JP2017111202A - Optical circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical circuit that employs a PLC, which allows for reducing pitch at a light exit end.SOLUTION: An optical circuit comprises an arbitrary number of input waveguides, and a multimode interference optical waveguide 3 having an input waveguide section 2 coupled to the input waveguides at one end and an output waveguide section 4 at the other end, where a width of the output waveguide section of the multimode interference optical waveguide is smaller than a width of the input waveguide section thereof. A length Lof the multimode interference optical waveguide in an optical waveguiding direction is set according to the following formula (1): L=4×n×Win×Wout/3λ ...(1), where Wrepresents the width of the input waveguide section, Wrepresents the width of the output waveguide section, λ represents a wavelength of incident light, and n represents a refractive index of a core of the multimode interference optical waveguide.SELECTED DRAWING: Figure 3

Description

  The present invention relates to an optical device, and more particularly to an optical circuit that converts a pitch between light sources.

  2. Description of the Related Art In recent years, research that applies an electromagnetic beam forming technique using an array antenna in an RF (Radio Frequency) band as a light deflection technique has attracted attention (for example, Non-Patent Document 1). In this technique, a plurality of light sources are periodically arranged, the phase of light emitted from each light source is controlled, and the light deflection is controlled by changing interference. At this time, in order to improve the characteristic of the deflection angle, it is necessary to reduce the distance between the light sources to the wavelength level. Since it is difficult to arrange the light sources at a narrow pitch of the wavelength level, a pitch conversion circuit using a quartz planar lightwave circuit (PLC) is often used.

  In PLC, a quartz layer that becomes a core and a clad is laminated on a planar Si substrate, an optical waveguide is produced by patterning by photolithography and the like, and etching, and a plurality of basic optical circuits (for example: Various functions are realized by combining a directional coupler, Mach-Zehnder interferometer, etc. FIG. 1 shows a conventional pitch conversion circuit. As shown in FIG. 1, the normal pitch conversion circuit 1 has a configuration that converts the waveguide pitch between the incident end and the output end from A to B using a bent waveguide and a straight waveguide.

Jie Sun, Erman Timurdogan, Ami Yaacobi, Ehsan Shah Hosseini, and Michael R. Watts, “Large-scale nanophotonic phased array” Nature, vol. 493, 195-199 Jan 2013 L. B. Soldano and E. C. M. Pennings, “Optical Multi-Mode Interference Devices Based on Self-Imaging: Principles and Applications”, Journal of Lightwave Technology, vol. 13, No. 4, pp.615-627, Apr. 1995. Y. Sakamaki, T. Saida, T. Hashimoto, and H. Takahashi, "New Optical Waveguide Design Based on Wavefront Matching Method", Journal of Lightwave Technology, vol. 25, No. 11, pp. 3511-3518, Nov. 2007.

  However, the pitch conversion circuit shown in FIG. 1 causes interference between adjacent waveguides as in a directional coupler and makes it difficult to process a narrow gap between waveguides. There is a problem that it is restricted. Here, “pitch” refers to the distance between the centers of adjacent waveguides indicated as A and B in FIG. 1, and “gap” refers to the boundary between adjacent waveguides indicated as G in FIG. The interval between each other. For example, the minimum gap between adjacent waveguides in a PLC that can be manufactured using a normal exposure machine and etching apparatus is about 3 μm, and interference between adjacent waveguides is avoided depending on the non-refractive index difference Δ between the core and the cladding. Therefore, a wider gap may be the minimum gap.

  FIG. 2 shows a simulation using a beam propagation method (BPM) for transmittance when a conventional pitch conversion circuit has an input waveguide width of 4 μm and an input waveguide gap of 1, 1.5, 2, 3 μm. It shows what was calculated by. The wavelength was 1.55 μm, which is a communication wavelength band, and Δ2%.

