cm, and C=4/~m.
The optical waveguide 9 having the core 9B
imprinted with the grating provides a low-reflection l0 spectrum property as shown in Figure 14, wherein a light power of appoximately -lOdB (100) is reflected with a rectangular-spectrum pattern.
Figure 15 shows another method of providing a grating imprinted in the core 9B of the optical waveguide 9 with no phase-grating mask and the same effect as a mono tonous grating having a negative value at the change of a refractive index.
In this method, an index grating is first imprinted in the core 9B of the optical waveguide 9 in accordance with UV light radiation via the phase-grating mask 4 to the core 9B. Then, laser beams each having a spot size of 50>a,m are surplusly radiated to the core 9B at positions where the value of "sin(C~Z)/(C~Z)" changes from positive to negative or vice versa, that is, points meeting the equation "Z=m~r/C(m=...,-3, -2,-1, 1,2,3,...)."
In accordance with this surplus light radiation, the phase of a transmission light is shifted by an amount determined from the equation (5).
B=ko~Neff ~WS~On (5) where B is a phase shift amount, ko is the number of waves in vacuum, Neff is the effective refractive index of the optical waveguide 9, Ws is the spot size of the laser beam, and ~n is the change of a refractive index in accordance with the surplus light radiation.
When the phase shift amount B is ~r, the same result as the provision of the phase-grating mask 4 having the phase shift portions 4B of ~t is obtained, even if the phase-grating mask 4 is not used.
In this method, the small the spot size of the laser beam is, the better the phase shift result is.
5 When the phase-shift amount B is ~r, equation (5) is modified according to equation (6) below.
Ws=~r/ (Ko~Neff ~On) (6) As is apparent from the equation (6), the spot size Ws of the laser beam is dependent on the refractive 10 index change On.
In a preferred embodiment, the spot size of 50~,m is determined in accordance with a wavelength ~ of 1.55~m, an effective refractive index Neff of 1.45, and a reflective index change of 0.0107.
Wo=6~M, AW=l~Cm, and C=4/~m.
The optical waveguide 9 having the core 9B
imprinted with the grating provides a low-reflection l0 spectrum property as shown in Figure 14, wherein a light power of appoximately -lOdB (100) is reflected with a rectangular-spectrum pattern.
Figure 15 shows another method of providing a grating imprinted in the core 9B of the optical waveguide 9 with no phase-grating mask and the same effect as a mono tonous grating having a negative value at the change of a refractive index.
In this method, an index grating is first imprinted in the core 9B of the optical waveguide 9 in accordance with UV light radiation via the phase-grating mask 4 to the core 9B. Then, laser beams each having a spot size of 50>a,m are surplusly radiated to the core 9B at positions where the value of "sin(C~Z)/(C~Z)" changes from positive to negative or vice versa, that is, points meeting the equation "Z=m~r/C(m=...,-3, -2,-1, 1,2,3,...)."
In accordance with this surplus light radiation, the phase of a transmission light is shifted by an amount determined from the equation (5).
B=ko~Neff ~WS~On (5) where B is a phase shift amount, ko is the number of waves in vacuum, Neff is the effective refractive index of the optical waveguide 9, Ws is the spot size of the laser beam, and ~n is the change of a refractive index in accordance with the surplus light radiation.
When the phase shift amount B is ~r, the same result as the provision of the phase-grating mask 4 having the phase shift portions 4B of ~t is obtained, even if the phase-grating mask 4 is not used.
In this method, the small the spot size of the laser beam is, the better the phase shift result is.
5 When the phase-shift amount B is ~r, equation (5) is modified according to equation (6) below.
Ws=~r/ (Ko~Neff ~On) (6) As is apparent from the equation (6), the spot size Ws of the laser beam is dependent on the refractive 10 index change On.
In a preferred embodiment, the spot size of 50~,m is determined in accordance with a wavelength ~ of 1.55~m, an effective refractive index Neff of 1.45, and a reflective index change of 0.0107.
