Highly efficient focusing waveguide grating coupler using leaky mode

Provided is a focusing waveguide grating coupler using a leaky mode which can form single output beam while relieving the dependency on manufacturing processes. The focusing waveguide grating coupler of the present research includes: a substrate having a first refraction index n1; a first core layer having a second refraction index n2, the first core layer being formed on the substrate; a second core layer having a third refraction index n3, the second core layer being formed on the first core layer apart from the first core layer with a space d in between; a first cladding layer having a fourth refraction index n4, the first cladding layer being formed on the second core layer; a second cladding layer having a fifth refraction index n5, the second cladding layer being formed on the first cladding layer and inserted between the first core layer and the second core layer; and a Fresnel lens positioned on the second cladding layer, wherein the refractive indexes satisfy conditions of n5>(n2, n3)>n1 and n5>n4; and light inputted through the first and second core layers to the Fresnel lens as radiated leaky beam by a leaky mode formed according to the conditions, and the leaky beam forms an optical focus by performing single directional coupling.

FIELD OF THE INVENTION

The present invention relates to a focusing waveguide grating coupler and, more particularly, to a focusing waveguide grating coupler having enhanced coupling efficiency.

DESCRIPTION OF RELATED ART

Focusing waveguide grating lenses become smaller and lighter than conventional lenses such as spheric lenses, aspheric lenses and compound lenses. Since a light source and a detector can be integrated in a waveguide, the miniaturized focusing waveguide grating lenses can be applied to the areas in need of miniaturized lenses, for example, optical disk storages and parts for optical communication systems.

Conventional Fresnel lenses diffract and converge parallel light with a simple Fresnel diffraction grating. The flat Fresnel lenses substitute thick spheric lenses, and they are mostly used for display equipment such as projectors.

However, a grating lens of the present invention is more like a miniaturized focusing light source device, considering that a grating is formed on a waveguide and a guided mode becomes an input light source, while the conventional Fresnel lenses are mere substitution for spheric lenses.

An application of the grating formed on a planar waveguide to an optical pickup device is disclosed in a paper by S. Ura, T. Suhara, H. Nishihara, and J. Koyama, entitled “An Integrated-Optic Disk Pickup Device,” J. Lightwave Technology, Vol. 4, pp. 913–918, 1986.

FIG. 1is a perspective view illustrating a conventional focusing waveguide grating lens. Referring toFIG. 1, the focusing waveguide grating lens includes a substrate11, a laser diode13, a core layer12, and a focusing grating14.

The substrate11is positioned on a plane formed of an x axis and a y axis. The laser diode13is attached on one side of the substrate11and couples lights into the waveguide. The core layer12is placed on the substrate11and forms a planar waveguide. The focusing grating14converges the light emerging from the laser diode13and forms an optical focus15out of the waveguide on a z axis.

Although not illustrated in the drawing, a photodiode and a beam splitter may be integrated additionally to form an optical pickup head. A reference character ‘b’ denotes a distance between input of light, which is outputted from the laser diode13and inputted to a waveguide, and the center of the focusing grating14.

The focusing waveguide grating coupler comes in the spotlight as a next-generation optical pickup head, because it can be miniaturized by integrating basic functions for detecting optical signals on the waveguide, which is different from a conventional pickup head.

However, to put the optical pickup head to practical use, problems of low optical coupling efficiency and large optical focus should be solved. The low optical coupling efficiency, which is a structural matter, should be improved necessarily.

FIGS. 2A and 2Bare phase matching diagrams depicting a representative coupling method in which a focusing waveguide grating coupler can radiate output beams. Referring toFIGS. 2A and 2B, a coupler includes a substrate11and a core layer12. With an air layer and the substrate11as a boundary, a mode is formed on the core layer12and propagates in an arrow direction. Here, the refraction index (nf) of the core layer12is larger than the refraction index (ns) of the substrate11. When the guided mode meets the focusing grating14, which is a surface-relief grating, output beams20,20′ and21having a smaller propagation constant than the guided mode can be obtained. Here, the focusing grating14is a staircase-type uneven grating.

