JPS60233525A - Reflected light measuring method of optical fiber and its device - Google Patents

Abstract

PURPOSE:To reduce an attenuation factor of a light quantity, and to improve the detecting accuracy of an abnormal point of an optical fiber to be measured, or the end face by making an optical pulse of a laser oscillator incident on one end of the fiber without irradiating to a total reflecting mirror. CONSTITUTION:An optical pulse 3 of a laser oscillator 1 passes through a center part through-hole 15 of a total reflecting mirror 16 and is made incident on an optical fiber 6, therefore, a light quantity is not attenuated. Also, as for a reflected light 4 from the fiber 6, its 99% is reflected by the mirror 16. Accordingly, the attenuation of the light quantity can be eliminated, the light quantity of the reflected light of the fiber 6 received by a photodetector 8 becomes large, and the detecting accuracy can be improved.

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Technical field to which the invention pertains The present invention relates to a method and apparatus for measuring reflected light of an optical fiber, and more specifically, the present invention relates to a method and apparatus for measuring reflected light of an optical fiber. The present invention relates to a method and apparatus for measuring.

(b) Prior art and its problems When optical fibers are broken, abnormally bent, or have local core diameter variations in the longitudinal direction, the amount of transmission through the optical fibers is attenuated. It is important to detect abnormalities in optical fibers during maintenance and the like. Pulse reflection method, backscattering loss measurement method, etc. are used as typical abnormal point detection methods. These technologies inject light pulses from a semiconductor laser, gas laser, etc. into an optical fiber, receive the reflected pulses reflected from each point of the optical fiber at the input end of the optical fiber, and detect abnormalities from the propagation time difference between the reflected pulses. It determines the distance to a point. Note that according to the above measurement method, reflected light from the end face of the optical fiber can also be detected, and the length of the optical fiber can also be checked.

A typical configuration of such a reflected light detection system is shown in FIG. In FIG. 1, a light pulse 3 from a laser oscillator 1 is transmitted through a nof mirror 2, which is a splitter, and then guided to an optical fiber 6 to be measured via a condenser lens 5. Light pulses from a laser oscillator may be guided through a polarizer, which is a type of polarizing plate. The reflected light 4 from the optical fiber 6 to be measured passes through a condenser lens 5, is reflected by a half mirror 2 serving as a splitter, and is guided to a photodetector 8.

This light reception signal is observed by the CRT 9 using time as a parameter. A half mirror 2, which is generally used as a splitter, is made by depositing a dielectric thin film on both sides of a parallel glass plate, and splits transmitted light and reflected light at a ratio of 1:1.

However, when such a half mirror is used as a splitter, the optical pulse 3 from the laser oscillator 1 and the reflected light 4 from the optical fiber 6 each pass through the splitter once, so the light intensity is attenuated by % each time. become. In reality, there is absorption loss in a portion of the half-mirror, so the amount of attenuation is greater than that. Such a decrease in the amount of light is disadvantageous in measuring backward several degrees of scattered light, which requires a high signal-to-noise ratio (S/N ratio) to maintain the reception value. In Rufuden 1 Nerf and Suba, the backscattered light level was about an order of magnitude lower than that of an optical fiber using Ge-doped quartz as the core material and pure quartz as the cladding material, which was extremely disadvantageous.

Another method is to use a polarizing beam splitter as a splitter instead of the half mirror 2. This is the second
As shown in the figure, the circularly polarized light component 11 incident on the polarizing beam splitter 14 is divided into two orthogonal components, namely, a P wave 12 and an S wave 13. In this case, if the incident laser light pulse is only a P wave, there is no light attenuation due to the incident light passing through the polarization beam splinter. However, the reflected light from inside the optical fiber is abnormal, and since the angle of incidence on a single point of reflection surface is rarely zero, it becomes scattered light, with a mixture of P waves and S waves, resulting in a state close to circularly polarized light. Therefore, when the reflected light passes through the splitter, it is inevitable that the amount of light will decrease by about 50 degrees. Furthermore, when using a polarizing beam splitter, the laser beam must be linearly polarized, and a certain arrangement is required between the polarization direction of the laser and the polarizing beam splitter. In other words, there are additional restrictions on the settings of the optical system.

