Note: Descriptions are shown in the official language in which they were submitted.
4S8z
MOLEt::ULAR BOhlDE~D FIBEP~ OPTIC COUPLERS
AND MET3 IOD OF FABRICATION
~ack~ro~L~d of the Inv~nIiqn
This invention relates generally to rne~hods and apparatus for
5 bonding optical fibers ltog~ther. Mor~ particularly, this invention relates
to methods and apparatus for forming fiber optic couplers. Still more
particularly, this invention relates to methods and apparatus for forming
a coherent molecular bond between two optical fibers.
Sorne familiarity with the propagation characteristics of light
10 within an optical fiber will facilitate an understanding of both the present
invention and the prior art. Therefore, a brief discussion of fiber optic
waveguides, normal modes of propagation of light in such waveguides
and polarization of light is presented.
The behavior of an optical wave at an interface between two
1~ dielectric materials depends upon the refractive indices of the two
materials. If the refractive indices of the two dielectrics are identical,
then the wave propagates across the interface without experiencing
any change. In the general case of different refractive indices,
however, there will be a reflected wave, which remains in the medium
20 in which the wave was first propagating, and a refracted wave, which
propagates beyoncl the dielectric interface into the second material with
a change in direction relative to the incident wave. The relative
intensities of the reflected and rcfract0d waves depend upon the angle
of incidence and the difference between the refractive indices of the two
2~ materials. If an optical wave ori~inally propagating in ~he higher index
material strikes the interface at an angle of incidence greater than or
equal to a critical angle, there will be nc refracted wave propagated
across the interface; and essentially all of the wave will be totally
internally reflected back into the high index region. An exponentially
30 decaying evanescent wave associated with the incident wav~ extends
a small distance beyond the interface.
Optical fiber has an elongated generally cylindrical core of
higher re7ractive index and a cladding of lower refractive index
surrounding the core. Optical fiber use the principle of total internal
1264SB2
reflection to confine the energy associated with an optical wave to the
core. The diameter of the core is so small that a light beam propagating
in the core strikes the core only at angles greater than the critical angle.
Therefore, a light beam follows an essentially zig-zag path in the core
5 as it moves between points on the core-cladding interface.
It is well-known that a light wave may be represented by a time-
varying electromagnetic field comprising orthogonal electric and
magnetic field vectors having a frequency equal to the fraquency of the
light wave. An electromagnetic wave propagating through a guiding
10 structure can be described by a set of normal modes. The normal
modes are the permissible distributions of the electric and magnetic
fields within the ~uiding structure, for example, a ~iber optic waveguide.
The field distributions are directly related to the distribution of energy
within the structure. The normal modes ar~ generally represented by
15 mathematical functions that describe the field components in the wave
in terms of the frequency and spatial distribution in the guiding
structure. The specific functions that d~scribe the normal modes of a
waveguide depend upon the geometry of the waveguide. ~or an
optical fiber, where the guided wave is confined to a structure having a
2Q circular cross section of fixed dimensions, only fields having certain
frequencies and spatial distributions will propagate without ssvere
attenuation. The waves having field components that propagate
unattenuat~d are the normal modes. A single mode ~ib~r will guide
only one energy distribution, and a multimode fiber will simultaneously
2~ guide a plurality of energy distributions. The primary characteristic that
determines the number of modes a fiber will guide is the ratio of the
diameter of the fiber core to the wavelength of the light propagated by
the fiber.
In describing the normal modesg it is convenient to refer to the
30 direction of the electric and magnetic ~ields relative to the direction of
propagation of the wave. If only the electric field vector is perpendicular
to the direction of propagation, which is usually called the optic axis,
then the wave is said to be a transverss electric (TE) mode. If only ~he
magnetic field vector is perpendicular to to the optic axis, the wave is a
3~ transverse magnetic (TM) mode. If both the electric and magnetic field
lZ6~S8~
vectors are perp0ndicular to the optic axis, then the wave is a
transverse electromagnetic (TEM~ mode. Nono of the normal modes
require a definite direction of the field components; and in a TE mode,
for example, the electric field may be in any direction that is
5 perpendicular to the optic axis.
