JP2006350226A - Optical coupler, optical waveguide device and optical waveguide coupling method using the optical coupler - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical coupler with which excellent optical coupling between an optical waveguide having a minute cross-sectional area and an external optical component is achieved, an optical waveguide coupling method excellent in optical coupling efficiency and an optical waveguide device. <P>SOLUTION: The optical coupler comprises at least a pair of electrode structure portions 12, 22 and a refractive index modulating portion 20 such as a liquid crystal, whose refractive index distribution is varied according to a voltage gradient caused by voltage application to the electrode structure portions 12, 22, and a phase of incident light is varied in the refractive index modulating portion 20 to optically couple the varied phase light to an external optical portion. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an optical coupling device that facilitates coupling with an optical waveguide having a minute waveguide cross section such as a silicon optical waveguide, an optical waveguide device using the same, and an optical waveguide coupling method.

In recent years, much research has been conducted on an optical waveguide having a silicon material as a core on a silicon substrate. An optical waveguide made of such a material is called silicon photonics, and is expected to be applied to, for example, a combination with a semiconductor device using a silicon substrate (see, for example, Non-Patent Document 1).
Since the refractive index of silicon material is close to 3.5, an optical waveguide having a silicon material as a core and silicon oxide as a cladding can obtain a very high refractive index difference. Therefore, it is possible to form an optical waveguide having a very small cross section, and it is also possible to form a fine curved optical waveguide having a radius of curvature of about 3 μm. Therefore, the entire optical waveguide can be reduced in size, and its application to various semiconductor devices described above is promising because of its compactness.

However, the small cross-sectional area of the optical waveguide is problematic in that high alignment accuracy is required when light is incident on the optical waveguide from the outside.
For this reason, for example, a tapered optical waveguide is provided on the end face of the optical waveguide to increase the cross-sectional area of the optical waveguide at the input / output portion of the optical waveguide element (see Non-Patent Document 1 above).
M. Salib et al., “Silicon Photonics”, Intel Technology Journal, Vol.08, Issue 02, May 10, 2004, pp.143-160

  By the way, in order to increase the cross-sectional area of the optical waveguide by providing a tapered structure on the end face of the optical waveguide, a tapered portion having a length corresponding to the ratio of the cross-sectional area is required. Therefore, the above-described advantage of downsizing is impaired. For this reason, a method of manufacturing a tapered optical waveguide for the purpose of remarkably relaxing the alignment accuracy is not practical.

  In view of the above problems, the present invention is a light capable of excellent optical coupling between an optical waveguide having an optical waveguide with a small cross-sectional area, such as the above-described silicon photonics, and an external optical component. An object of the present invention is to provide a coupling device and thereby provide an optical waveguide device and an optical waveguide coupling method excellent in optical coupling efficiency.

In order to solve the above problems, an optical coupling device according to the present invention has at least a pair of electrode structure portions and a refractive index modulation portion whose refractive index distribution changes corresponding to a voltage gradient caused by voltage application to the electrode structure portions. Thus, the refractive index modulation unit is configured to optically couple to an external optical unit by changing the phase of incident light.
Further, in the optical coupling device of the present invention, the above-described refractive index modulation unit is composed of liquid crystal.

Furthermore, the optical waveguide device of the present invention includes an optical waveguide portion, a condenser lens, and an optical coupling device, and uses the optical coupling device of the above-described configuration of the present invention as the optical coupling device. That is, this optical coupling device includes at least a pair of electrode structure portions and a refractive index modulation portion whose refractive index distribution changes corresponding to a voltage gradient caused by voltage application to the electrode structure portions. The input / output light to the optical waveguide unit is optically coupled to the external optical unit by changing the phase of the incident light in the unit.
In the optical waveguide coupling method of the present invention, a voltage gradient is generated by applying a voltage to change the phase of incident light, and the condensing position by the condensing lens is adjusted to optically couple with the optical waveguide.

  As described above, the optical coupling device of the present invention has a refractive index modulation section made of liquid crystal or the like whose refractive index distribution changes corresponding to the voltage gradient. With this configuration, when a voltage gradient is generated by applying a voltage from the electrode structure unit to the refractive index modulation unit and the refractive index distribution is changed, the phase of incident light passing through the refractive index modulation unit is changed. It is possible to produce an inclination in the equiphase wavefront. Therefore, when the light whose phase has changed is condensed by the condensing lens, the condensing spot position is shifted corresponding to the inclination of the equiphase wavefront. That is, according to the optical coupling device of the present invention, the focused spot position can be adjusted by voltage application. Thereby, input / output light to an optical waveguide or the like having a small cross-sectional area can be easily optically coupled to an external optical unit.

