CA2088081A1 - Out-of-pattern coating defect detection system - Google Patents
Out-of-pattern coating defect detection systemInfo
- Publication number
- CA2088081A1 CA2088081A1 CA002088081A CA2088081A CA2088081A1 CA 2088081 A1 CA2088081 A1 CA 2088081A1 CA 002088081 A CA002088081 A CA 002088081A CA 2088081 A CA2088081 A CA 2088081A CA 2088081 A1 CA2088081 A1 CA 2088081A1
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- Prior art keywords
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- defect detection
- coating
- fiber
- detection system
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- 230000007547 defect Effects 0.000 title claims abstract description 58
- 238000001514 detection method Methods 0.000 title claims abstract description 38
- 239000011248 coating agent Substances 0.000 title claims abstract description 32
- 238000000576 coating method Methods 0.000 title claims abstract description 32
- 238000012544 monitoring process Methods 0.000 claims abstract description 11
- 238000000034 method Methods 0.000 claims description 18
- 239000011247 coating layer Substances 0.000 claims description 13
- 238000012545 processing Methods 0.000 claims description 7
- 239000000835 fiber Substances 0.000 abstract description 33
- 230000003287 optical effect Effects 0.000 description 14
- 239000011521 glass Substances 0.000 description 6
- 239000013307 optical fiber Substances 0.000 description 6
- 238000007689 inspection Methods 0.000 description 5
- 238000004891 communication Methods 0.000 description 4
- 238000004519 manufacturing process Methods 0.000 description 4
- 239000003365 glass fiber Substances 0.000 description 3
- 239000000463 material Substances 0.000 description 3
- 230000035945 sensitivity Effects 0.000 description 3
- 238000004458 analytical method Methods 0.000 description 2
- 239000003990 capacitor Substances 0.000 description 2
- 238000013461 design Methods 0.000 description 2
- 230000007613 environmental effect Effects 0.000 description 2
- 239000000853 adhesive Substances 0.000 description 1
- 230000001070 adhesive effect Effects 0.000 description 1
- 230000003321 amplification Effects 0.000 description 1
- 230000015556 catabolic process Effects 0.000 description 1
- 239000002131 composite material Substances 0.000 description 1
- 230000001010 compromised effect Effects 0.000 description 1
- 238000010924 continuous production Methods 0.000 description 1
- 238000006731 degradation reaction Methods 0.000 description 1
- 230000001066 destructive effect Effects 0.000 description 1
- 238000011161 development Methods 0.000 description 1
- 238000010586 diagram Methods 0.000 description 1
- 238000005516 engineering process Methods 0.000 description 1
- 230000001747 exhibiting effect Effects 0.000 description 1
- CPBQJMYROZQQJC-UHFFFAOYSA-N helium neon Chemical compound [He].[Ne] CPBQJMYROZQQJC-UHFFFAOYSA-N 0.000 description 1
- 238000005286 illumination Methods 0.000 description 1
- 239000010410 layer Substances 0.000 description 1
- 238000005259 measurement Methods 0.000 description 1
- 238000003199 nucleic acid amplification method Methods 0.000 description 1
- 239000002245 particle Substances 0.000 description 1
- 239000011253 protective coating Substances 0.000 description 1
- 238000012827 research and development Methods 0.000 description 1
- 229920002379 silicone rubber Polymers 0.000 description 1
- 239000004945 silicone rubber Substances 0.000 description 1
- 239000007787 solid Substances 0.000 description 1
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N5/00—Analysing materials by weighing, e.g. weighing small particles separated from a gas or liquid
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/84—Systems specially adapted for particular applications
- G01N21/88—Investigating the presence of flaws or contamination
- G01N21/95—Investigating the presence of flaws or contamination characterised by the material or shape of the object to be examined
- G01N21/952—Inspecting the exterior surface of cylindrical bodies or wires
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/47—Scattering, i.e. diffuse reflection
- G01N2021/4704—Angular selective
- G01N2021/4707—Forward scatter; Low angle scatter
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/84—Systems specially adapted for particular applications
- G01N21/8422—Investigating thin films, e.g. matrix isolation method
- G01N2021/8427—Coatings
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/84—Systems specially adapted for particular applications
- G01N2021/8444—Fibrous material
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- General Health & Medical Sciences (AREA)
- Immunology (AREA)
- Life Sciences & Earth Sciences (AREA)
- Chemical & Material Sciences (AREA)
- Analytical Chemistry (AREA)
- Biochemistry (AREA)
- Health & Medical Sciences (AREA)
- General Physics & Mathematics (AREA)
- Physics & Mathematics (AREA)
- Pathology (AREA)
- Investigating Materials By The Use Of Optical Means Adapted For Particular Applications (AREA)
- Testing Of Optical Devices Or Fibers (AREA)
- Photometry And Measurement Of Optical Pulse Characteristics (AREA)
- Optical Fibers, Optical Fiber Cores, And Optical Fiber Bundles (AREA)
- Surface Treatment Of Glass Fibres Or Filaments (AREA)
Abstract
Out-of-Pattern Coating Defect Detection System Abstract A defect detection system (10) of this invention applies one or more orthogonally-aligned light beams (22 and 24) to a coated fiber (32) such that the light passes through the entire width of both the fiber (32) and its applied coating (31). As the light passes through a fiber (32) and coating (31) of good quality, a particular pattern, with a predictable intensity level, is produced by the forward scattered portions of the light beams directed through the coated fiber (32).
In accordance with the present invention, the intensity of the forward scattered patterns is continuously monitored by a series of photodiodes (44, 45, 46 and 47) each located adjacent to the expected forward scatter pattern. Various defects in the coating cause the path of the light passing through the coated fiber (32) to be altered, thereby directing some of the forward scattered light outside of the normally expected pattern and into the detection area of the photodiodes (44, 45, 46 and 47). Such distortion to the forward scatter patterns causes the light intensity level measured at the photodiodes (44, 45, 46 and 47) to increase.
Therefore, by continuously monitoring the light intensity level adjacent to the forward scatter patterns for an increase of light relative to the light intensity expected, the present invention provides reliable and accurate means for detecting, temporally short defects, such as air bubbles, which may be present within the coating (31) of a fiber (32).