  According to the results shown in FIG. 2, even when a relatively high Δ2% where light confinement is strong is set, the transmittance needs to be set to, for example, 0.99 or more in order to avoid interference between adjacent waveguides. It can be seen that the gap between the waveguides must be set to 3 μm or more. It is theoretically possible to avoid interference between adjacent waveguides by increasing Δ and further narrow the pitch. However, since it is difficult to manufacture a narrow gap of 3 μm or less, a conventional pitch conversion circuit is used. It can be said that a narrower pitch than this cannot be realized.

  Si-based planar waveguides are easier to process than quartz, and because Δ is high and interference between adjacent waveguides hardly occurs, it is possible to narrow the gap to less than the wavelength, but it is limited to the wavelength of transmitted light Because of the large polarization dependence, the application destination is limited. For this reason, an optical circuit that can emit light at a narrow pitch using a PLC is required.

  The present invention has been made in view of such conventional problems, and an object of the present invention is to provide an optical circuit using a PLC capable of realizing a narrow pitch at the light emitting end.

  In order to solve the above-described problem, the invention described in one embodiment has an arbitrary number of input waveguides and an input waveguide portion connected to the input waveguides at one end and an output at the other end. A multimode interference optical waveguide having a waveguide section, wherein the output waveguide section of the multimode interference optical waveguide is formed to have a width smaller than that of the input waveguide section. It is an optical circuit.

It is a figure which shows the conventional pitch conversion circuit. It is a figure which shows the relationship between the gap between waveguides in the conventional pitch conversion circuit, and the transmittance | permeability. It is a figure which shows schematic structure which looked at MMI used for the optical circuit of 1st Embodiment from the upper surface. It is a figure which shows schematic structure which looked at MMI which modulated the waveguide width along the advancing direction of light from the upper surface. It is a figure which shows schematic structure which looked at MMI used for the optical circuit of 2nd Embodiment from the upper surface. It is a figure which shows schematic structure which looked at MMI used for the optical circuit of 3rd Embodiment from the upper surface.

  Hereinafter, embodiments of the present invention will be described in detail.

An optical circuit of the present invention includes an arbitrary number of input waveguides and a multimode interference optical waveguide having an input waveguide connected to the input waveguide at one end and an output waveguide at the other end ( Multi-mode interference waveguide (MMI), and the width of the output waveguide section (W _MMIin ) of the MMI is smaller than the width of the input waveguide section (W _MMIout ). Normally, the optical circuit provided with the MMI has the same input / output end waveguide width of the MMI, but in the optical circuit of the present invention, the interval between the emitted light sources is changed by making the width different. Specifically, by _setting W _MMIin > W _MMIout , the pitch of the light source in the MMI having a predetermined length is multiplied by W _MMIout / W _MMIin, so that the pitch can be reduced.

In the optical circuit of the present invention, when the width of the MMI input waveguide section is W _MMIin , the width of the output waveguide section is W _MMIout , the wavelength is λ, and the refractive index of the core is n, the length of the MMI in the optical waveguide direction L is set to a value approximately given by (Equation 1) below.
L = (4/3 · n · W _MMIin · W _MMIout ) / λ (Formula 1)

  An optical circuit having an MMI generally includes an arbitrary number of input / output waveguides and a multimode interference waveguide (MMI) having a wide waveguide width, and light incident from the input waveguide is guided by a plurality of MMIs. Functions such as optical multiplexing / demultiplexing can be realized by utilizing the interference between the modes. That is, an optical multiplexing / demultiplexing circuit is realized by using a single MMI or a plurality of MMIs (see, for example, Non-Patent Document 2). When the length L satisfying the above (Formula 1) is set, the phase difference between modes becomes an integral multiple of 2π by MMI, and an image can be formed in the same fundamental mode as that at the time of incidence. This phenomenon is called self-imaging.

