U.S. patent application number 11/091106 was filed with the patent office on 2005-08-04 for raman fiber amplifier communication systems.
Invention is credited to Kalish, David, Kim, Jinkee, Lingle, Robert JR., Qian, Yifei.
Application Number | 20050168803 11/091106 |
Document ID | / |
Family ID | 34811022 |
Filed Date | 2005-08-04 |
United States Patent
Application |
20050168803 |
Kind Code |
A1 |
Kalish, David ; et
al. |
August 4, 2005 |
Raman fiber amplifier communication systems
Abstract
The specification describes an improved optical fiber design in
which the criteria for high performance in a Raman amplified
optical system, such as moderate effective area, moderate
dispersion, low dispersion slope, and selected zero dispersion
wavelength, are simultaneously optimized. In preferred embodiments
of the invention, the dispersion characteristics are deliberately
made selectively dependent on the core radius. This allows
manufacturing variability in the dispersion properties, introduced
in the core-making process, to be mitigated during subsequent
processing steps.
Inventors: |
Kalish, David; (Roswell,
GA) ; Kim, Jinkee; (Norcross, GA) ; Lingle,
Robert JR.; (Norcross, GA) ; Qian, Yifei;
(Alpharetta, GA) |
Correspondence
Address: |
Peter V. D. Wilde
301 East Landing
Williamsburg
VA
23185
US
|
Family ID: |
34811022 |
Appl. No.: |
11/091106 |
Filed: |
March 28, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11091106 |
Mar 28, 2005 |
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10397154 |
Mar 26, 2003 |
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6904217 |
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10397154 |
Mar 26, 2003 |
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10353762 |
Jan 29, 2003 |
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Current U.S.
Class: |
359/334 |
Current CPC
Class: |
G02B 6/02242 20130101;
C03B 2201/02 20130101; G02B 6/02009 20130101; Y02P 40/57 20151101;
G02B 6/02014 20130101; C03B 2203/22 20130101; C03B 2203/36
20130101; G02B 6/03611 20130101; G02B 6/0281 20130101; G02B 6/03644
20130101; C03B 2201/12 20130101; C03B 2203/23 20130101; C03B
2201/31 20130101; C03B 37/01228 20130101; G02B 6/02271
20130101 |
Class at
Publication: |
359/334 |
International
Class: |
H01S 003/00 |
Claims
1. An optical WDM system using Raman amplification comprising: a. a
length of optical fiber, b. lightwave signal means for introducing
a lightwave signal into the optical fiber, the lightwave signal
comprising at least three wavelength division multiplexed (WDM)
wavelengths, c. optical pump means for introducing lightwave pump
energy into the core of the glass fiber, whereby the lightwave pump
energy interacts with the lightwave signal to produce Raman
amplification of the lightwave signal, the invention characterized
in that the optical fiber comprises sequential regions of: i. a
core region extending from the center of the optical fiber, with
essentially all of the core region having a positive .DELTA., ii. a
trench region, with essentially all of the trench region having a
negative .DELTA., iii. a ring region having a positive .DELTA., and
the optical fiber is characterized by: Dispersion at 1550 nm: 5-8.5
ps/nm-km Dispersion slope at 1550 nm: <0.045 ps/nm.sup.2-km
Effective area at 1550 nm: >50 .mu.m.sup.2 Cable cutoff
wavelength: <1410 nm Macrobend loss (32 mm) at 1625 nm:
<5.times.10.sup.3 dB/km Zero dispersion wavelength: <1400
nm.
2. The system of claim 1 wherein the signal means includes
wavelengths above 1510 nm, and includes 1550 nm.
3. The system of claim 2 wherein: Dispersion at 1550 nm=7.3.+-.2
ps/nm-km
4. The system of claim 2 wherein: Effective area at 1550 nm=54-62
.mu.m.sup.2
5. The system of claim 2 wherein: Dispersion slope at 1550 nm:
<0.042 ps/nm.sup.2-km
6. The system of claim 2 wherein: Dispersion at 1550 nm=7.3.+-.2
ps/nm-km Effective area at 1550 nm=54-58 .mu.m.sup.2 Dispersion
slope at 1550 nm: <0.042 ps/nm.sup.2-km
7. The system of claim 1 wherein the system includes at least one
erbium-doped fiber amplifier.
8. The system of claim 1 wherein the at least three wavelength
division multiplexed (WDM) wavelengths operate over the S-, L- or
extended L-bands.
9. The system of claim 1 wherein the lightwave signal means
operates at 40 Gb/s.
10. The system of claim 1 further including means for dispersion
slope compensation.
