THEORETICAL ANALYSIS

When the input power density exceeds the SBS threshold, a strong SBS effect within Brillouin medium occurs, thus leading to a rapid energy transfer from input to the stokes light, and correspondingly a optical limiting trait in the transmitted light. Optical limiting based on SBS exhibits the characteristics of high optical limiting threshold, fast response speed and high damage threshold, thus especially suitable for safe protection of high power laser system. Both nonlinear material optical limiting and SBS optical limiting have similarly experimental conditions, namely, a beam of input light is focused into nonlinear material sample or Brillouin medium. The solvent of nonlinear material can be used as Brillouin medium to generate optical limiting due to SBS, so a compound optical limiting based on the combination of nonlinear material and SBS optical limiting can be prepared through making nonlinear material dissolved into Brillouin medium. With the double effects of both nonlinear materials and SBS, compound optical limiting presents much better optical limiting performance compared with single SBS.

The optical limiting threshold of nonlinear optical material (several μJ) is usually lower than that of SBS (hundreds of μJ to several mJ), however, some nonlinear optical limiting materials and their derivatives, such as fullerenes, MPC, metallic porphyrin complexes and nano materials, have high optical limiting threshold that is close match to the SBS threshold, thereby they can combine with SBS optical limiting to form a compound optical limiting.

Danilo et al. (Reference Danilo, Guo and Michael2003) reports that heavy MPC exhibits good optical limiting performance in input wavelength of 532 nm, so F₁₆PCCu is dissolved into acetone to prepare a compound optical limiting in this paper. Optical limiting mechanism of MPC mainly results from reverse saturable absorption (RSA), namely, absorption coefficient increases with the increasing input intensity. RSA usually takes place in the region of non-resonant or near-resonant of linear absorption spectrum (the valley value region of linear absorption spectrum of nonlinear material), and thus presenting a high linear transmissivity. At a low input intensity, the absorption of MPC mainly comes from the linear absorption of the ground state; in contrast, the excited singlet-singlet and the triplet-triplet state absorption of the MPC can be induced in the case of a high input intensity; therefore, the absorption coefficient of the MPC increases with the increasing input intensity.

According to the SBS optical limiting model (Lu et al., Reference Lu, Lu and Yang2003) and RSA model (Blau et al., Reference Blau, Byrne and Dennis1985), we numerically simulate the dependence of output energy of optical limiting based on single SBS and compound optical limiting scheme on input energy and the simulation results are shown in Figure 1. The parameters used in simulation are as follows: the input wavelength is 532 nm with a pulse width of 8 ns and divergence angle of 1.29 mrad. The Brilloun cell length is 10 cm and the focal length of the lens for it is 30 cm. The ratio of absorption cross section of excited state to ground state of nonlinear material is K = 12.5 and the linear transmissivity of the sample is T ₀ = 0.8.

Fig. 1. The simulation curves of the dependence of output energy on the input energy based on single SBS and compound optical limiting.

It can be seen from Figure 1 that the output clamp value of compound optical limiting (about 0.3 mJ) is lower than that based on single SBS optical limiting (about 0.6 mJ). In addition, the output energy of compound optical limiting shows much better optical limiting performance compared with that based on single SBS. The slope is used to denote the flat index of output energy of optical limiting, which is defined as the slope value of the straight line given by linear fit of output energy over the optical limiting threshold. Obviously, a small slope means flat output energy of optical limiting. According to the simulation results, the slope is 0.07 for single SBS and 0.03 for compound optical limiting, respectively.

EXPERIMENTAL INVESTIGATION

The experimental setup is shown in Figure 2. The Continuum's Nd:YAG seed-injected laser outputs s-polarized light, which becomes p-polarized after passing the 1/2 wave plate and circular polarized after passing the 1/4 wave plate and then injected into the SBS system. The SBS system is composed of lens 1 and SBS cell. The input light is focused into SBS cell to produce stokes light. Polarizer P together with a 1/4 wave plate forms a light isolator, preventing backward SBS light from entering the YAG oscillator. The Stokes light becomes s-polarized after passing the 1/4 wave plate, and then is reflected by polarizer P. The divergence beam after SBS optical limiting becomes parallel after passing the lens 2. The input energy is altered by adjusting the 1/2 wave plate in the experiment. The energies of input pulse, transmitted pulse and Stokes pulse are measured with energy meter MIN-E1000.

