JP6530061B2 - Grating coupler - Google Patents

Description

  The present invention relates to a grating coupler.

  In order to input and output optical signals to the optical circuit on the substrate composed of the optical waveguide, it has the function of emitting the propagation light of the optical waveguide to the outside of the optical waveguide or making the external light enter the optical waveguide. I have something I want to do. A grating coupler is an optical input / output element that can be used for this purpose, and the propagation light of the optical waveguide on the substrate is made to enter the end face of the optical fiber brought close to the substrate surface, or vice versa. Can be incident on the optical waveguide.

  FIG. 4A is a schematic cross-sectional view of a grating portion of an example of a grating coupler having a configuration for emitting guided light to the outside of an optical waveguide. Hereinafter, the structure, function, and the like of the grating coupler will be briefly described with reference to FIG. 4A.

  The grating coupler shown in FIG. 4A has the function of receiving an optical signal through an optical waveguide and converting the optical axis by diffraction. The cross-sectional structure illustrated in FIG. 4A is a BOX (Buried Oxide, buried oxide film) layer 402, a core layer 414 having a refractive index higher than that of the BOX layer 402, and an upper cladding having a refractive index similar to that of the BOX layer 402 (Overclad The core layer 414 has a diffraction grating formed thereon. As described above, the grating coupler enables optical path conversion only by, for example, processing for forming a diffraction grating on the core layer 414.

  The operation principle and the like of the grating coupler for emitting the guided light to the outside of the optical waveguide will be briefly described below.

In the core layer 414 of the grating coupler 400 shown in FIG. 4A, a diffraction grating having irregularities in the thickness direction (x direction) of the waveguide is formed periodically in the propagation direction of the guided light (z direction). . When such a diffraction grating is formed, a part of the guided light is diffracted to generate emitted light, but the guided light and the emitted light satisfy the phase matching condition in the propagation direction (z direction). That is, the propagation constants of the guided light beta _0, if the z-direction propagation constant of the emitted light and beta _q, the phase matching condition is β _{q =} β _{0 +} qK. Here, Λ is a period of the diffraction grating, K = 2π / π, and q is a value corresponding to the order of the emitted light (0, ± 1, ± 2,...).

In this case, the emission angle θ _q of the q-order radiation with respect to the normal to the diffraction grating is λ, where λ is the wavelength of light in vacuum, N is the effective refractive index of the waveguide, n _c is the refractive index of the upper cladding layer, Is given by the equation n _c sin θ _q = N + qλ / Λ.

In general, since the effective refractive index N of the waveguide takes a value between the refractive index of the core and the refractive index of the cladding, there is a relation of N> n _c , and the light emission is generated so as to satisfy q ≦ −1. Limited to order q. Furthermore, considering that the diffracted light with the smaller diffraction order has the higher diffraction efficiency, the radiation distribution (-1st radiation) satisfying q = -1 is used to maximize the power distribution ratio to the radiation. That's the case. Although the power distribution ratio is lower than that of the -1st radiation, -2nd radiation can also be used if necessary. In addition to the emitted light such as the above-described -1st radiation and -2nd radiation, the return light P _{ref in the} waveguide direction, the core layer transmitted light P _trans , and the radiation to the substrate 401 side via the BOX layer 402 The light P _down also exists at the same time.

Here, the emission angle θ _q can be arbitrarily designed according to the period Λ, the width w, the depth d, and the thickness D of the optical waveguide shown in FIG. 4A. The numerical range of the period Λ, the width w, the depth d, and the thickness D of the optical waveguide for light having a wavelength of 1.3 μm to 1.6 μm in vacuum is as follows.

Λ: 530 to 550 nm
FF (= 1−w / Λ): 0.3 to 0.6
d: 60 to 80 nm
D: 180 to 220 nm
Although the case has been described above where the guided light is emitted to the outside of the optical waveguide by the grating coupler, it has also been widely practiced in the past to make external light enter the optical waveguide by such a grating coupler.

  As described above, the grating coupler is basically based on the structure in which the diffraction grating is formed on the surface of the core, but in the process aiming to improve the performance, structures having various features have been proposed. For example, as the most basic grating coupler, it has a one-dimensional grating (grating) in which asperities are periodically arranged in the longitudinal direction of the waveguide, and the grating is linear and uniform in the lateral width direction of the waveguide. There is a certain one-dimensional grating coupler (see FIG. 4B).

