Method of measuring the viscosity of an optical fibre preform

Abstract: The method of measuring viscosity of a body exploits the scattering effects the body introduces into a substantially monochr­ matic radiation passing through it; more paticularly, the width of the spectral line due to Brillouin scattering is measured. The tem­ perature of the body can be determined too, by measuring also the width of the line generated by Rayleigh scattering. The method is of particular interest for measurements on glasses.

Description: The present invention concerns a method of measuring the viscosity of an optical fibre preform, which method serves for on-line monitoring of the characteristics thereof. The invention adapts principles known per se in physics ("Brillouin light scattering from polymer gels" by Marqusee and Deutch in J. Chem. Phys. 1981, 75(11), pages 5239-5245) to the technique of optical fibre drawing. When drawing optical fibre preforms which comprise layers with non-uniform composition (for instance because of different dopant concentrations), it is important to monitor viscosity throughout a whole preform cross-section in the softening region of the preform since deviations from a desired viscosity profile could alter the characteristics of the final material. In particular, an on-line, point-by-point monitoring of the material viscosity would be desirable. The literature does not disclose techniques for on-line monitoring of the viscosity of a body which is in fluid condition or whose behaviour can be assimilated with that of a fluid (e.g. a glass). Viscosity information could be obtained for instance from temperature values, measured by means of pyrometers. Yet temperature information supplied by a pyrometer cannot be used to obtain viscosity when the latter is to be determined near the softening temperature. In effect, considering for instance a vitreous body like an optical fibre preform, viscosity variations of the order of 10⁶ occur within an interval of few degrees centigrade. The precise determination of viscosity would require measurement of temperature variations of the order of 1°C in a body which is for instance at a temperature of about 2000°C, and industrial pyrometers do not have such a sensitivity. The invention aims to provide a method allowing direct measurement of the viscosity of an optical fibre preform with a very high measurement precision. The invention is based upon the consideration that, when an electromagnetic radiation (e.g. a light beam) is sent into a body such as a gel or also a glass (i.e. a fluid body), the scattered radiation exhibits frequency alterations depending, i. a., on body viscosity, as has been explained in the above mentioned literature by Marqusee and Deutch. Among scattering phenomena, Brillouin scattering is of particular interest for the present invention: this scattering gives rises to two spectral lines, at both sides of the spectral line of the incident radiation, with a width: where: η = viscosity of the body; k = 2πn/λ = wave vector of the incident radiation (n = refractive index of the body, λ = wavelength of the radiation); ρ o = density of the body; ϑ = angle of observation. Therefore, according to the present invention, the method of measuring the viscosity of a body is adapted, wherein: at least a first beam of substantially monochromatic light is sent into the body under test; a beam scattered by the body is collected according to a predetermined observation angle; the width of at least one line generated in the radiation spectrum due to the Brillouin scattering is measured; and the viscosity of the body region traversed by the beam is obtained from the measured linewidth; the invention being characterized in that said body is an optical fibre preform comprising a plurality of coaxial layers, which is submitted to the thermal treatments required for the drawing of an optical fibre, and in that said beam is first sent into the preform so as to pass through a first, substantially homogeneous preform layer and the viscosity of said layer is obtained, and then the measure is repeated by varying the beam position so that the beam passes also through a different, substantially homogeneous layer at each subsequent measurement, and the viscosity of each new layer is obtained from the difference with respect to the preceding measurement. For instance, the first measurement will be carried out on the outer preform layer and then the beam will be displaced step by step parallel to itself so as to pass through a different layer at each subsequent step, the displacement extent being such that the beam scans the whole area of a plane preform section. Information could also be simultaneously obtained in a plurality of sections taken in different planes, in particular in two perpendicular planes. To this end two or more beams are to be used, each beam being displaceable parallel to itself in one said plane. For instance, a measurement in axial direction and a measurement on a whole section perpendicular to the preform axis can be effected. Preferably, the measurements are carried out on the backscattered beam (ϑ=180°) as this makes value δν B maximum, as clearly shown by relation (1), thereby facilitating the measurement. The method as described has a very high measurement precision. In fact, as known, width δν B is of the order of 10 MHz and the commercially available apparatus for spectrometric measurements allow such a value to be measured with a precision that, depending on the greater or lesser apparatus sophistication, ranges from about 1 KHz to 1 Hz or a fraction of a Hz. Thus, viscosity variations ranging from 10⁻⁴ to 10⁻⁷, or even smaller, can be determined. It is also to be appreciated that the scattered radiation spectrum, besides the lines due to Brillouin scattering, exhibits a further line, centered on the incident radiation frequency and having a width: where k, η have the same meaning as in (1), and To and a are the temperature in °K and the radius of the particles forming the body, respectively. Thus, according to the invention, besides δν B , also δν RW could be measured to determine also the preform temperature. A further control parameter for the monitored process is thus available. The method disclosed can be carried out by means of the conventional apparatus for Brillouin spectrometry. To better illustrate said method, the accompanying drawing schematically shows one such apparatus employed for effecting measures on an optical fibre preform. In the drawing, the beam outgoing from a source 1, e.g. a monomode laser, is sent through a beam splitter 2 onto an optical fibre preform 3 located in a drawing furnace 4. The beam is so oriented as to impinge for instance at the softening region 3a. The backscattered beam is reflected by beam splitter 2 towards a Fabry-Perot interferometer 5, whose output signal is fed to a spectrum analyzer 6 sending the values of δν B to a computer 7 which computes the value of η according to relation (1). Source 1 is mounted on a support (not shown) allowing the source to be displaced parallel to itself to obtain the viscosity over a whole plane cross-section of preform 3. A second apparatus like that described above could be arranged to send a light beam parallel to the axis of preform 3, so as to supply viscosity information relevant to axial direction. A second measurement by the same apparatus may provide the width δν RW of the Rayleigh scattering line; by supplying computer 7 with such a value, also the body temperature can be obtained.

