U.S. patent application number 09/730650 was filed with the patent office on 2002-07-18 for dynamic gain equalizer for optical amplifiers.
Invention is credited to Huang, Tizhi, Liu, Jian-Yu, Mao, Chongchang, Wong, Charles, Wu, Kuang-Yi, Xu, Ming.
Application Number | 20020093725 09/730650 |
Document ID | / |
Family ID | 24936228 |
Filed Date | 2002-07-18 |
United States Patent
Application |
20020093725 |
Kind Code |
A1 |
Xu, Ming ; et al. |
July 18, 2002 |
DYNAMIC GAIN EQUALIZER FOR OPTICAL AMPLIFIERS
Abstract
An optical equalizer for use primarily with an erbium-doped
fiber amplifier has an initial polarizer that convert the input
beam to a predetermined polarization, followed by a series of
dynamically-adjustable sinusoidal filters that provide attenuation
as a sinusoidal function of beam wavelength. Each of the sinusoidal
filters has a first liquid crystal cell adjustably rotating the
polarization of the beam from the preceding polarizer. This is
followed by a second optical element that retards the beam as a
sinusoidal function of beam wavelength. For example, the second
optical element can be a birefringent crystal that provided a fixed
degree of retardance to the beam and a second liquid crystal cell
that provides a variable degree of retardance, thereby allowing
adjustment of the center frequency of the sinusoidal function.
Finally, a third liquid crystal cell adjustably rotates the
polarization of the beam. A final polarizer provides amplitude
control of the beam based on the polarization rotations introduced
by the first and third liquid crystal cells. A controller provides
control signals to the liquid crystal cells of each sinusoidal
filter so that their combined sinusoidal attenuation functions
produce a desired equalization curve.
Inventors: |
Xu, Ming; (Dallas, TX)
; Huang, Tizhi; (Plano, TX) ; Mao, Chongchang;
(Plano, TX) ; Liu, Jian-Yu; (Garland, TX) ;
Wu, Kuang-Yi; (Plano, TX) ; Wong, Charles;
(Richardson, TX) |
Correspondence
Address: |
BAKER BOTTS L.L.P.
2001 ROSS AVENUE
SUITE 600
DALLAS
TX
75201-2980
US
|
Family ID: |
24936228 |
Appl. No.: |
09/730650 |
Filed: |
December 5, 2000 |
Current U.S.
Class: |
359/337 ;
359/484.05; 359/487.02; 359/487.05; 359/487.06; 359/489.05;
359/489.07 |
Current CPC
Class: |
H01S 2301/04 20130101;
H01S 3/06754 20130101; H01S 3/10023 20130101; G02F 1/0136 20130101;
G02F 2203/05 20130101; H04B 10/2941 20130101; G02F 1/13471
20130101; G02F 2203/48 20130101 |
Class at
Publication: |
359/337 ;
359/501 |
International
Class: |
H01S 003/094 |
Claims
We claim:
1. An optical equalizer comprising: an initial polarizer converting
an input beam to a predetermined polarization; and a plurality of
sinusoidal filters receiving the polarized beam in series providing
attenuation as a sinusoidal function of beam wavelength, wherein
each of said sinusoidal filters has: (a) a first liquid crystal
cell adjustably rotating the polarization of the beam from the
preceding polarizer as determined by a control signal; (b) a second
optical element adjustably controlling the retardance of the beam
from said first liquid crystal cell as a sinusoidal function of
beam wavelength, wherein the degree of retardation is adjustably
controllable by a control signal, thereby allowing adjustment of
the center frequency of the sinusoidal function; (c) a third liquid
crystal cell adjustably rotating the polarization of the beam from
said second optical element as determined by a control signal; and
(d) a final polarizer passing that component of the beam from the
third liquid crystal cell having a predetermined polarization,
thereby providing amplitude control of the beam based on the
polarization rotations provided by said first and third liquid
crystal cells; wherein the combined sinusoidal attenuation
functions of said sinusoidal filters produce a desired equalization
curve as a function of beam wavelength.
2. The optical equalizer of claim 1 wherein said second optical
element comprises a liquid crystal cell.
3. The optical equalizer of claim 1 wherein said second optical
element comprises: a liquid crystal cell providing a variable
degree of retardance determined by the control signal; and a
birefringent element providing a fixed degree of retardance.
4. The optical equalizer of claim 1 comprising at least three of
said sinusoidal filters in series.
