JPH0658292B2 - Optical fiber splice loss measurement method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention is used for measuring an optical fiber for communication.

In particular, pulsed light is incident from one end of the measured optical fiber in which two or more optical fibers are connected in cascade, and by measuring the backscattered light generated in this measured optical fiber,
It relates to a method for measuring the longitudinal distribution of splice loss and light loss.

[Conventional technology]

Generates in the measured optical fiber by injecting pulsed light from one end of the measured optical fiber in order to measure the splice loss of the measured optical fiber including the splice part, the measurement of loss distribution, the detection of break points, etc. Then, the method of measuring the backscattered light returning to the incident end side is used.

That is, the power of the backscattered light is observed on the time axis and the amount of change is plotted as a logarithmic value. The waveform obtained by this is called a backscattering waveform. In this backscattering waveform, the position where backscattering occurs can be specified by the time after the pulsed light is incident. Further, since the backscattered light reciprocates in the same path in the optical fiber to be measured, when the backscattered light power P _bs returning to the incident end side is plotted as a logarithmic value, The calculation process is performed so that the loss amount of the optical fiber to be measured and the change amount of the backscattering waveform have a one-to-one correspondence.

The measurement method using such backscattered light has the advantage that the characteristics of the optical fiber to be measured can be evaluated by measurement from one direction, and an optical pulse tester using this principle is used for the construction and maintenance of optical communication lines. Has become essential.

However, the amount of change in the backscattering waveform and the amount of loss of the measured optical fiber do not completely correspond to each other, and the backward propagation factor S and the Rayleigh scattering loss α of each optical fiber are not matched.
affected by _s .

The backward propagation factor S is a value indicating the ratio of the power propagating to the incident side of the Rayleigh scattered light generated in the optical fiber. This value depends on the optical fiber parameters such as the numerical aperture NA of the optical fiber and the refractive index n ₁ of the core.
In the case of single mode fiber, Given in. Here, w ₀ is the beam spot size, a is the core diameter, and V is the normalized frequency.

For this equation, see E. Brinkmeyer, “Backscattering in Single-Mode Fibers,” Electronic Letters, Volume 16 (1980). ) Pp. 329-330.

Therefore, when optical fibers having different back propagation factors S or Rayleigh scattering loss α _s are connected in cascade,
The amount of step difference (in dB) of the backscatter waveform is different from the splice loss of the optical fiber under test.
It includes a component caused by the difference between the backward propagation factor S and the Rayleigh scattering loss α _s . Below, the product of the backward propagation factor S and the Rayleigh scattering loss α _s is referred to as the backward scattering coefficient. Due to this difference in backscattering coefficient, the true amount of splice loss cannot be obtained only by the backscattering waveform observed from one direction.

FIG. 9 shows an example of a backscattering waveform when connecting optical fibers having different backscattering coefficients.

In order to solve the above-mentioned problem, conventionally, the backscattering waveform was observed bidirectionally. That is, the first optical fiber f
_The pulsed light is propagated in the direction from ₁ to the second optical fiber f _{2 and} the backscattering waveform is observed.
Calculate ₁₂ [dB]. Next, the pulsed light is propagated in the opposite direction to observe the backscattered waveform, and the step amount D _{21 in} this direction is measured.
Calculate [dB]. Then, these two step amounts D ₁₂ , D
_The true connection loss amount is obtained by taking the average value of ₂₁ . Below, the case where there is a loss observed at the connection point is referred to as the positive step amount, and the opposite case is referred to as the negative step amount.

[Problems to be solved by the invention]

However, in the measurement method of the conventional example, it is necessary to transport the measuring instrument to the positions of both ends of the optical fiber to be measured, and it is necessary for the measurers to measure the connection loss while communicating with each other at the positions of both ends, which is very large. It has the drawback of requiring labor and time. In particular, the subscriber optical line connecting the telephone station and the subscriber's home requires one or more optical fibers for the subscriber's home, and the number of optical fibers is extremely large. Therefore, it is practically impossible to measure the splice loss from both ends of the measured optical fiber for all the optical fibers.

