JP7835286B2 - Optical element, optical integrated element, and method for manufacturing an optical element - Google Patents

Description

The present invention relates to an optical element for connecting optical elements, an optical integrated element, and a method for manufacturing optical elements.

With the advancement of optical communication networks, there is a growing demand for more sophisticated and cost-effective optical communication devices. Optical communication devices require electrical elements such as drivers, switches, and electrical amplification circuits, as well as optical elements such as semiconductor lasers, optical switches, and optical fibers.

In the optical connection process, where each optical element is discretely connected, precise positioning between the optical elements is crucial for achieving low-loss optical connections. Therefore, even in commonly used optical connectors, high-precision components are employed that minimize the optical axis misalignment between waveguide cores to less than 1 μm. Thus, in manufacturing optical communication devices, designs and precision components that consider strict tolerances are essential.

In particular, SiPh (Silicon Photonics) faces challenges in positioning accuracy during optical packaging. SiPh is an optical device with a semiconductor material core, enabling not only the fabrication of ultra-small and cost-effective optical circuits, but also high-density integration with electrical circuit elements.

However, in SiPh, higher positioning accuracy and tighter tolerances are required during connection than in conventional PLC (Planar Lightwave Circuit) devices with silica-based cores, which increases the process load for optical connection. This is because the smaller the mode field diameter (MFD), the stricter the tolerance requirements for optical connection. Therefore, optical connection of SiPh, a semiconductor-based optical circuit device with a minute MFD, requires more precise positioning technology.

On the other hand, increasing the MFD size can alleviate the positioning accuracy requirements. However, in SiPh, the film thickness of each layer during manufacturing is limited, and high losses occur due to the roughness of the waveguide sidewalls caused by the high refractive index unique to semiconductor waveguides. Therefore, compared to conventional optical elements such as PLCs, it is difficult to obtain an MFD with low loss and high yield comparable to that of general optical fibers.

Therefore, a method is used for low-loss optical connections to convert the MFD of an optical element (chip) to an MFD of about 4 μm, which is smaller than the MFD of conventional optical fibers of about 10 μm. However, when using this method, because the MFD is small, high positioning accuracy of the submicron level is required, which makes the positioning accuracy during optical connection and the tolerances required for optical components stricter.

As a technique that can alleviate the positioning accuracy required for this connection, a self-written waveguide (hereinafter referred to as "SWW") has been disclosed (Non-Patent Document 1).

This technology is an optical connection technology using a photocurable resin, and can connect waveguide cores as follows. Here, light used as signal light for optical communication (hereinafter referred to as "signal light") is emitted from at least one end face of the waveguide core.

In the formation of the SWW, first, a photocurable resin is dropped into the gap between the waveguide cores.

Next, resin curing light, which is light used to cure the photocurable resin, is irradiated from both or one of the waveguide cores. At this time, due to the property of the photocurable resin that it cures sequentially from areas with high light intensity, SWW cores (hereinafter referred to as "SWW cores") are formed sequentially from the end faces of each waveguide core. As a result, SWW cores are formed on the end faces of the waveguide cores.

Furthermore, since the SWW core is formed according to the propagation path of the resin-cured light, even if an optical axis misalignment occurs between waveguide cores, an S-shaped SWW core is formed to compensate for the optical axis misalignment, enabling low-loss optical connection.

Finally, if necessary, any uncured portions of the photocurable resin are removed by washing, etc. Then, cladding resin is dropped onto the removed area (around the SWW core) and cured as appropriate to form the SWW cladding (hereinafter referred to as "SWW cladding"), and the SWW connection is completed.

Here, the wavelength of the resin curing light used to form SWW is mostly in the wavelength range below visible light.

To form a SWW with a constant core diameter, it is necessary to lower the electric field intensity of the resin curing light below a predetermined threshold and make the light intensity distribution of the resin curing light close to a Gaussian function.

In detail, since photocurable resins harden from areas with high intensity of resin-curing light, if the intensity distribution of the resin-curing light is close to a Gaussian distribution, the resin-curing reaction proceeds rapidly in the center of that distribution. As a result, the tip of the SWW becomes lens-shaped. This lens-shaped tip of the SWW focuses the resin-curing light emitted from the tip of the SWW, curing the resin. Since the SWW is formed by the repetition of this phenomenon, the SWW is formed while maintaining its core diameter approximately constant by resin-curing light with an intensity distribution close to a Gaussian function (Non-Patent Literature 1).

Furthermore, increasing the intensity of the resin curing light causes the resin curing rate (= monomer polymerization rate) to saturate at a certain intensity. When this saturation occurs across the entire beam cross-section, the polymerization rate becomes almost constant across the beam cross-sectional area. As a result, the tip of the SWW does not become lens-shaped, and a tapered SWW is formed in which the core diameter of the SWW gradually increases with respect to the propagation direction of the resin curing light. Therefore, in order to form an SWW with a constant core diameter, the intensity of the resin curing light must be lower than a predetermined threshold that depends on the resin.

