Multimode interferometer

Abstract

To provide a multimode interference coupler capable of improving the wavelength dependence of coupling efficiency. [Solution] A multimode interference coupler comprising an input waveguide, an output waveguide, and a multimode interference waveguide connected to the input waveguide and the output waveguide and wider than the input waveguide and the output waveguide, wherein a plurality of holes are provided in the multimode interference waveguide, and the plurality of holes constitute a mosaic pattern in the plane of the multimode interference waveguide.

Description

[[TECHNICAL FIELD]]

[0001] The present disclosure relates to a multimode interference coupler. [[BACKGROUND ART]]

[0002] Mosaic-shaped optical passive elements have been developed. By arranging a plurality of holes in a plane, it is possible to split light at a predetermined intensity ratio, and a small multiplexer / demultiplexer is formed (Patent Document 1). Multimode Interference (MMI) couplers are known (Non-Patent Document 1 and Non-Patent Document 2). A multimode interference coupler can be used as an active MMI to amplify light. [[PRIOR ART DOCUMENTS]] [[PATENT DOCUMENTS]]

[0003] [[Patent Document 1]] Japanese Unexamined Patent Application Publication No. 2025-033327 [[NON-PATENT DOCUMENTS]]

[0004] [[Non-Patent Document 1]] Kiichi Hamamoto et.al.“First demonstration of novel active multi-mode interferometer(MMI) LDs integrated with 1st order-mode permitted waveguides”,IEICE Electronics Express,Vol.2,No.13,pp.399-403,2005 [[Non-Patent Document 2]] Zhingang Zang et al.“High-Power(>110mW) Superluminescent Diodes by Using Active Multimode Interferometer”,IEEE Photnic Technology Letters,Vol.22,No.10,May 15,2010 [Overview of the project] [Problems that the invention aims to solve]

[0005] A multimode interferometric coupler can be used, for example, as a semiconductor optical amplifier (SOA). By widening the width of the multimode interferometric waveguide (MMI) compared to other waveguides, the volume is increased. This increases the amplification width of the light.

[0006] Generally, a single mode (0th-order mode) is input to a multimode interfering waveguide, and a single mode is output from the multimode interfering waveguide. By widening the waveguide as described above, higher-order modes (2nd order and above) are also generated in addition to the single mode. The length of the multimode interfering waveguide is optimized so that a single mode is imaged at the output terminal of the multimode interfering waveguide.

[0007] However, the optimal length of a multimode interferential waveguide is wavelength-dependent. The length of the multimode interferential waveguide is determined so that the coupling efficiency is maximized at a specific wavelength. If the wavelength of light deviates from this specific wavelength, the coupling efficiency decreases, and reflected light may occur. Therefore, the objective is to provide a multimode interferential coupler that can improve the wavelength dependence of the coupling efficiency. [Means for solving the problem]

[0008] The multimode interference coupler according to this disclosure comprises an input waveguide, an output waveguide, and a multimode interference waveguide connected to the input waveguide and the output waveguide, having a width greater than that of the input waveguide and the output waveguide, wherein a plurality of holes are provided in the plane of the multimode interference waveguide. [Effects of the Invention]

[0009] According to this disclosure, it is possible to provide a multimode interference coupler capable of improving the wavelength dependence of coupling efficiency. [Brief explanation of the drawing]

[0010] [Figure 1] Figure 1 is a plan view illustrating a multimode interferometer according to the first embodiment. [Figure 2] Figure 2 is a cross-sectional view illustrating a multimode interference coupler. [Figure 3] Figure 3 is a cross-sectional view illustrating a multimode interference coupler. [Figure 4] Figure 4 is a cross-sectional view illustrating a multimode interference coupler. [Figure 5] Figure 5 is a plan view illustrating a multimode interference coupler in a comparative example. [Figure 6] Figure 6 illustrates an example of calculating the bonding efficiency in a comparative example. [Figure 7] Figure 7 illustrates an example of calculating coupling efficiency in the first embodiment. [Figure 8] Figure 8 is a plan view illustrating a multimode interferometer according to the second embodiment. [Figure 9] Figure 9 is a plan view illustrating a multimode interferometer according to the third embodiment. [Figure 10] Figure 10 is a cross-sectional view illustrating a modified multimode interference coupler. [Modes for carrying out the invention]

[0011] [Description of Embodiments in this Disclosure] First, the contents of the embodiments of this disclosure will be listed and explained.