11. An optical WDM system comprising: a. a length of optical fiber,
b. lightwave signal means for introducing a lightwave signal into
the optical fiber, the lightwave signal comprising at least three
wavelength division multiplexed (WDM) wavelengths, c. means for
dispersion slope compensation having a relative dispersion slope,
defined as dispersion slope divided by dispersion at a given
wavelength matched to the optical fiber, of: 0.0064 to 0.0082 per
nm at a wavelength of approximately 1510 nm (S-band application)
0.0046 to 0.0058 per nm at a wavelength of approximately 1550 nm
(C-band application) 00.42 to 00.54 per nm at a wavelength of
approximately 1570 nm (C+L band compensation with combined module)
0.0038 to 0.0048 per nm at a wavelength of approximately 1590 nm
(L-band application) the invention characterized in that the
optical fiber comprises sequential regions of: i. a core region
extending from the center of the optical fiber, with essentially
all of the core region having a positive .DELTA., ii. a trench
region, with essentially all of the trench region having a negative
.DELTA., iii. a ring region having a positive .DELTA., and the
optical fiber is characterized by: Dispersion at 1550 nm: 5-8.5
ps/nm-km Dispersion slope at 1550 nm: <0.045 ps/nm.sup.2-km
Effective area at 1550 nm: >50 .mu.m.sup.2 Cable cutoff
wavelength: <1410 nm Macrobend loss (32 mm) at 1625 nm:
<5.times.10.sup.3 dB/km Zero dispersion wavelength: <1400
nm.
12. The system of claim 11 wherein the C-band and L-band
compensation is achieved with a single means having an RDS of 00.42
to 00.54 per nm at a wavelength of approximately 1570 nm.
Description
RELATED APPLICATIONS
[0001] This application is a continuation-in-part of application
Ser. No. 10/353762, filed Jan. 29, 2003.
FIELD OF THE INVENTION
[0002] This invention relates to optical fibers having improved
optical transmission characteristics, methods for their production,
and communication systems incorporating the improved optical
fibers.
BACKGROUND OF THE INVENTION
[0003] Optical transmission systems employ Wavelength Division
Multiplexing (WDM) to increase information handling of an optical
fiber transmission line, typically a long haul transmission line.
Early WDM systems operated with a relatively narrow wavelength
bandwidth, centered around 1550 nanometers, e.g. 1530-1565
nanometers, referred to as the C-band. This is the wavelength
region where standard silica based optical fibers have optimally
low absorption.
[0004] In most WDM systems there is a trade-off between the number
of channels the system accommodates and the channel separation.
Higher bit rates generally call for an increase in channel spacing.
Both goals favor a wide operating spectrum, i.e. a wide range of
operating wavelengths.
[0005] Recently, systems have been designed that extend the
effective operating wavelength range well above the C-band
transmission band. In terms of wavelength, the new band, referred
to as the L-band, is variously defined, but for the purpose of this
description is 1570-1610 nanometers. Substantial work has also been
done in the S-band, defined as 1460-1530 nm. Use of these added
wavelengths substantially extends the capacity of WDM systems.
There is an ongoing effort to further extend the effective
operating wavelength window to above 1610 nm, for example to 1620
nm. Success of these efforts will depend on finding components, for
example amplifiers, that provide effective operation over this
broad wavelength range. It is now well appreciated that a
transmission fiber should have a minimum level of dispersion at
signal wavelengths to enable WDM transmission by suppressing four
wave mixing impairments. Since the dispersion of a non-zero
dispersion-shifted NZDF typically increases toward longer
wavelength, this requirement implies that the zero dispersion
wavelength should be 20-40 nanometers lower than the shortest
wavelength intended for WDM.
[0006] In WDM systems, it is important to have uniform gain over
the entire WDM wavelength band. This objective becomes more
difficult to reach as the operating wavelength range is extended to
longer and/or shorter wavelengths. Recently, new types of optical
fiber amplifiers have been developed that operate using stimulated
Raman scattering. The most prominent of these is a distributed
amplifier that operates over the normal transmission span as a
traveling wave amplifier. Raman scattering is a process by which
light incident on a medium is converted to light at a lower
frequency (Stokes case) than the incident light. The pump photons
excite the molecular vibrations of the medium up to a virtual level
(non-resonant state). The molecular state quickly decays to a lower
energy level emitting a signal photon in the process. Because the
pump photon is excited to a virtual level Raman gain can occur for
a pump source at any wavelength. The difference in energy between
the pump and signal photons is dissipated by the molecular
vibrations of the host material. These vibrational levels determine
the frequency shift and shape of the Raman gain curve. The
frequency (or wavelength) difference between the pump and the
signal photon is called the Stokes shift. In Ge-doped silica
fibers, the Stokes shift at which the maximum gain is obtained is
.about.13 THz. Due to the amorphous nature of silica the Raman gain
curve is fairly broad in optical fibers.
[0007] Since Raman scattering can occur at any wavelength; this can
be exploited to advantage in a telecommunication system that
contains multiple signal wavelengths by using Raman pumps at
several different wavelengths to amplify the signals. The gain seen
by a given wavelength is the superposition of the gain provided by
all the pumps, taking into account the transfer of energy between
the pumps due to Raman scattering. By properly weighting the power
provided at each of the Raman pump wavelengths it is possible to
obtain a signal gain versus wavelength profile in which there is a
small difference between the gain seen by different signal
wavelengths (this difference is called the gain ripple or gain
flatness). The use of Raman amplification thus enables dense WDM
(DWDM) outside the erbium window. Raman amplification is also an
enabling technology for the evolution from 10 to 40 Gb/s
transmission because it improves optical signal to noise ratio at
lower launch powers.