SWW technology offers axial misalignment compensation regardless of the gap between cores and optical axis misalignment, which are factors in connection loss between cores, enabling low-loss connections. Therefore, SWW-based optical connection technology can relax tolerance requirements in optical connections, enabling simple optical integration and high-yield, low-loss implementation.

SWWs are formed on the end face of a SiPh chip using resin-curing light (visible light) with a known specific waveguide structure. In this case, the waveguide structure must be transparent to visible light (the wavelength band of the resin-curing light) and long-wavelength light (the communication wavelength band). Materials such as SiN and SiON are transparent to light from the visible light to the communication wavelength band, and are therefore used for the waveguide structure of SWWs.

In a conventional optical element 40 for SWW formation, as shown in Figures 10A and 10B, a lower cladding 42 and a waveguide core 43 are formed in order on an SOI (Silicon-on-insulator) substrate (not shown), with the waveguide core 43 covered by an upper cladding 44. Here, the upper cladding 44 and lower cladding 42 are made of silicon oxide, and the waveguide core 43 is made of SiN. The waveguide core 43 may also be made of SiON.

The waveguide core 43 decreases in width towards the end of the optical element 40 (in the direction of the arrow x in Figure 10A), and has the function of expanding the MFD as a Spot-size converter (SSC). This expands the MFD at the end face of the optical element 40 to match other optical elements (not shown) to which it is connected, thereby reducing the optical connection loss between the optical element 40 and other optical elements.

Kagami, Manabu, "Self-formed optical waveguide technology for realizing optical wiring," Optics, vol.37, no.2, (2008).

However, when forming a SW using a conventional optical element 40, there was a problem in that connection loss occurred when the signal light propagated along the boundary between the end face of the optical element 40 and the SW. The details are explained below.

Figures 11A and 11B show the calculation results for the signal light and resin curing light in a conventional optical element. The x-coordinate indicates the direction of light propagation (arrow x direction in Figure 10A), and the y-coordinate indicates the direction perpendicular to the direction of light propagation (arrow y direction in Figure 10A). Here, in Figures 11A and 11B, the waveguide structure 431 used in the calculation is shown by a thin solid line.

The calculations were performed on a two-dimensional waveguide structure using the FDTD method (product name: "ANSIS Lumerical FDTD").

Furthermore, the waveguide structure was composed of tapered SiN, with one end (base) having a width of 800 nm and the other end (tip) having a width of 100 nm, and the length of the tapered structure being 350 μm. The parts other than the waveguide structure were composed of silicon oxide.

Furthermore, the structure used in the calculations has a waveguide with a constant width (800 nm) and a length of 10 μm at the base end of the tapered structure, and a waveguide with a constant width (100 nm) and a length of 20 μm at the tip. In this waveguide of constant width, the signal light and resin curing light propagate without changing the MFD.

Furthermore, the light source was positioned on the central axis of the waveguide structure, 1 μm from the base end of the waveguide structure into the waveguide (at x = 1 μm, y = 0 μm in the coordinates of Figure 11A), and calculations were performed assuming that the light emitted from the light source propagates in the base propagation mode relative to the waveguide cross-section.

Furthermore, the wavelength of the signal light was set to 1550 nm, and the wavelength of the resin curing light was set to 532 nm.

In the case of signal light, as shown in Figure 11A, the MFD gradually expands as it propagates through the waveguide and leaks out of the waveguide.

On the other hand, the resin curing light is confined throughout the entire waveguide, as shown in Figure 11B. This is because the wavelength of the resin curing light is shorter than that of the signal light.

In conventional optical elements, for signal light (long-wavelength light), the waveguide at the tip of a tapered SSC is narrowed, allowing the signal light to gradually leak out of the waveguide and expanding the MFD. In this case, the core size of the waveguide at the tapered tip is smaller than the core size in which the signal light can exist as a propagation mode within the waveguide.

On the other hand, resin curing light (short-wavelength light) is confined as a propagation mode within the core, even in smaller cores. As a result, as shown in Figure 11B, the resin curing light propagates within the waveguide without leaking out of the core in a waveguide of a size that functions as a SSC.

Thus, when the resin curing light propagates through the waveguide of the optical element, the resin hardens from the areas with higher intensity of the curing light, forming the SW. As shown in Figure 12, an SW core 461 having a width approximately the same as the width of the tapered waveguide core 43 is formed on the end face of the optical element 40. Subsequently, an SW cladding 462 is formed around the SW core 461.

On the other hand, as shown in Figure 11A, the signal light has an expanded MFD due to its tapered structure, resulting in an MFD larger than the width of the waveguide core. As a result, loss occurs in the signal light propagating within the optical element when it is connected to the SW.

