U.S. patent application number 10/639690 was filed with the patent office on 2004-03-25 for attenuator.
This patent application is currently assigned to Telefonaktiebolaget LM Ericsson (publ). Invention is credited to Adeback, Tomas, Hersoug, Ellef.
Application Number | 20040057680 10/639690 |
Document ID | / |
Family ID | 20282978 |
Filed Date | 2004-03-25 |
United States Patent
Application |
20040057680 |
Kind Code |
A1 |
Hersoug, Ellef ; et
al. |
March 25, 2004 |
Attenuator
Abstract
In the manufacture of an optical attenuator having a desired
value of the optical loss end regions of two optical fibers are
placed with an offset in the traverse direction in relation to each
other and having their end surface at each other. Thereafter the
region at end surfaces is heated to make the ends melt to each
other and the heating is then further continued. To achieve the
desired loss in the finished attenuating splice the further heating
is stopped for an optical loss exceeding the desired loss by a
calculated value. This value can be obtained from measurements in
real time of the loss for the splice during the continued heating.
The measurements can be made at the beginning and end of an
interrupt of the further heating. An attenuator manufactured in
this way obtains an attenuation that accurately aggres with the
desired value.
Inventors: |
Hersoug, Ellef; (Stockholm,
SE) ; Adeback, Tomas; (Jarfalla, SE) |
Correspondence
Address: |
NIXON & VANDERHYE, PC
1100 N GLEBE ROAD
8TH FLOOR
ARLINGTON
VA
22201-4714
US
|
Assignee: |
Telefonaktiebolaget LM Ericsson
(publ)
|
Family ID: |
20282978 |
Appl. No.: |
10/639690 |
Filed: |
August 13, 2003 |
Current U.S.
Class: |
385/97 ;
385/140 |
Current CPC
Class: |
G02B 6/2551 20130101;
G02B 6/266 20130101 |
Class at
Publication: |
385/097 ;
385/140 |
International
Class: |
G02B 006/255 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 14, 2001 |
SE |
0100488-6 |
Feb 14, 2002 |
WO |
PCT/SE02/00264 |
Claims
1. A method of manufacturing an optical attenuator from optical
fibers, end regions of two optical fibers being placed with a
lateral or transverse offset and having their end surfaces located
at each other, the region at the end surfaces being heated to make
the ends melt to each other and the heating thereafter being
continued until substantially a desired optical loss is obtained in
the melted region, whereafter finally the melted region is allowed
to cool, characterized in that the heating is interrupted for an
optical loss exceeding the desired loss by a value calculated from
measurements of the loss for this splice or for a splice between
identical fibers having the same initial offset.
2. A method according to claim 1, characterized in that the
measurements are made by temporarily interrupting the continued
heating during at least one time period before the heating is
finally stopped and by measuring the loss at this at least one time
period.
3. A method according to claim 2, characterized in that the
measurements are made at at least two interrupts of the continued
heating.
4. A method according to claim 2, characterized in that the
measurements are made by measuring the loss at the beginning of and
at the end of the at least one interrupt.
5. A method according to claim 1, characterized in that the result
of the measurements is used to determine at least one parameter or
constant characterizing an individual function in a group of
functions.
6. A method according to claim 5, characterized in that the group
of functions includes linear function characterized by two
constants.
7. A device for manufacturing an optical attenuator having a
desired optical loss from optical fibers comprising retainer and
alignment means for retaining and moving two end region of optical
fibers, heating means for heating the region at the end surfaces of
the fibers in the end regions, loss measuring means for measuring
optical loss for light propagating from one of the end regions to
the other one, and control means connected to the retainer and
alignment means, the heating means and the loss measuring means
arranged to first control the retainer and alignment means to place
the end regions with a lateral or transverse offset and with the
end surfaces thereof at each other, to thereafter control the
heating means to bring regions of the fibers at the end surfaces to
melt to each other and to thereafter continue the heating, to
receive during the continued heating measured values of the optical
loss from the loss measuring means and to control the heating means
to stop the continued heating depending on the measured values of
the optical loss, characterized in that the control means are
arranged to control the heating means to stop the continued heating
when the optical loss measured by the loss measuring means exceeds
the desired loss by a value calculated from previous measurements
of the optical loss for this splice or for a splice between
identical fibers having the same initial offset.
