GB1596757A

GB1596757A - Apparatus for and method of measuring the optical properties of an optical fibre while it is being drawn

Info

Publication number: GB1596757A
Application number: GB2008577A
Authority: GB
Original assignee: International Standard Electric Corp
Current assignee: International Standard Electric Corp
Priority date: 1977-05-12
Filing date: 1977-05-12
Publication date: 1981-08-26
Also published as: CH628142A5; AU3568778A; AU520895B2; ES469779A1

Abstract

The device is suitable for measuring optical characteristics of optical fibres as the latter are being produced. Light pulses from a laser are transmitted through the drawn fibre via the fibre blank (12) from which the fibre (11) is drawn. The light signals emerging from the fibre end are fed via a bandpass filter (16) to a photodetector (15), in order to be able to undertake measurements of the scattering and transit times of pulses. <IMAGE>

Description

(54) APPARATUS FOR, AND METHODS OF MEASURING THE OPTICAL PROPERTIES OF AN OPTICAL FIBRE WHILE IT IS BEING DRAWN (71) We, INTERNATIONAL STANDARD ELECTRIC COR PORATION, a Corporation organised and existing under the laws of the State of Delaware, United States of America, of 320 Park Avenue, New York 22, State of New York, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to the measurement of the optical properties of an optical fibre as it is drawn from a region of softened or molten glass.

Optical transmission fibres may be drawn from a rod preform which either has a refractive index profile providing an internal waveguiding structure or is isotropic necessitating the provision of a coating of a low refractive index material which also functions as the optical cladding of the fibre. Alternatively, the fibre may be drawn from the outfall of a bushing containing one or more glass melts. Fibre drawn from a single melt will normally require a protective coating that also functions as an optical cladding, whereas, in general, fibre drawn from a double or multiple bushing will have an integral internal waveguide structure which is not affected by the optical properties of any coating that may be added subsequently for the purpose of mechanical protection.

Whatever the method of manufacture of the fibre it is a considerable advantage to provide, during the drawing process, a measure of attenuation that can be expressed as a function of distance along the fibre. If a defect enters the drawing zone and produces a localised region of higher than average attenuation, this can be identified and the position of the defect determined so that, if necessary, it can be removed.

Our co-pending application No. 46845/76 (Serial No 1557154) describes an apparatus for the measurement of the optical attenuation of optical fibre as it is being drawn from a drawing zone preform or melt arrangement and reeled up on a drum, the apparatus including a motor driven drum winching gear incorporating a slip ring arrangement to provide electrical connection between a photo-detector mounted on a portion of the winding gear that rotates with the drum, and a photo detector output signal channel that does not rotate with the drum, the apparatus also including clamping means for holding the end of the drawn fibre opposite the photosensitive surface of the photo-detector such that the photo-detector is adapted to receive light transmitted through the fibre from the region of drawing.

According to the present invention there is provided an apparatus for the measurement of the optical properties of an optical fibre as the fibre is being drawn from a drawing zone preform or melt arrangement and reeled onto a drum, wherein the fibre being drawn when the apparatus is in use has a first end accessible via the drawing zone arrangement and a second end accessible at the drum, wherein a laser is so arranged as to launch light pulses into one of said ends of the fibre, wherein a photo-detector is so arranged as to receive light from the fibre after that light has traversed the fibre, the photo-detector being optically coupled to its end of the fibre via a band-pass filter whose pass band corresponds to the laser output, and wherein the output of the photo-detector is coupled to means for determining the transit time and dispersion of light pulses from the laser which have traversed the fibre.

Embodiments of the invention will now be described with reference to the drawings accompanying the Provisional Specification in which: Figure 1 is a schematic diagram of an apparatus for measuring the optical properties of a fibre as it is drawn from a rod preform; and Figure 2 and Figure 3 show alternative measurement arrangements.

Referring to Figure 1, optical fibre 11, which may be of the clad silica type, is drawn from a rod preform 12 and reeled up on a rotating drum 13. Note that for clarity the necessary furnace and control equipment for the fibre drawing process have been omitted from Figure 1.

The end 14 of the fibre 11 is cut perpendicular to the fibre axis and is led out through the spindle of the drum. The method of preparing the fibre is described more fully in our co-pending application No.

46845/76 (Serial No. 1557154). The fibre end 14 is secured in a fibre clamp (not shown) adjacent a high speed photo-detector 15, e.g. a photo-diode, mounted on the drum spindle and is optically coupled to the photo-detector 15 via a narrow band-pass interference filter 16. The pass band of the filter 16 is matched to the output spectrum of the laser. The output of the photodetector is fed via a rotatable connector arrangement 17 to an amplifier 10. A revolution counter (not shown) is provided on the drum winding gear to provide a record of fibre length enabling the output of the photo-detector to be related to the length of the fibre drawn. The two signals may then be fed into the X and Y inputs of the chart recorder 18 for display. The trace will be expected to be exponential if the pulled fibre has the same attenuation throughout its length.In practice however, the trace normally shows a sharper falling off at the beginning of the drawing operation. This is attributed to the rapid attenuation of high order modes which effectively disappear after the first few metres.

In the arrangement shown in Figure 1, the coupling 17 is of the rotatable coaxial type.

In some applications however, a slip ring and brush arrangement may be used. In a further application a photo-detector may be mounted on a stationary support adjacent the fibre end 14 and coupled thereto via a fibre optic taper device or large core fibre.

