JPWO2016170890A1 - Grating coupler - Google Patents

Abstract

Provided is a grating coupler that can be reduced in size and can be easily designed in accordance with a mode field to be optically coupled. The light incident on the light input side of the grating element 102 propagates through the tapered waveguide portion 101a and reaches the grating portion 101c, and −1st order radiation light is emitted from the grating portion 101c. The transmitted light that passes through the grating portion 101 c propagates through the tapered waveguide portion 101 b to reach the light transmission side of the grating element 101, further propagates through the waveguide 120, and enters the light input side of the grating element 102. Similarly to the grating element 101, the −1st order radiation light is emitted from the grating element 102 as well. As for the emission angle of the −1st order radiation light, the period Λ, the width w, the depth d, and the thickness D of the optical waveguide of the diffraction grating are set in the same manner as the grating element 101. It becomes equal to the light emission angle θ-1.

Description

  The present invention relates to a grating coupler.

  In order to input / output optical signals to / from an optical circuit on a substrate composed of an optical waveguide, the function to emit the light propagating through the optical waveguide to the outside of the optical waveguide or to allow external light to enter the optical waveguide is provided. There is something I want to do. A grating coupler is an optical input / output device that can be used for such a purpose. The propagation light of an optical waveguide on a substrate is incident on an end face of an optical fiber close to the surface of the substrate, or vice versa. Can be incident on the optical waveguide.

  FIG. 4A is a schematic cross-sectional view of a grating portion as an example of a grating coupler having a configuration for emitting guided light to the outside of the 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 an action of receiving an optical signal through an optical waveguide and converting an optical axis by diffraction. 4A includes a BOX (Buried Oxide) layer 402, a core layer 414 having a refractive index higher than that of the BOX layer 402, and an upper clad having an index of refraction comparable to that of the BOX layer 402 (Overclad). ) Layer 416 is laminated on the substrate 401 in this order, and a diffraction grating is formed on the core layer 414. Thus, the grating coupler enables optical path conversion only by forming a diffraction grating on the core layer 414, for example.

  Hereinafter, the operation principle of the grating coupler for emitting the guided light to the outside of the optical waveguide will be outlined.

In the core layer 414 of the grating coupler 400 shown in FIG. 4A, a diffraction grating that is periodic in the propagation direction (z direction) of the guided light and has irregularities in the thickness direction (x direction) of the waveguide is formed. . When such a diffraction grating is formed, part of the guided light is diffracted to generate radiated light. The guided light and radiated 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. Where Λ is the period of the diffraction grating, K = 2π / Λ, and q is a value corresponding to the order of the emitted light (0, ± 1, ± 2,...).

In this case, the emission angle theta _q of q Next emitted light relative to the normal of the diffraction grating, the wavelength in vacuum of light lambda, the effective refractive index of the waveguide N, the refractive index of the n _c upper cladding layer, lambda Is the period of the diffraction grating, it can be obtained from the equation: n _c sin θ _q = N + qλ / Λ.

In general, the effective refractive index of the waveguide N is takes a value between the refractive index and the refractive index of the cladding of the core, have a relationship of N> n _c, the emission of light occurs satisfies q ≦ -1 Limited to order q. Furthermore, considering that the diffraction efficiency with a smaller diffraction order is higher, the power distribution ratio to the emitted light can be maximized by using the emitted light satisfying q = −1 (−1st order emitted light). Is the case. Further, although the power distribution ratio is lower than that of the −1st order radiation light, the −2nd order radiation light can also be used as necessary. In addition to the emitted light such as the first-order emitted light and the second-order emitted light, 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. Light P _{down is} also present.

Here, the emission angle θ _q can be arbitrarily designed according to the period Λ, width w, depth d, and thickness D of the optical waveguide shown in FIG. 4A. An example of numerical ranges 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: 180-220 nm
As described above, the case where guided light is emitted outside the optical waveguide by the grating coupler has been described. However, it has been widely performed that external light is incident on the optical waveguide by such a grating coupler.

