Patent Description:
The present disclosure relates to an optical isolator and a light source device including the optical isolator.

In fields such as optical communication, if part of laser light emitted from a light source enters the light source as return light, there is a risk of damage to the light source, destabilization of the light source, or occurrence of noise caused by interference or the like. In view of this, an optical isolator that allows only propagation of light in one direction and prevents propagation of light in a reverse direction is used. As an optical isolator, one using a Faraday rotator having non-reciprocity is known (see, for example, Patent Literature <NUM>). PTL <NUM> discloses an optical circulator including a first optical isolator including a first port and a second port and a plurality of optical isolators coupled to the second port of the first optical isolator. Each of the plurality of optical isolators comprise a first port and a second port. PTL <NUM> discloses an optical nonreciprocal circuit formed by at least one lens through which a light propagating between two waveguides passes; and a polarization rotation circuit for sequentially applying a nonreciprocal rotation and a reciprocal rotation to a first half of the light propagating between two waveguides, and sequentially applying a reciprocal rotation and a nonreciprocal rotation to a second half of the light propagating between two waveguides, so that the first half and the second half of a light propagating forward are set to have an identical polarization to form an even guided mode and coupled, while the first half and the second half for a light propagating backward are set to have opposite polarizations to form an odd non-guided mode and dissipated. PTL <NUM> discloses a circulator array constructed in a planar substrate by forming a plurality of waveguide pair structures, each waveguide pair structure having first and second sections respectively coupled by first and second polarization multiplexers. A nonreciprocal polarization rotation element is positioned in the optical paths of the waveguide pair structures and is configured to rotate the polarization of light passing from the second sections of the waveguides to the first sections while leaving unchanged the polarization of light passing from the first sections to the second sections, such that optical signals received at one port of the circulator structure are routed along a predetermined path to another port of the circulator structure.

PTL <NUM>: <CIT> Summary of Invention; PTL <NUM>: <CIT>; PTL <NUM>: <CIT>; PTL <NUM>: <CIT>.

The present invention provides an optical isolator according to claim <NUM>, and a light source device according to claim <NUM>.

According to an optical isolator of the related art, constituent elements such as a polarizer, a Faraday rotator, and a half-wave plate are provided in an optical waveguide through which light emitted from a light source passes or in a space between the optical waveguide and an optical transmission path such as an optical fiber. In a case where an optical isolator is provided in an optical waveguide, a connection mechanism for connecting the optical waveguide to an external optical fiber, light source, or the like is needed. It is preferable that an optical isolator is easily connectable to an optical fiber or a light source and is constituted by a smaller number of components.

Embodiments of the present disclosure are described below with reference to the drawings. The drawings used for the descriptions below are schematic drawings. Dimensional ratios and the like on the drawings are different from actual ones.

As illustrated in <FIG>, an optical isolator <NUM> according to the first embodiment includes a substrate <NUM> and an optical waveguide <NUM> provided on the substrate <NUM>.

The substrate <NUM> is a plate-shaped member that is long in one direction. In the following description and the drawings, an X-axis direction is a longitudinal direction of a surface of the substrate <NUM>. A Y-axis direction is a direction orthogonal to the X-axis direction within the surface of the substrate <NUM>. A Z-axis direction is a direction normal to the surface of the substrate <NUM>. As illustrated in <FIG>, the substrate <NUM> may be a rectangle whose adjacent two sides extend along the X-axis direction and the Y-axis direction in plan view. A shape of the substrate <NUM> is not limited to this.

The substrate can be made of any of various materials. For example, the substrate <NUM> may be made of a material selected from a group consisting of a metal conductor, a semiconductor such as silicon, glass, and a resin.

The optical waveguide <NUM> includes a first end part <NUM> located at an end in a negative direction of the X-axis along the substrate <NUM> and a plurality of second end parts 14a to 14d located at an end in a positive direction of the X-axis. Hereinafter, the plurality of second end parts 14a to 14d are collectively referred to as second end parts <NUM>. The second end parts <NUM> are arranged in a one-dimensional array. In <FIG>, only four second end parts <NUM> are illustrated for simplification. The number of second end parts <NUM> can be any number equal to or more than <NUM>. For example, the number of second end parts <NUM> may be <NUM>, <NUM>, <NUM>, or the like.

The first end part <NUM> constitutes a first port <NUM> where light is input and output. The second end parts <NUM> constitute a second port <NUM> where light is input and output. The optical waveguide <NUM> extends substantially along the X-axis direction between the first port <NUM> and the second port <NUM>. "Light" as used herein encompasses not only light in a visible light region, but also light having any wavelength within a wavelength range from infrared radiation to ultraviolet radiation. Light that is input to the optical waveguide <NUM> from the first port <NUM> travels toward the second port <NUM>. Light that is input to the optical waveguide <NUM> from the second port <NUM> travels toward the first port <NUM>. The first port <NUM> and the second port <NUM> may be configured as end surfaces of the optical waveguide <NUM>.

As illustrated in <FIG>, the optical waveguide <NUM> (the first end part <NUM> thereof is illustrated in <FIG> and the second end parts <NUM> thereof are illustrated in <FIG>) is covered with a first medium 17a and a second medium 17b along a direction in which the optical waveguide <NUM> extends. The first medium 17a is provided on the substrate <NUM>. In a case where the substrate <NUM> is a dielectric body, the substrate <NUM> may also serve as the first medium 17a. The first medium 17a and the second medium 17b are in contact with an outer periphery of the optical waveguide <NUM>. The optical waveguide <NUM>, the first medium 17a, and the second medium 17b are dielectric bodies. The optical waveguide <NUM> has higher permittivity than the first medium 17a and the second medium 17b. The first medium 17a and the second medium 17b may be the same medium. The first medium 17a and the second medium 17b may be unified as a single medium.