5. The optical equalizer of claim 1 comprising at least five of
said sinusoidal filters in series.
6. The optical equalizer of claim 1 wherein at least one of said
liquid crystal cells comprises a twist nematic liquid crystal
cell.
7. The optical equalizer of claim 1 wherein at least one of said
liquid crystal cells comprises a mixed twist nematic liquid crystal
cell.
8. The optical equalizer of claim 1 wherein at least one of said
liquid crystal cells comprises a pi-cell liquid crystal cell.
9. An optical equalizer comprising: an initial polarizer converting
an input beam to a predetermined polarization; and a plurality of
sinusoidal filters receiving the polarized beam in series providing
attenuation as a sinusoidal function of beam wavelength, wherein
each of said sinusoidal filters has: (a) a first liquid crystal
cell adjustably rotating the polarization of the beam from the
preceding polarizer as determined by a control signal; (b) a
birefringent element causing retardation of the beam from the first
liquid crystal cell as a sinusoidal function of beam wavelength;
(c) a second liquid crystal cell providing a variable degree of
retardance to the beam as determined by a control signal, thereby
allowing adjustment of the center frequency of the sinusoidal
function; (d) a third liquid crystal cell adjustably rotating the
polarization of the beam from said second liquid crystal cell as
determined by a control signal; and (e) a final polarizer passing
that component of the beam from the third liquid crystal cell
having a predetermined polarization, thereby providing amplitude
control of the beam based on the polarization rotations provided by
said first and third liquid crystal cells; wherein the combined
sinusoidal attenuation functions of said sinusoidal filters produce
a desired equalization curve as a function of beam wavelength.
10. The optical equalizer of claim 9 comprising at least three of
said sinusoidal filters in series.
11. The optical equalizer of claim 9 comprising at least five of
said sinusoidal filters in series.
12. The optical equalizer of claim 9 wherein at least one of said
liquid crystal cells comprises a twist nematic liquid crystal
cell.
13. The optical equalizer of claim 9 wherein at least one of said
liquid crystal cells comprises a pi-cell liquid crystal cell.
14. The optical equalizer of claim 9 further comprising a
controller providing control signals to said liquid crystal cells
of said sinusoidal filters to produce sinusoidal attenuation
functions creating a desired equalization function.
15. An optical amplification system comprising: an optical
amplifier having an adjustable gain setting, and a resulting gain
spectrum that is a function of both the beam wavelength and said
gain setting; an optical equalizer having: (a) an initial polarizer
converting the beam from said optical amplifier to a predetermined
polarization; and (b) a plurality of sinusoidal filters receiving
the polarized beam in series providing attenuation as a sinusoidal
function of beam wavelength, wherein each of said sinusoidal
filters has: (i) a first liquid crystal cell adjustably rotating
the polarization of the beam from the preceding polarizer as
determined by a control signal; (ii) a second optical element
adjustably controlling the retardance of the beam from said first
liquid crystal cell as a sinusoidal function of beam wavelength,
wherein the degree of retardation is adjustably controllable by a
control signal, thereby allowing adjustment of the center frequency
of the sinusoidal function; (iii) a third liquid crystal cell
adjustably rotating the polarization of the beam from said second
optical element as determined by a control signal; and (iv) a final
polarizer passing that component of the beam from the third liquid
crystal cell having a predetermined polarization, thereby providing
amplitude control of the beam based on the polarization rotations
provided by said first and third liquid crystal cells; and a
controller providing control signals to said liquid crystal cells
of said sinusoidal filters to produce sinusoidal attenuation
functions creating an equalization function determined by the gain
setting of said optical amplifier.
16. The optical equalizer of claim 15 comprising at least three of
said sinusoidal filters in series.
17. The optical equalizer of claim 15 comprising at least five of
said sinusoidal filters in series.
18. The optical equalizer of claim 15 wherein at least one of said
liquid crystal cells comprises a twist nematic liquid crystal
cell.
19. The optical equalizer of claim 15 wherein at least one of said
liquid crystal cells comprises a pi-cell liquid crystal cell.
