Note: Descriptions are shown in the official language in which they were submitted.
Field of the Invention
The present invention relates generally to the field
of fiber optics, and i5 particularly advantageous when prac-
ticed in connection with single-mode and polarization-preserving
fiber-optic waveguides.
Description of the Prior Art
Fiber optics is generally concerned with the trans-
mission of light along a transparent fiber structure which has
a higher refractive index than its surroundings. Currently it
is possible to manufacture long, continuous strands of optical
fiber which may propagate signals without substantial attenua-
tion over long distances. It is also possible to manufacture
the fiber structure as an optical waveguide wherein only pre-
selected modes of light propagate in the fiber. By limiting
wave propagation through the fiber to a single mode, the
bandwidkh of the optical fiber may be exceedingly high to
provide a high information-trans~er capacity. Moreover,
optical-fiber transmission equipment is compact, lightweight,
and potentially inexpensive. Transmission over optical fibers
does not generate interference and is unaffected by external
interference.
While the development of optical fibers for tele-
communications systems is becomlng rather highly advanced, the
use of fiber optics for sensing and control systems is still
in its~early development. In sensing and control systems a
fiber-optic transducer is used that exploits ei~ther multimode
or single-mode~light~propagation in an optical fiber.
While multimode sensors use amplitude variations in
the~optlcal signals to sense~and transmLt; the deslred infor-
mation, single-mode sensors use phaise variations rather than
amplitude variations. The single-mode sensors usually involve
meohanisms for~alterlng such properties of the fiber as path
:
length or index of refraction to effect the desired ~hase ~ar-
iations in the optical signal. In the case of the fiber-optic
gyroscope, the single-mode sensor measures acceleration which
inherently alters the propagation of light even though the
fiber is not affected. Thus, in contrast to multimode sen-
sors, in single-mode sensors the uniformity and mechanism of
light propagation and hence the characteristics of the fiber
are especially critical.
Single-mode sensors are also sensitive to the state
of polarization of the lig'nt in the fiber. If the fiber is
not significantly polarization-holding or preserving, the
state of polarization at the detector will tend to fluctuate
randomly. Thus, for single-mode transducers, it is desirable
to use elliptical-core or other kinds of polarization-holding
fiber. See, e.g., McMahon et al., "Fiber-Optic Transducers,"
IEEE Spectrum, December 1981, pages 24-27.
There~are well-known techniques for making long,
continuous, single-mode optical fibers. Keck et al. U.S.
Patent 3,711,262 issued Januar~16, 1973, for example, des-
cribes the conventional method of producing an optical wave-
guide by first forming a film of glass with a preselected
index of refraction on the inside wall of a glass tube having
a different lndex of refractlon. The glass tube and glass
film combination is then drawn to reduce the cross-sectional
area and~ to collapse the film of glass to form a fiber having
a solld cross-section. As a result of this process, the core
is formed from the;glass film~, and the cladding lS formed from
the glass tube.
~ :
I~t is also known that mul~tiple core and cladding
:
layers may be~deposited on the inside o~ a preform which lS
then collapsed and drawn, so that the preform tube becomes a
support ~acket around the core and cladding layers. Light
,
,
~ 3
propagated through a fiber formed in this manner is confined
to the guiding region formed by the core and cladding layers
and does not significantly interact with the support jacke-t.
Consequently the optical properties of the support jacket can
be considerably inferior to the optical qualities of the core
and cladding. Details of this process for forming multiple
core and cladding layers is disclosed in ~acChesney et al.,
"A New Technique for the Preparation of Low-Loss and Graded-
Index Optical Fibers," Proceedings of the IEEE, 62, at 1280
(1974), and Tasker and Ench, "Low-Loss Optical Waveguides with
Pure Fused SiO2 Cores," Proceedings _ the IE~E, 62, at 1281
(1974).
