Optical attenuator

A passive optical attenuating device comprises an optical waveguide adapted to receive optical radiation and absorb, along its length, at least 0.2 dB/m of the optical radiation. The waveguide section may be coupled to a low-loss optical fiber so as to receive an optical signal to be attenuated therefrom. In accordance with one aspect of the invention, at least one region of the waveguide is doped with a transition metal to achieve a pre-selected absorptivity per unit length so that a controlled degree of attenuation can be achieved.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to the transmission of optical 
fiber signal communications and, more particularly, to an optical 
attenuator which employs a high-loss optical waveguide section. 
2. Description of the Prior Art 
The advantages of using optical transmission systems for communications are 
well recognized. Optical waveguides comprising dielectric fibers having a 
substantially transparent core coaxially surrounded by a cladding material 
of lower dielectric index may be used to guide and transmit optical 
signals over long distances in optical communications systems. Generally, 
great care is taken to minimize light losses due to absorption and 
scattering along the length of the filament, so that light applied to one 
end of the optical filamentary material is efficiently transmitted to the 
opposite end of the material. For this reason, low attenuation optical 
waveguides are commonly formed from fibers doped with rare earth elements. 
There are many situations, however, in which it is necessary to utilize 
optical attenuator devices to reduce the amount of power present in the 
optical signal. 
Two output characteristics are usually described for an optical 
transmission system: the transmission rate, e.g. in Mbit/s as a measure of 
the amount of transmitted data, and the system range, which indicates the 
maximum attenuation that may be placed between emitter and receiver in 
order to assume a certain minimum quality of transmission. However, 
further information is needed for practical use. A receiver is only able 
to function optimally within a certain range of the optical input level. 
Too low a radiation capacity, as well as too high a level, can impair the 
transmission quality. The path attenuation in optical transmission systems 
is a function of fiber length and the fiber attenuation coefficient. In 
addition, emitter output and receiver sensitivity have tolerances and may 
exhibit aging. For these reasons, an attenuation device for adapting the 
path attenuation to the receiver's optimum function range is needed. 
For example, disclosed in U.S. Pat. No. 5,187,610 entitled LOW NOISE, 
OPTICAL AMPLIFIER HAVING POST-AMPLIFICATION LOSS, issued to Habbab, et. 
al. and assigned to the assignee herein, AT&T, is a technique for 
improving the noise performance of an optical amplifier while concurrently 
meeting the amplifier design criteria for output signal power, amplifier 
gain, and compression. These benefits are obtained by combining an optical 
amplifier element with a post-amplification optical attenuator/loss 
element and pumping the optical amplifier to produce a higher gain and, 
therefore, a larger output signal power which is substantially compensated 
by the post-amplifier loss element. Compensation by the loss element 
allows the combination of elements to produce an output signal power which 
meets the design criterion. 
Habbab et al. provide several examples of conventional passive optical 
attenuation devices capable of serving as the means for introducing a 
post-amplifier loss in the inventive system. The loss introducing 
techniques suggested in that patent include the use of a fiber-to-fiber 
coupler having an intentional misalignment between the two fibers to cause 
the desired amount of loss or providing curvature or bending of an optical 
fiber or dielectric waveguide to subject the lightwave signal to 
controllable amounts of loss as a function of the radius of the curve or 
bend. In each of these loss introducing techniques, precise adjustments to 
the fiber gap or curvature are necessary in order to achieve the requisite 
amount of attenuation. 
Fiber optic attenuating devices are also employed as terminations for the 
ends of unused optical fibers of devices such as star couplers to 
eliminate deleterious reflections. As will be readily understood by those 
skilled in the art, an optical star coupler is a device which comprises a 
plurality of input optical fibers, a coupling region, and a plurality of 
output optical fibers. An optical star coupler typically operates to 
transmit a fraction of the optical power received at each input fiber to 
all output fibers and is particularly useful for implementing an optical 
bus which enables a plurality of terminals to communicate with one 
another. 
