KR101053057B1 - In-service monitoring method and apparatus for optical transmission system - Google Patents

Abstract

In an optical communication system, an optical signal modulated with data is transmitted from an optical transmitter via an optical fiber transmission link. The present invention provides a method and apparatus for performing in-service monitoring of an optical transmission system by measuring a spraying distribution along an optical fiber transmission link based on correlation detection using a pseudorandom noise signal superimposed on the optical signal. .

Description

In-service monitoring method and apparatus of optical transmission systems

The present invention relates to a fiber optic communication system, and more particularly to a method and apparatus for in-service monitoring of an optical transmission system.

The distribution of back reflected light along the optical fiber transmission link is an important parameter in identifying problems in external plants. For this problem identification purpose, Optical Time-Domain Reflectometers (OTDRs) are widely used to measure this distribution. In making OTDR-based measurements, short light pulses are introduced into the optical fiber transmission link and, like lidar (light detection and ranging) systems, the reflected signal is measured as a function of time. Modern OTDR can provide enough spatial resolution and dynamic range for the characterization of the transmission link. Accordingly, many efforts have been made to utilize OTDR in monitoring in-service for various types of optical fiber transmission systems. In these techniques, the supervisory channel is used for OTDR pulses at a wavelength different from that of the transmitted signal so as not to interfere with other signal channels in service. However, applying this technique based on OTDR to PON systems is not straightforward. This is because additional optical filters are required to block OTDR pulses in all optical network units (ONUs), which can cause cost problems. In addition, strong OTDR pulses may adversely affect the performance of the channels being served via Raman scattering.

The problem becomes more serious when using conventional OTDR in a WDM network implemented using WDM multiplexer and / or demultiplexer located on the transport link. For example, in WDM Passive Optical Networks (PON) implemented using Arrayed-Waveguide Grating (AWG) at remote nodes, this technology uses drop fiber (which serves to connect the remote node to each subscriber). Can not be monitored because the OTDR pulse is blocked at the remote node. Some proposals have been made to address this issue by installing additional couplers to bypass the AWG at the remote node, using variable OTDR, or generating a OTDR pulse for a specific drop fiber using a corresponding WDM transmitter. Was done. However, in all these techniques, the problem is that the service of the corresponding WDM channel must be terminated while monitoring the state of the drop fiber.

One way to deal with the above problem is to use the optical signal itself as an optical probe instead of using OTDR pulses. In digital transmission, each '1' bit is essentially a small OTDR pulse. The reflected signal is a time delay overlap of those reflected from all bits sent in the past round trip time. Thus, by calculating the correlation between the data signal and the back reflected signal, the reflection point can be found. This idea has already been discussed in the prior art. One of them points out that this method is very difficult to apply to systems operating with high-speed data communications because of the need for large amounts of high-speed memory and extremely fast correlators. The prior art also points out that this method has severe limitations on the dynamic range. In this method, it is preferable that the autocorrelation function of the optical signal has a form such as a delta function. However, since the optical signal is simply modulated with the transmission data, its autocorrelation function is accompanied by large background noise. This background noise introduces a lethal limitation on the dynamic range of reflectometry. In order to suppress background noise caused by finite data length, the prior art proposed a discrete component elimination algorithm, which estimates the background noise accompanying discrete reflection points and reduces it recursively. Although the dynamic range is somewhat improved by this algorithm, it is still not enough to detect Rayleigh backscattering in optical fibers.

For reliable operation of optical communication systems, it is very important to identify problems in optical fiber transmission links such as increased fiber loss, fiber breaks and poor connections. Accordingly, an object of the present invention is to provide a method and apparatus for monitoring an optical fiber transmission link using only an optical signal source for data transmission, without using an additional optical signal source. In this case, since the optical signal itself is used as the probe light, the monitoring according to the present invention can be performed without interrupting the transmission service, and as a result, in-service monitoring is possible.

