Nd:YAG激光器增大线偏振输出功率的实验研究

由热应力双折射产生的热退偏是限制Nd:YAG激光器线偏振输出功率的重要因素。本文理论分析并实验证明,相比于传统的[111]方向切割的Nd:YAG晶体棒,使用[100]方向切割的Nd:YAG晶体棒能够改善Nd:YAG激光器的线偏振态输出功率。在理论分析的基础上,实验中采用半导体侧面泵浦不同切割方向的Nd:YAG棒状晶体,比较[111]、[100]方向切割晶体的线偏振输出特性,实验证明[100]方向切割的Nd:YAG棒状晶体,有最小的晶体热退偏方向,可以提高激光器的线偏振输出功率。     

Polarization properties of vertical-cavity surface-emitting lasers

Polarization-state selection, polarization-state dynamics, and polarization switching of a quantum-well vertical-cavity surface-emitting laser (VCSEL) for the lowest order transverse spatial mode of the laser is explored using a recently developed model that incorporates material birefringence, the saturable dispersion characteristic of semiconductor physics, and the sensitivity of the transitions in the material to the vector character of the electric field amplitude. Three features contribute to the observed linearly polarized states of emission: linear birefringence, linear gain or loss anisotropies, and an intermediate relaxation rate for imbalances in the populations of the magnetic sublevels. In the absence of either birefringence or saturable dispersion, the gain or loss anisotropies dictate stability for the linearly polarized mode with higher net gain; hence, switching is only possible if the relative strength of the net gain for the two modes is reversed. When birefringence and saturable dispersion are both present, there are possibilities of bistability, monostability, and dynamical instability, including switching by destabilization of the mode with the higher gain to loss ratio in favor of the weaker mode. We compare our analytical and numerical results with recent experimental results on bistability and switchings caused by changes in the injection current and changes in the intensity of an injected optical signal     

YAG激光晶体热致双折射的非线性分析

为了克服线性模型在描述高功率运转的激光晶体时,热焦距和热致双折射计算值与实测值不符的缺点,采用非线性热传导模型,计算了常用[111]切割方向Nd:YAG激光晶体的热致双折射椭圆分布以及径向和切向热焦距。进行了平均热焦距数据测量和旋转线偏振光干涉实验,实验结果与理论分析吻合。结果表明,非线性模型对Nd:YAG激光晶体在高功率运转时的描述更符合实际情况,普适性更强。这一结果对于设计高功率径向或切向偏振固体激光器是有帮助的。     

双棒串接补偿热致双折射效应激光谐振腔

固体激光器中的热致双折射效应严重地限制了基模输出功率的提高,为了获得大功率高质量的激光输出,需要对热致双折射效应进行补偿.在理论上对双棒在腔内串接补偿热致双折射效应的条件进行了改进,通过设计合理的腔结构和腔内元器件,应用4f成像系统空间滤波器并考虑石英旋光器的厚度,使得腔内光束的退偏率降至2.5%以下.使用矩阵光学的方法简化了对含有这样一个复杂光学系统的谐振腔稳定性和腔内光束半径等特性的分析.通过在腔内加入一个正透镜来扩大基模体积,实验中在双氪灯连续抽运Nd∶YAG激光器中得到了61 W线性偏振的激光基模输出,表明在大基模体积谐振腔的设计中,双折射效应的补偿十分必要.     

808 nm近红外激光辐照三种不同的离体人组织的光传输特性

采用双积分球系统和光辐射测量技术的基本原理以及运用生物组织的光学模型 ,研究了 80 8nm激光及其线偏振激光辐照人正常小肠、人正常膀胱和人膀胱癌组织的光学特性。结果表明 ,Kubelka Munk二流模型下 ,人正常膀胱组织与人膀胱癌组织对 80 8nm激光及其线偏振激光的各个光学特性参量均有非常显著的差异 (P <0 0 1)。人正常膀胱组织对 80 8nm激光及其线偏振激光的光学特性参量均有差异 ,其中吸收系数和有效衰减系数有明显差异。人膀胱癌组织对 80 8nm激光及其线偏振激光的光学特性参量均没有显著性差异 (P >0 0 5 ) ,表明人膀胱癌组织对 80 8nm线偏振激光的退偏振较大。人正常小肠组织对 80 8nm激光及其线偏振激光的光学特性参量均有显著性差异 (P <0 0 1) ,表明人正常小肠组织对 80 8nm线偏振激光的退偏振较小。在Kubelka Munk二流模型下 ,80 8nm激光及其线偏振激光辐照三种人组织的光强分布均有明显的不同     

Birefringence effect of optical fiber laser with intracore fiberBragg grating

The spectra of Nd³⁺- and Pr³⁺-doped fibre lasers utilizing an intracore Bragg grating reflector written at 1.08-μm Bragg wavelength and a dichroic mirror for cavity feedback have been investigated under 0.6-GHz resolution. Two oscillating frequencies corresponding to two orthogonal linear polarization states of laser emission were observed. The frequencies and orientations of the two linearly polarized emissions could be changed by varying the fiber birefringence with an inline polarization controller. A phenomenological model is proposed to explain the experimental results     

