Active solid-state devices (e.g. – transistors – solid-state diode – Responsive to non-electrical signal – Electromagnetic or particle radiation
Reexamination Certificate
1999-05-21
2002-05-28
Tran, Minh Loan (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Responsive to non-electrical signal
Electromagnetic or particle radiation
C257S446000, C257S452000, C257S449000, C257S457000, C257S466000
Reexamination Certificate
active
06396115
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the field of optical storage systems and, more specifically, to a detection module used in optical storage systems.
BACKGROUND
Magneto-optical (MO) storage systems provide storage of data on rotating disks. The disks are coated with a magneto-optical material and divided into magnetic areas referred to as domains. The data is stored in the magneto-optical material as spatial variations in the magnetic domains.
In one type of MO storage system, a MO head is located on an actuator arm that moves the head along a radial direction of the disk. As the disk rotates, the head can be positioned over a particular domain. A magnetic coil on the head creates a magnetic field oriented perpendicular (i.e., vertical) to the disk's surface. Each vertically magnetized domain represents either a zero or one depending on the direction that the magnetic field is pointing.
The vertical magnetization is recorded in the MO material by focusing a beam of laser light to form an optical spot on a disk domain. The laser beam heats the MO material at the spot to a temperature near or above the Curie point. The Curie point is the temperature at which the magnetization in the material may be readily altered with an applied magnetic field. A current is then passed through the magnetic coil to orient the vertical magnetization in either the up or down direction indicating a one or zero. This orientation occurs only in the region of the optical spot where the temperature is sufficiently high, and remains after the laser beam is removed.
Information is read from a particular domain using a less powerful laser beam, making use of the Kerr effect, to detect a rotation of polarization of a beam reflected off the disk's surface. The magnetization of each domain causes a rotation of the optical polarization of the laser beam incident at the domain. The polarization of the reflected beam is rotated in a direction, clockwise or counter-clockwise, determined by the orientation of the domain's vertical magnetization. Measurement of the direction of rotation is performed by an optical detection system that converts the optical signals into electrical signals.
One particular MO storage system uses a radio frequency modulated Fabry-Perot laser source coupled to a single optical fiber to transmit the laser beam to the storage system's head. The optical fiber directs an incident laser beam to the head, which is then directed toward the rotating disk. The head also directs the reflected laser beam from the rotating disk to the same optical fiber. Discrete optical components are located remotely from the head for optically processing the rotated polarization components of the reflected laser beam. This system relies on the preservation of the polarization states of the reflected beam through the entire optical path. As such, a polarization maintaining optical fiber is used in the system. However, the birefringent nature of a polarization maintaining fiber combined with certain characteristics of the laser diode causes some undesirable results.
Birefringence is a characteristic of an optical material in which the index of refraction depends on the direction of polarization of light propagating in the material. Birefringence in the fiber material causes a phase shift between the orthogonally polarized light beams that are transmitted along the fiber. In addition, the use of a radio frequency modulated Fabry-Perot laser diode produces a relatively broad-spectrum, multi-wavelength incident light beam having wavelengths that fluctuate with time.
One problem with using a single optical fiber system is that noise, associated with the FP laser, is transmitted by the optical fiber to the discrete optical processing components located remotely from the head. Because of the birefringent characteristic of the optical fiber, the multiple fluctuating wavelengths of the incident light from the laser diode cause signal components in the reflected beam to have polarization states that also fluctuate. The competition between the multiple fluctuating wavelengths of the reflected beam appears as noise at the storage system's detectors. This noise may limit the achievable data rate at a given signal level. Furthermore, optical fibers exhibit polarization mode leakage that may cause one polarization mode to appear as another. This polarization leakage also appears as noise at the storage system's detectors.
Another problem with the single fiber optical system is that the use of optical processing components located remotely from the head require tight alignment tolerances between the discrete components. Such tight alignment tolerances cause the manufacturing of the system to be more difficult.
FIG. 1
illustrates another MO storage system
100
that uses multiple optical fibers and discrete optical components on a head. A single optical fiber is used to direct incident light and two optical fibers are used in the return path to direct reflected light. In system
100
, discrete optical processing components are placed directly on the head
10
in a particular relationship to each other in order to direct the incident light beam onto the disk and to convert the polarization information from a magnetized domain into two separate reflected light beams having differential intensity information. The discrete optical components may include a leaky beam splitter
20
, a phase plate
30
, and a polarizing beam splitter (or other polarization splitting element such as a Wollaston prism)
40
.
Linearly polarized light
50
is directed through leaky beam splitter
20
and then to the disk (not shown) by a mirror
60
. The return beam
70
is directed back to leaky beam splitter
20
with a Kerr rotation as discussed above. The linearly polarized light, in system
100
, is characterized by two plane-polarized beams: one beam with its electric field parallel to the plane of incidence (horizontal or p-polarized) and the other beam with its electric field perpendicular to the plane of incidence (vertical or s-polarized). The leaky beam splitter
20
reflects the return light beam
75
to phase plate
30
. Phase plate
30
introduces a phase shift between the p-polarized and s-polarized components of light
75
. The phase plate
30
rotates the polarization such that equal components of p-polarized and s-polarized light are received by polarizing beam splitter
40
when a Kerr rotation is not present on the disk. When a Kerr rotation is present on the disk, however, phase plate
30
rotates the polarization such that different components of p-polarized and s-polarized light are received by polarizing beam splitter
40
. The light passing through the phase plate
30
is directed to the polarizing beam splitter
40
.
The polarizing beam splitter
40
includes a glass plate having a multi-layer coating on its front surface that acts to separate the s-polarized and p-polarized components into spatially separate beams. The coating on the front surface of polarizing beam splitter
40
reflects light based on its polarization such that all of the s-polarized light
80
is reflected and all of the p-polarized light
85
is transmitted. The s-polarized light
80
is reflected to a lens
90
that focuses it into a multimode fiber which carries the light to a detector (not shown). The p-polarized light
85
is refracted to the back surface of polarizing beam splitter
40
. The back surface of polarizing beam splitter
40
acts as a mirror to reflect the p-polarized light
85
such that it is offset from the s-polarized light
80
. The p-polarized light
85
is transmitted to a second lens
95
that focuses it into a second multimode fiber which carries the light to a different detector. The detectors convert the light amplitude signals from each channel into electrical signals.
Once the electrical signals are produced, a difference signal between the s-polarized and p-polarized light is calculated. The difference signal is used to determine the sign of the Kerr rotation indicating the direction
Al-Jumaily Ghamin A.
Berger Jill D.
Dohmeier Steve C.
Durnin James E.
Gage Edward C.
Seagate Technology LLC
Simon Nancy R.
Simon & Koerner LLP
Tran Minh Loan
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