基于透射型体布拉格光栅的两通道2.5kW光谱组束输出

体光栅光谱组束是获得高功率激光输出的一种有效途径.在有限的可用带宽内,光谱通道间隔影响着组束光束数目以及最终的高功率组束输出.采用耦合波理论,建立了一个两通道高功率光谱组束模型.通过优化体光栅光谱通道间隔,可放宽对组束子束线宽和功率的限制,组束功率可大幅提升而光谱密度并无显著下降.基于此,实验上获得了2.5 kW组束输出,绝对效率超过85%,通道间隔5 nm,光谱密度为0.51kW/nm.组束功率1 kW时,组束输出能保持好的光束质量;组束功率1.5kW时光束质量恶化较明显,通过分析发现,组束光束质量的恶化主要受限于体光栅的色散及高功率下体光栅复杂的热畸变.     

Efficient power scaling of laser radiation by spectral beam combining

The possibility of achieving multikilowatt laser radiation by spectrally combining beams using volume Bragg gratings (VBGs) is shown. The VBGs recorded in a photothermorefractive glass exhibit long-term stability of all its parameters in high-power laser beams with power density >1 MW/cm2 in the cw beam of total power on a kilowatt level. We consider an architecture-specific beam-combining scheme and address the cross-talk minimization problem based on optimal channel positioning. Five-channel high efficiency spectral beam combining resulted in a >750 W near-diffraction-limited cw beam has been demonstrated experimentally.     

Efficient, high-brightness wavelength-beam-combined commercial off-the-shelf diode stacks achieved by use of a wavelength-chirped volume Bragg grating

We report a method of scaling the spatial brightness from commercial off-the-shelf diode laser stacks through wavelength beam combining, by use of a linearly wavelength-chirped volume Bragg grating (VBG). Using a three-bar commercial stack of broad-area lasers and a VBG, we demonstrate 89.5 W cw of beam-combined output with a beam-combining efficiency of 75%. The output beam has a propagation factor M2 approximately 26 on the slow axis and M2 approximately 21 on the fast axis. This corresponds to a brightness of approximately 20 MW/cm2 sr. To our knowledge, this is the highest brightness broad-area diode laser system. We achieve 81% coupling efficiency into a 100 microm, 0.22 N.A. fiber.     

基于体布拉格光栅的光谱合成的数值分析

 建立了分别用一个和两个反射式体布拉格光栅实现两路和三路光束合成的物理模型,模型中考虑了体布拉格光栅的吸收,并假设入射光束的光谱具有高斯线型。通过对两路和三路光谱合成的数值分析可知:当光谱宽度为0.1 nm时,两路和三路光谱合成的效率分别为98.76%和97.69%;当光谱宽度为0.3 nm的时候,对应的效率分别为97.34%和95.78%。随着光谱宽度的增加,合成的效率明显降低。当光谱宽度较窄时,实现多路光束较高的合成效率是可行的,这为高能激光的获得提供了一种确实可行的方法。     

Propagation characteristics of the Yb-doped fiber lasers after spectral beam combining by the VBGs

The propagation model for the spectral beam combining system of five Yb-doped fiber laser beams by the volume Bragg gratings (VBGs) has been built up. Considering the surface distortion by thermal effect, the intensity of an individual beam passing through the spectral beam combining system by the VBGs with deformation has been analyzed numerically and iteratively by using finite-different and sparse matrix methods, and the intensity of combined beam has been studied by the principle of incoherent superposition. Consequently, according to the intensity second-order moments method, the M²-factor of the combined beam has been calculated. The effects of deformation of the VBGs on the intensity, the combining efficiency, the M²-factor and the power in the bucket of the combined beam have been analyzed quantitatively. It can be shown that the effects of the deformation of the VBGs on the intensity distribution of combined beam become more and more obvious with the increasing of the deformation of the VBGs. The results show that the surface distortion of the VBGs due to high-power lasers dramatically degrades both the beam quality of the combined beam and the combining efficiency of the spectral beam combining system.     

Beam combining of lasers with high spectral density using volume Bragg gratings

Incoherent combining of multiple laser beams with offset wavelengths into a single near-diffraction-limited beam is an effective solution to increasing energy brightness and scaling output power of high-power lasers. Volume Bragg gratings (VBGs) recorded in photo-thermo-refractive (PTR) glass allow spectral beam combining with a remarkably high spectral density of channels. Spectral beam combining (SBC) of five channels within 1-2 nm bandwidth around 1064 and 1550 nm into a single near-diffraction-limited beam with absolute efficiency 92-94% is demonstrated. Scaling of this technique to multi-kW power level is discussed.     