[0008] A multiplicity of pumps has been used successfully in many
systems. However there is one persistent problem with multiple
pumps. An unwanted nonlinear effect called four-wave mixing (FWM)
may sometimes occur. In telecommunications systems, if FWM occurs
in the signal band this can lead to transmission errors. As the
number of pumps in a multi-pump wavelength Raman amplification
scheme increases, the likelihood of FWM increases.
[0009] The harmful effects of four-wave mixing have been
recognized. Recently one approach towards reducing these effects
has been proposed [EP 1 148 666 A2]. In this approach the pump
wavelengths are either time division multiplexed (TDM) together, or
the frequency of the pump source is modulated (FM). Since the
various pump wavelengths overlap for only a small distances along
the fiber, FWM between the pump wavelengths should be eliminated or
severely reduced.
[0010] While this approach would eliminate FWM, the nominal pump
power requirements in this system are relatively high. Moreover, to
TDM a relatively large number of pump wavelengths, some operating
at relatively high power, adds significantly to the cost and
complexity of the system. Other approaches to reducing FWM and
other non-linear effects would significantly advance the art.
[0011] At least equally as important as compatibility with
amplifier technology in the design of optical fibers for high bit
rate, wide-band, systems is management of chromatic dispersion.
This problem grows significantly as the data bit rate is increased.
An optical transmission line, comprising a cabled fiber and a
dispersion compensation element (typically a module but possibly a
cabled fiber), that transmits effectively at 10 Gb/s may show
excessive error rates at 40 Gb/s because of bit overlap. For
non-return-to-zero modulation, a 10 Gb/s system should accumulate
less than .about.1000 ps/nm chromatic dispersion over the total
link distance; for a 40 Gb/s system this requirement is tightened
to less than 60 ps/nm.
[0012] This requirement is met by a combination of two methods.
First, in NZDF fibers, dispersion is reduced in the C-band below
that of standard matched clad fiber. To gain this benefit over
multiple bands, it is advantageous that the slope of the dispersion
be low. Second, dispersion compensation technology is employed,
most commonly in the form of a dispersion compensating fiber (DCF)
in a module. For broadband operation, it is important that the
dispersion curve of the DCF "match" that of the transmission fiber
in the appropriate sense. In general, precise compensation of
chromatic dispersion over a broad band is achieved when the ratio
of the dispersion slope to the dispersion at band center is equal
for the fiber and DCF. Furthermore, the best results are obtained
when this ratio is low. This further emphasizes the advantage of
reduced dispersion slope.
[0013] A problem arises in designing optical fibers to meet this
general need: typical optical fiber profiles that are optimized for
low dispersion slope have reduced effective area due to bend loss
constraints. Optical fibers with reduced effective area generally
show increased and unwanted non-linear effects including four-wave
mixing as well as self- and cross-phase modulation (SPM, XPM). For
Raman amplified systems, too small effective area exacerbates the
issues of "Raman gain tilt" whereby shorter wavelength pumps
(signals) transfer energy to longer wavelength pumps (signals).
[0014] Thus the manufacture of optical fibers for high bit rate
(e.g. 40 Gb/s) systems and with both low dispersion slope and
medium or large effective area, while at the same time preserving
other performance characteristics such as low Polarization Mode
Dispersion (PMD), is a design challenge.
STATEMENT OF THE INVENTION
[0015] Trade-offs in the parameters just noted to yield improved
optical transmission performance have been achieved. The improved
optical fiber of the invention exhibits these characteristics:
[0016] Dispersion at 1550 nm: 5-8.5, pref. 6.5-7.8, and pref.
7.3.+-.2.0, ps/nm-km
[0017] Dispersion slope at 1550 nm: <0.045 ps/n m.sup.2-km,
pref. <0.042 ps/nm.sup.2-km
[0018] Effective area at 1550 nm: >50, pref. 54-62, and pref.
54-58 .mu.m.sup.2
[0019] Cable cutoff wavelength: <1410 nm
[0020] Microbend loss (32 mm) at 1625 nm: <5.times.10.sup.3
dB/km
[0021] Zero dispersion wavelength: <1400 nm
[0022] This set of properties in general represents optical fibers
with moderate chromatic dispersion and moderate effective area to
minimize non-linear effects, and low dispersion slope for ease of
precise, wideband, dispersion compensation. It also represents
fibers designed for distributed Raman amplification and/or S-, C-,
and L-band operation while at the same time being compatible with
erbium-doped fiber amplifiers (EDFAs).
[0023] A variety of optical fiber refractive index profiles that
produce these transmission characteristics have been designed. In
general these have a complex core comprising an up-doped central
core (usually referred to as the core), surrounded by a down-doped
region (usually referred to as the trench), further surrounded by
an up-doped region (referred to as the ring). A similar basic
profile (but with different performance characteristics) is
described and claimed in U.S. Pat. No. 5,878,182, and U.S. Pat. No.
5,905,838, which are incorporated herein by reference. In advanced
fiber designs, the profile may also have a second down-doped
trench, width a width of, for example, 2-10 microns, added either
just outside the ring, or farther out in the cladding, in order to
adjust the cutoff wavelength, and reduce microbending loss.
