A METHOD FOR MEASURING THE DIAMETER AND/OR DIAMETER CHANGE OF A GLASS FIBER ON THE BASIS OF FABRY-PEROT RESONANCES AND APPARATUS FOR CARRYING OUT THE METHOD

Abstract: Method for measuring the diameter and/or diameter change of a glass fiber on the basis of Fabry-Perot resonances, in the course of which the glass fiber is illuminated by a laser of constant wavelength from side direction making smnall angle with the direction being orthogonal to the fiber axis, the far-field intensity of light rays backscattered in the direction making the same but opposite angle with the surface normal is detected and the path length inside the glass fiber of light rays to be detected and backscattered from the inner back surface of the glass fiber is changed by changing the said small angle or by displacement of the glass fiber in axial direction. The apparatus capable to carry out the method comprises a laser light source illuminating the glass fiber from side direction and a photodetector sensing the far-field intensity of a ray boundle backscattered in a determined direction. At a first variant of the apparatus a direction changing device (beam deflector) is arranged between the laser and the glass fiber and a slit is placed between the photo detector and the glass fiber and a slit is placed between the photodetector and the glass fiber. At a second variant, the output beam of the laser light source illuminates the glass fiber at two different incidence angles through a beam splitter on one hand and through the beam splitter and a mirror on the other hand, and separate photodetectors are used for sensing the backscattered light rays arising from each of the two illuminating beams, and 90 degrees phase difference is produced between the photodetector signals. s

Description: A METHOD FCR MEASURING THE DIAMETER ANQ/CR DIAMETER CHANGE CF A GLASS FIBER ON THE 8ASIS OF FA8RY-FΞR0T RESONANCES AND APPARATUS FCR CARRYING OUT THE METHOD Technical Field The invention relates to a method for measuring the diameter and/or diameter change of a glass fiber on the basis of Fabry-Perot resonances. The main field of application of the invention includes the very accurate measurement of the diameter of unclad or clad glass fibers, plastic fibers and crystalline fibers in the 10 - 150 ,um diamater ran^e. The invention relates also to an apparatus for carrying out the method. Background Art Therj are a few methods KΠO-ΛΠ fcr measuring the diameter of thin glass fibers. In a paper /Proc. of IEEE, Vol. 70, iNo.o, dune 1382, pp.ό2c-634/ written by L.S. v*atkins anc titled as "Control of Fiber Manu acturing processes' a * nethod is described for diameter measurement, in which the glass fiber is illuminated by a laser ceam in a αirection making right angle with the fiber axis, and the angular distri¬ bution of the intensity of the forward scattered light is detected by a photodiode array comprising high number of photodiodes and saiα intensity α strieution is analized by a computer to calculate the diameter. This method makes possible to measure the diameter with +•_ 0,25 ,um accuracy, but in can be used only for fibers of small diameter because :h≤ number of the in¬ terference fringes in the angular distribution of the intensity increases very rβpioly when the diameter is increased, that maKβs the detection and evaluation difficult. Furthermore in order to reach high resolution a large photodiode array with 1CCC elements or so is reαuired and this together with the use of a microcomputer makes the method costly. In the cited paper an other method is also described, in which the fiber to be measured is scanned with a focused or unfocused laser beam in a plane being orthogonal to the fiber axis, the intensity of the scanned laser team is detected with a photodetector placed behind the fiber and the negative pulse generated by the "shadow" of the fibsr is evaluated. The ficer diameter is deter¬ mined from the time duration of this pulse knowing the scanning speed. The accuracy of this method is aoout _+ 0,5 ,um, but this value becomes considerably higher ,vhen the diameter is small /1C-2C ,um/. The fabrication and control of glass fibers requires higher accuracy than thoosβ ootainable by the methods described above. A method applicable to this task is described by A.Ashkin, 0.M.Dziedzic and R.H. Stolen in a paper titled "Outer diameter measurement of lew birefringence optical fibers by a new resonant backscatter technique" /Applied Cptics, Vol. 20 , No 13, duly 1981. pp. 22SS-2303/. This oethod is called as "near- field resonant backscatter raethoα" and it is based on the detection of the Fabry-Pβrot resonances and surface •~---'-- ( wave resonances observable in the backscattered from the fiber light intensity when the wavelength of the illuminating laser beam is tuned. An accuracy of + 0,01 urn was obtained using the detection of Faory- "" * / Perot resonances when performing comparative measu¬ rements of thick communication fibers. At the near-field resonant backscatter method