The present embodiment relates to an MCF having a low transmission loss and having a structure for reducing a transmission loss and effectively suppressing an inter-core XT. The uncoupled MCF includes alkali metal having a predetermined concentration in which each of a plurality of cores contributes to reduction in the transmission loss, and a core pitch is set so that a sum h_.sub.total of power coupling coefficients of a specific core and the remaining all cores of the plurality of cores is 2.3×10.sup.−4/km or less.

A single mode optical fiber is provided that includes a core region and a cladding region, the cladding region including a depressed-index cladding region, a first outer cladding region, and a second outer cladding region. The first outer cladding region has a lower relative refractive than the second outer cladding region. The single mode optical fiber has a bend loss at 1550 nm for a 15 mm diameter mandrel of less than about 0.75 dB/turn, has a bend loss at 1550 nm for a 20 mm diameter mandrel of less than about 0.2 dB/turn, and a bend loss at 1550 nm for a 30 mm diameter mandrel of less than about 0.005 dB/turn. Additionally, the single mode optical fiber has a mode field diameter of about 9.0 microns or greater at 1310 nm wavelength and a cable cutoff of less than or equal to about 1260 nm.

This MCF ensures sufficient manufacturing tolerance, is excellent in mass productivity, and is also capable of suppressing degradation of splice loss. The MCF includes four cores and a common cladding. Each core has adjacent relationships with two cores of remaining cores, an adjacent core interval Λ is from Λ.sub.nominal−0.9 μm to Λ.sub.nominal+0.9 μm, a common cladding diameter is from 124 μm to 126 μm, an MFD, λ.sub.cc and d.sub.coat at a wavelength of 1310 nm satisfy a predetermined relationship, the MFD is from a MFD-reference-value−0.4 μm to the MFD-reference-value+0.4 μm with the MFD-reference-value of from 8.6 μm to 9.2 μm, a zero-dispersion wavelength is from a wavelength-reference-value−12 nm to the wavelength-reference-value+12 nm with the wavelength-reference-value of from 1312 nm to 1340 nm, a dispersion slope at a zero-dispersion wavelength is 0.092 ps/(nm.sup.2.Math.km) or less, λ.sub.cc is 1260 nm or less, and a predetermined structural condition and an optical condition are satisfied.

A multimode coupling fiber for optical data links features low coupling loss to silicon photonics lasers, VCSELs, single mode transmission fibers, multimode transmission fibers, and high speed receivers. The coupling fiber includes a core, an optional inner cladding region, a depressed index cladding region, and an outer cladding region. The relative refractive index profile of the coupling fiber includes a core region with α profile and a depressed index cladding region that facilitates low bending loss and high bandwidth. The mode field diameter of the coupling fiber is well-matched to standard single mode transmission fibers and the etendue of the coupling fiber is high enough to couple efficiently to multimode transmission fibers.

An optical receptacle includes: a fiber stub including an optical fiber, a ferrule, and an elastic member; and a holder holding the fiber stub, the optical fiber including a cladding and a core for conducting light, the ferrule having a through-hole fixing the optical fiber, the elastic member fixing the optical fiber in the through-hole, the fiber stub having one end surface on the ferrule side, and another on an opposite side, the optical fiber including first and third portions on the opposite end surface sides and a second portion therebetween, a core diameter of the first portion being smaller than the third portion, and a core diameter of the second portion increasing from the first portion side toward the third portion side.

An optical fiber includes: a core; and a clad which is formed so as to surround an outer circumference of the core concentrically with the core, the clad having at least an inner cladding layer adjacent to the outer circumference of the core and an outer cladding layer formed on an outer circumference of the inner cladding layer, wherein a refractive index of the outer cladding layer is Δ3, and an outer circumference radius of the outer cladding layer is r3, a relationship of Δ1max>Δ3>Δ2min is satisfied, a relationship of Δ3−Δ2min≦0.08% is satisfied, a relationship of r1<r2<r3 is satisfied, a relationship of 0.35≦r1/r2≦0.55 is satisfied, a cable cut-off wavelength is less than or equal to 1260 nm, and an MFD at a wavelength of 1310 nm is 8.6 μm to 9.2 μm.

An MCF of the present embodiment has eight or more cores. A diameter of a common cladding is not more than 126 μm. 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 μm to 10.1 μm; 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; λ0 is from 1300 nm to 1324 nm; λcc is not more than 1260 nm; an XT or XTs at the predetermined wavelength is not more than 0.001/km.

A single mode optical fiber is provided that includes a core region and a cladding region, the cladding region including a depressed-index cladding region, a first outer cladding region, and a second outer cladding region. The first outer cladding region has a lower relative refractive than the second outer cladding region. The single mode optical fiber has a bend loss at 1550 nm for a 15 mm diameter mandrel of less than about 0.75 dB/turn, has a bend loss at 1550 nm for a 20 mm diameter mandrel of less than about 0.2 dB/turn, and a bend loss at 1550 nm for a 30 mm diameter mandrel of less than about 0.005 dB/turn. Additionally, the single mode optical fiber has a mode field diameter of about 9.0 microns or greater at 1310 nm wavelength and a cable cutoff of less than or equal to about 1260 nm.

The high-density FAU comprises a support substrate having a grooved front-end section that supports glass end sections of the small diameter low-attenuation optical fibers. A cover is disposed on the front-end section and secured thereto to hold the glass end sections in place. The substrate and the cover can be made of the same glass or glasses having about the same CTE. The glass end sections have a diameter d4 so that the pitch P2 of the fibers at the front end of the FAU can be equal to or greater than d4, wherein d4=2r.sub.4, with r.sub.4 being the radius of the glass end section as defined by the optical fiber cladding. The glass end section has a radius r.sub.4 less than 45 microns, allowing for a high-density FAU and a high-density optical interconnection device.

An optical fiber includes: a core portion made of glass; a cladding portion that is located on an outer periphery of the core portion and that is made of glass having a lower refractive index than a refractive index of the core portion; and a coating portion that covers an outer periphery of the cladding portion. Further, an average value of a relative refractive-index difference of a center core of the core portion is 0.1 to 0.5%, a fiber diameter including the coating portion is equal to or smaller than 220 μm, an effective cutoff wavelength is longer than 1260 nm and smaller than 1530 nm, and a mode field diameter of light at 1550 nm is equal to or larger than 9 μm.