Metropolitan area network using low insertion loss optical...

Details Metropolitan area network using low insertion loss optical... Metropolitan area network using low insertion loss optical...

2000-04-13

2003-07-01

Negash, Kinfe-Michael (Department: 2733)

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

C359S199200, C359S199200, C385S024000, C385S037000

Reexamination Certificate

active

06587237

ABSTRACT:

RELATED APPLICATIONS
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FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
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MICROFICHE APPENDIX
Not applicable
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the field of optical communication systems, and in particular, to a Metropolitan Area Network (MAN) using low insertion loss Optical Add-Drop Multiplexers (O-ADMs).
2. Description of the Prior Art
Fiber optic networks vary in size to accommodate different communication needs. Wide Area Networks (WANs) span the nation providing communications over long distances. Local Area Networks (LANs), in contrast, provide communications over short distances, such as in a building. In between WANs and LANs are Metropolitan Area Networks (MANs). MANs are smaller than WANs and larger than LANS, and typically range in size from approximately 25 km to 100 km. MANs are typically used for communications on a campus or in a city.
WANs use optical fiber amplifiers to boost optical signals transmitted over the network because of the expansive distances covered by the networks. MANs typically do not use optical fiber amplifiers in an attempt to keep costs and design complexity at a minimum. The smaller size and absence of optical fiber amplifiers could be features that distinguish MANs from WANs. Without optical fiber amplifiers, MANs are limited in how large they can grow.
Fiber optic networks, including MANs, often utilize multiplexing technologies to increase the volume of traffic upon the network. One such multiplexing technology is Wavelength Division Multiplexing (WDM). WDM is used to pass multiple data channels over one or more wavelengths of light simultaneously over a single fiber. As an optical signal is transmitted over the fiber, wavelengths can be dropped or added at nodes set out in the network. The nodes typically use Optical Add-Drop Multiplexers (O-ADMs) to add wavelengths to, and drop wavelengths from, the optical signal. Typically, a node is assigned a wavelength so that each node in the system drops and adds different wavelengths.
FIG. 1
shows a Metropolitan Area Network (MAN)
100
in the prior art. MAN
100
is comprised of a fiber
110
, a central node
112
, and O-ADMs
121
-
126
. Each end of fiber
110
is coupled to the central node
112
to form a ring. O-ADMs
121
-
126
are coupled to fiber
110
in series. Central node
112
is connected to a first system (not shown) and configured to transmit an optical signal comprised of wavelengths &lgr;
1
-&lgr;
n
over fiber
110
, transmit signals to the first system, and receive signals from the first system. O-ADMs
121
-
126
are configured to drop wavelengths from, and add wavelengths to, the optical signal.
In operation, central node
112
transmits the optical signal comprised of wavelengths &lgr;
1
-&lgr;
n
over fiber
110
. O-ADM
121
receives the optical signal from central node
112
. O-ADM
121
drops a wavelength &lgr;
1
from the optical signal and transfers &lgr;
1
to a second system (not shown). O-ADM
121
also receives &lgr;
1
from the second system and adds &lgr;
1
back to the optical signal. O-ADM
121
transfers the optical signal to O-ADM
122
. O-ADM
122
drops a wavelength &lgr;
2
from the optical signal and transfers &lgr;
2
to a third system (not shown). O-ADM
122
also receives &lgr;
2
from the third system and adds &lgr;
2
back to the optical signal. O-ADM
122
transfers the optical signal to O-ADM
123
. The same operation takes place over O-ADMs
123
-
126
. Central node
112
receives the optical signal comprised of wavelengths &lgr;
1
-&lgr;
n
from O-ADM
126
. Two common O-ADMs used in the art are single fiber grating O-ADMs and dielectric film filter type O-ADMs.
FIG. 2
shows a single fiber grating O-ADM
200