基于光纤光栅斯托克斯参量的压力传感测量研究

利用光纤光栅的偏振特性感知外界参量变化是光纤光栅传感的新研究热点。提出利用光纤光栅第一斯托克斯参量s1感知外界压力的新方法。推导了光纤光栅压力所致线双折射和s1参量的关系,建立了压力、线双折射和s1参量三者之间的关系模型,分析了线偏振光入射角度对s1参量的影响。仿真表明,偏振光入射角度影响s1曲线的基准值,同时也影响左右波长测量点的对称性和线性度。利用光纤光栅和高速在线检偏器搭建了实时压力测量系统,测量了不同偏振光入射角度下s1参量受压力所致线双折射的影响曲线,验证了理论仿真结果。     

光束偏振性对切割质量影响的研究

针对光束偏振态对切割质量的影响进行了实验探究。实验中,分别采用线偏振CO2激光和圆偏振CO2激光对6 mm低碳钢板进行切割,结果表明,使用线偏振光切割时,光束在不同切割方向上产生的切缝宽度不同,挂渣程度也不一样。而采用圆偏振光切割时,切缝宽度一致性良好,且无明显挂渣现象。从光能的吸收和反射两个方面解释了产生这种现象的原因。给出了在激光切割时,为了获得良好的切割效果应采取的措施。     

通过圆偏振光提高飞秒激光诱导击穿光谱的发射强度

激光诱导击穿光谱(LIBS)是一种快速、实时的元素成分分析技术。为了提高LIBS的灵敏度,人们已经提出多种方法来提高LIBS的光谱强度。本文采用飞秒脉冲激光烧蚀黄铜产生LIBS,对比了圆偏振和线偏振下LIBS光谱的强度,结果发现圆偏振下的光谱强度比线偏振下的强,光谱强度大约提高了15%。采用飞秒激光照射金属时,金属内部的自由电子吸收光子的能量。在线偏振飞秒激光场中,电子在脉冲的每个光学周期中经历交替的加速和减速;而圆偏振飞秒激光可以连续加速电子,因此电子可以获得更高的能量,这使得圆偏振飞秒激光产生的光谱强度不同于线偏振飞秒激光产生的光谱强度,圆偏振激光有助于改善飞秒LIBS信号的强度。     

铷775.9 nm激光感生色散光学滤波器

为提高自由空间通信、遥感、激光雷达等系统的信噪比,研究了铷775.9 nm激光感生色散光学滤波器(LIDOF).通过780 nm线偏振抽运光选择性激发基态原子,使其在激发态子能级间布居不均衡,从而引起激发态的圆二色向性和圆双折射,使与激发态共振的775.9 nm探测光偏振态发生旋转,实现滤波传输.获得的峰值透射率为4%,带宽约为605 MHz,同时,得到了峰值透射率随池温和抽运功率的变化关系.     

Polarization effects in the Sagnac-Michelson distributeddisturbance location sensor

An analysis, based on Jones calculus, is presented of polarization effects in the merged Sagnac-Michelson interferometric sensor for distributed disturbance detection. The nature of birefringence biasing in the Sagnac interferometer is clarified: the cosine-response of the Sagnac interferometer can be biased to a sine-response by introducing birefringence with retardation equal to π/2 in the Sagnac-loop. In addition, the input light should be linearly polarized and both the direction of polarization and the optical axes of the birefringence region should be oriented at π/4 with respect to the plane of the fiber-loop     

Study of thermal relaxation in a respectively pumped Nd:glass laser rod

The birefringence produced in a cylindrical 30 cm × 1.9 cm Nd:glass rod, pumped by close-coupled, six flashlamps, symmetrically placed in a circular cavity, is investigated. The expression for the phase difference caused by the birefringence for the two waves polarized in the radial and tangential directions is derived and calculated as a function of the input pump power, normalized rod-radius, elastooptical coefficients and other material parameters. The expression for the ratio of output intensity to input intensity for a linearly polarized beam passing through the thermally strained rod, placed between two crossed polarizers, is derived and calculated. The thermal time constant measured from the appearance and disappearance of the birefringence effect is found to agree reasonably well with the computed values. Observation of the negative lensing effect from the conoscopic interference pattern as well as from the output intensity profile from the crossed polarizers is reported.     

Picosecond polarization dynamics and noise in pulsed vertical-cavity surface-emitting lasers

We have measured on picosecond time scales the polarization behavior of vertical-cavity surface-emitting lasers (VCSELs) pumped with short optical pulses. Our data describe the temporal properties of the intensity and intensity noise for orthogonally polarized modes as well as noise correlations between the modes in both the circular and the linear detection bases. A variety of behavior is observed that depends on the polarization of the pump, the strength of the pump, the VCSEL operating temperature which determines the carrier spin-flip rates, and the degree of birefringence and dichroism in the laser. We develop an extension of the "split density" model which includes carrier relaxation processes that are important on picosecond time scales. Our numerical simulations illustrate how spin-flip processes lead to anti-correlated fluctuations in the circularly polarized modes and how anti-correlated noise can arise between the linearly polarized modes without switching between two "preferred" modes, as is commonly seen on longer time scales.     

