WO2018150813A1

WO2018150813A1 - Optical coupler and optical coupling method

Info

Publication number: WO2018150813A1
Application number: PCT/JP2018/001721
Authority: WO
Inventors: 毅小西
Original assignee: 国立大学法人大阪大学
Priority date: 2017-02-14
Filing date: 2018-01-22
Publication date: 2018-08-23
Also published as: JPWO2018150813A1; JP7141708B2

Abstract

This optical coupler for coupling input light (130) to an optical fiber (100), is provided with a diffraction optical element (10) formed in a core (110) of the optical fiber (100), wherein the input light (130) is incident on the diffraction optical element (10) from a direction perpendicular to the propagation axis of the optical fiber (100).

Description

Optical coupler and optical coupling method

The present invention relates to coupling of light to an optical waveguide.

Conventionally, an optical coupler that couples light to the optical waveguide by condensing the light on the end face of the core of the optical waveguide (for example, optical fiber, silicon waveguide (silicon photonics), etc.) has been used. A lens or a tapered waveguide is used for condensing light onto the end face of the core of the optical waveguide. For example, in patent document 1, the rod lens is provided in the end surface of the optical fiber.

JP-A-10-31128

However, in the above-described conventional technology, it is difficult to match the characteristics of the beam spot collected at the focal point of the lens with the parameters of the optical waveguide, and it is difficult to obtain good coupling efficiency. In particular, in optical coupling to a single mode optical fiber, it is necessary to match the intensity distribution of incident light with the distribution of the fiber mode, and it is difficult to obtain good optical coupling efficiency.

Moreover, in the said prior art, since incident light is condensed on the end surface of a core, an optical damage may be caused to the end surface of a core. The optical damage threshold per unit area of the core surface of the optical fiber is about 250 kW / cm ² . Therefore, when high-intensity light exceeding 0.5 W is coupled to a single mode optical fiber having a core diameter of about 10 μm, optical damage occurs on the end face of the core.

Therefore, the present invention provides an optical coupler capable of suppressing optical damage of the optical waveguide due to optical coupling and improving optical coupling efficiency.

An optical coupler according to an aspect of the present invention is an optical coupler that couples input light to an optical waveguide, the optical coupler including a region having a diffractive optical function formed in a core of the optical waveguide, Enters the region having the diffractive optical function from a direction perpendicular to the propagation axis of the optical waveguide.

According to this configuration, the input light can be coupled to the optical waveguide by making the input light incident on the diffractive optical element from a direction perpendicular to the propagation axis of the optical waveguide. Therefore, since it is not necessary to focus the input light on the end face of the core, it becomes possible to match the intensity distribution of the input light with the mode distribution of the optical waveguide by the region having the diffractive optical function, thereby improving the optical coupling efficiency. Can do. In addition, since it is not necessary to collect input light on the end face of the core, optical damage to the core can be suppressed. Further, since the diffractive optical element is formed inside the core, the optical coupling efficiency can be obtained no matter what direction the input light is incident on the region having the diffractive optical function from any direction as long as it is in a plane perpendicular to the propagation axis of the optical waveguide. The input light can be coupled to the optical waveguide while suppressing the variation of the above, and the degree of freedom of the environment in which the optical coupler is used can be increased. As a result, the optical coupler can be used even in an environment (for example, a light receiving portion of a microscope) in which restrictions on the installation location of the optical coupler are severe.

For example, the region having the diffractive optical function may satisfy a Bragg condition for the input light incident from a direction perpendicular to the propagation axis of the optical waveguide.

According to this configuration, the diffraction efficiency in the direction parallel to the propagation axis of the optical waveguide can be improved, and the optical coupling efficiency can be improved.

For example, the direction perpendicular to the propagation axis of the optical waveguide is a direction within a predetermined angular range from a direction strictly perpendicular to the propagation axis of the optical waveguide, and the predetermined angular range is a coupled wave. It may be in a range where the diffraction efficiency is 0.5 or more in theory (coupled-wave theory).

According to this configuration, input light can be incident on a region having a diffractive optical function within an angular range where the diffraction efficiency is 0.5 or more, and the optical coupling efficiency is improved while allowing a certain amount of error. be able to.

For example, the region having the diffractive optical function may be a fiber Bragg grating.

According to this configuration, a fiber Bragg grating can be used for the diffractive optical element. The fiber Bragg grating is most commonly used in an optical fiber sensor, and an optical coupler can be realized relatively easily.

For example, the lattice constant of the fiber Bragg grating may coincide with the wavelength of the input light.

According to this configuration, the lattice constant of the fiber Bragg grating can be matched with the wavelength of the input light. Therefore, the traveling direction of the diffracted light can be made closer to the direction parallel to the propagation axis of the optical waveguide, and the optical coupling efficiency can be further improved.