Therefore, the allowable value of the splice loss observed from one end (telephone station) of the optical fiber under test includes the spurious splice loss caused by the difference in the backscattering coefficient in addition to the spurious loss amount that can be truly allowed. A method is known in which the quality of the connection is determined by measuring from one end. For example, if the connection loss that can be actually tolerated is 0.3 dB and the apparent connection loss due to the difference in backscattering coefficient is 0.5 dB, the loss observed as a step in the backscattering waveform is up to 0.8 dB. Tolerate.

However, in this method, when observed from the optical fiber side with a small backscattering coefficient, the step difference at the connection point due to the difference in backscattering coefficient becomes negative. For this reason, there is a drawback that the judgment is passed even when the connection state of the optical fiber is bad and there is actually a connection loss of 0.3 dB or more.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above problems and provide a method for measuring the optical fiber connection loss with high accuracy by the measurement performed from one end of the measured optical fiber.

[Means for solving problems]

The present invention is an optical fiber connection loss measuring method, in which pulsed light is incident from one end of a measured optical fiber in which a plurality of optical fibers are connected in cascade, and on the time axis of backscattered light generated in the measured optical fiber. Of the backscattered light using the pulsed light of the first wavelength λ _{1 in} the optical fiber splice loss measuring method of observing the waveform of the above and measuring the splice loss of the optical fiber from the step difference at the splicing point of the observed waveform. The step difference of the waveform vs. the numerical value α _bs (λ ₁ ) is measured, and this first wavelength λ
₁ is different from the second wavelength lambda ₂ of the backscattered light waveform obtained by using pulse light step amount logarithm α _{bs (λ 2)} is measured, the first optical fiber under test wavelength lambda ₁ The connection loss logarithmic value α _J (λ ₁ ) in It is characterized in that it is estimated by the calculation of.

[Work]

The optical fiber splice loss measuring method of the present invention observes a backscattering waveform by using pulsed lights of two different wavelengths, and a logarithm of the step difference of each waveform (hereinafter simply referred to as “step difference”).
By removing the difference, the component due to the difference in the backscattering coefficient is removed. Furthermore, the true splice loss at one wavelength is estimated from the mutual relationship between the step amounts of the two wavelengths.

While two wavelengths can be selected arbitrarily, using the operating wavelength of the practical use of the optical fiber to be measured as the first wavelength lambda _1, the first wavelength as the second wavelength lambda ₂ lambda ₁
Different wavelengths are used. First wavelength λ ₁ and second wavelength λ
The difference from ₂ is determined by the measurement accuracy and the value of the wavelength dependence coefficient k (λ) of the axis deviation loss, and a difference of about 10% or more of the first wavelength λ ₁ is required. In particular, in normal optical communication, 1.
Since wavelengths in the 3 μm band and wavelengths in the 1.55 μm band are used, it is desirable to use these wavelengths in terms of the configuration of the measuring device.

〔Example〕

FIG. 1 is a block diagram of an optical fiber connection loss measuring instrument according to an embodiment of the present invention.

The pulse generator 2 generates a pulse signal synchronized with the timing signal from the signal processing unit 1. The switch 3 supplies this pulse signal to one of the light sources 4 and 5. With this pulse signal, the light source 4 generates pulsed light of wavelength λ ₁ ,
The light source 5 generates pulsed light of wavelength λ ₂ . This pulsed light is made incident on the measured optical fiber 8 in which the two optical fibers f ₁ and f ₂ are connected at the connection point J via the optical switch 6 and the optical directional coupler 7. The switch 3 and the optical switch 6 operate in synchronization.

A part of the Rayleigh scattered light generated in the measured optical fiber 8 returns to the incident end as the back scattered light, and the optical directional coupler 7
It is incident on the photodetector 9 via. The output of the photodetector 9 is amplified by the amplifier 10 and supplied to the signal processing unit 1.

