U.S. patent application number 10/605490 was filed with the patent office on 2005-02-03 for optical fiber transmission system with increased effective modal bandwidth transmission.
This patent application is currently assigned to OPTIUM CORPORATION. Invention is credited to Coylar, Mark, Ereifej, Heider, Gertel, Eitan, Hallemeier, Peter.
Application Number | 20050025416 10/605490 |
Document ID | / |
Family ID | 34108799 |
Filed Date | 2005-02-03 |
United States Patent
Application |
20050025416 |
Kind Code |
A1 |
Hallemeier, Peter ; et
al. |
February 3, 2005 |
OPTICAL FIBER TRANSMISSION SYSTEM WITH INCREASED EFFECTIVE MODAL
BANDWIDTH TRANSMISSION
Abstract
A multi-mode optical fiber link is described. The multi-mode
optical fiber link includes a first spatial mode converter that is
coupled to a first single mode optical fiber. The first spatial
mode converter conditions a modal profile of an optical signal
propagating from the single mode optical fiber to the first spatial
mode converter. A multi-mode optical fiber is coupled to the first
spatial mode converter. A second spatial mode converter is coupled
to an output of the multi-mode optical fiber and to a second single
mode optical fiber. The second spatial mode converter reduces a
number of optical modes in the optical signal. Both the first and
the second spatial mode converters increase an effective modal
bandwidth of the optical signal.
Inventors: |
Hallemeier, Peter; (North
Haven, CT) ; Coylar, Mark; (Fountainville, PA)
; Gertel, Eitan; (Gwynedd, PA) ; Ereifej,
Heider; (Chalfont, PA) |
Correspondence
Address: |
RAUSCHENBACH PATENT LAW GROUP, LLC
P.O. BOX 387
BEDFORD
MA
01730
US
|
Assignee: |
OPTIUM CORPORATION
500 Horizon Drive
Chalfont
PA
|
Family ID: |
34108799 |
Appl. No.: |
10/605490 |
Filed: |
October 2, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10605490 |
Oct 2, 2003 |
|
|
|
10605107 |
Sep 9, 2003 |
|
|
|
60481166 |
Aug 1, 2003 |
|
|
|
Current U.S.
Class: |
385/28 ;
385/24 |
Current CPC
Class: |
H04B 10/2581 20130101;
H04J 14/04 20130101; G02B 6/14 20130101 |
Class at
Publication: |
385/028 ;
385/024 |
International
Class: |
G02B 006/26; G02B
006/293 |
Claims
What is claimed is:
1. A multi-mode optical fiber link comprising: a first spatial mode
converter having an input that is coupled to an output of a single
mode optical fiber, the first spatial mode converter conditioning a
modal profile of an optical signal propagating from the single mode
optical fiber to the first spatial mode converter; a multi-mode
optical fiber having an input that is coupled to an output of the
first spatial mode converter; and a second spatial mode converter
having an input that is coupled to an output of the multi-mode
optical fiber and an output that is coupled to a second single mode
optical fiber, the second spatial mode converter reducing a number
of optical modes in the optical signal, wherein both the first and
the second spatial mode converters increase an effective modal
bandwidth of the optical signal.
2. The optical fiber link of claim 1 further comprising an optical
source having an output that is coupled to an input of the single
mode optical fiber, the optical source generating the optical
signal having a wavelength.
3. The optical fiber link of claim 2 wherein the optical source
comprises an intensity modulated optical source.
4. The optical fiber link of claim 2 wherein the optical source
comprises an electro-absorption modulated laser.
5. The optical fiber link of claim 2 wherein the optical source
comprises an integrated laser modulator.
6. The optical fiber link of claim 2 further comprising a second
optical source having an output that is coupled to the input of the
single mode optical fiber, the second optical source generating a
second optical signal having a second wavelength at the output.
7. The optical fiber link of claim 2 further comprising a second
optical source having an output that is coupled to the output of
the second spatial mode converter, the second optical source
generating a second optical signal having a second wavelength at
the output.
8. The optical fiber link of claim 1 wherein at least one of the
first and the second spatial mode converters comprise a fusion
splice between the multi-mode optical fiber and a respective one of
the single mode optical fiber and the second single mode optical
fiber.
9. The optical fiber link of claim 1 wherein at least one of the
first and the second spatial mode converters comprises a lens
imaging system having refractive and diffractive elements.
10. The optical fiber link of claim 1 wherein the second spatial
mode converter reduces a number of higher-order modes propagating
in the optical signal.
11. The optical fiber link of claim 1 wherein the second spatial
mode converter reduces a number of lower-order modes propagating in
the optical signal.
12. The optical fiber link of claim 1 wherein the second spatial
mode converter reduces both a number of lower-order and a number of
higher-order modes propagating in the optical signal.
13. The optical fiber link of claim 1 wherein the first spatial
mode converter optically couples the single mode optical fiber to
the multi-mode optical fiber so as to achieve a predetermined
offset between a core of the single mode optical fiber and a core
of the multi-mode optical fiber.
14. A method of increasing effective modal bandwidth of an optical
signal transmitting through a multi-mode optical fiber, the method
comprising: spatial mode converting an optical signal, thereby
reducing modal dispersion and increasing an effective bandwidth of
the optical signal; propagating the spatially mode converted
optical signal through a multi-mode optical fiber; and spatial mode
converting the spatially mode converted optical signal propagated
through the multi-mode optical fiber, thereby further reducing
modal dispersion and further increasing the effective bandwidth of
the optical signal.
15. The method of claim 14 further comprising generating the
optical signal.
16. The method of claims 15 wherein the optical signal is generated
with relatively low time varying phase and sideband
information.
17. The method of claim 14 wherein the spatial mode converting at
least one of the optical signal and the spatially mode converted
optical signal reduces changes in effective modal bandwidth of the
optical signal that are caused by thermal variations in the
multi-mode optical fiber.
18. The method of claim 14 wherein the spatial mode converting at
least one of the optical signal and the spatially mode converted
optical signal reduces changes in effective modal bandwidth of the
optical signal that are caused by polarization effects in the
multi-mode optical fiber.
19. The method of claim 14 wherein the spatial mode converting at
least one of the optical signal and the spatially mode converted
optical signal reduces changes in effective modal bandwidth of the
optical signal that are caused by mechanical stress in the
multi-mode optical fiber.
