High-frequency optical signal generation in a semiconductor laser by incoherent optical feedback

This work examines the feasibility of using incoherent optical feedback to generate high-frequency optical signals in a semiconductor laser. At a roundtrip feedback distance of 30 cm, a signal appears at around 1 GHz while the laser output is kept in single longitudinal mode and single polarization. The proposed method does not use any external modulation or saturable absorber. Some of the signal's properties are discussed as well.     

Processes limiting the rate of response in photorefractive composites

The processes limiting the rate of response of highly diffractive, reorientationally enhanced photorefractive polymer composite materials are identified from a series of degenerate four wave mixing and Mach–Zehnder interferometric measurements. In the regime of low intensity writing beams charge generation limits the rate of holographic grating formation, but at higher intensities charge transport or reorientation of dye molecules can restrict the rate of grating formation more strongly. A grating risetime of 540 ms is observed in a composite of high dye content with high reorientational mobility of the dye molecules. In this case it is proposed that the charge carrier mobility of the doped poly(N-vinylcarbazole):2,4,7-trinitro-9-fluorenone (PVK:TNF) matrix is the principal limiting factor in grating response rates.     

A novel quasi-phase-matching frequency doubling technique

A new quasi-phase-matching technique for efficient second-harmonic generation is reported. It is based on the spatial periodic modulation of the light intensity along the propagation direction, rather than the conventional spatial periodic modulation of the nonlinear optical coefficients. It can be realized by using a novel dual-channel waveguide frequency doubler structure for the desired light intensity distribution. This dual-channel waveguide device has major advantages including very small beam size, high light intensity within long nonlinear-waveguide interaction length, highly efficient second-harmonic generation, ease in fabrication of the nonlinear channel waveguides without any spatially periodic poling, and low waveguide propagation losses. The new quasi-phase-matching technique can also be applied to third-harmonic generation and other nonlinear optics processes.     

Focusing of electromagnetic waves into a uniaxial crystal

We derive integral representations suitable for studying the focusing of electromagnetic waves through a plane interface into a uniaxial crystal. To that end we start from existing exact solutions for the transmitted fields due to an arbitrary three-dimensional (3D) wave that is incident upon a plane interface separating two uniaxial crystals with arbitrary orientation of the optical axis in each medium. Then we specialize to the case in which the medium of the incident wave is isotropic and derive explicit expressions for the dyadic Green's functions associated with the transmitted fields as well as integral representations suitable for asymptotic analysis and efficient numerical evaluation. Relevant integral representations for focused 3D electromagnetic waves are also given. Next we consider the special case in which (i) the incident field is a two-dimensional (2D) TM wave and (ii) the optical axis in the crystal lies in the plane of incidence, implying that we have a 2D vectorial problem, and derive dyadic Green's functions, integral representations suitable for asymptotic and numerical treatment, and integral representations for focused TM fields. Numerical results for focused 2D TM fields based on these integral representations as well as corresponding experimental results will be presented in forthcoming papers.     

On timing Jitter in wavelength-division multiplexed soliton systems

Collision-induced timing shifts in a wavelength-division multiplexed soliton system are computed when damping, amplification, filtering and positive dispersion management following the loss profile are included. A statistical analysis is presented which takes into account the resulting effect of the large number of collisions occurring in the fiber. Analytic expressions are derived for the root mean square timing jitter and the maximum length of error-free transmission with an arbitrary number of channels. An extensive analysis of system performance corresponding to situations with and without filters and/or dispersion management is carried out.     

Asymptotics of the Interband Light Absorption Coefficient near the Band Edge for an Alloy-Type Model

We find the asymptotics of the interband light absorption coefficient of an alloy-type model in the case when the ground-state energies of the electron and the hole Hamiltonians are finite.     

Effective spectral emissivity measurements of superalloys and YSZ thermal barrier coating at high temperatures using a 1.6μm single wavelength pyrometer

A method which employs an integrating sphere and a single-wavelength (1.6 μm) pyrometer for measuring the spectral effective emissivities of superalloys in the temperature range (=650–1050°C) is described. The spectral effective emissivities for five superalloys, namely, MARM-247, MARM-509, CMSX-4, Inconel-718, N-155 and two René-N6 samples coated with YSZ thermal barrier coating were measured. Correcting the pyrometer measurements for the variations in the object emissivity would reduce the uncertainty in the temperature measurements to <±1%.     

Application of stain-etched porous silicon in light emitting diodes and solar cells

Porous silicon/c-Si heterostructures have been formed by the method of stain etching.The properties of light emitting diodes (LED) and solar cells have been studied. The transport mechanism of the diode has been investigated from the current–voltage characteristics measured at different temperatures (296–380 K). A model based on multi-step tunneling of carriers at reverse and low forward bias (<1 V) and on field tunneling across a narrow barrier at higher forward bias (>1.5 V) is proposed for the LED. In the case of the solar cells the porous silicon is formed in between the fingers of the front grid contact. Application of porous silicon in solar cells results in an increase of the short-circuit current and efficiency of the cells by about 30%.     

光照强度对蛇足石杉共生蓝藻细胞悬浮培养的影响

以蛇足石杉孢子囊为材料,分离出蛇足石杉共生蓝藻细胞,研究光照强度对液体培养的共生蓝藻细胞生长的影响.研究结果表明,在光强500 Lx下,共生蓝藻的生长速率最大;增加光照强度(> 2000 Lx)将抑制共生蓝藻的生长,菌体褪绿变白;在共生蓝藻生长过程中,蓝藻细胞内叶绿素a的含量随着光强的增加而显著减少,这与其在强光照下生物量的降低呈正相关.不同光质实验结果表明,绿光和蓝光条件下蛇足石杉内生藻生长快而红光条件下藻体增殖最慢,这与在绿光和蓝光条件下藻体细胞合成的色素含量增高有密切的关系.实验结论:蛇足石杉共生蓝藻细胞适宜的光照条件是弱光光照,适宜的光质是绿光或蓝光.     

光源稳定性对简并四波混频信噪比影响的实验研究

在碘蒸气中研究了染料激光稳定性对简并四波混频(DFWM)信噪比的影响。实验中采用多模Nd∶YAG激光器倍频的输出泵浦染料激光器(染料PM580溶于酒精)。在具有前向补偿分光系统和图像探测系统的帮助下,在常温常压条件下测得泵浦光在波长为554.013 nm时达到饱和光强为290 μJ。同时发现染料激光器的光稳定性(输出波长范围,光束质量和能量转换效率等)随使用时间,泵浦激光脉冲次数和能量的增加而逐渐减弱。另外通过对比发现在染料激光器输出不稳定时,光源单色性和指向性等指标的变化导致在同一泵浦强度不同时刻获得的DFWM信号强度相差很大,且DFWM信号信噪比低。染料激光器光源稳定性对DFWM信号影响的研究结果对物质的痕迹量探测以及无荧光物质光谱定标等方面的应用具有重要意义。