JP2672988B2 - Method of treating backscattered light in optical fiber - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial field of application> The present invention relates to a method of processing backscattered light in an optical fiber using an optical code string, and is applicable to, for example, a method of detecting an optical fiber break point.

<Prior Art> OTDR (Optical Time Domein) that performs code compression based on the autocorrelation of the code sequence to improve the dynamic range.
Reflectometr) is being studied, and the optical barker code (P HEALEY: 'Pulse compr
ession coding in optical time domeinreflectometl
y ', in Proc.7th Eur.Conf.Opt.Commun. (Copenhagen, De
nmark), 1981) is known. Optical Barker code is N
In the binary code string X = {x ₁ , x ₂ , ... x _N } of 0, 1 of bits, the autocorrelation function ρ (j) is a code string that satisfies the condition shown in the following equation (4). is there.

Here, n is the autocorrelation of j = 0, that is, 0 bit shift, and is the number of codes “1” in the code string X. j =
At least one of ρ (j) at the time of ± 1, ± 2, ..., ± (N−1) is 1. FIG. 9 shows an example of the autocorrelation of a 7-bit optical Barker code {1,1,0,0,1,0,1}. In this example, n = 4, and ρ (j) is 1 when j = ± 1, ± 2, ..., ± (N−1).

Next, the 7-bit optical Barker code {1, 1, shown in FIG.
OTDR using 1,0,0,1,0,1} is shown in FIG. As shown in the figure, in this OTDR, the pulse width is τ and the codes 0, 1
The corresponding amplitude 0, A _O of the light ASK (amplitude shift keying; Amplitud
e Shift Keying) A modulated optical pulse is incident on the measured fiber and backscattered light is observed. That is, the optical pulse emitted from the light source 1 provided in the OTDR is input to the measured fiber 13 via the optical connector 4, and the measured fiber 13
The backscattered light returning from is passed through the optical connector 4 and the directional coupler 2 and is received by the light receiver 3. Now the speed of light in the fiber and c _r, a longitudinal resolution of the optical fiber becomes c _r (τ / 2) is simply. The backscatter of the m-th point from the incident end when separated longitudinally resolution fiber c _r (τ / 2), a pulse width tau, when the s _m with respect to the incident pulse amplitude A _o, The backscattering amount p (mτ) after mτ seconds from the time when the pulse of the first bit is incident on the fiber is detected by the sampler as in the following expression (5). Incidentally, FIG. 11 is an explanatory diagram of the backscattered light when the code string {1,1,0,0,1,0} is incident.

p (mτ) + s _m + s _m-1 + s _m-4 + s _m-6 (5) Thereafter, the code string sent from the controller 8 {1,1,0,
Correlation between 0,1,0,1} and backscattered light that passed through the averager 6. Is obtained by the correlator 7 as shown in the following equation (6).

s _m-6 ＋ s _m-5 ＋ s _m-4 ＋ s _m-3 ＋ s _m-2 ＋ s _m-1 ＋ 4s _m ＋ s _{m + 1} ＋ s _{m + 2} ＋ s _{m + 3} ＋ s _{m + 4} ＋ s _{m + 5} ＋ s _{m + 6} (6) The correlation represented by the equation (6) coincides with the backscattered light from the m-th point from the incident end when the pulse based on the autocorrelation shown in FIG. 9 is incident on the fiber. Since the amplitude of the pulse shown in FIG. 9 is four times as large as the power transmitted in the original pulse train, that is, the number of the code 1, the dynamic range is expected to be improved by 6 dB. Further, the ratio of the peak intensity of the autocorrelation to the transmitted power (the number of code “1”) is 1, which is extremely efficient. In FIG. 10, 9 is an oscilloscope.

<Problems to be Solved by the Invention> However, since the pulse of FIG. 9 has a side lobe of amplitude 1 around 6τ seconds from the center, the backscattering amount at the n-th point from the entrance end of the fiber is , As shown in the equation (6), the total amount of backscattering is about 6 × c _r (τ / 2) in terms of the distance in the longitudinal direction of the fiber with that point as the center. Therefore, the backscattered light from the desired point of the fiber is hidden by the sum of the backscattered light around that point, and when trying to detect the break point position of the optical fiber by the backscattered light, the distance resolution is a single pulse. 13 times compared to the time of, that is, 13 ×
It falls to c _r (τ / 2).

