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CROSS REFERENCE TO RELATED APPLICATIONS
This application was concurrently filed with application Ser. No.
07/486,350 (now allowed, but not issued) entitled "Matte Finishes on
Optical Fibers and Other Glass Articles".
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a technique for fabricating an optical
fiber with a lensed endface and, more particularly, to a fiber lensing
technique which utilizes wet chemical etching operations to form the
desired shape of the lens.
2. Description of the Prior Art
In most optical communications systems, a lightwave signal must be coupled
between an optical fiber and a device (e.g., a laser, LED photodiode, or
another fiber). For most conventional arrangements, discrete lenses in
bulk form are inserted in the light path in front of the fiber to couple
the signal into or out of the core region of the fiber. Problems exist
with these arrangements with respect to alignment of components, the size
of the bulk optic components, signal loss, overall cost, etc. A partial
solution has been developed in which a discrete lens element is attached
(e.g., epoxied) directly to the endface of the fiber. This approach may
reduce signal loss and the overall complexity of the arrangement. However,
the operation involved in attaching the lens to the fiber is
time-consuming and may unnecessarily increase cost. Further, over extended
periods of time the adhesive may fail, and the lens may become misaligned
or completely detach itself from the fiber.
One alternative to the epoxied lens technique rests upon the recognition
that differently doped glass materials used to form fibers etch at
different rates when exposed to common fluorine-based etchants such as
hydrofluoric acid and buffered hydrofluoric acid. By controlling various
etching parameters (e.g., strength, time, temperature), workers have
discovered that, for example, the cladding can be preferentially etched
(removed) with respect to the core, leaving an exposed, protuberance.
According to P. Kayoun et al, Electronic Letters, 17(12), 400(1981), such
a "protuberance acts as a diffracting phase object with lens-like
properties." (p.401, col. 1). The lens, however, had a depression in the
center and exhibited a coupling efficiency of only 35%. See also P.
Kayoun, U.S. Pat. No. 4,406,732. In both references 40% concentration of
hydrofluoric acid was used to form the lens on MCVD single mode fibers
having pure silica cores and boron-doped claddings. Kawachi et al,
Electronic Letters, 18(2), 71(1982) recognized that the depression
prevented one from obtaining high quality microlenses. They described the
use buffered hydrofluoric acid to produce a mesa or "circular cone" (FIG.
1) on the end of a VAD single mode fiber having a Ge-doped core and a pure
silica cladding. Fire polishing converted the mesa or cone into a
"round-shaped" microlens (p.71, col.2), but no evidence of coupling
efficiency or reproducibility was reported.
Etching/polishing techniques, such as those described above, naturally
align the lens with the core, regardless of any deviation in core
placement within the fiber. Moreover, the shape of the lens immediately
after etching is not very important because, regardless of that shape,
surface tension effects during subsequent fire polishing convert that
shape into a sphere.
Although this chemical etching method of forming a fiber lens is a viable
alternative, some problems remain, including its relatively low coupling
efficiency (.eta. of approximately 40-50%) and the need to carefully
control the fire-polishing operation. In addition, fire polishing is
frequently incompatible with other aspects of the process (e.g., the
presence of plastic coating or epoxy on the fiber; or the availability of
only a short stub of fiber on which to operate).
More recently, H. Ghafoari-Sheraz, Optical and Quantum Electronics, 20,493
(1988) used buffered hydrofluoric acid to construct a conical microlens on
the end of an aluminum-coated VAD single mode fiber having a Ge-doped core
and a pure silica cladding. A minimum coupling loss of about 3 dB was
reported, which corresponds to a coupling efficiency of less than 50%.
Although the author did not use fire polishing in these experiments, he
explicitly allows for that possibility to form a round-shaped microlens.
Thus, a need remains in the prior art for a lensing technique which does
not include critical manufacturing operations, does not require fire
polishing, and is capable of reproducibly achieving coupling efficiencies
greater than 50%.
SUMMARY OF THE INVENTION
The need remaining in the prior art is addressed by one aspect of the
present invention, a technique for fabricating an optical fiber with a
lensed endface and, more particularly, a fiber lensing technique which
utilizes wet chemical etching operations to form the desired curvature of
the lens. Other aspects of the invention include the ability to produce
different desired lens shapes depending on the parameters of the etching
step and the doping profile of the fiber; the ability to do end point
detection by measuring the outside diameter of the fiber; and the ability
to metalize the fiber by an electroplating operation.
In general, the invention includes providing an optical fiber which has
first and second portions which etch at first and second rates when
exposed to an etchant, characterized by exposing those portions to a
treating agent, simultaneously with the etchant, which modifies at least
one of the etch rates relative to the other. The portions may be any two
zones of the fiber endface such as, for example, the core and cladding.