The present invention applies to a point-to-point system in which an optical signal from an optical transmitter is transmitted to a designated optical receiver. The invention also applies to point-to-multipoint networks, such as passive optical networks (PONs), where the optical signal from the optical transmitter is branched and distributed to many subscribers.

As mentioned above, the technical problem of the present invention is to provide a method and apparatus for monitoring the distribution of reflectivity in a more realistic way during service.

Unlike conventional OTDR, the proposed technique requires no source of short pulses at all, and only uses the data modulated optical transmitter itself. In order to utilize correlation detection, a pseudo-random-noise (PN) signal is superimposed on the optical signal to weakly modulate the amplitude of the optical signal into the PN signal. The distribution of reflectance along the light transmission system is obtained by calculating the cross correlation function between the PN signal and the back reflected signal. Unlike the method proposed in the related art, the present invention can use PN signals having excellent cross-correlation characteristics such as M-sequences (maximum-length sequences), which are shaped like delta functions having low background noise levels. Therefore, the dynamic range can be significantly improved. As a result, it is possible to solve the problem that occurs while using the conventional reflectance measuring system based on cross correlation detection.

The technique proposed in the present invention can utilize an existing optical transmitter with a slight change in superimposing a PN signal on the data. In addition, monitoring made during service can be realized at the signal wavelength and not at the monitoring wavelength. Therefore, the problem of using the OTDR of the prior art can be solved.

According to the present invention, an optical signal itself can be used as probe light to monitor an optical fiber failure during service. Thus, it is possible to diagnose a system failure even while providing services to other subscribers. Accordingly, the present invention has a superior effect compared to some prior arts in which service termination must be performed in advance and fault diagnosis must be performed.

In addition, in the present invention, since the optical transmitter is used as a source of probe light, a separate laser diode is not required. Thus, the present invention simplifies the construction of the device as compared to some prior art which must additionally have a variable laser source.

In the present invention, since the superimposed PN signal is used, any kind of PN signal can be used. As a result, it is possible to improve dynamic range and measurement accuracy by selecting PN signals with good correlation characteristics, such as M-sequences.

1 is a block diagram illustrating the method of the present invention. The optical signal is modulated into a data signal and a PN signal to enter the optical fiber transmission link. The back reflected signal and the PN signal are obtained using an A / D converter and then signaled to calculate the cross correlation function.
Figure 2 shows the waveform of the signals used in the present invention. (A) of FIG. 2 shows a data signal, (b) shows a PN signal, and (c) shows an optical signal generated from an optical transmitter.
3 is a block diagram illustrating another method of the present invention, wherein a reference signal for correlation detection is obtained using the output of an optical transmitter. The optical signal is modulated into a data signal and a PN signal to enter the optical fiber transmission link. The reference signal is obtained from the output of the optical transmitter using optical directional coupler-1. The light reflected back from the transmission link is extracted using light directional coupler-2. The back reflection signal and the reference signal are obtained using an A / D converter and then signaled to calculate the cross correlation function.
4 is a block diagram illustrating another method of the present invention, in which two optical couplers in FIG. 3 are integrated into one optical coupler.
5 shows a configuration diagram of a first embodiment of the present invention.
6 shows an eye diagram of the output of the optical transmitter obtained in the first embodiment.
FIG. 7 shows the bit error rate performance measured in the first embodiment. In FIG. 7, the rectangles and circles represent the results with and without the overlapping PN signals.
8 shows the measurement results obtained in the first example. Dotted lines represent cross correlation traces obtained without equalization. The solid line represents the cross correlation trace obtained by applying the FIR equalizer to the cross correlation trace.
9 shows a block diagram of a second embodiment of the present invention. The present invention is here implemented using the method shown in FIG.
10 shows the measurement results obtained in the second example.
Fig. 11 shows the construction of the third embodiment, in which the present invention is applied to a PON system. In a PON system, four ONUs, A, B, C and D, are connected to a remote node (RN) via drop fibers of different lengths. To simulate fiber failure, the drop fiber for ONU C is cut at point C '.
12 shows the measurement results obtained in the third example. The solid line represents the cross correlation measured before cutting the drop fiber of ONU C. The dashed line represents the cross correlation measured after cutting the drop fiber of ONU C at point C '.
13 shows the configuration of the fourth embodiment, where the present invention is applied to a WDM PON using an arrayed-waveguide grating (AWG) at a remote node.
14 shows the measurement results obtained in the fourth example.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following examples are only presented to understand the content of the present invention, and those skilled in the art will be capable of many modifications within the technical spirit of the present invention. Therefore, the scope of the present invention should not be construed as limited to these examples.

