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A fast Brillouin optical time-domain reflectometry (BOTDR) for dynamic strain measurement is proposed and experimentally demonstrated based on the frequency-agile technique. Employing the band-pass filter and envelope detection, the spontaneous Brillouin gain spectrum can be online demodulated in the time domain for a truly distributed, one-end-access and dynamic strain measurement. A strain sampling rate of 62.5 Hz is achieved for the proposed sensor over 172 m single-mode fiber with 100 sweeping frequencies and 64 times of averaging. In the experiment, the vibration frequencies of 6.82 Hz and 14.77 Hz have been measured with 2 m spatial resolution and 4000 μϵ dynamic range. The measurement accuracy is calculated to be ±30 μϵ by the standard deviation of the static fiber Brillouin frequency shift. Furthermore, the performance on the strain sampling rate and measurement accuracy is investigated with different averaging times. It should be noted that the upgraded schemes for the frequency-agile based Brillouin optical time-domain analysis can be also introduced into the proposed fast BOTDR.

Stimulated rotational Raman scattering (SRRS) limits the effective transmission distances of the high-energy and high-power laser pulses in laser–fusion systems and other applications. A simple and practical method of suppressing SRRS process by controlling the polarization direction of Stokes light is proposed. For a narrowband, linearly polarized, flat-topped laser pulse of 351 nm with intensity of 2 GW/cm2 and width of 3 ns, the SRRS threshold distance in air is lengthened to 30.0 m from 16.2 m easily using the method. Simulation results demonstrate that the method is also applicable for broadband laser.

It is necessary for high power ultraviolet lasers to propagate through a long air path during experiments of inertial confinement fusion (ICF). When the product of gain efficiency and propagation distance is beyond the stimulated rotational Raman scattering (SRRS) threshold, the high power ultraviolet laser beam is subject to loss of energy and a decrease in beam quality, further resulting in the reduction of conversion efficiency of triple-harmonic, and even causing the optical components to be destroyed. This paper studies the four-dimension numerical modeling of SRRS effects, with emphasis on the variation of beam aperture and corresponding beam distributions in near field. The research showed that the variation of the pump beam aperture made little effect on SRRS transform efficiency ratio with the same incident energy and the same propagation distance, the transverse intensity distribution of beams became more even with an increase in the beam aperture. We have made the deduction that high power laser's SRRS effects through a long air path could be depressed by increasing the beam aperture on the condition of equal incident energy and propagation distance.

The effect of smoothing by spectral dispersion (SSD) on stimulated rotational Raman scattering (SRRS) in air has been investigated both numerically and theoretically. The suppression effect of SSD on SRRS process is verified and it is demonstrated and proposed that the suppression effect is attributed to two aspects: the decreasing of the laser fluence modulation degree and the reducing of Stokes gain coefficient caused by the temporal and spatial variation of the phase of the incident laser pulse. The simulation results show that the SRRS threshold distance can be lengthened by choosing appropriate SSD parameters.

A prominent application of stimulated Brillouin scattering (SBS) is pulse sequence compression for obtaining high power and high intensity optical pulse. The characteristics and the stability of the compressed Stokes pulse were investigated using compact two-cell SBS phase-conjugation mirror. FC-770, having low absorption and high breakdown threshold at the wavelength of 1064 nm, was used as the SBS medium. Both the Stokes pulse width and the reflectivity were measured as a function of incident pump energy. In our work, a 8 ns laser pulse of 1064 nm generated by Q-switched Nd: YAG was optimally compressed to 116 ps for pump energies of 310 mJ, while the energy reflectivity is above 85%. The shortest pulses width obtained exhibit 99 ps duration. The stability of the temporal pulse widths after compression was investigated statistically. For non-optimal pump energies pulse widths become larger and the stability is found to decrease.

This paper demonstrates that the stimulated rotational Raman scattering (SRRS) threshold for high peak power laser pulses propagating through a long air path can be influenced strongly by the near-field quality of the laser beams, and the relationship between the SRRS threshold and the near-field beam quality (i.e., spatial intensity modulation index and contrast ratio) can be evaluated quantitatively. By using our three-dimensional numerical model, which can describe the spatial–temporal evolution behaviors of SRRS and is verified by previously published SRRS experimental data, the criteria of the safe transmission distance for high peak power nanosecond laser pulses are obtained, and the modified SRRS threshold criterion formulas considering the near-field beam conditions are presented.

