Study of narrow-band second harmonic generation from a broad-band fundamental pulse

This paper studies the type-I phase-matched second harmonic generation using 25-fs input laser pulses in a thick BBO crystal.The harmonic signal exhibits a narrow spectrum bandwidth,even though the input pulse has a broad bandwidth.The energy transfer efficiency and modulation of the fundamental spectrum are investigated.     

Visible femtosecond pulses by second-harmonic generation of a cw mode-locked KTP optical parametric oscillator

Efficient generation of femtosecond pulses in the wavelength range from 520 to 675 nm by external frequency doubling the signal wave of a non-critically phase-matched picosecond KTP Optical Parametric Oscillator (OPO) in a non-critically phase-matched temperature tuned LiB₃O₅(LBO) crystal is demonstrated. An average power of the second harmonic as high as 310 mW at 575 nm was generated. In the absence of group velocity mismatch of LBO for a wavelength of the OPO at about 1.3 µm the minimum second-harmonic pulse width was 400 fs at 645 nm.     

Tunable femtosecond frequency doubling in random domain structure of strontium tetraborate

Efficient tunable femtosecond phase-matched noncollinear second-harmonic generation in randomized nonlinear photonic crystal of strontium tetraborate is obtained. Spatial spectrum of nonlinear photonic structure is not flat but enables tuning of the harmonic frequency in the range from 355 to 460 nm. The narrowing of the bandwidth of second harmonic is found to be of order of 10-20%.     

LD泵浦355nm准连续紫外激光器

报道了一台LD侧面泵浦Nd:YAG晶体的内腔三次谐波转换的全固态准连续紫外激光器。在谐振腔内,1064nm的基频波通过对Ⅱ类相位匹配KTP晶体进行二倍频来产生532nm波长激光,二者再通过对Ⅱ类相位匹配LBO晶体进行和频来获得355nm紫外激光输出。355nm全固态紫外激光器在声光调Q重复频率为2.8kHz下,当输入电流为18A时可得到503mW的激光输出。     

Modal phase-matching second harmonic generation in Bragg fiber

Modal phase-matching second harmonic generation in uniformly poled Bragg fiber is theoretically proposed. The very low group velocity of the modes in Bragg fiber near the in-band cutoff frequencies leads to high nonlinear conversion efficiency comparable to that of the periodically poled conversional fiber. The subsequence phase-matched bandwidth reduction by slow light can be retrieved in a certain degree through structure parameters optimization.     

A frequency doubled dye laser with a servo-tuned crystal

Spectral tuning of the uv output of a frequency doubled dye laser has been successfully controlled by a servo motor system which tilts the nonlinear crystal appropriate for phase-matched second harmonic generation while the dye laser emission wavelength is tuned. The spatial direction of the generated uv beam is used as the regulating signal. The feasibility of this technique for spectroscopic applications has been successfully tested.     

Phase matching in monolithic Bragg reflection waveguides

We report what is believed to be the first observation of second-harmonic generation by type I phase matching the bulk chi(xyz)((2))(d(14)) nonlinear coefficient using Bragg reflection waveguides. Second-harmonic power of 0.7 microW was observed for a pump wavelength of 1587.8 nm with an average power of 25.2 mW and a pulse width of approximately 2 ps at a repetition rate of 75.6 MHz. An order of magnitude enhancement between the phase-matched and un-phase-matched second-harmonic conversion efficiency has been observed. Conversion efficiency at the phase-matched wavelengths was 0.001%. The bandwidth of the second harmonic was found to be equal to 0.43 nm, agreeing with the theoretical predictions.     

