WO2018087845A1

WO2018087845A1 - Planar waveguide

Info

Publication number: WO2018087845A1
Application number: PCT/JP2016/083244
Authority: WO
Inventors: 賢一廣澤; 柳澤　隆行
Original assignee: 三菱電機株式会社
Priority date: 2016-11-09
Filing date: 2016-11-09
Publication date: 2018-05-17
Also published as: JP6192883B1; JPWO2018087845A1

Abstract

A planar waveguide (10) is provided with: a flat plate-shaped core (11) via which light propagates; exterior claddings (12, 13) provided on both sides of the core (11), the exterior claddings (12, 13) having a higher reflectivity with respect to excitation light (31) than amplified light (21); and interior claddings (14, 15) provided between the core (11) and the exterior claddings (12, 13), the interior claddings (14, 15) having a lower refractive index than the core (11). In this configuration, the exterior claddings (12, 13) are multilayer films laminated from a plurality of films made of different materials.

Description

Planar waveguide

The present invention relates to a planar waveguide.

The double clad type planar waveguide is a waveguide in which clads are laminated on both sides of a core for propagating light, and functions as an optical amplifier by using a gain generating member for the core.
Note that the gain generating member is a member that amplifies the amplified light by propagating two types of light, that is, the amplified light and the pumping light, thereby absorbing the pumping light and forming an inverted distribution.

Some of the double clad type planar waveguides include an outer clad provided on both sides of the core and an inner clad provided between at least one surface of the core and the outer clad. In this planar waveguide, the amplified light propagating through the core is reflected by the inner cladding and returned to the core side, and the excitation light is reflected by the outer cladding and returned to the core side.

For example, the waveguide described in Patent Document 1 is the above-described double-clad planar waveguide. In this planar waveguide, the inner cladding is made of a material having a lower refractive index than the core, and the outer cladding is made of a material having a lower refractive index than the inner cladding.
Of the light propagating through the planar waveguide, the light totally reflected at the interface between the core and the inner cladding is confined in the core, and the light totally reflected at the interface between the inner cladding and the outer cladding is reflected between the core and the inner cladding. It is confined in the area formed by.

Of the light propagating through the planar waveguide, light having a small divergence angle is confined in the core, and light having a larger divergence angle is confined in a region formed by the core and the inner cladding.
In general, there is a desire to reduce the spread angle of the amplified light, and the higher the spread angle of the excitation light, the higher the output of the planar waveguide can be achieved.

International Publication No. 2009/016703

The degree of divergence of light that can be confined inside a planar waveguide is determined by the refractive index of the core and cladding, which is called the numerical aperture of the waveguide (NA) (hereinafter referred to as NA). ing.
However, when a material having a low refractive index such as fluoride glass is used for the core, considering the optical properties and physical properties, none of the materials for the outer cladding satisfy the NA required for the excitation light. For this reason, the material of a low refractive index cannot be used for a core, but the subject that the material which can be used for a core was restricted occurred.
Furthermore, when a planar waveguide functions as the optical amplifier, an inexpensive pumping light source having a large divergence angle cannot be used.

The present invention solves the above-described problems, and an object of the present invention is to obtain a planar waveguide having a high NA with respect to excitation light even if the core is made of a material having a low refractive index.

The planar waveguide according to the present invention includes a flat core that propagates light, a first clad that is provided on both sides of the core and has higher reflectivity than the amplified light with respect to the excitation light, and the core A second clad provided between at least one surface and the first clad and having a refractive index lower than that of the core. In this configuration, the first cladding is a multilayer film in which a plurality of films made of different materials are stacked.

According to the present invention, since the first clad is constituted by a multilayer film in which a plurality of films of different materials are laminated, even if the core is constituted by a material having a low refractive index, a high NA is obtained with respect to excitation light. There is an effect that a planar waveguide can be obtained.

It is a figure which shows the structure of the planar waveguide which concerns on Embodiment 1 of this invention. It is a figure which shows the outline | summary of the parasitic oscillation in a planar waveguide. It is a graph which shows the reflection characteristic of the external clad with respect to excitation light and amplification light. 4 is a diagram illustrating an arrangement example of a planar waveguide, an amplified light source, a coupling optical system, and an excitation light source according to Embodiment 1. FIG. FIG. 6 is a diagram illustrating another arrangement example of the planar waveguide, the amplified light source, the coupling optical system, and the excitation light source according to the first embodiment. It is a figure which shows the structure of the planar waveguide which concerns on Embodiment 2 of this invention. It is a figure which shows the outline | summary of the wave vector of amplified light. It is a figure which shows the outline | summary of the low order mode light and high order mode light of amplified light. It is a graph which shows the relationship between the amount of seepage of the amplified light to a clad, and the measurement result of waveguide loss. It is a figure which shows the electric field distribution in the 0th mode light of primary light and primary mode light, and the seepage to a clad. It is a figure which shows the multilayer film by which an optical path when amplified light reciprocates 1 time between films | membranes corresponds to the phase of 2 (pi). It is a graph which shows the reflection characteristic of the outer clad with respect to excitation light. It is a figure which shows the structure of the planar waveguide which concerns on Embodiment 3 of this invention.

