Note: Descriptions are shown in the official language in which they were submitted.
BACK~RoUND OF THE INYENTION
ThiS invention is directed to fiber op-tic
couplers and in part.icular to low loss access couplers for multi-
mode optial fibers and a method o~ produci~g these couplers.
An ernbodirnent o.E an inventi.o:n disclos~d in th.is application is
c:laimed in co-pend..ing ~anadian ~ppl~.cation 350,118, filed April
1, 1980 by B.S. Kawasaki et al.
The two main network topology systems for the
distribution of optical communication signals using single-
strand multimode fiber are the tree distribution system and thestar system. For networks with.many terminals the tree dis-
tribution scheme provides advantages o~ flexibility in the
number and location of the distribution paths or drops and
: minimizes the amount of ~iber used in comparison to a star
system~ However, a tree network can suffer from an ineffective
utilization of the total optical power launched in the trunk
feeder.if there are.many lossy access junctions along the trunk
which are encoun~ered in series. The tree scheme can be efect-
ively utiliz'ed only if the excess loss above furcation loss at
each access junction is made su~ficiently small.
Recently two methods for producing low loss
access couplers for multimode fibers have been demonstratedO In
t~e fi.rst which is described in a publication by Takesh.i Oæeki
and Brian S. Kawasakl entitled "Optical direc-tional coupler
using tapered sections in multimode fibers", -- Applied Physics
Letters, ~olO 28, No~.9, May 1, 1976 - pa~es 528 and 529, twin
biconlcal tapered sections o~ multimode optical ~ibe~ axe
~oi~ed by an optical epoxy to produce a directlonal coupler.
.~ In the second which is described in a publication b~ M.K~ Barnoski
and H~R. Friedrich entitled "Fabrication of an access coupler
~ith single strand multi-mode fiber waveguides", - Applied
Optics, ~cl. lS, No. llt November 1976, pages 2629 - 2630,
t:wo sections of multimode
fiber are fused side-by-side to form a low loss junction.
In both of these structures, the excess loss is iII the
order of 1 ctB.
SU~ RY OF THE INVENTION
It is t~erefore an object of this in~ention to
provide an access coupler having highly efficient coupling
action.
It is a further object of this inven~ion to
provide an access coupler in which considerable mode mixing
and mode coupling occurs.
It is another object o~ this invention to provide
an access coupler which is directional.
It is a urther object of this invention to
provide an access coupler which has isolation of the uncoupled
port~
It is another object Qf this invention to pro~ide
simple and inexpensive methods of producin~ low loss
access eouplersO
These and other objects are achieved in an access -
coupler comprising a fixst and a second multimode fiber,
each of the fibers having a biconical taper section~ the
biconical taper sections of the fibers being twisted around
one another and fused together along a predetermined length.
One method of producing an access coupler for
coupling optic energy between a first and a ~econd multimode
fiber, comprises fusing thP first and the second fiber
together along a predetermined length, applying a tensile
force to the length of fused fibars, and heating the fused
length of fibers to form biconical taper sections in the
fused fibers.
Another method of producing an access coupler for
coupling optic energy between a first and a second multimode
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fiber comprises twisting a portion of each o~ the fibers
around one another, applying a tensile force to the twisted
porkions of the ~ibers and heating a region of the twisted
fibers to soEten the fibers forming biconical taper sections
and to ~use a predetermined length o e the twisted fibers
together.
When the fibers already have biconical taper sections,
the access coupler may be produced by twisting the fibers
together along their taper sections and heating a region of
the taper sections to fuse them together.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:
Figure 1 illustrates an access coupler in
accordance with this invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Figure 1 illustrakes an access coupler 1 in
accordance with the present invention. The access coupler 1
consisks of a first fiber 2 with ends or ports Pl and P2
and a second fiber 3 with ends or ports P3 and P4. Each
fiber 2,3 may be a gradea index mulkimode ~iber or a stepped
index multimod~ fiber havlng an optical core 4 and a cladding
5. Each ~iber 2,3 also has a biconical tapered section 6
in which the diameter of the fiber 2,3 narrows and then
widens to the normal fiber cliameter. Within the tapered
section 6, the fibers 2,3 are fused to~ether along a
predetermined length Q which may be greaker than the entire
length of section 6. In addikion r within this fused length Q,
the fibers 2 and 3 may be twisted around one another as
shown in ~igure 1.
Generally, fibers 2 and 3 have similar diameters,
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however, fibers of different diameters may be used in an
access coupler 1 in which it is desired to have preferential
coupling. If fiber 3 has a greater diameter ~han fiber 2
optical energy will .be preferentially coupled to fiber 3
from fiber 2, that is to the larger diameter fiber. Optical
energy can thus be efficiently coupled into a trunk line
.: using a coupler 1 in which the larger diameter fiber is
connected into the line and optical energy is coupled into
the line via the smaller diameter fiber.
The access coupler 1 may be produced by fusing a
predetermined length of the fibers 2 and 3 together. Once
fused, the fibers are pulled by a tensile force and the
fused length is heated to soften t~e f:ibers 2 and3 ~uch
that they may be drawn to form biconical taper sections 6.
