We explored a number of factors affecting the properties relevant
to holographic optical data storage by using a two-color recording
scheme in reduced, near-stoichiometric lithium niobate. Two-color,
or photon-gated, recording is achieved by use of 852-nm
information-carrying beams and 488-nm gating light. Readout at 852
nm is nondestructive, with a gating ratio of
∼104. Recording sensitivity, gating ratio, dynamic
range, and dark decay were measured for crystals of differing
stoichiometry, degree of reduction, wavelength of the gating light,
temperature, and optical power density. The two-color sensitivity
per incident photon is still somewhat less than that of the one-color
process at 488 nm for ∼1 W/cm2 of gating light but is
essentially the same in terms of absorbed photons. Two-color
recording is an attractive way of achieving nondestructive readout in a
read–write material, and it allows selective optical
erasure.

Two-color holography in reduced near-stoichiometric lithium niobate.

A multiphoton photorefractive holographic recording material includes an electro-optic host material doped with at least one lanthanide.

Multiphoton photorefractive holographic recording media

A process and photoactive media for holographic recording and micro/nanofabrication of optical and bio-optical structures via the simultaneous absorption of two-photons by the photoactive media to induce a simultaneous photochemical change in regions of constructive interference within a holographic pattern is disclosed. The photochemical process of polymerization resulting from the simultaneous absorption of two-photons may be used for the microfabrication of micro and nanoscaled features, holographic data storage, and the formation of switchable diffraction gratings.

Holographic recording and micro/nanofabrication via ultrafast holographic two-photon induced photopolymerization (H-TPIP)

A description is provided of an optical information recording and reproducing apparatus. The apparatus comprises: a holographic memory made of a photorefractive crystalline material; a support portion for detachably supporting the memory; a reference beam supplying portion for supplying a coherent reference beam of a first wavelength incident on the memory; a signal beam generating portion for supplying a coherent signal beam of the first wavelength, which is modulated in accordance with image data, incident on the memory, intersecting therein the signal beam with the reference beam, and generating a three-dimensional interference pattern of the beams; a detecting portion for detecting a diffracted light from a refractive index grating of the interference pattern in the memory, due to irradiation of the reference beam; and a medium position adjusting portion for moving a position of the support portion in accordance with a signal corresponding to positioning images from the detection portion. The memory includes a nonvolatile refractive index grating corresponding to a three-dimensional interference pattern of a coherent reference beam and a coherent signal beam, the coherent signal beam being modulated in accordance with positioning images.

Optical information recording and reproducing apparatus having volume holographic memory

The object of the invention is a high data density holographic data storage method. The holograms are written into the volumetric data storage layer or layers, and during the writing process the accurate places of holograms in the data carrier structure are determined by the intersection domain of the object and reference beam or beams, and during the reading process the selection of holograms simultaneously illuminated by the reference beam or beams, the read-out of the addressed hologram, and the suppressing of un-addressed holograms are carried out by a spatial filter located confocally with the addressed hologram and/or by the satisfying of the Bragg condition. The optical arrangement for recording and reading out holograms has three dedicated plan in confocal arrangements, where the addressed hologram is in the middle dedicated plane in the storage material (8), and in the two outer dedicated planes there is spatial filter (95) and (304). The optical arrangement is a 12f optical System consisting of three pairs of different objectives: (321, 322 and 323).

High data density volumetric holographic data storage method and system

Rare earth doped ferroelectric materials are disclosed as reversible holographic recording medium (25) for use in two-photon recording systems. Such rare earth elements provide long-lived electronic states intermediate the ferroelectric material's valence and conduction bands. In some cases, these rare earth intermediate states have a sufficiently long life that low-power continuous wave ('cw') lasers (1) can be used to record interference patterns on them. Thus, two-photon holographic recording systems are also disclosed which do not require high-power, short pulse length, mode-locked or Q-switched lasers. Rather, the disclosed holographic recording systems employ cw lasers such as diode lasers. The rare earth dopants include praseodymium, neodymium, dysprosium, holmium, erbium, and thulium. These dopants provide ions having 4f excited states that give rise to absorptions in the near infra-red and visible spectral regions and typically have lifetimes on the order of 0.1 to 1 milliseconds. The disclosed two-photon holographic recording systems provide for absorption of a first photon which excites electrons of a holographic recording medium to a rare-earth intermediate state. Thereafter, upon absorption of a second photon, the electrons are promoted to the medium's conduction band where they are arranged according to the interference pattern provided by the recording system.

Roger M. Macfarlane

Papers

Gated recording of holograms using rare-earth doped ferroelectric materials