15 In this preferred embodiment in which the surplus radiation of the laser beam is used, a phase-grating mask 4 having no phase shift portion 4B of ~r, but having the ordinary striation 4A may be used for the scanning of light radiation in the direction of light propagation. In such a case, a periodically-striated distribution of refractive indices which is similar to that as shown in Figure 13A is obtained, having an envelope determined by the value of nlsin(C~Z)~/~C'Z~"~
Although the invention has been described with respect to specific embodiment for complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modification and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth.
Although the invention has been described with respect to specific embodiment for complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modification and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth.
Claims (15)
1. An optical device formed with a grating therein, comprising:
an optical waveguide comprising a core of a first refractive index and a cladding layer of a second refractive index lower than said first refractive index, said core being imbedded in said cladding layer;
a grating of a periodically-striated distribution of refractive indices to be formed in said optical waveguide in a direction of light propagation in accordance with UV
light radiation thereto;
wherein a width of said core is changed in said direction of said light propagation.
an optical waveguide comprising a core of a first refractive index and a cladding layer of a second refractive index lower than said first refractive index, said core being imbedded in said cladding layer;
a grating of a periodically-striated distribution of refractive indices to be formed in said optical waveguide in a direction of light propagation in accordance with UV
light radiation thereto;
wherein a width of said core is changed in said direction of said light propagation.
2. The optical device as defined in claim 1, wherein said width of said core is tapered along its length in such a way that a mean value of said refractive indices resulting from said UV light radiation along said core length is maintained generally constant, UV light of said UV
light radiation having a predetermined spatial distribution of light intensity in said direction of said light propagation along said core length.
light radiation having a predetermined spatial distribution of light intensity in said direction of said light propagation along said core length.
3. The optical device as defined in claim 1, wherein said core is doped with one of GeO2, P2O5 and B2O3.
4. The optical device as defined in claim 1, wherein a rate of changing said width of said core is set by a changing amount of less than 1µm for said width relative to a distance of 100µm in said direction of said light propagation.
5. The optical device as defined in claim 2, wherein said predetermined spatial distribution of said UV
light is a Gaussian-type.
light is a Gaussian-type.
6. An optical device formed with a low reflection grating therein, comprising:
an optical waveguide comprising a core of a first refractive index and a cladding layer of a second refractive index lower than said first refractive index, said core being imbedded in said cladding layer;
a grating of a periodically-striated distribution of refractive indices to be formed in said optical waveguide in a direction of light propagation in accordance with a UV
light radiation thereto;
wherein a width W of said core is changed at a position Z in said direction of said light propagation in accordance with an equation as defined below, W=W0-.DELTA.W~¦sin(C~Z)¦/¦C~Z¦
where W0 is an inherent and unchanged width of said core, .DELTA.W
is a difference between said unchanged width W0 and a minimum width of said core, and C is a constant, and wherein an envelope of said periodically-striated distribution of refractive indices for said grating is changed in accordance with a value of "¦sin(C~Z)¦/¦C~Z¦", whereby light of a specific wavelength to be transmitted through said optical waveguide is shifted in phase at said position Z meeting an equation as defined below, Z=m.pi./C(m=...,-3,-2,-1,1,2,3,...).
an optical waveguide comprising a core of a first refractive index and a cladding layer of a second refractive index lower than said first refractive index, said core being imbedded in said cladding layer;
a grating of a periodically-striated distribution of refractive indices to be formed in said optical waveguide in a direction of light propagation in accordance with a UV
light radiation thereto;
wherein a width W of said core is changed at a position Z in said direction of said light propagation in accordance with an equation as defined below, W=W0-.DELTA.W~¦sin(C~Z)¦/¦C~Z¦
where W0 is an inherent and unchanged width of said core, .DELTA.W
is a difference between said unchanged width W0 and a minimum width of said core, and C is a constant, and wherein an envelope of said periodically-striated distribution of refractive indices for said grating is changed in accordance with a value of "¦sin(C~Z)¦/¦C~Z¦", whereby light of a specific wavelength to be transmitted through said optical waveguide is shifted in phase at said position Z meeting an equation as defined below, Z=m.pi./C(m=...,-3,-2,-1,1,2,3,...).
7. The optical device as defined in claim 6, wherein said core is doped with one of GeO2, P2O5 and B2O3.
8. The optical device as defined in claim 6, wherein a rate of changing said width of said core is set by a changing amount of less than 1µm for said width relative to a distance of 100µm in said direction of said light propagation.