20. The method of claim 14 wherein the spatial mode converting at
least one of the optical signal and the spatially mode converted
optical signal reduces changes in effective modal bandwidth of the
optical signal that are caused by optical fiber splices in the
multi-mode optical fiber.
21. The method of claim 14 wherein the optical signal comprises
more than one optical wavelength.
22. A multi-mode optical communication system comprising: an
optical transmitter that generates an optical signal at an output;
a first spatial mode converter having an input that is coupled to
the output of the optical transmitter, the first spatial mode
converter conditioning a modal profile of the optical signal; a
multi-mode optical fiber having an input that is coupled to an
output of the first spatial mode converter; a second spatial mode
converter having an input that is coupled to an output of the
multi-mode optical fiber, the second spatial mode converter
reducing a number of optical modes in the optical signal, wherein
both the first and the second spatial mode converters increase an
effective modal bandwidth of the optical signal; and an optical
receiver having an input that is coupled to an output of the second
spatial mode converter, the optical receiver receiving the optical
signal.
23. The communication system of claim 22 wherein the optical
transmitter comprises an electro-absorption modulated laser.
24. The communication system of claim 22 wherein the optical
transmitter comprises an integrated laser modulator.
25. The communication system of claim 22 wherein the optical signal
comprises more than one optical wavelength.
26. The communication system of claim 22 wherein the second spatial
mode converter reduces a number of higher-order optical modes in
the optical signal.
27. The communication system of claim 22 wherein the second spatial
mode converter reduces a number of lower-order optical modes in the
optical signal.
28. The communication system of claim 22 wherein the multi-mode
optical fiber comprises at least one section of single mode optical
fiber.
29. The communication system of claim 22 wherein the optical
receiver comprises an active filter that reconstructs dispersed
optical signals received by the optical receiver.
30. The communication system of claim 22 wherein the optical
receiver automatically adjusts at least one receiver parameter in
order to compensate for changes in an average power of the received
optical signal.
31. The communication system of claim 30 wherein the optical
receiver automatically adjusts the at least one receiver parameter
so as to maintain a substantially constant bit error rate as the
average power of the received optical signal changes.
32. The communication system of claim 30 wherein the at least one
receiver parameter comprises receiver sensitivity.
33. The communication system of claim 22 wherein the optical
transmitter comprises an optical intensity modulator, at least one
parameter of the optical intensity modulator is chosen to suppress
at least one of phase and sideband information in the optical
signal.
34. The communication system of claim 33 wherein the at least one
parameter of the optical intensity modulator comprises a bandwidth
of the optical intensity modulator.
35. The communication system of claim 33 wherein the at least one
parameter of the optical intensity modulator comprises an
absorption spectrum of the optical intensity modulator.
36. The communication system of claim 33 wherein the at least one
parameter of the optical intensity modulator comprises an
extinction ratio of the optical intensity modulator.
37. The communication system of claim 33 wherein the at least one
parameter of the optical intensity modulator comprises an
absorption coefficient of the optical intensity modulator.
38. The communication system of claim 33 further comprising an
optical isolator that substantially eliminates reflected optical
signals from propagating into an output of the optical intensity
modulator.
39. A multi-mode optical communication system comprising: means for
spatial mode converting an optical signal, thereby reducing modal
dispersion and increasing an effective bandwidth of the optical
signal; means for propagating the spatially mode converted optical
signal through a multi-mode optical fiber; and means for spatial
mode converting the spatially mode converted optical signal
propagated through the multi-mode optical fiber, thereby further
reducing modal dispersion and further increasing the effective
bandwidth of the optical signal.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This patent application is a continuation-in-part of U.S.
patent application Ser. No. 10/605,107, filed on Sep. 9, 2003,
entitled "Optical Transmitter for Increased Effective Modal
Bandwidth Transmission," which claims priority to U.S. provisional
patent application No. 60/481,166, filed on Aug. 1, 2003, entitled
"Optical Fiber Transmission System with Increased Effective Modal
Bandwidth." The entire disclosure of U.S. patent application Ser.
No. 10/605,107 and U.S. provisional patent application No.
60/481,166 are incorporated herein by reference.
BACKGROUND OF INVENTION
[0002] Many existing optical fiber transmission systems use
multi-mode optical fiber. Multi-mode optical fiber is widely used
because it is relatively inexpensive, easy to install and because
it is suitable for use with low cost transmitter and receiver
components. The relatively large optical fiber core and numerical
aperture of multi-mode optical fibers allows more light to be
launched into the optical fiber, as compared to single-mode optical
fibers. Therefore, such systems can use lower power and lower cost
optical sources. For these reasons, local area networks have
employed multi-mode optical fiber for many years. Some data
communication systems, such as Fiber Data Distribution Interface
(FDDI) systems are specifically designed to use multi-mode optical
fiber. Known multi-mode optical fiber transmission systems,
however, have relatively low bandwidth-distance products for a
given bit error rate (BER) and, therefore, are not suitable for
many state-of-the art communication systems.
BRIEF DESCRIPTION OF DRAWINGS
[0003] This invention is described with particularity in the
detailed description. The above and further advantages of this
invention may be better understood by referring to the following
description in conjunction with the accompanying drawings, in which
like numerals indicate like structural elements and features in
various figures. The drawings are not necessarily to scale,
emphasis instead being placed upon illustrating the principles of
the invention.
[0004] FIG. 1 illustrates a block diagram of a multi-mode optical
fiber transmission system that includes two spatial mode optical
converters according to the present invention.
[0005] FIG. 2 illustrates a block diagram of a single-mode optical
fiber transmission system that includes a spatial mode optical
converter according to the present invention.
[0006] FIG. 3A is a schematic representation of the first spatial
mode converter that couples the single mode optical fiber and the
multi-mode optical fiber according to the present invention.
[0007] FIG. 3B illustrates an electric field diagram of an optical
signal passing from the single mode optical fiber to the multi-mode
optical fiber according to the present invention.
[0008] FIG. 4 illustrates a block diagram of an optical transmitter
that includes an electro-absorption modulator according to the
present invention that generates optical signals with improved or
optimal spectral and phase characteristics for transmission through
an optical fiber link.
[0009] FIG. 5 illustrates a block diagram of an optical transmitter
that includes an electro-absorption modulated laser (EML) according
to the present invention that generates optical signals with
improved or optimal spectral and phase characteristics for
transmission through an optical fiber link.
