Note: Descriptions are shown in the official language in which they were submitted.
CA 02739925 2011-05-11
Attorney Docket No. Patent Application
AUROR1300-1 Customer ID: 38396
DESCRIPTION
Compensation of Distortion from SBS/IIN Suppression Modulation
BACKGROUND INFORMATION
Field of the Invention
Embodiments of the invention relate generally to the field of optical
communications. More
particularly, an embodiment of the invention relates to compensation of
distortion from
SBS/IIN suppression modulation.
Discussion of the Related Art
Directly modulated lasers are commonly used in hybrid fiber-coax systems to
transmit RF
signals over long distances using optical fibers. These lasers are usually
single mode and
operate at wavelengths near 131 Onm or 1550nm. The advantage of 131 Onm is
most
installed fiber has zero dispersion around this wavelength. Zero dispersion is
important
because the wavelength of directly modulated lasers fluctuates or chirps with
modulation.
This can lead to non-linear transmission effects if fiber dispersion is not
zero.
The advantage of 1550nm is most installed fiber has minimum loss around this
wavelength.
So 1550nm can be used to transmit signals over longer distances than 131 Onm.
In addition,
the non-zero dispersion that most installed fiber exhibits around 1550nm helps
mitigate four
wave mixing effect, allowing 1550nm to be more easily used for wavelength
division
multiplexing. However, the non-zero dispersion interacts with laser chirp to
produce
intermodulation distortion of the transmitted signal. One way to mitigate the
effects of chirp-
dispersion induced distortion is to reduce the chirp of the optical source.
Directly modulated
optical sources with low chirp include, but are not limited to, special low
chirp DFB lasers as
well as external cavity lasers.
Reducing chirp too much or under-modulating the laser can lead to an increase
in noise and
distortion from SBS and/or IIN effects. To help compensate for this, a low
frequency SBS/IIN
suppression modulation can be added to the laser. This can be in the form of a
bias
modulation and/or a thermal modulation. A thermal modulation is preferred
because it will
not decrease clipping margin. However, obtaining a fast thermal modulation can
be difficult.
Therefore, a low frequency bias modulation may be more practical. Fortunately,
a low
frequency bias modulation will produce a similar effect as thermal modulation.
The chirp
from low frequency modulation is typically dominated by thermal effects caused
by junction
1
CA 02739925 2011-05-11
Attorney Docket No. Patent Application
AL ROR1300-1 Customer ID: 38396
heating and will provide a much larger increase in laser linewidth than can be
achieved for
the same modulation amplitude at RF frequencies. So a significant increase in
linewidth can
be obtained with only a small decrease in clipping margin.
Various predistortion schemes have been proposed to compensate for
intermodulation
distortion caused by chirp-dispersion interaction of the transmitted RF
signal. These
schemes all involve compensation of only intermodulation products of the RF
signal to be
transmitter i.e. distortion products of the RF signal mixing with itself. The
purpose of this
invention is to provide means compensate for intermodulation products of the
RF signal
mixed with a low frequency SBS/IIN suppression signal whether it be from
thermal or bias
modulation of the optical source.
SUMMARY OF THE INVENTION
There is a need for the following embodiments of the invention. Of course, the
invention is
not limited to these embodiments.
According to an embodiment of the invention, a process comprises: modulating a
laser that
is coupled to a fiber; modulating the laser with a member selected from the
group consisting
of low frequency thermal modulation or bias modulation to broaden a laser
linewidth,
increase an SBS threshold and reduce an IN and modulating the laser with a
predistorting
modulation selected from the group consisting of phase modulation or amplitude
modulation,
the predistorting modulation being of equal magnitude but opposite phase as
that produced
in at least one member selected from the group consisting of the laser or the
fiber as a result
of the low frequency thermal modulation or bias modulation. According to
another
embodiment of the invention, a machine comprises: a laser; and a fiber coupled
to the laser,
wherein the laser is i) modulated ii) modulated with a member selected from
the group
consisting of low frequency thermal modulation or bias modulation to broaden a
laser
linewidth, increase an SBS threshold and reduce an IIN and iii) modulated with
a
predistorting modulation selected from the group consisting of phase
modulation or
amplitude modulation, the predistorting modulation being of equal magnitude
but opposite
phase as that produced in at least one member selected from the group
consisting of the
laser or the fiber as a result of the low frequency thermal modulation or bias
modulation.
