Three Common Methods for OTDR Optical Fiber Testing

Optical fiber communication is a communication mode with optical wave as carrier and optical fiber as transmission medium. Optical fiber communication has become the main means of information transmission and the cornerstone of "information highway" because of its long transmission distance, large information capacity and high communication quality. Optical fiber testing technology is the most extensive and basic special technology in the field of optical fiber applications. OTDR is the main instrument in the field of optical fiber testing technology. It is widely used in the maintenance and construction of optical fiber lines. It can measure the length of optical fiber, transmission attenuation of optical fiber, joint attenuation and fault location. OTDR has the advantages of short test time, fast test speed and high test accuracy.

1. Two basic formulas supporting OTDR Technology

OTDR (optical time domain reflectometer) is a high-tech and high-precision photoelectric integrated instrument made by using Rayleigh scattering and Fresnel reflection when optical pulse is transmitted in optical fiber. The semiconductor light source (LED or LD) outputs light pulses modulated by the driving circuit, which are injected into the tested optical cable line through the directional optical coupler and movable connector to become incident light pulses.

When the incident light pulse is transmitted in the line, Rayleigh scattered light and Fresnel reflected light will be generated along the way. Most of the Rayleigh scattered light will be refracted into the cladding and attenuated. The back Rayleigh scattered light opposite to the propagation direction of the light pulse will be transmitted to the light inlet port of the line along the optical fiber, and will be transmitted to the photodetector through directional coupling shunt and converted into electrical signals, After low-noise amplification and digital averaging, the processed electrical signal is scanned synchronously with the trigger signal emitted from the back of the light source and becomes a reflected light pulse on the oscilloscope.

The returned useful information is measured by the OTDR detector, and they are regarded as time or curve segments at different positions in the measured optical fiber. According to the time from transmitting signal to returning signal, and then determine the speed of light in quartz material, the distance (fiber length) l (unit: m) can be calculated, as shown in formula (1).

In formula (1), n is the average refractive index and â–³ t is the transmission delay. The attenuation a (unit: dB / km) can be calculated by using the power level corresponding to the incident light pulse and the reflected light pulse and the length of the measured optical fiber, as shown in formula (2):

2. Five parameter settings to ensure OTDR accuracy

2.1 test wavelength selection

Since OTDR serves optical fiber communication, the test wavelength shall be selected before optical fiber test, and only 1310 nm or 1550 nm shall be selected for single-mode optical fiber. Since the influence of 1550 nm wavelength on the bending loss of optical fiber is much more sensitive than 1310 nm wavelength, 1550 nm wavelength is generally used to test the whole process optical fiber backscattering signal curve of an optical cable or an optical fiber transmission link, whether it is optical cable line construction, optical cable line maintenance or experiment and teaching.

The shapes of the test curves at 1310nm and 1550nm are the same, and the measured fiber connector loss values are basically the same. If no problem is found in the 1550 nm wavelength test, the 1310 nm wavelength test is certainly no problem.

If the 1550 nm wavelength is selected for testing, it is easy to find out whether there is excessive bending in the whole process of the optical fiber. If a large loss step is found somewhere on the curve, retest with 1310 nm wavelength. If the loss step disappears at 1310 nm wavelength, it indicates that there is excessive bending at this place, which needs to be further found and eliminated. If the loss step is also large at 1310 nm wavelength, there may be other problems in the optical fiber, which need to be found and eliminated. In the test of single-mode optical fiber line, 1550 nm wavelength should be selected as far as possible, so the test effect will be better.

2.2 optical fiber refractive index selection

The refractive index of the single-mode optical fiber used now is basically in the range of 1.4600 1.4800, which should be accurately selected according to the actual value provided by the optical cable or optical fiber manufacturer. For G.652 single-mode fiber, if 1310 nm wavelength is used in actual test, the refractive index is generally 1.4680; If 1550 nm wavelength is used, the refractive index is generally 1.468 5. Incorrect selection of refractive index will affect the test length.

In equation (1), if the refractive index error is 0.001, an error of about 35 m will be generated in the relay section of 50000 M. Small mistakes in optical cable maintenance and troubleshooting will bring obvious errors, which must be paid enough attention during testing.

2.3 selection of test pulse width

If the set light pulse width is too wide, strong Fresnel reflection will be generated, which will increase the blind area. Although the narrow test light pulse has a small blind area, the test light pulse is too narrow, the power must be too weak, the corresponding backscattering signal is also weak, the backscattering signal curve will fluctuate, and the test error is large. The set optical pulse width shall not only ensure that there is no strong blind spot effect, but also ensure that the backscattering signal curve has sufficient resolution and can see each point along the optical fiber.