Hereinafter, in order to describe the present invention in more detail, modes for carrying out the present invention will be described with reference to the accompanying drawings.
Embodiment 1 FIG.
FIG. 1 is a diagram showing a configuration of a planar waveguide 10 according to Embodiment 1 of the present invention, and shows a case where the planar waveguide 10 is an optical amplifier that amplifies amplified light 21.
The planar waveguide 10 includes an outer clad 12 and an outer clad 13 on both sides of the core 11, an inner clad 14 between the one surface of the core 11 and the outer clad 12, and the other surface of the core 11 and the outer surface. An internal clad 15 is provided between the clad 13 and the clad 13.

The core 11 is a flat member through which the amplified light 21 that is signal light propagates.
The amplified light 21 is light indicated by a solid line in FIG. On the other hand, the excitation light 31 emitted from the excitation light source 32 is light indicated by a dotted line in FIG.
In FIG. 1, the thickness direction of the core 11 is the x-axis direction, the direction perpendicular to the side surface of the core 11 is the y-axis direction, and the optical axis direction in which the amplified light 21 is propagated to the core 11 is the z-axis. Direction.
The refractive index of the core 11 is n ₁₁ , and the thickness of the core 11 is d ₁₁ .

The core 11 is composed of a gain generating member that has an action of amplifying the amplified light 21.
The gain generating member is a member that generates a gain by radiation transition by absorbing the excitation light 31 to form an inverted distribution.
As the gain generating member, for example, a glass to which a rare earth element such as Er, Yb, Tm, or Nd is added, a crystal to which a rare earth element such as Nd: YVO ₄ is added, or a rare earth element such as Yb: YAG is added. And ceramics made from such crystals.
Alternatively, a crystal to which a transition metal such as Cr: YAG or Ti: Sapphire is added may be used.

The polarization perpendicular to the plane including the thickness direction of the planar waveguide 10 and the propagation direction of the amplified light 21 or the excitation light 31 is called a TE (Transverse Electric field) mode, and the TE mode polarization is TE polarization. is there.
The polarization parallel to the plane including the thickness direction of the planar waveguide 10 and the propagation direction of the amplified light 21 or the excitation light 31 is called a TM (Transverse Magnetic Field) mode, and the TM mode polarization is TM. Polarized light.
In FIG. 1, the TE mode is set when y-polarized light and the TM mode is set when x-polarized light.

The outer cladding 12 and the outer cladding 13 are components embodying the first cladding, and are flat members that reflect the excitation light 31 and return it to the core 11 side. The outer clad 12 and the outer clad 13 each have a higher reflectance than the amplified light 21 with respect to the excitation light 31, and are arranged at positions symmetrical with respect to the yz plane.
Further, the outer clad 12 and the outer clad 13 are composed of a multilayer film in which a plurality of films made of different materials are laminated. The multilayer film includes a multilayer film in which the NA of the waveguide constituted by the core 11 and the

inner claddings

14 and 15 is 0.38 or more with respect to the excitation light 31 and 0.38 or less with respect to the amplified light 21. Used.

As shown in the enlarged view of FIG. 1, the outer cladding 12 is a multilayer film in which thin films 12a and thin films 12b are alternately stacked, and the outer cladding 13 is formed by alternately stacking thin films 13a and thin films 13b. It is a multilayer film. That is, the

outer claddings

12 and 13 are formed of a multilayer film in which one or more film sets made of different materials are stacked.

The thin film 12a and the thin film 12b are formed of different materials.
For example, two kinds of dielectric materials are selected from among the dielectric materials that can be formed, such as SiO ₂ , Ta ₂ O ₅ , MgO, Nb ₂ O ₅ , TiO ₂ , CaF ₂ , MgF _2, etc. 12a and thin film 12b are formed. The same applies to the thin film 13a and the thin film 13b.
Note that the combination of the materials of the thin film 12a and the thin film 12b and the combination of the materials of the thin film 13a and the thin film 13b may be the same, or may be different from each other.

In the planar waveguide 10, the refractive index of the thin film 12a is n _12a , the refractive index of the thin film 12b is n _12b , the film thickness of the thin film 12a is d _12a , and the film thickness of the thin film 12b is d _12b .
One of the refractive index or both of the refractive index between the refractive index _{n 12b} of the refractive index _{n 12a} and the thin film 12b of the thin film 12a may be higher than the refractive index _{n 11} of the core 11.
The refractive index of the thin film 13a is _n13a , the refractive index of the thin film 13b is _n13b , the film thickness of the thin film 13a is _d13a , and the film thickness of the thin film 13b is _d13b .
Similarly, one or both of the refractive index n _13a of the thin film _13a and the refractive index n _13b of the thin film 13b may be higher than the refractive index n ₁₁ of the core 11.

The inner cladding 14 and the inner cladding 15 are components embodying the second cladding, and are flat members that reflect the excitation light 31 and return it to the core 11 side.
The inner cladding 14 is provided between one surface of the core 11 and the outer cladding 12, and the inner cladding 15 is provided between the other surface of the core 11 and the outer cladding 13.
Further, the refractive index _{n 14} of the inner cladding 14 is lower than the refractive index _{n 11} of the core 11, the refractive index _{n 15} of the inner cladding 15 is lower than the refractive index _{n 11} of the core 11.