The first medium 17a and the second medium 17b are, for example, made of quartz glass (silicon dioxide: SiO<NUM>). The optical waveguide <NUM> is, for example, made of silicon (Si). Relative permittivity of silicon and relative permittivity of quartz glass are approximately <NUM> and approximately <NUM>, respectively. Silicon allows near-infrared light of approximately <NUM> to approximately <NUM> to propagate at low loss. This allows large part of light that enters from the first end part <NUM> or the second end parts <NUM> to propagate inside the optical waveguide <NUM>. In a case where the optical waveguide <NUM> is made of silicon, the optical waveguide <NUM> allows infrared light having a wavelength of a <NUM> band or a <NUM> band used for optical communication to propagate at low loss. The optical waveguide <NUM> is, in other words, a core. The first medium 17a and the second medium 17b are, in other words, clads.

Materials for the optical waveguide <NUM> and the first medium 17a and the second medium 17b are not limited to the materials described above. Some of the first medium 17a and the second medium 17b, for example, part of the second medium 17b may be air. That is, the optical waveguide <NUM> alone may be provided on the first medium 17a made of quartz glass.

The optical waveguide <NUM> has one or more branching parts <NUM> between the first end part <NUM> and the plurality of second end parts <NUM>. Each of the branching parts <NUM> divides a single path that forms part of the optical waveguide <NUM> on the first end part <NUM> side into two or more paths on the second end parts <NUM> side. For example, each of the branching parts <NUM> can be a Y-branching optical waveguide that divides a single path into two paths. Each of the branching parts <NUM> unifies a plurality of paths on the second end parts <NUM> side of the optical waveguide <NUM> into a single path on the first end part <NUM> side. The branching parts <NUM> may be disposed in multiple stages between the first end part <NUM> and the second end parts <NUM>. Each of the branching parts <NUM> can divide light entering from a path on the first end part <NUM> side of the optical waveguide <NUM> into light of substantially equal amounts passing through a plurality of paths on the second end parts <NUM> side. Each of the branching parts <NUM> can merge light entering from a plurality of paths on the second end parts <NUM> side of the optical waveguide <NUM> into light passing through a path on the first end part <NUM> side.

In the example of <FIG>, the optical waveguide <NUM> has a first branching part 18a, a second branching part 18b, and a third branching part 18c. The first branching part 18a is located between the first end part <NUM> and the second and third branching parts 18b and 18c. The second branching part 18b is located between the first branching part 18a and the second end parts 14a and 14b. The third branching part 18c is located between the first branching part 18a and the second end parts 14c and 14d. The number of branching parts <NUM> is not limited to three, and any number of branching parts <NUM> are provided according to the number of second end parts <NUM>. The first branching part 18a, the second branching part 18b, and the third branching part 18c are collectively referred to as branching parts <NUM> as appropriate.

At each of the branching parts <NUM> illustrated in <FIG>, a path on the first end part <NUM> side of the optical waveguide <NUM> is physically continuous with two paths on the second end parts <NUM> side. The path on the first end part <NUM> side and the paths on the second end parts <NUM> side of the optical waveguide <NUM> at each of the branching parts <NUM> need not be physically continuous as long as these paths are optically coupled through portions that are close and parallel like a known optical directional coupler. In a case where paths of the optical waveguide <NUM> are disposed close to each other, light transfer occurs between the path on the first end part <NUM> side and the paths on the second end parts <NUM> side due to evanescent waves.

The optical waveguide <NUM> includes a portion having non-reciprocity. The portion of the optical waveguide <NUM> that has non-reciprocity is also referred to as a phase shifter <NUM>. The phase shifter <NUM> is provided in any part on the optical waveguide <NUM> between at least one branching part <NUM> and the second end parts <NUM>. "Having non-reciprocity" means that an effect on light propagating through the optical waveguide <NUM> varies depending on a propagation direction of the light. The propagation direction of the light includes a first direction from the first end part <NUM> toward the second end parts <NUM> and a second direction from the second end parts <NUM> toward the first end part <NUM>. In the phase shifter <NUM>, a non-reciprocal phase shifting effect occurs in which light propagating in the first direction and light propagating in the second direction are different in amount of change of a phase due to a magneto-optical effect. An amount of change of a phase is referred to as a phase shift amount. The phase shifter <NUM> gives a non-reciprocal phase shift amount between the first end part <NUM> and the second end parts <NUM>.

The phase shifter <NUM> includes a first non-reciprocal member 20a and a second non-reciprocal member 20b each of which has non-reciprocity. Hereinafter, the first non-reciprocal member 20a and the second non-reciprocal member 20b are sometimes collectively referred to as non-reciprocal members <NUM>. The non-reciprocal members <NUM> are disposed in planar contact with part of the optical waveguide <NUM>. "Disposed in contact" as used herein encompasses being joined by any means. The non-reciprocal members <NUM> may, for example, contain a non-reciprocal material such as magnetic garnet, ferrite, iron, or cobalt. The non-reciprocal members <NUM> generate a non-reciprocal phase shifting effect on light propagating through the portions of the optical waveguide <NUM> that are in contact with the non-reciprocal members <NUM>.

The phase shifter <NUM> includes a first non-reciprocal path 21a between the first branching part 18a and the second branching part 18b and a second non-reciprocal path 21b between the first branching part 18a and the third branching part 18c. The first non-reciprocal path 21a and the second non-reciprocal path 21b are part of the optical waveguide <NUM>. The first non-reciprocal member 20a and the second non-reciprocal member 20b are disposed in contact with the first non-reciprocal path 21a and the second non-reciprocal path 21b, respectively. Portions where the first non-reciprocal member 20a and the second non-reciprocal member 20b are in contact with one side surface of the optical waveguide <NUM> are referred to as the first non-reciprocal path 21a and the second non-reciprocal path 21b, respectively. Hereinafter, the first non-reciprocal path 21a and the second non-reciprocal path 21b are sometimes collectively referred to as non-reciprocal paths <NUM>. The non-reciprocal paths <NUM> are portions of the optical waveguide <NUM> that have non-reciprocity.

In the sectional view of <FIG>, an area of each non-reciprocal member <NUM> is equal to or less than a half of an area of a corresponding non-reciprocal path <NUM>. That is, a volume of each non-reciprocal member <NUM> is equal to or less than a half of a volume of a corresponding non-reciprocal path <NUM>.

