JPH11271175A - Device and method for detecting defect in optical fiber surrounded by covering layer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To judge whether a defect such as air bubbles exists in an optical fiber by discharging light in a direction that is vertical to the axial direction of the fiber to the covering of the optical fiber and detecting the presence or absence of third reflection rays from the device of the covering in addition to the first and second reflection rays at the boundary between air and the covering. SOLUTION: Optical beams 12 are applied to a covering 14 for surrounding an optical fiber 3 in a direction that is vertical to the axial direction of the fiber 3. Due to the difference in the refractive index between air and the covering 14, first rays 15 and second rays 17 are reflected at a boundary 16 between air and the covering and at a boundary 18, respectively. When no defect exists in the fiber 3, only the first and second rays 15 and 17 are detected by a linear photosensor array 9. However, when a defect 35 exists in the fiber 3, third rays 36 are reflected due to the defect 35 and are focused on the array 9. In this case, all three rays 15, 17, and 36 are detected for the array 9, thus judging the presence of the defect. In this manner, by utilizing the optical characteristics of the fiber covering 14, a defect can be detected.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for detecting defects in an optical fiber. More particularly, the present invention relates to a method and apparatus for detecting defects in an optical fiber during manufacture of the optical fiber. The present invention relates to a detection system for optically detecting bubbles.

[0002]

2. Description of the Related Art Good implementation of an optical communication system requires a high quality optical fiber having mechanical properties sufficient to withstand the stresses experienced by the optical fiber. Each fiber must be able to withstand the maximum stress level that the fiber experiences during mounting and use, along its entire length. The importance of fiber strength is clear when considering that a single fiber failure results in the loss of hundreds of circuits.

[0003] Failure of an optical waveguide fiber in a stretched state is usually associated with surface flaws. This surface flaw causes stress concentration, resulting in a lower tensile strength than the original intact glass. The size of the flaw determines the level of stress concentration and the failure stress. Even scratches on micron-sized surfaces can cause stress concentration and significantly reduce the tensile strength of the fiber.

[0004] Optical fibers are usually manufactured in a continuous process involving the drawing of fine fiber glass fibers from a partially molten glass preform. A furnace is used to partially melt the preform and allow the fiber to be drawn. The heat of the furnace and the drawing speed of the fiber must be properly balanced so that the optical fiber can be drawn continuously under uniform conditions. Long optical waveguide fibers have considerable potential strength, but the strength is reduced by bubbles (bubble lines) or holes that are created in the optical fiber. In addition, bubbles in the optical fiber also affect the light propagation characteristics of the optical fiber.

As soon as the optical fiber is drawn, it is coated with a layer of a coating, such as a polymer. The coating prevents flying dust from impinging on and sticking to the surface of the drawn fiber. Dust not only weakens the fiber, but also degrades its transmission characteristics. The coating also protects the fiber from surface wear. This surface wear occurs as a result of subsequent manufacturing processes and handling during mounting.
The coating protects the cable from corrosive environments and opens up the fibers in the cable structure. However, this coating does not eliminate the problems caused by bubbles and pores present in the fiber itself. Nos. 08 / 815,180 and 08 / 814,673 mentioned above.
The issue is aimed at detecting defects in the optical fiber coating and detecting and discriminating between defects in the optical fiber coating, respectively.

It is well known to those skilled in the art to monitor an optical fiber as it is drawn during the manufacturing process to determine if a defect is present in the optical fiber. However, such known techniques require that the optical fiber be analyzed during the drawing process before the coating layer is applied, and that complicated hardware is required to detect defects contained in the optical fiber. Requires software and / or software.

For example, US Pat. No. 4,012,217 discloses that when an optical fiber is manufactured, a defect of the optical fiber is detected and the tensile strength of the optical fiber is detected before any coating layer is applied to the optical fiber. A system for determining is described. The device described in U.S. Pat. No. 4,012,217 projects a focused beam of monochromatic light onto an optical fiber as the fiber is drawn. A photodetector (e.g., a photomultiplier tube) is positioned off-axis with respect to the direction of light projected onto the optical fiber. Thus, the photodetector receives only scattered light that is significant for defects included in the optical fiber. The output of the detector is received by an electrometer band recording paper recorder and plots a scattering trajectory corresponding to the detected scattered light. The peak of the scattering trajectory corresponds to a defect in the optical fiber.

