JP2014002239A - Optical waveguide device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress cyclically wavelength dependent multi-stage crossing loss that occurs in a main optical waveguide having a plurality of crossing optical waveguides intersecting therewith.SOLUTION: An optical waveguide device, which includes a core 12a and a cladding 12b surrounding the core, has a main optical waveguide 14 and crossing waveguides 16-16which intersect with the main optical waveguide at first to N-th intersections C-Cthat exist serially along a light propagation direction P in the described order. When a distance between the i-th and j-th intersections is defined as D(I,j) (where i is an integer that satisfies 1≤i≤N-1, and j is an integer that satisfies i<j≤N), D(g,g+1)≠D(g+1,g+2) is true for at least one g (where g is an integer that satisfies 1≤g≤N-2).

Description

  The present invention relates to an optical waveguide device including a reference optical waveguide and crossed optical waveguides that intersect the reference optical waveguide at a plurality of intersections.

  In the optical communication field, silicon photonics technology that forms an optical circuit with silicon, which is a base material of an electronic circuit, and integrates the electronic circuit and the optical circuit has attracted attention. This technology is expected to be applied to an optical interconnection in which electronic circuits such as LSI (Large-Scale Integrated Circuit) are connected by optical communication with high speed and less interaction.

  An optical module used for optical interconnection includes an optical transmission unit and an optical reception unit that are connected to an electronic circuit, and an optical connection unit that connects the optical transmission unit and the optical reception unit. The optical waveguide constituting the optical connecting unit basically connects the light emitting element of the optical transmitting unit and the light receiving element of the optical receiving unit in a one-to-one relationship. However, in order to increase the degree of integration of the optical module, a technique for connecting the optical transmitter and the optical receiver in a “many-to-many” manner is required.

  As described above, when the optical transmission unit and the optical reception unit are connected in a many-to-many manner, it is inevitable that the optical waveguides intersect at the optical connection unit. At the intersection of the two optical waveguides, light is diffracted from one optical waveguide through which the light propagates to the other optical waveguide that intersects the optical waveguide, resulting in an intensity loss. Hereinafter, this loss of light intensity that occurs at one intersection is also referred to as a “single-stage intersection loss”.

  In order to reduce this one-stage crossing loss, a technique for devising the structure of the intersection of two optical waveguides has been reported (for example, see Patent Documents 1 and 2).

JP 2009-204736 A JP 2011-90223 A

  Certainly, according to Patent Documents 1 and 2, one-stage crossing loss can be reduced. However, the inventor has found that when the optical waveguide has a plurality of intersections, an optical loss of a mechanism different from the one-step cross loss occurs. More specifically, it has been found that a plurality of intersections function as grating points of the grating, and that the output light has a periodic optical loss with respect to wavelength (hereinafter also referred to as “multistage intersection loss”). Furthermore, the inventor has obtained knowledge that the multi-stage crossing loss cannot be sufficiently suppressed only by reducing the one-step crossing loss according to the above-mentioned literature.

  The present invention has been made with such a technical background. Accordingly, an object of the present invention is to obtain an optical waveguide device that can sufficiently suppress the multistage crossing loss practically.

  As a result of intensive studies, the inventor has come up with the idea that the above-described problems can be solved by changing the interval between the intersections formed in the optical waveguide. Accordingly, the optical waveguide device of the present invention includes an optical waveguide including a core and a clad surrounding the core. The optical waveguide includes a reference optical waveguide and a crossed optical waveguide that intersects the reference optical waveguide at first to Nth intersections (N is an integer of 3 or more) serially arranged in this order along the light propagation direction. Prepare.

  Here, when the length of the interval between the i-th and j-th intersections is D (i, j) (where i is an integer satisfying 1 ≦ i ≦ N−1, and j is i <j ≦ N) D (g, g + 1) ≠ D (g + 1, g + 2) for at least one g (g is an integer satisfying 1 ≦ g ≦ N−2).

  The present invention is configured as described above. Therefore, according to the optical waveguide device of the present invention, the multistage crossing loss can be sufficiently suppressed in practice.

