Highly rare-earth doped fiber

FIELD OF THE INVENTION

Various implementations, and combinations thereof, are related to using highly rare-earth doped fibers for Faraday rotation and more particularly to fiber isolators and fiber polarization rotators using highly rare-earth doped fibers.

BACKGROUND OF THE INVENTION

Faraday rotation, or the Faraday effect, is an interaction between light and a magnetic field. When linearly polarized light passes through a parallel magnetic field, the plane of the linearly polarized light is rotated. The rotation of the plane of polarization is proportional to the intensity component of the magnetic field in the direction of the beam of light. Light that is reflected back through the magnetic field is further rotated in the same direction.

The empirical angle of rotation is given by
β=VBd,
where β is the angle of rotation (in radians), V is the Verdet constant for the material, B is the magnetic flux density in the direction of propagation (in teslas), and d is the length of the path (in meters).

The Verdet constant reflects the strength of the Faraday effect for a particular material. The Verdet constant can be positive or negative, with a positive Verdet constant corresponding to a counterclockwise rotation when the direction of propagation is parallel to the magnetic field. The Verdet constant for most materials is extremely small and is wavelength dependent. Typically, the longer the wavelength the smaller the Verdet constant.

As can be seen from the relationship between the Verdet constant, the path length, and the angle of rotation, a desired angle of rotation can be achieved in a shorter distance where the Verdet constant is high. The highest Verdet constants are found in terbium gallium garnet (TGG), which has a Verdet constant of −40 rad/T·m at 1064 nm. Another material known to exhibit a large Verdet constant is terbium (Tb)-doped glass.

SUMMARY OF THE INVENTION

In one implementation, a multicomponent glass fiber is presented. The multicomponent glass fiber has a doping concentration of 55%-85% (wt./wt.) of a rare-earth oxide. The rare-earth oxide is selected from the group comprising: Pr2O3, Nd2O3, Pm2O3, Sm2O3, Eu2O3, Gd2O3, Tb2O3, Dy2O3, Ho2O3; Er2O3, Tm2O3, Yb2O3, La2O3, Ga2O3, Ce2O3, and Lu2O3.

In another implementation, an all-fiber Faraday rotator is presented. The Faraday rotator comprises a multicomponent glass fiber having a doping concentration of 55%-85% (wt./wt.) of a rare-earth oxide. The rare-earth oxide is selected from the group comprising: Pr2O3, Nd2O3, Pm2O3, Sm2O3, Eu2O3, Gd2O3, Tb2O3, Dy2O3, Ho2O3; Er2O3, Tm2O3, Yb2O3, La2O3, Ga2O3, Ce2O3, and Lu2O3. The Faraday rotator further comprises a magnetic tube surrounding the multicomponent glass fiber.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As will be appreciated by one of ordinary skill in the art, Faraday rotation can be used to build a Faraday rotating isolator and/or a Faraday rotator. Specifically, an optical isolator is an optical component which allows the transmission of light in only one direction. A Faraday isolator is a specific type of optical isolator that employs a Faraday rotator, whereas a Faraday rotator is a magneto-optic device that rotates the polarization of light as the light is transmitted through a medium exposed to a magnetic field.

Typically, a Faraday isolator is polarization dependent and consists of two optical polarizers at either end of a Faraday rotator. Polarized light traveling in the forward direction is aligned to be parallel to the polarization direction of the input polarizer and coupled into the Faraday rotator. The Faraday rotator will rotate the polarization by forty-five (45) degrees. The light then passes through the output polarizer, which is aligned to be parallel to the rotated beam in order to have a low attenuation. Back reflected light propagating in the opposite direction is rotated an additional forty-five (45) degrees when it passes through the Faraday rotator a second time, thereby resulting in an orthogonal polarization direction compared to the input laser beam polarization. The input polarizer thus blocks the reflected light.

Typically, Faraday rotators consist of terbium gallium garnet (TGG) crystal or terbium-doped glass inserted into a magnetic tube. The residual flux density of the magnetic tube should be strong enough to produce a forty-five (45) degree polarization rotation when the light passes through the Faraday rotator. In certain embodiments, the magnetic tube comprises a tube of ferromagnetic material. In certain embodiments, the magnetic tube comprises a tube of any material exposed to a magnetic field.

Common commercially available Faraday isolators are free-space isolators. As will be appreciated by one of ordinary skill in the art, free-space isolators have actual space between components.FIG. 1presents a schematic of an exemplary free-space Faraday isolator andFIG. 2presents a schematic of an exemplary fiber pigtailed free-space Faraday isolator.

The development of fiber isolators has become critical given recent advancements in high powered fiber lasers. Fiber lasers having as great as ten (10) kilowatts of output power have been demonstrated, enabling a wide range of new applications from laser welding, laser cutting, and laser drilling to military defense. While these fiber lasers have been successfully introduced into industry, much of their potential is retarded due to the limitations of the currently-available fiber isolators. For the moment, free-space fiber pigtailed isolators, such as depicted inFIG. 2, must be used. Such free-space isolators require fiber termination, lens alignment, and recoupling of the laser to fiber, all of which degrades performance of the fiber lasers. Not only does the use of a free-space isolator limit the power of a fiber laser to about 20 W, but it also lowers the ruggedness and reliability, two of the main advantages of a fiber laser over a free-space solid-state laser.

