U.S. patent application number 10/657742 was filed with the patent office on 2004-07-01 for dynamic gain equalizer.
This patent application is currently assigned to Xtellus, Inc.. Invention is credited to Cohen, Gil, Corem, Yossi, Suh, Seongwoo.
Application Number | 20040126120 10/657742 |
Document ID | / |
Family ID | 26324015 |
Filed Date | 2004-07-01 |
United States Patent
Application |
20040126120 |
Kind Code |
A1 |
Cohen, Gil ; et al. |
July 1, 2004 |
Dynamic gain equalizer
Abstract
A gain equalizer in which a multichannel input light signal is
split into its separate wavelength components by means of a
dispersive element such as a grating, and the spatially separated
wavelength components are passed through a linear array of variable
optical attenuators based on liquid crystal phase elements which
modulate the phase of part of the cross section of the light. The
separate attenuated wavelength components are then recombined and
output. The attenuation level of each variable optical attenuator
is adjusted according to the output of the light as a function of
its wavelength components, and in this way, the overall wavelength
profile of the output light signal can be adjusted to any
predefined form, whether a flattened spectral profile, as in gain
equalization applications, or a spectral compensating profile.
Inventors: |
Cohen, Gil; (Livingston,
NJ) ; Suh, Seongwoo; (Florham Park, NJ) ;
Corem, Yossi; (Beit Shemesh, IL) |
Correspondence
Address: |
DARBY & DARBY P.C.
Post Office Box 5257
New York
NY
10150-5257
US
|
Assignee: |
Xtellus, Inc.
Morris Plains
NJ
|
Family ID: |
26324015 |
Appl. No.: |
10/657742 |
Filed: |
September 5, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10657742 |
Sep 5, 2003 |
|
|
|
PCT/IL02/00187 |
Mar 8, 2002 |
|
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|
Current U.S.
Class: |
398/158 |
Current CPC
Class: |
G02F 2203/48 20130101;
G02F 2201/04 20130101; G02F 2201/17 20130101; G02F 1/134309
20130101; G02F 1/0115 20130101; G02F 1/1326 20130101; G02F 1/1396
20130101; G02F 1/2955 20130101; G02B 6/266 20130101 |
Class at
Publication: |
398/158 |
International
Class: |
H04B 010/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 8, 2001 |
IL |
141927 |
Apr 24, 2001 |
IL |
142773 |
Claims
We claim:
1. A gain equalizer comprising: an input port receiving light
comprising at least two wavelength components; a first dispersive
element receiving said light and spatially dispersing said
wavelength components of said light along a dispersion direction; a
plurality of variable optical attenuating elements disposed
generally along said dispersion direction, such that each of said
attenuating elements is traversed by a different wavelength
component of said light; a second dispersive element receiving
light after passage through at least part of at least one of said
plurality of variable optical attenuating elements, and operative
to combine said wavelength components of said light into an output
beam; and an output port receiving said output beam; wherein at
least one of said variable optical attenuating elements comprises a
variable phase changing element operative to change the phase of
part of the cross section of light passing through it.
2. A gain equalizer according to claim 1, and wherein at least one
of said attenuating elements is varied such as to vary the level of
light traversing said attenuating element.
3. A gain equalizer according to claim 1 and wherein at least one
of said input and output ports is an optical fiber.
4. A gain equalizer according to claim 2 and also comprising a
controller operative to vary the attenuation of at least one of
said variable attenuating elements, such that the light passing
through said attenuating element has a predefined level.
5. A gain equalizer according to claim 4 and also comprising a
spectrally selective detector providing to said controller at least
one signal corresponding to the power level of at least one of said
wavelength components.
6. A gain equalizer according to claim 5 and wherein said at least
one signal is utilized to adjust the attenuation of at least one of
said variable attenuating elements.
7. A gain equalizer according to claim 5 and wherein said detector
is located such that it measures the power level of at least one of
said wavelength components in said output beam.
8. A gain equalizer according to claim 5 and wherein said detector
is located such that it measures the power level of at least one of
said wavelength components of said light in said input port.
9. A gain equalizer according to claim 5 and wherein said
spectrally selective detector is a linear detector array utilizing
one of said dispersive elements for performing said spectral
selection.
10. A gain equalizer according to claim 5 and wherein said at least
one signal corresponding to the power level of at least one of said
wavelength components is obtained by means of a power splitter
located in the path of said wavelength components of said
light.
11. A gain equalizer according to claim 1 and wherein said phase
changing element is a liquid crystal element.
12. A gain equalizer according to claim 1 and wherein at least one
of said dispersive elements is a grating.
13. A gain equalizer according to claim 1 and also comprising a
half wave plate serially with said plurality of attenuating
elements, operative to reduce the polarization dependent loss of
said gain equalizer
14. A gain equalizer comprising: a port receiving light comprising
at least two wavelength components; a dispersive element receiving
said light and spatially dispersing said wavelength components of
said light along a dispersion direction; a plurality of variable
optical attenuating elements disposed generally along said
dispersion direction, such that each of said attenuating elements
is traversed by a different wavelength component of said light; and
a reflective surface operative to reflect light after passage
through at least part of at least one of said plurality of variable
optical attenuating elements back to said dispersive element, so as
to combine said wavelength components of said reflected light into
an output beam at said port; wherein at least one of said variable
optical attenuating elements comprises a variable phase changing
element operative to change the phase of part of the cross section
of light passing through it.
15. A gain equalizer according to claim 14, and wherein at least
one of said attenuating elements is varied such as to vary the
level of light traversing said attenuating element.
16. A gain equalizer according to claim 14 and wherein said port is
an optical fiber.
17. A gain equalizer according to claim 15 and also comprising a
controller operative to vary the attenuation of at least one of
said variable attenuating elements, such that the light passing
through said attenuating element has a predefined level.
18. A gain equalizer according to claim 17 and also comprising a
spectrally selective detector providing to said controller at least
one signal corresponding to the power level of at least one of said
wavelength components.
