Patent Description:
In general, when an optical fiber coupler is manufactured, a middle portion of each of the plurality of optical fibers is heated to melt a cladding of the optical fiber so that the middle portion may be stretched and joined to form a coupler portion. The coupler portion is, for example, inserted into a groove of a substrate, and is fixed inside the groove with an adhesive (see, for example, PATENT LITERATURE <NUM>). A coupler is provided which comprises a coupling part of a fusion coupler body formed of an optical fiber is fixed to a divided glass pipe by an adhesive, and a compound having a Shore D hardness of <NUM> or less is filled (see, for example, PATENT LITERATURE <NUM>). In addition, a protective structure is known which comprises a case for housing naked fiber parts not drawn at the time of producing an optical fiber coupler by melt bonding and drawing, first adhesive members for coating the naked fiber parts and second adhesive members for coating the first adhesive members and fixing the naked fiber parts in the case (see, for example, PATENT LITERATURE <NUM>). Further, PATENT LITERATURE <NUM> discloses an optical coupler which has a difference between a branching ratio determined immediately after fusing and drawing operation and a branching ratio determined after accommodating the optical coupler in a protective case and adhering it to the case, equal to or less than <NUM>%.

According to PATENT LITERATURE <NUM>, when the adhesive having a tensile shear adhesive strength of <NUM> MPa or more is used, it is possible to suppress variation of insertion loss of light passing through the optical fiber coupler. According to PATENT LITERATURE <NUM>, it is possible to reduce an influence of temperature, and according to PATENT LITERATURE <NUM>, it is possible to satisfactorily fix the optical fiber in a case and to reduce stress produced in naked fibers.

However, the optical fiber coupler does not consider factors other than the variation of optical insertion loss.

The optical fiber coupler according to the present invention has been invented in view of such circumstances. An object of the present invention is to suppress the variation of polarization state of the light passing through the coupler portion.

An optical fiber coupler according to the present invention includes: a substrate having a groove; a coupler portion which is inserted into the groove, wherein the coupler portion includes a plurality of optical fibers; a protective cylinder made of a metal member, wherein both end portions of the protective cylinder are sealed with a sealing portion, wherein the plurality of optical fibers penetrate the sealing portion and protrude outwardly; and adhesives for bonding the coupler portion to the substrate are respectively provided at both end portions of the groove, wherein Shore D hardness of the adhesives is <NUM> to <NUM>, each of the plurality of optical fibers includes a core, a cladding that covers a periphery of the core, and a covering portion that covers a periphery of the cladding, the coupler portion is formed by removing the covering portions of the plurality of optical fibers and heating middle portions of the plurality of optical fibers to melt the claddings and stretching and joining the middle portions, the adhesives fix the covering portions of the plurality of optical fibers and glass portions of the plurality of optical fibers from which the covering portions are removed to the groove, the coupler portion and the substrate are housed in the protective cylinder, and are fixed to the protective cylinder with a further adhesive.

In the optical fiber coupler according to an embodiment, a viscosity of the adhesives is <NUM> to <NUM> mPa·s.

In the optical fiber coupler according to the present invention, it is possible to suppress the variation of the polarization state of the light passing through the coupler portion by setting Shore D hardness of the adhesives to <NUM> to <NUM>.

Hereinafter, an optical fiber coupler <NUM> according to an embodiment will be described with reference to the drawings. <FIG> is a cross-sectional view schematically showing the optical fiber coupler <NUM>. <FIG> is a plan view schematically showing a substrate <NUM> and a coupler portion <NUM>. <FIG> is a schematic cross-sectional view taken along line III-III shown in <FIG>.

The optical fiber coupler <NUM> includes a columnar substrate <NUM>. As a material of the substrate <NUM>, quartz, Invar, Kovar, or the like can be used. In this example, quartz is used. The substrate <NUM> is formed with a groove 2a in its longitudinal direction. The coupler portion <NUM> is inserted into the groove 2a. The coupler portion <NUM> includes a first optical fiber <NUM> and a second optical fiber <NUM>. The first optical fiber <NUM> includes a core 11a, a cladding 11b that covers a periphery of the core 11a, and a covering portion 11c that covers a periphery of the cladding 11b. The second optical fiber <NUM> includes a core 12a, a cladding 12b that covers the periphery of the core 12a, and a covering portion 12c that covers the periphery of the cladding 12b. After removing the covering portions 11c and 12c of the first optical fiber <NUM> and the second optical fiber <NUM>, the claddings 11b and 12b are washed with alcohol or the like. Thereafter, middle portions of the first optical fiber <NUM> and the second optical fiber <NUM> are heated to melt the claddings 11b and 12b of the first optical fiber <NUM> and the second optical fiber <NUM> so that the middle portions may be stretched and joined to form the coupler portion <NUM>. Note that the coupler portion <NUM> may include three or more optical fibers.