As a countermeasure to the above-mentioned attenuation of the amount of light, it is possible to increase the brightness of the laser light source. Increasing the brightness of a laser light source is often difficult due to restrictions on the light receiving intensity of optical system components, such as polarizers, and safety issues.

Object of the Invention The present invention has been made in view of the above-mentioned conventional circumstances, and
A method and device for measuring abnormal points in an optical fiber or reflected light from the end face, which can prevent a decrease in the amount of light received by a photodetector without increasing the amount of light pulses from a laser oscillator and improve measurement accuracy. The purpose is to provide

B) Structure of the Invention The present invention injects a light pulse from a laser oscillator into one end of an optical fiber to be measured, and directs the reflected light from an abnormal point or end face of the optical fiber to be approximately coaxial with one end of the optical fiber to be measured. In the method and apparatus for measuring reflected light of an optical fiber, the reflected light is reflected by a mirror placed on the extension between the laser oscillator and the optical fiber to be measured and is received by a photodetector. mirror,
In addition, the optical pulse from the laser oscillator should not be applied to the total reflection mirror (it is special that the optical pulse should be made incident on one end of the optical fiber to be measured).

A method for measuring reflected light from an abnormal point or end face of an optical fiber, which prevents a decrease in the amount of light received by a photodetector without increasing the amount of light pulses from a laser oscillator and improves measurement accuracy. Equipment is provided.

(E) Embodiments of the Invention Preferred embodiments of the invention will now be described with reference to the drawings.

FIG. 3 shows a first embodiment of an optical fiber reflected light measuring device according to the present invention. It has a configuration similar to the conventional optical fiber reflected light measurement device shown in FIG. 1, but instead of the half mirror 2 serving as a splitter, a total reflection mirror 16 having a through hole 15 in the center is used. The difference is that

Normally, laser light has good convergence; for example, the diameter of a parallel beam in a YAG laser is about 2 degrees. The central through hole 15 of the total reflection mirror 16 is a hole for allowing the laser beam to pass through without being eclipsed or diffracted, and may have a diameter of, for example, 3 mm. The through hole 15 is provided parallel to the laser optical axis, that is, inclined with respect to the total reflection mirror surface that is inclined with respect to the laser optical axis. A light pulse from the laser oscillator 1 passes through the through hole 15 of the total reflection mirror, passes through the lens 5, and coaxially enters the optical fiber 6. After traveling along the optical fiber 6, the optical pulse reflected at the abnormal point 10 in the optical fiber travels in the opposite direction along the optical fiber as scattered light, and exits from the input end of the optical fiber at an angle equivalent to the numerical aperture. , the light beam is made into a parallel beam by the lens 5 and is incident on the total reflection mirror 16 . In order to minimize the attenuation of the amount of light, it is desirable that the reflected light beam be as large as possible on the total reflection mirror, that is, the focal length of the condenser lens 5 should be large.

- As an example, the focal length of the condensing lens 5 is 50+ut,
Use an optical fiber with a diameter of 50m1X, and the numerical aperture of the optical fiber is 0.
．． 3, the diameter of the luminous flux of the reflected light 4 will be approximately 30 mm.

Since the central through-hole 150 portion does not reflect light, the reflectance in terms of area is 1-(3/3o)2=0.99 (99%).

If ψ is the angle between the through hole 15 and the vertical axis of the total reflection mirror 16, the reflected light 4 will be refracted by 2ψ, so ψ must be set to a value other than 0° in order to separate the optical axes of the incident light and reflected light. There is. Considering the arrangement of the optical system, ψ=4de is generally used.

As the total reflection mirror, when using a YAG laser, Ar laser, etc. as a light source, a product equivalent to a flat surface mirror used in a laser resonator may be used.

According to the above embodiment, the laser resonator 1 and the optical pulse 3
Since the light passes through the central through hole 15 of the total reflection mirror 16 and enters the optical fiber 6, there is no attenuation in the amount of light during this time. Also, 99% of the reflected light 4 from the optical fiber 6 is reflected by a total reflection mirror.
reflected. Therefore, it is possible to almost eliminate the attenuation of the amount of light that was attenuated to H each time it passes through a half mirror in the conventional device. In this way, the amount of reflected light from the optical fiber received by the photodetector 8 is about four times that of the conventional device if the amount of laser light is the same, and the detection accuracy is improved accordingly.