Hamming切趾技术提高体光栅光谱合成功率的研究

体光栅光谱合成技术是获得高功率激光输出的一种有效途径,体光栅衍射旁瓣是影响合成光束数目的主要因素。采用了Hamming切趾技术对体光栅旁瓣进行抑制,建立了Hamming切趾体光栅的折射率分布模型,分析了Hamming切趾体光栅的衍射特性,给出了Hamming切趾体光栅光谱合成效率公式,分析了切趾光栅对光谱合成效率的影响。计算结果表明:体光栅切趾后有效减小了体光栅对相邻合成光束的衍射损耗,切趾后,在20 nm的带宽内,谱合成光束的数目由13束增加为20束,谱合成效率达75.3%,光谱合成功率提高为切趾前的1.5倍。     

Two-dimensional non-spatial filtering based on holographic Bragg gratings

The filter made up of two gratings performs as a two-dimensional non-spatial filtering. This paper reports that the volume Bragg gratings are fabricated by interfering two collimated coherent laser beams in photopolymer. Diffraction efficiency of a single grating is up to 78% in Bragg's condition, then a two-dimensional non-spatial filter, which consists of two volume Bragg gratings and a half-wave plate, enables the laser beam filtered in two dimensions with the diffraction efficiency of 54%. The Bragg's condition and effect of polarisation on performances of the two-dimension filter are also discussed.     

Six 38.7 W pulses in a burst of pulse-burst picosecond 1064 nm laser system at 1 kHz

A burst of six pulses with an average power of 38.7 W are obtained for a pulse-burst picosecond 1064 nm laser system at 1 k Hz. The six pulses have equal amplitudes and pulse spacings of 800 ps, the beam quality of the M2 factor is less than 2, and the pulse width is 67 ps. Seed pulses are broadened from 22 to 136 ps by single-pass volume Bragg gratings. A laser-diode end-pump Nd:YVO_4 beam-splitting amplifier is used to divide and amplify a single pulse into six pulses. An Nd:YAG regenerative amplifier and a single-pass high-gain amplifier are applied.     

Two-dimensional angular filtering by volume Bragg gratings in photothermorefractive glass

We propose and perform two-dimensional angular filtering on the basis of volume Bragg gratings (VBGs) in a series connection for cleaning up a laser beam. The dependence between angular selectivity and VBG parameters is analyzed with the coupled wave theory. Near-field modulation, contrast ratio, and power spectral density were used to evaluate the effect of filtering in both the spatial and frequency domain. The results from our initial experiments showed that the near-field modulation and contrast ratio of laser beam were remarkably improved.     

Experimental research on spatial filtering of deformed laser beam by transmitting volume Bragg grating

Using transmitting volume Bragg gratings (TVBG) as a basis,an experiment on one-dimensional spatial filtering of a deformed laser beam is designed.The deformed laser beam results from a He-Ne laser beam modulated by an amplitude modulation plate with a spatial frequency of 7.2 mm·1.Results show that when the central wave vector of the deformed beam satisfies the Bragg law of TVBG,the spatial profile of the -1st forward-diffracted order is similar to that of the undeformed He-Ne laser beam due to the TVBG with a spatial frequency selective bandwidth of less than 5.0 mm·1.The higher frequency components of the deformed beam are filtered out in the optical near field.Thus,the TVBG cleanup of the spatiallydeformed laser beam is realized experimentally.     

体Bragg衍射器件在高功率激光中的应用

记录在光热敏微晶玻璃(PTR)中的体Bragg衍射器件具有高衍射效率、可选择的角度和波长选择特性以及高损伤阈值等优点,成为先进激光技术发展中的一种新兴器件。综述了目前各型体Bragg衍射器件在激光技术中的应用,主要包括角选择近场滤波技术、光谱合成技术、外腔半导体稳频技术以及啁啾脉冲压缩与展宽技术。详细地阐述和分析了这些方法的原理、特点、适用范围以及它们的研究现状。     

基于一种新微细加工技术的亚波长光栅的研制

描述了一种新的亚波长光栅的微细加工技术，即电子束（EB）扫描曝光得到相应的亚微米级的线宽图形，再利用快速原子束刻蚀设备获得了高深宽比的立体结构。用此加工技术获得了100nm以下的刻蚀精度，并研制成功亚波长光栅。该亚微米线宽微细加工技术可用于布拉格光栅、半导体激光器、无反射表面等需要亚微米结构的器件中。     

Angular filtering by volume Bragg grating in photothermorefractive glass for nanosecond laser pulse

The two-dimensional angular filter based on volume Bragg gratings in photothermorefractive glass for a nanosecond(ns) laser pulse is demonstrated. The experimental results show that the near-field beam quality of the laser pulse was effectively improved. The near-field modulation and contrast ratio were improved by 1.75 and4.48 times, respectively. The power spectral density curves showed that the spatial frequencies more than0.9 mm~(-1)in the x direction and 1.2 mm~(-1)in the y direction were effectively suppressed.     

