U.S. patent application number 10/316972 was filed with the patent office on 2004-06-17 for optical fiber.
Invention is credited to Chiang, Kin Seng, Rastogi, Vipul.
Application Number | 20040114892 10/316972 |
Document ID | / |
Family ID | 32506032 |
Filed Date | 2004-06-17 |
United States Patent
Application |
20040114892 |
Kind Code |
A1 |
Chiang, Kin Seng ; et
al. |
June 17, 2004 |
Optical fiber
Abstract
An optical fiber is described in which the cladding is provided
with a refractive index that increases in a radially outward
direction. In particular embodiments the refractive index of the
cladding increases monotonically from a low value to a value close
to or higher Than the refractive index of the core. Such a fiber
can be formed that can be operated in an effective single mode
manner or in multimode operation and which is very suitable for use
in high-bit-rate communication systems
Inventors: |
Chiang, Kin Seng; (Kowloon,
HK) ; Rastogi, Vipul; (Uttar Pradesh, IN) |
Correspondence
Address: |
INTELLECTUAL PROPERTY GROUP
FREDRIKSON & BYRON, P.A.
4000 PILLSBURY CENTER
200 SOUTH SIXTH STREET
MINNEAPOLIS
MN
55402
US
|
Family ID: |
32506032 |
Appl. No.: |
10/316972 |
Filed: |
December 11, 2002 |
Current U.S.
Class: |
385/123 ;
385/124 |
Current CPC
Class: |
G02B 6/0283 20130101;
G02B 6/03688 20130101; G02B 6/03605 20130101; G02B 6/03627
20130101 |
Class at
Publication: |
385/123 ;
385/124 |
International
Class: |
G02B 006/16; G02B
006/18 |
Claims
1. An optical fiber comprising a central step-index or graded-index
core region surrounded by an annular cladding region, wherein said
cladding region is formed with a refractive index that increases in
a radially outward direction.
2. An optical fiber as claimed in claim 1 wherein the refractive
index of said cladding region increases from a low value to a value
close to or greater than the peak refractive index in the core.
3. An optical fiber as claimed in claim 1 wherein said refractive
index of said cladding region increases monotonically.
4. An optical fiber as claimed in claim 1 wherein said refractive
index of said cladding region increases in a step-like manner.
5. An optical fiber as claimed in claim 1 wherein said refractive
index of said cladding region increases in accordance with a power
law, wherein: 6 n 2 ( r ) = n 1 2 [ 1 - 2 ( b a - r a b a - 1 ) q ]
where n(r)=the refractive index at a radius r n.sub.1=the
refractive index in the core n.sub.2=the refractive index at radius
a a=the radius of the core b=the radius of the cladding region, and
e=the profile shape parameter (q>0), and 7 = n 1 2 - n 2 2 2 n 1
2 .
6. An optical fiber as claimed in claim 1 wherein the refractive
index of said cladding region increases exponentially or in a
Gaussian manner.
7. An optical fiber as claimed in claim 1 wherein said fiber is
adapted to be operated in effective single-mode operation.
8. An optical fiber as claimed in claim 1 wherein said fiber is
adapted to be operated in effective multimode operation.
9. A method of transmitting data through an optical fiber,
comprising providing a fiber having a central step-index or
graded-index core region surrounded by an annular cladding region,
wherein said cladding region is formed with a refractive index that
increases in a radially outward direction, and operating said fiber
in an effective single-mode manner.
10. A method of transmitting data through an optical fiber,
comprising providing a fiber having a central step-index or
graded-index core region surrounded by an annular cladding region,
wherein said cladding region is formed with a refractive index that
increases in a radially outward direction, and operating said fiber
in a multimode manner.
Description
FIELD OF THE INVENTION
[0001] This invention relates to a novel design for an optical
fiber, and in particular to designs for optical fibers that provide
a large core single-mode fiber or multimode fiber for high capacity
transmission.
BACKGROUND OF THE INVENTION
[0002] The most effective way of increasing the transmission
capacity of an optical fiber communication system is to use the
extremely wide bandwidth of a single-mode fiber and to feed a large
number of channels as is practicable into the fiber. As a
consequence of this, the trend has been to extend the communication
window from the C-Band to the L-Band and the S-Band so that the
communication window covers a total range of about 200 nm.