As described above, according to the optical coupling device of the present invention, optical coupling to a region having a relatively small cross-sectional area can be performed satisfactorily and easily.
In the optical coupling device of the present invention, a thin and practical optical coupling device can be provided by using liquid crystal as the refractive index modulation unit.
Furthermore, according to the optical waveguide device and the optical waveguide coupling method of the present invention, it is possible to provide an optical waveguide device and an optical waveguide coupling method that are easy in optical alignment and excellent in optical coupling efficiency.

Examples of the best mode for carrying out the present invention will be described below, but the present invention is not limited to the following examples.
FIG. 1 shows a schematic configuration diagram of an embodiment of an optical waveguide device using an optical coupling device according to the present invention. In this example, when the light L is input / output to / from the optical waveguide portion 1 by the condenser lens 2, an example is shown in which the optical coupling device 10 having the configuration of the present invention is used. By changing the phase of the light incident on the condenser lens 2 in the optical coupling device 10 having a refractive index modulation unit such as a liquid crystal, the position of the light spot condensed on the end face of the optical waveguide unit 1 is adjusted, and the light A state where the light is efficiently incident on the waveguide portion 1 is shown. In addition to these configurations, the optical waveguide device can be combined with various light source devices and various devices such as a semiconductor device including a light receiving unit.

Next, the principle of the optical waveguide coupling method using the optical coupling device of the present invention will be described with reference to FIGS.
When a liquid crystal such as a nematic liquid crystal material is used as the refractive index modulation unit, the refractive index of the liquid crystal changes according to the applied voltage, and the phase distribution of light transmitted through the liquid crystal is adjusted by the voltage distribution applied to the liquid crystal. Is done. When a liquid crystal material is used, it is advantageous for reducing the thickness of the device and has an advantage that it can be configured at a relatively low cost.
As shown in FIG. 2, when no voltage is applied to the optical coupling device 10, the incident light Li such as a laser does not change its phase distribution even if it passes through the optical coupling device 10, and is indicated by a solid line p. The equiphase wavefront is not inclined even after equalization, and a condensing spot is formed at a spot position on the optical axis c by the condenser lens 2.

In contrast, FIG. 3 shows how the phase of light changes when a voltage is applied to the refractive index modulation section of the optical coupling device 10. In FIG. 3, parts corresponding to those in FIG. For example, a voltage is applied to the refractive index modulation unit made of liquid crystal so as to generate a voltage gradient in one direction within a plane orthogonal to the optical axis, for example, a direction indicated by an arrow a in FIG. At this time, the refractive index of the refractive index modulator such as liquid crystal has a distribution in the direction indicated by the arrow a.
Corresponding to this refractive index distribution, the phase distribution of the light Lo after passing through the optical coupling device 10 has a phase difference d due to the optical path difference caused by the refractive index difference of the liquid crystal, and the equiphase wavefront p ′ It will incline with respect to an axis | shaft along the direction shown by the arrow a.
Then, the light of the tilted equiphase wavefront p ′ is shifted by, for example, δ from the optical axis c according to the tilt of the equiphase wavefront p ′ at the spot position collected by the condenser lens 2.
Accordingly, by appropriately selecting the magnitude of the voltage applied to the refractive index modulation unit of the optical coupling device 10 and the direction in which the voltage gradient is generated, the spot position condensed by the condenser lens 2 is shifted to an arbitrary position. You can see that
Note that the refractive index modulation unit may be configured using a ferroelectric crystal such as LiNbO ₃ in addition to the liquid crystal material, and the phase of light may be changed by an electro-optic effect. It goes without saying that other materials that cause a change in refractive index corresponding to a voltage gradient can also be used.

4 and 5 are diagrams showing a schematic plan configuration of an electrode structure portion of an embodiment of the optical coupling device according to the present invention.
In this example, by providing two pairs of electrode structures, voltage gradients are generated in two directions in a plane orthogonal to the optical axis, and the focused spot position can be adjusted two-dimensionally. Is.

  First, as shown in FIG. 4, a pair of electrodes, that is, in this case, a metal or other low-resistance material is formed on a substrate 11 made of one glass or the like along two opposite sides of the substrate 11. The low resistance conductive portions 13A and 13B are formed in a shape extending in parallel with each other in the direction indicated by the arrow y in FIG. 4 (left and right in FIG. 4). These are connected by a planar high-resistance conductive portion 14 made of thin-film ITO (indium-tin composite oxide) or the like to constitute the first electrode structure portion 12. In this case, when a voltage is applied from the terminals 13a and 13b of the low resistance conductive portions 13A and 13B, a voltage gradient is generated in the direction indicated by the arrow x (up and down in FIG. 4). In FIG. 4, a circular region D indicated by hatching indicates a region through which the light beam passes.