[0008] US Pat. No. 5,185,636 describes:
A method for detecting defects such as holes in a fiber is described. The device disclosed in U.S. Pat. No. 5,185,636 uses a laser to project a light beam onto an optical fiber. Two photodetectors are located on each side of the optical fiber. An interference pattern is created in the far field detected by the photodetector due to the coherence and blackness of the laser beam. The holes contained in the optical fiber cause a small number of fringes in the interference pattern generated in the far field. Multiple light sources are used to allow light to pass through the fiber so that there are no blind spots. This is to ensure that light is also reflected from a hole included in an arbitrary position in the optical fiber and detected by the photodetector. A spatial frequency spectrum is generated based on the output of the light exchanger. Then, the spectrum is analyzed to determine whether or not a hole exists in the optical fiber.

[0009] The systems disclosed in US Pat. Nos. 4,012,217 and 5,185,636 both use optical systems to detect defects in an optical fiber before any coating layer is applied to the optical fiber. Perform discovery. Moreover, both of these systems are fairly complex in terms of hardware and / or software. For example, U.S. Pat.
The system disclosed in U.S. Pat. No. 36 uses at least one photomultiplier tube for detecting light scattered by a defect and an electrometer recording paper recorder that records the scattering trajectory. The system disclosed in U.S. Pat. No. 4,012,217 includes a plurality of photodetectors for projecting light onto a fiber under inspection and a remote detector generated on each photodetector by reflection and refraction by the fiber. It is necessary to use a plurality of optical systems for projecting the field interference pattern. Then fast Fourier transform (FF
A complex technique for generating a frequency spectrum using T) is performed to determine the fiber diameter. The frequency of the outer diameter component of the fiber is determined, and it is determined whether the frequency spectrum of the fiber matches the frequency spectrum of a defect-free fiber having the same diameter.

[0010]

SUMMARY OF THE INVENTION In contrast, the present invention provides a system for detecting optical defects in an optical fiber that utilizes the optical properties of the optical fiber coating to reduce the complexity of the defect detection system. The purpose is to: In particular, the present invention takes advantage of the fact that there is little difference between the refractive index of the coating surrounding the fiber and the refractive index of the fiber itself. In the present invention, the light projected onto the optical fiber coating in a direction perpendicular to the fiber is the light that first strikes the boundary between the air and the coating and the light that strikes the fiber through the fiber and out of the coating. Each is reflected at the border. At the interface between the coating and the fiber, light is not reflected because the refractive index of the coating is very close to the refractive index of the fiber. Therefore, when there is no defect in the fiber,
Two rays are detected, one resulting from the reflection of the first defect at the air-coating interface and the other resulting from the second reflection at the air-coating interface. But,
When there is a defect in the fiber, three rays are detected: a first defect and a second reflected light at the boundary between the air and the coating, and a third reflected light caused by the defect. In the present invention, it has been determined that all three reflected lights are parallel to each other and perpendicular to the direction of the light projected onto the coating. An optical detector detects the reflected light, and a signal processor determines whether a defect exists in the fiber based on the number of detected reflected light.

[0011]

According to the present invention, there is provided an optical detection system for detecting defects in an optical fiber. The system includes a light source for projecting a light beam onto a coating layer of an optical fiber. A lens system focuses the reflected light from the coating layer and the reflected light from defects in the optical fiber onto the photodetector array. The photodetector array is
The light reflected at the boundary between the air and the coating layer when the light enters the coating layer surrounding the optical fiber, and the light reflected by the air and the coating layer when the light exits the coating layer after passing through the optical fiber. The light reflected from the boundary is received. For example,
If a defect, such as a bubble, is present in the optical fiber, a third light beam is reflected by the defect and detected by the photodetector array. A signal processing device, such as a microprocessor, electrically connected to the photodetector array, receives the output signal from the photodetector array and processes the output signal to determine whether a defect has been detected. judge.

In a preferred embodiment of the present invention, the defect detection method and apparatus of the present invention is incorporated into a fiber optic cable manufacturing process so that defects occurring within the fiber during fiber manufacture can be detected. The manufacturing process can be tailored to eliminate these defects and / or prevent them from occurring in the future. In a preferred embodiment of the present invention, laser light is projected from the laser onto a coating surrounding the optical fiber in a direction perpendicular to the fiber axis, i.e., perpendicular to the fiber flow direction. A lens system arranged perpendicular to both the axial direction of the fiber and the projection direction of the laser beam receives each reflected beam of the laser beam and focuses the received reflected beam on the photodetector array. The photodetector array converts each optical signal into an electric signal and outputs the electric signal to a signal processing device. The signal processor processes the electrical signals to determine whether a defect exists in the optical fiber.