  As described above, the grating coupler is basically based on a structure in which a diffraction grating is formed on the surface of the core. In the process of improving performance, structures having various characteristics have been proposed. For example, as the most basic grating coupler, it has a one-dimensional grating (grating) in which irregularities are periodically arranged in the length direction of the waveguide, and the grating is linear and uniform in the transverse width direction of the waveguide. There is a 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 and approximated to a two-dimensional structure in the length direction and the thickness direction. Therefore, when the radiated 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 a 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 the width of the optical waveguide on the substrate. It was necessary to insert a long and gentle taper waveguide section 420. This is to suppress the disturbance of the wavefront of propagating light at the connecting portion between the grating portion 410 and the optical waveguide portion 420. As a result, the overall structure of the grating coupler including the grating portion 410 and the tapered waveguide portion 420 is combined. However, it was difficult to increase the size (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 that is about 15 times 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.

  Thus, as another conventional technique, a grating coupler in which an arc-shaped grating is formed in a tapered waveguide having a large divergence angle has been proposed (see FIG. 4C) (Patent Documents 1 and 2). In such a grating coupler, since the grating is also formed in an arc shape, the entire structure including the grating portion and the tapered waveguide can be downsized.

  However, since the grating has an arc shape, the grating is not uniform in the transverse width direction of the waveguide, making it impossible to perform two-dimensional approximation in numerical calculations. For this reason, in order to know the characteristics of the arc-shaped grating coupler, it becomes necessary to perform three-dimensional calculation that requires enormous computer power and time, or to repeatedly perform time-consuming trial manufacture and verification by experiment. A new problem has arisen that it takes a lot of time to adjust the detailed structure in search of good properties.

US Pat. No. 7,245,803 US Pat. No. 7,260,289

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 above-mentioned problem of the large structure of the one-dimensional grating coupler and the difficulty of design of the arc-shaped grating coupler, it is small and easy to optimize the structure at the same time. It is a problem to provide a simple grating coupler.

  In order to solve the above-described problems, the grating coupler of the present invention is characterized by a configuration in which a plurality of grating elements having a one-dimensional grating are arranged adjacent to each other, and an embodiment thereof is roughly as follows. In the following, the names of “light input side” and “light transmission side” are assigned assuming that the light input from the waveguide is output from the grating coupler to the substrate surface side. It should be noted that if they are reversed, their name assignments are reversed.

  In the basic form of the grating coupler of the present invention, the grating coupler includes 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 having a characteristic one-dimensional grating 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 includes 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 width in total with respect to the width of a conventional one-dimensional periodic grating coupler A waveguide is provided between the two grating elements, and the waveguide is connected to the light transmission side of at least one grating element and connected to the light input side of another grating element (serial connection). Therefore, the width of each grating element is reduced in inverse proportion to the number of grating elements, and accordingly, the length of the tapered waveguide portion added before and after each grating element is also reduced.

  Since the grating coupler of this basic form 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. As a result, the conventional one-dimensional structure is reduced. It is possible to make it shorter than the total length of the grating coupler of the period. On the other hand, since each grating element is a one-dimensional grating, the ease of designing the light incident / exit characteristics is maintained. Needless to say, the same effect can be obtained in a mode in which light is incident on each grating element from the outside.

  In the first mode of the grating coupler of the present invention, in the basic mode, the interval between at least a pair of adjacent grating elements is configured to be equal to or smaller than the narrower width of the two grating elements. . By adopting such a configuration, it is possible to prevent a drop in the intensity of the emitted light due to a gap existing between adjacent grating elements from becoming a practical problem.

  In the second mode of the grating coupler of the present invention, in the basic mode and the first mode, two or more grating elements are provided with a one-dimensional grating having the same period and depth, and light is transmitted from the light input side. The direction toward the transmission side is the same. As a result, the emission angles of diffracted light (for example, −1st order radiated light) having the same diffraction order can be made uniform in each grating element, and radiated light with an appropriate intensity distribution can be easily realized.