[0010] FIG. 6 illustrates a block diagram of an optical transmitter
that includes an embodiment of a laser modulator according to the
present invention that generates optical signals with improved or
optimal spectral and phase characteristics for transmission through
an optical fiber link.
[0011] FIG. 7 illustrates a block diagram of one embodiment of an
optical receiver for the optical fiber transmission system with
increased effective modal bandwidth according to the present
invention that includes dynamic re-optimization and electronic
dispersion compensation.
DETAILED DESCRIPTION
[0012] The present invention relates to methods and apparatus for
increasing the effective modal bandwidth of optical fiber
transmission systems. The term "effective modal bandwidth" is
defined herein to mean the bandwidth-distance product of the
transmission system for a given Bit Error Rate (BER) and/or a
certain transmission specification. Increasing the effective modal
bandwidth of a multi-mode optical fiber transmission system will
allow providers to increase the data rate and will extend the
useful service life of many installed multi-mode optical fiber
transmission systems.
[0013] One aspect of the present invention is embodied in the
design of optical transmitters that have improved or optimum
spectral and phase characteristics for transmitting data in a
multi-mode optical fiber. Another aspect of the present invention
is embodied in the use of spatial filtering to reduce the number of
modes propagating in a multi-mode optical fiber. These aspects
alone or in combination increase the effective modal bandwidth of
multi-mode optical fiber transmission systems.
[0014] FIG. 1 illustrates a block diagram of a multi-mode optical
fiber transmission system 100 that includes two spatial mode
optical converters according to the present invention. The
transmission system 100 includes an optical transmitter 102, a
multi-mode optical fiber link 104, and an optical receiver 106. The
optical transmitter 102 generates optical signals for data
transmission through the multi-mode optical fiber link 104.
[0015] In one embodiment, the optical transmitter 102 includes an
intensity modulated optical source, an electro-absorption modulated
laser, an integrated laser modulator, or a laser modulator having
parameters that generate optical signals with improved or optimal
spectral and phase characteristics for transmission through an
optical fiber link as described herein. In one embodiment, the
optical transmitter 102 includes more than one optical source that
generates additional optical signals at different wavelengths. In
one embodiment, the optical source includes a WDM optical source
that generates a plurality of optical signals and each of the
plurality of optical signals has a different wavelength.
[0016] In some embodiments, the optical transmitter 102 includes
additional optical sources that are used to generate additional
optical signals that increase the data capacity of the multi-mode
optical fiber link 104. In some of these embodiments, the
multi-mode optical fiber transmission system 100 includes
additional optical transmitters 102 that are used to generate
optical signals that propagate in opposite directions in the same
multi-mode optical fiber. Separate optical carriers can be used to
minimize crosstalk between optical signals propagating in opposite
directions.
[0017] The optical transmitter 102 is optically coupled to a first
single-mode optical fiber 108. Optical signals generated by the
optical transmitter 102 propagate down the first single-mode
optical fiber 108. A first spatial mode converter 110 is optically
coupled to the first single-mode optical fiber 108. The first
spatial mode converter 110 conditions the modal profile of the
optical signal propagating through the first spatial mode converter
110. The first spatial mode converter 110 can condition the modal
profile in many ways in order to increase the effective bandwidth
of the optical signal or to increase other performance metrics of
the transmission system 100. For example, the first spatial mode
converter 110 can condition the modal profile of the optical signal
to reduce phase and sideband information, noise, or modal
dispersion in the optical signal.
[0018] An input 112 of the multi-mode optical fiber link 104 is
optically coupled to the first spatial mode converter 110. The
multi-mode optical fiber link 104 can include a single length of
multi-mode optical fiber or can include multiple lengths of
multi-mode optical fiber that are coupled together. The multiple
lengths of multi-mode optical fiber can be butt coupled together.
For example, the butt couplings can be tapered optical fiber
sections or polished optical fiber sections.
[0019] In one embodiment, the multi-mode optical fiber link 104
includes at least one single mode optical fiber link section. The
single mode optical fiber link section can be a long haul optical
fiber link that connects distant networks. In this embodiment, the
transmission system 100 of the present invention can be used to
link multiple enterprise networks that are separated by long
distances.
[0020] A second spatial mode converter 116 is optically coupled to
an output 114 of the multi-mode optical fiber link 104. The second
spatial mode converter 116 is also optically coupled to a second
single-mode optical fiber 118. The second spatial mode converter
116 reduces the number of modes in the optical signal that are
transmitted through the second spatial mode converter 116 and,
therefore, limits the number of dominant modes that are received by
the optical receiver 106.
[0021] The second spatial mode converter 116 can reduce the number
of higher-order modes, the number of lower-order modes or both the
number of higher-and lower-order modes in the optical signal
propagating through the second spatial mode converter 116. By
lower-order modes, we mean modes in which most of the energy is
localized around the center of the optical fiber core of the
multi-mode optical fiber. By higher-order modes, we mean modes in
which most of the energy is localized outside of the center of the
optical fiber core of the multi-mode optical fiber.
[0022] Both the first 110 and the second spatial mode converters
116 increase the effective modal bandwidth of the multi-mode
optical fiber transmission system 100. The first spatial mode
converter 110 can be any type of spatial mode converter that
conditions the modal profile. The second spatial mode converter 116
can be any type of spatial mode converter that reduces the number
of modes in the optical signal generated by the optical transmitter
102. For example, the first and second spatial mode converters 110,
116 can include a fusion splice or a butt coupling between the
multi-mode optical fiber link 104 and a respective one of the first
108 and the second single-mode optical fiber 118. The butt coupling
can be positioned at a bulkhead. The first and second spatial mode
converters 110, 116 can also include a lens imaging system having
refractive and diffractive elements.
[0023] The effective modal bandwidth of the multi-mode optical
fiber transmission system 100 according to the present invention
including the two spatial mode converters 110, 116 has a relatively
high-level of immunity to polarization effects, fiber stress,
vibration, and changes in temperature. In particular, there is
little or no change in the effective modal bandwidth due to changes
in laser polarization or changes in polarization caused by
mechanical stress on the multi-mode optical fiber link 104. Also,
there is little or no change in the effective modal bandwidth due
to temperature changes in the fiber environment.