In a preferred embodiment of the present invention, when there are no defects in the fiber, the photodetector array filters the reflected light of the first defect reflected at the air / coating boundary as light enters the coating. After detecting and the light has passed through the optical fiber,
It was determined to detect a second reflected ray reflected at the boundary between the air and the coating when exiting the coating layer. If there is a defect in the fiber, the photosensor array will detect more than two reflected rays. When the signal processing device receives an electrical signal related to three or more reflected light beams, the signal processing device determines that a defect exists in the optical fiber. The signal processor counts the number of reflected light rays detected by the photodetector array to determine the number of defects present in the optical fiber.

[0014]

FIG. 1 shows a preferred embodiment of an apparatus 1 according to the invention for detecting defects in an optical fiber. Generally, the coated optical fiber 2 comprises a fiber and a coating layer. These coating layers are typically made of a polymer that cures when irradiated with ultraviolet light. For purposes of describing the present invention, the coated optical fiber 2 will be described as consisting of a single coating layer surrounding it. The device 1 of the present invention comprises a light source 7 which is a laser, a photodetector array 9 which is a linear photosensor array, and a signal processing device 10 comprising an analog / digital converter and a microprocessor.

As shown in FIG. 2, a coherent beam 12 of substantially monochromatic light is applied to an optical fiber 3 by a laser 7.
Is projected in a direction perpendicular to the axial direction of the optical fiber 3 onto the coating 14 surrounding the optical fiber 3. Due to the difference between the index of refraction of air and the index of refraction of the coating 14, the first ray 15 is reflected at the interface 16 of air and coating. Coating 1 surrounding fiber 3
Upon exiting 4, the second ray 17 is reflected at the air / cladding interface 18. The first reflected light beam 15 and the second reflected light beam 17 are imaged by the objective lens 22 and focused on the linear photosensor array 9 by the cylindrical lens (cylindrical lens) 23 of the lens system 8. The cylindrical lenses 23 collect the reflected rays so that they appear on the linear photosensor array 9 at a higher intensity. The linear photosensor array 9 converts each optical signal into an analog electric signal, and the analog electric signal is converted into a signal processing device 10.
Is converted into a digital signal by an analog / digital converter (ADC) 25 included in. Subsequently, the digital signals are output from the ADC 25 of the signal processing device 10 to the microprocessor 30. Subsequently, the digital signals are analyzed by the microprocessor 30 to determine whether or not a defect exists in the optical fiber 3.

When there are no defects in the optical fiber 3, only the first and second rays 15, 17 are detected by the linear photosensor array 9. However, the optical fiber 3
When there is a defect 35 therein, the third light ray 36 is reflected by the defect 35 and focused on the photosensor array 9 by the lens system 8. In this case, the photosensor array 9 detects all three rays 15, 17, and 36. Microprocessor 30 receives the output of ADC 25 and determines that there is a defect in fiber 3 based on the number of rays detected by photosensor array 9.

There are many different signal processing devices suitable for this purpose. Microprocessors are used for this purpose, but various types of comparator circuits are used for linear photosensors.
It can be designed to detect when the analog signal generated by the array 9 exceeds a predetermined threshold and to indicate that reflected light has been detected from the coated optical fiber 2. These functions can be performed using analog or digital circuits. Therefore, the configuration of the present invention for analyzing the electric signal from the photosensor array 9 and determining whether a defect exists in the optical fiber 3 is not limited to any particular configuration.

Since the boundaries 16 and 18 between the air and the coating are cylindrical, the objective lens 22 has a very narrow field of view.
Only very small diameter images are linear photosensor arrays 9
Appear on top. The objective lens 22 is a microscopic objective lens having a narrow field of view. As a result, only a very limited diameter portion of the reflected light is subjected to the photo-
It contributes to focusing on the array 9. As a result, the light appearing in the image appears as a very thin line that is clearly separated. All of the axial length of the illuminated fiber 3 within the field of view of the objective lens 22 contributes to the formation of an image on the photosensor array 9. Therefore, the image formed on the photosensor array 9 is formed by two sharp parallel lines when no defect exists in the fiber 3.

