Telecommunication systems using optical waveguides

A method for transmitting light signals through optical waveguides is disclosed. The method comprises passing light emitted from a source of light, such as a laser, through a plurality of individual, longitudinally connected optical waveguides having progressively greater core diameters. Telecommunications systems for implementing the method are also disclosed.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention is directed to an improved method of transmitting 
light signals through optical waveguides and telecommunications systems 
which use the method. 
2. Description of the Prior Art 
The use of optical waveguides in telecommunications systems has become 
increasingly prevalent during the past several years. In brief, optical 
waveguides are extremely thin, flexible glass fibers, prepared by drawing 
glass preforms under controlled conditions. Although these conditions are 
subject to wide variation depending upon such factors as the composition 
of the glass and the drawing procedure, the optical waveguide so prepared 
will comprise, in its simplest form, a glass core of a given, constant 
diameter, surrounded by a glass cladding having an index of refraction 
less than that of the glass core. The cladding acts as a barrier which 
confines attenuation of the light traveling to the core. 
Thus, depending upon the specific physical and chemical properties of the 
waveguide, light signals may be transmitted through the core of the fiber 
with only minimal attenuation of light. 
However, attenuation poses a far greater problem when two optical 
waveguides are connected. Such connections are inevitable in any 
telecommunication system. To minimize this problem, connectors for optical 
waveguides have been developed. These devices attempt to precisely align 
the two waveguides so that the axis of the transmitting waveguide is 
coincident with the axis of the receiving waveguide and the ends of each 
waveguide are precisely adjacent to one another. Unfortunately, even with 
the use of such connectors, attenuation at the connection interface of the 
waveguides occurs. 
SUMMARY OF THE INVENTION 
Accordingly, it is a primary object of the present invention to provide a 
method for transmitting light signals through a plurality of optical 
waveguides which minimizes attenuation at connection points between two 
waveguides. 
Another object of the invention is to provide a telecommunication system 
for transmitting light signals through optical waveguides with minimal 
attenuation due to connections in the system. 
Still other objects and advantages of the invention will become apparent to 
those skilled in the art upon reviewing the description of the invention 
which follows. 
The foregoing objects and advantages are accomplished by providing a method 
for transmitting light signals from a source of light to a receiver which 
comprises passing light emitted from a light source through a plurality of 
individual, longitudinally connected optical waveguides having 
progressively greater core diameters. The method is adapted for use in 
telecommunication systems by providing a system for transmitting light 
signals comprising a source of light connected to a plurality of 
individual, longitudinally connected optical waveguides having 
progressively greater core diameters which terminate in a receiver for the 
light signals.

DETAILED DESCRIPTION OF THE INVENTION 
The optical waveguides used in operating the method and telecommunication 
system of the invention are not critical, although certain types, as more 
fully described below, have been found to be particularly effective. 
Hence, any of the conventional waveguides currently available, as well as 
those to be developed which function in an equivalent manner, may be used 
in the present invention. In general, such waveguides comprise extremely 
thin, cylindrical fibers containing a cylindrical core of a given diameter 
and refractive index, and a cladding which surrounds the core and 
possesses an index of refraction less than that of the core. Such fibers 
are usually grouped into one of two categories--graded or step index 
fiber. Graded index fiber is comprised of a core of substantially uniform 
composition and refractive index and a cladding having a non-uniform, 
radially varying composition and correspondingly, non-uniform, radially 
varying indices of refraction. This type of fiber is recognized as the 
preferred fiber for use in optical waveguide cables since it is capable of 
transmitting a large range of bandwidths. On the other hand, step fiber is 
comprised of a core of substantially uniform composition and refractive 
index and a cladding of a different, substantially uniform composition and 
refractive index. The refractive index is, of course, less than that of 
the core. 