An object of the present invention is to provide a method of processing backscattered light in an optical fiber which can improve the dynamic range without deteriorating the distance resolution.

<Means for Solving the Problem> In the present invention, a Golay code represented by a binary code of ± 1 is used.
An optical pulse compression code having a side lobe of 0 after autocorrelation is realized by providing a means for exchanging the optical pulse train represented by a binary code of 0,1.

Embodiments Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the following examples, the present invention is applied to the optical fiber breaking point detection method.

Embodiment 1 FIG. 1 shows a flowchart of signal processing in this embodiment. As shown in the figure, first, the Golay code A
= {A _i }, B = {b _i } (i = 1,2, ..., N) is generated. In the Golay codes A and B, the sum of the respective autocorrelations satisfies the condition shown in the following expression (7).

Here, {a _i } = {1,1,1, -1, -1, -1, -1, as a code string
Using 8-bit Golay codes of 1, -1,} and {b _i } = {1,1,1, -1,1,1, -1, -1,1}, their autocorrelation and their sum are shown in FIG. The result is shown in. That is, the side lobe of the sum of the two autocorrelations becomes 0, and the total number of A and B bits is 16
Since the central peak value after correlation is 16 times for a bit, the ratio of the ratio of the central peak value of autocorrelation to the transmitted power (the number of the code “1”) is 1, which is extremely efficient.

Next, for each element a _i , b _i of the code sequence A, B, 0, 1
Conversion processing to the code string of. First, as shown in the following equation (8), based on the code string A, the following two binary code strings V and X of 0 and 1
Generate

V = {v ₁ } = {a ₁ +1) / 2} = {1,1,1,0,0,0,1,0} X = {x ₁ } = {(1-a ₁ ) / 2} = {0,0,0,1,1,1,0,1} (8) Then, the backscattered light of each of these code strings V and X is observed. That is, when the light pulse ASK-modulated with the pulse width τ based on the code string V is incident on the observed fiber and the backscattered light is observed τ seconds later, the interval between the scattering points in the fiber is as described above. It looks like c _r (τ / 2) apparently. At this time, the backscattered light after mτ seconds by the code string V
p ₁ (mτ) is as shown in FIG. 12 (a).

p ₁ (mτ) = s _m + s _m-1 + s _m-2 + s _m-6 (9) Then, the correlation between the code string A and the backscattered light p ₁ Is in agreement with the correlation between the code strings A and V shown in FIG.

c ₁ (mτ) = s _m-6 ＋ s _m-5 ＋ s _m-4 −s _m-3 ＋ s _m-1 ＋ 4s _m −s
_{m + 2} −3s _{m + 3} −s _{m + 4} −s _{m + 5} −s _{m + 7} (10) Similarly, the backscattered light p ₂ (mτ) by the code string X is shown in FIG. 12 (b). Thus, p ₂ (mτ) = s _m-3 + s _m-4 + s _m-5 + s _m-7 . Correlation between code sequence A and backscattered light Is in agreement with the correlation between the code sequences A and X shown in FIG.

c ₂ (mτ) = s _m-7 + s _m-6 + 2s _m-5 −s _m-4 +2 _m-2 −4s _m −s _{m + 1} −s _{m + 2} −s _{m + 4} … (11) c The difference p _a between ₁ and c ₂ is in agreement with the autocorrelation of the code string A shown in FIG.

p _a (mτ) = c ₁ (mτ) −c ₂ (mτ) = −s _m-7 −s _m-6 −3s _m-3 + s _m-1 + 8s _m + s _{m + 1} −3s _{m + 3} −s _{m + 5} −s _{m + 7} (12) Next, similar processing is performed for one code string B of the Golay code according to FIG. Code string B = {1,1,1, −1,
When the following two code strings Y and Z are generated based on 1,1, −1,1} and their backscattered lights p ₃ and p ₄ are observed, FIG. 12 (c)
As shown in FIG.

