BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to an optical fiber, an optical fiber preform (i.e.,
base material), and a manufacturing method therefor, and in particular, relates to an
optical fiber whose refractive index distribution is controlled so as to perform
high-quality and high-speed transmission, a relevant optical fiber preform, and
manufacturing methods therefor.
Description of the Related Art
In the field of optical communication, optical fibers having a refractive index
distribution having a rectangular or triangular shape are mainly designed. Examples of
design values for such a refractive index distribution are shown in the graphs of Figs. 5
and 6. In each graph, the horizontal axis indicates the position in the radial direction of
the optical fiber, where the outer diameter of the cladding is defined as 100%, and the
position as percentage from the center (0%) to the outer periphery (50%) of the cladding
is shown (i.e., the left half (0 to -50%) is omitted in the graph). On the other hand, the
vertical axis shows the relative refractive index with respect t3 the refractive index of the
cladding.
In these examples, values for the refractive index distribution are symmetrical
with respect to the center of the core of the optical fiber. Therefore, in Figs. 5 and 6,
the left uid of each graph corresponds :o the center of the core, and the right end of the
graph corresponds to the periphery of the cladding. Accordingly, Fig. 5 shows a
rectangular-shaped refractive index distribution of an optical fiber, in which the
refractive index is constant in the core, and Fig. 6 shows a triangular-shaped refractive
index distribution of an optical fiber, in which the refractive index is maximized at the
center of the core and then decreases at a fixed rate in the core. The optical fibers
having the above-explained refractive index distributions are generally used because the
optical characteristics of these optical fibers can be easily estimated based on their
refractive index distributions.
That is, the optical fiber has a core having a highet refractive index and a
cladding having a lower refractive index. In order to obtai.i such a refractive index
distribution, the main component of the core and cladding of the optical fiber is highly
pure silica glass (SiO2), and a part or all of the entire material is doped with a dopant for
increasing the refractive index or a dopant for decreasing the refractive index.
The dopant for increasing the refractive index may be GeO2, TiO2, SnO2, ZrO2,
Nb2O5, Ta2O5, Yb2O3, La2O3, AI2O3. or the like. The dopam for decreasing the
refractive index may be B2O3, F, or the like. Additionally, ir> order to improve the
optical characteristics of glass (i.e., Si02) such as the softening point, the coefficient of
thermal expansion, the chemical resistance, the transition point, and the dispersion loss,
Si():l is doped with a dopant as explained above or P2O5, and further doped with another
known dopant if necessary. As a more concrete example relating to the composition of
the optical fiber, the core is made of Si02 which is doped with Ge02 and the cladding is
made of SiOi.
However, generally, the refractive index distribution of the actually
manufactured optical fiber has values different from the design values as shown in Fig. 5
in particular, when the optical fiber is manufactureu using a VAD (vapor-phase
axial deposition) method, the portion corresponding to the core is produced in a single
process; thus, the probability of producing an optical fiber whose refractive index
distribution has values different from the design values is high.
In addition, in the conventional process (including a heating process) of :
manufacturing the optical fiber, the concentration of the dopant for controlling the
refractive index, which should be added only to the core area, is inevitably irregular, and
such an additive (i.e., dopant) is also inevitably diffused towards the cladding area.
Therefore, irregular concentration of the additive tends to be produced in the
vicinity of the boundary between the core and the claddirg. Such irregular
concentration of the additive produces a portion where the refractive index steeply
changes (called the "refractive-index steep change portion" hereinbelow).
In conventional design of the refractive index profile, the presence of such a
refractive-index steep change portion has not been considered. However, in the
actually manufactured optical fiber, such a steep change in the refractive index affects
the optical characteristics of the fiber, in particular, the wavelength dispersion, so that
the wavelength dispersion has a value different from that anticipated in design. Such
an error in the wavelength dispersion causes a waveform distortion in the optical
transmission, thereby affecting the high-quality and high-speed transmission.