Prior to describing the preferred embodiment of the present invention, the configuration and overall operation of the present invention will be described below.

Detailed description of the working principle of the present invention

1 is a schematic structural diagram of an optical transmission system implemented using the present invention. The optical transmitter 110 is driven with a binary data signal from the data source 102 at a bit rate B to generate an optical signal. Also, for correlation detection, the PN signal generated using the PN-signal generator 104 is added to the data signal, which is superimposed on the optical signal. The chip rate of the PN signal is represented by C. (Hereinafter, one bit of the PN signal is represented by a chip to distinguish the data signal from the PN signal.) In principle, any kind of PN signal can be used in the method of the present invention. However, a finite length PN signal with good autocorrelation properties is suitable. An example of a PN signal that fits this method well is the M-sequence. Another example of a PN signal is the Gold sequence.

로 변환된다. 그 다음, 이 감지된 후방 반사 신호가 A/D 컨버터(150)를 이용하여 얻어진다. 마지막으로, 신호 처리 유닛(200)에서 교차 상관 함수에 기반을 둔 신호 처리에 의해 반사측정 기록이 얻어진다.(A), (b) and (c) of FIG. 2 are schematic diagrams showing the data signal D (t), the PN signal P (t), and the optical signal S (t) generated by the optical transmitter, respectively. In the optical signal, the PN signal is superimposed on the data signal, where the modulation amplitude of the PN signal is set to be small compared to the amplitude of the data signal so as not to disturb the data transmission. The optical signal then enters the optical fiber transmission link 120. To sense the back reflected signal, an optical directional coupler 130 is inserted between the optical transmitter 110 and the optical fiber transmission link 120. (This optical directional coupler 130 may be replaced by another optical element such as an optical circulator.) This back reflected signal is detected by a photo detector 140 to produce an electrical signal.

Is converted to. This sensed back reflected signal is then obtained using A / D converter 150. Finally, the reflection measurement record is obtained by signal processing based on the cross correlation function in the signal processing unit 200.

The signal processing sequence is explained in detail as follows. The principle of operation is similar to that of continuous wave random lidar. To illustrate this principle, consider first the case where an M-sequence of length m = 2 ⁿ −1 is used as the PN signal (where n is an integer greater than 2).

를 만족하는 정수이다), p_k의 자기 상관 함수는 다음 수학식 1을 만족한다.If you represent an M sequence of length m as p _k (where p _k has a value of +1 or -1, k is

), The autocorrelation function of p _k satisfies Equation 1 below.

Here, mod denotes a modulus function, and p _k is periodically defined as p _k = p _{k + m} . Referring to Equation 1, as the length m increases, the autocorrelation function approaches the delta function. By using p _k , the PN signal P (t) can be expressed as in Equation 2 below.

 Here, u (t) is a unit rectangular function defined by u (t) = 1 in -1/2 <t <1/2 and u (t) = 0 in another region. It should be noted that P (t) satisfies that the periodic function, that is, P (t) = P (t + m / C). In addition, the data signal D (t) is expressed as in Equation (3).

Here, d _k is binary data having a value of {-1, +1}.

The optical signal S (t) may be expressed by Equation 4 using D (t) and P (t).