In accordance with the present invention, the intensity of the forward scattered patterns is continuously monitored by a series of photodiodes (44, 45, 46 and 47) each located adjacent to the expected forward scatter pattern. Various defects in the coating cause the path of the light passing through the coated fiber (32) to be altered, thereby directing some of the forward scattered light outside of the normally expected pattern and into the detection area of the photodiodes (44, 45, 46 and 47). Such distortion to the forward scatter patterns causes the light intensity level measured at the photodiodes (44, 45, 46 and 47) to increase.
Therefore, by continuously monitoring the light intensity level adjacent to the forward scatter patterns for an increase of light relative to the light intensity expected, the present invention provides reliable and accurate means for detecting, temporally short defects, such as air bubbles, which may be present within the coating (31) of a fiber (32).
Description
1- 2~X$~
Out-of-Pattern Coatin~ l~efect Detection Sy~tern Technical Field This invention relates to an out-of-pattern coating defect detection system.
5 Background o~ the Inv~nt;on Primarily because of their increasing use in optical communication systems, the fabrication and subsequent processing of optical fibers continue to be subjects of intensive research and development.
Optical fibers are typically made in a continuous process which involves 10 drawing a thin glass strand or f~lber from a heat-softened glass preform. In addition, it is relatively common for the glass fibers thereafter to be coated with some type of polymeric material to provide protection from various environmental concerns, as well as increase the structural strength of the fiber.
Due to the relatively strict operational constraints under which a glass ~lber must perform when part of an optical communication system, the fiber manufacturing process should be precisely monitored and controlled.
In addition, the many operational characteristics of an optical fiber can be compromised if any defects are present in the protective coating applied to 20 the fiber during the drawing process. 'rherefore, monitoring should not be directed only at the drawing process of the glass strand, but to the coating application process as well.
In the past there have been various attempts to monitor and verify the accuracy with which a coating layer is applied to the optical ~lber.
~5 Parameters such as diameter, ellipticity, and fiber-to-coating concentricity each change relatively slowly during the drawing process. At present, such parameters are sometimes measured and controlled by various adjustments at the coating applicator. Generally, devices employing various scanning techniques are utilized to monitor the slow changing parameters identifled 30 above.
A particular method for monitoring the fiber-to-coating concentricity of the optical fiber polymeric coating by directing orthogonal light beams at the coated fiber has been disclosed. The speci~lc details o~
this concentricity monitoring method may be found in a paper by Ralph E.
35 Fra~ee, Jr. and David H. Smithgall entitled "High Speed Measurement and Control of Fiber-Coating Concentricity," the Bell Sys~em Technical Journal, :, - :
- :
Out-of-Pattern Coatin~ l~efect Detection Sy~tern Technical Field This invention relates to an out-of-pattern coating defect detection system.
5 Background o~ the Inv~nt;on Primarily because of their increasing use in optical communication systems, the fabrication and subsequent processing of optical fibers continue to be subjects of intensive research and development.
Optical fibers are typically made in a continuous process which involves 10 drawing a thin glass strand or f~lber from a heat-softened glass preform. In addition, it is relatively common for the glass fibers thereafter to be coated with some type of polymeric material to provide protection from various environmental concerns, as well as increase the structural strength of the fiber.
Due to the relatively strict operational constraints under which a glass ~lber must perform when part of an optical communication system, the fiber manufacturing process should be precisely monitored and controlled.
In addition, the many operational characteristics of an optical fiber can be compromised if any defects are present in the protective coating applied to 20 the fiber during the drawing process. 'rherefore, monitoring should not be directed only at the drawing process of the glass strand, but to the coating application process as well.
In the past there have been various attempts to monitor and verify the accuracy with which a coating layer is applied to the optical ~lber.
~5 Parameters such as diameter, ellipticity, and fiber-to-coating concentricity each change relatively slowly during the drawing process. At present, such parameters are sometimes measured and controlled by various adjustments at the coating applicator. Generally, devices employing various scanning techniques are utilized to monitor the slow changing parameters identifled 30 above.
A particular method for monitoring the fiber-to-coating concentricity of the optical fiber polymeric coating by directing orthogonal light beams at the coated fiber has been disclosed. The speci~lc details o~
this concentricity monitoring method may be found in a paper by Ralph E.
35 Fra~ee, Jr. and David H. Smithgall entitled "High Speed Measurement and Control of Fiber-Coating Concentricity," the Bell Sys~em Technical Journal, :, - :
- :
2 ~
Vol. 60, No. 9, No~ember, 1~)81, pp. ~065-2080, hereby expressly incorporated by reference herein.
I-Iowever, the particular devices presently used to measure the previously identified, slow-changing parameters are not capable of 5 accurately identifying and responding to signals ident;fying defects which are temporally short in duration at line draw speeds. Very often, such defects escape detection by the various coating monitors presently available which employ scanning techniques. Among the short duration signals generally not detectable with existing devices are signals indicative of 10 defects caused by inclusion of particulates which alter the coating diameter,entrapped bubbles which frequently erupt on the surface or high viscosity particles pulled through the applicator die. Each of these particular defects is exemplary of types of defects which can cause a loss of lightguide product during subsequent processing.
The only such defects that existing devices consistently and accurately detect are those which happen to be exceptionally large. The development of a system capable of dependably detecting short-duration defects as part of the on-line fiber processing would allow for reworking around the defect before placing the flber into a cable for operation. A
20 savings in both product materials and production time will be realized, in addition to the increase in the quality of the flber ultimately positioned within a communication system.
In light of the above-identifled problems with the present technology, what is needed and what seemingly is not provided by the prior 25 art is a dependable system capable of accurately detecting defects which may exist in a coating layer applied to an optical fiber. Furthermore, the sought-after coating defect detection system should utilize a non-destructive inspection technique which causes no degradation to the quality of a coated fiber as a result of the inspection. Still further, the sought-after coating 30 defect detection system should be operable to provide reliable detection of defects which are temporally short in duration and adaptable to be incorporated within the conventional on-line processing sequences used to ~-manufacture optical fibers.
