WO2024095457A1 - Optical device - Google Patents

Abstract

This optical device comprises a third core (103). The distance from the third core (103) to a first core (101) and a second core (102) is such that the third core (103) can be optically coupled to the other cores in an optical coupling region (151). The third core (103) is formed to be wider than the first core (101) and the second core (102). The third core (103) is formed so as to extend along a first optical waveguide (110) waveguiding direction and a second optical waveguide (120) waveguiding direction that coincide with each other.

Description

Optical Devices

The present invention relates to an optical device equipped with a waveguide-type optically active element.

The explosive increase in communication traffic has created a demand for faster, larger capacity, smaller, and less costly optical transceivers. In response to this demand, silicon photonics technology, which uses mature CMOS technology to form optical circuits on large-diameter Si wafers, has attracted attention. To date, small passive optical components and high-speed optical modulators using Si have been developed. Optical transceivers using these optical components are already in practical use.

On the other hand, Si is an indirect transition semiconductor, and it is not easy to realize highly efficient semiconductor lasers or semiconductor optical amplifiers using Si. Traditionally, direct transition semiconductors have been used for optical components such as semiconductor lasers and semiconductor optical amplifiers. For this reason, attempts have been made to epitaxially grow or directly bond compound semiconductors onto Si substrates, form optically active elements such as lasers or optical amplifiers using these compound semiconductors, and monolithically integrate them with Si optical circuits. Also, attempts have been made to form optically active elements such as lasers individually on InP substrates and hybrid integrate them with Si optical circuits.

In the latter hybrid integration, there is a method of butting together an optically active element chip and a Si optical circuit chip to connect them. There is also a method of transfer printing the optically active element onto the Si optical circuit (Non-Patent Document 1).

While hybrid integration requires high assembly costs, it has the advantage of being able to integrate after selecting non-defective products, and of being able to apply the optimal manufacturing process for each of the Si and compound semiconductor device manufacturing processes. In particular, transfer printing has the advantage of being able to achieve integration capabilities equivalent to monolithic integration while maintaining the aforementioned advantages.

R. KOU et al., "Inter-layer light transition in hybrid III-V/Si waveguides integrated by μ-transfer printing", Optics Express, vol. 28, no. 13, pp. 19772-19782, 2020.

However, this technology requires highly accurate alignment in order to integrate the optically active elements in a way that optically connects them to the optical waveguides of the fine Si optical circuit. If the alignment accuracy is poor, the optical output from the laser cannot be input to the Si optical circuit. Typically, alignment with an accuracy of a few hundred nanometers or less is desired, but achieving this level of alignment accuracy is not easy with current technology. For this reason, there is a demand for the realization of an optical connection structure with high tolerance to misalignment.

The present invention was made to solve the above problems, and aims to optically connect an optical active element and a Si optical circuit using an optical connection structure with high tolerance to misalignment.

The optical device according to the present invention comprises a first optical waveguide made of a first core that enters the optical coupling region from one side and terminates at the center of the optical coupling region, a second optical waveguide made of a second core that enters the optical coupling region from the other side and terminates at the center of the optical coupling region, a third core that is arranged at a distance in the optical coupling region that allows optical coupling to the first and second cores and is wider than the first and second cores, and a waveguide-type optical active element that is connected to the first optical waveguide and has an active layer made of a compound semiconductor, and the first core A is made of a compound semiconductor and has a first tapered portion tapered toward the center of the optical coupling region, the second core is made of Si and has a second tapered portion tapered toward the center of the optical coupling region, the waveguiding direction of the first optical waveguide and the waveguiding direction of the second optical waveguide are the same in the optical coupling region, the first core and the second core are formed without overlapping each other in the optical coupling region, and the third core is formed extending in the same waveguiding direction of the first optical waveguide and the waveguiding direction of the second optical waveguide.

As described above, according to the present invention, in the optical coupling region, a third core is provided that is arranged at a distance that allows optical coupling to the first core connected to the optical active element and the second core connected to the Si optical circuit, and that is wider than the first and second cores. This allows the optical active element and the Si optical circuit to be optically connected with an optical connection structure that has high tolerance to misalignment.

