Reasons for Fiber Splice Loss


I don't think I need to say more about how important the field test of fiber optic cable construction is. In fact, the test of optical cable construction is to prepare for the general adjustment of the connection of the optical transceiver. Then, how should we test the optical cable on site, and what are the reasons for the loss of optical fiber fusion?


Optical cable construction

How fiber optic cables are field tested

1. Measurement of field transmission attenuation

1.1 Attenuation of Optical Fiber

When the optical signal is transmitted along the optical fiber, the loss of optical power is the attenuation of the optical fiber, and the attenuation A is in decibels.


P1 and P2 are the optical power at the injection end and the output end, respectively.


1.2 Add injection system between optical cables

In order to obtain accurate measurement results, it is necessary to ensure that the power distribution is a steady-state mode, so an injection system is added between the light source and the cable under test. The injection system is a simulation device composed of mode scrambler, mode filter and cladding mode stripper; for multimode fiber, it can be used more than 1km, and the fiber is wound with a certain curvature radius.


1.33 test methods comparison

The CCITT recommendation G.651 recommends 3 test methods. That is, the shearing method and the backscattering method. The shearing method has high precision but is destructive; the insertion loss method is non-destructive, and the accuracy is not as good as the shearing method; and the backscattering method, that is, measuring with an optical time domain reflectometer, has full functions, high precision and non-destructiveness, and the measurement data can be directly printed come out.

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1.4 Advantages of measuring with optical time domain reflectometer

The optical time domain reflectometer test only needs to be carried out at one end of the optical fiber. The instrument can not only measure the attenuation coefficient of the optical fiber, but also provide the specific information of the attenuation characteristics along the length of the optical fiber, detect the physical defect or the position of the breaking point of the optical fiber, and determine the joint. Attenuation and position, as well as the length of the fiber under test, this instrument has a printer, which can print out the plotted curve.


The on-site fiber splicing is monitored by the OTDR. After the fusion splicer has spliced ​​a core, the estimated attenuation value of the splicing point will be given. The estimation is generally based on the visual monitoring of the local fiber core, that is, it is estimated by observing the quality of the core connection. Attenuation value.


Whether the connection work is in good condition will be notified by the supervisor after measurement. The advantages of this method: First, the OTDR is fixed. The vehicles and manpower and material resources required for the transfer of the meter are omitted; the second is that the test point is selected in a place where there is commercial power and no generator is required; the third is that the test point is fixed, which reduces the stripping of the optical cable.


1.5 Selection of OTDR measurement parameters

Select the appropriate range: OTDR has different ranges. The operator should select the appropriate range based on the length of the optical cable to be tested, so that the test curve is displayed in the middle of the screen as much as possible, so that the reading can be accurate and the error will be small.


Select the appropriate pulse width: OTDR can select the parameters of the optical pulse width injected into the fiber under test. In the case of the same amplitude, the energy of the wide pulse is greater than that of the narrow pulse, which can test longer distances, but the error is large. Therefore, the operator should select an appropriate pulse width according to the length of the fiber to be tested, so that it can test the longest distance possible while ensuring the accuracy.


Select the appropriate refractive index: Due to the different materials used for the optical fibers of different manufacturers, the transmission speed of light in the optical fibers is different, that is, different optical fibers have different refractive indices, so the appropriate refractive index should be selected during the test, so that the length of the optical fiber is measured. time to be accurate.


The selection of test points should be reasonable: At present, most OTDRs use the 5-point method to test the joint loss. During the test, the cursor should be positioned on the joint point as one point, and the remaining 4 points should correspond to the fiber characteristics on both sides of the joint point. In this way, the joint test can be accurate.


1.6 One-way test method of optical cable joint

This method is to place an OTDR at the beginning of the connection direction and perform one-way tests on all connection points.


When the hop length is short and there are not many optical cable joints, such as the local telephone trunk optical cable, when the attenuation requirements of the joints are not very accurate, the optical time domain reflectometer can be used to monitor from one end, and the operator can instruct the connector to adjust the connector to achieve a relatively optimal value, i.e. It can be formally continued. It can be observed from Figure 2 that the waveform of the point in the figure has a small "step", and the size of the attenuation can be estimated from the size of the "step".


The accuracy of this method is not as good as that of the comparison method, but it is simple. As long as one point is monitored and two points are coordinated, it is suitable for the construction of the optical cable section with a large attenuation margin in the relay section, which can speed up the progress.


1.7 Two-way loop test method of optical cable joint

This method is to short-circuit the two optical fibers at the beginning of the connection direction to form a loop, and the OTDR performs bidirectional testing on all the connection points at the point before the connection start point. Since the loopback point is added, the bidirectional value of the connection loss can be measured on the OTDR. The advantage of this method is that it can accurately evaluate the quality of the joint.