The present application is directed to one defect detection 35 method, namely an out-of-pattern analysis techniqlle, while a second commonly assigned application U. S. Serial No. 07/82635n, discloses another - ~
~ ~3 ~
Vol. 60, No. 9, No~ember, 1~)81, pp. ~065-2080, hereby expressly incorporated by reference herein.
I-Iowever, the particular devices presently used to measure the previously identified, slow-changing parameters are not capable of 5 accurately identifying and responding to signals ident;fying defects which are temporally short in duration at line draw speeds. Very often, such defects escape detection by the various coating monitors presently available which employ scanning techniques. Among the short duration signals generally not detectable with existing devices are signals indicative of 10 defects caused by inclusion of particulates which alter the coating diameter,entrapped bubbles which frequently erupt on the surface or high viscosity particles pulled through the applicator die. Each of these particular defects is exemplary of types of defects which can cause a loss of lightguide product during subsequent processing.
The only such defects that existing devices consistently and accurately detect are those which happen to be exceptionally large. The development of a system capable of dependably detecting short-duration defects as part of the on-line fiber processing would allow for reworking around the defect before placing the flber into a cable for operation. A
20 savings in both product materials and production time will be realized, in addition to the increase in the quality of the flber ultimately positioned within a communication system.
In light of the above-identifled problems with the present technology, what is needed and what seemingly is not provided by the prior 25 art is a dependable system capable of accurately detecting defects which may exist in a coating layer applied to an optical fiber. Furthermore, the sought-after coating defect detection system should utilize a non-destructive inspection technique which causes no degradation to the quality of a coated fiber as a result of the inspection. Still further, the sought-after coating 30 defect detection system should be operable to provide reliable detection of defects which are temporally short in duration and adaptable to be incorporated within the conventional on-line processing sequences used to ~-manufacture optical fibers.
The present application is directed to one defect detection 35 method, namely an out-of-pattern analysis techniqlle, while a second commonly assigned application U. S. Serial No. 07/82635n, discloses another - ~
~ ~3 ~
defect detecting method, namely an in-pattern analysis technique.
Summary of the Invention The foregoing problems of the prior art have been overcome by the out-of-pattern coating defect detection system set forth in the claims.
5 Brieî De~cription of the 1:1 rawin~
FIG. 1 is a top view of the opto-mechanical layout of one embodiment of the out-of-pattern coating dlefect detection system of the present invention;
FIG. 2 is a top view of the opto-mechanical layout of another 10 embodiment of the out-of-pattern coating defect detection system of the present invention;
FIG. 3 is a light beam ray trace of the refracted light through a cross-section of a coated fiber;
FIG. 4 is a block diagram representative of the over-all circuitry 15 utilized to perform the operation of the present invention;
FIG. 5 is an electrical schematic of the input amplifier and compensation circuit utilized in the present invention; and FIG. 6 is an electrical schematic of the fiber presence detection circuitry utilized in the present invention.
20 Detailed Description Two particular embodiments of the optical and mechanical design of the continuous, out-of-pattern coating defect detection system 10 of the present invention are illustrated in YIG. 1 and FIG. 2. In general, the detection system 10 incorporates an orthogonal illumination scheme to 25 apply at least one light beam through a coated rlber. More specifically, the present invention utilizes an out-of-pattern inspection techn;que to detect coating defects within the particular portion of coated fiber being illuminated. While the preferred employment of the present invention is to inspect coating applied to a glass optical l~lber, it should be noted that the 30 coating layers of other non-glass strand materials may also be inspected us~ng the present invention.
As used herein, the phrase out-of-pattern inspection refers to a technique wherein some type of photodetection means is precisely positioned just aside the forward scatter pattern generated when light is shone through 35 a properly coated fîber. To identify coating defects, the out-of-pattern technique continuously monitors the intensity of light just outside the - , .
~ ~ ~J~
- 'I -expected forwald scatter pattern for an increase in the light received. Such an increase of light intensity indicates the presence of a defect which caused a portion of the light shone t,hrough the coated rlber to be scattered outside of the normal forward scatter pattern and into the area being monitored by 5 the photodetectors.
There are two particular opto-mechanical arrangements or embodiments disclosed herein ~o carry out the out-of-pattern coating de~ect detection technique of the present inventiom The l~lrst embodiment is a one-laser configuration which is relatively similar to the opto-mechanical lO arrangement disclosed in the copending, commonly-assigned application identif~led above, with the exception of the physical positioning of the photodetectors. This first embodiment is illustrated in FIG. 1 and discussed immediately below. The second embodiment utilizes a multiple laser conf~lguration as illustrated in FIG. 2 and is discussed in greater detail later15 within this document.
As can be seen in FIG. 1, one embodiment of the present invention comprises a helium neon laser 12 which emits a beam 14 which is directed by a mirror 16 into a beam expander 18. In the preferred embodiment, a 6~c beam expander is utilized. The beam 14 is then split by a 20 beam splitter 20, preferably a 50/50 cube-type, and directed along two separate optical paths 22 and 24.
Through the use of three adjustable mirrors 26, 28 and 30, the split beams traveling along optical paths 22 and 24 are directed to orthogonally impinge on a fiber 32 having a coating layer 31. In addition, a 25 fiber-throughway opening 38 is designated as the cut-out portion of base plate 34 which the ~lber 32 passes through in a direction perpendicular to .,~
the base plate 34 as it travels along the fiber processing path. The fiber-throughway opening 38 creates two inward corners 40 and 42 within base plate 34 a~s shown in FIG. l.
In order to properly position the photodetectors just outside the expected forward scatter pattern, a series of photodetectors 44, 45, 46 and 47 are secured within inward corners 40 and 42 respectively of base plate 3~1.
In the preferred embodiment, the particular photodetectors used are BPX65 photodiodes manufactured by Centronics Incorporated which have good 35 high frequency response, sensitivity and relatively low cost. These specific photodiodes have a l mm square active area and are generally mounted in a TO-18 type, 2-pin header. When the laser 12 is on and a coated rlber 32 in position, a forward scattered light pattern should be formecl immediately adjacent to each photodiode 4'1, 95, 46 and 47. However, if slightly misaligned, mirror adjustments may be macle to position the pattern 5 adjacent to the active area of the photodiodes 44, 45, 46 and 47.