  In such a one-dimensional grating coupler, since the grating is uniform in the width direction of the waveguide, the structure in the width direction can be ignored to approximate a two-dimensional structure in the length direction and the thickness direction. Therefore, when the emitted light from the grating coupler is obtained by numerical calculation, the calculation scale can be reduced, and as a result, there is an advantage that the grating coupler with good optical coupling efficiency can be designed in a short time.

  On the other hand, in the one-dimensional grating coupler shown in FIG. 4B, the lateral width of the grating portion 410 is often 10 times or more of the width of the optical waveguide on the substrate, so the distance between the grating portion 410 and the optical waveguide is It is necessary to insert a long and gentle tapered waveguide 420. This is to suppress the disturbance of the wave front of the propagation light at the connection portion between the grating portion 410 and the optical waveguide portion 420, but as a result, the entire structure of the grating coupler combining the grating portion 410 and the taper waveguide portion 420 It is a drawback that the size is increased (Non-patent Document 1).

  For example, in a conventional one-dimensional periodic grating coupler as shown in FIG. 4B, it is necessary to provide a tapered waveguide 420 having a length S about 15 times or more the width U of the grating portion 410. If it is about 20 μm, S needs to be about 300 μm at the minimum. Incidentally, the length T of the grating portion 410 in that case is about 30 μm.

  Then, as another prior art, the grating coupler which formed the circular-arc-shaped grating in the taper waveguide with a large expansion angle was proposed (refer FIG. 4C) (patent document 1, patent document 2). In such a grating coupler, since the grating is also formed in an arc shape, it is possible to miniaturize the entire structure including the grating portion and the tapered waveguide.

  However, as the grating becomes arc-shaped, the grating is not uniform in the width direction of the waveguide, and two-dimensional approximation in numerical calculation can not be performed. Therefore, in order to know the characteristics of the arc-shaped grating coupler, it becomes necessary to perform three-dimensional calculations requiring a large amount of computer power and time, or to repeat time-consuming trial production and experiment verification similarly. A new problem arises in that it takes a lot of time to adjust the detailed structure in order to obtain good characteristics.

U.S. Patent No. 7245803 U.S. Patent No. 7260289

D. Taillaert et al, “An Out-of-Plane Grating Coupler for Efficient Butt-Coupling Between Compact Planar Waveguides and Single-Mode Fibers,” IEEE JOURNAL OF QUANTUM ELECTRONICS, Vol. 38, No. 7, JULY 2002, p. 949-955.

  In order to solve the problem of upsizing of the structure of the one-dimensional grating coupler and the problem of design difficulty of the arc-shaped grating coupler as described above, it is compact and it is easy to optimize the structure at the same time It is an object to provide a novel grating coupler.

  In order to solve the above problems, the grating coupler of the present invention is characterized by a configuration in which a plurality of grating elements provided with a one-dimensional grating are arranged adjacent to each other, and the embodiment thereof is roughly as follows. In the following, assuming that the light input from the waveguide is output from the grating coupler to the substrate surface, the names “light input side” and “light transmission side” are assigned, but the light propagation direction is It should be noted that in the opposite case, the assignment of their names is reversed.

  In the basic form of the grating coupler of the present invention, the grating coupler comprises a plurality of grating elements arranged adjacent to each other and a waveguide connecting between the grating elements, and the plurality of grating elements have periods in the same or substantially the same direction. A waveguide comprising a one-dimensional grating having a conductivity and connected to the light transmission side of at least one grating element is connected to the light input side of another grating element. Each grating element comprises a grating portion and a tapered waveguide portion added before and after the grating portion.

  In this basic mode, a plurality of grating elements arranged adjacent to each other in order to input light to adjacent grating elements having substantially the same total width with respect to the width of a conventional one-dimensional periodic grating coupler A waveguide connecting the two is configured to connect the waveguide to the light transmission side of at least one grating element and to connect (serial connection) to the light input side of another grating element. Therefore, the width of each grating element is reduced in inverse proportion to the number of grating elements, and the length of the tapered waveguide added before and after each grating element is also reduced accordingly.

  Since the grating coupler of this basic mode has a length substantially equal to that of the grating elements, the length of the tapered waveguide portion is shortened by increasing the number of grating elements, and as a result, the conventional one-dimensional structure can be obtained. It is possible to make it shorter than the total length of the periodic grating coupler. On the other hand, because each grating element is a one-dimensional grating, the ease of design of the light input / output characteristics is maintained. Needless to say, the same effect can be obtained in the mode in which light is externally incident on each grating element.