9. An ADD/DROP filter using an optical device formed with a grating therein, comprising:
first and second optical waveguides formed on a substrate, such that said first and second optical waveguides are arranged to be proximate to each other at first and second portions to provide first and second optical couplers, each of said first and second optical waveguides comprising a core of a first refractive index and a cladding layer of a second refractive index lower than said first refractive index, said core being embedded in said cladding layer;
a grating of a periodically-striated distribution of refractive indices to be formed in each of said first and second optical waveguides in a direction of a light propagation in accordance with a UV light radiation thereto;
wherein a width of said core is changed in said direction of said light propagation.
first and second optical waveguides formed on a substrate, such that said first and second optical waveguides are arranged to be proximate to each other at first and second portions to provide first and second optical couplers, each of said first and second optical waveguides comprising a core of a first refractive index and a cladding layer of a second refractive index lower than said first refractive index, said core being embedded in said cladding layer;
a grating of a periodically-striated distribution of refractive indices to be formed in each of said first and second optical waveguides in a direction of a light propagation in accordance with a UV light radiation thereto;
wherein a width of said core is changed in said direction of said light propagation.
10. The ADD/DROP filter as defined in claim 9, wherein said width of said core is tapered along its length in such a way that a mean value of said refractive indices resulting from said UV light radiation along said core length is maintained generally constant, UV light of said UV
light radiation having a predetermined spatial distribution of light intensity in said direction of said light propagation along said core length.
light radiation having a predetermined spatial distribution of light intensity in said direction of said light propagation along said core length.
11. The ADD/DROP filter as defined in claim 9, wherein said core is doped with one of GeO2, P2O5 and B2O3.
12. The ADD/DROP filter as defined in claim 9, wherein a rate of changing said width of said core is set by a changing amount of less than 1µm for said width relative to a distance of 100µm in said direction of said light propagation.
13. The ADD/DROP filter as defined in claim 10, wherein said predetermined spatial distribution of said UV
light is Gaussian-type.
light is Gaussian-type.
14. A method of fabricating an optical device formed with a low-reflection grating, comprising the steps of:
forming a core of a first refractive index having a width W which is changed at a position Z in a direction of light propagation in accordance with an equation as defined below, W=W0-.DELTA.W~¦sin(C~Z)¦/¦C~Z¦
where W0 is an inherent and unchanged width of said core, .DELTA.W
is a difference between said unchanged width W0 and a minimum width of said core, and C is a constant, said core being embedded in a cladding layer of a second refractive index lower than said first refractive index;
positioning a phase-grating mask over said core, said phase-grating mask having a grating striation which is shifted in phase at said position Z as defined below by .pi., Z=m.pi./C(m=...,-3,-2,-1,1,2,3,...);
scanning a UV light to be radiated via said phase-grating mask to said core in said direction of said light propagation in accordance with a speed determined by a value of "1/(¦sin(C~Z)¦/¦C~Z¦)", thereby providing a low-reflection grating of a periodically-striated distribution of refractive indices.
forming a core of a first refractive index having a width W which is changed at a position Z in a direction of light propagation in accordance with an equation as defined below, W=W0-.DELTA.W~¦sin(C~Z)¦/¦C~Z¦
where W0 is an inherent and unchanged width of said core, .DELTA.W
is a difference between said unchanged width W0 and a minimum width of said core, and C is a constant, said core being embedded in a cladding layer of a second refractive index lower than said first refractive index;
positioning a phase-grating mask over said core, said phase-grating mask having a grating striation which is shifted in phase at said position Z as defined below by .pi., Z=m.pi./C(m=...,-3,-2,-1,1,2,3,...);
scanning a UV light to be radiated via said phase-grating mask to said core in said direction of said light propagation in accordance with a speed determined by a value of "1/(¦sin(C~Z)¦/¦C~Z¦)", thereby providing a low-reflection grating of a periodically-striated distribution of refractive indices.