The beam expander 18 of this eImbodiment of the present invention may be assembled separately. Two plano-cylindrical lenses of different focal length, a 6.35 mm and a 40.0 mm respectively, are mounted with a silicone rubber type adhesive to the expander system rail. The lenses 10 are mounted plano side toward plano side and are initially spaced at ~6.35 mm. This particular degree of spacing produces a nearly recollimated 6.3 times expansion of the beam in one direction. The preferred configuration of the beam expander 18 allows slight spacing adjustments to be made after installing the module to bring the beam into collimation. The beam 15 expander 18 is positioned on the base plate 3'1 between the first mirror 16 and the splitter cube 20.
At present, the particular embodiment detailed immediately above is positioned on the draw tower base, where the flber path is horizontal and parallel with the base plate 34. With the physical 20 arrangement of the previously described embodiment, the fiber 32 travels through the thickness of the detection system 10, in the three-sided fiber-through-opening 38, where it is impinged by orthogonal laser beams 22 and 24. Unfortunately, such positioning of the detection system 10 leaves the length of the optics head sticking up vertically from the tower base into the 25 operator's work area, thereby making the optical alignment sensitive to accidental bumps.
An alternative embodiment of the present defect detection system is also disclosed herein and illustrated in FIG. 2. The pariicular alternative embodiment disclosed, allows the detection system 10 to be 30 physically located at a different point along the fiber processing path than the earlier disclosed embodiment, because it is substantially smaller than the size of the previously described embodiment.
The opto-mechanical design of the second embodiment of the present defect detection system is illustrated }n FIG. 2. This particular 35 arrangement employs two separate solid state lasers, 50 and 52 to generate light beams 54 ancL 56, respectively. The lasers used in the preferred 2 ~
embodiment are Melles Griot Model 06VBL 102/P. The output power Or each laser 50 and 52 is about 3mw at 670 nm. Two plano-convex cylindrical lenses 58 and 60 are positioned in each bearn 54 and 56, respectively, to expand the associated beam horizontally and recollimate it to a 6mm width.
5 Four mirrors 62, 64, 66 and 68, two in each beam, are utilized lo direct the light beam 54 and 56 to the f~ber 32. Speciflcally, light beam 54 is directed to the fiber 32 via mirrors 62 and 68, whereas mirrors 64 and 66 are used to direct light beam 56 to the fiber. The mechanical placement of these mirrors causes the beams to be substantially orthogonal to both the fiber 10 and each other when located at the position where they impinge the rlber.
As stated earlier, the optical detection technique described herein is based on monitoring the out-of-pattern scattering caused by defects. Five photodiode detectors may be used in the preferred arrangement of this second embodiment system. As stated with regard to 15 the earlier described embodiment, the particular photodiodes preferred are Model BPX65, which are manufactured by Centronics Incorporated, and have good high frequency response, sensitivity, and relatively low cost.
Four of the photodiodes, namely elements 44, 45, 46 and 47 of FIG. 2, are used to detect the out-of-pattern scattering caused by a coating 20 defect. Since the photodiodes utilized in both embodiments are identical, .
like reference numerals are used herein to identify the photodiodes throughout the discussions of both embodiments set forth. In particular, photodiodes 44 and 45 are precisely aligned immediately adjacent the expected forward scatter pattern of light beam 54, while photodiodes 46 and 25 47 are aligned aside the expected forward scatter pattern of light beam 56.
A fifth photodetector 78 is used to detect the presence of a laser illuminated coated fiber by monitoring a pattern from each axis generated when light is scattered through the cylindrical coating layer 31, as illustrated in FIG. 3.
The electronic circuitry to be used to implement either 30 embodiment of the coating defect detection system is illustrated in FIG. 4 - and FIG. 5. FIG. 4 depicts the overall circuitry utilized to operate the present defect detection system, while FIG. 5 details the specifics of an input amplifier circuit incorporated into the overall circuitry of FIG. 4 for each optical axis.
2 Q ~
A.C stated earlier and illustrated in FIC~. 4, each embodlllleIlt of the present invention incorporates four defect photodiodes with a pair of these photodiodes positioned in each scattering a~is. Electronically equivalent input amplifier stages 80 (see FI~:. 5 for detailed circuitry) are 5 provided for each optical axis. An output 82 of each input amplifier stage 80 is processed via comparator 84. Comparator 84 has as its inputs, output signal 82 and a reference voltage 86. Furthermore, the comparator 84 generates a defect signal channel 88, for the particular optical axis it is servicing.
As illustrated in FIG. 4, the reference voltage is adjusted during calibration and establishes the system sensitivity. As stated, a reference voltage 86 and the signal voltage 82 are differentially input to differential voltage comparator 84. An output 88 from the comparator 84 is input to a one shot multivibrator ~6 which stretches the triggering pulse to a sufficient 15 length of time for the control computer 98 to acquire the data. This stretched output 100 also illuminates a display LED 102 for operator information. An electronically identical circuit as described above and illustrated in FIG. 4 is utilized to process each of the optical axis generated.The following circuit description of the input amplifler 80, a 20 composite single axis defect signal channel, and the output stage of one axiswill be more easily understood by referencing the electronic schematic of FI~. 5. As shown in FIG. 5, the anode of photodiode 44 is connected to the inverting input of operational ampli~ler 104. A resistive feedback, via resistor 106, configures amplifier 104 as a transresistance amplifier or a 2~ current-to-voltage converter. An identical electrical configuration is employed around diode 45 with op-amp 108 and resistive feedback, via resistor 110, configuring the transresistance amplifier or the current-to-voltage converter.
~; The two voltage outputs 112 and 114 from input op-amps 104 30 and 108, respectively, are summed on the inverting input of amplifler 118.
A capacitor 118 in the feedback loop changes this circuit function to a low pass filter and resistor 120 holds the gain to unity. The output voltage 122 from this circuit is used to remove any quiescent voltage levels from getting into the defect signal channel.
~ . :
, ~.
