U.S. patent application number 09/827819 was filed with the patent office on 2001-09-13 for tuning of optical dispersion by using a tunable fiber bragg grating.
This patent application is currently assigned to University of Southern California, non-profit organization. Invention is credited to Cai, Jinxing, Feng, Kai-Ming, Khosravani, Reza, Lee, Sanggeon, Peng, Jiangde, Willner, Alan E..
Application Number | 20010021294 09/827819 |
Document ID | / |
Family ID | 22961132 |
Filed Date | 2001-09-13 |
United States Patent
Application |
20010021294 |
Kind Code |
A1 |
Cai, Jinxing ; et
al. |
September 13, 2001 |
Tuning of optical dispersion by using a tunable fiber bragg
grating
Abstract
Techniques and devices based on a wave-guiding element which has
a spatial grating pattern that is an oscillatory variation along
its optic axis. The wave-guiding element is configured to receive
an input optical signal and to produce an output optical signal by
reflection within a Bragg reflection band produced by the spatial
grating pattern so as to produce time delays of different reflected
spectral components as a nonlinear function of spatial positions
along said optic axis at which the different reflected spectral
components are respectively reflected. Such a wave-guiding element
may be a nonlinearly chirped fiber grating A control unit may be
engaged to the wave-guiding element and is operable to change a
property of the spatial grating pattern along the optic axis to
tune at least relative time delays of the different reflected
spectral components nonlinearly with respect to wavelength.
Inventors: |
Cai, Jinxing; (Ocean,
NJ) ; Lee, Sanggeon; (Union City, CA) ; Feng,
Kai-Ming; (Milpitas, CA) ; Willner, Alan E.;
(Los Angeles, CA) ; Peng, Jiangde; (Beijing,
CN) ; Khosravani, Reza; (Fremont, CA) |
Correspondence
Address: |
SCOTT C. HARRIS
Fish & Richardson P.C.
Suite 500
4350 La Jolla Village Drive
San Diego
CA
92122
US
|
Assignee: |
University of Southern California,
non-profit organization
|
Family ID: |
22961132 |
Appl. No.: |
09/827819 |
Filed: |
April 6, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09827819 |
Apr 6, 2001 |
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09253645 |
Feb 19, 1999 |
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09253645 |
Feb 19, 1999 |
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09027345 |
Feb 20, 1998 |
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5982963 |
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60069498 |
Dec 15, 1997 |
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Current U.S.
Class: |
385/37 ;
385/39 |
Current CPC
Class: |
H01S 3/0057 20130101;
G02B 6/2932 20130101; G02B 6/29395 20130101; G02B 6/29322 20130101;
G02B 6/02109 20130101; G02B 6/29394 20130101; G02F 1/0126 20130101;
G02F 1/0115 20130101; H04B 2210/258 20130101; G02B 6/02138
20130101; G02F 1/011 20130101; H04B 10/2519 20130101; G02B 6/4215
20130101; G02F 2201/307 20130101; G02B 6/022 20130101; G02F 1/0134
20130101; G02B 6/02085 20130101 |
Class at
Publication: |
385/37 ;
385/39 |
International
Class: |
G02B 006/34 |
Claims
What is claimed is:
1. A device, comprising: a wave-guiding element having (1) an optic
axis to transport optical energy along said optic axis and (2) a
spatial grating pattern which is an oscillatory variation along
said optic axis, said wave-guiding element configured to receive an
input optical signal and to produce an output optical signal by
reflection within a Bragg reflection band produced by said spatial
grating pattern so as to produce time delays of different reflected
spectral components as a nonlinear function of spatial positions
along said optic axis at which said different reflected spectral
components are respectively reflected; and a control unit engaged
to said wave-guiding element and operable to change a property of
said spatial grating pattern along said optic axis to tune at least
relative time delays of said different reflected spectral
components nonlinearly with respect to wavelength.
2. The device as in claim 1, wherein said control unit is
configured to control a length of said wave-guiding element along
said optic axis.
3. The device as in claim 2, wherein said control unit includes a
piezoelectric element.
4. The device as in claim 2 wherein said control unit includes a
magnetostrictive element that operates in response to a control
magnetic field.
5. The device as in claim 1, wherein said control unit is
configured to generate a varying control electrical field along
said optic axis and said wave-guiding element is configured to have
an index of refraction that changes in response to said varying
control electrical field so as to tune said relative time
delays.
6. The device as in as in claim 1, wherein said control unit is
configured to generate a varying control electromagnetic radiation
field along said optic axis and said wave-guiding element is
configured to have an index of refraction that changes in response
to said electromagnetic radiation field so as to tune said relative
time delays.
7. The device as in claim 1, wherein said control unit includes an
acoustic wave generator configured and coupled to produce a
frequency-tunable acoustic wave along said optic axis of said
wave-guiding element so that said acoustic wave alters a frequency
response of said wave-guiding element.
8. The device as in claim 1, wherein said control unit is
configured to control both a length and a refractive index of said
wave-guiding element along said optic axis.
9. The device as in claim 1, wherein said control unit is
configured to control a refractive index of said wave-guiding
element along said optic axis.
10. The device as in claim 1, further comprising: a dispersion
monitor unit configured and coupled to monitor information of
optical dispersion in said output signal and coupled to inform said
control unit of said information, wherein said control unit is
operable to adjust said property of said spatial grating pattern in
response to said information.
11. The device as in claim 1, wherein said wave-guiding element
includes an optical fiber having a fiber core and a fiber cladding
surrounding said fiber core.
12. The device as in claim 1, wherein said control unit is
configured to control a temperature of said wave-guiding
element.
13. The device as in claim 1, wherein said wave-guiding element
includes an optical waveguide formed on a substrate.
14. The device as in claim 1, where said wave-guiding element is
formed of an optical birefringent material to have two orthogonal
principal polarization axes that are substantially perpendicular to
said optic axis.
15. The device as in claim 1, wherein said wave-guiding element
further includes a spatial sampling pattern that spatially overlaps
with said spatial grating pattern along said optic axis and
includes a periodic modulation with a period greater than a grating
period of said spatial grating pattern so that said wave-guiding
element is operable to produce a plurality of Bragg reflection
bands at different wavelengths.
16. The device as in claim 1, wherein said spatial grating pattern
has a grating period that is nonlinearly chirped along said optic
axis.
17. The device as in claim 1, wherein said spatial grating pattern
includes a spatial nonlinear chirp in one aspect of an index of
refraction of said wave-guiding element along said optic axis.
18. A system, comprising: a plurality of optical devices connected
in series so that an optical transmission output from one optical
device is received by another adjacent optical device located in a
downstream of said optical output, wherein each optical device is
configured to be independently controlled and includes: a
wave-guiding element having (1) an optic axis to transport optical
energy along said optic axis and (2) a spatial grating pattern
which is an oscillatory variation along said optic axis, said
wave-guiding element configured to receive an input optical signal
and to produce (1) an output optical signal by reflection within a
Bragg reflection band produced by said spatial grating pattern so
as to produce time delays of different reflected spectral
components as a nonlinear function of spatial positions along said
optic axis at which said different reflected spectral components
are respectively reflected, and (2) an output transmission optical
signal having spectral components that are not reflected, and a
control unit engaged to said wave-guiding element and operable to
change a property of said spatial grating pattern along said optic
axis to tune at least relative time delays of said different
reflected spectral components nonlinearly with respect to
wavelength, wherein different spatial grating patterns in different
optical devices are configured to produce Bragg reflection bands at
different wavelengths.
19. The system as in claim 18, wherein said control unit in each
optical device is configured to control a length of said
wave-guiding element along said optic axis.
20. The system as in claim 18, wherein said control unit in each
optical device is configured to control a refractive index of said
wave-guiding element along said optic axis.
21. The system as in claim 18, wherein said control unit in each
optical device is configured to control both a length and a
refractive index of said wave-guiding element along said optic
axis.
22. The system as in claim 18, further comprising: a dispersion
monitor unit configured and coupled to monitor information of
optical dispersion in a final optical output signal from said
plurality of optical devices and coupled to communicate said
information to at least one of said plurality of optical devices,
wherein said control unit in said at least one optical device is
operable to adjust said property of said spatial grating pattern of
said wave-guiding element in response to said information to alter
optical dispersion in said final optical output signal.
23. The system as in claim 18, wherein said wave-guiding element in
each optical device includes an optical fiber having a fiber core
and a fiber cladding surrounding said fiber core, and further
comprising interconnecting optical fibers to interconnect said
plurality of optical devices.
24. The system as in claim 18, wherein said control unit in at
least one optical device is configured to control a temperature of
said wave-guiding element to achieve said nonlinear tuning.
25. The system as in claim 18, wherein said wave-guiding element in
at least one optical device includes an optical waveguide formed on
a substrate.
26. The system as in claim 18, where said wave-guiding element in
at least one optical device is formed of an optical birefringent
material to have two orthogonal principal polarization axes that
are substantially perpendicular to said optic axis.
27. The system as in claim 18, wherein said spatial grating pattern
has a grating period that is nonlinearly chirped along said optic
axis.
28. The system as in claim 18, wherein said spatial grating pattern
includes a spatial nonlinear chirp in one aspect of an index of
refraction of said wave-guiding element along said optic axis.
29. A system, comprising: an input optical fiber carry a plurality
of optical WDM channels; a WDM unit coupled to said input optical
fiber and configured to spatially separate said WDM channels; a
tunable dispersion module, connected to said WDM unit to receive
said WDM channels and operable to change dispersions of said WDM
channels to produce modified WDM channels, said dispersion module
comprising a plurality of optical devices which are coupled in
parallel with respect to one another to said WDM unit to receive
said WDM channels and to produce said modified WDM channels,
wherein each optical device includes: a wave-guiding element having
(1) an optic axis to transport optical energy along said optic axis
and (2) a spatial grating pattern which is an oscillatory variation
along said optic axis, said wave-guiding element configured to
receive an input optical signal and to produce an output optical
signal by reflection within a Bragg reflection band produced by
said spatial grating pattern so as to produce time delays of
different reflected spectral components as a nonlinear function of
spatial positions along said optic axis at which said different
reflected spectral components are respectively reflected, and a
control unit engaged to said wave-guiding element and operable to
change a property of said spatial grating pattern along said optic
axis to tune at least relative time delays of said different
reflected spectral components nonlinearly with respect to
wavelength, wherein different spatial grating patterns in different
optical devices are configured to produce Bragg reflection bands at
different wavelengths.
