Optical waveguides – Optical fiber waveguide with cladding – Utilizing nonsolid core or cladding
Reexamination Certificate
2001-11-14
2002-11-12
Healy, Brian (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
Utilizing nonsolid core or cladding
C385S123000, C385S126000, C372S006000
Reexamination Certificate
active
06480659
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a fiber lasers and, more particularly, to rare-earth element-doped fiber lasers operating at the three-level transition, and to fiber optic structures for fiber lasers.
BACKGROUND OF THE INVENTION
Optical communication networks demand high transmission speeds, wide bandwidth, and great channel capacity. Another important requirement is for reliable signal transmission at an appropriate optical power level for good signal detection after tens of kilometers of signal travel. Optical signals transmitted in a network are typically optically amplified after each 10 to 15 kilometers of transmission path. The amplification is performed by optical amplifiers, which represent a single or double clad fiber structure with the fiber core doped by rare earth elements. The optical signals carrying information travel through the fiber core. In order to perform the amplification, high power optical pumping radiation is also introduced into the fiber core by direct coupling to it or through the cladding. The pumping radiation raises the energy levels of the doping to enable amplification of the signal through stimulated emission. It is noted that the amplifiers are separate from the fibers used for signal transmission. Such amplifiers are made of fibers approximately ten to fifteen meters in length.
In conventional single-mode fibers, the wavelength of minimum loss is around 1.5 micrometers. The ability to amplify this wavelength is therefore extremely important in fiber optic networks. Erbium Doped Fiber Amplifiers (EDFA), when operated in the so-called “three-level mode” and pumped at a wavelength of approximately 980 nanometers, have the capability of amplifying signals of the required 1.5-micrometer wavelength. For efficient optical amplification, EDFA's in turn require high power single-mode coherent pumping at a wavelength close to 980 nanometers.
Ytterbium (Yb) doped fiber lasers are sources of high power, high brightness, single mode coherent approximately 980-nanometer optical radiation, provided they are operated at a three-level transition and not at their more-easily attained four-level transition. Operation of an Yb doped fiber laser at a three-level scheme presents a number of problems. One of the problems is the significant absorption loss of the three-level emission peak at about 980 nanometers. To overcome, these absorption losses and lase at approximately 980 nanometers the Yb doped fiber must be “bleached”; that is, more than 50% of all Yb ions must be excited to the upper excitation state.
Although various attempts have been made to produce Yb lasers operating at a three-level transition, there are no cost-effective, high-power, single transverse-mode, Ytterbium pump sources currently-available. Generally, three-level operation in an Ytterbium fiber laser is achieved in two ways:
a) by increasing the efficiency of the coupling of pumping radiation into the cladding; and
b) by improving absorption of the coupled pumping radiation into the Yb-doped core.
Increased coupling of the pumping radiation to the cladding is attained primarily by using air-clad fibers with a high numerical aperture (NA). Improving the absorption of the coupled pumping radiation by the doped fiber core is typically achieved by a fiber geometry that encourages the optical path of the pumping energy to cross the fiber core as much as possible. Additional pumping energy is absorbed by the core each time such a crossing occurs.
U.S. Pat. No. 4,815,079 to Snitzer et al. (herein denoted as “Snitzer”), which is incorporated by reference for all purposes as if set forth fully herein, discloses a clad pumped fiber laser where the improvement of absorption of the coupled pumping radiation by doped fiber core was obtained by placing the fiber core off-center relative to the cladding.
FIG. 1
illustrates this geometry. A fiber core
20
is surrounded by a first inner cladding
22
, a second inner cladding
24
, and an outer cladding
26
. The reasoning behind the off-center placement is that when pumping is characterized by multi-mode operation, it is well-known in the art that the radial distribution of the pumping energy in inner cladding
22
is such that a large part of the pumping energy is located away from the center of inner cladding
22
, and that to maximize the absorption of pumping energy, fiber core
20
should also be located away from the center. It is noted that the term “inner cladding” herein denotes any cladding that is interior to the outer cladding of a fiber. A fiber may have more than one inner cladding, as is illustrated in
FIG. 1
with a first inner cladding
22
and a second inner cladding
24
. The term “outer cladding” herein denotes a cladding whose outer surface is the exterior surface of the fiber. A fiber can have at most one outer cladding, as is illustrated in
FIG. 1
with outer cladding
26
.
Although the prior-art structure illustrated in
FIG. 1
is efficient in coupling pumping radiation into the core, an off-center fiber structure is not practical since optical transmission lines and networks typically have a coaxial structure, and connecting coaxial transmission lines to an off-center amplifier such as that of the prior art (
FIG. 1
) is extremely difficult.
Improving absorption of the pumping radiation in the Yb-doped fiber core is also achieved by increasing the number of places the pumping energy's optical path crosses the fiber core. U.S. Pat. No. 5,533,163 to Muendel (herein denoted as “Muendel”), which is incorporated by reference for all purposes as if set forth fully herein, teaches use of cladding having non-cylindrical shapes.
FIG. 2
shows such a geometry. A cylindrical single-mode fiber core
30
is surrounded by an inner cladding
32
in the form of a non-rectangular, convex polygon so that the propagating pump energy is induced to form an essentially uniform radiation field in which the various radiation modes comprising the pump energy are isotropically distributed. A cylindrical outer cladding
34
presents an overall cylindrical shape externally. A variety of additional cladding shapes, some of which are shown in
FIGS. 3A
,
3
B and
3
C, are also disclosed by Muendel. Muendel teaches criteria for proper convex polygon selection and generally states that good results may are obtained by use of any k-sided convex polygon that satisfies the condition:
&thgr;=360
°/k
(1)
where &thgr; is the central angle and k≳3.
Muendel discloses that fibers with off-center structure and irregular polygons are especially advantageous. Fabrication of a fiber optic structure in accordance with the invention is accomplished by machining a preform to the desired cross section and then drawing the preform according to techniques known in the art. Such preparation of a preform and machining of multiple facets on the preform, however, are operations that undesirably complicate fiber fabrication.
The term “cylindrical” herein denotes any surface describable as the normal (perpendicular) locus of a circle along an axis. As used herein, such an “axis” need not be a straight line segment, and may even be a closed curve. Thus, as the term “cylindrical” is used herein, a drawn fiber optic structure may have a cylindrical external surface even when deformed or bent so as not to correspond to the mathematically developable surface of a right cylinder of rotation. The term “coaxial” as used herein denotes a relationship between two or more cylindrical surfaces having the same axis and corresponding to loci of circles of different diameters. The cross-section of such a cylindrical surface normal to the axis is a circle.
U.S. Pat. No. 6,031,849 to Ball et al. (herein denoted as “Ball”), which is incorporated by reference for all purposes as if set forth fully herein, discloses a double-clad Yb fiber laser operating at a three-level transition at approximately 980 nanometers, along with a method of manufacturing the fiber. In order to enhance the pumping energy coupling efficiency, the form of the l
Patlakh Anatoly
Solodovnikov Vladimir V.
Healy Brian
Oliff & Berridg,e PLC
Rayteq Photonic Solutions Ltd.
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