The voltage outputs 112 and 114 from the op-amps 10~ and 108 are summed again along with the ~lltered inverted signal 122 ïrom ampl;rler 116 at the inverting input of amplifier 124. The gain o~ amplil~er 124 is set by input resistors 126, 128 and 130 and feedback res;stor 132 to a gain of 27 5 db. A small capacitor 134 in the feedback of amplifier 124 improves the stability of the circuit. The direct coupled s;gnals which are summed into amplifier 124 produce an output from amplirler 124 which is similar to that of a differentiator; that is, exhibiting relati~ely high gains for input voltagechanges (slopes), and low gains for stable levels. Therefore, the output 10 signal 136 from amplifier 124 will be pos;tive or negative going signal swings around a near ground (zero voltage) level.
The output signal 136 from amplifier 124 is further processed by a full wave rectifier (not shown) whose output is a unipolar signal with an amplitude directly related to the intensity change on the photodiode, and 15 therefore to the severity of the defect. This proportionality allows the output of the rectifier to be compared to an established unipolar reference voltage as done in comparator 84 of FIG. 4.
A separate independent signal channel is utilized for the fiber present photodiode 78. The flber presence detector circuit is specif~lcally 20 illustrated in FIG. 6. The fiber presence circuit function is to detect when a coated fiber 32 is in the inspection area and to determine if the fber 31 is being struck by sufficient laser beam power. The signal on the anode of photodiode 138 is input to a transresistance configured operational amplifier 140. The gain of this circuit is lower than the previously described inputs 25 because photodiode 138 resides in the normal scattered light pattern. The position of diode 138 also makes it susceptible to fluctuations in scattered light po~,ver due to parametric changes quite apart from coating defects.
Therefore, the output of op-amp 140 is heavily filtered in the next stage of amplification by op-amp 142 and its associated electrical components as 30 shown in FIG. 6. This circuit has a response time of 10 -12 seconds. The filtered output 146 is compared against a fixed reference voltage level at the differential voltage comparator 148. The output of the comparator 148 is used to illuminate LED 102 of FIG. 4 for operator information and is presented as data to the computer ~8 also shown in FIG. 4.
, $ ~
The above descriptions have detailed opt;cal, mechanical and electrical conrlgurations in accordance with the preferred embodiment of the present invention. The particular operation of the present coating defect detection system will now be discussed. As stated earlier, glass fibers 5 for use in optical communication systems are manufactured by a drawing proeess. During the drawing of a glass fiber, a coating layer i5 conventionally applied to provide additional strength enhancement for the fiber or to provide protection from various environmental concerns which may inhibit the operation of the fiber.
Generally, the draw process includes a series of on-line processes performed sequentially. The present invention provides a detection system whieh may be ineorporated on-line to deteet eoating defects to avoid installing a ~lber with unknown coating defects into a comrmunication cable.
The instant detection system is installed at a location after the application 15 of tne eoating layer but prior to the fiber take-up. As the fiber 32 passes through the system, light beams 22 and 24 of the first embodiment described, or light beams 54 and S6 of the alternate embodiment described herein, illuminate a port;on of the coated fiber 32 aeross its entire outer diameter. A forward seatter pattern as generally shown in FI~. 3 is 20 generated opposite each light source as the light passes through the coating layer 31 and/or the fiber 32.
A series of photodiodes 44,45,46 and 47is positioned immediately adjacent the forward scatter pattern expected to be produced as light passes through a properly coated fiber. 32 However, various defects 25 cause portion of the light to be scattered outside the expeeted forward seatter pattern. The present invention eontinuously monitors the light intensity level of an area adjacent the expected forward scatter pattern.
~hen the light intensity within the monitored area increases, an indication is generated such that a monitor computer ~8 or an LED display 102 can identify the presenee of a defect within the coating layer.
As stated earlier, the heart of the out-of-pattern detection technique utilized in the present invention relies on reeognizing an inerease in light intensity by photodetectors positioned immediately adjaeent an expeeted forward seatter pattern to indieate the presenee of a defeet. It 35 should be specifically noted that even though particular circuitry is disclosed herein, minor variations in the particular electronic monitoring :' .' ~, :.
,.~ .
2 ~
used to measure the light intensity aside the forward scatter pattern for the purpose of detecting coating defects is deemed to be within the scope of the present invention regardless of the exact electronic components or conrlguration employed.
Summary of the Invention The foregoing problems of the prior art have been overcome by the out-of-pattern coating defect detection system set forth in the claims.
5 Brieî De~cription of the 1:1 rawin~
FIG. 1 is a top view of the opto-mechanical layout of one embodiment of the out-of-pattern coating dlefect detection system of the present invention;
FIG. 2 is a top view of the opto-mechanical layout of another 10 embodiment of the out-of-pattern coating defect detection system of the present invention;
FIG. 3 is a light beam ray trace of the refracted light through a cross-section of a coated fiber;
FIG. 4 is a block diagram representative of the over-all circuitry 15 utilized to perform the operation of the present invention;
FIG. 5 is an electrical schematic of the input amplifier and compensation circuit utilized in the present invention; and FIG. 6 is an electrical schematic of the fiber presence detection circuitry utilized in the present invention.
20 Detailed Description Two particular embodiments of the optical and mechanical design of the continuous, out-of-pattern coating defect detection system 10 of the present invention are illustrated in YIG. 1 and FIG. 2. In general, the detection system 10 incorporates an orthogonal illumination scheme to 25 apply at least one light beam through a coated rlber. More specifically, the present invention utilizes an out-of-pattern inspection techn;que to detect coating defects within the particular portion of coated fiber being illuminated. While the preferred employment of the present invention is to inspect coating applied to a glass optical l~lber, it should be noted that the 30 coating layers of other non-glass strand materials may also be inspected us~ng the present invention.
As used herein, the phrase out-of-pattern inspection refers to a technique wherein some type of photodetection means is precisely positioned just aside the forward scatter pattern generated when light is shone through 35 a properly coated fîber. To identify coating defects, the out-of-pattern technique continuously monitors the intensity of light just outside the - , .
