JPH04276535A - Otdr apparatus - Google Patents

Abstract

PURPOSE:To decrease the coherency of the light having high intensity and to make it possible to decrease the noise of an OTDR waveform by casting the pulse light having the high intensity into one end of an optical fiber, generating stimulated Raman scattering, and taking out the wavelength component of the light of an original light source among the stimulated Raman scattered lights emitted from the other. end. CONSTITUTION:High output-power pulse light which is higher than a threshold value is emitted from a semiconductor-laser excited solid laser 11 into an optical fiber 12. Thus, stimulated Raman scattering is generated. At this time, forward Rayleigh scattered light, stimulated Brillouin scattered light and stimulated Raman scattered light having the small wavelength shifting amount are superimposed on the pulse light propagating in the optical fiber 12. Thus, the coherency of the pulse light cn can be decreased. The pulse light of high intensity whose coherency is decreased is made to be the size less than a threshold value where the stimulated Raman scattering is generated in an optical fiber 9 to be measured by passing an optical attenuator 2. Only the wavelength region of the original light generated in the laser 11 is taken out with an optical filter and cast into the optical fiber.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] This invention detects Rayleigh backscattered light or Raman backscattered light when pulsed light is input to one end of an optical fiber, and determines the intensity of the optical fiber from the time-based value of the backscattered light intensity. OTDR (Optical Time-Do) measures characteristics such as loss and temperature at each position in the length direction.
This invention relates to an improvement of a main reflectometry device.

0002

[Prior Art] In an OTDR device, pulsed light is generated from a light source, enters one end of an optical fiber to be measured, returns to the rear by Rayleigh scattering and Raman scattering generated in this optical fiber, and is emitted from one end. The separated light is separated. Then, the backscattered light is guided to a light receiving element and converted into an electrical signal, and data on the intensity of the backscattered light at each time is obtained. By arranging this time-based data based on the pulse light generation timing, the loss distribution in the length direction of the optical fiber can be obtained. Raman scattered light is light with a wavelength shifted from the wavelength of the incident light, and consists of anti-Stokes light shifted to a shorter wavelength and Stokes light shifted to a longer wavelength, and its intensity changes sensitively depending on temperature, so it is difficult to Temperature distribution at each position in the length direction can be measured.

[0003] Ley scattered light, which is detected as backscattered light, is weak at less than 1/10,000 of the incident light, and Raman scattered light is extremely weak at 1/100,000,000 of the incident light. need to use. Semiconductor laser (LD)
) as a light source, the Raman scattered light intensity is particularly weak, making it impossible to obtain sufficient temperature measurement accuracy. Therefore, a semiconductor laser (LD) pumped solid-state laser, which has been put into practical use recently, is used to inject strong light into the optical fiber to be measured to obtain strong Raman scattered light intensity.

0004

[Problems to be Solved by the Invention] However, when an LD pumped solid-state laser is used as a light source as in a conventional OTDR device, since this light source is a solid-state laser, the coherency of the oscillated light is extremely high, and the oscillated light is linearly polarized. As a result, there is a problem of noise generation. In other words, when using a multimode optical fiber as the optical fiber to be measured, it is easy to reduce the coherency because there are multiple modes, and the coherency of the light source is rarely a problem, but in the case of a single mode optical fiber The coherency of the pulsed light from the light source does not decrease even after it propagates over a long distance, so there is interference between the propagating pulsed light and the Rayleigh scattered light, interference with the polarization mode, interference with the Brillouin scattered light, and interference between the propagating pulsed light and the Rayleigh scattered light. Fluctuations such as differences in scattering generation efficiency occur in optical fibers. As a result, when an OTDR waveform of a single mode optical fiber is obtained using a light source with good coherency, the effects of fluctuations due to these interferences are observed as very large noise. Raman scattered light signals used for temperature measurement are also adversely affected as noise.

[0005] In view of the above, the present invention has been improved to reduce noise in Rayleigh and Raman OTDR waveforms by reducing the coherency of high-intensity light.
The purpose is to provide an OTDR device.

[0006]

[Means for Solving the Problems] In order to achieve the above object, in the OTDR device according to the present invention, a high-intensity pulsed light is incident on one end of an optical fiber to cause stimulated Raman scattering, and the light is emitted from the other end. The feature is that the wavelength component of the light from the original light source is extracted from the stimulated Raman scattered light, and this extracted light is used as light that enters one end of the optical fiber to be measured. When stimulated Raman scattering is generated in an optical fiber by inputting strong pulsed light, the pulsed light propagating in the optical fiber contains forward Rayleigh scattered light, stimulated Brillouin scattered light, and stimulated Raman scattered light with a small wavelength shift. It becomes possible to overlap and reduce the coherency of the pulsed light. By using pulsed light with reduced coherency, noise in Rayleigh and Raman OTDR waveforms can be reduced.

