As communication technologies and dense wavelength-division multiplexing (DWDM) systems develop rapidly, numerous optical cables are widely used in telecommunications. However, problems arise at the same time. Optical return loss is one of them. High bit rates in digital fiber-optic transmission systems and networks continue to increase, so measuring optical return loss (ORL) is a growing concern. But do you know well about ORL? Today, this article will help you to know more about optical return loss.
What Is Optical Return Loss?
Optical return loss of a connection is the loss of power in the signal returned/reflected by a discontinuity in a transmission line or optical fiber. It is a measure taken from one end of the total energy reflected back to the source by all interfaces due to a variation of the index of refraction (IOR), breaks, voids, backscatter, etc. In a word, optical return loss is the lost energy of a signal during transmitting process.
What Contribute to Optical Return Loss?
Optical return loss is the amount between the light launched into a device and light reflected by a defined length region. And sources of loss usually include reflections and scattering along the fiber network.
Reflections
Reflections are caused by a change in the index of refraction as light moves from one material to the next. In an optical connection, reflection can be caused by the mechanical interface, minimum air gaps, geometry misalignments and microscope oxide films on the surface of the fiber created during polishing process. Here is a picture showing a cross section of an optical connector. The region where the two fibers meet causes a much localized, discrete optical reflection.
Scattering
Scattering takes place when light interacts with small discrete particles. These particles can be impurities, defects, or even regions of mechanical stress. There are many types of scattering, but the most common type in optical fiber is Rayleigh scattering.
In addition, there are additional factors can cause high light loss. For example, a dirty connector, broken optical fiber and poorly mated connector. All of them may lead to light loss which affects the quality of transmission information.
Why Do We Need to Test Optical Return Loss?
With more and more high date rate networks applied, people have attached more importance to both the quality and speed of signals. While high optical return loss can increase the noise in transmitter and receiver, which doesn’t coincide with people’s will. From the perspective of technology, to ensure proper stability of the lasers and their central wavelength, it is essential to measure back reflection when installing and maintaining networks, which is important for network installers to decrease the back reflection to minimum.
Optical return loss testing measures the back-reflection of connectors and components for high-speed digital and analog systems used in telecommunication, CATV (Community Antenna Television), LAN (local area network) and WAN (Wide Area Network) applications. If the ORL is too high, the light signal in the network will not be transmitted correctly.
How to Measure Optical Return Loss?
Typically, there are two methods to measure optical return loss. One method is called optical CW reflectometer (OCWR). While the other method uses an optical time domain reflectometer (OTDR).
Measurement of OCWR
This method sends light from a source through a coupler to the connection which is being tested. That connection typically consists of a reference connector which is used to mate the connector under test. The light reflected from that connection is split by the coupler and part is calculated by the power meter.
Measurement of OTDR
The OTDR measures the amount of light that's returned from both backscatter in the fiber and reflected from a connector or splice. The amount of light reflected is determined by the differences in the index of refraction of the two fibers joined, a function of the composition of the glass in the fiber, or any air in the gap between the fibers, common with terminations and mechanical splices.
Conclusion
Optical fiber is the data carrier, and fiber optic networks have changed the way of transmission. However, optical return loss, as the one of the factors affecting transmission quality, should not be neglected. Knowing more about optical return loss will help us choose cables we want.