A typical, off-the-shelf star coupler is an 8.times.8 device, i.e., it 
comprises eight input fibers and eight output fibers. However, in a 
typical application not all of the input fibers receive optical signals 
and not all of the output fibers are connected to other fibers for 
transmitting optical signals to remote locations. For example, to provide 
a 4.times.4 coupler, four of the eight input fibers are not utilized and 
four of the eight output fibers are not utilized. These unused fibers 
conveying output signals give rise to undesired reflections that result 
from the fiber-air index of refraction mismatch at the ends of the unused 
fibers. Typically, the index of refraction mismatch at a glass fiber-air 
interface results in a reflection of four percent of the optical signal. 
Thus, in a 4.times.4 coupler formed by an 8.times.8 star coupler having 
four unused input fibers and four unused output fibers, the optical signal 
arriving on each of the used or connected input fibers is distributed by 
the coupling region to all eight output fibers. The radiation distributed 
to the used output fibers is transmitted via connector assemblies to other 
fibers for transmission to remote locations. At the ends of the four 
unused output fibers, reflections take place. The reflected radiation is 
then distributed by the coupling region to all the input fibers where 
reflection again takes place at the glass-air interfaces at the ends of 
the unused input fibers. This reflected radiation is then, in turn, 
transmitted by the coupling region back to the output fibers, and so on. 
Because glass-air interfaces at the ends of unused fibers cause multiple 
reflections in a device such as an optical coupler, a variety of 
reflection-less terminating devices have been proposed. For example, in 
U.S. Pat. No. 4,998,795 entitled "REFLECTION-LESS TERMINATOR" and issued 
to Bowen et al., a terminator comprising a length of optical fiber is 
described. The front end of the fiber is attached to a ferrule for mating 
with a connector plug attached to the end of the fiber to be terminated. 
The rear end of the fiber is crushed at an angle and inserted into an 
index matching opaque adhesive material. Because there are substantially 
no reflections at the fiber-adhesive material interface, substantially all 
of the radiation propagating in the fiber length is transmitted into the 
opaque index matching adhesive where this radiation is absorbed. While the 
device taught by Bowen et al. does appear to achieve a substantially 
reflection-less termination, its complex structure requires several 
labor-intensive processing steps and may degrade in performance over time. 
In view of the above, it would be advantageous to provide a passive optical 
signal attenuating element which may be flexibly configured to provide a 
controlled degree of attenuation such that it may be inserted at any point 
in an optical path to introduce a desired amount of loss or be utilized to 
provide a substantially reflection-less termination for an optical fiber. 
SUMMARY OF THE INVENTION 
A passive optical attenuating device constructed in accordance with the 
present invention includes a section of a waveguide having a core and 
cladding. The waveguide section is adapted to receive optical radiation 
and absorb at least 0.2 dB/m of the optical radiation along its length. In 
accordance with one aspect of the present invention, the waveguide may be 
configured to absorb between approximately 1 to 1000 dB/m in a 
substantially uniform manner along its length. The attenuation remains 
constant at optical powers of less than 100 mW. 
The waveguide may be formed from a fused silica fiber having a region doped 
with ions of a metal selected from the class consisting of Fe, Ni, Co, Cr, 
Cu, Ti, Mn, and V, in a concentration effective to provide a predetermined 
degree of absorption at a given wavelength. In accordance with one 
embodiment of the present invention, the doped region comprises the core 
of the fiber. In accordance with another embodiment, the doped region is a 
ring layer surrounding the core of the fiber. 
To facilitate connection to a low-loss, signal carrying waveguide, the 
optical attenuator of the present invention may also be provided with 
means for coupling the waveguide section thereto so as to enable optical 
signals to be received therefrom. 
The degree of attenuation provided by the waveguide section of the present 
invention is governed by the application. If desired, for example, the 
waveguide may be configured to absorb substantially all input radiation at 
a given wavelength. Thus, it may be utilized to provide a reflection-less 
terminating device for the unused, signal carrying fibers of a star 
coupler or similar device. 