The methods for preparing both graded and step index waveguides are well 
known to those of skill in the art, being described in numerous U.S. 
patents and other technical literature. In brief, these methods comprise 
forming a cylindrical preform, which is subsequently heat treated and 
drawn into fiber. By careful control of the composition of the glass in 
the preform, fiber is obtained which has the desired composition and 
refractive index. In general, the compositional variations are obtained by 
incorporating various dopants into the pure silica glass. Thus, dopants 
which increase the refractive index, such as titanium oxide, tantalium 
oxide, tin oxide, niobium oxide, zirconium oxide, aluminum oxide, 
lanthanum oxide and germanium oxide, may be incorporated in the core. 
Alternately, the core may be composed of pure silica and the cladding 
composed of silica doped with a component which decreases the refractive 
index, such as boron oxide. 
In constructing systems for transmitting light signals through optical 
waveguides, a relatively short (i.e., about one meter) transmitting 
optical waveguide is connected to a source of light, usually a laser. The 
end of the waveguide is equipped with a special lens to focus the light 
emitted from the light source into the core of the fiber. This 
transmitting waveguide is connected to the main transmission line which 
comprises an optical waveguide cable. The optical waveguide cable may 
comprise one or more optical waveguides connected longitudinally, 
depending upon the overall length of the main transmission line. At the 
receiving end of the line, the optical waveguide cable is connected to a 
relatively short receiving waveguide which is, in turn, connected to a 
receiving unit such as a pin diode. To facilitate construction and 
maintenance of these telecommunication systems it is imperative that the 
optical waveguides in the system have identical outer diameters. This 
insures that parts may be mass produced according to uniform 
specifications. 
The present invention modifies the foregoing conventional telecommunication 
system by requiring that each succeeding optical waveguide possess a 
greater core diameter than the transmitting waveguide to which it is 
connected. This arrangement reduces the amount of attenuation which occurs 
at the connection interfaces by increasing the cross-sectional area into 
which the light signals are transmitted across the interface. In this 
manner, light which would normally escape from the system across the 
interface is picked up by the expanded core area of the receiving 
waveguide. 
Thus, according to a preferred embodiment of the invention shown in the 
FIGURE of drawing, there is provided a telecommunication system comprised 
of a source of light 10 connected to at least one transmitting optical 
waveguide 11 having a core 12 of diameter d.sub.1 which connects the light 
source to at least one optical waveguide cable 13. The cable, which itself 
is comprised of an optical waveguide having a core 14 of diameter d.sub.2 
which is greater than diameter d.sub.1, is the main transmission line. The 
cable is, in turn, connected to at least one receiving optical waveguide 
15 having a core 16 of diameter d.sub.3 which is greater than both d.sub.1 
and d.sub.2. The receiving optical waveguide 15 is connected to receiving 
unit 17. Each optical waveguide core is surrounded by a cladding 18, which 
decreases in width as the core diameter in each succeeding waveguide 
increases, to maintain a constant, uniform outer diameter for the 
waveguides. In practice, the diameter d.sub.1 of the core of the 
transmitting optical waveguide is approximately 55 microns, the diameter 
d.sub.2 of the core of the optical waveguide cable is approximately 80 
microns, the diameter d.sub.3 of the core of receiving optical waveguide 
is approximately 110 microns and the outer diameters of all waveguides are 
approximately 125 microns. 
It is also preferred to construct the transmitting waveguide and waveguide 
cable of graded index optical fiber while employing a step index fiber as 
the receiving optical waveguide. This is because graded optical waveguides 
allow maximum bandwidth transmission while step index optical waveguides 
exhibit greater collecting power. Thus, by constructing the 
telecommunication system in this manner, both bandwidth transmission and 
receiving efficiency is maximized. 
While the present invention has been described in terms of various 
preferred embodiments, those of skill in the art will recognize that 
various additions, modifications, and omissions may be made without 
departing from the spirit thereof. Accordingly, it is intended that the 
scope of the present invention be limited solely by that of the claims 
which follow.