Where q _D0 is the dc level, q _D and q _P are the amplitudes of the data signal component and the PN signal component, respectively. If the reflectivity distribution including the round trip loss is represented by R (z) (where z is the distance from the optical transmitter), the power of the signal reflected back toward the transmitter can be expressed as Equation (5).

는 컨볼루션(convolution) 연산을 나타내며, v_c 는 광섬유 내에서의 광의 군속도이다. 이어서, 이 후방 반사된 광은 광 검출기(photo detector; 140)에 의해 검출된다. 검출 후의 신호 전압

은 다음 수학식 6과 같이 표현될 수 있다.here

Denotes a convolution operation, and v _c is the group velocity of light in the optical fiber. This back reflected light is then detected by a photo detector 140. Signal voltage after detection

May be expressed as Equation 6 below.

는 광 검출기의 변환효율이다.here

Is the conversion efficiency of the photo detector.

와 P(t) 사이의 교차 상관 함수

는 수학식 7과 같이 주어진다.For cross correlation calculation, the PN signal is used as the reference signal.

Cross-correlation function between and P (t)

Is given by equation (7).

Here, <> represents an ensemble average and w ₀ is a dc offset.

Since the data signal and the PN signal are not correlated with each other, the second term on the right side is ignored. In addition, the dc offset is removed by dc-cut. Therefore, Equation 7 may be written as Equation 8.

는 P(t)의 자기 상관 함수로서,

에 대해서는

의 값을,

에 대해서는

의 값을 갖는다.

는

같은 델타 함수를 이용하여 잘 근사화될 수 있기 때문에, 수학식 3은

로 표시될 수 있다. 이와 같은 방식으로, 반사도 분포는

같은 교차 상관 함수

를 이용하여 추출해 낼 수 있다.here

Is the autocorrelation function of P (t),

About

The value of,

About

Has the value of.

Is

Since it can be well approximated using the same delta function,

It may be represented as. In this way, the reflectivity distribution is

Same cross correlation function

Can be extracted using

Since P (t) has periodicity, the ensemble mean can be replaced by a simple integration in a finite interval of m / C as shown in Equation (9).

로서, 여기서 j는 정수이며,

는 A/D 컨버터의 샘플링 시간이다. 예를 들어서,

는 각각

로 표현되어야 한다. 이 경우, 수학식 9는 다음 수학식 10과 같이 이산시간으로 표시된다.By using this equation, the data length to be sampled can be extremely reduced. In fact, the above signal processing is performed using digital signal processing. Thus, all sensed signals are a function of discrete time

Where j is an integer,

Is the sampling time of the A / D converter. For example,

Respectively

Should be expressed as In this case, Equation 9 is expressed as discrete time as in Equation 10 below.

이다. 따라서,

를 얻고 수학식 10을 이용하여

를 계산함으로써, 반사도의 분포는

와 같이 얻어질 수 있는데, 여기서

이다. 또한,

가 주기적이기 때문에, 수학식 10의 계산은 FFT(fast Fourier transformation)를 이용하여 행해질 수 있다. 만약 FFT 및 inverse FFT의 연산자들을 각각 F 및 F^{- 1} 로 표시한다면, 수학식 10에서의

는 다음 수학식 11에 의해 얻어질 수 있다.here

to be. therefore,

And using Equation 10

By calculating the distribution of reflectivity

Can be obtained as

to be. Also,

Since is periodic, the calculation of equation (10) can be done using fast Fourier transformation (FFT). If the operators of FFT and inverse FFT are represented by F and F ^{- 1} , respectively,

Can be obtained by the following equation (11).

 Here, the over-bar represents a complex conjugate. Using Equation 11, the calculation time can be dramatically reduced.

가 고주파 영역에서의 데이터 신호를 포함하기 때문에,

의 SNR은 감지 신호

의 밴드폭을 제한함에 의해 개선될 수 있다. 따라서, 광 감지기 바로 다음 또는 A/D 변환 후에 디지털 신호 처리에서 저역 통과 필터링이 이용된다. 마찬가지로, 저역 통과 필터링이 PN 신호

에 적용될 수 있다.