基于光斑二阶矩的阵列光束倾斜相差自适应控制方法

针对阵列光束相干合成中存在的倾斜相差大的问题,提出了基于合成光束远场光斑二阶矩的阵列光束倾斜相差自适应控制方法。以合成光束远场光斑二阶矩作为评价函数,理论上模拟了采用随机并行梯度下降算法实现7路光束的倾斜闭环控制过程。实验上搭建了7路光纤激光相干合成系统,利用自适应光纤准直器对倾斜相差进行校正。以合成光束远场光斑的二阶矩作为评价函数,采用随机并行梯度下降算法,实现了7路光束的倾斜的闭环控制,合成光束模拟远场光斑的桶中功率由0.05 V提升至1.95 V。实验中将倾斜扰动的增益系数变为与二阶矩相关的函数,实现了自适应变增益系数的倾斜闭环,在一定程度上提升了倾斜控制的带宽。从理论上和实验上验证了基于光斑二阶矩的倾斜相差自适应控制方法在光束合成及合成孔径探测领域应用的可行性。     

Beam splitting, combining, and cross coupling through multiple superimposed volume-index gratings

We investigate the diffraction properties of multiple-superimposed index gratings from a vector synthetic viewpoint using coupled wave analysis with Bragg and non-Bragg readout. The vector synthetic gratings consist of two volume-index gratings with different grating wave vectors, which can be obtained in photorefractive materials, in optical fibers or in Bragg cells with two driving acoustic waves. This structure can realize beam splitting, beam combining, and beam cross coupling, etc. Analytic expressions for the diffraction efficiency as well as the amplitude of the waves involved are obtained. The results are presented and discussed.     

Diffraction of light from phase gratings at large modulation depths: transient-grating experiments in liquids at high laser powers

Generation of transient gratings in weakly absorbing liquids with a high-power laser gives rise to changes in the fluid's index of refraction that are sufficiently large to produce both multiple diffraction of a probe laser beam and a time response anomaly. The coupled-wave approach to the solution of the volume diffraction problem is shown to predict the existence of high-order diffraction of the probe beam and the time dependence of the diffraction intensity of each order. In addition, criteria for the Raman-Nath and Bragg diffraction regimes are derived from the first-order, coupled-wave equations.     

Wavelength stabilization of extended-cavity tapered lasers with volume Bragg gratings

The wavelength stabilization of high-brightness extended-cavity lasers at 810-nm by the use of volume Bragg gratings is described. Narrow linewidth (<?20 pm), high power (2.5 W) and good beam quality (M²<4) operation has been obtained in a robust and simple design. The impact of the beam focusing into the Bragg grating on the external cavity performance has been theoretically and experimentally investigated. Finally, second-harmonic generation of the infrared beam has been obtained in a ppKTP crystal, demonstrating a non-linear conversion efficiency of 0.8%/W and up to 8 mW at 405 nm.     

Frequency stabilization and output power undulations of diode lasers with feedback by volume holographic gratings

The mode-hop behavior and the power characteristics of a laser diode with wavelength-selective optical feedback are experimentally investigated. The feedback is provided by external volume holographic gratings, also called ‘Bragg mirrors’, at normal incidence. We demonstrate that a Bragg mirror forces a laser diode to operate only within a narrow wavelength range, and that the emission wavelength of the laser diode is stabilized against variations of the injection current. Moreover, we present periodic undulations of the power characteristics of the laser, depending on the driving current. They can be qualitatively explained with a simple model which takes into account that the threshold gain in the laser system strongly depends on the wavelength.     

全固态激光中的光谱合成技术

对多种全固态激光中的光谱合成技术进行了探讨和研究,包括光纤激光、Yb:YAG板条激光和半导体激光。对于光纤激光,探讨了基于单个多层介质膜（MLD）光栅、一对MLD光栅、多个体布拉格光栅三种衍射光学元件的光谱合成技术中色散造成的光束质量退化问题,指出子束光谱线型的二阶矩全宽决定了光束质量的退化量,但所允许的光谱宽度又依赖于具体的技术选择途径。进而比较了三种光谱合成方案的优缺点。对于固体激光,实验演示了基于Yb:YAG晶体的板条激光实现光谱合成的原理可行性。通过设计一个基于MLD光栅的振荡器内的光谱合成装置,实现了7束子激光最高241 W的光谱合成输出,合成后光束质量β因子约4.1,表明大功率Yb:YAG板条激光具有通过光谱合束技术实现功率进一步提升的潜力。对于半导体激光,提出并设计了大模场外腔半导体激光+快轴光谱合成的技术。实验演示了9个1 mm宽LD芯片沿快轴方向的光谱合成,用β因子评价合成后的光束质量,在慢轴方向β≈6.3,在快轴方向β≈1.6,表明快轴光谱合成造成的光束质量退化是完全可控的。