The refractive index n ₁₄ of the inner cladding ₁₄ and the refractive index n ₁₅ of the inner cladding 15 have a relationship of the following formula (1) and the following formula (2) between the refractive index n ₁₁ of the core 11. Yes.
(N ₁₁ -0.01) <n ₁₄ <n ₁₁ (1)
(N ₁₁ −0.01) <n ₁₅ <n ₁₁ (2)

The excitation light 31 radiated from the excitation light source 32 propagates through the core 11 and the

inner claddings

14 and 15 at propagation angles corresponding to these refractive indexes.
However, when the propagation angle θ ₃₁ is not near 90 °, the core 11 and the

inner claddings

14 and 15 have very close refractive indexes, as is apparent from the above formula (1) and the above formula (2). The refraction angle at each interface between the core 11 and the

inner claddings

14 and 15 is very small.
For this reason, it can be considered that the excitation light 31 propagates at the propagation angle θ ₃₁ both in the core 11 and in the

inner claddings

14 and 15.
The

outer claddings

12 and 13 have a reflectance of 99% or more with respect to the excitation light 31 incident at the propagation angle θ ₃₁ from the

inner claddings

14 and 15 side.

In a general double clad type planar waveguide, there is a limit to the realizable NA.
For example, when Er-added phosphate glass is used as the material of the core 11, the refractive index n11 of the core ₁₁ is 1.515. In this case, from the above formulas (1) and (2), it can be assumed that the refractive indexes n ₁₄ and n ₁₅ of the inner claddings ₁₄ and ₁₅ are approximately 1.51.
On the other hand, when fluoride glass, which is a low refractive index material, is used as the outer cladding, the refractive index n11 of the core ₁₁ is 1.47, and the NA for the excitation light in the planar waveguide is about 0.35.

The NA of the pumping multimode laser diode may exceed 0.35. For this reason, when a multimode laser diode is used as an excitation light source for a planar waveguide, an optical system such as a lens is required.
For example, when the longitudinal divergence angle of the light emitted from the multimode laser diode is 45 ° in total, in order to connect the multimode laser diode and the planar waveguide in close proximity, the NA is 0.38 or more. is required.

In contrast, Takamere a refractive index n ₁₁ of the core 11, but improves the NA of possible planar waveguide realized by total reflection, easily parasitic oscillation occurs.
FIG. 2 is a diagram showing an outline of parasitic oscillation in a planar waveguide. As shown in FIG. 2, when the path of the amplified light 21 is formed only by total reflection inside the waveguide, the laser oscillation occurs because the loss is very small. Such parasitic oscillation consumes energy for amplifying the amplified light 21 propagating through the core 11.

When the condition shown in the following formula (3) is satisfied, the path of the amplified light 21 shown in FIG. 2 is formed only by total reflection inside the waveguide. However, n _clad represents the higher one of the refractive indexes of the outer cladding 12 and the outer cladding 13.
When the refractive index n _clad of the outer cladding is 1.47, which is the same as that of fluoride glass, according to the following formula (3), when the refractive index n ₁₁ of the core 11 exceeds 1.78, parasitic oscillation is avoided. It becomes difficult.
arcsin (n _clad / n ₁₁ ) + arcsin (1 / n ₁₁ ) ≦ π / 2 (3)

Therefore, in the planar waveguide 10, multilayer films that have a reflectivity of 99% or more over a wide range with respect to the excitation light 31 and have a reduced reflectivity with respect to the amplified light 21 are used as the

outer claddings

12 and 13. ing. That is, a multilayer film in which the NA of the waveguide constituted by the core 11 and the

inner claddings

14 and 15 is 0.38 or more with respect to the pumping light 31 and 0.38 or less with respect to the amplified light 21 is used. . Thereby, it is possible to obtain the planar waveguide 10 having a high NA with respect to the excitation light 31 and suppressing the parasitic oscillation of the amplified light 21.

For example, in the planar waveguide 10, the refractive index n ₁₄ of the inner cladding ₁₄ and the refractive index n ₁₅ of the inner cladding 15 are both 1.42, and the refractive index n ₁₁ of the core ₁₁ is based on the refractive indices n ₁₄ and n ₁₅ . 3/1000 larger. Further, the total thickness of the core 11, the inner cladding 14 and the inner cladding 15 is 120 μm.
The multilayer film that is the outer cladding 12 includes a thin film 12a having a film thickness d _12a of 119 nm and a refractive index n _12a of 2.16, and a thin film 12b having a film thickness d _12b of 274 nm and a refractive index n _12b of 1.45. Assume that 10 layers are alternately stacked. Similarly, for the multilayer film as the outer cladding 13, a thin film _13a having a film thickness d _13a of 119 nm and a refractive index n _13a of 2.16, and a thin film 13b having a film thickness d _13b of 274 nm and a refractive index n _13b of 1.45. And 10 layers are alternately stacked.
The amplified light 21 is light having a wavelength of 1550 nm in vacuum, and the excitation light 31 is light having a wavelength of 940 nm in vacuum.

With this configuration, it is possible to realize the planar waveguide 10 having a high NA with respect to the excitation light 31 and a suppressed parasitic oscillation of the amplified light 21 as described above.
The following materials can be used to obtain the refractive indexes n ₁₁ , n ₁₄ , n ₁₅ , n _12a , n _12b , n _13a , and n _13b having the above values.
For example, Er-added aluminum fluoride glass can be cited as the material of the core 11, additive-free aluminum fluoride glass as the material of the

inner claddings

14 and 15, Ta ₂ O _{5 as} the material of the

thin films

12a and 13a, and the

thin films

12b and 13b. Examples of the material include SiO ₂ .

FIG. 3 is a graph showing the reflection characteristics of the

outer claddings

12 and 13 with respect to the excitation light 31 and the amplified light 21, and shows the relationship between the propagation angle and the reflectance. Here, the solid line curve a indicates the reflection characteristic of the excitation light 31 at 940 nm light, and the dotted line curve b indicates the reflection characteristic at the amplified light 21 of 1550 nm light.