Since contains a data signal in the high frequency region,

SNR signal detection

It can be improved by limiting the bandwidth of. Thus, low pass filtering is used in digital signal processing immediately after the light detector or after A / D conversion. Similarly, low pass filtering of this PN signal

Can be applied to

을 여러 번 측정하고 교차 상관기(160) 처리한 후에 이들을 평균화기/등화기(170)에서 처리함으로써 개선될 수 있다. 또한, 교차 상관을 적용하기 전에 평균화를 행함에 의해서도

의 SNR을 개선하는데 있어서 효과적이다. 이러한 평균화 기법은 A/D 변환에서 야기된 양자화 잡음 및 광 감지기의 잡음과 같은 원하지 않는 잡음을 억제하는 데 효과적이다.The SNR of the measured reflectivity distribution is cross correlated, as shown in FIG. 1.

Can be improved by measuring several times and processing the cross correlator 160 and then processing them in the averager / equalizer 170. Also, by averaging before applying cross correlation

It is effective in improving the SNR. This averaging technique is effective in suppressing unwanted noise such as quantization noise and light detector noise caused by A / D conversion.

또는

에 등화기(equalizer; 170)를 적용함으로써 이를 쉽게 보상할 수 있다.The measured cross correlation function may be distorted by the incomplete response of the light transmitter 110 and the light detector 140. However, as shown in FIG. 1,

or

This can be easily compensated by applying an equalizer 170 to the.

로 표시한다면, 교차 상관은 다음 수학식을 이용하여 얻어질 수 있다.3 shows another embodiment of the present invention wherein a reference signal for correlation detection is obtained from the light output of the optical transmitter 110. Except for the method of obtaining the reference signal, the configuration is almost the same as that of FIG. The optical transmitter 110 is driven with a binary signal from the data source at bit rate B to generate an optical signal. Also, for correlation detection, the amplitude of the optical signal is modulated into the PN signal at chip rate C by superimposing the PN signal on the data signal. In the optical signal, the PN signal is superimposed on the data signal, and the modulation amplitude of the PN signal is set to be small compared to the amplitude of the data signal so as not to disturb the data transmission. Referring to the configuration of the device shown in FIG. 3, a portion of the optical signal is sensed by the light detector 140 ′ tapped by the light directional coupler-1. (This optical directional coupler-1 can be replaced by another optical element, such as a half mirror.) In order to detect the back reflected signal, optical light is transmitted between the optical transmitter 110 and the optical fiber optical transmission link 120. Directional coupler-2 is inserted. (This optical directional coupler-2 can be replaced by another optical element, such as an optical circulator.) The optical signal enters the optical fiber transmission link 120, and at the same time the back reflected light is transmitted to the optical detector 140. Is detected by an electrical signal. Then, the branched light signal and the sensed back reflection signal are obtained using the A / D converter 150. Finally, the distribution of reflectivity is obtained by cross correlation calculation in the same manner as the method used in FIG. If the branched optical signal after A / D conversion

If expressed as, cross correlation can be obtained using the following equation.

를 만족하므로, 반사도의 분포는

로부터 구할 수 있다. 도 3의 구성을 이용하는 이점은, 이 방법이 PN 신호 성분의 왜곡에 대해 강인하다는 것이다. 심지어 변조 파형의 왜곡이 광 전송기(110)에 도입되고(이는 광 전송기 내에 있는 레이저 다이오드에 직접 변조를 이용하는 경우에 특별히 발생할 수 있다) 광신호 내의 PN 신호 성분이 PN 신호 생성기에서 생성된 원래의 파형과 다르더라도, 왜곡에 기인한 측정 에러는 상관 감지에서 자동적으로 소거된다. 결과적으로, 정확도가 향상되게 된다.

