Patent Description:
For general prior art, reference is made to patent document [<NUM>].

Patent document [<NUM>] discloses an optical device, including a light derivation section for deriving light from an optical waveguide in a direction crossing an axial direction of the optical waveguide for guiding light. The light derivation section includes one or more waveguide holes formed in a cladding section covering the optical waveguide, and an optical waveguide member that is disposed in the waveguide hole and guides light from the optical waveguide. The optical waveguide member is composed of light transmissive resin or an optical fiber.

As an optical branching technique, a wavelength multiplexing coupler or the like using an arrayed waveguide grating is known. Also, to realize optical sensing and monitoring of a transmission path, an optical side output technique using a tap waveguide has been suggested. By the optical side output technique, an optical waveguide is formed by laser processing in a fiber, and part of the power of light is output from the core (see Non Patent Literature <NUM>, for example).

A conventional wavelength multiplexing coupler is large in size, and has reflection and loss that are larger at connecting points. Therefore, it is difficult to dispose such wavelength multiplexing couplers at multiple points in a transmission path. Further, conventional tap waveguides are easily disposed at multiple points in a transmission path, but it is difficult to increase the wavelength selectivity of these tap waveguides.

Therefore, to solve the above problem, the present invention aims to provide an optical side input/output circuit that has wavelength selectivity and is easily disposed at multiple points in a transmission path, and an optical connector.

To achieve the above object, an optical side input/output circuit according to independent claim <NUM> is provided. Further embodiments are provided by the dependent claims.

The optical side input/output circuit has a fiber Bragg grating formed to give wavelength selectivity to the tap waveguide. Having the tap waveguide, the optical side input/output circuit is easily disposed at multiple points in a transmission path. Further, the optical side input/output circuit can input/output light of a desired wavelength with the fiber Bragg grating. Thus, the present invention can provide an optical side input/output circuit that has wavelength selectivity and is easily disposed at multiple points in a transmission path, and an optical connector.

With the optical side input/output circuit according to the present invention, light of a desired wavelength can be supplied to the optical fiber or light of a desired wavelength can be received from the optical fiber.

Note that the respective inventions described above can be combined as appropriate.

The present invention can provide an optical side input/output circuit that has wavelength selectivity and is easily disposed at multiple points in a transmission path, and an optical connector.

Embodiments of the present invention are described below with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to these embodiments. Note that components having the same reference signs in the present description and the drawings indicate the same components.

<FIG> is a diagram for explaining an optical side input/output circuit <NUM> according to this embodiment. The optical side input/output circuit <NUM> includes:.

The optical fiber <NUM> is a step-index fiber that is defined by the diameter dc of the core <NUM>, the diameter df of the optical fiber <NUM>, the refractive index ncore of the core <NUM>, and the refractive index nclad of a cladding <NUM>. In the optical fiber <NUM>, the tap portion <NUM> and the grating portion <NUM> are formed in this order in the longitudinal direction. The direction in which light entering from the tap waveguide <NUM> travels is the optical waveguide direction. In <FIG>, the optical waveguide direction is the direction from left to right. Further, the direction in which the tap waveguide <NUM> faces the side surface of the optical fiber <NUM> from the core <NUM> is the tap direction. In <FIG>, the tap direction is the direction inclined in the opposite direction to the optical waveguide direction.

The grating portion <NUM> reflects only the light of a desired wavelength in the light that has traveled in the optical waveguide direction and passed through the tap portion <NUM>, and returns the reflected light to the tap portion <NUM>. The coupling efficiency of light from the core <NUM> to the tap waveguide <NUM> in the tap portion <NUM> greatly depends on the light propagation direction. Specifically, light traveling in the optical waveguide direction in the core <NUM> is hardly coupled to the tap waveguide <NUM>. On the other hand, light traveling in the direction opposite to the optical waveguide direction in the core <NUM> can be coupled to the tap waveguide <NUM>, as appropriate α is set according to the mode coupling theory (see Non Patent Literature <NUM>, for example). Here, α is an angle (on the acute angle side) formed by the tap waveguide <NUM> and the core <NUM>.

The optical side input/output circuit <NUM> transmits light in the optical waveguide direction without being coupled to the tap waveguide <NUM> in the tap portion <NUM>, reflects only a desired wavelength in the grating portion <NUM>, returns the reflected light (light in the opposite direction to the optical waveguide direction) to the tap portion <NUM>, and couples the reflected light to the tap waveguide <NUM>.

