Research of Holographic Storage Properties of Phase Modulated Reference Beam in Photopolymer
|School||Beijing University of Technology|
|Keywords||photopolymer uniform diffraction efficiency exposure schedule phase-modulate random-phase modulate orthogonal phase code|
The improvement of data storage technology is the base of the progress ofinformation society, and also research focus today. The traditional magnetic recordingtechnology has come to its limit. Also the optical disc is going to its recording densitylimit gradually because of the effect of diffraction. By contrast, the holographicstorage technology with photopolymer has become the most competitive reserve datarecording method because of its great potential in storage density and relatively lowcosts.Photopolymer is a large area and relatively thin material, so it is suitable to apply inpartially-overlapping holograms storage with multiplexing method. And theholographic data storage systems based on the photopolymer have developed byinstitutes and companies all over the world. The Inphase Inc. has released its storagesystem by polytopic method. Stanford University and the Optware Inc. individuallyreleased their own coaxial system by speckle modulator. But there is much noise inthe system with the speckle modulated beams which influences the quality ofrecorded data. Additionally, it is also important to obtain the reconstructed images inuniform diffraction efficiency for all the overlapping recorded holograms.In order to implement high-density holographic storage with high reconstruct quality,the reference beam must be optimized. Our previous work has shown that thephase-modulated reference beams with amplitude distribution as plane wave recordedand reconstructed holograms in a high quality. And the multiplexing degree of phasemodulated reference beam is not confined by the thickness of the material, though itsbasic recording principle is based on the volume Bragg gratings which is positivelycorrelated to the thickness of the material. So it is meaningful to do research on theholographic storage properties of phase-modulated reference beams, and also theuniform diffraction holographic storage is going to be realized in the research on thephase modulated reference beam.For the random-phase modulated reference beam, the multiplexing properties havebeen discussed widely. We did further research to discuss about the recorded images’quality dynamic changes in the process of holographic storage. But before theexperimental research, it is necessary to test the holographic properties of the noveldual-monomer photopolymer based on the free-radical photopolymerization providedby the technical institute of physics and chemistry, Chinese Academy of Sciences (IPC_CAS). We did dark enhancement and holographic recording experiment. Andwith the simplified diffusion model, a data fitting has been done to obtain theparameters reflecting the holographic recording properties of the material with theexperimental data. Then we did research to the quality dynamic changes of hologramsrecorded in the material with random-phase modulated reference beam. The quality ofthe reconstructed images is evaluated by the loss of signal to noise (LSNR). Theexperimental results showed that the intensity and quality of the reconstructed imagesexperienced a dynamic change that the intensity and quality raised for a while andthen fell down until the images faded to disappear. It can be explained by the formingand developing of the scattering gratings. And it is defined that the time before thequality being less than3dB is character time of the material in dark enhancement anduniform post exposure. In the character time the quality and the intensity of thereconstructed images are acceptable and the intensity is descending less than20%. Soit is able to adjust and control the recording time to realize effective holographicstorage according to the character time. And it is also allowed to obtain the propercharacter time according to different demands of recording quality by material testexperiment, which also offers a method to adjust and control the quality ofholographic storage.After the research about the reconstructed images’ quality dynamic changes, takingshift-multiplexing by random-phase modulated reference beam for an example, webuilt up a uniform diffraction efficiency exposure schedule model for recordingpartially-overlapping holograms. Firstly we obtained the analytic expressionsdepicting the refractive index modulation and deduced the exposure schedule modelfor dual-monomer photopolymer. Then, the uniform diffraction efficiency model istransformed to an optimization problem with three constrictions. The exposureschedule was calculated by solving this problem through computer and we didnumerical simulation to calculate the diffraction efficiency of the recorded holograms.Then with the calculated schedule, multiplexing experiment was done to observe theintensity of the reconstructed images in the dual-monomer photopolymer. The resultsof numerical simulation and the experiments showed that following the exposureschedule, diffraction efficiency of all the recorded holograms were uniform, soreconstructed images also had a uniform intensity, which proved the exposureschedule model was effective. For the deterministic phase modulated reference beam, generally the orthogonalphase code modulated reference beam, the traditional multiplexing method actuallyapplied the Fourier transform of the reference beam to recording holograms. Thenumerical simulation results showed that there was a complex and serious fluctuationin both intensity and phase distribution. Moreover, the phase was no longer binary (0or) but varying continuously between–and, which would reduce the phaseorthogonality. Also the additional intensity distribution will influence the quality ofthe beam. A solution we suggested was an imaging setup applied for the orthogonalphase coded reference beam. And we have tested the holographic storage properties ofthe orthogonal phase code imaging reference beam. The numerical simulation andexperimental results showed that with orthogonal phase-coded reference beamposition shifting, the intensity of the reconstructed images is falling down very fastand becoming closely to a straight line, leading to a scanning peak. These come to aconclusion that the orthogonal phase code imaging reference beam has shiftselectivity.