19. A gain equalizer according to claim 18 and wherein said at
least one signal is utilized to adjust the attenuation of at least
one of said variable attenuating elements.
20. A gain equalizer according to claim 18 and wherein said
detector is connected such that it measures the power level of at
least one of said wavelength components in said output beam.
21. A gain equalizer according to claim 18 and wherein said
detector is connected such that it measures the power level of at
least one of said wavelength components of said light received at
said port.
22. A gain equalizer according to claim 18 and wherein said
spectrally selective detector is a linear detector array utilizing
one of said dispersive elements for performing said spectral
selection.
23. A gain equalizer according to claim 18 and wherein said at
least one signal corresponding to the power level of at least one
of said wavelength components is obtained by means of a power
splitter located in the path of said wavelength components of said
light.
24. A gain equalizer according to claim 14 and wherein said phase
changing element is a liquid crystal element.
25. A gain equalizer according to claim 14 and wherein at least one
of said dispersive elements is a grating.
26. A gain equalizer according to claim 14 and also comprising a
quarter wave plate serially with said plurality of attenuating
elements, operative to reduce the polarization dependent loss of
said gain equalizer
27. A gain equalizer according to claim 14 and wherein said light
received by said port and said output beam are separated by means
of a dual fiber collimator.
28. A gain equalizer according to claim 14 and wherein said light
received by said port and said output beam are separated by means
of a circulator.
29. A multichannel optical gain equalizer comprising: an input
fiber receiving a multi-wavelength input; a demultiplexer fed by
said input fiber, having a plurality of output wavelength channels;
an output fiber outputting a multi-wavelength output; a multiplexer
feeding said output fiber, having a plurality of input wavelength
channels; a plurality of variable optical attenuating elements,
individual ones of said attenuating elements being generally
disposed between individual output channels of said demultiplexer
and individual input channels of said multiplexer; and at least one
signal detector detecting the power in at least one of said input
wavelength channels of said multiplexer, and operative to adjust
the attenuation of said attenuating element associated with said at
least one input wavelength channel, according to the power of said
signal detected; wherein at least one of said variable optical
attenuating elements comprises a variable phase changing unit
operative to change the phase of part of the cross section of light
passing through it.
30. A multichannel gain equalizer according to claim 29, and
wherein said at least one signal detector detecting the power in at
least one of said input wavelength channels of said multiplexer is
a spectrally selective detector in series with said output
fiber.
31. A multichannel gain equalizer according to claim 29 and wherein
said at least one signal detector is located remotely from said
gain equalizer.
32. A multichannel gain equalizer according to claim 29, and
wherein said demultiplexer comprises a dispersive grating, such
that said plurality of output wavelength channels are spatially
dispersed.
33. A multichannel gain equalizer according to claim 32 and wherein
said multiplexer comprises a dispersive grating, such that said
spatially dispersed plurality of wavelength channels are combined
into one channel.
34. A multichannel gain equalizer according to claim 29 and wherein
said phase changing element is a liquid crystal element.
Description
[0001] This patent application is a continuation of International
Application No. PCT/IL02/00187 which was filed Mar. 8, 2002 and
designates the United States of America. This International
Application was published in English under International
Publication No. WO 02/071660.
FIELD OF THE INVENTION
[0002] The present invention relates to the field of gain
equalizers for use in fiber optical systems, especially those which
are dynamically controlled in real time.
BACKGROUND OF THE INVENTION
[0003] Gain equalizers are important components in fiber optical
communication systems. One of the main functions of an optical gain
equalizer is to bring all of the separate wavelength signals in a
data transmission to the same amplitude, in order to optimize
system spectral power budget. The functional requirements of an
optical gain equalizer for use in such a system are that it should
vary the intensity profile of the light transmitted as a function
of the wavelength of the light, and without appreciably altering
the spatial, temporal, or polarization distribution of the light
beam.
[0004] Many types of optical gain equalizers have been described in
the prior art. In U.S. Pat. No. 6,144,488, to H. Okuno, for
"Optically Amplifying Device with Gain Equalizing Function", there
is described a semiconductor optical amplifier, in which the gain
for each wavelength is varied to provide gain equalization
functionality. In U.S. Pat. No. 6,034,812, to T. Naito, for "Gain
Equalizer and Optical Transmission System having the Gain
Equalizer" there is described a system including three cascaded
gain equalizers, the first having a maximum loss at or near the
optical amplifier's peak gain, and the others having periodic loss
characteristics, arranged such that they fall at or near a
wavelength giving one of two gain peaks remaining when the gain
characteristic of the optical amplifier the has been equalized by
the first equalizer only.
[0005] Both of these prior art gain equalizers are comparatively
complex, involving a large number of components, and may require
careful alignment for good operation. Another type of gain
equalizer has been described in the co-pending U.S. Provisional
Patent Application No. 60/327,680 for "Fiber Optical Gain
Equalizer", and its corresponding PCT application, assigned to the
applicant of the present application, and herewith incorporated by
reference each in its entirety. This gain equalizer, however,
though very compact and simple in operation, utilizes a custom
stepped substrate construction, which is not a standard available
component, and so may make the gain equalizer less attractive for
small quantity production runs. Furthermore, the band resolution,
design flexibility and dynamic range may be limited. There thus
exists an important need for an electronically controllable optical
gain equalizer, of simple construction and operation, which can
perform spectral signal processing on an optical signal input to
it, including dynamic gain equalization, and which overcomes some
of the disadvantages of prior art gain equalizers.
[0006] The disclosures of each of the publications mentioned in
this section and in other sections of the specification, are hereby
incorporated by reference, each in its entirety.