It is known that elliptical-core, polarization-pre-
serving optical fibers may be drawn from elliptical-core pre-
forms. The preforms may be manufactured by collapsing a
cylindrical preform or tube, with a slight vacuum in the
center. Another method of manufacturing an elliptical-core
preform is to fabricate the substrate tube to have a wall of
non-uniform thickness and then collapse the tube by heating it
to the softening point. The surface tension in the shaped
wall, which occurs during the collapsing and subsequent draw-
ing steps, causes the resulting fiber core to be elliptical in
cross-section. See, e.g~, Pleibel et al. U.S. Patent 4,274,854
issued June 23, 1981~
Summary of the Invention
It is a principal object of the present invention to
provide an optical fiber which is easy to optically couple to
optical devices and other optical fibers. In particular, one
specific objective of the invention is to provide a polariza-
tion-holding optical fiber which is easy to align and couple
to other polarization-sensltive devices without a substantial
discontinulty in the preferred direction of polarization along
the optical path.
~ nother ob~ect of the present invention is to pro-
vide optical fibers which can be easily coupled to each other
at any desired locations along the lengths o~ the fiber.
~ further object of the present invention is to pro-
vide optical fibers in extremely long, continuous strands
having the above-mentioned attributes along their entire
lengths.
Yet another object of the invention is to provide a
method of drawing optical fibers having the above-mentioned
10 properties.
Still another object of the invention is to provide
a simple method of making a directional coupler from a pair of
optical fibers wherein the required alignment and desired
degree of coupling are easily obtained.
Other objects and advantages of the present inven-
tion will become apparent from the following detailed descrip-
tion and the accompanying drawings.
In accordance with the present invention, there is
provided an optical fiber comprising a core and cladding
~ having different refractive-indices and forming a slngle-mode
guiding region,~the guiding region being located sufficiently
close to the surface of the fiber, along a selected length of
the fiber, to allow coupling to a guided wave, and the outer
surface of the fiber having a non-circular cross-section with
a predetermined geometric relationship to the gulding region
so that the location of the guiding region can be ascertained
from the geometry of the outer surface. The guiding region of
the fiber preferably has a non-circular cross-section defining
two transverse orthogonal axes which, in combination wlth the
different refractlon indices of the core and cladding, permits
the de-ooupling of waves polarlzed along said axes, and the
non-circular cross-section ~of the outer surface of the fiber
:
. ,
~ x'~
preferably has a predetermined geometric relationship to the
transverse axes of the guidlng region so that the orientation
of those axes can also be ascertained from the geometry of the
outer surface. A directional coupler is formed by joining two
such fibers so that at least a portion of a wave propagated
through either guiding region is coupled into the other guiding
region.
There is also provided a method of making an optical
fiber of the type described above by forminy an optical preform
having a core and a cladding with different refractive indices,
the core being offset from the center of the preform and the
outer surface of the preform having a non-circular cross-
section with a predetermined geometric relationship to the
core, and drawing an optical fiber from the preform with the
drawing rate and temperature being controlled to produce a
fiber with a cross-sectional geometry similar to that of the
preform. The preform and drawn fiber preferably include a
support layer surrounding the guiding region (formed by the
core and the cladding) and forming the non-circular outer
surface of the fiber, with the guiding region located suffici~
ently close to the surface of the fiber so that removal of a
small amount of material from the fiber slliface allows coupling
to a guided wave. A directional coupler may be formed by
removing a portion of the support layer from selected segments
of two such fibers and then joinlng those segments of the
fibers. As an alternative, a unitary directional coupler may
be formed by lnsertlng~two such fibers into a tube with the
guiding regions of the fibers aligned with each other and with
the fiber surfaces closest to the respective guiding~regions
facing each other, and then drawlng the tube while heating
both the tube and the fibers so that the tube and fibers are
all fused together; the fibers as well as t.he tube may be
drawn sufficiently to reduce the diameters of the guiding
reyions therein, whereby thP fields of the fiber cores are
extended into the respective claddings to achieve a desired
degree of coupling between the two fibers. As the two fibers
are drawn, the coefficient of coupling ~ay be monitored and
the drawing process terminated when a desired degree of coup-
ling is obtained.
