JP7002092B2 - Dielectric optical waveguide with bends - Google Patents

Description

The present invention relates to a dielectric optical waveguide having a bent portion, and more particularly to a dielectric optical waveguide having a bent portion, which can significantly reduce a loss due to bending.

An optical waveguide is a circuit that is processed so as to guide an optical signal by providing an optical path on a substrate by utilizing the difference in the refractive index of light. Some optical waveguides have a linear portion in which an optical signal is propagated straight ahead and a bent portion in which the optical signal can change its propagation direction on the way. The optical waveguide having such a bent portion has a problem that a part of the light propagating in the core leaks to the outside and a bending loss occurs. That is, two types of losses occur, a pure bending loss related to the bending itself and a transition loss occurring at the connection portion with the straight portion. Further, it is necessary to reduce the bending radius in order to increase the density of the optical integrated circuit, but the smaller the bending radius is, the more the bending loss increases.

As a conventional typical method for reducing bending loss of an optical waveguide, a method of installing a trench (groove) in a substrate near the core (for example, Non-Patent Documents 1 and 2) and ARROW (Anti-Resonant Reflecting Optical Waveguide) ), A method for reducing bending loss (for example, Non-Patent Document 3) has been proposed.

However, although these loss reduction methods have an excellent loss reduction effect, they complicate the manufacturing process and require redesign according to the bending radius and the wavelength used. Further, the conventional method has a drawback that a polarization-dependent loss occurs in addition to requiring an additional manufacturing process. Further, regarding the reduction of the transition loss, a method of offsetting the waveguide axis (for example, Non-Patent Documents 4 and 5) has been proposed, but it is not effective in reducing the pure bending loss.

The present inventors have proposed a method of reducing bending loss only by adjusting the core position from the air interface in an embedded optical waveguide in which a core is embedded in a clad (Non-Patent Document 6). FIG. 1 is a perspective view showing an optical waveguide 10 having a bent portion by this method. The optical waveguide 10 is made of a quartz-based material having a structure in which a core 11 having a refractive index of n _co = 1.4675 is embedded in a clad 12 having a refractive index of n _cl = 1.46. The outside of the clad 12 is an air layer 13. The core width is 2ρ _x = 2ρ _y = 6.0 μm, the bending radius R = 7 mm, and the distance d _air from the surface (upper surface) of the core 11 to the air layer 13 is to prevent loss due to the expansion effect of the intrinsic mode world. Is set to, for example, 3.0 μm. It was confirmed that with such a configuration, bending loss (pure bending loss and transition loss) can be reduced to some extent only by adjusting the core position from the air layer 13.

However, in the structure shown in Non-Patent Document 6, the effect of reducing bending loss is limited.

E. G. Neumann, Low loss dielectric optical waveguide bend, Fiber and Integrated Optics, vol. 4, no. 2, pp. 203-211, 1982 M. Rajarajan et al., Design of compact optical bends with a trench by use of finite-element and beam-propagation methods, Applied Optics, vol. 39, no. 27, pp. 4946-4953, 2000 M. Galarza et al., Simple low-loss waveguide bends using ARROW effect, Applied Physics B, vol. 80, pp. 745-748m 2005 E. G. Neumann, Curved dielectric optical waveguide with reduced transition losses, IEE Proc. H, vol. 129, no. 5, pp. 278-280, 1982 E. C. M. Pennings et al., Low-loss bends in planar optical ridge waveguides, Electronics. Letters, vol. 24, no. 16, pp. 998-999, 1988 Y. Nito et al., Reduction in bend losses of a buried waveguide on a silicon substrate by adjusting the core location, IEEE / OSA Journal of Lightwave Technology, vol. 34, no. 4, pp. 1344-1349, Feb. 15 , 2016

The present invention has been made in view of the above-mentioned problems of the prior art, and can reduce the loss due to bending while suppressing the polarization-dependent loss over the entire optical communication wavelength range, and is easy to manufacture. It is an object of the present invention to provide a dielectric optical waveguide having a bent portion.