Another method by which couplers 1 may be produced
consists of taking two fibers 2 and 3, twisting or winding
the fibers 2 and 3 around one ariother and applying a tensile
force to the fîbers 2 and 3, as by putting them under spxing
tension in a clamping jig~ A desired region of the twisted
fibers 2 and 3 is then heated sufficiently to soften and
fuse the fibers 2 and 3~ The spring tension causes th
fibers 2 and 3 to be elongated in the softened ragion forming
the biconical tapers, and the twist in the fibers ~ and 3
causes the fibers 2 and 3 to stay together and to fuse during
the pulling process. The heating of the fibers 2 and 3 in
the desired region may be accomplish~d by an oxy-butane
microtorch flameO
However, when the two fibexs already have biconical
taper sections 6, the access coupler 1 may be produced by
twisting the two fibers 2 and 3 together along their taper
sections and heatirlg a xegion of the twisted biconical taper
'
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sections to fuse the fibers 2 and 3 together along a preae-
termined length Q o~ the taper section~. T~ese me~hods
o~ producing access couplers may be used to produce the
couplers in a plant or may be usecl in the field to produce
couplers direc-tly on a multimode fiber bus.
Some examples o~ twis-ted access couplers 1
fabricated using the second process describecl, are tabulatea
i~ table 1 below. The couplexs 1 wexe made ~rom Cornin~ *
sil.ica step-index fiber having an 85 Imicron core di~meter,
.0 a 20 micron cladding thickness and a numerical aper~ure of
0.175. The biconical section 6 was appxoximately 1 cm. long.
The access couplexs 1 were evaluated by illuminating poxt
P1 with a HeN~ beam coupled into port Pl o iber 2 w;~h ~
xS0 microscope objecti~e. Oil ba~h cladding mode strippers
were mounted on ports Pl, P2 and P4 and the power coupled
! from ports P2 and P~ was measured. The input power to the
coupler 1 through port Pl was measuxed by breaking the fiber
2 at a point located downstream ~rom the mode s~rippex and
upstream from section 6.
. TABLE 1
Coupler ._ . P2 P4 Couplingl Excess Insertion~Loss .
. Ratio P P4
. (m~) ~mW) (mW3 4/Pl 10 log 2 (dB~:
~ ... . ~ . ~ . .. . _ . .
a 7.05 6.53 0.27 0O038 ~0.16 -
. b 5.92 6.00 0.58 0.084 -0.22
c 6.30 5.~1 0.74 ~ -0.11
d 7.20 6012 0.87 0.12: -0.13
e 6.92 4.40 1.65 '~~ ~-
* Trademark
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~35~
Table 1 shows the measured power levels and the calculated
excess insertion loss for several couplers with various
values of the coupling ratio P4/Pl where Pl represents
the optical power in coupler port Pl. This coupling ratio
increases wit.h an increase of the ratio of the diameter of
the fiber to the diameter of th~ narrowest portion of each
taper. The coupling ratio can vary from near zero with
no taper section, to 50% with very high values of diameter
ratio, i.e. ~ 10:1. Generally, increasing values for
coupling ratios lead to increased insertion losses. The
values of excess insertion loss are very low with the better
couplers having values between 0.1 and 0.2 dB. This
efficient coupling action is the result of the operation of
the access coupler in accordance with the present invention.
As the light in port Pl enters the narrowing tapexed
section 6, the higher order modes are forced to radiate
out of the core 4 area to be guided as cladding modes. The
llght can cross the fused boundary between the two biconical
sections and is therefore guided in the overall structure.
As the ligh~ pxopagates beyond to the region of increasing
tapers associated with ports P2 and P4, the cladding modes
propagate at gradually decreasing angles to the fibex axis
and ar~ recaptured by the tapered core section to again
become core modes in the untapered portions of the fibers.
The coupling action is much ]ess lossy in this type of -
structure than in the structure described by Ozeki and
Kawasaki because of the high optical quality of the air-
cladding interface. In the~present access couplerr i is
difficult to discern the coupling region from a cursory
examination of the scat~ered light alone~ The coupling
action is also more efficient than in the coupler described
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by sarnoski and Friedrich because of the increasing -taper
section 6. In this region of the device, light which is
propa~ating in the cladding region can be recaptured by
the core because the effect of an increasing taper is to
r~duce the propagation angle of the light. It is .important
that the l.ight propagating in the taperecl sections 6 be
totally xeflected at the interface between the surace of
the tapers 6 and the surroundiny medium. Therefore, the
biconical taper sections 6 should be long and shallow and
without sharp angles.
Another significant characteristi~ of the present
device is that considerable mode mixing occurs. Though the
low order modes are predominantly in port P~ rather than port
P4 as expected, a fairly uniform filling of the modes is
observed in port P4. The mode mixing is enhanced due to the
twist asymmetry in the access coupler 1. This result,
~ thought unexpected is particularly advanta~eous when these
; couplers are used in series as a part of a tree distribution
network.
It is also noted that a high degree of directionality
or isolation is observed in the a~cess coupler 1 in
accordance with this inventionL For exampl~, when port P
lS illuminated, the light propagates in one direction
appearing at ports P2 and P4 with an excess insertion loss
of 0.1 to 0.2 dB loss and with virtually no light appearing
at port P3. Isolations in the order of -60 dB (10 ln P3/Pl)
have been measured for access coupler 1.
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