BRIEF ~ESCRIPTIONS OF DRAWINGS
Brief Description of Drawings
FIG. 1, labelled PRIOR ART, is a diagrammatic per-
spective view, in partial section, illustrating the electric
and magnetic fields in their preferred directions of polariza-
tion in the elliptical core of a single-mode optical fiber
waveguide;
FIG. 2 is an end view of the D-shaped optical fiber
waveguide according to one preferred embodiment of the present
invention;
FIG. 3 illustrates a method of interfacing the
D-shaped optical fiber to an optical device;
~0 FIG. 4, labelled PRIOR ART, illustrates an optical
beam splitter;
FIG. 5 is a perspective view of a fiber-optic dir-
ectional coupler embodying the present invention;
FIG. 6 lS a schematic perspective view of the active
area of:the d1rectional coupler o~ FIG. 5, showing the exchange
of electromagnetic energy from the core or one fiber of the
coupler to the core of the other fiber;
FIG. 7 is a schematic diagram of a single-mode
fiber-optLc sensor uslng continuous sensor and reEerence
fibers and directlonal couplers integral~with the fi~ers
accordlng~to one embodiment of the presen~ invention;
` :
6-
, :., , ~
FIG. 8 is a partially schematic side elevation of
apparatus for drawing optical fiber according to the present
invention.
FIG. 9 is a plan view or a unitary directional
coupler made according to a modified embodiment of the inven-
tion;
FIG. 10 is an enlarged section taken generally along
line 10-10 in FIG. 9;
FIG. 11 is a plan view of a miniature pulling machine
for fusing together the assembly of FIGS. 9 and 10 to form a
directional coupler; and
FIG. 12 iS a front elevation of the miniature pulling
machine o FIG. 11.
While the invention is susceptible to various modi-
~ications and alternative forms, specific embodiments thereof
have been shown by way of example in the drawings and will be
described in detail herein. It should be understood, however,
that it is not intended to limit the invention to the partic-
ular forms disclosed, but, on the contrary, the intention is
to cover all modifications, equivalents, and alternatives
~alling within the ~spirit and scope of the invention as defined
by the~appended claims.
~r Description of the~Preferred Embodiments
~TurnLng~now to FIG. 1, there is shown a dielectric~
core~20 for~supportLng the~propagation of electromagnetic
fields E, H~in~the axial dlrection. This particular core 20
has an ellLpt1ca~1 cros~s-section with a ma~or diame~ter 2a and~a
mino`r diamter 2b. ~A~single-mode~o~tical fiber :has :such a core
20 with a relatlvely~high~dielectrlc~constant/lndex of refrac-
tion which~tends~to conf~in~and guide electromagnetic energy ~ ;~
`(i.e`., light) a~long the~ax~is o~ the core. It is known that if
`` : ~ : : : ::
- '`' , ' :
tne index of refraction of the core 20 is properly chosen in
relation to the index of refraction of the surrounding medium,
the core dimensions a, b, and the wavelenyth of tlle liyht, the
distribution of the fields E, H will tend to occur in a single,
well-defined pattern, or mode. Shown in FIG. 1 is the field
pattern ~or the oHE11 mode.
Single-mode propagation has the advantage of pro-
viding well-defined field patterns for coupling the fiber to
optical devices. Another advantage is that the attributes of
the light propagation, such as phase velocity and group veloc-
ity, are relatively constant as the light propagates down the
~i~er. The group velocity specifies how fast modulation or
information travels down the fiber. Thus, for transmitting
information over long distances it is important that the group
velocity be relatively constant and in particular independent
of frequency so that the informatlon will be localized at a
speci~ic region rather than becoming "smeared out" as the
information travels down the fiber. A constant phase velocity
is important in fiber-optic sensor applications where the
phase of a signal in a sensor fiber is compared to the phase
of a reference signal in a reference fiber.
Single-mode propagation does not, however, guarantee
that the polarization of the signal is fixed in any definite
~- or constant angular relationship with respect to the core 20.
Polarizatlon is defined as the direction of the electric field
vector E.~ Thus, as shown ln FIG. 1, the light is pclarized a
vertical direction.
In single-mode fiber-optic sensors, the phase of the
optical signal at the end of~a sensor fiber is made a function
of an environmen~al parameter sought to be measured. Typically
this phase-shift is introduced by physically lengthening the
fiber, or by changing the index of refraction of the core 20.
-
~ 8-
But if the core 20 is not polarization-preserving, the polari-
zation of the light tends to change randomly as the light
propagates down the axis of the core 20. Such a random change
in polarization results in a fluctuation of the detected
signal since a 180 rotation o~ the direction of polarization
is equivalent, at the end of the fiber, to a 180 phase shift.