On the other hand, when the optical fiber is manufactured by the VAD method,
(i) the refractive index distribution of a manufactured ootical fiber preform is measured
and the amount of drawing of the optical fiber preform is determined based on the
measurement results, (ii) the refractive index distribution of the optical fiber preform
after the drawing is again measured and the amount of the outside deposition is
determined based on the measurement results, (iii) the refractive index distribution of the
optical fiber preform after the outside deposition is agt.in measured for confirmation, and
(iv) drawing of the optical fiber preform as produced above is performed so as to
produce an optical fiber.
Here, if the optical fiber preform includes a refractive-index steep change
portion as explained above, the above measurement of the refractive index distribution
cannot be accurately performed, so that it is difficult to produce an optical fiber having
target characteristics.
SUMMARY OF THE INVENTION
In consideration of the above circumstances, an object of the present invention
is to provide (i) an optical fiber having optical characteristics, such as the wavelength
dispersion, close to design values by controlling the amount of change in the refractive
index in the core, thereby realizing high-quality and high-speed transmission, (ii) a
relevant optical fiber preform, and (iii) manufacturing methods therefor.
Therefore, the present invention provides a method of manufacturing an optical
fiber preform having a core and a cladding, comprising the step of:
controlling the refractive index distribution in a manner such that at each
position in the area in which the relative refractive index of the core with respect to the
cladding is 80% or higher of the maximum value of the relative refractive index, the
absolute value of the rate of change of the relative refractive index with respect to the
position along the diameter of the cladding is 0.5 or less, where the position along the
diameter of the cladding is defined by percentage with respect to the diameter.
As a typical example, the step of controlling the refractive index distribution is
performed when a porous glass preform as a precursor of the optical fiber preform is
produced, and the step of controlling the refractive index distribution includes (i)
controlling the relative position of a burner for jetting a material for the core to a target
on which the material is deposited, or (ii) controlling the angle formed by a burner for
jetting a material for the core and a target on which the material is deposited.
The present invention also provides an optical fiber preform manufactured by a
method as explained above.
The present invention also provides a method of manufacturing an optical fiber,
comprising the step of:
drawing an optical fiber preform manufactured as explained above, so as to
produce the optical fiber in which at each position in the area in which the relative
refractive index of the core with respect to the cladding is 80% or higher of the
maximum value of the relative refractive index, the absolute value of the rate of change
of the relative refractive index with respect to the position along the diameter of the
cladding is 0.5 or less, where the position along the diameter of the cladding is defined
by percentage with respect to the diameter.
The present invention also provides an optical fiber manufactured by a method
as explained above.
According to the present invention, it is possible to manufacture an optical fiber
preform and an optical fiber in which at each position in the area in which the relative
refractive index of the core with respect to the cladding is 80% or higher of the
maximum value of the relative refractive index, the absolute value of the rate of change
of the relative refractive index with respect to the position along the diameter of the
cladding is 0.5 or less. Therefore, the optical characteristics, such as the wavelength
dispersion, can have values close to the design values, thereby realizing the high-quality
and high-speed transmission using optical fibers.
The present invention provides a method of manufacturing an optical
fiber perform having a core and a cladding , comprising the step of:
controlling the refractive index distribution in a manner such that at
each position in the area in which the relative refractive index of the core with
respect to the cladding is 80% or higher of the maximum value of relative
refractive index , the absolute value of the rate of change of relative refractive
index with respect to the position along the diameter of the cladding is 0.5 or
less, where such position along the diameter of the cladding is defined by
percentage with respect to the outer diameter of the cladding (100%),
wherein said step for controlling the refractive index is performed at
the time of production of a porous glass perform as a precursor of the optical
fiber perform and comprises controlling the relative position of a burner used
for jetting the material for the core to a target on which the material is
deposited.
The invention also provides a method for manufacturing an optical
fiber comprising the step of drawing the optical fiber perform as claimed in
claim 3, such that an absolute value for a ratio which indicates how a relative
refractive index varies should be 0.5 or less in each position in the area where
the relative refractive index of the core with respect to the cladding is 80 % or
higher with respect to the maximum value of the relative refractive index in
the core with respect to the cladding.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
fig. 1 is a diagram showing an example of the refractive index distribution
relating to the present invention.