The optical waveguide <NUM> is configured to propagate light in a single mode. An increase in a volume of the non-reciprocal members <NUM> disposed in contact with the optical waveguide <NUM> may generate an undesirable mode in the optical waveguide <NUM>, thereby degrading transmission characteristics of the optical waveguide <NUM>. The non-reciprocal members <NUM> are preferably small so as not to affect a mode of light propagating through the optical waveguide <NUM>. By making the volume of each non-reciprocal member <NUM> equal to or less than the volume of a corresponding non-reciprocal path <NUM>, degradation of the transmission characteristics can be reduced or suppressed.

The first non-reciprocal member 20a can be disposed in contact with a side surface, on a positive side in the Y-axis direction, of a portion (i.e., the first non-reciprocal path 21a) of the optical waveguide <NUM> between the first branching part 18a and the second branching part 18b. The second non-reciprocal member 20b can be disposed in contact with a side surface, on a negative side in the Y-axis direction, of a portion (i.e., the second non-reciprocal path 21b) of the optical waveguide <NUM> between the first branching part 18a and the third branching part 18c.

A magnetization direction of the non-reciprocal members <NUM> or a direction of an external magnetic field that generates non-reciprocity in the non-reciprocal members <NUM> and a polarization direction of incident light entering the optical waveguide <NUM> are substantially orthogonal to each other.

Specifically, the polarization direction of incident light entering the optical waveguide <NUM> is substantially parallel (i.e., the Y-axis direction) with a surface (substrate surface) of the substrate <NUM>. In this case, as illustrated in <FIG>, the first non-reciprocal member 20a and the second non-reciprocal member 20b generate different non-reciprocal phase shifting effects on the optical waveguide <NUM> by application of an external magnetic field having a component in the Z-axis direction. In other words, the first non-reciprocal path 21a and the second non-reciprocal path 21b have different non-reciprocal phase shift amounts. In a case where a magnitude of the external magnetic field is constant, a non-reciprocal phase shift amount is largest when the external magnetic field is applied in a substantially Z-axis direction.

In a case where the first non-reciprocal member 20a and the second non-reciprocal member 20b are ferromagnetic bodies, the first non-reciprocal path 21a and the second non-reciprocal path 21b have a non-reciprocal phase shifting effect even without application of an external magnetic field. In a case where the polarization direction of light entering the optical waveguide <NUM> is the Y-axis direction, the first non-reciprocal member 20a and the second non-reciprocal member 20b are disposed so that the magnetization direction has a component in the Z-axis direction. The first non-reciprocal member 20a and the second non-reciprocal member 20b may be disposed so that the magnetization direction becomes a substantially Z-axis direction.

For example, it is assumed that a difference of +<NUM>° in phase shift amount is generated between light propagating through the first non-reciprocal path 21a in the first direction and light propagating through the first non-reciprocal path 21a in the second direction. Furthermore, it is assumed that a difference of -<NUM>° in phase shift amount is generated between light propagating through the second non-reciprocal path 21b in the first direction and light propagating through the second non-reciprocal path 21b in the second direction.

Light entering the optical waveguide <NUM> from the first end part <NUM> is divided at the first branching part 18a. The divided light propagates through the first non-reciprocal path 21a and the second non-reciprocal path 21b. The light propagating through the first non-reciprocal path 21a is divided at the second branching part 18b and is output from the second end parts 14a and 14b. The light propagating through the second non-reciprocal path 21b is divided at the third branching part 18c and is output from the second end parts 14c and 14d. Light entering from the first end part <NUM> and output from the second end parts <NUM> can be caused to have the same phase, for example, by adjusting a length of the optical waveguide <NUM> from the first end part <NUM> to the second end parts <NUM>. In this case, light output from the plurality of second end parts <NUM> is output as a light beam having directivity in a narrow angular range in the X-axis direction, in which phases are aligned, due to a principle similar to a phased array antenna.

Meanwhile, light entering from the second end parts 14a and 14b is merged at the second branching part 18b and propagates through the first non-reciprocal path 21a. Light entering from the second end parts 14c and 14d is merged at the third branching part 18c and propagates through the second non-reciprocal path 21b. The light propagating through the first non-reciprocal path 21a and the light propagating through the second non-reciprocal path 21b are merged at the first branching part 18a and is then output from the first end part <NUM>. A phase difference of <NUM>° can occur between the light propagating through the first non-reciprocal path 21a and the light propagating through the second non-reciprocal path 21b due to non-reciprocity of the first non-reciprocal path 21a and the second non-reciprocal path 21b. Accordingly, an intensity of light entering the second end parts <NUM> from a positive side toward a negative side of the X-axis, propagating through the optical isolator <NUM> in the second direction, and being output from the first end part <NUM> is much weaker than a sum of intensities of light entering from the plurality of second end parts <NUM>. That is, light entering the second end parts <NUM> in a negative direction of the X-axis and the optical waveguide <NUM> are not easily coupled.

Accordingly, in the optical isolator <NUM>, light travelling in the first direction is easy to propagate, and light travelling in the second direction is not easy to propagate.

In the example illustrated in <FIG>, four second end parts <NUM> are provided, and two portions of the optical waveguide <NUM> have non-reciprocity for simplification. However, in a case where a large number of second end parts <NUM> that are different in non-reciprocal phase shift amount between the second end parts <NUM> and the first end part <NUM> are provided, light travelling in the second direction is further not easy to be coupled with the optical waveguide <NUM> at the second end parts <NUM>. Meanwhile, by making phase shift amounts of light propagating from the first end part <NUM> to the second end parts <NUM> uniform, directivity of light propagating through the optical isolator <NUM> in the first direction and being output from the second end parts <NUM> is further increased.

The first port <NUM> can be a port on a light incident side. The optical isolator <NUM> can be used in combination with a configuration for input of light. The optical isolator <NUM> and the configuration for input of light can be combined to constitute a light source device <NUM>. As illustrated in <FIG> and <FIG>, the light source device <NUM> includes the optical isolator <NUM>, a light source <NUM>, a power supply <NUM> that supplies power to the light source <NUM>, and a lens <NUM>. The light source <NUM> can be, for example, a semiconductor laser such as a laser diode (LD) or a vertical cavity surface emitting laser (VCSEL).