In order to solve the above problems, according to the present invention, first, a mesa-shaped upper clad is provided on the lower clad, and the core is from a position where the surface of the core is flush with the surface of the lower clad. The lower clad and / or the upper clad is embedded so as to be between the positions inside the upper clad, and at least the upper clad and the surface of the lower clad not covered by the upper clad are in contact with the air layer. Provided is a dielectric optical waveguide having a bend, characterized in that it is present.

Secondly, in the first invention, there is a dielectric optical waveguide in which the core is embedded in the lower clad so that the surface of the core is flush with the surface of the lower clad. Provided.

Third, in the first invention, the core is embedded in the lower clad and the upper clad so that the surface of the core is inside the upper clad and the lower surface of the core is inside the lower clad. Provided is a dielectric optical waveguide characterized by the above.

Fourth, in the first invention, the core is embedded in the upper clad so that the surface of the core is inside the upper clad and the lower surface of the core is flush with the surface of the lower clad. Provided is a dielectric optical waveguide characterized by the above.

Fifth, in the first aspect of the invention, there is provided a dielectric waveguide in which the core is embedded in the upper clad so that the surface and the lower surface of the core are inside the upper clad. Will be done.

Sixth, in any one of the first to fifth inventions, when the width of the core is w in the cross section of the dielectric optical waveguide, the distance s _R from the right end of the core to the air layer. Provided are dielectric optical waveguides characterized in that the distance s _L from the left end of the core to the air layer is 0.3 w to 1.5 w, respectively.

Seventh, in any one of the first to sixth inventions, there is provided a dielectric optical waveguide characterized in that the height h of the upper clad is 0.5 w or more.

Since the above configuration is adopted, the present invention provides an easy-to-manufacture dielectric optical waveguide having a bent portion, which can reduce the loss due to bending while suppressing the polarization-dependent loss over the entire optical communication wavelength. It will be possible to provide.

It is a perspective view which shows the structure of the optical waveguide which has a bent part which the present inventors took up in Non-Patent Document 6. It is sectional drawing which shows the structure of the bending part in the optical waveguide which has the bending part which concerns on Embodiment 1 of this invention. It is a figure which shows the result of having fixed the height h of the upper clad to ∞, and investigated the influence which the distance s _R and the distance s _L have on the pure bending loss, by contour lines. It is a figure which shows the result of having investigated the pure bending loss with respect to the height h of the upper clad when s _R = s _L = 4.0 μm. In (a), (b), (c), (d), and (e), when the distance d _air between the core and the air layer is 0.0 μm, the trench is a typical conventional bending loss reduction method. When the height h of the upper clad is 6.0 μm (= w) in the embodiment of the present invention, the distance d _air between the core and the air layer is 3 in the optical waveguide proposed in Non-Patent Document 6. It is a figure which compares and shows the eigenmode field distribution when the distance d _air between a core and an air layer is ∞ in the optical waveguide taken up in Non-Patent Document 6 when it is set to 0.0 μm. (A) and (b) are diagrams showing the wavelength characteristics of pure bending loss and polarization-dependent loss (PDL), respectively. It is a figure which shows the result (pure bending loss with respect to bending radius R) which investigated the dependence of the bending radius in the optical waveguide which has a bending part. It is sectional drawing which shows the structure of the bending part in the optical waveguide which has the bending part which concerns on Embodiment 2 of this invention. It is sectional drawing which shows the structure of the bending part in the optical waveguide which has the bending part which concerns on Embodiment 3 of this invention. It is sectional drawing which shows the structure of the bending part in the optical waveguide which has the bending part which concerns on Embodiment 4 of this invention. (A), (b), and (c) show the same diagram as in FIG. 5 when the surface of the core is shifted upward by 2 μm, 6 μm, and 8 μm with respect to the surface of the lower clad, respectively. It is a figure which shows the result of having investigated the pure bending loss when the value which shifted the surface of a core upward with respect to the surface of a lower clad was set as y _shift . The height h of the upper clad is fixed to ∞, and the results of investigating the effects of distance s _R and distance s _L on pure bending loss are shown by contour lines ((a) is TE mode, (b) is TM mode). Is. (A) and (b) are diagrams showing the wavelength characteristics of pure bending loss and polarization-dependent loss (PDL), respectively.