[0024] In the preferred manufacturing method, the size of the core
region is adjusted during manufacture to achieve the design
objectives. The optical parameters of the fiber are specifically
designed so that the dispersion characteristics of the fiber are
selectively dependent on the core radius. This allows the
dispersion characteristics to be selectively adjusted by changing
the core radius during manufacture. The preferred manufacturing
methods employ rod-in-tube techniques for making the optical fiber
preform. After forming the core rod, it rod may be subjected to a
plasma treatment, or other suitable process step, to modify the
core diameter.
[0025] In a preferred embodiment of the invention the core rod is
made using an MCVD method and the core consequently exhibits a
so-called alpha profile, with an index that varies with core
radius, and a maxima in the center half of the core (the first half
of the core radius measured from the core center). However, optical
fibers meeting the performance parameters of the invention, may
have any suitable refractive index profile, and may be manufactured
by any of a variety of methods, such as for example, OVD, VAD,
PCVD, POVD, MCVD. Combinations of the above methods may be used for
fabricating the various regions of the index profile and overclad
regions as appropriate, including the incorporation of
soot-on-glass, soot-on-soot, or glass-on-glass interfaces in the
preform as appropriate.
BRIEF DESCRIPTION OF THE DRAWING
[0026] FIG. 1 is a simplified diagram of an optical communications
system with a Raman optical fiber amplifier and an optical fiber
designed according to the invention;
[0027] FIG. 2 shows an alternative multiple pump arrangement for
the Raman amplifier of FIG. 1;
[0028] FIGS. 3-8 are optical fiber profile designs according to the
invention;
[0029] FIGS. 9 and 10 are schematic representations of a
rod-in-tube process for the manufacture of optical fiber
preforms;
[0030] FIG. 11 is a schematic representation of the step of
adjusting the core diameter according to the invention; and
[0031] FIG. 12 is a schematic representation of a fiber drawing
apparatus useful for drawing preforms made by the invention into
continuous lengths of optical fiber.
DETAILED DESCRIPTION
[0032] Referring to FIG. 1, an optical fiber communications system
is shown with a distributed Raman optical fiber amplifier. The
transmission span 11 represents a fiber of substantial length,
typically in excess of 1 km. It will be evident to those skilled
that the figures in this description are not drawn to scale, and
the elements are schematically shown. For purposes of illustration,
FIG. 1 shows a distributed amplifier where the amplifier medium is
the normal transmission span. For discrete amplifiers, a dedicated
length of optical fiber can be used for the amplification medium.
The length of fiber represented by 11 is typically at least 500 m
in length to allow for the optical interactions that produce signal
amplification. The amplifier is end pumped, and counter pumped, as
shown in the figure, by pump source 13 coupled into the core of the
fiber through coupler shown schematically at 12. The system
transmitter is shown at 14 and the system receiver at 16. A
dispersion compensating module may be included at 15.
[0033] Optical fiber Raman amplifiers operate on the principle that
light scattered in a silica based optical fiber has a wavelength
lower than that of the incident light. Pump photons excite
molecules up to a virtual level (non-resonant state). The excited
molecules quickly decay to a lower energy level (Stoke's case)
emitting signal photons in the process. Because the pump photon is
excited to a virtual level, Raman gain can occur for a pump source
at any wavelength. The difference in energy between the pump and
signal photons is dissipated by the molecular vibrations of the
host material. These vibrational levels determine the frequency
shift and shape of the Raman gain curve. The frequency (or
wavelength) difference between the pump and the signal photon is
called the Stokes shift. In Ge-doped silica fibers, the Stokes
shift at which maximum gain is obtained is .about.13 THz. Due to
the amorphous nature of silica the Raman gain curve is fairly broad
in optical fibers.
[0034] In a telecommunication system that contains multiple signal
wavelengths Raman pumps at several different wavelengths may be
used to amplify the signals, since Raman scattering can occur at
any wavelength. The gain seen by a given wavelength is the
superposition of the gain provided by all the pumps taking into
account the transfer of energy between the pumps due to Raman
scattering. By properly weighting the power provided at each of the
Raman pump wavelengths it is possible to obtain a signal gain
versus wavelength profile in which there is a small difference
between the gain seen by different signal wavelengths. This
difference is called the gain ripple or gain flatness, and may be
expressed in dB as (Gmax-Gmin).
[0035] FIG. 1 shows a Raman amplifier system using a single,
counter pump. The system may also be co-pumped, with pumps from
either direction. The advantages of this approach was recently
pointed out in (U.S. Pat. No. 6,163,630). Multiple co- and counter
pumps may be multiplexed to improve gain flatness as previously
described. In multiple order Raman pumping the signal light is
greater than 1.5 Stokes shift away from the maximum gain frequency
of the pump light. As an example in 2.sup.nd order pumping, a pump
wavelength 2 Stokes shifts away from the signal light is used to
pump a 1.sup.st order Stokes pump that is 1 Stokes shift away from
the signal light. This is illustrated in FIG. 2, where both a
1.sup.st and 2.sup.nd order pump are counter pumped relative to the
signal light. It takes a finite length of fiber for the 2.sup.nd
order pump to be converted to the 1st order pump. The 1.sup.st
order pump then pumps the signal. This then allows the signal
amplification to occur closer to the signal input end of the fiber.