30. The system as in claim 29, wherein said control unit in each
optical device is configured to control a length of said
wave-guiding element along said optic axis.
31. The system as in claim 29, wherein said control unit in each
optical device is configured to control a refractive index of said
wave-guiding element along said optic axis.
32. The system as in claim 29, wherein said control unit in each
optical device is configured to control both a length and a
refractive index of said wave-guiding element along said optic
axis.
33. The system as in claim 29, further comprising: a dispersion
monitor unit configured and coupled to monitor information of
optical dispersion in said modified WDM channels and coupled to
communicate said information to at least one of said plurality of
optical devices, wherein said control unit in said at least one
optical device is operable to adjust said property of said spatial
grating pattern of said wave-guiding element in response to said
information to alter optical dispersion in said modified WDM
channels.
34. The system as in claim 29, wherein said wave-guiding element in
each optical device includes an optical fiber having a fiber core
and a fiber cladding surrounding said fiber core, and further
comprising interconnecting optical fibers to interconnect said
plurality of optical devices to said WDM unit.
35. The system as in claim 29, wherein said control unit in at
least one optical device is configured to control a temperature of
said wave-guiding element to achieve said nonlinear tuning.
36. The system as in claim 29, wherein said wave-guiding element in
at least one optical device includes an optical waveguide formed on
a substrate.
37. The system as in claim 29, where said wave-guiding element in
at least one optical device is formed of an optical birefringent
material to have two orthogonal principal polarization axes that
are substantially perpendicular to said optic axis.
38. The system as in claim 29, wherein said spatial grating pattern
has a grating period that is nonlinearly chirped along said optic
axis.
39. The system as in claim 29, wherein said spatial grating pattern
includes a spatial nonlinear chirp in one aspect of an index of
refraction of said wave-guiding element along said optic axis.
40. A system, comprising: a laser configured to produce a laser
beam; a wave-guiding element having (1) an optic axis to transport
optical energy along said optic axis and (2) a spatial grating
pattern which is an oscillatory variation along said optic axis,
said wave-guiding element positioned to receive said laser beam
from said laser and to produce an output laser beam by reflection
within a Bragg reflection band produced by said spatial grating
pattern so as to produce time delays of different reflected
spectral components as a nonlinear function of spatial positions
along said optic axis at which said different reflected spectral
components are respectively reflected; and a control unit engaged
to said wave-guiding element and operable to change a property of
said spatial grating pattern along said optic axis to tune at least
relative time delays of said different reflected spectral
components nonlinearly with respect to wavelength.
41. The system as in claim 40, wherein said laser is a pulsed
laser.
42. The system as in claim 40, further comprising a substrate,
wherein said laser and said wave-guiding element are integrated on
said substrate.
43. The system as in claim 42, wherein said wave-guiding element is
an optical waveguide formed on said substrate.
44. The system as in claim 40, wherein said spatial grating pattern
has a grating period that is nonlinearly chirped along said optic
axis.
45. The system as in claim 40, wherein said spatial grating pattern
includes a spatial nonlinear chirp in one aspect of an index of
refraction of said wave-guiding element along said optic axis.
46. A method, comprising: designing a wave-guiding element to have
(1) an optic axis to transport optical energy along said optic axis
and (2) a spatial grating pattern which is an oscillatory variation
along said optic axis so that said wave-guiding element operates to
produce an output optical signal by reflection within a Bragg
reflection band produced by said spatial grating pattern with time
delays of different reflected spectral components as a nonlinear
function of spatial positions along said optic axis at which said
different reflected spectral components are respectively reflected;
directing an input optical signal into said wave-guiding element to
produce said output optical signal; and controlling said
wave-guiding element to change a property of said spatial grating
pattern along said optic axis so as to tune at least relative time
delays of said different reflected spectral components nonlinearly
with respect to wavelength.
47. The method as in claim 46, further comprising setting an amount
of change in said property of said spatial grating pattern to
control dispersion in said output optical signal.
48. The method as in claim 46, further comprising setting an amount
of change in said property of said spatial grating pattern so that
grating-induced dispersion in said output optical signal negates
original dispersion present in said input optical signal.
49. The method as in claim 46, wherein said property includes a
length of said wave-guiding element along said optic axis.
50. The method as in claim 49, further comprising engaging a
piezoelectric element to said wave-guiding element to control said
length.
51. The method as in claim 49, further comprising employing a
magnetostrictive element to control said length in response to a
control magnetic field.
52. The method as in claim 46, wherein said wave-guiding element is
configured to have an index of refraction that changes in response
to a varying control electrical field along said optic axis, and
further comprising: generating said varying control electrical
field; applying said varying control electrical field to said
wave-guiding element; and controlling said varying control
electrical field to tune said relative time delays nonlinearly with
respect to wavelength.
53. The method as in as in claim 46, wherein said wave-guiding
element is configured to have an index of refraction that changes
in response to a varying control electromagnetic radiation field
along said optic axis, and further comprising: generating said
varying control electromagnetic radiation field; applying said
varying control electromagnetic radiation field to said
wave-guiding element; and controlling said varying control
electromagnetic radiation field to tune said relative time delays
nonlinearly with respect to wavelength.
54. The method as in claim 46, wherein said property includes a
frequency response of said wave-guiding element, and further
comprising: generating and applying a frequency-tunable acoustic
wave along said optic axis of said wave-guiding element; and
controlling said acoustic wave to alter said frequency response of
said wave-guiding element to achieve said nonlinear tuning.
55. The method as in claim 46, wherein both a length and a
refractive index of said wave-guiding element along said optic axis
are controlled to achieve said nonlinear tuning.
56. The method as in claim 46, wherein a refractive index of said
wave-guiding element along said optic axis is controlled to achieve
said nonlinear tuning.
57. The method as in claim 46, further comprising: obtaining
information of optical dispersion in said output signal; and
adjusting said property of said spatial grating pattern in response
to said information.
58. The method as in claim 57, further comprising using said
information to dynamically adjust grating-induced dispersion
produced by said wave-guiding element in response to a
time-dependent change in dispersion in said input optical
signal.
59. The method as in claim 46, wherein said wave-guiding element
includes an optical fiber having a fiber core and a fiber cladding
surrounding said fiber core.
60. The method as in claim 46, wherein said controlling includes
controlling a temperature of said wave-guiding element to achieve
said nonlinear tuning.
61. The method as in claim 46, wherein said wave-guiding element
includes an optical waveguide formed on a substrate.
62. The method as in claim 46, further comprising: forming said
wave-guiding element by using an optical birefringent material to
have two orthogonal principal polarization axes that are
substantially perpendicular to said optic axis; and controlling
said wave-guiding element to control a polarization-mode dispersion
in said output optical signal.
63. The method as in claim 46, further comprising: designing said
wave-guiding element to include a spatial sampling pattern that
spatially overlaps with said spatial grating pattern along said
optic axis and includes a periodic modulation with a period greater
than a grating period of said spatial grating pattern so that said
wave-guiding element is operable to produce a plurality of Bragg
reflection bands at different wavelengths; and controlling said
wave-guiding element to control dispersion of WDM channels in said
input optical signal.
64. The method as in claim 46, wherein said spatial grating pattern
has a grating period that is nonlinearly chirped along said optic
axis.
65. The method as in claim 46, wherein said spatial grating pattern
includes a spatial nonlinear chirp in one aspect of an index of
refraction of said wave-guiding element along said optic axis.
66. The method as in claim 46, wherein said input optical signal is
a pulsed laser beam from a pulsed laser, and further comprising
controlling said wave-guiding element to control a pulse shape of
said pulsed laser beam.
67. A method, comprising: designing a fiber Bragg grating in a
fiber to have a spatial grating pattern which is an oscillatory
variation along said fiber to produce an output optical signal by
reflection within a Bragg reflection band produced by said spatial
grating pattern so that time delays of different reflected spectral
components are a nonlinear function of spatial positions along said
fiber at which said different reflected spectral components are
respectively reflected; directing an input optical signal into said
fiber Bragg grating to produce said output optical signal;
controlling a property of said spatial grating pattern of said
fiber Bragg grating so as to (1) shift said Bragg reflection in
frequency and (2) tune at least relative time delays of said
different reflected spectral components nonlinearly with respect to
wavelength.
68. The method as in claim 67, wherein both a length and a
refractive index of said fiber Bragg grating along said fiber are
controlled.
69. The method as in claim 67, wherein a refractive index of said
fiber Bragg grating along said fiber is controlled.
70. The method as in claim 67, wherein a length of said fiber Bragg
grating along said fiber is controlled.
71. The method as in claim 67, further comprising setting an amount
of change in said property of said spatial grating pattern to
control dispersion in said output optical signal.
72. The method as in claim 67, further comprising setting an amount
of change in said property of said spatial grating pattern so that
grating-induced dispersion in said output optical signal negates
original dispersion present in said input optical signal.
73. The method as in claim 67, further comprising: obtaining
information of optical dispersion in said output signal; and
adjusting said property of said spatial grating pattern in response
to said information.
74. The method as in claim 73, further comprising using said
information to dynamically adjust grating-induced dispersion
produced by said fiber Bragg grating in response to a
time-dependent change in dispersion in said input optical
signal.
75. The method as in claim 67, wherein said spatial grating pattern
has a grating period that is nonlinearly chirped along said optic
axis.
