Optical data storage has been interested in several decades because of the potential of fast access time and high-density storage. Recently volume holographic data storage technique has drawn much interest as new materials and improved techniques have developed.1 However, the large of hologram storage is restricted by low diffraction efficiency and mainly focused to read only memory because of the material's characteristic difficult to rewrite in short time interval. Another optical memory technique (photon echo optical memory)2 based on time-domain has developed since 1980's and made big progress for the area of real time image processing and dynamic random access memory. In this technique, data pulses interfere with timely separated write pulse (reference), and their interference pattern is written as population gratings. A read pulse (reference) retrieves each data pulse information as a four-wave mixing process. The write and read pulse separations are restricted by phase and population decay times of the material, respectively, and storage density is determined by the ratio of inhomogeneous to homogeneous linewidths. Even though photon echo technique has advantages of fast access time (≥ ns) and high-density (≥ Mbits/laser spot), its operating temperature is kept extremely low around liquid helium temperature. This is because that homogeneous decay time rapidly shortens as temperature increases due to phonon interactions, i.e., the ratio of inhomogeneous to homogeneous decay rates is ~ 1 at liquid nitrogen temperature (77 K) in a rare-earth doped solid, so that restricts its applications.
Optical data storage has been interested in several decades because of the potential of fast access time and high-density storage. Recently volume holographic data storage technique has drawn much interest as new materials and improved techniques have developed.1 However, the large of hologram storage is restricted by low diffraction efficiency and mainly focused to read only memory because of the material's characteristic difficult to rewrite in short time interval. Another optical memory technique (photon echo optical memory)2 based on time-domain has developed since 1980's and made big progress for the area of real time image processing and dynamic random access memory. In this technique, data pulses interfere with timely separated write pulse (reference), and their interference pattern is written as population gratings. A read pulse (reference) retrieves each data pulse information as a four-wave mixing process. The write and read pulse separations are restricted by phase and population decay times of the material, respectively, and storage density is determined by the ratio of inhomogeneous to homogeneous linewidths. Even though photon echo technique has advantages of fast access time (≥ ns) and high-density (≥ Mbits/laser spot), its operating temperature is kept extremely low around liquid helium temperature. This is because that homogeneous decay time rapidly shortens as temperature increases due to phonon interactions, i.e., the ratio of inhomogeneous to homogeneous decay rates is ~ 1 at liquid nitrogen temperature (77 K) in a rare-earth doped solid, so that restricts its applications.
The photon-echo memory[1], or the time-domain optical data storage, resembles the hole-burning memory [2] in that the inhomogeneous absorption line of a material is employed as a storage medium and the information is stored in it as a frequency-dependent absorption change with the resolution determined by the homogeneous width. Having the common memory capacity N given by the inhomogeneous-to-homogeneous width ratio, both types of memories share common problems like low-temperature operation, spectral diffusion and so on. Although a storage medium is common, storage techniques are definitely different for the two memories. The photon-echo memory utilizes the temporal interference effect between a reference pulse and a data pulse and their interference pattern is stored as a population grating in the frequency domain. In this sense this is a time-domain analog of holography where the spatial interference pattern between a reference beam and a data beam is stored in a medium as a population grating in the real space. Like the spatially diffracted beam from the read beam gives the information stored in the hologram, the “temporally diffracted pulse", or the photon echo, after the read pulse can give the information stored in the inhomogeneous distribution.
There has been much recent interest in the use of optical photon echoes for information storage and processing. Unfortunately, existing optical echo memories require doped crystals at liquid He temperature. The proposed scheme uses an optically excited microwave spin echo to store optical temporal information in the Raman excited ground state coherences. The advantage is that microwave echoes are known to exist at room temperature with decay times approaching 1 s in certain materials. We show that these spin echoes can be excited and detected optically, thereby allowing storage densities that are comparable to the optical echo techniques. Briefly, a three level A system is used, which can be excited by two near resonant optical transitions or one near resonant microwave transition. The Raman interaction first stores temporal and phase information in an inhomogeneously broadened ground states coherence. A microwave π-pulse is used to time-reverse the coherences, thus generating spin echoes that can be detected optically by a Raman probe. Preliminary experimental results will also be reviewed.