  In the first form of the grating coupler of the present invention, in the basic form, a distance between at least a pair of grating elements adjacent to each other is configured to be equal to or less than a narrow width of two grating elements. . By adopting such a configuration, it is possible to practically prevent a reduction in the intensity of the emitted light due to the gap existing between the adjacent grating elements.

  In the second form of the grating coupler of the present invention, in the basic form and the first form, the two or more grating elements comprise one-dimensional gratings of the same period and depth, and light from the light input side The directions toward the transmission side are configured to be the same. As a result, the emission angles of diffracted light (e.g., -1st order radiation) having equal diffraction orders can be made uniform at each grating element, and radiation of appropriate intensity distribution can be easily realized.

  In the third aspect of the grating coupler of the present invention, in the basic aspect and the first aspect, at least a pair of grating elements connected by a waveguide includes one-dimensional gratings having different periods, and light is transmitted from the light input side The direction toward the transmission side is configured to be opposite. As a result, diffracted light having different diffraction orders (for example, -1st radiation and -2nd radiation) is emitted from a pair of grating elements whose directions from the light input side to the light transmission side are opposite, The emission angles can be made uniform in the pair of grating elements, and radiation having an appropriate intensity distribution can be easily realized.

  In the fourth aspect of the grating coupler of the present invention, in the basic aspect and the first to third aspects, the light input side and / or the light transmission side is provided with the light branching portion connected to the grating element. It is configured as follows. By adopting such a configuration, the grating coupler can be made symmetrical with respect to the light branching portion, and an appropriate light intensity distribution excellent in symmetry in the width direction can be easily achieved.

  According to the present invention, it is possible to provide a grating coupler which can be miniaturized and which can be easily designed according to the mode field to which light is coupled.

FIG. 1A is a schematic plan view showing the configuration of an embodiment of the grating coupler of the present invention. FIG. 1B is a schematic plan view showing the configuration of a modification of the embodiment of the grating coupler of the present invention. FIG. 1C is a schematic plan view showing the configuration of another modification of the embodiment of the grating coupler of the present invention. FIG. 2A is a schematic plan view showing the configuration of another embodiment of the grating coupler of the present invention of the present invention. FIG. 2B is a schematic plan view showing the configuration of a modification of another embodiment of the grating coupler of the present invention. FIG. 2C is a schematic plan view showing the configuration of another modification of the other embodiment of the grating coupler of the present invention. FIG. 3A is a schematic plan view showing the configuration of still another embodiment of the grating coupler of the present invention of the present invention. FIG. 3B is a schematic plan view showing the configuration of a modification of still another embodiment of the grating coupler of the present invention. FIG. 3C is a schematic plan view showing the configuration of another modification of still another embodiment of the grating coupler of the present invention. FIG. 4A is a schematic cross-sectional view showing a configuration of an example of a grating coupler. FIG. 4B is a schematic plan view of a one-dimensional grating coupler. FIG. 4C is a schematic plan view of a grating coupler in which an arc-shaped grating is formed in a tapered waveguide with a large divergence angle.

  Hereinafter, embodiments of the grating coupler of the present invention will be described with reference to the drawings. However, the technical scope of the present invention is not limited by those embodiments, and should be interpreted based on the description of the claims. Also, it goes without saying that the "structure" of the grating coupler is strictly a "core structure" and the cladding is surrounded by the periphery. In the following description, although an aspect in which light is emitted from the grating element to the outside is mainly described, an aspect in which light is incident on the grating element from the outside can be realized by the same configuration.

  FIG. 1A is a schematic plan view showing the configuration of an embodiment of the grating coupler of the present invention. As shown in FIG. 1A, the grating coupler of this embodiment includes two grating elements 101 and 102, and a waveguide 120 connected to the light transmission side of the grating element 101 and the light input side of the grating element 102. However, they can be formed on a not-shown semiconductor substrate (for example, a silicon substrate), and can be configured as a buried optical waveguide having a cross-sectional structure as shown in FIG. 4A, for example.

  The grating element 101 includes tapered waveguides 101a and 101b and a grating 101c.

  The tapered waveguide portions 101a and 101b are each formed of a planar optical waveguide having a function of guiding light along the substrate surface, and the narrow end of the tapered waveguide portion 101a is the light of the grating element 101. The end on the wide side, which is the input side, is connected to the grating section 101c, and the wide end of the tapered waveguide section 101b is connected to the grating section 101c, and the narrow end Is the light transmission side of the grating element 101. The tapered shape of the tapered waveguide portions 101a and 101b is not limited to a linear tapered shape, and includes, for example, a curved tapered shape such as a quadratic curve or a semi-elliptic shape.