Typically, in the case of single-mode waveguides such as SiPh, the connection loss of light transitioning between optically different waveguides is determined by the difference in the MFD (Multi-Functional Diode) of the propagation modes of each waveguide. While the MFD is slightly affected by the refractive index difference between the waveguide core and cladding, it is generally of a similar magnitude to that of the waveguide core during propagation.

In SWW46, as shown in Figure 12, the MFD of the propagation mode of the signal light propagating through the SWW core 461 is approximately the same as the diameter of the SWW core 461. On the other hand, the MFD of the signal light at the end face of the optical element 40 is greater than that of the waveguide core 43. Thus, because the MFD at the SWW core 461 and the MFD at the waveguide core 43 are different for the signal light, connection losses occur between the respective waveguides.

As described above, when a SWW is formed using visible light in an optical element having an SSC where the waveguide narrows towards the end face of the conventional optical element, connection loss occurs due to the difference between visible light and signal light. As a result, optical communication devices that use conventional optical elements to form an SWW and connect them optically have the problem of connection loss.

To solve the problems described above, the optical element according to the present invention is connected to another optical element via a self-formed waveguide, which is a portion of a photocurable resin that is cured by irradiation with resin curing light, and comprises, in order, a substrate, a first cladding, a waveguide core having a refractive index higher than that of the first cladding and through which signal light and the resin curing light propagate, and a second cladding having a refractive index lower than that of the waveguide core , wherein the tip of the waveguide core that connects to the self-formed waveguide is positioned away from the end face of the second cladding such that the mode field diameter of the signal light and the mode field diameter of the resin curing light are approximately the same at the end face of the second cladding, and the second cladding is positioned between the tip of the waveguide core that connects to the self-formed waveguide and the end face of the second cladding.

Furthermore, the present invention relates to a method for manufacturing an optical element, which is connected to another optical element via a self-formed waveguide that is a portion of a photocurable resin that is cured by irradiation with resin curing light , and comprises a substrate, a first cladding, a waveguide core having a refractive index higher than that of the first cladding and through which signal light and the resin curing light propagate, and a second cladding having a refractive index lower than that of the waveguide core, comprising the steps of determining the structure of the waveguide core, the first cladding, and the second cladding, determining the wavelengths of the signal light and the resin curing light, and in the structure, the waveguide The method comprises the steps of: calculating the mode field diameters of the signal light and the resin curing light propagating and emitting from the waveguide core; determining the position of the end face of the second cladding at which the mode field diameters of the signal light and the resin curing light emitting from the waveguide core are approximately the same; depositing the material of the first cladding and the material of the waveguide core on the substrate in order; processing the material of the waveguide core into the waveguide core based on the structure and the position of the end face of the second cladding; and forming the second cladding so as to cover the waveguide core.

According to the present invention, it is possible to provide an optical element that can reduce connection losses with a self-forming waveguide (SWW) of signal light.

Figure 1A is a top-view schematic diagram showing the configuration of an optical element according to the first embodiment of the present invention. Figure 1B is a schematic cross-sectional diagram of IB-IB' showing the configuration of an optical element according to the first embodiment of the present invention. Figure 1C is a schematic cross-sectional diagram of an IC-IC' showing the configuration of an optical element according to the first embodiment of the present invention. Figure 2 is a top-view schematic diagram showing an example of the configuration of an optical element according to the first embodiment of the present invention. Figure 3A is a top-view schematic diagram showing the configuration of an optical integrated element according to the first embodiment of the present invention. Figure 3B is a schematic cross-sectional diagram of IIIB-IIIB' showing the configuration of an optical integrated element according to the first embodiment of the present invention. Figure 4A is a schematic diagram of a waveguide structure illustrating the mode field diameter (MFD) of the signal light and resin curing light propagating through the optical element according to the first embodiment of the present invention. Figure 4B is a diagram illustrating the mode field diameter (MFD) of the signal light and resin curing light propagating through the optical element according to the first embodiment of the present invention. Figure 4C is an enlarged view illustrating the mode field diameter (MFD) of the signal light and resin curing light propagating through the optical element according to the first embodiment of the present invention. Figure 5 is a flowchart illustrating the method for manufacturing an optical element according to the first embodiment of the present invention. Figure 6A is a top-view schematic diagram showing the configuration of an optical element according to a second embodiment of the present invention. Figure 6B is a schematic VIB-VIB' cross-sectional diagram showing the configuration of an optical element according to a second embodiment of the present invention. Figure 7A is a top-view schematic diagram showing the configuration of an optical integrated element according to a second embodiment of the present invention. Figure 7B is a schematic cross-sectional view of VIIB-VIIB' showing the configuration of an optical integrated element according to a second embodiment of the present invention. Figure 8 is a top-view schematic diagram showing the configuration of an optical element and a self-formed waveguide according to a third embodiment of the present invention. Figure 9 is a schematic top view showing the configuration of a mode filter in an optical element according to a third embodiment of the present invention. Figure 10A is a top-view schematic diagram showing the configuration of a conventional optical element. Figure 10B is a schematic cross-sectional diagram of the XB-XB' section showing the configuration of a conventional optical element. Figure 11A is a diagram illustrating the light intensity distribution of signal light in a conventional optical element. Figure 11B is a diagram illustrating the light intensity distribution of resin curing light in a conventional optical element. Figure 12 is a top-view schematic diagram showing the configuration of a conventional optical element and a self-formed waveguide.

