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OTDR stands for Optical Time Domain Reflectometer. An OTDR need scattered light in fibers to locate faults traditionally FTTH PON maintenance and troubleshooting is performed through external OTDR equipment. This approach requires new fibers to be connected to the optical Distribution network. It shows that OTDR record the reflected light from the fiber and test along the time. OTDR work from one end and produce “photo” of cable. The linked fiber is known as launch cable. We can function with a single launch cable and get some information with the receive cable.
The OTDR is the most important examination tool for optical fibers, which is applicable for the measurement of fiber loss, connector loss and for the determination of the exact place and the value of cable discontinuities. By means of very short pulses it is also possible to measure the modal dispersion of multimodal fibers. The structure of a typical OTDR equipment is shown below:
The principal of the OTDR analyzer is that a short light pulse is transmitted into the fibre under test and the time of the incidence and the amplitude of the reflected pulses are measured. The commonly used pulse width ranges from nanosecs to microsecs, the power of the pulse can exceed 10 mW. The repetition frequency depends on the fibre length, typically is between 1 and 20 kHz, naturally it is smaller for longer fibres. The division by 2 at the inputs of oscilloscope is needed since both the vertical (loss) and the horizontal (length) scales correspond to the one-way length.
There are three forms of OTDR in the market now.
1-Laboratory OTDRs: Laboratory OTDRs are usually used in test labs. They have the particularly long range with many options.
2-Mini OTDRs: Mini OTDRs are portable and design for fluid testing.
3-PC Computer Based OTDRs: PC Computer-based connect with personal computer and operate with windows based software.
Fast Automatic Test: Automatic test function give quick results without knowing much information about the equipment. First connect fiber and then press “Test” button, it will automatically set optimal test and shows accurate test results, such as test curve and event lists.
Fast Curve Analysis: It can find failure point in the test curve and also find the location information in the same curve test. Maintain an event list of all event occurred during the test.
Powerful Document Management: It provides the store view, deletes and saving function using the PCL-Language based software.
Convenient VFL function: Using this function we can easily find out the disconnecting points in short distance optical fiber links.
Manual Test Mode: There are two modes in it, real-time or average time. Real-time mode fast test comparatively of the dynamic test.
Average Test Mode: Average test mode is used to bear the noise in the test curves to the most extent, to obtain more accurate testing results.
OTDR uses Rayleigh scattering and Fresnel reflections to portion fibers’ characteristics. Rayleigh scattering refers to the rough scattering produced as optical signals transmitting in the fiber. OTDR only measures the scattered light back on the OTDR port. The backscatter signal shows the attenuation degree (loss/distance) of the optical fiber, and will be traced as a downward curve, showing the power of backscatter decreases. This is because both transmission signal and backscatter loss are attenuated. Rayleigh scattering power is related to the wavelength of transmitted signal: the shorter the wavelength, the stronger the power, which means the backscatter lose generated by the trajectory of 1310nm signal will be higher than that of 1550nm signals.
In the higher wavelength region (more than 1500 nm), the Rayleigh scattering will continue to decrease, and another phenomenon called infrared attenuation (or absorption) will appear to increase thus to cause an increase of the overall attenuation values. Therefore, 1550nm wavelength has the lowest attenuation, which testifies why it is a long distance communication wavelength. Similarly, OTDR of 1550nm wavelength also has low attenuation, therefore it can be used for long distance testing as well. Fresnel reflection falls into the group of distinct reflection that is produced by the individual point of the whole fibers. These points are the result of changes in inverse coefficient elements such as glass and air gap. At these points, a strong backscatter light will be reflected back. Therefore, OTDR uses the information of Fresnel reflection to locate the connection point, fiber optic terminal and breakpoints
The OTDR application is the finest resolution for testing long-haul backbone networks, optical MAN, and passive optical networks. Its speed is fast as compared to other solutions. It takes only 30 seconds to show dynamic range whereas another take at least 3 minutes with high costs. It displays test curves and event lists. It lists circuit events which later can help in maintenance. It can store all records which can be seen through PCs connecting via USB cable. It can measure faults like broken points, disconnecting points or big loss points in fiber optic link.
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