Thus, for sensor applications, the polarization of the light
should be maintained at a fixed angular relationship with
respect to the fiber as the light propagates down the core.
To maintain or preserve the polarization of a signal
in an optical fiber, the optical yroperties of the fiber must
be anisotropic, or in other words a function of the angle of
polarization with respect to the fiber. One method of making
the optical fiber anisotropic is to make the core 20 have a
cross-section which is elliptical or some other non-circular
shape which defines two transverse orthogonal axes permitting
the de-coupling of waves polarized along those axes. Signals
which are launched into such fibers in alignment with one of
the transverse axes tend to remaln aligned with that axls as
~0 the signals are propagated through the fiber, thereby preserv-
ing the polarization of the signal.
In accordance with one important aspect of the
present invention, the core of the gulding region in a single-
mode opti al flber has a non-circular~cross-section which
defines two transverse orthogonal axes for holding the~polari-
~zation of signals aligned wlth those axes, the gulding region
being located;sufficlentl~y close to the sur~ace of the fiber,
alon~ a selected length o~the fiber, to~allow coupling to a
guided wave,~and the outer surface~ of~the fiber has a non-
circular~cross-section with a predetermined geometric relation~
ship to`the guidi~g reglon~and the orthogonal transverse axes
of~the core so~that the~location of the guiding reyion and the
:
~ 9_
:::
~` orientation of the transverse axes can be ascertained from the
geometry of the outer surface. Thus, in the illus-trative
embodiment of FIG~ 2, an optical Eiber 25 has an elliptical
core 26 with a relatively high index of refraction surrounded
by a cladding 27 with a lower index of refraction. The dimen-
sions and the refractive indices of the core 26 and the clad-
ding 27 are selected to provide a single-mode guiding region.
Because of its elliptical shape, this guiding region will also
hold the polarization of optical signals propagated therethrough
in alignment with either axis of the ellipse. That is, the
major and minor axes of the elliptical cross-section represent
two transverse orthogonal axes which permit the de-coupling of
waves polarized along those axes.
Surrounding the guiding region formed by the core 26
and cladding 27 is a support layer 28 which provides the fiber
with increased mechanical strength and ease of manipulation.
Since this support layer 28 is not a part of the guiding
region, its optical properties are not nearly as critical as
those of the core 26 and the cladding 27. To prevent light
~0 from ~eing trapped in the cladding 27, the support layer has
an index of refraction higher than that of the~cladding 27.
As can be seen in FIG. 2, by removing a thin portion
of the support layer 28, and also a portlon of the cladding 27
~r if necessary to achieve the desired degree of coupling (e.g.,
by etching to the dashed contour~in FIG. 2), the guiding region
formed by the core~26 and cladding 27 can be located suffici-
ently close to~the surface of the fiber to allow coupling to
a guided wave~ As an alternative, a selected segment of the
fiber can be drawn to reduce the fiber diameter withln that
:
segment and thereby expand the field of the guidlng region to
permit the coupling of guided waves to and from~the guiding
, ~
region ln that segment of the fiber.
T~he outer surface of the~fiber as defined by the support
layer 28 in FIG. 2 has a D-shaped cross-section, with the ~lat
- --10--
.
~ 3
surface 29 of the D extending parallel to the major axis of
the elliptical guiding reyion on the side of the fiber closest
to the guiding region. This D-shaped optical ~i~er is easily
interfaced to a polarization-sensitive optical device b~ using
the flat surface 29 of the D as an indexing surface. As shown
in FIG. 3, an optical device 30, such as a solid state laser,
has a point source of horizontally-polarized light 31 which
may be coupled to the optical fiber 25 if the fiber 25 has its
core 26 aligned with the source 31 and if its preferred direc-
tion of polarization is also horizontal. To facilitate the
alignment, an indexing slide 32 may be scribed or photo-etched
on a horizontal surface such as the top of a chip 33. The
slide 32 may be fabricated using conventional photolithographic
and other microelectronic fabrication techniques. Similarly,
two fibers may be spliced coaxially using a chip with an
indexing slot to align the flats of the D's.
