Note: Descriptions are shown in the official language in which they were submitted.
7 ~P S
7910-2
FIBER OPTICAL MULTIPLEXER/DEMULTIPLEXER
Field of the Invention
This invention relates generally to optical
fiber communications, and more specifically to modules
for intercoupling of light from or to fibers and perform-
ing monitoring, splitting, 6witching, duplexing and
multiplexing functions.
Backqround Of The Invention
As existing communication systems have become
increasingly overloaded, optical tr.ansmission through
transparent fibers has been found to provide a means of
achieving a smaller cross-section per message, thus
enabling an increased capacity within existing conduit
constraints. The basic medium of t~ansmission is an
optical fiber. A first type of fiber is a 6tepped index
fiber which compri~es a transparent core member and a
transparent cladding, the core member having a higher
inde~ of refraction than the cladding. Light is transmit-
ted through the core, and contained within the core byinternal reflection. So long as the light does not
deviate from the fiber axis by more than the complement
of the critical anqle for the core-cladding interface,
total internal reflection with 6ubstantially no loss
results. A ~econd type of fiber i5 a graded index fiber
whose refractive index gradually decreases away from the
fiber axis. Transmi~sion i5 highly reliable, and is
7i~S
~ubstantially insensitive to electrical noise, ~ross
coupling between channel~, and the like.
As with any communication medium, once a ~uit-
able transmission line has been found, the need arises
for modules to couple sources and detectors to the line,
couple lines together, perform switching, splitting,
duplexing, and multiplexing functions. Ultimately, the
total system can be no more reliable than these modules.
When it is considered that the core of a typical optical
communication fiber is characterized by a diameter of
only 60 microns, it can be immediately appreciated that
such modules must be fabricated and installed to highly
precise tolerances.
In order to realize the inherent reliability of
optical fiber communication systems, the modules them-
selves must be highly reliable since they are typically
installed in relatively inaccessible locations (e.g.
within conduits running under city streets, etc.). Given
this requirement, it can be seen that it would be highly
desirable to have monitorinq signals that would verify
the operation of the modules and the integrity of the
fibers themselves. A further requirement for a satis-
factory optical communication system is that the modules
introduce a minimum of loss into the system. It has only
been with the development of extremely high transparency
fibers that optical fiber communication has become prac-
tical, and the introduction of lossy modules would con-
6iderably undercut the advantages and efficacy of such
fiystems .
Unfortunately, existing devices for interfacing
fibers to sources, detectors, and each other, have proved
to be lossy, bulky, delicate, and expensive. Thus, while
fiber optic communication sy~tems are proving to be
highly advanta~eous they are prevented from realizing
their fullest potential.
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Summarv of the Invention
The present invention provides modules for
interfacing optical fibers with very low light 106s and
with provision for monitoring of the optical 6ignal. The
modules according to the present invention are character-
ized by the precise tolerances required in high capacity
optical communication systems and yet may be mass pro-
duced at reasonable costs.
A device according to the present invention
comprises a transparent imaging element having a curved
reflective surface at one end and prealigned fiber inser-
tion holes at the other end. The transparent element is
characterized by an index of refraction equal to that of
the fiber core, and the fibers are glued in their respec-
tive holes with index matching cement. The holes facili-
tate precision alignment and provide mechanical strength.
The curved reflective surface is charactexized by a focal
plane having the property that a point source of light at
a first location in the focal plane is imaged at a second
complementary location in the focal plane, and the fiber
insertion holes maintain the ends of the fibers at suit-
able complementary locations within the focal plane. In
this context, the term "fiber insertion hole" should also
be taken to include a hole sized to maintain a light
60urce or detector at a given location within the focal
plane. In some applications, the source or detector
would be directly mounted to the transparent imaging
element, while in other applications the source or detec-
tor would communicate with the imaging element via a
3~ short length of fiber.
