JP6842377B2 - Planar optical circuit laminated device - Google Patents

Description

The present invention relates to a planar optical circuit stacking device that guides incident light to a nonlinear optical waveguide, and specifically to a planar optical circuit stacking device for optimally coupling light of different wavelength bands to a nonlinear optical waveguide. ..

Optical application technology using nonlinear optical effects is expected in the new field of optical communication and the field of quantum information communication using light. Wavelength conversion is known as a basic effect among nonlinear optical effects. In wavelength conversion, light incident on a nonlinear optical medium can be converted into light having a different frequency. Utilizing this characteristic, it is widely known as a technique for generating light in a wavelength band that is difficult to oscillate with a single laser. _{In particular, a periodic polarization inversion waveguide using lithium niobate (LiNbO 3} ), which is a second-order nonlinear material and has a large nonlinear constant, has been incorporated into a light source already on the market due to the high efficiency of its nonlinear optical effect. There is.

In the second-order nonlinear optical effect, light having wavelengths λ1 and λ2 is input to generate a new wavelength λ3.
1 / λ3 = 1 / λ1 + 1 / λ2 (Equation 1)
Wavelength conversion that satisfies is called sum frequency generation (SFG), and when λ1 = λ2, that is, by transforming (Equation 1),
λ3 = λ1 / 2 (Equation 2)
Wavelength conversion that satisfies the above conditions is called second harmonic generation (SHG). further,
1 / λ3 = 1 / λ1-1 / λ2 (Equation 3)
Wavelength conversion that satisfies the above conditions is called differential frequency generation (DFG). Further, there is also an optical parametric effect in which only light having a wavelength of λ1 is input to generate light having wavelengths λ2 and λ3 that satisfy (Equation 3). In particular, SHG and SFG are often used for generating a visible light region because they newly generate short-wavelength light, that is, high-energy light with respect to input light.

In order to efficiently cause these second-order nonlinear optical effects, it is required that the amount of phase mismatch of the interacting three wavelengths is zero. Therefore, conventionally, a method has been used in which the amount of phase mismatch is set to 0 by periodically reversing the polarization of the second-order nonlinear optical material. Assuming that the inversion period at that time is Λ, Λ that satisfies the following (Equation 4) may be set for the wavelengths λ1, λ2, and λ3 in the sum frequency generation represented by (Equation 1).
n3 / λ3-n2 / λ2-n1 / λ1-1 / Λ = 0 (Equation 4)
Here, n1 is the refractive index at the wavelength λ1, n2 is the refractive index at the wavelength λ2, and n3 is the refractive index at the wavelength λ3.

In addition to such a periodic polarization inversion structure, by making the periodic polarization inversion structure into a waveguide, wavelength conversion using a highly efficient second-order nonlinear optical effect becomes possible. In addition, the effect of the nonlinear optical effect increases as the overlapping density of light that causes the nonlinear interaction increases. Therefore, by adopting a waveguide structure capable of confining light in a small cross-sectional area and guiding light over a long distance, more efficient wavelength conversion becomes possible.

(Direct junction type ridge type waveguide)
In order to realize a waveguide structure using a nonlinear optical crystal, a method using Ti diffusion or proton exchange was common. However, in recent years, as a wavelength conversion element, a ridge-type optical waveguide that can utilize the bulk characteristics of a crystal as it is and has features such as high light damage resistance, long-term reliability, and ease of device design has been researched and developed (non-). See Patent Document 1). This ridge-type optical waveguide is formed by joining two substrates, thinning one of the substrates, and further performing ridge processing. When joining the substrates, a direct joining technique is known as a technique for firmly joining the substrates without using an adhesive or the like. The direct junction type ridge type waveguide using this technology can inject strong light, and has succeeded in reducing the core with the progress of the waveguide technology, and its nonlinear optical efficiency is steadily improving. (See Non-Patent Document 2).

(Light incident)
It is important to efficiently couple light of a plurality of wavelengths to the nonlinear optical waveguide in order to operate the waveguide device using the above-mentioned nonlinear optical effect. At that time, it may be necessary to combine light of a plurality of completely different wavelength bands at the same time. When a phase-sensitive amplifier is applied to a relay system of an optical communication network as described in Non-Patent Document 3, not only the communication wavelength band of 1.5 μm band but also the light of 780 nm band, which is a double wave of the communication wavelength band, is also nonlinear optics at the same time. It needs to be coupled to the waveguide.