[0024] In one embodiment, the receiver 106 includes electronic
dispersion compensation. In this embodiment, the receiver 106
includes at least one active electrical filter that is electrically
coupled to the output of a detector as described herein. Also, in
one embodiment, the receiver includes dynamic re-optimization that
automatically adjusts at least one receiver parameter in order to
compensate for changes in an average power of the received optical
signal as described herein.
[0025] The present invention features a method of increasing
effective modal bandwidth of an optical signal transmitted through
a multi-mode optical fiber. The method includes generating an
optical signal and propagating the optical signal through a
single-mode optical fiber. In one embodiment, the optical signal is
chosen to reduce phase corruption. The optical signal is then
spatially mode converted to an optical signal having a conditioned
modal profile. The spatial mode converting reduces modal
dispersion, which increases an effective bandwidth of the optical
signal.
[0026] The optical signal having the lower number of modes is then
propagated through a multi-mode optical fiber. The optical signal
propagating through the multi-mode optical fiber is then spatially
mode converted, which further increases the effective bandwidth of
the optical signal. The spatial mode conversions can reduce changes
in effective modal bandwidth of the optical signal that are caused
by physical effects, such as thermal variations in the multi-mode
optical fiber, polarization effects in the multi-mode optical
fiber, mechanical stress in the multi-mode optical fiber, optical
fiber splices in the multi-mode optical fiber, and optical
connector misalignment in the multi-mode optical fiber.
[0027] Some aspects of the present invention are described in
connection with a multi-mode optical fiber link that is typically a
local area fiber link. However, the present invention can also be
practiced with a single-mode optical fiber link that is typically a
long-haul optical fiber link.
[0028] FIG. 2 illustrates a block diagram of a single-mode optical
fiber transmission system 150 that includes a spatial mode optical
filter according to the present invention. The transmission system
150 includes an optical transmitter 102, a single-mode optical
fiber link 152, and an optical receiver 106. The single-mode
optical fiber transmission system 150 is similar to the multi-mode
optical fiber transmission system 100 that was described in
connection with FIG. 1.
[0029] The optical transmitter 102 generates optical signals for
data transmission through the single-mode optical fiber link 152.
In one embodiment, the optical transmitter 102 includes more than
one optical source that generates additional optical signals at
different wavelengths that increase the data capacity of the
single-mode optical fiber link 152. In some embodiments, the
single-mode optical fiber transmission system 100 includes
additional optical transmitters 102 that are used to generate
optical signals that propagate in opposite directions in the same
single-mode optical fiber.
[0030] The optical transmitter 102 is optically coupled to a first
single-mode optical fiber 108. Optical signals generated by the
optical transmitter 102 propagate down the first single-mode
optical fiber 108. A first spatial mode converter 110 is optically
coupled to the first single-mode optical fiber 108. The first
spatial mode converter 110 conditions the modal profile in the
optical signal propagating through the first spatial mode converter
110.
[0031] An input 151 of the single-mode optical fiber link 152 is
optically coupled to the first spatial mode converter 110. The
single-mode optical fiber link 152 can include a single length of
single-mode optical fiber or can include multiple lengths of
single-mode optical fiber that are fusion spliced or coupled
together. An optical coupler 154 is optically coupled to an output
156 of the single-mode optical fiber link 152. The optical coupler
154 is also optically coupled to a second single-mode optical fiber
118.
[0032] The first spatial mode converter 110 increases the effective
modal bandwidth of the single-mode optical fiber transmission
system 150. The effective modal bandwidth of the single-mode
optical fiber transmission system 150 according to the present
invention including the first spatial mode converter 110 has a
relatively high-level of immunity to polarization effects, fiber
stress, and changes in temperature.
[0033] In one embodiment, the receiver 106 includes an electronic
dispersion compensation circuit. In this embodiment, the receiver
106 includes at least one active electrical filter that is
electrically coupled to the output of a detector as described
herein. Also, in one embodiment, the receiver includes dynamic
re-optimization that automatically adjusts at least one receiver
parameter in order to compensate for changes in an average power of
the received optical signal as described herein.
[0034] FIG. 3A is a schematic representation 170 of the first
spatial mode optical converter 110 that couples the single mode
optical fiber 108 and the multi-mode optical fiber link 104
according to the present invention. The single mode optical fiber
108 is designed to propagate an optical signal having a wavelength
with only one type of spatial distribution (i.e. one optical mode).
Single-mode optical fibers typically have a core 172 that is
between about 8-10 microns in diameter.
[0035] The single mode optical fiber 108 is coupled to an input 174
of the first spatial mode converter 110. The first spatial mode
converter 110 can be any type of spatial mode converter that
conditions the modal profile of the optical signal that is applied
to the input 174. For example, the first spatial mode converter 110
can include a fusion splice, a butt coupling, or a lens imaging
system having refractive and diffractive elements. An output 176 of
the spatial mode converter 110 is coupled to the multi-mode optical
fiber link 104. The multi-mode optical fiber link 104 is designed
to support multiple spatial distributions. Multi-mode optical
fibers typically have a core 178 that is on the order of 50 microns
in diameter.
[0036] In one embodiment, the first spatial mode converter 110
couples the single mode optical fiber 108 and the multi-mode
optical fiber link 104 so as to achieve a predetermined offset
between a center of the core 172 of the first single mode optical
fiber 108 and a center of a core 178 of the multi-mode optical
fiber in the link 104. For example, in this embodiment, the center
of the core 172 of the first single mode optical fiber 108 can be
offset between about 15-25 micrometers from the center of the core
178 of the multi-mode optical fiber in the link 104. Offsetting the
center of the core 172 of the first single mode optical fiber 108
from the center of the core 178 of the multi-mode optical fiber in
the link 104 changes the launch conditions.
[0037] FIG. 3B illustrates an electric field diagram 180 of an
optical signal passing from the single mode optical fiber 108 to
the multi-mode optical fiber link 104 according to the present
invention. The electric field diagram 180 illustrates the magnitude
of the electric field in the optical signal as a function of
distance in microns from inside the single mode optical fiber 108
to inside the multi-mode optical fiber link 104.