20. An optical equalizer comprising: an initial polarizer
converting an input beam to a predetermined polarization; and a
plurality of sinusoidal filters receiving the polarized beam in
series providing attenuation as a sinusoidal function of beam
wavelength, wherein each of said sinusoidal filters has: (a) a
first liquid crystal cell adjustably rotating the polarization of
the input beam to the filter as determined by a control signal; (b)
a birefringent element introducing retardance of the beam from said
first liquid crystal cell as a sinusoidal function of beam
wavelength; and (c) a second liquid crystal cell adjustably
controlling the retardance of the beam from the birefringent
element, wherein the degree of retardation is adjustably
controllable by a control signal, thereby allowing adjustment of
the center frequency of the sinusoidal function; wherein the
combined sinusoidal attenuation functions of said sinusoidal
filters produce a desired equalization curve as a function of beam
wavelength.
21. The optical equalizer of claim 20 comprising at least three of
said sinusoidal filters in series.
22. The optical equalizer of claim 20 comprising at least five of
said sinusoidal filters in series.
23. The optical equalizer of claim 20 wherein at least one of said
liquid crystal cells comprises a twist nematic liquid crystal
cell.
24. The optical equalizer of claim 20 wherein at least one of said
liquid crystal cells comprises a pi-cell liquid crystal cell.
25. An optical equalizer comprising: a first birefringent element
spatially separating an input beam into a pair of
orthogonally-polarized beams; a first polarization rotator rotating
the polarization of at least one of the orthogonally-polarized
beams so that both beams have substantially the same polarization;
a circulator transmitting the polarized beams in a forward
direction, but routing light from the opposite direction in a
second direction; a series of sinusoidal filters, each sinusoidal
filter transmitting the polarized beams from the circulator along
parallel optical paths and providing attenuation as a sinusoidal
function of beam wavelength so that the combined sinusoidal
attenuation functions of the sinusoidal filters produce a desired
equalization curve as a function of beam wavelength, wherein each
of said sinusoidal filters has: (a) a first liquid crystal cell
adjustably rotating the polarization of the beams from the
preceding polarizer as determined by a control signal; (b) a second
optical element adjustably controlling the retardance of the beams
from said first liquid crystal cell as a sinusoidal function of
beam wavelength, wherein the degree of retardation is adjustably
controllable by a control signal, thereby allowing adjustment of
the center frequency of the sinusoidal function; (c) a third liquid
crystal cell adjustably rotating the polarization of the beams from
said second optical element as determined by a control signal; and
(d) a final polarizer passing those components of the beams from
the third liquid crystal cell having a predetermined polarization,
thereby providing amplitude control of the beams based on the
polarization rotations provided by said first and third liquid
crystal cells; a retro-reflector reflecting the beams exiting the
sinusoidal filters back through the sinusoidal filters, but with
their optical paths exchanged; wherein the circulator spatially
separates the reflected beams exiting the sinusoidal filters from
the polarized input beams and routes the reflected beams in the
second direction; a final polarization rotator rotating the
polarization of at least one of the reflected beams from the
circulator to produce a pair of orthogonally-polarized beams; and a
final birefringent element combining the orthogonally-polarized
beams from the final polarization rotator to produce an output
beam.
26. The optical equalizer of claim 25 wherein the circulator
comprises a half-wave plate, Faraday rotator, and
polarization-dependent routing element.
27. The optical equalizer of claim 25 wherein the second optical
element comprises: a liquid crystal cell providing a variable
degree of retardance determined by the control signal; and a
birefringent element providing a fixed degree of retardance.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates generally to the field of
equalizers for use in optical communications networks. More
specifically, the present invention discloses a dynamic gain
equalizer for use primarily with erbium-doped fiber amplifiers.
[0003] 2. Statement of the Problem
[0004] Erbium-doped fiber amplifiers (EDFA) are widely used in
optical communication systems. However, their gain spectrum is not
flat, which limits their applications in dense wavelength division
multiplex (DWDM) systems. To address this problem, some EDFAs use a
fixed filter in an attempt to flatten the gain spectrum. For
example, a typical commercial EDFA has a gain ripple of
approximately 1-3 dB using fixed filters. As shown in FIG. 1, a
typical EDFA gain spectrum will shift as a function of its gain
setting. While a fixed filter may be effective at a specific
operating gain for an EDFA, a fixed filter cannot accommodate
changes in the gain spectrum as illustrated in FIG. 1. Future WDM
systems will require EDFAs with operating windows of more than 30
nm and gain flatness within 1 dB peak-peak. It is also essential
for these high performances to be maintained over a range of
operating gains and over the lifetime of the EDFA.
[0005] The prior art in this field includes several approaches that
have been introduced to implement dynamically adjustable filters.