(A) is a schematic diagram schematically showing the structure of a conventional optical waveguide device, (B) is a characteristic diagram showing output characteristics of the conventional optical waveguide device, and (C) It is a schematic diagram which shows the structure of the optical waveguide element of this invention roughly, and (D) is a characteristic view which shows the output characteristic of the optical waveguide element of this invention. It is a schematic diagram which shows roughly the structure of the optical waveguide element of embodiment. (A) is a schematic diagram schematically showing the structure of the optical waveguide element of the most general mode, and (B) is a schematic diagram schematically showing the structure of a special mode of the optical waveguide device of (A). It is. (A) is a figure for demonstrating the pattern which determines the adjacent intersection space | interval of the optical waveguide element of FIG. 3 (B), (B) is a figure for demonstrating the reference | standard length of an intersection space | interval. . It is a schematic diagram which shows roughly the structure of the further special aspect of the optical waveguide element of FIG. 3 (B). (A) is a schematic diagram which shows an example of the optical waveguide element of FIG. 5, (B) is a figure for demonstrating the determination method of the crossing distance of the optical waveguide element of (A). (A) is a top view which shows typically the structure of an optical waveguide element, (B) is a cut end view along the AA line of (A). (A) is a characteristic view which shows the output characteristic of the conventional optical waveguide element, (B) is a characteristic figure which shows the output characteristic of the optical waveguide element of this invention.

  Embodiments of the present invention will be described below with reference to the drawings. In each figure, the shape, size, and arrangement relationship of each component are schematically shown to the extent that the present invention can be understood. Moreover, although the preferable structural example of this invention is demonstrated hereafter, the material of each component, a numerical condition, etc. are only a suitable example. Accordingly, the present invention is not limited to the following embodiments. Moreover, in each figure, the same code | symbol is attached | subjected to a common component and the description may be abbreviate | omitted. In addition, reference numerals of components that have a clear correspondence with other drawings may be omitted.

(Summary of Invention)
First, the outline of the present invention will be described with reference to FIG. FIG. 1A is a schematic diagram schematically showing the structure of a conventional optical waveguide element (hereinafter also referred to as a conventional element) having the same intersection spacing. FIG. 1B is a characteristic diagram showing output characteristics of a conventional element. FIG. 1C is a schematic view schematically showing a structure of an example of the optical waveguide device of the present invention in which the intersection intervals are not equal. FIG. 1D is a characteristic diagram showing output characteristics of the optical waveguide device of FIG. In FIGS. 1A and 1C, an optical waveguide element is simply drawn in order to contribute to an understanding of the invention. That is, the illustration of the substrate and the clad is omitted, and the core having a finite width is drawn by a simple straight line.

  Since the basic structure of both the conventional element 100 and the optical waveguide element 10 is the same, the structure of the optical waveguide element will be briefly described by taking the conventional element 100 of FIG. 1A as an example. The conventional element 100 includes an optical waveguide composed of a reference optical waveguide 14 and a crossed optical waveguide 26. This optical waveguide will be described later.

Crossed optical waveguides 26 _{1 to} 26 _N intersect the reference optical waveguide 14 at first to Nth intersections C _{1 to} C _N (N is an integer of 3 or more). Here, the length (distance) between adjacent intersections is referred to as an adjacent intersection interval. Since the total number of intersections is N, the total number of adjacent intersection intervals is N-1. Further, input light such as communication signal light is input from one end of the reference optical waveguide 14. This input light propagates through the optical waveguide 14 along the light propagation direction (see arrow P in the figure), and is output from the other end as output light of wavelength λ1.

The difference between the conventional element 100 and the optical waveguide element 10 is the distance between adjacent intersections. Specifically, in the conventional element 100, all adjacent intersection intervals are the same at x ₁ (x ₁ is a positive real number), whereas in the optical waveguide element 10, all adjacent intersection intervals are different values x ₁ and x _{2. ,...,} take the _{x (N-1) (x} 1 ~x (N-1) is a positive real number that satisfies _{x 1 ≠ x 2 ≠ ·· ≠} x (N-1)).

  FIG. 1B shows the output characteristics of the output light from the reference optical waveguide 14 in the conventional device 100, the horizontal axis is the wavelength of the output light (arbitrary unit), and the vertical axis is the light intensity of the output light (arbitrary Unit). As shown in FIG. 1B, in the conventional element 100, due to the equal interval between adjacent intersections, the output light from the reference optical waveguide 14 is periodic with respect to the above-described multistage crossing loss, that is, the wavelength. An intensity change occurs.

This periodic intensity change of the conventional element 100 is caused by the formation of a linear diffraction grating by regularly arranged intersections C _{1 to} C _N. The inventor has obtained knowledge that a multistage intersection loss with a wavelength period W of 20 to 30 nm occurs when the distance between adjacent intersections is about 10 μm. Since this wavelength period W is equivalent to the wavelength fluctuation (˜40 nm) of a light emitting element (LD: Laser Diode) generated in a normal use environment, it may adversely affect optical communication in the optical module.