<First Embodiment>
An optical element according to the first embodiment of the present invention will be described with reference to Figures 1 to 5B.

<Configuration of the optical element>
As shown in Figures 1A to C, the optical element according to this embodiment comprises, in order, a substrate 11, a first cladding 12, a waveguide core 13, and a second cladding 14.

The substrate 11 is a Si substrate.

The first cladding 12 is silicon oxide ( _SiO₂ ).

The waveguide core 13 is made of silicon nitride and is placed on the first cladding 12. The waveguide core 13 has a tapered shape that narrows towards the tip 131 of the waveguide core 13 (the tip on which the self-forming waveguide is formed). Here, for example, the length of the waveguide core 13 is 350 μm, and the width varies from 800 nm to 100 nm. The waveguide core 13 may also be made of SiON.

The second cladding 14 is made of silicon oxide and is arranged to cover the waveguide core 13.

The waveguide core 13 is not positioned near the end face 141 of the second cladding 14. That is, there is a gap (hereinafter referred to as the "MFD control gap") 15 between the end face 131 of the waveguide core 13 and the end face 141 of the second cladding 14, and the second cladding 14 is positioned in this gap.

In this embodiment, an example is shown in which the waveguide core 13 has a tapered shape, but it is not limited to this. The waveguide core 13 may have a waveguide 132 of a constant width at its tip 131. For example, as shown in Figure 2, the waveguide core 13 may have a shape of a constant width on the tip 131 side of the waveguide core 13 from the dotted line 133 in the figure, and a tapered shape on the opposite side. In this case, in the waveguide 132 of the waveguide core 13 with a constant width, the signal light and resin curing light propagate without changing the MFD.

<Configuration of an optical integrated element>
In the optical integrated element 1 according to this embodiment, as shown in Figures 3A and 3B, the optical element 10 is connected to other optical elements 10_2 via SWW 16.

SWW16 has an SWW cladding 162 around an SWW core 161.

Other optical elements 10_2 include waveguide elements and optical fibers, and for example, they comprise a substrate 11_2, a first cladding 12_2, a waveguide core 13_2, and a second cladding 14_2.

Light propagating through the waveguide core 13 of the optical element 10 is optically coupled with the waveguide core 13_2 of another optical element 10_2 via the SWW core 161.

Next, the MFD control gap 15 in the optical element according to this embodiment will be described based on the calculation results with reference to Figures 4A to 4C.

Figure 4A shows a schematic diagram of the waveguide structure used in the calculations.

The waveguide (shaded area in the figure) is made of SiN and consists of a waveguide with a constant width (width 800 nm) at the base end (x=0 to _xT1 ), a tapered waveguide (x= _xT1 to _xT2 ), and a waveguide with a constant width (width 100 nm) at the tip end (x= _xT2 to _xL ). In the tapered waveguide, the width at the base end (x= _xT1 ) is 800 nm, the width at the tip end (x= _xT2 ) is 100 nm, and the length (x= _xT1 to _xT2 ) is 350 μm. Here, the length of the waveguide with a constant width at the base end (x=0 to _xT1 ) is 10 μm, and the length of the waveguide with a constant width at the tip end (x= _xT2 to _xL ) is 20 μm. Thus, the total length of the waveguide (x = 0 to x _L ) is 380 μm.

The portion of the waveguide structure other than the waveguide itself, including the region outside the waveguide tip (x = x _L to x ₁ ), is composed of silicon oxide.

Furthermore, the light source was positioned 1 μm inward from the base end on the central axis C of the waveguide structure (x _R = 1 μm), and calculations were performed assuming that the light emitted from the light source propagates in the base propagation mode relative to the waveguide cross-section.

The calculations were performed on a two-dimensional waveguide structure using the Finite Difference Time Domain Method (FDTD) (product name: "ANSIS Lumerical FDTD"). In addition to the FDTD method, the Eigenmode Expansion (EME) method may also be used for the calculations.

The ground modes of the propagating light (signal light and resin curing light) were calculated using a signal light wavelength of 1550 nm and a resin curing light wavelength of 532 nm.