Acousto-optic tunable filters have been used to flatten an EDFA to
.+-.0.7 dB over a 6 dB dynamic range as taught by Hyo Sang Kim et
al., "Dynamic Gain Equalization of Erbium-Doped Fiber Amplifier
With All-Fiber Acousto-Optic Tunable Filters," OFC '98 Technical
Digest, WG4, pp. 136-138. However, acousto-optic tunable filters
have the drawbacks of significant polarization sensitivity,
intermodulation effects produced by the multiple drive frequencies,
and high RF power consumption.
[0006] Parry et al., "Dynamic Gain Equalisation of EDFAs with
Fourier Filters," Tech. Dig. OAA, 1999, paper ThD 22 (Nortel) have
suggested that a set of harmonic sinusoidal filter elements, (e.g.,
Mach-Zehnder devices) can be cascaded together to build a dynamic
gain flattener if each filter can tune center frequencies and
attenuation depths, as taught by Betts et al. "Split-Beam Fourier
Filter and its Application in a Gain-Flattened EDFA," OFC '95
Technical Digest, TuP4, pp. 80-81.
[0007] 3. Solution to the Problem
[0008] The present invention employs liquid crystal light modulator
technology to implement a dynamic gain equalizer consisting of a
series of sinusoidal filters. Nothing in the prior teaches or
suggests an equalizer using liquid crystal technology to implement
a sequence of sinusoidal filters with tunable depth and center
wavelength. In contrast to the prior art, the present approach
offers the following advantages:
[0009] 1. Dynamic. The filters are able to flatten the gain profile
at different input levels, different temperatures, and different
periods in the amplifier's life time.
[0010] 2. High performance. The present device has low insertion
loss, low polarization dependent loss, fast response and low power
consumption.
[0011] 3. Easy to implement. The present design consists of a
series of LC cells and crystals. Liquid crystal cells are a mature
technology and the requirements for the LC cells are within
industrial standards. Crystal polishing and cutting is also a
mature technology.
[0012] 4. Cost effective. All parts in the present design are
neither expensive nor difficult to obtain.
SUMMARY OF THE INVENTION
[0013] This invention provides an optical equalizer having an
initial polarizer that convert the input beam to a predetermined
polarization, followed by a series of dynamically-adjustable
sinusoidal filters that provide attenuation as a sinusoidal
function of beam wavelength. Each of the sinusoidal filters has a
first liquid crystal cell adjustably rotating the polarization of
the beam from the preceding polarizer. This is followed by a second
optical element that retards the beam as a sinusoidal function of
beam wavelength. For example, the second optical element can be a
birefringent crystal that provided a fixed degree of retardance to
the beam and a second liquid crystal cell that provides a variable
degree of retardance, thereby allowing adjustment of the center
frequency of the sinusoidal function. Finally, a third liquid
crystal cell adjustably rotates the polarization of the beam. A
final polarizer provides amplitude control of the beam based on the
polarization rotations introduced by the first and third liquid
crystal cells. A controller provides control signals to the liquid
crystal cells of each sinusoidal filter so that their combined
sinusoidal attenuation functions produce a desired equalization
curve.
[0014] These and other advantages, features, and objects of the
present invention will be more readily understood in view of the
following detailed description and the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The present invention can be more readily understood in
conjunction with the accompanying drawings, in which:
[0016] FIG. 1 is a graph showing the gain spectra for a typical
EDFA.
[0017] FIG. 2 is a schematic block diagram showing the manner in
which the present equalizer 20 is used with an optical amplifier
(e.g., an EDFA) 10 to dynamically equalize its gain spectrum.
[0018] FIG. 3 is a schematic block diagram of the present equalizer
20.
[0019] FIG. 4 is a graph illustrating the relationship between
attenuation depth and the voltages applied to the first and third
liquid crystal cells, LC.sub.i1 and LC.sub.i3, for a typical
filter. Attenuation depth increases with voltage.
[0020] FIG. 5 is a graph showing the relationship between the
center wavelength and voltage on the second liquid crystal cell,
LC.sub.i2, for a typical filter.
[0021] FIG. 6 is a graph showing a typical input gain spectrum from
a EDFA and the resulting output spectra for a dynamic equalizer
having from one to five cascaded filters.
[0022] FIGS. 7(a) through 7(d) are graphs of experimental results
showing sinusoidal amplitude tuning. Attenuation depth increases
with the voltage on the first and third liquid crystal cells,
LC.sub.i1 and LC.sub.i3.