  On the other hand, as shown in FIG. 1C, in the optical waveguide device 10 of the present invention, this multi-stage crossing loss is suppressed by arranging the crossing points where the crossing intervals are not equal in the reference optical waveguide 14.

Thus, by making the adjacent intersection intervals different from each other, as shown in FIG. 1D, output light in which a periodic intensity change is suppressed can be output from the reference optical waveguide 14. Note that the horizontal axis and the vertical axis in FIG. 1D are the same as those in FIG. Further, the cross optical waveguides 16 and 16 _{1 to} 16 _N in FIG. 1C correspond to the cross optical waveguides 26 and 26 _{1 to} 26 _N in FIG. 1A, respectively.

In FIG. 1C, in order to emphasize one of the features of the present invention that “adjacent intersection intervals are different”, a special mode in which all adjacent intersection intervals x _{1 to} x _(N−1) are different. Is shown. That is, the outer extension of the present invention is a general mode (FIG. 3A), which will be described later, “one or more adjacent intersection intervals are different”, and FIG. 1C corresponds to the special mode.

(Embodiment)
Subsequently, an embodiment of the optical waveguide device will be described with reference to FIGS. FIG. 2 is a schematic diagram schematically showing the structure of the optical waveguide device. FIG. 3A is a schematic diagram schematically showing the structure of the optical waveguide element of the most general aspect, and FIG. 3B is the structure of the optical waveguide element of the special aspect of FIG. It is a schematic diagram which shows schematically. 4A is a diagram for explaining a pattern of adjacent intersection intervals of the optical waveguide element of FIG. 3B, and FIG. 4B is a diagram for explaining a reference length of the intersection intervals. It is. FIG. 5 is a schematic diagram schematically showing the structure of the optical waveguide device of the more specific aspect of FIG. 6A is a schematic diagram illustrating an example of the optical waveguide element of FIG. 5, and FIG. 6B is a diagram for explaining a method of determining the intersection distance of the optical waveguide element of FIG. 6A. It is.

  2, 3, 4 (B), 5 and 6 (A) are simplified similarly to FIGS. 1 (A) and (C). In these drawings, the same components as those in FIG. 1C are denoted by the same reference numerals and description thereof may be omitted.

As shown in FIG. 2, in the optical waveguide device 10 similar to that in FIG. 1C, the length (distance) between the _i- th intersection C _i and the j-th intersection C _j is D (i, j). . Here, i is an integer that satisfies 1 ≦ i ≦ N−1, and j is an integer that satisfies i <j ≦ N. D (i, j) is a positive real number. Hereinafter, D (i, j) is also referred to as “interval D (i, j)” or “distance D (i, j)”.

If the notation D (i, j) is used, for example, the interval between the _second and fourth intersections C ₂ and C ₄ is represented as D (2,4). Further, the above-described adjacent intersection interval is represented as D (i, i + 1). Also, the total length of the optical waveguide element 10, i.e. the distance between the first and second N intersections _{C 1} and _{C N} is represented as D (1, N).

  N is 3 or more considering that a linear diffraction grating having a minimum configuration is formed at three consecutive intersections, that is, two adjacent intersection intervals D (i, i + 1) and D (i + 1, i + 2). And

  Next, with reference to FIGS. 3A and 3B, the conditions that D (i, j) constituting the optical waveguide device 10 should satisfy in order to suppress the multistage crossing loss will be described.

  FIG. 3A shows an optical waveguide element 10-A that can suppress the multistage crossing loss sufficiently practically. That is, the optical waveguide device 10-A in FIG. 3A is the most general mode that satisfies the minimum requirements for suppressing multi-stage crossing loss. On the other hand, FIG. 3B shows an optical waveguide element 10-B in a special case of FIG. 3A, that is, an aspect in which the effect of suppressing the multistage crossing loss is higher than that of 10-A.

  In the optical waveguide element 10-A, the adjacent intersection distance D (i, i + 1) is different in one or more i. In the example shown in FIG. 3A, the distance between adjacent intersections D (i, i + 1) is different for one i. Specifically, in the optical waveguide element 10-A, when the integer g satisfying 1 ≦ g ≦ N−2 is 2, D (g, g + 1) ≠ D (g + 1, g + 2) (D (2, 3) ≠ D (3,4)) holds. Further, when g ≠ 2, D (g, g + 1) = D (g + 1, g + 2) holds.