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

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

  According to the present invention, it is possible to provide a grating coupler that can be reduced in size and can be easily designed in accordance with a 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 variation of one embodiment of the grating coupler of the present invention. FIG. 1C is a schematic plan view showing the configuration of another variation 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. 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 a 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. FIG. 3B is a schematic plan view showing a configuration of a modification of still another embodiment of the grating coupler of the present invention. FIG. 3C is a schematic plan view showing a configuration of another modification of the further embodiment of the grating coupler of the present invention. FIG. 4A is a schematic cross-sectional view illustrating a configuration of an example of a grating coupler. FIG. 4B is a schematic plan view of a grating coupler having a one-dimensional period. FIG. 4C is a schematic plan view of a grating coupler in which an arcuate grating is formed in a tapered waveguide having 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 these embodiments, and should be interpreted based on the description of the claims. Strictly speaking, the “structure” of the grating coupler is “the structure of the core”, and it goes without saying that the clad surrounds the periphery. In the following description, an aspect in which light is emitted from the grating element to the outside will be mainly described. However, an aspect in which light is incident on the grating element from the outside can also be realized with 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 the present 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 are formed on a semiconductor substrate (not shown) (for example, a silicon substrate) and can be configured as, for example, a buried optical waveguide having a cross-sectional structure as shown in FIG. 4A.

  The grating element 101 includes tapered waveguide portions 101a and 101b and a grating portion 101c.

  The tapered waveguide portions 101a and 101b are each constituted by a planar optical waveguide having a function of guiding light along the substrate surface, and the narrow end portion of the tapered waveguide portion 101a is the light of the grating element 101. The wide end of the tapered waveguide section 101b is connected to the grating section 101c, and the narrow end is connected to the grating section 101c. Is the light transmitting 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 a curved tapered shape such as a quadratic curve or a semi-elliptical shape.

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

  The grating element 101 and the grating element 102 are provided with 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. This is because the emission angles of diffracted light having the same diffraction order (for example, −1st order radiated light) are aligned in each grating element, so that radiated light having an appropriate intensity distribution can be easily realized.

  The width of each grating element need not be equal. When the widths of the grating elements are made different, the intensity per unit area of light emitted from the narrow grating elements, that is, the power density is increased. 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, the light intensity distribution in the width direction of the grating part, that is, the shape of the mode field of the emitted light can be adjusted more finely. Therefore, for example, even when there are a plurality of types of optical fibers that are optical coupling partners and their mode fields are different, it is possible to perform an optimization design in the width direction in accordance with 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. Is.

  The interval between the adjacent grating elements 101 and 102 is set to be equal to or smaller than the narrower width of the grating elements 101 and 102. The reason for this setting is that if there is a gap between adjacent grating elements 101 and 102, the amount of emitted light is reduced at that portion, and the intensity distribution of light emitted from the grating coupler is affected. However, if the interval between the grating elements is set as described above, the influence can be reduced to a level that does not cause a problem in practice.

  With respect to the scale of the grating coupler shown in FIG. 1A, if the total size of the grating portions of a plurality of grating elements is substantially the same as the grating portion of the conventional grating coupler, the length of the grating coupler is calculated as follows. The

Assuming that the grating coupler is composed of p grating elements, the length of the tapered waveguide portion of the kth (1 ≦ k ≦ p) grating element is s _k , the length of the grating portion is t _k , and the grating portion Let u _k be the width of.

The length t _k of the grating portion is substantially equal to the length T of the grating portion 410 of a conventional grating coupler shown in Figure 4B. In addition, since 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, s _k / u _k = S / U = r (constant). Is established. Note that Σ _k u _k = U (where Σ _k represents the sum of 1 to p).

Accordingly, the length L of the grating coupler shown in FIG. 1A is determined by considering that there are two tapered waveguide portions in each grating element. L = T + 2 * Max (s _k ) = T + 2 * r * Max (U _k ). The conventional grating coupler shown in FIG. 4B is expressed as L ′ = T + S = T + r * U, where L ′ is the length.

Now, the width _{u k} of the grating elements when all _equal, since u k = U / p is satisfied, the L = T + (2 * r / p) * U. As is clear from the comparison of L and L ′ described above, L ≦ L ′ when p ≧ 2, and L decreases and approaches T as p increases.

  Therefore, in the grating coupler shown in FIG. 1A, the total length can be significantly reduced by increasing the number of grating elements as compared with a conventional one-dimensional period grating coupler.

As for the cases where the width u _k of grating elements are different, L is but slightly increases is compared with the above case, in which substantially the same conclusion holds.

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

Light incident on the light input side of the grating element 101 via an optical waveguide (not shown) propagates through the tapered waveguide portion 101a to reach the grating portion 101c, and the diffracted light having a diffraction order of −1 in the grating portion 101c. Is emitted. The outgoing angle θ ₋₁ of the −1st order radiation can be arbitrarily designed according to the period Λ, width w, depth d, and thickness D of the optical waveguide of the diffraction grating (see FIG. 4A).