In this paper, we designed an experiment to research the properties for the stimulated Brillouin scattering (SBS) medium of perfluorocarbon-compounds (PFCs) and perfluoropolyether (PFPE), then we proposed that the selection principle of the high load capacity SBS medium can be used in the high peak power SBS system. The results showed that, for PFCs, perfluorinated hydrocarbons (FC-72) has the highest optical breakdown threshold (OBT); for PFPE, the medium with average molecular weight (AMW) less than 1000 has small medium absorption coefficient (AC) and high OBT, for AMW, greater than 1000, the medium AC becomes high and the OBT becomes low. Further research shows that, for PFPE series SBS medium, the AC increases and the OBT decreases gradually with increasing of AMW. We find some SBS mediums can work stably under high peak power pump, which lays a good foundation for the application of SBS technology in a high-power laser system.

The current trends in stimulated Brillouin scattering and optical phase conjugation are overviewed. This report is formed by the selected papers presented in the “Fifth International Workshop on stimulated Brillouin scattering and phase conjugation 2010” in Japan. The nonlinear properties of phase conjugation based on stimulated Brillouin scattering and photo-refraction can compensate phase distortions in the high power laser systems, and they will also open up potentially novel laser technologies, e.g., phase stabilization, beam combination, pulse compression, ultrafast pulse shaping, and arbitrary waveform generation.

The effect of medium temperature upon characteristic of stimulated Brillouin scattering (SBS) and SBS optical limiting is investigated. The physical mechanism behind is analyzed theoretically and experimentally verified in Continuum's Nd: YAG Q-switched laser system using FC-72 as the SBS medium. The temperature affects the electrostrictive coefficient, refractive index, density and acoustic velocity of the medium weakly. In contrast, the kinematic viscosity, which is inversely proportional to the temperature, is related to gain coefficient and phonon lifetime and thus greatly affects the SBS characteristics. Therefore, in the low temperature, the kinematic viscosity is usually high, which can lead to a small gain coefficient and a short phonon lifetime. Therefore, the SBS characteristic can be changed by controlling the temperature to a great extent.

In order to improve the optical limiting performance based on stimulated Brillouin scattering (SBS), a combination of SBS and carbon nanotube/HT-270 suspension for optical limiting is proposed in this paper. The dependence of output energy of optical limiting based on this hybrid approach on the pump energy is numerically simulated, and validated in Continuum's Nd: YAG seed-injected laser. The results indicate that the output energy based on this hybrid approach shows much better performance compared with that based on single SBS. In this approach, the SBS threshold can be controlled by adjusting the gain coefficient of medium and focal length of the lens, and the threshold of suspension can be controlled by altering the concentration and focal length of the lens, therefore, this hybrid approach to realize optical limiting has great potential in practical application.

An overview on current trends in stimulated Brillouin scattering and optical phase conjugation is given. This report is based on the results of the “Second International Workshop on stimulated Brillouin scattering and phase conjugation” held in Potsdam/Germany in September 2007. The properties of stimulated Brillouin scattering are presented for the compensation of phase distortions in combination with novel laser technology like ceramics materials but also for e.g., phase stabilization, beam combination, and slow light. Photorefractive nonlinear mirrors and resonant refractive index gratings are addressed as phase conjugating mirrors in addition.

A 100-J-level Nd:glass laser system in nanosecond-scale pulse width has been constructed to perform as a standard source of high-fluence-laser science experiments. The laser system, operating with typical pulse durations of 3–5 ns and beam diameter 60 mm, employs a sequence of successive rod amplifiers to achieve 100-J-level energy at 1053 nm at 3 ns. The frequency conversion can provide energy of 50-J level at 351 nm. In addition to the high stability of the energy output, the most valuable of the laser system is the high spatiotemporal beam quality of the output, which contains the uniform square pulse waveform, the uniform flat-top spatial fluence distribution and the uniform flat-top wavefront.