~ ~ ~J~
- 'I -expected forwald scatter pattern for an increase in the light received. Such an increase of light intensity indicates the presence of a defect which caused a portion of the light shone t,hrough the coated rlber to be scattered outside of the normal forward scatter pattern and into the area being monitored by 5 the photodetectors.
There are two particular opto-mechanical arrangements or embodiments disclosed herein ~o carry out the out-of-pattern coating de~ect detection technique of the present inventiom The l~lrst embodiment is a one-laser configuration which is relatively similar to the opto-mechanical lO arrangement disclosed in the copending, commonly-assigned application identif~led above, with the exception of the physical positioning of the photodetectors. This first embodiment is illustrated in FIG. 1 and discussed immediately below. The second embodiment utilizes a multiple laser conf~lguration as illustrated in FIG. 2 and is discussed in greater detail later15 within this document.
As can be seen in FIG. 1, one embodiment of the present invention comprises a helium neon laser 12 which emits a beam 14 which is directed by a mirror 16 into a beam expander 18. In the preferred embodiment, a 6~c beam expander is utilized. The beam 14 is then split by a 20 beam splitter 20, preferably a 50/50 cube-type, and directed along two separate optical paths 22 and 24.
Through the use of three adjustable mirrors 26, 28 and 30, the split beams traveling along optical paths 22 and 24 are directed to orthogonally impinge on a fiber 32 having a coating layer 31. In addition, a 25 fiber-throughway opening 38 is designated as the cut-out portion of base plate 34 which the ~lber 32 passes through in a direction perpendicular to .,~
the base plate 34 as it travels along the fiber processing path. The fiber-throughway opening 38 creates two inward corners 40 and 42 within base plate 34 a~s shown in FIG. l.
In order to properly position the photodetectors just outside the expected forward scatter pattern, a series of photodetectors 44, 45, 46 and 47 are secured within inward corners 40 and 42 respectively of base plate 3~1.
In the preferred embodiment, the particular photodetectors used are BPX65 photodiodes manufactured by Centronics Incorporated which have good 35 high frequency response, sensitivity and relatively low cost. These specific photodiodes have a l mm square active area and are generally mounted in a TO-18 type, 2-pin header. When the laser 12 is on and a coated rlber 32 in position, a forward scattered light pattern should be formecl immediately adjacent to each photodiode 4'1, 95, 46 and 47. However, if slightly misaligned, mirror adjustments may be macle to position the pattern 5 adjacent to the active area of the photodiodes 44, 45, 46 and 47.
The beam expander 18 of this eImbodiment of the present invention may be assembled separately. Two plano-cylindrical lenses of different focal length, a 6.35 mm and a 40.0 mm respectively, are mounted with a silicone rubber type adhesive to the expander system rail. The lenses 10 are mounted plano side toward plano side and are initially spaced at ~6.35 mm. This particular degree of spacing produces a nearly recollimated 6.3 times expansion of the beam in one direction. The preferred configuration of the beam expander 18 allows slight spacing adjustments to be made after installing the module to bring the beam into collimation. The beam 15 expander 18 is positioned on the base plate 3'1 between the first mirror 16 and the splitter cube 20.
At present, the particular embodiment detailed immediately above is positioned on the draw tower base, where the flber path is horizontal and parallel with the base plate 34. With the physical 20 arrangement of the previously described embodiment, the fiber 32 travels through the thickness of the detection system 10, in the three-sided fiber-through-opening 38, where it is impinged by orthogonal laser beams 22 and 24. Unfortunately, such positioning of the detection system 10 leaves the length of the optics head sticking up vertically from the tower base into the 25 operator's work area, thereby making the optical alignment sensitive to accidental bumps.
An alternative embodiment of the present defect detection system is also disclosed herein and illustrated in FIG. 2. The pariicular alternative embodiment disclosed, allows the detection system 10 to be 30 physically located at a different point along the fiber processing path than the earlier disclosed embodiment, because it is substantially smaller than the size of the previously described embodiment.
The opto-mechanical design of the second embodiment of the present defect detection system is illustrated }n FIG. 2. This particular 35 arrangement employs two separate solid state lasers, 50 and 52 to generate light beams 54 ancL 56, respectively. The lasers used in the preferred 2 ~
embodiment are Melles Griot Model 06VBL 102/P. The output power Or each laser 50 and 52 is about 3mw at 670 nm. Two plano-convex cylindrical lenses 58 and 60 are positioned in each bearn 54 and 56, respectively, to expand the associated beam horizontally and recollimate it to a 6mm width.
5 Four mirrors 62, 64, 66 and 68, two in each beam, are utilized lo direct the light beam 54 and 56 to the f~ber 32. Speciflcally, light beam 54 is directed to the fiber 32 via mirrors 62 and 68, whereas mirrors 64 and 66 are used to direct light beam 56 to the fiber. The mechanical placement of these mirrors causes the beams to be substantially orthogonal to both the fiber 10 and each other when located at the position where they impinge the rlber.
As stated earlier, the optical detection technique described herein is based on monitoring the out-of-pattern scattering caused by defects. Five photodiode detectors may be used in the preferred arrangement of this second embodiment system. As stated with regard to 15 the earlier described embodiment, the particular photodiodes preferred are Model BPX65, which are manufactured by Centronics Incorporated, and have good high frequency response, sensitivity, and relatively low cost.
Four of the photodiodes, namely elements 44, 45, 46 and 47 of FIG. 2, are used to detect the out-of-pattern scattering caused by a coating 20 defect. Since the photodiodes utilized in both embodiments are identical, .
like reference numerals are used herein to identify the photodiodes throughout the discussions of both embodiments set forth. In particular, photodiodes 44 and 45 are precisely aligned immediately adjacent the expected forward scatter pattern of light beam 54, while photodiodes 46 and 25 47 are aligned aside the expected forward scatter pattern of light beam 56.
A fifth photodetector 78 is used to detect the presence of a laser illuminated coated fiber by monitoring a pattern from each axis generated when light is scattered through the cylindrical coating layer 31, as illustrated in FIG. 3.