The light source <NUM> is optically coupled to the first end part <NUM> of the optical waveguide <NUM> through the lens <NUM>. A positional relationship among the light source <NUM>, the lens <NUM>, and the first end part <NUM> of the optical waveguide <NUM> may be fixed so that displacement does not occur. The light source <NUM> and the lens <NUM> may be integrated on the substrate <NUM> together with the optical waveguide <NUM> and the medium <NUM>. The light source <NUM> may cause light linearly polarized so that a polarization direction becomes the Y-axis direction to enter the first port <NUM>. The light source device <NUM> may be configured not to include the lens <NUM>. In a case where the light source device <NUM> does not include the lens <NUM>, light emitted from the light source <NUM> may be directly input to the first end part <NUM>.

A method of input of light from the light source <NUM> to the first end part <NUM> of the optical waveguide <NUM> is not limited to the method of causing light of the light source <NUM> to be input directly or through the lens <NUM>. The light source <NUM> may be coupled to the first end part <NUM> through an optical fiber. Examples of a method for causing light propagating through the optical fiber to be input to the first end part <NUM> may include various methods such as a method of connecting a free space through a lens or the like, a method of directly joining an emission surface of the optical fiber and the first end part <NUM>, and a method using a connection waveguide.

The light source device <NUM> includes the light source <NUM> and the optical isolator <NUM>, and thus light emitted from the light source <NUM> propagates through the optical isolator <NUM> in the first direction. Meanwhile, the light source device <NUM> reduces or suppresses light returning in the second direction by the optical isolator <NUM>, and thus the light source <NUM> is not easily influenced by the return light.

As illustrated in <FIG>, the second port <NUM> including the second end parts <NUM> of the optical isolator <NUM> is optically coupled to an optical transmission path 40A. The optical transmission path 40A is a path for light transmission. Specifically, the optical transmission path 40A can be an optical fiber having a core <NUM> and a clad <NUM>. The second port <NUM> of the optical isolator <NUM> and the core <NUM> of the optical transmission path 40A face each other with a space interposed therebetween in the X-axis direction. Light output from the second port <NUM> and travelling in the X-axis direction is coupled with the core <NUM> with high coupling efficiency. A core diameter of the optical transmission path 40A of <FIG> is larger than a dimension of a cross section of the optical waveguide <NUM>. The core diameter of the optical transmission path 40A of <FIG> can be, for example, approximately <NUM>. The optical transmission path 40A allows transmission of light emitted from the optical isolator <NUM> in multiple modes.

In another embodiment, the second port <NUM> of the optical isolator <NUM> may be optically coupled to a core <NUM> of an optical transmission path 40B through a lens <NUM>, as illustrated in <FIG>. Light output from the second end parts <NUM> of the optical isolator <NUM> may be focused on an end surface of the core <NUM> of the optical transmission path 40B that is a single-mode optical fiber by the lens <NUM> disposed in a space and then enter the optical transmission path 40B. In this case, a core diameter of the optical transmission path 40B can be, for example, approximately <NUM>.

Ends of the second end parts <NUM> of the optical waveguide <NUM> can have end surfaces tapered in a longitudinal direction (i.e., the X-axis direction) of the optical waveguide <NUM>. <FIG> illustrates a shape (lower side in <FIG>) of end surfaces of the second end parts <NUM> of the optical waveguide <NUM> and an example of an intensity distribution (upper side in <FIG>) of output light in a radial direction. The end surfaces of the second end parts <NUM> have substantially cone-shaped side surfaces. In another embodiment, the end surfaces of the second end parts <NUM> may have a semispherical shape protruding in the X-axis direction. The end surfaces of the second end parts <NUM> of the optical waveguide <NUM> can have a shape that is longer than a wavelength of light propagating in the X-axis direction and is gradually tapered in a positive direction of the X-axis. In a case where the end surfaces of the second end parts <NUM> of the optical waveguide <NUM> are tapered in the longitudinal direction of the optical waveguide <NUM>, an intensity of light output from the individual second end parts <NUM> increases.

<FIG> illustrates, for comparison, a shape (lower side in <FIG>) of the end surfaces of the second end parts <NUM> of the optical waveguide <NUM> in a case where the end surfaces are flat and an example of an intensity distribution (upper side in <FIG>) of output light. In this case, larger part of single-mode light that has propagated through the optical waveguide <NUM> is reflected by the flat end surfaces of the second end parts <NUM> than in a case where the second end parts <NUM> have tapered end surfaces of <FIG>. Accordingly, an intensity of light output from the second end parts <NUM> having flat end surfaces of <FIG> is weaker than an intensity of light output from the second end parts <NUM> having tapered end surfaces of <FIG>.

In both <FIG> and <FIG>, the optical transmission paths 40A and 40B receive light traveling with directivity in the X-axis direction from the second end parts <NUM>. Phases of light output from the second port <NUM> of the optical isolator <NUM> are aligned at the second end parts <NUM>, and therefore the output light has high directivity in the X-axis direction. Furthermore, in a case where the second end parts <NUM> have tapered end surfaces, an intensity of light output from the second port <NUM> of the optical isolator <NUM> is further increased. This allows the light output from the second port <NUM> of the optical isolator <NUM> to be coupled with the optical transmission path 40A or 40B with high coupling efficiency. Meanwhile, as for light entering the second port <NUM> of the optical isolator <NUM> from the optical transmission path 40A or 40B side, light propagating from the second end parts <NUM> to the first end part <NUM> is different in phase shift amount due to non-reciprocity of the optical waveguide <NUM>. This breaks a condition for making phases of the return light identical at the first end part <NUM>, and thus it is difficult for return light entering the second port <NUM> to be coupled with the optical waveguide <NUM> of the optical isolator <NUM>.

As described above, the optical isolator <NUM> includes the optical waveguide <NUM> on the substrate <NUM>, and the non-reciprocal members <NUM> having non-reciprocity are disposed in contact with the optical waveguide <NUM> to give different non-reciprocal phase shift amounts between the first end part <NUM> and the plurality of second end parts <NUM>. With this configuration, the optical isolator <NUM> can have a function of an optical isolator that allows propagation of light in the first direction and reduces or suppresses propagation of light in the second direction.

