Method of measuring the refractive index profile of optical fibers

Claim 1

A method of measuring the refractive index profile of an optical fiber comprising a encased within a surrounding cladding and having a protective coating thereover, comprising the steps of: providing an uncoated end portion of the optical fiber for testing; surrounding at least the uncoated end portion of the optical fiber with a material having a refractive index less than the refractive index of the cladding; directing light from an optical source at the uncoated optical fiber so that only cladding modes will be excited along the length of uncoated fiber; focusing a limited portion of the light emergent from the test end of the optical fiber into a light detector means which is electrically connected to a computer; and computing the refractive index profile of the optical fiber from the measured near field intensity distribution of the light emerging from the test end of the optical fiber.

Claim 2

A method of measuring the refractive index profile of an optical fiber comprising a encased within a surrounding cladding and having a protective coating thereover, comprising the steps of: providing an uncoated end portion of the optical fiber for testing; surrounding at least the uncoated end portion of the optical fiber with a material having a refractive index less than the refractive index of the cladding; directing light from an optical source at the uncoated optical fiber so that only cladding modes will be excited along the length of uncoated fiber; focusing a limited portion of the light emergent from the test end of the optical fiber into a light detector means which is electrically connected to a computer; and computing the refractive index profile of the optical fiber from the measured near field intensity distribution of the light emerging from the test end of the optical fiber.

Claim 11

A method of measuring the refractive index profile of an optical fiber after removing coating adjacent the test end of the optical fiber and surrounding at least the test end of the optical fiber with a material having a refractive index less than the refractive index of the cladding comprising the steps of: directing light from an intense optical source substantially at the interface of the uncoated optical fiber and the coating so as to excite only cladding modes along the length of uncoated fiber, said cladding modes being uniformly excited by the light; focusing a limited portion of the light emergent from the test end of the optical fiber into a vidicon camera having an electrically connected computer, said selected portion determined by a lens system having a limited numerical aperture; and computing the refractive index profile of the optical fiber from the measured near-field intensity distribution of the light emerging from the test end of the optical fiber.

Claim 12

A method of measuring the refractive index profile of an optical fiber after removing coating adjacent the test end of the optical fiber and surrounding at least the test end of the optical fiber with a material having a refractive index less than the refractive index of the cladding comprising the steps of: directing light from an intense optical source substantially at the interface of the uncoated optical fiber and the coating so as to excite only cladding modes along the length of uncoated fiber, said cladding modes being uniformly excited by the light; focusing a limited portion of the light emergent from the test end of the optical fiber into a vidicon camera having an electrically connected computer, said selected portion determined by a lens system having a limited numerical aperture; and computing the refractive index profile of the optical fiber from the measured near-field intensity distribution of the light emerging from the test end of the optical fiber.

Claim 19

A method of measuring the refractive index profile of an optical fiber comprising a substantially cylindrical core encased within a surrounding cladding and having a protective coating thereover, comprising the steps of: providing an uncoated end portion of the optical fiber for testing; surrounding at least the uncoated end portion of the optical fiber with a material having a refractive index less than the refractive index of the cladding; directing light from an optical source at the uncoated optical fiber so that only cladding modes will be excited along the length of uncoated fiber and the excitation of the cladding modes is uniform; focusing a limited portion of the light emergent from the test end of the optical fiber into a vidicon camera through a lens system having a limited numerical aperture, said vidicon camera being electrically connected to a computer; and computing the refractive index profile of the optical fiber from the measured intensity distribution of the light emerging from the test end of the optical fiber according to the relationship N(r).sup.2 -No.sup.2 =-No.sup.2 sin.sup.2 θ.sub.1 +sin.sup.2 θ.sub.3 where N(r) is the local refractive index along the exit plane of the fiber No is the refractive index of the cladding θ 1 is the cladding mode propagation angle of incidence θ 3 is the angle at which light exits the cladding and the numerical aperture of the lens system is fixed at sin θ 3max , No is fixed and the relationship between θ 1 and intensity has been determined.

Claim 20

A method of measuring the refractive index profile of an optical fiber comprising a substantially cylindrical core encased within a surrounding cladding and having a protective coating thereover, comprising the steps of: providing an uncoated end portion of the optical fiber for testing; surrounding at least the uncoated end portion of the optical fiber with a material having a refractive index less than the refractive index of the cladding; directing light from an optical source at the uncoated optical fiber so that only cladding modes will be excited along the length of uncoated fiber and the excitation of the cladding modes is uniform; focusing a limited portion of the light emergent from the test end of the optical fiber into a vidicon camera through a lens system having a limited numerical aperture, said vidicon camera being electrically connected to a computer; and computing the refractive index profile of the optical fiber from the measured intensity distribution of the light emerging from the test end of the optical fiber according to the relationship N(r).sup.2 -No.sup.2 =-No.sup.2 sin.sup.2 θ.sub.1 +sin.sup.2 θ.sub.3 where N(r) is the local refractive index along the exit plane of the fiber No is the refractive index of the cladding θ 1 is the cladding mode propagation angle of incidence θ 3 is the angle at which light exits the cladding and the numerical aperture of the lens system is fixed at sin θ 3max , No is fixed and the relationship between θ 1 and intensity has been determined.