_{Y = {y 1} = {} (b 1 +1) / 2} = {1,1,1,0,1,1,0,1} Z = {z 1} = {(1-b 1) / 2 } = {0,0,0,1,0,0,1,0} ... (13 ) p 3 (mτ) = s m + s m-1 + s m-2 + s m-4 -s m-5 + s m _-7 … (1
4) p ₄ (mτ) = s _{m + 3} + s _{m + 6} (15) Correlations c ₃ and c ₄ between these and the code sequence B are shown in FIG. 3 (c).
Matches the correlation between the code examples Y and Z shown in (d) and the code string B,
It becomes like the following formula respectively.

c ₃ (mτ) = s _m-7 ＋ s _m-6 ＋ 2s _m-5 ＋ s _m-4 ＋ 3s _m-3 ＋ 2s _m-2 ＋ s _m-1 ＋ 6s _m ＋ s _{m + 1} ＋ 2s _{m + 3} ＋ s _{m + 4} ＋ s _{m +5} ＋ s _{m + 7} … (16) c ₄ (mτ) ＝ s _m-6 ＋ s _m-5 ＋ s _m-4 ＋ 2s _m-2 ＋ 2s _m-1 −2s _m ＋ 2s _{m + 1} ＋ s _{m + 2} −s _{m +3} + s _{m + 4} (17) The difference p _b between c ₃ and c ₄ is in agreement with the autocorrelation of the code string B shown in FIG.

p _b (mτ) = c ₃ (mτ) −c ₄ (mτ) = s _m-7 + s _m-6 + 3s _m-3− s _m-1 + 8s _m− s _{m + 1} + 3s _{m + 3} + s _{m + 5} + S _{m + 7} (18) Finally, the side lobes are canceled by the sum of p _a and p _b , and the following equation is obtained, which agrees with Fig. 2 (c).

p (mτ) = p _a (mτ) + p _b (mτ) = 16s _m (19) As is clear from the above signal processing results, by using a code string in which the side lobe of autocorrelation is 0,
It is possible to obtain the backscattered light only at a desired point in the fiber without including the components of the backscattered light from the area before and after that point, and to obtain the backscattered light for the transmitted bit number 8 × 4 = 32 bits. Since the intensity is 16 times as a result of pulse compression, the ratio of the ratio of the peak intensity of the autocorrelation to the transmitted power (the number of 1s) is 1, which is extremely efficient.

When such p (mτ) is continuously calculated for each point, it is reflected at the boundary between the air and the fiber at the breaking point of the optical fiber, and a large power of about 100 times is observed. Since the location and the fiber loss will be different,
It turns out that there is a break.

Embodiment 2 FIG. 4 shows a flowchart of signal processing in another embodiment. In the first embodiment, four types of code examples of 0 and 1 generated from Golay codes are time-division multiplexed.
This embodiment is characterized by wavelength multiplexing. In this way, as in the first embodiment, the backscattered light from only an arbitrary point of the fiber can be obtained without including the components of the backscattered light from the area before and after that point, and the number of transmitted bits is 8 Since the intensity of backscattered light is 16 times as a result of pulse compression for x4 = 32 bits, the ratio of the ratio of the peak intensity of the autocorrelation to the transmitted power (the number of 1) is 1, which is extremely efficient. In addition, this method can shorten the measurement time as compared with the first embodiment. The processes other than the frequency multiplexing and the frequency separation are the same as those in the first embodiment, and the description thereof will be omitted (the same applies hereinafter).

Embodiment 3 FIG. 5 shows a flowchart of signal processing in another embodiment. In the first embodiment, ASK modulation was performed based on four types of 0,1 code examples generated from Golay codes, but in the present embodiment, FSK (frequency shift keying: frequency shif) is used.
t keying) modulation. As in the first embodiment, the backscattered light from only an arbitrary point in the fiber can be obtained without including the components of the backscattered light from the regions before and after that point, and the number of transmitted bits is 8 × 4 = 32. Since the intensity of the backscattered light is 16 times that of the bit as a result of pulse compression, the ratio of the peak intensity of the autocorrelation to the transmitted power (the number of 1s) is 1, which is extremely efficient.

