Abstract: An optical module for connecting a photoelectric conversion device on a substrate to a ferrule connected to an optical fiber includes a body configured to be mounted on the substrate, a first lens disposed on the body at a side thereof connectable to the ferrule, a second lens disposed on the body at a side thereof facing the substrate, and a core disposed in the body between the first lens and the second lens, wherein a refractive index of the core is higher than a refractive index of the body.

Abstract: An apparatus for reading out X-ray information stored in a storage phosphor layer includes a light source for generating stimulating light which can stimulate the storage phosphor layer to emit stimulated emission, an optical fiber, in particular a single mode fiber, which is optically coupled to the light source and generates a stimulating light beam from the stimulating light, and a deflection element for deflecting the stimulating light beam to move over the storage phosphor layer.

Abstract: A quasi-single-mode optical fiber with a large effective area is disclosed. The quasi-single-mode fiber has a core with a radius greater than 5 ?m, and a cladding section configured to support a fundamental mode and a higher-order mode. The fundamental mode has an effective area greater than 170 ?m2 and an attenuation of no greater than 0.17 dB/km at a wavelength of 1530 nm. The higher-order mode has an attenuation of at least 1.0 dB/km at the wavelength of 1530 nm. The quasi-single-mode optical fiber has a bending loss of less than 0.02 dB/turn for a bend diameter of 60 mm for a wavelength of 1625 nm.

Abstract: A multicore fiber is provided. The multicore fiber includes a plurality of cores spaced apart from one another, and a cladding surrounding the plurality of cores and defining a substantially rectangular or cross-sectional shape having four corners. Each corner has a radius of curvature of less than 1000 microns. The multicore fiber may be drawn from a preform in a circular draw furnace in which a ratio of a maximum cross-sectional dimension of the preform to an inside diameter of the preform to an inside diameter of the draw furnace is greater than 0.60. The multicore fiber may have maxima reference surface.

Abstract: Optical transmission systems and methods are disclosed that utilize a QSM optical fiber with a large effective area and that supports only two modes, namely the fundamental mode and one higher-order mode. The optical transmission system includes a transmitter and a receiver optically coupled by an optical fiber link that includes at least one section of the QSM optical fiber. Transmission over optical fiber link gives rise to MPI, which is mitigated using a digital signal processor. The QSM optical fiber is designed to have an amount of DMA that allows for the digital signal processor to have reduced complexity as reflected by a reduced number of filter taps as compared to if the DMA were zero.

Abstract: Electro-optical circuit boards may include one or more recesses to provide stress relief between multiple layers of the electro-optical circuit boards during variations in applied temperature. The one or more recesses may be included in the substrate and/or the optical layer of the electro-optical circuit boards. The one or more recesses may also contain a compliant material disposed therein to improve the flexibility of the substrate and/or the optical layer. For example, the compliant material may disperse thermal expansion stress within the electro-optical circuit board.

Abstract: An optical fiber includes a core, and a clad surrounding an outer circumference of the core, in which a first relative refractive index difference ?1a is greater than 0, a second relative refractive index difference ?1b is greater than 0, the first relative refractive index difference ?1a is greater than the second relative refractive index difference ?1b, the first relative refractive index difference ?1a and the second relative refractive index difference ?1b satisfy a relationship denoted by the following expression: 0.20?(?1a??1b)/?1a?0.88, and a refractive index profile ? of the core in an entire region of a section of 0?r?r1 as a function ?(r) of a distance r from a center of the core in the radial direction is denoted by the following expression: ?(r)=?1a?(?1a??1b)r/r1.

Abstract: An MCF of the present embodiment has eight or more cores. A diameter of a common cladding is not more than 126 ?m. Optical characteristics of each core are as follows: a TL at a predetermined wavelength of 1310 nm is not more than 0.4 dB/km; an MFD at the predetermined wavelength is from 8.0 ?m to 10.1 ?m; a BL in a BR of not less than 5 mm or in the BR of not less than 3 mm and, less than 5 mm is not more than 0.25 dB/turn at the predetermined wavelength; ?0 is from 1300 nm to 1324 nm; ?cc is not more than 1260 nm; an XT or XTs at the predetermined wavelength is not more than 0.001/km.