The tap waveguide <NUM> and the grating <NUM> can be formed by locally modulating a refractive index of the optical fiber <NUM>, using femtosecond laser processing, for example. Here, the amounts of refractive index modulation (differences from the refractive index before modulation) in the core and the cladding are denoted by δncore and δnclad, respectively. That is, in the case of the core <NUM> (the overlapping portions of the grating <NUM>, the core <NUM>, and the tap waveguide <NUM>), the refractive index after the modulation is expressed as ncore + δncore. In the case of the cladding <NUM> (the tap waveguide <NUM> excluding the overlapping portions), the refractive index after the modulation is nclad + δnclad.

<FIG> is a graph for explaining the α dependence of the coupling efficiency at which light propagating in the opposite direction to the light propagation direction in the core <NUM> is coupled to the tap waveguide <NUM>. A tap waveguide <NUM> in which δncore = δnclad = <NUM>, and dt = <NUM> is formed for a fiber structure having a step-index refractive index distribution where dc = <NUM>, ncore = <NUM>, and nclad = <NUM>. The solid line indicates the efficiency of coupling to the core <NUM> (the ratio of light that is from the grating portion <NUM> and is propagating in the core <NUM>), and the dotted line indicates the efficiency of coupling to the tap waveguide <NUM> (the ratio of light that is from the grating portion <NUM> and is coupled to the tap waveguide <NUM>). As can be seen from the graph, the efficiency of coupling to the tap waveguide <NUM> changes with α, and the coupling efficiency is maximized when α = <NUM>°.

<FIG> is a diagram for explaining an example of an electrical field distribution in the tap portion <NUM> in a case where α = <NUM>°. Here, δncore = δnclad = <NUM>, and dt = <NUM>. Reflected light is supposed to propagate from right to left in the drawing. The wavelength of light is <NUM>. In the tap portion <NUM>, a state in which the light propagating in the core <NUM> is coupled to the tap waveguide <NUM> can be seen. However, the light is not coupled to a tap waveguide 53a extending in the opposite direction to the propagation direction. As can be seen from this drawing, the coupling to the tap waveguide <NUM> has great dependence (α dependence) on the orientation of the tap waveguide.

<FIG> is a graph for explaining the dt dependence of the coupling efficiency at which light propagating in the opposite direction to the light propagation direction in the core <NUM> is coupled to the tap waveguide <NUM>. A tap waveguide <NUM> in which δncore = δnclad = <NUM> is formed for a fiber structure having a step-index refractive index distribution where dc = <NUM>, ncore = <NUM>, and nclad = <NUM>. Here, α = <NUM>°. The solid line indicates the efficiency of coupling to the core <NUM> (the ratio of light that is from the grating portion <NUM> and is propagating in the core <NUM>), and the dotted line indicates the efficiency of coupling to the tap waveguide <NUM> (the ratio of light that is from the grating portion <NUM> and is coupled to the tap waveguide <NUM>). As can be seen from the graph, the efficiency of coupling to the tap waveguide <NUM> increases with the diameter dt of the tap waveguide <NUM>.

<FIG> is a graph for explaining the δn dependence of the coupling efficiency at which light propagating in the opposite direction to the light propagation direction in the core <NUM> is coupled to the tap waveguide <NUM>. A tap waveguide <NUM> in which dt = <NUM> is formed for a fiber structure having a step-index refractive index distribution where dc = <NUM>, ncore = <NUM>, and nclad = <NUM>. Here, α = <NUM>°. The solid line indicates the efficiency of coupling to the core <NUM> (the ratio of light that is from the grating portion <NUM> and is propagating in the core <NUM>), and the dotted line indicates the efficiency of coupling to the tap waveguide <NUM> (the ratio of light that is from the grating portion <NUM> and is coupled to the tap waveguide <NUM>). As can be seen from the graph, the efficiency of coupling to the tap waveguide <NUM> changes with the refractive index modulation amount δn, and the coupling efficiency is maximized when δncore = δnclad = <NUM>.

As can be seen from <FIG> and <FIG>, the efficiency of coupling to the tap waveguide depends on dt and δn, and it is possible to perform coupling about <NUM>% from the core <NUM> to the tap waveguide <NUM> by appropriately setting dt and δn.

