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.
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.
Fiber Optic Communications Systems