  The grating portion 101c is a one-dimensional diffraction grating formed of periodic grooves having a predetermined distance and depth, and has a function of emitting light propagating through the tapered waveguide portion 101a to the outside. The functions possessed by this are as outlined in the [Background Art] section using FIG. 4A. The one-dimensional lattice is not necessarily uniform, and may have an apodized structure in which, for example, the period and the depth of the groove gradually change.

  The grating element 101 and the grating element 102 have one-dimensional gratings of the same period and depth, and have the same direction from the light input side to the light transmission side. This is because the emission angles of diffracted light (e.g., -1st order radiation) having equal diffraction orders are aligned at each grating element to easily realize emitted light of an appropriate intensity distribution.

  The widths of the grating elements do not necessarily have to be equal. When the widths of the grating elements are made different, the intensity per unit area of the light emitted from the narrow grating element, that is, the power density becomes high. Therefore, if this characteristic is used, the intensity distribution of light in the width direction of the grating element can be adjusted, and as a result, the intensity distribution of light emitted from the grating coupler can be adjusted to a desired shape. That is, by using such a method, it is possible to more finely adjust the light intensity distribution in the width direction of the grating portion, that is, the shape of the mode field of the emitted light. Therefore, for example, even if there are a plurality of types of optical fibers that are to be coupled with each other and their mode fields are different, it is possible to perform optimization design in the width direction according to each mode field. .

  The waveguide 120 is disposed in the gap between the grating elements 101 and 102, and is connected to the light transmission side of the grating element 101 and the light input side of the grating element 102, and serially connects the grating element 101 and the grating element 102. It is a thing.

  The distance between the adjacent grating element 101 and grating element 102 is set to be equal to or less than the narrow width of grating elements 101 and 102. The reason for this setting is that, if there is a gap between adjacent grating elements 101 and 102, it is assumed that the amount of emitted light decreases at that portion and the intensity distribution of the light emitted from the grating coupler is affected. However, if the spacing between grating elements is set as described above, the effect can be reduced to a level that does not cause any practical problems.

  Regarding the scale of the grating coupler shown in FIG. 1A, assuming that the total size of the grating portions of the plurality of grating elements is substantially equal to that of the grating portion of the conventional grating coupler, the length of the grating coupler is calculated as follows Ru.

Assuming that the grating coupler is composed of p grating elements, the length of the tapered waveguide portion of the k-th (1 ≦ k ≦ p) grating element is s _k , the length of the grating portion is t _k , and the grating portion the width and _{u k.}

The length t _k of the grating portion is approximately equal to the length T of the grating portion 410 of the conventional grating coupler shown in FIG. 4B. Further, the shape of the tapered waveguide portion of the kth grating element is substantially similar to the shape of the tapered waveguide portion 420 of the conventional grating coupler, so s _k / u _k = S / U = r (constant) Is established. Note that _{k k} u _k = U (where Σ _k represents the sum of 1 to p).

From this, the length L of the grating coupler shown in FIG. 1A is L = T + 2 * Max (s _k ) = T + 2 * r * Max, taking into consideration that there are two tapered waveguide portions in each grating element. It will be expressed as (u _k ). In addition, about the conventional grating coupler shown by FIG. 4B, if the length is set to L ', it will be represented as L' = T + S = T + r * U.

Now, if the widths u _{k of the} grating elements are all equal, u _k = U / p holds, so L = T + (2 * r / p) * U. As apparent from the comparison of L and L 'described above, LLL' in the case of p ≧ 2, and L decreases and approaches T as p increases.

  Therefore, in the grating coupler shown in FIG. 1A, the overall length can be significantly reduced as compared to the conventional one-dimensional periodic grating coupler by increasing the number of grating elements.

In the case where the widths u _{k of the} grating elements are different, although L slightly increases as compared with the above-described case, substantially the same conclusion holds.

  Next, the operation of the grating coupler shown in FIG. 1A will be described in detail.

The light incident on the light input side of the grating element 101 through the optical waveguide (not shown) propagates through the tapered waveguide portion 101a and reaches the grating portion 101c, and the grating portion 101c emits radiation with a -1st-order diffraction order. Is emitted. The emission angle θ ₋₁ of the −1st order radiation can be optionally designed according to the period Λ, the width w, the depth d of the diffraction grating, and the thickness D of the optical waveguide (see FIG. 4A).