76. The method as in claim 67, wherein said spatial grating pattern
includes a spatial nonlinear chirp in one aspect of an index of
refraction of said fiber Bragg along said fiber.
77. The method as in claim 67, further comprising: deploying a
plurality of additional fiber Bragg gratings connected in series to
said fiber Bragg grating to receive a transmitted optical signal
from said fiber Bragg grating so that an optical transmission
signal from one additional fiber Bragg grating is received by
another adjacent additional fiber Bragg located in a downstream of
said optical transmission signal, wherein transmission of each
fiber Bragging includes spectral components outside a respective
Bragg reflection band which are not reflected, and wherein each
additional fiber Bragg grating is designed to produce time delays
of different reflected spectral components as a nonlinear function
of spatial positions along each fiber and to have a unique Bragg
reflection band at a center band wavelength different any other
fiber Bragg grating; and controlling each additional fiber Bragg
grating to tune at least relative time delays of said different
reflected spectral components nonlinearly with respect to
wavelength in each respective Bragg reflection band to control
dispersion in respectively reflected optical signals.
78. The method as in claim 67, further comprising: separating WDM
channels received from an input optical fiber; providing additional
fiber Bragg gratings connected in parallel to said fiber Bragg
grating to receive WDM channels from said input optical fiber,
respectively, wherein each additional fiber Bragg grating is
designed to produce time delays of different reflected spectral
components as a nonlinear function of spatial positions along each
fiber and to have a unique Bragg reflection band at a center band
wavelength different any other fiber Bragg grating; selecting a
first WDM channel as said input optical signal directed into said
fiber Bragg grating, wherein said first WDM channel is selected
within said Bragg reflection band said fiber Bragg grating;
selecting and directing other WDM channels to said additional fiber
Bragg gratings so that each WDM channel is within a Bragg
reflection band of a respective Bragg reflection band; controlling
said fiber Bragg grating and each additional fiber Bragg grating to
tune at least relative time delays of different reflected spectral
components nonlinearly with respect to wavelength in each
respective Bragg reflection band to control dispersion in
respectively reflected WDM channels; combining and exporting said
reflected WDM channels in an output optical fiber.
79. The method as in claim 67, wherein said spatial grating pattern
has a grating period that is nonlinearly chirped along said optic
axis.
80. The method as in claim 67, wherein said spatial grating pattern
includes a spatial nonlinear chirp in one aspect of an index of
refraction of said wave-guiding element along said optic axis.
Description
[0001] This application is a continuation application of a
copending U.S. application Ser. No. 09/253,645, filed Feb. 19,
1999, which is a continuation-in-part application of a copending
U.S. patent application Ser. No. 09/027,345, filed on Feb. 20, 1998
and issued as U.S. Pat. No. 5,982,963 on Nov. 9, 1999 which claims
the benefit of the U.S. Provisional Application No. 60/069,498,
filed on Dec. 15, 1997.
FIELD OF THE INVENTION
[0002] The present invention relates to optical dispersion
compensation and optical pulse manipulation, and more specifically,
to devices and systems having an optical grating capable of causing
wavelength-dependent delays.
BACKGROUND
[0003] Dispersion in optical waveguides such as optical fibers
causes optical waves of different wavelengths to travel at
different speeds. One parameter for characterizing the dispersion
is group velocity which is related to the derivative of the
propagation constant of an optical wave with respect to frequency.
The first-order group velocity dispersion is typically expressed as
a change in light propagation time over a unit length of fiber with
respect to a change in light wavelength. For many fibers used in
telecommunication, the first-order group velocity dispersion is on
the order of 10 ps/nm/km at 1550 nm.
[0004] In many applications, an optical signal is composed of
spectral components of different wavelengths. For example, a
single-frequency optical carrier may be modulated in order to
impose information on the carrier. Such modulation generates
modulation sidebands at different frequencies from the carrier
frequency. For another example, optical pulses, which are widely
used in optical data processing and communication applications,
contain spectral components in a certain spectral range. The
dispersion effect may cause adverse effects on the signal due to
the different delays on the different spectral components.
[0005] Dispersion in particular presents obstacles to increasing
system data rates and transmission distances without signal
repeaters in either single-channel or
wavelength-division-multiplexed ("WDM") fiber communication
systems. Data transmission rates up to 10 Gbit/s or higher may be
needed in order to meet the increasing demand in the marketplace.
Dispersion can be accumulated over distance to induce pulse
broadening or spread. Two adjacent pulses in a pulse train thus may
overlap with each other at a high data rate. Such pulse overlapping
can cause errors in data transmission.
[0006] One way to reduce the dispersion effect in fibers is to
implement a fiber grating with linearly chirped grating periods.
The resonant wavelength of the fiber grating changes with the
position due to the changing grating period. Therefore, different
spectral components in an optical signal are reflected back at
different locations and thus have different delays. Such
wavelength-dependent delays can be used to reduce the accumulated
dispersion in a fiber link.
SUMMARY
[0007] The present disclosure includes techniques and devices based
on a wave-guiding element which has a spatial grating pattern that
is an oscillatory variation along its optic axis. The wave-guiding
element is configured to receive an input optical signal and to
produce an output optical signal by reflection within a Bragg
reflection band produced by the spatial grating pattern so as to
produce time delays of different reflected spectral components as a
nonlinear function of spatial positions along said optic axis at
which the different reflected spectral components are respectively
reflected. A control unit may be engaged to the wave-guiding
element and is operable to change a property of the spatial grating
pattern along the optic axis to tune at least relative time delays
of the different reflected spectral components nonlinearly with
respect to wavelength. The dispersion of such a wave-guiding
element can be dynamically adjusted to produce a desired dispersion
with desired relative delays among different spectral components in
a controllable manner.
[0008] One embodiment of the above wave-guiding element is the
nonlinearly-chirped grating which may include a grating that has an
effective index n.sub.eff(x) and the grating period .LAMBDA.(x) are
configured to produce a grating parameter n.sub.eff(x).LAMBDA.(x)
as a nonlinear function of the position along the fiber optic axis.
Such a grating reflects optical waves satisfying a Bragg condition
of .lambda.(x)=2n.sub.eff(x).LAMBDA.(x). A single Bragg reflection
band is generated where the bandwidth is determined by the chirping
range of the grating parameter n.sub.eff(x).LAMBDA.(x).
[0009] A grating tuning mechanism may be implemented by using a
grating control unit to control either the effective index
n.sub.eff(x) or the grating period .LAMBDA.(x). This allows for
adjustment of the grating parameter n.sub.eff(x).LAMBDA.(x) and
thus to the relative delays for signals at different wavelengths
within the bandwidth of the reflection. A transducer, e.g., a
piezoelectric element, may be used as the control unit to compress
or stretch the overall length of the grating in order to produce a
tunable dispersion profile. A magnetostrictive element may also be
used to change the grating length according to an external control
magnetic field. If the grating material is responsive to a
spatially-varying external control field such as an electric field,
an electromagnetic radiation field, or a temperature field along
the grating direction, a control unit capable of producing such
conditions can be used to change effective index of refraction and
to produce a tunable dispersion profile.
[0010] In addition, the frequency response of a nonlinearly chirped
grating may be tuned by using an acoustic wave propagating along
the grating direction. The acoustic wave induces additional
modulation sidebands in the frequency response of the grating. Such
modulation sidebands are displaced from the baseband by a frequency
spacing that is dependent on the frequency of the acoustic wave.
Therefore, an adjustable dispersion can be achieved by tuning the
frequency of the acoustic wave.
[0011] The present disclosure also provides a sampled
nonlinearly-chirped grating for changing relative time delays of
signals at different wavelengths. This sampled nonlinearly-chirped
grating includes a wave-guiding element having a refractive index
that varies along its optic axis according to a multiplication of a
first spatial modulation and a second special modulation. The first
spatial modulation is an oscillatory variation with a
nonlinearly-chirped period along the optic axis. The second spatial
modulation is a periodic modulation with a period different than
the nonlinearly-changing period.
[0012] The first and second modulations effectuate first and second
gratings that spatially overlap each other in the wave-guiding
element along its optic axis. The first grating may be a
nonlinearly-chirped grating. The second grating may have a grating
period greater than the first grating. The first grating and second
gratings couple with each other and operate in combination to
produce a plurality of reflection bands at different wavelengths
and with a bandwidth determined by the first grating.
[0013] A nonlinearly-chirped grating can be further configured to
change relative time delays of two different polarization states in
an optical signal. One embodiment of such a grating comprises a
wave-guiding element formed of a birefringent material that exhibit
different refractive indices for the two polarization states. A
nonlinearly-chirped grating is formed in the wave-guiding element
along its optic axis and has a varying grating period that changes
as a monotonic nonlinear function of a position. The grating
operates to reflect two polarization states of an input optical
signal at different locations along the optic axis to cause a delay
between said two polarization states.
[0014] One aspect of the nonlinearly-chirped gratings is dispersion
compensation. A nonlinear chirped grating can be disposed at a
fiber link to reduce the effects of the dispersion. The dispersion
produced by such a grating is actively tunable to compensate for
varying dispersion in a fiber link which includes a dispersion
analyzer and a feedback control. This tunability can be
advantageously used in a dynamic fiber network in which
communication traffic patterns may change over time. For example, a
given channel may be originated at different locations in the
network from time to time so that the accumulated dispersion of
that given channel in a specific fiber link is a variable.
Therefore, the dispersion compensation required for that fiber link
needs to change accordingly. Also, the operating conditions for
point-to-point transmission may also change, resulting in
variations in the accumulated dispersion for signals in a fixed
fiber link.
[0015] Another aspects of the nonlinearly-chirp gratings include
dispersion slope compensation, polarization mode dispersion, chirp
reduction in directly modulated diode lasers, and optical pulse
manipulation.
[0016] These and other embodiments, aspects and advantages of the
invention will become more apparent in light of the following
detailed description, including the accompanying drawings and
appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a diagram illustrating a nonlinear chirped grating
in a wave-guiding element.