Furthermore, the optical isolator <NUM> can be easily connected to the optical transmission path <NUM> since directivity of light output from the second port <NUM> is high. Accordingly, the optical isolator <NUM> can be connected to the optical transmission path <NUM> by using a smaller number of components than in a case where an independent optical isolator is provided inside the optical waveguide <NUM> or in a space.

Furthermore, the optical isolator <NUM> according to the present embodiment can function as an isolator even in a case where the non-reciprocal phase shift amounts are small, by making an angle of a beam connecting the second port <NUM> and the optical transmission path <NUM> fall within a narrow angular range in the X-axis direction. In other words, even in a case where a difference in non-reciprocal phase shift amount among light in the second direction entering the second end parts <NUM> is small, the optical isolator <NUM> can be given a function as an isolator by breaking the condition for making phases of light entering from the second end parts <NUM> identical at the first end part <NUM>.

Specifically, in the above description, it is assumed that in a case where light propagates in the second direction, a phase difference of <NUM>° occurs between light propagating through the first non-reciprocal path 21a and light propagating through the second non-reciprocal path 21b due to non-reciprocity of the first non-reciprocal path 21a and the second non-reciprocal path 21b. However, this phase difference may be a value other than <NUM>°, for example, may be a value such as <NUM>° or <NUM>°. Even in such a case, light propagating in the second direction and then output from the first end part <NUM> is markedly reduced. Furthermore, in such a case, the second port <NUM> of the optical isolator <NUM> has a reception intensity distribution that expands in an angular direction within an XY plane rather than in a negative direction of the X-axis, depending on a phase difference occurring due to the phase shifter <NUM>, layout of the plurality of second end parts <NUM>, and the like. Accordingly, the optical isolator <NUM> has an effect of reducing or suppressing light entering the second port <NUM> in the negative direction of the X-axis.

In the optical isolator <NUM> according to the present embodiment, lengths of portions of the non-reciprocal members <NUM> that are in contact with the optical waveguide <NUM> can be made relatively short since non-reciprocal phase shift amounts can be reduced. This can reduce a loss of the optical waveguide <NUM> resulting from the lengths of the non-reciprocal members <NUM>.

Furthermore, since the light source device <NUM> according to the present embodiment has the optical isolator <NUM>, it is possible to prevent return light of light emitted from the light source <NUM> from entering the light source <NUM> and damaging the light source <NUM>, from destabilizing the light source <NUM>, or from generating noise or the like.

In the first embodiment, the first non-reciprocal member 20a and the second non-reciprocal member 20b are disposed in contact with two portions of the optical waveguide <NUM> to form the first non-reciprocal path 21a and the second non-reciprocal path 21b, which are two portions having non-reciprocity. However, even in a case where either the first non-reciprocal path 21a or the second non-reciprocal path 21b is not provided, the effects of the present embodiment can be obtained since a change occurs in relationship in phase shift amount between a case where light propagates in the first direction and a case where light propagates in the second direction.

An optical isolator <NUM> according to a second embodiment is described with reference to <FIG> is a plan view of the optical isolator <NUM>. The optical isolator <NUM> is similar to the optical isolator <NUM> according to the first embodiment, and therefore constituent elements identical or similar to those of the optical isolator <NUM> are given identical reference signs, and description thereof is omitted as appropriate.

A shape of an optical waveguide <NUM> of the optical isolator <NUM> is similar to the shape of the optical waveguide <NUM> according to the first embodiment. In the optical isolator <NUM>, a phase shifter <NUM> is not provided between a first branching part 18a and second and third branching parts 18b and 18c of the optical waveguide <NUM>, unlike the first embodiment. Instead, a portion between the second and third branching parts 18b and 18c of the optical waveguide <NUM> and second end parts 14a to 14d is a phase shifter <NUM> having non-reciprocity. The phase shifter <NUM> gives different non-reciprocal phase shift amounts between the first end part <NUM> and the second end parts <NUM>.

The phase shifter <NUM> includes non-reciprocal members 52a to 52d having non-reciprocity. Hereinafter, the non-reciprocal members 52a to 52d are sometimes collectively referred to as non-reciprocal members <NUM>. The non-reciprocal members <NUM> are disposed in planar contact with part of the optical waveguide <NUM>. The non-reciprocal members <NUM> generate a non-reciprocal phase shifting effect on light propagating through the optical waveguide <NUM>. The non-reciprocal members 52a to 52d are in contact with portions of the optical waveguide <NUM> that lead to the second end parts 14a to 14d, respectively. The portions of the optical waveguide <NUM> with which the non-reciprocal members 52a to 52d are in contact are non-reciprocal paths 53a to 53d, respectively. The non-reciprocal paths 53a to 53d are sometimes collectively referred to as non-reciprocal paths <NUM>.

The non-reciprocal members 52a and 52b are in contact with side surfaces of the non-reciprocal paths 53a and 53b on a positive side in the Y-axis direction. The non-reciprocal members 52c and 52d are in contact with side surfaces of the non-reciprocal paths 53c and 53d on a negative side in the Y-axis direction. The non-reciprocal paths 53a and 53b and the non-reciprocal paths 53c and 53d generate non-reciprocal phase shift amounts of opposite signs due to the difference in position of a side surface to which the non-reciprocal member <NUM> is joined.

The non-reciprocal member 52a and the non-reciprocal member 52b generate different non-reciprocity due to a difference in length in the X-axis direction. In a case where an identical magnetic field is applied, the non-reciprocal member 52a generates a larger non-reciprocal phase shift amount than the non-reciprocal member 52b. The non-reciprocal member 52c and the non-reciprocal member 52d generate different non-reciprocity due to a difference in length in the X-axis direction. In a case where an identical magnetic field is applied, the non-reciprocal member 52d generates a larger non-reciprocal phase shift amount than the non-reciprocal member 52c. The non-reciprocal member 52a and the non-reciprocal member 52d can have equal lengths. The non-reciprocal member 52b and the non-reciprocal member 52c can have equal lengths.