the unclad glass fiber is illuminated with a laser beam through a beam splitter /semitraπspareπt mirror/ and then the light rays backscattered in the same direction are directed by the beam splitter to an image forming optical system /e.g. microscope/ and the magnified image of the glass fiber is produced. In the image plane the light rays producing Fabry-f-erot resonances are spatially separated from thoose backsca tered light rays that produce surface wave resonances and light rays to be detected are selected with a slit. The selected light rays ..hich arise from the same type of the backseattering process are detected and the diameter is calculated from the wavelengths of the minima and maxima of the oacksca terec intensity when the wavelength is tuned. Although the accuracy of this method is very high the following difficulties arise at its application /particularly is case of small diameter/. Due to the required high magni ication of the imaging even a very small, and not avoidable in practice, displacement of the fiber results in a large displacement of the position of the selected backsca tered light rays in the image plane. Hence, the measurement can not be performed due to this relative to the slit displacement or false minima will be detected. This difficulty arises increasingly when measuring small diameter fibers because, on one hand, the susceptibility of very thin fibers to vibra¬ tions is larger and on the other hand, larger magnifi- . cation must be applied at small diameters. Sβsides the sensitivity to displacements neither the presence of the beam splitter is advanta¬ geous beacuse the otherwise small intensity of the detectable backscattered light is further reduced by a factor of 4 due to the overall two-way attenuation of the beam splitter. Due to the optical effects caused by the cladding layers the near-field resonant oackscatter method is applicable only in moderate extent to easu- rement of singly or pluraliy-clac fibers. Glass fibers used in image transmitting fiber boundles generally have double cladding and their diameter falls in the 10-20 ,um range. Due to its hi^h price the laser light source of tunable wavelength required to the impla.πeπ- tation of the nearfield backscattβr metnod, itself limits the application field of the mac od. The known diameter-measuring methods oescπ ed above are not capable to determine the fiber diameter with the required accuracy during the manufacturing /drawing/ process when it would be possible to prevent drawing a fiber of diameter differing from the prescribed value by proper control of the technological parameters of the manufacturing process. Object of the Invention The object of the invention is to provide a method and apparatus for the de:3.-0111-..ation of the dia¬ meter and/or diameter changes, that eliminates the use of a laser of tunable wavelength, the nearfield imaging of the fiber by the Fabry-Perot scattered light, and that provides sufficient accuracy in measuring the dia¬ meter and makes possible the application of "value-hold' control at manufacturing glass fibers. Disclosure of the Invention According to the invention the above object is solved by a method, in the course of which the glass fiber to be measured is illuminated with a laser beam from side direction, and light rays oackscattered in a predetermined direction are detected, and according to the invention the glass fiber is illuminated with a laser beam of constant wavelength making a small angle with a direction being orthogonal to the cylinder axis of the glass fiber, and light rays to ce detected are selected so that their direction makes the said small but negative angle with the direction oeing orthogonal to said cylinder axis, the far-field intensity of these selected backscattered light rays is detected, ano mean¬ while the path-length inside the glass fiber of light rays to be detected and backscattered from the inner back surface of the glass fiber is changed. In the case of a possible way for carrying out the method the pathlength of light rays inside the fiber is changed by continuous adjusting the direction of the illuminating laser beam between two extreme angular OMPI - 5 - positions, and the diameter is determined on the basis of the extreme values of the detected intensity and the angular positions of the illuminating las«r beam corresponding to said extreme values of intensity. The angle between the direction of the illumi¬ nating laser beam and the direction being orthogonal to the cylinder axis of the glass fiber is alowed to be in the 0,5o...30o angle interval. In the case of an other way of carrying out the method, the glass fiber is illuminated by two laser beams produced by splitting the illuminating laser beam and theese beams have different angles of incidence /χ' / f the backscattered light rays arising from the two illuminating laser beams are detected separately, and the two angles of incidence are adjusted so that there should be prescribed, prβferacly 9C phase diffe¬ rence between the intensities of the detected light rays, and the path-length inside the glass fiber is changed by continuous displacement of the glass fiber in axial direction. At this variant of the method, the axial speed of the glass fiber is controlled on the σasis αf one of the detected signals for keeping the fiber αiamβter constant, and furthermore the sign of the change of the speed is controlled on the casis of the second detected signal. C.'