[0038] The magnitude of the electric field intensity 182 in the
optical signal at Z=0 micrometers corresponds to the magnitude of
the electric field intensity inside the single mode optical fiber
108 and at the input 174 of the spatial mode converter 110. The
magnitude of the electric field intensity 184 in the optical signal
at Z=1,000 micrometers corresponds to the magnitude of the electric
field intensity at the mode conversion point inside the spatial
mode converter 110. The magnitude of the electric field intensity
186 in the optical signal at Z=2,000 micrometers corresponds to the
magnitude of the electric field intensity at the output 174 of the
spatial mode converter 110. The magnitude of the electric field
intensity 188 in the optical signal at Z=4,000 micrometers
corresponds to the magnitude of the electric field intensity inside
of the multi-mode optical fiber link 104.
[0039] The electric field diagram of an optical signal passing from
the multi-mode optical fiber link 104 to the second single-mode
optical fiber 118 is similar to the electric field diagram 180 of
FIG. 3B, but the distance scale is inverted. The spatial mode
converters 110, 116 condition and reduce the number of dominant
modes that are received by the optical receiver 106.
[0040] FIG. 4 illustrates a block diagram of an optical transmitter
200 that includes an electro-absorption modulator according to the
present invention that generates optical signals with improved or
optimal spectral and phase characteristics for transmission through
an optical fiber link. The optical transmitter 200 improves the
spectral and phase characteristics for transmission through
multi-mode optical fiber links, such as the multi-mode optical
fiber link 104 that is described in connection with FIG. 1. In
addition, the optical transmitter 200 improves the spectral and
phase characteristics for transmission through single-mode optical
fiber links, such as long-haul single-mode optical fiber links.
[0041] The optical transmitter 200 is designed to generate optical
signals that have specific characteristics which increase or
maximize immunity to variations in the phase of the optical signal
received by the optical receiver 106 (FIGS. 1 and 2). One
characteristic of the optical signal generated by the optical
transmitter 200 is a reduction in time varying phase or sideband
information in the transmission spectrum of the optical signal.
Another characteristic of the optical signal generated by the
optical transmitter 200 is a reduction in the phase information
that is required to transmit the data in the optical fiber links
104, 152 (FIGS. 1 and 2).
[0042] Another characteristic of the optical signal generated by
the optical transmitter 200 is a reduction or elimination of mixing
that is required at the optical receiver 106 (FIG. 1) to recover
the optical signal. Yet another characteristic of the optical
signal generated by the optical transmitter 200 is an increase in
isolation of optical signals reflected back towards the optical
transmitter 102. In one embodiment of the invention, the optical
transmitter 200 generates an optical signal with one or any
combination of these characteristics. Generating an optical signal
with one or more of these characteristics will increase the
effective modal bandwidth of the multi-mode optical fiber
transmission system 100 (FIG. 1) and the effective modal bandwidth
of the single-mode optical fiber transmission system 150 (FIG.
2).
[0043] One type of optical transmitter that can generate an optical
signal with one or any combination of these characteristics is an
electro-absorptively (EA) modulated optical transmitter. The
optical transmitter 200 illustrated in FIG. 4 is an exemplary EA
optical modulated transmitter. Numerous types of EA modulated
sources can be used in an optical transmitter according to the
present invention. In other embodiments, other types of intensity
modulators are used.
[0044] The optical transmitter 200 includes a laser 202 that
generates a continuous wave (CW) optical signal at an output 204.
In some embodiments, the laser 202 is a semiconductor diode laser.
However, other types of lasers can also be used. The transmitter
200 also includes a laser bias circuit 206. An output 208 of the
laser bias circuit 206 is electrically connected to a bias input
210 of the laser 202. The laser bias circuit 206 generates a
current at the output 208 that biases the laser 202.
[0045] The optical transmitter 200 also includes an
Electro-Absorption Modulator (EAM) 210 that modulates the CW
optical signal generated by the laser 202. In some embodiments, the
laser 202 and the EAM 210 are separate discrete components. In
other embodiments, the laser 202 and the EAM 210 are physically
integrated on a single substrate. The EAM 210 includes an optical
input 212, a bias and modulation input 214, and an optical output
216. The optical input 212 is positioned in optical communication
with the output 204 of the laser 202. A waveguide, such as an
optical fiber, can be used to optically couple the output 204 of
the laser 202 to the optical input 212 of the EAM 210.
[0046] The optical transmitter 200 including the EAM 210 generates
optical signals with improved or optimal spectral and phase
characteristics for transmission through a multi-mode optical fiber
link. The modulated optical signal that is generated by the optical
transmitter 200 including the EAM 210 has very little phase
information because EA modulators operate as efficient intensity
modulators.
[0047] In one embodiment of the invention, the EAM 210 is
specifically designed and fabricated to have at least one parameter
that causes the EAM 210 to modulate intensity so as to suppress
time varying phase and sideband information in the transmission
spectrum. EA modulators are relatively efficient intensity
modulators. Therefore, time varying phase and sideband information
in the transmission spectrum is generally suppressed. However, a
transmitter according to one embodiment of the invention can be
designed, fabricated, and/or operated to further reduce time
varying phase and sideband information in the transmission
spectrum.
[0048] There are numerous physical EA modulator parameters that can
be adjusted to change the amplitude and phase characteristics of
the modulated optical signal in order to suppress phase and
sideband information from the transmission spectrum. For example,
parameters, such as the extinction ratio or voltage swing of the EA
modulator, polarization properties, the 3-dB bandwidth, the facet
coating properties, the input third-order intercept (IIP3), and the
spurious free dynamic range (SFDR) can be adjusted during design
and fabrication to suppress phase and sideband information from the
transmission spectrum. Adjusting the extinction ratio of the EA
modulator has been shown to suppress phase and sideband information
from the transmission spectrum and, consequently, to increase the
signal-to-noise ratio of optical signals propagating through
multi-mode optical fiber. In one embodiment of the invention, the
extinction ratio of the EA modulator 210 is in the range of about
five to fifteen.
[0049] The optimal value of the extinction ratio is a function of
the length of the multi-mode optical fiber. The optimal value of
the extinction ratio can also be a function of the number of the
fiber connectors and the alignment of the fiber connectors in the
multi-mode optical fiber link 104 (FIG. 1) and the single mode
optical fiber link 152 (FIG. 2). In addition, the optimal value of
the extinction ratio can also be a function and many environmental
factors, such as the level of the vibration, mechanical strain,
thermal shock, and optical power fluctuations in the optical fiber
link.