The guiding region of the D-shaped fiber 25 of
FIG. 2 is preferably offset or displaced from the geometric
center 35 (i.e., centroid of mass or center of gravity of the
~0 transverse sectlon) toward the flat 29 of the D along the
perpendicular-bisector 36 of the flat. Preferably, the guid-
ing region is located within a few average core diameters of
the flat surface 29 so that the outer surface of a portion of
the fiber may be etched to expose the gulding region at the
surface 29, thereby permitting the transmission or gradual
exchange of light between the guiding region and the fiber
surface. For example, the guiding region can be located
within about three average core diameters of the flat surEace
29. For the elliptical core 20, the average core diameter is
the sum (a + b) of the~major and mlnor radii.
The fact that the flat surface 29 of the D is within
a few average core diameters of the guiding region does not
,
t~
~ 3
affect the attenuation or loss of the fiber since the flat
surface 29 is not within the cladding 27. Although there is
some light propagated within the cladding 27, substantially no
light reaches the support layer 28 which forms the flat 29 of
the D. But if an etchant such as hydrofluoric acid is applied
to the outer surface of the fiber along a selected length, the
flat surface 29 will be moved inwardly (e.g., to the dashed
contour shown in FIG. ~), thereby allowing light between the
core 26 and the flat surface 29 via the cladding 27. A fiber
with a central core is difficult to etch in this fashion since
there may not be any supporting layer remaining after the
etching process.
Perhaps the most important application which requires
the gradual exchange of light between the core and the surface
of an optical fiber is a directional coupler. Directional
couplers are the fiber-optic equivalents of optical beam
splitters and are indispensable elements for making single-mode
fiber-optic transducers. A beam splitter 40 is schematicaIly
shown in FIG. 4. The beam splitter 40 is essentially a parti-
~0 ally silvered mirror which transmits a portion of the incidentlight and reflects the rest. A source of incident light A' is
usually directed at 45 with respect to the plane of the bearn
splitter 40 so that the incident beam A' is split into a
transmitted beam C' and a reflected beam D'. In addition to
the beam-splltting function, the beam splitter 40 is also used
to combine two incident beams A', B'. The incident beam B',
ahown in dashed representation, may;also be directed at 45
with respect to the plane of the beam~spl1tter 40 and at right
angles to~ the incident beam A' so that the output beams C' and
D' are combinations of the incident beams A' and B'.
There lS shown in FIG. 5 an exemplary fiber-optic
direc~ional coupler comprised of two D-shaped optical fibérs
-12-
44, 45, like the fiber 25 of FIG. 2, positioned ad]acent each
other on a flat substrate 46. The flat surfaces of the ribers
44, 45 con~act each other along etched lengths 1, and their
guiding regions are aligned to permit the yradual exchange of
light between the guidlng regions along the length 1 of the
fibers. In the particular embodim~nt illustrated, alignment
of the guiding regions of the fibers 44, 45 is facilitated
because the D-shaped fibers 44, 45 are curved rather than
straight in their unstressed configuration, with the flats of
10 the D's located on the convex surfaces of the curved fibers~
The curving or curling of the fibers is achieved by forming
the flbers from materials which provide a large difference in
the coefficients of thermal expansion of the support layer 28
and th~ grinding region formed by the core 26 and cladding 27.
These two different portions of the fibers then contract at
different rates during cooling after the fibers are drawn~
producing the desired curvature illustrated in FIG. 5. With
this curvature, the flats of the D's are aligned vertically
when the curved fibers 44, 45 are laid flat on the substrate
20 46. Thus the fibers can simply be~moved into engagement with
each other along the etched lengths 1 and fastened together by
a drop of adhesivs 47.
The operation of the directional coupler of FIG. 5
t is best understood in terms of an exchange or transfer oE the
electromagnetic:fie:lds E, H propagating down the cores 48, 49
of the respective fibers 44, :45 as shown in FIG. 6. A portion
of the electromagnetic field energy in the~incident signaI A
is gradually trahsferred from one core 4:8 to the other~core
49. In general, the relative amount~of energy from signal A
30 that is transferred from one core 48 to the other core 4g is
proportional to~the amount of~coupling per unit length and the
length 1 over which the coupling occurs.