In the coupler portion <NUM>, a ratio (branching ratio) between an amount of light passing through the first optical fiber <NUM> and an amount of light passing through the second optical fiber <NUM> is a predetermined ratio. For example, when the branching ratio is set to <NUM>:<NUM>, <NUM>% of the amount of light introduced into the first optical fiber <NUM> moves to the second optical fiber <NUM> through the coupler portion <NUM>.

Adhesives <NUM> are respectively provided at two locations of the groove 2a. The adhesive <NUM> includes, for example, a visible light curable resin material or an ultraviolet curable resin material, and includes an epoxy resin material or an acrylate resin material. The adhesives <NUM> are arranged at both end portions of the groove 2a.

Shore D hardness of the cured adhesive <NUM> is, for example, <NUM> to <NUM>, and preferably <NUM> to <NUM>. If the Shore D hardness is less than <NUM>, when environmental temperature rises, the adhesive <NUM> is too soft and the coupler portion <NUM> is easily deformed. If the Shore D hardness exceeds <NUM>, stress tends to concentrate on a part of the coupler portion <NUM> and the coupler portion <NUM> is easily distorted. When distortion occurs in the coupler portion <NUM>, polarization state of the light passing through the coupler portion <NUM> is likely to vary.

A viscosity of the adhesive <NUM> before curing is, for example, <NUM> to <NUM> mPa·s. When the viscosity is less than <NUM> mPa·s, the adhesive <NUM> spreads too much into the groove 2a due to capillary action, and an area of the adhesive <NUM> adhering to the coupler portion <NUM> is too large. On the other hand, when the viscosity is more than <NUM> mPa s, the adhesive <NUM> is too hard and it is difficult to apply the adhesive <NUM> to the coupler portion <NUM> in the groove 2a.

Portions near both ends of the coupler portion <NUM>, in other words, the portions where the coupler section <NUM> branches into the first optical fiber <NUM> and the second optical fiber <NUM> are disposed at both end portions of the groove 2a. As described above, the adhesives <NUM> are respectively provided at both end portions of the groove 2a. The adhesive <NUM> fixes the covering portions 11c and 12c of the first optical fiber <NUM> and the second optical fiber <NUM> and glass portions of the first optical fiber <NUM> and the second optical fiber <NUM> from which the covering portions 11c and 12c are removed (portions near both ends of the coupler portion <NUM> of the claddings 11b, 12b and the cores 11a, 12a and where the first optical fiber <NUM> and the second optical fiber <NUM> are not thinned) to the groove 2a.

The coupler portion <NUM> and the substrate <NUM> are housed in a protective cylinder <NUM> made of a metal member, and are fixed to the protective cylinder <NUM> with an adhesive. As a material of the protective cylinder <NUM>, SUS, Invar, Kovar, or the like can be used. SUS is used in the example. Both end portions of the protective cylinder <NUM> are sealed with a sealing portion <NUM>. The sealing portion <NUM> contains, for example, a silicone resin material. The first optical fiber <NUM> and the second optical fiber <NUM> penetrate the sealing portion <NUM> and protrude outwardly.

By setting the Shore D hardness of the cured adhesive <NUM> to <NUM> or less, it is possible to suppress variation of the polarization state of the light passing through the coupler portion <NUM> with respect to temperature change. <FIG> is a block diagram schematically showing a configuration for measuring temperature dependence of the polarization state.

The optical fiber coupler <NUM> is provided in a heating/cooling device <NUM>. The heating/cooling device <NUM> includes a Peltier element. A light source <NUM> is attached to one end portion of the first optical fiber <NUM>, and a power meter <NUM> is attached to the other end portion. The power meter <NUM> measures the amount of light that has passed through the coupler portion <NUM> in the first optical fiber <NUM>. The power meter <NUM> can measure insertion loss of the light with respect to temperature change.

