U.S. patent application number 09/757414 was filed with the patent office on 2001-12-06 for tunable optical dispersion-slope compensation based on a nonlinearly-chirped bragg grating.
Invention is credited to Lee, Sanggeon, Pan, Zhongqi, Willner, Alan E., Xie, Yong.
Application Number | 20010048788 09/757414 |
Document ID | / |
Family ID | 22639104 |
Filed Date | 2001-12-06 |
United States Patent
Application |
20010048788 |
Kind Code |
A1 |
Xie, Yong ; et al. |
December 6, 2001 |
Tunable optical dispersion-slope compensation based on a
nonlinearly-chirped bragg grating
Abstract
Techniques for using a nonlinearly-chirped fiber Bragg grating
to produce tunable dispersion-slope compensation.
Inventors: |
Xie, Yong; (Fremont, CA)
; Lee, Sanggeon; (Union City, CA) ; Pan,
Zhongqi; (Los Angeles, CA) ; Willner, Alan E.;
(Los Angeles, CA) |
Correspondence
Address: |
FISH & RICHARDSON, PC
4350 LA JOLLA VILLAGE DRIVE
SUITE 500
SAN DIEGO
CA
92122
US
|
Family ID: |
22639104 |
Appl. No.: |
09/757414 |
Filed: |
January 8, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60175146 |
Jan 7, 2000 |
|
|
|
Current U.S.
Class: |
385/37 ;
385/123 |
Current CPC
Class: |
G02B 6/29394 20130101;
G02B 6/2932 20130101; H04B 10/2519 20130101; G02B 6/29395 20130101;
G02B 6/29322 20130101; G02B 6/022 20130101; G02B 6/29317 20130101;
H04B 10/25133 20130101; G02B 6/02085 20130101; G02B 6/29376
20130101 |
Class at
Publication: |
385/37 ;
385/123 |
International
Class: |
G02B 006/34 |
Claims
What is claimed is:
1. A system, comprising: a fiber grating formed in a fiber and
configured to have a spatial grating pattern that changes
nonlinearly along said fiber to exhibit up to at least a
third-order nonlinear dispersion effect so as to produce a tunable
dispersion slope; and a grating control unit coupled to said fiber
grating and operable to adjust a grating parameter of said fiber
grating to tune both dispersion and dispersion slope produced by
said fiber grating.
2. The system as in claim 1, wherein said grating control unit
includes a fiber stretcher that changes a length of said fiber
grating.
3. The system as in claim 1, wherein said fiber grating further
includes a spatial sampling pattern that is formed in said fiber to
overlap and modulate said spatial grating pattern to produce
multiple Bragg reflection bands at different band center
frequencies each with a tunable dispersion slope.
4. The system as in claim 3, wherein said spatial sampling pattern
is selected to produce a band spacing between adjacent Bragg
reflection bands that is different from a channel spacing between
adjacent channels in multiple wavelength-division multiplexed (WDM)
channels.
5. The system as in claim 3, wherein said spatial sampling pattern
is selected to produce a band spacing between adjacent Bragg
reflection bands that changes from channel to channel.
6. A system, comprising: a fiber having a receiving end to a
plurality of wavelength-division multiplexed (WDM) optical channels
with a constant channel spacing in wavelength; a fiber grating
formed in said fiber and configured to have a spatial grating
pattern that changes nonlinearly along said fiber to exhibit up to
at least a third-order nonlinear dispersion effect and a spatial
sampling pattern that overlaps with and modulates said spatial
grating pattern to produce a plurality of Bragg reflection bands
centered at different band center wavelengths that are spaced
differently from said constant channel spacing, said fiber grating
operable to produce a dispersion slope in each Bragg reflection
band that is adjustable when a grating parameter is changed; and a
grating control unit coupled to said fiber grating and operable to
adjust said grating parameter of said fiber grating to tune both
dispersion and said dispersion slope produced by said fiber
grating.
7. The system as in claim 6, wherein said Bragg reflection bands
are evenly spaced from one another.
8. The system as in claim 6, wherein said Bragg reflection bands
have a varying band spacing.
9. The system as in claim 6, wherein said grating control unit
includes a fiber stretcher that changes a length of said fiber
grating.
