The holographic recording characteristics of phenanthrenequinone-(PQ-) doped poly(methyl methacrylate) are investigated. The exposure sensitivity is characterized for single-hologram recording, and the M͞# is measured for samples as thick as 3 mm. Optically induced birefringence is observed in this material. © 1998 Optical Society of America OCIS codes: 090.2900, 090.4220, 210.2860.The characterization of phenanthrenequinone-(PQ-) doped poly(methyl methacrylate) 1,2 (PMMA) as a recording material for holographic memories is described in this Letter. This material consists of the polymer host matrix with added PQ molecules as photosensitive dopant. High-optical-quality samples of this material were made with variable thicknesses of up to 5 mm and in a variety of shapes. This material does not shrink after exposure and is lightweight, inexpensive, and durable, making it an attractive candidate for disk-based holographic memory systems.Sample preparation consists of dissolving PQ molecules in liquid methyl methacrylate together with a polymerization initiator. This solution is then poured into molds and allowed to polymerize in a pressure chamber at an elevated temperature. The molding process allows samples to be fabricated in a variety of geometries. Disks ranging from 2.5 to 10 cm in diameter with 1 -5-mm thickness were made. For a 1-mm-thick sample doped with a concentration of 0.7% of PQ molecules before exposure, the absorption reaches a maximum of 98.8% at 445 nm and is 58% for the 488-nm line of an argon laser, which was used in all experiments described in this Letter.A hologram was recorded by a pair of 488-nm beams, each incident upon the material at an outside angle of 21.5 ± . We monitored the growth of the hologram during recording by probing the sample with a Bragg-matched He-Ne laser beam. Figure 1 shows the diffraction eff iciency (diffracted power divided by the incident power) during recording in 1-mm-thick material. The diffraction eff iciency reached a maximum of 4.3% for an exposure energy of 2.5 J͞cm 2 . If exposure was allowed to continue, the diffraction eff iciency began to drop. After 20 J͞cm 2 of exposure with a single beam the hologram decayed to approximately 0.1%. At this point the material was completely exposed, and no more holograms could be recorded.We recorded permanent holograms that do not decay with subsequent illumination by stopping the exposure before saturation was reached and then baking the sample. Figure 2 shows the strength of a hologram as a function of baking time at a temperature of 55 ± C. The diffraction efficiency reached a maximum after 12 days and remained steady with continued baking. Figure 3 shows the selectivity curves for a weak and a strong hologram (2% and 35% diffraction eff iciency, respectively). The 2% hologram has a sinc-squared selectivity curve as expected for a 1-mm-thick hologram. The stronger hologram, on the other hand, has a selectivity curve that is distorted and shifted. For a holographic memory the diffraction eff iciency is relatively sma...

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The storage density of shift-multiplexed holographic memory is calculated and compared with experimentally achieved densities by use of photorefractive and write-once materials. We consider holographic selectivity as well as the recording material's dynamic range ͑M͞#͒ and required diffraction efficiencies in formulating the calculations of storage densities, thereby taking into account all major factors limiting the raw storage density achievable with shift-multiplexed holographic storage systems. We show that the M͞# is the key factor in limiting storage densities rather than the recording material's thickness for organic materials in which the scatter is relatively high. A storage density of 100 bits͞m 2 is experimentally demonstrated by use of a 1-mm-thick LiNbO 3 crystal as the recording medium.