There is also light that passes through the grating portion 101c (see FIG. 4A; _Ptrans ). The transmitted light propagates through the tapered waveguide portion 101b and reaches the light transmission side of the grating element 101, and further propagates through the waveguide 120. Then, it enters the light input side of the grating element 102. In the grating element 102, similarly to the grating element 101, radiation light having a diffraction order of −1 is emitted. As for the emission angle of the −1st order radiation light, the period Λ, the width w, the depth d, and the thickness D of the optical waveguide of the diffraction grating are set in the same manner as the grating element 101. It becomes equal to the light emission angle θ- ₁ .

Even when a further grating element is serially connected to the grating element 102 via a waveguide, the emission angle of the minus first-order radiation in the grating element is such that the period Λ, width w, depth d of the diffraction grating. If the thickness D of the optical waveguide is set in the same manner 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 manner, the emission angles of the −1st order radiation light can be made uniform in each grating element, and radiation light having an appropriate intensity distribution can be easily realized.

  FIG. 1B is a schematic plan view showing the configuration of a variation of one embodiment of the grating coupler of the present invention. The basic configuration of the form 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 an optical branching element 130 is provided on the light input side of the grating elements 103 and 105. It is in the point provided.

  The optical branching element 130 branches and outputs the light incident from the input end via an optical waveguide (not shown) to the grating elements 103 and 105 from the two output ends. The optical branching element 130 is composed of, for example, a 1-input 2-output multimode interferometer (MMI) having a core made of Si and a clad made of a silicon oxide film. In general, a multimode interferometer has a self-convergence effect that is periodically reproduced in the process of propagation of a light field at the incident end through the multimode interference waveguide, and is dependent on the width of the multimode interference waveguide. One or more converging fields can be obtained at a specific distance from the determined input end. By connecting the waveguide with the convergence position as the output end, a very low loss optical branch can be configured.

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

  In the present embodiment, the length of the optical branching element 130 from the input end to the output end needs to be equal to or longer than the specific distance, but can be set within about 15 μm. 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. By increasing the number of grating elements, the overall length of the grating coupler can be increased as compared with the conventional one-dimensional period grating coupler. The length can be significantly reduced.

  As for the action of the grating coupler according to the present embodiment, each of a set including the grating elements 103 and 104 and the waveguide 121 and a set including the grating elements 105 and 106 and the waveguide 122 that receive the output of the optical branching element 130, respectively. In this group, since it has the same function as the grating coupler of FIG.

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

  The configuration and operation of the optical branching element 140 are the same as those of the optical branching element 130 described with reference to FIG.

  According to this embodiment, similarly to the embodiment shown in FIG. 1B, the light branching ratio 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, outgoing light having a mode field symmetrical in the width direction can be obtained.

  Furthermore, the grating coupler of the present embodiment is the same as that of the embodiment shown in FIG. 1B. Even if the length of the optical branching element 140 is added, the entire number of grating elements is increased as compared with the conventional one-dimensional periodic grating coupler. The length of can be greatly reduced.

  Note that the action of the grating coupler of this embodiment is composed of a combination of the grating elements 107 and 108 and the waveguide 123, and the grating elements 107 and 109 and the waveguide 124, although the optical branching element 140 is interposed. Each set has the same action as the grating coupler of FIG.

  In the present embodiment, since the grating elements 108 and 109 divide the light transmitted through the grating element 107 by the light branching element 140 and input it, the intensity of the light emitted from the grating elements 108 and 109 decreases. 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 is increased. The light intensity distribution can be adjusted. As a result, the light intensity distribution 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 configuration shown in FIG. 2A is the same as the configuration shown in FIG. 1A in the basic configuration, but differs from the configuration shown in FIG. 1A in that the waveguide 120 is a grating element in the configuration shown in FIG. 1A. 2A, the waveguides 220 and 221 are installed so as to surround the grating elements 201 to 203. In the configuration shown in FIG.