In the dielectric optical waveguide having a bent portion according to the present invention (hereinafter, also referred to as an optical waveguide), a mesa-shaped upper clad is provided on the lower clad, and the core has a core whose surface is flush with the surface of the lower clad. It is embedded in the lower clad and / or the upper clad so as to be between the position to be inside the upper clad, and at least the upper clad and the surface of the lower clad not covered by the upper clad are air. It is characterized by being in contact with the layer.

Hereinafter, the present invention will be described in more detail with reference to embodiments.

FIG. 2 is a diagram showing a cross section of a bent portion in the dielectric optical waveguide 20 having a bent portion according to the first embodiment of the present invention. In this optical waveguide 20, the core 21 having a square cross section is embedded in the lower clad 22 so that the surface 21A of the core 21 is flush with the surface 22A of the lower clad 22. Therefore, the lower surface 21B of the core 21 is located in the lower clad 22. An upper clad 23 having a rectangular cross section is provided so as to cover the entire surface 21A of the core 21 and a part of the surface _22AL and 22A _R of the lower clad 22, and the upper clad 23 and the lower clad 22 not covered by the upper clad 23 are provided. The surface is in contact with the air layer 24. The upper clad 23 and the lower clad 22 may be integrally formed or may be formed separately. The formation of these claddings can be performed by a conventionally used method.

In the present embodiment, the cross-sectional shape of the core 21 is a square, but it can be a rectangle, a trapezoid, or any other appropriate shape. The upper clad can also be trapezoidal.

The optical waveguide 20 of this embodiment is made of a quartz-based material (SiO ₂ ). Here, the width (same as the height) of the core 21 is w, the refractive index of the core 21 is n _co , the refractive index of the lower clad 22 and the upper clad 23 is n _cl , the height of the upper clad 23 is h, and the core 21 Let s _L be the distance from the left end to the air layer 24, and s _R be the distance from the right end of the core 21 to the air layer 24.

As a numerical example of each parameter, the core width w is 6.0 μm × 6.0 μm, n _co = 1.4675, n _cl = 1.46, h = 6.0 μm, s _L = s _R = 4.0 μm. It has become. The operating wavelength λ used is 1.55 μm except for FIG. Although not shown, the radius R of the bent portion is 7 mm except for FIG. 7. Of course, the numerical example here exemplifies a preferable value, and is appropriately set to an appropriate value. Further, the shape and height of the upper clad 23 can be changed as long as the desired effect can be obtained. For example, a trapezoidal structure may be used.

Here, the height h of the upper clad 23 is fixed to ∞, and the influence of the distance s _R from the right end of the core 21 to the air layer 24 and the distance s _L from the left end of the core 21 to the air layer 23 on the pure bending loss. To find out. FIG. 3 shows contour lines of pure bending loss in TM mode with respect to distance s _R and distance s _L. The distance s _R is a parameter that not only affects the degree of spread of the field in the −y direction, but also affects the folding effect of the field. It is possible to suppress the spread of the field by selecting a large distance s _R , but if it is too large, the folding effect of the field decreases and the pure bending loss increases. Therefore, it is desirable to select the distance s _R smaller than the generation position of the leaked wave called the radiant point. The distance s _L is a parameter inside the bend and contributes only to the field spreading effect. From FIG. 3, it can be seen that the effect of expanding the field can be sufficiently suppressed by selecting the distance s _L ≧ 4.0 μm. Further, by selecting s _R = s _L = 4.0 μm, the minimum pure bending loss of 0.05 dB / cm can be obtained. Generally, s _R and s _L are 0.3w to 1.5w, more preferably 0.5w to w, respectively.