As shown in FIG. 3, the propagation angle θ ₃₁ of the excitation light 31 is 43 ° or more, and the reflectivity is 99% or more. At this time, when viewed from the outside of the planar waveguide 10, NA is 1 with respect to the excitation light 31, and light of an arbitrary angle can be confined.
Accordingly, even if the vertical spread angle of the pumping light 31 emitted from the pumping light source 32 is 45 °, the pumping light 31 can be provided with a sufficient design margin only by arranging the pumping light source 32 close to the planar waveguide 10. Can be propagated to the planar waveguide 10.
Incidentally, with respect to the amplified light 21, the propagation angle theta ₂₁ is the reflectance at 60 ° or less is suppressed to 50% or less.

Further, in an amplifying apparatus including the planar waveguide 10 as an optical amplifier, the following arrangement of components can be considered. Here, FIG. 4 is a top view showing an arrangement example of the planar waveguide 10, the amplified light source 22, the coupling optical system 23, and the excitation light source 32.
FIG. 5 is a top view showing another arrangement example of the planar waveguide 10, the amplified light source 22, the coupling optical system 23, and the excitation light source 32.
4 and 5, the amplified light source 22 is a light source that emits amplified light 21 that is signal light, and the coupling optical system 23 is an optical system for coupling the amplified light source 22 to the planar waveguide 10. is there.

In the amplifying apparatus shown in FIG. 4, the excitation light source 32, the planar waveguide 10, the coupling optical system 23, and the amplified light source 22 are arranged in this order in a certain direction. The certain direction is the optical axis direction (z-axis direction) in which the amplified light 21 propagates through the planar waveguide 10.
In the amplification device shown in FIG. 5, the direction in which the excitation light source 32 and the planar waveguide 10 are aligned (z-axis direction), and the direction in which the planar waveguide 10, the coupling optical system 23, and the amplified light source 22 are aligned ( and the y-axis direction) are orthogonal to each other.
Since the planar waveguide 10 has a high NA with respect to the excitation light 31, no optical system for coupling the excitation light source 32 and the planar waveguide 10 is required as shown in FIGS.

Up to now, as the

outer claddings

12 and 13, a multilayer film in which two types of thin films are alternately stacked has been shown, but it is not limited thereto.
For example, two types of thin films may be included in the multilayer film, or three or more types of thin films may be stacked.

As shown in FIG. 1, the multilayer film that is the outer cladding 12 and the multilayer film that is the outer cladding 13 are arranged in a symmetrical relationship with respect to the yz plane.
However, the outer clad 12 and the outer clad 13 may have different materials or film thicknesses used for the multilayer film.

Further, when the low refractive index glass or the like is not used as the material of the core 11, the outer cladding 12 or the outer cladding 13 is made of a material having a low refractive index. Thereby, the excitation light 31 may be confined inside the waveguide by total reflection at the outer cladding 12 or the outer cladding 13.

As described above, the planar waveguide 10 according to the first embodiment is provided on each of the flat core 11 that propagates light and both sides of the core 11, and is higher than the amplified light 21 with respect to the excitation light 31.

External claddings

12 and 13 having reflectivity, and

internal claddings

14 and 15 provided between the core 11 and the

external claddings

12 and 13 and having a refractive index lower than that of the core 11 are provided. The

outer claddings

12 and 13 are multilayer films in which a plurality of films made of different materials are laminated.
Since it is configured in this way, the planar waveguide 10 having a high NA with respect to the excitation light 31 can be obtained even if the core 11 is made of a low refractive index material.

In the planar waveguide 10 according to the first embodiment, the core 11 is a gain generating member that absorbs the excitation light 31 and amplifies the amplified light 21.
The amplified light 21 does not enter the

outer claddings

12 and 13, but when spontaneous emission light is emitted from the core 11 at a wavelength close to the amplified light 21, the spontaneous emission light is incident on the

outer claddings

12 and 13. Is done. At this time, if the light path shown in FIG. 2 is formed only by total internal reflection in the waveguide, parasitic oscillation occurs.
On the other hand, in the planar waveguide 10, since the

outer claddings

12 and 13 are formed of the multilayer film, the reflectance with respect to the amplified light 21 can be suppressed and parasitic oscillation can be suppressed.
Furthermore, since the planar waveguide 10 can enter the excitation light 31 with a high NA, it is possible to use a multimode excitation light source.
Note that multi-mode light sources are generally cheaper than single-mode light sources and can constitute high-power laser light sources.

Furthermore, in the planar waveguide 10 according to the first embodiment, the multilayer film constituting the

outer claddings

12 and 13 is formed by laminating one or more sets of films made of different materials. Furthermore, the NA is 0.38 or more for the excitation light 31 and 0.38 or less for the amplified light 21 depending on the characteristics of the multilayer film. Thereby, it is possible to obtain the planar waveguide 10 having a high NA with respect to the excitation light 31 and suppressing the parasitic oscillation of the amplified light 21.

Embodiment 2. FIG.
In the first embodiment, the double clad type planar waveguide in which the amplified light 21 is confined to the core 11 side by the inner clads 14 and 15 and the excitation light 31 is confined to the core 11 side by the outer clads 12 and 13 is shown. In the second embodiment, a configuration in which the inner cladding 15 is not provided and both the amplified light 21 and the pumping light 31 are confined on the core 11 side by the outer cladding 13 will be described.