[0018] FIG. 2 is a diagram showing a grating having a nonlinearly
chirped grating period.
[0019] FIG. 3A is a chart showing shift of reflective spectrum of a
nonlinearly chirped fiber grating due to fiber stretching.
[0020] FIG. 3B is a chart showing relative time delay of reflected
signals at two different wavelengths due to fiber stretching.
[0021] FIG. 4 is a diagram of one implementation of the system in
FIG. 2 using a piezoelectric element.
[0022] FIG. 5 is a schematic illustration of one approach to form a
nonlinearly chirped grating in a photosensitive fiber.
[0023] FIG. 6A is a chart showing measured wavelength shift in the
reflected signals due to fiber stretching in the system of FIG.
4.
[0024] FIG. 6B is a chart showing measured shift of the reflection
spectrum in the system of FIG. 4.
[0025] FIG. 6C is a chart showing nonlinear time delays of
reflected signals as a function of wavelengths that are measured in
the fiber grating of FIG. 4.
[0026] FIG. 6D is a diagram of a modulated nonlinearly chirped
fiber grating.
[0027] FIG. 6E is a chart showing a modulated voltage signal used
in FIG. 6D.
[0028] FIG. 6F is a chart showing reflected output signals as a
function of time at different modulation frequencies.
[0029] FIG. 7 is a diagram showing a nonlinearly chirped grating
based on electro-optic effects.
[0030] FIG. 8 is a diagram showing a photosensitive nonlinearly
chirped grating.
[0031] FIG. 9 is a diagram showing a nonlinearly chirped grating
having an acoustic tuning element.
[0032] FIGS. 10A and 10B are block diagrams of two dynamically
adjustable dispersion compensation systems.
[0033] FIGS. 10C, 10D, and 10E are diagrams showing three exemplary
implementations of the dispersion analyzer in FIGS. 10A and
10B.
[0034] FIG. 11A is a block diagram of a fiber communication system
based on the configuration in FIG. 10B using a nonlinearly chirped
fiber grating.
[0035] FIGS. 11B, 11C, and 11D are charts showing measured results
of the system in FIG. 11A.
[0036] FIG. 12 is a diagram illustrating a semiconductor laser have
a nonlinearly chirped waveguide grating for reducing
modulation-induced frequency chirps in the laser output.
[0037] FIG. 13 is a diagram showing a pulse shaping system based on
a nonlinearly chirped grating.
[0038] FIGS. 14A and 14B schematically show two implementations of
dispersion compensation in a WDM system by using multiple
nonlinearly-chirped fiber gratings.
[0039] FIG. 15 illustrates the fabrication and structure of a
sampled nonlinearly-chirped fiber grating according to one
embodiment of the disclosure.
[0040] FIGS. 16A and 16B show a periodic modulation on the
refractive index n(x) with a constant effective refractive index in
a fiber grating and the associated Bragg reflection peak in the
frequency space.
[0041] FIGS. 16C through 16F illustrate multiple reflection
spectral windows generated by modulating the refractive index n(x)
to produce two sets of gratings in two different modulation
schemes.
[0042] FIG. 17 schematically shows one embodiment of a tunable
multi-channel dispersion compensator for a WDM system by using a
single sampled nonlinearly-chirped fiber grating.
[0043] FIGS. 17A and 17B show measured shifts of the reflected
spectrum and the grating-induced time delay curves, respectively,
for an exemplary three-channel sampled nonlinearly-chirped fiber
grating under different stretching conditions.
[0044] FIGS. 17C and 17D are plots of the deviation of the
grating-induced nonlinear time delay from a linear time delay and
the dispersion as a function of wavelength for the same
three-channel sampled nonlinearly-chirped fiber grating of FIGS.
17A and 17B.
[0045] FIG. 18 schematically shows a test apparatus for
experimentally simulating tunable dispersion compensation in a WDM
system, where three WDM channels at 1551 nm, 1555 nm, and 1559 nm
are externally modulated at 10-Gb/s with a pseudorandom bit stream
(PRBS) 2.sup.15-1.
[0046] FIG. 18A are eye diagrams at -20 dBm channel input power for
the three WDM channels with and without the compensating grating at
different distances in the test apparatus of FIG. 18.
[0047] FIG. 18B shows measured bit-error-rate (BER) curves for the
1551-nm channel with and without the sampled compensating grating
at the two different distances in the test apparatus of FIG.
18.
[0048] FIGS. 19A and 19B respectively show reflectivity and
dispersion spectra of a tunable sampled nonlinearly-chirped fiber
grating having a spacing between adjacent Bragg reflection windows
that is different from the channel spacing in a WDM system.
[0049] FIG. 20A is a diagram showing a birefringent
nonlinearly-chirped fiber Bragg grating formed in a
high-birefringence optical fiber for compensating polarization mode
dispersion (PMD).
[0050] FIG. 20B shows time delays of two orthogonal states of
polarization as a function of wavelength from the birefringent
nonlinearly-chirped fiber Bragg grating of FIG. 20A.
[0051] FIGS. 21A and 21B show measured time delay curves of the
reflected signals as a function of wavelength and the respective
nonlinear dependence of the differential time delay on the
wavelength for each polarization direction from a birefringent
nonlinearly-chirped fiber grating with .DELTA..lambda. of around
0.6 nm at 1550 nm.
[0052] FIG. 22A shows the measured time delay as a function of the
relative amount of stretching of the fiber grating characterized in
FIGS. 21A and 21B.
[0053] FIG. 22B shows that the shape of reflection spectrum for
each polarization direction in remains substantially the same over
a wavelength tuning of about 2.32 nm by stretching in the
birefringent nonlinearly-chirped fiber grating with .DELTA..lambda.
of around 0.6 nm at 1550 nm.
[0054] FIGS. 23A, 23B, and 23C show the base-line eye diagram, the
eye diagrams for the 127-ps PMD emulation with and without
dispersion compensation, the eye diagrams for the 302-ps PMD
emulation without and with compensation that are measured from a
PMD emulation apparatus by using a birefringent nonlinearly-chirped
fiber grating.
DETAILED DESCRIPTION
[0055] FIG. 1 shows a nonlinearly chirped grating 100 in accordance
with one embodiment of the disclosure. The grating 100 is formed of
an optical wave-guiding element 104 such as a fiber or waveguide.
The grating period, .LAMBDA.(x), and the effective index of
refraction in the grating, n.sub.eff(x), are at least partly
dependent on the position, x, along the wave-guiding element 104.
The grating is effected by a modulation on the refractive index
n(x) of the wave-guiding element. The effective index n.sub.eff(x)
is a spatial average of n(x) and can be either a constant value or
a function of the position x depending on the n(x). An input
optical signal 102 enters the grating 104 at a nearly normal
incidence to produce a reflected signal 112 and a transmitted
signal 110.
[0056] A spectral component of a wavelength .lambda. in the input
optical signal 102 is reflected back at position x when the
wavelength .lambda., the grating period .LAMBDA.(x), and the
effective index of refraction n.sub.eff(x) satisfy a Bragg
phase-matching condition:
2n.sub.eff(x).LAMBDA.(x)=.lambda..
[0057] Therefore, the wavelength .lambda. of the reflected wave
varies with the position x according to the grating parameter
n.sub.eff(x).LAMBDA.(x). Different spectral components of different
wavelengths, e.g., the reflection 106 at .lambda..sub.1 and the
reflection 108 at .lambda..sub.2, are reflected at different
locations and have different phase delays. For example, when the
grating parameter n.sub.eff(x).LAMBDA.(x) increases with x,
spectral components at short wavelengths satisfying the
phase-matching condition are reflected back at locations before the
components at long wavelengths. A spectral component in the input
signal 102 that does not meet the above Bragg phase-matching
condition transmits through the wave-guiding element 104 as
indicated by a signal 110. The grating parameter
n.sub.eff(x).LAMBDA.(x) determines the spectral range of the
reflected signal from the grating 100. This forms the basis of
dispersion compensation and pulse shaping.
[0058] The grating 100 is generally configured to have a
nonlinearly chirped grating parameter n.sub.eff(x).LAMBDA.(x),
i.e., n.sub.eff(x).LAMBDA.(x) changes nonlinearly with the position
x. This may be achieved by a nonlinearly chirped n.sub.eff(x),
.LAMBDA.(x) or a combination of both.
[0059] The grating 100 can be adjusted to change the reflection
spectrum and the relative delays in the different reflected
spectral components. A grating control 120 is implemented to
control the grating parameter n.sub.eff(x).LAMBDA.(x) by varying at
least one of n.sub.eff(x) and .LAMBDA.(x) of the grating 100. This
provides a dynamically tunable reflection spectral range and
relative delays of different reflected spectral components.
[0060] FIG. 2 shows one implementation 200 of the nonlinearly
chirped grating 100. A fiber grating 204 has a constant effective
index of refraction n.sub.eff(x)=n and a nonlinearly chirped
grating period .LAMBDA.(x). Thus, a phase-matched wavelength
changes with the position x according to .LAMBDA.(x) only. A fiber
stretcher 220 is engaged to the fiber grating 204 to change the
overall length of the grating 204. This provides a control in the
reflection spectrum and the relative delays in different spectral
components.
[0061] When the fiber grating 204 is stretched, each grating pitch
increases. Accordingly, a phase-matched wavelength at each grating
position increases. Therefore, the reflection spectrum shifts
towards longer wavelengths. This effect is illustrated in FIG. 3A
in which curves 302 and 304 respectively represent the reflection
spectral profiles before and after the fiber stretching.
[0062] Since the grating period .LAMBDA.(x) is nonlinearly chirped,
the delay of the reflected spectral components also has a nonlinear
dependence on the position x. In addition, a change in the overall
fiber length produces different changes in .LAMBDA.(x) at different
positions along the fiber grating 204. This produces different
relative delays for different wavelengths that satisfy the Bragg
phase-matching condition. Such an effect can be used to produce
tunable dispersion compensation profiles.
