A novel analytical expression of saturation intensity of InGaAsPtapered traveling-wave semiconductor laser amplifier structures

We have proposed a novel approximate analytical expression for saturation intensity for tapered traveling-wave semiconductor laser amplifier structures. The application of this analytical expression of saturation intensity has been demonstrated by considering the effect of gain saturation on polarization sensitivity of two tapered amplifier structures, linear and exponential tapered amplifier structures. It is found that polarization sensitivity of the tapered amplifier structure is several decibels higher than that of passive tapered waveguides in unsaturated condition. Polarization sensitivity of the two tapered amplifier structures has also been investigated in a highly saturated condition. The combined effects of mode conversion and gain saturation on fundamental TE gain have also been investigated using the proposed analytical expression for saturation intensity     

Guiding Properties of Silica/Air Hollow-Core Bragg Fibers

The guiding properties of realistic silica/air hollow-core Bragg fibers have been investigated by calculating the dispersion curves, the confinement loss spectrum, and the field distribution of the guided modes through a full-vector modal solver based on the finite-element method. In particular, the silica bridge influence on the fundamental mode has been analyzed by comparing the properties of an ideal structure, without the silica nanosupports, and of two realistic fibers, with squared off and rounded air-holes. Simulation results have demonstrated the presence of anticrossing points in the dispersion curves, associated to the transition of the fundamental mode into a surface mode. It has been shown that surface modes are responsible for the sharp loss peaks, also experimentally measured, which pollute the loss spectrum of the fundamental mode and of the higher order modes. Then, the influence on the guiding properties of each geometric characteristic in the hollow-core Bragg fiber cross-section has been deeply investigated, thus showing which parameter it is better to change in order to properly modify the loss values or its spectral behavior. Moreover, in order to improve the loss properties of hollow-core Bragg fibers, the number of silica and air layers in the fiber cladding has been increased, and the layer thickness has been modified. Results have shown that the first change is more effective for the loss reduction, while the second is useful for a spectral shift. Finally, among the different possible applications, the feasibility of a DNA biosensor based on a hollow-core Bragg fiber has been demonstrated.      

Bending Effects on Highly Birefringent Photonic Crystal Fibers With Low Chromatic Dispersion and Low Confinement Losses

Photonic crystal fibers (PCFs) with elliptical air-holes located in the core area that exhibit high birefringence, low losses, enhanced effective mode area, and low chromatic dispersion across a wide wavelength range have been presented. The effects of bending on birefringence, confinement losses and chromatic dispersion of the fundamental mode of the proposed PCFs have been thoroughly investigated by employing the full vectorial finite element method (FEM). Additionally, localization of higher order modes is presented. Also, effects of angular orientation on bending loss have been reported. Significant improvement on key propagation characteristics of the proposed PCFs are demonstrated by carefully altering the desired air hole diameters and their geometries and the hole-to-hole spacing.     

High efficiency and output power from second- and third-harmonicmillimeter-wave InP-TED oscillators at frequencies above 170 GHz

InP TED (transferred electron device) oscillators have been experimentally investigated for frequencies between 170 and 279 GHz. It has been found that output powers of more than 7 and 0.2 mW are possible at 180 and 272 GHz using second- and third-harmonic mode operation, respectively. Conversion efficiencies of more than 13% and 0.3% between fundamental and second harmonic and fundamental and third harmonic, respectively, have been found. The conversion efficiencies are comparable to GaAs TEDs. The output powers, conversion efficiencies, and tuning ranges (more than 22%) are the biggest reported for InP TEDs at these frequencies. The output power at third harmonic was sufficient for supplying a superconducting mixer with local oscillator power     

Analysis of modal characteristics and thermal effects on photonic crystal fiber laser

In this paper, thermo-optics effects on fiber lasers based on index-guiding photonic crystal fibers (PCFs) are studied. The modal characteristics of the PCF lasers are discussed in terms of the thermal effects. Modal analysis of PCF has been done with help of plane wave expansion and supercell lattice methods. The thermal analysis is performed by obtaining the effective index of fundamental mode and core confinement factor (CCF) when the fiber laser was pumped. The results show that PCF laser structure has negligible modal sensitivity to thermal load in comparison to conventional fiber laser. Analyzing the mode performance of the fiber laser under thermal load shows that the assumed PCF laser at 80 W/m pump power show ∼500° rise in temperature and 0.34% propagation constant variation in fundamental mode index and 0.04% CCF variation in fundamental mode.     

