Title: Optical fiber with built-in grating and optical fiber for forming grating therein
Abstract: The present invention provides an optical fiber equipped with a grating that functions as a narrow-band loss filter. The optical fiber has a core, an inner cladding, an intermediate cladding, and an outer cladding, which have refractive indexes n.sub.0, n.sub.1, n.sub.2, and n.sub.3, respectively, the refractive indexes having a relationship of n.sub.0 >n.sub.3.gtoreq.n.sub.1 n.sub.2. At least a part of the inner cladding has a grating. The refractive index of the intermediate cladding is lower than the refractive indexes of the inner cladding and the outer cladding such that a recession is formed in the refractive index profile of the clad. The grating is provided on the inner side relative to the recession.
Patent Number: 6,842,566 Issued on 01/11/2005 to Ishikawa, et al.
| Inventors:
|
Ishikawa; Shinji (Yokohama, JP);
Taru; Toshiki (Yokohama, JP);
Shigehara; Masakazu (Yokohama, JP);
Oomura; Masaki (Yokohama, JP)
|
| Assignee:
|
Sumitomo Electric Industries, Ltd. (Osaka, JP)
|
| Appl. No.:
|
193733 |
| Filed:
|
July 12, 2002 |
Foreign Application Priority Data
| Jul 13, 2001[JP] | 2001-214256 |
| Current U.S. Class: |
385/37; 385/123; 385/124; 385/126 |
| Intern'l Class: |
G02B 006/34; G02B006/02 |
| Field of Search: |
385/123,124,126-128,37,141,144
|
References Cited [Referenced By]
U.S. Patent Documents
| 5721800 | Feb., 1998 | Kato et al. | 385/127.
|
| 6175680 | Jan., 2001 | Arai et al. | 385/127.
|
| 6314221 | Nov., 2001 | Riant et al. | 385/37.
|
| 6321007 | Nov., 2001 | Sanders | 385/37.
|
| 6400865 | Jun., 2002 | Strasser et al.
| |
| 2001/0026396 | Oct., 2001 | Enomoto et al. | 359/341.
|
| 2002/0122624 | Sep., 2002 | Jang | 385/37.
|
Primary Examiner: Kang; Juliana K.
Attorney, Agent or Firm: McDermott Will & Emery LLP
Claims
What is claimed is:
1. An optical fiber with a built-in rating, comprising:
a core having a refractive index n.sub.0 ;
an inner cladding that surrounds the core and has a refractive index
n.sub.1 ;
an intermediate cladding that surrounds the inner cladding and has a
refractive index n.sub.2 ; and
an outer cladding that surrounds the intermediate cladding and has a
refractive index n.sub.3,
wherein n.sub.0 >n.sub.3.gtoreq.n.sub.1 >n.sub.2,
grating is formed at least in a part of the inner cladding, and
the relative refractive index difference between the inner cladding and the
intermediate cladding, (n.sub.2 -n.sub.1)/n.sub.1, is more than -1.0% and
less than or equal to 0.3%.
2. The optical fiber with a built-in grating according to claim 1, wherein
the part in which grating is formed exists in an outer portion of the
inner cladding, the outer portion being in contact with the intermediate
cladding but not in contact with the core.
3. The optical fiber with a built-in grating according to claim 2, wherein
the outer portion of the inner cladding is a portion having an inside
diameter that is equal to 1.2 times or more and below double the outside
diameter of the core.
4. The optical fiber with a built-in grating according to claim 1, wherein
the inner cladding has an outside diameter b and the intermediate cladding
has an outside diameter c, and an outside diameter ratio c/b of the inner
cladding and the intermediate cladding is 1.2 or more and below 4.
5. The optical fiber with a built-in grating according to claim 1, wherein
the grating has equal refractive index planes that are inclined with
respect to an axis of the optical fiber.
6. An optical fiber for forming a grating therein, comprising:
a core having a refractive index n.sub.0 ;
an inner cladding that surrounds the core and has a refractive index
n.sub.1 ;
an intermediate cladding that surrounds the inner cladding and has a
refractive index n.sub.2 ; and
an outer cladding that surrounds the intermediate cladding and has a
refractive index n.sub.3,
wherein n.sub.0 >n.sub.3.gtoreq.n.sub.1 >n.sub.2,
at least a part of the inner cladding is photosensitive such that the
refractive index thereof increases when irradiated by light, and
the relative refractive index difference between the inner cladding and the
intermediate cladding, (.sub.2 -n.sub.1)/n.sub.1 is more than -1.0% and
less than or equal to 0.3%.