The electronic circuitry to be used to implement either 30 embodiment of the coating defect detection system is illustrated in FIG. 4 - and FIG. 5. FIG. 4 depicts the overall circuitry utilized to operate the present defect detection system, while FIG. 5 details the specifics of an input amplifier circuit incorporated into the overall circuitry of FIG. 4 for each optical axis.
2 Q ~
A.C stated earlier and illustrated in FIC~. 4, each embodlllleIlt of the present invention incorporates four defect photodiodes with a pair of these photodiodes positioned in each scattering a~is. Electronically equivalent input amplifier stages 80 (see FI~:. 5 for detailed circuitry) are 5 provided for each optical axis. An output 82 of each input amplifier stage 80 is processed via comparator 84. Comparator 84 has as its inputs, output signal 82 and a reference voltage 86. Furthermore, the comparator 84 generates a defect signal channel 88, for the particular optical axis it is servicing.
As illustrated in FIG. 4, the reference voltage is adjusted during calibration and establishes the system sensitivity. As stated, a reference voltage 86 and the signal voltage 82 are differentially input to differential voltage comparator 84. An output 88 from the comparator 84 is input to a one shot multivibrator ~6 which stretches the triggering pulse to a sufficient 15 length of time for the control computer 98 to acquire the data. This stretched output 100 also illuminates a display LED 102 for operator information. An electronically identical circuit as described above and illustrated in FIG. 4 is utilized to process each of the optical axis generated.The following circuit description of the input amplifler 80, a 20 composite single axis defect signal channel, and the output stage of one axiswill be more easily understood by referencing the electronic schematic of FI~. 5. As shown in FIG. 5, the anode of photodiode 44 is connected to the inverting input of operational ampli~ler 104. A resistive feedback, via resistor 106, configures amplifier 104 as a transresistance amplifier or a 2~ current-to-voltage converter. An identical electrical configuration is employed around diode 45 with op-amp 108 and resistive feedback, via resistor 110, configuring the transresistance amplifier or the current-to-voltage converter.
~; The two voltage outputs 112 and 114 from input op-amps 104 30 and 108, respectively, are summed on the inverting input of amplifler 118.
A capacitor 118 in the feedback loop changes this circuit function to a low pass filter and resistor 120 holds the gain to unity. The output voltage 122 from this circuit is used to remove any quiescent voltage levels from getting into the defect signal channel.
~ . :
, ~.
The voltage outputs 112 and 114 from the op-amps 10~ and 108 are summed again along with the ~lltered inverted signal 122 ïrom ampl;rler 116 at the inverting input of amplifier 124. The gain o~ amplil~er 124 is set by input resistors 126, 128 and 130 and feedback res;stor 132 to a gain of 27 5 db. A small capacitor 134 in the feedback of amplifier 124 improves the stability of the circuit. The direct coupled s;gnals which are summed into amplifier 124 produce an output from amplirler 124 which is similar to that of a differentiator; that is, exhibiting relati~ely high gains for input voltagechanges (slopes), and low gains for stable levels. Therefore, the output 10 signal 136 from amplifier 124 will be pos;tive or negative going signal swings around a near ground (zero voltage) level.
The output signal 136 from amplifier 124 is further processed by a full wave rectifier (not shown) whose output is a unipolar signal with an amplitude directly related to the intensity change on the photodiode, and 15 therefore to the severity of the defect. This proportionality allows the output of the rectifier to be compared to an established unipolar reference voltage as done in comparator 84 of FIG. 4.
A separate independent signal channel is utilized for the fiber present photodiode 78. The flber presence detector circuit is specif~lcally 20 illustrated in FIG. 6. The fiber presence circuit function is to detect when a coated fiber 32 is in the inspection area and to determine if the fber 31 is being struck by sufficient laser beam power. The signal on the anode of photodiode 138 is input to a transresistance configured operational amplifier 140. The gain of this circuit is lower than the previously described inputs 25 because photodiode 138 resides in the normal scattered light pattern. The position of diode 138 also makes it susceptible to fluctuations in scattered light po~,ver due to parametric changes quite apart from coating defects.
Therefore, the output of op-amp 140 is heavily filtered in the next stage of amplification by op-amp 142 and its associated electrical components as 30 shown in FIG. 6. This circuit has a response time of 10 -12 seconds. The filtered output 146 is compared against a fixed reference voltage level at the differential voltage comparator 148. The output of the comparator 148 is used to illuminate LED 102 of FIG. 4 for operator information and is presented as data to the computer ~8 also shown in FIG. 4.
, $ ~
The above descriptions have detailed opt;cal, mechanical and electrical conrlgurations in accordance with the preferred embodiment of the present invention. The particular operation of the present coating defect detection system will now be discussed. As stated earlier, glass fibers 5 for use in optical communication systems are manufactured by a drawing proeess. During the drawing of a glass fiber, a coating layer i5 conventionally applied to provide additional strength enhancement for the fiber or to provide protection from various environmental concerns which may inhibit the operation of the fiber.
Generally, the draw process includes a series of on-line processes performed sequentially. The present invention provides a detection system whieh may be ineorporated on-line to deteet eoating defects to avoid installing a ~lber with unknown coating defects into a comrmunication cable.
The instant detection system is installed at a location after the application 15 of tne eoating layer but prior to the fiber take-up. As the fiber 32 passes through the system, light beams 22 and 24 of the first embodiment described, or light beams 54 and S6 of the alternate embodiment described herein, illuminate a port;on of the coated fiber 32 aeross its entire outer diameter. A forward seatter pattern as generally shown in FI~. 3 is 20 generated opposite each light source as the light passes through the coating layer 31 and/or the fiber 32.
A series of photodiodes 44,45,46 and 47is positioned immediately adjacent the forward scatter pattern expected to be produced as light passes through a properly coated fiber. 32 However, various defects 25 cause portion of the light to be scattered outside the expeeted forward seatter pattern. The present invention eontinuously monitors the light intensity level of an area adjacent the expected forward scatter pattern.
~hen the light intensity within the monitored area increases, an indication is generated such that a monitor computer ~8 or an LED display 102 can identify the presenee of a defect within the coating layer.