-Fl At both variants of the method it is preferable, if the illuminating laser beam has a polarization plane being orthogonal to the cylinder axis of the gla93 fiber. The apparatus for oarrγing out the first variant of the method comprises a laser of constant wavelength , and in the path of the output laser beam a device changing continuously the direction of propagation of this laser beam between two extreme angular positions is arranged, and the deflected laser beams in the two extreme positi- ons make a first and second angles less than 30° with a direction orthogonal to the cylinder axis of the glass fiber and intersect the cylinder axis, and in the light path of the backscattered from the glass fiber rays a nontransparent plate is arranged on which a slit being parallel to the plane of the deflected laser beams is opened in the region of the two extreme positions of the backscattered light rays, and the detector is placed behind the plate. Here the device for changing the propa¬ gation direction of the laser beam is preferably an oscillating mirror. The realization of changing the propagation direction becomes simpler, if a first lens with optical axis falling in the bisectrix of the two extreme 1 angle positions of the deflected laser beas is placed between the direction changing device and the glass fiber, and both of the direction changing) device and the glass fiber are at a same distance being twice the focal length from the first lens, and furthermore, a second lens is placed - a - between the glass fiber and the detector, which second lens is at a same distance being twice the focal length from both of the glass fiber and the detector and its optical axis falls i/, e bisectrix of the two extreme angular positions of the backscat ered light rays. A further apparatus for carrying out the second variant of the method has also been realized, which apparatus is provided by a laser of constant wavelength, and in the light path of the output laser beam a beam splitter is placed, and in the path of one of the split beams a mirror is arranged and the reflected from this mirror first laser beam makes a first angle with the direction being orthogonal to the cylinder axis of the glass fiber, the split second aπo reflected towards the glass fiber laser beam -nakes a second angle oeing opposite to the said first angle, the value of this second angle differs from the value of t e first angle, and further¬ more, the first detector is arranged in the common plane of the first and second laser Dea s in a direction being opposite but equal to the first angle, and the second detector is placed in the same common plane in a direction being opposice but equal to the second angle. In order to determine the sign of the diameter change, it is preferable, if the first and second angles are choosen so, that there is a phase difference of given magnitude, preferably 90 , between the light intensities detected by the first and second detectors. ' OMPI Brief Description of Drawings Fig, 1. shows a magnified, cross-sectional View of a glass fiber, on which light rays of chβ illuminating laser beam and light rays scattered from the fiber in backward direction are depic¬ ted , Fig. 2. is a sketch illustrating a parallel crossection of the fiber and light, pathes of an illuminating light ray making a small /but exaggerated for the sake of clearness/ angle with the direction •being orthogonal to the fiber axis, Fig. 3. shows a characteristic intensity distribution as a function of the scattering angle at ortho¬ gonal and parallel polarization of the illumiπa- ting laser oeam illustrated in Fig. 1., Fig. 4. illustrates the generation of interference fringes at different inclination angles of the glass fiber in respect to the direction of the illuminating laser beam, Fig. 5/a are photos of the interference fringes taken at 5/b scanned inclination angle in case of different and 5/c polarizations and fiber diameters, Fig. 6. is a sketch of a measuring arrangement illust¬ rating the first variant of the method for carrying out the invention, Fig. 7. is a sketch of a measuring arrangement illust¬ rating tπe second variant of the method for carrying out the invention, and Fig. 8. shows the output signals of the first and second detectors sketched on Fig. 7 as a function of the fiber diameter OMPI Best Mode of Carrying Out the Invention In Fig. 1. light rays backscattered at 180 degree angle are illustrated when * ? ioubly-clad glass fiber 1C is illuminated with a laser beam. Illuminating light rays L being orthogonal to the axis of