Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates ~enerally to means for coupling energy
propagating in optical flbers and More particularly to a multi-port coupler
therefor.
For applications that require bidirectional communications over a
single multi-mode optical fiber such as fiber optic payout systems, there is
an explicit need for a bidirectional coupler that can provide low levels of
near end back scatter, effectively excite the single multi-mode fiber, and
- be economically manufactured and effectively ruggedized.
The coupler of the present invention is based on a well known
physical concept that an optical fiber which is longitudinally tapered will
exhibit conversion of guided modes into radiation modes. As the core of the
fibers is reduced in size from its nominal diameter, the highest order mode
will be cut off first and converted to radiationO As the core is made
smaller and smaller, more and more of the lower order modes are radiated.
F mally, for core sizes below a prescribed value, only the lowest order mode
will remain in the fiber. If the tapered fIber is surrounded by an optically
transparent cylinder larger than the fiber, the radiated energy will become
trapped within this outer cylinder. The process of converting the fiber
guided modes by wave rad~ation to the guided modés of the transparent
cylinder can be reversed by reversing the taper of the fiber.
If the fiber is tapered down to a waist region and then back to its
original size, there is provided what is called a biconical taper and higher
modes will leak out and be trapped in the outer cylinder which eventually are
relaunched into the fiber on the other side oi the waist region. If two or
more biconically tapered fibers are placed parallel and in close proximity
.. . .
within the transparent cylinder, the modes which are created in that cylinder
; will be recoupled back into the fibers. A four port coupler is thus
provided.~ Such a device is disclosed in an article entitled "Optical
~Directional Coupler Using Tapered Sections In Multi-Mode Fibers", by
T. Ozeki, et al. which appeared in the Applied Physics Letters, VolO 28,
No. 9, May, 1976 at pages 528, 529. Additionally in another article en~itled
i
:
" . ,
"Full Duplex Transmission Link Over Single-Strand Optical ~iber", by
B.S. Kawasaki, et al. appeared at Optical Letters,Vol. 1, I~O. 3, September,
1977., at pages 107, 108, there is also disclosed a biconically tapered
coupler having twin biconical sections wherein one unused port is terminated
remotely from the side fusion region by tapering the fiber to a point and
inserting the end in a small tube of oil which is carefully index matched
to the fiber material.
Briefly, the subject invention is described to a multi-port and,
in one configuration a three port coupler, including an optically trans-
parent cylinder in which there is located a biconically tapered section and
at least one parrallel adjacent single tapered secti.on whose taper is
selectively positioned with respect to the waist region of the biconical
section. In another configuration of the invention a plurality of single
tapered sections are arranged in an encircling relationship with one section
of the biconical section adjacent its waist region.
Figure 1 is a diagram illustrative of the conversion of optical
energy from guided modes into radia~ion modes which in turn couple into
guided modes of the cylinder;
Figure 2 is a diagram illustravtive of optical energy being
converted Erom radiation modes which are guided modes of the cylinder
to guided modes of the Eiber;
Figure 3 is a diagram illustrative of both types of conversion by
means of a biconical taper section located within an optically transparent
cyl~nder;
Figure 4 is a diagram illustrative of a four port coupler of known
prior art design;
Figure 5 is a diagram illustrative of a first embodlment of the
subject invention;
Figure 5 is a diagram illustrative of a second embodiment of the
; 30 subject invention;
Figure 7 is a diagram illustrative of a third embodiment of the
subject invention; and
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Figure 8 is a cross sectional view of the embodiment shown in
Figure 7 taken along the lines 8-8.
Figures 1 through 4 illustratively provide the background for the
subject invention and constitute known prior art concepts and devices.