Optical waveguide dispersion characteristics from the scalar wave equation

The electric and magnetic fields of an optical waveguide are well represented as derivatives of a linearly polarized vector potential. This vector potential obeys a scalar wave equation. Through the use of a variational principle one may obtain good approximations to the dispersion relation, however, without resolution of the birefringence. A simple perturbational approach yields the birefringence. Universal plots may be obtained in this way for TE and TM modes for rectangular channel waveguides with small index discontinuities.     

固体激光器腔型结构对热透镜焦距测量的影响

热透镜效应对固体激光器的性能影响很大,准确测量热透镜焦距对固体激光系统的设计有重要意义。考虑热致双折射效应,利用等效谐振腔的g参数坐标图进行分析,推导了三种不同腔型结构下径向热透镜焦距和切向热透镜焦距系数的计算方法,分析并得出了测量热透镜焦距系数所需的腔型结构。实验采用Nd∶YAG作为激光晶体,三组激光二极管互呈120°侧向抽运。实验结果表明,在非对称平-平腔中,当激光晶体相对于两面腔镜位置不同时,谐振腔输出功率会出现不同的变化趋势,测量出径向热透镜系数为31.76 m.W和切向热透镜系数为42.26 m.W。实验验证了理论分析的正确性。     

非线性双折射阶跃光纤中的极化调制不稳定性

利用光脉冲在光纤中传播时所遵守的相干非线性薛定谔耦合方程,研究了线偏振光在 弱双折射阶跃光纤中的传输特性。结果表明,由于线性双折射和色散的相互作用,导致光的偏振态发生变化,产生交叉相位调制(XPM) ,从而导致相位匹配参量的四波混频。这一过程不仅在反常色散区产生,在正常色散区也能产生,并且当表征距离的级数m发生变化时,也随之发生变化。     

激光诱导液晶定向研究

报道了一种新的激光诱导液晶定向技术。用偏振激光辐照两块涂有聚乙烯醇（ＰＶＡ）和甲基橙（ＭＯ）混合物薄膜的玻璃基片,再按激发光偏振态相互垂直的方向组装为液晶（ＬＣ）盒,用光诱导法获得盒内液晶分子的扭曲向列排列,该盒可使入射线偏振光旋转９０°。用１ＧＷ／ｃｍ２强激光辐照后,盒内ＬＣ分子取向不变,因而可望用于制备强激光系统中偏光控制器以改善光束质量。     

Modal birefringence measurements of polarization-maintaining single-mode fibers without and with stretching by optical heterodyne interferometry

Precision measurements of modal birefringence in polarization-maintaining single-mode fibers and its stretch-dependence are described. An orthogonally polarized two-frequency laser beam is launched into some length of a fiber in accordance with the principal axes of the fiber. After a passage through the fiber, its optical heterodyne detection processes are made to produce a beat-photocurrent signal whose phase is linear to modal birefringence. The modal birefringence is accurately measured by detecting the phase change of the beat signal resulting from a short cut of the fiber. The stretch-dependent change in modal birefringence is also examined in detail and is recognized to be in proportion to the longitudinal strain of the fiber.     

Linearly polarized polarization-maintaining Er3+-doped fluoride fiber laser in the mid-infrared

We demonstrated the ~2.8-μm and ~3.5-μm linearly polarized continuous wave (CW) laser outputs from a polarization-maintaining (PM) Er3+-doped fluoride fiber laser. By introducing a film polarizer into the cavity to select the laser polarization orientation, the ~2.8-μm linearly polarized CW laser with a high polarization extinction ratio (PER) of ~23 dB and maximum output power of 2.37 W was achieved under double-end pumping at 976 nm. By adding another 1981-nm pump source simultaneously, the ~3.5-μm linearly polarized CW laser was also obtained, giving higher PER of ~27 dB and maximum output power of 307 mW which is only limited by the available power of 1981-nm pump. To the best of our knowledge, this is the first report on a mid-infrared linearly polarized CW PM fiber laser in the >2.5-μm mid-infrared region. This work not only opens up opportunities for some new mid-infrared applications, but also provides a promising platform for developing high-stability and versatile mid-infrared laser sources.     

Repetition rate enhancement and mode shaping in dual-cavity stretched pulse fiber laser

A novel orthogonally polarized coupled-cavities configuration for stretched pulse fiber lasers, based on tunable birefringence, is presented. By alignment of the birefringence, we achieve envelope modulated mode patterns, which in turn are used to increase the repetition rate of the laser while maintaining the well-profiled femtosecond pulses. Experimental results and a supporting theoretical model are presented.