Generally, an appropriate test pulse width is selected according to the length of the measured optical fiber, and an optimal value is determined after one or two trials. When the distance of the measured optical fiber is short (less than 5000 m), the blind area can be less than 10 m; When the distance of the measured optical fiber is long (less than 50000 m), the blind area can be less than 200 m; When the distance of the measured optical fiber is very long (less than 2500 000 m), the blind area can be up to more than 2000 m.

In the single disk test, the blind area can be less than 10 m by properly selecting the optical pulse width (50 nm). If the average value is obtained through two-way test or multiple tests, the impact of blind area will be less.

2.4 selection of test range

The range of OTDR refers to the maximum distance that the abscissa of OTDR can reach. During the test, the measuring range shall be selected according to the length of the measured optical fiber, and it is better that the measuring range is 1.5 times the length of the measured optical fiber. If the range selection is too small, it can not be seen comprehensively on the display screen of the optical time domain reflectometer; When the range selection is too large, the abscissa compression on the display screen of the optical time domain reflectometer can not be seen clearly.

According to the practical experience of engineers and technicians, the selection of test range can make the backscatter curve account for about 70% of the OTDR display screen, whether it is length test or loss test, better direct viewing effect and accurate test results can be obtained.

In the test of optical fiber communication system, the link length is hundreds to thousands of kilometers, the relay section length is 40 60 km, and the single optical cable length is 2 4km. Good test results can be obtained by selecting the range of OTDR.

2.5 selection of averaging time

Because the backscattered light signal is extremely weak, the method of multiple statistical average is generally used to improve the signal-to-noise ratio. OTDR test curve samples the reflected signal after each output pulse, and averages the multiple samples to eliminate random events. The longer the averaging time is, the closer the noise level is to the minimum value, and the larger the dynamic range is. The dynamic range obtained with an average time of 3 min is 0.8 dB higher than that obtained with an average time of 1 min.

Generally speaking, the longer the averaging time, the higher the test accuracy. In order to improve the test speed and shorten the overall test time, the test time can be selected within 0.5 3 min.

In the connection test of optical fiber communication, satisfactory results can be obtained by selecting 1.5 min (90 s).

3 three common methods of OTDR testing

When OTDR tests optical cables and optical fibers, the test occasions include factory test of optical cables and optical fibers, construction test of optical cables and optical fibers, maintenance test and regular test of optical cables and optical fibers. The test connection of OTDR is shown in Figure 1.

The test connection method is: OTDR - optical fiber connector - the first optical cable - the second optical cable - the nth optical cable, and the terminal is not connected to any equipment. According to the actual test work, there are three main methods:

3.1 OTDR backward test method

This method is mainly used to monitor the optical cable connection. The optical cable connection must be equipped with a special optical fiber fusion machine and optical time domain reflectometer (OTDR). After welding a fiber core, the welding machine will generally give the estimated attenuation value of this contact. This method of testing has three advantages:

(1) OTDR is fixed, omitting the vehicles and a lot of manpower and material resources required for instrument transfer;

(2) The test point is selected in the place where there is mains power without gasoline generator;

(3) The test points are fixed to reduce the stripping of optical cables.

At the same time, this method also has two disadvantages:

(1) Due to distance and terrain constraints, it is sometimes impossible to ensure smooth communication;

(2) With the continuous increase of connection distance, the test range and accuracy of OTDR are limited.

At present, there are generally three methods to solve these problems:

â‘  The use of mobile phones in the city and suburbs can keep the testers and successors in touch at any time, facilitate organization and coordination, and improve work efficiency.

â‘¡ Contact by telephone. Make sure to connect one optical fiber (such as blue optical fiber unit and red optical fiber) to the optical telephone as the connecting line. Of course, the last optical fiber used for communication cannot be monitored because it cannot be contacted during fusion and fiber winding. Even so, the possibility of problems will be greatly reduced (if it is a 24 core optical cable, the probability of problems will be reduced to less than 1 / 24 of the original).

â‘¢ When the optical cable connection reaches a relay distance, the OTDR moves forward.

The test practice shows that these monitoring methods are effective to ensure quality and reduce rework.