Illustratively, the treating agent is an acid (e.g., an acetate-based acid
or a citrate-based acid) which, by itself, does not significantly etch the
fiber. An attractive feature of the invention is that the combination of
the etchant and the treating agent provides an additional degree of
freedom which allows lenses to be shaped from a variety of fibers of
different compositions.
The ability to shape such lenses is particularly important in the
fabrication of a lens on a fiber which has a nonuniformly doped core, a
common, albeit inadvertent, artifact of the predominant fiber fabrication
processes (CVD and MCVD). As noted earlier, a lens formed on the end of
such a fiber may have a generally conical shape but, undesirably, the apex
of the cone is obliterated by a cusp or depression (FIG. 1) which forms
there because the center portion of the core etches too fast relative to
the peripheral portion. In accordance with one embodiment of the
invention, the etch rate of the center portion is reduced by exposing the
fiber to a treating agent, simultaneously with the etchant, which alters
the etch rate of the center portion relative to the peripheral portion.
In exemplary embodiments the etchant comprises a fluorine-based etchant
(e.g., buffered hydrofluoric acid) and the treating agent comprises an
acid (e.g., acetic acid or citric acid). On single mode fibers this
etching technique advantageously produces a unique integral lens shape
comprising a frustum of a first cone having a cone angle .theta..sub.1,
and, on top of the frustum, a second cone having a cone angle
.theta..sub.2 <.theta..sub.1. This lens shape has led to average coupling
efficiencies of about 80%, and the inventive process has realized
extremely narrow statistical distributions (i.e., standard deviation of
less than .+-.2%) of the coupling efficiency. These characteristics,
together with considerable flexibility in the choice of etching parameters
(e.g., time, temperature, concentration), all point to a process with a
high degree of manufacturability.
Although the term cone implies a triangular cross section, in practice the
sides of the cone may be slightly curved (convex or concave) if so
desired. Such curvature may be useful in reducing reflections or enhancing
coupling efficiency.
The unique lens shape discussed above gives rise to another aspect of the
invention, an optoelectronic assembly comprising an optoelectronic device
(e.g., laser, photodiode or another fiber), an end portion of an optical
fiber coupled to the device, and an integral lens formed on the end
portion, characterized in that the lens comprises a frustum of a first
cone having cone angle .theta..sub.1, and, on top of the frustum, a second
cone having a cone angle .theta..sub.2 <.theta..sub.1.
Yet another aspect of the invention from a manufacturing standpoint arises
from our recognition that the end point of the etching process need not be
monitored by measuring time, nor by observing the lens shape directly
(since it is very small, and even small errors in judgment may lead to
large deviations in lens shape). Rather, we have found that the endpoint
can be determined by monitoring the much larger outside diameter (OD) of
the fiber. When the OD, which gradually decreases during the etching
process, reaches a predetermined dimension, the etching is terminated and
the desired lens shape is attained.
One other embodiment relates to forming a metal layer on a lensed fiber
using the technique described in our concurrently filed application,
supra. More specifically, a matte finish is formed on the outer surface of
the fiber by exposing it to a mixture of the etchant and a relatively high
concentration of a treating agent. The fiber is then electroplated with
metal, the matte finish being effective to ensure proper adhesion. Then
the end of the fiber may be lensed as described previously. This plating
procedure is much simpler and less costly than the prior art which
utilizes sputtering, or firing a metal ceramic onto the fiber, to deposit
the metal. In addition, electroplating produces more uniform layers than
presently practiced sputtering techniques.
BRIEF DESCRIPTION OF THE DRAWING
The invention, together with its various features and advantages, can be
readily understood from the following more detailed description taken in
conjunction with accompanying drawing, in which:
FIG. 1 is a sketch illustrating a prior art etched fiber lens having a cusp
or depression in the center;
FIG. 2 is a sketch illustrating an etched lens formed in accordance with
one embodiment of the present invention;
FIG. 3 is a graph illustrating a normal probability plot of coupling
efficiency of lensed fibers formed in accordance with one embodiment of
the present invention using a mixture of 3:1 BOE and acetic acid;
FIG. 4 is a graph illustrating coupled power as a function of the change in
separation (.DELTA.z) between the source and a lensed fiber fabricated in
accordance with the embodiment of the invention described in conjunction
with FIG. 3;
FIG. 5 is a graph illustrating how the etch rate of the fiber outside
diameter (OD) changes with temperature; and
FIG. 6 is a graph illustrating how the maximum coupling efficiency
.eta..sub.max changes as the fiber OD decreases during the etching
process; this graph, therefore, correlates .eta..sub.max and etching time.