The signal processor 1 displays the backscattering waveform on the CRT display device based on the detection signal level of the backscattered light from the optical fiber 8 to be measured. Further, the signal processing unit 1 _{compares the} step amount α _bs (λ ₁ ) at the connection point J obtained from the backscattering waveform when measured with the pulsed light from the light source 4 and the measured amount with the pulsed light from the light source 5. The step amount α _bs (λ ₂ ) is stored, the connection loss amount at the wavelength λ ₁ is calculated by the equation (8), and the result is displayed on the CRT display device.

The measuring instrument according to the present embodiment can estimate the connection loss amount with high accuracy only by switching the light source 4 or 5 by the switch 3 and the optical switch 6. Therefore, the measuring time is short and the measuring method is simple.

Here, the principle of the calculation of the connection loss amount by the signal processing unit 1 will be described in detail.

The backscattered light power P _bs (z) generated at the position z in the optical fiber to be measured, which is measured at the incident end of the pulsed light, has the power of the input pulsed light as P _in and the pulse width as W, and It is represented by. Here, V _{g is} a group velocity of light in the optical fiber, α _{R is a} Rayleigh scattering loss, and α is a loss of the optical fiber. Details of this equation are detailed in the aforementioned Brinkmeyer document.

Here, the Rayleigh scattering loss is α _R1 , α _R2 , the backward propagation factors are S ₁ and S ₂ , the group velocity of light in the optical fiber is V _g1 , V _g2 , and the loss of the optical fiber is α ₁ and α ₂ . Consider a case where two optical fibers f ₁ and f ₂ are connected. The backscattered light power from the point Δz before the connection point z _J is It is represented by.

In terms of distance from the connection point z _J contrary, the connection point z _J
The power of the propagating light changes in order to pass through. When the power change rate is P _J , the power of the incident pulsed light changes at this rate, and the backscattered light propagating in the opposite direction also changes at this rate. Therefore, the backscattered light power observed at the incident end of the pulsed light is It is represented by.

Therefore, a step occurs in the backscattering waveform at the connection point z _J. Here, since the backscattered light reciprocates in the same path in the optical fiber to be measured, when the backscattered light power P _bs returning to the incident end side is plotted as a logarithmic value, Note that the calculation process is performed.

As a result, the step amount α _bs of the backscattering waveform is given by the ratio of the values where Δz is set to “0” in the equations (2) ′ and (2) ″. It is represented by. In the step amount α _bs , the term in [] indicates the component due to the difference in the group velocity V _g and the backscattering coefficient α _R S, and 10 logP _J represents the actual loss amount. Here, it is defined that α _J = 10 log P _J.

In a normal optical fiber, between two optical fibers f ₁ and f ₂ , the group velocity ratio V _g2 / V _g1 , the Rayleigh scattering loss coefficient ratio α _R2 / α _R1, and the backward propagation factor ratio S ₂
/ S ₁ does not change even if the measurement wavelength is changed. Therefore, when measuring the backscattered light power distribution at different wavelengths,
The difference in the step amount α _bs [dB] of the backscattering waveform becomes the true difference in the connection loss amount [dB]. That is, α _bs (λ ₁ ) −α _bs (λ ₂ ) = α _J (λ ₁ ) −α _J (λ ₂ ) ... (4)

On the other hand, the factors of splice loss when connecting single-mode optical fibers are considered to be axial misalignment between two optical fibers, angular misalignment, electric field mismatch, and core deformation.

To connect a large number of optical fiber cores in a short time, connector connection or fusion splicing in which only the outer diameters of the optical fibers are matched is used. However, when the connection is made by such a method, an axis deviation occurs due to the eccentricity of the optical fiber core, and this is the main cause of the connection loss. The amount of connection loss due to axis deviation, angle deviation, electric field mismatch and core deformation is
When an ordinary optical fiber is connected by an ordinary method, it is about 0.1 dB, which is smaller than the amount of splice loss due to misalignment.

The axial displacement connection loss amount α _d caused by the axial displacement is determined by the axial displacement amount σ _d and the beam spot size w ₀ . It is represented by. For this, see D. Marcuse, “Loss Analysis of Singlemode Fiber Splice.
lice) ”, Bell Sys.Tech.J., Vol. 56 (1977) No. 5, pp. 703-718.