Figs. 2 A and 2B show an example of variation in the relative refractive index of
an optica] fiber.
Figs- 3 A and 3B show another example of variation in the relative refractive
index of an optical fiber.
Fig. 4A is a graph showing the wavelength dispersion at 1300 nm, with respect
to the gradient of the relative refractive index of the refractive-index steep change
portion which is closest to the cladding in the area in which the relative refractive index
is 80% or higher of the maximum relative refractive index. Fig. 4B is a graph showing
difference between estimated wavelength dispersion at 1300 nm of the optical fiber
preform and the wavelength dispersion measured for an optical fiber which is made of
the optical fiber preform, with respect to the gradient of the relative refractive index of
the refractive-index steep change portion which is closest to the cladding in the area in
which the relative refractive index is 80% or higher of the maximum relative refractive
index.
Fig. 5 shows an example of design values of the refractive index distribution of
an optical fiber.
Fig. 6 shows another example of design values of the refractive index
distribution of an optical fiber.
Fig. 7 is a diagram showing an example of the system for producing a porous
glass preform which is used for manufacturing an optical fiber according to the present
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Hereinafter, embodiments according to the present invention will be explained
with reference to the drawings.
Fig. 1 shows an example of the refractive index distribution relating to the
present invention. In Fig. 1, reference numeral 1 indicates a core portion and reference
numeral 2 indicates a cladding portion. The peak of the refractive index at the center of
the core is indicated by the reference symbol la, and a peak of the refractive index
produced at die boundary between the core and the cladding is indicated by the reference
symbol 1 b. The optical fiber of this example is manufactured in a manner such that the
rate of change of the refractive index in the core is within a predetermined range.
More specifically, the variation in the refractive index in the core is represented
by the variation in the relative refractive index with respect to each position along the
diameter of the cladding, and at each position in the area whose relative refractive index
is 80% or higher of the maximum value of the relative refractive index (i.e., the
maximum relative refractive index), the absolute value of the rate of change of the i
relative refractive index with respect to the diameter of the cladding is 0.5 or less. ,'
Here, the rate of change of the relative refractive index (%) corresponds to the
amount of change of the relative refractive index per 1% of (he diameter (100%) of the
cladding, in the radial direction of the fiber. The reason for defining the rate of change
of the refractive index in the area in which the relative refractive index is 80% or higher
ol the maximum relative refractive index, is that in most cases, the above-explained
refractive-index steep change portion is produced in such an area.
In order to measure the refractive index distribution of the optical fiber, a
measurement system such as "Refractive Index Profiler SI4" produced by York
technology Association, or an optical fiber analyzer "NR-9200" produced by EXrO
was used.
Figs. 2 A and 2B show the characteristics of an optical fiber whose refractive
index distribution includes no refractive-index steep change portion.
In Fig. 2A, the horizontal axis indicates the position along the diameter of the
cladding, indicated by percentage from the center (0%) to the outer periphery (50%) of
the cladding, and the vertical axis indicates the relative refractive index (%) with respect
to the refractive index of the cladding. In this graph, reference symbol lb indicates a
peak of the refractive index between the core and the cladding, the curve indicated by
reference symbol A shows the relative refractive index with respect to the cladding (as
the standard), and reference symbol B shows the 80% line with respect to the maximum
value of the relative refractive index.
Fig. 2B shows an enlarged view of Fig. 2A; that is, it shows the optical
characteristics of the core portion (the left end also corresponds to the center of core).
In Fig. 2B, curve A and line B are the same as those in Fig. 2A, and curve C indicates
differential values of first order of the relative refractive index (indicated by line A)
calculated with respect to the position along the diameter, that is, curve C indicates the
gradient of the relative refractive index, which numerically indicates the degree of
change in the refractive index.