10. The system as in claim 6, further comprising a dispersion
detection unit that is operable to measure dispersion and
dispersion slope of each optical channel reflected by said fiber
grating, wherein said grating control unit is operable to adjust
said fiber grating in response to said measurement.
11. A method, comprising: providing a fiber grating formed in a
fiber and configured to have a spatial grating pattern that changes
nonlinearly along said fiber to exhibit up to at least a
third-order nonlinear dispersion effect and a spatial sampling
pattern that overlaps with and modulates said spatial grating
pattern to produce a plurality of Bragg reflection bands centered
at different band center wavelengths, said fiber grating operable
to produce a dispersion slope in each Bragg reflection band that is
adjustable when a grating parameter is changed; directing multiple
optical channels into said fiber grating where channel wavelengths
of different optical channels are respectively in different Bragg
reflection bands and are respectively positioned differently with
respect to respective band center wavelengths; adjusting a grating
parameter of said fiber grating to change dispersion slopes
differently at respective different channel wavelengths to negate
different dispersions and different dispersion slopes with respect
wavelength in different channels.
12. The method as in claim 11, wherein said grating parameter
includes a length of said fiber grating.
13. The method as in claim 11, further comprising: measuring a
change in dispersions and dispersion slopes of in said optical
channels reflected from said fiber grating; and further adjusting
said grating parameter accordingly to negate said change in said
dispersions and said dispersion slopes.
14. The method as in claim 11, wherein said optical channels
transmit through a dispersive fiber system before entering said
fiber grating, wherein dispersions and dispersion slopes of said
dispersive fiber system are different for different channels and
change over time.
Description
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/175,146, filed on Jan. 7, 2000.
TECHNICAL FIELD
[0002] This application relates to compensation for optical
dispersion, and more specifically, to techniques and systems for
reducing chromatic dispersion in optical media such as optic fiber
links.
BACKGROUND
[0003] Many optical fibers and other optical transmission media can
exhibit chromatic dispersion when different spectral components at
different wavelengths in an optical signal travel at different
speeds. An optical pulse, which comprises different optical
spectral components, therefore, can be broadened or distorted in
shape after propagation through a distance in such a dispersive
optical medium. This dispersion effect can be undesirable and even
adverse for certain applications such as optical communication
systems where information is encoded, processed, and transmitted
through optical pulses. The pulse broadening caused by the
dispersion can limit the transmission bit rate, the transmission
bandwidth, and other performance factors of the optical
communication systems.
[0004] One way to mitigate the chromatic dispersion in dispersive
optical fibers and other optical transmission media is dispersion
compensation which introduces dispersion in an optical signal to
negate the dispersion accumulated in that optical signal. In a
wavelength-division multiplexed (WDM) optical systems, multiple WDM
optical channels at different wavelengths are simultaneously
transmitted through a fiber system. Since the dispersion in
different WDM channels may be different, it may be desirable to
provide different amounts of dispersion compensation to different
WDM channels at the same time. In addition, since the dispersion in
the WDM channels may vary over time, it may also be desirable to
adjust the dispersion compensation in time for different WDM
channels.
SUMMARY
[0005] One embodiment of the present disclosure includes a fiber
grating formed in a fiber and configured to have a spatial grating
pattern that changes nonlinearly along the fiber to exhibit up to
at least a third-order nonlinear dispersion effect so as to produce
a tunable dispersion slope. A grating control unit may be coupled
to the fiber grating and is operable to adjust a grating parameter
of the fiber grating to tune both dispersion and dispersion slope
produced by the fiber grating.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] FIGS. 1 and 2 illustrate two different modes of operation in
a sampled nonlinearly-chirped fiber grating.
[0007] FIG. 3 illustrates dispersion slope compensation with a
fixed dispersion slope but tunable amount of dispersion based on
the second order nonlinear chirp effect.
[0008] FIG. 4 illustrates tunable dispersion slope compensation
based on the third or higher order nonlinear chirp effect.
[0009] FIG. 5 shows one embodiment of a tunable dispersion slope
compensator.
[0010] FIGS. 6A, 6B, 7, 8A, 8B, 9A, and 9B show measured data from
a 3-channel sampled nonlinearly-chirped fiber grating.
