Physical origin of observed nonlinearities in Poly(1-naphthyl methacrylate):Using a single transistor–transistor logic modulated laser beam

A thermal lens technique is adopted using a single modulated continuous wave(cw)532-nm laser beam to evaluate the nonlinear refractive index n2,and the thermo-optic coefficient dn/dT,in polymer Poly(1-naphthyl methacrylate)(P-1-NM)dissolved in chloroform,tetrahydrofuran(THF),and dimethyl sulfoxide(DMSO)solvents.The results are compared with Z-scan and diffraction ring techniques.The comparison reveals the effectiveness and the simplicity of the TTL modulation technique.The physical origin is discussed for the obtained results.     

一种提高迈克尔逊干涉系统抗干扰能力的新方法

为了提高迈克尔逊干涉系统的抗干扰能力,取代传统的动镜扫描结构,设计了基于电光调制晶体折射率实现光程扫描的干涉系统。通过加载在可变折射率晶体上的调制电信号,使晶体折射率产生周期性变化,从而在原有系统光路中调制折射率改变光程差。通过理论计算获得了电光调制过程中系统可以产生的最大光程差,并仿真分析了晶体厚度及晶体衍射效率对调制过程的影响。经仿真分析可知,随着调制电压范围的增大,可获得的光程变化范围也增大,从而系统光谱分辨能力也相应增大。同时,在调制过程中设置调制范围使衍射损失的能量小于总能量的10%,从而保证较好信噪比。实验结果显示,随着调制电压的变化,干涉条纹产生周期性移动,但超过一定范围时会产生非线性误差。通过修正算法后系统光谱分辨率可达7.2 cm-1。在无抗震实验平台的条件下,传统干涉系统的相对误差超过20%,而本系统的相对误差低于3%,证明了系统采用静态电光调制后抗干扰能力显著增强。     

4k数字放映机广角放映镜头的设计

为满足4k电影的短距离放映需求,设计一款广角放映镜头,可适配于采用三芯片DMD技术的放映机,最大投射比可达0.75∶1,相对孔径为1/2.2,反远比为4.6∶1,在66 lp/mm的Nyquist频率处,MTF值为0.42,边缘视场MTF值为0.25。最大横向色差为3.7 m0.5 pixel,最大畸变为2.5%。分析了反远距形式的特点,给出了合理选型、分配光焦度和计算关键技术参数的方法,以及像差校正过程中应注意的问题。最终设计出了符合设计指标的具有高分辨率的广角放映镜头,该镜头结构简单、成本低。     

基于可调谐多模二极管激光吸收光谱的二氧化碳浓度测量

可调谐二极管激光吸收光谱技术(TDLAS)具有高分辨率、高灵敏度和快速测量等特点,是气体探测中的主流技术。使用多模二极管激光器作为光源,将多模二极管激光吸收光谱技术与关联光谱技术相结合有助于提高测试可靠性和稳定性,同时还可有效解决单模二极管激光器长时间工作时中心波长随外界温度或机械特性的变化而发生偏移的问题。以1 570 nm多模二极管激光器为光源,利用多模二极管激光关联光谱和波长调制的气体测量技术(TMDL-COSPEC-WMS),通过计算待测气体和参考池气体之间二次谐波信号峰值高度之间的关系,实现了对二氧化碳浓度的测量。实验中二氧化碳浓度测量范围在0.6%～30%之间,计算结果表明,二氧化碳浓度与真实浓度值之间具有良好的线性关系,其线性度为0.998 7,线性拟合的斜率为1.061±0.016 8。对二氧化碳与氮气混合气体的连续测量结果表明,系统的探测极限达到335 ppm·m,对同一样品在20 min内的20次连续测量的标准偏差为0.036 7%,表明了系统良好的稳定性,所有测量结果都显示了系统用于二氧化碳气体监测的有效性。     

Phase-matching analysis of four-wave mixing induced by modulation instability in a single-mode fiber

Four-wave mixing induced by modulation instability in a single-mode fiber is analyzed from the phase-matching point of view. For the two-channel transmission, a method is proposed to select the four-wave-mixing-induced sidebands, which is based on the proper use of a continuous-wave and a pulse as light sources. We find that a mass of sidebands are generated in the modulation instability resonance region, and the power of the sideband increases with not only the peak power of the pump pulse but also the continuous-wave power which acts as a seed. The research will provide guidance for fiber communication and sensing systems using wavelength division multiplexing technology.     