For example, the optical coupler may further include an input unit that makes the input light incident on a region having the diffractive optical function from a direction perpendicular to the propagation axis.

According to this configuration, the optical coupler can include an input unit that inputs the input light to the diffractive optical element from a direction perpendicular to the propagation axis of the optical waveguide. Thereby, the input light can be reliably incident on the diffractive optical element, and the optical coupling efficiency can be improved.

For example, the input unit may include a condensing element that condenses the input light in a linear shape extending in a direction parallel to the propagation axis.

According to this configuration, a condensing element that condenses input light in a linear shape extending in a direction parallel to the propagation axis can be used as the input unit. Therefore, the input light can be condensed linearly along the diffractive optical element formed in the core. As a result, the input light can be reliably incident on the diffractive optical element, and the optical coupling efficiency can be further improved.

For example, the input unit includes a dispersive element that disperses the input light in a direction parallel to the propagation axis, and the distribution of the lattice constant of the fiber Bragg grating is dispersed in the direction parallel to the propagation axis. It may correspond to the wavelength distribution of the input light.

According to this configuration, a dispersive element that disperses input light in a direction parallel to the propagation axis can be used as the input unit. Furthermore, in the direction parallel to the propagation axis, the distribution of the fiber Bragg grating lattice constant can be made to correspond to the distribution of the wavelength of the dispersed input light. Therefore, even for broadband input light, the wavelength of the input light can match the lattice constant of the fiber Bragg grating, and the shift between the traveling direction of the diffracted light and the direction of the propagation axis of the optical fiber can be reduced. Can do. That is, even for broadband input light, the diffracted light can be adapted to the mode of the optical fiber with higher accuracy, and the optical coupling efficiency can be improved.

For example, the optical coupler may further include an adjustment unit that adjusts a lattice constant of the fiber Bragg grating according to the wavelength of the input light.

According to this configuration, the fiber Bragg grating lattice constant can be adjusted according to the wavelength of the input light. Therefore, the range of wavelengths of input light that can be coupled can be expanded, and the versatility of the optical coupler can be improved.

For example, the optical coupler may further include a reflecting unit that reflects one of the lights diffracted in two directions parallel to the propagation axis by the region having the diffractive optical function in opposite directions.

According to this configuration, the light propagating through the core can be reflected. Therefore, one of the diffracted lights diffracted in both directions of the propagation axis can be reflected and combined with the other, and the optical coupling efficiency can be improved.

For example, the optical coupler may further include a phase compensation unit that compensates a phase of light coupled to the core by the region having the diffractive optical function.

According to this configuration, the phase of light coupled to the core can be compensated. Therefore, the phase shift of the combined light can be compensated, and for example, when the input light is pulsed light, a change in waveform due to optical coupling can be suppressed.

For example, the optical waveguide may be an optical fiber.

According to this configuration, input light can be coupled to the optical fiber.

The optical coupling method according to one aspect of the present invention is an optical coupling method for coupling input light to an optical waveguide, and is formed inside the core of the optical waveguide from a direction perpendicular to the propagation axis of the optical waveguide. An input step of inputting light to the region having the diffractive optical function, and a diffraction step of coupling light to the optical waveguide by diffracting the light input to the region having the diffractive optical function.

According to this, the same effect as the above optical coupler can be obtained.

The optical coupler according to one embodiment of the present invention can suppress optical damage of the optical waveguide due to optical coupling and improve optical coupling efficiency.

FIG. 1 is a perspective view showing the configuration of the optical coupler according to Embodiment 1. FIG. FIG. 2 is a diagram illustrating light diffraction by a general diffraction grating having a lattice constant d. FIG. 3 is a flowchart showing an optical coupling method using the optical coupler according to the first embodiment. FIG. 4A is a diagram showing an outline of an optical coupling experiment. FIG. 4B is a graph showing the relationship between the incident angle and the intensity of the combined light in the optical coupling experiment. FIG. 5 is a perspective view showing the configuration of the optical coupler according to the second embodiment. FIG. 6 is a perspective view showing the configuration of the optical coupler according to Embodiment 3. FIG. FIG. 7 is a diagram illustrating a simulation model and a simulation result of the optical coupler according to the third embodiment. FIG. 8A is a graph showing the relationship between the core diameter and the optical coupling efficiency in the simulation result of the optical coupler according to Embodiment 3. FIG. 8B is a graph showing the relationship between the core diameter and the optical damage threshold in the simulation result of the optical coupler according to Embodiment 3. FIG. 9 is a side view showing the configuration of the optical coupler according to the fourth embodiment. FIG. 10 is a side view illustrating the configuration of the optical coupler according to the fifth embodiment. FIG. 11 is a side view showing the configuration of the optical coupler in accordance with the sixth embodiment.