[0012] One embodiment of the present disclosure is a multimode interference coupler comprising (1) an input waveguide, an output waveguide, and a multimode interference waveguide connected to the input waveguide and the output waveguide, having a larger width than the input waveguide and the output waveguide, wherein a plurality of holes are provided in the plane of the multimode interference waveguide. The zeroth mode and higher-order modes unfold in the multimode interference waveguide. The length of the multimode interference waveguide is determined such that the multimodes interfere and image at the connection portion between the multimode interference waveguide and the output waveguide, resulting in an image with the same electric field distribution as the zeroth mode of the input waveguide. By providing a plurality of holes in the plane of the multimode interference waveguide, the wavelength dependence of the coupling efficiency is improved. (2) In (1) above, the first and second ends of the multimode interfering waveguide face each other, the input waveguide is connected to the first end, and the output waveguide is connected to the second end, and the plurality of holes between the first and second ends may form a mosaic pattern. The mosaic pattern is a pattern like that described in Patent Document 1. The length of the multimode interfering waveguide is determined so that the electric field distribution at the second end is the same as that of the 0th mode of the input waveguide and an image is formed. By providing a mosaic pattern of holes in the multimode interfering waveguide, the wavelength dependence of the coupling efficiency is improved. (3) In (2) above, a plurality of the mosaic patterns may be provided between the first end and the second end. The wavelength dependence of the coupling efficiency is improved. (4) In (2) above, multiple types of the mosaic pattern may be provided between the first end and the second end. The wavelength dependence of the coupling efficiency is improved. (5) In any of (1) to (4) above, the input waveguide, the output waveguide, and the multimode interference waveguide each have a first semiconductor layer, a second semiconductor layer, and a core layer, wherein the first semiconductor layer, the core layer, and the second semiconductor layer are stacked in this order, the first semiconductor layer has a first conductivity type, the second semiconductor layer has a second conductivity type, the core layer has optical gain, and the core layer may have the holes. The wavelength dependence of the coupling efficiency is improved. Light can be amplified by injecting carriers into the core layer. (6) In (5) above, the hole may be filled with air, or may be filled with the second semiconductor layer. The wavelength dependence of coupling efficiency is improved. (7) In (5) above, each of the input waveguide, the output waveguide, and the multimode interference waveguide has a mesa, the mesa includes the second semiconductor layer, the core layer extends from under the mesa to the outside of the mesa, and a width of the mesa in the multimode interference waveguide may be larger than the width of the mesa in the input waveguide and the width of the mesa in the output waveguide. Since current flows intensively through the mesa, light can be amplified efficiently. Light can be confined near the mesa. (8) In (5) above, each of the input waveguide, the output waveguide, and the multimode interference waveguide has a mesa, the mesa includes the core layer, block layers are provided on both sides of the mesa, and a width of the mesa in the multimode interference waveguide may be larger than the width of the mesa in the input waveguide and the width of the mesa in the output waveguide. Since current flows intensively through the mesa, light can be amplified efficiently. Light can be confined near the mesa. (9) In (5) above, the first semiconductor layer and the second semiconductor layer contain indium phosphide, and the core layer contains indium gallium arsenide phosphide or indium gallium aluminum arsenide, and may have a quantum well structure. The core layer has optical gain. Light can be amplified by injecting carriers into the core layer.

[0013] [Detailed Description of Embodiments of the Present Disclosure] Specific examples of the multimode interference coupler according to embodiments of the present disclosure will be described below with reference to the drawings. The present disclosure is not limited to these examples, and is defined by the claims, and it is intended that all modifications within the meaning and scope equivalent to the claims are included.

[0014] <First Embodiment> Figure 1 is a plan view illustrating a multimode interferometer coupler 100 according to the first embodiment. The top surface of the substrate 10 is parallel to the XY plane, which is composed of the X and Y axes in Figure 1. The Y axis is perpendicular to the X axis. The Z axis is perpendicular to the X and Y axes and represents the thickness direction of the substrate 10. The length L1 of the substrate 10 in the X direction is, for example, 1.2 mm. The length L2 of the substrate 10 in the Y direction is, for example, 0.4 mm.

[0015] The multimode interferometer coupler 100 is provided on the substrate 10 and has an input waveguide 12, an output waveguide 14, and a multimode interferometer waveguide 16. The multimode interferometer coupler 100 functions as an SOA, amplifying the light input from the input waveguide 12 and outputting it from the output waveguide 14.

