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Application Note 04

Understanding Fiber Optic Receiver Overloading and its Cure

One of the most important specifications pertaining to a fiber optic transmission system is the maximum allowable attenuation (or optical loss) it can tolerate from the optical transmitter to the optical receiver. This is an important parameter because it indicates maximum distance the fiber optic cable can be and still effectively pass a signal. If the fiber is too long, the system will not work and if it is too short, it may not work either, as we will see shortly. Optical loss is usually specified in terms of 0 to some value (in dB). While the upper value is easily understood, 0 dB is usually not. The purpose of this note is to clarify what 0 dB means in the real world because when too much light reaches a fiber optic receiver it can overload. When this effect occurs the result is usually evident in distorted signals, intermittent data or in many cases no operation at all!

To understand the phenomenon, we must first look at the operation of the basic optical fiber itself. In an optical fiber light travels through the ultra-pure glass core of the fiber as shown in figure 1. The reason this occurs is that the junction between the core glass and the cladding glass causes the light to continually “bounce” back and forth along the fiber much like a ball passing though a tube. The glasses chosen for each portion of the fiber are such that the refractive index of the core glass is lower than that of the cladding glass and it is this feature that “traps” the light. Additionally, there is another interface that is between the cladding layer and air. Since air has the lowest refractive index of all, it also causes light to be trapped, but this time within the actual cladding. The cladding is constructed of impure glass so that the trapped light is quickly absorbed and it normally never interferes with the light traveling through the core. However, when the length of fiber is short light trapped within the cladding can actually travel a reasonable distance. Figure 1 shows a step-index fiber. This simple construction is used for ease in explanation although more complex graded index fibers operate in a similar manner.

Light proppogation through a fiber

Normal fiber optic transmission systems are measured with an optical attenuator set for the maximum allowable attenuation on one hand and a so-called “standard 0dB fiber” for the zero reference point. The standard 0dB fiber usually consists of 30 meters of optical fiber wound around a 1 inch diameter form. This arrangement attempts to assure that only core light exits the fiber. The 30-meter length is usually long enough to absorb any cladding light and the 1-inch diameter form further attenuates light in the cladding. While reasonable care is taken to assure that only core light is used to establish the 0dB point there is always the possibility that a particular fiber optic cable will have a cladding layer that has good optical transmission. So good in fact that more light will exit than should. If this is the case, there is the possibility of overloading.

Solving the overloading problem can be accomplished in one of two ways. If the fiber length to be used is very short (less than 100 meters for example) an optical attenuator can be installed between either end of the fiber optic cable and the optical port. If the fiber length is longer than a 100 meters or so (which is usually the reason that a fiber optic system is used in the first place) then the problem will solve itself in the final installation. Care need only be taken when bench- testing the system to insert some attenuation. In the case of ST or FC type optical connectors sliding the tip of the connector a few millimeters out of the optical port can do this.

An overload of optical transmission power in a final installation is not a very common problem. This problem will appear when bench-testing a fiber optic transmission system using a fiber jumper without an attenuator. Now, that you understand mechanism present, you know the problem will correct itself in the final installation, or for extremely short distances only an attenuator has to be utilized.
 

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