Optical: systems and elements – Optical amplifier – Optical fiber
Reexamination Certificate
2002-01-23
2004-03-02
Moskowitz, Nelson (Department: 2874)
Optical: systems and elements
Optical amplifier
Optical fiber
C359S343000, C372S006000, C372S070000
Reexamination Certificate
active
06700697
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to reflective erbium-doped amplifiers (R-EDAs) and more specifically to compact reflective EDAs that use ultra-short high-gain waveguides.
2. Description of the Related Art
Significant and on-going efforts are being made on erbium-doped fiber amplifier (EDFA) schemes to improve amplifications characteristics such as gain, noise figure, saturation output power, and so on. One of the amplifier schemes used to achieve high signal gain is reflective-type EDFA (R-EDFA) as described by S. Nishi et al, “Highly efficient configuration of erbium-doped fiber amplifier”, ECOC'90, vol. 1 (Amsterdam), 1990, pp. 99-102. As shown in
FIG. 1
a
herein, R-EDFAs
10
with a 3-port optical circulator
12
and a mirror
14
placed at each end of the coiled silica erbium-doped fiber
16
, respectively, give double-path amplification to input optical signals. A single-mode pump
18
is coupled to fiber
16
via a WDM coupler
20
to pump the active material in the fiber core. An input optical signal is provided at port 1
22
, which directs the signal out of port 2
24
to the EDF. The reflected signal is returned to port 2, which then directs the signal out of port 3
28
.
A conventional 3-port circulator
12
of the type described in U.S. Pat. No. 4,650,289 by Kuwahara is illustrated in
FIG. 1
b
herein. This is a schematic depiction of a typical circulator, which can be implemented with many different combinations of optical elements, see for example U.S. Pat. No. 6,178,044. The conventional optical circulator includes four ports, port 1
22
, port 2
24
, port 3
28
, and port 4
30
, which is terminated to define a 3-port circulator. The optical circulator also includes polarizer prisms
32
and
34
, mirrored prisms
36
and
38
, Faraday rotators
40
and
42
, optically active elements
44
and
46
, and a collimating lens
26
. The polarizer prisms
32
and
34
transmit light in different directions depending on the polarization of the light.
The polarization of any optical signal can be divided into two mutually orthogonal directions, both of which are also perpendicular to the direction of propagation of the light. Light polarized in the first direction is transmitted undeflected by the polarizer prisms
32
and
34
. Light polarized in the second direction is transmitted at an angle of ninety degrees from the first direction. The mirrored prisms
36
and
38
merely reflect light without a change in polarization. The Faraday rotators
40
and
42
rotate the direction of polarization of incident light by forty-five degrees in a particular direction regardless of the direction in which light traverses the Faraday rotators. For example, the Faraday rotator
40
rotates the polarization of light from the prism
38
in the same direction as light from the optically active element
44
. Optically active elements
44
and
46
rotate the polarization of incident light by forty-five degrees. However, the direction that the polarization is rotated depends upon the direction in which the light traverses the optically active elements
44
and
46
. For example, optically active element
44
will rotate light from the Faraday rotator
40
and having one polarization by forty-five degrees in a particular direction. The optically active element
44
will rotate light from the polarizer prism
34
having the same polarization by forty-five degrees in the opposite direction Thus, an optical signal incident on the port 1
22
will travel a path through the mirrored prism
36
, a path through the optically active element
44
, of a path depending on the polarization of the optical signal. However, the elements of the conventional optical circulator
12
are chosen such that the portion of the optical signal from port 1
22
that is reflected from prism
38
will have a polarization such that it will be transmitted at ninety degrees by the polarizer prism
34
. Similarly, the elements of the conventional optical circulator
12
are chosen such that the portion of the optical signal from port 1
22
that is transmitted by the optically active element
44
will have a polarization such that it will be transmitted undeflected by the polarizer prism
34
. Thus, an optical signal from port 1
22
will reach port 2
24
, but not be transmitted to port 3
28
or port 4
30
and similarly for each of the ports except that port 4 is terminated.
R-EDFAs that incorporate optical circulators provide significant gain improvement primarily due to double passage of the signal through the erbium-doped fiber. Reflection of the pump results in higher average inversion ratio. However, only about 20% of the single-mode core-pumped radiation is not absorbed or scattered in the meters of silica fiber on the first pass, and is available for reflection through a second pass. Thus, the effect of reflecting the pump in a silica fiber amplifier is marginal.
Typically, tens of meters of silica fiber is coiled to obtain the desired amplification. The bend radius of the fiber is typically at least 50 mm to avoid attenuation. Integrated optical systems will require compact optical components, hence smaller bend radii. The induced attenuation due to bending a SMF28 single mode fiber is 0.5 dB per turn for a 16 mm bend radius with a single-mode core pumping at 1550 nm. In cladding pumped amplifiers the limitations on bend radius are even more severe since pump light can more readily escape the cladding than the core. In addition, the bending may redistribute the pump mode shape to favor modes with smaller or no overlap with the centrally-doped core, resulting in lower pump absorption and reduced gain for the amplifier.
Furthermore, Nishi's R-EDFA exhibits severely degraded noise figure compared with the conventional single-pass EDFA because the amplified signal and backward amplified spontaneous emission (ASE) make the population inversion in the input part of the EDF low. J. Ahn et. al. “Two-Stage reflective-type erbium-doped fiber amplifier with enhanced noise figure characteristics”, Optics Communications 197 (2000) pp. 121-125 Sep. 15, 2001, describes a two-stage R-EDFA to enhance noise figure. An positions the circulator to split the EDF into two segments, which prevents the amplified signal and backward ASE from propagating to the first segment. As a result, the population inversion in the first segment remains high and the noise figure is better than the conventional R-EDFA. A small amount of amplification is sacrificed.
U.S. Pat. No. 5,757,541 to Fidric entitled “Method and Apparatus for an Optical Fiber Amplifier” splits the pump light and input signal into two equal parts for simultaneous introduction into the two opposite ends of the active gain fiber. This bi-directional propagation results in a more uniform excitation along the entire length of the active fiber, providing uniform stimulation of photon emission at both ends, causing significantly reduced noise and higher gain of the signal. Fidric's OFA is not an R-EDFA and thus does not realize the enhanced gain associated with the signal passing through the active fiber twice.
U.S. Pat. No. 5,598,294 to Uno et al. entitled “Optical Fiber Amplifier and Optical Fiber Communication System” describes a R-EDFA in which the mirror is replaced by one or more wavelength selective reflectors to gain equalize different wavelength signals. This construct allows ASE outside the signal wavelengths to pass thereby improving noise figure. In one configuration, the last wavelength selective reflector is specified to reflect the pump wavelength.
U.S. Pat. No. 5,596,448 to Onaka et al. entitled “Dispersion Compensator and Optical Amplifier” provides an optical amplifier, which is not influenced by chromatic dispersion or polarization mode dispersion. As shown in
FIG. 8
therein, a dispersion compensation fiber
4
is connected in cascade with the EDF in an R-EDFA configuration. The dispersion compensation fiber has color dispersion of a sign opposite that of the silica telecom fiber, and the leng
Chavez-Pirson Arturo
Eradat Nayer
Jiang Shibin
Kaneda Yushi
Mendes Sergio Brito
Gifford Eric A.
Moskowitz Nelson
NP Photonics Inc.
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