  The magnitude of the multistage crossing loss behaves similarly to the intensity of reflected light from a general diffraction grating, and the loss increases with the number of crossing points. That is, the multi-stage crossing loss becomes apparent only when three crossing points are arranged at equal intervals in the reference optical waveguide 14, and increases as the number of adjacent crossing point intervals D (i, i + 1) having the same size increases. Therefore, if one or more of (N−1) adjacent intersection intervals D (i, i + 1) are different from each other, the multistage crossing loss of the optical waveguide element 10-A is reduced according to the number of different ones. Can be reduced.

FIG. 3B shows an optical waveguide element 10-B corresponding to a special mode of the optical waveguide element 10-A, in which the distance between adjacent intersections D (i, i + 1) is different for all i. In this example, the value of D (i, i + 1) x 1, x 2, ··, x i, ··, and _{x (N-1).} Specifically, in the optical waveguide element 10-B, when each of p and q is an integer satisfying 1 ≦ p ≦ N−1, 1 ≦ q ≦ N−1, and p ≠ q, all (p , Q), D (p, p + 1) ≠ D (q, q + 1) holds.

  Thus, in the optical waveguide element 10-B, since the adjacent intersection distance D (i, i + 1) is different for all i, the optical waveguide element 10-B includes the optical waveguide element 10-A (FIG. 3A). Multistage crossing loss can be further suppressed than in ()).

In the optical waveguide element 10-B, x ₁ , x ₂ ,..., X _i ,..., X _(N−1) may increase monotonically with i as long as they are different from each other. It may decrease monotonously in a narrow sense or change randomly. Here, narrowly monotonically increasing means that x _i <x _{i + 1} for all i. Similarly, narrow monotonic decrease means that x _i > x _{i + 1} for all i.

  Next, a specific example of the adjacent intersection distance D (i, i + 1) in the optical waveguide element 10-B will be described with reference to FIG. FIG. 4A is a diagram illustrating some of the methods for setting the adjacent intersection distance D (i, i + 1), and FIG. 4B is a diagram for explaining the reference length of D (i, i + 1). FIG.

  As shown in FIG. 4A, the adjacent intersection distance D (i, i + 1) may be set according to any of the four patterns illustrated in FIG. The four patterns are (1) prime type, (2) constant sum type, (3) equidistant type, and (4) geometric ratio type.

  Prior to the description of these patterns, the reference length U, which is a unit of measurement of the intersection distance D (i, j), will be described with reference to FIG. As shown in FIG. 4B, the adjacent intersection distance D (i, i + 1) is a natural number times the reference length U, that is, D (i, i + 1) = E × U (E is a natural number). Good. Here, U is a positive real number.

  By the way, there is a relationship of the following well-known formula (1) between the grating period Λ of the linear diffraction grating and the wavelength λ of the reflected light. Here, the grating period Λ is the distance between two adjacent grating points of the linear diffraction grating.

λ / n _e = λ ′ = 2Λ / M (1)
Here, _ne is an equivalent refractive index of the linear diffraction grating. M is a diffraction order, which is a natural number. Moreover, lambda 'is the light which is wavelength in vacuum lambda, the wavelength in the medium of the equivalent refractive index n _e.

Equation (1) is a grating period lambda, linear diffraction grating equivalent refractive index of n _e is, 2Λ, Λ, 2Λ / 3 , ···, can reflect the light of the wavelength lambda 'of 2 [lambda] / M Indicates that

  When the relationship between the grating period Λ and the wavelength λ ′ is applied to the optical waveguide element 10-B, in this element 10-B, each D (i, i + 1) is different from each other, but each has a reference length U. It is a natural number multiple. From this, it can be seen that the optical waveguide element 10-B can reflect light having a wavelength λ ′ of 2 U / M, in which U is substituted for Λ in the equation (1).

  Thus, in the optical waveguide element 10-B, reflection derived from the reference length U of D (i, i + 1) inevitably occurs. Therefore, it is preferable to set U so that the reference length U and the wavelength λ1 of the output light do not satisfy Expression (1).

  Moreover, in order not to satisfy Formula (1), it is preferable to set U to a value of 1/10 μm or more. This is because the diffraction grating generally reflects light of wavelength λ when the grating periods Λ are equal to each other with an accuracy of 1/100 μm or less. That is, if D (i, i + 1) is changed in units of length U of 1/10 μm or more, reflection of output light at wavelength λ1 can be suppressed.

  When the core 12a (FIG. 7) is made of Si, which will be described later, and the optical waveguide device 10-B is used for general optical communication (wavelength: 1.2 to 1.6 μm), the wavelength is about 1.1 μm or less. Therefore, it is not necessary to consider high-order reflection with M ≧ 2.

  When such a reference length U is used, each D (i, i + 1) can be set, for example, according to the four patterns described above. Each will be described below.

