High-efficiency pulse compression with intracavity Raman amplifiers

High-efficiency pulse compression using intracavity Raman amplifiers has been computed. The energy of a pump laser stored in a lossless cavity is extracted at the Stokes frequency by means of Raman amplification of an input Stokes pulse. Calculations are made for both long and short duration input Stokes pulses for different lossless cavities. As an example, we use a hydrogen-argon mixture as the Raman medium and 1.5 J/cm²energy fluence stored in the lossless cavity at the ruby frequency. By comparing amplified Stokes pulses to a 30 ns pulse duration conventional ruby laser delivering the same energy fluence, pulse shortening factors larger than 20 are computed with quantum conversion efficiencies higher than 80 percent. These values compare favorably to backward Raman amplification. Moreover, this technique is proved to be able to provide a pulse compression rate larger than 14, even for a broad-band laser, which is impossible with backward Raman amplification. This technique could be used with any laser, even with absorbing laser media (excimer lasers) provided pump energy is stored in the lossless cavity by shifting of the laser frequency with any nonlinear process.     

Multiple-order stokes pulse generation by nitrogen-laser-pumped fiber Raman laser

A multimode silica-core fiber Raman laser was pumped by a compact Blumlein-type nitrogen laser. Four orders of Stokes pulses were obtained in the range of 341-359 nm at 280 MW/cm²fiber input. All the Stokes pulses were expanded by modal dispersion. The threshold of each Stokes pulse was measured and the energy transfer to the next order was observed. Conditions for efficient conversion to the required-order Stokes pulse are discussed.     

Time resolved self-defocusing in InSb at room temperature

We report a detailed study of time resolved self-defocusing in InSb at room temperature, using 3 μs long CO2laser pulses. We have determined that the intensity dependence of the refractive index varies as intensity to the two-thirds power. The measured values correspond to an equivalentdn/dIof -0.11 cm²/MW at an incident intensity of 210 kW/cm².     

Parametric generation of 1-ps pulses between 5 and 11 μm with aZnGeP2 crystal

We use a novel picosecond source (a seeded optical parametric amplifier tunable near 3 μm) to pump a type-11 ZnGeP₂ traveling-wave optical parametric generator, With a rather simplified two-pass arrangement, the tunability of the driving Ti:sapphire regenerative amplifier is extended continuously up to 11 μm. As a result of the short pulse pumping and the pulse compression accompanying the parametric amplification, nearly bandwidth-limited pulses could be generated for the first time with this crystal. We report microjoule output energies with 20% quantum efficiency and unprecedentedly low (<100 MW/cm² for a crystal length of only 1 cm) parametric gain thresholds     

Optical power limiting and stabilization based on a novel polymercompound

Optical power limiting and stabilization based on the two-photon absorption (TPA) mechanism is performed in a polymer solution excited by ~810 nm and ~7-ns laser pulses. The solute is a novel polymer, a poly(2,5-dialkoxy-p-phenylene ethynylene) derivative (EBO-OPPE). Using 1-cm path-length EHO-OPPE solution in chloroform of d₀=0.03 mol of repeat unit/liter as the nonlinear absorptive medium, the dynamic transmission changes from T=0.92 to 0.28 when the input intensity of the ~810-nm laser beam is increased from I₀=15 to 600 MW/cm² . The measured nonlinear absorption coefficient is 14.5 cm/GW. Optical power stabilization is demonstrated at an average input intensity level of I₀≈400 MW/cm² with a Δ≈±25% peak-power fluctuation of the laser pulse. After passing through the nonlinear medium, the output peak-power fluctuation is reduced to Δ≈±8%. The spectral-width effect of the input laser beam on the nonlinear absorption of the EHO-OPPE solution is investigated. For three different spectral structures of the input laser beam (single narrow spectral line, multiple spectral lines, and broad spectral band), measured values of TPA cross section for EHO-OPPE are σ₂=66, 80, and 101×10^-20 cm⁴/GW, respectively. This means that EHO-OPPE is one of the best known nonlinear absorptive materials for power limiting purposes     

Dispersive subband-resonant nonlinearity in amorphous Si/SiO2 quantum well structures