3.2 OTDR forward one-way test method

OTDR is tested at the first joint point in the optical fiber connection direction, and construction vehicles are used to transfer the test instruments and testers in advance. Using this method for monitoring, the test point and connection point always have only one disc of optical cable length, the attenuation accuracy of the test joint is high, and it is convenient for communication. At present, the length of one optical cable is about 2 3 km. In general terrain, communication can be ensured by using walkie talkie. If the optical cable has a corrugated steel strip protective layer, you can also use the magnet telephone to contact.

The disadvantages of this test method are also obvious. Moving OTDR to each test point is labor-consuming and time-consuming, which is not conducive to the protection of instruments; The test points are also limited by the terrain, especially when the line is far away from the highway and the terrain is complex. Portable OTDR is selected for monitoring. The short-range test does not require high dynamic range of the instrument, and the small 0tdr has small volume, light weight and convenient movement, which can greatly reduce the workload of testers and improve the test speed and work efficiency.

3.3 OTDR forward two-way test method

The OTDR position is still the same as the "forward one-way" monitoring, but two optical fibers are respectively short circuited at the beginning of the connection direction to form a loop. This method can not only meet the optical fiber test of relay section, but also monitor the optical fiber connection. When testing the optical fiber in the relay section, the incident light pulse, reflected light pulse, joint point, fracture point, fault point and attenuation distribution curve can be clearly seen on the display screen of the optical time domain reflectometer. The OTDR test event type and display are shown in Figure 2, which can provide convenience for optical cable maintenance.

When monitoring optical fiber connection, due to the increase of loopback point, the bidirectional value of connection loss can be measured on 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, there will be false gain and false large attenuation in one-way monitoring with OTDR. For an optical fiber connector, the mathematical average of attenuation values in two directions can accurately reflect its real attenuation value. For example, the attenuation of a connector measured from a to B is 0.16 dB, and that measured from B to a is -0.12 dB. In fact, the attenuation of this connector is [0.16 (- 0.12)] / 2 = 0.02 dB.