  On the other hand, as shown in FIG. 5, on the other substrate 21 made of glass or the like, a second electrode structure portion that generates a voltage gradient in a direction orthogonal to the first electrode structure portion 12 shown in FIG. 22 is provided. That is, in this case, the low-resistance conductive films 23A and 23B extending in the direction indicated by the arrow x in FIG. The low resistance conductive films 23A and 23B are connected by a planar high resistance conductive film 24 made of ITO or the like to form the second electrode structure portion 22. In the illustrated example, in order to derive the terminals 23a and 23b of the low resistance conductive films 23A and 23B, the low resistance conductive films 23A and 23A are arranged in the direction indicated by the arrow y orthogonal to the direction indicated by the arrow x. The example which extended 23B is shown. Also in FIG. 5, a circular region D indicated by hatching indicates a region through which the light beam passes.

  An example of the waveform of the voltage applied to the low resistance conductive films 13A and 13B on one substrate 11 shown in FIG. 4 is shown in FIGS. In this example, the input signal is an AC waveform, and an AC voltage with an amplitude V1 is applied from the terminal 13a as shown in FIG. 6, and an AC voltage with an amplitude V2 is applied from the terminal 13b as shown in FIG. As described above, the reason why the AC voltage is applied is that, when a liquid crystal material is used as the refractive index modulation unit, the liquid crystal material is damaged when a DC (direct current) voltage is applied to the liquid crystal material. For example, if the thickness of the liquid crystal layer is about 10 μm, the refractive index of the liquid crystal material can be adjusted according to the amplitude of the applied voltage without being affected by the frequency of the AC waveform by applying a voltage of about 500 Hz to 1000 Hz. It is known that the refractive index is constant.

  A pair of substrates 11 and 12 having the structure shown in FIGS. 4 and 5 is deposited with the electrode structure portions 12 and 22 facing each other and sandwiching a refractive index modulation portion made of, for example, a liquid crystal material, A coupling device can be constructed. FIG. 8A shows a schematic side view of the optical coupling device having such a configuration as viewed from the direction indicated by the arrow b in FIGS. 4 and 5. Here, the case where the AC voltages having the waveforms shown in FIGS. 6 and 7 are respectively applied to the terminals 13a and 13b on the substrate 11 and the terminals 23a and 23b on the other substrate 21 are grounded (to 0V). Think. FIG. 8B and FIG. 8C show the voltage gradient and refractive index distribution in the refractive index modulation unit 20 in the plane of the side surface shown in FIG. 8A at this time. As shown in FIG. 8B, a linear voltage gradient is formed between both terminals 13a and 13b sandwiching the high-resistance conductive film, and as shown in FIG. 8C, the refractive index n changes, that is, the optical path length nd changes. I understand that That is, the refractive index changes so as to have a gradient according to the magnitude of each applied voltage difference V1-V2 between the terminals 13a and 13b. That is, since the refractive index modulation part such as a liquid crystal material has a refractive index gradient corresponding to the magnitude of V1-V2, the phase of the light transmitted through this optical coupling device is the magnitude of V1-V2. It will have a corresponding phase gradient.

  FIG. 9 shows how the phase of the light transmitted through the optical coupling device changes in this case. The equiphase wavefront p ′ of the light transmitted through the optical coupling device 20 is adjusted depending on the voltages V1 and V2. Therefore, when the transmitted light is condensed by the condensing lens, the condensing spot position moves in accordance with the inclination of the equiphase wavefront, so that the light applied to the optical coupling device is condensed with good accuracy. The spot position can be adjusted. In FIG. 9, parts corresponding to those in FIG.

In the above example, an example in which a voltage is applied only to the terminals 13a and 13b of the first electrode structure 12 formed on one substrate 11 to adjust the focused spot position is shown. It is also possible to apply a voltage to the terminals 23a and 23b of the second electrode structure part 22 formed in the above to move the spot in the direction of the voltage gradient generated by the second electrode structure part 22. An example of the waveform of the voltage applied to the terminal 23a is shown in FIG. When such an AC voltage is applied to the terminal 23a and the terminal 23b is grounded, for example, a voltage gradient is generated between the terminals 23a and 23b, thereby changing the refractive index of the refractive index modulation unit such as a liquid crystal material, thereby reducing the optical path length. It is possible to change and move the focused spot in the direction of the voltage gradient.
Note that the movement of the spot in this direction is a phenomenon that occurs regardless of whether the voltages V1 and V2 applied to the terminals 13a and 13b are equal or not. Therefore, the direction of the terminals 13a and 13b Independently, the spot movement in the direction of the terminals 23a and 23b can be performed.