[0016] In Figure 1, the input waveguide 12, the multimode interferential waveguide 16 (MMI), and the output waveguide 14 are arranged in order along the X-axis. The input waveguide 12, the multimode interferential waveguide 16, and the output waveguide 14 are, for example, parallel to the X-axis. The width W1 of the input waveguide 12 in the Y-axis direction is, for example, 2 μm. The width of the output waveguide 14 is, for example, equal to the width W1 of the input waveguide 12. The waveguide width is the width of the mesa 40, which will be described later.

[0017] A slab portion 18 is provided on the substrate 10. The slab portion 18 is provided on both sides of the input waveguide 12, the multimode interference waveguide 16, and the output waveguide 14 in the Y-axis direction. A recess 17 is provided between the input waveguide 12, the multimode interference waveguide 16, and the output waveguide 14 and the slab portion 18.

[0018] The planar shape of the multimode interfering waveguide 16 is, for example, rectangular. Of the four ends of the multimode interfering waveguide 16, two ends are parallel to the Y-axis. The input waveguide 12 is connected to the first end 16a of these two ends. The output waveguide 14 is connected to the second end 16b of these two ends. The other two ends of the multimode interfering waveguide 16 are parallel to the X-axis and face the slab portion 18. The length L3 in the X-axis direction of the multimode interfering waveguide 16 is, for example, 1 mm. The width W2 in the Y-axis direction of the multimode interfering waveguide 16 is wider than the width W1 of the input waveguide 12 and the width of the output waveguide 14, for example, 10 μm. The width of the waveguide is the width of the mesa 40, which will be described later.

[0019] Line CC in Figure 1 represents the centers of the input waveguide 12, the multimode interference waveguide 16, and the output waveguide 14 in the Y-axis direction. The centers of these waveguides coincide. The input waveguide 12 is connected to the center of the end 16a of the multimode interference waveguide 16 in the Y-axis direction. The output waveguide 14 is connected to the center of the end 16b of the multimode interference waveguide 16 in the Y-axis direction. The input waveguide 12, the output waveguide 14, and the multimode interference waveguide 16 are symmetrical with respect to line CC.

[0020] The multimode interference waveguide 16 has a plurality of holes 15. The planar shape of the holes 15 is, for example, circular. The plurality of holes 15 are provided in the plane of the multimode interference waveguide 16. The plurality of holes 15 are distributed in the plane of the multimode interference waveguide 16, forming a mosaic-like pattern. As shown in Figure 1, the plurality of holes 15 may be arranged together at each of the four corners of the multimode interference waveguide 16. The plurality of holes 15 may also be arranged together near each of the two ends of the multimode interference waveguide 16 that are parallel to the X-axis. In the example in Figure 1, the holes 15 are not located on the line C0C that passes through the center of the multimode interference waveguide 16. When the multimode interference coupler 100 is used as SOA, the optical amplification characteristics tend to deteriorate if unwanted reflected light is generated. Reflected light is likely to occur when the difference in refractive index between the holes 15 and the multimode interference waveguide 16 is large. With the arrangement of the holes 15 as described above, unwanted reflected light is less likely to occur inside the multimode interference waveguide 16.

[0021] Figures 2 to 4 are cross-sectional views illustrating a multimode interference coupler 100. Figure 2 is a cross-sectional view along line AA in Figure 1, illustrating a cross-section including the input waveguide 12. Figure 3 is a cross-sectional view along line BB in Figure 1, illustrating a cross-section including the multimode interference waveguide 16. Figure 4 is a cross-sectional view along line CC in Figure 1, illustrating a cross-section including the input waveguide 12, the multimode interference waveguide 16, and the output waveguide 14.

[0022] As shown in Figures 2 to 4, the substrate 10 is a semiconductor substrate and has a cladding layer 20 (first semiconductor layer), a core layer 22, a cladding layer 24 (second semiconductor layer), and a contact layer 26. Along the Z-axis direction, the cladding layer 20, core layer 22, cladding layer 24, and contact layer 26 are stacked in order from bottom to top.

[0023] As shown in Figures 2 and 3, the input waveguide 12 and the multimode interference waveguide 16 are ridge-mesa type waveguides. As shown in Figure 2, the input waveguide 12 has a mesa 40. As shown in Figure 3, the multimode interference waveguide 16 also has a mesa 40. The mesa 40 protrudes above the core layer 22 in the Z-axis direction. The output waveguide 14 has the same configuration as the input waveguide 12. The input waveguide 12 and the output waveguide 14 are single-mode waveguides.