Dispersive nonlinearity in amorphous Si/SiO₂ quantum well structures (QW's) has been investigated. The refractive index changes obtained from the intensity-dependent reflection spectra are nonlinearly dependent on the excitation intensity and can be described by the model of the saturating nonlinearity at low pump intensities. The nonlinear refractive index reveals resonant behavior associated with the subband structure of the QW's. The saturated nonlinear index and the saturation intensity have been obtained as Δn_s=-0.11 and I_s=1.9 MW/cm² at the transitions between the lowest subbands, and Δn_s~0.3 and I_s~0.5 MW/cm² at the transitions between the second subbands of the valence and conduction bands. The nonlinearity for the second subband transitions has been found high enough to provide potentially bistable operation, but the bistability is not expected at the transitions between the ground subbands. Carrier lifetime less than 1 ps restricting the switching time of the nonlinearity has been estimated from the saturation intensity     

布里渊放大修正陡前沿Stokes脉冲波形

研究了受激布里渊散射(SBS)放大池中．通过改变抽运光与Stokes种子光脉冲的延时．改变它们在池中的相遇位置．实现抽运光对陡前沿种子光脉冲不同部位的选择性放大．从而修正Stokes脉冲波形。理论和实验研究了Stokes光脉宽、脉冲波形前后沿时长比以及能量提取效率等参数随相遇时间的变化规律．在相遇时间2．5ns时，理论得到前后沿时长比为1，能量提取效率80%以上的Stokes放大光脉冲波形．理论和实验符合较好。     

光学差分参量放大布里渊时域分析优化研究

为了优化设计基于光学差分参量放大的布里渊光时域分析抽运的脉冲编码形式以及斯托克斯光与反斯托克斯光脉宽差,采用微扰法和小信号近似法,获得了探测光与斯托克斯光脉冲、反斯托克斯光脉冲在较小作用区域内的脉冲响应,对其性能进行了理论分析与数值验证。结果表明,相位斯托克斯光脉冲可有效压缩布里渊增益谱宽,提高布里渊信号信噪比;当斯托克斯光与反斯托克斯光传感脉冲脉宽差为8ns时,探测光布里渊增益最大。这一结果对进一步分析基于光学差分参量放大的布里渊光时域分析系统性能以及开展相关实验研究是有帮助的。     

Pulsed laser damage thresholds in vitro for intraocular lenses and membranes

Energy and power density damage thresholds were determined in air, for plastic IOL's and membranes at the focal point of several solid-state laser systems: 1) 694 nm,Q-switched single pulse (30 ns), multimode, 2) 1064 nm,Q-switched single pulse (20 ns), TEM00, 3) 1060 nm, mode-locked single pulse, 15 ps, TEM11, 4) 530 nm, mode-locked single pulse, 15 ps, TEM11, and 5) 1064 nm, mode-locked pulse train (9-11 pulses, 30 ps), TEM00. Pulse energies bracketing damage thresholds as well as focal diameter and pulse duration for each system were determined. Energy density thresholds are lower, and power density thresholds higher, for shorter duration pulses-e.g., 23 J/cm²(1.15 GW/cm²) versus 6 J/cm²(400 GW/cm²) at the same wavelength as in systems 2) and 3) (p = 0.005). Damage thresholds for glass IOL's are 37 J/cm²(1.9 GW/cm²) and 37 J/cm²(1235 GW/cm²) as in systems 2) and 5). Damage threshold values for plastic membranes (Saran Wrap^®) exposed to nanosecond and picosecond pulse trains of Nd:YAG at 1064 nm are about half that of plastic IOL's. When laser pulses with a cone angle of 14° from systems 2) and 5) are focused on plastic membrane next to the IOL, damage thresholds are 30 J/cm²(1.5 GW/cm²) for 20 nsQ-switched pulses and 20 J/cm²(670 GW/cm²) for trains of 30 ps mode-locked pulses. Damage thresholds of IOL's immersed in 0.9 percent saline are approximately the same as those obtained in air.     

Raman amplification of Stokes pulse in ultra-small silicon-on-insulator optical waveguide

The stimulated Raman amplification of picosecond Stokes pulse is numerically investigated in ultra-small silicon-oninsulator optical waveguide. Numerical results show that we obtain the gain of up to 30-dB for weak Stokes pulse in the copropagation configuration for 10 mm Jength waveguide using high intensity pump oPtical pulse. The peak gain, purse width, rise time, and fall time of Stokes pulse will experience the variation course of decaying then increasing with increasing waveguide length. The time delay of output Stokes pulse is controlled by adjusting the initial time delay of both pump and Stokes pulses.     