贝赛尔函数展开对空间方位失调角测量误差的影响

调制偏振光可以作为空间方位信息的载体,实现方位角度信息的测量,在军事、航天、生物医药等领域有广泛的应用前景。文章阐述了基于磁光调制偏振光的方位失调角测量原理,针对原理中贝赛尔函数展开带来的失调角测量误差,详细推导了截取不同项数时失调角的计算公式。仿真结果表明:随着贝赛尔函数展开式截取项数的增加,失调角的测量误差越来越小;截取二倍频信号与三倍频信号的测量误差相当,但符号相反;截取项数高于四倍频信号后,误差基本保持不变。因此利用磁光调制偏振光进行方位失调角测量时,贝赛尔函数展开式截取项数不易超过四倍频信号。     

Raman scattering studies of (GaP)n/(InP)n (n = 1, 1.7, 2) short‐period superlattice structures

We report Raman scattering from (GaP)_n/(InP)_n (n = 1, 1.7, 2) short‐period superlattice (SPS) structures to study the effect of lateral composition modulation (LCM) on the behavior of optical phonons. Cross‐sectional transmission electron microscope images revealed that LCM was formed with complex pattern in the n = 1.7 and n = 2 samples grown at 490 °C. Interestingly, both the InP‐ and the GaP‐like longitudinal optical (LO) phonon energies increased systematically as the number of monolayers was increased from n = 1 to n = 2. A significant broadening of the phonon line shapes was also observed for the n = 1.7 and n = 2 samples. In contrast, for samples grown at 425 °C, both the increase of the LO phonon energies and the broadening of the phonon line shapes were observed only when n = 1.7. Our results demonstrate that the optical phonons in the (GaP)_n/(InP)_n SPS structures are significantly affected in the occurrence of LCM related to the growth temperature and the number of monolayers.Copyright © 2009 John Wiley & Sons, Ltd.     

Influence of changing the forbidden-band gap on radiation frequency sweeping of quantum-well heterolasers

The principal features of changing (sweeping) the "instantaneous" emission frequency of quantum-well heterolasers as functions of the modulation frequency and depth and the constant component of the pump current when tuning the lasing frequency within the gain band are established using numerical modeling. The active medium is described in the framework of a two-band model with similar distributions of levels in subbands of electrons and holes assuming transitions between ground subbands with no k-selection rule. Emission frequency sweeping occurs because of changes in both the index of refraction of the active medium due to a variation in the concentration of non-equilibrium charge carriers and the forbidden-band gap. Emission frequency sweeping does not occur at low frequencies of current modulation that correspond with quasi-stationary lasing regimes. The modulation depth of the output radiation and, therefore, the emission frequency sweeping, also approach zero for the other limiting case of relatively high current modulation frequencies. The sweeping is greatest at intermediate current modulation frequencies. The sweeping value is approximately halved in certain instances by taking into account the change of forbidden-band gap of the semiconductor. In general, the sweeping value is determined by the combined effect of the modulation frequency and depth, the constant component of the pump current, and the spectral position of the laser emission frequency within the gain band. Presented at the VIIth International Scientific Conference "Laser physics and optical technologies," June 17–19, 2008, Minsk, and at the VIIth International Scientific Technical Conference "Quantum electronics," October 13–16, 2008, Minsk, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 76, No. 2, pp. 194–201, March–April, 2009.     

Characterization of phase modulated non-return-to-zero (PM-NRZ) format for DWDM long reach PONs

A technique which has been shown to be effective to mitigate the Rayleigh backscattering (RB) noise and potentially suited for use in the carrier distributed passive optical networks (PONs), uses a phase modulator (PM) at the reflective optical networking unit (RONU) to reshape the spectrum of the upstream non-return-to-zero (NRZ) signal, named PM-NRZ. Since the PM-NRZ signal has a much wider spectrum than the conventional NRZ signal, it would be very interesting to know its performance when applied to the multiple channels, or dense wavelength division multiplexed (DWDM) environment. Here, we, for the first time, show how close the neighboring channels can be located in the network by means of numerical simulations. We also provide solutions to reduce the crosstalk produced by neighboring PM-NRZ channels. Since in the WDM-PON system the distances between different ONUs and the head-end office vary, we also analyze the network performance when the powers of adjacent PM-NRZ signals change.     

Propagation and interaction of ultra-short pulses in quadratic crystals with controlled dispersion

Propagation of ultra-short optical pulses in an environment with quadratic nonlinearity and modulated dispersion is studied. It is shown that periodic modulation of the velocity-mismatch sign and of the third-order dispersion coefficient allows one to suppress pulse scattering and decompression, which noticeably increases their interaction efficiency. Original Russian Text ? A.A. Ivanov, B.B. Petrov, C.C. Sidorov, 2009, published in Vestnik Moskovskogo Universiteta. Fizika, 2009, No. 1, pp. 90–92.