A new multiplexing schedule is derived for multiplexing holograms in photorefractive polymers which do not exhibit mono-exponential recording behavior. An M-number (M/#) of 0.3 was measured experimentally by recording 20 holograms of roughly equal strength in a single location of 125-lm-thick material using peristrophic multiplexing. The eects of hologram dark-decay on the time-evolution of the M/# and the relative strengths of individual holograms is investigated. Ó 2000 Elsevier Science B.V. All rights reserved. Holographic data storage is a promising technology for the storage of large amounts of data. In order to be useful for holographic data storage applications, a material must be capable of achieving a high M/#, a property dependent on both the recording and erasure dynamics of the stored holograms. In this letter we report the recording of multiple holograms by peristrophic multiplexing in the novel material class of photorefractive (PR) polymers. A new recording schedule is devised to account for the variation of the erasure time constants as a function of exposure. The M/# measured from the recording of 20 equalized holograms was 0.3.The standard structures are sandwiches of the PR polymer between two glass slides coated with transparent electrodes made from indium tin oxide [1±6]. In particular, we used a composite derived from the ®rst high-performance PR polymer [7], consisting of (by weight) 42% poly-(N-vinylcarbazole) (PVK, polymer host), 7% N-ethylcarbazole (plasticizer), 25% each of the non-linear chromophores 2,5-dimethyl-4,4 H -nitrophenylazoanisole and 3-methoxy-4,4 H -nitrophenylazoanisole, and 1% 2,4,7-trinitro¯uorenone (sensitizer). The active layer thickness was d 125 lm. The holographic setup consisted of two plane-wave 633 nm recording beams at angles of 50°and 70°from the surface normal. They were both s-polarized and had equal intensities of approximately 500 lW/cm 2 at the sample surface. A weak (3.5 lW/cm 2 ) p-polarized readout beam counter-propagating to the 50°re-cording beam was used to perform a phase conjugate readout of the holograms. Under these conditions and at the externally applied ®eld used throughout these investigations (E 62 V/lm; necessary to induce bulk non-linearity), the logarithmically averaged holographic response time [8] of the material was 45 s and the internal diraction

We demonstrate holographic recording in a new photopolymer system. The recording material is created by copolymerization of an optically inert monomer, methyl methacrylate, and a second monomer that is optically sensitive. On exposure of the recording material to light, a portion of the optically sensitive component detaches from the polymer matrix and causes hologram amplif ication through diffusion of the free molecules. We measured postrecording grating amplif ications as high as 170% by this process. The recorded holograms are persistent at room temperature under continuous illumination at the recording wavelength. 

We report on a holographic method for recording fast events whose speed is limited by the laser pulse duration if the recording material has sufficient sensitivity to reliably record a frame of the fast event with a single pulse. The method we describe uses the angular selectivity of thick holograms to resolve frames that are recorded with adjacent pulses. Two specially designed cavities are used to generate the signal and reference pulse trains. We experimentally demonstrate the system by recording laser induced shock waves with a temporal resolution of 5.9 ns, limited by the pulse width of the Q-switched Nd:yttrium-aluminum-garnet laser used in the experiments. © 2002 American Institute of Physics. ͓DOI: 10.1063/1.1446205͔Since the early days when holography was invented, people have been studying high-speed events using holographic techniques. A well-known example is double exposure interferometry.1 Multiple frames can be stored and reconstructed separately using multiplexing techniques. Previous work has focused on spatial multiplexing 2-4 where holograms are recorded at different locations of the recording medium. Pulsed holograms have also been angularly multiplexed taking advantage of the thickness of the recording medium. In one method, three lasers are used to generate three reference beams with different angles and each laser fires a pulse in a different time.5 A rotating mirror 6 or electrooptic switches 7 have also been used to generate the reference beams. In these efforts, the speed is limited by electronics or mechanical scanning. In the system we describe the speed is limited by the pulse width of the laser ͑5.9 ns͒. With proper modification the method can also be used with subpicosecond pulses. During recording, a sequence of signal and reference pulses are incident on the holographic medium. The signal pulses all travel in the same direction while the reference beam direction changes from pulse to pulse in order to angularly multiplex holograms. After the recording, a cw laser at the same wavelength is used to readout individual frames. Depending on the incidence angle, different frames can be readout separately due to the angular selectivity of the thick hologram.In the experiments, both the signal and the reference pulse trains are generated by a single pulse from a frequency doubled Q-switched Nd:yttrium-aluminum-garnet ͑YAG͒ laser ͑wavelength 532 nm, pulse width 5.9 ns, energy per pulse 300 mJ, and beam diameter 9 mm͒. The system is shown in Fig. 1͑a͒. In the signal cavity, a polarizing beam splitter is used to couple the vertically polarized ͑perpendicu-lar to the paper͒ incident pulse into the cavity. The Pockels cell is timed to behave like a temporary /4 wave plate ͑ef-fectively a /2 wave plate since the pulse passes it twice each round trip͒ to rotate the polarization of the incident pulse to horizontal direction after it first enters the cavity. It is turned off afterwards while the pulse travels back towards the opposite mirror. The pulse is then trapped inside the cavity since the p...