[0003] However, a major obstacle to the development of
ultra-wide-band dense wavelength division multiplexing (DWDM)
systems are non-linear effects, and in particular the Raman effect,
which cannot be managed with dispersion. Non-linear effects can
cause distortion and cross-talk for example. A direct way to manage
such non-linear effects is to use a single-mode fiber with a large
effective core area, and therefore recently much work has been done
on developing such fibers.
[0004] At the same time, there is an increasing interest in
expanding the bandwidth of a multimode fiber to meet the demand for
short-distance broadband applications, such as broadband Internet
and local-area networks. For such applications, the very large core
size of a multimode fiber proves to be important because it can
ease the optical alignment and lower the fiber connection cost (and
hence, the system cost). Unfortunately, the bandwidth of a
conventional step-index multimode fiber, especially a large-core
multimode fiber, is very limited and cannot meet future demand. It
is desirable to design a multimode fiber that has a very large core
size yet provides a sufficiently large bandwidth.
PRIOR ART
[0005] The effective core area of a conventional dispersion-shifted
single-mode fiber is about 50 .mu.m.sup.2. In 1996 Corning Inc.
developed a large-effective-area dispersion-shifted fiber (LEAF)
which had an effective core area of about 80 .mu.m.sup.2, and there
arc other designs for large-effective-area fibers, one of which
shows an effective area of about 100 .mu.m.sup.2. A single material
photonic crystal fiber, a so-called holey fiber, is characterized
by a distribution of air holes in the cladding running through the
entire length of the cladding and has attracted considerable
attention in recent years because it is capable of single-mode
operation over a wide range of wavelengths. However, it is
difficult to keep the birefringence, and hence the polarization
mode dispersion in the fiber, low because of the large index
contrast introduced by the air holes.
[0006] The core area of a commercial multimode fiber is of the
order of 1000-100000 .mu.m.sup.2. The bandwidth problem is solved
by introduction of a suitable graded refractive-index profile in
the core of the fiber. However, to obtain a bandwidth that is much
wider than that of a step-index fiber of the same core size, an
accurate control of the profile shape in the core is required,
which is difficult to achieve, especially when the core is very
large.
SUMMARY OF THE INVENTION
[0007] According to the present invention there is provided an
optical fiber comprising a central step-index or graded-index core
region surrounded by an annular cladding region, wherein said
cladding region is formed with a refractive index that increases in
a radially outward direction.
[0008] An advantage of the present invention, at least in preferred
forms, is that the size of the core region can be small enough to
guarantee effective single-mode operation, or large-enough to
provide effective multimode operation.
[0009] Preferably the refractive index of the cladding region
increases from a low value to a value close to or greater than the
peak refractive index in the core region.
[0010] In preferred embodiments of the invention the refractive
index of the cladding region increases monotonically. For example,
in accordance with a power law, or alternatively, the refractive
index of said cladding region may increase in a step-like
manner.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] Some embodiments of the invention will now be described by
way of example and with reference to the accompanying drawings, in
which:--
[0012] FIG. 1 is a plot showing a selection of possible
refractive-index profiles for fibers in accordance with embodiments
of the invention,
[0013] FIG. 2 shows the effects the profile parameter and the
cladding-core radius ratio on the real part of the normalized
propagation constant for fibers in accordance with embodiments of
the invention,
[0014] FIG. 3 shows the effects of the profile parameter on the
loss of the fundamental mode and the first higher-order mode,
[0015] FIG. 4 is a comparison of the bandwidth-length product of a
conventional fiber with fibers according to embodiments of the
present invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0016] As will be seen from the following, the present invention
provides, at least in its preferred forms, an optical fiber with a
cladding refractive-index profile that increases monotonically away
from the center in the radial direction. The advantage of this
structure is that it is possible to provide a fiber with an
effective single-mode operation at a desired wavelength (e.g. 1550
nm communication window) with a very large effective mode area. A
radially rising cladding profile makes the fiber essentially a
leaky structure. An appropriate choice of the cladding profile
causes the first higher-order mode of the fiber to leak away very
quickly while offering a very low leakage loss to the fundamental
mode. The fiber, thus, stays effectively single-moded even with a
very large core size.