A method of fabricating an optical signal transmission system having at 
least one optical waveguide for defining an optical signal transmission 
path comprises the steps of providing an optical waveguide section having 
a core and cladding and defining an absorbing region adapted to receive 
optical radiation and to absorb at least 0.2 dB/m of the received optical 
radiation. The radiation is absorbed along the length of the waveguide 
section to provide attenuation which remains substantially constant below 
a predetermined optical power. The method further includes a step of 
coupling a first end of the optical waveguide section to a corresponding 
termination of the at least one optical waveguide. 
Where a single waveguide defines the optical transmission path, a 
substantially reflection-less, high loss termination can be fabricated 
utilizing the inventive method of the present invention. Alternatively, a 
desired degree of attenuation can be provided between two waveguides by 
coupling each end of the optical waveguide section to a corresponding end 
of the first and second optical waveguides. In accordance with an 
illustrative embodiment of the present invention, the at least one optical 
waveguide and the optical waveguide section are optical fibers and the 
coupling step comprises fusion bonding the respective optical fibers. 
Other features of the present invention will become apparent from the 
following detailed description considered in conjunction with the 
accompanying drawings. It is to be understood, however, that the drawings 
are designed solely for purposes of illustration and not as a definition 
of the limits of the invention, for which reference should be made to the 
appended claims.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS 
As indicated above, it is an object of the present invention to provide an 
attenuating element which may be utilized in a terminating assembly to 
provide a reflection-less termination for an optical fiber or in a 
coupling assembly to introduce a controlled degree of attenuation between 
two sections of fiber defining an optical signal path. For each of these 
applications, a section of fiber doped with a transition element (e.g,, 
Fe, Ni, Co, Cr, Cu, Ti, Mn, V) is utilized to provide the desired degree 
of loss/attenuation. 
It is, of course, well known that the fabrication of doped fused-silica 
glasses having extremely low optical losses must be virtually free of 
transition elements to reach the attenuation levels (&lt;20 dB/km) required 
for the successful operation of communication systems at wavelengths 
between about 600 and 1600 nm. An important aspect of the present 
invention, however, resides in the realization by the inventors hereof 
that a section of fiber doped with an appropriate concentration of 
transition metal ions may be utilized to provide a precisely controlled 
degree of attenuation at the operating wavelength. 
An illustrative embodiment of an attenuating fiber element 10 having an 
8-12 micron core region 12 and 125 micron thick cladding layer 14 is 
depicted in FIG. 1. In the embodiment of FIG. 1, core region 12 is doped 
with a refractive index raising element such as Ge and one of the 
aforementioned transition metals. For purposes of the present invention, 
it should be understood that techniques for forming doped fused silica 
glass are well known in the art and a detailed discussion of the same has 
been omitted. However, for a more detailed explanation of one technique 
which may be employed, reference may be had to U.S. Pat. No. 4,787,927 
issued to Mears et al. and entitled FABRICATION OF OPTICAL FIBERS, the 
entire disclosure of which is expressly incorporated herein by reference. 
In any event, it will be readily appreciated by those skilled in the art 
that the precise level of attenuation provided by the illustrative fiber 
section depicted in FIG. 1 will at least in part depend upon the 
concentration and absorption/loss characteristics of the transition metal 
selected. In an article by Peter Schultz entitled "Optical Absorption of 
the Transition Elements in Vitreous Silica", published in 57 Journal of 
the American Ceramic Society 309-313 (July 1974), practical absorptivity 
spectra for transition elements in fused silica prepared by flame 
hydrolysis are provided. Schultz reports that of the transition metals, 
vanadium is the strongest absorber in the 800 nm to 1000 nm range, with 
just 19 ppba V in fused silica being required to produce a 20 dB/km loss 
at 800 nm, while at wavelengths above 1300 nm chromium provides the 
strongest degree of absorption. 