DETAILED DESCRIPTION
It has been discovered that problems in coupling efficiency in many
conventional etched fiber lenses can, in part, be attributed to a cusp, or
depression, which forms in the extreme central portion of the core. FIG. 1
is a sketch of the end of a single mode fiber illustrating the etched
depression 12 formed on the tip of lens 10. When optical fiber preforms
are formed by collapsing a tube, the central region of the preform has
been found to have a much lower concentration of dopants (i.e., the
central region consists of essentially undoped silica glass). The drawn
fibers will then have the same characteristics. When this type of fiber is
exposed to a conventional buffered oxide etchant (BOE) to form a lens, the
undoped silica material in the extreme central portion of the core etches
at a much faster rate than the peripheral doped region of the core. This
differential etch rate results in the formation of the depression in the
central portion of the core (where the lens is formed) which in turn leads
to relatively low coupling efficiencies (typically less than about 50%).
Our investigation into this problem indicates that a generalized solution
includes adding a treating agent to the etchant in order to reduce the
etch rate of the central portion of the core relative to the peripheral
portion. We believe that the treating agent may bind preferentially to the
central portion, thus, to at least some extent, masking it from the
etchant and slowing down its etch rate. Suitable treating agents include
acids (e.g., acetic or citric) which, by themselves, do not significantly
etch the fiber. As mentioned previously, the presence of the treating
agent provides an additional degree of freedom which allows lenses to be
sculpted from a variety of fibers of different compositions.
EXAMPLE I
This example describes the formation of a lens on an end portion of an
optical fiber using a mixture of BOE and acetic acid.
The fiber itself was a standard, step-index, depressed cladding MCVD single
mode silica fiber having an 8 .mu.m core and a 125 .mu.m outside diameter
(OD). The core was doped with Ge such that the center core region had a
much lower dopant concentration than the peripheral core region. The
cladding included an inner cladding layer doped with P and F and an
essentially undoped outer cladding layer.
The end of the fiber was cleaved flat and dipped in a mixture containing
equal parts by volume of (3:1 BOE) and 99% acetic acid at 20.degree. C.
for 80 minutes. The BOE contained 1 part 49% HF to 3 parts 40% NH.sub.4 F
by volume. As a consequence, no depression or cusp was formed. Instead,
the etching process produced a unique lens 20 having a double conical
shape as shown by the sketch of FIG. 2. The lens 20 comprised a frustum 22
of a first cone having a cone angle .theta..sub.1, and, disposed on top of
the frustum, a second cone 24 having a cone angle .theta..sub.2
<.theta..sub.1. (e.g., .theta..sub.1 =50.degree. and .theta..sub.2
=20.degree. ). As shown, the base of the first cone is coextensive with
the top of the frustum, although this is not essential.
It is expected that other BOE compositions (e.g., 3:1 to 7:1) may produce
comparable results, although they might require different mixture
temperatures and/or treating agent concentrations.
Light at 1.3 .mu.m from a well-known covered mesa buried heterostructure
(CMBH) InP/InGaAsP laser emitting about 2600-2700 .mu.W of optical power
was coupled into the fiber via the lens. This lens exhibited an average
coupling efficiency of about 78%, with a standard deviation of less than
.+-.2%, without the need for any further lens shaping operations, such as
the fire polishing operation of the prior art. These aspects of the
invention are especially important in a manufacturing environment where
one goal is to provide a product of consistent quality and high yield.
More specifically, FIG. 3 is a normal probability plot of the coupling
efficiency .eta. for a statistically large number of lensed fibers formed
in accordance with this example. The data show that the lensed fibers of
the present invention have, in general, a high coupling efficiency
(average--78%) and tight statistical distribution. Subsequent experiments
repeating our prescription have yielded an even higher average coupling
efficiency of about 80% and a similar standard deviation.
EXAMPLE II
Using a fiber and etchant mixture as described in Example I, the etching
time at 20.degree. C. was varied from 70 to 85 minutes for a plurality of
fibers. For each lensed fiber a curve of the type shown in FIG. 4 was
plotted to determine the variation in coupling efficiency to a CMBH
semiconductor laser, as described above, as a function of the change in
axial distance .DELTA.z between the laser and the fiber lens. Although the
coupled power data of all of the lensed fibers was taken for etching times
of 70-85 minutes, suitable etching times range from 15 to 150 minutes,
depending on the specific etchant composition used.
The data showed that a maximum coupling efficiency of about 65% to 80%
occurred for .DELTA.z of about 4 to 8 .mu.m, depending on the particular
lensed fiber used in the measurements.
EXAMPLE III
Using a fiber as described in Example I and a BOE ratio of 3:1, the volume
percent of 99% acetic acid at 20.degree. C. was varied from 40% to 75%. We
found that approximately 40 to 55% acetic acid produced acceptable lenses,
but above about 55% the glass fiber began to exhibit a matte finish.