The direction of the electric field vector in an electromagnetic
wave is the polarization of the wave. In general, a wave will have
random polarization in which there is a uniform distribution of electric
field vectors pointing in all directions permissible for each mode. If all
10 the electric field vectors in a wave point in only one particular direction,
the wave is linearly polarizod. If the electric field consists of two
orthogonal electric field components of equal magnitude and 4~ out of
phase, the electric fieid is circularly polarized because the net electric
field is then a vector that rotates around the optic axis at an angular
15 velocity equal to the frequency of the wave. If the hAro linear
polarizations have unequal magnitudes and phases that are neither
equal nor opposite, the wave has elliptical polarization. In ~eneral, any
arbitrary polarization can be represented by either the sum of two
orthogonal linear polarizations, two oppositaly directed circular
20 polarizations or two oppositely directed elliptical polarizations having
orthogonal semi-major axes.
Propa~ation characteristics such as velocity, for example, of an
- optical wave depend upon the polarization of the wave and the index of
refraction of the medium through which the light propagates. Certain
2~ materials, including optical fiber, have different refractivs indices for
different polarizations. A material that has two refractive indices is said
to be birefringent.
The polarization of an optical signal is sometirnes referred to as
a mode. A standard single mode optical fiber will propagat0 two waves
30 of the same frequency and spatial distribution that have two different
polarizations. A multimode fiber will propagate two polarizations for
each propagation mode. Two different polarization components of ~he
same normal mode can propa~ate through a birefringent material
unchanyed except for a difference in velocity of the two polarizations.
35 Polarization is particularly important in interferometric sensors because
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only waves having the same polarization will produce the desired
interferenca patterns.
An optical coupler joins two fibers for transmitting optical
energy from one fiber to the other. Optical couplers are used in many
applications of optical fiber, including constructing fiber optic
inte~eromet~rs, resonators, sensor arrays and data buses.
Among the parameters that are considered in forming an
optical coupler are the polarizations of the waves before and after
coupling, the fraction of energy to be coupled from one fiber to the
other, the insertion loss of the coupler and whether the fibers guide only
a single mode or a multiplicity of rnodes.
Several methods have been employed for joining two fibers to
form a coupler for transrnitting optical energy from one fiber to the other.
A first technique for joining two fibers results in the biconical tapered
fiber optic coupler. The fibers are placed together, twisted and then
heated to near the meiting point to fuse them during application of a
force to stretch the fibers. The heating and twisting steps in the
~ormation of the biconical tapered fiber optic coupler result in alteration
of the molecular arrangements of the materials comprising the joined
fibers. In particular, the heating and twisting alters the distributions of
impurities used as dopants to control the refractive indices of the core.
The refractive index of a material depends upon its molecular
structure; therefors, forming the biconical tapered flber optic coupler, in
general, produces localized changes in tho refractive indices of the
fibers. These changes in refractive indices are uncontrollable and
cause undesirable, uncontrollable reflections and refraction at the
interfaces between the fibers. It is therefore difficult to fabricate
biconical tapered fiber optic coupler having coupling efficiencies
predetermined for specific applications.
Another technique for joining two fibers includes grinding flat
surfaces on facing portions of the fibers and joining them by
mechanically fixing the surfaces in juxtaposition with a refractive index
matching oil or other material ~hat enhances light transmission between
the fibers.
lZ6~S82
These and other prior methods of forming fiber optic couplers
have the disadvantage of suffering undesirably high loss of optical
signal intensity. These prior fabrication techniques generaily fail to
facilitate control of the amount of light transmitted by any particular
5 coupler and also fail to perrnit control of the arnounl of light that will be
coupled from one fiber to another. Such problems are caused by the
inherent intermolecular inhomogeneities and discontinuities that occur
- at the interfaces during prior fabrication processes for joining fibers.
Further, prior fabrication techniques are not conducive to producing the
10 : great numbers of optical couplers required in communieations; data
processing and sensor applications.
Many applications of fiber optic couplers require very low
signal loss in the coupler and also require couplers having critically
selected light transmission and coupling characteristics. Large
15 numbers of such couplers are required for practical applications of
optical fibers.
