APPARATUS FOR DETERMINING THE REFRACTIVE-INDEX PROFILE OF OPTICAL FIBRES AND OPTICAL-FIBRE PREFORMS

Abstract: The apparatus comprises an opaque screen (8), which is placed between a source and the fibre or preform (7) and has an oblique slit (9) which identifies a plane section of the fibre or preform (7), the index profile of which is determined, and an optical system (10, 11) which forms a near-field image of the beam incident on the fibre or preform and a far-­ field image of the refracted beam, in two perpendicular directions on a photosensitive element (12). The image is scanned line by line by a television camera the photosen­ sitive element (12) makes part of, and the deflection function and hence the refractive-index profile are obtained from the coordinates of the points with maximum brightness by using a computing system.

Description: APPARATUS FOR DETERMINING THE REFRACTIVE-INDEX PROFILE OF OPTICAL FIBRES AND OPTICAL-FIBRE PREFORMS De s c ription The present invention concerns an apparatus for determining the refractive-index profile of optical fibres or optical -fibre preforms. The refractive-index profile of optical fibres plays an essential role in the determination of a number of fundamental properties of the fibres, such as light collection efficiency, guiding properties and bandwidth. Such a profile is critically dependent on that of the preform the fibre is drawn from, and therefore it may be conveniently determined by performing the necessary measurements on the preform; in effect the preform is easter to handle than the fibre, owing to its bigger size. Moreover, generally a knowledge of the profile along the whole fibre or preform is desirable, thus non-destructive methods are to be used, i. e. methods which do not require a slab of fibre or preform to be taken off. Examples of non destructive methods are reported by I.I. Sasaki, D. N. Payne and M. J. Adams in the paper entitled: "Measurement of refractive-index profiles in optical-fibre preforms by spatial-fil tering technique , Electronics Letters, vol. 16, N. 16, 13 March 1980" and by I.I. Sasaki, D.N. Payne, R.J. Mansfield and M.J. J. Adams in the paper entitled "Variation of re- fractive-index profiles in single-mode fibre preforms measured using an improved high-resolution spatial-filtering technique" pre sented at the 6th European Conference on Optical Communications York, 16 - 19 September 1980. According to these methods a light beam illuminates a portion of the side surface of a preform which is displaced step by step in a plane perpendicular to the beam; the beam rays, refracted at angles dependent on the refractive-index profile, are collected by a lens which, for each position of the preform1 makes different positions along a straight line of its focal plane correspond to different deflection angles. Then, according to the first method, the deflection function, i. e. the function which establishes a relationship between the deflection angle and the incidence point of a ray on the preform, is imaged on a photosensitive surface; from said image, consisting of a dark and a bright zone, electrical signals are obtained and processed to supply the refractive-index profile. According to the second method, a rotating spatial filter, placed in the focal plane of the lens sweeps the different positions of the straight line and the values of the deflection angles, and hence the refractive-index profile, are obtained from the time elapsing between the instants the individual refracted rays are estinguished (instants signalled by a photodiode placed in a suitable image plane), and the instant the spatial filter passes a reference point. These known methods present a number of disadvantages. First,the image which appears on the screen does not present a sharp transition from the bright to the dark zone, but the two zones are separated by a half-light zone. Hence the boundary between the two zones is difficult to define. This difficultv is ercountered also when using the second method, because the zone in which the refracted beam is present is not distinctly separated from the dark zone. The difficulty is increased by the fact that the image to explore or the beam to intercept are not stationary but they are displaced together with the preform. In addition, the apparatuses carrying out these methods are made complicated and hence expensive by the presence of moving parts (the preform and the devices scanning the image or intercepting the refracted beam), which limit also the measurement accuracy, and by the necessity of exactly synchronizing the movements of such parts. It is the object of the present invention an apparatus for determining the refractive-inc2ex profile of an optical fibre or an optical-fibre prefontn, which does not require the detection of a transition from a bright to a dark zone and which does not present moving mechanical parts, so that accurate measurernents are obtained with a particularly simple equipment. According to the invention there is provided an aIparatus for determining the refractive-index profile of optical fibres or optical fibre preforms, in whicn a light source sends a light beam towards the fibre or preform, which beam, after refraction by the fibre or preform, is collected by a photosensitive element which supplies brightness information to a data acquisition system able to obtain the refractive-index profile from such information, characterized in that an opaque screen is placed between the fibre or preform and the source perpendicularly to the light beam propagation direction, said screen having an oblique slit identifying the plane section of the fibre or preform whose index profile is determined; in that an optical system is placed between the fibre or preform and the photosensitive element, which system forms on the photosensitive element, in two directions perpendicular to the light propagation direction, a near-field image, i. e. an image depending on the incidence points of said light beam on the fibre or preform, and a far-field image, i. e. an image depending on the deflection angles of the rays of the beam refracted by the fibre or preform; and in that said photosensitive element is the photosensitive element of a television camera connected to the data acquisition system and is scanned line by line by an electron beam, said data acquisition system obtaining from the signals resulting from the scanning the coordinates of the points of maximun brightness on the photosensitive element and, from such coordinates, the deflection function and hence the refractive-index profile. For a better understanding reference is made to the annexed drawing, in which: - Fig. 1 is a schematic representation of the apparatus according to the invention; - Fig. 2 shows a possible system of image formation on a larger scale; - Fig. 3 and 4 are diagrams showing the image formation. In Fig. 1 the apparatus comprises a light source 1, e. g. a He -Ne laser, which sends a light beam onto the fibre or preform, the refractive index of which is to be determined. An optical system, here schematized together with the fibre or preform by block 2 and depicted in detail in Fig. 2, forms an image on the sensitive element of a television camera, which image consists of a bright curve the points of which are in one-to-one correspondence with the incidence points of the light beam on the fibre or preform. The image is scanned by the electron beam of the television camera 3 which through a line discriminator 4 supplies a data acquisition and processing system 6 with analog electric signals representing the brightness of the points of the different lines of the sensitive element. In particular the points on the bright curve will be recognized due to the maximum bright ness. The image is sent also to a monitor 5 for checking purposes. The acquisition and processing system 6 converts the analog signals into digital form and obtains from the brightness information the deflection function and hence the refractive-index profile according to relationships well known to the skilled in the art; said relationships are disclosed e.g. in the already mentioned article from Electronics Letters. Referring to Figure 2, an opaque screen 8 perpendicular to the propagation direction z of the light beam is placed upstream the fibre or preform 7, shown for sake of simplicity with vertical axis y; screen 8 is equipped with an oblique slit 9 encompassing an angle !2 with horizontal axis x. An array of lenses or objectives 10, 11, such as a cylindrical lens with horizontal axis and a spherical lens, is placed downstream the fibre or preform and forms an image 13 on the photosensitive element 12 of television camera 3 (Fig. 1), said element being placed in the focal plane of spherical lens 11. The cell with refractive-index matching fluid in which the fibre is immersed has not been represented for sake of simplicity. Owing to the two lenses a far-field image, i. e. an image depending on the deflection angles of the beam refracted by the fibre or preform 7 (angular information) is formed in one direction (the horizontal direction with the already described arrangement) on element 12, and a near-field image, that is an image depending on the incidence points of the light beam on a plane section of the fibre or preform 7 (spatial information), is formed in the perpendicular direction. The angular content is discriminated either by the cylindrical lens or by the spherical lens, depending on the focal lengths of the two lenses, while the spatial content is discriminated jointly by the two lenses. Finally, the oblique slit 9 discriminates rays which would have the same deflection angle even though they are incident on different points, in particular non-deflected rays, discri minating the incidence points according to their vertical coordinates y and not only to their distances from the axis. The operation of the optical system is depicted also in Figures 3, 4. Here X, x, xl,x2, t (see also Figure 2) are the horizontal coordinates (perpendicular to direction z) of the points in screen 9, in fibre or preform 7, in lenses 10, 11 and in photosensitive element 12; Y, y, y1 , Y2,t1 are the corresponding ver tical coordinates; Cp ç (Fig. 3) is the deflection angle; f, f2 are 10,11 the focal distances of the optical system and of lens 11, respectively; d is the distance between the focal plane of the optical system and the image plane. Considering the horizontal plane (X, z; x, z; xl, z; x2, z; , z) the cylindrical lens has no effect; consequently a point of the fibre or preform with horizontal coordinate x (=X) will correspond to a point on element 12 whose coordinate (neglecting the refraction caused by the refractive-index matching fluid) is given by the relation: 5 f2 tan; (1) In the vertical plane, also cylindrical lens 10 is effective: optical system 10,11 presents as a whole a transverse a magnification f, and a point of the fibre or preform with coordinate y(=Y) will correspond to a point on element 12 with coordinate d d y (2) f The sign - indicates that the image is upside down with respect to the object. Taking into account that y= Y=X tanf = x tan substituting in (1), the equation of the bright image curve on the photosensitive element is obtained # = f2 tan #(x) = f2 tan # (-f# cot ss) (3)d (3) d Television camera 3 and line discriminator 4 supply the brightness values for each point of coordinate # of each line (coordinate #): the exact position of the bright curve is recognized, as said, because at that point there is a brightness maximum, easy to distinguish even from possible secondary maxima due to diffraction phenomena. There is no uncertainty in the detection of the image points, as in the kown methods based on investigation of the gradual transition from a bright area to a dark one. Morover, the system of the invention gives an image of a whole plane section of the fibre or preform, which therefore is not to be displaced parallel to its axis. It is clear that what described has been given only by way of non-limiting example and that variations and modifications are possible without going out of the scope of the invention.

