Position feedback system for volume holographic storage media

A method of holographic recording in a photorefractive medium comprising the steps of:

• recording a first servo pattern in the medium, the first servo pattern being defined by a first interference grating, the first grating being generated by illuminating the medium with a first reference beam and a first object beam to effect the migration of charges in the medium, the first servo block reference beam and the first servo block object beam being incident upon a face of the medium at a first servo block reference angle, the first servo block reference angle defining the angle between the first servo block reference beam and the first servo block object beam;
• recording a second servo pattern in the medium, the second servo pattern being defined by a second interference grating, the second grating being generated by illuminating the medium with a second reference beam and a second object beam to effect the migration of charges in the medium, the second servo pattern reference beam and the second servo pattern object beam being incident upon the face of the medium at a second servo pattern reference angle, the second servo pattern reference angle defining the angle between the second servo pattern reference beam and the second servo pattern object beam;
• recording a third servo pattern in the medium, the third servo pattern being defined by a third interference grating, the third grating being generated by illuminating the medium with a third reference beam and a third object beam to effect the migration of charges in the medium, the third servo pattern reference beam and the third servo pattern object beam being incident upon the face of the medium at a third servo pattern reference angle, the third servo pattern reference angle defining the angle between the third servo pattern reference beam and the third servo pattern object beam;
• recording a fourth servo pattern in the medium, the fourth servo pattern being defined by a fourth interference grating, the fourth grating being generated by illuminating the medium with a fourth reference beam and a fourth object beam to effect the migration of charges in the medium, the fourth servo pattern reference beam and the fourth servo pattern object beam being incident upon the face of the medium at a fourth servo pattern reference angle, the fourth servo pattern reference angle defining the angle between the fourth servo pattern reference beam and the fourth servo pattern object beam;
• recording a fifth servo pattern in the medium, the fifth servo pattern being defined by a fifth interference grating, the fifth servo pattern being generated by illuminating the medium with the fifth reference beam and the fifth object beam to effect the migration of charges in the medium, the fifth servo pattern reference beam and the fifth servo pattern object beam being incident upon the face of the medium at a fifth servo pattern reference angle, the fifth servo pattern reference angle defining the angle between the fifth servo pattern reference beam and the fifth servo pattern object beam;
• recording a first data page in the medium, the first data page being defined by a first data page interference grating, the first data page grating being generated by illuminating the medium with a first data page reference beam and a first data page object beam to effect the migration of charges in the medium, the first data page reference beam and the first data page object beam being incident upon the face of the medium at a first data page reference angle, the first servo pattern reference angle defining the angle between the first data page reference beam and the first data page object beam;
• recording a second data page in the medium, the second data page being defined by a second data page interference grating, the second data page grating being generated by illuminating the medium with a second data page reference beam and a second data page object beam to effect the migration of charges in the medium, the second data page reference beam and the second data page object beam being incident upon the face of the medium at a second data page reference angle, the second data page reference angle defining the angle between the second data page reference beam and the second data page object beam;
• and
• recording a third data page in the medium, the third data page being defined by a third data page interference grating, the third data page grating being generated by illuminating the medium with a third data page reference beam and a third data page object beam to effect the migration of charges in the medium, the third data page reference beam and the third data page object beam being incident upon the face of the medium at a third data page reference angle, the third data page reference angle defining the angle between the third data page reference beam and the third data page object beam,
• wherein adjacent servo patterns are recorded at angular increments of approximately one-half of a minimum angular spacing of the medium.

A method for retrieving holograms recorded within an image space in a photorefractive medium comprising the steps of:

• recording position feedback information within the image space in the medium, the position feedback information being defined by a plurality of patterns, each pattern being generated by illuminating the medium with a servo reference beam and a servo object beam, each servo reference beam and servo object beam being incident upon a face of the medium a servo block reference angle, the servo reference angle defining the angle between the servo reference beam and the servo object beam;
• converting each of the plurality of patterns into permanent spatially varying index of refraction patterns in the medium that cannot be erased by subsequent illumination;
• recording a plurality of data pages within the image space in the medium, each of the data pages being defined by illuminating the medium with a data page reference beam and a data page object beam to effect the migration of charges in the medium, each data page reference beam and data page object beam being incident upon the face of the medium at a data page reference angle, the data page reference beam defining the angle between the data page reference beam and the data page object beam;
• reconstructing one of the data pages and position feedback information on a detector array by propagating the data page reference beam at the face of the medium at a reference angle;
• and
• adjusting the data page reference angle in response to the position feedback information.

A method for retrieving holograms recorded within a plurality of image spaces in a photorefractive medium comprising the steps of:

• recording a plurality of patterns within the image spaces in the medium, each pattern containing position feedback information being defined by a servo reference beam and a servo object beam, each servo reference beam and servo object beam being incident upon a face of the medium at a servo reference angle, the servo reference angle defining the angle between the servo reference beam and the servo object beam;
• converting each of the plurality of patterns into permanent spatially varying index of refraction patterns in the medium that cannot be erased by subsequent illumination;
• recording an image space identifier within each of the image spaces, each identifier providing means for distinguishing each of the image spaces;
• recording a plurality of data pages within the image spaces in the medium, each of the data pages being defined by a data page reference beam and a data page object beam, each data page reference beam and data page object beam being incident upon the face of the medium at a data page reference angle, the data page reference beam defining the angle between the data page reference beam and the data page object beam;
• reconstructing a desired data page and position feedback information from an image space, on a detector array, by propagating the data page reference beam at the face of the medium at a reference angle;
• and
• continuously adjusting the reference angle in response to the position feedback information until the desired data page is reconstructed with a minimum crosstalk.