Surnmary of the Invention
The present invention provides a method and apparatus for
forrning fiber optic couplers that avoid the inharent problems of
20 couplers fabricated according to previous techniques. In the present
invention bonding of a pair of optical fibers occurs a~ the molecular
level In a critical fashion such that the molecular structure of the glass is
coherently homogeneous and uniform across th~ bondsd surfaces,
thereby preserving the physical characteristics of light transmission
25 across the coupling junction and avoiding unnecessary luss of signal
intensity.
The present invention permits precise control of the amount of
light coupl~d between fibars and is suitable for mass production of
optical couplers having precisely sel~cted coupling and transmission
30 characteristics.
The present invention provides a fi~er optic coupler for
coupling an optical wave between a pair of optical fibers each having a
central core surrounded by a cladding, th~ cores and claddihgs having
characteristic refractive indices, compr;sing a first planar surface
35 formed on a first optical fiber by removing at least a portion of the
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cladding therefrom and a second planar surface formed on a second
optical fiber by removing at least a portion of the cladding therefrom,
the first and second planar surfaces being juxtaposed to form an
interaction region wherein light is to propagate between the fibers. A
coherent molecular bond is formed between the first and second planar
surfaces without altering the core and oladding refractive indices
across the joined surfaces.
In on0 embodiment of the inv0ntion, ths planar surfaces are
formed by removing porRons of the cladding from curved lengths of the
optical fibers su~h that evanescent field coupling ~t the interaction
region between optical waves guided by a first one of the fibers and the
other fiber couples energy between the fibers.
In another embodiment of the invention the planar surfaces are
formed by removing portions of the cladding and a portions of the core
1~ from curved lengths of the optical fibers such tha~ juxtaposing the fiber
cores forms an intercepting core coupler.
The present invention inclwdes a fiber optic coupler for
coupling an optical wave between a pair of optical fibers each having a
central core surrounded by a cladding, the cores and claddings having
characteristic refractive indices. The coupler according to the invention
is formed by the process comprisin~ forming a first planar surface
~ormed on a first optical fiber by removing at teast a portion of the
cladding therefrom and forming a second planar surface on a second
optical fiber by rernoving at least a portion of the cladding ther~from,
the first and second planar sur~aces being juxtaposed to form an
interac~ion region wherein light propagates between the fibers; and
formin~ a coherent molecular intermolecular bond formed between the
first and second planar surfaces without altering the local core and
cladding refractive indices.
The coherent molecular bond that joins the fibers to form the
coupler may be formed by the process oomprising aligning the planar
surfaces relative to one another to provide a predetermined coup~ing
efficiency; and applying a controlled amount of energy to the planar
surfaces to cause them to bond together, forming a homo~eneous,
3~ coherent intermolecularly bonded s~ructure.
~264S8z
The method of the present invention for forming a fiber optic
coupler for coupling an optical wave between a pair of optical fibers
each having a central core surrounded by a cladding, th~ cores and
claddings having characteristic refractive indicas, comprises the steps
of: forming a first planar surface formed on a first optical fiber by
removing at least a portion of the cladding therefrom; forming a second
planar surface formed on a second optical fiber by removing at least a
portion of the cladding therefrom, the first and second planar surfaces
being juxtaposed to form an interaction region wherein light propagates
between the fibers; and forming a coherent molecular bond between
the first and second planar surfaces without altering the local core and
cladding refractive indices by causing intermolecular inhomogeneities
and discontinuities across the joined surfaces.
The step of forming the coherent molecular bond may comprise
ihe steps of aligning the planar surfaces relative to one another to
provide a predetermined coupling efficiency; and applying a controlled
amount of energy to the planar surfaces to cause them to bond
together, thereby forming a homogeneous, coherent interrnolecularly
bonded structure.
The step of applying energy to the planar surfaces may include
focusing a beam of coherent ligh~ thereon. The step of forrning a
coherent molecular bond may include heating the fibers to a transition
temperature belouv the melting temperature of the fibers.
