Splitter for use with an optical amplifier

Details Splitter for use with an optical amplifier Splitter for use with an optical amplifier

1999-04-06

2002-08-06

Bovernick, Rodney (Department: 2874)

Optical waveguides

With optical coupler

Particular coupling structure

C385S039000, C385S018000, C385S019000, C359S199200, C359S341100

Reexamination Certificate

active

06430343

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to optical communications systems, and more particularly, to optical amplifiers.
BACKGROUND OF THE RELATED ART
Optical communication systems typically include a variety of devices (e.g., light sources, photodetectors, switches, optical fibers, modulators, amplifiers, and filters). For example, in the optical communication system
1
shown in
FIG. 1
, a light source
2
, generates an optical signal. The optical signal comprises a series of light pulses. The light pulses are transmitted from the light source
2
to a detector
5
. Typically, an optical fiber
4
transmits the light pulses from the light source
2
to the detector
5
.
Many optical fibers are lossy in that they scatter (or absorb) portions of light pulses transmitted therealong (about 0.1-0.2 dB/km). When portions of the light pulses transmitted on an optical fiber are scattered (or absorbed), the optical power of such light pulses is reduced. To compensate for optical power losses attributable to the lossiness of optical fibers, optical amplifiers
6
are positioned along the length of the optical fibers
4
. The optical amplifiers
6
increase the optical power of the light pulses so light pulses with adequate signal strengths propagate along the length of the optical fiber
4
from the light source
2
to the detector
5
.
Optical amplifiers are also useful for transmitting optical signals through free space. Such free-space transmitters are useful for satellite communication links, building-to-building links, intra-city links, ship-to-pier links, and the like. The optical amplifiers provide the high power optical signals (about 100 milliwatts to about 10 watts) needed for transmission across such links.
A cut away view of an optical amplifier
6
useful for optical communication systems or optionally as a free-space transmitter is shown in FIG.
2
A. Optical amplifier
6
is a cladding pump optical amplifier. The cladding pump optical amplifier includes a cladding pump fiber
10
and a fiber bundle
20
.
The fiber bundle
20
has of a plurality of multi-mode fibers
22
(e.g., 6-19) and a single mode fiber
24
. The single mode fiber
24
is positioned at about the center of the fiber bundle
20
. The multi-mode fibers
22
transmit pump light. The single mode fiber
24
transmits optical signals.
The plurality of multi-mode fibers
22
and the single mode fiber
24
are fused together into a bundle. The bundle has a diameter which is tapered to match the size and numerical aperture (NA) of the cladding pump fiber
10
. The plurality of multi-mode fibers
22
proximate to the tapered end of the fiber bundle
20
are coupled to the cladding pump fiber
10
.
A pump diode
25
is coupled to each multi-mode fiber
22
in the fiber bundle
20
. The pump diode
25
is coupled to the distal end of the tapered fiber bundle
20
. Pump diodes are semiconductor devices designed to emit light at specified wavelengths (e.g., 915 nanometers (nm), 980 nm).
A cross-sectional view of the cladding pump fiber
10
is shown in FIG.
2
B. The cladding pump fiber
10
includes a single mode core
12
, first cladding
14
, and second cladding
16
. The single mode core
12
is made of silica that is doped with one or more ionized rare earth elements (e.g., Nd
3+
, Yb
3+
, Tm
3+
, and Er
3+
). The single mode core
12
typically has a diameter of about 4 &mgr;m to about 8 &mgr;m. The optical signal is transmitted from single mode fiber
24
to the single mode core
12
.
The single mode core
12
is surrounded by first cladding
14
. In
FIG. 2B
, the first cladding
14
is shown with a “star-shaped” cross-section. However, the first cladding
14
optionally has a “rectangular” (not shown) or a “D-shaped” (not shown ) cross-section. First cladding
14
is typically made of silica with an index of refraction suitable for transmitting the pump light.
Pump light from the pump diodes
25
is provided to the first cladding
14
from the multi-mode fibers
22
. As the pump light propagates through the first cladding
14
, it excites the ionized rare earth elements in the single mode core
12
, causing a population inversion. A population inversion is created when more electrons within the ionized rare earth elements are in the excited state than are in the ground state. The energy stored in the inverted population of excited rare earth elements is transferred to the optical signals propagating along the single mode core
12
, causing the optical signals to experience an increase in optical power (i.e., a gain). In order to create the population inversion of ionized rare earth elements in the single mode core
12
, the wavelength of the pump light must correspond to at least one absorption line for such ionized rare earth element. For example, pump light at 975 nm corresponds to an absorption line of erbium (Er
+3
).
First cladding
14
is surrounded by second cladding
16
. The second cladding
16
is made of a fluorinated, low index of refraction polymer or a low index of refraction glass. The second cladding
16
has an index of refraction that is different from the index of refraction for the first cladding
14
. The differences in the indices of refraction for the first cladding
14
and the second cladding
16
substantially confines the pump light within the first cladding
14
, preventing it from leaking out of the cladding pump fiber
10
. For example, when the index of refraction for the first cladding
14
is about 1.46 and the index of refraction for the second cladding
16
is about 1.38, the difference between the two indices confines about 90% of the pump light in the first cladding.
The electrical efficiency of an optical amplifier is calculated as the ratio of the net optical power output from the amplifier to the power used to operate the pump diodes. The net optical power of the amplifier is defined as the amplifier output power minus the preamplifier power. The power used to operate the pump diodes is defined as the number of diodes times the current-voltage product per diode. For example, when a Er
+3
/Yb
+3
cladding pump fiber pumped with six pump diodes (operated at about 1.7 volts and about 1.5 amps) amplifies a 1550 nm optical signal from a preamplifier power of about 120 mW to an output power of about 1.2 W, the Er
+3
/Yb
+3
cladding pump fiber has an electrical efficiency of about 7% (electrical efficiency=(1.2 W—0.12 W)/(6 diodes x 1.7 volts×1.5 amps)×100). This means that only about 7% of the electrical power is used for amplifying optical signals input to the optical amplifier.
In some communications systems (e.g., satellite communication systems), there is a limited amount of electrical power available for system operation. Optical amplifiers with low electrical efficiencies (less than about 10%) are undesirable for use in such communications systems because they consume a substantial portion of the available power, which potentially reduces the power available to operate other devices in the system. Additionally, the cost of some communication systems is directly related to the electrical power needed to operate the devices therein. In particular, the cost of the communications system increases as the electrical power needed to operate the devices in the system increases. Accordingly, optical communication systems that provide greater efficiency are desired.
SUMMARY OF THE INVENTION
The present invention is directed to an optical communication system in which a beam splitter is used to direct a portion of the pump light provided to an optical amplifier in an optical communication system to at least one other device in the optical communication system. The beam splitter is configured to receive optical signals as well as pump light. The beam splitter directs a portion of the pump light provided to one optical amplifier in the optical communication system to other devices (e.g., optical amplifiers, filters, modulators) of the optical communication system. Directing a portion of the

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