7. The optical fiber for forming a grating therein according to claim 6,
wherein the part that is photo-sensitive exists in an outer portion of the
inner cladding, the outer portion being in contact with the intermediate
cladding but not in contact with the core.
8. The optical fiber for forming a grating according to claim 7, wherein
the outer portion of the inner cladding is a portion having an inside
diameter that is equal to 1.2 times or more and below double the outside
diameter of the core.
9. The optical fiber for forming a grating therein according to claim 6,
wherein the inner cladding has an outside diameter b and the intermediate
cladding has an outside diameter c, and an outside diameter ratio c/b is
1.2 or more and below 4.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical fiber with a built-in grating
and an optical fiber used for the same.
2. Description of the Related Art
The structure of the fiber grating may be considered to be an array of
transverse planes having a constant refractive index, i.e., equal
refractive index planes, which are arranged at regular intervals in an
optical fiber. An interval of the equal refractive index planes is a
lattice interval, that is, a cycle in the fiber grating. The fiber grating
reflects light having a limited wavelength around Bragg wavelength defined
by the cycle and refractive index. The fiber grating can be formed by
applying light, such as ultraviolet light, which induces a change in the
refractive index, onto a photo-sensitive optical fiber, e.g., an optical
fiber formed of silica glass to which germanium oxide (GeO.sub.2) has been
added. An optical fiber with a fiber grating is used as a gain equalizer
of a multiplexer/demultiplexer or an optical amplifier in a wavelength
multiplex transmission system. The fiber grating used as the gain
equalizer induces the coupling between the fundamental mode and a cladding
mode of an optical fiber, causing strong power migration from the
fundamental mode to the cladding mode. As a result, the fiber grating
attenuates the intensity of light propagating in the fundamental mode over
a certain wavelength band, functioning as a loss filter.
The fiber grating working as a loss filter is required to precisely control
a loss spectrum. A narrower band of the loss filter is preferred because
gain equalization with less excess loss can be achieved.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an optical fiber with a
built-in grating that exhibits improved band characteristics, and an
optical fiber used for the same.
To this end, according to one aspect of the present invention, an optical
fiber is provided with a built-in grating, including a core having a
refractive index n.sub.0, an inner cladding having a refractive index
n.sub.1, an intermediate cladding having a refractive index n.sub.2, and
an outer cladding having a refractive index n.sub.3, wherein a grating is
formed at least in a part of the inner cladding, where n.sub.0
>n.sub.3.gtoreq.n.sub.1 >n.sub.2.
An optical fiber according to another aspect of the present invention, from
which the optical fiber with a built-in grating is fabricated, includes a
core, an inner cladding, an intermediate cladding, and an outer cladding,
their refractive indexes being n.sub.0, n.sub.1, n.sub.2, and n.sub.3,
respectively, where n.sub.0 >n.sub.3.gtoreq.n.sub.1 >n.sub.2, and at
least a part of the inner cladding being photo-sensitive such that the
refractive index thereof increases when irradiated by light.
The relative refractive index difference between the inner cladding and the
intermediate cladding, (n.sub.2 -n.sub.1)/n.sub.1, may be -1.0% or more
and below 0.3%. The part, in which the grating is formed, in the inner
cladding of the optical fiber having a built-in grating and the
photo-sensitive part, in which a grating is to be formed, in the inner
cladding of the optical fiber may be respectively an outer portion of the
inner cladding, i.e., the portion that is in contact with the intermediate
cladding but not in contact with the core. The inside diameter of the
outer portion may be 1.2 times or more but below double the outside
diameter of the core. The ratio of the outside diameter of the inner
cladding to the outside diameter of the intermediate cladding may be 1.2
or more but below 4.0. The grating formed in the inner cladding may
include the planes each having an equal refractive index that are inclined
with respect to the axis of the optical fiber.