As a method of photocoupling light of multiple wavelengths, a method using a dichroic mirror or a lens, a method of combining two wavelength bands using a planar optical circuit, and a method of directly attaching the planar optical circuit to a nonlinear optical waveguide can be considered. Be done. In the former case, it is necessary to accurately position and assemble a large number of optical components, which increases the mounting cost. On the other hand, the planar optical circuit has a feature that it is easy to realize the combine wave function in the planar optical circuit and it is easy to implement. Furthermore, in recent years, the processing accuracy of quartz-based planar optical circuits has increased, and the functions that can be realized on that plane have diversified. Therefore, optical coupling using a planar optical circuit contributes to the development of applied technology for nonlinear optical devices.

Y. Nishida, H. Miyazawa, M. Asobe, O. Tadanaga, and H. Suzuki, “Direct-bonded QPM-LN ridge waveguide with high damage resistance at room temperature,” Electronics Letters, Vol.39, No. 7, p.609-611, 2003. T. Umeki, O. Tadanaga, and M. Asobe, ‘Highly Efficient Wavelength Converter Using Direct-Bonded PPZnLN Ridge Waveguide,’ IEEE Journal of Quantum Electronics, Vol. 46, No. 8, pp. 1206-1213, 2010. T. Umeki, T. Kazama, O. Tadanaga, M. Asobe, Y. Miyamoto, and H. Takenouchi, “PDM Signal Amplification Using PPLN-Based Polarization-Independent Phase-Sensitive Amplifier,” J. Lightwave Technol., Vol. 33, No. 7, P.1326-1331, 2015.

As described above, in order to effectively utilize the nonlinear optical effect, it is necessary to efficiently inject light of completely different multiple wavelength bands into the nonlinear optical medium. FIG. 1 shows a perspective view of an optical waveguide made of a nonlinear optical medium. The optical waveguide 10 shown in FIG. 1 is a ridge waveguide having spatial asymmetry in which a core 12 is formed on a substrate 11, and the core 12 is an optical waveguide made of a nonlinear optical medium. In such an optical waveguide 10, the central position of the optical electric field distribution differs depending on the wavelength due to the wavelength dependence of the refractive index.

FIG. 2 shows the electric field distribution in the basic mode with respect to light having wavelengths of 780 nm and 1560 nm in the directly bonded lithium niobate waveguide. The solid line shown in FIG. 2 corresponds to a wavelength of 780 nm, and the broken line corresponds to a wavelength of 1560 nm. As shown in the figure, in different wavelength bands, the center position of the core having the maximum amplitude of the basic mode is directed in the vertical direction due to the asymmetry of the core, that is, from the surface of the core 12 in contact with the substrate 11 to the surface facing it. It is off in the direction. For such a ridge-type nonlinear optical waveguide, a quartz-based planar optical circuit is used in which the periphery is covered with a homogeneous cladding to have spatial symmetry and the center position of the core is not wavelength-dependent. When light of a plurality of wavelengths is combined to perform light incident, a problem is a decrease in coupling efficiency due to a shift in the center position of the core for each wavelength of the ridge type nonlinear optical waveguide.

The present invention is a method for solving the above problems, and an object of the present invention is to optimally couple light in two wavelength bands to a waveguide having an asymmetric core by using a plane optical circuit having symmetry. It is to provide a possible planar optical circuit stacking device.

In order to solve the above-mentioned problems, the present invention is a planar optical circuit laminated device, in which first and second first and second laminated devices are formed in which emission end faces inclined so that waveguide light is incident at a predetermined incident angle are formed and laminated. a planar lightwave circuit of the formed in the first and the inclined emitting end face on the second planar lightwave circuit, transmitted through the first first waveguiding light guided by the planar optical circuits, before serial is guided by the second planar optical circuits of, and a reflection film having a wavelength dependence which reflects the second guided wave of the first guided light with different wavelengths, the first guided light The second waveguide light is transmitted by the reflective film of the first planar light circuit and reflected by the reflective film of the second planar optical circuit and the reflective film of the first planar optical circuit. The optical axes of the first and second waveguide light emitted from the reflective film of the first planar optical circuit are separated from each other in the stacking direction of the first and second planar optical circuits. It is characterized by.