  Further, in this case, assuming that the refractive index modulation unit 20 such as a liquid crystal material has a threshold voltage Vth that causes a change in refractive index, the values of the smaller amplitudes of the voltages V1 and V2 applied to the first electrode structure unit 12 are The difference between the voltages V3 and V4 applied to the other second electrode structure 22 and the larger value of the amplitude needs to be Vth or more. By selecting the voltages V1 to V4 in this way, the equiphase wavefront of the light beam transmitted through the refractive index modulation unit such as a liquid crystal material can be changed linearly with respect to the voltage difference of the input voltage.

Next, each example of the optical waveguide part in which the optical coupling device of the present invention can be used is shown in the schematic sectional configuration diagram of FIGS. 11 to 13 and the schematic configuration diagram of FIG. Each example shows a configuration example of the above-described silicon photonics type optical waveguide portion.
FIG. 11 shows a so-called ridge-type optical waveguide in which a SiO ₂ portion 42 and a Si portion 43 having a lower refractive index are stacked on a Si portion 41, and a position corresponding to the waveguide of the Si portion 43 is a convex portion 43a. A waveguide part is shown.
Further, FIG. 12 shows an optical waveguide portion in which a SiO ₂ portion 44 and a Si portion 45 are further formed on the Si portion 43 having the convex portion 43a, and a so-called SIMOX (Separation by Implanted Oxygen) type can be formed. Each example of a strip type optical waveguide part having a so-called silicon wire optical waveguide in which a SiO ₂ part 4 is laminated on a Si part 41 and a line-like Si part 43 is formed thereon is shown.
FIG. 14 shows an inverse tapered optical waveguide type optical waveguide portion in which a tapered portion 43t is formed at the end portion of the Si portion 43 of the silicon wire optical waveguide, that is, the light input / output portion. In this case, the emitted light has a shape in which the light beam spreads outward from the tip of the taper portion 43t, as indicated by a broken line arrow Ld. By combining with the optical coupling device of the configuration of the present invention, more efficient coupling is possible, and by making the spot adjustable, the distance from the tip of the tapered portion 43t to the end face of the optical waveguide portion can be reduced. Is also possible.

Thus, when an optical waveguide unit using Si and SiO ₂ is provided in the optical waveguide device of the present invention, light can be coupled without impairing coupling efficiency even if the area of the optical coupling end face is extremely small. .
That is, in the optical coupling device of the present invention, it is possible to adjust the focal spot position with good accuracy, so that it is possible to form a fine waveguide pattern in various optical waveguides including such a silicon photonics type optical waveguide, In the case of using a silicon photonics type waveguide, an optical waveguide device that can be combined with various semiconductor devices using a Si substrate can be easily assembled and manufactured.

Next, as an example, the result of studying the amount of movement of the spot when a liquid crystal material is used as the refractive index adjusting unit will be described.
FIG. 15 shows the relationship between the numerical aperture NA of the condenser lens and the spot size when laser light having a wavelength of 1550 nm is used as incident light. 16 and 17 show the relationship of the amount of movement of the spot position according to the phase difference amount with respect to the numerical aperture NA of the condenser lens. In FIG. 16, one scale on the vertical axis is 1 μm, and in FIG. 17, one scale on the vertical axis is 0.2 μm.
From these results, in the optical coupling device according to the present invention, when a liquid crystal material is used as the refractive index adjusting unit, for example, in an optical system using a condensing lens having a numerical aperture NA of about 0.4, a collection with a diameter of about 5 μm is used. It can be seen that the light spot can be moved by about 5 μm, and in the optical system using a condensing lens having a numerical aperture NA of about 0.8, a condensing spot having a diameter of about 2 μm can be moved by about 2 μm.
As described above, even in an optical system having a very high numerical aperture NA of 0.8, the spot position can be moved by about 2 μm. It shows that the coupling with an external optical component such as a condenser lens can be set to an optimum value, that is, with a desired coupling efficiency.

  In the above-described embodiment, the example of adjusting the position of the light beam incident on the optical waveguide portion from the condensing lens is shown in the optical coupling device. It is also possible to adjust the position of the condensing spot by adjusting the phase of a light beam such as a laser beam that is arranged between the lens and the optical waveguide portion and is condensed.