FIG. 1A is a plan view showing a configuration of an optical device according to an embodiment of the present invention. FIG. 1B is a cross-sectional view showing a configuration of an optical device according to an embodiment of the present invention. FIG. 1C is a cross-sectional view showing a partial configuration of an optical device according to an embodiment of the present invention. FIG. 1D is a plan view showing a partial configuration of an optical device according to an embodiment of the present invention. FIG. 2 is a diagram showing the configuration of an optical connection structure for comparison. FIG. 3 is a characteristic diagram showing the calculation results of the optical coupling efficiency between the first optical waveguide 110 and the second optical waveguide 120. As shown in FIG. FIG. 4 is a characteristic diagram showing the calculation results of the change in light transmittance from the first optical waveguide 110 to the second optical waveguide 120, with the width (thickness) t in the first direction and the width W in the second direction of the third core 103 as parameters. FIG. 5 is a characteristic diagram showing the calculation results of the change in light transmittance from the first optical waveguide 110 to the second optical waveguide 120, with the width (thickness) t of the third core 103 in the first direction and the refractive index of the third core 103 as parameters. FIG. 6 is a diagram showing the configuration of an application example of an optical device according to an embodiment of the present invention. FIG. 7 is a diagram showing the configuration of another application example of the optical device according to the embodiment of the present invention. FIG. 8 is a diagram showing the configuration of another application example of the optical device according to the embodiment of the present invention. FIG. 9 is a diagram showing the configuration of another application example of the optical device according to the embodiment of the present invention. FIG. 10 is a cross-sectional view showing a partial configuration of another application example of the optical device according to the embodiment of the present invention. FIG. 11 is a diagram showing the configuration of another application example of the optical device according to the embodiment of the present invention. FIG. 12 is a cross-sectional view showing a partial configuration of another application example of the optical device according to the embodiment of the present invention.

Below, an optical device according to an embodiment of the present invention will be described with reference to Figures 1A, 1B, 1C, and 1D. Note that Figure 1B shows a cross section parallel to the waveguide direction, and Figure 1C shows a cross section perpendicular to the waveguide direction.

This optical device includes a first optical waveguide 110 that enters the optical coupling region 151 from one side and terminates at the center of the optical coupling region 151, and a second optical waveguide 120 that enters the optical coupling region 151 from the other side and terminates at the center of the optical coupling region 151. The waveguiding direction of the first optical waveguide 110 and the waveguiding direction of the second optical waveguide 120 are the same in the optical coupling region 151. A waveguide-type optical active element 141 having an active layer 104 made of a compound semiconductor is connected to the first optical waveguide 110.

The first optical waveguide 110 is composed of a first core 101 that terminates in the center of the optical coupling region 151, and the second optical waveguide 120 is composed of a second core 102 that terminates in the center of the optical coupling region 151. In addition, in the optical coupling region 151, the first core 101 and the second core 102 are formed without overlapping each other.

The first core 101 is made of a compound semiconductor. The first core 101 also has a first tapered section 111 that tapers toward the center of the optical coupling region 151. The second core 102 is made of Si. The second core 102 also has a second tapered section 121 that tapers toward the center of the optical coupling region 151.

The optical device also includes a third core 103 that is disposed at a distance in the optical coupling region 151 that allows it to be optically coupled to the first core 101 and the second core 102. The third core 103 is formed to be wider than the first core 101 and the second core 102. The third core 103 is formed to extend in the same waveguiding direction of the first optical waveguide 110 and the waveguiding direction of the second optical waveguide 120.

As shown in FIG. 1B, the third core 103 can be disposed between the first tapered portion 111 (first core 101) and the second tapered portion 121 (second core 102). In this example, the third core 103 is formed at a different height from the first core 101 and the second core 102 on the substrate 131 on which the first core 101 and the second core 102 are formed. Furthermore, in this example, the second core 102 is disposed below the first core 101 (on the substrate 131 side).

In this example, a first cladding layer 132 is formed on a substrate 131, a third core 103 is formed on the first cladding layer 132, and a second cladding layer 133 is formed on the third core 103. The second core 102 is formed so as to be embedded in the first cladding layer 132, and these constitute a second optical waveguide 120. The first core 101 is formed so as to be embedded in the second cladding layer 133, and these constitute a first optical waveguide 110. The first cladding layer 132 and the second cladding layer 133 can be made of, for example, silicon oxide (SiO ₂ ).

It is important that the third core 103 is formed at least in the optical coupling region 151. The first taper section 111 (first core 101) and the second taper section 121 (second core 102) can be positioned on the same side as seen from the third core 103.

The optically active element 141 is embedded in a semiconductor layer 105 made of a compound semiconductor, and an active layer 104 is formed therein. As shown in FIG. 1C, an n-type region 107a and a p-type region 107b are formed in the semiconductor layer 105 so as to sandwich the active layer 104. An n-electrode 108a is ohmic-connected to the n-type region 107a, and a p-electrode 108b is connected to the p-type region 107b. This allows current injection or voltage application to the active layer 104. In this example, a Separate Confined Heterostructure (SCH) structure is formed in which the semiconductor layers above and below the active layer 104 embedded in the semiconductor layer 105 are used as optical confinement layers, and a waveguide structure is formed with an optical waveguide having the active layer 104 as its core.

