Sunday, 27 September 2015

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OTDR - Optical Time Domain Reflectometer

The architecture and the operation of the OTDR system
The OTDR is the most important investigation tool for optical fibres, which is applicable for the measurement of fibre loss, connector loss and for the determination of the exact place and the value of cabel discontinuities. By means of very short pulses it is also possible to measure the modal dispersion of multimodal fibres. The structure of a typical OTDR equipment is shown below:
The principal of the OTDR analyzer is the following: 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.

The components of the fibre loss and their importance in the OTDR measurements

There are three reasons for the fibre loss:

• absorption

• radiation loss

• Rayleigh scattering

The absorption creates 10-20% of the fibre loss. It mainly originates from the OH- ions inside the fibre material (impurities). With modern technologies the number of these contaminants, so the loss can be kept at relatively low level. The fibre loss increases dinamically for wavelengths above 1700 nm, thus this is the lowest frequency for optical telecommunications. In practice the 1300 and 1550 nm wavelengths are used as the insertion loss shows minimal values at these wavelengths. Naturally, absorption does not induce reflection, so if this would be the only physical phenomena, the fibre loss could be measured by the means of OTDR only with a well known, calibrated termination

In practice, the fibre continuously radiates backwards due to the Rayleigh scattering, which will be described later, so the absorption loss is measured together with the other losses.
Radiation loss occurs when the geometrical parameters of the fibre abruptly change, or a mechanical tension is present in the fibre material due to fabrication failure or mechanical impact. Considering appropriate fabrication technologies and fibre jacket, the radiation loss can be neglected and, like absorption, it does not create reflections, so from the OTDR measurements point of view it can handle as absorption losses. A high level discontinuity originated by e.g. strong folding, can be shown by OTDR as it produces high loss.
During OTDR measurements the most important loss is the one caused by Rayleigh scattering. It generates the 80-90% of the total loss. The scattering is induced by the microscopic inhomogenity of the refractive index of the fibre. These inhomogenities cause diffraction, so a certain part of the light energy is radiated isotropically. The level of the diffrection reaches its maximum when the wavelength is in the same range as the dimensions of the microscopic inhomogenities. Thus the level of the scattering decreases when the wavelength is increased. Among others this is the reason for using the 1300 and the 1550 nm ranges instead of the 850 nm. A certain part of the diffracted light propagates backwards in the fibre which is, when measured, carries important information. In the following, we calculate the ratio of the diffracted and the backward propagating light.
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