Claim 21

A method of measuring the refractive index profile of an optical fiber comprising a substantially cylindrical core encased within a surrounding cladding and having a protective coating thereover, comprising the steps of: providing an uncoated end portion of the optical fiber for testing; surrounding at least the uncoated end portion of the optical fiber with a material having a refractive index less than the refractive index of the cladding; directing light from an optical source at the uncoated optical fiber so that only cladding modes will be excited along the length of uncoated fiber and the excitation of the cladding modes is uniform; focusing a limited portion of the light emergent from the test end of the optical fiber into a vidicon camera through a lens system having a limited numerical aperture, said vidicon camera being electrically connected to a computer; and computing the refractive index profile of the optical fiber from the measured intensity distribution of the light emerging from the test end of the optical fiber according to the relationship N(r).sup.2 -No.sup.2 =-No.sup.2 sin.sup.2 θ.sub.1 +sin.sup.2 θ.sub.3 where N(r) is the local refractive index along the exit plane of the fiber No is the refractive index of the cladding θ 1 is the cladding mode propagation angle of incidence θ 3 is the angle at which light exits the cladding and the numerical aperture of the lens system is fixed at sin θ 3max , No is fixed and the relationship between θ 1 and intensity for the optical fiber being tested can be computed according to the relationship θ 1 =sin -1 (1/No sin θ 3 ) where θ 1 is the cladding mode propagation angle of incidence, θ 3 is the angle at which light exits the cladding, and No is the constant value refractive index of the cladding.

Claim 22

A method of measuring the refractive index profile of an optical fiber comprising a substantially cylindrical core encased within a surrounding cladding and having a protective coating thereover, comprising the steps of: providing an uncoated end portion of the optical fiber for testing; surrounding at least the uncoated end portion of the optical fiber with a material having a refractive index less than the refractive index of the cladding; directing light from an optical source at the uncoated optical fiber so that only cladding modes will be excited along the length of uncoated fiber and the excitation of the cladding modes is uniform; focusing a limited portion of the light emergent from the test end of the optical fiber into a vidicon camera through a lens system having a limited numerical aperture, said vidicon camera being electrically connected to a computer; and computing the refractive index profile of the optical fiber from the measured intensity distribution of the light emerging from the test end of the optical fiber according to the relationship N(r).sup.2 -No.sup.2 =-No.sup.2 sin.sup.2 θ.sub.1 +sin.sup.2 θ.sub.3 where N(r) is the local refractive index along the exit plane of the fiber No is the refractive index of the cladding θ 1 is the cladding mode propagation angle of incidence θ 3 is the angle at which light exits the cladding and the numerical aperture of the lens system is fixed at sin θ 3max , No is fixed and the relationship between θ 1 and intensity for the optical fiber being tested can be computed according to the relationship θ 1 =sin -1 (1/No sin θ 3 ) where θ 1 is the cladding mode propagation angle of incidence, θ 3 is the angle at which light exits the cladding, and No is the constant value refractive index of the cladding.

Claim 23

A method of measuring the refractive index profile of an optical fiber after removing coating adjacent the test end of the optical fiber and surrounding at least the test end of the optical fiber with a material having a refractive index less than the refractive index of the cladding, comprising the steps of: directing light from an intense optical source substantially at the interface of the uncoated optical fiber and the coating so as to excite only cladding modes along the length of uncoated fiber, said cladding modes being uniformly excited by the light; focusing a limited portion of the light emergent from the test end of the optical fiber into a vidicon camera having an electrically connected computer, said selected portion determined by a lens system having a limited numerical aperture; and computing the refractive index profile of the optical fiber from the measured intensity distribution of the light emerging from the test end of the optical fiber according to the relationship N(r).sup.2 -No.sup.2 =-No.sup.2 sin.sup.2 θ.sub.1 +sin.sup.2 θ.sub.3 where N(r) is the local refractive index along the exit plane of the fiber No is the refractive index of the cladding θ 1 is the cladding mode propagation angle of incidence θ 3 is the angle at which light exits the cladding and the numerical aperture of the lens system is fixed at sin θ 3max , No is fixed and the relationship between θ 1 and intensity has been determined.

Claim 24

A method of measuring the refractive index profile of an optical fiber after removing coating adjacent the test end of the optical fiber and surrounding at least the test end of the optical fiber with a material having a refractive index less than the refractive index of the cladding, comprising the steps of: directing light from an intense optical source substantially at the interface of the uncoated optical fiber and the coating so as to excite only cladding modes along the length of uncoated fiber, said cladding modes being uniformly excited by the light; focusing a limited portion of the light emergent from the test end of the optical fiber into a vidicon camera having an electrically connected computer, said selected portion determined by a lens system having a limited numerical aperture; and computing the refractive index profile of the optical fiber from the measured intensity distribution of the light emerging from the test end of the optical fiber according to the relationship N(r).sup.2 -No.sup.2 =-No.sup.2 sin.sup.2 θ.sub.1 +sin.sup.2 θ.sub.3 where N(r) is the local refractive index along the exit plane of the fiber No is the refractive index of the cladding θ 1 is the cladding mode propagation angle of incidence θ 3 is the angle at which light exits the cladding and the numerical aperture of the lens system is fixed at sin θ 3max , No is fixed and the relationship between θ 1 and intensity has been determined.