Referring toFIG. 2A, if a grating vector17which is determined by a grating period fulfills a condition of ‘−nck<Nk−K<nck’ with respect to a propagation constant16of the guided mode, a propagation constant (nsk)18of the substrate11, and a propagation constant (nck)19of the air layer, the output beams20and21are formed towards the air layer and the substrate11. In the above condition, K denotes grating vector (K=2π/Λ) and Λ denotes the period of the grating.

The reference numeral ‘20’ indicates the output beams coupled and propagating towards the substrate11. The reference numeral ‘22’ denotes a propagation vector locus of the output beams coupled and propagating towards the substrate11. The reference numeral ‘21’ indicates the output beams coupled and propagating towards the air layer, and the reference numeral ‘23’ denotes a propagation vector locus of the output beams coupled and propagating towards the air layer. Here, the propagation constant24of the core layer12is nfk.

Referring toFIG. 2B, if the grating vector17fulfills a condition of ‘−nsk<Nk−K<nck’, the output beam20′ is formed only toward the substrate11. This is called single-beam coupling.

Generally, the focusing waveguide grating coupler has two-way output beams20and21, which is described above. Therefore, even if optical losses such as butt-coupling loss and waveguide absorption loss are minimized, it is very hard to obtain the power coupling efficiency to the air layer of over 50%. Methods for improving the low efficiency can be divided into four. Among the four, a structure applicable to a pickup head will be described herein with reference to a second prior art and a third prior art.

FIG. 3is a cross-sectional view showing a focusing waveguide grating coupler that produces a single beam output by using a prism in accordance with a second prior art. Referring toFIG. 3, a waveguide includes a substrate11and a core layer12, and a mode is formed on the core layer12with an air layer and the substrate11as a boundary and propagates in an arrow direction. Here, the refraction index (nf) of the core layer12is larger than the refraction index (ns) of the substrate11. When the guided mode propagates and comes across with the staircase-type uneven focusing grating14, the output beam20′ having a smaller propagation constant than the guided mode is obtained.

The structure ofFIG. 3utilizes the single output beam20′ radiated toward the substrate11. To minimize the reflection of the output beam in the boundary surface under the substrate11, a prism25is attached to the bottom of the substrate so that the output beam could go through the boundary surface vertically without suffering from any unwanted refractions.

This method has a shortcoming that the pickup head could not be miniaturized because the output beam20′ cannot be radiated vertically with respect to the surface of the waveguide. Also, since the grating period should fulfill the condition of ‘−nsk<Nk−K<−nck’ as described inFIG. 2B, the grating period should be shorter than a half of the input wavelength.

For example, when the input wavelength is 400 nm, the grating period should be about 150 nm to produce the single output beam20′. The single output beam20′ can also be produced by forming the surface grating not in the rectangular staircase-type, which is described above, but in another shape.

FIG. 4is a cross-sectional view illustrating a focusing waveguide grating coupler using a thick holographic grating (a) that fulfill a Bragg condition and a focusing waveguide grating coupler using a blazed surface-relief grating (b) in accordance with a third prior art, the focusing waveguide grating coupler producing a single output beam.

Referring toFIG. 4, the waveguide includes a substrate11and a core layer12. With an air layer and the substrate11as boundaries, a mode is formed on the core layer12and propagates in an arrow direction. Here, the refractive index (nf) of the core layer12is larger than the refractive index (ns) of the substrate11. An input light can fulfill a Bragg condition, Nkμ−Kμ=Nckμ, and produce output beam21toward the air layer in the upper part of the substrate11, when the guided mode propagates and meets the blazed surface-relief grating26or a grating27having a structure of slant refraction index modulation.

In other words, the reference numeral21indicates the output beams coupled and propagating toward the air layer, while the reference numeral23indicates a propagation vector locus of the output beams coupled and propagating toward the air layer. Here, the propagation constant24of the core layer12, the propagation constant26of the guided mode TE01or TM01, the propagation constant18of the substrate11, and the propagation constant19of the air layer are nfk, Nk, nsk and nck, respectively.