4 Conclusion

As the main instrument of optical fiber communication,

Three Common Methods for OTDR Optical Fiber Testing 1

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Why CCTV Monitor Testers Is the Better of the Other
CCV testers, also known as LCD field monitors or CCTV field monitors, provide installers with an ideal solution that allows them to view, calibrate and correct the camera's video feed in real time based on the additional features that they provide. The basic feature of a CCTV tester is an LCD display (25 inch x 3.5 inch diagonal) that allows the installer to view a camera feed on the spot. If the camera is set up in a separate area, the installer must run the feed to refine the settings.Other features of video calibration include video test patterns, the brightness, contrast and saturation adjustment, video signal intensity testing and support for PAL and NTSC video. Another aspect of the functionality that should not be overlooked by a CCTV tester is the ability to work with analog and IP cameras.If you are a professional installer, you probably have heard of closed circuit television testers (also called LCD field monitors, CCTV video testers, CCTV camera testers or IP camera testers). The ability of CCTV testers to work with both analogue and IP cameras is particularly important as digital surveillance cameras become increasingly popular. A closed circuit TV tester is a handy tool for professional CCTV installers and do-it-yourselfers to set up and configure CCTV cameras from any source.NKTech's 3.5-inch CCTV Tester is a portable device designed to install and maintain existing on-site CCTV video surveillance systems. The 1920x1200 resolution allows the 5-inch touchscreen IP camera tester to display network HD cameras and high resolution analog cameras. As with all our CCTV test monitors, we offer a one-year warranty of CCTV Camera Pro.All three options can be used together, but there is a higher tendency for IP camera systems than for digital video cameras. IP cameras have the ability to contain more cameras than one to cover wide angles captured by multiple cameras and camera systems to cover everything.The warranty does not cover operating system errors caused by third-party apps. On the Software Update screen, select Manual Updates and select Apps to Update. All features, including apps installed since the last factory reset, can be removed.Send your physical sample copy of your Dreiklang order to the address above. This allows you to store your videos in the cloud with EUFy's own subscription storage plan. For 10 days cloud video storage for a camera, you need to subscribe to a 200GB iCloud storage plan for $3 a month.Prisons use video surveillance to prevent drones from selling drugs and other contraband to prisoners. Disaster management uses CCTV cameras to allow emergency services and responders to assess and monitor events in real time and video the situation to the disaster management team.The CCV technology was first developed in 1942 by German scientists to monitor the launch of the V2 rocket. With road surveillance cameras and traffic lights, cities can monitor people, collect traffic statistics, evidence and speed. The legacy of the IoT (AOT) is the Chicago Initiative, which collects real-time weather and environmental data in Chicago.It is essential to constantly clean, pre-test and calibrate CCTV testers. To ensure accurate testing, it is important to study CCTV systems and conduct tests with tools that measure the performance of multiple indicators. A man on a ladder who manipulates a camera not only walks around, he measures the video signal and evaluates the running camera.This is a quick test for the effect of measuring the input and output video signal by examining a device. The same man who climbs a ladder and manipulates a camera can also do so with other video devices such as video splitters, disconnectors, switches, route switches, cable connectors, etc.With this connector we can connect the camera to its output and check the level of the video signal. It's a handy solution for men who have a ladder to manipulate the camera. You can connect the cable to the LED cable of the monitor to check the signal.Contrary to the impression, the 1.2V voltage causes many problems when we supply the camera with a long cable. The voltage loss caused by the greater resistance of the cable reduces the value of the camera clamp and interferes with proper operation.A tester with a USB port can charge mobile devices, but does not have a data interface. Android devices, mobile phones and tablets do not support hosting of USB sticks.As soon as the power switch is in position, press the power button to switch the tester to off mode and dim the backlight. The tester wakes up when the button is pressed. When the power switches are pressed and held down, the button selects the option Turn off to shut down the device.Brush - I could use two brass rods and a set of adapters for different brush sizes on a children's apron suitable for children 3-7 years. Bust 40, "Waist 38," Length 19 "(if you have smaller or larger sizes) Girl Nautical Miss Crab Fabric Personalized Crab.Note the handmade texture of the piece using hand stamping techniques using butterflies, butterfly cookie cutters, sunflowers and sugar. Hand cut and polished for participation in 2 Mineralogical exhibitions in Finland and Sweden, this is one favorite piece of ours that will be brought with you by Etsy if you wish a shipping offer for the purchase. The buyer of IP camera test equipment writes: "It is worth every penny.CCTV testers feature a colour LCD screen, video connection, rechargeable lithium batteries and several types of colour bars for signal output. A shopper who bought a CCTV camera tester used it to test his camera before he started installing it in his home.
The Basics of Using the CCTV Monitor Testers
The Basics of Using the CCTV Monitor Testers
Our security camera testers are designed to facilitate the installation, maintenance and testing of security cameras. All our CCTV test monitors are backed by a 1 year warranty from CCTV Camera Pro.CCV safety tester is a kind of multifunction test tool for CCTV security camera installation professionals. A CCTV tester (short for closed circuit television tester) is a tester and monitor that supports the installation and installation of CCTV security systems. It is used to check if the configured CCTV camera is in the right place, at the right angle, with free view and more.A CCTV tester can be used to display video, control the PTZ, generate images and acquire data from the RS485 test LAN cable. It also has a multi-meter feature to provide AC / DC voltage and resistance measurements to provide the installer with additional diagnostic capabilities to locate localized faults.The CCTV tester analyzes the video signal and displays peak, average video signal level, synchronization and pulse levels to determine the optimum level to display the best picture and synchronization to avoid cable loss. A multifunction tester can save you time and money on your next CCTV installation. In order to ensure accurate testing, it is important to study the CCTV system thoroughly and carry out a test with a tool that measures the performance of several indicators.Once the camera is set up in a separate area, the installer must run a video feed from the camera to refine the settings. Not only do you see the camera video and adjust the focus of the variofocal lens, you