According to the invention described in the second aspect , in the planar optical circuit stacking device according to the first aspect, the emission end faces of the first and second planar optical circuits are in the stacking direction of the first and second planar optical circuits. It is characterized by being inclined in the same direction.

According to the invention described in the third aspect, in the planar optical circuit laminated device according to the first or second aspect, the emission end surface of the first and second planar optical circuits has an incident angle of the first and second waveguide light. Is formed so as to be 45 ° or less.

The invention according to aspect 4 is the planar lightwave circuit laminated device according to any of embodiments 1 to 3, wherein said emitting end face of the first and second planar optical circuits, said first planar lightwave circuit It is characterized in that the optical axes of the first and second waveguide light emitted from the reflective film are formed to be parallel to each other.

The invention according to the fifth aspect is the first in the planar optical circuit stacking device according to any one of the first to fourth aspects, when the second planar optical circuit is arranged on the first planar optical circuit. Further includes an embedded layer formed so as to fill a gap between the reflective film and the second planar optical circuit on the emission end surface of the planar optical circuit of the above, and the embedded layer is formed by the first and second waveguide light. It is characterized by having an emission end surface that is transparent to the light and at least perpendicular to the emission direction of the first waveguide light.

The invention according to the sixth aspect is characterized in that, in the planar optical circuit laminated device according to any one of the first to fifth aspects , the first and second planar optical circuits are made of a dielectric or a semiconductor.

According to the present invention, light in two wavelength bands can be optimally coupled to a nonlinear optical waveguide by using a quartz-based planar optical circuit.

It is a perspective view which shows the optical waveguide which consists of a nonlinear optical medium. It is a figure which shows the state of the electric field distribution of the basic mode with respect to the light of the wavelength 780 nm, 1560 nm in the direct junction type lithium niobate waveguide. It is sectional drawing in the waveguide direction of the planar optical circuit laminated device and the nonlinear optical waveguide which concerns on one Embodiment of this invention. It is sectional drawing in the waveguide direction of the planar optical circuit laminated device and the nonlinear optical waveguide which concerns on one Example of this invention. It is sectional drawing which is perpendicular to the waveguide direction of the upper plane optical circuit 110 and the lower plane optical circuit 120 of the planar optical circuit laminated device which concerns on one Example of this invention. It is a perspective view which shows a general plane optical circuit and a nonlinear optical waveguide.

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

FIG. 3 shows a cross-sectional view of a planar optical circuit laminated device and a nonlinear optical waveguide according to an embodiment of the present invention in the waveguide direction. The planar optical circuit stacking device 100 is a stack of an upper planar optical circuit 110 and a lower planar optical circuit 120 that guide light in two different wavelength bands.

The exit end faces of the upper plane light circuit 110 and the lower plane light circuit 120 are inclined in the stacking direction of the upper plane light circuit 110 and the lower plane light circuit 120. Further, a reflective film 111 that reflects light having a wavelength of light propagating at least in the upper planar optical circuit 110 is provided on the exit end surface of the upper planar optical circuit 110. Further, the emission end face of the lower flat light circuit 120 reflects light having a wavelength of light propagating in the upper flat light circuit 110, and is reflected by transmitting light having a wavelength of light propagating in the lower flat light circuit 120. A film 121 is provided.

On the reflective film 121 of the lower planar optical circuit 120, a material transparent to a plurality of lights incident on the nonlinear optical waveguide 10 is used so as to fill the gap between the reflective film 121 and the upper planar optical circuit 110. It is desirable that the embedded layer 122 be formed. Further, it is desirable that the embedded layer 122 has an end face parallel to the incident end face of the nonlinear optical waveguide 10 for connection with the nonlinear optical waveguide 10. This is because, in the connection between the planar optical circuit laminated device 100 and the nonlinear optical waveguide 10, it is possible to realize the connection by the adhesive which is the conventional communication device mounting method. Further, it is desirable that the light transmitted or reflected from the reflective film 121 is vertically incident on the incident end surface of the nonlinear optical waveguide 10, and therefore, the embedded layer 122 is used as the effective refractive index of the lower plane optical circuit 120. It is desirable to have the same refractive index or to adjust the angle of the end face of the upper plane optical circuit 110.