For both the signal light and the resin curing light, the MFD at each x-coordinate was calculated by performing a Gaussian fitting on the function f(y), which represents the intensity distribution of the light at each x-coordinate in the propagation direction. Here, y represents the direction perpendicular to the propagation direction (x-direction) (arrow y in the figure).

Figures 4B and 4C show the calculated MFD results for the propagation direction of the signal light and resin curing light, respectively. Figure 4A shows the beam diameters of the signal light and resin curing light at position (coordinate) x in the region from the base end of the waveguide to the outside (x = 0 to _x¹ = 0 μm to 400 μm). Figure 4B shows a magnified view in the region x = 350 μm to 400 μm.

As shown in Figures 4A and 4B, as the signal light propagates through the SSC of the waveguide, its MFD expands and it is emitted from the tip of the waveguide (x = 380 μm). As the emitted signal light propagates through the cladding (silicon oxide), its MFD expands.

On the other hand, as the resin curing light propagates within the SSC, its MFD gradually decreases and is emitted from the tip of the waveguide (x = 380 μm). As the emitted resin curing light propagates through the cladding (silicon oxide), its MFD increases.

Here, with resin curing light, the MFD expands more rapidly than with signal light. This is because, as mentioned above, shorter wavelength light spreads more per unit propagation distance than longer wavelength light.

As a result, as shown in Figures 4A and 4B, the beam diameters of the propagating signal light and the resin curing light coincide at a predetermined position (coordinate x', in this case x' = 384 μm).

Therefore, if a predetermined position (coordinate x') is used as the output end face of the optical element, and a SW is formed from this end face, the size of the SW at the output end face will be approximately the same as the MFD of the resin curing light, and the MFD of the signal light will be approximately the same.

In other words, by setting the MFD control gap 15 in the optical element 10 to a length of x _L to x' (here, 4 μm), the MFD of the signal light emitted from the optical element 10 becomes approximately the same size as the SWW, thereby reducing the signal light connection loss.

Thus, in the optical element 10 according to this embodiment, by setting the MFD control gap 15 so that the beam diameters of the signal light and the resin curing light are approximately the same at the output end face 101 of the optical element 10, the connection loss of the signal light to the SWW caused by the difference in wavelength between the signal light and the resin curing light can be reduced.

Here, the range where the beam diameters are similar includes cases where the beam diameters are the same, and also includes cases where there is a difference of, for example, 1 to 2 μm. It is sufficient if the connection loss of the signal light between the optical element and the SW does not hinder the propagation of the signal light in optical communication, etc.

In this embodiment, calculation results for a two-dimensional waveguide structure are shown, but similar calculations can be performed for a three-dimensional waveguide structure using the FDTD method (product name: "ANSIS Lumérical FDTD"). By varying the waveguide core structure (thickness of 2 μm or less), the wavelength of the signal light, and the wavelength of the resin curing light, calculations were performed, and the results showed that the beam diameters of the propagating signal light and the resin curing light coincided at a predetermined position (MFD control gap: corresponding to 1 to 20 μm).

<Manufacturing method for optical elements>
A method for manufacturing an optical element according to this embodiment will be described with reference to Figure 5.

First, the MFD control gap 15 in the optical element is determined.

First, the structures of the waveguide core 13 and the first and second claddings 12 and 14 are determined. Also, the wavelengths of the signal light and the resin curing light are determined (step S1).

Next, in the determined structure, the MFD diameters of the signal light and resin curing light propagating through and emitting from the waveguide core 13 are calculated (step S2).

Next, the position on the end face of the second cladding is determined such that the MFD diameters of the signal light and the resin curing light emitted from the waveguide core 13 are approximately the same. That is, the MFD control gap 15 is determined (step S3).

Next, based on the above calculation results, the optical element is manufactured.

First, the material for the lower cladding (first cladding) 12 is deposited on the substrate. Subsequently, the material for the waveguide core 13 is deposited on top of the lower cladding (first cladding) 12 (step S4). Here, Si can be used as the substrate material, _SiO2 as the material for the lower cladding, and _SiNx as the material for the waveguide core 13.

Next, _SiNx is fabricated into the waveguide core 13 using photolithography (step S5). The mask pattern used for photolithography is fabricated based on the shape of the waveguide core 13 and the MFD control gap 15 obtained by calculation.

Finally, an upper cladding (second cladding) 14 is formed to cover the waveguide core 13 (step S6). For example, _SiO2 is used as the material for the upper cladding (second cladding) 14.

<Manufacturing method for optically integrated devices>
An example of a method for manufacturing an optical integrated element 1 in which the optical element 10 according to this embodiment and another optical element (waveguide element) 10_2 are connected via SWW16 will be described below.