:
:
: ~ ~
Directional couplers are used in devices such as
single-mode interferometer sensors and fiber gyros. A genera-
lized schematic diagram OL a single-mode interferometer sensor
using optical fibers and directional couplers accordiny to the
present invention is shown in FIG. 7. A coherent source of
light such as a laser 51 emits light into an incident port of
a directional coupler 52. The directional coupler 52 acts as
a beam splitter and sends half of the light into a reference
fiber 53 and the other half into a sensor fiber 54. The
sensor fiber 54 is coupled to the environment so that the
phase of the light in the reference fiber is modulated by a
desired environment signal 55 by the time the light reaches a
second directional coupler 56. This second directional coupler
56 accepts the light transmitted through the reference fiber
53 and the light transmitted through the sensor fiber 54 as
incident signals, and acts as a combiner. Combined signals
appear at two output ports 57, 58, one of which (57) is ter-
minated (preferably with substantially no reflections) and the
other of which (58) is fed to a photodetector 59.
The photodetector 59 is responsive only to the
amplitude of the detected signal at the output port 58. The
"beat phenomenon" generates a null in the relative response of
the photodetector 59 when the phase difference is 180, or one
half of a wavelength of the coherent light. For maximum
sensitivity, the null should be very sharp. But to get a
sharp null, the sensor fiber signal and the reference fiber
signal each must be phase coherent, and they must have equal
amplitudes. The~signals will be phase coherent if they propa-
gate as single modes in the sensor and reference fibers. The
amælitudes will be equal lf half of the llght 58 reaching the
photodetector 59 passes through the reference fiber 53 and the
other half passes through the sensor fiber 54. These propor-
tions are set predominantly by the coefficient of coupling of
~ 14-
the directional couplers 52 and 56, so it is important that
these couplers have coefficients of coupling that are precisely
defined and stable.
To some extent, the null can be sharpened by a null
signal generator 59a which generates a null signal on a feedback
line 59b to modulate the characteristics of t~e reference fiber
53. Thus the reference phase may be adjusted by the null
signal generator 59a so that the difference between the sensor
phase and reference phase is approximately 180. The relative
response is then always close to null.
It will be appreciated from the foregoing discussion
that the system of FIG. 7 measures ambient conditions by the
changes they effect in the sensor fiber 54 as compared to the
r~ference fiber 53. Thus, maximum sensitivity is affected by
the uniformity and mechanism of light propagation and the
characteristics of the fibers, the directional couplers, and
the connections between them. By using the D-shaped fiber of
the present invention, there are no connections or joints in
the reference fiber or the sensor fiber other than the coup-
lings at the directional couplers; thus, t~e propagation oflight through the system LS not affected by the reflectlons or
changes in optical properties that can occur at fibe~ termina-
tions. The directional couplers 52, 56 are easily fabrlcated
at any point along the length of the D-shaped fiber so that,
for example, they may be fabricated on:relatively~short lengths
of fiber extending from a large spool of fiber comprlsing the
sensor fiber 54.~ ~ ~
~ The maximum sensitivity of th~ fiber-optic sensor
shown in FIG. 7~is ultima~ely set by the relative change in
sensor phase~of~the sensor fiber 54 as a funotion of the
environment signal:55, and the~level of Iight received
at the end of the sensor fiber. Since the total phase change
is usually directly proportional to the length of the sensor
fiber 54, it may be desirahle to use an extremely long low-loss
sensor fiber.
In accordance ~ith another important aspect of the
present invention, the optical fiber is made by forming a
preform having a core and a cladding forming a guiding region
which is offset from the center of the preform and has a non-
circular cross-section defining two transverse orthogonal axes,
the outer surface of the preform also having a non-circular
cross-section with a predetermined geometric relatlonship to
the guiding region and its transverse axes; and drawing an
optical fiber from the preform with the drawing rate and temp-
erature being controlled to produce a fiber with a cross~
sectional geometry similar to that of the preform. The guiding
region of the drawn fiber is preferably located sufficiently
; close to one side of the fiber that the removal of a relatively
small amount of material from the outer surface of the fiber
allows coupling of guided waves to and from the guiding region.