FIG. 5is a cross-sectional view describing an optical focus in the blazed surface-relief grating ofFIGS. 4A and 4B. Theoretically, the gratings illustrated inFIGS. 4A and 4Bare the most desirable grating structures. However, the gratings should be fabricated to have an inclination angle changing at every location and a grating period of tens of nanometers, which is shorter than a wavelength to have the optical focus as shown inFIG. 5.

Current lithography technology has a limitation in the fabrication of the grating26satisfying the above conditions. Even if it is embodied successfully, it would hardly be reproducible.

Therefore, another approach is required other than the grating having a period shorter than a wavelength which is suggested in the structure of a coupler having a single output beam in accordance with the first through third prior arts.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide a focusing waveguide grating coupler radiating a single output beam out of the waveguide by using a waveguide having a single leaky-mode, instead of a grating having a short period, the grating which is required in a conventional focusing coupler having a single output beam. In accordance with the present invention, it is possible to fabricate a focusing waveguide grating coupler that has high coupling efficiency with reduced dependency on fabrication processes.

In accordance with an aspect of the present invention, there is provided a focusing waveguide grating coupler using a leaky mode, including: a substrate having a first refractive index n1; a first core layer having a second refractive index n2, the first core layer being formed on the substrate; a second core layer having a third refractive index n3, the second core layer being formed on the first core layer apart from the first core layer with a space d in between; a first cladding layer having a fourth refraction index n4, the first cladding layer being formed on the second core layer; a second cladding layer having a fifth refraction index n5, the second cladding layer being formed on the first cladding layer and inserted between the first core layer and the second core layer; and a Fresnel lens positioned on the second cladding layer, wherein the refractive indexes satisfies conditions of n5>(n2, n3)>n1 and n5>n4; and light inputted through the first and second core layers to the Fresnel lens as radiated leaky beams by a leaky mode formed according to the conditions, and the leaky beams form an optical focus by performing single directional coupling towards the lower part of the substrate by using beams refracted from the Fresnel lens.

DETAILED DESCRIPTION OF THE INVENTION

Other objects and aspects of the invention will become apparent from the following description of the embodiments with reference to the accompanying drawings, which is set forth hereinafter.

FIG. 6is a schematic diagram showing a leaky mode in a leaky mode waveguide in accordance with the present invention. It shows an example of a waveguide where the leaky mode can be formed.

Referring toFIG. 6, a depressed inner cladding, or inner cladding,63is added to a conventional asymmetrical planar waveguide. Specifically, a core layer62having a refraction index of nfis placed on a substrate61having a refraction index of nc. In the upper part of the core layer62, the inner cladding layer63having a refraction index of ndis formed. In the upper part of the inner cladding layer63, a top cladding layer64having a refraction index of ngis formed.

The inner cladding layer63helps to form a single leaky mode easily by lowering the effective refraction index ndthan the refraction index of ngof the top cladding layer64. Here, lowering the refraction index of the inner cladding layer63for generating the leakage effect can be considered equivalently to varying macro bending loss by the extent of bending of the optical fiber.

A guided mode is determined based on the refraction index structure of the planar waveguide. Therefore, the mode power propagating toward the cladding layer to the cover cladding layer64and its propagation angle are determined based on a designed refractive structure of the waveguide, and the properties of the leaky mode, such as a propagation constant and leakage efficiency, may be varied according to the designed structure of the waveguide. The left part ofFIG. 6shows the relative level of the effective refraction index in the leaky mode.

The leaky mode is generated when the effective refraction index Ndof the inner cladding layer63is smaller than the refractive index ngof the top cladding layer64or the refractive index nsof the substrate61, which is disclosed by D. Marcuse, entitled “Theory of Dielectric Optical Waveguides,” Academic Press, Chapter 1, pp. 31–59, 1991.

Since a bound mode is restricted by boundary conditions of the intermediate cladding layer between the two cores62aand62band the under cladding layer61, it has no loss by the radiation toward the top cladding layer64. However, since the leaky mode has an open boundary condition at the interface between inner cladding layer63and the top cladding layer64, it lose its power continuously as propagating along the waveguide.