Next, when s _R = s _L = 4.0 μm, the pure bending loss with respect to the height h of the upper clad 23 is investigated. The result is shown in FIG. In FIG. 4, for reference, the result of the height h = ∞ of the clad layer to be laminated is also shown by a broken line.

From FIG. 4, when the height h of the upper clad 23 is 0.0 μm (semi-embedded waveguide), the field spreading effect is large and leakage waves are likely to be generated. It can be seen that by alleviating the asymmetry, the field distribution is concentrated in the core and leakage waves are suppressed. Further, it can be seen that by selecting the height h of the upper clad 23 to ≧ 6.0 μm, the same bending loss reducing effect as in the case of the height h of the upper clad 23 = ∞ can be obtained.

In the case of (a), (b), (c), (d), and (e) of FIG. 5, when the distance d _air between the core and the air layer is 0.0 μm (semi-embedded waveguide), it is a conventional representative. In the case of a structure having a trench, which is a method for reducing bending loss, when the height h of the upper clad is 6.0 μm in the embodiment of the present invention, the optical waveguide proposed in Non-Patent Document 6 extends to the core and the air layer. When the distance d _air between the two is 3.0 μm (embedded waveguide) and the distance d _air between the core and the air layer is ∞ in the optical waveguide taken up in Non-Patent Document 6 (completely embedded waveguide). The boundary distributions are compared and shown. In FIG. 5B, the distance between the core and the trench is 3.0 μm, and the refractive index of the trench portion is 1.0 (air). In these figures, the real part Re {Hx} of the field is displayed to clarify the change of phase.

In the semi-embedded waveguide with a distance d _air = 0.0 μm between the core and the air layer in FIG. 5 (a), the center of gravity of the field localized in the core shifts in the −y direction, and a remarkable leak wave is observed. .. In the optical waveguide having the structure having the trench shown in FIG. 5B, the leakage wave is suppressed, but the shift of the center of gravity of the field in the −y direction is not improved. In the embedded waveguide of FIG. 5 (d), the leakage wave is suppressed, but the asymmetry of the field with respect to the core center (y = 0) axis remains. In the fully embedded waveguide of FIG. 5 (e), the fields are symmetrical with respect to the core central axis, but leakage waves also occur in the + y region. On the other hand, in the waveguide in which the height h of the upper clad is 6.0 μm in the embodiment of the present invention, it can be seen that the peak of the field appears at almost the center of the core and the leakage wave is suppressed at the same time. Further, it can be seen that in this structure, since the leakage wave is most efficiently canceled by the folding effect of the field, the pure bending loss can be reduced as compared with the semi-embedded waveguide, the embedded waveguide, and the fully embedded waveguide.

(A) and (b) of FIG. 6 show the wavelength characteristics of pure bending loss and polarization-dependent loss (PDL), respectively. For comparison, these figures also show the results of an embedded waveguide and a waveguide with a trench. From FIG. 6A, it can be seen that the optical waveguide of the present embodiment has a reduced pure bending loss over a wide band with a wavelength of 1.3 μm to 1.65 μm as compared with an embedded waveguide or a waveguide having a trench. .. Further, in the optical waveguide of the present embodiment, it can be seen that the polarization dependence loss (PDL) is also reduced because the field is concentrated on the core in a state where the symmetry of the field is improved by laminating the upper clad. .. Further, from FIG. 6B, it can be seen that in the optical waveguide of the present embodiment, the polarization-dependent loss (PDL) is suppressed to 0.007 dB / cm or less.

Next, the result of investigating the dependence of the bending radius in the optical waveguide having a bending portion (pure bending loss with respect to the bending radius R) is shown in FIG. From FIG. 7, it can be seen that the optical waveguide having a trench has a large difference between the polarizations of the pure bending loss, whereas the optical waveguide having an embedded structure and the optical waveguide of the present embodiment have almost no difference between the polarizations.