In Fig. 2B, the change of the peak lb of the refractive index is moderate, and
the refractive index distribution shows a small variation. Therefore, the differential
\ alues of first order are generally small. As a result, the above-explained condition is
satisfied; that is, at each position in the area in which the relative refractive index is 80%
or higher of the maximum relative refractive index of the optical fiber, the absolute
\alue of the rate of change of the relative refractive index with respect to the position
("o> along the diameter (100%) of me cladding is 0.5 or less.
A comparative example 'vuh respect to the refractive index distribution
according to the present invention is shown in Figs. 3A and 3B, which show a variation
in the refractive index of an optical fiber which includes a refractive-index steep change
portion.
In Figs.. 3A and 3B, the horizontal and vertical axes and each reference symbol
respectively correspond to those of Figs. 2A and 2B. As shown in Fig. 3B, the change
of the peak lb of the refractive index is steep; thus, in this refractive-index steep change
portion, the absolute value of the differential value of first order exceeds 1.0, which is
larger in comparison with the case shown in Fig. 2B.
The reason for determining the absolute value of the gradient of the relative
refractive index as explained above will be explained below.
Fig. 4A shows the optical characteristics of the refractive-index steep change
portion which is closest to the cladding in the area in which ihe relative refractive index
is 80% or higher of the maximum relative refractive index. The horizontal axis shows
the gradient of the relative refractive index, and the vertical axis indicates the
wavelength dispersion at 1300 nm. In Fig. 4A, the design value for the wavelength
dispersion is 1.33 ps/(nm-km). When the wavelength dispersion in the horizontal axis
closes to 1.33 ps/(nrrrkm), it means that an optical fiber having a wavelength dispersion
corresponding to the design value can be realized. However, as the wavelength
dispersion decreases from this design value, an optical fiber having a larger wavelength
dispersion error with respect to the design value is inevitably produced.
In Fig. 4A, in the vicinity of the gradient "0", no refractive-index steep change
portion is present and the refractive index does not change, which is an ideal state. If
an optical fiber having a refractive index distribution corresponding to the design value
can be manufactured, the optical fiber has wavelength dispersion corresponding to the
design value In ooier to realize an optical fiber for performing high-quality and
high-speed transmission, the wavelength dispersion should be approximately 95% or
higher of the design value. According to Fig. 4A, in the area in which the relative
refractive index is 80% or higher of the maximum relative refractive index, it is
preferable that the absolute value of the rate of change, that is, the gradient of the
relative refractive index, be 0.5 or less.
In the present embodiment, the optical fiber is manafactured in a manner such
that in the area where each point has a relative refractive index (of the core 1 with
respect to the cladding 2) 80% or higher of the maximum re ative refractive index, the
absolute value of the rate of change of the relative refractive index with respect to the
position (%) along the diameter (100%) of the cladding is 0.5 or less. Accordingly, it is
possible to realize an optical fiber having the optical characteristics, such as the
wavelength dispersion, close to the design values, thereby realizing the high-quality and
high-speed transmission.
The optical fiber preform as an embodiment of the present invention will be
explained below.
In Fig. 4B, the vertical axis indicates difference between (i) estimated
wavelength dispersion at 1.300 nm of the optical fiber preform and (ii) the wavelength
dispersion measured for an optical fiber which is made of the optical fiber preform, and
the horizontal axis shows the gradient of the relative refractive index of the
refractive-index steep change portion which is closest to the cladding in the area in
which the relative refractive index is 80% or higher of the maximum relative refractive
index.
The rate of change of the relative refractive index corresponds to the amount of
change of the relative refractive index per 1% of the diameter (100%) of the cladding, in
the r uiial direction of the optical filxr preform. Here, the refractive index distribution
of the optical fiber preform was measured at a measurement pitch of 40 um or less.
In Fig. 4B, in the vicinity of the gradient "0", no refractive-index steep change
portion is present and the refractive index does not change, which is an ideal state. If
an optical fiber preform having a refractive index distribution corresponding to the
design value can be manufactured, an optical fiber having wavelength dispersion
corresponding to the design value can be manufactured by drawing the above optical
fiber preform by using a known drawing system. In order to realize an optical fiber for
performing high-quality and high-speed transmission, the wavelength dispersion should
be approximately 95% or higher of the design value. According to Fig. 4B, in the area
in which the relative refractive index is 80% or higher of the maximum relative
refractive index, it is preferable that the absolute value of the rate of change, that is, the
gradient of the relative refractive index be 0.5 or less.