Saturable absorbers based on impurity and defect centers incrystals

Saturation of near-infrared absorption and transmission dynamics are investigated in tetravalent-chromium-doped Gd₃Sc₂ Ga₃O₁₂, Gd₃Sc₂Al₃ O₁₂, and Mg₂SiO₄ crystals, as well as in reduced SrTiO₃ using 20 ps 1.08 μm laser pulses. An absorption cross section of (5±0.5)×10^-18 cm² in garnets and (2.3±0.3)×10^-18 cm² in forsterite is estimated for the ³A ₂-³T₂ transition of tetrahedral Cr⁴⁺. Q-switched and ultra-short pulses are realized in neodymium lasers using chromium-doped crystals as the saturable absorbers. Saturation of free-carrier absorption with ultra-short relaxation time is observed in SrTiO₃ at 10⁸-10 ¹⁰ W/cm² pump intensities, while at 10¹⁰-10¹¹ W/cm² three-photon interband transitions predominate. The free-carrier absorption cross section is estimated to be (2.7±0.3)×10^-18 cm²     

Far-infrared raman laser with high intensity laser pumping

Theoretical and experimental results are presented for a pulsed far-infrared (FIR) molecular gas laser with high intensity laser pumping. In these FIR lasers, high intensity pumping is found to produce stimulated Raman emission at very large offsets (up to 30 GHz) from resonance with the intermediate state. A theoretical, density matrix model is developed for these lasers to account for simultaneous Raman emission on rotational levels in the ground and excited vibrational states (double Raman resonance). This theoretical approach is necessary in the case of off-resonant, high intensity pumping. Theory predicts the FIR emission frequency, the FIR laser gain, and the pump threshold intensity as a function of pump laser frequency. Experimental results are obtained onP-,Q-, andR-branch transitions in¹²CH3F and¹³CH3F using a single-mode, grating tuned CO2TEA pump laser with an intensity of up to 40 MW/cm². Good agreement is obtained between theory and experiment for the observed values of FIR emission frequency and pump threshold intensity. These results indicate that a widely tunable (150-1200 mum), pulsed FIR CH3F laser could be constructed with a tunable, multiatmospheric CO2pump laser of modest power (about 2-5 MW).     

双池布里渊放大器中Stokes波的高效放大

微小Stokes信号光在布里渊放大器中得到放大,当Stokes信号光强度为0.15mJ时,增益系数达到67。Stokes光增益决定于Stokes信号光强度及放大池中泵光与Stokes光的耦合程度,小信号Stokes光将得到较大的增益,并能得到较好的相位共轭效果。     

High peak power and high repetition rate characteristics in acurrent-pulsed Q-switched CO2 laser with a mechanical shutter

An electro-mechanical Q-switched (EMQ) CO₂ laser is Q-switched by a mechanical beam chopper in combination with a pulsed discharge current. Such a system can produce pulses with high peak powers (>10 kW) and high repetition rates (>1 kpps). In order to analyze the output characteristics, the peak power and the duration of the output pulses have been measured experimentally in detail over a wide range of Q-switching times up to 250 ns. For a low-pressure (<4 kPa) CO₂ gas system, the standard rate equations adequately explain the experimental results by introducing a new switching function for the form of the cavity loss for the mechanical chopper. In an EMQ-laser with a high initial inversion density (4.5·10¹⁵ /cm³ at 150 mA peak current), multiple peak pulses or pulse distortion have been observed. This is due to the plasma screening effect induced by the burning of the metal shutter blades placed inside the cavity. It is found that tungsten metal shutter blades can be used up to a power density of 259 MW/cm² for a focused beam without this effect occurring, The solutions of the rate equations show that optimum coupling can prevent the plasma screening effect even for a Q-switching time longer than the pulse buildup time. The EMQ-laser configured for optimum coupling has produced a peak output power of 30 kW for the 9P20 transition branch in the CO₂ spectrum without any pulse distortion. This value has been obtained even though the discharge length was only 1.3 meters     

Efficient third-harmonic generation of a picosecond laser pulsewith time delay

A new configuration based on the polarization-mismatching scheme with time delay for efficient frequency tripling conversion is proposed in this paper. The calculated results showed that the requirement for the efficient frequency tripling conversion of a 1-ps laser pulse is not only the optimization of peak intensity of the second-harmonic pulse, but also the optimization of the pulse duration ratio and temporal difference between the o-polarization second-harmonic pulse and the e-polarization first-harmonic pulse due to group-velocity mismatch among the interacting pulses. With the proposed scheme the group velocity mismatch can be compensated. Overall energy conversion efficiency increases from 55% to 75% under the optimized conditions at the intensity of 6 GW/cm². The temporal shape of the third-harmonic pulse with 1 ps pulse duration has no subpulses. The optimization of the efficient frequency tripling conversion for intensities of over 75 GW/cm² is also described. The results showed that the maximum tripling energy conversion efficiency is close to 80% with the optimized doubler and tripler     