Claims: Method of measuring the viscosity of a body (3), wherein at least a first beam of substantially monochromatic light is sent into the body under test; a beam scattered by the body is collected according to a predetermined observation angle; the width of at least one line generated in the radiation spectrum due to the Brillouin scattering is measured; and the viscosity of the body region (3a) traversed by the beam is obtained from the measured linewidth, characterized in that said body is an optical fibre preform (3) comprising a plurality of coaxial layers, which is being submitted to the thermal treatments required for the drawing of an optical fibre, and in that said beam is first sent into the preform so as to pass through a first, substantially homogeneous preform layer and the viscosity of said layer is obtained, and then the measure is repeated by varying the beam position so that the beam passes also through a different, substantially homogeneous layer at each subsequent measurement, and the viscosity of each new layer is obtained from the difference with respect to the preceding measurement. Method according to claim 1, characterized in that said angle is 180°. Method according to claim 1 or 2, characterized in that the measurement is carried out by displacing the beam parallel to itself, the displacement extent being such that the beam scans the whole area of a plane preform section. Method according to claim 3, characterized in that said plane section is perpendicular to the preform (3) axis. Method according to claims 3 or 4, characterized in that said plane section is a section taken in the softening region of the preform (3). Method according to any preceding claim, characterized in that at least a second monochromatic beam is sent into the preform along a second direction different from the first one, and the viscosity of the preform (3) at least in said second direction is also measured. Method according to claim 6, characterized in that the viscosity is determined also over a whole plane section comprising said second direction by changing the second beam position. Method according to any preceding claim, characterized in that the second beam is sent into the preform (3) parallel to the preform axis. Method according to any preceding claim, characterized in that the width of a line due to Rayleigh scattering is measured in the or each scattered beam, to determine the temperature in the region traversed by the beam.

Inventors: GREGO GIORGIO, GREGO, GIORGIO
Applicants: CSELT CENTRO STUDI LAB TELECOM, CSELT CENTRO STUDI E LABORATORI TELECOMUNICAZIONI S.P.A.
Application year: 1987
Main classification: G01N21
IPC codes: G01N21, G01J3, G01N11