Below, an embodiment for manufacturing the above-explained optical fiber
preform and optical fiber will be explained with reference to Fig. 7.
In Fig. 7, reference numeral 21 indicates a reaction container, reference numeral
22 indicates an exhaust pipe of the reaction container 21, reference numeral 23 indicates
a target having a rod shape, reference numeral 24 indicates a rotary lifting mechanism
for the target 23, reference numerals 25 and 26 indicate burners for synthesizing glass
particles, reference numerals 27 and 28 indicate glass material supply systems which
respectively have mass flow controllers 29 and 30.
As is known, the rotary lifting mechanism 24, positioned above the reaction
container 2 1, is provided so as to insert and pull out the target 23, which is vertically
held by the mechanism 24, into and from the reaction container 21.
The burners 25 and 26 have a known structure in which a plurality of gas
supply passages are arranged in a concentric-circle form. The gas supply passages m-supply a gas as main material (SiCl4), a gas as an additional material (i.e., a gas as
material for doping, such as GeCI4). a fuel gas (H2), an auxiliary fuel gas (02), a buffer
gas (At), or the like. In addition to the mass flow controllers 29 and 30, each of the
glass material supply systems 27 and 28 includes a liquefied material tank, a carrier gas
supply tank, a bubbling vessel for producing a material gas, and the like.
The above-explained burners 25 and 26 are attached to the reaction container 21
through the wail surface of the container, between a side face and the lowest portion of
the container, as shown in Fig. 7. The heads of the burners 25 and 26 are directed
towards the lower end of the target 23 which is lowered by the rotary lifting mechanism
2-4.
Fig. 7 shows the system for performing the VAD method so as to produce a
porous glass preform 10 which is a precursor of an optical fiber preform, where the
porous glass preform 10 includes a porous glass layer 11 for the core, and a porous glass
layer 12 for the cladding. The method of producing the porous glass preform 10 will
be explained below.
First, the target 23 is lowered and inserted into the reaction container 21 while
being rotated in a single rotation direction by the rotary lifting mechanism 24. The
above-explained burners 25 and 26 are respectively assigned to the core and the cladding,
and SiCLt, GeCLj, H2, O2, and Ar are supplied to the corresponding passages of the
burner 25 for the core, while SiCl4, H2, O2, and Ar are supplied to the corresponding
passages of the burner 26 for the cladding. These burners 25 and 26 are maintained in
a state of combustion.
In each of the burners 25 and 26 in the combustion sate, a known "flame
hydrolysis reaction" occurs, thereby producing soot-like glass particles. These glass
panicles are jetted from the head of e..'c'i burner towards the lower end of the target 23
so that the particles are deposited on the lower end.
Accordingly, at the lower end of the target 23, the porous glass preform 10 is
produced, which includes the porous glass layer 11 for ihe core and the porous glass
layer 12 for the cladding, where these layers are integrated in a concentric-circle form.
As the porous glass preform 10 grows along the axial direction of the target, the target
23 is raised by the rotary lifting mechanism 24.
Instead of the single burner 26 for the cladding, a plurality of burners may be
assigned to the cladding.
In the present embodiment, in order to produce the porous glass preform 10.
one or both of the following methods are performed: (i) the position of the burner 25
for the core is relatively shifted with respect to the position of the target 23, in the
vertical, longitudinal, and crosswise directions, and (ii) the angle formed by the burner
25 (for the core) and the target 23 is controlled and changed. According to these
methods, it is possible to obtain an optical fiber preform in which at each position in the
area in which the relative refractive index of the core with respect to the cladding is 80%
or higher of the maximum relative refractive index, the absolute value of the rate of
change of the relative refractive index with respect to the position along the diameter
(100%) of the cladding is 0.5 or less.