The present invention is further explained below by referring to the
accompanying drawings. The drawings are provided solely for the purpose of
illustration and are not intended to limit the scope of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graph showing the refractive index profile of an optical fiber,
which is suitable for forming a grating therein, in accordance with an
embodiment of the present invention.
FIG. 2 is a cross sectional view of the optical fiber shown in FIG. 1.
FIG. 3 is a graph showing the refractive index profile of an optical fiber
with a built-in grating in accordance with another embodiment of the
present invention.
FIG. 4 is a side view of the optical fiber shown in FIG. 3.
FIG. 5 is a graph showing a loss spectrum and a reflection spectrum of the
optical fiber shown in FIG. 3.
FIG. 6 is a side view of an optical fiber with a built-in grating in
accordance with still another embodiment of the present invention.
FIG. 7 is a graph showing a loss spectrum and a reflection spectrum of the
optical fiber shown in FIG. 6.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Embodiments of the present invention are explained below by referring to
the accompanying drawings. In the drawings, the same number refers to the
same part to avoid duplicate explanation. The ratios of the dimensions in
the drawings do not necessarily coincide with the explanation.
FIG. 1 is a graph showing the refractive index profile of an optical fiber
1 for forming grating therein, which is an embodiment in accordance with
the present invention. FIG. 2 is a cross sectional view of the optical
fiber 1. Referring to FIG. 2, the optical fiber 1 is constituted by a
substantially columnar core 10 and a substantially circular tubular clad
12 concentrically surrounding the core 10 in close contact with the
peripheral surface of the core 10. The core 10 and the clad 12 are both
formed of silica glass. The core 10 has a higher refractive index than the
clad 12.
The clad 12 is formed of three layers, namely, an inner cladding 14, an
intermediate cladding 16, and an outer cladding 18, all of which have
substantially circular, tubular shapes. The inner cladding 14
concentrically wraps the core 10, the inner surface thereof being in close
contact with the peripheral surface of the core 10. The intermediate
cladding 16 concentrically wraps the core 10 and the inner cladding 14,
the inner surface thereof being in close contact with the outer surface of
the inner cladding 14. The outer cladding 18 concentrically wraps the core
10, the inner cladding 14, and the intermediate cladding 16, the inner
surface thereof being in close contact with the outer surface of the
intermediate cladding 16.
The abscissa in FIG. 1 represents a radial distance from a point on the
central axis of the optical fiber 1, the point being represented by the
origin of coordinates. The ordinate represents a relative refractive index
difference based on a refractive index n.sub.1 of the inner cladding 14.
The relative refractive index difference of the portion having refractive
index n is expressed by (n-n.sub.1)/n.sub.1. As will be discussed
hereinafter, the inner cladding 14 has a refractive index that is
practically equal to the refractive index of pure silica glass.
The core 10 contains phosphorus oxide and has a relative refractive index
difference 0.5%. The radius of the core 10 is 3.3 .mu.m.
The outer radius of the inner cladding 14 is 9 .mu.m. The inner cladding 14
has a refractive index substantially equal to the refractive index of the
pure silica glass, and is constituted by two circular tubular layers 14a
and 14b having different compositions. More specifically, the inner
portion 14a (the circular tubular portion having an inner radius of 3.3
.mu.m and an outer radius of 5.1 .mu.m) of the inner cladding 14 is formed
of substantially pure silica glass. The outer portion 14b (the circular
tubular portion having an inner radius of 5.1 .mu.m and an outer radius of
9 .mu.m) of the inner cladding 14 is formed of silica glass that contains
GeO.sub.2 and boron oxide. The outer portion 14b is in contact with the
intermediate cladding 16, while it is not in contact with the core 10.
As it is well known, the additive GeO.sub.2 contained in the outer portion
14b serves as a photosensitive material reactive to ultraviolet light
having a wavelength of 260 nm or less. More specifically, the
characteristic of the silica glass containing GeO.sub.2 is that its
refractive index increases in response to the irradiation of the
ultraviolet light having a wavelength within the foregoing range. The
increase in the refractive index grows greater with increasing total power
of the applied light. For this reason, as will be discussed hereinafter,
grating can be formed on the outer portion 14b.
