Doubly doped crystal
Chapter 4 Chapter 4 System issues in two-center holographic recording
4.4 Reduction of fanning in two-center recording
scattering holograms are recorded. During the recording of a hologram by a reference beam and a signal beam, both beams get scattered inside the medium resulting in fanning. During read-out, the presence of the reading (or reference) beam causes fan- ning. The quality of the reconstructed beam during read-out of a hologram degrades in time as fanning builds up.
The buildup of the scattering holograms depends on the recording sensitivity and the dynamic range of the medium, especially during read-out. This is due to the fact that the average reading time in practical storage systems is much longer than the average recording time. The effect of fanning during read-out can be observed by recording a single hologram of a two-dimensional bit pattern (or a mask) and read the hologram while monitoring the reconstructed image over time.
Suppression or reduction of fanning without sacrificing sensitivity is a challenging task in normal recording. More sensitive materials fan more due to the fast recording of the scattering holograms. The sensitivity of the materials used in normal recording during read-out is the same as that during recording. However, fanning is highly reduced in two-center recording due to the inherent mechanisms involved in this type of recording. The presence of the incoherent homogeneous sensitizing (UV) beam during recording does not let scattering holograms buildup by erasing them. This can also be understood by recalling that the strength of a hologram in two-center recording depends on the ratio of the recording and sensitizing intensities. This intensity ratio is optimally chosen for the desired hologram, but it is far from the optimum for the scattering holograms since the recording intensity of the scattering hologram is much less than that of the desired hologram. Fanning occurs only in a short initial period during read-out of a hologram in two-center recording. This period consists of the time interval in which electrons are transferred from shallower to deeper traps. All shallower traps become empty after this initial period resulting in the insensitivity of the material to the reading beam. Therefore, fanning can not buildup after the initial period. Reducing the recording wavelength in two-center recording for improving the sensitivity can cause a very mild buildup of fanning due to the small sensitivity for recording scattering holograms from the deeper traps.
To compare fanning in normal and two-center recording, we performed holo- graphic recording experiments with two LiNb03:Fe:Mn crystals. The doping levels were 0.075 wt. % Fe203 and 0.01 wt. % MnO in both crystals. One crystal (XTAL1) was annealed properly to have all Fe traps as well as a portion of the Mn traps ini- tially empty. The other crystal (XTAL2) was highly reduced to have all Mn traps as well as a portion of the Fe traps occupied by electrons. Therefore, XTAL1 is ap- propriate for persistent two-center recording while XTAL2 is appropriate for normal single wavelength recording as it acts like a reduced LiNb03:Fe crystal.
We recorded a transmission geometry Fourier plane hologram of a two-dimensional data mask with 120 fJ,m x 120 fJ,m pixels in each case. Two-center recording in XTAL1 was performed by one sensitizing beam (wavelength 404 nm, intensity 3 mW/cm 2, homogeneous) and two recording beams (wavelength 514 nm, intensity of each beam at the crystal about 10 m W /cm2, ordinary polarization). Recording in XTAL2 was performed by the same two recording beams without any sensitizing beam. The diffraction efficiency of the recorded holograms in both cases was about 1%. Each hologram was then read by the corresponding reference beam, and the reconstructed image was monitored during read-out by a CCD camera. The signal-to-noise ratio (SNR) of the reconstructed hologram was then computed by measuring the intensities of the on and off pixels in the digitized image. Each bit on the data mask, a square measuring 120 microns on a side, imaged to occupy a square of roughly 14 x 14 CCD pixels. A 20 x 20 grid of these bits, from the center of the reconstructed image, was used for computing the SNR. The average of the pixel values were calculated for each bit, and the mean and standard deviation of these average pixel values were computed separately for the data bits which were supposed to be "on" and "off." The SNR was then calculated as:
( 4.20)
where m1 and mo are the mean values of the "on" and "off" pixels, respectively, while
0"1 and 0"0 are the standard deviations of the "on" and "off" pixels, respectively.
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Figure 4.9: Comparison of the variations of the signal to noise ratio (SNR) with time in normal and two-center recording. The details of the experiments are described in the text.
Figure 4.9 shows the loss in normalized signal to noise ratio (SNR) during read- out for the two cases. As Figure 4.9 shows, two-center recording suffers from fanning during read-out only in an initial period. The loss in SNR after that initial period is very slow as explained before. On the other hand, SNR in normal recording drops much faster and finally falls below the minimum acceptable value. Note that due to the loss in diffraction efficiency during read-out, we need to change the gain of the camera at different times. This results in the increase of the calculated SNR with time at a few data points that is not a physical effect.
Figure 4.10 shows a small portion of the reconstructed images at different times during continuous read-out in the two cases. It can be seen from Figure 4.10 that the quality of the reconstructed image in two-center recording after 7 hours of read-out is still comparable to the initial quality. However, the quality of the reconstructed image in normal recording is highly degraded after 80 minutes.
(a)
(b)
(c) (f)
Figure 4.10: Comparison of the evolution of the qualities ofreconstructed images with time in normal and two-center recording. The Figures (a), (b), and (c) show a portion of the reconstructed image in two-center recording at the end of recording (beginning of the read-out), after 40 minutes of continuous read-out, and after 420 minutes of continuous read-out, respectively. The Figures (d), (e), and (f) show a portion of the reconstructed image in normal recording at the end of recording (beginning of the read-out), after 45 minutes of continuous read-out, and after 80 minutes of continuous read-out, respectively. The details of the experiments are described in the text.
It can be concluded from Figures 4.9 and 4.10 that two-center holographic record- ing has an advantage over normal recording by suffering much less from fanning. It is important to note that the effect of fanning in two-center recording with red light would be even less due to the smaller sensitivity of holographic recording from Fe traps at red light.