Abstract: The invention relates to a method for producing a polarization-maintaining optical fiber, consisting of a core region and stress-generating elements embedded in the fiber body, having the following method steps: producing a core preform for the core region using internal deposition on a substrate tube, the internally coated substrate tube subsequently being collapsed, generating recesses on the core preform by virtue of the material on the outer surface of the core preform being removed parallel to the longitudinal axis of the core preform at diametrically opposed positions, filling the recesses with stress-generating rods, with the tightest possible rod packing, in a freely selectable first filling geometry, possibly filling the recesses in addition with non-stress-generating rods in a second filling geometry, sheathing the filled core preform with a jacketing tube, preparing the sheathed core preform for a fiber-drawing process, and drawing the sheathed arrangement to form the optical fiber.

Abstract: Structures and techniques are described relating to the alignment of multicore fibers within a multifiber connector. These structures and techniques include: multicore fibers having a number of different shapes, including, for example, circular, elliptical, D-shaped, double D-shaped, and polygonal; multifiber ferrules, having a plurality of fiber guide holes therein of various shapes; alignment fixtures for aligning multicore fibers within multifiber ferrules; and various multicore fiber alignment techniques.

Abstract: Some implementations described herein involve defining a viewing angle range, also referred to herein as a viewing cone. The viewing cone may be produced by an array of optical fibers on a display. The optical fiber array may include tapered optical fibers that are capable of increasing the amount of light transmitted through the optical fiber array. The optical fiber array may be a graded index optical fiber array, wherein the refractive index of the optical fiber cores varies along the axis of the optical fibers.

Abstract: A bend-insensitive multimode optical fiber includes a core layer, and cladding layers surrounding the core layer. The core layer has a parabolic refractive index profile with ? being 1.9-2.2, a radius being 23-27 ?m, and a maximum relative refractive index difference being between 0.9-1.2%. The inner cladding layer has a width being 1-3 ?m and a relative refractive index difference being between ?0.05% and 0.1%. The trench cladding layer has a width being 2-5 ?m and a relative refractive index difference being between ?1% and ?0.3%. The core layer is a Ge/F co-doped silica glass layer, where an F doping contribution at a central position of the core layer is less than or equal to 0%, an F doping contribution at an edge portion of the core layer is greater than or equal to ?0.45%. The outer cladding layer is a pure silica glass layer.

Abstract: An optical fiber containing an alkali metal and capable of reducing Rayleigh scattering loss is provided. An optical fiber has a core and a cladding made of silica glass and enclosing the core. The cladding contains fluorine and has a refractive index lower than the refractive index of the core. The core contains first group dopants selected from the group of Na element, K element, or a compound thereof at an average concentration of 0.2 ppm or more and 10 ppm or less. The core also contains second group dopants for reducing the viscosity of silica glass and having a diffusion coefficient of 1×10?12 cm2/s or more and smaller than the diffusion coefficient of the first group dopants, by an average concentration of 0.2 ppm or more at a temperature of 2000° C. to 2300° C.

Abstract: A multicore fiber alignment apparatus is described, having a chassis into which is mounted ferrule-holding means for holding a multicore fiber ferrule having one or more capillaries extending therethrough. Fiber-holding means for holding one or more multicore fibers in position to be mounted into the ferrule, such that each multicore fiber extends through a respective ferrule capillary. Means are provided for monitoring the rotation angle of each multicore fiber within its respective capillary, relative to a reference rotational orientation. Means are further provided for rotating each of the multicore fibers within its respective capillary. The rotational orientation of each multicore fiber is fixed when its rotation angle is equal to zero.

Abstract: A dual-slot waveguide receives energy from a coupling waveguide. The dual-slot waveguide includes first and second light propagating regions of low-index material located side-by-side in a direction normal to a light propagation direction. Inner sides of the first and second light propagating regions are separated by a first region of a high-index material. Second and third regions of the high-index material surround outer sides of the first and second light propagating regions. A near-field transducer receives portions of the energy from the first and second light propagating regions.

Abstract: A multicore fiber 1 includes a plurality of cores 3 disposed at predetermined intervals and surrounded by a cladding 5. The multicore fiber 1 also includes a marker 7 formed apart from the cores 3. The refractive index of the marker 7 is different from those of the cores 3 and the cladding 5. For example, the marker 7 may be made of a material having lower refractive index than that of the cladding 5. In this case, for example, the cores 3 may be made of germanium-doped quartz. The cladding 5 may be made of pure quartz. The marker 7 may be made of fluorine-doped quartz. Further, the marker 7 may be an empty hole.

