Note: Descriptions are shown in the official language in which they were submitted.
1065140
Background of the Invention ~
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1. Field of the Invention
The invention is concerned with fiber optical
communications.
2. Description of the Prior ~rt
Increasing demand for communications facilities ~
for voice, video, and data transmission has created interest ~-
in communications systems utilizing light as information
carrier. Compared with systems in current use, optical
communication promises to lead to a reduction in size of
transmission means combined with an increase in information
carrying capacity. Specifically, optical transmission
lines of a diameter of no more than 0.1 mm have a capacity
far greater than that of currently used microwave guides
of a diameter on the order of 1 cm.
The physical principles underlying the use of
optical fibers in communications are well established.
Light guiding in such fibers is achieved by a change in
refractive index from a higher index core portion to a
lower index cladding surrounding the core. This change in
refractive index need/not be large; for example, effective
light guiding can be achieved by a decrease in refractive
index of as little as 0.1 percent. The change in
refractive index may take the form of one or several steps
or it may occur gradually. For example, a gradual change
in refractive index with the grading profile chosen to
minimize mode dispersion has been advocated for use in
fibeFs intended for multi-mode transmission.
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Specific examples of optical fi~ers are the
pure silica clad, germania doped silica core fiber
disclosed in U.S. Patent 3,775,075, the pure silica core,
borosilicate clad fiber disclosed in U.S. Patent 3,778,132,
an~ the plastic-clad pure silica core disclosed by
L. Blyler, A. Hart and P. Kaiser at the Spring meeting
of the Optical Society of America, Washington, D.C. 1974.
U.S. Patent 3,778,132 also discloses the use of an
additional layer such as a pure silica layer over the
borosilicate cladding.
Whi~e design and development of transmission
lines have reached an advanced state, progress has also been
made in the development of devices such as light sources,
detectors, switches, and modulators. Miniature lasers,
light emitting and light detecting diodes, and a variety of
thin film optical devices have been proposed for use in
optical communications systems.
An important practical aspect of the development
of optical systems is the physical and chemical protection
~0 of devices, a purpose for which metal and glass encap- ,
sulations are suitable. Such use of encapsulations leads
- to the problem of making reliable seals between fibers and
device encapsulations without introducing undue optical
loss at the point of the seal.
Summar~ o~ the Invention_
- The invention provides for a seal between a
borosilicate glass encapsulation and an optical fiber lead-
ing through an orifice in the glass encapsulation.
Speclfically called for are an orifice located at the end
of a tubular protrusion of the glass body whose softening
temperature does not exceed that of the surface of the
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iber. A glass containing silica is specified as the ~-
surface glass of the fiber. The seal is made ~y heat
fusing the rim of the orifice to the surface of the fiber.
In accordance with one aspect of the present invention
there is provided a method for making a seal between a
device envelope and an optical fiber leading through an
orifice of said envelope characterized in that said fiber
has a silica containing glass surface, said envelope, at
least in the vicinity of said orifice, consists of a
borosilicate glass whose softening temperature is less
than or equal to the softening temperature of said silica
containing glass, said orifice is located at the end of a
protrusion of said envelope which at least in the vicinity
of said orifice closely fits said fiber, and said seal is
made by heating the rim of said orifice to the softening
temperature of said borosilicate glass whereby said rim is
deformed and fused to the surface of said fiber.
In accordance with another aspect of the present
invention there is provided article comprising a device
envelope and an optical fiber leading through an orifice
in said envelope characterized in that said fiber has a
silica containing glass surface, said envelope at least in
the vicinity of said orifice consists of a borosilicate
glass whose softening temperature is less than or equal to
the softening temperature of said silica containing glass,
said orifice is located at the end of a protrusion of said
envelope and the surface of said fiber is fused to the rim
of said orifice.
Brief Description of the Drawinq
The Figure shows, in cross section, an assembly of an
encapsulated optical device and an optical fiber fused to
the encapsulation as claimed.
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Detailed Description
l. The Figure
The Figure shows photodetector diode 10 with
electrodes ll and lead wires 12 mounted on insulator 20 ~`~
which is supported by base 30. Borosilicate glass
envelope or encapsulation 40 i5 sealed to base 30 and, by
; the claimed method, to optical fiber 50 which terminates
at diode 10 a~d is held in place by silicone positioner
21. In one embodiment of the invention, the borosilicate
~10 glass of the envelope contains B2O3, SiO2, and
Na2o in a combined amount of at least 90% by weight.