8. A device according to claim 7, characterized in that the control
means are arranged to control the heating means to temporarily
interrupt the continued heating during at least one time period
before the continued heating is finally stopped.
9. A device according to claim 8, characterized in that the control
means are arranged to temporarily interrupt the continued heating
during at least two different time periods.
10. A device according to claim 8, characterized in that the
control means are arranged to use as the previous measurements
values of the optical loss at the beginning of and at the end of
the at least one time period.
11. A device according to claim 7, characterized in that the
control means include calculating means to which the measured
values are provided and which are arranged to use the values to
determine at least one parameter or constant characterizing an
individual function in a group of functions.
12. A device according to claim 11, characterized in that the
calculating means are arranged to use as the group of functions
linear functions characterized by two constants.
Description
TECHNICAL FIELD
[0001] The present invention relates to a method of manufacturing
attenuating elements from optical fibers and a device for
manufacturing such attenuating elements.
BACKGROUND OF THE INVENTION
[0002] Optical attenuating elements can be manufactured by welding
two optical fibers to each other with a lateral offset of the
fibers, i.e. a splice having an intentionally produced lacking
alignment of the cores of the fibers is manufactured and thus
having a large loss. Then a welding device of the automatic type
having a modified control program can be used. The control of the
welding process can be performed in real time. The electronic
processor of the welding device can for example in real time get
information from an power meter measuring the power of light coming
from a light source and propagating through the splice during the
welding process, and use the information to control the electric
arc. The method comprises that first a desired lost is selected.
Then a splice having an offset is made. During the heating in the
splicing process a current loss is all the time read. The molten
glass material in the fibers has a surface tension reducing the
offset and the loss gradually falls during the heating. When the
loss has decreased to the desired loss the electric arc, is stopped
and thereby the heating.
[0003] This method is for example described in the published
International Patent Application No. WO 95/24665 corresponding to
U.S. Pat. No. 5,638,476, in U.S. Pat. No. 5,897,803 and in the
published European Patent Application No. 0594996.
[0004] It appears that several problems exist in this method. The
main problem is however that the splice loss in the resulting
splice does not become correct when using the method. It is thus a
basic problem that the loss determined in the splice during the
welding process according to the method differs from the loss that
is measured directly after finishing the welding process. Most
often the loss is lower after the end. The difference is about
0.5-2 dB for losses of about 3-15 dB for a reference point of about
200 .mu.W, i.e. an input light power of approximately this
value.
[0005] This effect could be explained by the fact that more light
hits the detector which has a broad spectral responsiveness due to
the fact that the fiber glows or that light from the electric arc
is transmitted in the fiber. However, from tests when the light
source is in-activated it has been possible to find that the light
emitted by the fibers and the electric arc contributes very little.
The power is in the magnitude of order of nW which corresponds to a
very small part of a measured difference of 0.5-2 dB in the case
where the reference point is about 200 .mu.W.
[0006] The explanation of the difference is more probably
associated with the fact that the optical character of the splice
is changed due to the large heat differences that exist. E.g., the
refractive index could be changed, this resulting in changed
conditions for total reflection or in chances of the mode field
diameters on which the loss depends. The steps could also be
thought of as being caused by a difference in lateral offset
between the fibers depending on whether the splice is hot or
cold.
SUMMARY OF THE INVENTION
[0007] It is an object of the invention to provide a method for
manufacturing optical attenuating elements having an optical
attenuation or loss closely agreeing with a predetermined or
desired value.
[0008] It is another object of the invention to provide a device
for welding two optical fibers to each other for manufacturing an
optical attenuating element having an attenuation closely agreeing
with a desired value.
[0009] Generally thus, an optical attenuator is manufactured from
optical fibers. In the conventional way end regions of two optical
fibers are placed to have an offset in the transverse direction in
relation to each other and having their end surfaces located at
each other. Thereafter the region at the end surfaces is heated to
bring the ends to melt to each other and the heating is then
further continued. The heating is stopped and finally the melted
and heated region is allowed to cool. To achieve a desired value of
the loss in the finished attenuating splice the further heating is
stopped for an optical loss exceeding by a calculated value the
desired loss. This value is obtained from measurements of the loss
for this splice made in real time during the continued heating or
made for a previously made splice between identical fibers having
the same initial offset. In particular, at least one and preferably
two temporary interrupts can be made during and of the further
heating and the loss be measured at the start of and at the end of
such an interrupt. These loss values are used in the calculation of
the value of the loss when the heating will be definitely
stopped.