[0050] In one example, an EA modulator with an extinction ratio of
about 11.5 has been shown to transmit optical signals through a
1250 foot multi-mode optical fiber link with relatively low phase
and sideband information and relatively high signal-to-noise ratio
compared with EA modulators having extinction ratios of about five
and about eight in the same optical fiber link under similar
environmental conditions. In another example, an EA modulator with
an extinction ratio of about ten has been shown to transmit optical
signals through a 4500 foot multi-mode optical fiber link with
relatively low phase and sideband information and relatively high
signal-to-noise ratio compared with an EA modulator having an
extinction ratio of about five in the same optical fiber link under
similar environmental conditions.
[0051] The optical transmitter 200 also includes a bias and data
multiplexing circuit 218 that generates the desired electrical bias
and data signals for the EAM 210. In some embodiments, the bias and
data multiplexing circuit 218 includes two physically separate
components. In other embodiments, the bias and data multiplexing
circuit 218 is one component as shown in FIG. 4. An output 220 of
the bias and data multiplexing circuit 218 is electrically
connected to the modulation input 214 of the EAM 210. The EAM 210
modulates the CW optical signal generated by the laser 202 with an
electronic data signal generated by the bias and data multiplexing
circuit 218. The modulated optical signal propagates from the
optical output 216 of the EAM 210.
[0052] In one embodiment of the invention, the operating conditions
of the EAM 210 are chosen so as to suppress phase and/or sideband
information in the transmission spectrum generated by the EAM 210.
For example, the operating temperature of the EAM 210 and the bias
voltage that is generated by the bias and data multiplexing circuit
218 and applied to the modulation input 214 of the EAM 210 can be
adjusted during operation to suppress phase and/or sideband
information from the optical signal.
[0053] In addition, parameters of the laser 202 that generates the
optical signal which is modulated by the EAM 210 can be adjusted to
suppress phase and/or sideband information from the transmission
spectrum. For example, parameters, such as the wavelength and the
optical mode structure of the optical signal generated by the laser
202 can be adjusted so as to suppress phase information and/or
sideband information from the modulated optical signal.
[0054] The modulated optical signal that is generated by the
optical transmitter 200 including the EAM 210 has certain
characteristics in its transmission spectrum that increase the
effective modal bandwidth of the optical fiber link. For example,
one characteristic of the transmission spectrum is that the optical
signal has minimal time varying phase. Another characteristic of
the transmission spectrum is that it has minimal sideband
information.
[0055] The modulated optical signal that is generated by the
optical transmitter 200 requires essentially no phase information
to transmit the data in an optical link, such as the multi-mode
optical fiber link 104 (FIG. 1) and the single-mode optical fiber
link 152 (FIG. 2). In addition, the modulated optical signal that
is generated by the optical transmitter 200 has good isolation from
optical signals reflecting back towards the optical transmitter
200.
[0056] Thus, the optical transmitter 200 improves the spectral and
phase characteristics for transmission through multi-mode optical
fiber links, such as the multi-mode optical fiber link 104 that is
described in connection with FIG. 1. In addition, the optical
transmitter 200 improves the spectral and phase characteristics for
transmission through single-mode optical fiber links, such as the
single-mode optical fiber link 152 that is described in connection
with FIG. 2.
[0057] There are numerous other types of optical transmitters that
when designed, fabricated, and operated according to the present
invention will generate optical signals with improved or optimal
spectral and phase characteristics for transmission through a
multi-mode and single-mode optical fiber link. These optical
transmitters include electro-absorption modulated lasers (EMLs),
laser modulators, and electro-optic modulators.
[0058] FIG. 5 illustrates a block diagram of an optical transmitter
250 that includes an electro-absorption modulated laser (EML) 252
according to the present invention that generates optical signals
with improved or optimal spectral and phase characteristics for
transmission through an optical fiber link. The EML 252 includes a
laser diode 254 section and an electro-absorption modulator (EAM)
256 section.
[0059] The laser diode 254 section is typically a distributed
feedback (DFB) laser. The EAM 256 is typically a device that
includes a semiconductor layer, such as a multi-quantum well
semiconductor layer. The semiconductor layer typically has a
slightly larger absorption band edge than the photon energy of the
light being modulated. The laser diode 254 section is optically
coupled to the EAM 256 section. The laser diode 254 section and EAM
256 section are typically integrated onto a single substrate, but
can be physically separate devices.
[0060] A laser bias circuit 258 has an output 260 that is
electrically coupled to a bias input 262 of the laser diode 254.
The laser bias circuit 258 generates a continuous wave (CW) current
that drives the laser diode 254, thereby causing the laser diode
254 to emit substantially monochromatic light of a predetermined
wavelength.
[0061] A modulator bias and data multiplexing circuit 264 has an
output 266 that is electrically coupled to a modulation input 268
of the EAM 256. The modulator bias and data multiplexing circuit
264 generates a voltage across the multi-quantum well semiconductor
layer that produces a reverse bias modulating electric field across
the semiconductor layer of the EAM 256. The reverse bias modulating
electric field causes the absorption edge of the semiconductor
layer of the EAM 256 to reversibly move to a longer wavelength,
which corresponds to a lower absorption edge. The lower absorption
edge causes the semiconductor layer of the EAM 256 to absorb the
light generated by the laser diode 254 section that propagates
through the semiconductor layer of the EAM 256.
[0062] Reducing the voltage across the multi-quantum well
semiconductor layer results in the elimination or reduction of the
reverse bias electric field, which causes the semiconductor layer
of the EAM 256 to allow light generated by the laser diode 254 to
transmit through the semiconductor layer of the EAM 256. Therefore,
light emitted from the laser diode 254 that propagates to the EAM
256 is modulated by modulating the voltage across the multi-quantum
well semiconductor layer of the EAM 256. The light emitted is
modulated between a sufficient reverse bias voltage across the
semiconductor layer that causes the layer to be substantially
opaque to the light emitted from the laser diode 254, and
substantially zero or a sufficiently positive bias voltage that
causes the layer to be substantially transparent to the light
emitted from the laser diode 254.
[0063] The resulting modulated light is emitted at an optical
output 270 of the EML 252. The optical output 270 is directly
coupled to the single-mode optical fiber 108 (FIGS. 1 and 2). The
wavelength of the modulated light can be controlled by adjusting
the amplitude of the CW current generated by the laser bias circuit
258 and applied to the laser diode 254. The wavelength of the
modulated light can also be controlled by adjusting the temperature
of the laser diode 254.
[0064] The EML 252 includes a thermoelectric cooler (TEC) 272 that
controls the temperature of the laser diode 254 and the EAM 256.