SUMMARY OF THE INVENTION
[0007] The present invention seeks to provide a new compact dynamic
gain equalizer in which a multichannel input light signal is split
into its separate wavelength components preferably by means of a
dispersive element such as a grating. The spatially separated
wavelength components are passed through a linear array of variable
optical attenuators based on variable phase changing elements which
modulate the phase of part of the cross section of the light. The
array of attenuators are preferably disposed along the same
direction as the direction of the spatial separation of the
wavelength components, such that different wavelengths preferably
pass through successive attenuators in the array. The separate
attenuated wavelength components are then recombined, preferably by
means of another dispersive element and the recombined,
multi-wavelength light signal is output. The attenuation level of
each variable optical attenuator is adjusted preferably according
to the output of the light as a function of its wavelength
components, and in this way, the overall wavelength profile of the
output light signal can be adjusted to any predefined form, whether
a flattened spectral profile, as in gain equalization applications,
or a spectral compensating profile, or a channel blocking profile.
The output level of the light as a function of its wavelength
components is preferably obtained from a spectrally selective
detector system, such as a sample of the output beam, dispersed
spatially and detected on a linear detector array, such that the
output for each wavelength range is determined.
[0008] The gain equalizer can preferably be constructed in a
transmissive embodiment, in which case separate dispersive and
recombining elements are used. Alternatively and preferably, it can
be provided in a reflective embodiment, in which case a single
dispersive element is used, and a reflective element is disposed in
the optical path after the array of variable optical attenuators,
and is operative to return the spatially separated wavelength
components back to the single dispersive element for recombining as
the multi-wavelength processed output signal.
[0009] There is thus provided in accordance with a preferred
embodiment of the present invention, a gain equalizer
comprising
[0010] (i) an input port receiving light comprising at least two
wavelength components,
[0011] (ii) a first dispersive element receiving the light and
spatially dispersing the wavelength components of the light along a
dispersion direction,
[0012] (iii) a plurality of variable optical attenuators disposed
along the dispersion direction,
[0013] (iv) a second dispersive element receiving the light after
passage through at least part of at least one of the plurality of
variable optical attenuators and operative to combine the
wavelength components of the light into an output beam, and
[0014] (v) an output port receiving the output beam, wherein at
least one of the variable optical attenuators comprises a variable
phase changing element operative to change the phase of part of the
cross section of light passing through it.
[0015] In the above described gain equalizer, the variable optical
attenuators may preferably be disposed along the dispersion
direction such that each of the attenuators is traversed by
different wavelength components of the light. At least one of the
attenuators is preferably varied so as to vary the level of light
traversing the attenuator. Furthermore, in any of the above
embodiments, either or both of the input and output ports may be an
optical fiber.
[0016] Furthermore, in accordance with still another preferred
embodiment of the present invention, the above mentioned gain
equalizer also comprises a controller operative to vary the
attenuation of at least one of the variable attenuators, such that
the light passing through the attenuator has a predefined level.
The controller is preferably provided with signals corresponding to
the power level of the wavelength components from a spectrally
selective detector. The signals are preferably utilized to adjust
the attenuation of any of the variable attenuators. The detector is
preferably located such that it measures the power level of
wavelength components in the output beam, so that closed loop
control of the wavelength profile of the output beam is
obtained.
[0017] Alternatively and preferably, the detector is located such
that it measures the power level of the wavelength components of
the light in the input port, in order to characterize the input
spectrum for correction or leveling.
[0018] The above-mentioned spectrally selective detector is
preferably a linear detector array utilizing one of the dispersive
elements for performing the spectral selection. Furthermore, in
accordance with a further preferred embodiment of the present
invention, the signals corresponding to power levels of the
wavelength components are obtained by means of a power splitter
located in the path of the wavelength components of the light.
[0019] Additionally, in any of the above-mentioned embodiments of
the present invention, the phase changing element is preferably a
liquid crystal element. Furthermore, either or both of the
dispersive elements are preferably gratings. The optical system can
preferably also include a half wave plate serially with the
plurality of attenuators, operative to reduce the polarization
dependent loss of the gain equalizer
[0020] There is further provided in accordance with yet another
preferred embodiment of the present invention, a gain equalizer
comprising:
[0021] (i) a port receiving light comprising at least two
wavelength components,
[0022] (ii) a dispersive element receiving the light and spatially
dispersing the wavelength components of the light along a
dispersion direction,
[0023] (ii) a plurality of variable optical attenuators disposed
along the dispersion direction, and
[0024] (iv) a reflective surface operative to reflect the light
after passage through at least part of at least one of the
plurality of variable optical attenuators back to the dispersive
element, so as to combine the wavelength components of the
reflected light into an output beam at the port, wherein at least
one of the variable optical attenuators comprises a variable phase
changing element operative to change the phase of part of the cross
section of light passing through it.
[0025] In the above described gain equalizer, the variable optical
attenuators may preferably be disposed along the dispersion
direction such that each of the attenuators is traversed by
different wavelength components of the light. At least one of the
attenuators is preferably varied so as to vary the level of light
traversing the attenuator. Furthermore, in any of the above
embodiments, the port may be an optical fiber.
[0026] Furthermore, in accordance with still another preferred
embodiment of the present invention, the above mentioned gain
equalizer also comprises a controller operative to vary the
attenuation of at least one of the variable attenuators, such that
the light passing through the attenuator has a predefined level.
The controller is preferably provided with signals corresponding to
the power level of the wavelength components from a spectrally
selective detector. The signals are preferably utilized to adjust
the attenuation of any of the variable attenuators. The detector is
preferably located such that it measures the power level of
wavelength components in the output beam, so that closed loop
control of the wavelength profile of the output beam is
obtained.
[0027] Alternatively and preferably, the detector is located such
that it measures the power level of the wavelength components of
the incoming light at the port, in order to characterize the input
spectrum for correction or leveling.
[0028] The above-mentioned spectrally selective detector is
preferably a linear detector array utilizing one of the dispersive
elements for performing the spectral selection. Furthermore, in
accordance with a further preferred embodiment of the present
invention, the signal corresponding to the power level of the
wavelength components are obtained by means of a power splitter
located in the path of the wavelength components of the light.
[0029] Additionally, in any of the above-mentioned reflective
embodiments of the present invention, the phase changing element is
preferably a liquid crystal element. Furthermore, either or both of
the dispersive elements are preferably gratings. The optical system
can preferably also include a quarter wave plate serially with the
plurality of attenuators, operative to reduce the polarization
dependent loss of the gain equalizer. Furthermore, the input light
received by the port and the output beam can preferably be
separated either by means of a dual fiber collimator or by means of
a circulator.