Thus, the preform can have the same cross-sectional
configuration as the fiber 25 illustrated in FIG. 2. Such a
preform can be made by first forming a cylindrical preform
with an elliptical core and cladding located in the center
thereof (using techniques known in the artj, and then grinding
one side of the preform to form a D-shaped~cross-section with
the flat surface of the D extending parallel to the major axis
of the elliptical core. An optical fiber is then drawn from
the D-shaped pre~orm at a drawing rate and temperature con-
trolled to produae the flber 25 of FIG. 2, i.e., with a cross-
seotional geometry substantlally the same as that of the preform.
A drawing machine suitable for precise control of the
drawing process is shown ln FIG. 8. In order to heat the preform
to approximately the softening temperature, the central component
.
~ -16-
of the drawing machine is an induction furnace generally
designated 60 comprising an external induction coil 61 and an
internal graphite toroid 62. The toroid g2 is approximately
an inch long, an inch in diamter, and has a core hole about a
quarter inch in diamter. The induction coil 61 is energized
by a radio-frequency power source 63 so that electrical heating
currents are induced in the graphlte toroid 62, the resulting
temperature being measured by an optical pyrometer 64 and
monitored by a control unit 65 adjusting the power source 63
In order to prevent the graphite toroid 62 from burning up,
the toroid 62 is disposed within a vertical glass cylinder 66
which is filled with a relatively inert gas such as argon from
a supply 67.
The preform 68 is fed from the top of the cylinder
66 and passes through the center of the graphite toroid 62.
The toroid 62 is heated white hot causing the preform 62 to
soften. The drawing of the fiber 69 from the pr~form 68
occurs approximately at the center of the toroid 62. The
toroid 62 has legs 71 which stand on a support ring 72
~0 attached to the glass cylinder 66.
The critical parameters affecting the drawing process
are the rate of Eeed Vp of the preform 68 toward the drawing
point, the temperature at the drawing point, and the rate Vf
at which the fiber 69 is drawn from the drawing point. The
temperature and~rate of drawing Vf set the tension at which
the fiber 69 is drawn, and this tension may be further regu-
lated by a series of tensioning rollers 73 which also assure
that the fiher 69 is drawn coaxially out of the bottom of the
glass cylinder 6~.~ The rate of feed of the preform V is
P
established by a vertical linear slide generally deslgnated 74
having a lead screw driven by a drive motor 75. A vertical
shaft 76 lS~ actuated by the slide 74 and extends into the
glass cylinder 66 through a gas seal 77. At the end of the
shaft 76 is a Phillips chuck 78 which grips the top end portion
of the preform 68. The rate of drawing Vf~ on the other hand,
is established by a horizontal take-up drum 79 below the lower
end of the glass cylinder 66. The take-up drum 79 is journaled
for rotation and driven by a take-up motor 81 through a speed
reduction gear assembly 82. To wind the fiber 73 i~ a helical
fashion on the take-up drum 79, the drum as well as the take-up
drive itself is mounted on a horizontal linear slide generally
designated 83 having a lead screw driven by a drum advance
motor 8~.
In one particular example, a preform was made by
depositing a pure silica cladding and a germania core on the
inside surface of a silica tube. The cladding and core were
formed by the thermal decomposition of silicon tetrachloride
and germanium tetrachloride, which were circulated through the
bore of the silica tube at approximately 1800C in an induction
furnace. Diametrically opposed portions o~ the outside~ surface
of the silica tube were then ground flat, after which the tube
was colIapsed and lightly dxawn to form a preform having an
outer surface with a cylindrical cross-section with a diameter
of about 2.8 mm. and a central core and cladding of~elliptical
cross-section. One side of the elliptical-cored preform was
then ground flat, wi~h the plane of the flat surface extending
parallel to the major axis of the elliptical core and located
within a few thousands o~ an inch of the cladding. The preform
thus had a D-shaped cross-section. Optlcal fiber was then
drawn from this D-shaped preform at a temperature of about
1790C while feeding the preform at a rate of about 0.3 mm/sec.
and while pulling fiber from~the preform at a rate of about
0.5 m/sec. These parameters were chosen to result in a drawing
tension~as high as practical without breaking the fiber. The
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resulting fiber had a D-shaped cross-section as illustrated in
FIG. 2, with a maximum outside diameter of about 85 microns.