[0010] A value of the loss is thus determined which the splice or
the welding will obtain in the continued heating and for which the
heating will be totally stopped. The heating is stopped at a time
somewhat before achieving the desired loss in the hot splice. When
the splice then is allowed to cool the manufacturing procedure of
the attenuator is finished and then the splice obtains an optical
loss closely agreeing with the desired one.
[0011] The advantage of manufacturing attenuators using this type
of real time control of the welding arc is among other things that
a model requiring knowledge of e.g. the lateral offset and the mode
field diameters of the fibers does not have to be used since
information of the loss is directly available.
[0012] Additional objects and advantages of the invention will be
set forth in the description which follows, and in part will be
obvious from the description, or may be learned by practice of the
invention. The objects and advantages of the invention may be
realized and obtained by means of the methods, processes,
instrumentalities and combinations particularly pointed out in the
appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] While the novel features of the invention are set forth with
particularly in the appended claims, a complete understanding of
the invention, both as to organization and content, and of the
above and other features thereof may be gained from and the
invention will be better appreciated from a consideration of the
following detailed description of non-limiting embodiments
presented hereinbelow with reference to the accompanying drawings,
in which:
[0014] FIG. 1 is a schematic picture of an automatic device for
welding optical fibers to each other,
[0015] FIG. 2 is a diagram of loss as a function of time when
manufacturing an optical attenuator according to prior art for a
welded splice having an initial offset and prolonged heating,
and
[0016] FIG. 3 is a diagram similar to FIG. 2 when manufacturing an
optical attenuator using basically the same method as in FIG. 3 but
including two interrupts during the prolonged heating.
PREFERRED EMBODIMENT
[0017] In FIG. 1 a schematic picture of a fiber welding device 1
for welding two fibers 3.sub.l, 3.sub.r to each other with a
simultaneous measurement of the transmission through the weld is
shown. The right fiber 3.sub.r is at its remote end connected to a
light source 5 issuing light into the fiber. The left fiber 3.sub.l
is at its remote end connected to a light detector in the shape of
a power meter 7.
[0018] The fibers 3.sub.l, 3.sub.r have their end regions located
between the points of electrodes 9 between which an electric
discharge is produced for heating the ends of the fibers, the
intensity of the electric discharge being determined by the
electric current between the electrodes 9. The fibers are retained
by retainers 11 which are movable in three orthogonal coordinate
directions, both in a direction parallel to the longitudinal
direction of the fibers and in two directions perpendicular to this
direction. The retainers 11 are thus operated to move along
suitable mechanical guides, not shown, by control motors 13.
Electrical cables to the light source 5, the electrodes 9 and the
motors 13 extend from an electronic circuit module 15 and from
driver circuits 16, 17 and 19 therein. The power meter 7 is
connected to a measurement interface 20 in the circuit module 15. A
video camera 21 can continuously take pictures of the welding
position, i.e. the region where the fibers 3.sub.l and 3.sub.r meet
each other. It is through an electrical line connected to a video
interface 23 in the electronic circuit module 15, from which a
suitable image signal is provided to an image processing and image
analysing unit 25. Pictures of the welding position which
advantageously include pictures simultaneously taken in two
direction perpendicular to each other can be displayed on a display
26 connected to the unit 25.
[0019] The different steps in the heating and welding process are
controlled by a control circuit 27, e.g. in the shape of a suitable
microprocessor or computer or a combination of processor and
computer that are also connected to the image processing and image
analysing unit 25. The control circuit 27 provides signals for
performing the different steps the welding process and is connected
to the electrodes, the motors and the camera through respective
drive circuits/interfaces. It thus controls the movement of the
fiber ends in relation to each other by activating the motors 13 in
suitable displacement directions and provides signals to the image
processing and image analysing unit 25 to start an analysis of
taken pictures. Furthermore the control circuit 25 controls the
time when a heating or welding is to start, by providing the
electrodes 9 with a suitable electric voltage, and controls the
time period during which this voltage is to be applied. The control
circuit also gives a signal to the light source to activate it to
emit light into the fiber 31. It receives information of measured
power values from the power meter 7.