Multiple order pumping is advantageous because in first order Raman
pumping the pump generally travels in the opposite direction of the
signal. Most of the amplification occurs near the signal output end
of the transmission span. At this position in the fiber the signal
power has already significantly degraded. If the Raman gain seen in
the fiber can occur closer to the signal input end of the fiber an
improved signal to noise ratio (SNR) and noise figure (NF) is
obtained. The power needed for a second order pump is fairly
modest. In one example of a dual order pumped system, the power
ratio for a 1366/1455 nm pump was 970/10 mW respectively.
[0036] The use of multiple pumps, however, introduces the problem
of four wave mixing (FWM). Four-wave mixing occurs when photons of
two or more waves combine to create photons at other frequencies.
The new frequencies are determined as such that total energy and
momentum (phase matching) is conserved. FWM may result from
non-linear interaction between two or more pump wavelengths.
[0037] In a telecommunications system, spurious wavelength
components resulting from FWM in the signal band may lead to
transmission errors. Unlike Raman scattering in which the phase
matching conditions are automatically satisfied, the efficiency of
FWM depends on a proper choice of frequencies and refractive
indices. There are three contributions to the phase mismatch;
material dispersion, waveguide dispersion and fiber nonlinearity.
By adjusting the location of the zero dispersion wavelength (hence
the waveguide dispersion) of the fiber, FWM can be controlled, and
in many cases, practically eliminated. In general, it is desirable
to have the zero of dispersion at a wavelength shorter than the
shortest wavelength pump, so that the dispersion is greater than or
equal to 1 or 2 ps/nm/km over the entire region of Raman pumps and
signals.
[0038] An effective approach towards reducing FWM, SPM, XPM, and
interband stimulated Raman effects is to minimize the non-linear
properties of the optical fiber itself. This may be achieved by
increasing the effective area, A.sub.eff, of the optical fiber. In
doing this, a variety of trade-offs should be considered, including
bend losses and cutoff. In general, there is a limit beyond which
increasing the effective area, while maintaining acceptable bend
losses and cutoff, will sacrifice lower dispersion slope and its
attendant benefits previously discussed.
[0039] It has been noted that the particular dispersion vs.
wavelength curve of an optical fiber design determines how
precisely chromatic dispersion can be compensated over a wideband,
especially in the case of single-mode dispersion compensating fiber
solutions. A relevant and useful parameter is the ratio of the
dispersion slope to the dispersion at the central wavelength of the
signal band (here called the "relative dispersion slope" or RDS).
If the RDS of the cabled transmission fiber (typically with
positive dispersion) is equal to the RDS of the negative dispersion
compensating fiber (typically housed in a module, but can be cabled
as well), then precise cancellation of dispersion can be achieved
over a wide range (e.g. .+-.15 to 20 nm). In general the best
results can be achieved when the transmission fiber has a
relatively low value of RDS. First, it is difficult to fabricate
dispersion compensating fibers with high RDS. Second, the
dispersion vs. wavelength relation for high RDS fibers are
typically more curved than for lower RDS fibers, leading to
significant error in compensation for 40 Gb/s transmission.
[0040] SSMF fiber has RDS .about.0.0033/nm at 1550 nm, defining the
lower end of the scale. Commercial NZDF fibers with Aeff>50 sq
microns have RDS ranging from 0.0065/nm to 0.02/nm at 1550 nm, with
full C-band compensation proving very difficult to realize for the
high end values of RDS. The present invention advances the state of
the art by reducing RDS across the S-, C-, and L-bands, supporting
innovation in wideband dispersion compensation solutions. In
addition to enabling more precise wide band compensation across the
C and L bands, the very low RDS .about.0.0050/nm at 1570 makes
possible combined C+L band module, while the low RDS
.about.0.0075/nm at 1510 nm enables compensation in the upper
S-band. A desirable prescription for RDS over the bands of interest
is:
[0041] 0.0064 to 0.0082 per nm at a wavelength of approximately
1510 nm (S-band application)
[0042] 0.0046 to 0.0058 per nm at a wavelength of approximately
1550 nm (C-band application)
[0043] 00.42 to 00.54 per nm at a wavelength of approximately 1570
nm (C+L band compensation with combined module)
[0044] 0.0038 to 0.0048 per nm at a wavelength of approximately
1590 nm (L-band application)
[0045] It is important not only to compensate the nominal chromatic
dispersion of the fiber, but also to minimize the usual
manufacturing variability that leads to dispersion non-uniformity
along the length of the fiber and between pieces of the same fiber
type. Typical commercial production specifications on dispersion at
1550 nm range from .+-.0.75 to 1.25 ps/nm/km. Dispersion varies
because the final index profile in the fiber varies from the ideal
targets, such as those shown in FIGS. 3-8. The impact over many
hundreds of kilometers in a transmission system is an accumulated
error in dispersion compensation, leading to greater cost expended
to trim the dispersion values to specification at the terminals of
the system.
