Optical: systems and elements – Optical amplifier – Optical fiber
Reexamination Certificate
2002-02-27
2004-04-20
Black, Thomas G. (Department: 3663)
Optical: systems and elements
Optical amplifier
Optical fiber
Reexamination Certificate
active
06724528
ABSTRACT:
FIELD OF THE INVENTION
The invention relates in general to a polarization-maintaining optical fiber amplifier that is fabricated from non-polarization-maintaining gain optical fiber. More specifically, the invention employs coiling under tension to create a stress-induced axis of linear birefringence within the fiber.
BACKGROUND OF THE INVENTION
A wide variety of applications in fiber optic photonics require the use of polarization-maintaining optical fibers. In such a fiber the polarization planes of linearly polarized light waves launched into the optical fiber are maintained during propagation with little or no cross coupling of optical power between the orthogonal polarization modes. In many of these systems there is also a need for polarization-maintaining optical fiber amplifiers. An optical fiber amplifier is a device that amplifies an optical signal directly, i.e., without the need to convert it to an electrical signal, amplify it electrically, and reconvert it to an optical signal.
An optical fiber amplifier uses an optical fiber having a rare-earth-doped core, which will be referred to hereinafter as a gain optical fiber. Although Er
3+
is most commonly used as a rare-earth element in gain optical fibers, different rare earth elements such as Nd
3+
, Yb
3+
, Pr
3+
, Ho
3+
, Sm
3+
, and Tm
3+
may be used. The rare-earth ion is optically excited, typically but not exclusively using the output of a diode laser; a signal beam propagating in the core experiences gain if a population inversion has been established by absorption of the pump beam by the rare-earth ions (and if the signal beam has a wavelength within the gain spectrum of the rare-earth dopant).
Optical fiber amplifiers generally out-perform conventional solid-state amplifiers in the following key areas: small-signal gain, tunability, beam quality (for single-mode optical fibers), immunity to mechanical and thermal disturbances, size, weight, cost, and electrical efficiency. One notable disadvantage of optical fiber amplifiers is their tendency to scramble the input polarization of the seed signal. This polarization-scrambling effect is a consequence of azimuthal asymmetry in the refractive-index distribution of the optical fiber, commonly referred to as optical fiber birefringence. A linearly polarized seed signal injected into the fiber will generally be converted to an unspecified, time-dependent elliptical polarization state, i.e., the fiber is not polarization maintaining.
In an ideal optical fiber having an azimuthally symmetric refractive-index profile, a signal injected into one end of the fiber will propagate through the optical fiber with its polarization state unchanged. Each of the transverse modes supported by the optical fiber waveguide can exist in two orthogonal polarizations (e.g., vertical and horizontal), and in a perfectly symmetric optical fiber these two polarization modes propagate at the same speed, independent of one another (i.e., the fiber is not birefringent). In practice, it is impossible to manufacture an optical fiber that has perfect azimuthal symmetry, and all real optical fibers thus exhibit non-zero birefringence. Core or cladding ellipticity and mechanical strain, which causes random refractive-index perturbations, are the main contributors to random birefringence in an optical fiber and thus to non-polarization-maintaining behavior.
There are several solutions for the problem of polarization scrambling due to random birefringence in an optical fiber. As described below, random birefringence may be corrected by utilizing a polarization controller, a Faraday mirror, or a polarization-maintaining optical fiber.
The simplest solution to the problem of polarization scrambling due to optical fiber birefringence is the use of a polarization controller. There are a number of different designs for polarization controllers, but in all cases the principle of operation is the same. The birefringent optical fiber is sandwiched between two waveplates whose orientation and retardation are independently adjustable; alternatively the fiber input or output beam may be directed through three waveplates whose orientations but not retardations are adjustable. It can be shown that for any fiber birefringence, it is always possible to set the adjustable waveplates such that there is no change in the polarization state for a signal passing through the entire system (fiber plus polarization controller). Unfortunately, the birefringence properties of the optical fiber are sensitive to environmental factors, such as changes in temperature or mechanical disturbances. Changes in the birefringence properties of the optical fiber over time necessitate readjustment of the polarization controller, making it unsuitable for most real-world applications.
In some optical fiber circuits, a device known as a Faraday mirror can be used to compensate for optical fiber birefringence. In a Faraday mirror, a signal passing through the birefringent optical fiber must retrace it's path through the optical fiber, traveling in the opposite direction on the return trip, thereby creating a folded optical path. The Faraday mirror never needs adjustment and is able to compensate for rapid changes in birefringence, limited only by the round-trip propagation in the fiber. The main disadvantage of the Faraday mirror is that it is applicable to only a small subset of optical fiber circuits and is therefore lacking in generality. In addition, commercially available Faraday mirrors suffer from one or more the following drawbacks: high cost, large size and weight, limited power-handling capability, and limited wavelength range (i.e., a wavelength range smaller than the range over which the fiber exhibits gain).
The best all-around solution to the problem of optical fiber birefringence is the use of a polarization-maintaining (PM) optical fiber. In a PM optical fiber, a very large azimuthal asymmetry is introduced intentionally during the manufacturing process. The goal is to create a controlled linear birefringence that is very large (compared with the random birefringence) and oriented along a well-defined axis. This birefringence can be generated by fabricating an optical fiber core with an elliptical cross-section, by subjecting an optical fiber to mechanical stress, or by a combination of both techniques. When this linear birefringence is much greater than the random birefringence due to optical fiber imperfections, good PM behavior is obtained.
FIGS. 1-4
show cross-sections of various conventional PM optical fibers
10
, looking down the optical fiber axes. The components of the PM optical fibers may include a core
20
, a cladding
22
, and stress elements
24
.
The stress elements
24
shown in
FIGS. 1-3
for the bow-tie
12
, Panda
14
, and oval-inner-clad
16
PM optical fibers are fabricated from a glass whose thermal expansion coefficient is different (usually greater) than that of the cladding
22
glass, which is usually silica. During manufacture, the optical fiber
10
is drawn from molten glass and therefore starts out stress-free. Solidification occurs several hundred degrees above room temperature, at which point the optical fiber
10
is capable of accumulating mechanical stress. As the optical fiber
10
cools further, the stress elements
24
contract differently (usually more) than the surrounding cladding, generating a stress field that is azimuthally asymmetric. Specifically, the stress distribution has two-fold bilateral symmetry, in which the mirror planes of minimum and maximum stress are perpendicular to each other. The stress-induced change in the refractive index has these same symmetry properties. Within each PM optical fiber
10
there is thus a fast axis
26
and a slow axis
28
that are mutually perpendicular (analogous to a waveplate). Because of the difference in index of refraction, a ray of light whose polarization direction is aligned along the fast axis propagates at a slightly faster speed than a ray of light whose polarization direction is alig
Goldberg Lew
Kliner Dahv
Koplow Jeff
Black Thomas G.
Hughes Deandra M.
Hunnius Stephen T.
Karasek John J.
The United States of America as represented by the Secretary of
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