[0017] FIG. 1 shows some typical refractive-index profiles for
fibers according to embodiments of this invention with the
refractive index increasing monotonically. In practice, however,
the manufacture of a fiber with a smoothly increasing refractive
index may not always be straightforward, and instead the
refractive-index may increase in a stop-like manner, and therefore
FIG. 1 also includes such a "stair-case" profile. Where the
refractive-index increases in a step-like manner the number of
steps can be varied, and it is not necessary for each step to be
the same height or the same length. The number of steps, their
length and their height can be varied as desired to approximate a
smoothly-increasing profile.
[0018] The refractive-index distribution profile in the cladding
can be of any shape (possibilities include exponential and Gaussian
profiles in addition to the following examples), provided that it
increases in the radially outward direction from a low value to a
value that is close to or larger than the peak index in the core.
For illustrative purposes in some embodiments of the invention, the
refractive-index distribution can be expressed as 1 n 2 ( r ) = n 1
2 r < a and r > b n 2 ( r ) = n 1 2 [ 1 - 2 ( b a - r a b a -
1 ) q ] a < r < b ( 1 )
[0019] where the region r<a represents the core and the region
a<r<b represents the cladding of the fiber. The cladding of
the fiber in truncated at r-b, beyond which there is a high-index
region n.sub.1. The profile in the core (r<a) or outside the
cladding (r>b) may either be graded-index or step index. For
simplicity, a step-index profile may be chosen in these regions.
The profile in the cladding is a power-law profile with profile
shape parameter q. 2 = n 1 2 - n 2 2 2 n 1 2
[0020] is the relative core-cladding index difference with n.sub.2
being the minimum value of the cladding index.
[0021] The fiber can be characterized by using the normalized
parameters 3 V = 2 a n 1 2 ,
[0022] which is the normalized frequency, and 4 B = n eff 2 - n 2 2
n 1 2 - n 2 2 ,
[0023] the normalized propagation constant, where n.sub.eff is the
mode index. Since the fiber is a leaky structure, B is complex,
whose real part B.sub.r gives the value of the mode index and
imaginary part B.sub.i measures the leakage loss. The actual
leakage loss of the fiber can be calculated from B.sub.i by 5 =
8.686 .times. 10 12 2 n 1 B i
[0024] in dB,km, where .lambda. is the free-space optical
wavelength in nm.
[0025] Fibers according to embodiments of the invention may be
analysed using the matrix method described by K. Thyagarajan, S.
Diggavi, A. Taneja, and A. K. Ghatak. Appl. Opt. 30 (1991) 3877 and
shown in FIG. 2 and FIG. 3 are the B.sub.r and B.sub.i of the
LP.sub.01 and LP.sub.11 modes as a function of the profile
parameter q for different values of V and b/a. FIG. 2 shows that
the effective index of the mode is hardly affected by the profile
shape parameter q. The value of b/a also has little effect on
B.sub.r. It can also be seen that the values of B.sub.r for a given
V are very close to those of the corresponding step-index fiber.
The fact that the radially rising profile in the cladding does not
greatly affect the effective indices of the modes indicates the
possibility of tailoring the chromatic dispersion characteristics
of the fiber with a suitable refractive-index profile in the core,
as in conventional fiber designs.
[0026] On the other hand, the radially rising profile in the
cladding has a significant effect on the leakage loss of the modes,
as shown in FIG. 3. It can be seen that, even for a large V (much
higher than the single-mode limit 2.4048 of a conventional
step-index fiber), with an appropriate choice of the cladding
profile, the leakage loss of the fundamental mode can stay very low
while that of the first higher-order mode is orders of magnitude
higher. The fiber, thus, shows effective single-mode operation. It
can also be seen that for a given profile q, an increase in b/a
increases the ratio of the leakage loss of the LP.sub.11 mode to
that of the LP.sub.01 mode by orders of magnitudes. For example,
this ratio for q=1 and V=4 increases from 10.sup.4 to 10.sup.5 when
b/a is increased from 5.0 to 6.25, and to 10.sup.6 for b/a=7.5,
clearly distinguishing the two modes in terms of guidance. A very
large V value or a very low q value results in a very low loss even
for the LP.sub.11 mode. Therefore, the values of V and q should be
chosen to give a sufficiently high leakage loss for the LP.sub.11
mode and, at the same time, a sufficiently low loss for the
LP.sub.01 mode.