These, and other, embodiments of the invention will be better appreciated and
understood
when considered in conjunction with the following description and the
accompanying
drawings. It should be understood, however, that the following description,
while indicating
various embodiments of the invention and numerous specific details thereof, is
given for the
2
CA 02739925 2011-05-11
Attorney Docket No. Patent Application
AUROR1300-1 Customer ID: 38396
purpose of illustration and does not imply limitation. Many substitutions,
modifications,
additions and/or rearrangements may be made within the scope of an embodiment
of the
invention without departing from the spirit thereof, and embodiments of the
invention include
all such substitutions, modifications, additions and/or rearrangements.
BRIEF DESCRIPTION OF THE DRAWINGS
The drawings accompanying and forming part of this specification are included
to depict
certain embodiments of the invention. A clearer concept of embodiments of the
invention,
and of components combinable with embodiments of the invention, and operation
of systems
provided with embodiments of the invention, will be readily apparent by
referring to the
exemplary, and therefore nonlimiting, embodiments illustrated in the drawings
(wherein
identical reference numerals (if they occur in more than one view) designate
the same
elements). Embodiments of the invention may be better understood by reference
to one or
more of these drawings in combination with the following description presented
herein. It
should be noted that the features illustrated in the drawings are not
necessarily drawn to
scale.
FIG. 1 is a schematic view of an exemplary transmitter with phase and
amplitude modulator
for compensating, representing an embodiment of the invention.
FIG. 2 is a schematic view of an exemplary broadband phase modulator,
representing an
embodiment of the invention.
FIG. 3 is a schematic view of an exemplary broadband amplitude modulator,
representing an
embodiment of the invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
Embodiments of the invention and the various features and advantageous details
thereof are
explained more fully with reference to the nonlimiting embodiments that are
illustrated in the
accompanying drawings and detailed in the following description. Descriptions
of well known
starting materials, processing techniques, components and equipment are
omitted so as not
to unnecessarily obscure the embodiments of the invention in detail. It should
be
understood, however, that the detailed description and the specific examples,
while
indicating preferred embodiments of the invention, are given by way of
illustration only and
not by way of limitation. Various substitutions, modifications, additions
and/or
rearrangements within the spirit and/or scope of the underlying inventive
concept will
become apparent to those skilled in the art from this disclosure.
3
CA 02739925 2011-05-11
Attorney Docket No. Patent Application
AUROR1300-1 Customer ID: 38396
Within this application one or more publications is(are) referenced by Arabic
numeral(s),
within parentheses or brackets. The disclosure(s) of this(these)
publication(s) in its(their)
entireties is(are) hereby expressly incorporated by reference herein for the
purpose of
indicating the background of embodiments of the invention and illustrating the
state of the
art.
A low frequency thermal and/or bias modulation can be added to a directly
modulated laser
to broaden the laser linewidth, increasing the SBS threshold and reducing IIN.
However, this
modulation can create itermodulation distortion products with the transmitted
RF signal. The
distortion can come from a variety of sources including, but not limited to,
chirp-dispersion
interaction in the fiber and thermal modulation of the laser slope efficiency.
Chirp-dispersion
interaction can result in phase modulation of the transmitted RF signal while
thermal
modulation of the laser slope efficiency modulation can result in amplitude
modulation.
Adding a predistorting phase and/or amplitude modulation of equal magnitude
but opposite
phase as that produced in the fiber and/or laser as a result of the SBS/IIN
suppression
modulation can reduce or eliminate this distortion.
FIG. 1 shows a block diagram of an exemplary transmitter incorporating
predistortion
hardware, firmware and/or software to compensate for a low frequency SBS/IIN
suppressing
modulation. In this particularly implementation, the SBS/IIN suppressing
modulation is in the
form of a bias modulation. The DC bias with superimposed low frequency
modulation is
shown as a separate input 100 to the laser 110 because directly modulated
laser typically
incorporate an internal bias T (not shown). However, it should be noted that
there are
multiple ways in which a low frequency modulation can be added to the laser
and someone
skilled in the art will realize that many suitable combiners can be used to
combine the RF
signal with the DC bias and low frequency modulation.