The temperature of the EML 252 can be stabilized by using a thermal
sensor 274 and a feedback circuit 276. The thermal sensor 274 is
thermally coupled to the laser diode 254 and is electrically
coupled to the feedback circuit 276. The feedback circuit 276 is
electrically coupled to the TEC 272. The feedback circuit 276
receives a signal from the thermal sensor 274 that is related to
the temperature of the laser diode 254 and generates a signal in
response to the temperature. The signal generated by the feedback
circuit 276 controls the thermal properties of the TEC 272 to
maintain the laser diode 254 at a predetermined operating
temperature (and thus the major portion of spectral energy of the
emitted light at the desired wavelength) independent of ambient
temperature.
[0065] In one embodiment of the invention, the EML 252 is
specifically designed and fabricated to have at least one parameter
that causes the EML 252 to generate a transmission spectrum with
suppressed phase and sideband information. There are numerous
physical EML parameters that can be adjusted to change the
amplitude and phase characteristics of the modulated optical signal
in order to suppress phase and sideband information from the
transmission spectrum.
[0066] For example, parameters of the EAM 256, such as the
extinction ratio, the polarization properties, the 3-dB bandwidth,
the modulator chirp, the optical mode structure, the input
third-order intercept (IIP3), the spurious free dynamic range
(SFDR), and the output facet coating properties can be adjusted
during design and fabrication to suppress phase and/or sideband
information from the transmission spectrum.
[0067] Also, parameters of the laser diode 254, such as the
wavelength, the optical mode structure, and the parameters of the
output facet coating can be adjusted during design and fabrication
to suppress phase and/or sideband information from the transmission
spectrum. In addition, parameters specific to EML devices, such as
the electrical isolation and the optical coupling between the laser
diode 254 and the EAM 256 can be adjusted during design and
fabrication to suppress phase and/or sideband information from the
transmission spectrum.
[0068] In one embodiment of the invention, the operating conditions
of the EML 252 are chosen so as to suppress phase and/or sideband
information in the transmission spectrum. For example, operating
conditions, such as the current generated by the laser bias circuit
258 and the resulting optical power received by the EAM 256, the
bias voltage swing that is generated by the bias and data
multiplexing circuit 264 and received by the EAM 256, and the
operating temperature of the laser diode 254 and the EAM 256 can be
adjusted during operation of the EML 252 to suppress phase and/or
sideband information in the transmission spectrum.
[0069] The present invention can also be practiced with numerous
types of laser modulators. FIG. 6 illustrates a block diagram of an
optical transmitter 300 that includes an embodiment of a laser
modulator 302 according to the present invention that generates
optical signals with improved or optimal spectral and phase
characteristics for transmission through an optical fiber link. The
optical transmitter 300 includes a laser section 304 and a
modulator section 306.
[0070] The laser section 304 of the laser modulator 302 shown in
FIG. 6 is a tunable three section Distributed Bragg Reflector (DBR)
laser. In other embodiments (not shown), a single section DFB laser
can be used if wavelength tuning is not desirable. The laser
section 304 includes a gain section 308, a phase section 310, and a
grating section 312 that are butted together. The gain section 308
generates an optical signal. The phase section 310 introduces an
optical phase shift to tune the laser wavelength. The grating
section 312 forms a DBR mirror.
[0071] A high reflection coating 314 is deposited on one side of
the gain section 308. A laser cavity is formed between the high
reflection coating 314 and the DBR mirror formed by the grating
section 312. The optical transmitter 300 includes a laser bias
circuit 316 having an output 318 that is electrically connected to
a bias input 320 of the gain section 308. The laser bias circuit
316 generates a current at the output 318 that biases the gain
section 308 to emit the desired optical signal. The design and
operation of such lasers are well known in the art.
[0072] The modulator section 306 of the laser modulator 302 is
positioned outside of the laser cavity beyond the DBR mirror in the
grating section 312. Forming the modulator section 306 external to
the laser cavity introduces relatively low wavelength chirp into
the modulated optical signal. An input 328 of the modulator section
306 is optically coupled to the grating section 312. An output
facet 330 of the modulator section 306 transmits the modulated
optical signal. An anti-reflection coating 332 is deposited on the
output facet 330 of the modulator section 306 to prevent undesired
reflection from entering the laser cavity.
[0073] A modulator bias and data multiplexing circuit 322 has an
output 324 that is electrically coupled to a modulation input 326
of the modulator section 306. The modulator section 306 is an
intensity modulator that modulates a CW optical signal that is
generated by the laser section 304 with the data generated by the
modulator bias and data multiplexing circuit 322. The modulated
optical signal is transmitted though the output facet 330 of the
modulator section 306 and the anti-reflection coating 332.
[0074] Many different types of modulator sections 306 can be used
with an optical transmitter 300. For example, the modulator section
306 can be a Franz-Keldysh-type electro-absorption modulator
section. Such a modulator section 306 includes a section of
waveguide with an active region of bulk semiconductor
heterostructure material having a slightly larger bandgap energy
than the photon energy of the optical signal being modulated. When
the modulator bias and data multiplexing circuit 322 applies a
reverse bias field to the modulation input 326 of the modulator
section 306, the absorption edge is lowered, thus reducing the
light emitted.
[0075] The modulator section 306 can also be a modulated
amplifier-type modulator. Such a modulator includes a gain section
that can be formed of the same material as the gain section 308 in
the laser cavity. Modulated amplifier-type modulators can achieve
relatively broad optical bandwidth. In addition, the modulator
section 306 can be a guide/antiguide-type modulator.
Guide/antiguide modulators use refractive index effects to achieve
intensity modulation. However, unlike other devices that use
refractive index effects, such as Mach-Zehnder type modulators,
these modulators do not generate large amounts of phase and
sideband information in the transmission spectrum because they do
not use interference effects.
[0076] In one embodiment of the invention, the optical transmitter
300 is specifically designed and fabricated to have at least one
parameter that causes the optical transmitter 300 to generate a
transmission spectrum with suppressed phase and sideband
information. There are numerous physical parameters of the laser
section 304 and the modulator section 306 that can be adjusted to
change the amplitude and phase characteristics of the modulated
optical signal in order to suppress phase and sideband information
from the transmission spectrum.