the glass fiber 10 are illustrated by parallel arrows in Fig. 1. Rays I of the illuminating laser beam reach orthogonally the central part of the surface of the glass fiber 10, and these rays are partially reflected from the outer front surface, meanwhile the entering into the fiber part of them is also partially reflected at the inner back surface of the fider at 180 degree angle thus producing light rays II. Rays III of the illuminating beam are also backscattered after multiple refractions at the claddings and reflection at the inner back surface. Finally light rays IV incident tangentially on the fiber are coupled into the fiber as surface light waves and they can be coupled out from the fiber 10 at the opposite side of the fiber thus producing 180 degree scattering. The intensities of rays I and II are the largest among the intensities of rays I-IV, but neither the intensities of rays III and IV is negligiole. The interference of rays I and II produces Fabry-Perot like interference, because the central part of the glass fiber can be regarded as an almost plane-parallel resonator Fig. 2 is a sketch showing the longitudinal cross-section of the fiber, where the illuminating laser beam L makes an angle ^ with the normal 3 to surface. The outer and inner claddings have an indices of refrac¬ tion of n-, and n_ and thicknesses of t, and t 2 , respec- tively. The innermost core of the fiber is characterized by a refractive index of n^ and a diameter of 2-r. The outer diameter of the whole fiber is d. The laser beam L incident at an angle * is refracted at the claddings and its direction of propagation in the core makes an angle fi> with the normal of the surface. The value of β is approximately valid for the claddings too, beacusβ the claddings are relatively thin in comparison to the fiber diameter. Investigating the resultant intensity of the backscat ered light rays I-IV as a function of the scattering angle θ, the distribution shown in Fig. 3. is obtained at given diame ers. In case of the continu¬ ous curve the polarization plane of the laser beam L is parallel to the ficer axis, while in case of the dotted curve the polarization plane is orthogonal to the a is. The changes in the angular intensity distribution come from the interference of rays I-IV^ and the effect of rays III and IV is seemingly much less in case of pola- rization being orthogonal to the axis. In the foilowing, our investigations a re carried out at & = C scattering angle or in the close vicinity of it. Now we refer to Fig. 2 again, and investigate that at what values of the angleA will the resultant intensity of rays I and II be minimal. To do so, we introduce the weighted average of the refractive index defined by the following expression: - /V In case of © ■ = 0 , a minimum in the intensity is obtainable if the condition is fulfilled. Here λ denotes the wavelength of the laser beam L, and is an odd integer, which gives the order of the interference. If the resultant intensity of light rays backscattered at 130 degree angle is investigated as a function of both of the 9 and •. angles, then a charac¬ teristic interference pattern schematically sketched in Fig. 4 is obtained. Fig. 5 a illustrates such an interference pattern produced cy continuous scanning the ancle of incidence - in case of a doubly-claα glass fiber of 12,2 ,um diameter, when the polarization direc¬ tion of the incident laser laser beam L is orthogonal to the fiber axis. Fig. 5/b and 5/c are similar to Fig. 5/a, but they were taken in case of a polarization plane parallel to the fiber axis. In Fig. 5/c the interference pattern obtained for a communication fiber of 120 um diameter can be seen. If the backscattered intensity is investigated along the θ = 0 line, then the diameter d of the glass fiber 10 can be determined from measuring the angl«s corresponding to two adjacent minima using the expression. Eguation /3/ can be used for measuring the diameter d, but due to -the error in the determination of the angles of minima /that is increased further by the effect of rays III and IV/ this kind of measurement gives the diameter by an error of 2-3 .urn. Now, we refer to Fig. 6, in which a sketch of the arrangement of the first exemplary embodiment of the apparatus capable to carry out the method accor¬ ding to the invention. A laser of constant frequency 11 is used as a light source which has an out-put laser beam 12 of small diameter ano in the light path of it a beam deflec¬ tor is arranged, which is an oscillating mirror 13 in the present case. The plane of the oscillating mirror 13 can be altered between two extreme angular positions, and hence the laser beam 12 leaves the oscillating mirror 13 in the two extreme positions of it as a first deflected beam 14 and a second deflected beam 14', respectively. The directions of the central rays of the first 14 and second 14 deflected beams make an angle with each other, and the value of this angle is usually between 15 and 30 degrees. In the light path of the