There is also light transmitted through the grating section 101c (see FIG. 4A; P _trans ), and the transmitted light propagates through the tapered waveguide section 101b to reach the light transmission side of the grating element 101 and further propagates through the waveguide 120. Then, the light enters the light input side of the grating element 102. Similarly to the grating element 101, also in the grating element 102, radiation with a -1st-order diffraction order is emitted. The radiation angle of the -1st order radiation is set to the period Λ, the width w, the depth d of the diffraction grating and the thickness D of the optical waveguide in the same manner as the grating element 101. It becomes equal to the light emission angle θ ₋₁ .

Even when the additional grating element is serially connected to the grating element 102 through the waveguide, the emission angle of the −1st order radiation light in the grating element is the period Λ, the width w, the depth d of the diffraction grating. If the thickness D of the optical waveguide is set to be the same as that of the grating element 101, it becomes equal to the emission angle θ ₋₁ of the −1st order radiation of the grating element 101.

  In this way, the emission angle of the -1st order radiation can be made uniform in each grating element, and radiation with an appropriate intensity distribution can be easily realized.

  FIG. 1B is a schematic plan view showing the configuration of a modification of the embodiment of the grating coupler of the present invention. The basic configuration shown in FIG. 1B is the same as that of the embodiment of FIG. 1A, but the difference from the embodiment of FIG. 1A is that the light branching element 130 is provided on the light input side of the grating elements 103 and 105. It is in the point provided.

  The light branching element 130 branches and outputs the light incident from the input end via an optical waveguide (not shown) from the two output ends to the grating elements 103 and 105. The light branching element 130 is configured of, for example, a 1-input 2-output multimode interferometer (MMI) whose core is made of Si and whose cladding is made of a silicon oxide film. In general, multimode interferometers have a self-focusing effect in which the field of light at the incident end is periodically reproduced in the process of propagating through the multimode interference waveguide, depending on the width of the multimode interference waveguide. One or more convergent fields can be obtained at a specific distance from the determined input end. By connecting the waveguides with the convergent position at the output end, it is possible to construct a very low loss optical branch.

  If the branching ratio of the light of the light branching element 130 is set to, for example, 1: 1, and the grating elements after the light branching element 130 are made symmetrical in the width direction, emitted light having a mode field symmetrical in the width direction is obtained. Can.

  The length of the light branching element 130 from the input end to the output end in the present embodiment may be set to about 15 μm or less, although it is necessary to set the length more than the specific distance. Therefore, the increase in the length of the grating coupler of the present embodiment by adding the length of the optical branching element 130 is limited, and by increasing the number of grating elements, the entire length of the grating coupler is improved as compared with the conventional one-dimensional period grating coupler. It is possible to reduce the length significantly.

  With regard to the function of the grating coupler of this embodiment, the set of grating elements 103 and 104 and waveguide 121 and the set of grating elements 105 and 106 and waveguide 122, which receive the output of the light branching element 130, respectively. In the case of the set of (2), since they have the same function as the grating coupler of FIG.

  FIG. 1C is a schematic plan view showing the configuration of another modification of the embodiment of the grating coupler of the present invention. The basic configuration shown in FIG. 1C is the same as the basic configuration shown in FIG. 1A, except that the light branching element 140 is disposed on the light transmitting side of the grating element 107 in that the configuration shown in FIG. It is in the point provided.

  The configuration and operation of the light branching element 140 are the same as those of the light branching element 130 described with reference to FIG. 1B, so the description will be omitted.

  According to this embodiment, as in the embodiment shown in FIG. 1B, the branching ratio of the light of the light branching element 140 is set to 1: 1, for example, and the grating elements after the light branching element 130 are made symmetrical in the width direction. Thus, it is possible to obtain outgoing light having a mode field symmetrical in the width direction.

  Furthermore, in the grating coupler of this embodiment, as in the embodiment shown in FIG. 1B, even if the length of the light branching element 140 is added, the number of grating elements is increased to increase the overall compared to the conventional one-dimensional periodic grating coupler. It is possible to significantly reduce the length of the

  Although the light branching element 140 intervenes in the function of the grating coupler of this embodiment, it comprises the set of the grating elements 107 and 108 and the waveguide 123, the grating elements 107 and 109, and the waveguide 124. Each set of sets has the same function as that of the grating coupler of FIG.