FIG. 6 is a diagram showing a configuration of a planar waveguide 10A according to Embodiment 2 of the present invention, and shows a case where the planar waveguide 10A is an optical amplifier that amplifies the amplified light 21. In FIG.
In FIG. 6, the same components as those in FIG.
As in FIG. 1, the polarized light perpendicular to the thickness direction of the planar waveguide 10A in the amplified light 21 and the excitation light 31 is TE polarized light. Also, the polarized light parallel to the plane including the thickness direction of the planar waveguide 10 and the propagation direction of the amplified light 21 in the amplified light 21 and the excitation light 31 is TM polarized light. In FIG. 6, the TE mode is obtained when y-polarized light and the TM mode is obtained when x-polarized light.

The outer clad 13A is a component that embodies the first clad, and is a flat member that reflects both the amplified light 21 and the pumping light 31 and returns it to the core 11 side. Further, the outer cladding 13 A has a higher reflectance than the amplified light 21 with respect to the excitation light 31.
In the planar waveguide 10 A, the inner cladding 15 is not provided on the other surface of the core 11, and the outer cladding 13 A is directly bonded to the other surface of the core 11.
As shown in the enlarged view of FIG. 6, the outer clad 13A is composed of a multilayer film in which thin films 13a and thin films 13b made of different materials are alternately stacked.

The amplified light 21 having the wavelength λ _{21 in the} vacuum propagates through the core 11 at the propagation angle θ ₂₁ . FIG. 7 is a diagram showing an outline of the wave number vector of the amplified light 21, and shows the wave vector of the amplified light 21 in the core 11 and the outer cladding 13A.
As shown in FIG. 7, the x direction component of the wave number of the amplified light 21 includes the x direction component k ₂₁₁₁ of the wave number in the core 11, the x direction component k _2113a of the wave number in the thin film 13a, and the x direction component k of the wave number in the thin film 13b. _2113b .

k ₂₁₁₁ , k _2113a and k _2113b are defined as in the following formulas (4) to (6).
Hereinafter, the x-direction component of the wave number is simply _{expressed as} wave numbers k ₂₁₁₁ , k _2113a, and k _2113b .
k ₂₁₁₁ = (2πn ₁₁ cos θ ₂₁ ) / λ ₂₁ (4)
k _2113a = 2π (n _13a ² −n ₁₁ ² sin ² θ ₂₁ ) ^1/2 / λ ₂₁ (5)
k _2113b = 2π (n _13b ² −n ₁₁ ² sin ² θ ₂₁ ) ^1/2 / λ ₂₁ (6)

Inside the planar waveguide 10A, takes a value propagation angle theta ₂₁ has discrete amplified light 21 shown in FIG. Here, it is assumed that the phase of the amplified light 21 incident on the interface between the core 11 and the outer cladding 13A and the phase of the amplified light 21 reflected at this interface are changed by π.
In this assumption, when the core 11 satisfies the relationship of the following formula (7), the propagation angle θ ₂₁ can be determined from the wave number k ₂₁₁₁ in the core 11 according to the above formula (4).
In the following formula (7), d ₁₁ is the thickness of the core 11 and m = 0, 1, 2, 3,.
k ₂₁₁₁ d ₁₁ = (m + 1) π (7)

The range of the excitation light 31 is propagated, core 11 and inner cladding 14 and has straddle the, the thickness _{d 14} of the thickness _{d 11} and inner cladding 14 of the core 11, _{usually, d} 14 >> and it has a d _11. Therefore, the propagation angle theta ₃₁ of the excitation light 31 may take continuous values in comparison with the amplified light 21.

Hereinafter, among the propagation angle theta ₂₁ to take discrete values, in the order of large angle propagation angle theta _21, the propagation mode is zero order, first order, referred to as the propagation angle theta ₂₁ of the secondary ....
A waveguide through which only the amplified light 21 having the 0th-order propagation angle θ ₂₁ can propagate is called a single mode waveguide. The amplified light 21 having the 0th-order propagation angle θ ₂₁ is referred to as 0th-order mode light.
Further, a waveguide capable of propagating the amplified light 21 at the low-order propagation angle θ ₂₁ but not capable of propagating the amplified light 21 at the high-order propagation angle θ ₂₁ is referred to as a low-order mode waveguide.
Here, the amplified light 21 of the low-order propagation angle theta ₂₁ is referred to as low-order mode light, it referred to as amplified light 21 high-order propagation angle theta ₂₁ and higher-order mode light.

FIG. 8 is a diagram showing an outline of the low-order mode light and the high-order mode light of the amplified light 21. In the planar waveguide 10A according to the second embodiment, the substantial operation is the same even if the inner cladding 14 is omitted. Therefore, for the sake of simplicity, the illustration of the inner cladding 14 is omitted in FIG. The low-order mode light and the high-order mode light are totally reflected at the interface between the core 11 and the outer cladding 12, and are totally reflected at the interface between the core 11 and the outer cladding 13 A and propagate through the core 11.

When the amplified light 21 is incident on the outer cladding 13A from the core 11, there are an infinite number of combinations of the material and film thickness of the multilayer film in which the reflectivity of the amplified light 21 is 99% or more, and it can be designed freely. it can. However, since the

thin films

13a and 13b included in the multilayer film are more scattered than general glass, the loss of the amplified light 21 propagating through the multilayer film may be increased.