[0063] FIG. 3B is a chart of the relative time delays of two
wavelengths before and after the fiber stretching. Curve 306
represents the time delay as a function of wavelength before the
fiber stretching. Two different wavelengths .lambda..sub.1 and
.lambda..sub.2 have a relative time delay .DELTA.t with respect to
each other. After the fiber grating is stretched, the time delays
of both wavelengths increase (curve 308) and the relative time
delay .DELTA.t' is in general different from .DELTA.t. In the
example shown, the relative time delay .DELTA.t' increases.
[0064] Referring to FIG. 2, any device capable of stretching the
grating 204 may be used as the stretcher 220. For example, a
piezoelectric element or a magnetostrictive element may be used to
produce a control over the length of the grating 204 according to
an external electrical voltage or a magnetic field. Piezoelectric
and magnetostrictive transducers are well known and will not be
described here.
[0065] A technique of using a magnetostrictive rod to stretch a
fiber in a non-uniform magnetic field is disclosed by Cruz et al.
in "Fibre Bragg gratings tuned and chirped using magnetic fields,"
Electronics Letters, Vol. 33(3), pp. 235-236 (1997), which is
incorporated herein by reference. This technique can be used in the
embodiment 200 of FIG. 2 to adjust the grating length. In
particular, since the fiber grating 204 is nonlinearly chirped, a
uniform magnetic field, rather than a gradient magnetic field, can
be used to uniformly stretch the fiber grating 204 for tuning the
dispersion response.
[0066] FIG. 4 shows an implementation of the embodiment 200 by
using a piezoelectric element. Two ends of a piezo element 410 are
respectively fixed at two sides of a nonlinearly chirped fiber
grating 406 by, for example, using an adhesive such as epoxy. A
voltage source 412 supplies a control voltage to the piezo element
410 to change the length of the piezo which in turn couples the
strain to the fiber grating 204. An optical circulator 404 is used
to couple an input optical signal 402 to the fiber grating 406 and
to route the reflected signal 408. An optional optical isolator may
be placed at the other end of the fiber grating 406 to reject any
optical feedback signal.
[0067] The nonlinearly-chirped fiber grating 204 may be made by a
near-UV technology that uses an interference pattern produced by a
phase mask with a light beam at 300 nm. The absorption of light in
the fiber core at the wavelength of 300 nm is sufficiently small to
avoid damage to the core-cladding interface in the fiber. A
photosensitive fiber (e.g., the type manufactured by QPS
Technology) is first soaked in a high-pressure molecular hydrogen
chamber under about 250 atm pressure at .about.60.degree. C. for
approximately 2 days to give the core an estimated hydrogen
concentration of about 2.5 mol. %.
[0068] FIG. 5 illustrates the formation of the nonlinearly-chirped
grating 204 in a hydrogen-loaded photosensitive fiber 500. A light
beam 502 from a UV argon laser operating on a group of spectral
lines near 300 nm is focused through a 50-mm long linearly-chirped
phase mask 504 onto the fiber core at an intensity of about 200
W/cm.sup.2. Two first-order diffraction beams 502a and 502b
interfere with each other to form an interference pattern in the
immediate vicinity of the phase mask 504 where the fiber core is
located. Each 1-mm spot on the fiber 500 is exposed for time
periods ranging from 5 to 100 sec. After each exposure, the fiber
500 and mask 504 are translated by 1 mm relative to the UV light
beam 502 and the process is repeated. The variable exposure time
induces the nonlinear chirp as shown in the insert of FIG. 5.
[0069] FIG. 6A shows the measured wavelength shift in the reflected
signal 408 as a function of the control voltage applied to the
piezo element 410. FIG. 6B shows the reflection spectrum shifts due
to fiber stretching for voltages on the piezo element 410 at 500 V
and 1000 V, respectively. When a control voltage of about 1000 V is
applied to the piezo element 410, the reflected band is shifted by
about 1.5 nm, and the wavelength shift is linear with respect to
the voltage. The bandwidth is about 1 nm and the reflectivity
varies from 85% to 100%, i.e. by approximately 0.7 dB. The
dispersion varies nonlinearly and smoothly from 300 ps/nm to 1000
ps/nm. While increasing the applied voltages, the time delay curves
shift to longer wavelengths without distorting the smooth shape.
Therefore, for a given transmitted channel wavelength, the channel
will encounter a different dispersion compensation corresponding to
different stretching of the nonlinearly-chirped fiber grating.
[0070] FIG. 6C further shows measured nonlinear time delays of
reflected signals as a function of wavelengths when the fiber
grating is stretched by different amounts under different control
voltages.
[0071] The length of the piezoelectric element 410 can be modulated
to provide dispersion switching. FIG. 6D shows a system using the
fiber grating 400 to produce a signal with a modulated dispersion.
A modulation signal generator 610 modulates the piezo control 412
so that the length of the fiber grating 406 is modulated. A
bandpass interference filter 620 with a bandwidth of 0.3 nm is used
to filter the reflected output from the fiber grating 406. A
photodetector 630 receives the transmitted signal from the filter
620. An oscilloscope 640 receives and displays the time response of
the signal from the photodetector 630.
[0072] FIG. 6E shows the modulated control voltage applied to the
piezo element 410. Measurements at modulation frequencies at 10 Hz,
50 Hz, 100 Hz, and 250 Hz are shown in FIG. 6F. The piezoelectric
element 410 may be modulated up to about 100 Hz using 0-500 Volts
modulation. The upper limit of the frequency response is limited by
the characteristics of the PZT. With this dynamic response,
dispersion compensation in less than 10 ms can be achieved in
circuit-switched optical networks.
[0073] The nonlinearly chirped grating 100 in FIG. 1 can also be
implemented by using a wave-guiding element that has an index of
refraction dependent on an external electrical field. One example
of such wave-guiding element is a dielectric waveguide or fiber
exhibiting electro-optic effects. LiNbO.sub.3 is a commonly used
electro-optic material. FIG. 7 shows a grating 700 with a
nonlinearly chirped grating period in such a wave-guiding element
704. The effective index of refraction n.sub.eff(x) of the
wave-guiding element 704 varies with an electrical field. A series
of pairs of electrodes 712, 714 are disposed along the wave-guiding
element 704 to produce adjustable local fields. An electrical-field
control module 710 controls the spatial variation of the field to
produce a desired nonlinear chirped n.sub.eff(x) and to adjust the
dispersion.
[0074] FIG. 8 shows another embodiment 800 that uses an
electromagnetic radiation to control the spatial variation of the
effective index n.sub.eff(x) of a wave-guiding element 804. The
wave-guiding element 804 responds to the radiation field 820 and
has a field-dependent effective index n.sub.eff(x).
[0075] For example, photosensitive materials such photorefractive
crystals and polymers may be used to implement the present
invention. The nonlinear chirping of the effective index
n.sub.eff(x) is formed by applying an electromagnetic radiation
field 820 with a nonlinear intensity distribution along the
grating. A radiation generator 810 is configured to control the
intensity variation I(x) of the field 820. In the optical frequency
range, the radiation generator 810 may be a laser.
[0076] It is further contemplated that an acoustic wave can be used
to modulate the response of any of the above nonlinearly chirped
gratings for tuning the output frequency. FIG. 9 shows a
nonlinearly chirped grating 900 with such an acoustic tuning
mechanism. An acoustic wave generator 910 produces a tunable
acoustic wave 912. An acoustic wave coupler 914, such as an
acoustic focusing horn, couples the acoustic wave into the grating
104.
[0077] In operation, the acoustic wave interacts with the grating
and induces two additional narrow-band peaks on either side of the
base band produced by the Bragg resonance condition. The frequency
components in either sideband has the same relative delays as in
the baseband but are shifted from the baseband in frequency by a
specified amount. This frequency shift is dependent on the
frequency of the acoustic wave. Thus, the frequency of a sideband
is adjustable by changing the frequency of the acoustic wave. Liu
et al. disclose such a technique in "Improved Efficiency
Narrow-Band Acoustooptic Tunable Reflector using Fibre Bragg
grating," post deadline paper PD4, Annual Meeting of Optical
Society of America, "Bragg Gratings, Photosensitivity, and Poling
in Glass Fibers and Waveguides: Applications and Fundamentals,"
Oct. 26-28, 1997, Williamsburg, Va., which is incorporated herein
by reference.
[0078] The nonlinearly chirped fiber gratings in accordance with
this embodiment are tunable in two aspects. First, the frequency
profile of the reflected and the transmitted signals can be shifted
as desired. Second, the relative delays of different frequency
components in an input pulse can be adjusted in a controllable
manner. The first aspect of tunability is useful in
multi-wavelength photonic systems such as wavelength-division
multiplexed fiber communications systems. The second aspect of the
tunability can be used for dynamic dispersion compensation in many
dispersive optical systems, especially in fiber communication
systems.
[0079] FIG. 10A shows a fiber system 1000 having a tunable
dispersion-compensating element 1020 in accordance with one
embodiment of the invention. The tunable dispersion element 1020
may be a nonlinearly chirped grating. A dispersive fiber system
1010 produces an optical signal 1012 with a certain amount of
dispersion. A dispersion analyzer 1030 measures the amount and the
sign of the accumulated dispersion in the output signal from the
tunable dispersion compensating element 1020. The tunable
dispersion-compensating element 1020 uses this information to
adjust the dispersion compensation of the element 1020 in such a
way that the dispersion in the signal 112 is compensated. As the
dispersion in the dispersive fiber system 1010 changes, the tunable
dispersion-compensating element 1020 adjusts accordingly in
response to the dispersion change to maintain the desired
dispersion compensation in output 1032.
[0080] FIG. 10B is a block diagram for a fiber communication system
1001 that uses a nonlinearly chirped fiber grating 1020a to
implement the system 1000 in FIG. 10A. A grating control 1040
adjusts the grating parameter n.sub.eff(x).LAMBDA.(x) in accordance
with the control command from the dispersion analyzer 1030 to
maintain the output 1032 properly compensated. The grating control
1040 may be any or a combination of the techniques shown in FIGS.