Hereinafter, embodiments will be specifically described with reference to the drawings.

It should be noted that each of the embodiments described below shows a comprehensive or specific example. Numerical values, shapes, materials, components, arrangement positions and connection forms of components, steps, order of steps, and the like shown in the following embodiments are merely examples, and are not intended to limit the scope of the claims. In addition, among the constituent elements in the following embodiments, constituent elements that are not described in the independent claims indicating the highest concept are described as optional constituent elements.

Each figure is a schematic diagram and is not necessarily shown strictly. Moreover, in each figure, the same code | symbol is attached | subjected about the substantially same structure, and the overlapping description is abbreviate | omitted or simplified.

In addition, in the following, the terms coincidence, vertical and parallel are used, but unless specifically limited, these are not strict and are used in a substantial sense. For example, “match” not only means that they are completely matched, but also includes a range that can be considered to be substantially matched. That is, “match” allows an error of about several percent. Further, for example, “vertical” means not only strictly vertical but also includes a range that can be regarded as substantially vertical. Further, for example, “parallel” not only means strictly parallel, but also includes a range that can be regarded as substantially parallel.

(Embodiment 1)
The first embodiment will be described below with reference to the drawings.

[Configuration of optical coupler]
FIG. 1 is a diagram illustrating a configuration of the optical coupler according to the first embodiment. In FIG. 1 and subsequent figures, the X-axis direction is a direction parallel to the propagation axis of the optical fiber, and the Y-axis direction and the Z-axis method are directions orthogonal to the propagation axis of the optical fiber.

The optical fiber 100 is an example of an optical waveguide and is a medium for transmitting an optical signal. In the present embodiment, the optical fiber 100 is a single mode optical fiber and includes a core 110 and a cladding 120.

The core 110 is made of, for example, quartz glass or plastic, and is surrounded by the clad 120. The diameter of the core 110 is, for example, about 10 μm to 100 μm.

The clad 120 is made of, for example, quartz glass or plastic, and has a lower refractive index than the core 110. The outer diameter of the clad 120 is, for example, about 30 μm to 500 μm.

A region having a diffractive optical function (hereinafter referred to as a diffractive optical element 10) is formed inside the core 110. The input light 130 enters the diffractive optical element 10 from a direction perpendicular to the propagation axis of the optical fiber 100 (Z-axis direction in FIG. 1). The input light 130 is diffracted by the diffractive optical element 10. As a result, the diffracted light 140a and 140b (also referred to as coupled light) propagates in a direction parallel to the propagation axis (X-axis direction in FIG. 1).

The direction perpendicular to the propagation axis of the optical fiber 100 may be a direction within a predetermined angle range from a direction strictly perpendicular to the propagation axis of the optical fiber 100. This predetermined angle range can be defined based on the coupled wave theory described in Non-Patent Document 1.

For example, the predetermined angular range may be a range in which the diffraction efficiency is equal to or higher than a predetermined threshold efficiency in the coupled wave theory. As the predetermined threshold efficiency, for example, 0.5 can be used, preferably 0.6 can be used, and more preferably 0.7 can be used. If a value equal to or greater than 0.5 is used as the predetermined threshold efficiency, it is possible to realize a higher coupling efficiency than before.

The diffractive optical element 10 satisfies the Bragg condition for the input light 130 incident from a direction perpendicular to the propagation axis of the optical fiber 100. The Bragg condition is expressed by Equation 1 below.

Here, d represents the lattice constant of the diffractive optical element 10, θ represents the angle formed by the propagation axis of the optical fiber 100 and the input light 130, λ represents the wavelength of the input light 130, and n represents an integer.

That is, in this embodiment, Formula 1 is satisfied at θ = 90 degrees. Note that satisfying the Bragg condition does not only mean that Expression 1 is strictly satisfied, but may mean that Expression 1 is substantially satisfied. In other words, in Formula 1, even when a minute error is included, it may be interpreted that the Bragg condition is satisfied.

In the present embodiment, the diffractive optical element 10 is a fiber Bragg grating (FBG). The FBG is manufactured by, for example, irradiating an optical fiber with ultraviolet laser light and periodically changing the refractive index inside the core having sensitivity to ultraviolet light in a direction parallel to the propagation axis.

The lattice constant d of the diffractive optical element 10 (FBG) matches the wavelength λ of the input light 130. By this diffractive optical element 10, the propagation direction of the input light 130 is converted from the incident direction (Z-axis direction in FIG. 1) to a direction perpendicular to the incident direction (X-axis direction in FIG. 1).