  Further, in the present embodiment, since the grating elements 108 and 109 divide and input the transmitted light of the grating element 107 by the light branching element 140, the intensity of the light emitted from the grating elements 108 and 109 is reduced. there is a possibility. However, if the width of the grating elements 108 and 109 is made narrower than the width of the grating element 107, the power density of the light emitted from the narrow grating elements 108 and 109 becomes high. The intensity distribution of light can be adjusted, and as a result, the intensity distribution of light emitted from the grating coupler can be adjusted to a desired shape.

  FIG. 2A is a schematic plan view showing the configuration of another embodiment of the grating coupler of the present invention. The form shown in FIG. 2A is similar in its basic configuration to the form shown in FIG. 1A, except that the form shown in FIG. 1A differs from the form shown in FIG. In the configuration shown in FIG. 2A, the waveguides 220 and 221 are disposed so as to surround the grating elements 201 to 203, while they are disposed in the gap between 101 and 102.

  With such a configuration, in the configuration shown in FIG. 1A, there is a restriction that the gap can not be made smaller than a certain limit because the waveguide 120 is placed in the gap between the grating elements 101 and 102. However, in the configuration shown in FIG. 2A, since there is almost no such restriction on the gap between the grating elements 201 to 203, the influence on the intensity distribution of light emitted from the grating coupler due to the presence of the gap is further reduced. It will be possible.

  The grating coupler of this embodiment, like the grating coupler shown in FIG. 1A, can significantly reduce the overall length as compared to the conventional one-dimensional periodic grating coupler by increasing the number of grating elements. is there.

  In addition, about the effect | action which the grating coupler of this form has, since it has an effect | action similar to the grating coupler of FIG. 1A, description is abbreviate | omitted.

  FIG. 2B is a schematic plan view showing the configuration of a modification of another embodiment of the grating coupler of the present invention. The basic configuration shown in FIG. 2B is the same as that of the embodiment shown in FIG. 2A, except that the light branching element 230 is provided on the light input side of the grating elements 204 and 207. It is in the point provided.

  The configuration and operation of the light branching element 230 are the same as those of the light branching element 130 described with reference to FIG.

  In the grating coupler of this embodiment, the mode field of the outgoing light can be made symmetrical, for example, in the width direction, and the number of grating elements can be increased even if the length of the light branching element 230 is added as in the embodiment shown in FIG. Thus, it is possible to significantly reduce the overall length as compared to the conventional one-dimensional periodic grating coupler.

  As to the function of the grating coupler of this embodiment, a set of grating elements 204, 205, 206, waveguides 222, 223 and grating elements 207, 208, 209, waveguides, which receive the output of the light branching element 230, respectively, In each set of the set consisting of 224 and 225, since it has the same function as the grating coupler of FIG. 2A, the description will be omitted.

  FIG. 2C is a schematic plan view showing the configuration of another modification of the other embodiment of the grating coupler of the present invention. The basic configuration shown in FIG. 2C is the same as the basic configuration shown in FIG. 2B, except that the light branching element 240 is provided on the light transmission side of the grating element 210 in the point different from the configuration of FIG. 2B. It is on the point.

  The configuration and operation of the light branching element 240 are the same as those of the light branching element 130 described with reference to FIG. 1B, so the description will be omitted.

  In the grating coupler of this embodiment, as in the embodiment shown in FIG. 1C, the mode field of the outgoing light can be made symmetrical, for example, in the width direction, and the number of grating elements is increased even if the length of the light branching element 240 is added. Thus, it is possible to significantly reduce the overall length as compared to the conventional one-dimensional periodic grating coupler.

  As to the function of the grating coupler of this embodiment, although the light branching element 240 is interposed, the set of grating elements 210, 211, 212 and waveguides 226, 227 and the grating elements 210, 213, 214, The respective sets of the waveguides 228 and 229 have the same function as that of the grating coupler of FIG.

  FIG. 3A is a schematic plan view showing the configuration of still another embodiment of the grating coupler of the present invention. The configuration shown in FIG. 3A is configured such that the pair of grating elements 301 and 302 connected by the waveguide 320 have one-dimensional gratings with different periods, and the direction from the light input side to the light transmission side is reversed. It is done. As a result, diffracted light (for example, -1st radiation and -2nd radiation) having different diffraction orders in the pair of grating elements 301 and 302 are emitted, and their emission angles are aligned in the pair of grating elements It is possible to easily realize radiation of appropriate intensity distribution.

  With such a configuration, the waveguides connected between the grating elements can be set short, and the formation of the waveguides can be facilitated.

  Also, the grating coupler shown in FIG. 3A, like the grating coupler shown in FIG. 1A, significantly reduces the overall length as compared to the conventional one-dimensional periodic grating coupler by increasing the number of grating elements. It is possible.