  With such a configuration, the configuration shown in FIG. 1A has a restriction that the gap cannot be reduced below a certain limit because the waveguide 120 is installed in the gap between the grating elements 101 and 102. However, in the form shown in FIG. 2A, since there is almost no restriction as described above regarding the gap between the grating elements 201 to 203, the influence on the intensity distribution of the light emitted from the grating coupler due to the existence of the gap is further reduced. It will be possible.

  As with the grating coupler shown in FIG. 1A, the overall length of the grating coupler of this embodiment can be greatly reduced by increasing the number of grating elements as compared to a conventional one-dimensional period grating coupler. is there.

  Note that the action of the grating coupler of this embodiment is the same as that of the grating coupler of FIG.

  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 of the form shown in FIG. 2B is the same as that of the embodiment of FIG. 2A. The difference from the embodiment of FIG. 2A is that an optical branching element 230 is provided on the optical input side of the grating elements 204 and 207. It is in the point provided.

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

  The grating coupler of this embodiment can make the mode field of the emitted light symmetric, for example, in the width direction, and can increase the number of grating elements even if the length of the optical branching element 230 is added as in the embodiment shown in FIG. 1B. As a result, the overall length can be greatly reduced as compared with a conventional grating coupler having a one-dimensional period.

  As for the action of the grating coupler of this embodiment, a set of grating elements 204, 205, 206 and waveguides 222, 223, and grating elements 207, 208, 209, waveguides that receive the output of the optical branching element 230, respectively. Each set of 224 and 225 has the same operation as the grating coupler of FIG.

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

  The configuration and operation of the optical branching element 240 are the same as those of the optical branching element 130 described with reference to FIG.

  The grating coupler of this embodiment can increase the number of grating elements even if the length of the optical branching element 240 is added, as well as the mode field of the emitted light can be made symmetrical in the width direction, for example, as in the embodiment shown in FIG. 1C. As a result, the overall length can be greatly reduced as compared with a conventional grating coupler having a one-dimensional period.

  As for the action of the grating coupler of this embodiment, although the optical branching element 240 is interposed, the set of the grating elements 210, 211, 212 and the waveguides 226, 227 and the grating elements 210, 213, 214, Each set of the waveguides 228 and 229 has the same function as 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 a pair of grating elements 301 and 302 connected by a waveguide 320 are provided with one-dimensional gratings having different periods and the direction from the light input side to the light transmission side is reversed. Has been. As a result, diffracted light (for example, −1st order radiated light and −2nd order radiated light) having different diffraction orders is emitted from the pair of grating elements 301 and 302, and their emission angles are aligned in the pair of grating elements. Therefore, it is possible to easily realize radiated light having an appropriate intensity distribution.

  With such a configuration, the waveguide connected between the grating elements can be set short, and the waveguide can be easily formed.

  Also, the grating coupler shown in FIG. 3A, like the grating coupler shown in FIG. 1A, greatly reduces the overall length compared to a 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, or there is light that passes through the grating element 301. The transmitted light reaches the light transmission side of the grating element 301 and further propagates through the waveguide 320. The light incident on the light input side of the grating element 302 is the same as that of 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, the emission angle of the −1st order radiation of the grating element 302 is assumed to be the same as that of the grating element 301 when the period Λ, width w, depth d of the diffraction grating and the thickness D of the optical waveguide are the same. The light output angle becomes −θ ₋₁ , and the inclination direction is opposite to the light output angle θ ₋₁ of the −1st order radiation light of the grating element 301.

  There are two means for matching the emission angle of the radiated light from the grating element 302 with the emission angle of the −1st-order radiated light of the grating element 301 including the tilting direction. One of them is to incline the −1st order radiation in the opposite 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 opposite to the -1st order diffracted light, as the radiated light of the grating element 302 (see FIG. 4A).

  In general, the use of −1st order diffracted light having a small diffraction order as the radiated light of the grating element 302 has higher radiation efficiency per unit length, but the period Λ of the diffraction grating becomes smaller. The difficulty level of increases. On the other hand, when -2nd order diffracted light is used as radiated light, the radiation efficiency per unit length decreases as the diffraction order increases, but the period Λ of the diffraction grating is the period of the diffraction grating of the grating element 301. Therefore, the difficulty of production does not increase.

