KR20020023983A - Submerged optical fiber detection sensor using a side-polished fiber optic coupler - Google Patents

Abstract

PURPOSE: A submersion sensor using a fiber optic coupler is provided to accurately detect the submerged position by checking the loss of optical power due to the transit of wavelength, to improve the reliability of a sensor by increasing the optical power when water invades, and to detect multiple points. CONSTITUTION: A slit(1) is formed on a rectangular parallelepiped quartz block to sustain a slight radius of curvature of an optical fiber(3). The optical fiber is fixed in the slit. After the optical fiber is completely adhered, the cladding layer of the optical fiber is polished by using polishing powder and sandpaper. The cladding layer is properly polished not to expose the core layer to the atmosphere. Glass pane is attached to the polished portion. Then, an upper planar waveguide(2) is prepared. Upon inputting a white light source to the optical fiber, the optical power of the optical fiber is transmitted to the upper waveguide. Then, the optical fiber is filtered at a specific wavelength range. The loss of optical power due to water invasion is measured by an optical time domain reflectometer. Since the optical time domain reflectometer indicates multiple positions on the optical fiber. Therefore, the submerged position is detected accurately.

Description

Submerged optical fiber detection sensor using a side-polished fiber optic coupler}

Optical cables are usually installed through underground pipelines or by using ground structures. It is common to use underground pipelines. In this case, when the length of the optical cable is to be extended, the coating is peeled off and the optical fibers are fused and connected. However, because the welded part is not completely waterproof, water may infiltrate the welded part when the pipe embedded with the fiber is flooded, which may significantly degrade the fiber transmission characteristics. Shorten. Therefore, it is very important to know the location of flooding in real time and to recover the flooded location. Currently reported fiber-optic immersion sensors are typical of microbending structures [1,2] and optical fiber gratings [3]. The optical fiber immersion sensor using micro banding is a structure in which an optical fiber is installed between two toothed gratings and a material having a large coefficient of expansion for water is wrapped around it. Therefore, it is a principle that absorbs water during immersion and expands gradually, narrowing the lattice spacing and causing bending loss in the optical fiber. This structure requires at least one hour to fully determine whether the water is inundated, so it is difficult to determine the location of water in real time, and it is difficult to recycle, which may cause serious maintenance and repair problems, which may incur additional costs. In addition, there is always a loss caused by natural or artificial factors of the buried optical fiber, so the reliability of the result is inferior. The optical fiber immersion sensor using an optical fiber grating is a structure in which a Bragg grating is formed on an optical fiber and a material having an absorption expansion coefficient is coated on the optical fiber grating to determine the location of the immersion by changing the Bragg wavelength. This structure also has a disadvantage in that the response time is slow and the shifted Bragg wavelength must be accurately detected in order to know the accurate submerged position information. It is also not economical because the sensor cannot be reused after flooding. Therefore, the present invention proposes an optical fiber immersion detection sensor having a simple structure, which has no polarization dependency, can grasp a immersion position in real time, can be used semi-permanently, and secures reliability.

Side polished fiber optic couplers have been studied for various applications for optical communication devices. Many studies have also been reported with fiber-optic sensors.

The present invention proposes a new type of immersion detection sensor using the basic principle of such a structure. It should be possible to observe the location of flooding in real time, and be able to identify the location of flooding at several points simultaneously. It is a device structure without polarization dependency and should be designed without affecting the sensor at the rear end by having an amplification effect when the sensor is submerged, rather than reducing the optical power.

In order to achieve this purpose, a device is fabricated by forming a flat glass waveguide having no polarization dependence on a side polished optical fiber, installing a device fabricated in an optical fiber junction box, and using a optical time domain reflectometer. By measuring the light output change of the sensor installed, it is characterized in that the step consisting in detecting the exact position of immersion in real time.

1 is a schematic view of an optical fiber immersion detection sensor using a side polished optical fiber coupler proposed in the present invention

2 is a light transmission spectrum for TE and TM polarization of the immersion sensor

Figure 3 shows the wavelength shift spectrum when not immersed and when immersed

4 is a view of installing the fabricated immersion detection sensor in the optical fiber splice enclosure

5 is an experimental diagram for measuring the operation of the optical fiber immersion detection sensor installed in multiple locations using the optical time domain reflectometer

6 is a result of measuring the optical power change according to the immersion or not with an optical time domain reflectometer (OTDR)

<Explanation of symbols for main parts of drawing>

1 Optical fiber warpage maintenance V-groove 2 Flat waveguide made of flat glass

3: optical fiber 4: fusion part of optical fiber

5: fused optical fiber splice enclosure 6: fabricated optical fiber immersion detection sensor

7 communication cable 8 optical time domain reflectometer

9: optical fiber immersion detection sensor 1 10: optical fiber immersion detection sensor 2

Hereinafter, described in detail by the accompanying drawings as follows.

