Gaussian-reflectivity mirror resonator for a high-power transverse-flow CO2 laser

A Gaussian-reflectivity mirror resonator is proposed to achieve high-quality laser beams. To analyze the laser fields in a Gaussian-reflectivity mirror resonator, the diffraction integral equations of a Gaussian-reflectivity mirror resonator are converted to the finite-sum matrix equations. Consequently, according to the Fox-Li laser self-reproducing principle, we describe the mode fields and their losses in the proposed resonator as eigenvectors and eigenvalues of a transfer matrix. The conclusion can be drawn from the numerical results that, if a Gaussian-reflectivity mirror is adopted for a plano-concave resonator, a fundamental mode can easily be obtained from a transverse-flow CO2 laser and high-quality laser beams can be expected.     

Numerical simulation of a high-average-power diode-pumped ytterbium-doped YAG laser with an unstable cavity and a super-Gaussian mirror

A numerical technique with which to compute the output characteristics of a solid-state laser with an unstable cavity and a super-Gaussian coupling mirror is proposed. This technique is applied to an Yb:YAG actively Q-switched laser. With this formalism, the mode formation for the fundamental mode is analyzed and the performance achievable by such a laser for various cavity parameters is determined. Then the results obtained with such a cavity are compared with those given for a stable cavity with graded phase output mirror that is also used for obtaining super-Gaussian mode.     

Electromagnetic theory of an open resonator.

Particular higher-order sources give rise to electromagnetic Gaussian beams, which are linearly polarized and have their maximum in the propagation direction. For this dipolar beam the cross-sectional shape changes in the propagation direction. Nodal surfaces exist on which the tangential component of the electric field vanishes in the standing wave that is formed by the two oppositely directed dipolar, electromagnetic Gaussian beams. These surfaces are identified as the mirror shapes for an open resonator that supports this standing wave. For standing waves that have a particular cross-sectional shape at the waist the cross section of the beam near the mirror surfaces is circular. The resonant frequencies for the fundamental transverse mode of such a resonator have been determined as a function of the geometry and the axial mode number. By a perturbation technique the resonant frequency of an open resonator with spherical mirrors has been obtained. This result is valid in only the paraxial approximation. Illustrative numerical results are included.     

Plate mode propagation losses in solidly mounted resonators

This paper investigates the acoustic losses of propagating eigenmodes through the acoustic mirror of a solidly mounted resonator (SMR) to clarify how resonator properties are influenced by reflection coefficients for the thickness shear (TS) wave as well as that for the thickness extensional (TE) wave. To this end, we analyze the effective acoustic admittance for several test structures with different mirror properties. Leaky modes are distinguished from plate-like modes and the propagation losses are quantified by calculating mode quality factors. The dependence of the propagation properties of leaky eigenmodes is compared with the mirror properties in terms of bulk wave transmission coefficients obtained by the one-dimensional Mason?s model. It is shown that the TE-like main mode couples with TS-like spurious modes, which then influence the leaky loss of the main mode as well. The coupling strength is strongly frequency-dependent and drastically changes with the mirror design. This result explains previous experimental results reported on SMR design.     

基于希尔伯特变换结构模态参数识别

应用HHT方法对GARTEUR飞机模型模态参数进行识别,通过采用多通带滤波器对信号进行滤波,较好的解决模态混叠问题,采用NExT法对信号预处理,由EMD分解获得较准确的各阶固有模态函数分量（IMF）,在EMD分解中使用镜像延拓方法对极值点进行处理来抑制端点效应,然后将分解得到的IMF分量进行希尔伯特变换并结合ITD法识别出各阶固有频率和阻尼比。最后对悬臂梁进行数值仿真模拟,并将模态参数识别结果和理论值进行对比,并运用此方法进一步识别GARTEUR飞机模型固有模态参数。     

Modes of resonators with dispersive phase-conjugate mirrors

We investigate the transverse and longitudinal modes of a resonator consisting of a spherical mirror, a Gaussian aperture, and a dispersive phase-conjugate mirror (PCM). The photorefractive PCM introduces spatial dispersion in the form of lateral and focal shifts along with temporal dispersion. For both degenerate and nondegenerate operation, the decentered Gaussian beam was found to be a mode whose peak intensity is displaced from the resonator axis. In the nondegenerate case, the components of a mode oscillating at a pair of frequencies that are up and down shifted from the pump frequency by the same amount have different spatial distributions, so that the intensity pattern moves periodically across the output mirror. The resonance frequencies of the longitudinal modes are calculated numerically.     