Abstract: An optical fiber having a large mode area for high power applications and geometrically configured for single-mode operation. One example of an optical fiber system includes a mandrill and an optical fiber helically coiled about the mandrill with a preselected bend radius. The optical fiber includes a core having a high aspect ratio elongated cross-section, wherein the core is narrower in a fast-axis direction and wider in a slow-axis direction, the core including an annular protrusion that is Gaussian in transverse cross-section and has a width in the slow-axis direction and an annular extension in the fast-axis direction, and wherein a ratio of the width, the annular extension, and the bend radius is selected for single-mode operation of the optical fiber.

Abstract: The present disclosure describes a method for optically powering transducers and related transducers with a photovoltaic collector. An optical fiber power delivery method and a free space power delivery method are also provided. A fabrication process for making an optically powered transducer is further described, together with an implantable transducer system based on optical power delivery.

Abstract: An optical device includes an SOI substrate, the embedded insulating layer having a thickness of 200 nanometers (nm) or less; an optical waveguide comprising a Group III-V compound semiconductor material formed on top of the SOI substrate; and an optical leakage preventing layer formed inside the SOI substrate on a bottom side of the optical waveguide to prevent leakage of light from inside the optical waveguide towards the SOI substrate.

Abstract: In an optical waveguide having plural cores including a pair of adjacent cores with an identical core structure, a minimum value D of center-center distance between the adjacent cores is 15 ?m to 60 ?m, each of the plural cores has a bent portion fixed in a radius of curvature Rb of not more than 7 mm, a bend supplementary angle of the bent portion is 58° to 90°, a height of the optical waveguide is defined as a height of not more than 10 mm, and a crosstalk of the adjacent cores is not more than 0.01.

Abstract: A method of designing multicore optical fibers is provided. A geometry for the core arrangement is selected. At least one of i) core width, ii) core position with respect to other cores, or iii) orientation with respect to incoming, outgoing, or at least partially traversing radiation such as an inscription beam are optimized. A design space is created in which no core shadows or blocks any other core with respect to incoming, outgoing, or at least partially traversing radiation. Optimization generally includes tracing tangents of core widths against an orthogonal axis and ensuring no overlap of space between said tangents on said axis. For twisted fiber, optimization also includes optimizing effective length and twist rate of the fiber. Devices entailing such fibers, such as multicore pump coupler and multicore fiber distributed feedback laser, are also contemplated.

Abstract: A photonic lantern spatial multiplexer that provides mode selectivity includes a multimode optical waveguide and a plurality of single mode optical waveguides. The single mode cores of the single mode optical waveguides merge with the multimode core of the multimode optical waveguide. At least two of the single mode cores have different respective single mode effective refractive indexes.

Abstract: A multi-core fiber (1) is a multi-core fiber including 10 or greater of even numbered cores and a cladding surrounding the core. In the even numbered cores, a half of cores (11a) are disposed in such a manner that centers are located on the apexes of a regular polygon (RP) whose center is at an origin point (O) in a cladding (20). In the even numbered cores, other cores (11b) are disposed in a manner that centers are located on perpendicular bisectors (LV) of the edges of a regular polygon on the inner side of the regular polygon (RP). The other cores (11b) are disposed in a specific range in the regular polygon (RP).

Abstract: A polarization-maintaining optical fiber of the invention includes: a core; a pair of stress-applying parts disposed at both sides of the core at a distance; and a cladding coat that surrounds the core and the paired stress-applying parts. The maximum refractive index of the core is greater than each of maximum refractive indexes of a first cladding coat, a second cladding coat, and a third cladding coat. The maximum refractive index of the second cladding coat is lower than each of maximum refractive indexes of the first cladding coat and the third cladding coat. The coefficient of thermal expansion of each of stress-applying parts is greater than a coefficient of thermal expansion of the cladding coat. Each stress-applying part is provided to cut the second cladding coat at a position in a circumferential direction.

Abstract: Provided is a method of producing a preform 10P for a coupled multi-core fiber including: an arranging process P1 for arranging a plurality of core glass bodies 11R and a clad glass body 12R in such a way that the plurality of core glass bodies 11R are surrounded by the clad glass body 12R; and a collapsing process P2 for collapsing a gap between the core glass bodies 11R and the clad glass body 12R, wherein the respective core glass bodies 11R have outer regions 16 having a predetermined thickness from the periphery surfaces and made of silica glass undoped with germanium, and the clad glass body 12R is made of silica glass having a refractive index lower than a refractive index of the outer regions of the core glass bodies 11R.