2, 7, and 8.
[0081] The dispersion analyzer 1030 may be implemented in a number
of ways. FIG. 10C shows a phase modulation to amplitude modulation
dispersion detector. A phase modulator 1051 is disposed in the
signal path to modulate the phase of the signal prior to
transmission through a dispersive fiber 1050. An envelop detection
circuit 1060 measures the converted amplitude modulation, whose
amplitude corresponds to the relative accumulated dispersion, in
the received signal by a photodetector 1070. More specifically, the
polarity of dispersion can be detected by including the total
dispersion of the group velocity dispersion in the fiber and the
self-phase modulation caused by the fiber nonlinearity. See,
Tomizawa et. al, "Nonlinear influence on PM-AM conversion
measurement of group velocity dispersion in optical fiber,"
Electronics Letters, Vol. 30(17), pp. 1434-1435(1994). The
amplitude of the converted amplitude modulation is then used to
determine the accumulated dispersion and to generate a control
signal to the tunable dispersion compensation element.
[0082] FIG. 10D shows another implementation of the dispersion
analyzer 1030. An electro-optic modulator 1052 is disposed in the
signal path to modulate the amplitude of the signal prior to
transmission through the dispersive fiber 1050. The relative
dispersion value can be determined by monitoring the amplitude of
the clock component extracted from the signal after a square wave
detection. This is done by a clock component monitor 1061. Since
the dispersion broadens the signal pulses and reduces the amplitude
of the signal, the magnitude of the clock component also decreases
according to the broadening. Therefore, by adjusting the dispersion
compensator to maximize the amplitude of the clock amplitude, the
accumulated dispersion can be reduced or canceled.
[0083] The dispersion analyzer 1030 can further be implemented by
directly measuring the bit error rate of the signal passing through
a dispersive fiber. This is shown in FIG. 10E. Since the dispersion
can broaden the data pulses, the bit error rate ("BER") is
degraded. A bit error rate testing device 1062 measures the bit
error rate and extracts a relative information of the accumulated
dispersion. With a feedback signal to the tunable dispersion
compensator, the dispersion compensation can be adjusted to reduce
or minimize the bit error rate.
[0084] FIG. 11A further shows a specific implementation of the
dynamic fiber system 1100 in FIG. 10B. An electro-optic modulator
1104 imposes data on a laser beam generated by a laser 1102 with a
data rate at 10 Gbit/s. In addition, a phase modulator 1106
modulates the phase of the optical signal prior to transmission. A
tunable dispersion compensator 1120 based on a nonlinearly chirped
fiber grating 400 as in FIG. 4 is implemented to perform the
dispersion compensation. An optical coupler 1112 splits the signal
into to different optical paths. The signal path passing through
the fiber loops 1110a, 1110b and acoustooptic switch 1116b is more
dispersive than the signal path passing through the fiber loop
1110a, the optical attenuator 1114, and the acoustooptic switch
1116a. Er-doped fiber amplifiers 1108a, 1108b, 1108c, and 1108d are
used to maintain the signal strength above a specified level. The
dispersion in the signal 1119 is detected by a dispersion analyzer
1122 by splitting a small portion 1120a of the signal 1119 (e.g.,
10%). The majority 1120b of the signal 1119 is fed to the fiber
grating 400 which produces a dispersion-compensated output 1120c.
An EDFA 1124 is used to amplify the output 1120c. An optical
receiver 1128 detects the amplified signal 1120c to produce the
received data.
[0085] The dispersion analyzer 1122 uses a PM-to-AM converter for
measuring the dispersion. Due to the different group velocity
dispersions of the different spectral components in the signal, the
phase modulation is converted to amplitude modulation after the
signal has traveled through a certain distance of fiber path. The
accumulated dispersion is measured by the dispersion analyzer 1122.
The dispersion analyzer 1122 further generates a corresponding
control signal to the tunable fiber grating 400.
[0086] A bit error rate test 1130 is used to measure the bit error
rate for evaluating the performance of the dispersion compensation
module 1120. The output 1120c from the module 1120 is amplified and
filtered by a bandpass filter 1126 with a bandwidth of 0.3 nm.
[0087] FIG. 11B shows measured results of the bit error rate as a
function of the signal power in dBm. FIG. 11C shows how the control
signal for the PZT tuning is generated in response to the
dispersion levels of the input signals. FIG. 11D shows the measured
eye diagrams indicating the significant improvements in the BER due
to the dynamic dispersion compensation.
[0088] The above described nonlinearly chirped gratings may also be
used in other applications such as chirp cancellation in directly
modulated lasers and pulse shaping.
[0089] FIG. 12 shows an integrated semiconductor laser module 1200
having a nonlinearly chirped waveguide grating 1230 for reducing
the modulation chirp. A laser diode 1210 is formed on a substrate
1202. A modulation signal 1212 is applied to the laser diode 1210
to modulate the driving current. Such direct modulation can cause
frequency chirps in the output of the laser diode 1210. A
nonlinearly chirped waveguide grating 1230 is formed on the
substrate 1202 to produce a dispersion for reducing the frequency
chirp.
[0090] The chirp in the laser output changes with the modulation
frequency of the modulation signal 1212. The relation between the
modulation frequency and the chirp in the laser output can be
determined, e.g., by measurements. Based on this relation, a
control circuit 1250 can be configured to generate a corresponding
dispersion control signal 1252 to adjust the dispersion of the
grating 1230. The control circuit 1250 may be located outside the
substrate 1202 as shown or alternatively integrated on the
substrate 1202. An optical circulator 1220 is located in the
optical path between the laser diode 1210 and the grating 1230 to
direct the reflected, chirp-reduced laser output from the grating
1230 to an output optical waveguide 1240.
[0091] FIG. 13 further shows a block diagram of a system 1300 for
pulse shaping. A nonlinearly chirped grating 1330 can produce a
variable dispersion to an input pulse 1312 from a laser 1310 so
that the output 1340 from the grating 1330 coupled to a grating
control unit 1332 has a desired pulse shape. An optical circulator
1320 is optically coupled between the laser 1310 and the grating
1330 to route and separate the original laser output pulse 1312 and
the reshaped output pulse 1340.
[0092] The above described nonlinearly-chirped fiber gratings are
configured so that the wavelength of a reflected spectral
component, .lambda.(x)=2n.sub.eff(x).LAMBDA.(x), is a nonlinear and
monotonic function of x. Because the length of the fiber grating is
limited, the chirping range of the grating spacings in practical
devices is also limited. This results in a reflection spectrum of
such fiber gratings with a limited bandwidth as illustrated in FIG.
3A. Such fiber gratings may not be able to compensate for
dispersion at two different wavelengths when the difference between
the two wavelengths is comparable to or greater than the reflection
bandwidth.
[0093] A WDM signal in a WDM fiber system has signals at different
wavelengths (WDM channels) which propagate in the same fiber. These
different wavelengths in the WDM signal can experience different
amounts of dispersion when transmitted through a dispersive fiber
link from one location to another. Such signals usually have a
wavelength difference of about 0.6 nm or greater (e.g., ITU uses
0.8 nm and its multiples at 1.6 nm, 3.2 nm, and so on for WDM
systems). The shortest wavelength and the longest wavelength of a
WDM signal may be too great for a single fiber grating to provide
proper dispersion compensation to both at the same time. For
example, the nonlinearly-chirped fiber grating shown in FIG. 6B at
a given bias voltage could not reflect two signals of 1551 nm and
1552 nm at the same time. Two such gratings, one with a control
voltage of about 0V on the piezo stretcher and one with a control
voltage of about 500V on the piezo stretcher, however, can be used
together to separately provide dispersion compensation to these two
signals. In the embodiments that follow, multiple
nonlinearly-chirped fiber gratings may be combined to respectively
compensate for dispersions of signals at different wavelengths (WDM
channels) in a WDM signal.
[0094] FIGS. 14A and 14B schematically show two implementations
1400A and 1400B of using multiple nonlinearly-chirped fiber
gratings 1410, 1420, and 1430 in a WDM system 1402. Each fiber
grating 1410, 1420, 1430, respectively has a designated grating
controller 1412, 1422, 1432 as a tuning mechanism. A grating
controller may be a fiber stretcher (e.g., a piezo element and a
voltage supply) or an other tuning device. Similar to the one in
FIGS. 10A and 10B, a dispersion detection device may be deployed in
each system to indicate dispersion information of an input WDM
signal 1404 so that each grating controller can respond accordingly
to provide a desired compensation in a respective fiber grating.
Alternatively, when the dispersion at different wavelengths in a
WDM signal is known at a given node in the WDM system 1402, the
dispersion detection device may be eliminated and each fiber
grating can be pre-configured to produce the desired compensation
at a respective wavelength.
[0095] In FIG. 14A, multiple nonlinearly-chirped fiber gratings
1410, 1420, and 1430 are connected in series. Each provides a
different compensation at a different wavelength in an input WDM
signal 1404. For example, the fiber grating 1410 can be configured
to compensate for dispersion within a limited spectral range around
a selected wavelength .lambda..sub.1. Due to the large separations
of the multiplexed signals in wavelength, signals at other
wavelengths such as .lambda..sub.2 and .lambda..sub.3 do not
satisfy Bragg conditions in the fiber grating 1410 and hence
transmit through the fiber grating 1410. These transmitted signals
may then be reflected by other fiber gratings in the series, e.g.,
1420 and 1430, to provide proper dispersion compensation. The
compensated signals are then reflected back to the input of the
first fiber grating 1410 and then routed by an optical circulator
1408 to generate a dispersion-compensated reshaped WDM signal
1406.