Br~f l:~escription Qf the Drawings
Figure 1 is a cross sectional view of a fiber optic evanescent
field coupler formed according to the present invention;
Figure 2 is a cross sectional view of the fiber optic evanescent
field coupler taken along lina 2-2 of Figura 1;
Figure 3 is a plan view showing an oval surface on a portion of
an optical fiber included in the fiber optic evanescent field coupler of
Figures 1 and 2;
Figure 4 is a cross sectional view of an intercepting core
coupler formed according to the present invention;
Figure 5 is a cross sectional view taken along line 5-~ of Figure
4;
~l2~458~
Figure 6 is a plan view showing core and cladding planar
`~ surfaces on a portion of an optical fiber included in the fiber optic
intercep~ing core coupler of Figures 4 and 5;
Figure 7 is a plan view illustrates a system for joining two
optical fibers to form either an evanescent field ~iber optic coupler as
shown in Figures 1-3 or an intercepting fieW coupler as shown in
Figures 4-6;
Figure 8 is an cross sectional view showing alignment of a pair
of opticai fibers with a jig and application of ener~y thereto for bonding
10 the ~ibers together to form a fiber optic coupler;
Figure 9 is a plan view of the apparatus of Figure 8; and
Figure 10 illustrates formation of an optically fiat surface on an
optical fiber to be used in forming a fiber optic coupler.
Desçription of the Prçferred Emb~diment
1~ Evanescent Field Coupler
As illustrated in Figure 1, a coupler 20 formed according to the
method of the invention includes a pair of optical fibers 22 and 24
mounted together. The fiber 22 has a core 26 and a cladding 28, and
the fiber 24 has a cor~ 30 and a cladding 32. As explained
20 subsequently, an oval shaped optically flat surface 34 is formed in the
cladding 28 of the fiber 22. Similarly, an oval shaped optically flat
surface 36 is formed in the cladding 32 of th0 fiber 22.
The oval surfaces 34 and 36 are juxtaposed. in facing
relationship to form an interaction region 42 wherein the evanescent
25 fields of light propagated by each of the fibers 22 and 24 interact with
the other fiber. The quality and quantity of light crossing or interacting
across the juxtaposed surfaces 34 and 36 are affected by
discontinuities, inhomogeneities and other local defects caused in the
interaction region by the manner of joining and the type of bond
30 produced between the surfaces 34 and 3B. The essence af the present
invention is a coupler and method of fabrication thereof that comprises
a bonded surface that is molccularly coherent, homogeneous and
continuous across and around the joined sur~aces 34 and 36 ~f the
fibers 22 and 24. Accordingly, local irregularities in the refractive
indices are avoided, with the resultant interaction region 46 of the
~2~i4~i82
joined region having well behaved refractive indices throughout as
expected for a rnolecularly consistent material.
The amount of fiber optic material removed increases gradually
from zero at the edges of the oval-shaped planar su~aces 34 and 36 to
5 a maximum amount at the centers thereof. The tapered rernoval of fiber
optic material snables the fibers 22 and 24 to eonverge and diverge
gradually, which is advantageous for avoiding backward reflection and
excessive loss of light energy at the interaction region 42. The gradual
curvature of the fibers 22 and 24 prevents sharp bends or other abrupt
10 changes in direction of the fibers 22 and 24 to avoid power loss through
mode perturbation.
Light is transferred between the fibers 22 and 24 by
evanescent field coupling at the interaction region 42. The core 26 has
a refractive index that is greater than that of the cladding 28, and the
1~ diameter of the core 26 is such that light propagating within the core 26
internally reflects at the core-cladding interface. Most of the optical
energy guided by the optical fiber 22 is confined to its core 26.
However, solution of the wave equations for the fiber 22 and applying
the well-known boundary conditions shows that th~ energy distribution,
20 although primarily in the core 26, includes a portion that extends into
the cladding and decays exponentially as the radius from the center of
the fiber increases. The exponentially decaying portion of the energy
distribution within the fiber 22 is generally called the evanescent field.
If the evanescent field of the optical energy initially propagated by the
2~ fiber 22 extends a sufficient distance into the fiber 24, energy will
couple from the fiber 22 into the fiber 24.