Abstract: In an optical waveguide having plural cores including a pair of adjacent cores with an identical core structure, a minimum value D of center-center distance between the adjacent cores is 15 ?m to 60 ?m, each of the plural cores has a bent portion fixed in a radius of curvature Rb of not more than 7 mm, a bend supplementary angle of the bent portion is 58° to 90°, a height of the optical waveguide is defined as a height of not more than 10 mm, and a crosstalk of the adjacent cores is not more than 0.01.

Abstract: A multi-core fiber for accommodating multiple single mode cores in one optical fiber is provided with multiple types of non-identical cores having different propagation constants. Each of the multiple types of non-identical cores includes a core part and a cladding part covering an outer periphery of the core part. The cladding part has a double cladding structure including a first cladding for surrounding an outer periphery of the core part and a second cladding on the outer side of the first cladding. In the multiple types of non-identical cores, an optical electromagnetic profile in the core part and the first cladding is confined within a range of the core part and the first cladding, restricting a leakage thereof to the second cladding, and the propagation constant of each non-identical core is made different using the refractive index of the first cladding as a parameter.

Abstract: An optical fiber includes a core region having a longitudinal axis. At least a portion of the core region has a substantially helical shape about a helical axis. The longitudinal axis may be substantially tangential to a helical bend in the optical fiber. A cladding region surrounds the core region. The core region and cladding region may be configured to support and guide the propagation of signal light in a fundamental transverse mode in the core region in the direction of the longitudinal axis. The fiber has a bend-induced gradient in its equivalent index of refraction over the portion of the core region. The fiber has a bend-induced equivalent index of refraction. At least a portion of cladding region has a graded refractive index opposite that of the bend-induced gradient. The cladding region may be configured to have a substantially flat equivalent index in response to a helical bend of the optical fiber.

Abstract: A multicore fiber according to an aspect of the present invention includes a plurality of cores and a cladding surrounding the plurality of the cores. In this multicore fiber, a pair of cores is arranged and disposed on a linear line passed through the center of the cladding, the pair of the cores being adjacent to each other and having refractive indexes varied differently from the cladding to the cores.

Abstract: Methods for making active laser fibers include the production of an optical fiber with disturbed (or deviated) cylindrical symmetry on the glass surface of the fiber. The methods include a preform containing a central core made of glass. In one embodiment, the preform is circular and surrounded by additional glass rods and an outer glass jacket tube. In a first alternative embodiment, this preform is merged during fiber drawing. In a second alternative embodiment, the preform merged in a process forming a compact glass body with disturbed cylindrical symmetry. This compact preform is drawn into a fiber under conditions maintaining the disturbed cylindrical symmetry.

Abstract: An optical waveguide has: a core layer which includes at least one core portion for transmitting a light signal and at least two side cladding portions respectively provided at lateral sides of the core portion so as to be opposed to each other; and two cladding layers respectively provided at vertical sides of the core layer. The core layer is configured to have a horizontal refractive index distribution curve W in a width direction of a cross-sectional plane of the core layer. The horizontal refractive index distribution curve W has a region including at least two local minimum values, at least one first local maximum value and at least two second local maximum values smaller than the first local maximum value. A refractive index in whole of the horizontal refractive index distribution curve W continuously varies.

Abstract: A multicore fiber includes a plurality of cores, a cladding that encloses the plurality of the cores, and a marker disposed in the cladding. The plurality of the cores is arranged and disposed on a linear line passed through the center of the cladding. The marker is disposed along the length direction of the cladding on a portion on which the marker does not overlap the cores in a first direction in which the plurality of the cores is arranged on the linear line and does not overlap the core in a second direction orthogonal to the first direction.

Abstract: A method and system for distributed spatial mode processing is disclosed where a plurality of optical signals is received via a plurality of spatial modes on a first optical link, spatial mode conversion is performed on the plurality of optical signals to switch the plurality of optical signals to different ones of the plurality of spatial modes and the plurality of optical signals is transmitted via the different ones of the plurality of spatial modes on a second optical link where spatial mode filtering may occur.