EXAMPLE IV
Using the fiber and etchant mixture as described in Example I, the
temperature was increased over the range of 20.degree. to 30.degree. C.,
whereas the corresponding etching time was decreased over the range of 150
to 15 minutes. Higher temperatures generally correspond to shorter times.
As shown in FIG. 5, the etch rate of the fiber OD varied from about 0.165
.mu.m/min. to 0.3 .mu.m/min. In addition, .eta..sub.max decreased with
increasing temperature from 75% at 20.degree. C. to 72% at 25.degree. C.
to 68% at 30.degree. C., yet .eta..sub.max occured at nearly the same OD
(e.g., 102 .mu.m.+-.1 .mu.m) despite the fact that the etch rate doubled
with only a 10.degree. C. change in temperature. This data demonstrates
the stability of our process; the fiber OD is a reliable predictor of a
desired lens shape or, equivalently, that maximum coupling efficiency has
been achieved.
EXAMPLE V
Using the fiber as described in Example I, citric acid was substituted for
acetic acid under the following conditions: a 100 ml solution was prepared
from 50 g of citric acid crystals and 100 ml water. This solution was
added to 3:1 BOE to make a mixture of 50% citric acid and 50% 3:1 BOE.
Experiments as described above at 20.degree. C. for 70 minutes
demonstrated that lensed fibers made this way had similar lens shapes and
a slightly higher average coupling efficiency (about 82%) with a
comparable standard deviation. The citric acid is so mild (low vapor
pressure above the liquid) that it has the added advantage of not
attacking the portion of the fiber or etching apparatus which is exposed
above the surface of the etching bath.
EXAMPLE VI
Following the procedures of Example V, similar results were obtained for
different amounts of citric acid and different etching times; e.g., 125 g
of citric acid and an etching time of 56 minutes, and 75 g of citric acid
and an etching time of 60 minutes. In general, citric acid as a treating
agent was found to be very attractive because 80% coupling efficiencies
were obtained with a variety of different types of fiber.
Another aspect of the invention relates to hermetic packaging in which a
metalized fiber is inserted through an opening in a metal package and
sealed thereto with solder. In particular, a matte finish is formed on the
outer surface of the fiber by exposing it to a mixture of 3:1 BOE and
acetic acid in which the concentration of acetic acid is relatively high
as described in Example III above and in our concurrently filed
application, supra. The textured fiber is inserted into a standard
electroplating bath in order to deposit metal layers such as Ni and Au. A
lens may then be formed on the end of the fiber as previously described.
The metal layers may be used, for example, to form a hermetic seal by
soldering.
One more aspect of the invention relates to end-point detection of the lens
etching procedure. Given that the lens itself has dimensions on the order
of the size of the core (e.g., <10 .mu.m for single mode fibers), it would
be extremely unreliable to measure the lens size directly--small errors in
measurement would be large percent errors relative to the small lens size.
Instead, because the outer surface of the fiber is being etched
simultaneously with the end face (FIG. 6), the fiber OD may be empirically
correlated to the desired lens shape. This correlation identifies a
predetermined OD corresponding to the desired lens shape. Thus, etching is
stopped when the predetermined OD is reached. Since the particular OD
(e.g., 100 .mu.m) is much larger than the lens (e.g., 8 .mu.m), this
procedure is much more tolerant to errors in measurement.
It is to be understood that the above-described arrangements are merely
illustrative of the many possible specific embodiments which can be
devised to represent application of the principles of the invention.
Numerous and varied other arrangements can be devised in accordance with
these principles by those skilled in the art without departing from the
spirit and scope of the invention. In particular, although the preceding
discussion emphasized lens formation on conventional single mode fiber, it
may also be applicable to multi-mode fiber, dispersion-shifted fiber, or
polarization-preserving fiber. In addition, it is well known in the art
that a trade off exists between coupling efficiency and alignment
tolerance; that is, a higher coupling efficiency generally requires
tighter alignment (smaller tolerance) and conversely. Tighter alignment,
of course, implies packages designed to provide a higher degree of
mechanical stability, especially where environmental conditions (e.g.,
temperature changes) are a problem. Because such packages may be more
difficult to manufacture reproducibly, it may be desirable, in some cases,
to sacrifice some degree of coupling efficiency in exchange for ease of
alignment and manufacture. The invention is particularly well suited to
such trade offs because the lens shape can be adjusted to give any
predetermined number within a relatively broad range of coupling
efficiencies (e.g., 65-80%) all of which are higher than the prior art
(e.g., 40-50%). Thus, the invention allows the designer to choose a
relatively high coupling efficiency yet design a package with acceptable
tolerances. This advantage is further enhanced by the fact that the
invention produces lensed fibers having tight statistical distributions
regardless of the particular coupling efficiency chosen.
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