15. A method of fabricating an optical device formed with a low-reflection grating, comprising the steps of:
forming a core of a first refractive index having a width W which is changed at a position Z in a direction of light propagation in accordance with an equation as defined below, W=W0-.DELTA.W~¦sin(C~Z)¦/¦C~Z¦
where W0 is an inherent and unchanged width of said core, .DELTA.W
is a difference between said unchanged width W0 and a minimum width of said core, and C is a constant, said core being embedded in a cladding layer of a second refractive index lower than said first refractive index;
radiating UV light onto said core to derive an envelope of a periodically-striated distribution of refractive indices, said envelope changing in accordance with a value of "¦sin(C~Z)¦/¦C~Z¦"; and, radiating UV light of a fine spot surplusly onto said core at said position Z to shift the phase of light to be propagated in said direction by n as defined below, Z=m.pi./C(m=...,-3,-2,-1,1,2,3,...) thereby providing a low reflection grating of said periodically-striated distribution of refractive indices.
forming a core of a first refractive index having a width W which is changed at a position Z in a direction of light propagation in accordance with an equation as defined below, W=W0-.DELTA.W~¦sin(C~Z)¦/¦C~Z¦
where W0 is an inherent and unchanged width of said core, .DELTA.W
is a difference between said unchanged width W0 and a minimum width of said core, and C is a constant, said core being embedded in a cladding layer of a second refractive index lower than said first refractive index;
radiating UV light onto said core to derive an envelope of a periodically-striated distribution of refractive indices, said envelope changing in accordance with a value of "¦sin(C~Z)¦/¦C~Z¦"; and, radiating UV light of a fine spot surplusly onto said core at said position Z to shift the phase of light to be propagated in said direction by n as defined below, Z=m.pi./C(m=...,-3,-2,-1,1,2,3,...) thereby providing a low reflection grating of said periodically-striated distribution of refractive indices.
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP8-30657 | 1996-02-19 | ||
| JP8-30656 | 1996-02-19 | ||
| JP8030656A JP3022300B2 (en) | 1996-02-19 | 1996-02-19 | Optical device formed with grating and Add / Drop filter using the same |
| JP8030657A JP2830819B2 (en) | 1996-02-19 | 1996-02-19 | Optical device having low reflection grating and method of manufacturing low reflection grating |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2184453A1 CA2184453A1 (en) | 1997-08-20 |
| CA2184453C true CA2184453C (en) | 2001-01-16 |
Family
ID=26369051
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2184453 Expired - Fee Related CA2184453C (en) | 1996-02-19 | 1996-08-29 | Optical device formed with grating therein, add/drop filter using same, and method of fabricating same |
Country Status (3)
| Country | Link |
|---|---|
| CA (1) | CA2184453C (en) |
| DE (1) | DE19634955A1 (en) |
| SE (1) | SE517466C2 (en) |
Families Citing this family (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5889899A (en) * | 1997-10-15 | 1999-03-30 | Lucent Technologies Inc. | Article comprising a Bragg reflective Mach-Zehnder filter of reduced coupled dependence |
| GB9722421D0 (en) * | 1997-10-24 | 1997-12-24 | Univ Southampton | Optical grating |
Family Cites Families (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5367588A (en) * | 1992-10-29 | 1994-11-22 | Her Majesty The Queen In Right Of Canada, As Represented By The Minister Of Communications | Method of fabricating Bragg gratings using a silica glass phase grating mask and mask used by same |
| US5459801A (en) * | 1993-10-29 | 1995-10-17 | Rutgers University | Coupler used to fabricate add-drop devices, dispersion compensators, amplifiers, oscillators, superluminescent devices, and communications systems |
-
1996
- 1996-08-27 SE SE9603098A patent/SE517466C2/en not_active IP Right Cessation
- 1996-08-29 CA CA 2184453 patent/CA2184453C/en not_active Expired - Fee Related
- 1996-08-29 DE DE1996134955 patent/DE19634955A1/en not_active Ceased
Also Published As
| Publication number | Publication date |
|---|---|
| SE9603098D0 (en) | 1996-08-27 |
| SE517466C2 (en) | 2002-06-11 |
| CA2184453A1 (en) | 1997-08-20 |
| DE19634955A1 (en) | 1997-08-21 |
| SE9603098L (en) | 1997-08-20 |
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