The waveguide light of the upper planar optical circuit 110 is reflected by the reflective film 111 of the upper planar optical circuit 110 and incident on the embedded layer 122, and is further reflected by the reflective film 121 of the lower planar optical circuit 120 to be further reflected by the non-linear optical waveguide 10. It is emitted from the embedded layer 122 toward. On the other hand, the waveguide light of the lower planar optical circuit 120 passes through the reflective film 121 of the lower planar optical circuit 120, enters the embedded layer 122, and is emitted from the embedded layer 122 toward the nonlinear optical waveguide 10. At this time, the optical axis of the light in the two wavelength bands emitted from the embedded layer 122 is in contact with the substrate 11 of the core 12 of the nonlinear optical waveguide 10 in the stacking direction of the upper planar optical circuit 110 and the lower planar optical circuit 120. The optical axis is adjusted so as to be separated from the surface toward the surface facing the surface. Further, it is desirable that the optical axes of the two wavelength bands of light emitted from the embedded layer 122 are parallel. By making the optical axes of the light in the two wavelength bands parallel to each other, both can be vertically incident on the incident end face of the nonlinear optical waveguide 10, and the connection loss can be reduced.

In the present invention, the central position of the light in the two wavelength bands emitted from the embedded layer 122 can be adjusted for each wavelength by the relative positional relationship between the upper planar optical circuit 110 and the lower planar optical circuit 120. That is, in the configuration shown in FIG. 3, when the upper planar optical circuit 110 is arranged so as to be shifted toward the nonlinear optical waveguide 10 side with respect to the lower planar optical circuit 120, the light embedded layer 122 is emitted from the upper planar optical circuit 110. The center position on the end face is lowered. On the contrary, when the upper planar optical circuit 110 is arranged so as to be offset from the lower planar optical circuit 120 to the side opposite to the nonlinear optical waveguide 10 side, the center of the light embedded layer 122 from the upper planar optical circuit 110 at the emission end surface. The position goes up. The upper planar optical circuit 110 and the lower planar optical circuit 120 are fixed at positions adjusted so that light of different wavelength bands is optimally coupled to the ridge-type nonlinear optical waveguide 10.

The emission end faces of the upper planar optical circuit 110 and the lower planar optical circuit 120 provided with the reflective films 111 and 121 are formed so that the incident angle in the optical circuit stacking direction of the waveguide light is smaller, so that the optical center is moved. The amount of deviation of the planar optical circuit with respect to the amount can be increased. As a result, the structure can be easily positioned on the upper plane optical circuit 110. However, as the incident angle in the optical circuit stacking direction is reduced, the optical path length to the exit end face of the embedded layer 122 becomes longer, and the diameter of the light beam incident on the incident end face of the nonlinear optical waveguide 10 becomes larger. Connection efficiency drops. Therefore, the angles of the emission end faces of the upper planar optical circuit 110 and the lower planar optical circuit 120 should be set in consideration of both the ease of positioning and the connection efficiency.

In this device, the materials constituting the upper planar optical circuit 110 and the lower planar optical circuit 120 can have different configurations, and it is desirable to change each material depending on the wavelength to be guided. The substances constituting the upper planar optical circuit 110 and the lower planar optical circuit 120 optical waveguide are dielectrics and semiconductors such as silicon, silicon dioxide, lithium niobate, indium phosphorus, and polymers, or compounds to which additives are added. , It may be transparent to the light used.

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

(Example)
FIG. 4 shows a cross-sectional view of a planar optical circuit laminated device and a nonlinear optical waveguide according to an embodiment of the present invention in the waveguide direction. The upper planar optical circuit 110 and the lower planar optical circuit 120 are made _{of a dielectric having SiO 2} as a main component, and the cores 112 and 123 contain additives as a waveguide structure, which is more than the surrounding claddings 113 and 124. It has a structure with a high refractive index. In this embodiment, the core shape of each waveguide is optimized for light in the 1.5 μm band in the upper planar optical circuit 110 and for light in the 780 nm band in the lower planar optical circuit 120. The upper planar optical circuit 110 and the lower planar optical circuit 120 were manufactured separately.