[0023] FIGS. 8(a) through 8(d) are graphs of experimental results
showing center wavelength tuning by varying the voltage on the
second liquid crystal cell, LC.sub.i2.
[0024] FIG. 9 is a schematic diagram of a polarization rotator
consisting of an anti-parallel liquid crystal cell with its optical
axis at an angle of 45 degrees to the x-axis and a quarter-wave
plate with its optical axis parallel to the x-axis.
[0025] FIG. 10 is a schematic block diagram of a second embodiment
of the equalizer.
[0026] FIG. 11 is a graph similar to FIG. 6 showing a typical input
gain spectrum from a EDFA and the resulting output spectra for an
equalizer as shown in FIG. 10 after each of the five cascaded
filter stages.
[0027] FIGS. 12(a) through 12(d) are graphs showing experimental
output spectra from the equalizer in FIG. 10 in response to input
signals of different power levels, and therefore different spectra
shapes.
[0028] FIG. 13 is a graph showing chromatic dispersion of the
equalizer in FIG. 10 as a function of wavelength.
[0029] FIG. 14 is a schematic diagram of another embodiment of the
equalizer.
[0030] FIG. 15 is a graph showing a typical input gain spectrum
from a EDFA and the resulting output spectra for an equalizer as
shown in FIG. 14 after each of the filter stages.
[0031] FIG. 16 is a graph showing the polarization dependent loss
of a one-stage equalizer when the amplitude of intensity
oscillation is 10 dB.
[0032] FIG. 17 is a graph showing the polarization dependent loss
of a five-stage equalizer when the input intensity curve is
flattened.
[0033] FIG. 18 is a schematic diagram of another embodiment of the
equalizer intended to minimize polarization dependent loss.
[0034] FIG. 19 is a graph showing the polarization dependent loss
of a one-stage equalizer using the reflective architecture in FIG.
18 when the intensity oscillation is 10 dB.
DETAILED DESCRIPTION OF THE INVENTION
[0035] FIG. 2 is a schematic block diagram showing an optical
amplifier 10, such as an EDFA, that amplifies WDM optical signals
received from an optical fiber. As previously discussed, the gain
spectrum of the EDFA 10 is typically not flat, and also varies as a
function of gain setting of the EDFA 10, as previously discussed
and illustrated in FIG. 1. The gain setting of the EDFA 10 is
determined by a controller 30 (e.g., a microprocessor) to achieve a
desired output power level. The present equalizer 20 receives the
amplified output signal from the EDFA 10 and dynamically equalizes
its gain spectrum based on the EDFA's gain setting supplied by the
controller 30. In an optical network, two EDFAs are often chained
together, one serves as a low noise pre-amplifier and the other as
a power booster. The present equalizer can be placed between these
two EDFAs to achieve spectrum flat output.
[0036] FIG. 3 is a schematic block diagram of the present equalizer
20. The equalizer 20 consists of an initial polarizer P.sub.0
(e.g., sheet dichroic or polarizing beamsplitter type) followed by
a series of N dynamically-adjustable sinusoidal filters (e.g., N is
approximately three to eight, and preferably about five). The
initial polarizer P.sub.0 converts the input beam to a
predetermined polarization (e.g., vertical polarization).
[0037] In this embodiment, each sinusoidal filter is comprised of
three liquid crystal cells followed by a final polarizer at a
predetermined orientation. The first and third liquid crystal cells
LC.sub.n1 and LC.sub.n3 are used to adjust the sinusoidal amplitude
of the n.sup.th filter, and LC.sub.n2 is used to tune the center
wavelength of the n.sup.th filter. All liquid crystal cells have
some twist angle .phi., with .phi. having a value between
-90.degree. to 90.degree.. Different cells may have different .phi.
values. If the cell is an ordinary twist nematic LCD, .phi. is
90.degree.. Mixed twist nematic cells having twist angles of
45.degree., 30.degree., etc. can also be used. If the cell is an
anti-parallel or pi-cell LCD, then .phi. is 0.degree.. The
alignment conditions for the cells can be homogeneous (i.e., the LC
molecules have a pretilt with a few degrees from the substrate
surfaces), homeotropic (i.e., the LC molecules have a pretilt
within a few degrees from the substrate normal), or tilt alignment
(i.e., between homogeneous and homeotropic).