  Next, the operation of the grating coupler shown in FIG. 3A will be described in detail.

  In the grating element 301, -1st order radiation light is emitted, and there is light transmitted through the grating element 301, and the transmitted light reaches the light transmission side of the grating element 301 and is further propagated through the waveguide 320. Incident on the light input side of the grating element 302 is similar to the grating coupler shown in FIG. 1A.

However, in the grating element 302, unlike the grating coupler shown in FIG. 1A, the direction from the light input side to the light transmission side is opposite to that of the grating element 301. Therefore, assuming that the emission angle of the -1st order radiation light of the grating element 302 is the same as that of the grating element 301, assuming that the period Λ, the width w and the depth d of the diffraction grating are the same as the grating element 301, The light emission angle is −θ _−1, and the direction of inclination is opposite to the emission angle θ ₋₁ of the −1st order radiation of the grating element 301.

  There are two means for matching the emission angle of the radiation light from the grating element 302 to the emission angle of the -1st order radiation light of the grating element 301, including the tilt direction. One of them is to tilt the −1st order radiation light in the reverse direction by making the period Λ of the diffraction grating of the grating element 302 smaller than that of the grating element 301. Another one is to use −2nd order diffracted light which is inclined reversely to −1st order diffracted light as emitted light of the grating element 302 (see FIG. 4A).

  In general, although radiation efficiency per unit length is higher if diffracted light of small diffraction order is used as the emitted light of grating element 302, the period Λ of the diffraction grating is smaller, so the fabrication is The degree of difficulty of On the other hand, when the −second-order diffracted light is used as radiation light, the radiation efficiency per unit length decreases by the amount of the diffracted order, but the period Λ of the diffraction grating is the period of the diffraction grating of the grating element 301 Because it is larger than it does not increase the difficulty of production.

  In the case of a grating coupler in which the core material is silicon and the cladding material is an oxide film, a grating designed to emit -1st-order diffracted light usually emits -2nd-order diffracted light Is not emitted, and conversely, the -1st order diffracted light is not emitted from the grating designed to emit the -2nd order diffracted light.

The emission angle of the light from the grating element 302 is the emission angle of the −1st order emission light of the grating element 301 regardless of which of the −1st order diffracted light and the −2nd order diffracted light is used as the emission light. In order to make it equal to the angle θ ₋₁ , the period Λ, the width w, the depth d, and the thickness D of the optical waveguide in the grating element 302 must be appropriately adjusted. However, among the above adjustment conditions, it is difficult to adjust the depth d of the diffraction grating and the thickness D of the optical waveguide, considering that the grating element 302 is formed simultaneously with the grating element 301, and it is difficult to It is considered realistic to make the emission angle of the radiation of the grating element 302 equal to the emission angle θ ₋₁ of the −1st order radiation of the grating element 301 by adjusting the period Λ and the width w of the grating. Therefore, in the present embodiment, the emission angle of the radiation light of the grating element 302 is set by particularly adjusting the period of the diffraction grating.

The transmitted light transmitted without being emitted from the grating element 302 reaches the light transmission side of the grating element 302, propagates through the waveguide 321, and enters the light input side of the grating element 303. At this time, light enters the grating element 303 in the same direction as the grating element 301. Therefore, if the period Λ, the width w, the depth d of the diffraction grating and the thickness D of the optical waveguide are set to be the same as those of the grating element 301, the emission angle of the radiation light from the grating element 303 is the −1st order of the grating element 301. It can be equal to the emission angle θ ₋₁ of the emitted light.

  If additional grating elements are connected serially via a waveguide behind the grating element 303, they can be handled in the same way as described above.

  In this way, in the pair of grating elements whose directions from the light input side to the light transmission side are opposite, the emission angles of the respective emitted light can be made uniform, and the emitted light of appropriate intensity distribution is easily realized it can.

  FIG. 3B is a schematic plan view showing the configuration of a modification of still another embodiment of the grating coupler of the present invention. The form shown in FIG. 3B is the same as the embodiment of FIG. 3A in its basic configuration, but differs from the embodiment of FIG. 3A in that the light branching element 330 is provided on the light input side of the grating elements 304, 307. It is in the point provided.

  The configuration and operation of the light branching element 330 are the same as those of the light branching element 130 described with reference to FIG.

  The grating coupler of this embodiment can make the mode field of the emitted light symmetrical in the width direction, for example, as in the embodiment shown in FIG. 1B, and the number of grating elements can be increased even if the length of the light branching element 330 is added. By increasing the length, it is possible to greatly reduce the overall length as compared to the conventional one-dimensional periodic grating coupler.