The use of a transparent imaging element char-
acterized ~y an index of refraction equal to that of the
fiber core has the important advantage that fresnel
reflec~ion at the fiber end, a significant potential
~ource of loss of the signal, i5 eliminated. Also,
~ I, l'7i~
refraction whi~h would spread the li~ht, thus necessitating a
larger reflective surface, is avoided. Moreover, the use of
prealigned fiber insertion holes wherein the fiber ends are
cemented into automatic registered position with index matching
cement results in a monolithic structure that is dimensionaLly
stable and sufficiently rugged to provide many years of trouble
free operation. A further advantage of the monolithic structure
wherein reflective light losses are avoided is that reflected
liaht pulses that could affect other communication line within
the system are avoided.
According to the present invention there is provided a
device for permitting multiple optical signals of differing
wavelength to be transmitted simultaneously on a single optical
fiber comprising:
imaging reflective means characterized by a focal plane
wherein a point in said focal plane is imaged in said focal
plane;
means for registering an end of said optical fiber at a
first location within said focal plane;
classifying means cooperating with said imaging
reflective means for imaging light of a first wavelength
emanating from said fiber end at a first image location in said
focal plane and imaging light of a second wavelength emanating
from said fiber end at a second image location within said focal
plane and displaced from said first image position.
1 I', 1'7~5
Preferably, said imaging reflective means and said
classifying means together comprise a concave reflection grating.
Preferably also, classifying means comprises a dichroic
beam splitter.
Preferably also, second means for registering a source
of light of said first wavelength at said first image location;
and means for registering a detector sensitive to light of said
second wavelength at said second image location, whereby said
device functions as a duplexer.
Preferably also, second and third means for registering
respective sources of light of said first and second wavelengths
at said first and second image locations whereby said device
functions as a multiplexer.
Preferably also, second and third means for registering
respective detectors sensitive to light of said first and second
wavelengths at said first and second image locations whereby
said device functions as a demultiplexer.
For a further understanding of the nature and
advantages of the present invention, reference should be had to
the remaining portions of this specification and to the attached
drawings.
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J. 1'7 1 7~5
Brief Description Of The Drawings
Fig. 1 is an isometric cut-away view of a fiber/fiber
coupler according to the present invention;
Fig. 2 is a simplified cross-sectional view of the
coupler of Fig. l;
Fig. 3 is a simplified cross-sectional view of a
source/fiber coupler;
Figs. 4A and 4B are simplified cross-sectional views of
different embodiments of a splitter according to the present
invention;
Figs. 5A and SB are simplified cross-sectional views of
alternate embodiments of a switch according to the present
invention;
Fig. 6 is an exploded view of the switch of Fig. SB
showing a mechanism for achieving increased precision;
Figs. 7A and 7B are simplified cross-sectional views of
alternate embodiments of two colored duplexers;
1 ~';'1 7VS
Fiqs. 8A, 8B and 8C are 6implified cross-sec-
tional views of multiplexer and demultiplexer embodi-
ments;
Figs. 9A and 9B 6how a directional monitor.
5Description Of The Preferred Embodiments
The present invention relates to modules for
interfacing optical fibers with each other, with light
~ources, and with detectors. This is generally accom-
plished by positioning detectors, ~ources, or respective
ends of 6uch fibers in a focal plane as will be described
below. It will be immediately apparent to one of ordi-
nary skill in the art that an input fiber and a light
source may be 6ubstituted for one another, that an output
fiber and a detector may be substituted for one another,
lS and that the 6ystem may be "time reversed" by inter-
changing input6 and outputs. Therefore, while the de-
scription that follows is in specific terms, such equiva-
lent systems will be made readily apparent.
Fig. 1 is an isometric cut-away view of a
fiber/fiber coupler 10 according to the present inven-
tion. Coupler 10 couples input and output fiber optic
cables 12 and 13 having respective fibers 14 and 15
therein 60 that optical information traveling within the
core of input fiber 14 is transmitted to the core of
output fiber 15 with low loss. An electrical output
signal proportional to the optical ~ignal power in fiber
14 i5 provided by monitor unit 16 at an electrical output
terminal 17 (preferably a "BNC" output connector).