Nothing is attached to one end portion of the second optical fiber <NUM>, and a polarimeter <NUM> is attached to the other end portion. As the polarimeter <NUM>, a free space polarimeter "PAX5710 series" manufactured by Thorlabs, Inc. can be used. In the example, "PAN5710IR-T" has been used. The polarimeter <NUM> can measure the variation of the polarization state of the light that has passed through the coupler portion <NUM> with respect to temperature change.

<FIG> is a diagram schematically showing a locus of an electric field vector projected on an XY plane when elliptically polarized light travels in Z direction. In <FIG>, X-axis and Y-axis are perpendicular to each other. Z-axis is perpendicular to the X-axis and the Y-axis and extends in a direction perpendicular to a paper surface. The light that has passed through the coupler portion <NUM> travels with elliptical polarization. When the light travels in the Z-axis direction, the locus of the electric field vector projected on the XY plane is an ellipse <NUM> as shown in <FIG>. Note that an angle formed by a major axis <NUM> of the ellipse <NUM> and the X-axis, that is, an azimuth angle is defined as θ. An angle between a line segment <NUM> connecting an intersection of the ellipse <NUM> and the major axis <NUM> and an intersection of the ellipse <NUM> and a minor axis <NUM>, and the major axis <NUM>, that is, an ellipticity angle is defined as η.

<FIG> is a graph showing the azimuth angle θ measured over time by the polarimeter <NUM>. <FIG> is a graph showing the ellipticity angle η measured over time by the polarimeter <NUM>. In <FIG> and <FIG>, a trace <NUM> shows a measurement result for the optical fiber coupler <NUM> using the adhesive <NUM> having a Shore D hardness of <NUM>. A trace <NUM> shows a measurement result for the optical fiber coupler <NUM> using the adhesive <NUM> having a Shore D hardness of <NUM>. Traces <NUM> and <NUM> show measurement results for the optical fiber coupler <NUM> using the adhesive <NUM> having a Shore D hardness of <NUM>.

A temperature at start of measurement is <NUM>. When <NUM> seconds have elapsed after the start of measurement, the temperature of the heating/cooling device <NUM> is changed to <NUM>, when <NUM> seconds have elapsed, the temperature of the heating/cooling device <NUM> is changed to <NUM>, and when <NUM> seconds have elapsed, the temperature of the heating/cooling device <NUM> is changed to <NUM>.

As shown by the trace <NUM> of <FIG> and <FIG>, when the adhesive <NUM> having a Shore D hardness of <NUM> is used, the azimuth angle θ and the ellipticity angle η vary greatly after the temperature change. On the other hand, as shown by traces <NUM> to <NUM>, when the adhesive <NUM> having a Shore D hardness of <NUM> or <NUM> is used, the azimuth angle θ and the ellipticity angle η hardly vary even after the temperature change.

Variation ranges of the azimuth angle θ and the ellipticity angle η in the traces <NUM> to <NUM> will be described. <FIG> is a graph showing a variation value Δθ of the azimuth angle θ with reference to a value at the start of measurement. As shown by the trace <NUM> of <FIG>, when the adhesive <NUM> having a Shore D hardness of <NUM> is used, the variation value Δθ is in a range of -<NUM> to +<NUM> degree. As shown by the traces <NUM> and <NUM> of <FIG>, the variation value Δθ is in the range of -<NUM> to +<NUM> degrees.

<FIG> is a graph showing a variation value Δη of the ellipticity angle η with reference to the value at the start of measurement. As shown by the trace <NUM> of <FIG>, when the adhesive <NUM> having a Shore D hardness of <NUM> is used, the variation value Δη is within the range of -<NUM> to <NUM> degrees. As shown by the traces <NUM> and <NUM> of <FIG>, the variation value Δη is within the range of -<NUM> to +<NUM> degrees.

As described above, when the adhesive <NUM> having a Shore D hardness of <NUM> or less is used, the azimuth angle θ and the ellipticity angle η hardly vary. That is, it is possible to suppress the variation of the polarization state with respect to temperature change in the light passing through the coupler portion <NUM>.

