JP5398790B2 - Optical line characteristic measurement system and optical line characteristic measurement method - Google Patents

Description

  The present invention relates to an optical line characteristic measuring system and an optical line characteristic measuring method using an OTDR (Optical Time-Domain Reflectometer).

  A wide variety of systems are used in the field of optical access networks, and representative examples include PON (Passive Optical Network) and CATV optical access networks. In particular, in an optical access network employing PON, an OLT (Optical Line Terminal) and ONU (Optical Network Unit) have a 1-to-N configuration (N is a positive number), and there is a problem with an optical fiber line of a certain user. Even if it occurs, the signal from the OLT cannot be stopped because communication with other normal users cannot be inhibited.

  There is a case where the OTDR test is performed in a situation where signal light from the OLT side exists in the optical line. In such a case, since the signal light from the OLT side becomes background light and affects the OTDR test, a wavelength filter having a characteristic of cutting the signal light and transmitting the OTDR signal is mounted inside the OTDR. Measure.

On the other hand, an apparatus for performing OTDR measurement has been proposed (see, for example, Patent Documents 1 to 3). The OTDR of Patent Document 1 makes test light modulated with a correlation code incident on an optical fiber to be measured and receives return light as backscattered light. At this time, by making the pulse width of the test light shorter than the gain saturation time of the transient response of the optical amplifier in the transmission line of the optical fiber to be measured, the signal light transmission characteristic is prevented from deteriorating.
The OTDR of Patent Document 2 makes test light modulated with a pseudo-random code incident on an optical fiber to be measured and receives return light that is backscattered light.
In the OTDR of Patent Document 3, test light modulated using two types of Golay codes is incident on an optical fiber to be measured, and return light that is backscattered light is received.

Japanese Patent Laid-Open No. 2004-32420 Japanese Patent Laid-Open No. 9-26376 JP-A-4-54429

  Since there are many measurement opportunities in an optical access network, it is necessary to deploy many OTDRs, and the demand for cost reduction required for OTDRs is increasing. However, the wavelength filter mounted inside the OTDR is expensive and has a problem of mounting costs, which hinders the cost reduction of the OTDR. Furthermore, when the wavelength filter is mounted, there is a problem that it is difficult to reduce the size of the OTDR.

  On the other hand, since it is necessary to detect a weak signal in order to increase the resolution of OTDR, it must be amplified with a high amplification factor. For this reason, when amplification is performed without using a wavelength filter, there is a problem that the apparatus is saturated by the background light and it is difficult to perform normal measurement.

In Patent Document 1, there is a description that modulation is performed with a correlation code so that the signal-to-noise ratio does not deteriorate even when the power of the test light is lowered. However, the return light and the signal light having different wavelengths are wavelength filters. It is separated.
In Patent Document 2, there is a description that modulation is performed with a random code in order to find the return light even from the noise level, but OTDR measurement at the time of in-service and separation of the return light and the signal light are not considered. .
In Patent Document 3, there is a description that modulation is performed using a Golay code in order to improve the dynamic range, but OTDR measurement at the time of in-service and separation of return light and signal light are not considered.

  As described in Patent Documents 1 to 3, conventionally, a wavelength filter is used as means for separating the return light and the signal light having different wavelengths, and the return light and the signal light having different wavelengths are separated from each other. No means has been proposed for separation without using a wavelength filter. For this reason, even if the inventions described in Patent Documents 1 to 3 are combined, only the invention for separating the return light and the signal light by mounting the wavelength filter can be configured, and the wavelength filter is mounted. Could not solve the problem.

  Therefore, an object of the present invention is to normally perform OTDR measurement under conditions with background light without using a wavelength filter.

The optical line characteristic measuring system of the present invention includes a PON in which one optical line (91) is branched into a plurality of optical lines (93_1 to 93_N) by an optical coupler (92), the one optical line and the plurality of optical lines. A wavelength filter (94_1 to 94_N, 95) that is disposed at an end of the optical line and transmits signal light transmitted by the PON and reflects light other than the signal light, the one optical line, and the optical line A test light that is arranged at one of the ends of a plurality of optical lines and that modulates light having a reflected wavelength of the wavelength filter with a random code is incident on the optical coupler, and the test light is incident on the wavelength filter or the light beam. the reflected or scattered return light road, by receiving without passing through the wavelength filter was cut background light component by extracting the AC component due to the random code from the electric signal by using a capacitor (17) To comprise, as OTDR (101) for performing correlation processing between the random code.