Embodiment 4 FIG. 6 shows a flow chart of signal processing in another embodiment. In the first embodiment, ASK modulation and time division multiplexing were performed based on four kinds of 0,1 code strings generated from Golay codes, but in the present embodiment, four FSK-modulated code strings are wavelength-multiplexed. And As in the first embodiment, the backscattered light from only an arbitrary point in the fiber can be obtained without including the components of the backscattered light from the regions before and after that point, and the number of transmitted bits is 8 × 4 = 32. Since the intensity of the backscattered light is 16 times that of the bit as a result of pulse compression,
The ratio of the ratio of the peak intensity of the autocorrelation to the transmitted power (the number of 1) is 1, and the efficiency is extremely high. In addition, this method can shorten the measurement time as compared with the first embodiment.

Fifth Embodiment FIG. 7 shows a flow chart of signal processing in another embodiment. Although using a code sequence of 0 and 1 of the four generated from Gore code In Example 1, the "+1" to f ₁ the frequency of light in Gore codes in the present embodiment, "-
1 ”is FSK modulated to f ₂ and backscattered light is frequency-separated after observation. Similar to the first embodiment, the backscattered light from only an arbitrary point of the fiber can be obtained without including the components of the backscattered light from the regions before and after that point, and the number of transmitted bits is 8 × 4 = 32. Since the intensity of the backscattered light is 16 times that of the bit as a result of pulse compression, the ratio of the ratio of the peak intensity of the autocorrelation to the transmitted power (the number of 1s) is 1, which is extremely efficient. In addition, this method can shorten the measurement time as compared with the first embodiment.

Sixth Embodiment FIG. 8 is a flow chart of signal processing in another embodiment. In the first embodiment, four kinds of 0,1 code strings generated from the Golay code were used, but in the present embodiment, the codes “+1” and “−1” in the Golay code are FSK-modulated to different light frequencies, In addition, two types of code strings are frequency-multiplexed, and backscattered light is frequency-separated after observation. As in the first embodiment, the backscattered light from only an arbitrary point in the fiber can be obtained without including the components of the backscattered light from the regions before and after that point, and the number of transmitted bits is 8 × 4 = 32. Since the intensity of the backscattered light is 16 times that of the bit as a result of pulse compression, the ratio of the peak intensity of the autocorrelation to the transmitted power (the number of 1s) is 1, which is extremely efficient. In addition, this method can shorten the measurement time as compared with the first embodiment.

<Effects of the Invention> As described above in detail with reference to the embodiments, the present invention uses the Golay code represented by the binary code sequence of ± 1 as 0,1.
The side lobe after autocorrelation becomes 0, because the optical pulse train is a binary code sequence of. Therefore, the dynamic range can be improved without deteriorating the distance resolution when detecting the broken position of the optical fiber by the backscattered light.

[Brief description of the drawings]

FIG. 1 is a flow chart showing a pulse compression procedure in the first embodiment of the present invention in which the Golay code is applied to an optical pulse tester, and FIGS. 2 (a), (b) and (c) are Golay codes used in the present invention. Graph showing an example of the autocorrelation of
FIGS. 3 (a), (b), (c), and (d) are graphs showing the correlation generated in the processing process when the Golay code shown in FIG. 2 is used in the present optical compression method, and FIG. FIG. 5 is a flow chart showing the pulse compression procedure in the second embodiment of the present invention in which the code is applied to the optical pulse tester, and FIG. 5 is the pulse compression in the third embodiment of the present invention in which the Golay code is applied to the optical pulse tester. Flowchart showing the procedure, No. 6
FIG. 7 is a flow chart showing the pulse compression procedure in the fourth embodiment of the present invention in which the Golay code is applied to the optical pulse tester, and FIG. 7 is the fifth embodiment of the present invention in which the Golay code is applied to the optical pulse tester. 8 is a flow chart showing a pulse compression procedure in FIG. 8, FIG. 8 is a flow chart showing a pulse compression procedure in a sixth embodiment of the present invention in which a Golay code is applied to an optical pulse tester, and FIG. 9 is used in a conventional optical pulse tester. Fig. 10 is a graph showing an example of the autocorrelation of the optical Barker code that is used, Fig. 10 is a block diagram of the OTDR that performs code compression based on the autocorrelation of the code sequence, and Fig. 11 is an optical pulse tester that uses the optical Barker code. Explanatory drawing showing backscattered light in a certain region when there is a light, FIG. 12 (a) (b) (c)
(D) is the code string V, X, used in the first embodiment of the present invention.
FIG. 6 is an explanatory diagram showing backscattered light in a certain Y, Z region. In the drawing, 1 is a light source, 2 is a directional coupler, 3 is a light receiver, 4 is an optical connector, 5 is a sampler, 6 is an averager, 7 is a correlator, 8 is a controller, 9 is an oscilloscope, and 13 is a fiber under test. Is.