Here, the principle of changing the light propagation direction to a direction perpendicular to the incident direction will be described. FIG. 2 is a diagram illustrating light diffraction by a general diffraction grating having a lattice constant d. As shown in FIG. 2, when light of wavelength λ is incident on the diffraction grating at angle θ ₁ , diffracted light of angle θ ₂ is generated. Since the wavefront phases of the incident light and the diffracted light are equal, the following Expression 2 is established for the + 1st order and −1st order diffracted lights.

By substituting θ ₁ = 0 and d = λ into Equation 2 above, θ ₂ = π / 2, −π / 2 is derived. That is, in FIG. 1, when the input light 130 having the wavelength λ (= d) from the Z-axis direction (θ ₁ = 0) is incident on the diffractive optical element 10 having the grating constant d, the diffracted

lights

140a and 140b are It proceeds in the positive and negative directions (θ ₂ = π / 2, −π / 2) in the axial direction.

Note that the incident direction of the input light 130 is not limited to the Z-axis direction. The input light 130 may be incident on the diffractive optical element 10 from a direction perpendicular to the X-axis direction (for example, the Y-axis direction). Even in this case, the diffracted

lights

140a and 140b travel in the positive and negative directions in the X-axis direction based on the above equation 2.

[Optical coupling method]
Next, an optical coupling method using the optical coupler configured as described above will be described. FIG. 3 is a flowchart showing the optical coupling method according to the first embodiment.

First, the input light 130 having a wavelength that matches the lattice constant is incident on the diffractive optical element 10 formed inside the core 110 of the optical fiber 100 (S110). The input light 130 is diffracted by the diffractive optical element 10 (S120). That is, the input light 130 is converted into diffracted light 140 a and 140 b that propagates in a direction parallel to the propagation axis of the optical fiber 100.

[effect]
As described above, according to the optical coupler in accordance with the present embodiment, the input light 130 is incident on the diffractive optical element 10 from the direction perpendicular to the propagation axis of the optical fiber 100 (for example, the Z-axis direction). Input light 130 can be coupled to the fiber 100. Accordingly, since it is not necessary to collect the input light 130 on the end face of the core 110, the intensity distribution of the input light 130 can be matched with the mode distribution of the optical fiber 100 by the diffractive optical element 10, and the optical coupling efficiency is improved. Can be made. In addition, since it is not necessary to collect the input light 130 on the end face of the core 110, optical damage to the core 110 can be suppressed. In addition, since the diffractive optical element 10 is formed inside the core 110, the input light 130 is transmitted from which direction (for example, the Y-axis direction and the Z-axis direction) 360 degrees within a plane perpendicular to the propagation axis of the optical fiber 100. Even if the light enters the diffractive optical element 10, the input light 130 can be coupled to the optical fiber 100 while suppressing variations in optical coupling efficiency, and the degree of freedom of the environment in which the optical coupler is used can be increased. As a result, the optical coupler according to the present embodiment can be used even in an environment (for example, a light receiving part of a microscope) in which restrictions on the installation location of the optical coupler are severe.

Further, according to the optical coupler according to the present embodiment, the diffractive optical element 10 can satisfy the Bragg condition for the input light 130 incident from a direction perpendicular to the propagation axis of the optical fiber 100. Therefore, the diffraction efficiency of the input light 130 in the direction parallel to the propagation axis of the optical fiber 100 can be improved, and the optical coupling efficiency can be improved.

Further, according to the optical coupler according to the present embodiment, the input light can be incident on the region having the diffractive optical function within an angle range where the diffraction efficiency is 0.5 or more, and a certain amount of error is allowed. However, the optical coupling efficiency can be improved.

In addition, according to the optical coupler according to the present embodiment, FBG can be used for the diffractive optical element 10. The FBG is most commonly used in an optical fiber sensor, and the diffractive optical element 10 can be formed in the core 110 relatively easily. Therefore, the optical coupler according to the present embodiment can be realized relatively easily.

Moreover, according to the optical coupler according to the present embodiment, the lattice constant of the FBG can be matched with the wavelength of the input light. Therefore, the traveling direction of the diffracted light can be made closer to the direction parallel to the propagation axis of the optical fiber 100, and the optical coupling efficiency can be further improved.

[Experimental result]
Here, an experiment will be described in which input light incident from a direction perpendicular to the propagation axis is coupled to an optical fiber using a diffractive optical element formed inside the core. FIG. 4A is a diagram showing an outline of an optical coupling experiment.

In this experiment, a slab waveguide was used as the optical waveguide as shown in FIG. 4A. A diffraction grating film sheet was used as the core and the diffractive optical element formed inside the core. The thickness of the diffraction grating film sheet was about 120 μm, and the lattice constant was 1.0 μm. Further, a glass plate was used as the cladding. The wavelength of the input light incident on the slab waveguide from the light source was about 1.6 μm. Since the refractive index of the diffraction grating film sheet is high, input light having a wavelength of 1.6 μm, which is slightly larger than the lattice constant of 1.0 μm, was used. Input light was incident on such a slab waveguide at a plurality of incident angles θ, and the intensity of the coupled light was measured with an intensity measuring device at each of the plurality of incident angles.