While forming an optical fiber doped with a transition metal is one 
technique for achieving an attenuating optical element in accordance with 
the present invention, other techniques may also be utilized to fabricate 
fibers having the desired absorption properties. For example, satisfactory 
results have also been achieved utilizing a post-processing technique such 
as impregnating a Ge-doped fiber with hydrogen. The hydrogen permeates the 
fiber and is reacted thermally or photolytically with the Ge to provide an 
absorptive region. An alternate post-processing technique which may be 
utilized to fabricate a fiber with suitable attenuating characteristics 
comprises gamma radiating the section of fiber. It should also be noted 
that hydrogen reaction may also be used to alter the oxidation state of 
the transition metals and thus vary the degree of absorption. A given 
fiber may then be tailored to provide a range of attenuation levels. 
In FIG. 2A, there is illustrated an attenuating fiber element constructed 
in accordance with another embodiment of the present invention. In the 
embodiment of FIG. 2A, fiber element 20 includes a Ge doped core region 22 
and a cladding layer 24 which defines a transition metal doped, absorptive 
ring layer 26. In accordance with this alternate embodiment, an optical 
signal having a short wavelength, .lambda..sub.1 avoids passage through 
the ring layer 26 and thus experiences low loss. In contrast, an optical 
signal having a long wavelength .lambda..sub.2 passes through ring layer 
26 and thus experiences a high loss. 
As seen in FIG. 2B, by providing fiber section 20 with a tapered region 28, 
it is also possible to cause signals at lower wavelengths to be attenuated 
by ring layer 26. Note that prior to tapered region 28, the mode field 
does not overlap the ring layer 26. At the taper the mode field expands, 
overlapping into the ring layer 26 to produce a lossy region at the taper 
28. 
As will be readily appreciated by those skilled in the art, a wide variety 
of substantially reflectionless, passive attenuating and terminating 
devices may be fabricated utilizing the attenuating fiber element of the 
present invention. For example, an attenuating device suitable for 
insertion into an existing optical path may be constructed by fusion 
splicing both ends of a lossy fiber section to respective pigtail sections 
of fiber having optical properties corresponding to those of the fiber 
comprising the existing optical path. These pigtails may then be fusion 
spliced or otherwise coupled to the corresponding ends of the optical 
fiber at the insertion site. A substantially reflection-less terminating 
device constructed in accordance with the present invention may include, 
for example, a section of lossy fiber fusion spliced to an unused fiber. 
The construction of an alternate terminating device utilizing a coupling 
assembly is depicted in FIG. 3. 
Illustratively, the terminating device 30 incorporates a bayonet type 
slotted coupling ring 32 which connects by means of a connector receptacle 
to a suitable bayonet connector (not shown) associated with an unused or 
other fiber to be terminated. The terminator 30 comprises a length of 
fiber 34 having a transition metal doped core in accordance with the 
embodiment depicted in FIG. 1. As will be readily appreciated by those 
skilled in the art, the degree of transition metal doping of fiber length 
34 may be selected such that substantially all optical radiation 
propagating therein is absorbed. 
The fiber length 34 has a front portion 36 and a rear portion 38. The front 
portion 36 of the fiber length 34 is attached to the bayonet type 
connector plug designated 32. The rear portion 38 of the fiber length 34 
is received in a housing 40 which supports a ceramic ferrule 42 having a 
bore 44 in which the front fiber portion 36 is inserted. Illustratively, 
the tip 46 of the ferrule is polished for physical contact with a similar 
ferrule (not shown) comprising part of the opposing bayonet connector (not 
shown) associated with the free end of an unused fiber. 
It will, of course, be understood that any suitable device may be utilized 
for coupling an attenuating optical fiber element constructed in 
accordance with the present invention to one or more fiber ends and that 
the specific type of coupling selected will generally depend upon the 
application involved. Thus, while there have been shown and described and 
pointed out fundamental novel features of the invention as applied to 
generally preferred embodiments thereof, it will be understood that 
various omissions and substitutions and changes in the form and details of 
the disclosed invention may be made by those skilled in the art without 
departing from the spirit of the invention. It is the intention, 
therefore, to be limited only as indicated by the scope of the claims 
appended hereto.