Fig. 2. Experiment setup.

In experiment, the output wavelength of Continuum Nd:YAG seed-injected laser is 532 nm with a repetition rate of 1 Hz, pulse duration 8 ns and divergence angle 1.29 mrad. The SBS medium is prepared by making F₁₆PCCu dissolved into acetone. The SBS parameters of acetone are listed in Table 1 (Jones, Reference Jones1997). The procedure in preparing the F₁₆PCCu/acetone solution is as follows: the F₁₆PCCu is first put into acetone and then oscillated for 24 h in the ultrasonic oscillator. The concentration of sample is 1.25 × 10⁻³ mg/ml. The focal length is 30 cm for lens 1 and lens 2, respectively, the Brillouin cell length is 10 cm.

Table 1. SBS parameters of acetone

Figure 3 gives the experimental curves of output energy versus input energy based on single SBS optical limiting and compound optical limiting, which shows similarly trend with the simulation results. It can be seen that the compound optical limiting shows a lower output clamp value and flatter output energy compared with that based on single SBS. In experiment, the output clamp value and slope of compound optical limiting is 0.3 mJ and 0.06 mJ, while the output clamp value and slope based on single SBS optical limiting is 0.5 mJ and 0.11 mJ, respectively. By comparing Figure 1 and Figure 3, we can find that there is some deviation between them. This is because that, in theory simulation, we did not consider the effects of beam quality, optical components and other nonlinear effects on optical limiting, but in experimental studies these factors have a certain influence on the results, which would lead to the deviation between theoretical simulation and experimental results. However, their changing trends will be same.

Fig. 3. The experimental results of the dependence of output energy on the input energy based on single SBS and compound optical limiting.

CHARACTERISTICS OF COMPOUND OPTICAL LIMITING

The proposed scheme in this paper is not only to present the characteristics of low output clamp value and flat output energy, but also has some excellent advantages, such as widely limiting energy range, broad limiting bandwidth, fast response speed, high limiting threshold, and damage threshold.

Optical limiting based on nonlinear material is more suitable for working in the case of a low input energy (the order μJ to mJ), while SBS optical limiting can produce good optical limiting effect in a high input energy (the order mJ to J) (Hasi et al., Reference Hasi, Lu, Gong, Li, Lin and He2008), so compound optical limiting can work in a low input energy as well as a high input energy. For example, when there is a low input energy, nonlinear material optical limiting plays a important role in the compound optical limiting, while when input energy is high, compound optical limiting mainly results from SBS optical limiting effect, furthermore, when input energy is at the same energy limiting range of nonlinear material optical limiting and SBS optical limiting, two kinds of optical limiting effect can work simultaneously, therefore, compound optical limiting exhibits the characteristic of a wide range of energy limiting.

SBS optical limiting can work in a waveband range from ultraviolet to infrared (Yoshida et al., Reference Yoshida, Fujita, Nakatsuka, Ueda and Fujinoki2007), so compound optical limiting with different wavebands can be prepared by the combination of SBS and nonlinear material optical limiting with different wavebands. For instance, an ultraviolet compound optical limiting can be made by making nonlinear materials with ultraviolet waveband dissolved into SBS medium, similarly, an infrared compound optical limiting can be prepared by making nonlinear materials with infrared waveband dissolved into SBS medium, thus the compound method presents a feature of broad limiting waveband.

Both SBS optical limiting and some nonlinear material optical limiting containing large π-conjugate system exhibits the characteristics of fast response speed, high limiting threshold and damage threshold (Liu et al., Reference Liu, Zhang, Hu, Guo, Ye, Dai and Zhu2002), therefore, the compound optical limiting based on the combination of SBS and those nonlinear materials also presents the same characteristics. Currently, we are carrying out some research related to these specific characteristics, and the details will be reported in another article.