8 and FIG. 9 shows an example in which one liquid crystal layer is provided as the refractive index adjustment unit of the optical coupling device. Since an unpolarized liquid crystal material is not easy to produce at this time, in this example, it is necessary to provide a polarizing optical system in which the alignment direction of the liquid crystal material matches the polarization direction of incident light.
On the other hand, when the optical coupling device is configured by using two liquid crystal layers having the same voltage change characteristics and orthogonal orientation directions as the liquid crystal material, the same voltage is applied to the light in the deflection direction in all directions. The phase of transmitted light can be adjusted by changing the refractive index by generating a gradient. That is, in this case, depolarization can be performed when the focused spot is moved.

  As described above, according to the present invention, since the spot of the condenser lens can be moved without using mechanically moving parts, the mounting accuracy of the condenser lens can be reduced in the mounting process. In addition, even when the position of the condenser lens or the light source has moved after mounting, if the amount of movement of the spot is within the movable range by the optical coupling device of the present invention, the deviation of the spot position is reduced. You will be able to recover. Therefore, it is possible to reduce the cost by reducing the system of the mounting process, to reduce the manufacturing cost and the device cost, and it is also possible to relax the use conditions of the optical waveguide device.

  The optical coupling device and the optical waveguide device according to the present invention are not limited to the above-described embodiments, and various arrangements, materials, configurations, etc. of optical components are possible without departing from the configuration of the present invention. Needless to say, modifications and changes are possible.

1 is a schematic configuration diagram of an embodiment of an optical waveguide device according to the present invention. It is explanatory drawing of the principle of the optical waveguide coupling method by this invention. It is explanatory drawing of the principle of the optical waveguide coupling method by this invention. It is a schematic plane block diagram of the principal part of one embodiment of the optical coupling device by this invention. It is a schematic plane block diagram of the principal part of one embodiment of the optical coupling device by this invention. It is a figure which shows an example of the waveform of the voltage applied to the optical coupling device by this invention. It is a figure which shows an example of the waveform of the voltage applied to the optical coupling device by this invention. A is a schematic side view of an embodiment of an optical coupling device according to the present invention. B is a figure which shows an example of the voltage gradient produced in the optical coupling device of this invention. C is a figure which shows an example of the refractive index distribution in the optical coupling device of this invention. It is a figure which shows the phase change of the light in one Example of the optical coupling device by this invention. It is a figure which shows an example of the waveform of the voltage applied to the optical coupling device by this invention. It is a schematic sectional block diagram of an example of an optical waveguide part. It is a schematic sectional block diagram of an example of an optical waveguide part. It is a schematic sectional block diagram of an example of an optical waveguide part. It is a schematic block diagram of an example of an optical waveguide part. It is a figure which shows the relationship of the spot diameter with respect to the numerical aperture of a condensing lens. It is a figure which shows the relationship of the beam shift amount with respect to the numerical aperture of a condensing lens. It is a figure which shows the relationship of the beam shift amount with respect to the numerical aperture of a condensing lens.

Explanation of symbols

  1. 1. Optical waveguide part Condensing lens, 10. 10. optical coupling device; Substrate, 12. 1st electrode structure part, 13A. Low resistance conductive part, 13B. Low resistance conductive part, 14. High resistance conductive portion, 21. Substrate, 22. Second electrode structure, 23A. Low resistance conductive part, 23B. Low resistance conductive portion, 24. High resistance conductive part

Claims (5)

At least a pair of electrode structures;
Comprising a refractive index modulation section in which a refractive index distribution changes corresponding to a voltage gradient caused by voltage application to the electrode structure section,
An optical coupling device characterized in that, in the refractive index modulation section, the phase of incident light is changed and optically coupled to an external optical section. The optical coupling device according to claim 1, wherein the refractive index modulation unit is made of liquid crystal. The electrode structure comprises first and second electrode structures;
The optical coupling device according to claim 1, wherein the phase of the incident light is changed in at least two directions. An optical waveguide unit, a condenser lens, and an optical coupling device;
The optical coupling device includes at least a pair of electrode structures;
Comprising a refractive index modulation section in which a refractive index distribution changes corresponding to a voltage gradient caused by voltage application to the electrode structure section,
An optical waveguide device characterized in that, in the refractive index modulation section, the phase of incident light is changed to optically couple input / output light to / from the optical waveguide section to an external optical section. Applying voltage causes a voltage gradient to change the phase of the incident light,
An optical waveguide coupling method characterized by adjusting a condensing position by a condensing lens and optically coupling with an optical waveguide.

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