Fiber6 14 and 15 optically communicate with a transparent
imaging element 20 within a housing 21 as will be de-
scribed below, the optical communication reguiring pre-
cise registration of the ends of the fiber~. Gross
mechanical positioning of the fiber optic cable~ is
accomplished by a clamping mechani6m 22 comprising
grooved mating body portions 25 for positioning and
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holding the cables. Elastomeric compression 6eals 27
provide ~train relief when mating portions 25 are tightly
fastened to one another, as for example by screwing.
Fig. 2 is a cross-sectional view of tran6parent
imaging element 20 with fibers 14 and 15 registered
thereto. Imaging element 20 comprises a body 30 of
transparent material, body 30 having a curved surface 32
at a first end and paired cylindrical fiber insertion
holes 35 and 37 at a second end. Surface 32 is a pol-
ished surface and coated with a reflective coating suchas a multilayer dielectric coating that reflects most of
the light incident on it from within transparent body 30,
but transmits a small fraction. Surface 32 is character-
ized by a focal plane 40 having the property that a point
source in focal plane 40 is imaged in focal plane 40.
Surface 32 is preferably spherical, in which case focal
plane 40 is perpendicular to a radial axis and passes
through the center of curvature. Fiber insertion holes
35 and 37 are of a diameter to accomodate fibers 14 and
15 and to maintain the fiber ends at precisely registered
locations in focal plane 40 such that the cone of light
emanating from the end of fiber 14 is imaged on the end
of fiber 15. Body 30 is preferably formed from a trans-
parent plastic ~y an injection molding process. The
transparent material i6 chosen to have an index of re-
fraction equal to that of the fiber core, and the fiber
ends are glued into their respective fiber insertion
holes with an index matching cement. ~he fiber insertion
holes themselves do not provide the precision alignment,
but rather facilitate such alig~ment which may be carried
out in a suitable jig or the li~e. Once the fibers have
been cemented into the holes, mechanical strength is
achieved.
Monitor unit 16 compri~es a ~hotodetector 45
and an associated protective window 47. Monitor unit 16
~'~
~ 1 ~ 17~5
is located outside transparent ~ody 30 in a position to
intercept the light that is transmitted by the reflective
coating on 6urface 32. Monitor unit 16 is ~ 6elf con-
tained unit which may be inserted into housing 21 if the
monitoring function is required. If no monitoring is
required, an opaque plug may close off the end of housing
21. -
~ he ends of fibers 14 and 15 are cleaved per-
pendicular to the re6pective axes and located symet-
rically about the center of curvature within focal plane40. In order to preserve modes, fiber insertion holes 35
and 37 are inclined with respect to one another ~o that -
the axes of the respective fibers are directed to a
common intersection point 42 on the axis of surface 32.
As discussed above, a light source may be
substituted for input fiber 14 without any change in the
functioning of the device. Fig. 3 shows a source/fiber
coupler 50 that differs from fiber/fiber coupler 10 only
in that a light ~ource 52 is substituted for input fiber
12. The purpose of coupler S0 is to transmit the light
from source 52 into a fiber 53. Source 52 may be a
metal/ceramic "pillbox" light emitting diode or a laser
having an optical coupling plastic window 55 and an oil
interface 57 to provide optical continuity and index
matching. Since light source 52 has a larger diameter
than that of a fiber, the complementary optical points
within the focal plane are moved farther away from the
center of curvature to accomodate the larger diameter
element. I ~ rder to maintain mode preservation and
minimize aberrations, fiber 53' is inclined at a cor-
responding larger angle with respect to the optic a~is.
Where a monitoring function i6 carried out, the current
from photodetector 45 may be used to provide feedback to
the power ~ource driving light source 52 to improve the
linearity of the dependence of light output on drive
current.
_ g _
117~7~ 5
Fig. 4A 6hows a first embodiment of a two-way
6plitter 60 for dividing the light carried by an input
fiber 61 between fir~t and 6econd output fiber6 62 and
65. As in the coupler, the basic element of 6plitter 60
i~ a transparent body 68 having a reflective surface at
one end and fiber insertion holes at the other end.