The emission end faces of the upper plane optical circuit 110 and the lower plane optical circuit 120 have an incident angle of 45 ° in the optical circuit stacking direction of the waveguide light with respect to the emission end faces of the upper plane light circuit 110 and the lower plane light circuit 120. It is formed. The inclined exit end faces of the upper planar optical circuit 110 and the lower planar optical circuit 120 were realized by dicing. Dry etching may be used as a method for producing the inclined exit end face.

Wavelength-selective reflective films 111 and 121 having wavelength selectivity are formed on the emission end face, in which light in the 1.5 μm band is transmitted but light in the 780 nm band is reflected.

An embedded layer 122 was formed on the wavelength-dependent reflective film 121 of the lower planar optical circuit 120 by using a general-purpose adhesive.

FIG. 5 shows a cross-sectional view perpendicular to the waveguide direction of the planar optical circuit laminated device according to the embodiment of the present invention. Clads 113 and 124 are embedded around the cores 112 and 123 of the upper planar optical circuit 110 and the lower planar optical circuit 120, but the cladding on the side where the upper planar optical circuit 110 and the lower planar optical circuit 120 are in contact with each other. It is desirable that the light is thin, and in this example, the device was manufactured so as to have a thickness of about 1.5 μm. Therefore, a clad layer of 3 μm exists between the core 112 of the upper planar optical circuit 110 and the core 123 of the lower planar optical circuit 120.

In order for each wavelength to take the optimum coupling position, the optical axis of the lower planar optical circuit 120 and the nonlinear optical waveguide 10 was first adjusted on the centering table. After that, the upper planar optical circuit 110 is arranged on the lower planar optical circuit 120, optimal positioning is performed for both waveguide lights, and the upper planar optical circuit 110 and the lower planar optical circuit 120 are provided with a general-purpose optical adhesive. Was fixed.

The connection loss in connection of the planar optical circuit laminated device 100 produced in this embodiment with the nonlinear optical waveguide 10 is -0.8 dB for 1.56 μm light and -0.9 dB for 0.78 μm light. Therefore, the maximum coupling efficiency could be obtained for both wavelengths at the same time.

Here, for comparison with the present embodiment, optical connection was performed using a general planar optical circuit shown in FIG. 6, that is, a planar optical circuit in which light of different wavelengths is emitted from the same core. In this case, a connection loss of −1.2 dB was generated for light of 1.56 μm and −1.2 dB was generated for light of 0.78 μm. At this time, the optical axis is adjusted so that the connection losses for both wavelengths are almost the same, and the optical axis is adjusted so that the maximum coupling efficiency is obtained for one wavelength, so that the coupling efficiency of the other light is obtained. Was prevented from dropping significantly.

The optical circuit that operates the basic wavelength is the upper plane optical circuit 110, and the optical waveguide that operates the harmonics is the lower plane optical circuit 120. That is, in the upper plane optical circuit 110, the light in the 780 nm band is the lower plane. The optical circuit 120 may be designed so that light in the 1.5 μm band is guided. In this case, the transmission / reflection characteristics of the wavelength-selective reflective films 111 and 121 may be inverted.

Further, the angle of the emission end face may be an angle such that the incident angle of the waveguide light in the optical circuit stacking direction is less than 45 °, and the angle of the emission end face of both the upper plane optical circuit 110 and the lower plane optical circuit 120 is derived. By setting the angle of incidence of the wave light in the optical circuit stacking direction to be smaller, the amount of shift of the upper planar optical circuit 110 with respect to the amount of movement of the optical center can be increased, and the positioning of the upper planar optical circuit 110 is easier. Structure. However, if the incident angle in the stacking direction is reduced, the optical path length to the optical coupling portion becomes longer, and the diameter of the light beam from the upper plane optical circuit 110 at the incident end surface of the asymmetric waveguide 10 becomes larger, resulting in light connection efficiency. Will fall. Therefore, the angles of the emission end faces of the upper planar optical circuit 110 and the lower planar optical circuit 120 should be set in consideration of both the ease of positioning and the connection efficiency.

