Optics: measuring and testing – For optical fiber or waveguide inspection
Patent
1981-10-01
1985-05-07
Rosenberger, R. A.
Optics: measuring and testing
For optical fiber or waveguide inspection
356128, G01N 2147
Patent
active
045154759
DESCRIPTION:
BRIEF SUMMARY
This invention concerns the measurement of refractive index profile across an object which is approximately cylindrical, such as an optical fibre, or an optical fibre preform, the measurement being made transverse to the cylindrical axis. Such objects ideally have circular symmetry and are invariant to the axial direction, but in practice major variations from the ideal conditions occur. Application of the present invention allows the variations to be sensed and quantified.
In this specification the term `light` means electromagnetic radiation at visible, ultraviolet and infrared wavelengths.
In Electronics Letters Nov. 24, 1977, volume 13, No. 24, pages 736 to 738, P. L. Chu describes a method of measuring the refractive index profile of an optical fibre preform by scanning a laser beam of very small diameter across the preform in the radial direction, i.e. transverse to the cylindrical axis of the preform, and sensing the deflection of the output beam as a function of radial position of the input beam. The deflection function measured in this way is numerically transformed to determine the refractive index profile. This method requires an input beam of very small diameter, which may be difficult to achieve, and use of a laser introduces spurious interference patterns which may be difficult to eliminate.
In another method described by H. M. Presby and D. Marcuse in Applied Optics, Mar. 1, 1979, Volume 18, No. 5, pages 671 to 677, an optical fibre preform is illuminated uniformly across its diameter and the intensity distribution of the transmitted light is sensed; the deflection function is determined by a first mathematical integration and the refractive index profile is then determined by a second integration. In this method to achieve high accuracy it is essential to provide an illuminating beam which has a precisely uniform intensity distribution across the radius or diameter of the fibre, and an intensity sensing arrangement which has a precisely uniform response in this direction. Another difficulty is that strict validity of the theory requires the plane in which intensity is observed to be placed at a distance from the preform which is large compared to its radius; in practice the plane in which intensity is observed must be close to the preform to eliminate the effect of cross or superimposition of beams transmitted through different sections or through opposite halves of the preform so that a single-valued output is achieved.
The object of the present invention is to provide an improved method of sensing the deflection function of a cylindrical object.
According to the invention, a method of sensing the optical deflection function of an approximately cylindrical object comprises:
illuminating the object over its width to be tested with a collimated beam of light;
focusing the light transmitted by the object so that in the focal plane the distance of transmitted light from the optical axis in a direction perpendicular to the cylindrical axis of the object is linearly proportional to the angle through which light has been deviated by the object;
optically modulating the focused light so that a property of the light varies as a function of said distance; and
receiving the modulated light in an image plane, whereby the deflection function of the object can be derived.
The focused light may be modulated so that either a temporal or a spatial property of the light varies in a direction parallel to said direction. In a temporal modulation, the light is pulsed, and the pulse width or pulse phase varies in said direction. In a spatial modulation, the intensity or the shadow height of the modulated light varies with said distance.
From the light received in the image plane an electrical signal related to the deflection function can be derived, and usually this signal will be mathematically transformed according to a known formula to derive the radial refractive index distribution of the object, i.e. the refractive index profile.
Also according to the invention, apparatus for sensing the optical deflection
REFERENCES:
patent: 3870415 (1975-03-01), Cornsweet
patent: 4168907 (1979-09-01), Presby
patent: 4181433 (1980-01-01), Marcuse
patent: 4348108 (1982-09-01), Shindow
Nondestructive Measurement of Index Profile of an Optical-Fibre Preform, Electronics Letters, vol. 13, No. 24, Nov. 24, 1977, pp. 736 and 738.
Refractive Index Determination by the Focusing Method, Applied Optics, D. Marcuse, vol. 18, No. 1, Jan. 1, 1979, pp. 9-12.
Optical Fiber Preform Diagnostics, H. M. Presby and D. Marcuse, Applied Optics, vol. 18, No. 1, Jan. 1, 1979, pp. 23-30.
Preform Index Profiling (PIP), H. M. Presby and D. Marcuse, Applied Optics, vol. 18, No. 5, Mar. 1, 1979, pp. 671-677.
Laser Beam Refraction Traversely Through a Graded-Index Preform to Determine Refractive Index Ratio and Gradient Profile, L. S. Watkins, Applied Optics, vol. 18, No. 13, Jul. 1, 1979, pp. 2214-2222.
Optical Method for Measuring the Radial Distribution of the Refractive Index, Ostrovskaya and Filippov, Soviet Physics Technical Physics, vol. 23, No. 11, published Nov. 1978 by America Institute of Physics, pp. 1364-1365.
Refractive Index Profile Determination of Optic Fibers from the Diffraction Pattern, Ernst Brinkmeyer, Applied Optics, vol. 16, No. 11, Nov. 1977, pp. 2802-2803.
Focusing Method for Nondestructive Measurement of Optical Fiber Index Profiles, D. Marcuse and H. M. Presby, Applied Optics, vol. 18, No. 1, Jan. 1, 1979, pp. 14-22.
Adams Michael J.
Payne David N.
Sasaki Issei
National Research Development Corporation
Rosenberger R. A.
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