Phase-matched high-order harmonics by interaction of Ar atoms with high-repetition-rate low-energy femtosecond laser pulses

High-order harmonics are generated by coherent interaction of an intense laser and atoms or molecules[1]. With the development of the intense ultrashort pulse laser, the research of high-order harmonic generation has reached the water-window in spectral region[2] and subfemtosecond in time domain[3]. Especially, the generation and application of subfemtosecond pulse led the study of high-order harmonic generation into a completely new world[4, 5]. It has made the study of ultrafast science fro…     

Phase-matched second harmonic blue light generation in ion implanted KNbO3 planar waveguides with 29% conversion efficiency

We report for the first time to our knowledge, noncritical phase-matched second harmonic generation in an ion-implanted KNbO₃ planar waveguide. The guided TM₀ mode of the fundamental wave (868 nm) is converted into the second harmonic TE₁ mode (434 nm). From 1.3 kW of internal fundamental peak power 385 W second harmonic blue light is generated giving a conversion efficiency of 29%. A comparison of the measured and the theoretical efficiencies implies that phase-matching is achieved over the full waveguide length. Based on these first experimental results we estimate that 100 mW of blue light can be generated from 400 mW of input power using an optimized KNbO₃ planar waveguide of 1 cm length.     

Noncritically phase-matched fourth harmonic generation of Nd:glass lasers in partially deuterated KDP crystals

Noncritically phase-matched (NCPM) fourth harmonic generation (FHG) of Nd:glass laser radiation in partially deuterated dihydrogen phosphate (KD*P) crystals has been demonstrated. At an Nd:glass laser wavelength of 1053.0 nm, NCPM FHG is achieved in 70% deuterated KD*P at a crystal temperature of 18.5±0.1 °C. Tuning the fundamental laser wavelength from 1052.9 to 1053.2 nm, FHG in KD*P is NCPM by changing the crystal temperature from 17.9 °C to 20.5 °C. When driven with 2.4 J of second harmonic radiation in a 3 ns flat-top pulse, corresponding to 1 GW/cm(2) 2ω drive intensity, 1.9 J of fourth harmonic radiation was generated in a 6 mm long KD*P crystal, yielding a second to fourth harmonic energy conversion efficiency of 79%.     

Interactive length of fundamental wave and second harmonic generated on the surface of anomalous dispersion medium

In this Letter, a new method is presented to calculate the interactive length between the fundamental wave and second harmonic generation(SHG) for the configuration of total internal reflection on the inner surface of a nonlinear crystal. Three independent experiments are designed to measure the bandwidths of this second harmonic wave. The theoretical expression of the intensity of SHG is obtained through a nonlinear coupled wave equation. The interactive length of this phase-matched SHG can be calculated mathematically by utilizing the measured bandwidths and the intensity equation. There is no existing method to obtain the interactive length either from theoretical calculations or by experimental measurement. This method can be applied to estimate the extremely short interactive volume in nonlinear processes.     

LD泵浦全固态355nm紫外皮秒脉冲激光器

利用激光二极管（LD）端面抽运Nd∶YVO4激光晶体皮秒三倍频355nm全固态紫外激光器,采用半导体可饱和吸收镜（SESAM）锁模技术及皮秒再生放大技术,对1 064 nm基波采用Ⅰ类相位匹配Li3B3O5（LBO）晶体二倍频和Ⅱ类相位匹配LBO晶体三倍频,获得了稳定性好、倍频效率较高的355 nm紫外激光输出。当二极管泵浦功率为5 W时,获得了脉宽为17 ps、重复频率为1 Hz、单脉冲能量为129.6 J的稳定三倍频紫外激光输出,基频光到二倍频光和三倍频光的转换效率分别达到60.3%和16.6%,3 h输出单脉冲能量的抖动在0.58%以下。     

Birth of solitons in quadratic spatial soliton collisions

We demonstrate experimentally and theoretically the creation of a third soliton in two soliton collision processes in type I noncritically phase-matched KNbO3. The output pattern in the collision process is phase dependent, but the total energy and the relative ratio of the fundamental to the second harmonic in each soliton remain essentially unchanged to within experimental accuracy.     

On the enhancement of the nonlinear frequency transformation in periodic media

It is theoretically shown that an enhancement of the nonlinear frequency transformation in a periodic medium occurs under definite conditions if it is accompanied by light diffraction in this medium. The enhancement of the phase-matched frequency transformation occurs if the harmonic frequency ω approaches the stop band frequency ω_e. In the nondepleted pump approximation the corresponding increase of the harmonic intensity is proportional to ω_e(ω?ω_e)^?1 and is restricted by a quantity proportional to the fourth power of the sample thickness. The formulas for the enhancement of second harmonic generation are presented.     