Claims (6)

(57) [Claims] 1. Two kinds of code strings A = {a _i }, B each consisting of N codes having a value of "+1" or "-1" and satisfying the condition of the following expression (1). = {B _i } (i = 1,2, ..., N), the four kinds of code strings V = {v _i }, X = {x _i } are calculated by the following formula (2). , Y ＝ {y _i }, Z ＝
{Z _i } (i = 1,2, ..., N) is generated, and based on the code string
An ASK-modulated optical pulse train having a pulse width τ is incident on the measured fiber, and the backscattered light quantity p ₁ (corresponding to each code sequence V, X, Y, Z obtained t seconds after the optical pulse is incident t), p ₂
A method for processing backscattered light in an optical fiber, which comprises subjecting (t), p ₃ (t), and p ₄ (t) to signal processing represented by the following expression (3). 2. The backscattered light which is wavelength-multiplexed with two or more kinds of the code strings V, X, Y, and Z of claim 1 and is incident on the fiber under test at the same time, and which returns the backscattered light. By separating the wavelengths by a filter, the backscattered light p ₁ (t), p
_A method for treating backscattered light in an optical fiber, which comprises obtaining backscattered light corresponding to a code string wavelength-multiplexed among ₂ (t), p ₃ (t), and p ₄ (t). 3. The code string V = according to claim 1.
{V _i }, X = {x _i }, Y = {y _i }, Z = {z _i } (i = 1,2, ...,
Based on N), an FSK-modulated optical pulse train with a pulse width τ is incident on the fiber under measurement, and each code sequence is incident, and after t seconds, coherent detection is performed to obtain the backscattered light amount p.
₁ (t), p ₂ (t), p ₃ (t), p ₄ (t) is subjected to signal processing represented by equation (3) Processing method. 4. The code string V = according to claim 1.
{V _i }, X = {x _i }, Y = {y _i }, Z = {z _i } (i = 1,2, ...,
Based on N), two or more types of code trains of FSK-modulated optical pulse trains of pulse width τ are wavelength-multiplexed and simultaneously incident on the measured fiber, and the backscattered light that returns is wavelength-separated by an optical filter, Also, the backscattered light amount p ₁ (t), p obtained by coherent detection t seconds after each code string is incident
_A method of processing backscattered light in an optical fiber, which comprises subjecting ₂ (t), p ₃ (t), and p ₄ (t) to signal processing represented by formula (3). 5. The Golay codes A and B according to claim 1.
, The optical pulse trains of the pulse width τ which are FSK-modulated at f ₁ and f ₂ at the optical frequencies respectively corresponding to the symbols “+1” and “−1” are independently incident on the measured fiber, and the symbol A frequency f ₁ of the light, f ₂ of the component, respectively p among the backscattered light by the FSK modulated pulse train on the basis of
₁ (t), p ₂ (t), and among the backscattered light by the FSK-modulated pulse train based on the code B, the light frequencies f ₁ and f ₂
The components are p ₃ (t), when p ₄ and (t), p ₁
For (t), p ₂ (t), p ₃ (t), p ₄ (t), equation (3)
A method of processing backscattered light in an optical fiber, characterized by performing signal processing represented by. 6. The Golay codes A and B according to claim 1.
In the code A, the code A is FSK modulated to f ₁ and f ₂ at the light frequency, and the code B is fSK modulated to f ₃ and f ₄ at the light frequency. An optical pulse train having a pulse width τ is simultaneously incident on the measured fiber, and components of optical frequencies f ₁ , f ₂ , f ₃ and f ₄ in the returning backscattered light are respectively p ₁ (t) and p ₂ (T), p ₃ (t), p ₄ (t)
, P ₁ (t), p ₂ (t), p ₃ (t), p ₄ (t)
A method for processing backscattered light in an optical fiber, characterized in that the signal processing represented by the formula (3) is applied to.