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

Regardless of which of the −1st order diffracted light and the −2nd order diffracted light is used as the radiated light, the emission angle of the light from the grating element 302 is determined by the emission of the −1st order radiated light from the grating element 301 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 of the diffraction grating in the grating element 302 must be appropriately adjusted. However, it is difficult to adjust the depth d of the diffraction grating and the thickness D of the optical waveguide in the above adjustment conditions in consideration of the fact that the grating element 302 is formed simultaneously with the grating element 301. It is considered realistic to adjust the period Λ and width w of the grating so that the emission angle of the emitted light of the grating element 302 is equal to the emission angle θ ₋₁ of the −1st order emitted light of the grating element 301. Therefore, in this embodiment, the emission angle of the radiated light of the grating element 302 is set by adjusting the period of the diffraction grating.

The transmitted light that is transmitted without being emitted from the grating element 302 reaches the light transmitting side of the grating element 302, further 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, and the thickness D of the optical waveguide of the diffraction grating are set similarly to the grating element 301, the emission angle of the radiated light from the grating element 303 is set to the −1st order of the grating element 301. It can be equal to the radiation output angle θ ₋₁ .

  Even when a further grating element is serially connected via a waveguide behind the grating element 303, it can be handled in the same manner as described above.

  In this way, in a pair of grating elements whose directions from the light input side to the light transmission side are reversed, the emission angles of the respective radiated light can be aligned, and radiated light with an appropriate intensity distribution can be easily realized. it can.

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

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

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

  As for the action 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 that receive the output of the optical branching element 330, respectively. Each of the pairs of 324 and 325 has the same function as the grating coupler of FIG.

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

  The configuration and operation of the optical branch element 340 are the same as those of the optical branch element 130 described in relation to FIG.

  As in the embodiment shown in FIG. 1C, the grating coupler of this embodiment has an overall length longer than that of a conventional one-dimensional period grating coupler by increasing the number of grating elements even if the length of the optical branching element 340 is added. It is possible to greatly reduce the length.

  In addition, regarding the operation of the grating coupler of this embodiment, although the optical branching element 340 is interposed, the set including the grating elements 310, 311, 312 and the waveguides 326, 327 and the grating elements 310, 313, 314, Each set of the waveguides 328 and 329 has the same function as the grating coupler of FIG.

  In each of the above-described embodiments, nothing is connected to the light transmission side of the grating element at the end of the serial connection. For example, in order to collect transmitted light, an optical multiplexing unit (with the same structure as the optical branching unit) is used. Or the like that operate in the opposite direction) may be connected, or bidirectional operation may be performed in such a structure.

  Further, the shape of the waveguide is not limited to the thin wire waveguide, but may be other forms such as a rib-type waveguide or a combination thereof.

  Furthermore, the optical branching unit can be configured as a multi-stage MMI having one input and two outputs, or an MMI having one input and four outputs can be used instead of a one input and two outputs MMI. Thus, the number of branches of the optical branching element is not limited to 2, and may be a larger branching ratio.

  Further, if there is a margin in the light branching ratio and the allowable value of light loss, the light branching unit 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.

  Also, by adjusting the length of the waveguide between the grating elements, it is possible to align the phase of the 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 the light emitted from each grating element is effective when the grating coupler of the present invention is to be optically coupled to a single mode waveguide such as a single mode fiber with high efficiency.

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

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

Claims (5)

  A grating coupler formed on a substrate, the grating coupler comprising a plurality of grating elements arranged adjacent to each other and a waveguide connecting between the grating elements, the grating elements being in the same or substantially the same direction A grating coupler comprising: a one-dimensional grating having periodicity; and a waveguide connected to a light transmission side of at least one grating element is connected to a light input side of another grating element.   2. The grating coupler according to claim 1, wherein an interval between at least one pair of adjacent grating elements is equal to or smaller than a narrower width of the two grating elements. 3.   The grating coupler according to claim 1 or 2, wherein the two or more grating elements have one-dimensional gratings having the same period and depth, and have the same direction from the light input side to the light transmission side. .   3. The grating coupler according to claim 1, wherein at least a pair of grating elements adjacent to each other includes a one-dimensional grating having a different period, and a direction from the light input side to the light transmission side is reversed.   The grating coupler according to any one of claims 1 to 4, further comprising an optical branching unit connected to the grating element on the light input side and / or the light transmission side.

Images