Handbook for Sound Engineers

Transmission Techniques: Fiber Optics 475

and reflection and unlimited shelf life, and it does not

require polishing or the use of adhesives. The splice,

Fig. 15-35, also enables the user to visually verify the

splicing process.

Another splice type is Fibrlok™ optical fiber splice

by 3M TelComm Products Division. After cable prepa-

ration, the two fibers are inserted into a Fibrlok splice.

The assembly tool is then used to close the cap, which

forces the three locating surfaces against the fibers. This

aligns the fibers precisely and permanently clamps them

inside the splice. Fibrlok is for both single- or multi-

mode fibers. The splice, Fig. 15-36, can be performed in

about 30 seconds after preparing the fiber ends.

15.10 Couplers

A fiber optic coupler is used to connect three or more

fibers together. The coupler is different from a connec-

tor or splice which joins only two entities. The fiber

optic coupler is more important in fiber optics than for

electrical signal transmission because the way optical

fibers transmit light makes it difficult to connect more

than two points. Fiber optic couplers or splitters were

designed to solve that problem.

There are five main concerns when selecting a

coupler:

1. Type of fiber used (single- or multimode).


2. Number of input or output ports.


3. Sensitivity to direction.


4. Wavelength selectivity.


5. Cost.

There are two types of passive couplers called the T

and the Star couplers, Fig. 15-37. The T coupler has

three ports connected to resemble the letter T. The star

coupler can employ multiple input and output ports and

the number of inputs can be different from the number

of outputs.

Couplers are quite simple to use. The following

calculation must be made:

Excess loss: The losses that are internal to the coupler

from scattering, absorption, reflections, misalignment,

and poor isolation. Excess loss is the ratio of the sum of

all the output power at the output ports to the input

power at the input ports. Usually it is expressed in dB.

Insertion loss: This loss is the ratio of the power

appearing at a given output port to that of an input port.

Thus, insertion loss varies inversely with the number of

terminals.

15.11 Fiber Optic System Design

15.11.1 System Specifications

When designing an FO system, it is often best to order

all the component parts from one manufacturer; this

way you can be sure of the parts being compatible.

Many manufacturers have developed complete systems

and have components available for the asking. The fol-

lowing are important things to consider when selecting

parts and designing a system:

1. If system is analog:
   A. Bandwidth in hertz (Hz) or megahertz (MHz).

Figure 15-35. Lucent CSL LightSplice. Courtesy Lucent
Technologies.

Figure 15-36. Fibrlok optical fiber splice.

Cap

End plug

(each end)

Jacket

Fiber entry port

(each end) Fiber sizedesignation circles

Figure 15-37. T and star couplers.

A. T coupler

B. Star coupler