[0024] In the Y-axis direction, recesses 17 are provided on both sides of the mesa 40. A slab portion 18 is provided at the position opposite the mesa 40 to the recesses 17. In the Y-axis direction, two recesses 17 and the mesa 40 are provided between the two slab portions 18. The mesa 40 is provided between the two recesses 17.

[0025] The mesa 40 and slab 18 include a cladding layer 24 and a contact layer 26. The recess 17 penetrates the cladding layer 24 and the contact layer 26 in the Z-axis direction and extends to the interface between the cladding layer 24 and the core layer 22. The core layer 22 and the cladding layer 20 extend below and outside the mesa 40, and also extend below the recess 17 and the slab 18.

[0026] The multimode interferometer 100 has an electrode 30 (first electrode), an electrode 32 (second electrode), and an insulating film 34. The insulating film 34 covers the surface of the multimode interferometer 100. The insulating film 34 covers the bottom surface and side walls of the recess 17, the top surface of the slab portion 18, and the sides of the mesa 40. The insulating film 34 has an opening on the top surface of the mesa 40. Of the contact layer 26, the portion that constitutes the top surface of the mesa 40 is covered by the electrode 32 and not covered by the insulating film 34.

[0027] An electrode 32 is provided on the contact layer 26 of the mesa 40, on the side opposite to the core layer 22 (the top surface). The electrode 32 is electrically connected to the contact layer 26 of the mesa 40. An electrode 30 is provided on, for example, the entire side of the cladding layer 20, on the side opposite to the core layer 22 (the bottom surface). The electrode 30 is electrically connected to the cladding layer 20. The insulating film 34 is made of an insulator such as silicon nitride (SiN). Electrodes 30 and 32 are made of metal.

[0028] As shown in Figure 4, the cladding layer 20, core layer 22, cladding layer 24, contact layer 26, electrodes 30 and 32 extend continuously to the input waveguide 12, the multimode interference waveguide 16, and the output waveguide 14.

[0029] The cladding layer 20 is composed of, for example, n-type (first conductivity type) indium phosphide (n-InP). The cladding layer 24 is composed of, for example, p-type (second conductivity type) indium phosphide (p-InP). The thickness of the cladding layer 20 is, for example, 150 μm. The thickness of the cladding layer 24 is, for example, 2 μm. The contact layer 26 is composed of, for example, p-type indium gallium arsenide (p-InGaAs).

[0030] The core layer 22 has, for example, a multiple quantum well (MQW) structure and includes multiple well layers and a barrier layer. The multiple well layers and the multiple barrier layers are stacked alternately. The well layers and barrier layers are formed of, for example, undoped InGaAsP.

[0031] The refractive indices of the cladding layer 20 and cladding layer 24 are lower than the refractive index of the core layer 22. The core layer 22 may have an optical confinement layer. The optical confinement layer is provided between the MQW and cladding layer 20, and between the MQW and cladding layer 24. The refractive index of the optical confinement layer is lower than the refractive index of the MQW and higher than the refractive indices of cladding layer 20 and cladding layer 24. The MQW and the optical confinement layer constitute the core layer 22 of the SCH structure (Separate Confinement Heterostructure).

[0032] As shown in Figure 3, the core layer 22 is provided with holes 15. The holes 15 penetrate the core layer 22 in the Z-axis direction, for example. The diameter of the holes 15 is, for example, 0.1 μm. The holes 15 are empty pores, and their interiors may be filled with, for example, air. When the holes 15 are filled with air, the refractive index of the holes 15 is 1. In this case, the difference in refractive index between the holes 15 and the core layer 22 is large. The interiors of the holes 15 may be filled with a cladding layer 24. In this case, the difference in refractive index between the holes 15 and the core layer 22 is small, so unwanted reflections due to this refractive index difference are less likely to occur. Multiple holes 15 form a mosaic pattern in the plane of the multimode interference waveguide 16. The mosaic pattern is designed to obtain the required characteristics. For example, multiple holes 15 may be arranged periodically along the X and Y axes to form a square grid. Multiple holes 15 may be arranged to form a triangular grid, or they may be arranged randomly.

[0033] Light is input to the input waveguide 12 from an external light source of the multimode interferometer coupler 100. Current is injected into the multimode interferometer coupler 100 using electrodes 30 and 32. In the mesa 40, an n-type cladding layer 20, an i-type core layer 22, a p-type cladding layer 24, and a contact layer 26 are stacked in the Z-axis direction, forming a PIN (positive-intrinsic-negative) junction. Current is selectively passed through the mesa 40 to inject carriers into the core layer 22. The core layer 22 has optical gain and emits light in response to the carrier injection. This amplifies the light. The amplified light is output from the output waveguide 14.