[0096] FIG. 14B uses multiple fiber gratings 1410, 1420, and 1430
in a parallel configuration. A demultiplexer unit 1440 is used to
receive and separate the input WDM signal 1404 into multiple
signals of different wavelengths. Each separate signal is then
reflected back to the demultiplexer unit 1440 by a corresponding
fiber grating in a way that compensates for the dispersion at that
wavelength. The demultiplexer unit 1440 then recombines the
reflected signals at different wavelengths into a
dispersion-compensated WDM signal 1406 that is output by the
circulator 1408.
[0097] Simultaneous compensation for dispersion at different
wavelengths of a WDM system may also be achieved by using a special
nonlinearly-chirped fiber grating. Such a fiber grating can replace
the multiple fiber gratings and their associated grating
controllers in FIGS. 14A and 14B.
[0098] FIG. 15 illustrates the fabrication and structure of such a
special fiber grating 1500. The fiber grating 1500 has a
nonlinearly-chirped, monotonic-valued grating period .LAMBDA.(x).
As described above, this nonlinearly-chirped grating may be formed,
according to one implementation, by producing a modulation on the
refractive index n(x) of the fiber in a nonlinearly chirped manner
along the fiber. When n(x) is modulated in a sinusoidal manner with
a constant amplitude, the effective index of refraction
n.sub.eff(x) is a constant along the fiber. In addition, the
refractive index n(x) is also modulated by a second index
modulation that has a modulation period greater than the
nonlinearly-chirped modulation. Hence, the reflective Bragg
wavelength, .lambda.(x), is no longer a monotonic-valued and
nonlinearly-chirped function of x but rather is a
nonlinearly-chirped periodic function of x. Two or more reflection
spectral windows centered at different wavelengths can be produced
by the two different modulations of the index n(x). Hence, a single
fiber grating of this kind can function as two or more fiber
gratings each having only one Bragg reflection window.
[0099] This special fiber grating 1500 may be formed by the
fabrication process illustrated in FIG. 15. A nonlinearly-chirped
phase mask 1510 is used to form the nonlinearly chirped index
modulation which has a nonlinearly-chirped period
.LAMBDA..sub.NC(X) . In addition, a periodical amplitude mask 1520
is used to sample the UV light during exposure and thus cause the
second index modulation of the index n(x) with a period of
.LAMBDA..sub.C. The two masks 1510 and 1520 are fixed to the fiber
1500 during fabrication. An UV light source and the fiber then are
moved relative to each other to expose the core of the fiber 1500
one section at a time.
[0100] The above process in effect produces two different gratings
in the fiber 1500: a nonlinearly-chirped grating .LAMBDA..sub.NC(X)
defined by the phase mask 1510 and a periodic grating
.LAMBDA..sub.C defined by the amplitude mask 1520. The coupling of
the two gratings forms multiple Bragg reflection windows or bands
at different wavelengths. The number of bands and the band spacing
are determined by the periodic modulation of the amplitude mask
1520. The bandwidth of each band is identical and is determined by
the chirping range of the grating .LAMBDA..sub.NC(X) defined by the
phase mask 1510. To distinguish from the nonlinearly-chirped
grating shown in FIG. 1, this special fiber grating will be
referred to as "sampled nonlinearly-chirped fiber grating".
[0101] The second periodic modulation of n(x) has a spatial period
.LAMBDA..sub.C greater than the grating period .LAMBDA..sub.NC(X) .
For example, .LAMBDA..sub.C may be in a range from about 0.1 mm to
about 2 mm, or more preferably from about 0.2 mm to about 1 mm,
while the average .LAMBDA..sub.NC(X) is about 0.5 .mu.m for fiber
systems near 1550 nm. FIGS. 16A through 16F illustrate the multiple
reflection spectral windows generated by the second periodic
modulation on the refractive index n(x). The reflected Bragg
wavelength .lambda.(x) is associated with the optical wavevectors
that satisfy the Bragg phase-matching conditions by Fourier
transforms of n(x), where n(x) is a function of the position x
along the optic axis of the fiber, the nonlinearly-chirped period
.LAMBDA..sub.NC(X), and the constant period .LAMBDA..sub.C. FIGS.
16A, 16C, and 16E show the spatial variations of the actual
refractive index along the fiber, n(x), and FIGS. 16B, 16D, and 16F
show respective reflection spectra satisfying the Bragg
conditions.
[0102] FIG. 16A shows a case where the index n(x) is only modulated
by a sinusoidal modulation with a constant period. The Fourier
transform of the sinusoidal function n(x) is a single value in the
wavevector space, i.e., only one wavevector matches the Bragg
condition and gets reflected (FIG. 16B). When the period of the
sinusoidal modulation is linearly or nonlinearly chirped, multiple
wavevectors of a limited range in the wavevector space can be
reflected at different locations along the grating. Hence, the
single peak in FIG. 16B becomes a reflection spectral window as
shown in FIG. 3A.
[0103] FIG. 16C represents a case where n(x) is modulated by a fast
sinusoidal modulation and a slow spatial square wave function with
a constant period. FIG. 16D shows multiple reflection bands that
are produced by the slow modulation of the index n(x). These bands
have different strengths due to the square-wave modulation. The
reflectivity of the band at the center wavelength is the highest
and reflectivities of other bands are reduced by a factor
determined by a sinc-function. When the slow modulation of n(x) is
formed of repetitive patterns of a portion of a spatial sinc
function, i.e., the amplitude of the slow index modulation is
highest at the center of a selected fiber segment and decays
towards both ends of the segment according to (sin x/x), the
multiple bands of substantially identical reflectivities can be
generated.
[0104] FIG. 16E shows one repetitive pattern of a slow modulation
of n(x). Each repetitive pattern includes first five lobes of a
sinc function. FIG. 16F represents 6 bands produced by the slow
modulation by n(x) in the frequency domain. The latter is preferred
in WDM applications in order to substantially reduce or minimize
signal distortion by the fiber grating. Sinc-sampled fiber gratings
are disclosed by Ibsen et al. in "Sinc-sampled fiber Bragg gratings
for identical multiple wavelength operation," IEEE Photonics
Technology Letters, Vol. 10, No. 6, p. 842-844 (1998).
[0105] FIG. 17 shows one embodiment 1700 of a tunable multi-channel
dispersion compensator for a WDM system 1402 by using a single
sampled nonlinearly-chirped fiber grating 1710. A grating
controller 1720 provides a tuning mechanism for the grating 1710 to
adjust the dispersions at different wavelengths. A dispersion
detection device may be incorporated to measure the actual
dispersion in the dispersive WDM signal 1404 and to provide a
control signal to the grating controller 1720.
[0106] This configuration of using a single fiber grating 1710
provides a number of advantages over a multi-grating configuration
shown in FIGS. 14A and 14B. For example, such a single-grating
compensator is relatively easy to fabricate and package at a lower
cost because only a single fiber grating and a single fiber control
are needed. Since the temperature of each grating can affect the
grating length and hence the dispersion caused by the grating, the
temperature of each grating may need be stabilized and controlled
at a desired constant temperature. The single-grating configuration
reduces complexity of such temperature stabilization. The
single-fiber configuration also has less insertion loss than that
of the multi-grating configuration.
[0107] Furthermore, in the single-grating configuration, the
desired channel spacing can be more easily and precisely set by the
manufacturing process and the reflectivities of different channels
can be made substantially the same.
[0108] The sampled nonlinearly-chirped fiber Bragg grating 1710 can
be fabricated as shown in FIG. 15 by using a sampling slit to
effectuate the periodic modulation onto the fiber's refractive
index. This sampling slit produces a square-wave modulation similar
to FIG. 16C with a period of 200 .mu.m. A 300-nm light source can
be used to avoid damage to the fiber's core-cladding interface. The
fiber grating 1710 may be 30 cm in length and sampled by the sample
slit to produce 3 principal channels separated by 4 nm. The channel
separation is determined by the sampling period: 1 = B 2 2 n neff c
,
[0109] where .DELTA..lambda. is the spacing between the centers of
adjacent channels, .lambda..sub.B is the Bragg wavelength of the
original grating without sampling, n.sub.eff is the effective
refractive index in the grating, and .LAMBDA..sub.C is the sampling
period of the slow modulation. By increasing the sampling period L
from 200 .mu.m to about 1 mm, the ITU standard channel spacing of
0.8 nm can be obtained.
[0110] FIGS. 17A and 17B show measured shifts of the reflected
spectrum and the grating-induced time delay curves, respectively,
for the above three-channel sampled nonlinearly-chirped fiber
grating under different stretching conditions. All channels exhibit
nearly identical optical and time-delay characteristics. The
reflectivity difference among the three channels is less than 2 dB
and can be reduced by using a sinc-shape modulation of the sampled
grating. Within one wavelength reflection band, the dispersion
changes smoothly from -200 ps/nm to -1200 ps/nm for different
wavelengths. By uniformly stretching the grating, the dispersion
varies nonlinearly and smoothly from about -200 ps/nm to about
-1200 ps/nm for a fixed wavelength within each band. As the grating
is tuned, the amplitudes and shapes of both the reflected spectrum
and induced delay curve remain relatively constant for all three
channels, allowing for robust operation. The grating ripple is
generally less than about 40 ps.
[0111] FIG. 17C shows the deviation of the nonlinear time delay
from a linear time delay, and the maximum deviation is
approximately 600 ps. FIG. 17D shows the grating-induced dispersion
of the three different bands as a function of wavelength.
[0112] FIG. 18 shows a test apparatus for experimentally simulating
tunable dispersion compensation in a WDM system. Three WDM channels
at 1551 nm, 1555 nm, and 1559 nm are externally modulated at
10-Gb/s with a pseudorandom bit stream (PRBS) 2.sup.15-1. Two
different amounts of fiber dispersion are introduced in the signals
by transmitting the data over distances of 60 km and 120 km in a
single-mode fiber segment, respectively. A small amount of
pre-chirping is applied to the signal at an electro-optic modulator
in order to increase the maximum usable transmission distance to
120 km with a single-mode fiber segment. The above 3-band sampled
nonlinearly-chirped fiber grating is placed at the end of the fiber
link for the data approximately after 60 km and is placed at the
mid-point of the link for the data approximately after 120 km.