As a specific example, in the conventional measurement system shown in FIG. 1, an Ar laser with a continuous wave output of 4 W is used as the laser oscillator 1,
The oscillation light with a wavelength λ-=0.5124 μm is used as a mode locker,
It is converted into pulsed light with a half-width of 500 ps by passing through a capity damper, and the pulsed light is transmitted through a half mirror 2 as a splitter, with a focal length of 50 m and an aperture of 40 m1.
Through the positive condenser lens 5, the core diameter is 50 μm, the outer diameter is 125
A pure quartz core fluorine-doped quartz clad fiber with a length of 100 μm and a length of 100 tatami was excited. When the backscattered light was measured using Photomaru as the photodetector 8, it was impossible to detect it. On the other hand, when the measurement was performed using the measurement system according to the present invention shown in FIG. 3, a reflected signal level curve as shown in FIG. 4 was obtained, and the measurement was possible. At this time, a total reflection mirror having a diameter of 50 W+ and having a 45° inclined through hole with a diameter of 3 in the center was used. FIG. 4 shows the output of the CRT 9 recorded using a recorder, where 17 is the backscattered light from inside the optical fiber, and 16 and 18 are the reflected lights at the input end and output end of the optical fiber, respectively. Further, 19 indicates noise.

FIG. 5 shows a second embodiment of the invention. The optical pulse 3' from the laser oscillator 1 is incident on one end of the optical fiber 6, but at this time, the optical pulse 3' passes through the side of the total reflection mirror 22 arranged on the generally coaxial extension of the optical fiber 6. , and is incident on the optical fiber 6 at an incident angle θ1 within an angle equivalent to the numerical aperture. Since the angle θ1 is less than the numerical aperture equivalent angle, the optical pulse 3' is completely incident on the optical fiber 6.

The incident light pulse travels along the optical fiber and is reflected at an abnormal point 10 within the optical fiber. As described above, the reflected light travels in the opposite direction along the optical fiber as scattered light, and is emitted from the input end of the optical fiber at an angle θ2 corresponding to the numerical aperture. The emitted reflected light reaches the total reflection mirror 22. The total reflection mirror 22 is arranged generally on the coaxial extension of the optical fiber 6, but so as not to interfere with the incident light pulse that is incident at an angle of θ1 with respect to the axis of the optical fiber 6. angle (θ2≧θl)
Although it is not possible to receive and reflect all of the reflected light emitted by the sensor, it can still be configured to receive and reflect most of the reflected light. The light reflected by the total reflection mirror 22 is guided to the photodetector 8 via the lens 7. The measurement result can be observed on the CRT 9 as a received light signal level with the propagation time 6 as a parameter.

According to the above embodiment, the optical pulse 3' from the laser oscillator 1
The light passes through the side of the total reflection mirror 22 and enters the optical fiber 6 at an angle less than the angle equivalent to the numerical aperture, so there is no attenuation of the amount of light during this time. Furthermore, the reflected light 4' from the optical fiber 6 cannot be reflected as it is received by the 100-chip total reflection mirror 22 as described above, but according to the configuration example of the measurement system shown in FIG. In view of the light receiving area for the reflected light 4' of the mirror 220, it is clear that a reflectance of 50- or more can be obtained. Therefore, it is possible to reduce the attenuation rate of the amount of light, which in the conventional measurement system was attenuated by 10% by passing through a nof mirror each time it is incident or reflected. In this way, as in the case of the first embodiment, it is possible to improve the accuracy of detecting an abnormal point or end face of an optical fiber.

(F) Effects of the Invention As described above, the present invention injects a light pulse from a laser oscillator into one end of an optical fiber to be measured, and transmits reflected light from an abnormal point or end face of the optical fiber to be measured. A method and apparatus for measuring reflected light of an optical fiber configured to be reflected by a mirror disposed on a substantially coaxial extension of one end and between the laser oscillator and the optical fiber to be measured and received by a photodetector, The present invention is characterized in that the mirror is a total reflection mirror, and the optical pulse from the laser oscillator is made incident on one end of the optical fiber to be measured without hitting the total reflection mirror.