[0007]

[Embodiment] Hereinafter, an embodiment in which the present invention is applied to an OTDR device that measures the longitudinal temperature distribution of an optical fiber to be measured by detecting Raman backscattered light will be described in detail with reference to the drawings. . In FIG. 1, a fiber Raman laser device 1 consisting of a semiconductor laser (LD) pumped solid-state laser 11 and an optical fiber 12 is used as a light source, and pulsed light generated therefrom is transmitted to an optical attenuator 2, an optical filter 4 and a spectroscopic device 3. The light is then input to the optical fiber 9 to be measured. The spectroscopic device 3 includes, for example, a diffraction grating, an optical filter film using a dielectric multilayer film, and the like. The wavelength components of the Raman backscattered light generated in the optical fiber 9 to be measured are separated by the spectroscopic device 3. Here, light of two wavelength components is extracted from the spectroscopic device 3,
The signals are sent to a digital averaging circuit 7 via a light receiving element 5 and an amplifier 6, respectively. This digital averaging circuit 7 is connected to a computer 8 and exchanges data with each other. The digital averaging circuit 7 performs sampling and A/D conversion of the input signal using the trigger signal for pulsed light generation sent to the LD pumped solid-state laser 11 as a starting point, and converts the data about the intensity of the backscattered light into the received signal. The S/N is obtained for each unit time from the time when the pulsed light is input into the measurement optical fiber 9, and the data obtained when the pulsed light is repeatedly input is added and averaged.
I'm trying to increase the ratio. The data thus obtained is sent to the computer 8, and the measured temperature at each position in the length direction of the optical fiber 9 to be measured is expressed by displaying, for example, the horizontal axis as the data sampling time and the vertical axis as the data size. It turns out.

[0008] High-power pulsed light having a magnitude above a threshold value at which stimulated Raman scattering occurs is input to the optical fiber 12 of the fiber Raman laser device 1. Therefore, high-output pulsed light of several watts or more is required, but it is difficult for a normal LD to emit light of such a high output. Therefore, here we introduce 11 LD-pumped solid-state lasers that have been put into practical use recently.
are using. This replaces the flash lamp, which is the excitation light source for conventional solid-state lasers, with an LD.
It is efficient and can be downsized. Currently commercialized solid crystals for laser oscillation are Nd-doped Y
It is made of AG or YLF and has an oscillation wavelength of 1.0.
Those with a diameter of 6 μm or 1.32 μm are known. Here, an LD excitation solid-state laser 11 with a wavelength of 1.321 μm is used.

[0009] The optical fiber 12 of the fiber Raman laser device 1 is a single-mode optical fiber with a length of several hundred meters to about 1 km, and a high-output pulsed light of a threshold value or higher is incident on this optical fiber 12. This causes stimulated Raman scattering. In other words, generally when pulsed light is introduced into an optical fiber, Rayleigh scattered light with the same wavelength as the input wavelength and Raman scattered light with a shifted wavelength are generated, and the intensity of these scattered lights increases in proportion to the intensity of the incident light. However, once a certain threshold is exceeded, a nonlinear effect occurs and stimulated Raman scattering suddenly occurs. At this time, forward Rayleigh scattered light, stimulated Brillouin scattered light, and stimulated Raman scattered light with a small amount of wavelength shift are superimposed on the pulsed light propagating in the optical fiber, making it possible to reduce the coherency of the pulsed light.

[0010] In this way, it is possible to obtain a high-intensity pulsed light with reduced coherency from the optical fiber 12, so by passing it through the optical attenuator 2, its intensity is adjusted, and by passing it through the optical filter 4,
Only the wavelength range (wavelength 1.321 μm in this case) of the original light generated from the LD excitation solid-state laser 11 is extracted. By adjusting the intensity in this manner, the intensity is set to be below the threshold value at which stimulated Raman scattering occurs in the optical fiber 9 to be measured. In addition, the light emitted from the optical fiber 12 includes the optical fiber 9 to be measured.
Since stimulated Raman forward scattered light with the same wavelength component as the Raman scattered light generated inside is included, this overlapping wavelength component is cut so that only the original incident wavelength component enters the optical fiber 9 to be measured. I have to. As a result, the spectroscopic device 3 can separate Stokes light with a wavelength of 1.403 μm, which is the backscattered light of the optical fiber 9 to be measured, and anti-Stokes light with a wavelength of 1.248 μm.

When an OTDR waveform was obtained with such a configuration, the one shown in FIG. 2 was obtained. Regarding the Rayleigh backscattered light, an optical switch (not shown) or the like is used instead of the spectroscopic device 3 to extract the same wavelength component as the incident light. For reference, FIG. 3 shows an OTDR waveform when the pulsed light from the LD excitation solid-state laser 11 is directly input to the optical fiber 9 to be measured without passing through the optical fiber 12. From the comparison between Figures 2 and 3, it is clear that by using pulsed light with reduced coherency, Rayleigh and Raman OTD
It can be seen that the noise of the R waveform is reduced.

0012

[Effects of the Invention] As described above with respect to the embodiments,
According to the OTDR device of the present invention, by lowering the coherency of high-power pulsed light, it is possible to reduce noise due to interference and obtain data with a high S/N ratio.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an embodiment of the present invention.

FIG. 2 is a graph representing an OTDR waveform.

FIG. 3 is a graph showing an OTDR waveform.

[Explanation of symbols]

1 Fiber Raman laser device 11 LD pumped solid-state laser 12 Optical fiber 2 Optical attenuator 3 Spectroscopic device 4 Optical filter 5 Photodetector 6 Amplifier 7 Digital averaging circuit 8 Computer 9 Optical fiber to be measured

Claims (1)

[Claims] Claim 1: A light source that generates high-power pulsed light;
an optical fiber into which the pulsed light is incident at one end and causes stimulated Raman scattering; an optical filter which extracts the wavelength component of the original light source light from the stimulated Raman scattered light emitted from the other end of the optical fiber; and this light. An OTDR device comprising: an optical device that inputs the output light of the filter into one end of an optical fiber to be measured and extracts backscattered light of the fiber to be measured that is emitted from the one end.