1 is a schematic diagram of an element for forming a planar waveguide made of flat glass on top of a side polished optical fiber to obtain characteristics of an immersion sensor. First, a groove is formed in the rectangular parallelepiped quartz block so that the optical fiber can maintain a slight radius of curvature and the optical fiber is fixed in this groove. When the optical fiber is completely bonded, the cladding layer of the optical fiber is appropriately polished using abrasive powder and sandpaper, but the core layer is polished so that the core layer is not exposed to air. Usually the thickness of the remaining cladding is on the order of 2 μm. The device is completed by pressing a flat glass of appropriate thickness to form the upper planar waveguide. When a white light source is incident on an optical fiber of the fabricated device, optical power of the optical fiber is transferred to the upper waveguide under the condition that the optical fiber mode and the upper waveguide mode satisfy the phase matching condition. When viewed from the output end side of the optical fiber, it is observed that optical power is filtered (filtered) in a specific wavelength region. Depending on the conditions of the flat glass used in the present invention, the bonding wavelength exhibits the characteristics of the comb filter having a period of 19 nm.

2 is a transmission spectrum of measuring the polarization dependence of the fabricated device. Although there is a slight difference between TE polarization and TM polarization, polarization dependence is a negligible result for use as a sensor and is a very important advantage to operate as an immersion detection sensor. 3 shows the transmission spectrum of the sensor during immersion. First, the case where the element is exposed to air is indicated by a solid line. When the device was immersed in water, the transmission spectrum shifted toward the longer wavelength by increasing the refractive index, which is indicated by a dotted line. From the results of FIG. 3, it can be seen that the device can detect whether or not the underground pipe in which the optical fiber is embedded is flooded. 4 is a diagram in which a sensor is installed in a fused optical fiber splice box. Dark fiber (extra optical fiber, which is not used for communication, among other optical fibers) can be used to measure inundation. Using a positive slope with respect to the horizontal axis (distance) in the transmission spectrum of FIG. 3 causes the output optical power to be attenuated when the sensor detects water, and using the negative slope increases the output optical power. In the present invention, the negative slope curve is used by fixing the wavelength of the light source at 1550 nm, in order to increase the reliability of the sensor. Although there are many loss factors that can occur in an optical fiber, increasing the output light power does not occur unless it intentionally amplifies the light from the outside. Thus, when the output light power is measured above the reference value, the information that the optical cable is submerged is much more reliable than conventional methods. The two optical fiber immersion detection sensors proposed in the present invention were connected to two optical fibers of 2km and 8.5km at the input terminal as shown in Fig. 5, and the optical power loss was measured by the optical time domain reflectometer. The optical time domain meter tells you the multi-position information on the fiber, so you know exactly where the submersion is. 6 shows the result. If sensor 1 and sensor 2 are not submerged, only the loss at a given wavelength will appear. If sensor 1 is not submerged and sensor 2 is submerged, sensor 1 has no change in optical power, but sensor 2 has less light loss than when exposed to air, so the graph goes up. In FIG. 6, the dotted line shows the result of measurement in the air and the solid line in the submerged state. As the sensor 2 is exposed to air and submerged, the power of the output light is increased by about 1 dB. From this, the installed position of the submerged sensor can be known. In general, the accuracy of the loss measurement of the wide-time domain reflectometer is ± 0.05dB for 1dB steps, so the value of the measured data is reliable. In addition, the result of flooding can be determined in real time, which saves time and money compared to existing technologies.

As described above, the present invention proposes a sensor capable of observing inundation of an underground pipe route in which an optical fiber is embedded in real time by using a thin plate glass as a planar waveguide at a polished portion of a side polished optical fiber. Due to the nature of glass, it has excellent physical and chemical durability against water, so it can be used semi-permanently and can be recycled. The structure is simple and responsive to the existing submersion sensor, and has the characteristics of a small optical fiber type sensor. By proposing a sensor structure that can increase the output light power when water is detected, it has superior reliability compared to the conventional method of measuring the amount of additional light loss when flooding.

Claims (4)

A method of fabricating a sensor that detects the inundation of an optical fiber by using a side polished optical fiber and a flat glass waveguide coupler and using it as a submersion sensor. When the sensor detects the inundation of the optical fiber, the operation of the sensor shows the amplification effect rather than the optical loss. This prevents the remotely installed sensor from masking when multiple point floods are detected simultaneously. The response of the sensor is instantaneous (within 1 second) to quickly identify the location Method to make time-independent by preventing the initial detected output characteristic from changing with time The optical fiber immersion sensor proposed in the present invention uses glass as a base material, and thus recovers the function of the initial state even after recovery after immersion. [references] 1. S. Tomita, H. Tachino, and N. Kasahara, "Water sensor with optical fiber," J. Lightwave Technol. Lett., Vol. 8, no. 12, pp. 1829-1832, 1990. 2. W. C. Michie, B. Culshaw, M. Konstantaki, S. Kelly, N. B. Graham, and C. Moran, "Distributed pH and water detection using fiber-optic sensors and hydrogels," J. Lightwave Technol., Vol. 13, no. 7, pp. 1415-1420, 1995. 3. Young-Bok Choi, Tae-Sang Park, Soo-Jin Park, Ki-Tae Jung, "Water Penetration Sensor Using Optical Fiber Grating Device," Photonics conference 2001, pp. 465-466, 2001.