<FIG> is a simplified example in which four second end parts <NUM> are provided. For example, the number of second end parts <NUM> may be larger than this and can be, for example, <NUM>, <NUM>, <NUM>, or the like. The second end parts <NUM> are disposed at a predetermined pitch at equal intervals. All of the second end parts <NUM> are connected to the non-reciprocal paths <NUM> having different non-reciprocal phase shift amounts. The non-reciprocal phase shift amounts generated by the non-reciprocal paths <NUM> can be amounts that regularly differ from one second end part <NUM> to an adjacent one.

Light input from the first end part <NUM>, propagating through the optical waveguide <NUM> in the first direction, and output from the second end parts <NUM> can be caused to have the same phase shift amount, for example, by adjusting lengths of the optical waveguide <NUM> from the first end part <NUM> to the second end parts <NUM>. Accordingly, light L<NUM> entering in the X-axis direction from the first end part <NUM> of the optical isolator <NUM> has an identical phase when being output from the second end parts <NUM>. Light output from the second port <NUM> of the optical isolator <NUM> becomes light L<NUM> having directivity in a narrow angular range in the X-axis direction.

Positions and lengths of the non-reciprocal members <NUM> are set so that light entering from the second end parts <NUM>, propagating through the optical waveguide <NUM> in the second direction, and output from the first end part <NUM> has different predetermined phase differences. For example, in a case where <NUM> second end parts <NUM> are arranged at equal intervals in the Y-axis direction, phase shift amounts of the optical waveguide <NUM> from the second end parts <NUM> to the first end part <NUM> can be made different from each other by <NUM>°. In a case where each of the second end parts <NUM> has a non-reciprocal phase shift amount different from that of an adjacent second end part <NUM>, a wave front of light entering the second end parts <NUM> that has an identical phase when being output from the first end part <NUM> is inclined from a direction perpendicular to a direction (i.e., the X-axis direction) in which the optical waveguide <NUM> extends. Accordingly, the second port <NUM> has high coupling efficiency with light L<NUM> entering the second port <NUM> from a predetermined direction inclined from the X-axis direction. Meanwhile, it is difficult for light entering from a direction different from the predetermined direction from which the light L<NUM> enters, to enter the optical isolator <NUM> from the second port <NUM>.

The solid line in <FIG> indicates an angular distribution, at the second port <NUM>, of an intensity of transmission light entering the first port <NUM>, propagating through the optical isolator <NUM> in the first direction, and transmitted from the second port <NUM> to an external space. The broken line in <FIG> indicates dependency, on an incident angle at the second port <NUM>, of an intensity of reception light entering the second port <NUM> from an external space, propagating through the optical isolator <NUM> in the second direction, and output from the first port <NUM>. In <FIG>, the horizontal axis indicates an angle with respect to the Y-axis direction on the XY plane formed by the X-axis and the Y-axis. The solid line and the broken line each indicate a result obtained by simulation.

In the simulation, it is assumed that the optical isolator <NUM> has <NUM> second end parts <NUM> arranged one-dimensionally in the Y-axis direction at a pitch of <NUM>. Furthermore, it is assumed that no phase difference is generated as for light propagating in the first direction from the first end part <NUM> to the second end parts <NUM>. Conversely, as for light propagating in the second direction from the second end parts <NUM> to the first end part <NUM>, it is assumed that a phase difference of <NUM>° is generated between light entering from adjacent second end parts. Furthermore, it is assumed that a wavelength of light propagating through the optical isolator <NUM> is <NUM>.

According to <FIG>, an intensity of the transmission light is highest when an angle θ is <NUM>° (i.e., the X-axis direction). A normalized intensity of the transmission light is indicated by dB assuming that the intensity is <NUM> dB when the angle θ is <NUM>°. An intensity of the reception light is highest when an angle θ of a light beam entering the second port <NUM> is <NUM>° (i.e., an angle shifted by <NUM>° from the X-axis direction). A normalized intensity of the reception light is indicated by dB assuming that the intensity is <NUM> dB when the angle θ is <NUM>°.

As is clear from <FIG>, light entering the optical isolator <NUM> from the first port <NUM> and output from the second port <NUM> has strong directivity in a narrow range close to the angle <NUM>° (the X-axis direction). Meanwhile, light entering the optical isolator <NUM> from the second port <NUM> is easiest to enter in a case where the light enters from a narrow range close to the angle <NUM>°. In a case where light enters the optical isolator <NUM> at an angle <NUM>° (the X-axis direction), an intensity of output light at the first port <NUM> is smaller by <NUM> dB or more than in a case where light enters the optical isolator <NUM> at an angle <NUM>°. The optical isolator <NUM> thus allows return light travelling in a direction opposite to the light L<NUM> in <FIG> to propagate only slightly. For example, in a case where the second port <NUM> is connected to the optical transmission path 40A or 40B through a space as illustrated in <FIG> and <FIG> of the first embodiment, return light travels through an optical path substantially opposite to output light and enters the second port <NUM> in a negative direction of the X-axis. Such return light can only slightly enter the optical isolator <NUM> from the second port <NUM>.

As described above, the optical isolator <NUM> according to the present embodiment can realize a function of an optical isolator of allowing propagation of light in the first direction and reducing or suppressing propagation of light in the second direction, as in the first embodiment. Furthermore, the optical isolator <NUM> is configured such that the second end parts <NUM> are arranged at a predetermined pitch and phase shift amounts that regularly differ from each other are given between the first end part <NUM> and the second end parts <NUM>. With this configuration, a direction of light that can enter the second port <NUM> from an outside is restricted in a narrow direction different from a direction of light output from the second port <NUM>. As a result, entry into the optical isolator <NUM> and propagation through the optical isolator <NUM> of return light of light output from the second port <NUM> can be reduced or suppressed at the second port <NUM> with more certainty.