  As to the function of the grating coupler of this embodiment, a set of grating elements 304, 305, 306, waveguides 322, 323 and grating elements 307, 308, 309, waveguides which receive the output of the light branching element 330, respectively, Each of the sets 324 and 325 has the same function as that of the grating coupler of FIG.

  FIG. 3C is a schematic plan view showing the configuration of another modification of still another embodiment of the grating coupler of the present invention. The basic configuration shown in FIG. 3C is the same as the basic configuration shown in FIG. 3B, except that the light branching element 340 is provided on the light transmitting side of the grating element 310 in the point different from the configuration of FIG. 3B. It is on the point.

  The configuration and operation of the light branching element 340 are the same as those of the light branching element 130 described with reference to FIG.

  The grating coupler of this embodiment, like the embodiment shown in FIG. 1C, increases the number of grating elements even if the length of the optical branching element 340 is added, and the overall length of the grating coupler is increased compared to the conventional one-dimensional periodic grating coupler. Can be significantly reduced.

  As to the function of the grating coupler of this embodiment, although the light branching element 340 is interposed, the set of the grating elements 310, 311, 312 and the waveguides 326, 327 and the grating elements 310, 313, 314, The respective sets of the waveguides 328 and 329 have the same function as that of the grating coupler of FIG.

  In each of the above embodiments, nothing is connected to the light transmission side of the grating element at the end of the serial connection, but for example, in order to collect the transmitted light, the light combining unit (same structure as the light branching unit , Etc. may be connected, or bidirectional operation may be performed with such a structure.

  Further, the shape of the waveguide is not limited to the thin wire waveguide but may be another form such as a rib waveguide or a combination thereof.

  Furthermore, it is also possible to use a multistage configuration of 1-input 2-output MMI, or to use 1-input 4-output MMI instead of 1-input 2-output MMI. As described above, the number of branches of the light branching element is not limited to 2, and may be a larger branching ratio.

  In addition, if there is a margin in the light branching ratio or the allowable value of the light loss, the light branching portion may be configured using a Y-branch or a directional coupler instead of the MMI.

  Further, the light branching portion may be provided on both the light input side and the light transmission side of the grating element.

  In addition, by adjusting the length of the waveguide between the grating elements, it is possible to align the phases of light emitted from each grating element constituting the grating coupler or to set an arbitrary phase difference. It is. In particular, aligning the phase of light emitted from each grating element is effective when it is desired to efficiently couple the grating coupler of the present invention to a single mode waveguide such as a single mode fiber.

  Although the embodiments of the present invention have been described above with reference to the drawings, those skilled in the art can use other similar embodiments and can appropriately select the embodiments without departing from the present invention. It should be noted that changes or additions of can be made.

101 to 109, 201 to 214, 301 to 314; grating elements 101a, 101b, 420; tapered waveguide portions 101c, 410; grating portions 120 to 124, 220 to 229, 320 to 329; waveguides 130, 140, 230, 240, 330, 340; light branching element 401; substrate 402; BOX layer 414; core layer 416; upper cladding layer

Claims (4)

A grating coupler formed on the base plate, said grating coupler comprises a waveguide connecting between a plurality of grating elements arranged adjacent each other in the width direction, the plurality of grating elements are the same as each other or comprising a one-dimensional lattice having a substantially periodic in the same direction, a waveguide which is connected to the light transmitting side of the grating element is connected to the optical input side of the grating elements adjacent to each other, at least a pair of grating elements that are adjacent to each other A grating coupler characterized in that the grating coupler comprises a one-dimensional grating having the same period and depth, and the direction from the light input side to the light transmission side is the same . A grating coupler formed on a substrate, the grating coupler comprising: a plurality of grating elements arranged adjacent to each other in the width direction; and a waveguide connecting the grating elements, the plurality of grating elements having periodicity The waveguide connected to the light transmission side of the grating element is connected to the light input side of the grating elements adjacent to each other, and at least a pair of grating elements adjacent to each other are one-dimensional gratings having different periods. A grating coupler characterized in that the direction from the light input side to the light transmission side is reverse. At least the distance between the pair of grating elements is characterized in that it is narrower in the width of the two grating elements following the grating coupler according to claim 1 or 2 adjacent to each other. The grating coupler according to any one of claims 1 to 3 , further comprising: a light branching portion connected to the grating element on the light input side and / or the light transmitting side.

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