In the event the low frequency SBS/IIN suppression modulation is a thermal
modulation, the
signal would be added to the thermal source connected to the laser and in
close thermal
contact with the laser. This thermal source could be in the form of a
resistive heater, a
thermo-electric cooler or some other appropriate means to provide a thermal
modulation of
the laser resulting in a wavelength modulation.
The same signal used to provide the low frequency bias or thermal modulation
also drives
predistortion circuits. The block diagram of FIG. 1 shows separate phase
modulation and
amplitude modulation predistortion circuits 120, 130. This is only an
exemplary arrangement.
The predistortion circuits can also be arranged in such a manner that a single
circuit
4
CA 02739925 2011-05-11
Attorney Docket No. Patent Application
AUROR1300-1 Customer ID: 38396
provides primarily phase, primarily amplitude and/or a combination of phase
and amplitude
modulation predistortion. It can also be arranged in such a manner that
multiple circuits
connected in series provide phase, amplitude and frequency dependent
predistortion. Also,
the low frequency input is shown as a separate input to the predistortion
circuit, however it is
also possible to feed the signal into the same input as the RF input by
appropriately
combining the RF signal with the low frequency modulation. In such cases, the
same
distortion generators used to provide predistortion of RF intermodulation
distortion can also
provide predistortion of the low frequency intermodulation distortion products
i.e. distortion
produced when the low frequency is mixed with RF frequencies.
Amplitude and phase adjust controls 150, 140 are provided for the low
frequency input to the
predistortion generators so the intermodulation products produced from the
generators will
have the same magnitude but opposite phase as that produced in the laser
and/or after
transmission over fiber. These controls may be fixed by design or adjustable
by manual or
automatic controls. In the event the magnitude and/or phase of distortion
varies from
transmitter to transmitter or from installation to installations, the
magnitude and/or phase of
the low frequency signal driving the predistortion generator can be
adjustable. In the event
the magnitude and/or phase of the distortion does not change from transmitter
to transmitter
or installation to installation, the magnitude and/or phase may be fixed by
design. In some
cases, the magnitude and/or phase may depend upon installation parameters such
as fiber
length. In such cases, the magnitude and/or phase may be best controlled by
automatic
means, such as with a microprocessor, in response to user parameter input such
as fiber
length.
FIG. 2 shows a simplified version of an exemplary broadband phase modulator in
which the
RF input 210 and the low frequency modulation input 220 are separate. This
circuit is a
modified version of a loaded line phase shifter in which the load is a pair of
varactors D1, D2.
The back to back nature of the varactors relative to the RF signal will
minimize capacitance
changes due to RF modulation and therefore minimize intermodulation distortion
from the
RF signal. The parallel nature of varactors relative to the low frequency
modulation input will
allow capacitance changes and therefore produce phase modulation. With
appropriate
choice of component values, this circuit can provide primarily phase
modulation of the RF
signal by a signal applied to the modulation input. As phase modulation is a
characteristic of
chirp interacting with fiber dispersion, this circuit can be useful to
compensate for chirp-
dispersion induced distortion.
5
CA 02739925 2011-05-11
Attorney Docket No. Patent Application
AUROR1300-1 Customer ID: 38396
FIG. 3 shows a simplified version of an exemplary broadband amplitude
modulator. This
circuit is a modified version of a PIN attenuator in which the attenuation is
modulated by a
signal applied to the modulation input 310. Diodes D1 through D4 are PIN
diodes. When
forward biased, these diodes act like current controlled resistors to RF
modulation. The
arrangement of the diodes and the bias network is such that the RF input and
output
impedance is not significantly affected by the attenuation or low frequency
modulation of the
attenuation. As amplitude modulation is a characteristic of slope efficiency
modulation of the
laser from a low frequency thermal or bias modulation, this circuit can be
used to
compensate for the resulting distortion.
Definitions
The term program and/or the phrase computer program are intended to mean a
sequence of
instructions designed for execution on a computer system (e.g., a program
and/or computer
program, may include a subroutine, a function, a procedure, an object method,
an object
implementation, an executable application, an applet, a servlet, a source
code, an object
code, a shared library/dynamic load library and/or other sequence of
instructions designed
for execution on a computer or computer system).