These two lines are always present, and they are sharp when the two lines are always at the same position on the photosensor array 9 and the photosensor array 9 is correctly placed in the image plane. Therefore, these two lines can be used as a display for continuously indicating the alignment state of the photosensor array 9. If the photosensor array 9 is not in the image plane, the image will be blurred, similar to the way the image projected on the screen by the overhead projector will be blurred when the lens system is not properly focused. Each line blurs.

In the presence of an airline, a third line appears between the first two lines. Further, the strength of the third line increases as the bubble diameter increases. Therefore, the intensity of the third line can be used as an indication signal indicating the size of the defect existing in the fiber 3.
The manner in which the size of a defect, such as a bubble, can be estimated from the intensity of this line is relatively straightforward.

Although the invention has been described with respect to particular embodiments, the invention is not limited to those embodiments. For example, components other than those described above can be used to construct the device of the present invention. The type of photodetector array used can be of any type suitable for detecting defects in optical fibers.

Other methods can be used to achieve the objects of the present invention. For example, an optical fiber is preferably inspected after both the primary coating and the secondary coating are applied, but after the primary coating is applied and before the secondary coating is applied, the optical fiber is Can be checked. Alternatively, the fibers can be inspected before any of the coatings are applied. However, if the system of the present invention is used to detect defects in the optical fiber before any coating is applied, the reflection caused by the defects in the outer region of the optical fiber will be reduced. Hidden or obscured by reflections caused by air-fiber boundaries, making it more difficult or impossible to detect defects. Performing the defect detection after one or more coating layers have been applied to the optical fiber ensures that the presence of those coatings ensures that both outer reflected lines that are always present correspond to the light reflected from the defect. Does not interfere with the above, so that a more reliable result can be obtained.

[0023]

As described above, according to the present invention,
A system is provided for detecting optical defects in an optical fiber that utilizes the optical properties of the optical fiber coating to reduce the complexity of the defect detection system.

The system of the present invention can be incorporated into a fiber optic cable manufacturing process. As a result, defects occurring in the fiber during its manufacture can be detected, and the manufacturing process can be rejected and / or prevent future defects from occurring in the manufacturing process. There is an advantage that can be adjusted.

[Brief description of the drawings]

FIG. 1 is a block diagram of an apparatus of the present invention for detecting defects in an optical fiber according to a preferred embodiment.

FIG. 2 is a block diagram of the apparatus shown in FIG. 1 illustrating the coating and air boundaries and defects that reflect light onto the lens system of the present invention.

[Explanation of symbols]

 Reference Signs List 1 Defect detection device in optical fiber 2 Coated optical fiber 3 Optical fiber 7 Light source 8 Condensing lens system 9 Photodetector array 10 Signal processing device 12 Light beam 14 Coating layer 15 First reflected light 16 First boundary 17 Second reflected light 18 Second boundary 22 Objective lens 23 Cylindrical lens 25 Analog / digital converter 30 Microprocessor 35 Defect 36 Third reflected light

 ──────────────────────────────────────────────────続 き Continuation of the front page (71) Applicant 596077259 600 Mountain Avenue, Murray Hill, New Jersey 07974-0636 U.S.A. S. A. (72) Inventor Fleming Peddersen DK-3520, Denmark, Pharam, Burke Hospital Rassan 405E

Claims (15)