[0030] There is further provided in accordance with still another
preferred embodiment of the present invention, a multichannel
optical gain equalizer comprising:
[0031] (i) an input fiber receiving a multi-wavelength input,
[0032] (ii) a demultiplexer fed by the input fiber, having a
plurality of output wavelength channels,
[0033] (iii) an output fiber outputting a multi-wavelength
output,
[0034] (iv) a multiplexer feeding the output fiber, having a
plurality of input wavelength channels,
[0035] (v) a plurality of variable optical attenuators disposed
between the output channels of the demultiplexer and the input
channels of the multiplexer, and
[0036] (vi) at least one signal detector detecting the power in at
least one of the input wavelength channels of the multiplexer, and
operative to adjust the attenuation of the attenuator in the at
least one input wavelength channel according to the level of the
signal detected, wherein at least one of the variable optical
attenuators comprises a variable phase changing unit operative to
change the phase of part of the cross section of light passing
through it.
[0037] In accordance with a further preferred embodiment of the
present invention, in the above described multichannel gain
equalizer, the at least one signal detector detecting the power in
at least one of the input wavelength channels of the multiplexer is
a spectrally selective detector in series with the output fiber.
This detector may preferably be located remotely from the gain
equalizer. Furthermore, the demultiplexer may preferably comprise a
dispersive grating, such that the plurality of output wavelength
channels are spatially dispersed. In addition, the multiplexer may
comprise a dispersive grating, such that the spatially dispersed
plurality of wavelength channels are combined into one channel. In
any of the above-mentioned multichannel gain equalizers, the phase
changing element is preferably a liquid crystal element.
[0038] It is to be understood that throughout this application, the
term gain equalizer is used, and is so claimed, to mean a system
which varies the intensity of the light transmitted as a function
of the wavelength of the light, whether the system is operative to
provide a flat wavelength profile, as implied by the term "gain
equalization" or whether the desired result is any other spectral
profile, such as a compensating profile, or a band blocking
profile.
[0039] Furthermore, it is to be understood that throughout this
application, the term light is used, and is so claimed, to mean
electromagnetic radiation, whether in the visible, ultra-violet or
the infra-red regions, or any other useful region of the
spectrum.
BRIEF DESCRIPTION OF THE DRAWINGS
[0040] The present invention will be understood and appreciated
more fully from the following detailed description, taken in
conjunction with the drawings in which:
[0041] FIG. 1 is a schematic illustration of a multichannel gain
equalizer, constructed and operative according to a preferred
embodiment of the present invention;
[0042] FIG. 2 is a schematic drawing of a face view of a preferred
embodiment of a single section of a variable attenuating element,
such as could preferably be used in the preferred embodiment of
FIG. 1;
[0043] FIG. 3 is a schematic graph of the relationship of the
attenuation obtained through the element of FIG. 2 as a function of
its applied voltage;
[0044] FIG. 4 schematically illustrates a dynamic gain equalizer
for use in a fiber optical system, constructed and operative
according to a preferred embodiment of the present invention;
[0045] FIGS. 5A and 5B are face views of two different multiple
pixel liquid crystal elements suitable for use as the pixellated
attenuating element in the dynamic gain equalizer illustrated in
FIG. 3;
[0046] FIG. 6 is a graph illustrative of the attenuation obtained
as a function of the wavelength of the traversing light from a
single pixel of the element of FIG. 5A or 5B;
[0047] FIGS. 7 and 8 are schematic graphs showing how a dynamic
gain equalizer according to the present invention, can be
preferably used to process an incoming signal with a non-uniform
wavelength profile;
[0048] FIG. 9 is a schematic block diagram showing the electronic
and optical components used in a gain equalizer according to one
preferred embodiment of the present invention;
[0049] FIG. 10 is a schematic illustration of a system, according
to a further preferred embodiment of the present invention, for
practically implementing the schematic block diagram of FIG. 9;
[0050] FIG. 11 schematically illustrates an alternative and
preferable method of handling the monitored power to that described
in FIG. 10;
[0051] FIG. 12 is a schematic drawing of the optical arrangement of
a reflective embodiment of a dynamic gain equalizer, constructed
and operative according to another preferred embodiment of the
present invention;
[0052] FIG. 13 schematically illustrates another preferred
reflective embodiment of the gain equalizer of the present
invention; and
[0053] FIG. 14 is a schematic cross section of another preferred
embodiment of the present invention, in which the gain equalizer is
constructed in a compact monolithic form.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0054] Reference is now made to FIG. 1, which is a schematic
illustration of a multichannel gain equalizer 10, constructed and
operative according to a preferred embodiment of the present
invention. The gain equalizer utilizes an array of variable optical
attenuator elements 16, preferably of a type which can be provided
in high density planar geometry. Particularly suitable for use in
the equalizer of the present invention are the preferred
attenuators described in co-pending Israel Patent Application Nos.
141927 and 142773, for "Fiber Optical Attenuator", and their
corresponding PCT application, both assigned to the applicant of
the present application, and both herewith incorporated by
reference, each in its entirety. However, any other suitable type
of variable attenuator may equally well be used in the present
invention, such as the commonly used liquid crystal attenuators
using polarizing effects. Other examples of such attenuators are
given in the background of the above-mentioned applications.
[0055] The input optical signal, composed of a number of separate
signals, each at its own characteristic wavelength, is input by
means of the input fiber 12. The function of the gain equalizer 10
is to bring all of the separate wavelength channels to the same
amplitude, in order to optimize system power spectrum.
Alternatively and preferably, the gain equalizer can be programmed
to provide a predetermined wavelength output profile from the input
signals. The input signal is input into a demultiplexer or a
wavelength dispersive component 14, which separates the individual
wavelength components of the signal into n separate channels,
.lambda..sub.1, .lambda..sub.2, .lambda..sub.3, . . .