[0038] LC.sub.n1 and LC.sub.n3 perform the task of amplitude tuning
by rotating the polarization of the light--the degree of rotation
changes with the voltage applied by the controller 30. The
intensity of light passing through the final polarizer P.sub.n will
change accordingly. The final polarizer P.sub.n also ensure proper
polarization of the beam entering the next filter.
[0039] To understand how these LC cells work, please consider an
embodiment in which the first and third liquid crystal cells,
LC.sub.n1 and LC.sub.n3, operate in anti-parallel mode. This is
perhaps the easiest mode to understand because one anti-parallel LC
cell plus a crystal quarter-wave plate form a linear polarization
rotator. FIG. 9 is a schematic diagram of a polarization rotator
consisting of an anti-parallel liquid crystal cell 91 with its
optical axis at an angle of 45 degrees to the x-axis and a
quarter-wave plate 92 with its optical axis parallel to the x-axis.
By adjusting the voltage on the LC cell 91, the input linear
polarization along the x-axis can be rotated to a linear
polarization along any direction on the x-y plane.
[0040] A theoretical analysis of this polarization rotation is
given below. The input polarization is: 1 E in = ( E 0 0 )
[0041] After passing through the LC retarder having birefringence
An and thickness d, in the principal frame of the retarder:
E.sub.mid=E.sub..parallel.e.sup.i.phi.+E.sub..perp.
[0042] with 2 E // = E = E 0 2 and = 2 nd
[0043] Back into the lab frame: 3 E mid = E 0 2 ( i + 1 i - 1 )
[0044] On exiting the quarter-wave plate: 4 E out = E 0 2 ( i + 1 i
2 i - i 2 ) = E 0 2 ( 1 + cos + i sin - sin - i + i cos ) = E 2 ( 1
+ cos i tan - 1 ( sin 1 + cos ) 1 - cos i tan - 1 ( 1 - cos sin ) )
= E 0 2 ( 1 + cos i 2 1 - cos i 2 )
[0045] E.sub.out is linear because its x and y components have the
same phase. It makes an angle of (E.sub.y/E.sub.x)=.phi./2 to the
x-axis. .phi. is controlled by the voltage on the LC cell.
[0046] Based on this understanding of how each polarization rotator
works, it is easier to understand how amplitude modulation is
achieved in the embodiment shown in FIG. 3. The input linear
polarization is rotated by LC.sub.n1 and the quarter-wave plate (as
part of the center crystal waveplate) so that it makes an angle,
.alpha., with respect to the crystal axis. When the parameter
.DELTA.nd of the crystal waveplate is an integer number of
wavelengths, the polarization remains at the angle .alpha. relative
to the crystal axis. The combination of a quarter-wave plate (as
part of the center crystal waveplate) and LC.sub.n3 would rotate
the polarization back to the x-axis. The relative intensity of the
of the transmitted beam equals one after passing through the second
polarizer. When the parameter .DELTA.nd of the crystal waveplate is
an integer number of wavelengths plus .lambda./2, the polarization
rotates to an angle of -.alpha. to the optical axis of the crystal.
Similarly, a quarter-wave plate and LC.sub.n3 rotate the
polarization to an angle of -2.alpha. with respect to the x-axis.
The intensity at these wavelengths is cos.sup.2(2.alpha.), which
varies with the applied voltage. The intensities at other
wavelengths is in between these two extremes.
[0047] As previously mentioned, the first and third liquid crystal
cells, LC.sub.n1 and LC.sub.n3, operate in anti-parallel mode in
the preferred embodiment of the present invention. However, twist
nematic liquid crystal cells or pi-cell liquid crystal cells could
also be employed.
[0048] The second liquid crystal cell is employed to tune the
center wavelength by controlling the retardation of light as a
function of beam wavelength. The effective thickness of the second
LC cell, and therefore the degree of retardation, is adjustably
controlled by the voltage provided by the controller 30. In this
system, the thickness of the second liquid crystal cell is
different from stage to stage, depending on required sinusoidal
wave's periods (free spectrum range, FSR, obtained from Fourier
analysis of the gain profile to be flattened). This might create
manufacturing difficulty, particularly since the second liquid
crystal cell must have a considerable thickness. To solve this
problem, the second liquid crystal can be replaced with a
combination of a solid birefringent crystal C.sub.n that provides a
fixed degree of retardance, and a liquid crystal cell LC.sub.n2
that provides a variable degree of retardance, as illustrated in
FIG. 3. These solid crystals C.sub.n are designed with different
thicknesses for each stages, while LC.sub.n2 can have the same
thickness for each stage, which should be large enough to ensure a
full FSR tuning. The solid birefringent crystal Cn typically has a
thickness on the order of many hundreds of wavelengths, which
greatly reduces the required thickness of the liquid crystal cell
LC.sub.n2 and thereby reduces manufacturing difficulties and
cost.