As stated earlier, the heart of the out-of-pattern detection technique utilized in the present invention relies on reeognizing an inerease in light intensity by photodetectors positioned immediately adjaeent an expeeted forward seatter pattern to indieate the presenee of a defeet. It 35 should be specifically noted that even though particular circuitry is disclosed herein, minor variations in the particular electronic monitoring :' .' ~, :.
,.~ .
2 ~
used to measure the light intensity aside the forward scatter pattern for the purpose of detecting coating defects is deemed to be within the scope of the present invention regardless of the exact electronic components or conrlguration employed.
Claims (10)
1. A system for detecting defects within a coating layer characterized by means for illuminating a coated strand with light so as to create an expected forward scatter pattern indicative of a forward path and intensity of the light when shone through a coating layer of acceptable quality; and means for monitoring the intensity level adjacent to the expected forward scatter pattern for an increase in light as an indication that a defect exists within the coating layer.
2. The defect detection system of claim 1 wherein the illuminating means further comprises beam splitter means which generates at least two substantially equivalent light beams from a single light beam emitted from the laser source.
3. The defect detection system of claim 1 wherein the monitoring means comprises at least two photodetectors positioned immediately adjacent the expected forward scatter pattern.
4. The defect detection system of claim 1 wherein the coated strand is illuminated by at least two orthogonally-aligned light beams.
5. The defect detection system of claim 1 wherein the light which illuminates the coated strand has a beam-width wider than the outer diameter of the coated strand.
6. A method of detecting defects within a coating layer characterized by the steps of:
illuminating a coated strand with light so as to create an expected forward scatter pattern indicative of a forward path and intensity of the light when shone through a coating of acceptable quality; and monitoring the intensity level adjacent to the expected forward scatter pattern for an increase in light as an indication that a defect exists within the coating layer.
illuminating a coated strand with light so as to create an expected forward scatter pattern indicative of a forward path and intensity of the light when shone through a coating of acceptable quality; and monitoring the intensity level adjacent to the expected forward scatter pattern for an increase in light as an indication that a defect exists within the coating layer.
7. The defect detection method of claim 6 wherein the coated strand is illuminated by at least two orthogonally-aligned light beams.
8. The defect detection method of claim 6 wherein the light which illuminates the coated strand has a beam-width wider than the diameter of the coated strand.
9. The defect detection method of claim 6 wherein the intensity level of the forward scatter pattern is monitored by at least two photodiodes positioned immediately adjacent to the expected forward scatter pattern.
10. The defect detection method of claim 6 wherein the intensity level monitoring has a response time which is sufficiently short to allow defects of relatively short duration to be accurately detected as part of an on-line processing scheme.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US83517292A | 1992-02-13 | 1992-02-13 | |
| US835,172 | 1992-02-13 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2088081A1 true CA2088081A1 (en) | 1993-08-14 |
Family
ID=25268805
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002088081A Abandoned CA2088081A1 (en) | 1992-02-13 | 1993-01-26 | Out-of-pattern coating defect detection system |
Country Status (5)
| Country | Link |
|---|---|
| EP (1) | EP0556987A1 (en) |
| JP (1) | JPH07119712B2 (en) |
| KR (1) | KR930018270A (en) |
| CN (1) | CN1076028A (en) |
| CA (1) | CA2088081A1 (en) |
Families Citing this family (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN1037027C (en) * | 1995-10-24 | 1998-01-14 | 南京航空航天大学 | Two-dimensional optical flaw detecting and displaying method and device |
| JP4808509B2 (en) * | 2006-02-21 | 2011-11-02 | 大日本スクリーン製造株式会社 | Substrate crack detection device and substrate processing device |
| JP4808510B2 (en) * | 2006-02-21 | 2011-11-02 | 大日本スクリーン製造株式会社 | Substrate crack detection device and substrate processing device |
| CN103292747B (en) * | 2013-05-20 | 2016-03-02 | 北京大学 | A kind of method and device measuring FinFET side wall surfaceness |
| CN110376203A (en) * | 2019-06-26 | 2019-10-25 | 阳程科技股份有限公司 | Glass fiber detects board and its detection method |
| CN116908211A (en) * | 2023-09-06 | 2023-10-20 | 山东华光新材料技术有限公司 | Defect detection device for manufacturing optical fiber perform |
Family Cites Families (8)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS54130487U (en) * | 1978-03-02 | 1979-09-10 | ||
| AU5856780A (en) * | 1979-05-21 | 1980-11-27 | United Glass Ltd. | Surface irregularity of translucent material |
| IT1176988B (en) * | 1983-11-02 | 1987-08-26 | Hauni Werke Koerber & Co Kg | DEVICE TO OPTICALLY CHECK CIGARETTES |
| GB8424084D0 (en) * | 1984-09-24 | 1984-10-31 | Sira Ltd | Inspection apparatus |
| EP0304477A1 (en) * | 1987-03-09 | 1989-03-01 | Battelle Memorial Institute | Optical inspection system for cylindrical objects |
| JPH0734365Y2 (en) * | 1989-05-23 | 1995-08-02 | 東芝硝子株式会社 | Foreign object detection and removal device |
| JPH0781963B2 (en) * | 1989-11-10 | 1995-09-06 | 松下電器産業株式会社 | Fine wire fine defect detection device |
| JPH04106448A (en) * | 1990-08-28 | 1992-04-08 | Fujikura Ltd | Detecting method of abnormality of optical fiber |
-
1993
- 1993-01-26 CA CA002088081A patent/CA2088081A1/en not_active Abandoned
- 1993-02-04 EP EP93300844A patent/EP0556987A1/en not_active Withdrawn
- 1993-02-12 CN CN93101802A patent/CN1076028A/en active Pending
- 1993-02-12 JP JP5046026A patent/JPH07119712B2/en not_active Expired - Lifetime
- 1993-02-13 KR KR1019930001996A patent/KR930018270A/en not_active Application Discontinuation
Also Published As
| Publication number | Publication date |
|---|---|
| JPH07119712B2 (en) | 1995-12-20 |
| CN1076028A (en) | 1993-09-08 |
| JPH0682394A (en) | 1994-03-22 |
| EP0556987A1 (en) | 1993-08-25 |
| KR930018270A (en) | 1993-09-21 |
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