Since the distribution of reflectivity is

Available from An advantage of using the configuration of FIG. 3 is that this method is robust against distortion of PN signal components. Even the distortion of the modulating waveform is introduced into the optical transmitter 110 (which can occur especially when direct modulation is used on the laser diode in the optical transmitter) and the PN signal component in the optical signal is generated by the PN signal generator. Although different from, measurement errors due to distortion are automatically canceled in correlation detection. As a result, the accuracy is improved.

로 평균화하고,

및

로 평균화함으로써 개선될 수 있다. 또한, 광송신기 및 광 감지기의 불완전한 응답에 의해 야기된 왜곡은

또는

, 또는 그 모두에 등화기(equalizer)를 적용함으로써 보상될 수 있다. 또한, 이 왜곡은 측정된

에 등화기를 적용함으로써 보상될 수도 있다.Similar to the method used in FIG. 1, the SNR of the measured reflectivity distribution uses a low pass filter for the tapped and back reflected optical signals,

Averaged to,

And

Can be improved by averaging In addition, the distortion caused by the incomplete response of the optical transmitter and the optical sensor

or

Can be compensated by applying an equalizer to, or both. This distortion is also measured

It may also be compensated by applying the equalizer to.

In FIG. 3, optical coupler 1 and optical coupler 2 may be integrated into one optical coupler 132 as shown in FIG. 4.

[First Embodiment]

Fig. 5 shows a schematic diagram of a first embodiment of the present invention, in which the present invention is applied to an optical line terminal in an optical fiber transmission link. This configuration corresponds to the configuration shown in FIG. A distributed-feedback (DFB) laser diode operating at 1550 nm was used as the optical transmitter and its output was directly modulated with 2.5-Gb / s non-return-to-zero (NRZ) data to demonstrate the service system. For correlation detection, the PN signal was added to the envelope of the signal and the optical transmitter as a pilot tone. 6 shows an example of an eye diagram of an optical signal. For correlation detection, the superimposed PN signals were M-sequences with lengths of 2 ¹⁵ -1 and chip speeds of 2.5 Mchip / s. Due to this superimposed signal, the high level of the eye diagram is divided into two. However, since the modulation depth of the overlapping signal was only ± 7%, the penalty for the corresponding receiver sensitivity was small. 7 shows receiver sensitivity measured for a downlink signal with and without an overlapping signal. The penalty incurred due to the overlap signal was measured as small.

The optical signal entered the optical fiber through the optical circulator. The average input power into the optical fiber was 0 dBm. To sense back reflected light, a conventional photodiode with a bandwidth of 20 MHz was used. The data and sense signals were filtered by 1.2-MHz LPFs and digitized with a 12-bit A / D converter at a sampling rate of 10 Ms / s. Then their cross-correlation function was calculated using a personal computer. In this example, spatial resolution was determined to be 66 m by the bandwidth of the LPF. The sampling point number of the A / D converter was 131068. The fiber optic transmission link consists of a 6.3-km single mode fiber and an optical receiver. To compensate for the incomplete response of the light detector, a fin-impulse response (FIR) equalizer was used. In FIG. 8, the solid line and the dotted line represent cross correlation functions with and without the FIR equalizer, respectively. As shown here, the results show that there are Fresnel reflections at both ends and Rayleigh scattering occurs in the optical fiber. In addition, it can be seen that the long tail caused by the imperfection of the light sensor response was cleanly removed by the use of an FIR equalizer.

Second Embodiment

9 shows an embodiment using the method shown in FIG. 1. A distributed-feedback (DFB) laser diode operating at 1550 nm was used as the optical transmitter and its output was directly modulated with 2.5-Gb / s non-return-to-zero (NRZ) data to demonstrate the service system. For correlation detection, a PN signal was added to the envelope of the optical transmitter and signal. For correlation detection, the superimposed PN signals were M-sequences with lengths of 2 ¹⁵ -1 and chip speeds of 2.5 Mchip / s. The optical signal entered the transmission link through the fiber coupler and the optical circulator. The average input power into the optical fiber was 0 dBm. In order to detect tapped light signals and back reflection signals, two conventional photodiodes have been used.