FIG. 9 is a graph showing the relationship between the amount of the amplified light 21 that leaks into the cladding and the measurement result of the waveguide loss. In FIG. 9, a core 11 made of Nd: YVO ₄ is used. As shown in FIG. 9, it can be seen that the waveguide loss increases as the amount of the amplified light 21 that leaks into the cladding increases, and the amount of the protrusion and the waveguide loss are directly proportional.

Note that the amount of the exudation indicates the ratio of the intensity of the amplified light 21 that has entered the multilayer film to the total intensity of the amplified light 21.
Even if the outer cladding 13A has a reflectivity of 100% with respect to the amplified light 21, a part of the amplified light 21 is reflected inside the multilayer film, so that a certain amount of energy is present inside the multilayer film. Is present.
At this time, the ratio of the energy existing inside the core 11 and the energy existing inside the multilayer film is defined as the amount of exudation. The amount of seepage varies depending on the mode described above.

FIG. 10 is a diagram showing the electric field distribution of the amplified light 21 in the 0th-order mode light and the first-order mode light and the seepage to the cladding. As in FIG. 8, in the planar waveguide 10A according to the second embodiment, the substantial operation is the same even if the inner cladding 14 is omitted. Therefore, for the sake of simplicity of explanation, the illustration of the inner cladding 14 is omitted in FIG.
A solid curve 21 a in FIG. 10 indicates the electric field distribution of the zero-order mode light of the amplified light 21, and a dotted curve 21 b in FIG. 10 indicates the electric field distribution of the primary mode light of the amplified light 21. Further, the amplified light 21 in these modes oozes out to the outer cladding 12 and the outer cladding 13A as indicated by the symbols A and B.

The assumption that the phase of the amplified light 21 incident on the interface between the core 11 and the outer cladding 13A and the phase of the amplified light 21 reflected at this interface are changed by π is a guide with a small amount of exudation of the amplified light 21. This is often true for waveguides.
For example, when the multilayer film satisfies the relationship expressed by the following formula (8), the amount of seepage of the amplified light 21 in a certain mode decreases. However, in the following formula (8), l is an arbitrary integer.
k _2113a d _13a + k _2113b d _13b = lπ (8)

FIG. 11 is a diagram showing a multilayer film in which the optical path when the amplified light 21 reciprocates once between the films corresponds to a phase of 2π. As shown in FIG. 11, in the multilayer film satisfying the relationship of the above formula (8), when the amplified light 21 reciprocates once through the pair of

thin films

13a and 13b among the plurality of thin films stacked on the multilayer film. The optical path corresponds to a phase of 2π.
And in the said multilayer film, since the reflected light in a thin film interface raises interference, even if it is a case where there are few layers of a multilayer film, a high reflectance can be implement | achieved.

However, a certain amount of error is allowed between the film thickness d _13a of the thin film _13a and the film thickness d _{13b of the} thin film 13b. For this reason, the amplified light 21 can be confined inside the core 11 even when the relationship of the following formula (9) is satisfied as a conditional expression for reducing the amount of the amplified light 21 oozing out. Accordingly, the film thicknesses d _13a and d _13b of the

thin films

13a and 13b are determined so as to satisfy the relationship of the following formula (9).
(L-1 / 4) π <k _2113a d _13a + k _2113b d _13b <(l + 1/4) π (9)

Thus, the outer cladding 13A in the planar waveguide 10A includes the

thin films

13a and 13b having the film thicknesses d _13a and d _13b that satisfy the relationship of the above formula (9) with respect to the propagation angle θ ₂₁ of the zero-order mode light. It is a multilayer film.
With this configuration, the bleeding of the zero-order mode light in the amplified light 21 is suppressed, and the waveguide loss can be reduced. In addition, since the amount of high-order mode light oozes out, only low-order mode light can be propagated.

Hereinafter, with respect to the propagation angle theta ₂₁ of the zero-order mode light of the amplified light 21 will be described design example of the planar waveguide 10A to satisfy the relation of the aforementioned expression (9). Here, it is assumed that the planar waveguide 10A is configured as follows.
The core 11 has a thickness d ₁₁ of 10 μm and a refractive index n ₁₁ of 1.42.
The multilayer film as the outer cladding 13A is a multilayer film in which thin films 13a and thin films 13b are alternately stacked. The thin film 13a is a thin film having a film thickness d _13a of 426 nm and a refractive index n _13a of 2.16, and the thin film 13b is a thin film having a film thickness d _13b of 229 nm and a refractive index n _13b of 1.45. Further, the amplified light 21 is light having a wavelength of 1.55 μm in vacuum, and the excitation light 31 is light having a wavelength of 940 μm in vacuum.

In addition, as a material from which the refractive indexes n ₁₁ , n _13a , and n _13b having the above values are obtained, for example, Er-added aluminum fluoride glass is used as the material of the core 11, and Ta ₂ O ₅ is used as the material of the thin film 13a. , SiO ₂ and the like as the material of the thin film 13b.

In the planar waveguide 10A having the above configuration, the propagation angle θ ₂₁ of the 0th-order mode light is 1.516 rad using the above formula (4) and the above formula (7).
The wave number k _2113a of the amplified light 21 in the thin film 13a is 6.61 × 10 ⁶ m ⁻¹ and the wave number k _2113b of the amplified light 21 in the thin film 13b is 1.23 × 10 ⁶ m ⁻¹ . Further, the optical path length of the amplified light 21 in the thin film 13a is 2.81 rad when converted into a phase, and the optical path length of the amplified light 21 in the thin film 13b is 0.28 rad when converted into a phase. Accordingly, since the total is 3.10 rad, the multilayer film satisfies the relationship of the above formula (9).