That is, when the porous glass layers 11 and 12 of the porous glass preform 10
are made transparent by using a known electric furnace, an optical fiber preform having
transparent glass layers for the core and cladding can be obtained.
An optical fiber can be obtained by drawing the above-produced optical fiber
preform by using a known drawing system, and the optical fiber immediately after the
drawing process is then coated with a primary coating, a secondary coating, or the like,
thereby producing a ccared optical fiber. This coating process is performed
simultaneously with the above drawing process.
The following variations may also be employed so as to produce the optical
fiber preform.
In the first variation, first, only the porous glass layer 11 for the core is
produced using the VAD method as shown in Fig. 7, and this layer is purified using a
system for producing transparent glass, so as to make a transparent glass layer for the
core. In the next step, a porous glass layer 12 for the cladding is formed around the
transparent glass layer for the core by using the known OVD (outside vapor deposition)
method, and this layer is purified using a device for producing transparent glass, so as to
make a transparent glass layer for the cladding.
In the second variation, first, a porous glass layer 11 for the core and a porous
glass layer 12 for the cladding (of a specific ratio to the porous glass layer 11) are
formed by using the VAD method as shown in Fig. 7. These layers are purified using a
system for producing transparent glass, so as to make a transparent glass layer for the
core and a transparent glass layer for the cladding. In the next process, an additional
porous glass laver 12 for the cladding is formed using the OVD method around the
transparent glass layer for the cladding, because only the former porous glass layer 12 is
insufficient. The additional porous glass layer 12 is then purified using a system for
producing transparent glass, so as to make a transparent glass layer for the cladding.
In each variation, the optical fiber preform is produced while satisfying the
above-explained condition (relating to the refractive index distribution) according to the
present invention.
WE CLAIM :
1. A method of manufacturing an optical fiber perform having a core and
a cladding , comprising the step of:
controlling the refractive index distribution in a manner such that at
each position in the area in which the relative refractive index of the core with
respect to the cladding is 80% or higher of the maximum value of relative
refractive index , the absolute value of the rate of change of relative refractive
index with respect to the position along the diameter of the cladding is 0.5 or
less, where such position along the diameter of the cladding is defined by
percentage with respect to the outer diameter of the cladding (100%),
wherein said step for controlling the refractive index is performed at
the time of production of a porous glass perform as a precursor of the optical
fiber perform and comprises controlling the relative position of a burner used
for jetting the material for the core to a target on which the material is
deposited.
2. The method as claimed 1 wherein said step of controlling the refractive
index distribution comprises controlling the angle formed by a burner for
jetting material for the core and the target on which the material is deposited
at the time of production of said porous glass perform used as the precursor
for said optical fiber perform
3. An optical fiber perform manufactured by a method as claimed in
any of the preceding claims.
4. A method for manufacturing an optical fiber comprising the step
of drawing the optical fiber perform as claimed in claim 3 , such that an
absolute value for a ratio which indicates how a relative refractive index
varies should be 0.5 or less in each position in the area where the relative
refractive index of the core with respect to the cladding is 80 % or higher with
respect to the maximum value of the relative refractive index in the core with
respect to the cladding.
5. An optical fiber manufactured by a method as claimed in claim 4.
6. A method of manufacture of an optical fiber perform substantially as
herein described and as illustrated in the accompanying drawings.
7. A method for manufacture of an optical fiber substantially as herein
described and as illustrated in the accompanying drawings.
Dated this 25th day of October, 2002.

An optical fiber and an optical fiber preform having optical characteristics, such
as the wavelength dispersion, close to design values by controlling the amount of change
in the refractive index in the core, thereby realizing high-quality and high-speed
transmission, and manufacturing methods therefor. The optical fiber or the optical
fiber preform is manufactured in a manner such that at each position in the area in which
the relative refractive index of the core with respect to the cladding is 80% or higher ot
The maximum value of the relative refractive index, the absolute value of the rate of
change of the relative refractive index with respect to the position along the diameter of
the cladding is 0.5 or less, where the position along the diameter of the cladding is
defined by percentage with respect to the diameter.