FIG. 4B is a graph showing the relationship between the incident angle and the intensity of the coupled light in the optical coupling experiment. As is clear from FIG. 4B, in this experiment, the intensity of the coupled light was highest when the incident angle θ was near 90 degrees. At this time, the optical coupling efficiency was 90% or more. That is, when the input light is incident on the diffractive optical element from a direction perpendicular to the propagation axis, the input light can be coupled to the waveguide with high optical coupling efficiency.

(Embodiment 2)
Next, a second embodiment will be described. The present embodiment is mainly different from the first embodiment in that the optical coupler includes a condensing element that condenses input light in a linear shape extending in a direction parallel to the propagation axis of the optical waveguide. Hereinafter, the optical coupler according to the present embodiment will be described in detail with reference to the drawings, centering on differences from the first embodiment.

[Configuration of optical coupler]
FIG. 5 is a perspective view showing the configuration of the optical coupler according to the second embodiment. As shown in FIG. 5, the optical coupler according to the present embodiment includes a condensing element 20 in addition to the diffractive optical element 10.

The condensing element 20 is an example of an input unit that inputs the input light 131a to the diffractive optical element 10 from a direction perpendicular to the propagation axis of the optical fiber 100, and is, for example, a cylindrical lens. The condensing element 20 condenses the input light 131a in a linear shape extending in a direction parallel to the propagation axis of the optical fiber 100 (X-axis direction). The linear light 131b enters the diffractive optical element 10 from a direction perpendicular to the propagation axis of the optical fiber 100 (Z-axis direction).

[effect]
As described above, according to the optical coupler in accordance with the present embodiment, the condensing element 20 that condenses the input light 131a in a linear shape extending in a direction parallel to the propagation axis (X-axis direction) can be provided. . Therefore, the input light 131a can be condensed linearly along the diffractive optical element 10 formed on the core 110 (linear light 131b). As a result, the input light 131a can be reliably incident on the diffractive optical element 10, and the optical coupling efficiency can be further improved.

(Embodiment 3)
Next, Embodiment 3 will be described. The present embodiment is mainly different from the first embodiment in that the reflection portion formed on the end face of the core is included in the optical coupler. Hereinafter, the optical coupler according to the present embodiment will be described in detail with reference to the drawings, centering on differences from the first embodiment.

[Configuration of optical coupler]
FIG. 6 is a perspective view showing the configuration of the optical coupler according to Embodiment 3. FIG. As shown in FIG. 6, the optical coupler according to the present embodiment includes a reflecting unit 30 in addition to the diffractive optical element 10.

The reflection unit 30 is provided on the end surface of the core 110, and reflects the diffracted light 140a propagating through the core 110 toward the reflection unit 30. That is, the reflecting unit 30 is one of the light (diffracted light 140a and 140b in the present embodiment) diffracted by the diffractive optical element 10 in two directions parallel to the propagation axis (diffracted light 140a in the present embodiment). ) Is reflected in the opposite direction. As a result, the traveling direction of the diffracted light 140a is reversed, and the diffracted light 140a is combined with the diffracted light 140b.

In FIG. 6, the reflection unit 30 reflects the light propagating in the negative direction in the X-axis direction in the positive direction in the X-axis direction. Specifically, the reflection part 30 is a metal film (for example, aluminum etc.) which covers the end surface of the core 110, for example.

[effect]
As described above, according to the optical coupler according to the present embodiment, the light propagating through the core 110 can be reflected by the end face of the core 110. Therefore, one (diffracted light 140a) of the diffracted light diffracted in both directions of the propagation axis can be combined with the other (diffracted light 140b), and the optical coupling efficiency can be improved.

Here, the effect will be described using the simulation result of the optical coupler according to the present embodiment. FIG. 7 is a diagram illustrating a simulation model and a simulation result of the optical coupler according to the third embodiment. Specifically, FIG. 7A is a perspective view of a three-dimensional model of the optical coupler in the simulation. FIG. 7B is a cross-sectional view of the three-dimensional model of the optical coupler in the simulation. FIGS. 7C and 7D are diagrams showing light intensity distributions after 130 femtoseconds and 1000 femtoseconds in the cross section shown in FIG. 7B. 7C and 7D show that the closer to white, the higher the light intensity, and the closer to black, the lower the light intensity.