deflected OMPI beams a lens 15 is placed at a distance of twice focal length from the oscillating mirror in a manner that the optical axis 16 of this lens 15 coincides with the direction of the deflected beam generated at the middle angular position of the oscillating mirror, that is with the bisectrix of the angle f". On the back side of the lens 15 the bisectrix, that is the optical axis 16 of the lens 15 crosses the glass fiber 10 to be measured at a distance being also egnal to twice the focal length from the glass fiber 10. The lens 15 transfers the deflected laser beams towards the glass fiber 10 at the same angle as the beams leave the oscillating mirror 13. In Fig. 6 it can be observed, that the axis of the glass fiber 10 is in an inclined position in respect tothe optical axis 16 of lens 15. The first deflected laser beam 14 is almost orthogonal to the fiber axis, the deviation of it is between -0,5 and -5 degree. The second deflec- tec laser beam 14' makes an other angle with the fiber axis and this angle differs by T " from the above menti¬ oned angle being slightly less than the right angle. Cut of the rays backscattered from the glass fiber 10 a second lens 17 collects thoose light rays having the same reflection angle as the instantaneous angle of incidence of the illuminating deflected beam is. *ϊfbe lers 17 is at a distance of twice the focal length from the glass fiber 10 and its optical axis intersects the glass fiber 1C at the same point where the optical axis 16 of the lens 15 does. The optical axes of the lenses 15 and 17 are in a common plane. The bisectrix of the angle between the optical axis of the lenses 15 and 17 is orthogonal to tha fiber axis . As the angle of incidence of the first deflec¬ ted laser beam 14 is less than 90 degree, the light pβthes of the illuminating and backscattered rays 24 are spatially separated. In the other extreme angular position the second deflected laser beam 14' is even more separated from the backsca tered rays 24. On the optical axis of the second lens 17 in the opposite to the fiber side a photodetector 13 is placed at a distance of twice the focal length from the lens, which detector detects the intensity of the backscattered from the glass fiber lϋ light rays collected by lens 17 onto the detector 13. In order to realize that only thoose lightrays should be ablii to reach the photodetector 18 that are oackscatterβd at θ = C degree or in the narrow vicinity of it, a nontransparent plate 9 is arranged preferably between the glass fiber lϋ and the lens 17, en this plate a slit is opened in the vicinity of 0 = G degree and the height of this slit is large enough to transmit the light rays 24 and 24' backsca tered at the two ex t reme angular positions. Light rays scattered at larger solid angle are not allowed to reach the photodetector 18 by the nontransparent plate 9. Using the aparatus sketched in Fig. b the diameter of the glass fiber 1C can be measured in the following way. The angular position of the oscillating ■* &TS A OMFI mirror 13 is continuously changed between the two extreme angular positions. Changing the angular position of the oscillating mi. or 13 the angle of incidence of the illuminating laser is also changed in a manner that the illuminating laser beam intersects the glass fiber 10 always at the same point. According ot Figs. 4 and 5 the intensity of the backscattered light rays transmitted by the slit changes periodically when the angle of iricidence x " of the illuminating laser beam is continuously changed beacuse the difference of the path lengthes of rays I and II also changes. Starting from the smallest value of the angle £ the maxi.-a and minima in the intensity of light rays is continuously monitored, and to every such an extreme value of intensity the coresponding angle derived from the known angular position of the oscillating mirror 13 is recorded. To determine the diameter d at least two minima or maxima must be detected, beacuse the use of Eq./3/ requires the knowledge of angles _S , and β> 2 ^(~ > l^ vvnicπ angles correspond to two adjacent maxima or minima. In orcsr to decrease th-e distortions in the detected signal chat arise from the presence of undesirable light rays the polarization plane of the illuminating laser eam is adjusted to be orthogonal to the axis of the class fiber 1C. The accuracy of the measurement can be increased if the angular data corresponding to more than two extreme intensity values are taken into account and a measured value of the diameter d is obtained using Eq. /3/ from every two angles J . and β , corresponding to two adjacent j — minima or maxima, and the averaged value of the measured values is calculated. The averaged value is basically independent from the fact r-s affecting thg angular positions corresponding ςu tne extreme values. From the point view of the determination of the angular positions corresponding to the minima and maxima in the intensity of the backscattered light rays the measuring arrangement