Further, the waveguide of the present embodiment has an advantage that a large loss reduction effect can be obtained with a bending radius of R = 6.0 mm to R = 8.0 mm as compared with the waveguide by other methods.

Next, the optical waveguide according to the second to fourth embodiments according to the present invention will be described with reference to FIGS. 8 to 10. In FIGS. 8 to 10, the same elements as those in FIG. 2 are designated by the same number, and the description thereof will be omitted.

FIG. 8 is a diagram showing a cross section of a bent portion in the optical waveguide 20 of the second embodiment according to the present invention. In this optical waveguide 20, the core 21 is embedded in the upper clad 23 and the lower clad 22 so that the surface 21A of the core 21 is inside the upper clad 23 and the lower surface 21B is inside the lower clad 22. FIG. 11A shows the same diagram as in FIG. 5 when the surface of the core 21A is shifted upward by 2 μm with respect to the surface of the lower clad 22. h = 6.0 μm. From FIG. 11A, it can be seen that the leakage wave is further suppressed and the loss reduction effect is remarkable as compared with the case of FIG. 5C.

FIG. 9 is a diagram showing a cross section of a bent portion in the optical waveguide 20 according to the third embodiment of the present invention. In this optical waveguide 20, the core 21 is embedded in the upper clad 23 so that the surface 21A of the core 21 is inside the upper clad 23 and the lower surface 21B is the same surface as the surface 22A of the lower clad 22. FIG. 11B shows the same diagram as in FIG. 5 when the surface of the core 21A is shifted upward by 6 μm with respect to the surface of the lower clad 22. h = 6.0 μm. From FIG. 11 (b), it can be seen that the leakage wave is further suppressed and the loss reduction effect is remarkable as compared with the cases of FIGS. 5 (c) and 11 (a).

FIG. 10 is a diagram showing a cross section of a bent portion in the optical waveguide 20 of the fourth embodiment according to the present invention. In this optical waveguide 20, the core 21 is embedded in the upper clad 23 so that the surface 21A of the core 21 is inside the upper clad 23 and the lower surface 21B is also inside the upper clad 23. FIG. 11C shows the same diagram as in FIG. 5 when the surface 21A of the core 21A is shifted upward by 8 μm with respect to the surface of the lower clad 22. h = 6.0 μm. From FIG. 11 (c), it can be seen that the leakage wave is further suppressed and the loss reduction effect is remarkable as compared with the cases of FIGS. 5 (c), 11 (a) and 11 (b).

FIG. 12 shows the results of investigating the pure bending loss when the value obtained by shifting the surface 21A of the core 21A upward with respect to the surface of the lower clad 22 is y _shift . R = 7.0 mm, λ = 1.55 μm, h = 6.0 μm, and TM mode. As shown in FIG. 4, the loss reduction effect is remarkable even when the y _shift is 0, but it can be seen that as the y _shift increases, the leakage wave is further suppressed and the loss reduction effect becomes more remarkable.

Next, the optical waveguide according to the fifth embodiment of the present invention will be described. In the present embodiment, the notation of each parameter is the same as that of the first to fourth embodiments.

The core of the optical waveguide of this embodiment is made of silicon. As a numerical example of each parameter, the core width w is 0.32 μm × 0.32 μm, n _co = 3.476, n _cl = 1.444, and the radius R of the bent portion is 2.0 μm.

