Pulse reflectometers OTDR (Optical Time Domain Reflectometer) operate on a similar principle, which is used in reflectometers for testing electrical cables. Reflectometer submits a powerful light pulse into the optical fiber, as well as an electrical reflectometer submits powerful probe pulse in the electrical cable. At the same time it carried out the measurement of power and pulse delay time that is returned back to the OTDR. The difference between the electric OTDR is that the metallic conductor returns only pulses that are reflected by strong inhomogeneities in the cable, that is, from the location of cable deformation, while the optical fiber return pulses are not only places of large irregularities, but also due to the Rayleigh effect scattering. Rayleigh scattering occurs due to changes in the refractive index of the optical fiber when it is stretched during manufacture. Such inhomogeneities arising in the manufacture of the optical fiber is extremely small, but have some effect on the scattering of the light beam that is scattered by these irregularities in all directions, including back to the OTDR.
- Passage of the light beam from the optical fiber OTDR
Due to the effect of Rayleigh scattering possible to determine information about the length of the optical cable, the distribution of non-reflecting irregularities of kilometricheskom average attenuation for all the length of the cable, as well as the local welding of optical fiber cable.
Rayleigh scattering coefficient is a very small quantity, hence in the OTDR, using photodetectors with high sensitivity down to -70 dB.
- OTDR Construction
Modern OTDR often have a modular design, as for the qualitative measurement of single-mode and multimode fiber requires appropriate optical emitters. Typically, the OTDR consists of two main modules:
- Basic module - is responsible for the post-treatment measurements and display the waveform on the oscilloscope screen, and for setting measurement parameters, etc.
- Interchangeable optical module - is responsible for measuring and digitizing the primary trace.
The OTDR uses laser emitters with high power level of up to 1000 mW. Laser transmitter sends the pulses into an optical fiber, which are reflected back into the optical connector removable optical module to which the optical fiber was connected. Inside the optical coupler module is installed, which acts as a filter to reverse emission did not fall on the radiator OTDR. Once the module converts the optical signal into an electrical and conduct primary processing, trace data are transferred to the base unit, which made the final processing and display of information on the OTDR screen.
Also, depending on the model, OTDRs may have internal and external drives for storing OTDR, interfaces for connecting to a PC, additional software, etc.