The focusing waveguide grating coupler using the leaky mode will be described herein with reference to an embodiment of the present invention.

FIG. 7shows a cross-sectional view of a waveguide focusing grating, which includes a symmetrical single mode waveguide70, a synchronous directional coupler71coupling the symmetrical single mode waveguide70and the leaky-mode waveguide, and a reflective Fresnel lens69.FIG. 7also shows a diagram presenting the refractive index structure of the directional coupler.

Referring toFIG. 7, the synchronous directional coupler71includes a substrate having a refractive index of ns; a first core layer62ahaving a refractive index of nf; a second core layer62bhaving a refractive index of nf; an inner cladding layer63having a refractive index of nd; a top cladding layer64having a refractive index of ng; and a Fresnel lens69.

The first core layer62ais formed on the substrate. The second core layer62bis formed on the first core layer62awith a predetermined space (d) apart from the first core layer62a. The inner cladding layer63is formed on the second core layer62b, and the top cladding layer64is formed on the inner cladding layer63. The Fresnel lens69is placed on the top cladding layer64. The symmetrical single mode waveguide70is formed of one core layer68.

Referring toFIGS. 6 and 7, the refractions of the layers are nf>ng>ns>nd. Light inputted through the first and second core layers62aand62bgoes into the Fresnel lens69as a radiated leaky beam65due to the leaky mode formed under the condition of nf>ng>ns>nd. The radiated leaky beam65forms a single directional coupling toward the lower part of the substrate61by the beam reflected from the Fresnel grating, thus forming an optical focus66. Here, the first core layer62aand the second core layer62bmay have the same or different refraction index.

Meanwhile, in the above examples, the refraction index ndof the inner cladding layer63has the least value among the refraction indexes. However, the leaky mode can be formed even when ndis larger than nf(nd>nf).

Therefore, if the refraction indexes of the layers fulfill the conditions of ng>nf>nsand ng>nd, the leaky mode can be formed.

Between the first core layer62aand the second core layer62bof the synchronous directional coupler71, the top cladding64is extended and inserted. The length of coupling is adjusted by controlling a space (d). The amount of radiated leaky beam65and the leakage angle are adjusted by controlling the refraction index ndof the depressed inner cladding63and the refraction index ngof the top cladding64. The leakage angle against a waveguide axis which is perpendicular to the x axis can be estimated approximately from cos−1(N/ng), wherein N denotes an effective refraction index of the leaky mode.

Here, the synchronous directional coupler71is located on a plane which is formed of the x and y axes. The optical focus66moves along the z axis. The space (d) is controlled to have the radiated leaky beam of the maximum leakage effect and Gaussian distribution within the range of b−L/2<y<b+L/2, wherein b denotes a distance from input light to the center of the Fresnel lens and L denotes a diameter of the Fresnel lens.

Referring toFIG. 7, which shows an example of a waveguide grating coupler using leaky mode coupling, light is inputted from the left of the waveguide through optical fiber (or laser diode). A single mode is formed on a symmetrical planar waveguide30having a structure where the refraction index ngof the top cladding layer64and the size of the core layer68are the same as those of the core layer68of the optical fiber.

Since the shape of the planar waveguide single mode, which is the first one of the transverse electric modes, (TEO) is the same as that of a one-dimensional single mode, which is the fundamental hybrid electric mode (HE11) of the optical fiber, insertion loss can be minimized due to mode-matching effect. A reference numeral ‘67’ denotes intensity distribution of the leaky mode in the direction of a cross section, and a reference numeral ‘72’ denotes intensity distribution of the guided mode in the direction of a cross section.

The single mode of the planar waveguide propagates from left to right along the symmetrical waveguide, and it is inputted to the synchronous directional coupler71at the y-axial starting point (y=b−L/2) of the Fresnel lens. The synchronous directional coupler71has a structure where the symmetrical single mode waveguide70is formed closely to a leaky mode waveguide, which includes the core layers62aand62b, the inner cladding layer63and the top cladding layer64.