  According to the optical circuit of the present invention, it is possible to solve the problems such as interference between adjacent waveguides in the conventional pitch conversion circuit and restriction of narrowing the light source by narrow gap processing, and further narrowing of the pitch at the emission end can be realized. . Specifically, an MMI having a taper that narrows the waveguide from the incident end to the output end is connected to the tip of the normal pitch conversion circuit (FIG. 1), and the output light at the output end of the MMI is narrowed. To do. Since the MMI consists of a single wide waveguide, narrow gap processing is not required, and interference with other light sources does not occur at the time of self-imaging, so that a further narrow pitch can be realized. In addition, this waveguide circuit shape is not dependent on the material of the planar waveguide and the non-refractive index difference Δ between the core and the clad, and therefore can be applied to all planar waveguides.

(First embodiment)
The optical circuit of the first embodiment will be described with reference to FIGS. FIG. 3 is a diagram showing a schematic configuration of the MMI used in the optical circuit of the present embodiment as viewed from above. The optical circuit of this embodiment is configured by providing a tapered multimode interference waveguide (MMI) 3 shown in FIG. 3 to the input waveguide shown in FIG. In the present embodiment, the case where the number of input waveguides 2 is five will be described as an example. Needless to say, any number of input waveguides can be used.

  The input waveguide 1 and the MMI 3 can be formed on the same substrate, and are formed by laminating a substrate, a lower cladding layer, a core layer, and an upper cladding layer. The lower cladding layer is provided on the substrate. The core layer having a refractive index higher than that of the cladding layer is provided on the lower cladding layer, and the upper cladding layer is provided so as to surround the core layer.

The optical circuit of this embodiment has a configuration in which the tapered MMI 3 shown in FIG. 3 is arranged at the output end of the pitch conversion circuit shown in FIG. MMI3 is waveguide length L, width W _MMIin input waveguide portion 2, is used as the width W _MMIout output waveguide section 4. By making the taper W _MMIin > W _MMIout , the pitch can be reduced. Since the length L in the waveguide direction of the MMI 3 is set based on (Equation 1), the propagating light can be self-imaged.

For example, when light having a communication wavelength band of 1.55 μm is used and the refractive index of the core is n = 1.474, MMI3 may set W _MMIin , W _MMIout , and L to 32 μm, 24 μm, and 3040 μm, respectively. it can. In this case, in the input waveguide portion 2 of the MMI 3, the width of each waveguide is 4 μm and the gap between the waveguides is 3 μm. In the output waveguide portion 4 of the MMI 3, the width of each waveguide is 3 μm, The pitch can be narrowed to the same light source pitch as the gap of 1 μm between the waveguides is formed. If the pitch is to be further narrowed, the width of W _MMIout needs to be made smaller. _However , if the pitch is made too small, light cannot be confined in the core and the loss increases.

(Modification of the first embodiment)
FIG. 4 is a diagram showing an MMI in which the waveguide width is modulated along the traveling direction of light. Since the effective refractive index of each mode developed by the MMI 3 is slightly different, the waveguide length necessary for self-imaging differs slightly depending on the input waveguide. Therefore, the waveguide width of the MMI 3 is not a simple taper as shown in FIG. 3, but the transmittance is improved by modulating the waveguide width along the light traveling direction as shown in FIG. be able to.

In order to calculate a specific shape of the waveguide width, a wavefront matching method (WFM) can be used here. The WFM determines the refractive index distribution so that the light wave propagating from the input side and the wavefront of the light propagating from the output side are matched to the optical circuit having a certain input / output. This is a simulation technique for calculating a structure that maximizes the transmittance, and has a great track record in PLC design using SiO ₂ as a material (see, for example, Non-Patent Document 3). At any point in the MMI, the wavefront of the forward-propagating optical field of the input field from the input waveguide section of the MMI matches the wavefront of the optical field propagated in the reverse direction of the output field from the output waveguide section of the MMI. Thus, the waveguide width of the MMI is changed along the light propagation direction with respect to a desired light wavelength range. In the output waveguide section of the MMI, an output field of light for which output is to be obtained is set in advance, and the wavefront of the light field propagated in the reverse direction of the set output field and the input of light actually input from the input waveguide section The waveguide width can be determined such that the wavefront of the optical field propagated in the forward direction of the field matches the desired wavelength range at any point in the MMI.