The term substantially is intended to mean largely but not necessarily wholly
that which is
specified. The term approximately is intended to mean at least close to a
given value (e.g.,
within 10% of). The term generally is intended to mean at least approaching a
given state.
The term coupled is intended to mean connected, although not necessarily
directly, and not
necessarily mechanically. The term proximate, as used herein, is intended to
mean close,
near adjacent and/or coincident; and includes spatial situations where
specified functions
and/or results (if any) can be carried out and/or achieved. The term distal,
as used herein, is
intended to mean far, away, spaced apart from and/or non-coincident, and
includes spatial
situation where specified functions and/or results (if any) can be carried out
and/or achieved.
The term deploying is intended to mean designing, building, shipping,
installing and/or
operating.
The terms first or one, and the phrases at least a first or at least one, are
intended to mean
the singular or the plural unless it is clear from the intrinsic text of this
document that it is
meant otherwise. The terms second or another, and the phrases at least a
second or at least
another, are intended to mean the singular or the plural unless it is clear
from the intrinsic
text of this document that it is meant otherwise. Unless expressly stated to
the contrary in
the intrinsic text of this document, the term or is intended to mean an
inclusive or and not an
6
CA 02739925 2011-05-11
Attorney Docket No. Patent Application
AUROR1300-1 Customer ID: 38396
exclusive or. Specifically, a condition A or B is satisfied by any one of the
following: A is true
(or present) and B is false (or not present), A is false (or not present) and
B is true (or
present), and both A and B are true (or present). The terms a and/or an are
employed for
grammatical style and merely for convenience.
The term plurality is intended to mean two or more than two. The term any is
intended to
mean all applicable members of a set or at least a subset of all applicable
members of the
set. The term means, when followed by the term "for" is intended to mean
hardware,
firmware and/or software for achieving a result. The term step, when followed
by the term
"for" is intended to mean a (sub)method, (sub)process and/or (sub)routine for
achieving the
recited result. Unless otherwise defined, all technical and scientific terms
used herein have
the same meaning as commonly understood by one of ordinary skill in the art to
which this
invention belongs. In case of conflict, the present specification, including
definitions, will
control.
Conclusion
The described embodiments and examples are illustrative only and not intended
to be
limiting. Although embodiments of the invention can be implemented separately,
embodiments of the invention may be integrated into the system(s) with which
they are
associated. All the embodiments of the invention disclosed herein can be made
and used
without undue experimentation in light of the disclosure. Although the best
mode of the
invention contemplated by the inventor(s) is disclosed, embodiments of the
invention are not
limited thereto. Embodiments of the invention are not limited by theoretical
statements (if
any) recited herein. The individual steps of embodiments of the invention need
not be
performed in the disclosed manner, or combined in the disclosed sequences, but
may be
performed in any and all manner and/or combined in any and all sequences. The
individual
components of embodiments of the invention need not be combined in the
disclosed
configurations, but could be combined in any and all configurations.
Various substitutions, modifications, additions and/or rearrangements of the
features of
embodiments of the invention may be made without deviating from the spirit
and/or scope of
the underlying inventive concept. All the disclosed elements and features of
each disclosed
embodiment can be combined with, or substituted for, the disclosed elements
and features
of every other disclosed embodiment except where such elements or features are
mutually
exclusive. The spirit and/or scope of the underlying inventive concept as
defined by the
appended claims and their equivalents cover all such substitutions,
modifications, additions
and/or rearrangements.
7
CA 02739925 2011-05-11
Attorney Docket No. Patent Application
AUROR1300-1 Customer ID: 38396
The appended claims are not to be interpreted as including means-plus-function
limitations,
unless such a limitation is explicitly recited in a given claim using the
phrase(s) "means for"
and/or "step for." Subgeneric embodiments of the invention are delineated by
the appended
independent claims and their equivalents. Specific embodiments of the
invention are
differentiated by the appended dependent claims and their equivalents.
8
CA 02739925 2011-05-11
Attorney Docket No. Patent Application
AUROR1 300-1 Customer ID: 38396
REFERENCE(S)
[1] "Multielectrode DFB Laser for Pure Frequency Modulation and Chirping
Suppressed
Amplitude Modulation" Yoshikuni et al, J. Lightwave Technology, Vol. 5, p. 516
(1987)
9