Coupling efficiency, which indicates the level of power transmission from the planar waveguide mode to the leaky mode, is different according to the space (d) between the two waveguides. Therefore, it is desirable to transmit power to the leaky mode sufficiently by controlling the space (d) while planar waveguide mode is propagating so that the power of the planar waveguide mode could be gone away at the end of the waveguide (y=b+L/2).

The power of the leaky mode, which is transmitted from the planar waveguide mode, propagates and generates radiated leaky beam65having a predetermined angle continuously. The radiated leaky beam65is diffracted by a reflective Fresnel lens69to thereby form the optical focus66.

In the conventional waveguide grating coupler ofFIG. 2, coupling is formed in the upper and/or lower directions due to evanescent field of the bound electric mode. However, in the present invention, the radiated beams are propagated in a single direction toward a focus out of the waveguide due to diffraction because the beam propagation toward the top cladding layer64is the propagation of electromagnetic waves. Therefore, in the focusing waveguide grating coupler of the present invention, a focus is formed by forming single directional coupling, instead of bi-directional coupling.

FIG. 8is a graph illustrating a power transmission property of the leaky mode shown inFIG. 7. Structures using the synchronous directional coupler71have following advantages: The input waveguide can be designed optimally, and the inputted power can be transmitted in the form of the leaky mode due to synchronous directional coupling effect.

Referring toFIG. 8, distribution of input beams radiated to the reflective Fresnel lens is formed in the shape80of Gaussian function. Generally, optical loss of the leaky mode tends to decay exponentially in a longitudinal direction. On the contrary, since the synchronous directional coupling is accumulated and increases along the longitudinal direction, the power distribution of the inputted beams of the reflective Fresnel lens, which is multiplication of sine function of the synchronous coupling and the exponential decaying function of the radiation decay is formed in the shape80of Gaussian function.

The distribution can reduce distortion of a focus point generated by asymmetrical distribution of input light and it is suitable for coupling a signal reflected by a recording medium at the focus point into the waveguide mode. A reference numeral ‘81’ indicates a leaky mode intensity curve that is declined as it proceeds, and a reference numeral ‘82’ indicates power of the leaky mode which is accumulated as it proceeds. The reference numeral ‘80’ indicates distribution of leaky beams radiated toward the reflective Fresnel lens.

The present invention suggests a method that maximizes output coupling efficiency of input light. The output generation efficiency can be improved when the insertion loss, which is generated during the input of the light to a head, is minimized and the transmission of power to the output beams which is propagated from the waveguide mode to the outside is maximized.

The coupling efficiency of the input beams is maximized when the power distribution of the output beams is matched with the power distribution of the reflective beams. Therefore, the coupling efficiency of the input beams is improved as the power distribution of the output beams is more symmetrical.

In the embodiment of the present invention which is described above, the synchronous directional coupler, a hybrid of the symmetrical single mode planar waveguide and the leaky mode waveguide, is used to improve the output coupling efficiency. The symmetrical planar waveguide has a structure that can minimize the insertion loss.

The synchronous directional coupler transmits the entire power of the input single mode to the leaky mode, and the entire power of the leaky mode is transmitted to the Fresnel lens. Therefore, the output coupling efficiency is determined based on the diffraction efficiency of the Fresnel lens. Generally, Fresnel lenses of a two-level structure have a diffraction efficiency of 35% and those of a four-level structure have a diffraction efficiency of 60%. This is all higher than the diffraction efficiency of conventional grating coupler.

Since the y-directional power distribution of the input beams transmitted from the leaky mode to the Fresnel lens is symmetrical, the input coupling efficiency is increased relatively.

The conventional waveguide grating couplers largely depend on the progress of lithography technology. However, the synchronous directional coupler of the present invention can relieve the dependency on the manufacturing process considerably, because the characteristics of the output beams are determined based on the design of the multi-layer waveguide and the Fresnel lens. Therefore, high-efficiency waveguide grating coupler can be embodied without high-resolution lithography.

The focusing waveguide grating coupler fabricated in accordance with the technology of the present invention has high coupling efficiency with a relatively simple process and, ultimately, the performance and productivity of the focusing waveguide grating coupler can be improved.