First, the end face 101 of the optical element 10 and the end face of the other optical element (waveguide element) 10_2 are placed facing each other with a predetermined distance between them.

Next, SWW material (photocurable resin) is filled (dropped) between the end face 141 of the second cladding 14 of the optical element and the end face of the other optical element (waveguide element) 10_2. By using a photocurable resin with high viscosity, the photocurable resin can be held in place.

Next, the resin curing light is propagated to the waveguide core 13 and emitted from the tip 131 of the waveguide core 13, and then propagated to the second cladding 14 and emitted from the end face 141 of the second cladding 14.

Next, the resin curing light is irradiated onto the photocurable resin. As a result, the area of the photocurable resin irradiated with the resin curing light hardens and becomes the SWW core 161.

Finally, after removing the uncured photocurable resin, a SWW cladding 162 is formed around it.

As a result, the optical element 10 according to this embodiment and another optical element (waveguide element) 10_2 are optically connected via the SWW 16 to manufacture the optical integrated element 1 (Figures 3A, B).

In the optical integrated element according to this embodiment, the SWW 16 functions as a waveguide if the refractive index of the core 161 is higher than the refractive index of the cladding 162. Therefore, the area around the SWW core 161 may be, for example, air, and an SWW cladding may not be provided. In this case, the SWW cladding may not be formed in the final step of the manufacturing method of the optical integrated element described above.

<Effects>
According to the optical element of this embodiment, the size of the SW and the MFD of the signal light can be made to be approximately the same, thereby reducing the connection loss of the signal light between the waveguide of the optical element and the SW.

The MFD of the signal light propagating through an actual SW is affected by the difference in refractive index between the SW cladding and core. Therefore, the refractive index of the SW cladding may be adjusted so that the MFD of the signal light matches the propagation mode of the signal light within the SW, thereby reducing losses that occur at the boundary between the SW and the end face of the optical element (chip).

Furthermore, the core diameter of the SWW depends somewhat on the intensity of the resin curing light. Therefore, the loss of signal light transitioning from the optical element to the SWW can be reduced by changing the intensity of the resin curing light.

Here, SWW can only be formed at an intensity of resin curing light above a predetermined level. On the other hand, if the intensity of the resin curing light is too high, bubbles will form due to the heat generated by the resin, which will adversely affect SWW formation. Therefore, it is necessary to form the SWW at an intensity of resin curing light lower than the intensity that adversely affects SWW formation.

Furthermore, in this embodiment, it is necessary to control the length of the MFD control gap 15. The optimal value for the MFD control gap 15 can be determined in advance by numerical calculation or the like during the design of the optical element. Based on that design value, polishing marks can be made on the optical element (chip) along with dicing markers using waveguide patterns or the like to adjust the MFD control gap 15 of the fabricated optical element (chip). This allows for efficient control of the length of the MFD control gap 15 by polishing the end face of the optical element while checking the polishing marks with an optical microscope or the like.

Furthermore, the optical element according to this embodiment can improve yield in the manufacturing process and reduce waveguide loss.

In more detail, conventional optical elements require narrowing the tip width to enlarge the MFD. However, narrowing the waveguide width makes it difficult to control the waveguide width during the manufacturing process, and also increases optical loss due to the roughness of the waveguide sidewalls.

On the other hand, in the optical element according to this embodiment, the MFD expands when not only resin curing light but also signal light propagates through the MFD control gap. Therefore, even if the width of the waveguide tip of the optical element (width of SSC) is set wider than the width of the waveguide tip in a normal optical element, the MFD at the end face of the optical element can be made equivalent.

Thus, with the optical element according to this embodiment, the waveguide width can be set to be wider, which improves the yield in the manufacturing process and reduces optical loss due to the roughness of the side walls of the manufactured waveguide.

In this embodiment, an example is shown in which the length of the MFD control gap in the optical element is set to 4 μm. However, the length of the MFD control gap depends on the shape of the waveguide core, the wavelength of the signal light, and the wavelength of the resin curing light. For example, when the wavelength of the signal light is 1550 nm and the wavelength of the resin curing light is 532 nm, the length of the MFD control gap is 2 to 10 μm when the length of the tapered structure of the waveguide core is 200 to 500 μm, the width of the base end is 500 to 1000 nm, and the width of the tip is 50 to 200 nm. Also, when the length of the tapered structure of the waveguide core is 350 μm or more, the width of the base end is 500 to 1000 nm, the width of the tip is 500 nm or less, and the thickness of the waveguide core is 2 μm or less, the length of the MFD control gap is 1 to 150 μm when the wavelength of the signal light is 1250 to 1350 nm and the wavelength of the resin curing light is 385 to 1550 nm.

<Second Embodiment>
A second embodiment of the present invention, an optical element, will be described with reference to Figures 6A to 7B.