Moreover, by setting the viscosity of the adhesive <NUM> to <NUM> to <NUM> mPa·s, it is possible to prevent the adhesive <NUM> from adhering to unnecessary portions of the coupler portion <NUM>. Further, the adhesive <NUM> can be smoothly applied to the coupler portion <NUM> in the groove 2a. Therefore, manufacturing operation of the optical fiber coupler <NUM> can be performed smoothly.

Further, as characteristics of the optical fiber coupler <NUM>, absolute values of the variation ranges of the azimuth angle θ and the ellipticity angle η are desirably <NUM> degrees or less, and more desirably <NUM> degrees or less. As described above, the absolute value of the variation range of the azimuth angle θ is <NUM> degrees or less and the absolute value of the variation range of the ellipticity angle η is <NUM> degrees or less.

The optical fiber coupler <NUM> can be used in an optical interferometer that requires stability of polarization characteristics. The optical fiber coupler <NUM> is used, for example, in an apparatus using OCT (Optical Coherence Tomography). In general, the apparatus is provided with a polarization controller. When the apparatus is used, degree of polarization is adjusted by operating the polarization controller depending on the temperature of an environment where the apparatus is installed. When the polarization controller is driven manually, user operation is required. However, a user may forget the operation. Typically, when the user has not operated the polarization controller and has not adjusted the degree of polarization despite the fact that the temperature has changed significantly, the polarization state of the optical fiber coupler <NUM> has changed, and thus there has been a problem in using the apparatus.

In the optical fiber coupler <NUM> according to the embodiment, the absolute values of the variation ranges of the azimuth angle θ and the ellipticity angle η with respect to temperature change are <NUM> degrees or less. Therefore, once the polarization is adjusted, it is not necessary to perform readjustment due to the change in polarization by the temperature change, so that convenience for the user can be improved.

Even when the temperature is changed in a range of <NUM> to <NUM>, the absolute values of the variation ranges of the azimuth angle θ and the ellipticity angle η are <NUM> degrees or less. That is, the variation of the polarization state can be suppressed at the environmental temperature assumed to be normally used.

As the optical fiber used in the present invention, an optical fiber that transmits the light having a wavelength band of <NUM> to <NUM> in a single mode is selected, and the optical fiber having a cladding diameter of <NUM> and a coating diameter of <NUM> is generally selected. In the embodiment, Coming HI780 has been used as an example of the optical fiber.

Claim 1:
An optical fiber coupler (<NUM>) comprising:
a substrate (<NUM>) having a groove (2a);
a coupler portion (<NUM>) which is inserted into the groove (2a), wherein the coupler portion (<NUM>) includes a plurality of optical fibers (<NUM>, <NUM>);
a protective cylinder (<NUM>) made of a metal member, wherein both end portions of the protective cylinder (<NUM>) are sealed with a sealing portion (<NUM>), wherein the plurality of optical fibers (<NUM>, <NUM>) penetrate the sealing portion (<NUM>) and protrude outwardly; and
adhesives (<NUM>) for bonding the coupler portion (<NUM>) to the substrate (<NUM>) are respectively provided at both end portions of the groove (2a), wherein
Shore D hardness of the adhesives (<NUM>) is <NUM> to <NUM>,
each of the plurality of optical fibers (<NUM>, <NUM>) includes a core (11a, 12a), a cladding (11b, 12b) that covers a periphery of the core (11a, 12a), and a covering portion (11c, 12c) that covers a periphery of the cladding (11b, 12b),
the coupler portion (<NUM>) is formed by removing the covering portions (11c, 12c) of the plurality of optical fibers (<NUM>, <NUM>) and heating middle portions of the plurality of optical fibers (<NUM>, <NUM>) to melt the claddings (11b, 12b) and stretching and j oining the middle portions,
the adhesives (<NUM>) fix the covering portions (11c, 12c) of the plurality of optical fibers (<NUM>, <NUM>) and glass portions of the plurality of optical fibers (<NUM>, <NUM>) from which the covering portions (11c, 12c) are removed to the groove (2a),
the coupler portion (<NUM>) and the substrate (<NUM>) are housed in the protective cylinder (<NUM>), and are fixed to the protective cylinder (<NUM>) with a further adhesive.