.lambda..sub.n, one for each wavelength band. Such a demultiplexer
function can preferably be provided by a dispersive grating,
transmissive or reflective, though it is to be understood that any
other suitable dispersive element may be used for the execution of
the present invention. Each of these channels is input into its own
variable optical attenuator 16, VOA.sub.1, VOA.sub.2, . . .
VOA.sub.N, preferably of the type described in the above-mentioned
patent applications. The levels of the signals in each channel 1, 2
. . . n, are detected, preferably by means of in-line signal
detector elements, 18, and a feedback signal from each detector
element is used to control the level of attenuation of each VOA.
The resulting signals from all of the separate channels are thus
preferably brought to the same level, if this is the desired
outcome, and are recombined in a multiplexer unit 20, into a
multi-channel, gain-equalized, output signal for outputting through
the output fiber 22. The multiplexer unit 20 can preferably be a
second grating, or any other suitable wavelength combining
component.
[0056] In order to enable the gain equalizer to operate
bidirectionally, each of the "input" channels can optionally be
provided with its own in-line signal detector elements 19, such
that when the unit is used in the reverse direction, or when the
input comes from the reverse direction, these detector elements 19
are operative to detect the attenuated output power level, instead
of the detector elements 18 on the other side of the VOA's. Means
should preferably be provided to determine in which direction the
signal is travelling, since the equalizer must be controlled by the
light signal detected after attenuation. The direction of use can
either be predetermined by the system configuration, or can be
determined on-line during use by comparing the signal levels
detected by detector elements 18 and 19. Alternatively and
preferably, the input detectors 19 can be used for monitoring the
input power, if so required.
[0057] The gain equalizer preferably utilizes an array of
integrated variable optical attenuators as this enables the
construction of a particularly compact gain equalizer.
Alternatively and preferably, it can be constructed of separate
variable optical attenuators of the type shown in the above
mentioned Israel Patent applications, in a free-space embodiment.
Such embodiments can include both transmissive and reflective
embodiments of variable optical attenuators.
[0058] According to further preferred embodiments of the present
invention, detector elements such as those labeled 18 and 19 in
FIG. 1, can be installed anywhere in the optical channel where the
power level is to be monitored. The signal monitoring can even be
performed remotely from the gain equalizer, such as at the receiver
station or at an en-route repeater unit, either of which could be
at a considerable distance from the gain equalizer itself. In this
way, the gain equalization is performed on the basis of the signal
strengths of each channel detected at the destination point of the
signal, where the gain equalization is important for the system
power budget.
[0059] In order to illustrate the operation of the attenuator,
reference is now made to FIG. 2, which is a schematic drawing of a
face view of a preferred embodiment of a single section of a
variable attenuating element, such as could preferably be used as
the element 16 shown in FIG. 1. The attenuating element may be
similar to any of those described in co-pending Israel Patent
Application Nos. 141927 and 142773, and their corresponding PCT
application, both assigned to the applicant of the present
application, and both herewith incorporated by reference, each in
its entirety. The operation of such a variable optical attenuator
can be illustrated by reference to FIG. 2, which is a view in the
direction of the light propagation, of the cross-section of a
preferred embodiment of such a variable optical attenuator 16. The
cross section of the light beam of one wavelength passing through
the attenuator is shown in dotted outline 38. The light is input
from a single mode fiber, and is thus incident on the face of the
attenuator element as a low order mode. In the preferred embodiment
shown, a liquid crystal element is used, with pixelated electrodes
on the surfaces of the element, preferably dividing the element
into two halves. A control voltage is applied, preferably to a pair
of electrodes on one half 32 of the element, and at its maximum
designed value, this voltage modifies the liquid crystal under the
electrode, such that the light passing through that part of the
element is phase shifted by .pi. relative to its phase without any
voltage applied. The liquid crystal under the other part 34 of the
element is unchanged. The light beam 38 passing through the liquid
crystal element is therefore transformed from a low-order symmetric
mode to a mode having an anti-symmetric field distribution, such
that it cannot couple to the symmetric fundamental lowest order
mode of the output fiber. The light is converted to the HE.sub.12
and higher order asymmetric modes, which cannot propagate in the
output fiber. In this voltage state, the attenuator is at its
maximum value, ideally fully blocking the light passage. For
intermediate values of applied voltage, when the additional phase
shift for the light passing through pixel 32 is less than .pi.,
only part of the light is transformed to the anti-symmetric, higher
order mode, and the light is thus partially attenuated. The level
of light transmitted is thus a function of the applied voltage.
[0060] Though FIG. 2 shows a simple pixel geometry, a number of
different pixel designs can equally well be used, such as are
described in the above-mentioned co-pending Israel patent
applications. A particularly convenient geometry is a stripe
geometry, in which the liquid crystal is divided into a number of
thin pixelated strips, since this geometry is less susceptible to
misalignment of the optical path through the element. An example of
the use of such a geometry is shown in FIG. 5B hereinbelow.
[0061] The relationship of the attenuation obtained through the
element as a function of the applied voltage V is shown in the
schematic graph of FIG. 3. It is to be understood that though the
electrode structure of FIG. 2 has been described in terms of two
separate electrodes on each surface, each covering half of the
element surface, in practice, there is need for a phase control
electrode preferably only on one half of the element in order to
generate the higher order anti-symmetric mode.