[0077] For example, parameters of the laser section 304, such as
the wavelength, the optical mode structure, and the parameters of
the output facet coating can be adjusted during design and
fabrication to suppress phase and/or sideband information from the
transmission spectrum. In addition, parameters specific to DBR and
DFB laser devices, such as the grating parameters and the
properties of the waveguides in the gain section 308, the phase
section 310, and the grating section 312, as well as the coupling
parameters between these sections, can be adjusted during design
and fabrication to suppress phase and/or sideband information from
the transmission spectrum.
[0078] Also, parameters of the modulator section 306, such as the
extinction ratio, the polarization properties, the 3-dB bandwidth,
the modulator chirp, the optical mode structure, the input
third-order intercept (IIP3), the spurious free dynamic range
(SFDR), the lateral index guide and antiguide profiles (for
guide/antiguide-type modulators), and the output facet coating
properties can be adjusted during design and fabrication to
suppress phase and/or sideband information from the transmission
spectrum.
[0079] In one embodiment of the invention, the operating conditions
of the optical transmitter 300 are chosen so as to suppress phase
and/or sideband information in the transmission spectrum. For
example, the current generated by the laser bias circuit 316 and
the resulting optical power received by the modulator section 306,
the bias voltage that is generated by the bias and data
multiplexing circuit 322 and received by the modulator section 306,
and the operating temperature of the laser section 304 and the
modulator section 306 can be adjusted during operation to suppress
phase and/or sideband information from the transmission
spectrum.
[0080] The optical transmitters described herein that generate
optical signals with improved or optimal spectral and phase
characteristics for transmission through an optical fiber link can
be used for transmitting 10GEthernet data in multi-mode optical
fiber transmission systems greater than 300 meters long. Error free
transmission of optical signals having a 1310 nm wavelength over
300 meters of multi-mode optical fiber using such optical
transmitters has been demonstrated.
[0081] FIG. 7 illustrates a block diagram of one embodiment of an
optical receiver 350 for the optical fiber transmission system with
increased effective modal bandwidth according to the present
invention that includes dynamic re-optimization and electronic
dispersion compensation. The dynamic re-optimization and electronic
dispersion compensation alone or in combination increase the
bandwidth-distance product of the optical fiber transmission system
according to the present invention.
[0082] The optical receiver 350 includes a photo-detection circuit
352 that is optically coupled to the output of the second
single-mode optical fiber 118 (FIGS. 1 and 2). The photo-detection
circuit 352 includes a photo-diode that converts the received
optical signal into an electrical signal. The optical receiver 350
also includes a pre-amplifier 354 having an input 356 that is
electrically connected to a signal output 358 of the
photo-detection circuit 352. The pre-amplifier 354 amplifies the
electrical signal generated by the photo-detection circuit 352 to a
signal level that is suitable for electronic processing.
[0083] The optical receiver 350 also includes a voltage sensing
amplifier 360 having an input 362 that is electrically connected to
a control output 364 of the photo-detection circuit 352. The
voltage sensing amplifier 360 generates a feed forward signal
having a voltage that is proportional to the average optical power
level of the received optical signal. A decision threshold circuit
366 has a signal input 368 that is electrically connected to an
output 370 of the pre-amplifier 354 and a control input 372 that is
electrically connected to an output 374 of the voltage sensing
amplifier 360. The decision threshold circuit 366 adjusts the
decision threshold in the optical receiver 350 to the optimal
threshold for the received power level in order to maximize the
signal-to-noise ratio of the optical receiver 350.
[0084] The optical receiver 350 includes an electronic dispersion
compensation circuit 376 having an input 378 that is electrically
connected to an output 380 of the decision threshold circuit 366.
The electronic dispersion compensation circuit 376 compensates for
the effects of dispersion by reconstructing the dispersed optical
signals. Dispersion can severely degrade signals in the optical
fiber transmission systems 100, 150 that are described in
connection with FIG. 1 and FIG. 2. Several different types of
dispersion can occur in these optical fiber transmission
systems.
[0085] For example, chromatic dispersion can occur in WDM optical
fiber transmission systems. Chromatic dispersion is caused by
differences in the speed at which signals having different
wavelengths travel in the optical fiber link. Chromatic dispersion
generally decreases the acceptable transmission distance as the
square of the bit rate.
[0086] Polarization mode dispersion (PMD) occurs when the
orthogonal polarization components of the optical signal travel at
different rates in the optical fiber link. Polarization mode
dispersion results from asymmetries in the optical fiber core.
Polarization mode dispersion causes a statistical disruption in
network operation and, consequently, limits the transmission
distance.
[0087] Signal degradation caused by these dispersions, if
uncompensated, can corrupt the received optical signal by
broadening the pulses in the optical signal, which causes Inter
Symbol Interference (ISI). The ISI will eventually degrade the
signal quality enough for the signal to fall below the acceptable
threshold for service. Thus, these dispersions can limit the
possible bandwidth-distance product in the optical fiber links and
can cause service interruptions.
[0088] The dispersion compensation circuit 376 includes at least
one active filter. There are many different types of active filters
know in the art that are suitable for electronic dispersion
compensation. For example, the active filter can be a Finite
Impulse Response (FIR) filter, such as a Feed Forward Equalizer
(FFE) filter. Such filters sample the received signal, after
electro-optic conversion by the photo-detection circuit 352.
Different delayed samples are scaled and then summed once per
sample clock. The length of the FIR filter (i.e. the number of
taps) is related to the amount of ISI that is incurred during
transmission.
[0089] The dispersion compensation circuit 376 can also include a
Decision Feedback Equalizer (DFE) filter that is used with the FFE
filter to further reduce the ISI in the optical signal. The DFE
filter takes the decisions from the FFE filter as its input. The
output of the DFE filter is combined with the output of the FFE
filter and is fed back to the input of the DFE filter. The clock
and data are then recovered from the dispersion compensated
signal.
[0090] The optical receiver 350 also includes a demodulator 382
having an input 384 that is electrically connected to an output 386
of the dispersion compensation circuit 376. The demodulator 382
demodulates the reconstructed optical signal and recovers the
transmitted data. The demodulator 382 generates the recovered data
at an output 388.
[0091] Equivalents
[0092] While the invention has been particularly shown and
described with reference to specific preferred embodiments, it
should be understood by those skilled in the art that various
changes in form and detail may be made therein without departing
from the spirit and scope of the invention as defined by the
appended claims.
* * * * *