In the equation (5), since the spot size w ₀ differs depending on the wavelength, the axial displacement connection loss amount differs depending on the wavelength λ ₁ and the wavelength λ ₂ .

FIG. 2 shows the calculated values of the axial displacement connection loss amount with respect to the axial displacement amount at wavelengths of 1.3 μm and 1.55 μm. Wavelength 1.
The amount of misalignment connection at 3 μm is larger than the amount of misalignment connection at wavelength of 1.55 μm.

FIG. 3 shows the calculated values of the axial displacement connection loss amount when the axial displacement amount is changed, the horizontal axis represents the axial displacement loss amount at a wavelength of 1.3 μm,
The vertical axis shows the amount of misalignment loss at a wavelength of 1.55 μm.

These calculated values are generally used for optical communication in the 1.3 μm band, the core diameter is 10 μm, the outer diameter is 125 μm, and the relative refractive index difference is 0.
It is a value for an optical fiber of 3%.

As shown in FIG. 3, the relationship between the axial misalignment connection loss α _d when the wavelength is changed is α _d (λ ₂ ) = k (λ) α _d (λ ₁ ) ... (6) There is. This coefficient “k (λ)” is referred to as a wavelength-dependent coefficient of the axis deviation loss. The wavelength dependence coefficient k (λ) changes depending on the values of the wavelengths λ ₁ and λ ₂ , and also changes depending on the parameters of the optical fiber.

FIG. 4 shows the relationship between the wavelength dependence coefficient k (λ) and the wavelength ratio λ ₂ / λ ₁ when the wavelength λ ₁ is 1.3 μm for an optical fiber used for 1.3 μm band optical communication. The calculated results are shown. The long dependence coefficient k (λ) is the ratio of wavelengths λ ₂ / λ ₁
Is smaller and has a certain width due to variations in optical fiber parameters. For example, when λ ₂ = 1.55 μm, the wavelength dependence coefficient k (λ) of the axis deviation loss is a value in the range of k (λ) = 0.75 to 0.8. This value, as shown in FIG.
It can be calculated from the parameters of the optical fiber. Therefore, this value is stored in the signal processing unit 1, and the corresponding value is called and used at the time of actual measurement.

As described above, the true connection loss from the axial displacement connection loss is a major cause of the true connection loss, the true connection loss α _{J (λ 1)} at the wavelength lambda _1, the wavelength lambda ₂ is in a normal connection between the amount α _J (λ _2), the relationship of _{α J (λ 1) ≒ k} (λ) α J (λ 2) ......... (7) is satisfied. Therefore, the true connection loss α _J (λ ₁ ) at the wavelength λ ₁ is calculated by the equations (4) and (7) as follows. Can be obtained by

FIG. 5 shows an example of observation of the backscattering waveform by the measuring instrument of this example.

In this observation example, the measured optical fiber 8 is 1.3 μm.
Designed for m band communication, core diameter: approx. 10 μm, outer diameter: 125 μm
The two optical fibers f ₁ and f ₂ are used by being connected. A pulsed light having a pulse width of 500 ns was incident on the optical fiber 8 to be measured, and the backscattering waveform was observed for each of the wavelength of 1.3 μm and the wavelength of 1.55 μm. Step amount α at connection point J
_bs is 1.0 dB (α _bs (1.
3μm) = 1.0dB), 0.89dB (α for wavelength 1.55μm)
_bs (1.55 μm) = 0.89 dB). Assuming that the value of the wavelength dependence coefficient k (λ) of the axis deviation loss is k (λ) = 0.75, the true splice loss α _J (λ ₁ ) at a wavelength of 1.3 μm can be estimated to be 0.44 dB from Eq. (8). .

The same optical fiber 8 to be measured was measured by the conventional method. That is, wavelength 1.3 μm, pulse width 500 ns
The pulsed light of was incident from the opposite direction, and the step amount of the backscattering waveform was measured. The step amount was 0 dB, and the average value of the step amounts observed from both ends was 0.50 dB. in this way,
With the measuring instrument of this embodiment, an estimated value of splice loss that is in good agreement with the value obtained by the conventional method can be obtained.