[0020] By arranging the closely located end regions of the fibers
3.sub.l, 3.sub.r held by the retainers 11 with a predetermined
initial offset between their longitudinal axes or between the cores
of the fibers and thereafter perform a controlled welding with a
following prolonged heating a fiber-optical attenuator can be
manufactured, compare the Patent Application No. WO 95/24665
corresponding to U.S. Pat. No. 5,638,476, U.S. Pat. No. 5,897,803
and the European Patent Application No. 0594996 cited above.
[0021] The values obtained from the power meter 7 of the received
light power can be easily recalculated to an optical loss in the
splice between the fibers 3.sub.l, 3.sub.r provided that the light
power injected from the light source 5 in the fiber 3.sub.l is
known. During all of the following prolonged heating process after
the very welding step the optical loss can thus be determined. In
the diagram of FIG. 2 thus the measured loss in a splice having an
initial offset between cores/claddings as a function of time during
a prolonged welding period with constant electrical current in the
electric arc is shown. The light arc between the electrodes 9 has
been shut off when the value read by the power meter 7 for the
first time becomes lower than 22.5 dB. In the diagram is clearly
seen how the loss is after the shutting off rapidly decreased by
about 2 dB.
[0022] In tests several interrupts have been made when heating a
splice having an initial offset with the same current intensity in
the electric arc, see the diagram of FIG. 3 that shows basically
the same heating procedure as the diagram of FIG. 2 but with two
extra interrupts of the electric arc. The same current intensity
has been used during all the periods when the electric arc is
activated after the initial welding of the fiber ends to each
other. It appears that the "hops" or "steps" in the graph depends
on the present optical loss in the splice, i.e. the loss existing
exactly when the electric arc is stopped.
[0023] The value of the offset during the prolonged heating
decreases exponentially with time provided that viscosity, surface
tension and fiber diameters are constant, see the patent
applications/patents cited above and references to other documents
given therein. This is probably even more true if the temperature
or the current also is constant. According to the butt-joint theory
which is a good model if the lateral offset is large, the loss in
dB is a quadratic function of the offset and then also the loss
should decrease exponentially with time. The magnitude of the steps
could therefore also be exponentially decreasing with the heating
time. However, the conditions during the heating time depends on
the welding current used, the state of the electrodes, etc. and are
often not very repeatable.
[0024] Therefore it is better to consider the instantaneous loss in
the splice during the prolonged heating process and to assume that
the magnitude of the steps is a function of this loss or
equivalently of the attenuation or transmission of the splice. It
appears that in many cases a linear model that presupposes that the
magnitude of the steps is linearly dependent on, such as
proportional to, the instantaneous loss in the splice, can be used
with a good accuracy. Such a model could in principle possibly be
considered as equivalent with an exponential dependence on
time.
[0025] The linear model is generally given by the formula, compare
FIG. 3:
.DELTA.L=kL+m (1)
[0026] where .DELTA.L is the magnitude of the step or hop, L is the
loss at the start of the step and k and m are constants. They can
be determined from experimentally determined measured values. For
the determination measurements for a number of interrupts equal to
the number of constants or parameters in the model, i.e. in this
case two interrupts, are required. For the case shown in FIG. 3 the
magnitude of the step .DELTA.L1 for the loss L1 and of the step
.DELTA.L2 for the loss L2 can be measured from which values of k
and l are calculated.
[0027] In the linear model according to the discussion above two
constants, k and l, are used which need to be determined. However,
if either one of the constants k and l can be assumed to have a
value known in advance, only a determination of the other constant
is required. A determination of only one constant requires only a
measurement of the loss at a single interrupt. Also other models
can be conceived that use a suitably selected group of functions
from which a specially selected function is selected by
measurements at one or more interrupts in real time. Such a group
of functions could comprise suitably selected exponential
functions.
[0028] The value L* of the loss measured in real time for which the
electric arc is to be stopped in order that the final result will
be the desired loss L.sub.des can for the linear model according to
the description above be calculated from:
L*+.DELTA.L*=L.sub.des (2)
[0029] where .DELTA.L* is the magnitude of the step obtained when
the heating is interrupted for the loss L*. From (1) and (2) is
obtained
L*=(L.sub.des-m)/(l+k)
[0030] The small circles in the diagram of FIG. 3 represent the
times at which the electric arc has been shut off and has been
started respectively. The time during which the electric arc is
shut off should have a length of 1.5 till 3.0 s in order that the
splice loss will have time to adopt a stable value.
