Zhiwei Lu

Nonlinear propagation and optical limiting of Nd:YAG nanosecond laser pulses by stimulated Brillouin scattering

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%.

High pulse-shape fidelity in stimulated Brillouin scattering generator with seed injection

Though phase-conjugation fidelity of stimulated Brillouin scattering (SBS) has been widely investigated, seldom attention is paid to pulse-shape fidelity. However, some applications of SBS phase conjugate mirror (PCM) require nearly identical SBS pulse shapes with pump pulses, i.e. high pulse-shape fidelity. To avoid the pulse compression effect in common SBS PCM, a novel scheme with a Stokes seed injection into the SBS generator is suggested and investigated experimentally. Experimentally results show that SBS duration becomes wider with increasing of the seed intensity, but shorter focal length can get wider duration. Pulse-shape fidelities at points of the widest duration are calculated as about 92% and 73% with focal lengths 5cm and 10cm respectively. Compared with 40% and 48% of cases without the Stokes seed, it is showed that seed injection improves the shape fidelity.

New parametric emissions in diatomic sodium molecules

By excitation of sodium vapor with an ultra-violet laser, tunable coherent emissions around 688 and 770 nm generated from diatomic sodium molecules have been observed for the first time. We demonstrate that the emission around 688 nm is due to an axially phase-matched four-wave-mixing process involving cascade emissions of ≈2.54 µm and ≈910 nm radiation from sodium dimers.

Temporal profiles reshaping and power limiting characteristics of nanosecond laser pulses in four-Brillouin media

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.

Power and energy limiting of nanosecond laser pulses by thin stimulated Brillouin scattering cell

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 waves 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.

The influence of seed on optical limiting pulse reshaping in stimulated Brillouin scattering process

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.

Investigation on the stimulated Brillouin scattering threshold with broadband KrF laser

A 1.5m long cell, different focusing lens, a KrF laser which bandwidth is 0.5cm-1, and SF6 gas as Brillouin medium are used in the experiment. The influences of focus length and SF6 pressure on threshold of stimulated Brillouin scattering excited by broadband KrF laser were investigated, respectively. The SBS threshold increases with increasing of the focus length and decreasing of the SF6 pressure. The results show that the threshold of broadband SBS is various with the different focus length and SF6 pressure. For more precisely to explain the experimental results, a novel model considering optical breakdown and pump linewidth was presented. The model we proposed here assumes that the KrF laser spectrum is made up of a seriues of narrowband spectra, each generating one Stokes wave. In addition, it is assumed further that the neighbors of one narrowband spectrum also contribute to the generation of its corresponding Stokes wave to some degree. Optical breakdown is assumed to lead to the loss of pumping energy only. The experimental results are consistent with the numerical theory.

Investigation on stimulated Brillouin scattering pumped by broadband KrF laser

A 1.5m long cell, different focusing lens, a KrF laser which bandwidth is 0.5cm-1, and SF6 gas as Brillouin medium are used in the experiment. The influences of SF6 pressure on threshold, energy reflectivity and pulse compression of stimulated Brillouin scattering excited by broadband KrF laser were investigated, respectively. For more precisely to explain the experimental results, a novel model considering optical breakdown and pump linewidth was presented. Optical breakdown is assumed to lead to the loss of pumping energy only. The experimental results are consistent with the numerical theory.

Generation of high-quality pulses with controllable pulse shape by one-cell SBS phase-conjugating mirror

The influence of configuration parameters in one-cell system on pulse shape of stimulated Brillouin scattering was investigated experimentally. Stokes pulse duration was easily changed from 1.5 ns to 8 ns by varying the depth of focusing. Stokes pulse shape was numerically simulated by a theory model of stimulated Brillouin scattering initiated by thermally excited acoustic waves. The theoretical calculation was well coincident with experimental results.

Generation of high-quality tunable-duration pulses by two-cell SBS phase-conjugating mirror

The pulse compression of two-cell SBS system in CCl4 liquid was experimentally investigated. The duration of Stokes pulse was tunable by simply changing the separation between two cells and the tuning range depended on the pump energy. By increasing the input energy, the wider tuning range of pulsewidth could be achieved. The near-diffraction limited output pulses have achieved and the shot to shot stability on far-field divergence angle was also measured. The numerical simulation on pulse shape of two-cell SBS system was made and agreed with the experimental results well.