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Apparatus for a Holographic-Tracking-Servo using an Adjustable-Wavelength Source

IP.com Disclosure Number: IPCOM000176250D
Original Publication Date: 2008-Nov-10
Included in the Prior Art Database: 2008-Nov-10
Document File: 7 page(s) / 52K

Publishing Venue

IBM

Abstract

This invention describes a method for providing a tracking servo system for a holographic read/write data storage drive supporting an underlying DVD Layer. The novel tracking system disclosed uses a wavelength shift of the laser source in the holographic drive to provide a tracking control system.

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Page 1 of 7

Apparatus for a Holographic-Tracking-Servo using an Adjustable-Wavelength Source

This invention describes a method for providing a tracking servo system for a holographic read/write data storage drive supporting an underlying DVD Layer. The novel tracking system disclosed uses a wavelength shift of the laser source in the holographic drive to provide a tracking control system.

Referring to Figure 1, a first embodiment 100, of the present invention is described. A variable wavelength reference and reconstruction source 116, is used to provide tracking control by adjusting the wavelength of source 116 in a closed loop feedback tracking control system.

By use of Eq. 5 described in detail below, the angle of the signal beam 113 impinging upon the data detector can be modified:

1

(

R

λ

C

SinI Sin λ

α ±

=

), Eq. 5.

Servo 180 in combination with processor 150 controls the output wavelength of reference and reconstruction source 116 to shift the image of Hologram 121 on Detector 130 to produce the optimum alignment of the data pattern on Detector 130. This method inventions supports obtaining tracking signals obtained from an underlying DVD Layer that is part of the holographic data storage media.

The image distance shift for recording of a hologram at wavelength, R

α

C

λ and reconstruction at

wavelength, C

1

λ, may be derived from Eqs. 1& 2:

=

±

1λ⎛ −

1

1

C

R

I R

R

R

λ

C

R

O

R

, Eq. 1,

Sin α

α −

±

I Sin

Sin

=

Sin

α

C

λ( )

α

C

λ

O

R

R

, Eq. 2,

where I

R is the distance from the hologram to the image, C

R is the distance from the hologram

to the reconstruction source, O

R is the distance from the hologram to the object, R

R is the

distance from the hologram to the reference wave, where I

α is the angle from the hologram to

the image, C

α is the angle from the hologram to the reconstruction source, O

α is the angle from

the hologram to the object, R

α is the angle from the hologram to the reference wave and the

±

is

for the real and conjugate images.

For reference and reconstruction plane waves (i.e. collimated laser beams), ∞

R and

C

R ,

R

1

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Eq. 1 reduces to Eq: 3:

RI R

λ

=

±

R

λ( )

    O C

, Eq. 3, this shows how we can shift the image position with wavelength.

For data storage applications as shown in Figure 1, C

α the angle from the hologram to the

reconstruction source = R

                                                α, the angle from the hologram to the object = 0 because the object is typically on axis to the normal to the media surface.

Eq. 2 reduces to Eq: 4:

α, the angle from the hologram to the reference wave and O

Sin α

λ

α ±

=

I Sin

Sin

α

C

λ( )

C

R

R

, Eq. 4, which can be further simplified to Eq. 5,

λ

SinI Sin λ

α ±

=

α

1

(

C

C

R

), Eq. 5.

From Eq. 5, it is apparent that the angle of optical beam leaving the hologram, I

α can be altered

by changing the wavelength of the reconstruction source, C

λ

.

Referring to Figure 1, a first embodiment 100,...