2. Method and Resultin~ y
The optical fiber whose surface is a silica containing
glass is fed through the orifice at the end of a tubular
borosilicate protrusion which closely fits the fiber in
the vicinity of the orifice. The inner diameter of the
orifice should exceed the diameter of the fiber by a
~ factor of at most 10, and preferably, at most three or
; even less. The softening temperature of the glass of the
tubular protrusion is required not to exceed that of the
cladding in order to prevent heat damage to the fiber
during fusing. Microscopic observation may be helpful in
ascertaining central alignment of the fiber through the
orifice.
Sealing of the fiber to the tubular protrusion is
effected by localized short-duration heating of the end of
the tubular protrusion to its softening point to cause the
r~m of the orifice to deform until it comes into contact
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with an~ is fused to the surface of the fiber. Heating
by means of infrared laser radiation from a CO2-laser was
found to be effective for sealing. If a single
directional heat source such as an unmodified laser beam
is used, uniformity of the seal may be enhanced by rotating
the assembly during heating. Prolonged heating should be
avoided, however, to prevent damage to the cladding such
as by extensive mixing of the cladding material with the ~ -
material of the protrusion because such mixing generally
leads to a weakened seal and to optical loss at the point
of the seal.
To prevent undue strain at the point of the seal
a close match of thermal expansion coefficients of the
glasses being fused is desirable. Fiber surface glasses
with a thermal expansion coefficient in the range of
from 2 x 10 6 to 6 x 10 6 inches per degree centigrade
are particularly suited in this respect for sealing to
commercially available borosilicate glasses.
While not directly applicable to seal a plastic-
sheathed fiber to an encapsulation, the claimed method can
~ be adapted for this purpose by splicing the sheathed fiber
- to a so-called pigtail, i.e., a short section of an
unsheathed fiber whose surface is of a suitable glass. In
order to maintain continuity in light gathering ability
between the fiber and the pigtail, care has to be taken in
the selection of core and cladding materials for the pigtail.
Specifically, materials have to be chosen so as to produce
a close match between the numerical aperture of the sheathed
fiber and the numerical aperture of the pigtail as
discugsed, for example in N.S. Kapany, Fiber Optics,
A~ademic Press, 1967, on page 9. For example, a pure
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- silica core, fluorinated polymer sheathed fiber has a
numerical aperture of 0.58. To match this numerical
aperture in a borosilicate clad fiber whose cladding has
a refractive index of 1.448, a core material with a
refractive index of at least 1.57 has to be chosen. By
applying the claimed method to make a seal between the
pigtail and the glass body the plastic-sheathed optical
fiber is effectively connected to the encapsulation. The
foregoing procedure is important because plastic-sheathed
fibers are particularly suited for the transmission of
light emitted by a light emitting diode. This suitability
is specifically based on the large difference of refractive
index between a glass core and a plastic coating, a fact
which allows such fibers to efficiently gather and guide
light emitted in a broad angular range.
Due to reliance on a silica containing fiber
surface glass on the one hand and independence of the
internal structure of the fiber on the other, the claimed
method is applicable to a great variety of light guiding
structures in addition to the pure silica core and germania
doped silica core fibers mentioned above. The method is
equally applicable with fibers whose core is doped with
substances such as the oxides of germanium, sodium, phos-
phorus, lead, lanthanum, calcium, tantalum, tin, niobium,
zirconium, aluminum and titanium as well as with fibers
with doped claddings. The silica containing surface glass
may contain substantial amounts of B2O3 and preferably in a
combined amount of at most 10 percent by weight, substances
such as the oxides of lead, bismuth, phosphorus, germanium,
3C aluminum, or magnesium as may be present as impurities or
as added, e.g., to enhance workability or lower the refractive
index of the surface glass.
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While an essentially tubular shape of the
- encapsulation is called for in the vicinity of the orifice
to be sealed, the shape of the protrusion away from the
vicinity of the orifice and the overall shape of the
encapsulation are not restricted by this requirement.
Similarly, the composition of the encapsulation away from -
the orifice may vary from the borosilicate composition at
the orifice and may be partly composed of non-glassy
substances such as metals. ' -
3. Examples
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Example 1
A silica-core, (3SiO2-B2O3)-clad fiber was sealed
to commercially available borosilicate glass by exposing
the orifice to 5 watt CO2-laser radiation for ten seconds.
No appreciable optical loss was introduced into the fiber
by making this seal.
Example 2
A pure silica-clad fiber was sealed to a
(6SiO2 B2O3)-tube. In spite of a considerable difference
in *hermal expansion coefficients between the tube and
cladding materials, a satisfactory seal was obtained.
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