It has been found that to ensure proper evanescent field
coupling, the amount of material removed from the fibers 22 and 24
must be carefully controlled so that th~ spacing between the cores of
30 the fibers 22 and 24 is within a predetermined critical zone. The
evanescent field extends a short distance into the cladding and
decreases rapidly in magnitude with distance outside the fiber core.
Thus, sufficient fiber optic material should be removed to permit overlap
between the evanescent fields of waves propaga~ed by the ~wo fibers
35 22 and 24. If too little material is removed, the cores will not be
~aZ6
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sufficiently close to permit the evanescent f;elds to cause the desired
interaction of the guided waves; and therefore, insufficient coupling will
result.
Conversely, removal of too much material alters the
5 propagation characteristies of the fibers, resulting in loss of light energy
from the fibers due to mode perturbation. However, when the spacing
between the cores of the flbers 22 and 24 is within the critical zone,
each fiber 22 and 24 receives a significant portion of the evanescent
field energy ~rom the other to achieve good coupling without significant
1C energy loss. The critical zone includes the region in which the
evanescent fields of the fibers 22 and 24 overlap sufficiently to provide
- good evanescen~ field coupling with each core being within the
evanescent field of light guided by the o~her core. It is believed that for
weakly guided modes, such as the HE11 mode guided by single mode
15 fibers, mode perturbation occurs when the fiber core is exposed.
Therefore, the critical zone is the core spacing that causes the
evanescent ~ields to overlap sufficiently to cause coupling without
causing substantial mode perturbation induced power loss.
The extent of the critical zone for a particular coupler depends
20 upon a number of factors, such as the physical parameters of th0 fibers
and the geometry of the coupler. The critical zono also is affected by
the type of bond formed betwe0n the joined fibers. The critical zone
rnay be quit0 narrow for a single mode fiber having a step index profile.
The center-to-center spacing of the fibers 22 and 24 is typically less that
25 two to three core diameters.
The fibers 22 and 24 preferably have substantially identical
core and cladding diameters, the same raclius of curvature at the
interaction zone 42, ~nd the same amount of fiber optic material
removed therefrom to form the interaction region 42. The fibers 22 and
30 24 are symmetrical through the interaction region 42 in the planes of
the surfaces 34 and 36, respectively, so that the facing planar oval
surfaces of the fibers 22 and 24 are coextensive when they are
superimposed. The two fibers 22 and 24 therefore have identical
propagation ~haracteristics at the interaction region, thereby avoiding
3~ reduction in coupling that is associated with dissimilar propagation
~Z6~5~3Z
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characteristics. Each of the optical fibers 22 and 24 has a propagation
constant that determines the parameters, such as wavelength,
reflection at interfaces and attenuation of waves propagating therein. It
is well-known that energy couples between media having substantially
identical propagation constants more easily than between media
having different propagation constants. There~ore, in forming the
coupler 20, it is desirable to minimize variations of the propagation
constant a~ the interaction region 42.
The coupler 20 of Figures 1 and 4 includes four ports labeled
20A, 20B, 2ûC and 20D. Ports 20A and 20B, which are on the left and
right sides, respectively, of the coupler 20 correspond to the fiber 22;
The ports 20C and 20D, which are on the left and right sides,
respectively, of the coupler ~û correspond to the fiber 24. For purposes
of explanation it is assumed that an optical signal input is applied to
pcrt 2ûA through the fiber 22. The signal passes through the coupler
20 and is output at sither one or both of ports 20B or 20C depending
upon the amuunt of coupling between the fibers 22 and 24. The
"coupling constant" is defined as the ratio of the coupled power to the
total output power.
In the above example, the coupling constant is the ratio of the
powsr output at port 20D divided by the sum of the power output at the
ports 20~ and 20D. This ratio is sometimes called the "couplin~
efficiencyN, which is typically expressed as a percent. Thsrefor0, when
the term "coupling constant~ is used herein, it should be understood
that the corresponding coupling efficiency is equal to the coupling
constant times 100.