This reduces the attenuation rate of the amount of light when the light enters the optical fiber from the laser oscillator and when the reflected light is emitted from the optical fiber to the photodetector, thereby increasing the accuracy of detecting abnormal points or end faces of the optical fiber by the photodetector. Improved. In addition, by achieving the above-mentioned reduction in the light intensity attenuation rate, it is possible to maintain or improve the detection accuracy of an abnormal point or end face of an optical fiber even by using a laser oscillator with a smaller output than the conventional one. Furthermore, since there is no large attenuation of the incident light and reflected light, there is no need to set the light intensity damage tolerance level range of the entire optical fiber reflected light measuring device at widely different levels for each component, making it easier to configure the device. In addition to being easy to use, there is also less risk that components that should receive weak light will be damaged by the entry of abnormally strong light, resulting in damage to the device.

[Brief explanation of drawings]

Fig. 1 is a schematic diagram showing the configuration of a conventional optical fiber reflected light measuring device, Fig. 2 is a conceptual diagram showing the action of a polarizing beam splitter, and Fig. 3 is an example of an optical fiber reflected light measuring device according to the present invention. FIG. 4 is a diagram showing an example of the reflected light signal level curve of an optical fiber; FIG. 5 is a diagram showing the configuration of another embodiment of the optical fiber reflected light measuring device according to the present invention. Schematic diagram shown. 1... Laser oscillator 2... Half mirror 6...
Optical fiber 8... Photodetector 10... Abnormal points in the optical fiber 16, 22... Total reflection mirror Fig. 4 Fig. 5

Claims (6)

[Claims] (1) A light pulse from a laser oscillator is incident on one end of the optical fiber to be measured, and the reflected light from the abnormal point or end face of the optical fiber to be measured is directed on a generally coaxial extension of one end of the optical fiber to be measured and In a method for measuring reflected light of an optical fiber configured to be reflected by a mirror disposed between a laser oscillator and the optical fiber to be measured and received by a photodetector, the mirror is a total reflection mirror, and the laser A method for measuring reflected light of an optical fiber, characterized in that the optical path from an oscillator is not applied to the total reflection mirror (it is made incident on one end of the optical fiber to be measured). (2) A light pulse from the laser oscillator is made to pass through a through hole provided in the center of the total reflection mirror and coaxially enter the optical fiber to be measured. Method for measuring reflected light of an optical fiber as described in . (3) The light pulse from the laser oscillator is passed through the external side of the total reflection mirror and is made incident on the optical fiber to be measured at an angle of incidence equal to or less than an angle equivalent to the numerical aperture. A method for measuring reflected light of an optical fiber according to scope 1. (4) A laser oscillator for injecting a light pulse into one end of the optical fiber to be measured, and a generally coaxial extension of one end of the optical fiber to be measured to reflect the reflected light from an abnormal point or end face of the optical fiber to be measured. An apparatus for measuring reflected light of an optical fiber, including a mirror disposed above the laser oscillator and the optical fiber to be measured, and a photodetector that receives the reflected light reflected by the mirror. is a total reflection mirror, and the laser oscillator and the total reflection mirror are aligned with each other with respect to the optical fiber to be measured so that the optical pulse from the laser oscillator reaches one end of the optical fiber to be measured without hitting the total reflection mirror. An optical fiber reflected light measuring device characterized in that: (5) The total reflection mirror has a through hole in the center, and the optical pulse from the laser oscillator passes through the through hole of the total reflection mirror and is coaxially incident on the optical fiber to be measured. 5. The apparatus for measuring reflected light of an optical fiber according to claim 4, wherein the laser oscillator and the total reflection mirror are mutually arranged with respect to the optical fiber to be measured. (6) Connect the laser oscillator and the total reflection mirror so that the light beam from the laser oscillator passes through the external side of the total reflection mirror and enters the optical fiber under test at an angle of incidence equal to or less than the angle equivalent to the numerical aperture. 5. The apparatus for measuring reflected light of an optical fiber according to claim 4, wherein the reflecting mirrors are disposed relative to each other with respect to the optical fiber to be measured.