Extended phase matching of high harmonics driven by mid-infrared light

We demonstrate that phase-matched frequency upconversion of ultrafast laser light can be extended to shorter wavelengths by using longer driving laser wavelengths. Experimentally, we show that the phase-matching cutoff for harmonic generation in argon increases from 45 to 100 eV when the driving laser wavelength is increased from 0.8 to 1.3 microm. Phase matching is also obtained at higher pressures using a longer-wavelength driving laser, mitigating the unfavorable scaling of the single-atom response. Theoretical calculations suggest that phase-matched high harmonic frequency upconversion driven by mid-infrared pulses could be extended to extremely high photon energies.     

非共轴三阶自相关中的倍频过程影响分析

针对非共轴三阶自相关法测试中对二倍频光束的要求,采用分步傅里叶变换方法对入射高斯光束,使用KDP晶体I类匹配的二倍频过程时间波形演变规律进行数值模拟,给出二倍频光束波形仍为高斯分布的入射基频光能量范围.不同二倍频效率下基频光与倍频光脉宽比值不同,所得的三阶自相关函数脉宽则不同,导致与脉冲形状有关的半高宽度换算因子不是固定值.分析结果对三阶自相关仪的参数设计和三阶自相关法测试中的误差分析有一定理论指导意义.     

101 W of average green beam from diode-side-pumped Nd:YAG/LBO-based system in a relay imaged cavity

A 52-W green laser at 532 nm by extra-cavity second-harmonic generation in a coupled-cavity configuration is demonstrated. The fundamental laser is a diode-side-pumped acousto-optic (AO) Q-switched Nd:YAG rod laser producing 84 W of average power at 1064 nm at 8 kHz repetition rate. Type-II phase-matched polished KTP crystal is used as the nonlinear crystal for second-harmonic generation. The individual green pulse width is 50 ns and the fundamental to second harmonic conversion efficiency is 61.8%.     

Efficient and high-power green beam generation by frequency doubling of acousto-optic Q-switched diode-side pumped Nd:YAG rod laser in a coupled cavity

A 52-W green laser at 532 nm by extra-cavity second-harmonic generation in a coupled-cavity configuration is demonstrated. The fundamental laser is a diode-side-pumped acousto-optic (AO) Q-switched Nd:YAG rod laser producing 84 W of average power at 1064 nm at 8 kHz repetition rate. Type-II phase-matched polished KTP crystal is used as the nonlinear crystal for second-harmonic generation. The individual green pulse width is 50 ns and the fundamental to second harmonic conversion efficiency is 61.8%.     

聚片多畴LiNbO3晶体的倍频效应

通过对聚片多畴LiNbO₃晶体倍频效应的研究,从实验上对Bloembergen等人所提出的准位相匹配理论进行了细致的检验。首先运用Maker条纹法在实验上直接测定了单畴LiNbO₃晶体在e^ω·e^ω→e^2ω非位相匹配下的倍频光强,并采用角度扫描和温度扫描方法,以获得满足准位相匹配条件时的不同片数的聚片多畴晶体的最大倍频强度,从而验证了聚片多畴晶体倍频相对光强与片畴数N成平方关系。在N<200时,取得 关键词：     

Correction of walk-off-induced wavefront distortion for continuous-wave laser

We theoretically and experimentally investigate the wave front distortion in critically phase-matched continuous-wave (CW) second harmonic generation (SHG). Due to the walk-off effect in the nonlinear crystal, the generated second harmonic is extremely elliptical and quite non-Gaussian, which causes a very low matching and coupling efficiency in experiment. Cylindrical lenses and walk-off compensating crystals are adopted to correct distorted wave fronts, and obtain a good TEM₀₀ mode efficiently. Theoretically, we simulate the correction effect of 266-nm laser generated with SHG. The experiment results accord well with the theoretical simulation and an above 80% TEM₀₀ component is obtained for 266-nm continuous-wave laser with a 4.8°-walk-off angle in beta barium borate (BBO) crystal.