Numerical calculations of diffraction losses in advanced interferometric gravitational wave detectors

Knowledge of the diffraction losses in higher-order modes of large optical cavities is essential for predicting three-mode parametric photon-phonon scattering, which can lead to mechanical instabilities in long-baseline gravitational wave detectors. We explore different numerical methods in order to determine the diffraction losses of the higher-order optical modes. Diffraction losses not only affect the power buildup inside the cavity but also influence the shape and frequency of the mode, which ultimately affect the parametric instability gain. Results depend on both the optical mode shape (order) and the mirror diameter. We also present a physical interpretation of these results.     

空间相机主镜加工状态下的有限元分析

采用轻量化结构的空间相机主镜,因为镜体力学分布较传统的实心镜体复杂得多,因而轻型镜面的加工较之实心镜面复杂得多。镜子在加工中的支撑方式和受力状态是影响镜面加工精度的主要因素之一,没有严格准确的数学分析难以保证镜子的加工精度,本文用有限元法首次对正在加工中的空间相机主镜进行力学分析,根据变形规律设计了几种支撑方案,从中选定了主镜的最终支撑结构。镜面面形的加工精度实现PV值λ／10,MS值λ／62。满足使用要求。     

Focusing characteristics of a planar solid-immersion mirror

The focusing characteristics of a planar waveguide solid-immersion mirror with parabolic design have been investigated. The solid-immersion mirror is integrated into an optical waveguide, and light focusing is achieved with a parabolic mirror parallel to the waveguide plane and waveguide mode confinement normal to the waveguide plane. Optical-quality tantala silica planar waveguides can be obtained by evaporation. The parabolic sidewall reflects over 50% of the incident waveguide mode and generates a diffraction-limited focus. The measured spot size for the solid-immersion mirror described here is less than one third of the wavelength. Polarization analysis shows that the electric field near the focal region has components parallel and normal to the polarization state of the incident beam. The planar solid-immersion mirror is essentially free of chromatic aberration, and the alignment of the illumination beam is within a fraction of degrees.     

Design and performance of diffractive optics for custom laser resonators

Diffractive optical elements are used as end mirrors and internal phase plates in an optical resonator. A single diffractive end mirror is used to produce an arbitrary real-mode profile, and two diffractive mirrors are used to produce complex profiles. Diffractive mirror feature size and phase quantization are shown to affect the shape of the fundamental mode, the fundamental-mode loss, and the discrimination against higher-order modes. Additional transparent phase plates are shown to enhance the modal discrimination of the resonator at the cost of reduced fabrication tolerances of the diffractive optics. A 10-cm-long diffractive resonator design is shown that supports an 8.5-mm-wide fundamental mode with a theoretical second-order mode discrimination of 25% and a negligible loss to the fundamental mode.     

新型静电驱动双向平动MEMS变形镜(英文)

设计了一种新型静电驱动双向平动的MEMS变形镜,其结构包括3部分:中央下电极,4个静电驱动杠杆结构以及反射镜面(上电极).该变形镜有两种驱动模式:向下驱动模式和向上驱动模式.在向上驱动时,4个杠杆结构实现了位移的放大;向下驱动时,利用了非线性弹性系数法扩展了镜面的平动范围.采用表面硅工艺完成了变形镜的加工.通过白光干涉仪对变形镜的测试表明:在向上驱动模式下,变形镜在驱动电压为31 V时位移达到1.1μm;在向下驱动模式下,变形镜在6 V的驱动电压下位移达到1.1μm.变形镜的总行程为2.2μm,达到了同样工艺下传统变形镜的3倍左右.     

Modal properties of an external diode-laser-array cavity with diffractive mode-selecting mirrors

Coupled mode theory is used to describe the behavior of an external laser cavity consisting of a diode laser array and a diffractive mode-selecting mirror. The mirror is designed to establish a uniform-amplitude, uniform-phase fundamental mode. Coupled mode theory is then used to study the behavior of higher-order modes. We show that the maximum discrimination against higher-order modes occurs when the round-trip cavity length satisfies certain Talbot relations. In addition, this high modal discrimination can be maintained for arrays with large numbers of lasers without incurring significant loss in the fundamental mode.     

Properties of a phase-conjugate etalon mirror and its application to laser resonator spatial-mode control

The concept of a phase-conjugate etalon mirror consisting of one flat and one aspheric surface is introduced. This new element can be used as an end mirror of a conventional resonator to promote spatial-mode selection and mode shaping. A phase-conjugate etalon designed for the fundamental Gaussian mode is experimentally implemented and tested with a single-mode He-Ne laser.     