[0046] It is thus of great advantage to minimize manufacturing
variability in dispersion. Most fiber refractive index profile
non-idealities originate in the core rod fabrication. However,
there are practical techniques to correct certain types of errors
prior to the draw process. The present invention facilitates this
goal by designing the refractive index profile to have a pattern of
sensitivities to core index errors that is suitable to correction
by these techniques.
[0047] The index profile parameter most susceptible to control by
these techniques is the core radius. It is generally impractical or
impossible to adjust the refractive index of the core rod after
fabrication. However, it is possible to adjust the core diameter at
an intermediate stage in the core rod fabrication step sequence
prior to overcladding. The latter mechanism is described and
claimed in U.S. patent application Ser. No. 09/567,536, filed May
9, 2000, which is incorporated herein by reference.
[0048] Thus it is beneficial to determine a set of parameters in
which the transmission property of dispersion, the ultimate
parameter to be controlled, will vary strongly with core radius,
but only weakly with other design parameters.
[0049] Generally speaking, assume that Y is a variable denoting an
optical property of the fiber, and X is a variable denoting a
refractive index profile parameter. The derivative of Y with
respect to X, denoted .differential.Y/.differential.X, indicates
the sensitivity of fiber property Y to the index parameter X. The
objective is for dispersion to vary more strongly with core radius
than with any other index profile parameter, and for no other
optical property to be strongly dependent on core radius. Thus:
.differential.D/.differential.w.sub.1>4.times..differential.Y/.differen-
tial.X, where X.noteq.outer radius of core region
[0050] where D denotes dispersion and w.sub.1 denotes outer radius
of the fiber core region. With this condition in place, the core
radius can be adjusted as mentioned to effectively control the
dispersion characteristics of the optical fiber without introducing
variation along the fiber into other target optical properties,
such as dispersion slope or mode field. At the same time it is
desired that other properties, such as refractive index in the
core, have substantially less effect. The condition of relatively
high dependence of dispersion D on core radius a.sub.1 may be
expressed, as just stated, as the ratio
.differential.D/.differential.r, where, with D in ps/nm-km and r in
microns, the ratio desired is at least 5 and preferably 10 or
more.
[0051] Examples of index profiles meeting the requirements of the
invention are shown in FIGS. 3-7. The profiles are shown as preform
design profiles (the preform OD is typically 63 mm). However,
optical fibers produced from these preforms essentially replicate
these profiles, but with smaller dimensions. In all cases, the
properties of optical fibers produced using these preforms fall
within the following prescription:
[0052] Dispersion at 1550 nm: 7.3.+-.1.0 ps/nm-km
[0053] Dispersion slope at 1550 nm: <0.042 ps/nm.sup.2-km, 0.041
typical
[0054] Effective area at 1550 nm: 54-62 .mu.m.sup.2
[0055] Cable cutoff wavelength: <1410 nm
[0056] Microbend loss (32 mm) at 1625 nm: <5.times.10.sup.3
dB/km
[0057] Zero dispersion wavelength: <1400 nm
[0058] The optical fiber profiles basically comprise four regions.
These are shown in FIG. 3, for example, as core region 21, trench
region 22, ring region 23 and cladding 24.
[0059] Core Region
[0060] The core consists of a raised index region extending from
the central axis of the preform to radius a, with the radial
variation of the normalized index difference, .DELTA.r, described
by the equation:
.DELTA.r=.DELTA.(1-(r/a).sup..alpha.)-.DELTA..sub.dip((b-r)/b).sup.y
(1)
[0061] where
[0062] r is the radial position,
[0063] A is the normalized index difference on axis if
.DELTA..sub.dip=0,
[0064] a is the core radius,
[0065] .alpha. is the shape parameter,
[0066] .DELTA..sub.dip is the central dip depth,
[0067] The parameters .DELTA..sub.dip, b, and .gamma., i.e. the
central dip depth, b the central dip width, and the central dip
shape are artifacts of MCVD production methods, and these factors
may be used when MCVD methods are the production choice for the
optical fiber preform. When using other preform fabrication
techniques, for example VAD, there will be no central dip.
[0068] The equation describing the core shape consists of the sum
of two terms. The first term generally dominates the overall shape
and describes a shape commonly referred to as an alpha profile. The
second term describes the shape of a centrally located index
depression (depressed relative to the alpha profile). The core
region generally consists of silica doped with germanium at
concentrations less than 10 wt % at the position of maximum index,
and graded with radius to provide the shape described by equation
(1).
[0069] Nominal values for the above parameters that yield fiber
with the desired transmission properties are:
.DELTA.=0.50%, a=3.51 .mu.m, .alpha.=12, .DELTA..sub.dip=0.35%,
b=1.0 .mu.m, y=3.0
[0070] In general, the range of variation for these parameters may
be:
.DELTA.=0.30.about.0.70%
a=2.0.about.4.5 .mu.m
.alpha.=1.about.15
[0071] The Trench Region
[0072] The trench region is an annular region surrounding the core
region with an index of refraction that is less than that of the
SiO.sub.2 cladding. The index of refraction in this region is
typically approximately constant as a function of radius, but is
not required to be flat. The trench region generally consists of
SiO.sub.2, doped with appropriate amounts of fluorine and germania
to achieve the desired index of refraction and glass defect
levels.