The grating portion <NUM> reflects only the wavelength to be extracted by the fiber Bragg grating (FBG) <NUM> from the light traveling in the light propagation direction in the core <NUM>, and returns the reflected light to the tap portion <NUM>. The grating pitch Λ is calculated according to the expression shown below, from the wavelength λ in vacuum and the average value neff of the effective refractive indexes in the grating portion <NUM> at the wavelength λ (see Non Patent Literature <NUM>, for example). [Mathematical Expression <NUM>] <MAT>.

<FIG> is a graph for explaining the wavelength dependence of the grating pitch of the grating portion <NUM>. Note that this graph shows a calculation of a structure in which a grating having a refractive index modulation amount δn of <NUM> is provided for a fiber structure having a step-index refractive index distribution where dc = <NUM>, ncore = <NUM>, and nclad = <NUM>. For example, where a grating pitch Λ of <NUM> is set, the grating portion <NUM> can selectively reflect light having a wavelength of <NUM>.

<FIG> is a graph for explaining the wavelength dependence of the reflectivity of the grating portion <NUM>. Note that this graph shows the calculation result in a case where the effective refractive index neff = <NUM>, the refractive index modulation amount = <NUM>, the grating length Lg = <NUM>, and the grating pitch Λ = <NUM>. Further, the reflectivity is shown as normalized output power of the backward transmission light at the end of entrance to the grating portion <NUM>.

Note that it is possible to adjust the passband width (the wavelength range of reflected light) by changing δn and the grating length Lg. <FIG> is a graph for explaining the wavelength dependence of the reflectivity of the grating portion <NUM> when the grating length Lg is <NUM>. As can be seen from the graph, the wavelength range of reflected light is narrower than the waveform of the grating length Lg = <NUM> in <FIG>.

Note that the FBG can be formed by a CO2 laser or a femtosecond laser.

<FIG> is a diagram for explaining an optical side input/output circuit <NUM> according to this embodiment. The optical side input/output circuit <NUM> differs from the optical side input/output circuit <NUM> illustrated in <FIG> in that a plurality of sets of the tap portion <NUM> and the grating portion <NUM> is continuously arranged in the optical fiber <NUM>.

As the sets of the tap portion <NUM> and the grating portion <NUM> are arranged in the light propagation direction, the optical side input/output circuit <NUM> can perform tapping (which is taking out light of a desired wavelength from the optical fiber <NUM>) at any position in the transmission path. Further, as illustrated in <FIG>, the optical side input/output circuit <NUM> can form a multistage optical feed system by attaching optical feed elements <NUM> onto output portions on the side surface.

<FIG> and <FIG> are diagrams for explaining an optical connector <NUM> of this embodiment. The optical connector <NUM> includes an optical side input/output circuit <NUM>. The optical side input/output circuit <NUM> is the same as the optical side input/output circuit <NUM> described with reference to <FIG>, except for further including a light receiver <NUM> that is disposed on the side surface of the optical fiber <NUM> and receives the reflected light output from the tap portion <NUM>. Reference numeral <NUM> indicates the coating of the optical fiber <NUM>.

The optical connector <NUM> includes a ferrule <NUM> that has the optical side input/output circuit <NUM> therein, and a connector plug <NUM> that serves to connect to another optical connector. The shape of the connector plug <NUM> is of SC type, FC type, LC type, MPO type, or the like, which is widely used. By inserting the optical side input/output circuit <NUM> into the optical connector <NUM>, it is possible to easily connect to another optical fiber 50a, and realize optical side inputs/outputs from the optical fiber <NUM>.

Claim 1:
An optical side input/output circuit (<NUM>, <NUM>, <NUM>) comprising:
a grating portion (<NUM>) in which a fiber Bragg grating (<NUM>) that reflects light of a desired wavelength is formed in a core (<NUM>) of an optical fiber (<NUM>), wherein the optical fiber (<NUM>) is a step-index fiber, and the light of the desired wavelength being of light propagating in the core (<NUM>); and
a tap portion (<NUM>) that is disposed at a stage before the grating portion (<NUM>) in a propagation direction of the light, and is provided with a tap waveguide (<NUM>) that outputs a reflected light reflected by the grating portion (<NUM>) from a side surface of the optical fiber (<NUM>), wherein the tap waveguide (<NUM>) of the tap portion (<NUM>) is connected to the core (<NUM>) by passing through a cladding (<NUM>) of the optical fiber (<NUM>), and an acute angle (α) is formed by the tap waveguide (<NUM>) and the core (<NUM>).