In this simulation, a finite element time difference method (FDTD) is used, and a linear (two-dimensional) input light from a light source is a propagation axis of an optical fiber as shown in FIG. The light was incident on the diffractive optical element formed inside the core through the clad from the direction perpendicular to. At this time, the length in the direction of the propagation axis of the core region (that is, the region where the diffractive optical element is formed) on which the input light is incident was set to 1 cm. In addition, a reflection portion was provided on one end face of the core, the wavelength of the input light was 1.485 μm, and the lattice constant of the diffractive optical element was 1 μm.

Referring to FIG. 7D, the input light incident on the diffractive optical element from the direction perpendicular to the propagation axis of the optical fiber has been converted by the diffractive optical element into coupled light traveling in a direction parallel to the propagation axis. I understand.

This simulation was performed a plurality of times while changing the diameter of the core. FIG. 8A is a graph showing the relationship between the core diameter and the optical coupling efficiency. FIG. 8B is a graph showing the relationship between the core diameter and the intensity of input light causing optical damage. In FIG. 8B, 250 kW / cm ² was used as the optical damage threshold.

FIG. 8A shows that an optical coupling efficiency of 60% or more can be realized at a core diameter of 10 μm or more. FIG. 8B shows that input light having a higher intensity than that of the prior art can be coupled to the optical fiber without optical damage.

As described above, this simulation shows that the optical coupler according to the present embodiment can achieve high optical coupling efficiency and can increase the intensity of input light that can be coupled without optical damage as compared with the prior art. It was.

(Embodiment 4)
Next, a fourth embodiment will be described. The present embodiment is mainly different from the first embodiment in that the optical coupler includes a dispersive element that disperses input light in a direction parallel to the propagation axis of the optical waveguide. Hereinafter, the optical coupler according to the present embodiment will be described in detail with reference to the drawings, centering on differences from the first embodiment.

[Configuration of optical coupler]
FIG. 9 is a side view showing the configuration of the optical coupler according to the fourth embodiment. As shown in FIG. 9, the optical coupler according to the present embodiment includes a diffractive optical element 11, a dispersion element 40, and a lens 41.

The dispersion element 40 is an example of an input unit that inputs the input light 132a to the diffractive optical element 11 from a direction perpendicular to the propagation axis of the optical fiber 100, and is, for example, a diffraction grating. The dispersion element 40 disperses the input light 132a in a direction (X-axis direction) parallel to the propagation axis of the optical fiber 100. That is, the dispersive element 40 separates the input light 132a at different positions in the X-axis direction according to the frequency of the input light 132a. The dispersed light 132b includes, for example, red dispersed light 132bR, green dispersed light 132bG, and blue dispersed light 132bB.

The lens 41 refracts the dispersed light 132b from the dispersion element 40 and makes the dispersed light 132b enter the diffractive optical element 11 from a direction perpendicular to the propagation axis of the optical fiber 100 (Z-axis direction).

The diffractive optical element 11 is an FBG. In the direction parallel to the propagation axis of the optical fiber 100 (X-axis direction), the distribution of the lattice constant of the FBG corresponds to the wavelength distribution of the dispersed light 132b. That is, the lattice constant of the FBG continuously changes in the X-axis direction so that the wavelength of the dispersed light 132b incident on the FBG matches the lattice constant of the FBG. For example, as shown in FIG. 9, the lattice constant of the region where the red dispersed light 132bR is incident is larger than the lattice constant of the region where the blue dispersed light 132B is incident.

[effect]
As described above, according to the optical coupler in accordance with the present embodiment, the dispersive element 40 that disperses the input light 132a in the direction parallel to the propagation axis (X-axis direction) can be provided. Furthermore, in the direction parallel to the propagation axis (X-axis direction), the distribution of the lattice constant of the FBG can correspond to the wavelength distribution of the dispersed light 132b. Therefore, even for broadband input light, the wavelength of the dispersed light 132b and the lattice constant of the FBG can be matched with high accuracy, and the deviation between the traveling direction of the diffracted light and the direction of the propagation axis of the optical fiber is reduced. can do. That is, even for broadband input light, the diffracted light can be adapted to the mode of the optical fiber 100 with higher accuracy, and the optical coupling efficiency can be improved.

(Embodiment 5)
Next, a fifth embodiment will be described. When the input light is not a single wavelength, the input light incident on the diffractive optical element is diffracted at different angles depending on the wavelength. Therefore, the phase of the diffracted light is shifted depending on the wavelength. As a result, when the input light is pulsed light, the waveform of the pulsed light changes. Therefore, the optical coupler according to the present embodiment includes a phase compensator for compensating for a phase shift due to wavelength. Hereinafter, the optical coupler according to the present embodiment will be described in detail with reference to the drawings, centering on differences from the first embodiment.

[Configuration of optical coupler]
FIG. 10 is a side view illustrating the configuration of the optical coupler according to the fifth embodiment. As shown in FIG. 10, the optical coupler according to the present embodiment includes a phase compensation unit 50 in addition to the diffractive optical element 10.