光纤基模光强分布等容性质的广义证明

应用高斯近似理论,本文解析分析了任意折射率剖面光纤的基模的光强分布性质,导出了关于等折射率剖面容量的任意光纤之间,基模的光强分布一致地趋于重合的普适性判据,即２∫＾１ｆ＾２（Ｒ）ＲｄＲ≤０．２,这种情形下基模的归一化模斑半径为「２ｌｎ（Ｖ√Ω／π／ρ）」＾１／２。     

A wave approach to hollow cylindrical electron cyclotron masers

A wave method has been applied to analyze and derive the dispersion equation for hollow cylindrical ECM's, which features in simpler mathematics and clearer physical interpretation. Besides, the effects of space charge Columb field have been taken into account. For the time being, the result is more likely suitable for fundamental mode interaction.     

Investigations of complex modes in a generalized bilateral finlinewith mounting grooves and finite conductor thickness

A generalized bilateral finline with mounting grooves and finite conductor thickness is analyzed by a full-wave mode-matching method. The final nonstandard eigenvalue equation is derived from the unknown coefficients in the slot regions to reduce the size of the matrix equation. The convergence studies of the mode-matching method is first studied for the fundamental mode of a symmetric bilateral finline. Both the propagation constant and the characteristic impedance as defined by the power-voltage relationship are analyzed and compared to the existing data. Excellent agreement is obtained. The effects of metallization thickness and mounting grooves are discussed. The accurate results obtained for the fundamental mode by the mode-matching method with respect to both relative and absolute convergence were also obtained for the complex modes of the finline. The dispersion characteristics of the fundamental, higher order, evanescent, and complex modes are presented for an asymmetric bilateral finline. The effects of mounting grooves and metallization thickness on the complex mode propagation constants are investigated and discussed     

高重复频率飞秒激光烧蚀熔融石英制作单偏振微结构波导

利用中心波长为1040nm、脉宽为190fs、重复频率在200～5000kHz之间可调的飞秒激光对熔融石英进行微加工。研究了烧蚀阈值随脉冲重复频率、扫描速度的变化规律,阐明不同参数下热扩散效应及热累积效应对烧蚀过程的主导作用。在最优化条件下,制作了双线波导,可以对1040nm激光实现圆形基模传输。进一步制作了椭圆晶胞的六角微结构波导,对1040nm激光可以输出近高斯强度分布的基模,模场面积达到247.48μm2。该微结构波导可实现单偏振传输,消光比达9.05,波导数值孔径约0.017。     

Circular groove guide for short millimeter and submillimeter waves

A new type of groove guide, circular groove guide, has been designed for use as a low losses, high power handling, low dispersion and single mode transmission medium in the short millimetric and submillimetric waves ranges. The characteristic equations of TE modes and TM modes have been developed and the propagation characteristics of the fundamental mode have been discussed. Experimental measurements are in good agreement with theoretical results     

Polarization behavior of birefringent multitransverse mode vertical-cavity surface-emitting lasers

An analysis has been undertaken of the effect of birefringence on the selection of polarization states of a weakly index guided vertical-cavity surface-emitting laser (VCSEL) supporting both a fundamental and a first-order transverse mode. It is shown that for small index steps polarization switching due to spatial-hole burning effects can occur. For larger index steps it is found that higher order modes can emerge which are orthogonally polarized to the dominant polarization of the fundamental mode.     

Reduction of mode Q-factor due to coupling between fundamental and first order whispering-gallery modes in coupled microdisks

The mode characteristics for twin coupled microdisks are investigated by finite-difference time-domain technique. In the coupled microdisks,the same order whispering-gallery(WG) modes can form coupled modes with split mode wavelengths. We find that the coupled fundamental and first order WG modes can have anticrossing mode coupling as their wavelengths approach the same value in some case,which prevents the cross of the coupled mode wavelengths. The anticrossing mode coupling greatly reduces the coupled mode Q-factor,because the coupled mode field distribution transfers between the fundamental and the first order WG modes.     