The coupler 20 may be tuned to adjust the coupling constant to
any desired value between zero and 1.0 by offsetting the confronting
surfaces 34 and 36 to control the dimensions of the region of overlap of
the evanescent fields of waves guided by the fibers 22 and 24. Tuning
may be accompiished by sliding the substrates 34 and 36 laterally or
longitudinally relative to one another during assembly of the coupler
20.
The coupler 20 is highly directional with substantially all of the
power applied at one side thereof being output at the ports on the other
1264S8Z
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side. Substantially all of the light applied as an input to sither ports
20A or 20C is delivered to ports 20B and 20D without appreciable
contra-directional coupling. The directional charactaristic is symme~rical
in that light applied to either ports 20B or 20D is delivsred to ports 20A
and 20B. The coupler 20 is essentially non-discriminatory with respect
to polarizations and preserves the polarization of light input thereto.
Light ~ha~ is cross-coupled from one of the fibers 22 and 24 to
the other undergoes a phase shift of 1l/2, but light that passes straight
through the coupler 20 wlthout being cross-coupled is not shifted in
phase. For example, if the coupler 20 has a-coupling constant of 0.5,
and an optical signal is input to port 20A, then the outputs at ports 20B
and 20D will be of equal magnitude; but the QUtpUt at port 20D will be
shifted in phase by ~/2 relative to the output at port 2ûB.
The coupler 20 is a low loss device, having typical insertion
1~ losses of about 0.1% to 0.2%. The term "insertion loss" as used herein
refers to the real scattering losses of light energy passing through the
coupler 20. For example, if light energy is input to port 20~; and the
light energy output at ports 20B and 20D totals 97% of the input energy,
the insertion loss is 3%. The term "coupler transmission~ is defined as
one minus the insertion loss and is typically expressed an a decimal
fraction.
Intercepting Core Coupler
Referring to Figure 4, an intercepting core fiber optic coupler 48
formed according to tho methods of the present inv~ntion includes a
pair of optical fibers 50 and 52.
The fiber 50 is polished to remove sufficient fiber optic material
to expose a core portion 62 and a cladding portion 64 on the fiber 50.
Similarly, the fiber 52 is polished to remove sufficient fiber optic
material to expose a cors portion 66 and a ciadding portion 68 on the
fiber 52. The fibers 50 and 52 preferably have identical diameters, and
the cora and cladding portions 62 and 64 have dimensions
substantially identical to the core and cladding portions 66 and 68,
respectively. The dimensions of the core portion 62 the cladding
portion 64 are determined by the diameters of the cors and cladding of
the fiber 50 and the depth to which fiber op~ic material is removed
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therefrom. Similarly, the dimensions of the core portion 66 the cladding
portion 68 are determined by the diameters of the core and cladding of
the fiber 52 and the depth to which fiber optic material is removed
therefrom.
The exposed core portions 62 and 66 and the ~ladding
portions 64 and 68 are juxtaposed in facing relationship. The ~ibers ~0
and 57 are then bonded together as described subsequently herein.
The coupling efficiency of the ceupler 48 is determined primarily by the
depths to which material is removed from the cores of the fibers ~0 and
52 and the relative positions of the flat regions thereon when the fibers
are bonded together.
If the fibers 50 and 52 have different diameters, then they vvill
ordinariiy be polished to different depths so tha$ the exposed core and
cladding portions, respectively, will have essentially the same
dimensions. If the fiber 50 has a smaller core diameter than the fiber
52, the coupling efficiency of the coupler 4B depends on which fiber
carries the optical wave to the region where the cores intercept. The
coupling efficiency will in general be higher when the light is incident
upon the coupler 48 from within the fiber having the smaller diameter.
Method of Fabricatien
The ensuing description of the processes for fabricating th~
coupiers 20 and 48 begins with a description of the steps used for
fabricating the evanescent field coupler ~0. Many identical steps are
involved in fabricating the evanescent field coupler 20 and the
intercepting core coupler 48. The process for fabricating the
intercepting core coupler 48 will point out the similarities and
differences between the fabrication methods.