  Specifically, to calculate the waveguide width shape for the purpose of reducing loss using WFM, for example, using BPM (beam propagation method), the input waveguide section side, the output waveguide section side In each of the above, light having a desired mode field is propagated, and the width of the waveguide is changed so that the wavefronts of the two lights are matched at the center of the circuit. By repeating this procedure a plurality of times, the waveguide width shape that reduces the loss can be determined.

  As described above, by using the tapered MMI, the light source can be emitted with a narrower pitch than the conventional pitch conversion circuit.

(Second Embodiment)
FIG. 5 is a diagram illustrating a schematic configuration of an MMI used in the optical circuit of the second embodiment as viewed from above. Although the MMI of the first embodiment has a line-symmetric shape with respect to the light traveling direction, the MMI of the present embodiment is such that the center of the input waveguide portion and the center of the output waveguide portion are shifted. In addition, it is configured such that they are translated in parallel with each other perpendicular to the light traveling direction. Other configurations are the same as those of the MMI used in the optical circuit of the first embodiment.

  Since the length of the MMI 3 in the optical waveguide direction depends on the width of the input waveguide portion and the width of the output waveguide portion, it may be set based on (Equation 1) as in the first embodiment. However, the parallel movement distance between the center of the input waveguide section and the center of the output waveguide section is set within a range where light can be confined so that light is not emitted from the side wall of the waveguide due to a tight taper angle. The

  With this configuration, it is possible to reduce the pitch even when the structure is not symmetrical.

(Third embodiment)
FIG. 6 is a diagram illustrating a schematic configuration of an MMI used in the optical circuit of the third embodiment as viewed from above. In the present embodiment, a plurality of MMIs 3a, 3b, 3c, and 3d of the second embodiment in which the distance of parallel movement differs from the output of the input waveguide can be arranged in parallel. Specifically, MMIs 3a, 3b, 3c, and 3d having different parallel movement distances in which a plurality of blocks are arranged in parallel with the output of the input waveguide with the MMI of the second embodiment as one block can be used. In the output waveguide portions 4a, 4b, 4c, and 4d, it is desirable to make the interval between the blocks as small as possible.

  Increasing the number of inputs / outputs with a single MMI increases the length required for self-imaging, leading to an increase in the size of the element. According to the present embodiment, the number of inputs and outputs can be increased with the element length unchanged. However, a gap of 3 μm or more is required between MMIs in order to avoid interference between adjacent waveguides.

DESCRIPTION OF SYMBOLS 1 Pitch conversion circuit 2 Input waveguide part 3 Multimode interference waveguide (MMI)
4 Output waveguide section

Claims (5)

Any number of input waveguides;
A multimode interference optical waveguide having an input waveguide portion connected to the input waveguide at one end and an output waveguide portion at the other end;
An optical circuit, wherein a width of the output waveguide portion of the multimode interference type optical waveguide is smaller than a width of the input waveguide portion. The length L in the optical waveguide direction of the multimode interference optical waveguide is such that the width of the input waveguide section is W _MMIin , the width of the output waveguide section is W _MMIout , the wavelength of input light is λ, 2. The optical circuit according to claim 1, wherein the optical circuit is set based on the following (Equation 1), where n is a refractive index of the core of the mode interference optical waveguide.
L = (4/3 · n · W _MMIin · W _MMIout ) / λ (Formula 1)   The multi-mode interference type optical waveguide propagates the wave field of the light field that has been forward-propagated through the input field of the light input from the input waveguide section and the output field of the light to be output to the output waveguide section. The waveguide width is determined so that a wavefront of an optical field coincides with an arbitrary point in the multimode interference optical waveguide with respect to a desired wavelength range of light. 2. The optical circuit according to 2.   2. The multimode interference type waveguide according to claim 1, wherein the center of the input waveguide portion and the center of the output waveguide portion of the multimode interference waveguide are translated from each other perpendicular to the light traveling direction. 4. The optical circuit according to any one of 3.   An optical circuit comprising a plurality of multimode interference waveguides according to claim 4 arranged in parallel.