sketched in Fig. 6 is equivalent to the case as if the wavelength of the laser II had been changed. This latter solution is considerably more expensive than a beam defflection performed by the single oscillating mirror 13. The laser beam 12 can also be deflected in another way instead of the oscillating mirror 13, for example by using an acousto- optic deflector, and furthermore tne same result can be obtained if the inclination angle of the glass fiber 10 is changed and all of the other elements are kept in constant position. The scanning of the inclination angle of the glass fiber 10 can not be realized in a simple way in case of very thin fioers, therefore the solution sketched in Fig. 6 may be regarded to be the most preferable and most easily realizable from the point of view of changing the relative to each other angular positions of the illumina ing laser beam and the glass fiber to be measured, because all of the elements of the measuring arrangement are in static state except for the oscillating mirror 13. The method descrioed aoove is useful for measuring the dia tere d. Bu thers are such measuring and, in the first place, control problems, where there WtfO ^ is no need to know the value of the diameter d, but it is sufficient to sense or measure the changes of it. This is the case, for example, at mai. ractu- t-^g glass fibers, where there in a possibility to keep the diameter constent by proper control of the angular speed of the fiber diameter well /e.g. in case of 14 .urn thick fiber with a 2-3 ,um accuracy:/, but in many cases the diameter must be kept within much less tolerance limits. The post-measurement of the manufactured fibers is appli- cable only to post-control when the technological pa¬ rameters can not be affected. In Fig. 7 a sketch of the arrangement of another emcodiment of the apparatus according to tne invention is illustrated, which is capable to measure the small diameter chances during the manufacturing process and makes possible to use valu^-hold control during manufac¬ turing. The glass fiber 10, for example, moves down- wares along a vertical line at a speed determined by the speed of rotation of the pulling crurπ / ot iil^s cra¬ ted on Fig. 7/. Similarly to Fig. b the laser 11 of constant wavelength generates a laser beam 12 of small diameter and in the light path of it a beam splatter 19 is placed. In the light path of the laser beam passed through the beam spli * ter 19 without changing its direc¬ tion a mirror 20 of fixed inclination angle is arran¬ ged and t reflects the laser beam 21 towards the glass fiber 10. The cirection of the laser beam 21 makes an angle o -, with the direction 22 being orthogonal to the fiber axis. In the direction making the angle -αC-iWith OMPI the line 22 and being coplanar with the laser beam 21 a first photodetector 23 is placed * vhich detects the intensity of the backscattered to tis direction light rays generated by the sss ti..-sr 10 due to the illumination with the laser beam 21. The laser beam 25 reflected by the beam splitter 19 towards the glass fiber 10 makes an angle - -at 2 with the direction 22 and it reaches the glass fiber 10 at the intersection point of the glass fiber 10 and the direction 22. In the direction making the angle * , ' 2 with the direction 22 and being coplanar with the laser Deam 25 a second photodetector 26 is arranged in the light path of the backscat ered rays and this second pho odetector 26 αetects the intensity of the backscat ered light rays arising due to the illumination with the laser beam 25. The position o the elegants of the arrangement cescriced here is unchanged during the measurement /o r control/, and the only moving element is the glass fiber 10 itself moving downwards during manufac¬ turing . The operation of the apparatus sketched in Fig. 7 is based on the interference of rays I and II backscattered towards the photodetectors 23 and 26 and the phenomenon is not dependent on the diameter d of the glass fiber 10. If the diameter of the manu¬ factured glass fiber 10 is not constant then the inten¬ sities detected by the photodetectors 23 and 26 periodi¬ cally oscillate when the diameter d is continuously changing. The minima and maxima of the intensities can be detected by any of the photodetectors 23 and 2-.. and knowing the constant wavelength and the angle of incidence the diaπeter change can be determined on the basis of the resonance condition desert bed by Ξq./2/. This calculation provides information about the appearance of change ano the value of change, but it does not give information about the cireccion /sign/ of change. Accorcing to the invention the sign of the chance of the ciaπeter is aαe to :e datar inable in that way that the glass fi-ar 10 is illumina ed with not a single laser be≤ but with two laser eoms 21 an 25, the polarization plane of chem is orc ogonal to the fiber axis, and provision is made to realize 90 degree phase difference between the detected sig¬ nals which oscillate sinuscidally when the diameter