[0034] The input waveguide 12 and the output waveguide 14 are single-mode waveguides. Single-mode (fundamental mode, 0th-order mode, TE0 mode) light propagates through the input waveguide 12 and enters the multimode interference waveguide 16. In the multimode interference waveguide 16, the 0th-order mode and higher-order modes (TE2 mode, etc.) unfold. The 0th-order mode and higher-order modes propagate toward the end 16b of the multimode interference waveguide 16. At the connection point between the end 16b and the output waveguide 14, the 0th-order mode and higher-order modes of the multimode interference waveguide 16 are imaged by interference, resulting in the same electric field distribution as the 0th-order mode of the input waveguide 12. The imaged 0th-order mode propagates through the output waveguide 14 and is output outside the multimode interference coupler 100.

[0035] The propagation speed of a mode depends on the order of the mode. At the connection point with the output waveguide 14, the length L3 of the multimode interference waveguide 16 is set to an optimal value so that the phases match between each mode. This increases the coupling efficiency between the multimode interference waveguide 16 and the output waveguide 14. In the first embodiment, since the multimode interference waveguide 16 is provided with a mosaic pattern of holes 15, high coupling efficiency can be obtained over a wide wavelength band.

[0036] (Comparative example) Figure 5 is a plan view illustrating a multimode interference coupler 110 according to a comparative example. The multimode interference waveguide 16 does not have holes 15.

[0037] (binding efficiency) Figure 6 illustrates an example of calculating coupling efficiency in a comparative example. Figure 7 illustrates an example of calculating coupling efficiency in the first embodiment. The horizontal axis represents the wavelength of light propagating through the multimode interference coupler. The vertical axis represents the coupling efficiency from the input waveguide 12 to the output waveguide 14. In the examples of Figures 6 and 7, the length of the multimode interference waveguide 16 is determined so that high coupling efficiency is obtained at a wavelength of 1.55 μm. The wavelength of light is within the range that includes the C band (1530 nm to 1565 nm) in optical communication.

[0038] As shown in Figure 6, in the comparative example, the coupling efficiency is close to 1.0 in the wavelength range of 1.55 μm to 1.56 μm. However, the coupling efficiency decreases at wavelengths longer and shorter than this range. For example, at wavelengths shorter than 1.53 μm, the coupling efficiency falls below 0.9. This decrease in coupling efficiency can result in a light loss of about 10%.

[0039] As shown in Figure 7, in the first embodiment, the coupling efficiency is higher than 0.9 and around 1.0 throughout the entire wavelength band from 1.52 μm to 1.58 μm. High coupling efficiency is obtained throughout the entire C band. Compared to the comparative example, the light loss is smaller.

[0040] According to the first embodiment, an input waveguide 12 and an output waveguide 14 are connected to a multimode interfering waveguide 16. Holes 15 are provided in the plane of the multimode interfering waveguide 16. Multiple holes 15 form a mosaic pattern in the plane of the multimode interfering waveguide 16. By providing a mosaic pattern of holes 15, the wavelength dependence of the coupling efficiency is improved, as shown in Figure 7.

[0041] The input waveguide 12 and output waveguide 14 are single-mode waveguides. The width W2 of the multimode interference waveguide 16 is wider than that of the input waveguide 12 and output waveguide 14. The increased volume of the core layer 22 allows for effective amplification of light. On the other hand, higher-order modes are also deployed in the multimode interference waveguide 16 along with the zero-order mode. By setting the length L3 of the multimode interference waveguide 16 to an appropriate size, the zero-order mode is imaged at the connection point between the multimode interference waveguide 16 and the output waveguide 14. For example, the length L3 is optimized so that the zero-order mode is imaged at one wavelength in the wavelength band used by the multimode interference coupler 100. By providing a mosaic pattern of holes 15 in the multimode interference waveguide 16, the coupling efficiency is increased over a wide bandwidth including the aforementioned one wavelength. The zero-order mode is imaged at the connection point between the multimode interference waveguide 16 and the output waveguide 14 over a wide bandwidth. Unwanted reflected light is reduced. The output of the multimode interference coupler 100 increases.

[0042] The number and position of the holes 15 are determined according to the characteristics required of the multimode interferometer 100. In the example in Figure 7, the coupling efficiency is around 1.0 in the band including the entire C band (1530 nm to 1565 nm). The mosaic pattern may be adjusted so that the coupling efficiency is around 1.0 in the L band (1565 nm to 1625 nm). Because the coupling efficiency is high, the multimode interferometer 100 functions as a high-saturation output SOA over a wide wavelength range, for example, 35 nm or more.

