1.

Introduction

Volume holograms are capitalized in optical tomography,¹ holographic data storage² and optical communication³ due to its wavelength and wavefront selectivity. Although in the last decade, various photopolymers have been invented for volume holography, photorefractive $\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_3$ crystals are still wildly studied because they are rewritable, do not shrink during exposure, and are easy to be grown thicker. However, there are still some shortcomings in the $\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_3$ crystals. Higher storage capacity and higher recording speed are expected in holographic data storage. In most practical applications, nonvolatile recording is also required. Accordingly, various fixing methods, such as thermal fixing,^{4, 5} electrical fixing,⁶ and two-center fixing,⁷ are proposed. Most of them, however, are still impractical in a commercial system because of their complicated processes or low diffraction efficiency. On the other hand, some novel materials, such as periodic poled $\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_3$ ,^{8, 9} stoichiometric $\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_3$ ,^{10, 11} and doubly doped $\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_3$ ,^{7, 12} have been proposed to improve the holographic abilities of crystals. Although there are specific advantages by using these novel materials, the global holographic properties of these materials are always worse than those of general iron-doped $\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_3$ . In this letter, we propose another simple method to record holographic gratings in a nominally iron-doped $\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_3$ crystal. In our demonstration, the storage capacity and sensitivity are remarkably improved by the proposed method. Because the proposed method only requires an additional low-coherence, short-wavelength light source for exciting, it is easy to be achieved in most of present holographic systems.

2.

Experimental Setup

The setup for two-color holographic recording is shown schematically in Fig. 1. We used a He–Ne laser with wavelength $633\mathrm{nm}$ as the light source for interference. The laser beam was collimated and divided into two, the signal beam and the reference beam, by a polarizing beamsplitter. The two beams with polarization state normal to the tabletop crossed each other to form an interference fringe pattern in a photorefractive $\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_3$ : Fe crystal. The size of the crystal to be used is $10\times 10\times 2{\mathrm{mm}}^3$ , and the doping concentration is 0.1%. The optic axis ( $c$ -axis) of the crystal is parallel to the tabletop, as shown in Fig. 1. Thus, the interference beams are ordinarily polarized in the crystal. Finally, a diode-pumped solid state (DPSS) laser with wavelength $532\mathrm{nm}$ was used for carrier exciting. The exciting laser beam was also collimated to illuminate the crystal surface uniformly.

Fig. 1

Schematic of experimental setup for two-color recording.

In the recording process of two-color holographic recording, the shutter (SH) in Fig. 1 was opened so that both the signal beam and the reference beam reached the crystal to interfere. In the crystal plane, the intensity of each interference beam is $15\mathrm{mW}∕{\mathrm{cm}}^2$ , and the intensity of the exciting light beam varied from $0.2\text{to}2\mathrm{mW}∕{\mathrm{cm}}^2$ in different attempts. During the recording, the diffracted light power was measured by a photodetector at intervals of $1\min$ . It is noted that the shutter in Fig. 1 was closed as the photodetector was taking data for avoiding the disturbance of directly transmitting signal light. In the retrieving process, the shutter was always closed, and the intensity of the reference beam doubled to simulate the illumination due to another recoding. Finally, we also performed one-color holographic recording in the same geometry by a single He–Ne laser and by a single DPSS laser, respectively, for comparison. The total intensity of He–Ne laser and DPSS laser are respectively 30 and $2.9\mathrm{mW}∕{\mathrm{cm}}^2$ .

3.

Experimental Results

After the relationship between diffraction efficiency and the recording (retrieving) time was obtained, the corresponding holographic parameters were found as well. $M$ number $\left(M\right)$ is a parameter that is directly relative to the storage capacity of a holographic system.¹³ The system with larger $M$ will record more holograms with the same diffraction efficiency or will record the same number of holograms with higher diffraction efficiency. $M$ can be easily found by^{2, 13}

Fig. 2

Relationship of $M$ and $I_{\mathrm{R}}∕I_{\mathrm{E}}$ ratio.

