The influence of pump energy on the dynamics of Brillouin fiber-optical ring laser by the CW
(continuous wave) DFB fiber laser was studied experimentally and theoretically. Using a 1Km-length
single model 10 μm-diameter fiber in our experimental regime, half-periodic, serial identical Stokes
pulses were found in the experiments when pump energy was low (less than 1.5 Pth ); then they became
quarter-periodic or even smaller periodic with further increase of pump energy; until the periodic
disappeared as pump power is great enough (more than 4Pth ), the output of Brillouin fiber-optical ring
laser is tend to give continuous wave. At mean time, the duration of Stoke pulses became narrow
gradually, and the peak power of Stoke pulses increased gradually with the increase of pump energy.
Based on coupled wave equations of SBS process, using a seed of loons, theoretical simulation of
temporal characteristics of pulses by SBS process in single model fiber has been given numerically and
computationally. By computer modulation of dynamics of Brillouin fiber-optical ring laser by the CW
(continuous wave) DFB fiber laser, the characteristic of periodic, pulse duration and peak power of
Stoke were analyzed. Using both pump exhausts effect and multilevel SBS nonlinear process, we tried
to explain some experimental phenomena.
Characteristics of the output pulse shape of stimulated Brillouin scattering optical limiting process induced by the seed field is numerically simulated. Rules of limiting pulse shape controlled by
the seed field are obtained as follows: When seed pulse duration is chosen to be five times as that of the pump pulse duration, and delay time is just the same as that of pump pulse duration, the best limited
temporal profile can be obtained. At the same time, it is affected by the pump power: as the pump power increases, it falls endlessly before it becomes zero. Study indicates that controlling of SBS optical limiting pulse shape can be achieved by using a seed pulse.
Theoretical simulation and experimental studies of optical limiting characteristics in thin stimulated Brillouin scattering (SBS) cell of CCl4 have been given. Temporal coupling equations of stimulated Brillouin scattering initiated by thermally excited acoustic wave's field-uniformity existing in thin SBS cell are used. In order to reveal optical limiting performance, several limiting parameters are discussed. The dependences of SBS limiting parameters on pump energy and length of thin SBS cell are studied numerically firstly. The results showed that non-focusing thin SBS cell can limit energy as high as several Joule magnitudes. Power and output energy limiting behaviors have been studied experimentally in our experimental regime. For 1cm thin cell, when laser pulses transmit through the SBS medium CCl4; the output power and energy are limited fainlyt: the output peak power is limited and pulse narrowed without evident power limiting in the following edge; as for 2cm and 4cm cell, both show evident power limiting "step" and energy limiting characteristics, when pump energy increases from 5mJ to 100mJ, the output energy is limited under 20mJ for 2cm SBS cell and 12mJ for 4 cm SBS cell, which are quite agree with theoretical calculations in low input energy level.
Theoretical simulations and experimental studies of power limiting effect in four Brillouin media have been given in this paper. One-dimensional coupling equations are used to stimulate the propagation of nonlinear laser pulse in four SBS media. The transmitted temporal pulse profiles through SBS medium with various pump energies are numerically simulated, then temporal pulse reshaping, power limiting dependent on SBS Gain of medium in four SBS media are investigated theoretically. By using Nd:YAG laser pulses with 20ns pulse duration and four Brillouin media such as CCl4, Fc-72, CS2 and acetone, Temporal profiles reshaping and power limiting characteristics are given experimentally. Those SBS limiting effects have characteristics as follows: CCl4, Fc-72 give higher limited power, CS2 and acetone give lower limited power. The control rule of power limiting dependent on Gain of medium is exhibited in theoretical calculations and experiments as follows: Smaller gain of medium system will give higher limited power, at mean time; higher gain of medium will give lower limited power. The above study makes SBS suitable to be applied in the protection and stabilization of high-power laser system.
In this paper, optical limiting effect based on stimulated Brillouin scattering (SBS) in a nonlinear medium has been demonstrated. First, nonlinear propagation has been numerically analyzed and computationally treated in the transient SBS progress. Optical limiting, stabilization and output pulses have been shown theoretically when high-power laser pulses enter a SBS medium. Experimental investigation of optical limiting performances predicted by theory has been conducted within a nonlinear medium CCl4. The input optical signa is Nd:YAG nanosecond laser pulse with width 2ns. The limiting characteristics depending on varied physical parameters such as focus, cell length of SBS medium CCl4 and separation between cell and lens have been investigated. The good pulse shape of envelope of the transmitted pulse train and a superior constant output energy have been obtained when the SBS occurs. The relative fluctuation for transmitted signal is about ± 7% while relative fluctuation for laser pulse is ± 13%.
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