Investigation On Control And Improvement Of Sbs Optical Limiter

For Laser, especially large laser system with high power, it's important to work expeditiously, safely and stably. Studying for a method of optical limiting which can work at high power and features quick response time and high transmittance is crucial for high power laser system.A method of optical limiting based on stimulated Brillouin scattering (SBS) is proposed in this paper. We investigated the characters of energy limiting of SBS and found that there are several merits as follows. Firstly, SBS has a high power threshold, and the transmittance is high under the threshold. When the intensity of the incident light excesses the SBS threshold, the output energy will be clamped at a value. Secondly, the clamped value of output energy can be controlled by changing the medium or the focal length. Thirdly, the damage threshold is higher. Fourthly, it features sub-nanosecond response time. To sum up, it's hopefully for optical limiter of SBS to provide protection for the high-power laser system.Firstly, we found that the clamped value of output energy in the SBS optical limiter can be controlled by choosing medium with different gain coefficient and lens with different focal length, which presents great convenience in terms of practical application of SBS optical limiter. For example, if a high clamped value of output energy is preferred, medium with small gain coefficient or lens with a long focal length can be chosen; in contrast, medium with a large gain coefficient or lens with a short focal length can be used if a low clamped value of output energy is required.Secondly, although the energy limiting can be readily realized by SBS optical limiting, the energy limiting performance is not so satisfactory due to the insufficient energy transfer from the pump to the Stokes, thus presenting a disadvantage to practical application of SBS optical limiter. To solve the problem, several methods of improving the optical limiting performance based on SBS are proposed. One is based on double SBS; the other is based on a combination of SBS and optical limiting materials for optical limiting. The method based on double SBS, namely there is another SBS system behind the first one. When the input power density is low, the output energy of optical limiting based on double SBS shows similar performance with that based on single SBS. As the pump energy increases further, the second SBS system is activated. The output energy of optical limiting based on double SBS shows much better optical limiting performance than that based on single SBS. The method based on hybrid approach, namely there is a kind of optical limiting materials behind the SBS system. In the case of a low pump energy, only the SBS optical limiting works. However, as the pump energy increases, the material limiting process can be activated if the transmitted energy of SBS exceeds the threshold of the optical limiting material. Therefore, this hybrid optical limiting approach has an improved performance compared with that based on single SBS in terms of the output energy.