(1) Prime type In the prime type, each D (i, i + 1) is a prime number multiple of the reference length U. For example, in the prime type, each D (i, i + 1) is set as shown in Table 1.

(2) Constant sum type In the constant sum type, each D (i, j) is divided into a constant C (C is a natural number) that is a natural number multiple of the reference length U and a d ( i) and C + d (i). For example, in the constant sum type, each D (i, i + 1) is set as shown in Table 2.

In this example, d (i) added to the constant C is a prime number times the reference length U. However, d (i) may be the product of the difference type (a + dn) and the ratio ratio br ^m described below and the reference length U.

(3) Equal Difference Type In the equal difference type, each D (i, i + 1) is a product of an arbitrary term (a + dn) of the difference sequence and the reference length U. The item number n of the arithmetic progression is an integer greater than or equal to 0, and a and d are natural numbers. For example, in the differential type, each D (i, i + 1) is set as shown in Table 3.

(4) Geometric Type In the geometric type, each D (i, i + 1) is a product of an arbitrary term br ^m of the geometric sequence and the reference length U. The term number m in the geometric sequence is an integer of 0 or more, and b and r are natural numbers. For example, in the ratio ratio type, each D (i, i + 1) is set as shown in Table 4.

  As described above, in the optical waveguide device 10-B in which the interval D (i, i + 1) is different for all i, the wavelength λ (equation (1)) of the reflected light changes for each interval D (i, i + 1). . As a result, in the optical waveguide element 10-B, the reflected light of a specific wavelength does not intensify, and the periodic multistage crossing loss derived from this specific wavelength can be suppressed.

  In this example, the case where D (i, i + 1) is set according to the above four patterns has been described. However, the setting method is not limited to these patterns, and D (i, i + 1) having a different value for each i can be set using various numerical sequences, random numbers, and the like.

  Subsequently, with reference to FIG. 5, a further special aspect of the optical waveguide device 10 -B of FIG. That is, in the optical waveguide element 10-C shown in FIG. 5, in addition to the adjacent intersection distance D (i, i + 1), D (i, j) is used for all i and j that may act as the grating period Λ. Are different.

  This means that two D (i, j) are the first and second intervals D (i1, j1) and D (i2, j2) (where i1 ≠ i2 or j1 ≠ j2), and all i1, i2 , J1 and j2 means that D (i1, j1) ≠ D (i2, j2). That is, it means that D (i, j) is made different in all combinations of the two sections including the case where the number of intersections in the section is different. Specifically, for example, a first section D (1,2) including two adjacent intersections and a second section D (3,7) including five consecutive intersections are represented by D (1, 2) It means that D ≠ 3 (7).

  Note that “i1 ≠ i2 or j1 ≠ j2” is a condition that excludes that the first and second sections are the same section (i1 = i2 and j1 = j2).

  With such a configuration, the optical waveguide element 10-C can further reduce the multistage crossing loss as compared with 10-B.

  Next, an example of a method for designing the optical waveguide device 10-C will be described with reference to FIGS. 6 (A) and 6 (B). FIG. 6A is a schematic diagram illustrating an example of the optical waveguide element 10-C. FIG. 6B is a diagram for explaining a design method of the optical waveguide device 10-C in FIG.

As shown in FIG. 6 (A), the optical waveguide device 10-C with the first to fifth intersection _C 1 -C _5, consider the case of adding a sixth intersection _{C 6.} Here, the distance D (i, j) between the existing intersections C _{1 to} C ₅ is D (1,2) = 1 μm, D (2,3) = 3 μm, D (3,4) = 5 μm and D (4, 5) = 7 μm. In the following description relating to FIG. 6B, the notation of the unit “μm” of D (i, j) is omitted, and only numerical values may be shown.

FIG. 6B is a list of all intervals D (i, j) included in the optical waveguide device 10-C. The corresponding D (i, j) is written in the cell where the row direction corresponds to the index i of the interval D (i, j), the column direction corresponds to the index j, and the row i and the column j intersect. According to this table, the total combination D of the existing intersection _{C 1 ~C 5 (1,2) ~D} (4,5), D (i1, j1) ≠ D (i2, j2) is satisfied.

This optical waveguide device 10-C, while satisfying the condition of D (i1, j1) ≠ D (i2, j2), consider adding new sixth intersection _{C 6.} Adding a sixth intersection _{C 6,} the list of FIG. 6 (B), the new section D of the bottom line surrounded by a dotted line (1,6) ~D (5,6) is applied. That is, these new sections D (1,6) to D (5,6) and the existing sections D (1,2) to D (4,5) are D (i1, j1) ≠ D (i2, j2) so as to satisfy, may it can be seen that by adding a sixth intersection C _6. As described above, the comparison between the new section and the existing section may be performed in a section that satisfies i1 ≠ i2 or j1 ≠ j2.