[0049] The controller 30 is typically a microprocessor that has
been programmed to output control voltages to each of the liquid
crystal cells in each sinusoidal filter as a function of the gain
of the EDFA 10, so that the combined sinusoidal attenuation
functions of the sinusoidal filters produce a desired equalization
curve. The appropriate magnitude of the control signal for each LC
cell can be derived from a combination of empirical data and
Fourier analysis of the output spectra of the EDFA 10 at each gain
setting.
[0050] The results of a simulation of a gain equalizer 20 with a
single-filter are shown in FIGS. 4 and 5. FIG. 4 is a graph
illustrating the relationship between attenuation depth and the
voltages applied by the controller 30 to the first and third liquid
crystal cells, LC.sub.n1 and LC.sub.n3, for a typical filter stage.
Attenuation depth increases with voltage. FIG. 5 is a graph showing
the relationship between the center wavelength and the voltage
applied by the controller 30 to the second liquid crystal cell,
LC.sub.n2, for a typical filter stage. In other words, the center
wavelength is tunable by varying the voltage on LC.sub.n2. While
tuning the depth, the center wavelength may shift but this is no
problem because we can tune the center wavelength easily by
controlling the voltage on LC.sub.n2.
[0051] FIG. 6 is a graph showing the simulation result a typical
gain spectrum from a EDFA and the resulting output spectra for a
dynamic equalizer having from one to five cascaded filters. The
gain profile is flattened gradually after passing each filter. It
can flatten the gain profile to within 0.4 dB, and with further
effort, perhaps to within 0.3 dB.
[0052] To verify the simulation, we built a single-filter equalizer
using two polarizers, three liquid crystal cells, and a solid
crystal wave plate. By varying the control voltage applied to the
first and the third liquid crystal cells, LC.sub.n1 and LC.sub.n3,
we can change filtering amplitude. FIGS. 7(a) through 7(d) are
graphs of experimental results showing sinusoidal amplitude tuning.
Attenuation depth increases with the voltage applied on the first
and third liquid crystal cells, LC.sub.n1 and LC.sub.n3 In
particular, the amplitude change is about 7 dB when the applied
voltage increases from 0 V to 2 V.
[0053] FIGS. 8(a) through 8(d) are graphs of experimental results
showing center wavelength tuning by varying the voltage on the
second liquid crystal cell LC.sub.n2. These results demonstrate
that we can shift the center wavelength over more than one free
spectrum range (FSR) and agree very well with the simulation
results for one filter.
[0054] FIG. 10 is a schematic block diagram of a second embodiment
of the equalizer. At the input port 101, the input beam is
spatially separated into two orthogonally polarized beams by a
birefringent element 102. A polarization rotator 103 rotates the
polarization of one of the beams by 90 degrees, so that both beams
have the same polarization entering the first stage of the
equalizer. Thus, the birefringent element 102 and polarization
rotator 103 eliminate the need for the initial polarizer P.sub.0 in
FIG. 3. In each of the first four stages, the final polarizer
(P.sub.1 through P.sub.4 in FIG. 3) has been replaced by a
polarized beamsplitter (PBS) to increase isolation between stages.
Light having a first polarization is transmitted by the PBS, but
any light having a second, orthogonal polarization is reflected by
the PBS. Thus, the PBS can be considered to be a "polarizer" in
that it passes only that component of each beam having a desired
polarization. In the fifth stage, a polarization rotator 113
rotates the polarization of one of the beams to create an
orthogonally-polarized beam pair. A final birefringent element 112
combines this beam pair into a output beam at the output port 111.
It should be understood that this combination of polarization
rotator 113 and birefringent element 112 also serves as a
"polarizer" because only that component of each beam having a
desired polarization are routed to the output port 111. Light of
any other polarization will follow an optical path through the
birefringent element 112 that is not aligned with the output port
111.
[0055] FIG. 11 is a graph similar to FIG. 6 showing a typical input
gain spectrum from a EDFA and the resulting output spectra for the
equalizer in FIG. 10 after each of the five filter stages. FIGS.