The outer radius of the intermediate cladding 16 is 28 .mu.m. The
intermediate cladding 16 is formed of silica glass that contains fluorine
and has a relative refractive index difference of -0.76%.
The outer radius of the outer cladding 18 is 62.5 .mu.m. The outer cladding
18 is formed of silica glass provided with dehydration treatment using a
chloride gas. Since chlorine is added to silica glass during the
dehydration treatment, the outer cladding 18 has a refractive index
slightly higher than the refractive index of pure silica glass, the
relative refractive index difference thereof with respect to that of pure
silica glass in this embodiment being 0.05%
Thus, in the optical fiber 1, when the refractive indexes of the core 10,
the inner cladding 14, the intermediate cladding 19, and the outer
cladding 18 are denoted as n.sub.0, n.sub.1, n.sub.2, and n.sub.3, then
there is a relationship expressed by n.sub.0 >n.sub.3 >n.sub.1
>n.sub.2. The optical fiber 1 can be fabricated by a publicly known
method, such as the MCVD method or the OVD method.
As mentioned above, the photo-sensitive material has been added only to the
outer portion 14b in the inner cladding 14, so the refractive index of the
outer portion 14b increases when irradiated by ultraviolet light. In
contrast to this, since the inner portion 14a contains no photosensitive
material, the irradiation of ultraviolet light does not cause a change in
the refractive index thereof. This means that when ultraviolet light is
applied by a known method, such as the phase masking method, grating will
be formed only on the outer portion 14b.
FIG. 3 is a graph illustrating the refractive index profile of the optical
fiber 2 with built-in grating, which is an embodiment of the present
invention. FIG. 4 is a side view of the optical fiber 2. In FIG. 4, for
the convenience of illustration, the boundary between the core 10 and the
inner cladding 14 and the boundary between the inner portion 14a and the
outer portion 14b of the inner cladding 14 are respectively indicated by
dashed lines.
Referring to FIGS. 1 and 3, in comparison with the refractive index profile
of the optical fiber 1, the refractive index profile of the optical fiber
2 shows that the refractive index of the outer portion 14b of the inner
cladding 14 increases, which indicates the presence of a grating 20. In
the optical fiber 2, the amount of the increase in the refractive index
periodically changes along an axis 5 of the optical fiber 2. In the
optical fiber 2 shown in FIG. 4, equal refractive index planes 22 of the
grating 20 are perpendicular to the axis 5 of the optical fiber 2.
The optical fiber 2 has two characteristics, one of which is that the
intermediate cladding 16 has a refractive index that is lower than those
of the inner cladding 14 and the outer cladding 18 and the refractive
index profile of the clad 12 has a relatively deep recession 19. The other
characteristic is that the grating 20 is provided on the inner side
relative to the recession 19 in the clad 12 rather than on the core 10.
Because of the presence of the recession 19, the optical fiber 2 has two
different cladding modes. One cladding mode is an inner cladding mode in
which power distribution shows the concentration on the inner side
relative to the recession 19, and the other cladding mode is an outer
cladding mode in which the power is distributed over the entire cladding
12. The inner cladding mode has power distribution close to that of a
fundamental mode, so it has high efficiency of coupling to the fundamental
mode. In contrast to this, the outer cladding mode has lower efficiency of
coupling to the fundamental mode.
For this reason, the grating 20 formed on the inner side of the recession
19 of the refractive index profile has a narrower loss band that allows
the fundamental mode to be coupled only to the inner cladding mode with
high efficiency. Hence, the optical fiber 2 incorporating the grating 20
can be used ideally as a narrow-band loss filter or a gain equalizer of an
optical amplifier. On the other hand, a conventional fiber grating couples
the fundamental mode to many cladding modes, so that it has a wider loss
band.
The optical fiber 2 provides another advantage in that less reflection is
caused in the fundamental mode because no grating is formed on the core
10.
The optical fiber 2 with a built-in grating can easily be fabricated from
the optical fiber 1 by forming a grating therein. The inventors have used
a phase mask having a cycle of 1070 nm to form an apodized grating 20
having a refractive index cycle of 535 nm over a length of 5 mm in the
optical fiber 1. Then, the inventors have measured the loss spectrum and
the reflection spectrum of the grating 20.