[0027] To provide an estimate of the practical values of the
leakage losses, we consider a silica fiber with .DELTA.=00023. The
loss of this fiber at 1550 nm in terms of B.sub.i is given as
.alpha.=1.17.times.10.su- p.8 B.sub.i in db/km. Therefore, in a
silica fiber with q=4.0, a=20 .mu.m (an effective core area of
.about.1000 .mu.m.sup.2), and b=100 .mu.m, which correspond to V=8
and b/a=5, for the LP.sub.01 mode with B.sub.1=3.7.times.10.sup.-9,
the leakage loss is as small as 0.43 dB/km, while for the LP.sub.11
mode with B.sub.1=4.6.times.10.sup.-7, the leakage loss is as large
as 54 dB/km. The fiber is practically a single-mode fiber.
[0028] For a typical silica fiber with core radius 10 .mu.m (an
effective core area of .about.200 .mu.m.sup.2) cladding radius 62.5
.mu.m, and a radially rising stair-case profile, the leakage losses
at the wavelength 1550 nm are 0.06 dB/km and 2000 dB/km for the
fundamental mode and the first higher-order mode, respectively, and
the fundamental mode has a Gaussian-like mode pattern of a
conventional step-index fiber. It should be mentioned here that it
is possible to design a fiber to give a much larger mode area, hut
bending loss limits the maximum mode area that can be achieved in
practice.
[0029] The same design as shown in FIG. 1 can be used to provide
multimode operation with a wide bandwidth. By increasing the core
radius or the index contrast sufficiently (e.g., a=100 .mu.m and
.DELTA.=0.01), the value of V can become very large (V>>1)
and the number of modes in the fiber increases at a rate
proportional to the square of V. As implied by the results in FIG.
3, the leakage loss increases generally with the mode order.
Therefore, a leakage loss distribution can be defined, which is a
function that characterizes how the leakage loss varies with the
mode order. Obviously, the form of the leakage loss distribution
depends on the refractive-index profile of the cladding. In
general, a higher-order mode, because of its larger leakage loss,
leaks out from the fiber at a shorter distance than a lower-order
mode. The fiber thus has the effect of stripping off the modes
continuously along the transmission distance, starting from the
modes of the highest orders. In other words, the effective number
of modes propagating in the fiber decreases as the length of the
fiber increases, This is equivalent to say, using the concept of
geometric optics, that the effective numerical aperture of the
fiber decreases as the length of the fiber increases because of the
leakage mechanism. By reducing the effective numerical aperture
(through gradual elimination of higher-order modes), the
bandwidth-length product of the fiber increases with the length of
the fiber.
[0030] On the other hand, the bandwidth-length product of a
conventional multimode fiber, which has a uniform cladding, is a
constant, which is equal to the initial bandwidth-length of the
corresponding leaky fiber, as shown in FIG. 4. For the leaky fiber,
the dependence of the bandwidth-length product on the fiber length
is governed by the refractive-index profile of the cladding. It is
possible to obtain a linear dependence by using a suitable index
profile. In that case, the bandwidth (not the bandwidth-length
product) of the fiber becomes a constant, which implies that the
fiber can promise the same bandwidth regardless of its length.
[0031] It should be pointed out that the leaky multimode fiber is
inherently more lossy than the conventional multimode fiber. The
gain in bandwidth is traded with an increase in optical loss. The
leaky fiber is therefore most suitable for short-distance
high-capacity applications, where the transmission distance is
limited by The dispersion instead of the attenuation in the
fiber.
[0032] The present invention, at least in its preferred forms,
provides a fiber that is easy to fabricate with the existing
well-established technology for making graded-index fibers and
should be very useful for high-bit-rate communication systems.
* * * * *