The sensed signals were filtered by 1.2-MHz LPFs and digitized with a 12-bit A / D converter at a sampling rate of 10 Ms / s. Then their cross-correlation function was calculated using a personal computer. In this example, spatial resolution was determined to be 66 m by the bandwidth of the LPF. The sampling point number of the A / D converter was 131068. The fiber optic transmission link consists of an 11-km single-mode fiber and an optical receiver. To compensate for the incomplete response of the light detector, a fin-impulse response (FIR) equalizer was used. The solid and dashed lines in FIG. 10 show the cross correlation function measured after averaging 400 trajectories. As shown here, the results show that there are Fresnel reflections at both ends and Rayleigh scattering occurs in the optical fiber.

Third Embodiment

11 shows a configuration used to verify monitoring in service state in a PON system. The dotted box indicates the construction of an optical line terminal, which is almost similar to that shown in FIG. A DFB laser diode operating at 1550 nm was used as the downlink optical transmitter and its output was directly modulated with 2.5-Gb / s NRZ data. For correlation detection, a PN signal was added to the envelope of the optical transmitter and signal. PN signals were superimposed for correlation detection. For correlation detection, the superimposed PN signals were M-sequences with lengths of 2 ¹⁵ -1 and chip speeds of 2.5 Mchip / s. The optical signal enters the PON through the fiber coupler and the optical circulator. The average input power into the feeder fiber was 0 dBm. In order to detect tapped light signals and back reflection signals, two conventional photodiodes have been used. The sensed signals were filtered by 1.2-MHz LPFs and digitized with a 12-bit A / D converter at a sampling rate of 10 Ms / s. Then their cross-correlation function was calculated using a personal computer. The PON consists of a 6.4-km feeder fiber, a 1x4 star coupler (used as a remote node), four connected drop fibers, labeled AD, and optical network units. The solid line in FIG. 12 shows the measured cross correlation function. From the steps in the measured Rayleigh scattering component, the remote node and four drop optical fibers can be clearly identified. Subsequently, the optical fiber C was intentionally cut to simulate the optical fiber failure at the point C 'as shown in FIG. Using conventional OLT, this failure would cause signal loss in the ONU connected to fiber C, which could not be distinguished by simply turning off the ONU or by a fiber failure. On the other hand, according to the present invention, the OTDR trace can be obtained without ending the service for the ONUs connected to the optical fibers A, B, and D, thereby identifying the root cause. For example, the dashed curve of FIG. 12 shows the OTDR trajectory measured after fiber cutting. The difference between before and after fiber cutting is also shown. The results clearly indicate that fiber C has a problem and reveal the location of the fiber cut.

[Example 4]

13 shows a configuration used to verify monitoring in service in a WDM PON system. The system under test has a central office (CO) consisting of optical transmitters, AWG, 6.3-km long feeder fiber, and remote node, where the remote node consists of AWG and 2.4-km long drop fiber. ONU was connected to the end of the drop fiber. These optical fibers had a propagation loss of 0.21 dB / km. The OLT is the same as used in the third embodiment. In general, in order to monitor failures in drop optical fibers using conventional OTDR, it was necessary to implement a bypass route for OTDR pulses or utilize tunable OTDR. In comparison, the present invention can obtain the OTDR trajectory as long as the downlink signal arrives. Thus, there is no need to use expensive variable OTDRs or change devices outside the WDM PON. FIG. 14 is an example illustrating the OTDR trajectory measured in the configuration shown in FIG. 13. From the measured OTDR trajectory, it is easy to identify the drop fiber connected to the RN.