A laser pulse temporal compression technique combining stimulated Brillouin scattering (SBS) and laser-induced breakdown (LIB) is proposed in which the leading edge of the laser pulse is compressed using SBS, and the low intensity trailing edge of the laser pulse is truncated by LIB. The feasibility of the proposed scheme is demonstrated by experiments in which a pulse duration of 8 ns is compressed to 170 ps. Higher compression ratios and higher efficiency are expected under optimal experimental conditions.

The influence of focal length on optical limiting power characteristics based on stimulated Brillouin scattering in CCl4 medium is investigated numerically and experimentally. The simulation results show that, the characteristics of limiting power and pulse shape are controlled mainly by focal length when other parameters are fixed. When focal length is short, for example 5cm, the characteristic of limiting power and pulse shape is close to ideal. In experiment, focusing 8ns, 1064nm, 16mJ pulses into CCl4 medium, the rules of limiting power and pulse shape versus focal length are investigated. The experimental results are in good agreement with the conclusions of theoretical simulations.

Focusing Q-switched laser pulses into FC-72 medium, the transverse profiles and spatial evolution of beam in stimulated Brillouin scattering (SBS) are investigated experimentally by CCD camera and digital image processing technology, and the rules of spot size versus pump energy are obtained. The results show that the transverse profiles of backward SBS spots with different pump energies are Gaussian or quasi-Gaussian profiles when pump beam is Gaussian. And the spatial profiles of SBS spots change from quasi-Gaussian to Gaussian when the incident energy increases from the threshold to several times the threshold. At the same time, the size of SBS spot decreases with the increasing pump energy on the whole.

Focusing Q-switched laser pulses into FC-72 medium, the transverse profiles and spatial evolution of backward stimulated Brillouin scattering (SBS) beam are investigated experimentally by CCD camera and digital image processing technology.

It is investigated that the duration of stimulated Brillouin scattering (SBS) amplification of Stokes pulses is smaller than the acoustics lifetime and the oscillation period of hypersound with the nanosecond pulse as pump light. In the experiment, the Fluorinert (FC-72) is used as the Brillouin amplification medium, the Stokes pulse duration less than 1/4 of oscillation period of hypersound of FC-72 as seed pulse is achieved by two-stage SBS compressor to compress 8ns laser pulse. The experiment and simulation show that, in this case, their energy amplification is efficient, but the duration of amplified pulses is longer than the initial seed pulse. Simulation results of higher energy than the maximum energy in the experiment show that at sufficient high pump or initial seed pulse power intensity, the duration of amplified pulses can be no larger than the initial seed pulse duration., even less than the seed pulse duration.

Serial coherent laser beam combination is studied using one-dimensional transient model of stimulated Brillouin scattering (SBS). Beam combination is achieved by transferring the energy of several pump beams to one Stokes beam. The pulse-width of the Stokes beam, which is approximately equal to that of the pump, can be kept constant by adjusting the meeting position between Stokes beam and pump beams in the cell. Numerical simulation is made for the condition that power density of Stokes beam is larger than that of the pump beam. Simulation results show that the major factors that determine the energy extraction efficiency and the critical value of beam combination are the absorption of the medium and the length of SBS cell. The absorption loss is found to play a leading role, so that the overall energy extraction efficiency and the critical value of beam combination can be increased by using shorter SBS cells and/or medium with lower absorption rate. The simulation chooses CCl4 as target medium. The SBS cell length is 20cm; 20 beams (wavelength 1064nm, energy 1J, pulse-width 10ns, beam-area 1cm2) are combined in serial, one of which generates the Stokes seed, others are the pump beams. In order to keep the power density below 100MW/cm2 to avoid being more than threshold of SBS, the Stokes beams and pump beams must be expanded. The output beam of ~10J energy, ~10ns pulse-width, ~10cm2 beam area is obtained.© (2006) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Based on the multi-axial theory of stimulated Brillouin scattering (SBS), a group of coupled wave equations without limitation to the mode separation of a laser is deduced when the random initial phases of the axial modes are introduced for SBS pumped by a free operation laser. The coupling coefficient between two axial modes is obtained, and the dependences of the threshold and reflectivity of broadband SBS on the bandwidth, the mode number, the mode separation of the laser, are simulated numerically. The SBS threshold and reflectivity are dependent not only on the laser bandwidth, but also on the mode number and the mode separation of the 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.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

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.