[0073] The nominal trench parameters are:
.DELTA.=-0.21% and width=2.51 .mu.m.
[0074] In general, the range of variation for these parameters may
be:
.DELTA.=-0.25.about.-0.10%
a=4.0.about.8.0 .mu.m
[0075] The Ring Region
[0076] The ring region is an annular region surrounding the trench
region with an index of refraction that is greater than that of the
SiO.sub.2 cladding. The index of refraction in this region is
typically constant as a function of radius, but is not required to
be flat. The ring region generally consists of SiO.sub.2, doped
with appropriate amounts of germania to achieve the desired index
of refraction.
[0077] The nominal ring parameters are:
.DELTA.=0.18% and width=2.0 .mu.m
[0078] In general, the range of variation for these parameters may
be:
.DELTA.=-0.10.about.-0.60%
a=7.0.about.10.0 .mu.m
[0079] The Cladding Region
[0080] The cladding region is an annular region surrounding the
ring, usually consisting of undoped SiO.sub.2. However, internal to
the cladding region may also exist an additional region of fluorine
doped glass, of the appropriate index level and radial dimensions,
to improve bending loss characteristics. The cladding region
generally extends to 62.5 .mu.m radius.
[0081] An idealized preform profile meeting the requirements of the
invention is shown in FIG. 8. Here the core region is shown at 31,
the trench region at 32, the ring region at 33, and the undoped
cladding at 34. The characteristic center dip, not an ideal
feature, is represented by the dashed lines 35.
[0082] The variations of the major transmission properties over the
variation of the index profile parameters for this design are as
follows:
1 (f = D) (f = Aeff) (f = DS) df (df/f .times. 100) df (df/f/
.times. 100) df (df/f/ .times. 100) df/dN1 0.40(5.6) -1.237(-2.2)
0.000(-0.7) df/dW1 1.56(22.2) -0.017(0.0) 0.001(1.6) df/dN2
0.16(2.3) 0.497(0.9) 0.001(3.0) df/dW2 0.05(0.7) -0.580(-1.0)
-0.001(-3.6) df/dN3 -0.17(-2.4) 0.380(0.7) 0.000(0.9) df/dW3
-0.19(-2.7) 0.301(0.5) 0.000(0.1) dD/dW = change in D resulting
from a 0.1 micron change in width (W) dD/dN = change in D resulting
from a 0.0001 change in relative delta (N) D = dispersion at 1550
nm Aeff = effective area at 1550 nm DS = dispersion slope at 1550
nm
[0083] It is evident that the core radius is the dominant parameter
that affects the transmission property of dispersion, while
variations in other profile parameters do not have as much effect.
This means that an intelligent core diameter adjustment to the
fabricated rod may be applied, after it is measured, to correct
errors in the profile. Such adjustments may result in improved
manufacturing yields, lower costs, and better system performance.
The choice of 0.1 micron width variation and 0.0001 as the scale
for index variation in this example places the derivatives with
respect to these two different parameters on equal footing for
comparison, since these levels of variation correspond to the
typical standard deviations for real manufactured fibers.
[0084] It is also evident that the manufacturing expedient just
described, i.e. adjusting core diameter during preform manufacture,
is applicable to a rod-in-tube preform manufacturing process. Those
skilled in the art may develop techniques for adjusting core
diameter in other manufacturing techniques, but making the
adjustment in a rod-in-tube process is the preferred case. This
approach also allows the core and inner cladding regions to be
formed using MCVD, a preferred choice from the standpoint of
quality and performance of the finished fiber.
[0085] Typical rod-in-tube methods are described in conjunction
with FIGS. 9 and 10. It should be understood that the figures
referred to are not necessarily drawn to scale. A cladding tube
representative of dimensions actually used commercially has a
typical length to diameter ratio of 10-15. The core rod 42 is shown
being inserted into the cladding tube 41. The tube 41 may represent
a single tube or several concentric tubes. The rod at this point is
typically already consolidated. The tube may be already
consolidated or still porous. Normally, there exist several common
options for the make-up of the core rod. It may be just the center
core, or it may include one or more of the layers 32, 33 in FIG. 8.
In the embodiment of the invention where the rod consists of just
the core, 31 in FIG. 8, the remaining doped layers may be formed by
one or more cladding tubes. Cladding tubes made with very high
quality glass-forming techniques may be used for trench and ring
layers, as well as the cladding layers.
[0086] After assembly of the rod 42 and tube 41, the combination is
sintered to produce the final preform 43, shown in FIG. 10, with
the core rod 44 indistinguishable from the tube or tubes except for
a small refractive index difference. This may occur either prior to
or during the draw process.
[0087] Since the rod 42 either has only the core region, or
contains the core region, the diameter of the core region may be
adjusted before insertion in the tube. The adjustment is determined
by measuring of the optical characteristics of the core rod to find
the actual deposited core radius characterizing region 31 in FIG.
