Optical waveguides – With optical coupler – Particular coupling function
Reexamination Certificate
1999-07-21
2001-11-27
Ullah, Akm E. (Department: 2874)
Optical waveguides
With optical coupler
Particular coupling function
Reexamination Certificate
active
06324319
ABSTRACT:
SCOPE OF THE INVENTION
The present invention relates generally to altering the mode field pattern exhibited by single-mode optical fibers and more particularly to altering the mode field pattern exhibited by optical fibers that are used in optical storage drives.
BACKGROUND
In magneto-optical storage systems that use magneto-optical (MO) recording material deposited on a rotating disk, information may be recorded on the disk as spatial variations of magnetic domains. During readout, a magnetic domain pattern modulates an optical polarization, and a detection system converts a resulting signal from optical to electronic format.
In one type of a magneto-optical storage system, a magneto-optical head assembly is located on an actuator that moves the head to position the head assembly over data tracks during recording and readout. A magnetic coil is used to create a magnetic field that has a magnetic component in a direction perpendicular to the disk surface. A vertical magnetization of polarity, opposite to that of the surrounding magnetic material of the disk medium, is recorded as a mark indicating zero or a one by first focusing a beam of laser light to form an optical spot on the disk. The optical spot functions to heat the magneto-optical material to a temperature near or above a Curie point (a temperature at which the magnetization may be readily altered with an applied magnetic field). A current passed through the magnetic coil orients the spontaneous vertical magnetization either up or down. This orientation process occurs in the region of the optical spot where the temperature is suitably high. The orientation of the magnetization mark is preserved after the laser beam is removed. The mark is erased or overwritten if it is locally reheated to the Curie point by the laser beam during a time the magnetic coil creates a magnetic field in the opposite direction.
Information is read back from a particular mark of interest on the disk by taking advantage of the magnetic Kerr effect so as to detect a Kerr rotation of the optical polarization that is imposed on a reflected beam by the magnetization at the mark of interest. The magnitude of the Kerr rotation is determined by the material's properties (embodied in the Kerr coefficient). The sense of the rotation is measured by established differential detection schemes and, depending on the direction of the spontaneous magnetization at the mark of interest, is oriented clockwise or counter-clockwise.
Conventional magneto-optical heads tend to be based on relatively large optical assemblies which make the physical size and mass of the head rather bulky (typically 3-15 mm in a dimension). Consequently, the speed at which prior art magneto-optical heads are mechanically moved to access new data tracks on a magneto-optical storage disk is slow. Additionally, the physical size of the prior art magneto-optical heads limits the spacing between magneto-optical disks. Because the volume available in standard height disk drives is limited, magneto-optical disk drives have not been available as high capacity commercial products.
N. Yamada (U.S. Pat. No. 5,255,260) discloses a flying optical head for accessing an upper and lower surface of a plurality of optical disks. The flying optical head disclosed by Yamada describes an actuating arm that has a static (fixed relative to the arm) mirror or prism mounted thereon, for delivering light to and receiving light from a phase-change optical disk. While the static optics described by Yamada provides access to both surfaces of a plurality of phase-change optical disks contained within a fixed volume, Yamada is limited by the size and mass of the optics. Consequently, the performance and the number of optical disks that can be manufactured to function within a given volume is also limited.
Utilization of optical fibers to deliver light to a storage location within an optical disk drive allows for a lower profile optical path which can increase the number of disks that can be vertically positioned within a given form factor. In a magneto-optical storage drive, polarization-maintaining optical (PM) fiber is typically used to convey laser light delivered by inexpensive laser diodes. The mode field that is exhibited by single-mode optical fiber is typically circularly symmetric. Because the mode field of a Fabry Perot laser diode is typically asymmetric, the mode field mismatch may create optical inefficiencies that result in a reduced signal to noise ratio in the detected data signal.
What is needed, therefore, is a method and apparatus that enables a laser source exhibiting an asymmetric mode field to be used with polarization maintaining optical fiber so as to efficiently convey light between a laser source and a storage location of an optical data storage system.
SUMMARY
The present invention includes an optical apparatus for directing a laser light exhibiting a first mode field along an optical path between a source and a data storage location. The optical apparatus may comprise: a first optical element, wherein the first optical element is disposed in the optical path, wherein the first optical element exhibits a second mode field; and a second optical element, wherein the second optical element is disposed in the optical path, and wherein the second optical element matches the first and second mode fields to each other. The first mode field may comprise a asymmetric mode field. The second mode field may comprise an symmetric mode field. The asymmetric mode field may be elliptical. The symmetric mode field may be circular. The first optical element may comprise a single mode polarization maintaining optical fiber. The second optical element may comprise a multi-mode optical fiber or a anamorphic prism. The source may comprise a Fabry Perot laser diode.
The present invention may also comprise a method for directing a laser light along an optical path between a source and a data location that comprise the steps of: providing a source of light that exhibits a first mode field in the optical path; providing a first optical element that exhibits a second mode field in the optical path; and matching the first and second fields to each other by providing a second optical element in the optical path.
In the present invention the coupling efficiency into an optical fiber is maximized when a focused spot size and shape are matched to the size and shape of the optical fiber mode field. The light emerging from a laser diode is generally asymmetric (in that light diverges more quickly along one axis of the beam), and if a simple lens is used to focus the laser light without any beam-shaping optics, the focused spot is asymmetric as well. In general, the mode field of a single-mode PM fiber is symmetric. The present invention modifies the mode field of the single-mode PM fiber, and generates an asymmetry that matches that of the focused spot in order to achieve efficient coupling.
In the present invention, a short piece of multi-mode fiber is fusion spliced to a single-mode PM fiber. (A longer piece of multi-mode fiber may be used during splicing, and then cleaved back.) Typical lengths of the multi-mode fiber piece vary between 50% and 100% of the cladding diameter, and the operation of the invention does not depend critically on the exact length.
In order to achieve efficient coupling, the light from a laser source should be focused onto the splice between the multi-mode fiber piece and the single-mode PM fiber. Because the fiber materials on either side of the splice are nearly identical in refractive index, there is very little light reflected back to the laser from the splice. This helps reduce feedback into the laser, and reduces laser noise. The reflection from the other end of the piece of multi-mode fiber is out of focus with respect to the laser; most of the reflected light does not re-enter the laser cavity, and does not significantly contribute to the laser noise.
In the present invention, it is possible to align the polarization axes of the PM optical fiber while the optical fiber tip is bein
Gage Edward C.
Gerber Ronald E.
Mowry Gregory S.
Tselikov Alexander
Moser Patterson & Sheridan LLP
Seagate Technology LLC
Ullah Akm E.
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