DETAILED DESCRIPTION
[0011] The techniques of this disclosure are based on
nonlinearly-chirped Bragg gratings. See, U.S. Pat. No. 5,982,963 to
Feng et al. A nonlinearly-chirped Bragg grating is a grating that
is formed along an optical waveguide such as an optical fiber and
has a grating parameter n.sub.neff(z) .LAMBDA. (z) that changes
nonlinearly with the position z along the fiber optic axis, where
n.sub.neff (z) is the effective index of refraction and .LAMBDA.(z)
is the period of the grating. In general, each of n.sub.neff(z) and
.LAMBDA.(z) may vary with the position z along the fiber. One way
to implement a nonlinearly-chirped grating parameter
n.sub.neff(z).LAMBDA.(z), for example, is to modulate the amplitude
or phase of the refractive index of the fiber.
[0012] In operation, this nonlinearly-chirped grating reflects
optical waves satisfying a Bragg condition of
.lambda.(z)=2n.sub.neff(z).LAMBDA.(- z). Hence, different spectral
components are reflected back at different positions in grating to
produce different group delays, .tau..sub.gg, which is a function
of z and .lambda.:
.tau..sub.g=.tau..sub.g(z, .lambda.). (1)
[0013] A single Bragg reflection band centered at a center
wavelength .lambda..sub.0 can be generated and the bandwidth,
.DELTA..lambda..sub.FBG, is determined by the chirping range of the
grating parameter n.sub.neff(z).LAMBDA.(z).
[0014] One distinct feature of the nonlinearly-chirped grating is
that, the relative group delays for different spectral components
at different wavelengths are different, that is, the dispersion D
of the grating is a function of the wavelength and is tunable by
adjusting the grating parameter n.sub.neff(z).LAMBDA.(z). For
example, a fiber grating may be stretched or compressed to adjust
its overall length to change the relative group delays of different
reflected spectral components.
[0015] The nonlinear chirp in the grating parameter can be used to
cause the grating dispersion, D, to vary with the wavelength:
D(.lambda.)=D.sup.(1)(.lambda..sub.0)+D.sup.(2)(.lambda..sub.0)(.lambda.-.-
lambda..sub.0)+D.sup.(3)(.lambda.)(.lambda.-.lambda..sub.0).sup.2+
(2)
[0016] where D.sup.(1)(.lambda..sub.0) is the linear dispersion of
the grating, D.sup.(2)(.lambda..sub.0), D.sup.(3)(.lambda..sub.0),
and other terms represent nonlinear dispersion terms of the grating
caused by the nonlinear chirping. The dispersion slope with respect
to wavelength, hence, can be written as: 1 slope = D ( ) = D ( 2 )
( 0 ) + 2 D ( 3 ) ( 0 ) ( - 0 ) + ( 3 )
[0017] The 2.sup.nd order nonlinear dispersion
D.sup.(2))(.lambda..sub.0) provides a constant dispersion slope for
all different spectral components, the 3.sup.rd order nonlinear
dispersion D.sup.(3))(.lambda..sub.0) (or other high order
nonlinear dispersion) provides a varying dispersion slope that
changes with wavelength. The following describes techniques that
use one or more such nonlinear dispersion effects to produce
tunable dispersion at multiple wavelengths.
[0018] A nonlinearly-chirped grating can be sampled to produce
multiple Bragg reflection bands at multiple center wavelengths.
See, e.g., U.S. patent application Ser. No. 09/253,645 by Jin-Xing
Cai et al. In essence, a spatial sampling pattern is also formed in
the fiber to overlap with the underlying nonlinearly-chirped
grating structure. The sampling pattern has a sampling period
greater than the varying grating period .LAMBDA.(z) and may also be
spatially chirped. The coupling of the nonlinearly-chirped grating
and the sampling pattern produces multiple Bragg reflection windows
or bands at different wavelengths. The number of bands and the band
spacing are determined by the modulation of the spatial sampling
pattern. The bandwidth of each band is determined by the chirping
range of the grating parameter n.sub.neff(z).LAMBDA.(z).