also have to supply the camera with its lithium-ion battery. This means a ride up and down the ladder to mount the camera, test the cables and adjust the settings.CCV testers, also known as LCD field monitors or CCTV field monitors, provide installers with an ideal solution that allows them to view, calibrate and correct the video feed of the camera in real time based on the additional features they provide. The basic features of a CCTV tester are an LCD screen (2.5 inch x 3.5 inch x 0.5 inch diagonal) that allows the installer to display a camera feed in place. Other features for video calibration include video test patterns, brightness, contrast and saturation adjustment, video test of signal intensity and support for PAL and NTSC video.The built-in 24-inch TFT LCD display enables the display and adjustment of camera settings and functions. The display monitor can also be connected to a DVR display to allow viewing of a CCTV camera with multiple cameras. The closed-circuit TV tester is a handy tool for professional installers of CCTV security systems and do-it-yourselfers to set up and configure a CCTV camera source.The NKTech 35-inch CCTV tester is a portable device designed to install and maintain existing on-site CCTV video surveillance systems. If you are a professional installer, you may have heard of closed circuit TV testing (also known as IP camera testers, CCTV video / LCD field monitors, CCTV camera testers or IP / CCTV camera testers ).The device can be used for TVI cameras as well as cameras in HD, SDI cameras and network IP cameras. IP cameras have the ability to record more than one camera and cover a wider angle, and can record multiple cameras so that the camera system is covered. The CCTV tester is handheld for good reason, it is for installation and convenience.Prisons use video surveillance to prevent drones from delivering drugs and other contraband to prisoners. Video encoders enable migration from an analog CCTV system to a network system, allowing users to take advantage of cheaper hardware with advanced features. There are three possibilities, which are used with high tendency: IP camera systems, digital video cameras or both.Perform real-time camera settings such as focus, zoom, optimization of advanced settings such as BLC, send-up, lighting adjustment, etc. Disaster management uses CCTV cameras for emergency services and responders to assess and monitor events in real time and report the situation to the disaster management team via video. City and community streets monitor cameras and traffic lights in cities to monitor people, collect traffic statistics, evidence and speed.A CCTV tester is a portable handheld service monitor that helps installers configure the camera at the touch of a finger. Market-buying reviewers looking for a video tester or an IP camera monitor kit can get an angle on where the camera is set up and how it should work. A shopper who bought a CCTV camera tester used it to test his camera before he started installing it in his home.The completion of the CAPTCHA proves that you are a human being and gives you temporary access to the web property. It takes 10-25 working days at best, and I cannot keep it in stock according to your order quantity. Your order has been put on hold for 14 days while we await the Woods trial.
Main Features of Cable Fault Tester
Main Features of Cable Fault Tester
Cable fault tester is a comprehensive cable fault detection instrument. It can test the cable flashover fault, low resistance grounding, short circuit, cable disconnection, poor contact and other faults. If equipped with an acoustic point detector, it can accurately determine the position of the fault point. It is especially suitable for testing power cables and communication cables of various models and different levels of voltage. The cable fault tester has the following remarkable characteristics: 1. It has complete functions and can test faults safely, quickly and accurately. The instrument adopts low-voltage pulse method and voltage flashover method for detection, which can test various faults of cables, and directly test the flashover and resistance faults of power cables without burning through. If equipped with a sound point meter, the position of the fault point can be measured accurately. 2. Test accuracy. The instrument adopts fast data sampling technology with a reading resolution of 1m. Degree of intelligence. The test results are automatically displayed on the large screen LCD with small and data, and the fault judgment is intuitive. It is also equipped with menu display operation function, without special training for operators. 3. It has the functions of wave opening, parameter storage and call out. Nonvolatile devices are adopted, and the waveform and data are not volatile after shutdown. 4. With dual trace display function. The test waveform of the faulty cable can be compared with the normal waveform, which is conducive to further judgment of the fault. 5. With waveform scaling function. By changing the waveform proportion, the waveform can be expanded for testing. 6. Control the measurement cursor to automatically search along the line and stop automatically at the inflection point of the fault waveform. 7. The position of the double cursor can be changed arbitrarily to directly display the direct distance or relative distance between the fault point and the test point. 8. With printing function. Print and archive the test results.
Cable Fault Detector_ Medical Devices Will Maintain High Growth in the Coming Years
Cable Fault Detector_ Medical Devices Will Maintain High Growth in the Coming Years
The medical and pharmaceutical chamber of Commerce of the all China Federation of industry and Commerce established the medical device Professional Committee yesterday and issued the 2012 analysis statement of the medical device industry. The statement shows that the dependence of some domestic high-end medical equipment markets on imports has decreased slowly last year. According to the deployment of deepening medical reform and the spirit of the 2012 national health work conference, the statement implies that China's medical device industry achieved a total sales revenue of 68.7 billion yuan in the first half of 2012, a year-on-year increase of 20.27%, The overall requirements of this year's work are: closely focus on the central task of deepening medical reform. It is predicted that China's medical devices will maintain rapid growth in the next 5-10 years, and the industry as a whole will maintain a growth rate of more than 20%. Among them, the focus is on food safety, occupational disease prevention and drinking water safety. According to the statistics of the statement, at present, although the number of medical device enterprises across the country has reached more than 14000, the competition pattern of the medical device industry is relatively scattered. In 2012, the food safety and health supervision of the health system should pay close attention to the following aspects: first, do a good job in food safety, More than 80% of them are small and medium-sized enterprises. The scale of enterprises is small, and there are problems such as insufficient R & D investment and weak technical ability. Because of fierce competition and low profitability, enterprises can only produce medium and low-end products or provide spare parts for foreign enterprises. " The head of the association hinted.
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