Exact revision of the elliptically bent mirror theory

One of the main hurdles for nanometer focusing by a bending mirror lies in the theoretical surface errors by its approximations used for the traditional theory. The impacts of approximations and analytical corrections have been discussed, and the elliptically bent mirror theory has been described during exact mathematical analysis without any approximations. These approximations are harmful for the focusing system with bigger grazing angle, bigger mirror length, and bigger numerical aperture. The properties of equal-moment and single-moment bent mirrors have been described and discussed. Because of its obvious advantages, a single-moment bending mirror has high potential ability for nanometer focusing.     

卡塞格伦望远镜系统杂散光分析

白天强背景光将通过望远镜系统对后续观测系统产生很强的杂散光。在这种条件下，卡塞格伦望远镜系统杂散光成对称分布，主要来源于望远镜筒壁的漫反射光、马蹄镜的直接反射光和次镜支架的漫反射光。理论分析和数值模拟表明，系统结构中马蹄镜和次镜的作用最大，而观测视场角的增大将导致杂散光通量的急剧上升。     

等厚双层涂层材料受切向集中力作用的三维理论解

基于三维弹性问题Papkovitch一般解和半无限体表面受切向集中力作用的基本解,通过利用镜像点方法和Dirichlet等值性原理,推导了等厚双层涂层材料受切向集中力作用的显式理论解.该理论解是以固定在各镜像点上的局部坐标系下的位移函数的形式给出的.由于载荷点通过涂层自由表面和界面的反复映射,可产生无穷多个镜像点,但最后的数值验算表明,我们只需考虑有限个镜像点,即可获得足够精度的解,这不仅说明推导的正确性,而且也表明只有前面几个镜像点的位移函数才对结果有较大影响.该理论解还可用作格林函数,进一步求解复杂问题的理论解.     

Diffractive optical elements for intra-cavity beam-shaping of laser modes

Abstract

Diffractive optical elements (DOE) are applied as intra-cavity mode selection devices for customizing the fundamental mode of laser resonators for high power laser systems. Using a phase-conjugating mode selecting element (MSE) in a laser oscillator, we are able to produce a good approximation to a super-Gaussian mode with a near flat intensity profile. This offers higher energy extraction from any following laser amplifiers compared to an unmodified Gaussian TEM₀₀ mode. Two different designs for operation in a 1 m cavity length Nd:YAG master oscillator are presented. Both designs are surface relief phase elements fabricated in fused silica using photolithography with reactive-ion etching to produce 16 level elements for use in transmission. One element is designed to replace the cavity end mirror, while the other stands off an arbitrary distance from the end mirror. A novel iterated design for these transmissive elements is introduced. Numerical results and experimental measurements are presented and discussed.     

Reverse-wave suppressor mirror effects on cw HF unstable ring laser performance

The effect of a reverse-wave suppressor (RWS) mirror on the performance of a multiline cw HF ring laser was measured and theoretically discussed. Forward-wave far-field brightness was found to be very sensitive to the tilt of a conventional RWS mirror, being reduced by a factor of 2 for a 200-microrad RWS tilt. An aberrated mirror was found to reduce this tilt sensitivity by nearly an order of magnitude. Also discussed is a possible mechanism whereby the tilt of the conventional RWS mirror reinforced a higher-order mode in the forward-wave output, yet the tilt of the aberrated mirror did not.     

Picosecond pulse generation and its simulation in a nonlinear optical mirror mode-locked laser

A nonlinear mirror composed of a lithium triborate crystal and a dichroic output coupler is used to passively mode lock an Nd:YVO4 laser that is pumped by a diode laser array. A mode-locked output power of 3.2 W, a repetition rate of 178 MHz, a pulse width of 8.4 ps, and a beam quality parameter (M2) of 1.27 are obtained at 1064 nm for a pump power of 10.0 W. The numerical simulation for the steady-state pulse width agrees well with the bandwidth-limited value. A double-pass average gain g(ave) is defined by considering the constancy of the output energy. In the simulation g(ave) is kept as a free parameter, and its value required for the bandwidth-limited pulse is found to be 0.047, whereas its calculated value, based on our definition, is 0.057.     

A novel temperature sensor and variable optical attenuator based on a high-birefringence fiber loop mirror

A novel variable optical attenuator based on a high-birefringence fiber loop mirror is described. The fiber loop mirror is placed in a controlled-temperature chamber. As the temperature is increased, the transmission spectrum exhibits a shift, while virtually not changing its shape. The degree of attenuation at a wavelength of 1558 nm exceeds 30 dB, while the introduced losses are about 2.5 dB. The temperature-induced shift of the spectrum is strictly linear. The response of the temperature sensor based on this effect is linear with a correlation coefficient of 0.9997. The obtained experimental data are interpreted based on the results of numerical modeling.