The phase compensation unit 50 compensates the phase of the light coupled to the core 110 by the diffractive optical element 10. That is, the phase compensation unit 50 compensates for the phase shift of the diffracted light due to the wavelength. In the present embodiment, the phase compensation unit 50 includes an optical circulator 51 and an FBG 52.

The optical circulator 51 is a 3-port type optical circulator. The optical circulator 51 guides light that has entered the optical circulator 51 from the optical fiber 100 to the optical fiber 101. Further, the optical circulator 51 guides the light that has entered the optical circulator 51 from the optical fiber 101 to the optical fiber 101. That is, the diffracted light 140 b from the diffractive optical element 10 is guided to the FBG 52, and the reflected light from the FBG 52 is guided to the optical fiber 102.

Note that the

optical fibers

100, 101, and 102 may be referred to as first, second, and third optical fibers, respectively.

The FBG 52 is formed inside the core of the optical fiber 101. The FBG 52 has a lattice constant that changes in the direction of the propagation axis of the optical fiber 101, and reflects light having a wavelength corresponding to the lattice constant at that position at each position in the direction of the propagation axis. That is, the optical path length in the optical fiber 101 differs depending on the wavelength of the diffracted light 140b. As a result, the phase shift of the diffracted light 140b is compensated.

[effect]
As described above, according to the optical coupler according to the present embodiment, the phase of the light coupled to the core 110 can be compensated. Therefore, the phase shift of the combined light can be compensated, and for example, when the input light is pulsed light, a change in waveform due to optical coupling can be suppressed.

(Embodiment 6)
Next, a sixth embodiment will be described. The present embodiment is mainly different from Embodiment 1 in that an adjustment unit that adjusts the lattice constant of the FBG according to the wavelength of input light is included in the optical coupler. Hereinafter, the optical coupler according to the present embodiment will be described in detail with reference to the drawings, centering on differences from the first embodiment.

[Configuration of optical coupler]
FIG. 11 is a side view showing the configuration of the optical coupler in accordance with the sixth embodiment. As shown in FIG. 11, the optical coupler according to the present embodiment includes an adjustment unit 60.

The adjusting unit 60 adjusts the lattice constant of the diffractive optical element 10 (FBG) according to the wavelength of the input light 130. For example, the adjustment unit 60 is a heater, and adjusts the lattice constant of the FBG by heating the optical fiber 100. Further, for example, the adjustment unit 60 is an actuator, and adjusts the lattice constant of the FBG by applying an external force to the optical fiber 100.

[effect]
As described above, according to the optical coupler according to the present embodiment, the lattice constant of the FBG (diffractive optical element 10) can be adjusted according to the wavelength of the input light 130. Therefore, the range of wavelengths of input light that can be coupled can be expanded, and the versatility of the optical coupler can be improved.

(Other embodiments)
Although the optical coupler according to one or more aspects of the present invention has been described based on the embodiment, the present invention is not limited to this embodiment. Unless it deviates from the gist of the present invention, one or more of the present invention may be applied to various modifications that can be conceived by those skilled in the art, or forms constructed by combining components in different embodiments. It may be included within the scope of the embodiments.

For example, both the light collecting element in the second embodiment and the dispersion element in the fourth embodiment may be included in the optical coupler. Thereby, further improvement in coupling efficiency can be expected.

Further, both the dispersion element in the fourth embodiment and the phase compensation unit in the fifth embodiment may be included in the optical coupler. Thereby, even if the input light is a broadband pulse light, the input light can be coupled to the optical waveguide while suppressing a change in waveform.

In each of the above embodiments, the case where the optical waveguide is an optical fiber has been described. However, the optical waveguide is not limited to an optical fiber. For example, the optical waveguide may be a silicon waveguide formed by silicon photonics. In this case, the cladding of the optical waveguide may be replaced by an air layer.

In each of the above embodiments, the diffractive optical element is FBG. However, the present invention is not limited to this. For example, the diffractive optical element may be realized by a hologram. Hereinafter, the fact that a hologram can be applied to a diffractive optical element will be described.

The amplitude transmittance H (x, y, z) of the hologram formed by the superposition (interference) of the reference wave A _in (x, y, z) and the object wave A ₀ (x, y, z) is This can be expressed by the following formula 3.

Here, γ represents a constant. When this reference wave A _in (x, y, z) is used as a reproduction wave, the transmitted wave A _out (x, y, z) from the hologram is expressed by the following Expression 4.