We describe a new approach to develop a high-speed, high-density random access cache memory. This new technique is based on a novel approach using, in essence, a hybrid of the time domain stimulated echo (SE) concept and the frequency domain scheme. It has been demonstrated that the rare-earth doped crystals, can at low temperatures store large amounts of information for up to 24 hours using SE. The basic approach we have developed is the partitioning of the absorption frequency domain into smaller bins, so that each frequency bin stores a smaller portion of information independently. The bins are distinguishable by their different absorption frequencies and they are accessed by changing the laser frequency (color). However, information is still stored and retrieved using the time-domain pulse sequency used in SE. Specifically, with the laser frequency set at the particular absorption of a frequency bin, the laser is externally modulated by an acousto-optic modulator to produce amplitude modulated pulses representing the digital data, the write pulse, and the read pulse, during the write and retrieve operation, respectively. The advantages of this approach are that memory can be stored in space, time, and frequency domain, allowing us to tailor a flexible memory architecture to match those of the computational processor.
We have observed excitation of spin echoes and spin free induction decay (FID) by electromagnetically induced transparency (EIT) in an optically dense solid sample. The experiments are done in a double-lambda system of 605.7 nm 3H4 - 1D2 transition of Pr3+:Y2SiO5, where the 10.2 MHz ground state spin coherence is excited by low-power resonant Raman pulses. It has been shown that the spin coherence, including spin echo, is equivalent to the transparent state of EIT, and therefore a high efficiency is expected for such resonant Raman-excited spin echo. The observed efficiency of spin echo is as high as 75% of the FID signal at 5K. A background-free detection scheme is used based on EIT and enhanced nondegenerate four-wave mixing. The technique is applied in the frequency-selective time-domain optical data storage, that utilizes the spin as well as the optical inhomogeneous spectral widths. The data storage scheme is analogous to the stimulated spin echo with resonant Raman excitation of the spin coherence. We verify that the write window is determined by the spin T2 which is much longer than the optical T2, especially at higher temperature. We find that the spin dephasing time T2 is almost constant at approximately 500 microseconds in the range of 2 to approximately 6 K, whereas the optical T2 decreases rapidly, by a factor of approximately 50, above 4 K. These results will be useful in the development of high capacity time-domain optical data storage operating at higher temperature.
Spectral hole burning and photon echoes are the techniques for frequency-selective optical memories, which have the potential to increase areal data storage density by a factor 10**7. They also have the capability to perform in-memory data processing. In this thesis, several materials and techniques for optical memories in the frequency- and time domain have been investigated. In the frequency domain, ways to increase the storage density and the working temperature were studied using photon-gated spectral hole burning materials of the metal-tetrabenzoporphyrin derivatives. A selection rule for the electron acceptors in the donor-acceptor electron transfer systems was deduced. By doping the material with two different donors a maximum inhomogeneous line width of more than 30 THz was achieved. A polarization holographic technique was established for the formation and detection of spectral holes. Our results indicate that this technique can provide a better signal-to-noise ratio than that achieved by a conventional holographic technique, in particular for samples of poor optical quality. In the time domain, magnetic field-induced and the intensity-induced dephasing processes were studied in Pr3+ doped in a YAlO3 crystal and a Y2SiO5 crystal, respectively. As a step along the route to make photon echo based optical processing possible, the amplification of photon echo signals by the use of a fiber amplifier (Pr3+-doped ZBLAN) was demonstrated. A gain of 45 was achieved. The erasure of stored data using photon echoes has also been investigated. A technique to diagnose the phase and frequency stability of a light source by the photon echo erasure process was proposed and illustrated. An approach to bit-selective data erasure, that is free from laser phase and frequency fluctuations, is also suggested. A concept for an arbitrary shape pulse generator based on photon echoes and hole-burning was experimentally and theoretically studied. Finally, a comparison between the properties and capabilities of spectral hole burning and photon echoes was made.
The speed of modern supercomputers is primarily limited by the relatively slow speed of memory. Research over the past years has focused on developing optical memory systems that, by nature of optics, promise to achieve high speed and high memory capacity. Three- dimensional optical memory systems are well known for their storage capacity and achievable short access time. But the early attempts to exploit 3-dimensional media were limited to photorefractive crystals such as lithium niobate, due to the unavailability of any other suitable medium (alternative media are precluded by their diffraction limited storage densities and non- erasable characteristics). Although volume phase holograms can be recorded in photorefractive crystals, these materials have many obvious flaws such as strong scattering, phase distortion, self diffraction, fast decay, and low refractive index modulation. There have also been continuous efforts to develop new 3-D media (such as photon echo, photorefractive polymers, bacteriorhodusphin, memory access cache, etc., but none can be applied in real world 3-D device because these materials are still in the early stage of development.