The first step in formation of the coupler 20 is to remove any
jacketing material that may bs ~n a length of a few centimeters of the
fibers to provida access to the bare cladding thereof. Referring to
Figures 7 and 8, the fibers 22 and 24 are then attached to conv~xly
curved surfaces of a pair of substrates 70 and 72, respectively, by the
use of any suitable adhesivc. The substrates 70 and 72 may be made
of any generally rigid material, such as glass, metal or plastic. The radii
of curvature of the substrates 70 and 72 preferably are identical and are
~;~6~ ;J
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sufficiently large to preclude the risk of fracturing the fibers as they are
bent to conform to the curvature.
The central portions of the fib0rs 22 and 24 rnay then be
lapped using conventional lapping methods to a predetermined depth
to remove cladding material thersfrom and form the planar suriaces 34
and 36, respectively. The planar surfaces 34 and 36 should be
sufficiently flat that they $end to adhere to one another when they are
placed together with no adhesive or index rnatching oil therebetween.
A lapping technique that has been found to b0 advantageous
10 in forming the planar surfaces 34 and 36 to be optically flat is illustrated
in Figure 10. The fiber 22 is shown mounted to the substrate 70, which
is connected to a retainer 73 by any suitable means, such as clamping
or gluing. The retainer 73 is supported on a shaft 75 that extends from
a motor 77, which controls the elevation of tha shaft 77 and retainer 75
1~ relative to an optically flat lapping surface 79. Tha lapping begins with
the central portion of the convexly curved fiber 22 being tangent to the
lapping surface 79. As the lapping procaeds, the motor 77 lowers the
shaft 75, the retainer 73 and the substrate 70 with the fiber 22 attached
thereto such that the planar surface 34 is formed by lapping radially
20 inward from the point of initial contact between the fiber 22 and the
lapping surface 79.
A preferred rnethod for determining the depth to which the fiber
is lapped is to apply an optical signal to one end thereof fronn a laser 81
and to monitor the optical output at the other end with a photodetector
2~ 83. A lapping slurry having a refractive index near that of the fiber 22 is
maintained on the lapping surface 79 during the lapping process. As
the lapping proceeds, light couples from the fiber 22 into the lapping
slurry, thereby reducing the optical throughput of the fiber 22. Tha
optical throughput may be correlatsd to the lapping depth. In general,
30 the relation between the opticaJ throughput and lapping depth depends
upon the specific type of fiber being lapped. Therefore, it is necessary
to perform a series of measurements of the lapping depth as a function
of optical throughput to produce a calibration curve for the par~icular
fiber being lapped and the lapping slurry used ~herewith.
3~
64L58~
As explained previously, care must be taken to avoid removal
of too much cladding material from the fibers used to form the
evanescent field coupier 20. Using ordinary single mods flber, forming
the coupler 20 to have a coupling efficiency of one to two percent
5 requires removal nf about ten percent of the core depth at the center of
the oval surfaces.
After the fibers 22 and 24 are mounted on the corresponding
substrates 70 and 72 and the oval surfaces are formed using, for
example, the rnethods described above, the planar surfaces 34 and 36
10 are placed in juxtaposition as shown in Figures 7-9. A preferred
method for holding the surfaces 34 and 36 in the desired positions is to
place the substrates 70 and 72 are placed in a jig 82 as shown in
Figure 9. The jig- 82 includes a pair of retainers 84 and 86 for holding
the substratÆs 70 and 72, respectively, so that they are movable with
1~ respect to one another. The substra~es 70 and 72 are placed in desired
positions relative to one another and then the fibers 22 and 24 are
bonded together. Ordinarily, the fibers 22 and 24 will be position0d t
maximize the coupling of optical signals therebetween.