Due to the test principle and the structure of the fiber, the phenomenon of false gain will appear in one-way monitoring with OTDR, and correspondingly, the phenomenon of false large attenuation will also appear. For a connector, use the mathematical average of the attenuation values ​​in the two directions. In order to accurately reflect its real attenuation value.


The standard of fiber attenuation constant is: at 1310mm wavelength, the average attenuation value should be less than or equal to 0.36dB/km, and the maximum attenuation value should be less than or equal to 0.4dB/km; at 1550mm wavelength, the average attenuation value should be less than or equal to 0.22dB/km, The maximum attenuation value should be less than or equal to 0.25dB/km; when the optical fiber is connected, its bidirectional average joint loss should not be greater than 0.08dB.


After completion, use the light source and optical power meter to conduct two-way test for the whole process, and the attenuation value must meet the design requirements. And use the OTDR to check whether the backscattering curve meets the requirements.


2. Baseband response test of field fiber

Multimode fiber is an indirect characterization of the fiber's dispersion based on the baseband response. The baseband response test of optical fibers in a single-reel cable can use either the frequency domain method or the time domain method. The frequency domain method is now introduced as follows.


2.1 Sweep the modulated light source with the test bandwidth

The wavelength of the light source should be the working wavelength of the optical fiber to test the bandwidth scanning modulation light source. If the bandwidth of the fiber under test is, it should be a detector at the end of the fiber under test from a low frequency, and connect it to the spectrum analyzer. ,As shown in Figure 3.


2.2 Connect sending and receiving with short fiber

Before the test, connect the sending and receiving with a short optical fiber, and record its waveform. Insert the fiber under test and record its waveform. The frequency at which a 6dB point is obtained by subtracting the two waveforms is the bandwidth of the fiber under test. The baseband response per unit length is then converted.


3. System commissioning after installation of optical transceiver

The installation of the equipment shall be carried out in accordance with the installation requirements provided by the equipment manual and engineering design. Before the equipment is powered on, check whether the power cord is short-circuited. The rack should be powered on after all the circuit boards are unplugged, and then plug in the power board, the alarm board, test whether the various voltages on each terminal are appropriate, and then plug in various circuit boards.


What are the causes of fiber splicing loss

First, the main factors affecting the loss of optical fiber fusion

There are many factors that affect the loss of optical fiber fusion, which can be roughly divided into two categories: intrinsic factors and extrinsic factors of optical fibers.

1. The intrinsic factor of the fiber refers to the factor of the fiber itself, and there are four main points.

(1) The fiber mode field diameter is inconsistent;

(2) The core diameters of the two optical fibers are mismatched;

(3) The core section is not round;

(4) The concentricity between the core and the cladding is not good.


Among them, the inconsistency of fiber mode field diameter has the greatest impact. According to the recommendation of CCITT (Consultation Committee for International Telegraph and Telephone), the tolerance standard of single-mode fiber is as follows:

Mode field diameter: (9—10μm)±10%, that is, the tolerance is about ±1μm;

Cladding diameter: 125±3μm;

Mode field concentricity error ≤6%, cladding out-of-roundness ≤2%.


2. The extrinsic factor that affects the optical fiber splice loss is the splice technology.

(1) Axial misalignment: The single-mode fiber core is very thin, and the axial misalignment of two butt-jointed fibers will affect the splice loss. When the misalignment is 1.2μm, the splicing loss reaches 0.5dB.

(2) Inclination of the axis: When the optical fiber section is inclined by 1°, a splice loss of about 0.6dB will be generated. If the splice loss is required to be ≤0.1dB, the inclination angle of the single-mode fiber should be ≤0.3°.

(3) End face separation: The connection of the movable connector is not good, and it is easy to cause end face separation, resulting in large connection loss. When the discharge voltage of the fusion splicer is low, end face separation is also likely to occur, which can generally be found in fusion splicers with a tensile test function.

(4) End face quality: When the flatness of the fiber end face is poor, loss and even air bubbles will occur.

(5) Physical deformation of the optical fiber near the splice point: the tensile deformation of the optical cable during the erection process, and the pressure of clamping the optical cable in the splice box is too high, which will affect the splice loss, and even several times of fusion can not be improved.


3. The influence of other factors.

The operation level of the splicer, the operation steps, the fiber coil process level, the cleanliness of the electrodes in the fusion splicer, the splicing parameter settings, and the cleanliness of the working environment will all affect the value of the splicing loss.

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