When the amount of exudation of the amplified light 21 in the planar waveguide 10A having the above configuration is calculated by simulation, the amount of exudation of the 0th-order mode light is 0.11%, and the amount of exudation of the first-order mode light is 0.44%. The amount of seepage of the secondary mode light is 1.1%.
Therefore, the loss of the 0th-order mode light in the amplified light 21 is suppressed to a value lower than that of the first-order mode light and the second-order mode light, and the planar waveguide 10A is a waveguide capable of propagating low-order mode light. It has become.

FIG. 12 is a graph showing the reflection characteristics of the outer cladding 13A with respect to the excitation light 31, and shows the reflection characteristics of the outer cladding 13A with respect to the excitation light 31 in the planar waveguide 10A having the above-described configuration. As shown in FIG. 12, the propagation angle θ ₃₁ of the excitation light 31 has a high reflectance of 53% or more and 99% or more.
For example, as in the structure shown in FIG. 8, when the outer cladding 12 is symmetrical with the outer cladding 13A, the NA of this waveguide is 0.85.
Therefore, as in the first embodiment, the waveguide can propagate the excitation light 31 emitted from the excitation light source 32 with a longitudinal spread angle of 45 ° with a sufficient design margin.

In addition, although the case where the outer clad 12 has a symmetric structure with the outer clad 13A has been described, for example, as long as the outer clad 12 is highly reflective with respect to the excitation light 31 as in the structure of FIG. .

Up to now, a multilayer film in which two types of thin films are alternately laminated as the

outer claddings

12 and 13A has been shown. However, the present invention is not limited to this.
For example, two types of thin films may be included in the multilayer film, or three or more types of thin films may be stacked.

Further, in the amplifying apparatus including the planar waveguide 10A as an optical amplifier, the planar waveguide 10A, the amplified light source 22, the coupling optical system 23, and the excitation light source 32 may be disposed as shown in FIGS. .

As described above, the planar waveguide 10 A according to the second embodiment is provided with the core 11 and the inner cladding 14 interposed on one surface of the core 11. An outer cladding 12 having a high reflectance, and an outer cladding 13 A which is provided directly joined to the other surface of the core 11 and has a higher reflectance than the amplified light 21 with respect to the excitation light 31. In this configuration, the

outer claddings

12 and 13A are multilayer films in which a plurality of films of different materials are laminated.
Since it is configured in this manner, even if the core 11 is made of a material having a low refractive index, the planar waveguide 10A having a high NA with respect to the excitation light 31 can be obtained.
Further, since the planar waveguide 10A can enter the excitation light 31 with a high NA, it is possible to use a multimode excitation light source.
Note that multi-mode light sources are generally cheaper than single-mode light sources and can constitute high-power laser light sources.

In the planar waveguide 10A according to the second embodiment, the multilayer film that is the

outer claddings

12 and 13A is formed by laminating one or more sets of films made of different materials. In this configuration, in the multilayer film, the amount of light leaking into the multilayer film varies depending on the propagation mode. In particular, in a multilayer film, the amount of seepage increases as the propagation mode becomes higher order.
By configuring in this way, the amount of the 0th-order mode light in the amplified light 21 can be suppressed and the waveguide loss can be reduced.
Further, since the amount of the high-order mode light in the amplified light 21 increases, the planar waveguide 10A capable of propagating only the low-order mode light in the amplified light 21 is obtained.

Embodiment 3 FIG.
Embodiment 3 describes a planar waveguide that propagates linearly polarized amplified light emitted from an amplified light source to the core while maintaining the polarization.
FIG. 13 is a diagram showing a configuration of a planar waveguide 10B according to the third embodiment of the present invention, and shows a case where the planar waveguide 10B is an optical amplifier that amplifies the amplified light 21.
In FIG. 13, the same components as those in FIGS. 1 and 6 are denoted by the same reference numerals, and description thereof is omitted.

For example, when the core 11, the outer cladding 13A, and the inner cladding 14 are configured using an isotropic medium such as glass as in the conventional planar waveguide, the amplified light 21 is incident on the core 11 with TE polarization. However, the polarization may not be maintained. This is because birefringence occurs in the core 11 due to heat or stress.

Therefore, in the planar waveguide 10B, the amount of the TE-polarized amplified light 21 and the amount of the TM-polarized amplified light 21 are changed to change the propagation constant of the TE-polarized amplified light 21 and the TM-polarized amplified light. 21 to control the propagation constant. Thereby, the polarization of the amplified light 21 incident on the core 11 is maintained.

Hereinafter, the propagation constant of the TE-polarized light and TM-polarized amplified light 21 is controlled, and the planar waveguide 10B satisfying the relationship of the above formula (9) with respect to the propagation angle θ ₂₁ of the 0th-order mode light of the amplified light 21 A design example will be described. Here, it is assumed that the planar waveguide 10B is configured as follows.
The core 11 has a thickness d ₁₁ of 10 μm and a refractive index n ₁₁ of 1.42.
The multilayer film as the outer cladding 13A is a multilayer film in which thin films 13a and thin films 13b are alternately stacked.
The thin film 13a is a thin film having a film thickness d _13a of 426 nm and a refractive index n _13a of 2.16, and the thin film 13b is a thin film having a film thickness d _13b of 229 nm and a refractive index n _13b of 1.45. Further, the amplified light 21 is light having a wavelength of 1.55 μm in vacuum, and the excitation light 31 is light having a wavelength of 940 μm in vacuum.