In addition, by arranging a diffraction grating 106, which is made of gratings arranged in the waveguiding direction, near the active layer 104, the optical active element 141 can be a distributed feedback (DFB) laser. The diffraction grating 106 can be composed of, for example, a plurality of grooves 106a formed in the third core 103. The plurality of grooves 106a are arranged periodically in the waveguiding direction.

Next, the coupling efficiency of the optical connection between the first optical waveguide 110 and the second optical waveguide 120 of the optical device according to the embodiment will be described. In the following, as shown in FIG. 2, an optical coupling structure in which the first tapered portion 311 of the first core 301 constituting the first optical waveguide and the first tapered portion 331 of the second core 302 constituting the second optical waveguide are arranged to overlap in the optical coupling region 351 will be used for comparison.

Figure 3 shows the calculation results of the coupling efficiency in the optical coupling region 151. The width (thickness) of the third core 103 in the first direction from the first core 101 to the second core 102 is 450 nm, the width of the third core 103 in the second direction perpendicular to the first direction is 4000 nm, and the refractive index of the material constituting the third core 103 is 2. The third core 103 can be composed of, for example, silicon oxynitride or silicon nitride.

(a) of FIG. 3 shows the coupling efficiency between the first optical waveguide 110 and the second optical waveguide 120 in the optical coupling region 151 of the optical device according to the embodiment. (b) of FIG. 3 shows the coupling efficiency of the optical coupling structure shown in FIG. 2. In FIG. 3, the horizontal axis represents the positional deviation (Error) between the two optical waveguides, and the vertical axis represents the optical transmittance from one optical waveguide to the other optical waveguide.

As shown in Figure 3, when there is no misalignment, optical coupling is achieved at a high rate of nearly 100%. However, in the configuration of Figure 2 without the third core 103, as the misalignment increases, as shown in (b), the transmittance decreases and the optical coupling loss increases. On the other hand, according to the embodiment in which the third core 103 is provided, as shown in (a), there is almost no transmission loss even if there is a misalignment of about 1 μm. Thus, according to the embodiment, even if there is some misalignment between the first optical waveguide 110 and the second optical waveguide 120 in the optical coupling region 151, it can be seen that the two can be optically connected with high efficiency.

Figure 4 shows the calculation results of the change in light transmittance from the first optical waveguide 110 to the second optical waveguide 120, using the width (thickness) t in the first direction and the width W in the second direction of the third core 103 in the optical coupling region 151 as parameters. In this calculation, the positional deviation between the first optical waveguide 110 and the second optical waveguide 120 in the optical coupling region 151 is set to 1.5 μm. The refractive index of the third core 103 is set to 2. As shown in Figure 4, it can be seen that the transmittance is high when the width W in the second direction is 4 μm, regardless of the width (thickness) t in the first direction. It can also be seen that the width (thickness) t in the first direction of the third core 103 is appropriate to be 250 nm or more.

Figure 5 shows the calculation results of the change in light transmittance from the first optical waveguide 110 to the second optical waveguide 120, with the width (thickness) t of the third core 103 in the first direction in the optical coupling region 151 and the refractive index of the third core 103 as parameters. In this calculation, the positional deviation between the first optical waveguide 110 and the second optical waveguide 120 in the optical coupling region 151 is set to 1.5 μm, and the width W of the third core 103 in the second direction is set to 4000 nm. It can be seen that the refractive index of the material constituting the third core 103 is high at about 2.2 when t is 400 nm, and about 1.9 when t is 550 nm. Examples of materials with this refractive index include SiON and SiN.

From the above results, it can be seen that it is appropriate for the third core 103 to be made of silicon oxynitride or silicon nitride, have a refractive index of 1.5 to 3.0, have a width in the first direction from the first core 101 to the second core 102 of approximately 100 to 500 nm, and have a width in the second direction perpendicular to the first direction of approximately 2000 to 6000 nm.

Next, an application example of the optical device according to the embodiment will be described. For example, as shown in FIG. 6, a Mach-Zehnder modulator can be configured by combining a phase shifter 201, which is an optically active element having an active layer 104, and a multimode interference (MMI) 202 made of Si, which is an optical circuit. A Mach-Zehnder modulator can be configured by combining two MMIs 202 with two arms, each of which has a phase shifter 201. In each arm, a first optical waveguide formed by a first core 101 connected continuously to the phase shifter 201 and a second optical waveguide formed by a second core 102 of the MMI 202 are optically connected via a third core 103 in the optical coupling region 151.

Also, as shown in FIG. 7, a smaller directly modulated laser array can be constructed by combining a plurality of optically active elements 141 each having an active layer 104 with an arrayed waveguide grating (AWG) 203 made of Si. A second optical waveguide made of a second core 102 optically connected via a third core 103 in an optical coupling region 151 to a first optical waveguide made of a first core 101 connected continuously to the optically active elements 141 constituting the laser is connected to an optical fiber 204 of the AWG 203. A highly efficient multi-wavelength laser array can be constructed by changing the photoluminescence (PL) wavelength of the active layer 104 in each optically active element 141 according to the oscillation wavelength.