For example, consider the additional possibility of sixth intersection _{C 6} of adjacent intersection distance D (5,6) = 8. In this case, the new sections in FIG. 6B are D (1,6) = 24, D (2,6) = 23, D (3,6) = 20, D (4,6) = 15 and D (5,6) = 8. When these new sections and existing sections are compared, D (5,6) = D (2,4) = 8, which does not satisfy the above condition. Therefore, additional sixth intersection _{C 6} of adjacent intersection distance D (5,6) = 8 is impossible.

However, when the adjacent intersection interval D (5,6) is changed to 10, the new sections are D (1,6) = 26, D (2,6) = 25, D (3,6) = 22, D, respectively. (4,6) = 17 and D (5,6) = 10. All of these new sections satisfy the above conditions. Therefore, if adjacent intersection distance D the (5,6) and 10 [mu] m, it can be added a sixth intersection _{C 6.}

Thus, when a new intersection C _{(N + 1)} is added to form 10-C, a newly generated new section and an existing section are sections in which i1 ≠ i2 or j1 ≠ j2, and D ( i1, j1) ≠ D (i2, j2) may be satisfied.

(Specific examples of optical waveguide elements)
Subsequently, the structure of the optical waveguide device will be described with reference to FIGS. FIG. 7A is a plan view schematically showing the structure of the optical waveguide device 10, and FIG. 7B is a cut end view taken along the line AA in FIG. 7A. 8A is a characteristic diagram showing the output characteristics of the conventional element 100, and FIG. 8B is a characteristic chart showing the output characteristics of the optical waveguide element 10. As shown in FIG. In FIG. 7A, the core 12a is embedded in the clad 12b and cannot be directly seen, but is shown by a solid line for emphasis.

  First, referring to FIGS. 7A and 7B, directions and dimensions used in the following description are defined. A direction perpendicular to the light propagation direction P of the reference optical waveguide 14 and parallel to the main surface 8a is referred to as a width direction, and a geometric length measured along the width direction is referred to as a “width”. A direction perpendicular to the main surface 8a is referred to as a height direction or a thickness direction, and a geometric length measured along the height direction is referred to as "height" or "thickness". Similarly, the geometric length measured along the light propagation direction is referred to as “length”. A section perpendicular to the light propagation direction of a predetermined structure is referred to as a “cross section”.

  The optical waveguide element 10 includes an optical waveguide 12. The optical waveguide 12 includes a core 12a provided on the main surface 8a side of the substrate 8 and a clad 12b surrounding the core 12a.

  The width S and thickness H of the core 12a are preferably selected from values in the range of about 200 to 500 nm. By setting the width S and the thickness H of the core 12a within this range, the optical waveguide 12 can be set to a single mode in both the width and thickness directions. In this example, the optical waveguide 12 is set to a single mode by setting the width S of the core 12a to about 480 nm and the thickness H to about 220 nm. In this example, the constituent material of the core 12a is Si having a refractive index na of about 3.47.

  The clad 12b is a film body provided on the main surface 8a which is a flat surface. The thickness of the cladding 12b is about 4 μm in this example. The distance between the lower surface of the core 12a embedded in the clad 12b and the main surface 8a is about 2 μm. In order to prevent undesired coupling of light to the substrate 8, a cladding 12b having a thickness of 1 μm or more is preferably interposed between the core 12a and the main surface 8a.

In this example, the constituent material of the clad 12b is SiO ₂ having a refractive index nb of about 1.47. It is preferable that the refractive indexes nb and na of the clad 12b and the core 12a satisfy the relationship of nb ≦ (1 / 1.4) na (≈0.714na). By using the clad 12b in this refractive index range, the light confinement in the optical waveguide 12 can be improved. As a result, it is possible to realize a curved optical waveguide that bends light with a small curvature radius of about 5 μm. Furthermore, if the core 12a is made of Si, a processing technique using a Si electronic device can be used, so that the optical waveguide element 10 having a submicron order cross-sectional structure can be formed.

  Further, any suitable material can be selected for the substrate 8, but Si is preferably used. If the substrate 8 is made of Si, an SOI (Si On Insulator) substrate can be used for manufacturing the optical waveguide device 10 as in the following manufacturing process, so that the manufacturing process can be simplified.