Claims: Patent Claim An apparatus for determing the refractive-index pro file of optical fibres or optical-fibre preforms, in which a light source (1) sends a light beam towards the fibre or preform (7), which beam after refraction by the fibre or preform (7) is collected by a photosensitive element (12) which supplies brightness information to a data acquisition system (6) able to obtain the refractive-index profile from such information, characterized in that an opaque screen (8) is placed between the fibre or preform (7) and the source (1) perpendicularly to the light-beam propagation direction (z), said screen having an oblique slit (9) identifyiing the plane section of the fibre or preform (7) whose index profile is determined; in that an optical system (10, 11) is placed between the fibre or preform (7) and the photosensitive element (12), which system (10, 11) forms on the photosensitive element (12), in two directions (t,7) perpendicular to the light propagation direction (z), a near-field image, depending on the incidence points of said light beam on the fibre or preform (7), and a far -field image, that is an image depending on the deflection angles of the of the raystbeam refracted by the fibre or preform (7); and in that said photosensitive element (12) is the photosensitive element of a television camera (3) connected to the data acquisition system (6) and is scanned line by line said data acquisition system (6) obtaining from the signals resulting fro das scanning the coordinates of the pointe of maximum brightness on the photosensitive element (12) and, from such coordinates, the deflection function and hence the refractive-index profile.

Inventors: COPPA, GIANNI, DI VITA, PIETRO, POTENZA, MARCELLO, ROSSI, UMBERTO
Applicants: CSELT CENTRO STUDI E LABORATORI TELECOMUNICAZIONI S.P.A.
Application year: 1983
Main classification: G01M11
IPC codes: G01N21, G01M11, G02B6