Optical fiber measurement technology OTDR),

Optical fiber measurement technology OTDR) 2016 - 04 - 01 optical fiber communication technology is the new technology rapid development over the past 20 years, due to large amount of information, optical fiber communication transmission speed, digital and information, is the digital signal transmission, so that wide band image signal and computer networking, information transfer is possible. The fiber loss measurement is very important, it is directly related to the quality of the optical fiber communication, and timely found potential point of failure. Fiber loss measurements are mainly truncation method, insertion method and backward reflection method. In construction and maintenance of optical fiber is often used after the reflection method, it has the characteristics of non destructive and single-ended measurement. Its measuring principle is, if in the input end of the fiber into a narrow pulse of light, the light within the narrow pulse in optical fiber transmission, optical fiber due to the internal uniformity will not produce a Rayleigh scattering ( In optical fiber connector, breakpoint will produce scattering) 。 Part of the scattered light along the fiber to return to the input transmission, to input the continuous transmission of scattered light is called the reflected light. Transmission loss is small, close to the input light scatters back to have a stronger signal from the input from the light transmission loss is big, back scattering signal is weak. As long as the light scattering can measure two return power and the distance between two points, can calculate the average attenuation coefficient. Generally based on the principle of loss measurements was done by optical time domain reflectometer. Optical time domain reflectometer ( OTDR） Principle is that the standard clock signal generated by the master clock, the clock pulse generator based on narrow pulse produced to meet the requirements, and use it to light source modulation. In optical coupler light source the light coupling to the optical fiber to be tested, and the scattering and reflection signal coupling photodetector, after amplifying signal processing into the oscilloscope display output waveform and output the data in the data output system. Due to the reflected light is very weak, submerged in the noise, therefore, to use sampling integrator, to a certain time interval sampling and summation of weak scattering light waves. In this process, because the noise is random, when the sum is offset, so as to remove the scattering signal. 1, the instrument parameter is set correctly for the average number of times: OTDR test curve is reflected signal sampling, after each output pulse and do multiple sampling average processing to eliminate some random event, the longer the average time, noise level, the more close to the minimum value, the greater the dynamic range. Average, the longer the higher measurement precision, but no longer raise enough precision. In order to improve the test speed, in some qualitative measurement does not need accurate data, can reduce the average number of appropriately, shorten the overall test time. Range and resolution: ranges are decided by the distance range of the fiber, the range should be set at least twice the tested fiber, the analytic software provides a curve endpoint enough after the noise of the clean area. For accurate analysis, can double the length of the fiber, when choosing the next available range of distance. Resolution values specify the data sample points of distance, the higher the resolution, the closer the distance of the sampling point, to reflect the details of the fiber clearer, but high resolution will reduce the average number per unit time, to achieve the ideal signal-to-noise ratio will need to increase the measuring time, reduce the measurement speed. Pulse width: used to specify by input by measuring the optical pulse width of the fiber. Under the same pulse amplitude, pulse width, pulse energy is, the greater the to measure fiber for larger range, large pulse width will increase the measuring blind area. Refractive index: the value is used to calculate the distance measurement, the refractive index values affect all distance measurements, different manufacturers, different types of optical fiber optical fiber refractive index is different, the measurement should be set up correctly before. 2, the matters needing attention when using OTDR to accurately measure should ensure that optical fiber links to connect the adapter to be tested in good condition. Connector of the system under test shall be carried out before the connection to the universal connector and adapter is clean, to avoid hand contact with the connector. Optical fiber, especially the single mode fiber, vulnerable to stress caused by microbend or other loss. In order to ensure the correct and repeatable measurement, optical fiber wire connected to the OTDR minimizing mechanical tension must be placed in position. When optical fiber is measured with OTDR, in between there is a blind spot ( End reflection area) Single-mode optical fiber, multimode fiber in the blind area is lesser, the blind area is larger, the equivalent of about 100 m long fiber. So measuring single-mode fiber, the first connection length is more than 100 m reference fiber. Optical fiber connector, mechanical joint and fracture in optical fiber will cause loss and reflection, end at the end of the fiber at the ends of the fracture surface due to the irregularity can produce various kinds of Fresnel reflex Fresnel reflection peak or not. If the cursor is set not accurate enough, can produce a certain error, the application of the function of amplification of the OTDR is it can be set in the corresponding cursor accurate inflection point. Stead in measuring point, in the connecting the two measurements, and pan fiber after the first time can be found in a timely manner in place of the stand or fall of quality, the second time can be caused the loss of pan fiber. Using OTDR measurement fiber loss on both ends of the disadvantage is that the measured attenuation values differ, this is because the backward scattering field of unable to control the mode field distribution, which can lead to measure fiber attenuation and scattering loss values are not really equal, usually the average of the measured on both ends. In short, as long as we seriously sum up experience in the work, understand the characteristics and limitations of OTDR, can make it play a bigger role in the optical fiber to maintain.