[0019] FIG. 1 illustrates the dispersion of four adjacent Bragg
reflection bands 101, 102, 103, and 104 produced by a sampled
nonlinearly-chirped grating that are centered at wavelengths
.lambda..sub.B1, .lambda..sub.B2, .lambda..sub.B3, and
.lambda..sub.B4, respectively. Curves 110, 120, 130, and 140 are
essentially identical and represent the dispersion curves of the
Bragg reflection bands 101, 102, 103, and 104 according to Eq. (2).
This sampled nonlinearly-chirped grating can be used to
simultaneously control the dispersion at multiple WDM channels by
placing different WDM channels into different Bragg reflection
bands in the wavelength domain. As illustrated by the example in
FIG. 1, four adjacent WDM channels .lambda..sub.1, .lambda..sub.2,
.lambda..sub.3, and .lambda..sub.4 are respectively fit into the
different Bragg reflection bands 101, 102, 103, and 104.
[0020] If each different WDM channel were located at the same
relative spectral position from the center of the respective Bragg
reflection band as with other WDM channels, then the dispersion
produced at different channels would be the same (e.g.,
D1=D2=D3=D4) because the dispersion curves in different bands are
essentially the same. When the grating is stretched or compressed
or otherwise tuned by controlling the grating parameter
n.sub.neff(z).LAMBDA.(z), although the dispersions of spectral
components within each WDM channel are changed, the overall
dispersions of different WDM channels are changed by approximately
the same amount (e.g., D1'=D2'=D3'=D4'). This result may be
undesirable in some applications because the dispersions of
different WDM channels may be different and hence need to be
compensated differently. In particular, different WDM channels,
after transmission through some fiber systems, may accumulate
different fiber dispersion slopes in additional to different
dispersions.
[0021] A sampled nonlinearly-chirped fiber grating, when properly
designed, can produce both tunable dispersion and tunable
dispersion slope for different WDM channels. First, the fiber
grating is designed to place different WDM channels at different
spectral positions with respect to the respective centers of the
different Bragg reflection bands. Secondly, the nonlinear chirp of
the fiber grating is designed to produce the third-order dispersion
D.sup.(3))(.lambda..sub.0) or higher order nonlinear dispersion
effects.
[0022] FIG. 2 shows an example of the first condition where 4
different WDM channels .lambda..sub.1, .lambda..sub.2,
.lambda..sub.3, and .lambda..sub.4 are at 4 different positions
relative to the respective band centers to produce different
dispersions: D1.noteq.D2.noteq.D3.noteq- .D4. When the input WDM
channels are evenly spaced in the wavelength domain, this condition
can, be achieved by using sampled nonlinearly-chirped grating with
evenly-spaced Bragg reflection bands that has a band spacing
.DELTA..lambda..sub.FBG different from the channel spacing
.DELTA..lambda..sub.Ch. Alternatively, the sampling pattern of the
grating may be designed to produce Bragg reflection bands that are
not uniformly spaced in the wavelength domain to have a varying
band spacing that change from channel to channel.
[0023] The second condition is illustrated by FIGS. 3 and 4 where
different nonlinearly-chirped dispersion terms in the above sampled
fiber grating are shown to produce different effects on the
tunability of dispersion.
[0024] The first nonlinear term, the 2.sup.nd order nonlinear
dispersion D.sup.(2))(.lambda..sub.0) represents a constant
dispersion slope for all wavelengths. When the nonlinear chirp of
the fiber grating is specifically designed to primarily exhibit the
D.sup.(2)) (.lambda..sub.0) effect and the higher nonlinear
dispersion effects are negligible, the dispersion curve in the
wavelength domain represented by Eq. (2) is a liner curve with a
constant dispersion slope. Hence, as illustrated in FIG. 3, when
the fiber grating is stretched or compressed, the Bragg reflection
band positions are shifted in the wavelength domain. This shifts
the dispersion produced on each channel. When the relative
positions of different channels with respect to respective band
centers are different, the total dispersions at different channels
are different. The dispersion slope for different channels,
however, remains as a constant. Therefore, the
D.sup.(2)(.lambda..sub.0) effect can be used to produce a tunable
dispersion with a constant dispersion slope. The dispersion slope
is independent of the stretch or compression and is not
tunable.