In Equation 4, the first term (A1) represents straight light, the second term (A2) represents a direct image (virtual image), and the third term (A3) represents a conjugate image (real image). Therefore, by appropriately setting the reference wave A _in (x, y, z) and the object wave A ₀ (x, y, z), the traveling direction of the reproduced wave A _in (x, y, z) Holograms can be formed that produce images and conjugate images directly in the vertical direction. In other words, the hologram formed inside the core is diffracted light that travels in a direction parallel to the propagation axis when input light (reproduced wave) is incident from a direction perpendicular to the propagation axis of the core. A direct image and a conjugate image) can be emitted.

In the second embodiment, the optical coupler includes the light condensing element, but the light condensing element is not necessarily provided. For example, when the input light is linear light, it is not necessary to collect the input light. Also, the clad can collect the input light, and when the input light is not linear light, the optical coupling efficiency can be improved even when there is no condensing element.

In Embodiment 3 described above, the reflective portion is provided on the end surface of the core, but the reflective portion is not necessarily provided on the end surface of the core. For example, the reflecting portion may be an FBG formed inside the core. Even in this case, the diffracted light propagating through the core can be reflected in the opposite direction. The same effect as that of the reflecting portion can be realized by using an optical coupler that couples two optical waveguides (optical fibers) to one optical waveguide (optical fiber).

As is clear from the simulation result of the third embodiment (FIG. 8A), the optical coupling efficiency increases as the diameter of the core on which the diffractive optical element is formed increases. Therefore, the diameter of the portion where the core diffractive optical element is formed (referred to as the first portion) may be larger than the diameter of the portion where the core diffractive optical element is not formed (referred to as the second portion). In this case, a tapered coupling portion may be formed between the first portion and the second portion. Thereby, the optical coupling efficiency can be further improved.

The optical coupler according to one embodiment of the present invention can be used as an optical coupler for coupling input light to an optical fiber or the like.

10, 11 Diffractive optical element 20 Condensing element 30 Reflecting part 40 Dispersing element 50 Phase compensation part 51 Optical circulator 52 FBG
60

Adjustment unit

100, 101, 102 Optical fiber 110 Core 120

Clad

130, 131a, 132a Input light 131b Linear light 132b Dispersed light 140a, 140b Diffracted light

Claims

An optical coupler that couples input light to an optical waveguide,
A region having a diffractive optical function formed inside the core of the optical waveguide,
The input light enters the region having the diffractive optical function from a direction perpendicular to the propagation axis of the optical waveguide.
Optical coupler.
The region having the diffractive optical function satisfies a Bragg condition for the input light incident from a direction perpendicular to the propagation axis of the optical waveguide.
The optical coupler according to claim 1.
The direction perpendicular to the propagation axis of the optical waveguide is a direction within a predetermined angle range from a direction strictly perpendicular to the propagation axis of the optical waveguide,
The predetermined angle range is a range where the diffraction efficiency is 0.5 or more in coupled-wave theory.
The optical coupler according to claim 1 or 2.
The region having the diffractive optical function is a fiber Bragg grating,
The optical coupler according to any one of claims 1 to 3.
The lattice constant of the fiber Bragg grating matches the wavelength of the input light.
The optical coupler according to claim 4.
The optical coupler further includes:
An input unit that inputs the input light to a region having the diffractive optical function from a direction perpendicular to the propagation axis;
The optical coupler according to any one of claims 1 to 5.
The input unit includes a condensing element that condenses the input light in a linear shape extending in a direction parallel to the propagation axis.
The optical coupler according to claim 6.
The input unit includes a dispersive element that disperses the input light in a direction parallel to the propagation axis,
The region having the diffractive optical function is a fiber Bragg grating,
The lattice constant of the fiber Bragg grating matches the wavelength of the input light,
In the direction parallel to the propagation axis, the lattice constant distribution of the fiber Bragg grating corresponds to the wavelength distribution of the dispersed input light,
The optical coupler according to claim 6 or 7.
The optical coupler further includes:
An adjustment unit for adjusting a lattice constant of the fiber Bragg grating according to the wavelength of the input light;
The optical coupler according to claim 4 or 5.
The optical coupler further includes:
A reflection unit configured to reflect one of the lights diffracted in two directions parallel to the propagation axis by the region having the diffractive optical function in opposite directions;
The optical coupler according to any one of claims 1 to 9.
The optical coupler further includes:
A phase compensator for compensating the phase of light coupled to the core by the region having the diffractive optical function;
The optical coupler according to any one of claims 1 to 10.
The optical waveguide is an optical fiber.
The optical coupler according to any one of claims 1 to 11.
An optical coupling method for coupling input light to an optical waveguide,
An input step of inputting light from a direction perpendicular to the propagation axis of the optical waveguide to a region having a diffractive optical function formed inside the core of the optical waveguide;
A diffraction step of coupling light into the optical waveguide by diffracting light input to the region having the diffractive optical function,
Optical coupling method.