FIG. 5 is a graph showing the measurement results, in which the loss
spectrum is indicated by the solid line, while the reflection spectrum is
indicated by the dashed line. As shown in FIG. 5, the loss spectrum shows
the loss peak around 1544.8 nm, and the half-value width thereof is 0.5
nm, which is satisfactorily narrow. The reflection in the band having the
loss peak therein is controlled to -50 dB or less, and the grating 20 can
be used as a narrow-band, low-reflection loss filter. There is, however,
the reflection of -4 dB at 1550.8 nm, which is a Bragg wavelength.
The reflection at the Bragg wavelength is effectively reduced by forming,
in the optical fiber, a grating having equal refractive index planes that
are inclined with respect to the axis 5. FIG. 6 is a side view showing the
construction of an optical fiber 3 having a built-in grating, in which a
grating 30 is provided in place of the grating 20 shown in FIG. 4. The
optical fiber 3 can also be fabricated easily by forming the grating 30 on
the outer portion 14b of the inner cladding 14 of the optical fiber 1.
A tilt angle .theta. of the grating 30 is the angle formed by the plane
orthogonal to the axis 5 and an equal refractive index surface 32. To
sufficiently reduce the reflection, a tilt angle of 120 or more should be
imparted to the grating 30.
The inventors have used a phase mask having a cycle of 1069 nm to form the
grating 30 having the tilt angle .theta. of 2.5.degree. over a length of 5
mm in the optical fiber 1. Then, the inventors have measured the loss
spectrum and the reflection spectrum of the grating 30.
FIG. 7 is a graph showing the measurement results, in which the loss
spectrum is indicated by the solid line, while the reflection spectrum is
indicated by the dashed line. As shown in FIG. 7, the grating has the
narrow-band loss peak, whose half-value width is 0.5 nm, at a wavelength
of 1544.8 nm. Moreover, the reflection at 1550.8 nm, which is a Bragg
wavelength, is controlled to a low value of -32 dB. Thus, it has been
verified that the Bragg reflection can be reduced by providing an optical
fiber with a grating having equal refractive index planes that are
inclined with respect to the axis of the optical fiber.
In the optical fibers 2 and 3 discussed above, the depth of the recession
19 provided in the refractive index profile of the clad 12 can be
represented by a relative refractive index difference (n.sub.2
-n.sub.1)/n.sub.1 of the intermediate cladding 16. This value is -0.76% in
the above embodiments. The refractive index n.sub.1 of the inner cladding
14 is the refractive index of the inner cladding 14 excluding the grating
20, and is equivalent to the refractive index of pure silica glass in this
embodiment.
Setting the relative refractive index difference of the intermediate
cladding 16 to -0.3% or less makes it possible to restrain the efficiency
of coupling between the outer cladding mode and the fundamental mode so as
to sufficiently limit the loss band of the grating 20 or 30. Preferably,
the relative refractive index difference between the inner cladding 14 and
the intermediate cladding 16 is set to -1.0% or more so as to permit easy
fabrication.
In order to satisfactorily restrain the reflection in the band of the loss
peak, the outer portion 14b preferably has an inside diameter that is 1.2
times or more the outside diameter a of the core. If the outer portion 14b
is larger than a mode field diameter, then the mode transformation
efficiency deteriorates considerably. For this reason, it is desirable to
set the inside diameter of the outer portion 14b to a value below twice
the outside diameter a of the core.
In order to prevent the coupling of the fundamental mode with the outer
cladding mode, it is preferable that the ratio of the outside diameters
c/b (where b denotes the outside diameter of the inner cladding 14 and c
denotes the outside diameter of the intermediate cladding 16) be set to
1.2 or more. Furthermore, in order to sufficiently restrain the bend loss
caused by the cutoff of the fundamental mode, the ratio of outside
diameters c/b between the inner cladding 14 and the intermediate cladding
16 is preferably set to below 4.
The refractive index n.sub.1 of the inner cladding 14 and the refractive
index n.sub.3 of the outer cladding 18 may be the same.
The entire disclosure of Japanese Patent Application No. 2001-214256 filed
on Jul. 13, 2001 including specification, claims drawings and summary are
incorporated herein by reference in its entirely.
*