[0062] Reference is now made to FIG. 4, which illustrates
schematically a dynamic gain equalizer for use in a fiber optical
system, constructed and operative according to a preferred
embodiment of the present invention. The preferred embodiment shown
in FIG. 4 uses the conceptual construction shown in the embodiment
of FIG. 1 with the variable optical attenuator elements described
in FIG. 2. The signal from the input fiber 40 to be equalized, is
input through a fiber optical collimator 42, such as a GRIN rod
lens, and is directed onto a wavelength dispersive element 44,
which is preferably a reflective grating element. It is to be
understood that any other suitable dispersive element may also be
used for the execution of the present invention, including
transmissive elements, provided that the correct geometrical
arrangements are made for handling the signals therefrom. The
grating is operative to disperse the signal spatially into its
wavelength components, each specific wavelength range being
directed at a different angle. The beams are imaged by means of a
first focussing lens 46, preferably located at a distance equal to
its effective focal length from the diffraction grating onto a
pixellated attenuating element 48, whose construction will be
described hereinbelow. For the sake of clarity, only two dispersed
wavelengths .lambda..sub.1 and .lambda..sub.2 are shown in FIG. 4,
but it is to be understood that the number of wavelength channels
is limited only by the resolution of the system, which is
dependent, inter alia, on the number and spacing of the pixels and
the optical design. The light from each dispersed wavelength range
is imaged to pass through a different region of the pixelated
element 48. The pixels are switched by means of control voltages V
applied through an array of transparent electrodes on the liquid
crystal surfaces, as described below, though for reasons of
clarity, only one applied voltage is shown in FIG. 4.
[0063] After passing through the liquid crystal element 48, the
light signals are imaged, preferably by means of a second focusing
lens 50, onto another dispersive grating 52. According to a
preferred embodiment of the present invention, the attenuating
element 48 is preferably located at the back focal plane of the
focusing lens 46, and at the front focal plane of the lens 50, such
that the overall assembly has a 4-f configuration, for optimum
optical performance. Grating 52 is operative to recombine the
different wavelengths .lambda..sub.1, X.sub.2, X.sub.3, . . .
.lambda..sub.n coming from their respective wavelength-dispersed
directions, into the output fiber collimator 54, and from there,
into the output fiber 56. According to a further preferred
embodiment, a half wave plate 51 can be inserted after the liquid
crystal attenuating element in order to minimize the polarization
dependent loss of the grating or to allow the use of two identical
liquid crystal cells, as is known in the art.
[0064] As is known, light transmitted through optical fibers
generally has randomized polarization directions. It is therefore
important that the elements used in the present invention be
polarization independent. A number of methods are available for
rendering the liquid crystal attenuator element 48 insensitive to
the polarization of the incoming light, as described in the
above-mentioned Israel Patent Application No. 142773 and its
corresponding PCT application.
[0065] Comparing the preferred embodiment shown in FIG. 4 with the
preferred embodiment outlined in FIG. 1, the demultiplexer
operation of item 14 in FIG. 1 is performed by the grating 44 of
FIG. 4, while the multiplexing operation of item 20 in FIG. 1 is
performed by the grating 52 of FIG. 4.
[0066] Reference is now made to FIG. 5A, which is a face view of a
multiple pixel liquid crystal element 60, suitable for use as the
pixelated attenuating element 48 in the dynamic gain equalizer
illustrated in FIG. 3. The element shown in FIG. 5A has 6 pixels,
though it is to be understood that in practice, many more pixels
may be used. Each of the pixels is preferably operative according
to the pixel embodiment shown and described in FIG. 2, and the
individual circular cross sectional beams for each wavelength
stretched into an ellipse-like continuum of overlapping spots,
depending on the spectral content of the input light. Furthermore,
the circular shape of the beam at a single wavelength may be
changed to an elliptic form depending on the design or the geometry
of the optical system. The element is disposed vertically in the
embodiment shown in FIG. 4, such that successive wavelengths of the
spatially dispersed light traverse the element at different
successive locations on the element, as indicated by the arrow
marked .lambda.. By way of example, if at a wavelength
.lambda..sub.1, the dispersed input beam falls exactly on the
center of a pixel 62, when the correct control voltage V is applied
to the electrodes of pixel 62, the light of wavelength
.lambda..sub.1 is attenuated maximally. At different control
voltages, the attenuation can be selected accordingly. As the
wavelength of the light changes, the dispersed beam moves away from
the center of the pixel 62, and the attenuation decreases. At
wavelength .lambda..sub.2, where the light falls exactly between
two pixels, the attenuation is minimal, and at wavelength
.lambda..sub.3, where the light falls at the center of a pixel 64,
the attenuation is maximum again. This explanation assumes that all
of the pixels have applied voltages which provide attenuation for
the individual pixel elements.
[0067] Reference is now made to FIG. 5B which is a schematic
drawing of a multiple pixel liquid crystal element 66, similar to
that of FIG. 5A, except that the individual pixels 68 have a
striped electrode pattern over the whole of their area, thus
providing better uniformity and better freedom from optical
misalignment.
[0068] Reference is now made to FIG. 6, which is a graph
illustrative of the attenuation obtained as a function of the
wavelength of the traversing light from a single pixel of the
element of FIG. 5A or 5B, while the other pixels are in a
non-attenuating state. As is seen in FIG. 6, the attenuation is
maximum at the wavelength corresponding to the center of the pixel
profile. As the wavelength changes such that the light does not
traverse the center of the pixel, the attenuation decreases until a
minimum attenuation level is reached when the wavelength is such
that the beam does not illuminate the pixel at all.
[0069] Reference is now made to FIGS. 7 and 8, which are schematic
graphs showing how a dynamic gain equalizer, according to the
present invention, can be preferably utilized to process an
incoming signal with a non-uniform wavelength profile, such that
the output signal has a flattened wavelength profile, in the
process known in the art as gain equalization. In FIG. 7 there is
shown the uneven wavelength profile of an input optical signal 70,
which it is desired to flatten. The variation in power is given by
the value of the gain-tilt 72. The wavelength spectrum is divided
up into bands, each band .DELTA..lambda. being represented by a
pixel on the attenuating element 60 of FIG. 5A, or 66 of FIG. 5B.
The attenuation of each wavelength band is individually adjusted by
means of the voltage applied to the electrodes of each pixel, such
as to bring the overall level of the power transmitted to just
below the minimum power level 74 of the wavelength profile. The
resulting wavelength profile obtained after this gain equalization
process is shown in FIG. 8, where it is observed that the output
profile 76 is substantially flatter than the initially input
profile. The error level 78, which is the departure of the output
profile from a truly flat level, is a result of the discrete nature
of the pixel structure in the attenuating element, and of the
spread of the individual pixels. The greater the number of pixels,
and the smaller the spread, the flatter the possible output
profile.