  Since the optical line characteristic measurement system of the present invention uses test light modulated with a random code, in OTDR, the background light is removed from the received light signal of the return light without removing the background light using a wavelength filter. can do. Thereby, the optical line characteristic measuring system of this invention can perform an OTDR measurement normally on conditions with background light, without using a wavelength filter.

In the optical line characteristic measuring system according to the present invention, the OTDR extends over a predetermined time longer than the time required for light to propagate through the optical line from the end of the one optical line to the ends of the plurality of optical lines. The test light may be modulated with a random code having a code length.
According to the present invention, the waveform of the return light can be made a periodic waveform around a certain offset level. Thus, the background light can be removed by AC coupling after converting the return light into an electrical signal.

In the optical line characteristic measuring method of the present invention, one optical line (91) of the PON branched into a plurality of optical lines (93_1 to 93_N) by an optical coupler (92) and the plurality of optical lines (91) Light having a wavelength reflected by the wavelength filters (94_1 to 94_N, 95) disposed at the ends of the one optical line and the plurality of optical lines is directed from one of the ends of the optical line toward the optical coupler. A test light incident procedure for entering test light modulated with a random code, and return light reflected or scattered by the test light at the wavelength filter or the optical line without passing through the wavelength filter to be converted into an electrical signal. a light receiving procedure for converting the periodic signal extraction procedure for cutting a background light component to extract the AC component due to the random code from said electrical signal using a capacitor (17), said random code and the periodic signal It has a correlation processing procedure for performing correlation processing, in sequence.

  Since the optical line characteristic measuring method of the present invention uses test light modulated with a random code in the test light incident procedure, the periodic signal extraction procedure and correlation processing are performed without removing background light using a wavelength filter in the light receiving procedure. In the procedure, the signal due to the background light can be removed from the light reception signal of the return light. Thereby, the optical line characteristic measuring method of this invention can perform an OTDR measurement normally on conditions with background light, without using a wavelength filter.

In the optical line characteristic measuring method of the present invention, in the test light incident procedure, the time required for light to propagate through the optical line from the end of the one optical line to the ends of the plurality of optical lines is longer than the time required for the light to propagate. The test light may be modulated with a random code having a code length over a predetermined time.
According to the present invention, the waveform of the return light can be made a periodic waveform around a certain offset level. Thus, the background light can be removed by AC coupling after converting the return light into an electrical signal.

  According to the present invention, the optical line characteristic measurement system and the optical line characteristic measurement method of the present invention can normally perform OTDR measurement under conditions with background light without using a wavelength filter.

An example of the optical-line characteristic measurement system which concerns on this embodiment is shown. An example of OTDR which concerns on this embodiment is shown. An example of test light modulated with a random code is shown. An example of the characteristics of the optical fiber to be measured is shown. An example of the electrical signal output from the light receiver 16 is shown. An example of an output waveform from the correlator 20 is shown.

  Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. In the present specification and drawings, the same reference numerals denote the same components.

FIG. 1 shows an example of an optical line characteristic measurement system according to this embodiment. The optical line characteristic measurement system according to the present embodiment includes a PON, wavelength filters 94_1 to 94_N, a wavelength filter 95, and an OTDR 101. In the PON, one optical line 91 is branched by an optical coupler 92 into a plurality of optical lines 93_1 to 93_N. Thereby, communication is performed between the OLT 96 and the plurality of ONUs 97_1 to 97_N. Wavelength filter 94_1~94_N and wavelength filter 95 is arranged at an end portion of the optical path 91 and optical lines 93_1～93_N, by passing the signal light _{S C} transmitted by the PON reflects the signal light _{S C} other light .