[0043] When optical amplification is performed, the multimode interference waveguide 16 generates heat. The outer edge of the multimode interference waveguide 16 is in contact with the recess 17. Generally, the temperature is higher in the center and lower in the outer edge during operation. The refractive index will also be different in areas with different temperatures. The pattern of the holes 15 may be designed to cancel out the effect of the difference in refractive index due to the temperature distribution.

[0044] As shown in Figure 1, the multimode interference waveguide 16 has ends 16a and 16b. Ends 16a and 16b face each other in the X-axis direction. The input waveguide 12 is connected to end 16a. The output waveguide 14 is connected to end 16b. The length L3 from end 16a to end 16b is adjusted so that the 0th mode is imaged at end 16b. A mosaic pattern of holes 15 is provided between end 16a and end 16b. Light propagating between end 16a and end 16b is affected by the mosaic pattern. The coupling efficiency between the multimode interference waveguide 16 and the output waveguide 14 is increased over a wide wavelength band.

[0045] The planar shapes of the input waveguide 12, the output waveguide 14, and the multimode interference waveguide 16 are rectangular. These waveguides may, for example, have tapered sections.

[0046] In the input waveguide 12, output waveguide 14, and multimode interference waveguide 16, the n-type cladding layer 20, core layer 22, p-type cladding layer 24, and contact layer 26 are stacked in this order. Electrode 30 is an n-electrode and is electrically connected to the cladding layer 20. Electrode 32 is a p-electrode and is electrically connected to the contact layer 26. By applying a forward voltage to electrodes 30 and 32, current can be passed through the multimode interference coupler 100. Carriers can be injected into the core layer 22 to amplify light. As shown in Figure 3, holes 15 are provided in the core layer 22. This improves the wavelength dependence of the coupling efficiency.

[0047] Electrodes 30 and 32 are provided in the input waveguide 12, the output waveguide 14, and the multimode interference waveguide 16. Current is injected into any of these to amplify the light.

[0048] In the example in Figure 3, the hole 15 is hollow and filled with air. The air in the hole 15 may include the atmosphere and gases used in manufacturing. The air pressure in the hole 15 may be equal to atmospheric pressure or reduced to atmospheric pressure. The hole 15 may be filled with a cladding layer 24. The refractive index of the hole 15 is different from that of the core layer 22. The planar shape of the hole 15 may be, for example, circular, elliptical, or polygonal, such as a triangle or quadrilateral.

[0049] The input waveguide 12, output waveguide 14, and multimode interference waveguide 16 each have a mesa 40. The mesa 40 includes a cladding layer 24 and a contact layer 26. Recesses 17 and slabs 18 are provided on both sides of the mesa 40. The core layer 22 extends from below the mesa 40 to the recesses 17 and slabs 18. The mesa 40 is sandwiched between the recesses 17, and a pin junction is formed in the mesa 40. By selectively injecting current into the core layer 22 below the mesa 40, light can be efficiently amplified. Furthermore, because the mesa 40 is sandwiched between the recesses 17, light can be confined near the mesa 40 in the Y-axis direction (lateral direction).

[0050] The width W1 of the mesa 40 in the input waveguide 12 and the output waveguide 14 is narrow. Single modes propagate in the input waveguide 12 and the output waveguide 14. The width W2 of the mesa 40 in the multimode interference waveguide 16 is wide. Zero-order modes and higher-order modes unfold. By setting the length L3 of the multimode interference waveguide 16 to an appropriate value and providing multiple holes 15 in the core layer 22, the coupling efficiency can be increased over a wide wavelength range. The modes unfolding in the multimode interference waveguide 16 are imaged as zero-order modes and output from the output waveguide 14.

[0051] The cladding layer 20, core layer 22, cladding layer 24, and contact layer 26 are composed of, for example, a III-V compound semiconductor. Cladding layers 20 and 24 contain InP, or may be InP layers. Core layer 22 contains indium gallium arsenide phosphide (InGaAsP), and may include barrier layers and well layers of InGaAsP. Core layer 22 contains indium gallium aluminum arsenide (InAlGaAs), and may include barrier layers and well layers of InAlGaAs. The barrier layers and well layers have different compositions from each other. Core layer 22 may have an MQW structure. Contact layer 26 may contain InGaAs.