12(a) through 12(d) are graphs showing experimental output spectra
from the equalizer in FIG. 10 in response to input signals of
different power levels, and therefore different spectra shapes. The
equalizer in FIG. 5 is characterized by good performance
parameters, such as polarization mode dispersion (PMD) and
chromatic dispersion. The average PMD is less than 0.1 pico-second,
which is very good. Chromatic dispersion is also small, as shown in
the graph provided in FIG. 13.
[0056] FIG. 14 is a schematic diagram of another embodiment of the
equalizer in which all of the polarized beamsplitters and the third
LC cells have been removed so that the phases of the stages are now
coupled together. In other words, the phase of the output beams
from the second LC cell is the phase of the input beams to first LC
cell in the next stage. This approach has the advantage of
requiring fewer components and therefore costing less. It is also
smaller in size and has a lower insertion loss. However, this
embodiment is more difficult to tune and requires a more
complicated control algorithm due to the coupling between stages.
FIG. 15 is a graph showing a typical input gain spectrum from a
EDFA and the resulting output spectra for an equalizer as shown in
FIG. 14 after each of the filter stages.
[0057] The embodiments depicted in FIGS. 10 and 14 demonstrate
excellent functionality in terms of gain equalization, but may have
an undesirably large polarization dependent loss (PDL). PDL is very
sensitive to non-uniformity of all of the optical components
employed in the device. In the embodiments in FIGS. 10 and 14, the
beam pair passes through the optical components in each filter
stage with a separation of about 1 mm. If all of the optical
components are absolutely uniform, both beams experience the same
thicknesses and indices of components, so that PDL is zero. In the
real world, PDL may be very large due to non-uniformity of the
crystals and liquid crystal cells. FIG. 16 is a graph showing the
polarization dependent loss of a one-stage equalizer when the
amplitude of intensity oscillation is 10 dB. Similarly, FIG. 17 is
a graph showing the polarization dependent loss of a five-stage
equalizer when the input intensity curve is flattened. Both graphs
show large PDL at certain wavelengths.
[0058] FIG. 18 is a schematic diagram of another embodiment of the
equalizer intended to minimize polarization dependent loss. Both
beams pass through the filter stages twice, but their respective
optical paths are exchanged for the second pass. The total optical
path for both beams are then substantially identical. This design
is essentially insensitive to any non-uniformity of the optical
components. FIG. 19 is a graph showing the polarization dependent
loss of a one-stage equalizer using the reflective architecture in
FIG. 18 when the intensity oscillation is 10 dB. Compared to the
embodiment in FIG. 10, the PDL of the reflective scheme in FIG. 18
is much smaller. The insertion loss is, however, larger due to the
fact that the optical path length is almost doubled.
[0059] Turning to FIG. 18, the beam from the input port 101 is
separated by a birefringent element 102 into two orthogonally
polarized beams, which are converted to a pair beams having the
same polarization by the polarization rotator 103. These beams pass
through a Faraday rotator 185 and a half-wave plate 186 without
changing polarization. The beams initially pass in the forward
direction (i.e., from left to right) through a series of filter
stages, as previously discussed. In the specific embodiment
illustrated in FIG. 18, each stage consists of a polarized
beamsplitter PBS.sub.n, three liquid crystal cells LC.sub.n1,
LC.sub.n2, and LC.sub.n3, and a birefringent crystal C.sub.n, which
function as previously described. The beam pair are then reflected
by a retro-reflector 190 back through the filter stages in the
opposite direction (i.e., from right to left), but with their
respective optical paths exchanged. The filter stages are
inherently bidirectional. At the end of the return pass, the
half-wave plate 186, Faraday rotator 185, and rhomboid prism act as
a circulator to spatially separate the reflected beams from the
polarized input beams. In particular, the half-wave plate 186 and
Faraday rotator 185 rotate the polarization of the reflected beam
pair by 90 degrees so that their polarization is orthogonal to that
of the polarized input beams propagating in the forward direction.
The reflected beam pair are reflected twice within the rhomboid
prism 181, routed through a polarization rotator 113, and combined
by a birefringent element 112 at the output port 111. It should be
noted that a polarized beamsplitter or any other type of
polarization-dependent routing element could be used in place of
the homboid prism 181 to separate the reflected beam pair from the
polarized input beams.
[0060] The above disclosure sets forth a number of embodiments of
the present invention. Other arrangements or embodiments, not
precisely set forth, could be practiced under the teachings of the
present invention and as set forth in the following claims.
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