[0031] Summarizing thus, by in the same way as in determining the
diagram of FIG. 3 interrupting the electric arc twice before
achieving the desired loss, the constants k and m in the linear
model can be determined and therefrom L*. It can be made in real
time to manufacture an attenuator having a desired attenuation so
that when L* is achieved the electric arc is finally stopped.
[0032] A plurality of tests has been made and the set values and
the obtained losses appear from Table 1. Here current2 is the value
of the current intensity that is used during the welding operation
and that is also used during the prolonged heating for obtaining
the desired loss in the splice in several cases, this being
indicated by the fact that current3 equals zero. In other cases a
lower current intensity is used after the very splicing operation
during the prolonged heating for obtaining the desired loss in the
splice, this current velocity being indicated by current3 when this
quantity is different from zero. The initial offset can be set so
that it gives approximately twice the loss compared to the desired
one, i.e. approximately equal to 2.multidot.L.sub.des. Table 1
demonstrates that in many cases finished attenuators are obtained
having attenuation values very close to the desired values.
[0033] In the method performed in real time the following steps are
executed:
[0034] 1. Place the end surfaces of the fibers quite at each other
having the longitudinal directions of the fibers parallel to each
other.
[0035] 2. Align the fibers in the transverse direction with a
lateral or transverse offset which, if the fibers were welded to
each other for this offset, would give a loss that is much larger
than the desired one, for example substantially equal to twice the
desired one, i.e. 2.multidot.L.sub.des.
[0036] 3. Bring in the longitudinal direction the end surfaces of
the fibers against each other using some so called overlap, i.e. so
that the fiber ends are somewhat pressed against each other.
[0037] 4. Start the electric arc using a large welding current and
finish the welding in a short period of time.
[0038] 5. Reduce, if desired, the current intensity through the
electric arc to a lower constant value and measure all the time the
loss in the splice.
[0039] 6. Stop and start the electric arc at least once and
preferably twice. Each interrupt must have a sufficient length to
allow that the attenuation at the end of the interrupt will have
reached a constant value, i.e. so that the welding position has had
time to sufficiently cool. Record the values of the loss at
beginning and end of each interrupt. A first interrupt can be made
when the loss for example has decreased to a value exceeding the
desired loss by about 70-80% such as about 70%, i.e. approximately
for the loss of 1.7.multidot.L.sub.des. From the measurements of
loss directly before and after this interrupt .DELTA.L.sub.1 is
calculated. If a further interrupt will be made, it can be made
when the loss in the splice is approximately measured to be twice
this value, ie. for the loss of L.sub.des+2.DELTA.L.sub.1. From the
measured values of loss the loss L* is calculated for which the
continued heating obtained by the electric arc will be stopped.
[0040] 7. Stop the electric arc when the loss L* is obtained.
[0041] 8. Allow the fibers welded to each other to cool.
[0042] To control these steps the processor 27 contains various
modules. A module 31 handles positioning the fiber ends and
therefor receives information from the unit 25 and produces signals
to be transferred to the setting motors 13. Another module 33
controls the current through the electrodes and includes submodules
35-39 for determining electrical current for welding, of current
for the continued heating and of times for interrupts during the
continued heating respectively. A third module 41 calculated the
present loss in the splice departing from the signal from the power
meter 7. A fourth module 43 uses the calculated loss values and
includes submodules 45-49 in which at least some of the calculated
loss values are stored, the parameters k and l are calculated and
the stop value L* of the loss is calculated.
[0043] If the initial alignment of the fibers is accurately
determined, the determination of L* can be made for a first fiber
splice and attenuating element after which the same value of L* is
used for a series of attenuators manufactured from fibers of the
same type having the same initial offset. However, the same good
accuracy of the loss of the manufactured attenuating elements
cannot always be obtained because the heating conditions in
splicing operations are not repeatable. In the preferred method
including real time measurements and real time control these
conditions have no influence since a determination of L* is made
for each splice dependent on measurements during the prolonged
heating period.
[0044] While specific embodiments of the invention have been
illustrated and described herein, it is realized that numerous
additional advantages, modifications and changes will readily occur
to those skilled in the art. Therefore, the invention in its
broader aspects is not limited to the specific details,
representative devices and illustrated examples shown and described
herein. Accordingly, various modifications may be made without
departing from the spirit or scope of the general inventive concept
as defined by the appended claims and their equivalents. It is
therefore to be understood that the appended claims are intended to
cover all such modifications and changes as fall within a true
spirit and scope of the invention.
* * * * *