[0052] Other elements may be monolithically integrated on the substrate 10 of the multimode interferometer 100. For example, a semiconductor laser element may be provided on the substrate 10 and optically coupled to the input waveguide 12. The semiconductor laser element may be a DFB (Distributed Feedback) laser element. The DFB laser element, like the multimode interferometer 100, has a cladding layer 20, a core layer 22, a cladding layer 24, a contact layer 26, etc., and also has a diffraction grating.

[0053] <Second Embodiment> Figure 8 is a plan view illustrating a multimode interference coupler 200 according to the second embodiment. The same configuration as in the first embodiment will not be described. Compared to the first embodiment, the multimode interference waveguide 16 is longer. The length of the multimode interference waveguide 16 is, for example, 4 × L1. The multimode interference waveguide 16 has four regions 50, 52, 54, and 56. Regions 50, 52, 54, and 56 are arranged in this order between end 16a and end 16b. The dotted lines in Figure 8 represent the boundaries between regions. The length of each region is L1.

[0054] The multimode interferential waveguide 16 has multiple mosaic patterns. One mosaic pattern is provided in each of the regions 50, 52, 54, and 56. The mosaic pattern formed by the holes 15 is of the same type. The same mosaic pattern is provided in the four regions.

[0055] According to the second embodiment, the multimode interferential waveguide 16 is long and has a large volume. By amplifying light in the large-volume multimode interferential waveguide 16, the optical output can be increased. Multiple mode imaging occurs between end 16a and end 16b of the multimode interferential waveguide 16. Multiple mosaic patterns are provided between end 16a and end 16b. The wavelength dependence of the coupling efficiency is improved. Specifically, the multimode interferential waveguide 16 has four regions. The length of each region is determined so that the 0th mode is imaged at the region boundary and end 16b. A mosaic pattern is provided in each region. Unwanted reflected light is less likely to occur at the region boundary and end 16b.

[0056] <Third Embodiment> Figure 9 is a plan view illustrating a multimode interferometer coupler 300 according to the third embodiment. Descriptions of the same configuration as in the first or second embodiment are omitted. Multiple types of mosaic patterns are provided in the multimode interferometer waveguide 16. The same mosaic pattern is provided in regions 50 and 56 of the multimode interferometer waveguide 16. The mosaic pattern in regions 50 and 56 is denoted as pattern A. The same mosaic pattern is also provided in regions 52 and 54. The mosaic pattern in regions 52 and 54 is denoted as pattern B.

[0057] According to the third embodiment, the optical output can be increased by amplifying light in a large-volume multimode interferential waveguide 16. Multiple mode imaging occurs between end 16a and end 16b of the multimode interferential waveguide 16. Multiple types of mosaic patterns are provided between end 16a and end 16b. The wavelength dependence of the coupling efficiency is improved. The length of each region is determined so that the 0th mode is imaged at the region boundary and end 16b. A mosaic pattern is provided in each region. Unwanted reflected light is less likely to occur at the region boundary and end 16b.

[0058] The length of the multimode interference waveguide 16 is an integer multiple of the length L1 over which one imaging occurs, and may be 2 times or more, or 4 times or more of L1. As in the second and third embodiments, the multimode interference waveguide 16 is divided into regions of length L1, and a mosaic pattern is provided in each region. The wavelength dependence of the coupling efficiency between regions is improved. The wavelength dependence of the coupling efficiency between the entire multimode interference waveguide 16 and the output waveguide 14 is also improved.

[0059] One mosaic pattern is provided for each region of the multimode interfering waveguide 16. The number of mosaic patterns is equal to the number of regions in the multimode interfering waveguide 16. There may be one, two or more, or three or more types of mosaic patterns. As shown in Figure 8, the same mosaic pattern may be provided for multiple regions. As shown in Figure 9, there may be four regions and two types of mosaic patterns. The number of types of mosaic patterns may be equal to the number of regions. Different mosaic patterns may be provided for multiple regions.

[0060] (modified version) Figure 10 is a cross-sectional view illustrating a modified multimode interference coupler, illustrating a cross-section of the multimode interference waveguide 16. As shown in Figure 10, the multimode interference waveguide 16 is an embedded waveguide. The central part of the cladding layer 20 in the Y-axis direction protrudes more in the Z-axis direction than the part of the cladding layer 20 that is outside the central part. A core layer 22 is laminated in the central part of the cladding layer 20. The mesa 40 includes the central part of the cladding layer 20 and the core layer 22. The cladding layer 20 extends from below the mesa 40 to outside the mesa 40.