For example, as shown in FIG. 8, by extending the third core 103 up to the substrate end 111a, optical coupling with the optical fiber 204 is possible. If the refractive index of the third core 103 is about 2 and the core width of the third core 103 in the first direction is about 500 nm, it becomes possible to achieve highly efficient optical coupling to the optical fiber 204 at the substrate end 111a.

9 and 10, the optical fiber 204 can be optically connected to the second optical waveguide formed by the second core 102 through the third optical waveguide 205 having a core 205c made of _SiOx or a polymer material having a refractive index equivalent to that of the core of the optical fiber 204. The third optical waveguide 205 includes, for example, a clad 205b formed on a Si substrate 205a and a core 205c embedded in the clad 205b. The third optical waveguide 205 can be combined by connecting the core 205c partially exposed by removing a part of the clad 205b to the third core 103 extended to the substrate end 111a. In this way, the use of the third optical waveguide 205 enables coupling with the optical fiber 204 with lower reflection and higher efficiency.

By setting the thickness of the cladding 205b between the Si substrate 205a and the core 205c of the third optical waveguide 205 to 3 to 20 μm, it is possible to reduce light leakage to the Si substrate 205a. It is desirable to set the thickness of the first cladding layer 132 between the second core 102 and the substrate 131 to 1 to 2 μm. This is because if the first cladding layer 132 is too thick, the thermal resistance increases and the optical device characteristics deteriorate.

Also, as shown in Figures 11 and 12, the optical fiber 204 can be optically connected to the second optical waveguide formed by the second core 102 via the third optical waveguide 205. In this configuration, the optical connection is made to the second optical waveguide formed by the second core 102 without going through the third core 103. As shown in Figure 12, a part of the cladding 205b of the third optical waveguide 205 is removed to partially expose the core 205c, which is then connected to the top of the second optical waveguide 120 at the substrate end 111a.

As described above, according to the present invention, the optical coupling region includes a third core that is arranged at a distance that allows optical coupling to the first core connected to the optical active element and the second core connected to the Si optical circuit, and that is wider than the first and second cores, so that the optical active element and the Si optical circuit can be optically connected with an optical connection structure that has high tolerance to misalignment. According to the present invention, even if there is some misalignment between the first core and the second core in the optical coupling region, for example, the optical active element and the Si optical circuit can be optically connected with high efficiency, and for example, a large proportion of light compared to the laser output can be input to the Si optical circuit.

The present invention is not limited to the embodiments described above, and it is clear that many modifications and combinations can be implemented by those with ordinary skill in the art within the technical concept of the present invention.

101...first core, 102...second core, 103...third core, 104...active layer, 105...semiconductor layer, 106...diffraction grating, 106a...groove, 107a...n-type region, 107b...p-type region, 108a...n-electrode, 108b...p-electrode, 110...first optical waveguide, 111...first tapered portion, 120...second optical waveguide, 121...second tapered portion, 131...substrate, 132...first cladding layer, 133...second cladding layer, 141...optically active element, 151...optical coupling region.

Claims (4)

a first optical waveguide including a first core that enters an optical coupling region from one side and terminates at a center of the optical coupling region;
a second optical waveguide including a second core that enters the optical coupling region from the other side and terminates at a center of the optical coupling region;
a third core that is disposed at a distance capable of being optically coupled to the first core and the second core in the optical coupling region and has a width greater than that of the first core and the second core;
a waveguide-type optical active element connected to the first optical waveguide and having an active layer made of a compound semiconductor;
the first core is made of a compound semiconductor and has a first tapered portion tapered toward a center of the optical coupling region;
the second core is made of Si and has a second tapered portion tapered toward a center of the optical coupling region;
a waveguiding direction of the first optical waveguide and a waveguiding direction of the second optical waveguide are made the same in the optical coupling region;
In the optical coupling region, the first core and the second core are formed without overlapping with each other,
an optical device according to claim 1 , wherein the third core is formed extending in a same waveguiding direction of the first optical waveguide and a same waveguiding direction of the second optical waveguide; 2. The optical device according to claim 1,
An optical device, comprising: a third core disposed between the first tapered portion and the second tapered portion; 3. The optical device according to claim 2,
An optical device, characterized in that the first core and the second core are formed at different heights above a substrate on which the first core and the second core are formed. The optical device according to any one of claims 1 to 3,
the third core is made of silicon nitride or silicon oxynitride having a refractive index of 1.5 to 3.0;
The width of the third core in the first direction from the first core to the second core is 100 to 500 nm;
a width of the third core in a second direction perpendicular to the first direction is 2000 to 6000 nm.

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