Applicant's invention discloses a rare-earth doped fiber having a doping concentration greater than 55% (wt.). In certain embodiments, the doping concentration is greater than 65% (wt.). In certain embodiments, the doping concentration is greater than 70% (wt.). In certain embodiments, the doping concentration is between 55%-85% (wt./wt.).

Applicant's invention further discloses a rare-earth doped fiber, used as a Faraday rotating element, that is fusion spliced with a fiber-based polarizer to form an all-fiber isolator. One of ordinary skill in the art will appreciate that fusion splicing is the act of joining two optical fibers end-to-end using heat in such a manner that light passing through the fibers is not scattered or reflected back by the splice.

In certain embodiments, the throughput power of Applicant's Faraday rotating element is greater than 100 W.

Turning now toFIG. 3, an exemplary schematic of Applicant's all-fiber isolator is presented. As can be seen in the illustrated embodiment ofFIG. 3, Applicant's all-fiber isolator comprises first fiber-based polarizer102, Faraday rotating fiber106inside magnetic tube108, and second fiber-based polarizer112, wherein either end of Faraday rotating fiber106is fusion spliced (depicted by fiber fusion splicing joints104and110) and acts as the Faraday rotating element.

In certain embodiments, Faraday rotating fiber106is doped with Tb2O3. The transmission spectra of terbium-doped glass is presented inFIG. 4. As can be seen in the illustrated embodiment ofFIG. 4, while Tb2O3exhibits the largest Verdet constant out of the rare-earth oxides, it also absorbs light near 1.5 microns and 2 microns. This is significant as near 1.5 micron and near 2 micron fiber lasers have a high transmission in air and are considered to emit an eye-safe wavelength.

Turning now toFIG. 6, an exemplary schematic of an alternative embodiment of Applicant's all-fiber isolator is presented.FIG. 7depicts the magnetic field distribution of the all-fiber isolator ofFIG. 6. In the illustrated embodiment ofFIG. 6, the Faraday rotation of Applicant's all-fiber isolator is increased by using two types of fibers having Verdet constants with opposite signs, wherein one type of fiber is place inside of a magnetic tube and the other fiber is placed at one or both ends outside the magnetic tube. Thus, in the illustrated embodiment ofFIG. 6, fibers202and204have a Verdet constant having a first sign while fiber206within magnetic tube208has a different sign. In certain embodiments, fibers202and204are fusion spliced with fiber206within magnetic tube208. In certain embodiments, fiber206within magnetic tube208has a negative Verdet constant while fibers202and204have a positive Verdet constant. In other embodiments, fiber206within magnetic tube208has a positive Verdet constant while fibers202and204have a negative Verdet constant. In certain embodiments, the fiber having a positive Verdet constant is doped with Yb2O3, Sm2O3, Gd2O3, and/or Tm2O3. In certain embodiments, the fiber having a negative Verdet constant is doped with Tb2O3.

Returning toFIG. 3, in certain embodiments, Faraday rotating fiber106is doped with La2O3, Ga2O3, Yb2O3, Ce2O3. In such embodiments, the fiber laser may be a near 1.5 micron or a near 2 micron fiber laser.

In certain embodiments, the multicomponent glass of Faraday rotating fiber106further comprises glass network formers, intermediates, and modifiers. As will be understood by one of ordinary skill in the art, the network structure of glass allows for the accommodation of different types of atoms which can significantly change the properties of the glass. Cations can act as network modifiers, disrupting the continuity of the network, or as formers, which contribute to the formation of the network. Network formers have a valence greater than or equal to three and a coordination number not larger than four. Network intermediates have a lower valence and higher coordination number than network formers. In certain embodiments, one or more glass network formers of the multicomponent glass of Faraday rotating fiber106comprise SiO2, GeO2, P2O5, B2O3, TeO2, Bi2O3, or Al2O3.

Table 1 presents examples of terbium-doped silicate glasses, erbium doped glasses, and ytterbium-doped silicate glasses. One of ordinary skill in the art will appreciate that Table 1 is meant to be illustrative and not limiting.

Turning now toFIG. 5, a cross sectional view of an exemplary highly rare-earth doped fiber for use as a Faraday rotating fiber according to Applicant's invention is presented. As can be seen in the illustrative embodiment ofFIG. 5, core glass rod116is surrounded by cladding glass tube114. In such embodiments, the diameter of core glass rod116is the same as the inside diameter of cladding glass tube114, such that there is no space between the core and the cladding.

In certain embodiments, Applicant's Faraday rotating fiber, as depicted in the illustrated embodiment ofFIG. 5, is manufactured using a rod-in-tube fiber drawing technique. As will be appreciated by one of ordinary skill in the art, in the rod-in-tube method, a glass rod having a higher refractive index is placed in a glass tube of lower refractive index of compatible material and is then heated until the tube shrinks around the rod. In such embodiments, core glass rod116is drilled from a bulk highly rare-earth doped glass and the outside of the core glass rod116is polished to a high surface quality. In such embodiments, cladding glass tube114is fabricated from another piece of rare-earth doped glass with a slightly lower refractive index. In such embodiments, the inner and outer surfaces of cladding glass tube114are polished to a high surface quality.