13 (a) and 13 (b) show the results of investigating the effects of distance s _L and distance s _R on pure bending loss when the cross-sectional structure of the optical waveguide of this embodiment is shown in FIG. FIG. 13 (a) shows contour lines of pure bending loss in TE mode for distance s _L and distance s _R , and FIG. 13 (b) shows contour lines of pure bending loss in TM mode for distance s _L and distance s _R. show. Here, h is fixed at ∞, and the operating wavelength λ used is 1.55 μm. In the quartz-based optical waveguide used to obtain the result shown in FIG. 3, the difference in refractive index between the core and the clad was small, so that the results in both modes were the same. Because of the large size, the characteristics differ depending on the polarization. It should be noted that there are positions where the loss is as low as that. From FIG. 13, it can be seen that the effect of expanding the field can be sufficiently suppressed by selecting s _L ≧ 0.22 μm. Further, the minimum pure bending loss can be obtained by selecting s _L = s _R = 0.22 μm. In the silicon optical waveguide, s _L and s _R are generally 0.3 w to 1.5 w, more preferably 0.5 w to w, respectively.

14 (a) and 14 (b) show the wavelength characteristics of pure bending loss and polarization-dependent loss (PDL), respectively. Here, s _L = s _R = 0.22 μm and h = 0.2 μm. For comparison, these figures also show the results of an optical waveguide with a trench. From FIG. 13 (a), it can be seen that the optical waveguide of the present embodiment has a lower pure bending loss than the optical waveguide having a trench over a wide band wavelength. Further, in the optical waveguide of the present embodiment, it can be seen that the polarization dependence loss (PDL) is also reduced because the field is concentrated on the core in a state where the symmetry of the field is improved by laminating the upper clad. .. Further, from FIG. 14B, it can be seen that in the optical waveguide of the present embodiment, the polarization-dependent loss is suppressed to 0.01 dB / cm or less. Even if the cross-sectional structure of the silicon optical waveguide is shown in FIGS. 8, 9 and 10, the bending loss reducing effect can be obtained as in the quartz optical waveguide.

20 Dielectric Optical Waveguide 21 Core 21A Core Surface 21B Core Bottom 22 Bottom Clad 22A Bottom Clad Surface 22A _L , 22A _R Partial Surface of Bottom Clad 23 Top Clad 24 Air Layer

Claims (6)

A mesa-shaped upper clad is provided on the lower clad, and the core is located between a position where the surface of the core is flush with the surface of the lower clad and a position inside the upper clad. And / or at least the upper clad and the lower clad surface not covered by the upper clad, which are embedded in the upper clad, are in contact with the air layer.
In the cross section of the dielectric optical waveguide, when the width of the core is w, the distance sR between the right end of the core and the air layer and the distance sL from the left end of the core to the air layer are 0.3w to 1, respectively. .5w and the height h of the upper clad is w or more .
A dielectric optical waveguide having a bent portion, wherein the core is made of a quartz-based material and the bent portion has a bending radius of 6 to 8 mm . A mesa-shaped upper clad is provided on the lower clad, and the core is located between a position where the surface of the core is flush with the surface of the lower clad and a position inside the upper clad. And / or at least the upper clad and the lower clad surface not covered by the upper clad, which are embedded in the upper clad, are in contact with the air layer.
In the cross section of the dielectric optical waveguide, when the width of the core is w, the distance sR between the right end of the core and the air layer and the distance sL from the left end of the core to the air layer are 0.3w to 1, respectively. .5w and the height h of the upper clad is w or more.
A dielectric optical waveguide having a bent portion, wherein the core is made of silicon and the bent portion has a bending radius of 2 μm. The dielectric optical waveguide according to claim 1 or 2 , wherein the core is embedded in the lower clad so that the surface of the core is flush with the surface of the lower clad . Claim 1 is characterized in that the core is embedded in the lower clad and the upper clad so that the surface of the core is inside the upper clad and the lower surface of the core is inside the lower clad. Or the dielectric optical waveguide according to 2 . Claim 1 or claim 1 , wherein the core is embedded in the upper clad so that the surface of the core is inside the upper clad and the lower surface of the core is flush with the surface of the lower clad. 2. The dielectric optical waveguide according to 2. The dielectric optical waveguide according to claim 1 or 2, wherein the core is embedded in the upper clad so that the surface and the lower surface of the core are inside the upper clad.

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