~owever, the reflective 6urface is continuous but not
mathematically ~mooth, compri6ing abutting 6pherical
6urface segments 70 and 72. Spherical 6urface 6egments
70 and 72 are characterized by the 6ame radius but have
respective center~ of curvature 75 and 77 that are dis-
placed from the axi6 of input fiber 61. In particular,
center of curvature 75 i~ midway between the end of fiber
61 and the end of fiber 62; center of curvature 77 i~
midway between the end of fiber 61 and fiber 65. Gen-
erally, for an N way 6plitter, N pie-shaped surface
6egments having wedge angles 360 and respective 6phere
centers in a circular array 6urrounding the end of the
input fiber would be required.
Fig. 4B is a cross-6ectional view of an alter-
nate embodiment of a two-way 6plitter 80 for dividing the
- light from an input fiber 82 evenly between output fibers
85 and 87. ~hi6 embodiment differ6 from the embodiment
of Fig. 4A in that each fraction of the input light cone
is intercepted by a plane reflecting surface before
encountering the corresponding focusing 6egment. In
particular, a transparent bo~y ~2 is configured with a
wedge-shaped depression 92 which defines respective plane
interfaces 95 and 97 tha~ come together at an apex 100 on
the axi6 of input fiber 82. ~he half cone that reflects
from plane 6urface 95 impinges on a fir6t curved reflec-
tive 6egment 102 and i6 focused on the end of output
fiber 85. Similarly the other half cone i6 incident on a
~econd curved reflective 6egment lOS and focused on
output fi~er 87. Thi6 embodiment i~ typically ea6ier to
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t l'; 1'7~5
fabricate than ~e embodiment of Fig. 4A 6ince all the
curved 6egments, if 6pherical, may be located with a
common center of curvature. The differing point6 of
focus are achieved by providing a wedge angle of slightly
more than 90. Generally, for an N-way 6plitter with N >
2, an N-sided pyramid rather than a wedge is u6ed.
Fig. 5A i5 a cross-sectional view of a two-way
(single-pole/double-throw) ~witch 110 for 6electively
directing light traveling along an input fiber 112 to
either of paired output fiber6 115 and 117. Switch 110
comprises a transparent body 120 having respective fiber
insertion holes 122, 125, and 127 at one end, and a
continuous, mathematically 6mooth focusing surface 130 at
the other end. Selective 6witching is accomplished by
providing pivoting means to permit reflective 6urface 130
to rotate relative to the fiber insertion holes about a
point 132 intennediate the fiber ends and the reflective
surface and located along the axis of input fiber 112.
~hi6 is accomplished by fabricating body 120 out of a
flexible transparent material and providing the body with
a necked portion 135 proximate pivot point 132 of rela-
tively ~mall diameter to permit flexing without deforma-
tion of the remaining portions of body 120. In particu-
lar, when body 120 is flexed about pivot point 132, a
body portion 137 moves relative to a body portion 138 to
permit the center of curvature of spherical 6urface of
segment 130 to be ~electively directed to a point midway
between the ends of fiber~ 112 and 115 or between the
ends of fibers 112 and 117.
The rotation is effected by electromagnetic
deflection. A soft ~;teel sleeve 140 surrounds body por-
tion 137 having reflective surface 130 thereon and car-
ries tapered wedge sections 142 and 143. For an N-way
~witch, there are N ~uch wedge 6ections. Corresponding
electromagnets 145 and 146 are mounted to the fixed
housing corresponding to each ~witch position. Each
electromagnet includes a yoke 147 and a coil 148. The
yoke has portions defining a tapered depression ~ith
6urfaces adapted to mate with the outer ~urfaces of its
respective wedge 6ection on 61eeve 140 in order to index
movable body portion 137 to the desired position. Ma~-
netic latch elements 150 may be provided to maintain a
given 6witch position after the respective electromagnet
current has been turned off.