That is, in manufacturing the optical waveguide element 10, the above-described SOI substrate in which the SiO ₂ layer and the Si layer are laminated in this order on the Si substrate can be used. In other words, the core 12a is formed using the uppermost Si layer, and the SiO ₂ layer, which is a BOX (Buried-OXide) layer, is used for the lower cladding 12b. More specifically, the uppermost Si layer is patterned by a conventionally known dry etching method or the like to form the core 12a. Then, an SiO ₂ layer corresponding to the upper clad 12b is formed by CVD (Chemical Vapor Deposition) or the like so as to embed the core 12a, and the optical waveguide device 10 is obtained.

The optical waveguide 12 includes the reference optical waveguide 14 and the intersecting optical waveguide 16 as described above. Crossing optical waveguide 16 is provided with a cross waveguide ₁₆ 1 ~ 16 _N intersecting at first to N intersection _C 1 -C _N in series in this order. The reference optical waveguide 14 and the intersecting optical waveguide 16 are provided on a common plane parallel to the main surface 8a. That is, at the i-th intersection C _i between the reference optical waveguide 14 and the intersecting optical waveguide 16 _i , both the optical waveguides 14 and 16 _i are integrated without changing the thickness. As a result, the i-th intersection C _i is composed of a planar optical waveguide capable of propagating a plurality of lights having different modes.

In this example, all the crossed optical waveguides 16 _{1 to} 16 _N are orthogonal to the reference optical waveguide 14, but the crossed optical waveguides 16 _{1 to} 16 _N may cross the reference optical waveguide obliquely. .

  In this example, the case where the reference optical waveguide 14 and the intersecting optical waveguide 16 are single-mode optical waveguides has been described. However, both optical waveguides 14 and 16 may be multimode optical waveguides as long as they are linearly formed.

In this example, the case where one intersection C _i is formed at the intersection of the reference optical waveguide 14 and one intersection optical waveguide 16 _i has been described. However, two or more intersections may be formed by one crossed optical waveguide by using a curved crossed optical waveguide.

  The optical waveguide element 10 can be applied to an optical waveguide structure of an optical connection unit that optically connects an optical receiving unit and an optical transmission unit in the optical module described above, and includes an optical waveguide structure in which three or more intersections are in series. It can be applied to various optical elements.

  Next, the effect of the optical waveguide device 10 will be described with reference to the simulation result of FIG. FIG. 8A shows the output characteristics of the conventional element 100 (FIG. 1A) already described, and FIG. 8B shows the output characteristics of the optical waveguide element 10 of the present invention. 8A and 8B, the horizontal axis indicates the wavelength (nm) of the output light output from the reference optical waveguide 14 and the crossed optical waveguide 16i (or 26i), and the vertical axis indicates the output. The light intensity is shown in dB. The intensity of output light is an intensity ratio with respect to input light.

The curve III of FIG. 8 (A), the output light intensity from the reference optical waveguide 14 of a conventional type device 100, curve IV is the output light intensity from the cross waveguide 26 _i conventional element 100 . Similarly, the curve I in FIG. 8B is the output light intensity from the reference optical waveguide 14 of the optical waveguide element 10, and the curve II is the output light intensity from the intersecting optical waveguide 16 _i of the optical waveguide element 10. is there.

  Both the conventional element 100 and the optical waveguide element 10 have a common intersection number N of ten. Further, in the conventional element 100, the adjacent intersection interval D (i, i + 1) is set to a constant value of 10 μm. On the other hand, in the optical waveguide element 10, the adjacent intersection distance D (i, i + 1) is a prime number times the reference length U. Specifically, in the optical waveguide element 10, the reference length U is 1 μm, and a prime number of 11 or more is used in ascending order with respect to i. That is, D (1,2) = 11 μm, D (2,3) = 13 μm, D (3,4) = 17 μm, D (4,5) = 19 μm, D (5,6) = 23 μm, D (6 7) = 29 μm, D (7,8) = 31 μm, D (8,9) = 37 μm and D (9,10) = 41 μm. The optical waveguide element 10 in which the adjacent intersection distance D (i, i + 1) is set in this way corresponds to the above-described optical waveguide element 10-B. This is clear from D (1,5) (= 11 + 13 + 17 + 19) and D (6,8) (= 29 + 31) being equal at 60 μm.

  The simulation was performed by a two-dimensional FDTD (Finite Difference Time Domain) method using an effective refractive index method. In the calculation, the materials and dimensions of the core 12a and the clad 12b of the optical waveguide element 10 are the same as those described above, and the same material and the same dimensions are used in the conventional element 100.