<Configuration of the optical element>
In the optical element 20 according to this embodiment, as shown in Figures 6A and 6B, the end faces 121 and 141 of the first cladding 12 and the second cladding 14 are located inward from the end face 111 of the substrate 11 (towards the tip 131 of the waveguide core 13). That is, the end of the optical element 20 has a step, and the surface of the step is the surface of the substrate 11. Hereinafter, this region 21 will be referred to as the "SWW region". The other configurations are the same as in the first embodiment.

<Configuration of an optical integrated element>
In the optical integrated element 2 according to this embodiment, as shown in Figures 7A and 7B, the optical element 20 and the other optical element 10_2 are connected via the SWW 16. Thus, the SWW 16 is formed in the region enclosed by the end face 121 of the first cladding 12, the end face 141 of the second cladding 14, the upper surface of the substrate 11, and the end face of the other optical element 10_2, i.e., the SWW region 21.

The SWW region 21 of the optical element 20 according to this embodiment can be fabricated by protecting the region other than the SWW region 21 with a mask pattern fabricated by a wafer process, for example, and then etching it. This mask pattern is fabricated using a high-precision (1 μm or less) positioning technique used in wafer processes.

Therefore, the gap 15 for MFD control can be controlled with higher precision than dicing or marker processing in the first embodiment, and the MFD can be controlled with a higher yield.

Furthermore, on the surface of the substrate 11 in the SWW region 21 of the optical element 20 according to this embodiment, the gap between the end face 101 of the optical element 10 (the end face 121 of the first cladding 12 and the end face 141 of the second cladding 14) and the end face of the other optical element 10_2 to be connected can be filled with SWW material (resin).

As a result, compared to structures without an SWW region, the SWW material (resin) can be easily filled and held in the gap between the end face of the optical element 20 and the end face of the other optical element 10_2 during the formation of the SWW 16.

In this embodiment, an example is shown where the position of the step in the SWW region is on the surface of the substrate, but it may also be located inside the substrate or inside the first cladding.

<Third Embodiment>
A third embodiment of the present invention, an optical element, will be described with reference to Figures 8 and 9.

<Configuration of the optical element>
As shown in Figure 8, the optical element 30 according to this embodiment has a mode filter 31 that functions with respect to resin curing light on the base end side (opposite the tip end 131) of the waveguide core 13. Here, the mode filter 31 can be placed on the waveguide core 13.

Furthermore, in the optical integrated element 3 according to this embodiment, the optical element 30 is connected to other optical elements (not shown) via the SWW 16.

The mode filter 31 includes, for example, a width modulation structure of a Bragg grating on both sides of the waveguide core 13, as shown in Figure 9. The grating configuration is, for example, such as a waveguide width of 800 nm and a grating depth of 45 nm. The grating period is 320 nm and the duty cycle (ratio of the length of the protrusion to the period) is 0.5.

<Effects>
In the optical element 10 according to the first embodiment, when forming the SWW, the multimode effect of the resin curing light may adversely affect the conditions for forming the SWW.

The structure of the optical element 10 is the same as that used in SiPh and the like, and the waveguide core 13 is single-mode under signal light conditions. Therefore, the waveguide core 13 behaves as a multimode waveguide with respect to resin curing light, which has a shorter wavelength than the signal light.

In the case of a multimode waveguide, multiple transverse modes of light are emitted from the edge of the optical element (chip). In this case, resin curing light of different modes propagates, compared to the case where only single-mode resin curing light propagates.

As a result, the length of the MFD control gap differs between single-mode and multi-mode applications. Therefore, even if the MFD control gap is formed based on the value designed for single-mode, the SWW is formed by the multi-mode resin curing light, making it impossible to match the size of the SWW to the MFD of the signal light, and thus preventing a reduction in the signal light connection loss.

Furthermore, perturbations such as differences in the roughness of the waveguide sidewalls and refractive index distribution caused by manufacturing errors can alter the generation of transverse modes in multimode waveguides. Therefore, it is difficult to control the MFD control gap with high yield in multimode waveguides.

Furthermore, when forming SWWs, resin curing light with a unimodal Gaussian intensity distribution allows for the formation of long SWWs (approximately mm in length) that grow with a constant core diameter. Therefore, in order to form long SWWs, it is necessary to suppress the multimode nature of the resin curing light.

On the other hand, in the optical element according to this embodiment, the effects of multimode amplification can be suppressed by using a mode filter.

In the optical element according to this embodiment, the higher-order transverse modes of the resin-cured light propagating through the waveguide core 13 are strongly scattered by the mode filter. This makes it possible to make the light intensity of the resin-cured light at the exit end face of the waveguide core 13 a Gaussian distribution or close to a Gaussian distribution (single-mode emission).

