An optical fiber preform includes: a columnar portion having an approximately constant radius of r; and a taper portion located adjacent to the columnar portion in a lengthwise direction and having a radius decreasing along the lengthwise direction. The taper portion includes: a first taper portion including a portion having a radius varying between 0.9r and 0.6r; and a second taper portion including a portion having a radius varying between 0.4r and 0.15r. A diameter of the first taper portion in the portion having the radius varying between 0.9r and 0.6r decreases so as to form a maximum angle θ1 between 40 degrees and 60 degrees with respect to the columnar portion, a diameter of the second taper portion in the portion having the radius varying between 0.4r and 0.15r decreases so as to form an average angle θ2 between 5 degrees and 30 degrees with respect to a central axis in the lengthwise direction, and a volume of the taper portion is smaller than or equal to 45% of a volume of a column having a same outer diameter as a maximum outer diameter of the taper portion and having a same length as the taper portion.

A preform for making a vortex optical fiber comprises a glass cylinder formed substantially of silicone dioxide that defines a core portion along a longitudinal axis of the glass cylinder and a cladding portion surrounding the core portion. The glass cylinder further defines a plurality of holes running parallel to the longitudinal axis from a first end of the glass cylinder to a second end of the glass cylinder.

The present invention, as disclosed and described herein, in one aspect thereof comprises a preform for making a vortex optical fiber includes a glass cylinder formed substantially of silicone dioxide that defines a core portion along a longitudinal axis of the glass cylinder and a cladding portion surrounding the core portion. The glass cylinder further defines a plurality of holes running parallel to the longitudinal axis from a first end of the glass cylinder to a second end of the glass cylinder.

An optical preform manufacturing process is disclosed in which an alkali dopant is deposited between an optical fiber core rod and an optical fiber cladding jacket. Depositing the alkali dopant between the core rod and the cladding jacket permits diffusion of the alkali dopants into the core during fiber draw when the core and the cladding are at their respective transition (or vitrification) temperatures. Introduction of the alkali dopants between the core rod and the cladding jacket also permits decoupling of the alkali doping process from one or more of other optical preform manufacturing processes. The optical preform manufacturing process can also include placing alkali dopants between an optical fiber inner cladding jacket and an optical fiber outer cladding jacket to reduce the glass viscosity during fiber draw.

There is provided a method for producing a multicore optical fiber while depressurizing holes in a common cladding tube. A production method for a multicore optical fiber includes a preform forming step of forming a common cladding tube having a plurality of holes extending between a first end and a second end, an end-face working step of digging the common cladding tube from the second end to a predetermined depth to forming a third end, a connection step of connecting a glass tube to the second end, an insertion step of inserting core rods into the holes to the third end, a sealing step of sealing the first end, and a drawing step of spinning the multicore optical fiber while depressurizing the holes through the glass tube and combining the common cladding tube and the core rods from the first end.

There is provided a method for producing a multicore optical fiber while reducing the mass of a glass block to be connected to a common cladding tube. A production method for a multicore optical fiber includes in order, a preform forming step of forming a common cladding tube having a plurality of holes extending between a first end and a second end, an insertion step of inserting core rods in the holes in a state in which end portions of the core rods are recessed from the first end, a heat shrinkage step of reducing a diameter of the first end by heating, a sealing step of sealing the holes by connecting a glass block to the first end, and a drawing step of depressurizing insides of the holes from the second end and performing spinning from the first end while combining the common cladding tube and the core rods.

An optical preform manufacturing process is disclosed in which an alkali dopant is deposited between an optical fiber core rod and an optical fiber cladding jacket. Depositing the alkali dopant between the core rod and the cladding jacket permits diffusion of the alkali dopants into the core during fiber draw when the core and the cladding are at their respective transition (or vitrification) temperatures. Introduction of the alkali dopants between the core rod and the cladding jacket also permits decoupling of the alkali doping process from one or more of other optical preform manufacturing processes. The optical preform manufacturing process can also include placing alkali dopants between an optical fiber inner cladding jacket and an optical fiber outer cladding jacket to reduce the glass viscosity during fiber draw.

The present invention, as disclosed and described herein, in one aspect thereof comprises a preform for making a vortex optical fiber includes a glass cylinder formed substantially of silicone dioxide that defines a core portion along a longitudinal axis of the glass cylinder and a cladding portion surrounding the core portion. The glass cylinder further defines a plurality of holes running parallel to the longitudinal axis from a first end of the glass cylinder to a second end of the glass cylinder.

A preform for making a vortex optical fiber comprises a glass cylinder formed substantially of silicone dioxide that defines a core portion along a longitudinal axis of the glass cylinder and a cladding portion surrounding the core portion. The glass cylinder further defines a plurality of holes running parallel to the longitudinal axis from a first end of the glass cylinder to a second end of the glass cylinder.

An optical fiber manufacturing method includes supplying power to a drawing furnace to cause T/V to decrease to T.sub.target/V.sub.target along a quadratic function having a value of the T.sub.target/V.sub.target at an apex with lapse of time, where T is a tension applied to an optical fiber when an optical fiber preform is heated by the drawing furnace and the optical fiber is drawn, V is speed of withdrawing the optical fiber when the optical fiber is heated by the drawing furnace and the optical fiber is drawn, T.sub.target is a target value of the tension, and V.sub.target is a target value of the speed.