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A method of generating flat-top waveform by double optical limiting based on stimulated Brillouin scattering (SBS) is proposed. The waveforms are numerically simulated by the theoretic model of double optical limiting based on SBS, and the experimental results are in good agreement with the theoretical simulations. There is a peak in the front and a platform in the back of the waveform by single optical limiting, while the top is almost a platform by double optical limiting.
©2006 Optical Society of America
1. Introduction
In high power laser system, the output power density is usually so high that kinds of optical materials, such as optical crystal, lens, and optical film, are easily damaged. Therefore, providing security protection to the laser system becomes necessary. In addition, the laser system is generally operated under rated power, so the efficiency of laser system can be very low. If the optical protection is proper, the laser system can work in comparatively high power in order to improve the efficiency of laser system. The optical limiter can provide good protections. For high power laser system, the optical limiting based on SBS process (the transmission light after SBS process) is investigated in reference [1]. Although the optical limiting effect has been showed in single SBS process, the front of transmission pulse shape has a peak, and the back is flat. Apparently, the peak in the front of pulse shape does great harm to the optical limiting. To solve this problem, a method of generating flat-top waveform by double optical limiting based on SBS process is proposed. As a result, the peak in the front of transmission pulse shape after the first SBS process is eliminated by optical limiting effect in second SBS process, and the flat-top waveform is generated ultimately.
Besides the application in high power laser system, the flat-top waveform has been applied extensively, such as laser material processing (for example, laser welding and laser boring), laser clinic medicine (for example, excimer laser myopia therapy), laser offset, detecting array laser radar, laser scanning, optical information processing, storage and record, etc. T. Kanabe obtained flat-top waveform in the time domain by pulse stacking method [2]. In this paper, a laser pulse with a flat-top waveform in the time domain is obtained by optical limiting based on stimulated Brillouin scattering (SBS).
2. Theory simulation
Based on SBS optical limiting model [1], the transmission waveform by double SBS optical limiting is simulated numerically. For the transmission waveform after first SBS process, there is a peak in the front and a platform in the back, as shown in Fig. 1(b). When the front edge of pump pulse is entering the medium and the power is not strong enough to produce SBS, the front edge of pump pulse is transmitted completely, at this time the front edge of transmitted pulse is the same as that of pump pulse, showing as quasi-Gauss profile. When the power of pump pulse exceeds the threshold of SBS, the building of photon field needs some relaxing time, so the pump still keeps high transmissivity. At this time the front edge of transmitted pulse is similar with that of pump pulse, despite a little distortion owing to the SBS. When the power of pump pulse far exceeds the threshold of SBS, the strong SBS process takes place, leading to the energy transferring from pump to Stokes quickly. The Stokes light rises rapidly, so the pump energy is exhausted rapidly, and the back edge of transmitted pulse starts to descend suddenly, leaving a steep decline. Continuously, the back edge of transmitted pulse maintains a platform because the power of following portion is below the threshold of SBS,.
Fig. 1. the numerical simulation curve of transmitted pulse shape in SBS optical limiting process (a) pump pulse (b) transmitted pulse after first optical limiting (c) transmitted pulse after second optical limiting
The transmitted pulse after second SBS optical limiting is almost flat-top, as shown in Fig. 1(c). In the pulse by double SBS optical limiting, only the peak achieves the threshold of SBS, and the pump energy transfers to the Stokes light quickly, so the peak is removed. The power of original platform can not reach the threshold of SBS, still keeping original state, so the whole pulse becomes a platform. The fluctuation of the top is caused by modulation phenomenon [3].
Through theoretical simulation, it is known that, the pump energy together with the structure parameters and medium parameters have an effect on waveform and energy of transmitted pulse by double SBS optical limiting. Reference [4] shows the influence of structure parameters on the characteristic of SBS optical limiting. The results show that, the energy and waveform of transmitted pulse can be adjusted by the focal length. In order to generate high energy optical limiting pulse with a flat back edge, longer lens should be chosen in the structure of first SBS optical limiting. In order to eliminate the peak in the front edge, shorter lens should be chosen in the structure of second SBS optical limiting. Reference [5] shows the influence of medium parameters on the characteristic of SBS. The results show that, the smaller the gain coefficient and the absorption coefficient of medium are, the higher the transmitted energy is. In order to generate comparatively high transmitted energy of optical limiting, the medium with small gain coefficient and small absorption coefficient should be chosen in the first generator cell. In order to eliminate the peak in the front edge, the medium with large gain coefficient and small absorption coefficient should be chosen in the second generator cell.