Besides $M$ , sensitivity $\left(S\right)$ is also an important parameter that reflects the energy requirement for a holographic system. A system with higher sensitivity will record a hologram faster, which is expected in most applications. Sensitivity can be found by²

Fig. 3

Relationship of $S$ and $I_{\mathrm{R}}∕I_{\mathrm{E}}$ ratio.

4.

Discussion

Although two light sources are involved in the proposed two-color holographic recording, it differs from the two-center nonvolatile recording because no nonvolatile effect has been observed. To interpret the experimental results, charge migration and interference visibility should be taken into account. For general one-color holographic recording in a photorefractive $\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_3$ crystal, charges are excited from impurity levels due to doping by the interference light. These excited charges move in the conduction band via diffusion and photovoltaic effect, and are trapped some time later. In quasi-steady state, the charges being resident in the dark region of the crystal are more than those in the bright region. Finally, the space-charge field due to nonuniform space charges results in a modulation of the refractive index via Pockels effect. In two-color holographic recording, the whole process is similar except for the additional exciting light. The exciting light excites more charges from the impurity levels in the first step of photorefractive effect. Therefore, either the formation speed or the dynamic range of the photorefractive gratings becomes larger. In our demonstration, the absorption coefficients of the crystal at 633 and $532\mathrm{nm}$ are 0.786 and $5.96{\mathrm{cm}}^{-1}$ , respectively. Thus, there is a substantial difference between the exciting efficiencies of the two wavelengths. This helpful effect can be easily found, as shown in the right side of experimental curves in Figs. 2 and 3 $(I_{\mathrm{R}}∕I_{\mathrm{E}}>40)$ . On the other hand, the exciting light is uniform so that it cannot help to generate nonuniform distribution of charges. Thus, the grating strength as well as the fringe visibility will deteriorate if too much exciting light is added. This effect is the reason for the drooping of $M$ in the left side in Figs. 2 and 3 $(I_{\mathrm{R}}∕I_{\mathrm{E}}<40)$ .

In addition to above mechanisms, the exciting light is also associated with the effect of incoherent erasure. In previous research, it was found that suitable incoherent light or ordinary light will help the formation of phase grating in a $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3$ crystal.^{14, 15} It is reasoned that the incoherent light will suppress the fanning light (the source of optical noise) so that the principal gratings form without disturbance. In our experiment, the exciting light and the interference light are mutually incoherent. Thus, the exciting light will suppress the fanning light due to the interference light. This effect will affect the crystal performance in steady state, resulting in better maximum diffraction efficiency. However, the effect should not be the only mechanism in two-color recording because it cannot enhance the sensitivity, which only relates to the transient response in the beginning of recording.

For comparison, we listed the performance characteristics of two and one-color recordings in Table 1. Because the holographic performance of photorefractive crystals depends on the illuminating intensity, the total intensity for one-color recoding is similar to that used in two-color recording, as described in Section 2. According to Table 1, all the performance characteristics of two-color recording are better than those of one-color recording by He–Ne laser. On the other hand, the performance characteristics of two-color recording are generally better than those of one-color recording by DPSS laser because the intensity of the DPSS laser is low. However, the sensitivity of the latter is about 1.5 times higher than that of the former, which reflects the high absorption coefficient at $532\mathrm{nm}$ . Finally, in either one- or two-color recording, the performance characteristics depend on the intensity of interference light. Therefore, the enhancement effect achieved by applying two-color recording will become insignificant in saturation interference intensity.

Table 1

Performance characteristics of one- and two-color recordings.

Conclusion

In summary, we proposed a two-color holographic recording method to improve the holographic properties of a single-doped $\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_3$ crystal. In our demonstrations, a light with wavelength $633\mathrm{nm}$ is used for interference and a light with wavelength $532\mathrm{nm}$ is used for exciting. In the optimized intensity ratio, the $M$ and the sensitivity are enhanced by 43 and 35%, respectively. Because the crystal we used is a typical iron-doped $\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_3$ crystal, the proposed method is easy to be achieved in most holographic systems. Moreover, an exciting light source in two-color recording can be low power and low coherence, such as an inexpensive light-emitting diode. Therefore, the proposed method provides a good balance between the system performance and the cost.