FIG. 6 shows the correlation between the connection loss values measured by the conventional measuring method and the present embodiment.

In this measurement, as in the observation example above, 1.
Core diameter of about 10 μm and outer diameter of 125 designed for 3 μm band communication
A large number of optical fibers 8 to be measured were prepared by fusion splicing μm optical fibers, and the amount of splice loss was measured for each of them by the conventional example measuring method and the present example. In FIG. 5, the solid line shows the perfect agreement between the two values, the ∘ mark shows the measured value according to this example, and the Δ mark shows the step amount of the backscattering waveform as it is at the wavelength of 1.3 μm. From the extrapolated values when the splice loss is “0” in the measured values from both ends, it is found that the step difference component of the backscattering waveform due to the difference in the backscattering coefficient of the fiber used for the measurement is 0.48 dB. Recognize.

In the calculation method used in the present embodiment, factors other than the axis shift, that is, the angle shift, the electric field mismatch, and the connection loss due to the core deformation are ignored. The amount of connection loss due to these factors is
There is about 0.1 dB, and the wavelength dependence coefficient k of the axis deviation loss
(Λ) varies in the range of 0.75 to 0.80. Therefore, it is considered that this calculation method causes an error of about ± 0.2 dB. However, due to the difference in the backscattering coefficient of the optical fiber, the step difference amount of the backscattering waveform easily becomes 0.2 dB or more, and thus the value approximate to the true splice loss can be obtained by the calculation method of the equation (8).

It is also possible to carry out the measurement method used in the measuring instrument of the present embodiment using an optical pulse tester which has been conventionally used. However, since the conventional optical pulse tester cannot select the measurement wavelength, it is necessary to use two optical pulse testers with different wavelengths or replace the light source, optical directional coupler and photodetector parts. There is.

FIG. 7 is a block diagram of the optical fiber splice loss measuring device according to the second embodiment of the present invention.

The pulse generator 2 generates a pulse signal synchronized with the timing signal from the signal processing unit 1. The switch 3 supplies this pulse signal to one of the light sources 4 and 5. With this pulse signal, the light source 4 generates pulsed light of wavelength λ ₁ ,
The light source 5 generates pulsed light of wavelength λ ₂ .

When the light source 4 is selected by the switch 3, the optical fiber 8 to be measured is attached to the optical directional coupler 7, and the pulsed light generated by the light source 4 is transmitted through the optical directional coupler 7 to the optical fiber 8 to be measured. Incident on. Further, the backscattered light generated in the measured optical fiber 8 is incident on the photodetector 9 via the optical directional coupler 7. As the optical directional coupler 7, one that can most efficiently split the optical path at the wavelength λ ₁ emitted by the light source 4 is used. As the photodetector 9, an element that can detect the backscattered light most efficiently at this wavelength λ ₁ is used.

When the light source 5 is selected by the switch 3, the optical fiber 8 to be measured is attached to the optical directional splicer 7 ', and the pulsed light generated by the light source 5 is transmitted via the optical directional coupler 7'. It is incident on the fiber 8. Furthermore, the measured optical fiber 8
The backscattered light generated inside is incident on the photodetector 9'through the optical directional coupler 7 '. As the optical directional coupler 7 ′, one that can most efficiently split the optical path at the wavelength λ ₂ emitted by the light source 5 is used. As the photodetector 9 ', an element that can detect backscattered light most efficiently at this wavelength λ ₂ is used.

In the measuring instrument of this embodiment, it is necessary to reattach the optical fiber 8 to be measured to the optical directional coupler 7 or 7'when switching the switching devices 3 and 11, but the optical switch 6 of the measuring instrument of the first embodiment is used. Is unnecessary. Therefore, the loss due to the optical switch 6 (about 2d
The measurable length of the measured optical fiber 8 can be increased by the amount of B). For example, the loss of the measured optical fiber 8 is 0.
In the case of 5 dB / km, the length of the measured optical fiber can be increased by 2 km.