Figure 1, for example, discloses an optical wave-guide in the form of a
fiber 10 which has optical energy 12 inputted thereto and which is
transmitted in guided modes up to the region 14 where the fiber 10 is long-
itudinally tapered to a pointO The region 14 at least is surrounded by an
optically transparent cylinder 16 which is larger in cross section than
the fiber itself. The optical energy transmitted through the riber 10 at
the tapered region 14 will be converted from a guided mode into radiation
modes. As the cross sectional diameter of the fiber is reduced in size
from its nominal diameter, the highest order modes will be cut off and
converted to radiation 18. As the core is made smaller and smaller, more
and more of the lower order modes are radiated. Finally for core sizes
below a prescribed value, only the lowest order mode will remain in the
fiberO What is significant, however, is that the radiated energy 18 will be
trapped as reflected energy 20 within the cylinder 16 and become guided modes
of the cylinder. The reverse process is shown in Figure 2 wherein another
optical fiber 22 having a tapered region 24 is located within the transparent
cylinder 16. Such an arrangement will receive the reflected energy 20 and be
relaunched into the fiber 22 in the form of guided modes~
As is well known, if a biconically tapered optical fiber 26 as
shown in Figure 3 i8 located within the cylinder 16 and has two opposing
tapered sections 28 and 30 separated by a waist region of reduced finite
diameter higher order modes will be converted to radiation in tapered section
28 which will then be reflected back to the tapered secti~ 30. Proceeding
now to Figure 4, there is disclosed a four port coupler in accordance with
the teachings of the Ozeki, et al. pulbication referenced above. In this
- 30 instance two doubly tapered or biconical fibers 34 and 35 are located in
close parrallel proximity within the transparent cylinder 16 and radiation
modes that are created in the cylinder 16 from energy inputted into one of
-- 3 --
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the fibers are reco~le~ back into both fibers past the ,laist regions 35
and 37. For example, an optical signal fed into port 1 will be effectively
split into ports 3 and 4. The optical signal coming out of port 2 will
consist of back scatter, which if proper coupling coefficients exist, will
be negligible. It can be seen that a device such as shown in Figure 4 is
bidirectional in that any input coupled to one of the ports will be cou?led
to the remaining ports, depending upon the coupling coefficients.
For certain applications, a three port coupler is required since
the splitting of power into a fourth port is unwanted~ Accordingly,
reference is now made to the first embodiment of the subject invention as
depicted in Figure 5. There reference numeral 38 designates an optically
transparent cylinder having two optical fibers 40 and 42 therein in parrallel
relationship and in relatively close proximity to one another. The fiber 40
is tapered to an infinitesimal diameter or point 43 providing a single
tapered section 44 while the second fiber 42 includes a biconical taper
including the tapered sections 46 and 48 and having a waist region 50 of
finite smaller diameter. In this embodiment, the tip 43 of the single
tapered section 44 extends substantially to the mid-point of waist region 50
of the biconical fiber 42. In such a configuration, substantially all of
the optical energy incident into port 1 will emerge through port 3, while the
; back scatter output of port 2 is substantially zero i.eO less than 20db below
the input power. When a signal, however, is inputted to port 3, a splitting
of power into ports 1 and 2 occurs, and if the point 43 extends laterally to
substantially the mid-point of the waist region 50 as shown in Figure 5, a
substantially equal splitting of output energy will occur between ports
1 and 2.
- Variations in the values of the coupling coefficients for power
incident to port 3 being coupled to ports 1 and 2 can be obtained by
laterally displacing the single tapered fiber 40 away from the mid-point of
the waist region 50 of the biconical tapered fiber 42 as shown in Figure 6.
Ihis type of configuration allows more power to be trapped by the tapered
region 46 of the biconical fiber 42 before the effect of the single tapered
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region 44 of the fiber 40 is realized.
Proceeding now to the embodiment shown in Figure 7 and 8, whereas
in the first embodiment one single tapered fiber 40 was utilized in
combination with the biconical tapered fiber 42, in the present embodiment
three single tapered fibers 40, 40' and 40" are located symmetrically
around fiber 42 providing ports lA, lB and lC surrounding port 2 with port
3 being located on the other side of the waist region SO. In such an
embodiment, input energy coupled to ports LA, lB and lC will be combined
and will exit out of port 3. In the other direction, optical energy coupled
to port 3 will be coupled to ports LA, lB and lC as well as port 2 depending
upon the lateral displacement of the tapered regions 40, 40' and 40" with
respect to the mid-point of the waist region 50 of the biconical tapered
fiber 42. The embodiment shown in Figures 7 and 8 is particularly adapted
for combining the optical power generated from three strips of a triple
stripe injection laser.
Thus what has been shown and described is an improved bi-directional
optical power coupler consisting of a combination of a biconical tapered
section and a single tapered section located within a common optically
transparent cylinder.
