Method of and apparatus for determining refractive-index profiles of cylindrical transparent bodies

Claim 1

A method of determining the refractive-index profile of a generally cylindrical test object transparent to luminous radiation, the refractive index of said test object being constant at a given distance ρ from the axis thereof, comprising the steps of: (a) generating a first and a second monochromatic beam of luminous radiations of constant frequencies differing slightly from each other; (b) deriving from said first and second beams an electrical reference wave of a beat frequency corresponding to the difference between the beam frequencies; (c) transluminating the test object over the entire width thereof with parallel rays of said second beam in a plane transverse to the axis of said test object; (d) mixing the transluminating rays of said second beam with rays of said first beam, avoiding the test object, in a field of radiation within said plane which lies downstream of the test object and extends over the entire width thereof; (e) photoelectrically sampling said field of radiation at a multiplicity of closely spaced locations to produce respective electrical signals of the same beat frequency as said reference wave but differing in phase therefrom to an extent dependent on the refractive index of the parts of the test object traversed by the respective transluminating rays; (f) determining the phase differences between said signals and said reference wave at said locations; (g) computing the refractive-index profile of the test object from said phase differences; and (h) interposing a phase-shift compensator of known refractive-index profile in the path of said transluminating rays whereby the profile computed in step (g) represents deviations from the complement of the known profile of said compensator.

Claim 2

A method of determining the refractive-index profile of a generally cylindrical test object transparent to luminous radiation, the refractive index of said test object being constant at a given distance ρ from the axis thereof, comprising the steps of: (a) generating a first and a second monochromatic beam of luminous radiations of constant frequencies differing slightly from each other; (b) deriving from said first and second beams an electrical reference wave of a beat frequency corresponding to the difference between the beam frequencies; (c) transluminating the test object over the entire width thereof with parallel rays of said second beam in a plane transverse to the axis of said test object; (d) mixing the transluminating rays of said second beam with rays of said first beam, avoiding the test object, in a field of radiation within said plane which lies downstream of the test object and extends over the entire width thereof; (e) photoelectrically sampling said field of radiation at a multiplicity of closely spaced locations to produce respective electrical signals of the same beat frequency as said reference wave but differing in phase therefrom to an extent dependent on the refractive index of the parts of the test object traversed by the respective transluminating rays; (f) determining the phase differences between said signals and said reference wave at said locations; (g) computing the refractive-index profile of the test object from said phase differences; and (h) interposing a phase-shift compensator of known refractive-index profile in the path of said transluminating rays whereby the profile computed in step (g) represents deviations from the complement of the known profile of said compensator.

Claim 10

An apparatus for determining refractive-index profile of a generally cylindrical test object transparent to luminous radiation with a refractive index which is constant at a given distance from the axis thereof, comprising: a holder for the test object to be examined; a source of monochromatic first and second beams of luminous radiation of constant frequencies differing slightly from each other; first photoelectric means positioned to receive rays from said first and second beams to derive therefrom an electrical reference wave of a beat frequency corresponding to the difference between the beam frequencies; optical means for guiding rays of said first and second beam around said test object and for guided parallel rays from said second beam through said test object in a plane transverse through the axis thereof for combination with the rays of said first beam in a field of radiation downstream of said test object extending over the entire width of the latter; second photoelectric beams disposed in said field at a multiplicity of closely spaced location for intercepting rays from both beams and deriving therefrom a multiplicity of electrical signals of the same beat frequency as said reference wave but differing in phase therefrom at said locations to an extent depending on the refractive index of the parts of said test object traverse by the rays of said second beam respectively arriving thereat; phase comparison means is connected to said first and second photoelectric means for determining the phase differences between said electrical signals and said reference wave; calculating means connected to said phase comparison means for computing the refractive-index profile of the test object from said phase differences, said source being a Zeeman-effect laser emitting a composite beam containing said first and second monochromatic beams, said optical means including the quarter-wave plate positioned for translumination by said composite beam and a polarizer inserted between said quarter-wave plate and said holder for separating rays of the first and second beams from each other; another polarizer disposed in said field between said holder of said second photoelectric means for reconfining the rays of said first and second beams guided around and through the test object, said holder comprising a cell field with a transparent fluid whose refractive index matches a constant refractive index n c of a cladding of a workpiece of vitrious material constituting said test object and being used for the manufacture of an optical fiber and having a core whose refractive-index profile is to be determined; and a phase-shift compensator in said cell immersed in said fluid translumination by the rays of said second beam traversing the workpiece, said compensator having a known refractive-index profile, said calculating means being programmed to take said known profile into account.

Claim 11

An apparatus for determining refractive-index profile of a generally cylindrical test object transparent to luminous radiation with a refractive index which is constant at a given distance from the axis thereof, comprising: a holder for the test object to be examined; a source of monochromatic first and second beams of luminous radiation of constant frequencies differing slightly from each other; first photoelectric means positioned to receive rays from said first and second beams to derive therefrom an electrical reference wave of a beat frequency corresponding to the difference between the beam frequencies; optical means for guiding rays of said first and second beam around said test object and for guided parallel rays from said second beam through said test object in a plane transverse through the axis thereof for combination with the rays of said first beam in a field of radiation downstream of said test object extending over the entire width of the latter; second photoelectric beams disposed in said field at a multiplicity of closely spaced location for intercepting rays from both beams and deriving therefrom a multiplicity of electrical signals of the same beat frequency as said reference wave but differing in phase therefrom at said locations to an extent depending on the refractive index of the parts of said test object traverse by the rays of said second beam respectively arriving thereat; phase comparison means is connected to said first and second photoelectric means for determining the phase differences between said electrical signals and said reference wave; calculating means connected to said phase comparison means for computing the refractive-index profile of the test object from said phase differences, said source being a Zeeman-effect laser emitting a composite beam containing said first and second monochromatic beams, said optical means including the quarter-wave plate positioned for translumination by said composite beam and a polarizer inserted between said quarter-wave plate and said holder for separating rays of the first and second beams from each other; another polarizer disposed in said field between said holder of said second photoelectric means for reconfining the rays of said first and second beams guided around and through the test object, said holder comprising a cell field with a transparent fluid whose refractive index matches a constant refractive index n c of a cladding of a workpiece of vitrious material constituting said test object and being used for the manufacture of an optical fiber and having a core whose refractive-index profile is to be determined; and a phase-shift compensator in said cell immersed in said fluid translumination by the rays of said second beam traversing the workpiece, said compensator having a known refractive-index profile, said calculating means being programmed to take said known profile into account.