All fiber gain flattening optical filter

Details All fiber gain flattening optical filter All fiber gain flattening optical filter

2001-12-12

2004-11-23

Moskowitz, Nelson (Department: 3663)

Optical: systems and elements

Optical amplifier

Correction of deleterious effects

C359S199200, C359S199200, C385S127000

Reexamination Certificate

active

06822786

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to the field of fiber optic communications. More particularly, the present invention relates to the field of filtering of amplified signals used in fiber optic communications systems.
BACKGROUND OF THE INVENTION
Fiber optic communication systems use optical fibers to carry a modulated lightwave signal between a transmitter and a receiver. A cross-section of a typical optical fiber is illustrated in FIG.
1
. The optical fiber
2
includes a core
4
and a cladding
6
. Optionally, the optical fiber
2
includes a jacket
8
. In a typical optical fiber, the core
4
has an index of refraction greater than the cladding
6
, thereby forming an optical waveguide. By maintaining the core diameter within an allowed range, light traveling within the core
4
is limited to a single mode. If included, the jacket
8
protects the outer surface of the cladding
6
and absorbs stray light traveling within the cladding
6
. A typical single mode optical fiber intended for use in communication systems operating at a 1300 mn wavelength band or a 1550 nm wavelength band has a core diameter of about 8 &mgr;m and a cladding outside diameter of 125 &mgr;m. If the jacket
8
is included, the jacket
8
typically has an outside diameter of 250 &mgr;m.
In Wavelength Division Multiplexing (WDM) systems, multiple signals are carried by various wavelengths of light through a single optical fiber. A typical WDM system is shown in FIG.
2
. The WDM system
10
includes a transmission system
11
, which includes a series of transmitters
12
,
14
, and
16
, each coupled to a multiplexer
18
. The multiplexer
18
provides an output, which is coupled to an optical fiber
20
. Over long distances amplifiers
22
are included along the optical fiber
20
. The optical fiber
20
is then also coupled to a receiving system
23
, which includes a demultiplexer
24
and a series of receivers
26
,
28
, and
30
. The optical fiber
20
is coupled to an input of the demultiplexer
24
of the receiving system
23
. Outputs of the demultiplexer
24
are coupled to the series of receivers
26
,
28
, and
30
.
In the WDM system
10
, a first transmitter
12
transmits a light signal at a first wavelength (&lgr;
1
), a second transmitter
14
transmits a light signal at a second wavelength (&lgr;
2
) and so forth until an nth transmitter
16
transmits a light signal at an nth wavelength (&lgr;
n
). The shortest wavelength signal and the longest wavelength signal form a wavelength band. The signals are combined by the multiplexer
18
, which then transmits the light signals along the optical fiber
20
. Over distance the power of the light signals decrease due to attenuation. The light signals are typically amplified about every 50-100 km. For the 1550 nm wavelength band, this amplification is generally performed by an Erbium Doped Fiber Amplifier (EDFA)
22
. When the light signals reach their destination they are separated by the demultiplexer
24
. The light signals are then received by the receivers
26
,
28
, and
30
. Light signal intensity versus wavelength for a typical wavelength band of WDM light signals is illustrated in FIG.
3
.
Flat gain response for EDFAs is crucial to the performance of the WDM system
10
, since small variations in gain for various wavelengths will grow exponentially over a series of in-line EDFAs
22
. Agrawal in “Fiber Optic Communication Systems,” (Wiley, 2nd ed., 1997, pp 414-415) teaches that numerous methods can be used to flatten the gain response of these amplifiers. One method of flattening this gain response is to use channel filters to equalize the gain for various wavelengths. Another method is to adjust the input powers of different wavelengths so that amplification results in uniform intensity for various wavelengths. A third method is to use inhomogeneous broadening of the EDFA gain spectrum to equalize wavelength intensity. A fourth method is to use multiple EDFAs tuned to different wavelength ranges and configured with feedback loops. A final method is to use a filter or series of filters to selectively attenuate the gain response of an EDFA.
A typical gain versus wavelength response for an EDFA is shown in FIG.
4
A. When utilizing a filter or series of filters to flatten gain response, an optical filter, with an attenuation curve as shown in
FIG. 4B
, can be used to selectively attenuate the gain response. The resulting attenuated EDFA gain is shown in FIG.
4
C. As shown in
FIG. 4C
, this attenuated EDFA gain is substantially flat over a range of wavelengths including 1530 nm to 1560 nm. Without a substantially flat gain, the quality of the signal received by the receivers
26
,
28
, and
30
will be poor.
There are many different known methods for selectively attenuating the gain response of an EDFA in order to improve the signal quality of the signals received by the receivers
26
,
28
, and
30
. U.S. Pat. No. 5,260,823 to Payne et al. entitled, “Erbium-Doped Fibre Amplifier with Shaped Spectral Gain,” teaches that a wavelength-selective resonant coupling between a propagating core mode to a cladding leaky mode can be used for filtering a wavelength band for EDFA gain flattening. A periodic perturbation of the core forms a grating and the selected wavelength is attenuated by the resonant coupling between the core and the cladding. By varying the perturbation length, various selected wavelengths can be attenuated. Payne et al. also teach that multilayered dielectric coatings can be used for making an optical filter for EDFA gain flattening. A multilayered filtering apparatus includes two coupling lenses and a multilayered dielectric filter. The two coupling lenses connect to an optical fiber and sandwich the multilayered dielectric filter. The multilayered dielectric filter is designed to cancel out the larger gain around the peak wavelength and to be transparent elsewhere.
U.S. Pat. No. 5,473,714 to Vengsarkar entitled, “Optical Fiber System Using Tapered Fiber Devices,” teaches that tapered fiber devices can be used for filtering in an optical telecommunications system. Vengsarkar teaches that by tapering an optical fiber, light can be attenuated by wavelength cutoff and direct coupling from a core to a cladding. The tapered fiber device is formed from the optical fiber by heating the optical fiber and stretching it. The taper reduces the diameter of the core to a value close to the cutoff wavelength. Light with wavelengths near and above the cutoff wavelength are coupled directly to the cladding.
U.S. Pat. No. 5,583,689 to Cassidy et al. entitled “Filter With Preselected Attenuation/Wavelength Characteristic,” teaches that a fiber grating, with spatially separated parts having different attenuation characteristics, can perform filtering for EDFA gain flattening. The fiber grating is preferably a side-tap Bragg fiber grating. By varying the pitch along the fiber grating an appropriate attenuation profile can be provided for flattening the EDFA gain response.
U.S. Pat. No. 5,067,789 to Hall et al. entitled, “Fiber Optical Coupling Filter and Amplifier,” teaches that a light-attenuating light path adjacent to a first core within a cladding can be used to filter wavelengths about a specific wavelength for EDFA gain flattening. The light attenuating light path is preferably one or more lossy cores that are evanescently coupled to the first core. The evanescent coupling between the first core and the light attenuating light path is greatest where the effective index of refraction of the first core equals the effective index of refraction of the light attenuating light path. By choosing a single mode or a higher multimode optical waveguide structure for the light attenuating light path, the effective index of refraction for the light attenuating light path can be varied. Hall et al. teach that the index of refraction for the material for the light attenuating light path should be greater than the index of refraction for the material for the first core. Hall et al. further teach that as an alternative embodi

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