changes. This condition gives a relation between the angles - , and 2 . that is He re λ denotes the wavelength of the laser 11, π is the refraccive index of the glass fiber 10 /or in case of clad fibers the averaged refractive index/ anc d is the diameter of the fiber. This condition must be filfilled at tne practical implementation of the arrangement by aαjustiπg the positions of the beam splitter 19 anc the mirror 2C. For better understanding the operation of the arrangement sketched in Fig. 7 we refer to the curves in Fig. 3, in wnich the curve drawn by continuous line shows the output signal of the photodetector 23 and the dotted curve represents the output signal of the photodetector 25. The independent variable on the abscissa is the fiber diameter. The two curves shifted in phase by 90 degree in respect to each other havi DC components V Qr and an AC components V _ /if the output signals of the detectors 23 and 26 are given in voltage/. For measurement or control action it is sufficient to in¬ vestigate the AC components V ^ of both signals. If the output signals of the detectors 23 and 26 are regarded as measuring and reference signals, respectively, the theese two signals can be written - \ . r ef * - \ ^ co^ l 'l i to .) ' ** -, = '7l»l As the X cos = COS ^ 8 πd equation is valid in a small vicinity of the diameter d /for example in a _+ 5 ,um interval/ we may write r_e_f Λ = i- V ^U ! Ikr. ;J cos A|) AC To obtain this reference signal the limiting conditions coming from the arrangement sketched in Fig. 7 must bo choosen in an appropriate manner. If the diameter d changes in the intervel of length of Δ d then the nominal value of the diameter which must be kept constant by the speed control of the pulling drum must preferably be choosen to a value which corresponds to the working point M~ . In this αd interval the sign of the signal V , is unchanged and positive and the sign of the signal V is changes at this working point M 2# From the decresase or increase of the signal V one can determine the sign of the change of the diameter. Similarly the point M, can also serve for working point where the signal V also changes its siqπ but the signal V , is negative. From the sign of the signal V - one knows that what range of control is effective that is in which working point the system works /M-, or M 2 /« The points P-, and P 2 correspond to diameters where the s-gπal V , changes its sign and where the signal Vm reaches its maximum and minimum value, If the control is sufficiently fine thon the diameter of the glass fiber 10 can be kept within the control region Δd defined by the points P-, and P 2 shown in Fig. 8» Obviously, if the change of the diameter d is larg 3 er than this then the sig 3 nal and Vre £ f chang 3 e periodically and the sign of the signal V . determines whether the diameter d increases or decreases. In case of a larger change of the diameter the number of half periods of the sig a nal V_m is counted and this number is OMPI multiplied by the diameter change Λd/2 which corres¬ ponds to a h- If period and then the increment determined in the remaining not full half period is added to it. In Fig. 8. a full period of the signal V corresponds to 0,198 ,um at λ- 0,633 ,um and n = 1,6. One can see from this that using a proper control the diameter d can be kept constant with an accuracy better than 0,1 After forming a proper error signal from the sig a nals Vm_ and Vref, on the basis of the log 3 ic described above it is preferable to control the angular speed of the pulling drum.

Claims: Claims : 1. A method for measuring the diameter and/or diameter change of a glass fiber on the basis of Fabry- Perot resonances, comprising the steps of illuminating the glass. fiber to be measured with a laser beam from side direction and detecting light rays backscattered in a predetermined direction from the glass fiber, c h a r a c t e r i z e d in, that comprising additionally the steps of illuminating the glass fiber /10/ with a laser beam /12, 14, 14', 21, 25/ of constant wavelength from a direction making small angle with a direction /22/ being orthogonal to the longitudinal axis of the said fiber /10/ selecting the di ' rection of light rays /24, 24'/ to be detected to make the said small but negative angle with the said direction /22/, detecting the fαr- field intensity of theese rejected light rays /24, 24'/ and changing the path-length inside the glass fiber /10/ of light rays to be detected and backscattered from the inner back surface of the glass fiber /10/. 2. The method as claimed in claim 1, c h a r a c t e r i z e d in, that adjusting continuously the direction of said illuminating laser beam /14, 14'/ between two extreme angular positions to change the said path length inside the glass fiber /10/, and determining the diameter /d/ on the basis of the extreme values of the detected intensity end of the angular positions of the illuminating laser beam /14, 14'/ corresponding to βaid extreme values of intensity. _ OMPI 3. The method as claimed in claim 2, c h a r a c t e r i z e d in, that adjusting said angle / / at most between 0,5 and 30 degrees. 