In the second embodiment, it is assumed that the phase of the incident light and the phase of the reflected light of the amplified light 21 are changed by π at the interface between the core 11 and the outer cladding 13A.
However, in practice, the phase change amount does not become a constant value, but the phase change amount varies depending on the amount of the amplified light 21 that has leaked into the outer cladding 13A.
For example, when the phase rotation amount of the amplified light 21 in the outer cladding 13A is _φ2113 and the phase rotation amount of the amplified light 21 in the inner cladding 14 is _φ2114 , the above equation (7) is exactly the following equation (10) Can be expressed as However, m = 1, 2, 3,.
k ₂₁₁₁ d ₁₁ + φ ₂₁₁₃ + φ ₂₁₁₄ = mπ (10)

The phase rotation amount φ ₂₁₁₄ corresponds to the Goose Henschen shift in total reflection at the interface between the core 11 and the inner cladding 14. However, when the angle is near the critical angle, the Goose Henschen shift is small. Therefore, the above formula (10) can be expressed as the following formula (11).
k ₂₁₁₁ d ₁₁ + φ ₂₁₁₃ = mπ (11)

When the phase change amount of the amplified light 21 is different between the TE mode and the TM mode in the core 11, that is, when the phase rotation amount φ ₂₁₁₃ is different depending on the polarization, the amplified light 21 in the core 11 is obtained from the above equation (11). It can be seen that the wave number k ₂₁₁₁ of the laser beam varies depending on the polarization.

For example, when maintaining the polarization of incident light in an optical fiber, stress is applied to the optical fiber to change the refractive index for each polarization. Thereby, since the propagation constant of incident light changes, the polarization of incident light is maintained.
On the other hand, in the planar waveguide 10B, the multilayer film that is the outer cladding 13A is configured so that the amount of phase change of the amplified light 21 varies depending on the polarization. By configuring in this way, the amount of the amplified light 21 oozing out to the outer cladding 13A changes according to the polarization, and the propagation constant of the amplified light 21 changes according to the polarization. Thereby, the polarization of the amplified light 21 incident on the core 11 can be maintained.

When the phase rotation amount φ ₂₁₁₃ of the amplified light 21 is calculated by simulation in the planar waveguide 10B having the above configuration, the difference between the phase rotation amount φ ₂₁₁₃ between the TE mode amplified light 21 and the TM mode amplified light 21 is about 0. 0. 077 rad. For this reason, the propagation constant can be changed by about 2.5% for each mode.

In the third embodiment, a multilayer film in which two types of thin films are alternately stacked as the

outer claddings

12 and 13A is shown. However, the present invention is not limited to this.
For example, two types of thin films may be included in the multilayer film, or three or more types of thin films may be stacked.

Further, in the amplifying apparatus including the planar waveguide 10B as an optical amplifier, the planar waveguide 10B, the amplified light source 22, the coupling optical system 23, and the excitation light source 32 may be arranged as shown in FIGS. .

As described above, in the planar waveguide 10B according to the third embodiment, in addition to the configuration shown in the second embodiment, in the multilayer film that is the

outer claddings

12 and 13A, light leaks into the multilayer film due to polarization. The amount is different, and the amount of phase change of the amplified light differs depending on the polarization.
With this configuration, the same effect as in the second embodiment can be obtained, and the propagation constant of the amplified light 21 can be controlled in the TE mode and the TM mode, and the polarization of the amplified light 21 can be maintained. .

In the present invention, within the scope of the invention, a free combination of each embodiment, a modification of an arbitrary component of each embodiment, or an omission of any component in each embodiment is possible.

The planar waveguide according to the present invention is suitable for a laser medium of a laser device, for example, because it has a high NA with respect to excitation light even if the core is made of a material having a low refractive index.

10, 10A, 10B planar waveguide, 11 core, 12 outer cladding, 12a, 12b thin film, 13, 13A outer cladding, 12a, 12b, 13a, 13b thin film, 14, 15 inner cladding, 21 amplified light, 22 amplified light source , 23 coupled optical system, 31 excitation light, 32 excitation light source.

Claims

A flat core that propagates light;
A first clad provided on each side of the core and having a higher reflectance than the amplified light with respect to the excitation light;
A second clad provided between at least one surface of the core and the first clad and having a lower refractive index than the core;
The planar waveguide according to claim 1, wherein the first cladding is a multilayer film in which a plurality of films of different materials are laminated.
2. The planar waveguide according to claim 1, wherein the core is a gain generating member that absorbs excitation light and amplifies amplified light.
The planar waveguide according to claim 1, wherein the multilayer film is formed by laminating one or more sets of a plurality of films made of different materials.
2. The planar waveguide according to claim 1, wherein the numerical aperture is 0.38 or more with respect to the excitation light and 0.38 or less with respect to the amplified light.
The plurality of films belonging to the same set have a film thickness determined based on the relationship between the wave number of the amplified light propagating through each of the plurality of films and the optical path when the amplified light makes one round trip through the set. 4. The planar waveguide according to claim 3, wherein
2. The planar waveguide according to claim 1, wherein the amount of light oozing out into the multilayer film varies depending on the light propagation mode.
The planar waveguide according to claim 6, wherein in the multilayer film, the amount of protrusion increases as the light propagation mode becomes higher.
The planar waveguide according to claim 1, wherein the multilayer film has different amounts of light leaking into the multilayer film in accordance with polarization.
The planar waveguide according to claim 1, wherein the multilayer film has a different amount of phase change of the amplified light according to polarization.