[0113] FIG. 18A shows the eye diagrams at about -20 dBm channel
input power for the three WDM channels with and without the
compensating grating at different distances. After transmission
over a fiber segment by 60 km, the eye diagrams for the 3 channels
are fairly open without compensation, and the grating was tuned to
provide a relatively small amount of dispersion compensation. The
eye diagrams of the 3 channels after about 120 km of propagation
are fairly closed without compensation, and the grating was
stretched to shift the resonance bands by about 2 nm to provide
sufficient dispersion and open the eye diagrams.
[0114] FIG. 18B shows the bit-error-rate (BER) curves for the
1551-nm channel with and without the sampled compensating grating
at the two different distances. Due to the initial chirp of the WDM
signal, the sensitivity at 60 km without compensation is slightly
better than the back-to-back measurement. Comparing the BER curves
with and without the grating after 60 km, the power penalty induced
by the grating is .about.0.5 dB. After 120 km, the power penalty of
the sampled grating compensator is less than 0.5 dB after 120 km,
compared with back-to-back BER curve. Without compensation by the
fiber grating, the bit error rate was much larger than 10.sup.-9
after transmission over 120 km. The BER curves for the other two
channels show similar results at both transmission distances.
[0115] A sampled nonlinearly-chirped fiber grating may be
configured in a way so that the frequency spacing between two
adjacent bands in the reflected spectrum of the grating is
different from the channel spacing in a WDM signal. Since spectral
components of different wavelengths in a band experience different
dispersion compensations (FIGS. 17B and 17D), the dispersions of
two different signals in two different bands at different relative
locations with respect to the centers of bands are different. This
feature of a sampled nonlinearly-chirped fiber grating can be used
to provide different dispersion compensations to different channels
in a WDM signal. For example, dispersion of optical fiber can vary
significantly over the gain bandwidth of an Er-doped fiber
amplifier (EDFA). In conventional fibers, the dispersion slope,
(dD/d.lambda.), of the dispersion (D) with respect to the
wavelength (.lambda.) is about 0.08 ps/nm.sup.2/km. This wavelength
dependence of chromatic dispersion presents special problems in
long-haul WDM systems because signals of different wavelengths may
undergo different dispersions. Therefore, it is desirable to
provide different dispersion compensations to signals with
different wavelengths.
[0116] FIGS. 19A and 19B illustrate the operation of a single
sampled nonlinearly-chirped fiber grating for producing a tunable
dispersion slope compensation. FIG. 19A shows that the band spacing
of the fiber grating is less than the channel spacing so that each
channel of the WDM signal is then located at a different position
in each reflected band of the fiber grating relative to the center
of each band. FIG. 19B shows a different dispersion compensation is
so generated for a different channel in an example where the
dispersion compensation increases with wavelength.
[0117] In addition to dispersion compensation, the above sampled
nonlinearly chirped fiber grating may be used for chirp
cancellation in directly modulated multi-wavelength semiconductor
laser and simultaneous tunable compression of multi-channel ultra
short pulses. Device implementations for such applications are
similar to FIGS. 12 and 13 except that the laser source 1210 or
1310 is replaced by a source that produces a laser signal of
multiple wavelengths.
[0118] A nonlinearly-chirped fiber may also be modified to
compensate for polarization mode dispersion (PMD) in fibers. Many
fibers are known to exhibit some birefringence caused by factors
such as imperfect circular core or unbalanced stress of the fiber.
Optical fiber can accommodate two different states of polarization
of light in a fiber. Since the effective indices of refraction of
the two polarization states are not the same, the transmission
speeds of the two polarization states are different. This
polarization mode dispersion is undesirable and can distort the
signal.
[0119] PMD can be compensated by delaying one polarization state
with respect to the other by a proper amount to cancel the delay
between the two polarization states in the fiber link. Since the
amount of PMD at any given location in a fiber network often
changes due to environmental disturbances such as vibrations and
fluctuations in temperature, it is highly desirable to have a
tunable PMD compensator that can dynamically adjust the relative
delay between two states of polarization in a signal. Such
polarization-dependent dispersion compensation can be achieved by
introducing birefringence in the above nonlinearly-chirped fiber
gratings.
[0120] One embodiment of a nonlinearly-chirped fiber grating for
PMD compensation is formed by writing nonlinearly-chirped grating
into a high-birefringence photosensitive fiber. The difference in
the indices of refraction for the two principal polarization axes
may be on the order of 10.sup.-4 or greater (e.g.,
5.times.10.sup.-4) at or near 1550 nm. The high-birefringence fiber
provides different time delays for different states of
polarization. The nonlinear chirp allows tuning of relative delays
of different spectral components in each state of polarization and
a frequency shift in the reflective spectral band.
[0121] FIG. 20A illustrates a birefringent nonlinearly-chirped
fiber Bragg grating formed in a high-birefringence optical fiber.
The high-birefringence optical fiber may be formed of a
polarization-maintaining fiber. This allows a large difference in
refractive indices between fast and slow polarization axes. The
reflection position from the nonlinearly-chirped grating is
different for each polarization of an input optical signal at one
fixed wavelength within the grating bandwidth. This difference in
reflection positions, .DELTA.L, causes a differential time delay
(.DELTA.t) between the two polarization states (FIG. 20B). The
differential time delay is dependent of the wavelengths of
different spectral components within the grating bandwidth due to
the nonlinear chirping of the grating period. This combination of
the birefringence of the fiber and the nonlinear chirping of the
grating provides a tuning mechanism for adjusting the relative
delays between two polarization states by mechanical stretching of
the grating.
[0122] Optical signals having two different polarization states can
be combined at the output of the grating without interference
because of their orthogonal polarization states. In an actual
implementation, a fiber stretcher may be used to control the length
of the birefringent nonlinearly-chirped fiber grating. A dispersion
detection module is used to monitor the PMD and to control the
fiber grating accordingly in order to produce the proper dispersion
compensation.
[0123] An exemplary nonlinearly-chirped grating may be written on a
photosensitive highly birefringent fiber through a
nonlinearly-chirped phase mask using near-UV light at about 300 nm.
The grating may be 15 cm long and nonlinearly chirped from 1547.2
nm to 1550.5 nm for two polarization directions. At a given
location in the fiber grating, the reflected signals of the
orthogonal polarization directions have two different wavelengths
that are separated by .DELTA..lambda.: 2 = n s - n f n - n cl g
,
[0124] where n.sub.s, n.sub.f, n, n.sub.cl, and .lambda..sub.g
respectively represent slow axis, fast axis, core, cladding
refractive indices and average of the fast and slow polarization
resonant wavelengths.
[0125] FIG. 21A shows measured time delay curves of the reflected
signals as a function of wavelength for each polarization direction
from a birefringent nonlinearly-chirped fiber grating with
.DELTA..lambda. of around 0.6 nm at 1550 nm. Note that almost
identically-chirped gratings are written for both polarization
directions. FIG. 21B shows the respective nonlinear dependence of
the differential time delay on the wavelength. The time delay
.DELTA.t changes from 320 ps to 100 ps when wavelength changes from
1547.03 nm to 1550.34 nm. The solid line provides the expected time
delay between the two polarization states, obtained by fitting the
experimental data.
[0126] FIG. 22A shows measured time delay as a function of the
relative amount of stretching of the same fiber grating. The
measurements were performed by mounting the birefringence fiber
grating on a translational stage. The time delay .DELTA.t of the
two polarizations for a signal at 1549.33 nm changes due to
stretching of the fiber grating. A tuning .DELTA.t of approximately
170 ps is achieved by 0.22% stretching of the grating at 1549.33
nm. FIG. 22B shows that the shape of reflection spectrum for each
polarization direction does not change significantly over a
wavelength tuning of about 2.32 nm by stretching.
[0127] Stretching of the fiber grating provides tunable
compensation of PMD on long distance, high-speed optical data
transmission. This is because .DELTA.t is tunable and the
polarization does not change. To demonstrate this application, a
DBR laser at 1550.2 nm is externally modulated at 10 Gb/s PRBS in a
non-return-to-zero data format using a 16 GHz electro-optic
intensity modulator. Delays of about 127 ps and 302 ps are
respectively introduced between the two orthogonal polarizations of
the signal to simulate the effect of PMD by using a PMD emulator.
The PMD emulator includes two polarization beam splitters, optical
delay and mechanical attenuator. The power ratio into one of the
paths is adjusted to be the same for each path to simulate the
worst condition of PMD. A polarization controller is used before
the birefringent nonlinearly-chirped fiber grating to align the
polarization directions to the grating.
[0128] FIG. 23A shows the base-line eye diagram of the signal at
the output of the intensity modulator. FIG. 23B shows the eye
diagrams for the 127-ps PMD emulation with and without dispersion
compensation being performed by the grating. The emulated eye is
completely closed because emulation is larger than one bit period.
The three-level eye comes from the fact that optical delay from the
PMD emulator is almost multiple times of the bit time.
[0129] FIG. 23C shows the eye diagrams for the 302-ps PMD emulation
without and with compensation of HN-FBG with tuning by 0.215%
stretching. The eye is completely recovered after compensation, and
bit-error-rate measurements confirm error free operation for both
compensated cases.
[0130] Although the present invention has been described in detail
with reference to a few embodiments, various modifications and
enhancements may be made. For example, a sampled
nonlinearly-chirped fiber grating may be formed in a highly
birefringent fiber to combine the multiple bands of the fiber
grating in FIG. 15 and the PMD compensation of the fiber grating in
FIG. 20A. This hybrid fiber grating can compensate the PMD in a WDM
signal and wavelength-dependent PMD. Also, while fiber stretchers
are described in the disclosure, it should be understood that a
fiber compressor or a device that changes any other characteristics
of the fiber, could alternatively be used. These and other
embodiments are intended to be encompassed by the following
claims.
* * * * *