8, and calculating the difference between the actual and desired
core radius. The desired core radius may be taken from one of the
profiles shown in FIGS. 3-7.
[0088] The core adjustment step is represented by FIG. 11, where
the rod 42, with initial diameter D.sub.1, is processed to change
the rod diameter and produce a rod with diameter D.sub.2. Diameter
D.sub.2 is the desired core rod outer diameter, which produces the
correct diameter of deposited core (region 31 in FIG. 8). The
diameter change may be produced by any suitable method. The outer
core material may be removed by machining or by plasma etching. In
a preferred embodiment, the diameter of the core region of the rod
is modified by traversing the rod with a plasma torch with the rod
under longitudinal stress. This step is procedurally similar to
that used in standard MCVD for tube collapse, and is well known in
the art. The stress may be compressive or tensile, depending on the
result of the measurement just described. This produces a strain
.DELTA.L in the rod length L. Since the amount of material in the
rod is fixed, any strain .DELTA.L will produce a change in the core
diameter. In the case illustrated in FIG. 11, the core radius is
reduced from D.sub.1 to D.sub.2 by stress S applied in a tensile
mode.
[0089] For optimum preform manufacture, essentially all of the
preforms will be processed to "trim" the core diameter to the
values of the specification. The method involves preparing a
"conditional" rod, with diameter D.sub.1, measuring the conditional
rod to determine the rod diameter adjustment value
(D.sub.1-D.sub.2), making the adjustment, and completing the
rod-in-tube method. In some cases variations in optical properties
along the length of the rod will be found. These may be corrected
by changing the stress on the rod as the plasma traverses the
length of the rod, or by changing the plasma torch conditions, or
traverse speed, during traverse.
[0090] When the method used for core diameter adjustment involves
removal of material from the outer surface of the rod (for example,
the shell region represented by D.sub.1-D.sub.2 in FIG. 9) and it
is anticipated that all rods will require adjustment, the diameter
D.sub.1 is preferably made deliberately larger than the
specification, to eliminate the possibility of producing undersized
rods. If the rod is undersized, the rod diameter cannot be
corrected by material removal. However, even in that event, preform
rods can be salvaged by depositing additional core material on the
undersized rod.
[0091] It should be mentioned that plasma treatment of core rods is
desirable, in addition to being the method of choice for diameter
adjustment by stretching/compression, because at least some
material will be removed from the outer surface of the core rod.
This surface typically is contaminated with OH.sup.- and removing
the contaminant by plasma treatment is desirable regardless of
other processing choices.
[0092] The optical fiber preform, as described above, is then used
for drawing optical fiber. FIG. 12 shows an optical fiber drawing
apparatus with preform 81, and susceptor 82 representing the
furnace (not shown) used to soften the glass preform and initiate
fiber draw. The drawn fiber is shown at 83. The nascent fiber
surface is then passed through a coating cup, indicated generally
at 84, which has chamber 85 containing a coating prepolymer 86. The
liquid coated fiber from the coating chamber exits through die 91.
The combination of die 91 and the fluid dynamics of the prepolymer
controls the coating thickness. The prepolymer coated fiber 94 is
then exposed to UV lamps 95 to cure the prepolymer and complete the
coating process. Other curing radiation may be used where
appropriate. The fiber, with the coating cured, is then taken up by
take-up reel 97. The take-up reel controls the draw speed of the
fiber. Draw speeds in the range typically of 1-30 m/sec. can be
used. It is important that the fiber be centered within the coating
cup, and particularly within the exit die 91, to maintain
concentricity of the fiber and coating. A commercial apparatus
typically has pulleys that control the alignment of the fiber.
Hydrodynamic pressure in the die itself aids in centering the
fiber. A stepper motor, controlled by a micro-step indexer (not
shown), controls the take-up reel.
[0093] Coating materials for optical fibers are typically
urethanes, acrylates, or urethane-acrylates, with a UV
photoinitiator added. The apparatus of FIG. 12 is shown with a
single coating cup, but dual coating apparatus with dual coating
cups are commonly used. In dual coated fibers, typical primary or
inner coating materials are soft, low modulus materials such as
silicone, hot melt wax, or any of a number of polymer materials
having a relatively low modulus. The usual materials for the second
or outer coating are high modulus polymers, typically urethanes or
acrylics. In commercial practice both materials may be low and high
modulus acrylates. The coating thickness typically ranges from
150-300 .mu.m in diameter, with approximately 245 .mu.m
standard.
[0094] It should be emphasized that, while the invention has been
described largely in the context of MCVD and rod-in-tube
processing, the actual method used to achieve the results that form
a basis for one aspect of the invention may be selected from a wide
variety of choices. These include, but are not limited to, the use
of OVD, VAD, PCVD, POVD, MCVD and combinations thereof; the use of
different refractive index profiles to achieve the end properties
claimed, and other similar alternatives. These and other additional
modifications of this invention will occur to those skilled in the
art. All deviations from the specific teachings of this
specification that basically rely on the principles and their
equivalents through which the art has been advanced are properly
considered within the scope of the invention as described and
claimed.
* * * * *