The mode filter in the optical element according to this embodiment can impose a large loss on all modes except the lowest-order transverse mode of the resin curing light propagating through the waveguide core 13. Therefore, even if the resin curing light becomes multimode due to manufacturing errors, only the lowest-order transverse mode can be emitted from the tip 131 of the waveguide core 13 with a high yield. In other words, it is possible to provide an optical element that emits resin curing light having a distribution in which the light intensity is high at the center of the core and decreases towards the sides of the core.

According to the optical element of this embodiment, in addition to the effects of the first embodiment, the light intensity distribution of the resin curing light can be made to be a Gaussian distribution or close to a Gaussian distribution, so that a SW can be formed with high precision, and a low-loss SW core can be formed in a long shape with a constant core diameter. Therefore, the connection loss of signal light between the waveguide of the optical element and the SW can be further reduced.

In this embodiment, an example is shown where the grating is formed on the side surface of the waveguide core as a mode filter, but it may also be formed on the top surface of the waveguide core.

Furthermore, a grating made of a material other than the waveguide core 12, such as a metal diffraction grating made of aluminum (Al), may also be used.

Furthermore, in the grating, the bumps and dips do not have to be arranged periodically; the bumps and dips may be arranged randomly, and the waveguide width may be randomly modulated. Also, instead of bumps and dips, the shape may be a waveform.

Alternatively, a bent waveguide may be used as a mode filter. In a bent waveguide, the radiation loss of higher-order modes of the propagating light is large, so multimode propagation can be suppressed. In addition, a structure that suppresses multimode propagation of resin-cured light propagating through the waveguide may be used.

The optical element according to the embodiment of the present invention can be applied to a structure in which an optical waveguide made of a material transparent to resin-cured light is formed on a substrate. For example, it can be applied to a planar lightwave circuit (PLC).

In the embodiments of the present invention, examples of the structure, dimensions, materials, etc., of each component in the configuration and manufacturing method of the optical element are shown, but the invention is not limited to these examples. Any configuration that allows the optical element to perform its function and produce the desired effect is acceptable.

The present invention relates to an optical element for connecting optical elements, an optical integrated element, and a method for manufacturing an optical element, and can be applied to optical communication devices and optical communication network systems.

10 Optical element 10_2 Other optical element 11 Substrate 12 First cladding 13 Waveguide core 131 The tip of the waveguide core,
14 Second cladding 141 End face of the second cladding 16 Self-forming waveguide

Claims (6)

An optical element connected to another optical element via a self-formed waveguide , which is a portion of a photocurable resin that is cured by irradiation with resin curing light, comprising, in order, a substrate, a first cladding, a waveguide core having a refractive index higher than that of the first cladding and through which signal light and the resin curing light propagate, and a second cladding having a refractive index lower than that of the waveguide core ,
The tip of the waveguide core that connects to the self-forming waveguide is positioned away from the end face of the second cladding such that the mode field diameter of the signal light and the mode field diameter of the resin curing light are approximately the same at the end face of the second cladding.
An optical element characterized in that the second cladding is positioned between the tip of the waveguide core that connects to the self-forming waveguide and the end face of the second cladding. The end face of the first cladding and the end face of the second cladding are located on the tip side of the waveguide core relative to the end face of the substrate.
The optical element according to claim 1, characterized in that the self-forming waveguide is formed in a region surrounded by the end face of the first cladding, the end face of the second cladding, the upper surface of the substrate, and the end face of the other optical element. The optical element according to claim 1, characterized in that the waveguide core includes a mode filter. The optical element according to claim 1, characterized in that the waveguide core includes a mode-field conversion unit. The optical element according to claim 1,
An optical integrated element comprising the other optical elements connected via the self-forming waveguide. A method for manufacturing an optical element, comprising a substrate, a first cladding, a waveguide core having a refractive index higher than that of the first cladding and through which signal light and the resin curing light propagate, and a second cladding having a refractive index lower than that of the waveguide core, wherein the optical element is connected to another optical element via a self-formed waveguide, which is a portion of a photocurable resin that is cured by irradiation with resin curing light, and the optical element comprises a substrate, a first cladding, a waveguide core having a refractive index higher than that of the first cladding and through which signal light and the resin curing light propagate,
The steps include determining the structure of the waveguide core, the first cladding, and the second cladding, and determining the wavelengths of the signal light and the resin curing light,
In the above structure, the steps include calculating the mode field diameters of the signal light and the resin curing light propagating through the waveguide core and being emitted,
A step of determining the position on the end face of the second cladding where the mode field diameters of the signal light and the resin curing light emitted from the waveguide core are approximately the same,
The process involves sequentially depositing the material for the first cladding and the material for the waveguide core onto the substrate,
A step of processing the material for the waveguide core into the waveguide core based on the above structure and the position of the end face of the second cladding,
A method for manufacturing an optical element, comprising the step of forming the second cladding so as to cover the waveguide core.