3. Experimental investigations
The experimental setup is shown in Fig. 2. The Nd:YAG Q-switched laser contains a 100% reflective mirror M1, a Q-switch dye plate, a polarizer P1, an Nd:YAG rod, a small hole, and a partial reflective mirror M2. Polarizer P2 is parallel to P1, which forms a light isolator together with a 1/4 wave plate, in order to prevent backward SBS light from YAG oscillator. The oscillator outputs p-polarized light, which becomes circular polarized when passing the 1/4 wave plate. The double SBS optical limiting system comprises two generator cells 1 and 2, two focus lens L1 and L2. Firstly the pump light is focused into generator cell 1 and produces first SBS; then the transmitted light is focused into generator cell 2 and produces second SBS. The two Stokes lights both become s-polarized light after passing the 1/4 wave plate, and are reflected by polarizer P2. The energies of pump light, transmitted light and SBS light are measured by energy meter ED200. The pulse shape is detected by PIN photodiode, and recorded by digital oscillograph TDS684A.
The output wavelength of Nd:YAG Q-switched laser is 1.064 µm, with repetition rate 1 Hz, pulse width 10 ns, maximum energy 50 mJ, divergence angle 1.6 mrad. The variation of incident energy is realized by inserting attenuator. The length of first generator cell is 60 cm, focal length of lens L1 is 30 cm, medium is FC-72 [6]. The length of second generator cell is 30 cm, focal length of lens L2 is 7.5 cm, medium is CS2. The SBS parameters of above media are listed in table 1.
Table 1. the SBS parameters of two media
The experimental pulse shape of transmitted light by double SBS optical limiting is shown in Figs. 3(b) and 3(c). For the transmission waveform by first SBS optical limiting, there is a peak in the front and a platform in the back. The transmission waveform by double SBS optical limiting is nearly flat-top. The experimental results are consistent with the theoretical results. Because the transmitted energy of SBS optical limiting is decided by system exponential gain coefficient (G=gIL, G is system exponential gain coefficient, g is gain coefficient of medium, L is effective interaction length), the transmitted energy of SBS optical limiting is controlled by changing the medium parameter and structure parameter. In the experiment, when pump energy is 30 mJ, the transmitted energy is 18 mJ after first SBS optical limiting and 10 mJ after second SBS optical limiting.
Fig. 3. the experimental pulse shape of SBS optical limiting (a) pump waveform (b) transmission waveform after first SBS optical limiting (c) transmission waveform after second SBS optical limiting
4. Analyses and discuss
In the experiment, the front peak can be easily eliminated by three methods: increasing incident energy, choosing medium with high gain coefficient or using lens with short focal length. The three methods can lead the enhancement of the exponential gain coefficient, therefore, the building of photon field advances and the front peak is eliminated. Thus, the flat-top waveform can be easily generated. For example, the peak appears when the focal length of lens L2 is 7.5 cm, as shown in Fig. 4(a); the peak disappears when the focal length of lens L2 is 5 cm, as shown in Fig. 4(b).
Fig. 4. The transmission waveform of second SBS optical limiting, with focal length (a) f=7.5 cm and (b) f=5 cm, respectively.
It is found in the experiments, the flat-top waveform can not be generated by single SBS optical limiting. The reason is that, when a lens with long focal length is used, the power density near the focus is comparatively low, and then the photon field builds late, so the peak of transmission waveform appears, as shown in Fig. 5(a). On the other hand, when a lens with short focal length is used, the power density near the focus is comparatively high, and the photon field builds early, so the peak of transmission waveform disappears. However, the effective interaction length of pump and Stokes becomes short, so flat-top does not appear in the back of transmission pulse, as shown in Fig. 5(b).
Fig. 5. The transmission waveform of first SBS optical limiting, with focal length (a) f=30 cm and (b) f=5 cm, respectively
5. Conclusions
A method of generating flat-top waveform by double optical limiting based on stimulated Brillouin scattering is proposed. The feasibility of this method is investigated in theory and experiment. For the transmission waveform of first SBS optical limiting, there is a peak in the front and a platform in the back. The transmission waveform of second SBS optical limiting is nearly flat-top. The generation of flat-top waveform could be designed for the application of SBS optical limiting in high power laser system.
Acknowledgments
This work is supported by National Natural Science Foundation of China (Grant No. 10476009, 60478020).
References and links
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