In <FIG>, the non-reciprocal members 52a and 52b and the non-reciprocal members 52c and 52d are disposed on different sides, in the Y-axis direction, of the non-reciprocal paths <NUM>, which are part of the optical waveguide <NUM>. It is possible to employ a configuration of an optical isolator <NUM> illustrated in <FIG> in which a phase shifter <NUM> includes non-reciprocal members 62a to 62c that have different sizes and are disposed only one side of non-reciprocal paths 63a to 63c, which are part of the optical waveguide <NUM>. The non-reciprocal members 62a to 62c are configured to give non-reciprocal phase shift amounts that regularly differ between adjacent ones of the second end parts 14a to 14c. In the example illustrated in <FIG>, a portion having non-reciprocity is not provided between the first end part <NUM> and the second end part 14d. Even according to such a configuration, light propagating in the second direction from the plurality of second end parts <NUM> to the first end part <NUM> can have different phase differences. This can produce operation and effects similar to those of the optical isolator <NUM> of <FIG>.

The optical isolators <NUM>, <NUM>, and <NUM> according to the first embodiment and the second embodiment are configured such that light in the first direction entering from the single first end part <NUM> and output from the plurality of second end parts <NUM> is allowed to propagate and light in the second direction opposite to the first direction is not allowed to propagate. However, an optical isolator may also be configured such that light in the second direction is allowed to propagate and light in the first direction is not allowed to propagate. A light source device <NUM> using such an optical isolator <NUM> is described with reference to <FIG> and <FIG>.

As illustrated in <FIG>, the light source device <NUM> includes the optical isolator <NUM>, a light source <NUM> that emits light to be input to the optical isolator <NUM>, a power supply <NUM> that supplies power to the light source <NUM>, and at least one lens <NUM> that collimates light from the light source <NUM> and causes the light to enter a second port <NUM>. The light source <NUM> can be a semiconductor laser similar to the light source <NUM> of the light source device <NUM> according to the first embodiment. In a case where light from the light source <NUM> is a parallel light beam, the lens <NUM> need not be provided. Furthermore, a plurality of lenses <NUM> may be used in combination in order to adjust a beam diameter of light from the light source <NUM>. The light source <NUM> and the lens <NUM> may be integrated on a substrate <NUM> of the optical isolator <NUM>, as in the light source device <NUM> according to the first embodiment. The light source <NUM>, the power supply <NUM>, and the lens <NUM> can be constituent elements separable from the optical isolator <NUM>.

The optical isolator <NUM> is configured such that non-reciprocal phase shift amounts that regularly differ from each other are given between a first end part <NUM> and second end parts <NUM>, as in the optical isolators <NUM> and <NUM> according to the second embodiment. In <FIG> and <FIG>, description of an internal structure of the optical isolator <NUM> is omitted. Unlike the optical isolators <NUM> and <NUM> according to the second embodiment, lengths of the optical waveguide <NUM> between the first end part <NUM> and the second end parts <NUM> of the optical isolator <NUM> are adjusted so that light of an identical phase input from the second end parts <NUM> is output from the first end part <NUM> as light of an identical phase. This allows light emitted from the light source <NUM> and then collimated to easily enter the second port <NUM> of the optical isolator <NUM>. That is, the light source <NUM> and the optical isolator <NUM> can be easily connected. The light source <NUM> and the optical isolator <NUM> can be configured so that coupling efficiency becomes high.

<FIG> illustrates a configuration of the light source device <NUM> and a light beam, indicated by the broken lines, of light emitted from the light source <NUM> and entering the optical isolator <NUM> from the second port <NUM>. Light that enters the plurality of second end parts <NUM> of the second port <NUM> in the X-axis direction is coupled by passing through the optical waveguide <NUM> in the optical isolator <NUM> and is output as light L<NUM> traveling in the X-axis direction from the first end part <NUM> of the first port <NUM>.

<FIG> illustrates a configuration of the light source device <NUM> and a light beam, indicated by the broken lines, of return light L<NUM> entering the first end part <NUM> of the first port <NUM> and returning to the light source device <NUM>. The return light L<NUM> enters the first end part <NUM> in a negative direction of the X-axis, which is opposite to the traveling direction of the light L<NUM> output from the first end part <NUM>. The return light L<NUM> is divided into a plurality of portions by passing through the optical waveguide <NUM> in the optical isolator <NUM> and is then output from the plurality of second end parts <NUM>. Phases of light output from the second end parts <NUM> regularly differ from each other due to non-reciprocity. For example, phases of light emitted from adjacent ones of the second end parts <NUM> differ from each other by <NUM>°. Accordingly, the plurality of light portions output from the second end parts <NUM> are output so that a direction normal to a wave front thereof is inclined from the negative direction of the X-axis. Therefore, the return light L<NUM> travels from the second end parts <NUM> in a direction inclined with respect to the negative direction of the X-axis and does not enter the light source <NUM>. This makes it possible to prevent the return light L<NUM> of light emitted from the light source <NUM> from entering the light source <NUM> and damaging the light source <NUM>, from destabilizing the light source <NUM>, or generating noise or the like due to interference.

An optical isolator <NUM> according to a fourth embodiment is described with reference to <FIG>. <FIG> is a plan view of the optical isolator <NUM>. <FIG> is a view illustrating the optical isolator <NUM> viewed from a first port <NUM> side. <FIG> is a view illustrating the optical isolator <NUM> viewed from a second port <NUM> side. <FIG> is a side view of the optical isolator <NUM>. The optical isolator <NUM> is partially similar to the optical isolator <NUM> according to the first embodiment, and constituent elements identical or similar to those of the optical isolator <NUM> are given identical reference signs, and description thereof is omitted as appropriate.

An optical waveguide <NUM> of the optical isolator <NUM> branches not only in the Y-axis direction on the XY plane, but also in the Z-axis direction. The optical waveguide <NUM> may branch in the Z-axis direction at any position other than at portions in contact with non-reciprocal members 20a to 20d. For example, as illustrated in <FIG> and <FIG>, the optical waveguide <NUM> may branch into two paths in each of the Y-axis direction and the Z-axis direction, that is, into four paths in total, at a first branching part 18a. Each of the four branch paths of the optical waveguide <NUM> branching at the first branching part 18a branches into two paths in the Y-axis direction at second to fifth branching parts 18b to 18e. As a result, eight second end parts <NUM> of the optical waveguide <NUM> are arranged in two-dimensional arrays in two layers at an end of the optical isolator <NUM> on the positive side in the X-axis direction, as illustrated in <FIG>. Furthermore, non-reciprocal members 20a to 20d are disposed in contact between the first branching part 18a and the second to fifth branching parts 18b to 18e, respectively.