[0025] In contrast, the D.sup.(3))(.lambda..sub.0) effect or higher
nonlinear effects can provide not only a tunable dispersion as by
the D.sup.(2)(.lambda..sub.0) effect but also a tunable dispersion
slope as indicated by Eq. (3). FIG. 4 illustrates the operation of
the tunable dispersion slope by the 3.sup.rd order or higher order
nonlinear effect. Notably, the grating dispersion is now a
nonlinear curve due to the D.sup.(3)(.lambda..sub.0) effect or
higher nonlinear chirp effects. As a result, when the fiber grating
is stretched or compressed, the dispersion at each channel changes
due to the shifts of the reflection bands. In addition, the
dispersion slope changes with the fiber stretching or compression.
The D.sup.(3)(.lambda..sub.0) effect or higher nonlinear chirp
effects hence can be used to adjust the dispersion slope produced
by the fiber grating to negate the varying dispersion slope
accumulated in a received WDM channel after transmission through a
dispersive fiber system.
[0026] FIG. 5 shows one example of a dispersion-slope compensator
500 for a WDM system based on a sampled nonlinearly-chirped fiber
grating 501. The input port 1 of the circulator 520 is coupled to
receive an input signal 510 of multiple WDM channels from a
dispersive WDM fiber system 511. An optical coupler or a beam
splitter 530 is used to tap a small fraction of the reflected
signal 512 with multiple channels for dispersion measurements in a
dispersion monitor device 532. The device 532 measures the
dispersion and the dispersion slope in the reflected channels and
produces a dispersion indicator. A grating control 534 is coupled
to control the nonlinearly-chirped grating parameter nA as a
function of the position z. In addition, the control 534 is
operable to control the grating parameter nA according to the
dispersion monitor from the device 532. A number of implementations
of the grating control 534 are described in U.S. Pat. No. 5,982,963
to Feng et al., including a fiber stretcher engaged to the fiber
grating and a control circuit that supplies a control signal to the
fiber stretcher to vary the total length of the fiber. As the
dispersion in the input signal 510 varies, the device 500 can
respond to the changing dispersion in the fiber system 511 by
dynamically adjusting the nonlinearly-chirped sampled grating 501
accordingly to change the amount of dispersion and associated
dispersion slope for each reflected WDM channel.
[0027] A 3-channel sampled nonlinearly-chirped grating was used to
demonstrate the above dispersion slope compensation. FIGS. 6A and
6B show the measured reflection bands and dispersion of the
grating, respectively. The band spacing .DELTA..lambda..sub.FBG of
the three Bragg reflection bands is about 4 nm. Three dispersive
optical channels at 1559.7 nm, 1554.8 nm, and 1549.2 nm with a
channel spacing .DELTA..lambda..sub.Ch of about 5 nm were generated
with a pseudorandom bit stream (PRBS) 2.sup.15-1 and modulated at a
bit rate of 10 Gb/s. FIG. 7 shows the dispersion map of the optical
channels at different travel distances in a dispersive fiber
line.
[0028] FIG. 8A show compensated output channels from the fiber
grating after the channels transmitted through a 900-km dispersive
fiber line. When the transmission fiber line was increased to 1200
km to add additional dispersion to the transmitted channels, the
fiber grating was stretched to increase the grating dispersion by
shifting each Bragg reflection band by about 0.8 nm. FIG. 8B shows
the compensated output channels. FIGS. 9A and 9B respectively show
the measured bit error rates. The measurements show that the
3-channel grating is used to produce a high dispersion compensation
for the most severely dispersed channel at 1549.2 nm and a low
dispersion for the 1559.7 nm channel close to the zero-dispersion
wavelength for the fiber line.
[0029] Therefore, sampled nonlinearly-chirped fiber gratings may be
designed to have desired nonlinear chirp and band spacing to allow
sensitive dispersion tuning of dispersion at different channels.
The third or higher order nonlinear effects of such gratings can be
used to further improve the tuning sensitivity. In real fiber
systems where the zero dispersion wavelength is usually allocated
near the center of the transmission band, two sampled
nonlinearly-chirped gratings with opposite dispersions may be used
to compensate for channels below and above the zero dispersion
wavelength, respectively. Hence, an optical filter may be used to
separate the input channels and direct the proper channels to
proper gratings for compensation.
[0030] Although the present disclose only includes a few examples,
it is understood that various modifications and enhancements may be
made without departing from the following claims.
* * * * *