Abstract: An optical fiber has two or more core regions disposed within a common cladding region. Each of the core regions is configured to guide a respective light transmission comprising at least one optical mode along the length of the fiber. The cores are arranged within the common cladding region according to a core configuration that substantially prevents crosstalk between modes of neighboring cores in the fiber, in a deployment of the fiber in which cross-coupling between neighboring cores is affected by perturbations arising in the deployed fiber.

Abstract: Field-flattening strands may be added to and arbitrarily positioned within a field-flattening shell to create a waveguide that supports a patterned, flattened mode. Patterning does not alter the effective index or flattened nature of the mode, but does alter the characteristics of other modes. Compared to a telecom fiber, a hexagonal pattern of strands allows for a three-fold increase in the flattened mode's area without reducing the separation between its effective index and that of its bend-coupled mode. Hexagonal strand and shell elements prove to be a reasonable approximation, and, thus, to be of practical benefit vis-à-vis fabrication, to those of circular cross section. Patterned flattened modes offer a new and valuable path to power scaling.

Abstract: A multi-core optical fiber according to an embodiment of the present invention is provided with a plurality of core parts, a common cladding, and a coating. Particularly, in order to improve a spectral efficiency per unit sectional area, optical properties typified by the number of core parts, a sectional area of the entire multi-core optical fiber, the sum of power coupling coefficients to a core part n from all the other core parts, and a transmission loss, a non-linear refractive index, an effective area, and a chromatic dispersion of the core part n with the largest crosstalk from other core parts are set so as to satisfy a predetermined relation.

Abstract: Photonic bandgap fibers are described that can be solid across the core and clad and have a large core diameter with little loss in the fundamental mode. In addition, the mode loss of the higher order modes can be much greater than that of the fundamental mode, providing high power fibers with high effective mode area. Excellent single mode output can be obtained from the fibers in length scale close to what is required for fiber laser and amplifiers.

Abstract: There is used an acoustic signal receiving apparatus including a wavelength-tunable light source for irradiating measurement light, a controller for controlling a wavelength of the measurement light, a Fabry-Perot probe having a first mirror on a side where the measurement light enters, a second mirror on a side where an elastic wave from an object enters, and a spacer film positioned between the first and second mirrors and deforms in response to the elastic wave, an array photosensor for detecting a reflected light amount of the measurement light by the Fabry-Perot probe, and a signal processor for acquiring an intensity of the incident elastic wave based on a change in the reflected light amount. The controller sweeps the wavelength of the measurement light, and the signal processor determines the wavelength based on the reflected light amount at each wavelength subjected to the sweep.

Abstract: In some embodiments, coupled multi-core fiber is used for optical transmission. The coupled multi-core fiber includes multiple cores each supporting a spatial mode. The cores are positioned close enough to cause coupling between their modes that generates supermodes, that are used to transmit data.

Abstract: A multi-core fiber includes a plurality of cores, a marker which is disposed to be parallel to the cores, and a clad which surrounds outer peripheral surfaces of the cores and the marker. The marker may propagate light having a wavelength which is the same as a wavelength of light which propagates in the core as single mode light.

Abstract: A semiconductor device includes a substrate supporting a plurality of layers that include at least one modulation doped quantum well (QW) structure offset from a quantum dot in quantum well (QD-in-QW) structure. The modulation doped QW structure includes a charge sheet spaced from at least one QW by a spacer layer. The QD-in-QW structure has QDs embedded in one or more QWs. The QD-in-QW structure can include at least one template/emission substructure pair separated by a barrier layer, the template substructure having smaller size QDs than the emission substructure. A plurality of QD-in-QW structures can be provided to support the processing (emission, absorption, amplification) of electromagnetic radiation of different characteristic wavelengths (such as optical wavelengths in range from 1300 nm to 1550 nm).

Abstract: Structures and techniques are described relating to the alignment of multicore fibers within a multifiber connector. These structures and techniques include: multicore fibers having a number of different shapes, including, for example, circular, elliptical, D-shaped, double D-shaped, and polygonal; multifiber ferrules, having a plurality of fiber guide holes therein of various shapes; alignment fixtures for aligning multicore fibers within multifiber ferrules; and various multicore fiber alignment techniques.