  The OTDR 101 is disposed at either one of the optical line 91 and the ends of the optical lines 93_1 to 93_N, and enters test light obtained by modulating the light having the reflected wavelengths of the wavelength filters 94_1 to 94_N and 95 with a random code toward the optical coupler 92. To do. Then, the return light reflected or scattered by the wavelength filters 94_1 to 94_N and 95 or the optical lines 91 and 93_1 to 93_N is received without passing through the wavelength filter, and correlation processing with the random code is performed. For example, the OTDR 101 is connected to the optical line 93_3.

  FIG. 2 shows an example of the OTDR according to the present embodiment. The OTDR is correlated with a random code generator 11, a current controller 12, a light source 13, an optical coupler 14, a port 15, a light receiver 16, a capacitor 17, an amplifier 18, and an A / D converter 19. And a container 20. In the present embodiment, a case where the port 15 is connected to the optical line 93_3 and the optical fiber characteristics of the optical fiber to be measured including the wavelength filter 95, the optical coupler 92, the optical line 93_3, and the optical line 91 will be described.

  The optical line characteristic measuring method according to the present embodiment includes a test light incident procedure, a light receiving procedure, a periodic signal extraction procedure, and a correlation processing procedure in this order.

  In the test light incident procedure, test light is incident from the end on the ONU 97_3 side of the optical line 93_3 toward the optical coupler 92. Specifically, the random code generator 11 generates a random code. The random code is, for example, an M sequence code. The current controller 12 changes the current supplied to the light source 13 in accordance with the random code from the random code generator 11. The light source 13 is, for example, an LD (Laser Diode), and generates 1.65 μm light that is a reflection wavelength of the wavelength filters 94_1 to 94_N and 95. The light source 13 generates light with an output intensity corresponding to the current from the current controller 12 and emits it. Thereby, the test light modulated by the random code can be emitted from the light source 13. The optical coupler 14 guides the test light to the port 15. Thereby, the test light enters the optical line 93_3. FIG. 3 shows an example of test light modulated with a random code.

  Here, in the present embodiment, the light source 13 continues for a predetermined time that is longer than the time required for light to propagate through the optical line from the OLT 96 end of the optical line 91 to the ONU 97_3 end of the optical line 93_3. And output test light. The end of the optical line 91 on the OLT 96 side may be a wavelength filter 95, and the end of the optical line 93_3 on the ONU 97_3 side may be a wavelength filter 94_3.

In this case, the random code generator 11 has a random code having a code length over the predetermined time. For example, the code length N of the random code is expressed by the following equation using the pulse width Pw, the measured optical fiber length L _fiber, and the propagation velocity v of light in the optical fiber.
N> (2 / v) · (L _fiber / Pw)
However, N satisfies 2 ⁿ −1.
Therefore, in this embodiment, when the optical fiber length L _fiber to be measured is 500 m and the 1-bit pulse width Pw of the random code is 10 nsec, the code length N is 511 bits or more.

  Further, in this embodiment, the random code generator 11 continuously generates a random code having a code length over the predetermined time four times, and the light source 13 continuously outputs test light over a time four times the predetermined time. Output.

  In the light receiving procedure, the return light reflected or scattered by the test light by the wavelength filter or the optical line is received without passing through the wavelength filter and converted into an electrical signal. Specifically, the test light passes through the optical coupler 92 and enters the optical line 91 and is reflected by the wavelength filter 95. Return light reflected or scattered by the optical fiber under measurement including the wavelength filter 95, the optical coupler 92, the optical line 93_3, and the optical line 91 is incident on the port 15. The optical coupler 14 guides the return light to the light receiver 16. The light receiver 16 receives the return light and converts it into an electrical signal.

  From the optical fiber to be measured, background light and backscattered light corresponding to the output random code are received by the light receiver 16. The backscattered light modulated by the random code becomes a signal modulated around a certain level. On the other hand, the background light is modulated by the communication signal, but the bit rate is very high compared to the modulation frequency of the light source 13, so that it is limited to the OTDR reception band when converted into an electric signal by the light receiver 16. It becomes. For this reason, when the optical fiber to be measured has optical fiber characteristics as shown in FIG. 4, the electrical signal output from the light receiver 16 is as shown in FIG.