[0061] The multimode interferometer coupler has a block layer 60 and a block layer 62. The block layer 60 is provided on the upper surface of the cladding layer 20. The block layer 62 is laminated on the side of the block layer 60 opposite to the cladding layer 20. The block layer 60 and the block layer 62 are provided on both sides of the mesa 40 in the Y-axis direction.

[0062] The cladding layer 24 is provided on the upper surface of the core layer 22 and the upper surface of the block layer 62. The contact layer 26 is laminated on the upper surface of the cladding layer 24. The insulating film 34 covers the upper surface of the contact layer 26 but does not cover the upper surface of the contact layer 26 of the mesa 40. The electrodes 32 are provided on the upper surface of the insulating film 34 and the upper surface of the mesa 40 and are electrically connected to the contact layer 26 of the mesa 40.

[0063] Block layer 60 is formed of, for example, p-type InP. Block layer 62 is formed of, for example, n-type InP. On both sides of the mesa 40, an n-type cladding layer 20, a p-type block layer 60, an n-type block layer 62, and a p-type cladding layer 24 are stacked. Current does not easily flow to either side of the mesa 40, but flows concentratedly within the mesa 40. Carriers can be efficiently injected into the core layer 22. Because the mesa 40 is sandwiched between block layers 60 and 62, the lateral optical confinement coefficient can be increased. The input waveguide 12 and output waveguide 14 can also be embedded waveguides as shown in Figure 10.

[0064] The modified form is applicable to any of the first to third embodiments. In the first to third embodiments, the waveguide is a ridge mesa type waveguide, as shown in Figure 3. The waveguide may also be an embedded type waveguide, as shown in Figure 10.

[0065] Although embodiments of this disclosure have been described in detail above, this disclosure is not limited to these specific embodiments, and various modifications and changes are possible within the scope of the gist of this disclosure as described in the claims. [Explanation of symbols]

[0066] 10 circuit boards 12 Input waveguides 14 Output waveguide 15 holes 16. Multimode Interferential Waveguide 17 Recess 18 Slab Section 20. Cladding layer (first semiconductor layer) 22 core layers 24. Cladding layer (second semiconductor layer) 26 Contact Layer 30, 32 electrodes 34 Insulating Film 40 Mesa 50, 52, 54, 56 areas 60, 62 block layers 100, 110, 200, 300 multimode interferometer coupler

Claims

1. Input waveguide and, Output waveguide and The system comprises a multimode interference waveguide connected to the input waveguide and the output waveguide, and having a width greater than that of the input waveguide and the output waveguide, A multimode interferometer coupler having multiple holes in the plane of the multimode interferometer waveguide.

2. The first and second ends of the multimode interference waveguide face each other, The input waveguide is connected to the first end, The output waveguide is connected to the second end, The multimode interferometer according to claim 1, wherein the plurality of holes between the first end and the second end form a mosaic pattern.

3. The multimode interferometer according to claim 2, wherein a plurality of the mosaic patterns are provided between the first end and the second end.

4. The multimode interferometer according to claim 2, wherein a plurality of types of the mosaic pattern are provided between the first end and the second end.

5. The input waveguide, the output waveguide, and the multimode interference waveguide each have a first semiconductor layer, a second semiconductor layer, and a core layer. The first semiconductor layer, the core layer, and the second semiconductor layer are stacked in this order. The first semiconductor layer has a first conductivity type, The second semiconductor layer has a second conductivity type, The core layer has optical gain, The multimode interference coupler according to claim 1 or claim 2, wherein the core layer is provided with the holes.

6. The multimode interference coupler according to claim 5, wherein the hole is filled with air or the second semiconductor layer.

7. Each of the input waveguide, the output waveguide, and the multimode interference waveguide has a mesa. The mesa includes the second semiconductor layer, The core layer extends from below the mesa to outside the mesa, The multimode interference coupler according to claim 5, wherein the width of the mesa in the multimode interference waveguide is greater than the width of the mesa in the input waveguide and the width of the mesa in the output waveguide.

8. Each of the input waveguide, the output waveguide, and the multimode interference waveguide has a mesa, The mesa includes the core layer, Block layers are provided on both sides of the mesa. The multimode interference coupler according to claim 5, wherein the width of the mesa in the multimode interference waveguide is greater than the width of the mesa in the input waveguide and the width of the mesa in the output waveguide.

9. The first semiconductor layer and the second semiconductor layer contain indium phosphide. The multimode interference coupler according to claim 5, wherein the core layer comprises indium gallium arsenide phosphate or indium gallium aluminum arsenide and has a quantum well structure.

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