The shape of the cross-section is retained as the preform is
drawn into a fiber due to the high drawing tension, the rela-
tively small diamter of the preform, and the precise tempera-
ture and localized heating of the induction furnace.
As another feature of the present invention, a
unitary directional coupler is formed by inserting two of the
fibers of FIG. 2 into a tube with the guiding regions of the
two f1bers aligned with each other and with the fiber surfaces
closest to the respective guiding regions facing each other,
and then drawing the tube while heating both the tube and the
fibers so that the tube and fibers are all fused together.
The fibers are preferably drawn along with the tube so as to
reduce the diameters of the guiding regions therein, whereby
the fields of the fiber cores are extended further into the
respective claddings to achieve a selected degree of coupling
between the two fibers. If desired, the coefficient of coup-
ling may be monitored during the drawing operation and the
process terminated when a desired degree of coupl1ng is
obtained.
Thus, according to an exemplary method illustrated
in FIGS. 9 and 10, a pair of fibers 9l, 92 are threaded into
the bore of a sil1aa tube 93. The fibers 91, 92 are prefer-
ably the D-shaped fibers of FIG. 2 and are inserted into the
tube 113 w1th the flat surfaces of the D's facing each other.
The bore of the sillca tube 93~is just slightly greater in ~ ;
diameter than the diameter of the combined pair of fibers so
~hat the guiding regions of the fibers 9l, 92 are automatically
aligned when the~fibers are 1nserted into the tube 93. Altern-
atively, the pair of fibers 91, 92 may be aligned and tacked
together at an intermediate location by an electric arc so
~ 3
that alignment will be provided by the tacking rather than
relying solely on a close fit between the fibers and the bore
of the silica tube 93. The end portions of the silica tube 93
are then clamped into a miniature pulling ~achine generally
designated 100 in FIGS. ll and 12.
The pulling machine 100 is comprised of a linear
slide base 101 receiving a block 102 fixed to the base, and a
sliding block 103 also received by the base. The slide base
101 and blocks 102, 103 are optical bench components. One end
portion of the silica tube 93 is clamped to the fixed block
102 and the other end portion is clamped to the sliding block
103. The linear motion of the sliding block 103 with respect
to the fixed block 102 is limited by a stop 105 fastened to
the fixed block cooperating with a micrometer 104 fixed to the
sliding block. Thus, after the end portions of the silica
i tube 103 are clamped between the blocks 102, 103, the gap
between the micrometer 104 pole face and the cooperating
surface of the stop 105 may be adjusted to set the degree of
extension of the silica tube 93 when it is drawn. The drawing
tension is regulated by a spring 106 and is further set by a
weight 107 strung over a pulley 108.
The drawing of the silica tube 93 occurs when it is
heated to the softening point by a platinum coil 109 which is
energized by a power supply 110. The cross-sectional area of
the tube 93 decreases as it is drawn, thereby closing in on
and squeezing the D fibers together. The coefflcient of
coupling between the optical ~fibers 91, 92 is monitored during
the drawing process by exciting an input port A of the coupler
with a laser 11l and measuring the response at output ports C
and D with a photodetector 112. The apparatus shown in FIGS.
11 and 12 is semi-automatic, it being understood that the
drawing process may be repeated a number of times while the
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coefficient of coupling is monitored until the desired coeffi-
cient of coupling is obtained.
While the method shown and described in conjunction
with FIGS . 9-12 uses D-shaped fibers to ~orm a directional
coupler, it should be understood that other off-center core
optical fibers may be coupled using this technique. To accom-
modate an off-center core fiber having an arbitrary external
cross-section, the cores may be first positioned together and
then tacked. If an off-center core fiber is used which has a
relatively large spacing between the outer surface of the
fiber and the core, the portions of the fibers to be coupled
should first be etched to expose the guiding regions on the
fiber surfaces closest to the guiding regions.
The method of fabricating directional couplers
illustrated in FIGS. 9~12 results in a unitary assembly where-
in the fibers 61, 62 and the sillca glass tube 63 are fused
together at an intermediate location. In contrast to the
design of FIG. 5, the unitary directional coupler is more
suited for applications such as the fiber gyro which require
the coefficient of coupling~to be stable over long periods of
time and also insensitive to mechanical shocks and other
environmental stresses.
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