The decay of holograms stored in photorefractive polymer composites based on poly͑N-vinyl-carbazole͒ with and without extrinsic deep traps is investigated. The photorefractive phase shift is identified as one of the key parameters determining the dark decay dynamics. This has important implications for all kinds of photorefractive imaging applications including holographic data storage. A trade off will be required between accepting a certain degree of hologram distortion due to two-beam coupling on the one hand and achieving high hologram stability during idle periods in the dark with the external field applied on the other. © 2002 American Institute of Physics. ͓DOI: 10.1063/1.1492848͔The photorefractive ͑PR͒ effect is one of the most promising reversible holographic storage mechanisms. 1 Under nonuniform illumination, the refractive index of the photosensitive material is modulated due to the generation of mobile charge carriers in the bright regions, their subsequent redistribution, and eventual trapping in the dark areas. This gives rise to a space-charge field E SC , which modulates the refractive index of the material through the linear electrooptic effect and orientational effects. 2,3 Photorefractivity in amorphous polymers has been intensively investigated, 4,5 and these systems have been widely recognized as potential active media in rewritable holographic optical memories for security applications, 6 in associative memories, 7 or in adaptive ultrasound sensors. 8 Due to the rather low dielectric constants of polymers ( Ͻ10), oppositely charged carriers show a rather strong tendency to recombine. As a result, only rather short storage times are anticipated. However, so far, the dark decay ͑referred to as ''dd'' hereafter͒ of the holograms in periods when the system is idle, i.e., held in the dark with the external field still applied, has been mostly neglected in literature on organic PR materials, even though it is important for the aforementioned applications. In this letter, we present systematic investigations of the dd of PR gratings in PR polymers. Our results will give evidence that the phase shift between the interference pattern and the recorded index grating, the commonly accepted fingerprint of photorefractivity, is one of the key parameters, yielding slower dd for a larger phase shift.The investigated materials contained the photoconductor poly͑N͒vinylcarbazole ͑PVK, 39 wt %͒, the plasticizer N-ethylcarbazole ͑10 wt %͒, the eutectic mixture of two EO chromophores 2,5-dimethyl-4͑p-nitrophenylazo͒-anisole ͑25 wt %͒, and 3-methoxy-4͑p-nitrophenylazo͒-anisole ͑25 wt %͒, and finally the sensitizer 2,4,7-trinitro-fluorenone ͑TNF, 1 wt %͒. We also prepared a similar material doped with 0.82 wt % ͑replacing PVK͒ of the commonly used hole conductor N,NЈ-bis͑3-tolyl͒-N,NЈ-diphenyl-benzidine ͑TPD͒, whose highest occupied molecular orbital levels are situated about 0.5 eV below those of PVK. Thus, TPD moieties constitute deep traps within the carbazole transport manifold, and therefore a longer storag...

Abstract. We propose and demonstrate a widely tunable optical filter, realized by angle tuning a volume holographic grating. The volume holographic grating selectively drops a narrow portion of the signal bandwidth into a fiber while passing through the rest of the signals. The demonstrated 1510-to 1590-nm tuning range covers the entire erbiumdoped fiber amplifier (EDFA) C band, with small bandwidth variation and low insertion loss (Ͻ1 dB). Group delay, polarization-dependent loss, and polarization mode dispersion are measured and investigated for optimizing the filter characteristics.