The present invention is effective not only in the combination of light in the 1.56 μm and 780 nm bands, but also in all wavelength bands. In particular, it exerts a great effect when optimally coupling two wavelengths having greatly different wavelength bands. At that time, a reflective film having a wavelength selectivity that transmits light passing through the lower plane light circuit 120 and reflects light passing through the upper plane light circuit 110 may be formed on the exit end face.

The embedded layer 122 may be SiO ₂ or SiO ₂ doped with Ge, F, B, or P as an additive. From the viewpoint of ease of manufacturing the device, it is preferable to form the embedded layer 122 with a general-purpose adhesive.

Further, in order to reduce the connection loss, it is desirable that the light emitted from the planar optical circuit stacking device 100 is vertically incident on the incident end surface of the nonlinear optical waveguide 10. In order to make the waveguide light transmitted from the lower plane optical circuit 120 through the reflection film 121 perpendicular to the incident end face of the nonlinear optical waveguide 10, the embedded layer 122 is placed with the effective refractive index of the lower plane optical circuit 120. It is desirable to have a refractive index equal to. Further, it is desirable to adjust the angle of the exit end surface of the upper plane optical circuit 110 so that the waveguide light from the upper plane optical circuit 110 is vertically incident on the incident end surface of the nonlinear optical waveguide 10.

In the present invention, the materials constituting the upper planar optical circuit 110 and the lower planar optical circuit 120 can have different configurations, and it is desirable to change the respective materials depending on the wavelength of the waveguide light. The material constituting the optical waveguide may be transparent to the light used, such as a dielectric such as silicon, silicon dioxide, lithium niobate, indium phosphorus, or a polymer, a semiconductor, or a compound obtained by adding an additive to them. ..

10 Nonlinear Optical Waveguide 11 Substrate 12 Core 20 Plane Optical Circuit 100 Plane Optical Circuit Laminated Device 110 Upper Plane Optical Circuit 111, 121 Wavelength Selective Reflective Film 112, 123 Core 113, 124 Clad 120 Lower Plane Optical Circuit 122 Embedded Layer

Claims (6)

The first and second planar optical circuits in which the exit end faces inclined so that the waveguide light is incident at a predetermined angle of incidence are formed and laminated, and
It is formed on the inclined emitting end face on the first and second planar lightwave circuit, transmitted through the first first waveguiding light guided by the planar optical circuits, before Symbol second planar light is guided by the circuits, a reflective film having a wavelength dependence which reflects the second guided light of wavelength different from the first guided wave,
The first waveguide light is transmitted through the reflective film of the first planar optical circuit, and the second waveguide light is the reflective film of the second planar optical circuit and the first one. The optical axes of the first and second waveguide light reflected by the reflective film of the flat light circuit and emitted from the reflective film of the first flat light circuit are the first and second flat light. A planar optical circuit stacking device characterized in that the circuits are separated in the stacking direction. The planar light according to claim 1, wherein the emission end faces of the first and second planar optical circuits are inclined in the same direction in the stacking direction of the first and second planar optical circuits. Circuit stacking device. According to claim 1 or 2, the emission end faces of the first and second planar optical circuits are formed so that the incident angle of the first and second waveguide light is 45 ° or less. The planar optical circuit stacking device described. The exit end faces of the first and second planar optical circuits are formed so that the optical axes of the first and second waveguides emitted from the reflective film of the first planar optical circuit are parallel to each other. The planar optical circuit laminated device according to any one of claims 1 to 3, wherein the device is characterized by the above. When the second planar optical circuit is arranged on the first planar optical circuit, the gap between the reflective film and the second planar optical circuit on the emission end surface of the first planar optical circuit is filled. With an additional embedded layer formed in
Claims 1 to 4, wherein the embedded layer is transparent to the first and second waveguides and has at least an emission end face perpendicular to the emission direction of the first waveguides. The planar optical circuit stacking device according to any one of. The planar optical circuit laminated device according to any one of claims 1 to 5, wherein the first and second planar optical circuits are made of a dielectric or a semiconductor.

Images