102: data source 104: PN-signal generator
110: optical transmitter 120: optical fiber transmission link
130: optical directional coupler 132: optical directional coupler
140, 140 ': Light Detector 150: A / D Converter (Converter)
160: cross correlator 170: averager / equalizer
200: signal processing unit

Claims (14)

A method for in-service monitoring an optical transmission system by measuring a distribution of reflectance along an optical fiber transmission link coupled to an optical transmitter, the method comprising:
Generating a pseudo random noise (PN) signal;
Superimposing a pseudo random noise signal on an output optical signal of the optical transmitter;
Detecting a back reflected optical signal back to the optical transmitter and converting it into an electrical signal;
Cross-correlation function between the back-reflected optical signal detected using the pseudorandom noise signal as a reference signal

And a fourth step of calculating

Is time;
Time the horizontal axis of the calculated cross-correlation function

Is converted into the distance z from the optical transmitter

A fifth step of obtaining the reflectance distribution as

Is the group velocity of light in the optical fiber;
In-service monitoring method of an optical transmission system comprising a. The in-service monitoring method of claim 1, wherein the pseudorandom noise signal used in the fourth step is directly obtained by dividing the signal generated in the first step. The in-service monitoring method of claim 1, wherein the pseudorandom noise signal used in the fourth step is obtained by dividing an output of the optical transmitter and sensing the power of the divided signal. The in-service monitoring method of claim 1, wherein in the first step, M-sequences are used as pseudorandom noise signals. The method of claim 1, wherein the Gold sequence is used as a pseudorandom noise signal in the first step. The method of claim 1, further comprising the step of averaging the measured cross-correlation function or reflectance distribution to improve the SN ratio of the measured reflectivity. The method of claim 1, further comprising the step of averaging the sensed back reflected light signal to improve the SN ratio of the measured reflectivity. An apparatus for in-service monitoring of a failure in an optical transmission system including an optical fiber transmission link connected to an optical transmitter modulated with a data signal by measuring reflectivity distribution along the link,
Signal generating means for generating a pseudo random noise (PN) signal;
Optical signal generating means for generating an optical signal and modulating the optical signal into a data signal and a pseudo random noise signal;
Optical directional coupling means positioned between the optical signal generating means and the optical fiber transmission link to pick up a back reflection signal from the optical fiber transmission link back to the optical transmitter;
Optical sensing means for sensing the back reflection signal as an electrical signal;
Signal sensing means for obtaining a pseudo random noise signal as an electrical signal;
Data acquisition means for converting the detected back reflection signal and the pseudorandom noise signal;
Cross-correlation function between pseudorandom noise signal and detected back reflection signal

, Computed cross-correlation function

The horizontal axis of time

Is converted into the distance z from the optical transmitter

Signal processing means for obtaining a reflectance distribution as

Is the group velocity of light in the optical fiber;
In-service monitoring device of the optical transmission system comprising a.
Claim 9 was abandoned upon payment of a set-up fee. 9. The in-service monitoring apparatus of the optical transmission system according to claim 8, wherein the optical directional coupling means is an optical circulator. Claim 10 was abandoned upon payment of a setup registration fee. 9. The in-service monitoring apparatus of the optical transmission system according to claim 8, wherein the optical directional coupling means is an optical coupler. The apparatus of claim 8, wherein the signal sensing means is an electrical divider connected to an output of the optical signal generating means. 9. The apparatus of claim 8, wherein the signal sensing means comprises an optical directional coupler located between the optical signal generating means and the optical fiber transmission link, and optical sensing means for sensing a pseudo random noise signal superimposed on the optical signal. In-service monitoring device of the optical transmission system. Claim 13 was abandoned upon payment of a registration fee. 9. The apparatus of claim 8, wherein the optical fiber transmission link comprises an optical network. Claim 14 was abandoned when the registration fee was paid. 10. The apparatus of claim 8, wherein the optical fiber transmission link comprises a passive optical network (PON) having a drop optical fiber, a feeder optical fiber coupled to a remote node and a subscriber.

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