4. The method as claimed in claim 1, δ c h a r a c t e r i z e d in, that illuminating the glass fiber /10/ at two different angles of incidence / • *-,# ,itn two Iss 91* beams /21,25/ produced by splitting the illuminating laser beam /12/, detecting separately the backscattered light rays arising from the two illu i- 0 πating laser beams /21,25/ f selecting the two angles of incidence /. < .-,, .. p / so that producing a prescribed, preferably 90 degree phase difference between the separa¬ tely detected light rays, aπd changing the path length inside the glass fiber /10/ by continuous displacement of the glass fiber /10/ in axial direction. 5. The method as claimed in claim 4, c h a- r a c t e r i z e d in, that controlling the axial speed of the glass fiber /10/ and the sign of the change of speed on the basis of one of the detected signal /V / and the other detected signal /V ./ respec¬ tively, to keep constant diameter. 6. The method as claimed in any of the claims 1-5, c h a r a c t e r i z e d in, that adjusting the plane of polarization of the illuminating laser beam /12, 14, 14', 21, 25/ to be orthogonal to the fiber axis. 7. An appa. atus for carrying out the method of claim 2, comprising a laser and a pfiotodetector, c h a r a c t e r i z e d in, that the apparatus comprises a laser of constant wavelength /ll/, and in the path of the output laser beam /12/ a direction changing device changing continuously the direction of propagation of said laser beam /12/ between two extreme positions is arranged, and the deflected by this device laser beam ' 14, 14'/ in the two extreme positions make a first and second angles / <. ,, ι " 2 / less than 30 degree with the direction /22/ being orthogonal to the cylinder axis of the glass fiber /10/, and said deflected laser beams /14, 14'/ intersect the cylinder axis, and in the light path of the backscattered from the glass fiber /10/ light rays /24,24 / a nontranspa¬ rent plate /9/ is arranged on which a slit being parallel to the plane of the deflected laser beams /14, 14'/ i≤ opened in the region of the two extreme positions of the backscattered light rays /24, 24'/, and the photo- detector /18/ i3 placed behind the nontransparent plate /9/. 8. The apparatus as claimed in claim 7, c h a r a c t e r i z e d in, that the direction changing device is an oscillating mirror /13/. 9. The apparatus as claimed in claims 7 or 8, c h a r a c t e r i z e d in, that a first lens /15/ with optical axis /16/ being collinear with the bisectrix of the two extreme angle positions of the deflected laser beams /14 ι 'l-4'/ is arranged between the direction changing device and ch- * - ** - . iber /10/, and both of the direction . . .. changing device and the glass fiber /10/ are at the same distance being twice the focal length from the first lens /15/, and furthermore a second lens /17/ with optical axis being collinear with the bisectrix of the two extreme angle positions of the backscattered light rays /24, 24'/ is palced between the photodetektor /18/ and the glass fiber /10/, and the second lens /17/ is at the same distance being twice the focal length from both of the glass fiber /10/ and the photodetector /18/. 10. - An apparatus as claimed in claim 4, that camprises laser and photodetector c h a r a c t e r i- z e d in, that the apparatus is provided by a laser /ll/ of constant wavelength, and in the path of its output beam /12/ a beam spiller /19/ is arranged, and in the path of one of the split beams a mirror /20/ is placed and the reflected from this mirror /20/ first laser beam /21/ makes a first angle / \. ,/ with a direction being orthogonal to the cylinder axis of the glass fiber /10/, the split and reflected towards the glass fiber /10/ second laser beaπa /25/ makes a second angle / <_ / with said direction /22/, this second angle / " _/ is opposite to and different in value from the first angle / and furthermore, a first photodetector /23/ is arranged in the common plane of the first and second laser beams /21, 25/ in a direction being opposite but equal to the first angle / <, -, / in reference to said direction /22/, and in this common plane a second photodetector /26/ is placed in a direction being opposite but equal to the second angle / ^ / in reference to said direction /22Λ 11. The apparatus as claimed in claim 10, c h a r a c t e r i z e d in, that the first and second angles /•*• " -t 2 / ar "β choosen so that there is a phase difference of prescribed magnitude, preferably 90 degree, between the light intensities detected by the first and second photodetectors /23, 26/.

Inventors: BARABAS MIKLOS, BEHRINGER TIBOR, GIBER JANOS, MARKUS LENK ATTILA, PODMANICZKY ANDRAS, BARABAS, MIKLOS, BEHRINGER, TIBOR, GIBER, JANOS, MARKUS LENK, ATTILA, PODMANICZKY, ANDRAS
Applicants: BUDAPESTI MUESZAKI EGYETEM
Application year: 1984
Main classification: G01B11
IPC codes: G01B11