Picosecond CO2laser-induced self-defocusing in InSb

Self-defocusing of 45 ps CO2laser pulses was observed in InSb at intensities as low as 1 MW/cm². This self-defocusing was caused by the two-photon absorption generated free carriers and the resulting self-phase modulation. In addition to demonstrating spatial beam modulation in this material at 10.6 μm, the present experiment also confirms directly that such a process occurs on a picosecond time scale. This information is important in order to increase the switching speed of infrared optical bistable devices to the picosecond regime. Furthermore, the present self-defocusing experiment also provided an accurate determination of the two-photon absorption coefficient in InSb. The value of 2.5 cm/MW obtained agrees exactly with a previous study. Other applications to continuum and ultrashort pulse generation will also be discussed.     

脉冲光布里渊散射信号的拉曼放大研究

实验研究了前向拉曼泵浦方式下脉冲信号光产生的自发布里渊散射信号和受激布里渊散射(SBS)信号的拉曼放大规律。拉曼泵浦放大自发布里渊散射信号时,随泵浦功率增大会出现SBS现象,对散射信号的放大由拉曼放大和布里渊放大两部分引起,因此增益较大,当拉曼泵浦功率为1 000 mW时Stokes光增益可达54 dB。拉曼泵浦放大SBS信号时,放大过程中只存在拉曼放大。且当泵浦功率增大至600 mW时,会引起多级布里渊散射,致使一级Stokes和泵浦能量会转移到下一级布里渊散射,一级Stokes光增益饱和并下降。     

Stimulated Raman shifting of the Nd:YAG fourth harmonic (266 nm) inH2, HD, and D2

The dependence of the first Stokes stimulated Raman conversion efficiency of fourth-harmonic radiation from a Nd:YAG laser at 266 nm has been studied for the isotopic species H₂, HD, and D₂ as a function of gas pressure and laser energy using a low numerical aperture (~4.5×10^-3) pumping geometry. While the laser energy threshold for first Stokes conversion is seen to vary significantly between the species it has been found that photon conversion efficiencies of at least 50% can be achieved for all of them for laser pump energies at 266 nm ⩽50 mJ/pulse. This study provides a new measurement of the differential cross section for stimulated Raman scattering in HD of 8.1±2.4×10^-29 cm²/sr at 266 nm and at high pressures, and agreement is found with previous measurements of the cross sections for H₂ and D₂. The results have been used to optimize the laser transmitter system for a differential absorption lidar (DIAL) system to measure tropospheric ozone concentration profiles     

基于缓变上升沿泵浦光的布里渊慢光研究

为了增加布里渊慢光的时间延迟,减小脉冲展宽,利用迎风格式的有限差分法对光纤中受激布里渊散射耦合方程组进行数值求解,分析了具有缓慢变化上升沿的连续光作为泵浦光时,光纤输出端斯托克斯光的时间延迟和脉冲展宽因子随增益的变化情况。数值结果表明:具有缓慢变化上升沿的泵浦光可使高斯长脉冲FWHM(半宽全高)为120ns的时间延迟增加到52ns,高斯短脉冲(FWHM为20ns)的时间延迟增加到64ns,同时可减小高斯长短脉冲的展宽。     

High average and high peak brightness slab laser

A high average and high peak brightness Nd:YAG MOPA laser system composed of a laser-diode-pumped Nd:YAG master oscillator, flash-lamp-pumped slab power amplifiers and a phase conjugated mirror was developed. The system demonstrates an average output power of 235 W at a repetition rate of 320 Hz and a peak power of 30 MW at a pulse duration of 24 ns with M²=1.5. Both an average brightness of 7×10⁹ W/cm²·sr and a peak brightness of 1×10¹⁵ W/cm2·sr are achieved simultaneously. The system design rules that we confirmed suggest that by replacing lamp pumping in the amplifier with laser-diode pumping, an average output power of ~1 kW can be obtained at ~1 kHz with a higher average brightness of ~3×10¹⁰ W/cm²·sr and a higher peak brightness of ~3×10¹⁵ W/cm²·sr