[0070] Reference is now made to FIG. 9, which is a schematic block
diagram showing the electronic and optical components used to
provide the necessary gain equalizing voltage signals to each
pixel, according to one preferred embodiment of the present
invention. The input signal 80 to be equalized is input at P.sub.in
to the dynamic gain equalizer 82, constructed according to one of
the above-described preferred embodiments. The output signal passes
through a broadband power splitter 84, where a portion of the
signal is monitored. This monitor signal is analyzed as a function
of wavelength either in an optical spectrum analyzer or in a
wavelength sensitive optical power monitor 86. Such spectrally
sensitive power detectors, though generally thought of as stand
alone laboratory instruments, can preferably be provided in
miniature dedicated form, such that they can be integrated into the
casing of a dynamic gain equalizer according to various preferred
embodiments of the present invention. The output signal from such a
monitor is input to a controller board 88, where the signal
representing the optical output power level at each wavelength is
compared with the desired power level, according to a predetermined
function defining the desired output level as a function of
wavelength. Feedback correction signals are generated in the
control board, which are applied 89 as the pixel voltages for each
pixel in the dynamic gain equalizer. The controller board can
preferably be programmed to provide as flat a response as possible.
According to another preferred embodiment of the invention, the
control voltages are adjusted to be such as to enable the synthesis
of a predefined spectral profile. This application is useful for
generating power spectra which precompensate for the spectral
response of the system into which they are working. Thus, for
instance, if it is known that higher frequencies are attenuated in
a certain transmission network, it is possible, using this
preferred embodiment to taper the power profile to provide more
power output at the high frequency end of the wavelength range
used. Alternatively and preferably, the output profile can be
tailored to block a predefined wavelength band if so required. Such
applications are further described in the co-pending U.S.
Provisional Patent Application No. 60/327,680 for "Fiber Optical
Gain Equalizer" and its corresponding PCT application, assigned to
the applicant of the present application, and herewith incorporated
by reference in its entirety.
[0071] Reference is now made to FIG. 10, which illustrates a
system, according to a further preferred embodiment of the present
invention, for practically implementing the schematic block diagram
of FIG. 9. The preferred embodiment shown in FIG. 10 shows
components for internally determining the wavelength profile of the
input signal, such that the correction function necessary to gain
equalize or to otherwise process the wavelength profile of the
input signal can be performed, and also components for determining
the wavelength profile of the output signal, such that the
equalizer can be operated in closed loop configuration to maintain
the desired output profile without the need to know the input
profile. The monitoring components are incorporated respectively at
the input and the output to the dynamic gain equalizer, such as
that illustrated in FIG. 4. Items common to the embodiment shown in
FIG. 4 are labeled with the same reference characters as in FIG. 4.
The input signal power P.sub.n 90 is fed into a power splitter 92,
where typically 5% of the power is split off for monitoring
purposes, though values of less than 5% can also equally
effectively be used. Both the 5% monitoring power and the remaining
95% of the main power are input, by means of collimating units 94,
96, into the dynamic gain equalizer for processing. The collimator
of the main power 94 is aligned at an angle such that the light is
incident onto the dispersive grating 44 so that it is incident on
the input lens 46, traverses the remainder of the optical path of
the gain equalizer and is processed optically as shown in the
embodiment of FIG. 4. For the sake of clarity, only one input
wavelength path .lambda..sub.1 is shown.
[0072] The collimator unit 96 of the 5% monitoring power input is
aligned at a different angle, such that the monitoring power is
incident on the dispersive grating at such an angle that the
reflected beams for the different input monitored wavelengths are
directed into an auxiliary imaging lens 98, which focuses the
different beams onto a linear detector 99, such as an InGaAs linear
array. Two wavelengths .lambda..sub.10 and .lambda..sub.11 are
shown, and each is incident on the linear array at a position which
is specific to the particular wavelength. The output signals from
the linear array can thus be used to define the wavelength profile
of the incident signal P.sub.in which it is desired to process or
equalize.
[0073] Though the embodiment shown in FIG. 10 utilizes a power
coupler to divide off part of the incident power for monitoring,
this function can equally well be performed by using a beam
splitter, or even a partially reflecting coating to divide off a
small part of the power as is known in the art. These components
are then angled such that the divided off power sample is directed
to a separate part of the dispersive element, as shown in FIG.
9.
[0074] The embodiment shown in FIG. 10 is described as an open loop
system, in which it is the input power which is sampled in order to
determine its spectral shape for correction. According to further
preferred embodiments of the present invention, a similar
arrangement can be used at the output of the dynamic gain
equalizer, in an output power feedback loop, to fulfil the
functions of the power splitter 84 and the wavelength sensitive
detector 86 shown in the schematic block diagram of FIG. 9. Such a
preferred arrangement is shown on the output side of the equalizer,
whereby a second power coupler 91 is located in the output line 56,
and splits off a portion 93 of the output power. This output power
can then be spectrally analyzed, preferably by using one of the
gratings of the equalizer, and a linear detector array, similar to
the arrangement shown for the input power monitor. The output
signals from the detector array can then be used for input to the
pixellated attenuator element so as to process the output signal
profile as desired, as explained in relation to the block diagram
of FIG. 9.
[0075] Reference is now made to FIG. 11, which schematically
illustrates an alternative and preferable method of handling the
monitored power to that described in FIG. 10. According to the
embodiment of FIG. 10, the main beam and the split-off monitor
beams lie in the same plane, and an additional focusing lens 98 is
thus required to project the monitored beam onto its detector
array. In the embodiment shown in FIG. 11, which is a side view of
the system, viewed from within the plane of the paper of FIG. 10,
the monitor collimator 96 is located below the optical plane of
FIG. 10, and is aligned such that the monitor beam is projected at
an angle to that plane. The advantage of this geometry is that the
dispersed monitoring beam can be imaged onto its linear detector
array 99 by means of the same lens 46 that is used to image the
main beam onto the pixelated attenuator element 48. The linear
detector array 99 can then be conveniently located just beneath the
attenuator element 48. Such an arrangement not only allows a more
compact system to be constructed, but it is also economical on
component utilization.