双包层椭圆光纤的保偏特性

本文对双包层椭圆光纤进行了解析求解，得到了模式特征方程。研究了基模的保偏特性，给出了不同椭圆比下的归一化双折射和模间色散随归一化频率，纤芯内包层相对折射率差，内包层和纤芯半长轴比等参数的变化关系曲线。     

Circular grating-coupled surface-emitter with high-quality focusedoutput beam

We have investigated the effect of resonator size and geometry on the performance of focusing circular grating-coupled surface-emitting lasers. Both the effects of reducing the resonator diameter and introducing an unpumped indentation in the center of the resonator have been studied. We have found that inclusion of such an unpumped central indentation can greatly improve the spatial-mode behavior, resulting in a very uniform near-field intensity, single wavelength emission, and a focused spot size only 1.5 times the diffraction limit for a laser oscillating in the fundamental circular mode. Reducing the resonator diameter of lasers without an unpumped central indentation does not offer similar improvement     

失调腔之间的模式耦合与转换

本文理论分析了谐振腔之间调节不好,它们的光轴之间同时存在平行移位和倾斜时,光腔之间的模式耦合和转换.用相对失调参数的函数形式给出了基模到低阶横模之间的模式耦合系数.结果发现,当腔之间失调存在时,厄米-高斯光束奇偶模之间存在耦合;当腔失配和失调满足一定条件时,基模到低阶横模之间的功率耦台系数达到极大值.     

Output mode spectra, comparative parametric operation, quenching, photolytic reversibility, and short-pulse generation in atomic iodine photodissociation lasers

The advantages and disadvantages of the atomic iodine photodissociation laser for high-energy operation have been considered. Laser excitation by both slow and fast flashlamps has been investigated; in slow flash excitation, self-mode-locking occurs frequently and the output-mode characteristics of the laser operated in this way have been studied. The comparative performance of different parent materials for photodissociation has been investigated in the fast flash mode of excitation. This mode of excitation gives higher energy laser output and efficiency and less pyrolysis of the parent material. These observations, and the photolytic reversibility characteristics of different materials in repetitively photolyzed operation, have been discussed in the light of the most recently available kinetic data; i-C3F7I is the material of choice for laser operation. Finally, limits to the ability of atomic iodine photodissociation laser/amplifier(s) systems to generate and amplify short duration, high-energy pulses set by intralevel relaxation effects and coherent interactions are discussed.     

Multimode interference couplers for the conversion and combining ofzero- and first-order modes

Optical waveguide mode-combiners for fundamental and first-order modes, based on multimode interference (MMI) couplers are presented. These devices convert a fundamental mode into a transversal first-order mode and combine it in lossless fashion with a second fundamental mode. They can separate zero- and first-order modes in a common waveguide and allow the splitting and combining of zero- and first-order modes with nonuniform power splitting ratios. Realizations in InGaAsP-InP are demonstrated. These new components have successfully been integrated into all-optical switches and were found to have advantageous characteristics in all-optical devices     

Mode coupling at arc-fusion splices in graded-index fibers

Mode coupling at an arc-fusion splice has been investigated theoretically and experimentally. It has been certified experimentally that fiber parameters change at an arc-fusion splice. A mode transfer matrix has been derived which describes mode coupling at splices with fiber parameter changes along the fiber axis. The mode-coupling effects on the frequency response of spliced graded-index fibers have been investigated. Length dependence of 3 dB bandwidth has been measured for 10 km long graded-index fiber both with and without a splice. It has been clarified that the mode-coupling effect at the splice broadens the 3 dB bandwidth by 10 percent in the long fiber link in comparison with that for the fiber without a splice.     

Harmonic emission from high-power high-frequency gyrotrons

The results of a study of second-harmonic emission from a gyrotron designed for high-power high-frequency operation at the fundamental of the cyclotron frequency are presented. Stable, very narrow bandwidth second-harmonic cavity emission from 209 GHz to 302 GHz has been observed. An output power of 25 kW and efficiency of 6·5% in the TE_{11, 2, 1}, mode at 241 GHz is reported; this represents the highest power obtained to date from a high-frequency (> 100 GHz) harmonic gyrotron. These experiments have been carried out in a cavity for which the mode density is very high; the cavity diameter is approximately six free-space wavelengths for emission at the second harmonic. Mode competition between fundamental and second-harmonic modes is discussed. It is also shown that, in general, gyrotrons designed for high-power low-Q operation in overmoded cavities at the fundamental will also have high efficiencies and strong emission in second-harmonic modes. Prospects for high-frequency harmonic gyrotrons for plasma diagnostics and other applications are described     

An improved multimode horn for Gaussian mode generation at millimeter and submillimeter wavelengths

An improvement on the dual-mode horn for Gaussian mode generation has been developed. Previous designs have used a sudden transition or linear taper to generate the required TE/sub 11/ and TM/sub 11/ mode mixture. In the improved design presented here, an optimized nonlinear taper is used to generate multimodes, which have a fundamental Gaussian mode power fraction of 99% that exceeds that of the corrugated horn (98%). The bandwidth and return loss are both improved over other dual-mode horn designs.