The configuration of the optical isolator <NUM> illustrated in <FIG> is merely an example. The optical waveguide <NUM> of the optical isolator <NUM> may branch into a plurality of optical waveguides <NUM> in the Y-axis direction and the Z-axis direction which are orthogonal to the X-axis direction, which is a light propagation direction, at a plurality of branching parts <NUM> disposed at any positions. The number of layers of the second end parts <NUM> in the Z-axis direction is not limited to two and may be three or more. The number of second end parts <NUM> is not limited to eight and can be any number.

According to the above configuration, the second end parts <NUM> are arranged two-dimensionally at the second port <NUM> of the optical isolator <NUM>. Accordingly, in the optical isolator <NUM>, a larger number of second end parts <NUM> can be disposed per unit area at the second port <NUM>. For example, in the optical isolator <NUM>, the second end parts <NUM> can be arranged in arrays of m second end parts <NUM> in the Y-axis direction and n second end parts <NUM> in the Z-axis direction (m and n are any integers of two or more). This makes it possible to dispose the second end parts <NUM> at a density n times as high as that in a case where only m second end parts <NUM> are arranged in the Y-axis direction in a region of the same area in the optical isolator <NUM> according to the first embodiment. Accordingly, in a case where light output from the second port <NUM> of the optical isolator <NUM> is caused to enter an optical transmission path 40A as in <FIG>, more light can be caused to enter even in a case where a core having the same diameter is used. Furthermore, in a case where light output from the second port <NUM> of the optical isolator <NUM> is coupled with a core <NUM> of an optical transmission path 40B through a lens <NUM> as in <FIG>, a diameter of the lens <NUM> can be made smaller even in a case where the same number of second end parts <NUM> are provided. Furthermore, in a case where the second port <NUM> of the optical isolator <NUM> is an input side and the first port <NUM> of the optical isolator <NUM> is an output side as in the third embodiment, loss of light at the second port <NUM> can be reduced since the second end parts <NUM> are provided at a high density at the second port <NUM>.

An optical isolator <NUM> according to a fifth embodiment is described with reference to <FIG>. In the drawings, constituent elements identical or similar to those in the fourth embodiment are given identical reference signs.

In the fourth embodiment illustrated in <FIG>, the plurality of second end parts <NUM> are arranged in a grid pattern in the Y-axis direction and the Z-axis direction when viewed in the X-axis direction. In the fifth embodiment, an array of second end parts <NUM> arranged in the Y-axis direction on a positive side in the Z-axis direction and an array of second end parts <NUM> arranged in the Y-axis direction on a negative side in the Z-axis direction are displaced from each other. That is, the second end parts <NUM> are displaced in at least one direction from the grid pattern in a two-dimensional direction of the Y-axis direction and the Z-axis direction orthogonal to each other. <FIG> illustrates an example of a way in which the second end parts <NUM> are arranged when viewed from a positive side in the X-axis direction in the fifth embodiment. In a case where the second end parts <NUM> are arranged in this way, occurrence of interference between upper and lower second end parts can be reduced.

As illustrated in <FIG>, in the present embodiment, a first optical waveguide 12a and a second optical waveguide 12b can be provided as an optical waveguide <NUM>. The first optical waveguide 12a has a first end part <NUM>. The second optical waveguide 12b is coupled to the first optical waveguide 12a by a directional coupler <NUM> on a first end part <NUM> side of the optical isolator <NUM>. The directional coupler <NUM> constitutes one of branching parts <NUM>. As illustrated in <FIG>, the first optical waveguide 12a and the second optical waveguide 12b each branch two times between the directional coupler <NUM> and four second end parts <NUM>. Accordingly, eight second end parts <NUM> are arranged in two layers in the Z-axis direction at an end of the optical isolator <NUM> on a positive side in the X-axis direction. The first optical waveguide 12a and the second optical waveguide 12b have different shapes as illustrated in <FIG> so that a plurality of second end parts <NUM> arranged on a positive side in the Z-axis direction and a plurality of second end parts <NUM> arranged on a negative side in the Z-axis direction are displaced from each other.

Portions of each of the first optical waveguide 12a and the second optical waveguide 12b that lead to the second end parts <NUM> are a phase shifter <NUM> having non-reciprocity. The phase shifter <NUM> has a plurality of non-reciprocal members <NUM> having non-reciprocity. Each of the non-reciprocal members <NUM> is disposed in contact with the first optical waveguide 12a or the second optical waveguide 12b. The plurality of non-reciprocal members <NUM> give different phase shift amounts and therefore have different positions and different lengths in the X direction. The position and the length in the X direction of each of the non-reciprocal members <NUM> are determined so that a desired phase shift amount is given according to a position of a corresponding second end part <NUM>.

According to the present embodiment, not only effects similar to those of the optical isolator <NUM> according to the fourth embodiment are obtained, but also occurrence of interference between the plurality of second end parts <NUM> can be reduced. Note that the number of layers of the second end parts <NUM> in the Z-axis direction is not limited to two and can be three or more.

Claim 1:
An optical isolator (<NUM>) comprising:
a substrate (<NUM>); and
an optical waveguide (<NUM>) provided on the substrate (<NUM>),
the optical waveguide (<NUM>) including:
a first end part (<NUM>),
a plurality of second end parts (<NUM>) arranged in an array,
at least one branching part (<NUM>) located between the first end part (<NUM>) and the plurality of second end parts (<NUM>), and
a portion having non-reciprocity that provides different non-reciprocal phase shift amounts between the first end part (<NUM>) and at least two second end parts (<NUM>) of the plurality of second end parts (<NUM>),
characterized in that the non-reciprocal phase shift amounts differ between adjacent second end parts (<NUM>) of the plurality of second end parts (<NUM>).