Referring to curve III in FIG. 8A, multistage crossing loss occurs in conventional element 100 in the wavelength range of the horizontal axis (about 1.45 to 1.65 μm). That is, the curve III shows an intensity variation of about ± 1.5 dB around the average intensity of −12 dB with a period of about 20 to 30 nm. Note that the periodic intensity fluctuation occurs not only in the reference optical waveguide 14 (curve III) but also in the intersecting optical waveguide 26 _i (curve IV) at the intersection C _i from the reference optical waveguide 14 to the intersecting optical waveguide 26 _i. This is because the light is diffracted to the back.

In addition, the output from the reference optical waveguide 14 (curve III) becomes −12 dB because the one-stage crossing loss (about −0.15 dB / intersection) at each of the intersections C _{1 to} C ₁₀ and the output to the cladding 12 b. By light emission.

On the other hand, referring to the curve I in FIG. 8B, in the optical waveguide device 10, the multistage crossing loss disappears in the wavelength range of the horizontal axis, and the intensity of the output light is constant at about −12 dB. As a result of the disappearance of the multistage crossing loss in the output light from the reference optical waveguide 14, periodic intensity fluctuations have disappeared in the output light from the crossing optical waveguide 16 _i (curve II).

  As is clear from FIGS. 8A and 8B, the optical waveguide device 10 can sufficiently suppress the multistage crossing loss practically without reducing the one-step crossing loss.

  Moreover, although the optical waveguide element 10 of FIG. 8 (B) is not the optical waveguide element 10-C in which the effect of suppressing the multistage crossing loss is the maximum, it is 10-B (FIG. 3 (B)). Multistage crossing loss can be sufficiently suppressed in practical use.

8 Substrate 8a Main surface 10 Optical waveguide device 12 Optical waveguide 12a Core 12b Clad 14 Reference optical waveguides 16, 16 ₁ , 16 ₂ ,..., 16 _i ,, 16 _j ,, 16 _N , 16 _k , 16 _{( k + 1)} , 16 _{(k + 2)} , 16 _{(k + 3)} , 16 _{(k + 4)} , 16 _{(k + 5)} , 26, 26 ₁ , 26 ₂ ,..., 26 _i , 26 _N crossed optical waveguides C ₁ , C ₂ , , C _i ,..., C _j ,..., C _N , C _k , C _{(k + 1)} , C _{(k + 2)} , C _{(k + 3)} , C _{(k + 4)} , C _{(k + 5)} first to Nth intersections

Claims (10)

An optical waveguide including a core and a clad surrounding the core is connected to the reference optical waveguide and the reference optical waveguide in series in this order along the light propagation direction (N is an integer of 3 or more). And an intersecting optical waveguide intersecting at
When the distance between the i-th and j-th intersections is D (i, j) (where i is an integer satisfying 1 ≦ i ≦ N−1 and j is an integer satisfying i <j ≦ N) ,
An optical waveguide element characterized by D (g, g + 1) ≠ D (g + 1, g + 2) for at least one g (g is an integer satisfying 1 ≦ g ≦ N−2).   For all (p, q) pairs (p and q are integers satisfying 1 ≦ p ≦ N−1, 1 ≦ q ≦ N−1, and p ≠ q), D (p, p + 1) ≠ D ( The optical waveguide according to claim 1, wherein q, q + 1).   3. The optical waveguide element according to claim 2, wherein the D (i, j) is a prime number multiple of a reference length U (U is a positive real number).   3. The D (i, j) is (a + dn) times a reference length U (U is a positive real number) (n is an integer of 0 or more, and a and d are natural numbers). 2. An optical waveguide device according to 1. The D (i, j) is br ^m times a reference length U (U is a positive real number) (m is an integer greater than or equal to 0, and b and r are natural numbers). The optical waveguide device described.   D (i, j) is a constant C (C is a natural number) that is a natural number multiple of a reference length U (U is a positive real number), and d (i) is a prime number multiple of the reference length U. The optical waveguide device according to claim 2, wherein C + d (i) is used.   The optical waveguide element according to any one of claims 3 to 6, wherein the reference length U is a value having a size of 1/10 µm or more. When the two D (i, j) are D (i1, j1) and D (i2, j2), respectively.
8. All of i1, i2, j1, and j2 that satisfy i1 ≠ i2 or j1 ≠ j2, and D (i1, j1) ≠ D (i2, j2). The optical waveguide device according to item.   9. The optical waveguide device according to claim 1, wherein the D (i, i + 1) increases monotonously in a narrow sense or decreases monotonously with i. It said core and Si, the optical waveguide device according to any one of claims 1-9, characterized in that said cladding and SiO _2.

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