Fig. SB is a 6implified cross-sectional view
showing an alternate embodiment of a two-way 6witch.
This embodiment differs from that of Fig. 5B in that the
body comprises two relatively movable portions 155 and
157 having a spherical interface 160 therebet~een to
define an optical ball bearing. The variable region
between body portions 155 and 157 is filled with a sili-
cone oil reservoir 162 being bounded by a suitable bel-
lows 165. The two mating parts are maintained in tension
against one another by a magnet or spring (not 6hown).
While Figs. 5A and 5B illustrate two-way switches, it
will be immediately appreciated that an N-way switch is
achieved by the provision of additional input fiber
insertion holes, additional indexing electromagnets, and
corresponding tapered wedge 6ections on the 61eeve.
Fig. 6 illu6trates an additional embodiment of
an indexing system suitable for either of the two 6witch
embodiments described above, but illustrated for the
embodiment of Fig. SB for definiteness. It will be
i~mediately apparent that the angular positioning of
movable body portion 157 with respect to fixed body
portion 155 having fiber insertion holes therein i6
extremely critical to proper operation of the 6witch. In
particular, thi6 translates into precise tolerance6 on
the fabrication of the 61eeve 6urrounding the movable
body portion and the location of the electromagnet6. It
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1 7
has been found that increased precision of angular orien-
tation can be achieved by 6eparating the wedges and
electromagnet~ from the movable body portion along the
axial direction. In particular, an axial lever arm 170
rigidly couples a 61eeve 172 ~urrounding movable body 157
with a soft ~teel ring 175 having tapered wedged portion
177 mounted thereon in the 6ame fashion that tapered
wedged portion 142 apd 143 were mounted to sleeve 140 in
Fig. 5A. Sleeve 172, lever arm 170 and ring 175 are
coaxially aligned. Electromagnets, not 6hown, cooperate
with wedges 177 and precisely the 6ame manner that elec-
tromagnets 145 and 146 cooperated with wedges 142 and 143
in Fig. 5A.
Fig. 7A is a simplified cross-sectional view of
a duplexer 180 according to the present invention. The
purpose of duplexer 180 is to permit optical information
to be transmitted ~imultaneously in both directions on a
single fiber 182. This is accomplished by using optical
~ignals of differing wavelengths for the different direc-
tional transmission, and incorporating classification~eans to 6eparate the optical signals. In particular,
duplexer 180 couple~ a source 185 of light of a first
wavelength and a detector 187 sensitive to light of a
6econd different wavelength to fiber 182. While source
185 and detector 187 are shown communicating to duplexer
180 by short fibers 190 and lg2, 6uch sources and/or
detectors could be directly mounted to the duplexer.
Duplexer 180 itself comprises a transparent bod~ 195
having a curved surface at one end and fiber insertion
holes at the other end. However, in contrast with the
devices described above, the curved surface carries a
concave reflection grating 197. Grating 197 has the
property that light emanating from a point in a curved
focal 6urface i6 imaged at different location6 in the
focal 6urface depending on the wavelength of the light.
7~s
.
Different image points are determined by the ~pacing of
the grating lines ~nd the particular wavelength~ in-
volved. Thu6, fiber 190 has its end at the complementary
position with respect to the end of fiber lB2 for the
first wavelength and fiber 192 has it~ end at a comple-
mentary position with respect to the end of fiber 182 for
the ~econd wavelength. ~hus, light from source 185 is
imaged onto the end of fiber 182 and transmitted away
~rom duplexer 180 while light of the second wavelength
traveling along fiber 182 in a direction toward duplexer
180 is imaged onto the end of fiber 192 and thus trans-
mitted to detector 187.