The coupler 20 may be formed to have a desired coupling
2Q constant. One preferred method for assuring achievernent of a desired
coupling constant includes the step of inputting an optical signal frorn a
laser 90 into an end 92 of the fiber 22. The intensities of the optical
signals emanating from the fibers 22 and 24 after the inpu~ beam has
impinged upon the interaction re~ion 42 are monitored using suitable
25 photodetectors 94 and 96, respectively, while the substrates are
manipulated to achieve a desired coupling efficieney. The amount of
coupling may be varied by moving the substrates 28 in the jig 72 to
adjust the amount of overlap of the planar surfacss 30 and 32. The
coupling efficiency is
Tl = 1 - It/i
lC(lt + ~C)-1 ~
where 1l is the coupling efficiency, Ij is the light intensity input to fiber
22, It iS the light transmitted through fiber 22 beyond the interaction
region 42 and Ic is the light intensity coupled from fiber 22 to fiber 24.
~ZG9~5~3;Z
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After the fibers 22 and 24 have been positioned to provide the
desired coupling constant, energy is applied to the interface oF the
planar surFaces 30 and 32. As shown in Figure 8, the energy source
may be a laser 98. The laser ~8 is p~ferably a CO2 laser, and it should
produce an sutput beam that will heat the fibers 22 and 24 to a
temperature near the glass transition temperature. The energy source
may also be an ultrasonic wave genera~or, an induction heating source
or other suitable device for providing the desired amount of heat to the
fibers 22 and 24. As shown in Figures 8 and 9, a forc~ F may be
applied to compress the fibers 22 and 24 together during the bonding
process. Generally it has been found that a compressive force of about
a pound facilitates formation of the coherent molecular bond
The transition temperatura is below the melting point of the
glass from which ~he fibers 22 and 24 are formed. Ths transition
temperature depends upon the materials comprising the fibers 22 and
24. Most optical fiber is formed from silicon dioxide with a dopant such
as germanium dioxide or boron added thareto to control the refractive
index. Such fibers typically have transition temperatures in thc range of
1100 C to 1200 C. The transition temperature should be determined
~o ~xperimentally for the fibers to be joined, and the energy output from
the laser 98 should be controlled to assure that the temperature in the
bonded region does not excsed the transition temperature. The
transition temperaturc of an optical fiber is attained when the fiber
begins to soften as the temperature increases.
Applying the outpwt of the laser 98 over the juncture of the
surfaces 30 and 32 fuses the fibers 22 and 24 together. It has been
found that the above described method forms a junction of the surfaces
30 and 32 that results in a coherent molecular bonded r2gion having
the same physical structure and the samc optical characteristics as the
bulk material comprising ~ibers 22 and 24.
The processes of forming and joining optically flat surfaces on
fiber 22 an~l 24 described herein provide a fiber optic coupler having a
bonded surfaoe that is molecularly coherent, homogeneous and
continuous across and around ths joined surfaces 34 and 36 of the
3~ fibers 22 and 24. Accordingly, local irregwiarities in the refractive
~z~
-17-
indices are avoided, with the resultant interaction region 46 of the
joined region having well behaved refractive inclices throughout as
expected for a molecularly consistent material. The step of coupling
light from the fibers 22 and 24 while they are lapped to form the
optically flat surfaces 34 and 36, respectively, permits sufficient control
of the lapping depth ~abrication to form the coupler 20 to have a
predetermined coupling efficiency.
The primary difference between the steps for fabricating the
evanescent field coupler 20 and the intercepting cors coupler 48 is the
- depth to which the fibers 22, 24, 50 and 52 are lapped. The amount of
coupling depends upon the length of the interaction zone, which is a
function of the lapping depth and the radius of curvature of the flber
being lapped. It has been found that in general a 3 dB intercepting
core coupler should have the fibers lapped to remove 5û% of the core
15 to provide full modal mixing. It is well known from electromagnetic
theory that the energy distributions of tha normal modes varies with the
radial distance from the center of an optical fiber. The energy in the
lower orcler modes tends to be primarily in the ~entral region of the
core, whereas the higher order modes tend to have more snergy near
20 the core/cladding interface. Since most of the energy in a multimode
fiher is in the lower order modes, good coupling is facilitated by having
the central regions of the cores in contact.
Couplers formed according to the absve described method
have been cut into sections and examined with high power
25 microscopes and a mireau interferometer. These instruments show that
the bond interface has no distortion of the molecular arrangements of
the cores or the claddings of the fibers and that the rnolecular structure
of the inter~ace is identical to that of the fibers.
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