commonly used in MANs in the prior art. O-ADM
200
is comprised of a first optical circulator
220
coupled to a second optical circulator
222
by a fiber
214
. Between optical circulators
220
and
222
, a Bragg grating
230
is written into fiber
214
. The Bragg grating
230
, which is based on the Bragg effect,is a periodic perturbation of the effective refractive index of fiber
214
, wherein fiber
214
is photo-sensitive. The Bragg grating
230
is configured to reflect a narrow or broad range of wavelengths of light while passing all other wavelengths. The Bragg grating
230
is written into fiber
214
with a laser beam of Ultra-Violet (LV) light. The UV light permanently changes the refractive index of fiber
214
.
In operation, optical circulator
220
receives an optical signal comprised of wavelengths &lgr;
1
-&lgr;
n
over a fiber
210
. The optical signal passes through optical circulator
220
to the Bragg grating
230
. The Bragg grating
230
drops a wavelength &lgr;
1
from the optical signal by reflecting &lgr;
1
back to optical circulator
220
. Optical circulator
220
prevents &lgr;
1
from propagating over fiber
210
and transfers &lgr;
1
over a fiber
212
. The optical signal comprised of wavelengths &lgr;
2
-&lgr;
n
passes through the Bragg grating
230
. Optical circulator
222
receives thepoptical signal comprised of wavelengths &lgr;
2
-&lgr;
n
over fiber
214
and &lgr;
1
over a fiber
216
. Optical circulator
222
adds &lgr;
1
back to the optical signal. O-ADM
200
transfers the optical signal comprised of wavelengths &lgr;
1
-&lgr;
n
over a fiber
218
. Optical circulators
220
and
222
typically have an insertion loss between 0.8 and 1.0 dB. Thus, the insertion loss of O-ADM
200
is typically above 1.6 dB. The strength of the optical signal is appreciably diminished by the insertion loss of O-ADM
200
.
FIG. 3
shows a dielectric film filter type O-ADM
300
also commonly used in MANs in the prior art. O-ADM
300
is comprised of a first dielectric WDM add-drop filter
320
coupled to a second dielectric WDM add-drop filter
322
by a fiber
314
.
In operation, dielectric WDM add-drop filter
320
receives an optical signal comprised of wavelengths &lgr;
1
-&lgr;
n
over a fiber
310
. Dielectric WDM adddrop filter
320
drops a wavelength &lgr;
1
from the optical signal by filtering &lgr;
1
and transfers &lgr;
1
over a fiber
312
. The optical signal comprised of wavelengths &lgr;
2
-&lgr;
n
passes through dielectric WDM add-drop filter
320
and over fiber
314
. Dielectric WDM add-drop filter
322
receives the optical signal comprised of wavelengths &lgr;
2
-&lgr;
n
over fiber
314
and &lgr;
1
over a fiber
316
. Dielectric WDM add-drop filter
322
adds &lgr;
1
back to the optical signal. O-ADM
300
transfers the optical signal comprised of wavelengths &lgr;
1
-&lgr;
n
over a fiber
318
. Dielectric WDM add-drop filters
320
and
322
typically have an insertion loss between 0.8 and 1.0 dB. Thus, the insertion loss of O-ADM
300
is typically above 1.6 dB. The strength of the optical signal is appreciably diminished by the insertion loss of O-ADM
300
.
Fused fiber O-ADMs have been disclosed that have a lower insertion loss than O-ADM
200
in FIG.
2
and O-ADM
300
in FIG.
3
.
FIG. 4
shows a fused fiber O-ADM
400
. O-ADM
400
is comprised of a first fiber
410
coupled to a second fiber
412
. A portion of first fiber
410
is fused to a portion of second fiber
412
to form a fused region
414
. The fused region
414
has a first side
421
and a second side
422
. A Bragg grating
416
is written into the fused region
414
as discussed in FIG.
2
. First fiber
410
is configured to couple to a fiber optic network:(not shown) carrying optical signals. Second fiber
412
is configured to couple to a system (not shown, wherein the system is configured to transmit and receive a wavelength &lgr;.
In operation, O-ADM
400
receives an optical signal comprised of wavelengths &lgr;
1
-&lgr;
n
over first fiber
410
on the first side
421
of the fused region
414
. The optical signal travels into the fused region
414
and the Bragg grating
416
drops a wavelength &lgr;
1
from the optical signal by reflecting &lgr;
1
back over the second fiber
412
. Wavelength &lgr;
1
does not reflect back over

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