[0076] Though the above described preferred embodiments have been
described in terms of a transmissive liquid crystal pixellated
attenuating element, it is also possible to execute the present
invention, according to more preferred embodiments, using a
reflective mode liquid crystal pixelated attenuating element.
Reference is now made to FIG. 12, which is a schematic drawing of
the optical arrangement of such a dynamic gain equalizer,
constructed and operative according to another preferred embodiment
of the present invention, using a reflective pixelated attenuating
element. A dual fiber collimator 100 is preferably used as the
input/output device. The signal to be attenuated is input by the
fiber 102, and is preferably converted by the dual fiber collimator
100 into an approximately collimated output beam 104. A dispersive
reflective grating 114 separates the input signal beam 104 into
separate wavelength channels .lambda..sub.1, .lambda..sub.2,
.lambda..sub.3, . . . , each of which falls onto a different
spatial location 115, 116, 117, on a pixelated reflective
attenuating element 113, which operates in the same manner as the
transmissive pixelated attenuating element 48 of the embodiment of
FIG. 4, except that it is reflective instead of transmissive. Each
spatial location is associated with a separate pixel of the
phase-changing liquid crystal element, such that each wavelength
channel can be individually controlled by a separate pixel. The
reflected, attenuated channels are recombined by the same grating
114 as was operative to disperse the input signal, and are input to
the dual fiber collimator 100, from which the recombined gain
equalized signal is output on fiber 109.
[0077] In the free-space schematic embodiment illustrated in FIG.
12, no signal strength detectors are shown, and the individual
channel signal levels may preferably be determined at a remote
site. Alternatively and preferably, the signal levels may
preferably be determined by any of the signal monitoring methods
shown in the transmissive embodiments described hereinabove.
Alternatively and preferably, the reflective pixelated
phase-changing element 113 may be of an integrated type containing
an array of detectors, such as those described in co-pending Israel
Patent Application Nos. 141927 and 142773, for "Fiber Optical
Attenuator", and their corresponding PCT application, and the
individual channel signal strengths adjusted on-line. This
embodiment thus enables a compact and cost-effective
gain-equalizing unit.
[0078] In the optical embodiment shown in FIG. 12, an approximately
collimated beam 104 is shown at the output of the dual-fiber
collimator 100. It is to be understood that this embodiment is only
one possible configuration of this aspect of the invention. The
above described reflective dynamic gain equalizer can be equally
well constructed using alternative optical set-ups, such as with a
dual fiber collimating coupler outputting a point source, and a
beam expander to diverge the output beam, or with any other
suitable combination of optical elements for beam forming and
focusing purposes. Likewise, a lens can preferably be used at the
output of the dual fiber collimator 100 to ensure that the
collimated beam 104 properly fills the aperture of the dispersive
grating 114. Furthermore, in the embodiment shown in FIG. 12, a
concave grating 114 is used as the dispersive element, such that
the light from each channel is focussed by the grating itself onto
the reflective pixelated phase-changing element.
[0079] Reference is now made to FIG. 13, which schematically
illustrates another preferred configuration of this embodiment,
which is essentially a reflective embodiment of the gain equalizer
shown in FIG. 4. A dual fiber collimator 100 is preferably used as
the input/output device. The signal to be attenuated is input by
the fiber 102, and is preferably converted by the dual fiber
collimator 100 into an approximately collimated output beam 104. A
preferably plane dispersive reflective grating 44 separates the
input signal beam 104 into separate wavelength channels, only two
of which are show for clarity .lambda..sub.1, .lambda..sub.2. The
separate wavelength channels are imaged by a lens 46, each onto a
different spatial location on the pixelated attenuating element
118, which is rendered reflective preferably by virtue of a
reflective coating or mirror 120 situated downstream of the
attenuating element 118. The pixelated attenuating element operates
in the same manner as the transmissive pixelated attenuating
element 48 of the embodiment of FIG. 4, except that it is
reflective instead of transmissive. Each spatial location is
associated with a separate pixel of the liquid crystal element,
such that each wavelength channel can be individually controlled by
the voltage applied to a separate pixel. The reflected, attenuated
channels are refocused by the lens 46 onto the same grating 44 as
was operative to disperse the input signal, and are there
recombined into one beam which is input to the dual fiber
collimator 100, from which the gain equalized signal is output on
fiber 109. If desired, a quarter wave plate 119 can preferably be
located behind the pixelated attenuating element in order to
compensate for polarization dependent loss in the various
components of the system, as is known in the art. Since the beam
makes two traverses through this quarter wave plate, the effect is
that of a half wave plate, as described in relation to the
embodiment shown in FIG. 4.
[0080] According to further preferred embodiments of the present
invention, instead of a dual fiber collimator, a circulator can be
used at the input/output port to separate the input from the output
beams, as is well known in the art.
[0081] Reference is now made to FIG. 14, which is a schematic cross
section of another preferred embodiment of the present invention,
in which the gain equalizer is constructed in a particularly
compact monolithic form by juxtaposing together micro-manufactured
parts, such as could be produced by micro-lithographic processes.
In the preferred embodiment shown, the input fiber 130 is
preferably attached to either a micro-prism or to a transmission
grating 132, and the light beam is transferred to the pixelated
attenuator element 136 by means of a GRIN lens or a diffractive
optical element 134. After traversing the attenuator element 136, a
similar GRIN lens or a diffractive optical element 134 is used to
image the output beam back down onto the output dispersive element
132, and from there, into the output fiber 138.
[0082] It is appreciated by persons skilled in the art that the
present invention is not limited by what has been particularly
shown and described hereinabove. Rather the scope of the present
invention includes both combinations and subcombinations of various
features described hereinabove as well as variations and
modifications thereto which would occur to a person of skill in the
art upon reading the above description and which are not in the
prior art.
* * * * *