Causes of fiber splice loss

Measurement of field transmission attenuation

fiber attenuation

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, A=10lgP1/P2

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

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 conditions of the attenuation characteristics along the length of the optical fiber, detect the physical defect or the location of the fracture point of the optical fiber, and determine the joint The attenuation and position, as well as the length of the measured optical fiber, this instrument is equipped with a printer, which can print out the measured curve.

On-site optical fiber splicing is monitored by OTDR. The fusion splicer will give the estimated attenuation value of the fusion point after splicing a core. 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 fiber core butt joint. attenuation value.

Whether the connection work is intact or not, the supervisor will notify the connection worker after measuring. The advantages of this method: First, the OTDR is fixed. The vehicle, manpower and material resources required for instrument transfer are omitted; the second is that the test point is selected in a place with mains power and does not need to be equipped with a generator; the third is that the test point is fixed, which reduces the stripping of optical cables.

Selection of OTDR measurement parameters

Select the appropriate range: OTDR has different ranges, the operator should choose a more appropriate range in combination with the length of the optical cable to be tested, so that the test curve can be 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 choose the optical pulse width parameters injected into the fiber under test. In the case of the same amplitude, the energy of the wide pulse is greater than the energy of the narrow pulse, which can test a longer distance, but the error is larger. Therefore, the operator should select an appropriate pulse width in combination with the length of the fiber to be tested so that it can test as long a distance as possible under the premise of ensuring accuracy.

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

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

Optical cable joint one-way test method

This method is to place an OTDR at the beginning of the splicing direction to perform a unidirectional test on all joint points.

When the length of the relay section is short and there are not many optical cable connectors, such as the local telephone relay optical cable, when the attenuation of the connector is not very accurate, you can use the optical time domain reflectometer to monitor from one end, and direct the connector to adjust the connector to achieve a relatively optimal value. Can be formally followed, the size of the attenuation can be estimated from the size of the "step".

This method is not as accurate as 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 an optical cable section with a large attenuation margin in the trunk section, and can speed up the progress.

Optical cable joint two-way loop test method

This method is to short-circuit two optical fibers at the beginning of the splicing direction to form a loop, and the OTDR will perform a bidirectional test on all splicing points at the point before the starting point of the splicing. Due to the increase of the loopback point, the bidirectional value of the connection attenuation 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 optical fiber structure, false gain and large attenuation will appear when OTDR is used for one-way monitoring. For a joint, the mathematical average of attenuation values in two directions is used. In order to accurately reflect its real attenuation value.

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

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

Baseband Response Test of Optical Fiber in Field

The dispersion of multimode fiber is characterized indirectly by baseband response. The baseband response test of the optical fiber in the single-reel optical cable can use the frequency domain method or the time domain method. Now the frequency domain method is introduced as follows.

Sweep the modulated light source over the test bandwidth

The wavelength of the light source should be the working wavelength of the optical fiber, and the light source is modulated by scanning the test bandwidth. If the bandwidth of the optical fiber under test is 1000 MHz. ,As shown in Figure 3.

Connect sending and receiving with short fiber optics

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

System commissioning after installation of the 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 powering on the equipment, check whether the power cord is short-circuited. The rack should be powered on after the circuit boards are unplugged, then plug in the power board and 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

1. The main factors affecting the fiber splicing loss

There are many factors affecting the fiber splicing loss, which can be roughly divided into two types: intrinsic factors and extrinsic factors.

1. The intrinsic factors of optical fiber refer to the factors of optical fiber itself. 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 the fiber mode field diameter has the greatest impact. According to the recommendation of CCITT (Consultative 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 affecting the fiber splicing loss is the splicing technology.

(1) Axis misalignment: The core of a single-mode fiber is very thin, and the misalignment of the axes of two butt-connected optical fibers will affect the splice loss. When the misalignment is 1.2μm, the connection loss reaches 0.5dB.

(2) Axis tilt: When the fiber section is tilted by 1°, about 0.6dB splice loss occurs. 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 the end-face separation is easy to occur, resulting in a large connection loss. When the discharge voltage of the fusion splicer is low, it is also easy to cause end face separation. This situation can generally be found in fusion splicers with tensile test functions.

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

(5)Physical deformation of the optical fiber near the splicing point: the tensile deformation of the optical cable during the erection process, too much pressure to clamp the optical cable in the splicing box, etc., will affect the splicing loss, and even several times of welding cannot be improved.