FIG. 8 is a block diagram of an optical fiber connection loss measuring device according to a third embodiment of the present invention.

In the measuring instrument of the present embodiment, the operations of the switching unit 3, the light sources 4, 5 and the optical switch 6 in the measuring instrument of the first embodiment are carried out by the variable wavelength light source 12. As the variable wavelength light source 12,
It is possible to use a semiconductor light source whose emission wavelength is variable by changing the temperature and the applied current, a YAG laser, a light source combining a Raman scattering excitation optical fiber and a spectroscope.

Since the measuring instrument of this embodiment uses the variable wavelength light source 12,
The step amount of the backscattering waveform at any wavelength can be observed. Since the true splice loss can be estimated from the measured values of the step difference of many backscattered waveforms, a more accurate value can be obtained than the splice loss estimated from the measured values of two wavelengths. Also in the measuring instruments of the first and second examples, the same effect can be obtained by changing the light source and the configuration associated therewith.

〔The invention's effect〕

As described above, the optical fiber splice loss measuring method of the present invention is highly accurate only by observing the backscattering waveform with pulsed light of two wavelengths for one optical fiber to be measured and performing simple calculation. The connection loss amount can be measured. This measurement can be performed only from one end of the optical fiber to be measured, and the splice loss amount can be obtained with the same accuracy as the measurement by observing the backscattering waveform from both directions.

Therefore, when the present invention is used for the measurement of the optical fiber line, it is not necessary to move the measuring device, the measurement time is shortened, and the optical fiber line can be quickly constructed and maintained.

Further, since the connection loss can be measured with high accuracy even by measurement from one end of the optical fiber line, it is possible to set the criterion for judging the quality of the connection when the backscattered light is used to be stricter than the conventional criterion. Therefore, the defective connection point can be removed,
There is an effect that the quality of the optical fiber line can be improved.

[Brief description of drawings]

FIG. 1 is a block configuration diagram of an optical fiber connection loss measuring device according to a first embodiment of the present invention. FIG. 2 is a diagram showing a calculated value of the shaft misalignment connection loss amount. FIG. 3 is a diagram showing a calculated value of the axial displacement connection loss amount when the axial displacement amount is changed. FIG. 4 is a diagram showing the calculated value of the wavelength dependence coefficient of the axis deviation loss. FIG. 5 is a diagram showing an example of observation of a backscattering waveform by the measuring instrument of this embodiment. FIG. 6 is a diagram showing the correlation of the measured values between the conventional measuring method and this embodiment. FIG. 7 is a block configuration diagram of an optical fiber connection loss measuring device according to a second embodiment of the present invention. FIG. 8 is a block configuration diagram of an optical fiber connection loss measuring device according to a third embodiment of the present invention. FIG. 9 is a diagram showing an example of a backscattering waveform. 1 ... Signal processing unit, 2 ... Pulse generator, 3 ... Switch, 4,5 ... Light source, 6 ... Optical switch, 7,7 '...
Optical directional coupler, 8 ... Measured optical fiber, 9, 9 '...
… Light detector, 10 …… Amplifier, 11 …… Switcher, 12 …… Tunable wavelength light source.

Claims (1)

[Claims] 1. A pulsed light is incident from one end of a measured optical fiber in which a plurality of optical fibers are connected in cascade, and a backscattered light waveform generated in the measured optical fiber on a time axis is observed. In the optical fiber splice loss measuring method for measuring the splice loss of the optical fiber from the step amount at the splicing point of the observed waveform, the step difference logarithmic value α of the waveform of the backscattered light is used by using the pulsed light of the first wavelength λ _{1. bs} (λ ₁ ) is measured, and the step difference of the wavelength of the backscattered light obtained by using the pulsed light of the second wavelength λ ₂ different from the _first wavelength λ _1
bs (λ ₂ ) is measured, and the splice loss logarithmic value α _J (λ ₁ ) of the measured optical fiber at the first wavelength λ ₁ is calculated as the wavelength dependence coefficient k of the axis deviation loss.
Using (λ), An optical fiber splice loss measurement method, which is characterized by the calculation of