In the periodic signal extraction procedure, an AC component based on a random code is extracted from the electrical signal. Specifically, by AC coupling electrical signals from the light receiver 16 with a capacitor 17, removes the signal light S _C as a background light. The amplifier 18 amplifies the analog signal from the capacitor 17. The A / D converter 19 converts the analog signal from the amplifier 18 into a digital signal.

  In the correlation processing procedure, correlation processing between a random code and a periodic signal is performed. Specifically, the correlator 20 correlates the digital signal from the amplifier 18 and the random code from the random code generator 11.

  In the periodic signal extraction procedure, the low-frequency background light is cut by the capacitor 17. A part of the modulated background light is converted into a digital signal by the A / D converter 19 together with the backscattered light modulated by the random code. However, since the correlation processing procedure is performed, by acquiring the correlation between the digital signal and the random code generated by the random code generator 11, only the backscattered light modulated by the random code can be extracted. At this time, since some modulated background light has no correlation with the random code, only the backscattered light characteristic having the correlation is extracted. For this reason, even in an environment containing background light, it is possible to perform OTDR measurement without the influence of background light.

  FIG. 6 shows an example of an output waveform from the correlator 20. The characteristics of the optical fiber to be measured shown in FIG. 4 can be reproduced in two times except for the first and last of the four consecutive generations of random codes. In this way, by using test light in which random codes are generated three or more times consecutively, signal amplification is performed after AC coupling even when strong background light is present, so no wavelength filter is used. It is possible to perform an OTDR test at the time of in-service.

  In this embodiment, the example in which the test light is incident from the ONU side toward the OLT side has been described. However, the test light may be incident from the OLT side to the ONU side. In this case, the OTDR 101 is inserted between the optical coupler 92 and the wavelength filter 95.

  The present invention can be applied to the information communication industry.

11: Random code generator 12: Current controller 13: Light source 14: Optical coupler 15: Port 16: Light receiver 17: Capacitor 18: Amplifier 19: A / D converter 20: Correlator 91, 93_1 to 93_N: Optical line 92 : Optical couplers 94_1 to 94_N, 95: Wavelength filter 101: OTDR
96: OLT
97_1 to 97_N: ONU

Claims (4)

PON (Passive Optical Network) in which one optical line (91) is branched into a plurality of optical lines (93_1 to 93_N) by an optical coupler (92);
Wavelength filters (94_1 to 94_N, 95) that are disposed at end portions of the one optical line and the plurality of optical lines, and pass signal light transmitted by the PON and reflect light other than the signal light. ,
The test light that is arranged at any one of the one optical line and the ends of the plurality of optical lines and that is modulated with a random code of light having a reflected wavelength of the wavelength filter is incident on the optical coupler, and the test light The return light reflected or scattered by the wavelength filter or the optical line is received without passing through the wavelength filter, and the AC component by the random code is extracted from the electrical signal using the capacitor (17). OTDR (Optical Time-Domain Reflectometer) (101) for performing correlation processing with the random code after cutting the background light component ;
An optical line characteristic measuring system comprising: The OTDR is
Modulating the test light with a random code having a code length over a predetermined time longer than the time required for light to propagate through the optical line from the end of the one optical line to the ends of the plurality of optical lines. The optical line characteristic measuring system according to claim 1. One optical line (91) of the PON branched into a plurality of optical lines (93_1 to 93_N) by an optical coupler (92) from any one of the one optical line and the ends of the plurality of optical lines To the optical coupler, the test light obtained by modulating the light having the reflected wavelength of the wavelength filters (94_1 to 94_N, 95) arranged at the ends of the one optical line and the plurality of optical lines with a random code is incident. Test light incidence procedure;
A light receiving procedure for receiving the test light reflected or scattered by the wavelength filter or the optical line without passing through the wavelength filter and converting it into an electrical signal;
A periodic signal extraction procedure for extracting an AC component of the random code from the electrical signal using a capacitor (17) to cut a background light component ;
A correlation processing procedure for performing correlation processing between the random code and the periodic signal;
The optical line characteristic measuring method which has these in order. In the test light incident procedure,
Modulating the test light with a random code having a code length over a predetermined time longer than the time required for light to propagate through the optical line from the end of the one optical line to the ends of the plurality of optical lines. The optical line characteristic measuring method according to claim 3.

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