Hi everyone!
Today I will write about reflectance testing. In this first article about this theme, I will talk about problems and solutions.
Reflection is the phenomenon of returning a signal to a source, as a consequence of the interaction of light with a change in the refractive index. Reflection occurs at sites of glass-air refractive index change (from 1.5 to 1). These are connector pairs, mechanical splices and rarely bad fusion splices – air bubbles or other problems in the splice.
Figure 1. Reflectance
The reflection is represented in dB. It is calculated as the logarithmic ratio of reflected power and emitted power. The calculation formula is as follows:
Reflectance (dB) = 10*log(Preflected power/Pemitted power).
This phenomenon leads to problems in optical systems. Especially in singlemode systems. The reflected signal can lead to noise and nonlinearities in the laser. A much smaller problem is in systems with multimode fibers – background noise in the fiber. We need to minimize the reflection in the SM system in order to get maximum performance and high transfer speeds. This is especially a problem in systems where AM modulated CATV signals are transmitted.
There are several ways to reduce reflection. Like I said, reflection is mostly a problem with optical connectors. So the solution is making a convex polish on the end of the connector ferrule. It is a PC (Physical Contact), as we can see in figure 2. In this way, it can reduce reflectance to about -30 to -50 dB. The best method for singlemode fibers is used APC (Angled Physical Contact) optical connectors. APC optical connectors are connectors in which the ferrule is polished at an angle of 8 degrees. In this way, a typical reflectance of -60 dB or higher is achieved. For systems that use CATV, APC optical connectors must be used. That it why ONTs that have an RF port for CATV, the input connector/adapter is SC/APC, for example, HG8247H, HG8247H5, EG8143A5, HG8010H, etc.
Figure 2. Types of optical connector ferrule polish
This is the end of this first article. This phenomenon is very important, it can cause big problems. This requires the use of PC and APC optical connectors, cleaning the optical connectors, and of course testing reflectance.
In the next article I will write about methods for measuring and testing reflectance >> Reflectance testing (2): OCWR method for measuring reflectance.
Thank you!
