Field Applications OTDR And ORL Measurements
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Today’s high-speed systems are comprised of many highly sensitive components, and great care must be taken to ensure that proper operating conditions are created and maintained. Failure to control optical return loss (ORL) in systems will cause high bit error rates resulting from multipath interference, degraded optical signal-to-noise ratio (OSNR) and transmitter instability. This application note will focus on a detailed description of this phenomenon and on the importance of accurately measuring ORL and identifying its main components.
When light is injected into a fiber-optic component, such as a connector, a multiplexer or the fiber itself, some of the energy is transmitted, some is absorbed and some is reflected. The total light that comes back (i.e., reflected) is what we call ORL.
ORL is caused by two fundamental effects:
Rayleigh scattering effect; specifically, the part that goes back to the source point, known as backscattering
Rayleigh scattering is intrinsic to the fiber itself. It consists of the light interacting with the density fluctuations of the fiber. It can be caused by a variation in the material density and composition that give rise to a variation of the fiber’s refractive index. This causes part of the wave to escape from the waveguide. The phenomenon is called scattering when the size of the defect is less than one tenth of the wavelength’s incident light, while backscattering refers to the part that is captured in the fiber and that propagates in the backward direction.
Because it is intrinsic to the fiber, backscattering cannot be eliminated. The intensity will depend on many factors such as the incident light wavelength and the refractive index and length of the fiber, etc. Over long distances, ORL caused by Rayleigh backscattering can be as high as 32 dB. Therefore, it is very important to consider this phenomenon during network design.
As another important component of ORL, Fresnel reflections are also related to a variation in the index of refraction. This phenomenon will typically occur at discrete interfaces (connectors, adapters, etc.) as a result of air gaps, misalignment, and mismatched refractive indices. Because it results from discrete interfaces, Fresnel reflections have to be optimized during fiber and system component installation in order to ensure proper conditions.
There are many different ways of testing ORL. The method chosen will depend greatly on the scope of the test. For example, the way a field technician tests when commissioning a new span may vary greatly from the way a central office technician tests during system turn-up or troubleshooting. Here are four methods supported by the IEC 61300-3-6 standard:
Optical Continuous-Wave Reflectometer (OCWR): OCWRs directly measure the incident power and reflected power. This method is very accurate and provides the nearest value to the theoretical definition of ORL. However, it cannot spatially resolve many different reflections on the line. Back reflection meters are based on this approach.
Optical Time-Domain Reflectometer (OTDR): OTDRs measure return loss from reflection points on the optical line with nanometer spatial resolution. Today, most OTDRs also allow an operator to extract an ORL measurement from the OTDR trace.
Optical Low-Coherence Reflectometer (OLCR): OLCRs measure reflection profiles of singlemode optical devices with micrometer spatial resolution.
Optical Frequency-Domain Reflectometer (OFDR): OFDRs measure the return loss of single optical devices with a centimeter spatial resolution.
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