Abstract: A fiber having a large effective area at 1550 nm of at least 130 ?m2 and a wire mesh drum microbending loss of less than 0.4 dB/km at a wavelength of 1550 nm. The fibers may include a core, a cladding, and a coating. The core may include a central core region and a surrounding first core region. The cladding may include a depressed index inner cladding region and an outer cladding region. The coating may include a primary coating surrounding the cladding and a secondary coating surrounding the primary coating. The primary coating may be formed from a primary composition that may include an acrylate monomer or an N-vinyl amide monomer in combination with an acrylate oligomer, where the acrylate oligomer is present at 35 wt % to 55 wt %. The secondary coating may be formed from a secondary composition including one or more acrylate or diacrylate monomers and an acrylate or methacrylate oligomer, where the oligomer is present at 3 wt % or less.

Abstract: A computer system has an optical data distributing device for transmitting and distributing optical signals. A laser source generates light for forming the optical signals, and an optical fiber with a graded index of refraction couples the light from the laser source to the optical data distributing device. A lens is disposed to image light generated by the laser source into an input end of the optical fiber. The magnification of the lens is selected as a function of a ratio of a numerical aperture and diameter of the laser source divided by a ratio of a numerical aperture and diameter of the optical fiber.

Abstract: The invention relates to optical fiber communications. A multicore optical fiber comprises at least two light-guiding cores made of doped fused silica with refractive indices nc1, nc2, nck, each light-guiding core of the at least two light-guiding cores being surrounded by a respective arbitrarily shaped inner reflecting cladding made of fused silica or doped fused silica with refractive indices nc11, nc12, nclk, which are less than the refractive indices nc1, nc2, nck of respective light-guiding cores; a continuous or intermittent barrier region made of fused silica and having an arbitrary cross-sectional shape, the barrier region being formed in the space between the inner reflecting claddings and an outer cladding of fused silica with refractive index n0, the barrier region having refractive index nb, which is less than the refractive index of each of the inner reflecting claddings; and an external protective coating.

Abstract: A multicore fiber includes a plurality of cores, a cladding that encloses the plurality of the cores, and a marker disposed in the cladding. The plurality of the cores is arranged and disposed on a linear line passed through the center of the cladding. The marker is disposed along the length direction of the cladding on a portion on which the marker does not overlap the cores in a first direction in which the plurality of the cores is arranged on the linear line and does not overlap the core in a second direction orthogonal to the first direction.

Abstract: A method of designing multicore optical fibers is provided. A geometry for the core arrangement is selected. At least one of i) core width, ii) core position with respect to other cores, or iii) orientation with respect to incoming, outgoing, or at least partially traversing radiation such as an inscription beam are optimized. A design space is created in which no core shadows or blocks any other core with respect to incoming, outgoing, or at least partially traversing radiation. Optimization generally includes tracing tangents of core widths against an orthogonal axis and ensuring no overlap of space between said tangents on said axis. For twisted fiber, optimization also includes optimizing effective length and twist rate of the fiber. Devices entailing such fibers, such as multicore pump coupler and multicore fiber distributed feedback laser, are also contemplated.

Abstract: A multicore fiber includes a plurality of cores and a cladding surrounding the plurality of cores. The plurality of cores is arranged and disposed on a linear line passed through the center of the cladding. A pair of cores is included. The pair of the cores is located adjacent to each other, and has different core diameters in a first direction in which the plurality of cores is arranged on the linear line. A ratio of a core diameter in the first direction to a core diameter in a second direction orthogonal to the first direction is different between the pair of the cores.

Abstract: A multicore fiber includes a plurality of cores and a cladding that encloses the plurality of the cores. The plurality of the cores is arranged and disposed on a linear line passed through the center of the cladding. A difference in the cutoff wavelength between an outer core located at the outermost position and an inner core located next to the outer core is set at a wavelength of 100 nm or less.

Abstract: An easily producible optical fiber preform which is drawn to an optical fiber having a core containing a sufficient concentration of alkali metal is provided. An optical fiber preform 10 is composed of silica-based glass and includes a core portion 20 and a cladding portion 30. The core portion 20 includes a first core portion 21 including a central axis and a second core portion 22 disposed on the perimeter of the first core portion 21. The cladding portion 30 includes a first cladding portion 31 disposed on the perimeter of the second core portion 22 and a second cladding portion 32 disposed on the perimeter of the first cladding portion 31. The core portion 20 contains an alkali metal at an average concentration of 5 atomic ppm or more. The concentration of the OH group in the perimeter portion of the first cladding portion 31 is 200 mol ppm or more.