Optical cable construction site and inspection methods for acceptance
This paper introduces the test methods and standard requirements for the construction site of the optical cable and the project acceptance, as well as the problems that should be paid attention to in the test, the correct, reasonable and standardized quality inspection of the optical cable project, and the timely solution of hidden dangers, so as to prolong the service life of the optical cable and reduce the number of optical cables. The amount of maintenance of the line to ensure smooth communication will play its due role.
The on-site test of the optical cable construction is very important, it is to prepare for the general commissioning of the connection optical transceiver. The test items for the fiber in the cable are the measurement of transmission attenuation, and for multimode fiber, the baseband response is tested when needed.
The purpose of the single-reel optical cable test is the quality of the factory products; the test after construction and deployment is to check whether there is any damage during the deployment process, and as a check before the connection; the test during the connection is to check whether the joint reaches low loss; after the connection The purpose of the test of the optical cable section of the unit is to check whether the design requirements for total transmission attenuation and total baseband response are met, as a preparation for the overall commissioning of the optical transceiver.
Single mode fiber is characterized by dispersion coefficient. The dispersion coefficient of single-mode fiber is originally very low, and the limit for a 140Mbit/s system is 300ps/nm, so when the length of the hop is less than 50km, the limit has a large margin, and the construction process does not need to be measured; 565Mbit/s five times The swarm is limited to 120ps/nm, so it is necessary to consider it in design, and perform verification measurements after construction.
Optical cable construction inspection
1. Measurement of field transmission attenuation
1.1 Optical fiber attenuation
When the optical signal is transmitted along the fiber, the loss of optical power is the attenuation of the fiber, the attenuation A is in decibel (dB), A=10lgP1/P2 (dB)
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.
1.3 Comparison of three test methods
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, measured with an optical time domain reflectometer (OTDR), has full functions, high accuracy and non-destructiveness, and the measurement data Can be printed directly.
1.4 Advantages of Measurement with Optical Time Domain Reflectometer (OTDR)
The optical time domain reflectometer (OTDR) test only needs to be carried out at one end of the fiber, as shown in Figures 1 and 2. This instrument can not only measure the attenuation coefficient of the fiber, but also provide details of the attenuation characteristics along the length of the fiber. Detect the location of physical defects or breaking points of the optical fiber, determine the attenuation and position of the joint, and the length of the optical fiber under test. This instrument has a printer, which can print out the plotted curves.
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 shall be measured by the supervisor and notified to the connection worker. 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
(1) Select the appropriate range: The OTDR has different ranges. The operator should select a more appropriate range based on 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.
(2) Select the appropriate pulse width: The 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.
(3) 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 in the It is only accurate when measuring fiber length.
(4) The selection of test points should be reasonable: At present, most of the OTDR test joint loss adopts the 5-point method. During the test, the cursor should be positioned on the joint point as one point, and the remaining 4 points should correspond to the two sides of the joint point respectively. Fiber characteristics. 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 at point (c) in the figure appears 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 optical 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 modulated light source. If the bandwidth of the optical fiber under test is 1000MHz.km, it should be from a low frequency (for example, 100 to 1000MHz or higher), and the end of the fiber under test should be a detector. , 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 (km) 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, alarm board, test whether the various voltages on each terminal are appropriate, and then plug in various circuit boards.
3.1 Local transceiver self-loop test
First perform the self-loop test of the local transceiver. The self-loop is only a preliminary verification of whether the various components of the terminal machine can work normally.
3.2 Transmitting optical power test of terminal
The optical power injected by the laser of the optical transceiver into the optical fiber can be tested as shown in Figure 5. The digital generator uses pseudo-random code as the information source, the optical power meter is connected to the output end of the laser through a 1km fiber, and the optical power meter displays the value of Pd.
Laser power Ps=Pd+α±δd
α is the attenuation of the 1km fiber and the live joint, and δ is the measurement error of the optical power meter.
The actual transmitted optical power should deduct the insertion loss of the optical fiber connector, which is about 1dB or less. The measured value shall conform to the CCITT specified value standard.
3.3 Optical receiver sensitivity test
Test according to Figure 4. The receiving sensitivity refers to the minimum power that the optical detector of the optical transceiver needs to receive under the condition of the bit error rate (10-9) specified by the equipment.
Gradually increase the attenuation value of the optical attenuator until the bit error detector reaches the specified bit error critical value, then disconnect the optical input end of the optical transceiver, and measure the optical power output by the attenuator with an optical power meter. The optical power meter shows Pd. Receiver sensitivity S=Pd
δ is the measurement error of the optical power meter.
3.4 Jitter test
Most of the jitter values of the intranet and short-distance optical systems can meet the CCITT regulations. Generally, a system can be tested randomly or from several systems. The maximum gain of the jitter transfer function for each digital segment should not exceed 1dB. Use a dedicated jitter tester for testing.
3.5 Bit error performance test
The required instrument is a bit error analyzer (transmit, receive). The test method is similar to the optical receiver sensitivity test.
The test of the long-term average bit error rate is to divide the total number of accumulated bit errors measured by the measurement time (seconds) over a long period of time to obtain the long-term average bit error rate. The long-term average bit error rate is the most important aspect to reflect the bit error performance of the system. CCITT's detailed error performance requirements also include Errored Seconds (ES), Severely Errored Seconds (SES), and Degraded Points (DM). The meanings are Errored Seconds: 1 or more errored seconds; SES Serious Errored Seconds: Seconds with a bit error rate greater than 10-3, Deterioration points: Errored seconds with a bit error rate greater than or equal to 10-6 minutes.
3.6 Testing of the alarm system
The alarm system includes emergency and non-emergency alarms. At least the following warnings should be present.
(1) Power failure: main and backup power conversion;
(2) The bit error rate exceeds 10-6;
(3) The bit error rate exceeds 10-3 and is switched to the standby system;
(4) There is no optical signal at the sending and receiving ends.