To observe the pulse shape at the fibre end 14, the amplifier output is coupled to an oscilloscope 19 set on a fast time scale.

From measurements of the pulse width and by comparison with the fibre length, the pulse dispersion can be determined. The transit time of the light pulses through the fibre, and hence the core refractive index in the case of a step index fibre or the effective core refractive index in the case of a graded index fibre, is determined by measuring the time delay between the input pulse to the preform and the output pulse from the fibre end by means of a digital delay line. The fibre attenuation is determined from the change in the mean light pulse power output from the end 14 and thus displayed on the chart recorder with reference to the length of fibre drawn.

An alternative measurement apparatus is shown in Figure 2. In this arrangement reference signals are provided by a beam splitter device 21 disposed between the laser launch optic system and a further optical system 22 at the light input to the fibre preform 12. Light from the laser passes down the fibre and a portion of the light is reflected back from a partially reflecting mirror 2 arranged at the fibre end 14. The reflected light passes back through the optical system 22 and is reflected by the beam splitter 21 via a further band pass filter 23 to a photodetector 24, the output of which is fed to a second amplifier 25. As before the light output from the fibre end 14 is fed via a band pass filter 16 and the photodetector 15 to the amplifier 10.The output from the second amplifier 25 is fed to the oscilloscope 19 to display the pulse dispersion and transit time as previously described.

The arrangement shown in Figure 3 provides for the use of a high speed sampling oscilloscope for display of the output signals. The high speed electronic input system 31 of the oscilloscope is mounted together with a preamplifier 32 on the drum 13. In this way a relatively low speed output can be fed to the oscilloscope display 32 via a slip-ring and brush arrangement 33.

In some cases, the direction of light transmission through the fibre in the arrangements described herein can be reversed. Thus the laser, plus its launching optical arrangements and drive circuitry may be mounted on the drum 13 adjacent the end 14 of the fibre. Power is applied to the laser via the rotatable connector arrangement (or slip ring and brush arrangement). The laser output is then launched into the fibre end 14 and passes therefrom through the fibre to emerge at the preform where it is filtered and detected by the photodetector, now at the preform end of the fibre.

To extend the sensitivity of the arrangement, an integrated output from the oscilloscope can be obtained by setting the oscilloscope on a slow scan and recording the output trace on an X-Y plotter.

WHAT WE CLAIM IS: 1. An apparatus for the measurement of the optical properties of an optical fibre as the fibre is being drawn from a drawing zone preform or melt arrangement and reeled onto a drum, wherein the fibre being drawn when the apparatus is in use has a first end accessible via the drawing zone arrangement and a second end accessible at the drum, wherein a laser is so arranged as to launch light pulses into one of said ends of the fibre, wherein a photo-detector is so arranged as to receive light from the fibre after that light has traversed the fibre, the photo-detector being optically coupled to its end of the fibre via a band-pass filter whose pass band corresponds to the laser output, and wherein the output of the photo-detector is coupled to means for determining the transit time and dispersion of light pulses from the laser which have traversed the fibre.

2. An apparatus as claimed in claim 1 wherein the laser is at the drawing zone end of the fibre, and wherein the photo-detector is at the drawn end of the fibre.

3. An apparatus as claimed in claim 1 or 2, and wherein light pulses from the photodetector are applied to an oscilloscope.

4. An apparatus as claimed in claim 1, wherein the laser is at the drawing zone end of the fibre the light pulses reaching the end of the fibre via a beam splitter device located between the laser and the fibre end, wherein at the drum end of the fibre the light pulses leaving the fibre reach a further photo-detector via a partially-reflecting mirror which reflects part of the light reaching it from the fibre back into the fibre, wherein the light thus reflected back into the fibre traverses the fibre to its drawing zone end, from which it passes via said beam splitter device to the firstmentioned photo-detector, and wherein the output of the first-mentioned photodetector is applied to an oscilloscope.

5. An optical measurement apparatus substantially as described herein with reference to Figure 1, 2 or 3 of the drawings accompanying the Provisional Specification.

6. Optical fibre when produced via an apparatus as claimed in any one of claims 1 to 5.

7. A method of measuring the optical properties of optical fibre substantially as described herein with reference to Figure 1, 2 or 3 of the drawings accompanying the Provisional Specification.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. the preform where it is filtered and detected by the photodetector, now at the preform end of the fibre. To extend the sensitivity of the arrangement, an integrated output from the oscilloscope can be obtained by setting the oscilloscope on a slow scan and recording the output trace on an X-Y plotter. WHAT WE CLAIM IS:

1. An apparatus for the measurement of the optical properties of an optical fibre as the fibre is being drawn from a drawing zone preform or melt arrangement and reeled onto a drum, wherein the fibre being drawn when the apparatus is in use has a first end accessible via the drawing zone arrangement and a second end accessible at the drum, wherein a laser is so arranged as to launch light pulses into one of said ends of the fibre, wherein a photo-detector is so arranged as to receive light from the fibre after that light has traversed the fibre, the photo-detector being optically coupled to its end of the fibre via a band-pass filter whose pass band corresponds to the laser output, and wherein the output of the photo-detector is coupled to means for determining the transit time and dispersion of light pulses from the laser which have traversed the fibre.