Fig. 7B illustrates an alternate embodiment of
a duplexer 200 wherein the classification means and the
imaging means are separated. In particular, a dichroic
beam splitter interface 202 is reflective ~ith respect to
light of the first wavelength and transmissive with
respect to light of the econd wavelength. Beam splitter
interface 202 is disposed at approximately 45 from the
axis of fiber 182 so that light of the first wavelength
is significantly deviated from its original path. Sep-
arate reflective imaging elements 205 and 207 cooperate
with beam splitter 6urface 202 in order to couple light
of the first wavelength between source 185 and fiber 182
and light of the second wavelength between fiber 182 and
detector 187. In a duplex ~ystem, a similar duplexer
would be employed at at remote end of fiber 182, except
that 60urce 185 and detector 187 would be replaced by a
detector 6ensitive to light of the first wavelength and a
source of light of the second wavelength, respectively.
Fig. 8A ~hows a first embodiment of a three-
color multiplexer for 6imultaneously trasmitting optical
information from three sources 212, 215, and 217 along a
6ingle fiber 220. Multiplexer 210 comprise~ a transpar-
ent body 222 having a concave reflection grating 225 as
l ~ JS
de~cribed in connection with duplexer 180. In fact,
duplexer 180 could be converted to a two color multi-
plexer by 6ub~tituting a 60urce of light of the 6econd
wavelength or detector 187.
Fig. 8B 6hows a three color demultiple~er 230
for receiving 6imultaneous transmission of light at three
wavelengths along a fiber 232 and sending the light to
three detectors 235, 237 and 240. Since the light from
the different wavelengths is spatially 6eparated, de-
tectors 235, 237 and 240 could be detectors ~hat are
6ensitive to all three wavelengths, although selective
wavelength detector6 may be preferable. Demultiplexer
230 is substantially identical to multiplexer 210 and
comprises a transparent body 242 having a concave reflec-
tion grating 245 at one end and fiber insertion holes at
the other.
Fig. 8C ~hows an alternate embodiment of a
three color multiplexer 250 for transmitting light at
three wavelengths from respective sources 252, 255 and
257 along a ~ingle fiber 260. This is accomplished by
two dichroic beam 6plitter surfaces 262 and 265 and 6epa-
rate reflective imaging elements 270, 272, and 275. This
embodiment functions substantially the same as duplexer
200 shown in Fig. 7B.
The couplers described above have the property
that they are bidirectional, that is, that the direction
of light travel can be reversed and the device will still
function in the same way. ~owever, it sometimes happens
that directionality i~ reguired in the monitoring or
splitting operation. Figs. 9A and 9B illustrate a cou-
pler 280 having a directional monitoring feature. In
particular, a directional coupler 2B0 comprises a body of
graded index (~elf focusing) material 2B2 for coupling
fir~t and 6econd fibers 2B4 and 285. Graded index mater-
ial has the property that a point source at a fir6t axial
l ~ ~ 17~s
location i~ imaged at a 6econd axial location. Thus, inorder to couple fibers 284 and B25, respective fiber ends
are located at complementary axial positions 287 ~nd 288.
A beam ~plitter 6urface 290 i6 interpo~ed at ~n obligue
S angle in the path of the light and reflects a 6mall
fraction to a suitable detector 292. Due to the oblique
inclination, detector 292 only receives light ~hen the
light is traveling from fiber 284 to fiber 285.
In 6ummary it can be 6een that the present
invention provide6 a 6urprisingly effective ~eries of
modules for interfacing optical fibers with a very low
liqht 106s and with provisions for monitoring the optical
6ignal. While the above provides a full and complete
disclosure of the preferred embodiment of the present
invention, variou~ modifications, alternate construc-
tions, and equivalents may be employed without departing
from the true spirit and 6cope of the invention. For
example, the 6plitters and switches described were geo-
metrically 6ymmetric devices. ~owever, there is no need
for 6uch geometrical symmetry, nor is there any absolute
requirement that the fractions of light transmitted be
egual or that the 6witching be total. Rather, a 6witch
could employ feature6 of a 6plitter as well in order to
provide partial switching and partial splitting. More-
over, while a common focal plane is 6hown, this is not anabsolute prereguisite. Therefore, the above descriptions
and illustrations should not be construed as limiting the
scope of the invention which is defined by the appended
claims.