[Claims] An apparatus for detecting defects in an optical fiber (3) surrounded by a coating layer (14), comprising: a light source that projects a light beam (12) onto the coating layer (14); A first reflected light (15) corresponding to light projected on the coating layer and reflected at a first boundary (16) between air and the coating layer is received, and is projected on the coating layer to form the coating. Receiving second reflected light (17) corresponding to light reflected at a second boundary (18) between air and the coating layer after the light projected on the layer has passed through the fiber; A condensing lens system (36) for receiving a third reflected light (36) corresponding to light reflected by the first defect (35) contained in the optical fiber and projected on the coating layer by a light source; 8), disposed adjacent to the lens system (8); The first, second, and third reflected lights (1) focused by the lens system
5,17,36) and a photodetector array (9) for generating an electrical output signal responsive to the focused light.
And electrically connected to the photodetector array (9), receiving the electrical output signal from the photodetector array, processing the electrical output signal and allowing the photodetector array to receive the first,
Determining whether the second and third reflected lights have been detected; if it is determined that the photodetector array has detected the first, second and third reflected lights, a defect is detected in the optical fiber; A signal processing device (10) for generating a display signal indicating the presence of the defect within the optical fiber. 2. The light beam (12) is projected onto the coating layer (14) of the optical fiber (3) in a direction perpendicular to a longitudinal axis of the optical fiber (3). The apparatus of claim 1. 3. The apparatus according to claim 1, wherein said light source is a laser. 4. The position of the photodetector array (9) perpendicular to the direction of projection of the light beam (12) onto the cover layer (14) and perpendicular to the longitudinal axis of the optical fiber (3). The device according to claim 1, wherein the device is arranged at 5. The signal processing device according to claim 1, wherein
Digital converter (25) and microprocessor (3
0), said analog / digital converter (25)
Receives the electrical output signal from the photodetector array (9) and converts the electrical output signal to a digital signal, and the microprocessor (30) receives the digital signal and processes the digital signal Device according to claim 1, characterized in that it is determined whether a defect is present in the optical fiber (3). 6. The signal processing device (10) counts the number of reflected lights detected by the photodetector array (9), and determines whether three or more reflected lights have been detected, If the signal processing device (10) determines that three or more reflected lights have been detected, the signal processing device (10) outputs a display signal indicating this. An apparatus according to claim 1. 7. A display signal indicating a magnitude of the first defect (35) based on a magnitude of the electric signal output from the photodetector array (9). And the magnitude of the electrical signal output from the photodetector array (9) increases as the magnitude of the first defect (35) increases. The described device. 8. The method according to claim 1, wherein the determination as to whether a defect has been detected is used in an optical fiber manufacturing process to prevent or minimize the occurrence of defects in the optical fiber created in the manufacturing process. The device according to claim 1. 9. A method for inspecting an optical fiber surrounded by a coating layer and detecting bubbles formed in the optical fiber, comprising: a light beam (1) applied to the coating layer (14) of the optical fiber;
Projecting 2); and a first reflected light () corresponding to the light projected by the light source onto the cover layer (14) and reflected at a first boundary (16) between air and the cover layer. 15) focusing the light onto the photodetector array (9); and projecting the light projected onto the coating layer (14) onto the coating layer after passing the optical fiber through the optical fiber. A second reflected light (17) corresponding to light reflected at a second boundary (18) with the coating layer is applied to the photodetector array (9).
Focusing on the first defect (3) projected onto the coating layer (14) by the light source and contained in the optical fiber (3).
The third reflected light (3) corresponding to the light reflected by 5)
6) focusing the light on the photodetector array (9); and the first, second, and third reflected lights (15, 17) focused on the photodetector array (9). Generating an electrical output signal containing information about the photodetector array (9) as an output of the photodetector array (9); processing the electrical output signal in a signal processing device (10); ) Has detected the first, second, and third reflected light (15, 17, 36), and if the photodetector array (9) has the first, second,
When the third reflected light is detected, the signal processing device (1
0) generating an indication signal indicating that a defect (35) is present in said optical fiber (3), wherein the optical fiber is surrounded by a coating layer. Defect detection method. 10. The light projecting step, wherein the light beam (12) is projected onto the coating layer (14) in a direction perpendicular to a longitudinal axis of the optical fiber (3). The method according to claim 9. 11. The method according to claim 10, wherein the light beam (12) projected onto the covering layer (14) is generated by a laser. 12. The photodetector array (9) is perpendicular to the direction of projection of the light beam (12) onto the cover layer (14) and perpendicular to the longitudinal axis of the optical fiber (3). 2. The device according to claim 1, wherein:
The method according to 0. 13. The signal processing device (10) comprises an analog / digital converter (25) and a microprocessor (3).
0), said analog / digital converter (25)
Receives the electrical output signal from the photodetector array (9) and converts the electrical output signal to a digital signal, and the microprocessor (30) receives the digital signal and processes the digital signal The method according to claim 10, characterized in that it is determined whether a defect is present in the optical fiber (3). 14. The signal processing device (10) counts the number of reflected lights detected by the photodetector array (9), and determines whether three or more reflected lights are detected, If the signal processing device (10) determines that three or more reflections have been detected, the signal processing device (10) outputs a display signal indicating this. The described method. 15. The signal processing device (10) outputs a display signal indicating the size of the first defect based on the size of the electric signal output from the photodetector array (9). The method of claim 10, wherein the magnitude of the electrical signal output from the photodetector array increases as the magnitude of the generated first defect increases.